Photoinitiators

Photoinitiators are chemical compounds that absorb light and generate reactive species capable of starting polymerization or crosslinking reactions. They enable rapid curing of UV- or visible-light‑curable resins used in coatings, inks, adhesives, and dental materials, allowing processing at ambient temperatures and on demand with light exposure.

Photoinitiators are commonly categorized by their activation mechanism. Type I initiators undergo cleavage upon light absorption to form radicals directly. Type II systems require a co-initiator, typically an amine, and rely on energy or electron transfer from a photosensitizer to produce initiating radicals. In practice, many formulations combine a photoinitiator with a co-initiator to optimize curing speed and efficiency, especially under visible light.

Chemistries and representatives include α-diketones and related carbonyl compounds (often used in Type I systems), benzoin ethers, camphorquinone used with amine co-initiators in dental resins, and acylphosphine oxides (such as TPO) that are valued for strong initiation and visible-light activity. The choice of photoinitiator depends on the light source, desired cure depth, and the monomer system, with common formulations tailored for UV lamps or LED emitters.

Applications span UV- and visible-light-curable coatings, inks, and adhesives, as well as dental composites and 3D-printing resins (stereolithography and Digital Light Processing). Important performance factors include the absorption spectrum and matching light source, initiation quantum yield, diffusion of radicals, monomer compatibility, and resistance to oxygen inhibition. Safety and environmental considerations address potential toxicity, odor, and color changes, leading to ongoing development of lower-toxicity, less yellowing, and more efficient initiators.