Tensile properties improvement of a unidirectional-mat flax/epoxy composite via click chemistry, nanocellulose addition and surface refining of short mat fibres

The mechanical characteristics of natural fibre composites (NFCs) are closely linked to fibre-matrix and fibre-fibre interactions. This work investigates the improvement of tensile properties of a flax/epoxy composite through the application of click chemistry reaction to a unidirectional-mat (UDM) reinforcement with modifications made on the short fibre mat and unidirectional flax fibre phases of the reinforcement. The surface of short flax fibres was fibrillated to increase the accessibility of hydroxyl groups for all preliminary reactions and the final click chemistry cross-linking, which creates stable covalent triazole bonds between azide and alkyne groups. A small percentage of treated nanocellulose was incorporated to further enhance the reinforcement properties. FTIR and EDX analysis confirmed the presence of the various functional groups on the surface of nanocellulose and flax fibres with very high degrees of substitution. The treatment significantly improved the mechanical properties of the dry reinforcement, including a 220% mean increase in the tensile strength. However, the treatments, particularly the addition of nanocellulose, resulted in a reduction in the permeability to liquid resin of the reinforcements, highlighting the need for compromises in their manufacture. Nonetheless, marked improvements in tensile strength and Young’s modulus were obtained for composites made of pre-compacted and cross-linked fibre preforms. Increases in elastic modulus, strength and strain at break of up to 50.1%, 53.8% and 10.1% were obtained, respectively.