The Effect of Poly[methylene(polyphenyl isocyanate)] and Maleated Polypropylene Coupling Agents on New Zealand Radiata Pine Fiber–Polypropylene Composites

The effects of poly[methylene (polyphenyl isocyanate)] (PMPPIC) and maleated polypropylene (MAPP) coupling agents, separately and combined, on the strength and Young’s modulus of New Zealand radiata pine-reinforced polypropylene (PP) composites were assessed. A modest improvement compared to the unreinforced matrix strength of 4% and much greater improvement of 123%, substantially greater than reported elsewhere in the literature, were obtained using PMPPIC and MAPP respectively. Young’s modulus improvements compared to the unreinforced matrix of 77 and 177% were obtained using PMPPIC and MAPP respectively. The MAPP coupling agents with the longest chain lengths and lower functionality gave the greatest benefit, supporting chain entanglement as the major mechanism for enhanced bonding with PP. Relatively high moulding temperatures and low injection pressures, bringing about enhancement of fiber orientation in the test direction, also appear to have contributed to such large improvements in composite properties in the current work. Combination of PMPPIC and MAPP coupling agents produced mixed results: benefits were obtained by combining PMPPIC and MAPP with the longest chain lengths which can be explained by PMPPIC bonding with hydroxyl groups not accessible to further coupling with MAPP once saturation is achieved.