The rubber particle size to control the properties-processing balance of thermoplastic/cross-linked elastomer blends

The influence of the number-averaged rubber particle size (d(n)) on the mechanical and rheological properties of thermoplastic/cross-linked elastomer blends was studied, with the focus on thermoplastic vulcanizates (TPVs) based on poly(propylene) (PP) and ethylene-propylene-diene (EPDM) rubber. Time-resolved small-angle X-ray scattering measurements revealed that the criteria to obtain tough behavior are very different for TPVs than for traditional rubber-toughened thermoplastics, since the deformation mechanism of TPVs under tensile conditions is dominated by yielding of the semi-crystalline, thermoplastic matrix without the occurrence of matrix crazing and internal rubber cavitation. The formation of interlamellar voids, as occurs in the unfilled thermoplastic during deformation, is more effectively suppressed with decreasing d(n), which leads to a significant enhancement of the ultimate tensile properties. Additionally, the decrease in d(n) leads to an enhanced elastic recovery, but also increases the melt viscosity. These results demonstrate that d(n) is an important parameter to control the balance between the mechanical properties and the melt processability of blends based on thermoplastics and cross-linked elastomers.