Preparation of cardanol-based curing agent for underwater drainage pipeline repairs

The underwater curing agents have demonstrated promising potential in various applications, especially in underwater repair engineering, yet have proven considerable challenging. Here, we report a cardanol-based curing agent for epoxy resin that achieves robust adhesion to steel and concrete substrates both in air and underwater. Cardanol, paraformaldehyde, and m-phenyldimethylamine are selected as the polymeric monomers to synthesize curing agent by Mannich reaction in the absence of chemical cross-linker agents. The coating is completely cured within 46 ± 1 min in air and 54 ± 2 min under water with an adhesion of 0 or 1 and a hardness of 5H. The impact strength, shear strength, and tensile strength of coating on underwater concrete were 9.58 ± 0.41 kJ·m−2, 13.1 ± 0.3, and 10.5 ± 0.2 MPa, respectively, demonstrating exceptional flexibility and mechanical strength as well as favorable hydrophobicity. This work paves the way for the rehabilitation of underwater drainage network for urban infrastructure and water conservancy projects.