In the field of packaging, barrier layers are functional films, which can be applied to polymeric substrates with the objective of enhancing their end-use properties. In the case of food packaging applications, the packaging material is required to preserve packaged foodstuffs and protect them from a variety of environmental influences. Amongst others, the impermeability of the packaging material to substances including water vapour, oxygen and aromas is an important requirement for successful food packaging. Polymer films, vacuum coated with thin transparent barrier layers of aluminium oxide or silicon oxide, are very attractive candidates for food packaging applications due to the oxide film imparting attractive properties, including good barrier performance, transparency, microwaveability and recyclability.
In this project, aluminium oxide barrier layers were deposited onto various commodity grade BOPP films via reactive evaporation of aluminium, using a modified industrial ‘boat-type’ roll-to-roll metalliser. Optimisation of the deposited coating, in some cases together with potential surface modifications of the BOPP films, was the main focus of the work. The effects of different film treatments (in-line and off-line); surface properties of the polymer film, such as topography and chemistry; coating stoichiometry and thickness; as well as conversion processes; on barrier properties were investigated using a broad variety of analytical techniques. Furthermore, critical parameters for the convertibility of vacuum coated films, including coating adhesion and coating surface energy, were assessed.
This project has demonstrated that the barrier performance of aluminium oxide coated BOPP is heavily dependent on the plain film surface and the growth/nucleation conditions of the deposited film, both of which can vary to a large extent on standard packaging grade BOPP film. Whilst acceptable oxygen barrier levels were achieved on some of the standard BOPP film types, others did not match the requirements, despite investigating a wide range of coating parameters. This was found to be due to the presence of defects (permeation pathways) in the coating, which were reproduced from defects in the underlying polymer film surface. With regards to the barrier performance after aluminium oxide coating, the polymer film surface chemistry was identified as an important parameter. Furthermore, oxygen barrier performance was significantly enhanced when a high surface energy polymer skin layer was co-extruded onto the BOPP film. Nevertheless, water vapour barrier improvement for aluminium oxide coated BOPP films was only achieved through the use of different polymer skin layers or via depositing coatings with reduced oxygen content, thus obtaining grey coatings that can no longer be classified as transparent. Peel tests indicated very high levels of adhesion of the aluminium oxide coating to the BOPP film, with cohesive failure taking place within the polymer, rather than adhesive failure at the coating-substrate interface. Examination of the time related change of surface energy revealed a distinct decay with ageing time, most probably due to transfer of polymeric material and film additives from the reverse side of the film onto the coating and also migration through defects in the coating. Finally, the application of acrylate under- and topcoats, as well as adhesive lamination, was found to have the capability to significantly enhance the barrier performance of the aluminium oxide coated BOPP film. In the case of acrylate undercoats, this was attributed to the change in surface chemistry, whilst for topcoats and lamination processes, the barrier properties of the acrylate/adhesive play an important role, together with a possible ‘pore filling’ effect
