New Insights in Adhesive Properties of Hybrid Epoxy-Silane Coatings for Aluminum Substrates: Effect of Composition and Preparation Methods

The sol–gel system has attractive properties both in terms of adhesion to metal substrates and of its great potential to replace the toxic treatments of metal surface currently used, such as chromating and anodization. In this work, hybrid epoxy-silane coatings prepared by the sol–gel method with epoxy resin and tetraethoxysilane (TEOS) were evaluated for the metal–metal bonding of aluminum substrates, based on the influence of different compositions/preparation methods of the coatings in their physical–chemical and adhesive properties. Five variables were studied: time after preparation of the adhesive systems prior to the deposition, the composition of the coating layers, the addition of a silane compound (3-glycidoxypropyltrimethoxysilane-GPTMS), the addition of epoxy resin to the adhesive systems (with or without curing agent) and the pressing method (type C clamp or by marble plates). The samples were characterized by infrared spectroscopy (FT-IR), rheological analyzes (viscosity) and shear test. The FT-IR analysis showed that the presence of epoxy resin in the sol–gel system delayed the gel transition, at the same time that the presence of the sol–gel accelerated the epoxy resin curing process. The rheological analyses showed similar trends, and the viscosity of the epoxy resin and epoxy-sol systems increased after 60 min at 70 °C, while the sol–gel system showed an increase in viscosity after 20 min at 70 °C, indicating that this is the best moment for the deposition. The shear results showed that the system adhesion is stronger when silane is used as a coupling agent between the aluminum substrate and the epoxy resin, probably due to the Si–O–Al bonding and the good cohesion of the epoxy resin with the curing agent.