LED sources are rapidly gaining in popularity in the UV-cured coatings market as they offer reduced cost, longer life and environmentally-friendly alternatives to conventional lamps. Transitioning to UV LED, however, often requires more than just a simple equipment change. Modifications to the chemistry also might be needed to effectively compensate for the lower energy levels and narrower wavelength range. This is particularly true in coatings applications, where oxygen inhibition can have a detrimental effect on cure properties. In this paper, we will examine how proper photoinitiator selection and concentration can be used to optimize the performance of LED-cured coatings.
Poor surface cure, due to oxygen inhibition, is one of the most challenging aspects associated with LED-cured coatings. Oxygen, in its ground state, has a “diradical” nature and is highly reactive toward radical species. As a result, oxygen can scavenge radicals to form less reactive peroxy compounds, which can terminate the growing chain via radical-to-radical interaction. The result of oxygen inhibition is observed as a decreased rate of polymerization and, ultimately, compromised coating performance.
Increased energy is another way of improving curing of coatings under UV LED. Upon inception, LED lamps were limited in the amount of energy they could produce, but as the technology evolved, higher energy lamps capable of producing more free radicals and faster cure speeds were developed. While this significantly improves surface cure, the higher rate of polymerization can have a detrimental effect on depth of cure, depending on the photoinitiator package the formulator has chosen. This is particularly true with thicker coatings, where the poor depth of cure can lead to poor coating performance in the field.
Similarly, increasing the concentration of photoinitiator in the coating allows for more free radical formation, which, in turn, provides for better through-cure. Depending on the type of photoinitiator chosen, this approach can have a detrimental effect on depth of cure, as well as obvious economic implications. Care also should be taken that the concentration of free radicals produced does not exceed available sites, as this could result in a reduction of cure speed.
One variable that has yet to be completely explored is the effect of the type of photoinitiator used in the formulation. Formulators frequently use combinations that have succeeded in the past, but photoinitiator performance under conventional UV lamps is not necessarily indicative of how it will do under UV LEDs. In some cases, particularly thick coating applications, entirely new combinations that work using a completely different mechanism can produce significantly better results. In this paper, we will evaluate not only how the concentration of photoinitiators can affect coating performance but, more importantly, why the choice of material also is critical.