The relative significance of biaxial stretch ratio effects on the permeability of oriented PET film

In this paper the relative influence of the stretch ratio in machine direct ion (MD) and transverse direction (TD) on the permeability of tubular polye thylene terephthalate (PET) films was studied. The biorientation process wa s realized on a laboratory scale by means of a double-bubble tubular film a pparatus. In order to evaluate interaction between the stretch ratios in MD and TD on barrier properties, an experimental design to measure the intera ction of two independent variables was applied. The study was also performe d on annealed PET films. The results have shown that some degree of interac tion exists between the stretch ratio in MD and TD. In particular, for anne aled PET films, the relative influence of the stretch ratio in MD and TD de pends largely on the size of the penetrant gas: the lowest permeability res ults were obtained using the highest second draw ratio (SDR) values for CO2 and the lowest second blow-up ratio (SBUR) values for N-2, respectively. H owever, it was verified that annealing strongly influences the barrier prop erties of PET films. Copyright (C) 2000 John Wiley & Sons, Ltd.