High‐Performance Epoxy Adhesives for Underwater Application: Hydrophobic Modification of Curing Agents and Reinforcement With Fumed Silica

ABSTRACT Fiber‐reinforced composites are widely used in underwater equipment, requiring rapid and stable repairs for local damage. Epoxy adhesives, favored for their strength and compatibility, are commonly used for such repairs. In this paper, we developed a novel epoxy resin (EP) underwater adhesive by incorporating hydrophobic‐modified curing agents and hydrophobic fumed silica. The curing agents were formulated by blending phenolic amine 810 with polyamide 651(PA651), ensuring a balanced combination to enhance the adhesive's performance. The mechanical performance of fumed silica‐modified PA651/phenolic amine 810/EP adhesives was evaluated through tensile strength, flexural properties, and SEM analysis under different curing conditions, while FTIR analyses tracked changes in functional groups. Additionally, tensile lap shear strength tests were also performed to evaluate the bonding properties of the obtained underwater epoxy adhesives. The results showed that the modified epoxy resin adhesive cured effectively in both air and underwater environments, demonstrating superior performance. Compared to pure epoxy resin adhesive, the obtained silica‐modified PA651/phenolic amine 810/EP adhesives exhibited a 72.59% and 60.37% increase in tensile lap shear strength when cured in air and water, respectively. Additionally, its tensile and flexural strength improved by 72.63% and 74.56% when cured underwater. The obtained epoxy adhesive is suitable for underwater and water‐rich environments.