WO1994014170A1

WO1994014170A1 - Insulated electrical wire

Info

Publication number: WO1994014170A1
Application number: PCT/US1993/001019
Authority: WO
Inventors: John E. Bacino
Original assignee: W.L. Gore & Associates, Inc.
Priority date: 1992-12-10
Filing date: 1993-02-04
Publication date: 1994-06-23
Also published as: ITTO930931A1; ITTO930931A0

Abstract

This invention is an insulated electrical wire in which an electrical conductor is wrapped with a tape made of a substrate of expanded porous polytetrafluoroethylene which has adhered on one side thereof a film of a thermoplastic polymer having a melting point below 342 °C and having a thickness of 9 microns or less.

Description

TITLE OF THE INVENTION

Insulated Electrical Wire

FIELD OF THE INVENTION

This invention is directed to an electrical wire and cable construction. More specifically, this invention is directed to electrical conductor wire wrapped with a special insulation.

BACKGROUND OF THE INVENTION

Polytetrafluoroethylene (PTFE) has chemical inertness, heat resistance, good electrical insulation properties, and a low coefficient of friction in a wide temperature range. However, when used as an insulating material on an electrical wire or cable construction, PTFE has low cut-through resistance, which is defined as the amount of force required for a sharp edge to penetrate through an insulation layer on a conductor and make electrical contact with the conductor.

PTFE may be produced in an expanded porous form as taught in U.S. Patent No. 3,953,566 to Gore. This material, called expanded porous polytetrafluoroethylene (ePTFE), is of a surprisingly higher strength than ordinary PTFE while maintaining the chemical inertness and heat resistance of ordinary PTFE. However, ePTFE exhibits the same relatively poor cut-through resistance that ordinary PTFE does.

A compressed densified ePTFE is taught in U.S. Patent No. 4,732,626 to Cooper, et al . This compressed densified ePTFE is expanded through the method taught in the Gore patent and is subsequently densified through calendaring to a density of about 1.6 g/cm³. This compressed densified ePTFE is subsequently wrapped on a conductor wire and heated to a temperature above 385°C. The resultant compressed densified ePTFE insulated conductor exhibits the high strength of ePTFE and has increased resistance to cut- through. However, the compressed densified ePTFE does not consistently produce inter-layer bonds of sufficient strength when wrapped on a conductor.

SUMMARY OF THE INVENTION

This invention is an insulated electrical wire in which an electrical conductor is wrapped with a tape made of a substrate of expanded porous polytetrafluoroethylene which has adhered, i.e. joined, to it on one side thereof a film of a thermoplastic polymer, which polymer has a melting point below 342°C and which film has a substantially uniform thickness of less than about 9 microns.

In a preferred aspect the thickness of the thermoplastic film is 7 microns or less, more preferably 5 microns or less.

Preferably the thermoplastic film is a fluoropolymer and most preferably a thermoplastic copolymer of tetrafluoroethylene.

Depending on the degree of stretching, the thermoplastic film can form a very thin, i.e., 9 microns or less thick, film on the surface of the expanded porous PTFE which is continuous and nonporous.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view partly in section of a composite article of the instant invention wrapped on a conductor.

Figure 2 is a schematic representation of a dynamic cut- through tester.

DETAILED DESCRIPTION OF THE INVENTION

The expansion, i.e., stretching, of polytetrafluoroethylene is a well-known procedure and is -.escribed in USP 3,953,566. Preliminarily, the type of PTFE called fine-powder, which may contain a filler material, is mixed with a sufficient amount of a hydrocarbon extrusion aid, usually an odorless mineral spirit ur - ^' a paste is formed. The paste is compressed into a billet and subsequently extruded through a paste extruder to form a coherent PTFE shape which can be in a the form of a rod, filament, tube or sheet which may be a thin film or membrane. The coherent PTFE shape is optionally compressed and then dried by volatilizing the hydrocarbon extrusion aid with heat. Volatilization of the extrusion aid results in a coherent PTFE shape having a small degree of porosity. The resulting porous PTFE material is now ready to be coated with the thermoplastic polymer and the coated material expanded. However, if a highly porous expanded PTFE product is desired, a pre-conditioned stretching step can be carried out by stretching at 200°-300°C, preferably for about 1.5 to 5 times the original length.

The coherent PTFE shape is combined with a thermoplastic polymeric layer by contacting the thermoplastic polymeric layer with a surface of the coherent PTFE shape. Ordinarily the PTFE is in sheet or membrane form and the thermoplastic polymer is in sheet or film form and the polymer sheet is placed on the PTFE sheet. The combination is heated to a temperature between the melt point of the thermoplastic polymeric layer and 365°C. The coherent PTFE shape is kept under tension while being heated thereby maintaining its dimensions while the thermoplastic polymeric layer is combined with the coherent PTFE shape. The means for heating the coherent PTFE shape may be any means for heating commonly known in the art including, but not limited to, a convection heat source, a radiant heat source or a conduction heat source. The conduction heat source may be a heated surface such as a heated drum or die or roll or curved plate. As the coherent PTFE shape is heated to a temperature above the melt point of the thermopl stic polymeric layer, the thermoplastic polymeric layer in contact with the coherent PTFE at least partially softens and adheres to the surface of the coherent PTFE shape thereby forming a composite precursor, i.e, a coated PTFE material ready to be expanded. When a conduction heat source is used as the means for heating the coherent PTFE shape, the surface of the coherent PTFE shape should be against the conduction heat source so as to prevent sticking and melting of the thermoplastic polymeric layer upon the conduction heat source. The thermoplastic polymeric layer is composed of a thermoplastic polymer that has a melting point of 342°C or less. Melting points of thermoplastic polymers are determined by Differential Scanning Calorimetry. The thermoplastic polymer is a polymer that will bond to the substrate and may be polypropylene, polyamide, polyester, polyurethane, or polyethylene. Preferably, the thermoplastic polymer is a thermoplastic fluoropolymer. Representative thermoplastic fluoropolymers include fluorinated ethylene propylene (FEP), copolymer of tetrafluoroethylene and perfluoro(propylvinyl ether)(PFA), homopolymers of polychlorotrifluoroethylene (PCTFE) and its copolymers with tetrafluoroethylene (TFE) or vinylidene fluoride (VF2), ethylene- chlorotrifluoroethylene (ECTFE) copolymer, ethylene- tetrafluoroethylene (ETFE) copolymer, polyvinylidene fluoride (PVDF), and polyvinylfluoride (PVF). Thermoplastic fluoropolymers are preferred as the thermoplastic polymer since thermoplastic fluoropolymers are similar in nature to PTFE, having melt points near the lowest crystalline melt point of PTFE, and therefore are relatively high temperature thermoplastic polymers. Thermoplastic fluoropolymers are also relatively inert in nature and therefore exhibit resistance to degradation from many chemicals.

The coated material is expanded by stretching it according to the method taught in U.S. Patent No. 3,953,566 to Gore. The temperature range at which expansion of the material is performed is between a temperature at or above the melt point of the thermoplastic polymer layer and a temperature at or below 342°C.

The material may be stretched uniaxially, only in a longitudinal direction; biaxially, in both longitudinal and transverse directions; or radially, .in both longitudinal and transverse directions simultaneously.. It may be stretched in one or more steps.

The coherent PTFE shape forms an expanded porous PTFE (ePTFE) article as it is stretched. The ePTFE article is characterized by a series of nodes interconnected by fibrils. As the coherent PTFE shape is expanded to form the ePTFE article, the thermoplastic polymer layer adhered to the coherent PTFE shape is carried along a surface of the coherent PTFE shape while in a melted state theret, forming a thin thermoplastic polymer film on the ePTFE article. The thin thermoplastic polymer film is less than 9 micron thick, and preferably has a thickness of one half, preferably one tenth, of the thermoplastic polymer layer's original thickness. For example, a thermoplastic polymer layer originally having a thickness of 1 mil (25.4 microns) could produce a thin thermoplastic polymer film having a thickness as low as 0.1 mil (2.54 microns) or less after expansion of the coherent PTFE shape into the ePTFE article.

The thermoplastic polymer layer is in contact with and is carried on a surface of the coherent PTFE shape as the coherent PTFE shape is expanded at a temperature at or above the thermoplastic polymeric film's melt point.

A second thermoplastic polymeric layer may be present on the other side of the expanded porous PTFE. This second thermoplastic polymeric layer may be simultaneously stretched with the first thermoplastic polymeric layer. Alternatively, the second thermoplastic polymeric layer may be stretched sequentially to the first thin thermoplastic polymeric film, but if re elting of the first thin thermoplastic polymeric film is to be avoided, the second thermoplastic polymeric layer should have a melt point below the melt point of the thin thermoplastic polymeric film, and the stretching of the second thermoplastic polymeric layer should be accomplished at a temperature at or above the melt point of the second thermoplastic polymeric layer but below the melt point of the first thin thermoplastic polymeric film.

In still another embodiment a second coherent PTFE layer can be applied to the thermoplastic polymeric layer thereby encapsulating or sandwiching the thermoplastic polymeric layer between two PTFE layers. The second PTFE layer may be stretched, by the previously described process, simultaneously with or subsequent to the expansion of the first coherent PTFE article.

A particulate filler may be incorporated into the fine powder PTFE resin by steps taught in U.S. Patent NO. 4,985,296 to Mortimer, Jr. at column 2, lines 60-68 and column 3, lines 1-10. The term "particulate" is meant to include individual particles of any aspect ratio and thus includes fibers and powders. The particulate filler may be an inorganic filler which includes metals, semi-metals, metal oxides, graphite, carbon, ceramics and glass. Alternatively, the particulate filler may be an organic filler which includes thermoplastic resin. The thermoplastic resins can include polyether ether ketone (PEEK), fluorinated ethylene propylene (FEP), a copolymer of tetrafluoroethylene and perfluoro(propylvinyl ether) (PFA), or the like.

Referring to Figure 1, the film of expanded layered composite article 42 is cut to a desired width and then helically wrapped around an electrical conductor 41 or around other layers of dielectric insulative materials previously applied to the conductor to produce a wrapped wire or cable 40. The expanded layered composite article in film form may be positioned so that the thin thermoplastic polymeric layer 43 is either toward the conductor or away from the conductor. The expanded layered composite may also be used as a wrap for optical fiber. The expanded layered composite in film form exhibits good resistance to cut-through and greater break strength when compared to other ordinary PTFE wrapped conductors. When the thin thermoplastic polymeric layer in the expanded layered composite film is continuous, the wrapped wire or cable exhibits good resistance to attack by low surface tension liquids.

The expanded layered composite article in film form also acts as a barrier to low surface tension liquids by itself, when laminated to other sheet-like articles.

The following examples exemplify the present invention, they are illustrative only and are not intended to limit the scope of the present invention in any way.

TEST DESCRIPTIONS

FILM THICKNESS

Thickness of the films used herein was determined with a micrometer. Thickness of layers in the composites of the Examples was determined by SEM cross-section or infrared analysis. Uniformity of thickness was observed visually. CUT-THROUGH RESISTANCE

To compute cut-through resistance, a 30.5 cm. long sample of an insulated wire is placed in a "Dynamic Cut-Through Testing Machine" as shown in Figure 2. A sample wire 1 is supported and held in place by an anvil 3. A block 5 with a 0.025 + 0.001 mm. radius is positioned at an angle perpendicular to the axis of the sample wire \ and against the outer surface of the insulation. The machine has a threaded lever arm 7 and weight 9 with a hole through it, having matching threads so that when the threaded lever arm is rotated, the weight moves traversely along the lever arm away from a fulcrum 10. As the weight moves, the force on the blade 5 is increased. The speed of rotation of the threaded lever arm is constant such that the force of the blade is increased at a rate of 10 kg/minute. An electrical detection circuit senses when the blade has pierced the insulation and touches the conductor and stops the rotation of the threaded lever arm. This is considered the end of the test. A timer measures the amount of time which has elapsed between the start and end of the test. This time measurement along with the known fixed rate at which the force increases allows the calculation of the force on the blade at failure. This is the cut-through resistance measurement. Dynamic cut-through resistance measurements are made on each sample and the results are averaged.

Test results are highly dependent on the local sharpness of the blade. This puts strict requirements on blade hardness and uniformity across the entire cutting surface. Care must be taken to use only blades that are uniformly sharp and durable. The blade should be calibrated before each test.

One method to check the calibration of the blade is to test an AWG 30 solid wire insulated with a 0.12 mm. thickness of an extruded copolymer of ethylene and tetrafluoroethylene, (Tefzel^® insulation available from E. I. du Pont de Nemours & Company, Wilmington, DE). When the failure occurs at 1.0 kilograms plus or minus 0.1 kilograms, the blade is at the desired sharpness. FILM TENSILE PROPERTIES

To determine the tensile properties of a film, a sample of film is obtained. Thickness of the film is determined with a micrometer and width of the film is determined with a linear gauge. A constant rate-of-jaw separation-type machine (Instron testing machine, Model 1122) is used to test samples to break. The gauge length of the machine is 10.16 cm. The jaw separation rate employed is 25.4 cm/min.

Load at maximum load and percent elongation at maximum load are recorded. Stress at maximum load is determined by dividing the load at maximum load by the smallest cross-sectional area measured on sample. The matrix tensile strength is obtained by multiplying the stress at maximum load by the ratio of the density of solid PTFE (2.18 g/cc) to the density of the sample. A population of three samples is given for each value listed herein.

INSULATED WIRE BREAK LOAD PROPERTIES

To determine the force to break property of a sample of insulated wire, ASTM method entitled "Standard Test Methods for Hookup Wire Insulation" designation D 3032-91 is used with a few alterations as noted below.

Test samples includes the conductor as well as the insulation. A 25.4 cm gap rather than a 5.1 cm gap is used in the tension testing apparatus. Also, the rate of crosshead separation of the tension testing apparatus is 25.4 cm.

A minimum of five tests were performed on each sample.

SCRAPE ABRASION RESISTANCE

To determine the scrape and abrasion resistance of an insulation layer applied to a conductor, a sample of insulated wire is affixed to an abrasion testing device. The abrasion testing device contains a holding anvil upon which a length of double-sided tape containing pressure sensitive adhesive has been placed. A pair of clamps positioned distally and proximally to the holding anvil hold the sample against the anvil.

A reciprocating head containing a 0.41 mm (410 micron) diameter needle contacts the outside surface of the sample held against the holding anvil. A 50 g weight is placed upon the head to apply a 0.49 N downward force on the sample. The head is attached to a means for reciprocating the head which, when activated, causes the head to travel back and forth upon the length of the wire for a distance of 0.5 mm (500 micron) at a speed of approximately one cycle per second.

The abrasion testing device is allowed to cycle until enough of the insulation layer is scraped or abraded so that an electrical connection is made between the needle contained on the reciprocating head and a conductor contained within the sample. An electrical detection circuit senses when the needle has scraped away the insulation thereby stopping the action of the reciprocating head. An automatic counter records the number of cycles the reciprocating head has performed.

ELECTRICAL INSULATION RESISTANCE

To determine the insulation resistance (IR) which is a measure of the ratio of applied voltage to the total current between two electrodes in contact with a specific insulator, the following test is performed: Twenty five (25) feet of sample of an insulated conductor is wrapped on a spool. The spool is lowered into a bath of a test solution containing 2% by weight ammonium perfluoro-octanoate (FC 143 available from 3M Inc.) and 5% by weight of sodium chloride. Both ends of the insulated conductor are removed from the spool and kept above the surface of the test solution in the bath.

The sample is allowed to condition for a period of four (4) hours in the bath prior to application of an electric current. A current leakage is measured between the conductor and the test solution from a five hundred (500) volt source. A value of 5.9 x . . 3/01019

10⁹ ohm/1000 ft or above is considered a passing value on this test.

EXAMPLES

Example - Preparation of Tape

This example describes the preparation of a tape of layered composite. The layer of thermoplastic polymer film forms a continuous layer.

A fine powder PTFE resin was combined with a quantity of an odorless mineral spirit and mixed until a paste was formed. The paste was compressed under a vacuum to form a billet, and the billet was subsequently extruded through a die thereby forming a coherent PTFE extrudate.

The coherent PTFE extrudate was compressed between a pair of rollers until a coherent PTFE sheet, 430 micron (0.43 mm) thick, was obtained. The coherent PTFE sheet still contained an amount of the odorless mineral spirit.

The odorless mineral spirit was volatilized from the coherent

PTFE sheet by heating yielding a dry porous coherent PTFE sheet.

Subsequently, the dry coherent PTFE sheet was stretched while still hot two (2) times its original length by passing the sheet over a series of gapped rollers driven at differing speeds. A sheet of a thermoplastic polymer, a copolymer of tetrafluoroethylene and perfluoro(propylvinyl ether) (PFA) 51 micron (0.051 mm) thick was obtained (200 LP available from E.I. du Pont de Nemours, Co.). The PFA sheet was slit so that its width was slightly less than the width of the dry coherent PTFE sheet.

The PFA sheet was fed on top of the dry coherent PTFE sheet which in turn was fed across a heated curved plate, heated to a temperature of 340^CC which is above the melting point of the PFA sheet. The two sheets were stretched together 1.2 to 1 to form a laminate. The speed with which the sheets were passed over the heated curved plate was 12.19 m/min.

The laminate was stretched in two sequential heating zones. one set at 330°C, the other set at 340°C. The laminate was stretched fifty-four (54) times its initial length thereby causing the dry coherent PTFE sheet to form an expanded porous polytetrafluoroethylene layer and resulting in an expanded layered composite film of the instant invention. The expanded layered composite film had a total thickness of 61 micron. The PFA layer was determined to be 6.3 micron thick through infrared analysis of the expanded layered composite film. Its surface was glossy, indicating a smooth surface of the PFA layer. The PFA layer has a substantially uniform thickness. Tensile properties were determined for a population three samples. The results of this testing were as follows:

Load at max. load (N) 342.99 + 10.36 .

Stress at max. load (N/m^z) 2.840xl0⁸ ± 8.584x10°

% elongation at max. load (%) 8.562 + 0.563 Matrix tensile (N/m^z) 5.605xΪ0⁸ ± 6.902xl0⁵

Example 1

An expanded layered composite article as produced Example 1 was subsequently slit lengthwise and helically wrapped according to conventional tape wrapping techniques onto a 36(7/44) SPC conductor. Four (4) layers were applied resulting in a wall thickness of about 0.098 mm. These layers were applied with the thermoplastic film layer outermost.

A comparative insulated conductor was produced using a film of only expanded porous PTFE, i.e., without the thermoplastic polymeric layer, by the method described in U.S. Patent No.

4,732,629 to Cooper, et al .

Prior to testing, the comparative insulated conductor was passed through a molten salt bath heated at a temperature of about

390°C for a period of about ten (10) seconds to sinter the PTFE. The inventive insulated conductor was passed through an air oven at approximately 370°C for a period of about twenty seven (27) seconds to sinter the ePTFE. The resultant insulated conductors were subsequently tested for various physical properties. The results of these tests are as follows:

Test Example 3 Comparative Example

As is seen, the insulated conductor of the invention exhibited improvement over the comparative example in scrape abrasion resistance, and break load.

The comparison in cut-through resistance showed a marked improvement in the insulated conductor of the invention over the comparison.

Claims

I CLAIM:

1. An electrical conductor comprising: (a) a conductor, and (b) a layer of a composite article in film form comprising a layer of expanded porous polytetrafluoroethylene adhered to a smooth thermoplastic polymeric film, said polymeric film having a melt point of 342°C or less and a substantially uniform thickness of less than about 9 microns; said layer being in film form wrapped on the conductor.

2. A wrapped conductor as in Claim 1 wherein the conductor is a wire.

3. A wrapped conductor as in Claim 1 wherein the conductor is an optical fiber.