Effect of mixing sequence in the preparation of white rice husk ash filled polypropylene/ethylene–propylene–diene monomer blend

Abstract

White rice husk ash (WRHA) filled polypropylene (PP) and ethylene–propylene–diene monomer (EPDM) blends were prepared using five different mixing sequences in two types of internal mixers. The mixing sequences studied were, addition of; (i) WRHA followed by EPDM to molten PP; (ii) EPDM followed by WRHA to molten PP; (iii) master batch of EPDM and WRHA prepared with a laboratory two roll mill to molten PP in the internal mixer; (iv) PP followed by WRHA to molten EPDM and (v) WRHA followed by PP to molten EPDM. In all cases, the material charged first was preheated for one minute. Mixing torque developments and the effect of mixing sequences on mechanical properties of the blends were studied. Completion of mixing was indicated by stabilising torque between eight to nine minutes mixing for each sequence except for the third system, which attained equilibrium torque early. Therefore, a mixing time of ten minutes including one minute preheating time is sufficient. Results indicate that all the blends prepared have similar ultimate tensile strength, tear strength, and hardness except the blend prepared following the fourth mixing sequence. The blend prepared following the fourth mixing sequence has the highest tensile strength although it exhibited the lowest elongation at break. Of the other mixing sequences investigated, the sequence in which WRHA followed by PP were added to molten EPDM rubber gave the blend with the highest elongation at break (%) and comparable tensile strength. SEM photomicrographs of the tensile fracture surfaces indicated this system also has good filler dispersion. As such, this mixing sequence was considered the best among the candidate methods.