Photocurable coatings with self-initiating and self-healing properties via dynamic disulfide bonds

In the field of photocurable coatings, disulfide-containing monomers are gaining importance due to the unique properties and responsiveness of S–S bonds, which make them suitable for various applications, including photoinitiator-free and self-healable materials. In this work, a photocurable disulfide-containing polyurethane diacrylated monomer (DS-IPDA) was synthesized through a simple two-step addition reaction. First, the initiating ability of thiyl radicals, which are photogenerated by cleavage of disulfides by irradiation at wavelengths around 250 nm, was demonstrated by investigating the photopolymerization kinetics of the monomer in the presence or absence of a photoinitiator by FTIR analysis. Secondly, the dynamic nature of disulfide bonds and hence the self-healing ability of the coatings was assessed. Clear coatings were obtained by photocuring DS-IPDA alone or together with polyethylene glycol diacrylate at various weight ratios under a broadband (200 – 600 nm) ultraviolet (UV) light or under a UV LED light at 365 nm. The latter is safer and more energy efficient than traditional UV sources and helps preserve a higher content of unreacted disulfide bonds in the cured coatings for further use in dynamic covalent reactions. However, it requires the presence of a photoinitiator in the photocurable formulation. The disulfide exchange reaction proved effective in repairing surface damages after quick heat treatment. The incorporation of disulfide bonds imparting self-initiating or self-healing properties to coatings thus may contribute to improved sustainability reducing the number of components in the formulation by avoiding the use of potentially migrating photoinitiators and by extending product lifetime.