US2000428A - Electrical conductor - Google Patents

Description

May 7, 1935.

A. c. WALKER 2,000,428

ELECTRICAL CONDUCTOR Fild Jan. 9, 1932 f/a/ l A 7' TURA/EV Patented May 7, 1935 I UNITED STATES 2,000,428 ELEc'rmcsr. CONDUCTOR Albert C. Walker, East Orange, N. J., asslgnor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 9, 1932, Serial No. 585,759

7 Claims.

This invention relates to insulated conductors and particularly to ilexible insulated conductors and cables and the method of their manufacture. The invention is especially applicable to insulated conductors such as are used in telephone switchboard cables and to distributing frame wire, both of which are used extensively in the telephone l plant. f v

It is welllknown that different insulating materials vary widely in their electrical properties, such as insulation, resistance and capacity. Fi'- brous materials such as cotton and tussah oss silk which are used extensively in the insulation of f iiexible conductors possess good v electrical properties when dry but when these materials are exposed to moist air their insulation resistance is considerably lowered and their capacity increased. Regardless of their relative insulating properties the insulation resistance of all such materials falls oi rapidly under rising humidity conditions. While the amount of water absorption is not necessarily a direct and proportionate measure of the insulation resistance of brous insulating materials, nevertheless the presence of water in all casesy produces a deleterious effect upon the insulation resistance properties. The presence of absorbed water in switchboard cable insulation causes unbalancesv and short-circuit paths between the various conductors, thus destroying their efficiency for purposes of telephonie communication, and also sets up an electrolytic action which tends to decompose the conductors and may ultimately render the cable unfit for service.

The principal object ofl the present invention is to improve the electrical characteristics of insulated conductors and to maintain these improved characteristics even when the conductors are exposed to moist air. y

Another object of the invention is to maintain the improved electrical. characteristics \with a structure which is relatively flexible and one that I has an improved appearance and can be manufactured economically.

To attain these objects and in accordance with a feature of the invention, the conducting core is provided with a plurality of coatings of cellu- 4 lose acetate of different derivatives. The inner coating may comprise one or more layers of fibrous material such as cotton which has been .converted into the lower derivatives of cellulose acetate and in which the individual fibres are similar in many of their physical characteristics to the iibr'es of the original material. By thus acetylating the fibrous material, its electrical properties are greatly enhanced and its attraction for atmospheric moisture is diminished. After being provided with a coating of cellulose acetate of the lower derivatives, the insulated wire is preferably nrst treated to expel any moisture present and is then treated with a cellulose acetate lacquer comprising a suitable solvent suchas acetone and a homogeneous cellulose acetate of a higher derivative such as cellulose triacetate, after which it is passed through a drying chamber which dries oiI all traces of the solvent. fibrous material are both forms of cellulose acetate, they possess a high ainity for each other and are bonded together in such a way as to form a coating about the conductor which is ilexible, has a good appearance and is water repellent to a high degree. As the coating of cellulose acetate lacquerdries, there is a shrinking down of the-fibres 'and as a result the diameter' of the completely insulated conductor is less than that of the conductor before the nal treatment and individual loose fibre ends are matted down and .therefore do not present leakage paths. Accordingly the insulation resistance of a conductor made in this manner is far superior to that of any iiexible conductor made heretofore where brous materials are employed and the capacity is decreased accordingly.

The invention may be more clearly understood Since the lacquer and the acetylatedv by reference to the accompanying drawing in- Fig. 1 discloses an electrical cable embodying the features of this invention;

Fig. 2 is a view illustrating the method employed in impregnating the individual conduc-` tors; and

Fig. 3 illustrates two portions of an insulated conductor one of which is untreated and the other of which is treated in accordance with this invention.

Referring to Fig. 1, tl'ere is disclosed an electrical cable in which a'. plurality of bare conductors 5, 5 each of which is insulated with servings 6 and 1 of acetylated cotton wound on with reverse twists and the conductors as'thus formed Atheir electrical characteristics are greately improvedn` Preferably the cotton or other iibrous material is rst soda-boiled and subjected to a bleaching process which reduces the ash content of the material and removes the natural waxes present, thereby permitting more uniform acetylation. Particularly good results have been obtained when thefollowing process has been employed prior to acetylation:

(1) Heat 21/2 parts of cotton with 40 parts by weight of 112% to 2% caustic soda solution for 1 in 40 parts water containing .01 part sodium bisuliite (NazHSO) to eiect dechlorination.

(8) Wash. g (9) Dry quickly at room temperature (120 FJ. The bleached material is next acetylated to yield a product varying in acetyl value from 10% to about 35% depending upon the variation in the time of acetylation and the temperature and combustion of the acetylating bath. In general, the most desirable results both electrically and mechanically are obtained when the acetylation process is not carried beyond the stage where the material is converted into mono-acetate the theoretical acetyl value of which is 21.1% but good results are obtained when the product consists of di-acetate, the acetyl value of which is 34.9%, or a combination of mono-acetate and diacetate. If the acetylation is carried beyond the di-a'cetate stage destruction of the fibres occurs to such an extent that the mechanical strength of the product is decreased and it is much less satisfactory. Furthermore, the mono-acetate and di-acetate obtained by partial acetylation are insoluble in ordinary organic solvents such as acetone, chloroform, the benzene, the alcohols and also the acetylating mixture itself.

A typical example of the acetylating process and one which has been found to give particularly good results is as follows: The bleached cotton yarn is immersed in 8.8 times its weight' of acetylating mixture which contains 4 parts cornmercial glacial acetic acid, 4 parts commercial acetic anhydride (90 to'95%) and.8 part commercial anhydrous zinc chloride, for 7 hours at 25 C. The excess acid is removed by wringing or centrifuging and the cotton is immersed in or ilooded with a large volume of water, washed acid iree, and dried. The material obtained by following this process has an acetyl value offabout 17%. It is necessary to control the temperature Within very narrow limits as, for example, .60/2

yarn acetylated under the same conditions as described above at various temperatures yields th following acetyl values: f

Temperaturedeges C' Acetyl values While cotton has been cited as an example and l it is believed that this material. iends itself most readily for the insulation of electrical conductors,

the process may also be employed on other celluhemp, etc., the essential feature being to insure that the acetylating process is not carried beyond the di-acetate stage and preferably not beyond the mono-acetate stage.

'After the conductor has been provided with one or more servings of partially acetylated cellulose material it is coated or' impregnated with a cellulose acetate of a higher derivative (cellulose tri-acetate). Both of these coverings being formed of cellulose acetate, they possess a high `anity for each other andas a result are bonded together so as to 'form an insulating coating about the conductor which is not' only exible but one which is water repellent to a high degree and, therefore, one which possesses superior electrical characteristics when exposed.

:to moisture. In applying the outer coating of cellulose tri-acetate'it is preferable to maintain the temperature of the coated wire above the dew point until the solvent has evaporated, thus leaving an outer -glossy tube-like coating 'which possesses high insulation resistance under substantially all humidity conditions up to the dew point.

Referring to Fig. 2 of the drawing the supply reelJ I5, upon whichIis wound the conductor provided with a serving of cellulose acetate of the lower derivatives, is heated to expel all moisture prior to the impregnating process. From. the supply reel the'conductor passes through an inlet l provided with a stumng box I into the impregnating tank il containing a bath of cellulose tri-acetate dissolved in acetone or other suitable solvent. As the conductor enters tank il it passes over` guide roller i3 and is then directedvthrough the impregnating bath by guide rollers i9, 20, 2l

The coated conductor emerges from the tank through a small vent 22 in the air-tight cover 23 and into the drying chamber 2d. If desired, the

vent 22 may be equipped with a die or wiper for removing all excess solution from the impregnated conductor.l A strong current of hot dry air is circulated through the chamber 2li from an inlet pipe 25 to an outlet pipe 26 which may be associated with condensing apparatus (not shown) for recovering the solvent. The hot dry air is preferably maintained at or above the temperature of the boiling pointof acetone or other solvent employed and the temperature' may be observed at all times bymeans of a thermometer 2l associated with the inlet pipe 25.

The acetone content in the insulation is rapidly evaporated in chamber 253 leaving a glossy coating upon the iibrous insulation. After passing over a suitable guide roller 28 and through an outlet 2Q- provided with a stumng box, the conductor emerges from chamber 26 and is wound upon the take-up reel 30.

a moisture resisting, insulated oeverk upon the wire in which the b'res oi cellulose acetate of the lower derivatives are enclosed in a `matrix of cellulose tri-acetate and intimately bonded thereto. The cellulose `tri-acetate in drying causes a shrinking down oi the iibres and produces a solid tube-like insulation which is considerably smaller in diameter than the original unimpregnated loose insulation. This is illustrated more clearly'in Fig. 3 in which 3l is a magfrom which form leakage paths when the conductors are grouped together in a cable. Upon being impregnated with cellulose tri-acetate these fibres are matted down as shown at 32, thus materially reducing the space occupied by the insulation and permitting a greater buik of conductors to be enclosed in a given space in a cable sheath when capacity considerations admit of it. At the same time the area and points of contact betweenthe insulated conductors are decreased, thus decreasing the leakage paths between conductors.

The results of a 'large number of tests made with conductors insulated with partially acetylated cotton indicate that the resulting insulation resistance under similar conditions is from 500 to 1000 times that of purified cotton and about 10 times that of puriiied silk. For use in toll ca' bles it is required that the insulating material not only have a high insulation resistance but also that it have a low capacitance and conductance.

The tests show that a toll conductor provided' ing acetylated cotton is only 80% that of the standard cable while the conductance is 30%' of the standard cable and the capacitance unbalance is only 30% of that obtained from the standard cable. These results indicate that not only is it possible to replace the highest-grade of silk insulation employed on toll cables with acetylated cotton but that a far superior product results.

For other types of cables the conductance and capacitance unbalance requirements are not so severe and, therefore, in certain cases it may be Iound desirable to replace the outer serving I with la serving of puriedcotton. By this means it is possible to obtain a cablehaving electrical characteristics superior to those now available and at a somewhat lower cost since the purified greater than 35% surrounding said core and an outer coating of cellulose acetate having an acetyl value not less than 37% in intimate contact with and bonded to' said inner coating.

2. Anelectrical conductor comprising a conducting core, a coating of fibrous cellulose acetate of the lower derivatives surrounding said core and a coating of non-librous cellulose acetate.

ducting core, a coating about said core of partially acetylated cellulose nonso1uble in the ordinary organic solvents, and an outer coating of cellulose acetate of a form soluble in said organic solvents surrounding and bondedV to said inner coating.

5. An electrical conductor comprising a vconducting core, a coating aboutsaid core-of partially acetylated cotton bres having an acetyl value of approximately and an outerv coating of cellulose tri-acetate enclosing said iibres in a matrix of cellulose trithereto.

6.'A'n electrical conductor comprising a conducting core, a coating of partially acetylated tate and bonded vcotton helically wrapped about said core, a second coating of acetylated cotton. -heiically wrapped about said rst coating with a reverse twist, and .a coating of cellulose tri-acetate bonded to and surrounding said second coating.

7. An electrical cable comprising'a plurality o! pairs of twisted conductors each of said conductors being insulated with an inner coating of partially acetylated cellulose 'and an outer coating of cellulose tri-acetate bonded to said iirst coating, and shrinking down the outer ilbers of the partially acetylated cellulose to prevent leakage paths between adjacent conductors, and a spaced serving of fibrous cellulose binding said conductors in the form of a cable.

ALBERT C. WALKER.

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