Adhesion of lacquers to tinplate

A set of passivated tinplates coated with organic lacquers as used for internal coatings on cans is examined. Adhesion is measured using a butt-joint test and the results of failure load and percent area lacquer removal analysed in order to grade adhesion performance. Fracture behaviour is determined by the surface structure of the tinplates where a defect distribution common to all types is identified by Weibull analysis as responsible for failure initiation. Differences in fracture behaviour seen in the mechanical test results are revealed by examination of the fracture surfaces using optical microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. The samples with good lacquer adhesion show a positive relationship between failure load and lacquer removal which is not evident in the samples with poor lacquer adhesion. The latter exhibit weak bonding between the passivation layer and tinplate and on examining those areas of fracture surface where tinplate is revealed the fracture is seen to have propagated at this interface whereas those samples with good adhesion had failed near the lacquer-passivation layer interface within the lacquer.Passivation layers are characterised using X-ray photoelectron spectroscopy and transmission electron microscopy. Differences in composition relating to specific tin oxides and chromium oxides are correlated with adhesion performance and models for the structure of passivation layers and failure mechanisms are proposed.