Wood Coatings with UV LED Curing: A Focus on Heat

Light-emitting diodes for ultraviolet curing applications (UV-LEDs) have been commercially available for more than a decade . However , their unique output characteristics work best with newly formulated UV chemistries that take advantage of UV-LEDs ' many benefits. Initially, UV-LEDs were used in applications such as medical device assembly and inkjet printing, but, with increasing energy outputs and lower initial costs, wood-coating applications became commercially viable. Coating heat-sensitive wood  substrates is especially challenging, but recent testing shows that UV-LEDs provide a significant advantage. This paper discusses the characteristics and benefits of UV-LED curing systems for wood coating applications, including recent temperature test comparisons, and reviews formulating strategies for optimizing the performance of UV-LED systems.

As electrons move through a semiconductor device (called a diode), it emits energy in the form of photons. The specific materials in the diode determine the photon wavelengths and, in the case of UV-LEDs, the output is typically in a very narrow band +/-10nm.

UV curing is a photopolymerization process that uses UV energy to change a liquid to a solid. Upon absorption of the UV energy, the photoinitiator (PI) produces free radicals which initiate crosslinking with binders (monomers and oligomers) in a polymerization reaction to cure or solidify the wood coating. UV formulations also incorporate various additives such as stabilizers, wetting agents, adhesion promoters, defoamers and pigments to provide desirable characteristics or color of the cured material.

UV-LED lamps offer significant advantages over traditional UV lamps for wood finishing.

The electrical to optical conversion efficiency of UV-LEDs is much better, plus the instant on/off and no warm-up time results in a combined savings of 70% on electricity. They are a cool source largely due to no output in the infrared range. This reduced heat eliminates complicated cooling mechanisms such as cooling air (quieter operation) and external shutters, and enables applications on heat-sensitive wood substrates such as glued veneers and resinous woods.

Taking a deeper dive into wood coating lines, a significant challenge is the heat generated throughout the process.

Heat is particularly challenging for resinous/oily woods such as pine, fir, spruce and mahogany. When a resinous wood becomes too hot, the resins or pitch come to the surface or "bleed," causing problems with coating adhesion and discoloration.

The use of traditional UV lamps to cure coatings on heat-sensitive substrates such as glued veneers
and resinous woods (such as pine) causes  damage  to  the  substrate and increases scrap rates. UV-LEDs produce significantly less heat, enabling coating of heat-sensitive wood substrates.