FTIR Analysis of Protective Coatings

FTIR analysis has the following uses:

  • To determine the generic identity of a coating where the identity is unknown or uncertain, eg. to determine if a coating is an alkyd, epoxy, urethane, or acrylic. This is useful when records are lost or in disputes regarding the identification of a coating on a structure.
  • To identify the specific brand (fingerprinting) of coating applied to a structure, where the brand is uncertain or under dispute, eg. to confirm that an epoxy is brand A and not brand B.
  • To identify organic-based contaminants in a coating, eg. to identify overspray droplets, foreign material, or in some cases, surface contaminants.
  • To identify manufacturing problems, eg. the coating supplied has precipitated resins, or the wrong resins.
  • To determine if an applied coating was improperly or incompletely mixed (not applicable to all coatings).
  • To ensure, for QC purposes, that the supplied coating (or ingredient) is the specified coating (or ingredient) that was originally tested, qualified, and specified.

What is FTIR analysis?

FTIR is the acronym for Fourier Transform Infra-Red spectroscopy. FTIR analysis is based on the interaction of IR radiation with the organic resins in a coating.

The Fourier Transform part of FTIR refers to a mathematical operation used in data acquisition and analysis that significantly reduces the time needed to obtain the results. In FTIR analysis, a coating sample is subjected to IR radiation and the result is an FTIR spectrum, which shows the absorption (or transmittance) of IR radiation as a function of wavelength. Each coating has a unique spectrum (or pattern of absorption bands), which depends upon the specific chemical composition of the sample.

The manner of IR absorption by the coating is related to the various motions (vibrations, stretches, bends) occurring within and between the various chemical bonds of the coating molecules. Chemical groups such as aromatics, aliphatics, esters, amines, and isocyanate groups all produce specific, characteristic absorption patterns. A large computer searchable library of FTIR spectra is used in the identification and analysis of a spectrum.

The following types of samples can be used in FTIR analysis:

  • Coating chips, coating scrapings, coated test panels.
  • Liquid coating components, liquid coating immediately after mixing, powder coatings, solid or liquid resin ingredients, solvents.
  • Samples of plastics, elastomeric coatings and composites.
  • Control samples are commonly required to enable direct comparison of the test or suspect sample with a sample of known composition.

Microscopic techniques:

Particles as small as 0.010 inches in size can be analyzed. This technique is normally used to identify foreign material (eg. overspray particles) within a specified coating, undesirable precipitated resins, or un-mixed areas.

Example of Generic Coating Identification by FTIR Analysis

Each coating has a unique composition, and therefore has a unique FTIR spectrum. Among the various bands within the spectrum, however, is a group of bands that is specific for the generic type of coating, such as an epoxy. These groups of bands are used in generic identification. For example, a bis-phenol epoxy, Figure 1, can be readily differentiated from an aliphatic polyurethane (used in outdoor painting applications), Figure 2, Both spectra have a sharp band at about 1725 cm-1, due the carbonyl group, C=O, that they both contain.

However, the epoxy also has a group of bands related to its aromatic content (eg. bands at 3000 to 3100, 1590, 1490 and 830 cm-1). The aliphatic polyurethane has a tight cluster of bands at 2800 to 3000 cm-1 due to its aliphatic groups, but no aromatic bands.

Example of Fingerprinting by FTIR

In addition to the main bands used in generic identification, each spectrum contains a complex combination of minor bands, fine structure, and minor frequency shifts of some bands. The result is a unique spectrum, or fingerprint, for each coating. This attribute can be used to determine, for example, that a particular coating is manufacturer A’s epoxy (and not manufacturer B’s epoxy).

Spectra for this type of analysis are shown in Figure 2. The spectrum on the left was obtained for a coating sample which was thought to be manufacturer A’s aliphatic polyurethane. Comparison with a spectrum obtained for a control sample known to be manufacturer A’s polyurethane confirmed the identification, as the pattern of bands was virtually identical between the two samples.

 


Figure 1. FTIR spectrum of epoxy coating

 


Figure 2. FTIR spectra of aliphatic polyurethane coating (Field Sample)

 


Figure 3. FTIR spectra of aliphatic polyurethane coating (Control Sample)

 

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