Photoinitiators are small molecules that are sensitive to light. Upon light absorption they undergo photochemical cleavage to produce reactive species (either free radicals or a Bronsted or Lewis acid) that will interact with the active components in formulations. In other words, one could say that photoinitiators take the energy from light and transform it into chemical energy to induce chemical reactions. This process where a liquid formulation is transformed into a cross-linked polymer induced by a photoinitiator and its reaction with light is called photopolymerization (also known as radiation curing).
There are 2 classes of photoinitiators: Type I and Type II
- Type I photoinitiators are those that undergo unimolecular bond cleavage after absorption of light to render the reactive species. No other species are necessary in order for these photoinitiators to work.
- Type II photoinitiators undergo a bimolecular reaction. After absorption of light, the photoinitiator reaches excited state from which reacts with another molecule (co-initiator or synergist) to create the reactive species.
Regardless of the photoinitiator used and for a successful cure, it is extremely important that the absorption bands of the photoinitiator will overlap with the emission spectrum of the light source used for curing. Also, there should be minimum competition for light absorption from other species in the formulation. A poor match between absorption bands and light emitted by the source will result in a poor cure or no cure at all.
As mentioned above, the photopolymerization process can undergo two different mechanism: Free radical or cationic. Due to the different nature of the reactive species formed in each process, it is also key to choose the photoinitiator capable to induce the right kind of photopolymerization.
Free radical photoinitiators are used in the photopolymerization of acrylate or styrene based resins. This is probably the most common type of photopolymerization used. The radiation curing can be performed using UV or Visible light, and in some instances, near IR photoinitiators have been developed (check our H-Nu Blue and H-NU near IR series). 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.
A main drawback of free radical photopolymerization is the susceptibility to oxygen inhibition.
Cationic photoinitiators are used to initiate the photopolymerization of epoxy resins. Just as the free radical process, the curing process can be carried out with UV or visible light. Unlike free radical, the photopolymerization process in cationic systems continues even when the irradiation of light has stopped. Cationic systems are free of oxygen inhibition (unlike free radical systems), but a drawback could be the longer times required for the cure to take place.
In conclusion, proper selection of photoinitiator is key to obtain the perfect cured photopolymer regardless of the mechanism used.