Effect of GPTMS Passivation on Adhesive Bonding Performance of Aluminum Substrates Using an Epoxy Adhesive

This study investigates the effect of (3-glycidyloxypropyl)trimethoxysilane (GPTMS) passivation time on the adhesive bonding performance of aluminum substrates using an epoxy adhesive. Alkaline etching was used to generate a chemically active surface prior to silane treatment. GPTMS passivation led to the formation of silane-derived species on the aluminum surface. SEM/EDS indicated the presence of silicon-containing species. ATR-FTIR analysis showed the progressive development of siloxane (Si–O–Si) bonding with increasing passivation time. The mechanical performance of the bonded joints was evaluated using single-lap shear (SLS) testing. The SLS strength increased from 4.3 ± 1.0 MPa in the as-received substrate to 5.5 ± 1.2 MPa after etching. After GPTMS passivation, the strength reached a plateau beginning at 3 min, with a value of 13.5 ± 1.8 MPa. This corresponds to increases of 28% after etching and 223% after GPTMS passivation. This plateau behavior indicates a self-limiting interfacial process. The improved adhesion is attributed to siloxane formation within the silane layer and the chemical compatibility between GPTMS and the epoxy adhesive. A first-order conceptual semi-quantitative model was developed to relate silane surface coverage to adhesion strength. The results demonstrate that adhesion depends on both surface coverage and the development of siloxane bonding within the silane layer. This study highlights the importance of controlled passivation time in improving adhesion performance under the present experimental conditions.