Laser-induced curing

A critical and most important problem that still remained to be solved was to find an adequate analytical methcxl that would enable us to follow in real time the kinetics of such ultra-fast polymerizations. Different techniques have already been used to study the laser-induced curing of acrylic monomers (DSC, IR spectroscopy, interferometry,...) but none of them provide conversion versus exposure time curves for polymerization that occur in a fraction of a second. The results obtained by some of these techniques are now briefly reported for comparison with those obtained by using real-time infra-red (RTIR) spectroscopy, a powerful method newly developed in our laboratory to investigate the kinetics of ultrafast photopolymerizations (0. 

The objective of the present work was to determine the influence of the light intensity on the polymerization kinetics and on the temperature profile of acrylate and vinyl ether monomers exposed to UV radiation as thin films, as well as the effect of the sample initial temperature on the polymerization rate and final degree of cure. For this purpose, a new method has been developed, based on real-time infrared (RTIR) spectroscopy 14, which permits to monitor in-situ the temperature of thin films undergoing high-speed photopolymerization, without introducing any additive in the UV-curable formulation 15. This technique proved particularly well suited to addressing the issue of thermal runaway which was recently considered to occur in laser-induced polymerization of divinyl ethers 13>16. 

Stereolithography (SL) uses a semp similar to SLS, it, a computer-assisted laser source over a platform. In SL, though, the laser is used to induce curing of a liquid layer of polymer resin via photopolymerization. The platform then moves downward together with the first cured layer, allowing for fresh liquid resin to flow over the first cured layer. The process then repeats, with each new layer cured onto the previous layer. Once the SL process is complete, the scaffold is rinsed in solvent to remove the uncured resin. Then, the scaffold can be postcured to increase the degree of polymerization and the mechanical properties. ... 

This technique proved particularly well suited to follow ultra-fast polymerizations induced by laser beams and Aus to obtain directly the kinedc profile of the reaction, as shown by Figure 10. One of the unique advantages of this method is that it now becomes possible, based on these kinetic curves, to determine instantly and at any moment, both the true rate of polymerization and the amount of residual unsaturation in the cured polymer, i.e., its crosslink density. 

Like IR spectroscopy, Raman spectroscopy can provide qualitative, quantitative, and structural information about a variety of materials. The potential of laser Raman for distinguishing critical structure differences in cured urea-formaldehyde (UF) resins has been demonstrated from studies made on model compounds [30]. Fourier transform-Raman studies of thermally and photochemically induced epoxy curing reactions have been reported [31]. 

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