Micromechanisms of deformation and recovery in thermoplastic vulcanizates

Abstract

The micromechanisms of deformation and recovery in thermoplastic vulcanizates (TPVs) are studied using a series of micromechanical models. TPVs are a class of composite material consisting of a relatively large volume fraction of elastomeric particles ( v p=0.40–0.90) in a thermoplastic matrix. A representative TPV with v p=0.77 is selected for the study. Six five-particle representative volume element (RVE) models are constructed where the symmetry of particle distribution and the relative thickness of the matrix ligament bridging particles are systematically varied. The macroscopic stress–strain behavior of the TPV during loading and unloading is successfully predicted by the simulation study as shown by direct comparison with experimental data. The simulation study reveals the important role of relative matrix ligament thickness as well as geometric asymmetry in the formation of a pseudo-continuous rubber phase which provides the rubber-like behavior of TPVs during loading. The study shows the important role of matrix ligament thickness in controlling the initial stiffness and flow stress of the TPV; thinner ligaments lead to earlier matrix yielding and thus earlier formation of the pseudo-continuous rubber phase. Upon formation of the pseudo-continuous rubber phase, the matrix material is seen to accommodate the large straining of the rubber phase by nearly rigid body motion (rotation and translation) of the bulky domains of the matrix; the rubber phase is seen to undergo large contortions as it attempts to deform as an almost continuous network around the “rigid” domains of matrix material. Furthermore, the asymmetry together with the thin matrix ligaments greatly aids the recovery of the material during unloading. Upon unloading, the rubber phase attempts recovery in a rubber-like manner. The bulkier regions of matrix material simply rotate and translate with the recovering rubber domains. The thin ligaments also rotate, but eventually also undergo bending and buckling which enables the large amount of recovery observed in thermoplastic vulcanizates.