Adhesion performance of an epoxy clear coat on aluminum alloy in the presence of vinyl and amino-silane primers

The influence of amino and vinyl-silane-based treatments on the performance of an epoxy coated AA 1050 aluminum has been studied. The surface energy of the silane treated samples was determined using static contact angle measurement. Chemical interactions between the silane layer and aluminum substrates were also examined using FTIR-ATR spectroscopy. Pull-off adhesion was used under wet, dry and recovery conditions. The contact angle measurements showed a relative increase in the surface free energy of the silane treated specimens. For specimens treated with silane solutions below the IEP of aluminum, adhesion strengths were higher than those for pHs higher than IEP. This may suggest that an acid-base interaction occurs between the negatively charged silanol species and the positively charged AA surface. FTIR revealed two modes of interaction, one between silane and Al3+ (Si-O-Al), and the other indicative of a siloxane network (Si-O-Si). Thickness measurement showed that the silane layer is 100-200 nm thick indicating that the silane formed at the surface is not a monomolecular layer. Direct pull-off adhesion measurements of silane treated samples showed a significant improvement in initial bond strength compared with degreased aluminum substrates. However, those silane solutions having pHs higher than 8.4 showed inferior adhesion in comparison to those having lower pHs. The adhesion improvement was maintained, to some extent, after exposure to salt spray cabinet. In spite of good dry, wet and recovery adhesion, silane treated surfaces are not comparable with chromated AA aluminum in terms corrosion resistance. (C) 2006 Elsevier B.V. All rights reserved.