Effect of fillers on the degradation of automotive epoxy adhesives in aqueous solutions

The effects of water and NaCl solutions on four different automotive epoxy adhesives bonded on electrodeposited zinc-iron steel substrate were investigated as a function of the degree of microhardness change in the adhesives under water and NaCl solution immersion tests. On the basis of the microhardness measurements, it was found that the soft and elastic adhesives were more susceptible to adhesive softening, swelling and microdamages than the hard and brittle adhesives upon exposure to the test solutions. The degree of microhardness decrease in the adhesives increased with the concentration of the NaCl solution. In the presence of sodium chloride, the same degree of microhardness decrease was obtained in the adhesives although less water was absorbed from the NaCl solution than from the distilled water. In addition, delamination of the adhesives from the steel substrate was observed after each microhardness measurement. Oxygen was found to play an important role in the process of adhesive delamination, although it had no effect on the change in the microhardness of the adhesives.