I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
April 1952
used a difference between means in rubber hydrocarbon recoveries of about 3y0 would be required for statistical significance a t the 5y0level. If only four replicates were used, and it is assumed that the standard deviations were the =me, then a difference of about 5Y0 between means would be required to establish significant differences at the 5% level. Inasmuch as most pilot plant work will o hydrocarbon probably be done in the range of 90 to 1 0 0 ~ rubber recovery, eight replications should be used to establish the effect of various treatments. CONCLUSIONS
These experiments are believed to demonstrate, though in a limited way, the applicability of biological experimental design and statistical analysis to process development research. Utilization of these principles enables the researcher to speak with more accurate knowledge of the meaning of his results, and a t the same time permits the elucidation of factors bearing on his work which might require years of trial and error experimentation to uncover. They emphasize the necessity for elimination of human bias wherever possible, and accentuate the fact that the nontechnical workers (who must be relied upon in much pilot plant research) must be very carefully trained in each operational step to achieve the maximum accuracy of results.
887
ACKNOWLEDGMENT
The authors are grateful to I. C. Feustel for his encourngement and leadership in this work, to the analytical section of this station-particularly to R. V. Crook and W. J. Gowaiis for running the chemical analyses, and to E. C. Taylor for statistical analyses. LITERATURE CITED
Allen, P. J., and Emerson, Ralph, IND.ENQ.CHEM.,41, 346-65 (1949). Bainbridge, J. R., Ibid., 43, 1300-6 (1951). Gumming, J. M., and Chubb, R. L., %hem. & Met. Eng., 53, 1256 (1946). Gore, W. L., IND.ENG.CHEM.,42, 320-3 (1950). Huhndorff, R. F., Ibid., 41, 1300-3 (1949). Jones, E. P., U. S. Patent 2,434,412 (Jan. 13, 1948). Lloyd, F. E., Carnegie Inst. Wash. Pub., No. 139 (1911). Nishimura, M. S., Hirosawa, F. N., and Emerson, Robe1t , IND. ENG.CHEM.,39, 1477 (1947). Snedeoor, George W.., “Statistical Methods,” 4th ed., Ames, Ia., Iowa State College Press, 1946. Spence, D., and Caldwell, M. L., IND.ENG.CHEM.,5, 371 (1933). Taylot, K. W., Econ. Botany, 5, 255-73 (~$951). Tint, H., and Murray, C. W., U. S. Patent 2,459,369 (Jan. 18, 1949). RECEIVED for review February 9, 1961.
ACCEPTED October 31, 1951.
Moisture in Oil-Treated Insulation FRANK M. CLARK General Electric Co., Schenectady, N. Y.
T
HE efforts of the engineer to obtain and maintain electrical
equipment in a moisture-free condition are severely handicapped by the chemical properties of the insulations which are present. Since the essential dielectric properties of the insulating materials are adversely affected by the presence of moisture, the solution of the problem may appear to lie in the exhaustive drying of the equipment during its manufacture. It has been demonstrated, however, that the excessive drying of the cellulose insulation results in the chemical degradation of the cellulose, leading to acid formation, gas evolution, increase in dielectric loss, and a decrease in the mechanical and dielectric strength of the insulation
(4,8). On the other hand, it may be suggested that the moisture problem may be alleviated by the selection of insulating materials having the least ability to attract and to hold moisture. Chemical and dielectric considerations, however, frequently prevent the engineer from applying the materials having the greatest water repellency. For example, the ability of mineral oil to dissolve moisture decreases with the increased severity of its refinement, and such a suggestion would lead to the application of highly refined mineral oil as the preferred impregnant for cellulose insulation. It has, however, become widely recognized that the chemical and dielectric instability of the highly refined mineral oil presents a major hazard in itself to the safe operation of oilfilled equipment in which such oil may be used ( 7 , 9 ) . Laboratory studies and practical experience unite in the conclusion that moisture in the insulation of electrical machines presents a dielectric hazard which is reflected in decreased dielectric strength, increased power factor, increased chemical deterioration of the cellulose and the mineral oil and, in general, a thoroughly undesirable chemical and electrical condition. Laboratory studies and practical experience also indicate, however, that the approach to the moisture problem must be made with full recognition of its complexity. The type and design of the equipment, its susceptibility to heat accumulation, its rated
.
voltage and the voltage stress on the insulation during operation, the temperature range of operation, and the possibility and degree of hazard presented by the oxidation and chemical deterioration of its insulation are engineering factors which must ultimately determine the moisture limitations necessary for the safe operation of the equipment and the type of test best suited for its evaluation and control. EXAMINATION OF THE MOISTURE PROBLEM
Both dry mineral oil and dry cellulose will adsorb or dissolve moisture from the surrounding medium, whether this medium be gaseous or liquid. Given sufficient duration of exposure, a n equilibrium condition will be established between the oil or cellulose and the surrounding moisture-containing medium. T h e equilibrium conditions established in mineral oil which is in contact with air of varying degrees of humidity are illustrated in Figure 1 (6). Figure 2 illustrates typical data which have been reported for the moisture content of oil-free cellulose in contact with moist air. Because of its hygroscopicity, cellulose has been described as a n excellent dehydration agent. Immersed in mineral oil, cellulose will adsorb moisture from the oil and establish a moisture equilibrium with the oil (Figure 3). It has been suggested (16) that the presence of the oil medium merely slows up but does not change the ultimate moisture equilibrium of the cellulose-humid air system illustrated in Figure 2. This suggestion, if true, appears to be of little practical value because of the slowness with which the moisture equilibrium is established. The moisture content of vacuum-dried and oil-impregnated kraft paper immersed in mineral oil whose surface is exposed to air with a relative humidi6y of 70%, approaches its equilibrium conditions very slowly, and the moisture content of the oil-immersed paper after long periods of exposure is lower (approximately 1.1%moisture) than the moisture content (9 to 11%) which is established when
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or in cylindrical rolls as described in the test data presented. Dielectric tests on this paper have been made at both 60 and 1000 cycles. Aluminum foil electrodes have been used. Where the paper has been impregnated with minera1 oil, the oil used was of naphthenic base having a viscosity of about 58 seconds Saybolt Universal when tested a t 37.8' C . (100' F.) and prepared with the care usually taken in the manufacture of electrical oils.
WOOD 110)
GLASSINE PAPER 1 I I KRAFT PAPER I 1 6 1
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In studies relating the electrical properties of cellulose with its moisture content it has not been found possible to express the moisture value in terms of the vapor pressure of water or the dew point of the air in contact with the cellu-
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In further consideration of the ultimate moisture equilibrium which is set up between oil-immersed cellulose insulation and humid air, it must be remembered that the solubility of moisture iu mineral oil varies widely with the type of oil and its degree of refinement as well as with the extent of its oxidation. The marked effect of only moderate oxidation on the separation of moisture from water-saturated mineral oil is illustrated in the data of Figure 4,which compares the cloud point values of new and oxidized mineral oil. The ability of the oil to hold water in clear solution is greatly increased by the mild oxidation of the oil described. Laboratory studies have shown that those factors which affect the degree of oxidation and the type of oil oxidation products formed also affect the solubility of water in the oxidized oil as compared to new oil and, therefore, not only affect the equilibrium established between the oil and humid air but affect as well the rate at which the moisture equilibrium is established in the oil-immersed cellulose. There are also indications that the water content of the dried and oil-treated cellulose a t equilibrium is materially affected by the increased solubility of water in the oxidized oil in which it may be immersed. I n view of the differences which are encountered in the use of mineral oil and the widely varying possibilities of oil oxidation in the various types of oil-filled electrical equipment, the studies of the present paper are limited to the dielectric properties of the oilcellulose system involving only new and unoxidized mineral oil. Because of the hygroscopicity of cellulose (1.2, IS, 20) and, consequently, its ability to concentrate moisture in areas of the highest dielectric stress, the present analysis is directed primarily t o the evaluation of the moisture problem as it cbncerns the cellulose member of the composite oil-treated cellulose dielectric. Consideration of the moisture problem as It applies to the mineral oil taken separately (6),or as it applies to the arrangement of oil ducts in series with the insulation, as is customary in
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penetration into the paper assembly with the measured water content expressed in terms of the weight of the dry paper. It is recognized that this procedure of study generally results in
,
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1952
0
100
'200
300
400
500
600
700
WATER IN OIL SOLUTION -F? F! M.
Figure 4. Solubility of Moisture in Mineral Oil Is Increased a s Result of Oil Oxidation 4cidity of oxidized oil, 0.04 mg. of potassium hydroxide per gram of oil
'
ical changes occur which lead to further evolution of moisture. When oil-treated insulation is tested, it has been found best to immerse the insulation in new mineral oil of known water content during the 110" 6.treatment with nitrogen gas which is bubbled slowly through the mineral oil. I n either case, the moisture evolved is evaluated in accordance with the usual analytical technique. The results are expressed in terms of moisture content
30
889
value of permissible moisture content can be broadly established. If, however, such moisture values are in the range below about 3y0,the adequacy of the 1000-cycle power factor teat for this purpose may well be questioned. In Figure 5, typical results are illustrated showing the 1000-cycle power factor of kraft paper as related to its water content. It is only for water content values above 3% that the power factor rises rapidly. The fact that the paper with a water content as high as 5 or 6yo gives a substantially lower power factor when tested a t 75" C. as compared t o 30" C. throws further doubt on the value of the 1000-cycle test for moisture, in view of the general experience in commercial practice that an increase in temperature to as high as 75" C. results in a marked increase in the power factor of moist paper when tested a t commercial frequencies. Sixty-cycle voltage tests for power factor offer greater promise for analytical use in the moisture problem. As shown in Figure 6, the 60-cycle power factor of unimpregnated kraft paper increases with its increasing moisture content. For water contents greater than 0.2% the 75" C. power factor rises rapidly and exceeds the 30" C. value in an increasing ratio as the water content of the paper is increased. But such test data need further exploration in order to demonstrate the effect of temperature and voltage stress.
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Figure 6. 60-Cycle Power Factor of Unimpregnated Kraft Paper as Affected by I t s Water Content
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3
4
5
6
X MOISTURE BY WEIGHT
Figure 5. Effect of Moisture on Power Factor of Unimpregnated Kraft Paper Tested a t 1000 Cycles
0.0005-inch kraft pa er Testing gradient, 29volts per mil
0.0005-inch kraft paper
EFFECT OF TEMPERATURE AND VOLTAGE STRESS ON POWER FACTOR
by weight of the dry paper. When mineral oil-treated paper is examined, the original weight is determined after the oil has been removed by extraction with benzene or other suitable volatile solvent. POWER FACTOR AS A TEST FOR MOlSTURE
The power factor and insulation resistance have been suggested as nondestructive analytical tests for the presence of moisture in electrical equipment (17, Sl-%$). With unimpregnated paper, it is usually preferred to examine the power factor using a 1000-cycle bridge measurement (13) or with 60-cycle measurements at low voltage stress in order to avoid ionization effects. Because of the wide variations in the design and the technical requirements of the numerous types of oil-filled electrical equipment, it is probable that no limiting
As indicated in Table I, the 60-cycle power factor test a t 30" C. is of no value for elucidating changes in the water content of unimpregnated kraft paper having less than 0.2%moisture. Under carefully controlled conditions of test, however, the power factor value measured a t 75" C. rises slowly from that which is characteristic of the driest condition tested, but the increase in power factor (0.17 to 0.3070)is not sufficiently great to warrant the belief that the test may have significant value under the more practical conditions of use. However, under such conditions, water contents of greater magnitude are envisaged. With an increasing water content above 0.274, the power factor of the unimpregnated kraft paper becomes increasingly more sensitive to the temperature and voltage stress applied during it9 measurement. The test assembly used for this evaluation consists of tightly wound, cylindrical rolls containing a dielectric of three sheets of
INDUSTRIAL AND ENGINEERING CHEMISTRY
890
TABLE I.
P O W E R FACTOR OF ~ T N I M P R E G X A T E D 0.0005-INCH I h - 4 F T P A P E R AS . h F E C T E D B Y TT-ATER C O N T E N T , TERfPER.4TURE2 AND VOLTAGE STRESS
Toits per mil Water Content, '.7c
30' C. Power Factoi.. yo 25 53 100
0,006
0.26
0.045 0.124 0.182
0.27 0.28
0'27
o
26
0.27 0.28 o.27
75' C . Power Factor,-Yo 25
0.17
0.27 0 27 0 28 28
0.17
0 17
0.21 o.30
0 21 o.3"
0.19
0.19 0.21 29
0.19
Voi. 44, No. 4
observed that a t 75" C. the power factor (Figure 8) increases with vo]t,age stress when the Rater content of the paper exceeds 0.88%. The becomes pronounced with further increase in the water content of the paper. Power factor tests a t 30" C. show no effect of voltage stress within the test range covered. These effects are illustrated in greater detail in the data of Figure 9. The results described map be taken to support the use of the insulation power factor as an evaluation test for moisture in unimpregnated paper when the test is made a t power frequency. Unimpregnated paper is used only seldom as the insulation in electrical equipment, but the relationships described are of value tor gaging the efficiency of the drying process which is used in the manufacture of paper-insulated electrical machines preparatory to oil impregnation. NIOISTURE IN OIL-TREATED PAPER
n 2.5%
2L
I
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0
.
20
1.32%
3 1.07 %
-
88 ..%
40 60 BO TESTING GRADIENT 1V.P. M.1
50 X i . 0 0 6 -.l82%
100
Figure 7. Effect of Moisture and Voltage Gradient o n Power Factor of Unimpregnated Kraft Paper Tested a t 30' C. 0.0005-inch kraft paper Voltage applied at 60 cycles Water eontent of paper shown on curves
0.0005-inch kraft paper between continuous tapes of thin aluminum foil which function as electrodes. The assembled rolls of paper and foil were 1 3 / 4 inches long and 3/4 inch in diameter. Each assembly as carefully dried a t 100 C. for 48 hours under a good vacuum, cooled to room temperature while still under varuum, and exposed to a moisture-controlled atmosphere a t 25 C. A t intervals, the 60-cycle power factor of each assembly was measured a t 30" and 75" C. under a voltage stress of 25, 50, and volts per mil, Careful attention given to the testing technique in order to prevent the drying out of the insulation during the power factor test. The results obtained are the subject of this analysis.
Becaurr of the widely varying conditions under which oilimpregnated cellulose insulations are used in electrical equipment, it is doubtful that a moisture content determination of the oil in which the insulation is immersed can be used with certainty RS a gage of the moisture content of the cellulose. However, it is usually impractical to perform the moisture measurement on cellulose insulation during the commercial operation of the equipment. Measurement of the moisture content of the mineral oil, therefore, is of importance as an indication of the moisture content which may be possible in the cellulose and which normally n.ould be expected to be present under equilibrium conditions. However, the effect of physical structure and the chemical variations introduced as a result of the varying conditions of paper riianufacture and oil oxidation cannot be ignored in their possible cffects on the moisture equilibrium. There is even some evic1eI1ce that the final moisture equilibrium set up between paper and O i l is affected the previous history Of the paper. Whm dried and oil-treated kraft paper in the form of tightly wound cylindrical rolls is immersed in mineral oil, the surface of ,,-hi& is exposed at 25" C. to air of 40% relative humidiLy, there is immediately no significant change in the temperaturepower factor relation (Figure 10). After a more prolonged exposure, the penetration of the moisture into the dielectric assembly becomes evident only in the increased power factor a t the higher teqting temperature (75" C. and higher). In this respect the oil-
The Dower factor of the unirnpregnated kraft paper of Figure 6 was measured a t 25 volts per mil, 60-cycle frequency. From a practical standpoint, the slow increase in the 30" C. power factor which accompanies the increase in water content of the paper is misleading, for 30 when test,ed a t 7 5 " C. the insulation shows a discontinuity and a rapid increase in power factor when its moisture content exceeds about 0.2%. This pronounced effect of moisture warrants further study and suggests that as an analytical test for the evaluation of the dryness of paper, the power l o t factor should be measured at 5the higher temperatures. : 1 In Figures 7 and 8 is illus0 trated the effect of voltage stress on the power factor of the unimpregnated kraft paper containing varying 0.0005-inch kraft paper amounts of moisture up to Voltage applied at 60 cycles 3.38% by weight. I t will be Water content of paper indicated
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15
20
25
30
35
% WATER (WEIGHTI
Figure 9. Power FactorTemperatureVoltage Gradient Relation for Unimpregnated Kraft Paper as Affected by Its Water Content on curves
0.0005-inch kraft paper Voltage applied at 60 cycles Testing temperatures indicated on curves
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1952
treated paper behaves like the unimpregnated paper described in Table I, and the adsorption of any substantial amount of moisture by the paper under the conditions given may well be questioned. Exposed similarly to air a t 75% relative, humidity, the paper gives more rapid change in power factor (Figure 11). Here again, however, the 30" C. power factor value rises only after 0.4
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100
Figure 10. 60-Cycle Power FactorTemperature Relation for VacuumDried, Oil-Immersed 0.00U5-Inch Kraft Paper a s Affected by Exposure of Oil Surface to Wet Air Weeks of exposure at 25' C. to air having 40% relative humidity indicated on curves
pTolonged exposure. As in the case of the unimpregnated paper, the power factor measured at the higher temperature is more sensitive to the penetration of moisture into the dielectric and the power factor-temperature relation ultimately takes on a dl runaway" characteristic. The conclusion seems justified that for detecting moisture in oil-treated, oil-immersed' paper, the power factor test should be applied at temperatures higher than room temperature and preferably as high as 75" C. These data a r e indicative of the dielectric problem presented as the result of moisture adsorption by oil-treated and oil-immersed paper but become of greater significance when the power factor of the insulation is associated with actual moisture content values. The first examination concerns the moisture content of the immersion oil. When dried and oil-treated kraft paper is immersed in wet oil, i t s adsorption of moisture from the oil (Figure 3) is accompanied
14
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by an increase in power factor which becomes more pronounced as the water content of the immersion oil exceeds 50 p.p.m. For lower water content values of the immersion oil under equilibh u m conditions, the power factor of the paper is but little affected. This is illustrated in the data of Figure 12. Because of the difficulty with which a moisture equilibrium i6 established between a practical paper dielectric assembly, such as a taped conductor, and the oil in which it is immersed, dielectric effects based on the moisture content of the immersion oil may be grossly misleading. It is preferred, therefore, to relate the moisture effect in insulation to the actually measured moisture content values of the oil-treated cellulose dielectric. I n Figure 13, the power factor of vacuum-dried and oil-impregnated kraft paper is related to its moisture content after immersion in mineral oil of selected moisture content values until a stable power factor has been established. As in the case of the unimpregnated paper of Figure 6, the 75" C. power factor of the oil-treated paper is more greatly affected by the increase in its water content than is the power factor tested a t 30" C. However, there is a marked difference in the degree of change in the power factor of the paper as its water content is increased after drying and subsequent oil impregnation and oil immersion (18). The power factor of the oil-impregnated paper is substantially more stable in the presence of moisture than is the power factor of the unimpregnated paper. As illustrated in Figure 13, the power factor of the oil-treated and oil-immersed paper shows marked increase only as its water content exceeds about 0.85%. With a water content varied in the range lower than about 0.5%, the power factor of the oil-treated paper even when tested a t the higher temperature (75 C.) is but little affected. These test data would indicate that the power factor of dried and oil-treated, oil-immersed kraft paper is of little practical value for the evaluation of its moisture content unless the moisture present exceeds about 0.85%. This is in agreement with the conclusions drawn by Piper (16), based on the work of Hirshfeld, Meyer, and Connell (14) as illustrated in Figure 14. Despite the differences in testing techniques and the insulation tested, the relationships established by Hirshfeld, Meyer, and Connell are in substantial agreement with those established by the data of Figure 13 and led Piper to a conclusion which also seems ineicapable on the basis of the present data. Piper, referring to the results of Hirshfeld, Meyer, and Connell, concluded that "these data showed that power factor measurements even
-
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5 D
0
20
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40 60 80 WATER IN OIL I P P M 1
,
IO0
Figure 11. 60-Cycle Power FactorTemperature Relation for VacuumDried, Oil-Immersed 0.0005-Inch Kraft Paper as Affected b y Exposure of Oil Surface to Wet Air
Figure 12. 60-Cycle Power Factor of Vacuum-Dried, Oil-Treated 0.0005-Inch Kraft Paper i n Relation to Water Content of the Oil in Which I t Is Immersed
Weeks of exposure at 25' C. to air having 75%
Tests made at 3O0 C. after equilibrium conditions have been established
relative humidity indicated on curves
891
0 0
0.2 0.4 0.6 0.8 1.0 X WATER IN PAPER ( W E I G H T ) ,
1.2
Figure 13. 60-Cycle Power Factor of Oil-Treated 0.0005-Inch Kraft Paper a s Related to I t s Water Content
INDUSTRIAL AND ENGINEERING CHEMISTRY
892
at 80" C. are not a reliable criterion of the moisture content of impregnated paper when the moisture content is less than approximately 0.7% based on the weight of the dry paper." The adequacy of the power factor test as an analytical tool f6r the evaluation of moisture contaminatjon in oil-filled equipment will be reviewed in greater detail in subsequent paragraphs. DIELECTRIC STRENGTH IMPREGNATED PAPER WITH APPROX
For many practi2 8 PER CENT MOISTURE cal applications, the change in the pon'er 1.4 PER CENT MOISTURE factor value with its e information concerning possible heat ac0 7 PER CENT MOISTURE, cumulation is a more 2 ~ t ' o R y ' significant way of evaluating dielectric 0 0 20 40 60 80 100 deterioration than is TEMPERATURE "C the short-time dielecFigure 14. Effect on Power Factric strength. Howtor of Moisture i n Impregnated ever, the latter is not Cable Paper (14) without its own significance. Data relating the breakdown of oil-impregnated paper with the water content of the immersion oil bear strong relation to the power factor characteristic which has already been described. Thus in Figure 15, whether gaged by rapidly applied or by minute step-up testing procedure ( d ) , the 30"C. dielectric strength of 0.0005-inch kraft paper pads is substantially unchanged with the increasing moisture content of the immersion oil up to approximately 50 p.p.m. With higher moisture content in the oil, the dielectric strength of the paper drops. I n like manner, the power factor of the oil-treated paper has been found to deteriorate with an increase in the water content of the oil above 50 p.p.m. (Figure 12). When the minute test dielectric strength of the oil-treated paper is related to its measured water content, the dielectric strength is found to decrease slomdy with the increase in the moisture content of the insulation from the dry state (Figure 16). The decrease in the dielectric strength becomes more pronoupced when the moisture content of the impregnated paper exceeds about 0.85%. The effect on the power factor of the oil-treated insulation which is produced by moisture accumulation above 0.85% has been discussed with respect to the data of Figure 13.
g
POWER FACTOR TEST FOR MOISTURE
Practical experience appears to substantiate the use of the 60cycle power factor test as an analytical means for detecting the presence of an excessive amount of moisture in the insulation of
12
RAPIDLY A P P L I E D
$5
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Vol. 44, No. 4
oil-filled equipment. Most frequently, the operator in his use of the power factor test for this purpose, places reliance on an increase in the power factor from a base value typical of the power factor of the equipment under examination rather than on comparison with an absolute standard of power factor. This necessarily results because of the wide difference in the design and materials which are used in the construction of the many different t-ypes of equipment tested. The data of the present paper indicate that the power factor test is of little value in the detection of moisture in the insulation for quantities less than about 0.85% b\ weight of the paper, and even with greater quantities of moisture the power factor becomes significantly large only when the insulation is tested a t higher than room temperature. With vacuumdried paper impregnated with and immersed in new (unoxidized) mineral oil, such water content values are encountered only wliei~ the oil approaches its water saturation point. It would appcm ~ practical ~ from experience that the insulation which is declared "wet" on the basis of the power factor test made a t approximately room temperature contains considerably more moisture than that which has been found in the insulations of the present Ytudy, despite the fact that these insulations have been exposed to moisture-laden atmosphere under such conditions that further picliup of moisture was not obtained despite prolonged exposure. The greater moisture content of commercial insulations is attributed to the fact that deteriorated mineral oil in its various stages and degrees of oxidation holds an increasingly large amount of mokture in solution as the oxidation progresses. The data of the present s t u d y support the use of the power factor test a t commercial frequencies aB a 2 1 b means for the 010" KRAFT d e t e c t i o n of moisture in the cellulose insulation of new and unoxidized oilY filled equipment 0005" KRAFT only as it apL proaches t h e T moisture equi"0 02 04 0.6 OB IO 12 librium which is WATER I N PAPER ( % W E I G H T ) establishedwhen Figure 16. Dielectric Strength of Oilthe insulation Treated Kraft Paper in ReIation to Its is immersed in Water Content water saturated Test thickness for 0.010-inch kraft, 0.020 inch; for 0.005-inch kraft, 0.010 inch; and for (new) oil. I t 0.0005-inch kraft, 0.004 inch remains for furTest voltage, 60 cycles Test temperature, 25' C. ther studies t o Test procedure, minute stepup of voltage demonstrate the effect of deteriorated oils in producing the larger water content values which appear necessary in the insulation in order to give the more pronounced power factor increases a t room temperature on which the evaluation of the moisture problem is usually based on commercial practice. Laboratory studies in this field are in progress.
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~~~
SUM M A R Y '0
20
BO WATER IN O I L ( R P M )
40
60
100
Figure 15. Dielectric Strength of OilTreated 0.0005-Inch Kraft Paper in Relation to the Water Content of the Oil i n Which I t I s Immersed Test thickness of paper, 0.0004 inch Teat temperature, 30' C. Test voltage, 60 cycles
AIineral oil is a good waterproofing agent but it will absorb moisture from humid air and will transmit this moisture to the dried cellulose insulations with which i t may be in contact, thereby setting up a moisture equilibrium with the moist air or other gas on the one hand and the oil-immersed cellulose on t h e other. Available data indicate that this moisture equilibrium between dried and oil-treated cellulose and the mineral oil i n
April 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
which it is immersed is established slowly under practical conditions and a t a rate which is greatly affected by theempirical conditions of the dielectric assembly. Data also indicate that the moisture contained in the oil-treated cellulose in practical equilibrium with the water-saturated immersion oil, whose surface is exposed to humid air, is smaller than the water content which i s established for the same insulation in direct contact with air of like humidity. The power factor of cellulose insulation is increased by unlimited water adsorption but in the case of dried, oil-treated cellulose immersed in new oil, the power factor is substantially unaffected by water adsorption up to about 0.85% of the weight of the paper. This condition is attained with new and unoxidized oil only after prolonged exposure of the oil-immersed insulation to highly humid air. It is suggested that the greater moisture adsorption characteristic of commercial experience is promoted by the greater solubility of moisture in oxidized mineral oil. The power factor test as an analytical method for the evaluation of the moisture in cellulose inmlation impregnated with new and unoxidized oil is of little value for water contents in the range up to 0.85%. For higher water contents, the power factor m e w ured a t a temperature in the range of 75 O C. is more sensitive to changes in the moisture content of the insulation than is the room temperature power factor value. Because of its ability to adsorb moisture and thereby concentrate it in areas of high voltage stress, and in view of the limited moisture adsorption which has been obtained in the exposure of dried cellulose impregnated with and immersed in new and unoxidized mineral oil, further studies are needed to evaluate the effect of oxidized oil on the moisture equilibrium established in oil-treated cellulose. Presently available data indicate that the utility of the power factor test as a gage of insulation deterioration in commercial service is closely associated with the presence or absence of ioniz-
893
able contaminants, including the products of oil oxidation, the dielectric effects of which are made more pronounced by the presence of moisture. LITERATURE CITED
Abrams, A,, &ndBrabender, G. J., Paper Trade J., 102,204-13 (1936). “A.S.T.M. Standards,” p. 411, Designation D 149-44, Philadelphia, American Society for Testing Materials, 1949. Brabender, G. J., Paper TradeJ., 110,27 (1941). Clark, F. M., Elec. Eng., 54,1088-94 (1935). Ibid., 59,433-41 (1940). Ibid., 61,742-49 (1942). Clark, F. M.,IND. ENQ.CHEM.,31, 327 (1939). Clark, F.M.,Trans. EEectrochem. Soc., 83, 143-60 (1943). Clark, F.M.,and %ab, E. L., IXD. ENG.CHEM.,34,110(1942). Davideon Chemical Corp., Philadelphia, Pa., “Silica Gel,” 1939. Evans, R. N., Davenport, J. E., and Revikas, A. J., IND. ENG. C H ~ MANAL. ., ED.,11,553-65 (1939). Hasselblatt, M., 2.anorg. u. allgem. Chem., 154,375 (1936). Hill, C. F.,Elec. J . , 35, 195 (1938). Hirshfeld, C. F.,Meyer, A. A., and Connell, L. H., minutes of the Association of Edison Illuminating CompanieB, 1929. “International Critical Tables,” 1st ed., pp. 2,322, New York, McGraw-Hill Book Co., Ino., 1926 Piper, J. D., Ekc. Eng., 65,791-97,1152-5 (1946). Rawls, J. A., Am. Inst. Elec. Engrs., paper 51-37 (1950). Rottmann, C. J., J. Frunklin Inst., 409,188 (1919). Schwaiger, A., Science Abstracts, B 18, Item 654, 334-5 (1915); Archiv.-Elektrotech., 3, 332-34 (1915). Simril, V. L., and Smith, Sherman, IND. ENG.CHEW,34,22&30 (1942). I Smith, L. W., Doble Engineering Co., Rept. 2,601 (1950). (22) Urguhart, A. R., and Williams, A. N., J. Teztile Inst., 15, 138 (1924). (23) Walker, A. C., J. Applied Phys., 8,261-8 (1937). (24)Williams, R. R.,and Murphy, E. J., Bell System Tech. J.,8 , 225 (1929). RECEIVED for review 1Meroh 16, 1551.
ACCEPTED December 15, 1551.
Basic Degree of Polymerization
of Cellulose Acetate L. A. COX AND 0. A. BATTISTA American Viscose Corp., Marcus Hook, Pa.
K
NOWN solvents for cellulose, for example cuprammonium
.
hydroxide or cupriethylenediamine, are sufficiently alkaline t o de-esterify completely certain cellulose esters. When a cellulose ester such as cellulose acetate, therefore, is dissolved in cuprammonium hydroxide, the resulting specific viscosity of the sample is due to homologous chains of regenerated cellulose. I n other words, a solvent such as cuprammonium hydroxide is suitable for the measurement of the viscosity or weight-average molecular weight of both cellulose and cellulose acetate. Various investigators (3, 6-10,14) already have considered deesterifying cellulose esters for the purpose of measuring the molecular weights of the resulting regenerated cellulose in a cellulose solvent such as cuprammonium hydroxide. For example, Staudinger and Daumiller (14) showed that cellulose acetates may be dissolved directly in cuprammonium hydroxide without degradation when the proper precautions are taken. Heuser and coworkers (6, r) as well as Howlett and his associates (8-10) have considered both the direct solution of cellulose acetates in cuprammonium hydroxide as well as a predgacetylation by ammonium hydroxide followed by the solution of the regenerated cellulose in c u p
rammonium hydroxide. Harrison (5) used cuprammonium hydroxide as a solvent for measuring the degrees of polymerization of cellulose acetates and other esters, such as cellitaose butyrates, which he found were insoluble in organic solvents because all of them had been incompletely esterified by heterogeneous methods. In a classical piece of work, Howlett et al. (IO),determined a series of K,,, values for a number of soluble cellulose acetates having a wide range of acetyl values in various organic solvents. The reason Howlett did this was that if the viscosities of cellulose acetates in organic solvents are t o be used for calculating molecular weights, different K , values are necessary for each acetyl composition in each solvent system. Such a situation is not desirable or practical for most studies. The use of a single solvent, such as cuprammonium hydroxide, is the best way of eliminating the need ’ for a series of different K , values. As has already been pointed out, however, Howlett and coworkers (8-10) did use cuprammonium viscosities in lieu of organic solvent viscosities in some of their work. More recently Malm and his associates (12) found that in the case of a series of aliphatic acid esters of cellulose, the intrinsic viscosities of these esters varied with the acyl group and the solvent
