Amélie Tribot
email: amelie.tribot@uca.fr

Dan Batalu
email: dan.batalu@upb.ro

Clément Brasselet
email: clement.brasselet@uca.fr

Cédric Delattre
email: cedric.delattre@uca.fr

Lu Wei
email: weilu@tongji.edu.cn

Jonathan Lao
email: jonathan.lao@clermont.in2p3.fr

Petre Badica
email: badica2003@yahoo.com

Hélène Philippe Michaud
email: philippe.michaud@uca.fr

Hélène De Baynast
email: helene.de_baynast@uca.fr

Green polymers filaments for 3D-printing

Keywords: Biopolymer, green polymer, 3D-printing, Fused deposition modeling 1

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Synthetic polymers have become indispensable to our daily life. They are at the basis of a large range of modern applications including materials, packaging consumer's products, food and other. Their commercial and technical success emerged during the second part of the 20 th century. It was based on their low costs but above all on their resistance to chemical, physical and biological degradations. This success is today at the origin of researches for green polymers as an alternative to synthetic ones to satisfy the conditions of biodegradability, low-toxicity (including that of degradation products) and sustainability. Indeed, the resistance of synthetic polymers to degradation and their mass consumption produce more and more resistant wastes becoming less and less acceptable. What is a green polymer? According to the International Union of Pure and Applied Chemistry (https://iupac.org/), green polymers are manufactured by the green chemistry, green chemistry relating to the "design of chemical products and processes that reduce or eliminate the use or generation of hazardous substances for animals, plants and environment". The main characteristic of green polymers is their biodegradability which can be defined as a degradation occurring through the action of enzymes and /or biochemical compounds produced by living microorganisms. It is often associated with abiotic degradation due to natural environment (oxidation or photodegradation for examples). This biodegradation leads to the formation of H 2 O, CO 2 , CH 4 and low molecular weight products. Natural polymers or biopolymers such as nucleic acids, proteins or polysaccharides are usually green. So in this chapter natural polymers will be design as green polymers. Biopolymers are abundant in nature and diverse in respect of their structures. They exhibit a wide range of properties depending on their biological functions. At this time, biochemists divide biopolymers into 8 major classes: polyphenols such as humic acids or lignins, polyisoprenoids, inorganic polyesters (polyphosphate), polythioesters, cutin and polymalic acid, polyoxoesters, polysaccharides, polyamides and nucleic acids. They are abundant, sustainable and eco-friendly polymers. They have chemical groups, compatible with chemical modifications. However, these biopolymers have low printing performances and this issue has to be overcome for production of 3D object at large scale. 3D printing also known as additive manufacturing has drastically developed in recent years. Additive manufacturing can produce everything such as food, clothes, bikes, cars, whashing machines or houses. The process includes constructing a 3D object through a successive layer-by-layer depositions of materials under a computer control. The two early 3D-printing methods have been invented by Kodama of the Nagoya Municipal Industrial Research Institute, to manufacture 3D plastic materials in 1981 using a photosensitive polymer [START_REF] Kodama | Automatic method for fabricating a three-dimensional plastic model with photo-hardening polymer[END_REF]. The term 3D-printing designs polymer technologies whereas that of additive manufacturing was used in metal working. The materials used are various: plastic, metal, ceramic, living cells or food. There exist few 3D printing methods: Fused Deposition Modeling (FDM), direct ink printing, continuous or drop-on-demand inkjet printing, Electron Beam Direct manufacturing, Stereolithography (SLA), Selective Laser Sintering (SLS), depending of the material and the application researched [START_REF] Liu | Current advances and future perspectives of 3D printing natural derived biopolymers[END_REF]. This chapter will be restricted to the Fused Deposition Modeling technology. It will be treated only of the manufacturing and the implementation of biodegradable green polymers through their printability. It will be focused on their thermal, rheological and mechanical properties.

3D printing technology

2.1 Short history of 3D printing technology 3D printing technology appeared at the right moment, because of the inspiration of the inventor and the development of the computer assisted design and manufacturing (CAD and CAM, respectively). It was probably the next natural step to integrate previous technologies into one programmable machine, that can fabricate 3D objects with high reproducibility and accuracy. For a better understanding of this "natural" step forward we present the following short timeline of the inventions that led to Fused Deposition Modelling (FDM) technology.

In 1936 was published the patent entitled "Soldering iron" by Henry Joseph Leitsch, describing an electric device used for soldering [START_REF] Leitsch | Soldering iron[END_REF]. The principle of feeding the soldering alloy through a tube with a hot area inspired the invention of the hot glue gun. In 1938, a similar device, but using wax for sealing was patented by John [START_REF] Pesark | Device for applying sealing wax[END_REF]. The patent was called "Device for applying sealing wax". In 1951, the patent "Plastic welding device" of Richard Smurthwaite [START_REF] Arkless | Plastic welding device[END_REF], introduced the idea of using thermo-plastic materials for welding.

In 1965 was issued the patent "Portable thermoplastic cement dispenser" , a precursor of the current (hot) glue gun, where we can find very similar elements with the extruder head of a 3D printer, and of the later 3D pen [START_REF] Paulsen | Portable thermoplastic cement dispensers[END_REF]. It was designed to use thermoplastic filaments. In 1986 was introduced the 3D printing concept by stereolithography, through the patent "Apparatus for production of three-dimensional objects by stereolithography" [START_REF] Hull | Apparatus for production of three-dimensional objects by stereolithography[END_REF]. The patent claimed obtaining 3D objects from liquid resins by solidifying selected areas, layer by layer, using various sources depending on materials (X-ray, ultraviolet light, an electron beam, laser, a jet of reactive chemical, etc). However, it is worthy to mention here the scientific paper of Hideo Kodama, published 5 years earlier on a similar topic, where he proposed ultraviolet light guided through an optical fiber for hardening a polymer into a 3D shape [START_REF] Kodama | Automatic method for fabricating a three-dimensional plastic model with photo-hardening polymer[END_REF]. Other even earlier patents, not mentioned here, refer to 3D images using photopolymerization.

In 1987 the STL file format was developed for 3D Systems, and it is still in use as a main format to communicate with 3D printers. In 1992 was published the patent entitled "Apparatus and methods for creating three-dimensional objects", assigned to Stratasys Inc, where the inventor is Steven Scott [START_REF] Crump | Apparatus and method for creating three-dimensional objects[END_REF]. The patent was filed in October 30, 1989, and the story tells that the idea came out when the inventor created a toy for his daughter using a glue gun. Anyhow, as we can see from the evolution of 3D printing, the main merit is introduction in use of thermoplastics for 3D printing. The technology is known today as Fused Filament Fabrication (FFF, name proposed by RepRap community project) or Fused Deposition Modeling (FDM, Stratasys trademark name).

An important change in the status of FDM happened when the patent of the technology expired in 2009 October 30. Since then a real explosion of 3D printer companies and DIY (Do It Yourself) projects occurred, as the electronics and mechanical components (inexpensive, reliable, and easy to use microcontrollers, sensors, motors, and other components) became easily available. Also, the enthusiasm of hobbyists made the FDM 3D printers very accessible, and their improvement is continuing, sometimes surpassing the commercial technology. Some future directions of 3D printing technology are related to:

(i) increase the equipment's accuracy, speed, resolution, temperature and materials range;

(ii) creation of new functional materials for FDM that can provide ready to use and smart objects;

(iii) development of more advanced software for 3D printing;

(iv) recycling aspects, making 3D printing a green technology.

Based on classification proposed in ISO/ASTM 52900:2015, new directions can be envisioned for improving or developing new ceramics, metals, polymers, and composites (ISO/ASTM 52900, 2015, p. 1-9.). Currently, new technologies are already on the market or proposed, such as 3D bioprinting, polyjet 3D printing, rapid liquid printing (using gel suspension), two-photon polymerization 3D printing (working at micro/nanoscale). Future development of new materials and technologies will surely go beyond the limits of what we know or imagine today.

2.2

Principle of the fused deposition modeling

Looking at the evolution of FDM, the main idea is to use a programmable machine (additive manufacturing machine (ISO/ASTM 52900, 2015, p. 1-9.) that will deposit, following a certain pattern, a melted polymer on a build platform and will continue adding layers on previous ones. The melted state of the polymer is necessary both for easily changing the filament shape along the path, and for the thermal fusion between polymeric strings. In this way, a 3D object is made of 2D layers bonded together, and 2D layers are made of bonded thin strings.

The dispensing head is an important component of the 3D printer, having the role to melt and deliver the material for the 3D printed object (Fig. 1). The dispensing head ends with a conic-like nozzle that has a small hole in the tip (Fig. 2). a.

b. Fig. 1 Dispensing head of the polymeric strings (a -encapsulated, b -opened). The hole's diameter ranges between 0.1 to 1.4 mm, with an increase step of 0.1 or 0.05 mm. The smaller the size of the hole, the higher the resolution of the 2D layer, but at the expense of increasing printing time. A smaller hole also requires a stronger pulling/pu drive gear of the filament. Most of 3D printers work fine with a 0.4 mm nozzle. Different materials are used for making the nozzles, to provide either affordable price, or higher wear resistance, such as brass, steel, hardened steel, ruby, tungsten c nozzle made of hard materials should be considered when printing with abrasive composite filaments. The nozzle must be heated to melt the filament. A cartridge heater (Fig used to raise the temperature, usually to around 25 materials to be printed, and a thermistor measures the temperature of nozzle (Fig. 3.b An electronic circuit is used for amplifying the signal and transmitting the data (cartridge heater temperature, fan on/off, etc tube, down to the hot nozzle. Depending on 3D printer design, the drive separated by the hot-end or embedded together. The components are assembled on a frame, and cooled by a fan, which are numerous designs, depending on producer. The challenge is to increase the working temperature to 500 o C or even above, for printing with Polyether Ether Ketone (PEEK), Polyetherimide (PEI), or other high

The dispensing head is fixed on a support which is guided in a limited build space. Depending on mechanism, the header position can be controlled by combining 3 linear movements along X, Y, and Z axis ( and independent movements (delta system,Fig. 4.b). Some proposed (e.g., polar, scara, robotic arm), but they remained part of more conceptual printers. diameter ranges between 0.1 to 1.4 mm, with an increase step of 0.1 or 0.05 mm. The smaller the size of the hole, the higher the resolution of the 2D layer, but at the expense of increasing printing time. A smaller hole also requires a stronger pulling/pu drive gear of the filament. Most of 3D printers work fine with a 0.4 mm nozzle. Different materials are used for making the nozzles, to provide either affordable price, or higher wear resistance, such as brass, steel, hardened steel, ruby, tungsten carbide, and others. The nozzle made of hard materials should be considered when printing with abrasive composite filaments. The nozzle must be heated to melt the filament. A cartridge heater (Fig used to raise the temperature, usually to around 250-270 o C, depending on 3D printer and materials to be printed, and a thermistor measures the temperature of nozzle (Fig. 3.b An electronic circuit is used for amplifying the signal and transmitting the data (cartridge heater temperature, fan on/off, etc). The solid filament is pushed by a drive tube, down to the hot nozzle. Depending on 3D printer design, the drive end or embedded together. The components are assembled on a frame, and cooled by a fan, which can also have the role to cool the melted filament. There are numerous designs, depending on producer. The challenge is to increase the working C or even above, for printing with Polyether Ether Ketone (PEEK), ther high-performance thermoplastics. The dispensing head is fixed on a support which is guided in a limited build space. Depending on mechanism, the header position can be controlled by combining 3 linear movements along X, Y, and Z axis (cartesian system, Fig. 4.a) or combining 3 linear vertical and independent movements (delta system,Fig. 4.b). Some other exotic mechanisms are proposed (e.g., polar, scara, robotic arm), but they remained part of more conceptual a. heater for filament melting (a) and thermistor for temperature measurement (b).

A . Tribot et al. diameter ranges between 0.1 to 1.4 mm, with an increase step of 0.1 or 0.05 mm. The smaller the size of the hole, the higher the resolution of the 2D layer, but at the expense of increasing printing time. A smaller hole also requires a stronger pulling/pushing drive gear of the filament. Most of 3D printers work fine with a 0.4 mm nozzle. Different materials are used for making the nozzles, to provide either affordable price, or higher wear arbide, and others. The nozzle made of hard materials should be considered when printing with abrasive composite filaments. The nozzle must be heated to melt the filament. A cartridge heater (Fig. 3.a) is C, depending on 3D printer and materials to be printed, and a thermistor measures the temperature of nozzle (Fig. 3.b).

An electronic circuit is used for amplifying the signal and transmitting the data (cartridge ). The solid filament is pushed by a drive-gear through a tube, down to the hot nozzle. Depending on 3D printer design, the drive-gear can be end or embedded together. The components are assembled on a can also have the role to cool the melted filament. There are numerous designs, depending on producer. The challenge is to increase the working C or even above, for printing with Polyether Ether Ketone (PEEK),

The dispensing head is fixed on a support which is guided in a limited build space. Depending on mechanism, the header position can be controlled by combining 3 linear , Fig. The z resolution is given by the thickness of each layer, reaching down to 0.01 mm according with producers' specifications. In practice, the layer thickness is recommended to be about 80% of nozzle diameter. For example, for a nozzle of 0.4 mm diameter, the layer thickness should be 0.32 mm, and for a nozzle of 0.1 mm diameter, the layer thickness should be 0.08 mm. A certain resolution of a 3D printed object can be accepted if the size of the printed object is large enough. By FDM technology, very small objects cannot be printed at the highest printer resolution, e.g. a cube of 0.5x0.5x0.5 mm 3 . Also, thin walls of an object will have a thickness equal to multiple of string thickness, otherwise empty space will be left (still the gaps can be filled using certain software options). On the other hand, a compromise between optimal resolution and time of printing is often made.

It is worthy to mention that recent 3D printers with two or more dispensing heads started to become popular. For dual dispensing heads one filament can be used for permanent deposition, and the second one for deposition of soluble supports. In other configurations, bicolor/multicolor objects can be printed. Regarding the physical parameters, for best results the operator should tune the working temperature of the extruder and the feed rate of the filament.

There are two main approaches for obtaining a 3D printed object. First route is to design a 3D virtual object using CAD (Computer Assisted Design) software, such as AutoDesk Inventor, Fusion 360, SolidWorks, Catia, Rhino, FreeCad etc, either commercial or opensource (Fig. 5). where loss of details can be noticed,

Though each software has its own file type/extension, usually the virtual object can be exported as a stl file. STL stands for stereolithography, and the format carry the information about surfaces of a 3D object, each surface being composed of triangles. STL files can be saved/exported as ASCII or binary representations, the latter being more compact (much smaller size). STL files only represent the shape of t textures. The second route of obtaining STL files is by 3D scanning a real objec its shape into a digital one (Fig.

Specialized software is used to analyze and enhance the STL file (Fig. 7), Meshmixer, Autodesk Netfabb, Meshlab, Materialise Magics, and many others. These applications are very useful to identif redundant surfaces, self-intersections) that can occur in the process of obtaining the STL file. Finally, once the clean/repaired STL file is obtained, it is imported in another software for slicing and converting it into a g many of them work only with g and this is useful for advanced modifications or research purpose. Advanced so integrate all functions, from importing STL to repairs and printing.

2.3

Production of filaments: technical approach,

Plastic industry is at a new crossroad in a world where white pollution became a real menace to our environment. It was reported an estimated quantity of 275 million metric tons of plastic waste in 2010, with 4.8 [START_REF] Jambeck | Plastic waste inputs from land into the ocean[END_REF]. United Nations Environment Program reports an increase of for limiting single-use plastics and microplastics, and even ban the use of some products. While there is a decrease of using single counterbalancing the plastic industry by introducing on the market a of various materials for FFF/FDM 3D printing. Nevertheless, users of 3D printing technology a. Roman lamp from third century (a) and the corresponding 3D scanned image (STL file, where loss of details can be noticed, b).

has its own file type/extension, usually the virtual object can be file. STL stands for stereolithography, and the format carry the information about surfaces of a 3D object, each surface being composed of triangles. STL files can be exported as ASCII or binary representations, the latter being more compact (much smaller size). STL files only represent the shape of the object, not the original colors or textures. The second route of obtaining STL files is by 3D scanning a real objec its shape into a digital one (Fig. 6).

Specialized software is used to analyze and enhance the STL file (Fig. 7), Meshmixer, Autodesk Netfabb, Meshlab, Materialise Magics, and many others. These applications are very useful to identify and repair some errors (e.g., holes in the surface, intersections) that can occur in the process of obtaining the STL file. Finally, once the clean/repaired STL file is obtained, it is imported in another software for converting it into a g-code file. Some 3D printers can directly print STL files, but many of them work only with g-code files. Anyhow, g-code files are easy to read and modify, and this is useful for advanced modifications or research purpose. Advanced so integrate all functions, from importing STL to repairs and printing.

of filaments: technical approach, present status and future directions Plastic industry is at a new crossroad in a world where white pollution became a real menace to our environment. It was reported an estimated quantity of 275 million metric tons of plastic waste in 2010, with 4.8 -12.7 million metric tons found in the oceans . United Nations Environment Program reports an increase of use plastics and microplastics, and even ban the use of some products. While there is a decrease of using single-use plastics, the new 3D printing industry is counterbalancing the plastic industry by introducing on the market a new product: filaments of various materials for FFF/FDM 3D printing. Nevertheless, users of 3D printing technology A. Tribot et al.

b.

) and the corresponding 3D scanned image (STL file, has its own file type/extension, usually the virtual object can be file. STL stands for stereolithography, and the format carry the information about surfaces of a 3D object, each surface being composed of triangles. STL files can be exported as ASCII or binary representations, the latter being more compact (much he object, not the original colors or textures. The second route of obtaining STL files is by 3D scanning a real object and convert Specialized software is used to analyze and enhance the STL file (Fig. 7), such as Meshmixer, Autodesk Netfabb, Meshlab, Materialise Magics, and many others. These y and repair some errors (e.g., holes in the surface, intersections) that can occur in the process of obtaining the STL file. Finally, once the clean/repaired STL file is obtained, it is imported in another software for code file. Some 3D printers can directly print STL files, but code files are easy to read and modify, and this is useful for advanced modifications or research purpose. Advanced software and future directions Plastic industry is at a new crossroad in a world where white pollution became a real menace to our environment. It was reported an estimated quantity of 275 million metric 12.7 million metric tons found in the oceans . United Nations Environment Program reports an increase of interest use plastics and microplastics, and even ban the use of some products.

use plastics, the new 3D printing industry is new product: filaments of various materials for FFF/FDM 3D printing. Nevertheless, users of 3D printing technology should be aware of toxicity that comes from decomposition of polymers, volatile organics and ultrafine particles [START_REF] Stephens | Ultrafine particle emissions from desktop 3D printers[END_REF] The technology for manufacturing plastic products is mature enough to easily shift from making bags, straws, etc to making 3D filaments. There is already a great boom in this industry, and very often a new type of filament (new colors, improved mechanical properties, new composites, new functions) is released on the market. The research in this direction is also spreading fast. From a research perspective, simple techniques for creating and testing new filaments are of main interest. Though sophisticated, full extrusion machines already exist, for research purposes miniaturization is more desirable, as low quantities are required. Moreover, when nano the best option. There are two main technical approaches laboratory, considering the mixing of the starting raw materials.

The first one is a mechanical premixing of the pellets with the filler in a container, and then pour them in the extruder for a second mechanical mixing in the ext mechanical mixing will disperse the filler into the molten polymer, so the obtained filament will be a composite of the filler homogenously dispersed in the polymer. If a better dispersion is required, the composite filament can be cut in A second approach consists in dissolving the polymer in a solvent (e.g., chloroform), and add the filler in the resulted solution. Then slowly evaporate the while mixing the solution. Depending o techniques (ultrasonication, decreasing the viscosity of the solutions to avoid sedimentation of the filler). When dispersion of the filler is considered homogenous, the liquid composite will be spread in thin layers on a large support for a faster evaporation of the remnant acetone, and lower sedimentation. eliminated in the extrusion step. It is important that all acetone to be evaporated before extrusion, otherwise the first filament will be porous. The dry thin layers are cut in small pellets for extrusion. Green Polymers for 3D printing should be aware of toxicity that comes from decomposition of polymers, volatile organics [START_REF] Stephens | Ultrafine particle emissions from desktop 3D printers[END_REF]. technology for manufacturing plastic products is mature enough to easily shift from making bags, straws, etc to making 3D filaments. There is already a great boom in this industry, and very often a new type of filament (new colors, improved mechanical perties, new composites, new functions) is released on the market. The research in this direction is also spreading fast. From a research perspective, simple techniques for creating and testing new filaments are of main interest. Though sophisticated, full extrusion machines already exist, for research purposes miniaturization is more desirable, as low quantities are required. Moreover, when nano-fillers are studied large extruders are not the best option. There are two main technical approaches for making filaments in a laboratory, considering the mixing of the starting raw materials.

The first one is a mechanical premixing of the pellets with the filler in a container, and then pour them in the extruder for a second mechanical mixing in the ext mechanical mixing will disperse the filler into the molten polymer, so the obtained filament will be a composite of the filler homogenously dispersed in the polymer. If a better dispersion is required, the composite filament can be cut in small pellets and extruded again.

A second approach consists in dissolving the polymer in a solvent (e.g., ), and add the filler in the resulted solution. Then slowly evaporate the while mixing the solution. Depending on the filler, dispersion can require more advanced techniques (ultrasonication, decreasing the viscosity of the solutions to avoid sedimentation of the filler). When dispersion of the filler is considered homogenous, the liquid composite thin layers on a large support for a faster evaporation of the remnant acetone, and lower sedimentation. The dried composite will show porosity, but this will be eliminated in the extrusion step. It is important that all acetone to be evaporated before rusion, otherwise the first filament will be porous. The dry thin layers are cut in small Errors (holes) found with the Meshmixer software in the 3D scanned surface.

should be aware of toxicity that comes from decomposition of polymers, volatile organics technology for manufacturing plastic products is mature enough to easily shift from making bags, straws, etc to making 3D filaments. There is already a great boom in this industry, and very often a new type of filament (new colors, improved mechanical perties, new composites, new functions) is released on the market. The research in this direction is also spreading fast. From a research perspective, simple techniques for creating and testing new filaments are of main interest. Though sophisticated, fully automated extrusion machines already exist, for research purposes miniaturization is more desirable, as fillers are studied large extruders are not for making filaments in a

The first one is a mechanical premixing of the pellets with the filler in a container, and then pour them in the extruder for a second mechanical mixing in the extruder. The second mechanical mixing will disperse the filler into the molten polymer, so the obtained filament will be a composite of the filler homogenously dispersed in the polymer. If a better small pellets and extruded again. A second approach consists in dissolving the polymer in a solvent (e.g., PLA dissolves in ), and add the filler in the resulted solution. Then slowly evaporate the acetone, n the filler, dispersion can require more advanced techniques (ultrasonication, decreasing the viscosity of the solutions to avoid sedimentation of the filler). When dispersion of the filler is considered homogenous, the liquid composite thin layers on a large support for a faster evaporation of the remnant The dried composite will show porosity, but this will be eliminated in the extrusion step. It is important that all acetone to be evaporated before rusion, otherwise the first filament will be porous. The dry thin layers are cut in small Errors (holes) found with the Meshmixer software in the 3D scanned surface.

A second extrusion could be applied for elimination of the porosity or for a better dispersion of filler in the polymer.

For a good quality of 3D printing the obtained filament must have a constant diameter along its whole length. This is not an easy task even for an experienced researcher, especially when a new composite material is studied. Obtaining a filament with a constant diameter right from the extruder is almost impossible. Auxiliary devices are necessary, such as a cooling system (in air or water), a puller device to control de diameter of the wire (usually 1.75 mm), and a spinner to roll the wire. Also, various sensors are useful to control and tailor the parameters of the chained devices, such as the temperature of the extruder (with one to four heating areas), the temperature of the cooling system, the speed of the extruder motor, speed of the pulling device, speed of the roller. Some auxiliary devices for tensioning the filament, especially when working at high speed, could be also desirable. In the simplest configuration, at least 4 parameters should be adjusted for obtaining an almost constant diameter (temperature and speed of the extruder, temperature of the cooling system and the speed of the pulling system). The system can be automatically or manually operated, but in both cases many tests are necessary to optimize the process. Since the laboratory work is usually aiming the proof of concept, scaling to the industrial fabrication can totally change the working conditions.

Making the new filaments, though a time and effort consuming work, is just a small part of a research. Finding new or highly improved properties of the polymers/filaments, and finally of the 3D printed object, especially taking into the consideration the polymer-filler(s) long term reaction/adhesion and behavior, is the main work/research and the real challenge [START_REF] De Franca Da | Natural and synthetic fillers for reaching high performance and sustainable hybrid polymer composites (chapter 7)[END_REF][START_REF] Ulfah | Influence of Carbon Black and Silica Filler on the Rheological and Mechanical Properties of Natural Rubber Compound[END_REF][START_REF] Wang | Effect of Polymer-Filler and Filler-Filler Interactions on Dynamic Properties of Filled Vulcanizates[END_REF].

Currently there are many commercial thermoplastic filaments used for 3D printing, such as Acrylonitrile Butadiene Styrene (ABS), Polylactic Acid (PLA), Polyethylene Terephthalate Glycol-modified (PETG), Polycarbonate (PC), Acrylonitrile Styrene Acrylate (ASA), Thermoplastic Polyurethane (TPU), Nylon (Table 1), just to mention few.

From Table 1 we can observe that tuning the 3D-printer can sometimes be a trial and error method. The range of setting parameters (nozzle temperature, printing speed) are quite large and the experience of the operator and preliminary tests can hopefully tune the machine-material parameters for optimal results. Composite filaments are also an attractive option on the market, either for esthetical purpose (e.g. PLA + wood fiber, stone/gypsum powder, bronze/steel/copper/brass powder), or because of their enhanced mechanical properties (e.g. carbon/glass fiber reinforced).

The challenge for 3D printing industry is to go beyond the hobby or the conventional approach of materials and to create new materials that can open new research directions and new applications by combining materials properties with complex shapes. Some current researches are about advanced composites materials for 3D printing, materials that exhibit shape memory effect [START_REF] Yang | 3D printing of shape memory polymer for functional part fabrication[END_REF], dielectric [START_REF] Castles | Microwave dielectric characterisation of 3D-printed BaTiO 3 /ABS polymer composites[END_REF]), actuator (Correa et al., 2015;[START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF] or magnetic properties [START_REF] Khatri | A 3D-Printable Polymer-Metal Soft-Magnetic Functional Composite[END_REF].

There is a large spectrum of shape memory materials, such as cross-linked PE, transpolyisoprene, styrene-based polymers, segmented polyurethane [START_REF] Hu | Recent advances in shape-memory polymers: Structure, mechanism, functionality, modeling and applications[END_REF]). Yang et al. reported DiAPLEX MM-4520 (a thermoplastic polyurethane elastomer type) pellets used for making the filament for 3D printing [START_REF] Yang | 3D printing of shape memory polymer for functional part fabrication[END_REF]. The 3D printed objects exhibited shape memory effect at different temperatures. A dielectric composite based on ABS and BaTiO 3 addition was proposed by [START_REF] Castles | Microwave dielectric characterisation of 3D-printed BaTiO 3 /ABS polymer composites[END_REF] , that exhibited promising properties in different geometrical configurations. [START_REF] Copper-White | Theological properties of poly(lactides). Effect of molecular weight and temperature on the viscoelasticity of poly(L-lactic acid)[END_REF] and Le [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF] presented an interesting approach: a wood fiber-polymer composite behaves as 4D material/printed object, where the 4 th dimension is time, as the 3D printed object changes its shape or physical properties over time. The new material is hygroscopic and sensitive to humidity and temperature, being able to show different geometries influenced by external conditions. In [START_REF] Khatri | A 3D-Printable Polymer-Metal Soft-Magnetic Functional Composite[END_REF], an ABS-stainless steel composite filament was obtained, being useful for magnetic sensing applications.

2.4

Control parameters for high quality printing.

3D printing is a promising future technology, as part of the fourth industrial revolution.

Conventional or unconventional 3D printers and dedicated materials are becoming more mature, meaning that we can acquire ready to print equipment. PLA and ABS are already easy printable materials, and finding optimal parameters is no longer a challenge, as the 3D printing software also offers optimized settings. Things are much different when it comes to print new materials, even pristine ones. Composite materials though are designed to enhance mechanical properties or deliver functional properties can require many tests for finding the adequate parameters. These parameters depend both on equipment performance, and materials properties.

The first condition to have a successful 3D printing is to provide a clean stl file, designed keeping in mind the possibilities and limits of a 3D printer. The next important condition is to provide good adherence of the material to the build platform (BP). First step, or the first layer is probably the most important. If the printed object does not firmly adhere to the build platform, it can be moved from the original position and the 3D printing fails. There are still many options to improve the adherence, such as (i) level the build platform to be in a horizontal position and the distance from the nozzle to be close enough, (ii) always adjust the temperature of the build platform according with the polymer type (e.g., 60 o C for PLA, 100 o C for ABS, 80 o C for PVA or Nylon, 100 o C for PMMA, but temperatures can strongly depend on each commercial filament, as different additions are used) and the melting temperature of the filament, (iii) increase the roughness of the building platform by grinding it with a fine sandpaper, spread a sticking film on the BP, use a special tape, clean the BP, (iv) tune the cooling fan speed, (v) tune the printing speed. Another solution is to increase the contact area with the BP by building rafts (an intermediate support between BP and 3D object), brims (outlines attached to the object) or skirts (outlines around the rafts that clean the nozzle and provide a constant flow). Sometimes, using BP of a specific material can help for a better adhesion, for example borosilicate glass, with a very low thermal expansion, works fine with ABS. Large shrinkage of 3D printed objects leads to warping effect (Fig. 8). Hence an improved adhesion of the first layers is critical. For avoiding large shrinkage of 3D objects, 3D printers are usually encased in a chamber for a better temperature distribution.

Once the adherence is solved, next essential parameter to set is the melting temperature of the filament. The melting temperature can depend not only on the material (Table 1), but also on the filament thickness. If the filament is thicker, the temperature should be slightly higher. The operator should test within the temperature range recommended by the producer (usually start with the average) and depending on the print quality or equipment decide the optimal temperature. An optimal temperature will provide a smooth flow of the filament. If the temperature is too high the filament tends to spread, while if it is too low the filament will tend to cool down too fast to properly stick to previous strings/layers. In PMMA case, if the temperature is too low, the strings will start curling.

Though each software will correlate the nozzle diameter with the filament thickness and printing speed, still some parameters can be manually modified for a fine tuning of printing quality. One important parameter is the extrusion multiplier, that can tune the flow rate of the printed string. If some gaps are observed between the melted strings (Fig. 9), then we have an under-extrusion, and the flow rate should be increased. If the flow rate is too high, then we have an over-extrusion, which usually will affect the quality of the walls.

The gaps between walls can occur due the assigned thickness of the walls in the CAD software. For a good quality of the 3D object, the designer should correlate the dimensions of the digital object with the performance and resolution of the 3D printer.

Green Polymers

Green polymers are numerous, a proposed to class the biodegradable polymers in function of their synthesis and their origin in four groups (Fig. 11).

-Group 1: Polymers derived from biomass. On the one hand one, it is polysaccharides such as starch, cellulose, lignin, … and on the -Group 2: Polymers produced by microorgan polyhydroxyalkanoates (PHA)

Green Polymers for 3D printing

Under-extrusion of deposited melted filament.

For avoiding the gaps between outline/perimeter shells (Fig. 10.a, b), either we can adjust the wall thickness using the CAD software, or just check some additional 3D printing options 10.c, d). A preview of the printing simulation is useful, and it is important to attentively read and analyze the simulated result. This can save much effort and time. There are different calibration tools or stl files designed by passionate 3D printer operators, as well as a large 3D printing community, to help the operator to print and figure r obtaining high quality objects, with smooth surface, good , excellent mechanical or functional properties. It is important to understand that 3D printing technology is not a "one-click" technology, and constant effort, lures, analyzes are required to obtain the desired object or material b. c.

Brim (1) around the raft (2) and gaps (3) between the outline shells. printed object with gaps between outline walls. c. Filled gaps (4) using advanced option (allow single extrusion wall) in Simplify3D software. d. Filled gaps between outline walls. polymers are numerous, and they are obtained with various ways. Avérou proposed to class the biodegradable polymers in function of their synthesis and their origin derived from biomass. On the one hand one, it is polysaccharides such as starch, cellulose, lignin, … and on the other hand, it is animal or vegetable proteins, Group 2: Polymers produced by microorganisms and obtained by extraction polyhydroxyalkanoates (PHA) 13), either we can adjust the wall thickness using the CAD software, or just check some additional 3D printing options A preview of the printing simulation is useful, and it is ulated result. This can save much effort files designed by passionate 3D printer operators, as well as a large 3D printing community, to help the operator to print and figure r obtaining high quality objects, with smooth surface, good , excellent mechanical or functional properties. It is important to technology, and constant effort, lures, analyzes are required to obtain the desired object or material.

d.

Brim (1) around the raft (2) and gaps (3) between the outline shells. b. The 3D advanced option Filled gaps between outline walls. nd they are obtained with various ways. Avérou ( 2004) have proposed to class the biodegradable polymers in function of their synthesis and their origin derived from biomass. On the one hand one, it is polysaccharides such other hand, it is animal or vegetable proteins, isms and obtained by extraction:

-Group 3: Polymers derived from biotechnology. They are produced by conventional synthesis from momnomers derived from renewable resources, such as polylactic acid (PLA) -Group 4: Polymers whose monomers come from fossil resources and which are obtained by conventional synthesis. They are polyesters such as poly (-caprolactone) (PCL), polyesteramides and aliphatic copolyesters (PBSA) or aromatic.

In this chapter only PCL will be discussed. In the production of these polymers, green principles are: a clean production process, no use of additional substances such as organic acids, high content of raw material, energy efficiency of the manufacturing, use of renewable resource and/or energy, lack of environment hazards, controlled lifecycles, low carbon footprint and high safety standards.

3.1

Polymers from petrochemical synthesis (group 4): Polycaprolactone (PCL)

Poly(-caprolactone), often simply termed polycaprolactone and abbreviated as PCL, is a biodegradable aliphatic polyester that is semi-crystalline. It is composed of hexanoate repeated units (Fig. 12). There are two methods to synthesize PCL: the polycondensation of 6-hydroxycaproic (6-hydroxyhexanoic) acid and the ring-opening polymerisation (ROP) of caprolactone (-CL, a cyclic monomer). Both 6-hydroxyhexanoic acid and -CL can be obtained as intermediate products during the oxidation of cyclohexanol into adipic acid by a number of microorganisms, such as Acinetobacter sp. strain SE19 [START_REF] Thomas | Biocatalysis: applications and potentials for the chemical industry[END_REF]. But at an industrial scale, -CL is produced from the oxidation of cyclohexanone by peracetic acid (Labet and Thielemans, 2009). Of the two synthesis routes, the main one is the ROP of -CL since it allows higher molecular weight PCL chains and lower polydispersity. Four main mechanisms for the ROP of lactones exist, and they depend on the catalyst: anionic, cationic, monomer-activated and coordination-insertion ROP, a large number of catalysts and catalytic systems, spanning virtually the whole periodic table having been investigated. The catalyst is mainly chosen for the particular application and the targeted reaction condition, the different ROP mechanisms affecting the resulting molecular weight, molecular weight distribution, end group composition [START_REF] Okada | Chemical syntheses of biodegradable polymers[END_REF]. More informations on the reaction mechanisms can be found in the review from Labet and Thielemans (2009). The chain termination step usually requires the addition of an acid or of low molecular weight alcohols to control the molecular weight of the obtained polymer chains.

Fig
PCL is semi-crystalline with a degree of crystallinity reaching up to 69 % depending on the thermal treatment applied -slow crystallization or melt quenched cooling, even higher values having been obtained by rearrangement of the chains in solution after slow solvent evaporation (Más [START_REF] Estellés | Physical characterization of polycaprolactone scaffolds[END_REF]. PCL has a density of around 1.2 g.cm -3 . Its number average molecular weight (Mn) may generally vary from 3000 to 80,000 g/mol, with crystallinity decreasing with increasing molecular weight. Depending on its crystallinity and Mn, PCL melting point ranges between 59 and 64°C, making it a "safe" thermoplastic to print. It has a low glass transition temperature (Tg) of -60°C meaning PCL is always in a rubbery state at room temperature, an unusual property among aliphatic polyesters, making the product extremely flexible and tough [28]. PCL starts decomposing at 250 -350°C, with full decomposition being obtained above 500°C according to thermogravimetric analyses (PCL Mn = 50k) [START_REF] Lepoittevin | Polycaprolactone/clay nanocomposites prepared by melt intercalation: Mechanical, thermal and rheological properties[END_REF]. PCL is hydrophobic owing to its large water contact angle and it is insoluble in water, alcohols, petroleum ether, diethyl ether. On the other hand, it is soluble in chloroform, dichloromethane, carbon tetrachloride, benzene, toluene, cyclohexanone, 2-nitropropane, THF; slightly soluble in acetone, 2-butanone, ethyl acetate, dimethylformamide, aceto-nitrile…. PCL demonstrates quite a unique ability to form compatible blends with a wide range of other polymers, e.g. to improve stress crack resistance, dye-ability and adhesion [START_REF] Engelberg | Physico-mechanical properties of degradable polymers used in medical applications: A comparative study[END_REF]. It has been used in association with cellulose propionate, cellulose acetate butyrate, PLA and PLGA [START_REF] Chandra | Mechanical and dynamic viscoelastic properties of hydroxyapatite reinforced poly(ε-caprolactone)[END_REF]. -caprolactone can also be copolymerized with a variety of monomers including ethylene oxide, polyvinylchloride, chloroprene, polyethylene glycol, polystyrene, urethanes, tetrahydrofuran, diglycolide, dilactide, valerlactone, substituted caprolactones, 4vinyl anisole, styrene, methyl methacrylate and vinyl acetate, yielding PCL copolymers [START_REF] Okada | Chemical syntheses of biodegradable polymers[END_REF][START_REF] Woodruff | The return of a forgotten polymer-Polycaprolactone in the 21st century[END_REF].

The rheological properties of PCL are depending whether temperature is above or below melting point (60°C). Chen et al. (2005) performed the dynamic mechanical thermal analysis

(CH 2 ) 5 O O n
(DMTA) of PCL (Mn = 80k) in tensile mode, within a range of temperature investigated from -100 to +50°C. At an oscillation frequency of 1 Hz, they observed the storage modulus E' decreasing from 3 GPa at -100°C to 0.3 GPa at + 50°C, while the loss modulus E'' decreased from 0.15 GPa at -100°C down to 0.05 GPa at + 50°C. The peak transition temperature in the tan  curve (maximum value of tan  associated to the glass transition resulting in an increase in the viscous behavior) is found to be higher than the peak temperature in the E'' curve, which is characteristic of semi-crystalline polymers. PCL exhibits a sharp decrease in the modulus values above 55°C, due to the onset of melting of the polymer [START_REF] Mittal | Mechanical, Thermal, Rheological and Morphological Properties of Binary and Ternary Blends of PLA, TPS and PCL[END_REF]. The effect of the molecular weight on the rheological properties of PCL melts were examined in a reference study by [START_REF] Grosvenor | The effect of molecular weight on the rheological and tensile properties of poly(ϵ-caprolactone)[END_REF] who investigated Mw in the range 17k to 36k with polydispersity between 1.7 and 1.9. PCL melts with low Mw (< 25k) showed only a slight deviation from a Newtonian behavior, with viscosity remaining nearly constant with increasing shear rate. In contrast, for Mw> 30k a non-linear shear stress/shear rate relationship typical of a pseudoplastic flow behavior was observed, with the viscosity decreasing with increasing shear rate. Apparent melt viscosity at 70°C ranged from 30 Pa.s up to 300 Pa.s, and at 100°C ranged from 20 Pa.s up to 125 Pa.s, the lowest values being obtained for low Mw, poorly crystalline PCL, an intuitive result since less thermal energy is required to disentangle short polymer chains compared to larger ones; since the degree of entanglement is generally a function of polymer chain length, the shorter the length (low Mw) the lower the viscosity [START_REF] Graessley | Viscosity of Entangling Polydisperse Polymers[END_REF]. Concerning the dynamic rheological properties of high molecular weight PCL melts, at low frequencies the melt behaves like a Newtonian fluid while at relatively high frequencies it exhibits a pseudo-plastic behavior. Hence for oscillation angular frequency increased from 1 to 100 rad/s, Chen and Sun (2005) observed the storage modulus G' to increase from 4 kPa to 400 kPa and the loss modulus G'' to increase from 15 kPa to 300 kPa, while the viscosity decreased from 15 kPa.s to 5 kPa.s (PCL Mn = 80 k at 110°C). Similarly, for PCL Mn=50k at 80°C, [START_REF] Lepoittevin | Polycaprolactone/clay nanocomposites prepared by melt intercalation: Mechanical, thermal and rheological properties[END_REF] found G' to increase from 100 Pa to 80 kPa and G'' to increase from 2 kPa to 100 kPa with increasing angular frequency (from 1 to 60 rad/s).

PCL is compostable under home composting conditions, a reason why it has been used as a biodegradable packaging material (Engelberg ad Kohn, 1991). Depending on its molecular weight, degree of crystallinity and conditions, its degradation can take several months to years, being significantly slower than that of PGA or PLA [START_REF] Gross | Biodegradable Polymers for the Environment[END_REF]. Many microbes in nature are able to completely biodegrade PCL. The amorphous phase is degraded first, resulting in an increase in the degree of crystallinity while the molecular weight remains constant [START_REF] Lam | Comparison of the degradation of polycaprolactone and polycaprolactone-(β-tricalcium phosphate) scaffolds in alkaline medium[END_REF] . Then, cleavage of ester bonds results in mass loss [START_REF] Peña | Long term degradation of poly(ɛ-caprolactone) films in biologically related fluids[END_REF]. At low temperature, the polymer chain degrades by random scission while at higher temperatures end chain scission occurs [START_REF] Joshi | Degradation of polycaprolactone in supercritical fluids[END_REF]. High molecular weight PCL takes much longer to degrade: the larger the chain length, the greater the number of ester bonds to be cleaved before obtaining water-soluble monomers/oligomers allowing surface erosion to proceed. Like other polyesters, PCL degradation is auto-catalysed by the carboxylic acids liberated during hydrolysis, but several enzymes can also act as catalyst and speed up the decomposition in the environment (Labet and Thielemans, 2009). PCL is also bioresorbable, meaning that it is entirely eliminated from the body through natural pathways, including its degradation by-products, with no residual side-effects. After the non-enzymatic hydrolytic cleavage of ester groups, once the polymer has become more highly crystalline and of low molecular weight (Mn < 3000), PCL in vivo undergoes intracellular degradation as evidenced by the uptake of PCL fragments in phagosomes of macrophages, giant cells and within fibroblasts [START_REF] Woodward | The intracellular degradation of poly(ε-caprolactone)[END_REF]. Indeed, the hydrolysis of PCL yields 6-hydroxyl caproic acid and acetyl coenzyme A which enter the citric acid cycle and through this metabolic pathway are eliminated from the body [START_REF] Albertsson | Controlled degradation by artificial and biological processes[END_REF]. A 3-years follow-up study showed that PCL with an initial Mw = 66 kg/mol remained intact in shape during the first 2 years of implantation, then broke into low Mw (8 kg/mol) pieces after 30 months. The Mw continued to linearly decrease with time. Using radioactive labelling, it was shown that more than 90% of the labelled-PCL was excreted from feces and urine, while radioactivity in the organs remained close to background level, evidencing that PCL did not accumulate in body tissue and could be completely excreted [START_REF] Woodruff | The return of a forgotten polymer-Polycaprolactone in the 21st century[END_REF][START_REF] Sun | The in vivo degradation, absorption and excretion of PCL-based implant[END_REF].

The properties of PCL make it particularly suitable for additive manufacturing and especially for FDM which the most commonly used and affordable printing technique is. PCL has high adhesion between layers, improving the impact resistance, strength, durability of the fabricated object. Due to the low melting point of PCL, the objects can be re-shaped after printing to fit specific application by simply softening it in warm (60°C) water. It can be processed at much lower and safer temperatures (70°C to 140°C) compared to the commonly used PLA or ABS polymer and does not require a heated bed. Details on the setup parameters depending on the printing technique are given below in Table 2.

PCL filaments are also commercially available under a 1.75 mm diameter; their molecular weight is rarely specified, although it should presumably be comprised between 50k to 80k. Some suppliers are Polyfluor Plastics, Filaments.ca and commercial brand names include FACILAN™ PCL100 and FACILAN™ ORTHO by ElogioAM, "Flexible Filament" by Makerbot, eMorph and eMate by eSun. For most of FDM printers equipped with a 400 μm nozzle, PCL prints best at approximately 140°C with a bed temperature between RTP to 38°C and a speed of 30 mm/s. Since PCL flows extremely easily through the nozzle at 140°C it is necessary to cool it down quickly to fix the print, therefore best results are obtained with fans at 100% (from Elogio AM tech sheet recommendation https://cdn.shopify.com/s/files/1/0108/1881/4009/files/TDS_Facilan_PCL100_ElogioAM_V1 .4.pdf?4463025051778730948).

FDM generally involves the use of a filament, although a variant of the technique (sometimes denoted as Precision Extrusion Deposition, PED) allows the printing from raw PCL powders. Fabrication of the filament implies the hot extrusion of PCL through a 1.75 mm nozzle using a screw extruder at a temperature above 70°C. This can lead to small changes in the mechanical properties of the extruded PCL filament. For information, the properties of a commercially available filament are provided on the technical data sheet from the supplier (https://cdn.shopify.com/s/files/1/0108/1881/4009/files/TDS_Facilan_PCL100_ElogioAM_V 1.4.pdf?4463025051778730948 ). Overall, the mechanical properties of the source PCL before and after extrusion remain in the range shown in Table 3, although it is advisable to monitor the possible change in the PCL molecular weight resulting from the additive manufacturing process, which can be deduced e.g. from Gel Permeation Chromotography. : Woodruff, et al. 2010 ; (5) : Neufurth et al., 2017; (6) : [START_REF] Szojka | Biomimetic 3D printed scaffolds for meniscus tissue engineering[END_REF] 3D printing temperature 75°C (1) -115°C (2) 3D printing speed 0.5 mm.s -1 -5 mm.s -1 (3) Heated bed temperature Ambient (4) Extrusion pressure 1-8 bars (5) Pre-flow delay 0.3 s (5) Post-flow delay 0.5 s (5) Wait time between layer 5 s (5) Interlayer overlap 20 -33% (5) Needle gauge (inner diameter) 0.178 µm -800 µm (3) Needle length 3 mm ( 6) Polylactic acid is generally synthesized from -hydroxyacids [START_REF] Garlotta | A Literature review of Poly(Lactic Acid)[END_REF]. Lactic acid or 2-hydroxypropanoïc acid is obtained from sugar or starch fermentation (Fig. 13). The basic building block for PLA is lactic acid which can be manufactured either by carbohydrate fermentation or chemical synthesis. The most commonly used industrial method is the ring opening polymerization (Fig. 14). In a first step, a prepolymer is produced by polycondensation of lactic acid (high temperature and low vacuum conditions). This prepolymer has a low molecular weight (Mw< 1000 -5000 Da). The second step consists in the depolymerisation of the prepolymer to obtain lactide. Lactide is formed by different isomers: L-Lactide, D-lactide and meso-lactide. Ring opening polymerization of lactide leads to high molecular weight PLA (Mw>100 000 Da). PLA forms a chiral molecule with two stereoisomers: L-lactic acid and D-lactic acid (Fig. 15). PLA properties depend principally of the ratio between L-and D-lactic acids. PLA which contain a large part of L-lactic acid (>93%) are semi-crystalline while PLA which contain between 50% and 93% of L-lactic acid are totally amorphous [START_REF] Auras | An overview of polylactides as packaging materials[END_REF]. Semicrystalline PLA and amorphous PLA have glass transition temperature (Tg) and melting temperature (Tm) slightly different. Glass transition temperature and melting temperature are function of the initial monomeric composition and the molecular weight. They also depend of the molecular flexibility and geometry and the intermolecular forces. Its leads to higher values for semicrystalline PLA. In his review, [START_REF] Garlotta | A Literature review of Poly(Lactic Acid)[END_REF] said the melting temperatures of pure D lactide and pure Lactide were the same (207°C) but typical melting temperatures of PLA were in the 170-180°C range. He attributed this to small and imperfect crystallites, slight racemization and impurities.

Poly -L-Lactic acid (PLLA) has a crystallinity of around 37%, a melting temperature between 170 -183°C [START_REF] Gupta | New emerging trends un synthetic biodegradable polymers -Polylactide: a critic[END_REF] Fang [START_REF] Hanna | Rheological properties of amorphous and semicrystalline poly lactic acid polymers[END_REF] determined glass transition and melting temperatures of 58°C and 148°C respectively for amorphous PLA (82% of L-lactide and 18% of D-lactide) while they are of 62°C and 151°C, respectively for PLA (95% of L-lactide and 5% of D-lactide). J. Dorgan et al. worked with commercial grade of lactic acid (Cargill-Dow Polymers) (96% of L-lactide and 4% of D-lactide) and they obtained a Tg of 58.4 °C and a Tm of 151.7°C [START_REF] Dorgan | Thermal and Rheological properties of commercial grade poly(lacti acid)s[END_REF]. For [START_REF] Jamshidi | Thermal characterization of polylactides[END_REF], glass transition temperature is not affected by the molecular weights of PLA. They found a Tg of 55°C for PLA with a molecular weight of 22,000 g/mole and the predicted value of PLA with infinite molecular weight is only to 4-5°C upper. On the contrary, Dorgan et al. have shown the PLA glass transition temperature depended of L-isomer content and molecular weight [START_REF] Dorgan | Melt rheology of variable L-content poly(lactic acid)[END_REF] [START_REF] Peinado | Effect of extrusion on the mechanical and theological properties of a reinforced Poly(Lactic Acid) : reprocessing and recycling of biobased materials[END_REF]. Rheological properties are strongly depended of PLA structure and conditions of implementation (temperature and shear rates). PLA is a thermoplastic polymer, so it exhibits two behaviors in function of the shear rates. For low shear rate (<10 s -1 ), high molecular weight PLA presents a Newtonian behavior whereas at high shear rates (>10 s -1 ), it presents a non Newtonian behavior [START_REF] Hamad | Properties and medical applications of polylactic acid:A review[END_REF]. As the shear rates increase, the viscosity of high molecular weight PLA melt decreases significantly. An explication is a strong shearing action can break the molecular chains of the polymer (Fang and [START_REF] Hanna | Rheological properties of amorphous and semicrystalline poly lactic acid polymers[END_REF]. On the other hand, low molecular weight has a quasi Newtonian behavior at shear rates typical of film extrusion [START_REF] Copper-White | Theological properties of poly(lactides). Effect of molecular weight and temperature on the viscoelasticity of poly(L-lactic acid)[END_REF]. Temperature influences too the viscosity. This one decreases when the temperature increases. The effects of the composition (L and D lactides) influence also the results. It has been shown that the shear viscosity of PLA increases when the % of L lactide increased i.e. when the crystallinity increased [START_REF] Dorgan | Thermal and Rheological properties of commercial grade poly(lacti acid)s[END_REF]Fang and Hanna, 1999).

When exposed to elevated temperature, PLA is known to undergo thermal degradation. The decrease of the molecular weight depends in the process temperature, the residence time in the extruder and the shear stress. The reactions are numerous: hydrolysis by trace amounts of water, depolymerization according to back-sitting mechanism, oxidative, random main-chain scission, intermolecular transesterification to monomer and oligomeric esters and intramolecular transesterification resulting in formation of monomer and oligomer lactides of low molecular weight [START_REF] Kopinke | Thermal decomposition of biodegradable polyesters -II. Poly(lactic acid)[END_REF][START_REF] Lim | Processing technologies for poly(lactic acid)[END_REF]. Also, to reduce PLA thermal degradation, it is recommended to correctly dry the PLA before its implementation to avoid the hydrolysis reaction during the melt processing. Natureworks LLC, one of the main suppliers for PLA polymers, recommended that PLA is dried to below 0.025% w/w. moisture content before extrusion [START_REF] Lim | Processing technologies for poly(lactic acid)[END_REF]. Drying conditions are dependent on temperature, air flow rate and dew points (Jamshidian et al., 2010). For example, amorphous PLA in the form of pellets must be dried below the glass transition temperature. Above, pellets can stick together. For semicrystalline PLA, drying conditions are recommended with temperatures range between 80 to 100°C and times range between 4 to 2h. After drying, PLA pellets must be conserved in sealed packages to prevent moisture from atmosphere.

Comparatively to traditional polymers, PLA has good properties and close to polystyrene. PLA tensile strength and Young's modulus are high. For example, Young's modulus of PLA is 3.8 GPa while Young's modulus is 3.39 GPa for polystyrene and 1.4 GPa for polypropylene. PLA has a good flexural strength but a very low elongation at break: 4 % for PLA versus 2% for polystyrene and 400 % for polypropylene [START_REF] Dorgan | Thermal and Rheological properties of commercial grade poly(lacti acid)s[END_REF]. PLA is a relatively brittle plastic that limits its utilization. In fact, mechanical properties depend of the molecular weight and the treatment after implementation, annealing for example. [START_REF] Perego | Effect of molecular weight and crystallinity on poly(lactic acid) mechanical properties[END_REF] results are summarized in the Table 4. Mechanical properties of higher molecular weight PLA were better than those of lower molecular weight PLA. After an annealing at 105°C during 90 min under nitrogen, the strength increased probably because of the increase of the crystallinity. Izod impact is defined as the kinetics energy needed to initiate facture and continue the fracture until the specimen is broken. Two tests exist: notched and unnotched Izod Impact tests according to the preparation of the specimen. Nature of PLA (L or L/D PLA) and thermal treatment (annealing or not) influence the material resistance to impact as it is summarized in the Table 5. Valorization of PLA wastes were studied by Piemonte (2011). Four scenarios were studied: incineration, composting, anaerobic digestion and mechanical recycling. The last scenario was estimated the best alternative from an environmental point of view. Much of commercial filaments come from recycling. However, literature reports an important decrease of the PLA molecular weight, an increase of crystallization [START_REF] Brüster | Thermomechanical degradation of plasticized poly(lactide) after multiple reprocessing to simulate recycling: multi-scale analysis and underlying mechanisms[END_REF][START_REF] Pillin | Effect of thermomechanical cycles on the physico-chemical properties of poly(lactic acid)[END_REF]) , a decrease of the tensile stress after few mechanical recycling processes (extrusion process and injection process) [START_REF] Zenkiewicz | Characterisation of multi-extruded poly(lactic acid)[END_REF]. But in these studies, degradation PLA during its service life was not considered. Beltran et al. have studied the effect of degradation during use from an accelerated ageing and introduced a demanding washing process before a second melt processing step.

They have observed i) a decrease of the molecular weight which has limited effect on mechanical properties [START_REF] Hopmann | Influence of recycling of poly(lactic acid) on packaging relevant properties[END_REF]Perega et al., 1996) , ii) an increase in the mobility of the polymer chain and iii) no change for the degree of crystallization. To improve the properties of recycled PLA, additives can be added during the mechanical recycling step.

Additive can be a chain extender or an organic peroxide, which react with the PLA residues giving rise to cross-linking, branching or chain extension reactions [START_REF] Beltran | Mechanical recycling of poly(lactic acid): Evaluation of a chain extender and a peroxide as additives for upgrading the recycled plastic[END_REF]. To improve the mechanical properties of recycled PLA, [START_REF] Zhao | Enhanced mechanical properties of selfpolymerized polydopamine coated recycled PLA filament used in 3D printing[END_REF] proposed to use polydopamine (PDA) as adhesion promotor. Mechanical properties of recycled PDA/PLA filaments obtained from extruded PDA coated PLA pellets and recycled PLA are summarized in the Table 6.

PLA degradation depends of few factors such as molecular weight, crystallinity, purity, temperature, pH, water permeability. But it is known that PLA rapidly degraded under composting conditions. The hydrolytic degradation products are totally assimilated by microorganisms.

The object manufactured by 3D printing presents poor interfacial strength at interfilamentous junctions. The mechanical properties of parts are as high as those of parts manufactured by injection molding. To increase its mechanical performance, PLA can be reinforced with a particles or fibers. It exists a wide choice of reinforcements: metal particles, wood or plant fibers, beer, coffee or algae residues. PLA is a clear, colorless thermoplastic but it is offered for sale in various colors. There are more than 15 PLA filaments manufacturers and more than 500 references. Filaments are available unto several categories depending on diameter (1.75 mm or 2.85 mm), conditioning ( 0.5 to 6 kg), color ( more than 10), translucence ( zero, low medium and high), minimum extrusion temperature ( [180-190°C] to [220-230°C]), durability ( medium and high), rigidity (flexible to rigid) etc… The most conventional printing conditions are summarized in the Table 7. Heated bed is not necessary for PLA printing. They can vary with manufacturers and product qualities. Some like Volcano PLA of Formfutura ask for post-print annealing. It is design for high printing speeds up to 120 mm.s -1 . After extrusion, Volcano PLA has a high crystallinity content; it is rigid and temperature resistant. 3D printing temperature is recommended between 220 to 255°C and an annealing step is optional after annealing, the temperature resistance is up to 95°C.

Among the different commercial PLA filament, we find the LW-PLA of ColorFabb. This filament uses an active foaming technology which is triggered by temperature. At its peak, the filament will expand 3 x its volume. The filament density is variable. Before impression, density is in range of [1.21 -1.43 g.cm -3 ] and after expansion, decreases in range of [0.403 -0.476 g.cm -3 ]. The expansion will produce when temperature is around 230°C. it is necessary to decrease the printing speed, of 65%. Its leads to layer height important, until 0.6 mm with a nozzle of 0.4 mm. the manufacturer guidance indicates the stringing/oozing phenomena are inevitable.

Wood fiber or wood flour are the main reinforcement used but wood flour composites obtained by 3D printing have lower mechanical properties than those objects produced by traditional manufacturing processes [START_REF] Auras | An overview of polylactides as packaging materials[END_REF]. The incorporation of wood fibers in bio-composite reduces the cost of final products. In their study, Tao et al. have compared pure PLA and wood flour (WF)/ PLA composite with 5 %wt wood content [START_REF] Wang | Effect of Polymer-Filler and Filler-Filler Interactions on Dynamic Properties of Filled Vulcanizates[END_REF]. Before extrusion, PLA and WF were dried during 4h to eliminate the moisture. The device consisted in a single screw extruder with a maximum extrusion speed of 2 m.min -1 for a diameter of 1.75 mm. The nozzle diameter was 0.4 mm, and nozzle temperature was set at 210 •C during print. The addition of WF has changed the fraction surface: flat and smooth for pure PLA and rough for WF/PLA composite. Visible clusters of wood particles have been observed for higher wood content [START_REF] Kariz | Effect of wood content in FDM filament on properties of 3D printed parts[END_REF]. Without coupling agent, a poor interfacial bonding between PLA and WF has been observed. Glass transition and cold crystallization temperatures decreased with the addition of the wood flour, from 67 to 60°C and from 101 to 97°C respectively. On the other hand, the melting temperature of pure PA and WF/PLA composites were the same at 167°C. When strain was low (<1.5%) rigidity of wood flour/PLA composite was more important than pure PLA but for higher strains, it was the opposite. The 3D printed composite showed lower tensile mechanical properties. The same behavior was observed for cork/PLA composite [START_REF] Daver | Cork-PLA composite filaments for fused deposition modeling[END_REF]. M. [START_REF] Kariz | Effect of wood content in FDM filament on properties of 3D printed parts[END_REF] studied too the impact of wood content on the mechanical properties. They used beech wood particles of 0.237 mm size and PLA matrix. Tensile strength increased from 55 MPa to 57 MPa with the addition of wood fibers from 0% to 10%. But beyond that, tensile strength decreased, until 30 MPa for 50% wood content. Commercial wood fiber filaments have higher wood fiber rates ranging 10 to 40%wt (Le [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF]. Manufacturers announce that fibers adding does not change the printing condition. However, as Le [START_REF] Duigou | 3D printing of wood fibre biocomposites: From mechanical to actuation functionality[END_REF] pointed out, mechanical properties are strongly dependant on printing orientation due to fiber anisotropy. Wood fiber orientation follows the printing orientation of the filament, weakening inter filament and interlayers interactions [START_REF] Kariz | Effect of wood content in FDM filament on properties of 3D printed parts[END_REF]. The printing width also influences the mechanical properties by changing the porosity of the printed object.

Butanediol Polyesters

The butanediol polyesters are promising biodegradable aliphatic polyesters. They are used in various forms such as films, packaging, injection-molded products and in biomedical applications [START_REF] Bai | Correlation between the chemical structure and enzymatic hydrolysis of poly(butylene succinate), poly(butylene adipate) and poly(butylene suberate)[END_REF]. Among butanediol polyesters, let us note poly(butylene succinate) (PBS), poly(butylene adipate) (PBA) and poly(butylene succinate adipate) (PBSA). Poly(butylene succinate) (PBS) and its co polymers are synthetized via polycondensation of succinic acid and 1,4-butanediol. Generally, succinic acid is manufactured by hydrogenization of maleic anhydride. This one is itself obtained by oxidation of butane or benzene. It is the petroleum way (Fig. 16, Fig. 17). Since 2010, the biologic way from renewable feedstocks is developed and commercialized [START_REF] Xu | Poly(butylene succinate) and its copolymers: research, development and industrialization[END_REF]. Succinic acid can be then obtained from fermentation of microorganisms on glucose, starch, xylose… Most of the time, the production of 1,4-butanediol is from petroleum feedstocks. There exists an alternative from a biobased process. In the case, succinic acid is obtained from fermentation of renewable feedstocks such as corn-derived glucose or dextrose. After a purification by electrodialysis, succinic acid is reduced catalytically to 1,4-butanediol. Succinic acid and 1,4-butanediol are esterified to obtain oligomers which are polycondensed to remove 1,4-butanediol to form high molecular weight PBS. In the case of poly(butylene adipate) (PBA) 1,4-butanediol is esterified with adipic acid. PBS is soluble in various solvents: chloroform, dichloromethane, 1,1,1,3,3,3-hexafluoro-2-propanol, o-chlorobenzene. Glass transition temperature of PBS is around of -32 °C and its melting point is 114°C (at peak). Properties of PBS depend of its molecular weight, but it is comparable to polypropylene. With high molecular weight, PBS is ductile since its elongation at break can reach 270% and its Izod impact strength 73 J.m -1 [START_REF] Xu | Poly(butylene succinate) and its copolymers: research, development and industrialization[END_REF]. It can be used for blowing processes. But to be extruded or injection molded, PBS needs a molecular weight less than 100 000. In this case, PBS is brittle with an elongation at break around 10% and the Izod impact strength less than 40J.m -1 . PBS is sensitive to water content. Before thermal processing (extrusion or printing) PBS pellets can be dried until a water content lower than 0.1 % to avoid hydrolysis at high temperature. Note that when processing temperature is upper that 200°C, the shear viscosity decreases drastically [START_REF] Xu | Poly(butylene succinate) and its copolymers: research, development and industrialization[END_REF]. Melt flow rate (MFR) of PBS depends of the grade of PBS and its co-polymers. Properties of typical grades of bionolle PBS are summarized in Table 8 [START_REF] Rudnik | Compostable polymer properties and packaging applications in Plastic Film in Food packaging[END_REF]. PBS can be mixed with PLLA. The expected properties are tensile strength, elastic modulus, elongation at break and ductility. [START_REF] Xu | Poly(butylene succinate) and its copolymers: research, development and industrialization[END_REF] compared in their review mechanical properties of PBS/PLA and PBSA/PLA blends bars obtained by injection molded (Table 9). Poly(butylene succinate-co-butylene adipate) (PBSA) properties are function of the butylene adipate content. PBSA with 5-15 mol% butylene adipate content is interesting for its crystallinity and its tensile strength higher than those of PBS.

C. Boudouard et al have obtained PBS/PLLA 50/50 wt% composite obtained by injection and PBS/PLLA 50/50 wt% with 10% of flax fibers filaments. The comparison is interested to observe the loss of properties between composites obtained by injection and by 3D printing. Pellets of PBS/PLLA50/50 wt%-10% of flax fibers were obtained in two steps to remove water. In a first step, formulations were compounded with a twin screw extruder at 190°C and 20 rpm. Extruded composites were the granulated and dried at 50°C. Some of them were injected to produce tensile specimens and the others were extruded using a singlescrew extruder to produce the filament. 3D printer was equipped with a 1.0 mm nozzle and an extrusion temperature of 190°C. Printing speed was in range 0.8-1.5 m.min -1 with a Z amplitude varied from 0.6-1.0 mm. Parameters used by Q. Ou- [START_REF] Ou-Yang | Preparation and characterization of poly(butylene succinate)/polylactide blends for fused deposition modeling 3D printing[END_REF] to print PBS/PLA material is summarized in Table 10. (Fig 18) is synthetized from the polymer of adipic acid, 1.4 butanediol and the polymer of dimethyl terephthalate with 1,4 butanediol. It is a random co polymer known to be flexible and tough with a wide melting point. The random structure means that it cannot crystallize. Its density is 1.26 g.cm 3 . The glass transition of PBAT is -28 °C melting temperature is in range of 110-120 °C (Table 11) whereas its VICTA temperature is 89°C). Its melting flow rate is more than 5g/10 min at 190°C and 2.16 kg.

PBAT is biodegradable: Zhao et al. have estimated its degradation around 90% after 80 days in testing [START_REF] Zhao | Preparation, Mechanical and thermal properties of biodegradable polyesters/poly(Lactic acid) blends[END_REF]. PBAT is very interested to drastically improve the toughness of PLA. Notched impact strength of PLA increased from 5.1 kJ.m -2 for pure PLA to 37.2 kJ.m -2 for PLA with PBAT20wt%. With this PBAT content, PLA/PBAT blend elongation at break reached more than 600 %. S. Wang et al. are not so optimistic but they confirmed the increase of elongation at break: PLA/PBAT (80-20 wt%) printed bar obtained a strain at break in tensile mode of 38% versus a value of 4% for PLA printed bar [START_REF] Liu | Current advances and future perspectives of 3D printing natural derived biopolymers[END_REF]. In the same time, the Young modulus was 2471 MPa for PLA/PBAT (80-20 wt%) printed bar versus 3282 MPa for PLA printed bar. S. [START_REF] Singamneni | Extrusion 3D printing of polybutyrate-adipate-terephthalate polymer composites in the pellet form[END_REF] have realized successfully PBAT filament reinforced by wood floor in range of 0-20 wt%. They used a homemade extrusion 3D printer. 3D printing parameters are summarized on Table 12. Mechanical properties of 3D printed samples were significantly lower than that of injection molded samples. For example PBAT/woof flour (95-5 wt%) 3D printed samples have a strength of 53MPa and a flexural modulus of 403MPa while the same composition sample obtained by injection have a strength of 49MPa and a flexural modulus of 483MPa. Polyhydroxyalcanoates are accumulated biopolymers in bacterial cells in the form of granules. This accumulation is caused by the lack of a nutrient in the presence of an excess of carbon substrate. PHAs can be produced by a large number of microorganisms for energy storage purposes. Chemical nature and characteristics of PHAs are dependant of culture conditions, carbon substrate ad selection of bacterial species. [START_REF] Menčik | Effect of Selected Commercial Plasticizers on Mechanical, Thermal, and Morphological Properties of Poly(3-hydroxybutyrate)/Poly(lactic acid)/Plasticizer Biodegradable Blends for Three-Dimensional (3D) Print[END_REF] announced the number of 150 different PLA momoners (Table 13).

Polyhydroxyalcanoates can be divided into 3 categories: i) short chain length (scl) PHA constituted of hydroxyalcanoïc acid with a maximum of 5 atoms; ii) medium chain length (mcl) PHA which monomeric units are in range of 6-12 atoms and iii) long chain length (lcl) PHA with 12-16 atoms. Short chain length PHAs are rigid and brittle and they are considered as thermoplastics. Mcl et lcl PHAs are considered as elastomers and adhesives.

Poly(3-hydroxybutyrate) PHB and poly(3-hydroxybutyrate-co-3hydroxyvalerates) PHBVs are the most common family members of PHAs (Fig. 19). Poly(3-hydroxybutyrate) is a linear saturated polyester. PHBV is obtained by copolymerization with hydroxyvalerate (HV). PHB and its co polymers PHBV are biodegradable and biocompatible but they present a poor thermostability and a relatively low impact resistance [START_REF] Avella | Review: properties of blends and composites based on poly(3-hydroxybutyrate (PHB) and poly(3-hydroxybutyrate-hydroxyvalerate) (PHBV) copolymers[END_REF]. PHBs are insoluble in water, but they are soluble in almost apolar solvent, such as chloroform.

PHB is a semi crystalline polymer with high melting temperature (170°C) and high degree of crystallinity (80%) [START_REF] Menčik | Effect of Selected Commercial Plasticizers on Mechanical, Thermal, and Morphological Properties of Poly(3-hydroxybutyrate)/Poly(lactic acid)/Plasticizer Biodegradable Blends for Three-Dimensional (3D) Print[END_REF][START_REF] Vaidya | Integrating softwood biorefinery lignin into polyhydroxybutyrate composites and application in 3D printing[END_REF]. Glass transition temperature is sometime no discernible [START_REF] Vaidya | Integrating softwood biorefinery lignin into polyhydroxybutyrate composites and application in 3D printing[END_REF][START_REF] Zhang | Blending Polylactic Acid with Polyhydroxybutyrate: The Effect on Thermal, Mechanical, and Biodegradation Properties[END_REF]. PHBV is more ductile than PHB, with lower melting point and lower strength and rigidity. The melt flow rate of PHB (NaturePlast, France) is estimated to 15-30 g/10min tested at 210°C and 2.16kg loading [START_REF] Armentano | Processing and characterization of plasticized PLA/PHB blends for biodegradable multiphase systems[END_REF].

PHA filaments for fused deposition modeling are generally added with natural fibers [START_REF] Chiulan | Thermal and rheological properties of polycaprolactone and polystyrene blends[END_REF]. To increase the interfacial adhesion between PHA and fibers, a fiber treatment is used with a coupling agent or a reactive compatibilizer [START_REF] Zhang | Effect of interfacial modifiers on mechanical and physical properties of the PHB composite with high wood flour content[END_REF][. Silane or acylation are used a coupling agent. Silane coupling is tetrathoxysilane (TEOS) (Wu et al., 2017). For acylation of the hydrophilic hydroxyls, Shibata et al have selected butyric because the carbon number of the acid components in ester groups of PHBV is t same than those of treated fibers reactive compatibilizer. Effectiveness of maleaic anhydride grafted PHBV (PHBV shown in various studies [START_REF] Zhang | Effect of interfacial modifiers on mechanical and physical properties of the PHB composite with high wood flour content[END_REF]Wu and Lioa, 2017) made a PHBV-g-MA/palm fiber filament. Tensile strength was 7 MPa and Young modulus was 65 MPa with 20 wt% incorporation of fibers the prepared palm fiber-reinforced polyhydroxyalkanoate is a good c filament, due to its low cast and excellent characteristic.

In general, commercial PHA filaments are mixed with PLA. For some authors, PLA and PHA are immiscible but they exhibit molecular interactions others, PHB is miscible with low molecular weight PLA (Mw< 18000) but for high molecular weight PLA, PHB/PLA blends present biphasic separation is the major phase, PLA is miscible with low molecular weigth (Mw <9400) high molecular weight PHB. To increase the compatibility between PHB and PLA, compatibilizers can be used: glycidyl methacrylate, poly(ethylene glycol) or poly(vinyl acetate) [START_REF] Abdelwahab | Thermal, mechanical and morphological characterization of plasticized PLA-PHB blends[END_REF] In PHB/PLA blends, PHB particles mechanical properties. In their study, Zhang and Thomas have observed than glass transition Various types of PHA and PLA m value

R nature m = 0 R=CH 3 Polylactic m = 1 R=H hydroxypropionate) m = 1 R=CH 3 m = 1 R=C 2 H 5 m = 1 R=C 2 H 7 hydroxyhexanoate)
19. Chemical structure of PHB and PHBV.

acylation are used a coupling agent. Silane coupling is tetrathoxysilane (TEOS) . For acylation of the hydrophilic hydroxyls, Shibata et al have selected butyric because the carbon number of the acid components in ester groups of PHBV is t same than those of treated fibers [START_REF] Shibata | Biodegradable polyester composites reinforced with short abaca fiber[END_REF]. Maleated polymer are used as reactive compatibilizer. Effectiveness of maleaic anhydride grafted PHBV (PHBV Anderson et al., 2013;Wu and Lioa, 2017). Wu et a MA/palm fiber filament. Tensile strength was 7 MPa and Young modulus was 65 MPa with 20 wt% incorporation of fibers (Wu et al., 2017). They have concluded than reinforced polyhydroxyalkanoate is a good candidate for 3D printing filament, due to its low cast and excellent characteristic.

In general, commercial PHA filaments are mixed with PLA. For some authors, PLA and PHA are immiscible but they exhibit molecular interactions [START_REF] Zhang | Blending Polylactic Acid with Polyhydroxybutyrate: The Effect on Thermal, Mechanical, and Biodegradation Properties[END_REF] others, PHB is miscible with low molecular weight PLA (Mw< 18000) but for high molecular weight PLA, PHB/PLA blends present biphasic separation [START_REF] Koyama | Miscibility of binary blends of poly[(R)-3-hydroxybutyric acid] and poly[(S)-lactic acid[END_REF] is the major phase, PLA is miscible with low molecular weigth (Mw <9400) high molecular weight PHB. To increase the compatibility between PHB and PLA, compatibilizers can be used: glycidyl methacrylate, poly(ethylene glycol) or poly (vinyl 2012). In PHB/PLA blends, PHB particles are disperse as fillers in PLA, which should improve the mechanical properties. In their study, Zhang and Thomas have observed than glass transition In general, commercial PHA filaments are mixed with PLA. For some authors, PLA and PHA [START_REF] Zhang | Blending Polylactic Acid with Polyhydroxybutyrate: The Effect on Thermal, Mechanical, and Biodegradation Properties[END_REF] For some others, PHB is miscible with low molecular weight PLA (Mw< 18000) but for high molecular [START_REF] Koyama | Miscibility of binary blends of poly[(R)-3-hydroxybutyric acid] and poly[(S)-lactic acid[END_REF]. When PHB is the major phase, PLA is miscible with low molecular weigth (Mw <9400) and immiscible with high molecular weight PHB. To increase the compatibility between PHB and PLA, compatibilizers can be used: glycidyl methacrylate, poly(ethylene glycol) or poly(vinyl are disperse as fillers in PLA, which should improve the mechanical properties. In their study, Zhang and Thomas have observed than glass transition temperature of PHB/PLA blend is always 5°C below PLA glass transition temperature (i.e. 50°C instead of 55°C for pure PLA) [START_REF] Zhang | Blending Polylactic Acid with Polyhydroxybutyrate: The Effect on Thermal, Mechanical, and Biodegradation Properties[END_REF]. Whatever PHB/PLA ratio, blends have presented brittle fracture. Both tensile stress at break and elongation at break have decreased with the PHB content. However, the PHB/PLA 25/75 blend has presented better mechanical properties than pure PLA. Tensile strength at break has reached 32 MPA with an elongation near 17%. The thermal history of the PHB/PLA blend is also important. [START_REF] Zhang | Blending Polylactic Acid with Polyhydroxybutyrate: The Effect on Thermal, Mechanical, and Biodegradation Properties[END_REF] have compared mechanical properties of quenched and annealed PHB/PLA 25/75 blends. After dynamic mechanical analysis, they have concluded that annealed PHB/PLA 25/75 blend showed better mechanical properties and higher thermal distorsion temperature than quenched PHB/PLA 25/75 blends.

To improve to mechanical properties of PHB/PLA filament, a solution is to add a plasticizer. Some of them lower the glass transition temperature and increase the elongation of the polymer. A comparison is made between a polyester-type plasticizer (lapol108) and citrate palstifiers. [START_REF] Abdelwahab | Thermal, mechanical and morphological characterization of plasticized PLA-PHB blends[END_REF] have compared mechanical properties of pure PHB and PHB with lapol 108. Their results are summarized in Table 14. They have tested with PHB/PLA blends with PHB/PLA ratio of 25/75. [START_REF] Menčik | Effect of Selected Commercial Plasticizers on Mechanical, Thermal, and Morphological Properties of Poly(3-hydroxybutyrate)/Poly(lactic acid)/Plasticizer Biodegradable Blends for Three-Dimensional (3D) Print[END_REF] have compared another type of plasticizers based on esters of citric acid. The best results were obtained with tributyl citrate (C-4) and acetyl tributyl citrate (A-4) as plasticizers with an improvement of ductile properties. However, it is interesting to note a decrease of the elastic properties of plasticized PHB/PLA filaments after 40 days. Printing has modified the mechanical properties of the initial filaments (Table 15) with a lower elongation at break.

Before printing, it is necessary to dried the filament. Balogová et al. ( 2018) have compared dried and undried PHB/PLA 25/75 blends during and after printing. Parameters of 3D printing are summarized in Table 16. Undried material showed high variable viscosity values compared to dried material. They concluded that, after drying at 80°C during 60 min, the PHB/PLA filament was more stable during printing. The printed object was degraded much less quickly than the one made with an undried filament. PHB can also be mixed with PCL. PHB and PCL are considered as immiscible in the melt and the amorphous state [START_REF] Kumagai | Enzymatic morphologies of binary blends microbial poly(3hydroxybutyrate) with poly(-caporolactone), poly(1,4-butylene adipate and ply(vinylacetate)[END_REF]. It is possible than a small amount of solubilization from one to this other occurs. For 60% PLC content and above; PLC forms a continuous matrix with PHB spherulites embedded it. For less than 60% PHB forms a continuous phase [START_REF] Avella | Review: properties of blends and composites based on poly(3-hydroxybutyrate (PHB) and poly(3-hydroxybutyrate-hydroxyvalerate) (PHBV) copolymers[END_REF]. In both cases, the mechanical properties are closed to the continuous phase. [START_REF] Kumagai | Enzymatic morphologies of binary blends microbial poly(3hydroxybutyrate) with poly(-caporolactone), poly(1,4-butylene adipate and ply(vinylacetate)[END_REF] have studied the properties of PHB/PCL blends films. They observed two glass transition temperatures that confirmed PHB and PCL are not miscible. They have concluded than the mechanical properties of PHB/PCL blend films decreased with the increased of PCL ratio until 50 wt% (Table 17). Above 50 wt% PCL ratio, mechanical properties increased with the increased of PCL ratio. To increase the miscibility between PHB and PCL, the two polymers can be melted mixed by adding peroxide. Dibenzoylperoxide (DBPO) or dicumylperoxide (DCPO) can be used [START_REF] Avella | Review: properties of blends and composites based on poly(3-hydroxybutyrate (PHB) and poly(3-hydroxybutyrate-hydroxyvalerate) (PHBV) copolymers[END_REF]. 3-4 Polymers from biomass (group 1)

Starch (TPS)

Starch is a complex natural polymer. It presents naturally in plant tissues, in form of waterinsoluble granules. The granules contain ordered region which are semi-crystalline and the granules sizes are ranging from 1 to 100 µm. It consists to a mixture of two homologous polysaccharides, amylose and amylopectin plant origin [START_REF] Keetels | Gelation and retrogradation of concentrated starch systems : 1 Gelation[END_REF][START_REF] Nessi | Multi-scale characterization of thermoplastic starch structure using Second Harmonic Generation imaging and NMR[END_REF] whose short chains are organized into double helices to form crystallites. Amylose is an essentially linear molecule consisting of (1 highly branched molecule. It contains chins of 1 linked branches. Content of amylase and amylopectin depends of the starch origin.

For native starch, thermal degradation occurs before melting, which prev implementation by melt-process. The melting of pure dry starch is 220 and its decomposition starts at 220°C. Native starch is converted into TPS under heat and shear in the presence of plasticizers (Fig.

the hydroxyl groups of starch that means to a reduction of hydrogen bonding among the So, the melting temperature of starch is lower than the degradation temperature. The formed thermoplastic starch can be melt (extrusion and injection-molding). Used plasticizers are polyols (glycerol, sorbitol, polyethylene glycol) or nitrogen compounds (urea, amines, ammonium derived) [START_REF] Mitrus | TPS and Its Nature[END_REF]. Water is often added as a destructur Halley, 2009). The combination of heat and water leads to the gelatinization of starch that lead to the disruption of the highly granular organization and the starch swells. A viscous paste is forming with destruction of most of hydrogen links. Melting and glass transition temperature are reduced. Green Polymers for 3D printing polysaccharides, amylose and amylopectin (Fig. 20), in various proportions depending of the [START_REF] Keetels | Gelation and retrogradation of concentrated starch systems : 1 Gelation[END_REF][START_REF] Nessi | Multi-scale characterization of thermoplastic starch structure using Second Harmonic Generation imaging and NMR[END_REF]. Amylopectin is the main component whose short chains are organized into double helices to form crystallites. Amylose is an essentially linear molecule consisting of (1-4)--linked D-glucan chains. Amylop highly branched molecule. It contains short chins of 1-4)--linked D-glucan with (1 linked branches. Content of amylase and amylopectin depends of the starch origin. native starch, thermal degradation occurs before melting, which prev process. The melting temperature of pure dry starch is 220 and its decomposition starts at 220°C. Native starch is converted into TPS under heat and shear in the presence of plasticizers (Fig. 21) [START_REF] Ghanbari | Preparation and characterization of thermoplastic starch and cellulose nanofibers as green nanocomposites: Extrusion processing[END_REF]. Plasticizers interact with the hydroxyl groups of starch that means to a reduction of hydrogen bonding among the So, the melting temperature of starch is lower than the degradation temperature. The formed thermoplastic starch can be melt processed with conventional technologies molding). Used plasticizers are polyols (glycerol, sorbitol, polyethylene glycol) or nitrogen compounds (urea, amines, ammonium derived) [START_REF] Mitrus | TPS and Its Nature[END_REF]. Water is often added as a destructuring agent and a volatile plasticizer (Avérou and Halley, 2009). The combination of heat and water leads to the gelatinization of starch that lead to the disruption of the highly granular organization and the starch swells. A viscous estruction of most of hydrogen links. Melting and glass transition Structure of amylose (a) and amylopectin (b).

Schematic transformation of native starch to thermoplastic starch.

33

, in various proportions depending of the Amylopectin is the main component whose short chains are organized into double helices to form crystallites. Amylose is an glucan chains. Amylopectin is a glucan with (1-6)-linked branches. Content of amylase and amylopectin depends of the starch origin.

For native starch, thermal degradation occurs before melting, which prevents its direct process. The melting temperature of pure dry starch is 220-240°C and its decomposition starts at 220°C. Native starch is converted into TPS under heat and l., 2018). Plasticizers interact with the hydroxyl groups of starch that means to a reduction of hydrogen bonding among the So, the melting temperature of starch is lower than the degradation temperature. processed with conventional technologies molding). Used plasticizers are polyols (glycerol, sorbitol, polyethylene glycol) or nitrogen compounds (urea, amines, ammonium derived) (Mitrus, ing agent and a volatile plasticizer (Avérou and Halley, 2009). The combination of heat and water leads to the gelatinization of starch that lead to the disruption of the highly granular organization and the starch swells. A viscous estruction of most of hydrogen links. Melting and glass transition Properties of TPS depend of the plasticizer/starch ratio. [START_REF] Avérous | Biocomposites based on plasticized starch[END_REF] have presented the impact of glycerol and water contents on glass transition temperature and mechanical properties of plasticized starches (Table 18). Three types of starches (potato, rice and corn) were studied in the aim to evaluate their rheological properties and their printability [START_REF] Chen | Effect of rheological properties of potato, rice and corn starches on their hot-extrusion 3D printing behaviors[END_REF]. Their amylose content and moisture are summarized in the Table 19. Steady shear rheological measurements were conducted. The conclusion was rice starch was much rigid than potato and corn starches and would show a gelling behavior during the heating. Gelling behavior is critical for the printing medium to be dispensed as a free-standing filament so, rice starch was more preferable as printing medium. However, the three types of starches are printable according to the conditions summarized in Table 19. They have excellent printability, shape retention and resolution.

The chemical structure of starch is composed of numerous hydroxyl group (OH) which is referred to as a hydrophilic group, so the thermoplastic starch is more sensitive to water in ambient condition and lose its properties. Ageing of TPS depends of the temperature. At temperature inferior to glass transition temperature, physical ageing occurs versus time and TPS presents a densification. At a temperature above Tg, retrodegradation phenomena modify the crystallinity and plasticizer molecules rearrangements (Avérou and Halley, 2009). To reduce hydrophilic character of TPS, structure of starch can be modified by acetylation but it leads to lower mechanical properties. Another solution is mixing plasticizer starch with another biopolymer: PCL, PLA, PHBV [Avérou et al., 2000, Kalambur and[START_REF] Kalambur | on overview of starch-based plastic blends from reactive extrusion[END_REF].

Cellulose

Cellulose is the most abundant renewable biopolymer on Earth. It is the main structural component of plant cell wall (35 to 50%). Cellulose consists of a repetition of -Dglucopyranose units (Fig. 22), that are covalently linking through acetal function between the C1 carbon atom and the equatorial -OH group of C4 [START_REF] Dai | 3D printing using plantderived cellulose and its derivatives : a review[END_REF]. Cellulose is insoluble in water and in common organic solvents due to the preferential formation of intra-and intermolecular hydrogen bond [START_REF] Peng | The evolution behavior and dissolution mechanism of cellulose in aqueous solvent[END_REF]. The manufacturing of cellulose is done in several steps. The first step is the separation between cellulose and hemicellulose/lignin by Cellulose is infeasible for extrusion because its degradation temperature is inferior of its melting temperature [START_REF] Wang | Effect of Polymer-Filler and Filler-Filler Interactions on Dynamic Properties of Filled Vulcanizates[END_REF]. Cellulose glass transition temperature is in range of 200 -230°C and thermal degradation starts 260°C [START_REF] Oksman | Review of the recent developments in cellulose nanocomposite processing[END_REF]. However, nanocellulose hydrogels showing a shear-thinning behavior might be considered as extrudable precursors for 3D printing. Cellulose is promising ink which focuses on pharmaceutical products and biomedical devices. Its characteristics are excellent such as low density, biodegradability, ecofriendlyness, toxicological harmlessness.

Other characteristics are high specific stiffness, good thermal and acoustical insulation. For these reasons, lignocellulose materials were mainly used as reinforcement component to improve the mechanical properties. The hydrophilicity and less thermal stability of cellulose micro/nanoparticles restrict the range of choice of polymer matrix [START_REF] Oksman | Review of the recent developments in cellulose nanocomposite processing[END_REF]. The mechanical properties of PLA, PCL and PA were modified using cellulosic fibers [START_REF] Bai | A novel high mechanical strength shape memory polymer based on ethyl cellulose and polycaprolactone[END_REF]. For example, Murphy and Collins have used microcrystalline cellulose (MCC) as reinforcement in PLA 3D printing composites [START_REF] Murphy | Microcrystalline cellulose reinforced polylactic acid biocomposite filaments for 3D printing[END_REF]. MCC was surface modified using a titanate coupling agent (Lica 38) to improve its compatibility with PLA matrix. Only 3wt% of MCC increased significantly the crystallinity and the storage modulus of the composite.

Low amount of nanocellulose is used, about 1-5% as reinforcement phase. The process for nanocellulosic preparation is complex and expensive, so the application of such composites is still restricted [ [START_REF] Wang | Preparation of 3D printable micro/nanocellulose-polylactic acid (MNC/PLA) composite wire rods with high MNC constitution[END_REF]. However, Wang et al. have shown that it is possible to add until 30% of cellulose fibers in a PLA matrix using a mixing of micro and nano cellulose. To increase the interface compatibility between cellulose and PLA, silane (KH550) was used as a coupling agent (1%). Incorporation of PolyEthylene Glycol (PEG) in PLA increased the chain mobility and improved the elongation at break. For mixture of 30% cellulose, 65%PLA and 5% PEG, elongation at break was 12%, tensile strength was 59.7 MPA and flexural strength was reached 50 MPa. Cellulose can be chemically modified to obtain a thermoplastic. The best transformation is esterification rather than etherification. The most common cellulose esters in this field are cellulose acetate (CA), cellulose acetate propionate (CAP) cellulose butyrate (CAB), nitrocellulose [START_REF] Dai | 3D printing using plantderived cellulose and its derivatives : a review[END_REF]. Its hypoallergenic properties and its mold resistance make it possible prints parts that will be in contact with the skin. Cellulose acetate is natural resistance to UV and to chemical agents such as chlorine. Its properties are summarized in Table 20. Cellulose has an excellent print quality with or without hot bed and presents no shrinkage and warping. During the print, acetate evaporates but some advice to immerge the final piece in a sodium hydroxide to remove the acetate and restore the cellulose with its full network of hydrogen bonds (www.sculpteo.com/blog/2018/12/12/faster-3d-printer-and -antibactterials-3d-printedcellulse-mit-is-going-further/). [START_REF] Paggi | Structure and mechanical properties of 3D-printed cellulose tablets by fused deposition modeling[END_REF] have studied the influence of printing parameters on mechanical properties of the piece (Table 21). They used corn starch/cellulose acetate (SCA blend 50/50 wt%, Mater-Bi Y1010). Filament was produced by hot melt extrusion with a single screw extruder. The optimum conditions were: extrusion temperature 175°C, 60 rpm. 2.16 mm 3. s -1 2.43 mm 3. s -1 2.16 mm 3. s -1 2.43 mm 3. s -1 437. 72 ± 44.76 431.00 ± 38.03 475.53 ± 4.04 428.10 ± 19.87 41.19 ± 6.7 48.13 ± 6.18 53.61 ± 9.57 40.74 ± 10.72 Nozzle size: 0.6 mm, layer height: 0.15 mm, print speed: 30 mm.s -1

Lignin

Lignin is a biopolymer commonly found in plant cell walls linked to carbohydrates. It is the second most abundant macromolecule on Earth's surface after cellulose. Natural lignin extraction according different processes can give technical lignins with specific Mw, structure, and properties. The pulp and paper industry makes use of the cellulose fraction leaving lignin behind as a by-product with low-value. Today, although lignin raises scientists' interest for a wide range of applications, only 2 % of industrial lignin is valorized. Yet, with its high carbon content and its variety of functional groups (aromatic rings, methoxy, hydroxyl, etc.), lignin has the potential to substitute to some fossil-based materials, therefore playing a key-role in the development of environmental-friendly products. Some achievements can be reported for 3D difficulties in melt processing rough surface of the filament when used fibers is possible from swollen gels carbon fibers from lignin fibers extrusion line were developed especially for extruding lignin fibers. In this process, lignin monofilaments can be crosslinked for a further conversion to carbon fibers (http://www.xplore-together.com/products/lignin Lignin was not used as a mono other polymers, melt processing difficulties were overcome. Addition of fibers could enhance 3D-printed specimens' stiffness. scavenger) and fire retardant. The use of lignin fillers in 3D printing would have the potential to reduce flammability and preserve the filaments from printing market is the reduction of filaments cost since lignin i raw material.

To improve melting of the filaments for 3D other thermoplastic polymers. Fused filament fabrication (FFF) process is generally used. Some processing parameters of d of lignin plus poly(lactic acid) (PLA) or biodegradable. Materials made of lignin plus polyolefins or (ABS) are not biodegradable.

Blending lignin with PLA gives possibilities in the area of 3D homogeneous filaments could sometimes be challenging since a good compatibility between lignin and PLA was not always exhibited. Gkartzou when 3D-printing specimens [1 Green Polymers for 3D printing coumaryl alcohol (a), coniferyl alcohol (b) and sinapyl alcohol (c) One acknowledged obstacle to valorization is lignin's complex and heterogeneous structure.

Lignin is formed by the polymerization of 3 oxidized monolignols, ie. p coniferyl alcohol, and sinapyl alcohol (Fig. 23) giving rise to a variety of linkages 4 linkage which is present in larger quantity are ma Some achievements can be reported for 3D-printing of lignin. The main limitations are difficulties in melt processing the lignins (Nguyen et al., 2018a), their brittle behavior, or a rough surface of the filament when used as a filler (Ghartzou et al., 2017) from swollen gels or melts. main application is the preparation of fibers. Some apparatus such as Xplore Instruments BV eloped especially for extruding lignin fibers. In this process, lignin monofilaments can be crosslinked for a further conversion to carbon fibers together.com/products/lignin-line/about-micro-fiber-Lignin was not used as a mono-constituent for 3D-printing. But when blending it with other polymers, melt processing difficulties were overcome. Addition of fibers could printed specimens' stiffness. Lignin is known as anti-oxidant (free radical scavenger) and fire retardant. The use of lignin fillers in 3D printing would have the potential preserve the filaments from oxidation. An interesting fact for 3D printing market is the reduction of filaments cost since lignin is a very cheap and abundant To improve melting of the filaments for 3D-printing, lignin was blended or formulated with other thermoplastic polymers. Fused filament fabrication (FFF) process is generally used. Some processing parameters of different studies are presented in Table 22 of lignin plus poly(lactic acid) (PLA) or polyhydroxybutyrate (PHB) are bio biodegradable. Materials made of lignin plus polyolefins or acrylonitrile Blending lignin with PLA gives possibilities in the area of 3D-printing. Obtaining homogeneous filaments could sometimes be challenging since a good compatibility between lignin and PLA was not always exhibited. Gkartzou et al. observed Kraft lig [128]. One acknowledged obstacle to valorization is lignin's complex and heterogeneous structure.

Lignin is formed by the polymerization of 3 oxidized monolignols, ie. p-coumaryl alcohol, to a variety of linkages. Ether 4 linkage which is present in larger quantity are mainly cleaved during printing of lignin. The main limitations are , their brittle behavior, or a (Ghartzou et al., 2017). Extrusion of lignin main application is the preparation of s such as Xplore Instruments BV lignin eloped especially for extruding lignin fibers. In this process, lignin monofilaments can be crosslinked for a further conversion to carbon fibers by pyrolysis -line-copy). printing. But when blending it with other polymers, melt processing difficulties were overcome. Addition of fibers could oxidant (free radical scavenger) and fire retardant. The use of lignin fillers in 3D printing would have the potential . An interesting fact for 3Ds a very cheap and abundant printing, lignin was blended or formulated with other thermoplastic polymers. Fused filament fabrication (FFF) process is generally used.

22. Materials made (PHB) are bio-based and acrylonitrile-butadiene-styrene printing. Obtaining homogeneous filaments could sometimes be challenging since a good compatibility between observed Kraft lignin aggregation Observation of lignin-PLA interface by SEM showed a separation lignin's aggregates and the PLA matrix. This resulted in nozzle clogging for highest lignin loading and in roughness of the filament' surface compared to pure PLA.

Therefore, the blend containing 5 wt.% wt Kraft lignin was recommended for 3D-printing (Ghartzou et al., 2017). The authors suggest that lignin could affect nucleation and crystal growth in the PLA phase with the formation of thicker crystalline structures during PLA cold crystallization. 3R3D Technology Materials developed its Lignina® product made of PLA with 10 wt.% micronized lignin powder showing in this case, a smooth filament' surface (https://www.3r3dtm.com/producto/lignina-filamento-3d-175-mm-0-75-kg ). FPinnovations investigated the use of a coupling agent to improve lignin-PLA homogeneity by chemically bonding them [START_REF] Li | Lignin-containing filaments for fused deposition modelling 3D printing[END_REF]. The authors exposed that they could reduce agglomeration of lignin particles allowing the fabrication of 3D-printed specimens that contain up to 20 wt.% lignin. Lignin addition did not impact tensile strength properties. Obielodan et al. claimed that they successfully filled a PLA matrix with organosolv lignin up to 25 wt.%. With organosolv lignin, no agglomeration effect was observed. Very good processability of the blend as well as smooth surface of the extruded filament were reported [START_REF] Obielodan | Development of a Thermoplastic Biocomposite for 3D Printing[END_REF]. As for [START_REF] Gkartzou | Polycaprolactone-and polycaprolactone/ceramic-based 3D-bioplotted porous scaffolds for bone regeneration: A comparative study[END_REF] tensile strength of specimens containing lignin decreased. Contrary to Kraft lignin, organosolv lignin improved the material's ductility. With Kraft lignin a decrease of elongation at break and a brittle behavior was reported. Domínguez-Robles et al. ( 2019) used a coating method on PLA with castor oil so that they could add 0.5 to 3 wt.% lignosulfonates powder on the surface. The goal of this study was to highlight antioxidant properties brought by lignosulfonates for biomedical purpose. Extrusion and 3D-printing of such material was demonstrated.

Lignin could also be used as filler in biodegradable polyesters such as PHB (polyhydroxybutyrate). Scientists of Scion institute produced filaments that contained up to 20 wt.% biorefinery lignin produced by the enzymatic saccharification of pretreated Pinus radiata pulp in a PHB matrix [START_REF] Vaidya | Integrating softwood biorefinery lignin into polyhydroxybutyrate composites and application in 3D printing[END_REF]. Lignin addition induced a melt rheology suitable for 3D-printing, especially for 20 wt.% lignin ratio. The authors noted after FTIR and SEM analyses that lignin particles are more likely to be present in the filament core rather than in the surface that is PHB rich. Thermal analyses assessed that lignin acted as non-reacting filler. Yet, reduced shrinkage of printed specimens and better inter-layers adhesion were highlighted in the presence lignin compared to neat PHB. Consequently, wrapping was also reduced in the presence of lignin-filler, making 3D-printing much more convenient. Shrinkage moderation could be explained by the formation of micro-voids in the microstructure that reflects interfacial incompatibility between lignin and PHB. [START_REF] Nguyen | Confinement and alignment of miscible amorphous lignin in polypropylene and polyethylene by 3D-printing and melt-spinning[END_REF] prepared composites with lignin and polypropylene (PP) or polyethylene (PE) matrix. They investigated the influence of lignin on 3D-printing, melt-spinning, and mechanical performances of the composites. It was observed that amorphous lignin chains were aligned between PP and PE crystalline zones and that PP and PE crystallization rate was moderated in the presence of lignin. Some materials containing lignin and fibers were developed. Natural fibers are often used for reinforcing 3D-printed materials. They are hydrophilic and relatively cheap but sensitive to temperature changes. Natural fibers confer a wood-like appearance to the product, especially when combined with lignin. Carbon fibers can also be added when high mechanical performances are looked for.

The German company Tecnaro launched more than 3,500 composite products containing lignin and natural fibers under the tradenames ARBOBLEND®, ARBOFILL®, and ARBOFORM®. All the formula can be processed by injection-molding, and some of them can be 3Dprinted(https://www.tecnaro.de). ARBOFORM® materials are especially filled in lignin. They exhibit a thermoplastic behavior as well as potentially interesting melt characteristics towards printing [START_REF] Nägele | Arboform® -A Thermoplastic, Processable Material from Lignin and Natural Fibers, in Chemical Modification, Properties, and Usage of Lignin[END_REF][START_REF] Nedelcu | Microindentation and differential scanning calorimetry of ''liquid wood[END_REF]. Mechanical, thermal, and rheological properties of typical materials are reported in A relatively high amount of lignosulfonates, i.e. 49 wt.% of the whole compostion, was therefore valorized. The hydrogels were 3D-printed, and pyrolyzed to give a material with high electrical conductivity [START_REF] Shao | Use of lignocellulosic materials and 3D printing for the development of structured monolithic carbon materials[END_REF]. A good adhesion between layers was obtained. After pyrolysis, crosssections of the materials revealed porosities of various diameters up to 50 µm resulting in a low-density of 0.74 g / cm 3 . Mechanical properties remained interesting with an elastic modulus of 6.6 GPa for the sample submitted to a 900°C temperature.

Researchers of Oak Ridge Laboratory successfully 3D-printed a material containing up to 40 wt.% organosolv hardwood lignin (Nguyen et al., 2018b). They made use of acrylonitrilebutadiene-styrene (ABS) polymer matrix with its well-known properties suitable for 3Dprinting. Hydrogen bonding between lignin and ABS was reported improving interfaces quality. At high lignin loading composites were relatively brittle with an ultimate tensile strain of 1.2% (Nguyen et al., 2018c). The authors believed that low molecular weight lignins enhance the brittleness in ABS matrix. They also tried to explain stiffness variations at macro-scale by the difference in lignin structure / functional groups. Rigid segments like biphenyl and biphenyl ethers of Kraft softwood lignin increased the thermal transition temperature causing a flow resistance detrimental to 3D-printing (Nguyen et al., 2018a). These phenolic units also provided stiffness enhancement of the composites. On the contrary, aliphatic ether groups, aliphatic chains, and β-O-4 linkages within hardwood lignin favored the 3D-printing characteristics. In order to reduce the composites brittleness, acrylonitrile butadiene rubber (NBR) compatibilizer was added to the mix. Hydrogen bonding as well as covalent bonding were formed between lignin hydroxyl groups and NBR nitrile groups [START_REF] Nguyen | Anacrylonitrilebutadiene-lignin renewable skin with programmable and switchable electrical conductivity for stress/strain-sensing applications[END_REF]. The authors explained that free radicals from the less stable groups may form at high temperatures in the one-pot mix allowing the formation of new covalent bonds. Plasticity was enhanced thanks to NBR, therefore it was possible to add up to 40 wt.% lignin. Because of their intrinsic properties, carbon fibers reinforced the composite but they also acted as bridges between printed layers. Finally, carbon fibers may breakdown lignin particles during the mixing preparation step. Thus, carbon fibers contributed to a better adhesion of consecutive printed layers. The overall material exhibited strong mechanical performances with tensile strengths up to 65 MPa and elastic modulus of 2.6 GPa. Printing parameters are presented in Table 22.

Other materials with excellent 3D-printability were developed by mixing hardwood lignin with nylon 12 (Nguyen et al., 2018a). The dispersed lignin particles reduced the melt viscosity, improving the flow characteristics of nylon. Material containing 12 wt.% carbon fibers and 40 wt.% lignin had a tensile strength over 80 MPa and a tensile modulus of 6 GPa. Mixtures containing up to 60 wt.% lignin were 3D-printed.

Chitosan

Chitosan (CS) is a polysaccharide made up with two different monomers Nacetylglucosamine and glucosamine linked through β-1,4-glycosidic bonds (Fig. 24) [START_REF] Brasselet | Modification of Chitosan for the Generation of Functional Derivatives[END_REF]. This polysaccharide is derived from another polysaccharide named chitin which is one of most earth abundant polysaccharide after cellulose. Chitin is present in a huge range of living organisms like lobsters, shrimps but also insects and fungi. Two main allomorphic forms of chitin exist, α and β-chitin. A third form γ-chitin which is a combination of α and β-chitin [START_REF] Kaya | Differentiations of Chitin Content and Surface Morphologies of Chitins Extracted from Male and Female Grasshoper Species[END_REF]. Chitin is insoluble in water or other common solvents however it can be converted into CS under hot and alkaline conditions. After this treatment, chitosan became soluble in slightly acidic medium which allows its formatting and its modification more easily. Chitosan and chitosan derivatives have many applications, one of the most promising opportunities is its use as biomaterial for production of tissues, skin and other materials. Several studies have shown that it's possible to use chitosan inks for bio 3D printing. Wu et al. have shown in their study that it was possible to produce chitosan inks adapted for 3D printing [START_REF] Wu | Processing and properties of chitosan inks for 3D printing of hydrogel microstructures[END_REF]. In this case, production of chitosan inks was made up by dissolving chitosan in an acidic medium composed of 40% acetic acid, 40% water and 20% lactic acid, with various chitosan concentrations from 6 to 10 % wt. After centrifugation at 3000 rpm to remove air bubbles CS solution was placed into printer cartridge and extruded through 3D printer micronozzles of 150 µm at a speed of 1 to 2 mm/s. Wu et al. have demonstrated that the optimal chitosan concentration for production of chitosan scaffold was around 8% due to rheological properties of chitosan. Gelation process can be achieved via neutralization in NaOH [START_REF] Bergonzi | Study of 3D-Printed Chitosan Scaffold Features after Different Post-Printing Gelation Processes[END_REF] or KOH [START_REF] Liu | Preparation and Properties of 3D Printed Alginate-Chitosan Polyion Complex Hydrogels for Tissue Engineering[END_REF] but also by Na 2 CO 3 , ammonia vapours or by freeze gelation. Some studies, shown the possibility to make composites with chitosan in association of other polymers or bioplastics to reinforce their properties or their mechanical resistance. A brief summary, of some of the most important chitosan-based composites is presented at Table 24. Some of them have never been used yet in 3D printing processes.

For instance, for the preparation of PLA-g-MA/CS composites chitosan is dried several times at 50°C and mixed with PLA grafted maleic acid (PLA-g-MA) at 180°C until melting, then the mixture is slowly extruded and cooled to obtain 3D printed strips- [START_REF] Tanase | PLA/chitosan/keratin composites for biomedical applications[END_REF]. Various mass ratio could be used from 5/95 to 20/80 (chitosan/PLA-g-MA). Better mechanical and antimicrobial properties were found for 20/80 mass ratio. PLA grafted maleic acid allow a better compatibility with CS [START_REF] Correlo | Properties of melt processed chitosan and aliphatic polyester blends[END_REF].

Fig. 24 Structure of chitosan

Table 24 Main green chitosan composites for potential applications in 3D printing

For medical applications, chitosan can be also blended with PCL [START_REF] Thuaksuban | Biodegradable polycaprolactone -chitosan three-dimensional scaffolds fabricated by melt stretching and multilayer deposition for bone tissue engineering ; assessment of the physical properties and cellular response[END_REF]. In this case, chitosan microparticles were mixed with PCL but immiscibility of PCL and CS was observed. It is the same behaviour with PBS, PBAT and PBSA (Harris et al., 2019) [155]. PCL-20%CS blend obtained optimum results in term of physical properties. Chitosan could also be blended with hydroxyapatite and biocompatible plastics such as PVC, PP to reinforce physical properties of 3D printed scaffolds [START_REF] Nishant | Biocompatible Thermoplastic Composite Blended With HAp and CS for 3D Printing in Reference Module in Materials Science and Materials Engineering[END_REF]. As a conclusion, chitosan is a promising polymer for production of biocompatible and biodegradable green materials with enhanced mechanical properties and strong biological activities.

Conclusion

3D printing has well earned its place in the fabrication of green material using green polymers and is probably at the first steps of its industrial development. However even if this technology is currently becoming an industrial tool in the field of synthetic materials (or metallic material for additive manufacturing) its use remains very limited applied to green polymers for the construction of 3D bio-based and/or biodegradable materials despite the abundance in nature of biopolymers and the progress of the green chemistry applied to polymers. This observation can be explained as the majority of low costs biopolymers such as polysaccharides have low printing performances. Moreover, other non-natural green polymers with better printing ability have often a cost too high for material applications even in the 3D printing area. The formulation of biopolymers as chemically modified biopolymers or their use in combination with other polymers obtained from green chemistry (polycaprolactone for example) having better printing properties could be the first step of future developments. In parallel, the increase in performance of new generation of 3D printers will probably open the way to the gradual utilization of biopolymers. The requirements of the regenerative medicine for patient specific prosthetic devices, biocompatible hydrogels and dried scaffolds with high levels of biocompatibility and/or biodegradability will be probably the greatest opportunity for increase the market share of green polymers. If this field of application could constitute an opportunity in a near future for 3D printing applied to green polymers, the contribution of 3D printing and green polymers to the urgent and necessary eco-industrial revolution of the 21 st century appears as evident on longer terms. Indeed, in 2050, 9 billion of people will be living on earth consuming considerable amounts of raw materials. Many of these resources and notably fossil fuel and metals are not infinitely available and limited. In contrast, biosourced raw materials including the major part of green polymers are renewable and can fulfill the needs of next generations if we succeed to change for a bio-based economy. In this economy the promise of 3D printing with green polymers is mainly its ability to create a sustainable and circular manufacturing process to manufacture goods locally when people need them. For that, 3D printing have to use renewable bioresources such as green polymers but also renewable energies and not fossil ones, the development of 3D printing, green polymers and renewable energy going hand in hand.

Fig. 3 .

 3 Fig. 3. Cartridge heater for filament melting (

  4.a) or combining 3 linear vertical other exotic mechanisms are proposed (e.g., polar, scara, robotic arm), but they remained part of more conceptual b. ) and thermistor for temperature a. b. Fig. 4 Cartesian (a) and delta (b) systems of most used 3D printers.

Fig. 5

 5 Fig. 5 Plate with slots for orthopedic use, designed with Autodesk Inventor 2020.

Fig. 6

 6 Fig. 6 Roman lamp from third century (where loss of details can be noticed,

Fig. 7 .

 7 Fig. 7. Errors (holes) found with the Meshmixer software in the 3D scanned surface.

Fig. 8 .

 8 Fig. 8. Large warping effect on a PLA 3D printed cuboid due to shrinking and poor adhesion.

Fig. 9 .

 9 Fig. 9. Under

Fig 12 .

 12 Fig 12. Polycaprolactone polymer chain.

  and a glass transition temperature between 50-80°C while Poly (D,L-lactic acid) (PDLLA) has a glass transition temperature between 55 -65°C (Madhavan Nampoothriri al al., 2010).

Fig 14 .

 14 Fig 14. PLLA manufacturing process.

Fig 16 .

 16 Fig 16. Synthesis of Succinic Acid.

Fig 17 .

 17 Fig 17. Synthesis of PBS from succinic acid and 1,4-butanediol.

Fig 18 .

 18 Fig 18. of PBAT.

A

  a coupling agent. Silane coupling is tetrathoxysilane (TEOS) . For acylation of the hydrophilic hydroxyls, Shibata et al have selected butyric because the carbon number of the acid components in ester groups of PHBV is the . Maleated polymer are used as reactive compatibilizer. Effectiveness of maleaic anhydride grafted PHBV (PHBV-g-MA) was Wu et al. (2017) have MA/palm fiber filament. Tensile strength was 7 MPa and Young modulus . They have concluded than andidate for 3D printing

Fig 20 .

 20 Fig 20. Structure of amylose (a) and amylopectin (b).

Fig 21 .

 21 Fig 21. transformation of native starch to thermoplastic starch.

Fig. 22

 22 Fig. 22 of cellulose

Fig. 23

 23 Fig. 23 Structure of p-coumaryl alcohol (a), coniferyl alcohol (b) and sinapyl alcohol (c)

  37 coumaryl alcohol (a), coniferyl alcohol (b) and sinapyl alcohol (c)

Table 1 .

 1 Commercial polymers and recommended 3D properties (based on commercial Polymaker data, average values).

	Polymer	Nozzle	Printing	Build platform	Cooling	Filament	Young	Tensile	Bending	Charpy
		temperature,	speed,	temperature,	fan	drying	Modulus,	strength,	strength,	impact
		[ o C]	[mm/s]	[ o C]		conditions	[MPa]	[MPa]	[MPa]	strength,
											[kJ/m 2 ]
	ABS	245-265	30-50	90-100	OFF	8h		2174	33	59	13
						at 80	o C
	PLA	190-230	40-60	25-60	ON	8h		1879	28	48	12
						at 80	o C
	PETG	230-240	30-50	70-80	ON	8h		1472	32	54	5
						at 70	o C
	PC	250-270	30-50	90-105	OFF	8h		2048	60	94	25
						at 80	o C
	ASA	240-260	30-50	75-95	OFF	8h		2379	44	73	10
						at 80	o C
	TPU	210-230	20-40	25-60	ON	12h	-	29	-	-
						at 70	o C
	Nylon	250-270	30-50	25-70	OFF	12h	2223	66	97	10
						at 80	o C

Table 2

 2 Print parameters investigated an suitable for PCL(1) :Hendrikx et al., 2016; (2) :Kosik-Kozioł et al., 2019; (3) :Gómez-Lizárraga et al., 2017; (4)

Table 3

 3 Summary of PCL properties[START_REF] Bossard | Polycaprolactone / bioactive glass hybrid scaffolds for bone regeneration[END_REF] Chun et al., 2015; Labet and Thielemans, 2009;[START_REF] Kosik-Kozioł | Surface Modification of 3D Printed Polycaprolactone Constructs via a Solvent Treatment: Impact on Physical and Osteogenic Properties[END_REF][START_REF] Salin | Kinetic analysis of high-resolution TGA variable heating rate data[END_REF][START_REF] Wang | Biomimetically Ornamented Rapid Prototyping Fabrication of an Apatite-Collagen-Polycaprolactone Composite Construct with Nano-Micro-Macro Hierarchical Structure for Large Bone Defect Treatment[END_REF]

	Density				1.07 -1.2
	Crystallinity				up to 70%
	Lattice parameter			a = 7.496 Å, b=4.974 Å, c (fibre axis) =17.297 Å
	Number average molecular weight Mn = 530 -630 000 g/mol
	Tensile strength				 = 4 -780 MPa
	Tensile modulus				E = 210 -440 MPa
	Elongation at break			 = 20 -1000 %
	Water contact angle			74 -79°
	Glass transition Tg			(-65 °C) -(-60 °C)
	Melting point Tm			56 -65 °C
	Decomposition temperature		250 -350 °C
	Crystallization temperature Tc		20 -37°C
	Theoretical heat of fusion for 100%	136 J/g
	crystalline PCL
	Degradation	time	in	the	months to years
	environment
	Resorption time in the body		years
	Solvents				highly soluble in chloroform, dichloromethane, carbon
					tetrachloride, benzene, toluene, cyclohexanone, 2-
					nitropropane, THF; slightly soluble in acetone, 2-butanone,
					ethyl acetate, dimethylformamide, aceto-nitrile
	Non-solvent				alcohols, petroleum ether, diethyl ether, water
	Commercial brand names			Capronor, CAPA, Polymorph, Plastimake, NiftyFix,
					Protoplastic,	InstaMorph,	Shapelock,	ReMoldables,
					Plastdude, TechTack…

Poly (lactic acid) or polylactide (PLA) is probably the most used of green polymers for filaments. This polymer is vegetable origin (corn starch, beets…), biodegradable and nontoxic. It is used in various applications: biomedical, packaging, textile fibers, technical items… PLA is hydrophobic and water resistance. All PLA are insoluble in water, some alcohols and alkanes. In general, PLA is soluble in dioxane, acetonitrile, chloroform, methylene chloride, 1,1,2-trichloroethane and dichloroacetic acid

[START_REF] Nampoothiri | An overview of the recent developments in polylactide (PLA) research[END_REF]

).

  . Among the commercial filaments, Nature works PLA has a glass transition temperature of 55-56 °C et Biomer L9000 PLA has a VCAT softening point of 56°C[START_REF] Nampoothiri | An overview of the recent developments in polylactide (PLA) research[END_REF]. The disavantage of PLA is its low heat distortion temperature with behavior above 60°C.Melt flow rate (MFR) depends of PLA. For example, PLA Biomer L9000 (Biomer, Germany) has a MFR in range 3-6 g/10 min. MFR forNature Works PLA  (NatureWorks LLC is between 2.3 and 4.3 according to Madhavan[START_REF] Nampoothiri | An overview of the recent developments in polylactide (PLA) research[END_REF]. while it is estimated to 6g/10 min (2,6 kg, 210°C) according to

Table 4

 4 Mechanical properties of Poly(L-lactide), annealed Poly(L-lactide) and Poly(D-L Lactide) specimens[START_REF] Perego | Effect of molecular weight and crystallinity on poly(lactic acid) mechanical properties[END_REF]

	Sample	Low Mw PLLA	High Mw PLLA	Low Mw annealing PLLA	High Mw annealing PLLA	Low Mw PDLLA	High Mw PDLLA
	Mw	23 000	67000	20 000	71 000	47 500	114 000
	Yield
	strength	-	70	-	70	49	53
	(MPa)
	Tensile
	strength	59	59	47	66	40	44
	(MPA)
	Yield
	elongation	-	2.3	-	2.0	1.7	1.5
	(%)
	Elongation at break (%)	1.5	7.0	1.3	4.0	7.5	5.4
	Young modulus	3550	3750	4100	4150	3650	3900
	Flexural
	strength	64	106	51	119	84	88
	(MPA)
	Maximum strain (%)	2.2	4.7	1.6	4.6	4.8	4.2
	Modulus of
	elasticity	3650	3650	4200	4150	3500	3600
	(MPa)

Table 5

 5 Influence of nature and thermal treatment on unnotched and notched Izod Impact[START_REF] Garlotta | A Literature review of Poly(Lactic Acid)[END_REF].

		L-PLA	Annealed l-PLA	D,L-PLA
	Unnotched Izod Impact (J/m)	195	350	150
	Notched Izod Impact (J/m)	26	66	18

Table 6

 6 Mechanical properties of recycled PLA and Recycled PDA/PLA filaments(Zhao et al.improve adhesion between two separated polymer surfaces is to use mending agent.[START_REF] Davidson | Design Paradigm Utilizing Reversible Diels-Alder Reac ons to Enhance the Mechanical Proper es of 3D Printed Materials[END_REF] have proposed a furanmaleimide Diels Alder agent. While cross-section of pure PLA parts has shown a success of filaments layers with spaces at interfilamentous junction, remendable PLA has presented observable voids and interfilamentous boundaries.

	,

Table 7

 7

	3D printing commercial PLA filaments specifications
	Density	1.2 -1.45 g.cm -3
	3D printing temperature	180 -220 °C
	3D printing speed	40 -100 mm.s -1
	Heated bed temperature	20 -60 °C

Table 8

 8 Properties of typical grades of bionolle PBS[START_REF] Rudnik | Compostable polymer properties and packaging applications in Plastic Film in Food packaging[END_REF] Yang et al. remarked that few studies are made on PBS because of the difficulty of forming monofilament (Ou-[START_REF] Ou-Yang | Preparation and characterization of poly(butylene succinate)/polylactide blends for fused deposition modeling 3D printing[END_REF]

				Bionolle TM 1000		series	Bionolle TM 3000 series
				(PBS)			(PBSA)
	Properties		1001 MD	1020 MD	3001 MD	3020 MD
	Density (g.cm -3 )		1.26			1.23
	Degree of crystallinity		35-45		20-35
	Melting point (°C)		114			94
	Glass transition temperature (°C)		-32			-45
	MFR (g/10min)		1-3	20-34	1-3	20-34
	Table 9 Mechanical properties of PBS/PLA and PBSA/PLA blends bars obtained by injection
	molded (Xu et al., 2010)
		Elongation	at	Tensile strength	Tensile strength	Izod	impact
		break (%)		at yield (MPa)		at break (MPA)	strength (J/m)
	PBS	275±3.5		35±1		29±2	73±28
	PLA/PBS 60/40	112±82		39±3		12±5	29±4
	PLA/PBS 80/20	277±47		45±4		22±9	28±2
	PBSA	476±124		30±1		28±3	132±32
	PLA/PBSA 60/40	225±64		37±1		17±11	39±4
	PLA/PBSA 80/20	319±17		49±1		23±8	24±2
	Q. Ou-

Table 10

 10 Parameters for 3D printing of PBS/PLA material in Q.Ou-Yang et al. study (2018)

	Printing parameter	Amount
	Temperature	200-230 °C
	Speed	40 mm.s -1
	Nozzle diameter	0.4 mm
	Layer height	0.1 mm
	Heated bed temperature	Room temperature
	3.2.3. Polybutyrate adipate terephthalate (PBAT)
	Polybutyrate adipate terephthalate

Table 11

 11

	Thermal characteristics of PBS, PBA and PBAT
	polyester	Tg (°C)		Tm (°C)
	PBS	-36		114
	PBA	-68		55
	PBAT	-28		114
	Table 12 Printing parameters used by Singamneni et al. (2018)
	Material	Extrusion	Bed Temp.	Spindle	Print	Strand
		Temp. (°C)	(°C)	Velocity	Velocity	Gap
				(RPM)	(mm/min)	(mm)
	PBAT (100wt%)	120	110
	PBAT/ WF (95/5wt%)	135	110
	PBAT/ WF	130	110
	PBAT/ WF (80/20wt%)	140	110
	3-3 Polymers from microorganisms obtained by extraction (group 2):
	Polyhydroxyalcanoates (PHA)

Table 13

 13 Various types of PHA and PLA

	Fig 19.

Table 14

 14 Mechanical properties of pure PHB and PHB/Lapol 108(Abdelwahab et al.

	, 2012)

Table 16

 16 Parameters for 3D printing of PHB/PLA material inBalogová et al. study (2018)

	Printing parameter		Amount
	Temperature			180-230 °C
	Pressure			5 Bars
	Speed			3 mm.s -1
	Interval			10 minutes
	Nozzle diameter		0.6 mm
	Needle offset			0.48
	Volume of high temperature cartridge	4 g
	Table 17 Thermal and mechanical properties of PHB/PLC blend films (Kumagai and Doi,
	1992)
	PHB/PCL	Tg (°C)	PHB Tm	PCL Tm	Young's	Tensile	Elongation
	Blend			modulus	strength	at break
	composition				(MPa)	(MPa)	(%)
	(wt% ratio)
	100/0	1	179	-	1560	38	5
	77/23	4	178	59	730	21	9
	49/51	2	178	59	110	4	18
	25/75	1	178	60	220	8	11
	0/100	-70	-	60	220	15	24

Table 18

 18 Thermal and physical properties of plasticized starches in function of glycerol and water contents(Avérou and Halley, 2009)

	Initial	Glycerol/dry	Water	Density Tg	transition	Modulus	Max	Elongation
	formulation	starch ratio	(wt%)		(°C)	(°C)	(MPa)	tensile	at break
		(wt/wt)						strength
								(MPa)
	S74G10W10	0.14	9	1.39	43	63	1144	21.4	3
	S75G18W12	0.25	9	1.37	8	31	116	4.0	104
	S67G24W9 S65G35W0	0.35 0.50	12 13	1.35 1.34	-7 -20	17 1	45 11	3.3 1.4	98 60

Table 19

 19 Amylose and moisture contents of corn starch, potato starch and rice starch and conditions of printing (concentrations and printing temperature)

		Amylose	(%)	Concentrations	Printing
		content		%(w/w)	Temperature
					(°C)
	Corn starch		14.590.01	20-25	75
	Potato starch	34.5 0.4	15.540.03	15-20	70
	Rice starch	26.50.3	14.970.05	15-25	80

Table 20

 20 Physical and thermal properties of cellulose acetate (Tag3D FilAcetate)

	Density (g.cm -3 )	1.27
	Glass transition temperature (°C)	70
	Melting point (°C)	200

Table 21

 21 Mechanical properties of SCA specimen 3D printed under different conditions of nozzle temperature of nozzle temperature and flow rate[START_REF] Paggi | Structure and mechanical properties of 3D-printed cellulose tablets by fused deposition modeling[END_REF]

	Nozzle temperature Flow rate	Flexural	modulus	Stress at 10% (MPa)
		(MPa)
	230
	230
	240
	240

Table 22

 22 Review of several processing parameters / characteristics

		Extrusion	Nozzle	Printing	Nozzle	Filament
	Reference	temperature	temperature	speed	diameter	diameter
		(°C)	(°C)	(mm/s)	(mm)	(mm)
	Ghartzou et al., 2017	180 -190	205	20	0.2 -0.3 -0.4	1.78
	Obielodan et al., 2018	170 -175	190 -210	-	-	1.77
	Domínguez-
	Robles et al. ,	170 -190	185 -205	-	0.4	2.85
	2019
	Vaidya et al., 2019	160 -170	190	25	-	-
	Shao et al., 2018	-	-	9	0.5	0.6
	Nguyen et al.,2018b	210	230	50	0.5	2.5

Table 23 .

 23 The company Extrudr provides filaments made of lignin, natural fibers, and other thermoplastic polymer under the trade name extrudr Wood® dark or nature which

	properties	are	reported	in	Table	23
	(https://www.extrudr.com/en/products/catalogue/fichte-natur_2263/ ).
	Shao et al. published a study on the fabrication of carbon-based electrodes composed of
	microfibrillated cellulose, lignosulfonate, and cellulose powder.

  Table 23 of extruder Wood®, ARBOFORM®, ARBOBLEND®, and ARBOFILL® materials. https://www.tecnaro.de; (2) : https://www.extrudr.com/en/products/catalogue/fichtenatur 2263/)

	(1)
			ARBOFORM® (1)		ARBOBLEND® (1)[	ARBOBLEND® (1)[	ARBOFILL® (1)	Extrudr Wood ®(2)
		Tensile modulus (MPa)	6000	4800 2200 3100 3200
		Tensile strength (MPa)	15		55	45	33	40
		Strain (%)	0.3		2	4.5	3.5	-
	Mechanical	stress at break (MPa)	15		51	20	32	-
	properties	Tensile strain at break (%)	0.3		4	18.7	4.1	2
		Flexural stress (MPa)	42		-	-	-	43
		Charpy impact strength (kJ / m 2 )	2		51	130	-	-
		Ball indentation hardness (MPa)	122		-	-	-	-
	Thermal	Linear expansion coefficient (K -1 )	1.8.10	-5	-	-	-	-
	properties	Vicat temperature (°C)	83		-	-	-	48
		Heat conductivity (W/m/K)	0.384	-	-	-	-
	Rheological	MVR (cm 3 /10 min)	-		8	32	6	-
	properties	MFR (g / 10 min)	-		-	-	-	22
		Shrinkage (%)	0.2		2	-	-	-
	Others	Density (g / cm 3 )	1.29				1.03 1.23
		Water content (%)	6		-	-	-	-

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