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The flax stems are a source of fibre. Long fibres are suitable for textiles. Short fibres are incorporated into other polymers to make composites. Short fibres, which are the valorisation of intermediate quality batches, are produced by grinding processes to obtain the desired size for each industrial application. Depending on the parameters of the process used, this grinding also produces a greater or lesser proportion of very fine particles (called 'fines').

As part of an international collaboration, we have recently shown the impact of grinding processes on the morphology of the fibres and the mechanical properties of the composites formed.  Thus, we have shown that 'fines' are 6 times less efficient than calibrated short fibres for the mechanical reinforcement of composites. Thus, the grinding process, which determines the particle size and shape, must be adapted according to the desired use of the short flax fibres (i) for their mechanical reinforcement properties of the composites or (ii) for their economical filling function (in the case of 'fines').

In addition, by comparing different grinding processes, we have shown that the grinding of fibres also induces changes in the polysaccharides contained in these fibres. In particular, the content and crystallinity of cellulose is affected by the grinding intensity, opening up new perspectives for the conversion of polysaccharides into bioenergy.

This research opens many perspectives on the understanding of composition-structure-mechanical property relationships. In the future, the investigations will be extended to other biomasses, in particular those of the French hemp industry.

This study was carried out by the INRA BIA and IATE units in collaboration with the University of Southern Brittany - IRDL, INSA Rouen, and the Centre for Natural Material Innovation, University of Cambridge UK.