The Effect of Contamination on Adhesive Strength: Wettability Characterization by the CSC Method

Abstract

Abstract The correlation between the adhesive strength of joints of aluminum alloys and the wettability of their surfaces was studied. Two alloys, Al-1100 and Al-2024, were used, with two commercial epoxy adhesives, FM-73 and FM-300K, and the BR-127 primer. The wettability of the surfaces was modified by applying coatings of silicone oil and stearic acid at various concentrations. Wettability was characterized by the “complete spreading concentration” (CSC) method and by advancing contact angles. The CSC method was proven to be more reliable than contact angle measurements in detecting variations in the surface energy of the aluminum alloys studied. The adhesive strength was measured by the lap shear strength (LSS) and by the T-peel strength. The adhesive strength is only mildly sensitive to the concentration of silicone oil for the two adhesive systems. For FM-300K, the LSS decreases when the concentration of stearic acid in the coating increases. For FM-73, the LSS is only mildly sensitive to the concentration of stearic acid, but the T-peel strength shows appreciable sensitivity. The failure mode becomes more adhesive as the concentration of stearic acid and thereby the CSC increase. It is only mildly dependent on the concentration of silicone oil. The results indicate that silicone oil probably interacts with either the primer or an adhesive component in a way which counteracts the expected decrease in adhesive strength due to the reduction in surface energy and wettability of the adherends. However, the effect of stearic acid on the adhesive strength is associated with decreased wettability. All cases for which a pronounced decrease in the adhesive strength was measured are associated with contact angles larger than 90° and with either high CSC values or nonspreading situations.