Role of metal alkoxides as primers in polymer/metal adhesion

Steel is a material with a high tensile shear strength and is relatively inexpensive to produce. Further, it is widely used in industry and its use in adhesive bonding applications is increasing. Adhesively bonded structures offer several advantages over mechanical fastening such as reductions in cost and weight and also provide uniform stress distribution. The goal of this research is to study the effect of four metal alkoxides on the adhesive bonding of steel. The topography and chemical composition of gritblasted, alkoxide primed surfaces were determined using primarily scanning electron microscopy and x-ray photoelectron spectroscopy. Gritblasted steel was primed with the metal alkoxide solutions and then bonded with both polyethersulfone (thermoplastic) and FM 300U, a structural epoxy (thermoset). Bond durabilities were studied using wedge specimens. The two aluminum alkoxide primed steel surfaces generally showed slower crack propagation than titanium and silicon alkoxide primed surfaces. Bond strength information was obtained from lap shear joints that were primed with aluminum tri-sec-butoxide, titanium (IV) butoxide and control samples where no primer was used. Statistical analysis showed that there were significant differences in bond strengths, with the aluminum alkoxide primed surfaces showing higher breaking strengths. Further, the effect of applied stress and temperature on bond strength was determined. Lap shear joints that were primed and unprimed and bonded with PES and FM 300U were loaded to 10-40%, depending on the ultimate breaking strength and placed in a stress durability tester (immersion in deionized water at 80°C). A significant decrease in breaking strength was observed between the stress durability samples and the lap shear joints tested at room temperature. The failure surfaces of wedge, lap shear and stress durability samples were analyzed using scanning electron microscopy, energy dispersive x-ray analysis, x-ray photoelectron spectroscopy and Auger electron spectroscopy.