UV Curing

A UV ink or coating contains no water and no solvent. It is a liquid of oligomers, monomers, pigment and photoinitiator that becomes a cross-linked solid the moment it passes under the lamp. Solids are almost 100 %, so nothing evaporates and almost no volatile organic compound is released on cure.

A medium-pressure mercury lamp emits a broad spectrum from 200 to 450 nm with strong lines at 254, 313 and 365 nm, plus visible light and a great deal of infra-red. The short-wave lines cure the surface: they are absorbed in the top micrometre and overcome the oxygen inhibition that would otherwise leave the surface tacky. That is why a conventional UV ink cures surface-first under mercury and why it may not cure at all under LED.

Cure is a dose, not a switch. Under-cure leaves unreacted monomer and photoinitiator in the film, which is a migration problem on food packaging and an adhesion and resistance problem everywhere else. We specify cure by measured dose and provide on-press cure checks with every data sheet.