The formulator's guide to anti-oxygen inhibition additives

Photo-curing has become increasingly popular in wood coating applications relative to thermally initiated processes due to increased curing rate with reduction in solvent and energy requirements. Stressing the last advantage, light emitting diodes (LEDs) utilize less energy and last longer than traditional Hg lamps and are commercially available now in wavelengths below 400 nm. Although photo-curing does have its advantages, an additional difficulty is encountered when this is performed in open-air since molecular oxygen inhibits radical polymerization. This leads to insufficiently cured films that remain tacky at the surface. Although nitrogen gas inerting can be highly successful in excluding oxygen, chemical additives are often preferable to the small and medium-sized enterprise (SME) end user. A variety of additives have been introduced over the last 30 years both in the scientific and patent literature. We have chosen to experimentally reinvestigate the applicability of some of these additives (hydrogen donors, functional monomers, reducing agents, CO₂ producing agents, and singlet oxygen scavengers), applying them to a urethane acrylate base formulation. Curing was performed with LED irradiation and effectiveness assessed by FTIR in transmission mode.