Development of wood-substituted composites from highly filled systems of polypropylene and Hevea brasilliensis woodflour

Abstract

In recent years, the natural fibers as reinforcement have been increasingly used to replace the conventional inorganic filler in polymer matrix composites. Especially, natural filler reinforced thermoplastics have a good potential in the future as a substitution for wood-based material in many applications. Natural fibers render several advantages, i.e. the filler cost is usually very low. Also, the natural fillers are biodegradable (thus contributing to an improved environmental impact). In addition, this kind of filler is naturally renewable as well. Moreover, many mechanical and thermal properties can be enhanced. In this work, Hevea Brasiliensis woodflour filled polypropylene composites were prepared by twin screw extruder followed by injection molding. In the first part of this study, the aims are to determine the effect of polypropylene types (PP-HP644T, PP-HP648N and PP-HP740H), average particle sizes, filler contents and sized distributions on the mechanical, thermal and some related physical properties. The experimental results revealed that the composites based on high melt flow rate polymer matrix (PP-HP644T) had higher mechanical properties than those of wood flour composites with PP-HP648N and PP-HP740H. For the effect of particle sizes, filler contents and sizes distributions, it was found that the particle sizes and woodflour contents played a significant role in the mechanical properties of wood flour composites. Flexural and tensile modulus increased, while flexural and tensile strength decreased with increasing woodflour contents. Furthermore, the average particle size of woodflour that was suitable for improving the mechanical properties of the composite was about 200-300 µm. The appropriate bimodal particle sizes that rendered the greatest packing density of wood flour were 275 µm and 49 µm at a mass ratio of 73:27. Additionally, it was found that the thermal stability and degradation temperature (at 5% weight loss) of wood flour composites decreased with increasing woodflour mass fraction. In the second part, maleic anhydride grafted polypropylene (MAPP) was used as a coupling agent to improve the interfacial bond strength between woodflour filler and polymer matrix. The influence of the amount of MAPP on mechanical and thermal properties of woodflour composites has been investigated. This work recommends MAPP content of 5 wt% (of woodflour) to be introduced into the woodflour composites for optimization of the composite mechanical properties. It was reported that the flexural and tensile strength were 110% and 87% higher than those of the woodflour composite without MAPP, respectively. The thermal stability and degradation temperature of woodflour composites containing the coupling agent were slightly increased with increasing coupling agent content.