An instrument for characterization of the thermal and optical properties of charring polymeric materials

Twenty-Fifth Symposium (International) on Combustion/The Combustion Institute, 1994/pp. 1447-1453

AN I N S T R U M E N T FOR CHARACTERIZATION OF T H E THERMAL A N D OPTICAL PROPERTIES OF C H A R R I N G POLYMERIC MATERIALS MICHAEL A. SERIO, DAVID S. PINES,* ANTHONY S. BONANNO AND PETER R. SOLOMON Advanced Fuel Research, Inc. 87 Church Street, East Hartford, CT 06108, USA AND

GIRARD A. SIMONS Simons Research Associates 3 Juniper Road, Lynnfield, MA 01940, USA A test instrument was developed and used to characterize the changes in the thermal and optical properties of charring polymeric materials when exposed to radiative heat fluxes. The system is based on a bench-top emissometer apparatus, which was developed originally to simultaneously measure the surface temperature and spectral properties of materials at elevated temperatures and was modified in this work to study charring polymers. An advantage of using a modified emissometer is that the front surface temperature is measured optically instead of with a thermocouple. Time-resolved measurements of the front surface temperature, along with thermocouple measurements of the interior and back surface temperatures can provide information on the changes in thermal conductivity and thermal diffusivity with the extent of charring, assuming that the heat of gasification is known. In addition, this apparatus measures the changes in the material's spectral emissivity and functional group composition as it chars. Data are presented for experiments on two different samples of 3.2-mm-thick polyurethane, heated using radiant fluxes from 22 to 32 kW/m2. Analysis of the surface temperature and emissivity data indicates that the samples undergo a transition from a volume to a surface absorber during initial irradiation. Quantification of this behavior will be a critical element in predicting the flammability limits of these materials.

Introduction An important consideration in the combustion of polymers is the pyrolytic degradation processes that lead to the production of volatile fuel, smoke, toxic gases, and char [1,2]. While test methods allow reasonable assessments for noncharring polymers, test methods for charring polymers have not been as well developed, and there is no accepted methodology for prediction of their behavior in a fire [3]. The current polymer decomposition models have also focused primarily on the predictions of relatively simple, noncharring polymers such as PMMA [4-8]. Recently, the study of charring polymers under fire conditions has begun to receive more attention [9-13]. However, there is no currently available model for the pyrolysis of charring polymers that provides a good description of the char physical properties (thermal conductivity, heat capacity, emissivity, and oxygen reactivity). The development of such a model would be of significant benefit in modeling the behavior of *Current address: Department of Environmental Engineering, University of Massachusetts, Amherst, MA 01002, USA. 1447

charring polymers in fires. These models require input data on the thermal and optical properties at combustion temperatures. This paper describes an apparatus that has been developed to obtain the relevant data for charring polymers.

Experimental Samples:

The apparatus was developed and tested using two samples of polyurethane (Mobay 110-25) obtained from General Motors in flexible sheets of - 3 . 2 mm in thickness. The monomeric components of the Mobay 110-25 polyurethane are diphenylmethane diisocyanate and polyether polyol. The samples were cut into discs that were 7.1 mm in diameter. The initial density of the material was estimated to be - 1 . 0 8 g/cc. One of the samples was reinforced with 15% mica. Apparatus:

A bench-top emissometer was developed at Advanced Fuel Research (AFR) to simultaneously mea-

1448

FLAME SPREAD, FIRE AND HALOGENATED FIRE SUPPRESSANTS Paraboloidal Mirror

Front Surface M e a s u r e m e n t

Hemi-ellipsoidal Mirror - Selector Mirror

./ Water-Cooled Chopper Blade

FT-IR Interferometer

Sample Radiating N e a r Black.

Detector (0.8-x.6~m~) B a c k Surface M e a s u r e m e n t [ Mirror to Direct Top H a l f o f B e a m after it E x i t s t h e I n t e r f e r o m e t e r to MCT D e t e c t o r (1.6-20~.m)

FIG. 1. Schematic diagram of the Bench-top emissometer. sure the surface temperature and spectral properties of materials at elevated temperatures [14-16]. A schematic of the bench-top emissometer is shown in Fig. 1. The hemiellipsoidal mirror has both foci inside the mirror, with a source located at one of the foci and the sample located at the other focus. This mirror geometry, combined with the radiating characteristics of the blackbody source, provides a means of irradiating the sample both hemispherically and diffusely. Therefore, the measurement of reflected radiation from the front surface in a given direction is that of directional-hemispherical reflectance since the sample is irradiated uniformly from all directions. Likewise, for transmissive samples, the back surface measurement is of directional-hemispherical transmittance. An integral part of the optics is the rotating chopper system that moves either an aperture or a cold blackbody element in front of the source. The Fourier transform infrared (FT-IR) data collection system is synchronized with these two states and allows for the discrimination of sample radiation from reflected/transmitted radiation as follows. For the reflectance measurement, the IR beam originates at the blackbody source at one focus of the hemiellipsoidal mirror. The radiation reflects from the hemiellipsoidal mirror and is focused onto the sam-

ple at the other focus, where it is reflected (scattered) by the sample into the interferometer. The reflectance and the sample radiance are measured together when the aperture on the chopper rotor is in place over the source (chopper open condition). When a cold blackbody is substituted for the aperture over the source (chopper closed condition) it is the sample radiance alone that is measured. Both the radiance and directional-hemispherical reflectance can be obtained from these two spectra and their difference. For the transmission measurement, radiation from the source is collected by the hemiellipsoidal mirror and directed to the sample. In this case, the radiation directly and diffusely transmitted by the sample is directed by the selector mirror into the FT-IR spectrometer for measurement. Again, the directionalhemispherical transmittance and radiance from the sample's back surface is obtained by comparing the spectra for the two chopper positions. In the bench-top instrument, two separate detectors are normally utilized to measure the near- and mid-IR energy. A room-temperature indium-gallium-arsenide detector is sensitive to near-IR energy (0.8-1.6 pm), and a liquid nitrogen-cooled mercurycadmium-telluride detector is used for longer wavelengths (1.6--20/~m). Both spectral regimes are mea-

CHARACTERIZING PROPERTIES OF CHARRING POLYMERIC MATERIALS

Calcium Silicate Sample Holder

T

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Ill L Polyurethane Ill Sample ~ ~ ~ L ~ - -

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sured simultaneously. However, for the work done on the current project, the former detector was not installed. In order to provide the information required to characterize the properties of a charring polymer that is exposed to a radiant heat source, several changes were made. The emissometer was modified to include an inert gas purge over the sample's front surface, temperature measurements of the interior and back surface using thermocouples and insulation of the sample with a block of calcium silicate to ensure one-dimensional heat transfer. It is planned to include a load cell (for weight loss measurements) and a provision for analysis of evolved gases in the final version of the instrument. The thermocouples used were type K (Omega no. CHAL-005) of 0.13-mm diameter that were inserted through predrilled holes in the calcium silicate block and covered (except at the tip) with a ceramic insulator. A schematic of the sample holder is shown in Fig. 2. In order to simplify the interpretation of the measured thermal and optical properties, it is desirable to have a uniform heat flux. In a hemiellipsoid mirror, the magnification of an image at one focus onto the other focus depends on the distance between the two foci. In order to determine the heat flux profile of the bench-top emissometer at the sample focus from a 750 ~ source furnace (25 • 25 mm), a power gauge with a 3.6-mm aperture was used to measure the heat flux at various locations around the sample's focus [17]. The ellipsoidal shape of the profile was consistent with the experimental and analytical ray tracing done by Neu [18]. The heat flux profile indicates that the energy is more constant in the vertical direction than the horizontal. In order to keep the heat flux over the entire sample within 10% of the average flux, a sample size of approximately 7-mm diameter was selected. To verify the one-dimensional heat-transfer assumption, the temperatures, at the same depth (1.6 mm from the front sur-

FIG. 2. Configuration of sample holder, thermocouples, and nitrogen purge flow.

face) but at two different radial positions (2 mm right of center and 2.8 mm left of center), were measured using thermocouples for a polyurethane sample with a 750 ~ source furnace. Since the measured temperatures were within 2 ~ of each other, the onedimensional assumption was assumed to be valid.

Theoretical Background: For the front surface sample measurements, the cold blackbody element of the rotor is in place over the cavity radiation source. The measurement M1 will be the sample's directional spectral radiance: Mlv = ev(T~) R~(Ts)

(1)

where ev is the emissivity of the sample at temperature, Ts, and R~b is the Planck function at temperature Ts. The subscript v indicates spectral quantities. With the aperture of the rotor placed over the blackbody source, the measured radiation, M2, will include that emitted by the sample (M1 v) and the blackbody source radiation reflected by the sample in a spectral directional-hemispherical mode, M2v = Mlv + pv(T~)R~(Tbb)

(2)

where Tbb is the constant blackbody temperature of the source radiation and pv is the spectral directionalhemispherical reflectivity. The difference of the two measurements is thus M2~ - MI~ = p~(T~)R~(Tbb).

(3)

The constant source radiation is quantified by replacing the sample with a perfect reflector (a gold mirror, Pv = 1.0) and measuring the spectrum in the chopper open condition: Mref = pgoldRb(Tbb) = Rb(Tbb).

(4)

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FLAME SPREAD, FIRE AND HALOGENATED FIRE SUPPRESSANTS

The ratio of Eq. (3) to Eq. (4) results in the measurement of the directional-hemispherical reflectance of the sample, p , at the unknown temperature,

T,: (M2v- MI~)/M~ef = [p~(T,)a~(Tbb)]/R~(Tb~)

= p~(T~).

(5)

For an opaque sample, the spectral emittance, ev, can now be determined from closure: e~ = 1 - pv

(6)

Once e~ is determined, the precise sample temperature is found by rearrangement of Eq. (1):

R~(Ts) = [MlJev(Ts)].

(7)

For nonopaque samples, the directional-hemispherical transmittance, rv, is measured by flipping the selector mirror and measuring the back surface radiance (M3v) and back surface radiance plus transmittance (M4v). The source radiation is quantified with the sample absent, and the analysis to determine z~ follows that for Pv. The more extensive closure relationship is then used to determine e~: e~=

1-p~-r~.

(8)

Advantages of Emissometerfor Charring Polymer Applications: An advantage of using the modified emissometer for studying the behavior of charring polymers exposed to radiant heat is that the front surface temperature is measured optically instead of with a thermocouple. Front surface temperatures are difficult to measure accurately with a thermocouple because they are affected by the radiant heat source. It is also true that small changes in the placement of the thermocouple (slightly above or below the surface) can give a significant difference in the temperature measurement. In addition to measuring the thermal properties in cases where the heat of gasification is known, this apparatus will measure the change in the material's emissivity as it chars. Knowing the emissivity is necessary in accurately predicting a material's flammability since radiation is usually the main heattransfer mechanism in a fire.

Results

Preliminary testing of the polyurethane samples was conducted using the existing emissometer prior to making any modifications. The sample without mica reinforcement was tested at blackbody source temperatures ranging from 600 ~ to 850 ~ corre-

sponding to radiant heat fluxes ranging from 22 to 32 kW/m2. The sample was initially heated with a 600 ~ blackbody source. When the sample had reached steady state, radiance and radiance plus reflection measurements were taken from the front surface (Fig. 3a). The selector mirror was then flipped, and radiance and radiance plus transmission measurements were taken from the back surface (Fig. 3b). The two back surface measurements were identical below tile 4500 cm- 1 wave number, which indicates that the sample is opaque in this region. By closure (1 - p - r), the emittance was calculated and is presented in Fig. 3c. Finally, the normalized radiance of the front surface (radiance/emissivity) along with a Planck blackbody curve at 229 ~ is presented in Fig. 3d. These preliminary tests demonstrated the feasibility in using the bench-top emissometer to measure the surface temperature and the spectral changes in a charring polymer. Consequently, the bench-top emissometer was modified to include a nitrogen purge, and a special sample holder was designed and constructed, as described in the experimental section (see Fig. 2). The first tests performed with the modified apparatus were to measure the front, middle, and back surface temperature as well as the spectral properties of the front surface as the sample was being heated at different radiant heat fluxes (22, 26.5, and 32 kW/m2). The 15% mica-reinforced sample was selected because it is opaque. Therefore, transient temperature measurements can be made using a single scan without having to flip the selector mirror. The front, middle, and back surface temperatures as a function of time at the middle heat flux are shown in Fig. 4. In addition, the average emissivity between 6500 and 500 cm- 1 wave numbers is shown (right-hand scale). The results at the low (22 kW/m2) and high (32 kW/m2) heat fluxes are qualitatively similar [17]. In all three cases, the polyurethane became more reflective as it was charred. The spectral changes in the emissivity as the sample was heated at the 26.5 kW/m2 heat flux are shown in Fig. 5. Initially, the virgin material is a gray body due to self-absorption. As the surface is charred, it becomes more reflective and exhibits very strong spectral features with an isocyanate peak ( N ~ C ~ O antisymmetric stretch at 2275 cm-1), a N--H-free stretch at 3350 cm -1, and an O - - H stretch at 3625 cm-1. There is initially an aliphatic peak at 2950 cm-1, which then disappears as the polymer becomes charred, with the appearance of a strong aromatic peak at 3079 cm -1. The change from an aliphatic to a more stable aromatic structure is consistent with previous studies involving the pyrolysis of charring polymeric materials [2]. These changes are not only a function of temperature, but are also time dependent, which is consistent with the finite-rate chemistry of the charring process. The ability to see the spectral detail of the char is

CHARACTERIZING PROPERTIES OF CHARRING POLYMERIC MATERIALS

1451

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FIG. 3. Spectral emittance and temperature determination of polyurethane: (a) spectral radiance (chopper closed) compared to radiance + reflectance (chopper open); (b) back surface radiance (chopper closed) compared to back surface radiance + transm,ission (chopper open); (c) spectral emittance by closure, ev = 1 - pv - rv; and (d) temperature determination by overlaying normalized radiance with theoretical blackbody temperature curves.

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FIG. 4. Polyurethane (15% mica) temperature profile at 26.5 kW/m 2.

~ 3 m e = 0 s; Temp = 94~ "Time = 200 s; Temp = 379~ "rl'lme = 230 s; Temp = 418~ "-Time = 1372 s; Temp = 392~

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FIG. 5. Spectral emittance of polyurethane (15% mica) exposed to 26.5 kW/m z.

1452

FLAME SPREAD, FIRE AND HALOGENATED FIRE SUPPRESSANTS TABLE 1 Exponential decay constants for temperature profiles in polyurethane slabs

Heat Flux (kW/m2) 22

26.5 32

Location Front surface

1/16" depth

Back surface

0.022 0.023 0.027

0.012 0.013 0.012

0.008 0.009 0.008

quite surprising since it was expected that the char would exhibit gray body behavior similar to the virgin material. More work is needed to understand why the spectral features are so strong. However, it is likely that this is related to the change of the material from a volume absorber to a surface absorber as charring proceeds. Further evidence for this phenomenon is obtained from an analysis of the temperature profiles, as discussed below. This measurement technique provides the important advantage of being able to monitor the chemical changes in the polymer's surface during the charring process. The 25% decrease in the average emissivity indicated in Fig. 4 reduces the radiation absorbed and, therefore, inhibits further pyrolysis of the interior. Knowing how the emissivity changes with charring is an important parameter in being able to accurately predict the behavior of a polymer that is exposed to a radiant heat source. The temperature profiles of the front, middle, and back surfaces of all three tests showed an exponential decay response from room temperature to an asymptotic maximum temperature. The profiles were fitted using the following formula, and the exponential factors were compared for the three cases: T = (Ts~ - T,)-[1 - exp(-mt)] + T,

(9)

where T is the temperature at time t (~ Tss is the asymptotic steady-state temperature (~ Ti is the initial temperature at t = 0 s (~ t is time (s), m is the exponential constant. It is interesting to note that the exponential factors for the interior and back surfaces are approximately the same for the three different heat fluxes. As expected, the exponential factor for the front surface increased as the heat flux increased. Recent studies conducted on the ignition of solids have treated the optical depth as a constant or as zero for the case of a pure conductor (e.g., see Ref. 11). While in-depth pyrolysis has been described as the source of the combustion products, the subsequent effects of the internal pyrolysis have not been studied extensively. The temperature profiles of the virgin materials, shown in Fig. 4, indicate that temperature

overshoot of the front surface temperature is characteristic of the virgin material. However, there is no such observation for the precharred material [17]. The temperature rise of the virgin material is nearly linear, whereas that of the precharred material is approximately the square root of time. This observation suggests that the virgin material is a volume absorber, whereas the precharred material is a surface absorber: relating the heat flux I to the temperature rise AT (t), we write It = pCp AT5

(10)

where p ~ 1 g/cc9 c,,E ~- 2 J/g K and 5 is either the optical or the thermal depth of the energy deposition. The surface temperature data for the virgin material (liT(t) = 4t) suggests that 5 = constant optical depth = 4 mm, while that for the precharred material (AT(t) = 50 t 1/2) suggests that ~ is a thermal diffusion depth (4at) 1/2 corresponding to a = 2 • 10 s m2/s. A value of 3 mm was independently measured for the optical absorption depth of the virgin material in the near IR (1.5 to 2.7/lm) and a value of 2 to 4 x 10 -8 m2/s for the diffusivity of the char [17]. These data clearly demonstrate that the charring is controlling the evolution of the material from a volume to a surface absorber and a thorough understanding of this aspect of the charring process is necessary to predict the temperature profile and ignition characteristics.

Conclusions A test instrument was developed and used to characterize the changes in the thermal and optical properties of charring polymeric materials when exposed to radiative heat fluxes. The system is based on a bench-top emissometer apparatus, which was developed originally to simultaneously measure the surface temperature and spectral properties of materials at elevated temperatures and modified in this work to study charring polymers. Time-resolved measurements of the front surface temperature, along with thermocouple measurements of the interior and back

CHARACTERIZING PROPERTIES OF CHARRING POLYMERIC MATERIALS surface temperatures, provide information on the changes in thermal conductivity and thermal diffusivity with the extent of charring, assuming that the heat of gasification is known. These data can be used for characterization of polymeric materials or as inputs to a model of a charring polymer surface. In addition, this apparatus measures the changes in the material's spectral emissivity and functional group composition as it chars. The results for polyurethane indicate that it becomes more reflective as it is charred and that the chemical composition becomes more aromatic. It is relatively straightforward to use the instrument to determine the thermal properties of already charred materials. For the virgin materials, the effects of the heat of gasification and the change in the material from a volume absorber to a surface absorber must be included in the analysis.

Acknowledgments The authors arc grateful for the support of the National Science Foundation under Grant No. Ill-9101585. The authors also wish to thank Karen Kinsella of Advanced Fuel Research for assistance with the experimental work and Professor Eric Suuberg of Brown University for helpful discussions on the behavior of charring polymers. Professor Mark Petrich of Northwestern University supplied the polyurethane samples, which were obtained from General Motors. REFERENCES 1. Gann, R. G., and Dipert, R., "Polymer Flammability," Encyclopedia of Polymer Science and Engineering, John Wiley and Sons, New York, 1986. 2. Cullis, C. F., and Hirschler, M. M., The Combustion of Organic Polymers, Clarendon Press, Oxford, 1981. 3. Di Blasi, C., Prog. Energy Combust. Sci. 19:71 (1993). 4. Kashiwagi, T., Inaba, A., Brown, J. E., Hatada, K., Kitayama, T., and Masuda, E., Macromolecules, 19:2160 (1986).

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5. Inaba, A., Brown, J. E., and Kitayama, T., Eur. Polym. J. 23:871 (1987). 6. Kashiwagi, T., and Omori, A., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1329 (1988). 7. Meeki, K., Atreya, A., Agrawal, S., and Wichman, I., Twenty-Third Symposium (International)on Combustion, The Combustion Institute, Pittsburgh, 1701 (1990). 8. Hertzberg, M., and Zlochower, I. A., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1247 (1990). 9. Parker, W. J., in Heat Releasefrom Fires (V. Babranskas and S. J. Grayson, Eds.), Elsevier Applied Science, New York, 1992, pp. 333~56. 10. Kashiwagi, T., and Nambu, H., Combust. Flame 88:345 (1992). 11. Nakabe, K., Bantu, H. R., and Kashiwagi, T., "Ignition and Subsequent Flame Spread over a Thin Cellulosic Matetial," Proceedingsof the Second International Microgravity Combustion Workshop, NASA Conference Publication 10113, 1992. 12. Milosavljevic, I., and Suuberg, E. M., ACS Div. Fuel Chem. Preprints 37(4):1567 (1992). 13. Ohlemiller, T. J., "Assessment of the NASA Flammability Screening Test and Related Aspects of Material Flammability," Final Report, NASA CR-189226, Aug., 1992. 14. Markham, J. R., Solomon, P. R., and Best, P. E., Rev. Sci. Instr. 61:3700 (1990). 15. Markham, J. R., Best, P. E., Solomon, P. R., and Yu, Z. Z., ASME]. Heat Trans. 114:458 (1992). 16. Markham, J. R., Kinsella, K., Carangelo, R. M., Brouillette, C. R., Carangelo, M. D., Rest, P. E., and Solomon, P. R., Rev. Sci. Instr. 64(9):2525 (1993). 17. Serio, M. A., Pines, D. S., Bonanno, A. S, and Solomon, P. R., "Measurements and Modeling of the Behavior of Charring Polymers in Fires," Annual Report for National Science Foundation, Grant No. III9101585, Dec., 1993. 18. Neu, J. T., NASA CR--73193, 1968.