Enzymatic modification of flax fibres for composite applications: Improved moisture durability and reduced processing time

Renewable reinforcements such as flax fibres provide a sustainable and lightweight alternative to synthetic fibres like glass, supporting the development of eco-efficient composites. However, their hydrophilic nature limits broader use in structural applications. While enzymatic retting of flax fibres has been widely investigated for fibre extraction and mechanical performance, its influence on moisture-driven durability for composite applications has not been systematically investigated. This study investigates the relationship between pectin removal and moisture-induced degradation in flax fibre composites through a multi-scale approach combining fibre sorption analysis, surface chemistry characterisation, and composite durability testing. Polygalacturonase was used to selectively degrade pectin in manually extracted green flax (GFlax) and industrial hackled flax (HFlax). Accelerated ageing on HFlax composites was performed via prolonged water immersion to evaluate moisture-induced degradation. Dynamic vapour sorption revealed a reduction in moisture uptake of up to 43% for GFlax and 17% for HFlax at 95% relative humidity. At the composite level, enzymatically treated fibres exhibited reduced water absorption and improved retention of flexural properties after 30 days of immersion. Chemical analyses confirmed reduced pectin content and surface hydrophilicity, supported by contact angle measurements, Fourier-transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR), and X-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM) revealed enhanced fibre individualisation and cleaner surfaces. These results highlight enzymatic treatment as an effective approach to simultaneously improve moisture durability and reduce processing time in flax fibre composite applications.