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Effect of high temperature and humidity treatment on adhesion properties and bulk properties of structural adhesive

To investigate effects of high temperature and humidity treatment on adhesion properties and bulk properties of structural adhesive, a tensile test was conducted and the results were compared with those of a lap shear test. In the stress-strain curves, the changes in fracture strain and elastic modulus with increasing aging time at 80 oC in 95% R. H. were negligible. On the other hand, the extension of adhesive was 1.0% before aging, and it decreased to 0.5% by aging over 200 h. The reduction of the extension with aging was caused by hydrolysis of urethane which is added for flexibility. In aging at 80 oC in 95% R. H., the tensile strength decreased with increasing aging time until 200 h, after that the strength was stable. In the fracture surfaces of bulk specimen, fracture with elongation of matrix resin occurred in the unaged specimen. On the other hand, the crack mainly progressed at the boundary between the filler and the matrix in the aged specimen. Comparing the results with those of a lap shear test, the decrease of the lap shear strength was much larger than that of tensile strength of adhesive itself in aging at 80 oC in 95% R. H. The difference of deterioration of both strength depends on the change of the fracture mode. In the lap joint specimen, the main fracture mode changed from cohesive fracture to interfacial fracture upon aging. The result shows that the deterioration of interface was faster than that of adhesive resin itself.