1104
INDUSTRIAL AND ENGINEERING CHEMISTRY
(17) Hildebrand. J. H., and Buehrer, T. F., J . Am. Chem. SOC.,42, 2216 (1920). (18) International Critical Tables, Vol. 111, pp. 394-7, New York, McGraw-Hill Book Co.,1928. (19) Jacobs, W. A., and Heidelberger, M., J . Am. Chem. Soc., 39,1448 (1917). (20) Jaeger, A.,Brennstoff-Chem., 4,259 (1923). (21) Janecke, E., 2. Elektrochem., 38,860 (1932): 2. physik. Chem., 184, 61 (1939). (22) Koch, H., and Steinbrink, H., Brennstoff-Chem., 19,282 (1938). (23) Kraus, C.A.,and Zeitfuchs, E. H., J . Am. Chem. SOC.,44,1250 (1922). (24) Kruyt, H.R., 2. physik. Chem., 65,497 (1909). (25) Landolt-Bornstein-Roth-Scheel,Tabellen, pp. 760-1. (26) Zbid., Erg. I, 301. (27) Zbid., Erg. IIa, 474-8. (28) Zbid., Erg. IIIa, 678-9. (29) Lecat, M., J . chim. phys., 27,79 (1930). (30) Lecat, M., Rec. trau. chim., 46,244 (1927); 47,16 (1928); Ann. soc. sci. Bruzelles, 45,284 (1926); 47B,i, 149 (1927); 49B,ii, 17,109 (1929). (31) Mair, B. J., Willingham, C . B., and Streiff, A. J., J . Research Natl. Bur. Stanclards, 21,599 (1938). (32) Moles, E.,and Jimeno, E., Anales soc. espufi. fds. qudm., 11, 393 (1913). (33) Mulliken, 5. P.,and Wakeman, R. L., IND. ENG.CHEM.,ANAL. ED., 7,276 (1935);Rec. true. chim., 54,370-1 (1935). (34) Ormandy, W. R., Pond, T. W. M., and Davies, W. R., J . Znst. Petroleum Tech., 20,324,328 (1934). (35) Petrov, A. D , and Andreev, D. N., J . Gen. Chem. (U.S.S.R.), 12,95 (1942). (36) Rice, H. T., and Lieber, E., IND. ENG.CHEM.,ANAL.ED., 16, 109 (1944).
Vol. 36, No. 12
(37) Rothmund, V.,2. physik. Chem., 26,467,475(1898). (38) Schiessler, R. W., and co-workers, Petroleum Refiner, 22, 392 (1943);Proc. Am. Petroleum Inst., 24,111,50 (1943). (39) Seidell, A., “Solubilities of Organic Compounds”, 3rd ed,, Vol. 11,p. 27,New York, D.Van Nostrand Co., 1941. (40) Zbid., Vol. 11, pp. 46,47,53. (41) Zbid., Vol. H, pp. 66,92,121. 198,341,342,346,355,356,367, 389,516,551,584,667,700. (42) Zbid., Vol. 11, p. 121. (43)Zbid., Vol. 11, p. 365. (44)Zbid., Vol. 11,p. 394. (45) Shepherd, F. M.E., J . Inst. Petroleum Tech., 20,299 (1934). (46) Shtkkarev, A,, 2. physik. Chem., 71,94 (1910). (47) Sidgwick, N. V., and Neill, J. A., J . Chem. SOC.,123,2813 (1923). (48) Tilicheev, M,D., Khim. Tverdogo Topliva, 9,No. 2, 181 (1938). (49) Tilicheev, M. D., and Kuruindin, K. S., Neftyunoe Khoz.,’lS, 686 (1930); Chem. Zentr., 1931,I, 2561. (50) Timmermans, J., J . chim. phys., 20,506 (1923). (51) Timmermans, J., thesis, Brussels (1911); Proc. A d . Sci. Amsterdam, 13,507 (1910). (52) Timmermans, J., and Hennaut-Roland, Mme., Zbid., 27, 420 (1930). (53) Tropsch, H., and Simek, B. G., Mitt. Kohlenforsch. Inst. Prug., 1931,62. (54) VrevsW, M. S., Held, N. A., and Shchukarer, S. A., J . RUE. Phys. Chem. Soc., 59,625 (1927); Z.physik. Chem., 133,386 (1928). (55) Woodburn, H. M., Smith, K., and Tetewsky, H., IND.ENQ. CHEM.,36,588 (1944). (56) Wratschko, F.,Pharm. Presse, 34,143 (1929). PRESENTED before the Division of Petroleum Chemistry at the 108th MeetYork, N. Y.
ing of the AMERICAN Cxshrrcar. SOCIETY, New
OIL-PAPER DIELECTRICS Power Factors and Related Properties of Impregnated Cable and Filter Papers JOHN D. PIPER AND N . A. KERSTEIN The Detroit Edison Company, Detroit, Mich.
I
N SERVICE, oil-impregnated paper dielectrics often deteriorate with resultant increase in power factor and conductivity. 1n.this respect they are similar to oil dielectrics. As Clark (6) pointed out, however, the oils that are most resistant to deterioration in themselves do not necessarily produce the most resistant oil-paper dielectrics. The reason for the apparent difference in behavior is probably twofold. First, the paper acts as a catalyst that partially determines the types of degradation products formed; second, the same type of degradation product affects the power factor, conductivity, and other dielectric properties of oils differently from the corresponding properties of oil-impregnated papers. T o these must be added the third possibility that the type of degradation undergone by the paper depends on the type of degradation product (from oil) with which it is in contact. There is evidence, however, that the latter is unimportant a t service temperatures below 100” C. , It ispossible that both oil and paper may sometime be displaced by materials that are intrinsically more stable. However, continuous efforts are still being made by manufacturers of insulating oils, equipment, and cables, and by utility companies to select and improve insulating oils in order to obtain superior stability in both liquid dielectrics and oil-impregnated paper dielectrics. These efforts have been expended principally in various service and accelerated life tests on finished insulation, and in tests designed to subject experimental samples of oil or oil-impregnated paper to controlled deterioration treatments such as oxidation,
corona discharge, high temperature, catalysts, and combinationa of these. Significantly more stable dielectrics have resulted from such efforts; but reasonably accurate predictions based upon the degree to which one oil resists given treatments cannct be made concerning the degree to which a different oil will resist the same treatment or the degree to which the same oil will resist altered treatments. Results of such tests are often chaotic. It is probable that this condition will persist until the fundamentals of the changes that take place in dielectrics are better understood. ADDITIVE STUDIES
For a number of years the authors and their colleagues (7-18) have been endeavoring to obtain fundamental data for the limited field that concerns the physicochemical nature of substances that can cause significant increases in the power factors and conductivities of insulating oils and impregnated papers. The kinds of materials studied have been limited to those that could conceivably be formed by the deterioration of oil and paper in contact with materials with which they are used in service; the maximum concentrations have been limited to those that cauld be formed during severe service conditions. The method used consists simply of adding to an oil with a very low conductivity, various concentrations of highly purified materials of the types selected to’represent constituents or deterioration products of commercial insulating oils, and of determining
December, 1944
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 ~
Cable and filter papers, impregnated with liquid paraffin solutions containing a few t h o u s a n d t h s millimole of lauryl sulfonic acid per g r a m of oil, have alternating c u r r e n t conductivities that are m a n y t i m e s greater than the conductivities of the solutions. The acid is strongly sorbed by the paper. If a small fraction of the total sulfur c o m p o u n d s in most commercial impregnating oils should become oxidized in service to sulfonic acids, s o m e of the h i g h power factors that impregnated insulations often acquire in service could readily be explained. The technique of Balsbaugh, Assaf, and co-workers (1, 3) for differentiating between the types of degradation products that exert t h e i r influence t h r o u g h preferential sorption by the paper and those that exert their influence in the impregnant m a y become useful analytically if the technique is improved to include values a t very low frequencies.
the resulting changes in dielectric properties. When insulating paper is involved, the oil and the mixtures derived from it are used to impregnate the paper. These additive studies were designed only to show the effect of a given type of material while it exists as such, an$ not to show the effect such a material may have on the ultimate stability of a dielectric as the result of any chemical reactions it may enter or affect catalytically. The authors do not imply that all substances of the types that increase the conductivities and power factors of insulating oils and cable papers only insignificantly can be tolerated in these insulations. Selection of the additives has been based upon available information concerning the constituents and deterioration products of oil and the deterioration products of paper. Whenever possible, compounds of the types that have actually been isolated have been used. I n other cases additives have been selected to resemble certain properties of deterioration products whose structures are unknown. Up to the present the classes of additives have been: oil-soluble oxidation products in oil (If) and in impregnated paper (I!?); copper and lead soaps (7); sulfur and nitrogen compounds (8); and products resulting from subjecting oil to corona discharge (13) in oil. NATURE OF ADDITIVES CAUSING HIGH POWER FACTOR AND CONDUCTIVITY
All the additives thus far found to cause high power factors in either insulating oils or impregnated papers belong in one of two classes, termed A and B. Class A additives contain a t least one constituent that is incompletely soluble in the oil. This constituent is stabilized in suspension either by other constituents or by ita own soluble portion. I n the former case the suspensions are often so stable that they resemble true solutions. Most of the additives that cause high power factors in oils appear to be of class A. Most of these materials are essentially neutral in character or only weakly acidic. They include asphaltic materials, “soluble” sludge, cuprous compounds, eta. The class is far from completely described, however, for many colloidal systems in oil have low power factors and conductivities. A working hypothesis concerning class A additives has been described previously (7, IS). .The effect of class A additives on impregnated paper has been investigated in only a preliminary manner, but it is indicated that they are less significant in impregnated paper than in oil. Class B additives are the types of materials that disaociate electrolytically in water, notably acids. Most of those that are truly soluble in oil, however, cause only mild increases in conductivity, apparently because their degree of electrolytic dissociation is very low in media of low dielectric constant. In impregnated
110s
paper, however, those that have a strong polar group become sorbed on the paper (9)and cause high alternating-current conductivi ties. The present paper deals with an additive of class B, lauryl sulfonic acid. A sulfonic acid was selected for two reasons. First, commercial insulating oils usually contain traces of sulfur conipounds, and some of them, such as the mercaptans and the sulfides, may upon oxidation be converted to sulfonic acids. Second, Evans and Davenport (6) discovered a ‘‘strong acid” in addition to organic acids in a sample of insulating oil that had been subjected to ultraviolet radiation. Benzene sulfonic acid was tried first and found to be practically insoluble in insulating oil. Lauryl sulfonic acid, however, was adequately soluble in the base oil for experimental purposes. Its solutions had higher power factors for a given concentration of additive than any of the other homogeneous solutions of additiies that had been investigated (8). Nevertheless, the power factors were not sufficiently high to indicate that the formation of soluble sulfonic acids might by itself be a major cause of the increase in power factor of insulating oils during oxidation. From the information that had been obtained with carboxylic acid additives (8), it was expected that the lauryl sulfonic acid would be sorbed from the oil by the paper and that the effect of a given concentration of lauryl sulfonic acid would be greater in oil-impregnated paper than in oil alone. The principal object of the experimental work was to determine whether small concentrations of sulfonic acids, such as might be formed by the oxidation of sulfur compounds in the impregnant, could cause high power factors in impregnated insulation. A second object was to determine the differences in the dielectric properties of a dense kraft cable paper and a highly purified but,loose filter paper when impregnated with solutions of lauryl sulfonic acid in oil. A third object was to provide information concerning the dielectric behavior of the impregnated papers over a range of frequencies; with similar information from class A additives, it may subsequently be determined whether contaminants of class A can be differentiated from those of class B by measuring the dielectric properties of impregnated insulations over a range of frequencies. MATERIALS
The base oil was a liquid paraffin having a viscosity of 285 Saybolt Universal seconds at 40” C. and 50.5 seconds a t 100” C. It waa more fully described previously (11). The cable paper was highly calendered kraft cable paper of American manufacture made from Swedish wood pulp. It had a specific gravity of 0.968 and a Gurley air resistance (1 square inch orifice) of 243 seconds per mil. The average thickness was 5.4 mils. The filter paper wae Whatman 41H, the same paper used by Balsbaugh and coworkers (I, I) in studies on the deterioration of oil and oil-impregnated paper. The lauryl sulfonic acid was received from Stanford University through the cooperation of J. W. McBain. It was carefully dried and purified in the manner described previously (IO). That used for the dielectric investigation was water-white in both the crystalline and melted state. That used for the sorption experiments was the material recovered from the solvent used for crystallization. It was slightly pink. APPARATUS AND PROCEDURE
The variable-frequency bridge used for the electrical measurements was constructed essentially according to the design of Balsbaugh, Howell, and Dotson (4). It has a range of 20 to 20,000 cycles. The input to the bridge was furnished by a General Radio beat frequency oscillator, Type 713-B, with the voltage so set that the total potential across the sample was slightly less than 100 volts. The voltage stress was between 2 and 2.6 volts per mil. Three-terminal, platinum-glass cells (8) were used in determining the dielectric properties of the oil samples; they were eon-
1106
F
'K LI DETAIL OF GUARDED ELECTRODE ASSEMBLY
G
Vol. 36, No. 12
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
A. Pyrex diah B. Unwarded atainleassteel electrode ground into A C. Chrome1 wire contacting B through hole in A and welded to tungaten wire fuaed through D to form contact with bridge D. Glaas insulator with ground joint at cell cover E. Cell cover F. Suapenaion for A during aasemblyi A resta on G after aaaembly G. Cell case H. Paper aample (diameter same as A) I. G u a r d e d electrode made of Kovar J. Guard electrode made of Kovar K. Corning 7OSAO glaaa fund to I and J L,M. Teleaco ic tube. N. Silphon befiows 0. Rod connecting I to P within Q P. Electrical eontact from guarded electrode to Lrid e Q. Slidtng fitting, to raise and lower I and J and form grounded guard circuit with bridge R. Electrical insulation in Q and L S. Spring u a e d to a ply pressure to I and T., Adjustment .crew U. Gage to ahow electrode preasure I
I
.
.f
structed under the supervision of J. C. Balsbaugh. The cell for the impregnated-cable paper samples was designed so that the "oil-paper ratio" of the sample (weight of oil/weight of paper) was approximately 0.6, which is the average oil-paper ratio found in the insulation of solid-type high-voltage underground cables. (This does not include the weight of oil in the copper strands, fillers, etr.) This condition was desirable not only to reproduce service conditions, but also to eliminate excess oil from space beyond the outline of the paper sample. I n the cell previously used (11, 1 8 ) , which required a minimum oil-paper ratio of 2, excess oil, which was needed to prevent flashover a t the potentials used, was in contact with both ends of the cylindrical paper sample. The paper sorbed the sorbable additives not only from the oil in its immediate vicinity but also from that in contact with the ends. The sorption from the oil a t the ends was a slow process as compared with the sorption from the oil in close proximity; the result was that the concentrations of additive in the ends of the cylindrical sample were higher than that in the center, and they kept increasing over periods so long that true equilibrium could not be reached. Inasmuch as the ends were in the guard circuit of the cell, the dielectric effect was minimized; nevertheless, the need for a better cell was indicated. The cell and impregnating apparatus are shown in Figure 1. The essential features of the cell are: the ground-in electrode and the closely fitting paper sample which eliminated need for excess impregnant; the raisable guarded electrode which allowed the paper to be dried and impregnated while in a loose stack but which compressed it during electrical measurements; the trap at the base of the ground joint on the filling tube which prevented contamination of the impregnant by the special high-temperature lubricant used in the joint; and the displacement vessel (9, Figure 1) by which the desired volume of impregnant was caused to flow into the cell. Table I summarizes the data on the impregnated samples. The concentration of acid in the impregnant was determined by titrating the solution remaining in the impregnating apparatus. The sorption experiments were carried out essentially as was described previously (9). NOTATION
I n previous papers (7, 8, 1 1 , 12, IS), where all a.c. electrical measurements were made a t 60 cycles only, the values were reported in terms of the power factor. When measurements made a t several frequencies are to be compared, some unit that measures over-all effect rather than effect per cycle is preferable. Balsbaugh and co-workers ( 1 ) use the conductance factor d'f. The authors believe that for most chemists the specific parallel conductance in mhos per cm. has more physical significance. The following parameters were determined by the bridge measurements: Tan 6, or dissipation factor, where 6 is dissipation angle C, capacitance of electrode arran ement containing sample capacitance of same electrode arrangement with nitrogen as dielectric
tio,
V.
Cam to raiae I and J
W. Vacuum connection
X. Stainlems-ateel-lined 511ing tube; joint equipped with tra to catch exceaa lubrfcant Y. Lead a8ket 1. sampfe 2. Glaaa window S. Magnetically operated hammer 4. Connection to vacuum or dry nitrogen 5,6. Chambet. 7. Displacement vase1 to bring level of .ample in chamber 6 to outlet 8. Outlet tube 9. Dimplacement veasel to transfer l.% ml. to paper sample
Figure 1. Impregnation Equipment and Measuring Cell
The essential relationships are:
-- c/co
g4 = 0.556 (1O-la)e"f d tan 6
€('
e
where g1
-
specific conductance, mhos/cm. loss factor = dielectric constant f = frequency, cycles/ sec. Power factor A = cos 0 e = 90-8 e'' e'
=:
For values below 0.2, power factor 5~ tan 6.
December, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
1107
ELECTRICAL PROPERTIES
The effect at 80" C. of various concentrations of lauryl sulfonic acid on the 60cycle power factors of its solutions in liquid paraffin, and on cable paper and Whatman 41H filter paper impregnated with these solutions is shown in Figure 2. Curve A represents the power factors of the oil solutions. The values obtained were nearly identical with those previously determined (8) at 50 volts per mil. Curve B is for cable paper. In c-oncentrations below 0.07% the lauryl wlfonic acid produced only a mild inrrease in power factor. Beyond 0.07%, however, the effect was very marked. Curve C shows that the effect on the filter paper was even greater and that voncentrations as low as 0.02% produccd high power factors. The corresponding conductivity values art' givcn in Figure 3. Figures 4 and 5 show, respectively, thc variation in conductivity of the impregnttted cable and filter papers with change in frequency, temperature, and concentration of acid. The concentrations arc expressed in terms both of concentration of acid in impregnant and concentrations per unit weight of paper. (The means by which the latter were computed is described in the next paragraph.) At all temperatures and frequencies investigated, the lauryl sulfonic acid caused marked increases in conductivities. These increases were greater, the higher the frequency, but the percentage increase was higher a t the lower frequencies. The conductivities of the impregnating solutions were independent of frequenry.
- -
SORPTION
From previous experience with carboxylic acids (9) and from the limited solubility of lauryl sulfonic acid in liquid paraffin (8), it was expected that the lauryl sulfonic acid would be strongly sorbed from the oil by the paper. Figure 6 shows that this expectation was substantiated. The acid was strongly sorbed by both papers, and although the degree of sorption was greater for the filter paper than for the cable paper, the difference was insufficient t o account for the difference in the conductivities of the two papers when impregnated with solutions of the same concentration. I n working with papers as dissimilar as the dense cable paper and the porous filter paper, choice had t o be made between volume and weight relations. As stated previously, a n oil-paper ratio of 0.6 by weight was desired for the cable paper. At the time of making the selection, i t seemed more logical to choose the number of disks of filter paper required to occupy the same volume as the eight disks of cable paper, and t o use the same volume of impregnant rather than to maintain the same oil-paper ratio. Micrometer measurements indicated that seven disks of filter paper would occupy the same space as eight of cable paper. However, it was found that the capacitance of the cell with eight rings of filter paper in the outer half of the guard circuit was nearly tho same as that with eight rings of cable paper. Hence, except for one set of experiments, eight disks of paper were used and all samples were impregnated with the same volume of impregnant. T o determine the effect of a given concentration of acid per unit weight of paper, the relation between. the concentration in the original impregnant and the final concentration in the paper waa first determined from the oil-paper ratios and the sorption
Concentration in Oil, Per Cent by Weight
data. This relation is shown in Figure 7. The concentrations so found are used in Figure 8 to compare the 80' C., BO-cycle conductivities of cable and filter papers containing equal concentrations of acid sorbed per unit weight of paper. Even on this basis the acid exerts a stronger influence upon the highly purified but porous filter paper than upon the dense kraft paper. No effort has yet been made to determine whether the density or the kind of paper is primarily responsible for the difference. The small effect caused by the lower concentrations in cable paper (note the hflection a t the base of curve A, Figure 8) may possibly indicate that low concentrations in the rable paper are converted to insoluble salts, such as calcium lauryl sulfonate, by means of bases or base- acting salts in the paper. On the other hand, it may simply mean that most of the acid was sorbed by the top layers of the dense cable paper in spite of efforts to impregnate the loose pile of disks quickly and evenly, thus causing the dielectric to consist of a series of layers of progressively decreasing conductivity. Comparison between the effect of an equal concentration of acid in the two papers is further given in Figures 4 and 5 at all
TABLE I. DATAON IMPREGNATED SAMPLES Number of aper layers Conditions Buring drying Temp of oil bath * C. Time ht temp.. h;. Final preasure within oell, microns Hg Av. wt. of dry paper, grams Wt. of imprepnant, grams (1.96 ml.) Oil-paper ratio Temp. of impreqnation. C. Period between im regnation & meaaurement (amrox.). Rr. Approx. temp. to which samples were cooled O C. Pressure'of cell after impregnation Electrode. preasure, Ib. sq. in. Immediately after ekctrode WBI) lowered During electrical meesurementa Av. capacitance of empty' cell, rwf
Cable PaDer
Filter PaDer
100 2 to 3 20-100 2.77 1.72 0.62 100
100 2 to 3 20-100 1.73 or 1.51 1.72 1.00 or 1.13 100
16
16
30
30
6.6 4.5 10.97
6.5 4.5 10.82 or 12.16
Atm., filled with dry N
Determined by replacing pa er disks by e ual number of paper rings having widths approximately halfthat of guard &&rode.
1108
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 36, No. 12
1000
1000
I00
I.
IO0
WHATMAN 41H FlLT IMPREGNATED WITH S OF LAURYL SULFON
.O
000
0
00
IO
$
0
t ' 0
4E
8
.O
I.
0.1
100
10
I.o
Concentration of Acid NAL IMPREGNANT IN PAPER Wt. Millimoles/Cm. Millimoles/Gm
0.1
0 01
20
60
I I VIII~I
I
200 600 2000 Frequency, Cycles per Second
I I, 6000 /II/I/
I
0 01
20000
0.1
20
60
200 600 2000 Frequency, Cycles per Secmd
6000
20000
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
D e c e h r , 1944
1109
build up in oil-paper dielectrics t o account for some of the cmes in which high power factors develop in serviceaged insulation. 5.0
0.02
DIFFERENTIATION BETWEEN CLASS A AND B
E 5%
;
go km
-!
8 E2.0 3B 0.008 Q
il.0
$0.004
z
5 0.5
0.002
0.3
0.0012 0.0004
a o w a0012 1x02 0.002s
Mlllimoleo per Gramot Oil
0.I 0.2 0.3 0.5 0.7 Milligrams per Gromof Oil EOUlLlERlUM CONCENTRATION IN THE OIL
the temperatures and frequencies by the coincidence that the concentrations represented by cupve F of Figure 4 and E of Figure 6 are the same. This comparison shows that the greater effect of the acid in the filter paper over that of the same concentration in the cable paper persists at all the temperatures and frequencies investigated, SIGNIFICANCE
As stated previously, a n object of this investigation wm to acquire information t h a t may lead t o the development of a technique whereby electrical measurements may be used t o differentiate between class A and class B degradation products in impregnated paper. An approach t o such technique was described by Balsbaugh, Assaf, and co-workers ( 1 , 3) who plot the conductivity (or values t h a t are proportional t o it) against the frequency and extrapolate the eonductivities t o zero frequency. The conductivity represented by the intercept is supposed t o represent the conduction loss, whereas any additional conductivity represents dielectric absorption. Balsbaugh, Assaf, et al. used this technique t o interpret the natures of degradation products of different oils that were oxidized in the presence of copper and paper; some of the latter was part of the dielectric in one of the measurement cells. According t o their interpretation, materials that are selectively sorbed by the paper increase the dielectric absorption loss of the paper but decrease the conduction loss of the impregnant; and the conduction of impregnated paper at zero frequency is simply the conduction of the impregnant which contains whatever concentration of conducting matter is not removed by the Paper. Through this technique they interpreted their data t o mean that, for some of the impregnated samples, the “impregnated Paper is merely exhibiting the large conduction 105s of the impregnating oil”, whereas for others “the paper has selectively absorbed some oxidation products which produce absorption and possibly conduction in addition t o t h a t contributed by the impregnating oil”. The authors believe that in the first of these cases the degradation products were primarily of class A whereas in the second they were Of *’ Inasmuch both kinds were probably formed in both cases although in different proportions, i t seemed desirable t o test the technique on impregnated Papers in which class A and class B materials are present separately. The data obtained in this study afford a n opportunity t o apply the technique t o papers containing a highly purified material of class B. Figure 9 shows representative data from Figures 4 and 5 plotted on linear scales instead of log-log $tales. For the lower conductivity values at the bottom, the conductivity is nearly proportional to the frequency and the intercept represents a very low
Inasmuch as commercial insulating oils usually contain traces of sulfur, it is instructive to speculate concerning the effect these compounds could have on the power factors of impregnated insat.on they become oxidized to sulfonic acids. F~~example, of eight such oils discussed in the previous work (81, the minimum Concentration was 0.09% of sulfur, which corresponds to 0.028 millimole of sulfur compound per gram of oil. If all of that sulfur were oxidized t o sulfonic or other strong sulfur acids, the resulting concentration would be higher than any used in this investigation, and paper impregnated with the oil would have very high power factors and conductivities. Just what proportion of the sulfur compoundsin a commercial insulating oil would oxidize in service t o sulfonic or other strong acids is a matter of conjecture. Evans and Davenport (6) found 0.00025 millimole per gram of a “strong acid” in a 5 0.02 sample of highly purified, oil-filled cable type of oil t h a t had been subjected 0.016 through Pyrex t o ultraviolet 8, B radiation in air for 30 hours at room temperature. The concentration formed by this mild treatment, on a n oil t h a t probably had a 2 Eo.005 very low sulfur content, is less than t h a t a t which highly purified lauryl sulfonic acid 0,004 caused marked increases in the power factors and conductivities of impregnated 0. I 0.2 0.3 0.4 Per Cent by Weight paper; however, it seems CONCENTRATION IN ORIGINAL OIL probable t h a t sufficient concentrations of such acids may
%OO
B
g
14 8
3
f 0
E
10
E I
0 *
.
:
I: 0
:1.0
s
I 0.5
0.6
I
0.094 0.008 0.012 0.016 Mill i mole8 per Gram of Paper I 2 3 4 Mil liaram Acid per Gram of Pwer CONCENTRATION OF ACID
INDUSTRIAL AND ENGINEERING CHEMISTRY
1110
Vol. 36, No. 12 h a v e a c o n d u c t i v i t y of 10-12 mho per cm., a value indistinguishable from zero on a plot having the scale of the top part of Figure 9. Although the data support the hypothesis, they indicate t h a t the means of finding the zero interc e p t will h a v e t o be improved before this technique c a n be used t o e s t i m a t e t h e r e l a t i v e significance of c l a s s A a n d class B d e g r a d a t i o n products in a particular sample. Possibly t h e t e c h n i q u e may be useful if values are obtained a t considerably lower f r e q u e n c i e s o r if c o m b i n e d w i t h directcurrent conductivity measurements. Investigation of the technique is contemplated for systems containing class A materials. 0.2 X
ACKNOWLEDGMENT
200
400
600
BOO 1000 1200 Frequency, Cycles per Second
1400
1600
1800
2C
Figure 9. Application to Lauryl Sulfonic Acid, Liquid Paraffin, Paper Systems of the “Intercept” Method of Balsbaugh, Assaf et al. (I, 3) for Characterizing the Nature f Degradation Products in Impregnated Paper
conductivity; this is in agreement with the similar curves of Balsbaugh, Assaf, et aE. for unoxidized or mildly oxidized samples consisting of the same kind of filter paper but a different impregnant. For the intermediate and higher conductivity values, however, the relation between conductivity and frequency is far from linear, as shown at the middle and top of Figure 9. The intercept values probably approach zero conductivity in accordance with the Balsbaugh-Assaf hypothesis that the conductivity of impregnated paper a t zero frequency is a function of the concentration of ions left in the impregnant. For example, consider the conductivity of the impregnant for the sample represented by the top curve of Figure 9. The concentration of acid in the paper was 0.014 millimole per gram of paper. Figure 6 shows that 0.014 millimole per gram in the paper is in equilibrium with 0.00155 millimole in the impregnant; according to Figure 3, this would
The authors are pleased to credit their colleagues, H. A. Bolts and C. D. Robb, with the construction of the measurement bridge, a n d t h e former with the c o n s t r u c t i o n of t h e guarded electrode of the cell used for the impregnated s a m p l e s . Portia Treend made part of the electrical m e a s u r e m e n t s a n d computations. The work was made possible by the interest and support of Henry S. Walker, director of research. LITERATURE CITED
(1) Balsbaugh, J. C . , Assaf, A. G . , and Pendleton, W . W . , I N D .E N Q . CHEM.,33,1321(1941). (2) Balsbaugh, J. C.,and Howell, A. H., Rev. Sci. Instruments, 10, 194 (1939). (3) Balsbaugh, J. C.,Howell, A. H., and Assaf, A. G., Ibid., 32,1497 (1940). (4) Balsbaugh, J. C . , Howell, A. H., and Dotson, J. V., Trans.Am. Inst. Elec. Enma.,59, 590 (1940). (5) Clark, I?. M., IND.ENQ.CHEM.,31,327 (1939). ( 6 ) Evans, R. N., and Davenport, J. E., IND. ENG.CHEM.,ANAL. ED.,9, 321 (1937). (7) Piper, J. D., Fleiger, A. G., Smith, C . C . , and Kerstein, N. A., IND.ENG.CHEM.,31,307 (1939). (8) Ibid., 34,1505 (1942). (9) Piper, Kerstein, and Fleiger, Ibid., 29, 1040 (1937). (10) Kerstein, and Fleiger, IND. ENQ.C H E M .ANAL. , ED., 14, . Piper, 738 (1942). (11) Piper, J. D.,Thomas, D. E. F., and Smith, C. C., IND.ENG. CHEM.,28, 309 (1936). (12) Ibid., 28, 843 (1936). (13) Sticher, Joseph, and Piper, J. D., Ibid., 33,1567 (1941).
