Investigation of the differences in the lap-shear strengths of sulphuric acid anodised aluminium alloy AA6060-T6 joints bonded with hot and cold cure adhesives

The automotive industry is becoming increasingly reliant on adhesives for joining components, due to their versatile applications and high-performing mechanical properties. Conventional adhesives are one-part, epoxy-based systems that require a high-temperature cure (180°C–200 °C), resulting in a substantial energy consumption and restricting the variety of materials that can be bonded. These drawbacks can be eliminated through adopting cold-cure adhesives ($\sim$ 20 °C), however these adhesives lack the high-performing mechanical properties of their hot-cure counterparts. Addressing this, the differences in structure and chemistry between hot and cold curing adhesives were investigated, focusing on bonding anodised aluminium joints. The structure of the bonding interface was analysed using Scanning Electron Microscopy (SEM), whereas the chemistry involved was determined through Fourier Transform InfraRed (FT-IR) and X-ray Photoelectron Spectroscopy (XPS). Lap-shear results showed that significant adhesion was achieved between the aluminium oxide coating and the hot- and cold-cure adhesives. The cold-cure adhesive was found to contain 10.6 % fewer hydroxyl groups at the bonding interface. Functional bonding groups corresponding to aluminium silicate bonds were detected at interface of anodised aluminium joints bonded with cold cure adhesives, but these bonds were absent at interface of joints bonded with the hot cure adhesive. Although differences in functional bond groups were found to exist at the interface between the anodised aluminium and cold or hot cure adhesives, all tested lap-shear joints exhibited cohesive failure regardless of the adhesive being hot or cold curing. Results from this investigation indicates that the lower lap-shear performance of anodised joints bonded with the cold cure adhesive compared to those bonded with the hot cure adhesive stems from differences in the bulk properties of the adhesives themselves, suggesting that the cold cure adhesives have inferior mechanical bulk properties compared to the hot cure adhesive.