Morphology development in thermoplastic vulcanizates (TPV): Dispersion mechanisms of a pre-crosslinked EPDM phase

The fragmentation and dispersion in molten polypropylene (PP) of several pre-crosslinked and plasticized ethylene–propylene–diene terpolymer (EPDM) networks was studied. Thus, the morphologies and mechanical properties of PP/EPDM blends having similar compositions but made from either un-crosslinked, pre-crosslinked or dynamic-crosslinked EPDMs were compared. The results first highlight the importance of the gel fraction of the pre-crosslinked EPDMs, as well as the impact of the thermoplastic matrix proportion on the quality of the dispersion of such networks. As a result, pre-crosslinked EPDM having a gel fraction below gEPDM = 0.7 can be finely and homogeneously fragmented and dispersed in presence of PP. It can be then admitted a collision–coalescence–separation type erosion mechanism of the EPDM domains. Nevertheless, contrarily to some theoretical model expectations, a partial fragmentation of the chemical networks was always observed even at very high crosslink density (gEPDM > 0.7). Finally, the blends crosslinked under shearing (dynamic-crosslinked) showed a clear mechanical property synergy due to their fine and homogeneous morphology coupled with the full crosslinking of the elastomer. In the end, these results brought significant information on TPV morphology stabilization and their related mechanical properties.