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Radical photopolymerization PDF Print E-mail

Since most monomers do not produce enough radical initiators when exposed to UV radiation, it is necessary to add a photosensitive compound which effectively absorbs the incident light and produce free radicals or ionic species with a high quantum yield (the quantum yield of a photochemical reaction is defined by the ratio of the number of molecules converted to the number of photons absorbed). Once initiated, the chain reaction develops as in a conventional polymerization. The whole process can be represented schematically as follows:

A resin formulation photoréticulable has three basic components:

  • A photoamorceur: photolytic release of reactive species to the functional group of the monomer
  • an oligomer telechelique: consisting of a short polymer chain terminated at each end by a reactive function, which in polymerising, give rise to a three-dimensional polymer network
  • a monomer having one or several reactive functions, which allows to adjust the viscosity of the resin and, after curing, will be incorporated into the polymer network.
  • Depending on the type of application considered, various additive have to be introduced in the formulation in view to improve the final properties of the material, such as stabilizers, wetting agents, fillers, pigments, etc..


Photoamorceur

To be effective, a photoamorceur must meet a number of criteria:

  • it must have a strong absorption of light emission used, usually a mercury vapor lamps;
  • excited singlet and triplet states must have a short lifespan (a few nanoseconds) to avoid deactivation (quenching) by molecular oxygen or the monomer;
  • ions or radicals from the excited states must be produced with a quantum yield as high as possible and be responsive to the monomer grouping.

The different photophysical processes that occur after absorption of a photon by a molecule of initiator are shown schematically in Figure 1. Boot speed is given by:



The absorbance depends on the concentration of photoamorceur [PA] and the thickness of the irradiated sample:


There are two main types of photoamorceur, as their releases photolysis radical or ionic species.

Figure 1: different ways of deactivation of an excited photoamorceur molecule