Limitations of Reptation Theory for Modeling the Stress‐Dependent Rheological Behavior of Polyethylene Terephthalate Above the Glass‐Transition Temperature

Abstract

The biaxial stretch blow molding is an established process for manufacturing plastic containers, in which preforms are stretched both in circumferential and axial directions while being blown into a mold. In the development phase of these products, computer‐aided analysis tools are extensively used to increase the material and process efficiency. The accuracy of these tools depends on the underlying material models and parameters. The aim of this article is to investigate the suitability of reptation theory for the prediction of the strain‐dependent rheological behavior of polyethylene terephthalate (PET) in the stretch blow molding process. Reptation theory has already been successfully applied to a number of polymer melts in the past decades. However, the practical applicability of reptation theory for predicting the strain‐dependent rheological behavior of highly viscous polymers slightly above the glass‐transition temperature, as is the case with stretch blow molding, has not yet been fully investigated. In the first step, the constitutive material model equation of reptation theory is implemented and the necessary model parameters are determined using various measurement methods. However, the measurements could not be conducted with the same accuracy as in the case of polymer melts, because the measurement methods used showed instabilities in the glass‐transition temperature range, which led to high measurement uncertainties. Consequently, the application of the material model does not match quantitatively to biaxial stretch tests. Qualitatively, on the other hand, the material model successfully reproduces the stress–strain behavior of PET films at low strains. In case of temperature dependence, the model results are neither qualitatively nor quantitatively satisfactory. The temperature dependency of the material model has been further investigated in the second step. It was shown that the derivative of the Doi–Edwards memory function with respect to the temperature has an inflection point if the stretching duration is equal to the disengagement time. For very small disengagement times compared to the stretching duration, the results of the model match the experimental observations. For high disengagement times induced by the large viscosities near the glass‐transition temperature and for low stretching times induced by high strain rates; however, the Doi–Edwards memory function cannot predict the experimental observations correctly. The investigations show that reptation model qualitatively predicts the strain behavior of biaxial stretched PET films at low strains correctly. However, different measurement approaches for a more accurate and reproducible determination of the material properties and a modification of the model are required in order to adapt the model to highly viscous melts above the glass‐transition temperature. The results have shown that the process conditions of the two‐stage stretch blow molding, such as high strain rates and low processing temperatures, exceed the validity limits of reptation theory. POLYM. ENG. SCI., 60:765–772, 2020. © 2020 The Authors. Polymer Engineering & Science published by Wiley Periodicals, Inc. on behalf of Society of Plastics Engineers.