Alkali treatment of flax fibres: effects on tensile strength, thermal performance, and moisture absorption

Alkali treatment is commonly used to improve the fibre-matrix bond of plant fibre polymer composites. Still, there needs to be more consensus on the optimal protocol. This study seeks to identify the most effective alkali treatment protocol for flax fibres that enhances their mechanical performance without compromising the fibre strength. Flax fibres were treated with 2% and 5% sodium hydroxide at different soaking times. Single fibre tensile strength test was performed on 243 fibre samples, and the results were analysed using the Weibull distribution model. The effect of the alkali treatment on the fibre morphology was examined using microscopic imaging, functional groups using Fourier-transform infrared spectroscopy, thermal behaviour by thermogravimetric analysis and the fibre moisture absorption behaviour was also investigated. The results indicate that treatment with 2% NaOH for 30 and 60 minutes enhanced the tensile strength of the fibres. Fibre imaging confirmed the breakdown of the fibre bundles using the treatment method. FTIR analysis showed that the treatment reduced the fibres' hemicellulose content, enhancing the fibres' thermal and moisture absorption behaviour. The results highlight flax fibres treated with 2% NaOH for 60 minutes as the optimal treatment protocol for flax fibres used as the reinforcing phase in polymer composites.