Multilayer, Flexible Food Packaging Structures: Design and Response of Plastic Films to Accelerated Ageing

The effect of accelerated ageing on the structure and properties of single, metallized and multilayer films used in food packaging was studied, by exposing specimens of the above films to repeated ageing cycles in a weatherometer, under the combined action of UV, humidity and heat. The investigated films were: biaxially oriented polypropylene film (BOPP), metallized BOPP film, polyethylene (PE), polyethylene terephthalate (PET), laminated PE with PET film (PE//PET) and coextruded PE/EVOH/PE film. The aged specimens were tested for their mechanical properties, water vapor transmission and color stability characteristics and the results were compared to those obtained from measurements with the original specimens. The property changes introduced to the films by ageing were further explored by Attenuated Total Reflectance Spectroscopy (ATR) and Differential Scanning Calorimetry (DSC), in an attempt to correlate the changes of properties with structure characteristics. The results of this work showed that polyolefins are susceptible to degradation and photo-oxidation when exposed to ageing. PE in contrast to PP, forms cross-linked structure and, therefore, it shows a modest decrease of mechanical strength. On the other hand, PET shows good UV stability, whereas its resistance to water is low and undergoes hydrolytic degradation. EVOH was also found vulnerable to this attack. The use of multilayer films requires special attention on the stability of each one of the layers because, after ageing, the failure mechanism can be initiated and further promoted at sites of weak components of the systems. Finally, UV and water stability of interfacial tie resins, placed between the layers of laminate films to enhance adhesive bonding, must also be taken into consideration for acceptable outdoor performance of final products.