Explaining the abnormally high flow activation energy of thermoplastic polyurethanes

The rheological properties of a thermoplastic polyurethane (TPU) were studied at small and large deformation via three different types of rheometry: dynamic shear, capillary, and torque (an instrumented batch mixer). The effect of degradation during TPU processing on the melt viscosity was investigated and several factors, such as temperature, time, shear stress, and flow type that may affect the degradation were studied. Apparent activation energy of flow (E_a) was determined to be 328 kJ/mol, much larger than expected. A simple model was derived to describe the relationship of molecular weight and thermal dissociation of urethane linkages. Contributions of flow and the degradation reaction of TPU to overall activation energy were found to be additive: E_a = Eη + 1.7ΔH_deg. True activation energy of flow (Eη) was estimated to be 144 kJ/mol. While the high apparent flow activation energies in dynamic shear and capillary rheometry can be explained by simple thermal degradation, melt viscosities interpreted from the instrumented batch mixer showed a much lower apparent activation energy (186 kJ/mol). This low value may be due to a combination of effects: errors in the relation between viscosity and mixer torque for TPU, side reactions resulting from air exposure, high stress level during the melting, and extensional stresses.