Pressure-assisted thermal sterilization effects on gas barrier, morphological, and free volume properties of multilayer EVOH films

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.