Preparation, Compatibilization and Characterization of Low Environmental Impact Polymer Composites Containing Natural Fibres

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UNIVERSITÀ DI PISA

Scuola di Dottorato in Ingegneria “Leonardo da Vinci”

Corso di Dottorato di Ricerca in

INGEGNERIA CHIMICA E DEI MATERIALI

Tesi di Dottorato di Ricerca

Preparation, Compatibilization and

Characterization of Low Environmental Impact

Polymer Composites Containing Natural Fibres

Autore:

Md. Minhaz-Ul Haque _________________________

Relatore:

Dott. Mariano Pracella _________________________ Dott. Giovanni Polacco _________________________

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Dedicated to

“My Heaven-born Father &

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I

INDEX

SOMMARIO V

ABSTRACT VII

CHAPTER 1. GENERAL INTRODUCTION

1.1 Preface 1

1.2 Polymer composite materials 3

1.2.1 Polymer composites with natural fibres 3

1.2.1.1 Natural fibres 4

1.2.1.1.1 Structure of natural fibres 4

1.2.1.1.2 Chemical composition of natural fibres 5 1.2.1.1.3 Physical and mechanical properties of natural fibres 7

1.2.1.2 Composites with thermoplastic polymer matrices 8

1.2.1.3 Applications of natural fibre reinforced polymer composites 9 1.3 Statement of the problems and possible solutions 9

1.4 Objectives of the thesis 11

1.5 Outline of the thesis 13

CHAPTER 2. BINARY COMPOSITES OF POLYMERS WITH CELLULOSE FIBRES

2.1 Introduction 15

2.2 Experimental 16

2.2.1 Materials 16

2.2.2 Preparation 18

2.2.2.1 Preparation of cellulose butanoate 18

2.2.2.2 Synthesis of PCL-g-MAGMA 18

2.2.2.3 Modification of cellulose microfibres (CF1) 18

2.2.2.4 Processing of composites 19

2.3 Results and discussion 20

2.3.1 Composites with PCL matrix 20

2.3.1.1 Characterization of modified fibres and PCL 20

2.3.1.2 Characterization of composites 22

2.3.1.2.1 Morphology 22

2.3.1.2.2 Crystallization behaviour 24

2.3.1.2.3 Thermal properties 27

2.3.1.2.4 Tensile mechanical properties 28

2.3.2 Composites of EVA with functionalized cellulose 30

2.3.2.1 Characterization of modified cellulose 30

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II

2.3.2.3 SEM and FT-IR analyses of composites 34

2.3.2.4 Thermal behaviour 35

2.3.2.5 DMTA analysis 37

2.3.2.6 Mechanical and rheological behaviour 38

2.3.3 Composites with functionalized EVA 41

2.3.3.1 Morphology 41

2.3.3.2 Thermal behaviour 42

2.3.3.3 FT-IR analysis 44

2.3.3.4 DMTA and tensile mechanical tests 45

2.4 Conclusion 49

CHAPTER 3. COMPOSITES OF POLYMER BLEND MATRICES WITH CELLULOSE FIBRES

3.2.2 Processing of blends and composites 53

3.3.1 PLA/EVA-GMA blends 54

3.3.1.1 Morphology of blends 54

3.3.1.2 Crystallization behaviour of PLA matrix 56

3.3.1.3 Tensile mechanical behaviour of blends 59

3.3.2 PLA/EVA-GMA/CF composites 61

3.3.2.1 Morphology 61

3.3.2.2 DSC and TGA analyses 62

3.3.2.3 Tensile mechanical properties 65

3.4 Conclusion 68

CHAPTER 4. TERNARY COMPOSITES OF MATER-BI AND POLYOLEFINS WITH HEMP FIBRES

4.2.2 Processing of composites 70

4.3.1 Thermal behaviour 72 4.3.2 Morphology 76 4.3.3 Thermogravimetric analysis 78 4.3.4 Mechanical tests 81 4.3.5 Rheological behaviour 84 4.4 Conclusion 85

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III

CHAPTER 5. BIO-COMPOSITES OF POLY(LACTIC ACID) WITH NANOCELLULOSE 5.1 Introduction 87 5.2 Experimental 89 5.2.1 Materials 89 5.2.2 Preparation 90 5.2.2.1 Nanocellulose preparation 90 5.2.2.2 Functionalization of PLA 90 5.2.2.3 Functionalization of nanocellulose 90 5.2.2.4 Processing of nanocomposite 91

5.3.1 Characterization of NC, functionalized NC and PLA 92 5.3.2 Characterization of nanocomposites 95

5.3.2.1 Morphology of PLA/NC nanocomposites 95

5.3.2.2 DSC analysis 96

5.3.2.3 Thermogravimetric analysis 99

5.3.2.4 Mechanical properties 100

5.4 Conclusion 103

CHAPTER 6. OVERALL CONCLUSIONS AND FUTURE DEVELOPMENTS 6.1 Overall Conclusions 105 6.2 Future developments 107 REFERENCES 109 APPENDIX 123 LIST OF SYMBOLS 127 ACKNOWLEDGEMENTS 129 LIST OF PUBLICATIONS 131

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V

SOMMARIO

Il presente lavoro è mirato allo sviluppo di nuovi materiali compositi polimerici a basso impatto ambientale e con struttura e proprietà controllate per potenziali applicazioni in settori tecnologicamente avanzati (quali imballaggio, componentistica, bio-ingegneria, trasporti, etc). In particolare, in questa tesi vengono discussi metodi di preparazione e compatibilizzazione, e le proprietà di vari tipi di compositi di polimeri termoplastici e loro miscele - quali policaprolattone (PCL), copolimeri etilene-vinil acetato (EVA), acido polilattico (PLA), Mater-Bi (MB) e poliolefine (PE, PP, PS) - contenenti fibre naturali di vario tipo (canapa, cellulosa micro- e nano-fibrillare).

Allo scopo di migliorare le interazioni tra le matrici polimeriche (idrofobe) e le fibre cellulosiche (idrofile), i sistemi esaminati sono stati compatibilizzati mediante modifica funzionale dei componenti o mediante aggiunta di copolimeri e agenti interfacciali. La funzionalizzazione è stata effettuata impiegando reazioni di esterificazione o innesto radicalico con monomeri bi-funzionali - come glicidilmetacrilato (GMA) e anidride maleica (MA) - in grado di reagire sia con il polimero che con le fibre di cellulosa. I compositi, ottenuti mediante miscelazione nel fuso in miscelatore Brabeder Plasti-Corder, sono stati caratterizzati mediante analisi microscopiche (OM, SEM, TEM, AFM), diffrattometriche (WAXS), spettroscopiche (FT-IR, NMR), termiche (DSC, TGA, DMTA), prove meccaniche tensili e misure reologiche nel fuso. I risultati hanno dimostrato che la morfologia, le proprietà termiche, il comportamento reologico e meccanico dei compositi sono strettamente influenzati dalla struttura dei componenti, dalla composizione e dai processi di miscelazione nel fuso (e in soluzione).

Per compositi binari PCL/CF e EVA/CF è stato esaminato l’effetto della modifica funzionale della matrice (con GMA e MA) e delle fibre (con acido butanoico) sulle proprietà finali dei materiali. Per compositi a base di miscele di polimeri, i risultati ottenuti hanno evidenziato che la morfologia, il comportamento di fase e le proprietà meccaniche di miscele PLA/EVA-GMA contenenti microfibre di cellulosa dipendono dalla miscibilità dei polimeri e quindi variano con il rapporto di composizione PLA/EVA-GMA. Nel caso di sistemi ternari costituiti da una matrice biodegradabile (MB), poliolefine e fibre di canapa (H), il comportamento alla cristallizzazione, la morfologia e le proprietà meccaniche risultano largamente influenzati dal grado di dispersione dei componenti e dalla compatibilizzazione. Per compositi ternari MB/PE/H e MB/PS/H è stato osservato un miglioramento significativo del modulo di elasticità e della resistenza rispetto ai compositi binari (MB/H). I risultati indicano che l'incorporazione di poliolefine nella matrice biodegradabile può comportare notevoli vantaggi in termini di proprietà, processabilità e costi.

Inoltre, sono stati messi a punto nuovi metodi di preparazione di bio-compositi di PLA con cellulosa fibrillare (NC) per promuovere la dispersione delle nano-fibre nella matrice, mediante innesto di GMA su NC o inclusione di NC in polivinil acetato (PVAc) - polimero miscibile con PLA. Tali sistemi, rispetto ai compositi PLA/NC non modificati, mostrano una accentuata dispersione delle fibre, con conseguente miglioramento delle proprietà meccaniche e della stabilità termica.

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VII

ABSTRACT

The present work is focused on the development of innovative and low environmental impact polymer composites with controlled structure and properties for potential applications in several advanced sectors (i.e.,food packaging, bio-engineering, electrical & electronics and automotive applications). In this thesis, the preparation, chemical modification and characterization of different types of thermoplastic polymers and their blends - such as poly(ε-caprolactone) (PCL), ethylene-co-vinyl acetate (EVA), poly(lactic acid) (PLA), Mater-Bi (MB) and polyolefins (PE, PP, PS) composites – containing various natural fibres (hemp, micro- and nano-fibrillated cellulose) have been reported.

Owing to the incompatibility of polymer matrices (hydrophobic) with cellulosic fibres (hydrophilic), either polymer or fibres have been functionalized, or a compatibilizer has been added during processing of composites. The functionalization was carried out by esterification or by grafting with bi-functional monomers - such as glycidyl methacrylate (GMA) and maleic anhydride (MA) - able to react with both polymer and fibres during melt mixing.

The composites were prepared by melt mixing in Brabeder Plasti-Corder and characterized by microscopic analyses (OM, SEM, TEM, AFM), diffractometry (WAXS), spectroscopy (FT-IR, NMR), calorimetry (DSC, TGA), tensile mechanical and rheological tests. Results demonstrated that morphology, phase behaviour, interfacial interactions, rheological and mechanical properties of composites were strictly affected by the component structure, composition and mixing processes. For binary PCL/CF and EVA/CF composites the effect of functional modification of matrix (with GMA and MA) and fibres (with butanoic acid) on the final properties of materials was examined. Results obtained for composites with polymer blend matrix (PLA/EVA-GMA/CF) demonstrated that phase morphology and mechanical properties were influenced by the polymer-polymer miscibility, and properties varied with blend composition. In the case of ternary composites of biodegradable MB, polyolefins and hemp fibres (H), the crystallization behaviour, morphology and mechanical strenght of the composites were found to be markedly affected by the phase dispersion and compatibilizer type. A significant improvement of tensile modulus and strength was recorded for ternary MB/PE/H and MB/PS/H composites as compared to binary composites (MB/H). The results indicated that incorporation of polyolefins in the biodegradable matrix may have significant advantages in terms of properties, processability and cost.

Moreover, a novel approach to the preparation and functionalization of PLA bio-composites with nano-fibrillated cellulose (NC) has been reported. The effect of different processing strategies on the phase dispersion of NC, as well as on the thermal and mechanical properties of these systems has been analysed. To improve the dispersion of NC in PLA either functionalization of NC or PLA by GMA grafting, or inclusion of NC in poly(vinyl acetate) (PVAc) – which is miscible with PLA - were applied. Better dispersion of NC was observed for PLA nanocomposites containing functionalized components and/or NC included in PVAc. These systems – as compared to the unmodified PLA/NC composites - display a higher degree of fibre dispersion with improved tensile properties and thermal stability.

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CHAPTER 1. GENERAL INTRODUCTION

1.1 Preface

As early as 1908, probably the first polymer composite materials -made of cotton reinforced phenol or melamine-formaldehyde resins- were applied for the fabrication of large quantities of sheets, tubes and pipes for electronic purposes. The initial reason for the use of fillers in polymers was to reduce the cost of the compound. The most familiar composite materials is Fibreglass_{(introduced in}

1940), which is widely used to form large lightweight reinforced structure, such as the body of a Corvette, or the hull of a cabin cruiser. Fibreglass is the trade name for a composite consisting of glass-fibre-reinforcement of an unsaturated-polyester matrix. Since the 1970s, the application of composites has widely increased due to development of new fibres such as carbon or graphite, aromatic polyamide (Kevler), boron, aramids etc. These fibres-reinforced composites show high mechanical and thermal properties, so they are widely used in various applications such as construction of components for boats, cars and aeroplanes, as well as sports equipment e.g. tennis racquets and golf clubs [1, 2].

Fig. 1.1- Consumption of natural fibres tonnes in the european automotive industry.

However, these advantages, on the other hand, cause environmental problems in disposal. Therefore, in order to overcome these problems, environmentally friendly composites are keenly required utilizing natural fillers (e.g. natural fibres) as reinforcements. In addition, the unsustainable consumption of petroleum and new environmental regulations have led to a renewed interest concerning natural materials with the focus on renewable raw materials.

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2 Natural fibres are important and potential renewable raw materials, which are used in diverse applications. Recently natural fibre-reinforced-polymer composites became an important branch in the field of composite materials and it has been studied for decades [3-12]. Incorporation of the tough and light-weight natural fibres into polymer (thermoplastic and thermoset) matrices produces composites with a high specific stiffness and strength [13]. Moreover, addition of natural fibres into synthetic polymers (biodegradable or non-biodegradable) offers the possibility to design new class of eco-friendly composite materials with reduced environmental impact. As compared to conventional inorganic fillers e.g., glass fibre and carbon fibres, natural fibres have many advantages such as abundance and therefore low cost, biodegradability, renewability, flexibility during processing and less resulting machine wear, minimal health hazards, low density, desirable fibre aspect ratio, and relatively high tensile and flexural modulus. The renewable and biodegradable characteristics of natural fibres facilitate their ultimate disposal by composting or incineration, options not possible with most industrial fibres. In addition, the recycling by combustion of cellulosic fibres filled composites is easier as compared to inorganic fillers systems. Therefore, the possibility of using cellulosic fillers in the plastic industry has received considerable interest. Automotive applications display strong promise for natural fibre reinforcements [14-16]. Fig 1.1 shows the consumption of natural fibres by the European automotive industry (upto 2000) and projections of (upto 2010) total consumption [17]. Potential applications of agrofibre based composites in railways, aircraft, irrigation systems, furniture industries, and sports and leisure items have been researched [18]. Despite these attractive properties, cellulosic fillers are used only to a limited extent in industrial practice due to difficulties associated with surface interactions. Recently, it has been reported by Seppala in a review paper [19] that nanocellulose is an important renewable material with a very bright future. Gilberto et al reported in a review based on cellulosic bio-nanocomposites that cellulose nanoparticles can be used as fillers to improve the mechanical and barrier properties of bio-composites. The use of cellulosic bio-nanocomposites for industrial packaging is being investigated, with continuous studies to find innovative solutions for efficient and sustainable systems [20].Cellulose nanofibres and nanowhiskers derived from renewable biomass have attracted much interest as an alternative to micro-sized reinforcements in composite materials [21-35]. Polymer nanocomposites based on nanocellulose are a relatively new class of composites that exhibit ultrafine phase dimensions of 1-1000 nm. Due to the nanometric size effect, these composites have some unique outstanding properties with respect to their conventional microcomposite counterparts.

In recent years, potential emphasis has been given in the preparation of cellulose nanofibres reinforced polylactic acid (PLA) nanocomposite materials with the goal of obtaining fully bio-based composites [29-35, 36-39]. However, the development of more flexible and industrially viable processing techniques is necessary to promote commercialization of these materials.

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3

1.2 Polymer composite materials

Composite materials are defined as engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties, which remain separate and distinct at the macroscopic or microscopic scale within the finished structure. There are two categories of constituent materials: matrix and reinforcement. The matrix material surrounds and supports the reinforcement materials by maintaining their relative positions. The reinforcements impart their special mechanical and physical properties to enhance the matrix properties [40].

There are two classification systems of composite materials. One of them is based on the matrix material (metal, ceramic, and polymer) and the second is based on the material structure. [41]

Classification of composites based on matrix materials:

(i) Polymer matrix composites (ii) Metal matrix composites (iii) Ceramic matrix composites (iv) Carbon-carbon composites

Classification of composite materials based on reinforcing material structure: (i) Particulate composites

(ii) Fibrous composites (iii) Laminate composites

Composites are a versatile and valuable family of materials that can solve problems of different applications, improve productivity, lower cost and facilitate the introduction of new properties in materials.

The most common advanced composites are polymer matrix composites. These composites consist of a polymer thermoplastic or thermosetting reinforced by fibre (glass, carbon or graphite, boron, cellulosic fibres etc.). These materials can be fashioned into a variety of shapes and sizes. They provide great strength and stiffness along with resistance to corrosion. The reason for these being most common is their low cost, high strength and simple manufacturing principles.

1.2.1. Polymer composites with natural fibres

When natural fibres or cellulosic fibres are used as reinforcements and a polymer is used as matrix then the resultant composite material is said to be natural fibres reinforced polymer composite.

The role of matrix in a fibre-reinforced composite is to transfer stress between the fibres, to provide a barrier against an adverse environment and to protect the surface of the fibres from mechanical abrasion. The matrix plays a major role in the tensile load carrying capacity of a composite structure. The binding agent or matrix in the composite is of critical importance. Four major types of matrices have been reported: polymeric, metallic, ceramic and carbon. Most of the composites used in the industry today are based on polymer matrices. Polymer resins have been divided broadly into two categories: thermosetting and thermoplastics.

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4

Thermoset is a hard and stiff crosslinked material that does not soften or become moldable when heated. Thermosets are stiff and do not stretch the way that elastomers and thermoplastics do. Several types of polymers have been used as matrices for natural fibre composites. Most commonly used thermoset polymers are epoxy resins and other resins (unsaturated polyester resins (as in fibre glass) vinyl ester, phenolic epoxy, novolac and polyamide)) [42-44]. Unsaturated polyesters [45] are extremely versatile in properties and applications and have been a popular thermoset used as the polymer matrix in composites. They are widely produced industrially as they possess many advantages compared to other thermosetting resins including room temperature cure capability, good mechanical properties and transparency. The reinforcement of polyesters with cellulosic fibres has been widely reported. polyester-jute [46, 47], polyester-sisal [48], polyester-coir [49], polyester-straw [50], polyester-pineapple leaf [51], and polyester-cotton-kapok [52], are some of the promising systems. Systems with thermoplastic matrix have been described in later section.

1.2.1.1 Natural fibres

The term “natural fibres” covers a broad range of vegetable, animal, and mineral fibres. However, in the composites industry, it usually refers to wood fibre and agrobased bast, leaf, seed, and stem fibres. These fibres are also referred to as cellulosic fibres, related to the main chemical component cellulose. These fibres often contribute greatly to the structural performance of the plant and, when used in plastic composites, can provide significant reinforcement.

1.2.1.1.1. Structure of natural fibres

Fig. 1.2 - Structure of natural fibre [54].

Natural fibres can be considered as composites of hollow cellulose fibrils held together by a lignin and hemicellulose matrix [53]. The cell wall in a fibre is not a homogenous membrane (see Fig. 1.2) [54]. Each fibre has a complex, layered

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5 structure consisting of a thin primary wall which is the first layer deposited during cell growth encircling a secondary wall. The secondary wall is made of three layers and the thick middle layer determines the mechanical properties of the fibre. The middle layer consists of a series of helically wound cellular microfibrils formed from long chain cellulose molecules.

Fig. 1.3 - Structure model of cellulose given by Gardner and Blackwell [55].

The angle between the fibre axis and the microfibrils is called the microfibrillar angle. The characteristic value of microfibrillar angle varies from one fibre to another. Such microfibrils have typically a diameter of about 10–30 nm and are made of 30-100 cellulose molecules in extended chain conformation and provide mechanical strength to the fibre. The basic elementary fibrils are composed of successions of elementary crystallites. The internal cohesion between the crystallites is achieved through polymer molecules extending from less ordered interlinking and non-crystalline regions (Fig. 1.3). The amorphous matrix phase in a cell wall is very complex and consists of hemicellulose, lignin, and in some cases pectin. The hemicellulose molecules are hydrogen bonded to cellulose and act as cementing matrix between the cellulose microfibrils, forming the cellulose-hemicellulose network, which is thought to be the main structural component of the fibre cell. The hydrophobic lignin network affects the properties of other network in a way that it acts as a coupling agent and increases the stiffness of the cellulose/hemicellulose composite [56]. Some of the important natural fibres with their chemical compositions used as reinforcement in composites are listed in Table 1.1.

1.2.1.1.2 Chemical composition of natural fibres

The reinforcing efficiency of natural fibre depends on the nature of cellulose and its crystallinity. The main components of natural fibres are cellulose (α-cellulose), hemicellulose, lignin, pectins, and waxes. Cellulose is a linear homopolysaccharide made of repeating units of β-(l,4)-D-glucopyranose. The carbon atoms in the pyranose ring are numbered 1 to 5, and the carbon in the attached methanolic group is numbered 6 (Fig. 1.4). Depending on the plant source, native cellulose, which did not undergo any alteration after biosynthesis, can have average degrees of polymerisation (DP) higher than 10,000, in other words average molar masses above 1.5xl06 g mol-1.

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6 Fig. 1.4 - Cellulose molecule, planar (left) and 3-D (right) representations

Each repeating unit contains three hydroxyl groups. These hydroxyl groups and their ability to hydrogen bond play a major role in directing the crystalline packing and also govern the physical properties of cellulose. Solid cellulose forms a microcrystalline structure with regions of high order, i.e. crystalline regions, and regions of low order, i.e. amorphous regions. 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.

Hemicellulose is not a form of cellulose and the name is a misnomer. They comprise a group of polysaccharides composed of a combination of 5- and 6-carbon ring sugars. Hemicellulose differs from cellulose in three aspects. Firstly, 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 containing pendant side groups giving rise to its noncrystalline nature, whereas cellulose is a linear polymer. Thirdly, DP of hemicellulose is around 50-300, whereas that of native cellulose is 10-100 times higher than that of hemicellulose. Hemicellulose forms the supportive matrix for cellulose microfibrils. Hemicellulose is very hydrophilic, soluble in alkali, and easily hydrolyzed in acids. Lignin is a complex hydrocarbon polymer with both aliphatic and aromatic constituents. They are totally insoluble in most solvents and cannot be broken down to monomeric units. Lignin is totally amorphous and hydrophobic in nature. It is the compound that gives rigidity to the plants. It is thought to be a complex, three-dimensional copolymer of aliphatic and aromatic constituents with very high molecular weight. Hydroxyl, methoxyl, and carbonyl groups have been identified in lignin. Lignin has been found to contain five hydroxyl and five methoxyl groups per building unit. It is believed that the structural units of lignin molecule are derivatives of 4-hydroxy-3-methoxy phenylpropane. The main difficulty in lignin chemistry is that no method has been established by which it is possible to isolate lignin in its native state from the fibre. Lignin is considered to be a thermoplastic polymer exhibiting a glasstransition 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 [57]. Pectins are a collective name for heteropolysaccarides. They provide flexibility to plants. Waxes make up the last part of fibres and they consist of different types of alcohols.

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7 Table 1.1. Chemical composition of various natural fibres [58-60].

Fibre Cellulose (wt%) Hemicellulose (wt%) Lignin (wt%) Waxes (wt%) Abaca 56-63 20-25 7-9 3 Bamboo 26-43 30 21-31 - Coir 32-43 0.15-0.25 40-45 - Cotton 85-90 5.7 - 0.6 Flax 71 18.6-20.6 2.2 1.5 Hemp 68 15 10 0.8 Jute 61-71 14-20 12-13 0.5 Kenaf 72 20.3 9 - Ramie 68.6-76.2 13-16 0.6-0.7 0.3 Sisal 65 12 9.9 2 Sun hemp 41-48 8.3-13 22.7 -

1.2.1.1.3. Physical and mechanical properties of natural fibres

Table 1.2. Mechanical properties of various natural fibres as compared to conventional reinforcing fibre [58, 59, 61, 62].

Fibre Tensile strength

(MPa) Young’s modulus (GPa) Elongation at break (%) Density (g/cm3) Abaca 400 12 3-10 1.5 Bamboo 140-230 11-17 - 0.6-1.1 Coir 175 4-6 30 1.2 Cotton 287-597 5.5-12.6 7-8 1.5-1.6 Flax 345-1,035 27.6 2.7-3.2 1.5 Hemp 690 70 1.6 1.48 Jute 393-773 26.5 1.5-1.8 1.3 Kenaf 930 53 1.6 - Ramie 560 24.5 2.5 1.5 Sisal 511-635 9.4-22 2.0-2.5 1.5 E-glass 2000-3500 70.0 2.5 2.5 S-glass 4570 86.0 2.8 2.5 Aramide 3000-3150 63.0-67.0 3.3-3.7 1.4 Carbon 4000 230.0-240.0 1.4-1.8 1.4

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8 The structure, microfibrillar angle, cell dimensions, defects, and the chemical composition of fibres are the most important variables that determine the overall properties of the fibres [63]. Generally, tensile strength and Young’s modulus of fibres increase with increasing cellulose content. The microfibrillar angle determines the stiffness of the fibres. Plant fibres are more ductile if the microfibrils have a spiral orientation to the fibre axis. If the microfibrils are oriented parallel to the fibre axis, the fibres will be rigid, inflexible, and have high tensile strength. Table 1.2 presents the important physical and mechanical properties of natural fibres [64, 65].

1.2.1.2 Composites with thermoplastic polymer matrices

Thermoplastics are polymers that can be heat-soften in order to process into a desired form. After cooling, such materials retain their shape. In addition, these polymers may be reheated and reformed, often without significant changes in their properties. Waste thermoplastics can be recovered and refabricated by application of heat and pressure. Examples of thermoplastics composites which have been used with natural fibre reinforcements are reported in Table 1.3.

Table 1.3 List of thermoplastics used in polymer composites

Polymer matrix Filler Reference

HDPE Wood flour 66,67

LDPE Wood fibres 68, 69

CPE Kenaf 70

PP Wood, Hemp 67, 71-74

PS Wood, Cellulose 75, 76

PVC Wood pulp 77, 78

Mixed polymers Cellulose 79

Recycled (PE, PP) Wood fibres 80, 81

EVA Sisal 82

PLA Cellulose, Hemp, Cotton, Kenaf

83-87

PHB Wood fibres 88

PCL Abaca, Flax 89, 90

Only those thermoplastics are useable for natural fibre reinforced composites, whose processing temperature (temperature at which fibre is incorporated into polymer matrix) does not exceed 230°C. These are, most of all, polyolefines, like polyethylene and polypropylene. Technical thermoplastics, like polyamides and polycarbonates require processing temperatures > 250°C and therefore, are not useable for such composite processing without fibre degradation.

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9

1.2.1.3 Applications of natural fibre reinforced polymer composites

Fig. 1.5 – Applications of PP/natural fibres composites in different sectors [91].

Fig. 1.5 shows some applications of composities of polypropylene (PP) with natural fibres. Natural fibres used in composites offer several advantages, such as no net carbondioxide release and 40% less weight compared with glass fibre. Natural fibre/plastics have found many applications replacing natural wood or plastics, such as (i) Automotive applications: there are remarkable trends towards the replacement of established materials by several types of plants fibre embedded in plastic matrices e.g. the automotive industry is using flax, hemp, jute, sisal, kenaf, wood or grain based products as reinforcement and the most commonly used plastic matrix is PP, although polyurethane applications are emerging less weight in comparison to glass fibre filled marterials and no net carbondioxide release make the new materials attractive for car manufacturers; (ii) Building applications: in contrary to Asia, where natural fibre reinforced materials have been used for buildings and similar applications for many years, application in the western world started in the mid-nineties in Japan, and in the US followed by Europe; (iii) Furniture and panels: furniture applications based on wood flour filled materials have been reported from Canada, Japan and Germaney; (iv) Aerospace applications: due to advantages of weight, mechanical stability and price, interest in the application of natural fibre reinforced materials is growing in the aerospace industry in the US and Europe; (v) Others: Natural fibres composites are being used in benches, dog kennels, sheds, flower pots, partitions, and fences on the Japanese market along with decks, pavements and balcony boards [17].

1.3 Statement of the problems and possible solusions

Although natural fibres have many advantages, such as sustainability, recyclability, low density, biodegradability, low cost - which offer greater opportunities to develop a new class of environmental friendly structural composites – but, the usually polar fibres have inherently low compatibility with non-polar polymer matrices [92, 93]. The incompatibility may cause problems in the composite processing and material properties. Hydrogen bonds may form among the hydrophilic fibres, and thus the fibres tend to agglomerate into bundles and unevenly distribute throughout the non-polar polymer matrix during compounding processing [94, 95]. There is also

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10 insufficient wetting of fibres by the non-polar polymer matrices, resulting in weak interfacial adhesion. As a result, the stress transfer efficiency from the matrix to the reinforcing fibres is reduced. Although, mechanical properties of composites depend on several factors, such as the properties of constituent reinforcement and matrix, their relative volume fraction, the shape, size and architecture of reinforcement phase, but to a great extent on the reinforcement/matrix interfacial shear strength. A relatively strong interfacial bond is needed for an effective transfer of the applied load, since a weak interface will probably lead to a premature failure of the composite [96].

Another drawback of cellulosic fillers is their high moisture absorption and the resulting swelling and decrease in mechanical properties. The moisture absorption of the natural fibres may cause dimensional changes of the resulting composites and weaken the interfacial adhesion [97, 98]. Therefore, to increase the compatibility among the composite components improving fibre dispersion and the interfacial adhesion, either surface modification of fibres and/or matrix or addition of suitable copolymers (coupling agent) can be exploited.

Among the processes of chemical modification of natural fibres (such as alkaline, silane, permanganate treatments, or peroxidation, acetylation, benzoylation, etc.), those concerning grafting of maleic anhydride (MA) [99] and glycidyl methacrylate (GMA) [100]are very versatile. In particular, GMA functionalization has been used to effectively modify both the polymer matrix and natural fibres achieving enhanced interfacial bonding and mechanical properties of their composites [73]. The chemical modification may make the fibre cell walls more dimensionally stable, reduce water sorption, or increase resistance against fungal decay. Additionally, polymer functionalised with reactive monomers such as (MA and GMA) can be used as compatibilizer to improve the interfacial interactions between polymer matrix and cellulose. Extensively used compatibilizer or coupling agents for natural fibres polymer composites are copolymers containing maleic anhydride such as maleated polypropylene (MAPP) or maleated polyethylene (MAPE) [95, 101-103] and copolymer containing GMA such as PP-g-GMA [73]. The anhydride groups or epoxy groups of the copolymers may react with the surface hydroxyl groups of natural fibres forming ester bonds whilst the other end of copolymer entangles with the polymer matrix due to their similar polarities [104].

In the case of cellulose nanocomposites, the main problem is the aggregation or non homogeneous dispersion of cellulose nanofibres in polymer matrix. As, it is difficult to disperse them uniformly in a non-polar medium, the processing of cellulose nanocomposites was first limited to solvent casting [21-29]. Recently, melt compounding processing method has been developed [30-35, 36]. The nanocelluloses have a very high surface area and have a tendency to form aggregates when dried. To avoid aggregation during processing, nanocellulose aqueous suspension can be dispersed in the aqueous solution or emulsion of a matrix polymer or a polymer, which is miscible with polymer matrix. The mixture can be dried and followed by melt-mixing with polymer matrix. In such way, nanocellulose can be dispersed properly into a polymer matrix by melt compounding processing.

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11

1.4 Objectives of the thesis

The main goal of the present thesis was the development of eco-friendly natural fibre reinforced polymer composites with cotrolled properties - for packaging and other applications – using different types and compositions of polymer matrices and fibres. Within this framework we started a research study with the following objectives:

(i) development of innovative biodegradable materials for packaging applications containing polymers and natural fillers derived from biogenic resources;

(ii) application of tailored functionalization and compatibilization methods, aimed at the control of interface properties, morphology and mechanical performances of the composites;

(iii) design of multicomponent composite systems based on polymer blend matrices and natural components;

(iv) preparation of bio-composites systems containing nanocellulose by solution dispersion followed by melt blending (for biomedical applications).

Fig. 1.6 - Flow diagram for composite processing and studies

The Fig. 1.6 shows the flow diagram for composite processing and characterization. Owing to the incompatibility of polymer matrices (hydrophobic) with cellulosic fibres (hydrophilic) either polymer or cellulose have been functionalized or a compatibilizer has been added during processing of composites. The functionalization of polymers (or fibres) were carried out basically with bi-functional monomers such as glycidyl methacrylate and maleic anhydride so that

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Chapter 1………...………...………

General Introduction

12 one functional group of monomer can react with polymer (or fibres) during functionalization and another group remain available to react with cellulose fibre (or polymer) during melt mixing respectively ( see Fig. 1.7). The components were melt mixed in an internal mixer by melting. Finally, interfacial interaction between components, morphology, thermal and crystallization behaviour, mechanical properties and rheological behaviour of the obtained composite materials were studied by different analytical techniques such as FT-IR, DSC, TGA, SEM, OM, XRD, DMTA, tensile mechanical tests and rheological tests.

In most of the composite systems, cellulose fibres - the main constituent of natural fibres - have been used as reinforcement to facilitate the reaction of hydroxyl group of cellulose with epoxy/anhydride group of a functionalized copolymer.

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General Introduction

13 Fig. 1.7b - Functionalization of polymer and reactive mixing with fibres

1.5 Outline of the thesis

In this thesis the preparation, compatibilization and characterization of cellulosic fibres reinforced polymers (polyesters and polyolefins) composites and nanocomposite will be discussed.

For a better discussion and understanding of the activities carried out, the work has been divided in the following four chapters:

1. Binary composites of polymers with cellulose fibres (PCL/CF, EVA/CF) 2. Composites of polymer blend matrices with cellulose (PLA/EVA-GMA/CF) 3. Ternary composites of mater-Bi and polyolefins with hemp fibres

4. Bio-composites of poly(lactic acid) with nanocellulose

In chapter 2, the compatibilization and properties of binary composites of polymers (PCL, EVA) with cellulose microfibres have been described. In particular, the effect of different functionalization procedures on the properties of composites has been analysed. In chapter 3, the results of PLA/EVA-GMA blend and their composites with cellulose have been reported. For this composite systems, the effect of phase

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Chapter 1………...………...………

General Introduction

14 morphology and cellulose content on the properties of blends and composites have been examined. Chapter 4 deals with the characterization of Mater-Bi/Polyolefin/Hemp ternary composites. In this composite systems, the effect of polyolefin and compatibilizer content on the properties of composites has been discussed. Chapter 5 deals with the development of PLA bio-composites with cellulose nanofibre. In chapter 6, the main conclusions from this research have been summarised and an outlook has been given regarding possible future research on this subject.

To avoid the repetition of the description of analytical techniques in each chapter, their description has been given seperately in the appendix.