Barrier properties of ethylene vinyl alcohol films in thermal processing

Ethylene vinyl alcohol (EVOH) copolymers are widely used in food packaging for shelf stable foods because of their good oxygen barrier. However, EVOH is very sensitive to moisture which adversely affects its oxygen barrier. The overall objective of this research was to understand the influences of thermal processing on oxygen barrier properties of multilayer EVOH films, with emphasis on influence of water absorption by packaging films when exposed to conditions of high moisture and high temperature during processing. The research focused on two multilayer films containing EVOH as the oxygen barrier which were used as lidstock for rigid trays. Film A was a laminated structure of oriented polyethylene terephthalate (PET), EVOH and polypropylene (PP). Film B consisted of PET laminated to a co-extruded structure of PP/tie/Nylon 6/EVOH/Nylon 6/tie/PP. Oxygen transmission rates were measured on films after retort and combined microwave-hot water heating under conditions necessary to produce shelf-stable low acid foods. For both films, the retort treatments resulted in more deterioration of the oxygen barrier than microwave treatments. This was attributed to the amount of moisture absorbed during retort treatment, which was thought to be directly related to duration of direct exposure of films to high temperature humid environment during processing. Storage of packages for 2 months under ambient conditions resulted in recovery of more than 50% of the oxygen barrier. Further studies were carried out to investigate the relationship between the amount of water absorbed during retort processing and oxygen transmission through the films. Results confirmed dramatic increases in oxygen transmission at high film water contents which was attributed to plasticization and stresses created by high moisture and temperature conditions. Water absorption behavior of multilayer films was also studied at different temperatures and compared to monolayer films representing the individual components of the multilayer films. A preliminary study on use of split post dielectric resonator (SPDR) technique as a method for measuring amount of water in polymers was carried out. The measured dielectric property data correlated well with water absorption data, indicating that the method can be used to rapidly and non-destructively measure water absorption in these films.