Multivariable Rheological Models for Commercial Polypropylene with High Molar Masses

This work investigates the fitting performance of conventional rheological models and the development of multivariable rheological models to reproduce experimental rheological data of different industrial grades of linear isotactic polypropylene (iPP) having high mass average molar masses, Mm (164–404 kg mol⁻¹), at three temperature values (180–220 °C) over a wide range of shear rates (10⁻¹–10⁴ s⁻¹). A shear thinning behavior is found in all investigated conditions. However, a low shear rate primary Newtonian plateau for a short shear rate range is only identified for the smallest Mm among those investigated, and for higher Mm such primary plateaus are even found at shorter shear rate range. Among the investigated models, only Cross and Carreau–Yasuda models are effective to reproduce the data for a specific PP grade. Two modified models are proposed that incorporate three variables. In the modified Cross Model, it has been shown that the characteristic time (λ) between the Newtonian plateau at the low shear rates and the shear-rate range with shear-thinning behavior depends exponentially on the Mm, and it does not depend on the temperature. Both proposed models fit very well with the experimental data with shear thinning behavior for a wide range of Mm.