Corona Discharge on Liquid
Dielectrics Materials Responsible for Increases in Power Factor JOSEPH STICHER AND JOHN D. PIPER The Detroit Edison Company, Detroit, Mich.
ORONA discharge has
C
This paper describes the nature of the products
experiment to ensure the samples
against contamination by oxythat are formed when hydrocarbon liquids are sublong been known to take gen. Neither subjected the jected to corona discharge and that cause the power place in engineering disamples t o possible contaminafactors of these hydrocarbon liquids to increase. tion by mercury vapor. I n both, electrics. The significance of the sample came in contact only The products were found to be amber-colored the discharge in deteriorating with glass while being subjected polymers that are essentially unsaturated hydroorganic dielectrics, particularly t o discharge. The samples were carbons of a highly reactive nature. The average “solid-type” h i g h - v o l t a g e transferred from either cell orders of polymerization of these polymers are low, t o the measurement cell under underground cables, has been vacuum. but the effect of given concentrations increases with investigated by The Detroit increasing order of polymerization. Edison Company over a period One of these, designated as of years. A previous paper Synthetic products have been made whose orders cell 2, was described previously of polymerization and physical properties are simi( l a ) showed that corona dis(la). The measuring equiplar to those of the products formed by corona discharge causes changes in the ment, which was a n integral charge. Addition of these products to hydrocarbon chemical, electrical, and physipart of the cell, was also decal properties of liquid dielecliquids, however, caused relatively small changes in scribed together with the major power factor. trics consisting of hydrocarbon techniques involved. oils. An increase in power It is believed that an insoluble component, which The other cell, designated factor resulted whenever the is formed in very small proportions during the disas cell 3, was designed to hydrocarbon liquids were subcharge, is the product primarily responsible for be rotated in order to expose jected to the discharge. The the power factor increases. This component, a continuously renewed oil present paper describes the which may not be exclusively hydrocarbon, is surface to the corona discharge progress made in attempting stabilized in a large proportion of the original and to dissolve continuously to determine the nature of the hydrocarbon by polymers having orders of polyany soluble reaction products materials that cause these merization and other properties intermediate bethat formed in the vapor phase power factor increases. tween those of the original hydrocarbon and the and deposited on the cell walls. T h e p r o b l e m was a p insoluble component. The resulting suspension Because this cell had a larger proached in two different ways: resembles true solutions. Just why such suspencapacity than cell 2 (normal The properties of bombarded sions should have high power factors is not well working capacity, 20 ml. as hydrocarbons were studied understood. Two hypotheses are discussed. compared with 10 ml.), cell 3 analytically; then materials was generally used for the synthesized to resemble those experiments designed to informed by bombardment were dissolved in oils having low vestigate the chemical properti& of the reacti& products. power factors, and the resulting power factor changes were The essential characteristics of the cell are shown in Figure 1. studied. Samples for electrical measurements were transferred from cell 3 to cell 2 by sealing the communication tubes together ANALYTICAL STUDIES and breaking the glass partition with a magnetically operated The investigation was carried out as a series of individual hammer. experiments. Each consisted of two parts-the bombardMost of the bombardment studies reported were made on ment of purified hydrocarbons under conditions designed Decalin (decahydronaphthalene) for the following reasons: to prevent contamination, and the determination of the elecAdequate supplies of this hydrocarbon were available; i t trical, physical, and chemical effects produced by the bomcould be purified to a degree at which it had a low initial power bardment. factor; it could be distilled at room temperature under high vacuum, which allowed the Decalin to be removed from the Equipment, Materials, and Procedure. Two difreaction products at room temperature; its vapor pressure ferent cells were used for subjecting the hydrocarbons to was not too great to prevent discharge from taking place; and corona discharge. These cells and the procedures used with i t was a good solvent for its reaction products. them had the following characteristics in common: The Decalin for some of the earlier experiments was prepared from the technical product by drying it over sodium and The hydrocarbons were thoroughly degassed, usually by revacuum distilling it. For later experiments the most effective fluxing under a good vacuum at room tern erature. Both celIs were tested for vacuum tightness before &e beginning of each method for preparing batches of Decalin having low initial
156’2
1568
Vol. 33, No. 12
INDUSTRIAL AND ENGINEERING CHEMISTRY
power factors was as follows: Treat Decalin with 98 per cent sulfuric acid in the absence of air until fresh portions of the acid remain nearly colorless; separate the Decalin layer and wash it repeatedly with water, dilute permanganate solution, and again water; dry the Decalin layer over sodium, shake with mercury, and distill a t atmospheric pressure in contact with mercury or sodium amalgam; finally, distill the treated Decalin fractionally several times under vacuum and collect the middle portion of the distillate. The sole purpose of the purification was to remove nonhydrocnrbon material. No effort was made to separate the Decalin into the cis and trans isomers.
nx
11
CONTACT-MAKING
Products Formed by Corona Discharge. I n all cases in which hydrocarbons that could be fractionally distilled were bombarded, a part was converted to gas, another part was converted to polymer, and a part appeared to remain unchanged. (The term "polymer" is here used in the broad sense which includes condensation products.) Figure 2 shows a typical composition of OIL-BOYBARDMEKT CELL3 FIQKJRE 1. ROTATWG matter resulting when Decalin was subA. Bombardment chamber B. Gaa accommodation bulb jected to nine 4-hour periods of bombardC. Separating wall between bombardment chamber and gas accommodation bulb ment. As shown, only 1.1 per cent by 0. Communication orifice in separating wall weight of the original Decalin was changed U. High.voltage electrode: Aquadag coating on outer surfaoe of tube to gas and 11.9 per cent to polymer. The V. Grounded electrode: silver coating on inner surfaoe of tube X. Offtake for sample oontainer and for initial evacuation (aealed off) remaining 87 per cent, which had the same Y. Communications tube for removal of contents (showing thin glaas window) distillation range as Decalin, ha I In iodine number of 11.2 which is e q u i d e n t to 5.2 per cent of octahydronaphthalene or smaller percentages of more liiglily unsaturated products. Over 80 whether the cell was rotated a t about 1.5 r. p. m. or was still per cent of the Decalin remained unchanged. The conditions during the bombardment. under which these particular samples were bombarded are The composition of the distillate obtained in this experiment was investigated by fractional distillation and deterdescribed in Table I, experiment 1. mination of iodine numbers. The iodine numbers of the three fractions obtained were approximately the same; this indicated that, upon bombardment, part of the Decalin loses DETAILS OF BOMBARDMENT EXPERIMENTS" TABLEI. ESSEXTIAL hydrogen to form unsaturated molecules having otherwise Experiment No. 1 2 3 4 6 nearly the same molecular size and structure as Decalin. Bombardment cell No. 3 2 3 3 2 The polymers resulting from the discharge consisted of a Volume of sample. ml. 20 10 20 20 10 Bombardment period, hr. 4 4 24 25 4 series of products of different molecular weights. The averNo. bombardment periods 9 1 3 a 9 age order of polymerization of the polymer obtained from the The bombardment pntential was 15.000 volts 60 cycles: the temperaexperiment was less than 3. The average order of polyture of the samples was -2prnximately 25 C. (ro:m temperature). 0
The composition of the gases resulting from corona discharge on hydrocarbons was discussed in the previous paper (13) in which some of the constituents were referred to simply as condensed gases. A more detailed study showed the composition of the gases evolved from Decalin to be as follows: Total VOl. of Gas, M1. 899.6" 0
I
H z
CHI
63.8
6.7
Per Cent by Volume C I H ~ , CZH4, etc. etc. CsHa CO 17.6 0.9 9.8 0.0
0.8
Na 0.6
(
Corrected to standard conditions.
I n addition to hydrogen and methane previously shown to be formed (I$), significant concentrations of ethane and its higher homologs and acetylene, as well as traces of other gases, were found This experiment, which was part of that described in experiment l, Table I, showed that the composition and rate of evolution of gases were essentially unchanged
DECALIN
4 % ..-
% GAS (MAINLY HYDROGEN)
-5.2%
81.8 % UNCHANGED
COS
1.1
UNSATURATED HYDROCARBONS ( M O L wr. SIMILAR TO DECALINI
11.9% POLYMERIZED HYDROCARBONS (UNSATURATED. AV. MOL.WT. T O 12001
FIGURE2. PRODUCTS FROM NINE C H o m BOMBARDMENTS OF DECALIN Composition is expressed in per oent b y weight of origind Decalin.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
December, 1941
1569
merization of even the highest molecular material isolated was less than 12. These are number averages rather than weight averages, for they were obtained from cryoscopic molecular weight determinations. Viscosities of solutions of these polymers were not markedly greater than the viscosities of the respective solvents, which indicates that the polymers did not consist of long linear chains. All of the fractions were more or less unsaturated. Upon exposure to air, the polymer oxidized in a manner similar to that of drying oils. The fact that a drying varnish is obtained by the action of an electrical discharge on Decalin was reported previously by Becker (9).
FIGURE 3. POWER FACTOR OF DECALIN SOLUTIONS CONTAINING POLYMEM FRACTIONATED BY DISTILLATION OF THE POLYMER FORMED FROM DECALIN BY CORONA DISCHARGE
Isolation of Materials that Cause High Power Factors. One or more of the kinds of products discussed must be responsible for the high power factors of hydrocarbons that have been subjected to corona discharge. Removal of the gas was found by repeated experiments not to change the power factor of bombarded Decalin. They showed also that the power factor of the distillate from bombarded Decalin was not materially greater than the power factor of the Decalin itself; thus the unsaturated hydrocarbons that distilled in the same range as Decalin were not the materials primarily responsible for the power factor increases. These findings indicated by elimination that the polymer formed in the discharge must contain the material or materials that cause the power factor increases. Actual tests showed this to be true. To determine whether the effect on power factor produced by a given concentration of the polymer in Decalin varied with the order of polymerization of the polymer, the polymer
r Distillate 14.7 ml., power factor
(All power factor values at 50° C. and 60 oycles.) 10 ml. Decalin (power factor 0.0004) bombarded as described in Table I, expt. 2
10 ml. Decalin (power factor 0.0020) bombarded as described in Table I, expt. 2
8.0 ml., power factor 0.319
6.9 ml., power factor 0.514
I
I
L i J United; distd. a t 25’ C., 0.01p Hg 1
II
Residue 0.2 ml.: distd. in closed system at 293’ C.
0.0093
.I
r
II
Distillate 0.1 ml.: stored 18 months
Residue 0.1 ml.; stored 18 months
I
Made hp to 10 ml. in Decalin (power factor 6.0034); resulting power factor, 0.0012
I
Made up to 10 ml. in Decalin (power factor 0.0059); resulting power factor, 0 612
was fractionated, and the effect of a known concentration of each fraction on the power factor of Decalin was determined. Figure 3 gives the essential details of a fractional distillation
FACTORS OF DECALIN SOLUTIONS CONTAINING POLYMERS WITH DIFFERENT ORDERS OF POLYMERIZATION OBTAINED FIGURE4. POWER BY SOLVENT FRACTIONATION OF THE POLYMER FORMED FROM DECALIN BY CORONA DISCHARGE (All atepe involved in the separations were oarried out in closed evacuated glass ayetems in atmospheres of the solvents used. Power factor values marked with an asterisk were determined with a concentric-electrode stainlesa-steel cell and a modified Schering br2dge (9). All power faotor values at 50° C. and 60 cycles.)
20 ml. Decalin (power factor 0.001) bombarded as described in Table I, expt. 3; gas removed a t end of each 24 hours
I I
Part of volatile liquid distd. off at 25O C., 0.01~Hg 7
I
II
Distillate (10 ml.) discarded
Residue (IO ml.) added to 40 ml. anhydrous isopropanol and centrifuged; soln. decanted from ppt.
II
Ppt. dissolved in 10 ml benzene; soh. poured into 80 ml. anhydrous isopropanol and treated as above ”
r Disthlate disoarded
II
Residue (fraction 1). 6.?% by wt. of original Decalin; amber-colored oil, iodine No. 177, mol. wt. (cryoucopic in camphene) 390, av. ordor of polymerization 2.8
r I
Distillate discarded
Dissolved in Decalin to form 0.5% solns. Power factor: Decalin, 0.0006*,soh. of fraction 1, 0.052*
I
r
Soh,, distd. at 0.01p Hg, 251000 c.
If
Residue freed from adhering solvent by evacuating to 0 . 0 1 ~ Hg and heating to 1000
I
1
Residue (fraction 2). 2.2% by wt. original Decalin; light amber oil, iodine No. 141. mol. wt. (camphene) 440, av. order of polymerization 3.2
Dissolved in Decalin to form 0.5% solns. Power factor: Decalin, 0.0006*, soln. of fraction 2. 0.048*
c.
I
Solvent disoarded
l
Residue (fraction 3). 1.5% by wt. of original Decalin; cream-colored powder, iodine No. 108, mol. wt. (camphene) 1200, av. order of polymerization 8.7
I
Dissolved in Decalin to form 0.5% solns. Power factor: Decalin, 0.0006*,soln. of fraction 3, 0.10*
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1510
experiment. It showed that the material producing the high power factor was contained in the fraction having the highest order of polymerization, as judged by the fact that it was less volatile than the fraction distilled. Because of other work these fractions were not added to Decalin immediately but were stored for 18 months in a dark place in evacuated glass containers. Figure 4 gives the results of similar tests in which the fractionation was carried out by selective solvation. This experiment also showed that the material primarily responsible for high power factors of bombarded Decalin was contained in the fraction having the highest order of polymerization. Inasmuch as further experimentation showed that fractions 1 and 2 contained traces of material resembling fraction 3, i t seems possible that formation of the low-molecular polymer was not directly related to the resulting power factor increases. Figure 4 shows that each fraction was unsaturated, as indicated by the iodine number, but that the degree of unsaturation decreased as the molecular weight increased. Further, the fraction with the highest order of polymerization dissolved in Decalin less readily than the fractions with lower orders of polymerization and also less readily than the unfractionated polymer.
Effect of Hydrogenating Polymer from Bombarded Decalin. Because the polymer that caused the increase in the power factor of bombarded Decalin was found to be unsaturated, an attempt was made to determine whether saturation of the polymer would change its effect on the power factor of Decalin. The polymer was dissolved in Decalin, and power factor measurements were made before and after the solution was hydrogenated over Raney nickel a t the University of Wisconsin. The essential details are outlined in Figure 5. Hydrogenation removed not only the unsaturation and the color, but also reduced the power factor of the solution to a value only slightly greater than that of Decalin itself.
O FSI)EC4LIN SOLUTIONS BEFORE LND FIGURE5. POWER F ~ F T O R AFTER HYDROGENATION OF POLYMER DISSOLVED IN THEM
(Power factor values a t 50' C. and 80 cycles.)
20 ml. Decalin (power factor 0.001) bombarded
under conditions described in Table I, expt. 4: gas removed, distd. a t 25' C., 0 . 0 1 Hg ~
I
r--
I
Distillate (14 ml.) discarded
I/
Residue (6 ml.) dissolved in 30 i d . Decalin (power factor 0,001); amber-colored soln., iodine No. 26.0, power factor 0.0715 I
r
-1
Soln. hydrogenated ok.er 2 successive portions of Raney nickel for 5 lir. a t 225-250° C. and 175-250 atm. of H::
I
Resulting soln. colorle'ss, iodine number 0.0, power factor 0.0098
Change in Power Factor with Time of Corona Discharge. To obtain sufficient polymer for the chemical tests, it was necessary to bombard the Decalin for a much longer period than the 4 hours described in the preceding paper (IS). The rate of power factor increase diminished as the time of bombardment was extended. This phenomenon was investigated under the conditions described in Table I, experiment 5 . Power factor measurements were made before
Vol. 33, No. 12
and after each bombardment period. The power factor increased during the first five periods and then decreased during the remaining four periods (Figure 6). Removal of some of the Decalin by distillation caused further decreases in power factor, probably because the effect of the decreasing mobility of the charge carriers, resulting from increased viscosity, became greater than the effect of increasing the concentration of the carriers.
Dielectric Tests Indicative of Colloidal Character of Bombarded Decalin. Many experiments have indicated that the polymers responsible for the high power factors of bombarded Decalin are not in true solution but in some sort of colloidal suspension; the degree of dispersion of the latter is affected by conditions such as application of heat, time of standing, and application of the potentials used in measurements.
BOMBARDMENT
PERIODS
FIGURE6. EFFECTO F NINE 4-HOUR PERIODSO F CORONADISCHARGE ON THE POWER FACTOR OF DECALIN
All samples were heated for short periods in order to bring them and the equipment to several constant temperatures in the range between 25" and 60" C. The effect of heat was determined during cooling by rechecking at specified temperatures the power factor values that had been measured a t those same temperatures during heating. The values a t 50" and 60" C. were apparently unaffected either by the heating necessary for obtaining constant temperatures or by prolonged heating a t these temperatures. The values a t points below 50" C., however, were higher after heating than a t the corresponding temperatures before heating. These higher values remained fairly constant, a t least for several days. The change produced by heat was apparently eliminated by the bombardment during the next period. Although standing did not affect the power factor of bombarded Decalin when the concentration of polymer was low, standing usually resulted in a decrease of power factor when the concentrations were high. In the latter case the original power factor was restored by shaking the sample. The original decrease in power factor was not accompanied by visible precipitation of the polymer. Ultramicroscopic particles were not usually perceivable in bombarded Decalin when viewed through the cardioid ultramicroscope. In a few cases, however, ultramicroscopic particles were observed. They were found to bear either positive or negative charges when the sample was subjected to potential. Sometimes the direction of motion changed as the particle approached one of the electrodes of the measuring cell. It is believed that colloidal particles were present in all the
December, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY 0
Fu 0 0
B9
0
z
x
0
sx
bombarded samples but that they were not individually perceivable because their refractive indices were nearly the same as that of the medium in which they were suspended. The possibility of perceiving theseparticlesindividually by an electron microscope is being considered. Application of directr current potential invariably resulted in a marked decrease of the power factor of the bombarded samples. T h e power f a c t o r rapidly attained its former value, however, upon removal of the potential. This phenomenon was discussed in the preceding paper (IS). It is believed that the application of the d.c. potential caused the colloidal particles to migrate toward the electrodes near which they formed space charges similar to those observed by Whitehead (16).
2 9
0
p.l
8
x
? m
.a $
a
AdL I
3
3
d
ti
d cj
P
*
B 8
v
SYNTHETIC STUDIES
v
Analytical studies alone are insufficient to reveal the nature of the materials that cause insulating oils to acquire high power factors when they are subjected t o corona discharge. C o r r e 1a t i o n between the results of power factor tests and of any other test used to characterize a constituent of deteriorated oil does not necessarily mean that the constituent caused the increase in the power factor of the oil. The constituent actually causing the increase may have been formed simultaneously with that characterized and may have remained undetected. One method of determining whether a material, suspected of
1572
Vol. 33, No. 12
INDUSTRIAL AND ENGINEERING CHEMISTRY
causing abnormally high power factors actually does cause them is to determine the Jhange in power factor that results when the material is added to oil having a low power factor. For the present study the method had the following limitations: 1. Individual compounds, other than simple gases, have not been isolated from the products resulting from corona discharge on hydrocarbon oils. Even the types of compounds formed have been only roughly classified,as described in the preceding section. Much of'this classificationwas done simultaneously with the work to be described. The materials selerted for study, therefore, were usually chosen to represent types of compounds that conceivably might be formed by corona discharge on hydrocarbon oils rather than to represent the types of compounds that actually are formed. 2. Hydrocarbons of the types that might be formed by corona discharge cannot usually be purified by the drastic methods often necessary t o reduce the conductivities of stable hydrocarbon liquids to less than, say, IO-'* mho. per centimeter-cube. Nevertheless, the method was used for two groups of materials. One group consisted of individual hydrocarbons of known chemical structure, and the other consisted of hydrocarbon polymers most of which had known type structures. The solvent oils to which the hydrocarbon materials were added were Decalin and liquid paraffin. Purification of the former was described under "Analytical Studies". The liquid paraffin was used untreated except for drying and degassing. It was described previously (9) as were the test equipment and procedure.
Effect of Hydrocarbons of Known Structure on Power Factor. The following types of compounds were either known to be formed or believed possible of being formed by corona discharge on hydrocarbons: 1. Compounds that are electroIytically dissociabIe to the extent that under suitable conditions one or more hydrogen atoms are replaceable by metal. 2. Hydrocarbons that are easily polymerizable. 3. Unsaturated hydrocarbons whose unsaturation is so active that the hydrocarbons behave like drying oil. 4. Colored hydrocarbons whose color depends upon the presence of chromophoric groups. 5. Fluorescent hydrocarbons. 6. Free radicals. Data on the seven hydrocarbons investigated are given in Table 11. The power factors (80' C.) of the oils before the compounds were added and of the mixtures of these oils and the hydrocarbons are compared in Table 11. Power factor measurements were also made a t lower temperatures, usually 60°, 40°,and 30' C., and the values found to be progressively lower than those shown. The data indicate that in no case was the power factor of the oil increased markedly by the addition of one of these hydrocarbons; the following conclusions were drawn: 1. The degree of electrolytic dissociation of hydrocarbons, such as phenylacetylene and indene, that have hydrogen atoms replaceable by metal is too small to increase markedly the power fartor of oils in which such hydrocarbons might be formed by corona discharge. 2. The resence of highly polymerizable compounds, such as styrene anjindene, likewise cannot cause high power factors. 3. The presence of compounds with such a reactive type of unsaturation that they behave like drying oils does not of itself cause marked increases in power factor. 4. Color in oils does not increase markedly the power factor of oil. Drovided the color is caused bv the formation of chromophoric groups. 5. Fluorescence in oils is not necessarily related to any increase in the ower factor. 6. Pree radicals of the stable type represented by triphenylmethyl do not cause marked increases in power factor.
Development of color has long been associated with the deterioration of oil that is evidenced by increase in power fac-
tor. The relation between color changes and power factor changes of oils subjected t o oxidation has been studied systematically by Balsbaugh and eo-workers (I). Sticher and Thomas (IS)showed that color developed in nearly all of the hydrocarbons, including Decalin, that were subjected by them to corona discharge. We believe that development of color in deteriorated oils may be caused by the formation of either chromophores or colloidal dispersions. The fact that the highly colored solutions of carotene and dimethylfulvene in Decalin had low power factors shows that the mere presence of compounds containing chromophores does not cause high power factors.
:I
0.10,
r
I ULTRAVIOLET (UNTREATED)
0.GB
:I j
0.05
/
/
0.04 ULTRAVKILET RESIDUE
POLYMERS
ANQ D I S T I L L A T E
0 00
CONGENTRATION OF ADDED MATERIAL PERCENT BY WEIGHT
FIGURE 7. POWER FACTORS OF SOLUCONTAINING INDENE POLYMERS IN DECALIX
TIONS
Experiments with carotene indicated that solutions containing colored hydrocarbons may have high power factors even though the colored hydrocarbon was not the direct cause of the higher power factor. The power factor values of solutions of carotene, as received from the vendor except that it had been dried, were many times higher for equal concentrations than were those shown in Table 11; the latter were determined on solutions containing carotene purified by fractional precipitation from octane followed by a single crystallization. Further crystallization would undoubtedly have resulted in even lower values. Although the unpurified carotene seemed readily soluble in Decalin, a Decalin-insoluble amorphous fraction consisting of approximately 8 per cent of the weight of the original sample was isolated from it by fractional precipitation from octane. Apparently this insoluble impurity was the material causing the high power factor. According to a hypothesis developed in the "Summary", the sole function of the carotene was to stabilize the colloidal dispersion of this insoluble material in the Decalin.
Effect of Polymeric Hydrocarbons on Power Factors of Oil. Because the analytical studies indicated that the high power factors of bombarded oils were caused by high-molecular hydrocarbons, seven of these compounds were added to oil and the resulting effect on the power factor was determined. Table I11 lists the high-molecular hydrocarbons investigated and shows the increases in power factors and conductivities resulting from the addition of various amounts of these hydrocarbons to liquid paraffin or Decalin at 80' C. Power factor and conductivity measurements were also made at lower temperatures, usually 60°, 40°, and 30" C. I n all cases these values were progressively lower than those obtained at 80" C, Some of the mixtures of polymer and oil were prepared for the definite purpose of investigating colloidal systems. Un-
INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1941
1513
HYDROCARBONS ON POWER FACTORS AND CONDUCTIVITIES OF INSULATING OILS TABLE 111. EFFECTOF HIGH-MOLECULAR
Hydrocarbon Viskanol 314
How Obtained Esso Laboratories
Av. Mol. Weight 10,000~
Viskanol320
Esso Laboratories
180,0000
Polyisoprene
Isoprene heated 100-105° C. in sealed evacuated tube 11 days rened heated at looo C. 24 r. in sealed evacuated tube; unchanged styrene distd residue dissolved in beneen;: ptd. with alcohol, dried I n 9em&/ heated at 175O C. 23 days in sealed evacuated tube; about S5Y0 polymer-
Not known
Polystyrene
Polyindene
stx
60,000'
Solvent Appearance Oil Colorleas Liquid gummy oil paraffin Colorless Decalin rubber; Viacous color- Decalin less liquid Colorless solid Decalin
Hydro-
Conductivityb Mho/Cm.a X 1614 Oil Oil hydroalone carbon 0.27 4.94
+
1.26
Oil alone 0.0002
0.51
0.0005
0.0015
0.31
1.66
4.0
0.0024
0.0022
1.11
1.39
1.0
0.0038
0.0040
0.82
0.82
O % ::$
0.0012
. About 1,OOOp Viscous yellow oil
Decalin
1.1 3.9
0.0006 0.0006
0.0036 0.0083
0.38 0.38
1.54 6.56
Decalin
3.9
0.0003
0.0072
0.27
2.72
0.83 2.23
0.0006 0.0000
0.0018
0.0009
0.38 0.38
0.77 1.16
Polyphenylacetylene
6600
Reddishamber solid
Polydimethylfulvene
6000
Yellow oil, Decalin c.: 1000 yellow solid 2h0 C.
At 60 cycles 50 volts per mil 80' C. ap lication at'2.5 volts per mil and 80" C. Information from %sso Laboratories. d Monomer purified as described in Table 11. From specific viscosity: I Sample 2 of Table 11. 0 Cryosoopically in camphene or oamphor. 5
b After l-min;te e
fortunately, the polymers that could be obtained and dispersed readily were of colloidal size in one dimension only. The Viskanols, which are reputedly polyisobutylenes, were selected to represent saturated aliphatic linear macromolecules. Polyisoprene was used to represent similar unsaturated compounds, and polystyrene represented linear macromolecules having aromatic side chains. The results of the tests show that the presence of a colloidal dispersion of linear macromolecular hydrocarbons in oils does not cause high power factors. Polyindene, polyphenylacetylene, and polydimethylfulvene were prepared to investigate polymers having physical properties similar to those of the polymer formed by corona discharge on Decalin. Polyindene represented short-chain aromatic polymers that are essentially saturated with respect to the chain and that may have one dissociable hydrogen atom per molecule. Polyphenylacetylene represented short-chain aromatic polymers having an unsaturated chain. Polydimethylfulvene was prepared in the hope that the polymer would exhibit the marked chemical reactivity of the monomer and of the polymer resulting from corona discharge on Decalin; the polymer, however, was chemically inert. These polymers resembled the bombardment product of Decalin in that they had approximately the same molecular weights as were determined cryoscopically and they influenced the viscosity of their solvent only slightly. The polymer of indene was fractionally precipitated in much the same way as was the bombardment product of Decalin. The highest molecular fraction had an average molecular weight of 1800. Dielectrically, however, these polymers behaved quite differently from the Decalin polymer; the power factor values of Decalin containing these polymers were much less than the power factor of Decalin containing similar concentrations of the polymer resulting from corona discharge. Inasmuch as the polyindene used was colored, instead of being colorless as was the polymer described by Dostal and Raff (a), the presence of degradation products of unknown structure is indicated. Even the small increases in power factor resulting from the addition of the indene polymer to Decalin are believed to hpve been caused more by the presence of this unkpown impurity than by the presence of the indene polymer itself.
Effect of Polymers Formed by Ultraviolet Light. Besides thermal polymers, a few polymers formed by the action of ultraviolet light upon hydrocarbons were investigated. For these experiments the samples were irradiated in quartz tubes in atmospheres of their own vapors. The tubes were immersed in cold running water to prevent the samples from being heated by the source of the irradiation, which was a General Electric Uviarc mercury vapor lamp placed about 4 inches from the sample. The principal product formed by the irradiation of dimethylfulvene resembled "Cable X" in appearance and in its insolubility in all common solvents. This material formed on the inside of the tube on the side exposed to the radiant energy and apparently protected the remainder of the contents. Irradiation of indene sample 2 (Table 11) for 45 days changed the fluid colorless hydrocarbon to a viscous amber polymer in which were suspended flakes resembling Cable X. This mixture mas divided into two portions. The first was added to Decalin to produce two different concentrations, and the resulting changes in power factor were determined. The results are shown on the upper curve of Figure 7. The second portion was distilled at rooin temperature under a pressure less than 1 mm. of mercury absolute. Eighty per cent by weight of the original material was recovered as residue and 20 per cent as distillate. Power factor measurements were made on the solution containing the residue in Decalin and then on the solution resulting when the distillate was also added. The results of these meamrements are shown on the middle curve of Figure 7. Distillation and recombination of the products resulting from the irradiation diminished greatly the extent to which these products affected the power factor of the solution. Comparison of the upper and middle curves with the lower curve shows that the power factors of the solutions containing the products formed by ultraviolet irradiation were much greater than the power factors of solutions containing equal concentrations of the thermal polymer. No explanation of these phenomena will be ventured at this time. In addition, preliminary experiments with Decalin indicate that ultraviolet light may transform hydrocarbons into products that resemble those formed by corona discharge. Products from corona discharge were previously shown b y
1574
INDUSTRIAL AND ENGINEERING CHEMISTRY
Schoepfle and Fellows (11) to produce chemical effects similar to those caused by alpha particles. The experiments with ultraviolet light were discontinued because the nature of the products formed was known no better than that of the products formed by corona discharge. SUMMARY AND DISCUSSION
The analytical studies indicate that of the materials formed from hydrocarbons by corona discharge, the one responsible for the high power factors of the product is a colored polymer. This polymer is hydrocarbon in nature, as far as has been ascertained, and is unsaturated. The extent of the unsaturation is not particularly high, but the reactivity of the unsaturation is great. Although the polymer appears to be soluble in the hydrocarbon from which it was derived, a part of it seems to be in the colloidal state. Individual colloidal particles are not usually perceivable, however, with the aid of a cardioid ultramicroscope. The polymer seems not to be of the linear type. If it is, its chain lengths must be short. The properties of the polymer indicate that it may be classified as hemicolloid. As the synthetic studies have shown, i t is improbable that electrolytic dissociation of any known hydrocarbons, present in a continuous oil medium in concentrations which could reasonably be produced by deterioration, can cause high power factors and high conductivities in such a medium. [This and other statements in this paper refer to power factor and conductivity measurements made at the usual potential of not over 50 volts per mil. The mechanism of conduction of hydrocarbon liquids at high field strengths, which Plumley (IO) ascribes to electrolytic dissociation of hydrocarbons, is outside of the scope of this paper.] Furthermore, in concentrations up to several per cent in a continuous hydrocarbon medium, the presence of readily polymerizable hydrocarbons, deeply colored hydrocarbons, fluorescent hydrocarbons, and hydrocarbon polymers, even those whose molecular size extends into the field of colloidal dimensions, does not cause the power factors and conductivities of the medium to be high. Although the data seem conflicting in many respects, all of the systems studied so far and found to have high power factors have one characteristic in common; namely, all of these systems contain a component which by itself is insoluble in the hydrocarbon liquids that constitute the major component of the systems. This insoluble component seems to be kept in suspension by a third component which makes the system so stable that it resembles a true solution. The third component consists of polymers having molecular weights and other properties intermediate between those of the other two. This common characteristic seems to hold whether the system is formed by corona discharge on hydrocarbons or by adding impure carotene, certain copper and lead soaps (6),or certain oxidation products (8) to dielectric liquids. Hypotheses to explain why such liquids should have high power factors are none too satisfactory. I n a previous publication (8) a tentative hypothesis was given to explain the high power factors of systems in which the insoluble components had dielectric constants materially greater than those of hydrocarbons. I n this hypothesis i t was assumed that: In nearly every hydrocarbon medium the concentration of potentially dissociable materials is sufficiently high so that all of these media would have high power factors if the materials were completely dissociated, The degree of electrolytic dissociation of these materials in continuous media of low dielectric constant is, however, very smalI. Formation of a finely dispersed phase with a high dielectric constant would provide a medium in which, or on which, the potentially dissociable impurities could dissociate. The ions thus formed would probably be distributed unequally between the two phases and thus cause the colloidal particles to acquire charges of one sign and furnish ions having a preponderance of the other sign to the medium.
Vol. 33, No. 12
Use of this hypothesis to explain the high power factor of hydrocarbons that had been subjected to corona discharge encounters difficulty. If the insoluble constituent consists of hydrocarbons, it probably does not have a dielectric constant greater than 3. Carbon seems to be the only material having a dielectric constant greater than 3 that might originate from a hydrocarbon under the conditions of the experiments. I n the case of Decalin, bombarded as described, there was no evidence that carbon was formed. An alternative hypothesis is suggested, based upon the possibility that the charge carriers involved were formed by the bombardment rather than by electrolytic dissociation. By means of a mass spectroscope Linder (6) showed that the initial products formed by corona discharge on hydrocarbons are fragments bearing positive charges. Under ordinary circumstances these ions would be expected to have exceedingly short lives; what actually happens, however, between the time the initially charged bodies are formed and the time the final product is isolated as a polymer is not known. Possibly when the initially charged fragments are caught in an insulating medium, not all of the charges are discharged, and thus charged bodies are left suspended in the insulating liquid. Perhaps neither of these hypotheses approaches the truth; certainly neither is complete. Further investigation of the problem is being undertaken. ACKKOW LEDGMENT
This work formed part of an investigation on the deterioration of high-voltage underground cable conducted by The Detroit Edison Company under the direction of the late C. F. Hirshfeld, chief of research. It was made possible only by the cooperation of the staff of the Research Department, and the aid and advice of many persons not connected with that department. The authors are pleased to acknowledge especially the assistance of their colleagues, C. D. Robb, A. G. Fleiger, C. C. Smith, and N. A. Kerstein in obtaining experimental data; of C. S. Schoepfle of the University of Michigan as consultant; of P. L. Cramer of General Motors Research Laboratory for information concerning dimethylfulvene; of Homer Adkins of the University of Wisconsin for arranging to have samples of bombarded Decalin hydrogenated; and of Floyd L. Miller and the Esso Laboratories for furnishing the Viskanols. LITERATURE CITED (1) Balsbaugh, J. C., and Oncley, J. L., IND.EXG.CHEM.,31, 318 (1939). (2) Becker, Brit. Patent 275,813 (Sept. 29, 1926) ; Ellis, Carleton, “Hydrogenation of Organic Substances”, 3rd ed., p. 482, par. 4232, New York, D. Van Nostrand Co., 1930. (3) Dostal, H., and Raff, R., 2.phg5i.k. Chem., B32, 417 (1938). (4) Kuhn, R., and Brockmann, H., B e y . , 67, 885 (1934). (5) Linder, E. G., Phys. Rev.,41, 149 (1932). (6) Piper, J. D., Fleiger, A. G., Smith, C. C., and Kerstein, N. A , , IND.ENG.CHEM.,31, 307 (1939). (7) Piper, J. D., and Kerstein, N. A., IND. ENG.CHEM.,ANAL.ED., 9, 403 (1937). (8) Piper, J. D., Smith, C. C., Kerstein, N. A,, and Fleiger, A. G., IND. ENG.CHEM.,32,1510 (1940). (9) Piper, J. D., Thomas, D. E. F., and Smith, C. C., Zbid., 28,309 (1936). (10) Plumley, H. J., Research Council, Conf. on Electrical Insulation, Washington, D. C., 1940. (11) Schoepfle, C. S., and Fellows, C. H., IND. ENQ.CHEM.,23, 1396 (1931). (12) Smith, J. H. C., and Milner, H. W., J . Biol. Chena., 104, 437 (1934). (13) Sticher, Joseph, and Thomas, D. E. F., TTalas. Am. Inst. Elcc. Engra., 58, 709-22 (1939). (14) Stobbe, H., and FBrber, E., Bet-., 57, 1838-51 (1924). (15) Thiele, J., Zbid., 33, 666 (1900). (16) Whitehead, J. B., and Marvin, R. H., Trans. Am. Inst. Eke. Engrs., 49, 647 (1930).
