Coating formulations are a complicated mixture of inorganic and organic chemical compounds. Most coatings typically consist of a carrier solvent that is meant to evaporate, leaving behind a homogenous dispersion of binders, resins, and pigments. From a basic chemical perspective, each of these components can be further subdivided into configurations of atoms bonded together in particular combinations. These atomic configurations, called functional groups, are the basic building blocks for defining the chemistry of a particular molecule. For instance, the commonly encountered term acrylic (defined chemically as COO-R), found on the exterior label of paint cans, refers to the functional groups included in the polymeric binders in the coating.

Often there is a need, from a failure analysis perspective, to determine the original formulation used for an application. FTIR spectroscopy is an analytical technique that utilizes the infrared radiation of the electromagnetic spectrum to excite molecular motions. The wavelength of absorption is proportional to the energy required to initiate intramolecular oscillations. Typical oscillations found in molecules include stretching, bending, rocking, wagging, and twisting, and these various motions are recorded as “peaks” or “absorbances” by the FTIR spectrometer. These motions are specifically quantized to the functional group oscillations and are commonly found in the same location of the infrared spectrum regardless of the compound.

The spectrometer output includes a plot of absorbance (or % transmittance) as a function of energy, typically given in units of wavenumber (cm^–1). The infrared spectrum is commonly defined as 500–4,000 cm^–1, but is practically limited by the type of infrared detector used in the instrument.

All spectrometers will provide the same spectrum for the same coating sample. However, the number of absorbances used for identification is limited by the spectrometer’s detector. The area of the spectrum between 1,600–4,000 cm^–1 is commonly referred to as the “functional group region,” and is primarily composed of peaks corresponding to the stretching motions of the functional groups. This area is easily interpreted, as the associated absorbances are evenly spaced and readily identified. Common functional groups identified in this area include: alcohols, amines, amides, aliphatics, alkenes, aromatics, acids, esters, and acrylonitriles. As seen in Table 1, these functional groups allow for the identification of the resin used in the coating.

The region from 500–1,600 cm^–1 is referred to as the “fingerprint region.” Within this region, the peaks commonly overlap, requiring mathematical deconvolution for identification. Despite the apparent complexity, this spectral region is extremely useful (just as a fingerprint is distinctive for a person, the pattern of peaks in this region of the FTIR spectrum is distinctive for the chemical compounds in the coating formulation).

As seen in Figure 1, different commercial paint formulations can be distinguished from each other when using FTIR. Furthermore, comparison to two polymer resins, a polyacrylate and a polyurethane, illustrates that the functional group region is not the same. Specifically, the commercial paint samples in Figure 1 are more consistent with polyacrylates because there are no urethane resonances between 3,300–2,900 and 1,600–1,500 cm^–1. Furthermore, the broadness in the region between 700–500 cm^–1 is indicative of inorganic filler in the formulation. In this stack plot, a comparison of flat, semi-gloss, and gloss paint is shown. Here, an examination of the region between 1,000–1,100 cm^–1 reveals distinguishable differences.

Changes in pigment composition and pigment/resin ratios, which alter the reflectance of light on the coating, are common for flat paints. The increase in the overall absorbance intensity in this region is consistent with an increase in the concentration of extender (e.g., an inorganic carbonate) as the formulations are modified from high-gloss to flat. Therefore, within a given brand of coatings, the type or grade of coating can be identified through a comparative examination of the fingerprint region of the spectrum.

Both bulk and surface analysis can be performed with FTIR depending on the mode of the analysis performed. There are three commonly available modes of analysis for FTIR: transmission, attenuated total reflectance (ATR) and reflectance. Each of these analytical variants of the FTIR technique has its advantages and disadvantages, as well as specific sample requirements. Transmission FTIR requires a thin sample, typically 1–10 microns (0.04–0.4 mils) in thickness. This sampling technique is considered a bulk analysis because the infrared beam is transmitted through the sample.

Due to the requirement for extremely thin samples, transmission FTIR is generally limited to the analysis of liquids or solids that can be prepared with the appropriate dimensions. Reflectance spectroscopy and ATR are more suitable for coating analysis because they are confined to the sample surface. Reflectance spectroscopy requires the sample to be on the order of 1–5 microns (0.04–0.2 mils) thick in order to allow the infrared beam to penetrate through the sample to a reflective substrate underneath. To meet this requirement for a coating, a paint sample could be placed onto a reflective mirror surface. The entire sample is analyzed in this instance, although the sample thickness defines this technique as a surface analysis. Therefore, this method is confined to liquid and solid samples that can be spread onto a reflective substrate.

ATR requires direct physical contact of a reflective crystal onto the surface of the substrate. The infrared beam penetrates the first 1–4 microns (0.04–0.16 mils) of the surface and is angled back into the spectrometer by the optical pathway of the crystal. This technique is commonly applied to samples in their natural state, when a non-destructive analysis is required. Virtually any sample shape or configuration can be used, including sample thicknesses on the order of 1–5 inches. Liquids, powders, and solids can all be analyzed with the ATR technique.

The absorbances, denoted by spectral peaks, of infrared radiation found in a typical FTIR spectrum are governed by Beer’s Law, which holds that the intensity or amplitude of the observed absorbance (i.e., the height of the peak) is necessarily proportional to the concentration of the absorbing species. Although the functional form is dependent on the mode of analysis (e.g., ATR, reflectance, or transmission), in general, absorbances in FTIR spectra are proportional to the concentration of the organic compound.

As an example, Figure 2 denotes the reflectance FTIR spectra of wet and dry Glidden Evermore semi-gloss paint applied onto a reflective mirror surface. The reflectance-mode spectrum of the wet paint was taken immediately after application; the “dry” spectrum was taken two hours later. When first applied, the Glidden paint contains a higher concentration of carrier solvent (e.g., water), which makes the coating spreadable. After drying, a solid film forms and the resonances associated with the carrier solvent are no longer found.

The time required for film formation can be determined by quantifying the decrease in water absorption as a function of drying time. This is helpful, for example, when drying time needs to be established at multiple temperatures. An added benefit to FTIR, over calorimetric and gravimetric techniques, is the ability to determine if there has been any chemical change in the resins once the coating dries. Thus, FTIR can confirm some reactive functionality is still available to react at a particular time during the application of a layered coating.

FTIR can also be used in forensic failure analysis on recovered paint samples to determine if particular types of paint/primer systems were employed in the original coating. In some cases, specific brands can also be identified. Figure 3 shows spectra of a recovered primer-coated surface along with two reference spectra. As the figure demonstrates, the recovered primer coat sample is consistent with an oil-based primer formulation available from Sherwin-Williams, the use of which was suspected based on other evidence in the case.

Figure 4 shows a similar analysis with a recovered top-coat sample. The recovered latex coat is consistent with a reference latex-based satin finish paint available from Sherwin-Williams. Although not illustrated in Figure 4, FTIR can also be used to characterize aging and degradation due to environmental exposure.

Summary

FTIR is a powerful analytical technique that can readily be applied to the analysis of coating systems. An FTIR spectrum contains a wealth of information about the basic functional chemical groups in a sample that can be used for identification and for evaluation of chemical changes that occur as a result of environmental exposure or other phenomena. Although other techniques such as energy dispersive spectroscopy (EDS) or X-ray diffraction (XRD) may be necessary to confirm the identity of mineral pigments—and more detailed structural information can be obtained using techniques such as gas chromatography–mass spectrometry (GC-MS)—FTIR should be the technique of choice for the initial identification of resins and other organic additives in paint systems.

About the Authors

Jason Clevenger, Ph.D., is a managing scientist with Exponent Failure Analysis Associates in Natick, Mass.

Michelle Poliskie works for Solyndra, Inc., of Fremont, Calif. She may be reached by calling (510) 440-2400.