Formation of Rubber Particle Agglomerates During Morphology Development in Dynamically Crosslinked EPDM/PP Thermoplastic Elastomers. Part 1: Effects of Processing and Polymer Structural Parameters

Abstract

Abstract Formation of agglomerate structure by the rubber particles through flocculation or networking mechanism during dynamic crosslinking of thermoplastic elastomers based on EPDM rubber and polypropylene has been evidenced. Scanning electron microscopy (SEM) examination performed on the crosslinked blend samples which had been etched by hot xylene suggested that agglomeration occurs mainly through a joint shell mechanism. Reduction of the mixing torque after passing the peak maximum at the dynamic crosslinking stage was concluded to be due to the shear induced breaking down of agglomerates leading to a more defined morphology. Samples removed after the maximum mixing torque showed higher dynamic loss tangent (tan δ) above the PP glass transition. This is attributed to the broadening of the retardation time spectra for the PP matrix in the blend system. Higher mixing torque, higher tensile strength, as well as better extensibility were found for the blend samples based on PP with low MFI value as a result of higher density of aggregates and more extent of their interfacial adhesion with the PP matrix. More defined morphology and higher rate of network breakdown was observed at high mixing shear rate. Mixing torque increased significantly with increasing the rubber content of the blend system from 40% to 60% (W : W) as a consequence of higher interaction of rubber aggregates with the PP matrix. Based on the obtained results, the structure of the rubber aggregates and associated networks as well as extent of interaction between the two phases play an important role in controlling the final morphology, processing behavior and therefore mechanical properties of the dynamically cured blend system.