Photo-curing composite paint containing urushi (Oriental lacquer), and wrinkled coating caused by phase separation

To improve the production of traditional lacquerwares by urushi (Oriental lacquerwork) through accelerated drying, we mixed photo-curing monomers with urushi lacquer. We used commercial monomers bearing epoxy, oxetane, or vinyl ether functional groups in conjunction with a photo-cationic initiator [bis(4-tert-butylphenyl)iodonium hexafluorophosphate] and a high-pressure mercury lamp as a UV source. Even in the presence of a large amount of urushi (50 wt% of raw urushi content in the composite paint), photo-cationic polymerization proceeded and gave cured films within a few hours. Photo-differential scanning calorimetry showed negligible retardation of the reaction rate and conversion. Whereas efficient exposure (3.2 J/cm2) gave cured paint films with glossy smooth surfaces, less exposure unexpectedly gave lusterless surfaces. Microscopy revealed phase separation and wrinkling of the surface with minute ripples, several tens of micrometers in width and height. The outer surface consisted of cross-linked hard polymer, but the inner part under the ripples consisted mainly of monomeric urushi components that solidified slowly. A photo-mask for UV exposure could be used to create a pattern based on variations in the gloss of the paint surface.