Question

Explain why photoinitiators are needed in most commercial VP resins. Explain what these photoinitiators do. Please summarize the answer

Answer 1

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.Some small molecules in the atmosphere can also act as photoinitiators by decomposing to give free radicals (in photochemical smog). For instance, nitrogen dioxide is produced in large quantities by gasoline-burning internal combustion engines. NO₂ in the tropospheregives smog its brown coloration and catalyzesproduction of toxic ground-level ozone. Molecular oxygen (O₂) also serves as a photoinitiator in the stratosphere, breaking down into atomic oxygen and combining with O₂ in order to form the ozone in the ozone layer.