MIL-HDBK-1110
referenced in the specification, the paints can be tested for
conformance to manufacturer's listed composition or properties.
11.3.6.1 Microscopic Examination. Samples of paint collected at
the job site should be examined under a light microscope. An
edge examination may reveal the number of coats and the thickness
of each coat. An examination of the surface may reveal fine
cracking or other irregularities. Examination under a scanning
electron microscope (SEM) can reveal much more detailed
information about the paint film. Also, the SEX often has an
attachment for energy dispersive x-ray analysis which can
identify the metals and other elements in the pigment portion of
small areas of the coating.
11.3.6.2 Spot Tests. There are several simple laboratory spot
tests that can be run on samples of deteriorated paint collected
at the job site. They generally provide specific information
about the paint binder (ASTM D 5043, Field Identification of
Coatings) or pigment. Special chemicals and training are usually
required by the analyst.
11.3.6.3 Infrared Spectrophotometric Analysis. The most widely
used technique in laboratory analysis of paint failures is the
infrared spectrophotometry. The use of new Fourier transform
infrared (FTIR) spectrophotometers permits much more versatility
and precision than earlier instruments. The technique can
identify the resin components of paints from the shapes and
locations of their characteristic spectral peaks. It is highly
desirable to separate the resin from the paint pigment before
analysis, because the pigment may cause spectral interference.
This is easy to do with thermoplastic but not thermosetting
paints. Thermoplastic resins can be dissolved in solvents that
are transparent in part or all of the infrared region, filtered
to remove the pigment, and the solution placed in standard liquid
cells or cast as films onto sodium chloride or other plates that
are transparent in the infrared region. Thermosetting coatings
can be pressed into potassium bromide pellets or their spectra
measured using diffuse reflectance equipment. Although the
pigment is not separated in these procedures, the spectrum of the
pigment can often be separated from that of the total coating by
the FTIR spectrometer. Spectra of field samples are compared
against published standards or authentic samples. It should be
remembered that exterior weathering may cause oxidation that may
appear in spectral analyses.
11.3.6.4 Other Specialized Instrumentation. There are other
specialized instruments that can be very helpful in failure
analysis. These include emission spectroscopy, atomic absorption
spectroscopy, and x-ray fluorescence instruments that can
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