UV Curing

Short answer: UV curing is photopolymerization. A UV-curable ink, coating or adhesive is a liquid mix of small reactive molecules (monomers and oligomers) plus a few percent of photoinitiator.

When the photoinitiator absorbs UV light, it breaks apart into reactive species that link the molecules into a solid, crosslinked network, often within a fraction of a second to a few seconds. There are two main chemistries. Free-radical systems, mostly acrylates, form radicals and stop curing almost as soon as the light goes off, and oxygen in the air can slow them at the surface. Cationic systems, mostly epoxies, form a strong acid and can keep curing after the light is gone, which is called 'dark cure'. The light source matters. Traditional mercury-vapor lamps emit a broad spectrum that includes a lot of infrared heat. UV LEDs emit a narrow band, often near 365-395 nm, and produce much less heat, but the formula has to include a photoinitiator that absorbs at that wavelength. Because there's little or no solvent to dry off, UV curing is fast and usually uses less energy than heat drying. The trade-off is that the light has to reach the material, which is why UV-cured pipe liners need translucent glass-fiber tubes (see CIPP).