Statistical WVTR Models for Extrusion-Coated Webs in Various Atmospheric Conditions

The target of this study was to establish statistical prediction models for the water vapour transmission rate (WVTR) of extrusion-coated papers and paperboards commonly used in food packaging. The models bring along reliable and easily available information about materials’ moisture barrier, estimate packaging costs and optimise food quality at specific packaging applications. Furthermore, the models can help laboratories to reduce the expenditure of the rather time-consuming WVTR testing. Another target of the study was to investigate the fundamentals concerning the influences of thermo-hygrometric conditions on the water vapour barrier properties of extrusion-coated webs. Also, the effect of intensive heat treatments (or processes made at elevated temperatures) on the barrier properties were planned to investigate. The study indicates that experimental data can be used in developing accurate prediction models for WVTR. For non-polar coating polymers, it is possible to establish a comprehensive model that calculates WVTR as a function of the following external factors: temperature, humidity and the multilayer profile of the coating. When the coatings contain a water-sensitive ethylene vinyl alcohol copolymer (EVOH) layer, the WVTR can be predicted indirectly by characterising the resistance effect caused by a non-polar skin layer on EVOH water sorption. In such case, atmospheric conditions were not introduced into the model as variables because of the too complex correlation with WVTR. According to heat treatment experiments, the over-melting-point treatments considerably improved the water vapour and oxygen barrier properties of polyolefin-coated papers, namely low and high density polyethylenes. The improvement was mainly controlled by the crystallisation kinetics in the coating during a slow cooling period. Chemical reactions in polyethylene were also involved with the treatments above 200°C. Concerning polylactide (PLA) coatings, water vapour barrier properties were already improved at lower temperatures. The heat treatments between 100-150°C were able to reorganise the PLA’s crystalline and amorphous regions leading to considerably reduced transmission levels.