The aim of the project was to characterize installation parameters, raw material properties and properties of the cured coatings in order to optimally combine these parameters and to achieve good coating properties with the still relatively-new UV LED technology.
Due to their very short curing time, coating systems that are UV-cured allow high efficiency in industrial coating processes. As a result, they are often used in paint shops with short throughput times, such as in the parquet and furniture industry or in the graphic arts sector. The development of new light sources in the form of UV LEDs makes it possible to supplement or replace today’s mercury vapor lamps with an even more energy-saving alternative. Further advantages include minimal heat radiation (enabling temperature-sensitive substrates to be coated) and the avoidance of mercury-containing device components.
A spectrally resolving radiometer is best suited for characterizing light sources, especially LED lights. Broadband radiometers are only useful for monitoring the stability of existing installations, especially if these are fitted with Hg lamps.
The absorption spectra of the initiators are accurately described in the respective technical data sheets of the manufacturers. At certain wavelengths, the cleavage products of the initiators absorb UV light more intense than the initiators themselves, thus causing yellowing with some initiator types. Using a 2-component polyurethane clearcoat with low UV absorption, it was possible to unaffectedly study the initiators and their decay products after irradiation.
Below 320 nm, binders and reactive thinners absorb a great amount of UV light. Therefore, these wavelengths do not contribute to curing in depth. In the range of the wavelengths emitted by UV LEDs, binders and reactive thinners are transparent.
The dose required to ensure sufficient curing depends on the type of radiator and on the type of the initiator. Small initiator amounts (1%) resulted in insufficient curing at the surface level, regardless of the irradiation source (oxygen inhibition).
The curing depth depends on the type and concentration of the initiator; higher concentrations tend to result in insufficient deep curing, which can be explained by shadowing by the initiators according to Lambert-Beer.
With one type of initiator, a rise in the glass transition temperature (Tg) as the dose increases was observed at low doses; with other types, the dose had no effect on Tg. Different initiators in otherwise identical formulations resulted in different Tg values, suggesting that the structure of the formed networks also depends on the type of initiator.
Residual C=C double bonds that had not reacted during the UV curing process dissipated rapidly during artificial weathering.
The IGF project 20127N of the research association “Forschungsgesellschaft für Pigmente und Lacke e. V.” was funded by the Federal Ministry for Economic Affairs and Energy via the German Federation of Industrial Research Associations AiF within the framework of the program to promote joint industrial research (IGF) on the basis of a resolution passed by the German Bundestag.
Dr. Katharina Weber, Dr. Rolf Nothhelfer-Richter,
Dr. Norbert Pietschmann
Department of Coating Systems and Painting Technology
Analytics and Material Tests Group and Applied Coating Technology Group