Adhesives for automotive windscreen replacement: Kinetics of cure and bond strength

Abstract

Cure depth (z) has been measured for three water-hardening isocyanate adhesives, which are used in replacing automotive windscreens, at a range of temperatures. Swelling the cured materials in toluene showed that temperature does not affect the extent of crosslinking. Depth of cure conforms to the equation z=(2PpVt)1/2, where P is the water permeability coefficient of the cured adhesive, p the vapour pressure of water, V the volume of adhesive which reacts with 1 mol of water, and t is time. The actual vapour pressure of water is the dominant factor in causing the rate of cure to vary with temperature. There was a tendency for joint strength to increase with cure time; however, this was subject to much scatter and generally the increase in joint strength was less than would be expected from consideration of the increase in cured adhesive area. This was attributed to the complex failure paths observed, with combinations of failure in the adhesive, in the metal primer and at the aluminium–metal primer interface. The lowest joint strengths were associated with high proportions of failure at the aluminium–metal primer interface. Joint strength prediction was reasonable for those joints failing predominantly in the adhesive, but strength was over-predicted for joints failing predominantly at the aluminium–metal primer interface. This was because the failure criteria used were based on failure in the adhesive.