Abstract
Pressure-assisted thermal sterilization (PATS) alters the morphology and free volume distributions of polymers leading to a decrease in gas-barrier properties of polymer packaging materials, and hence compromising the quality and shelf life of PATS processed foods. Two multilayer ethylene–vinyl alcohol (EVOH) films, intended for high pressure applications, were investigated to understand the influence of free volume characteristics and film morphology on gas-barrier properties of PATS processed EVOH films. X-ray diffraction (XRD) and positron annihilation lifetime spectroscopy (PALS) were applied to investigate film morphology and free volume characteristics, respectively. Film A was comprised of polyethylene terephthalate (PET)/EVOH/polypropylene (PP). Film B consisted of PET laminated to a co-extruded structure of PP/tie/Nylon6/EVOH/Nylon6/tie/PP. Both oxygen and water vapor transmission rates increased in the two films after the selected treatments. However, the increase in film A is much larger which can be understood from the change in free volume distributions measured by positron lifetime and overall crystallinity observed from X-ray diffraction. This work shows that PALS and XRD are suitable tools to reason out the gas-barrier changes in multilayer EVOH films after PATS treatment.
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