Paint Pigments and Extenders
The pigments in paints are responsible
for the final colors. Paints may contain single pigments or multiple
pigments. Even white paint contains pigments. The most common
pigment for white paint is rutile (TiO2). (12) Other pigments
may be organic or inorganic in nature.
Raman spectroscopy has the advantage over FTIR for identifying pigments in paint samples. Both organic and inorganic paints can be infrared active, but their absorptions are so low that it is difficult to distinguish between two samples using only the IR spectra. The pigments, however, have strong absorption peaks in the Raman spectra. Many of these extend below 600 cm-1 where the IR does not strongly absorb. (12) The capability of Raman spectroscopy to detect these low frequency vibrations makes Raman a useful tool for differentiating paint samples by their pigments.
Raman spectroscopy has another important advantage over the FTIR. Since the Raman peaks are sharp and do not overlap, it is much easier to resolve mixtures of pigments than it is using FTIR. Also, Raman is sensitive to even small differences in the intensities of various peaks. Since paints are a complex mixture of many components, Raman can be used to distinguish between two similar paint samples that have slightly different amounts of each component. (11)
Reference 12
Raman spectroscopy has the advantage over FTIR for identifying pigments in paint samples. Both organic and inorganic paints can be infrared active, but their absorptions are so low that it is difficult to distinguish between two samples using only the IR spectra. The pigments, however, have strong absorption peaks in the Raman spectra. Many of these extend below 600 cm-1 where the IR does not strongly absorb. (12) The capability of Raman spectroscopy to detect these low frequency vibrations makes Raman a useful tool for differentiating paint samples by their pigments.
Raman spectroscopy has another important advantage over the FTIR. Since the Raman peaks are sharp and do not overlap, it is much easier to resolve mixtures of pigments than it is using FTIR. Also, Raman is sensitive to even small differences in the intensities of various peaks. Since paints are a complex mixture of many components, Raman can be used to distinguish between two similar paint samples that have slightly different amounts of each component. (11)
Reference 12
Above are comparisons of two
automobile paints (the primer layer) using both Raman and FTIR. (12) DeGelder
et al. were studying the efficacy of using Raman spectroscopy to analyze
all layers of automobile paint. The two paints have very similar peaks
in the IR spectra and it would be difficult to make an accurate identification
using only this technique. Notice the Raman spectra, however. Though
many of the peaks are of the same frequency, the relative intensities are
different enough to distinguish the two.
Below are the (Web MO) IR spectra of two common pigment extenders (molecules that enhance the properties of the pigments):
In the IR spectra above, there are some strong peaks for these molecules, but the peaks are in a region where they are masked by the absorbances of other molecules. Since IR peaks can overlap, this shows that it is not an ideal method for showing the presence or absence of these extenders.
Overall, Raman has proved to be a better technique for differentiating paint samples by their pigments. The combination of Raman and FTIR is needed in order to obtain a more complete spectroscopic picture of the unknown paint samples.
Below are the (Web MO) IR spectra of two common pigment extenders (molecules that enhance the properties of the pigments):
Sulfate- showing presence
of barium sulfate
Carbonate- showing presence of calcite
In the IR spectra above, there are some strong peaks for these molecules, but the peaks are in a region where they are masked by the absorbances of other molecules. Since IR peaks can overlap, this shows that it is not an ideal method for showing the presence or absence of these extenders.
Overall, Raman has proved to be a better technique for differentiating paint samples by their pigments. The combination of Raman and FTIR is needed in order to obtain a more complete spectroscopic picture of the unknown paint samples.
