Conductivity of Rubber Tread Stocks EFFECT OF PROPERTIES OF CARBON BLACK LEONARD H. COHAN AND JAMES I?. MACKEY Continental Carbon Company, Chicago, Ill. A previously described appaONSIDERABLE interest The conductivity of a rubber tread stock ratus (S),equipped with a dehas been shown in semicompounded with 50 parts carbon black invice for applying sustained conducting rubber comcreases exponentially as the particle size of pressure, was used. The measpounds. These compounds are the black decreases. As the crystal strucurements were made under particularly useful where it is ture of the carbon approaches a more pressure in order to duplicate as desirable to have a conducting far as possible the conditions path for the dissipation of static graphitic form, the conductivity increases under which a tire tread actuelectricity. Examples of such sharply. Noncarbon surface constituents, ally operates. Within the range application are: shoe soles and either of the hydrocarbon or carbon oxide of pressures used, a limiting flooring in certain plants, bus type, decrease the conductivity. This effect value was reached beyond and automobile tires, etc. Semiwhich increasing the pressure is less important than particle size or conducting rubber can be predecreased the resistance very pared by the addition to the crystal structure. However, increases in little; a constant pressure of 36 rubber formula of certain carconductivity of more than tenfold can be pounds per square inch was bon blacks manufactured both brought about by removal of surface conused which was well above this from natural gas and acetylene. limiting value. The average deUnloaded ribber or rubber stituents. viation of measured resistance loaded with other pigments for a given black run in the than carbon blacks has a spestandard formula under standard conditions was found t o be cific resistance of 1012 to 1016 ohm-cm. By the addition of 25 per cent. proper types of carbon black, resistances as low as 30-500 Degree of dispersion of the carbon black in the rubber ohm-cm. can be readily obtained. compound has been shown by other investigators to effect Several publications (6,7,8, 10, 11) have reported the effect conductivity (4, 11). However, rubber tests such as aged of different carbon blacks on the conductivity of rubber stocks. and unaged tensile, hardness, resilience, and abrasion resistI n the present work an effort has been made to determine the ance indicated that for the stocks reported here, the degree of influence of various fundamental properties of carbon black dispersion was very similar and could therefore have only a on the conductivity of a typical rubber tread stock test formnegligible influence on the general trends. ula. The properties considered are ultimate particle size, crystal form, and surface structure. EFFECT OF PARTICLE SIZE
C
PROCEDUREANDAPPARATUS
The blacks were milled into the following standard tread formula : Smoked sheet Zino oxide Pine tar Stearic acid
100
7.85 3.00 3.30
Sulfur Captax (mercaptobenzothiazole) Carbon black
2.81 0.743 50.5
Tensile sheets were cured a t 280" F.; a soap solution was used for mold lubrication, and reasonable care was exerted in curing and handling to prevent defacing the surface of the sheets. Resistance measurements were made before tensile strips were cut and broken. The specific resistance, R,, listed for each black was taken on a tensile sheet cured a t or near the cure giving the maximum tensile value. Resistance was determined by placing the tensile sheet between brass electrodes and measuring the current I and voltage E. All measurements were made a t about 15 volts. Specific resistance R, was calculated from the formula:
Table I gives the effect of particle size of the carbon black on the conductivity of the rubber stock. Since the color of a rub-up of carbon black in oil varies principally with particle size, the color index as given in Table I is taken as a rough measure of surface area per unit weight. Also included in the table are estimates of actual particle diameters made from
TABLEI. EFFECT OF PARTICLE SIZEOF CHANNEL BLACKON RESISTIVITY Black Continental AA Continental E Continental G Low-color High-color (Continex)
Estd. Particle Diam., mp 33 25
20 17 10
Color 60 80 98
127 222
Volatile Matter,
%
5.5 5.7 5.4 6.0 4.7
R Ohmhn. 7 x 108 5 x 105 3 x lo& 6 X 10s 2 x 101
published electron microscope (16) and adsorption (14) measurements on similar blacks. In Figure 1 the logarithm of R, is plotted against color index and estimated particle size.
where t = thickness of the rubber sheet, cm. 31.0 = area of the electrode, sq. cm. 806
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1943
As Table I shows, the specific resistance of a rubber stock varies exponentially with the diameter of the ultimate particles of carbon black used; these are channel blacks prepared from almost identical gases and permit the reasonable assumption that their crystal structure is very similar. The volatile content is a measure of the amount of volatilizable surface constituents of the black and is sufficiently constant to have little influence on conductivity.
The position of acetylene black in Table I1 is in qualitative agreement with these considerations; however, in comparison with Graphon, acetylene black is slightly better conducting than might be expected considering the x-ray data of Biscoe
TABLE11. EFFIWTOF CRYSTAL STRUCTURE OF CARBON BLACKS ON RESISTIVITY Estd. Particle Diam., mM
Black
I1
1
PARTICLE
xDIAMETER 0- C U O R INDM
251
5
Kemp and Hermann (11) and Habgood and Waring (6) stated that the conductivity of rubber loaded with carbon probably occurs through chains of carbon particles which act as leakage paths for the current. As the particle size decreases, the number of particles in a given weight increases and the probability of the formation of carbon chains increases enormously. The results of Table I are in full agreement with these considerations.
Volatile Matter,
%
R.,
Ohm-Cm.
Series 1 Continental AA (devola33 1.o a x 105 tilized) Statex 34 (I6 ) 0.8 2 X 108 Graphon 33 (14) 0.4 5 x 102 Shawinigan acetylene 43 (18) 0.9 1 x 102 black Series 2 P-33 74" 0.6 2 x 1010 Gastex 8Ia 0.2 2 x 108 Lampblack devolatiliaed 97a 0.6 7 x 104 a Values were determined,with the electron miaroscope by Wiegand and Ladd (16). Smith, Thornhill, and Bray (24) give t h e following particle diameters as calculated from adsorption measurements: Gastex 75 m,u, lampblack 107 mp, P-33 200 mp.
-
-
-
I
807
and Warren (2) and the particle size estimates of Table 11. Statex is prepared by partial combustion at rather high temperatures. A second series of blacks of similar particle size and volatile content appears in Table 11. Owing to the different production conditions (P-33 by thermal decomposition of natural gas diluted with hydrogen, Gastex by partial combustion of natural gas and lampblack by partial combustion of higher hydrocarbons), we would again expect differences in crystal structure among the products.
6
I
I
I
I
I
,
EFFECT OF CRYSTAL STRUCTURE
Blacks listed in series 1, Table 11, have similar particle size and fairly similar volatile content but different crystal structures. Continental AA is a commercial channel black prepared from natural gas. Materials of this type give an x-ray pattern with diffuse bands. Graphon is prepared by heating channel black for several hours a t about 2000' C. (12). As a result of this treatment the x-ray pattern becomes much sharper and more nearly approximates that of graphite (9); the conductivity as indicated in Table I1 is markedly increased, Shawinigan black is made by the thermal decomposition of acetylene gas, and the higher temperature prevailing during formation of this black makes it probable that a greater extent of crystal growth should occur. The results of heat of combustion determinations indicate that acetylene black is closer to graphite than channel black (IS). Likewise, x-ray photographs indicate a more nearly graphitic structure for acetylene black than for ordinary channel black (2, 6). However, Biscoe and Warren (2) indicated that channel black heated a t high temperature, as in the case of Graphon, increases in crystal dimensions and becomes similar to acetylene black.
R, (OHM-CM.)
Figure 2.
Relation of Specific Resistance to DPG Adsorption and Volatile Content
Both lampblack and acetylene black were reported to differ from other blacks in their appearance under the electron microscope (16). Ultimate particles of lampblack and acetylene black group together as irregular networks in the specimens prepared for examination with the electron micro-
INDUSTRIAL AND ENGINEERING CHEMISTRY
808
scope, whereas other blacks, such as ordinary channel black, Statex, Gastex, and P-33, appear either as individual particles or floes. These tendencies are probably due to crystal structure factors which distinguish acetylene black and lampblack from the other blacks. The conductivity values in series 2, Table 11,shorn that, like acetylene black, lampblack has a higher conductivity than other carbons of similar diameter and volatile content. EFFECT O F SURFACE STRUCTURE
The samples listed in Table I11 were prepared by partially removing the volatilizable surface constituents from a moderately small particle size (17 mp) channel black (Continental
c-151. OF SURFACE STRUCTURE OF CARBON TABLE 111. EFFECT BLACKS ON RESISTIVITY
%
Volatile Matter 70DPG R. Medium-Color Black (9) (1 7% I** Ohmbm. riginal 4 3 56 62 6000 i r t l v devolatilized 3 3 36 65 3500 devolatilized 1 8 23 70 3300 latilized 1.2 12 75 1100 Cnmnletelv devolatilized < 0.15 4 73 400 .. * Adsorbed by 1 gram black (to whiph, 10 cc. 10% HzSO4 has been added) from 100 cc. of 1 2 solution in KI contaming 2.7 grams Iz and 4.05 grams K I per liter of solution.
The volatile matter was reduced by heating the black for various periods in a nonoxidizing atmosphere. Since the temperature in all cases was kept below 800” C., no significant change in either particle size or crystal structure could have occurred. Thus the differences in conductivity can be attributed solely to the surface structure of the blacks. The conductivity increases appreciably as the volatile content decreases (Figure 2 ) . The per cent DPG is a measure of the amount of oxygen containing complexes present on the surface of the black. Figure 2 shows that there is even closer correspondence between per cent DPG and conductivity than between volatile content and conductivity. The iodine adsorption corresponds roughly to the bare carbon surface. As the volatile content is removed, the extent of carbon surface is increased and the per cent iodine adsorbed is also increased. The iodine adsorption of the completely devolatilized sample is lower than would be expected, owing possibly to the higher temperature of devolatilization.
TABLEIV. EFFECT OF CHANGING VOLATILECONTENTO F
STANDARD RUBBER-GRADE CHANNEL BLACK
Continental E Black (25 ma) 1. Volatile increased 2. Cntreated 3. Volatile decreased
Volatile Matter,
DPG,
Iz
9.7 5.7 1.8
86 50 12
28 44
%
%
d
72
Resistance of Uncom ounded Ra, Bfack, Ohm-Cm. Ohm-Cm. 4 X 106 5000 2500 5 X 10‘ 750 1 X 105
The influence of volatile matter content is further illustrated in Table IV. Sample 2 was the untreated black, 3 was the same black with the volatile partially removed, and 1was prepared by treating 2 in an oxidizing atmosphere to increase the oxides present on the surface. The results are in complete agreement with those of Table 111. Also shown in Table IV is the effect of changes in volatile content on the conductivity of the uncompounded carbon black. This type of measurement depends on physical factors such as the size of pellets or
Vol. 35, No. 7
TABLE V. RESULTSON LAMPBLACK Lampblack Original Devolatilized
Volatile Matter,
%
4.5
0.6
DPG,
12,
%
Yo
21 5
16 28
Re9 Ohm-Cm. 1 X 10: 7 X 10
aggregates present in the black and the degree of packing of these aggregates, so that the results are only of relative importance. The conductivity of a commercial lampblack (estimated particle diameter 97 mp) before and after the volatilizable surface constituents had been removed is shown in Table V. As indicated by the low DPG adsorption, the volatile content of lampblack consists chiefly of hydrocarbons rather than oxides of carbon. I n this case, also, removal of the surface film leads to strikingly higher conductivity. The foregoing indicates that noncarbon surface constituents on carbon blacks act as an electrically insulating layer. Since the electric current is probably carried through rubber loaded with carbon black along conducting chains of the black, a nonconducting layer preventing good contact between the carbon black particles will lower the conductivity considerably. For example, in the casg of the Continental E series of Table IV, a surface layer 2.5 A. thick on the carbon particle is roughly equivalent to a nonconducting film of 0.2 mm. on one side of a test sheet 1 em. thick.o (A surface layer 2.5 A. thick causes an insulating film of 5 A. between carbon particles. Since for Continental E the particle diameter is about 250 A., the thickness of the insulating film is about 2 per cent of the total thickness. Ignoring variation in specific resistance with thickness, this would be equivalent to a 0.2mm. film on a sheet 1 em. thick.) LITERATURE CITED
(1) Amon, F. H., a n d Estelom, R. K., IND.ENC. CHEM.,24, 579 (1933). (2) Biscoe, J., and Warren, B. E., J . Applied Phys., 13, 364-71 (1942). (3) C a b o t , G o d f r e y L., Inc., Carbon Black Bull., M a r c h , 1941. (4) Columbian C a r b o n Co., “Columbian Colloidal Carbons”, pp. 69 ff. 11938). Downey; W. C., G. E. C. Journal (London), Nov., 1931; Kaufm a n n , C., Can. Chem. Met., 1 7 , 9 3 4 (1931). Habaood, J., a n d Warin., J. R. S., Trans. I n s t . Rubber Ind., 17, 50-64 (1941). Heering, H., Proc. Rubber Tech. Conf., London, 1938, 1086-91. Heering, H., Gisycki, I., a n d Kirceck, A., Kautschuk, 17, 55-62 (1941). Johnson, C. R., IND. ENG.CHEM.,20, 904 (1928). K e m p , A. R., a n d H e r m a n n , D. B., Proc. Rub6e) Tech. Conf., London, 1938, 893-91 1. K e m p , A. R., and H e r m a n n , D. B., Rubber Chem. Tech., 12, 317 (1939). Offutt, H. H., U.5. Patent 2,134,960 (Nov. 1, 1938). R o t h , W. A,, 2.anorg. alloem. Chem., 41, 273-B(1928). S m i t h , W. R., Thornhill, F. S., a n d Bray, R. I., IND.E x o . CHEU., 33, 1303-7(1941). Wiegand, W. B., India Rubber World. 105, 270-2 (1941). Wiegand, W. B., a n d Ladd, W. A,, Rubber Age, 50,431-6 (1942).
