Photopolymerization thick samples

Fouassier Rabek, 1993, show that there is an optimum initiator concentration for the efficient photopolymerization of thick samples (Fig. 10). As the initiator concentration is increased, the initiation rate at the surface is increased, but the rate of propagation of the front through the sample is decreased. 

It is interesting that there is little change in the time required to cure the polymer as the concentration of BP is increased from 0.2 to 1.0 wt.%. These results suggest that the reaction proceeds to completion even before the light penetrates deep into the sample, perhaps by the propagation of a thermal front created by the initial photopolymerization at the leading surface. In any case, these results illustrate that the addition of a thermal initiator drastically reduces the polymerization time for thick systems. 

Clarke and Shanks have examined the influence of sample thickness on the benzoin photoinitiated polymerization of butyl acrylate. They found that as the photoinitiator concentration increases so the extent of polymerization become less susceptible to changes in sample thickness. Grauchak et al. have successfully photopolymerized acrylic monomers in polyamide matrices with aromatic carbonyl compounds. In the photocycloaddition of olefins to poly(4,-vinylbenzo-phenone) and its copolymers with styrene, the rate of addition was found to be independent of the glass transition temperature suggesting that large-scale molecular motion is unimportant in this photoreaction. 

As a result, O Brien and Bowman developed a comprehensive photopolymerization model. It incorporates heat and mass transfer effects, diffusion-controlled propagation and termination, and temporal and spatial variation of species concentration, temperature, and hght intensity. This model is applied to systems with varying diermal and optical properties. The absorbance of the polymerizing system is varied by altering either the initiator concentration, sample thickness, or molar absorption coefficient of the initiator. Based on simulations they concluded that the choice of initiator and sample thickness limits the initiator concentration usable to achieve complete monomer... 

In this respect, it should be mentioned that dual-cure systems, combining UV irradiation and a thermal treatment, have been developed to address the issue of the lack of cure in shadow areas of 3D objects, as well as in thick pigmented samples." " A few centimeter thick nanocomposite materials can be produced by this two-step process which associates the photopolymerization of the acrylate resin and the thermally-induced polyaddition of isocyanate and alcohol groups ... 

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