Chapter 26
Fire and Polymers II Downloaded from pubs.acs.org by UNIV OF MASSACHUSETTS AMHERST on 08/10/18. For personal use only.
Analysis of Soot Produced from the Combustion of Polymeric Materials Kent J . Voorhees Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO 80401
Soot is composed of a carbonaceous matrix and often condensed products of incomplete combustion. Liquid chromatography, GC/MS, and pyrolysisGC/MS have been used to analyze soot for the primary polymer degradation products, degraded polymers similar to the original structure, plus many of the additives of polymer systems. Both intact soot and extractable materials from the soot have been utilized. Specific examples of soot analyses from fires are discussed along with the success of applying the results to fuel source identification, fire growth, fire models, and environmental contamination. The combustion of a polymer is a complex process that involves many parallel and sequential reactions. A simplified mechanism for this process can be represented in two primary steps: polymer -> polymer fragments + pyrolysates pyrolysates stable products (low and high molecular weight) + soot. The initial reaction takes place as a pyrolysis process while the second reaction occurs in the combustion zone. Therefore, soot is composed of a highly condensed carbon material, an amorphous polymeric fraction, and condensed volatile substances. Early studies showed that for polyurethanes, the amorphous fraction was a degraded (lower molecular weight) material that had a similar repeating unit to the original polymer (1,2). In addition, many commercial polymer systems contain additives for stabilizing, processing, or property modifications (3). When these polymer systems are exposed to combustion conditions, many additives are distilled and eventually are condensed onto a soot particle (4,5). The thermal degradation products of many polymer systems have been extensively studied (6-9). The portion of these compounds that survive the combustion process as pyrolysates is usually a small percentage of the total isolated material. Some compounds in the pyrolysate fraction can be very diagnostic for establishing a particular polymer system as a fuel source. For example, polyvinyl chloride is known to quantitatively liberate hydrogen chloride at about 250°C (10) and has been identified in PVC soot (11,12). Other polymers such as polymethylmethacrylate "unzip" during pyrolysis to produce high percentages of monomer (4). Styrene monomer and dimer are observed from the degradation of polystyrene (13). 0097-6156/95/0599-0393$12.00/0 © 1995 American Chemical Society
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Several papers have been published describing the use of volatile compounds trapped in the soot matrix to identify a hydrocarbon fuel source (14-17). Takatsu and Yamamoto have examined the solvent extracts from laboratory soot samples produced from the combustion of benzene, toluene, xylenes, ethyl benzene, styrene, and cumene (14-17). The various chromatograms showed specific changes that varied according to the original fuel. Tsao and Voorhees (18) examined the volatile compounds in soot produced from the combustion of five polymeric materials with gasoline present as an accelerant. Pyrolysis-mass spectrometry was used with charcoal glued to the Curie-point wire (19) to fingerprint the volatile compounds. The spectra were successfully classified using factor analysis and the presence of gasoline established. Characterization of the aerosol material which had been heated under vacuum to remove the volatile compounds has been studied by Voorhees and Tsao for both flaming and non-flaming combustion using both natural and synthetic polymers (12,20). These researchers found, using pattern recognition procedures on pyrolysismass spectrometry data, that the non-volatile material in soot could be used to identify the fuel producing the aerosol. Successful identification of 12 polymers in mixtures containing up to three of the individual polymers was above 70%. The following paper summarizes the application of some of the previously described laboratory studies to the analysis of soot from actual fire sites. Applications to fuel source identification, fire growth, fire models, and environmental contamination are highlighted. Experimental Soot Collection Soot samples were collected by scraping a soot coated surface with a razor blade followed by carefully placing the soot into a glass vial equipped with an aluminum lined cap. Except for Formica® surfaces, non-organic surfaces such as glass were chosen for scraping. Gas Chromatographv/Mass Spectrometry (GC/MS) Analysis Extraction of the soot for GC/MS analysis was done using 20 to 65 mg of soot suspended in 0.15 mL of hexane. Suspensions were sonicated for 10 min, allowed to partition over a two-hour period, and then an aliquot of the hexane layer injected into the gas chromatograph. One-microliter injections were typically used for most of the samples. A DB-5 fused capillary column (0.25 mm X 30 m) temperature programmed from 50° to 280°C at 10°C per minute was used in the GC/MS analysis. Detection of the eluents was accomplished using a Hewlett Packard MSD or a Finnigan TSQ70 mass spectrometer in the electron ionization mode scanned from 40 to 450 Da. All chromatographic peaks were searched against the EPA/NIST mass spectral library. Homologous series of compounds were represented as selected ion plots. Both mass spectrometers were tuned with fluorinated standards to EPA specifications. Blank runs of the GC/MS system and the solvents were made before the initiation of the analysis. Hydrocarbon standards were run to establish a hydrocarbon number scale for the retention times. Pvrolvsis-Gas Chromatographv/Mass Spectrometry Analysis (Pv-GC/MS) The pyrolysis of soot was performed using a Curie-point Pyrolyzer (University of Utah) with a Fisher Power supply (1.5 kW, 1.1 MHz) connected to a Finnigan TSQ70 GC/MS-MS system. Samples (~10 μg of material) were coated as methanol
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suspensions onto 510°C Curie-point wires. The gas chromatography and mass spectrometer operating parameters were identical to those described for the GC/MS extract analysis. Ion Chromatographic Analysis Both soot and wipe samples have been analyzed by ion chromatography. The collection of soot has been previously described. The wipe samples were taken by wiping a 100 cm area with a Whatman 40 filter paper wetted with 10 mL of isopropyl alcohol. Following collection, the filter paper was sealed in a glass vial using an aluminum lined cap. Appropriate field blanks, where the filter paper was wetted and then handled and placed in the container, were also collected. Soot samples weighing approximately 20 mg were dissolved with sonication (10 min) in 10 ml of doubly distilled deionized water. Plastic laboratory equipment was exclusively used for the analysis. A Dionex ion chromatograph equipped with a 15 cm HPIC A54A analytical column was used for the chloride analysis with a mobile phase (flow rate = 2 mL/min) containing 1.8 mmoles Na C0 and 1.7 mmoles NaHC0 . A 50 μL sample, previously filtered through a 0.45 μπι filter, was injected through a 1.0 /xL sample loop. The regenerate contained 25 mmoles of H S0 . The retention time and the mass response for the chloride ion peak were determined by injecting standard sodium chloride solutions. Sodium ion concentrations were determined for the same solutions by ion chromatography using a 15 cm HPIC CS3 analytical column. The regenerate was a 70 mmole solution of tetrabutylammonium hydroxide. Appropriate standards were used for retention times and mass response. The sodium and chloride levels for the soot samples are reported as weight percent. The wipe sample filters were individually extracted using deionized water volumes from 0.5 mL to 15 mL. The ion chromatographic analysis scheme was identical to that previously described for the soot analysis. Because the actual amount of total material on the wipe samples is not known, the sodium and the chloride levels are reported as micrograms per wipe sample. 2
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Polvchlorinated Biphenvls (PCB) Analysis The method used for the PCB wipe sampling was similar to that described by Ness (22). Gauze pads 3" X 3" (7.6 cm X 7.6 cm) saturated with 8 mL of Nanograde hexane were used to wipe a 100 cm surface area. Following exposure, the pads were placed in a glass jar sealed with a metal foil lined screw cap. Blank samples were also taken where the gauze was saturated with hexane, handled and then returned to the glass jar. Extraction of the gauze pad was done by sonicating for 5 min with 25 mL of Nanograde hexane followed by vacuum distillation of the solvent to a 1 mL final volume. The extracts were immediately analyzed on a gas chromatograph equipped with a Hall electrolytic conductivity detector. A 1 ptL sample was injected onto a 30 m SPB-5 megabore column maintained at 180°C for one minute followed by a temperature program from 180 - 250°C at 10°C/min. The final temperature was held for 35 min. Other parameters for the analysis were: injector temperature - 220°C, detector temperature - 910°C, reaction gas - hydrogen, and carrier gas - helium at 17 ml/min. All chromatograms were electronically integrated and were compared to the appropriate PCB standards. Samples with the highest PCB levels were run by gas chromatography/mass spectrometry for verification. Chromatographic conditions for the GC/MS analysis were: column- 0.25 mm X 12 m, temperature programmed from 70 - 270°C at 10°C/min and scan range m/z 170 to 450. Mass spectral identifications were made by comparing the spectra to PCB mass spectra. 2
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Results and Discussion Ion Chromatography Polyvinyl chloride is extensively used in commerce and has been involved in many structural fires. The following discussion presents the historical events and the chemical analyses of two fires that involved PVC combustion with the release of hydrogen chloride. A fire occurred in a multistory office building that had been undergoing a refmishing project on the wood paneling in the executive offices located on one floor. The construction occurred on a Saturday and was finished for the day in the late afternoon. Oil, rags and other supplies were left in an office so that work could begin the next morning. A single company occupied the entire floor of origin. The floor was laid out such that a series of executive offices surrounded a secretarial pool which was located in the center of the complex. The secretarial area was subdivided into stations and was nicely furnished. Polyvinyl chloride wall covering was present on the secretarial pool side of the divider wall between the executive offices and the secretarial stations. A fire alarm from this floor was received on a central alarm board on the first floor and was observed by two security officers. One of the security people took an elevator to the fire floor where he was met by thick smoke. The guard later stated that he was overcome by the smoke and had to radio the other security person to override the elevator controls and return the elevator car to the first floor. Several fire sources were proposed. The best scenario that fit the eye witnesses' accounts who were outside the building and the security guard, placed the fire origin in an executive office where the refmishing supplies had been left. The ignition source was speculated as occurring from spontaneous combustion of linseed oil soaked rags left in the office. Burned rags were recovered in this office during the post-fire investigation. The PVC wall covering, known from purchase records to be in the secretarial pool, provided a probe to determine whether or not the fire had spread into this area at the time the security guard arrived on the floor. A smoke coating was visible on the inside surfaces of the elevator car that was used by the security guard. Wipe samples were collected from the inside of the car when scrapping provided insufficient quantities of soot. Table I summarizes the results of the ion chromatography analysis for the elevator wipe samples. The major background contributor to the chloride level would primarily be sodium chloride. To gauge this contribution, it has been assumed that any sodium ion present was associated with sodium chloride. The measured sodium level was multiplied by a factor of 1.54 to obtain the chloride level that would be present if all sodium ions were in the form of sodium chloride. If large chloride levels and low sodium levels were obtained for a sample, it was assumed that the chloride was associated with hydrogen chloride released from the PVC combustion. Two soot samples collected from the fire floor during the post-fire investigation had the following sodium and chloride levels in weight percent: Sample #1 Na-0.28% Cl-1.7% Excess CI-1.3% Sample #2 Na-0.23% Cl-3.8% Excess Cl-3.4% Both samples show the presence of large quantities of chloride ion and low levels of sodium; therefore, significant quantities of hydrogen chloride were present. The chloride values listed in Table I, except for sample #1, are near the background levels. Enough sodium is present to account for all chloride as sodium chloride. Sample #1 has excess chloride ion. Based on the results from a
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Table I. Ion Chromatography Results from Elevator Wipe Samples +
Na Levels
Sample
1
1
CI- Levels
Deionized water blank
13.7
31.1
Field Blank #1 (Wetted) Field Blank #2 Field Blank #3
26.3 15.3 5.0
33.0 29.3 24.3
Elevator Elevator Elevator Elevator Elevator
43.5 44.0 31.8 20.3 25.8
149 39.6 45.6 36.1 33.7
1
Sample #1 Sample #2 Sample #3 Sample #4 Sample #5
All levels for sodium and chloride ions are ug/wipe.
substantial number of samples analyzed in our laboratory, this value is very small and in most analyses would be considered as a negative for hydrogen chloride. The conclusion from this analysis was that the PVC in the secretarial pool was not involved in the fire at the time the smoke was deposited on the elevator car's interior surfaces. Furthermore, these results support the hypothesis that the linseed rags in the office provided the combustion source for the fire. Another application of ion chromatographic analysis has been to assess smoke movement associated with PVC combustion. A vinyl wallpaper was the major source of PVC in a recent hotel fire. This wall became involved in the fire immediately after flashover from the room of origin. The smoke from the burning PVC along with a portion of the smoke from the combustibles in the room of origin, was vented through the hotel casino and lobby where multiple deaths occurred. Soot was collected by scraping non-organic surfaces throughout the lobby, casino, shops, and guest rooms. The results of the ion chromatographic analysis showed chloride levels from 4.6 wt % to a high of 32.7 wt %. Since this hotel was near the ocean, the question of sea spray was an important factor. In all but one sample, the ratio of sodium to chlorine clearly defined the high chloride levels to be from hydrogen chloride. The highest chloride level was found in a room off the casino area where very little fire or heat damage occurred. The lower chloride levels were in the casino and lobby areas that had suffered high heat damage. It became clear in correlating chloride levels to fire and heat damage that high heat fluxes cause thermal desorption of the adsorbed hydrogen chloride on deposited soot. The other major application of chloride analysis we have used has been for directing cleanup after a PVC fire. Both soot and wipe sampling methods have been employed in these investigations. Gas Chromatographv/Mass Spectrometry Analysis of Soot The development of GC/MS has provided the capability to analyze complex organic mixtures such as those obtained from extraction of soot. Figure 1 illustrates a total ion chromatogram of a soot solvent extract from the same hotel fire described previously. In general, the two compound classes in the greatest concentration in most samples are phthalate plasticizers (marked with an O) and polynuclear aromatic hydrocarbons (PAH) (marked with a X). Depending upon the characteristics of a fire, these two compound classes can dominate the chromatogram. However, even
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when this occurs as illustrated in Figure 1, marker compounds are often observed that can be used to identify a particular fuel source. For example, some of the minor peaks in this chromatogram were identified as phosphate plasticizers and fatty acids commonly used in PVC. The following example illustrates and expands the concept of using marker compounds to identify a commercial product present in a horse stable fire. The manager of a horse training facility awoke early one morning to see an orange glow near his large horse barn. He saw that he could not extinguish the fire and immediately removed a tractor and then went into the stable portion of the barn and started to release the horses. While he was removing his prized Arabian stallion, the fire rapidly spread across the ceiling of the stable which resulted in the manager being overcome by the smoke and the horse eventually being killed. In total, 32 horses were killed in the fire. The vapor barrier system associated with the ceiling appeared to provide the fuel for the rapid spread of the fire. Because of the legal aspects of the fire, it became important to identify the manufacturer of the barrier system. The legal discovery process provided composition information on several potential manufacturers' products and served as a database for the following investigation. Figure 2 shows the total ion chromatogram of a hexane extract from a soot sample removed from a window in an area of the stable where only minor fire damage had occurred. Because this window had only minor fire impact, it had served as a cool surface for collection of the soot and distilled products. Table II summarizes the product identification associated with Figure 2. Polynuclear aromatic hydrocarbons in the extract are common combustion products; however, in this case, an asphalt binder used in the vapor barrier system was also a potential contributor. It should be noted that almost all soot sample extracts studied in our laboratory have contained some polynuclear aromatic hydrocarbons. The normal hydrocarbons observed in the stable soot sample are not a result of combustion, but of a distillation from one of the fuels. After reviewing product information from the various manufacturers of vapor barrier materials that could have been in the barn, one was found that contained an asphalt material and a hydrocarbon wax. Figure 3 represents the total ion chromatogram of the hexane extract of the asphaltic-wax materials. Table III lists the compounds identified in the study. Note that a similar normal-hydrocarbon pattern of evenly spaced peaks with the same retention times exists in both Figures 2 and 3. The change in relative intensity occurs during distillation because of the differing vapor pressures and boiling points. A survey of the materials involved in this fire and the chemical formulations allowed for an identification of this product as being present in the stable barn. Polymer additives have also been observed in soot extracts from other fires studied. Table IV lists the various types of organic additives that have been identified. Based on the complexity of the fire, select additives can be traced to a particular polymer system. The fire retardants used in polyurethanes are quite specific in identifying these materials as fuels. Compounds of environmental interest are often found in soot extracts. We have observed poly chlorinated biphenyls (PCB), chlorinated phenols, and insecticides in various fire samples. The primary sources of PCBs are light ballasts and paint. Frequently, the type of PCB (i.e., Araclor types) can be determined by comparing the soot extract data against PCB standards. Pentachlorophenol has been used as a wood preservative and is effectively distilled from treated wood prior to its combustion. A surprising class of compounds that have been detected in soot extracts is pesticides. These compounds were not the result of stored pesticide, but from the extended use of pesticides in an area later impacted by fire.
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Figure 1. A Total Ion Chromatogram of a Hexane Extract of Soot Removed from a Hotel Fire Scene (0= plasticizer compounds, X = poly nuclear aromatic hydrocarbons).
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Table II. A Summary of Products Identified in a Hexane Soot Extract Peak Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Compound Az-Hexadecane Az-Heptadecane Phenanthrene or anthracene Az-Octadecane Az-Nonadecane Phthalate ester AZ-Icosane Dimethylphenanthrene or anthracene Fluoranthene Az-Henicosane not identified AZ-Docosane Az-Tricosane Az-Tetracosane Benzofluoranthene Triphenylene Chrysene /z-Pentacosane AZ-Hexacosane Az-Heptacosane Benzopyrene Benzofluoranthene Az-Octacosane Benzofluoranthene Benzopyrene Az-Nonacosane Az-Triacontane
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Table III. Compounds Identified in the Hexane Extract of the Asphaltic Materials : Number
Compound
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Ai-Heptadecane Λ-octadecane Phenanthrene or anthracene C hydrocarbon C hydrocarbon Λ-nonadecane Methoxyphenylmethylbenzene Methylphenanthrene or anthracene Methylphenanthrene or anthracene n-Icosane Hexahydropyrene Dimethylphenanthrene or anthracene Dimethylphenanthrene or anthracene Dimethylphenanthrene or anthracene Az-Henicosane Dimethylphenanthrene or anthracene C i hydrocarbon unknown unknown Trimethylphenanthrene or anthracene n-Docosane Trimethylphenanthrene or anthracene Trimethylphenanthrene or anthracene Phthalate Hexacosane Silicone peak Silicone peak Heptacosane 18 18
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Table IV. Organic Additives Identified in Soot Extracts Additive Type
Examples
Fire Retardants Plasticizers
Chlorinated phosphates Phthalate Esters Trialkyl Phosphates Triaryl Phosphates Fatty Acid Esters Glycerides Hindered Phenols Fatty Acids
Antioxidants Lubricants
Direct Pvrolvsis-GC/MS Analysis of Soot Direct Curie-point pyrolysis-GC/MS (Py-GC/MS) from many of the soot samples that were subjected to extraction has also been conducted. Figure 4 represents a pyrolysis total ion chromatogram of the soot that was extracted to produce the chromatogram represented in Figure 2. In addition to the Λ-alkane series, many new peaks were observed in the pyrolysis chromatogram (Table V) when compared to the Table V. A Summary of Compounds Identified from the Py-GC/MS of Soot Peak Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
Compound Acetic Acid Phenol Benzyl Alcohol Isopropylbenzene Hydroxymethoxybenzene l,2-Dimethyl-2-pyrrolidine Carboxylic Acid 2-Ethoxynaphthalene Trimethylnaphthalene 2,5-dimethylphenylbutanoic acid Benzophenone Pentachlorophenol Phenanthracene Ai-Nonadecane Oxacycloheptadecanone Hexadecanoic Acid /i-Icosane Fluoranthene Λ-Henicosane Octadecanoic Acid /z-Docosane /z-Tricosane Λ-Tetracosane n-Pentacosane rt-Hexacosane Az-Heptacosane Benzopyrene
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extract. The small peaks near baseline were mostly polynuclear aromatics. In general, pyrolysis-GC/MS analysis has not been as successful as the extraction approach. This results from the added complexity that occurs from the degradation of the amorphous polymeric material remaining in the soot. It had been postulated that the polymeric material could be used to identify the presence of degraded polymer (12), but this has not occurred in most samples analyzed. The solvent extraction procedure has definitely been the more successful of the two approaches. Conclusions Soot has been shown to be an effective adsorbent material for both thermal polymer degradation products and polymer additives that are distilled from a polymer during combustion. These studies have shown that both organic and inorganic compounds in extracts can be detected using a variety of analytical procedures. The use of the data has been applied to solve a number of different problems associated with actual fire cases. For volatile organic analyses, data obtained from soot pyrolysis-GC/MS were more complex than the data obtained from soot extracts and generally contained less information about the fuel source of a fire. The author would like to thank David N. Osborne for his assistance in conducting the analyses and Dr. Steven C. Packham for his interest in soot analysis applied to combustion toxicity. Literature Cited 1. Hileman, F.D.; Voorhees, K.J.; Wojcik, L.H.; Birky, M . M . ; Ryan, P.W.; Einhorn, I.N. J. Polym. Sci. Chem. Ed., 1975, 13, 571. 2. Wooley, W.D. Br. Polym. J., 1972, 44, 27. 3. Gachter, R.; Muller, H. In Plastic Additives, Hanser Publishers: New York and others, 1987. 4. Mordechai, P.; Zinn B.T.; Browner, R.F. Comb. Sci. and Tech., 28, 263 (1982). 5. Takatsu, M . ; Yamamoto, T. Bunslei Kagaku, 1993, 42, 543. 6. Grassie, N . In Chemistry in High Polymer Degradation Processes. Interscience Publications: New York, 1964. 7. Madorsky, S.L. In Thermal Degradation of Organic Polymers, Interscience: New York, 1964. 8. Montaudo, G.; Puglisi, C. In Comprehensive Polymer Science. Pergamon Press: New York, 1992, p. 227. 9. Bryk, M.T. In General Degradation of Filled Polymers at High Temperatures and Thermal Oxidation Processes, Ellis Harwood: New York and others, 1991. 10. Boettner, E.A.; Weiss, B. J. Appl. Polym. Sci., 1969, 13, 337. 11. Stone, J.P.; Hazlett, R.N.; Johnson, J.E.; Carhart, H.W. J. Fire and Flam., 1973, 4, 42. 12. Tsao, R.; Voorhees, K.J. Anal. Chem., 1984, 56, 368. 13. Wu, B.; Yang, M . ; Liu, G.;Pan X.; Han S. Sepu., 1993, 223. 14. Takatsu, M . ; Yamamoto, T. Nippon Kagaku Kaishi, 1990, 1749. 15. Takatsu, M . ; Yamamoto, T. Nippon Kagaku Kaishi, 1990, 880. 16. Takatsu, M . ; Yamamoto, T. Nippon Kagaku Kaishi, 1991, 235. 17. Takatsu, M . ; Yamamoto, T. J. Anal. Appl. Pyrol., 1993, 26, 53. 18. Tsao, R. Voorhees, K.J. Anal. Chem., 1984, 56, 1339. 19. Colenutt, B.A.; Thornburn, S. Chromatographia, 1979, 12, 12. 20. Voorhees, K.J.; Tsao, R. Anal. Chem., 1985, 57, 1630. RECEIVED November 28,
1994
