Nonflammable Dielectric Organic
Compounds F. M.CLARK General Electric Company. Pittsfield, Mass.
material is inversely proportional to its dielectric constant. Since paper and pressboard have dielectric constants in the range of 4 to 5 sa. compared to 1 for air, it is obvious that the major proportion of the electric stress for the “air impregnated” dielectric will be placed on the air film itself. Since tile dielectric strength FACTORY INSTALLATION OF TRANSFORMER, C s r ~ oCALOBIVATED HYDROCARBON DIELECTRIC LIQUID of air is low, such composite dielectrics are not suited to meet the demands of high-voltage use. The substiVault constrt,etion is not nroesaary. tution for air or other gas of a liquid dielectricimpreguant with its higher dielectric constant gives a more equitable HE introduction of mineral oil as a coolingand dielectric voltage stress distribution. The more nearly the dielectric liquid impregnant was of major iiigiiificance in tlie deconshnt of the liquid approaches that of the cellulosic insulavelopnient of modern: tiigl~-voltageelectrical apparatus. tion itself, the greater is the elimination of the stress differIts introduction, however, gives rise bo clieniical problems, on ence. Mineral oil has a dielectric constant of approximately the solution of which depcrids the siicccssful commercial use 2.25. of the equipmelit. The foremost of these prohlcms roiicerns With certain apparatus such as capacitors, where the largthe cheniical stalility and the flamrnability oi the niinerol oil itself. The development of syntlietic, nonRarnmat,lc, est electrical capacity per unit of physical size is an important organic comlmunds successfully s o l ~ e stlieae prohieins mid consideration, economic demands also dictate tlie use oC an impregnant with a dielectric constant higher than that constitutes another imiiirrtant advance in t.Iit: dcctrical art. po&s&d by air. Necessity of a Liquid Impregnant The development of synthetic organic liquid dielectric compounds, properly selected and prepared, allows the use of The insulation of modern, liigli-voltage electrical equipan irnpregnant with a dielectric constant of approximately 5 ment, such as trausfomnx?rs, capacitors, and cables, consists and a resultant high degree or stress equalization not obtainof a solid barrier and a liquid diclectric. Tlie solid is genable even with the use of mineral oil. A liquid with such a erally a cellulosic material such as paper or pressboard. To high dielectric constant, obtained without sacrifice oi other maintain such an insulat,ion with the Iiighcst degree of dielecessential dielectric propert.ies, has resulted in a reduction in tric properties, not only must it be carefully dried in the the physical size of capacitors of approximately 50 per cent, factory hut it must be kept dry during use as ffinintegral as compared to the corresponding mineral-oil-treated unit. part of the apparatus. Because of the hygroscopic nature of cellulosic materials, it. is necessary that such insulations Flammability Characteristics of Mineral Oil be protected from atmospheric hriniidity effects. This is accoinpIished by the treatment of tlie solid with a nonlivgroThe flammabilitv of mineral insulatina oil is so low that it ._ scopic impregnant. In high-voltage apparatus a liquid does not involve more than a negligibfe hazard in normal impregnant which later servcs as the operating medium is dielectric service. The flammability hazard, however, preferred in order to dissi1jat.ethe lieat,generated in the operabecomes of major importance a t tlie time of dielectric hreaktion of the device. Such an apparatus is tlic high-voltage down. The number of electrical failures of apparatus in transfonner. service is only a small percentage of the total nunilxr of eleoThe liquid impregnant serves also another higlily important trical machines in use. Yet because of the complexity of dielectric purpose. Cellulosic insulation is never a homogenedielectric phenomena the operation of high-voltage electrical ous material. It consists of the solid cellulose in series with equipment such as transformers is always carried out with a film which, in the nhsencc of a specially applied impregnant, full recognition of the flammability dangers accompanying is the air itself. When voltage is apl~liedto such a composite the failure of the apparatus. Tlie restrictions surrounding dielrctric. tlie potential s t r e r niwrlml by each insulating the use of mineraluil-filled transformers, as set np by the
T
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Mineral hydrocarbon oils oxidize, thereby producing sludge, organic acids, water, and volatile gases, each of which contributes to dielectric deterioration. Mineral hydrocarbon oils are flammable and, under the arc, evolve explosive gases. The nonflammable chlorinated cyclic hydrocarbons described in this paper eliminate these defects. They are nonoxidizing, are of good dielectric strength, and evolve only nonexplosive gas mixtures when decomposed by the arc. Such liquid dielectric materials include the chlorinated polyphenyl compounds, such as the chlorinated derivatives of diphenyl, diphenyl oxide, diphenyl ketone, and diphenyl methane. Their use has made available for the electrical industry synthetic dielectric liquids of high dielectric constant and marked dielectric stability. The elimination of the fire and explosion hazard has been recognized by the National Board of Fire Underwriters with the modification of the National Electric Code. Many of the restrictions heretofore necessarily associated with the installation and use of oil-filled electrical equipment have been removed.
National Electric Code, is the result. Because of the increasing commercial desire to install transformers a t strategic locations indoors, these restrictions present many difficulties, especially in the larger cities. When the high-voltage electric machine fails, the dielectric is subjected to a power arc of varying intensity, depending upon the conditions of service and the nature of the electrical failure. The temperature of the arc may reach values as high as 4000" C. When subjected to such conditions, mineral oil is thermally decomposed. Gases are formed with a corresponding increase in internal tank pressure. The transformer tank may rupture either from gas accumulation or from the explosion of the gas mixture formed. In either case the rupture of the tank releases flammable gases and insulating oil which, under the proper chemical and electrical conditions, will ignite with explosive force. The fire hazard with mineral insulating oil, therefore, involves both the flammability of the gases evolved under arc decomposition and the flammability of the oil itself,
Arc-Formed Gases from Mineral Insulating Oil When mineral insulating oil is decomposed by the electric arc, the chemical nature of the gas mixture formed depends in part on the type of oil present and in part on the electrical characteristics of the arc. With a Gulf Coast mineral transformer oil, not previously degassed, the approximate percentage composition of the arc-formed gas mixture is as follows: Hydrogen Carbon monoxide Carbon dioxide Unsatd. hydrocarbons
60 3 0 16
Satd. hydrocarbons Oxygen Nitrogen
10 2 9
699
Such a gaseous mixture is highly explosive in air. The practical problems connected with its presence are considered by the designer and operating engineer as overshadowing the hazards arising from the flammability of the oil itself. Chemically the problem of hazard elimination begins with the gaseous products of arc formation, since the substitution of liquids of low order of flammability does not eliminate or even reduce the explosiveness of the arc-formed gases.
Elimination of Arc-Formed Explosive Gas Mixtures At the temperature of the power arc, evidence obtained indicates that the hydrocarbon molecule present in mineral oil is broken down into carbon and hydrogen. In the cooler zones surrounding the arc, this decomposition together with possible free radical formation occurs to a varying degree, depending on the zone temperature. The result is essentially the formation of carbon and hydrogen in a h i g h l y active 2I f chemical state; upon passing to a suitable 5 l o w e r temperature, 5 a these gases react in whole or in part to 5 4 p 6 form volatile hydrocarbons and hydro8 gen molecules, as are11 '!E as smaller amounts of E24 the oxides of carbon , arising from the prese ence of oil-dissolved * air. The successful , e l i m i n a t i o n of the gaseous fire and ex40 45 50 55 €4 plosive h a z a r d de% C H L O R I N E ( W T ) I N MOLECULE mands the substitution of a chemical FIGERE1. EFFECTO F CHLORINATIOK ON HYDROGEN EVOLUTION FROM action giving nonCHLORIKATED DIPHENYL UNDER THE flammable products ELECTRIC ARC i n p l a c e of t h e normally occurring reaction between hydrogen atoms, carbon, and hydrocarbon residues. The strong chemical affinity of chlorine for hydrogen and the thermal stability and nonflammability of the reaction product, hydrogen chloride, prove an adequate answer to the demand presented. STABILITY O F HYDROGEN CHLOTABLE I. RELATIVE THERMAL RIDE ARC-FORMED G.4SES OF HYDROCARBON MOLECULES Gas
Decompn. T e m r .
Reference
Type of Chlorinated Molecule Demanded iis a dielectric for high-voltage transformers, capacitors, and cables, the highest chemical stability is demanded. Chlorinated paraffin or chlorinated olefin hydrocarbons, including the chlorinated mineral oils of various types, are recognized to possess varying degrees of chemical instability. Carbon tetrachloride, derived from the simplest paraffin hydrocarbon, shows chemical instability (11, 14) to a degree not suited for dielectric application except t,o a limited extent in certain specific electrical machines. The chlorinated cyclic hydrocarbons, however, are recognized to possess the demanded high degree of chemical stability and to resist successfully oxidation and hydrolysis effects under the tem-
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manufacturing process. Since the capacity of the dielectric is influenced by the dielectric constant of the impregnating material, that impregnant having the highest dielectric constant consistent with other electrical and physical properties is desired. For the higher voltage applications where heat dissipation becomes important, TABLE11. CORROSIVE CHARACTERISTICS OF CARBONTETRAa liquid impregnant is preferred. PentachlorodiCHLORIDE AND TRICHLOROBENZENE phenyl (6,9),prepared by the direct chlorination Hydrocarbon Metal Temp. Time Corrosion of Metal of diphenyl a t elevated temperatures in the presc. Hr. ence of a catalsst, in addition to being a nonA1 76 14 Completely dissolved (0.4 gram) CClr vapor flammable liquid and giving off only nonfl&nmable A1 76 Completely dissolved (0.7 gram) cc14 liquid and nonexplosive gas mixtures when decomposed A1 210 14 l4 No corrosion CaHsCla vapor A1 210 14 No corrosion CeHsCls liquid by the electric arc, has a dielectric constant of A1 1405 24 No corrosion CsHsCls liquid approximately 5. I t s use as an impregnant Fe 140a 24 No corrosion CaHsCla liquid cu 140" 24 No corrosion CaHsCla liquid not only gives a more equal stress distribution 5 These tests were made in a sea!ed bomb at, 250 p,ounds per square inch (17.6 kg. per throughout the dielectric pad, but also results in sq. cm.) oxygen pressure in order to include possible oxidation effects. a marked reduction in the physical size per microfarad as compared to a similarunit treated with the usual mineral oil having benzene, is illustrated in Table 11. The stability required a dielectric constant of the dielectric liquid dictates the use of the chlorinated 5000 of approximately 2.25. cyclic compounds. The characteristics of Degree of Chlorination pentachlorodiphenyl are given in Table 111. The hydrocarbon molecules selected must be chlorinated, Figure 2 shows its visa t least to the degree that there is a chemical equivalency of 2ooo cosity c h a n g e w i t h chlorine and of hydrogen in the molecule. Given such an temperature from 25 O equivalency, the reactivity of hydrogen and chlorine, toto looo gether with the thermal stability of its reaction product, Other similar chlorinhydrogen chloride, ensures that the arc decomposition reacated compounds may tion will occur in almost theoretical proportions. The formabe used to advantage. tion of hydrogen chloride in practically theoretical amounts is Among these are the further evidence that at the arc temperature, decomposition 4oo pentachlorodiphenyl of the molecule into its elements occurs with but little free oxide (6), pentachloro;300 radical formation. Figure 1 shows the relation between the diphenyl ketone (7), theoretical and the actual formation of hydrogen and hydrogen and h e x a c h l o r o d ichloride under arc decomposition conditions as a function of phenyl methane (8); the degree of chlorination of the hydrocarbon diphenyl. The their properties are also following is a typical analysis of the gases formed from pentadescribed in Table 111. chlorodiphenyl when decomposed by the electric arc:
perature conditions applying in the normal use of electrical apparatus. The relative chemical stability of the chlorinated paraffins, as represented by carbon tetrachloride, and the chlorinated cyclic hydrocarbons, as represented by trichloro-
c.
Hydrogen Volatile hydrocarbons: Saturated Unsaturated
0%
0 0
Chlorine Carbon monoxide Carbon dioxide Hydrogen chloride
Nonflammable Transformer Liquids
:2 3 0-0.3
99-100
Pentachlorodiphenyl contains a chemical equivalency of hydrogen and chlorine in its molecule.
Nonflammable Capacitor Compounds
II11 1 1I I11
In transformer use, the insulating liquid functions both as a dielectric and a cooling medium. As a dielectric, it e n d o w s t h e solid insulation with increased electrical strength ~ and the ability ~ to w i t h s t a n d high-
The chief object of the capacitor is to supply capacitance for the correction of the power factor characteristics of disto 90 30 40 50 60 70 80 90 100 tribution circuits and of electric motors. I n addition, the TEMPERATURE - ' 0 . capacitor is widely used to furnish torque in motor-startFIGURE 2. TEMPERATURE us. VISing service. Although the total voltage applied to the COSITY (IN c ~FORPENT*.. ~ capacitor is equaled and in many instances surpassed in CHLORODIPHENYL other forms of electrical apparatus, because of the demand for high capacitance per unit of physical size the dielectric thickness is reduced TABLE111. CHARACTERISTICS OF TECHNICAL NONFLAMMABLE AND to such a n extent that this type of device operates NONEXPLOSIVE CHLORINATED, DIELECTRIC LIQUID COMPOUNDS Pentaohlorodi- Pentachlorpdi- Pentachlorodi- Hexachlorodiwith one of the highest (if not the highest) potential stresses of all electrical equipment. The Property phenyl phenyl Oxlde phenyl Ketone phenyl Methane careful selection of materials used in its construction is therefore of dominant importance. Insulation stability under voltage stress and temperatqre cannot be sacrificed. The dielectric of the capacitor usually consists of built-up paper pads, each sheet of which is closely controlled as to thickdess and porosity. I n order to improve the dielectric properties and to increase the electrical capacity, these insulating pads are carefully impregnated in the
Nonflammable
~ ~ ; ; ~ ~ 2 ~c.) , , 1 5 ~1.51 5 (65) 0 dscosity, Saybolt Uni-
45 (98O C.) A . ~ ~ ~ . e point ~ o u0 c. r 10 1.6380 Refractive index (25' "2.) Pale yellow Color Dielectric strength kv. 40 D1electric (25' C.) 6.1 Nonexplosive gases None Free acid None 360-400 Distn' ' *'
i;;;znyd
+
Nonflammable Nonflammable Nonflammable 1.590 (100) 1.43 (100) 1.52 (100) 390 (37.8' C.) 0 1.6220 Pale yellow 40 5.0 Nonexplosive None 23 0-2 None 50 (15 mm.)
54 (100' c.) +15 1,6370 Pale yellow 40
8.0 Nonexplosive None None 250-300 (25 mm.)
84 (100' c.) f20 1.6370 Pale yellow 40
4.3 Nonexplosive None None 290-340 (25 mm.)
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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When blended with any of the chlorinated capacitor liqui& already described, the viscosity and the A. S. T. M. pour point are reduced (3). This effect is utilized in the preparation of nonflammable and nonexplosive chlorinated cyclic hydrocarbons for transformers. A typical and successfully applied transformer liquid is that prepared by the blend3 ing of trichlorobenzene and QAP DISTANCE INCHES chlorinated diphenyl with a FIGURE4. COMPARATIVE DIELECTRIC 'Ontent Of 6o per cent STRENGTH CHARACTERISTICS OF MINERAL TRANSFORMER OIL AND TRANSFORMER(4). The characteristics of CHLORINATED HYDROCARBON LIQUID such a mixture are as follows:
-
30
-10 -m
0
10
20
Y)
TEMPERATURE
-'9
50
eo 70
8 0 so
loo
C.
FIGURE 3. COMPARATIVE CHANGE IN VISCOSITY WITHTEMPERATURE FOR MINERAL TRANSFORMER OILAND NONFLAMMABLE TRANSFORMER LIQUID
Burn point Nonflammable A. S. T. M pour point, C. Lower than -32 Sp. gr (156/15,5O C.) 1,563 Viscosity (37.8'' C.), Saybolt Uruversal 8ec. 54 Free acidity None Color Pale yellow
voltage application without disastrous ionization effects. As a coolant, it absorbs and dissipates the heat generated in the working parts of the apparatus. Therefore the best transformer liquids must, to a high degree, combine stability in dielectric properties (especially resistance to those factors tending to lower the dielectric strength) with chemical stability and the resistance to any change which would prevent heat absorption and heat dissipation from the core and coils of the transformer. Since the coolingof a transformer largely involves heat dissipation by means of liquid circulation, the best material is one of low viscosity which undergoes no change in service that will alter the viscosity characteristic, or deposit on the transformer coils or on the transformer tank wa11s, a solid (sludge) precipitate of high thermal resistance. Although hydrocarbon liquids are widely used, they do not fully meet these severe requirements; not only do they oxidize and deposit solid precipitates of high thermal resistance, but, in their oxidation, gases (including carbon dioxide) are formed along with water with a resultant decrease in dielectric strength. Because of the nonflammability, the high dielectric strength, and the marked stability of the chlorinated cyclic hydrocarbons against oxidation, they are peculiarly well suited to meet the requirements for successful transformer use. The low-viscosity requirements for the proper dissipation of heat generated in the transformer eliminate the chlorinated compounds which have been found highly satisfactory for capacitor use. Pentachlorodiphenyl, for example, has a viscosity a t 37.8" C. of approximately 2400 Saybolt Universal seconds as compared to the usual transformer oil viscosity in the range of 55 to 60 seconds. I n addition, its pour point is considered too high. Resort, therefore, is made to the marked effect produced when such materials are blended with trichlorobenzene (6). Trichlorobenzene prepared through the direct chlorination of benzene in the presence of a catalyst a t elevated temperature is a material of low viscosity. Its properties are as follows : Freezing point, O C. Burn point Color Sp. gr. (25' C.) Coe5cient of expanaion (25-100' C.)
0-10 Nonflammable Water-white 1.46 0.0007
When decomposed by the electric arc, this mixture evolves only nonflammable and nonexplosive gases as follows (in per cent) : Hydrogen Hydrocarbons: Saturated Unsaturated Carbon monoxide
0 0
0 50 0.25
Carbon dioxide Oxygen Nitrogen Hydrogen chloride
0 0.26 2.0 97.0
The viscosity change with temperature of such a mixture closely parallels the corresponding changes of a typical transformer oil (Figure 3). The effect of increased gap distance on the dielectric strength of the liquid as compared with corresponding changes in the testing of a typical transformer oil is illustrated in Figure 4. These liquids show a high resistance to chemical oxidation. When heated for 42 hours at 140" C. under 250 pounds per square inch (17.6 kg. per sq. cm.) oxygen pressure, no substantial chemical change occurs: Liquid Property Condition Reaction Free chlorides
Original Characteristics Clear Neutral None
Characteristice after Oxidation Clear Neutral None
The chlorine in the molecule of these transformer compounds is not hydrolyzed even under conditions far more severe than are met in transformer use. These products
Viscosity (37.8O C.), Saybolt Universal sec. drc-formed gases Nonexplosive Sludginp None 1,5700 Refractive index (25O Dielectric strength (25 ) kv. 40 Dielectric constant (25' C.j 4.8
--
9.
Coefficient of expansion (25100' C.) 0.0007 Sludging oharacteristlcs None Dielectric strength (25' C ) kv. 40 Dielectric constant ( 2 5 O C.j 4 3 Refractive index (25' '2.1 1 6140
DAYS
FIGURE 5. COMPARATIVE STABILITY OF MINERALTRANSFORMER OILAND CHLORINATED TRANSFORMER HYDROCARBON LIQUID
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VOL. 29, NO. 6
show no free chloride generation when sealed in glass and heated with water for 6 hours or more at 150°C. No free chlorides are generated ?hen these compounds are refluxed with water, even when oxygen gas is passed through the liquid-water mixture. One of the most sensitive tests for the chemical instability of chlorinated organic compounds is the attack of the unstable chlorine, if present, on clean aluminum foil. The transformer liquids here described show no corrosive action on aluminum even though heated in contact with it in the presence of air and water at temperatures as high as 260" C. After having been heated a t 260" C, for 6 hours in contact with aluminum, the stability of the chlorinated compound suggested for transformer use is demonstrated not only by lack of metallic corrosion but by the substantially unchanged condition of the compound itself, the only change is a slight and almost imperceptible darkening in color. The high dielectric strength of these transformer compound mixtures is maintained even when directly exposed to atmospheric humidity changes over long periods. Figure 5 illustrates the stability in dielectric strength of a trichlorobenzenechlorinated diphenyl mixture exposed to atmospheric changes under conditions which were known to have caused the accumulation of water in the mixture itself. The water accumulated on the surface of the heavier chlorinated mixture without noticeable effects on the dielectric strength.
pounds of the type described has been successfully carried 0ut.l The practical elimination of the fire hazard heretofore associated with oil-filled electrical transformers has been recognized by the National Board of Fire Underwriters to the extent that the National Electric Code has been modified (12). The stringent restrictions and expensive vault requirements for the use of oil-filled transformers have been removed. This action appears fully justified in the light of the successful commercial use of these products during the past five years.
Industrial Application
RRCEIVED March 10, 1937. Presented before the Division of Industrial and Engineering Chemistry at the 93rd Meeting of the American Chemical Society, Chapel Hill, N. C., April 12 to 15, 1937.
The commercial introduction of the chlorinated cyclic, nonflammable, and nonexplosive dielectric hydrocarbon com-
Literature Cited (1) Bone and Coward, J. Chem. SOC.,93, 1197 (1924). (2) Cantelo, R. C., J. Phye. Chem., 28, 1036 (1924). (3) Clark, F. M., Trans. Electrochem. SOC.,65, 5 9 (1934). (4) Clark, F. M., U. 8.Patents 1,931,373 and 1,931,455 (1933). (5) Ibid., 1,944,730 (1934). (6) Ibid., 2,041,594 (1936). (7) Clark, F. M., and Kuta, W. M., Ibid., 2,012,301 (1935). (8) Ibid., 2,012,302 (1935). (9) Jenkins, R. L., and Sikarski, J. A., Ibid., 1,892,400 (1932). (10) Mellor, J. W., "Comprehensive Treatise on Inorganic and Theoretical Chemistry," Vol. I, p. 492, Vol 11, p. 173, Vol. V, p. 819, New York, Longmans, Green and Co., 1922-4. (11) Milbauer, J., Collection Czechoslov. Chem. Commun., 3 , 73 (1931). (12) Natl. Board Fire Underwriters, Natl. Electrio Code, pp. 282-8 (1935). (13) Norrish, R. J. W . , Proc. Roy. SOC.(London), 150, 36 (1935). (14) Rhodes, F. H., and Carey, J. T., IND. ENG. CHEM.,17, 909 (1926).
, Commercially
designated as Pyranol.
Lubricating Properties of Lime-Base Greases F. H. RHODES AND THOMAS ELLIOTT WANNAMAKER Cornell University, Ithaca, N. Y.
T
H E cup greases in common use consist essentially of petroleumlubricating oils thickened with calcium salts of the fatty acids. Water is also a normal component of commercial cup greases, the amount of water usually varying from 3 to 1 per cent. In most cases the lime-base greases are prepared commercially by direct saponification with calcium hydroxide of a fat in solution in a petroleum oil. Products thus prepared contain a t least a small amount of glycerol, which may have some effect upon the structure and properties of the grease. Since the conditions under which greases are used are frequently such as to make it difficult to maintain an excess of lubricant on the bearing, the characteristics of greases under conditions of film lubrication are of particular importance. It has been found (2) that in the case of the soda-base greases the lubricating characteristics of the base oil are greatly modified by the presence of soap, water, and glycerol. It is to be expected that, in the lime-base greases, corresponding effects although not necessarily similar ones, should be observed.
I
The ratio of water to calcium soap in a limebase grease has a very pronounced effect upon the consistency and upon the lubricating characteristics of the grease. The lubricating power may also be affected by the glycerol that is present.
I
Preparation of Greases The petroleum oil used in all of these greases was a distilled lubrication oil from paraffin-base (Pennsylvania) crude. It showed the following characteristics: density a t 20" C., 0.88; Saybolt viscosity a t 100" F., 445 seconds, and a t 210" F., 64 seconds. Preliminary experiments showed that pure dry calcium oleate, free from oleic acid, is practically insoluble in the petroleum oil. No true grease could be obtained by warming and stirring a mixture of the soap and oil to which a small amount of water had been added. On the other hand, when 7Qparts of the oil were warmed with 30 parts of a somewhat
