LED-UV Curing

Light-emitting-diode UV sources have gone from curiosity to mainstream on label presses, sheet-fed offset and screen lines within a decade. They cure instantly at a fraction of the electricity, contain no mercury, produce no ozone and put almost no heat into the substrate. They also change the chemistry: an ink formulated for a mercury-vapour lamp may not cure under LED at all.

LED-curable formulations need initiators that absorb at 385–405 nm. TPO’s reclassification as reprotoxic category 1B under CLP in 2025, and its inclusion in the EuPIA Charter exclusion list, removed the industry’s workhorse LED initiator. Reformulation moved to BAPO, polymeric phosphine oxides and multifunctional acylphosphine derivatives that offer the same absorption with far lower migration and no restricted classification.

LED light penetrates deeply and cures from the bottom up, so the surface is the last thing to cure and the first thing inhibited by oxygen. Surface cure depends on radical flux beating oxygen diffusion: high irradiance close to the substrate, amine synergists or, for low-migration grades, polymeric amine-free co-initiators. Yellow and white absorb or scatter near-UV strongly, so opaque white and dense black remain the limiting colours and set the lamp requirement.