Evaluation of primers for adhesively-bonded aircraft repair

This paper contains the results of an investigation to determine the effectiveness of primers in promoting adhesion and improving the environmental durability of epoxy adhesive bonds on grit-blasted aluminum substrates. The control for this study consisted of phosphoric acid anodization of substrates followed by the application of EC 3924 B primer. The following primers were evaluated on grit-blasted substrates: (1) EC 3924 B; (2) Metlbond 6726; (3) Dow Corning Z-6040; and (4) EA 9203. In addition, grit blasting followed by Alodine application was compared with the other primers. Studies include shear strength evaluations of each primer/adhesive system at 23 and 105°C. In order to assess the relative durability of each system, shear strength testing at 93°C of samples exposed for 2 weeks to a 93°C/condensing humidity environment and wedge crack extension testing of samples exposed for a 2-week period to a 60°C/condensing humidity environment were performed. The peel strengths of the systems were determined by conducting floating roller peel studies at -55, 23, and 105°C. Statistical analysis of the data revealed that the level of performance provided by EC 3924 B, solvent-borne, corrosion inhibiting control primer on phosphoric acid-anodized surfaces exceeded that of all the primers on grit-blasted substrates. However, when comparing the primers applied after only a grit-blast treatment, y-glycidoxypropyltrimethoxysilane primer showed statistically better results than the other primer systems.