Effects of shear during injection molding on the anisotropic microstructure and properties of EPDM/PP TPV containing rubber nanoparticle agglomerates

Thermoplastic vulcanizate (TPV) have been widely used in industry, and injection molding is one of the most widely used technique to produce TPV products. In this study, effects of shear during injection molding on the anisotropic microstructure and properties of EPDM/PP TPV containing rubber nanoparticle agglomerates was systematically studied by using quantitative nanomechanical mapping of atomic force microscopy (AFM-QNM) for the first time. The shear of injection molding induced break-up of the EPDM nanoparticles agglomerates, and a significant decrease in the size of the latter with increasing shear rate was observed. Meanwhile, the shear orients both PP crystals and EPDM nanoparticle agglomerates, and their orientation degree along the shear direction, and the crystallinity of PP increase with increasing the shear rate. As a result, whatever the anisotropy, the Young's moduli at nanoscale of PP obtained by AFM-QNM, and the tensile strength and elastic modulus of TPV increase with increasing shear rate till the shear-induced PP chain degradation becomes important. Moreover, the tensile strength and elastic modulus of TPV parallel to the shear direction are much higher than those perpendicular to it, while the elongation at break and elasticity are the opposite. Interestingly, for TPV perpendicular to the shear direction, despite of the higher crystallinity and elastic modulus of TPV prepared under moderate shear rate than that under very low shear rate, the elasticity of the former is even much higher than the latter. The mechanisms on effects of shear on the anisotropy of microstructure, mechanical properties and elasticity of EPDM/PP TPV were revealed. This study provides guidance for optimizing process conditions in order to control the microstructure of the TPV and best balance their effects on mechanical properties and elasticity of TPV.