ENVIRONMENTAL RESISTANCE OF FLAX/BIO-BASED EPOXY AND FLAX/POLYURETHANE COMPOSITES MANUFACTURED BY RESIN TRANSFER MOULDING

In recent years, many car manufacturers such as Audi, BMW and Ford have developed natural fibre composites components. The driver behind developing interior parts with natural fibres as reinforcements is to save weight without compromising mechanical properties, with the additional benefit of moving towards a more sustainable part production. Current biocomposites are based on the application of short, non-woven natural fibres. However, bonding hydrophilic natural fibres to a hydrophobic polymeric matrix can sometimes lead to a relatively poor interface, mostly because of the different polarities of the two components. In addition, natural fibre composites are highly sensitive to water absorption. The aim of this study was to compare the physical properties two biocomposites: (1) a flax/bio-based epoxy and (2) a flax/polyurethane. In fact, polyurethane is synthesized by polyaddition of an isocyanate and a polyol, an alcohol containing several hydroxyl groups. On the other hand, flax fibres are mainly composed of cellulose, a natural polymer that presents multiple hydroxyl groups. A crosslinking reaction between the polyurethane matrix and the cellulose in the fibres could increase the interface strength. Both materials were manufactured using a resin transfer moulding (RTM) process in order to maximize the fibre volume fraction and were reinforced with the same flax woven fabric. Post-cured composite samples were aged at 90%RH and 30 degrees C. The results showed that both composites followed a Fickian diffusion behaviour and that flax/polyurethane composites were less sensitive to moisture ageing than flax/bio-epoxy composite. The chemical bonds between the hydroxyl groups of the fibres and the isocyanate lead to a stronger interface which improved the mechanical properties as compared to the flax/bio-epoxy composites. When exposed to moisture, the flax/polyurethane composite have more stable mechanical performances, especially the short beam strength, the compressive strength and the compressive modulus.