Electric Conductor Adapted for Use in Process Instrumentation

This invention relates generally to electrical conductors and more particularly to an insulated wire having improved properties at elevated temperatures.

Electric wires having a synthetic resinous insulation are used extensively in power plants, refineries and chemical plants for connecting instruments used in process control. For example, wires of this type are used to connect a potentiometer or the like to a remote thermocouple. Such a wire may also be used for electrically connecting a transmitter or differential cell with a process control room. The heretofore available insulated wires have not been entirely satisfactory because they are not dimensionally stable in the sense that the insulation tends to melt and flow off the wire at elevated temperatures.

It is therefore an object of this invention to provide an improved electrically insulated wire for use in instrumentation in chemical process plants, power plants, nuclear generating plants and the like. Another object of the invention is to provide an insulated conductor having dimensional stability and improved resistance to stress cracking which adapt it for use at elevated temperatures of 400.degree. F. or higher. Still another object of the invention is to provide a method for making an electrical conductor adapted for use as the connecting link in instrumentation in power plants, chemical process plants and the like.

Other objects will become apparent from the following description with reference to the accompanying drawing wherein:

FIG. 1 is a perspective view of an embodiment of the invention; and

FIG. 2 is an enlarged fragmentary perspective view of a metallic shield used in the embodiment of FIG. 1.

The foregoing objects and others are accomplished in accordance with this invention, generally speaking, by providing an electrical conductor having at least one pair of electrical conducting wires which are electrically insulated with an irradiation cured ethylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer, or polyvinyl chloride polymer or copolymer coating and a jacket of an irradiation cured halosulfonated polyethylene, polychloroprene, or chlorinated polyethylene. Preferably, an electrically conducting shield is disposed about the electrical conductors. An uninsulated drain wire may be disposed on either side of the shield in electrical contact therewith. A rip-cord may be disposed between the jacket and shield for facilitating the stripping of the jacket from about the electrical conductors. The electrical conductor provided by the invention may be made by a process wherein an electrical insulating coating of a composition containing ethylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene, or polyvinyl chloride polymer is extruded on a suitable wire, the resulting electrical insulated wire is associated with a ground wire and a metallic foil is folded about or helically wrapped about the wires, a composition containing a halosulfonated polyethylene, polychloroprene or chlorinated polyethylene is extruded over the wrapped wires and the resulting electrical conductor is cured by subjecting it to a source of high energy electrons.

Any suitable source of high energy electrons may be used for curing the polymer composition such as, for example, a Van de Graff accelerator or the like. Preferably, the energy of the electrons should be from about 100,000 electron volts to 3 mev. (million electron volts). The electrical conductor may be wound on a suitable spool or the like after the jacket has been extruded thereon for storage and may be later unwound and passed through a source of electrons when it is desired to cure the jacket and insulation. It has been found that the jacket and the insulation on a pair of wires can be cured simultaneously in one pass through the electron source but it is preferred to irradiate the insulation on the wires before the wires are enclosed in the jacket. A dosage of electrons should be from about 2 Mrads (megarads) to about 20 Mrads, preferably, from about 5 Mrads to about 12 Mrads. Curing may be at ambient temperature and atmospheric pressure. The wire may be passed through the electron source at any suitable speed with linear speeds of 500 ft. per minute usually providing a dosage sufficient to cure the polymers and improve the heat resistance properties of the insulated conductor.

One suitable ethylene-tetrafluoroethylene copolymer is sold by E.I. duPont de Nemours and Co. under the trade name "TEFZEL" and is described in U.S. Pat. No. 3,738,923. "HALAR" is the trade name for a suitable ethylene-chlorotrifluoroethylene copolymer sold by Allied Chemical Co. Ethylene-chlorotrifluoroethylene copolymers are also disclosed in U.S. Pat. No. 5,738,923.

The electrically conductive shield protects the conductors from electrostatic noise which would interfere with the performance of instruments connected by the conductors. The shield may be copper, aluminum or other metal foil or other electrically conductive layer such as electrically conductive polyethylene which is flexible. A metal foil coated with Mylar is preferred.

Referring now to the drawing, the illustrated embodiment of a cable 10 has a pair of electrically insulated conductors 24 and 28 of wires each having an electrical insulating coating 22 and 26, respectively, about 10 mils thick of an irradiation cured ethylene-tetrafluoroethylene copolymer, twisted with a tinned copper drain wire 30, a shield 16 having a flexible aluminum foil layer 20, a poly(ethylene terephthalate) ester coating 18 helically wound at a lay of about two inches about the wires 24, 28 and 30, a nylon 6 rip-cord 14 and a jacket 12 of an extruded chlorosulfonated polyethylene polymer sold by E.I. duPont de Nemours and Co. under the trade name "HYPALON". The shield 16 may be an aluminum foil about 0.1 to 2 mils thick having a coating of "Mylar" about 0.5 mil to about 5 mils thick adhered thereto. "MYLAR" is a poly(ethylene-terephthalate)ester sold by E.I. duPont de Nemours and Co.

Cable 10 is fabricated by extruding an unfilled ethylene-tetrafluoroethylene copolymer containing about 50 mol percent ethylene and about 50 mol percent tetrafluoroethylene mixed with about 3 parts triallylisocyanurate sensitizer per 100 parts polymer as a coating about 10 mils thick over twisted wires 24 and 28. The resulting electrically insulated conductors 24 and 28 are then twisted with a tinned copper drain wire 30 at a lay of about 2 inches and the shield 16 is helically wrapped thereabout. The resulting jacketed wire assembly is then enclosed in jacket 12 by extruding a coating about 20 - 80 mils thick thereover. The cable 10 is then passed through an electron accelerator at a rate which provides a dosage of 2 - 20 Mrads. to cure coatings 22, 26 and jacket 12.

The jacket 12 may be formed by extruding a mixture containing in parts by weight

100 parts Hypalon 45

30 parts litharge

60 parts hydrated alumina

10 parts antimony trioxide

5 parts chlorinated paraffin wax

3 parts petrolatum

2 parts paraffin wax

2 parts HVA-2 m-phenylene dimaleimide (sensitizer)

15 parts carbon black

Any other suitable compounds which sensitize a polymer to irradiation curing may be used in either the polymer of the jacket 12 or the polymer used for insulation 22 and 26. A triallyisocyanurate sensitizer or the like may be used with the ethylene-chlorotrifluoroethylene copolymer, polyvinyl dichloride polymer as well as with ethylene-tetrafluoroethylene. It is preferred to include a sensitizer in the composition to accelerate curing but suitable curing can be obtained with increased exposure time without the sensitizer.

Any suitable filler such as, for example, litharge, Kaolin, calcinated clay, carbon black, zinc oxide, talc, hydrous magnesium silicate, lead oxide, hydrated aluminum silicate or the like may be used in the polymer composition from which jacket 12 is extruded. The amount of filler should be sufficient to provide an extruded thermoplastic composition which can be handled without substantial distortion before curing. Preferably, the uncured jacket should have a hardness of at least Shore 45A and a minimum tensile modulus or tensile strength at 100% elongation of at least about 300 p.s.i. After curing, the jacket should have a hardness of at least about Shore 50A and a tensile strength at 100% elongation of about 400 p.s.i. Usually, from about 50 to 250 parts filler per 100 parts by weight polymer will provide a composition of sufficient hardness to facilitate extrusion of the jacket.

The composition from which the jacket is extruded may also contain a plasticizer such as a vinyl silane, paraffin oil or the like. An antioxidant may also be included such as, for example, poly(trimethyl-dihydroquinoline).

Conventional extrusion equipment may be used to form the insulation on the wires and to shape the jacket.

Conductors having wires insulated with an ethylene-chlorotrifluoroethylene copolymer or polyvinyl dichloride polymer and having a jacket shaped from compositions containing a polychloroprene or a chlorinated polyethylene can be substituted for the one described in detail in the foregoing description with reference to the drawing.

Although the invention is described in detail for the purpose of illustration it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.