US2377630A - Stabilized dielectric composition - Google Patents

Description

June 5, 1945. J. L. HYDE I STABILIZED DIELECTRIC COMPOSITION Filed June 14, 1945 ol IMES L.HY.DE

ATTORNEY Patented June 5, 1 945 2,377,630 STABILIZED DIELECTRIC COMPOSITION James L. Hyde, Williamstown, Mass., assignor to Sprague Specialties Company, North Adams, Mass, a corporation of Massachusetts Applicatlon June 14, 1943, Serial No. 490,777

' (Cl. 252--63.2i

2 Claims.

This invention relates to new dielectric compositions which may be used for extended periods of time and under widely varying conditions without appreciable deterioration. More particularly it refers to chemical dielectric compositions stabilized with a class of organic chemical compounds which have been found to be of outstanding value for this purpose.

Heretofore, electrical condensers, cables, transformers, etc., have been manufactured with dielectric compositions which deteriorate under an electric field, thereby producing undesirable prodnets and causing serious breakdowns and inconvenience. This deterioration is in large part due to the formation of gases in the dielectric composition. Gas formation is particularly pronounced under high field strength, This evolution of gas produces bubbles in the insulation through which an arc may be established causing failure of the condenser or other electrical element containing the gas-evolving dielectric composition V It is an object of the present invention to oversome the foregoing disadvantages and others which directly or indirectly result therefrom.

A further object is to produce dielectric compositions which may be used for extended periods of time and under widely varying conditions without appreciable deterioration in their dielec trio characteristics.

A still further object is to produce stabilized dielectric compositions wherein the evolution of gas during use is reduced markedly and wherein such gas as is evolved is rendered substantially harmless by suitable treatment. Additional ob-' Jects will become apparent from a consideration of the following description and claims,

These and other objects are attained in accordance with the present invention wherein a'dielectrio composition is produced containing a dielectric and a stabilizer adapted to retard the deterioration of the dielectric. In a more restricted sense this invention is concerned with stabilized dielectric compositions wherein the stabilizer is adapted to retard the formation of deleterious gases and to reduce the detrimental effects of such gases as may be formed. In a still more restricted sense this invention is directed to dielectric compositions stabilized with hydrocarbons of the aromatic series. In a still more restricted sense this invention pertains to dielectric compositions containing one or more liquid organic dielectric materials anda stabilizer therefor which is adapted to retard the formation of gases under conditions of'use and to reduce the deleterious effect of such gases as are formed, such stabilizer comprising a hydrocarbon predominantly aromatic in character and having a molecular weight less than 300, said aromatic hydrocarbon containing preferably at least two rings of carbon atoms. In its preferred embodiment this invention pertains to dielectric compositions containing one or more liquid organic dielectric materials intimately admixed with a stabilizer therefor comprising a hydrocarbon predominantly aromatic in character having a molecular weight less than 300 and having at least two rings, all but one of which rings may be nonbenzenoid, but all non-benzenoid rings should preferably each contain a double bond in alphaposition to a benzene ring or otherwise conjugated thereto; for example, hydrocarbons of the naphthalene and indene series with side chains which are either comparatively short or nonexistent. Additional embodiments of this invention concern condensers, transformers, cables and related electrical elements containing the fore-' going dielectric compositions as well as others suggested thereby or related thereto.

Numerous dielectric compositions are known which in use, particularly under high field strengths, produce gases such as hydrogen, low molecular weight hydrocarbons and the like. The evolution of the foregoing and related gases produces gas bubbles in the condenser or other electrical element and causes a high power factor or high leakage current therein. Furthermore, dielectric compositions consisting of halogenated hydrocarbons, when used under high field strengths, will evolve a certain amount of hydrogen halide which causes the conductivity of the dielectric material to increase.

It has now been found that by adding to said dielectric compositions or mixtures thereof a small amount of an aromatic hydrocarbon selected from the types previously mentioned, the evolution of these deleterious gases is appreciably reduced. Furthermore, such gases as are evolved are prevented from detrimentally afiecting the electical element by the presence of the compounds herein described.

Compounds which produce the desirable stabilizing effect previously referred to are, for example, aromatic hydrocarbons such as anthracene, benzene, biphenyl, diphenylene, indene, naphthalene, diphenylmethane, triphenylmethane, phenanthrane, stilbene, and their comparatively low-molecular-weight hydrocarbon derivatives, including such partially hydrogenated derivatives as may retain the predominantly aromatic character of their parent compounds. Ex-

2 I as'mcso amples of such applicable derivatives are toluene,

particularly advantageous for use under a wide range of conditions.

Those substances which are the most unsaturated and which have the highest vapor pressure are frequently of more value in this connection than their homologues and derivatives which are more saturated or have lower vapor pressures. In employing these compounds it is advisable, from the standpoint of economy, to select one or a mixture thereof having a sumciently low vapor pressure to permit impregnation of the condenser or other electrical element at an elevated temperature without undue loss of the stabilizing agent or agents. Furthermore, it is advantageous to so select the conditions of impregnation that the stabilizing agent will not on evaporation condense in locations wherein it cannot be of maximum effectiveness.

Naphthalene has been found to be of outstanding value for this purpose since it fulfills the foregoing requirements to a surprising degree and possesses dielectric properties which are sufficiently satisfactory to prevent undue alteration of the dielectric characteristics of the final composition. It has a high degree of unsaturatic-n, a sufficiently high vapor pressure to be effective in reducing gas formation, and in addition, the vapor pressure is not so high that efficient impregnation of the condenser or other electrical element is defeated.

Impregnation of condensers, cables, etc., has been heretofore described and for the sake of brevity will not be referred to herein in detail. Sifilce it to say that the condenser, cable or other electrical element is dipped in a molten dielectfic composition or subjected to vapors of tially eliminate formation of gas in the condenser, cable or other electrical element. If a high dielectric constant impregnant is used, the stabilizer should be present in an amount small enough to prevent any marked decrease in the effective dielectric constant of the impregnant. As a general rule, amounts of stabilizing agents varying from about 1% to about 10% by weight of the impregnant should be sumcient, although it is to be understood that the invention is not restricted thereto. In the case of naphthalene, amounts within the range of about 4% to about 5% by weight of the impregnant have been found to give surprisingly effective results.

In order to avoid excessive loss of the stabilizing agent or agents where they have appreciable vapor pressures at the temperature of impregthe dielectric compositions, preferably under re'- I duced pressures. The resulting product may be subsequently treated under such conditions that polymerization of polymerizable materials present in the composition may be accomplished. Particularly satisfactory methods of impregnation are referred to in U. S. application Serial No. 489,475, filed on June 3, 1943, by Nelson E. Beverly, entitled Method of impregnating electrical capacitors. By following the instructions of this copending patent application, condensers and other electrical elements having excellent impregnation with consequent improvements in their dielectric properties may be obtained.

The instructions of the foregoing copending patent application as well as any other method of impregnating condensers and electrical elements may be applied to the present invention. In following such instructions it is to be understood that the impregnating composition has admixed with it but a small quantity of the stabilizing agents embraced herein. The amount of stabilizing agent or agents utilized should be sufiicient to effectively decrease or to substannation, as in the case of naphthalene, as an illustration, the impregnating liquid should advisably be dried thoroughly under vacuum at an elevated temperature before adding the stabilizer thereto. This addition may be accomplished by opening a small container filled with the proper amount of stabilizer and passing the contents thereof into an evacuated tank containing the dried impregnating fluid, while excluding air, moisture, etc., from the system. The tank containing the impregnating fluid should advisably be equipped with suitable stirring means for obtaining intimate and homogeneous. admixture of the dielectric composition and the stabilizer. Alternatively, a concentrated solution of the stabilizer in the dried impregnating liquid may be first prepared and an appropriate amount of this solution passed through a valve into the evacuated tank containing the main body of impregnating liquid, care being taken to exclude air and moisture during the process. After the impregnating liquid and the stabilizer have been intimately and homogeneously admixed they are run into the tank containing the condensers or other electrical element which are to be impregnated, or in lieu thereof the electrical elements are lowered into the tank containing the stabilized impregnating liquid.

Where the stabilizer is a solid, such as naphthalene, it is desirable to prevent it from condensing on the outer, cooler portions of the finished condenser or other electrical element withthe resulting impoverishment of the inner warmer portions thereof. An addition to the impregnant 01 a small amount of a suitable solvent having sufficiently high vapor pressure, will prevent this undesirable result. The solvent, condensing with the stabilizer, e. g., naphthalene, in solution maintains it in a liquid state, so that it flows down the walls of the comparatively c001 outer can or other container to the bottom where it comes in contact with the rolled condenser sections or other electrical elements and thereby is effectively retained in the dielectric composition.

The action of the described solvent also aids in impregnation of condensers by preventing certain portions of the apparatus from accumulatin solid naphthalene or other stabilizer; for instance, pipes of a vacuum apparatus cannot become plugged by solid material. In this connection a suitable solvent for naphthalene and its derivatives is tetralin. The amount of solvent may vary from about 5% to about 20% of the amount of naphthalene or other stabilizer. While it is contemplated that numerous other solvents or mixtures thereof besides tetralin may be used, this latter has been found to itself exert a certain stabilizing effect upon the dielectric composition. It is therefore particularly adaptfrom ten to twenty parts of naphthalene has been found to give excellent results when added to about 150 to 300 parts of the dielectric composition.

Dielectric compositions which may be advantageously stabilized as aforesaid are those which upon use produce deleterious gases. Organic dielectrics, a great many of which are unstable when subjected to high field strengths, are especially adaptable to this treatment. Furthermore, dielectric compositions which, while not particularly unstable, nevertheless undergo a certain amount of deterioration when used for particular purposes involving severe conditions, may be advantageously improved by incorporating therewith the stabilizing agents or mixtures thereof previously referred to.

Since most organic dielectrics in common use contain one or more liquid organic materials such as mineral or vegetable oils, chlorinated hydrocarbons, liquid polymers, liquid waxes, liquid resinous materials, etc., nearly all of which materials will give off hydrogen and/or other gases when subjected to the action of a sufllciently strong electric field, which may include the action of gaseous discharge and/or bombardment by charged particles, the addition of a stabilizer as 'described herein may be applied to any organic dielectrics such as these. In particular, dielectrics containing cellulose, such as paper, cotton cloth, regenerated cellulose, etc., are benefited thereby. Those dielectrics which are at least partially in the liquid phase under conditions of actual use are also especially benefited by treatment with the foregoing stabilizing agents. The preferred embodiment of this, invention therefore contemplates stabilization of dielectrics conforming to at least one and particularly more than one of the foregoing categories.

This invention in one of its embodiments is illustrated in the accompanying diagrammatic drawing, partly in section, of a' condenser, stabilized in accordance herewith. The condenser shown in the drawing comprises an open-ended container In of terneplate, aluminum or the like sealed by a cover I I through which protrude insulated terminals Within the container is a condenser section comprising interwound foils H and I5 consisting of aluminum, lead, tin or the like. Interposed between the foils I4 and I5 are insulating spacers l6 and ll of kraft paper, linen paper or the like and thoroughly impregnating the spacers l6 and ll is a dielectric l8, stabilized in accordance with the foregoing description, said dielectric also preferably filling all voids in the container I! as shown. The interwound foils l4 and i5 and interspacers l6 and I! are enclosed by an insulating paper wrapping l9 which electrically insulates the condenser elements from the container I0. Electrical connections to the electrode foils l4 and 15 are provided by tabs 20 and 2t respectively, said tabs passing through the terminals i2 and I3 and being soldered thereto as shown at 22 and 23 respectively. The con-- denser may be impregnated after assembly and this takes place through an aperture 24 of the cover H which is subsequently sealed by a solder fillet 25. a

As many widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except asdeflned in the appended claims.

I claim:

1. A new dielectric composition particularly adapted for the impregnation of cellulosic material comprising a liquid halogenated hydrocarbon dielectric stabilized with from about 1% to about 10% by weight of naphthalene to which has been added an amount of tetralin ranging from about 5% to about 10% by weight of the naphthalene.

2. A new composition comprising a cellulosic material impregnated with a liquid halogenated hydrocarbon dielectric stabilized with from about 1% to about 10% by weight of naphthalene to which has been added an amount of tetralin ranging from about 5 to about 10% by weight of the naphthalene.

I JAMES L. HYDE.

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