Nonlinear viscoelastic properties of molten thermoplastic vulcanisates: An insight on their morphology

Abstract A series of thermoplastic vulcanisates (TPVs) in the molten state were submitted to large amplitude oscillating strain tests at different frequencies, to investigate their nonlinear viscoelastic properties. A purposely modified torsional harmonic tester with a closed cavity was used to run such experiments, whose results were treated with a Fourier transform (FT) algorithm to extract main torque and strain components, and harmonics if any. Quarter cycle integrations of (averaged) torque signal were also performed, to supplement FT analysis, namely to distinguish extrinsic and intrinsic nonlinear viscoelasticity. The nonlinear viscoelastic character of TPVs was found substantially differing from the one of a molten polypropylene (PP) used for comparison. Within the strain window investigated, no linear behavior is observed with TPVs, in contrast with the pure PP. However, extrapolated “linear” complex modulus G * 0 tends to decrease with increasing hardness of TPVs, but subtle differences between the various grades are clearly detected, when using simple models to fit experimental data. While the (room temperature) hardness is somewhat related with the viscoelastic behavior in the molten state, the strain sensitivity of TPVs appears essentially affected by the extractible content (mainly oil). TPVs nonlinear viscoelastic character appears to depend on the strain amplitude: mainly intrinsic, i.e., due to their morphology, at low strain, and essentially extrinsic, i.e., due to the large strain amplitude, at high strain, and in this respect, qualitatively similar to pure PP. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4193–4205, 2006