- .
1
L
c' C
1
Bertsil B. Baker, Jr. E. I. du Pont de Nemours & Company, Inc. Polymer Products Department P.O. Box 80269 Wilmington, DE 19880-0269
Mary A. Kaiser E. I. du Pont de Nemours & Company, Inc. Engineering Department P.O. Box 6094 Newark, DE 19714-6094
Fires. Everyone knows the devastating damage they can cause. Today, our homes and offices are filled with new synthetic materials that have replaced wood and other natural products. The fire properties of these materials, many of which are polymers, are just now being defined. In 1982the National Bureau of Standards (NBS, now the National Institute of Standards and Technology
[NIST]) conducted a small-scale test to evaluate the toxicity of polymer combustion products (1). In this test certain perfluoro polymers generated products that were unusually toxic (by
only slightly more toxic than wood and less toxic than certain synthetic materials and natural fibers. Because of these contradictions and the lack of any reports of unusual toxic-
ANALYTICAL APPROACH 2 orders of magnitude when compared with other materials) when combusted alone and in the presence of continuous heat. These results contradicted earlier observations by scientists at the University of Pittsburgh (2), where poly(tetrafluoroethylene) (PTFE) was found to be 100 times more toxic than Douglas fir (compared with 400 times more toxic as reported by the NBS test), and the University of San Francisco (3), where PTFE was rated as
ity of polymers during the past halfcentury, we decided to investigate the significance of the NBS results. We confirmed the NBS results in our laboratory but found that when the fluoro polymer was burned in a large-scale fire, no unusual toxicity was observed. This was puzzling because nearly all chemical plants are built based on pilot plant models or operations that are 1/100 or smaller in size. However, in this situation results of the small-scale +
0003-2700/9 1/0363-079A/$02.50/0 @ 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 2, JANUARY 15, 1991
79 A
.
'4
ANALYTICAL APPROACH
Figure 1. Furnace and exposure chamber Tor me NBS test metnoa. (Adapted from Reference 4.)
laboratory experiments contradicted those of the real-life, full-scale experiments. In addition, work by NIST investigators with cotton fabric and polyurethane foam supported our results (4).
In this article, we will describe the analytical techniques used to understand this phenomenon and to find what caused the unusual toxicity. The little fire The NBS test chamber consisted of a 180-L box constructed of poly(methy1 methacrylate) (Figure 1).Holes in the side provided for nose-only exposure of the male rats (Charles River Laboratories). Three milligrams of polymer were burned in a crucible in an electric furnace (in the case of fluoro polymer to 700 "C). Animal exposure was for 30 min with an approximate lethal concentration (ALC) of 20 mg/m3, based on the weight of the polymer burned. (Approximate lethal concentration is defined as the concentration at which one or more animal deaths occurs during exposure or a subsequent observation period, usually two weeks.) This test provided results for many polymers, including polystyrene, acrylonitrile/butadiene/styrene,and poly(vinyl chloride), that generally agreed with previous studies. However, the toxicity levels for PTFE and tetrafluoroethylene-hexafluoroprop ylene copolymer were 2 orders of magnitude higher than expected. Extensive studies reported in 1987 (5) could not explain the cause of the unusual toxicity. In those studies, the gaseous combustion products were 80A
ANALYTICAL CHEMISTRY, VOL.
drawn into an evacuated 20-m cell and analyzed by IR spectroscopy.The principal products detected were tetrafluoroethylene (TFE) and COF2 (Table I). TFE is relatively nontoxic, and the COF2 concentration was far below the lethal level. We carefully examined spectra for smaller amounts of more toxic compounds, but only relatively nontoxic products were found. We decided to look for the very toxic compound perfluoroisobutylene (PFIB), which has been found in fluoro polymer degradation in inert atmospheres. We did not detect any and, furthermore, PFIB has never been observed as a major degradation product in air. Even if all the PTFE burned during the NBS test produced a 100% yield of PFIB, there still would have been an insufficient amount to account for the test animals' deaths (the 30-min lethal concentration for rats is 50 mg PFIB/m3). No HF was detected by using IR spectroscopy, but HF has a low molar absorptivity and a propensity for adsorption on cell walls. If we presumed that HF was present (for the purpose of calculation) and calculated its concentration by measuring the total HF with an ion-selective electrode and subtracting the COF2 detected by IR spectroscopy, the difference would be equivalent to only a few percent of the fluorine present. A careful material balance resulted in 100% recovery of fluorine with -5% estimated error. On the chance that the toxic entity might be short-lived or lost in transfer lines or on cell walls, an optical system was devised to examine the products in the NBS chamber itself. The IR beam from a Nicolet 7199 FT-IR spectrome-
63, NO. 2,
JANUARY 15, 1991
ter was directed into the chamber by using a mirror/detector arrangement that provided a several-meter pathlength through the combustion products and allowed real-time monitoring of the atmosphere in the chamber without any chance of transfer loss. No new compounds were identified. Collection and examination of the chamber gases by GC/MS was equally unproductive. Initial studies using electron spin resonance (ESR) spectrometry implicated free radicals (6),but further ESR studies showed that they did not contribute to the unusual toxicity (7). We identified particle-bound peroxy radicals in some acutely toxic perfluoro polymer fume aerosols, but the concentration of radicals was below the level found in polluted urban air. In other equally toxic perfluoro polymer fume aerosols, no radicals or reactive functional groups were detected. Although we still had not identified the unusual toxic compound, two factors discovered in the initial NBS studies lessened our concern for the toxicity of smoke from building fires where fluoro polymer products might be present. First, the toxicity was present only when the combustion products were regenerated by the continued
Table 1. Concentration (ppm) of gaseous combustion products in the NBS ch and the large-scale fir NBS hambera
20,000 2,000
eo HF COF2 CF4 c2F6 c3F8 C C4F8 (cyclic) CHFS
Full-scale fireb
300 . ..
IO0 200
NDC
10
40 5
20 I50 20
NDC NDC NDC
I50
60
a Initial concentration from burning 30 mg PTFE in a 65-L chamber. Present for several minutes at height of le burn. Not detected. The detection limit for all these compounds in the 20-m gas cell was well less than 1 ppm. dHF and COFp were not independently measured in the full-scale fire. The HF value I the sum of HF and COF2, expressed as HF.
8
More For Less. 3 100 Atomic Absorption Spectrometcr delivers exceptionalperforman Add the Enhanced Data Systemfor advanced data handling and accessory controlfeatures.
p
I
With the Stabilized Temperature Platform Graphite Furnace you get unrivaled detection limits and freedom from interferences. ~
ing information. All in eas
The FIAS-~OOJ~OW injection system provades new levels of speed, performance and cost effectiveness for automated sample pretreatment.
A
1 Perkin-Elmer combines experience, expertise, and worldwide resources to provide unmatched products, service, and s u pfor analytical la oratories. See us o l the Pittsburgh Conference Booth 4162. For Information Only # I 10 For Sales Call #111
ANALYTICAL APPROACH presence of the heat source (e.g., 700 "C cup furnace in the test chamber). Second, if almost any other material (wood, paper) was burned with the fluoro polymer, no unusual toxicity was observed. The need for a continuous heat source may explain the difference between the results observed during the NBS test and the Pittsburgh and San Francisco tests. In the latter two studies, the sample was heated in a tube furnace and the products were swept into the animal chamber without recirculation of the air through the hot zone.
The big fire Because we were unable to identify the toxic compound, we decided to undertake the expense and effort to simulate a full-scale fire to assure users of perfluoro polymer products that there are no unusual hazards caused by the polymers. This experiment was carried out at the fire research facility of the State University of Ghent, Belgium (8). We chose to combust plenum cables with fluoro polymer insulation and jackets, such as those used for telephone and computer lines in office buildings, using as a heat source a large burning fuel bed usually consisting of wood. The cables were contained in a tray suspended in the upper part of the burn room. The room was connected to a U-shaped corridor a t the end of which a portion of smoke was removed, diluted, and cooled with fresh air to prevent overheating of the test animals (Figure 2). The intent was to provide exposure to toxic products without fatal exposure to heat. In a typical experiment there was a dilution factor of 2 between the corridor and the first animal exposure chamber and a dilution factor of 4 for the second chamber. The results showed no usual toxicity. The rodent deaths that did occur were attributed to the combined effects of HF (or COF2) and CO. In our simulated large-scale fire, no unusual toxicity was observed-as predicted by the results when paper and wood were included in the NBS tests. Thus the little fire did give the same results seen in the big fire, but only if the sample was burned under the same conditions. In the experiments at Ghent, researchers had to be certain that a reasonable amount of the fluorine-containing combustion products reached the test animals. Smoke samples were collected from the corridor and the animal exposure chambers in evacuated bottles containing 0.1 M NaOH. The solution was analyzed for fluoride from HF and COF2 by using an ion-selective 82 A
Figure 2. Layout of burn facility at the State University of Ghent, Belgium.
electrode. The presence of any hydrolyzable organic compounds, such as perfluoroacetyl fluoride, was excluded by examination of the solution with NMR spectroscopy. The only fluorine species present was fluoride ion. The gases above the NaOH solution were transferred to a 20-m IR cell, and all the organic fluorine-containing components were measured (Table I). When all of these components and the fluoride ion were added, we could account for only 40% of the fluorine. In contrast, we had obtained essentially 100% recovery in the NBS chamber. Where was the missing 60% of the fluorine? We checked all our calibrations and calculations, to no avail. Then we realized that this fire was a lot wetter than the one burned in the NBS chamber. There was water in the wood, water from combustion of the wood, and even water released from dehydration of the gypsum board that lined the burn room and smoke corridor. A quick calculation showed that when the smoke cooled to near-ambient temperature at the animal chambers, it was at or above the saturation point. In the presence of water, the equilibrium partial pressure of HF was only a tiny fraction of its molar proportion, leading to condensation of HF (9, 10). Experiments were carried out in the laboratory using water and HF vapor streams mixed in varying proportions. At the expected HF concentrations in the smoke, we were in the condensation range. Even in a clean
ANALYTICAL CHEMISTRY, VOL. 63, NO. 2, JANUARY 15, 1991
system consisting of mostly polyethylene lines and vessels, when a fog formed it collected on the walls almost quantitatively. It evaporated and was recovered when dry gas was passed through the system.
The remaining problem Where did this leave us? We had demonstrated nearly 100%recovery of fluorine in the NBS chamber, but the products found could not account for the extremely high toxicity observed. In the NBS chamber with co-combustion of wood or paper we could also account for all of the fluorine, but there were no animal deaths. In the large fire, we demonstrated that the animals were exposed to a significant level of fluorine combustion products, but there was no unusual toxicity. We felt that the answer to the problem lay in identifying the extremely toxic entity in the NBS test. No highly toxic gases were observed, and the influence of free radicals was considered, studied, and discarded. During combustion in the NBS test, a piezobalance had shown the presence of particulate in the air equivalent to 1 or 2% of the total mass burned, but because of the small amount present, we did not believe at that time that it could explain the observed toxicity. Because the 30min lethal concentration of the polymer was 20 mg/m3, this level of particulate would amount to only 0.2-0.4 mg/ m3 in the chamber.
Table II
Seidel. F. B. Clarke of Benjamin-Clarke Associates provided fire consulting on the work at the State University of Ghent.
Comparison of toxicities
References (1) Levin, B. C.;Fowell, A. J.; Birky, M. M.; Paabo, M.; Stolte, A.; Malek, D. Technical Report No. NBS IR-82-2532; National Bureau of Standards: Gaithersburg, MD, 1982. (2) Anderson, R. C.; Alarie, Y. C. J. Combust. Toxicol. 1978,5,54. (3) Hilado, C. J.; Schneider, J. E. J. Combust. Toxicol. 1979,6,91. (4) Braun, E.; Gann, R. G.; Levin, B. C.; Paabo, M. J. Fire Sciences 1990,8,63. ( 5 ) Williams, S. J.; Baker, B. B.; Lee, K-P. Food & Chemical Toxicology 1987, 25, 177-85.
a Some of these values are LCsOrsome are ALC, and some are less rigorously established le concentrations (i.e., the lowest concentration where animal deaths occurred). They a here as approximate values for comparisonover the wide range that they illustratf * Analytically determinedat the animal exDosure chambers,expressedas HF. Corrected for the toxic effect of the measured amount of CO at the animal exposure chamber:
However, concurrently with the large-scale fire studies a t Ghent, further work on the NBS system showed that filtered air was no longer toxic, thus implicating the particulate. Additional work using IR and NMR spectroscopies, ionization smoke detection, and photographic analysis showed that the particulate was a PTFE oligomer, 4.03 pm in diameter with no discernible end groups. Follow-up work (7,1I, 12) has shown that
