Property enhancement of PP-EPDM thermoplastic vulcanizates via shear-induced break-up of nano-rubber aggregates and molecular orientation of the matrix

Abstract

In this study, in order to investigate the influence of melt shearing on the phase morphology and mechanical performance of PP/EPDM thermoplastic vulcanizates (TPVs), the oscillatory melt shear was successively applied on the TPV melt in the mold cavity during the packing stage of injection molding by a modified facility, called dynamic-packing injection molding. For a clear comparison, conventional injection molding was also used. It is shown that conventional injection-molding results in large EPDM rubber aggregates with dimensions of ∼1.5 μm dispersed in the PP matrix. To our surprise, well-dispersed nanoscale EPDM particles with dimensions of ∼40 nm have been achieved for the first time when introducing oscillatory melt shearing, in additional to a largely enhanced molecular orientation for PP matrix. As a result, a simultaneous improvement of tensile strength and elongation has been achieved for dynamic packing injection molded samples. In addition, step cycle tests showed typical “Mullins effect” for both conventional injection-molded samples and dynamic packing injection molded samples but improved fatigue resistance for dynamic packing injection molded ones. To isolate the contribution of PP matrix orientation and nanoscale EPDM particle on the tensile behavior of PP/EPDM TPVs, thermal annealing of injection molded samples were carried out to eliminate the orientation of PP matrix. It was found that the tensile strength is decreased but the elongation at break is maintained after annealing of dynamic packing injection molded samples. By comparing the property change before and after annealing, it is concluded that the orientation of PP matrix is in great favor of Young's modulus and tensile strength while the enhanced elongation at break is directly corresponding to the decreased rubber size and improved dispersion in TPVs. Our work demonstrates the strong effect of melt shearing on the breakup of rubber particles and matrix orientation of TPVs as well. The isolation of the contribution of PP matrix orientation and rubber particle size on the tensile behavior of TPVs provides guidance for the preparation of new TPVs with high performance.