Environmental Durability Testing of Structural Adhesives. Part I. AF- 143-2/EC-3917; PL-729-3/PL-728

Abstract : A program has been conducted to investigate the durability of two modified epoxy adhesives in an elevated temperature, high humidity environment while under stress. For shear stress levels low enough to preclude fracture of the adherend surface oxide layer, and for aging times of less than 2400 hours, no significant difference exists in the time-to-failure behavior of the two adhesives. At higher stress levels, however, where fracture of the adherend surface oxide layer is likely, the adhesive system containing a rubber-filled primer (PL-729/PL-728) produced significantly longer times-to-failure than the adhesive system containing a non-rubber-filled primer (AF-143/EC-3917). Evidence was also developed to indicate that the presence of the rubber filler in the PL- 728 primer gives rise to a thin boundary layer along the adherent oxide surface along which fracture occurs on specimens in which the adherends have been phosphoric acid anodized.