Cellulose as an Insulating Material G. T. KOHMAN Bell Telephone Laboratories, New York, N. Y.
The general nature of the behavior of cellulose as a dielectric in directand alternating-current fields is discussed, and some of the more important dielectric properties are tabulated. The uses of cellulosic insulation are discussed under the following five classifications: paper capacitors, high-voltage power cables, telephone cables, textiles, and reinforced and laminated insulation. The more important unsolved problems suggested by the literature and by various authorities are listed.
HE statement has been made that no man who has stayed within the recognized boundaries of his own field has contributed fundamentally to science. This remark applies particularly well to the field of insulation research. The insulation engineer has had it impressed upon him repeatedly that the phenomena involved in the electrical breakdown of cellulosic insulation cannot be understood without going beyond the bounds of dielectric theory and physics and considering the chemistry involved in the process. It is well recognized that insulation of this type is continually changing in service as the result of chemical action and that the initial dielectric properties of the material are of very little value in predicting the conditions under which it can safely be used. For this reason insulation engineers in general welcome the opportunity of describing the difficulties which arise in the use of cellulose as an insulating material to cellulose chemists with the hope of stimulating their interest in the fundamental problems. The importance of cellulose as an insulating material cannot be determined by a consideration of the quantities consumed annually but must be determined by the extent to which we depend upon apparatus in which this type of insulation is used. Whitehead (@) states: “In spite of its limitations, paper has proved itself by far the superior material for the insulation of high-voltage cables. Both history and product are truly remarkable. The impregnated paper cable is unique, i t cannot be even approached in performance by any other type and without it we would still be a t a very early stage in the remarkable expansion in transmission and distribution of electric power.” Very little information is available concerning the total quantity of cellulosic insulation produced annually. The available data indicate that the annual consumption in the United States of high-voltage cable paper is of the order of 10 million pounds. An approximately equal quantity of cellulosic insulation is used in each of the four other types of insulation to be discussed. The purpose of this paper is to describe the nature of some of the unsolved problems of a chemical nature rather than to present the results of completed investigations or to describe in detail the electrical circuits and apparatus in which the material is used. A brief description of cellulose as a dielectric will be given, some of the more important applications as an insulating material will be discussed, and finally some of the more important problems of a chemical nature will be mentioned which are encountered in its use.
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Cellulose as a Dielectric Although the use of cellulose as an insulating material does not go back to the Garden of Eden as do some of its applications, it is nearly as old as the electrical industry itself. The question is frequently asked, ((Why do we continue to use paper for insulation when its use involves so many complicated problems?” The answer becomes clear when we focus our attention on the remarkable combination of dielectric properties which it possesses; some of these are given in Table I. For example, the dielectric constant, E, of the chief constituent, cellulose, is exceptionally high for a solid dielectric (approximately 7 for the material as such; see note to Table I). Yet it has physical properties which enable it to be used in a form having very low dielectric constants only slightly greater than one. Its power factor over a wide range of frequencie8 is a small fraction of one per cent, and its resistivity when well ohms per cm. Its dielectric dried is of the order of strength is higher than that of most dielectrics, reaching values of the order of 2.5 million volts per cm. in thin films. I n addition to this remarkable combination of dielectric properties, i t has an unusual combination of other physical and chemical properties such as high chemical stability, no cold flow at elevated temperatures, and high tensile strength and flexibility in thin films. . TABLE I.
DIELECTRIC PROPERTIES OF CELLULOSIC INSULATING MATERIALSa
DielecResis- Dielectric trio tivity Strength, ConPower Ohms) Volts Sample Form stant, e Factor Cm. D . C. Density 1 0 0 ~ cellulose o 8.1 ... lo’* 2.5 X 106 1.56 Regenerated cellulose 7.5(7) 0.01 101’ 2 X 10s 1.42 PaDer 1.2-4 0.001-0.002 >lo’s 2 x 10‘ 0.2-1.2 a The value of e for 100 per cent cellulose was calculated from Stoops’ value, taking into consideration density differences. It is probably high as the result of hydration. DeLuca, Campbell, and Maass @A) report a value of 6.1. The dielectric strength values -are maximum values dedermined on films less than 1 mil in thickness. A. c. measurements were made at 1000 cycles per second and 25’ C. The resistivity values are final values corrected for short a’nd long time polarizations.
We frequently find that we know least about some of the most common materials which we must use daily and upon which our existence depends. We think of water as HzO, yet we know that if it were as simple as this, life could not exist. We know exactly in what class of dielectrics nitrobenzene and chloroform fall, but we can do no more than guess as to the classification of cellulose as a dielectric. Let us consider 807
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FIGURE 1. CHARGE AND DISCHARGE CURVESSHOWING REVERSIBLE FLOW OF CURRENT one of its simplest dielectric properties, its d. c. conductivity. It is common practice to express the conductivity of cellulose in terms of the current flowing after one minute application of potential. Yet as Figure 1 shows, most of this current is not a true conduction current; but even after much longer periods of application of potential, most of it is reversible. The charge transported can be recovered much as the current which charges a storage battery, except that its potential during charge and discharge is not constant. McLean and Kohman (17) find t h a t the nature of the charge and discharge curves depends in a complicated manner upon moisture content, voltage gradient, and temperature. Not only does the current increase with voltage, but under certain conditions large increases may be observed with time of application of a constant potential, as Figure 2 shows. According to Evershed (9) the applied field alters the distribution of moisture contained in isolated pores and causes it to form continuous conducting paths; the increase in conductivity with time and potential observed a t high humidity is thus accounted for. McLean and Kohman have found, however, that similar changes occur a t low humidities a t high potential gradients and temperature where it is improbable that the adsorbed water can be regarded as forming conducting liquid paths. The explanation is offered that ions which are strongly bound to the cellulose structure move against restraining forces when a potential is applied. At low potential gradients these ions finally reach equilibrium positions in which case the leakage current reaches a constant low value and when the potential is removed a current flows in the opposite direction. At high potential gradients the restraining force may break down and some of the ions become free, thus causing an increase in current which is not reversible. Similar phenomena may be observed on a crystal surface containing adsorbed moisture; as Figure 1 indicates, this behavior may be characteristic of surfaces containing adsorbed moisture or ions. McLean and Wooten (18) have recently shown that insulating papers exhibit the phenomenon of ionic interchange described by Stamm (39) in other cellulosic materials, and it is probable
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FIGURE 2. INFLUENCE OF MOISTURE ON CURRENTFOR PAPER TIMECURVES
that the exchangeable ions such as calcium, magnesium, sodium, potassium, and hydrogen play an important part in the dielectric behavior of cellulosic materials. The a. c. phenomena in cellulosic materials are still more complicated. I n addition to these d . c. phenomena there are many others to be considered, for the d. c. conductivity is sufficient to account for only about one per cent of the a. c. conduction in the dry condition. Studies of the a. c. behavior over a sufficiently wide range of temperatures and frequencies reveal two regions of dispersion of dielectric constant and absorption corresponding to widely different relaxation times. One region occurs a t approximately room temperature in the low-frequency range and has been studied by Whitehead and Greenfield (61) ; they propose a method based upon power factor measurements for the determination of the amount of moisture a.dsorbed by cellulose. The second region occurs a t a temperature of approximately -40" C. and has been reported by Morgan (21). This second region is shown in Figure 3. The polarization responsible for the dispersion a t -40" C. has a relaxation time of the order of second a t room temperature, and the other greater than 1 second a t the same temperature; the longtime polarization is responsible for the capacity and loss increase a t low frequencies and high temperatures, and the shorttime polarization, at low temperatures and high frequencies. The long-time polarization contributes practically nothing to the dielectric constant of a well-dried paper a t or above audio frequencies and at temperatures below 0" C. At temperatures above 60' the short-time polarization is contributing a practically constant amount to the capacitance and dielectric loss below lo6 cycles, and any changes in these values with frequency under these conditions are due to the long-time polarization. The absence of a measurable free ion conductivity a t low temperatures shows that the short-time polarization is not due to free ions or electrons. Morgan finds that, after correcting the capacitance for dimensional changes in the electrodes, there is still an increase in capacity with temperature, contrary to the findings of
JULY, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY K R A F T PAPER
0 TEMPERATURE IN DEGREES CENTIGRADE
FIGURE 3. LOW-TEMPERATURE REGIONO F DISPERSION IN DRYPAPER($1)
Whitehead and Greenfield that thermal expansion of the electrodes is sufficient to account for the observed capacitance changes. Morgan gives the following temperature coefficients of dielectric constant for cellulosic materials: Change of Capacity per Material Cable paper Kraft condenser paper Linen condenser paper Cellophane (Stoops, 48) Cable paper (graphite electrodes)
c., % 0.056 0.039
O
0.043 0.078 0.055
Stoops (42) also finds a positive temperature coefficient of capacitance for cellulose which he explains by the rotation of glucose units about the bonds linking them t o other glucose units in the chain. Morgan points out that there is as yet no possibility of differentiating between this explanation and that of rotation of groups such as the hydroxyl. He finds, however, that chemical action which alters the hydroxyl groups has a marked effect upon the low-temperature region of dispersion, and it seems likely that the high-frequency losses in cellulose result from the oscillation of the hydroxyl groups. It seems unlikely that cellulose falls in the class of polar dielectrics which exhibit dipole rotation in the solid state. It is more probable that the high dielectric constant is due to oscillation of some group such as the hydroxyl or to strongly adsorbed moisture. I n cellulose impregnated with oils and waxes, additional c o m p l i c a t i o n s a r i s e . Whitehead (50) has shown that the dielec-
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tric loss in impregnated paper cannot be estimated from a knowledge of the loss in dry paper and oil, but that empirical factors whose underlying explanation is not understood must be used for a pre-estimate of final values. He states: "If i t can be assumed that on impregnation the free ions in the oil join, through the process of adsorption, those ions which are already bound to the cellulose structure, we have a t least a tentative picture which is in accord with the facts; that is, the increase in loss due to irreversible adsorption would be proportional to the number of new ions added, and we may assume that the motion of the adsorbed ions on the surface of the cellulose fibers is different from the motion of the ions in the body of the liquid." More recently Piper (29) and his co-workers have found that organic acids are adsorbed from oil to a degree which drops off rapidly with increasing molecular weight, and that the value of the 60-cycle power factor of oil-impregnated paper is markedly affected by the amount of acid sorbed by the paper although the presence of the same amount of acid dissolved in the oil increases the power factor of the oil only slightly (Figure 4). We have found that the dielectric constant of impregnated condenser tissue cannot be accurately estimated from the dielectric constant of the components. It is not only necessary to take into consideration differences in degree of pore filling dependent upon differences in molecular shape and size of the impregnant, but it also appears that impregnating compounds act as if they formed a series rather than a parallel circuit with the paper. A consequence is that the gain in capacity of paper capacitors obtainable by increasing the
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O.O10 0 2 4 6 8 1 0 1 2 0 2 4 6 8 1 0 CONCENTRATION OF OXIDATION PRODUCT IN OIL-PER CENT BY W T . OF ORGANIC ACIDSDISSOLVED IN OIL ON FIGURE 4. INFLUENCE FACTOR OF OIL AND OIL-IMPREGNATED PAPER (99)
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manufacturers. This situation is commonly found, not only in paper capacitors but also in high-voltage cables; therefore, it must indicate either that the performance of paper dielectrics is dependent upon no single property but on a combination of many, or that there exists an important variable now unrecognized which completely conceals the effect of the variables which are recognized and controlled. Our experimental results support the latter contention and indicate that the variable is to be found in electrochemical ac- tion occurring at the electrodes in which Dax)er impregnant &d electrode material are involved. TABLE11. QUALITATIVE DATAON INSULATIKG PAPERS ’ The outstanding problem in connection with Paper ThickWater-Sol. the use of cellulosic insulation is that of life. Paper Type Fiber Density ness Acidity Content Ash MiEliThis is particularly true of power cables and equivalent/ paper capacitors. Whitehead states in his reMils Qram % % Condenser Linen sulfate wood 0.9-1.1 0.3-2.0 0.001-2 0.4 0.5-0.8 port of progress in insulation research for 1937 Tela tissue hone Hemp sulfate wood 0 . 6 - 0 . 7 1.5-15 0 . 0 0 5 2.5 3, that “in recent years we have passed through a c a k e aper cotton series of fashions in our ideas of the causes of High-&age Sulfate wood 0.7-1.1 2-10 0.001-2 0.5 0.5-2.0 cable deterioration. We have noted as chief cable paper Laminated Sulfate wood, sulfite 0.5-0.8 5-7 0 . 0 0 5 1.0-3.0 suspect in successive periods high inherent power insulation wood, cotton rag factor, gaseous ionization due to temperature cycles, wax formation, and oxidation. At the moment, we appear to be leaving oil oxidaRace, low-frequency losses in undried paper are caused by contion and reverting to gaseous ionization through new duction in water adsorbed on paper fibers. High-frequency methods of studying free gas space in cables.” Studies in the losses in impregnhed paper are much too large to be caused Bell Telephone Laboratories strongly suggest that the accuby simple polar orientation in the liquid impregnating commulation of gas in impregnated paper dielectrics is a fundamenpound and may occur within the fiber where the oil does not tal cause of failure, and studies of the generation of gas in penetrate. Race suggests that the increase in loss over the condensers by such processes as electrolysis, thermal breaks u m for the paper and oil separately may be accounted for in down of cellulose, and ionic bombardment are in progress. Preliminary results of this investigation were presented a t the large part by interfacial effects associated with the wetting of National Research Council’s Dielectric Symposium in Pittsthe fiber by the oil, and that a more comprehensive theory is necessary which shall include interfacial forces between the burgh (24). They indicate that the principal constituents are hydrogen, carbon dioxide, carbon monoxide, and water oil and the cellulose fibers. Attempts have been made to investigate these interfacial formed by a combination of electrolysis and ionic bombardment. effects by Kanamaru and Nakamura (14), Briggs ( 2 ) ) McBain and Foster (16)) and Urban, White, and Strassner (43). There are signs that those interested in high-voltage paper According to Briggs, the OH- is held more closely to the celluinsulation are again becoming concerned with the effects prolose surface than the H+ ion, and a zone of potential differduced by small quantities of moisture. W. A. Del Mar sugence is thus created. Measurements of the potential indigests that the problem of defining and determining the moisture cate that a potential difference of approximately 0.2 volt content of cellulosic materials be studied. He states that if it exists across the interfacial water layer approximately 10+ is true that no sharp distinction between mechanically and em. thick. This high potential gradient is accompanied by chemically held water exists, the basis for a practical demarcaa lowering of the dielectric constant of water adsorbed on the tion should be found and standardized so that we may know cellulose surface and by a high interfacial conductivity. what we are talking about when we speak of dry cellulose. Measurements of this interfacial conductivity have been Murphy and Kohman (24) report results which suggest a posmade by Stock (41) and by Stamm (38). They indicate that sible means for accomplishing this. If the gases which are the conductivity of this interfacial layer of water is perhaps pumped from undried paper are analyzed, the composition of two or three times the normal conductivity of water. the gases appears to offer a means for following the degree of dryness of the paper. During the early stages of pumping as Paper Capacitors moisture evaporates, it carries along considerable quantities of residual air. The moisture content of the gas pumped off The most severe conditions under which cellulose is regradually increases until it consists almost entirely of water. quired to operate are found in paper capacitors and in highFinally the percentage of moisture begins to decrease, and apvoltage cable, and it is in these two fields that we find the preciable quantities of carbon dioxide and permanent gases most urgent need for more information. Table I1 gives a are found. As drying proceeds, the ratio of water to carbon summary of the various properties for the important classes of dioxide approaches a constant value a t which point it seems cellulosic insulation used for control purposes and their reasonable to suppose that the physically adsorbed moisture approximate limits. I n most cases, these limits represent the may be considered zero and any further moisture removal is best opinion formed as a result of general experience and are supplied by the pyrolysis of the cellulose. not based upon experimental relations between the various I n considering the problem of drying, the problem of pyvariables and performance. Frequently when attempts are rolysis should be studied a t the same time. It seems possible made to investigate such a relation, none can be found. For that the conditions under which it is safe to dry paper could example, it is reasonable to suppose that the content of elecbe greatly extended if the thermal decomposition of cellulose trolyte in condenser tissues is closely related to such characwere more thoroughly understood. The boiling temperature teristics as insulation resistance, breakdown strength, and of water (100’ C.) is sometimes given some special significance life. Although it can be demonstrated that if sufficient elecwhen drying of cellulose is considered, and it appears to have trolytic material is present it does influence these characterisled to the practice of removing water at a temperature not tics, no relation appears when the electrolytic material falls far from the boiling point. But in relation to the drying procwithin the limits found in the product as supplied by reliable dielectric constant of the impregnant is limited by the dielectric constant of the paper sheet. Several attempts have been made to explain dielectric absorption in impregnated paper. I n particular, the theories advanced by Wagner (45) using the Maxwell relationship in two-layer dielectrics, and those of Boning ( I ) , Murphy and Lowry (26), and Race @ I ) , based upon ions bound to the surface of the cellulose fiber, are of interest. According to
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JULY, 1939
ess the boiling point is no different from any point on the vapor pressure-temperature curve, and the temperature to be used should be determined by consideration of the drying rate and final moisture content desired and the extent of thermal decomposition allowable. A large amount of apparently conflicting information exists in the literature concerning the process of thermal decomposition of cellulose and its influence on dielectric behavior. F. M. Clark (8) finds thermal decomposition to be an autocatalytic reaction with a marked effect on dielectric life. Under somewhat different conditions we have found the decomposition of condenser tissue to be a reaction which proceeds a t a decreasing rate with time and which has no easily detectable effect on dielectric properties. The conditions under which these two types of behavior may be found require further investigation. There is no doubt that the thermal breakdown can be catalytic, for it is well known that acids and certain alkali metal carbonates greatly accelerate the pyrolysis of organic materials such as cellulose and sugars. Little work appears to have been carried out on the possible influence of traces of impurities and residual salts on the thermal breakdown of cellulosic insulation. Yet, it is general practice to limit drying temperatures to 100-120" C. to avoid thermal breakdown. The rate of drying is a problem of importance. Because of the rapid rate a t which vapor pressure and diffusion increases with temperature it appears possible greatly to accelerate drying rates by drying a t the highest temperature permitted by thermal decomposition. *-50°C. X-15OoC. INSULATION RESISTANCE IN MEGOHpS/MF
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of condenser paper. It was experimentally found that a t 60" C. the rate of decomposition may be considered zero, whereas above 150" C. it becomes very rapid. A large number of experimental paper condensers were dried, pumped for various periods of time up to 20 hours at these two temperatures, and then impregnated and tested. After the longest period of heating a t 150" C., the paper was badly discolored and very brittle, and large changes in dielectric properties were anticipated. As the data in Figure 5 show, however, there appears to be no significant change in dielectric properties accompanying this extreme treatment. I n fact, some improvement in insulation resistance and breakdown voltage is indicated. Much has been written concerning the presence of strongly bound water in cellulose, and there can be little doubt that a certain m o u n t of water is strongly held to cellulose. Although few, if any, factors can account for greater changes in cellulosic insulation than water, the assumption often made that these effects may be extrapolated to very low moisture contents does not appear to be warranted. We have found that when the moisture content is reduced to less than 0.1 per cent, its influence on the properties of paper condensers, if any, is concealed by other variables. The satisfactory performance of paper condensers appears to depend upon complete displacement of gases in the structure with the impregnating compound and the maintenance of this condition. This latter requirement, however, involves numerous considerations; many of them are incompletely understood, such as the electrochemical reactions which occur a t the electrodes and the chemical changes brought about by ionic and electronic bombardment.
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