Environmental resistance of flax/bio-based epoxy and flax/polyurethane composites manufactured by resin transfer moulding

Abstract

In recent years, many car manufacturers such as Audi, BMW and Ford have developed natural fibre composite components. The driver behind developing interior parts with natural fibres as reinforcement 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 poly-isocyanate and a polyol, an alcohol containing several hydroxyl groups. 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.Composite samples were then aged at 90% RH and 30°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 composites. 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 composites show more stable mechanical performances, especially the short beam strength, the compressive strength and the compressive modulus.Scanning electron micrographs were taken to look at the interface between the flax elementary fibres and the polymer matrix and clearly showed a better fibre/matrix adhesion in the case of the flax/polyurethane composites. Finally, dynamic mechanical analysis (DMA) was performed to study the evolution of the glass transition temperature ( ) in function of the water uptake for composite samples immersed in distilled water at 30°C. III ABREGEDepuis plusieurs années, de nombreux constructeurs automobiles dont Audi, BMW et Ford ont développé des pièces en composite renforcés par des fibres naturelles. Le développement de composants pour les intérieurs de voiture renforcé par des fibres naturelles permet d'économiser du poids sans pour autant compromettre les propriétés mécaniques, avec l'avantage supplémentaire de produire des pièces dans un matériau plus durable. Les biocomposites actuels contiennent surtout des fibres naturelles courtes, non-tissés. Toutefois, l'adhésion de fibres naturelles hydrophiles à une matrice polymère hydrop...

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