Pickering Properties and Performance of Natural-Fibre Composites

1. Auflage 2008
ISBN: 978-1-84569-459-3
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 576 Seiten

Reihe: Woodhead Publishing Series in Composites Science and Engineering

Concern about global warming has led to renewed interest in the more sustainable use of natural fibres in composite materials. This important book reviews the wealth of recent research into improving the mechanical properties of natural-fibre thermoplastic composites so that they can be more widely used.The first part of the book provides an overview of the main types of natural fibres used in composites, how they are processed and, in particular, the way the fibre-matrix interface can be engineered to improve performance. Part two discusses the increasing use of natural-fibre composites in such areas as automotive and structural engineering, packaging and the energy sector. The final part of the book discusses ways of assessing the mechanical performance of natural-fibre composites.With its distinguished editor and team of contributors, Properties and performance of natural-fibre composites is a valuable reference for all those using these important materials in such areas as automotive and structural engineering. - Provides an overview of the types of natural fibres used in composites - Discusses fibre-matrix interface and how it can be engineered to improve performance - Examines the increasing use of natural-fibre composites in automotive and structural engineering and the packaging and energy sector

Autoren/Hrsg.

Pickering, Kim

Leseproben

2 Matrices for natural-fibre reinforced composites
P.A. Sreekumar    National Institute of Technology Calicut, India S. Thomas    Mahatma Gandhi University, India 2.1 Introduction
Composite materials are the most advanced and adaptable engineering materials. A composite is a heterogeneous material created by the synthetic assembly of two or more components constituting reinforcing matrix and a compatible matrix, in order to obtain specific characteristics and properties.1 The matrix may be metallic, ceramic or polymeric in origin. The matrix gives a composite its shape, surface appearance, environmental tolerance and overall durability while the fibrous reinforcement carries most of the structural loads, thus giving macroscopic stiffness and strength. It is the behaviour and the characteristics of the interface that generally control the properties of a composite. Development of advanced composite materials having superior mechanical properties opened new horizons in the engineering field. Advantages such as corrosion resistance, electrical insulation, reduction in tooling and assembly costs, low thermal expansion, higher stiffness and strength, fatigue resistance, such as greater stiffness at lower weight than metals, etc., have made polymer composites widely acceptable in structural applications. However, the disadvantages of composite materials cannot be ignored: their complex nature, designers’ lack of experience, little knowledge of material databases and difficulty in manufacturing are barriers to large-scale use of composites. Composites can be classified based on the form of their structural components: fibrous (composed of fibres in a matrix), laminar (composed of layers of materials) and particulate (composed of particles in a matrix). The particulate class can further be subdivided into flake (flat flakes in a matrix) and skeletal (composed of a continuous skeletal matrix filled by a second material). In general, the reinforcing agent can be fibrous, powdered, spherical, crystalline or whiskered and either an organic, inorganic, metallic or ceramic material. 2.2 Natural-fibre reinforced polymer composites
Concern about the preservation of natural sources and recycling has led to renewed interest in biomaterials with the focus on renewable raw materials. As a resuit, new types of composites based on plant fibres have been developed in recent years. Natural-fibre reinforced composites offer a good mechanical performance and eco-friendliness. The application of natural-fibre-based composites is increasing rapidly. This is especially related to certain problems concerning the use of synthetic fibre reinforced composites. As far as synthetic polymer composites are concerned, waste disposal and recycling are major issues worldwide. Landfill disposal is being increasingly excluded around the world due to growing environmental sensitivity. Therefore, in recent years environmentally compatible alternatives have been examined by researchers. This research covers factors such as efficient cost-effective and environmentally friendly recovery of raw materials, CO2-neutral thermal utilisation or biodegradation in certain circumstances. That is why composites based on renewable resources consisting of either natural fibres or so-called biopolymers, or both, are economically and ecologically acceptable. Natural fibres such as flax, hemp, banana, sisal, oil palm and jute have a number of techno-economical and ecological advantages over synthetic fibres such as glass fibres. The combination of interesting mechanical and physical properties together with their environmentally friendly character has aroused interest in a number of industrial sectors, notably the automotive industry. The advantages and disadvantages of using natural fibres in composites are given in Table 2.1. Lignocellulosic fibres have an advantage over synthetic ones since they buckle rather than break during processing and fabrication. In addition, cellulose possesses a flattened oval cross-section that enhances stress transfer by presenting an effectively higher aspect ratio. Table 2.1 Advantages and disadvantages of using natural fibres in composites Advantages Disadvantages Lowspecific weight, compared with glass reinforced composites Enormous variability Renewable resource with production requiring low CO2 emissions Poor moisture resistance The processing atmosphere is worker friendly with better working conditions Poor fire resistance High electrical resistance Lower durability Good thermal and acoustic insulating properties Lack of fibre-matrix adhesion Biodegradability 2.3 Different matrices
2.3.1 Thermoplastic matrices
Polymers that soften or melt upon heating, called thermoplastic polymers, consist of linear or branched chain molecules having strong intramolecular bonds but weak intermolecular bonds. Melting and solidification of these polymers are reversible and they can be reshaped by application of heat and pressure. They are either semicrystalline or amorphous in structure. Examples include polyethylene (PE), polystyrene (PS), nylons, polycarbonate (PC), polyacetals, polyamide-imide, polyether-ether ketone (PEEK), polysulphone polyphenylene sulphide and polyether imide. 2.3.2 Thermosetting matrices
Thermosetting plastics have crosslinked or network structures with covalent bonds between all molecules. They do not soften but decompose on heating. Once they have been solidified by crosslinking, they cannot be reshaped. Common examples of thermosetting polymers include epoxies, polyesters and phenol formaldehyde. 2.3.3 Rubber matrices
The principal classes of rubber composites that have been used for the preparation of composites are: natural rubber (NR), styrene butadiene rubber (SBR), butyl rubber (IIR), butadiene rubber (BR), nitrile rubber (NBR), chloroprene rubber (CR), ethylene propylene diene rubber (EPDM), polyurethane rubber and silicon rubbers. The most widely used rubber matrix is natural rubber. 2.3.4 Biodegradable matrices
At a time when the world is switching to everything natural from organic farming to vegetarian diets, terms such as ‘green chemistry’ and ‘biocomposites’ seem to be the mantra of the times. Research efforts are currently being harnessed in developing a new class of fully biodegradable ‘green’ composites by combining natural/biofîbres with biodegradable resins. The major attractions about green composites are that they are environmentally-friendly, fully degradable and sustainable. At the end of their life they can be easily disposed of or composted without harming the environment. Green composites may be used effectively in many applications such as mass-produced consumer products with short life cycles or products intended for one-off or short-term use before disposal. A number of natural and biodegradable matrices available to use in green composites are listed in Table 2.2. Starch and modified resins have also been used as matrix to form green composites. The reinforcement of biofibres in green composites has been highlighted by Bismarck et al.2 Figure 2.1 shows a classification of biodegradable polymers in four families.3 Table 2.2 Natural and biodegradable matrices Natural Synthetic Polysaccharides Poly(amides) Starch Poly(anhydrides) Cellulose Poly(amide-enamines) Chitin Poly(vinyl alcohol) Proteins Poly(vinyl acetate) Collagen/gelatin Polyesters Casein, albumin, fibrogen, silks Poly(glycolic acid) Polyhydroxyalkanoates Poly(lactic acid) Lignin Poly(caprolactone) Lipids Poly(orthoesters) Shellac Poly(ethylene oxides) Natural rubber Poly(phosphazines) 2.1 Classification of biodegradable polymers.3 Except the fourth family, which is of fossil origin, most polymers (families 1–3) are obtained from renewable resources (biomass). The first family is agropolymers (e.g. polysaccharides) obtained from biomass by fractionation. The second and third families are polyesters, obtained respectively by fermentation from biomass or from genetically modified plants (e.g. polyhydroxyalkanoate, PHA) and by synthesis from monomers obtained from biomass (e.g. polylactic acid, PLA). The fourth family comprises polyesters, totally synthesised by the petrochemical process (e.g. polycaprolactone, PCL; polyesteramide, PEA; aliphatic or aromatic copolyesters). Another important biocomposites category is based on agro-polymer matrices, mainly focused on starchy materials. Plasticised starch, the so-called ‘thermoplastic starch’ (TPS), is obtained after disruption and plasticisation of native starch, with water and plasticiser (e.g....