Compatibilization of Polyolefin/Hydroxyapatite Composites Using Grafted Polymers

The objective of the present work is to evaluate the compatibilization of composites of polyolefins (high-density polyethylene and polypropylene) with 30 wt.% hydroxyapatite of nanoscale particle size, employing grafted polymers to improve the interaction between phases and hence, the composite's properties. The functionalization of a high-density polyethylene and a polypropylene was performed in solution, employing acrylic acid as the grafting monomer and dicumyl peroxide as the initiator. The grafting degree was determined through volumetric titration. The nanoscale hydroxyapatite was synthesized through a precipitation method using calcium hydroxide and ammonium phosphate. Polyolefin composites were melt-blended in an internal mixer, varying the amount of compatibilizing agent at 4, 6, 8, and 10 phr. The grafting degrees obtained were 0.72 and 0.38wt% for the high-density polyethylene and polypropylene, respectively. On the other hand, the melt flow rate values of the composites slightly decreased as the grafted polymer contents increased. The crystallinity degree also decreased in the presence of grafted polymers. The incorporation of the compatibilizing agent favored the dispersion and distribution of the hydroxyapatite particles. The Young's modulus and the tensile stress slightly increased in all composites. The values of activation energy decreased with the addition of the grafted polymers, which is attributed to the less thermally stable functionalized polymer, which is prone to thermal degradation at relatively lower temperatures due to the presence of a large content of carboxylic groups. Nonetheless, the activation energy increased with the amount of functionalized polymer incorporated into the composites due to the interaction between components at the polymer/filler interface.