The effect of humidity on the durability of aluminium-epoxide joints

Single lap joints of sandblasted aluminium alloy have been prepared with anepoxide adhesive based on dgeba and 1, 3-diaminobenzene. They have been exposed to air at 50°C and to a range of humidities for up to 10080 h, and then tested. After 10080 h ageing the results give some evidence for a critical relative humidity at 65%, such that joints weaken to a greater extent if this value is exceeded. Measurement of mass uptake of films of the adhesive suspended in air at various humidities has provided the water absorption isotherm and diffusion coefficients. The isotherm is gently curved and of BET type IV; there are no sharp changes that might account for a critical relative humidity. The presence of salt hydrates at the interface is a feasible explanation for this.