Radiation crosslinking of pigmented coating material by UV LEDs enabling depth curing and preventing oxygen inhibition

LED light sources emit in a small wavelength region, e.g. in the blue light and UV-A. These LEDs can be applied for the radiation curing via radical polymerization, which is a growing sector in the coating industry. LEDs will in prospect substitute mercury arc lamps as the light sources, but development of higher intense LEDs and also in the UV-B or UV-C range is still in progress. In this report the efficiency of initiator systems was analyzed for the photopolymerization of UDMA. The chosen systems are usually used in the coating industry and others were invented in the dentistry and holography. The aim is the depth curing of urethane dimethacrylate (UDMA) with the help of UV LEDs at 365 nm, 395 nm, and 405 nm. The overlap of the absorbance of the initiator systems and the emission of the LED is discussed regarding the reactivity in the top layer and a thickness up to 20 mm hereby giving insights in the influence of the light source and the reactivity or spectral properties of the initiator system on the photopolymerization process. Surprisingly, the initiator systems like Ivocerin, and HABI-mercapto, which are unusual as initiators for the curing of coatings, show a high efficiency. Even pigmented and filled systems are curable up to a depth of 4–20 mm, respectively. Furthermore, applying a UV-C LED at 278 nm and using these initiators also decrease the tackiness on the surface caused by oxygen inhibition.