Thioxanthone compounds are widely used to photoinduce the free radical polymerisation and curing of monomers and prepolymers. Compared with many other aromatic ketone photoinitiators these compounds exhibit extended absorption in the near UV-Visible region of the electromagnetic spectrum (380-420 nm) and consequently are effective for the curing of pigmented coating systems. The efficiency of the thioxanthone chromophore is dependent upon a number of factors. These include the nature of the substitution, environment (monomer/prepolymer) and the presence of an amine cosynergist. The substitution may include other variables such as water solubilising groups or polymeric structures while the latter is an inherent drawback from the complexity and cost-efficiency of the final formulation. However, the presence of an amine cosynergist is often crucial with regard to the nature of the overall curing mechanism. Radical or ions derived from the amine play a major role in the mode of action and efficiency of the thioxanthone chromophore. Over the last twenty years thioxanthone photochemistry has come a long way in many of these developments and this article will hopefully shed light on old and new developments in the field.
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