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To determine the significance of fluorescence and similar phenomena upon the optical measurements, solutions of different concentrations of Shell sample 100 in Nujol were made, and their optical densities (log 1/T) were measured on the Hardy spectrophotometer. The results showed that the oils obeyed Beer’s law. Further work in which the transmission curves are being expressed by an equation is being carried on to provide constants that are a function of the color of the oils. I n this manner it is hoped that a more definite correlation between the color and other properties of the oils may be obtained.
Acknowledgment The data presented in this paper have been obtained in a long-term program of research on electrical insulating oil deterioration which until September 1, 1939, had been carried on in cooperation with the Committee on Insulating Oils and Saturants, Utilities Co-ordinated Research Inc., Association of Edison Illuminating Companies. Since that date it has been carried on in cooperation with a committee sponsored by the Engineering Foundation and the American Institute of Electrical Engineers. We wish to express our appreciation to the Gulf Research & Development Corporation and to the Shell Petroleum Corporation for making available the oil samples. We also wish
VOL. 32, NO. 11
to acknowledge the cooperation of our colleagues, W. C. Hollibaugh and It7.W. Pendleton.
Literature Cited (1) (2) (3) (4) (5)
Am. Soc. Testing Materials, Standards, D92-33. Ibid., D129-34. Ibid., D155-34T. Ibid., D445-37T. Assaf and Gladding, IND.EXG. CHEM., Anal. Ed., 11, 164 (1939). (6) Assaf and Hollibaugh, data to be published. (7) Balsbaugh and Howell, Rev. Sei. Instruments, 10,194 (1939). (8) Balsbaugh and Oncley, IND. EXG.CHEM.,31,318 (1939). (9) Clark, Ibid., 31,327 (1939). (10) Dornte, Ibid., 28, 26; Dornte and Ferguson, Ibid., 28, 883; Dornte, Ferguson, and Haskins, Ibid., 28, 1342 (1936). (11) Fuchs, von, and Anderson, Ibid., 29,319 (1937). (12) Hardy, J. Optical SOC.Am., 18,96 (1929); 25,305 (1935). (13) Harrison, Proc. 6th Conf.Spectroscopy, 1938,91 (1939). (14) Hersh, Fisher, and Fenske, IND.ENG.CHEM.,27, 1441 (1935). (15) Hill and Coates, Ibid., 20, 641 (1928). (16) Keith and Roess, Ibid., 29,460 (1937). (17) Larsen, Ibid., Anal. Ed., 10,195 (1938). (18) McCluer and Fenske, ISD. ENG.CHEM.,27,82 (1935). (19) Rossini, Refiner Natural Gasoline M f r . , 16,545 (1937). (20) Weiland, Natl. Petroleum News. 31,R334, 36 (1939). PHEBENTED as part of t h e Symposium on Electrical Insulation before t h e Division of Industrial a n d Engineering Chemistry a t t h e 99th Meeting of t h e American Chemical Society, Cincinnati, Ohin.
LIQUID DIELECTRICS Electrical and Physical Properties of Systems Containing Sparingly Soluble Oxidation Products in Liquid Paraffin’ JOHN D. PIPER, C. C. SMITH, N. A. KERSTEIN, AND A. G. FLEIGER The Detroit Edison Company, Detroit, Mich.
HE work to be described is part of an investigation de-
T
signed to determine which of the types of products that may be formed by the service degradation of insulating oils cause serious dielectric losses in such oils a t 60 cycles and which do not. Although many investigators have studied the over-all effect on the power factor and conductivity of an insulating oil caused by a given kind of deterioration, such as oxidation, little information is available concerning the relation between the types of compounds formed by the deterioration and the changes in power factor and conductivity observed. Acid numbers, saponification numbers, hydrophile numbers, and Grignard numbers all give some measure of the degree of oxidation of a mineral oil, and apparent correlations between any of these numbers and the power factor or conductivity values of given insulating oils have been observed. Such studies, valuable as they may be, do not show which of the varieties of products that are simultaneously formed during oxidation actually cause the observed increases in power factor and conductivity. One method for obtaining the types of oxidation products to be used for investigating their effect on the power factor 1
Three papers in this serips have already been published (8.10. 1 1 ) .
and conductivity of an insulating oil is to oxidize the insulae ing oil and isolate all of the types of oxidation products formed. This method has the disadvantage that it is difficult or impossible to isolate, from oxidized oil, oxygen-containing compounds of known structure in sufficient purity for an investigation on dielectric properties. The method adopted consisted of the following steps: The types of products that may be formed by oxidation of insulab ing oils were deduced from available information. Compounds representing these types were prepared in a high degree of purity. The compounds were then added to an oil having a low power factor and conductivity, and the resulting changes in power factor, dielectric constant, and d. c. conductivity were determined. Use of this method, as of the one previously mentioned, is limited by the scarcity of available information concerning the chemical and physicochemical structures of products formed by the oxidation of hydrocarbons, and also by the availability of compounds of the type desired. The compounds selected for study are not intended to represent the authors’ interpretation of the chemical compounds that are actually formed by the oxidation of hydrocarbons, but rather
NOVEMBER, 1940
INDUSTRIAL AKD ENGINEERING CHEMISTRY
1511
liquid paraffin in such concentrations that a given c o p ound but dissolved completely at a certain temperature up to 100 separated to form a visibly heterogeneous system at some lower temperature. The temperatures at which the systems became heterogeneous were determined, except in the case of a few of the earlier experiments in which the temperatures were roughly estimated, by cooling the systems, contained in glass tubes suspended in a liquid-paraffin bath, a t the rate of 0.5"t o 1' C. per minute. In some cases the transition was also observed microscopically under polarized light and recorded photographically. I n these instances the microscope and sample were contained in a thermostatically controlled air bath during the observations. It was not possible t o observe the systems while the electrical measurements were being made. The techniques involved in making the electrical measurements and the equipments used were essentially the same a8 those described previously (IO), except that the plastic insulators in the electrical-measurements cell were replaced durin the course of the investigation by quartz insulators of slightly iifferent design. The reason for this change will be discussed later. Power factor measurements were made at selected temperatures, usually as the systems were being cooled, at 60 cycles and 1970 volts per mm. (50 volts per mil). Dielectric constant values were computed from the data obtained in determining the power factor. Direct-current conductivity measurements were made a t 98.5 volts per mm. (2.5 volts per mil), the readings being taken one minute after the application of voltage. These determinations were made a t each temperature immediately after the power factor values had been determined. The systems studied fell naturally into two classes, depending upon whether the suspended phase which formed was liquid or solid.
8.
Various compounds, selected to represent types of sparingly soluble oxidation products such as may be formed by the service degradation of insulating oils, have been added to liquid paraffin in such concentrations that the systems appeared homogeneous at the higher temperatures employed and beterogeneous at the lower temperatures. The 60-cycle power factor and dielectric constant, and the direct-current conductivity of the systems were measured a t several temperatures as the systems were cooled. At temperatures well above the transition temperatures, all the systems had low power factors and conductivities. A t temperatures at which the systems appeared to be heterogeneous, those in which the suspended phase consisted of droplets of formic acid, moist acetic acid, ethyl alcohol, and phenol had power factors that were much higher than could be accounted for by Wagner's model of a heterogeneous dielectric. The power factor and conductivity of systems i n which the suspended phase consisted of crystalline particles-namely, the systems containing palmitic acid, cr-hydroxyisobutyric acid, 10-hydroxystearic acid, cerotic acid, carnauba wax, laurone, and c h o l e s t e r o l d i d not increase as the systems became heterogeneous. The power factor of systems containing cetyl alcohol, ceryl alcohol, and octadecyl alcohol increased markedly, however, as the systems became heterogeneous. The similarity between the dielectric behavior of these systems and the dielectric behavior of cetyl alcohol observed by Smyth and Baker ( 1 4 ) is discussed.
to represent certain of the types of chemical and physicochemical structures that are probably formed. The first papers of this series (IO,11) showed that the power factor and the d. c. conductivity of liquid paraffin was only slightly changed by the presence of moderately large proportions of many compounds, selected to represent acids, aldehydes, ketones, esters, and peroxides, as long as those compounds remained in homogeneous solution. A few of the systems described in those studies became visibly heterogeneous on cooling. One of the systems in which a gel formed, and others in which emulsions formed, developed high power factors and conductivities a t the transition temperatures. Other systems, in which the phase that developed was composed of crystalline particles, did not. The present paper deals with an extension of the investigation on systems that became visibly heterogeneous on cooling. The investigation was limited to systems in which the added components had molecular weights no greater than that of insulating oil. Sludgelike compounds were not included, principally because the chemical structure of sludge is not sufficiently established to warrant attempts to prepare compounds simulating it.
Preparation and Treatment of Systems The compounds used for preparing the systems are listed in Table I, together with information concerning the methods by which they were purified and their solubility in liquid paraffin. Except for the castor oil and carnauba wax, the original compounds were the best quality obtainable from the Eastman Kodak Company. The liquid paraffin was described previously (IO). The systems were prepared by adding the compounds t o
Systems in Which Suspended Phase Was Liquid Systems in which the suspended phase, which forms as the systems are cooled, is composed of liquid droplets may be subdivided into several classes, depending upon the nature of the droplets formed. In this work two such classes were investigated, one in which the droplets were composed of a mobile, strongly polar liquid and the other in which the droplets were composed of a viscous, weakly polar liquid. Figure 1 shows curves representing the results of tests made on five systems in which the suspended phases which formed as the systems were cooled were composed of droplets of mobile, strongly polar liquids. The temperatures a t which each system appeared homogeneous are to the right or shaft side of the half vane shown with each set of curves; the tempera-
TABLEI. COMPOUNDS USEDTO REPRESENTSPARINQLY SOLUBLE OXIDATION PRODUCTS Soly. i n Liquid Paraffin, % by Wt. Greater Less than than 0.01 0.04 4.4 ... 1.3 2.5 ... 8
Method of Temp., c. Compound PuriEoationo Formic acid 40 Acetic acidb 20 30 E t h y l alcohol A (111 40 Castor oil B, C 30 7.8 ,.. Methyl n-amyl ketoneb A (10) Phenol D, E 20-30 1.7 , 3.9 Palmitic acid F , G. E 30 6 Hydroxyisobutyric acid H 40 0.1 10-Hydroxystearic acid A (8) 40 . 0.2 Cerotic acid R. G so 1 .o 5 .. 35 4.9 Laurone F; I, E Cholesterol R. I 20 2 Carnauba wax B; c 60 ... 1.8 Cetyl alcohol A (10) 30 3.8 7.5 Ceryl alcohol F, G 45 1.8 F. I, E 35 ... 3.5 Octadecyl alcohol 0 Explanation of symbols: A. Previously described (see references indicated). B. Untreated. C . Light yellow-green. D. Dried over P201, fractionated under reduced pressure, middle portion collected. E . Crystallized three times from mother liquor in closed system (8) F. Dried by heating below boiling point under reduced pressure. G. Distilled in all-glass system. H. Dried and purified by repeated sublimation under reduced pressurr. I. Distilled in molecular still. b Used only in connection with a less soluble component in this study.
::!E)
.. . ... .. .... ..
...
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t u r e a t which the systems appeared heterogeneous are to the left or feathered side of t h e half v a n e . Curves not accompanied b y vanes refer to systems which appeared homogeneous a t all the temperatures investigated. I n the case of each of the systems which became visibly heterogeneous on cooling (Figure 1, A , C, and D), the power factor was low when the system appeared homogeneous a n d high when the system appeared heterogeneous. As the temperatures of some of the systems were being l o w e r e d , the power factors began to rise a t tempera tu r es l ! l higher than those 1 a 4 so i o a t which a n y T€URZRAN~M6MESC! visible evidence of heterogeneity FIGURE 1. CHASGES WITH TEMPERATURE 11; DIELECTRIC PROPERTIES OF could b e obSYSTEMS C O N T A I N I N G STRONGLY POL.4R served. This is LIQUIDS illustrated in * Transition temperature estimated Figure 1, B and E, where t h e power factor values of both systems are shown to have increased as the temperature was lowered, although neither system became visibly heterogeneous a t the lowest temperatures shown. The d. c. conductivity curves had the same general shape as the power factor curves for the system containing formic acid (Figure 1A) and for the system containing moist acetic acid (Figure IC) although the d. c. conductivity values were lower than the corresponding a. c. conductivity values. (The a p proximate ratios between the d. c. values and the calculated a. c. values are shown in Figure 1 and other figures by the ratio between the distance at which the d . c. conductivity values and the respective power factor values are plotted above the axes of abscissas.) I n the case of the system containing ethyl alcohol (Figure 1D)the curves were of different shape, the conductivity curve not being upturned a t temperatures below the transition point. The changes in the dielectric constants of the several systems with temperature changes were only slightly different from those of liquid paraffin itself. Several other systems from which droplets of mobile, strongly polar liquids separate as the systems are cooled have also been investigated. They include systems having different proportions of the constituents shown in Figure 1, liquid
b
io
VOL. 32, NO. 11
paraffin containing ethylene glycol monoethyl ether (chosen only because of its solubility behavior), and a commercial insulating oil containing moist propionic acid. The power factors of these systems increased greatly as they became visibly heterogeneous. Inasmuch as deterioration products of oil may separate from the oil as droplets that are not necessarily mobile and highly polar-for instance, droplets of an inert condensation product-it seemed desirable to investigate a system from which viscous, nonpolar, or weakly polar droplets separate. To form such a system with liquid paraffin, castor oil was selected because i t fulfilled the physical requirements. It is recognized that the probability of any glyceride being formed by the deterioration of a mineral oil is remote. The temperature a t which the castor oil separated from the liquid paraffin could not be determined in the usual manner. The systems remained clear during cooling, but the castor oil separated as a layer a t the bottom of the containing vessel. Measurement revealed that the index of refraction of the castor oil and the liquid paraffin differed by only 0.0005. Inasmuch as the light scattered from the castor oil was blue, whereas what little was scattered from liquid paraffin was white, i t was found that when a system containing 8.0 per cent castor oil was cooled to 40" C , it was possible to perceive globules by examining the mixture, illuminated through a cardioid condenser, with a low-power microscope. (Percentages are expressed on a weight basis.) The globules appeared as dull blue circles against a gray field. Owing to lack of contrast between the phases, attempts to photograph the appearances of the emulsions were unsuccessful. When the mixture was heated to about 45" C. the globules disappeared. Figure 2A shows the results of dielectric measurements made during the cooling of such a system. There was no increase in the power factor a t temperatures near the transition. The slope of the dielectric constant curve between 80" and 60°C. was the same as for liquid paraffin, but from 60" to
k
FIGURE2 . DIELECTRIC PROPERTIES DURING COOLINGOF Two SYSTEMS CONTAINING CASTOR OIL 8.0 per cent castor pi1 8.4 per cent castor oil, 6.3 per cent methyl n-amyl ketone Transition temperature estimated
A. B.
*
40' C. the slope was slightly steeper.
Below 40" C. the slope diminished, owing probably to settling of the emulsion below the active electrodes of the cell. Because the castor oil separated from the liquid paraffin so readily, several attempts were made to obtain more stable emulsions by adding a third component. I n no case was the transition from a homogeneous to a heterogeneous system
NOVEMBER. 1940
INDUSTRIAL AND ENGIIVEERING CHEMISTRY
FIGURE 3. DIELECTRIC PROPERTIES OF A SusTEM CONTAINING 5.11 PERCEXTPHENOL A.
A t various temperatures during cooling
B. Upon being cooled rapidly from 70” C. t o 30’ C . , t h e n cooled further t o 25’ C.
accompanied by an increase in power factor except when the third component was a mobile, highly polar liquid. The increase in power factor of such a system a t the transition point is shown in Figure 2B. The third component of the system there represented was methyl n-amyl ketone. It was shown previously (10) that when this compound is in solution in liquid paraffin, it causes only a small increase in the power factor of the latter.
Systems in Which Suspended Phase Was Either Liquid or Solid Systems containing phenol in liquid paraffin were examined, both because such systems may become visibly heterogeneous on cooling and because they offered an opportunity to investigate an apparent contradiction between some of the conclusions of Sommerman (15) and those of Piper, Thomas, and Smith (10, 11).
From the results of experiments made on systems prepared by adding phenol or stearic acid to a mixture of paraffins, Sommerman in 1935 (15) concluded that “although great increases in the final coizductivities are brought about on adding organic acids, the true short time conductivities increase little, if at all”. The italicized clause, in so far as it relates to homogeneous solutions, is in opposition to our observation made in 1936 (10, 11) that organic acids, as well as many other chemical types of oxygen-containing compounds, have only a
1513
small influence on the power factor and conductivity of a n insulating oil as long as such compounds are in true solution in the oil. Some of the properties of Sommerman’s systems are contrasted with similar properties of the systems prepared by us in Table 11. At the temperature a t which the comparison is made, all the systems were visibly homogeneous. Table I1 shows that the power factor and conductivity values for Sommerman’s paraffi as well as those for each of the systems prepared from i t were much higher than the corresponding values for our liquid paraffin and the systems prepared from it. Only part of this difference can be accounted for by the fact that the velocity of ion migration is inversely proportional to the viscosity of the medium. The last column shows the product of the viscosity and the conductivity. It is evident that in Sommerman’s tests the addition of either stearic acid or phenol increased not only the power factor and the conductivity of liquid paraffin but also the product of the conductivity and the viscosity more than it did in our tests. The tests on the systems containing phenol lend further support for our belief that oxidation products of oils, when in true solution in insulating oils, do not cause pronounced increases in the power factor or the conductivity of such oils. Apparently, these oxidation products do not dissociate electrolytically to a marked degree in a continuous oil medium. It is believed that the increase in the power factor and conductivity of Sommerman’s paraffin when either stearic acid or phenol was added, may have been due to the mutual action of an impurity, which Sommerman knew to be in one of his paraffin constituents, and the added constituents. The nature of this material and the mutual influence exerted by it and the added constituents to produce ions, is as yet a matter for conjecture. That a small proportion of certain impurities may markedly affect the dielectric properties of a system was found when a system containing 4.46 per cent phenol was examined in the electrical-measurements cell which had been used up to that time. The power factor and conductivity values of this system were considerably higher than those of systems containing similar concentrations of the other compounds investigated (10, 11). These values are shown on the last line of Table 11. When the cell was taken apart, i t was found that the plastic insulators had been visibly a,ttacked and that the phenol which separated from the paraffin was colored. For this reason all the plastic insulators were replaced by quartz.
TABLE 11. EFFECT OF ORGANIC ACIDSO N POWER FACTOR AND CONDUCTIVITY OF PARAFFINS AT 70” C. Powei Facto] Sample (Cycles) Liquid paraffin 0.0002 (GO) Sommerman’s paraffin 0.0105b (65) 10% stearic acid in: 0.0006 (GO) Liquid paraffin Sommerman‘s 0 . 0 1 7 b (65) paraffin 5.117 phenol in liqzid paraffin 0.001 (60) 3% phenol in Sommerman’s paraffin
0.035 (65)
Conductivity, X, Mhos/Cm Cube X 1014 1 min.0 Final0
Initial
...
Viscosity, 9 , Poises
7X
0.22
0.04
0.05d
0.04
.. .
0.2
79C
...
. ..
1.5
...
‘
130C
...
36C
e
.. .
3.1
...
265C
...
0.W
170c
4 46Y0 phenol in
bppror. 0.03
1.8 0.7
e
8.5
liquid paraffin plastic insu4.4 lators in cell 0 . 0 0 7 (60) , 20 a Used for calculating t h e values of b Figure 7 of citation 15. c Table I11 of citation 15. d Figure 8 of citation 16. * T h e values for the parent paraffins were used in calculating t h e results shown in t h e last column.
.. +..
...
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Plienol-contaiinig systems uncontaminated by material extracted from the plastic had low power factor and Conductivity values, as shown by the data in Table 11for the Tystem containing 5.11 per cent phenol. The dielectric behavior of phenol-containing systems as they became heterogeneous upon cooling will now be considered. Several such systems have been investigated. For example, power factor, dielectric constant, and d. c. conductivity values of a system containing 5.11 per cent phenol in liquid paraffin are plotted against temperature in Figure 3A. At temperatures above 38" C. the power factor and d. e. conductivity were low, Belor this temperature the system hecame heterogeneous, whereupon the power factor rose to very high values. The dielectric constant also rose. The d. c. conductivity, however, did not increase until the temperature bad been lowered to about 25" C. The power factor a t this temperature WRS much lower than a t 30' C., and it decreased progresaively as the temperature was further lowered. The dielectric constant values also dropped at temperatures helow 30" C. The rise and fall in the power f x t o r and dielectric constant values was a function of time as well as of temperature. Neither function was regular. Figure 3 8 shows the change in dielectric properties with time after the same system was reheated to 70" C. and quickly cooled to 30" C. The power factor values a t 30"C. increased from 0.15 to > 0.37 in 40 minutes; the d. e. conductivity remained a t the lower limit of the measuring apparatus-namely, 0.3 X lo-" mho per cm. cube. The temperature WBS then lowered to 25' C. In 10 minutes the power factor decreased to 0.044 and the conductivity increased to 6 X 10-" mho per cm. citbe.
VOI.. 32, NO. 11
Inasmuch as the para& used in our experiments was a clear liquid, it was possible to examine the systems optically while they were becoming heterogeneous. Figure 4 shows the appearance of the system containing 5.11 per cent phenol when viewed with polarized light; A was taken shortly after the system first became heterogeneous. The phase which a p peared was composed of droplets, as shown. B was taken after crystals began to appear. These crystals always started from the oily phase. Never, however, in any of the cases studied, did the crystals appear before the droplets. As the crystals grew, globules in the vicinity of the crrjtals gradually dissolved. The crystals seemed never to enter a globule.
FIGURE5.
VARIATIONS
WITH
TEM-
T E R ~ A LIN SUSPENSION
A
B
FIGURE 4. MICROW~PIC APPEARANCEOF A SusTEM CONTAINING 5.11 PER CENT PHENOL ( X 85) A.
8
Shortly sfter the system beosme heterogeneous Later
A similar rise and fall in power factor and dielectric constant values was observed by Sommerman (26) when be cooled his system containing 3 per cent phenol in mixed paraffis to 20" C. Sommerman suggested the following mechanism for the phenomenon: "Upon cooling, mme small phenol crystals, having large electric moment, form. The orientation of these polar aggregates increases the dielectric constant at low frequencies, hut since the dipoles are of larger than molecular size, they are restricted from orientation a t a relatively low frequency and give rise to a power factor maximum there. As the crystals continue to grow, they become so large that they cannot rotate at all, and the whole effect disappears."
From the results of experiments made before and after adding a seed crystal to the heterogeneous systems, both in the electrical-measurements cell and on microscope slides, it is believed that the high power factors and dielectric constants result when the systems exist aa emulsions. The lowering of the power factor and dielectric constant is due to the gradual disappearance of the emulsion as the phenol dissolves in the oil to replace that being removed by crystal growth. These crystals, which remain suspended in the oil, are long enough to bridge the electrodes of the measurements cell. The increased conductivity of the mass of crystals over the condnctivity of the emulsion may thus he due to the formation of conducting bridges.
Systems in Which Suspended Phase Was Solid Numerous systems which, upon cooling, formed a new phase composed of solid particles were examined. In most of these cases there was no increase in the power factor of the systems near the temperatures a t which they became heterogeneous. Figure 5 shows the change in power factor with change in temperature for several such systems. The system containing 1.85 per cent carnauba wax (Figure 5A) became cloudy at 68" C. Between 68" and 41" C., in which region the system appeared to be composed of masses of solid, probably crystalline particles (3) suspended in oil,
INDUSTRIAL AND ENGINEERING CHEMISTRY
NOVE3IBER. 1940
the power factor was low. Below 41" C. the system was too rigid to pour. The power factor values increased as the temperature was decreased below the pour point. This region was not more completely examined because the present investigation is concerned with the behavior of liquid rather than of solid dielectrics. Similarly, the system containing cerotic acid (Figure 5B) became cloudy a t 74" C., but there was no increase in the power factor until the system became solid below 60" C. Both a-hydroxyisobutyric acid and 10-hydroxystearic acid were sparingly soluble in liquid paraffin even a t 100"C. The small amounts of each which did dissolve remained in solution until the temperature was lowered to about 40" C., whereupon each separated in crystalline form. Figure 5, C and D,shows that there is no inflection in the curve representing the power factor values near the transition temperature. Systems containing laurone (Figure 523) palmitic acid (F), and cholesterol (G) were examined and photographed in polarized light as each became heterogeneous on cooling. I n each case the phase which appeared was definitely crystalline. The system containing cholesterol became supersaturated in the cell upon cooling. Later i t crystallized to a mass that could not be poured. The power factor of this mass was higher than that of the solution but was still low. Upon heating the system, the power factor values rose to a low maximum at 30" C. and then fell as the crystals melted. As the temperature was further increased, the power factor again increased in the usual manner, owing probably to the increased conductivity resulting from reduction of viscosity with temperature increase. Palmitic acid was selected for study be,
_ ,
,
I , I
, 2.260
0
,
1
I
5 f0 IS 20 2 TIM€, MINUTES
FIGURE 6.
VARI.4TION
IN
DI-
CONSTANTOF Two SYSTEMS AS THEY BECAMEHETEROGENEOUS AT CONSTAYT TEMELECTRIC
PERATURE
cause Piekara (7) had observed that the freezing of palmitic acid or its separation from hexane solution was accompanied by a sudden increase in the dielectric constant a t wave lengths of 200 to 3000 meters. This was followed by the normal fall. A system containing 6.26 per cent palmitic acid was prepared to determine whether a similar rise in dielectric constant a t 60 cycles would occur when the system became heterogeneous and whether this increase would be accompanied by an increase in the power factor. Figure 5 F shows that there was no increase in the power factor as the system became heterogeneous. The dielectric constant did increase, however, as
1515
FIGURE 7. CH.4NGE I N DIELECTRIC PROPERTIES A S A SYSTEM COXTAINING CETYLALCOHOLBECAME HETEROGENEOUS
shown in Figure 6A, the data for which were obtained as the system became heterogeneous a t 30" C. As illustrated in Figure 6B for the system containing 4.88 per cent laurone, the dielectric constant of the systems usually dropped as a component of the systems crystallized. The initial increase in this case was probably caused by the cell not being as cool as the bath when the initial measurement was taken. For both sets of measurements represented in Figure 6, the systems were cooled rapidly to the desired temperature. Initial measurements were made without waiting for temperature equilibrium to be attained because the transition progressed rapidly. The time required for the systems to become heterogeneous was roughly estimated by making observations through the quartz top of the cell. Like the other systems in this group, the system containing laurone had a low power factor a t all temperatures near the transition point (Figure 5E). The dielectric behavior of three systems containing certain of the higher alcohols was markedly different from the behavior of the systems discussed in connection with Figure 5. Two of the previous papers (10, 1 1 ) showed that the power factor of a system composed of liquid paraffin and cetyl alcohol became very high when the system became heterogeneous and that the system formed a gel-like structure. The change in the power factor and the dielectric constant of such a system as it becomes heterogeneous has been further investigated. The left-hand curve in Figure 7 shows the change in power factor of a system containing 8.52 per cent cetyl alcohol as the temperature was lowered by progressive steps both above and below the transition temperature. The curve a t the right shows the increase in the power factor and the dielectric constant as the transition was in progress a t constant temperature. For this curve the scales of ordinates were so selected that the initial and final values of power factor and dielectric constant, respectively, were plotted as coincidental points. The intervening points followed the same curve closely, as shown. As systems containing cetyl alcohol became heterogeneous on cooling, they congealed to masses resembling gels except that they were quite opaque. These systems were repeatedly examined with a polarizing microscope during and after the transition. The material which first separated from the oil was rod-shaped (Figure 8A). These rods were often curved. Shortly afterward ribbon-shaped bodies appeared which were often twisted as well as curved (Figure 8B). When the system was slowly cooled on a microscope slide, the ribbonshaped pieces grew very large as shon-n in C and D. In addi-
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VOL. 32, NO. 11
alcohols and unlike the other systems which formed masses of crystals on cooling.
Dielectric Behavior of Heterogeneous Liquid Dielectrics Because the primary object of this work was to determine the total effect of varions chemical and physical structures on the power factor and conductivity of insulating oil, no effort was made to design an experimental technique by which the mechanisms for the observed phenomena could he interpreted RCcurately. However, an inquiry was made concerning whether the dielectric behavior of the heterogeneous systems could be explained on the basis of the Maxwell-Wagner mechanism (13). For this reason the values for the dielectric properties c a l c u l a t e d a c c o r d i n g t o Wagner's formulas (13)for five systems representing the types studied are contrasted (Table 111) with APPEARANCE IN POLARIZEW 1,ir.n~01 A NETER~OENE~VS the observed v a l u k FIGURE 8. MICROSCOPIC The high power factors of the 8YSTEM CONTAININc 8 . 5 2 PER CENT (lSTY1, ALCUHOI. A. Rod-shaped bodies (X 85) systems in which the suspended R T w i s t 4 ribbons ( X 1821 p h a s e s were composed of the ~nnb.4sln-1~ mm r n i r r , ~ ~ , ,slide ~ ~ ) t o aOo C . ( x 85, .~ C. ( X 5 5 ) . C mid 1) show X ~ ~ ~ X I I Iui highly polar liquids could not be 'z): note t r i a t in 2' exvlained in terms of Waener's static model of a heterog&eous dielectric (13). This is shown in Table 111, columns 2 tion to the ribbons, nutsues of crystals and vinelike structure and 3, where a comparison is made between the observed were found. After one of the systems had been left standing and calculated properties of two systems, one containing about a year, it was found that the gel-like property of the formic acid and other phenol. For these calculations the system had disappeared and left a mass of crystals in the oil. In order to determine whether systems containing other higher alcohols have high power factors, several systems containing ceryl alcohol were prepared and examined. Figure 9 shows that the power factor and dielectric constant of a system containing 3.55 per cent ceryl alcohol rose to a sharp maximum a t 50" C. as the system was cooled, and then dropped. The d. e. conductivity values, however, were low a t dl temperatures. Below 30' C. the system congealed to an opaque gel-like structure. After several months the gel-like structure broke down. Photomicrographs were made a t varions temperatures to show the appearances of the system containing 3.55 per cent ceryl alcohol when viewed with polarized light as the system was being slowly cooled on a microscope slide. Figure 10 shows the appearance a t 30" C . At 56" C. the crystal growths, A , began to appear. Between 53" and 49" C. the butterfly-shaped patterns, B, developed. Below 49" C., which was approximately the temperature at which the power FIGURE9. DIELECTRIC PROPERTIES factor was a maximum, there was little change in the microDURINQ COOLING OF A SYSTEM CONscopic appearance. It should be stated that the sizes of the TAINING8.55PERCENT CERYL ALCOHOL crystals shown in Figure 10 were much larger than the sizes of those found in the wwer factor cell a t the conclusion of a conductivities, dielectric constants, and volume fractions meeaurement. of the two eomponent,s have been taken to represent the reI n contrast with the systems containing cetyl and ceryl spective conductivities, dielectric constants, and volume fracalcohols, a system Containing 3.55 per centbctadecyl alcohol tions of the two phases (6, 16). The calculated values are had no gel-like properties, nor were bodies other than definite believed to besufficiently accurate for the purpose. The only crystals found on a microscope slide as the system cooled. manner in which the high observed values could be explained The power factor, dielectric constant, and d. e. conductivity by the Wagner model would be to assume that the volume of this system increased markedly, however, as the system fraction of the suspended phase was many times larger than the became heterogeneous on cooling (Figure 11). In this respect volume fraction of the polar component. Observations indithe system behaved like the systems containing cetyl and ceryl ,I
2
IKDUSTRIAL AKD ENGINEERING CHElIISTRY
1518
FIGURE 11. VARIATION IN THE DIELECTRIC PROPERTIES AS A CONTAINING 3.55 P E R C E N T OCTaDECYL ALCOHOL BECAME HETEROGENEOUS
,SYSTEM
A.
B.
Variation with temperature Variation with t i m e during transition a t 35O C.
ties of cetyl, ceryl, and octadecyl alcohols are such (about 10-10 mho per cm. cube) that high power factors for systems containing them may be predicted on the basis of the Maxwell-Wagner mechanism. On the other hand, there are several alternative mechanisms. First, the systems containing cetyl and ceryl alcohols resembled gels. The authors showed ( 8 ) that gels of copper and lead soaps in liquid paraffin have high power factors. The concentrations of soap necessary to produce this effect were too low to create the observed effects by the Maxwell-Wagner mechanism. Secondly, in the Maxwell-Wagner mechanism it is assumed that the phases do not move with respect to each other. By means of a n a p TABLE IV. DIELECTRIC PROPERTIES OF OCTADECYL ALCOHOL (KOTHIGHLY PURIFIED) Temp.,
c.
so
70
Power Factor
Dielectric Constant
Conductivity, Mhos/Cm Cube X 1010 Direct Equivalent current a . c.
> 0.:‘ .”
YOL. 32, S O . 11
dielectric constants of the systems in which cetyl alcohol separated from liquid paraffin on cooling was a property of cetyl alcohol more than that of the system cetyl alcohol-liquid paraffin. An attempt was made to determine whether octadecyl alcohol alone would also exhibit a marked increase in the power factor, dielectric constant, and conductivity as it crystallized upon cooling. Table IV shows the results. Above and a t the transition temperature the power factor was too high to be measured with the apparatus employed. Because the rather large (40-gram) sample necessary for this purpose was not highly purified, the values of the power factor and conductivity are probably high. Xevertheless, the conductivity did increase as the octadecyl alcohol crystallized, then decreased again a t a temperature about 10” C. below the freezing point. Although a similar experiment was not performed for ceryl alcohol, i t is probable that the unusual dielectric properties of the system containing the ceryl alcohol at the transition temperature was due to the nature of the ceryl alcohol and that the principal functions of the liquid paraffin were to act as a diluent and to lower the temperature of the transition.
Summary The data obtained on all the systems a t temperatures well above their transition temperatures support the conclusion (10, 11) that the power factor and conductivity of liquid paraffin are not markedly increased by the presence in true solution of moderately large concentrations of oxidation products. The data obtained on the systems a t temperatures below the transition temperatures show that liquid paraffin having a n oxidation product suspended in i t may or may not have a high power factor and conductivity, depending upon the nature of the suspended material. Systems in which the suspended phase is composed of mobile, polar liquids appear to have high power factors which are many times higher than those that can be accounted for on the basis of the MaxwellWagner inhomogeneity mechanism. Many systems in which the suspended phase consists of crystalline oxidation products seem to have very low power factors and conductivities. Systems in which the suspended phase consists of certain of the higher alcohols, however, have high power factors.
Acknowledgment Through the courtesy of R. A. Millikan of the California Institute of Technology, part of the apparatus used by Pugh and Schwartz was made available for our use.
Literature Cited paratus similar to that described by Pugh and Schwartz ( l a ) , crystals of octadecyl alcohol were observed to rotate about a n axis under the influence of a rotating field. Thirdly, and probably most important, the high power factors of the systems containing the three alcohols may be due to the nature of the alcohols themselves rather than to the nature of the mixture of alcohol and paraffin. During the time this investigation was in progress, Smyth and Baker (14) showed that the dielectric constant and the a. c. conductivity of cetyl alcohol rise sharply on solidification a t 47.8” C. and that the extent of the rise increases with decreasing frequency between 50,000 and 500 cycles. They also showed that strong anomalous dispersion occurred in the region between the freezing point and a n alpha-beta transition at 32.3” C. They believed that the phenomenon was not due to a Maxwell-Wagner effect. They concluded that the molecules in the alpha form have a freedom of rotation which, like that of a viscous liquid, decreases with falling temperature. From the work of Smyth and Baker i t seems probable that the high power factors and
Baker, E. B., and Bolts, H. A,, Phys. Rev., 51,275 (1936). Edler, H., and Zeier, O., 2.Phusik., 84,356 (1933). Emanueli, L., “High Voltage Cables”, p. 44, New York, John W’iley & Sons, 1930. Gemant, A., “Liquid Dielectrics”, p. 107, New York, John Wiley & Sons, 1933. International Critical Tables, Vol. VI, pp. 83, 143, New York, McGraw-Hill Book Go., 1929. Piekara, A.,Kolloid-2.. 49,99 (1929). Piekara, B., Physik. Z., 37,624 (1936). Piper, J. D., Fleiger, A. G., Smith, C. C., and Kerstein, N. A., I N D .EKG.CHEM., 31,307 (1939). Piper, J . D., and Kerstein, N. A., Ibid., Anal. Ed., 9,403 (1937). Piper, J. D.,Thomas, D. E. F., and Smith, C. C., IND.ENQ. CHEM., 28,309 (1936). Zbid., 28,843 (1936). Pugh, E. M., and Schwartz, C. A,, Phys. Rev., 36, 1495 (1930). Schering, H.,“Die Isolierstoffe der Elektrotechnik”, Chsp. 1, esp. p. 20,Berlin, Julius Springer, 1924. Smyth, C. P., and Baker, W. O., J . Chem. Phys., 5 , 666 (1937); Baker, W. O.,and Smyth, C. P., J . Am. Chem. SOC.,60,1229 (1938). Sommerman, G . M. L., J.Franklin Inst., 219,433 (1935). Weissberger, A , and Proskauer, E . , “Organic Solvents”, pp. 42,30,London, Oxford Univ. Press, 1935.
