Co-authors​ :

 Ali Al Hadi NOUR EL DEIN (FRANCE), Nicolas LE MOIGNE (FRANCE), Monica PUCCI (FRANCE), Antoine BARBULEÉ , Olivier GAMOND , Anne-Sophie CARO (FRANCE) 

Flax shive (FS) is the predominant by-product of flax fibre production, representing around 50% of the weight of the dry flax stem. Its low price and relatively low bulk density (100 to 140 kg/m3) [1] make it an economically viable option for reinforcing injection-moulded thermoplastic biocomposites. Recent studies [2,3] have shown that FS initial granulometry influences the tensile properties of the reinforced composite. Therefore, our study aims to investigate the influence of FS granulometry on the microstructure of the composite and understand its effect on the composite tensile properties. Various granulometries of FS (FS-0-200, FS-200-500, FS-500-800 and FS-800-1600 µm) starting from the same batch were prepared industrially, supplemented with lab-scaled prepared fractions FS-200-315 and FS-315-500 derived from FS-200-500. Polypropylene (PP) composites reinforced with 10% to 40% wt. of FS for each granulometry were manufactured using twin-screw extrusion and injection-moulding processes with an optimal amount of Maleic Anhydride Polypropylene (MAPP) as a compatibilizer. X-ray tomography was conducted on composites reinforced with 10% wt. of FS-200-500 (Figure 1) and FS-800-1600, providing insights into the impact of FS granulometry on FS orientation and distribution in the composite microstructure. The dimensions and aspect ratio of the FS particles were measured using a 2D scanner after solvent extraction. Combining X-ray tomography and the 2D scanner results allowed us to consider the elliptical shape of the fibres for aspect ratio measurement. The 3D microstructural parameters (3D fibre orientation, fibre aspect ratio, fibre volume fraction and porosity volume fraction related to intrinsic porosity of flax shives) were used to estimate the FS Young’s Modulus by modelling the elastic properties of the composites using the Mori-Tanaka model. The microstructural analysis results reveal that the maximum aspect ratio of the FS after processing is reached by the medium-size particles FS-200-315, with deviations in particle size leading to a decrease in its aspect ratio. Additionally, it was noticed that FS-200-500 tends to be more oriented toward the injection flow compared to larger particles. These findings align with the tensile properties of the composites. PP composites reinforced with medium-size particles (FS-200-315) exhibit a 20% and 25% increase in Young’s modulus and ultimate tensile strength, respectively, compared to those reinforced with larger particles (FS-800-1600). The Mori-Tanaka model shows that there is a variation in Young’s Modulus among the different FS granulometry within a range between 14 and 29 GPa (Figure 2). These results will be discussed in relation with the biochemical composition and ultrastructure of flax shives.