Fourier Transform Infrared Spectrometry ASTM E168, ASTM E1252

If the material does not absorb infrared light a spectral scan cannot be obtained. Example: metals do not absorb infrared light.

FTIR is the first logical step in identifying a polymer. FTIR is also used for quality control of materials and for contamination analysis (surface or internal).

Test Procedure:
A material’s absorbance of infrared light at different frequencies produces a unique "spectral fingerprint" based upon the frequencies at which the material absorbs infrared light and the intensity of those absorptions. The resulting spectral scan (absorbance or transmittance) is usually specific to a general class of material. Example: the spectral scan of Polycarbonate does not look like the scan of Nylon but all Nylon scans have unique similarities.

Unknown spectral scans can be analyzed to determine the base material of the unknown by comparing their scan to spectral scans of known materials that are stored in a computer-based library.

A typical infrared scan is generated in the mid-infrared region of the light spectrum. The mid-infrared region is from 400 to 4000 wavenumbers, which equals wavelengths of 2.5 to 25 microns (10-3mm).

Matching the unknown infrared spectrum to known spectra can be done manually or with the help of a computerized program. Computerized spectral searches can quickly compare an unknown spectrum to a very large number of spectra located in multiple databases in a very short period of time.

Computerized spectral matches to the unknown spectral scan are presented from best to worst with assigned certainty ratings. Computer programs are very helpful for comparing unknown spectral scans to those of known materials, but computer selected matches can be misleading. A skilled FTIR analyst is needed to examine the computer selected spectral matches to ensure that sample identifications are both accurate and complete. Computer matching programs have difficulties with subtle differences that can be critically important.

Sample size required:
Samples the size of a single resin pellet can be scanned by reflective FTIR. Samples, which can be easily tested by reflective FTIR, include polymer pellets, parts, opaque samples, fibers, powders, wire coatings, and liquids.

Materials with large quantities of carbon (carbon black or carbon fiber) are difficult to obtain a usable spectral scan from because carbon strongly absorbs infrared light in a broad range of frequencies. This results in an FTIR spectrum without the minute details necessary to identify the unknown material.

Basic Identification:
As previously noted, FTIR polymer identification of an unknown is done by matching the material’s infrared peaks, either transmittance or absorbance, to the peaks of similar infrared scans of known materials. The better the match, the higher the certainty for a correct identification of the unknown polymer.

An FTIR spectral analysis can easily identify classes of polymers such as Nylons, Polyesters, Polypropylenes, Polycarbonates, Acetals, or Polyethylenes. However, an FTIR spectral scan alone should not be expected to identify the type of Nylon or Polyester, identify a Polypropylene or Acetal as a homopolymer or copolymer, or determine whether the Polyethylene is a high density or low density material.

Further identification may be aided by DSC.
Further identification may be aided by an Ash Test.

Quality Control:
A spectral scan of a reference material can be generated and stored in a spectral library database. A stored reference scan will allow all future material scans to be compared back to the same earlier scan. The objective is to look for material differences. Differences noted in a newly generated spectral scan could indicate a change in processing or a possible contamination problem.

Internal Polymer Contamination:
FTIR spectral subtractions are used to look for internal contamination in polymers. A computer program is used to subtract the peaks associated with the base polymer from the spectral scan and then an analysis of the remaining spectral scan is performed.

The amount of contamination that can be detected depends on the spectral scans of the base polymer and the contaminant. Contamination involving materials with very different infrared spectra can usually be detected at a level of about 1-2%. Contamination involving materials with similar infrared spectra may not show up at even the 10% level.

Surface Contamination:
Obvious surface contamination of polymers can be analyzed by normal reflective FTIR because the infrared beam only enters a few microns into the sample surface. Another method to look for a possible surface contamination involves a solvent wash of the sample’s surface.

A solvent wash involves using a solvent that is nondestructive to the sample. A solvent wash of the sample’s surface is collected and evaporated to dryness on the FTIR reflective sample area. Once the solvent is evaporated off an FTIR analysis is performed on the solvent wash residue.

**Please note that this test description is intentionally generic in nature and aimed at providing a descriptive summary to enhance test understanding. Standards can be obtained from appropriate standards authorities.