The State of UV-LED Curing: An Investigation of Chemistry and Applications

Light-emitting diodes for ultraviolet-curing applications (UV-LEDs) have been commercially available for nearly 10 years. However, their unique output characteristics require newly formulated UV chemistries in order to take advantage of UV-LEDs’ many benefits. This paper discusses the characteristics of UV-LED lamps; the importance of properly formulating chemistries; the benefits to end-users; commercial applications of UV-LEDs; and future expected developments.

The benefits of UV-LED as compared to traditional mercury-arc UV lamps are numerous and significant.

Traditional UV arc lamps produce UV energy by generating an electric arc inside an ionized gas (typically mercury) chamber to excite atoms, which then decay and emit photons. The emitted photons cover a broad range of the electromagnetic spectrum, including some infrared and even visible light. Only about 25% is in the safer UV-A range.

A UV-LED generates UV energy in an entirely different way. As an electric current (or 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 wavelengths of these photons and, in the case of UV-LEDs, the output is typically in a very narrow band +/- 20 nm. The wavelength is dependent on the band gap between excited state and the ground state of the semiconductor material. 

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 that initiate crosslinking with binders (monomers and oligomers) in a polymerization reaction to cure or solidify the ink, coating or adhesive. 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.

The benefits of UV-LED as compared to traditional mercury-arc UV lamps are numerous and significant. UV-LEDs are more environmentally friendly because they do not generate ozone and contain no mercury as arc lamps do. They are a cool source compared to arc lamps, largely due to no output in the infrared range. This reduced heat eliminates complicated cooling mechanisms such as chill rolls and external shutters, and enables applications on heat-sensitive substrates. The electrical-to-optical conversion efficiency of UV-LEDs is much better and the ability to instantly turn the unit off and on enables saving about 50-75% on electricity.