Low molecular weight molecules diffusion in advanced polymers for food packaging applications

The effect of chain modification on the transport properties of two types of polyethylene-based polymers is reviewed in this work. Attention is focused on a polyketone terpolymer (0.93 : 0.07 : 1 ethylene/propylene/carbon monoxide) and on an ethylene-acrylic acid ionomer in order to determine their gas and water vapor transport properties and to relate them to the polymer structure. In fact, both polymers can be envisaged as modified polyethylene, in which some of the olefinic units have been replaced. Permeability tests have been performed on the polyketone terpolymer at several temperatures (from 25 to about 65 degrees C) with two different gases (oxygen and carbon dioxide). The investigated polymer was found to be rubbery at the test temperatures (glass transition temperature is about 17 degrees C), nevertheless the detected permeabilities are comparable to those of glassy polymers widely used for packaging applications. Water transport was analyzed performing both sorption and permeation experiments. Water vapor sorption tests were conducted at four temperatures (35, 45, 55 and 65 degrees C) and al several activities. Water permeation experiments performed at 35 degrees C at upstream pressures ranging from 4 to 25 torr showed a reduction of water diffusivity as a function of water sorbed concentration as would be expected if penetrant molecules did form aggregates. Transport properties of O-2 and CO2 in two ethylene-acrylic acid ionomers characterized by a different amounts of acrylic acid groups and percentages of neutralization have also been investigated. Sorption and permeation experiments have been performed in the 25-65 degrees C range. Water vapor permeation tests have been performed at 30 degrees C in the 5-25 torr pressure range. Data presented in this investigation were compared with analogous results reported in the literature for polyethylene in order to better highlight the effect on the gas transport mechanism of backbone modifications due to the presence of carbon monoxide moiety in one case and of ionic aggregates in the other.