Aging and performance of structural film adhesives. IV. A modified epoxy with improved ambient temperature shelf life

It appears that improved latency in nitrile epoxy adhesives can be achieved by replacing the commonly used urea accelerator with one of much reduced solubility but similar chemical activity. A new, commercially available film adhesive claimed to possess a shelf life of 1 year at 22°C, excellent resistance to humidity prior to cure, and a service temperature range of −55 to 120°C has been examined. Areas studied included characterization, cure kinetics, adhesive performance, and shelf life. Its composition was found to be a variant of the 120°C curing nitrile—epoxy systems using a urea‐accelerated dicyandiamide cure. Two novel features were noted: the presence of bisphenol S and the urea 1,1'‐p‐phenylene‐bis(3,3‐dimethyl) urea. The relatively high shear strength found at elevated temperatures could be related to the adhesive's high Tg of 155°C resulting from the addition of bisphenol S and the improved room‐temperature latency was due to the ultralow solubility of the urea accelerator. Good durability was obtained from aluminum joints prepared using a simple silane pretreatment. Aging experiments indicated a shelf life of at least 3 months at 23°C under both dry and wet conditions, and it is predicted that adhesives of this type of composition would be stable for at least 1 year at normal domestic refrigeration temperatures of 5–10°C. © 1993 John Wiley & Sons, Inc.