Light-Curing Technology Series — Light Source 2

UV LEDs, or ultraviolet light-emitting diodes, are a new type of light source that, compared with traditional medium-pressure mercury lamps and electrodeless lamps, offer advantages such as longer lifespan, lower heat generation, reduced energy consumption, and flexible configuration options.

Although UV LED light sources offer numerous advantages, they currently also have certain drawbacks that are not easily overcome:

1. Severe oxygen inhibition: Conventional mercury‑lamp light sources emit across the ultraviolet, visible, and infrared spectra, with UV radiation in the 200–350 nm range playing a critical role in counteracting oxygen inhibition. In contrast, UV LED light sources have a much narrower spectral distribution, and the most common wavelengths—365 nm, 385 nm, 395 nm, and 405 nm—are matched by relatively few photoinitiators. Consequently, oxygen inhibition remains one of the most challenging issues in current LED‑based formulations, particularly in industries where yellowing is a critical concern.

2. High cost: At present, UV LED light sources generally command a significantly higher price than mercury lamps. This is not only due to the high manufacturing costs of UV LEDs but also stems from technological monopolies held by upstream manufacturers. Patent protection restricts large-scale production, further driving up prices.

3. Limited selection of UV LED photoinitiators: For today’s mainstream UV LED light sources, the most suitable photoinitiators include TPO, 819, ITX, and DETX. Traditional short-wavelength photoinitiators such as 1173 and 184 deliver suboptimal performance under current UVA LED irradiation.

4. Yellowing issue: Most photoinitiators currently available for UVA curing exhibit some degree of yellowing. TPO shows relatively limited yellowing, but its resistance to oxygen inhibition and polymerization is poor. Adding an amine co-initiator can mitigate the impact of oxygen inhibition on surface drying to a certain extent, yet it often introduces significant yellowing.

In recent years, as the COVID‑19 pandemic has spread, UVC disinfection technology has gained increasing recognition and seen intensified R&D efforts. With advances in UVC technology, current UVC irradiance fluxes can now exceed 200 mW (e.g., Nichia’s NC4U334BR—280 nm). The 250–310 nm wavelength range corresponds precisely to the absorption peak of conventional short‑wavelength photoinitiators (such as 1173 and 184), and the oxygen‑induced inhibition of polymerization is significantly mitigated when these photoinitiators are employed.

Advances in UVC LED technology have opened up new possibilities for its application in photopolymerization. The growing variety of photoinitiators now available, coupled with the use of short-wavelength photoinitiators that enhance surface drying while reducing yellowing, is driving progress. Amid the pandemic, as leading UV‑LED manufacturers continue to ramp up R&D investment and expand production capacity, costs are expected to decline gradually.

Due to the significant attenuation of UVC LEDs in air, they are currently limited to certain specialized photopolymerization applications—such as UV flatbed printing and label printing—where the light source is positioned close to the coating. As UVC LED power continues to increase, UVC and UVA LEDs will increasingly be able to replace traditional mercury lamps, and we expect them to find broader adoption across many photopolymerization sectors in the future.

About UVC   Applications and Development of LEDs in Photocuring ， From an application‑oriented perspective, we have several points to consider, which will be elaborated in the next related article.