Hydrothermal Stability of Titanium/Epoxy Adhesive Joints

γ-Aminopropyltriethoxysilane (γ-APS) was shown to be an extremely effective primer for improving the hydrothermal stability of titanium/epoxy adhesive joints when applied to the adherends by adsorption from dilute aqueous solutions prior to adhesive bonding. The breaking strength of lap joints prepared from unprimed adherends decreased slowly during immersion in water at 60°C and was only about 960 psi after 60 days. The breaking strength of joints prepared from adherends primed with γ-APS at pH 10.4 and 8.0 decreased very little during similar hydrothermal aging and was about 1750 psi after 60 days. Joints prepared from adherends primed with γ-APS at pH 5.5 were somewhat less durable and had a breaking strength of about 1180 psi after 60 days in water at 60°C. The performance of γ-APS as a primer was very different for titanium and iron adherends even though the molecular structures of the primer films formed on the two substrates were very similar. As a result, the performance of γ-APS primer films could not be related to the overall molecular structure of the films but was related to acid/base interactions between γ-APS and the oxidized surfaces of titanium and iron.