Inorganic Primers for Adhesive Bonding: A Status Report

Abstract

The use of inorganic primers in adhesively bonded joints offers an alternate approach to standard organic primers or coupling agents for improving the environmental resistance of bonded structures. Up to a tenfold improvement in resistance to crack growth, as measured by wedge crack tests, has been found in specific cases using the inorganic primer. In addition, application of inorganic primers on aluminum adherends has resulted in elimination of the differences normally associated with varying metal surface treatments when organic primers are employed. The inorganic primers investigated, formed by hydrolysis of metal alkoxides on a treated adherend surface, are equally effective after room or elevated temperature conversion to their oxide forms. As with organic primers, a thickness effect has been demonstrated. A review of the results obtained with inorganic primers in bonded joints at United Technologies Research Center (UTRC) and other laboratories is presented, and reasons are discussed for the demonstrated improvements in environmental stability.