Flax for composite reinforcement: Impact of middle lamella degradation on flax fibre bundle mechanical properties

Flax fibre bundles behaviour with two different retting degrees were studied Steam explosion and gamma irradiation, as retting complements, were explored The morphology of flax fibre bundles was analysed using micro-tomography Mechanical characterisation at two different scales: elementary fibres and bundles The impact of middle lamellae degradation was assessed through tensile tests Flax fibre bundles, widely used in composite reinforcement, consist of elementary fibres bound by the middle lamella. They can be considered as unidirectional composite materials reinforced with discontinuous fibres. Their mechanical properties strongly depend on the quality of retting and subsequent treatments. This study analyses the sensitivity of bundle-scale tensile tests at two different gauge lengths to the degradation of the middle lamellae and cortical residues by comparing an under-retted (UR) and a well-retted (WR) batch with and without a chemical treatment using ethylenediaminetetraacetic acid (EDTA). Additional treatments with gamma irradiation and steam explosion were applied to evaluate their potential to improve bundles division especially in case of under-retting, without compromising the structural integrity and the mechanical properties of elementary fibres. Morphological and thermogravimetric analyses showed that treated bundles have a lower moisture sorption and a higher cellulose content. Mechanical results revealed a 30% drop in stress and strain at break with increasing retting, attributed to the degradation of cortical residues and middle lamellae due to improved bundles division. Tensile tests at higher gauge lengths (75 mm) revealed a higher sensitivity to the state of the middle lamellae. In particular, steam explosion caused up to 80% reduced stress at break at a gauge length of 75 mm, demonstrating its great potential as a complement to retting. Conversely, gamma irradiation had minimal impact on the mechanical properties and the division of the bundles. These results are relevant for optimising the properties of flax fibre bundles and composite materials.