Aluminium oxide barrier films on polymeric web and their conversion for packaging applications

• Effect of conversion on barrier levels of AlOx coated polypropylene was studied.
• Acrylate coatings and lamination can significantly enhance barrier properties.
• Pore-filling is proposed as an important mechanism for barrier improvement.
• Barrier properties of the acrylate/adhesive also play a critical role.

In recent years, inorganic transparent barrier layers such as aluminium oxide or silicon oxide deposited onto polymer films have emerged as an attractive alternative to polymer based transparent barrier layers for flexible food packaging materials. For this application, barrier properties against water vapour and oxygen are critical. Aluminium oxide coatings can provide good barrier levels at thicknesses in the nanometre range, compared to several micrometres for polymer-based barrier layers. These ceramic barrier coatings are now being produced on a large scale using industrial high speed vacuum deposition techniques, here, reactive evaporation on a ‘boat-type’ roll-to-roll metalliser. For the thin barrier layer to be useful in its final packaging application, it needs to be protected. This can be either via lamination or via an additional topcoat. This study reports on acrylate topcoats, but also undercoats, on aluminium oxide coated biaxially oriented polypropylene films. The effect of the acrylate layer on barrier levels and surface topography and roughness was investigated. The acrylate was found to smooth the substrate surface and improve barrier properties. Furthermore, the activation energy for water vapour and oxygen permeation was determined in order to investigate barrier mechanisms. The oxide coated film was, additionally, converted via adhesive lamination, which also provided improvement in barrier levels.