Composites reinforced with cellulose based fibres

This review article concerning natural and man-made cellulose fibre reinforced plastics, introduces possible applications of this material group. The physical properties of natural fibres are mainly determined by the chemical and physical composition, such as the structure of fibres, cellulose content, angle of fibrils, cross-section, and by the degree of polymerization. Only a few characteristic values, but especially the specific mechanical properties, can reach comparable values of traditional reinforcing fibres. This physical structure can be modified by using alkali treatment and acetylation processes. The application of natural fibres as reinforcements in composite materials requires, just as for glass-fibre reinforced composites, a strong adhesion between the fibre and the matrix, regardless of whether a traditional polymer (thermoplastics or thermosets) matrix, a biodegradable polymer matrix or cement is used. Further this article gives a survey about physical and chemical treatment methods which improve the fibre matrix adhesion, their results and effects on the physical properties of composites. These different treatments change among others the hydrophilic character of the natural fibres, so that moisture effects in the composite are reduced. To bring about hydrophobic properties to natural fibres, a special treatment, termed acetylation, can be used. The effectiveness of this method is strongly influenced by the treatment conditions used. The mechanical and other physical properties of the composite are generally dependent on the fibre content, which also determines the possible amount of coupling agents in the composite. The influence of such treatments by taking into account fibre content on the creep, quasi-static, cyclic dynamic and impact behaviour of natural fibre reinforced plastics are discussed in detail. For special performance requirements, hybrid composites made of natural and conventional fibres can be prepared with desired properties. The processing conditions play, next to the mechanical properties of natural fibres, an important role for the industrial use of these materials. The results presented in this paper show, that natural fibres can be processed with the already commonly applied methods: glass mat thermoplastic matrix (GMT), sheet moulding compound (SMC) or bulk moulding compound (BMC). For the processing of thermoplastics reinforced with natural fibres, new methods (e.g. the "EXPRESS" processing) are of increasing importance. Natural fibres seem to have little resistance towards environmental influences. This can be recognized in the composite and can be advantageously utilized for the development of biological degradable composites with good physical properties. (C) 1999 Elsevier Science Ltd. All rights reserved.