Injection moulded short hemp fibre polypropylene composites

Injection moulded short hemp fibre polypropylene composites

Natural fibre reinforced polymer composites are attracting the attention of various industrial fields due to both their environmental and economic advantages. Bio-composites, which refer to composites that combine natural fibres with either biodegradable or non-biodegradable, provide numerous benefits. The natural fibres in the bio-composites could be kenaf, jute, hemp or sisal.

Investigations on the use of hemp fibres as reinforcement, to increase polypropylene performance, have introduced many applications for hemp fibre polypropylene composites in automotive and construction industries.

The aim of this project was to utilise cheap waste hemp fibres (noil) to produce short fibre polypropylene composites and to carry out detailed investigations into the various parameters that contribute to composite performance characteristics.

The microstructural, chemical and tensile characterizations of noil hemp fibre and normal hemp fibres were first studied using scanning electron microscopy (SEM), fourier transform infrared analysis (FTIR) and Dynamic Mechanical Analyser (DMA). Noil hemp fibre reinforced polypropylene composite samples with different noil fibre contents (10-60 wt%) were fabricated using an intermixer/extrusion and injection moulding machines. Maleic anhydride grafted polypropylene (MAPP) and maleic anhydride grafted polyethylene octane (MAPOE) were used as coupling agents for modifying the matrices.

X-ray micro-tomography, image analysis and Weibull statistical methods were employed to characterise the size distributions of noil hemp fibres in the
polypropylene matrices. X-ray micro-tomography provided direct observations and accurate measurements of length and width of noil hemp fibres within the hemp-polypropylene composites. The effects of the weight content of the noil hemp fibre and the addition of compatibilisers on the fibre breakage, due to the manufacturing process of the composites, were studied using X-ray micro-tomography in this project.

Tensile, impact and flexural tests were carried out to study the mechanical properties of samples. Free vibration testing and dynamic mechanical analysis methods were also used to study the damping and thermo mechanical properties of the composites. Furthermore, the influence of fibre content and compatibiliser addition on interfacial shear strengths (IFSS) was evaluated by means of the modified Bowyer and Bader model. Finally, the influence of the type and initial length of the hemp fibre (0.2, 0.5, 1 and 2 mm) on mechanical properties of the composites was studied.

The results indicated that the tensile strength of the noil hemp fibre reinforced composites without the coupling agents was lower than that of pure polypropylene. High noil hemp fibre content caused more fibre breakage due to the fibre-fibre interaction mechanism.

The addition of coupling agents improved the tensile strength of the composites by the enhanced fibre/matrix interfacial adhesion. This was confirmed by SEM observations. It was also shown that the addition of MAPP reduced the fibre breakage due to the better dispersion of fibres.

DMA revealed no noticeable changes in the α-transition temperature when the fibre content increased or coupling agents were added. The composites revealed better temperature resistance at higher fibre content. However, the increase in storage modulus was negligible in composites reinforced with more than 40 wt% hemp fibres due to the agglomeration of the fibres. The results of the damping ratio analysis revealed that higher interfacial bonding was achieved by the addition of MAPP coupling agent in comparison with the addition of MAPOE coupling agent. The storage modulus of the composites increased with the increase in hemp fibre content. However, the maximum damping ratio was obtained from the composite with 30 wt% noil hemp fibre. The addition of coupling agents reduced the damping capacity of all composites. However, 30 wt% noil hemp fibre reinforced polypropylene coupled with 2.5 wt% MAPOE revealed the highest damping ratio among coupled composites.

Finally, the noil hemp fibre composites indicated slightly lower tensile properties than the alkali-treated ones. However, the difference was not significant. The analysis of the tensile, flexural and impact results indicated the optimum initial fibre length of 0.2 mm produced the ideal composites due to better dispersion of fibres (powders). This test methodology can be extended to different types of natural fibres.