The transport mechanisms of gases through metallized films intended for food packaging applications

Metallized polymeric films are amongst the most favoured candidates for replacing aluminum foil-based multilayer films. The main advantages in using the former instead of the latter are: weight saving of about 40%; much less aluminum (320 time less in weight); improved recyclability; and the possibility of optimizing package barrier properties. It is generally assumed that metallization decreases the permeability of the polymeric substrate by reducing the total area available for permeation, even though the validity of such a hypothesis has never been proved experimentally. In the present paper an attempt has been made to elucidate the role of metal coating in reducing the permeation flux of polyolefine films. For this purpose, permeability tests were performed using oxygen, nitrogen and carbon dioxide at temperatures of 20-40°C. Permeation tests have been performed on two different metallized films, on their substrates alone and on two laminates. From the analysis of the experimental results, it has been concluded that, contrary to what generally thought, gas molecules can permeate through the metallized films investigated due to both the permeable porous structure of the deposited aluminum layer and the presence of pin-holes uniformly dispersed on the metallized film surface