US2967986A - Electrical condenser - Google Patents

Description

Jan. 10, 1961 J. c. BALSBAUGH 2,967,986

ELECTRICAL CONDENSER Filed March 28, 1957 INVENTOR Jayson CBa/sa y/z ATTO EYS ELECTRICAL CONDENSER Jayson C. Balsbaugh, Marshfield, Mass, assignor to AMP Incorporated, Harrisburg, Pa.

Filed Mar. 28, 1957, Ser. No. 649,257

6 Claims. (Cl. 317-258) This invention relates to electrical condensers and a method of making them. This application is a continuation-in-part of Serial No. 172,272, filed July 6, 1950, now abandoned, which was replaced by Serial No. 534,076, now Patent No. 2,535,030; Serial No. 534,077, now Patent No. 2,817,880; Serial No. 534,078; and Serial No. 534,079, now Patent No. 2,882,036, all filed May 4, 1944.

Prior to my invention it has been the practice to make electrical condensers by assembling sheets of metal adapted to serve as electrodes of the condenser with interposed pre-formed sheets of dielectric materials with or without a filling of an impregnating material such as wax, oil, etc. In some cases it has been suggested to use preformed sheets of plastic such as polystyrene alone or laminated with sheets of mica; in other cases it has been suggested to coat mica with shellac or polystyrene and to use sheets of such composite dielectric for Condensers; in still other cases plastics such as polystyrene have been molded to forms such that a mass of the plastic enters between and surrounds the electrodes of a condenser.

Whereas structures such as those known and used in the prior art have proven adequate for many purposes, there has been a persistent demand for condensers of higher quality especially for a condenser which would be capable of operating at high temperatures, e.g, in the neighborhood of 100 C. or higher, and at high voltages (e.g. many hundreds or even thousands of volts) at high frequencies (from thousands of cycles up into the megacycle range) without ionization, internal discharge, or breakdown. Known condensers have not proven suitable for such uses because of the development of internal discharge, internal heating due to power factor of the dielectric, or variations in capacity due to physical changes of the condenser upon heating-to mention only a few of the more important defects.

I have found that surprisingly superior condensers satisfactory for severe service under high temperature conditions can be made by use of polyvinyl carbazole in the dielectric layer between the electrodes. Vinyl carbazole and its polymers have been known chemically for many years. A product of this nature has been sold in Germany since before the war under the trade name Luvican and in this country by General Aniline and Film Corporation.

Although polyvinyl carbazole, like many other plastics, has obvious properties desirable in a condenser dielectric its chemistry seemed to put it in the class of plastics which are unsuitable for high frequency high voltage use, and it is surprising that it has, in combination with thin sheets of refractory insulating material, given better performance in condensers for such severe service than any other comparable dielectric and displays extraordinary electrical characteristics.

Alsifilm is the suitable material for the refractory sheets which I have chosen in this combination. Alsifilm is a dielectric film made from bentonite type clay gelled and indurated by a process in accordance with the teachings of Hauser, for example, as described in US. Patents 297,936 Patented Jan. 10, 1961 2,317,685 and 2,401,348 and advantageously in accordance with the application of Fox, Kerr and Zimmerman Serial No. 594,189, filed June 27, 1956. A gel may be formed of purified white California bentonite of particle size classified to between 10 and 250 millimicrons. This gel is formed in a layer about 0.02" thick and shrinks on drying to a thickness of a few thousandths of an inch. The clay is treated with a solution containing an exchangeable ion which, by ion exchange, enters into the chemical structure of the clay and converts the inorganic gel-forming hydrous oxides into a hydrophobic compound. For this purpose one may use a resin-forming ion, e.g., a water solution of ethylene diamine diacrylate or phthalate. The hydrophobic film is washed in distilled water until free from soluble salts and is then dried and subjected to heating.

In this specification and the accompanying drawings, I am setting forth a number of particular examples and suggesting various modifications and alternatives thereof, it should be understood, however, that these are not intended to be exhaustive or limiting of the invention; but, on the contrary, are chosen and presented for purposes of illustration in order to make clear the principles of the invention and the practical employment of those principles in applying the invention to practical use; and thus so fully to instruct others skilled in the art that they will be enabled readily to modify and to select and substitute alternatives, each as may be best suited to the particular conditions of any given application or use.

In the accompanying drawings:

Fig. l is a view in cross section of two coated electrodes ready for assembly into a condenser; and

Fig. 2 is a cross sectional diagrammatic representation of a condenser made with a pre-formed dielectric coated with polyvinyl carbazole assembled between the electrodes before molding.

In the drawings, I have shown a simple condenser made for service with high voltage at high frequency which is able to serve satisfactorily under relatively high temperatures. In the making of this condenser, the electrodes 10 and 12 are cut to appropriate shape, with integral lead strips 14-, 16 from thin sheets of metal or foil, e.g. of copper. Each of these plates 10 and 12 is covered with a coat 18 of polyvinyl carbazole,.which coat has thoroughly wet, and adheres to, the entire surface of the p ate.

This coat 18 is best applied as a lacquer made up of the polymer dissolved in three times its weight of toluene to give a varnish consistency. To this is added 2% to 15% of a suitable plasticizer to give a smooth plastic flow during the subsequent molding step and reduce the brittleness of polyvinyl carbazole, but which does not substantially increase the internal heating of the condenser e.g., b y ionization or polarization under conditions of operation.

In a condenser to be used on direct current or low frequency A.C., any of the plasticizers known to be effec tive with polyvinyl carbazole, e.g. tricresyl, phosphate, dioctyl phosphate, triphenyl phosphate, etc., could be used. For high frequency use, however, it is advantageous to use non-polar compounds as the plasticizer. For this purpose I have found particularly suitable higher hydrocarbons, particularly amyl naphthalene, diphenyl or terphenyl, and I have used especially such a plasticizer which is sold commercially under the designation HE-40.

The metal sheets 10 and 12 are thoroughly cleaned before coating to assure thorough wetting by the polyvinyl carbazole lacquer. Thus, for example, the sheets are first washed with benzene and then with a solution of hydrochloric acid and are then etched in a nitric acid solution made by dissolving one part of concentrated nitric acid in three parts of water. The etching continues until any burrs at the edges of the sheet have been dissolved away leaving a microscopically blunt edge. In practice this is indicated when the surface gloss is cut to a fine matte appearance. The foil is then removed from the etching solution and washed thoroughly with distilled water, dried in warm, dry air and coated with a thin film, 0.0002 or less, of the polyvinyl carbazole lacquer.

This lacquer is brushed or sprayed and flowed onto the surface of the metal plates and 3.2 so that the surfaces are thoroughly wet by the lacquer and the plates are then supported in a horizontal position allowing the lacquer to flow into a level film and are thoroughly dried in this position, removing all traces of the solvent. if desired, additional coats of the lacquer may be applied to assure complete and perfect covering.

The Alsifilm is then similarly coated with the polyvinyl carbazole e.g., using a lacquer made by dissolving in toluene about one-third of its weight of the polyvinyl carbazole and enough plasticizer, e.g. 2% to to assure a smooth flow under the molding temperature and pressure. This is applied in a thin, uniform coat to give when dry, a film of 0.0001 or 0.0002 inch. The solvent is thoroughly removed and with their coatings thus dried,

a number of the coated sheets are assembled with the coated metal sheets 12 (which serve as electrodes, etc.) in a desired sequence and relationship, e.g., 3 to 10 sheets of coated Alsifilm between each two adjacent sheets of metal. The assembly is placed in a mold, heated to a temperature sufficiently high to soften the coating to a condition capable of smooth, plastic flow under high pressure, and subjected to the very high molding pressure, e.g. 500 pounds per square inch.

It has been found that the results achieved in use with condensers embodying this invention exhibit some substantial variation depending upon the particular polyvinyl carbazole polymer used and the degree of polymerization thereof. This degree of polymerization is designated in the art by K-value or characteristic visin which c=grams of polymer in 100 cc. of solvent, the K parameter is independent of the concentration and is named characteristic viscosity. For writing convenience, K is designated as K 10 Considering various grades of polyvinyl carbazole polymers thus designated by their K-values, it has been found that condensers made according to the teachings hereof by using the lower polymers such as K or others below approximately K-40 may produce condensers superior to those previously known, yet when, tested for high frequency high voltage use, are not free from ionization as are those made with the higher polymers according to this invention. These enhanced results are achieved in condensers embodying the polymers of K- and higher, and especially by using polymers of K-53 to K- and even as high as K450, 14-155 and higher provided such polymers are mixed with suitable plasticizers or otherwise treated to have a moldable plastic flow within the range of molding temperatures utilized, under the pressure conditions of the molding operation; and provided also that such plasticizing is accomplished without substantial interference with the dielectric properties of the material or substantial increase of the sus- 4 ceptibility of the finished product to ionization. Within the broader range, the polymers in the range K-45 to K-70 are surprisingly superior.

As illustrative of the variations and results achieved by using polymers of varying degrees of polymerization, the following examples are given:

Five capacitors of a size 1 /2 x 1 /2 each comprising 5 electrodes and 24 sheets of Alsifilm were molded using as the bonding agent a mixture of 7 grams K-42 polyvinyl carbazole, 1.4 grams HB-40 plasticizer, 86.6 grams toluene solvent. All five showed ionization or internal discharge with 60 cycles per second, at room temperature and 3.0 kv. (R.M.S.). At 1000 cycles they exhibited a 0.27% power factor and capacitances of from 749 to 796 rnicromicrofarads. All lower polymers tried in this combination likewise showed internal discharge.

By contrast, 5 3" x 3 capacitors were molded identical with those above except for using as the bonding agent a mixture of 14.0 grams l;44.5 polyvinyl carbazole, 218 grams HB-40 plasticizer and 200 ml. toluene solvent. All five of these K44.5 capacitors when subjected to the same test were clear of internal discharge, exhibited from 0.295% to 0.325% power factor and capacitances of from 3489 to 3653 micromicrofarads;

Another 5 capacitors of the 3" x 3" size with 5 electrodes and 24 sheets of Alsifilm were molded using as a bonding agent 12 grams of polyvinyl carbazole having a K-value of 42 dissolved in 172 grams of toluene but without the addition of a plasticizer. All 5 showed internal discharge at 60 cycles per second and 3.0 kv. (R.M.S.) and at 1000 cycles, they exhibited a power factor of 0.17 and capacitances of from 2900 to 3050 micromicrofarads.

Capacitors of a size 3 x 3 also including 5 electrodes and 24 sheets of Alsiiilm were molded with a bonding agent comprising 12 grams K-69 polyvinyl carbazole, 2.4 grams HB-40 plasticizer and 144 grams of solvent. These capacitors were clear of internal discharge under the same test, exhibited 0.26% power factor and capacitance of approximately 2809 micromicrofarads.

Illustrative of still higher degrees of polymerization of polyvinyl carbazole, 1 /2 x 1 /2 capacitors were molded using as the bonding agent 7 grams K-150 polyvinyl carbazole 1.4 grams HB40 plasticizer (20% concentration) and 174 grams toluene solvent. These exhibited a power factor of 0.13% and capacitance of 587 micromicrofarads, and all ionized under the same test, showing, on the oscilioscope screen, extensive departures from the proper line and of wide amplitude. By contrast, similar capacitors of 3 x 3" size and embodying a bonding agent of 10.5 grams sf-150 polyvinyl carbazole, 3.2 grams HB-40 (30% concentration) and 3 cc. toluene, as solvent, molded under similar conditions exhibited power factors ranging from 0.295%.to 0.33% and capacitances ranging from 2926 to 3328 micromicrofarads; but, of those produced from this composition, were free of ionization or internal discharge under the above test, thereby indicating that even these very high polymers are susceptible of use with this invention provided they are plasticized sufficiently for adequate plastic flow in the molding step.

in considering the foregoing data in terms of, for example, capacitance per unit area, it should be noted that the 3 x 3" capacitors mentioned had electrodes which are actually 2 /2 x 2 /2 in effective area, with 5 electrodes in each capacitor. In the case of the 1 /2 x 1 /2" capacitors, the effective areas of each of the 5 electrodes included therein was approximately one square inch. Also regarding the foregoing data, it should be noted that satisfactory condensers made in accordance with this invention may have capacitance ratings of, for example, to 200 rnicroinicrofarads per square inch of effective area, that the capacitance persquare inch will vary depending upon the thickness of Alsifilm and/or the bonding agent utilized, as well as, perhaps, on other factors. The unexpected results achieved by using polyvinyl carbazoles of a K-value at least as high as about 45 and adequately plasticized are indicated particularly in the foregoing data by the freedom from internal discharge or ionization under the conditions to which the capacitors are subjected in use.

It should be noted that the molding temperatures utilized according to this invention are within ranges of from 150 C. to 300 C. and preferably within the range of 225 C. to 250 C. Accordingly, the amount of plasticizer is selected so that the polyvinyl carbazole bonding agent will, at these molding temperatures and under a feasible pressure, have a sufficient degree of plastic flow, to form a uniform and continuous uninterrupted bond with the various layers of the condenser being molded, and to squeeze out excess polymer from between the layers of Alsifilm and electrodes yet will not be so fluid as to permit the layers of Alsifilm and metal to come into direct contact and leave a discontinuous interface by excessive extrusion during molding.

The presence of ordinary polymerization catalysts in the polyvinyl carbazole is generally undesirable when the polymer is used for condensers in that it may cause an increase in the electrical losses. Accordingly, it is preferred to use previously formed polyvinyl carbazole polymer as the catalyst and avoid the addition of the ordinary non-volatile polar compound catalysts.

One of the important advantages of the polyvinyl carbazole is the effectivene s with which it wets and adheres to the surfaces of the electrode, thus assuring that no interface defect occurs which may result in ionization or local variations in capacity or other defects in the action of the condenser. There is evidence that the polyvinyl carbazole reacts to some extent with the copper at the surface, when copper plates are used as the electrodes. If, in any case, this proves objectionable, the copper can be thinly coated with amalgam or a flash of gold, or othere corrosion resistant metal or a thin alloy may be formed at the surface of the copper. In most cases, however, this does not appear to be necessary and such reaction as may occur appears to'assure adhe ion. With silver, tin, aluminum, etc., no reaction is apparent.

The known theories about the present conden er do not appear to account entirely for the improved characteristics. It may be that a certain amount of interfacial polymerization occurs at the boundary of the polyvinyl carbazole and the Alsifilm due to reaction between the polyvinyl carbazole and the surface molecules of the Alsifilm, or it may be that the surfaces of the polyvinyl carbazole and the refractory film have complete miscibility at the higher polymerization, which is reached only during the heating of the condensers.

Condensers made in accordance with the present invention are operated constantly at temperatures as high as 125 C., voltages from 6000 to 25,000 volts, and at high frequencies of the order of kilocycles or megacycles. The condensers are characterized by stability, relatively low energy absorption due to power factor of the dielectric material, relatively high capacity, and absence of ionization at high stresses.

I claim:

1. An electrical condenser which comprises overlapping spaced electrodes, a plurality of layers of indurated bentonite of uniform thickness between said electrodes and extending beyond the edges of said electrodes, and an adequately plasticized polyvinyl carbazole resin covering and pressed into intimate conformity and adhesion with the adjacent surfaces of the electrodes and of said indurated bentonite layers, forming therewith a dense body of high electrical breakdown strength and low loss, said polyvinyl carbazole resin including polyvinyl carbazole having a softening range providing plastic flow under pressure within a temperature range of approximately to 300 C. and having a K-value as herein defined at least about 45.

2. An electrical condenser as defined in claim 1 in which said polyvinyl carbazole resin has a softening range providing pl stic flow under molding pressure of approximately 225 to 250 C.

3. An electrical condenser as defined in claim 1 in which said polyvinyl carbazole resin has a K-value within the range of approximately 45 to 155.

4. An electrical condenser as defined in claim 1 in which said polyvinyl carbazole resin has a K-value within the range of approximately 45 to 70.

5. An electrical condenser which comprises a continuous layer of indurated bentonite of uniform thickness throughout the effective area of said condenser, a continuous bond of an adequately plasticized polyvinyl carbazole resin coating each of said layers and thereby cementing each of said layers to the adjacent layers, and at least two spaced electrodes bonded thereto with said polyvinyl carbazole resin. said indurated bentonite layers being located between said electrodes, and said polyvinyl carbazole resin including polyvinyl carbazole of a degree of polymerization designated by a K-value as herein defined at least about 45.

6. An electrical condenser which comprises a continuous layer of indurated bentonite of uniform thickness throughout the effective area of said condenser, a continuous bed of an adequately plasticized polyvinyl carbazole resin coating each of said layers and cementing each of said layers to the adjacent layers, and at least two spaced electrodes bonded thereto with said polyvinyl carbazole resin, said indurated bentonite layers being located between said electrodes, and said polyvinyl carbazole resin including polyvinyl carbazole of a degree of polymerization designated by a K-value as herein defined of at least about 45 and higher than that which will give substantial plastic flow under molding pressure at 150-350 C. and a plasticizer therefor giving said polyvinyl carbazole plastic flow under said pressure at molding temperature and pressure.

References Cited in the file of this patent UNITED STATES PATENTS 2,535,030 Balsbaugh Dec. 26, 1950

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