Strength of Epoxy/Glass Interfaces after Hygrothermal Aging

Abstract

ABSTRACT The stability of epoxy/glass interfaces subjected to hygrothermal aging was assessed using a fracture-mechanics approach. An epoxy system consisting of diglycidyl ether of bisphenol F cured with 2-ethyl-4-methyl-imidazole was bonded to borosilicate glass adherends that were treated with various types of adhesion promoters to provide a variety of interfaces. Adhesive strength was measured under dry, as-processed conditions and as a function of exposure time to an 85°C/85% relative humidity (RH) environment. As expected, the strain-energy-release rate, G c , dropped significantly with aging time for the bare epoxy/glass interface. The drop in G c is assumed to be due to a loss of interfacial forces. The use of two silane-based adhesion promoters, 3-aminopropyltriethoxysilane (APS) and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (ECH) resulted in improved adhesive strength both before and after hygrothermal aging. The improvement in adhesive strength can be explained by the introduction of chemical bonds at the interface. The drop in G c is assumed to be due to a loss of interfacial forces and hydrolysis of siloxane bonds. In addition to the use of organosilane-based adhesion promoters, a series of polyhydroxyaminoethers (PHAE) thermoplastic adhesive resins was also investigated as potential adhesion promoters. It was found that 2% PHAE in Dowanol® PM, a hydroxyl-group-containing solvent, was the best system for the PHAE-based adhesion promoters. Interestingly, both the acetic acid concentration in the solvent and maleic anhydride content in the PHAE resin were shown to affect the adhesive strength.