Abstract:
An electrical cable of conductive wire having an insulating layer of expanded, microporous sintered polytetrafluoroethylene around it, followed by a coating of a vinylidene fluoride-hexafluoropropylene copolymer surrounding the insulating layer, and an outer coating of polyvinylidene fluoride.

Description:
FIELD OF THE INVENTION 
     This invention relates to electrical cable and to a method for preparing it. 
     BACKGROUND OF THE INVENTION 
     Heretofore, conductive electrical wire, such as copper wire, has been coated with film of microporous, expanded polytetrafluoroethylene (PTFE) to provide an insulated wire. The PTFE coating provided a low dielectric, flexible, chemically resistant protective covering surrounding the conductive wire. This assembly is then ordinarily provided with an outer insulative covering of full density, non-expanded polytetrafluoroethylene to provide a covering that is heat resistant, is chemically inert, is resistant to being cut (called cut-through resistance) and is of long flex life. This configuration is particularly useful for flat or ribbon cable, such as is described in U.S. Pat. No. 4,443,657. 
     It is desirable to provide a cable that has the attributes of the above-described cable and has substantially improved abrasion resistance, further improved cut-through resistance, high coefficient of kinetic friction, and increased stiffness. 
     The invention herein possesses these desirable features. 
     SUMMARY OF THE INVENTION 
     An electrical cable comprising: 
     (a) at least one conductive wire, 
     (b) an insulating layer surrounding the conductive wire, said insulating layer comprising expanded, microporous sintered polytetrafluoroethylene, 
     (c) a primer coating of a vinylidene fluoride-hexafluoropropylene copolymer surrounding the insulating layer, 
     (d) an outer coating covering and surrounding the primer coating comprising polyvinylidene fluoride. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 represents a three dimensional perspective view of one embodiment of the cable of this invention. 
     FIG. 2 represents a cutaway enlarged view of the cable of FIG. 1 taken along line 2--2 of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The cable of the present invention is particularly adaptable for use where ribbon or flat cable containing a plurality of parallel wire conductors in coplanar configuration are desired. One advantage of the cable of this invention is the excellent abrasion resistance that is achieved, while further improving the good cut-through resistance of previous cable constructions. The cable of this invention also shows higher coefficient of kinetic friction than other cables with the above properties, and shows increased stiffness than other cables with the above properties. 
     With reference to FIGS. 1 and 2, there is provided a plurality of center wire conductors 1, surrounded by insulation of low dielectric 2 which is expanded, microporous polytetrafluoroethylene made generally as described in U.S. Pat. No. 3,953,566. 
     Surrounding the insulation 2 is a layer of primer coating of vinylidene fluoride-hexafluoropropylene copolymer 3. A representative vinylidene fluoride-hexafluoropropylene copolymer is Kynar Flex 2850 provided by Pennwalt. This layer is ordinarily applied by solution coating, as for example, dip-coating the insulated wire in a solution of the vinylidene fluoride-hexafluoropropylene copolymer. A typical solution of such vinylidene fluoride-hexafluoropropylene copolymer is a 5-20% by weight solution in a suitable organic solvent, as for example, 1-methyl-2-pyrrolidinone. Temperature and pressure are not critical. 
     The coating 3 is applied as a primer solution to enable the outer coating 4 to be applied with ease and good adherability. The primer coating 3 works its way partially into the pores of the microporous, expanded polytetrafluoroethylene and provides a firm interlocking bond therewith, thus, providing a firm foundation for the outer jacket coating 4. 
     The outer coating 4 is a layer of polyvinylidene fluoride (PVDF). A representative PVDF is Solef 11010 supplied by Soltex Polymer Corp. The jacket may contain suitable filler material, such as flame retardants and fibrous materials. The outer coating 4 is applied as a melt extrudate and is bonded to vinylidene fluoride-hexafluoropropylene copolymer primer coating 3 with the aid of vacuum. 
     The fabrication of the conductor cable includes the initial steps of embedding the conductors in separate top and bottom inner films of microporous unsintered PTFE and compressing the films. The cable is fabricated using initially unsintered layers of PTFE; the insulation layers are made out of unsintered, expanded microporous PTFE. The flexible PTFE insulated ribbon cable can be produced in an advantageous manner in a one-step continuous process in that the conductors are embedded in two inner films of porous expanded, unsintered PTFE by combining those elements at room temperature in a roll nip under pressure. This assembly is then subjected to heat to sinter the PTFE. 
     The resulting insulated wire is then subJected to a primer coating solution of vinylidene fluoride-hexafluoropropylene copolymer by any usual means. One such means is by immersing the insulated wire into the solution and passing the wire continuously through the solution. Room temperature and pressures are conveniently used. The resulting insulated wire now is coated with primer coating 3. The coated wire is dried to remove solvent. 
     Next, a coating of PVDF is extruded on the cable by passing the cable through an extrusion head. Temperature of the PVDF extrudate may be 200° to 300° C. The extrudate comes into contact with the cable while still molten and makes a bond with the vinylidene fluoride-hexafluoropropylene copolymer primer. The resulting assembly is then cooled. 
     EXAMPLE 1 
     20 conductors, each of 28 gauge 7 strand bare copper wire obtained from Hudson International Conductors, Inc., spaced on 0.050 inch centers, were continuously coated with 2 layers of expanded microporous 0.006 inch thick PTFE tape obtained from W.L. Gore &amp; Associates, Inc., Newark, Delaware, by passing the wires and the tape on each side thereof through the nip of 2 compression rolls at 80 pounds pressure at a pull weight of about 20 pounds, and then the PTFE layers were sintered by feeding into a bath of molten salt at about 400° C. at a line speed of about 15 feet per minute and then cooled by subjecting to water at 15° C. This procedure embeds the conductors between the 2 layers of PTFE tape. The two PTFE layers are bonded by the sintering process. 
     The laminated wire was then dipped in a solution of vinylidene fluoride-hexafluoropropylene copolymer (Kynar Flex 2850) and 1-methyl-2-pyrrolidinone, at a line speed of about 4 feet per minute, at room temperature and pressure. The cable was then dried by means of a hot air oven at temperatures up to 380° F. 
     The primer coated cable was then passed through the head of a screw extruder, with zone temperatures of 225, 262, 270, 274, 275, 276 and 276° C., at a line speed of about 27.5 feet per minute, and withdrawn under vacuum. PVDF was extruded about the cable to form an outer coating around the exposed cable surfaces. 
     The abrasion resistance of the cable assembly was determined by MIL-T-5438. The cable was too wide to fit the testing machine and was slit to provide 8 in the cable assembly. For the side of the assembly that was about 13.4 mils thick (average), the amount of abrasive tape used before the tape wore through the cable was 129 inches (average). 
     In the Tabor stiffness test, the stiffness values obtained resulted in an average stiffness of 131.6 grams centimeters. 
     The coefficient of friction, as determined by ASTM D1894 was 3.371. The cut-through resistance was good.