Effectiveness of water-borne primers for structural adhesive bonding of aluminum and aluminum-lithium surfaces

Three commercially available water-borne structural adhesive primers (two of which contain chromates) were evaluated for their effectiveness on both aluminum and aluminum-lithium alloy surfaces. The water-borne primers evaluated were American Cyanamid's BR 250 WBP and BR 250 WBP-NC, and 3M's EC 3982. The study control was 3M's EC 3924B, a solvent-borne epoxy primer. Studies included shear strength evaluations for each primer system using 3M's AF-163-2K film adhesive at 24°C and 104°C for both metals, at -55°C for aluminum-lithium, and at 93°C after 2 weeks exposure to a 93°C/ condensing humidity environment for aluminum samples. Wedge crack extension tests were performed for samples exposed to 60°C and condensing humidity for 2 weeks. Floating roller peel studies were conducted on aluminum test samples only at -55, 24, and 104°C. A statistical analysis was performed to determine if the performance of the water-borne primers was as good as the performance of the solvent-borne primer. Statistical analyses of the data obtained indicated that only EC 3982 water-borne primer performed as well as the solvent-borne control when tested on aluminum-lithium. However, the water-borne primers performed as well as the solvent-borne primer when tested on aluminum in all cases except lap shear testing at 93°C (which was preceded by sample conditioning at 93°C and condensing humidity).