No rinse Cr free Surface Treatments on Aluminium Foil for Flexible Packaging: Opportunities, Advantages and Electrochemical Measurements for Performance Evaluation

摘要

Surface treatments of Aluminium alloys for laminates used for rigid and semi-rigid packaging sectors are well established since many years. In such applications the use of surface treatments enhances the adhesion of the polymer coating to the metallic substrate in order to increase its corrosion resistance. In flexible packaging, however, such treatments are hardly to be applied due the micrometer thickness of the laminates (normally in the range 6.3÷40 mm) that are, currently, subject to a thermal degreasing for the evaporation of fluids used in the cold rolling process. The reason for the almost "no knowledge" of surface treatments of rolled aluminium with micrometric thickness arises from the fact that these treatments, up to a decade ago, were made only in immersion baths with processes requiring subsequent rinsing in demineralised water (rinse process). This process, for the complexity and length of installations, is not achievable on thin rolled laminates. In the last years, however, the spread and consolidation of the treatments applied by coater not requiring any rinsing (no rinse surface treatments) opens up opportunities in the field of flexible packaging. Moreover, electrochemical techniques, such as: (i) potentiodynamic polarization measurements (used to assess the degree of protection offered by the pre-treatment), (ii) Electrochemical Impedance Spectroscopy EIS (used on rolled after coating application of polymeric coatings, in order to evaluate the corrosion resistance), (iii) capacity measurements at fixed frequency (which allow us to monitor the kinetics of absorption of water and electrolytes in the protective polymer coating), in addition to innovative methodologies such as cathodic delamination test (test carried out in continuous current through which is possible to obtain information on the adhesion of paint to the metal substrate) allow to measure accurately (and rapidly) the advantages offered by the chemical pre-treatment of the surface and to evaluate the improvements both in terms of corrosion and adhesion resistance.