Environmental Health PerspectivesVol. 11, pp. 197-202, 1975

An Industrial Approach to Evaluation of Pyrolysisand Combustion Hazardsby R. S. Waritz*

In addition to the usual toxicology studies necessary for the safe manufacture anduse of polymers at room temperature, special studies are needed for polymers whichwill be used at elevated temperatures. This paper discusses various areas to beinvestigated and principles for deciding on test materials, tests, and test conditions,polytetrafluoroethylene (PTFE) and fluorinated polyethylene-propylene (PFEP) pyrolysisstudies being used as an illustrative case history. Some limitations of animal testing alsoare mentioned. A toxicological spectrum relating toxicological determinants to PTFEtemperature is developed.

The industrial approach to the evaluation ofpyrolysis and combustion hazards of polymersshould not be unique from an academic orconsulting or governmental laboratory ap-proach to the same problem. All four shouldbe interested in evaluating these hazards formanufacturing, processing, and transportationworkers, the consumer, and the environment.The approach that I will develop can be sub-

divided into four, usually interrelated, areas:evaluation, simulation, education, and follow-up.

Evaluation consists of examining the pro-posed manufacturing process, proposed secon-dary fabrication processes, proposed uses andprobable disposal procedures, seeking placeswhere the polymer could be exposed to excessiveheat.

Simulation is the attempted laboratory simu-lation of these situations to determine theirhazards. Ideally, the laboratory simulationshould also simulate reasonable abuse or givesome idea how much abuse is possible beforetoxic products are evolved. Representative com-

* Haskell Laboratory for Toxicology and IndustrialMedicine, E. I. du Pont de Nemours and Co., Inc.,Wilmington, Delaware 19899. Present address: HerculesInc., Medical Division, Hercules Tower, 910 Market St.,Wilmington, Del. 19899.

mercial material should be used for theseevaluations.The education phase covers educating manu-

facturing personnel and users about safe manu-facturing and use conditions and the hazards ofabuse.The follow-up aspect consists of maintaining

continuous liaison with all aspects of the poly-mer from manufacture to disposal or recycle.Manufacturing processes change, fabricationmethodology changes, uses and abuses changeand disposal or recycle methodology changes.Any of these changes may result in new op-portunities for the polymer to be exposed toexcessive heat. A good intelligence networkthat will promptly inform the toxicologist ofthese new opportunities for thermal degrada-tion is extremely important.

Polytetrafluoroethylene (PTFE) and fluor-inated polyethylene-propylene polymers (FEP),because of their high thermal stability as wellas other unique properties, are particularlysusceptible to thermal abuse. These polymerswill be considered in this paper to illustratethis approach.The manufacture of these polymers offers no

opportunity for excessive heating. Howeverfabrication, the many uses and incineration dooffer many such opportunities. Uses or opera-

June 1975 197

tions where thermal exposure or thermal abusemight occur are: reclaiming clad metal, wirecoating and stripping, nonstick cookware,scavenging melts, fabricating, coatings, fires,incineration, and machining.The PTFE resins were the first perfluoro-

carbons developed commercially, and they willbe discussed first. They are not thermoplastic,so instead of fabricating structures with in-jection molding techniques, methodology similarto powder metallurgy was used. In thismethodology, a mold was filled with the resingranules, compacted and then heated to ca.3000C for several minutes. At this tempera-ture, the granules coalesced to form a more orless rigid mass in the shape of the mold. Thisis called sintering. The mold and shape werethen cooled and, if necessary, the shape wasthen machined to the final shape or dimensions.There are thus two opportunities in this

methodology for overheating-the sinteringprocess and machining operations.

Studies were undertaken to determine thetemperature at which toxicants were evolvedfrom PTFE and the nature of these products(1-5). Variables such as air flow rate andvolume over the sample while heating, relativehumidity of the air, sample size, and surfacearea probably would affect the results and itwas necessary to pick a set of standard condi-tions. We decided to model the system afterconditions used by the Underwriter's Labora-tory to study the toxicity of pyrolysis productsfrom structural materials.The weight loss for PTFE resins varies

typically from about 1 X 10-3g%/hr at 2900Cto about 49/hr at 4500C (6). Under the con-ditions of our test, the pyrolysis products werenot lethal to rats in a 4-hr exposure when thesample was heated at 4000C, but killed all ratswhen the sample was heated at 450°C. Analysisof the off-gases by a gas chromatographicprocedure revealed only tetrafluoroethylene(TFE) at 4500C. At a sample temperature of4600C, hexafluoropropylene (HFP) was alsodetected and at 4750C, perfluoroisobutylene(PFIB) was detected (5). The concentrationsof these gases as a function of temperature areshown in Figure 1. Since the 5-hr approximatelethal concentration (ALC) of TFE is 45,000ppm (v/v) (7) and the 4-hr ALC of HFP isca. 3000 ppm (v/v) (3,8,9) these off-gases couldnot account for the toxicity seen. The 4-hr ALC

EaE

I&IM'.I.-

0

O--OTFEA-AHFP9*-H-PFIB

T E MP.(*C)500

15EC

E

ILU.

FIGURE 1. Evolution of (0) tetrafluoroethylene, (i\)hexafluoropropylene, and (0) perfluoroisobutylenefrom polytetrafluoroethylene resin as a function oftemperature.

of octafluoroisobutene (PFIB) is 0.5 ppm '(2),but PFIB was not detected at 450°C. Otherinvestigators had reported octafluorocyclo-butane (OFCB) and hexafluoroethane (HFE)as pyrolysis products from PTFE under variousconditions (2). We did not detect them. Ouranalytical method for these five fluorocarbonswas reliable to 0.1 ppm with amounts as lowas 0.03 ppm detectable, so if they had beenpresent, we would have detected them. OFCBand HFE have 4-hr ALC's for rats of >800,000ppm (10,11), i.e., if all the nitrogen in theatmosphere they breathed for 4 hr was replacedwith either of these, it would not be lethal.Obviously, these possible pyrolysis productswould be toxicologically insignificant, comparedto HFP and PFIB.We did detect some hydrolyzable fluoride,

but it was not present in sufficient concentra-tion to account for the toxicity seen, if it was

Environmental Health Perspectives198

any. known material containing hydrolyzablefluoride. Other experiments also indicated itwas not the lethal agent.

Further experiments indicated that the lethalagent at this temperature was a particulatematerial which could be removed by filters witha 0.2 ,tm pore size (5).From these experiments, it appeared to be

safe to sinter PTFE at 300°C. Ventilation andrespirators would provide adequate protectionin the event of any overheating; at least attemperatures up to ca. 500°C.As most toxicologists are aware, animal

studies do not reveal everything about thetoxicological effects of a chemical and thetoxicity of PTFE and FEP pyrolysis productsis a dramatic illustration of this fact.Workers carrying the hot sintered shapes

from the ovens to cooling benches found thatif they carried them close to their chest, theydeveloped a condition which came to be knownas the "shakes" (J. H. Foulger, Haskell Lab-oratory project report, E. I. du Pont deNemours and Co., Inc., unpublished). If theycarried them at arm's length, they developedno symptoms. The "shakes" were characterizedby typical influenza symptoms: chills, spikingfever, achy feeling, tightness of chest, headache,cough, weakness in legs, and malaise. Theylasted 18-48 hr. Recovery was complete andwithout any residual effects or aftereffects. Noanimal species has yet been found that respondsto PTFE or poly-FEP fume the same way ashumans, so definitive studies on the etiologicagent(s) have not yet been possible. Cavagnaet al. (12) have reported some of the clinicalsigns in rabbits pretreated with an aerosol ofdilute acetic acid and then exposed to thepolymer fume. We have not been able to dupli-cate their findings. We also have evaluatedsquirrel monkeys, dogs and cats as possiblemodels for "polymer fume fever" as the"shakes" are now called, but have not yet founda suitable model (13).

The etiologic agent is not known, but sub-limate or particulate has been suggested [J. H.Foulger, Haskell Laboratory project report,E. I. du Pont de Nemours and Company, Inc.,unpublished; and (12)]. Particulate is formedwhenever PTFE or FEP is heated to tempera-tures where weight loss occurs. It probably ispartially degraded polymer.The similarity of symptoms, clinical signs

and their onset and duration between metalfume fever and polymer fume fever is consistentwith particulate as the etiologic agent for poly-mer fume fever (14). In the former, the syn-drome is caused by inhaling fume from a hotmetal such as zinc. In the latter, it is caused byinhaling the fume from a hot polymer. Theobvious common element is a small, hot, pos-sibly "activated" particle (12). No facileexplanation is presently available for the ab-sence of similar fume fevers when fumes fromother polymers are inhaled. We do not knowof any deaths from polymer fume fever.Although the causative agent is not known,

protective steps are known: ventilation, use ofrespirators, or use of furnace cooling.

This problem was thought to be solved afterpinpointing the probable cause of polymer fumefever and prescribing the appropriate protec-tive steps, but sporadic outbreaks of polymerfume fever continued to occur, even whenproper precautions were taken. Thorough in-vestigation of these incidents showed that theyoccurred only in pipe and cigarette smokersworking with the powdered resin or in otheroperations where their tobacco could becomecontaminated with polymer particles. Followingthis lead, controlled human studies were carriedout in which volunteers smoked cigarettesspiked with various amounts of a low molecularweight PTFE in a cigarette. It was found thatas little as 0.4 mg of this polymer caused poly-mer fume fever in nine of ten volunteers (11).The educational phase of the program has

almost eliminated polymer fume fever from theindustrial scene, but occasional cases are stillreported to us. They are always found to re-sult from not following handling precautionsin du Pont's freely distributed bulletin on safehandling practices for these resins (6). Thereis also an American Industrial Hygiene Associa-tion Hygienic Guide on Teflon (15).

Machining operations using high speeds anddull tools could possibly heat the PTFE enoughto cause polymer fume fever in a machinistbending over his work (6). However, suchmachining conditions also give parts that willbe out of specification when they cool. This, plusthe educational approach have virtuallyeliminated polymer fume fever from this area.Even under the worst imaginable machiningconditions, the PTFE would not be heated to450°C, so the other toxicological aspects areunimportant here.

June 1975 199

Wire coating with FEP enamels involvesa heat-curing step. However, the knowledgealready discussed is directly applicable andthere are no new hazards. There are, however,two areas in wire technology where exposureto serious toxicants could possibly occur:thermal wire stripping and short circuits oroverloads.

Because of the cost of copper wire, scrap isstripped of its insulation and recycled. Theusual process is to overheat the insulation sothat it separates from the wire and is easilystripped mechanically. No new toxicants areproduced from FEP by this procedure, and theindustrial hygiene answer is again to use ade-quate spot ventilation and to educate the work-ers carrying out this operation.A short circuit or overload, particularly with

some of the exotic alloy wires, could conceivably

FIGURE 2. Typical exposure arrangement for evalu-ating the toxicity of pyrolysis products from PTFEcoated frypans. The vacuum pump (A) draws aknown volume of air through the annular space(B) between the frypan and the stainless steelfunnel. The air then enters the exposure chamber(C). The air is then passed through two gasscrubbing towers containing O.1N aqueous NaOH(D). Samples for gaseous fluorocarbon analyses aretaken at (E).

result in localized overheating of an FEP in-sulated wire and the release of toxicologicallyimportant pyrolysis products. Our studies indi-cate that it is indeed possible to evolve productswhich are lethal to rats from these wire enamelsat elevated temperatures. However, the thermalsafety margin is greater for these enamels thanfor alternative insulations (6).

Nonstick cookware is another application ofPTFE resins where thermal degradation is pos-sible. Before these resins could be used in thisapplication, it was necessary to carry out ex-tensive pyrolysis tests to be sure that no onecould be injured from this use. Rats were ex-posed for 4 hr to the fumes from frypanscoated by prototype commercial processes withprototype resins and heated to various tempera-tures. A 4-hr exposure seems excessive if onlyhome use is considered, but it is not an exces-sive exposure if commercial use is also con-sidered. This, in fact, is the usual inhalationexposure time for studies on commercialchemicals. A typical exposure arrangement isshown in Figure 2. A measured volume of air,drawn through the annular space between thestainless steel funnel and the frypan carriesthe pyrolysis products to the inhalation cham-ber. Aliquots of the chamber atmosphere areanalyzed for gaseous fluorocarbons and hydro-lyzable fluoride.

Typical temperatures for frying variousfoods range from 1300C for fish fillet to 2800Cfor steak (16). The approximate lethal tempera-ture (ALT) for rats for a 4-hr exposure to thepyrolysis products from a PTFE-coated fry-pan is 425-4500C. Thus there is a wide safetymargin under typical use conditions. The plastichandle of a frypan would decompose if the panwas kept at 2800C for long. The odor from thedecomposing handle would alert the user to aproblem long before there was any problemfrom pyrolyzed PTFE.

Considering the amount of PTFE resin ona pan, the usual frying temperatures, the pos-sible abuse temperatures, the size and ventila-tion of home and commercial kitchens, we didnot feel the use of PTFE-coated cookware pre-sented any real hazard from thermal degrada-tion products. Actual human tests indicated thateven gross thermal abuse of a coated frypanwould not cause polymer fume fever underuse conditions.Use experience indicates that these assess-

Environmental Health Perspectives200

ments were correct for humans. However, afterthe pans had been on the market for severalyears, we began to get sporadic reports thatpet birds kept in kitchens had died after acoated frypan had been left unattended for longperiods of time on a stove set at a high heat.As a result we carried out several experi-

ments with Japanese quail and parakeets.These are summarized in Table 1 (17). Becauseof a short supply of parakeets, fewer experi-ments could be carried out with these birds thanwith the Japanese quail. Enough experimentscould be carried out, however, to demonstratethat both bird species were more sensitive tothe fumes from PTFE resin than are rats. How-ever, they are not more sensitive than rats tothe fumes from the naturally occurring fryingmedia (11), and both rats and these birds aremore sensitive to the fumes from these mediathan the fumes from PTFE. The safety balanceadditionally favors the PTFE-clad frypan, inthat corn oil or butter left unattended in afrypan on a stove, and reaching temperatureswhere PTFE resins would emit fumes lethalto a parakeet, would probably flash near thattemperature and not only kill the bird, butpossibly involve the kitchen and the entirehouse in a fire.

This sensitivity of some birds to inhaledmaterials is not a new finding. It has been longknown. Probably the best known example isthe classic use of a caged canary in mines todetect low oxygen levels or detect toxicants atlevels below a human effect level.The data already presented can be applied

to the other possible resin overheating situa-

Table 1. Pan temperatures at which the pyrolysis productsfrom PTFE-coated frypans, uncoated pans or uncoated panscontaining butter or corn oil were lethal to Japanese quail,

parakeets or rats exposed for 4 hr (ALT).

ALT, 0CTest material -

Quail Parakeets Rats

Cast iron frypan (CI) > 450 > 330Aluminum frypan (AL)a >450AL+corn oils 310bCI+corn oil 290bAL+butters 290 < 260 250CI +butter 260 < 260PTFE-clad pan 330 280 425-450

* Handle removed.b Media flashed.

PTFE INHAlATION TOXICANT SPECTRUM

50 150 250 350 450 550 650 750

- PARTICULATE -POLYMER

_I FUME I4-FEVER ?'

D- HF -

TFE COF2 *

1+HFPIPFIB lC2

L * * K50 150 250 350 450

TEMP. (C)550 650 750

FIGURE 3. PTFE inhalation toxicant spectrum as afunction of temperature (heated in air).

tions listed above and those uses will not bediscussed further.The toxicological spectrum of PTFE as

developed in du Pont and other laboratories(1-5) is summarized in Figure 3 and can beapplied to all known PTFE uses. As can be seen,the industrial hygiene ramifications of thisspectrum are complex.For temperatures up to about 350°C, the

resin dust is the toxicological determinant.Studies have shown this can be considered anuisance dust (15).

Particulate, whatever its chemical composi-tion, is probably the principal toxic agent fromabout 350°C to 450°C. However, at tempera-tures ca. 25°C above the ALT, PFIB is formedand, at temperatures ca. 30°C above the ALT,it may be present in concentrations lethal torats (5). At temperatures 100-300°C above theALT, carbonyl fluoride is probably the principaltoxicant (18,19). At these elevated tempera-tures, a further reaction, yielding the less toxicmaterials CF4 and CO2 is possible (18).

The pyrolysis product boundaries shownshould be considered a range rather than adefinite value. The toxicological importance ofeach material in the spectrum will be minimalat either end of its region and will become maxi-mal in the center.The relative abundance of the respective

toxicants at various temperatures is, of course,a function of the reaction kinetics for the vari-ous reactions involved.A similar spectrum exists for FEP polymers,

but the evolution of the various gaseous ma-terials commences about 500C lower than forPTFE resins.

June 1975 201

A strictly analogous toxicological situationpertains in the case of the pyrolysis in air ofpolymethylene (also known as oil) or its con-stituent hydrocarbons. Its toxicity at roomtemperature is due solely to the toxicity of theparticular hydrocarbon molecule. As the tem-perature is raised, thermal "cracking" andrearrangement occur, and the toxicity of theseproducts becomes important. As the tempera-ture is increased further, carbon monoxide isformed and its toxicity usually dominates. Withfurther increases in temperature, carbondioxide is formed, and its toxicity is predomi-nant. Thus, here also, the toxicological deter-minant depends upon the temperature of thesystem.A similar temperature-dependent toxicologi-

cal spectrum also exists for the naturally occur-ring cellulosic polymer wood, and a similartoxicological spectrum almost certainly existsfor other synthetic polymers. However, mostother polymers differ from PTFE and FEP inthat they contain adjuvants, as does wood,which may also contribute to the toxicity ormodify the pyrolysis products at any tempera-ture.The amount of pyrolysis and the pyrolysis

products may also vary with the pyrolysisatmosphere. For example, we have found thatthe weight loss of PTFE heated in nitrogen at450'C was less than half that of PTFE heatedin air. As might be expected, the pyrolysate,on a weight-of-polymer-heated basis, was lessthan half as toxic to rats (5). We also haveevidence that particulate evolved from PTFEheated in a nitrogen atmosphere is chemicallydifferent from particulate evolved from PTFEheated in an oxygen atmosphere. Since we donot yet have an animal model for polymer fumefever, we have not been able to determinewhether or not this particulate can cause poly-mer fume fever.

In summary, to evaluate all the pyrolysis andcombustion hazards from a polymer on a con-tinuing basis, one must: try to anticipate all(human) exposures; simulate these in the lab-oratory; use the actual product in all tests, ifpossible; maintain close liaison with themarket; keep an open mind; and rememberthat pyrolysis products vary with temperatureand additives and also with pyrolysis atmos-phere.

REFERENCES

1. Treon, J. F., et al. Wright Air Development CenterTechnical Report 54-301. Wright-Patterson AirForce Base, Ohio, June 1954.

2. Zapp, J. A., Jr., Limperos, G., and Brinker, K. C.Toxicity of pyrolysis products of Teflon tetrafluoro-ethylene resin. Paper presented at American In-dustrial Hygiene Association Annual Meeting, 1955.

3. Danishevskii, S. L., and Kochanov, M. M. On thetoxicology of some fluoro-organic compounds. Gig.Trud. Prof. Zabolev. 5: 3 (1961).

4. Coleman, W. E., Scheel, L. D., and Gorski, C. H.The particles resulting from polytetrafluoroeth-ylene (PTFE) pyrolysis in air. Amer. Ind. Hyg.Assoc. J. 29: 54 (1968).

5. Waritz, R. S., and Kwon, B. K. The inhalationtoxicity of pyrolysis products of polytetrafluoro-ethylene heated below 500 degrees centigrade.Amer. Ind. Hlg. Assoc. J. 29: 19 (1968).

6. E. I. du Pont de Nemours, and Co., Inc. Teflonfluorocarbon resins-safety in handling and use,1970.

7. Clayton, J. W., Jr. The mammalian toxicology oforganic compounds containing fluorine. In: Hand-book of Experimental Pharmacology, New Series,Vol. 21, F. A. Smith, Ed., Springer-Verlag, NewYork, 1966, Chap. 9.

8. Paulet, G., and Debrousses, S. The toxicity ofhexafluoropropylene (1). Arch. Mal. Prof. Med.Trav. 27: 509 (1966).

9. Clayton, J. W., Jr. Haskell Laboratory Report 50-60, E. I. du Pont de Nemours and Co., Inc,unpublished.

10. Clayton, J. W., Jr., Delaplane, M. A., and Hood,D. B. Toxicity studies with octafluorocyclobutane.Amer. Ind. Hyg. Assoc. J. 21: 382 (1960).

11. Clayton, J. W., Jr. Fluorocarbon toxicity and bio-logical action. In: Fluorine Chemistry Reviews,P. Tarant, Ed., Dekker, New York, 1967.

12. Cavagna, C., Finulli, M., and Vigliani, E. C.Experimental study on the pathogenesis of thefever caused by inhalation of Teflon vapors (poly-tetrafluoroethylene). Med. Lavoro 52: 251 (1961).

13. Stephens, S. S. Animal models for polymer fumefever studies. Paper presented at American In.dustrial Hygiene Association Annual Meeting,1971.

14. Harris, D. K. Polymer fume fever. Lancet 261:1008 (1951).

15. American Industrial Hygiene Association. TeflonFluorocarbon Resin Hygienic Guide, AmericanIndustrial Hygiene Association, Akron, Ohio 44313,1963.

16. Lehman, A. J. Quart. Bull. Assoc. Food Drug Offic.U.S., 26: 109 (1962).

17. Griffith, F. D., Stephens, S. S., and Tayfun, F. 0.Exposure of Japanese quail and parakeets to thepyrolysis products of frypans coated with Teflonand common cooking oils. Amer. Ind. Hyg. AssocJ. 34: 176 (1973).

18. Coleman, W. E., et al. The identification of toxiccompounds in the pyrolysis products of polytetra-fluoroethylene (PTFE). Amer. Ind. Hyg. Assoc. J.29: 33 (1968).

19. Scheel, L. D., Lane, W. C. and Coleman, W. E.The toxicity of polytetrafluoroethylene pyrolysisproducts-including carbonyl fluoride and a re-action product, silicon tetrafluoride. Amer. Ind.Hyg. Assoc. J. 29: 41 (1968).

202 Environmental Health Perspectives