US 3705257 A
In an electric cable having a semiconducting jacket applied directly over a wall of polymeric insulation, excessive bonding of the jacket to the insulation is prevented by applying the curing agent for the jacket only to its outer surface after extrusion, and then effecting a vulcanization such that the inner jacket surface remains thermoplastic.
Description (OCR text may contain errors)
United States Patentv Wade v 1451 Dec. 5, 1972 1541 ELECTRIC CABLE AND METHOD OF, I
MAKING 1 v  Inventor: 'RobertM. Wade, Wabash, Ind.
 Assignee: Anaconda Wire and Cable Company, New York, NY.
 Filed: March 6, 1972  Appl. No.: 231,840
 US. Cl. ..174/115, l17/75, l74/l 10 PM,
, '174/12osc  -lnt.Cl. ..H01b7 /02,11o1b9/02-  Field of Search. 174/120 R, 1 20 C,v 1-20 SC, 120 SR,l74/l 15, 110 PM; 117/69, 72,7 5; 252/637,
 References Cited UNITED STATES PATENTS 3,666,877 5/1972 Arnaudimlr.etal....,. ;.,.i7 i/l2OSC Ling et al ..174/12o sc 3,646,248 2/1972 3,614,300 10/1971 Wilson .......l74/l20R 3,571,613 3/1971 Plate ..174/11sx 3,474,189 10/1969 Plate et al ..174/11s 3,541,228 11/1970 Lombardi ..174/12o SC Arnaudin, Jr. et al. 174/120 R Primary Examiner-Lewis H. Myers Assistant Examiner-A. T. Grimley AttorneyVictor F. Volk  ABSTRACT In an electric cable having a semiconducting jacket applied directly over a wall of polymeric insulation, excessive bonding of the jacket to the insulation is prevented by applying the curing agent for the jacket only toits outer surface after extrusion, and then effecting a vulcanization such that the inner jacket surface remains thermoplastic.
, v 24 Claims, 2 Drawing Figures 1 j ELECTRIC CABLEAND METHOD OF MAKING BACKGROUND OF THE INVENTION Economies in high-voltage cables have recently been effected by eliminating the separate'shielding system, making the protective jacket semiconducting,'and applying it directly onto the wall of cable dielectric. Such a cable construction, wherein the jacket comprises embedded drain wires is described'in Plate et al. U.S. Pat. No. 3,474,189, the disclosures of which are included herein by reference. Although it would be-desirable to increase the toughness and tensile strength of the jacket material by vulcanization, this has not proven feasible because of the necessity .of avoiding adhesion between the semiconducting jacket and the outer surface of the wall of insulation. Such adhesion would make it difficult to free stripped ends of cablefrom residues of the electrically conducting jacket composition at terminations and joints.
SUMMARY BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a section of a cable made to my invention FIG. 2 shows .the steps ofa method of my invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, a cable of my invention, indicated generally by the numeral 10, has a conductor 11, a heavy wall 12 of insulation, and a semiconducting jacket 13 in which are embedded a plurality of drain wires 14. The wall 12 comprises cross-linked polyethylene, but my invention has application to cables with insulation walls of other compositionssuch,
for example, as ethylene-propylene rubber, polyvinyl chloride, and unvulcanized polyolefin. However, my invention has particular advantages for cross-linked polyethylene insulated cables since cross-linked polyethylene has a particularly high thermal coeff cient of expansion, and this places an extra strain on the jacket 13. The jacket 13 comprises chlorinated polyethylene blended with ethylene ethyl acrylate and a high proportion of carbon black to increase its electrical conductivity. A preferred thermoplastic jacket has been described in application Ser. No. 167,741 assigned to the assignees of the present invention, which has the advantage, that, being thermoplastic, it does not bond unduly to the wall 12. In the present invention an inner surface 16 of the jacket 13 remains thermoplastic but an outside surface 17 has been vulcanized to provide greater toughness and tensile strength. vulcanization of the outer portion of the jacket 13 is achieved by diffusion of vulcanizing agent through the surface 17 so that the jacket 13. is homogeneous except for radial differences in the degree of vulcanization or cross linking. This vulcanization decreases gradually through a radial-section of the jacket 13 so as to leave the inner surface 16 free from vulcanization. Chlorinated polyethylene has been emin two ways. The higher tensile strength of the vulcanized composition resists radial expulsion of the wires by electrical forces during surges of current, and the higher softening temperature of the vulcanized composition prevents the surface from melting during short periods of high ambient temperature.
In FIG. 2 I have diagrammed a method of making the cable 10 whereby a reel 21 of the conductor 11 is continuously paid through an extruder 22, a vulcanizing tube 23, and cooling section 24 for application of the dielectric wall 12. The insulated core then passes through a second extruder 26 wherein it receives the jacket 13 and drain wires 14.
So far the description of my method has been previously disclosed for the manufacture of cables with thermoplastic jackets. I now provide a novel step of coating the jacket 13 with a suitable curing agent by passing the cable 10 through a chamber 28 wherein it is sprayed or otherwise coated with di-a-cumyl peroxide. Although I prefer to use di-a-cumyl other peroxides are known for curing compositions comprising chlorinated polyethylene and polyethylene and these may be used within the scope of my invention, including tertiary peroxides in which the valences of the carbon atoms not linked to oxygen are attached to alkyl, cycloalkyl, alkylcycloalkyl, cycloalkyl-alkyl, aryl or aralkyl radicals, and also quinhydrone dimerides as disclosed in U.S. Pat. No. 3,036,982. Solvents other than acetone may be used, less volatile solvents having the advantage of holding the vulcanizing agent on the surface 17 through the entrance to a curing chamber. It is not essential to my invention, however, to apply the curing agent in solution. As detailed in an example hereinbelow, I have successfully applied the molten agent directly to the jacket. Curing agents can also be applied as suspensions or emulsions, such as water suspensions or emulsions.
- condensate drain 341 but a hot inert pressurized gas such as nitrogen or helium may be used instead of steam within the scope of my invention. The latter method has the advantage that it is easier to keep the application chamber relatively cool. To counteract the pressure in the tube 29 a pump 36 is used to meter the vulcanizing agent into he chamber 28. In lieu of the continuous process shown, the entire cable can be removed to a curing chamber after the surface has been coated with peroxide in solution. I have found that acetone works well as a solvent for di-a-cumyl peroxide in thisapplication' but other solvents, which have been known, can also be used. v
EXAMPLE 1 A cable made in accordance with the description of FIG. 1, hereinabove, with a jacket 13, 0.080 inch thick was brush coated with a 50 weight percent solution of di-a-cumyl peroxide in acetone. The acetone permitted to evaporate. Thereupon the cable was vulcanized in a steam chamber at 225 pounds per square inch in pressure for one minute. Specimens cut from the outer surface of the jacket with thicknesses from 0022-0032 inch were found to have an average-tensile strength of 1,586 psi compared to atensile strength of 1,373 psi for an unvulcanized jacket specimen and an average elongation of 183 percent compared to 327 percent for an unvulcanizedspecimen.
A length of the cable was coated with molten dia-cumyl peroxide at 75 psi. Specimens 0.020-0.024 inch thick were slicedfrom. the outer surface of the jacket and found to average 2,783 psi tensile strength and have 120 2 percent elongation. I Specimens 0.0l80.020 inch thick were then sliced from the underlying thickness of jacket for testing to determine the depth of diffusion of the curing agent. These averaged 2,870 psi in tensile strength and 207 percent elongation. The much greater elongation indicates presence of some thermoplastic composition at the deeper level.
EXAMPLE 3 A cable with 250 MCM conductor, a 0.175 inch wall of cross-linked polyethylene insulation and an 0.080 inch thick chlorinated polyethylene, ethylene ethyl acrylate jacket was coatedwith di-a-cumyl peroxide and cured .in steam at 200 psi for 1 minute. A specimen 0.0l50.020 inch thick was sliced from the outer surface of the jacket and found to have a tensile strength of 2,056 psi and an elongation of 162 percent. The conductor of a length of this cable was raised to 135C for 24 hours to determine if the expansion of the insulation would cause cracks to appear in the jacket. No cracks appeared. The conductor temperature was raised to 155C for 24 hours without causing any cracks to appear in the jacket. The conductor was raised to 200C for 1 hour without causing any cracks to appear over the drain wires. A circumferential crack did appear next to a holding clamp. For comparison, cracks appeared over the drain wires of a cable with an unvulcanized jacket at a conductor temperature of 135C after 2 hours.
l have invented a new and useful cable and method of making the same of which the foregoing description has been exemplary rather than definitive and for which I desire an award of Letters Patent as defined in the following claims.
1. An electric cable comprising:
A. a conductor,
B. a wall of dielectric polymeric composition surrounding said conductor,
C. a jacket of polymeric composition surrounding said wall, comprising:
1'. a thermoplastic inner surface directly adjacent to said wall,'and' 2. a vulcanized outer surface, said jacket being homogeneous save for a decreasing degree of vulcanization from said outer toward said inner surface. I
2. The cable of claim 1 wherein said wall comprises polyethylene.
3. The cable of claim 1 wherein said wall comprises vulcanized polyethylene.
4. The cable of claim 1 wherein said jacket is semiconducting.
Su'lhe cable of claim 1 whereinsaid jacket comprises chlorinated polyethylene.
6. The cable of claim 3 wherein said jacket comprises chlorinated polyethylene.
7. The cable of claim 4 comprising drain wires embedded within said jacket.
8. The method of making'a jacketed cable comprising the steps of; v
A. extruding a polymeric jacket free from vulcanizing agent around an insulated cable core,
B. applying a vulcanizing agent to the surface of said jacket, said agent being sufficient to vulcanize said surface but insufficient to vulcanize the entirety of said jacket,
C. heating said cable thereby vulcanizing said surface and diffusing said agent into said jacket.
9. The method of claim 8 wherein said vulcanizing agent is applied to said jacket in solution.
10. The method of claim 8 wherein said jacket comprises chlorinatedpolyethylene blended with ethylene ethyl acrylate copolymer.
11. The method of claim 8 wherein said agent comprises an organic peroxide.
12. The method of claim 8 wherein said agent comprises di-a-cumyl peroxide.
13. The method of claim 9 wherein said agent comprises di-a-cumyl peroxide.
14. The method of claim 9 wherein said solution comprises acetone and said agent comprises di-acumyl peroxide.
15. The method of claim 8 wherein said heating is effected under pressure.
16. The method of claim 15 wherein said heating is effected by pressurized steam.
17 The method of claim 15 wherein said agent is applied to said jacket in solution.
18. The method of claim 15 wherein said jacket comprises chlorinated polyethylene blended with ethylene ethyl acrylate copolymer.
19. The method of claim 15 wherein said agent comprises in organic peroxide.
20. The method of claim 15 wherein said agent comprises di-a-cumyl peroxide.
21. The method of claim 17 wherein said agent comprises di-a-cumyl peroxide.
22. The method of claim 17 wherein said agent comprises di-a-cumyl peroxide and said solution comprises acetone.
23. The method of claim 15 wherein said agent is ap plied within a chamber containing said pressure.
24. The method of claim 16 wherein said agnt is applied within a chamber folf said steam.
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