JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Infrared Transmittance and Emittance of Polytetrafluoroethylene*

TUNIS WENTINK, JR.,t AND WALTER G. PLANET, JR.t

A vco-Everett Research Laboratory, Everett, Massachusetts

(Received September 27, 1960)

The infrared absorption spectrum of polytetrafluoroethylene at 2980K from 2 to 15 1 and the surface-

emission spectrum at a minimum temperature of 700'K from 2.7 to 6.0 1 are reported. Corresponding

absorption coefficients and emissivity are discussed. The emissivity varies with wavelength in accordance

with the absorption, as expected, so that assignment of a color temperature or description of the hot surface

as a graybody are not valid.

I. INTRODUCTION

W E have reported previously a study of the infraredNV (ir) emissivity of quartz.' The following is areport of a similar investigation on polytetrafluoro-ethylene (Teflon)2 under ablating conditions.3

The plastic Teflon is of interest because of its high-temperature (for organic materials) stability. Thematerial is very stable up to 650°K,4'5 and decomposesrapidly and smoothly in the 700-800'K range.'6' Wepresent here some results on the ir emission spectra of

Teflon above 700°K and discuss the emissivity (e).

II. EXPERIMENTAL

The instrumentation and experimental approach wereidentical to that described previously.' Our method wasto record the emission spectra, with the absolute radiantemittance known by comparison with a calibratedblackbody, and in a spectral region where the assump-tion of emissivity e near unity was made. Thus the truesurface temperature could be estimated. Hence, in

adjacent spectral regions where the e was clearly less

than unity, it can be deduced from the known tempera-ture and absolute emittance.

A study of the ir absorption of Teflon was necessary

*Sponsored by the Ballistic Missile Division, Air Research andDevelopment Command, U. S. Air Force.

t Present address: Avco Research and Advanced DevelopmentDivision, Avco Corporation, Wilmington, Massachusetts.

$ Present address: Barnes Engineering Corporation, Stamford,Connecticut.

1 T. Wentink, Jr., and W. G. Planet, Jr., J. Opt. Soc. Am. 51,595 (1961).

2 For convenience and briefness we use Teflon loosely as ageneric term. However, it should be noted that Teflon is theregistered trademark of the DuPont Company, covering fluoro-carbon resins. These include the TFE-fluorocarbon resins(polymers of tetrafluoroethylene) and FEP-fluorocarbon resins(copolymers of hexafluoropropylene and tetrafluoroethylene).Here we deal only with the TFE-resins.

3 S. Georgiev, H. Hidalgo, and M. C. Adams, Heat Transfer andFluid Mechanics Institute (Stanford University Press, Stanford,California, 1959), p. 171. Enthalpy conditions are discussed andphotographs of the ablating surfaces are presented therein.

4J. C. Siegle and L. T. Muus, "Pyrolysis of Polytetrafluoro-ethylene," paper presented at 130th National Meeting of Am.Chem. Soc., September 17, 1956.

5 S. L. Madorsky and S. Straus, J. Research Natl. Bur. Stand-ards 63A, 261 (1960).

6 S. L. Madorsky, V. E. Hart, S. Straus, and V. A. Sedlak, J. Re-search Natl. Bur. Standards 51, 327 (1953); R. E. Florin, L. A.Wall, D. W. Brown, L. A. Hymo, and J. D. Michaelsen, J. Re-search Nat]. Bur. Standards 53, 121 (1954).

for interpretation of the emission data. Liang and Krimm7

have reported absorption spectra for films (0.004-0.0041mm thickness) in the spectral range 3-13 A, and ingreater thicknesses from 11 to 140 p. The room-temperature absorption spectra of sections (0.013-0.76mm) that we measured in the 2 to 15 tt region are shownin Fig. 1. The strong absorption at X<3 A is due tooptical scattering in the opaque crystalline milky whitepolymer.

However, above 3240C, a transition temperature,Teflon is optically clear so that this scatter "absorption"is not expected to contribute to the high temperatureemissivity. The only significant absorption in thinsections out to about 6 ,u is that due to a C-F vibrationalovertone at 4.2 /. Hence, one would predict in theregion 2-6 A the emittance from Teflon would be char-acterized by a pronounced peak at 4.2 p.

An emission spectrum of Teflon is shown in Fig. 2.

The presence of a sharp peak at 4.2 , is in agreementwith the prediction of the emissivity from the absorptionspectrum. The absorption noted in the emission spec-trum is attributed to CO2 outside the test chamber.Normally the gases C2F4 (the monomer from Teflondecomposition) and CO2 (from C2F 4 oxidation) 4 6 wouldbe a problem due to the large absorption coefficients ofthese molecules. However, these can be ignored becauseof the low gas density and short gas path lengths of thetest conditions.

III. DISCUSSION

The surface temperature deduced from the emittancewas 700430'K, on the assumption of e= 1 at the peak,and no correction for the CO2 absorption. Both of theseapproximations tend to lower the calculated tempera-ture and so set a lower bound of the temperature duringablation. The wavelength of measurement is near theblackbody maximum at the temperature involved, asituation favorable in determining the temperaturefrom the measured radiance. For example, at 4.2 theblackbody emittance increases a factor of 2 in goingfrom 650 to 750°K. Hence, the experimental error in

the temperature is expected to be small.However, the assumption of e = 1 is more fraught with

uncertainty. This is particularly questionable if, as is

7 C. Y. Liang and S. Krimm, J. Chem. Phys. 25, 563 (1956).

601

JUNE, 1961VOLUME 51, NUMBER 6

. WENTINIK, JR., AND W. G. PLANET, JR.

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indeed expected, the low conductivity of Teflon willmake for large temperature gradients below the surface,that is, the radiating surface of interest is very thin andthe transmittance becomes appreciable. Then theassumption is poor, and the measurement interpretationbecomes more complicated. Thus, a more desirableexperiment is to measure the emittance of the band at8-9 u, where the emissivity assumption is better.

The scatter effect in the transmission spectra at theshorter wavelengths prevents accurate calculation ofabsorption coefficients (a), although we have estimatedsuch near the absorption band at 4.2 p.. The data fit inmost cases the usual expression I/lo = exp (-a C). Thesecoefficients are 9.08 and 1.61 (in mm-') at the peak andlowest absorption wavelengths, 4.2 and 4.8,u, respec-tively. Beyond 5 , the data should permit a betterdetermination of the coefficients. Some typical valuesare 4.90, 2.60, and 12.15 (in mm-') at 5.6, 6.1, and 7.5 u,respectively.

An estimate of the surface-layer thickness needed togive an e close to 1.0 can be made with the approxima-tion that the a is insensitive to temperature. Thus, at4 .2 ,u where =9.08 mm-', .C=0.1, 0.2, and 0.5 mmcorrespond to transmittances of 40, 16, and 1%. It istempting to estimate an emissivity for our case. How-ever, the temperature distribution normal to the surfaceunder our test conditions is sufficiently uncertain (theaverage ablating layer thickness 0.2 to 0.5 mm) so thatwe refrain from this estimate and correspondingcorrection to the measured minimum temperature.

The ablation3 of the Teflon models, monitored with

, . . . . . . . ~ ~~~. I 6 5 4 3 2.7 3 4 5 6

FIG. 2. Infrared emission spectrum of Teflon (Au-Ge detector;A1 20 3 window; CaF2 spectrometer prism; 0.1 v/large division).

\M;LE FIG. 1. Infrared trans-_ smittance of Teflon

(room temperature).The 0.025-mm samplewas cast film and trans-parent in the visible; the

_ .__________ thicker samples were"skived" (shaved) frombulk milk-white stockand opaque in the

E H i ~~~~~~~~visible.I 11 12 13 14 15

motion pictures, proceeded smoothly and with no indi-cation of a liquid layer. The surface temperature for"rapid and complete" decomposition of Teflon to themonomer is given as 780'K by i\Madorsky. Our data arereasonable if we assume there is equilibrium at the sur-face during ablation.

Since completion of the present set of experiments wehave learned of several results that indicate the tem-perature of ablating Teflon is in the 900-1000 0 K range.Jennings and Easton8 have calculated a surface tem-perature 8330 to 11270 K at heating rates of 1 Btu/sec ft2

to 1000 Btu/sec ft2, and cite the pyrometer measure-ment of Zirinsky 9 of 9220K (in a cyanogen-oxygen flameat 450 Btu/sec ft2). The most extensive work known tous is that of Hanst, 10 which is based on measured spec-tral emissivity and covered the 8-9 1u band. This givestemperatures in the 950-1000'K range.

Thus, calculation of the emissivity in the ir regionfrom our data is not warranted, since our operating truetemperature is not known sufficiently accurately. How-ever, the most important result is confirmation that thesurface emissivity of Teflon varies with wavelength inaccordance with the absorption spectrum. Thus, non-spectral measurements leading to a "color" temperaturecan be misleading; that is, treatment as a graybody isnot justified.

ACKNOWLEDGMENTS

Our thanks to Dr. J. C. Siegle of the Du Pont Com-pany and Dr. S. L. Madorsky of the National Bureauof Standards. The former supplied thin film samples,and both provided much useful information. Dr. P.Hanst of Avco RAD contributed considerably invalu-able discussions, and particularly in supplying hisunpublished data and calling other results to ourattention.

8 R. L. Jennings and C. R. Easton (unpublished data), DouglasAircraft Company, Inc., Santa Monica, California (May, 1959).

9 S. Zirinsky (unpublished data), General Electric Company,MSVD, Philadelphia, Pennsylvania (1957).

10 P. -lanst (un)ublished data), Avco Research and AdvancedDevelopment Division, Wilmington, Massachusetts (January,1960).

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