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S. Ligon, B. Husar, H. Wutzel, H. Hoffmann, J. Torgersen, R. Liska:
"An overview of anti-oxygen inhibition strategies for LED-based wood coatings";
Talk: RadTech Asia 2013, Shanghai; 2013-05-20 - 2013-05-24; in: "Guide Book", (2013).

Photoinduced polymerizations are often preferred to thermally initiated processes due to increased speed and spatial resolution with reduction in solvent and energy requirements. Stressing the last advantage, light emitting diodes (LEDs) utilize less energy than traditional Hg lamps and are available in wavelengths appropriate for wood coatings. LEDs are much more efficient light sources than Hg lamps but since their spectral output is also narrower, photoinitiators with well overlapping absorbances must be chosen. For wood coating formulations photocured in open air, an additional difficulty is encountered 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 providing proper cure, the method is impractical for most SME end users. Chemical additives which combat oxygen inhibition are preferred instead. Efficiency of cure in urethane acrylate coatings can be quantified by monitoring functional group specific IR absorbance bands. With a rapid MCT detector, multiple scans are collected every second to allow real time (RT) double bond conversion. Final double bond conversion is dependent on a variety of parameters including sample thickness, viscosity, and intensity of light. Without adjusting these parameters, double bond conversion can be increased by employing chemical additives that reduce the degree of oxygen inhibition. A variety of additives have been introduced both in the scientific and patent literature. We have chosen to experimentally reinvestigate the applicability of such additives (hydrogen donors, singlet oxygen scavengers, molecular inerting agents, thiol-ene chemistry, and others) for LED based coatings. Bulk cure was monitored by FTIR in transmission mode and the final surface cure by ATR-IR. Coupling of the RT-IR with the LED power source through a multichannel relay trigger allows precise control of multiple LED lamps for wavelength specific reactions.

Wood Coatings, Oxygen Inhibition, Photo Polymerization, Acrylates