WO1994013469A1

WO1994013469A1 - Composite article

Info

Publication number: WO1994013469A1
Application number: PCT/US1993/005424
Authority: WO
Inventors: John E. Bacino
Original assignee: W.L. Gore & Associates, Inc.
Priority date: 1992-12-10
Filing date: 1993-06-04
Publication date: 1994-06-23
Also published as: AU4408693A; ITTO930933A1; ITTO930933A0

Abstract

A composite PTFE article and a process for producing the article is described. A PTFE article is stretched while a thermoplastic polymeric layer is adhered to the PTFE article thereby causing the thermoplastic polymer layer to be stretched and become thin. The expanded composite PTFE article may be in the shape of a film, tube, rod or filament. The expanded composite PTFE article exhibits high cut-through resistance and increased resistance to penetration by low surface tension liquids.

Description

TITLE OF THE INVENTION

Composite Article

FIELD OF THE INVENTION

This invention is directed to a composite article. More specifically, this invention is directed to a composite article having a thermoplastic film adhered to an expanded microporous polytetrafluoroethylene article and a process for making the composite article.

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. Its high thermal stability is due to the strong carbon-fluorine bond and characterizes PTFE as a very useful high temperature polymer. Commercial PTFE is manufactured by two different polymerization techniques that result in two different types of polymers. Suspension polymerization produces a granular resin, and emulsion polymerization produces a coagulated dispersion that is often referred to as a fine-powder PTFE dispersion. Because of its chemical inertness and high molecular weight, PTFE does not flow and cannot be fabricated by conventional techniques. Therefore, an extensive processing technology had to be developed.

The suspension-polymerized PTFE polymer (referred to as granular PTFE) is usually fabricated by modified powder metallurgy techniques. Emulsion-polymerized PTFE behaves entirely differently from granular PTFE. Coagulated dispersions are processed by a cold extrusion process. Stabilized PTFE dispersions, made by emulsion polymerization, are usually processed according to latex processing techniques. However, these ordinary PTFE polymers exhibit low tensile strength and low cold flow resistance. When used as an insulating material on a wire or cable construction having a conductor, 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 absorbs through its pores liquids which have a low surface tension, e.g. less than 50 dynes/cm and, therefore, is not useful as a barrier to these liquids for any extended period of time. Thus, when used as an insulating material in wire and cable applications, ePTFE must be used in conjunction with a non- permeable material to prevent absorption of low surface tension liquids into the insulation material which is especially important when the low surface tension liquid is conductive in nature and has the potential to disrupt the current through a wire or cable covered by ePTFE insulation. However, in situations where a wire or cable must have a small outside diameter, inclusion of both an ePTFE layer and a non-permeable layer material may not be possible because of the bulk. Also, ePTFE ordinarily exhibits the same relatively poor cut-through resistance that ordinary unexpanded PTFE does.

Laminates of expanded, porous PTFE membrane or film with thermoplastic films are known, but the thermoplastic films are thick. For example, EP 211505-A states that protective layers of a fluoroplastic such as perfluoroalkylvinyl ether copolymer (PFA) and tetrafluoroethylene-hexafluoropropylene resin (FEP) have been placed on porous PTFE. But the EP publication then states that it is difficult to form a very thin protective layer. The layer must be thick, and the publication states that the flexibility of such porous PTFE tapes is reduced.

USP 4,547,424 describes a laminate of a layer of expanded porous PTFE affixed to a solid substrate, which can be a solid layer of a fluoroplastic such as tetrafluoroethylene- hexafluoropropylene or perfluoroalkylvinyl ether polymer.

USP 4,946,736 describes a four layer laminate of a layer of expanded porous PTFE, a layer of FEP, a second layer of expanded porous PTFE, and a layer of woven fibers of PTFE. The FEP is applied in the form of a film.

Films of fluoropolymers such as FEP or PFA are ordinarily relatively thick, usually on the order of 50 micron or more. In some instances films of FEP or PFA are available on the order of 0.5 mil (or 12.7 microns) thick.

Dispersions of thermoplastic fluoropolymers can be used as coatings, but they do not produce thin, i.e., 1 mil (25.4 microns) or less, continuous films. However, there is a need for expanded porous PTFE films that have even thinner uniform layers of protective fluoropolymers and heretofore such thin films have been unavailable. This invention provides such materials.

SUMMARY OF THE INVENTION

A composite article comprising a substrate of expanded porous polytetrafluoroethylene which has adhered, i.e. joined, to at least a portion thereof a film of a thermoplastic polymer having a melting point below 342°C wherein the thermoplastic film has a substantially uniform thickness of less than about 9 microns.

In one embodiment, the substrate is in the form of a membrane or film and the thermoplastic polymer is in the form of a continuous film adhered over one surface of the substrate. In another embodiment the substrate is in the form of a membrane or film and the thermoplastic polymer is in the form of a discontinuous film, i.e., a film which has openings through the film at various locations, which are usually random locations. In a subembodiment the openings are rents in the film.

In a preferred aspect the thickness of the thermoplastic film is 7 microns or less, more preferably 5 microns or less. The film can be so thin that it forms discontinuities in it, i.e., it has openings or rents in the film. Preferably the thermoplastic film is a fluoropolymer and most preferably a thermoplastic copolymer of tetrafluoroethylene.

The process of the invention is a process for making the composites and comprises a) contacting a porous polytetrafluoroethylene substrate, usually in the form of a membrane or film, with a layer, usually a film, of a thermoplastic polymer; b) heating the composition obtained in step a) to a temperature above the melting point of the thermoplastic polymer; and c) stretching the heated composition of step b) while maintaining the temperature above the melting point of the thermoplastic polymer; d) and cooling the product of step c). Depending on the degree of stretching, the thermoplastic film can form a very thin, i.e., 9 micron or less thick, film on the surface of the expanded porous PTFE which is continuous and nonporous. Or, if the degree of stretching is great enough, the thermoplastic film will eventually rend and form rents.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures la, lb and lc are photomicrographs (taken at 5000x magnification) depicting the composite article of Example 1.

Figures 2a, 2b and 2c are photomicrographs (taken at 5000x magnification) depicting the composite article of Example 2. Figures 3a, 3b and 3c are photographs (taken at 5000x magnification) depicting the composite article of Example 3.

DETAILED DESCRIPTION OF THE INVENTION

The expansion, i.e., or stretching of polytetrafluoroethylene is a well-known procedure and is described 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 until a paste is formed. The paste is compressed into a billet and subsequently extruded through a die in a ram-type extruder to form a coherent PTFE shape which can be in a the form of a rod, filament, tube or, preferably a sheet or a film.

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-conditioning 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 about 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 thermoplastic 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 of a thermoplastic polymer that has a melt point of 342°C or less. Melting points of thermoplastic polymers were determined by Differential Scanning Calorimetry Techniques. 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^CC.

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 thereby 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. In one embodiment, the thin thermoplastic polymeric layer is continuous and has a smooth, high-gloss surface and is non-porous. In another embodiment, in which the coated PTFE is stretched until the thermoplastic layer rents or tears, the thin thermoplastic polymer layers is discontinuous and some pores of the PTFE substrate are unblocked and open from one side to the other. There are several ways to prepare the discontinuous coating. The thin thermoplastic polymeric layer may be discontinuous prior to contact with the coherent PTFE shape as, e.g., a screen. Another method, as stated above, is through the expansion of the coherent PTFE shape to an extent exceeding the amount of expansion the thin thermoplastic polymeric film may withstand and remain continuous. In this case, the film rends or tears, and the thermoplastic film may be concentrated on the nodes of the ePTFE. In the discontinuous embodiment, the thin film covering the surface of the ePTFE may cover only 10% or even less of the total surface area. Once the discontinuous variation is obtained, stretching transversely at below the melting point of the thermoplastic will produce a material with greater areas of exposed substrate.

The coated material may be heat set, if desired, to amorphously-lock the expanded porous PTFE structure.

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 remelting 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 first 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 expanded, 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.

The expanded layered composite article of this invention in film form is useful as a low-dielectric insulation in electrical wire or cable. The film of expanded layered composite article is cut to a desired width and then helically wrapped around an electrical conductor or around other layers of dielectric insulative materials previously applied to the conductor to produce a wrapped wire or cable. The expanded layered composite article in film form may be positioned so that the thin thermoplastic polymeric layer 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 thermoplastic film can act as an adhesive to adhere the ePTFE substrate to the surfaces of other materials. 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.

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 gauge 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 include 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./min.

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 microns) 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 microns) 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: A twenty five (25) feet long 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 10⁹ ohm/1000 ft or above is considered a passing value on this test.

Melting Point

Melting point is determined by differential scanning calorimetry.

EXAMPLES

Example 1

This example describes the preparation of a layered composite article in film form. 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°C 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 PTFE 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 microns. The PFA layer was determined to be 6.3 microns 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 had a substantially uniform thickness as seen in the microphotographs.

Micrographs of the expanded layered composite film of Example 1 can be seen in Figures la, lb and lc. Figure la is a view of the side of the expanded layered composite film containing the ePTFE layer. Figure lb is a view of the smooth PFA surface of the expanded layered composite film. Figure lc is a cross section of the article depicting a portion of both the ePTFE layer and the PFA sheet.

Tensile properties were determined for a population of three samples. The results of this testing were as follows:

Load at max. load (N) , 342.994- 10.36 Stress at max. load (N/m²) 2.840x10^s ± 8.584x10°

% elongation at max. load (%) 8.562 ± D.563

Matrix tensile (N/m²) 5.605xl0⁸ ± 6.902x10°

Example 2

This example describes preparation of a continuous thin film of thermoplastic polymer on a film of expanded porous PTFE.

A dry coherent PTFE sheet was obtained as in Example 1. A sheet of a thermoplastic polymer, fluorinated ethylene propylene (FEP) 0.025 mm (25 microns) thick was obtained (100 A available from E. I. du Pont de Nemours & Co.). The FEP sheet was combined with the dry coherent PTFE sheet and the laminate was stretched as in Example 1.

The expanded layered composite film had a total thickness of 0.048 mm (48 microns). The FEP layer was determined to be 2.4 micron thick through infrared analysis of the expanded layered composite film. The surface was glossy, indicating a smooth surface of the FEP layer.

Micrographs of the expanded layered composite film of Example 2 can be seen in Figures 2a, 2b and 2c. Figure 2a is a view of the side of the expanded layered composite film containing the ePTFE layer. Figure 2b is a view of the smooth FEP surface of the expanded layered composite film. Figure 2c is a cross section of the article depicting a portion of both the ePTFE layer and the FEP sheet.

Tensile properties were determined for a population of three samples. The results of this testing were as follows: Load at max. load (N) 310.90 ±₀12.81

Stress at max. load (N/m^z) 2.898x10** ± 1.195X10⁷

% Elongation at max. load (%) 7.915 ± 1.320

Matrix tensile (N/m²) 6.077xlO^b ± 1.922x10'

Example 3

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 expanded porous PTFE produced by the method described in U.S. Patent No. 4,732,629 to Cooper, et al . The expanded porous PTFE of Cooper without the thermoplastic layer was wrapped on a conductor exactly as specified in the paragraph above.

Prior to the 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 ePTFE. The inventive insulated conductor was then passed through an air oven at approximately 370^CC 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

Break Load (N) 48.9 28.9

Scrape Abrasion (to failure) 325 cycles 38 cycles

IR (ohm/1000 ft) 5.6xlO^π <5.9xl0⁹

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

A comparison in cut-through resistance showed a marked improve ent in the insulated conductor of the invention over the comparison.

Example 4

This example describes preparation of a discontinuous film of thermoplastic polymer on a film of expanded porous PTFE.

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, 0.43 mm (430 micron) thick, was obtained. The coherent PTFE sheet still contained an amount of odorless mineral spirit. The odorless mineral spirit was volatilized from the coherent PTFE sheet by heating, yielding a dry coherent PTFE sheet.

A sheet of a thermoplastic polymer, fluorinated ethylene propylene (FEP) 0.0125 mm thick (12.5 micron) was obtained (50A available from E.I. du Pont de Nemours & Company). The FEP sheet was slit so that its width was slightly less than the width of the dry coherent PTFE sheet. The FEP sheet was fed on top of the dry coherent PTFE sheet which in turn was fed across two heated curved plates, heated to a temperature of 300°C which is above the melting point of the FEP sheet. The two sheets were stretched together 1.33 to 1 forming a laminate. The speed with which the sheets were passed over the heated curved plates was 12.19 m/min.

The laminate was expanded in two heating zones at 300^'C. The laminate was expanded twenty four (24) times its length at a speed of 10.67 m/min. The expanded laminate was then expanded a second time in two heat zones one at 365°C, the other at 350°C. The laminate was expanded 6.5 times its length at a speed of 4.57 m/min. The total expansion was 206 times its original length causing the dry coherent PTFE sheet to form an ePTFE composite resulting in an expanded composite film of the instant invention. The FEP layer has a substantially uniform thickness.

The expanded composite film had a total thickness of .0225 mm (22.5 micron). The FEP layer was determined to be discontinuous where the FEP was randomly deposited over the surface of the ePTFE primarily on the nodes of the ePTFE, as evidenced by SEMS.

The laminate was air permeable.

Figures 3a, 3b and 3c are views of the discontinuous FEP layer on the ePTFE substrate. Figure 3a is magnified lOOx; Figure 3b is magnified 500x; and Figure 3c is magnified lOOOx.

Claims

I CLAIM:

1. A composite article comprising a substrate of expanded porous polytetrafluoroethylene which has adhered to at least a portion thereof a thermoplastic polymer having a melt point of

342°C or less wherein the thermoplastic polymer has a substantially uniform thickness of less than about 9 microns.

2. A composite article as in Claim 1 wherein the substrate and the thermoplastic polymer are both films and the film of thermoplastic polymer is a continuous film.

3. A composite article as in Claim 1 wherein the substrate and the thermoplastic polymer are both films and the film of thermoplastic polymer is discontinuous.

4. A composite article as in Claim 1, 2 or 3 wherein the thermoplastic polymer is a thermoplastic fluoropolymer.

5. A composite article as in Claim 4 wherein the thermoplastic fluoropolymer is fluorinated ethylene propylene.

6. A composite article as in Claim 4 wherein the thermoplastic fluoropolymer is a copolymer of tetrafluoroethylene and perfluoro(propylvinyl ether).

7. A composite article as in Claim 1 wherein the layer of expanded porous polytetrafluoroethylene contains a particulate filler.

8. A composite article as in Claim 1 wherein the uniform thickness is less than about 7 microns.

9. A composite article as in Claim 1 wherein the uniform thickness is less than about 5 microns.

10. A process for producing a composite comprising a layer of expanded porous polytetrafluoroethylene adhered to a thermoplastic polymer having a melt point of 342°C or less wherein thermoplastic polymer has a thickness of 9 micron or less, comprising the steps of:

(a) contacting a surface of a porous polytetrafluoroethylene shape with a thermoplastic polymeric film; (b) heating the composition obtained in step a) while under tension to a temperature above the melt point of the thermoplastic polymeric layer; and (c) stretching the heated composition of step b) while maintaining the temperature at or above the melt point of the thermoplastic polymeric layer; d) and cooling the product of step (c).