Effects of Brief Elevated Temperature Exposures on Adhesive Pot (Work) Lives
The use of frozen-premixed adhesives is pervasive in high tech manufacturing. When removed from freezers, there is a short ‘wait time’ for adhesives to thaw and be flowable or pliable, and thus be usable. Production personnel sometimes eliminate this wait time by placing the packaged adhesive (syringe or sheet) into a convection oven for a brief, elevated temperature exposure. This will both quickly thaw the adhesive and accelerate its cure. This procedure is fraught with the danger that the quickly-warmed adhesive will have cured sufficiently that it will exhibit degraded adhesion, resulting in test or field failures, if it has remained in the oven too long, or if application is delayed. This is a bad practice and is properly forbidden by Engineering. This paper will discuss this situation, and will discuss the factors that determine when it can be safely done, and when it cannot. The principal factors determining this are extent of cure, with accompanying adhesive viscosity increase, and how these vary with time and temperature. Adhesive cure curves, when available, are used for this analysis, assisted by chemical reaction kinetics, including Arrhenius theory, and heat transfer theory.
Author: Thomas H. Sutherland
Conference: SAMPE Seattle 2017