Mechanism of morphology development in dynamically cured EPDM/PP TPEs. I. effects of state of cure

Attempts were made to follow and correlate morphological development with t he crosslinking density, or state of cure (SOC), and the surface tension (g amma) of the rubber phase in dynamically cured thermoplastic elastomers (TP Es) based on ethylene propylene diene rubber and polypropylene (PP) with 60 /40 (w/w) ratios. Samples were taken from a hot running mixer without inter ruption and quickly quenched in liquid nitrogen both before and after the o nset of vulcanization at various SOCs to carry out this process. The quick cooling of the samples prevented the coalescence and agglomeration of the d ispersed rubber particles. A two-phase morphology with the rubber particles dispersed throughout the PP matrix was observed for the uncured but frozen samples, whereas unfrozen blend samples showed a particulate cocontinuous morphology in the uncured state. An increase in the mixing torque with the SOC was observed after the addition of a curing system. This was understood to be caused by the increase in the rubber crosslinking density and also b y the enhancement of the interfacial adhesion between the cured rubber phas e and the PP matrix, leading to the better wetting of the two phases. Above a certain crosslinking density (SOC), gamma of the rubber particles decrea sed through elastic shrinkage. This phenomenon, together with the breakdown of the agglomerate structure formed by the cured rubber particles, led to a decrease in the mixing torque after a maximum was passed and, finally, to a defined morphology. Based on the obtained results, a four-stage model is proposed to describe the microstructural development in dynamically vulcan ized TPEs. Dynamic mechanical thermal analysis and differential scanning ca lorimetry results are also used to support the model. (C) 2001 John Wiley & Sons, Inc.