Rheological modeling of thermoplastic vulcanizates (TPVs) using the Kaye-Bernstein, Kearsley, Zapas (K-BKZ) constitutive law
Thermoplastic vulcanizates (TPVs) comprise a thermoplastic polypropylene (PP) matrix and a dynamically vulcanized ethylene propylene diene monomer (EPDM) rubber. Several model TPVs of known composition were studied rheologically in both shear and elongation to understand and model their response using the Kaye-Bernstein, Kearsley, Zapas (K-BKZ) rheological law. A new technique is proposed to capture the effects of yielding and wall slip using the K-BKZ model. Moreover, the linear viscoelastic behavior of the TPVs was investigated, and it was found that all the TPVs exhibit a universal relaxation modulus (power-law with added yield). All TPVs exhibited a yield stress due to the presence of curing at the PP/EPDM interface evident from simple shear and uniaxial extensional experiments. The damping function was also determined, and it was shown to follow the universal Zapas relation. Finally, due to their elastomeric nature, TPVs slip significantly. Overall, it is shown that the use of the K-BKZ with yield stress and slip can capture well their rheological response.