Hydration of oxide films on aluminium and its relation to polymer adhesion

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Abstract
In the flexible packaging industry, aluminium foil is used to fabricate metal–polymer complex systems without any prior surface treatment (e.g. anodic oxide, chemical conversion coating). The inevitable delay between foil rolling and subsequent application of the polymer, in this case polyethylene, allows ageing of the aluminium oxide to occur, which may significantly reduce the adhesion strength of the metal/polymer joint.To enable a better understanding of the ageing phenomena occurring and hence to improve the industrial control of the key parameters, controlled ageing of mini‐coils of a 1200 aluminium alloy has been performed in H2O and D2 atmospheres, with some material being subsequently stored under dry conditions. The resultant hydration of the surface oxide has been followed by XPS, SIMS and electrochemical impedence spectroscopy (EIS).The combination of XPS and EIS indicates that loosely bound physisorbed water, partly removed by dry storage, plays a key role in the adhesion strength of the metal/polymer joint. SIMS imaging of deuterium incorporated in the film as Al(OD)3 shows clearly the heterogeneous nature of the hydration process, with preferential sites being associated with iron‐rich intermetallic phases. For storage periods of >50 days, the formation of these friable bayerite islands predominates and will cause catastrophic loss of adhesion. For shorter periods, however, the loosely bound water plays a major role, and its reversible nature points to the possibility of at least partial recovery of the joint strength by dry storage of the foil prior to application of the polymer.