Published: Jan 1992
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The value of infrared spectroscopy to the coatings industry has long been recognized. Now Fourier transform infrared (FTIR) equipment is replacing dispersive instruments because it is much faster, more accurate, and more sensitive. Of even greater impact is its data processing capabilities. Together, these advances enable the paint analyst to generate information which would have been impossible a few years ago. Applications of this technique to problem solving, cure and weathering studies, and quality control will be reported.
The extraction and identification of minor components and contaminants is one of the most difficult and time-consuming problems for the coatings analyst. The computational power of the FTIR spectrometer permits the point-by-point subtraction of one spectrum from another. The differential spectrum reveals minor differences, which are the other components in the system. This spectral stripping detects constituents which would be difficult, if not impossible, to isolate by chemical means. If the samples are skillfully prepared and subtractions carefully performed, even the concentration can be estimated. Further, the differential spectrum can be identified via computer searching of spectral libraries. This makes problem solving, such as contaminant detection and identification, rapid and effective.
Another difficult task for the coatings chemist is determining the chemical mechanisms of cure, weathering, and degradation. FTIR is providing new, valuable information about these processes. By taking and storing high-precision spectra at frequent, regular intervals, a moving picture of the process can be recorded. We call this technique “Time Lapse IR spectroscopy.” Changes in band intensity are an indication of the chemical reactions taking place within the film. These spectral shifts can be measured quantitatively so that not only the path but also the rate can be determined. Weathering studies are now being carried out after the development of a durable substrate. This provides a useful chemical picture of what is taking place during cure, weathering, or degradation.
FTIR will likely make its greatest contribution to the coatings industry as a quality assurance tool. Du Pont's Finishes Division has developed and deployed a unique instrumental package, called CATS© (Compositional Assurance Testing System). By combining a horizontal attenuated total reflectance (ATR) sampling attachment with a FTIR spectrometer and proprietary software, we have a most effective test for raw materials, intermediates, and finished products. An operator with minimum training can run the spectrum of an incoming material and decide on its acceptability within minutes. If the material is mislabeled or contaminated, it can further be identified and dealt with before a serious problem develops. This eliminates much wasted manufacturing time and material.
spectroscopy, infrared—Fourier transform, chemical analysis, coatings, cure measurement, durability measurement, quality assurance
Research associate, E. I. du Pont de Nemours & Co., Marshall R & D Laboratory, Philadelphia, PA