Characterization and evaluation of primer formulations for bonding silicone rubber to metal

Abstract The aim of this study was to explore the chemistry and adhesive properties of industrial primer formulations used for bonding silicone elastomers on AA6061 aluminum alloy. The composition of different primer formulations was determined using conventional chemical analyses. Adhesion strength was measured using 90° peel test. The formulation comprising partially condensed vinyltrimethoxysilane (VTMS) and tetrapropoxysilane (TPOS) and titanium tetrabutoxide (TTB) catalyst showed a strong adhesion to metal but a low adhesive strength of the primer-elastomer interface. 29Si NMR spectroscopy results have shown that exchange reactions of alkoxy groups of VTMS, TPOS and TTB has occurred in the formulation. Surprisingly, one formulation was free of silane and consisted of a one-component addition curing system comprising a SiH oligosiloxane, a vinyl-PDMS with a Pt catalyst and silica filler. This formulation enabled high cohesive strength of the primer and primer-elastomer interphase but it showed low adhesion to the metal. The more complex formulation containing a mixture of silanes (vinyltriethoxysilane, methyltriethoxysilane and tetraethoxysilane), a vinyl silicone gum, a hydroxy-terminated PDMS and silica filler resulted in the highest adhesive strength with 100 % cohesive failure in the elastomer. We found that the reinforcement effect of silica filler plays an important role in the promotion of adhesion.