Photopolimerisation

Photopolimerisation, also spelled photopolymerisation or photopolymerization, is a chemical process in which monomers and oligomers cross-link to form a solid polymer network upon exposure to light, typically in the ultraviolet or visible range. The process is driven by photoinitiators that absorb light and generate reactive species, such as free radicals or cations, which initiate polymer chain growth and subsequent propagation.

There are two main mechanisms: radical photopolymerisation and cationic photopolymerisation. Radical systems are common with acrylates and methacrylates and use initiators that produce radicals upon light absorption; these may be Type I initiators that cleave to form radicals or Type II initiators that generate radicals through interaction with a co-initiator. Cationic photopolymerisation, used with epoxies and vinyl ethers, proceeds via positively charged reactive species. Oxygen can inhibit radical polymerisation by scavenging radicals, affecting surface cure and layer uniformity.

Key parameters include the wavelength and dose of illumination, the type and concentration of photoinitiator, the monomer/oligomer formulation, and the presence of inhibitors. The penetration depth of light and the diffusion of reactive species influence cure depth, conversion, and potential shrinkage stress.

Applications span UV- and visible-light curable coatings and inks, dental resins and adhesives, photo-patterning, holographic recording materials, and various forms of 3D printing (such as stereolithography and digital light processing). Advantages of photopolimerisation include rapid curing at ambient temperature and spatial control through patterned illumination, while limitations involve oxygen inhibition, shrinkage and stress, limited penetration in thick layers, and the need for compatible light sources and safe handling of initiators.