ELASTIC-PLASTIC ANALYSIS OF THE DEFORMATION MECHANISM OF PP-EPDM THERMOPLASTIC ELASTOMER - ORIGIN OF RUBBER ELASTICITY

Polyolefinic thermoplastic elastomer (TPE) is a two-phase material in which cured EPDM (ethylene-propylene-diene rubber) particles are densely dispersed in a PP (polypropylene) matrix. It can be melt-processed at high temperatures and behaves like a vulcanized rubber at ambient temperature. The question is on its strain recovery, i.e., why the TPE can shrink back from the highly deformed states, even though the matrix consists of the ductile polymer. We constructed a two-dimensional model with four rubber inclusions in ductile matrix and carried out the elastic-plastic analysis on the deformation mechanism of the two-phase system by FEM (finite element method). FEM analysis revealed that, even at the highly deformed states at which almost the whole matrix has been yielded by the stress concentration, the ligament matrix between rubber inclusions in the stretching direction is locally preserved within an elastic limit and it acts as an "in-situ formed adhesive" for interconnecting rubber particles. It will provide a key mechanism of the strain recovery in the two-phase system.