3. Cure of an acrylic coating

The sampling depth variability of FTIR-PAS measurements is especially useful in analysis of coatings where it is desirable to enhance the coating spectrum relative to the substrate spectrum. A typical example is monitoring of cure due to ultraviolet-induced polymerization of an acrylic coating on polycarbonate. The spectra of Fig. 27 show the cure monitoring as indicated by reduction of the cure band height with increasing polymerization. All of the spectra in Fig. 27 are scaled so that the polycarbonate peak heights at 1600 cm^-1 are equal. Considerably more contrast is observed in the cure bands measured by shallow-sampling than by deep-sampling. Consequently, the more coating-specific shallow-sampling FTIR-PAS analysis provides the most sensitive monitor of the degree of coating cure.

Fig. 27. FTIR-PAS spectra of an acrylic coated polycarbonate with high and low degrees of cure that are measured with shallow and deep sampling. The intensity of the cure band decreases going from a low to a high degree of cure. Note that shallow sampling is most sensitive to the cure process because it enhances the coating versus the substrate spectrum. All spectra are scaled so that the substrate band at 1600 cm^-1 are of equal height.