Capsanthin/Bismuththiol/Epoxidized Soybean Oil/Eugenol: A Green Synthesis of Biobased Antifungal Coatings for Wood under LED Irradiation

2,5-Dimercapto-1,3,4-thiadiazole (DMTD) was used as a cross-linking agent and photoinitiator in conjunction with capsanthin and iodonium salt to construct a photoinitiating system suitable for UV-LED irradiation. In a fully green synthesis method, this photoinitiating system enables the thiol-epoxy polymerization and thiol-ene polymerization of biobased monomers, such as epoxidized soybean oil and eugenol, with final conversion rates exceeding 70%. The structure of the prepared coating becomes denser with increasing DMTD content, leading to improved hydrophobicity, wear resistance, thermal stability, and hardness. At the optimal DMTD content of 20 wt %, the coating EP20 exhibited an adhesion level of 0, an Aspergillus niger (A. niger) infection value of 1, and a water contact angle of 109.7°. The addition of eugenol further enhanced the coating’s antifungal properties, reducing the A. niger infection value to 0. This approach provides significant guidance for achieving sustainable development through “sustainable (biobased prepolymers) + eco-friendly (bioinitiating systems) + green (environmentally friendly curing processes)”.