Fibers pyrograms

Py-GC can therefore be used as an effective means of determining the chemical composition of man-made homopolymer and copolymer fibers, natural fibers, fiber blends, or partly degraded fibers. Pyrograms of polyester, acrylic, and cotton fibers show how these fibers can readily be identified (Figure 8.11). 

Another application of peptide pyrolysis is the study of silk. Pyrograms of minute pieces of fiber can be pyrolysed to identify the fingerprint of natural silk. 

Fibers are another important source of forensic material. Both synthetic and natural fibers were analyzed by pyrolysis, the pyrograms or Py-MS data generating a fingerprint that can be diagnostic for each type of fiber [13]. 

FIGURE 8.11 Pyrograms of (a) polyester, (b) acrylic, and (c) cotton fibers. 1 = benzene, 2 = vinyl benzoate, 3 = benzoic acid, 4 = biphenyl, 5 = acrylonitrile, 6 = dimethyl formamide, 7 = methyl vinyl pyridine, 8 = acrylonitrile dimers, 9 = acrylonitrile trimers, 10 = 2-furalde-hyde, 11 = dihydromethylfuranone, 12 = levoglucopyranose. 

Many acrylic copolymers are currently used in the textile industry as binders for nonwoven fabrics. The purpose of these fibers is to stabilize the material. In many instances, these copolymers are used in conjunction with amino resins. Casanovas and Rovira have done a study of methyl methacrylate-ethyl acrylate-N-methylol-acrylamide by PY/GC-MS. Among the products identified were methane, ethylene, propene, isobutene, methanol, propionaldehyde, ethanol, ethyl acetate, methyl acrylate, methyl isobutyrate, ethyl acrylate, methyl methacrylate, n-propyl acrylate, and ethyl methacrylate. In this sample, clearly monomer reversion is the primary degradation process occurring however, several other degradation mechanisms are at work. When the sample contains an amino resin in the mixture, acrylonitrile is observed in the pyrogram. Another effect of the amino presence was a marked increase in the amount of methanol detected. Other products detected were meth-oxyhydrazine, methyl isocyanate, and methyl isocyanide. 

Figure 16.23 shows a pyrogram resulting from the pyrolysis of a polyester fiber. The conditions used to analyze this particular fiber (listed in Table 16.16) are typical of conditions used to analyze a variety of different fibers and are similar to the conditions used for PGC of paints (Table 16.16). 

FIGURE 16.23 Pyrogram of a 1-cm length of a polyester fiber on a 30-m HP-5 fused-silica capillary column (0.32-mm-i.d. x 0.25- xm film) GCFID conditions see Table 16.17. (Courtesy of New Jersey State Police.). 

PGC has been applied to the characterization of a number of different types of polymers besides the traditional paint chips and fiber evidence submitted to the crime laboratory. Synthetic plastics have been readily identified and differentiated by PGC (253). Hume et al. (265) applied PGC to the characterization of motor vehicle body fillers utilizing a 15-m Carbowax 20 M gas chromatographic column and a computer-based system to compare pyrograms. 

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