Photoinitiators are small molecules that are sensitive to light. Upon light absorption, they undergo a photoreaction and produce reactive species.They initiate or catalyze chemical reactions that result in significant changes in the solubility and physical properties of suitable formulations. Therefore, a photoinitiator is a compound that can transform the physical energy of light into appropriate chemical energy in the form of reactive intermediates. In polymer chemistry, the process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks is called polymerization.
Catalysis: application of a photoinitiator
Photophysics: direct absorption of light
The photoinitiator absorbs one quantum of light of the energy E = hn.
The molecule is promoted by this energy from its ground state (SO) into the short-lived lowest excited singlet state (S1).
The excited singlet state, S1, undergoes intersystem crossing to the longer lived triplet state, T1.
For an efficient photoinitiator, other deactivation pathways of the S1 state, such as fluorescence or thermal deactivation, are negligible: the quantum yield for T1 formation is almost unity.
The excited state of the photoinitiator can also be populated by a sensitization process.
Benefits of polymerization include:
- Able to cure at lower temperatures
- Limits amount of VOCs
- Faster than thermal curing
- Offers more control over curing process
The mechanism of photopolymerization process should be divided into two types: free radical and cationic. Free radical generating photoinitiators are used to cure through double bonds. They are usually used within styrene-based or acrylate-based formulations for polymerization and have a strongly developed product line with many different applications. This is probably the most common type of photopolymerization used. The photopolymerization process immediately stops once the irradiation ceases. For this reason, it is extremely important to know the exposure time required to obtain a fully cured photopolymer. Free radical photopolymerization tends to be a “faster cure” compared to cationic photopolymerization. Cationic photoinitiators are often used for formulations with epoxy or certain resins, unlike free radical, it can continue curing after being exposed to the light and is unaffected by oxygen. This allows for highly efficient curing times and can be an advantage over free radicals during the photopolymerization process. Cationic photoinitiators are widely used in printing ink and overprint varnishes such as can coating, industrial metal coating, coil coating.
Photoinitiators offer a lot of advantages to the curing and formulation processes. Their versatility of attributes and applications make them highly useful products. Okchem has efficient supply chains and offer competitive pricing with professional technical support. If you have any questions or concerns about photoinitiators or would like a complete literature, samples or quotes, please do not hesitate to contact us.