Self-matting waterborne polyurethane acrylate wood coating by 222 nm far-UVC irradiation in ambient air

Self-matting coatings, featuring micro-winkles for low gloss and a soft tactile feel, were obtained by employing 222 nm far-UVC radiation emitted by a Krypton-Chloride excimer lamp applied to UV-curable waterborne polyurethane acrylate wood coatings. The 222 nm far-UVC penetrates the wet film's top layer (a few micrometers) to induce C--C to & sdot;C- C & sdot; biradical, participating in the free radical polymerization process and resulting in the top layer completing gradient curing first. Subsequently, the thicker lower layer begins gradient curing due to the radical produced by the photoinitiator, and volume cure shrinkage induces internal stress, resulting in the formation of micro-wrinkles. This study systematically investigated the effects of varied photoinitiator concentrations (2.0 wt% and 3.0 wt%), film thicknesses (60 mu m and 110 mu m), and curing temperature (low: 28.3 degrees C, high: 37.4 degrees C) on the coating films' properties. At low curing temperature, the optimal matte film is characterized by gloss values of 3.6 GU at 60 degrees and 5.6 GU at 85 degrees. When maintaining consistent coating thickness and curing temperature, gloss values increased with higher photoinitiator concentration, while roughness values decreased. Conversely, with the same photoinitiator concentration and curing temperature, gloss values decreased as coating film thickness increased, while roughness values increased. At high curing temperature, the coating film exhibits the best matte effect with gloss values of 2.8 GU at 60 degrees and 3.6 GU at 85 degrees. Film gloss values experienced a slight reduction with increased film thickness or photoinitiator concentration. Scanning electron microscope observations indicated that coating films cured at high temperature displayed higher micro-wrinkles with folding than films cured at low temperature. This study demonstrates that self-matting coating films exhibit favorable physical and mechanical properties, and the curing process can occur directly in ambient air. The utilization of 222 nm far-UVC irradiation holds the potential for the large-scale production of matte coatings for wooden products.