FUNCTIONAL POLYSILOXANES AS PHOTOINITIATORS FOR UV-CURABLE COMPOSITIONS - EVIDENCE FOR A POLYMER EFFECT

Functional silicone copolymers containing one or several types of pend ant ester groups including a benzophenone or a thioxanthone chromophor e, and/or a tertiary amine were used to prepare novel photo-initiating systems based on the phenone - amine photogeneration of radicals. The influence of the nature and relative amounts of the functional pendan t units on the overall efficiency of the various initiating systems wa s evaluated by measuring the rate of polymerization of an acrylate bas ed composition. By comparing the results obtained from different combi nations of polymeric or low molecular weight reactants, a neat polymer effect inducing a greater initiation efficiency is evidenced, especia lly when a polysiloxane containing only aromatic carbonyl ester groups is associated with free 4-dimethylaminobenzoic esters as hydrogen don ors. Time-resolved spectroscopy performed with thioxanthone functional systems indicates that the favourable effect on the apparent polymeri zation rate is not correlated with the rate of quenching of the triple t excited state by a tertiary amine. The polymer effect can rather be explained by the microheterogeneity of the distribution of the partner s in the complex initiation process.