Crosslink densities and phase morphologies in thermoplastic vulcanizates

The degree of EPDM crosslinking during dynamic vulcanization of a PP/ EPDM thermoplastic vulcanizate (TPV) was modified by varying its phenolic curative content. The rise in TPV viscosity, the drop in its swelling, the change in its NMR MAS lineshape, and the increase in its EPDM domain AFM force modulation hardness verified the increase in EPDM crosslink density with increasing curative content. Further, the EPDM crosslink extent in TPV was measured by either the bound phenolic or the residual diene content both determined by solid state NMR. SEM morphologies of cryo-faced and ruthenium-stained TPVs with varying curative contents were analyzed to determine the effects of cure on EPDM domain sizes and PP ligament thickness. A narrowing of the EPDM domain size distribution, with a decrease in the third moment of the distribution, was observed with increasing EPDM crosslink density. Correspondingly, the PP ligament number-average thickness was raised slightly.