The chemical photoinitiators are sensitive to UV light, which changes the chemical bond structure of the photoinitiators, forming free-radical groups that trigger resin cross-linking. Curing happens in a 2-step sequence; first a photoinitiator absorbs UV rays and forms free radicals. These interact with resin molecules to form resin free radicals, then the small amount of heat from the infrared (IR) component in UV lamps accelerates the polymerization crosslinking reactions of the resin molecule free radicals. This IR heat is minimal due to the brief dwell time of parts in the UV cure zone, but it is enough to give a fully-cured coating. Some radicals often remain for a brief time (1-2 minutes) after UV exposure which give a minor degree of added postcuring to the film. Abrasion, mar, and scratch resistance of UV coatings are therefore excellent.

UV coatings may or may not require solvent (or other fluidizing media) to reduce their viscosity and promote flow-out. If solvent is used, a flash-off time is allowed after application prior to UV cure. If the fluidizing media is also a cross-linker, it is called a “reactive diluent”. For reactive diluents, no flash-off time is required since they become part of the cured film. Rapid, extensive resin cross-linking can be initiated with UV light, so that often extremely low-molecular weight resins with very low viscosities are possible in the coating formulation. For this reason, the UV coating cures to a more stress-free and smoother film with less orange peel than possible with most heat-cured coatings. The UV resins may flow out so well by themselves that little solvent is required, allowing a low VOC coating, and in some cases even a zero VOC coating. The minimal solvent content in UV coatings results in only minor shrinkage from wet to dry film and considerable less induced film stresses compared to 2-component forced-curing and heat-curing coatings. Forced heat-curing coatings most often contain 40% or more solvent content.

UV curing is fast – usually in 10-60 seconds, which permits UV ovens to be confined and compact, and which enables faster production rates than cure methods that require substantial oven dwell times. The quick cure also minimizes substrate heating, which is a great advantage when curing films on heat-sensitive substrates such as printed circuit boards, wood, and many thermoplastics. Since the UV lamps (usually mercury vapor) become rather hot, it is necessary that they be turned off whenever the production line stops to avoid harming the product being coated. In the past, many UV lamps could not be restarted quickly once they had been turned off. Fortunately, modern technology irradiator systems utilize fast “on-off” UV lamps that cool off rapidly to enable starting and stopping the coating line quickly.

Cure by UV is accomplished in shielded and enclosed chambers saturated with high intensity electrically generated UV light. For total curing to take place, the UV light must activate all of the photoinitiator molecules, which means that the light must “see” them. This is fine for unpigmented coatings, but only about 1-2 mil dry film thicknesses of pigmented coatings can be UV cured because the pigment molecules will block UV light from some of the photoinitiator.

The energy of UV light decreases with the square of the distance between the light source and the surface receiving the light. So doubling the lamp to paint distance drops UV light intensity to 25%; tripling the distance cuts the light intensity to just over 10%. Therefore, the UV light source must be kept close to the painted part. For this reason, UV cure is ideally suited for use on flat surfaces which can be kept very close to the light source. However, highly polished parabolic reflectors enable a variety of 3-dimensional components to be UV-coated. These include metal wall emblems, golf balls, guitar bodies, and decorative plastic items.