Photoinitiator

A photoinitiator is a chemical compound that absorbs light and generates reactive species to start polymerization, crosslinking, or other photochemical reactions without the need for external heat. In photopolymerization, the initiator converts light energy into reactive radicals or ions that propagate chain growth in monomers or oligomers, enabling rapid curing of coatings, inks, adhesives, and dental resins.

Photoinitiators are commonly classified into Type I (cleavage-type) and Type II (sensitizer-type) systems. Type I initiators absorb light and undergo homolytic cleavage to produce radicals directly, examples include benzoin ethers (such as benzoin methyl ether) and α-hydroxyketones (e.g., 2-hydroxy-2-methylpropiophenone). Type II initiators require a co-initiator or photosensitizer to generate radicals; in these systems, the excited photosensitizer transfers energy or hydrogen to a donor, forming initiating radicals. A well-known Type II system uses camphorquinone with amine co-initiators.

Many modern photoinitiators are designed for visible-light curing. Visible-light systems often employ camphorquinone-based Type II mechanisms or acylphosphine oxide-based Type I initiators, which are efficient under LED irradiation and favored for low-heat curing and deeper penetration.

Applications include UV- and visible-light curing of coatings, inks, 3D printing resins, adhesives, and dental composites. Important considerations include initiator efficiency, light absorption spectrum, cure depth, color effects, toxicity, storage stability, and compatibility with monomers and additives.