Study of the fibre morphology stability in polypropylene-flax composites

The objective of this paper is to understand the impact of the fibre morphology and process parameters on the final stability and microstructure of polypropylene-flax fibres composites. In a first part, we showed that after various extrusion or injection cycles, fibre length and aspect ratio are quite similar, no matter the initial length. This point is crucial for the choice of initial fibre length and fibre incorporation easiness. Moreover, the values of elastic modulus and stress at break have revealed the importance of the fibre dispersion to obtain performing mechanical properties. In the second part, we studied the influence of the vegetal fibre volume loading on the fibre morphology and on the composite mechanical properties. The aspect ratio of the plant fibre after extrusion and injection was measured. After a good stabilization for low fibre loadings, it decreases, from a critical threshold around 30-vol %, as fibre content increases due to the shear rate increase. In the same time, we showed that the mechanical properties of the injected composites are optimal around this threshold. Finally, the study of the injected composites morphology evidenced differences in orientation between flax and glass fibres, which could explain the shrinkage properties of the different composites.