FEP CABLES
United States Patent 3650827
Electric cables of the type known as composite electrical insulated cables are insulated with a first inner layer of insulation material such as polyolefin and second outer layer of an irradiated cross-linked co-polymer of tetrafluoroethylene and hexafluoropropylene known in the trade as FEP to form a superior high temperature cable.
US Patent References:
Process for promoting adhesion to difficultly wettable polymer surface
Greyson - May 1959 - 2888367
Irradiated polyethylene and products therefrom
Mathes et al. - March 1960 - 2929744
Extrusion of cross-linked polyethylene and process of coating wire thereby
Vostovich et al. - March 1960 - 2930083
Method for marking a perfluorocarbon resin surface and composition therefor
Osdal - August 1961 - 2998332
Coated article comprising a substrate of polyethylene or polyamide and a grafted coating of polytetrafluoroethylene or polymethyl methacrylate
Burk et al. - September 1961 - 2999772
Process for promoting adhesion to difficultly wettable polymer surface
Greyson - May 1959 - 2888367
Irradiated polyethylene and products therefrom
Mathes et al. - March 1960 - 2929744
Extrusion of cross-linked polyethylene and process of coating wire thereby
Vostovich et al. - March 1960 - 2930083
Method for marking a perfluorocarbon resin surface and composition therefor
Osdal - August 1961 - 2998332
Coated article comprising a substrate of polyethylene or polyamide and a grafted coating of polytetrafluoroethylene or polymethyl methacrylate
Burk et al. - September 1961 - 2999772
Inventors:
Brown, Chester A. (Andover, MA)
Rossetti, Louis F. (Arlington, MA)
Rossetti, Louis F. (Arlington, MA)
Application Number:
04/877269
Publication Date:
03/21/1972
Filing Date:
11/17/1969
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Export Citation:
Assignee:
Electronized Chemicals Corp. (Burlington, MA)
Primary Class:
Other Classes:
427/118, 427/388.200, 427/379, 428/421, 428/920
International Classes:
B05D3/06; B05D7/20; H01B3/44; H01B7/28; H01B7/17; B44D1/42
Field of Search:
117/128.4,232,218,161H,93.31
US Patent References:
Primary Examiner:
Katz, Murray
Assistant Examiner:
Speer, Raymond M.
Claims:
I claim
1. A structural article of manufacture comprising at least one electric conductor and an insulating material surrounding at least a portion of said conductor, said insulating material comprising an inner layer comprising an irradiated cross-linked polyolefin and an outer layer comprising an irradiated cross-linked co-polymer of tetrafluoroethylene and hexafluoropropylene.
2. A structural article comprising a first layer consisting a polyolefin and a second layer consisting of a co-polymer of tetrafluoroethylene and hexafluoropropylene, each of said layers being irradiated and thereby cross-linked throughout and each of said layers being irradiated and thereby cross-linked at least to an extent that when heated above its respective crystalline melting temperature it has form stability.
3. The structural article of claim 2 wherein said polyolefin is polyethylene or its copolymers.
4. An electric cable comprising at least one conductor, a plurality of layers of insulation surrounding said conductor, one of said layers comprising an extruded cross-linked polyolefin intimately in contact with said electric conductor and a sheath surrounding said cross-linked polyolefin material and in intimate contact with said cross-linked polyolefin, said sheath comprising an irradiated cross-linked co-polymer of tetrafluoroethylene and hexafluoropropylene.
5. The method of forming an electrically insulated current carrying body comprising selecting electrical current carrying bodies whose lengths are substantially greater than their widths, forming a polyolefin layer over substantially the entire length of one of said electrical current carrying bodies, irradiating and thereby cross-linking said cross-linked polyolefin, coating said polyolefin layer with a co-polymer of tetrafluoroethylene and hexafluoropropylene and, irradiating and thereby cross-linking said co-polymer, said cross-linking being accomplished above the glass transition temperature of the co-polymer.
6. The method of claim 5 wherein said irradiation comprises high energy electrons sufficient to penetrate said co-polymer.
7. The method of claim 6 wherein the irradiation is accomplished at temperatures above the internal friction temperature of the co-polymer.
1. A structural article of manufacture comprising at least one electric conductor and an insulating material surrounding at least a portion of said conductor, said insulating material comprising an inner layer comprising an irradiated cross-linked polyolefin and an outer layer comprising an irradiated cross-linked co-polymer of tetrafluoroethylene and hexafluoropropylene.
2. A structural article comprising a first layer consisting a polyolefin and a second layer consisting of a co-polymer of tetrafluoroethylene and hexafluoropropylene, each of said layers being irradiated and thereby cross-linked throughout and each of said layers being irradiated and thereby cross-linked at least to an extent that when heated above its respective crystalline melting temperature it has form stability.
3. The structural article of claim 2 wherein said polyolefin is polyethylene or its copolymers.
4. An electric cable comprising at least one conductor, a plurality of layers of insulation surrounding said conductor, one of said layers comprising an extruded cross-linked polyolefin intimately in contact with said electric conductor and a sheath surrounding said cross-linked polyolefin material and in intimate contact with said cross-linked polyolefin, said sheath comprising an irradiated cross-linked co-polymer of tetrafluoroethylene and hexafluoropropylene.
5. The method of forming an electrically insulated current carrying body comprising selecting electrical current carrying bodies whose lengths are substantially greater than their widths, forming a polyolefin layer over substantially the entire length of one of said electrical current carrying bodies, irradiating and thereby cross-linking said cross-linked polyolefin, coating said polyolefin layer with a co-polymer of tetrafluoroethylene and hexafluoropropylene and, irradiating and thereby cross-linking said co-polymer, said cross-linking being accomplished above the glass transition temperature of the co-polymer.
6. The method of claim 5 wherein said irradiation comprises high energy electrons sufficient to penetrate said co-polymer.
7. The method of claim 6 wherein the irradiation is accomplished at temperatures above the internal friction temperature of the co-polymer.
Description:
BACKGROUND OF THE INVENTION
It has long been known that polyolefins, such as polyethylene, are excellent insulating materials for electric wires, electrical components and the like. Generally, such wires consist of polyolefins covered with an outer layer of polyvinylidene fluoride (tradename Kynar) in which the polymer comprising each one of these layers is cross-linked. Cables of this type have been described in U.S. Pat. No. 3,269,862. Difficulties with such prior art cables have arisen since the dielectric properties of polyolefins are offset by their relatively low melting point and their low resistance to flame and oxidation while the polyvinylidenes have been noted for their poor mechanical strength, low operating temperatures and degradation during extrusion under elevated temperatures.
It is important therefore that when cables are to be used at high temperatures or in mechanically abrasive areas that they be coated with a material which has high mechanical strength, high operating temperature and can be easily fabricated.
SUMMARY OF THE INVENTION
I have now discovered that a superior cable of this type will result from the use of a co-polymer of tetrafluoroethylene and hexafluoropropylene which has been cross-linked by either irradiation or chemical action. Unexpectedly, such a wire provides not only elevated structural strengths and elevated operating temperatures, but also permits the wire construction to be reduced in thickness and further provides a wire which becomes essentially non-flammable and chemically inert. The cable of the invention therefore essentially comprises a conductor, preferably of copper because of its high conductivity, an inner coating of insulation, such as cross-linked polyolefin, coated with a cross-linked co-polymer of tetrafluoroethylene and of hexafluoropropylene (FEP) surrounding the polyolefin inner layer.
BRIEF DESCRIPTION OF THE DRAWING
A more thorough understanding of our invention can be gained from the appended drawing where the FIGURE shows a cut-away prospective view of the end of a sheet cable made in accordance with our invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The cable indicated in the drawing has a central copper conductor which may be in stranded or unstranded form, coated with a cross-linked polyolefin extruded thereon with a sheath of cross-linked FEP extruded over this insulating material. As noted above this outer sheath is in accordance with the present invention composed of a co-polymer of tetrafluoroethylene and hexafluoropropylene (FEP) either in its natural state or radiation cross-linked. Although the drawing discloses but a single conductor device, the invention is not to be limited to such a single conductor device but may also be applied to multi-conductor cables where one or more conductors are included within a single sheath.
In each of these known general constructions, however, the cable of the present invention is characterized by the novel use of a thin sheath of cross-linked FEP over the cross-linked polyolefin material. A suitable FEP material is sold by E. I. duPont de Nemours and Company of Wilmington, Delaware under the designation Teflon 100 FEP Fluorocarbon resin. The method of making the preferred cable consists of coating a central copper conductor with a composition consisting of a mixture of high density, high molecular weight polyethylene composition, plus ingredients such as antioxidants, cross-linking promoters and flame retardants to provide various desired characteristics as is now known to the art. This insulated wire is then subjected to an irradiation dose of approximately 10 megarads using high energy electrons as the irradiation source. Following irradiation of the initial polyolefin coating, a thin layer of FEP is extruded over the irradiated polyolefin by well-known techniques. The FEP material utilized for the purpose preferably comprises the Teflon 100 FEP fluorocarbon resin noted above. Following the extrusion of the FEP sheath, high energy electron, X-rays or ultraviolet light is used to induce cross-linking in the FEP sheath. It is important to note that it is necessary this irradiation of this jacket occur at temperatures above the glass (internal friction) transition temperature of the FEP where cross-linking predominates over degradation. By proper selection of radiation dose and temperature, modified FEP resins with wide ranges of properties can be made.
Large doses of radiation cross-linked the Teflon 100 FEP resin co-polymer such that its resistance to high temperature cut-through is significantly improved. Small amounts of radiation alter the resin's melt-flow characteristics by changing its molecular weight and molecular weight distribution. This shift in molecular weight and molecular weight distribution also has a significant effect on the dependence of a shear rate on shear stress. Generally, electron irradiations for such operations is carried out using 2 Mev electrons. Such high energy electrons can easily be realized from a 3 Mev Van de Graaff accelerator.
To irradiate cable coated in the described manner, it is preferable that the cable be passed under a 250 microamp - 2 Mev beam at a rate such that the beam energy exposure of the completed cable is approximately 11 watts-seconds/cm. 2 /pass. The cable will thus receive a total dose of approximately 1.3 megarads. When the described cable is irradiated in this fashion at room temperature, net degradation will occur. However, if the temperature is raised to the glass (internal friction) transition temperature of the FEP resin (approximately 80° C.) the cross-linking of the resin becomes predominant and net degradation does not occur. At this glass (internal friction) transition temperature, under a constant rate dose of irradiation, the melt viscosity of the resin increases with temperature beyond the crystalline melting point. At higher temperatures (greater than 300° C. for FEP resin) thermal degradation again becomes a factor and net increases in viscosity in the resin are smaller. Thus it is important that during the irradiation of the FEP sheath that the temperature of the cable be maintained above 80° C. but less than 300° C.
Cable sheath irradiated above the glass (internal friction) transition temperature exhibited not only the normal excellent electrical properties associated with fluorocarbons but also exhibited increased aging resistance, improved elongation and deformation resistance properties, increased stress, resistance, decreased flow rate at low stress levels with no change in flow rate at high stress levels, increased tensile strength with increasing radiation up to a dose of about 6.5 megarads as set forth in the tables below: ##SPC1##
At dose rates greater than 2.6 megarads, improved elongation, resistance to deformation under load at elevated temperatures, increased stress resistances with only some slight loss in toughness. When the cable is irradiated at less than 1 megarad, the sheath retains its full characteristics at high stresses while at low stresses there is an advantageous decrease in flow rate. Thus, the FEP resin of the sheath increases the cable's mechanical strength at high operating temperatures while simultaneously preventing oxidation of the underlying polyolefin thus preventing degradation of the product. ------------------------------------------------------------ --------------- TABLE II
fep fep irradiated Unirrad- 2.6 mr. iated at 250° C. ____________________________________________________________ ______________ Cut Through, time in hours at 250° C., 1/4" mandrel 40 500
Dielectric Constant after 1000 hours exposure 2.04 2.06 Dissipation Factor after 1000 hours exposure 0.0001 0.0001
Yield Stress p.s.i. at 23° C. 1,920 2,030 at 225° C. 333 290
tensile Strength p.s.i. at 23° C. 3,330 3,010 at 225° C. 333 316
ultimate Elongation % at 23° C. 354 383 at 225° C. 160 498
flexural Modulus p.s.i. at 23° C. 102,000 110,000 at 100° C. 15,900 20,900 at 200° C. 6,310 5,290 ____________________________________________________________ ______________
the method of cable construction thus described permits a significant increase in the operating temperature of cables by creating a wire construction which utilizes one of the best insulations known today, namely irradiated polyolefin. It further provides elevated structural strengths, elevated operating temperatures, reduction in thickness, reduction in weight and prevents combustion of the irradiated polyolefin wire in normally combustible situations. All of this is achieved without sacrificing the desirable electrical properties of the irradiated cross-linked polyolefin insulating layer.
Additionally the outer layer of cross-linked FEP provides a highly adherent oxygen barrier preventing oxygen degradation at high temperatures of the inner polyolefin insulating layer. This permits a great increase in the allowable noted temperature of the wire with a reduced thickness which is essentially non-flammable and electrically inert.
It should be noted that the irradiated polyolefin inner layer can be compounded with antioxidants, cross-linking promoters and flame retardants.
It has long been known that polyolefins, such as polyethylene, are excellent insulating materials for electric wires, electrical components and the like. Generally, such wires consist of polyolefins covered with an outer layer of polyvinylidene fluoride (tradename Kynar) in which the polymer comprising each one of these layers is cross-linked. Cables of this type have been described in U.S. Pat. No. 3,269,862. Difficulties with such prior art cables have arisen since the dielectric properties of polyolefins are offset by their relatively low melting point and their low resistance to flame and oxidation while the polyvinylidenes have been noted for their poor mechanical strength, low operating temperatures and degradation during extrusion under elevated temperatures.
It is important therefore that when cables are to be used at high temperatures or in mechanically abrasive areas that they be coated with a material which has high mechanical strength, high operating temperature and can be easily fabricated.
SUMMARY OF THE INVENTION
I have now discovered that a superior cable of this type will result from the use of a co-polymer of tetrafluoroethylene and hexafluoropropylene which has been cross-linked by either irradiation or chemical action. Unexpectedly, such a wire provides not only elevated structural strengths and elevated operating temperatures, but also permits the wire construction to be reduced in thickness and further provides a wire which becomes essentially non-flammable and chemically inert. The cable of the invention therefore essentially comprises a conductor, preferably of copper because of its high conductivity, an inner coating of insulation, such as cross-linked polyolefin, coated with a cross-linked co-polymer of tetrafluoroethylene and of hexafluoropropylene (FEP) surrounding the polyolefin inner layer.
BRIEF DESCRIPTION OF THE DRAWING
A more thorough understanding of our invention can be gained from the appended drawing where the FIGURE shows a cut-away prospective view of the end of a sheet cable made in accordance with our invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The cable indicated in the drawing has a central copper conductor which may be in stranded or unstranded form, coated with a cross-linked polyolefin extruded thereon with a sheath of cross-linked FEP extruded over this insulating material. As noted above this outer sheath is in accordance with the present invention composed of a co-polymer of tetrafluoroethylene and hexafluoropropylene (FEP) either in its natural state or radiation cross-linked. Although the drawing discloses but a single conductor device, the invention is not to be limited to such a single conductor device but may also be applied to multi-conductor cables where one or more conductors are included within a single sheath.
In each of these known general constructions, however, the cable of the present invention is characterized by the novel use of a thin sheath of cross-linked FEP over the cross-linked polyolefin material. A suitable FEP material is sold by E. I. duPont de Nemours and Company of Wilmington, Delaware under the designation Teflon 100 FEP Fluorocarbon resin. The method of making the preferred cable consists of coating a central copper conductor with a composition consisting of a mixture of high density, high molecular weight polyethylene composition, plus ingredients such as antioxidants, cross-linking promoters and flame retardants to provide various desired characteristics as is now known to the art. This insulated wire is then subjected to an irradiation dose of approximately 10 megarads using high energy electrons as the irradiation source. Following irradiation of the initial polyolefin coating, a thin layer of FEP is extruded over the irradiated polyolefin by well-known techniques. The FEP material utilized for the purpose preferably comprises the Teflon 100 FEP fluorocarbon resin noted above. Following the extrusion of the FEP sheath, high energy electron, X-rays or ultraviolet light is used to induce cross-linking in the FEP sheath. It is important to note that it is necessary this irradiation of this jacket occur at temperatures above the glass (internal friction) transition temperature of the FEP where cross-linking predominates over degradation. By proper selection of radiation dose and temperature, modified FEP resins with wide ranges of properties can be made.
Large doses of radiation cross-linked the Teflon 100 FEP resin co-polymer such that its resistance to high temperature cut-through is significantly improved. Small amounts of radiation alter the resin's melt-flow characteristics by changing its molecular weight and molecular weight distribution. This shift in molecular weight and molecular weight distribution also has a significant effect on the dependence of a shear rate on shear stress. Generally, electron irradiations for such operations is carried out using 2 Mev electrons. Such high energy electrons can easily be realized from a 3 Mev Van de Graaff accelerator.
To irradiate cable coated in the described manner, it is preferable that the cable be passed under a 250 microamp - 2 Mev beam at a rate such that the beam energy exposure of the completed cable is approximately 11 watts-seconds/cm. 2 /pass. The cable will thus receive a total dose of approximately 1.3 megarads. When the described cable is irradiated in this fashion at room temperature, net degradation will occur. However, if the temperature is raised to the glass (internal friction) transition temperature of the FEP resin (approximately 80° C.) the cross-linking of the resin becomes predominant and net degradation does not occur. At this glass (internal friction) transition temperature, under a constant rate dose of irradiation, the melt viscosity of the resin increases with temperature beyond the crystalline melting point. At higher temperatures (greater than 300° C. for FEP resin) thermal degradation again becomes a factor and net increases in viscosity in the resin are smaller. Thus it is important that during the irradiation of the FEP sheath that the temperature of the cable be maintained above 80° C. but less than 300° C.
Cable sheath irradiated above the glass (internal friction) transition temperature exhibited not only the normal excellent electrical properties associated with fluorocarbons but also exhibited increased aging resistance, improved elongation and deformation resistance properties, increased stress, resistance, decreased flow rate at low stress levels with no change in flow rate at high stress levels, increased tensile strength with increasing radiation up to a dose of about 6.5 megarads as set forth in the tables below: ##SPC1##
At dose rates greater than 2.6 megarads, improved elongation, resistance to deformation under load at elevated temperatures, increased stress resistances with only some slight loss in toughness. When the cable is irradiated at less than 1 megarad, the sheath retains its full characteristics at high stresses while at low stresses there is an advantageous decrease in flow rate. Thus, the FEP resin of the sheath increases the cable's mechanical strength at high operating temperatures while simultaneously preventing oxidation of the underlying polyolefin thus preventing degradation of the product. ------------------------------------------------------------ --------------- TABLE II
fep fep irradiated Unirrad- 2.6 mr. iated at 250° C. ____________________________________________________________ ______________ Cut Through, time in hours at 250° C., 1/4" mandrel 40 500
Dielectric Constant after 1000 hours exposure 2.04 2.06 Dissipation Factor after 1000 hours exposure 0.0001 0.0001
Yield Stress p.s.i. at 23° C. 1,920 2,030 at 225° C. 333 290
tensile Strength p.s.i. at 23° C. 3,330 3,010 at 225° C. 333 316
ultimate Elongation % at 23° C. 354 383 at 225° C. 160 498
flexural Modulus p.s.i. at 23° C. 102,000 110,000 at 100° C. 15,900 20,900 at 200° C. 6,310 5,290 ____________________________________________________________ ______________
the method of cable construction thus described permits a significant increase in the operating temperature of cables by creating a wire construction which utilizes one of the best insulations known today, namely irradiated polyolefin. It further provides elevated structural strengths, elevated operating temperatures, reduction in thickness, reduction in weight and prevents combustion of the irradiated polyolefin wire in normally combustible situations. All of this is achieved without sacrificing the desirable electrical properties of the irradiated cross-linked polyolefin insulating layer.
Additionally the outer layer of cross-linked FEP provides a highly adherent oxygen barrier preventing oxygen degradation at high temperatures of the inner polyolefin insulating layer. This permits a great increase in the allowable noted temperature of the wire with a reduced thickness which is essentially non-flammable and electrically inert.
It should be noted that the irradiated polyolefin inner layer can be compounded with antioxidants, cross-linking promoters and flame retardants.