Rotationally moulded polyethylene reinforced with alkali treated hemp fibre

Abstract

The increasing demand for rotationally moulded products highlights the necessity to develop cost-effective and sustainable materials to expand the application of such products. Natural fibres are a potential reinforcement for rotationally moulded products due to their high specific strength and stiffness. However, few attempts have been reported using natural fibres in rotational moulding. The most-reported challenges in incorporating natural fibres in this process are poor adhesion between fibre and matrix, fibre agglomeration and porosity, which require improvement. Accordingly, this research proposes methods to address these issues to increase rotationally moulded composites' tensile properties. Initially, hemp fibres were alkali-treated to improve the fibre-matrix interface along with the coupling agent, maleic anhydride polyethylene (MAPE). The effect of alkali treatment on hemp fibre was assessed by single fibre tensile testing, X-ray diffraction, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). Subsequently, materials were melt-compounded and chopped into pellets of different sizes to produce rotationally moulded composites with increased fibre length and uniform fibre distribution. The porosity within the final composites was assessed using optical microscopy, Archimedes density tests and micro-CT. To mitigate porosity within these composites, additives including mineral oil, stearic acid and recycled carbon fibre were tested. Fibre orientation in optical microscopy cross-sectional images of composites produced with pellets of aspect ratio larger than 1 were assessed using ImageJ. The results showed that the alkali treatment removed the non-cellulosic components from hemp fibre, improving fibre separation, fibre resistance to thermal degradation, and fibre-matrix adhesion along with MAPE. In addition, composite pellets of up to 1.5 mm size were found suitable to produce rotationally moulded composites with increased fibre length and moderate porosity, with the addition of 3 wt.% of stearic acid. It was also observed that using pellets with an aspect ratio higher than 1 improved fibre orientation parallel to the composite mould wall, resulting in higher tensile properties than pure polyethylene. Finally, the addition of recycled carbon fibre improved the melting of composite pellets of hemp-PE which reduced the void size in the final hybrid composites. This reduction in composite porosity, combined with the high stiffness of RCF, resulted in higher tensile properties with increasing RCF content. In conclusion, this research showed innovative methods and materials to produce reinforced rotationally moulded polyethylene composites with superior strength and stiffness, and moderate porosity.