Paint & Coatings Industry - February 2009 - (Page 42) Analysis of Coatings ecause they are complex formulations of both organic and inorganic materials, coatings offer a special challenge to the analytical chemist. The high concentration of opaque materials that give paints their ability to cover a surface may also make the application of some typical analytical techniques impossible. Further, the organic constituents are generally very high molecular weight, and so not volatile, making both GC and mass spec inapplicable. When dry, the paint is also insoluble, ruling out many other solvent-based analytical procedures. Analytical pyrolysis is a technique that creates volatile compounds from organic macromolecules, making samples like polymers, adhesives and dried paint suitable again for analysis using gas chromatography, mass spectrometry and FT-IR. Application is simple; a small piece of the solid material is heated rapidly to a high temperature (generally 600 – 800 °C), causing it to degrade into smaller organic compounds, which then are delivered to the analytical device for processing as with any other sample. Interpretation of the resulting fragments requires an understanding of how various polymers behave at high temperatures, but once the basic theory of pyrolytic degradation is understood, the information is invaluable in understanding the molecular composition of these complex systems. B Introduction Processing polymerized materials by pyrolysis for the purpose of analytical scrutiny has been commonplace for decades.1 Based essentially on free radical-induced degradation, the chemistry follows basic rules of chemistry, and the resulting products are both instructive of the original polymer and reproducible. It is imporFIGURE 1 | Separation of various compounds by the GC column, producing a chromatogram. Capillary GC Column Liquid Phase Recorder/Data Station R E S P O N S E Detector Time tant to use parameters that produce a wide array of products, if possible, which retain much of the identity of the original polymer,2 rather than creating very small, but undiagnostic fragments. Many polymers used in the coating industry produce monomers and other small oligomers upon pyrolysis, so the identity of the original polymer is evident. For example, polystyrene produces considerable styrene, poly(methyl methacrylate) makes methyl methacrylate, and so on. When using gas chromatography as the analytical tool,3 these volatile compounds should be produced quickly and transferred rapidly to the column inlet. Once the pyrolysis volatiles are on the GC column, they behave in the same way as compounds introduced by any other technique (Figure 1). The column contains a liquid phase that interacts with the volatiles as they proceed through the length of the column, separating the mixture of compounds into peaks for individual components. These compounds then pass into the detector, producing a signal presented as a chromatogram. In practice, the pyrolysis step is conducted at relatively high temperatures so that the polymer degrades in seconds. The transfer interface from the pyrolyzer to the gas chromatograph must have a small volume so that the peaks do not spread out, producing poor chromatography. Molecularly, polymers are generally comprised of a long chain backbone (or crosslinked network), onto which are attached groups that make one polymer different from another. Degradation by pyrolysis is initiated by breaking the weakest bond in the polymer molecule. If that bond is part of the chain or backbone, the length of the chain is reduced until the pieces are volatile and can be analyzed by GC. These fragments frequently include the monomer of the polymer, and some polymers unzip to produce little but monomer. Poly(methyl methacrylate), as well as the other methacrylates, and Teflon are examples of polymers that unzip in this way. Other polymers degrade to produce monomer and also higher oligomers. Figure 2 shows a pyrogram of polystyrene, which generates a substantial amount of styrene monomer, but also the dimer and trimer. Many polymers and copolymers used in coatings behave in this fashion, including the acrylates like butyl acrylate, polyolefins like polypropylene, natural rubber and silicones. Some polymers degrade by chain bond breaking without producing much of the monomers, but do make characteristic and reproducible products. Condensation polymers like polyamides and polyesters By T.P. Wampler, K.D. Jansson and C.P. Zawodny | CDS Analytical, Inc., Oxford, PA 42 FE BRUARY 2009 | W W W . P C I M A G . C O M http://WWW.PCIMAG.COM
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