Flax fiber treatment by an alkali solution and poly(dopamine) coating: Effects on the fiber physico-chemistry and flax/Elium? composite interfacial properties

The use of natural fibers to reinforce polymer-based composites is gaining interest to replace conventional synthetic fibers due to their sustainability. In this frame, optimizing the adhesion between natural fibers and the polymer matrix is crucial to ensure high mechanical properties. To improve the adhesion between flax fiber and an acrylic-based thermoplastic resin, the combining of alkalization and poly(dopamine) (PDA) coating as fiber treatments has been investigated in this paper. It is found that applying an alkalization with sodium hydroxide (3 wt%, 2 h, 21 +/- 2 degrees C), followed by dopamine polymerization (2 g/L, pH of 8.5, 24 h, 21 +/- 2 degrees C) in the presence poly(ethylene imine) (600 g/mol, 1 g/L) as a polyelectrolyte, provides the highest increase in composite interfacial shear strength (+22.0 %) and interlaminar shear strength (+63.2 %) compared to the composite with untreated fibers. The results are discussed based on the fiber physico-chemical properties and composite inter-facial aspects.