Characterization studies of aluminum oxide barrier coatings on polymeric substrates

Abstract

Aluminum coatings are widely used as oxygen and water barrier layers on food packaging materials to protect the product from contamination and to extend its shelf life. Thin layers of aluminum, of the order of a few tens of nanometers, are thermally evaporated onto polymeric web in high speed roll-to-roll vacuum metallizers. More recently, transparent barrier layers, based on silica or aluminum oxide, have been introduced to allow product visibility and provide retortable, sterilizable, or microwaveable packaging. In all cases, though, the barrier characteristics are critically dependent on both the structure and properties of the coating and the nature and surface properties of the polymer film substrate. This paper, therefore, reports on characterization trials of aluminum oxide (AlOx) coatings deposited onto polyethylene terephthalate (PET) and biaxially oriented polypropylene (BOPP) film substrates. Coating structures and properties have been investigated by scanning electron microscopy and atomic force microscopy; the influence of process parameters on barrier levels has been determined; surface energies have been determined through contact angle measurements; and coating-to-substrate adhesion has been assessed using “pull-off” tests. The main findings of this study are that the barrier performance of AlOx coated PET and BOPP films strongly depends on the surface properties of the plain film and the aluminum oxide film growth. While AlOx coated PET films deliver consistent barrier results, BOPP reveals considerable variations, depending on the individual substrate.