WO1997003812A1

WO1997003812A1 - Moldable articles of ptfe/elastomer composites and processes for molding

Info

Publication number: WO1997003812A1
Application number: PCT/US1996/011523
Authority: WO
Inventors: Edward A. Rubin; Mark S. Spencer
Original assignee: W.L. Gore & Associates, Inc.
Priority date: 1995-07-14
Filing date: 1996-07-11
Publication date: 1997-02-06

Abstract

The present invention is an improved moldable composite comprising an open microporous unsintered ePTFE substrate containing a resilient material, such as curable silicone, fluorosilicone rubber, or fluorocarbon elastomer, on the walls of the micropores. The present invention also includes molded, shaped articles of the moldable composite. This substrate may take any shape, such as tubes, beads, rods, tapes, or sheets. The substrate is permeated with a curable resilient material in such a way as to envelope the walls, e.g., fibrils and nodes of ePTFE. By shaping and compressing into a desired shape and then curing the resilient material, a highly resilient article is created with significantly improved operational properties.

Description

TITLE OF THE INVENTION

MOLDABLE ARTICLES OF PTFE/ELASTOMER COMPOSITES AND PROCESSES FOR MOLDING

FIELD OF THE INVENTION

The present invention relates to shaped, molded articles made from moldable composites of polytetrafluoroethylene (PTFE) and elastomers.

BACKGROUND OF THE INVENTION

Polytetrafluoroethylene (PTFE) is well known for its chemical inertness and for its resistance to decomposition at elevated temperatures. A specialized form of PTFE is also well known in which ordinary coagulated dispersion produced PTFE resin is processed and stretched under certain conditions to form a film or membrane of microporous, expanded PTFE that is characterized by a morphology consisting of a microstructure of a myriad of micro-nodes interconnected by a network of microfibrils. These nodes and fibrils form the network of pores present in the films or membranes. Microporous, expanded PTFE (ePTFE) can be made by the procedures described in U.S. Patents 4,187,390 and 3,953,566 to Gore.

Silicones are also well known materials and consist of a chain of altemating oxygen and silicon atoms with organic side chains.

Silicones are generally elastomeric, and can be cured by heat or by moisture when a cross-linking agent is present, in order to set the silicone. Silicones are frequently available in blends of several silicone compositions. They can ordinarily be processed in liquid or solution form and then cured to form the elastomer.

Composites of ePTFE and silicones have been experimented with in the past. For example, in Silicone and Polytetrafluoroethylene Interpenetrating Polymer Networks, Adv. Chem. Ser., 1994, Volume 239, pp. 393-404, there is described a procedure for mixing PTFE dispersion grade resin with a curable silicone resin blend, and then processing by extending and then stretching the extrudate to obtain a fibrillated PTFE network containing the silicone in the fibrillated network.

Japanese Publication 61-40328 describes imbibing a silicone elastomer into the pores of a microporous expanded PTFE sheet and then curing the elastomer.

It has proven difficult to mold PTFE however, and thus shaped articles of PTFE are not readily available. However, it would be desirable to provide a composite of ePTFE and silicone resin that is easily shaped or molded, so that a variety of molded shapes can be obtained. It is a purpose of this invention to provide moldable or shapable composites. These and other purposes of the present invention will become evident based upon the following description of the invention.

SUMMARY OF THE INVENTION

The present invention is an improved moldable composite comprising an open microporous stretched, or expanded, polytetrafluoroethylene (ePTFE) substrate containing a resilient material on the walls of the micropores, such as curable silicone rubber, including fluorosilicone rubber, or crosslinkable fluoroorganic elastomers. The present invention also includes molded, shaped articles of the moldable composite. The ePTFE can be either sintered or unsintered.

Preferably, the present invention comprises a substrate of microporous expanded polytetrafluoroethylene (PTFE) having a matrix of interconnecting fibrils and nodes. This substrate material may take virtually any shape, such as tubes, beads, rods, tape, or sheets. The substrate is permeated with a curable rubber material in such a way as to envelope and encompass the fibrils and nodes of the polymeric matrix. By shaping and, optionally, compressing this structure into a desired shape and then curing the rubber material, a highly resilient structure is obtained with significantly improved properties. Further, a number of properties can be provided by adding other elements to this basic material, such as thermally or electrically conductive elements.

By "open" is meant that the pore volume is high, e.g., on the order of 20-95% or so. The process is a process for providing a shaped article which comprises:

(a) imbibing a solution of a curable elastomeric material, such as a silicone, or fluoroorganic elastomer, into the pores of an expanded porous polytetrafluoroethylene structure in which the polytetrafluoroethylene structure has a morphological microstructure comprising an interpenetrating network of nodes interconnected by fibrils;

(b) removing any solvent, if present, leaving the elastomeric material as a coating or covering encompassing the nodes and fibrils;

(c) shaping the structure obtained in step (b) into a desired shaped structure;

(d) optionally compressing at least a portion of the shaped structure to force air within the pores out of the structure; and (e) curing the elastomer by heating the structure above the curing temperature of the elastomer.

The sheet used in step (a) may be sintered or unsintered.

DESCRIPTION OF THE DRAWINGS

The operation of the present invention will become apparent from the following description when considered in conjunction with the accompanying drawings, in which:

Figure 1 is an SEM of one composition of the present invention. The SEM is a cut-through view, 470 times magnified, of a polymeric PTFE having nodes and fibrils covered with an elastomeric silicone material.

Figure 2 is a three-quarter isometric view of an O-ring shape made of the material shown in Figure 1.

Figure 3 is a view of a bellows made of the material shown in Figure 1. Figure 4 is a view of a conical shaft seal made of the material shown in

Figure 1.

Figure 5 is a view of a pump diaphragm made of the material shown in Figure 1.

Figure 6 is a three-quarter isometric view of a diaphragm made in accordance with Example 9.

Figure 7 is a side view along lines B-B' of the diaphragm of Figure 6, showing both compressed and uncompressed regions.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is an improved moldable or shapable material suitable for use in a wide variety of applications. The invention also comprises molded articles made from the moldable material. The cured resilient material of the present invention may be used in a wide variety of applications ranging from sporting goods and medical and surgical devices, to a number of automotive and industrial applications. An example of a desirable use for the material of the present invention is in sealing applications, such as gaskets for a wide variety of joints, sealants around openings, and the like. It is particularly suitable for use as shaft seals, oil well packing boxes, pump diaphragms, or the like.

The material of the present invention comprises a composite of a porous substrate of expanded microporous polytetrafluoroethylene (PTFE) imbibed with an elastomer material in the pores. This composite is formed from an expanded PTFE material, such as the materials made through the methods described in United States Patents 3,953,566 to Gore; 3,962,153 to Gore; 4,096,227 to Gore; and 4,187,390 to Gore, each specifically incorporated herein by reference. For instance, an expanded PTFE sheet, e.g. a film or a membrane, may be formed from a mixture of PTFE resin (having a crystallinity of about 95% or above) and a liquid lubricant (e.g., a solvent of naphtha, white oil, mineral spirits, or the like). The mixture is thoroughly blended and then dried and formed into a pellet. The pellet is extruded through a ram-type extruder into a sheet structure. Subsequently, the lubricant may be removed through evaporation in an oven. The resulting sheet material is then subjected to uniaxial or biaxial stretching at a temperature of less than 327^CC to impart the desired amount of porosity and other properties to the tube. Stretching may be performed through one or more steps, at amounts varying from 1 :1 or less up to 45:1 or more. A microporous structure is obtained as described above. The expanded

PTFE has a microstructure comprising an interpenetrating network of polymeric nodes interconnected by fibrils. Typical properties of such a structure comprise an average fibril length between nodes of 0.05 to 30 μm (preferably 0.2 to 10 μm), and a void volume of 20 to 95% (preferably 30 to 50%). The precise properties and dimensions of the microporous expanded PTFE structures employed in the present invention will depend on the properties desired. The general properties suitable for use with the present invention should include medium to high porosity, e.g., 60-95% pore volume, to provide space for the elastomer to be imbibed therein. The fibril length of expanded PTFE that has been expanded in a single direction is defined herein as the average of ten measurements between nodes connected by fibrils in the direction of expansion. The ten measurements are made on a representative micrograph of an expanded PTFE sample. The magnification of the micrograph should be sufficient to show at least five sequential fibrils within the length of the micrograph. Two parallel lines are drawn across the length of the micrograph so as to divide the image into three equal areas, with the lines being drawn in the direction of expansion and parallel to the direction of orientation of the fibrils. Measuring from left to right, five measurements of fibril length are made along the top line in the micrograph beginning with the first nodes to intersect the line near the left edge of the micrograph, and continuing with consecutive nodes intersecting the line. Five more measurements are made along the other line from right to left, beginning with the first node to intersect the line on the right side of the micrograph. The ten measurements obtained by this method are averaged to obtain the average fibril length of the material.

In another embodiment of the present invention, the expanded PTFE structure may also comprise one or more fillers, such as through the methods described in U. S. Patent 4,985,296, which is specifically incorporated herein by reference, to enhance one or more properties. For instance, the material may include conductive shielding properties by including electrically conductive particles within the microporous expanded PTFE. In one preferred embodiment, the material may include particles, fibers, or other fillers of one or more of the following: carbon, graphite, aluminum, silver plated aluminum, copper, copper alloy, iron, iron alloy, nickel, cobalt, gold, silver or silver plated copper, or the like. Filler content preferably comprises 5-85% by volume of the PTFE/filler composition. Material made in this manner will also provide shielding against electrical and electromagnetic effects. It has unexpectedly been found that if carbon particles are present in the expanded PTFE, while silicone is being imbibed, penetration of the silicone is facilitated, and less shrinkage occurs during curing.

Stretched porous PTFE material made through one of the above described methods and suitable for use in the present invention is commercially available in a wide variety of forms from a number of sources, including under the trademark GORE-TEX® from W. L. Gore & Associates, Inc., Newark, DE.

Once a suitable microporous, expanded PTFE substrate material is obtained, it is used to produce the composite material of the present invention. Depending on, for example, the composition of the elastomer, the desired properties of the material formed, etc., the elastomer may be dissolved in a solvent prior to incorporation with the expanded PTFE or may be incorporated directly without the requirement for a solvent. Suitable elastomers of the present invention include ethylene-propylene-diene monomer (EPDM), nitrile rubbers, neoprene, silicones, flouroelastomers, and the like.

In one embodiment, a solution may be formed by dissolving the elastomeric material in an organic solvent. The ratio of elastomeric material to solvent, when used, typically should be in the range of 4:1 to 1 :10 parts by volume, and preferably is in the range of 3:1 to 1 :3 parts by volume. The solution is formed through any conventional means, such as by blending in a mechanical mixer under ambient conditions.

Preferred solutions comprise a silicone material comprising a material soluble in one or more solvents capable of permeating and wetting out an expanded microporous PTFE structure. The solution preferably has a solids content of 95-100%, a specific gravity of between 0.95 to 1.5, and a viscosity between 300 and 150,000 centipoise. Further, the solution preferably employs a one or two part cure system to later cure the liquid silicone into a rubber-like mass. In one embodiment, it is preferred to use a silicone with a platinum-type cure system that is activated at elevated temperatures to cross-link into a rubber-like substance.

The silicone material can be selected from a wide variety of silanes, polysiloxanes having reactive groups, and copolymeric siloxanes containing reactive functional groups. Fluorinated silicones are useful also.

Representative curable silicone rubber material compositions include low temperature curing types and high temperature curing types. Suitable silicones for use in the present invention include methyl hydrogen siloxane, dimethylhydrogen siloxane, dimethyl siloxane, dimethylvinyl-terminated siloxane, dimethoxy siloxane, methylphenylmethoxy siloxane and the like. Additionally, the silicone can contain dimethylvinylated silane, trimethylated silane, methyltrimethoxysilane, and the like. Commercially available silicones for use with the present invention include Q3-6611 , X1-4105, and Q1-4010, all available from Dow Corning, Inc., Midland, Michigan. Fluoroorganic elastomers, or fluorocarbon elastomers as they are sometimes called, are well known in the art. Ones useful herein include ones that can be imbibed either from an organic solvent or without the requirement for such a solvent. Commonly they are copolymers of vinylidene fluoride (VDF). Other useful fluorocarbon elastomers include fluorovinylethers, perfluoroelastomers, perfluoropolyethers, and the like. One example of a fluorocarbon elastomer which is useful in the present invention is a solution of a crosslinkable fluorocarbon elastomer comprising polyvinylidene fluoride (PVDF) in methyl ethyl ketone (20%) available from Pelmor Labs, Inc., Newton, PA, as PLV-2000. Comonomers useful with VDF for making elastomers include perfluoropropylene, chlorotrifluoroethylene, perfluoro (methyl vinyl ether), and propylene. Terpolymers of VDF, TFE (tetrafluoroethylene) and any of the above comonomers are also useful. In addition, fluoroalkoxyphosphazines are also useful.

Room temperature curing and high temperature curing compositions of silicone include two-pack type materials. Available two-pack type materials deliver a silicone rubber having cross-linked structure by means of a reaction between siloxanes having reactive groups (e.g., SiOH, SiO-R (where R is an alkyl group), SiH, SiCH=CH2 or the like) in the presence of a catalyst. The two-pack compositions are divided into condensation reaction types and addition reaction types.

The condensation reaction types include those employing: dehydration- condensation reactions between silanol and alkoxy siloxane; a de-alcoholation condensation reaction between silanol and hydroxy siloxane; and a dehydrogenation condensation reaction between SiH and silanol. The addition reaction types include those employing addition reactions between vinyl groups, or other unsaturated groups and SiH.

A suitable curing catalyst may be selected depending on the type of curing reaction desired. For example, metal, organic-metal salts, organic amines, quaternary ammonium salts, and the like are employed in reactions of condensation reaction types. Palladium or platinum black, platinum asbestos, chlorplatinic acid or other forms of platinum are employed in reactions of addition-reaction types. The above-mentioned compositions may also contain other materials, such as silicone oil, Siθ2, or fumed silica as property altering agents.

In embodiments where the use of a solvent is desired, a preferred solvent comprises a solvent that both actively dissolves the elastomer and is readily absorbed into the structure of the microporous expanded PTFE scaffold substrate. A halogenated solvent, such as methylene chloride, acetone, or toluene, is particularly useful for the silicones, as are commercially available mineral spirit solvents such as NORPAR-12 and the ISOPAR- solvents. The choice of solvent, however, may vary depending on, for example, the elastomer chemistry, the composition of the solvent, and the like. One preferred solution comprises a mixture of 10-75% by volume of

Dow Corning Q1-4010 silicone elastomer and 25-90% methylene chloride, acetone, ISOPAR-C, or toluene solvent. This mixture is formed by stirring the solvent while adding the silicone elastomer at room temperature (about 22°C) until the mixture has achieved a homogenous color. With an acetone mixture, the mixture should be re-stirred prior to each use due to precipitation of solids. Dow Corning 4010 Silicone Conformal Coating contains dimethyl, methylhydrogen siloxane copolymer, dimethyl siloxane, dimethylvinyl- terminated silica and trimethylated silica.

Once the silicone or silicone/solvent solution is prepared, it can then be applied to the microporous expanded PTFE substrate.

For example, a solution can be applied to the microporous PTFE material by spreading the solution evenly over the material and then allowing it to become absorbed therein. Preferably, the PTFE material is immersed within the solution until the micropores are filled to a desired extent, such as by submerging the PTFE material in a bath of the solution over a period of 1 to 5 minutes. The entire procedure may be carried out under reduced pressure, such as in a vacuum chamber, to facilitate a desirable amount of filling of the microporous polymeric PTFE scaffold.

Once the micropores are filled with the silicone solution, the silicone filled material is exposed to an energy source, such as a heated oven set at 50 to 60°C for a period of 2 to 24 hours or more to evaporate away any solvent which may be present. Ideally, evaporation comprises employing an oven heated at 50°C, or above or below and exposing the composition for at least 5 minutes. The evaporation of solvent can also be performed, for example, by air drying. When produced in this manner, it has been found that the microporous PTFE nodes and fibrils can become thoroughly coated, i.e., encompassed by the silicone between the top and bottom surfaces of the PTFE. Depending on the desired morphology of the final article, the pore volume between the nodes and fibrils may be completely filled with elastomer or, alternatively, it may be desirable to leave some pore volume available for compression during the molding step.

The composite material may then be subjected to appropriate conditions to shape, mold and cure the articles of the present invention in any desirable manner. For example, the materials may be shaped using any conventional molding techniques, such as placing the PTFE composite sheets or forms into molds, around mandrels, or in desired lay-up configurations, blow molding, vacuum forming, and the like. Depending on the end use for the material, it may be desirable to compress at least a portion of the structure in order to, for example, change the spatial orientation of the nodes and fibrils, minimize or remove voids, etc. Such compression may be done uniformly over the entire structure or selectively on only a portion of the structure, such as along an outer or inner surface or in a selected discontinuous pattern over the structure.

Curing the silicone while the structure is under compression may lead to significant advantages in the properties of the final article. For example, it has been unexpectedly discovered that articles formed in this manner tend to exhibit a longer flexural life, better creep and cold flow, less swelling, Iower deflection force, Iower deflection force (as measured by Taber testing) and enhanced resistance to creasing when the material is folded over on itself. Since polymers of VDF typically cure at a Iower temperature than silicones, the VDF polymer filled material can be placed in the mold before solvent is evaporated. Upon opening the mold, the solvent evaporates as curing takes place.

As mentioned earlier herein, a complete filling of elastomer is not necessary, and in some embodiments complete filling of the pore volume may be undesirable. Figure 1 shows a microporous PTFE structure in which the nodes and fibrils are covered with silicone. This material was the result of placing an expanded PTFE membrane in a solution of 75% by volume of Q1- 4010 silicone and 25% by volume of ISOPAR-C solvent for 1 minute.

The expanded PTFE composites of the present invention are suitable for a wide variety of uses.

In one preferred embodiment, the uncured composites may be used as formable shapes which may be custom shaped to suit a desired application, then cured, for example by heating, to permanently set the customized shape. For example, such customizabe shapes may be useful in sporting goods, such as custom golf club grips, custom footbeds for athletic shoes, custom athletic helmets, or other sporting good applications where the beneficial properties of expanded PTFE can be combined with the shapable, resilient properties of the elastomer. Moreover, such customizable shapes may be beneficial in the medical and surgical devices field for such uses as orthopedic footbeds, orthopedic supports, casts, and other anatomically formable devices.

In another preferred embodiment, the material may be used as a gasket or other form of sealant material in, for example, automotive applications, industrial applications, and the like. It may be shaped and compressed into various shapes such as O-rings depicted in Figure 2, bellows as depicted in Figure 3, seals as in Figure 4, pump diaphragms as in Figure 5, as well as shrouds, gaskets, and the like, and then set by heating above the curing temperature of the elastomer. The curing step locks the structure in the desired shape. The resulting structure has the high strength and abrasion resistance of expanded PTFE with the elasticity and flexibility of elastomers, such as silicones.

In particular, O-rings, conically shaped pump seals, spinning shaft seals, and other shaped articles can be made by layering up a number of plies of the uncured silicone or fluoroelastomer treated sheet composite and stamping out washer shapes. The stamped out shapes can then be compressed in an O-ring mold and cured. If desired also the plies can be silicone impregnated after layering up, or after the washers are stamped out. Alternatively, a mandrel can first be wrapped with ordinary PTFE tape (full density) and then wrapped with expanded porous unsintered or sintered PTFE. Elastomer can be added either before or after wrapping. By slicing perpendicularly through the mandrel, washer or tube structures are obtained which have a layer of full density PTFE on the exposed inner face to promote ease of slippage in use in pumps. The wrapped, sliced washer shapes can be placed in an O-ring mold to compress and cure or crosslink the material to a true O-ring shape.

Alternatively, the mandrel can be wrapped with untreated ePTFE tape, which is then sintered to fuse the layers and then the elastomer imbibed and the assembly sliced. Or, if desired, the untreated wrapped mandrel can be sliced into washer shapes which are then imbibed with the silicone solution. After slicing, the shapes can be placed in a mold, such as an O-ring mold, and compressed and cured. Conically shaped pump seals can also be molded by these procedures.

In a further preferred embodiment of the present invention, one or more oleophobic coatings, such as TEFLON AF^® or the like, may be applied by any conventional means to at least a portion of the surface of the material in order to provide a material which possesses both resiliency due to the presence of the elastomer, and an inert character due to the presence of the oleophobic coating. Without intending to limit the scope of the present invention, the following examples illustrate how the present invention may be made and used:

EXAMPLE 1

Forty-eight layers of expanded partially sintered PTFE membrane obtained from Japan Gore-Tex, Inc. having a combined thickness of approximately 0.064 inches (1.6 mm) were assembled. A sample weighing 134.645 grams measuring 48 inches by 4 inches (121 by 10.2 cm) was cut from this and placed into a vat containing a solution of Dow Corning Q1-4010 silicone and Isopar-C solvent in a ratio of 1 part silicone to 3 parts solvent. The vat was then placed into a vacuum chamber and the chamber was subsequently evacuated using a vacuum pump. When a gauge measuring the vacuum within this chamber read 28 inches of mercury, the pump was turned off and the vacuum was allowed to leak off to approximately 20 inches in about 5 minutes. The pump was again turned on until the gauge read 28 inches and again shut off. The vacuum was allowed to leak off over a period of approximately 1 hour. The vat was then removed from the vacuum chamber and the sample was removed from the solution, placed onto a paper towel and blotted dry. The sample was placed under an oven at 60^°C to drive off the solvent contained within the sample, thereby leaving only the silicone as a covering on the nodes and fibrils of the ePTFE structure. The weight of this sample increased from its original weight of 134.645g to 241.6872g or 179.5% of its pretreated weight. Three specimens 4 inches X 5 inches ( 102 by 127 mm) and 60 mil (0.025 mm) thick were cut from the sample. Each sample was placed between 1 inch (25.4 mm) thick steel plates at varying initial temperatures. Sample 1 used no stops or shims between the plates to control final thickness. In Samples 2 and 3, 0.035 inch (0.89 mm) shims were used on either side of the specimen so as to control molding by controlling the amount of compression. The sets of plates and specimens were then placed one at a time into a hydraulic press (Pasadena Hydraulics Inc., Model No. PW22J-C- XS-J) heated to 180°C to compress and to cure the silicone. The press was hand jacked to apply pressure to the samples. Pressure, time and temperature were recorded and were as follows:

SAMPLE 1

ELAPSED TIME TEMPERATURE °C RAM FORCE of sandwich plates (Pounds x 100)

0 24 45

20 110 38

40 120 40

55 160 44

70 175 48

90 175 48

In the above example both the press plates and the additional plates used to sandwich the material were at room temperature when pressure was first applied. As can be seen, it took 75 minutes to reach the peak temperature from this condition. In the following example the press plates were already heated to the peak temperature when the sandwich plates and material were placed in the press. This reduced the amount of cold flow of the material by allowing a more rapid heating and curing of the silicone material. Evidence of this can be seen in the difference in pressure drop off in the press after reaching the initial pressure.

SAMPLE 2

ELAPSED TIME TEMPERATURE RAM FORCE Min. of sandwich plates (Pounds x 100)

0 75 42

2 125 40

4 140 40

5 150 40

10 160 42

13 175 44

20 175 45.5 SAMPLE 3

ELAPSED TIME TEMPERATURE°C RAM FORCE Min. of sandwich plates (Pounds x 100)

0 -17 40

5 125 39

7 135 40

15 155 44

25 175 46

30 175 46

The final thickness of sample 1 varied from .036 inch to .043 inch (0.91 to 1.1 mm) across its length while the shims used in samples 2 and 3 produced a consistent .040 inch and .041 inch (1.02 and 1.04 mm) thickness respectively. Sample 3 was measured to be 4.35 by 6 inches (11.1 by 15.2 cm) prior to pressing and measured 5.1 by 7.3 inches (13 by 18.5 cm) at the widest and longest dimensions after pressing.

The material formed by the process in the above examples is a semi- translucent, flexible, resilient membrane which is easily bent and flexed. Average percentage creep and average percentage recovery for these materials were determined to be 25.8% and 64%, respectively.

EXAMPLE 2

An O-ring having dimensions of standard AS-568-A size 16 was manufactured by the following procedure. The same material as used in Example 1 was produced as follows. First, a forty-eight layer piece of expanded PTFE membrane having a combined thickness of approximately 0.064 inches (1.6 mm) was assembled. A sample weighing 134.645 grams was cut from this and placed into a vat containing a solution of Dow Corning Q1-4010 silicone and Isopar-c solvent in a ratio of 1 part silicone to 3 parts solvent. The vat was then placed into a vacuum chamber and the chamber was subsequently evacuated using a vacuum pump. When a gauge measuring the vacuum within this chamber read 28 inches of mercury the pump was turned off and the vacuum was allowed to leak off to approximately 20 inches in about 5 minutes. The pump was again turned on until the gauge read 28 inches and again shut off. The vacuum was allowed to leak off over a period of approximately 1 hour. The vat was then removed from the vacuum chamber and the sample was removed from the solution, placed onto a paper towel and blotted dry. The sample was placed in an oven at 60°C to drive off the solvent contained within the sample thereby leaving only the silicone in the expanded PTFE structure. The weight of this sample increased from its original weight of 134.645 grams to 241.6872 grams or 179.5% of its pre-treated weight. A circular pattern having a .750 inch (19 mm) outside diameter and a .615 inch (15.6 mm) inside diameter was stamped out of the above sample. This was then placed into a split cavity mold having the dimensions of a number 16 0- ring. The two halves were assembled together and the mold was placed into a heated press at 180°C at 500 psi for five minutes to shape the material and cure the silicone. The mold was removed and cooled in water. Upon opening the mold it was found that the material formed an O-ring shape. Several other O-rings were prepared using this same process.

The above sample was tested in a pressurized O-ring test fixture to determine its ability to seal against compressed air. This fixture comprises a cylinder having a length of 2 inches (51 mm) with a 0.75 inch (19 mm) diameter bore 1.625 inches (41.3 mm) deep with .625 inch (15.9 mm) at the outer most portion of said bore having a 0.5 inch (13 mm) pipe thread and the opposite end of the bore having a 0.25 inch (6.4 mm) opening. A second piece of the fixture comprises a dowel like part having an outside diameter of .745 inch

(18.9 mm), a length of 0.5 inch (13 mm) and an O-ring groove positioned at the center of this length. This fixture is operated by placing the subject O-ring over the dowel piece and into the groove, placing this into the bore of the first piece, threading a 0.5 inch (13 mm) pipe into the threaded portion of the bore, applying air pressure through this pipe while holding under water to detect air bubbles exiting the 0.25 inch (6.4 mm) hole. No bubbles were detected which means the O-ring provided a good seal.

EXAMPLE 3

Another procedure was used to produce a similar size O-ring. A length of sintered expanded PTFE film 3 inches (76.2 mm) wide and having a thickness of .003 inch (0.076 mm) and a density of approximately 0.656 g/cc was submerged in a solution of 3 parts (by volume) Dow Corning Q1-4010 silicone and 1 part Isopar-c solvent. The solution completely penetrated the membrane and the treated membrane was then allowed to set in an exhaust hood to evaporate off the Isopar-c. This membrane was tightly wrapped onto a hollow mandrel having a .610 inch (15.5 mm) diameter and a slot cut through its center along the longitudinal axis. A pin was placed into the center of the mandrel in order to hold the slot open during this process. This mandrel was then placed into a lathe and a razor blade was used to perpendicularly section the film into thicknesses of approximately .070 inch (1.8 mm). The pin was removed from the center of the mandrel to facilitate the removal of these section pieces. These pieces were placed into the O-ring mold as above and cured. Several good quality O-rings were produced this way but suffered from delamination of the layers.

EXAMPLE 4

In order to improve the delamination problem of Example 3, the membrane, prior to silicone treatment, was wrapped onto the mandrel and placed into an oven at 360°C for one hour to sinter the membrane and promote bonding between its layers. It was then sectioned as in Example 3 and individual sectioned pieces were placed into a silicone/lsopar-c solution having a ratio of 1 :1 by volume, under a vacuum bell jar at 28 inches Hg. The silicone solution permeated the pieces completely. These pieces were then molded into O-rings as in Example 2. These O-rings had significantly reduced delamination over samples from Example 2. It was observed that these samples were more difficult to permeate with the silicone solution. This is probably due to the increase in density (about 6.5 gm/cc) of the material which takes place during sintering.

EXAMPLE 5

In order to improve the permeability of the O-rings produced under Example 4, an expanded porous PTFE that was carbon filled to about 25% by weight having a thickness of .020 inch (0.51 mm) and a low density of 3.4 g/cc was wrapped onto a mandrel having a diameter of 3.25 inches (82.6 mm) and was sintered at 360°C for 20 minutes. This was slit along its longitudinal axis and removed from the mandrel. Layers were peeled away to produce a sheet of approximately 0.070 inch (1.8 mm) thick material. This material was then soaked in a solution of 1 part by volume Dow Corning Q1-4010 and 1 part Isopar-c under a vacuum bell jar at 28 inches Hg until completely permeated. This material did permeate more easily than standard membrane, but had reduced adhesion between layers. Pieces were stamped out of this material as in Example 2 and placed into the O-ring mold for curing. These samples were prone to delamination but otherwise looked good with improved rubber-like qualities.

EXAMPLE 6

Example 5 was repeated except the material which the samples were stamped from was a single layer of .080 inch (2.0 mm) thick expanded porous PTFE used in Example 5. This material produced satisfactory O-rings.

EXAMPLE 7

Effective penetration into PTFE membrane with elastomers such as silicone depends on pore size and volume. Although under vacuum conditions almost any size pores will eventually become permeated, the time involved may be prohibitive. It was however discovered that by addition of small amounts of fillers such as carbon black to the PTFE prior to coagulation, extrusion, and expansion, the material was rendered much easier to permeate. This phenomenon is seen in this example by using the membrane of Example 5 to produce a relatively large sealing device having a 3.5 inch (89 mm) outside diameter, a 1.24 inch (31.5 mm) inside diameter and a 1 inch (25.4 mm) thickness. Membrane material having a .010 inch (0.25 mm) thickness and 4 inches (102 mm) wide was wrapped onto several different size mandrels having diameters from 1 1/16 inch to 1.25 inches (27 to 31.8 mm). These mandrels used were as in Example 3 being hollow with a slot cut through them along the longitudinal axis and a pin placed in the center to hold the slot open until removal of the section pieces from the mandrel. The material was wrapped tightly onto the mandrels to a diameter of 3.5 inches to 4.25 inches (89 to 108 mm) and then placed in an oven for 2 hours at 360°C for sintering. They were then placed into a lathe and sectioned at approximately 1 inch (25.4 mm) thicknesses and removed from the mandrels. Shrinkage occurred during the sintering process but the samples were still relatively low in density. Several of each size were soaked in the above described silicone/lsopar-C solution in a 1:1 ratio by volume. This process was done under vacuum and took 24 hours for complete penetration. Other pieces were placed into a solution having a 1 :3 ratio by volume and took only 2 to 3 hours for complete penetration. Vacuum was applied for several minutes and then bled off several times. These pieces were then placed into a conically shaped mold and placed under 10,000 to 20,000 pounds of force at temperatures from 150°C to 200°C for 15 to 30 minutes. The resulting article is conical in shape with surprisingly smooth surfaces. A test oil pumping rig was built having several of these cone shaped stuffing box seals. These were run for about 12 hours with lubrication. Upon inspection of the seals no degradation was found. Without lubrication, the seal leaked after about 2 hours.

EXAMPLE 8

Another area of performance where the material of this invention excels is in flexure life. In order to test this property a strip of material 6 inches wide was cut from a 4.35 inches by 6 inches (110 by 152 mm) piece of .041 inch (1.04 mm) thick material produced as in Example 2, with a 1 :3 ratio by volume of Dow Corning Q1-4010 and Isopar-C. This piece was placed in a flex life testing apparatus known as a Shoe and Allied Trade Research Association STM 129 fiber board flexing machine (rev. June 1992). This device causes the material to be flexed through a 180° motion at 66 cycles per minute. Since this new material has an extremely long flex life, the time necessary to test the material to failure may be a year or more. The sample has flexed a multitude of times with no signs of degradation. For purposes of comparison both Viton elastomer and neoprene rubber samples were also tested on the STM 129 fiber board tester.

The material of the invention performed better in flex life compared to the two materials. One application which takes advantage of the strength of this material and long flex life of this material is in the area of pump diaphragms. A diaphragm was produced as follows. A piece of material was produced as in Example 2 with a 1 :1 ratio of Dow Corning Q1-4010 and Isopar-c and uncured was placed into a two part mold having a mirrored image of a specific pump diaphragm. The mold was heated to 163°C and was closed with only slight pressure for one minute. This step is necessary to allow any trapped air out of the mold. Then 20 tons of pressure was applied for six minutes to cure the silicone. Finally the mold was allowed to cool to 120°C and the sample was removed. The resulting product was a strong yet easily flexed diaphragm. Several more were produced using this same method.

EXAMPLE 9

A 32 layer sample of ePTFE having an overall thickness 0.053 inch (1.3 mm) was placed into an open canister which was then placed into a vacuum chamber having an access port through which liquid materials could be introduced. The vacuum chamber was evacuated using a vacuum pump. When a gauge measuring the vacuum within this chamber read 28 inches of mercury, a volume of Dow Corning Q 1*4010 silicone oil was introduced through the access port until the sample was completely submerged. The pump was then turned off and the chamber was allowed to retum to atmospheric pressure over approximately 15 minutes. The sample was permitted to remain submerged in the silicone oil for approximately 20 hours to completely fill the pores with silicone, then it was retrieved and excess oil was removed by blotting with paper towels. A portion of the imbibed ePTFE material was formed into a diaphragm using the following steps. Specifically, a split two- piece mold was preheated to about 115-120°C. A 4 inch (101 mm) diameter piece of material was centered in the mold so that an approximately 0.125 inch (3.2 mm) section protruded from the mold. The mold was then placed into a heated hydraulic press and pressurized to 2500 psi for about 12 minutes. The mold was then removed from the press, and the diaphragm was removed.

Figure 6 shows a three-quarter isometric view of the diaphragm, and Figure 7 is a side view along lines B-B' of the diaphragm depicted in Figure 6, showing compressed region 11 and uncompressed region 12.

Claims

WE CLAIM:

1. Process for providing a shaped article which comprises, in sequence:

(a) imbibing a solution of a curable elastomeric material into the pores of an expanded porous polytetrafluoroethylene structure in which the polytetrafluoroethylene structure has a morphological microstructure of nodes interconnected by fibrils;

(b) removing the solvent, leaving the elastomeric material as a coating encompassing the nodes and fibrils; (c) shaping the structure obtained in step (b) into a desired shaped structure;

(d) compressing the shaped structure to force any air inside the pores out of the structure; and

(e) curing the elastomeric material by heating the structure above the curing temperature of the elastomer.

2. The process of Claim 1 wherein the polytetrafluoroethylene structure used in step (a) is unsintered, and the elastomeric material is a silicone.

3. The process of Claim 1 wherein the polytetrafluoroethylene structure used in step (a) has been sintered, and the elastomeric material is a silicone.

4. The process of Claim 2 wherein the polytetrafluoroethylene structure used in step (a) contains carbon particles distributed within the structure.

5. The process of Claim 3 wherein the polytetrafluoroethylene structure used in step (a) contains carbon particles distributed within the structure.

6. The process of claim 1 , wherein said curing of the elastomer is carried out during compression of the shaped structure.

7. A shaped article preparable by the process of Claim 1.

8. The shaped article of Claim 7 in the form of an O-ring.

9. The shaped article of Claim 7 in the form of a bellow.

10. The shaped article of Claim 7 in the form of a conical shaft seal.

11. The shaped article of Claim 7 in the form of a pump diaphragm.

12. Process for providing a shaped article which comprises, in sequence:

(b) removing the solvent, leaving the elastomeric material as a coating encompassing the nodes and fibrils; (c) shaping the structure obtained in step (b) into a desired shaped structure; and

(d) curing the elastomeric material by heating the structure above the curing temperature of the elastomer.

13. The process of claim 12, further comprising selectively compressing at least a portion of the structure to force air inside the pores in selected regions of the structure prior to curing the elastomeric material.

14. The process of claim 13, wherein said selective compressing comprises compressing at least a portion of the outer surface of said structure.

15. The process of claim 13, wherein said selective compressing comprises compressing at least a portion of the inner surface of said structure.

16. The process of claim 13, wherein said selective compressing comprises compressing said structure in a discontinuous pattern over the entire surface of the structure.

17. The process of Claim 12 wherein the polytetrafluoroethylene structure used in step (a) is unsintered, and the elastomeric material is a silicone.

18. The process of Claim 12 wherein the polytetrafluoroethylene structure used in step (a) has been sintered, and the elastomeric material is a silicone.

19. The process of Claim 17 wherein the polytetrafluoroethylene structure used in step (a) contains carbon particles distributed within the structure.

20. The process of Claim 18 wherein the polytetrafluoroethylene structure used in step (a) contains carbon particles distributed within the structure.

21. A shaped article preparable by the process of Claim 12.

22. A shaped article preparable by the process of Claim 13.

23. The shaped article of Claim 21 in the form of an O-ring.

24. The shaped article of Claim 21 in the form of a bellow.

25. The shaped article of Claim 21 in the form of a conical shaft seal.

26. The shaped article of Claim 21 in the form of a pump diaphragm.

27. The shaped article of Claim 22 in the form of an O-ring.

28. The shaped article of Claim 22 in the form of a bellow.

29. The shaped article of Claim 22 in the form of a conical shaft seal.

30. The shaped article of Claim 22 in the form of a pump diaphragm.