Enhancement of tensile properties of flax-mat epoxy composites via click chemistry with surface fibrillation and compaction of the fiber preforms

Abstract

The mechanical properties of composite materials are strongly related to the fiber–matrix interface properties. This study focuses on the click chemistry modification of short flax fibers using the Cu(I)-catalyzed Huisgen cycloaddition type, to strengthen the fiber–fiber interface for composite applications. The flax fibers are functionalized in three steps: a mechanical fibrillation pre-treatment of the fibers surface, followed by a chemical cleaning treatment to eliminate pectin, lignin, hemicelluloses and waxes, allowing exposure of the hydroxyl groups in flax fibers in view of the final treatment of click chemistry. The chosen strategy allows the adaptation of propargylation and tosylation reactions to flax fibers in aqueous media. FTIR and EDX analysis of fibers treated at intermediate stages confirmed the presence of various surface functions of modified fibers with a very high degree of substitution. The properties obtained are strongly improved for reinforcements containing covalent fiber–fiber contacts. Tensile, tearing and bursting tests performed on dry mat reinforcements showed increases in the tensile index, elongation at break, tensile stiffness, burst and tear indexes of 519%, 355%, 201%, 304% and 421%, respectively. Resin transfer molding (RTM) was used to fabricate epoxy composites made of click chemistry-treated short fiber flax mats at a fiber volume content (V_f) of 40%. Tensile tests results showed the positive effect of the click chemistry treatment, with increases in the tensile modulus, strength and strain at break of 41.5%, 64.3% and 30.8%, respectively. Marked improvements in strength and Young's modulus were obtained for composites made of pre-compacted and cross-linked flax-mat preforms.