Mechanical Performance of Protective Epoxy Coatings with Bio-based Ingredients for Flax-Fiber Composites

With its long and continuous cellulose fibers, flax offers excellent specific tensile strength and stiffness relatively to other natural fibers such as hemp or jute and it is widely used as fiber reinforcement in composites with relevance in industries such as automotive, sports and maritime environment. However, the use of natural fibers poses additional challenges relative to synthetic fibers to ensure functional lifetime of composites: in particular, water resistance and resistance against UV conditions should be improved for outdoor use. Therefore, a protective coating offering high resistance against environmental conditions and mechanical damage can be applied to avoid direct surface exposure of the natural fibers. The linseed oil or wax coatings increase hydrophobic surface properties and limit water ingress, but they have drawbacks such as extended curing periods through oxidative crosslinking and weak mechanical performance. Seeking alternatives for natural fiber composites, the potential for biobased crosslinked coatings to enhance mechanical robustness, surface protection and durability was explored by screening various coating grades including bio-based epoxy resin, diluents and crosslinkers. The epoxy coatings with a bio-based phenalkamine crosslinker offer higher hardness and scratch resistance and the water resistance was improved in presence of the amine crosslinker with long alkyl chains. In parallel, the mechanical abrasion resistance of crosslinked coatings hugely increased in relation with the intrinsic mechanical properties and crosslinking density of the coating. The processing of the epoxy coatings was further enhanced by adding a biobased trifunctional diluent with low viscosity, while providing limited shrinkage and good compatibility with the composite substrate. Moreover, the UV resistance was better for epoxy coatings with a biobased diluent likely through migration effects and formation of a protective layer at the outer surface.