Prof Alain Baley

Dr Frederic Dufresne

Dr Darshil U Ruch

Shah

LIST OF ABBREVIATIONS (cont.)

ABSTRACT

Thailand has a lot of pineapple leaves left in the field after the fruits are harvested and they are not utilized. In this thesis, the potential of PALF in bundle and microfiber (PALMF) forms was assessed as performance reinforcing fibers for natural rubber (NR) by comparing with aramid fiber in the low strain regions where fiber pullout has not taken place. In addition, the fiber was oriented unidirectionally to observe clearly the reinforcement effect. Attempts were also made to improve the reinforcing efficiency of the fibers by surface modifications to enhance either chemical interaction or physical interaction between PALF and the matrix. The content of PALF was varied from 2 to 10 part per hundred (phr) by weight of rubber. The composites were characterized using moving die rheometer (MDR), dynamic mechanical thermal analysis (DMTA), and tensile testing. The morphologies of the tensile and the cryogenic fractured surfaces were observed using scanning electron microscopy (SEM). Surface modifications were achieved with alkali and silane-69 treatment and characterized by Fourier-Transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), and SEM. The results showed that rubber reinforced with 10 phr alkali and silane-69 treated PALMF had higher storage modulus than that reinforced with the same amount of aramid fiber due to higher aspect ratio of PALMF. Chemical interaction between the fiber and the matrix was obtained by using low molecular weight functionalized NR and amino group on silane treated fiber to form carbinolamine, while physical interaction was obtained by using liquid carboxylated isoprene rubber. The liquid carboxylated isoprene rubber gave much higher modulus than carbinolamine formation and other reinforcing materials. The mechanical properties were further improved with modified fibers. It is clear that PALF can be used as a high performance reinforcement for advanced applications. However, there is still much room for adhesion improvement at high strains where fiber pull-out can occur.

INTRODUCTION

Short fiber reinforced rubber composites are very interesting composites because they have enhanced mechanical properties such as tensile modulus and strength without losing the flexibility of rubber. Enhancing the mechanical properties of composites can be done by using fiber with different mechanical properties, aspect ratios, dispersion, orientation and adhesion between the fiber and the matrix. Moreover, using short fiber in composites is much more convenient in the term of mixing and processing as compared with continuous fiber or fabric. Their applications can be found in tires, hoses, and belts [START_REF]Aramid-short-fiber reinforced rubber as a tire tread composite -Kashani[END_REF]. In recent years, natural fiber has received a lot of attention as reinforcing material in polymer composites due to several advantages. It can be used as alternative replacement for synthetic fibers such as glass fiber, carbon fiber, and aramid fiber in different polymer matrices.

Natural fibers have been now recognized as important materials worldwide.

In many parts of the world, plants are cultivated in order to get their fibers. In Thailand, there are various sources from which natural fiber can be obtained. Among that, there are large areas of pineapple cultivation and pineapple leaf fiber (PALF), if extracted, can be regarded as a byproduct. PALF has high cellulose content and high mechanical properties. In my previous research, the strength of Thai pineapple leaf fiber was found to be around 400 MPa. Although the strength of PALF depends on where it is taken and the position on a leaf, the difference is not large to worth the separation before using.

In addition, fiber separation methods also have an effect to fiber strength. To be able to scale up for industrial uses, a novel method for PALF separation has been proposed and tested to produce reinforcement for polymer matrix composites [START_REF] Kengkhetkit | Utilisation of pineapple leaf waste for plastic reinforcement: 1. A novel extraction method for short pineapple leaf fiber[END_REF]. The fiber displays very effective reinforcement for rubber [START_REF] Wisittanawat | Mechanical properties of highly aligned short pineapple leaf fiber reinforced -Nitrile rubber composite: Effect of fiber content and Bonding Agent[END_REF]. When the samples of these composites were tested under tensile, the fibers were pulled out from the matrix [START_REF] Prukkaewkanjana | High performance hybrid reinforcement of nitrile rubber using short pineapple leaf fiber and carbon black[END_REF]. This indicates that there is room for the improvement of the adhesion between the fiber and the matrix to

Budsaraporn Surajarusarn

Introduction / 2 produce stronger composites. In order to develop pineapple leaf fiber reinforced composites for commercial utilization, it is important that we have industrial composite as standard reference. Aramid fiber is one of important synthetic fiber that is widely used in many applications and it might be a standard fiber for industry in the future.

Thus, it will be used as a benchmark in the project.

In this research, the potential of PALF both in bundle and microfiber (PALMF) forms will be assessed as performance reinforcing fibers for natural rubber (NR) by comparison with aramid fiber. To demonstrate the fiber full potential, comparison will be focused mainly in the low strain regions where fiber pullout has not taken place. In addition, methods to improve the interfacial adhesion between the fiber and the matrix will be investigated.

CHAPTER II

OBJECTIVES

From previous researches on pineapple leaf fiber (PALF) reinforced natural rubber, especially that from our group, mechanical properties of composites in the low strain regions were quite high and impressive. Their mechanical properties were, however, still low when compared with that of Kevlar (aramid) fiber reinforced natural rubber [START_REF] Pittayavinai | Comparative study of natural rubber and acrylonitrile rubber reinforced with aligned short aramid fiber[END_REF]. However, when failure of PALF-rubber composite is considered, it is found that PALF did not break but pulled-out from the matrix indicating poor interfacial adhesion between the two phases. Therefore, PALF still has the capability to give the high reinforcing performance as the aramid fiber. In this respect, we aim to determine the potential of PALF composite in the low strain region before pulling-out occurs and find the way to improve the adhesion.

In this research, two types of fibers, i.e. PALF and Kevlar (aramid), will be used to reinforce natural rubber and oriented in uniaxial direction. The research will be divided into three parts. The first part will involve Kevlar-natural rubber composites whose properties will become the target to be compared with PALF composites. Kevlar masterbatch will be acquired from a company and composites containing different amounts of Kevlar will be prepared and their properties determined. Pineapple leaf microfiber (PALMF) will be prepared from PALF and both fibers will be used to reinforce natural rubber. Different amounts of PALF and PALMF will be used. The composite properties will be compared with those of Kevlar composites.

In the second part, low molecular weight functionalized natural rubber will be prepared by using periodic acid and used as coating on fiber surface to form chemical bonds and improve the properties of either PALF and PALMF reinforced rubbers. The methods will include preparation of pre-dispersed PALF or PALMF, and surface modification of fiber.

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Objectives / 4

In the third part, functionalized liquid rubber will be used as adhesion promoter (AP) to improve the properties of PALF reinforced natural rubber prepared by incorporation in the masterbatch form. The following objectives will be fulfilled.

1. To assess the potential of PALMF as rubber reinforcement by comparing the mechanical properties of Kevlar (aramid) fiber reinforced rubber composites and those of PALMF reinforced rubber composites at low strain.

2. To study the effect of functionalized low molecular weight natural rubber (PANR) between fiber and the rubber matrix on reinforcing performance of the fiber at both low and high strain.

3. To evaluate the use of functionalized liquid rubber as adhesion promoter (AP) on properties of the PALF and surface modified PALF reinforced rubber composites at both low and high strains.

CHAPTER III

LITERATURE REVIEW

This research was aimed to study the reinforcement of natural rubber with natural fiber, namely, pineapple leaf fiber (PALF). In particular, the potential of PALF both in bundle and microfiber (PALMF) forms will be assessed as performance reinforcing fibers for natural rubber (NR) in comparison with aramid fiber. Thus, backgrounds relating to the NR matrix, market applications, modification, vulcanization and carbon black reinforcement will be reviewed. Moreover, backgrounds relating to natural fiber especially environmental impact of natural fiber production, PALF, properties and pineapple leaf structure and methods of treatments for natural fiber will be also reviewed. Then, some detail on properties of the bench marked fiber, i.e. aramid fiber, will be given. Afterward, previous works and current knowledge on the use of PALF in rubber composites will be presented and parameters affecting properties of the composites shown. Finally, opportunity in developing further knowledge regarding the reinforcement potential of PALF for rubber system will be put forward.

Natural rubber (NR)

Natural rubber is one of the important natural resource. There are many types of plants which can produce natural rubber. However, commercial natural rubber is always produced from the para rubber tree. This species is Hevea brasiliensis. The main natural rubber producers are located South-east Asia. Time for harvesting is ready in 5-7 years, and rubber latex could be continuously tapped for a period of 20-25 years [START_REF] Kurian | Natural Rubber: Production, Properties and Applications[END_REF].

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Literature Review / 6

Figure 3.1 Tapped rubber latex from rubber tree [START_REF] Alam | Vietnam, USA Origins of Natural Rubber Latex ~ Suzuki Latex Industry Malaysia Sdn Bhd in Malaysia[END_REF].

Natural rubber is considered as a biomaterial with high performance such as high tensile, high tear strength, high elasticity, good crack growth resistance and low heat build-up [START_REF] Liu | New evidence disclosed for networking in natural rubber by dielectric relaxation spectroscopy[END_REF]. The tendency of natural rubber consumption has been increasing. The natural rubber consumption and production are shown in Figure 3.2. It can be used in variety applications such as engineering, medical, sport and household. However, it is mainly used in rubber tires (see Figure 3.3). The rubber hydrocarbon of natural rubber contains 94 % of cis-1, 4polyisoprene whose structure is shown in Figure 3.4 [START_REF] Sakdapipanich | CHAPTER 2:Natural Rubber: Biosynthesis, Structure, Properties and Application[END_REF]. The average molecular weight of the polyisoprene varies from 200,000 to 400,000 g.mol -1 . There are 3000 to 5000 isoprene units per polymer chain. Storage hardening phenomenon in solid natural rubber occurs after prolonged storage. Crosslinking of active functional groups in phospholipids at the chain end of molecules have an important role for this phenomenon

Budsaraporn Surajarusarn

Literature Review / 8 [START_REF] Sakdapipanich | CHAPTER 2:Natural Rubber: Biosynthesis, Structure, Properties and Application[END_REF]. Tanaka Y. [START_REF] Tanaka | Structural Characterization of Natural Polyisoprenes: Solve the Mystery of Natural Rubber Based on Structural Study[END_REF] studied the NR structure that it composed of long chain of cis-1,4polyisoprene connected with two trans-1,4-isoprene. Another end chain of cis-1,4polyisoprene can be linked with mono-or di-phosphate groups. The ω-terminal part is associated with protein while α-terminal part connects to phospholipid by H-bonding or ionic bonds. The proposed linear rubber chain and networking in NR structure is shown in Figure 3.5 [START_REF] Liu | New evidence disclosed for networking in natural rubber by dielectric relaxation spectroscopy[END_REF], [START_REF] Sriring | Viscoelastic and mechanical properties of largeand small-particle natural Université de Haute Alsace & Mahidol University Ph.D. (Polymer Science and Technology) / 157 rubber before and after vulcanization[END_REF]. Normally, there are different standard Thai rubber (STR) of natural rubbers, which depend on the amount of dirt, ash, and nitrogen. The rubber grades are shown in Table 3.2.

Modification of natural rubber

Natural rubber has excellent physical properties, however it has some limitation such as low heat resistance, low abrasion resistance, low flame resistance, low oil resistance, and low flame resistance. It has also poor ageing properties. These drawbacks have an effect on its range of applications and uses. To solve undesirable properties, the modification of natural rubber was mainly required. There are many methods to modify natural rubber by changing the molecule structure and introducing of new atoms or molecules such as cyclisation, epoxidation, halogenation, hydrogenation, ozone, metathesis, photochemical reaction.

In particular, degradation reaction is categorized as modification reaction in the aim to decrease the molecular weight. NR has a very high viscosity, which may induce technical problems during processing or compounding in solid rubber form.

Degradation of natural rubber can be done either by mechanical or chemical processes, which depend on the required applications.

Degradation of natural rubber by mechanical processes

Mechanical process by milling rubber in two-roll mill mixer or in an internal mixer is known as mastication. Mastication of rubber is an important step for rubber processing. It can provide good dispersion of chemicals and fillers induced by a viscosity decrease. Moreover, the rubber rheological properties also improve such as die swell, calendar shrinkage, and building tack [START_REF] Fries | Mastication of Rubber[END_REF]. Normally, synthetic rubbers do not require the mastication step because they can be synthesized with various viscosities by polymerization techniques. Therefore, mastication step is necessary for natural rubber. The possible mechanism of mastication of rubber [START_REF] Fries | Mastication of Rubber[END_REF].

Free radicals can be generated during mastication at chain end. The free radical can be recombined in absence of oxygen. However, free radicals can form peroxide radical in presence of oxygen. The peroxide radical can react by abstracting an H-atom from methylene group of hydroperoxide or cyclic diperoxide groups [START_REF] Fries | Mastication of Rubber[END_REF].

The mastication efficiency on rubber depends on the temperature and catalysts. The presence of catalysts or mastication agents directly influences the mastication efficiency. Temperature during mastication of rubber, especially low temperature has higher effect on breaking rubber chains. Indeed, the rubber chains at high temperature have higher mobility and slide past to each other. Therefore, shear forces cannot be directly passed through rubber chain. There are other factors that effect on mastication of rubber such as rate of shear and initial molecular weight [START_REF] Ceresa | Mastication of rubber. VII. Mechanical degradation of polymers during mastication[END_REF]. Shear rate depends on rotor speed of the masticator, design of rotor, and chamber. Higher shear rate has more effect on mastication of rubber. The higher initial molecular weight has higher viscosities. The probability for breaking or rupturing long chain molecules is higher when those molecules are subject to high applied shear forces.

Degradation of natural rubber by chemical processes

Chemical processes can be used (with the help of chemical reagents) to reduce molecular weight of natural rubber beside mechanical force. The breaking natural rubber chain induces not only molecular weight decrease but also induces a functionalization of the chain with specific group on natural rubber chain.

Using potassium persulphate

Chaikumpollert [START_REF] Chaikumpollert | Low temperature degradation and characterization of natural rubber[END_REF] studied the use of potassium persulfate (K2S2O8, KPS) as radical initiator for degradation of natural rubber latex by varying reaction time and amount of K2S2O8 from 0.1 phr to 2 phr. A high amount of KPS was used and induced a strong decrease of the NR Mooney viscosity. The molecular weight and the gel content also decreased after oxidative degradation occurred. Phetphaisit and

Phinyocheep [START_REF] Phetphaisit | Kinetics and parameters affecting degradation of purified natural rubber[END_REF] also studied the degradation of purified natural rubber (PNR) by using a combination between potassium persulfate and propanol. The intrinsic viscosity of PNR depended on the concentration of initiator and propanol, the reaction time, the temperature, and the dry rubber content. Macromolecular weight of NR can be controlled by phenylhydrazine ratios. The mechanism is shown in Figure 3.10, where phenylhydrazine reacts with oxygen and provides phenyl radicals. The radicle can react at carbon-carbon double bonds and form methylketone and phenylketone terminal groups. However, phenylhydrazine can be oxidized with repetitive reaction cycles [START_REF] Brosse | Chemical modifications of polydiene elastomers: A survey and References / 158[END_REF]. Phinyocheep and Duangthong used phenylhydrazine/O2 system for degradation of natural rubber latex in order to prepare ultraviolet-curable liquid natural rubber [START_REF] Phinyocheep | Ultraviolet-curable liquid natural rubber[END_REF]. Abdullah [START_REF] Abdullah | Liquid Natural Rubber: Preparation and Application[END_REF] studied the use of phenylhydrazine to create liquid natural rubber (LNR). The molecular weight of LNR decreases inversely of the concentration of phenylhydrazine. [START_REF] Phinyocheep | Chemical degradation of epoxidized natural rubber using periodic acid: Preparation of epoxidized liquid natural rubber[END_REF] reported the use of periodic acid in order to prepare epoxidized liquid natural rubber. Degree of degradation of rubber depended on the amount of epoxide on epoxidized natural rubber ENR and the amount of periodic acid. After degradation of rubber, carbonyl functional groups appear at the chain ends.

The mechanism of this reaction was proposed in Figure 3.11. The authors also studied the effect of periodic acid with NR and PNR. The results of epoxide content and average molecular weight with different reaction time were shown in Table 3.3 and 3.4, respectively. Use of periodic acid to degrade ENR, NR, and PNR was quite successful.

Aldehyde and ketone groups are found at the chain ends of rubber chain and they were confirmed by IR and 1 H NMR.

Table 3.3 Epoxide content and average molecular weight of NR degradation by using periodic acid [START_REF] Phinyocheep | Chemical degradation of epoxidized natural rubber using periodic acid: Preparation of epoxidized liquid natural rubber[END_REF].

Table 3.4 Epoxide content and average molecular weight of PNR degradation by using periodic acid [START_REF] Phinyocheep | Chemical degradation of epoxidized natural rubber using periodic acid: Preparation of epoxidized liquid natural rubber[END_REF].

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Literature Review / 16

Sadaka et al. [START_REF] Sadaka | Controlled chemical degradation of natural rubber using periodic acid: Application for recycling waste tyre rubber[END_REF] used periodic acid to degrade natural rubber.

They also found that the rubber contained ketone and aldehyde groups at the chain ends.

The NR final molecular weight was in the range of 3000-7000 g.mol -1 . Gillier-Ritoit et al. [START_REF] Gillier-Ritoit | Telechelic cis-1,4-oligoisoprenes through the selective oxidolysis of epoxidized monomer units and polyisoprenic monomer units in cis-1,4-polyisoprenes[END_REF] also used periodic acid for the cleavage of epoxidized cis-1,4-polyisoprenes (EPIs) and isoprenic units (I) in cis-1,4-polyisoprene (PI). They obtained aldehydic and ketonic chain ends in both cases.

Vulcanization

Un-vulcanized natural rubber has limitation in its applications due to its antagonist behavior due to elastic but also strong properties obtained under different sollicitations. It is soft and sticky at high temperature and it is hard and brittle at low temperature. Thus, vulcanization process represents an important step for improving its properties, i.e. high tensile strength, high tear strength, low hysteresis, high fatigue resistance, by forming linkage between the rubber chains. The property of vulcanized natural rubber depends on the cross-linking density as shown in Figure 3.12.

Nonetheless, natural rubber has poor resistance to ozone, sunlight, oxygen, solvents, and oils due to the presence of unsaturated carbon-carbon double bonds in backbone of natural rubber. There are 3 types of sulfur vulcanization system by varying ratio of sulfur and accelerator. The ratio of sulfur and accelerator is shown in Table 3. The types of sulfur vulcanization system influence network structure and properties of vulcanizated rubber. Their effects are displayed in phr. They found that the effect of crosslinking was obvious in the high strain region.

Increasing cross-link density has also affected the increase of the stress rate. carbon-carbon cross-links, (g and j) cyclic sulfides, (h and i) conjugated segments of elastomer chains, (l) pendant side-group (Acc-accelerator residue) and, (k) chemically non-bonded additives [START_REF] Kruželák | Sulphur and peroxide vulcanisation of rubber compounds -overview[END_REF].

The strain-induced crystallization (SIC) is the partially reason of outstanding properties of natural rubber. This phenomenon can be explained by the high regularity of the molecular structure. In addition, non-rubber components such as protein and lipids enhance its capacity to crystallize. However, different other factors may have an effect to strain-induced crystallization, such as crosslink density, carbon black fillers, temperature, fatigue, and crack tip [START_REF] Huneau | Strain-induced crystallization of natural rubber: a review of x-ray diffraction investigations[END_REF].

Carbon black reinforcement

Carbon black is produced from incomplete combustion of petroleum. It is categorized as reinforcing material. Normally, some organic and inorganic are incorporated on CB during manufacturing. Organic and mineral impurities do not have an effect on ability of carbon black reinforcement. However, mineral impurity has an effect on the rate of vulcanization. There are many types of carbon black depending on their properties, particle size, structure, and surface activity. Normally, carbon black for rubber is defined by ASTM Standard D1764-04 (see Table 5). The amounts of carbon black affect the properties of the composites as shown in Figure 3.15.

Figure 3.15

Effect of carbon black level on composite properties [START_REF] Rodgers | Chapter 9 -The Science of Rubber Compounding[END_REF].

Carbon black predominantly consists of particle aggregates and agglomerates. Their size varies from 1 nm up to hundreds of microns as shown in Figure occluded rubber [START_REF] Roland | Reinforcement of Elastomers[END_REF].

The filler-filler interaction has a strong impact on modulus of composites. impact on high strain region compared to low strain regions. Increasing amount of carbon black reduces the initial slope of the stress-strain curve and also the elongation at break of the composite.

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Natural fiber

There are many meaning and types of fibers, and their use mainly depends on their research and applied fields. In botanic field, fiber is defined as a cell with spindle shape and long cell. It is responsible for supporting element in plant and there is no longer possibility of elongation [START_REF]Sclerenchyma[END_REF]. Normally, fibers can be divided into 2 parts which are natural fiber and synthetic fiber. The fiber types were shown in Figure 3.21.

Natural fiber reinforcement

Natural fiber can be used as alternatives reinforcing material for composite.

The interesting one is automotive application due to low cost and low density.

Moreover, natural fiber is particularly attractive for researcher because of its reduced dependence on non-renewable energy/material sources, lower pollution, and green house emission. Energy requirements for production of natural fiber and glass fiber are shown in Table 3.6. Production of glass fiber and china reed also have also an impact on environment as shown in Table 3.7. The use natural fiber reinforcement has been reported in many publications.

Wu et al. studied improving mechanical properties and water resistance of kenaf fiber in order to replace glass fiber composite in unsaturated polyester resin [START_REF] Wu | Development of natural fiberreinforced composite with comparable mechanical properties and reduced energy Université de Haute Alsace & Mahidol University Ph.D. (Polymer Science and Technology) / 161 consumption and environmental impacts for replacing automotive glass-fiber sheet molding compound[END_REF]. Yantaboot and Amornsakchai have studied pineapple leaf fiber reinforced natural rubber composites by varying mastication time from 2 to 16 min. It was found that, longer mastication time can induce improvement of stress at low strain by use of PALF without adhesion promoters [START_REF] Yantaboot | Effect of mastication time on the low strain properties of short pineapple leaf fiber reinforced natural rubber composites[END_REF]. Hariwongsanupab et al. have also studied pineapple leaf fiber reinforced natural rubber with the addition of acrylonitrile butadiene rubber. Due to the low compatibility between pineapple leaf fiber and natural rubber, the use of acrylonitrile butadiene rubber, which has a higher polarity compared to NR, was found to be a solution to enhance adhesion between fiber and matrix. They found that the different mixing sequences of NR and NBR has a direct effect onto mechanical properties of composites.

Université de Haute Alsace & Mahidol University

Ph.D. (Polymer Science and Technology) / 29

Pineapple leaf fiber (PALF)

Most of the pineapples are produced in Thailand, Philippines, China, Brazil, Indonesia, and India [START_REF] Leão | 7 -The use of pineapple leaf fibers (PALFs) as reinforcements in composites[END_REF]. Normally, growing of pineapple plant takes time around 6-9 months to produce first fruit. After the pineapple fruit was cut, their leaves and stem are buried under soil or destroyed. The leaves can be classified as agricultural waste.

Utilizing pineapple leaf in many applications is very interesting. Nowadays, there are products that are produced from pineapple leaf fiber such as leather from PINATEX.

This leather can be fabricated into footwear, accessories, fashion, and furnishing.

Mechanical properties of pineapple leaf fiber (see Table 8) are quite high, when compared to other plants. So the easiest method for separating pineapple leaf fiber is scraping by hand. Currently, there have been developments of knowledge and technology for fiber separation. The different extraction methods provide PALF with different fiber yield and average fiber diameter as shown in Table 3.8. Novel extraction method for separation short pineapple leaf fiber has also been proposed and is able to be scaled up at large scale for industry applications [START_REF] Kengkhetkit | Utilisation of pineapple leaf waste for plastic reinforcement: 1. A novel extraction method for short pineapple leaf fiber[END_REF]. The fiber which is obtained from this technique, has 6 mm in length and 3-15 µm of elementary fiber diameter. However, in order to use pineapple leaf fiber for fabricating material with specific properties, they need to be more investigated and characterized.

Budsaraporn Surajarusarn

Literature Review / 30 smaller microfibril angle shows higher mechanical properties [START_REF] Tabet | Cellulose Microfibril Angle in Wood and Its Dynamic Mechanical Significance[END_REF]. In particular, PALF

shows quite small micro-fibril angle.

Table 3.12 Micro-fibril angle of natural fiber [START_REF] Yimer | Development of Natural Fiber composites for Automotive Applications[END_REF]. Prukkaewkanjana et al. [START_REF] Prukkaewkanjana | High performance hybrid reinforcement of nitrile rubber using short pineapple leaf fiber and carbon black[END_REF] incorporated pineapple leaf fiber (PALF) into acrylonitrile butadiene rubber. PALF can be used as reinforcing material to improve moduli of composites. The moduli have increased after increasing the amount of fiber.

Surajarusarn et al. [START_REF] Surajarusarn | Revisiting the morphology, microstructure, and properties of cellulose fiber from pineapple leaf so as to expand its utilization[END_REF] studied pineapple leaf fiber structure morphology and properties.

Pineapple leaf fiber can be segmented in 2 regions, which are vascular fiber (fibrovascular) bundles and mesophyll fiber bundles (see Figure 3.24). Each fiber bundles are composed of elementary fibers (fiber cells) and their size is approximately 3 µm. The middle of elementary fiber is called lumen as shown in Figure 3.25.

Natural fiber treatment

The utilization of natural fiber as reinforcing material in composites is very interesting due to its low cost, low density, and high specific strength. However, there are some limitation, due to their poor adsorbability and wettability on rubber. The polarity difference between natural fiber and rubber represents a barrier of adhesion between fiber and matrix. Natural fiber treatment or surface modification is necessary to improve the interfacial bonding between both components.

Alkali treatment

The use alkali treatment in natural fiber (mercerization) is a common method to break down fiber bundle (higher aspect ratio) and clean fiber surface (increase of possible reactive sites). Alkali treatment has an effect on the chemical composition by removing lignin and hemicellulose [START_REF] Kalia | Pretreatments of natural fibers and their application as reinforcing material in polymer composites-A review[END_REF]. Moreover, alkali treatment also provides better physical properties and thermal stability [START_REF] Chandrasekar | A review on the characterisation of natural fibres and their composites after alkali treatment and water absorption[END_REF]. Gassan et al. studied the effect of alkali treatment on jute fiber. Tensile strength and modulus increased after Normally, natural fiber is more hydrophilic due to the presence of hydroxide groups on its surface compared to the matrix, so the combination between fiber and matrix is incompatible. The silane molecule presenting a bifunctional group is chosen: one side reacts with the matrix and the other side reacts with the fiber. Different other types of silane coupling agents can be used such as azide, methacryl, amino, triethoxy, glycidoxy, mercapto, trimethoxy, vinyl, chlorine, and alkylsilanes [START_REF] Ouarhim | 3 -Mechanical performance of natural fibers-based thermosetting composites[END_REF]. Anju et al. [START_REF] Anju | Impact of Bis-(3triethoxysilylpropyl)tetrasulphide on the properties of PMMA/Cellulose composite[END_REF] used Bis-(3-triethoxysilylpropyl) tetrasulphide (Silane-69) as coupling agent for improving the interfacial properties between microcrystalline cellulose and poly(methyl methacrylate) (PMMA). This treatment has a strong impact on the strength, flexural strength, tensile strength, and storage modulus of composite. Rao et al. [START_REF] Rao | Revealing the Interface Structure and Bonding Mechanism of Coupling Agent Treated WPC[END_REF] also studied the Silne-69 for improving the compatibility and interfacial adhesion in the constituents of wood plastic composites (WPC). silane treated stober silica, which was synthesized by sol-gel method [START_REF] Kapgate | Rubber composites based on silane-treated stöber silica and nitrile rubber: Interaction of treated silica with rubber matrix[END_REF].

Tetraethoxysilane, 98% (TEOS) was used at 1%, 2%, and 3% of γ -MPS to reinforce the NBR. Modulus and tensile strength of composite has increasde as a function of the increase of the amount of silanes (TEOS). This behavior has been observed in dynamic mechanical analysis. Table 3.13 Sulfur rank and chemical structures of silane coupling agents [START_REF] Brinke | Mechanistic aspects of the role of coupling agents in silica-rubber composites[END_REF].

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Literature Review / 42

Sulfuric acid treatment

Sulfuric acid is the common method for pretreatment of switchgrass, corn stover, Spruce, and poplar [START_REF] Kumar | Recent updates on different methods of pretreatment of lignocellulosic feedstocks: a review[END_REF]. Liu and Wyman [START_REF] Liu | The Effect of Flow Rate of Very Dilute Sulfuric Acid on Xylan, Lignin, and Total Mass Removal from Corn Stover[END_REF] flowed very dilute sulfuric acid (0.05 -0.1 wt %) in order to remove xylan, lignin and total mass from corn stover. Lignin can be removed by about 12% , but no specific effect has been observed with and without the use of flowing sulfuric acid. However, xylan can be removed up to 25% by using an acid flow rate 10 mL/min in a 3.8 mL of reactor.

Kristiani et al. [START_REF] Kristiani | Effect of Pretreatment Process by Using Diluted Acid to Characteristic of oil Palm's Frond[END_REF] used diluted sulfuric acid as a pretreatment process on oil palm's frond. Sulfuric acid concentrations were varied from 0 -2% v/v. Results show that hemicellulose content has been highly decreased after addition of sulfuric acid concentrations 2% v/v at 121 °C for 30 min. Concentrated sulfuric acid can also be used in lignocellulosic compound to hydrolyze cellulose. However, its drawbacks are corrosive, toxic and hazardous [START_REF] Sun | Hydrolysis of lignocellulosic materials for ethanol production: a review[END_REF]. Yasuda et al. [START_REF] Yasuda | Formation and chemical structures of acid-soluble lignin I: sulfuric acid treatment time and acid-soluble lignin content of hardwood[END_REF] used 72% sulfuric acid in order to form soluble lignin at different treatment time. The results showed that lignin can be highly dissolved in 72% sulfuric acid at the initial stage, but it did not evolve and improve for prolonged time.

Sodium chlorite treatment

Sodium chlorite (NaClO2) is used for bleaching fibers. It could be used to delignify lignocellulosic products [START_REF] Li | Chemical Treatments of Natural Fiber for Use in Natural Fiber-Reinforced Composites: A Review[END_REF]. Muensri et al. [START_REF] Muensri | Effect of lignin removal on the properties of coconut coir fiber/wheat gluten biocomposite[END_REF] reported on the treated coconut fiber with 0.7% sodium chlorite. Amount of lignin decreased from 42 to 21 wt.%. Coconut fiber was used to reinforce wheat gluten biocomposite. Their mechanical properties were not changed after treatment of the fiber. Partial reduced lignin content had no effect on the improvement of the properties of this biocomposite. used. There was also no strong effect on hardness of the composites. However, notched impact strength has decreased as a function of higher delignification levels.

Aramid fiber

Aramid fiber is a long-chain of synthetic polyamide. 11. Temperature influences Nomex fiber that has been exposed to different temperatures for 5 min in air. Fiber tenacity and modulus decrease, while elongation at break increases after increasing temperature (see Figure 3.34). Nomex is suitable for applications that do not require special highly tensile properties. However, if some applications have to require very high tensile strength, Kevlar represents a more appropriate candidate. Manchado and Arroyo studied effects of aramid, cellulose, and glass fibers on properties of natural rubber (NR), styrene-butadiene rubber (SBR), and ethylene propylene-diene terpolymer rubber (EPDM). Aramid fiber shows significant increase in strength and tensile modulus. It has also a higher abrasion and tear resistance [START_REF] Manchado | Short fibers as reinforcement of rubber compounds[END_REF].

Increasing adhesion between aramid fiber and matrix has been studied. Aramid fiber surface was modified by chemical grafting with polyethyleneimine (PEI) and by formation of a complex with Fe 3+ . The thickness of grafting layer depended on reaction time. It was found that polarity matching, chemical interlocking, and fiber/epoxy interfaces were improved as a function of the reaction time [START_REF] Cheng | Grafting degradable coordination polymer on aramid fiber surface to improve its interfacial properties[END_REF]. Lin et al. [START_REF] Lin | Isolation of Aramid Nanofibers for High Strength and Toughness Polymer Nanocomposites[END_REF] incorporated aramid nanofiber in an epoxy resin. The weight fractions of aramid nanofiber were varied from 0.5% up to 2%. Elastic modulus, strength and fracture toughness have been improved after addition of nanofiber.

The use of pineapple leaf fiber in rubber composites

Amornsakchai's research group has been developing the short fiber extraction method and the use of pineapple leaf fiber (PALF) to reinforce the polymer matrix. Kengkhetkik et al. [START_REF] Kengkhetkit | Utilisation of pineapple leaf waste for plastic reinforcement: 1. A novel extraction method for short pineapple leaf fiber[END_REF] reported the novel PALF extraction method. The fresh leaves were chopped and then mechanical force was applied to crush the leaves into a paste. Finally, the fiber and non-fiber were obtained and separated by sieving process after drying.

The improvement of PALF in Amornsakchai's research group started with Wisittanawat et al. [START_REF] Wisittanawat | Mechanical properties of highly aligned short pineapple leaf fiber reinforced -Nitrile rubber composite: Effect of fiber content and Bonding Agent[END_REF], [START_REF] Wisittanawat | Remarkable improvement of failure strain of preferentially aligned short pineapple leaf fiber reinforced nitrile rubber composites with silica hybridization[END_REF]. The researchers studied the effect of pineapple leaf fiber (PALF) content and bonding agent to reinforce nitrile rubber composites with silica hybridization. The PALF significantly improves tensile strength, modulus, and tear strength. The use of a bonding agent further improves mechanical properties (see Figure 3.35). However, the bonding agent does not improve mechanical properties after addition of silica (see Figure 3.36). Prukkaewkanjana et al. [START_REF] Prukkaewkanjana | High performance hybrid reinforcement of nitrile rubber using short pineapple leaf fiber and carbon black[END_REF] also studied the effect of carbon black on PALF reinforced nitrile rubber composites. The amount of PALF was fixed at 10 phr while the amount of carbon black was varied from 0 to 30 phr. Modulus at low strain was slightly improved and highly improved at high strain after the addition of carbon black. The elongation at break of composites was expanded as the increasing amount of carbon black (see Figure 3.37). Yantaboot et al. [START_REF] Yantaboot | Effect of mastication time on the low strain properties of short pineapple leaf fiber reinforced natural rubber composites[END_REF] reported the method to improve PALF reinforced NR composite by varying the mastication time of the rubber and modified PALF surface with alkali treatment (TPALF). The time was varied from 2, 4, 8, up to 16 min. The modulus of composite has been significantly improved by about 370% at 10% strain after an increase of mastication time from 16 min to 2 min. The modulus further increases when PALF was treated with alkali treatment as shown in Figure 3.39. This result indicates that the adhesion between fiber and matrix has been improved. The distribution of carbon black was studied to improve the mechanical property of composites by modeling the carbon black in the composite as shown in Figure 3.40.

Model c shows the highest improvement of stress at the medium strain of composites by divided carbon black into 2 portions, one part was mixed in the fiber masterbatch and then the second during the "regular" mixing. Hariwongsanupab et al. [START_REF] Hariwongsanupab | Improving the mechanical properties of short pineapple leaf fiber reinforced natural rubber by blending with acrylonitrile butadiene rubber[END_REF] reported the use of polar acrylonitrile butadiene rubber by replacing some amount of the NR matrix to improve adhesion between PALF and matrix. The amount of acrylonitrile butadiene was varied from 0% to 20% by weight. The acrylonitrile butadiene and NR are immiscible and a frictional stress between the two rubbers occurred. To get better adhesion, the acrylonitrile butadiene should be mixed with PALF before being its dispersion in the NR matrix. It was found that the modulus at low strain region was increased due to the immiscibility of rubbers and better adhesion between fiber and acrylonitrile butadiene rubber (NBR).

From the review above, it is clear that PALF can be used as a very effective reinforcing material. However, the real potential of PALF had not been illustrated yet. In addition, there is a clear sign of fiber pulling-out at high strains which can be improved.

According to short fiber composite theory, reinforcement efficiency of the fiber depends on its modulus and aspect ratio. Thus, to illustrate the potential of PALF, fiber of high aspect ratio is required. Previous study in our group on PALF structure has shown that PALF is a bundle of microfibers. Pineapple leaf microfiber (PALMF) will be prepared and is expected to have a very high aspect ratio (when fiber of the same length is used).

PALMF will be used for the assessment of the fiber potential and will be compared with aramid fiber. To avoid the problem of interfacial debonding, the assessment will be made at very low strain region using dynamic mechanical analysis (DMA). In addition, two methods for improving the interfacial adhesion between the fiber and the rubber matrix will be investigated. These are based on both chemical reactions between fiber and modified rubber molecules but also on physical interaction through the use of high polarity adhesion promoter. Pineapple leaf fiber (PALF) was obtained from Bang Yang District, Phitsanulok Province, Thailand. The process of separation of pineapple leaf fiber followed the method developed by Kengkhetkit and Amornsakchai.

Pineapple leaf microfiber (PALMF) was prepared from defibrillated PALF.

PALF was treated with 10% w/v sodium hydroxide solution for 30 min and was stirred using a homogenizer for 30 min. The fiber was washed with distilled water until the pH mains types of pre-treatment. For all pre-treatments, the ratio between fiber and solution was 1 g into 30 ml. PALMF was washed with distilled water for several time until pH of solution was equal to 7 in order to remove residual components. Then PALMF was dried at 80 C for 24 hours.

Sodium hydroxide

Number of treatment cycles

PALMF was treated with 10% w/v sodium hydroxide solution.

The treatment has been repeated three times and referred to first, second and third cycles.

In each cycle, there are different possibilities of duration of treatments: either 15 or 30 min. This PALMF was called ACxTyPALMF. The letter x is the cycle number and y is treatment time. triethoxysilane was used with 3% wt. of fiber. Silane was mixed and stirred with absolute ethanol (analytical grade) for 1 hour. The ratio of fiber and solution was 1 g to 40 ml. Then, the fiber was treated with silane for 1 hour and solvent was removed with rotary evaporator. The fiber was dried in oven at 80 C for 24 hours.

Degradation of natural rubber (NR) and reaction with fiber

Natural rubber latex containing 60%DRC was diluted into 30%DRC by distilled water. 5 phr of tergitol as a nonionic surfactant was added to stabilize the solution at room temperature for 24 hours. Periodic acid (H5IO6) at a concentration of 0.02 mole of latex rubber was used to initiate degradation reaction. The reaction was conducted at 50 C for 2 hours. The rubber was called PANR.

After the degradation reaction of natural rubber latex by using periodic acid, fibers were added into NR latex for 2 hours. The fibers were then washed with distilled water for several time and dried at 80 C for 24 hours.

Composite preparation

The composites were mixed by using two-roll mill with a rotor speed of 50 rpm and initial temperature around 40 C. The nip gap of two-roll mill was approximately 0.25 mm. The amount of mixing compound was fixed at 40 g. The compound was sheeted out in specific way to force the orientation of the fiber in the same direction.

Then, compound was determined curing time (tc90) by using MDR measurement. The compound was cut into mold shape in order to do compression and kept the fiber orientation with 1 mm thickness sheet. The compound was compressed with the molding temperature and hydraulic pressure at 150 C and 10.3 MPa respectively.

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Materials and Methods / 58

Preparation of fiber masterbatch

The fiber masterbatches were prepared at 30% of fiber using two-roll mill with a rotor speed of 50 rpm. Rubbers were masticated for 16 min. After that, fiber was slowly added and mixed for 12 min. The masterbatch was made into sheet with two-roll mill and kept them in a refrigerator at around 4˚C.

Preparation of carbon black masterbatches

Masterbatches of carbon black in NR were prepared at 40% wt. with the fill factor of 0.75 by using 2 machines which were internal mixer and two-roll mill. First, rubbers were masticated with internal mixer for 1 min with a rotor speed of 40 rpm at 50 C. After that, carbon black and paraffin oil (10% of carbon black) were slowly added during 6 min. Then, the masterbatch was continuously mixing for 3 min. Finally, the masterbatch was made into sheet with two-roll mill and kept them in a refrigerator at around 4˚C.

Preparation of carbon black and adhesion promotor masterbatches

Masterbatches of carbon black in NR were prepared at 40% wt. with the fill factor of 0.75 of internal mixer. First, rubbers and adhesion promoter (LIR-410) were masticated for 16 min with a rotor speed of 40 rpm and 50 C. After that, carbon black and paraffin oil (10% of carbon black) was slowly added for 6 min and it was continuously mixing for 3 min. Finally, the masterbatch was rolled into sheet with tworoll mill and kept in a refrigerator until use.

Characterizations

Pineapple leaf microfiber (PALMF) was characterized by using ATR-FTIR, X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM) in order to observe functionalization on the surface and morphology of fiber. For rubber composites reinforced with aramid fiber and carbon black, the cure curves of composites after mixing with two-roll mill were characterized by using moving die rheometer (MDR). Then, the composites were compressed and they were characterized by using tensile tester, dynamic mechanical-thermal analysis (DMTA), scanning electron microscope (SEM), and nuclear magnetic resonance spectroscopy (NMR).

Dynamic mechanical-thermal analysis (DMTA)

Rubber composites were characterized with Viscoanalyzer (METRAVIB VA 4000). The rubber composite specimens were cut into 30 x 10 x 1 mm 3 . Experiment was performed with fixed gauge length 20 mm. First, the specimen was set up in the closed chamber with a control of temperature. The specimen was carried on at 1% prestained, 0.5% dynamic strain, and 10 Hz frequency. There were 2 modes for performing which were tension and shear mode. The temperature was decreased from room temperature to -90 C at the scan rate 10 C/min with liquid nitro gen. The specimen was held at -90 C for 15 min. Then, temperature was increased from -90 C to 100 ̊ C at the scan rate 2 C/min.

Tensile tester

Rubber composites were cut into dumbbell shape along the fiber alignment with die cut type C. In each sample, there were five specimens for testing. The specimen was performed with tensile testing machine (Instron 5566) with a long travel contacted style extensometer. This testing was performed according to ISO 37 but thinner specimen. This performing used a crosshead speed at 500 mm/min and fixed load cell at 1 kN.

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Materials and Methods / 62

Scanning electron microscope (SEM)

Morphology of fiber and rubber composite was observed by using scanning electron microscope at different magnifications. Samples were stuck with carbon tape and coated with gold before observation.

ATR-FTIR spectroscopy

ATR-FTIR spectroscopy was used to investigate fiber surface before and after treatment. Ge crystal has a high refractive index and its angle is 45˚. The characterization was performed with 32 scans at resolution 4 cm -1 over the range of 4000 down to 600 cm -1 .

X-ray photoelectron spectroscopy (XPS)

This technique was used to measure the element composition on the surface.

Moving die rheometer (MDR)

For observation of the curing behaviors of composites, moving die rheometer (MDR) was used according to ISO 6502. Biconical shape disc with the cone angle of ± 1.3% was used. This performing required the weight of sample around 5 g.

The sample was pressed with forced and dynamic frequency at 11 kN ± 0.5 kN and 1.7

Hz ± 0. CHAPTER V

RESULTS AND DISCUSSIONS

The results and discussions section of this research manuscript is divided into three main parts. The first part presents the possibility of using pineapple leaf microfiber (PALMF) for the reinforcement of natural rubber (NR), and aramid fiber reinforced rubber was used as a standard of reference. This part also studies the effect of silane-69 treatment on PALMF to reinforce rubber composites.

The second main part focuses on the use of low molecular weight natural rubber (PANR) as thin coating of natural fibers to improve their interfacial adhesion (fiber/matrix) into the composite. This part presents the pre-treatment of fiber (PANR)

and post-treatment with (3-aminopropyl) triethoxysilane on pineapple leaf fiber (PALF)

and PALMF. The effect of fiber treatment on hybrid composites with carbon black is also displayed.

The third part focuses on the use of Kuraray as an adhesion promoter (AP)

for PALF. Indeed, it provides low molecular weight and high polarity able to improve interfacial adhesion between fiber and NR matrix. This part also deals with the effect of alkali and silane-69 treatment on PALF reinforced rubber hybrid composites properties.

Evaluating the potential of pineapple leaf microfiber (PALMF) as performance reinforcement of natural rubber comparing to aramid fiber

Pineapple leaf fiber had been using for reinforcing rubber material.

However, the reinforcement efficiency of PALF is still under development. The PALF reinforced composites will be prepared in unidirectional orientation. The important factor for improving the mechanical properties of fiber-reinforced composites is the increase of the fiber aspect ratio. In this part, PALF has been defibrillated into pineapple leaf microfiber (PALMF). Fiber surface treatment is the common pathway for improving adhesion between fiber and matrix. Silane-69 was used and grafted onto Budsaraporn Surajarusarn

Results and Discussions / 64 PALMF surface. Two types of fiber were used, i.e. PALMF and Si69APALMF. Kevlar (aramid fiber) is a synthetic fiber. It is used to reinforce rubber in various applications.

The Kevlar composite was selected as the benchmark to compare its mechanical properties with pineapple leaf fiber reinforced rubber composites.

Silanization of pineapple leaf microfiber

Silane-69 treatment was used to improve the interfacial adhesion between PALMF and NR. PALMF was modified with silane-69 at a ratio of 3%wt of fiber. The fiber was grafted with silane-69 for 72 hr and dried at 80 C for 24 hr. Scanning electron microscopy (SEM), FTIR spectroscopy, and x-ray photoelectron spectroscopy (XPS)

were used for characterization.

Morphology of pineapple leaf microfiber

The morphologies of fiber surface were investigated by using SEM. Unfortunately, no IR peak corresponding to the presence of the grafting of silane-69 onto fiber has been observed. The specific other peaks of FTIR were identified and shown in Table 5.1. However, the silanization of PALMF was confirmed by XPS characterization and was successful as displayed in the following section. 1734 cm -1 1243 cm -1

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Results and Discussions / 66

XPS analysis

The silanization method used a concentration of silane equal to 3% weight of fiber. Absolute ethanol was used as a solvent. The ratio between fiber and solution was 1 g for 40 ml of solvent. The reaction time was 72 hours and then, fibers were dried at 80 C for 24 hours. The amount of atomic content is shown in

Intensity

Comparison between fiber surface treatment and Kevlar (aramid fiber) composites

PALMF was incorporated in NR at different amounts varying from 2, 5, and 10 phr and with different surface treatment. PALMF was prepared into masterbatch form and then it was diluted into composites. Compounds were mixed by using a tworoll mill. Cure characteristics of composites were investigated with a moving die rheometer (MDR). The performance of PALMF as reinforcement material was tested by DMA and tensile tester. Morphologies of cryogenic fracture of composites were observed by SEM.

Dynamic mechanical properties

The mechanical property of PALMF and Kevlar reinforced natural rubber at very low strain was observed with 0.5% dynamic strain with a range of temperature varying from -90 to 100 °C. Storage modulus and tan delta of composites containing different amounts of fiber varying from 2, 5, and 10 phr are shown in Figure 5.4.

All the composites seem to have very closed storage modulus (Eʹ) which remain constant at low temperature. The storage moduli of all composites drop at approximately -44 °C. When temperature further increases, the modulus of composites remains constant. However, the storage modulus (Eʹ) decreases while the fiber content is increased. Unfilled NR has the lowest modulus values for all the range of temperature. In order to easier consider and compare the value of storage modulus and tan delta of fiber composites, illustrations at a specific temperature are shown in are still no differences of storage modulus at fiber content 2 phr and 5 phr. Kevlar composites seem to have a similar behavior than PALMF composite. Nevertheless, the storage modulus of PALMF composite is higher than the one containing Kevlar fibers at a content of 10 phr. The corresponding results of tan delta also show a decrease in the value, when composites display an increase in storage modulus.

The fiber content has an effect on the tan delta of composites.

Unfilled NR shows the highest peak of the tan delta. Then, tan delta decreases after the

Tensile properties

Tensile properties of different amounts of fiber and surface treatment were investigated. The stress-strain curves of composites are shown in Figure 5.7. At the beginning of the stress-strain curves, they display a higher slope than at large strain region. An increasing amount of fiber has an effect on the curve in the low strain region by increasing the modulus. The effect of reinforcement can be indicated by an interfacial strength between fiber and matrix, which can be observed in the low strain region. Indeed, silane-69 treatment of PALMF has an effect on the final composite's mechanical properties, especially for composite containing 10 phr of fiber. At low strain, the modulus of NR reinforced with PALMF is higher than Kevlar, which can be seen in fiber-containing 5 phr. In order to confirm modulus values at a low strain of composite, DMTA was used to characterize this effect shown in paragraph 5.1.2.1. However, the interface failure of PALMF composites occurs at approximately 25% of strain. This phenomenon is not observed for Kevlar composites. The modulus was still remained constant and was even higher after the application of additional force. Kevlar composite with the same fiber content displays a higher value compared to the PALMF composite. There are no significant differences between PALMF and Silane-69 treated PALMF composites. The tensile strength of composites and elongation at break decreased after the addition of fiber, but the modulus of composites increases for all the different samples. The effect of the silane-69 treatment can be observed in the region of modulus at 10% strain. Silane-69 treated PALMF has a higher modulus than PALMF composites. However, the fiber de-bonding and pull-out from the matrix can be still seen from the tensile fracture surface (see Figures A1 in appendix A).

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Results and Discussions / 64

Morphology of cryogenic-fractured composites

The cryogenic fracture of composites is shown in Figure 5.9.

The fiber is oriented in the uniaxial direction of composites. The SEM pictures also display a relatively good dispersion of fibers, especially observed at low magnification images. High magnification images allow verifying the high wettability between fiber and matrix.

Discussion

Pineapple leaf fiber (PALF) was extracted by using the milling method [START_REF] Kengkhetkit | Utilisation of pineapple leaf waste for plastic reinforcement: 1. A novel extraction method for short pineapple leaf fiber[END_REF] and provided 2.8% of fiber yield (calculated by weight percentage of dried fiber based on a fresh leaf). Their average diameter and range of diameter are 18.70 µm and 3-68 µm, respectively. The cellulose content of PALF was reported by Kengkhetkit et. al [START_REF] Kengkhetkit | A new approach to 'Greening' plastic composites using pineapple leaf waste for performance and cost effectiveness[END_REF] and Berzin et al. [START_REF] Berzin | Processing and properties of pineapple leaf fibers-polypropylene composites prepared by twin-screw extrusion[END_REF] and was evaluated at 70.98±0.93 and 69.0±2.5, respectively.

Alkali treatment and mechanical force can defibrillate fiber bundle (PALF) into elementary fiber (PALMF). Pineapple leaf fiber (PALF) has relatively high mechanical properties in a similar way as flax and hemp fiber [START_REF] Kalia | Pretreatments of natural fibers and their application as reinforcing material in polymer composites-A review[END_REF]. PALF has 3 -68 m of diameter and 6 mm lengths, while PALMF has approximately 3 m of diameter and 6 mm lengths [START_REF] Kengkhetkit | Utilisation of pineapple leaf waste for plastic reinforcement: 1. A novel extraction method for short pineapple leaf fiber[END_REF]. Figure 5.9 confirms the removal of hemicellulose from PALMF after alkali treatment. In addition, other amorphous materials were also removed at the same time. Hemicellulose, pectin and waxes removal by alkali treatment was reported by Marques et al. [START_REF] Marques | Improvement of mechanical properties of natural fiber-polypropylene composites using successive alkaline treatments[END_REF]. This phenomenon can be confirmed by the increase of the O/C ratio of the PALMF surface compared to the PALF surface by XPS measurements.

However, hemicellulose cannot be directly characterized by XPS due to the similarity of cellulose and hemicellulose structures. Cellulose is mainly composed of β-glucose while hemicellulose is composed of several sugar monomers. Cellulose contains a majority of linear chain molecules, whereas hemicellulose contains more molecules made of branched chains. Moreover, the degree polymerization of cellulose is higher

mm mm

Budsaraporn Surajarusarn

Results and Discussions / 66 than hemicellulose, of the order of 100 times [START_REF] Marques | Improvement of mechanical properties of natural fiber-polypropylene composites using successive alkaline treatments[END_REF]. The theoretical C/O ratio values of cellulose are 0.83, while its values for lignin is 0.33 [START_REF] Johansson | Evaluation of surface lignin on cellulose fibers with XPS[END_REF]. By comparison of theoretical values and XPS measurements, the O/C ratio values of the PALMF surface seem closer to lignin composition more than cellulose one. As a result, the presence of lignin on the surface fiber can be assumed due to C/O value and the presence of C-C bondings. Lignin removal can be done by deliginification, however, it highly affects the mechanical properties of the fiber. Their mechanical properties strongly decreased after bleaching [START_REF] Muensri | Effect of lignin removal on the properties of coconut coir fiber/wheat gluten biocomposite[END_REF], [START_REF] Tran | Wetting analysis and surface characterisation of coir fibres used as reinforcement for composites[END_REF].

PALMF surface was modified by silane-69grafting. The silanization of fiber cannot be observed by SEM and FTIR, due to the weak molecules content. The presence of this molecular film does not affect the topography of the fiber surface. This result has also been investigated by Zhou et al. [START_REF] Zhou | Effect of silane treatment on microstructure of sisal fibers[END_REF], who did not observe any morphology change after silane grafting on sisal fiber. In addition, they could not observe the presence of silane peak by IR characterization. Normally IR peaks of silane-69 should appear at around 1500 -400 cm -1 , but fiber peaks [START_REF] Zhu | Structural characterization of bis-[triethoxysilylpropyl]tetrasulfide and bis-[trimethoxysilylpropyl]amine silanes by Fourier-transform infrared spectroscopy and electrochemical impedance spectroscopy[END_REF] overlap silane ones. However, silane-69 on the fiber surface was confirmed by using XPS characterization. Overall, the physicochemical analysis made on treated fibers allows confirming the success of the silanization of the natural fibers.

The aim of this part is to compare the mechanical properties of PALMF and Kevlar filler reinforced natural rubber composites. From DMTA results, at a low amount of fiber, Kevlar shows a slightly higher mechanical effect on composites than PALMF.

However, when increasing the amount of PALMF, this effect is inverted: PALMF composite shows higher performance compared to Kevlar fiber and represents a better candidate for reinforcing rubber material.

This specific phenomenon raises a question: how a fiber with a low modulus (PALMF) can better reinforce a rubber material compared to a composite containing fiber with a higher modulus (Kevlar)?

𝜂 𝑙 = 1 - tanh(𝛽𝑙/2) 𝛽𝑙/2
(2)

𝛽 = √ 8𝐺 𝑚 𝑑 2 𝐸 𝑓 ln( √ 𝜋 4𝑉 𝑓 ) (3) 𝐸 (𝑠ℎ𝑜𝑟𝑡) = 𝜂 𝑙 𝐸 (𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠) (4)
Where, From equation 1, the main factors that affect the modulus of composites are the modulus (Ef) and the aspect ratio (𝜂 𝑙 ) of the fiber. PALMF has a lower modulus than Kevlar. Therefore, to get a higher modulus, longer fibers should be used. The latter 𝜂 𝑙 can be calculated using equations 2 and 3. When the modulus of fiber is extremely higher than the matrix modulus, the modulus of fiber-reinforced composites follows equation 4. The reinforcing efficiency at the end of the fiber is lower than in the middle of the fiber length. Therefore, longer fiber shows a higher reinforcing part, when compared to the fiber ends. If the aspect ratio of fiber is very high, the value of 𝜂 𝑙 is equal to 1. Therefore, short fiber can behave like a continuous fiber. The modulus of Kevlar depends on the grade. It is of the order of 64 GPa for as-spun fiber. For other grades of Kevlar, moduli are of the order of 71 GPa for K-24 grade and 112 GPa for K-

E1

Budsaraporn Surajarusarn

Results and Discussions / 68 [START_REF] Azwa | A review on the degradability of polymeric composites based on natural fibres[END_REF]. PALMF is 6 mm long and its diameter is approximately 3 µm, while Kevlar is 1-2 mm long and its diameter about 10 µm according to product specification. Therefore, the aspect ratio of PALMF and Kevlar are around 2000 and 100-200, respectively. The 𝜂 𝑙 value of Kevlar is 0.22 and 𝜂 𝑙 of PALMF reaches the value of 0.74 due to its high aspect ratio: this represents an increase of more than 70.5% for this parameter. Indeed, Figure 5.10 shows fiber that has been extracted from uncured rubber. PALMF shows longer fiber than Kevlar. Therefore, PALMF displays a high potential of reinforcing material in rubber composites compared to Kevlar.

Fibers are well dispersed in the matrix and can be uniaxially oriented by using a two-roll mill. For both Kevlar and PALMF, storage modulus (E') of the composites increased, while tan delta decreased with increase of fiber content. At very low temperatures, the matrix of composites is in a glassy stage. Storage moduli of all composites are very close together with a maximum difference of 5%. When temperature further increases, the matrix reaches the rubbery stage. The deformation of composites includes both fiber and matrix. The stress transfer through friction and traction between the surface of fiber and matrix can be observed. As a result, the higher aspect ratio of fiber induces a better effect of reinforcement in composites.

Other possible factors may affect the reinforcement of PALMF. For instance, reinforcement of material in composites can be explained according to the hexagonal model. In such a model, the hypotheses are the following: (1) all elementary fiber cells are identical; (2) a cell is a prism with a hexagonal section (such a volume filling configuration is obtained); (3) overlapping between adjacent cells is such that each cell has the same environment. The cell arrangement is organized in hexagonal packing, and they overlap between adjacent cells. Overlapping of fiber ends has a strong positive effect on the mechanical properties of composites [START_REF] Berthelot | Stress Distribution and Effective Longitudinal Young's Modulus of Unidirectional Discontinuous Fibre Composites[END_REF]. Therefore, increasing the fiber amount and inducing overlapping of fiber might have a strong impact on the final modulus of the composite (in comparison with the lower amount of fiber).

Silane-69 treated PALMF improves modulus by providing better filler dispersion and better adhesion between fiber and matrix. The silane-69 also reduces the polarity difference of fiber surface and matrix. The increase of storage modulus and decrease of the tan delta of PALMF treated with silane-69 is similar to the phenomenon observed on cotton fiber reinforced natural rubber [START_REF] Zeng | Effect of bis(3triethoxysilylpropyl) tetrasulfide on the crosslink structure, interfacial adhesion, and mechanical properties of natural rubber/cotton fiber composites[END_REF].

Tensile results show that PALMF and Si69PALMF displayed high efficiency for reinforcing material. Modulus at 10% strain and yield point of the composites have increased after using the silane-69 treatment. The improvement of mechanical properties results was similar to those that were reported by Hanafi [START_REF] Ismail | The Effect of Bis-(3triethoxysilylpropyl) Tetrasulphide (Si-69) as a Coupling Agent on Properties of Natural Rubber/Kenaf Fibre Composites[END_REF].

However, they used kenaf fiber and treated with silane-69 to reinforce natural rubber.

Modification of PALMF with silane-69 provides free sulfur during curing compound, which allows co-crosslinking with rubber [START_REF] Ansarifar | Using a sulfur-bearing silane to improve rubber formulations for potential use in industrial rubber articles[END_REF]. Normally, silanization has been reported in the use of silica. Lopattananon [START_REF] Lopattananon | Hybridized reinforcement of natural rubber with silane-modified short cellulose fibers and silica[END_REF] reported the use of silane-69 in hybrid filler between cellulose fibers and silica. Their results were also similar to silane-69 treated of PALMF.

Budsaraporn Surajarusarn

Results and Discussions / 70

Summary

Pineapple leaf microfiber (PALMF) could be prepared from PALF by passing through the alkali process and hemicellulose could be removed at the same time.

The fiber was then successfully treated by silanization using silane-69 molecules.

Kevlar and pineapple leaf fiber (PALMF) have both high reinforcing efficiency. The elastic moduli increase and tan delta decreases after an increase of the fiber volume. PALMF and silane-69 treated PALMF show higher reinforcement than Kevlar at high fiber content in the composite. The fiber aspect ratio is the key factor to have an effect on the modulus of the composites at a high amount of fiber, while the modulus of fiber becomes the main factor affecting the composite modulus at the low fiber content. The mechanism of reinforcement of short fiber is different between low and high fiber content. In addition, silane-69 modification on fiber surface further improves the mechanical properties of the composites.

Effect of functionalized low molecular weight natural rubber on NR/PALF hybrid composite

The tackiness of rubber has been used to improve mechanical properties by increasing mastication time [START_REF] Yantaboot | Effect of mastication time on the low strain properties of short pineapple leaf fiber reinforced natural rubber composites[END_REF]. Moreover, increasing matrix polarity by incorporating NBR can improve polar-polar interaction between fiber and matrix of rubber composites [START_REF] Hariwongsanupab | Improving the mechanical properties of short pineapple leaf fiber reinforced natural rubber by blending with acrylonitrile butadiene rubber[END_REF].

In this part, the molecular weight of NR was decreased to increase tackiness on the fiber surface. NR was functionalized by introducing aldehyde functional group at the end of the NR chain. The fiber was functionalized with amino silane. Aldehyde functional group on the NR chain reacts with an amine group of the fiber surface to form a carbinolamine bond. Functionalized low molecular weight NR was prepared in latex form by using periodic acid (H5IO6). To make sure that the fibers were coated with functionalized low molecular weight NR, the fibers were mixed with functionalized low molecular weight NR latex first.

To obtain high grafting efficiency of (3-aminopropyl) triethoxysilane on the fiber surface, different methods of cleaning the fiber surface were used such as sodium hydroxide solution, delignification, and sulfuric acid solution. The temperature and reaction time of cleaning were also studied. Five types of fiber were used, i.e. untreated pineapple leaf fiber (PALF), alkali-treated pineapple leaf fiber (APALF), pineapple leaf microfiber (PALMF), amino silane treated pineapple leaf fiber (aSiAPALF), and amino silane treated pineapple leaf microfiber (aSiPALMF). The PALMF was used to compare with PALF.

Characterization of functionalized low molecular weight natural rubber

Natural rubber latex was degraded to obtain functionalized low molecular weight by using 0.02 mol periodic acid per mole of polyisoprene. It is called PANR. NR and PANR were characterized by using ATR-FTIR spectroscopy, gel permeation chromatography (GPC), and nuclear magnetic resonance spectroscopy (NMR).

Budsaraporn Surajarusarn

Results and Discussions / 72 5.2.1.1 FTIR analysis Figure 5.12 shows the FTIR spectra of natural rubber (NR) and PANR. Both NR and PANR were prepared from latex and were coagulated with methanol. The vibrational bands according to the functional group are assigned in Table 5.5. The common vibrational bands of natural rubber are attributed to carbon-carbon and carbon-hydrogen bonds. The rubber is mainly composed of cis-1,4 polyisoprene and its peak occurs at 834 cm -1 corresponding to C-H out-of-plane bending. The appearance of the peaks at 1633 cm -1 and 1543 cm -1 correspond to amide I and amide II respectively. These bands are peptide bonds that are linked to protein in NR. The peak at 1740 cm -1 is specific to ester and it is mostly present on lipids. Amine is also present in protein and it appears at 3281 cm -1 [START_REF] Rolere | Investigating natural rubber composition with Fourier Transform Infrared (FT-IR) spectroscopy: A rapid and non-destructive method to determine both protein and lipid contents simultaneously[END_REF]. There is no significant difference between NR and PANR.

Sodium hydroxide (1 st method)

The first one was the use of 10%w/v sodium hydroxide.

PALMF was soaked in the alkali solution. This pre-treatment was performed for different times and cycles. SEM micrographs in Figure 5.15 display that, there was no strong difference in fiber surface morphology before and after this treatment. Despite the longer time and higher number of cycles that were used, PALMF external morphology seems not to be cleaner. All the residues observed on the microfibers seem not to be removed. From these results, AC3T15 was selected to study the influence of the cleaning temperature on PALMF, and SEM micrographs are shown in Figure 5.16.

Temperatures of 25 °C, 50 °C and 80 °C of alkali treatment were studied. However, it was found that temperature has no effect on the cleaning of the fiber surface. The third method was done to delignify PALMF by using a mixing solution between 17 wt% sodium chlorite and glacial acetic acid. Removal of lignin has been performed for different times of treatment. The color of the fiber after delignification seems to be brighter but the fibers were unfortunately not completely cleaned as XPS results could have been shown in Table 5. The fifth method was based on the use of highly concentrated sulfuric acid. The solution was made of 61% w/v sulfuric acid and the time of treatment was varied from 5 to 30 min. It was found that PALMF was destroyed after acid treatment. Even though PALMF was treated with a strong acid, fibers were not cleaned.

Sulfuric Acid Concentrate (AcidCx)

x is treatment time (min) Independently of all proposed pre-treatment methods, the fiber surface could not be completely cleaned. There was not significant changing in fiber surface. However, SEM images are the easiest way to observe the fiber surface. To more investigate and confirm the surface cleaning, XPS was used and shown in section 5.2.3.

Budsaraporn Surajarusarn

Results and Discussions / 88

XPS analysis

PALMF treated with AC3T30, ASC3T30 and D120 process have been characterized by XPS and compared to the reference raw PALMF. Figure 5.21 displays the different XPS spectra of all these samples. The percentages of the atomic content corresponding to the fiber surface are shown in Table 5.7. XPS results show that the variation of the atomic content of carbon and oxygen after each pre-treatment method did not significantly change. After a strong cleaning using the AC3T30 treatment, the O/C ratio values of PALMF remain the same O/C ratio values of raw PALMF. In a similar way, the O/C ratio values of ASC3T30 and D120 were also of the order of 0.50.

All the O/C values are in the range of 0.50-0.52. However, PALMF was passed through alkali treatment during the defibrillation of PALMF from PALF. Therefore, other cleaning processes could not be used to make it cleaner. Finally, untreated PALMF was selected for silanization.

Results and Discussions / 90

Silane treatment and characterization

Silanization of pineapple leaf microfiber (PALMF) and pineapple leaf fiber (PALF) were characterized by X-ray photoelectron spectroscopy (XPS). Surface morphology was observed with scanning electron microscope (SEM). Two types of fiber were used, i.e., APALF and PALMF. Before silanization of the fiber, the fiber surface was characterized by FTIR and SEM.

FTIR analysis

FTIR spectra of untreated fiber (PALF), alkali-treated fiber (APALF), and untreated microfiber (PALMF) are shown in Figure 5.22. The specific peaks of FTIR were identified and shown in Table 5.8. Untreated PALF is mainly composed of cellulose and hemicellulose and their structure are similar. However, the wavenumbers of hemicellulose appear at 1734 cm -1 and 1243 cm -1 . They correspond to C=O and C-O stretching in the acetyl groups, respectively. After PALF has been treated with 10% w/v sodium hydroxide solution, these peaks have disappeared. A similar response was obtained for PALMF. PALMF was prepared from defibrillated PALF by the action of alkali solution and mechanical force. The hemicellulose peaks have also disappeared. The fiber surface morphologies were investigated by using SEM. Figure 5.23 shows untreated PALF, alkali-treated PALF, and untreated PALMF.

The PALF displays fiber bundle and it has a rough surface with cementing material covering. After PALF was cleaned with sodium hydroxide, this cementing material was removed. APALF has a smaller bundle and it shows a smooth surface compared to PALF. PALMF was prepared from defibrillated PALF by sodium hydroxide treatment and mechanical force. PALMF was called elementary fiber. It has a lower diameter and smoother surface compared to PALF and APALF.

XPS analysis

Aminosilane 3% weight of fiber was used and mixed with absolute ethanol for 1 hour. The ratio between fiber and solution was fixed to 1 g for 40 ml of solvent. The reaction time was 1 hour and then, the solvent was evaporated with a rotary evaporator. Fibers were dried at 100 C for 24 hours. The elemental compositions are shown in Table 5.9. Wide spectra scan and high-resolution spectra of C1 of amino silane treated PALMF and amino silane treated APALF are shown in Figure 5.24.

PALMF and PALF display a successful grafting of amino silane onto the fiber surface.

The elemental compositions allow the appearance of silicon and nitrogen atoms on PALMF at 3.95% and 2.78 %, while the APALF surface displays the presence of silicon and nitrogen atoms at 1.13% and 1.65% respectively. PANR was deposited in thin coating on the fiber surface. Five types of fiber were used and coated, i.e. untreated fiber (PALF), alkali-treated (APALF), untreated microfiber (PALMF), amino silane treated fiber (aSiAPALF), and amino silane treated microfiber (aSiPALMF). The fiber was mixed in the latex form of PANR and washed with water several times. Figure 5.26 shows the fiber after coated with PANR and it will be used for compounding.

Results and Discussions / 96

Effect of fiber surface treatment on composite properties

Five types of fiber were used, i.e. PALF, APALF, PALMF, aSiAPALF, and aSiPALMF. Fibers were coated with PANR in latex form. Then fibers and carbon black were incorporated in NR in masterbatch form and diluted into 10 phr and 30 phr respectively. Compounds were mixed with curative agent by a two-roll mill and their cure characteristics were investigated with a moving die rheometer (MDR). The mechanical property of composites was tested by tensile testing and DMTA.

Morphologies of cryogenic fracture of composites were observed by SEM.

Dynamic mechanical properties

The dynamic mechanical property of composites containing PANR and different treatment of fiber and microfiber was observed along a range of temperatures from -90 °C to 100 °C, and "reference" values were taken at the very low strain: 0.5% dynamic strain. The storage modulus and tan delta of composites are shown in Figure 5.27. At very low temperatures, all the composites show high storage modulus whose values were quite similar. When temperature increases up to -60 °C, the storage moduli of all composites drop and remain constant when temperature further increased.

Storage modulus increases after the addition of fiber. The value of storage modulus and tan delta at specific temperatures of composites are illustrated and are shown in Figure 5.27 and Table 5.10. The storage modulus of composites at temperatures lower than glass transition temperature (Tg) is not hugely different. Hence, at temperature of -75

°C, the addition of PANR has not improved the storage modulus of all the composites.

Only the storage modulus containing PANR of PALF and aSiPALMF composites are a bit higher than PALF without PANR. When temperature further increases above the glass transition temperature, (25 °C and 60 °C), the modulus of the composite containing PANR is even lower than composite without PANR. These behaviors are also correlated with tan delta values. Tan delta of the composites containing PANR are higher than composite without PANR. However, tan delta values and temperature at peak positions are not significantly changed after the presence of PANR. The cryogenic fracture of composites is shown in Figure 5.32.

The fiber is oriented in uniaxial direction and is well dispersed in composites as observed at low magnification images. At high magnification, images show high wettability between fiber and matrix. In the case of PALF composite, a large fiber bundle can be seen while APALF composite shows a smaller fiber bundle. PALMF composites show elementary good fiber dispersion and good wettability by NR.

However, no significant differences of wettability and fiber dispersion within composites can be observed independently of the fiber treatment.

Discussion

The molecular weight of Natural rubber (NR) has been decreased by chain scission in order to increase its tackiness and improving adhesion between fiber and matrix. Moreover, NR presented a functionalized aldehyde functional group at the end of the NR chain by oxidation reaction (see Figure 5.33). Natural rubber (NR) and functionalized low molecular weight natural rubber (PANR) will be discussed first. PANR was prepared from natural rubber by treatment with periodic acid. The chemical structure of NR and PANR was analyzed by infrared spectroscopy and nuclear magnetic spectroscopy. The infrared spectra of NR and PANR displayed in Figure 5.12 and Table 5.5 shows the appearance of characteristic amide bands at 3,281 cm -1 (N-H stretching), 1,633 cm -1 (amide I vibration), and 1,544 cm -1

(amide II vibration). This signal could be derived from peptide bonds of protein [START_REF] Rolere | Investigating natural rubber composition with Fourier Transform Infrared (FT-IR) spectroscopy: A rapid and non-destructive method to determine both protein and lipid contents simultaneously[END_REF].

The 1 H-NMR spectra of NR and PANR in Figure 5.13 display the chemical shift at 1.67 and 2.04, which correspond to methyl and methylene protons, respectively. The triplet signal at 9.77 ppm can be assigned to aldehyde [START_REF] Phinyocheep | Chemical degradation of epoxidized natural rubber using periodic acid: Preparation of epoxidized liquid natural rubber[END_REF], [START_REF] Grange | Functionalization of polyisoprene : toward the mimic of natural rubber[END_REF]. These first analysis suggest the fact that the preparation of PANR was successful. The 𝑀 𝑛 ̅̅̅̅ value from the NMR spectrum can be calculated by using equation 5.

𝑴 𝒏 ̅̅̅̅ = [ 𝐼 (𝐶=𝐶-𝐻) 𝐼 (𝐶𝐻𝑂) × 68] + 100 (5)
𝐼 (𝐶=𝐶-𝐻) corresponds to the integration of the signal from the alkene proton 𝐼 (𝐶𝐻𝑂) corresponds to the integration of the signal from the aldehyde proton

The integration of alkene protons and aldehyde protons are 2346 and 1, respectively. Thus, the calculated 𝑀 𝑛 ̅̅̅̅ value is 159,528 daltons, while 𝑀 𝑛 ̅̅̅̅ value from GPC is 37,021 daltons. This inconsistency can be accounted by very low dilution of aldehyde proton compared to hydrocarbon backbone, despite the use of 5,000 number of scans. It means that the NR chain was cut about 1 to 7 times if we compare to 𝑀 𝑛 ̅̅̅̅ values of NR obtained from Swanson et al. [START_REF] Swanson | Molecular weights of natural rubbers from selected temperate zone plants[END_REF]. These results show that the cleavage of double bond to generate aldehyde functional group on the NR chain via periodic acid treatment was successful. This is also in good agreement with the previous report on the degradation of epoxidized natural rubber [START_REF] Phinyocheep | Chemical degradation of epoxidized natural rubber using periodic acid: Preparation of epoxidized liquid natural rubber[END_REF]. However, the amount of aldehyde functional groups is not as high as we expected.

Pineapple leaf microfiber (PALMF) and pineapple leaf fiber (PALF) will be discussed in this section. PALMF was prepared from defibrillated pineapple leaf fiber of coconut fiber has no effect on the mechanical properties of the fiber itself and biocomposites [START_REF] Muensri | Effect of lignin removal on the properties of coconut coir fiber/wheat gluten biocomposite[END_REF]. However, PALMF without any pre-treatment is cleaned enough to allow grafting of amino silane. Modification of fiber with alkali treatment induces a higher volume fraction of fiber due to hemicellulose removal in comparison with untreated PALF. Research suggests that polymer can penetrate into fiber bundle and mechanical properties are improved due to a better interlocking and better adhesion between matrix and fiber [START_REF] Sreekala | Oil Palm Fibre Reinforced Phenol Formaldehyde Composites: Influence of Fibre Surface Université de Haute Alsace & Mahidol University Ph[END_REF], [START_REF] Mwaikambo | Mechanical Properties of Hemp-Fibre-Reinforced Euphorbia Composites[END_REF]. However, in the case of PALF, APALF has a smoother surface than untreated PALF. But, enhancement of mechanical properties of the composite can be explained by the increase of tackiness induced by PANR coating. PANR can stick on the fiber surface and induces an increase of friction along the fiber length. Stress-strain curves show that not only PANR but also alkali treatment can be used to improve tensile properties. The tensile properties of PALF composites further improve after silanization.

This can be explained by the fact that the polarity between fiber and matrix has been reduced. This result is similar to those observed in the case of the preparation of aminosilane modified rice straw fiber reinforced poly(butylene succinate) composites. It has also been reported by Zhao et al. [START_REF] Zhao | The interfacial modification of rice straw fiber reinforced poly(butylene succinate) composites: Effect of aminosilane with different alkoxy groups[END_REF] that tensile properties have increased after modification of the fiber. Ramle et al. has reported the improvement of tensile and modulus properties by using amino silane treated rice husk [START_REF] Ramle | Tensile Properties of Aminosilane Treated Rice Husk/ Recycled PVC Composite[END_REF]. Nevertheless, the presence of PANR can also be used in order to increase tackiness on the fiber surface, in a similar way of the effect of the mastication process of NR rubber. Increasing of mastication time of NR rubber has an effect on improving the mechanical properties of composites [START_REF] Yantaboot | Effect of mastication time on the low strain properties of short pineapple leaf fiber reinforced natural rubber composites[END_REF]. This indicates that the stress transfer in APLAF and aSiAPALF composites is better than for untreated PALF composites.

For microfiber composite, the results are different of those of fiber bundle composite. The tensile properties of microfiber are slightly lower than those of fiber bundle: their properties decrease after modification of the microfiber with amino silane.

A possible reason is again the weakening of the fiber and a lower dispersion in the composite. Even though PALMF has an advantage in terms of smaller diameter, the "average" length of PALMF has decreased after mixing as it has been shown in section 5.1.

The storage modulus of fiber shows different results compared to stressstrain curves. DMTA experiment was used to study the adhesion between fiber and matrix at very low strain. Untreated PALF and aSiPALMF containing PANR show a high storage modulus along with the temperature. However, the storage modulus of

Budsaraporn Surajarusarn

Results and Discussions / 112 composites is not hugely different. The scattering of results may be due to the different amount of PANR on the fiber surface which could not be easily controlled. Figure 5.26

shows the aggregation of fiber after its coating with PANR, as a result, it leads to a difficulty to homogeneously re-disperse the fiber in the composites.

Another factor that should help the improvement of mechanical properties is the chemical bond between fiber and matrix. This can be explained by the reaction between the aldehyde functional group on the NR chain and the amino group on the fiber surface. This chemical bond may easily occur after the reaction depicted in Figure 5.35. This linkage is called carbinolamine linkage [START_REF] Ciaccia | Mechanisms of imine exchange reactions in organic solvents[END_REF]. As mentioned above, the amount of the aldehyde group is not high enough for carbinolamine formation. Therefore, the mechanical properties of composites are not as high as expected.

In order to better understand all these phenomena, additional studies are required. The reaction of decreasing molecular weight and increase of aldehyde group in NR with periodic acid should be more investigated. NR should do epoxidation before oxidation in order to increase the amount aldehyde group and decrease molecular weight. To obtain a stronger adhesion between fiber and matrix, PANR might contain more functional groups along the chain. In addition, the different types of functional groups should be studied. The amount of PANR on the fiber surface should also be more controlled. The use of fiber should be considered and well prepared.

Summary

Pineapple leaf fiber (PALF) and pineapple leaf microfiber (PALMF) were used to reinforce NR composites. The alkali treatment process of fiber is enough for post-silanization of the fiber. PANR improves the mechanical properties of the composite, but the amount of aldehyde group on the NR chain should be increased.

Moreover, the preparation of fiber composite should be considered. Indeed, composite reinforced with PALMF required a specific preparation process to avoid a low fiber dispersion in the composite. As a result, PALF will be used for further study in subject 5.3.

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Results and Discussions / 114

Effect of low molecular weight polar rubber in NR/PALF hybrid composite

For an effective reinforcement of NR by PALF, a good compatibility between fiber and matrix is required. Since PALF has a higher polarity than rubber, this bînome leads to a low reinforcing efficiency of the final composite. In order to improve the mechanical property of PALF reinforced rubber, various methods had been proposed and applied. Hariwongsanupab et al. [START_REF] Hariwongsanupab | Improving the mechanical properties of short pineapple leaf fiber reinforced natural rubber by blending with acrylonitrile butadiene rubber[END_REF] reported that by blending acrylonitrile butadiene rubber (NBR) with NR, better interaction between fiber and matrix was obtained. The mixing sequence was also important. By introducing fiber in NBR first and then mixed with NR, a strong response of reinforcement was obtained. Moreover, increasing the mastication time of NR has a direct effect on the stress transfer of fiberreinforced rubber composites by decreasing the molecular weight of NR. NR can flow and fully cover on the fiber surface easier than starting with high molecular weight NR;

in fine, the mechanical properties of final composites could be improved with such mastication process [START_REF] Yantaboot | Effect of mastication time on the low strain properties of short pineapple leaf fiber reinforced natural rubber composites[END_REF].

In this part, the effect of low molecular weight polar polymeric adhesion promoter (AP), i.e. Kuraray LIR-410 on the composite has been studied. This product has both a lower molecular weight and a higher polarity than normal NR. It can also cocrosslink with the NR matrix. correspond to C=O stretching of ester and carboxylic groups, respectively. The signal assignment of the specific peak values is shown in Table 5.12.

3500 3000 2500 2000 1500 1000 500

Wavenumber (cm -1 ) Figure 5.37 FTIR spectrum of AP. The AP was analyzed using GPC to evaluate its molecular weight. Table 5.13 shows the number average molecular weight (𝑀 ̅ n), the weight average molecular weight (𝑀 ̅ w), the size average molecular weight (𝑀 ̅ z), and polydispersity. The polydispersity value indicates the broadness of the molecular weight distribution and it is equal to 1.36.

Effect of fiber surface treatment on composite properties

Three types of fiber were used, i.e. PALF, APALF, and Si69APALF. The NR composites were prepared with fiber and carbon black content of 10 phr and 30 phr, respectively. AP was also added to masterbatch. The cure behavior of the composite was studied by MDR. Properties of the composite were characterized by tensile tester and DMTA. Fiber surface and surface fracture morphologies of composites were observed by SEM. This behavior is also further enhanced after modification of fiber surface with sodium hydroxide and silane-69. AP not only improves modulus at low deformation but also high deformation. The tensile strength of composites decreases after addition of fiber and AP by comparison with unfilled NR. However, there are no significant differences between fiber treatment and addition of AP. Elongation at the break of composites decreases after the addition of fiber and it kept decreasing after both incorporation of adhesion promotor and modified fiber. Even though the modulus of the composite is high in the low strain region, the de-bonding and pull-out of fiber from the matrix can still be found from the tensile fracture surface (see Figures A3 in appendix A).

Morphology of pineapple leaf microfiber

Discussion

Results show that the reinforcement efficiency of PALF can be improved by addition AP and surface modification of fiber. Section 5.1 shows the reinforcement efficiency of PALF is higher than aramid fiber. This can be explained by stress transfer.

The reinforcing mechanism in short fiber-reinforced composite occurs from stress transfer between matrix and fiber via shear stress. Shear stress increases from fiber's end to the center of the fibers. Thus, the average stress can be calculated for a different strain of composite. There are different models to calculate the modulus of uniaxial short fiber composites. These models are assumed that they have perfect bonding and no slip occur during deformation between fiber and matrix. However, the shear lag model is a useful one that can be used to describe the modulus of composites. It is originally proposed by Cox and written in a simple form by following in equation ( 1) [START_REF] Cox | The elasticity and strength of paper and other fibrous materials[END_REF].

𝐸 𝑙 = 𝜂 𝑙 𝑉 𝑓 𝐸 𝑓 + (1 -𝑉 𝑓 )𝐸 𝑚 (1)
𝜂 𝑙 = 1 - tanh(𝛽𝑙/2) 𝛽𝑙/2 (2)
Where El is elastic modulus of composite, Em is modulus of matrix, Ef is modulus of fiber, Vf is fiber volume fraction, d is diameter of fiber, l is fiber length, Gm is shear modulus of matrix, 𝜂 𝑙 is shear lag factor or modulus adjustment variable for short fiber, 𝛽 is a constant value that depends on shape, orientation and arrangement of the fiber.

For composite that has uniaxial orientation of cylindrical fibers with square arrangement, 𝛽 would assume the value given by Equation (3).

𝛽 = √

From equation (1), 𝜂 𝑙 𝑉 𝑓 can be considered as an effective fiber volume fraction. From the results, all composites have similar behavior by increasing modulus because of an increase in effective fiber volume fraction ( 𝜂 𝑙 𝑉 𝑓 ) or simply (𝜂 𝑙 ). There are two parameters that have an effect on the value of 𝜂 𝑙 . The first parameter is the aspect ratio of fiber. The bundle fiber is composed of elementary fiber [START_REF] Kengkhetkit | Utilisation of pineapple leaf waste for plastic reinforcement: 1. A novel extraction method for short pineapple leaf fiber[END_REF], [START_REF] Surajarusarn | Revisiting the morphology, microstructure, and properties of cellulose fiber from pineapple leaf so as to expand its utilization[END_REF]. If it is defibrillated into microfiber, the fiber aspect ratio is very high as shown in section 5.1.

The second parameter is the stress build-up pattern on the fiber length. It depends on the interfacial interaction. If there is a perfect bonding, the stress will be increased to the maximum value with the shortage distance from the fiber's end. If there is no perfect Budsaraporn Surajarusarn Results and Discussions / 132 bonding or poor interfacial interaction, the stress will slower increase and take longer distance before reaching the highest value.

The low molecular weight polar polymeric adhesion promoter (AP), i.e. Kuraray LIR-410 contained carboxylic and ester functional groups. It is more polar and has a lower molecular weight than natural rubber. AP improves compatibility between cellulosic PALF and rubber phases and it is also miscible with natural rubber matrix. Three types of fiber, i.e., untreated fiber (PALF), alkali-treated fiber (APALF), and silane-69 treated fiber (siAPALF) have been incorporated with carbon black 10 phr and 30 phr, respectively. The low molecular weight of AP provides adhesiveness or tackiness of the matrix.

The PALF bundle is defibrillated into microfiber by friction tearing of AP as it can be seen in Figure 5.45. Defibrillation of the fiber bundle increases the number of fiber and fiber aspect ratio significantly. In addition, the increase of polarity of the matrix also improves wettability and induces a better adhesion between the fiber surface and the matrix. The improved adhesion may arise from hydrogen bond and chemical bond between fiber and the AP as shown in Figure 5.46. Therefore, these would enhance the reinforcing efficiency of the composites by modifying the stress built-up pattern and causes 𝜂 𝑙 in equation 1 to increase and hence increase effective fiber volume fraction. Alkali treated fiber (APALF) shows a smooth surface because the cementing material has been washed with 10% w/v sodium hydroxide solution. Thus, the fiber bundle has been easily defibrillated, especially in the matrix containing AP which has lead to increase tackiness and polarity during mixing. The fiber bundle can be broken into microfiber and its aspect ratio can be reached to the maximum value of about 2000. Even though its aspect ratio is not equal to the maximum value, the interfacial bonding between fiber and matrix is still high due to their high tackiness and polarity.

Silane-69 treated fiber (siAPALF) displays a smooth surface such as APALF. This system should be similar to the APALF system. However, the storage modulus of siAPALF composite keeps on increasing as compared to APALF composites. This is assumed that this additional increase in modulus occurs from the change in stress profile along the fiber length. This change occurs because the polarity of the fiber surface is slightly lower due to the silane-69 modification of the fiber surface that matches better with the rubber matrix containing AP. Therefore, it induces an effect on the stress built-up pattern and increases the effective fiber volume fraction.

However, it is difficult to know from the highest modulus value if the best interfacial adhesion has been achieved. In addition, it is also difficult to measure the accurate value of fiber diameter and length and fiber distribution into the composite.

Pineapple leaf fiber clearly shows high performance as reinforcement material with low carbon footprint. As it is used with natural rubber, this composite very green and represents a sustainable high performance material.

The comparison between the use of PALF and other nano crystalline material such as cellulose and chitin is worth to discuss. Even though nano filler has excellent mechanical properties, they have limitation due to their low loading capacity.

There would not help much in term of the replacement with organic carbon material.

However, the aspect ratio of nano material is limited, it still has an effect on reinforcement efficiency. The filler can be loaded up to 15% but the achievable modulus is lower than that provided by 10% PALF [START_REF] Yu | Reinforcement of Natural Rubber: The Use of in Situ Regenerated Cellulose from Alkaline-Urea-Aqueous System[END_REF][START_REF] Mariano | Preparation, structure, and properties of montmorillonite/cellulose II/natural Budsaraporn Surajarusarn References / 174[END_REF]. Except chitin whisker composite (20 % wt.) which shows higher elastic modulus [START_REF] Nair | Crab Shell Chitin Whisker Reinforced Natural Rubber Nanocomposites. 2. Mechanical Behavior[END_REF]. However, the fabrication of transparent sample is the only disadvantage of PALF. Thus, PALF composites should be concerned in term of sustainability and performance more than nanomaterials.

Budsaraporn Surajarusarn

Results and Discussions / 134

The result can be modeled by assuming that there is a perfect adhesion between fiber and matrix at -75 °C. Thus, the shear lag factor (𝜂 𝑙 ) may be assumed to take a value of 1. The modulus of fiber (Ef) is calculated by equation 1, and its value is 39.19 GPa, which is not too far off the reported values for PALF. At higher temperatures (25°C and 60 °C), the shear lag factor values drop significantly to 0.026 and 0.020, respectively. The decrease in shear lag factor occurs from poor stress transfer from the matrix to the fiber. This remains a tough challenge for further investigation.

From section 5.1, PALF and PALMF show high potential as reinforcing material compared to aramid fiber. The development of surface adhesion between fiber and matrix is very important. The use of functionalized low molecular weight natural rubber (PANR) and AP can improve interfacial adhesion between fiber and matrix.

Incorporation of AP in rubber composites displays better performance than PANR due to its low molecular weight and a high amount of functional group along the AP chain.

However, the action of AP is more a physical interaction, while PANR induces more a chemical interaction with fiber. The use of PANR should provide higher mechanical properties but it still contains a too low amount of aldehyde group. Therefore, to improve the use of PANR, the molecular weight of NR should be further decreased. It should increase the strength of the reaction between the aldehyde function of PANR and amino groups grafted on the treated fiber surface.

Summary

AP can be used as an adhesion promoter and it has been shown to be very effective for improving the modulus of uniaxial short PALF reinforced NR. Moduli of composites containing AP and fiber treated with sodium hydroxide and silane-69 solution, are even higher than single AP. This result indicates that the increase of the polarity of NR matrix has a direct effect on the overall modulus of the composite. In addition, it can also cocrosslinked with the rubber network. The matrix interacts with fiber by polar interaction and this phenomenon can be clearly observed in the case of modified fiber. These effects change the stress profile along the length of the fiber. This finding does not only show the improvement of mechanical properties by modification of interfacial adhesion between fiber and matrix but also demonstrates the potential of using PALF as high performance and green reinforcing fiber.

CHAPTER VI

CONCLUSIONS

The present research was concerned with the assessment of PALF as reinforcing element for natural rubber and benched mark with Kevlar. The main criterion for the evaluation was modulus at very low strain where the fiber-rubber interface remains perfect. To achieve this objective, the work was divided into 3 parts.

The first part was started by breaking down PALF into microfibers constituent or PALMF, to get a high aspect ratio of fiber and hence to induce a high reinforcing efficiency of the filler. Uniaxial composites with different PALMF contents were prepared and their low strain properties were determined and compared with Kevlar counterparts. The second part dealt with improving the interfacial adhesion between the fiber and the rubber matrix by chemical interaction via the formation of carbinolamine bond with functionalized low molecular weight natural rubber (PANR). Different types of surface treatments were carried out in order to prepare the fiber surface for effective chemical bonding. In the third part, low molecular weight and high polarity adhesion promoter (AP) and fiber surface modification were used to improve the interfacial adhesion.

Results have shown that, by using PALMF without special treatment, the fiber gave similar reinforcing efficiency at low loading of 2 and 5 phr. The fiber provided better reinforcing efficiency than does Kevlar at high loading of 10 phr. The use of PANR to form carbinolamine bond on the fiber did not give the expected improvement. Significant improvement was obtained when low molecular weight and high polarity adhesion promoter AP was used. Fiber treatment with alkaline solution and silane-69 provided further improvement after each step only in combination with the AP. The highest level of improvement was obtained with silane69 treated PALF combined with AP and this was well above that obtained from the same content of Kevlar.

Budsaraporn Surajarusarn

Conclusions / 150

The high reinforcing efficiency of PALF can be illustrated by comparison with other fillers reported in literature such as cellulose xanthate, chitin whisker, and regenerated cellulose. The PALF composites have higher elastic modulus with the same matrix and the same or even higher amount of filler except chitin whisker as it can be seen in Table 6.1. NR/Chitin Whisker 20 wt. [START_REF] Nair | Crab Shell Chitin Whisker Reinforced Natural Rubber Nanocomposites. 2. Mechanical Behavior[END_REF] 7000 800

Considering all the results summarized above, the following conclusions can be drawn.

1. Pineapple leaf fiber composites with high fiber content and high aspect ratio have higher elastic modulus than Kevlar composites in low strain region. However, the average length of pineapple leaf microfiber (PALMF) has decreased after their mixing in NR: this is the consequence of a difficulty to homogenously disperse microfiber in the matrix.

2. The fiber modified with alkali treatment enhances the mechanical properties of the composite and further improvement was obtained after their treatment with silane-69 by increasing stress transfer between fiber and matrix. improves adhesion between fiber and matrix by the action of tack and carbinolamine bond. However, the fiber dispersion and amount of PANR in composites have to be controlled.

4. The use of tack and polarity of AP (Kuraray) can further increase the mechanical properties of the composite. This improvement may occur from hydrogen bonds and transesterification between fiber and the AP. The modification of fiber such as alkali and silane treatment in combination with AP represents additional steps to improve interfacial adhesion between fiber and matrix and then increase the reinforcing efficiency of the composite.

5. All systems of PALF composites display higher performance than Kevlar composites for fiber content 10 phr in low strain region (see Figure 6.1). The AP system with modified fiber shows the best performance for fiber reinforced natural rubber.

6. The strength and the aspect ratio of the fiber including the adhesion between the fiber and the matrix are very crucial. The modulus of the composites in the high strain at 10% strain is not as high as the theoretical prediction of modulus's composites due to the poor adhesion between matrix and fiber.

It is anticipated that the conclusions reached above would encourage the use of pineapple leaf fiber as reinforcement in high-performance applications and this would, in turn, instigate the use of pineapple leaf agricultural waste in Thailand and around the world. There is, however, still room for improving the adhesion of fiber and the matrix to develop more advanced materials. If these materials can be used in daily life or wider area of applications, it will have a high impact on the world by sequestering carbon dioxide and reduce our dependence on petroleum-based materials.

Budsaraporn Surajarusarn

Conclusions / 152

Suggestion for the future work

1. The number of breaking chains of the functionalized low molecular weight natural rubber (PANR) done by periodic acid treatment to decrease molecular weight was not high. Therefore, the amount of aldehyde group at the chain end of NR was low. To increase the carbinolamine formation for improving surface adhesion, NR should realize epoxidation before oxidation in order to increase the reaction rate of chain cleavage by periodic acid.

2. The functionalized low molecular weight natural rubber (PANR) should be designed to contain a lot of additional functional groups along the chain like the AP (Kuraray) to get similar behavior than AP.

3. The molecular weight of PANR should be better controlled to compare and understand the results.

4. Finding the method to confirm the reaction between the amine and the aldehyde functional group would help to better understand the reinforcing performance of the green composite.

KEY

3. 1 7 3. 2 9 3. 3 LIST 5 . 1 66 5. 2 68 5. 3 73 5. 4 87 5. 6 89 5. 7 4 Figure Page 3 . 4 . 1 Figure Page 5 . 2 FTIRFigure Page 5 .Figure Page 5 .Figure Page 5 .

 1729351662683734876897434152555 Figure Page 3.1 Tapped rubber latex from rubber tree 6 3.2 The relation of production and consumption of natural rubber during 1995 to 2020

Figure 3 . 2

 32 Figure 3.2 The relation of production and consumption of natural rubber during 1995 to 2020 [9].

Figure 3 . 3

 33 Figure 3.3 Natural rubber market [10].

Figure 3 . 4

 34 Figure 3.4 Cis-1,4-polyisoprene chemical structure of natural rubber.

Figure 3 . 5

 35 Figure 3.5 Proposed network structure of NR in solid form [8].

Figure 3 . 6

 36 Figure 3.6The possible mechanism of mastication of rubber[START_REF] Fries | Mastication of Rubber[END_REF].

Figure 3 . 7

 37 Figure 3.7 Influence of temperature on the efficiency of mastication [15].

Figure 3 . 8

 38 Figure 3.8 Influence of temperature and mastication time on the relative viscosity of NR in the internal mixer [15].

Figure 3 . 9 2

 392 Figure 3.9 Mechanism of oxidation degradation of natural rubber [19].

Figure 3 . 10

 310 Figure 3.10 Natural rubber latex degradation by using phenylhydrazine and oxygen (A Process) [20].

Figure 3 . 11

 311 Figure 3.11 Proposed of oxidative degradation of epoxidized rubber (PNR) by using periodic acid [23].

Figure 3 . 12 Literature Review / 18 (

 31218 Figure 3.12 Relation of cross-linking density and vulcanizate property [26].

Figure 3 . 13

 313 Figure 3.13 Reaction sequences in sulfur vulcanization [30].

Figure 3 . 14

 314 Figure 3.14 General chemical structure of sulfur vulcanization (a) monosulphidic crosslinks, (b) disulphidic cross-links, (c) polysulphidic cross-links (x = 3-6), (d) polysulphidic cross-links connected several elastomer chains, (e) vicinal cross-links, (f)

Figure 3 .

 3 Figure 3.18 Double-layer interface model represented by Fukahori [35].

Figure 3 . 19

 319 Figure 3.19 The relation of surface area of carbon black with bound rubber and

Figure 3 .

 3 Figure 3.20 Stressstrain curves of PALF/CB-NBR and CB-NBR composites in longitudinal direction at fiber content 10 phr with different amount of carbon black content varying from 10-30 phr [4].

Figure 3 . 21

 321 Figure 3.21 Types of fiber [41].

  compositions of natural fiber are made of cellulose, hemicellulose, lignin, pectin, etc. as shown in Figure 3.22. The amounts of chemical compounds depend on the type of natural fiber. PALF has a high content of cellulose. The structure of cellulose, hemicellulose, lignin, and pectin is shown in Figure 3.23.

Figure 3 . 22 Literature Review / 32 Figure 3 . 23

 32232323 Figure 3.22 Pineapple leaf fiber and other natural fibers chemical composition [49].

Figure 3 .Literature Review / 34 Figure 3 .

 3343 Figure 3.24 OM micrograph of stained pineapple leaf in transverse section consisting of vascular fiber bundles (A) and mesophyll fiber bundles (B) [51].

  Université de Haute Alsace & Mahidol University Ph.D. (Polymer Science and Technology) / 35 treatment. The changes of mechanical properties occurred from the modification of the fiber structure such as crystallinity ratio, degree of polymerization, and orientation (Hermans factors) [54]. Sreekala et al. found that 10-30% sodium hydroxide solution provided the best effects on natural fiber properties [55]. Nopparut et al. has been studied alkali treatment of PALF with 10% sodium hydroxide solution for 30 min and the fiber was used in nylon 6 composites. Tensile, flexural strength and flexural moduli have also increased after treatment [56]. Lopattananon et al. has been also studied the effect of sodium hydroxide concentration (1, 3, 5, and 7%) on PALF fiber for 18 h at room temperature. An appropriate concentration of 5% provides the best tensile strength in the final rubber composites.

Figure 3 . 26

 326 Figure 3.26 Illustrate of the effect of natural fiber surface with alkali treatment.

Figure 2 . 37 3. 5 . 3 . 2

 237532 Figure 2.27 SEM micrograph of pineapple leaf fiber (A and B) and washing with NaOH solution (C, D and F) in longitudinal direction with different magnifications [51].

Figure 3 . 28

 328 Figure 3.28 Interaction of grafting silane on natural fiber by hydrolysis process [62].

Figure 3 . 29

 329 Figure 3.29 Proposition of silane-69 reaction on silica particles and NR [65].

Figure 3 . 41 3. 5 . 3 . 3

 341533 Figure 3.30 Stress-strain curve of SBR/BR/silica composites with different coupling agents. TESPT ( ); TESPD ( ); TESPM ( ); ETES ( ); HTES ( ); DTES (x); MPTES ( ) [65].

Figure 3 . 31

 331 Figure 3.31 Scheme of formation of lignosulfonate by sodium sulfite and lignin [66].

  (p-phenylene terephthalamide)(PPTA) or Kevlar is a type of aramid fiber. It was first developed by Dupont and its chains contained p-disubstituted benzene rings. Twaron based on PPTA and Technora based on copoly(p-phenylene-3,4-diphenyl ether terephthalamide) (ODA-PPTA) were produced by Teijin company. The aramid polymer chemical structures, preparation are shown in Figure 3.32 and 3.33. Aramid fiber can be classified into two branches. The first branches display aramid material with lower deformability and dynamic deformation modulus reaching values in the range of 130-160 GPa. This group is designed as a high-strength structure. The second branch has a lower dynamic deformation with modulus value of the order of 80-120 GPa. The mechanical properties of aramid fiber are shown in Table

Figure 3 . 32 Literature Review / 44 Figure 3 . 33

 33244333 Figure 3.32 Chemical structure of aramid polymer [81].

Figure 3 .

 3 Figure 3.34 Stressstrain curves of Nomex fiber at different temperatures [80].

Figure 3 .Literature Review / 48 Figure 3 .

 3483 Figure 3.35 Stress-strain curves of different amounts of PALF reinforced NBR composites. Open symbols and close symbols represent composites without and with bonding agent, respectively [87].

Figure 3 .

 3 Figure 3.37 Stress-strain curves of PALF reinforced NR hybrid carbon black composites with varying amounts of carbon black (a) and focus on the low strain region (b) [89].

Figure 3 .

 3 Figure 3.38 Stress-strain curves of PALF reinforced NR hybrid carbon black composites with varying amounts of sulfur. The composites contain carbon black content at 30 phr. Close symbols and open symbols represent composites without and with PALF at 10 phr [90], respectively.

Figure 3 . 51 Figure 3 . 40

 351340 Figure 3.39 Stress-strain curves of PALF reinforced NR with 10 phr UPALF (a) and TPALF (b) with different mastication time [44].

reached 7 ,

 7 and it was dried at 80 °C for 24 hours. Extraction of PALF and PALMF are shown in Figure 4.1.

Figure 4 . 1

 41 Figure 4.1 Extraction of pineapple leaf fiber from fresh leaf and pineapple leaf microfiber processing method [2].

Figure 4 . 2

 42 Figure 4.2 Pre-treatments of pineapple leaf micro fiber (PALMF)

4. 2 . 4

 24 Sulfuric acid treatment PALMF was treated with 10%w/v sulfuric acid solution at 85 C. The treatment time could vary from 20-120 min. Sampling was observed every 20 min. This PALMF was called Acidx. The x letter denotes to treatment time (min).PALMF could be both treated with acid at 120 min, and underwent a delignification treatment. This PALMF was called AcidxDx. The x letter denotes to treatment time (min).

4. 2 . 5 57 4. 3

 25573 Concentrated sulfuric acid treatment PALMF was treated with high concentrated sulfuric acid at 61 % w/v. The temperature of this experiment was 45 C. This fiber was called AcidCx. The x letter denotes to treatment time (min). Université de Haute Alsace & Mahidol University Ph.D. (Polymer Science and Technology) / Silanization of pineapple leaf fiber (PALMF) and pineapple leaf fiber (PALF) Bis-[γ-(triethoxysilyl)-propyl]-tetrasulfide (silane-69) or (3-Aminopropyl)

  It used double anode (Al Kα = 1486.6 eV and Mg Kα = 1253.6 eV) and hemispherical analyzer. 2 types of scan have been carried out: wide-scan and high resolution scan. The sample was measured under high vacuum. The x-ray can penetrate the surface of sample over a thickness of around 10 nm.

Figure 5 .

 5 1 shows the PALMF and Si69-treated PALMF surface. Si69-treated PALMF surface did not significantly change after treatment in comparison with bare PALMF.

Figure 5 . 1

 51 Figure 5.1 SEM micrographs of untreated PALMF (a) and Si69-treated PALMF (b).

Figure 5 . 2

 52 Figure 5.2 FTIR spectra of PALMF and silane-treated PALMF.

Figure 5 . 3

 53 Figure 5.3 XPS wide scan and high resolution spectra of C1s of (a and b) PALMF and (c and d) silanized PALMF.

Figure 5 . 4 and

 54 Figure 5.4 and Table 5.3. Storage moduli of composites reinforced with PALMF and Kevlar fibers, at -75 °C, (temperature lower than glass transition temperature (Tg)) are significantly equivalent. When temperature increases and reaches 25 °C and 60 °C, there

Figure 5 . 4 Figure 5 . 5

 5455 Figure 5.4 Temperature dependences of E' and tan delta of NR containing various amounts of Kevlar and PALMF and for different surface treatments of PALMF.

Figure 5 . 6

 56 Figure 5.6 Cure curves of NR/PALMF composites containing different fiber content and fiber treatment.

Figure 5 . 7

 57 Figure 5.7 Stress-strain curves of different treatment of PALMF and Kevlar [93] reinforced natural rubber composites with fiber content at 2phr (a), 5 phr (b), and 10 phr (c).

  Université de Haute Alsace & Mahidol University Ph.D. (Polymer Science and Technology) / 63 Tensile characteristics of composites are shown in Figure 5.8, especially modulus, tensile strength, and elongation at break. The tensile strength of

Figure 5 . 8

 58 Figure 5.8 Tensile strength, modulus and elongation at break of PALMF and Kevlar reinforced NR composites with different surface treatments of PALMF.

Figure 5 . 9 Figure 5 .

 595 Figure 5.9 SEM micrographs (high and low magnification) of cryogenic-fractured composites containing Kevlar (a and d), PALMF (b and e) and Si69-PALMF (c and f) reinforced NR composites.

  factor or modulus adjustment variable for short fiber𝛽= Constant value depends on shape and orientation of fiber It is assumed that the composite is composed of oriented cylindrical fibers in uniaxial direction and it has perfect adhesion between fiber and matrix. It is arranged with square shape.

69 Figure 5 . 11

 69511 Figure 5.11 Scheme of hexagonal model [104].

  Université de Haute Alsace & Mahidol University Ph.D. (Polymer Science and Technology) / 71

Figure 5 . 12 FTIRTable 5 . 5 Figure 5 .

 512555 Figure 5.12 FTIR spectra of NR and PANR.

Figure 5 .Figure 5 . 13 (

 5513 Figure 5.13 NMR spectra of (a) NR and (b) PANR.

5. 2 . 2

 22 Pre-treatments of fiber and characterization Pre-treatment of pineapple leaf microfiber (PALMF) represents an important step for a post-silanization process. The purpose of this action is to clean fiber surface to expose hydroxyl groups at the fiber surface. Different methods have been proposed to clean the fibers before silanization and are shown in Figure 5.14. PALMF was observed with Scanning electron microscope (SEM).

Figure 5 .

 5 Figure 5.14 Pre-treatments of pineapple leaf microfiber (PALMF).

Figure 5 .Figure 5 .Figure 5 . 2 ASC2T15Figure 5 .ASC3T30Figure 5 .

 555255 Figure 5.15 SEM micrographs of pre-treated PALMF with alkali solution at ambient temperature.

7 and Figure 5 Figure 5 . 83 D120PALMFFigure 5 . 4 .Figure 5 .Figure 5 . 5 .

 558354555 Figure 5.18 SEM micrographs of pre-treated PALMF with delignification solution at different treatment time at ambient temperature.

Figure 5 .Figure 5 .

 55 Figure 5.20 SEM micrographs of pre-treated PALMF with concentrated sulfuric acid solution at different treatment time at ambient temperature.

Figure 5 .

 5 Figure 5.21 XPS wide scan and high resolution spectra of C1s of (a and b) PALMF, (c and d) AC3T30 PALMF, (e and f) ASC3T30 PALMF and (g and h) D120 PALMF.

Figure 5 .

 5 Figure 5.22 FTIR spectra of PALF, APALF, and PALMF.

1

 1 Université de Haute Alsace & Mahidol University Ph.D. (Polymer Science and Technology) / 91

2

 2 Morphology of pineapple leaf fiber (PALF) and microfiber (PALMF)

Figure 5 .

 5 Figure 5.23 SEM micrographs of untreated PALF (a), alkali treated PALF (b) and untreated PALMF (c).

Figure 5 .

 5 Figure 5.24 XPS wide scan spectra of C1s of aminosilane treated PALMF (a) and aminosilane treated APALF (b) surface.

4 Figure 5 .

 45 Figure 5.25 shows the morphology of alkali-treated PALF, amino silane treated APALF, untreated PALMF, and amino silane treated PALMF. The fiber surface did not significantly change between before and after the silanization of PALF and PALMF.

Figure 5 .

 5 Figure 5.25 SEM micrographs of alkali treated PALF (a), aminosilane treated APALF (b), untreated PALMF (c) and aminosilane treated PALMF (d).

Figure 5 . 26

 526 Figure 5.26 Images of fiber after coated with PANR. PALF (a), APALF (b), PALMF (c), aSiAPALF (d), and aSiPALMF (e).

Figure 5 . 27 Figure 5 . 28

 527528 Figure 5.27 Elastic moduli (Eꞌ) and tan  of natural rubber and its composites containing PANR, 10 phr of fiber and 30 phr of carbon black with various types of fibers. The composite containing PANR was represented with the letter c.

2 Figure 5 . 29

 2529 Figure 5.29 Cure curves of natural rubber and its composites containing PANR, 10 phr of fiber and 30 phr of carbon black with various types of fibers. The composite containing PANR was represented with the letter c.

Figure 5 .

 5 Figure 5.30 shows the stress-strain curves of composites containing PANR hybrid filler with fiber and carbon black. The initial stage of stressstrain curves displays a high slope when force started to be applied. Then, the interface failure of all composite occurs at approximately 25% of strain by slightly dropping of stress. When stress was applied further, the modulus remained constant and it kept on increasing until the end of strain. PANR improves the modulus in the low strain region for modified PALF with sodium hydroxide and amino silane. At the beginning of the stress-strain curves, APALF composite shows the improvement of modulus compared to PALF and it kept on improving after amino silane treatment onto fiber. However, PANR has no effect on PALMF and modified PALMF with amino silane. In order to clearly observe the difference, the illustration of relative modulus, tensile strength, and elongation at break are shown in Figure5.31. Modulus at 10% strain of composites containing PALF with and without PANR are not significantly different. However, the modulus of APALF composite increases compared to PALF and further increases for aSiAPALF. The use of PANR has no effect on the modulus of PALMF and aSiPALMF composites. The tensile strength of composites is slightly increased after using fiber coated with PANR except for aSiAPALF composite. Elongation at break of composites decreases after addition of fiber, and it further decreases with the addition of modified fiber coated with PANR except for PALMF and aSiAPALF composite. All composites clearly display fiber pulled out in the tensile fracture surface (see FiguresA2 in appendix

Figure 5 . 30 Figure 5 . 31 105 5. 2 . 6 . 4

 530531105264 Figure 5.30 Stressstrain curve of natural rubber and its composites containing PANR, 10 phr of fiber and 30 phr of carbon black with various types of fibers. The composite containing PANR was represented with the letter c.

Figure 5 . 32

 532 Figure 5.32 Cryogenic fractures at low and high magnification of hybrid filler of carbon black 30 phr and 10 phr of PALF reinforced NR composites. PALF (a and b), PALFc (c and d), APALFc (e and f), PALMFc (g and h), aSiAPALFc (i and j), and aSiPALMFc (k and l).

  Université de Haute Alsace & Mahidol University Ph.D. (Polymer Science and Technology) / 107

Figure 5 .

 5 Figure 5.32 (cont.) Cryogenic fractures at low and high magnification of hybrid filler of carbon black 30 phr and 10 phr of PALF reinforced NR composites. PALF (a and b), PALFc (c and d), APALFc (e and f), PALMFc (g and h), aSiAPALFc (i and j), and aSiPALMFc (k and l).

Figure 5 . 33

 533 Figure 5.33 Oxidation of natural rubber [19].

(

  PALF) with sodium hydroxide and mechanical force. PALMF without any pretreatment shows partially cementing cover on the fiber surface. The cleaning of PALMF fiber surface was investigated with different treatments such as sodium hydroxide, mixing solution between sodium hydroxide and sodium sulfite, delignification, diluted sulfuric acid, and concentrated sulfuric acid. However, cementing layer still covers PALMF surface after application of this cleaning and it was further confirmed by XPS analysis. The results show that the O/C ratio was not significantly changed after the application of the different cleaning processes. The O/C values of PALMF after treatments are in the range of 0.50-0.52 while the theoretical C/O ratio values of cellulose and lignin are 0.83 and 0.33 respectively[START_REF] Johansson | Evaluation of surface lignin on cellulose fibers with XPS[END_REF]. PALMF surface does not correspond to neither cellulose nor lignin. A possible reason is that PALMF surface might still be partially covered with lignin. The O/C ratio of PALMF is similar to untreated coir fiber whose ratio is 0.29[START_REF] Tran | Wetting analysis and surface characterisation of coir fibres used as reinforcement for composites[END_REF]. Muensi et al. reported that delignification

Figure 5 .

 5 Figure 5.10 from part 5.1 shows that pineapple leaf microfiber (PALMF)has been extracted from uncured rubber and the length of PALMF is shorter than 6 mm. One of the reasons to explain the breaking of PALMF might be a possible weakening of the fiber during its preparation (treatment). Therefore, pineapple leaf fiber (PALF) was preferred to reinforce NR composite. PALF was treated with 10% w/v sodium hydroxide solution and was characterized by FTIR. FTIR signal that corresponding to

Figure 5 . 34

 534 Figure 5.34 Illustration of grafting of amino silane onto fiber.

Figure 5 . 35

 535 Figure 5.35 Illustration of carbinolamine formation between fiber and PANR.

Figure 5 .

 5 Figure 5.36 Kuraray LIR-410 chemical structure [122].

Figure 5 .

 5 Figure 5.38 NMR spectrum of AP.

Figure 5 .Figure 5 .c 123 5. 3 . 2 . 2 Figure 5 . 40 Figure 5 . 41

 55123322540541 Figure 5.39 shows untreated PALF, alkali-treated PALF, and silane-69 treated PALF. The PALF displays cementing material covering the fiber surface. After treatment of PALF with 10% w/v sodium hydroxide for 30 min, the cementing material was removed. The size of the fiber bundle also decreased. Silane-69 PALF surface did not significantly change after silane-69 treatment compared with APALF.

Figure 5 . 42

 542 Figure 5.42 Cure curves of natural rubber and its composites containing 10 phr of fiber and 30 phr of carbon black with various types of fibers.

Figure 5 .Figure 5 . 44 Figure 5 .

 55445 Figure 5.43 Stressstrain curve of natural rubber and its composites containing 10 phr of fiber and 30 phr of carbon black with various types of fibers.

Figure 5 . 45

 545 Figure 5.45 Cryogenic fractures at low and high magnification of hybrid filler of carbon black 30 phr and 10 phr of PALF reinforced NR composites. NR+PALF (a and b), NR+PALF+AP (c and d), NR+APALF+AP (e and f) and NR+SiAPALF+AP (g and h).

Figure 5 .

 5 Figure 5.45 (cont.) Cryogenic fractures at low and high magnification of hybrid filler of carbon black 30 phr and 10 phr of PALF reinforced NR composites. NR+PALF (a and b), NR+PALF+AP (c and d), NR+APALF+AP (e and f) and NR+SiAPALF+AP (g and h).

  Université de Haute Alsace & Mahidol University Ph.D. (Polymer Science and Technology) / 131

Figure 5 . 46

 546 Figure 5.46 Proposed adhesion mechanism between the fiber and the AP.

  Alsace & Mahidol University Ph.D. (Polymer Science and Technology) / 133

  Université de Haute Alsace & Mahidol University Ph.D. (Polymer Science and Technology) / 149

  Université de Haute Alsace & Mahidol University Ph.D. (Polymer Science and Technology) / 151 3. The functionalized low molecular weight natural rubber (PANR)

Figure 6 . 1

 61 Figure 6.1 Comparison elastic modulus of PALF, PALMF, and Kevlar (dot lines) composites of all systems at different temperature.

FiberFigure 6 . 1 (

 61 Figure 6.1 (cont.) Comparison elastic modulus of PALF, PALMF, and Kevlar (dot lines) composites of all systems at different temperature.

Fiber

  content (phr) some recent results," Journal of Applied Polymer Science, vol. 78, no. 8, pp. 1461-/1097-4628(20001121)78:8<1461::AID-APP20>3.0.CO;2-V.

Figure A1 (Figure A2

 A1A2 Figure A1 (cont.) Tensile fractures of composites containing Kevlar 2 phr (a), Kevlar 5 phr (b), Kevlar 10 phr (c), PALMF 2 phr (d), PALMF 5 phr (e), PALMF 10 phr (f), Si69-PALMF 2 phr (g), Si69-PALMF 5 phr (h), and Si69-PALMF 10 phr (i) reinforced NR composites.

Figure A3

 A3 Figure A3 Tensile fractures of hybrid filler of carbon black 30 phr and 10 phr of PALF reinforced NR composites. NR+PALF (a), NR+PALF+AP (b), NR+APALF+AP (c) and NR+SiAPALF+AP (d).

Table 3 .

 3

	Component	Percentage (w/w)
	Rubber	30-40
	Proteins	1-1.5
	Resins	1.5-3
	Mineral matter	0.7-0.9
	Carbohydrates	0.8-1
	Water	55-60

[START_REF]Aramid-short-fiber reinforced rubber as a tire tread composite -Kashani[END_REF]

Components of fresh natural rubber latex

[START_REF] Kurian | Natural Rubber: Production, Properties and Applications[END_REF]

.

Table 3 . 2

 32 Natural rubber comparison of standard Thai rubber (STR)[START_REF]Standard Thai Rubber | Thaihua[END_REF].

			TSR CV	TSR L	TSR 5	TSR 10	TSR 20
	Parameter	Unit	STR 5 CV	STR XL	STR 5L	STR 5	STR 10	SRT 10CV	STR 20	STR 20CV
	Dirt (max)	% wt	0.04 0.02 0.04 0.04 0.08 0.08 0.16 0.16
	Ash (max)	% wt	0.60 0.40 0.40 0.60 0.60 0.60 0.80 0.80
	Nitrogen (max)	% wt	0.60 0.50 0.60 0.60 0.60 0.60 0.60 0.60
	Volatile Matter (max)	% wt	0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80
	Initial Plasticity (min)		NA	35	35	30	30	NA	30	NA
	Plasticity
	Retention Index (PRI)		60	60	60	60	50	50	40	40
	(min)
	Colour
	Lovibond Scale (individual		NA	4.0	6.0	NA	NA	NA	NA	NA
	value, max)
	Moonery Viscosity** (ML, 1+4, 100 ̊C		60 +7 /-5	NA NA	NA	NA	60 +7 /-5*	NA	65 +7 /-5*

Table 3

 3

	.4. The general chemical structure is shown

Table 3 .5

 3 Sulfur and Accelerator level of sulfur vulcanization system[START_REF] Kruželák | Sulphur and peroxide vulcanisation of rubber compounds -overview[END_REF].

	Type	Sulfur (S)/ phr	Accelerator (A)/	A/S ratio
			phr
	Conventional	2.0-3.5	1.2-0.4	0.1-0.6
	Semi-EV	1.0-1.7	2.4-1.2	0.7-2.5
	EV	0.4-0.8	5.0-2.0	2.5-12.0
	phr -Parts per hundred rubber.

Table 3 .6

 3 The properties of sulfur vulcanization system[START_REF] Kruželák | Sulphur and peroxide vulcanisation of rubber compounds -overview[END_REF].

	Parameter	Conventional	Semi-EV	EV
	Poly and disulphidic cross-	95	50	20
	links/ %
	Monosulphidic cross-links/	5	50	80
	%
	Cyclic sulphide	high	medium	low
	concentration
	Heat-ageing resistance	low	medium	high
	Reversion resistance	low	medium	high
	Fatigue resistance	high	medium	low
	Tear resistance	high	medium	low
	Compression set	high	medium	low

Figure 3.17 Effect of particulates filler on modulus or viscosity of rubber [34].

	Budsaraporn Surajarusarn Université de Haute Alsace & Mahidol University	Literature Review / 22 Ph.D. (Polymer Science and Technology) / 23
	Figure 3.16 Carbon black structure and size in primary particle, aggregate, and
	agglomerate forms [33].
	3.16. Primary particles or nodules have diameters approximately 15 -300 nm. CB
	aggregates are the smallest dispersible units. They act as temporary structure because
	they are held together by van der Waals forces (physical bond). Their size is of the order
	of 10-30 µm, which is larger than intrinsic flaw size; however, they can also play a role
	as defects. But, there are also other mechanisms that might affect to modulus or viscosity
	(see Figure 3.17).

Table 3 .7

 3 Characteristics of carbon blacks[START_REF] Roland | Reinforcement of Elastomers[END_REF].

ASTM type Particle size (nm) Aggregate size (nm) Surface area (m 2 g -1 )

	Budsaraporn Surajarusarn			Literature Review / 24
	N110	17	54	143
	N220	21	65	117
	N330	31	86	80
	N339	26	75	90
	N351	31	89	75
	N550	53	139	41
	N660	63	145	34
	N762	110	188	21
	N990	246	376	9
	A new model of carbon black was proposed by Fukahori [35]. This model
	consists of 2 layers displayed in Figure 3.18. The inner layer is rubber in the glassy state
	and it is called Glassy Hard layer (GH). Its thickness is around 2 nm. The outer layer is
	rubber with more mobility compared to GH layer. It is called Sticky Hard layer (SH
	layer) and its thickness is approximately 3-8 nm. The GH layer plays the main role in
	stress increases at low extensions, while the SH layer increases at high extensions. These
	rubber layers cover on the surface of carbon black are called bound rubber (rubber -
	filler interaction). They are insoluble in good solvent due to strong interaction. As a
	results, the increase particle surface area of carbon black has an effect on the increase
	of bound rubber as depicted in Figure 3.19. Carbon black with smaller size increases
	bound rubber content but also reinforcement in composite (due to their high specific
	surface area) [36].

Table 3 .8

 3 Energy requirements for fibers production[START_REF] Joshi | Are natural fiber composites environmentally superior to glass fiber reinforced composites?[END_REF].

Nonrenewable energy requirements (MJ/kg) Glass fiber mat Flax fiber mat China reed fiber

	Budsaraporn Surajarusarn				Literature Review / 28
	Raw materials	1.7	Seed production	0.05	Cultivation	2.50
	Mixture	1.0	Fertilizers	1.0	Transport plant	0.40
	Transport	1.6	Transport	0.9	Fiber extraction	0.08
	Melting	21.5	Cultivation	2.0	Fiber grinding	0.40
	Spinning	5.9	Fiber separation	2.7	Transport fiber	0.26
	Mat production	23.0	Mat production	2.9
	Total	54.7	Total	9.55	Total	3.64

Table 3 .9

 3 Environmental impacts from production of glass fiber and china reed fiber[START_REF] Joshi | Are natural fiber composites environmentally superior to glass fiber reinforced composites?[END_REF].

	Environmental impact	Glass fiber	China reed fiber
	Energy use (MJ/kg)	48.33	3.64
	Carbon di-oxide emissions (kg/kg)	2.04	0.66
	CO emissions (g/kg)	0.80	0.44
	SOx emissions (g/kg)	8.79	1.23
	NOx emissions (g/kg)	2.93	1.07
	Particulate matter (g/kg)	1.04	0.24
	BOD to water (mg/kg)	1.75	0.36
	COD to water (mg/kg)	18.81	2.27
	Nitrates to water(mg/kg)	14.00	24481
	Phosphates to water (mg/kg)	43.06	233.6

Table 3 .

 3 [START_REF]Finding Flexible Solutions in the Rubber Industry[END_REF] Mechanical properties of natural fiber[START_REF] Mohanty | Biofibres, biodegradable polymers and biocomposites: An overview[END_REF].

	Fiber	Density	Diameter	Tensile	Young's	Elongation at
		(g/cm3)	(μm)	strength	modulus	break (%)
				(MPa)	(GPa)
	Jute	1.3-1.45	20-200	393-773	13-26.5	1.16-1.5
	Flax	1.5-3	-	345-1100	27.6	2.7-3.2
	Hemp	-	-	690	-	1.6
	Ramie	1.5	-	400-938	61.4-128	1.2-3.8
	Sisal	1.45	50-200	468-640	9.4-22.0	3-7
	PALF	-	20-80	413-1627	34.5-82.51	1.6
	Cotton	1.5-1.6	-	287-800	5.5-12.6	7.0-8.0
	Coir	1.15	100-450	131-175	4-6	15-40
	E-glass	2.5	-	2000-3500	70	2.5
	S-glass	2.5	-	4570	86	2.8
	Aramid	1.4	-	3000-3150	63-67	3.3-3.7
	Carbon	1.7	-	4000	230-240	1.4-1.8

Table 3 .

 3

	Method	Fiber yield	Range of diameter	Average diameter
		(%)	(μm)	(μm)
	Retting	1.8	5-166	58.98
	Scraping	1.4	5-129	57.36
	Ball milling	2.9	3-95	8.66
	Milling	2.8	3-68	18.70
	Milling of dried leaf	3.0	5-194	63.43

[START_REF] Sakdapipanich | CHAPTER 2:Natural Rubber: Biosynthesis, Structure, Properties and Application[END_REF]

Fiber yields and fiber diameter of PALF obtained from different extraction methods

[START_REF] Kengkhetkit | Utilisation of pineapple leaf waste for plastic reinforcement: 1. A novel extraction method for short pineapple leaf fiber[END_REF]

.

The important factors that have an effect onto mechanical properties of natural fiber are micro-fibril angle (MFA) of natural fiber. MFA is the orientation of the constituent cellulose. It was found that it has a direct effect on its stiffness and tensile strength. Variations of MFA depend on type of fiber as displayed on Table

10

. The Université de Haute Alsace & Mahidol University Ph.D. (Polymer Science and Technology) / 31

  It has at least 85% of the amide linkage and attached directly to two aromatic rings. Its properties are excellent with high thermal stability, chemical resistance, higher tensile strength, and modulus compared to aliphatic polyamide fibers. Due to their excellent properties, it has been

used as reinforcing material in many applications, especially automotive and optical fiber cables. It was first developed by Nomex from DuPont in 1960s

[START_REF] Akato | 10 -High performance fibers from aramid polymers[END_REF]

. There are many types of aramid fibers depending on producers such as Dupont and Teijin. Poly

Table 3 .

 3

	Commercial	Kevlar	Twaron	Technora	Teijinconex	Nomex
	name
	Polymer	PPTA	PPTA	ODA/PPTA	MPDI	MPDI
	Fiber type	K-24 K-	Std HM	Std	Std	HT	430
			49
	Density	1.44 1.44 1.44 1.45	1.39	1.38	1.38	1.38
	(g/cm3)
	Strength	2.9	3	2.9	2.9	3.4	0.61-	0.73-	0.59
	(GPa)						0.68	0.86
	Elongation	3.6	2.4	3.6	2.5	4.6	35-45 20-30	31
	(%)
	Modulus	71	112	70	110	72	7.9-9.8 11.6-	11.5
	(GPa)							12.1

[START_REF]Standard Thai Rubber | Thaihua[END_REF]

Aramid polymers properties

[START_REF] Ertekin | 7 -Aramid fibers[END_REF]

.

1.2 Chemical for fiber surface treatment

	Budsaraporn Surajarusarn Université de Haute Alsace & Mahidol University	Materials and Methods / 52 Ph.D. (Polymer Science and Technology) / 53
	4.1.
	CHAPTER IV
	MATERIALS AND METHODS
	4.1 Materials and instruments
	Bis-[γ-(triethoxysilyl)-propyl]-tetrasulfide (silane-69), (3-Aminopropyl)
	triethoxysilane and sulfuric acid were purchased from Sigma-Aldrich. Sodium
	hydroxide (NaOH), sodium chlorite (NaClO2), glacial acetic acid (CH3COOH), and
	sodium sulfite (Na2SO3) were purchased from Alfa Aesar. Toluene and ethanol were
	purchased from Carlo Erba.

4.1.1 Rubbers, carbon black and curing additives

Natural rubber (NR) was STR5L grade and it was purchased from M.B.J.

Enterprise

Co., Ltd. Polar liquid rubber (Kuraray LIR-410) was kindly supported by Kuraray Co., Ltd., Japan. High ammonia natural rubber latex (HNRL) is commercial grade and was purchased from Thai Rubber Latex Corporation (Thailand) Public Co. Ltd. Tergitol-15s-15 is commercial grade and was purchased from East Asiatic Company (Thailand). Curing additives, i.e., zinc oxide (ZnO), stearic acid, sulfur and CBS (N-Cyclohexyl-2-benzothaiazolesulfenamide) were commercial grade. Carbon black was N330 grade manufactured by Thai Carbon Black Public Co., Ltd. The aramid fiber was Kevlar® (DuPont, USA) and it was produced in masterbatches with fiber content of 40 wt% in NR. It was supplied by Polymer Innovation Co., Ltd. (Bangkok, Thailand).

4.

3 Pineapple leaf fiber (PALF) and pineapple leaf microfiber (PALMF)

Table 4 . 1

 41 The instruments used in this work are shown in table 4.1. This table showed the name of instrument, model, manufactures and place. The list of all instruments.

	Budsaraporn Surajarusarn

2 Sodium hydroxide and sodium sulfite treatment PALMF

  was treated with an oxidant solution composed of 10% w/v sodium hydroxide and 17% w/v sodium sulfite. This pre-treatment was similar to sodium hydroxide treatment. It could have been repeated three times and referred to first, second and third cycles. In each cycle, there are different times of treatment which were 15 or 30 min. This PALMF was called ASCxTyPALMF. The letter x is the cycle number and y is treatment time.

	Budsaraporn Surajarusarn	Materials and Methods / 56
	4.2.1.2 Treatment temperature
	PALMF from the third cycle with a treatment duration of 15 min
	was studied at different temperatures. Solution was heated at either 25 C, 50 C, or 85 C.
	This PALMF was called ACxTyPALMF-z. The letter x is the cycle number, y is
	treatment time and z is temperature.
	4.2.

4.2.3 Delignification treatment

PALMF was mixed with 17 wt% sodium chlorite and glacial acetic acid at the ratio 1:1 by volume at 80 • C with varies time. PALMF was washed with distilled water until pH was equal to 7 to remove chemical residues. PALMF fibers were called DxPALMF, where x is treatment time. DxPALMF was dried at 80 • C for 24 hours.

4 Preparation fiber rubbers composites

  The compounds were mixed by using two-roll mill. Sequences of mixing are depicted in Tables 4.2 and 4.3. All chemicals and sequences were used and shown in Table4.4.

	Université de Haute Alsace & Mahidol University	Ph.D. (Polymer Science and Technology) / 59
	4.5.

Table 4 . 2

 42 Sequences of natural rubber reinforced with aramid fiber mixing.

	Step	Ingredients	Time (min)
	1	NR	0-2
	2	Master batch of fiber	2-6
		Such as aramid fiber, PALMF, and
		PALF
	3	ZnO and stearic acid	8-10
	4	Sulfur and CBS	10-13
	5	Sheet out	13-14
	Total time	14

Table 4 . 3

 43 Sequences of natural rubber reinforced with fiber and carbon black mixing.

	Step	Ingredients	Time (min)
	1	NR	0-2
	2	Master batch of fiber	2-6
		Such as aramid fiber,
		PALMF, and PALF
	3	Carbon black	6-8
	4	ZnO and stearic acid	8-10
	5	Sulfur and CBS	10-13
	6	Sheet out	13-14
	Total time	14

Table 4 . 4

 44 Formulation of rubber mixing composites with aramid fiber.

		Amount (phr)
	Compound/Ingredient	Rubber CB	Aramid fiber	ZnO	Steric acid	CBS Sulfur
	K0	100	-	-	5	2	1
	K2	100	-	2	5	2	1
	K5	100	-	5	5	2	1
	K10	100	-	10	5	2	1
	K2CB30	100	30	2	5	2	1
	K5CB30	100	30	5	5	2	1
	K10CB30	100	30	10	5	2	1

Table 4 . 5

 45 Formulation of rubber mixing composites with PALF or PALMF.

		Amount (phr)
	Compound/Ingredient	Rubber CB	PALMF or PALF	ZnO	Steric acid	CBS Sulfur
	PALMF 0	100	-	-	5	2	1
	PALMF 2	100	-	2	5	2	1
	PALMF 5	100	-	5	5	2	1
	PALMF 10	100	-	10	5	2	1
	PALF10CB30	100	30 10	5	2	1

  1 Hz respectively. The curing temperature was 150 C.

	Université de Haute Alsace & Mahidol University	Ph.D. (Polymer Science and Technology) / 63
	cure rate index =	100 t c90 -t s2	Equation 1
	4.6.7 Nuclear magnetic resonance spectroscopy (NMR)
	Nuclear magnetic resonance spectra were obtained using Bruker Advance
	spectrometer for 1 H (400 MHz) nuclei. Residual solvent peaks were used as references

and chemical shifts was reported in ppm (part per million).

Table 5 . 1

 51 Assignment of FTIR signal position peaks to functional groups on different treatment fiber surface.

	Signal position (cm -1 )	Assignment
	3278	O-H stretching
	2904	C-H stretching
	1734	C=O stretching
	1635	Adsorbed water
	1425	CH 2 asymmetric deformation
	1370	CH 3 symmetric deformation
	1335	CH 2 wagging
	1316	O-H vibration

Table 5

 5

	.2.

Table 5 . 2

 52 Extracted elemental compositions of silanization of PALMF.

		Name		% Atomic content
			Control	Silane-69	Si69PALMF
			PALMF
	C1s	C-C	26.43	27.16	47.39
		C1 (C-OR)	33.58	19.10	20.5
		C2 (C=O,	6.72	-	2.84
		O-C-O)
		C-Si	-	8.07	2.59
		C-S	-	-	1.23
	O1s	O1	19.92	-	7.64
		O2	13.35	-	5.12
		Si-O (silane)	-	19.37	7.7
	S2p	S-S	-	6.08	1.21
		C-S	-	6.08	1.21
	Si2p	Si(-O) 3	-	8.05	2.57
	O/C		0.50		0.27
	ratio

Table 5 . 3

 53 The moduli and tan delta values of natural rubber composites at specific temperature.

	Name/ Temperature	Elastic modulus (MPa)	Tan delta at peak position	Temperature (C) at peak position
	(C)	-75 (C) 25 (C) 60 (C)
	NR	2783.9	1.7	1.4	2.8	-45
	K2NR	3351.3	3.5	3.1	1.7	-41
	PALMF2	3485.3	3.0	2.5	1.9	-44
	Si69PALMF2	3367.2	2.6	2.4	2.1	-44
	K5NR	3667.	9.2	7.9	1.1	-45
	PALMF5	3602.7	8.1	7.1	1.2	-45
	Si69PALMF5	3914.6	9.5	8.0	1.2	-44
	K10NR	4512.4	20.4	17.1	0.8	-45
	PALMF10	4275.5	31.0	23.7	0.8	-46
	Si69PALMF10	4076.9	35.0	25.4	0.7	-45

Table 5 . 4

 54 Cure characteristics of NR/PALMF composites containing different fiber content and fiber treatment.

		ML (N.m)	MH (N.m)	ts2 (min)	tc90 (min)
	NR	0.13	4.85	6.2	11.53
	PALMF2	0.14	6.02	5.84	13.85
	Si69PALMF2	0.16	6.48	5.79	10.51
	PALMF5	0.14	8.33	5.06	10.43
	Si69PALMF5	0.13	7.96	5.13	12.58
	PALMF10	0.13	10.1	4.77	10.75
	Si69PALMF10	0.14	9.74	4.72	12.38

  Université de Haute Alsace & Mahidol UniversityPh.D. (Polymer Science and Technology) / 67To answer this question, the model of modulus of composites proposed by Shear lag, but originally proposed by Cox[START_REF] Cox | The elasticity and strength of paper and other fibrous materials[END_REF] should be considered. The simple form of the equation is shown in the following.

	𝐸 𝑙 = 𝜂 𝑙 𝑉 𝑓 𝐸 𝑓 + (1 -𝑉 𝑓 )𝐸 𝑚	(1)

Table 5

 5

	𝑤 ̅̅̅̅̅ and 𝑀 𝑛 ̅̅̅̅ , values of NR are

.6 displays number average molecular weight (𝑀 ̅ n), weight average molecular weight (𝑀 ̅ w), and size average molecular weight (𝑀 ̅ z) of PANR and their values are 37,021, 66,279, and 103,119 daltons, respectively. Its polydispersity is 1.79. However, NR could not be observed because it was poorly dissolved in THF. Swanson et al.

[START_REF] Swanson | Molecular weights of natural rubbers from selected temperate zone plants[END_REF]

reported that the 𝑀

Table 5 . 6

 56 Molecular weight and polydispersity of PANR.

		𝑴 ̅ n (Daltons)	𝑴 ̅ w (Daltons)	𝑴 ̅ z (Daltons)	Polydispersity
	PANR	37021	66279	103119	1.79

Table 5 . 7

 57 Extracted elemental compositions of different pre-treated PALMF.

				% Atomic content
		Name	Control PALMF	AC3T30	ASC3T30	D120 PALMF
		C-C	26.43	26.37	25.18	23.86
		C1 (C-OR)	33.58	33.62	34.15	34.76
	C1s	C2 (C=O, O-C-O)	6.72	6.73	6.84	6.96
		C-Si	-	-	-	-
		C-S	-	-	-	-
		O1	19.92	19.93	20.26	20.61
	O1s	O2	13.35	13.35	13.58	13.81
		Si-O (silane)	-	-	-	-
	O/C ratio		0.50	0.50	0.51	0.52

Table 5 .

 5 8 FTIR signal position peaks corresponding to functional groups on PALF, APALF, and PALMF.

	Signal position (cm -1 )	Assignment
	3335	O-H stretching
	2919	C-H stretching
	1734	C=O stretching
	1640	Adsorbed water
	1425	CH 2 asymmetric deformation
	1370	CH 3 symmetric deformation
	1335	CH 2 wagging
	1316	O-H vibration

Table 5 . 9

 59 Extracted elemental compositions of silanization of PALMF and PALF.

		% Atomic content
		aSiPALMF	aSiAPALF
	C 1s	64.69	66.64
	O 1s	27.26	29.50
	N 1s	2.78	1.65
	Ca 2p	1.32	1.07
	Si 2p	3.95	1.13

Table 5 .

 5 [START_REF]Finding Flexible Solutions in the Rubber Industry[END_REF] The moduli and tan delta values of natural rubber and its composites containing PANR, 10 phr of fiber and 30 phr of carbon black with various types of fibers at specific temperature.

		Elastic modulus (MPa)	Tan delta	Temperature
	Name				at peak	(C) at peak
		-75 (C)	25 (C) 60 (C)	position	position
	NR	2780.7	1.8	0.9	3.2	-48
	PALF	4968.5	63.8	48.5	0.6	-47
	PALFc	5087.5	1245.0	95.4	0.5	-49
	APALFc	4106.7	41.1	29.2	0.7	-51
	PALMFc	4123.9	37.3	27.8	0.7	-51
	aSiAPALFc	4544.1	38.9	30.7	0.7	-49
	aSiPALMFc	5028.5	63.2	46.6	0.7	-48

* The composite containing PANR was represented with the letter c.

Table 5 .

 5 11 Cure characteristics of different compounds.

		ML (N.m) MH (N.m) ts2 (min) tc90 (min)
	NR	0.1	5.89	5.18	10.78
	PALF	0.24	13.32	3.33	6.51
	PALFc	0.59	16.03	2.52	6.06
	APALFc	0.71	13.69	2.76	5.92
	PALMFc	0.25	11.82	3.35	6.78
	aSiAPALFc	0.41	12.71	3.14	6.66
	aSiPALMFc	0.27	12.46	3.27	6.56

* The composite containing PANR was represented with the letter c.

Table 5 .

 5 [START_REF] Tanaka | Structural Characterization of Natural Polyisoprenes: Solve the Mystery of Natural Rubber Based on Structural Study[END_REF] FTIR signal position peaks corresponding to functional groups on AP.

	Signal position (cm -1 )	Assignment
	835 and 888	=C-H and =CH2 out of plane bending
	1000 -1300 (1033, 1091, 1166, and 1204)	O-C stretching (ester)
	1644	C=C symmetry
	1743	C=O stretching (ester)
	1708	C=O stretching (carboxylic)
	2850 -3000 (2853, 2914, and 2960)	CH3, CH2 and CH stretching

Table 5 .

 5 [START_REF] Sriring | Viscoelastic and mechanical properties of largeand small-particle natural Université de Haute Alsace & Mahidol University Ph.D. (Polymer Science and Technology) / 157 rubber before and after vulcanization[END_REF] Molecular weight and polydispersity of AP.

		𝑴 ̅ n (Daltons)	𝑴 ̅ w (Daltons)	𝑴 ̅ z (Daltons)	Polydispersity
	AP	28854	39353	64415	1.36

Table 5 .

 5 [START_REF]Standard Thai Rubber | Thaihua[END_REF] The moduli and tan delta values of liquid rubber and different type of fiber reinforced natural rubber composites at specific temperature.

					Tan
		Elastic modulus (MPa)	delta at peak	Temperature (C) at peak position
	Name				position
		-75 (C) 25 (C) 60 (C)
	NR	4181.2	5.8	2.5	1.36	-43
	NR+PALF	4968.5	63.8	48.5	0.62	-47
	NR+PALF+ AP	5099.3	135.5	102.8	0.44	-51
	NR+APALF+ AP	4799.8	172.4	123.0	0.43	-51
	NR+SiAPALF+ AP	5927.3	226.9	160.6	0.39	-52

Table 5 .

 5 [START_REF] Fries | Mastication of Rubber[END_REF] Cure characteristics of different compounds. The fiber and carbon black were used to achieve the content of 10 phr and 30 phr, respectively. At low strain region, all composites except unfilled NR reacts strongly against the applied stress. The stresses of each composite reach up to maximum and then slightly decrease. After prolonging strain, stresses remain constant and increase with a strong slope as a function of the strain increase. At low deformation, addition of PALF improves modulus at 10% strain compared to unfilled NR. The composites moduli are enhanced after the incorporation of adhesion promotor into the composites.

		ML (N.m) MH (N.m) ts2 (min)	tc90 (min)
	NR	0.18	8.49	3.89	6.62
	NR+PALF	0.24	13.32	3.33	6.51
	NR+PALF+AP	0.72	17.85	3.1	7.05
	NR+APALF+AP	0.78	17.04	3.2	7.05
	NR+SiAPALF+AP	0.66	16.71	3.12	6.85

Table 6 . 1

 61 Comparison of PALF, Kevlar, and other fillers reinforced NR composite

		Elastic modulus (MPa)
	Composite
		-75 (C)	25 (C)
	NR+SiAPALF+AP	5927	227
	NR/Kevlar	4512	20.37
	NR/Cellulose xanthate15phr/clay [124]	4000	10.8
	NR/Regenerated cellulose 10 phr [123]	3000	10

μm (a)

ACKNOWLEDGEMENTS

Firstly, I would like to express my sincere gratitude to my advisor Assoc. Prof. Dr. Taweechai AMORNSAKCHAI and Dr. Karine MOUGIN whose support, guidance and encouragement have been invaluable throughout this research. Furthermore, the success of this research required a lot of collaboration between Mahidol University and Université de Haute Alsace. I also would like to thank my coadvisor, Assoc. Prof. Sombat THANAWAN and Assoc. Prof. Pranee PHINYOCHEEP for suggestions and comments for complete my research. I would like to thank my dissertation committee. Prof. Christophe

APPENDIX A TENSILE FRACTURE OF COMPOSITES