Paint & Coatings Industry - February 2009 - (Page 44) Analysis of Coatings Using Pyrolysis-GC/MS the methyl methacrylate, styrene and butyl acrylate also seen in the chromatogram. Polyurethanes Polyurethane polymers are made by reacting a diisocyanate with a di- or poly-functional polyol. When they are pyrolyzed, the diisocyanate is regenerated and becomes one of the dominant peaks in the pyrogram. Other peaks provide information about the nature of the polyol used in making the polymer. The polyols may be polyester based or polyether based, and information is generated that helps distinguish the two. Polyester polyols are frequently made using adipic acid and, upon pyrolysis, this portion of the molecule produces cyclopentanone. Polyether polyols will not make cyclopentanone, but will produce small compounds that retain the ether structure. A piece of dried polyurethane finish was pyrolyzed to produce Figure 5. This was a typical solvent-based FIGURE 5 | Pyrogram of a typical oil-based polyurethane finish, showing a peak from the polyol and both toluene diisocyanate isomers. MCounts 15.0 12.5 10.0 2 7.5 5.0 2.5 0.0 5 10 15 Minutes 20 25 1 3 3 1 2 finish, made with toluene diisocyanate. Toluene diisocyanate is usually a mixture of two isomers, the 2,4- and 2,6- configurations. Not only is the diisocyanate regenerated – a marker for polyurethanes – but in the case of isomers, the isomeric configuration is preserved in the pyrogram. This is true of complex formulations in which the polyurethane content is relatively small in addition to materials that are essentially pure polyurethane. As a consequence, the relative amount of polyurethane in a paint or finish may be assayed using pyrolysis. Recently, waterborne polyurethane finishes have become available. These formulations use different diisocyanates than products using organic solvents, and the pyrograms may be quite different. The example shown in Figure 6 contains considerable styrene and 2-ethylhexyl acrylate, and at first may not be recognized as a polyurethane. Two of the larger peaks are plasticizers, but at about 23 minutes there are three peaks that elute close to each other and have very similar mass spectra. These peaks have an area ratio of about 20:30:50, and are the three principal isomers of H12MDI (4,4´-methylene dicyclohexyl diisocyanate), the diisocyanate used in this preparation. Again, the relative amounts of the three isomers found in a commercial preparation of H12MDI is regenerated and revealed in the peak areas in the pyrogram. Epoxies Epoxies are considered rugged and indestructible, but they are organic polymers and may be analyzed using Py-GC/ MS like any other polymer.6 Most epoxies are made using Bisphenol A, which is one of the main products when the sample is pyrolyzed. Figure 7 shows results for a sample of the coating used as a liner inside a food can. A small piece was scraped off the inner surface of the can and pyrolyzed, revealing phenol and Bisphenol A among other things. Although the presence of Bisphenol A is important in identifying the epoxy, it is actually not diagnostic for epoxies since polycarbonates also use Bisphenol A. In these cases, the presence and relative abundance of the other pyrolysis products are important to make the distinction. FIGURE 6 | Pyrogram of a waterborne polyurethane. Peak #1 = styrene, 2 = 2-ethylhexyl acrylate, 3 = dibutyl phthalate, 4 = diisocyanate isomers, 5 = benzylbutyl phthalate. MCounts 2.5 2.0 1.5 1.0 0.5 0.0 5 10 15 Minutes 20 25 3 4b 2 4a 4c 1 5 Additives Not all of the peaks in a pyrogram are necessarily pyrolysis products from the polymer. If the sample has been heated just once, volatile and semi-volatile compounds may also be present, which are desorbed from the polymer before it pyrolyzes. Residual solvents, contaminants and additives7 are likely to be present among the peaks from the polymer itself, and may be important in the analysis of the paint as a finished product. These compounds may be present in small quantities, making only small peaks, or they may be substantial as in the case of plasticizers. When the additives are present at low levels, it is helpful to adopt a multi-step approach8 to the analysis. In this case, the sample is heated to one or more temperatures below the pyrolysis temperature of the polymer, releasing the volatiles intact but not 44 FE BRUARY 2009 | W W W . P C I M A G . C O M http://WWW.PCIMAG.COM
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