THERMAL CONDUCTIVITY OF CARBON BLACKS The thermal conduativity of three samples of commercial carbon black has been measured. The thermal conductivity appears to be independent of the ultimate particle si- and, in two aamples, is lower than that of still air. Agglomeration of carbon blaok into free-flowingpellets increases the conductivity t o a value slightly higher than that of still air.
Smo1uohowL.i (19) reported APPARATUS The thermal values reported here were that “soot” had a lower thermal 91 W. R. SMITH determined in the Heat Measurements conductance than still air. Other Godfrey L. Cabot, Inc., Boston, Mass. Laboratory at Massachusetts Institute values reported in standard tables are of Technology. The apparatus was similarly low and equally vague as t o G. B. WILKES essentially the standard guarded plate the identification of the carbonaceous Massachusetts Institute ofTechnology, (1). While the apparatus described (1) material examined. They are usually Cambridge, Mass. is sufficient for determining the thermal referred t o as either soot or lampblack. conductivitv of the usual tvDes of inIn recent years carbon black has become sulating materials with K values varythe most familiar and best defined of the ing between 0.26 and 1.0, certain added refinements are essenh e l y divided carbons. The surface area, particle size, and p r o p tial when making measurements on materials with coefficients erties of well defined samples of these materials have been reported less than 0.17. in the literature (9, 10, 11). The present paper gives values for The guarded plate equipment, as employed, consisted of a 10the thermal conductivity of standard commercial carbon blacks. inch-diameter heater with an 18-inch-diameter guard ring. For many years the lower limit for the coefficient of thermal Both the heater and the guard ring consisted of two copper conductivity of insulating materials was considered to be that plates, */, inch thick. Electrically insulated resistance wire of still air-namely, 0.17 B.t.u., hr.-1, ft.-3, inch, O F.-l Since was between the plates. Asbestos paper covered both the heater no known solid has a lower conductivity than still air and most and the guard ring t o ensure a high emissivity surface in contact thermal insulators consist of porous or granular materials such as with the aample. The water-cooled plates were covered with cork, rock wool, 85% magnesia, etc., i t seemed reasonable t o asblotting paper for the same purpose. Four copper-constantan sume that the conductivity of air established the minimum thermocouples (No.30 wire) were attached t o the sample side of value that one could hope to attain with thermal insulation. the above papers by thin paper labels for measuring the temThe National Bureau of Standards (8) in 1929 suggested that perature drop across the sample. The thickness of the sample still air was the ideal insulator. Fishenden and Saunders (7) under test was determined by small balsa wood spacer Mocks in 1932 stated: “In conclusion, the above considerations applaced near the outer edge of the guard ring. They also served pear t o show that the effective conductivity of porous or granular t o carry the load of the equipment so that the sample would not materiala must always exceed that of air which may be regarded as be under comp&on. The water for cooling was supplied from the ideal insulator, barring a vacuum.” an overhead tank with the temperature thermostatically conDuring the past twenty-five years there have been occasional trolled. references in the literature t o insulating materials with a lower The power input to the heater was maintained constant by conductivity than air. Smoluchowski’s theory (18) for these low means of a voltage stabilizer and was measured by a calibrated values was based on the assumption that heat flowing from one wattmeter. With the small power inputs used, it is important wall t o another through an air space of thickness L should not be that the wattmeter be connected so that the power in the current calculated by the usual conduction formula,
I’
“ 1
K(Ts
- Ti)
L
but by
- TI) L - 2d
K(Tz
where d represents a %urface resistance” that is independent of L. With minute air spaces the value of d becomes a n important factor in determining the rate of heat flow which would be much less than that calculated from the usual formula for conduction. Kistler (8) in 1934 reported. that silica aerogel also had a thermal conductivity less than air. Hie explanation of the phenomenon was that the pores were probably less than 0.1~diameter and thus less than the mean free path of the air molecules. Consequently the thermal conductivity would be less than the &ccepted value of still air. Since that time various investigators have established fairly well that it is possible to have a thermal insulation with a K value less than that of air?
TABLE I. ANQLE OF RBPOSEAND TXZRMAL CONDUCTIVITY OF INSULATINQ
Processed wood 5ber Expanded vermiculite Granulated oork Carbon black (Spheron) Still air
MATERIALS
Angle of Repore
Thermal Conduotivity K B.T.U.,Hr.-1, Ft.-s, In., F:-r
64O 399
0.30 0.480 0.310 0.182 0.17
38’ 28’
..
TABLE 11. THERMAL CONDUCTIVITY OF CARBON BLACK Carbon Black Grade 6 Qrade 0 Spheron Monaroh 71
1111
Density Lb./Cu. st. 16.2
20.2 12.0
A, O
F.
192 162
189
?$.
Mean Ttmp
74 74 77
133 118 183
0.141 0.182 0.148
VoL 3s. No. 12
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1112
coil b not messured as heater input. A amdl oorreotion must be made for the power used in the voltage coil of the meter. Whh materink of such low conductivity, it becomes inoreasingly i m p r t m t to maintain the guard ring and the haater at ex~ t l the y same temperature in order to prevent any trmminsion
P
n
d "ood I l k
A third sample of carbon hlaek, identi6ed ae Monarch 7 1 , ~ s dso included in the study. This material had s n apparent density of 12 pounds per cubic fwt and WM not pelletied. The number average diameter of this carbon black is 150 A. The mens&
Exwnded rarmisuIIto
Vkigure 1.
surface arm b 330 square meters per gram.
Gr.mdatd
Carbon b1e.k
e0.k
Demonstration of Angle of Repose of Insdating Materiale
of heat from o~ieto the other. Automatic control of this temperature differential was established by a multiple differential thermocouple (eight couples in series) attached to the guard ring and heater. This multiple couple was connectedto a galvanometer which deflected with the slightest temperature difference. A ham of light was reflected from the galvanometer to a photoelectric relay which altered the current in the guard ring by sbout 0.1 ampere. With this arrangement the temperature difference between the heater and gusrd ring oodd be maintained to better than *O.Ola C. The plate equipment was used in a horizontal pasitian to facilitate the introduction ofpowdercd materink. The ssmple was held in place by an ashestas cloth ring fastened to the balm r w d pacers to make two chambers approximstely 17.75 inches in diameter and 1.5 inches thick. The total volume of sample required for a test was approximately 0.43 cubic foot. Carbon black, wei&ed 80 tbst it would fill one of these chembem to the density mentioned (with B slight excess) was csrefully introduced by pouring. When the u p w plate was plaeed in p i t i a n , it compressed the sample slightly and -lured no air pocketa. With material of such low conductivity. it is important to run the test long enough to establish thermal equilibrium. The standard test requiremente are not su5cient to give g o d equilibrium, and it WEE not u n d to apend 5 days for a determination nt on? mean temperature. T h e change in calculated K vslue might be lea than 1% over a %hour period, an specified in the tentative code, hut over a period of 2 or 3 days the change was much greater than this amount. Without these precautions considerable error may result. The Monarch 71 carbon h h k contained 2% moisture and the Grade 6 material. 3%. The materials were tested without drying since these are the conditions under which they are normally available in commercial quantities. Moisture tends to increase the conductivity somewhat; hence it is &e to m u m e thst the thermal conductivity of bonedry carbon black would be slightly lower than the test vdues dven here. MATERIALS
Three samples of carbon black were selected for measurement of thermal Conductivity. These included a ample of standard
Grade 6 rubber carbon black with an apparent density of 16.2 pounds per cubic foot and a sample of the same material in wllet ized form (Grade 6 Spheron) with an apparent density of 20.2 pounds. The latter sample is illustrated in Figure 1. The surface area of the Grade 6 material as measured by the low-temperature nitrogen isotherm method (4,6,6)was 110square meters per gram, and the number aver* diameter from electron mierascope count was ZRO A.
RESULTS A N D DISCUSSION
Aa initially produced, oarbcu blsck is an extrmely fluffymaterid with an apparent density between 5 sud 6 pounds; in this condition ita flow properties are extremely poor. However, a number of methods are employed for converting this material into dustless, free-flowingpellets (S) with an apparent density of around 20 pounds per cubic foot. This is the form in which it is usually mpptied to the rubber tire industry. In this freeflowing condition it should provide an excellent insulating material, particularly for lilling difficult shapea or forms. Figure 1 demonstrate the flow properties of pelletized carbon hlwk as compsred to three stsndard insulating materists. The vslues for the engle of repwe of these materials arc reported in Tsble I. The results of thermal oonductivity messurementa are prewentad in Table II. The K values of Grade 8 and Monarch 71 are lower than that of still air. The values far these two carbon hlseks indicate that therms1 conductivity is independent of the ultimate particle size over the range studied. The thermal conductivity of these two samples also ~ppearsto be independent of Rpparent density. However, when the physical stste of the material is altered by agglomeration into free-flowingpellets shout 0.4 mm. in diameter, as in Grade 6 Spheron, a definite incram in thermal conductivity is noted. The value. however, i% still only alightly higher than that of still air and about half thst reported for other standard insulsting materials.
LITERATURE CITED and Ventilating Engra., Tentstive Code (l944), Teat for Thermal Conductivity of Materiala by
( I ) Am. Soo. of Heating
Guarded Hot Plate. F. R.. Smith. W. R.. and Thornhill. F. S.. IND. Ewe. CH& A&. Eo.;15. 256(1943). 131 Billings, E.. and Offutt, H. (to Godfrev L. Cabot. Inc.) U. 8. Patent 1.957.314 (May 1.1934). 141 . . Bruneuer. S.. Emmett. P. H.. and Teller.. E... J . Am. C k .Sm..
(21 . . Amon.
M), 309
(1938).
(51 Emmett. P. H.. and Bmnauer, 8..Ibid.. 56. 36 (1934). (8) Emmett. P. H., and De Witt. T.,IND.ENO.Caex.. ANAL.Eo., 13. 28 (1941). (7) Fishendah, M:, and Ssundem, 0.A,. "Calculation of Heat (81
Tranamission". London, A. M.Stationery Offioe. 1932. Kiatler, 9. S., and Caldwell. A. G.. IND. ENO.Ctrsx.. 26, 658 (1934).
(VI Natl. Bur. of Standsrds, Bull. 376 (1929). (IO) Rosaman. R. P.. and Smith, W. R.. IND.END.Cas.%., 35, 972 (1943). (11)
Smith. W. R.. Thornhill, F. S.. and Bray. R. I., Ibid.. 33, 1303
(12)
Smolurhowski. M., PTOC. 2nd Re/?
