ACKNOWLEDGMENT
The authors are indebted to the Pittsburgh Consolidation Coal Co. for their generous support of the present work in the form of a fellowship. LITERATURE CITED
(1) Beebe, R. A., Biscoe, J., Smith, W. R., and Wendell, C. B., J . Am. Chem. SOC.,69, 95 (1947).
(2) Brunauer, S., “Adsorption of Gases and Vapors,” Vol. I, p. 150, Princeton, Princeton University Press, 1942. (3) Corrin, M. L., J. Am. Chem. SOC.,73, 4061 (1951). (4) Dryden, I. G. C., “Review of Present Ideas about Physical Structure of Coal,” presented a t 131st Meeting of Coal *
Research Club, Paris, France, 1951. (5) Emmett, P. H., A S T M Tech. Pub. 51, pp. 95-105 (1941). (6) Zbid., private communication (1952). (7) Emmett, P. H., and Love, K. S., J . Am. Chem. SOC., 55, 4043 (1933).
(8) Gulbransen. E. A.. and Andrew. K. F..
[email protected], ,
I
1039 (1952). ~ 0. A., ~ and ~Watson, ~ K. ~M., “Industrial , Chemical Calculations,” 2nd Ed., p 127, New York, John Wiley 8: Sons, Inc., 1945. (10) Lecky, J. A , , Hall, W. K., and Anderson, R. B., A’afure, 168, 124 (1951). i l l ) Long, F. J.. and Sykes, K. W., Pioc. Rog. SOC. (London), Ai 193, 377 (1948). (12) iMaggs, F. A. P., and Bond, R. L., Fue2, 28, 172 (1949). (13) Malherbe, P. Le R., Ibid., 30, 97 (1951). (14) Malherbe, P. Le R., and Carman, P. C., Ibid., 31, 210 (1952). (15) Polley, M. H., Schaeffer, W. D., and Smith, W. R., presented before the Division of Colloid Chemistry, 122nd Meeting, AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J. (16) Seitr, F., “Modern Theory of Solids,” 1st ed., p. 109, New York, McGraw-Hill Book Co., Inc., 1940. (17) Walker, P. L., Jr., Foresti, R. J., Jr., and Wright, C. C., IND. ENG.CHEM.,45, 1703 (1953). (9) H
RECEIVED for review January 26, 1953.
ACCEPTEDApril 27, 1983.
M
Structure and Properties of Carbon Black CHANGES INDUCED BY HEAT TREATMENT W. D. SCHAEFFER, W. R. SMITH, AND M. H. POLLEY Godfrey L. Cabot. Znc.. Boston, Mass.
T h e rubber reinforcing properties of carbon black are definitely associated with particle size. It has been suggested that properties are also associated with “degree of graphitization.” Degree of graphitization, as evaluated from x-ray diffraction data, is enhanced by heating in the range 1000” to 3000” C. A series of commercial carbon blacks has been heat treated to varying degrees of graphitization, and complete x-ray, surface area, and particle size data collected. The degree of graphitization, as judged from dimensions of the parallel layer groups composing the particle, is a function of temperature and particle size of the carbon black. A prerequisite for growth appears to be release of combined hydrogen within the particle. The bulk electrical resistance of all carbon blacks passes through a minimum in samples heated to 1000” C. Crystal growth sets in above this temperature and resistance increases. Graphitization changes the shape of the particle from spherical to well defined polyhedra. Surface heterogeneity is greatly decreased as judged from adsorption isotherms and heat of adsorption measurements. While tests i n rubber are as yet incomplete, i t appears that increasing degree of graphitization of carbon black is accompanied by progressive decrease in modulus properties.
c
ARBON black is a particulate form of elemental carbon
prepared by the partial combustion of gaseous or liquid hydrocarbons (13). Depending upon the process of manufacture-i.e., channel or furnace-they analyze 95 to 99% carbon, the remainder consisting chiefly of combined oxygen or hydrogen. Under the electron microscope all carbon blacks appear essentially spherical, and those grades most widely used in the rubber tire industry range from some 200 to 1600 A. in diameter (4). All carbon blacks yield the same general x-ray powder pattern, consisting of two rather diffuse bands approximately in the position of the 002 and 100 reflections of graphite. On heating above 1000” C. the diffuse bands sharpen and new reflections appear. Apart from the 001 reflections, however, only two-dimensional (hk) reflections are present. The hkl reflections characteristic of the three-dimensional crystallinity of graphite are generally
August 1953
absent, These facts led Biscoe and Warren ( 1 )to conclude that in carbon black there are well-established graphite layers stacked roughly parallel but in random orientation about the layer normal. From the breadth of the appropriate 001 and hk reflection at half intensity (IS), i t is possible to obtain the values Lo and L,, which are interpreted as the dimensions of the stacks or parallel layer groups of graphite platelets within the carbon black particle. The dimensions so obtained are generally less than a tenth the diameter of the carbon black particle. Consequently, a particle of carbon black may be visualized as a n essentially spherical aggregate of parallel layer groups. A standard reinforcing furnace black particle some 225 A. in diameter may contain 1000 such groups. As pointed out previously, heat treatment in the region 1000” t o 3000’ C. leads t o a sharpening of the diffraction pattern and appearance of further 001 and hk reflections, indicating growth
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Table I. Black Spheron 6 Vulcan 3 Sterling S P-33 a Arithmetic
Type MPC
Process Gas channel Oil furnace HAF Gas furnace SRF Gas thermal FT mean diameter.
Electron Micrograph, (&)e., A. 280 228 800 1600
Nt Area,
Sq. M./Gram
115 72 27 15
n
A-SILICA
D-GRAPHITE CRUCIBLE
SHELL
B- SIGHTING TUBE
E-CRUCIBLE CHARGE F- INDUCTION COIL
C-THERMAX INSULATION Figure 1.
anism of carbon black formation as well as the temperature under which the black Volatility, % L.. A. L ~A,, was formed must, in fact, be 6 23 13 very similar, w ln $
16
I
QCARBON MONOXIDE
I2
0.6
I 0
gL
e 4
-tn
0.4.
W
0
260 400
600
800
a
1000 1200
TEMPERATURE, OC. 0.2
Figure 6
0 STERLING S
As mentioned above, carbon black contains some chemisorbed oxygen on the surface as well as 0.5% by &-eight of combined hydrogen distributed throughout the particle. The hydrogen is residual hydrogen left from the hydrocarbon raw material and as such must be located in part a t the edge planes of the parallel layer groups. As shown in Figure 6, maximum evolution of hydrogen occurs in the neighborhood of 1000" C. From Figures 4 and 5 it may be noted that this is also the temperature a t which growth of parallel layer groups is initiated. Liberation of hydrogen appears to be a prerequisite for graphitization.
Figure 7.
1724
Electron Micrographs of p-33 (25,000 x) after 2700" C. Treatment
0 P-33
1000
2000
3000
TEMPERATURE 'C.
Figure 8
entropy requirements would demand a decreased rate, such as noted in the temperature range 2000" to 3000" C. The rate of graphitization, as well as the parallel layer group size a t equilibrium, increases with increasing size of the original carbon black particle. I t is obvious that the dimensions of the particle must limit the final size which the parallel layer groups can attain. If one assumes the growth rate to be uniform throughout the particle, then the larger particles which display minimum surface-to-volume ratio should have the maximum rate of graphitization. The rate should be less in finer carbon blacks in which a far greater proportion of parallel layer groups are located a t the surface and, hence, presumably would have less opportunity for growth than those located in the interior of the particle. A rather striking change in the shape of the carbon particle can be noted after graphitization (11 ). Figure '7 reproduces electron micrographs of P-33 before and after heating two hours a t 2700" C. Originally the particles are essentially spherical. After graphitization, however, they produce sharply defined six. and eight-sided electron microscope images. The effect is probably common to all graphitized blacks, before and but it is most strikingly observed in the larger particle size grades which attain larger L, and L, values and are
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Vol. 45, No. 8
Effect of Heat Treatment on Particle Size and Surface Area
Table 111.
Before Heat Treatment Electron Nz Area, micrograph Black aq. m./gram (&)a, A. Spheron 6 115 280 Vulcan 3 72 225 Sterling S 27 800 P-33 15 1600 0 Arithmetic mean diameter. b Tentative values. ,
After Treatment at 2700’ C. Electron Nz Area, micrograph 8q. m./gram ( d d , A. 84 212 62 (215) b 25 (800) 13 (1900)
u
also more fully resolved in the electron microscope. The sharpness of the edges and angles indicates a high degree of orientation. The faces on these samples are, in our opinion, primarily 001 planes. The remarkable homogeneity of these surfaces is further indicated by the fact that adsorption of argon or nitrogen at -195’ C. yields stepwise isotherms (IO) as predicted by Halsey (7). The specific resistivity of carbon black is a property of considerable significance in some applications. I n dry cell manufacture, for example, it is essential that the carbon black employed possess as high a conductance as possible in order to provide a cell of minimum internal resistance; also rubber stocks of high conductivity are often desirable to permit static dissipation. It has been recognized for many years t h a t the resistivity of bulk carbon black could be decreased by calcining a t elevated temperatures (8). It has not been clear whether this was due t o the removal of combined oxygen and hydrogen or to an increased degree of graphitization. The present series of heat treated blacks provided opportunity t o investigate this point. The direct current resistivity was measured with the black under 150 pounds per square inch compression and a t room temperature. Figure 8 shows the relation between direct current resistivity and temperature of heat treatment of the four blacks described. I n all cases, a striking minimum in resistivity is noted in the 1000’ to 1200” C. region. From 1200” to 2500” C. the resistivity increases and then starts to fall somewhat on heating to 3000” C. From these data it is obvious that the reRistivity of carbon black in bulk is associated primarily with combined oxygen and hydrogen. On freeing the black of these constituents, resistivity
reaches a minimum. As indicated previously, significant graphitization does not occur until 1000” c. The onset of.graphitization is reflected in an increase in resistivity and continues up to 2500” C. The maxima noted at 2500” C. may be associated with three-dimensional ordering since these samples display incipient hkl reflection. The minimum resistivity attained on heat treatment appears to be inversely related to the particle size of the carbon black. Thus P-33, when completely free of hydrogen and oxygen, is more conductive in bulk than a similar sample of the much finer Vulcan 3. This is quite the reverse of what is found when standard rubber stocks containing 50 parts of original P-33 and Vulcan 3 are evaluated in rubber. The resistivity of a P-33 stock may be some 106 times higher than that of a Vulcan 3 loaded stock. The authors believe this relation will persist even when the heat treated blacks in rubber have been evaluated. It is apparent that in rubber the number of through-going carbon paths influence conductivity t o a greater degree than the actual conductivity of the carbon particles in the path. At sufficiently high loadings, however, rubber stocks should duplicate the data of Figure 8. ACKNOWLEDGMENT
The authors are indebted to B. E. Warren for supervising the x-ray diffraction studies and for his advice in their interpretation We also wish to thank C. Houska for collecting the x-ray data. LITERATURE CITED
(1) Biscoe. J.. and Warren. B. E.. J . Aaalied Phus..’ 13.’ 364 11942) (2j Brennan, R . O., J . Cheh. Phys., 20,YO (1952): (3) Brewer, L., Ibid., 20,758 (1952). (4) Cabot, Godfrey L., Inc., “Cabot Carbon Blacks Under the .
I
Electron Microscope,” Boston (1950). ( 5 ) Emmett, P. H., J . Phys. & Colloid Chem., 51, 1329 (1947). (6) Franklin, R. E., Proc. Roy. SOC.( L o n d o n ) ,209A, 196 (1951). (7) Halsey, G. D . , J . Chem. Phys., 16,931 (1948). (8) Offutt, H. H., U. S.Patent 2,134,950 (Nov. 1, 1938).
(9) Parkinson, D., in “Advances in Colloid Science,”Vol. 11,p. 389, New York, Interscience Publishers, Inc., 1946. (10) Polley, M. H., Schaeffer, W. D., and Smith, W. R., J . Phys. Chem., 57,469 (1953). (11) Ragoos, A., Hofmann, U., Holst, R., Kolloid-Z., 105, 118 (1943). (12) Smith, W. R., in “Encyclopedia of Chemical Technology,” Vol. 3, p. 34, New York, Interscience Publishers, Inc., 1946. (13) Warren, B. E., Phys. Rev.,59,693 (1941). RECEIVED for review October 3, 1952.
ACCEPTEDMarch 18, 1953.
END OF SYMPOSIUM
August 1953
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