Characterization of the Rheological Behavior of Mixed Miscible Polymers with a High Pressure Capillary Rheometer

In this work, the rheological behavior of two types of virgin polypropylene and blends of these materials were studied using a high pressure capillary rheometer. Furthermore, it was analyzed, if the viscosity of a mixture of two materials can be estimated from the viscosity of the pure materials by applying mixing rules. Therefore, existing mixing rules were adapted.
Mechanical recycling of polymers often requires mixing polymer types with the same chemical structure but different properties, e.g., two different polypropylene grades with diverging melt flow rates. This leads to the question how the physical properties of the blended polymers change. In this paper, the rheological behavior of blends of two types of polypropylene with different weight fractions (0.1/0.9, 0.3/0.7, 0.5/0.5, 0.7/0.3 0.9/0.1) and the pure materials was studied experimentally and modeled by the Bird-Carreau-Yasuda model. Different mixing rules (Heitmiller, Bersted, Friedman and Porter, Tsenoglou) were adapted to consider the shear rate-dependent rheological behavior of polymers. The viscosity of the 0.1/0.9 and 0.9/0.1 mixtures can be described rather well by all models. In the other cases, the experimentally determined viscosity is in general higher than predicted by the mixing rules with deviations below 13 %. A comparison of the applied mixing models shows that the model of Heitmiller predicts the lowest values, while that of Friedman and Porter predicts the highest ones resulting in the lowest mean deviation of only 3.66 %.