Raman spectroscopy in coatings research and analysis: Part II. Practical applications

JCT CoatingsTech, Sept, 2005 by Neil J. Everall

Cure

UV CURE: Raman spectroscopy is ideally suited to studying the cure of reactive C=C groups; Figure 11 provides a simple example. In this case, an unsaturated polyester resin was cured using UV light and a cationic initiator, and a microtomed cross-section was then mapped with a Raman microscope to monitor consumption of the C=C groups as a function of distance from the air interface. The cis and trans C=C isomers can be easily distinguished by their stretching frequencies (an advantage over infrared spectroscopy, where the trans band is almost invisible). We can see that the cis isomer is almost fully consumed irrespective of the distance from the air surface, whereas the trans isomer only cures near the surface. The UV light is attenuated as it penetrates into the coating, and this adversely affects the consumption of the trans C=C groups. The cis isomers are apparently more labile, and so cure to completion despite the UV attenuation. The practical conclusion is that one could maximize the cure in this system by increasing the level of cis isomer in the resin. In this example, the UV penetration controlled the cure profile, whereas in radical-initiated systems one often finds that oxygen inhibition inhibits cure near the surface. In such cases, the 1 [micro]m lateral resolution of the Raman microscope can be vital in quantifying significant changes in cure in the top few microns of a sample. (7)

There is extensive literature on studying UV-cure using Raman spectroscopy; only a few examples are quoted here. Schrof et al. combined confocal Raman and confocal laser scanning microscopy (CLSM) to study filler distribution and cure of urethane-acrylate and ester-acrylate resins, although they neglected the effect of spherical aberration on the Raman depth resolution. (39) Kim et al. combined Raman and ATR-FTIR spectroscopy to study the cure of a polyester acrylate to which an acrylate-functionalized poly(dimethylsiloxane) (AF-PDMS) was added, and showed that there was an optimum level of AF-PDMS which resulted in maximum surface cure and hardness. (40) Unfortunately, it was not clear whether the authors could differentiate acrylate functionality in the PDMS from that in the ester-acrylate copolymer. Nichols and colleagues discussed several UV clearcoats in which spectral overlap made it difficult to resolve bands adequately for quantitation. (41) They showed how second derivatives could be used to improve band resolution and permitted quantitative cure profiling. They cut cross-sections and mapped cure at 10 [micro]m intervals through the coat thickness, and compared situations in which cure was limited by either UV penetration (as in Figure 8), or by oxygen inhibition. It was claimed that sufficiently high UV dose could overcome [O.sub.2] inhibition at the surface, but their spatial resolution (10 [micro]m) was not adequate to resolve any cure gradients which might be present within the first few microns of the coat.

[FIGURE 9 OMITTED]

Raman microscopy is pre-eminent for monitoring spatial cure profiles, but cure kinetics can often be measured more easily using IR spectroscopy. For example, real-time IR spectroscopy has been pioneered by Decker for monitoring UV cure on the ms timescale. (42) It would be difficult to obtain adequate spectral quality as rapidly using Raman spectroscopy.