UV-induced polymerization of urushiol without photoinitiator

Urushiol, the basis of lacquer, can be cured under UV irradiation without any photoinitiator in the presence of air. In this paper, we used a high-pressure mercury lamp as a light source to dry urushiol rapidly and to study the curing mechanism of urushiol under UV irradiation. The surface of the films has been characterized by pencil hardness method, Fourier transform infrared spectroscopy (FT-IR), electron spin resonance (ESR) and elemental analysis. Pencil hardness method showed that the hardness of the films increased with the irradiation time. The oxygen content of the films did not increase considerably with the decrease in unsaturated groups with UV curing time. The peak due to CO at 1722 cm−1 was assigned to urushiol quinones. The ESR spectroscopy detected an intense broad monotonous signal that was probably associated with phenoxy radicals, therefore the spectra indicated the whole change of the film. The results suggested that UV curing of urushiol was different in mechanism from lacquer polymerized by laccase-catalyzed oxidation. It was not an aerobic oxidative polymerization, but initiated by radicals under UV irradiation.