Primary processes occurring in the excited state of a UV radical photoinitiator

Primary processes occurring in the excited state of a UV radical photoinitiator. (Adapted from Jablonski, A., Z. Physik, 94, p. 38 (1935).) 

The result of a photochemical reaction involving monomers, oligomers, and polymers depends on the chemical nature of the material, wavelength of the light, and other components of the system. Ultraviolet, visible, and laser light can polymerize functional monomers, cross-link polymers, or degrade them, particularly in the presence of oxygen.  

There are essentially two types of compounds that are used in the UV curing process to absorb the light and generate reactive species. These are photoinitiators and photosensitizers. 

photosensitizers are useful mainly by being capable of extending the spectral sensitivity of certain photoinitiators under specific conditions. 

In the UV curing process, photons from the UV source are absorbed by a chromophoric site of a molecule in a single event. The chromophore is a part of the photoinitiator. The light absorption by the photoinitiator requires that an emission light from the light source overlap with an absorption band of the photoinitiator. 

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