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

JCT CoatingsTech, Sept, 2005 by Neil J. Everall

Thermal Cure

In the paint industry, the drying (i.e., room temperature cure) of alkyd resin is a complex process involving cis-trans isomerization, C=C bond migration and crosslinking, and formation of peroxide and hydroperoxide species. While Raman can follow the C=C consumption and O-O production, FTIR is far better suited to detecting O-O-H production. Claybourn et al. have used Raman and FTIR spectroscopy to follow the drying process in the bulk of paint films and model systems, and observed features such as cis-trans isomerization and the effect of Ti[O.sub.2] filler on the rate of cure. (43,44) Mirone et al. used confocal Raman to depth profile the cure of C=C in alkyd films, and monitored the drying front as it moved into the film. (45) The data was used to develop a model to predict the degree of crosslinking and Young's Modulus as a function of film thickness and drying time. Erich et al. combined NMR imaging and confocal Raman microscopy to monitor the penetration of drying fronts into alkyd films. (46) The NMR data were used to calibrate the true sampling depth of the Raman probe, which was shown to be about 1.6 times deeper than the nominal focal point. This confirmed that refraction must be accounted for to correctly analyze the position of the drying front.

Perhaps surprisingly, it is also possible to use Raman spectroscopy to predict the viscosity of paint emulsions. Ito et al. used partial least squares (PLS) regression to predict viscosity over the range 20-350 mPaS with a precision of about 25 mPaS. (47) The model used spectral features relating to chain length and styrene content which had positive and negative correlations with viscosity. Combined with the more obvious measurements of composition and extent of polymerization, (44) this could be an interesting tool for on-line analysis and production control, particularly given the ease with which Raman spectrometers can be configured for process analysis. (48)

Despite Raman spectroscopy's advantages in terms of sampling and spatial resolution, sometimes the basis spectroscopy of the system dictates that IR spectroscopy is the preferred approach. For example, isocyanates are common components of coating materials which cure by reaction with alcohols or water to form urethane (-NCOO-) and urea linkages. Cure of NCO is easily monitored using IR spectroscopy (loss of antisymmetric stretching band near 2280 [cm.sup.-1]), but this band is completely invisible in the Raman spectrum (Figure 12). The symmetric NCO stretch, which is Raman active, falls at lower frequency and is often masked or overlapped by other bands, notably the amide II (coupled NH bend and CN stretch) bands of the urethane and urea groups that are formed during cure, so it is much harder to locate and quantify. In simple cases, one can still quantify the cure with this peak, (49) but in situations where the urethane is present in both H-bonded and non H-bonded forms there are multiple overlapping spectral features and quantification becomes very difficult. Hence, quantification of NCO cure is usually far more straightforward using IR spectroscopy. In other cases, there is little spectroscopic reason for preferring IR or Raman spectroscopy over the alternative, and either technique can be used. The cure of epoxies is a good example, since the oxirane ring has strong bands in both the IR and Raman. Here, the choice of technique might depend on other factors, such as whether remote sampling is required (e.g., fiber-optic coupled), or whether another component masks the IR or Raman band of interest.