Correlation of surface chemistry and shear strength in epoxy-bonded float and low-iron glass joints

The increasing use of glass as a load-bearing material in modern architecture and structural engineering has intensified reliance on adhesive bonding, making the interfacial properties of bonded surfaces critical to structural performance. Indeed, the chemical and physical properties of the glass surface, particularly the distinction between the tin and air sides, significantly influence bond strength and long-term joint durability. Therefore, this study systematically investigates the adhesive bonding behaviour of two widely used architectural glass types, soda-lime-silica float glass and low-iron glass, with a focus on differences between their tin-side and air-side surfaces. Experimental results reveal significant variations in bond strength related to both glass type and bonded sides. Low-iron glass consistently outperformed standard float glass, reaching peak shear strengths of 12.65 MPa on the tin-side, whereas the soda-lime-silica float glass showed lower values of 7.60 MPa and 6.32 MPa on its air and tin sides, respectively. To assess the mechanisms underlying these differences, surface characterisation was conducted using water contact angle (WCA) measurements, X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS). Enhanced adhesive performance was found to correlate with lower WCA values and higher surface concentrations of non-bridging oxygen (NBO) and negatively charged SiO 2 - ions. Notably, the tin-side of low-iron glass exhibited the highest NBO content (19.9%) and the strongest SiO 2 - signal. These findings underscore the crucial role of glass composition and surface orientation in determining interfacial adhesion and mechanical properties. They provide practical insights for optimising adhesive bonding strategies in structural glazing, laminated glass, and hybrid assemblies.