Photoinitiator.

The component of a UV ink or varnish that absorbs ultraviolet light and turns it into the free radicals or cations that crosslink the resin; without it the ink sits under the lamp as a liquid. It absorbs over a defined band of wavelengths, which is the detail that catches shops moving from mercury lamps to LED: an ink is matched to the lamp it was formulated for, not to UV in general, and the wrong pairing leaves it uncured under a lamp that looks like it is working. Photoinitiator is a small share of the formula by weight, and free-radical types are robbed of radicals by atmospheric oxygen at the exposed surface, which is why a print can be hard underneath and tacky on top. That soft skin is easy to walk past on a fast line, it offsets onto whatever gets stacked on it, and the job fails a rub test at the customer and comes back as a reprint the shop pays for.