811
N° d’ordre 2578
THÈSE DE DOCTORAT
Présentée parFatima Ezzahra Arrakhiz
Discipline : Physique Spécialité : MécaniqueMise en œuvre et caractérisation de Matériaux composites :
Polymères thermoplastiques / fibres naturelles
Soutenue le : 23 juin 2012, devant le jury : Président :
Pr. Omar Fassi Fehri Université Mohammed V-Agdal Rabat
Examinateurs :
Pr. Mohammed Ouadi Benssalah Université Mohammed V-Agdal Rabat Pr. Mohamed Jamal Eddine Sebbani Université Mohammed V-Agdal Rabat
Pr. Noureddine Damil Université Hassan II Mohammedia-Casablanca
Pr. El mokhtar Essassi Université Mohammed V-Agdal Rabat
Dr. Abou El Kacem Qaiss Responsable du laboratoire de mise en œuvre, Fondation MAScIR
FACULTÉ DES SCIENCES
Rabat
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Les recherches qui ont fait l’objet de cette thèse de doctorat ont été effectuées au Laboratoire de mécanique et des matériaux à la Faculté des Sciences de Rabat sous la direction du Pr. Omar Fassi Fehri, de la faculté des sciences de Rabat Agdal, je tiens à le remercier d’avoir accepté la direction de ce travail et de présider mon jury de thèse.
La présente thèse a été effectuée en collaboration avec Moroccan Foundation For Advanced Science, Innovation and Research (MAScIR), Institut des Nanomatériaux et Nanotechnologies, laboratoire de mise en œuvre des polymères dirigé par Docteur Abou El Kacem Qaiss ; je tiens à le remercier d’avoir encadré mes travaux de recherches, je veux aussi le remercier pour sa disponibilité, son attention, sa confiance en mon travail, et bien évidemment pour les connaissances qu’il a su me transmettre tout au long de cette période. Qu’il trouve ici l’expression de toute ma gratitude. Dr. Qaiss m’honore par sa présence dans mon jury de thèse.
Mes remerciements vont aussi au Docteur Khalid Benmoussa, chercheur mathématicien à NANOTECH pour son support et soutient tout au long de cette période de thèse.
Les tests et analyses chimiques ont été réalisé au Laboratoire de caractérisation de NANOTECH, je souhaite exprimer ma gratitude au Docteur Rachid Bouhfid qui a veillé sur les analyses et l’aide aux interprétations.
Je tiens à exprimer mes sincères reconnaissances au Pr. El Mokhtar Essassi Professeur à l’Université Mohammed V-Agdal d’avoir accepté de juger ce travail comme membre du jury.
Je tiens à exprimer mes vifs remerciements au Pr. Mohammed Ouadi Benssalah, de l’Université Mohammed V-Agdal pour avoir accepté de participer au jury de cette thèse.
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soit assuré de ma gratitude d’avoir accepté de juger mon travail comme rapporteur.
Je remercie également le Pr. Noureddine Damil, Vice-président de l’université Hassan II Mohammedia-Casablanca qui me fait l’honneur de participer à ce jury et pour avoir accepté d’effectuer le travail de rapporteur.
Je remercie tous mes collègues à MAScIR-NANOTECH pour l’ambiance agréable qu’ils ne cessent de créer.
Je remercie vivement pour le soutien financier: les dirigeants de MAScIR dont le directeur Mr. Lasry et Mme. Bayahya la DRH.
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A mon père My Hafid, A ma mère Ait lyazidi Sabah, A ma grand-mère, Et à mes frères Mohammed Amine et Abdessamia.
iv ABSTRACT ... 1 RÉSUMÉ ... 3 INTRODUCTION ... 5 CHAPTER 1 LITERATURE REVIEW 1. Natural Fibers ... 7
2. Applications of Natural Fibers ... 7
3. Chemical Composition of Plant Fibers ... 9
3.1. Cellulose ... 10
3.2. Hemicelluloses ... 11
3.3. Lignins ... 11
3.4. Thermoplastics as Matrices ... 13
4. Interface between Polymer and Fibers ... 13
5. Problematic ... 14
6. Improvement of Interface Adhesion Techniques ... 14
6.1. Physical Methods ... 14 6.2. Chemical Methods ... 15 7. Used Materials ... 15 7.1. Fibers: ... 15 7.2. Matrices ... 17 8. Material Preparation: ... 18
8.1. Fiber Size Reduction ... 18
8.2. Fiber Pre-Traitement ... 19
8.3. Processing Techniques for Polymer Composites ... 19
9. Fiber Characterization ... 21
9.1. Thermogravimetric Analysis (TGA) ... 21
9.2. Fourier Transform Infrared Spectroscopy (FT-IR) ... 22
9.3. X-Ray Diffraction (DRX) ... 22
9.4. Scanning Electron Microscope (SEM) ... 22
10. Composite Characterization ... 23
10.1. DSC Measurements ... 23
10.2. Tensile Tests ... 23
10.3. Three Point Bending ... 23
v CHAPTER 2
MECHANICAL AND THERMAL PROPERTIES OF POLYPROPYLENE REINFORCED WITH ALFA FIBER UNDER DIFFERENT CHEMICAL
TREATMENT
1. Abstract ... 36
2. Introduction ... 36
3. Materials and Methods ... 38
3.1. Materials ... 38
3.2. Chemical Treatments of Alfa Fibers ... 38
3.3. Experimental Procedure ... 40
3.4. Characterization ... 40
4. Results and Discussion ... 41
4.1. Structural Characteristics of the Alfa Fibers ... 41
4.2. Thermal Stability ... 42
4.3. Tensile Properties ... 44
5. Conclusion ... 46
CHAPTER 3 MECHANICAL PROPERTIES OF HIGH DENSITY POLYETHYLENE REINFORCED WITH CHEMICALLY MODIFIED COIR FIBERS: IMPACT OF CHEMICAL TREATMENTS 1. Abstract ... 51
2. Introduction ... 51
3. Materials and Methods ... 52
3.1. Materials ... 52
3.2. Treatment of Coir Fibers ... 53
3.3. Chemical Analysis ... 55
3.4. Mechanical Testing ... 55
4. Results and Discussion ... 56
4.1. Chemical Analysis ... 56
4.2. Tensile Properties ... 58
4.3. Torsion Properties ... 62
5. Conclusion ... 63
CHAPTER 4 TENSILE, FLEXURAL AND TORSIONAL PROPERTIES OF CHEMICALLY TREATED ALFA, COIR AND SUGARCANE BAGASSE REINFORCED POLYPROPYLENE 1. Abstract ... 68
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3.1. Materials ... 70
3.2. Sample Preparations ... 71
3.3. Characterization ... 73
4. Results and Discussion ... 74
4.1. Tensile Properties ... 74
4.1. Flexural Properties ... 76
4.2. Torsional Properties ... 78
5. Conclusion ... 82
CHAPTER 5 MECHANICAL AND THERMAL PROPERTIES OF NATURAL FIBERS REINFORCED POLYMER COMPOSITES: DOUM/LOW DENSITY POLYETHYLENE 1. Abstract ... 87
2. Introduction ... 87
3. Material and experimental details ... 89
3.1. Material ... 89
3.2. Sample Preparations ... 89
3.3. Characterization ... 90
4. Results and Discussions ... 93
4.1. Infrared Analysis of Alfa Fibers ... 93
4.2. Thermal Stability of Doum composites ... 94
4.3. Thermal Properties of Alfa Composites ... 95
4.4. Tensile Properties of the Natural Fiber Composites ... 96
4.5. Flexural properties of the Natural Fiber Composites ... 99
4.6. Torsional Properties of Natural Fiber Composites ... 100
5. Conclusion ... 101
CHAPTER 6 EVALUATION OF MECHANICAL AND THERMAL PROPERTIES OF PINECONE FIBERS REINFORCED COMPATIBILIZED POLYPROPYLENE 1. Abstract ... 108
2. Introduction ... 108
3. Experimental details ... 109
3.1. Materials ... 109
3.2. Chemical Treatment of PineCone and PP Modification Procedure ... 111
3.3. Experimental procedure ... 112
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4.2. Structural Characteristics of the Fibers ... 117
4.3. XRD analysis of untreated and Alkali treated Pinecone fibers ... 118
4.4. Thermal Stability ... 119
4.5. Tensile properties ... 121
5. Conclusion ... 125
SUMMARY AND CONCLUSIONS ... 129
SUMMARY ... 130
CONCLUSIONS ... 133
GLOBAL CONCLUSION ... 135
CONCLUSION GENERAL ... 136
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Tableau 1.1: Chemical composition of selected natural agricultural fibers ... 12
Tableau 1.2: important physical and mechanical properties of selected polymer... 17
Tableau 2.1: Composition and properties of some natural fibers from literature ... 38
Tableau 4.1: Composition and properties of some natural fibers from literature ... 70
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Figure 1.1: classification of natural fibers. ... 7
Figure 1.2: application of natural fibers composites ... 9
Figure 1.3: fibers main constitution [18] ... 9
Figure 1.4: schematic drawing of cellulose molecules including the hydrogen bonds ... 10
Figure 1.5: Hemicelluloses Example Structure ... 11
Figure 1.6: lignin structure ... 12
Figure 1.7: photographs of the used fibers ... 17
Figure 1.8: (a) a draw of a knife mill, (b) a draw of a hammer mill ... 18
Figure 1.9: the alkali treatment ... 19
Figure 1.10: a draw of twin screw extruder. ... 20
Figure 1.11: drawing of injection molding machine ... 21
Figure 1.12: drawing of compression hot molding machine ... 21
Figure 1.13: three point bending system ... 24
Figure 1.14: torsion rectangular mode ... 24
Figure 1.15: An exemple of stress-strain curve ... 29
Figure 2.1: reaction of the chemical modification of alfa etherification with dodecyl bromide ... 39
Figure 2.2: reaction of the chemical modification of alfa esterification with palmitic acid n-succinimidyl ester. ... 39
Figure 2.3: FTIR spectra of untreated (alfa), alkali-treated (alfa-NaOH), etherified (alfa-C12) and esterified (alfa-Palm). ... 42
Figure 2.4:TGA curves for polypropylene, raw alfa/PP composites, estherified alfa/PP composites, etherified alfa/PP composites and alkali treated alfa/PP composites. ... 43
Figure 2.5: DTG curves for polypropylene, raw alfa/PP composites, estherified alfa/PP composites, etherified alfa/PP composites and alkali treated alfa/PP composites. ... 43
Figure 2.6:stress-strain curves of neat PP, composites alfa /PP. alfa composite with NaOH treatment, alfa-Palm composite, alfa-C12 composite, untreated alfa composite. ... 45
Figure 2.7: effect of chemical treatment on young’s modulus and tensile strength of PP composites with 20 wt% of alfa Fiber. ... 45
Figure 3.1: schematic illustration of the reaction involved in producing the coir-ether (C12 Coir) ... 54
Figure 3.2: schematic illustration of the reaction involved in producing the coir-silane ... 54
Figure 3.3: test specimen pictures. (a) tensile specimen, (b) torsion specimen. ... 56
Figure 3.4: FT-IR spectra of coir fibers (untreated, alkali treated, etherified, and silane treated) (4000-600 cm-1) ... 57
Figure 3.5: young’s moduli and tensile strength of HDPE composites made with the various chemically treated fibers. ... 58
Figure 3.6: stored energy for neat HDPE and its composites as determined by the stress-strain diagrams. ... 61
Figure 3.7: evolution of the torsion modulus under a dynamic sweep frequency for the various composites. ... 62
Figure 4.1: fibers length and width distribution ... 71
Figure 4.2: extrusion process ... 72
Figure 4.3: young’s modulus at various loading content for each fiber type. ... 74
Figure 4.4: tensile strength at various loading for different fiber type... 75
x
fiber content, (b) loss modulus factor of PP/ alfa fiber ALK natural fiber composite as a
function of fiber content. ... 81
Figure 4.8: (a) Torsion modulus of PP/ coir fiber ALK natural fiber composite as a function of fiber content, (b) loss modulus factor of PP/ coir fiber ALK natural fiber composite as a function of fiber content. ... 81
Figure 4.9: (a) torsion modulus of pp/ sugarcane bagasse fiber ALK natural fiber composite as a function of fiber content, (b) Loss modulus factor of PP/ sugarcane bagasse fiber ALK natural fiber composite as a function of fiber content. ... 82
Figure 5.1: doum fiber length and width distribution ... 89
Figure 5.2: FT-IR spectrum of doum fiber raw and with NaOH treatment ... 93
Figure 5.3: TGA results of different percentage of treated doum in LDPE matrix composites. ... 95
Figure 5.4: Enthalpy variation vs. doum fiber content in LDPE matrix ... 96
Figure 5.5: Stress-strain curves of neat LDPE, composites LDPE/doum. ... 96
Figure 5.6: Young’s modulus of LDPE composites with fiber loading of 0-30 wt %. ... 97
Figure 5.7: Experimental results of tensile strength and elongation at break of alfa reinforced LDPE composites. ... 98
Figure 5.8: Stored energy for neat LDPE and its composites as determined by the stress-strain diagrams. ... 99
Figure 5.9: Flexural modulus of LDPE/ treated doum composite as a function of fiber content by weight. ... 100
Figure 5.10: Torsional modulus of LDPE/ treated doum composite as a function of fiber content by weight. ... 101
Figure 6.1: photographs of the pinecone plants ... 110
Figure 6.2: fiber length and width distribution ... 110
Figure 6.3: Scheme of (a) PP modification with FG-1901X/ Alkali pinecone composite, (b) PP modified with D1152/ alkali pinecone composite. ... 112
Figure 6.4: SEM images of fracture surface of 30wt.% pinecone reinforced PP composites. Figures (a), (b), (c), and (d) are took in different magnitudes. ... 116
Figure 6.5: image processing of the pinecone fiber ... 116
Figure 6.6: FT-IR spectra’s of raw, NaOH, and modified pinecone fibers. ... 117
Figure 6.7: X-ray diffraction spectra’s of (a) raw pinecone, (b) Alkali treated pinecone. ... 118
Figure 6.8: comparative DTG curves for the three system to the pinecone fibers and the matrix; (a) polypropylene/ pinecone (ALK), (b) FG1901X/ pinecone (ALK), (c) PP-D1152/pinecone (ALK). ... 120
Figure 6.9: (a) tensile strength of the three composites, (b) young’s modulus of the three composites ... 122
Figure 6.10: FT-IR of 30wt. % fiber content composites of the three systems ... 123
Figure 6.11: plastic energy of the three system of composite ... 123
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This study is related to the use of some local crops fibers and its effect on polymer composite with varying fiber reinforcement content or improving the adhesion between polymer matrix and surface fiber.
Actually, vegetable fibers as reinforcement in plastic composites can be used as a potential replacement for synthetic fibers, due to their low cost, good mechanical properties and biodegradability.
In this study the used fibers were alfa, doum, coir,sugarcane bagasse and pinecone fibers reinforcing different thermoplastic through extrusion and injection molding processes, or using a heated two roll mill followed by pressing the composite samples with a hot press molding machine.
The properties of the fibers/polymer composites were defined by using tensile, flexural, torsional, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) tests. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) has been used to study the surface composition of treated and untreated natural fibers and its composites.
The mechanical properties of the composites were mostly enhanced with the addition of fibers compared to the neat polymer matrix. Optimal properties were reached and calculated by mathematical models. Also, some optimal properties were observed when varying the surface chemical treatments of the fibers and the use of coupling agent.
As it can be remarked in the following works all changes in the composites were compared to a basic chemical treatment called Alkali treatment. This treatment cleans the surface of fibers leading to the hydroxyl groups the opportunity to be exposed on the surface of the fibers to get good wettability with the polymer matrix. Resulting good mechanical properties
2 to reach their full commercial potential.
Keywords: Natural fibers, polymers, composites, coupling agents, chemical treatment, macanical properties, thermal properties, processing.
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Cette étude s’intéresse à l’utilisation de certaines fibres sur une matrice polymère en variant leur pourcentage ou en améliorant leurs surfaces d’adhésion avec la matrice polymère.
Actuellement, les fibres végétales utilisées comme renfort dans les composites plastiques peuvent remplacer les fibres synthétiques, grâce à leurs faibles couts, leurs bonnes propriétés mécaniques et leur biodégradabilité.
Dans cette étude les fibres utilisées sont l’alfa, le doum, le coco, la bagasse de la canne à sucre et la pomme de pin, elles renforcent différents thermoplastiques par les procédés d’extrusion et de moulage par injection, sinon par des procédés à l’échelle du laboratoire tel que le mélangeur interne et la presse chauffante.
Les propriétés des composites polymères / fibres sont définies par les tests de traction, flexion, torsion, analyse thermogravimétrique (ATG), Calorimétrie différentielle à balayage (DSC).
La diffraction par rayon X (XRD) et la spectroscopie Infrarouge à Transformée de Fourier (FT-IR) ont été utilisées pour l’étude des composites et des éléments de la surface des fibres naturelles traitées et non traitées.
Les propriétés mécaniques des composites ont augmenté avec l’adjonction des fibres en comparaison avec la matrice polymère vierge. Aussi, de bonnes variations des propriétés ont été observées par la variation des traitements chimiques des surfaces des fibres et avec l’utilisation d’agents comptabilisant.
Dans les travaux de cette thèse, tous les composites élaborés sont comparés au composite traité à la soude (traitement Alkali). Ce traitement a pour rôle de nettoyer la surface des fibres
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bonne mouillabilité avec la matrice polymère. On obtient alors des composites avec de bonnes propriétés mécaniques et qui ne coûtent pas cher. Toutefois, des connaissances fondamentales des surfaces des fibres peuvent être bénéfiques pour une bonne exploitation des possibilités qu’elles offrent.
Mots clés : fibres naturelles, polymères, composites, agents comptabilisant, traitements chimiques, propriétés mécaniques, propriétés thermiques, mise en œuvre.
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A composite material can be defined as a heterogeneous mixture of more than one element. The phases that have been bonded together can be any materials as polymer, ceramics, metals or even fluids. The resulting material can be regarded as a homogeneous material with its own properties; these properties are usually not shared with any of its constituent element alone.
Among the phases making the composite material, there is the reinforcement, which is the strongest phase of the composite and it is used as discrete elements as fibers or particles inside the composite. The second phase which is less strong than the reinforcement is called the matrix. The matrix serves to distribute the fibers and also to transmit the load to the fibers. The composites are thus classified according to the nature of their matrix: polymer, ceramic, and metal composite.
The aim of this thesis will be focused on studying the mechanical and thermal properties of natural fibers reinforcing thermoplastics matrix.
A review of the literature has shown that during the last few decades research efforts have centered in the incorporation of vegetable materials in synthetic matrix [1-3]. Compared to glass fibers, aramid or other synthetic reinforcement, natural fiber offers good properties and can be used as an alternative way in such industrial field [4]. The natural fiber composite may eventually be recycled or burned to recover heat without production of toxic by-products [5]. Moreover, they display a good set of mechanical properties, provide better working conditions and are much less abrasive than the common synthetic fibers [5]. All these aspects make their use very attractive to the manufacture of polymer matrix composites and making it an interesting product for low wage countries.
However, one difficulty that can prevent the wide usage of natural fiber in composites is the lack of good adhesion to the polymeric matrix. The hydrophobic nature of most polymers and
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properties [6]. To hold or to minimize these problems, fibers and/or polymers are subjected to some modification treatments in their surface in order to promote adhesion [7].
Several reports concerning the surface modification of even the natural fibers or the polymers shows that in natural fiber filled polymer composites, the properties of the composites were improved when the surface properties of the natural fibers or the polymer were modified, even with chemical or physical treatments [5].
This research was to integrate the natural plant fibers such as alfa, doum, coir, sugarcane bagasse, pinecone, and thermoplastics into a new biocomposite material.
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1. CHAPTER
CHAPTER 1
LITERATURE REVIEW
1. Natural Fibers
Natural fibers can be derived from many sources; they can be plants, animals or minerals as illustrated in the figure 1.1. However, plant fiber has recovered in many applications to be the most popular reinforcement as natural fiber [8].
Figure 1.1: classification of natural fibers [9].
Plant fibers are divided on bast fibers, leaf fibers, seed, fruit, wood, cereal straw, and other grass fibers. Their principal composition is cellulose, lignin and hemicelluloses [10].
2. Applications of Natural Fibers
For years, natural resources are used for the production of commodities and a large amount of technical products. Textiles, ropes, canvas and also papers were made of natural fibers,
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such as alfa, hemp, coir, and sugarcane bagasse. Some of these are still used and other has appeared. As early as 1908, the first composite materials were manufactured to be used in tubes and pipes for electronic purposes [11].
Nowadays, the industry of natural fiber composite has widely invaded the world; the automotive industry is the prime driver of “green composites” because the industry is faced with issues for which green materials offer a solution [12]. Many components for the automotive sector are now made from natural fiber composite materials [13] (figure 1.2). In Europe, car makers are using mats made from abaca, flax and hemp in press-molded thermoplastic panels for door liners (figure 1.2), parcel shelves, seat backs, engine shields and headrests. For consumers, natural fiber composites in automobiles provide better thermal and acoustic insulation than fiber glass and reduce irritation of the skin and respiratory system.
Natural fiber composites offer also vast opportunities for an increasing role as alternate materials, especially wood substitutes in the construction market [14]. Various natural fiber-based composites products such as laminates, panels, partitions, door frames, shutters, and roofing have been produced as an alternative to existing wood materials.
Other applications of the use of fiber in the polymer matrix are noted as soil conservation, [11], they are also used extensively in motorsport and increasingly in the automotive sector because of their potential to reduce mass. Impact structures in motorsport are required to act as both load-bearing members and energy absorption devices [15].
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Figure 1.2: application of natural fibers composites [16,17] 3. Chemical Composition of Plant Fibers
The morphological microstructure of plant fibers is very complex due to the hierarchical organization of the different compounds present at various compositions. The chemical composition varies with the type of fibers. However, the main composition is the cellulose polymer, hemicelluloses and lignin. These constituent contribute to the overall properties of the fibers.
10 3.1. Cellulose
Cellulose forms the basic material of all plant fibers. It is generally accepted that cellulose is a linear polymer consisting of D-anhydroglucopyranose units joined together by β-1,4-glycosidic linkages. Cellulose is thus a 1,4- β-Dglucan [5]. The molecular structure of cellulose, which is responsible for its supramolecular structure, determines many of its chemical and physical properties. In the fully extended molecule, the adjacent chain units are oriented by their mean planes at the angle of 180° to each other (figure 1.3). Thus, the repeating unit in cellulose is the anhydrocellobiose unit (figure 1.4), and the number of repeating units per molecule is half of the degree of polymerization (DP). This may be as high as 14.000 in native cellulose [5].
Figure 1.4: schematic drawing of cellulose molecules including the hydrogen bonds [18]
The mechanical properties of natural fibers vary with cellulose type. Each type of cellulose has its own cell geometry, and the geometrical conditions determine the mechanical properties. Solid cellulose forms a microcrystalline structure with regions of high order, and regions of low order. Cellulose is also formed as microfibrils (figure 1.2). The crystal nature (monoclinic sphenodic) of naturally occurring cellulose is known as cellulose I. Cellulose is resistant to strong alkali (17.5 wt%) but is easily hydrolyzed by acid to water-soluble Sugars. Cellulose is relatively resistant to oxidizing agents [19].
11 3.2. Hemicelluloses
Hemicellulose is not a form of cellulose at all. It comprises a group of polysaccharides (excluding pectin) that remains associated with the cellulose after lignin has been removed (figure 1.5). The hemicellulose differs from cellulose in three important aspects [20]. In the first place, they contain several different Sugar units, whereas cellulose contains only 1,4-
β-D-glucopyranose units. Secondly, they exhibit a considerable degree of chain branching,
whereas cellulose is strictly a linear polymer. Thirdly, the degree of polymerization of hemicellulose is 10–100 times lower than that of native cellulose. Unlike cellulose, the constituents of hemicellulose differ from plant to plant [19-20].
Figure 1.5: Hemicelluloses Example Structure [21]
3.3. Lignins
Lignins are complex hydrocarbon polymers with both aliphatic and aromatic constituents [22,23] (figure 1.6). Their mechanical properties are lower than those of cellulose. Lignin is totally amorphous and hydrophobic in nature. It is the compound that gives rigidity to the plants. Lignin is considered to be a thermoplastic polymer, exhibiting a glass transition temperature of around 90°C and melting temperature of around 170°C. It is not hydrolyzed by acids, but soluble in hot alkali, readily oxidized and easily condensable with phenol [24].
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Figure 1.6: lignin structure [25]
The table 1.1 summarized the composition and properties of some natural fibers against glass fiber.
Tableau 1.1: Chemical composition of selected natural agricultural fibers [13, 26] Type of fiber Cellulo se (%) Lignin (%) Hemicelluloses (%) Density (g/cm3) Elongation at break (%) Tensile strength (MPa) Young’s modulus (GPa) Flax 71 2.2 18.6-20.6 1.5 2.0-3.2 345-1100 27.6 Hemp 57-77 3.7-13 14-22.7 --- 1.6 690 --- Jute 41-48 21-24 18-22 1.3-1.45 1.16-1.8 393-773 13-26.5 Sisal 47-78 7-11 10-24 1.45-1.5 2-2.5 468-640 9.4-22 Coir 36 - 43 41 - 45 0.15 - 0.25 1.2 15-40 131-175 4-6 E-glass --- --- --- 2.5 2.5 2000-3500 70 S-glass --- --- --- 2.5 2.8 4750 86
E-glass is common electrical grade fiber glass S-glass is modified higher strength fiber glass
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The tensile strengths as well as the young’s modulus of plant fiber like hemp, coir, are lower than glass fibers commonly used in composites. However, the density of glass fiber is higher (~ 2.5g/cc) while that of natural fibers is much lower (~ 1.4g/cc). The specific strength and specific young’s moduli of some of these natural fibers are quite comparable to glass fibers. This becomes particularly important where the weight of the structure needs to be reduced. Table 1 compares the mechanical properties of some natural fibers to glass fibers.
Therefore the natural fibers could potentially be substituted for synthetic fibers in the composites. In the chapters bellow we focused our attention to the uses of some local crops as alfa, doum, sugarcane bagasse, pinecone and on Ivory Coast coir to reinforced thermoplastics polymers.
3.4. Thermoplastics as Matrices
Thermoplastics as matrix are a plastic material they can be categorized by their physical and molecular structures. This type of polymer can be melted, remelted at a higher temperature, and then solidified in a mold by cooling without a significant change in their properties [27]. They are also the most widely used binding agents in natural fiber-reinforced composites because of their low cost, lightweight, stiffness, and moldability [27]. Among thermoplastics we can call the low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high-density polyethylene (HDPE), and polypropylene (PP). Other types of thermoplastics have also been used as the matrix in fiber-reinforced composites. 4. Interface between Polymer and Fibers
Generally properties of a composite are dependent on the interface between polymer matrix and fibers. Thus, it is dependent on the strength of the bond between fiber and matrix. This interface is the limiting factor of fiber-reinforced composite performance as it ultimately defines the amount of load that can be transferred from one fiber to the next by the matrix.
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The strength of a fiber–matrix interface is dependent upon the degree of mechanical, chemical and electrostatic bonding and level of inter-diffusion between the matrix and fibers. The lack of chemical bonding between fiber and polymers is due to the chemistry of natural fibers that is strongly hydrophilic and to the polymer matrices which are mostly hydrophobic and apolar. 5. Problematic
All natural fibers are hydrophilic and polar in nature. However, the polymer matrices are hydrophobic in nature. The addition of this reinforcement to the polymer will reduce the mechanical properties of the composite and will results in high moisture sorption in natural fiber reinforced the polymer. The moisture if not removed from fiber before use leads to voids in the fiber matrix interface causing deterioration in mechanical properties and loss dimensional stability.
6. Improvement of Interface Adhesion Techniques
The quality of the fiber–matrix interface is important when using natural fibers as reinforcement in plastics matrix. Some physical and chemical methods are developed to optimize this interface. These modification methods are of different efficiency for the adhesion between matrix and fiber [28].
6.1. Physical Methods
Such physical methods are used to divert the adhesion problem between vegetable fibers and polymers matrix. Stretching, calandering, thermotreatment [5], and the production of hybrid yarns are considered as an example of physical treatment. Generally physical treatment changes structural and surface properties of the fibers but do not change the chemical compositions of them [5], which influence the mechanical bondings to the polymeric chains.
15
Other types of physical treatment are also finding in the literature as the electric discharge (corona, cold plasma).the corona and cold plasma treatment have the same effects on the fiber surface, the surface energy could be increased or decrease depending on the type of the used gases, and a new free reactive radicals could be produced [5].
6.2. Chemical Methods
The hydrophilic fibers are incompatible with the hydrophobic character of polymers, chemical treatments are developed to bring the problem of incompatibility between the two materials.
Amoung chemicals that have been screened in laboratory experiments to enhance the potential fiber/matrix interface, the sodium hydroxide, peroxide, organic and inorganic acids, Silane, anhydrides, acrylic monomers [10,19,20,22,23]. An important chemical modification method is also efficient is the chemical coupling method, which improves the interfacial adhesion. The fiber surface is treated with a compound, which forms a bridge of chemical bonds between fiber and matrix. Chemical treatment can remove lignin and hemicellulose from the fiber to enhance its roughness and porosity [2]. Fiber surface treatment may also increase the strength of the fiber, reduce the water absorption, and improve the mechanical properties of the reinforced composites.
7. Used Materials
7.1. Fibers
The specific fibers alfa, doum, coir, sugarcane bagasse and pinecone were selected due to the wide amount of research material currently available and due to the worldwide availability of such plants. These fibers will be introduced in the following text to know more about them and their characteristics.
16
Alfa fiber (Stipa tenacissima L.) (see figure 1.7) is one of the most suitable leaf fibers, the alfa fiber used is collected from Moroccan north areas in the period of september to february. This species was used as raw material for manufacturing paper, thread, baskets and jute. The alfa plant measure about one meter in height and its stalks vary between 1 and 2 mm in diameter. Alfa fiber is obtained from the stalks through retting, drying, and crushing. Doum fiber (Chamaerops humilis) was collected from a rural area in the west of morocco. Coir and pinecone plant, however, are fruit fibers and their fiber strands are shorter. The pinecone (figure 1.7) was collected from a plantation in Morocco, after falling to the ground and releasing their seeds, pinecones were collected between august and september in recent times. Pinecone biomass has been applied as biosorbent for metal and dye waste waters by several authors [13]. The presence of chemical compounds such as cellulose and lignin provides hydroxide functional groups which accounts for polymer matrix.
Coir fiber (see figure 1.7) is a versatile lignocellulosic fiber obtained from coconut trees (Cocos nucifera), which grow extensively in tropical countries. Because of its hard-wearing quality, durability and other advantages, it is used for making a wide variety of floor furnishing materials, yarn, rope, e t c. But these traditional coir products consume only a small percentage of the potential total world production of coconut husk. Hence, apart from the conventional uses of coir as mentioned above, research and development efforts have been under way to find new use areas for coir, including use of coir as reinforcement in polymer composites.
The last used fiber in this thesis was sugarcane bagasse (figure 1.7). This is obtained from a local company. The bagasse fibers were a byproduct from sugar-cane processing. In general it is composed with approximately 50% cellulose, 25% hemicelluloses, and 25% lignin. Also it is distinguished by its high annual regeneration capacity compared to the other agricultural residues [14].
17
Figure 1.7: photographs of the used fibers 7.2. Matrices
Table 1.2 bellow shows a comparative data of the physical and mechanical properties of different thermoplastic polymers used in this thesis. PP has a higher maximum tensile strength than LDPE and HDPE. Young’s modulus (or modulus of elasticity) and flexural modulus of PP are higher than HDPE, and LDPE.
Tableau 1.2: important physical and mechanical properties of selected polymer [13] Thermo-plastic category Density (g/cm3) Melt flow index (g/10min) Max tensile strength (MPa) Elongation at break (%) Young’s modulus (GPa) Flexural modulus (GPa) LDPE 0.915-1.13 0.25-14 9-17.9 226-750 0.15-0.29 0.145-0.276 HDPE 0.48-1.46 0.057-26 13.1-37.7 25-2200 0.8-1.0 0.5-1.65 PP 0.886-1.04 0.25-1480 16-460 8-700 0.68-2 0.62-2.46
18
These polymers have a melting temperature points below the thermal degradation temperature of plant fibers, which make their use with natural fibers as a matrix easier and practical. One other point to choose these thermoplastics is their lower melting temperature that offers an advantage on energy consumption during manufacturing the composites.
8. Material Preparation
8.1. Fiber Size Reduction
The size of natural fibers is usually reduced by a mechanical method such as knife mill (figure 1.7a) or hammer mill (figure1.7 b). In this study fibers were grinded with a precision grinder (FRITSCH Pulverisette 19). However, to determine the optimal fiber length required for the maximum performance of the composites, sieves with defined sizes have been used. Size reduction is the first step used when fibers are collected.
19 8.2. Fiber Pre-Treatement
Due to the hydrophilic character of natural fiber they are found incompatible with the hydrophobic polymeric matrix. Besides the different chemical treatment that will be added to the fibers surface or the added compatibilizer to couple these two unlike materials, we used to treat fiber surface with sodium hydroxide (NaOH) treatment (figure 1.8) to improve the mechanical interlocking with the polymeric matrix followed by neutralization with acetic acid, these fibers were filtered and finally dried.
Figure 1.9: the alkali treatment [18]
8.3. Processing Techniques for Polymer Composites
There is a wide variety of methods or techniques of processing polymeric composites. The most common processing techniques for polymer manufacturing include: extrusion, injection and compression molding.
Ext rusi on
In most extruders, polymers in the form of pellets, powders or flakes are drawn from a hopper in the gap between a rotating screw or twin screw and a heated barrel (figure 1.9). They are transported forward, compacted, melted and eventually the melt is pumped through
20
the die where it gets shaped into the desired form before solidification by cooling outside. However, this process offers the possibility of adding charge into the polymer to get new composite material. The extruded composites were in semi-finished product and were used for injection process.
Figure 1.10: a draw of twin screw extruder [31].
Inj ection and compr ession molding
The injection molding (Figure 1.10) and compression molding (Figure 1.11) operations, all entail forcing the polymer into a cavity and reproducing its shape. In the process of compression molding, a polymer solid mass is heated up or melted and forced to undergo a squeezing flow by hot mold surfaces that close to form a final shape. In the injection molding process, a polymer melt is forced through an orifice (gate) into a closed, cold mold, where it solidifies under pressure in the shape of the mold cavity.
21
Figure 1.11: drawing of injection molding machine [32]
Figure 1.12: drawing of compression hot molding machine 9. Fiber Characterization
9.1. Thermogravimetric Analysis (TGA)
Thermogravimetric Analysis (TGA) measures the amount and rate of change in the weight of fibers and their composites as a function of temperature in a controlled atmosphere.
22
Measurements are used primarily to determine the composition of materials and to predict their thermal stability at temperatures up to the composite manufacturing. The technique can characterize materials that exhibit weight loss due to decomposition, oxidation, or dehydration.
9.2. Fourier Transform Infrared Spectroscopy (FT-IR)
Fourier transform infrared spectroscopy (FTIR) is most useful for identifying chemicals that are either organic or inorganic. It can be utilized to evaluate qualitatively some components of an unknown mixture. It can be applied to the analysis of solids, liquids, and gases. The term Fourier Transform Infrared Spectroscopy refers to a fairly recent development in the manner in which the data is collected and converted from an interference pattern to a spectrum. In this work, FT-IR was used to characterize the effect of chemical treatment to the fibers surface compared to non treated ones also this technique was used to define the new apparent bonds in the composites before using coupling agents.
9.3. X-Ray Diffraction (DRX)
X-ray diffraction method is especially significant for the analysis of solid material as plant fibers in our case, whichreveal information about the crystal structure, chemical composition of the used fibers even treated and untreated.
9.4. Scanning Electron Microscope (SEM)
The scanning electron microscope (SEM) is routinely used to generate high-resolution images of shapes of objects and to show spatial variations in chemical compositions; thus we used it to reveal the fibrillar character of the used plant, also to show the dispersion and the distribution of these fibers in the polymer matrix.
23 10. Composite Characterization
10.1. DSC Measurements
To determine the melting temperatures of both the virgin polymer and the different composites, differential scanning calorimetry (DSC) was performed on 5mg samples of polymer and polymer composites taken from the pellet form, using a TA Instruments DSC Q 100 differential scanning calorimeter. To remove the effect of thermal history on the DSC results, samples were heated in the DSC from ambient temperature to 200°C at 10°C min-1 and then cooled back to -30°C, also at 10°C min-1. Immediately, the samples were reheated to 200°C at 10°C min-l and the endothermic data taken from this second heating stage. The data obtained from this method describes the crystalline melting temperature, which is shown as a peak endotherm.
10.2. Tensile Tests
Tensile tests to determine tensile modulus and tensile strength were performed using an Instron Tensile testing machine equipped with a 5 kN load cell, standard grips and Merlin data acquisition software. A crosshead displacement of 3mm.min-1 and a gauge length of 35mm were used. Modulus of each specimen is calculated as the gradient of the stress vs. strain curve between 0.025% and 0.05% strain, which is seen to be linear for these tapes. The values presented subsequently for moduli, strength and strain to failure are the average of at least 5 repetitions of each test.
10.3. Three Point Bending
The flexural test measures the force required to bend a beam under 3 point loading conditions. The data is often used to select materials for parts that will support loads without flexing. Flexural modulus is used as an indication of a material’s stiffness when flexed. Since
24
the physical properties of many materials (especially thermoplastics) can vary depending on ambient temperature. The used three bending system in this work (figure 1.12) was a homemade conception and realization system to be adequate to the tensile test machine
Figure 1.13: three point bending system 10.4. Torsional Tests
Torsional tests were carried out with an ARES-LS rheometer, operating with a rectangular torsion system. The tests defined the complex modulus G* and the loss factor tanδ. The discussion and the interpretation of the data are well developed in the following chapters.
25
ORIGINALITY AND OBJECTIVES
The subject dealt with this thesis is the mechanical and thermal behavior of composites as influenced by fiber content and fibers chemical treatments. A large overview of the literature has shown that good composites properties require a good adhesion between fibers and polymer matrix. Such work has done in this research area and various treatments have been tested to improve the wettability of the fibers in the matrices. In this work, the studies have been applied by an experimental research on composite material, reinforced by plant fibers from local crops and abundant ones. The used processing was not a common field of research in our country, even so many objects made with composite polymer and fibers were exported from other countries. The development of this new material could be a big jump for our national industry.
The global objectives of this work were to study the behavior of the composites reinforced with fibers, under different tests to be compared with the neat matrices. The evaluation of the use of some abundant fibers in the polymer matrix even when varying the chemical treatment or the fiber content was also fixed.
Three main points were developed as specific objectives of this thesis, the census of the vegetable fibers candidate to be used as reinforcement in different thermoplastic matrices, the development of a pilot process to manufacture easily a variety of natural fiber composites and to optimize the main conditions of their manufacturing, the last point was to enhance the compatibility between the selected fibers and the polymer sweeping many chemical treatments.
The following chapters describe the used materials and methods in this work, which includes: the choice of materials, a description of the detailed preparation, the processing procedure, and the test methods. The results of all experiments that were done will be
26
presented and discussed, followed by a summary of the work and the conclusions that may be drawn from this research. In the final part, some recommendations for future works are suggested.
27
SUMMARY OF CHAPTERS
The following chapters will describe the materials and methods used in this research, which includes: the choice of materials, a description of the detailed preparation, the processing procedure, the test methods, and the obtained results. The results of all tests will be presented and discussed, followed by a conclusion that may be drawn from this research. In the final chapter, some recommendations for future works are suggested. The summary of all chapters will be presented here bellow.
Chapt er 1
In this part a general introduction and a global literature review were developed, to present the main constituent of the studied composites, some research that were carried out in this filed. This chapter shows also the main tests and used analyses that have been done during the processing and the characterization of the composites.
Chapt er 2
In this part 20wt. % of alfa fibers were optimized to be impregnate in the polypropylene matrix. Two chemical treatments were investigated in this work as a comparative treatment with alkali treated alfa and raw alfa fibers composites. The composites were prepared using a two roll mill and a hot compressing machine. This study treats the effect of chemical modification (alkali treatment, etherification treatment and esterification treatment) to the mechanical and thermal properties of the composites. DTG curves showed a best increase in the thermal stability when the etherified fibers were used and a slight decrease in this property with palmetic acid fonctionalization. Also the used treatment possesses good mechanical properties when compared to the alkali treatment or the non-treated composite. However the alkali treated fiber composite had shown a modest thermal property compared to the neat polymer and even in the mechanical properties.
28
Chapt er 3
Another kind of fiber was investigated in this study, coir fibers were exported from ivory cote and compounded with high density polyethylene. The fixed percentage was 20wt.% and the collected fibers were treated with Silane and Dodecane bromide (C12) to be compared to the alkali treatment and raw fibers composites. In this chapter it is found that the tensile properties of all treated composites are higher than the neat HDPE and non-treated composite. The alkali treatment removes the impurities from the surface and also appeared to cause a decrease in the microfibril angle, when the two other treatments favor physical and chemical reactions with the polymer to get more tensile properties. An energy study was also developed in this chapter; the energies were concluded from strain-stress curves to give us an idea about the stored energy in each composite after deformation. Thus, the ductility of the various composites was evaluated through the plastic energy (figure 1.14) and it was clear that C12 coir composite and Silane coir composite store more energy compared to raw coir composite, while the alkali coir composite shows a slight decrease. This is due to the use of these coupling agents that took the same orientation as the stretching.
The calculated energies were as follow:
t Pl el
E E E
Where Et is the total energy measured under the strain stress curves; Epl is the plastic
energy
And Eel is the elastic energy that is represented by air in the right triangle:
2 max 2
el
29
Figure 1.15: An exemple of stress-strain curve E is the young’s modulus and σmax the maximal tensile strength.
In this study torsional test were also carried out it shows that the NaOH treatment gives more flexibility to the fibers when the used Silane treatment makes the composite more rigid under the used stress.
Chapt er 4
We used to compare in this section three composites based on three fibers alfa, coir and sugarcane bagasse. Composites are prepared with the extrusion and injection process. The prepared samples were mechanically tested to be compared with each other. As a result, the young’s moduli to all composites have shown a significant increase with fiber loading when the marked tensile strength showed a decrease with fibers content. However, the highest tensile strength was reached in coir composite followed by alfa and bagasse, which is due to the lower cellulose content and a higher lignin content of coir fiber compared to alfa and sugarcane bagasse fibers. Torsional tests were also carried out in this paper and have showed a good response in all composites under the applied deformation, thus the torsion modulus
30
increases with frequency and fiber content which make us think about the solid state as the elastic character. A numerical study based on the experimental data was performed to model the torsion modulus of the three composites and to define the exact percolation threshold of the three systems.
Chapt er 5
In this chapter we treat doum fiber with sodium hydroxide to remove the impurities from its surface. The prepared fibers were impregnate at different content in the low density polyethylene matrix to improve the mechanical properties of the composites. This first work allows the development of the pilot process to manufacture composites. Thus, the composites were prepared using a twin screw extruder machine and injected in an industrial injection molding machine. An overview of the results shows that an important increase of the tensile modulus were reached with adding fiber in the polymer matrix, when the tensile strength showed a decrease with adding fibers which is due to the poor cohesion between fibers and the matrix under the applied strain. Fibers are considered as a stress concentration zone and leads to a speed rupture when tensile tests are in progress.
Also a crystalline study was done by a Differential Scanning Calorimetry (DSC) characterization to see the evolution of crystallites when fibers are incorporated in the polymer matrix. The DSC curves show that the fibers at 5wt. % are as nucleating agent when beyond this percentage; fibers became barriers and prevent the spherolites growth.
Chapt er 6
A new use of pinecone fiber in the polymer matrix was used with two coupling agent to be compared with the alkali treated fiber without compatibilizer .The SEM analysis illustrate clearly the fibrillar structure of the pinecone fiber that we analysed using image software to determine the cellulose and holocellulose component from the picture. Moreover we studied
31
the tensile, torsional and thermal properties of the composites and characterize the fibers before and after the alkali treatment with X-Ray diffraction and FT-IR analysis.
The obtained results showed that the tensile properties enhance with adding fibers and it is found a good adhesion when the SEBS-g-MA was used as a coupling agent which is noticeable in the tensile strength curves. Moreover, the addition of SEBS-g-MA enhanced noticeably the torsional modulus compared to the binary and the other compatibilized composite.
Summar y and conclusion
This section is presented in the end of this work, and it resumes the whole experiments and the collected data since we started this thesis project.
Recommendati ons for fut ure wor k
32 Reference
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4. Wambua P, Ivens J, Verpoest I. Natural fibers: can they replace glass in fiber reinforced plastics?. Compos Sci Technol 2003;63: 1259–1264
5. Bledzki AK, Gassan J. Composites reinforced with cellulose based fibers. Prog. Polym. Sci. 1999; 24:221–274.
6. Yanjun X, Hill CAS, Xiao Z, Militz H, Mai C. Silane coupling agents used for natural fiber/polymer composites: A review. Compos Part A-Appl S 2010; 41: 806–819 7. Saiful Islam Md, Hamdan S, Jusoh I, Rezaur Rahman Md, Ahmed AS, The effect of
alkali pretreatment on mechanical and morphological properties of tropical wood polymer composites. Mater Design 2012; 33: 419–424.
8. Brahmakumar M, Pavithran C, Pillai RM, Coconut fiber reinforced polyethylene composites: effect of natural waxy surface layer of the fiber on fiber/matrix interfacial bonding and strength of composites. Compos Sci Technol 2005,65: 563–569
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35
2. CHAPITR E
CHAPTER 2
MECHANICAL AND THERMAL PROPERTIES OF
POLYPROPYLENE REINFORCED WITH ALFA FIBER UNDER
DIFFERENT CHEMICAL TREATMENT
Résumé
L’intérêt d’utiliser les fibres naturelles comme renfort dans des thermoplastiques polymères a suscité beaucoup d’études dans le domaine des sciences des matériaux et de la technologie verte. L’utilisation de plante fibreuse requiert la compatibilité entre la matrice et les fibres. Cette étude traite de l’effet des modifications chimiques sur la fibre d’alfa notamment le traitement alkali, l’éthérification et l’estérification, et son influence sur les propriétés mécaniques, thermiques des composites. Pour cela, le pourcentage des fibres a été fixé à 20% en masse, et on a évalué l’effet de chaque traitement chimique sur l’alfa renforçant le polypropylène (PP) et sur l’ensemble des propriétés mécaniques et thermiques des composites. Les composites qui contiennent les fibres traitées chimiquement possèdent de bonnes propriétés mécaniques et thermiques en comparaison avec les composites à fibres non traitées. Dans ce sens le plus haut module de Young a été relevé pour les fibres ayant subis une estérification, avec une hausse de 35%. Pour la stabilité thermique le meilleur résultat a été enregistré pour les fibres ayant subi une éthérification avec plage plus large de 80°C.
36 1. Abstract
The interest in using natural fibers as reinforcement for thermoplastic polymers was attracted several studies covering both material science and green technology. The use of plant fiber requires the issue of compatibility between matrix and fibers. This study treats the effect of chemical modification (alkali treatment, etherification treatment and estherification treatment) on the alfa fiber surface, and its impact on mechanical and thermal properties of composites. To this end, the percentage of fibers was fixed at (20wt. %), and to evaluate the effect of each chemical modification in alfa reinforced polypropylene (PP), based on the mechanical and thermal properties of composites. Composites containing chemically modified alfa fibers were found to possess improved mechanical and thermal properties when compared to non treated composite. The highest improvement in Young’s modulus was observed with esterified fibers, with a 35% increase. Thermal stability is best increased using etherification-treated fiber, with gains in the temperature up to 80°C.
2. Introduction
Nowadays, a lot of attention is dedicated to the use of natural fibers as reinforcement for thermoplastic polymers [1]. Composites reinforced with fibers received increasing interest from industries in a wide field of application, such as automotive, construction, aerospace and packaging [2-4]. The advantages of using natural fibers are their low cost, abundance, are renewable and have good specific properties due to their low densities. Another advantage that appears during processing is that natural fibers do not damage the processing equipment the way glass or carbon do [5,6].
The main difficulties in the use of natural fibers are generally related with the highly polar surface, this induces an interfacial in compatibility with non-polar matrices like polypropylene and polyethylene and causes inter-fibers aggregation by hydrogen bonding
37
[7].Moreover, the hydrophilic character of natural fibers created by the hydroxyl group in the anhydroglucose repeating unit of the natural fiber cellulose structure [8] could be another problem to solve.
The main composition and properties of some natural fibers are summarized in table 2.1. To get the good adhesion with a hydrophobic polymer, all waxy material and pectin covering the fiber’s surface should be removed beforehand [8]. One goal of the chemical treatment is to remove these non cellulosic components while adding functional groups to increase bonding in polymer composites [9,10].
Among all natural fibers, alfa is considered to be one of the strongest and easily available in the dry region of North Africa. This xerophilous plant belongs to the family of grasses and grows in the form of cylindrical rods to a maximal height of 1 m [11]. The Young’s modulus of elementary alfa fiber is found to be near of 13, 4GPa, while a tensile strength of 944MPa.
This work focuses on three chemical treatments used for short natural fibers to enhance mechanical properties in composites. Modified and unmodified alfa fibers were mixed in a polypropylene matrix before the tensile specimens were fabricated by hot press molding.
The effectiveness of the fiber’s chemical treatments was evaluated thorough changes in to mechanical and thermal properties of composites. Modified and unmodified alfa fibers were added at fixed fiber content (20%wt.). Thorough this, the effects of chemical treatment can clearly be evaluated.
Alfa fibers were treated with three chemical reactions: i) A sodium hydroxide aqueous solution (NaOH), ii) palmitic acid (CH3 (CH2)14COOH), and dodecyl bromide. To check the
38
Tableau 2.1: Composition and properties of some natural fibers from literature
Types of fibers
Composition (wt.%) Properties
Cellulose Lignin Waxes Density
(g/cm3) Tensile strength (MPa) Tensile modulus (GPa) Alfa 45 [12] 23 [12] 5 [12] 1,2 [18] 134-220 [19] 13-17,8 [19] Hemp 56,1 [13] 6 [13] 7,9 [13] 1,48 [3] 550-900 [3] 70 [3] Sisal 78 [14] 8 [14] 2 [14] 1,45 [14] 530-630 [14] 17-22 [14] Coir 43 [16] 45[16] - 1,15-1,25 [15] 120-304 [15] 4-6 [15] Flax 71 [17] 2,2 [17] 1,7 [17] 1,5 [18] 345-1100 [17] 27,6 [17] Kenaf 45-57 [20] 8-13[20] - 1,4[20] 930[20] 53[20]
3. Materials and Methods
3.1. Materials
PP (polypropylene) was used to prepare a reinforced composite (ExxonMobil chemical, a density of 0,9 g/cm3, and melting temperature of 165°C). Raw alfa fibers (Stipa tenacissima) were collected from rural areas of Morocco. The chemical used for treatment are NaOH (sodium hydroxide, Sigma Aldrich,98%), CH3COOH (Acetic acid, Riedel- de haёn,
99-100%), Palmitic acid N-succinimidyl ester and dodecane bromide are from sigma Aldrich. 3.2. Chemical Treatments of Alfa Fibers
Preparation Of Al fa -Alkali
Crushed alfa fibers are first washed with water and then kept for 48 hours in a 1.6mol/l sodium hydroxide aqueous solution [21]. The resulting fibers are then removed from the NaOH solution and treated with acetic acid (100mL) to neutralize the remaining hydroxide [22]. These fibers were finally air-dried for 24 hours before further use.
39
Preparation of Alf a -Ether
Dodecane bromide (3ml ) was added to a solution of alfa fibers (5g) and NaOH (20mL) in isopropanol. The solution was stirred at room temperature for 12 hours. The alfa fibers were recovered by filtration, washed with isopropanol and ethyl ether, and dried for 12 hours at 50°C. The obtained etherified fibers were denoted alfa-C12 (fig. 2-1).
Figure 2.1: reaction of the chemical modification of alfa etherification with dodecyl bromide
Preparation of Alf a -Est er
Alfa fibers (2g) were added in CH2Cl2 (40mL ) under stirring. Palmitic acid
N-succinimidyl ester (0.30g) was added as a solid and the reaction mixture was stirred at room temperature for 5 hours. The alfa fibers were recovred by filtration, washed sequentially with CH2Cl2, and then dried under reduce pressure. The obtained esterified fibers were denoted
alfa-Palm (fig.2-2).
Figure 2.2: reaction of the chemical modification of alfa esterification with palmitic acid n-succinimidyl ester.
40 3.3. Experimental Procedure
Composit e Pr epar ati on
Alfa fibers-filled Polypropylene (PP) composites were blended using a heated two rollmill mixer (thermo Haak Rhoomix,Germany); the mixing conditions were set at 220 °C for 5 minutes in each case. These conditions have been selected in order to homogenize the dispersion and the distribution of fibers in the PP matrix. For each blend, the neat PP matrix was filled onto the rolls. Heated rotating, after which the fibers were carefully added onto the melted PP and milled at a constant rotational speed of (60 rpm). For 5 minutes, at that point the torque measured was constant, rolls were then stopped and the composite removed from the heated rolls, before being cutted into small pieces for hot press molding.
Hot press molding was done in an automatic press CARVER, under the following operating conditions: both upper and lower plates were heated to 200°C, after which a force of 1200 LB was applied to the composites. Samples were molded in a dogbone shape as prescribed in the ISO 527-3 norm [23].
3.4. Characterization
ATR-FTIR Anal ys is
Fourier Transform-Infrared spectra were recorded on an ABB Bomem FTLA 2000-102 spectrometer (using SPECAC GOLDEN GATE). ATR accessory with a resolution of 4cm-1 was used.
Diff erenti al Ther mal and Ther mogr avimetric Analysis (DTG)/ (TGA )
The thermal decomposition of polymer and natural fibers was evaluated by thermogravimetric analysis (TGA), using a Q500 instrument from TA Instruments. Samples
