Numerical Simulation of Bowing Distortion Formation of BOPET Film During Tentering Process

Abstract

When preparing biaxially oriented polyethylene terephthalate (BOPET) films through sequential biaxial stretching, bowing distortion occurs within the film plane, leading to inhomogeneous optical and mechanical properties between the edge and central regions of the films. This quality degradation renders the films unsuitable for optical applications. Considering that polyethylene terephthalate (PET) macromolecules are in a rubbery state and have crystalline physical crosslinking points when the film is stretched above its glass transition temperature T_g but below the melting point T_m, this study establishes a visco-hyperelastic model. The model parameters are obtained by numerically fitting the experimental results of uniaxial stretching and stress relaxation tests. These fittings yield the second-order coefficient of the Mooney-Rivlin polynomial (describing hyperelasticity) and the third-order coefficient of the Prony series (describing viscosity). Then, ANSYS finite element simulations of the tentering process of PET films are carried out with the isotropic elastic, the orthotropic elastic, the plastic, and the visco-hyperelastic material models, respectively. The simulated bowing distortion obtained from the visco-hyperelastic model shows better agreement with experimental results, demonstrating that this model can accurately capture the mechanical response behavior of PET films during biaxial stretching.

Keywords

biaxially oriented polyethylene terephthalate (BOPET) film / tentering process / bowing distortion / visco-hyperelastic model / numerical simulation

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