US2708215A - Jacketing material for high frequency cables - Google Patents

Description

May 10, 1955 S. KAGANOFF JACKETING MATERIAL FOR HIGH FREQUENCY CABLES Filed May 51, 1951 //V/VER C'OIVOUCTOR BRA/D INVENTOR $6M GNU V KAGA/VOF'F BY AGENT United States Patent I ACKETIN G MATERTAL FOR HIGH FREQUENCY CABLES Solomon Kaganoff, Bloomfield, N. J., assignor to International Telephone and Telegraph Corporation, a corporation of Maryland Application May 31, 1951, Serial No. 229,289

6 Claims. (Cl. 174 -110) This invention relates to a jacketing material for high frequency cables and, more particularly, to a low temperature jacketing material noncontaminating to polyethylene, comprising a polyvinyl chloride resin and a polymeric ester of 1,3-but-anediol and sebacic acid.

Two types of noninflammable thermoplastic synthetic resin jacketing materials have heretofore been specified for sheathing high frequency coaxial cables having a polyethylene dielectric, primarily because no one material has been available which meets all conditions of use. The specifications for these two types of materials are set forth in joint Army-Navy specification JAN-C-17A, entitled Cables, Coaxial and Twin-Conductor, For Radio Frequency.

Type I is required to pass a special low temperature test at -40 C., but compositions which pass this test are contaminating to polyethylene dielectrics. The contamination ordinarily is due to the plasticizer which must be incorporated with the synthetic resin to increase its flexibility. Inasmuch as this specification does not require that the material be noncontaminating to polyethylene, it can be met with known materials, but the life of cables coated with contaminating jacketing materials is very short. Within a few weeks such cables can become completely useless.

- Type II is required to be noncontaminating to the polyethylene, but hitherto such packets have had low temperature flexibility only to a temperature of C. There are known noncontaminating compositions which meet this specification, although they do not meet the specification for Type I because of their lack of low temperature flexibility.

The need remains for a jacketing material for radio frequency coaxial cables which is both noncontaminating to polyethylene and flexible at temperatures as low as C. with no sacrifice in other desirable characteristics. This problem has recently become urgent due to the extreme weather conditions under which such cables must be used in certain parts of the World.

It is an object of this invention to provide a noninflammable thermoplastic jacketing composition useful for sheathing high frequency coaxial cables having a polyethylene dielectric, which composition is noncontaminating to polyethylene at temperatures as high as C. and remains flexible at temperatures as lowas -40 C.

In accordance with the foregoing object, a coaxial cable of well-known construction is illustrated in the accompanying drawing. Such polyethylene coaxial cables are well-known in the art and have been described, for example, in an article entitled Coaxial cable design, by N; D. Kenney, which appeared in Electronics, May 1945, pages 124-128, and also in an article entitled Jacketing materials for high frequency transmission lines, by A. J. Warner, which appeared in Electrical Communication, volume 23,'No. 1, March 1946, pages 63-69.

v This drawing shows one embodiment of a high frepletion.

2,708,215 Patented May 10, 1955 quency coaxial cable which may be sheathed with a thermoplastic jacketing composition of the subject invention. Such a cable essentially consists of an inner conductor, a polyethylene dielectric disposed about the inner conductor, an outer braid conductor about the polyethylene dielectric and a low temperature non-contaminating jacketing material, as set forth herein, about the conductive braid.

The composition provided by the invention which meets these specifications consists essentially of a polyvinyl chloride resin as the synthetic resin component and a polyester of 1,3-butanediol and sebacic acid as a plasticizer for the resin.

Polyvinyl chloride resins, including vinyl chloride-vinyl acetate copolymers, are employed in preference to other polyvinyl resins because when plasticized with the polymeric ester of the invention they are not contaminating to polyethylene, are not brittle at low temperatures, are readily available, and are easy to process. A variety of polyvinyl chloride resins may be used, including Geon 101 and 202 (available from the B. F. Goodrich Chemical Co.), Marvinol resins FG56(), FG-425W and FG-425V (available from the Glenn L. Martin Co.), Resin 152 #1812, Resin #1813 and Ultron (available from the Monsanto Chemical (30.). All of these are commercial polyvinyl chloride polymers. Copolymers of vinyl chloride and vinyl acetate containing up to 10% vinyl acetate may also be employed, such as VYCM (copolymer of a mixture of 91% vinyl chloride and 9% vinyl acetate) and VYNW (copolymer of a mixture of 95% vinyl chloride and 5% vinyl acetate). The latter resin is preferred because it has low temperature properties superior to other polyvinyl chloride resins and can be easily plasticized with a minimum quantity of plasticizer. This is important in view of the relative high cost of the plasticizer or" the invention, compared to the resin.

The plasticizer employed in accordance with the in vention is a linear polyester prepared from 1,3-butanediol and sebacic acid. The reaction can be carried out at the reflux temperature of the reaction mixture at atmospheric pressure in the presence of zinc chloride as a catalyst. Under these conditions, a polymeric liquid reaction product is obtained having a refractive index i'Z ZS in the range from 1.4686 to 1.4696. Those skilled in the art will recognize other methods of preparing this reaction product. It will be understood that the term polyester as employed herein refers to this product. The term fully condensed as applied to this product is intended to mean that the polymerization or condensation has been carried as far as it will go under these or comparable conditions, but does not necessarily imply that a further polymerization or condensation might not be effected under more rigorous conditions, for example, at higher temperatures or in the presence of other condensation catalysts. It is important to employ proper reaction conditions, such as those set forth above, both because they ensure the obtention of a liquid having plasticizing properties, and inasmuch as it obviously is desirable that the composition of the invention be uniform in properties.

The physical properties of this polyester uniquely suit it for the purposes of the invention. Polyesters of sebacic acid and other dihydric alcohols which have been prepared and tried are not satisfactory plasticizers. It is thought that this is because either the condensation or polymerization proceeds too far or stops short of com- For example, the polyesters of sebacic acid and 1,3-propanediol and of sebacic acid and Z-methyl- 2,4-pentanediol are solids, showing that in these cases condensation proceeds too far. This ditficulty cannot be obviated by attempting to arrest condensation short of completion, primarily because the condensation reaction is too difficult to control. The polyesters of sebacic acid and 1,2-propanediol and of sebacic acid and 1,3- octanediol are liquids, but considerably less effective than the 1,3-butanediol-sebacic acid polyester of the invention.

In addition to the polyvinyl chloride resin and the polyester, the composition may contain a stabilizer for the resin and a lubricant for facilitating processing the composition as jacket on wire or a cable. As stabilizers, basic silicate of lead and dibasic lead phthalate give the best results; the former is preferred because it displays a lesser tendency to form clouds of dust during handling. The most satisfactory lubricant is stearic acid.

The amount of plasticizer employed will depend to some extent upon the particular polyvinyl chloride or vinyl aeetatevinyl chloride copolyrner whose flexibility is to be increased and will be sufficient to increase its flexibility to such a point that the composition will pass the Cold Bend Test described hereinafter. Ordinarily from 30 to 50% plasticizer based on the total weight of the resinous composition of matter should be present. A composition of the copolyrner of 95% vinyl chloride with vinyl acetate containing approximately 42% polyester of 1,3-butanediol and sebacic acid, based on the total weight of the resinous composition of matter, gives optimum results.

The lubricant may be present in an amount in the range from 0 to 3%, preferably /2 to 1%, based on the weight of the composition, and the stabilizer in an amount within the range from 0 to 7%, preferably 3%, based on the weight of the resinous composition of matter.

The components of the composition are blended in conventional ways and can be extruded as cable sheathing in the manner well known to the art. One mode of preparation of the composition is illustrated in Example 1.

The flexibility of the compositions of the invention at low temperatures is determined by the Cold Bend Test. This test can be carried out on either strips or cables and coated wire. Ten to twelve or more specimens are tested each time the test is made and the results are reported as the per cent of the samples Whose surfaces are free from cracks and breaks at the test temperature of -40 C. In order for a composition to pass this test, at least 80% of the specimens tested must be free from cracks and failures.

The apparatus employed in making the test is installed in a special chamber equipped to permit the bending of the specimens therein without opening the chamber. The apparatus consists of a rigid steel frame supporting a mandrel, which is connected by a /1 inch shaft to a horsepower motor fitted with a variable speed gear box located outside the chamber. The shaft is adjusted to a speed of R. P. M. When testing inch strips cut from a standard test slab, the mandrel is inch in diameter and is fitted so that the strips can be clamped thereto. When used for testing cables and coated wires, the mandrel is 2 /2 inches in diameter and fitted so that the wire can be rolled up thereon.

In testing cable and wire, five sheaves 2 inches in diameter hold the cables tightly against the mandrel, but are free to move along the shaft as the cables are being wound up. Brass guide tubes keep the cable straight prior to winding on the mandrel. In testing strips, the small guide sheaves are replaced by aluminum sheaves 3 7 inches in diameter with inch wide flat faces to hold the strip against the %1 inch mandrel.

The test procedure for strips is as follows: Strips inch Wide and 2 /2 long are cut from the standard test slabs. The mandrel is rotated until the clamping strip clears the sheaves by /5 inch. Two strips are slipped under each sheaf and into their respective places along the clamping strip. Five or six sheaves are used. The clamping strip is then secured tightly and the chamber brought down to the required test temperature and kept there for the specified time interval. Then the mandrel is rotated until the brass fastening strip is just about to pass under the sheaves. The chamber is warmed to room temperature, the samples are removed and the cracks and failures noted.

The test procedure for cables is as follows: Five samples each 31 inches long are cut from the cable to be tested. The mandrel is then allowed to rotate until the cable clamps are approximately of a revolution past the sheaves. Each sample is then passed under the proper sheaf, threaded through the proper brass guide tube, clamped in position, and the mandrel rotated until the strip clears the sheaves by Vs inch. The test chamber is then brought to the desired temperature and this temperature maintained for the time interval of the test. The motor is then switched on and the mandrel allowed to rotate until the cables are entirely free of the guide tubes but still held in place by the guide rolls and have made 3 revolutions around the mandrel. The chamber is then warmed to room temperature and the samples examined for cracks and failures.

The test for contamination of polyethylene, referred to hereafter as the Contamination Test, is performed as follows: A polyethylene specimen is molded in the form of a rod suitable for measurement with the Massachusetts Institute of Technology CO-AX Instrument. The dissipation factor at 3000 megacycles is measured for an initial value. In general this value is approximately 0.0003. The polyethylene sample is then wrapped with a sheet of the composition and heated at C. for one week 168 hours). At the end of this time the dissipation factor of the polyethylene sample is measured again. The increase in dissipation factor is the measure of the contaminating property. An increase to 0.0006 or less is regarded as negligible and indicates no contamination under this test. A test at one temperature, i. e., 100 C., is sufiicient, for if no contamination is found at 100 C., none will be found at 40 C. or lower, the test being more rigorous at the elevated temperatures.

In testing the plasticizer for contamination, the polyethylene specimen is immersed in a bath of the plasticizer and heated at 100 C. for one week (168 hours). The dissipation factor of the polyethylene is measured before and after the test.

The flammability of the compositions of the invention is determined by ASTM Procedure D635-44.

Example 1 The polyester of 1,3-butanediol and sebacic acid was prepared as follows.

1,3-butanediol (482 g., 5.3 moles), sebacic acid (830 g., 4.1 moles) and zinc chloride (0.9 g.) were placed in a flask equipped with an oil-sealed mechanical stirrer and gas inlet tube and an upwardly extending condenser to the head of which was attached an adapter holding a thermometer and a Liebig air condenser. A stream of nitrogen was passed slowly over the surface of the reaction mixture while it was refluxed for 9 hours under atmospheric pressure. Thereafter the water and other volatiles were distilled off under a pressure of 40 min.

(water pump) at a temperature from 54 to 168 C.

and the excess glycol removed by fitting the reaction vessel with a short Vigreux column and heating at 97 to 200 C. under 0.7-4.8 mm. pressure (vacuum pump). This required 13 hours. The last traces of glycol were removed by heating for 6 hours at to 220 C. at a pressure of 1.6 to 1.9 mm.

The polyester prepared by this procedure was labelled Batch No. 1.

Similarly, two additional batches, labelled Batch Nos. 2 and 3, respectively, were prepared. Batch No. 2 was made from 547.8 g. (6.1 moles) of 1,3-butanediol, 830 g. (4.1 moles) of sebacic acid and 0.9 g. of zinc chloride, and Batch No. 3 from 414.4 g. (4.6 moles) of 1,3-

butanediol, 830 g. (4.1 moles) sebacic acid and 0.9 g. zinc chloride.

The refractive indices of the polyester products at 25 C. were:

Batch No. 1 1.4694 Batch No. 2 1.4692 Batch No. 3 1.4696

Each of the polyesters was then separately made up in a cable jacketing composition. VYNW resin (a polyvinyl resin prepared by copolymerization of 95 parts of vinyl chloride and 5 parts of vinyl acetate, available from the Bakelite Corporation), 58 parts polyester (42 parts) and basic silicate of lead, PbSiO3Pb(OH)2 (3 parts) and stearic acid (0.5 part) were weighed out to a batch size one liter in volume, and premixed by hand to a pasty consistency. The mix was then charged in a Banbury mixer and mixed until the temperature reached 320 F. Mixing was continued at 320 Fri-5 for 5 minutes.

The mix was then milled at 280 F.i5, the rolls in the mill being spaced to give a sheet approximately mils thick. During the first two minutes of milling, the mix was allowed to form a continuous sheet around the front roll. Then it was taken off in the form of a roll and given eight complete end passes through the rolls, after which it was allowed to cool.

' A 70 g. sample of the milled sheet was placed in a 6 x 6 inch mold which had been preheated to 320 F.i10 and then molded at a pressure of 5 tons, degassing the mold at zero pressure 10 times in the course of the operation. After the last degassing, the pressure was increased to 10 tons and held for 8 minutes. Thereafter the pressure was released and the mold transferred to the cooling press as quickly as possible, after which it was cooled to 150 F. for at least 4 minutes under 6 tons pressure. The finished material was stored in a desiccator until evaluation of its physical properties.

The hardness of the jacketing compound was determined by means of a Shore durometer using the A scale and a temperature of C. The tensile strength, per cent elongation and stress (at 100% elongation) were determined in accordance with ASTM Designation D-638-46T.

The determination of flammability was made according to ASTM Procedure D635-44.

The heat deformation was determined as follows: The test specimen was molded to /2 inch in diameter and 007520.005 inch thick. Prior to testing the specimen 51 was conditioned for 24 hours at room temperature and the test instrument was preheated for one hour at the test temperature. Tests were made at 120 C. with a Randall-Stickney gauge having a /1. inch diameter presser foot. The original thickness was measured using a 3 ounce weight at room temperature. The specimen was then placed under the pressure foot of a preheated gauge at 120 C., which was loaded with a 500 gram weight. The gauge dial reading was recorded at 15 minute intervals. The final reading was taken one hour after the application of the load. The heat deformation under load is reported as a percentage change in the original thickness.

The brittle point temperature was determined according to American Society for Testing Materials, ASTM Standard D-746-44T, and the flex temperature by the test described in paragraph F6 of JAN-C-17A (July 25, 1946). By standard brittleness temperature I refer to that brittle point temperature as determined in accordance with ASTM Standard Test Method D746 44T. These tests are not quite accurate in their correlation with actual use conditions at very low temperatures, and tend to show a higher flex and brittle temperature than is actually found in use.

The following data was taken:

Composition 1 2 3 VYNW Resin 58.0 58.0..... 58.0. Polyester:

Batch No 1 Batch No 2...

Batch No. 3

Basie Silicate of Lead Stearie Acid Properties:

Hardness Elongation Tensile Strength Stress (100% Elong.)..

Heat Deformation....

Brittle Point, C

Flex Temperature, C

Flammability Test..-

Contamination Test.-

Cold Bend Test 0 90% Passed. 55 C Passed.

The outstanding properties of the composition, particularly its flexibility at low temperatures, are evident from this data. The relative consistency of the data for each composition shows the polyester can be repeatedly condensed to the same stage of polymerization in sep arate batches, an important factor as regards reproduce-- ability of the results. 1

Example 2 1,3-butanediol (482 g., 5.3 moles), sebacic acid (830 g., 4.1 moles) and zinc chloride (0.9 g.) were reacted together as set forth in Example 1 to form a polyester which had a refractive index at 25 C. of 14686-14690.

The polyester was combined with VYNW resin, basic silicate of lead and stearic acid, and formed into test strips using the proportions and procedure set forth in Example 1.

From the test strips thus prepared 4 inch strips 5 inches long were cut with a die and each of these inch strips was then cut in two and tested by the Cold Bend Test and the Contamination Test. Ninety per cent of the samples passed the Cold Bend Test at 40 C., and all of the samples passed the Contamination Test.

A second set of samples prepared from identical compositions but containing 1,2-propanediol-sebacic acid polyester as the plasticizer were considerably less flexible. Only 15% passed this test at 40 C., but all of the samples passed the Contamination Test.

Although the composition of the invention has been shown to be particularly useful as a sheathing for high frequency coaxial cables, it will be evident to those skilled in the art that it may be used in sheathing any type of wire or cable having a polyethylene dielectric. It is also useful for other purposes where the problem involves the contact of polyethylene with a material which must be flexible over a wide range of temperatures and be noncontaminating to the polyethylene. It will be understood that the invention is not to be limited except as set forth in the claims.

The expression consisting essentially of as used herein is intended to refer to the components which are essential to the composition, namely, the polyvinyl chloride resin and the polyester of 1,3-butanediol and sebacic acid, and the expression does not exclude other components from the composition which do not render it unsuitable for a jacketing composition which is noncontaminating to polyethylene at temperatures in the range from to C. and which remains flexible at temperatures to below -40 C.

All parts and percentages are by weight unless otherwise indicated.

I claim:

1. A high-frequency cable jacketing material comprising an extrudable plasticized polymeric composition of matter containing as sole components of polymeric material and polyester a polymeric material selected from the group consisting of homopolymers of vinyl chloride and copolymers of vinyl chloride and vinyl acetate, said copolymers containing up to 10% by Weight of vinyl acetate, and a polyester obtained by reacting together 1,3-butanediol and sebacic acid to a reaction stage to give a polyester liquid reaction product having a refractive index n ZS C. in the range from 1.4686 to 1.4696, said jacketing material having a standard brittle point temperature of at least minus forty degrees Centigrade and being substantially. non-contaminating to polyethylene.

2. A jacketing material in accordance with claim 1 wherein said polymeric material consists essentially of a copolymer of 95% vinyl chloride and vinyl acetate.

3. A jacketing material in accordance with claim 2 wherein said jacketing material includes 58 parts of said polymeric material, 42 parts of said polyester, 3 parts of basic silicate of lead and 0.5 part of stearic acid.

4. A high-frequency coaxial cable having a center conductor, a metal braid surrounding said center conductor, a polyethylene dielectric layer disposed between said center conductor and braid and lying in close contact with both and an outer thermoplastic jacketing layer nondeleterious to said dielectric tightly fitting around said metallic braid, the material constituting said jacketing layer comprising an extrudable plasticized polymeric composition of matter containing as sole components of polymeric material and polyester a polymeric material selected from the group consisting of homopolymers of vinyl chloride and copolymers of vinyl chloride and vinyl acetate, said copolymers containing up to by Weight of vinyl acetate, and a polyester obtained by reacting together 1,3-butanediol and sebacic acid to a reaction stage to give a polyester liquid reaction product having a refractive index n C. in the range from 1.4686 to 1.4696, said jacketing material having a standard brittle point temperature of at least C. and being substan-,

standard brittle point temperature of at least -40 C.

and being substantially non-contaminating to polyethylene, comprising mixing 58 parts of a polymeric material selected from the group consisting of homopolymers of vinyl chloride and copolymers of vinyl chloride and vinyl acetate, said copolymers containing up to 10% by weight of vinyl acetate, 3 parts of basic silicate of lead, 0.5 part of stearic acid and 42 parts of a polyester reaction product, said polyester reaction product being formed by reacting together as the essential reacting ingredients 1,3- butanediol and sebacic acid to a reaction stage to give a polyester liquid reaction product having a refractive index #1 25 C. in the range from 1.4686 to 1.4696.

References Cited in the file of this patent UNITED STATES PATENTS 2,555,062 Small et al May 29, 1951 FOREIGN PATENTS 586,826 Great Britain Apr. 1, 1947 615.256 Great Britain Jan. 4, 1949

Claims (1)

1. A HIGHLY-FREQUENCY COAXIAL HAVING A CENTER CONDUCTOR, A METAL BRAID SURROUNDING SAID CENTER CONDUCTOR, A POLYETHYLENE DIELECTRIC LAYER DISPOSED BETWEEN SAID CENTER CONDUCTOR AND BRAID AND LAYING IN CLOSE CONTACT WITH BOTH AND AN OUTER THERMOPLASTIC JACKETING LAYER NONDELECTERIOUS TO SAID DIELECTRIC TIGHTLY FITTING AROUND SAID METALLIC BRAID, THE MATERIAL CONSTITUTING SAID JACKETING LAYER COMPRISING AN EXTRUDABLE PLASTICIZED POLYMERIC COMPOSITION OF MATTER CONTAINING AS SOLE COMPONENTS OF POLYMERIC MATERIAL AND POLYESTER A POLYMERIC MATERIAL SELECTED FROM THE GROUP CONSISTING OF HOMOPOLYMERS OF VINYL CHLORIDE AND COPOLYMERS OF VINYL CHLORIDE AND VINYL ACETATE, SAID COPOLYMERS CONTAINING UP TO 10% BY WEIGHT OF VINYL ACETATE, AND A POLYESTER OBTAINED BY REACTION TOGETHER 1,3-BUTANEDIOL AND SEBACIC ACID TO A REACTION STAGE TO GIVE A POLYSTER LIQUID REACTION PRODUCT HAVING A REFRACTIVE UNDEX ND25* C. IN THE RANGE FROM 1.4388 TO 1.4696, SAID JACKETING MATERIAL HAVING A STANDARD BRITTLE POINT TEMPERATURE OF AT LEAST -40* C. AND BEING SUBSTANTIALLY NON-CONTAMINATING TO POLYETHYLENE.

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