Hybrid Sol–Gel Epoxy–Silane Adhesives: Thorough Physicochemical and Mechanical Studies by Statistical Analysis

Aluminum alloys are commonly subjected to chromatization to improve adhesion and protect against corrosion. However, this process is harmful to health and the environment. In this context, hybrid organic–inorganic silane films obtained by the sol–gel process come out as alternatives, with the advantage of acting as coupling agents. In this study, a factorial design was used to optimize the sol–gel process of a tetraethoxysilane (TEOS)/3-glycidoxypropyltrimethoxysilane (GPTMS) system with the curing of an epoxy resin, seeking adhesive efficiency. The samples were characterized by a shear test, scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, goniometry, and viscosity. Both the resin curing and sol–gel transition processes began during the system preparation, before deposition. The most important factor for the adhesive efficiency of the coatings was the interaction between two factors: TEOS:GPTMS molar ratio and interval time to deposition. The coating with the highest amount of GPTMS showed the best adhesive efficiency for the shortest deposition interval, while the opposite occurred with the sample prepared with the lowest amount of GPTMS. The results were explained by the complex reactions involving the sol–gel system summed to the resin curing process overtime. The statistical study provided information for the processing optimization of this environmentally friendly adhesive.