Epoxy adhesive with high underwater adhesion and stability based on low viscosity modified Mannich bases

ABSTRACT A series of Mannich base curing agents were synthesized by using different structured phenols and amines. The phenols were phenol, o ‐cresol, and nonylphenol, and the amines were diethylenetriamine, m ‐xylylenediamine, and JEFFAMINE D230 polyetheramine. The chemical structures of Mannich bases were confirmed by 1 H nuclear magnetic resonance and Fourier transform infrared spectroscopy. The curing agents had low viscosities in the range of 19.9 to 1549 mPa s. The curing behavior of diglycidyl ether of bisphenol A with these curing agents was studied by differential scanning calorimetry. The mechanical properties and thermal properties of the cured epoxies were also investigated and compared. Lap‐shear adhesion in air and underwater was studied by using stainless steel substrates, and the adhesion after ageing in hot water (80 °C) was also investigated. Adhesive cured with phenol modified m ‐xylylenediamine showed the highest underwater adhesion around 5.9 MPa, which was comparable to other ones tested in air. In addition, the high adhesive strength was kept after ageing in hot water for 7 days. The structural influence of the curing agents on mechanical property and adhesive performances were discussed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135 , 45688.