Industrial refining impacts on the structure and mechanical performance of flax fibres for polymer reinforcement

For composite applications, flax fibre bundles must be divided to optimise processing and mechanical performances. Industrially, they are divided using mechanical treatments such as opening, carding and combing. In a context of climate change, retting (natural process to facilitate bundles extraction) can be insufficient, and further steps or additional treatments could be required. This study aims to evaluate the influence of industrial processes: mechanical refining and a complementary treatment using ultrasound to achieve high bundle division, on the microstructure, biochemical composition and mechanical properties of flax fibre bundles and elementary fibres, using a multiscale approach. X-ray microtomography was first used to produce three-dimensional renderings of bundles from each treatment, providing clear evidence of the effectiveness of the different refining methods (mechanical refining with and without additional ultrasonic treatment). However, despite the efficiency of mechanical refining, polarised light microscopy revealed a higher proportion of structural defects, as well as a reduction in the average length of both fibre bundles and elementary fibres. Biochemical analyses also indicated that elementary fibres composition changed following ultrasonic treatment, which is likely related to the use of aggressive accessory chemical agents. Furthermore, tensile tests at both bundle and elementary fibre scales demonstrated a decrease in mechanical properties after all refining treatments. Calculations of aspect ratios and critical lengths show a decrease in the aspect ratio after refining treatments. These findings highlight the need for alternative flax reinforcement preparation methods that are less damaging to ensure effective bundle division while preserving the mechanical performance of flax.