Hot-drawing of poly(ethylene terephthalate) under biaxial stress: Application of a three-dimensional glass-rubber constitutive model

A study was made of the hot-drawing of amorphous poly(ethylene terephthalate) (PET) under biaxial stress in the temperature/strain-rate regime relevant to industrial film drawing. Film specimens were drawn at constant width at temperatures 75-120 degrees C and strain-rates 1-16 s(-1), using a specially developed testing machine-the Flexible Biaxial Film Tester. The stress-strain data obtained, together with similar results from uniaxial tests, allowed a recently-proposed three-dimensional constitutive model for amorphous polymers to be tested and applied to PET. It was found that the yield, flow and strain-stiffening observed in drawing could be described well by the model at the lower temperatures. A procedure was devised for systematically fitting data to the model, and produced values of the model parameters which were interpreted in terms of the physical mechanisms of flow and rubber-like strain stiffening. The major inadequacy of the model was that it did not include flow resulting from entanglement slippage, modified in PET by stress-induced crystallization, that intervenes at the upper end of the temperature range. (C) 1996 Elsevier Science Ltd.