Injection moulding of plant fibre composites

The use of ligno-cellulosic fibres to reinforce polymer matrices has been most frequently experimented on during last years, because these fibres offer a number of benefits such as a reduction in weight and carbon footprint, limited dependence on oil sources, and recyclability. In most cases they are intended to replace glass fibres, which are mostly applied for that purpose in a number of sectors, including for example the automotive and construction industries. In other applications, such as those dealing with packaging and furniture, the use of short fibres is often preferred, which leads to the extensive application of the injection moulding process for manufacturing, often in combination with other techniques, such as melt compounding or extrusion. The most commonly used ligno-cellulosic fibres are bast-extracted (hemp, flax, jute, kenaf, etc.), leaf-extracted (e.g., sisal or abaca) and obtained from fruit hair, like coir. The different characteristics of these fibres, together with the inherent variability of biological materials, may create substantial problems connected with the processing of plant fibre composites. In particular, it may be cumbersome to obtain a real homogenization of the fibre’s properties together with adequate control over polymerization and crystallization and effective adhesion between fibre and matrix, especially at fibre lengths that are not far, if not even lower, than their critical length. This chapter deals first with the general characteristics of plant fibres (Section 1), then moves to a discussion of the variables which may affect injection moulding of these fibres into polymers (Section 2) and subsequently passes to presenting different attempts performed with plant fibres (Section 3). The conclusions (Section 4) briefly deal with prospective developments and possible improvements of the effectiveness of the process of injection moulding for plant fibre composites aimed at semi-structural applications.