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Natural fibres, and in particular lignocellulosic fibres from hemp and flax, are increasingly used to produce eco-composite materials to strengthen and lighten equipment, particularly in the automotive industry. The quality of eco-composite materials depends heavily on the mechanical properties of the fibre elements, but the reasons for their improved performance are not well understood. In addition, unlike synthetic fibres traditionally used in composites, natural fibres can have tortuous and strongly branched shapes, which impact the final properties of the materials. This morphology depends on the physical and/or chemical pre-treatments applied to the fibres.

The major challenge of this study was to better understand the impact of hydro-mechanical treatments widely used for natural fibres, on their physical integrity.

The morphology of the fibre elements was quantified by automated image analysis tools coupled with statistical descriptions of all fibre populations generated at each stage of material processing, i.e. on thousands of objects of different shapes and morphologies. These statistical descriptions were analyzed in conjunction with other analytical methods conventionally used to describe the microstructure of the fibre elements. For example, porosimetry provides information on the quantities and size distribution of pores in fibrous objects. But also the chemical composition, X-ray crystallography and nanoindentation, a nanometric tip that will penetrate the material only a few micrometers (the latter provides information on certain local properties of the fibers at the scale of the plant cell wall).

We measured an increase in the number of branches of plant fibre elements with the intensity of a thermomechanical treatment. This can bring a benefit in the physical attraction of the fiber elements. On the other hand, we have quantified a decrease in fibre stiffness with this treatment. Finally, the data set collected in the study will make it possible to decide on the best trade-off solution for a desired application.