Docteur De L'

Khaoula Bouzidi

Thèse Dirigée Par

Monsieur Davide Beneventi

Directeur De Recherche

Monsieur Antoine Le Duigou

Monsieur Didier Chaussy

Roberta Madame

Bongiovanni

Didier

THÈSE Pour obtenir le grade de

Formulation d'un biocomposite thermodurcissable à base de l'alcool polyfurfurylique et de cellulose pour le procédé d'impression 3D.

Formulation of a thermosetting biocomposite based on poly(furfuryl alcohol) and cellulose for 3D printing.

pour autant me laisser me soucier des problèmes administratifs que tu as toujours pu résoudre tout seul. Merci aussi pour ta confiance dans mes idées et de m'avoir laissé gérer le projet et prendre des décisions ce fut un plaisir de travailler à tes côtés. Didier je te remercie d'avoir été là quand j'avais besoin de toi, de ton expérience et de ton sens de l'organisation. Tu as assuré un très bon équilibre dans cette équipe de projet puisque tu as toujours su donner des conseils précis et de bien m'encadrer surtout pendant ma dernière ligne droite de thèse. Malgré tes occupations et tes responsabilités pendant la direction du labo tu étais toujours présent quand il le fallait. Roberta je te remercie aussi pour tes réponses et tes encouragements même depuis un autre pays.

Je voudrais remercier plus particulièrement Pr. Alessandro Gandini pour tout ce qui a apporté pour ce projet. Une chose est sûre, cette thèse n'aurait jamais été pareille sans vos conseils et votre assistance. La chimie n'a jamais été mon point fort et encore moins la chimie des furanes mais votre patience et passion pour la science (même avec un membre cassé !) ont été suffisants pour que je comprenne tous ces spectres RMN et les réactions de réticulation. Je vous serai à jamais reconnaissante pour l'aide et l'encouragement que vous m'avez apporté.

Un grand merci à la région Auvergne Rhône Alpes et l'UGA qui ont financé ce projet et qui ont permis de le prolonger dans le contexte sanitaire particulier. Grâce à eux, j'ai pu finir ma thèse sereinement. Je voudrais également remercier nos différents collaborateurs industriels et Qui dit LGP2 et vie de laboratoires, dit un beau groupe des doctorants et post doctorants soudés (bon aller je rajoute aussi les ingénieurs qui monopolisent LGP2 dernièrement). Merci à vous tous pour tous les moments partagés. Je n'oublierai jamais nos pause-café à 16h et nos discussions parfois futiles et parfois profondes et scientifiques. Je n'oublierai pas votre soutien pour la préparation de la soutenance et vos remarques toujours constructives et bienveillantes.

Nos soirées BIM et nos afterwork dans le PPM seront à jamais gravés dans ma mémoire. Je remercie en particulier mes collègues de bureau Fanny, Amélie, et Myriam j'ai eu de la chance de pouvoir partager mes journées avec vous dans le C104 (la vue de notre bureau sur Belledonne aide aussi hein !). Un grand merci à Malek pour son soutien pendant toutes mes années de thèse mais surtout pendant la soutenance, merci aussi pour les heures de rälement dans ton bureau.

Une thèse de doctorat n'est pas uniquement le fruit d'un travail scientifique et ne se fait pas seulement entre les murs d'un laboratoire et sur un campus. Une thèse c'est aussi le résultat des expériences cumulées et grâce au soutien apporté au quotidien et pendant des longues années d'études. Ainsi je voudrais remercier mes amis extérieurs au labo pour leur soutien. Je remercie en particulier Nahla ma « compagnon de route » tu as toujours été à mes côtés pour me soutenir et m'encourager.

Enfin, le plus grand des mercis à ma grande famille. Je suis incapable de vous remercier assez pour tout ce que vous avez fait (et vous continuez à faire) pour moi. Un paragraphe, une page voire un autre mémoire de 300 pages ne serait suffisant pour exprimer ma profonde gratitude.

Cette thèse et tous mes succès passés et futurs sont dédiés à ma chère maman Halima Bouzidi qui malgré tous les défis de la vie a su me donner tout ce dont j'ai besoin…une affection profonde et un amour pur. Tu es une femme forte, douce et intelligente et tes sacrifices ne seront jamais en vain. Je remercie chacun de mes oncles, mes tantes et mes cousins qui sont certes (très) nombreux mais qui ont tous d'une façon ou d'une autre une influence positive sur ma vie personnelle et mon cursus académique. Je voudrais tout de même exprimer mes profonds remerciements à mes deux soeurs Intiss(ar) et Tess(nime).

dditive manufacturing, also referred to as 3D printing or additive fabrication, is a revolutionary manufacturing technology receiving a significant interest in recent years. It is indeed revolutionary and also evolutionary, provoking manufacturers to reconceptualize the approach used to develop, produce and design objects. Due to its adaptability and controllable parameters, a myriad of applications is possible. What is more, in a time where environmental concerns are increasing, this process has low consumables, offers customizable object geometry using the same machines, and is also cost-effective [START_REF] Yang | Cellulose, hemicellulose, lignin, and their derivatives as multi-components of bio-based feedstocks for 3D printing[END_REF].

More recently, the increasing use of 3D printing has been promoted by a series of research and development works conducted over the last several decades. As one can see in Figure A-0-1, the number of publications dealing with additive manufacturing has been increasing since 2000 to reach over 45,000 publications by 2020. A closer look at these publications suggests that the majority of the references displayed by the scientific database, scifinder, were patents. This trend highlights the innovation capacity of additive manufacturing and also the growing interest this new fabrication process generated among scientists. For instance, composite printing is one of the less explored tracks. However, it can enable the development of high performance materials outstanding conventional single-phase materials. On the same graph in Figure A-0-1, the evolution of publication numbers dealing with 3D printing of composite is following the same tendency as that of additive manufacturing with almost 30% of the total publication mentioning composite. However, biocomposite additive manufacturing is not yet fully explored representing less than 0.5% of published papers.

Therefore, a handful of researchers are focusing their attention on the development and study of new biocomposites to fill this gap (Kariz et al., 2016;[START_REF] Kucherov | Three-Dimensional Printing with Biomass-Derived PEF for Carbon-Neutral Manufacturing[END_REF]Li et al., 2017;[START_REF] Montalvo Navarrete | Thermal and mechanical behavior of biocomposites using additive manufacturing[END_REF]. Combining bio-based thermoplastics with natural fillers and natural fibers has been studied, highlighting that natural fillers can effectively increase the mechanical performances of most thermoplastics used in fused filament fabrication (Nguyen et al., 2018;[START_REF] Tao | Development and Application of Wood Flour-Filled Polylactic Acid Composite Filament for 3D Printing[END_REF].

Moreover, polymers derived from the vegetal biomass are interesting candidates for the development of novel printing 'inks' by direct ink writing as it is one of the more abundant resources in nature [START_REF] Liu | Current advances and future perspectives of 3D printing natural-derived biopolymers[END_REF]. For instance, the use of cellulose was studied by several researchers and it has been shown to be highly interesting in tissue engineering and many other bio-medical applications. However, several challenges emerged such as poor/no shape fidelity and a high process cost [START_REF] Wang | 3D printing with cellulose materials[END_REF].

On another note, alongside the evolution of direct ink writing as a reliable additive manufacturing technique, the use of thermosetting polymers as matrix offering better thermal stability and high mechanical performances has been recently considered as a viable option [START_REF] Peerzada | Additive Manufacturing of Epoxy Resins: Materials, Methods, and Latest Trends[END_REF].

A

General introduction

Khaoula BOUZIDI 2022 17 Despite the high performance of these composite materials, the most commonly used materials are fossil based. Therefore, the use of bio-based thermosetting resin and natural fillers remains rather unexplored. In due course, the work conducted in this PhD project proposed the development of a biocomposite with the use of a bio-based thermosetting resin alongside cellulose. This would enable the development of a fully biomass-based composite with high thermal stability conferred by the matrix and adequate shape fidelity. The experimental approach and main findings obtained during this PhD project are presented in 5 chapters.

The thesis begins with a literature review in Chapter 1, which evaluates the current state of the art of additive manufacturing by material extrusion and more specifically composite material manufacturing and their ensuing performances. This chapter ends with an explication of the production and polymerization of the thermosetting resin used in this work, i.e. poly(furfuryl alcohol), and its use in composites production.

The used materials and methods are detailed in Chapter 2, where a detailed description of the commercial products is provided along with the employed characterization techniques and conditions.

General introduction

Khaoula BOUZIDI 2022 18 To gain a better insight on the composition of used polymers, they were analyzed and the ensuing results are displayed in Chapter 3.

Chapter 4 is dedicated to the development, characterization and optimization of composite slurries for direct ink writing. The first part of this chapter exposes the results of the development of a structural ink and the characteristics of the ensuing cured composite. The second part deals with the development of a functional ink yielding electrically conductive prints.

Chapter 5 is dedicated to the study of the process. In the first part of this chapter, a general study to understand the curing and solidification step of the used polymer was conducted. In the second part, the effect of different printing parameters on the mechanical performances and printability are investigated.

For a clear and quick overview of the thesis content, the general layout and structure are displayed in 1.1 Composite materials

Generalities

Composite materials are defined as a combination of several macro or micro components that differ in size, morphology, composition and that are not soluble one in another [START_REF] Smith | Principles of materials science and engineering[END_REF]. A more recent definition considers composite materials as the result of a consolidated mixture of two or more macro, micro, or nano components after optimal processing. The components can have different morphology, size, and chemical composition. One constituent plays the role of the matrix while the other constituents provide reinforcement or special functionalities "ex-situ" or "in-situ" (Pech-Canul and Valdez-Rodriguez, 2015). The author here eliminates the "insolubility" restriction as one can identify many composite materials with components that are soluble in the matrix or the creation of new components in-situ.

Composites are not new materials; they have been created by humans for 5,000 to 6,000 years. In ancient Egypt, in some parts of Asia and Africa, and America, composite materials based on wattle and daub were used in building monuments and structures. Composites in more advanced form were used by Mongols around 1200 AD especially in the fabrication of more performant reinforced bows which helped the empire to spread across Asia [START_REF] Todd | What is the Definition of a Composite Material?[END_REF].

Composite as we know today were first used in the 20 th century with the invention of plastics and in particular with the discovery of glass fibers in 1935. By 1945, over 3 million kg of glass fiber were used in composites, essentially for military uses because of wartime. The advantages of composites especially fiber-reinforced plastics (FRP), such as their lightweight and corrosion resistance, allowed them to spread and gain the interest of other markets like transportation, automobile, and construction [START_REF] Ngo | Introduction to Composite Materials[END_REF].

In general, a composite material is constituted of two components:

• The first is the discontinuous phase usually fibrous or in form of particles and provides mechanical strength (or other functions depending on the use). This component is called fillers.

• The second is the continuous phase which is in the case of FRP a polymer that plays the role of binder and provides also efforts distribution and protects fillers from the external environment. This component is called the matrix.

In addition to these two main regions, we can add the interphase region or also known as the interface which also plays an important role in the composite final properties [START_REF] Cantwell | The impact resistance of composite materials -a review[END_REF][START_REF] Kalpakjian | Manufactura, ingeniería y tecnología[END_REF].

Any material can be used as a matrix but the most used matrices are ceramics, metals, and polymers.

In reality, the vast majority of matrices are polymer-based by either using thermoplastics or thermosetting which present very different properties [START_REF] Haddout | Cours Les Matériaux composites à matrices polymériques 76[END_REF][START_REF] Ngo | Introduction to Composite Materials[END_REF]. The classification into thermoplastics and thermosets defines the processability of material in respect to temperature changes [START_REF] Swallowe | Mechanical Properties and Testing of Polymers[END_REF].

Thermoplastics can be melted and flow when heating because of either exceeding the glass transition temperature (Tg) or because of crystal melting. This makes the thermoplastics easily shaped and processed as they maintain the desired shape by dropping the temperature below their melting point to obtain a glass-like solid. This process can be repeated many times but as these materials melt and solidify repeatedly they can lose some of their properties hence the limitation of recyclability of these thermoplastics [START_REF] Jones | Mechanics Of Composite Materials[END_REF][START_REF] Swallowe | Mechanical Properties and Testing of Polymers[END_REF].

Thermosets on the other hand, are materials that can be transformed from a liquid to a solid state after undergoing chemical changes and reactions called curing or crosslinking reactions. The uncured liquid state is due to the small unlinked molecules forming the uncured resin, these molecules are called monomers. The curing reaction can be triggered by a thermic treatment (heat, radiation) or by physio-chemical treatment by introducing a catalyst or a hardener. During this reaction, monomers would polymerize forming longer molecules called macromolecules or chains which crosslink to form a three-dimensional network causing the material solidification. Unlike thermoplastics, this solidification or crosslinking step is irreversible and the solid-state is permanent causing a permanent deterioration of the material under high temperature [START_REF] Jones | Mechanics Of Composite Materials[END_REF][START_REF] Ngo | Introduction to Composite Materials[END_REF][START_REF] Swallowe | Mechanical Properties and Testing of Polymers[END_REF].

Broadly, thermoplastics are considered as linear or branched polymers, and thermosets as rigid crosslinked molecules. Interestingly, these two polymer-based materials can have extremely varied and different mechanical and physico-chemical properties which are summarized in Table 1-1. The manufacturing of composite and the matrix choice is based on these properties related to the final use of the composite. Both thermoplastics and thermosets have their places in the market, dominated by thermosets with a 60% of the composites market [START_REF] Haddout | Cours Les Matériaux composites à matrices polymériques 76[END_REF][START_REF] Swallowe | Mechanical Properties and Testing of Polymers[END_REF].

Chapter 1: State of the art Khaoula BOUZIDI 2022 26

Table 1-1: comparison between Thermoplastics and thermosets [START_REF] Haddout | Cours Les Matériaux composites à matrices polymériques 76[END_REF][START_REF] Ngo | Introduction to Composite Materials[END_REF]. Currently, thermosets count for roughly 75% of plastics mass used in matrices versus 25% for thermoplastics [START_REF] Biron | Chapter 6 -Thermoplastic Composites[END_REF]. The former polymers are classified into polyester resins, epoxy resins, vinyl ester resins, phenolic, polyurethane, and other high-performance resins like cyanate esters.

When it comes to the latter polymers we can classify them by their use:

• Commodities use: Polyethylene (PE), Polypropylene (PP), polyvinyl chloride (PVC), and polystyrene (PS).

• Technical thermoplastics: Polyamide (PA), polyacrylics (PMMA), polyacetal (POM), polycarbonate, polyphenylene oxide (PPO), or polyphenylene ether (PPE) and thermoplastic polyesters (PET).

• Specialties thermoplastics: polysulfone (PSU), polyphenylene sulfide (PPS), fluoroplastics, polyetheretherketone (PEEK), polyetherimide (PEI), and polyamide-imide.

Nowadays, we can find many plastic materials on the market The properties and cost of some of the most commonly used plastics are summarized in Table 1-2.

As for fillers, we can find the use of different fibers and particles, and perhaps the most used and the most efficient are glass fibers, carbon fibers, and aramids fibers. Recently, besides these synthetic fibers, one start also noticing the use of natural fibers which are less efficient but their environmental impact makes them attractive especially in recent years.

Composite materials offer many advantages over conventional materials such as steel or aluminum.

Perhaps the most obvious one is the high strength to weight ratio easily noticeable in 60 MPa•cm 3 /g for the steel. The mechanical performances of these plastics can be considerably increased by coupling them with fibers such as glass fiber and carbon fibers making the strength reach until 500 MPa (Çeçen et al., 2008). This is definetly an asset for the aerospace and transport industry. Composites have also good fatigue properties and corrosion resistance and high anisotropy.

Coupled with these assets, an easier and more cost-effective manufacturing process makes FRP more attractive. Composites, like conventional materials, have limits that depend on the used matrix, filler, and the compatibility between them. More general known drawbacks of FRP, comparing to more conventional materials, are limited fire resistance, recycling issues, limited resistance under heat, and their response to localized impact loading [START_REF] Cantwell | The impact resistance of composite materials -a review[END_REF][START_REF] Km | Fiber Reinforced Composites -A Review[END_REF].

Table 1-2: Properties of some plastic matrices [START_REF] Biron | Chapter 6 -Thermoplastic Composites[END_REF][START_REF] Haddout | Cours Les Matériaux composites à matrices polymériques 76[END_REF][START_REF] Ngo | Introduction to Composite Materials[END_REF]. Furthermore, these multi-component materials make the reuse or recycling very challenging due to the difficulty in separating different components even when using thermoplastic (La [START_REF] Mantia | Green composites: A brief review[END_REF][START_REF] Netravali | Composites get greener[END_REF]. Unfortunately, this makes these materials environmentally unfriendly. Moreover, plastic, the base materials of most composites, is a fossilbased material that is non-renewable (La [START_REF] Mantia | Green composites: A brief review[END_REF][START_REF] Marsh | Composites that grow in fields[END_REF].

Polymer
Chapter 1: State of the art Khaoula BOUZIDI 2022 28

Biocomposites in virtue of a growing interest in sustainability

As discussed previously, the majority of plastic matrices and fibers used in FRP are not renewable and has a high environmental impact. This issue was starting to be more conspicuous in the last 20 years. Thusly, scientific researcher started to get more interested in finding bio-based substitutes to petroleum-based polymers traditionally used in composite manufacturing. These fully or partially bio-based composites referred to as biocomposites or 'green composites' have a lower impact on the environment and are renewable. 'Greener' substitutes can have limited mechanical properties comparing to traditional FRP, especially when compared to glass and carbon fibers. However, these materials can be great candidates for manufacturing secondary and tertiary structures (packaging, panels, casing, etc.) (La [START_REF] Mantia | Green composites: A brief review[END_REF][START_REF] Netravali | Composites get greener[END_REF]. As we can see in Figure 1-1, the scientific interest in composites has never declined over the years and especially during these last 5 years. On the other hand, researchers only started giving green composite a chance in the late nineties, and ever since the publications number on environmentallyfriendly composites kept increasing. However, the ratio between the research concerning composites in general and those concerning green composites is decreasing in this last decade despite the raising environmental consciousness among consumers, manufacturers, and governments. To increase the potential of the use of natural fibers and matrices, researchers need to develop enhanced green composites with better mechanical properties and with a wider range of applications [START_REF] Misra | 11 -Creep and fatigue of natural fibre composites[END_REF]. Nonetheless, as ambitious as this may be, it is a difficult task, especially when using natural fibers with complex structures.

Chapter 1: State of the art Khaoula BOUZIDI 2022 29

1.1.2.1 Natural fibers for 'greener' composites

Carbon dioxide neutrality of natural fibers that grow in crop fields and their renewable nature provide good potentials to use them as reinforcement in FRP [START_REF] Ilcewicz | Interlaminar fracture toughness testing of composite mode I and mode II DCB specimens[END_REF]. Natural fibers, as their name suggests, can be found in nature but from different sources, it may be plants, animals, or minerals. According to literature reviews, plant fibers are the most popular among the natural fibers in fiber-reinforced composites [START_REF] Akil | Kenaf fiber reinforced composites: A review[END_REF]. Lignocellulosic fibers can also be subdivided according to their origin from the plant itself. It is also possible to find a more general classification into wood-fibers and non-wood fibers. Therefore, nature provides a wide range of natural fibers classified in Figure 1-2 alongside some examples of the most used fibers in composites.

Figure 1-2: A general classification of lignocellulosic fibers adapted from [START_REF] Akil | Kenaf fiber reinforced composites: A review[END_REF].

Chapter 1: State of the art Khaoula BOUZIDI 2022 30 These fibers are designed by nature and have a rather complicated structure. Structure-wise, these fibers are considered like natural composite themselves; the reinforcement component is the crystalline cellulose micro-fibrils and the matrix is the amorphous lignin and hemicellulose.

Composition-wise, they are composed (except for cotton) of cellulose, lignin, and hemicelluloses as the main constituents along with waxes and some water-soluble compounds. From a chemical point of view, cellulose (the main constituent) is a linear polysaccharide generated from the repetition of β-D-glucopyranose monomers that are covalently linked through acetal functions (Abdul [START_REF] Khalil | Green composites from sustainable cellulose nanofibrils: A review[END_REF]. Figure 1-3 shows a summary of the different forms of cellulose usable in composite reinforcement such as macro-sized fibers, microfibrillated cellulose (MFC), cellulose nanocrystal (CNC), nanofibrillated cellulose (NFC), or bacterial cellulose (BC) [START_REF] Games | Wood. Chemistry, Ultrastructure, Reactions[END_REF].

Throughout the bibliographic research, one can notice that the most used lignocellulosic fibers in green composites development are essentially the bast fibers. One of the reasons for this growing interest is their cost, sustainability, and low density. Indeed Natural fibers are also significantly lighter than glass, with a density of 1.15-1.50 g/cm³ against 2.4 g/cm³ for glass fiber [START_REF] Pickering | Properties and Performance of Natural-Fibre Composites[END_REF]. Kenaf fibers, for example, were largely used in the reinforcement of thermoplastics thanks to their toughness and high aspect ratio. For instance, kenaf was added to PE [START_REF] Chen | Composite of polyethylene and kenaf, a natural cellulose fiber[END_REF][START_REF] Maldas | Composite Molded Products Based on Recycled Thermoplastics and Waste Cellulosics. II. Kenaf Fiber-Recycled PE Composites[END_REF][START_REF] Verma | Characterization of Mechanical Properties of Short Kenaf Fiber-HDPE Green Composites[END_REF], to PP (Deka et al., 2013;[START_REF] Sukri | Green composites based on recycled polyamide-6/recycled polypropylene kenaf composites: mechanical, thermal and morphological properties[END_REF], and to PLA [START_REF] Ben | Examination of heat resistant tensile properties and molding conditions of green composites composed of kenaf fibers and PLA resin[END_REF][START_REF] Chen | Preparation, characterization and crystallization kinetics of Kenaf fiber/multi-walled carbon nanotube/polylactic acid (PLA) green composites[END_REF] which provided an enhancement in the mechanical properties of these different thermoplastics. Furthermore, these bast fibers were added to several thermosetting polymers recently such as poly(furfuryl alcohol) (PFA) (Deka et al., 2013) and epoxy [START_REF] Krishna | The effect of treatment on kenaf fiber using green approach and their reinforced epoxy composites[END_REF]. Flax fibers are also largely used in composites reinforcement for thermoplastics [START_REF] Modniks | Evaluation of the apparent interfacial shear strength in short-flax-fiber/PP composites[END_REF]Wang et al., 2019a), thermosets [START_REF] Bagheri | Biomechanical properties of an advanced new carbon/flax/epoxy composite material for bone plate applications[END_REF][START_REF] Coroller | Effect of flax fibres individualisation on tensile failure of flax/epoxy unidirectional composite[END_REF], and biodegradable matrices [START_REF] Liu | Green" Composites from Renewable Resources: Preparation of Epoxidized Soybean Oil and Flax Fiber Composites[END_REF]Xia et al., 2016). Studies showed that these fibers have good specific mechanical properties which have the potential to replace glass fibers as reinforcement in composite [START_REF] Coroller | Effect of flax fibres individualisation on tensile failure of flax/epoxy unidirectional composite[END_REF][START_REF] Dittenber | Critical review of recent publications on use of natural composites in infrastructure[END_REF]. However, their incompatibility with these matrices can result in a poor fiber/matrix adhesion [START_REF] Modniks | Evaluation of the apparent interfacial shear strength in short-flax-fiber/PP composites[END_REF] yet it has been proven that adequate chemical treatment can limit this issue [START_REF] Chandrasekar | A review on the characterisation of natural fibres and their composites after alkali treatment and water absorption[END_REF][START_REF] Modniks | Evaluation of the apparent interfacial shear strength in short-flax-fiber/PP composites[END_REF]Xia et al., 2016). Jute fibers are one of the most abundant bast fibers worldwide that can be valorized in composite reinforcement [START_REF] Alves | Ecodesign of automotive components making use of natural jute fiber composites[END_REF]. Nevertheless, these fibers as well suffer from the same limits as flax fibers and many studies have been conducted to overcome these challenges by performing surface chemical modifications [START_REF] Corrales | Chemical modification of jute fibers for the production of green-composites[END_REF][START_REF] Mohanty | Surface modification of jute and its influence on performance of biodegradable jute-fabric/Biopol composites[END_REF][START_REF] Mohanty | Effect of MAPP as a Coupling Agent on the Performance of Jute-PP Composites[END_REF][START_REF] Sinha | Influence of fibre-surface treatment on structural, thermal and mechanical properties of jute fibre and its composite[END_REF]. [START_REF] Miyashiro | A Review of Applications Using Mixed Materials of Cellulose, Nanocellulose and Carbon Nanotubes[END_REF].

More recently, the use of cellulose nanofibers has overstepped natural fiber-reinforced composite.

These nanofibers (10-60 nm) can be extracted from any natural plant-based fibers using different methods. The high stiffness of nanofibers can make the development of high performances biobased composite possible. Scientist believing in the potential of these nano/micro-sized natural fibers started their investigation 25 years ago [START_REF] Favier | Nanocomposite materials from latex and cellulose whiskers[END_REF].

Composites with natural fibers reinforcement offer many advantages like a low machine abrasion, a low density, a lower cost, and renewability [START_REF] Wambua | Natural fibres: can they replace glass in fibre reinforced plastics?[END_REF]. Yet the main motivation to use natural fibers in composite manufacturing is their positive impact on the environment [START_REF] Mansor | Life Cycle Assessment of Natural Fiber Polymer Composites[END_REF].

On the other hand, these naturally-produced fibers have non-uniform dimensions and mechanical properties which make the design of composite challenging. Another major drawback of the use of cellulosic fibers is their polarity. Their hydrophilic nature makes the adhesion with the hydrophobic matrix lower. Moreover, cellulosic fibers' polarity leads to high moisture sorption (Mehta et al., Chapter 1: State of the art Khaoula BOUZIDI 202232 2004). Another considerable limitation to the use of natural fibers in composites is their limited thermal stability and thus they cannot be processed over 200°C [START_REF] Akil | Kenaf fiber reinforced composites: A review[END_REF].

Natural matrices for 'fully green' composites

For better sustainability, polymers produced from natural raw material were used along natural fibers to process fully green composites [START_REF] Wambua | Natural fibres: can they replace glass in fibre reinforced plastics?[END_REF]. In the literature, we can find many studies and reviews on bio-based polymers such as starch, furan resins, and epoxy. Many others are still under investigation and development [START_REF] Doroudgarian | High performance bio-based composites : mechanical and environmental durability[END_REF]. In Table 1-3, a non-exhaustive list of commercially available bio-based thermoset resins is presented. Lately, research in bio-based composites has intensified and many countries start to consider the potential of these materials. The European Union, in particular, has been funding many research projects namely, ANACOMPO, ECOFINA, BIOCOMP, and WOODY aiming to explore more the potential of green composites [START_REF] Doroudgarian | High performance bio-based composites : mechanical and environmental durability[END_REF].

Natural oils are used as a feedstock for many thermosetting resins thanks to their availability and low cost. After several chemical modifications, these oils can yield different resins such as polyesters, polyamides, polyurethanes, and epoxy resins [START_REF] Llevot | Sustainable Synthetic Approaches for the Preparation of Plant Oil-Based Thermosets[END_REF][START_REF] Xia | Bio-based Thermosetting Polymers from Vegetable Oils[END_REF].

For instance, epoxidized vegetable oil is largely used to replace or to toughen fossil-based thermosetting resins, and soybean and castor oils are the most used [START_REF] Liu | Green" Composites from Renewable Resources: Preparation of Epoxidized Soybean Oil and Flax Fiber Composites[END_REF][START_REF] Llevot | Sustainable Synthetic Approaches for the Preparation of Plant Oil-Based Thermosets[END_REF][START_REF] Xia | Bio-based Thermosetting Polymers from Vegetable Oils[END_REF]. Furan-based resins are also a possible matrix to produce bio-based composites as they are produced from biomass and more particularly agricultural waste. These resins and more particularly furfuryl alcohol polymers will be explained and further detailed in section 1. 3 (p.63) of this chapter.

Notwithstanding their renewability, these composites undergo the same end-of-life treatment as the fossil-based ones because of the no-degradability of the matrix and the difficulty to separate the encapsulated natural fibers. These composites unfortunately can only be downcycled but a chemical modification is possible to make them biodegradable or to make the crosslinking reversible [START_REF] Netravali | Composites get greener[END_REF][START_REF] Quirino | Thermosetting polymers from renewable sources[END_REF].

More recently, research of a 'truly green' composite increased. These materials are composed of natural biodegradable fibers and natural biodegradable resin thus at their end of life (usually short life cycle) these composites can be composted.

For instance, Biopol, a commercial poly(hydroxybutyrate-co-hydroxyvalerate) resin from Monsanto company, was used in green composite development. Biopol was combined with natural fibers such as pineapple and henequen for a composite with good mechanical properties with Young's modulus of 3 GPa in the longitudinal direction [START_REF] Luo | A study of physical and mechanical properties of poly(hydroxybutyrateco-hydroxyvalerate) during composting[END_REF], 1999a, 1999b). Moreover, this biodegradable resin was also successfully combined with jute fabric which resulted in 80% enhancement in tensile properties [START_REF] Khan | Study on Physical and Mechanical Properties of Biopol-Jute Composite[END_REF]. Starch is another promising material for biodegradable and available thermoplastic matrices which was used combined with hemp and bamboo fibers and resulted in excellent flexural properties and high tensile strength [START_REF] Takagi | Effect of Fiber Length on Mechanical Properties of "Green" Composites Using a Starch-Based Resin and Short Bamboo Fibers[END_REF]. The critical limit to the use of these plastics is their water sensitivity and poor mechanical performance. To overcome these limits, starch is usually used blended with other thermoplastics [START_REF] Averous | Biocomposites based on plasticized starch: thermal and mechanical behaviours[END_REF].

Recently, protein-based thermosettings are gaining the attention of the research community. Soy protein in particular has been used as a feedstock for biodegradable matrices. For instance, wood fibers were mixed with soy protein for the production of biocomposite by hot compression molding.

Mechanical properties of these composites turn out to be even more efficient than PE/wood fibers composite [START_REF] Netravali | Composites get greener[END_REF]. By the same token, composites with soy protein isolate and chopped ramie fibers yielded composite with an improved strength [START_REF] Lodha | Characterization of interfacial and mechanical properties of "green" composites with soy protein isolate and ramie fiber[END_REF].

Lactic acid, the main monomer of PLA, was first isolated in 1780 from milk by the Swedish chemist Scheele. PLA is aliphatic polyester made from a-hydroxy acids. This thermoplastic is not only biobased (made from renewable waste) but biodegradable as well. Furthermore, PLA has good mechanical properties compared to other biodegradable polymers and can be easily processed by using common techniques such as extrusion and injection molding [START_REF] Kaplan | Biopolymers from Renewable Resources, Macromolecular Systems -Materials Approach[END_REF]. This aliphatic polyester starts to chemically decompose at 200 °C and has a low Tg temperature of only 55°C [START_REF] Spinu | Material Design in Poly(Lactic Acid) Systems: Block Copolymers, Star Homo-and Copolymers, and Stereocomplexes[END_REF]. In the literature, we can find many biodegradable composites based on PLA matrices.

For instance, up to 30% of chopped flax fibers were added to PLA yielding a composite similar to that of polyester/glass fibers in terms of mechanical properties [START_REF] Bodros | Could biopolymers reinforced by randomly scattered flax fibre be used in structural applications?[END_REF]. In another study, a plasticizer was added to PLA/ flax fibers composite and was found that despite the enhancement of mechanical properties with the addition of flax fibers the plasticizer harmed these properties [START_REF] Oksman | Natural fibres as reinforcement in polylactic acid (PLA) composites[END_REF]. Kenaf fiber is also a good candidate for the production of high performances PLA-based composites. It was reported that kenaf fibers' addition improved the storage modulus and thermal properties of the composite and its mechanical properties with Young's modulus of 6.3 GPa with 70% kenaf reinforcement [START_REF] Nishino | Kenaf reinforced biodegradable composite[END_REF].

Different manufacturing techniques

Composite materials are very different composites need different manufacturing techniques. The first developed manufacturing processes were manual techniques. However, as the industry grows and composites start to get popular in many markets, a massive production was needed. Thusly, the development of different manufacturing techniques was essential to cope with this industrial growth. The method choice mainly depends on the materials, the part design, and the final use.

Manual techniques

The hand lay-up or wet lay-up method is an open contact molding technique. The resin is impregnated manually into a textile structure (woven, non-woven, knitted) after laying it in a mold coated with mold release and the number of these fiber layers depends on the desired thickness.

Rollers and brushes are often used in the impregnation step to minimize air bubbles in the composite. The structure is then left to cure at room temperature before adding more layers if needed. Instead of impregnating the fibrous structures with the resin manually, it is also possible to use a pre-impregnated fibers called 'prepeg'. [START_REF] Elkington | Hand layup: understanding the manual process[END_REF] The spray lay-up is another similar method where workers use a handgun to spray simultaneously resin and chopped fibers on the mold. The final step is to compact the thick layer with rollers and wait until it is cured at room temperature.

Autoclave processing (Compression molding)

It is a close molding technique by placing the material manually or robotically in the mold. The two halves of the mold are then closed and pressure is applied to impregnate the resin into the fibers and especially to eliminate voids in the composite [START_REF] Tatara | 14 -Compression Molding[END_REF]. Heat can also be applied with a varied amount depending on the composite size and thickness. However, in some cases, the heating is not required and the pressure is applied at room temperature, we talk here about cold press molding.

Vacuum-assisted method (The vacuum bagging technique)

Prepregs with viscous resin are placed in a mold in the desired sequence topped with a film layer, air bleed layer, blocked film layer, breath layer, and then the vacuum bag layer. A vacuum is then applied to make the resin (even with high viscosity) liquefy and drawn for uniform impregnation. The vacuum will also help to remove the trapped air in the mold [START_REF] Pearce | Cause and effect of void formation during vacuum bag curing of epoxy film adhesives[END_REF].

Injection molding

This is another close molding process used more commonly for thermoplastics filled with chopped fibers but can be also used with thermosets. This process has been used for over 150 years [START_REF] Frizelle | Injection Molding Technology[END_REF]. The fibers and pellets go through a funnel-shaped hopper with a screw into a heated barrel for liquefaction. The molten polymer is then injected in a closed mold until solidification. This is one of the fastest manufacturing processes [START_REF] Balasubramanian | Manufacturing techniques of composites for aerospace applications[END_REF].

Resin transfer molding (RTM)

In this method, the two halves of the mold are treated with a releaser, and the dry reinforcement is placed in the lower half before closing the mold. The thermosetting resin along with a catalyst is pumped into the mold at low pressure. The used resin usually has low viscosity for a quicker injection before reaching the gel point. This technique has been used for natural fibers-based composites [START_REF] O'dell | Natural Fibers in Resin Transfer Molded Composites[END_REF].

Pultrusion

The name of this technique comes from the combination of the words pull and extrusion and is a continuous fabrication method highly automated. In this process, continuous fibers are pulled through a die that contains a resin bath. The resin is cured in the die and the final shape of the composite, usually in form of bars, is given by the die's shape. This technique is largely used for synthetic composites and was also successfully used for natural fibers such as kenaf [START_REF] Zamri | Development of green pultruded composites using kenaf fibre: influence of linear mass density on weathering performance[END_REF], kenaf/glass [START_REF] Hashemi | Volumetric composition and shear strength evaluation of pultruded hybrid kenaf/glass fiber composites[END_REF], and jute/glass [START_REF] Uawongsuwan | Long jute fiber-reinforced polypropylene composite: Effects of jute fiber bundle and glass fiber hybridization[END_REF].

Filament winding

This is another example of a highly automated continuous fabrication. This technique is used to produce circular-shaped components using filaments winding process on an open cylindrical mold called a mandrel. The continuous fibers are pulled from the creel and impregnated in a resin bath before being wounded in the rotating mandrel. The fibers are equally distributed along the mold.

Additive manufacturing

Additive manufacturing (AM), also known as 3D printing, is an uprising technology in the manufacturing techniques introduced more than 20 years ago. This technology is also used in composite manufacturing not only for thermoplastics but also for thermosetting polymers. An introduction to this technology and scientific progress in composite production using this technology will be presented and detailed in section 1. 2 (p.40).

A comparison of different manufacturing processes for composites production is presented in Table 1-4 [START_REF] Balasubramanian | Manufacturing techniques of composites for aerospace applications[END_REF]. The images included in Table 1-4 are taken from a recent book by [START_REF] Ngo | Introduction to Composite Materials[END_REF]. were launched in these last two decades to find a natural and renewable substitute to synthetic composites, however many challenges appeared when using natural feedstocks. Hence, the presence of these composite on the market is still humble thus more research and development are needed to overcome its limits. New developments are also being done in the manufacturing techniques to produce more efficient and eco-friendly materials such as additive manufacturing. This new technology is gaining more popularity these last years and should be fully exploited in composites production.

Chapter 1: State of the art Khaoula BOUZIDI 2022 40

1.2 Composites and additive manufacturing

Additive manufacturing -overview

According to ISO standard (ASTM 52900:2015) "AM is the process of joining materials to make parts from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing and formative manufacturing methodologies". AM is also referred to as layer manufacturing or freeform fabrication. Usually, objects are fabricated according to three main techniques either by subtraction, by combination, by deformation, or by combining more than one technique. When using these classic techniques, the material loss is so important and more than one machine is needed which demands more workers more maintenance, and more costs. Furthermore, the time which is the main capital in the industry, is not optimized adding to that all the complicated logistics and production management. Meanwhile, the use of AM is different; the object is built layer by layer using one machine with almost zero material loss. AM methods can be different but the main common point is the fact of adding layer by layer the material according to the object pattern to obtain a solid 3D fabricated object. The main steps of AM are summarized in Table 12345.

Table 1-5: A summary of AM main steps [START_REF] Zhang | From 3D Sensing to Printing: A Survey[END_REF]. In 2012, according to the Economist magazine, this technology was considered the third industrial revolution (The Economist, 2012). Indeed, AM size market is growing rapidly and steadily to pass from less than $5 Billion in 2014 to over $12 Billion in 2020 [START_REF] Wohlers | Wohlers report 2021 3D prinitng and additive manufacturing: Global state of the industry[END_REF]. AM market has been growing with an average growth of 27.4% over the previous 10 years but in the most recent Wohlers

The step

Associates' report, 3D printing market only grew by 7.5% (was expected to be $15.8 billion in 2019)

which was still considered to be satisfactory owing to the impact of COVID-19 crises [START_REF] Wohlers | Wohlers report 2021 3D prinitng and additive manufacturing: Global state of the industry[END_REF].

In 2018, this new manufacturing technology was used the most to build end-use parts and functional prototypes as we can see in Figure 1-5 [START_REF] Wohlers | Modification of flax fiber surface and its compatibilization in polylactic acid/flax composites[END_REF]. Despite the recent low growth rate, another report forecasted a $51 billion market size by 2030 with a larger share for end-use parts [START_REF] Research | Lux Research Forecasts the 3D Printing Market Will Reach $51 Billion in 2030[END_REF]. As a consequence of the rapid growth of AM popularity these last years and by dint of the development of new methods and techniques, many new terminologies have appeared. Thus, the ASTM decided to give a clear classification and universal terminologies for the different techniques.

We list below a short description of the main used methods. [START_REF] Wohlers | Modification of flax fiber surface and its compatibilization in polylactic acid/flax composites[END_REF].

.2.1 Material extrusion

The main technique is Fused Filament Fabrication (FFF) where a thermoplastic filament is melted after heating the nozzle until the melting temperature of the used plastic. Formerly, the liquefied plastic is deposited according to the coordinates on the GCode generated to print the specific object.

Next, the plastic is cooled at room temperature to solidify before depositing the next layer. This technique is the most used for personal 3D printers thanks to its simplicity, yet a very precise configuration of the height, thickness, and filament orientation is required for a successful printing without weakening the mechanical properties of the object [START_REF] Mohamed | Optimization of fused deposition modeling process parameters: a review of current research and future prospects[END_REF][START_REF] Sood | Parametric appraisal of mechanical property of fused deposition modelling processed parts[END_REF]. Under the same category, we can also find cold material extrusion, also called direct ink writing (DIW) or Liquid Deposition Modeling (LDM) [START_REF] Lewis | Direct Ink Writing of 3D Functional Materials[END_REF], largely used in bioprinting using gels and pastes instead of molten plastic.

Powder bed fusion

This method consists of depositing layers of a very thin powder of polymers, metal, or ceramics on a platform and then a source of thermal energy selectively fuses the thin particles according to the desired pattern. This action is repeated for the next layers. Powder bed fusion is similar to the bed jet binding where the particles are assembled with glue jetting. After building the object the excess powder is eliminated and further processing is performed if necessary. The object is determined by the size distribution and the packing density of the used powder which are the main factors of a successful printing [START_REF] Utela | A review of process development steps for new material systems in three dimensional printing (3DP)[END_REF]. Moreover, the difference between the Selective Laser Sintering (SLS) and Selective Laser Melting (SLM) should also be highlighted; in the first case the material choice is larger and the high temperature will make only the surface of the particle melt and fuse whereas in the latter case the particles are melted and fused which limits the material choice ( aluminum, steel) but also results in better mechanical properties [START_REF] Lee | Lasers in additive manufacturing: A review[END_REF]. The main interest of this method is its simple process with no required post-processing, decent resolution, and mechanical properties along with a variety of raw materials. However, this method has many challenges like the impossibility to build closed hollow cavities, as illustrated in Figure 1-6. Moreover, when using some material, the speed of laser scanning can be too high to melt correctly the particles which can lead to high porosity and fragile structure. Dust generation is another issue of this technique.

Chapter 1: State of the art Khaoula BOUZIDI 2022 44

Binder jetting

Binder jetting, or 3DP, is one of the oldest methods of AM. This method has the same principle as powder bed fusion but instead of using a laser, the particles are glued by jetting a liquid binder. In this case, the essential parameters to control for good manufacturing are the binder rheology, deposition speed, and powder-binder interaction [START_REF] Wang | 3D printing of polymer matrix composites: A review and prospective[END_REF]. The porosity is higher than the other methods leading to fragile objects and also poor surface finish.

Sheet lamination

Laminated object manufacturing (LOM) is based on the use of sheets of various materials such as paper, ceramics, polymers, and metallic sheets. This technique is also based on a layer-by-layer building by spreading the sheet and then cutting it precisely using either a mechanical cutter or laser according to the desired pattern. Then, the platform is lowered by one layer height and another fresh layer is bonded to the latter and the cycle is repeated until the object is built. After the building, postprocessing is required to remove the rest of the non-used material (that beneficially insured the structure support) which can be recycled afterward [START_REF] Gibson | Sheet Lamination Processes[END_REF]. LOM is recommended for the simple objects and is also time-effective however this technique is not recommended for the complex shape as the elimination of laminates becomes complex and time-consuming besides the low surface quality.

Vat photo-polymerization

Stereolithography (SLA) is the very first AM method used. This method is based on layer-per-layer solidification of photo-curable monomers using a light source. After the selective crosslinking of the photopolymer vat, the platform is lowered by a thin layer to repeat the process several times until the object is built. At the end of the printing, the object should then be finished by washing it with a solvent and further crosslink it in a UV oven. SLA is considered one of the most precise AM methods.

Another benefit of this method is the possibility to print large objects. However, like all the AM methods, the SLA has numerous drawbacks like its high cost and process complexity. The very limited material choice is also a major limitation of this method in addition to the low manufacturing speed.

General remarks and comparaison of available techniques in AM

Despite the common principle of manufacturing by adding layer-by-layer material, every AM is different as it needs its specific materials and has its limitations. So, the right AM technique should be chosen depending on the object structure, its application, and the budget tolerance. In Figure 1-6, the structural limitations of every technique are revealed where one can notice that evry technique has its limitations in terms of design structure and geometry. The availability of material currently used for this manufacturing technology is still limited as we can find mainly thermoplastics, photocurable monomers, some metals powders and alloys, ceramics, and paper. Despite all of these limitations, AM is still an object of development and researchers are working to overcome these challenges to offer more materials and to optimize its techniques. 3D printing has multiple benefits to offer. First of all, personal product customization becomes easy and possible even in the biggest companies where large series production is no longer a necessary condition to make profits.

Moreover, the manufacturing cost is significantly decreased thanks to the use of merely one machine instead of using many that consume a lot of energy and need specific skills and human resources. In addition, the necessity of outsourcing, transport can be considerably reduced (Berchon and Luyt, 2014;[START_REF] Horvath | Mastering 3D Printing[END_REF][START_REF] Kamran | A Comprehensive Study on 3D Printing Technology[END_REF]. Furthermore, AM is a great technology to optimize the material used with a significant waste reduction. A summary of these different techniques along with their main benefits and limits is presented in Table 1-6. [START_REF] Ngo | Additive manufacturing (3D printing): A review of materials, methods, applications and challenges[END_REF]. (Feilden, 2017).

Method

Composites in AM by material extrusion 1.2.3.1 An overview

Before tackling the composite additive manufacturing state of the art, we will start with an overview of the steps involving AM by extrusion and its forms for a better understanding of the research challenges.

Based on the extrusion type, the printable material, or also called 'ink', can be divided into three main categories [START_REF] Godoi | 3d printing technologies applied for food design: Status and prospects[END_REF]:

• Hot-melt extrusion: thermoplastics whose solidification takes place after a phase change (solidification).

• Gel extrusion: their extrusion can be either at room temperature or higher temperature but their shape maintenance is due to a phase change after gelation.

• Cold extrusion: for this type of ink, the extrusion, and the deposition take place at ambient temperature and no phase transition occurs for supporting the printed structure. In this case, rigorous rheological behaviors are required so the material can be extrudable, easily flows through the nozzle, and resists deformations after printing to form a self-supporting structure.

Depending on the used material, the extrusion can take place either at high temperature using a liquefier as described before or at room temperature when the used material is not a thermoplastic.

We can find different extruder systems on the market for the FFF (high-temperature extrusion) or DIW (ambient temperature extrusion).

In the FFF 3D printer available on the market two types of extruder systems can be distinguished:

• The pinch roller system is the most used. The pinch roller pushes the filament toward the liquefier with a constant linear speed. To ensure a constant flow rate, the adherence between the pinch roller and the plastic filament should be perfect and without any slipping.

• Another less popular extruder system is the screw-driven technique used for the pellets. The screw drives the melted pellet and pushes the molten rod through the nozzle.

In the case of cold extrusion, we can distinguish three extrusion systems to push the material through the nozzle, as illustrated in Figure 1-7-(c) [START_REF] Sun | Extrusion-based food printing for digitalized food design and nutrition control[END_REF]:

• Screw-driven system with a stepper motor where the material is pushed by the rotation of the screw. The flow rate is regulated by the stepper motor and the material is initially stored in a syringe and pushed with pneumatic pressure until it comes in contact with the screw.

This technique is largely used for concentrated suspensions and pastes (W. Li et al., 2017).

• The pneumatic extrusion technique is wildly used in bio-inks and food extrusions [START_REF] Siqueira | Cellulose Nanocrystal Inks for 3D Printing of Textured Cellular Architectures[END_REF]. The driving force of the ink in this system is a compressed gas (commonly the air).

This system can be used for large inks types [START_REF] Hölzl | Bioink properties before, during and after 3D bioprinting[END_REF] but is still limited to material with low viscosity at the extrusion shear rate, and with low yield stress. This extrusion technique is simple and easy to use but the flow is not instantly controlled because of the compressed gas.

• The piston system is a syringe with a plunger connected to a step motor that controls the extrusion and the flow rate as shown in Figure 1-7-(a). This system is used in many commercial printers (such as Choc Creator). Contrarily to the pneumatic solution this technique allows a rapid adjustment of the extrusion rate.

Chapter 1: State of the art Khaoula BOUZIDI 2022 49

Figure 1-7: Extruder for cold extrusion [START_REF] Sun | Extrusion-based food printing for digitalized food design and nutrition control[END_REF].

Independently to the used type of extrusion, AM by extrusion process passes by 4 main steps;

(i) Material preparation: this step is depending on the nature of the material. For instance, in the case of FFF the material is usually fed as a filament with a normalized diameter. The extrusion can also take place at ambient temperature in the case of ceramic, bio-inks, and many other formulations that should be compatible with AM requirements. In the latter case, the material preparation is complex because the formulation should have convenient rheology, should be homogeneous, and with no air bubbles.

(ii) Extrusion step: This is a crucial step in which the main objective is to insure a continuous and homogeneous rod flow through the nozzle. This step not only depends on the material nature but also on the extrusion system and the nozzle morphology.

(iii) Layers deposition: The quality of the raster deposition highly depends on the previous steps and the process parameters chosen by the user which will be detailed later.

(iv) Solidification: this is the last step before any eventual post-processing. The solidification step once again depends on the nature of the used material. The solidification of the melted thermoplastics takes place by simple cooling to room temperature while in the case of other pastes and suspensions this step can be more complicated and needs more control. The consolidation can be achieved by air drying, polymers crosslinking, or freeze-drying.

The final properties of the printed object are not only depending on the used material but also the hardware specifications such as the extruder type which usually is not controlled by the user, (Kuznetsov et al., 2019). However, there are many parameters, presented in detail below, that can be controlled by the user through the software that generates the GCode.

• Air gap: the air gap is the distance between the deposited roads. This air gap can be either positive in the case where the roads do not touch or positive when they are overlapped. This parameter can also affect the dimensional accuracy and mechanical performance of the printed object. A negative air gap can reduce the porosity and improve the bonding between the roads [START_REF] Rodriguez | Characterization of the mesostructure of fuseddeposition acrylonitrile-butadiene-styrene materials[END_REF].

• Build orientation and raster orientation: the build orientation depends on how the object is placed on the printing platform in the X-Y-Z space. This parameter has a major effect on the mechanical performances as well as on the dimensional accuracy [START_REF] Alafaghani | Experimental Optimization of Fused Deposition Modelling Processing Parameters: A Design-for-Manufacturing Approach[END_REF][START_REF] Raju | An experimental investigation on surface quality and dimensional accuracy of FDM components[END_REF]Chacón et al., 2017;[START_REF] Panda | Optimization of Fused Deposition Modelling (FDM) Process Parameters Using Bacterial Foraging Technique[END_REF]. The raster orientation which is the deposit inclination with respect to the X-axis is also one of the parameters affecting the tensile strength. It has been proved that an angle of 45/-45° [START_REF] Kung | Evaluation of tensile strength of 3D printed objects with FDM process on RepRap platform[END_REF][START_REF] Rinanto | Simultaneous Optimization of TensileStrength, Energy Consumption and Processing Time on FDM Process Using Taguchi and PCR-TOPSIS[END_REF] and 0/90° [START_REF] Leite | Study of the influence of 3D printing parameters on the mechanical properties of PLA[END_REF] is the best for improving tensile properties. However, the effect of this parameter should be further studied especially for specimens printed in the same direction (0°).

• Extrusion temperature: The temperature of the liquefier is fixed mainly according to the used material. This parameter can influence the quality of the printed part. As the road leaves the nozzle from the heated liquefier to the printing environment, it solidifies. Hence, the material is subjected to thermal stresses that might distort the printed part and thus affect the shape fidelity and the mechanical performances [START_REF] Alafaghani | Experimental Optimization of Fused Deposition Modelling Processing Parameters: A Design-for-Manufacturing Approach[END_REF].

• Infill density: is defined as the fraction of the space occupied by the material in a layer over the total internal surface of the layer. The infill excludes the shells, which is also considered an important parameter that can affect tensile and compressive strengths [START_REF] Croccolo | Experimental characterization and analytical modelling of the mechanical behaviour of fused deposition processed parts made of ABS-M30[END_REF]. This parameter affects directly the properties of the printed part, especially its mechanical performances. The lower is the infill density, the higher is the porosity in the final object, and the poorer are the mechanical performances [START_REF] Mahmood | Tensile strength of partially filled FFF printed parts: experimental results[END_REF][START_REF] Raju | A hybrid PSO-BFO evolutionary algorithm for optimization of fused deposition modelling process parameters[END_REF].

• Road width is the distance between the edges of deposited rods. Although this parameter can be fixed on the software, it cannot be less than 1.2 to 1.5 times the nozzle diameter when printing with thermoplastics [START_REF] Agarwala | Structural quality of parts processed by fused deposition[END_REF] and certainly not less than the nozzle's diameter.

• Layer thickness is the height of one layer and is another parameter constrained by the nozzle dimension. The layer thickness cannot be more than the nozzle diameter and usually is chosen to be less. This parameter influences the printing resolution. Thinner layers improve the surface roughness and the dimensional accuracy but increase the built time [START_REF] Agarwala | Structural quality of parts processed by fused deposition[END_REF][START_REF] Han | Process speeding up via deposition planning in fused deposition-based layered manufacturing processes[END_REF][START_REF] Sood | Improving dimensional accuracy of Fused Deposition Modelling processed part using grey Taguchi method[END_REF][START_REF] Alafaghani | Experimental Optimization of Fused Deposition Modelling Processing Parameters: A Design-for-Manufacturing Approach[END_REF]. The layer thickness influence also the tensile properties, it is believed that thin slices improve tensile strength [START_REF] Deng | Mechanical Properties Optimization of Poly-Ether-Ether-Ketone via Fused Deposition Modeling[END_REF][START_REF] Leite | Study of the influence of 3D printing parameters on the mechanical properties of PLA[END_REF][START_REF] Li | The effect of process parameters in fused deposition modelling on bonding degree and mechanical properties[END_REF] but certain researchers proved the contrary [START_REF] Tontowi | Optimization of 3D-Printer Process Parameters for Improving Quality of Polylactic Acid Printed Part[END_REF].

• Printing speed: is the extruder movement speed in the XY plane expressed by mm/s which also affects the dimensional accuracy of the printed part and directly affects the building time [START_REF] Anitha | Critical parameters influencing the quality of prototypes in fused deposition modelling[END_REF][START_REF] Sood | Improving dimensional accuracy of Fused Deposition Modelling processed part using grey Taguchi method[END_REF].

Composite manufacturing by hot-melt extrusion

According to a report published by Statista in 2020, FFF is the most used 3D printing technology in 2018. FFF is also the most common personal 3D printer thanks to its simplicity and cost-effectiveness and its office-friendly environment. However, 3D printed parts using this technique suffers from poor mechanical properties. Therefore, the idea of reinforcing the polymer with nanoparticles, short fibers, or continuous fibers was imposed to improve mechanical, thermal, and electrical properties. Plentiful research works have been carried out since last two decades to print quality products through FFF by using composite materials which have been the subject of many reviews these last years [START_REF] Mohan | A review on composite materials and process parameters optimisation for the fused deposition modelling process[END_REF][START_REF] Parandoush | A review on additive manufacturing of polymer-fiber composites[END_REF][START_REF] Mazzanti | FDM 3D Printing of Polymers Containing Natural Fillers: A Review of their Mechanical Properties[END_REF][START_REF] Kabir | A critical review on 3D printed continuous fiberreinforced composites: History, mechanism, materials and properties[END_REF][START_REF] Penumakala | A critical review on the fused deposition modeling of thermoplastic polymer composites[END_REF][START_REF] Wickramasinghe | FDM-Based 3D Printing of Polymer and Associated Composite: A Review on Mechanical Properties, Defects and Treatments[END_REF][START_REF] Angelopoulos | Functional fillers in composite filaments for fused filament fabrication; a review[END_REF][START_REF] Mogan | A review of FDM and graphene-based polymer composite[END_REF][START_REF] Lee | Potential for Natural Fiber Reinforcement in PLA Polymer Filaments for Fused Deposition Modeling (FDM) Additive Manufacturing: A Review[END_REF][START_REF] Krajangsawasdi | Fused Deposition Modelling of Fibre Reinforced Polymer Composites: A Parametric Review[END_REF][START_REF] Mazurchevici | Additive manufacturing of composite materials by FDM technology: A review[END_REF]. Hereby is an overview of composite printing by FFF highlighting the diverse types of fillers (continuous, discontinuous, natural, Nano-sized) that can be used and their mechanical properties.

Continuous fibers-reinforcing phase

Continuous fibers reinforcement offers significant improvement in mechanical properties compared to discontinuous fibers. However, this technique is still challenging 3D manufacturers and researchers in the development of a standard paradigm. In 2016, the team of Matsuzaki published in the scientific journal Nature an innovative technique for in-nozzle impregnation of continuous fiber within a thermoplastic matrix. In this study, PLA was used as the matrix while carbon fibers, or twisted yarns of natural jute fibers, were used as reinforcements. PLA reinforced with carbon fibers exhibited a net enhancement in mechanical properties with 500% amelioration in tensile modulus and 400% in tensile strength. Despite the increase of mechanical properties with the use of natural fibers, their impact stays limited comparing to carbon fibers. Moreover, comparing the continuous carbon-fiber composites with composite fabricated with discontinuous fillers using FFF or other printing techniques, one can notice that this innovative technique showed a superior Young's modulus and strengths as we can see on the graph in Figure 1-8 [START_REF] Matsuzaki | Three-dimensional printing of continuous-fiber composites by in-nozzle impregnation[END_REF]. In fact, continous carbon-fiber composite outperform the filled and unfilled composite produced by different techniques in terms of strength with values approaching 250 MPa.

To overcome the poor matrix-carbon fiber bonding, carbon fibers were processed to enhance adhesion between the two phases. The produced composite had improved mechanical and thermal properties (N. Li et al., 2016). This trend of mechanical properties enhancement was supported with many other studies using PLA [START_REF] Tian | Interface and performance of 3D printed continuous carbon fiber reinforced PLA composites[END_REF], ABS [START_REF] Ning | Additive manufacturing of carbon fiber reinforced thermoplastic composites using fused deposition modeling[END_REF], and Nylon [START_REF] Dickson | Fabrication of continuous carbon, glass and Kevlar fibre reinforced polymer composites using additive manufacturing[END_REF][START_REF] Klift | 3D Printing of Continuous Carbon Fibre Reinforced Thermo-Plastic (CFRTP) Tensile Test Specimens[END_REF] but it was also found that high fiber content eventually deteriorated the mechanical performances because of the increasing porosity.

Figure 1-8: Young's moduli and strengths of continuous carbon-fibers composites (CFRTP) with composites fabricated by FFF, SLS, and SLA [START_REF] Matsuzaki | Three-dimensional printing of continuous-fiber composites by in-nozzle impregnation[END_REF].

Short fibers and particles -reinforcing phase

Short fibers, macro, and nanoparticles were also added to enhance mechanical properties or to add specific functionalities such as thermal and electrical conductivity. Examining the literature, one can notice that the most used fibers are carbon fibers which enhanced the mechanical performances of thermoplastic matrices such as ABS [START_REF] Ning | Additive manufacturing of carbon fiber reinforced thermoplastic composites using fused deposition modeling[END_REF][START_REF] Tekinalp | Highly oriented carbon fiber-polymer composites via additive manufacturing[END_REF], PLA [START_REF] Ferreira | Experimental characterization and micrography of 3D printed PLA and PLA reinforced with short carbon fibers[END_REF], and PEEK [START_REF] Han | Carbon Fiber Reinforced PEEK Composites Based on 3D-Printing Technology for Orthopedic and Dental Applications[END_REF][START_REF] Wang | Preparation of short CF/GF reinforced PEEK composite filaments and their comprehensive properties evaluation for FDM-3D printing[END_REF]. Despite an improvement in mechanical properties, the adhesion between the two phases was poor and fiber pull-out from the matrix was highlighted by SEM analysis (Figure 123456789). On one hand, it was found that the average size of the triangular channels between the rods decreases with the presence of carbon fibers as compared to neat matrices which can be attributed to the decrease in die-swell. On the other hand, voids inside the rods increased. This porosity caused a limited mechanical properties improvement compared to compressed molded composites. Furthermore, nanoparticles with a high specific surface, such as graphene nanoplatelets [START_REF] Caminero | Additive Manufacturing of PLA-Based Composites Using Fused Filament Fabrication: Effect of Graphene Nanoplatelet Reinforcement on Mechanical Properties, Dimensional Accuracy and Texture[END_REF]Y. Wang et al., 2020;[START_REF] Yaragalla | Comprehensive Enhancement in Thermomechanical Performance of Melt-Extruded PEEK Filaments by Graphene Incorporation[END_REF] and carbon nanotubes [START_REF] Berretta | Fused Deposition Modelling of high temperature polymers: Exploring CNT PEEK composites[END_REF][START_REF] Gonçalves | Electrically Conductive Polyetheretherketone Nanocomposite Filaments: From Production to Fused Deposition Modeling[END_REF], were combined with many thermoplastics in order to develop high-performance nanocomposites.

In general terms, the incorporation of nanoparticles improved the mechanical and thermal properties and changed the mechanical behavior of thermoplastic from brittle to ductile. However, the improvement in the performances of these nanocomposites starts to decline when adding a large quantity of nano-fillers because of the creation of aggregates due to the dispersion difficulties of these nano-sized particles. Another commonly reported phenomenon is the alignment of fibers and nanoparticles in the printing direction as a consequence of shearing in the nozzle during extrusion.

This alignment makes the printed parts anisotropic with tunable properties. In Figure 1-9, one can notice the shear-induced alignment of carbon fibers on the cross-section SEM images of the printed composites.

Figure 1-9: SEM micrographs of carbon fibers in (A) PLA matrix [START_REF] Ferreira | Experimental characterization and micrography of 3D printed PLA and PLA reinforced with short carbon fibers[END_REF] and (B) ABS matrix [START_REF] Tekinalp | Highly oriented carbon fiber-polymer composites via additive manufacturing[END_REF] highlighting fibers pullout and the resulting pores.

Natural fillers for composites produced by the FFF technique

Thermoplastics were also combined with natural fibers for greener or fully bio-based composites when blended with PLA. The use of bio-based fibers could be a great substitute for fossil-based fibers to enhance the performances of many matrices. However, the use of these natural fibers with a rather complex structure raises some challenges such as higher porosity, visible in Figure 1-10, and higher water uptake which leads to swelling and defects in the dimensional stability sought in 3D printed parts [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF][START_REF] Perez | Fracture Surface Analysis of 3D-Printed Tensile Specimens of Novel ABS-Based Materials[END_REF]. On the other hand, when flax fibers were blended with PP the shrinkage upon solidification decreased by more than 80% [START_REF] Stoof | Sustainable composite fused deposition modelling filament using recycled pre-consumer polypropylene[END_REF]. [START_REF] Perez | Fracture Surface Analysis of 3D-Printed Tensile Specimens of Novel ABS-Based Materials[END_REF].

In addition to natural fibers, some works have been conducted to valorize bio-refinery byproducts to blend them in 3D printing filament to boost their performances and their environmental impact. For instance, lignin was added to ABS and Nylon which increased the stiffness and decreased the melt viscosity making it a good material for 3D printing by melt extrusion (Nguyen et al., 2018). In another study, 25 wt% of wood hemicellulose were blended to PLA to produce a fully green composite filament for 3D printing. The added hemicellulose slightly improved the thermal properties of the resulting composite but no significant improvement in mechanical performances was reported.

Nevertheless, the authors stressed the importance of hemicelluloses active sites which may be used as carriers or molecular anchors to introduce tuned functionality (Xu et al., 2018a). Different composites developed in the last 5 years for 3D printing by hot-melt extrusion are summarized in Table 1-7 with a focus on their mechanical properties. As one can notice, adding fillers to thermoplastics has significantly improved mechanical properties but their effect on thermal properties is rarely reported. With no surprise, PLA was the most used matrix thanks to its easy printability. When reinforcing this plastic with short fiber the maximum stiffness was 7.5 GPa and the strength was 67 MPa. The composite tested with ABS and PP had stiffness under 3 GPa and a maximum strength of 65 MPa.

The major limitation to the use of thermoplastics is their weak thermal stability and low Tg which led to the development of more technical thermoplastic such as PEEK. These costly and highly performant plastics can only be printed by heating the nozzle over 350°C which makes the printing process rather complicated. As highlighted previously in Table 1-1, thermosets are more resistant to high temperature, have better wettability without being over-priced. In the next section, recent studies for the development of 3D printing inks using thermosetting resin will be presented. facilitating respectively the extrusion and the shape maintaining [START_REF] Abouzeid | Biomimetic Mineralization of Three-Dimensional Printed Alginate/TEMPO-Oxidized Cellulose Nanofibril Scaffolds for Bone Tissue Engineering[END_REF][START_REF] Håkansson | Solidification of 3D Printed Nanofibril Hydrogels into Functional 3D Cellulose Structures[END_REF][START_REF] Sultan | 3D printed scaffolds with gradient porosity based on a cellulose nanocrystal hydrogel[END_REF]. However, for the solidification of the printing parts, the hydrogels are usually combined with biopolymers to ensure the consolidation of the material by crosslinking such as alginate [START_REF] Abouzeid | Biomimetic Mineralization of Three-Dimensional Printed Alginate/TEMPO-Oxidized Cellulose Nanofibril Scaffolds for Bone Tissue Engineering[END_REF][START_REF] Leppiniemi | 3D-Printable Bioactivated Nanocellulose-Alginate Hydrogels[END_REF][START_REF] Markstedt | 3D Bioprinting Human Chondrocytes with Nanocellulose-Alginate Bioink for Cartilage Tissue Engineering Applications[END_REF] Chapter 1: State of the art Khaoula BOUZIDI 2022 57

and chitosan [START_REF] He | Fabrication of silk fibroin/cellulose whiskers-chitosan composite porous scaffolds by layer-by-layer assembly for application in bone tissue engineering[END_REF]. The combination of the hydrogel and these biopolymers enables a successful 3D printing of scaffolds as shown in Figure 1-11-A and Figure 1-11-B. Although MFC hydrogel is a very interesting candidate for AM of biomaterials, its high water content (up to 98 wt %) harms the shape maintenance after drying. As we can see in Figure 1-11-C2, after air-drying the water evaporates leaving a shapeless fibers residue. Air drying is not an option when printing hydrogels so the most used technique is freeze-drying for shape maintenance and the generation of a porous structure ideal for tissue engineering (Figure 1-11-C1). Freeze-drying is an expensive technique that is worth using for high-performance and high-impact materials such as biomaterials.

However, in order to use cellulose fibers in more general applications, it is obvious that water content has to be limited. Accordingly, recent studies have been conducted to develop cellulosebased 'inks' for AM by extrusion with higher cellulose content and limited water load. A paste based on high content enzymatically fibrillated cellulose nanofibers was used for AM by extrusion. Indeed, the high cellulosic content limited the shrinkage upon drying but cracking of the printed parts was resulted from water evaporation. Moreover, clogging in the nozzle (1.2 mm) was reported when extruding fibrous pastes with high solid content [START_REF] Klar | Shape fidelity and structure of 3D printed high consistency nanocellulose[END_REF]. The approach of increasing cellulose content was also used to formulate a paste based on softwood microfibers combined with carboxymethyl cellulose which played the role of a matrix and also lubricant to limit nozzle clogging.

Figure 1-11: Printed parts of (A1) 3% alginate and (A2) 2.5% cellulose nanofibers (A3) cellulose nanofibers and alginate. (B) human ear 3D printed with alginate and cellulose [START_REF] Håkansson | Solidification of 3D Printed Nanofibril Hydrogels into Functional 3D Cellulose Structures[END_REF][START_REF] Markstedt | 3D Bioprinting Human Chondrocytes with Nanocellulose-Alginate Bioink for Cartilage Tissue Engineering Applications[END_REF]. Photographs of printed parts: MFC hydrogel after freeze-drying (C1) and after air drying (C2) [START_REF] Håkansson | Solidification of 3D Printed Nanofibril Hydrogels into Functional 3D Cellulose Structures[END_REF] and of high content cellulose paste after air drying (Thibaut et al., 2019).

Chapter 1: State of the art Khaoula BOUZIDI 2022 58

The paste's overall solid content was over 40 wt% which further limited shrinkage and deformation of the printed parts. However, the fibers' anisotropic shrinkage upon drying was still important as we can notice in Figure 1-11-C3 (Thibaut et al., 2019). These bio-based printed materials exhibited good mechanical performances compared to thermoplastics but the major limitation is unsurprisingly their water and humidity sensors (Le Duigou et al., 2019). Nevertheless, this limitation can also be a big advantage for the recycling step.

Some other approaches were also proposed such as the combination of wood powder with an adhesive mixture which presented many limitations and needs more optimization (Kariz et al., 2016).

Another approach consisting of printing a solution of cellulose acetate by extrusion and solidification by solvent evaporation was adopted [START_REF] Pattinson | Additive Manufacturing of Cellulosic Materials with Robust Mechanics and Antimicrobial Functionality[END_REF].

Thermosetting-resin-based composites

Thermoset polymers exhibit superior thermal, chemical, and mechanical stability, making them more adequate for demanding applications comparing to thermoplastics and cellulosic materials. However, 3D printing of crosslinkable polymers is still limited to photocurable resins because thermosetting resins do not have rapid solidification after extrusion. As mentioned previously in the DIW technique the ideal 'ink' should exhibit shear-thinning and thixotropic behaviors. Thus, thermoset polymers are usually never printed alone but in a combination with fillers which play the role of rheology modifiers. When printing thermoset polymers by DIW, the most important step is the formulation and preparation of the ink. Indeed, tuning the rheological parameters is important to make the thermosetting feedstock printable.

In a study published in 2017, epoxy resin was mixed with 30 wt% alginate that played the role of thickener and rheology tuner (Yield shear stress 2000 Pa) and dimethyl methyl phosphonate for increasing the solid load. The purpose of this study was to find the universal property and printing parameters for AM of thermoset by extrusion. The authors reported that the first important parameter is the rheological behavior of the extrudate which has to be pseudoplastic. Alginate powder transformed the Newtonian behavior of the pure resin to a shear-thinning ink and also increased the yield stress. The effect of different extrusion and printing parameters such as nozzle height and extrusion rate was studied with respect to the cross-section geometry. A new parameter called nozzle critical height was reported that results in an extruded line with a near 90° contact angle [START_REF] Shang | Rheological and Printable Behavior of Epoxy Resin-based Materials for Extrusion 3D Printing Applications[END_REF]. In 2015, Compton and his team [START_REF] Compton | 3D printing of composites with controlled architecture 30[END_REF] have already mixed epoxy resin with nano-clay and SiC whiskers particles to make it printable. The nanoparticles addition modified the rheology by increasing the viscosity and the shear-thinning abaility of the slurries to [START_REF] Compton | 3D printing of composites with controlled architecture 30[END_REF].

Furthermore, and in the same reported work, less than 4 vol% of carbon fibers were added which enhanced Young's modulus to reach 25 GPa. It was also found that during extrusion, high aspect ratio fillers become highly aligned, resulting in superior properties along the print direction as we can clearly see in Figure 1-13. The shear inducing anisotropy of printed parts due to high aspect ratio particles alignment in the extrusion direction is a relatively old discovery compared to AM history [START_REF] Peng | Orientation effects in freeformed short-fiber composites[END_REF]. This approach was more explored for 3D printing of structures with specific stiffness by mimicking balsa cell walls. Carbon fibers were added to epoxy which made the material stiffer with Young's modulus of 57 GPa [START_REF] Malek | Lightweight 3D cellular composites inspired by balsa[END_REF].

Figure 1-13: Shear-induced alignment of carbon fiber visible in printed rods [START_REF] Compton | 3D printing of composites with controlled architecture 30[END_REF].

In 2018, graphene flakes were blended with epoxy resin to 3D print electrically conductive material.

It was found that besides increasing electrical conductivity, graphene particles modified the viscosity to enable the AM of the thermoset polymers. However, the printability of the ink was limited thus a low concentration of nanoclay was added to further increase the yield stress of the ink. The resulting composite was as strong as neat casted epoxy (flexural strength 80 MPa) and stiffer (flexural modulus 4.8 GPa) [START_REF] Compton | Electrical and Mechanical Properties of 3D-Printed Graphene-Reinforced Epoxy[END_REF]. Once again, the same research team conducted a recent study to investigate the effects of filler morphology, nozzle size, and print speed on the mechanical anisotropy in 3D printed epoxy composites. 10 wt% of fumed silica and nanoclay pellets were added to epoxy resin and successfully modified the rheology to obtain a printable ink (yield shear stress of about 1000 Pa) with preference to nanoclay in the case of multi-fillers composites. In this study, it was also demonstrated that SiC whisker insured a better enhancement in mechanical properties with flexural strength values up to 215 MPa and a 4 GPa flexural modulus while fumed silica composite had 107 MPa flexural strength and a 3.1 GPa modulus. Furthermore, the mechanical properties of these composites containing high-aspect-ratio fillers (like nanocalay and unlike fumed silica) display pronounced dependence on both nozzle size and print speed [START_REF] Hmeidat | Mechanical anisotropy in polymer composites produced by material extrusion additive manufacturing[END_REF]. An attempt to 3D print continuous carbon fiber with epoxy resin was also made and inspired by the FFF process.

The printed material exhibited high mechanical properties over-performing composites of thermoplastic and continuous carbon fiber. The elastic modulus and tensile strength of these composites reached respectively 162 GPa and 793 MPa [START_REF] Hao | Preparation and characterization of 3D printed continuous carbon fiber reinforced thermosetting composites[END_REF].

Usually, when manufacturing composite the porosity is not desired but in some applications such as regenerative medicine or sensors, voids are essential. Accordingly, in a recent study, porous printed structures of different thermosets were developed by tuning the rheology with sacrificial particles such as sodium chloride. The added particles play a double role of rheology modifier to enable the printing of the structure and also as porosity precursors as they will be removed after curing of thermosets by water dissolving and freeze-drying [START_REF] Lei | A general strategy of 3D printing thermosets for diverse applications[END_REF].

In recent years, AM of thermoset polymers gained more popularity and many techniques were developed to tune the rheology and to enhance mechanical performances. However, the major limitation of these resins is their non-recyclability. Accordingly, some researchers developed a recyclable 3D printing approach for thermally cured polymers by using a recyclable vitrimer epoxy by solvent dissolution [START_REF] Shi | Recyclable 3D printing of vitrimer epoxy[END_REF] and Diels-Alder reversible thermoset [START_REF] Yang | Diels-Alder Reversible Thermoset 3D Printing: Isotropic Thermoset Polymers via Fused Filament Fabrication[END_REF]. To tune the rheology of the vitrimer epoxy, 18 wt% of nanoclay were added and the blend was precured to increase the viscosity. The printed parts had limited mechanical properties with tensile modulus and strength respectively 11 MPa and 6 MPa. Besides of epoxy, many other thermoset polymers were also used to bestow higher performances to the printed part. For instance, cyanate ester resin was an ideal candidate for DIW of high thermal performance composite [START_REF] Chandrasekaran | 3D printing of high performance cyanate ester thermoset polymers[END_REF]. Indeed, the printed composites (containing 20 wt% silica nanoparticles) exhibited excellent thermo-oxidative stability up to 420°C and high Tg temperature. Furthermore, owing to its high char residue the printed part was carbonized and yielded a stiff carboneous material while conserving its original printed structure. More recently in the frame of a master's degree thesis [START_REF] Stellato | Renewable Monomers for 4D Printing[END_REF], a fully bio-based printed composite was developed based on a blend of PFA resin and lignin. The ink was prepared by blending 74% of PFA with 18% lignin and 7 % fumed silica which played the role of rheology modifier. In this work, the curing and consolidation step was conducted in-situ by loading two different feedstocks of the curing agent and the ink. Afterward, the two feedstocks are mixed in the extrusion head and the crosslinking of the thermoset starts as soon as the material is extruded and deposited. This in situ curing technique, illustrated in Figure 1-14, was also reported before by [START_REF] Rios | 3D printing via ambient reactive extrusion[END_REF] without mentioning the used polymer. The temperature of curing and the catalyst ratio were carefully chosen and optimized to avoid the foaming of the furanic resin. The printed furan-based material had flexural modulus and strength respectively 3.8 GPa and 88 MPa.

Figure 1-14: Schematic of the printing process for reactive polymers [START_REF] Rios | 3D printing via ambient reactive extrusion[END_REF].

Conclusions

Notwithstanding, additive manufacturing of polymer composites has known significant developments in recent years, it is still not ready for larger industrial use. Several limitations of this technology need to be overcome such as the poor material choice. The polymer selection for composites additive manufacturing is limited to printable materials. Currently, the majority of 3Dprinted composites are thermoplastic-based owing to the easy process of hot-melt extrusion by the FFF technique. However, these materials have low Tg and poor thermal properties which do not meet As discussed previously, sustainability gained a large interest in the manufacturing of products due to the dwindling of fossil resources. The use of bio-based polymers along with natural fibers is the key to shift towards materials with a carbon-neutral footprint. One such bio-based and available polymer is PFA. In this section, the production and the polymerization steps of this renewable polymer will be reviewed.

1.3.1.1 Furfural monomers -one of the bases of furan polymers

1.3.1.1.

Source and synthesis of furfural

For a better understanding of the preparation and properties of the PFA macromolecules, is wiser to deal with the synthesis and production of the furfural molecule, i.e the precursor of the monomer which represents the base of the used material in the present project.

For the sake of terminologies, it is worth mentioning that furfur is actually a Latin word that means bran [START_REF] Fink | Chapter 7 -Furan Resins[END_REF]. Furfural molecule is not a recent discovery as it was first isolated in the early 19 th century by Döbereiner when producing formic acid. However, industrial production only started in the early 20 th century [START_REF] Dunlop | The furans[END_REF]. Manufacturers were and still are very interested in the production of this molecule because of the availability and the cost-effectiveness of the raw materials and the production process. Indeed, this molecule can be readily produced from any agricultural and forest waste containing pentoses. Therefore corn cobs, oat, and rice hulls, sugarcane bagasse, cotton seeds, olive husks, wood chips can all be used as a feedstock for the production of this monomer (Choura et al., 1996;Gandini, 1997). 2-furancarboxaldehyde, more commonly called furfural, is produced from hemicellulose and more specifically from pentoses. These polymeric pentoses (pentosans) are hydrolyzed to break them down into small units. These monosaccharide units, such as xylose, will be the base for the synthesis of furfural molecules. A general overview is presented in Figure 123456789101112131415. [START_REF] Istasse | Mechanistic aspects of saccharide dehydration to furan derivatives for reaction media design[END_REF].

After hydrolysis of xylane (pentosan), the resulted xylose (monosaccharide) is progressively dehydrated and then cyclized to give the furfural molecule (Gandini, 1997). The reactions included in this step are schematized in Figure 12345678910111213141516.

Industrial production of furfural

There are two processes for furfural production, either a one-stage or two-stage process. In the onestep process the depolymerization of pentosans by acid and the dehydration of the resulted pentoses, happen simultaneously in a single reactor. The two-stage process starts first with the dissolution of pentosans in the mild condition in one reactor. Then, in another reactor, the pentoses are dehydrated to yield furfural [START_REF] Machado | Literature Review on Furfural Production from Lignocellulosic Biomass[END_REF][START_REF] Mesa | Restructuring the processes for furfural and xylose production from sugarcane bagasse in a biorefinery concept for ethanol production[END_REF].

The first furfural production was a two-stage process originally developed by Quaker Oats Technology. The hydrolysis step is performed in an aqueous acid batch of sulphuric or phosphoric acid (2.2 wt%) and with steam at 153°C. After the second stage of cyclodehydration, furfural is recovered by steam stripping from the solution. This process has many drawbacks such as high effluent production and high energy consumption for a low furfural yield [START_REF] Dashtban | PRODUCTION OF FURFURAL: OVERVIEW AND CHALLENGES[END_REF].

Many studies have been carried out to improve furfural production to develop a more environmentally friendly process. For further reading, Mehdi Dashtban has published a detailed review treating this subject [START_REF] Dashtban | PRODUCTION OF FURFURAL: OVERVIEW AND CHALLENGES[END_REF].

These drawbacks and environmental concerns led to the closing of many furfural production plants.

Nevertheless, the furfural global market size is in constant growth with about $479 million in 2019

and is expected to grow by 6.2% from 2020 to 2027 according to Grand View Research online data (Grand view research, 2020). China is the leading producer with over 200,000 ton/year in 2005 using corncobs as feedstock. Dominican Republic is also another major actor in the furanic production with 40,000 tons per year of furfural produced from sugarcane bagasse which is transformed to furfuryl alcohol by Transfurans Chemicals in Belgium [START_REF] Istasse | Mechanistic aspects of saccharide dehydration to furan derivatives for reaction media design[END_REF].

By simple chemical reactions, it is possible to obtain many derivatives of furfural (Gandini, 1997).

However, the majority of furfural is industrially converted into furfuryl alcohol (FA) because the latter has found many applications. Polymerization and brunching mechanisms as well as the usage of this industrialized product will be summarized in the following section.

1.3.1.2 Furfuryl alcohol (FA)

1.3.1.2

.1 The synthesis of furfuryl alcohol from furfural

FA is made from furfural by reduction with hydrogen as shown in Figure 1-17. FA can be further converted into 2,5-bis(hydroxymethyl)furan (BHMF) by formylation as illustrated in Figure 1-17 (Gandini, 1997). This molecule is majorly used in form of polymer or oligomer of poly(furfuryl alcohol). The polymerization requires an acid catalyst and leads to a black crosslinked solid material. To understand the reactions involved in the formation of this network, many studies have been conducted since 1953.

When in contact with an acidic medium FA goes through a polymerization process in which the first step is a condensation reaction. The condensation reaction can take place following two paths as illustrated in the simplified schema in Figure 1-18. In reaction [1], the condensation occurs between the OH of one monomer with the free hydrogen (H5) atom in the furan ring of another monomer to eliminate one water molecule. In this case, the structure is called head-to-tail and yields a methylene bridge between furan rings. When following reaction [2], the hydroxyl group of one monomer condensates with the same group of another monomer with the elimination of one molecule of water. This structure is called head-to-head and yields an ether moiety between the two monomers.

Generally, reaction [1] is more frequent, so that in the ensuing oligomers it is more probable to find methylene bridges because the reaction is faster and under strongly acidic conditions the ether moieties can lose a formaldehyde molecule to revert to the former moiety, as shown in Figure 1-18.

Therefore, in the macromolecular structures methylene bridges are in general much more frequent than their ether counterparts (Choura et al., 1996;Gandini, 1997).

Crosslinking of FA -Puzzles of a complex structure collected over years

Condensation reactions can only lead to a linear structure and do not explain the crosslinking of this polymer. In fact, the condensation reactions discussed above, would lead to a colorless thermoplastic material. The polymerization of FA instead yields a yellowish resinous oligomeric compound when the reaction is stopped at its early stages. As the reaction evolves, the material starts to develop a darker color and higher viscosity until turning into a black solid product (Gandini, 1997;Iroegbu and Chapter 1: State of the art Khaoula BOUZIDI 2022 67

Hlangothi, 2018). Therefore, the PFA structure and the involved crosslinking reactions are more complex than a simple linear polycondensation.

This complex structure has puzzled chemists for decades. As analysis techniques evolve, scientists had more data and hence more information about the structure emerged. The first investigation was conducted in 1953 where the linear structure by condensation, discussed before, was explained and the crosslinking was associated with the formation of methylene bridges thanks to the formaldehyde released after the reversion of di-methylene ether moieties into methylene. A simplified illustration of this structure is given in Figure 1-19 [START_REF] Dunlop | The furans[END_REF]. A decade later, thanks to the use of FTIR spectroscopy, a furan ring-opening, as shown in Figure 1-20, was identified after the detection of γ-diketones [START_REF] Conley | An investigation of the structure of furfuryl alcohol polycondensates with infrared spectroscopy[END_REF]. In 1971, a paper was published proposing a polymerization pathway for the termination of the chains when using alumina catalysts [START_REF] Wewerka | Study of the γ-alumina polymerization of furfuryl alcohol[END_REF]. Over a decade later, a new analysis using 13 C-NMR, confirmed the ring-opening and the crosslinking through methylene bridges as suggested back in the fifties. The published work also confirmed the lack of di-methylene ether bridges comparing to their methylene counterparts (Chuang et al., 1984). In addition to 13 C-NMR, 1 H-NMR was used in analyzing furfuryl acetate structure. This study identified for the first time aside reaction leading to a conjugated structured as represented in Figure 1-21 [START_REF] Buchwalter | The polymerization of furfuryl acetate in acetonitrile[END_REF]. In 1996, a systematic study was conducted using different model compounds and using FTIR spectroscopy and 1 H-NMR for polymer characterization. This study confirmed the linear polymerization and formation of methylene bridges as the first phase of curing. The second phase according to the research group involved many side reactions in particularly the conjugation of the furan-methylene chain which is at the origin of the coloration of the cured product. Authors attributed the brunching to Diels-Alder reactions between conjugated and unconjugated chains as highlighted in Figure 1-22 (Choura et al., 1996).

Figure 1-22: Diels-Alder reaction interchains. adapted from [START_REF] Falco | Opening Furan for Tailoring Properties of Biobased Poly(Furfuryl Alcohol) Thermoset[END_REF].

Over the last two decades, many analysis techniques were used such as chemo-rheological kinetics (Guigo et al., 2007a) Attenuated total reflectance (ATR), FTIR spectroscopy [START_REF] Barsberg | Poly(furfuryl alcohol) formation in neat furfuryl alcohol and in cymene studied by ATR-IR spectroscopy and density functional theory (B3LYP) prediction of vibrational bands[END_REF], Raman spectroscopy [START_REF] Kim | Effects of solvent on the furfuryl alcohol polymerization reaction: UV Raman spectroscopy study[END_REF], and 13 C solid-state NMR spectroscopy [START_REF] Falco | Opening Furan for Tailoring Properties of Biobased Poly(Furfuryl Alcohol) Thermoset[END_REF]. These studies mainly confirmed the structures revealed before with some more specifications about their frequencies.

More recently, a new study investigated this complicated structure of PFA by complementing 13 C-NMR, FTIR, and UV-Raman spectroscopies with spectra modeling. The author confirmed again the presence of most of the structures mentioned before but their NMR analysis showed a limited signal for the conjugated and the Diels-Alder structures. This study suggested for the first time a new structure resulting from the furan ring opening and the linear structure presented in Figure 1-23. This new structure was supported by 13 C-NMR and FTIR techniques [START_REF] Tondi | Understanding the Polymerization of Polyfurfuryl Alcohol: Ring Opening and Diels-Alder Reactions[END_REF]. [START_REF] Tondi | Understanding the Polymerization of Polyfurfuryl Alcohol: Ring Opening and Diels-Alder Reactions[END_REF].

This rather complicated structure confers to the cured plastic interesting properties and assets.

These Furanic macromolecules have no functional groups. Therefore, the crosslinked material has outstanding alkali resistance, solvent resistance, and good corrosion resistance to inorganic acid.

Another well-known asset of PFA is its good heat resistance, low inflammability, and low smoke release (Gandini, 1997). The crosslinked material can indeed be used at 120°C to 140°C for a long time and in some cases at 180°C [START_REF] Wang | 3 -Matrix materials[END_REF]. However, its oxidation resistance is poor because of the possible furan ring-opening under certain conditions. Another limitation to the use of PFA is its brittleness which requires the utilization of plasticizer or blending it with other polymers. In the literature, one can find the usage of this product in numerous and different areas, such as the historical metal cores, fragrances production, and of course composites.

Some applications of furfuryl alcohol monomer and polymer

FA is historically often used in the foundry as a binder for the fabrication of molds and cores. This still holds nowadays, as in 2019 foundry industry monopolized FA market share by over 80% (Grand view research, 2020). What makes this furanic binder popular in the foundry industry is its lower volatile organic compound, less formaldehyde, and less smoke release during casting comparing to phenolic urethane thermosets [START_REF] Fink | Chapter 7 -Furan Resins[END_REF]. Preservation, or as commonly called "furfurylation" of wood is another usage of low-viscosity FA. This furanic compound is a great eco-efficient substitute to toxic and hazardous compounds such as salts of copper [START_REF] Goldstein | Stable Furfuryl Alcohol Impregnating Solutions[END_REF][START_REF] Lande | Furfurylation of Wood: Chemistry, Properties, and Commercialization[END_REF].

FA is also used in form of polymers for the fabrication of resins. Thanks to their excellent resistance to corrosive chemicals these resins find applications as cementitious grouts, coating, impregnating materials, and sealants [START_REF] Schmitt | Polyfurfuryl Alcohol Resins[END_REF]. FA can also be used in its liquid state as a solvent serving as a cleanser, paint softener, or dye dispersant for textile applications (Pizzi and Mittal, 2003).

Pyrolysis of PFA and the ensuing materials gained also much interest in many applications such as carbon/carbon composites (brakes and clutches, rocket motors, heatshield, aero-engine components), electrodes, etc... The pyrolysis process was studied by many researchers (Wang et al., 1997;[START_REF] Zarbin | Preparation, characterization and pyrolysis of poly(furfuryl alcohol)/porous silica glass nanocomposites: novel route to carbon template[END_REF] and it was found that until 300°C the condensation and crosslinking reactions dominated. When exceeding 350°C, loss of hydrogen and oxygen led progressively to carbonization which is accomplished at about 900°C (Gandini, 1997).

FA finds also use in many different fields such as flavoring and fragrances [START_REF] Centi | Chapter 16 -Catalytic Transformation of CO2 to Fuels and Chemicals, with Reference to Biorefineries[END_REF], pharmaceuticals [START_REF] Mohamed | Revaluation of biomass-derived furfuryl alcohol derivatives for the synthesis of carbocyclic nucleoside phosphonate analogues[END_REF], and rocket fuels [START_REF] Kulkarni | Theoretical Evaluation and Experimental Validation of Performance Parameters of New Hypergolic Liquid Fuel Blends with Red Fuming Nitric Acid as Oxidizer[END_REF]. As mentioned previously, cured PFA has excellent fire resistance properties and low smoke emission but is a brittle material. Accordingly, this polymer would be a good matrix to use combined with different fillers providing it with better mechanical properties and different functionalities. Indeed, this was the idea of some research groups who used PFA as a matrix for composites which will be further detailed in the next section.

1. Under higher temperature and longer thermal treatment, the aromatic microdomains enlarge and rearrange. Therefore, the structure becomes more ordered and the pores shrink as illustrated in the conceptual model in Figure 1-24 [START_REF] Mariwala | Evolution of Ultramicroporous Adsorptive Structure in Poly(furfuryl alcohol)-Derived Carbogenic Molecular Sieves[END_REF]. Furthermore, the electrical conductivity of the resulting PFA-derived carbon increases when increasing the carbonization temperature. At above 850°C the pyrolysis yields a highly conductive material (Wang et al., 1997).

Figure 1-24: Conceptual model for adsorptive structure evolution in PFA-derived carbon [START_REF] Mariwala | Evolution of Ultramicroporous Adsorptive Structure in Poly(furfuryl alcohol)-Derived Carbogenic Molecular Sieves[END_REF].

Accordingly, PFA was used as a precursor for many carbon-based composites. A nanocomposite of carbon and silica was prepared by using a sol-gel process to obtain mesoporous carbon. The precursor was prepared using tetraethyl orthosilicate in the presence of FA followed by the polymerization of FA and its subsequent carbonization. The removal of the silica framework from such nanocomposite led to the formation of porous carbon. The size of the pores was directly influenced by the sol-gel reaction conditions [START_REF] Kawashima | Preparation of Mesoporous Carbon from Organic Polymer/Silica Nanocomposite[END_REF]. PFA was also used as precursors for the fabrication of films with high electrical conductivity (sheet resistances 13 Ω/sq).

PFA resin was mixed with 1 wt% of graphene and FA microspheres, obtained after emulsion polymerization, were mixed with graphene oxide to yield porous film. This porous film was then selectively carbonized according to the desired pattern. This carbonaceous porous film was demonstrated to be an excellent material for the fabrication of micro-supercapacitors with a maximum capacitance of 16.0 mF/cm² at 0.05 mA/cm². The micro-supercapacitor had also an impressive life cycle, hence it could retain up to 97.4% of its capacitance after 10000 cycles. This PFA/graphene oxide composite has many potential uses especially in wearable electronics (Hawes et al., 2019).

Lignocellulosic fillers

As discussed before, recently the interest in composites based on renewable materials has grown because of greater environmental awareness. When coupling PFA with natural fillers in particularly lignocellulosic fillers we can obtain a fully biomass-based composite. One popular natural fiber used in green composites is flax fiber and which was unsurprisingly combined with PFA. Examples include the use of PFA as copolymer in cellulose acetate butyrate. The copolymers mixture was added to a flax nonwoven mat to reach a 50 wt% fiber fraction. The well-impregnated flax mat thanks to the effect of FA was cured by compression molding following many curing steps to release byproduct from FA poly-condensation and thus avoid foaming and possible defects of the composite. High pressure of about 2.2 MPa had to be applied and temperature cycles up tol 155 °C had to be used for a long time which makes the manufacturing rather complicated and energy-consumer. The ensuing composite had an improved storage modulus compared to the neat matrices and the addition of 25 wt% thermoplastic improved the stiffness of PFA at temperatures higher than 80°C. Changes in mechanical properties were evaluated for different PFA fractions in the matrix phase of the composite and it was found that the thermosetting resin promoted the increase of both stiffness and strength at rupture to reach respectively 12.2 GPa and 95 MPa when adding 100 wt% PFA against 4

GPa and 70 MPa without PFA. However, these improveming were at the expense of impact strength.

Nevertheless, in this study, the impact of the reinforcement with flax nonwoven fibers was not evaluated as compared to neat matrices [START_REF] Toriz | Novel cellulose ester-poly(furfuryl alcohol)flax fiber biocomposites[END_REF].

In a similar approach, humins were combined with PFA which enhanced the mechanical properties thanks to the plasticization role of humins [START_REF] Pin | Valorization of Biorefinery Side-Stream Products: Combination of Humins with Polyfurfuryl Alcohol for Composite Elaboration[END_REF]. In another study, aligned natural fiber textiles, randomly oriented natural-fiber textiles, and glass fiber textiles have been hand laid up and impregnated with a commercial furan resin to form prepregs. The composite prepregs were cured into composite plates by compression molding. During compression molding, the mold was lifted several times to evaporate the water resulting from the poly-condensation. The curing process lasted less than 8 min, since the prepregs were precured for 8 hours, and 2 MPa pressure was applied. The approximate void content in the different composites was measured with a µCT scanner, and it was found that the aligned flax fibers composites had the highest void content spatially compared to glass fiber composites which could be caused by the unfilled lumen of natural fibers as shown in the SEM image in Figure 1-25. Accordingly, the specific Young's modulus of glass fiber and aligned flax fiber composites were similar of respectively 12.4 GPa•m 3 /kg and 11.4 GPa•m 3 /kg except for strength which was higher for composite with the synthetic fibers. Moreover, the authors showed that the sensitivity of fiber orientation to load angle was less in the case of flax fibers comparing to glass fibers. These interesting mechanical properties of the fully green composite were explained with a high matrix fiber adhesion owing to a similar chemical composition between the two phases. The fire resistance properties were also evaluated and contrary to the glass fibers base composites the fully natural composite was ignited but the burning rate was low revealing good fire retardance [START_REF] Pohl | Properties of compression moulded new fully biobased thermoset composites with aligned flax fibre textiles[END_REF]. [START_REF] Pohl | Properties of compression moulded new fully biobased thermoset composites with aligned flax fibre textiles[END_REF].

As one can notice, voids in composite, created because of the morphology of natural fibers and of steam bubbles released during curing, present a major challenge in the development of PFA-based green composite. In another study presented in the 15 th European conference on composite materials, aligned flax fibers were impregnated by PFA and were cured using vacuum molding and casting which was found to be not possible because of the boiling of water solvent as shown in Figure 1-26. The elimination of solvent was reported to be quasi-impossible because of the exponential increase of viscosity. It was found that a low-temperature cure to prevent the exothermal peak from exceeding the boiling point of the solvent could stop the foaming of the resin as visible in Figure 1-26-D. The adapted technique was found to be a beta-stage compression molding which insured good mechanical properties with a tensile strength of 211 MPa [START_REF] Crossley | The development and processing of a sustainable fully bio derived polyfurfuryl alcohol matrix flax fibre prepreg 8[END_REF]. [START_REF] Crossley | The development and processing of a sustainable fully bio derived polyfurfuryl alcohol matrix flax fibre prepreg 8[END_REF].

The development of more eco-friendly materials is also interesting to the European countries which have launched a European program project called BIOCOM for the development of high-performance composites. In the frame of this project, an adapted PFA commercial resin for the composite was developed by Transfurans Chemicals. This natural matrix was combined with flax woven textile and glass fibers for the fabrication of vehicle panels. These composites were manufactured using the vacuum bag technique and compression molding for the consolidation step. By characterizing mechanical properties it was found that Young's modulus of the composite with glass fibers and flax fibers were similar (7.2 GPa and 8.5 GPa respectively) but the tensile strength of the latter was 30% lower than the former. In virtue of their end-use, the mechanical properties were also evaluated after aging in different fluids and it was shown that the use of natural fibers caused a drop in mechanical performances with a remarkable loss in stiffness caused by their hydrophility with more than 50% drop in Young's modulus after mmersion in water [START_REF] Giannis | Development of high performance bio-composites based on furan bio-resins for vehicle panels[END_REF].

In another study, 0% to 20 wt% of bleached kenaf fibers were added to the PFA matrix. The optimum properties of the green composites were achieved by 20 wt% fiber loading which showed significant increments in tensile strength (310%), storage modulus (123%), and flexural strength (48%) compared to neat PFA. In addition, the introduction of kenaf fibers improved thermal stability with an increase in storage modulus values. Mechanical properties were also reevaluated after boiling-water absorption. These mechanical performances were retained up to 83% for tensile, 89%

for flexural, and 82% for impact strength (Deka et al., 2013). When used with low fractions, PFA or FA can also be used for reinforcement of cellulosic fibers such as microfibrillated lignocellulose (MFCL) (Lems et al., 2019) and bamboo fibers (Wang et al., 2019b). In both cases, it was found that the furfurylation of the lignocellulosic fibers enhances mechanical and thermal performances especially when the PFA fraction approaches 50 wt%. On the other hand, the authors mentioned a degradation of the mechanical properties of the cellulosic material caused by the use of acidic catalyst which should have been avoided (Wang et al., 2019b). Furthermore, it was proven that cellulosic fibers containing a higher amount of residual noncellulosic polymers had better wetting and better compatibility with FA during the in-situ polymerization (Lems et al., 2019).

To replace the use of bast fibers with more sophisticated fibers, the use of high tenacity rayon fibers was considered [START_REF] Malaba | Unidirectional Cordenka Fibre-Reinforced Furan Resin Full Biocomposite: Properties and Influence of High Fibre Mass Fraction[END_REF]. In this research work aligned rayon fibers were compressed to form sheets and then PFA resin was added. The composite was cured by mold compression with a fibers mass fraction ranging from 51 to 75%. First of all, the study of the laminate cracking behavior revealed the two materials to be compatible as a composite system but the matrixfibers adhesion deteriorated at fibers fraction as high as 75 wt%. Moreover, for composite with not more than 64% fibers, the mechanical and thermal properties enhanced which make them a lighter substitute to glass fibers. Despite the long curing process and the pressure used, porosity in these composites was really high (38% for 51 wt% of fibers) which believed to be caused by steam bubbles during curing. The voids volume decreased when adding more fibers to reach 13%. In spite of the sustainability of this bio-based composite rayon fibers are artificial and not fully natural fibers and undergo a polluting production process which might harm the overall environmental impact of this composite. Bio-based nanocomposites of PFA and 2 wt% freeze-dried sisal whiskers were prepared by an in-situ polymerization method. After the dispersion of nanofibers by sonication a "resinification" was conducted at 85°C and then the composite was cured in open mold by gradually increasing curing temperature. The nanocomposite had improved viscoelastic properties by increasing the storage modulus of PFA. However, thermal stability studied by TGA was almost unchangeable compared to neat matrix (Ahmad et al., 2013). Recently, Menager et al. reported a first approach for preparing a fully bio-based composite using micro (50-200 µm) and macro (2-4 mm) particles of cork [START_REF] Menager | Green" composites prepared from polyfurfuryl alcohol and cork residues: Thermal and mechanical properties[END_REF]. PFA insured a good wetting of the empty cork cells which was observed by SEM images and also testified with lower water absorption. Moreover, cork incorporation enhanced thermal stability and improved the rigidity of the matrix by increasing the storage modulus. Tensile modulus and strength were optimized with 25 wt% added cork powder. In fact, cork powder over-performed cork granules by making the material more rigid owing to a higher density (storing more energy per unit of volume).

PFA and fillers

Among the most widely used fillers added to thermosetting resins, we can find carbon nanotubes (CNT) multi-walled (MWCNT), single-walled (SWCNT), or double-walled (DWCNT). These nanomaterials were also added to PFA to achieve different functions and properties. For instance, MWCNT with diameter distribution of 100-300 nm and length of about 10 µm were dispersed in PFA resin with ultrasonication for 30 min. MWCNT concentration in the polymer ranged from 5 to 15 wt%. To characterize the resulting material, the PFA-MWCNT mixture along with an acidic curing agent was either poured in a plastic tray for the production of bulk specimens or in the doctor blade apparatus for the fabrication of film specimens. It was found that composites prepared with the doctor blade technique had a preferentially aligned MWCNT in the blading direction as clearly visible in field emission scanning electron microscope (FE-SEM) micrographs in Figure 1-27. Unsurprisingly, this alignment of the nanotubes caused an anisotropic electrical conductivity with an electrical conductivity parallel to the blading direction of 10 -5 S/m which is 4 orders of magnitude higher than in the perpendicular direction and 6 orders of magnitude higher than the bulk specimen. However, this electrical conductivity in the two directions converged to the magnitude order as MWCNT concentration gets high (15 wt%). Furthermore, when using the doctor blade technique, there was a slight tilting of the nanotubes in film creating additional contacts between nanotubes resulting in higher electrical conductivities in the film than in the bulk (Lanticse et al., 2006).

Figure 1-27: FE-SEM micrographs of the cross-sections of (A) bulk (B) the film viewed at the direction perpendicular to the blading direction (Lanticse et al., 2006).

Besides improving electrical conductivity, MWCNT were also used to improve the tribological properties of PFA composite coatings by mean of functionalization by grafting FA onto their surface.

The used MWCNT had a diameter of 8 nm and length of 50 nm and they were first oxidized to obtain MWCNT-COOH then treated with thionyl chloride to obtain MWCNT-COCl which were treated with FA to finally obtain MWCNT-FA. After this grafting, it was found that the overall dispersion of these nanotubes was enhanced. It was also reported that the functionalization of the MWCNT reduced friction and anti-wear ability of the poly(furfuryl alcohol) composite coatings which were explained by the self-lubrication property of MWCNTs and the dispersion enhancement (X. H. Men et al., 2008).

The same research group has also successfully modified MWCNT by perfluorooctanoic acid (PA) to obtain fluorocarbon-modified MWCNTs. PFA/MWCNT films were then prepared with a superhydrophobic surface. This hydrophobicity was attributed not only to the low surface energy of the fluorocarbon-modified MWCNTs but also to the roughness structures on the surfaces. This hydrophobicity was confirmed with water contact angle and water sliding angle measurements. And the surface morphology was evaluated with FE-SEM (X.-H. Men et al., 2008).

Moreover, fully bio-based nanocomposites were prepared using PFA as a matrix and cellulose whisker and montmorillonite clay as nano-fillers. These nanocomposites were prepared by dispersing nanoparticles in FA then in-situ polymerized. The polymerization was catalyzed by the fillers because of their acidic character. These fillers improved the thermal stability of the plastic matrix. For instance, the addition of 0.8 wt% of cellulose whiskers increased the onset of degradation by 80°C while the addition of montmorillonite clay increased the residual weight above 400°C (Pranger and Tannenbaum, 2008). In another work later in 2021, the researchers confirmed the results of their first work and explained the better thermal stability of cellulose fillers as compared to montmorillonite clay by a stronger matrix-nanoparticles interaction. The same research group evaluated the mechanical properties of bio-based composites with added cellulose whiskers. This composite had a limited stiffness with an elastic modulus of about 1 GPa and over 30 MPa breaking strength [START_REF] Pranger | Mechanism of the nanoparticle-catalyzed polymerization of furfuryl alcohol and the thermal and mechanical properties of the resulting nanocomposites[END_REF]. In addition to organic nanoparticles, inorganic nanofillers were also used combined with PFA to yield a nanocomposite with high thermal stability. A hybrid material based on the dispersion of silica in PFA was prepared. The nanoparticles of silica were homogeneously dispersed in the furanic matrix which outputs an improvement of the Tg temperature and the overall thermal stability thanks to the confinement of furanic chains. By detecting and analyzing volatile organic compounds during thermal degradation, the authors proposed a multi-step degradation pathway for the neat PFA. According to their analysis, this degradation starts first by the scission of methylene and methyne links then at a higher temperature the scission of the furanic links starts (Guigo et al., 2009).

An overview of the composites developed with PFA highlighting their mechanical and thermal properties is presented in Table 1-8. According to this data, one can notice that PFA has been mainly coupled either with CNT or with different classes of natural non-wood fibers such as flax, sisal, and bamboo. These composites were manufactured using different methods but compression is always applied and if not PFA was used to produce thin films pobably due to high porosity content which this resin can generate when manufacturing bulk structures. In terms of mechanical properties, it was reported that the neat PFA has a stiffness below 1 GPa (tensile mode) and tensile strength of about 10 MPa (Deka et al., 2013;Kherroub et al., 2015). After the addition of continuous reinforcing phase, for instance, aligned flax fiber woven textile, the stiffness of the resulting composite reached a value of 25 GPa and its strength attained 211 MPa. However, when adding discontinuous filler we notice a limited improvement in Young's modulus which remains under 2 GPa but a considerable improvement in the strength of about 400% compared to the neat polymer. In terms of thermal properties, the results can be quite dispersed depending on the used polymer and filler with a Tg ranging from 80°C to over 130°C and a storage modulus over 1 GPa at 160°C in some cases. The addition of some of these fillers (especially nanoparticles) served to confer the polymer with new properties such as electrical conductivity, long wear life, and superhydrophobic surface.

How green is PFA?

Although it is true that PFA is a fully renewable and bio-based resin, the production of furfural and FA, as already explained, seems to be an energy-consuming process and generating polluting wastes.

One cannot abstain from considering the actual environmental impact of such production. In 2015 a life Assessment Analysis (LCA) was conducted to quantify the impact of furfural and furfuryl alcohol production from corncob by identifying the main pollution processes and substances [START_REF] Hong | Environmental and economic impact of furfuralcohol production using corncob as a raw material[END_REF]. According to this LCA, the corncob production, transport, and electricity consumption stages had the greatest impact on the environment because of direct heavy metal, phosphate, and phosphorus emissions. After data analysis, the authors suggest increasing furfural yield, optimizing wastewater and electricity consumption efficiencies, and decreasing corncob transport distance for more cost-effective and eco-friendly production. A comparative LCA study between a PFA-based composite and a fossil-based matrix using natural fibers for reinforcement would be interesting to compare the real environmental impact enhancement of the use of this kind of bio-based resins.

Conclusions

In short, furfural is a cheap and sustainable utility chemical. Its manufacturing is simple and calls upon large variety agriculture wastes available in every part of the world. This chemical is described by David Win Tin as "Gold from Garbage [START_REF] Win | Furfural -Gold from Garbage[END_REF]. Furfural is mostly used to produce furfuryl alcohol used in various applications in different fields. The production of bio-based composites is among these applications. When combined with natural fibers and fillers this polymer can yield a fully green composite and one can find many examples in the literature. However, compared with this polymer benefits, its use in composites stays humble. The reason behind this poor exploitation might be the unclear understanding of its curing process during which many defects can occur such as voids creation. Nevertheless, this polymer confers excellent properties sought-after in composites such as thermal stability, low smoke release, fire resistance, and resistance to chemical attacks.

Chapter 1: State of the art Khaoula BOUZIDI 2022 83

Chapter Conclusions

To conclude, composites are interesting materials used in many sectors which can be engineered according to their final use by selecting the adapted matrix and fillers. As environmental concerns are increasing and the fossil resources are dwelling, the need for more eco-friendly and sustainable materials starts to be a priority for their futur use. Many research projects have been conducted to replace the synthetic fibers and plastic usually used in composite production with natural feedstocks such as biomass. Despite the promising enhancement in mechanical properties of the composites, many challenges were faced when using natural fibers such as poor fiber/matrix adhesion, humidity sensitivity, and high porosity. Recently, various research works are being done for composite production using innovative additive manufacturing technology. The majority of these printed composites were produced using the fused filament fabrication by blending thermoplastics with continuous or short fiber. This technique enabled a rather simple way for composite printing but the matrices choice is limited to thermoplastics. Thermoplastic printed composites had greater mechanical properties in respect to the neat polymers used in FFF (until x50 better) but their thermal properties remain limited to the matrix thermal stability which can rarely be used under temperature exceeding 100°C. Although some printing filament with high thermal stability exit on the markets they are usually overpriced and skill-demanding.

To overcome these limits, composites using thermosets polymers were printed with the DIW technique by tuning the rheology with nano and macroparticles. These thermosetting composites exhibited higher thermal properties and significant mechanical properties enhancement. However, the most used polymers were petroleum-based and non-sustainable. A schematic summary of the state of art presented in this chapter is presented in Figure 1-28. Khaoula BOUZIDI 2022 108

Chapter introduction

To fulfill the PhD project goals many materials, processes, and characterization methods were used and will be detailed in this chapter to provide essential information before tackling the results discussed in the following chapters. This chapter is divided into 5 main sections related to the composite's production phase (Figure 2-1):

• Section 2.2: Description of the used commercial materials in the development and optimization of the composites.

• Section2.3: Detailing the slurry preparation step with an overview of the different tested formulations.

• Section2.4: Explication of the composite production step including a description of the used 3D printer, the software, the bulk composite molding, and the curing process.

• Section 2.5: Description of the carbonization step of the cured composites.

Cellulose powder and fibers

In this project, the used cellulose is a commercial product by CFF from their Technocel® pure series and was kindly supplied by Rossow Industry. The mainly used cellulose is the finest powder in this product line, known under the trade name Technocel® FM8. According to the supplier's datasheet, this cellulose has a powdery aspect, with over 99.5% purity, and 80% whiteness. Moreover, the particle size was reported to be between 6-12 µm and the specific density 1.5 g/cm 3 . The density was further confirmed by experimentally measuring the apparent density of cellulose pellets (obtained by powder compression at 1.2 GPa) which is in line with solid cellulose density values. According to the supplier's information, the water content of this powder is vaguely determined to be below 8 wt% and was experimentally measured to be 4.58 ±0.05 wt% using the standard method to determine the dry content of paper, board, and pulps (ISO 638:2008). The Technocel®200, made of 300 µm long fibers, was also used in some preliminary tests. According to the little information provided by the supplier and the limited information published, the used raw material is wood fibers (hardwood) which were cleansed, bleached and hot steam dried before ending up in the CFF production sites in Germany. The fibers are then milled multiple times and, if necessary, finely milled and sieved to reach the desired particle size [START_REF] Lg Hemija | CFF[END_REF]. A more detailed characterization of the cellulose morphology, size distribution, and structure will be presented in the next chapter.

Carbon nanotubes

Two sources of Carbon NanoTubes (CNT) were used during this project. First, for preliminary tests, DWCNT were kindly provided by Centre Interuniversitaire de Recherche et d'Ingénierie des Matériaux of Toulouse, France (DWCNT-L). These CNT were synthesized at a lab-scale by a catalytic chemical vapor deposition method using a removable MgO-based catalyst [START_REF] Flahaut | Gram-scale CCVD synthesis of double-walled carbon nanotubes[END_REF]. In Figure 2-2-

A one can see the CNT bundles thanks to the high-resolution transmission electron microscopy image provided by Dr. Flahaut Emmanuel. The length of these CNT was in the range of 100 µm and their average external diameter of 2 µm [START_REF] Flahaut | Gram-scale CCVD synthesis of double-walled carbon nanotubes[END_REF]. As lab-scale production of CNT is limited, for further experimentation, a larger quantity of commercial CNT was purchased from Nanoamor (TX, USA). [START_REF] Flahaut | Gram-scale CCVD synthesis of double-walled carbon nanotubes[END_REF], (B) commercial DWCNT, and (C) commercial MWCNT short (Nanoamor, 2021).

Different types and morphologies of CNT were used to select the most suitable, their main characteristics, given by the supplier, are listed in Table 2-1. The form factor, defined as the ratio between the length and diameter, ranged from 33 to 2500 and their bulk electrical conductivity (EC)

given by the supplier is estimated to be 100 S/cm. The EC was experimentally measured by the 4

probes technique on a thin layer of CNT and was found to be in line with the supplier's indications.

The morphological difference between the short and long CNT is visible on the TEM photos published by the supplier ( particles have a specific surface area of around 750 m²/g and EC of the order of 10 5 S/cm (STREM, 2021).

The graphite powder, known under the trade name TIMCAL TIMREX® SLP 30, was supplied by Timcal Group (Bodio, Switzerland) consists, according to the technical datasheet, of more than 99.5% graphite with a bulk density of 2.2 g/cm 3 . According to the analysis found in the literature, the size of the analyzed particles was 90% below 31 μm [START_REF] Ghimbeu | Influence of Graphite Characteristics on the Electrochemical Performance in Alkylcarbonate LiTFSI Electrolyte for Li-Ion Capacitors and Li-Ion Batteries[END_REF].

Poly(furfuryl alcohol) polymer

The main polymer matrix used in this project is a commercial poly(furfural alcohol) resin known under the trade name Furolite (i.e. Furolite 050915A RF-2ST) that was kindly supplied by Transfurans Chemicals (Geel, Belgium). On the market, it is possible to encounter different PFA resins and Furolite was selected for its pH neutral nature (pH 4-5) which was developed to be compatible with cellulosic fillers [START_REF] Arnold | Next generation sustainable composites: development and processing of furan-flax biocomposites[END_REF]. Indeed, the action of acids can cause hydrolysis in cellulose chains thereby degrading the fiber's mechanical properties (Thygesen et al., 2007).

According to the supplier's datasheet, this PFA resin contains between 5 and 7.5 wt% water and less than 1% of the FA free monomers. To determine the exact water content, around 15 g of the Furolite

were placed in a vacuum oven at 60°C and left for 120 h to eliminate water. When the resin mass was stabilized the water content was calculated as the difference between the initial and final mass and it was found to be 7.5 wt%. The apparent density of the Furolite is 1.21 g/cm 3 . The apparent viscosity given by the producer is below 5 Pa•s, i.e.3.8 ±0.3 Pa•s as experimentally determined.

Preliminary tests aimed at identifying the PFA/water miscibility limit (carried out by preparing several aqueous solutions and centrifuging them for 20 min at 4020 rpm) showed phase separation when the water content was higher than 24 wt%. Therefore, it was concluded that the resin could be diluted with water up to 24 wt% (including the catalyst). Moreover, the supplier's information claim that the polymer can be stored for a long time (days/months) at temperatures between 20°C and 25°C without risking its polymerization, and according to the literature, the shelf life of this kind of resin can reach 18 months at room temperature [START_REF] Behzadfar | Shelf-life of polyfurfuryl alcohol resin: an accelerated rheokinetics study[END_REF]. The viscous uncured Furolite resin has a brownish color, shown in Figure 234, and is stored in a plastic container in a bunker where the temperature hardly exceeds 25°C even on the hottest days.

A latent hardener that will only catalyze the curing reaction at hot temperature was opted for the resin crosslinking. This latent catalyst will prevent the untimely crosslinking of the formulations and prevent the use of strong acid catalysts that would attack the cellulosic fillers. This latent hardener is a salt-based latent catalyst (M 1448) which consists of a 65 wt% aqueous solution of (2-hydroxyethyl) ammonium nitrate (Wiz chemicals). It has a density of 1.2 g/cm 3 and a boiling temperature of 100°C.

Other chemicals

In addition to distilled water, for dilution and cleansing purposes some organic solvents were used such as denatured ethanol C2H6O at 99% (boiling point 78.4°C) and acetone C3H6O (boiling point 56°C) (PubChem, 2021).

Ink preparation-Formulation step 2.3.1 Paste mixing method

The slurries or 'pastes' were prepared by first adding the catalyst to the resin and manually mixing the two fluids. The mixture was then poured into a planetary mixer (KitchenAid 5KSM3311X) and the cellulose powder was gradually added for 5 min while mixing at 120 rpm. The slurry was further blended for 15 minutes at 180 rpm and then 5 min at 200 rpm. The resulted paste is then manually loaded in 55 cm 3 syringes with precaution to avoid creating any air bubbles and voids. These In this study, different formulations were tested with different cellulose fractions and water content.

As a first step, preliminary formulations were tested using different fiber morphology, different solvent content, and different resin/cellulose ratios. After a better definition of the concentration ranges of every component, more formulations were prepared with and without added water. All tested formulations are placed on the Ternary diagram in Figure 23456.

The preliminarily tested formulation zone highlighted in the red triangular zone and the detailed weight ratio of the components are detailed in Table 2-2. Cellulose weight fraction ranged from 10 wt% to 32 wt% and solvent from at least 19 wt% to 28 wt%. Two different solvents were tested, water and ethanol to compare their effect on drying and solidification of the slurries containing from 6 to 7.5 wt% of water content depending on the PFA polymer content and cellulose content from 16 to 29 wt%. The latter formulations were the most studied and characterized in the present study. The weight (W) fractions of the cured composite were calculated considering that during the crosslinking 100% of water was evaporated (both added and condensation water).

Although it has been reported in the literature that only half of the water is supposed to evaporate, the water content in the slurries is low (under 7.5 wt%), which allows neglecting the weight contribution of residual water in case of a partial evaporation [START_REF] Domínguez | Development of new biomass-based furan/glass composites manufactured by the double-vacuum-bag technique[END_REF].

CNT suspensions preparation

CNTs were received in powder form; hence a suspension preparation step was needed. In this work, we chose to prepare the suspension using ethanol as carrier liquid for optimal dilution of the PFA resin and an easier solvent evaporation. Suspensions of 1 wt% of CNT in ethanol were prepared by hand stirring with added dispersant agent provided by the supplier which is a mixture of nonionic surfactants in polymerized form (Nanoamor 8011YJ, USA). The dispersant was added at different concentrations following the supplier's recommendations depending on the used CNT type and morphology. The dispersion of these nanoparticles was carried out at first with sonication. This ultrasonic treatment was applied by introducing a titan base probe powered by a Branson 250 ultrasounds generator. All the suspensions were sonicated in discontinues mode for 30 min with 10 min intervals to avoid over-heating. Moreover, the suspensions were covered and placed in an ice bath to limit solvent vaporization. Afterward, the CNT suspensions were further homogenized using an ultra-turrax for 5 min at 10,000 rpm, the speed and duration were limited to prevent CNT breakage. The resulted 1% CNT suspensions were then stored in well-sealed bottles to prevent any solvent evaporation for further use. A visual illustration of the suspension preparation process is presented in Figure 234567.

Electrically conductive ink formulations

To prepare printable inks with suitable electrical conductivity several conductive fillers were added into the PFA resin. First, graphite and graphene powders were directly added with different concentrations to PFA resin and the cellulose powder was then added when needed. Therefore, CNTbased formulations were prepared using the previously prepared CNT-ethanol suspensions. To do so, PFA resin and catalyst were added to CNT and then hand mixed. The cellulose was gradually added while mixing using the ultra-turrax at 7000 rpm for 10 min. The mixture is then placed on a hot plate with magnetically continuous stirring. The hot plate temperature was fixed at 78°C to evaporate the excess solvent.

The different prepared formulations are displayed on the zoomed Ternary diagram in Figure 2345678. For comparison reasons, similar formulations were prepared using graphene and graphite powders.

The preliminary formulation prepared using the DWCNT-L adding different cellulose and CNT concentrations. For further optimization, CNT supplied by Nanoamor were used, and different formulations using either DWCNT or MWCNT-short were prepared with different concentrations of conductive fillers and cellulose. These formulations will allow to study the effect of CNT morphology and concentration and cellulose addition on the ink rheology and electrical conductivity. The formulations contained from 1% to 24% of nanoparticles (graphene, graphite, CNT) and from 0 to 21 wt% of cellulose powder. The detailed formulations are listed in Although in the literature the PFA-based composites are usually molded under pressure to limit the porosity and the foaming, in this study, we aimed for a simple hand casting with no applied pressure to be able to compare with printed composites which are cured under ambient pressure.

3D printing by cold extrusion 2.4.2.1 The used modified 3D printer

The main used machine for additive manufacturing is a commercial 3D printer Artillery sidewinder (China). The printing platform moves in the Y-axis whereas the printing head moves in x and z axes.

This printer is designed for fused filament printing and was upgraded with a Direct Ink Writing (DIW) extruder fabricated by WASP (Lombardia, Italy) originally designed for clay extrusion and compatible with many concentrated pastes. This extruder was fixed on the printing head thanks to a designed support which can be easily eclipsed and when needed disconnected when needed (Figure 2-9-1).

The paste formulation is fed to the extruder using a pressure syringe of 55 cm 3 with a modified straight Luer taper with an internal diameter of 4 mm to avoid constrictions and flow direction The different described parts in the printer and the printing head are shown in Figure 2-9 and the designation of the spare part is detailed in Table 2-6. Slicer software is properly named as it virtually "cuts" the 3D digital model into 2D layers which will be later printed by the 3D printer. In other words, the slicer software is our tool to communicate with the machine and the used language is G-code. In this work the used slicer was Simplify 3D. The inputs of this software are; (i) the CAD models in format .stl, (ii) different printing parameters introduced by the user, and (iii) information about the used printer and material. The single output of the software is a digital file with detailed instructions in form of G-code commands. The G-code commands define the x,y,z tracks coordinates, printing speed, and the extrusion flow of the materials. The extrusion flow is an important parameter that must be controlled and verified to have a coherent printing with continued well-aligned roads. The extrusion flow calculated by the software is directly related to the road's width, the layer's height, and the printing speed. The needed material volume per unit of time is then translated to a linear velocity of the plastic filament fed to the extrusion head (length per unit of time) which is calculated based on its diameter (usually 1.75 mm). As in our work the used extrusion head is an Archimedes screw pump, a relation between the linear extrusion velocity and the rotation speed of the extruder was determined and an extrusion correction factor was determined after performing several extrusion calibrations on the used pump which will be detailed in chapter 5. As mentioned before, the user can program several printing parameters, illustrated in Figure 2-10, such as;

• Nozzle diameter.

• Layer height: the thickness of the layer deposited by nozzle tip.

• Road width: is the width of the material bead used for rasters. The value of raster width varies based on nozzle tip size.

• Raster angle: refers to the angle of the raster pattern with respect to the X-axis on the bottom part layer.

• Infill percentage: is the percentage of the layer area covered with the deposited material.

• Outline Overlap: is the percentage of the distance of superposition between the outlines and the infill roads.

• Outline Shell: the number of outlines.

Optimization of flow extrusion and printing parameters

Ink formulation is an important step to obtain a printable materials but an adaptable 3D printer with optimized printing parameters and calibrated extrusion flow is also a primordial variable for successful printing.

Extrusion flow calibration

As mentioned previously in section 2.4.2 (p.120), the FFF commercial printer was upgraded with a DIW extruder suitable for paste extrusion. On the other hand, the software is designed for managing the extrusion flow of a conventional thermoplastic extruder fed with a continuous filament with known diameter. In the case of volumetric/screw pumps, the filament diameter must be provided in couple with the calibration factor just to allow the slicer to correlate the volume flow of extruded material with the pump angular speed. The calibration should be done for a specific material, using a specific nozzle and extruder head. In the present study, the calibration is done on the PFA-Cell28-B formulation using 0.72, 1 and 1.2 mm nozzle diameters. Using the software control panel, or by directly coding an extrusion command (G1 Ex Fy) it is possible to fix the number of revolutions and the rotation speed of the screw pump to obtain its characteristic flow curve by weighing the mass of extruded material when varying the rotation speed with a constant number of revolutions and fluid feed pressure. The mass flow can be calculated and therefore the volume flow can be concluded based on the paste's density (measured according to the method described in 2.6.2.1 p.130). To do so, the rotation speed ranged from 25 rpm to 400 rpm, and for extrusion velocity, the extruded filament was weighed. Every mass measurement was repeated at least 3 times and weighed to within ±0.0001 g. The calculation details will be detailed further in chapter 5.

Printing parameters optimization

Besides developing and optimizing the rheology and the printability of the paste and calibrating the extrusion flow for a continuous and homogeneous flow, the printing parameters have a major effect on the printing quality and the final material's mechanical performances. Therefore, the effect of road parameters and raster angle on mechanical performance and the effect of infill percentage and printing height on dimensional accuracy was studied.

Effect of Layer and road parameters

To optimize the effect of layer and road parameters as well as studying the independent effect of each of the parameters, bending bars specimens were printed with different layer heights, road width, and extrusion flow. For every different parameter combination of at least 10 specimens were prepared for further mechanical testing according to the method described in section 2.6. 4.8 (p.135) of this chapter. The parameters that were held constant during the printing process of this set of specimens are given in

Effect of Infill percentage on dimensional accuracy

DIW is a challenging technique when it comes to unsupported areas and high object printing as the object is not yet solidified and is only supported by viscous forces. To assess the dimensional accuracy, a rectangular cuboid with a 2.00 cm square cross-section with a height from 2.5 to 7 cm was printed with a varied infill from 25% to 75% as shown in Figure 2-12. The shape fidelity to the 3D model is evaluated and the quality and horizontality of the solid top layer are assessed.

Curing process

The crosslinking step is done by a thermal treatment in an open-air lab oven. The thermal treatment starts at a temperature inferior to 90°C overnight followed by a step post-curing for 3 hours at 130°C.

Pyrolysis step

After formulating inks, the resulting cured composites can be carbonized thanks to the use of poly(furfuryl alcohol) polymers which have a char residue of at least 50 wt% as discussed in chapter 1. The pyrolysis step is performed to yield a carbon/carbon composite which increases the EC of both conductive and structural materials.

Characterization methods

In every step of the research and development, process characterization and assessment is the key to a better understanding and a wiser progress. In the next paragraph, the different techniques used for analysis and characterization will be described.

Raw materials characterization 2.6.1.1 Nuclear magnetic resonance spectroscopy (NMR)

To analyze the structure and composition of the commercial PFA and cellulose powder, 13 C and 1 H NMR investigations were done on both liquid and solid polymers. Spectroscopic measurements were recorded on a Bruker AVANCE400 spectrometer (Billerica, MA, USA) equipped with a 5 mm BB/19F-1H/d Z-GRD probe operating at 100.612 MHz for 13 C, and 400.130 MHz for 1 H. For solid-state RMN the sample was compacted in 4 mm ZrO2 rotors while the liquid NMR measurements were performed on PFA samples dissolved in DMSO-d6 and placed in a 10 mm tube.

Fourier transform infrared spectroscopy (FTIR)

Chemical structure characterization is conducted with a PerkinElmer FT-IR spectrometer (Perkin Elmer, USA). For solid samples analysis, each sample was grounded and then mixed with KBr powder with a ratio of 1:125 for pellets preparation. The spectra were acquired in a frequency range of 4,000-500 cm -1 with a resolution of 4 cm -1 and then corrected with the environmental spectrum. For liquid resin analysis, the sample was placed between two NaCl plates after removing the water.

Chapter 2: Materials and methods Khaoula BOUZIDI 2022 129

Polymerization degree determination

For the characterization of molecular weight and polymerization degree (DP) of PFA, the matrixassisted laser desorption/ionization mass spectrometry (MALDI-TOF-MS) was used. It is an ionization technique applied for the analysis of large organic molecules. This technique produces ions from large molecules using a laser energy-absorbing matrix. The ions are then accelerated and the time of flight is analyzed which is directly related to their molecular weight (heavier molecular longer time of flight). In this work, the PFA sample was analyzed by Autoflex Speed (Brüker Daltonics, USA) and the scan is realized between 50 to 2,000 m/z.

The DP of cellulose was measured according to ISO 5351:2010 standard. Intrinsic viscosity (ɳint) of cellulose dissolved in copper (II) ethylenediamine was measured and then the DP was deduced using the Mark-Houwink-Sakurada equation. Two measurements were made.

Crystallinity index

Wide-angle X-ray diffraction diagrams (XRD) spectra were used to determine the crystallinity index (CI) of cellulose. The powder sample is placed above a zero background holder and a X'Pert PRO MPD diffractometer was used to make the measurements at room temperature with a monochromatic

CuKα radiation source (radiation λ = 1.5419 Å) in step-scan mode with a 2θ angle ranging from 5° to 56° with a scanning time of 10 min.

The cellulose CI (%) was determined according to the peak height method as described in eq 1:

CI = (I 002 -I am ) • 100 I 002 eq 1
where I002 represents the diffraction intensity of the main crystalline peak at 2θ ~ 22.5˚ and Iam the intensity at 2θ ~ 18.7˚.

Fibers morphology and size distribution

The fiber length and width distributions were measured using the Morfi® approach (Techpap®, Grenoble, France) [START_REF] Passas | Morfi as a novel technology for morphological analysis of fibers[END_REF]. Furthermore, the size distribution of the particles was measured using a granulometer (Cilas particle size analyzer 1190) based on the Fraunhofer method. To measure the density of the paste, a 5 cm long filament was first extruded using a 1 mm diameter nozzle. The filament was then weighed and its real diameter was further measured using a binocular microscope. The density was measured as the ratio between the weight and the volume of the extruded filament. The test was done on 5 filaments.

Rheology

To assess the rheological behavior of the pastes and the resin, a parallel plate of 25 mm in diameter was used on an MCR 301 rheometer (Anton Paar, Austria). The gap between the plate and the platform was set to 1 mm and the temperature was fixed at 25°C. Before any rheological measurements, the fluids were sheared at a shear rate of 100 1/s for 200 seconds, followed by a rest of 200 seconds to remove any previous shear history. The samples were covered during the tests to avoid any solvent evaporation.

To perform the flow curve experiments, the shear rate was increased from 0.1 to 100 1/s or 1000 1/s depending on the paste nature during which the viscosity was recorded.

To assess the thixotropic behavior of the pastes, the shear rate was fixed at 100 1/s for 60 s before a sudden drop to 0.1 1/s. The rheological behavior in terms of viscosity trends as well as stress response was evaluated as a function of time.

Oscillatory tests were also conducted to evaluate the viscoelastic properties of the prepared pastes.

To do so, amplitude sweep tests were performed at the frequency of 1 Hz and strain ranging from 0.001 to 100%. The evolution of the storage modulus (G') and loss modulus (G"), as a function of strain, or stress was analyzed.

To check the reproducibility of the rheological measurements especially when using concentrated slurry the tests were duplicated at least 3 times.

Curing characterization 2.6.3.1 Differential Scanning Calorimetry

The thermal analysis of the thermosetting PFA resin and the paste was performed by Differential Scanning Calorimetry (DSC) using TA DSC Q-100 apparatus (TA Instruments, USA), equipped with a liquid nitrogen cooling system unit. An average of 10 mg of each sample was ppplaced in a closed

Soxhlet extraction

To better understand the curing kinetics and behavior of the PFA resin, samples of PFA cured with different crosslinking degrees were prepared. The crosslinking degree was varied mainly via the crosslinking time while keeping the curing temperature and the composition constant. Later, the crosslinked polymer ratio (gel content) was determined by soxhlet extraction. Approximately 6 g of the liquid resin with 5 wt% catalysts were introduced in an aluminum pan to form a thin disc. After precisely weighing the sample, the pan was introduced in a previously heated oven where the temperature is controlled thanks to a thermocouple device. After curing for a variable time, the sample is weighed to conclude the water evaporated. This procedure is repeated for different temperatures and different duration (from few minutes to few hours depending on the curing temperature). The cured or semi-cured samples are then milled using a cryomiller (Retsh) for 4 min at 30 Hz frequency to obtain a fine powder. Then, about 500 mg of the powder are introduced into cellulosic soxhlet thimble; the sample was weighed to within ±0.0001 g (M0). The sample was then loaded in a 100 ml soxhlet using acetone as a solvent heated using a heating mantle. The extraction lasted 8 hours per extraction for every sample. Then the sample was dried in an oven heated at 60°C for few hours to evaporate the remaining solvent. The insolubilized sample fraction was then weighed to within ±0.0001 g (M1). The gel fraction G% (unsolubilized) was then calculated as in equation eq 2 which is directly related to the crosslinking rate [START_REF] Hirschl | Determining the degree of crosslinking of ethylene vinyl acetate photovoltaic module encapsulants-A comparative study[END_REF]. The soxhlet extraction steps are illustrated in Figure 2-14.

G% = M0-M1 M0 •100 eq 2

DMA shear mode

To study the viscoelastic properties of the paste during crosslinking, a shear specimen holder for pasty materials PEC1013000 was used on a DMA apparatus Metravib50 (METRAVIB-ACOEM, France).

The measurements were conducted in an isothermal mode at temperatures between 70 and 120°C.

The paste was placed in a hollow aluminum cup fixed to the bottom part of the DMA apparatus and a plunger was fixed on the top part as showed in Figure 2-15. The reactive paste was placed in the air gap of the cup-piston system. The DMA applies uniaxial stress from top to bottom, so the reaction mixture is sheared. On a temperature level, we can therefore follow the progress of the polymerization as a function of time by analyzing the evolution of the modulus G', and G''.

In the present work, the hollow part of the cup has a height of 5 mm and a diameter of 10 mm whereas the plunger has an 8 mm diameter. Therefore the gap volume (paste volume) between the two parts is 142 mm 3 . The dynamic vertical displacement (D dyn) was fixed at 1 µm with a 1 Hz frequency.

.1 Dimension measurement

To evaluate the shrinkage upon crosslinking of the neat PFA and the biocomposites, the French standard NF 51-401 was followed. 5 specimens of 80 mm × 10 mm × 4 mm were prepared for each sample according to the protocol described in section 2.4.1 (p.120). The samples were then placed in a normalized atmosphere for 24 hours before measuring their length L' after curing, using a digital caliper (±0.01 mm). The measurement was repeated at least 5 times on every specimen for a total of 5 specimens per sample. The shrinkage upon curing R% was calculated according to equation eq 3.

R (%) = L′ L • 100 eq 3 2.6.4.

Porosity and density measurement

To measure the closed porosity in the composite and the Neat cured PFA, 2.5 cm square specimens with 2 mm thickness were cured overnight at 90°C and 130°C. The Buoyancy method was used

Humidity and water sensitivity

To assess the composite sensitivity to humidity, samples of the cured composite and neat PFA along with cellulose powder were placed in a Varimass® apparatus developed in the LGP2. The relative humidity (RH) was controlled and the mass variation was recorded during the entire test via an integrated precision balance. The humidity program consisted of 3 cycles. In the first cycle, the humidity was fixed at 50% for 2 hours. The second cycle started with two hours at low humidity (20%) followed by two hours at 50%. The third cycle started with two hours at a high humidity level (90%) followed by stabilization at 50% for two extra hours.

Microscopic observations

The morphology of both fillers and the composite surface and cross-section were investigated by scanning electron microscopy (SEM). The specimens were gold coated and examined using a QuantaFEG250 instrument. The accelerating voltage was fixed at 2.50 kV and the magnitude between 60 and 2000.

The morphology of the cellulose fibers and powder was further investigated via optical microscopy.

The homogeneity of different CNT suspensions (in ethanol with and without PFA) was assessed using

Zeiss Axio Imager M1 optical microscope.

Thermogravimetric analysis

Thermogravimetric analysis (TGA) of the different composites and raw materials were performed on TGA-DSC3+ Mettler Toledo. Samples of about 13 mg were placed in an open 70 μL ceramic crucible and then heated from room temperature to 1000°C at a heating rate of 10 °C/min. The thermooxidative degradation of the composites and the neat PFA were studied under a 40 mL/min oxygen flow and further tests were performed under nitrogen flow of 40 mL/min.

Electrical conductivity measurements

The conductivity of dried extruded filament or bulk composites was measured using an ohmmeter (HP 34401A multimeter). Hook-type probes were used for an optimized contact with the filament.

The electrical resistance between the two ends of the 2 to 5 cm long filament is then measured. The Electrical Conductivity (EC), was calculated following eq 5; with L (m), the length between the two probes, R (ohm), the electrical resistance and S (m²) the cross-section area of the filament. At least triplicate was performed for every composite.

Dynamic mechanical analysis: bending mode

The thermo-mechanical dynamic properties, such as storage modulus G', were studied with a Dynamic Mechanical Analyzer (DMA) Metravib50 (METRAVIB-ACOEM, France). Tests were realized using 3 points bending mode with a temperature scan ranging from 25°C to 300°C at a 2°C/min heating rate with 1 Hz frequency. The static force had an average value of 25 N and the dynamic deformation was ±15 μm. The specimen's dimensions were 40×10×4mm (length, width, and thickness) and were measured precisely for every sample before the scan. Tests in the isothermal mode were also performed to evaluate the thermal stability of composite for prolonged exposure at high temperatures in isothermal mode. At least 3 specimens per sample were analyzed.

Mechanical characterization

Before any mechanical testing, most of the composites were conditioned at 23°C and 50% relative humidity for at least 16 h. The mechanical properties were tested with an Instron Universal testing machine (Instron, USA), and the data analyzed with the help of the Blue Hill software. The tensile test was inspired by the ISO 527 standard using the 5A specimen dimension and a crosshead speed of 2 mm/min. The flexural properties were measured according to ISO 178 at a crosshead speed of 2 mm/min. The obtained data were the average of 10 to 15 measurements. The specimens were prepared by open-air casting molding or by 3D printing, as described above. The surfaces of the samples were polished by sandblasting and the exact dimensions were measured before every test.

Chapter conclusions

In this chapter, a detailed description of the used materials and methods was provided and the general approach was exposed.

In the second part of this manuscript, the obtained results will be presented and described.

Therefore, the next part contains 4 chapters: Chapter 5: Dealing with process study and composed of two parts:

• Part 1: Understanding the curing process toward a defect-free cured composite.

• Part 2: Optimization of printing parameters for improved additively manufactured composites.

Chapter introduction

Developing a new material requires the utilization of several raw materials or components. These materials can either be synthesized in the laboratory or acquired from a commercial supplier. In this work, the materials are commercially available and were provided by different suppliers around the world as mentioned before. As the main focus of this project is the development and the optimization of a bio-based composite, the use of commercial materials readily available and produced by experts was privileged. This choice comes with several limitations such as poor information and data provided by the supplier about the origin, the structure and the production process of these materials.

In research, knowing the materials which are the building blocks of our composite is primordial. Thus, a complete characterization of these materials is fundamental to identify their main characteristics.

This step is primal for a potential material replacement and also to identify the chemical reactivity between the composite components. Therefore, in this chapter, we will provide a thorough chemical and morphological structure and characteristics of the used PFA polymer and cellulose using several analytical techniques. The first part is focusing on characterizing the cellulose powder, while the second part deals with the PFA oligomer. 3.2 Cellulose fibers and powder

Size distribution

A correct evaluation of the size distribution of the cellulose powder (Cell8) is challenging; although it is considered as a powder, the particles have different morphology with an aspect ratio superior to 1.

They can neither be considered as fibers because of their low aspect ratio. The evaluation of their size distribution was done according to two techniques; first, the Morfi apparatus was used to evaluate the content of long fibers and their size distribution and that of the 'fines' which are the fibers with a length <80 µm. The Morfi only considers the particles with fiber morphology in its analysis. Therefore, the powder was further evaluated by a granulometer which is mainly used for the estimation of the size of spherically shaped powders to estimate the overall size distribution of the powder. Unsurprisingly, according to Morfi analysis data, Cell8 is composed of mainly 'fines' (98%) with 2% of long fibers which are the non-milled fibers in the powder, with an average length of 117 µm and an average width of 14 µm. On the other hand, the short fibers in Cell8 have a length distribution ranging from 10 to 74 µm with an average of 27 µm. Morfi does not provide data regarding the width of the 'fines' elements but it can be considered similar to the width of the long fibers as these long fibers are nothing but the non-milled fibers. The size distributions are displayed in Figure 3-2. According to these results, we can conclude that the aspect ratio of cell8 is superior to 1 but is still low to be considered as fibers.

In Figure 3-3, one can see the overall size distribution of cellulose powder Cell8 ranging from 1 to 50 µm which is in alignment with Morfi data. 90% of these particles have a size below 20 µm and 50%

below 10 µm making it one of the finest commercial MCC powders. The non-spherical shape and low size of these particles would lead to a better surface area for bonding during powder compression and eventually improve their liquid bonding [START_REF] Khan | The Effect of Particle Size and Moisture on the Tensile Strength of Microcrystalline Cellulose Powder[END_REF][START_REF] Levis | Production and Evaluation of Size Reduced Grades of Microcrystalline Cellulose[END_REF].

Morphology

For further characterization of the cellulose particles morphology, the cellulose fibers Cell200 and the powder Cell8 were dispersed in water and observed under an optical microscope as shown in Figure 3-4. The long fibers (0.2 mm long) morphology revealed most likely hardwood-like fibers while the Cell8 particles exhibited the shape of short sticks, retaining the morphological characteristics of plant fibers with good dispersion in water. The dry Cell8 was further observed with a SEM, as one can notice on the bottom images of Figure 3-4, the observation with the optical microscope is confirmed since the powder contains rod-shaped particles along with some fine particles. The powder is well dispersed and contains limited agglomerates with an aspect ratio (L/d) ranging from 1 to 5.

Crystallinity index and DP

According to the information provided by the supplier, Cell8 is MicroCrystalline Cellulose (MCC)

which is a purified partly depolymerized cellulose prepared by treating cellulose pulp with a mineral acid. In this process, the amorphous region is preferentially hydrolyzed so that cellulosic crystals are released. The acid hydrolysis causes dramatic depolymerization of the cellulose until reaching a constant level of DP [START_REF] Battista | Microcrystalline cellulose[END_REF]. To confirm the supplier's information and obtain precise information about the cellulose form and crystallinity level, an XRD test was conducted.

On the XRD pattern in Figure 3-5, we can first confirm that the cellulose is well crystallized and that the different planes' peaks of crystalline polymorph cellulose are visible [START_REF] Park | Cellulose crystallinity index: measurement techniques and their impact on interpreting cellulase performance[END_REF]. Moreover, we can notice the well-known diffraction peaks at 2θ around 14.5°, 16.5°, and 22.5° which are in agreement with the characteristic diffraction peaks of cellulose I-β [START_REF] French | Idealized Powder Diffraction Patterns for Cellulose Polymorphs[END_REF]. Thus the manufacturing process of this fine MCC powder was mild enough to maintain the cellulose in its native form. Crystallinity has an important effect on the physical, mechanical, and chemical properties of cellulose. For instance, with increasing crystallinity, tensile strength, dimensional stability, and density increase, while chemical reactivity and swelling decrease. The CI was calculated using the peak height method [START_REF] Segal | An Empirical Method for Estimating the Degree of Crystallinity of Native Cellulose Using the X-Ray Diffractometer[END_REF] and gave a value of 76.6%. Although the CI is high, it is still lower than that of commercially available MCC such as Avicel PH-101 which has a CI higher than 88%

(calculated with the same technique) [START_REF] Terinte | Overview on native cellulose and microcrystalline cellulose I structure studied by X-ray diffraction (WAXD): Comparison between measurement techniques[END_REF]. The latter MCC has coarser particles so the CI drop in cell8 could be caused by longer mechanical milling which deteriorated the crystals [START_REF] Zheng | Effects of Ball Milling Processes on the Microstructure and Rheological Properties of Microcrystalline Cellulose as a Sustainable Polymer Additive[END_REF]. Furthermore, the DPv was measured by the intrinsic viscosity method which gave a low value of 102 ±3. Thus, the biopolymer was either severely depolymerized during the different industrial processes or the used hardwood fibers had a very limited starting DP. The DP drop can be caused by chemical treatments of the fibers such as a harsh pulping process to obtain pure cellulose and a stiff bleaching process, but can also be related to severe mechanical treatment in the milling step to obtain such fine particles. These results are consistent with some previous findings that mechanical ball milling of microcrystalline cellulose decreases particle size, reduces the DP of cellulose, and increases the amorphous content of cellulose [START_REF] Paakkari | Structure of Amorphous Cellulose[END_REF][START_REF] Zhao | Effects of Crystallinity on Dilute Acid Hydrolysis of Cellulose by Cellulose Ball-Milling Study[END_REF][START_REF] Zheng | Effects of Ball Milling Processes on the Microstructure and Rheological Properties of Microcrystalline Cellulose as a Sustainable Polymer Additive[END_REF]. [START_REF] Poletto | Crystalline Properties and Decomposition Kinetics of Cellulose Fibers in Wood Pulp Obtained by Two Pulping Processes[END_REF][START_REF] Tserki | Novel Biodegradable Composites Based on Treated Lignocellulosic Waste Flour as Filler. Part I. Surface Chemical Modification and Characterization of Waste Flour[END_REF][START_REF] Viera | Synthesis and Characterization of Methylcellulose from Sugar Cane Bagasse Cellulose[END_REF]. The infrared spectrum shows that the MCC Cell8 sample has the characteristic absorption peaks of cellulose I. To conclude, the used Cell8 cellulose is microcrystalline cellulose with rod-shaped particles 90% of which have a size below 20 µm. According to XRD this MCC has a CI of about 77% and still conserving its native form. These results are further confirmed with 13 C NMR and FT-IR spectra which revealed a typical signature of rather pure cellulose I without evidence of neither chemical transformation nor functional groups grafting. Cell8 powder can be considered as an inert cellulosic micro-sized particle filler.

Furan oligomer: Furolite

The commercial oligomeric furfuryl resin was characterized with different techniques such as NMR, MALDI-ToF, and FT-IR.

We start first by defining the following abbreviations:

• Furan ring is symbolized by FU (Figure 3-8-A)

• 2-Monosubstituted furan rings will be symbolized by FU-(Figure 3-8-B)

• 2,5-Disubstituted furanic rings will be symbolized by -FU-(Figure 3-8-C) The 1 H NMR spectrum of the commercial OFR in Figure 3-9 shows a clear peak around 6.2 ppm, which reflects the resonance of the H3 and H4 protons of the furan ring. The peak of the H5 proton around 7.5 ppm is surprisingly very weak because, in the case of the oligomerization of exclusively FA monomers, those protons will be present at least at one of the extremities of the formed oligomers (Choura et al., 1996). On the other hand, if one compares the signal of the -CH2-methylene bridge protons between two furan rings around 3.8 ppm and the signal of CH2-O protons, which can be either those derived from the terminal CH2OH groups or those in the ether bridge formed after the condensation according to reaction [2], shown and detailed in section 1. 3.1.2 (p.65) of the first chapter, one can notice that the latter is uncommonly more important (Choura et al., 1996;Deka et al., 2013;[START_REF] Principe | An NMR Study of Poly(Furfuryl Alcohol) Prepared with p-Toluenesulphonic Acid[END_REF]. Examining the FT-IR spectra in Figure 3-11, the first observation relates to the important signal around 3400 cm -1 of the OH groups of the "dry" product which we have verified on the 1 H NMR spectrum, probably partially arising from the residual moisture in the resin. However, these hydroxyls are mainly due to the terminal groups of the oligomers, which are the reason behind the partial solubility in water of the commercial resin thanks to the polarity of its molecules. The presence of the furan ring is proven by the two signals around 1000 and 800 cm -1 . One can also distinguish the typical double peak around 2900 cm -1 of the CH2 groups. The peak around 1200 cm -1 is indicative of ether bonds (CH2-O-CH2). Around 1720 cm -1 a peak of the carbonyl groups is due to the furan ring opening during the oligomerization, as discussed above (Choura et al., 1996;[START_REF] González | Furfuryl Alcohol Polymerisation by Iodine in Methylene Chloride[END_REF]. According to the NMR data of the commercial oligomer, the ether bridges are more frequent than the methylene ones, and terminal CH2OH groups are much more frequent than unsubstituted H5 end groups, both facts being entirely uncommon for typical FA oligomers, in view of the established mechanism shown in the first chapter. Indeed, in the case of the predominance of reaction [2] without loss of formaldehyde, and hence the important presence of ether bridges, the condensation would be soon blocked for lack of CH2OH moieties, i.e. one would only be faced with oligomers ending with 2 unreacted protons at the C5 positions, which is clearly not the present situation.

Conversely, the strong presence of hydroxyl groups on the NMR and FTIR spectra versus a very limited sign of the proton at the C5 position, suggests that these oligomers are ending predominantly with hydroxymethyl groups rather than a furan ring with an unreacted H5 proton.

Interestingly, a MALDI-ToF spectrum of the PFA oligomer, confirmed these conclusions, showing the same pattern of moieties within a mixture of oligomers bearing two-to-four monomer units. Indeed on the spectrum in Figure 3-12, we can first notice that the molecular mass (m/z in Dalton)

distribution is around the average of 350 Dalton corresponding to an average DPn of 4 units per oligomer. After the electrons shelling the fragmentation of the oligomer took place with a sequence of 80 Dalton, which corresponds to the molecular weight of the repetition unit of the oligomer (FA (98) -H2O (18) = 80). According to this data, the pre-polymers in the commercial product are short and hence the frequency of the terminal hydroxyl groups detected in the RMN and FTIR spectrums. We can as well detect some fragments of 110 Dalton corresponding to the fraction of the furan ring attached to an ether bridge, viz. FU-CH2-O-CH2. The average distribution of the fragment is around 351, which corresponds to about 3 units of a furan attached to the ether bridge. The DPn of this prepolymer ranges between 2 and 5.

It seems logical to suggest therefore that the synthesis of the commercial PFA required the concomitant use of FA and another furan monomer capable of generating a high frequency of ether bridges and terminal CH2-OH groups, most likely 2,5-hydroxymethylfuran.

This structural investigation showed that the interest of the product developed by Transfurans in that it allows water compatibility (hence a reduced viscosity and therefore better processability)

while maintaining the potential to polymerize and crosslink following essentially the same mechanisms discussed in Chapter 1 for FA alone, using a temperature activation catalyst which allows the process to occur upon heating the system, as will be discussed in Chapter 5.

Cured polymer

According to the data provided by the RMN 13 C of the product after crosslinking in Figure 3-13, we can recognize the structure of the PFA. For instance, the peaks around 150-160 ppm and 110 ppm prove the presence of furan rings. Moreover, the peaks at around 27 and 65 ppm prove the presence of the methylene and ether bridges after the curing of the resin. At around 205 ppm we can also notice the presence of the carbonyl groups after the furan ring opening. As we can notice in Figure 3-13, the majority of suggested structures of the crosslinked PFA in the literature, detailed earlier in section 1.3 of Chapter 1 (p.63), could be detected which also demonstrates how complex the structure of the crosslinked product is.

Conclusions

According to the data provided by NMR and FTIR spectra, it was obvious that not only furfuryl alcohol (FA) monomers were involved in the pre-polymerization of the PFA oligomer. The most probable comonomer used was the 2,5-hydroxymethylfuran (BHMF).

During the polymerization step, 3 polymerization reactions are possible FA-FA, FA-BHMF, or BHMF-BHMF but the oligomer produced is always the poly(furfuryl alcohol). As for the proportions of the two monomers in the initial mixer, we suggest that the BHMF is in excess because of the quasi absence of the non-substituted protons H5 and the domination of the ether bridges. According to the data of the MALDI TOF, the average polymerization degree is around 3 with a maximum of 5. In conclusion, we can suggest a basic structure of the commercial Furolite used in this work as:

OH-CH 2 -(FU-CH 2 -O-CH 2 ) n -(FU-CH 2 ) m -FU
with n>m because the ether bridges are more frequently present in the polymer than the methylene bridges.

Chapter conclusions

In a conclusion, on one hand, we can consider that the used cellulosic fillers are micro-sized rodshaped chemically inert particles. On the other hand, we can propose that the commercial Furolite oligomer is a sequence of an average of 2 to 5 furan rings connected predominantly by ether bridges and ending with hydroxyl groups making it rather soluble in water. In the light of these assignments, the combination of these two components should yield a mechanical reinforcement of the furanic matrix without any chemical interaction between the two components (Lems et al., 2019).

Development and characterization of new bio-based materials for 3D printing by cold extrusion

Khaoula BOUZIDI 2022 164

Chapter introduction

Recently, 3D printing is getting more and more popular in many fields and for many applications that require different materials. In AM by material extrusion one can notice the dominance of thermoplastics which are for the most part petroleum-based. Under the upraising environmental concerns, as already stated in chapter 1, many research works have been conducted to find a substitute to the use of thermoplastics and among the suggested materials one can notice cellulose and, more generally, lignocellulosic materials. The latter bio-based material is particularly interesting for 3D printing as it is available, renewable, cost-effective, and lightweight. However, these advantages come with a major limitation: cellulose cannot be melted and hence cannot be used as a matrix for AM by melt extrusion. Therefore, researchers suggested several formulations using different forms of cellulose for printing by cold extrusion in particular aerogels of NFC, MFC, and CNC which were compatible with this AM technique. Nonetheless, the use of these cellulose-based aerogels, and in some cases suspensions and solutions, raised additional challenges and limitations to its printability such as an important shrinkage upon drying due to a limited solid content in the formulation or a complicated and expensive freeze-drying process.

From a more technical point of view, thermoplastics have limited thermal stability making the use of the 3D printed parts limited to low-temperature environments. Despite the existence of technical thermally stable printable filaments such as PEEK and PEI, they are still expensive (upto 790 $/kg) and rather complicated to print. To overcome these thermal performances limitation when using thermoplastics, several articles have been published about formulations compatible with AM by extrusion of composites with the use of thermosetting resin with better thermal and mechanical performances. Nonetheless, the majority of the suggested composites (chapter 1) are still fossilbased. To overcome these limitations the intention of the present work is to develop a 3D printable material by AM by cold extrusion using cellulose fibers as bio-based and cost-effective filler along with a bio-based thermosetting resin which will play the role of binder with excellent thermal performances. This mixture would yield a fully bio-based printable composite with performances over-stepping currently existing materials. However, an optimization step should be done to define the appropriate proportions to obtain a printable 'ink' and optimal performance of the cured composite which is the main goal of this chapter. As mentioned in chapter 2, two different 'inks' are developed in this work:

• A structural ink based on cellulose fibers and PFA oligomers and eventually a solvent.

• An electrically conductive ink based on conductive particles, cellulose, and PFA oligomers. The different formulations and their optimization for both inks will be detailed in this chapter along with a rheological study and an investigation of the most relevant properties for the resulting cured composites. Figure 4-1 is an illustration of the chapter's content and layout highlighting the main conducted studies and short conclusions which will be detailed in the present chapter.

Introduction

In no uncertain terms, direct ink writing is inseparable from rheology. One cannot begin DIW without first questioning the rheological properties of the formulation. Indeed, these properties determine the range of structures that can be printed, the optimal printing parameters, and ultimately the quality of the printed part. Despite this, the manner in which rheology ties in with these considerations has to date not been specified well in the literature. Some work has been done to identify some of the important rheological requirements of the ink formulation and suggested a range of values these properties should take for a successful print. This chapter aims to advance the understanding of how each key rheological property affects different aspects of the printing process and to begin to refine the quantitative requirements of these properties.

Formulation optimization

The 'ink' formulation is a very important step towards successful additive manufacturing by material extrusion. A simple suspension containing cellulose particles as the solid phase and PFA oligomers as the liquid phase is aimed to be prepared. The ink suspension will thus contain 3 main components, cellulose particles as fillers, PFA oligomers as the binder, and water or ethanol as resin diluents. To define an optimal ink that is extrudable, printable and with a defect-free solidification, several formulations with different liquid and solid contents were prepared. An early estimation of the liquid content required for an extrudable suspension is advantageous. Theoretically, the maximum liquid content in a powder to obtain a paste-like highly filled suspension can be estimated by predicting the free space in a body of powder but even doing so the yielding paste can present deagglomeration.

The simpler way is to perform a subsidiary experience by adding gradually the liquid phase until obtaining a smooth workable paste. Regulating the liquid content in the suspension is crucial to have the optimum amount of liquid filling the space between the filler particles (depends on the packing properties of the powder) and lubricating the relative particle movement along the wall of the die which will enable the extrusion of the paste. There is a narrow range of liquid content with which extrusion is possible, as too much liquid content will lead to a drop in the pressure needed to extrude and no shape maintenance in the extrudate, whilst an exaggerated increase in solid content would lead to the formation of a wet powder causing the die's clogging. To restrict the solid content range in the 'ink', a preliminary testing step was carried out.

The preliminary formulations were prepared with fibrous and powdery cellulose and by varying the cellulosic content, the binder content, and the solvent ratio. The assessment of these formulations was done in two steps, the first by assessing its extrudability (through 2 mm and then 1 mm nozzles)

and the second step by a visual assessment of the drying and curing quality of the formulations. The formulation that can be both extrudable and crosslinked without any visible major flaws and instabilities can be qualified as possibly printable. In chapter 2 (p.115) an exhaustive list of the tested formulations was presented which are also presented on the ternary diagrams in In Table 4-1, the aspect of the extrudates and the crosslinked disks of the most important formulations are presented. The formulations IF-01-L and IF-01 contain the same weight fractions of the 3 components, but the former was prepared with cellulose long fibers (Cell200) and the latter with cellulose powder (Cell8). As we can see in Table 4-1, despite a defect-free crosslinking for both forms of cellulose, the extrusion of the long fibers suspension was not possible through a 2 mm nozzle. We can notice a phase migration phenomenon due to a phase separation in the nozzle. This phase separation mechanism during extrusion can occur by filtration in the die, where the pressure exerted on the liquid phase causes it to flow along the die more quickly than the solids. This phenomenon is exacerbated by the formation of solid mats which causes an ultimate clogging of the nozzle [START_REF] Yaras | Flow instabilities in capillary flow of concentrated suspensions[END_REF][START_REF] Yilmazer | Mat formation and unstable flows of highly filled suspensions in capillaries and continuous processors[END_REF]. Indeed, under high shear stress, the slip velocity (Vs) that occurs on the walls of the die becomes significant and starts to be comparable to the overall velocity of the suspension (V) and in the case of high filled suspensions, the extrusion flow takes place only by slip as it flows as a plug (V=Vs). However, the pressure drop across the suspension does not only move the suspension as a plug, but also generates a driving force for the suspending liquid to filter through the particles leading to a velocity of the liquid with respect to the fillers (Vm) which should be minimized to avoid the liquid filtration to occur [START_REF] Yilmazer | Mat formation and unstable flows of highly filled suspensions in capillaries and continuous processors[END_REF]. Moreover, a study on the effect of the formulation parameters on the extrusion flow was done and it was found that in order to avoid any phase migration, the matrix viscosity should be maximized and the particle size should be minimized in order to maximize the Vs and minimize Vm. Thus, the use of cellulose powder with limited particle size was more advantageous to avoid the phase migration phenomenon.

Furthermore, limiting the particle size allows reaching better printing resolution by using small nozzle sizes as the absolute maximum dimension of the particles must be a tenth of the smallest die diameter to prevent particles bridging during flow [START_REF] Benbow | Paste Flow and Extrusion[END_REF]. [START_REF] Benbow | Paste Flow and Extrusion[END_REF].

As the experimental solid content limit of the cellulose powder was quickly reached, the viscous binder was further diluted with water in IF-04 and with ethanol in IF-04-E to reach a total cellulose weight fraction of 31 wt%. These highly concentrated suspensions were hardly extruded through a 2mm nozzle and their extrusion flow was not stable with a noticeable surface fracture phenomenon.

Surface fracture, as the name suggests, is when the extrudate shows a rough and uneven surface as we can see on the photograph of the extrudate of IF-04. This phenomenon can be influenced by the limited liquid phase content in paste formulation as shown in Figure 4-2 [START_REF] Benbow | Paste Flow and Extrusion[END_REF]. Moreover, we can notice cracking upon crosslinking of these composites whether using water or ethanol as solvent. These cracks can be due to a very limited polymeric binder (45 wt%) and an important added solvent. The added solvent volume has to be removed and leaves interparticle pores which lead to the cracking of the composite [START_REF] Benbow | Paste Flow and Extrusion[END_REF]. Not only is the quantity of the liquid phase, but also its viscosity are critical to obtain an extrudable paste. The matrix should have a high viscosity to generate stresses during mixing to produce consolidation of the paste and for shape retention of the extrudate, but should not be too high to limit extrusion pressure and to ensure homogeneous mixing. Furthermore, the evaporable liquid volume should be limited to avoid cracking upon drying and crosslinking [START_REF] Benbow | Paste Flow and Extrusion[END_REF].

The formulations IF-01 and IF-03 are the only pastes exhibiting a stable extrudate flow, through 2 and 0.7mm nozzles. The formulation containing 25.5% cellulose weight fraction is the most filled suspension being extrudable through narrow nozzles. 79 wt% is then the approximate liquid phase weight fraction that can be added in the suspension to have a stable extrusion to fill the voids in the cellulose powder to insure enough lubrication between the particle and the wall of the die for a steady and easy extrusion.

In the light of the preliminarily tested formulations, plotted on the ternary diagram in Figure 4-3, we can limit the testing area of the fraction of the 3 different components of the printable ink. As highlighted on the diagram, the total solvent content in the formulation should not exceed 20% (32% of the polymeric matrix) to avoid any cracking and defect in the composite upon crosslinking. On the other hand, the cellulose weight fraction cannot exceed a maximum of 30%, which leads to a powdery crumbly aspect of the mixture. Moreover, it was found that less than 10 wt% of cellulose powder yields a diluted suspension with a law bulk yield stress which is needed in the formulation so that the ink maintains the shape of the die after extrusion, which is very important for 3D printing.

Finally, the use of fine cellulose particles is privileged to avoid any extrusion defects and to make possible the use of small nozzles without clogging. These conclusions leave us with a better-targeted formulation composition range highlighted in blue in Figure 4-3. The next step is the optimization of the formulation. As discussed before, the solid content is a very important parameter in the formulation of a suspension compatible with AM by material extrusion.

The target is to define the optimal cellulose content for an extrudable, printable, and shapemaintaining ink. To do so, 6 formulations were prepared with cellulose weight fractions ranging from In the next section, the ink formulation will be further optimized by assessing the printability. The printability assessment is carried out through the characterization of the different formulations according to a rheological approach.

Effect of cellulose concentration on the rheological behavior and the printability of the suspensions 4.2.3.1 Basic Rheological Properties and printability concept

In most of AM techniques, the used material undergoes an instant phase transition such as solidification in the case of fused melt deposition which insures the shape maintenance of the part during printing. In contrast, in the case of cold extrusion, the material does not undergo any phase transition so the printed part is only held by the properties of the extrudate. Thus, the rheological behavior of the ink plays a crucial role in the printability and the quality of the printed object.

In the following subsection, general rheology principles, as well as printability concept definition according to the state of art, will be briefly presented.

Dynamic rheology

The most used parameter to describe how a material flows is its viscosity which can be defined as the resistance of the fluid to flow. The dynamic viscosity (ƞ) can be measured according to eq 6 where τ

(Pa) is the shear stress and γ̇ (1/s) is the shear strain rate.

ƞ = τ γ̇ eq 6

The sheared material can undergo different flow types as a function of the shear rate and their flow curves can be summarized as illustrated in Figure 4-6. The flow can be either with or without yield stress and we can distinguish three different flow possibilities:

• Newtonian: shear stress evolves linearly with the shear rate and the viscosity is insensitive to the shear rate.

• Shear-thickening (Dilatant): viscosity increases with increasing shear rate.

• Shear-thinning (Pseudoplastic): viscosity decreases with increasing shear rate. [START_REF] Kubo | Chapter 1 Rheological Properties of Tomato Products 1-25[END_REF].

The rheological behavior of non-newtonian fluids can be described by the simple power-law relating the shear rate and the shear stress presented in eq 7.

τ = K γ̇n eq 7

Where τ is the shear stress (Pa), 𝛾̇ is the shear rate (1/s), K is the consistency coefficient (Pa•s n ) and n is the flow behavior index (dimensionless). By arranging eq 6 and eq 7, the shear rate can be related to the apparent viscosity according to eq 8.

𝜇 = 𝐾 𝛾̇1 -𝑛 eq 8
The flow behavior index (n) assesses the relation between the shear stress and the shear rate.

• n> 1, the flow is shear thickening.

• 0 <n< 1, the flow is shear thinning.

• n= 1 Newtonian or Bingham flow.

Oscillatory rheology

In addition to the viscosity measurement and flow ramp measurements, an assessment of the viscoelastic behavior of the material is also important. The viscoelastic properties of the flowing material can be characterized with oscillatory tests which can be performed on a rheometer by subjecting the fluid to sinusoidal oscillatory stress and measuring the elastic and the viscous response. The shear complex modulus (G*) is the most important parameter provided, which is the resistance of the ink against the subjected deformation with its two components, viz. the storage modulus G' (or elastic) and the loss modulus G" (or plastic) [START_REF] Mckenna | Texture in Food: Semi-Solid Foods[END_REF].

The storage modulus G' represents the elastic behavior of the material. This modulus, given by eq 9, expresses the ability of the material to return to its original shape after stress removal. In the case of a pure viscous behavior, G' is zero because the deformation is out of phase with the shear stress. The loss modulus G" represents the viscous behavior and energy dissipation when the material starts to flow. The loss modulus G" is given by eq 10. Another important parameter is the loss factor (eq 11) which helps to understand if the material is behaving more as a solid or as a liquid. When the material is solid-like, the loss factor approaches zero and when the material is behaving liquid-like, the loss factor approaches infinity [START_REF] Barnes | An Introduction to Rheology[END_REF][START_REF] Wildemuth | Viscosity of suspensions modeled with a shear-dependent maximum packing fraction[END_REF].

G' = G * cos(δ) eq 9

𝐺" = 𝐺 * 𝑠𝑖𝑛(𝛿) eq 10

tan δ = G" G′ eq 11
To study the viscoelasticity of a material, one of the used tests is the amplitude sweep where the deformation increases whilst G' and G" are being measured. The modulus is constant in the linear viscoelastic region (LVER) and G' starts to decrease when the material structure breaks down, this point being nothing else than an estimation of the yield stress, which can be approximated as the crossover of G" over G' [START_REF] Smay | Directed Colloidal Assembly of 3D Periodic Structures[END_REF].

The importance of the ink formulation

These rheology changes induced by the flow generate different macroscopic behaviors. The interaction between solid particles at rest affects the flow of these suspensions. In fact, when a shear force is applied to a complex fluid, energy is supplied to the elements and this energy varies from one element to another since the forces are distributed in a complex way through the network of disordered elements. Thus, this fraction of displaced elements allows the fluid to flow macroscopically [START_REF] Coussot | Rheophysical classification of concentrated suspensions and granular pastes[END_REF]. The interaction between the solid particles would be influenced by the arrangement of the particles relative to each other, by their size, their shape, and possibly by the chemical potential of the suspending fluid. As discussed before, and confirmed by the primary tested formulation, these parameters should be taken into consideration when preparing ink suspension for AM by cold extrusion. As one can see in Figure 4-7, the ink suspension can be in three different regimes, depending on the filler volume fraction. The first is a diluted regime, where the solid particles have no interaction. This regime is not interesting for the formulation of printable and extrudable inks. The second is a concentrated regime, where the particles can still move in the suspension, but particle-particle contacts are frequent. The latter regime is limited by a critical volume fraction when the suspension starts to be too brittle and crumbly. The final regime is the solid regime when the maximum possible packing of the filler is attained [START_REF] Coussot | Rheophysical classification of concentrated suspensions and granular pastes[END_REF].

In this work, the selected suspensions are in the concentrated regime recognizable in Figure 45. In a general manner, printability is a measure of how suitable a suspension's properties are for AM by a certain technique, which in our case is cold material extrusion. As much as this printability concept can seem to be almost innate, especially for researchers that have been working on AM by material extrusion (called also direct ink writing or robocasting in the literature), up to now there is not a clear and standardized method to define this emergent concept. However, in the literature, we can find few attempts to define some criteria and figures of merit.

In 2002, a first attempt simply considers the minimum G' required for printing a spanning structure (James E. Smay et al., 2002). The minimum G', given by eq 12, was determined by setting a maximum spanning rod deflection to be 0.05D, where D is the rod's diameter.

G′ min = 1.4 • ρ • g • D • s 4 eq 12
Where ρ is the density of the fresh paste, g the gravitational constant, and with (s=d/D) is the normalized span distance between rods. The stiffness of the paste (expressed by the storage modulus G') is a very important rheological parameter to print spanning structure, but is not the only crucial parameter to establish a general printability understanding.

In another work, a printability criterion was set, based on the yield stress required to withstand both forces of gravity and surface tension (M' Barki et al., 2017). The printability criterion Ξ is given by eq 13, where τy is the paste's yield stress, ρ the density of the paste, ϒs the surface tension, h the height of the part, g gravitational acceleration, and R the radius of curvature of the smallest printed feature.

This criterion takes into account not only the intrinsic properties of the ink, but also includes the printing part properties.

Ξ = τy ρgh + γs R -1 eq 13
That is why in 2017 a new figure of merit was suggested by (Feilden, 2017) which is only based on the rheological properties of the paste formulations, thus allowing a rapid selection of a promising formulation for AM by material extrusion. This figure of merit was developed based on the rheological properties required, experimental work and the state of art, for a successful printing, such as:

• Maximizing G' to minimize the elastic deformation during printing, hence shape maintainance.

• Minimizing the flow index n, the consistency coefficient K and the yield stress τ 𝑦 to reduce the force required to bend the extruded filament, to deposit it gently without deforming the printed layers.

• Nonetheless, τ 𝑦 should not drop from a minimum value in order to resist the surface forces and gravity.

• Minimizing the yield shear rate γy to reduce the paste compressibility during extrusion for an instant break of extrusion, when needed.

Accordingly, a paste-specific figure of merit (Φ) was suggested and a minimum value of 20 was found to be required for a successful robocasting (eq 14).

Φ = G′ τ y eq 14

In the most recent work [START_REF] Duty | What makes a material printable? A viscoelastic model for extrusion-based 3D printing of polymers[END_REF], a more general viscoelastic model was proposed based on the rheological and thermophysical properties of the ink and the processing parameters. This model establishes a methodical approach to characterize printability by providing different equations serving printing criteria for a series of conditions that must be satisfied in order to successfully print a part. These equations depend not only on the material properties, but also on the used hardware and time-depending printing parameters. In the literature, we also come across other attempts to define the concept of printability for AM by material extrusion, but they focus more on assessing the printing quality, with no clear definition of a figure of merit for the ink itself [START_REF] Gillispie | Assessment methodologies for extrusion-based bioink printability[END_REF][START_REF] Naghieh | Printability -a Key Issue in Extrusion-based Bioprinting[END_REF]).

In the present work, the figure of merit (Φ), coupled with maximization of paste stiffness G', were considered as criteria to optimize the ink formulation as this approach is paste-based without compassing printing parameters, which will be assessed more in detail in chapter 5.

Rheological study and optimization of the ink formulation -figure of merit

In the absence of a clear definition of printability with material extrusion, if we rely on the different criteria leading to a successful printing in the recent literature [START_REF] Beach | New "all-in-one" silicone pastes for additive manufacturing of elastomer parts[END_REF][START_REF] Chu | Development of extrudable high strength fiber reinforced concrete incorporating nano calcium carbonate[END_REF][START_REF] Duty | What makes a material printable? A viscoelastic model for extrusion-based 3D printing of polymers[END_REF][START_REF] Fahmy | 3D printing and additive manufacturing of cereal-based materials: Quality analysis of starch-based systems using a camera-based morphological approach[END_REF][START_REF] Gillispie | Assessment methodologies for extrusion-based bioink printability[END_REF][START_REF] Montoya | Impact of viscoelastic and structural properties from starch-mango and starch-arabinoxylans hydrocolloids in 3D food printing[END_REF] 2021; [START_REF] Panda | Extrusion and rheology characterization of geopolymer nanocomposites used in 3D printing[END_REF][START_REF] Panda | Investigation of the rheology and strength of geopolymer mixtures for extrusion-based 3D printing[END_REF][START_REF] Panda | Additive manufacturing of geopolymer for sustainable built environment[END_REF][START_REF] Rieger | Slurry development for lithography-based additive manufacturing of cemented carbide components[END_REF][START_REF] Zhang | Rheological and harden properties of the high-thixotropy 3D printing concrete[END_REF], we can define 3 main rheological properties:

• Shear-thinning behavior to facilitate the extrusion of the ink through different nozzles.

• Optimized yield stress and stiffness of the ink formulation to ensure maximum shape fidelity and minimum structure slumping without affecting the extrudability.

• Thixotropic behavior for a good viscosity recovery upon extrusion to avoid layer slump.

These properties will be assessed in the next 3 subsections.

Shear-thinning behavior

The evolution of the slurries' apparent viscosity with increasing cellulose volume fraction is given in Figure 4-8. The viscosity of each slurry was measured at low shear rates (0.1 1/s) over at least 150 s. To assess the slurries flow under different shear rates, rotational rheological measurements were performed. Figure 4-9 shows that the neat PFA resin (red curve) has a Newtonian flow where the viscosity is independent of the applied shear rate, whilst the addition of cellulose powder led to a shear-thinning flow. This shear-thinning behavior is more or less marked for each formulation. Thus, in order to quantify the shear-thinning degree, a fitting of the apparent viscosity versus shear rate curves with the power-law (eq 8) was done. To do so, a log-log plot of the apparent viscosity vs. On the other hand, the thinning exponent n decreases with increasing filler fraction, dropping from 0.65 for the formulation containing 16 wt% cellulose, to 0.1-0.3 when exceeding 19 wt% of cellulose.

Over a certain filler fraction, no clear correlation can be concluded between the shear-thinning ability and the content of the filler. This can be explained by the fact that, adding only 16 wt% of particle, the network is not strong enough to resist shearing forces and could easily be broken and flow, thus leading to a negligible dependence between the viscosity and the shear stress. However, when reaching a critical fraction of the fillers, the network starts to be stronger and the inter-particle interactions becom dominant, thus making the structure flow more dependent on the shear stress applied. After this critical point, the shear-thinning mechanism does not change, thus making the thinning exponent insensitive to the filler content. The values of n are in agreement with those of other printable mixtures of viscoelastic materials [START_REF] Feilden | Robocasting of structural ceramic parts with hydrogel inks[END_REF][START_REF] Li | Rheological Study on 3D Printability of Alginate Hydrogel and Effect of Graphene Oxide[END_REF][START_REF] Barki | Linking Rheology and Printability for Dense and Strong Ceramics by Direct Ink Writing[END_REF]Thibaut et al., 2019).

Oscillatory rheology tests

In order to assess the viscoelastic properties of the different formulations, an amplitude sweep between 1% and 100% was performed at a 1 Hz frequency. Figure 4-10 shows the storage and loss moduli (G' and G", respectively) of different slurries. As one can notice, in the Linear ViscoElastic Region (LVER), the G' value is more or less stable, which defines the stiffness of the formulation (G' LVER) which is, as discussed before, rucial to printability assessment. Then, as stress increases, the storage modulus drops until below the loss modulus. The crossover point, associated with the breakdown of the particle network within the slurry, can serve as an approximation of the yield stress τy. The stiffness expressed by the G' (LVER) and the yield stress values of the tested slurries are plotted in Figure 4-11-A. One can notice that with the addition of cellulose particles, there is an important increase of several orders of magnitude in the stiffness and yield stress.

Therefore, to select the most printable formulations, the merit figure (eq 14) suggested by (Feilden, 2017) was used. It was recommended that the value of Φ be superior to 20, based on the analysis of different printable formulations in the literature that were declared printable by material extrusion.

A plot of stiffness (G' LVER) against the yield stress (τy) of each formulation is presented in Figure 4-11-B. The first criterion to classify printable and unprintable formulation is the absolute value of Φ, the line delimiting the value Φ= 20 is highlighted, but all the tested formulations were above this minimum value and the highest value is that of the formulation PFA-Cell28-B.

The second criterion is the minimum yield stress necessary to bear the gravitational forces for a certain build height. By neglecting the effect of the surface tension and considering only the effect of gravitational forces, the criterion suggested by (M' Barki et al., 2017) (eq 13) can be simplified and the yield stress can be expressed in terms of the building height according to eq 15. If we consider a minimum build height of h= 10 mm the yield stress value should be above 146 Pa. The upper limit of the yield stress can be determined by the extruder force.

Respecting these criteria, the formulations with the most printability potential are those containing over 25 wt% of cellulose (highlighted in green on Figure 4-11-B). Furthermore, by applying printability criteria (J. E. Smay et al., 2002), which consist of maximizing the stiffness of the suspension for better-spanning structures (eq 12), the formulations containing 28 wt% and 29 wt% are more advantaged.

Shear recovery behavior

During 3D printing with material extrusion, the inks experience shear forces and undergo physical changes, especially for complex fluid. The thixotropic rheological component of the different suspensions should be examined because it has crucial importance in the extrusion-deposition process. Indeed, ideally, the viscosity of the developed ink should decrease at high shear rates to facilitate the extrusion process (shear-thinning) and increase immediately after a total recovery at the outlet of the nozzle [START_REF] Kraut | Influence of shear thinning and material flow on robotic dispensing of poly(ethylene glycol) diacrylate/poloxamer 407 hydrogels[END_REF][START_REF] Lille | Applicability of protein and fiber-rich food materials in extrusion-based 3D printing[END_REF]. To investigate the thixotropy of formulations, a 5-step experiment was applied to model the high shear deposition process by alternating high shear rates and low shear rates at 60 s intervals. An estimation of the shear rate in the nozzle was first realized using the Rabinowitsch-Mooney equation, according to which a shear rate (Υ ̇) through a circular orifice of radius (R) for shear-thinning materials can be expressed according to eq 16 [START_REF] Dealy | Melt Rheology and Its Role in Plastics Processing: Theory and Applications[END_REF].

𝛾̇= 4Q πR 3 ( 3n+1 4n ) eq 16
Where Q is the volume flow rate and n the flow behavior index of the shear-thinning material. To determine the maximum shear rate which could be reached in our work, we can consider the formulation PFA-Cell28-B (n=0.16) printed using a nozzle with a minimum diameter of 0.5 mm (10 times larger than the largest particle of the powder) with a flow rate of 4.5 mm 3 /min (corresponding to a printing speed of 1800 mm/min). In these conditions, the shear rate would be 848 1/s.

Unfortunately, shearing such complex suspensions at this value of rate on the rotational rheometer was not possible because of edge fracture phenomenon. This kind of sample instability is characterized by a deformation of the sample's surface at the free edges between the upper and lower part of the geometry, as shown in Figure 4-12. Indeed, a 3-step thixotropic behavior test was performed on the formulation PFA-Cell28-B by shearing the slurry at 1000 1/s and the recorded viscosity change was unstable at high shear rate, as one can see in Figure 4-12-A. In Figure 4-12-B, a flow curve from 0.1 to 1000 1/s displayed the same phenomenon at around 200 1/s, i.e. a sudden drop in the viscosity values and a change in the flow behavior. This rheological characterization limitation is caused by a deformation of the surface and a secondary flow propagating radially as a function of both time and applied deformation which increases measuring errors at high shear rates [START_REF] Hemingway | Edge Fracture in Complex Fluids[END_REF][START_REF] Meyer | Optimizing process conditions and ensuring end product requirements of plastics with rheological analysis 7[END_REF]. For a relevant thixotropic characterization, the formulations were sheared at 100 1/s and 0.1 1/s rates. wt% of cellulose powder one can notice a drop in viscosity of 2 orders of magnitude reaching up to 3 orders of magnitude when adding more than 25 wt% of particles. When shifting to a low shear rate after 60 s of high rate shearing, the viscosity starts to build up again. In the viscosity build-up phase, a difference between the formulations occurs. The formulation containing the least cellulose builds up more quickly than those containing over 19 wt% cellulose, which can be explained by the difficulty of the highly filled structures to build up again after an important decrease in viscosity and breakage of the particles network. This might also be related to an increase in the temperature of the suspension due to particles friction, which decreases the resin viscosity. To quantify the overall recovery of the viscosity, a recovery percentage R(%) is calculated according to eq 17 where ƞin is the average viscosity during the last 10 s of step 3 and ƞfin the average of the viscosity during the last 10 s of step 5. Cellulose contains hydroxyl groups that can interact with water molecules which makes this biopolymer highly sensitive to humidity. As one can notice in Figure 4-15, after mass stabilization at 50% of relative humidity, the cellulose lost up to 1.5% of its weight upon desorption at low humidity levels. At a high humidity level (90%), cellulose gained weight rapidly to reach 5% in two hours. On the other hand, cured PFA is a hydrophobic matrix insensitive to water. Therefore, it is expected that the number of OH groups in FA, available for hydrogen bonding with water molecules, was reduced during PFA polymerization, resulting in a non-polar polymer. As shown in Figure 4-15, the weight of the specimen was stable with virtually no mass variation. The composite with up to 27 wt% of cellulose remained very little sensitive to water with a mass variation that does not exceed 0.1%, even at high humidity levels. The composite nevertheless is slightly more sensitive to humidity compared with that of the neat matrix. These curves suggest that the ensuing composite is quasihydrophobic despite the high cellulose fraction, a fact that can be explained by the encapsulation of cellulose particles in the hydrophobic polymer matrix.

These findings are in agreement with a previous work [START_REF] Mokhothu | Bio-based coatings for reducing water sorption in natural fibre reinforced composites[END_REF] where the coating performances of PFA for a flax/phenolic composite were assessed. Coated samples with different polymers were tested for water absorption properties according to ASTM D570 for 7 days and the moisture content was evaluated after aging for 3 days according to ASTM D5229. It was found that the PFA coated laminate had the best performance, as highlighted in Figure 4-16, with a reduction of 75% in water absorption and of 70% in moisture content after aging with respect to the uncoated laminates which also agrees with other works in the literature [START_REF] Dong | Wood/Polymer Nanocomposites Prepared by Impregnation with Furfuryl Alcohol and Nano-SiO2[END_REF][START_REF] Kumar | Biofilms from soy protein isolate and polyfurfuryl alcohol[END_REF].

The PFA coating showed also preservation in mechanical properties and prevention of thermal degradation [START_REF] Deka | Bio-based hyperbranched polyurethanes for surface coating applications[END_REF][START_REF] Dong | Wood/Polymer Nanocomposites Prepared by Impregnation with Furfuryl Alcohol and Nano-SiO2[END_REF]. Hence, the important cellulose content in the composites developed in the present work should not disrupt the outstanding performance of PFA polymers such as insensitivity to moisture and aging, thermal stability, and water resistance.

Effect of cellulose on porosity and dimensional stability

One of PFA major issues is the high water content in the resin and the water generated by the condensation reactions during curing. This issue is particularly noticeable in composite manufacturing because it leads to high porosity and low dimensional stability. Other parameters can trigger pores formation in the furanic resin, but moisture content and viscosity are the dominant factors [START_REF] Oishi | Viscosity, pH, and moisture effect in the porosity of poly(furfuryl alcohol)[END_REF].

The used commercial PFA contains 7.5 wt% of water and generates over 2 wt% during curing (Domínguez and Madsen, 2015), added to over 1.75 wt% of water brought by the catalyst. Therefore, at least 11 wt% of the resin's weight should be eliminated during the curing phase, which promotes porosity when the gas release is blocked. This important water content can lead to foaming caused by the boiling of the water inside the curing process especially for samples with thick walls which blocks the evacuation of gases through the structure. The foaming is particularly noticeable when the curing kinetic is fast, leading to a low gel time which interferes with gas elimination and hence the bubbles can be trapped inside the rubbery polymer matrix. To further investigate the effect of cellulose on the porosity caused by the water evaporation during curing, the prepared formulations were cured in the oven, but at a higher temperature (around the exothermic peak of the curing reaction) to favor resin foaming and to assess the effect of cellulose on this phenomenon. Some samples fluffed up and the porosity could not be accurately measured.

However, the effect of cellulose is clearly shown in Figure 4-17. One can notice that when curing at 90°C, below the water boiling temperature, the composite with or without cellulose did not foam, but still displayed micro and macro-porosity. This porosity might be mainly due to manual molding and mixing and not to vapor bubbles which were probably, as discussed above, drained through the fibrous network. On the micrograph in Figure 4-19-D one can visualize the fibers network. However, pores formation by vapor generation can still be possible and we suppose that they partially coalesced with the pre-existing air bubbles in the composite.

The porosity content of different composites is displayed in Figure 4-20-A. The pure PFA resin cured at 90°C had 8.8% porosity which is in agreement with the porosity found in a previous work for the same commercial PFA resin using the double-vacuum-bag technique (Domínguez and Madsen, 2015).

The porosity percentage doubled after the addition of 18 wt% of cellulose. Above 18 wt% of cellulose, the porosity decreased steadily when increasing the cellulose weight fraction. Owing to the particularly viscous and sticky nature of the PFA-Cell16-B slurry, the sudden increase in porosity when adding 18 wt% cellulose might be ascribed to air inclusion during the mixing and casting step and not only to water vapor generation during the thermal curing. In fact, this specific slurry was not as fluid as the pure PFA, which flowed easily in the molds. The stickiness of this particular slurry against the spatula further complicated the molding process leading to porosity embedding. For this reason, when the formulation started to be less sticky, casting was easier and voids inclusion was avoided. However, the porosity percentage of all the composites containing cellulose is higher than the neat PFA sample which is inconsistent with water content 30% lower than that of the bare resin.

Therefore, the porosity volume (Vp) in the composites was mainly associated with the mixing and casting process (Vp mix). For a better understanding, the hypothesized internal composite structure is illustrated in Figure 4-18-C where red circles represent the porosity creating during the mixing and molding and those represented with blue circles present the porosity created later during curing.

Accordingly, to estimate the porosity created during paste mixing and molding step (Vp mix), few assumptions should be made, namely:

• The porosity volume created in the neat resin in the curing step (Vp cure) is not affected by the cellulose addition (neglecting the draining effect of fibers).

• Any porosity in the fibers is neglected.

In this case, the Vp cure can be estimated based on the porosity of the neat PFA, which was found to be 8.9% using eq 18: Where Vm is the volume fraction without considering any eventual porosity in the composite calculated using eq 19.

V p cure (vol%) = V p neat PFA • Vm
Vm (Vol%) = V PFA (cm 3 ) • 100 V PFA (cm 3 ) + V cellulose (cm 3 ) eq 19
Where 𝑉 𝑃𝐹𝐴 (𝑐𝑚 3 ) = 𝑊 𝑃𝐹𝐴 (𝑔) 𝜌 𝑃𝐹𝐴 is the added oligomers volume inside the formulation, and

𝑉 𝐶𝑒𝑙𝑙𝑢𝑙𝑜𝑠𝑒 (𝑐𝑚 3 ) = 𝑊 𝑐𝑒𝑙𝑙𝑢𝑙𝑜𝑠𝑒 (𝑔) 𝜌 𝑐𝑒𝑙𝑙𝑢𝑙𝑜𝑠𝑒
the added cellulose powder volume inside the formulation.

The porosity volume caused by the mixing can be therefore estimated as the difference between the total porosity and the porosity created in the resin phase during curing as:

V p mix (vol%) = V 𝑝 -V p cure eq 20
The effective volume fractions in the composite considering the pores volume can therefore be recalculated and the results are given in Table 4-2. PFA, like the majority of thermosetting polymers, shrinks during curing, which, according to the supplier data, can reach 7%. The dimensional stability is important for composite production and 3D

printing and it is, therefore, essential to minimize it. As shown in Figure 4-20-B, the shrinkage decreased when increasing the volume fraction of cellulose. Shrinkage upon curing of the PFA went from 2.9% for neat PFA to 1.8% when adding at least 24 wt% of cellulose fibers. The shrinkage of the resin is due the densification of the network and the evaporation of water and other volatiles during the curing process. However, the cured composites have up to 35% of their volume occupied by dimensionally stable components (cellulose and pores), which could explain the shrinkage decrease.

A theoretical shrinkage percentage was estimated by considering the shrinkage to be strictly related to the matrix phase shrinkage (which is estimated to be 2.9 %) using eq 21 and the resulting values As one can see in Figure 4-20-B, the calculated theoretical shrinkage estimates well the experimental data except for the composite containing 21 wt% of fibers which might be related to an experimental issue (like incomplete curing of the specimens inside the oven).

Overall, cellulose addition has a major role in improving the composite dimensional stability during the curing process as shown by the fact that, under the tested conditions, cellulose decreased by 35% the shrinkage of the neat PFA down to 1.8% for the formulation PFA-Cell28-B (i.e. 3 times the usual shrinkage of ABS and PLA but less than the shrinkage of PP and PA which is over 2%). Cellulose addition can also prevent the foaming of the resin minimizing the curing porosity. It follows that, when avoiding air bubbles inclusion during the processing of the pasty formulation, the composite porosity can be decreased to less than 6%.

Thermal stability

The thermal stability of the crosslinked PFA, cellulose and the composites was assessed by thermogravimetric analysis (TGA). The TGA curves given in Figure 4-21, show that in an oxidative atmosphere, cellulose loses 3% of its weight between 90 and 200°C, which is considered to be the residual moisture, followed by a classical abrupt degradation starting at 300°C, before a total degradation of the material when the temperature reaches 530°C. In the case of PFA, one notices a three-step degradation process. The first ramp starts at 200°C with only 10% weight loss, which could be explained by a further curing and consequent water evaporation [START_REF] Ma | Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review[END_REF]. The second step starts when the decomposition rate increases around 400°C to reach a 50% weight loss at 600°C, which is attributed to the decomposition of the furan polymer (Guigo et al., 2009). The final degradation step leads to the total burning of the organic polymer at 700°C.

The degradation of the composites with 31 wt% cellulose content goes through the same 3 steps as for the neat PFA, with an increase in the cellulose decomposition temperature when the degradation rate increases, to reach a 25% weight loss at 400°C.

Under inert atmosphere (N2), the degradation curves (Figure 4-21) show the same degradation steps, but the degradation rate decreases considerably starting from 500°C to reach a plateau at 800°C. After the stabilization of the weight loss, the char residue is about 50 wt%, which is in agreement with values given in the literature (Gandini, 1997). This residue decreases when increasing the thermally unstable cellulose fraction that loses up to 90% of its initial weight. The char residue of the composite with more than 30 wt% of cellulose is around 40 wt%, which therefore does not exclude the possibility of carbonization of the molded composites.

Thermomechanical behavior

The storage modulus E' of composites as a function of temperature is given in Figure 4-22-A.

Cellulose addition increased the storage modulus of the composites, which is higher than that of the neat cured PFA over the whole temperature region. This could be ascribed to the tension transfer from the matrix to the high-modulus filler, which enhanced the composite stiffness.

At room temperature, the modulus E' increased when increasing the cellulose fraction, which was maximized for Wf= 30 wt%. Cellulose addition also improved the composite thermomechanical stability by increasing the stiffness at high temperatures, as highlighted in Figure 4-22-B. For instance, with a 30 wt% weight fraction of cellulose, the modulus remained higher than 4 GPa up to 150°C and higher than 2 GPa up to 200°C, whereas, for the neat cured PFA, it dropped below 2 GPa when reaching 150°C. The thermal stability and the rigidity of both the cured neat PFA and the composites were higher than those reported in the literature when incorporating sisal whiskers and lignin in conventional PFA (Ahmad et al., 2013;[START_REF] Guigo | Eco-friendly composite resins based on renewable biomass resources: Polyfurfuryl alcohol/lignin thermosets[END_REF]. In addition to the typical thermomechanical stability of cellulose fibers, cellulose microparticles are supposed to hinder polymer chains mobility thus further enhancing the composite thermomechanical behavior. This thermomechanical stability is competitive with those of PEEK and some other technical polymers used in 3D printing [START_REF] Diez-Pascual | The influence of a compatibilizer on the thermal and dynamic mechanical properties of PEEK/carbon nanotube composites[END_REF][START_REF] Cao | The surface modifications of multi-walled carbon nanotubes for multi-walled carbon nanotube/poly(ether ether ketone) composites[END_REF][START_REF] Qu | Elevated-Temperature Wear and Friction of PTFE/PEEK composite[END_REF][START_REF] Papageorgiou | Hybrid poly(ether ether ketone) composites reinforced with a combination of carbon fibres and graphene nanoplatelets[END_REF][START_REF] Niu | Mechanical and thermal properties of PEEK composites by incorporating inorganic particles modified phosphates[END_REF]. Unlike the PFA/cellulose composite, these technical thermoplastics can be extremely expensive and also rather complicated to print, apart from the fact that they are fossil-based materials.

In order to evaluate composite mechanical properties at high temperature and under prolonged solicitations, composite samples were tested during 6 h at 200°C. Figure 4-23 shows the storage modulus of the composite PFA-cell28-B (containing 30 wt% cellulose) under a static force of 10 and 20 N with a dynamic displacement of ±15 µm at a frequence of 1 Hz. As one can see on the figure, under 10 N of static force (dashed red curve) the storage modulus is around 5 GPa at room temperature and, when heating at 200°C, it drops to 1 GPa to raise again until stabilizing at 2.8 GPa.

This decrease-increase of the storage modulus when heating at 200°C was related to further crosslinking of the polymeric structure. Accordingly, another sample was heat-treated in an oven for 2 hours at 200°C and the same DMA test was run again. In this case, the storage modulus at room temperature increased by 20% with respect to the untreated sample and when the temperature increased to 200°C the storage modulus dropped to 3 GPa and the increase of the modulus was also remarkably limited comparing the untreated sample. Furthermore, the storage modulus decreased only by 20% when increasing the temperature from 25°C to 200°C and was stable for more than 6 hours. The same test was carried out under 20 N static force and the same behavior was observed with a 30 % decrease in the storage modulus.

Mechanical performances

As can be seen in Figure 4-24, the composite stiffness improved after the addition of cellulose powder. The tensile modulus increased from 2527 to 2880 MPa and the flexural modulus from 3092 to 4309 MPa by adding 30 wt% of cellulose. This modest improvement suggests a good wetting by the resin of the cellulose particles [START_REF] Ku | A review on the tensile properties of natural fiber reinforced polymer composites[END_REF] and agrees well with the DMA results.

On the other hand, the tensile and flexural strength decreased when adding cellulose with an opposite evolution of porosity. This can be explained by the fact that high porosity content in the composite is deteriorating the strength because pores act as stress concentrators and fracture nucleation sites [START_REF] Pfister | Green composites from a conjugated linseed oil-based resin and wheat straw[END_REF]. Therefore, when recalculating the specific modulus by normalizing the measured values to their density, one can notice that cellulose is improving the mechanical properties of the composite by increasing the flexural modulus by 40% and tensile modulus by 10%. Therefore, voids generated during the paste preparation and manual casting are supposed to be at the origin of the limited enhancement of the mechanical performance of the tested samples. 197

Conclusions

To sum up this section, the following points can be retained.

i.

The conducted preliminary tests showed that:

• The use of microscopic powdery cellulose is preferred for a stable extrusion through narrow nozzles usually used in 3D printing to maximize the printing resolution.

• An addition of at least 10 wt% of cellulosic fillers is needed to convert the fluid PFA oligomers to a more concentrated suspension with a stable extrusion flow whilst the addition of more than 30 wt% cellulose results in highly concentrated solid-like paste, which is hardly extrudable.

• The solvent content in the formulation should be limited as much as possible to prevent cracking of the composite upon curing and pores creation usually caused by the exothermic curing of the PFA oligomers.

ii.

The conducted rheology tests revealed that:

• The shear-thinning behavior of all the slurries is maximized with the addition of at least 25 wt% fillers.

• The yield stress and stiffness increased with the increase of fillers content, to reach respectively 1.46 kPa and 270 kPa when adding 29 wt% of cellulose powder.

• The slurries containing cellulose powder exhibited a thixotropic behavior, but the viscosity recovery decreased when increasing the cellulose content and it was limited to 60% when adding 28 wt% of fillers, against 98% when adding only 16% of cellulosic particles.

• According to these rheology features, the ink formulation containing at least 25 wt% of cellulose and less than 6.6 wt% of solvent (imported by the resin and the catalyst) have the highest potential for 3D printing by material extrusion.

iii.

Finally, the physical, thermal, and mechanical characterization of the cured composite showed that:

• Cellulose particles thwart the PFA foaming when curing under fast kinetics thanks to steam diffusion through the fibrous network, but, on the other hand, they promote pores creation during mixing and hand-casting because of the sticky nature of the slurries.

• The dimensional shrinkage of the cured composite containing 30 wt% of cellulose (PFA-Cell28-B)

decreased by 40% with respect to that of neat PFA.

• The neat PFA and the ensuing composites exhibited good thermal stability until 400°C with a mass loss of less than 10%.

• The thermomechanical performance of PFA enhanced with cellulose addition in order to reach a storage modulus of about 5 GPa at room temperature and more than 2 GPa at 200°C which remained stable for at least 6 hours.

• In agreement with the DMA tests, the flexural and tensile modulus of the composites increased with increasing cellulose content to reach respectively 4.3 and 2.9 GPa for the composite containing 30 wt% cellulose, referred to as PFA-Cell28-B. However, the strength of the composite decreased when adding cellulose which was in line with samples porosity. Thus, the drop of composite strength is believed to be caused by the internal pores which act as stress concentrators.

Development and characterization of 3D printable material with electrical conductivity

Introduction

To upgrade the printable material, the idea of adding electrically conductive filler was explored. In the literature, various printable conductive inks were developed for 3D printing by adding conductive particles such as graphene, carbon fibers, silver wires, and carbon nanotubes to different kinds of polymers. To successfully formulate this conductive printable ink the formulation should fulfill the printability requirements discussed before but also conduct as many electrons as possible after solidification. In this section, we start with preliminary tests to select the most convenient conductive filler, then a study of the effect of fillers morphology on both the dispersion ability in ethanol and on the rheology is studied, and finally, different formulations were prepared to optimize the rheological behavior and the electrical conductivity.

Fillers effect on rheology and electrical conductivity -preliminary tests

As a first step, different formulations were prepared with three different conductive fillers described in chapter 2; graphene pellets (Gr), graphite powder (G), and double-walled CNT dispersed in water (DWCNT-L). The first attempt was to directly add Gr to the PFA and study the effect on the rheological properties. As we can see in Figure 4-25-A, the suspension with 14 wt% Gr (green curve) has a perfectly Newtonian flow behavior, and the suspension containing 24 wt% Gr (orange curve) start to have a slightly shear-thinning behavior (n=0.85) which can be a proof of the onset of particles percolation.

The limited viscosity and shear-thinning ability of these formulations are probably due to poor dispersion of the Gr particles which was done mechanically and were difficult to disperse in the viscous PFA resin. To overcome the poor rheological performances, formulations with 19 wt% of cellulose powder with the addition of different weight fractions of Gr and G were prepared with a simple mechanical stirring. The flow behavior of these formulations was shear-thinning (n=0.4) with higher apparent viscosity, dominated mainly by the effect of cellulose powder as can be seen in and DWCNT-3 have a highly shear-thinning behavior, appreciated for a 3D-printable ink. The formulation DWCNT-1 contains less CNT and cellulose compared with DWCNT-3, which can explain the smaller shear-thinning behavior compared with that of DWCNT-3. However, the apparent viscosity of the latter is lower, which might be mainly due to the important water content which comes with the addition of CNT (27 wt% vs. 17 wt% water in the formulations). Another important parameter to assess is the electrical conductivity of these formulations after curing. The conductivity of the formulations containing graphite and graphene particles along with 19% cellulose was not detected so these materials are considered to be non-conductive.

In Figure 4-26-A, one can see that the electrical conductivity of a casted cylinder of Gr-24 is around 3.4 10 -2 S/m which exceeds that of the formulation containing 1.23 wt% of DWCNT-L (after water evaporation). However, the electrical conductivity of the formulation containing DWCNT-L increases with the decrease of nozzle diameter. This opposite proportionality is due to the increase of the shear stress applied during the extrusion when decreasing the nozzle size which favors the alignment of the CNT. The shear-induced alignment of CNT in the nozzle during extrusion is a well-known phenomenon that increases the contact probability among CNT in the extrusion direction, increasing accordingly the electrical conductivity in the same direction (Lanticse et al., 2006;[START_REF] Tang | Shear Flow and Carbon Nanotubes Synergistically Induced Nonisothermal Crystallization of Poly(lactic acid) and Its Application in Injection Molding[END_REF][START_REF] Zheng | Dispersion and shear-induced orientation of anisotropic nanoparticle filled polymer nanocomposites: insights from molecular dynamics simulation[END_REF]. In the case of graphene pellets, the electrical conductivity decreased (Figure 26-A), which can be due to a breakage of the contact between the particles during shearing or the formation of disconnected clogs of the particles due to the high friction between the pellets. In the photographs presented in Figure 4-26-B/C, one can notice the poor printability of the formulation with the graphene pellets as the printed lines tend to spread on the plastic substrate. On the other hand, the lines printed with ink formulation DWCNT-3, containing 0.9 wt% CNT, 27 wt% water, 14 wt% cellulose, and 58 wt% PFA, tend to retain the shape of the extruded filament with no visible spreading on the substrate.

In light of these first tests, the use of CNT seems to be more convenient because: (i) it confers a shear-thinning flow behavior and a certain shape maintaining compatible with the requirements of 3D printing by extrusion, (ii) it can be aligned with shear applied in the nozzle outlet improving the electrical conductivity, and (iii) it has a low percolation threshold in PFA/cellulose composites.

Suspension of CNT in ethanol

Based on the results of the preliminary formulation tests, carbon nanotubes seem to be the most suitable filler for a functional printable ink formulation. Nonetheless, one of the challenges encountered is the high water content added to the formulation when increasing the CNT fraction in the formulation, which not only affects the rheological behavior of the formulation, but also can lead to phase separation since the PFA resin is only soluble in 24 wt% of water. In addition, defaults can be caused by the high water content during curing. As explained in chapter 1, the exothermal reaction of PFA oligomers can cause high porosity and foam upon curing. To overcome these challenges, it was decided to use CNT powder and disperse it in ethanol (a solvent of PFA resin which can be easily evaporated) using the protocol described in chapter 2. The already tested doublewalled nanotubes with a high aspect ratio are known for their good electrical conductivity, but their dispersibility can be tricky [START_REF] Li | Correlations between Percolation Threshold, Dispersion State, and Aspect Ratio of Carbon Nanotubes[END_REF]. Therefore, a choice of CNT type should be made according to their dispersibility. Thereafter, 6 stock dispersions of 6 different CNT types (whose properties are given in chapter 2 p.111) with different aspect ratios were prepared and their contents are detailed in Table 4-3. As already mentioned in chapter 2, a dispersing agent provided with the CNT was added according to the fraction recommended by the manufacturer. despite the important amount of added dispersant. After 24 hours, the sedimentation of SWCNT starts to be more noticeable and the same phenomenon was detected for MWCNT1 and DWCNT (these 3 CNT types have a high aspect ratio of 1500-2500). After 10 days, the same dispersion behavior was observed for the high aspect ratio CNT, whilst the suspensions with low aspect ratio CNT (<333) had a stable dispersion.

To further assess the homogeneity of the CNT suspensions, the 1 wt% CNT solutions were then mixed with the PFA resin, which results in an overall 0.3 wt% concentration of CNT. The suspensions were placed on glass supports to form a film to be observed under a fluorescence microscope. These observations are displayed in Figure 4-27-B. One can notice that the dispersion of SWCNT and DWCNT is not uniform and inhomogeneous with the formation of important aggregates and bundles, which is a classic issue when using high aspect ratio nanotubes [START_REF] Ma | Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review[END_REF]. The suspensions with an aspect ratio ranging from 33 (MWCNT-short) to 333 (MWCNT-2 and MWCNT-Gr) were easily dispersed despite the existence of some visible aggregates which could be broken with a longer sonication. Despite the advantage of the use of a high aspect ratio of CNT to decrease the percolation threshold, these long CNT have seriuous dispersion issues which can impair the electrical conductivity and the printability of the composite paste [START_REF] Huang | Dispersion and rheology of carbon nanotubes in polymers[END_REF][START_REF] Valentino | Influence of the polymer structure and nanotube concentration on the conductivity and rheological properties of polyethylene/CNT composites[END_REF][START_REF] Mcclory | Electrical and rheological percolation of PMMA/MWCNT nanocomposites as a function of CNT geometry and functionality[END_REF].

Accordingly, the flow behavior of the different suspensions in ethanol under a steady shear rate was studied. For every type of CNT, three suspensions with 1, 0. Given the latter results, the short nanotubes (MWNCT-short) tend to have the best morphology in order to provide good and stable dispersions, resulting in a stable flow, higher viscosity, and a shearthinning behavior needed for the future 3D printable ink. For the development of the printable formulation, different suspensions were prepared using MWCNT-short and DWCNT with a concentration between 0.5 wt% (concentration from which a clear entanglement is noticed) and 4 wt%.

Conducive ink formulation

To obtain a conductive and 3D printable ink, different suspensions of DWCNT and MWCNT-short in PFA with or without cellulose powder were prepared using the protocol detailed in chapter 2. These suspensions are referred to as DCNT-x-y for the DWCNT and MCNT-x-y for MWCNT-short with x being the CNT weight fraction and y the cellulose weight fraction in the formulation.

Once again, a rough rheological characterization is necessary to assess the printability and thus to determine the most adapted ink for 3D printing by material extrusion. To this end, the variation of the apparent viscosity with shear rate of the 0.5 wt% and 4 wt% CNT dispersions was measured and shown in Figure 4-29-C. A concentration of 0.5 wt% CNT in PFA results in a Newtonian flow behavior characterized by a constant viscosity. The viscosity of the suspension containing short MWCNT is higher than that with high aspect ratio CNT, which correlates well with the results presented in the former section where the viscosities of the different stock suspensions were compared (Figure 4-28-D). However, the shear-thinning behavior observed in the case of ethanol suspension is not detectable here, which is most likely due to a poorer dispersion in the viscous PFA resin. When increasing the CNT content to 4 wt%, a shear-tinning behavior of the suspension starts to appear with a clear increase in the apparent viscosity, especially for the formulation MCNT-4. We can notice that in the case of low aspect ratio CNT, the addition of 15 wt% cellulose powder was enough to confer the suspension (MCNT-4-15) flow a pronounced highly shear-thinning behavior. On the other hand, when using high aspect ratio nanotubes, 15 wt% cellulose (DCNT-4-15) only increased the apparent viscosity of the suspension without any noticeable shear-thinning in its flow.

The addition of an extra 5 wt% cellulose (DCNT-4-20) was needed to form a more solid cellulose/CNT network to detect a shear-thinning flow triggered by the breakage of the network under shear stress.

An amplitude sweep test of DCNT-4-15 and MCNT-4-15 was performed and the corresponding results are shown in Figure 4-29-D. The storage modulus of the two formulations containing 15 wt% of cellulose is slightly higher than the loss modulus before a cross-over point corresponding to yield stresses of 770 ±170 Pa for MCNT-4-15 and 524 ±125 Pa for DCNT-4-15. However, the stiffness of both formulations cannot be easily concluded based on these oscillatory rheology results, as the G' starts to decrease in the tested strain range (from 1 to 100%). Nevertheless, the storage modulus of the formulation containing short nanotubes is higher, which can be explained by their better dispersion.

According to this rheological assessment, the addition of at least 15 wt% of cellulose is needed for yielding a shear-thinning flow needed for a stable extrusion through nozzle without any clogs formation. Morover, the additional cellulose can increase the viscosity and the stiffness of the material in order to resist the printing force and also to obtain a viscoelastic material with high yield stress sufficient to bear the gravitational forces. The use of short nanotubes is also preferred because of their better dispersion. To determine the effect of the addition of CNT to the PFA matrix, the volume electrical conductivity (EC) of CNT/PFA nanocomposites was evaluated from resistance measurements performed on nanocomposite filaments extruded from a 2mm diameter nozzle with increasing CNT content (0.5-4 wt%). Each resistivity measurement was done on 3 different filaments.

As shown in Figure 4-30, the resistivity of the nanocomposite with 0.5 wt% CNT was out of the range to be measured with the multimeter and hence the EC was considered to be zero. The EC starts to increase substantially upon the addition of 1.5 wt% CNTs to reach 1.6 10 -4 S/m for DWCNT and 6.1 10 -4 S/m for MWCNT-short. Furthermore, an abrupt increase in EC is observed when increasing the MWCNT concentrations to 4 wt%, reaching values of 5 10 -3 S/m and 4 10 -2 S/m for, respectively, DWCNT and MWCNT-short. The difference in EC values about an order of magnitude between the two types of CNT is once again mainly due to the dispersion quality which rules the percolation of the conductive particles and hence favors the EC for the better-dispersed nanotubes which are in the present case the short CNT (MCNT-4). Indeed, by examining the cross-section SEM images in Figure 4-31, one can clearly notice the presence of large aggregated DWCNT (highlighted with red circles) suggesting a poor dispersion of the CNT. On the other hand, the short CNT diaplays a good dispersion in the PFA, with no visible aggregates. The overall EC is higher than that reported by Lanticse et al.

which was about 10 -5 S/m for 5 wt% of MWCNT in the PFA matrix (Lanticse et al., 2006). It is also worth mentioning that the measured electrical conductivity is underestimated, since it does not consider the contact resistance between the probe placed on the filament surface and the material. The electrical conductivity of the composites containing an equal content of CNT (4 wt%) and 15wt%

and 20 wt% of cellulose powder was measured and is shown in Figure 4-32. After the incorporation of 15 wt% of cellulose, the electrical conductivity increases by one order of magnitude for both CNT types. The EC increases further when adding 5 wt% more cellulose powder to reach 4.5 10 -1 S/m. This phenomenon is explained by a decrease in the percolation threshold of the CNT after the incorporation of inert filler. The filler occupies a certain volume in the composite which makes the space occupied by the PFA/CNT dispersion less important. This increases the contact probability among the nanotubes which therefore decreases the percolation threshold with the same overall CNT concentration in the composite and thus increases the electrical conductivity. The CNT arrangement in the composite with and without cellulose powder is illustrated in Figure 4-33 for a better understanding. This phenomenon is also explained by Bao et al., where they introduced the concept of the effective concentration of CNT as the ratio between the CNT volume and the sum of the CNT and the polymer volumes [START_REF] Bao | Effect of electrically inert particulate filler on electrical resistivity of polymer/multi-walled carbon nanotube composites[END_REF]. The effective concentration in our case is calculated on the weight base according to eq 22 and found to be respectively 0.048 and 0.050, when adding 15 and 20 wt% cellulose against 0.04 without any inert particles, which explains the increase in EC.

CNT eff = W CNT W (CNT+PFA) eq 22 Figure 4-32: Electrical conductivity of composite containing 4 wt% of CNT with different cellulose content. Therefore, we can conclude that:

• The CNT addition increases the viscosity of the PFA resin without changing its Newtonian and viscous flow behavior.

• An addition of at least 1.5 wt% of CNT in the PFA matrix is needed to effectively increase the electrical conductivity.

• The dispersion of low aspectratio CNT in PFA is better than that with high aspect ratio, which resulted in higher viscosity, better shear-thinning behavior, and higher electrical conductivity.

• The addition of cellulose enhances the printability of the nanofluid by enhancing the shearthinning behavior and transforming the viscous flow to a viscoelastic behavior with a yield stress of 770 ±170 Pa. • The added cellulose increases the electrical conductivity by one order of magnitude at a constant CNT concentration with respect to the cellulose-free nanocomposite. The cellulose addition enables a reduction in the CNT content, thus limiting the material cost and environmental impact.

In the light of these findings, the formulation MCNT-15-4 was selected to be the most relevant to our application as, on the one hand, its rheological properties fulfill 3D printing by extrusion's requirements and, on the other, it exhibits good electrical conductivity values. The addition of 20 wt% cellulose was found to deteriorate the mechanical properties of the extruded filament which was found to be too fragile.

Towards a better electrical conductivity-pyrolysis

Organic polymers with high carbon content and limited oxygen content can usually be carbonized and in the case of some polymers even graphitized. Furan resin fulfills these carbonization requirements and, as shown previously, the used PFA resin can yield up to 50 wt% char residue.

Thus, this polymer is largely used as a precursor for a non-graphitizing carbon called glassy carbon (same for cellulose) resulting in a lower electrical conductivity compared to graphite, but in an improved hardness [START_REF] Manoharan | Elastic Properties of 4-6 nm-thick Glassy Carbon Thin Films[END_REF][START_REF] Rajagopalan | Molecular sieving platinum nanoparticle catalysts kinetically frozen in nanoporous carbon[END_REF][START_REF] Sharma | Glassy Carbon: A Promising Material for Micro-and Nanomanufacturing[END_REF]. A study of the Raman spectra of PFA pyrolyzed at different temperatures revealed certain critical temperatures. At 400°C carbonization starts, at 550°C graphite nucleation appears, at 700°C crystallite growth is observed, and above that temperature crystallite size increases [START_REF] Li | Comparative Study on Different Carbon Nanotube Materials in Terms of Transparent Conductive Coatings[END_REF]. These critical temperatures can be also recognized in the TGA and heat flow curves of Figure 4-21. It was also proven that the increase of heat treatment (above 850°C) raised the efficiency of creation of charge carriers by a better degradation of PFA molecules which resulted in an improvement in electrical conductivity (Wang et al., 1997). Based on these findings, and the TGA curves obtained in this work, the PFA-based composites (with and without CNT) were pyrolyzed under inert gas under a low heat rate and until reaching the high temperature of 950°C in order to increase the electrical conductivity of the furanic material.

Filaments of composite MCNT-4-15 and the CNT-free composite (PFA-Cell25-B) were pyrolyzed and their electrical conductivity was measured and displayed in Figure 4-35. As expected, the electrical conductivity increased by 5 orders of magnitude from 4 10 -2 to 4.7 10 3 S/m for the composite containing CNT and 2 10 3 S/m for the composite without CNT. These EC values surpass the value reported by Hawes et al. of laser-induced carbonization of PFA polymer doped with graphene oxide, which was in the range of 9 10 2 S/m (Hawes et al., 2019). The increase in EC conductivity when adding CNT can be explained by the higher conductivity of the percolated CNT (> 10 4 S/m) compared with that of the vitreous carbon formed after PFA and cellulose pyrolysis. From a morphological point of view, one can see on the SEM cross-section images of fractured filaments in Figure 4-36 that the PFA-Cell25 composite conserved its main circular form and that the rough cross-section transformed into a smooth glass-like material after pyrolysis. Many macroscopic pores can also be observed before and after pyrolysis, which is most likely caused by volatile evaporation difficulties during the curing step, as we can see on the filament image before pyrolysis which was discussed more in detail in the previous section of this chapter. In addition to the macropores, the formation of open pores was suspected to facilitate the transport of volatiles and tar products formed during the pyrolysis of PFA [START_REF] Fitzer | The formation of glasslike carbon by pyrolysis of polyfurfuryl alcohol and phenolic resin[END_REF][START_REF] Fitzer | The effect of crosslinking on the formation of glasslike carbons from thermosetting resins[END_REF]. This hypothesis was confirmed later to show the formation of meso and micropores when heating below 400°C and a complete collapse of the mesoporous structure and the exclusive residue of microspore with a size of 4-5 Å [START_REF] Burket | Genesis of porosity in polyfurfuryl alcohol derived nanoporous carbon[END_REF]. In the case of the present work, the remaining micropores after the heat treatment at 950°C were out of the limit of the SEM resolution.

For large samples, pure glassy carbon is only obtained using temperatures as high as 2000°C or more to ensure effective annealing of volatiles from the core of the sample [START_REF] Hancox | Sample size dependence in pyrolysis: An embarrassment, or a utility?[END_REF]. However, it was demonstrated that for samples with dimensions <3 mm, which is the case of the used filament in this work, a temperature as low as 900°C was sufficient to yield a glassy carbon with some oxygen impurities [START_REF] Maleki | Determining the shortest production time for glassy carbon ware[END_REF]. The lower electric conductivity obtained in the present study (2 10 3 S/m), with respect to the electrical conductivity of pure glassy carbon, might be due to the not compensated contact resistance when the EC conductivity was measured.

Conclusions

In this section, a printable formulation for the manufacturing of a semi-conductive composite was successfully developed. The formulation containing 4 wt% of MWCNT-short and 15 wt% cellulose, referred to as MCNT-4-15, exhibited the best rheological performances and electrical conductivity values reaching at least 3.85 10 -1 S/m.

Furthermore, the carbonization of the pristine electrically insulating biocomposite containing 30 wt% cellulose (PFA-cell28-B), yields an increase of its electrical conductivity to attain 2 10 3 S/m. This electrical conductivity doubles when carbonizing the biocomposite containing 4 wt% of carbon nanotube (MCNT-4-15) to reach 4.7 10 3 S/m. These different printable biocomposites with variant electrical conductivity levels, highlighted in Figure 4-37 by a yellow star, can allow the direct fabrication of conductive structures with tailored 3D objects and enable the integration of electronic functionalities into complex 3D structures.

Chapter conclusions

In this chapter, two types of printable ink formulation by extrusion were developed and the resulting cured composites were characterized.

For the development of the structural ink, a rheological study of different slurries based on a PFA thermosetting resin and microcrystalline cellulose powder was conducted. Accordingly, the rheological data singled out two ink formulations with potential for 3D printing. These two formulations contain from 25 to 28 wt% cellulose powder, respectively referred to as PFA-cell25-B and PFA-cell28-B. These identified slurries exhibited a shear-thinning behavior, satisfactory yield stress, and a thixotropic behavior, which are crucial parameters for a successful 3D printing by cold extrusion.

The obtained composite materials after the thermal curing of the prepared slurries were characterized for a better understanding of the resulting physical and mechanical properties. The tensile and flexural moduli of the composites increased with increasing cellulose content, to reach 2.9 GPa and 4.3 GPa, respectively. On the other hand, the strengths decreased after the addition of cellulose particles with respect to the pure PFA because of the high porosity caused by air inclusion during the mixing and casting steps of the composites. The tensile and flexural strength of the PFA-cell28-B composite are respectively 40 and 20 MPa.

Shrinkage is another parameter to supervise when 3D printing, especially when using shrinkable amorphous polymers. The neat PFA had a dimensional shrinkage of 2.9 %, which was decreased by almost 40% for PFA-cell28-B (30 wt% cellulose in the cured composite).

Furthermore, cellulose addition enhanced the thermomechanical properties of the composite with a storage modulus of over 5 GPa at room temperature. At high temperature (200°C), the modulus decreased by only 20% and stayed stable for hours. The thermal stability of the composite was also assessed by TGA tests, which showed a limited weight loss (<10%) at temperatures below 300°C.

The development of conductive ink started with a comparison of the dispersion quality and the rheology of suspensions of different types of carbon nanotubes, which revealed a better dispersion and hence a better rheology behavior of the CNT with a low aspect ratio.

The formulation containing 4 wt% of MWNT-short and 15 wt% of cellulose powder showed the best rheological performances for a better printability and an electrical conductivity of at least 2 10 -2 S/m.

Furthermore, the carbonization of the two different composites (conductive and structural) further increased the electrical conductivity thanks to the formation of a glassy-carbon structure or, in the case of the addition of CNT, a carbon/carbon composite with an electrical conductivity of at least 4.7 10 3 S/m, enough to turn on a LED in a simple electric circuit.

An optimized ink for LDM additive manufacturing is not enough to achieve a defect-free and accurate solidified printed object. The optimization and the comprehension of both the 3D printing process and the curing step of the object, especially when handling an oligomer with a challenging curing process such as the poly(furfuryl alcohol). Optimization of the printing and curing process of the optimized structural ink formulations will be detailed in the following chapter.

Chapter introduction

As described in chapter 1, among other additive manufacturing technologies, material extrusion remains the most widespread in terms of the printed volume. Resultantly, if one looks into the scientific production since the flourishing of the additive manufacturing technology, one will notice that most of the studies on the print quality, the effect of printing parameters are related to FFF and the most commonly used thermoplastics such as PLA and ABS. When it comes to direct ink writing, the effects of these parameters are poorly studied and also can differ from one material to another due to their different properties. Accordingly, when developing a new material it is important to first understand and optimize every step in the printing process.

Subsequently, the objective of this chapter is to study the printing and solidification process of the PFA/cellulose paste with the aim to optimize the process and the ensuing performances of the cured composite. The chapter is divided into two distinct but complementary parts. The first part deals with the understanding of the curing kinetics of the used PFA commercial polymer and the water evaporation step. These insights would permit to better program the solidification step during or after the printing process in order to obtain a defect-free-fully-crosslinked composite. The focus of the second part, is the printing step. In order to master the printing step, several key parameters need to be controlled and understood. Thereafter, the extrusion step was optimized by the calibration of the extrusion head to match with the flow calculation. Then, the effect of the main printing parameters was assessed to better quantify the printability and the limitations of the used material. Finally, a customized printer was developed. The general layout of the present chapter is shown in Figure 5-1. On the same figure, one can see the addressed issue along with the main used characterization techniques.

Part 1 -Solidification and crosslinking optimization

Introduction

The crosslinking of PFA is a complex process as explained in chapter 1 which should be optimized to avoid any defects during curing and to ensure a total crosslinking in order to yield high mechanical and thermal properties. The optimization of the curing process of the oligomer can only be achieved when the different events during curing are identified and the crosslinking kinetics are investigated.

Accordingly, in this first part of the fifth chapter, DSC scans were first performed to have a close-up of the events occurring during the crosslinking of PFA oligomers, then the kinetics of both curing and water evaporation during crosslinking were studied in order to optimize the solidification step.

Finally, shear tests were performed on the PFA/cellulose paste to monitor the changes in the viscoelastic properties during the crosslinking process.

Polymer curing and crosslinking understanding -Differential

Scanning Calorimetry

Effect of catalyst concentration

Different PFA/catalyst ratios were analyzed by DSC at a 5 °C/min heating rate. The PFA samples were prepared by diluting the commercial product with 15 wt% water and adding the catalyst at 0, 2, 5, and 15 wt%. On the DSC thermograms at different catalyst content, shown in Figure 5-2 and summarized in Table 5-1, the variation of the exothermic network-polymerization peak can be observed. The higher the catalyst concentration, the more pronounced the exothermic peak and the lower the exothermic peak temperature. In the same Figure, one can also see that the addition of the catalyst is essential for curing since, in the absence of catalyst, one cannot observe any exothermic peak, but only the endothermic peak of water evaporation at around 100°C. The increase in catalyst concentration up to 15 wt% induced a more exothermic (the maximum heat flow at the peak increases) and rapid reaction (lower onset temperature) which, in the case of PFA, can lead to the quick evaporation of residual water and foaming/high porosity. When adding 5 wt% of catalyst the curing heat has a similar value as when adding 15 wt% catalyst (292-309 J/g), thus suggesting that 5 wt% of catalyst is enough for a total crosslinking of the polymer in the tested conditions. On the other hand, the use of only 2 wt% of the catalyst seems to be insufficient for rapid curing because the reaction curing heat is around 262 J/g, well below the enthalpy of 300 J/g measured when adding 5-15 wt% of catalyst. This reduction is supposed to be due to the fact that this low amount of catalyst is not capable of reaching all the reactive sites in the resin network leading to an incomplete crosslinking reaction [START_REF] Montserrat | Influence of the accelerator concentration on the curing reaction of an epoxy-anhydride system[END_REF][START_REF] Ooi | DSC studies of the curing mechanisms and kinetics of DGEBA using imidazole curing agents[END_REF][START_REF] Domínguez | Study on the non-isothermal curing kinetics of a polyfurfuryl alcohol bioresin by DSC using different amounts of catalyst[END_REF]. It can be concluded that 5 wt% seems to be the best catalyst concentration to minimize the lower water content (the catalyst is water-based) while keeping a rapid curing rate with high conversion and a limited exothermic reaction peak.

Effect of heating rate

To assess the effect of the heating rate on the curing of the polymer when adding 5 wt% catalyst DSC tests with 2, 5, and 10 °C/min heating rates were realized and their respective thermograms are displayed in Figure 5-3. As one can observe in Figure 5-3, the faster the heating rate, the more pronounced the exothermic peak, and the peak shifts to higher temperatures as highlighted in Table 5-1. This behavior is described in the literature for the curing of PFA/FA [START_REF] Domínguez | Study on the non-isothermal curing kinetics of a polyfurfuryl alcohol bioresin by DSC using different amounts of catalyst[END_REF]Guigo et al., 2007b;Lopez de Vergara et al., 2014) and similar thermosetting polymers systems [START_REF] Gaisford | Principles of Thermal Analysis and Calorimetry[END_REF][START_REF] Montserrat | Influence of the accelerator concentration on the curing reaction of an epoxy-anhydride system[END_REF]. In terms of curing heat, at 2°C/min and 5°C/min the heat was in the range of 282-294 J/g whilst at 10°C/min the curing heat was limited to 230 J/g which suggests an under-curing of the resin because of the lower curing time caused by a decrease in the reactivity of the system which can be overcome by the addition of more catalyst if needed. However, a fast curing rate is not wanted when using this kind of polymers because this might suppress water degassing and induce resin foaming, as explained in chapter 4.

Effect of cellulose addition

The latter DSC scans have all been conducted on a sample of PFA polymer with catalyst and eventually added water. However, the developed formulation for 3D printing (Chapter 4) contained from 15 to 30 wt% cellulose powder. Despite that, the analysis of the cellulose powder showed neither transformation nor functionalization which might provide to cellulosic particles a catalytic effect on the polymerization of PFA (Pranger and Tannenbaum, 2008). Therefore, a scan with the addition of 10 wt% cellulose and 5 wt% catalyst with a heating rate of 2°C/min was done to deny or confirm any eventual catalytic effect of the cellulose on the system. The scan (red curve on Figure 5-4) shows a similar thermogram to that without any cellulose and the occurrence of the exothermic peak around the same temperature, indicating that the used cellulose does not interfere with the catalyst activity. However, the total heat of the reaction decreased by 10%. An explanation for this slight decrease of the reaction total heat was suggested by [START_REF] Harsch | Influence of fillers and additives on the cure kinetics of an epoxy/anhydride resin[END_REF]; these fillers adsorb the reacting monomers at the surface which can affect the stoichiometry and thus create sterical hindrance to the forming network.

Furthermore, a second run was performed on the same cured sample with a 5 wt% catalyst and without any cellulose. The thermogram is displayed in Figure 5-4 and shows no heat flow variation, which indicates that complete curing had occurred in the first reaction. According to the previous data 5 wt% of added catalyst seems to be enough to increase the reactivity of the system while avoiding extra water brought by the catalyst solution. The onset reaction temperature is in the range of 90-115°C. It was also found that the addition of inert cellulose particles has no catalytic effect on the curing of the resin and might even act as inhibitors but this effect can be neglected because the onset reaction temperature remains unchanged. The curing kinetics of PFA has been studied in the literature with different techniques such as DSC, DMA, and rheology using different types and concentrations of catalyst and curing methods [START_REF] Domínguez | Study on the non-isothermal curing kinetics of a polyfurfuryl alcohol bioresin by DSC using different amounts of catalyst[END_REF][START_REF] Guigo | Chemorheological analysis and model-free kinetics of acid catalysed furfuryl alcohol polymerization[END_REF]Iroegbu and Hlangothi, 2018;[START_REF] Ooi | DSC studies of the curing mechanisms and kinetics of DGEBA using imidazole curing agents[END_REF][START_REF] Sadler | Kinetics studies and characterization of poly(furfuryl alcohol) for use as bio-based furan novolacs[END_REF][START_REF] Seyedlar | Curing of poly(furfuryl alcohol) resin catalyzed by a homologous series of dicarboxylic acid catalysts: Kinetics and pot life[END_REF]. In this work, our focus is to monitor the evolution of the curing of the commercial oligomers using a fixed amount of latent catalyst (5 wt%) under variable curing temperatures. The degree of crosslinking (α) can be directly correlated with the gel fraction (G%)

which was experimentally measured with the soxhlet extraction method described in chapter 2.

Extraction time

Although soxhlet extraction method of the uncrosslinked fraction of the polymer is a well-known method for a direct and accurate assessment of the crosslinking degree, it can be time-consuming and can resort to the use of hazardous solvents. In our work acetone was used as a proven appropriate solvent for uncured PFA resin [START_REF] Seyedlar | Curing of poly(furfuryl alcohol) resin catalyzed by a homologous series of dicarboxylic acid catalysts: Kinetics and pot life[END_REF]. Furthermore, to optimize the extraction time (the number of cycles) a poorly crosslinked sample was left in the soxhlet from 5 to 16 hours, and the dissolved fraction was carefully determined after every extraction. Furthermore, a visual assessment of the extraction solvent, presented as well in Figure 5-5, is in agreement with what was discussed previously. It is also worth mentioning that the samples were later further extracted for 8 hours in ethanol and no further dissolution was observed.

Crosslinking of oligomers-kinetics

The experimentally measured values of the crosslinking degree (α) of the PFA resin crosslinked at temperatures ranging from 80°C (around the onset temperature of reaction) to 120°C (around the peak temperature) for a time ranging from 0 to 500 min are plotted in Figure 5-6. The curing of resin especially at higher temperatures follows a logarithmic evolution form typical for such crosslinking reactions. The curing process of a thermosetting polymer is commonly described by a single-step kinetic equation (eq 23).

dα dt = k(T)f(α) eq 23
Where α represents the curing degree of the polymer, t is the time, k(T) is the rate constant, which depends on temperature, and f(α) is the process mechanism function. In thermosets curing, we can simply consider n th order equation as a kinetic model which is shown below in eq 24.

dα dt = k(T)(1 -α) n
eq 24

Here (1-α) represents the concentration of reactants which varies from 1 at the start of the reaction to zero at the completion of the reaction. In our case, the reaction of polymerization by condensation of poly(furfuryl alcohol) oligomers follows second order kinetics (n=2) [START_REF] Sadler | Kinetics studies and characterization of poly(furfuryl alcohol) for use as bio-based furan novolacs[END_REF], so eq 24 can be finally written as:

dα dt = k(T)(1 -α) 2
eq 25

To determine the rate constant k(T), dα/dt was plotted in Figure 5-7 versus the square concentration of the remaining reactant (1-α) 2 using the equation eq 25. The constant rate for every temperature can be determined as the slope of the best fit line in Figure 5-7. The constant rate is, as expected, proportional to the curing temperature and increases from 4.5 10 -3 1/min to 1.1 10 -1 1/min when curing at 80°C and 120°C respectively.

It is possible to calculate the activation energy considering that the rate constant (k) follows the Arrhenius law shown in eq 26 where R is the universal gas constant, T is the absolute temperature, Ea is the energy of activation, and A0 is the pre-exponential function.

k(T) = A 0 e ( -E a RT ⁄ )
eq 26

The log of the different rate constant (k) was plotted versus the inverse of the curing temperature T as shown in Figure 5-8. The slope of the fit line was used to determine the energy of activation (Ea)

and the pre-exponential factor A0 by fitting it to equation eq 26. The calculated energy of activation (Ea) for 5 wt% added catalyst is 93.6 kJ/mol which is similar to the 55-80 kJ/mol found in the literature calculated with model-free kinetics model and stronger acid catalysts [START_REF] Domínguez | Study on the non-isothermal curing kinetics of a polyfurfuryl alcohol bioresin by DSC using different amounts of catalyst[END_REF]Lopez de Vergara et al., 2014;[START_REF] Sadler | Kinetics studies and characterization of poly(furfuryl alcohol) for use as bio-based furan novolacs[END_REF].

Water evaporation during curing

Water and volatile evaporation rate during curing is also another important event conditioning the solidification quality of the objects. In Figure 5-10, one can clearly notice the effect of water evaporation kinetics on the porosity and the overall dimensional stability of the cured PFA. When curing at 120°C, both curing and water evaporation rates are high which results in foam formation whilst when curing under 100°C water evaporation seem to be less sudden especially since the water boiling point was not reached. In order to better understand the kinetics of water evaporation through the cured sample, about 5 g of PFA/catalyst were weighed within ±0.0001 g before curing (M0) and after curing (M1) for a certain time, and the mass loss was calculated according to the following equation eq 28.

W% = M 0 -M 1 M 0 • 100 eq 28
To follow the evolution of water evaporation during curing, the total water originally in the system PFA/catalyst should be calculated. This total water in the uncured mixture resin/catalyst can be divided into 3 sources:

i. Physical water brought by the PFA resin can be estimated with equation eq 29 elaborated by (Domínguez and Madsen, 2015).

W PFA (%) = CW PFA • (1 -C catalyst ) eq 29 i.
Physical water brought by the catalyst is alike estimated with the equation eq 30.

W catalyst (%) = CW catalyst • C catalyst eq 30
Where Ccatalyst is the concentration of the catalyst (5 wt%), CWPFA is the concentration of water in the resin (7.5 wt%) and CWcatalyst is the water content in the catalyst (45 wt%). ii. Chemical water from the condensation reaction between two PFA oligomers can be estimated with eq 31.

W cond (%) = 𝑎 • M H 2 O M PFA (1 -W catalyst ) • (1 -W PFA ) eq 31
Where MH2O is the molecular mass of water, MPFA the average molecular mass of PFA oligomer and a is the number of water molecular released for every PFA macromolecule during the condensation reaction.

Accordingly, the physical water yields a value of 9.37 wt%. The chemical water on the other hand cannot be precisely calculated because the exact molecular mass of the oligomer is not exactly known as discussed in chapter 2. However, as the maximum water evaporation experimentally measured was 17.4 wt% the chemical water should be at least 7 wt%. To achieve this value the molecular mass should not exceed an average value of 230 g/mol which is hardly achievable if we consider only the structure suggested in chapter 3 (structure I bellowed) with values of n>2. It is then believed that there is another type of macromolecule as result of homo-condensation of two BHMF (structure II) which allows the release of two water molecules (a=2).

OH-CH2-(FU

-CH2-O-CH2)n -(FU -CH2)m -FU (I) OH-CH2-FU-CH2-O-CH2 -CH2-OH (II)
However, no conclusions can be made as to the average molecular weight of the oligomers, but it is believed that the chemical water released during curing is more than that calculated by (Domínguez and Madsen, 2015) which did not exceed 4 wt%. The mass loss during the condensation measured experimentally exceeded 8 wt% and known that free furfuryl alcohol monomers in the resin are lower than 1 wt%, the possibility of the release of other volatile at this range of temperature can be neglected and hence we can suppose that the total water in the resin/catalyst system to be 17.4

wt%.

The water evaporation extent can be therefore be calculated according to eq 32 and the respective experimental values are plotted in As one can see the evolution of the evaporation extent follows nearly a logarithmic tendency especially when curing at high temperature and the evaporation kinetics can be fitted with a firstorder kinetic law. To verify this, in Figure 5-12-A, the natural log of (1-α') was plotted virus time and the best fit line was plotted. By fitting the Arrhenius law (eq 26), to the equation of the best fit line of ln(K) versus the inverse of temperature the activation energy can be calculated which yields a value of 72 kJ/mol, as expected, is higher than that of evaporation of bulk water (44 kJ/mol) [START_REF] Prado | Activation energies of water vaporization from the bulk and from laponite, montmorillonite, and chitosan powders[END_REF]. Finally, by comparing the first-order kinetic model to the experimentally determined data we can see in Figure 5-13 that water evaporation follows the model when heating at a temperature higher than 90°C but when heating at lower temperatures the model seems to be no longer valid as it is visible in Figure 5-13. This is highlighted by the MAE values calculated and displayed in Table 5-3 where the error of α' reaches 0.125 when crosslinking at 80°C. One can also notice that, when curing at temperature below 100°C the evaporation rate decrease significantly when the curing gel time is reached ( α=0.5-0.7 ) so around 150 min at 80°C and 80 min at 90°C until reaching a plateau when only 60% of water is evaporated. This can be due to water molecules entrapping in the compact polymer structure which hinders the water evaporation. On the other hand, when curing at a higher temperature, the water evaporation continues even after the gel time which can be explained by the gel deformation and foaming induced by the higher vapor pressure which results in a faster and complete vapor release. As was shown in chapter 4, the cellulose addition can help to solve this issue by promoting the water vapor migration through the gelled structure which can help to eliminate the totality of water at a temperature under 100°C and hinder foaming and shape deformations when curing at higher temperatures by decreasing the vapor pressure in the structure.

Survey of rheology behavior during paste curing

To simulate and better understand the viscoelastic behavior during printing of the developed paste PFA-Cell25-B, it was sheared according to the protocol explained in chapter 2 in the linear viscoelastic region at different temperatures while the evolution of storage and loss modulus was tracked. In Figure 5-14, the evolution of G', and G" over 8 hours is plotted under isothermal curing at 90°C. As one can see, when 5 wt% of catalyst was added, the storage modulus start to increase after 70 min which suggests an increase in the stiffness of the paste caused by the beginning of the gelation of the resin. This gel time is in agreement with the curing kinetics shown in Figure 5-9. On the other hand without the addition of catalyst the modulus remains stable for 8 hours with a slight increase after 120 min which can be related to solvent loss. These results prove that the changes in the viscoelastic properties, and more precisely the storage modulus G,' can be directly correlated with the curing of PFA oligomers and can be used to determine experimentally the gel time for the developed paste containing 25 wt% cellulose. The G' and G" evolution over time at temperature ranging from 70°C to 80°C is shown in Figure 5-15.

One can notice that both moduli start to increase when the condensation reaction advances and the onset time of this increase is proportional to the curing temperature. When curing at high temperature, the increase rate of G' is high and quickly reaches a plateau whilst when curing at low temperature this increase is gradual and the plateau was not yet reached when curing at 80°C. These observations are directly related to the curing kinetics of the resin/catalyst discussed previously.

Moreover, as one can see in Figure 5-15, under the different temperatures G' slightly decreases before starting to increase; this drop is believed to be caused by the decrease in the viscosity of the resin when heating before the polymerization takes place. The time around which the modulus starts to increase can be considered as a gelation time (tgel), when the network of polymeric chains densifies and chain ramification decreases their flexibility thus inducing the increase of G'. The influence of the curing temperature on tgel, ie. the onset time of G' exponential increase, is clearly shown in Figure 5-15-B and summarized in Table 5-4. On the same table, the correspondent crosslinking degree (αgel) was calculated based on the second-order kinetic model (eq 25) as well as the theoretical time needed to cure 90% of the resin (t0.9). Therefore, according to the data in Table 5-4, the extent of the reaction should be around 0.68 ±0.012 to reach the gelation state of the paste. Furthermore, the t0.9 correlates well with the evolution of the G' as we can see in Figure 5-15 (dashed lines) after this time the G' starts to reach plateau.

Conclusions

The results of this first section of chapter 5 can allow us to optimize the catalyst type, concentration, and the curing thermal program. The main conclusions are listed below:

• The use of latent catalyst is preferred to expand the pot life of the developed pasty 'ink' and to avoid any eventual unwanted curing in the printing head.

• According to DSC findings, 5 wt% of latent catalyst seem to be enough for effective curing of the resin at intermediate temperature (80°C-120°C) while avoiding increasing the water content in the final paste.

• According to the curing kinetics of the resin at a temperature higher than 100°C, the curing rate is higher but the foaming of the resin takes place. Whilst when curing under lower temperature the foaming can be avoided but the curing kinetics slows down remarkably and the water evaporation cannot be completed.

• The cellulose addition can facilitate the total water evaporation and hinder the dimensional deformation of the eventually solidified composite.

• The gelation of the paste occurs when reaching ca. 68% of resin crosslinking extent which varies from over 2 hours when curing at 80°C to only 6 min when curing at 120°C.

Accordingly, a 3-steps crosslinking strategy to solidify the printed object with good shape fidelity, low porosity, and optimized thermomechanical properties can be proposed;

1. Pre-curing stage to gently evaporate the water from the 'fresh' composite until reaching the gelation. This stage is conducted under 70-90°C; The temperature in the first stage should be as low as possible for a massive compacted structure as the water evaporation can be more difficult. However, when curing a thin-walled structure or with low infill water evaporation is easier and the temperature can be increased to minimize the curing time.

5.3 Part 2 -3D printing process optimization

Introduction

In the first part of the present chapter, some insights were provided concerning the solidification step for the sake of obtaining a defect-free printed object. In the second part, the optimization and the understanding of the printability of the developed paste will be discussed. To do so, a calibration of the extrusion head was first carried out as it is one of the key steps for accurate printing.

Afterwards, the effect of the main printing parameters on the mechanical properties is investigated.

Finally, the printability of different forms is assessed.

Calibration of extrusion flow

As described in the material and method chapter, the paste is provided through an air pressurized cartridge to the inlet cavity of the extruder which permits the adjustment of the feed flow rate. The rotation of the screw conveys the paste at a set rotation velocity until the nozzle. The paste is extruded through the nozzle at a certain extrusion flow, which is a key parameter for accurate printing.

Calculation method of the extrusion multiplier

Screw pump calibration

In the printer firmware, the screw extruder has a nominal calibration factor ecal = 400 µstep/mm. This value is valid for a « virtual » filament with a diameter of 1.75 mm, a stepper motor with 200 steps/round and a stepper driver with 16 µstep/step. In the case of volumetric/screw pumps, the filament diameter must be provided in couple with the calibration factor 'e' just to allow the slicer to correlate the volume flow of extruded material with the pump angular speed.

Calibration of the screw pump (extruder)

The typical flow vs. rotation speed of a screw pump can be obtained by converting the G-Code command Ex (where x is the length of extruded filament) and Fy (where y is the extrusion speed in mm/min) into the corresponding number of revolutions and rotation speed.

An extrusion G-Code command G1 Ex Fy can be therefore converted into the corresponding pump rotation parameters, like the number of revolutions (ER) which can be calculated according to eq 33. Where x is the length of the 1.75 mm "virtual" filament to extrude, ecal is the extruder calibration factor (i.e. 400 µstep/mm) and Nrev is the number of µsteps for a complete revolution (i.e. 3200 for a stepper with 200 steps/rev and a 16-µstep driver).

Similarly, the rotation speed can be calculated according to eq 34 where y is the extrusion speed of the "virtual" filament.

F R = y • e cal N rev
eq 34

Using the machine control panel, or by directly coding an extrusion command (G1 Ex Fy), and eq 33, it is, therefore, possible to fix the number of revolutions and the rotation speed of the screw pump to obtain its characteristic flow curve by weighing the mass of extruded material when varying the rotation speed FR with constant number of revolutions ER and fluid feed pressure (therefore the feed flow rate). The mass flow (M) ̇ can be therefore calculated using eq 35 where M is the weighed mass of extruded material, and ER/FR is the extrusion time.

M ̇= M • F R E R eq 35
Since all calculations performed by the slicer to synchronize the extrusion and tool-head moving speed, in the XY-plane, are based on a volume balance, the mass flow obtained by weighing needs to be converted into the corresponding volume flow (V e ̇) using eq 36, where M ̇ is the mass flow and the material's density.

V e ̇= M ρ eq 36

The objectives of extracting the pump characteristic are: i) to precisely calibrate the extrusion flow in order to avoid any under-extrusion or overextrusion which would generate printing defect and shape fidelity issues.

ii) to identify a linear region where the extrusion flow proportionally varies with the pump rotation speed in order to identify an operational window for the printing speed.

The pump characteristic curve volume flow vs. rotation speed can be correlated to the virtual filament volume flow which can be calculated according to eq 37 where Sf is the section of the "virtual" filament.

V f ̇= S f • y eq 37

By using eq 34, the filament volume flow can be plotted as a function of the stepper motor rotation speed and directly compared to the screw pump characteristic curve (eq 38).

V

f ̇= S f • F R • N rev e cal eq 38
The comparison (ratio) between the slope of the linear region in the flow vs. rotation speed of the screw pump and the linear increase of the "virtual" filament flow given by eq 37, is used to extract a multiplicative factor α in order to superpose the pump flow linear region to the virtual flow. This extrusion multiplier (α) can be obtained using the slope of the parity plot V ̇e vs. V ̇f (plotted for similar rotation speeds) in a linear region (eq 39).

α = 𝑉 ̇𝑒 𝑉 ̇𝑓 eq 39

It is important to mention that the calibration protocol (pump characteristic volume flow vs. rotation speed) is specific for a given material formulation; pump inlet pressure and nozzle type (i.e. diameter and geometry). Variation of one of these parameters requires pump re-calibration.

Chapter 5: Process optimization and study Khaoula BOUZIDI 2022 251 Correlation between the pump flow and the printing speed Printing parameters such as the layer thickness (H), the deposed filament width (w), and the printing speed (z) are used to calculate the material volume flow necessary to print lines with the desired dimensions according to eq 40.

V ̇f = H • w • z eq 40
The slicer then calculates the corresponding "virtual" filament extrusion speed (y) using eq 37 and eq 40 according to eq 41.

y = h • w • z S f eq 41
The rotation speed of the extruder stepper motor to obtain the required flow of "virtual" filament can be therefore calculated using eq 34 and eq 41 thanks to eq 42.

F R = h • w • z S f • e cal N rev eq 42
Whereas, the rotation speed of the screw pump to obtain the required flow of fluid can be calculated using eq 39 and eq 42 with the following equation.

F R = α • h • w • z S f • e cal N rev
eq 43

Influence of the rotation speed

As explained previously, the printing speed window using the screw pump extruder with the optimized ink formulation PFA-Cell28-B has to be determined. To do so the evolution of the experimental extrusion flow (V e ̇) was plotted vs. the rotation speed of the extruder stepper motor (FR) as shown in Figure 5-16. The extrusion flow was measured for rotation speed ranging from 25 to 275 rpm using a nozzle of 0.72 mm and under a feed pressure of 3 bars. The higher limit of FR was constrained by a maximum extrusion speed in the firmware of the used 3D printer. As one can notice

Influence of the feed pressure

The influence of the feed pressure at the outlet of the cartridge on the measured extrusion flow (V e ̇)

through a nozzle with a diameter of 0.72 mm was evaluated for pressure varying from 1.5 to 3 bar.

The minimum pressure corresponds to the minimum pressure allowing the extrusion of the paste through the nozzle and 3 bars were set as a maximum pressure to avoid any leakage in the extruder.

The plot of the values of V e ̇ as a function of the extrusion speed for different feed pressures, is shown in Figure 5-17. As one can notice, for the three applied pressures, (V e ̇) increases linearly with the extrusion speed with no levelling which is thanks to the important supply rate from the outlet of the cartridge. However, the proportionality coefficient between V e ̇ and FR increases by increasing the feed pressure. For instance, this proportionality coefficient triples when doubling the feed pressure to pass from 0.05 under 1.5 bars to 1.5 under 3 bars. This increase in the proportionality can be explained by a lower pressure loss in the nozzle allowing a higher extrusion flow. Therefore, it is more interesting to maximize the feed pressure to widen the printing speed window.

Influence of the nozzle

The extrusion efficiency was evaluated for three different nozzle diameters ranging from 0.72 to 1.2 mm with extrusion velocity (FR) ranging from 25 to 275 rpm (Figure 5-18). As one can notice in Figure 5-18-A, the extrusion flow was larger for larger nozzles and it displayed a linear evolution over all the range of extrusion velocity of the 3 nozzles. Nevertheless, the proportionality coefficient increased when increasing the nozzle diameter. Accordingly, and as explained in section 5. 3.2.1 (p.248), the extrusion multiplier (α) can be obtained using the slope of the parity plot V ̇e vs. V ̇f shown in Figure 5-18-B for the three nozzles. The extrusion multiplier has the value of the slope of the best fit line which is 2.12, 1.71, and 1.35 for nozzles size 0.72, 1, and 1.2 mm, respectively. These values of α can be used in the G-code as an extrusion multiplier which will allow regulating the extrusion flow for different printing speeds and printing parameters for accurate printing. When printing the paste PFA-Cell28-B, with 3 bars pressure, with a layer height of 0.6 mm and road width of 0.72 mm the theoretical printing speed (calculated from the extrusion flow) can reach 100 mm/s. On the other hand, when fixing the printing parameter and increasing the extrusion flow by 10% and 20%, we can notice that the extruded weight per sample increased accordingly by 9% and 21% which proves the accuracy of the calibration of the extrusion head.

Effect of 3D printing parameters on mechanical performances

The influence of the main extrusion settings on the mechanical performances and the morphology of the fractured sample was assessed. To do so, as detailed in chapter 2, different flexural bars were printed by varying the layer height, the road width, the extrusion flow, and the raster angle while keeping the rest of the parameters fixed such as the nozzle size and the printing speed. The layer height, the road width, and the extrusion flow ranged from 0.36 to 0.65 mm, 0.72 to 1 mm, and 100 to 120%, respectively. Meaning that the layer height values vary from 50% to 90% of the nozzle diameter (H%) and the road width values vary from 100% to 150% of the nozzle diameter (W%).

Layer height effect

In the literature, the effect of layer height is not clearly understood and contradictory results can be found of whether the layer height has an effect on mechanical performances and which proportionality there between these values. Many studies were conducted with different printing techniques and parameters and some authors found that an increase in layer height increases the mechanical properties and some others found the contrary. Moreover, it was stated that the layer height can have a different effect on flexural and tensile performances [START_REF] Vaezi | Effects of layer thickness and binder saturation level parameters on 3D printing process[END_REF] and might change by changing the printing parameters such as raster angle, the building orientation or even the flow rate (Chacón et al., 2017). These different results are caused by the absence of a common methodology to assess printed parts strength when modifying printing parameters (Kuznetsov et al., 2019).

In the case of our study, as one can notice in Figure 5-20, the variation of the ultimate bending strength and flexural modulus in the range of layer thickness H% = {50, 65, 80, 90} % were negligible.

Nevertheless, one can notice a slight increase in the flexural performance when decreasing the layer height which might be due to a better binding of layers due to filament squeezing. This slight increase of the flexural strength can also be due to a more oval filament (less circular) as we can see on the SEM images in Figure 5-21 which limits the 'triangular' inter filament porosity. The difference in the porosity cannot be quantified because of a high intrinsic porosity generated by the water evaporation during the curing process and the presence of air bubbles embedded in the fresh paste.

The layer height is directly related to the number of printed layers for a given object and hence to the printing time which should be minimized. Therefore, from a mechanical point of view, the effect of layer height can be neglected and a larger layer can be preferred for the printing of this material.

Moreover, when decreasing the printing layer height the squeezing of the layer against each other can increase the printing force on the already-printed fresh object which might cause the deformation of the object especially when printing objects with a high form factor. However, one should keep in mind that the decrease in layer height is recommended to print objects with overhangs in order to increase the section of the filament supported by the previous layer (Thibaut et al., 2019).

Road width effect

Road width is another parameter that interested many researchers. Similarly to the layer height, the effect of this parameter on mechanical performances of a printed specimen can be different from one study to another and its effect can be related to many other printing parameters such as raster angle and air gap between roads. When printing with thermoplastics by FFF technique, it is usually advised to increase the printing width by at least 10% because of the die-swallow phenomenon of the thermoplastic at the outlet of the nozzle. But this phenomenon is not applied to the extrusion of the developed paste where the extruded filament has the exact diameter of the used nozzle.

However, when increasing the road width, the slicer will automatically increase the extrusion flow rate to fulfill the programmed road dimensions which might decrease the printing time. However, the effect of this parameter on the mechanical performance is unknown when fixing the other printing parameters (H%=65%, raster angle=45°, extrusion flow=100%). In Figure 5-22, one can notice that the Flexural modulus decreases when increasing the road width, whereas the ultimate strength decreases when W% increases to 120% and then increases again when W% reaches 150%. However, this difference is marginal considering the errors of the values.

The eventual increase in the flexural performances of the specimens can be explained by a higher inter-filamentary and intra-filamentary porosity for the samples with high road width compared to the lowest road width, as we can see on SEM images in Figure 5-23. This increase of porosity might be due to a more difficult water evaporation in thicker filaments or also to the fact that the extrusion flow was not enough to reach the programmed road width which is 50% larger than the nozzle size.

Overall, when printing the developed paste is preferred to use a road width equal to the nozzle size.

Extrusion flow effect

The extrusion flow is calculated and regulated by the slicer for a set of given printing parameters (layer height, road width, and printing speed) thanks to the extrusion multiplier α to match with the real flow supplied by the screw extruder. An extrusion flow of 100% corresponds then to an extrusion multiplier equal to α value previously calculated, which is 2.13 for the used printing parameters. The extrusion flow was increased to 110% and 120% (extrusion multiplier, in this case, is respectively 1.1α and 1.2α) in order to assess its impact on the mechanical performances. As shown in Figure 5-24, there are no significant changes in the stiffness of the printed sample when increasing the extrusion flow. However, the ultimate strength slightly increases by 14% when increasing the extrusion flow from 100% to over 110%. This minor increase is believed to be due to the decrease of the inter-filamentary porosity which was filled with the 10% excess of extruded paste. However, when the extrusion reaches 120% we notice signs of over-filling of the sample and a 'swallowing' of the printed bar as one can notice in Figure 5-26. The calibration of the extrusion is then accurate and permits a proper filling of the prints. In order to optimize the mechanical performances of 100% filled prints it is possible to increase the extrusion to 10%, however to maintain the shape fidelity of the object the extrusion flow should not exceed 110%. 260

Raster angle effect

Raster angle is one of the most important parameters of 3D printing which makes the printed object highly anisotropic. Most of the work in the literature agreed that a raster angle of 90° (in the load direction) provided the best mechanical performances as the printed filaments are in the direction of the load. Moreover, when printing composites with high-aspect-ratio fillers, the anisotropy of the material is even magnified. To assess the anisotropy of the printed composites in the present work, the flexural performances of bars printed with a raster angle of 90° and 0° are compared. It is worth highlighting that in bars printed with a raster angle of 0°, the load is perpendicular to the oriented filaments and parallel to the inter-filamentary interface. As one can observe in Figure 5-26, the flexural modulus along the printing direction is slightly higher than the modulus measured on samples with raster printed at 0°, which can probably be due to the effect of the shear-induced cellulose particles or to an imperfect inter-filamentary adhesion in the bars printed at angles 0°.

However, in terms of flexural ultimate strength, no significate difference can be detected between the bars printed in both directions except for a higher dispersion in the values of the strength obtained when the load is in along the printing direction. The ultimate strength in the printing direction could reach 50 MPa with an average of 41 MPa. On SEM images of the fractured cross-section in Figure 5-27, no significant difference can be detected between the two specimens except for a slightly more important pore size which seems to be propagated across the surface which might be a sign of a poor layer adhesion in the core of the specimen. However, when zooming on the edge of both specimens, the inter-layer adhesion seems to be adequate with no signs of delamination. The water evaporation during curing is therefore the main responsible for the cracks embedded between the layers which might be the reason behind the slight decrease in stiffness. The equality in the strength of both types of specimens suggests also that there is no effect of particles alignment along the raster direction. Usually, the alignment of the particles is proportional to the shear rate (ϒ ̇) applied on the extrudate which also depends on the nozzle size (R) and type, and the extrusion rate as suggested by Rabinowitsch-Mooney in equation (eq 44) where Q is the volume flow rate and n is the thinning exponent of the shear-thinning material. A filament was extruded using a large nozzle of 1 mm and a more restrictive nozzle with 0.5 mm diameter with a low and high extrusion rotation speed of respectively 25 and 275 rpm (which is directly correlated with extrusion flow rate). When extruding with the large nozzle, Figure 5-28 shows the presence of pores in the core of the filament, whereas nearly no pores are observed in the first 150-200 µm below the filament surface. This pores distribution was associated to an effective vapor degassing close to the filament surface and vapor segregation in the filament core. Particles seem to be randomly oriented with no clear orientation in the extrusion direction as we can see on the surface of the filament in the zoomed image on the cross-section where one can notice that some of the cellulose fibers are parallel to the surface of the fractured region and some of them are even broken transversally which can explain the isotropy of the resulting material. No difference was noticed when increasing the extrusion flow rate except of a slight decrease in the porosity but this difference is marginal. Whilst for the filament extruded with a 0.5 mm nozzle, the first change one can notice is the decrease in the porosity of the filament, which is due to the decrease in the thickness of the filament which facilitates the water evaporation and degassing during the curing phase. In terms of particles alignment, no effect could be noticed as one can see in Figure 5-29, when further increasing the shear rate, the surface of the filament displays the same roughness with no visible alignment, as confirmed by the zoomed observation on the cross-section. This non-alignment of cellulose particles is not surprising as the aspect ratio of the used particle is limited. However, in the case of the addition of a larger fraction of longer fiber, the anisotropy of the printed object should be taken into consideration.

3D printing quality and limitations

Printing with DIW, as highlighted before, can be very restrictive and limiting as for the printable forms, the infill level, the maximum height, and the resolution. Therefore, the printability of the spanning structure with a low infill was inspected using the optimized paste formulation (PFA-Cell28-B). Moreover, the maximum printing height was also evaluated using the current printing process.

Spanning structure and infill

As already explained in chapter 4, the printability of the spanning structure was assessed (James E. Smay et al., 2002) by determining the minimum G' required for printing a spanning structure with a filament deflection inferior to 0.05D, where D is the filament diameter. Therefore, the normalized maximum span distance(s) can be calculated for a known value of G' by rearranging eq 12 (eq 45) with (s=d/D). For printing the paste with a storage modulus of 0.2 MPa, the density of 1.46 g/cm 3 and nozzle diameter of 0.72 mm, the maximum span distance with a deflection inferior to 5% is ~1.45 mm. Nevertheless, it is possible to print span distance up to 10 mm if deflection is accepted until 25%. As one can see in Figure 5-30-A, the freshly printed filament was only slightly deflected between the edges of the support when reaching a span distance of 10 mm.

s 4 = G′ 1.4 • ρ • g eq 45
Furthermore, to assess the performance of the spanned structure, a two-layer circular grid was printed with different infill levels ranging from 10% to 80% which corresponds to a span distance ranging from <0.1 mm to 6 mm. In line with the theoretically calculated span distance, the bestspanned structure with the best quality was obtained when the span distance was inferior to 2 mm which corresponds in our case to 40% rectilinear infill. Moreover, to investigate the effect of the extrusion flow on the quality of the spanned structure, the extrusion flow was decreased by 10% in order to possibly enhance the spanned structure. As one can see in Figure 5-30-B, this caused either a break of the extruded filament because of the under-extrusion or a very thin deposited filament that does not correspond to the nozzle diameter. Therefore, when printing spanned structure, it is preferred to not exceed a spanning distance with a deflection percentage that does not exceed a maximum of 5% of the nozzle's diameter which can be estimated using eq 45. It is important to highlight that this distance is proper to every printing parameter, and printed paste. By the same token, the printability of these spanning structures was evaluated on 3D objects by stacking multiple layers and assessing the shape fidelity and the overall stability of the printed part. To do so, towers with a base of 20 mm² and height ranging from 25 mm to 60 mm were printed with 40% infill and with no surrounding contour. As one can notice in Figure 5-31 when reaching a height of 25 mm the printed structure was neat and stable with no noticeable defects. When reaching 50 mm the structure was still dimensionally stable and the span structure was also neat as we can see on the top view. But we can notice that the tower is slightly tilted because of the stress exerted on the freshly printed part during the transfert to the oven. When reaching 60 mm, the structure collapsed during the travel as well, nevertheless, the tower was not stable and was continuously moving during the printing because of the force applied by the moving nozzle which created many printing defects. As explained in chapter 4, the maximum height, which can be supported by the fresh paste without collapsing under its weight, can be estimated using eq 46. The maximum height (Hmax) is estimated to be around 55 mm when printing with the optimized ink. These results are in agreement with the observations done previously.

When the form factor of the printed part exceeds 2, destabilization can take place because of the printing forces and also the displacement at the end of the printing process. When curing ex-situ, is preferred to not exceed a form factor of 2 and to let the object dry in the open air at room temperature to increase its stiffness before displacing it. Printing by direct ink writing or also referred as to DIW presents different challenges than printing thermoplastics by fused filament fabrication. These challenges are mostly related to the latent solidification when printing thermosets as in our case. Indeed, deposing an N th layer on top of a soggy N-1 layer can cause many issues and limits the possible printing forms and features. For instance, bridging and overhangs can be particularly challenging with the current rheological performances of the paste which is commonly reported by other authors when printing thermoset-based inks where we can rarely see any printed complex structures. However, the printability of more complex parts as shown in Figure 5-32 was possible and successful such as the ring with a low overhang angle, a thinwalled vase (2 shells) with detailed textures. Nevertheless, several printing defects can be detected.

Indeed, a distinct mark of z-seam when the nozzle starts a new layer can be observed in Figure 5-32 (photo on the left). This defect is not particular to the printed paste and is a classic issue that can be faced when printing thermoplastics as well, but this issue can be widened when the sticky filament sticks to the nozzle tip when changing positions especially at higher acceleration values. This same phenomenon affected also the quality of corners when printing rectangular objects where the corner get round which is a common challenge when printing with the DIW technique (Feilden, 2017a;Thibaut et al., 2019). Furthermore, the air bubbles embedded during the mixing and the loading in the cartridge caused the filament (road) to break as one can notice in Figure 5-32 (photo on right). Interestingly, the delamination issue classically faced when printing with thermoplastic polymers and especially technical filaments such as PEEK, was not noticed when printing with the developed paste.

Indeed the layers are perfectly merged as highlighted before in SEM observations and as we can notice on the printed object. This good adhesion is particularly interesting for the first-layer adhesion on the printing bed and also for good adhesion with an eventual bi-material support printing.

Warping is another issue when printing with thermoplastics with high Tg, which is caused by an uneven crystallization of the different layers caused by a fast solidification of the first layers compared to the subsequent ones. Favorably, printing by DIW technique and curing ex-situ prevents this phenomenon despite an overall shrinkage of about 2% after curing. When curing in-situ, the curing kinetics should be slowed to prevent a fast shrinkage of the printed layers and by the same token prevent bubbles formation, as explained in the first part of this chapter. However, an arching effect can take place when curing ex-situ due to very fast solidification as one can notice in Figure 5-33. Indeed, when curing at a high temperature around 120°C for 2 hours and then at 200°C for 2 hours (cycle I) the fast curing causes warping of the extremities of the bar upwards whilst when curing at 70°C for 8 h then at 100°C for 2 hours to finally post-cure at 200°C for 2 hours (cycle II) no deformation was noticed on the bar which was straight as highlighted in Figure 5-33. This arching is believed to be due to the slower curing of the first layers which were in contact with the printing film support. Cycle II is therefore advised to avoid this distortion.

Towards curing in-situ and more complex objects

Until present, the described 3D printer all along this work is a commercial FFF printer with a modified printing head which composition is detailed in chapter 2. This approach only allows an ex-situ curing of the printed object as the printer is not upgraded with a heating chamber. Moreover, the use of a commercially available high-temperature 3D-printing machine is not possible as there is a risk to solidify the paste in the nozzle. It was decided to develop a customized machine by collaborating with a 3D-printer fabricator Qualup (France) allowing a better printing quality of thermoset-based pastes revealed in Figure 5-34. This 130-kg-machine is made up of:

i. a heated chamber that can reach 270°C isolated with a hood.

ii.

A moving printing head unit.

iii. a case to stock thermoplastic filament feedstock.

iv. an integrated computer serving as a control panel. The machine can be as well controlled with computer thanks to an installed rooter.

v.

A fix printing bed of PEEK fixed with vacuum thanks to the action of an integrated vacuum pump. The printing unit is presented in Figure 5-35, which contains:

i.

Wiping brushes to clean the nozzle before and after every printing and when needed after printing several layers to minimize the printing defects related to the filament drugged with the sticky nozzle.

ii.

A video camera to inspect the printing process, especially of the first layer, since the printing chamber is isolated.

iii.

A double extrusion head containing a FFF extrusion head and LDM extrusion can be switched thanks to a motion-balance system. The LDM extrusion head is similar to that used in this work and detailed in chapter 2 with minor changes in the connector shape between the cartridge and the extruder chamber for space management reasons. During the curing phase, the printing nozzle retracts towards a ventilated upper compartment to stay isolated thanks to a silicon sheath highlighted in Figure 5-34. This 3D printer helps to overcome some of the printing issues discussed before such as structure collapsing during transport when curing ex-situ. Furthermore, the printing of higher objects can be improved by increasing the stiffness of the printed layers by initiating a partial-curing and evaporating the solvent in the fresh paste during printing. After printing a few layers the object can also be cured until its gel time by retracting the nozzle to the cold 'safe' zone to avoid any unwanted curing in the nozzle before continuing the printing. When adapting the second strategy it is important to only prime the curing to enhance the rheological properties of the paste without completing the curing to avoid any uneven shrinkage in the object. Furthermore, to print more complex features such as bridges and overhangs it is possible to use the FFF printing head to print support which will be eliminated at the end of the curing but the support should resist a curing temperature of at least 80°C. The printability of the paste using the developed printer was tested after a recalibration of the extrusion flow because of the changes brought to the extrusion parameters. In Figure 5-36, one can see the results on a printed tube using a 1mm nozzle which demonstrates a good calibration of the extrusion flow and the printing parameters. The curing was done in-situ after the printing step.

Conclusions

In this part of the chapter, the printing process was optimized and closely studied to obtain accurate 3D printing. Successful printing of the object can only be obtained when mastering first the extrusion flow which has to be carefully calibrated and then the printing parameters have to be adapted to the object form, required performances and the used material. Accordingly, the extrusion flow was first calibrated by determining an extrusion multiplier which we called α. This calibration factor changes when changing extrusion parameters such as the nozzle's size and shape, the extrudate rheology, and the extrusion head settings. This calibration factor permitted accurate printing with a good calibration of the extrusion flow when changing printing parameters. By the same token, the effect of printing parameters such as layer height, road width, extrusion flow, and raster angle on mechanical performances was evaluated and we concluded that:

i. A thick printing layer might slightly decrease the composite strength because of an increase in the inter-filamentary porosity.

ii. Wide printing roads might deteriorate the mechanical performance which is believed to be caused by an insufficient extrusion flow.

iii.

Extrusion flow has little effect on mechanical performances but for more compact bulk objects it is possible to increase by 10%. An increase of more than 10% will cause an overfilling effect which should be avoided.

iv.

The raster angle had the most effect on the mechanical performances by enhancing the stiffness when the load was applied along the printing direction. However, the effect remains limited compared to other printed materials which suggest a good inter-filamentary cohesion. Overall, the effect of all printing parameters was little on mechanical performances, thus these parameters should be optimized according to the object morphology and requirements. The average flexural modulus of the tested printed specimen ranged from 3 to 3.5 GPa and its ultimate strength was between 40 and 50 MPa.

Furthermore, the printing of different objects with relatively simple form was possible with some defects caused by the sticky nature of the paste, and the instability of the fresh prints during the curing step. To overcome these challenges, a customized printing machine was designed for ex-situ curing and bi-material printing, allowing thusly the printing of more complex objects.

Chapter 5: Process optimization and study Khaoula BOUZIDI 2022 273

Chapter Conclusions

At the outcome of this last chapter, the printing and curing process of the developed ink formulation are more understood which allowed a better optimization of the printing quality and material performances. First, 5 wt% of latent catalyst was fixed for an adequate crosslinking reaction of the resin and it was found that cellulose has little to no effect on the crosslinking reaction. Based on these findings, an understanding of crosslinking evolution was attempted when adding 5 wt% catalysts under different temperatures. It was found that the curing rate was proportional to the curing temperature. On the other hand, it was demonstrated that curing at high temperatures causes foaming and bubble formation in the material which should be avoided. Hence, curing at lower temperatures is privileged. Accordingly, the following curing program was recommended:

1. Pre-curing stage: evaporate water at a temperature lower than 90°C until reaching gel point;

2. Curing stage: reach high crosslinking degree under 90°-100°C allowing the manipulation of the printed object;

3. Post curing stage: maximize the crosslinking of the polymer under 150°C-200°C.

The pre-curing temperature can be increased for a faster solidification for an object with low infill as they dispose of a thin wall facilitates the water evaporation and hence limiting the foaming risk.

The extrusion step was optimized by calibrating the printing head to match the extrusion flow calculated by the slicer. This calibration was done by defining an extruding multiplier which is specific for every extrudate, nozzle shape, nozzle size, and feeding rate. The optimized extrusion flow guaranteed an accurate road deposit and neat printing quality.

Furthermore, the effect of layer height, road width, raster angle, and extrusion flow on mechanical performances was addressed. It was found that these parameters have no significant effect on the mechanical properties of the tested specimens. However, it is recommended to limit the layer thickness, not exceed the nozzle size for the road's width, and to increase by a maximum of 10% the extrusion flow to guarantee an optimized printing quality and mechanical performance. The interlayer and inter-filamentary adhesion were shown to be optimum which discarded the delamination issues. The printed specimens exhibited a flexural modulus ranging from 3 to 3.5 GPa and ultimate strength between 40 and 50 MPa.

Finally, the printing quality of the developed material was demonstrated to be adequate for the fabrication of simple and relatively more complex objects. Nevertheless, several defects and limitations in the printing of this material were relieved what motivated the idea to develop a customized printer for thermoset-based materials.

he introduction section of this thesis highlighted a research gap in the development of thermoset biocomposites for additive manufacturing. Accordingly, the overarching goal of this thesis was to fill this gap by developing a new biomass-based thermoset composite that can be processed by Direct Ink Writing (DIW).

DIW of thermoset-based composite is a rather new 3D printing approach that can yield new materials with interesting properties and performances permitting to diversify the material panels for additive manufacturing. This approach was and still to this date underexplored. We also demonstrated a more thorough analysis of interests and approaches to printability when it comes to the use of bio-based sustainable thermosets in 3D printing than previous research in the field. What is more, this dissertation focused on understanding the effect of ink's formulation on its rheology and subsequently its printability. Later, the effect of different process parameters (curing and printing parameters) on the printing quality and the material overall performances were studied. The findings of this work can be divided in two major sections whose main conclusions will be sumed-up below.

Overall conclusions

Biocomposite formulation and characterization

At the outset, the characterization of the commercial PFA resin and cellulose powder used in the thesis project was carried out. The characterization revealed that the used cellulosic fillers are microsized rod-shaped chemically inert particles. As for the thermosetting polymer, it was proposed that the oligomer is a sequence of an average of 2 to 5 furan rings connected predominantly by ether bridges and ending with hydroxyl groups.

Once the main characteristics of raw material were known, the next step was the formulation of a printable ink for additive manufacturing which was detailed in Chapter 4. This chapter was divided into two main parts, the first dealing with the development and characterization of structural material and the second addressing the possibility to develop a conductive ink. The optimized structural ink formulation contains 25 to 28 wt% cellulose powder with PFA matrix which exhibited shear-thinning behavior, high yield stress, and thixotropic behavior thus making it compatible with DIW. By the same token, the ensuing cured composite was characterized. The stiffness of the composite increases when increasing cellulose load whilst the strength decreases, which is correlated with the porosity content in the casted specimens and is believed to be the reason behind the deterioration of the strength. Likewise, cellulose addition enhanced the thermomechanical properties with a 60% increase in storage modulus and enhanced stability at 200°C with less than 10% decrease in storage modulus. Cellulose additionally allowed a 40% decrease in the mold T

General conclusions

Khaoula BOUZIDI 2022 281 shrinkage of the thermosetting resin. The physical and mechanical properties of this material are detailed in the datasheet provided below (p.282). As for the second part of the chapter, a formulation containing 4 wt% of short ultiwalled Carbon NanoTubes (CNT) and 15 wt% of cellulose powder was selected for an eventual printing of conductive material with electrical conductivity of at least 2 10 -2 S/m. To further enhance the electrical conductivity, the carbonization of the two developed materials was tested which yielded an electrical conductivity of at least 4.7 10 3 S/m when adding CNT.

Biocomposite 3D printing

The setup and optimization of the printing process was explained in Chapter 5. In the first part of this last chapter, the curing process of the used resin was investigated when adding 5 wt% of a latent catalyst using different analysis techniques such as DSC, DMA, and soxhlet extraction. It was determined that the curing rate was proportional to the curing temperature and so is the water evaporation rate. However, when curing at a high temperature (>100°C) the foaming and porosity risk is high whilst curing at a lower temperature (<100°C) hinders total evaporation of the solvent.

Accordingly, a step-by-step curing process was suggested; starting with a pre-curing stage at a low temperature (<90°C) followed by a curing step at a temperature ranging from 90-100°C and ending with a post-curing stage carried out at a temperature between 150 and 200°C.

The second section of the same chapter was dedicated to a comprehensive study of the printing process of the optimized ink formulation. To do so, the extrusion step was optimized for accurate printing by identifying an extrusion multiplier specific for extrusion parameters (nozzle, material, extruder, feed rate, etc.). Once the extrusion step was successfully completed, the printing parameters' effect on printing quality and mechanical performances was analyzed. It was found that layer height, road width, and extrusion flow have little to no effect on the mechanical performances of the printed specimens. However, the stiffness in the printing direction was slightly higher than that measured perpendicular to the filament direction. Even so, the difference is marginal which showed a good inter-filamentary adhesion of the prints. This adequate adhesion was also visible on different printed objects. Nevertheless, several issues were pointed out in the printability of the developed material such as limited object printing height and aspect ratio because of the forces which may be applied during printing and transport on the fresh print. Furthermore, some of the complex features cannot yet be printed because of the delayed solidification step. To overcome these challenges, a customized 3D printer was developed for an in-situ-curing with the possibility of support printing.

Conductive ink

An increase in the CNT content to further increase the electrical conductivity of the ink can be tested without limiting its printability in restrictive nozzles. The replacement of cellulose fillers with higher carbon char material can be also conceived for a lower shrinkage upon carbonization.

Furthermore, the printability of the developed ink was not fully addressed. Accordingly, in future work, the printability of these inks should be further studied and the printing process should be optimized.

Process

The future work will be more focused on the optimization and the set-up of a reliable and stable printing process on the customized 3D printer.

Short term

A thermal camera will be installed for real-time monitoring of the temperature in the heated chamber, the printed layers, and the nozzle. This information will permit a better optimization of the in-situ curing to first avoid curing in-nozzle, to prime the crosslinking of the first layers while avoiding fast curing to ensure an adequate interlayer adhesion.

Moreover a larger 'ink cartridge' will be installed on top of the printer to enable the printing of bigger parts. A 'purge cartridge' will also be installed for a faster and more efficient extruder cleaning to avoid the paste crosslinking inside the system in the long run. The feeding system can be visualized in

Long term

The manual loading of the cartridge should be optimized to avoid the embedding of air bubbles in the paste.

Printing of more complex parts should be tested with the use of a supporting printing material which can be eliminated after curing. This can be achieved thanks to the already installed FFF printing head.

geometries. Characterization of the carbonized parts should be conducted especially to assess the mechanical performances of the carbonized material. To go further

• Assessment of the composites' fire and chemical resistance can provide a better idea of their possible use and application.

• A deeper study of composite's thermal stability under different types of solicitation and different levels of load.

• Bi-material printing of both the PFA-based conductive ink and the PFA/cellulose paste can be done to yield a functional object.

• One cannot talk about a bio-based renewable material without thinking about its end of life.

It is no secret that the crosslinking of thermosetting polymers is an irreversible reaction limiting its recyclability which was pointed out in the first chapter. The grinding of the composite might be one solution to obtain a powder useful as filler in future formulations.

Another conceivable solution is the synthesis of a furan-based copolymer with the use of bismaleimides to achieve a reversible crosslinking reaction at higher temperatures. The latter solution is currently being investigated in the frame of another Ph.D. project conducted with cooperation between the University of Grenoble and the University of Torino.

General conclusions

Khaoula BOUZIDI 2022 288

Scientific out-puts of the thesis

• Patent: Beneventi, D., Chaussy, D., Bouzidi, K., Gandini, A., 2021. Thermosetting Composite Material and Associated Printing Process.

• Oral presentation in an international conference: Bouzidi, K., Chaussy, D., Gandini, A., Beneventi, D. "3D printable fully biomass-based composite", EPNOE 2021, Nantes,

France.

• Oral presentation in an international conference: Bouzidi, K., Chaussy, D., Gandini, A., Beneventi, D. "3D printing of biocomposite by cold material extrusion", addfabcomp 2021, Online.

• Publication submitted to ASC Applied Polymer Materials: Bouzidi, K., Chaussy, D., • Publication submitted to carbohydrate polymers: Bouzidi, K., Chaussy, D., Gandini, A., Bongiovanni, R., Beneventi, D. "3D printable fully biomass-based composite using poly(furfuryl alcohol) as binder and cellulose as a filler"

• Publication in preparation: Bouzidi, K., Chaussy, D., Gandini, A., Bongiovanni, R., Flahaut, E., Beneventi, D. "Bio-based formulation of an electrically conductive ink with high potential for additive manufacturing by direct ink writing ".

• A Review paper in preparation: Mohammed, Z., Shaqour, B., Bouzidi, K., Barnett, P., Hmeidat, N. "Progress in use of Sustainable Materials for Polymeric 3D Printing:

Challenges and Future Perspectives". Figure 1-11: Printed parts of (A1) 3% alginate and (A2) 2.5% cellulose nanofibers (A3) cellulose nanofibers and alginate. (B) human ear 3D printed with alginate and cellulose [START_REF] Håkansson | Solidification of 3D Printed Nanofibril Hydrogels into Functional 3D Cellulose Structures[END_REF][START_REF] Markstedt | 3D Bioprinting Human Chondrocytes with Nanocellulose-Alginate Bioink for Cartilage Tissue Engineering Applications[END_REF]. Photographs of printed parts: MFC hydrogel after freeze-drying (C1) and after air drying (C2) [START_REF] Håkansson | Solidification of 3D Printed Nanofibril Hydrogels into Functional 3D Cellulose Structures[END_REF] and of high content cellulose paste after air drying (Thibaut et al., 2019) Objective: Evaluation of the possibility to produce a bio-composite using PFA resin along with in-lab produced Hemp MFC.

List of figures

Hemp MFC production

Hemp bleached and pre-refined pulp was used to prepare to produce a highly concentrated MFC (>30 wt%). These micro-fibrillated cellulosic fibers were successfully produced with the use of the low energy consuming method Twin-Screw Extrusion. The followed process of MFC production is

Printable formulation

The produced Hemp MFC were later mixed with the same afore-described PFA resin at different concentrations ranging from 5 to 35 wt% as detailed in Table 0-1. As suspected, the dispersion of these mico/ nanofibers in the viscous oligomer was not efficient using only Ultra-Turrax high-speed mixer. Therefore, a three-roll milling machine was used to break the fibrous agglomerates formed in the paste and thusly enhance the homogeneity of the paste as one can see in Overall, both the produced hemp MFC and the developed ink have a high potential to produce bionano-composite for 3D printing or composite manufacturing in general and need to be more investigated. Toutefois, la fabrication additive de biocomposite n'a pas encore été pleinement explorée, comptant pour moins de 0,5 % des articles publiés.

Introduction générale et contexte

Par conséquent, certains chercheurs se sont concentrés sur le développement et l'étude de nouveaux biocomposites (Kariz et al., 2016;[START_REF] Kucherov | Three-Dimensional Printing with Biomass-Derived PEF for Carbon-Neutral Manufacturing[END_REF]Li et al., 2017;[START_REF] Montalvo Navarrete | Thermal and mechanical behavior of biocomposites using additive manufacturing[END_REF]. Dans le cadre de cette recherche, des thermoplastiques biosourcés ont été couplés à des charges naturelles et à des fibres naturelles (Nguyen et al., 2018;[START_REF] Tao | Development and Application of Wood Flour-Filled Polylactic Acid Composite Filament for 3D Printing[END_REF]. De plus, les polymères issus de la biomasse végétale sont l'une des ressources les plus abondantes dans la nature ce qui les rendent des candidats intéressants pour le développement de nouvelles « encres » d'impression par écriture directe à l'encre (DW) [START_REF] Liu | Current advances and future perspectives of 3D printing natural-derived biopolymers[END_REF]. En particulier, l'utilisation de la cellulose a été étudiée par plusieurs chercheurs pour l'impression de matériaux à haute valeur dans de nombreuses applications biomédicales. Cependant, plusieurs défis ont émergé, comme une fidélité à la forme faible/inexistante et un coût de fabrication élevé [START_REF] Wang | 3D printing with cellulose materials[END_REF]. The developed ink consists of a blend of bio-based furanic resin and cellulosic fillers and eventually carbon nanotubes. The thermosetting resin provides dimensional stability, the cellulose particles enable the tuning of the rheological properties and the carbon nanotubes permit an increase in the electrical conductivity. First, the rheological requirements for a printable ink compatible with the direct ink writing technique, were studied and displayed a shear-thinning behavior and a yield stress superior to 10 5 Pa. Later the ensuing crosslinked composites were extensively characterized highlighting high thermomechanical stability permitting to reach a material processing temperature of up to 200°C. The DIW process and the curing step were thoroughly studied and optimized in order to obtain a better printing quality and excellent interlayer cohesion. Lastly, an in-situ curing strategy was suggested to overcome some of the challenges encountered in the paste DIW. This strategy was implemented on a 3D printer designed as part of this thesis work and equipped with a thermoregulated printing chamber between 30 and 250°C.

Key-words: Additive Manufacturing, Biocomposite, Furan, Cellulose, DIW

  académiques. Merci à Qualup pour leurs aide et intérêt pour le développement de l'imprimante et surtout leur patience malgré les difficultés administratives rencontrées. Merci à Emmanuel Flahaut pour nous avoir fourni les nanotubes de carbone et son expertise. Merci au Cermav pour les belles images au MEB et à tous les autres laboratoires qui m'ont aidé à réaliser des tests dans leurs locaux avec un accueil toujours chaleureux. Je cite donc Isterre, Nanobio, LEPMI et CMTC. Je ne peux pas oublier le personnel scientifique, technique, administratif et informatique de notre cher laboratoire qui m'ont toujours soutenu et aidé. Je voudrais remercier en particulier Chu qui j'ai sollicité pendant toute ma thèse et je l'ai embêté avec mes moules et mes pièces bizarres mais surtout pour m'avoir laissé finir mes expériences après 19h. Je remercie ensuite Cécile, Maxime Mikael et Nadège pour leurs disponibilités. Je pense en particulier à Cécile Sillard qui a toujours étais là pour me former et trouver de l'azote liquide. Merci à Maxime Terrien pour les longues heures passées sur la nouvelle DMA et ta patience. Enfin merci à Denis Curtil pour sa disponibilité malgré ses engagements pour nous avoir aidé avec toutes nos idées parfois un peu exotiques. Je tiens aussi à remercier Giulia Lannello pour son implication et motivation pendant son stage malgré tous les obstacles. Ce fut un plaisir de travailler avec vous tous.

Figure A- 0

 0 Figure A-0-1: The evolution of publications number concerning additive manufacturing, composite in additive manufacturing and biocomposite in additive manufacturing over the 2 last decades found in the database of scifinder. Source: Scifinder, 2021

Figure

  Figure A-0-2, where the different questions answered by the respective chapters are highlighted.

Figure A-0- 2 :

 2 Figure A-0-2: Schematic representation of the manuscript organization.

Figure 1

 1 Figure 1-1: The evolution of publications number concerning composites and green composites over the 5 last decades. Source: Scifinder, 2021

Figure 1 - 3 :

 13 Figure 1-3: Schematic illustration of (a) the supramolecular organization of cellulose (b) the reaction between cellulose and strong acid to obtain cellulose nanocrystals and (c) bacterial cellulose nanofibers.[START_REF] Miyashiro | A Review of Applications Using Mixed Materials of Cellulose, Nanocellulose and Carbon Nanotubes[END_REF].

  To conclude, nowadays, composite materials are widely used in different sectors, owing to the scientific research contribution and development of new efficient materials and manufacturing techniques. The majority of the used materials are petroleum-based which does not align with the recent increasing environmental concerns. Accordingly, many research and development projects

  popularity of 3D printing, this technology has existed for almost four decades during which it has been improved and developed for larger use. The engineer Chuck Hull is the author of the first patent in AM explaining the stereolithography (SLA) technique in 1984, which as a matter of fact, was also the origin of the STL designation of the files used nowadays for the 3D printers. In this US4575330B1 patent, Chuck Hull described how to build an object from an ultraviolet (UV) ray-sensitive material by successive layering and curing[START_REF] Charles | Priority to US06638905[END_REF]. The engineer called the machine a 'Stereolithography Apparatus' and then the very first 3D printer, called SLA-250, was developed in 1988 by the 3D Systems Startup. One more patent has been deposited in 1989 by Scott and Lisa Crump, the creators of Stratasys company, for the development of the Fused Deposition Modeling (FDM) printer. The FDM® printing technique is based on the deposition of layers of a melted thermoplastic. This technique has rapidly gained its place in the market. In 1993, Massachussets Institute of Technology developed a new 3D printer based on a new technique called Three Dimensional Printing (3DP) which enables object building by depositing a thin layer of powder and jetting a few drops of glue according to the object pattern. This technique did not stay in the labscale since enterprise Z Corporation (later redeemed by 3D Systems) has fabricated the first 3DP machines for industry use. Until 2006, Stratasys, 3D Systems, Z Corporation were the leaders of the 3D-printers market. These companies developed and sold many AM machines that made this technology more popular and used especially for rapid prototyping in the industry. In 2007, in Europe, several achievements took place such as the elaboration of the online service for public users to 3D print. A timeline of some key achievements in AM is presented in Figure 1-4.

Figure 1 - 4 :

 14 Figure 1-4: General timeline of important events in AM.

Figure 1 - 5 :

 15 Figure 1-5: Circle chart of the share of the industrial application of AM in 2018[START_REF] Wohlers | Modification of flax fiber surface and its compatibilization in polylactic acid/flax composites[END_REF].

Figure 1

 1 Figure 1-6: Structural limitations of different AM methods(Feilden, 2017).

Figure 1

 1 Figure 1-10: SEM images of the fractures of (a) ABS and (b) ABS and jute fibers (Torrado Perez et al., 2014).

  ink for AM. As we can see in Figure1-12, nanoclay platelets provided the shear thinning, stiffness, and the yield strength required for 3D printing. As for SiC whiskers, they provided additional stiffness and yield strength to the ink and acted as effective structural reinforcement in the final composite.

Figure 1 -

 1 Figure 1-12: Effect of particle addition on resin rheology. (a) Curves of viscosity function of shear rate and (b)Storage and loss moduli function of shear stress[START_REF] Compton | 3D printing of composites with controlled architecture 30[END_REF].

Figure 1

 1 Figure 1-15: General overview of furfural production from biomass feedstock. Adapted from[START_REF] Istasse | Mechanistic aspects of saccharide dehydration to furan derivatives for reaction media design[END_REF].

Figure 1 -

 1 Figure 1-16: Dehydration of xylose to produce furfural.

Figure 1 -

 1 Figure 1-17: Yielding FA and BHMF from Furfural.

Figure 1 -

 1 Figure 1-18: Acid-catalyzed condensation of furfuryl alcohol.

Figure 1 -

 1 Figure 1-19: Crosslinking by Methylene Bridge.

Figure 1 -

 1 Figure 1-20: Furan ring opening.

Figure 1 -

 1 Figure 1-21: Conjugated structure of PFA.

Figure 1 -

 1 Figure 1-23: New Diels-Adler reaction suggested by[START_REF] Tondi | Understanding the Polymerization of Polyfurfuryl Alcohol: Ring Opening and Diels-Alder Reactions[END_REF].

Figure 1 -

 1 Figure 1-25: Images (SEM) of flax fiber non-crimp composite fracture area[START_REF] Pohl | Properties of compression moulded new fully biobased thermoset composites with aligned flax fibre textiles[END_REF]

Figure 1 -

 1 Figure 1-26: Foaming during cure of furan resin samples at [A] 150°C ambient pressure, [B] 140°C RTM closed mold pressure tested to 8 bar and [c] 140°C 6 bar autoclave [D] 60°C at 1°C/min[START_REF] Crossley | The development and processing of a sustainable fully bio derived polyfurfuryl alcohol matrix flax fibre prepreg 8[END_REF].

Figure 1

 1 Figure 1-28: State of the art: key facts and limitations.

•

  Section2.6: A general description of the used characterization methods and techniques. This section is divided into 4 subsections: Description of the methods and apparatus used in the characterization of the raw commercial polymers. Explication of rheological methods used in the characterization of different developed uncured pastes. Description of the methods used in the characterization of the curing process. Explication of the different techniques employed in the characterization of mechanical properties, thermal stability, dimensional stability, morphology, and electrical conductivity of the cured/carbonized final composite.

Chapter 2 :

 2 Materials and methodsKhaoula BOUZIDI 2022 109

Figure 2 - 1 :

 21 Figure 2-1: General layout of the project's approach.

Chapter 2 :

 2 Materials and methodsKhaoula BOUZIDI 2022 111

Figure 2 - 2 :

 22 Figure 2-2: HRTEM image of (A) DWCNT produced in lab-scale[START_REF] Flahaut | Gram-scale CCVD synthesis of double-walled carbon nanotubes[END_REF], (B) commercialDWCNT, and (C) commercial MWCNT short (Nanoamor, 2021).

Figure 2 - 2 -

 22 B and Figure 2-2-C).

Figure 2 - 3 :

 23 Figure 2-3: (A) SEM picture of TIMCAL TIMREX® SLP 30 (Ghimbeu et al., 2013) and (B) TEM picture of a nearly transparent platelet of graphene (STREM, 2021).

Chapter 2 :

 2 Materials and methodsKhaoula BOUZIDI 2022113

Figure 2 - 4 :

 24 Figure 2-4: Photos of the used viscous PFA resin (left) and the catalyst (right).

Chapter 2 :

 2 Materials and methods Khaoula BOUZIDI 2022 114 cartridges are sealed and then placed in a refrigerator at 4-5°C before further processing. An illustration of the different slurry preparation steps is given in Figure 2-5. The mixing time was fixed based on microscopic visualization of the slurry homogeneity. Further mixing can help the vaporization of water however in the present study the mixing was limited to 30 min.

Figure 2 - 5 :

 25 Figure 2-5: Illustration of the paste preparation method.

Chapter 2 :

 2 Materials and methodsKhaoula BOUZIDI 2022 115

Figure 2 -

 2 Figure 2-6: Ternary diagram with weight fraction (wt%) of cellulose, solvent, and PFA resin. The red zone is presenting the preliminary testing area and the blue is the optimization zone formulation. On the top is presenting 100 wt% of PFA on the right 100 wt% cellulose and the left 100 wt% solvent.

Figure 2 -

 2 Figure 2-7: Illustration of CNT/PFA/Cellulose mixing and preparation.

Figure 2 -

 2 Figure 2-8: Ternary diagram with weight fraction (wt%) of conductive fillers (graphene aggregates in red, graphite in blue, and CNT in green), cellulose, and PFA resin.

4

 4 Composite manufacturing-Process step2.4.1 Bulk composite moldingTo study the effect of cellulose addition and the overall composite behavior and characteristics, bulk composites specimens were prepared. First, the cold slurries were placed in an oven at 50°C for 5 min to decrease their viscosity and facilitate their processing. The biocomposites were prepared using a simple hand casting in open poly(tetrafluoroethylene) molds. The molding was done manually and without any additional constraints, yet avoiding creating any voids because of the sticky nature of the slurries. The used open molds are designed for every specific specimen shape.

Chapter 2 :

 2 Materials and methodsKhaoulaBOUZIDI 2022 121 change (faced in the original syringe system). The paste is fed thanks to air pressure applied with a plastic plunger and the adapter (Figure2-9-2).The WASP extruder is composed of an Archimedes screw-driven device, an extruder body tipped with a cylindrical barrel where the screw can rotate thanks to the action of a step motor. At the end of the barrel, there is an interchangeable nozzle with different outlet diameters. The X-ray tomography images of the nozzles revealed an inner shape with two successive constrictions from a diameter of approximately 5 mm to the final outlet diameter of 1.2-0.5 mm (Thibaut, 2020) (Figure2-9-3). According to the supplier's information, this extruder can apply up to 40 bars of pressure to the paste.

Figure 2 -

 2 Figure 2-9: Photo of the modified 3D-printed Artillery sidewinder and the WASP extruder.

Chapter 2 :

 2 Materials and methodsKhaoula BOUZIDI 2022123

Figure 2 -

 2 Figure 2-10: Simulation of printed cube done with Simplify3D.

Chapter 2 :

 2 Materials and methodsKhaoula BOUZIDI 2022124

Figure 2 -

 2 Figure 2-11: 3D sliced model of tensile specimens with different raster angles.

Chapter 2 :

 2 Materials and methodsKhaoula BOUZIDI 2022 127

Figure 2 -

 2 Figure 2-12: 3D model of a sliced cuboid with low infill and important height (on right) and with high infill and low height (on left).

Figure 2 -

 2 Figure 2-13: Carbonization oven Carbolite® Type 3216. The pyrolysis of cured composites was done in a tubular oven (Carbolite®, type 3216) under nitrogen flux (Figure 2-13). The heating program is determined based on the TGA test results. This heating

Chapter 2 :

 2 Materials and methods Khaoula BOUZIDI 2022 131 aluminum crucible and heated from 40°C to 200°C at 5 °C/min under airflow. The enthalpy values were calculated based on the effective PFA weight in the sample without considering the water and catalyst weight ratios.

Figure 2 -

 2 Figure 2-14: Soxhlet extraction experiment steps illustration.

Chapter 2 :

 2 Materials and methodsKhaoula BOUZIDI 2022132

Figure 2 -

 2 Figure 2-15: DMA Metravib50 and paste shearing specimen holder.

( 4 Figure 2 -

 42 Figure 2-16: Density measurement with Buoyancy method. (A) measuring the dry weight (B) measuring the wet weight.

Chapter 3 :

 3 Investigation and characterization of the used commercial raw materials. Chapter 4: Development and characterization of new bio-based materials for 3D printing by cold extrusion.

Chapter 2 :

 2 Materials and methodsKhaoula BOUZIDI 2022136

Figure 3

 3 Figure 3-1: Chapter 2 graphic abstract.

Figure 3 - 2 :

 32 Figure 3-2: Morfi analysis of length distribution of Cell8.

Chapter 3 :

 3 Characterization of commercial polymers Khaoula BOUZIDI 2022 146

Figure 3 - 3 :

 33 Figure 3-3: Granulometer analysis of size distribution of Cell8.

Chapter 3 :

 3 Characterization of commercial polymers Khaoula BOUZIDI 2022 147

Figure 3 - 4 :

 34 Figure 3-4: Microscopic observations of the used celluloses under different magnitudes.

Chapter 3 :

 3 Characterization of commercial polymers Khaoula BOUZIDI 2022 148

Figure 3 - 5 :

 35 Figure 3-5: X-ray diffraction pattern of cellulose Cell8.

Chapter 3 :

 3 Characterization of commercial polymersKhaoula BOUZIDI 2022 1493.2.4 Composition and chemical structureSolid-state 13 C NMR was performed on the Cell8 cellulose powder to study the purity and its chemical structure. The ensuing spectrum is displayed in Figure3-6.First of all, we can notice that the carbon contributions of the cellulose backbone are clearly identified on the spectrum. The intense peak between 100 and 110 ppm is attributed to the carbon C1 (as shown on the cellulose structure), the two peaks between 80-90 ppm come from the amorphous (86-92 ppm) and crystalline (79-86 ppm) C4. The intense peaks around 70-80 ppm aregiven by carbon C2, C3, and C5, and the last peaks around 60 ppm were attributed to the C6[START_REF] Atalla | The Role of Solid State 13C NMR Spectroscopy in Studies of the Nature of Native Celluloses[END_REF].

Figure 3 -

 3 Figure 3-6: Solid-state 13 C NMR spectrum of Cell8.In order to further analyze the chemical composition of the MCC Cell8, it was characterized by FT-IR analysis. The results are shown in Figure3-7. First, a broad band can be observed in the region of 3600-3000 cm -1 , attributed to stretching of the hydroxyl (OH) groups, and 3000-2800 cm -1 , attributed to CH and CH2 stretching. According to the Infrared band assignments for cellulosic materials found in the literature, the bands at 1429, 1369, 1338, and 1319 cm -1 are attributed to CH2, in-plane CH deformation, in-plane OH bending, and CH2 wagging, respectively. In the fingerprint region of the spectrum, the bands at 1160, 1113, 1061, 1033, 896, and 668 cm -1 are associated with the asymmetric C-O-C bridge stretching, the anhydroglucose ring, C-OR stretching, C-O-C pyranose ring skeletal vibration, CH deformation and out-of-plane OH bending, respectively(Poletto et al.,

Chapter 3 :

 3 Characterization of commercial polymers Khaoula BOUZIDI 2022 150

Figure 3 -

 3 Figure 3-7: FT-IR spectra of MCC Cell8.

Chapter 3 :

 3 Characterization of commercial polymers Khaoula BOUZIDI 2022 151

Figure 3

 3 Figure 3-8: Different representations of furanic ring.3.3.1 Polymer structure

Figure 3 -

 3 Figure 3-9: 1 H NMR spectrum of the commercial PFA oligomers in DMSO-d6.

Chapter 3 :

 3 Characterization of commercial polymersKhaoulaBOUZIDI 2022 152 Around 5.3 ppm is the signal of the CH2OH hydroxyl protons which is a much more important peak than that of the free H5 protons. The strong peak at 3.3 ppm is related to the water protons, assigned to the remaining moisture in the commercial product.On the 13 C NMR spectrum of the commercial oligomers, shown in Figure3-10, the signals between 156 ppm and 149 ppm match the C3 and C4 carbons, and those of the C2 and C5 are found between 110 and 113 ppm, which reveal the presence of furan rings. Comparing the methylene bridge and the ether bridge signals, at respectively 27 ppm and 56 ppm, one can see again a stronger presence of the latter. Around 175 ppm, one can also detect a modest signal of carbonyl groups which is due to the furan ring-opening posterior the acid hydrolysis of the furan ring during the oligomerization (Chuang et al., 1984). The NMR spectra are in agreement with a recent investigation of the same commercial resin (Lopez de Vergara et al., 2014).

Figure 3 -

 3 Figure 3-10: 13 C NMR spectrum of the commercial PFA oligomers in DMSO.

Chapter 3 :

 3 Characterization of commercial polymers Khaoula BOUZIDI 2022 153

Figure 3 -

 3 Figure 3-11: FT-IR spectrum of the commercial OFR.

Chapter 3 :

 3 Characterization of commercial polymers Khaoula BOUZIDI 2022 154

Figure 3 -

 3 Figure 3-12: Maldi-TOF spectrum of the PFA oligomers.

Figure 3 -

 3 Figure 3-13: Solid-state RMN of the cured Furolite.

Chapter 3 :

 3 Characterization of commercial polymers Khaoula BOUZIDI 2022 156

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusion Khaoula BOUZIDI 2022 165

Figure 4 -

 4 Figure 4-1: Graphic abstract of chapter 4.

Figure 4

 4

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusionKhaoula BOUZIDI 2022 168

Figure 4 - 2 :

 42 Figure 4-2: Surface fracture phenomenon influenced by the composition[START_REF] Benbow | Paste Flow and Extrusion[END_REF].

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusion Khaoula BOUZIDI 2022 169

Figure 4 - 3 :

 43 Figure 4-3: Ternary diagram indicating the weight fractions of PFA, cellulose fibers, and water for the preliminarily tested formulations.

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusionKhaoula BOUZIDI 2022 171 16 to 29 wt%. No extra solvent was added to the PFA oligomers in order to avoid any defects during crosslinking. As explained in chapter 1, water can generate many defects during the exothermic crosslinking reaction such as high porosity and possible foaming. However, as mentioned in chapter 2, the oligomeric product and the added solution of the catalyst contain 7.5 wt% and 40 wt% water, respectively, which sums up to be upto 7.5 wt% of total free water in these formulations.

Figure 4 - 4 :

 44 Figure 4-4: Ternary diagram of the optimized printable formulation.These 6 tested formulations are placed on the ternary diagram in Figure4-4. In Figure4-5, photographs of the suspensions with varying cellulose content are displayed and a change of the rheological behavior and aspect can be be noticed, ranging from a diluted consistency with a limited bulk stress suspension (Figure4-5-A) to a highly filled paste-like suspension with starting of deagglomeration (Figure4-5-F). This progressive change can be explained by the bridging of viscous PFA oligomers between the cellulosic particles formed by a diffusion process or by the formation of particle network via a contact sticking thanks to the matrix layer around fillers particles.

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusionKhaoula BOUZIDI 2022 172

Figure 4 - 5 :

 45 Figure 4-5: Photographs of different suspensions aspect.

Figure 4 - 6 :

 46 Figure 4-6: Classification of fluids according to their shear stress/shear rate relationship[START_REF] Kubo | Chapter 1 Rheological Properties of Tomato Products 1-25[END_REF].

Figure 4 - 7 :

 47 Figure 4-7: Illustration of different dispersion regimes as a function of particles concentration.The Printability concept

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusion Khaoula BOUZIDI 2022 176

Figure 4 -

 4  shows that the addition of cellulose powder increases very considerably the apparent viscosity of the neat PFA. For instance, the apparent viscosity increases to about 500 Pa•s for 13 vol% cellulose and exceeds 6.5 10 4 Pa•s when filling 24% of the slurry volume with cellulose particles. This exponential viscosity increase can be explained by an increase in the interaction energy between the particles which increases the fluid resistance to flow, hence an increase in the viscosity under the same shear strain.

Figure 4 -

 4 Figure 4-8: Slurry apparent viscosity as a function of cellulose volume fraction.

  shear rate was used to compute the values of n and K by linear regression over the linear portions. The values of n and K of different formulations are given in Figure 4-9-B. One can notice a clear interdependency between the cellulose fraction in the different suspensions and their consistency index K which increases from barely 155 Pa•s n when adding only 16 wt% of cellulose to over 2.5 10 3 Pa•s n with 28 wt% cellulose.

Figure 4 -

 4 Figure 4-9: Rotational rheology of different slurries. (A) Flow curves and (B) values of the consistency index K and thinning exponent n.

Figure 4 -Chapter 4 :

 44 Figure 4-10: Storage and loss moduli vs. shear stress obtained by amplitude sweep at 1 Hz for different suspensions.

Figure 4 -

 4 Figure 4-11: (A) Evolution of the yield stress and stiffness as a function of cellulose weight fraction and (B) formulation selection map showing a minimum merit figure and minimum printable object without collapsing.

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusion Khaoula BOUZIDI 2022 183

Figure 4 -

 4 Figure 4-12: Edge fraction problem at a high shearing rate of highly filled formulations. The results of the thixotropic shear recovery behavior are illustrated in Figure 4-13. All formulations' viscosity decreased significantly at high shear rate and recovered more or less rapidly at low shear rate until reaching stabilized values at the end of the shearing interval. When adding from 16 to 22

Figure 4 - 4 :

 44 Figure 4-13: A 5-step thixotropic behavior study of different suspensions.

Figure 4 -

 4 Figure 4-14: Thixotropic recovery rate as a function of cellulose weight fraction in different slurries.

Figure 4 -

 4 Figure4-14 shows the recovery percentage of the inks within 60 s after the removal of the high shear rate (100 1/s) as a function of cellulose powder content. The recovery percentage is dependent of the cellulose weight fraction, which is inversely proportional. This tendency might be due to a permanent disruption of the more entangled network. The viscosity recovery is between 65 and 100%, which is an acceptable value, but for an optimized shape fidelity, this value should be maximized for a total recovery of the structure during layer deposit.

4. 2 . 4

 24 Effect of cellulose particles incorporation on physical and mechanical properties of bulk composite 4.2.4.1 Water absorption under controlled relative humidity The capacity of water absorption and desorption of the neat cured PFA, cellulose powder, and the composite was assessed with the Varimass device. Mass gain and loss during the different relative humidity levels are gathered in Figure 4-15.

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusionKhaoula BOUZIDI 2022 186

Figure 4 -

 4 Figure 4-15: Water absorption of composites, neat PFA, and cellulose under controlled relative humidity.

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusion Khaoula BOUZIDI 2022 187

Figure 4 -

 4  water absorption and (B) moisture content after aging, of composite with a different polymeric coating (PFA in green)[START_REF] Mokhothu | Bio-based coatings for reducing water sorption in natural fibre reinforced composites[END_REF].

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusion Khaoula BOUZIDI 2022 188

Figure 4 -

 4 Figure 4-17: Photographs of different composites aspect cured overnight at 90°C and 130°C. When curing at 130°C, above the water boiling point, and with a lower gel time, the resin foamed because of the internal pressure caused by the generated gases and blocked by the external rubbery resin layer which is illustrated in Figure 4-18-B. This foaming was mitigated by the addition of cellulose, particularly in the PFA-Cell25-B composite. This effect can be related to the percolation of highly hydrophilic cellulose fibers which helped steam diffusion through the fibrous network without causing any noticeable deformation in the composite showed in Figure 4-17-B and explained in Figure 4-18-B.

Figure 4 -

 4 Figure 4-18: Illustration of the foaming and porosity formation phenomenon under different conditions. (A) Representation of the water evaporation and porosity formation in the mold at 90°C of the neat PFA resin and the cellulose-based slurry. (B) Water evaporation at 130°C through neat PFA with foam formation under the evaporation forces and through cellulose-based slurry. (C) a representation of the different types of forms porosity in molded cured neat PFA and in the biocomposite.

Figure 4 -

 4 Figure 4-19: SEM images of several cured (90°C) PFA surface: (A) surface of the cured specimen of neat PFA,(B) fractured surface of neat PFA, (C) PFA-Cell25-B composite fractured surface, (D) zoom on the pores in PFA-Cell25-B composite.

4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusionKhaoulaBOUZIDI 2022 191

Figure 4 -

 4 Figure 4-20: (A) Porosity percentage in composites with different cellulose contents, cured at 90°C in the oven, (B) Shrinkage percentage of composites with different cellulose contents cured at 90°C in the oven.

4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusionKhaoulaBOUZIDI 2022 193

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusionKhaoulaBOUZIDI 2022 194

Figure 4 -

 4 Figure 4-21: TGA scans collected at 10°C/min showing the degradation of cured neat PFA, cellulose powder, and biocomposites with different cellulose weight content under N2 (continuous curves) and under an oxygen atmosphere (dashed curves).

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusionKhaoulaBOUZIDI 2022 195

Figure 4 -

 4 Figure 4-22: Storage (continuous curves) and loss (dashed curves) moduli of composites with different cellulose weight fraction plotted as a function of temperature.

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusionKhaoulaBOUZIDI 2022 196

Figure 4 -

 4 Figure 4-23: Storage modulus evolution at room temperature and at high temperature.

Figure 4 -

 4 Figure 4-24: Tensile and flexural modulus and strengths of neat cured PFA and different biocomposites.

Figure 4 -

 4 Figure 4-25-A/B. Indeed, the formulation containing cellulose without any added Gr/G (Ref-cell19) has the same flow behavior (dashed red curve) as the other formulations containing the carbonbased particles. The second type of conductive particle widely used in enhancing electrical conductivity are carbon nanotubes (CNT). A suspension of DWCNT-L in water (3 wt%) was tested with

Figure 4 -

 4 Figure 4-25: Flow curve of PFA-based formulations containing (A) 14 to 24 wt% graphene pellets and 1 to 5 wt% graphene pellets with 19 wt% cellulose powder, (B) 1 to 5 wt% graphite powder with 19 wt% cellulose powder, and (C) from 0.55 to 0.85 wt% DWCNT with 11 to 15 wt% cellulose powder and 17 to 27 wt% water.

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusion Khaoula BOUZIDI 2022 201

Figure 4 -

 4 Figure 4-26: (A) Electrical conductivity of extruded filament through different nozzle sizes, (B) printed tracks of Gr-24, and (C) printable tracks of DWCNT-3.

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusion

  5, and 0.2 wt% concentrations were prepared and their flow curves are shown in Figure4-28-A, B, and C, respectively. By analyzing the flow curves, one can first notice a shear-thinning behavior explained by an alignment of the CNT in the shear direction which breaks the network interaction resulting in lower viscosity values. This shear-thinning behavior is proportional to the CNT concentration. Similarly, the viscosity measured at a shear rate of 0.1 1/s, summarized in the graph of Figure4-28-D, increases when increasing CNT concentration with a sharp increase when the concentration exceeds 0.5 wt%, thus suggesting a stronger entanglement. An exception should be made for the MWCNT-1 suspension which exhibits constant viscosity values. This can be explained by the poor dispersion (Figure4-27) and weak network interactions compared wth the other suspensions. Furthermore, as one can notice in Figure4-28-C, for low CNT concentration (0.2 wt%) the flow is unstable with noticeable disruptions, especially for the SWCNT, which might be due to the important aggregates and the non-uniform dispersion shown in Figure4-27. On the other hand, the suspensions that showed fewer aggregates and more uniform dispersion exhibit a more stable flow curve over the whole range of shearing rates. The flow of the suspension gets more stable when increasing the CNT concentration, probably due to a stronger network interaction which diminishes the aggregates effect. The short CNT (MWCNT-short) and Long DWCNT exhibit the highest viscosity values for the concentration higher than 0.5 wt% triggered in the first case by a good dispersion and in the second by the high aspect ratio.

Figure 4 -

 4 Figure 4-27: (A) photograph of CNT dispersions after sonication and mechanical shearing; (B) Optic microscopy images of 0.3 wt% CNT films in diluted PFA resin under the same magnification. Scale bar: 100 µm.

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusion Khaoula BOUZIDI 2022 205

Figure 4 -

 4 Figure 4-28: Viscosity and flow curves for CNT dispersions in ethanol at different concentrations.

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusion Khaoula BOUZIDI 2022 206 For a better assessment of the printability, an amplitude sweep test was carried out on the formulation MCNT-4 presented in Figure 4-29-D, which shows a dominance of the loss modulus over all the range of the shear amplitude declining the printability of this fluid. Therefore, more formulations were prepared with the addition of cellulose powder to tailor the rheology and enhance printability. The flow behavior of suspension containing 0, 15, and 20 wt% of cellulose at a constant CNT content (4 wt%) for both short and long nanotubes was analyzed and shown in Figure 4-29-A/B.

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusionKhaoula BOUZIDI 2022 207

Figure 4 -

 4 Figure 4-29: A), (B), and (C): Flow curves of conductive ink formulationsand (D): amplitude sweep tests.

Figure 4 -

 4 Figure 4-30: Electrical conductivity (EC) of extruded filaments of nanocomposite as a function CNT concentrations and type.

Figure 4 -

 4 Figure 4-31: SEM images at different magnitudes of the cross-section of a cured filament of nanocomposite containing 4 wt% of MWCNT-short (right) and DWCNT (left).

Figure 4 -

 4 Figure 4-33: Illustration of the influence of cellulose powder on the arrangement of CNT in the composite: (left) PFA/CNT composite without inert particles, (right) PFA/CNT/cellulose powder composite. SEM cross-section images of a fractured MCNT-4-15 filament are shown in Figure 4-34. First, one can notice a perfect circular shape of the filament in Figure 4-34-A, suggesting a higher stiffness and better resistance to the gravitational forces compared with the cellulose-free nanocomposites shown

Figure 4 -

 4 Figure 4-34: SEM images of the cross-section of a cured extruded filament of a nanocomposite containing 4 wt% of MWCNT-short and 15 wt% of cellulose powder at three different magnifications.

Chapter 4 :

 4 Development and characterization of new bio-based materials for 3D printing by cold extrusion Khaoula BOUZIDI 2022 212

Figure 4 -

 4 Figure 4-35: (A) Electrical conductivity of the composite MCNT-4-15 before and after pyrolysis and (B) after pyrolysis of composite with and without added CNT.

Figure 4 -

 4 Figure 4-36: SEM images of the cross-section of (A) a cured extruded filament of PFA-Cell25-B composite, (B) a carbonized filament of PFA-Cell25-B composite, and (C) a carbonized filament of nanocomposite containing 4 wt% of MWCNT-short and 15 wt% of cellulose powder at three different magnifications.

Figure 4 -

 4 Figure 4-37: Materials classification based on their electrical conductivity levels and the positioning of biocomposite developed in this work.

Figure 5 -

 5 Figure 5-1: Graphic abstract of chapter 5.

Figure 5 - 2 :

 52 Figure 5-2: Curves of DSC scans of PFA containing different catalyst concentrations at a heating rate of 5°C/min. The given numbers corresponding to different codes of samples detailed inTable 1-5.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 232

Figure 5 - 3 :

 53 Figure 5-3: Curves of DSC scans of PFA containing 5 wt% catalyst at different heating rates. The given numbers corresponding to different codes of samples detailed inTable 1-5.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 233

Figure 5 - 4 :

 54 Figure 5-4: Curves of DSC scans of neat PFA with 5 wt% catalyst with and without cellulose at a heating rate of 2°C/min. The given numbers corresponding to different codes of samples detailed inTable 1-5.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 234 5.2.3 Curing and water evaporation kinetics investigation for a defect-free curing 5.2.3.1 Curing kinetics of the resin -soxhlet extraction

Figure 5 - 5 :

 55 Figure 5-5: Evolution of dissolved oligomer fraction as a function of soxhlet extraction time As one can see in Figure 5-5, after only 5.5 hours of extraction time (corresponding to 24 cycles) 68 wt% of the oligomer has been dissolved, and then after 10 hours the total extracted fraction stabilized at 70 wt%. The extraction time was then fixed at 8 hours (35 cycles) for optimum extraction. Another more crosslinked sample was extracted for 8 hours and then 14 hours which

Figure 5 -

 5 Figure 5-6: The evolution of the experimental degree of crosslinking (α) of PFA resin with 5 wt% catalyst under different temperatures over time.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 236

Figure 5 - 7 :

 57 Figure 5-7: The differential rate law plot for PFA crosslinking under different cure temperatures using 5 wt% catalyst.

Figure 5 -

 5 Figure 5-8: Arrhenius Law plot of experimental results with the best fit line (dashed line).The degree of crosslinking α versus time for both experimental and estimated values calculated with the general integrated kinetic second-order law equation (eq 27) are plotted in Figure5-9. The performance of this general model start to decline at low temperatures as highlighted in Table5-2.Indeed the calculated Mean Absolute Error (MAE) exceed a value of 0.1 for α values predicted when crosslinking at 80°C.

Figure 5 - 9 :

 59 Figure 5-9: Evolution of the degree of crosslinking (α) of PFA resin experimentally measured (points) and estimated with second-order kinetic law (dashed lines) under different temperatures over time.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 239

Figure 5 -

 5 Figure 5-10: Effect of the curing temperature on the dimensional stability and aspect of cured neat resin with 5 wt% catalyst.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 240

Figure 5 -

 5 Figure 5-11 as a function of time for temperature ranging from 80°C to 120°C.

Figure 5 -

 5 Figure 5-11: Water evaporation extent (α') over time under different temperatures.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 242

Figure 5 -

 5 Figure 5-12: Natural logarithmic of the integrated first-order rate law plot for water evaporation during crosslinking (A) and Arrhenius Law plot (B).

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 243

Figure 5 -

 5 Figure 5-13: Water evaporation experimental normalized amount (α') plotted over time under different temperatures and the theoretical values estimated by the first-order kinetic model.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 244

Figure 5 -

 5 Figure 5-14: Evolution of G' and G" during curing of PFA/cellulose paste at 90°C with and without catalyst addition.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 245

Figure 5 -

 5 Figure 5-15: Evolution of G' (A) and G" (B) during curing of PFA/cellulose paste under different temperatures.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 246

2 .

 2 Curing stage to favor the crosslinking of the printed object under a temperature ranging from 90-100°C to avoid any porosity caused by the remaining water; 3. Post curing stage under high temperature 150°C-200°C to guarantee a total crosslinking and densification of the structure as it was demonstrated previously in chapter 4.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 250

Chapter 5 :

 5 Process optimization and studyKhaoula BOUZIDI 2022 252 in Figure5-16, the extrusion flow increases linearly with the extrusion speed until the maximum speed without leveling in this wide range of extrusion speed. This means that the optimized paste can be printed at a large printing speed window under 3 bar pressure which minimizes the pressure loss in the nozzle. However, the evolution of this extrusion flow can vary when changing the feed pressure and the used nozzle which is the main interest of the next two sections.

Figure 5 -

 5 Figure 5-16: Influence of the rotation velocity (FR) on the extrusion flow through a nozzle of 0.72 mm and under a feed pressure of 3 bars.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 253

Figure 5 -

 5 Figure 5-17: Influence of the feed pressure for different rotation speeds on the extrusion flow through a nozzle of 0.72 mm.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 254

Figure 5 -

 5 Figure 5-18: Influence of the nozzle size for different rotation speeds on the extrusion flow under 3 bars feed pressure. (A) A plot of extrusion measured extrusion flow as a function of the rotation velocity (B) Plot of the extrusion flow of a virtual thermoplastic filament calculated by the slicer vs. the experimental extrusion flow 𝑉 ̇𝑒.

  Finally, to assess the calibration of the extrusion flow and the efficiency of the extrusion multiplier α in the regulation of the extrusion flow, samples with different printing parameters were carefully weighed to evaluate the accuracy of the extrusion calibration. As one can see in Figure5-19-A, the overall weight is similar for the different samples (detailed parameters corresponding to the codes are provided in chapter 2 p.124). When changing printing parameters, the slicer automatically manages the layers number and the exact volume of material to be deposited which might explain the slight difference in the weights.

Figure 5 -

 5 Figure 5-19: Weight changes of printed samples when changing (A) printing parameters and (B) extrusion flow.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 256

Figure 5 -Chapter 5 :

 55 Figure 5-20: Effect of layer height on the mechanical performances of printed bars. (A) Evolution of flexural modulus as function of layer height. (B) Evolution of the ultimate flexural strength as function of the layer height.

Figure 5 -

 5 Figure 5-21: SEM images of a fracture surface of bending specimens with a layer height of 0.65 mm (top) and 0.36 mm (below) printed with road width of 0.72 mm and 100% extrusion flow and with 45° infill raster angle.

Figure 5 -

 5 Figure 5-22: Effect of layer height on the mechanical performances of printed bars. (A) Evolution of flexural modulus as a function of layer height. (B) Evolution of the ultimate flexural strength as a function of the layer height.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 259

Figure 5 -

 5 Figure 5-23: SEM images of a fracture surface of bending specimens with a road width of 1 mm (top) and 0.72 mm (below) printed with a layer height of 0.47 mm and 100% extrusion flow and with 45° infill raster angle.

Figure 5 -

 5 Figure 5-24: Effect of the extrusion flow on the mechanical performances of printed bars. (A) Evolution of flexural modulus as function of extrusion flow. (B) Evolution of the ultimate flexural strength as function of the extrusion flow

Figure 5 -

 5 Figure 5-26: Effect of raster angle on the flexural modulus (A) and the ultimate strength (B).

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 262

Figure 5 -

 5 Figure 5-27: SEM images of a fracture surface of bending specimens with a raster angle of 0° (top) and 90° (bottom).

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 263

Figure 5 -

 5 Figure 5-28: SEM images of an extruded filament through 1 mm nozzle with two different levels of extrusion flow rate.

Figure 5 -

 5 Figure 5-29: SEM images of an extruded filament through 0.5 mm nozzle with two different levels of extrusion flow rate.

Figure 5 -

 5 Figure 5-30: Visual assessment of the spanning structure quality (A) printed filament on plastic support with triangular-shaped hollow part (B) circular spanning structure with different infill percentages.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 266

Figure 5 -

 5 Figure 5-31: Printed tower of spanned structure with 40% infill with no supporting shell with height ranging from 25 mm to 60 mm.

Figure 5 -

 5 Figure 5-32: A selection of printed objects with a zoom of broken filament on the right and visible z-seam of the left.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 268

Figure 5 -

 5 Figure 5-33: Arching of a printed thin bar when changing the thermal curing cycle.

Figure 5 -

 5 Figure 5-34: Developed personalized 3D printer for thermoset printing and curing in-situ fabricated by Qualup.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 270

Figure 5 -

 5 Figure 5-35: Composition of the printing unit and detailed composition of the LDM extrusion head.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 271

Figure 5 -

 5 Figure 5-36: Freshly printed tube observed on the control panel screen thanks to the installed camera.

Chapter 5 :

 5 Process optimization and study Khaoula BOUZIDI 2022 272

Figure

  Figure C-0-1.

Figure C- 0

 0 Figure C-0-1: Diagram of the Ink feedstock and purge system on the extruder head of the customized printer.

  Gandini, A., Beneventi, D. "Development of biomass-based slurry for the manufacture of furan/cellulose composite by open air cast molding".

Figure A- 0 - 1 :

 01 Figure A-0-1: The evolution of publications number concerning additive manufacturing, composite in additive manufacturing and biocomposite in additive manufacturing over the 2 last decades found in the database of scifinder. Source: Scifinder, 2021 ................................................................................ 17

Figure A-0- 2 :Figure 1 - 3 :

 213 Figure A-0-2: Schematic representation of the manuscript organization. ............................................ 18 Figure 1-1: The evolution of publications number concerning composites and green composites over the 5 last decades. Source: Scifinder, 2021 ........................................................................................... 28 Figure 1-2: A general classification of lignocellulosic fibers adapted from (Akil et al., 2011)............... 29 Figure 1-3: Schematic illustration of (a) the supramolecular organization of cellulose (b) the reaction between cellulose and strong acid to obtain cellulose nanocrystals and (c) bacterial cellulose nanofibers. (Miyashiro et al., 2020). ..................................................................................................... 31 Figure 1-4: General timeline of important events in AM. ..................................................................... 42 Figure 1-5: Circle chart of the share of the industrial application of AM in 2018 (Wohlers, 2019). ..... 42 Figure 1-6: Structural limitations of different AM methods (Feilden, 2017). ....................................... 47 Figure 1-7: Extruder for cold extrusion (Sun et al., 2018). .................................................................... 49 Figure 1-8: Young's moduli and strengths of continuous carbon-fibers composites (CFRTP) with composites fabricated by FFF, SLS, and SLA (Matsuzaki et al., 2016). .................................................. 52 Figure 1-9: SEM micrographs of carbon fibers in (A) PLA matrix[START_REF] Ferreira | Experimental characterization and micrography of 3D printed PLA and PLA reinforced with short carbon fibers[END_REF] and (B) ABS matrix[START_REF] Tekinalp | Highly oriented carbon fiber-polymer composites via additive manufacturing[END_REF] highlighting fibers pullout and the resulting pores. ............................... 53 Figure 1-10: SEM images of the fractures of (a) ABS and (b) ABS and jute fibers (Torrado Perez et al., 2014). ..................................................................................................................................................... 54

  Figure1-11: Printed parts of (A1) 3% alginate and (A2) 2.5% cellulose nanofibers (A3) cellulose nanofibers and alginate. (B) human ear 3D printed with alginate and cellulose[START_REF] Håkansson | Solidification of 3D Printed Nanofibril Hydrogels into Functional 3D Cellulose Structures[END_REF][START_REF] Markstedt | 3D Bioprinting Human Chondrocytes with Nanocellulose-Alginate Bioink for Cartilage Tissue Engineering Applications[END_REF]. Photographs of printed parts: MFC hydrogel after freeze-drying (C1) and after air drying (C2)[START_REF] Håkansson | Solidification of 3D Printed Nanofibril Hydrogels into Functional 3D Cellulose Structures[END_REF] and of high content cellulose paste after air drying(Thibaut et al., 2019). ............................................................................................................................ 57 Figure 1-12: Effect of particle addition on resin rheology. (a) Curves of viscosity function of shear rate and (b) Storage and loss moduli function of shear stress (Compton, 2015). ........................................ 59 Figure 1-13: Shear-induced alignment of carbon fiber visible in printed rods (Compton, 2015). ........ 59 Figure 1-14: Schematic of the printing process for reactive polymers (Rios et al., 2018). ................... 61 Figure 1-15: General overview of furfural production from biomass feedstock. Adapted from (Istasse and Richel, 2020). .................................................................................................................................. 64

Figure 2 - 2 :Figure 2 - 3 :Figure 2 - 4 :Figure 2 - 5 :Figure 2 - 6 :Figure 2 - 7 :Figure 2 - 8 :Figure 2 - 9 :Figure 2 -Figure 2 -Figure 2 -Figure 2 -Figure 2 -Figure 2 -Figure 2 -

 22232425262728292222222 Figure2-2: HRTEM image of (A) DWCNT produced in lab-scale[START_REF] Flahaut | Gram-scale CCVD synthesis of double-walled carbon nanotubes[END_REF], (B) commercial DWCNT, and (C) commercial MWCNT short (Nanoamor, 2021)......................................................... 111 Figure 2-3: (A) SEM picture of TIMCAL TIMREX® SLP 30 (Ghimbeu et al., 2013) and (B) TEM picture of a nearly transparent platelet of graphene (STREM, 2021). ................................................................ 112 Figure 2-4: Photos of the used viscous PFA resin (left) and the catalyst (right). ................................ 113 Figure 2-5: Illustration of the paste preparation method. .................................................................. 114 Figure 2-6: Ternary diagram with weight fraction (wt%) of cellulose, solvent, and PFA resin. The red zone is presenting the preliminary testing area and the blue is the optimization zone formulation. On the top is presenting 100 wt% of PFA on the right 100 wt% cellulose and the left 100 wt% solvent. 115 Figure 2-7: Illustration of CNT/PFA/Cellulose mixing and preparation. .............................................. 117 Figure 2-8: Ternary diagram with weight fraction (wt%) of conductive fillers (graphene aggregates in red, graphite in blue, and CNT in green), cellulose, and PFA resin. .................................................... 118 Figure 2-9: Photo of the modified 3D-printed Artillery sidewinder and the WASP extruder. ............ 121 Figure 2-10: Simulation of printed cube done with Simplify3D. ......................................................... 123 Figure 2-11: 3D sliced model of tensile specimens with different raster angles. ............................... 126 Figure 2-12: 3D model of a sliced cuboid with low infill and important height (on right) and with high infill and low height (on left). .............................................................................................................. 127 Figure 2-13: Carbonization oven Carbolite® Type 3216. ..................................................................... 127 Figure 2-14: Soxhlet extraction experiment steps illustration. ........................................................... 131 Figure 2-15: DMA Metravib50 and paste shearing specimen holder. ................................................ 132

Figure 3 - 1 :

 31 Figure 3-1: Chapter 2 graphic abstract. ...............................................................................................

Figure 3 - 2 :

 32 Figure 3-2: Morfi analysis of length distribution of Cell8. ...................................................................

Figure 3 - 3 :

 33 Figure 3-3: Granulometer analysis of size distribution of Cell8. .........................................................

Figure 3 - 4 :

 34 Figure 3-4: Microscopic observations of the used celluloses under different magnitudes. ...............

Figure 3 - 5 :

 35 Figure 3-5: X-ray diffraction pattern of cellulose Cell8. ......................................................................

Figure 3 -

 3 Figure 3-6: Solid-state 13 C NMR spectrum of Cell8..............................................................................

Figure 3 -

 3 Figure 3-7: FT-IR spectra of MCC Cell8. ...............................................................................................

Figure 3 - 8 :

 38 Figure 3-8: Different representation of furanic ring. ..........................................................................

Figure 3 -

 3 Figure 3-9: 1 H NMR spectrum of the commercial PFA oligomers in DMSO-d6. ..................................

Figure 3 -

 3 Figure 3-10: 13 C NMR spectrum of the commercial PFA oligomers in DMSO. ....................................

Figure 3 -

 3 Figure 3-11: FT-IR spectrum of the commercial OFR. .........................................................................

Figure 3 -

 3 Figure 3-12: Maldi-TOF spectrum of the PFA oligomers. ....................................................................

Figure 3 -

 3 Figure 3-13: Solid-state RMN of the cured Furolite. ...........................................................................

Figure 4 - 1 :

 41 Figure 4-1: Graphic abstract of chapter 4. ..........................................................................................

Figure 4 - 2 :

 42 Figure 4-2: Surface fracture phenomenon influenced by the composition (Benbow and Bridgwater, 1993). ...................................................................................................................................................

Figure 4 - 3 :

 43 Figure 4-3: Ternary diagram indicating the weight fractions of PFA, cellulose fibers, and water for the preliminarily tested formulations........................................................................................................

Figure 4 - 4 :

 44 Figure 4-4: Ternary diagram of the optimized printable formulation.................................................

Figure 4 - 5 :

 45 Figure 4-5: Photographs of different suspensions aspect. ..................................................................

Figure 4 - 6 :

 46 Figure 4-6: Classification of fluids according to their shear stress/shear rate relationship (Kubo et al., 2019). ...................................................................................................................................................

Figure 4 - 7 :

 47 Figure 4-7: Illustration of different dispersion regimes as a function of particles concentration. .....

Figure 4 - 8 :

 48 Figure 4-8: Slurry apparent viscosity as a function of cellulose volume fraction................................

Figure 4 - 9 :

 49 Figure 4-9: Rotational rheology of different slurries. (A) Flow curves and (B) values of the consistency index K and thinning exponent n. .......................................................................................................

Figure 4 -

 4 Figure 4-10: Storage and loss moduli vs. shear stress obtained by amplitude sweep at 1 Hz for different suspensions. .........................................................................................................................

Figure 4 -

 4 Figure 4-11: (A) Evolution of the yield stress and stiffness as a function of cellulose weight fraction and (B) formulation selection map showing a minimum merit figure and minimum printable object without collapsing. ..............................................................................................................................

Figure 4 -

 4 Figure 4-12: Edge fraction problem at a high shearing rate of highly filled formulations. .................

Figure 4 -

 4 Figure 4-13: A 5-step thixotropic behavior study of different suspensions. .......................................

Figure

  Figure 4-14: Thixotropic recovery rate as a function of cellulose weight fraction in different slurries. ............................................................................................................................................................. 185

Figure 4 -Figure 4 -Figure 4 -Figure 4 -Figure 4 -Figure 4 -Figure 4 -Figure 4 -Figure 4 -Figure 4 -Figure 4 -Figure 4 -Figure 4 -Figure 4 -Figure 4 -Figure 4 -Figure 4 -Figure 4 -Figure 5 - 1 :Figure 5 - 2 :

 4444444444444444445152 Figure 4-20: (A) Porosity percentage in composites with different cellulose contents, cured at 90°C in the oven, (B) Shrinkage percentage of composites with different cellulose contents cured at 90°C in the oven. .............................................................................................................................................. 192 Figure 4-21: TGA scans collected at 10°C/min showing the degradation of cured neat PFA, cellulose powder, and biocomposites with different cellulose weight content under N2 (continuous curves) and under an oxygen atmosphere (dashed curves). .................................................................................. 194 Figure 4-22: Storage (continuous curves) and loss (dashed curves) moduli of composites with different cellulose weight fraction plotted as a function of temperature. ......................................... 195 Figure 4-23: Storage modulus evolution at room temperature and at high temperature. ................ 196 Figure 4-24: Tensile and flexural modulus and strengths of neat cured PFA and different biocomposites. .................................................................................................................................... 197 Figure 4-25: Flow curve of PFA-based formulations containing (A) 14 to 24 wt% graphene pellets and 1 to 5 wt% graphene pellets with 19 wt% cellulose powder, (B) 1 to 5 wt% graphite powder with 19 wt% cellulose powder, and (C) from 0.55 to 0.85 wt% DWCNT with 11 to 15 wt% cellulose powder and 17 to 27 wt% water. ..................................................................................................................... 200 Figure 4-26: (A) Electrical conductivity of extruded filament through different nozzle sizes, (B) printed tracks of Gr-24, and (C) printable tracks of DWCNT-3. ....................................................................... 201 Figure 4-27: (A) photograph of CNT dispersions after sonication and mechanical shearing; (B) Optic microscopy images of 0.3 wt% CNT films in diluted PFA resin under the same magnification. Scale bar: 100 µm. ........................................................................................................................................ 204 Figure 4-28: Viscosity and flow curves for CNT dispersions in ethanol at different concentrations. . 205 Figure 4-29: A), (B), and (C): Flow curves of conductive ink formulationsand (D): amplitude sweep tests. .................................................................................................................................................... 207

Figure 5 - 5 :Figure 5 - 6 :Figure 5 - 7 :Figure 5 - 8 :Figure 5 - 9 :

 5556575859 Figure 5-5: Evolution of dissolved oligomer fraction as a function of soxhlet extraction time .......... 234 Figure 5-6: The evolution of the experimental degree of crosslinking (α) of PFA resin with 5 wt% catalyst under different temperatures over time. .............................................................................. 235 Figure 5-7: The differential rate law plot for PFA crosslinking under different cure temperatures using 5 wt% catalyst. ..................................................................................................................................... 236 Figure 5-8: Arrhenius Law plot of experimental results with the best fit line (dashed line). ............. 237 Figure 5-9: Evolution of the degree of crosslinking (α) of PFA resin experimentally measured (points) and estimated with second-order kinetic law (dashed lines) under different temperatures over time. ............................................................................................................................................................. 238

Figure 5 -Figure 5 - 11 :Figure 5 -Figure 5 - 13 :Figure 5 - 14 :Figure 5 - 15 :Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -Figure 5 -

 55115513514515555555555555555555555 Figure 5-10: Effect of the curing temperature on the dimensional stability and aspect of cured neat resin with 5 wt% catalyst. .................................................................................................................... 239 Figure 5-11: Water evaporation extent (α') over time under different temperatures. ....................... 241 Figure 5-12: Natural logarithmic of the integrated first-order rate law plot for water evaporation during crosslinking (A) and Arrhenius Law plot (B). ............................................................................ 242 Figure 5-13: Water evaporation experimental normalized amount (α') plotted over time under different temperatures and the theoretical values estimated by the first-order kinetic model. ....... 243 Figure 5-14: Evolution of G' and G" during curing of PFA/cellulose paste at 90°C with and without catalyst addition. ................................................................................................................................. 244 Figure 5-15: Evolution of G' (A) and G" (B) during curing of PFA/cellulose paste under different temperatures. ...................................................................................................................................... 245 Figure 5-16: Influence of the rotation velocity (FR) on the extrusion flow through a nozzle of 0.72 mm and under a feed pressure of 3 bars. .................................................................................................. 252 Figure 5-17: Influence of the feed pressure for different rotation speeds on the extrusion flow through a nozzle of 0.72 mm. ............................................................................................................. 253 Figure 5-18: Influence of the nozzle size for different rotation speeds on the extrusion flow under 3 bars feed pressure. (A) A plot of extrusion measured extrusion flow as a function of the rotation velocity (B) Plot of the extrusion flow of a virtual thermoplastic filament calculated by the slicer vs the experimental extrusion flow 𝑉𝑒.................................................................................................... 254 Figure 5-19: Weight changes of printed samples when changing (A) printing parameters and (B) extrusion flow. ..................................................................................................................................... 254 Figure 5-20: Effect of layer height on the mechanical performances of printed bars. (A) Evolution of flexural modulus as function of layer height. (B) Evolution of the ultimate flexural strength as function of the layer height. ................................................................................................................ 256 Figure 5-21: SEM images of a fracture surface of bending specimens with a layer height of 0.65 mm (top) and 0.36 mm (below) printed with road width of 0.72 mm and 100% extrusion flow and with 45° infill raster angle. ................................................................................................................................. 257 Figure 5-22: Effect of layer height on the mechanical performances of printed bars. (A) Evolution of flexural modulus as a function of layer height. (B) Evolution of the ultimate flexural strength as a function of the layer height. ................................................................................................................ 258 Figure 5-23: SEM images of a fracture surface of bending specimens with a road width of 1 mm (top) and 0.72 mm (below) printed with a layer height of 0.47 mm and 100% extrusion flow and with 45° infill raster angle. ................................................................................................................................. 259 Figure 5-24: Effect of the extrusion flow on the mechanical performances of printed bars. (A) Evolution of flexural modulus as function of extrusion flow. (B) Evolution of the ultimate flexural strength as function of the extrusion flow .......................................................................................... 260 Figure 5-25: Photos of printed bending bars with extrusion flow ranging from 100% to 120%. ....... 260 Figure 5-26: Effect of raster angle on the flexural modulus (A) and the ultimate strength (B). ......... 261

  given in Figure A-0-1. The use of hemp as starting pulp is interesting in terms of the possible mechanical properties of the resulting MFC as well as in terms of sustainability and costs of the plant cultivation. Moreover, the high solid content of these fibers enables the possibility of using them as fillers in the PFA.

Figure A- 0

 0 Figure A-0-1: Hemp MFC production process. Later, paper sheets were prepared using the produced Hemp MFC (Figure A-0-2). The mechanical properties of these papers were characterized with a tensile test and yielded Young's modulus of 6.65 ±0.25 GPa and ultimate strength of 15.6 ±2.94 MPa.

Figure A-0- 2 :

 2 Figure A-0-2: Photo of the produced sheets and the conducted mechanical test.

Figure A-0- 3 .

 3 Indeed, the homogeneity of the paste was enhanced which yielded a smoother and homogenous paste visible to the naked eye. It is also worth mentioning that the paste viscosity increased when enhancing the paste homogeneity however this was not quantified.

Figure A-0- 3 :

 3 Figure A-0-3: The aspect of the concentrated paste after a 0 to 3 passes through the three-roll milling machine.

Figure A-0- 5 :

 5 Figure A-0-5: Photos of the extruded paste formulation containing 35 wt% of hemp MFC (on the left) and a printed cube (on the right).

  La fabrication additive, également appelée impression 3D, est une technologie de fabrication révolutionnaire qui suscite un intérêt considérable ces dernières années. Elle est également évolutive, incitant les fabricants à reconceptualiser l'approche employée pour développer, produire et concevoir de nouveaux objets. Par son adaptabilité et la contrôlabilité de ses paramètres, elle permet une multitude d'applications. De plus, à une époque où les préoccupations environnementales augmentent, ce procédé nécessite peu de consommables et offre une géométrie d'objet personnalisable tout en utilisant une seule machine[START_REF] Yang | Cellulose, hemicellulose, lignin, and their derivatives as multi-components of bio-based feedstocks for 3D printing[END_REF].Plus récemment, le recours croissant à l'impression 3D a été favorisé dans une multitude de projets de recherche réalisés ces dernières décennies. Comme on peut le voir sur la figure R-0-1, le nombre de publications sur la fabrication additive s'est accru depuis 2000, atteignant plus de 45 000 publications en 2020. De plus, la plupart des références fournies par la base de données scientifique Scifinder sont des brevets, ce qui met en évidence la capacité d'innovation de ce nouveau procédé de fabrication. La fabrication additive de composites figure parmi les domaines d'innovation car elle permet, entre autres, le développement de matériaux à hautes performances en dehors des matériaux monophasés conventionnels. Sur le même graphique de la figure R-0-1, l'évolution du nombre de publications sur l'impression 3D de composites suit la même tendance que celle de la fabrication additive avec près de 30% du total de ces publications mentionnant des composites.

Figure

  Figure R-0 -1 : L'évolution du nombre de publications concernant la fabrication additive, composite dans la fabrication additive et biocomposite dans la fabrication additive au cours des 2 dernières. Source : Scifinder, 2021.Par conséquent, l'utilisation comme matrice de polymères thermodurcissable avec une stabilité thermique améliorée et un rendement mécanique élevé a récemment été considérée comme une option viable.[START_REF] Peerzada | Additive Manufacturing of Epoxy Resins: Materials, Methods, and Latest Trends[END_REF]. En dépit de la haute performance de ces composites, les matériaux les plus couramment utilisés sont pétro-sourcés et l'utilisation de résine thermodurcissable biosourcée et de charges naturelles demeure assez inexplorée.

Figure R-0- 2 :[Chapitre 4 ]

 24 Figure R-0-2 : Représentation schématique du plan du manuscrit de thèse.Résumé des travaux de thèse

Figure R-0- 3 : 4 : 5 ]

 345 Figure R-0-3: Evolution de la contrainte seuil d'écoulement et du module de stockage en fonction de l'ajout de poudre de cellulose. Les matériaux composites obtenus après réticulation ont été caractérisés pour mieux comprendre leurs propriétés physiques et mécaniques. Les modules de traction et de flexion des composites ont augmenté avec l'accroissement de la teneur en cellulose, pour atteindre respectivement 2,9 GPa et 4,3 GPa. D'autre part, après l'ajout de particules de cellulose, les contraintes à la rupture ont diminué par rapport au PFA pur en raison de la forte porosité causée par l'inclusion d'air lors des étapes de mélange et de moulage des composites. Les résistances à la traction et à la flexion du composite PFA-cell28-B sont respectivement de 40 et 20 MPa. Le retrait est un autre paramètre important pour l'impression 3D, notamment lors de l'utilisation de polymères amorphes. Les expériences réalisées dans ce travail ont montré que le PFA pur a un retrait dimensionnel de 2,9% qui a été réduit de près de 40% pour le PFA-cell28-B (contenant 30% cellulose). En outre, l'ajout des particules cellulosique a amélioré les propriétés thermomécaniques du composite avec un module de stockage supérieur à 5 GPa à température ambiante. A haute température (200°C), le module ne baisse que de 20% et reste stable pendant plusieurs heures comme le montrent les résultats de DMA présentés Figure R-0-4. De plus, la stabilité thermique du composite a été évaluée par des tests TGA, qui ont montré une dégradation thermique limitée, avec une perte en point inférieure à 10% pour des températures inférieures à 300°C.

Figure R- 0

 0 Figure R-0-8: Propriété mécanique en flexion des éprouvettes imprimées ayant différentes orientations de trame.

Table 1

 1

-2 of different possible matrices with a specific strength ranging from 40 to 100 MPa•cm 3 /g against around Chapter 1: State of the art Khaoula BOUZIDI 2022 27

Table 1 -

 1 3: Examples of commercially available thermosetting resins. Adapted from[START_REF] Doroudgarian | High performance bio-based composites : mechanical and environmental durability[END_REF].

	Trade name	Raw material	Manufacturer
	Palapreg® ECO	Unsaturated polyester (55% bio-based)	DSM
	ENVIROLITE	Unsaturated polyester and soy oil (25% bio-based)	Reichhold
	Envirez®	Unsaturated polyester and soybean oil (18% bio-based)	Ashland
	Biorez furfuryl resin	Furfuryl alcohol from biomass	Tranfurans chemicals
	Furolite	Furfuryl alcohol from biomass	Tranfurans chemicals
	LAIT-X/POLLIT	Lactic-acid-based	JVS-Polymers Ltd

Table 1 -

 1

4: Comparison between composite manufacturing techniques.

Table 1 -

 1

6: Summary of the main AM methods

Table 1 -

 1

8: A summary of research work on PFA-based composites and their mechanical and thermal properties.

Table 2 -

 2

	Labeled	Supplier's reference	Internal diameter (nm)	External diameter (nm)	Length (nm)	Aspect ratio L/d	EC (S/cm)
	SWCNT	1283YJ		1-2	5000-30000	2500	>100
	DWCNT	1280YJD	1-3	2-4	50000	1250-2500	>100
	MWCNT 1	1229YJ	5-10	20-30	10000-30000	333-1500	>100
	MWCNT 2	1233YJ	5-15	30-80	<10000	125-333	>100
	MWCNT Gr	1223YJ	5-15	30-80	<10000	125-333	>100
	MWCNT short	1235YJS	3-5	8-15	500-2000	33-250

1: The commercial CNT characteristics. >100

2.2.2 Graphene aggregate and graphite powder

Some other nano and micro conductive particles were tested during the present thesis such as graphene and graphite (Figure

2-3

). Graphene nanoplatelets were supplied in form of powder by Strem Chemicals (MA, USA). These are sub-micron platelet aggregates with a diameter smaller than 2 μm, a thickness of few nanometers and an apparent density ranging from 0.2 to 0.4 g/cm 3 . These Chapter 2: Materials and methods Khaoula BOUZIDI 2022 112

Table 2 -

 2 Thereafter, a new set of formulations without any added water are done and detailed in Table2-4

	Formulation	PFA wt%	Solvent wt%	Cellulose wt%
	IF-01-L	58.42	20.53	21.05
	IF-01	58.42	20.53	21.05
	IF-02	53.37	27.40	19.23
	IF-03	55.11	19.36	25.53
	IF-04	43.82	24.61	31.58
	IF-04-E	43.81	24.61	31.58
	IF-05-E	53.37	27.40	19.23
	IF-06	76.60	23.40	0.00
	IF-07	68.94	21.06	10.00
	After the preliminary tests, more formulations were prepared to optimize and study the effect of
	cellulose addition on the 'ink'. These formulations are in the blue zone in Figure 2-6. First
	formulations set was conducted with PFA resin diluted with 25% water (the dilution limit of PFA)

2: Preliminary testing formulations.

yielding solvent weight fraction in the formulation ranging from 15 to 19% and the cellulose weight fraction was between 20 and 32%. The respective weight fractions of this set of formulations are detailed in

Table 2 -

 2 3: Optimization of paste composition with 25% resin dilution.

		Slurry component by weight	Slurry component by volume	Composite by weight
	Formulation	PFA wt%	Solvent wt%	Cellulose wt%	PFA vol%	Solvent vol%	Cellulose vol%	PFA wt%	Cellulose wt%
	PFA-Cell20-A	61.28	18.72	20.00	62.52 20.44	17.04	74.75	25.25
	PFA-Cell25-A	57.44	17.55	25.01	59.15 19.34	21.51	68.93	31.07
	PFA-Cell28-A	55.16	16.85	28.00	57.11 18.68	24.21	65.56	34.44
	PFA-Cell30-A	53.61	16.38	30.01	55.73 18.22	26.05	63.31	36.69
	PFA-Cell32-A	52.07	15.91	32.02	54.33 17.77	27.90	61.10	38.90

Table 2 - 4 :

 24 Optimization of the formulation without added water.

		Slurry component by weight	Slurry component by volume	Composite by weight
	Formulation	PFA wt%	Solvent wt%	Cellulose wt%	PFA vol%	Solvent vol%	Cellulose vol%	PFA wt%	Cellulose wt%
	PFA-Cell16-B	76.60	7.40	16.00	81.84	4.95	13.20	82.22	17.78
	PFA-Cell19-B	73.65	7.12	19.23	79.23	4.79	15.98	78.72	21.28
	PFA-Cell22-B	70.93	6.85	22.22	76.78	4.64	18.58	75.51	24.49
	PFA-Cell25-B	68.39	6.61	25.00	74.47	4.51	21.03	72.55	27.45
	PFA-Cell28-B	66.03	6.38	27.59	72.30	4.37	23.33	69.81	30.19
	PFA-Cell29-B	65.14	6.29	28.57	71.46	4.32	24.21	68.77	31.23

Table 2 -

 2 5.

	Chapter 2: Materials and methods

Table 2 -

 2 5: Conductive formulations detailed weight ratios and fillers type.

	Filler type	Formulation reference	PFA	Fillers	Cellulose
	Graphene aggregates	Gr-9	91.0	9.0	0.0
	Graphene aggregates	Gr-14	86.0	14.0	0.0
	Graphene aggregates	Gr-24	76.0	24.0	0.0
	Graphene aggregates	Gr-1.3	78.2	1.3	20.5
	Graphite Powder	G-1.3	78.2	1.3	20.5
	Graphene aggregates	Gr-2.5	78.3	2.5	19.1
	Graphite Powder	G-2.5	78.3	2.5	19.1
	Graphene aggregates	Gr-4.7	76.7	4.7	18.6
	Graphite Powder	G-4.7	76.7	4.7	18.6
	DWCNT-L	DWCNT-1	86.10	0.65	13.25
	DWCNT-L	DWCNT-2	81.52	1.18	17.30
	DWCNT-L	DWCNT-3	79.30	1.20	19.50
	DWCNT	DCNT-4-19	76.5	4.0	19.5
	MWCNT-Short	MCNT-4-19	76.5	4.0	19.5
	DWCNT	DCNT-4-15	81	4.0	15.0
	MWCNT-Short	MCNT-4-15	81.0	4.0	15.0
	DWCNT	DCNT-4	96.0	4.0	0.0
	MWCNT-Short	MCNT-4	96.0	4.0	0.0
	DWCNT	DCNT-1.5-19	79.0	1.5	19.5
	MWCNT-Short	MCNT-1.5-19	79.0	1.5	19.5
	DWCNT	DCNT-1.5	98.5	1.5	0

Table 2 -

 2

	image	Part number	Description	Supplier
		A	PLA filament coil
		B	Printer chassis	Artillery
	1	C	Extruder head	Artillery
		D	Platform (printing bed)	Artillery
		E	Manometer
	2	A	Extruder support	3D Printed and designed in LGP2

6: Modified DIW 3D-printer spare parts designation.

Table 2

 2 -7.

	Chapter 2: Materials and methods

Table 2 -

 2 7: Constant parameters for studying the effect of road's parameters.

	Parameter	value
	Printing speed	900 mm/min
	Nozzle diameter	0.72 mm
	Formulation	PFA-Cell28-B
	Raster angle	45°/-45°
	infill	100%
	Shells number	2
	Overlap	0%
	Feeding pressure	3 bar

The studied variable parameters are detailed in Table

2

-8. The layer height (H%) ranged from 50% to 90% and the width (W) was varied from 100% to 120% of the nozzle's diameter. The last parameter to be studied is the extrusion flow (E%) which was varied by increasing the extrusion multiplier by 10 to 30% to have an over-extrusion.

Table 2 -

 2

	Code	H (%)	W (%)	E (%)
	H0.6	90	100	100
	H0.5	80	100	100
	H0.4	65	100	100
	H0.3	50	100	100
	W0.8	65	120	100
	W1	65	150	100
	E1.1	65	100	110
	E1.2	65	100	120
	E1.3	65	100	130
	Effect of raster angle
	The raster orientation is another printing parameter. The effects of 3 raster angles on flexural
	properties were chosen to be studied. At least 10 specimens were printed (3D sliced models are

8: Set of variable rod parameters. presented in Figure 2-11) for each raster angle while maintaining several printing parameters constant (listed in

Table 2-9).

Table 2 -

 2 9: Constant parameters for studying the effect of raster angle.

	Parameter	value
	Printing speed	900 mm/min
	Nozzle diameter	0.72 mm
	Formulation	PFA-Cell28-B
	Shells number	0
	Infill	100%
	Feeding pressure	3 bar

Table 2 -

 2 10, is slow and meticulous to avoid any deformation or break caused by violent volatiles release.

	Chapter 2: Materials and methods

Table 2 -

 2 10: Carbonization heat treatment program.

	T range (°C) Heating rate (°C/min) Dwell time at final temperature (min)
	Tamb-150	5.0	15
	150-300	10.0	30
	300-450	0.1	60
	450-800	0.1	60
	800-950	0.2	15

  Thygesen, A., Madsen, B., Thomsen, A.B., Lilholt, H., 2007. Effect of acidic conditions on interface and strength of cellulose fibres. Interface Des. Polym. Matrix Compos. -Mech. Chem. Model. Manuf. 289-296. Chapter introduction ...................................................................................................... 3.2 Cellulose fibers and powder ........................................................................................... 3.2.1 Size distribution ............................................................................................................ 3.2.2 Morphology .................................................................................................................. 3.2.3 Crystallinity index and DP ............................................................................................. 3.2.4 Composition and chemical structure ........................................................................... 3.3 Furan oligomer: Furolite ................................................................................................. 3.3.1 Polymer structure......................................................................................................... 3.3.1.1 Oligomer .................................................................................................................. 3.3.1.2 Cured polymer ......................................................................................................... 3.3.2 Conclusions .................................................................................................................. 3.4 Chapter conclusions ........................................................................................................ 3.5 References ......................................................................................................................

	Chapter content
	3.1 Khaoula BOUZIDI 2022	144

Table 4 -

 4

1: Example of the assessed preliminary tested formulations

Table 4 -

 4

			Without porosity			With porosity	Porosity origin	Shrinkage
	Reference	Wm (wt%)	Wf (wt%)	Vf (vol%)	Vm (vol%)	Vm eff (vol%)	Vf eff (vol%)	Vp (vol%)	Vp cure (vol%)	Vp mix (vol%)	Shrinkag e exp	Shrinkag e theo
	Neat PFA	100.0	0.0	0.0	100.0	91.2	0.0	8.8	8.8	0.0	2.9	2.9
	PFA-Cell16-B	82.2	17.8	14.9	85.2	67.7	11.8	20.5	7.5	13.0	2.3	1.9
	PFA-Cell19-B	78.7	21.3	17.9	82.1	67.0	14.6	18.4	7.2	11.2	1.3	1.9
	PFA-Cell22-B	75.5	24.5	20.7	79.3	69.6	18.2	12.2	6.9	5.2	1.8	2.0
	PFA-Cell25-B	72.6	27.5	23.4	76.6	68.4	20.9	10.8	6.7	4.0	1.9	1.9

2: Summary of the weight and volume fractions of different components and their respective shrinkage rates.

Table 4 - 3 :

 43 Summary of components mass and weight fractions in the prepared CNT dispersions.

	Reference	CNT (g)	Dispersant (g) (Nanoamor)	Ethanol (g)	Dispersant fraction	CNT (%)
	SWCNT	2.0	10.0	188	05:01	1.0
	DWCNT	2.0	4.0	188	02:01	1.0
	MWCNT 1	2.0	1.5	188	0.75:01	1.0
	MWCNT 2	2.0	1.0	188	0.50:01	1.0
	MWCNT Gr	2.0	1.0	188	0.50:01	1.0
	MWCNT short	2.0	2.0	188	01:01	1.0

As a first assessment, the stability of these dispersions was visually evaluated. As one can see in Figure

4

-27-A, after few hours, the majority of the suspensions are visibly stable with no clear sedimentation with the exception of SWCNT, where one can start noticing signs of sedimentation

Table 5 -

 5

	Sample	Catalyst content (wt%)	Cellulose content (wt%)	Heat ramp (°C/min)	Total water content (wt%)	Onset Temperature (°C)	Peak temperature (°C)	Reaction enthalpy (J/g)
	1	2	0	5	21.75	119	137	262
	2	5	0	5	24.75	109	124	309
	3	5	0	5	9.75	115	128	282
	4	5	0	10	9.75	112	134	231
	5	5	0	2	9.75	91	110	294
	6	5	10	2	9.75	95	113	262
	7	15	0	5	29.25	100	114	292

1: Summary of DSC scans data

Table 5 -

 5

	T (°C)	MAE
	120	0.055
	105	0.053
	90	0.066
	80	0.101

2: MAE values for the predicted crosslinking degrees at different temperatures.

Table 5 - 3

 53

	T (°C)	MAE
	120	0.028
	105	0.046
	90	0.071
	80	0.125

: MAE values for the predicted normalized amount of evaporated water at different temperatures.

Table 5 - 4 :

 54 Summary of different gelation times and correspondent crosslinking degrees when curing under different temperatures

	T (°C)	tgel (min)	αgel	t0.9 (min)
	70	>346	0.664	1600
	80	142.5	0.668	650
	90	72	0.709	280
	105	18	0.674	80
	120	6	0.680	30

  ). .................................................................................................................................. 64 Crosslinking by Methylene Bridge. .................................................................................... 67 Figure 1-20: Furan ring opening. ........................................................................................................... 67 Figure 1-21: Conjugated structure of PFA. ............................................................................................ 68 Figure 1-22: Diels-Alder reaction interchains. adapted from (Falco et al., 2018). ................................ 68 Figure 1-23: New Diels-Adler reaction suggested by (Tondi et al., 2019). ............................................ 69 Figure 1-24: Conceptual model for adsorptive structure evolution in PFA-derived carbon (Mariwala and Foley, 1994). ................................................................................................................................... 70 Figure 1-25: Images (SEM) of flax fiber non-crimp composite fracture area (Pohl et al., 2011). ......... 72 Figure 1-26: Foaming during cure of furan resin samples at [A] 150°C ambient pressure, [B] 140°C RTM closed mold pressure tested to 8 bar and [c] 140°C 6 bar autoclave [D] 60°C at 1°C/min (Crossley et al., 2012). ........................................................................................................................................... 73 Figure 1-27: FE-SEM micrographs of the cross-sections of (A) bulk (B) the film viewed at the direction perpendicular to the blading direction (Lanticse et al., 2006). ............................................................. 75 Figure 1-28: State of the art: key facts and limitations. ........................................................................ 84 Figure 2-1: General layout of the project's approach. ........................................................................ 109

Figure 1-16: Dehydration of xylose to produce furfural. ...................................................................... 64 Figure 1-17: Yielding FA and BHMF from Furfural................................................................................. 65 Figure 1-18: Acid-catalyzed condensation of furfuryl alcohol. ............................................................. 66 Figure 1-19:

  4-14: Thixotropic recovery rate as a function of cellulose weight fraction in different slurries. ............................................................................................................................................................. 185 Figure 4-15: Water absorption of composites, neat PFA, and cellulose under controlled relative humidity. ............................................................................................................................................. 186 Figure 4-16: (A) water absorption and (B) moisture content after aging, of composite with a different polymeric coating (PFA in green) (Mokhothu and John, 2017)........................................................... 187 Figure 4-17: Photographs of different composites aspect cured overnight at 90°C and 130°C. ........ 188 Figure 4-18: Illustration of the foaming and porosity formation phenomenon under different conditions. (A) Representation of the water evaporation and porosity formation in the mold at 90°C of the neat PFA resin and the cellulose-based slurry. (B) Water evaporation at 130°C through neat PFA with foam formation under the evaporation forces and through cellulose-based slurry. (C) a representation of the different types of forms porosity in molded cured neat PFA and in the biocomposite. ...................................................................................................................................... 189 Figure 4-19: SEM images of several cured (90°C) PFA surface: (A) surface of the cured specimen of neat PFA,(B) fractured surface of neat PFA, (C) PFA-Cell25-B composite fractured surface, (D) zoom on the pores in PFA-Cell25-B composite. ............................................................................................ 189

  Table 1-5. 231 Figure 5-3: Curves of DSC scans of PFA containing 5 wt% catalyst at different heating rates. The given numbers corresponding to different codes of samples detailed in Table 1-5. ................................... 232 Figure 5-4: Curves of DSC scans of neat PFA with 5 wt% catalyst with and without cellulose at a heating rate of 2°C/min. The given numbers corresponding to different codes of samples detailed in Table1-5. ............................................................................................................................................. 233

Table 0 -

 0 1: Main composition of the tested paste formulations.

		1	2	3	4	5
	MFC (wt%)	5	10	20	25	35
	PFA (wt%)	95	90	80	75	65

Table 0 -

 0 2: Computed shear-thinning exponent and constancy of the different formulations.

		0% MFC	5% MFC	10% MFC	20% MFC
	K	7,05	21,18	81,61	204,02
	n	0,979	0,694	0,473	0,318
	R 2	0,361	0,966	0,974	0,988

  RésuméAu cours des deux dernières décennies, la fabrication additive est devenue une technologie révolutionnaire. Historiquement, la stéréolithographie est la première technique utilisée en fabrication additive donnant plus tard naissance à de nombreuses nouvelles techniques basées sur différentes approches. L'écriture directe à l'encre est l'une de ces techniques. Elle est utilisée pour imprimer différents matériaux comme la céramique, les pâtes de ciment et, plus récemment, les résines thermodurcissables petro-sourcées. Dans un contexte où le développement durable devient un sujet clé, une nouvelle encre imprimable biosourcée a été développée en utilisant cette nouvelle approche de FA dans cette thèse. L'encre développée est constituée d'un mélange d'une résine furanique biosourcée avec des charges cellulosiques et éventuellement de nanotubes de carbone. La résine thermodurcissable assure la stabilité dimensionnelle, les particules de cellulose permettent la modification des propriétés rhéologiques, et les nanotubes de carbone une augmentation de la conductivité électrique. Au préalable, les propriétés rhéologiques ont été optimisées pour aboutir une encre imprimable, compatible avec la technique d'écriture directe à l'encre, caractérisée par un comportement rheofluidifiant et un seuil d'écoulement élevé supérieure à 10 5 Pa. A posteriori, les composites réticulés résultants ont été largement caractérisés, mettant en évidence une stabilité thermomécanique élevée permettant d'atteindre une température de mise en oeuvre du matériau pouvant atteindre les 200°C. Le procédé DIW et l'étape de réticulation ont été étudiés et optimisés pour une meilleure qualité d'impression avec une excellente cohésion intercouche. Enfin, une stratégie de réticulation in-situ en cours d'impression a été proposée pour surmonter certains des défis rencontrés en cours de l'impression de la pâte. Cette stratégie a été implémentée sur une imprimante 3D conçue dans le cadre de ce travail de thèse et équipée d'une chambre d'impression thermoregulable entre 30 et 250°C.Mots-clés: Fabrication Additive, Biocomposite, Furan, Cellulose, DIWAbstractIn the last two decades, additive manufacturing has emerged as a revolutionary and influential technology. Historically, stereolithography is the first technique used in additive manufacturing giving later birth to many new techniques based on different approaches. Direct ink writing, is one such technique, used to print different materials like ceramics, concrete, and more recently fossilbased thermosetting resins. In the context of the increasing interest in sustainable development, in this thesis, a new bio-based printable ink was developed using the aforementioned AM approach.

Khaoula BOUZIDI 2022

Characterization of commercial polymers

Table 0-2: Computed shear-thinning exponent and constancy of the different formulations. ......... 304 Khaoula BOUZIDI 2022

Suggested curing thermal program

General conclusions

Khaoula BOUZIDI 2022 284

Perspectives and future work

Despite the progress attained in 3 years by transforming brainstormed concepts into a concrete product, many other ideas and approaches have been developed through a better understanding of the process and the material. A perspective of future work is given below.

Raw material

A decision has been made to use readily available commercial products as raw material during this project to focus more on formulation and process development. However, this choice limited the understanding of basic crosslinking mechanisms and the tuning of the product and formulations.

Accordingly, it is recommended to:

• Use water-free PFA oligomers to reduce the issues caused by water evaporation.

• Use different types of catalysts with less water content.

• Use of cellulose fibers with a higher aspect ratio with better reinforcing-ability.

Ink formulation

Structural ink

The choice of a simple 3-component formulation was sought for as a base formulation. Now, one can envision the addition of additives to enhance this printable paste such as:

• Adding nanoparticles of clay or fumed silica to boost the rheological performances and thus the printability.

• Adding carbon nanofibers to increase the mechanical performances of the composite.

• Adding flame retardant agents to complement the low flammability and low smoke release of PFA matrix.

It is also possible to replace the used cellulose powder with cellulose micro/nanofiber which was already tested in the frame of an internship work in this project. The printability and extrudability of the developed paste were limited because of an insufficient dispersion of the nanofibers in the furanic resin. Therefore, in-situ mixing is conceivable in a twin-screw mixer. An overview of these findings is annexed to the present report. Accordingly, the extrudability of the formulation ( 5) was evaluated through a 1mm nozzle which was possible as one can see in Power-law fitting of viscosity curves

List of tables