Semiconducting tape for electric cable

March 16, 1948.

C. E. PLASS ET AL

2,437,708

SEMI-CONDUCTING TAPE FOR ELECTRIC CABLE

Filed Feb. 18, 1944 gra/d •5-SEMA CONOUCTANG C0AT/N6 3-PAPER

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2-O/LEO PAPER

1-COPPER WARES

CLOTH ί COPPER WARES

SEMT CONDVCT/NG COATING

RUBBER /NSUiATiON

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^^aed Plass

BY

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ATTORNEY

Patented Mar. 16,1948

2,437,708

UNITED STATES PATENT OFFICE

2,437,708

SEMICONDUCTING TAPE FOR ELECTRIC CABLE

Curtis Edward Plass, Worcester, Mass., and David Joseph Sullivan, Fairfield, Conn.; said Sullivan assignor to E. I. du Pont de Nemours &amp; Company, Wilmington, Del., a corporation of Delaware; said Plass assignor to The American Steel and Wire Company of New Jersey, a corporation of New Jersey

Application February 18,1944, Serial No. 522,980

Claims.

This invention relates to a semi-conducting oil impervious coating material for use as an electrostatic shield on high tension insulated electrical conductors where it is desirable to maintain the voltage gradient at the surface of the insulation below a value above which harmful electrical disturbances occur.

Heretofore it has been common to dissipate high voltage charges which surround electric cables carrying high voltages, that is, of the order of 2000 volts or more, by means of a metallic conducting shield over an insulating medium which may be in the form of oil saturated paper, rubber, varnished cambric and the like. In ordinary use it is necessary to dissipate or discharge the charges which surround such a cable and this has been done .by various means, such as surrounding the cable with the thin metallic sheath mentioned above. Metals, however, have the disadvantage that frequent bendings of the cable tend to develop air gaps because of the separation of the metal shield from the adjacent insulation. When such gaps are formed, arcing across the air gap may occur which eventually causes a failure of the insulating medium or ozone may be formed in the air gap which deleteriously affects the insulation. It has also been proposed to provide a shield which is more flexible than metal. Such shields have been prepared by a conducting paint, compositions of rubber, or other film-forming agents in combination with metallic particles. None of these expedients has been entirely satisfactory when used in combination with oil impregnated insulation due primarily to the attack of the oil on the filmforming agents whereby the conducting properties of the films are deleteriously affected.

In U. S. application for Letters Patent Ser. No. 373,181 of Peterson, now Patent No. 2,322,702, granted June 22, 1943, a method for producing satisfactory shielded cables is disclosed. The present invention relates to a modification of the method therein disclosed which offers several desirable advantages, specifically a type of conducting coating or film that embodies enhanced resistance to the solvent action of materials common to cable insulation, permanence of properties when exposed to the effects of aging and electrical discharges, and a means of providing a (Cl. 174—102) readily controlled degree of electrical conductivity.

We have now found that many disadvantages of prior art rubber compositions and similar 5 semi-conducting shields have been overcome by the use of a tape having a composition containing a special type of polyamide and a special type of carbon black on its surface or impregnated therewith, especially when used in combination 10 with oil impregnated insulation.

The special types of polyamides are those which are soluble in a lower aliphatic monohydric alcohol, such as those disclosed in U. S. Patent 2,285,009 to Brubaker et al„ granted June 2,1942, 15 and N-alkoxymethyl polyamides disclosed in Cairns application Ser. No. 445,635, filed June 3, 1942 (now abandoned).

The latter are a new class of nitrogen-substituted polyamides formed by reacting with an 20 aldehyde and a formaldehyde-reactive organic compound having a hydrogen attached to an element of groups V and VI of series 2 and 3 of the periodic table, in the presence of an oxygencontaining acid catalyst, polyamides of the gen25 eral type described in U. S. Patents 2,071,250, 2,071,253 and 2,130,948. The polyamides of this kind, generally speaking, comprise the reaction product of a linear polymer-forming composition containing amide-forming groups, for example, 30 reacting material consisting essentially of bifunctional molecules each containing two reactive groups which are complementary to reactive groups in other molecules and which include complementary amide-forming groups. 35 These polyamides can be obtained by the methods given in the above mentioned patents and by other methods, for example, by self-polymerization of a mono-aminomonocarboxylic acid, by reacting a diamine with a dibasic carboxylic acid 40 in substantially equimolecular amounts, or by reacting a monoaminomonohydric alcohol with a dibasic carboxylic acid in substantially equimolecular amounts, it being understood that reference herein to the amino acids, diamines, 45 dibasic carboxylic acids, and amino alcohols is intended to include the equivalent amide-forming derivatives of these reactants. The preferred polyamides obtained from these reactants have a unit length of at least 7, where “unit length” is 50 defined as in United States Patents 2,071,253 and

3,437,708

2,130,948. The average number of carbon atoms separating the amide groups in these polyamides is at least two.

These linear polyamides include also polymers, as for instance the polyester-amides, obtained by 5 admixture of other linear polymer-forming reactants, as for instance glycol-dibasic acid mixtures or hydroxy acids, with the mentioned polyamide-forming reactants. Both the simple and modified linear polyamides contain the recurring io amide groups conductor I. Over this oiled paper layer Is applied a semi-conducting tape made up of a base 3 which may be paper 3 as shown in Figure 1, or the paper may be replaced by a suitable fabric. The tape with the paper or fabric base is provided with a semi-conducting coating 5. The tape is prepared by impregnating or coating the base with a semi-conducting composition and cut into strips of suitable width, which is then wound around the insulated cable. Figure 2 is a modification of Figure 1 showing a rubber insulation 4 next to the conductor I instead of the oiled paper shown in Figure 1. The semi-conducting tape also differs from Figure 1 in the cloth base 7 instead of the paper base 3. In Figure 2 the semi-conducting coating is covered with a conventional braid 6.

Figure 3 represents a modification of Figures 1 and 2 in which the semi-conducting tape is applied directly to the copper conductor ί. An insulation 8 is provided over the semi-conducting coating 5. This insulation may be rubber shown as 4 in Figure 2 or oiled paper shown as 2 in Figure 1 or other types of electrical insulation. Figure 4 illustrates a section of the semi-conducting tape which has a cloth base 7 and a semi-conducting co'ating 5, the composition of which will be more fully described in the following examples.

In some cases it may be desirable to encase the improved cable or a plurality of such cables in a lead sheath or equivalent protective means according to common practice to make the assembly more impervious to water and more resistant to mechanical abrasion and to function also as a fault current carrying medium to ground. .·

The resistance of the tape or unsupported layer should be of the order of 300 to 1000 ohms per inch square. Under some conditions the tape may have a resistance of up to 2000 ohms or even more. When a cloth base is used, it is preferable to coat the same with about 2 to 4 ounces per square yard of polyamide-carbon black composition. When a paper base is used, about 1 ounce per square yard is satisfactory. The composition applied to the fabric or paper base may be prepared in any known manner or by methods illustrated by the following examples.

Example I

The following ingredients were heated to about 80° F. with stirring for 12 hours in a suitable mixer, such as a Werner &amp; Pfleiderer:

_ Parts by weight

Polyamide prepared according to Example I in which X is oxygen or sulfur and R is hydrogen 15 or a monovalent hydrocarbon radical, as an integral part of the main chain of atoms in the polymer. On hydrolysis with hydrochloric acid the amino acid polymers yield the amino acid hydrochloride, and the diamine-dibasic acid 20 polymers yield the diamine hydrochloride and the dibasic carboxylic acid, and the amino alcoholdibasic acid polymers yield the amino alcohol hydrochloride and the dibasic carboxylic acid.

These nitrogen-substituted polyamides are 28 typified by N-me,thoxymethyl polyhexamethylene adipamide, which was prepared as follows:

A solution of 1 part of polyhexamethylene adipamide (intrinsic viscosity=1.0) in 3.7 parts of formic acid, (commercial grade, specific 80 gravity=1.20) was prepared at 60° C. in a stainless steel reaction vessel equipped with a mechanical stirrer and an opening for the addition of reagents. To this was added at 60° C. a solution of 1 part paraformaldehyde dissolved in 35 1.33 parts methanol containing 0.0006 part sodium hydroxide to depolymerize the paraformaldehyde and render it soluble in the methanol. The resulting solution was maintained at 60° C. for 30 minutes during which time 40 an additional 0.3 part of methanol was added. This solution was poured into a mixture of 9 parts acetone and 11.5 parts of cold water with stirring. The resulting solution was then made basic to litmus by the addition of concentrated 45 aqueous ammonia, the mixture being maintained below 40° C. and stirred vigorously during this step. The reaction product, N-methoxymethyl polyhexamethylene adipamide, separated from the mixture as a white granular solid. This 50 product was removed by filtration, washed thoroughly and dried in vacuum at 50° C.

The carbon black which is used may be prepared according to an article published in Canadian Chemistry &amp; Metallurgy,” May, 1933, 55 pages 93 to 95 by Charles Kaufmann.

In addition to this carbon black, there are now on the market many blacks with high conducting properties. Chapter 7, page 67 of the book “Columbian Colloidal Carbons,” published in 1938 60 by the Columbian Carbon Company, defines a suitable type of black as having an electrical resistance in ohms per 1 cube of .152 to .217, although a somewhat higher resistance up to about 1.0 ohm per 1” cube is satisfactory. In the cla.&lt;ms 65 such blacks are termed “low resistance carbon blacks.”

In the drawing Figures 1, 2, and 3 are diagrammatic partial sections of a high tension cable prepared according to the present inven- to tion. Figure 4 is a diagrammatic section of a piece of semi-conducting tape. In the drawing, 1 represents a conductor which may be solid or made up of a bundle of wires as shown. In Figure 1 an oiled paper layer 2 surrounds th? 75 of U. S. Patent 2,285,009io

Isopropyl alcohol_________“I 30

Water___10

Low resistance carbon black 3

The water, alcohol, and polyamide were placed in the mixer first and after about 12 hours’ mixing the low resistance carbon black was added and mixing continued for an additional 12 hours. Immediately before applying the composition to the fabric or paper base, it was stirred vigorously in a high speed mixer for another hour.

Example II

While the coating composition of Example I functions well when the semi-conducting tape is formed from paper, it may penetrate an open weave fabric unduly. To prevent penetration of

3,437,708 the base support, the following composition may be applied:

Parts by weight

Polyamide¹ 25

Ethyl alcohol110

Water 45 ¹ The polyamide used in thia example was an N-alkoiymethyl polyhexamethylene adipamide as disclosed above. A single coat of this composition is applied by any suitable means to a moderately open weave fabric sheeting. After the base coat has dried, it is treated as in Example I with about 4 ounces per square yard of the composition therein disclosed.

Example III

It -was found that tape with either paper or fabric backing, more particularly with fabric backing, produced from the coating compositions described in Examples I and II, while highly serviceable, could be improved as to some of the desired characteristics, such as shortness, conductivity, elasticity, extensibility, and absence of flakiness of the coating, if the resin base included the N methoxy methyl polyhexamethylene adipamide type disclosed above. The compounding of the Brubaker composition has already been described. The N-alkoxymethyl polyamide base composition is produced as follows:

Parts by weight N - alkoxymethyl polyhexamethylene adipamide 25

Ethyl alcohol___________________‘90

Water36

The resin and solvents are heated and slowly agitated for 12 hours at 140° F. or higher. 75 parts of low resistance carbon black were then added and slow agitation is continued for 12 hours. The mixture is agitated for 1 hour with a high speed agitation. Equal parts of the two compositions, those from the Brubaker patent and the above described N-alkoxymethyl polyamide type, are mixed together, after which coating proceeds as in Examples I and II. While equal parts yield highly serviceable products, we are not limited to such proportions; our invention comprehends mixtures in all proportions.

The time of mixing is important since if the carbon black is entirely surrounded by a film of polyamide, the composition is not sufficiently conducting. The grinding of the carbon black and polyamide mixture should be controlled to give the desired resistance. This time is usually between 6 to 15 hours. In the examples, the supporting base is coated on one side only. If desired, both sides may be coated or the fabric may be impregnated by drawing it through a bath of the polyamide-carbon black mixture suitably diluted. The paper base may be any suitable type but cable Insulating paper sold by Union Mills having a thickness of about .003 to .0035 inch is preferred. Suitable fabric is a cotton sheeting of 40 inch width weighing about 6.25 yards per pound. Usually three coats are applied to obtain the necessary thickness although a fewer or greater number may be applied as desired. After the paper or fabric base is coated, it is cut into convenient widths in the form of a tape which is then used for wrapping about the conductor in any conventional desired manner. The particular type of polyamide used is critical and only those which are soluble preferably In an alcoholic solution may be used. The ratio of polyamide to low resistance carbon black should be between .25 and 4 parts of polyamide to each part of carbon black.

In addition to the soluble polyamides illustrated in the examples, any of those described in U. S. Patents 2,252,554, granted August 12, 1941, 2,252,555, granted August 12, 1941, and 2,252,556, granted August 12, 1941, are likewise suitable. Those melting below about 175° C. are particularly preferred. Examples of these are the multi-component polyamides prepared from hexamethylenediamine, adipic acid, sebacic acid, and caprolactam as described in U. S. Patent 2,285,009, and the alkoxymethyl polyamides such as methoxymethyl hexamethylene adipamide described above. These latter materials may be prepared by the condensation of a polyamide with formaldehyde and an alcohol. While the invention has been illustrated with certain polyamides, the polyesteramides, formed from combinations of ester- and amide-forming reactants, including dibasic acids, diamines, glycols, hydroxy acids; amino acids, and amino alcohols which are described in U. S. Patents 2,071,250, grafted February 19, 1937, and 2,312,879, granted March 2, 1943, may be used. Polyesteramides which have been converted to higher molecular weight products by reaction with a diisocyanate as described in U. S. Patent 2,333,639, granted November 9, 1943, are also suitable. Such products may be subsequently converted after calendering to insoluble, infusible products by treating them with formaldehyde or formaldehyde liberating substances.

Cables prepared according to the present invention have been found to have an exceptionally long life and maintain uniform electrical characteristics much longer than any heretofore tested. The compositions applied to the tape are particularly resistant to the oil with which the sublayers of paper have been impregnated. The breakdown of the older type materials, rubber, etc. is accompanied with an emanation of decomposition products which contaminate the insulating oil and ruin its insulating properties. The stability of the nylon base in oil contributes to the longevity of the insulating phase. While the composition of matter which we have discovered is particularly adaptable to use as a conducting shield for high tension cables, it is also equally useful for other related applications where it is desirable to obtain an electrically conducting surface or film.

It is apparent that many widely different embodiments of this invention may be made without departing from the spirit and scope thereof; and, therefore, it is not intended to be limited except as indicated in the appended claims.