Chemical modification of flax fibres with amino acids for enhanced interfacial bonding in epoxy composites

Abstract

Natural fibre-reinforced epoxy composites have attracted increasing attention due to their sustainability and potential for lightweight structural applications; however, their performance is often limited by insufficient fibre–matrix interfacial adhesion. In this work, flax fibres (FF) were successfully modified using glycine (Gly) as a natural, environmentally friendly reinforcement agent to improve their compatibility with an epoxy resin matrix. To the best of our knowledge, the modification of natural fibres with amino acids, particularly in epoxy-based composites, appears to be a novel approach. Fibres were characterised using FTIR, EDS, TGA, and SEM. The effectiveness of the surface modification was confirmed by FTIR and EDS analyses, which indicated the formation of ester bonds between FF and Gly. After the modification, N% content increased from 0.00 (FF) to 6.7 ± 0.7%wt (FF-Gly-NaHCO₃). Adhesion between epoxy resin and fibres was evaluated using the transverse fibre bundle test (TFBT). Gly-modified fibres exhibited a significant enhancement in interfacial properties, with σ_TFBT increasing up to 38% (22.0 ± 1.5 MPa) for FF-Gly and 22% (19.6 ± 2.7 MPa) for FF-Gly-NaHCO₃ compared to the untreated FF/epoxy system. The observed improvement is attributed to enhanced interfacial adhesion, likely resulting from chemical interactions between the amino groups of Gly and the epoxy groups of the resin. SEM observations of fractured TFBT specimens further supported these findings, revealing resin residues on the Gly-modified fibre surfaces, indicative of stronger fibre–matrix bonding. These preliminary results demonstrate that Gly is an effective and sustainable modifier for natural fibres, leading to improved interfacial and mechanical performance in epoxy-based composites. The proposed approach offers promising potential for expanding the range of applications of natural fibre-reinforced epoxy composites in engineering materials.