Thermal Effects on the Gas Transmission Properties of Ionomers Used in Food Packaging Applications

Abstract

Polyethylene-based ionomers, due to t h e presence of ionic aggregates, display a time-dependent crystalline morphology if submitted t o thermal treatments. As a consequence gas permeability, which is strongly affected by physical morphology, is also found t o be time dependent in previously thermally treated ionomer samples. The effect of sample exposure a t 65°C and o f successive ageing a t 35°C on gas transmission properties of three different ethyleneacrylic acid ionomers (characterized by a different amount of acrylic acid groups, percentage of neutralization and type of counterions) has been investigated. In particular the changes in oxygen and carbon dioxide permeabilities due to thermal treatments a t 65°C and subsequent ageing a t 35°C have been monitored. X-Ray diffraction analysis and differential scanning calorimetry have been used t o elucidate these changes in transport properties.

Effect of the thermal treatment of ionomer films on permeability and permselectivity

“…It is not clear why the O 2 permeability coefficient was more affected by the higher formation temperature than the CO 2 permeability coefficient. However, when testing the O 2 and CO 2 permeability of ethylene ionomers, Mensitieri et al14 observed a similar pattern and alluded to a decrease in the tortuosity factor (diffusive path length) and immobilization factors (blocking effect), which could lead to an increase in gas permeability. Because diffusion is affected by the size of the permeating molecules, O 2 could permeate faster than CO 2 ; therefore, the relative increase in O 2 permeability was higher.…”

“…In our view, the ratio of CO 2 /O 2 permeability (i.e., permselectivity) is a limiting factor in extending the shelf life of packaged respiring fruits and vegetables. 12 Following the suggestions of Del Nobile et al 13 and Mensitieri et al 14…”

“…In our view, the ratio of CO 2 /O 2 permeability (i.e., permselectivity) is a limiting factor in extending the shelf life of packaged respiring fruits and vegetables 12. Following the suggestions of Del Nobile et al13 and Mensitieri et al14 that annealing conditions [below the melting temperature ( T m )] could affect the permeability of ionomers, we set out to determine if thermal treatments of ionomers would favorably affect permselectivity.…”