The effect of particle size and content on the properties of hemp-reinforced polypropylene composites

In this study, hemp particle (HP)-reinforced polypropylene (PP) matrix composites containing two sieve-defined HP fractions, HP_90 (≤90 µm) and HP_150 (>90 µm and <150 µm), at weight ratios of 10-30% were produced, and the effects of HP addition on the mechanical, tribological, thermomechanical, and thermal properties of PP were investigated. Tensile test results showed that increasing the content of the HP_150 fraction increased the stress at yield and elastic modulus but decreased the strain at yield. The highest mechanical performance was obtained for the composite containing 30 wt. % of the HP_150 fraction. SEM images supported the lower agglomeration of the HP_150 fraction. The results of the adhesive wear test showed that the lowest and most stable COF was obtained for the composite containing 30 wt. % of the HP_150 fraction. The DMA results showed that the storage modulus values increased over the entire temperature range with the addition of HP, with the highest values obtained for the composite containing 30 wt. % of the HP_150 fraction. DSC analysis showed that the addition of HP did not cause a significant change in the T m of PP but affected the X c,rel of the PP phase in the composites. At the same HP ratios, higher X c,rel was obtained with the HP_150 fraction. The results revealed that the particle-size range is a determinant of reinforcement efficiency and that the finer fraction does not always lead to superior composite performance. Although additional processes to obtain the finer HP_90 fraction are costly and energy-intensive, composites containing the HP_150 fraction were found to exhibit superior performance. Therefore, it was concluded that the optimum particle-size range in natural particle-reinforced composites should be determined not only by the surface area approach but also by considering the agglomeration tendency and stress transfer efficiency together.