Oct. 17, 1950 F. M. CLARK 2,526,330 ACETATE COATED DIELECTRIC FOR ELECTRICAL DEVICES Filed March l, 1946

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UNITED STATES PATENT OFFICE 2,526,330 CELLULOSE ACETATE COATED DELECTRC PAPER FOR ELECTRICAL DEVICES Frank M. Clark, Pittsfield, Mass, assignor to Gen eral Electric Company, a corporation of New York Application March 1, 1946, Serial No. 651,134 2 Claims. (C. 174-113) 1. The present application is a continuation-in breakdown will occur in such void spaces. Also, part of my prior application Serial No. 523,475, tapes or sheets of this material slide and wrinkle fled on February 22, 1944, now abandoned, and during application, resulting in regions of weak comprises improved insulating and dielectric ma ened dielectric strength. terial for electrical devices, To improve their machanical properties, cellu In particular, my invention comprises sheet lose acetate or similar cellulose lacquer products products which consist of cellulosic material, such commonly are loaded with plasticizers. When as manila, linen, cotton or , pressboard, brought into contact with impregnating or cool or other stock made of cellulose , which has ing liquids, the plasticizers become dissolved and, adherent on opposite surfaces thereof thin, super 10 being poor dielectrics, result in deterioration of ficial films consisting of non-fibrous and non-por the insulation. Also, regardless of its state of ous cellulose lacquer. These superficial films are plasticity, cellulose acetate dielectric and insulat separable from the paper while being strongly ing elements are not readily impregnated due to adherent. their dense structure and absence of capillary The accompanying drawing shown in Fig. 1 is s pOres. a slide elevation of a capacitor containing dielec I have discovered that cellulosic sheet material, tric sheet material embodying my invention; Fig. Such as ordinary insulating paper, when coated 2 conventionally represents a transformer as ill on exposed surfaces with Superficial films consist lustrative of a device containing insulation em ing of cellulose lacquer which are so thin as to bodying the present invention; Fig. 3 is a graph be almost imperceptible and still possessing the showing aging characteristics at elevated ten desirable physical properties of paper, possesses perature; and Fig. 4 is an enlarged sectional view marked advantages both over uncoated paper and of a capacitor structure fabricated under heat over sheet material consisting solely of cellulose and pressure. lacquer. The term "cellulose lacquer" will be In electric devices, as for example transformers, used herein to include cellulose ester, as, for ex capacitors and cables, it has been found desirable ample, cellulose acetate and cellulose ether, such to employ fibrous cellulosic sheet material, such as ethyl or benzyl cellulose. as paper, for spacing, insulating purposes and In its more specific aspect, the improved di dielectric purposes. Although paper in many re electric material embodying the present invention spects is well Suited for such purposes, it possesses 30 comprises paper or similar fibrous cellulosic sheet SOe disadvantageous properties. In particular material which is provided on opposite sides with the electrical and physical properties of paper de adherent but separable films of cellulosic lacquer. teriorate rapidly at elevated temperatures. For ex Such films preferably should have a thickness ample, electric capacitors containing dielectric within the range of about 0.00001 to 0.001 inch, Spacers consisting of paper when operated at ten 35 and should be unplasticized or otherwise unmodi peratures above about 85 C. are characterized by fled. The protective effect of the cellulose lacquer undesirably short life due to deterioration of di coatings on the paper permits the operation of electric properties of the paper. The mechanical coated dielectric and insulating material at sub strength of cellulosic material also is rapidly re stantially higher temperatures with less deteriora duced by continued exposure to elevated tempera O tion than resulting when uncoated paper is oper tures. This fact has imposed a maximum operat ated at the lower temperatures heretofore main ing temperature of about 85 C. on electric trans tained. formers containing paper, pressboard, or other Surprisingly, the apparently impermeable sur form of fibrous cellulose as insulating and spacing face coating of Such composite cellulose sheet ma material. terial does not prevent subsequent impregnation Sheet material consisting of cellulose acetate with liquids. Impregnating liquid passes through has been found to be more temperature-resistant the films into the pores of the paper even though than fibrous cellulosic material. However, the ap the films appear to be continuous in structure, plication of cellulose acetate sheet material to that is, free from openings. electric devices is accomplished by mechanical 50 As another consequence of my invention, the and other difficulties. In the first place, cellu total thickness of dielectric and insulating ela lose acetate sheets or tapes are difficult to wind ments may be reduced. or otherwise handle. It is almost impossible to In the preparation of my improved insulation, guard against the presence of voids or "air pock the paper sheet, tape, or other cellulosic product, ets" between sheets. As is well known, electrical is passed through a suitable solution of cellulose

2,520,880 3 4. ester or cellulose ether. Acetone is satisfactory The cause of such early and progressively ac as a solvent. The paper base should have a den celerated deterioration is not understood but may sity greater than about.85 and commonly in the be due to a minute amount of hydrogen chloride range of .9 to 1.1. The space of the pores should being disengaged from the chlorinated hydro amount to less than one hundred per cent of the carbon. Whatever its cause may be, the deter space occupied by the alone. For use in ioration is prevented or, in any event, is mate capacitor manufacture, paper having a thickness rially reduced, by the presence of thin adhering in the range of 0.0003 inch to 0.001 inch is satis films of cellulose lacquer on the surfaces of the factory. For use in transformers and cables, paper spacers. paper having a thickness of 0.001 inch to 0.005 O Paper coated in accordance with my invention inch may be used. The is passed also initially has greater dielectric strength than through an air-drying column of sufficient height uncoated paper. Tests made with alternating to completely remove the solvent. I have found Current for sheets comprising .00035 inch kraft that with the paper sheet or tape traveling at a capacitor tissue as a base material and being speed of three feet per minute in a drying column coated with cellulose acetate show the following three to four feet long which is maintained at a improvement in dielectric strength:

temperature of 125 to 160° C., satisfactory coat ings are obtained. The thickness of the cellulose Thickness of Dielectric acetate coating on the paper is in part deter Coating for Strength Per Cent mined by the speed with which the paper is 20 Each Side in Wolts Increase passed through the solution and drying tower of Sheet per Mill and in part by the concentration of the solution. Inches Assuming a fixed speed of three feet per no coating 1,000 minute, the thickness of the coating applied to 0.000017 1,050 0.00025 1,200 the paper is determined by the concentration of 0.000045 1,300 an acetone solution as indicated by the following 0.000050 1,500 illustration: It has been the practice to supply capacitors Per Cent Thickness of Cellulose Coating for the 330 volts A.-C. field having a dielectric Acetate in Opposite of .00095'' in total thickness (two sheets of .0003' Acetone Surfaces and one sheet of .00035' kraft capacitor tissue). Utilizing .0003' kraft paper as a base, coated with Inches 1.0 00001. cellulose acetate film .000015' On Opposite sides 20 .000025 (total sheet thickness .00033'), an equivalent 3.0 .000035 dielectric pad consists of only two sheets of in 4.0 .000050 sulation. The total thickness, therefore, is only .00066', a reduction of approximately 33 per cent A coating of 0.0005 inch, which is preferred for from the thickness heretofore found to be re most purposes, may be obtained by employing a quired. As a consequence, capacitors embodying speed of two feet per minute with a ten per cent 40 such dielectric material have approximately 20 concentration. per cent greater electrical capacity per cubic inch The coated paper, after being prepared and of physical bulk than conventional capacitors. dried, is susceptible to impregnation with a liquid Capacitors containing lacquer-coated paper in accordance with the usual and well-recognized 45 dielectric, when vacuum dried and impregnated practice. with pentachlor diphenyl, are characterized Fig. 1 shows a conventional wound-type elec with a dielectric loss above room temperature tric capacitor comprising metal foils , 2 and substantially lower than from that of ordinary interleaved dielectric spacers 3, 4 of sheet mate paper spacers similarly treated. Typical data rial embodying my invention. The metal arma are given in the following tabulation showing the tures , 2 are provided with terminals 5, 6. Such dielectric loss (expressed as power factor) of assembly is commonly placed in a casing, which, kraft paper sheets coated with cellulose acetate however, has not been illustrated, and is in to a thickness of .000015' per side: pregnated with a suitable dielectric material which normally is liquid Or Solid. 55 For some applications it is necessary, or in any Powerover FactorTac it."incoate event desirable, to operate electric capacitors at Temperature Nii Paper temperatures in the range of 100 to 125 C., and Insulation at even higher temperatures. At such elevated oC. Per Cen Per Cent temperatures conventional types of paper-spaced 2S .36 .36 s .31 . 44 capacitors rapidly deteriorate as heretofore in 100 .33 .62 dicated. For example, capacitors containing 12S .56 1. 05 kraft paper spacers and impregnated with chlo rinated aromatic hydrocarbon compositions as Although the advantages resulting from in described in my prior Patent 2,041,594 of May 19, proved dielectric material embodying the pres 1936, have good stability and a long life of thou ent invention have been stated with particular sands of hours at operating temperatures up to reference to capacitors, improved results are also about 85' C. When such capacitors are operated obtained in other electrical apparatus, for ex at an impressed potential of about 1000 volts ample in transformers and cables. A trans D.-C. at 110° C., they have an average life of only 70 former thus insulated is conventionally illus about 100 hours. trated in Fig. 2. During operation at elevated temperatures, the Transformer windings are subject to severe power factor increases and the electrical resist mechanical stresses and strains during opera ance decreases. These changes are an indication tion due to high voltages imposed momentarily of deterioration of the paper dielectric elements. 5 by accidental conditions. As paper is commonly 8,520,880 used as insulating and spacing elements in trans loss of mechanical strength in eight weeks, formers, it is necessary to impose an operating coatings on opposite sides of 0.0005 inch cellu temperature limitation to prevent excessive loss lose acetate films will extend the life to 25 weeks. of mechanical strength of such elements. Ac As contrasted with the short life of uncoated cording to well established engineering practice, kraft paper as indicated by graph 7, kraft paper it is necessary to limit the maximum tempera coated on opposite sides with films of cellulose ture of the oil or other cooling liquid to about acetate of 0.0005 inch thickness would have a 85° C. in order to insure that cellulosic insula life calculated by extrapolation as long as 6000 tion of the device will not become prohibitively weeks (nearly 120 years) before complete loss deteriorated. It has been found that the rate 0 of tensile strength occurred. of deterioration of mechanical strength of paper In transformers containing chlorinated hy insulation increases approximately twofold for drocarbons of the nature described in my prior each temperature rise of 10 C. above 85 C. Patent 1931,373, patented October 17, 1933, the Another source of deterioration in the dielec occurrence of an electric arc in contact with tric strength and insulating qualities of paper s the halogenated insulating and cooling liquid insulation is traceable to the migration of cop would form corrosive products which would at per particles from the conductor into the dielec tack the cellulosic insulation of the transformer. tric media. This migration is accelerated by Although electric arcs in transformers indicat temperature rise above the 85 C. top limitation ing a breakdown or failure in the transformer now applied. fortunately are extremely rare, the occurrence Composite cellulosic insulation embodying my of such an arc with its accompanying genera present invention possesses at elevated temper tion of halogenated compounds might, when or atures a surprisingly high mechanical stability dinary paper is employed as insulation, render and also resistance to metal migration. For ex the apparatus entirely unfit for further use. ample, transformers containing electrical insu Such occurrence, however, would not destroy in lation (not shown in Fig. 2) consisting of kraft sulation consisting of paper coated with cellu or manila paper which has been coated on op lose lacquer in accordance with my present in posite sides with cellulose acetate films having vention. A thin layer of cellulose acetate pro a thickness of about .0005 inch can be operated tects the underlying paper from an attack of at temperatures at least as high as 105 C. with 30 acid. The repair of the transformer, therefore, out greater loss of mechanical strength of the becomes a simple operation such as commonly insulation than characteristic of ordinary un- . carried out with transformers containing min filmed paper at 85° C. Fig. 3 shows graphs il eral oil as insulating and cooling medium. lustrating the progressive loss with time of the In electrical apparatus such as transformers, mechanical strength of ordinary and lacquer 35 bushings and electric cables, it is highly desir coated kraft paper when submerged in trans able, that the insulation should be protected former oil at an even higher temperature, from access of moisture, . It has been found that namely, 140° C. The coated kraft paper is pro paper coated in accordance with the present in vided on opposite sides with films of cellulose vention with thin layers of cellulose acetate or the acetate having a thickness of about 0.0005 inch. 40 like so materially retards the absorption of The graphs are shown on semi-log paper, the moisture that a substantial advantage is ob ordinates showing the decrease of tensile strength tained. and the abscissa the time in weeks plotted in It is not necessary in all cases that the com logarithms. r posite dielectric material constituting my in Graph T shows that ordinary uncoated kraft 45 vention should be impregnated with a liquid or paper at a temperature of 140 C. loses one-half liquefied material. For various uses, as for ex of its tensile strength in about two weeks. Sin ample for low voltage, direct current filter cir ilar kraft paper provided with superficial lac cuits such as employed in radio devices, it is quer films of 0.0005 inch thickness as shown by desirable that capacitors should be used which graph 8, under the same conditions and time, 50 do not contain a liquid impregnant. Hereto loses about 30 per cent of its tensile strength. fore, considerable difficulty has been encountered Kraft paper coated with lacquer films of 0.001 to provide capacitors which have properties well inch thickness, as indicated by graph 9, loses a adapted for Such field of use. When capacitors little over 20 per cent of tensile strength in two for this type of service are impregnated with weeks. However, in about thirty weeks the un- 5, conventional waxy materials, they are not coated paper has undergone complete disinte adapted for Operation at temperatures exceed gration while the lacquer-coated paper has lost ing the melting points of the impregnants which approximately half of its tensile strength. normally fall in the range of about 80 to 100° C. It is permissible to carry the loss of tensile For many fields of application it is desired that strength of paper insulation in transformers to 50 capacitors should operate at temperatures in the about a 75 per cent loss. As shown by graphs range of 90 to 100° C., and even as high as 150° C. 8 and 9, this would permit operation for about Capacitors containing ordinary unimpregnated 300 weeks (or six years) at even the relatively paper are undesirable because the paper has a high temperature of 140° C. No advantage is relatively high air content and resulting low obtained by a film thickness of 0.001 inch over electrical capacity. a thickness of 0.0005 inch. For temperatures in In preparing electrical capacitors containing the range below 140° C. and down to 85 C., the the herein-described new dielectric materials, permissible operating period would be corre the armatures 0, -f, Fig. 4, are assembled with spondingly longer. interleaved coated layers of paper 2, 3, the The advantages accruing from my present in U number of layers depending upon the field of vention are very much more marked in the case use for which such capacitors are designed to of lacquer-coated kraft paper than in the case operate. The elements so assembled are wound of lacquer-coated manila paper. Under elevat as usual upon themselves, then clamped tightly ed temperature conditions causing uncoated and heated to a temperature of about 150 to 160 manila paper submerged in oil to suffer complete C. for a period dependention the size of the 2,520,830 7 capacitors. In general, a heating period of about ductors from one another, said material con six hours is satisfactory. At a Somewhat higher sisting of cellulosic paper having a density with temperature of 175 to 180° C., a shorter period, in the range of about 0.85 to 1.1 and being pro for example two to four hours, is satisfactory. vided on opposite sides with adherent coatings This heat treatment causes the film of cellulose consisting of cellulose acetate having no modi acetate or the like on the paper to Soften and fying ingredients and having a thickness within cement the layers of paper together as indicated a range of 0.00001 to 0.001 inch and a liquid in on a magnified scale by Fig. 4. The result is a sulating material permeating said sheet material. solid product of interleaved armatures and di 2. An electric device containing Conductors electric material in which each turn is cemented O charged normally with unlike electric potentials, to adjacent turns by the films of cellulose lacquer. sheet insulation therebetween, Sald insulation Capacitors thus made operate with low power consisting of kraft paper having a density of factors, namely about .15 per cent at room tem about .85 to 1.1 and being provided on opposite perature and no more than about .22 per cent at surfaces with adherent films consisting of cellu 150° C. The insulation resistance of such capaci s lose acetate having no modifying ingredients and tors is relatively high, being about 15,000 having a thickness of about 0.0005 inch and an megohm-microfarads at room temperature. insulating, impregnating liquid permeating said Capacitors thus made, when possessing a di paper. electric element consisting of two sheets of 0.0004 RANK. M. CARK, inch kraft paper coated on opposite sides with thin layers of cellulose acetate as above-de 20 REFERENCEs CITED scribed, function indefinitely either on 600 volts D. C. at 100° C. or at 5 volts O. C. at 150 C. The following references are of record in the Capacitors which are otherwise similar but in fle of this patent: UNITED STATES PATENTS which the dielectric consists of paper impreg 25 nated with wax or oil, operate at 150° C. with Number Name Date a continuously deteriorating dielectric char 1,554,895 Sutherland ------Sept. 22, 1925 acteristic and with correspondingly short life. 1,607,090 Leonard ------Nov. 16, 1926 What I claim as new and desire to Secure by 1921,086 Macdonald ------Aug. 8, 1933 Letters Patent of the United States is: 30 2,295,958 Lutz ------Sept. 15, 1942 1. An electric device containing conductors 2,307,488 Clark ------Jan. 5, 1943 charged normally with unlike electric potentials, 2,320,922 Ford ------June 1, 1943 sheet material Spacing and insulating Said con

Certificate of Correction Patent No. 2,526,330 October 17, 1950 FRANK M. CLARK It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Column 1, line 50, for the word “accomplished” read accompanied; - and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Office. Signed and sealed this 20th day of March, A. D. 1951.

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THOMAS F. MURPHY, Assistant Commissioner of Paten.