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.

中文翻译：

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在玻璃化温度以上建模聚对苯二甲酸乙二酯应力依赖的流变行为的束缚理论的局限性

双轴拉伸吹塑是用于制造塑料容器的既定方法，其中，将预成型坯在吹入模具的同时在周向和轴向上均被拉伸。在这些产品的开发阶段，广泛使用计算机辅助分析工具来提高材料和过程效率。这些工具的准确性取决于基础材料模型和参数。本文的目的是研究蠕变理论对预测拉伸吹塑过程中聚对苯二甲酸乙二酯（PET）应变依赖的流变行为的适用性。在过去的几十年中，剥脱理论已经成功地应用于许多聚合物熔体。然而，蠕变理论在预测稍高于玻璃化转变温度的高粘度聚合物的应变相关流变行为方面的实际适用性（如拉伸吹塑方法）尚未得到充分研究。第一步，实施代表理论的本构材料模型方程，并使用各种测量方法确定必要的模型参数。但是，由于所使用的测量方法在玻璃化转变温度范围内不稳定，因此测量不确定性高，因此无法以与聚合物熔体相同的精度进行测量。因此，材料模型的应用不能与双轴拉伸试验定量匹配。另一方面，定性地，材料模型成功地再现了低应变下PET薄膜的应力-应变行为。在温度依赖性的情况下，模型结果在质量和定量上都不令人满意。在第二步中进一步研究了材料模型的温度依赖性。结果表明，如果拉伸持续时间等于脱离时间，则Doi–Edwards记忆函数相对于温度的导数具有拐点。对于与拉伸持续时间相比非常小的脱离时间，该模型的结果与实验结果相符。对于玻璃化转变温度附近的大粘度引起的高脱离时间，以及由于高应变速率引起的低拉伸时间；然而，Doi–Edwards记忆功能无法正确预测实验观察结果。研究表明，复制模型定性地预测了低应变下双轴拉伸PET薄膜的应变行为。但是，为了使模型适应玻璃化转变温度以上的高粘度熔体，需要采用不同的测量方法来更准确，可重复地确定材料性能并修改模型。结果表明，两阶段拉伸吹塑的加工条件，例如高应变速率和低加工温度，超出了蠕变理论的有效性极限。POLYM。ENG。SCI。，60：765-772，2020.©2020作者。研究表明，复制模型定性地预测了低应变下双轴拉伸PET薄膜的应变行为。但是，为了使模型适应玻璃化转变温度以上的高粘度熔体，需要采用不同的测量方法来更准确，可重复地确定材料性能并修改模型。结果表明，两阶段拉伸吹塑的加工条件，例如高应变速率和低加工温度，超出了蠕变理论的有效性极限。POLYM。ENG。SCI。，60：765-772，2020.©2020作者。研究表明，复制模型定性地预测了低应变下双轴拉伸PET薄膜的应变行为。但是，为了使模型适应玻璃化转变温度以上的高粘度熔体，需要采用不同的测量方法来更准确，可重复地确定材料性能并修改模型。结果表明，两阶段拉伸吹塑的加工条件，例如高应变速率和低加工温度，超出了蠕变理论的有效性极限。POLYM。ENG。SCI。，60：765-772，2020.©2020作者。为了使模型适应玻璃化转变温度以上的高粘度熔体，需要采用不同的测量方法来更准确，可重复地确定材料性能并修改模型。结果表明，两阶段拉伸吹塑的加工条件，例如高应变速率和低加工温度，超出了蠕变理论的有效性极限。POLYM。ENG。SCI。，60：765-772，2020.©2020作者。为了使模型适应玻璃化转变温度以上的高粘度熔体，需要采用不同的测量方法来更准确，可重复地确定材料性能并修改模型。结果表明，两阶段拉伸吹塑的加工条件，例如高应变速率和低加工温度，超出了蠕变理论的有效性极限。POLYM。ENG。SCI。，60：765-772，2020.©2020作者。Wiley Periodicals，Inc.代表塑料工程师协会出版的《聚合物工程与科学》。