Infrared Spectroscopy in the Analysis, Characterization, and Testing of Coatings

By John M. Chalmers

In this JCT COATINGSTECH series of in­troductory tutorial articles, the empha­sis sought is on the applications of modern analytical, characterization, and testing methods for coatings. And while it is easy to cite many examples of re­cent applications of mid-infrared spec­troscopy, the most common sampling techniques employed have been well es­tablished for a long time and, as a con­sequence, mid-infrared spectroscopy has been at the forefront of analytical meth­ods for characterizing coatings and sur­faces for many years.

In 1980, the Federation of Societies for Coatings Technology (FSCT) published an Infrared Spectroscopy Atlas for the Coatings lndustry,¹ which contained over 1400 mid-infrared spectra, covering categories such as polymers, monomers, solvents, inorganic pigments and extenders, or­ganic pigments, and various additives. A fourth edition of Infrared Spectroscopy Atlas for the Coatings Industry, which contains ca. 2500 mid-infrared spectra, was published in two volumes by the FSCT in 1991.2.

In addition to its estab­lished position as a favored technique for characterizing and analyzing poly­mers, 3·6 the long tradition for using mid-infrared spectroscopy to study and characterize in situ coatings stems es­sentially from two sampling technique developments in the 1960s.

The first, known as ATR (Attenuated Total Reflection) mid-infrared spectroscopy, is an internal reflection spectroscopy tech­nique. 7·11 The second, more specialized technique is an external reflection ap­proach undertaken at grazing angle of incidence, which is often referred to as RAIRS (Reflection-Absorption Infrared Spectroscopy),12 or some other similar acronym.

Mid-infrared ATR spectroscopy, which is discussed in more de­tail later, interrogates a surface layer of a sample to a depth in the approximate range of 0.3 µm to 3 µm . RAIRS is used to study thin films, typically in the range 100 A -100 nm, such as Langmuir-Blodgett and vacuum evapo­rated films and self-assembled mono­layers supported on a smooth metal re­flective surface such as that of gold, silver, and copper.13 RAIRS remains largely a more specialized research tool with one of its other main areas of ap­plication being in the study of species adsorbed on metal catalysts.

However, since the 1980s, with the introduction of high sensitivity modern Fourier transform infrared spectrometers (FTIR), and in particular FTIR microscope sys­tems, mid-infrared reflection-absorption spectroscopy at near normal angles of in­cidence has become quite a commonly used direct sampling technique for fingerprinting coating layers on reflective sub­strates. It is commonly referred to as the transflec­tion sampling tech­nique.

Other mid-in­frared sampling methods that have been used to exam­ine coatings include transmission spec­troscopy of material abraded from a sur­face, emission spec­troscopy, and the more recently ap­plied methods of photoacoustic and diffuse reflection spectroscopy. Almeida et al.14 have dis­cussed and compared the relative merits of a range of FTIR sampling techniques used for the study of surfaces and coat­ings. Sampling methods will be consid­ered in more detail later.