Raw Material
Selection for UV/LED Until recently, UV curing applications required the use of high power mercury (Hg) lamps to provide the energy and wavelength distribution necessary for effecting complete cure of UV formulations (e.g., inks, coatings, adhesives and composites). Traditional photoinitiators work well with mercury lamps due to many peak wavelengths in a typical Hg spectrum being matched to the many possible photoinitiator choices available commercially and these lamps continue to do the bulk of work in the industrial UV world.
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ver the past ten years, development of high power UV/LED lamps has provided another option for UV curable applications. UV/LED lamps are more energy efficient, quieter and easily repositioned, provide ‘instant on/off’ capability and last much longer with little decay of the lamp output. They are different from mercury lamps in one key respect: spectral output is very narrow and centered on the specific LED chosen, most commonly 395, 385 or 365 nm lamps. Work continues to move toward other high intensity wavelengths, but for now, UV/LED requires careful formulation with fewer photoinitiator choices. Significant work in RAHN’s UV laboratories has quantified, and optimised to some extent, both photoinitiator and oligomer choices that will work with commercial UV/LED lamps. (This work has been limited to 395 and 385 nm lamps.) It is the purpose of this communication to summarize the highlights of that work in order to guide material choices for use in UV/LED curing. Of critical importance is to understand the assessment paradigm within which evaluations were done. The majority of work was completed with high power 15 W/cm2 lamps. Emphasis of this work was on speed of cure, i.e., how fast printed or coated substrate could move under the lamps while still achieving full cure (mar-free surfaces). Work was done in two phases, one emphasising photoinitiator choice and levels and the other focused on oligomer choices. The ‘lock and key’ nature of photoinitiators – i.e., creation of adequate free radicals only occurs when a PI absorbs light of specific wavelength(s) – limits PI choices when the lamp output spectrum is narrow. Based on a 395 nm UV/LED lamp, DETX/ITX, TPO, EMK, TPO-L, BAPO and BDMM are the most logical photoinitiator candidates, but the quantum efficiency of those products is not addressed simply by choosing absorption spectrum. Combinations of photoinitiators can participate in radical transfer reactions that sometimes prove more efficient than one PI alone, but the efficacy of those combinations can change from formula to formula. Furthermore, required PI levels can vary significantly from clear coatings, where 3-4 per cent PI may be adequate, to black or white inks where 15-20 per cent PI may be required. 54 Industry Europe
RAHN oligomers were tested in a clear formulation that was held constant across all experiments and applied to white card stock at a film thickness of 12 microns. Photoinitiators were TPO at five parts and DETX at 0.1 part. Drawdowns were assessed for cure by both ‘thumb twist’ and ‘fingernail scratch’ qualitative assessments to determine the maximum speed at which full cure was reached. Nearly all products cured faster at 385 nm versus 395 nm and lamp distance, while possibly optimal around 25-35 mm, accounted for little variation in cure response. Among the several dozen oligomers assessed, results clearly followed one specific trend: greatest curing efficacy is realised when oligomers combine moderate to high amine value with moderate to
