One-Part Heat Curing Paste Adhesives

One-part heat curing paste epoxy adhesives generally contain latent catalysts, liquid epoxies, toughening agents, fillers, thixotropes, and adhesion promoters. 

Room temperature latency of the nitrogen-containing curatives is typically achieved in one of two ways. One method is to employ a curative which is insoluble, or only marginally soluble, in the adhesive mixture at room temperature, but which becomes soluble at the cure temperature. The classic example of this is dicyandiamide (10). The second method is to employ a curative precursor which is inactive at room temperature, but which converts to the active curative at the cure temperature. Examples include Monuron (11) and the di-urea adduct (12) of toluene diisocyanate and dimethylamine. Full cure in one hour at about 250 F is achieved. These solid curatives are incorporated into the epoxy under high-shear conditions. Efforts to replace the water-soluble dicyandiamide with adipic dihydrazide and solid adducts of ethylenediamine such as HY-940, sold by Ciba-Geigy, have been successful. Most latent curatives typically have some activity at room temperature, which often requires the storage of the uncured adhesive at subambient temperatures. 

Latent anhydride catalysts are not used extensively in structural adhesives because the cured products tend to be brittle and exhibit low peel strength. Electrical properties are improved over amine-cured epoxies, however. 

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