photoinitiation

Photoinitiation is the initiation step in photopolymerization reactions driven by light. When a photoinitiator absorbs photons, it generates reactive species such as free radicals or cations that begin the polymerization of monomers, often producing crosslinked networks in coatings, inks, adhesives, or dental resins.

There are two general classes: Type I photoinitiators undergo homolytic cleavage upon light absorption to form radicals directly, whereas Type II systems rely on a co-initiator that reacts with the excited photosensitizer to produce initiating radicals. Sensitizers can extend absorption into visible light, enabling curing with blue or green LEDs and reducing the need for UV radiation.

The efficiency of photoinitiation depends on the photoinitiator's light absorption spectrum, quantum yield, and the monomer's reactivity. Oxygen can quench radical species, reducing cure speed, while diffusion limits cure depth in thicker films. Depth of cure is enhanced by higher initiator loading, more efficient initiators, and the use of suitable sensitizers.

Common photoinitiators include benzoin ethers and related α-hydroxy ketones, acylphosphine oxides, and thioxanthones. Type II systems, such as camphorquinone with an amine co-initiator, are widely used in dental resins. Visible-light systems using blue LEDs are increasingly favored to reduce UV exposure and improve safety and handling.

Applications span UV- and visible-light curing of coatings, inks, 3D printing resins, and dental materials. Advantages include rapid curing at ambient temperatures and spatial control through light exposure; limitations involve oxygen inhibition, potential yellowing, and the need for compatible light sources and initiators.