The Application of Natural Fibre Reinforcements with Bio-based Polyester Resins for Liquid Composite Moulding Processes

Abstract

There is growing interest in the development of materials based on renewable, natural resources, to reduce the environmental impact of traditional materials. In the composites field this has resulted in increased interest in the use of natural fibres such as flax, hemp, sisal and others as alternatives to glass, carbon fibre and synthetics. This project investigates the use of flax, which is available in a range of woven forms suitable for use as composite reinforcement, with a bio-based polyester resin currently under development as a composites matrix material. Initial work investigated two key features of a potential reinforcement, compaction response and permeability, which directly affect the mould clamping forces necessary to achieve higher volume fractions, and also the fluid pressure needed to force the resin into the compressed reinforcement material. Results of this investigation show that both these rise sharply as volume fractions greater than about 0.5 are attempted. Two Liquid Composite Moulding processes were then used to trial the materials, VARTM (Vacuum Assisted Resin Transfer Moulding) and RTM (Resin Transfer Moulding). In the VARTM process the mould pressure is applied by atmosphere, so higher volume fractions are not possible and consequently permeability is not a problem. Sample from panels produced by this process were mechanically tested, and samples prepared for microscopy were also examined for voids and other defects. The RTM process provided the opportunity to produce panels with higher volume fractions, up to 0.55, using air pressure to force the resin into the compressed reinforcement. A range of panels was produced, including some using glass fibre reinforcement and others a similar but synthetic polyester resin to allow comparisons to be made. The same mechanical testing was carried out as on the VARTM panels, and significant improvements in performance were obtained. Although flax, like other natural fibres, absorbs moisture and can have variable characteristics, it has similar strength to glass fibre but significantly lower weight. There is assuredly a place for it in the composites industry. On the basis of the development product used here, bio-based resins are not yet quite ready for commercial use.