Silane-Functionalized Covalent Organic Framework-Epoxy Nanocomposite Coatings for Advanced Concrete Sewer Protection

Concrete sewer infrastructure deteriorates rapidly due to biogenic sulfuric acid corrosion. This study developed a covalent organic framework (COF)-reinforced epoxy composite coating to enhance durability under such aggressive conditions. A boronate ester-linked COF was synthesized, functionalized with (3-aminopropyl)triethoxysilane (APTES) to improve hydrolytic stability, and incorporated into epoxy resin. Coatings with varying APTES-COF loadings (0–1.5 wt %) were assessed for water uptake, acid resistance, adhesion, and hardness. At the optimal loading of 1.5 wt %, water absorption decreased from 1.6% to 0.09%, while the static contact angle increased from 70.9° to 91.2°, indicating improved hydrophobicity. Mass loss under acid exposure was limited to 4.05%, representing 75.91% and 83.85% improvements compared to neat epoxy and uncoated concrete, respectively. Adhesive strength increased to 6.7 MPa, a 59.52% improvement over unmodified epoxy. Nanoindentation confirmed enhanced hardness and modulus due to a denser cross-linked network. Density functional theory (DFT) simulations revealed a reduced HOMO–LUMO gap and increased dipole moment, supporting stronger molecular interactions. Overall, APTES-COF incorporation significantly improved chemical resistance, moisture barrier performance, and mechanical integrity, offering an effective long-term protective solution for concrete sewer assets. The combined experimental and theoretical approach provides key insights into the rational design of durable coatings for harsh environments.