I N D U S T R I A L A N D ENGINEERING CHEMISTRY
March, 1931
273
Effect of Carbon Black on Insulating Oils' W. B. Wiegand,2 C. R. Bog@,*and D. W. Kitchina BISNEY
8r
SXITII
CO.,N E W
YORK,
N. Y.,A N D
SIMPLEX WIRE 82
CABLE
CO.,
BOSTON, MASS.
treatment followed by desiccaHE most satisfactory Carbon black has been successfully used to improve tion with charcoal, silica gel, dielectric available for the electrical properties of insulating oils. etc. the insulation of cerThe electhcal breakdown or dielectric strength of a L'. S.1,752,238, D . C. Cox. Filters oil through a 40-inch tain types of cables, switches, series of transformer oils was raised by an average of bed of silica or charcoal, etc., and transformers is, at the 40 per cent after treatment by various methods with followed by the addition of new present time, a specially recarbon black. oil. fined petroleum fraction comA series of experiments designed to illustrate the U . S. 1,747,161, L. H . Clark. Treats oil with activated carmonly known as transformer mechanism of this action of carbon black is described. bon followed by alkali treatoil. I n certain respects, howThe results indicate that the action is one of removing ment. ever, the transformer oils are moisture and/or electrolytes. References (2 to 18)include still defective. Under l o n g A brief summary of other methods for treating new the more important literature exposure to elevated temperaand used insulating oils and of the more important relating to such subjects as the ture certain oxidation prodliterature is included. determhation of small traces ucts are formed, which are eventually thrown out of solution (sludging), causing deterio- of moisture, the mechanism by which moisture and other impurities cause deterioration of the dielectric properties, ration of dielectric strength, resistivity, p!wer factor, etc. Another active cause of deterioration is the presence of various means of treatment including heat, vacuum, and traces of moisture. Thus, Clark (1) in 1928 showed that various absorbents. I n no case has the employment of car2 parts in 100,000by volume may lower the dielectric strength bon black to improve insulating oils, either before or after their deterioration through use, been noted. sufficiently to destroy the usefulness of the oil. Previous work by two of the present writers (19)has shown BREAKDOWN T E S T S that the addition of small amounts of an active carbon black Experimental Methods (such as Micronex) to rubber compounds markedly improves The standard method of determining electrical breaktheir insulating properties-in particular, the power factor, dielectric strength, and resistivity. This phenomenon is down-namely, breakdown between circular disks 0.1 inch remarkable because the expected effect of carbon black is apart in a standard test c u p w a s used to demonstrate the to injure the insulation value of dielectric materials. improvement, or otherwise, due to the treatment. It is evident, therefore, that because of its high adsorptive To obtain the dry carbon black, the carbon black as s u p powers the carbon black renders inactive something whose plied was sifted through an 80-mesh screen and then dried presence is more harmful than that of the required amount for at least 1 day a t 120' C. This, of course, applies to of adsorbent. This consideration led to two distinct lines the small quantities required for laboratory experiments.4 of experiment: (1) the study of the increase in breakdown Two methods of filtering were used. I n the first, carbon strength of transformer oil on treatment with carbon black black was stirred into the oil, which was then kept a t least where all or most of the black was removed; (2) the study of 24 hours, with frequent vigorous agitation, in Pyrex flasks the effect of carbon-black additions on the highfrequency sealed with tinfail-coated rubber stoppers. The black was power factor of suspensions of moist whiting in highly re- filtered out by a layer of fine asbestos fibers supported on a fined mineral oil. The first study was made on account Buchner funnel and dried a t high temperature. This filtraof its commercial vrilue; the second was undertaken to tion completely removed the suspended black. I n the second investigate the mechanism of power factor reduction via car- method untreated oil was drawn by suction through a 2-foot bon black in a medium free from the variables prevailing in (61-em.) column of dried, screened carbon black in a 1-inch rubber compounds. (2.5-em.) diameter Pyrex tube. I n the lower part of the tube a layer of glass wool supported a mat of asbestos fibers. Previous Work The tube with the asbestos filter was dried a t high temperaThe published literature relates principally to the appli- ture before the carbon black was introduced. This method cation of various materials to the removal of sludge from was too slow for regular tests, but it showed the interesting reoils already deteriorated through use. The following patents sult that oil thus filtered was optically empty. A Zeiss slit are illustrative of purifying methods: ultra-microscope was used for this test. Both Transil oil and U. S. 1,540,218, W. 2". Maloney. Describes blowing with air Nujol before treatment showed large numbers of bright specks or oxygen followed by alkaline treatments with or without clay. in the field when a slow stream of liquid was passing through U . S. 1,553,481, L. H. Clark. Describes the hot treatment the cell, whereas after filtration they were almost completely of oil with alkalies, agitation, separation of impurities, followed absent. This removal of suspended particles by the column by treatment with fuller's earth. U . S. 1,591,744, R. Cross. Describes the filtration of wet filtration doubtless tends to improve the dielectric strength of the oil. The asbestos layer was essential to obtain this oil through bentonite. U . S.1,606,406, H. K. Moody. Clarifies the oil with a mixture result; filter papers introduced fibers into the oil.
T
of bone char, fuller's earth, and ferric oxide; followed by hydrogenation. U. 5'. 1,609,546, F. W. Harris. Separates water by treatment with kieselguhr, etc. U. S. 1,665,845, D . C. Cox Filters the oil through fuller's earth. U. S.1,685,681, Rodman and Hecht. Employs vacuum-heat 1
Received October 16, 1930 Binney h Smith Co.
8
Simplex R'ire h Cable Co.
Filtration Series-Comparative Effectiveness of Carbon Black and Metallic Sodium
A control sample, tested as described, showed a breakdown of 26,400 volts. A sample of the same oil was then dried for
'
For larger scale experiments not here described, drying was carried out a t temperatures as high as 250' C. for shorter periods of time, but with agitation of the black to insure uniform drying.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
2 weeks over metallic sodium; the breakdown rose to 30,700 volts. The sodium treatment caused some darkening and sludging of the oil. The oil was then treated by column filtration;6 the breakdown rose to 35,100 volts. As a control on the above, the step of dehydration with metallic sodium was omitted, the treatment otherwise being identical; the result was 36,500 volts. 4.0
Vol. 23, No. 3
breakdown strength of oil falls off markedly a t higher temperatures. This behavior was attributed to the decreased affinity of carbon black for water with rising temperature. This idea was confmned by the results (Figure 1) of a study of the moisture pickup of Micronex a t different temperatures from a stream of moist air in which the partial pressure of water vapor was constant a t 20 mm. Hg. At each temperature equilibrium was approached from both sides. The moisture pickup was determined from the increase in weight of the Micronex. a The pickup is greatest a t low temperature. This is an advantage in the treatment of transformer oils, which are liable to deterioration a t elevated temperatures. Moisture Pickup of Carbon Black and Whiting at Room Temperature
I n securing conditioned samples for some of the work of the second part of this paper, data were obtained which are included here for their possible interest. The moisture pickup us. humidity a t room temperature of Micronex and Chalk Cliff whiting was determined by a static method in which samples were exposed for 2 months in closed desiccator jars to atmospheres of different relative humidities obtained from solutions of sulfuric acid of the proper strength. (Figure 2) 1
I
20
60 TEMPERATURE
40
Figure 1
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POWER FACTOR OF PIGMENT SUSPENSIONS IN OIL
I00
80 OC.
Method of Test
Sedimentation Series
When oil samples containing as low as 0.5 per cent of screened carbon black were tested, their dielectric strength was negligible. That this effect Fas due to poor dispersion could be inferred from the results obtained with carbon black in rubber where i t remained properly dispersed. To test this idea in the case of oil, experiments were made in which the carbon black was not removed by filtration, but allowed t o settle until only the finest particles remained in suspension. The sample was shaken frequently for 1 day in a stoppered flask with 3 per cent of dry carbon black, settled, and decanted twice. The time of treatment was 100 hours. The amount of carbon black left in suspension was 0.0056 per cent by weight. The breakdown results were: blank, 24,160 volts; treated sample, 36,625 volts. Repetition of the treatment with another sample for 76 hours gave the following results: 0.0073 per cent carbon black in suspension; blank, 27,350 volts; treated sample, 41,550 volts.
The current for the power-factor tests was supplied by a vacuum-tube oscillator set at 580 kilocycles by means of a precision wavemeter. The circuit is shown in Figure 3. izI is a grounded, untuned circuit coupling oscillator and measuring circuit. The measuring circuit consists of a
I-5
z W
r
124 a.
e3 W
I Y
22
b,
8
20 Sedimentation Treatment with Various Grades of Carbon Black
Carbon blacks apparently differ in their effectiveness for the electrical improvement of insulating oils. Studies to date have not fully explained the reasons for these differences; further work on this aspect is in progress. Table I illustrates these differences. Table I-Effect of Various Grades of Carbon Black GRADEO F CONTROL BREAKDOWN AFTER BREAKDOWX TRE.4TMENT IMPROVEMENT VOllS Volts % F . 7 3% 20 796 32,476 +66 S.S. 3% 20:796 29,075 +40 FU 37 20,796 33,000 59 WE. 3% 20,796 31,578 52 U.M. 3% 20,796 30,360 +46 us. 377 33,200 23,000 -31 R.S. 3 d 33,200 26,233 -21
CARBON BLACK
++
Effect of Temperature of Treatment
I n a series of tests a t various temperatures it was found t h a t the effectiveness of carbon black in improving the 5 A special grade of the brand known as Micronex, sold by Binney & Smith Co., was employed for these experiments.
40
60
80
% RELATIVE HUMIDITY Room Temp. Figure 2
coil, L, in series with decade resistance box, R, and precision condenser, C, across which the test condenser, C,, is connected. A bunched coil of fine wire a t the low-potential end of L couples the resonant circuit to the vacuum-tube voltmeter, G is a galvanometer protected bya universal shunt, S. Test condenser C, consists of two co-axial silver-plated copper cylinders supported in a Pyrex tube in such a way that only the liquid to be tested is in the electric field. The condenser was taken apart and cleaned after each test. The method of testing is as follows: With C, connected across C, and R set a t zero, the circuit is tuned to resonance by varying C,. With C, disconnected C, is increased until resonance is again reached. Then C, = Cz - C1. Resistance is added until the response shown a t G is the same as when C, was connected. Then the losses in the circuit due to the poor dielectric in C, are equal to those due to the known added resistance, R. The power factor is calculated by the following equations :
INDUSTRIAL, AND ENGINEERING CHE;MISTRY
March, 1931
(2)'
where! =
R, = R ,-*, cycles per second, R,
P. F. = 2~fR,(; = ohms, C, = capacity in farads
Carbon Black i n Oil-Effect
of Dispersion
At room temperature and 580 kilocycles the power factor of the neutral white oil used was zero. Preliminary tests showed that the power factor of oil-carbon black mixtures depends greatly on the dispersion. With Micronex dried but not screened, a mix containing only 4 per cent by weight showed 6.20 per cent power factor, although it was ground
-F
Figure 3-Circuit
for Power-Factor Measurements
into the oil in a mortar a little a t a time. Mixes containing dry Micronex screened through an 80-mesh sieve and merely stirred thoroughly into the oil showed only 0.8 per cent power factor at 4 per cent by weight and 2.6 per cent a t 10.7per cent by weight. ill1 tests were accordingly made with screened carbon black. The blacks for this work were dried by passing a slow stream of air through calcium chloride, phosphorus pentoxide, and then through the black in a flask in an oil bath a t 150" C. The drying was continued for a t least 24 hours. The results with dry screened Micronex are plotted in Figure 4. They furnish a control for other tests involving Micronex, because the power factor here is due entirely t o the black and represents the minimum for a given percentage of black. Similar tests on W-4,another type of carbon black, showed what great differences in electrical behavior exist between different kinds of black. I n the light of the pronounced effect of dispersion in the case of Micronex, it seems very probable that the bad electrical properties of T - 4 suspensions are due to poor dispersion. System Oil-Pigment-Moisture
After some preliminary search for a mixture of high power factor in which the influence of carbon-black additions could be studied, it was found that the power factor of a suspension of whiting in oil is extremely sensitive to the moisture content of the whiting. This is shown in Figure 5, where the power factor of suspensions in oil of 23.1 per cent by weight of Chalk Cliff whiting conditioned to various moisture contents is plotted against the percentage of moisture on the total weight of mix. This striking effect is probably enhanced by the presence of water-soluble electrolytes in the whiting. A similar curve (Figure 5) for 5.65 per cent of LIicronex, in which the moisture content of the Micronex ranged from 0 to 8.85 per cent is nearly horizontal, shows how relatively slight is the effect of moisture in the presence of Micronex. It might be thought that the difference in behavior of the two powders was due only to the greater tenacity with which moisture is held by Micronex; i. e., that the addition of moist whiting introduced water into the oil, giving rise to the high
275
power factor, whereas the carbon black did not give up its water. The following experiment disproved such an explanation: A mixture of 100 grams oil with 30 grams of conditioned whiting showed 11.5 per cent power factor. A portion of this mix, after being thoroughly shaken, was allowed to stand and the clear supernatant oil decanted and tested. The power factor was negligible. Hence the bad effect of moist whiting is not produced merely by its carrying water into the oil, but requires the presence of the whiting. Effect of Carbon-Black Additions on Power Factor of Moist Whiting Suspensions
When carbon black was added to rubber, the minimum in the power factor could not be very striking because the power factor of the base mixture was not high (1 to 2 per cent). In studying this effect in oil, it was possible to reproduce it on an exaggerated scale by starting with a base mixture of very high power factor. Moist whiting in oil served very well for this purpose. The most convenient method of conditioning the whiting consisted in blowing a stream of saturated air through it, with frequent stirring. The stock for a run was then tumbled in a tightly stoppered flask for over 12 hours for the sake of uniformity. Increasing amounts of dried, screened Micronex were added t o equal quantities of base mix. The samples were kept in rubber-stoppered flasks for about 50 hours with frequent vigorous shaking. It was found that the base mixture, if left open to the air, lost moisture in spite of the layer of oil, and the power factor dropped. It was also necessary to allow time for the establishment of equilibrium in the moisture distribution The 50-hour period of storage allowed the entrapped air to escape. I
1
1
I
I
I
I
a
2 2
4
6
8
10
O/oCARBON GLPCK added t o O I L Figure 4
Each sample contained 50 grams of saturated whiting in 120 grams of oil plus the added Micronex. It is interesting to note that, although dry whiting and oil in these proportions gave a fluid mix, when saturated whiting was used, the mix was rather stiff and difficult to pour. When an active carbon black such as Micronex was added, the mix became much more fluid, in spite of the added powder, showing that the Micronex robs the whiting of its water. When W-4 was added in a similar way it did not thin the mix, a behavior which is directly related to the difference in effectiveness of the two blacks in reducing power factor. The striking power-factor reduction brought about by the addition of active black is shown in Figure 6. Mechanism of Power-Factor Reduction
Inspection of Figure 2 shows that carbon black has much greater avidity for moisture than whiting, since it picks up
276
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 23, No. 3
mum in Figure 6, could not be explained on the basis of conductivity of particles. Preliminary tests with materials not suitable for use in rubber compounds, but whose particles were both insulators and strong adsorbents of moisture, produced similar behavior. I n these cases there could be no question of electrostatic shielding due to conductivity. Thus, active carbon black markedly reduces power factor in the oil-moist whiting mix, because i t renders inactive the water which on the whiting produces a contribution to power factor very large compared with that of the amount of black necessary to rob the whiting of its moisture. Conclusions
I
0
1
l 2
l
l
I
l
4 '10 MOISTURE IN M I X T U R E
3
l
l
(
5
Figure 5
larger amounts from atmospheres of the same humidity. It is therefore to be expected that in an oil suspension of originally moist whiting and bone-dry carbon black the carbon black will rob the whiting of most of its water. The curves of Figure 5 show that, while water on the whiting produces a very large contribution to power factor, the same amount adsorbed by carbon black causes negligible increase in power factor over that due to bone-dry carbon. The pronounced reduction in power factor shown in Figure 6 is therefore readily explained in the light of these properties as due to the transfer of the moisture from the whiting, where its effect is very bad, to the carbon, where its effect is negligible.
1-New transformer oils, when treated either by filtration or by sedimentation with dried carbon black of suitable quality, have shown an average improvement in dielectric strength of 40 per cent. 2 4 u c h treatment has been found more effective than that with metallic sodium, which also tended to darken the oil. 3-Active carbon black tends to remove moisture, electrolytes, and suspended particles. 4-The importance of substantial removal of carbon black after treatment is shown by data on power factor of various carbon blacks in oil mixtures. &The effectiveness of carbon-black treatment of insulating oils is a t its maximum at the lower temperatures, thereby avoiding the ill effects of existing high-temperature drying procedures. &The power factor of suspensions of whiting in mineral oil is extremely sensitive to the presence of moisture. 7-Active carbon blacks, such as Micronex, have great avidity for water and moderate amounts of adsorbed water cause only slight power-factor increase in suspensions of them. &Active carbon black is able to deprive suspended moist whiting of its water. %The transfer of water in a mix from the whiting, where it gives large power factor, to the carbon black where its effect is negligible, produces a striking decrease in the power factor of the mix. 10-The electrical conductivity of the carbon black particles plays no important role in this effect. 11-These results show that in insulating compounds such as rubber, where whiting is used as a filler, it is essential to have the whiting as dry as possible. The addition of small amounts of active carbon black tends to reduce the power factor of a compound in which the whiting or other pigment is not absolutely water-free. Pigments other than whiting are less sensitive to moisture in insulating compounds, but the addition of carbon black in small amounts will probably bring about an improvement even then. Literature Cited
Although the carbon black in itself always contributes to power factor, it may, as in this case, render inactive some material whose contribution is relatively much greater, giving a net reduction in Dower factor. InTooking for a cause Tor the difference in behavior of the two materials shown in Figure 5, one of the most obvious differences is the fact that whiting is an insulator, carbon black a conductor. It is conceivable that the moisture on the carbon black was electrostatically shielded from the alternating field. But it was proved experimentally that the flatness of the curve of power factor us. per cent moisture in the case of carbon black in Figure 5, and the striking mini-
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(1) Clark, F. M . , Gen. Elec. Reo., 31, 174 (1928). (2) Davey and Wilson, Ibid., 28, 770 (1925). (3) Friese, R . M . , Wiss. Verdfentl. Siemens-Koneem, 1, 41 (1921). (4) Green, C. F., Teztile Colorisl, 47, 644 (1925). (5) Gyemant, A,, Physik. Z., 26, 686 (1925). (6) Gyemant, A,, Naturwissenschuften, 13, 726 (1925). (7) Harvey, D . , Elec. J., 2S, 96-99, la7-130 (1928). (8) Hayden and Eddy, J. Am. Inst. Elec. E m . , 41, 138 (1922). (9) Monkhouse, A,, "Electrical Insulating Materials," p. 150. (10) Oertel, H . , Chem.-Ztg., 52, 92 (1928). 68# I3O1 (1916). (11) Ogawa and Kubo$ Elec. (12) Rengade and Clostre, Compt. rend., 173, 311 (1921). (13) Rodman, c, ,., J. IND, ENO. CHEM., 13, 1149 (1921). (14) Shrader, J. E., J . Franklin Inst., 199, 513 (1925). (15) Soccart, F., Rev. gen. l'elect., 23, 749 (1928).
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" , ; k ~ o ~ ~ ~ " ~ ~ ~ ~ ~ 86n ~ (1928). ~8~8, c, A,,
(19) Wiegand,
World, B2, 840 (192s),
w. B., and Boggs, c. R., IND. ENG.CHEM.,22,
822 (1930).
