WO1997022661A1

WO1997022661A1 - Improved polytetrafluoroethylene article

Info

Publication number: WO1997022661A1
Application number: PCT/US1996/019520
Authority: WO
Inventors: Mark S. Spencer; James R. Hanrahan
Original assignee: W.L. Gore & Associates, Inc.
Priority date: 1995-12-15
Filing date: 1996-11-27
Publication date: 1997-06-26
Also published as: AU1283697A

Abstract

A resilient composite article is provided having a polytetrafluoroethylene matrix and a predetermined volume. The composite article comprises PTFE energy expandable or expanded hollow polymeric particles, and silicone elastomer particles. The article is useful as gasketing material.

Description

TITLE OF THE INVENTION

IMPROVED POLYTETRAFLUOROETHYLENE ARTICLE

BACKGROUND OF THE INVENTION

1 Field of the Invention

The present invention relates to compositions of microporous polymers such as expanded polytetrafluoroethylene and methods of producing such compositions More particulariy, the present invention relates to compositions of microporous polymers having elastomeric properties

2 Description of Related Art

With the development of expanded polytetrafluoroethylene (PTFE), a host of improved polymeric compositions have been achieved As is disclosed in numerous patents relating to such compositions, such as United States Patent 3 953,566 issued April 27, 1976, to Gore, these compositions comprise a microporous structure that includes many long sought properties, including selective liquid repellency and vapor permeability, low dielectric constant, biocompatibility, high tensile strength, and low density, for example

While there has been continuous interest in improving upon these basic compositions and processes in numerous respects, an effective method of providing a truly elastic expanded PTFE membrane has not been easily achieved For example, a number of investigators have proposed the addition of some form of silicone to PTFE resin during the initial mixing process to impart elastic properties Various examples of this concept are disclosed in United States Patents 4,859,383 issued August 22, 1989, to Dillon, 4,891 ,407 issued January 2, 1990, to Mitchell, 4,945, 125 issued July 31 , 1990 to Dillon et al , 4 962, 136 issued October 9, 1990, to Peters, and 5,071 609 issued December 10, 1991 , to Tu et al Unfortunately, the common requirement of initial mixing of PTFE resin and silicone in these processes limits the properties and possible uses of these compositions Moreover, most of these processes provide incomplete coating and/or penetration of the elastomer within the PTFE matrix, thus seriously constraining the elastomeric properties of the final product

Another approach which has been repeatedly tried in different forms calls for the introduction of the elastomer into an existing expanded PTFE structure In United States Patent 4,972,846 issued November 27, 1990, to Owens et al , it is taught that a heart defibulator patch can be produced by mechanically impregnating silicone into an expanded PTFE membrane through use of a vacuum While this process is successful for the disclosed purpose the resulting product does not provide the full degree of elasticity desired and 5 believed possible for wider uses Additionally the disclosed use of a vacuum system to impregnate a membrane is considered to be too cumbersome and inexact for practical application in other contexts Further, this material has a tendency to "ooze" silicone material when placed under pressure

To address some of these concerns, a number of researchers have

10 attempted to find a method of chemically impregnating a microporous polymer membrane with an elastomer One approach is to spread the silicone solution on to the expanded PTFE surface such as with a doctor's knife and wait for the solution to penetrate the substrate Examples of this technique are disclosed in Japanese Laid-Open Patent Applications 61 -40 328 to Japan l ^ Gore-Tex Inc , laid open February 26, 1986, and 62-100.539 to Japan Gore-

Tex Inc , laid open May 1 1 , 1987 Unfortunately, these techniques provide only limited penetration, and require too much processing time

Since a silicone material will not readily penetrate an expanded PTFE membrane in a raw form, it has been proposed to mix a silicone with some 0 form of solvent or wetting agent One proposed composition employing an

RTV silicone and a solvent of xylene produced only incomplete silicone penetration

A further solvent penetration process is proposed in United States Patent 5.066 683 issued November 19, 1991 , to Dillon et al With such a process it is ^ observed that at least partial penetration of a microporous PTFE membrane with a silicone will provide improved resistance to surfactant activity To achieve this result, a mixture of kerosene and silicone is employed Regrettably, this process appears to achieve only partial coating of the PTFE structure and apparently does not produce the highly elastic material presently 0 sought

Yet another approach to achieve improved resilient properties in an expanded PTFE product is taught in Japanese Laid-Open Patent Application JP 4335044, of Junkosha Company Ltd , published 24 November 1992, as well as WIPO Patent Applications WO 93/00163 and WO 93/00390 of 5 Minnesota Mining and Manufacturing Co , each published 7 January 1993

Each of these references teaches that a dimensionally stable expanded PTFE can be formed by mixing unexpanded thermoplastic expandable microspheres into a resin of PTFE and then applying heat to expand the microspheres and the resin into an expanded mass of nodes and fibrils This process produces an expanded mass which is resilient to deformation and imparts some degree of resilient loft to the PTFE material as well as a number of other unique properties Despite considerable promise of the PTFE/microsphere products these processes are replete with shortcomings which detract from their widespread use Perhaps the most substantial constraint of the poiymer/microsphere compositions is that the expandable microspheres presently commercially available under the trademark EXPANCEL® from Nobel Industries Sweden Sundsvall, Sweden, have temperature limitations above which the microspheres breakdown This eliminates the use of such products in high temperature applications and in applications where sintering of the PTFE is desired

SUMMARY OF THE INVENTION

The present invention relates to a resilient composite article having a polytetrafluoroethylene matrix Energy expandable hollow particulates are embedded within the matrix and a silicone elastomer material is disposed within the composite article in a discontinuous fashion

This article provides a microporous membrane such as an expanded PTFE membrane which has distinct elastic and resilient properties and provides usefulness as gasketing material

A process is also provided for producing an elastic membrane which can impart elastic properties to existing membrane structures

These and other purposes of the present invention will become evident from review of the following specification

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises improved compositions of highly elastic microporous membranes, such as expandable polytetrafluoroethylene (PTFE) and processes for producing such unique compositions In one embodiment of the present invention, a composition is provided which comprises a microporous polymer membrane having a top and a bottom surface, which is completely impregnated with an elastomeric silicone and which has energy expandable hollow particulate, such as microspheres in the polymer matrix

In accordance with the present invention a preferred embodiment is a precursor material comprised of PTFE, in the form of paste dispersion or powder and expandable microspheres in the form of a dry powder or solution is mixed in proportions of 3 to 15 volume percent microspheres, and 5 to 70 volume percent PTFE, with or without elastomeric silicone in the material Mixture may occur by any suitable means including dry blending of powders wet blending, co-coagulation of aqueous dispersions and slurry filler, high shear mixing, etc As the term is used herein "volume percent shall mean a percentage of the volume of the precursor material

The PTFE aqueous dispersion employed in producing the PTFE precursor of the present invention may be a milky-white aqueous suspension of PTFE particles Typically, the PTFE aqueous dispersion will contain about 20% to about 70% by weight solids, the major portion of such solids being PTFE particles having a particle size in the range of from 0 05 micrometers to about

5 0 micrometers Such PTFE aqueous dispersions are presently commercially available from the E I duPont de Nemours Company, for example under the tradename TEFLON® 3636, which is 18-24% by weight solids being for the most part PTFE particles of about 0 05 micrometers to about 5 0 micrometers A thickness of the above described precursor material in tape form may range from about 5 mils to about 125 mils for example

Upon heating the precursor material the thickness increases due to the expansion of the energy expandable hollow particulate The amount of expansion observed is dependent on several factors including the weight percent of energy expandable particulate present, the type of energy expandable particulate, the molecular weight of the polymeric shell of the energy expandable particulate, and the toughness of the PTFE matrix holding the precursor material together Typical thickness of the material of Ihe invention can be in the range of from about at least 10 mils, and preferably from 10 to 100 mils, and most preferably from 20 to 60 mils Other thicknesses can be achieved

Temperatures needed for the thermal expansion step to occur are dependent on the type of polymer comprising the shell of the microsphere and on the particular expandable gas inside the shell Typical temperatures range from about 40°C to about 220°C, preferably from 60°C to 200°C, most preferably from 80°C to 190°C

An elastomer material, such as a silicone elastomer material (e g dimethyl siloxane) can be added to the PTFE before expansion In one 5 embodiment this is achieved by compounding the PTFE/particulate with an elastomeric silicone such as dimethyl siloxane A suitable dimethyl siloxane is type X1-4105, or Q1 -4010, which may be obtained from Dow Corning [It may also be suitable to use a silicone dioxide reinforced silicone material such as Q3-6611 which may also be obtained from Dow Corning ] The siloxane is 10 added on a weight per weight basis, and may be diluted with a solvent, such as mineral spirits, for example In general the siloxane may be added in amounts ranging from 1 to about 50 percent, preferably from 5 to about 20 percent and most preferably from 10 to about 15 percent of the total weight Other suitable elastomer materials include but are not limited to silicone rubbers, ι fluorosilicone elastomers, fluorocarbon elastomers, perfluoro elastomers, other fluoroelastomer materials, or polyureathane

Subsequently the precursor material is heated in a range from about 130°C to about 190°C, to not only achieve expansion of the precursor material, but also to catalyze the siloxane into a cured state The resulting article is an 0 easily compressible, PTFE composite, including a silicone elastomer disposed within the composite article in a discontinuous fashion

The addition of the elastomer material yields a composite with increased z-strength, tensile strength and elongation It also provides some degree of resilience and increases the usable temperature range of the material These 5 desired properties are achieved without sacrificing electrical conductivity or softness/compressibility of the composite article

When the silicone is added after expansion, it can be added in a solution of silicone and solvent which is mixed in the broad range of 0 1 1 to 3 1 percentage by weight The preferred silicone for use with the present invention 0 comprises a material soluble in one or more solvents capable of permeating and wetting out an expanded PTFE structure The mateπal 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 The material is preferably translucent in color Further, the material preferably employs a one or two part 5 cure system ideally at an elevated temperature, to cure the liquid silicone into a rubber-like mass Suitable silicones for use in the present invention include but are not limited to, Q3-661 1 X1 -4105, and Q1 -4010, all available from Dow Corning, Inc , Midland, Michigan

A preferred solvent for use with the present invention comprises a solvent which both actively dissolves the silicone and is readily absorbed into a structure of the intended polymeric substrate For use with a PTFE substrate structure, a solvent, such as methylene chloride, isopar or toluene s particularly useful

A preferred mixture of silicone and solvent is a mixture of 10-75% by weight of Q1 -4010 silicone elastomer and 25-90% methylene chloride, isopar, or toluene solvent This mixture is formed by stirring the solvent while adding the silicone elastomer at room temperature (about 22°C) until the m ixture has achieved a homogenous color With an isopar mixture, the mixture should be re-stirred prior to each use due to precipitation of materials Once the silicone/solvent composition is formed it can then be applied to the unexpanded precursor but preferably is applied to the stretched PTFE While the particular properties of the membrane will vary according to the requirements of any given application the general membrane properties should include medium to high porosity, and wettability by various solvents such as methylene chloride, toluene, and/or isopar

The silicone/solvent can be a composition applied to the membrane by spreading it evenly over the membrane and then allowing the composition to become absorbed therein Alternatively, the membrane may also be immersed within the composition until it becomes saturated Preferably, the membrane and composition is exposed to an energy source, such as a heated oven set at

70 to 75°C or above for a period of 2 to 5 minutes or more to evaporate away any solvent Ideally, evaporation comprises employing an oven healed to 85°C or above and exposing the composition for at least 5 minutes The evaporation of solvent can also be performed in one of the following manners air drying for about a 5 hour period, or about 1 hour at about 50°C in an explosion-proof oven The membrane can be made according to U S Patent 3,953,566 When applied in this manner it has been found that the membrane becomes thoroughly impregnated with the silicone between its top and bottom surfaces In fact, the bottom surface of the membrane (i e the surface opposite the side where the composition is applied) tends to have a l acky feel to it which may be desirable if the membrane is to be used as an adhesive layer

By contrast, with some applications the top surface of the membrane has been found to have a powder-like material on it This is believed to be a 5 coating of silicon dioxide found as a filler in some commercial silicone materials This material may be left in place for ease in handling or may be removed through any suitable means, such as through use of a solvent and/or mechanical scraping

To further aid in the impregnation process the above described process ι o may be combined with other processes to achieve a material having specific properties For example, for some applications, such as applications requiring very fine porous membranes, it may be desirable to impregnate the membrane with the silicone/solvent composition with the aid of a mechanical vacuum process Other possible methods include use of mechanical pressure through 15 either a pressure or vacuum process

The other additive is energy expandable hollow particles Such particles comprise a plastic coating surrounding an expandable liquid or gaseous volatile fluid As is explained in United States Patent 3,615,972 issued October 26, 1971 , to Morehouse et al , thermoplastic microspheres are 0 adapted to expand dramatically when exposed to heat These microspheres are monocellular particles comprising a body of resinous material encapsulating a volatile fluid When heated, the resinous material of thermoplastic microspheres softens and the volatile material expands, causing the entire microsphere to increase substantially in size On cooling, the resinous material 5 in the shell of the microspheres ceases flowing and tends to retain its enlarged dimension, the volatile fluid inside the microsphere tends to condense, causing a reduced pressure in the microsphere

Such thermoplastic microspheres are commercially available from Nobel Industries Sweden, Sundsvall, Sweden, under the trademark 0 EXPANCEL® These microspheres may be obtained in a variety of sizes and forms, with expansion temperatures generally ranging from 80 to 130° A typical EXPANCEL microsphere has an initial average diameter of 9 to 17 microns and an average expanded diameter of 40 to 60 microns According to Nobel Industries, the microspheres have an unexpanded true density of 1250- 5 1300 kg/m³ and an expanded density below 20 kg/m³ 2661 PC 17US96/19520

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It should be understood that the use of the term "energy expandable hollow particules" herein is intended to encompass any hollow resilient container filled with volatile fluid which is adapted to expand Although presently available microspheres are essentially ball-shaped particles adapted to expand when exposed to an energy source, it should be understood that such microspheres are quite resilient in their expanded form and can be compressed and released (e g through extrusion) to achieve the expansion required for the present invention Additionally, it may be possible to form a variety of other shapes, such as tubes, ellipsoids, cubes, particles etc As such, the term "energy expandable particulate" in the context of the present invention is intended to include all applicable forms and uses of these products now known or later developed

A wide variety of expandable fluids may be enclosed within the polymeric shell of the expandable microspheres They can be volatile fluid- forming agents such as aliphatic hydrocarbons including ethane ethylene propane, butane, isobutane, isopentane, neopentane, acetylene hexane heptane, or mixtures of one or more such aliphatic hydrocarbons preferably having a number average molecular weight of at least 26, and a boiling point at atmospheric pressure about the same temperature range or below the range of the softening point of the resinous material of the polymeric shell when saturated with the particular blowing agent utilized

In one embodiment of the present invention EXPANCEL type 091 DU ts employed This product comprises an off-white dry powder with a particle size between 5 and 50 microns The shell of the microsphere comprises polyacrylonitrile or polymethacrylonitπle The volatile liquid comprises isopentane

It has been found that by mixing a dry preparation of EXPANCEL microspheres with a dispersion of PTFE or similar polymer and then heating the resulting composition, the polymer will undergo expansion in three- dimensions to achieve a fibπllated PTFE matrix A mixture of PTFE, in the form of paste, dispersion or powder and microspheres, in the form of dry powder or solution, are mixed in proportions of 1 to 90% by weight microspheres with 5 to 85% by weight of microspheres being preferred It should be appreciated that a wide range of products may be created even with a percentage of microspheres of merely 0 1 to 5% by weight, similarly, for some uses, filled products may be created with a percentage of microspheres and/or other fillers between 90 to 99 or more percent by weight Mixture may occur by any suitable means, including dry blending of powders wet blending, co- coagulation of aqueous dispersions and slurry filler high shear mixing, etc Without intending to limit the present invention the following serve as examples of how the present invention may be employed

EXAMPLE 1

39g of EXPANCEL 091 DU microspheres were slurried with 10 3 liters of de-ionized water and 9 7 liters of isopropyl alcohol The slurry was then co- coagulated with 2858g of a PTFE dispersion at 18 7% solids (type TE-3636 from E I duPont de Nemours and Company) The resulting coagulum was dried at 95°C for 16 5 hours The coagulum was frozen at -10°C for 7 hours, and hand screened into a powder form through a 1/4 inch mesh metal screen

The material was then lubricated at a level of 0 2 pounds per pound with a lubricant comprising a mixture of 75% silicone monomer (Sylgard® type 4105 obtained from Dow Corning Inc ) and 25% mineral spirits The material was then re-frozen at -10°C for 17 hours, and re-screened through the 1 /4 screen The resulting powder was then preformed into a 2 5 inch diameter pellet and extruded into a tape 45 mils thick and 4 inches wide The extrudate was calendered to a thickness of about 20 mils and dried at 105°C for 5 minutes

A gasket material was made by heating the tape, with the lubricant not removed to a temperature of 190°C for 2 minutes The heating not only has the effect of expanding the structure of the tape, but additionally, the heating will cure the siloxane to create a network of silicone elastomer and expanded PTFE along with the conductive particles This creates a material with a high Z strength a high tensile strength a high usable temperature range and excellent recoverability EXAMPLE 2

In a 1500 cc vessel, 2 Og of EXPANCEL 091 DU type microspheres is slurried with 785g de-ionized water and 87 3g isopropyl alcohol The mixture is stirred at 500 rpm This mixture is co-coagulated with 38 25g of a PTFE dispersion at 26 5% solids (type TE-3636 from E I duPont de Nemours and Company) The resulting coagulum is dried at 105°C for 24 hours The coagulum was then frozen at -10°C for 6 hours, and then screened into powder form through a 1/4 inch metal screen The material is then lubricated with mineral spirits at a level of 0 125 pounds per pound The material is then re- frozen for 24 hours, and hand screened through the 1/4 inch metal screen to remove any large lumps of lubed coagulum The resulting material is then allowed to dwell at room temperature for at least 4 hours The lubed powder was then extruded into a tape form with a rectangular cross section, and calendered to a typical thickness of 25 mils The tape is dried at 105°C for five minutes This same material is heated to 190°C for five minutes to expand the composite

The material is treated with a mixture of 1 part by volume Dow Corning Q 1 -4010 silicone and 1 part mineral spirits The material is soaked in this mixture for 30 minutes and then cured at 1 10°C

Although a few exemplary embodiments of the present invention have been described in detail above, those skilled in the art readily appreciate that many modifications are possible without materially departing from the novel teachings and advantages which are described herein Accordingly, all such modifications are intended to be included within the scope of the present invention, as defined by the following claims

Claims

WE CLAIM

Having described the invention, what is claimed is 1 A composite article comprising a polytetrafluoroethylene matrix, energy expandable hollow particles embedded within said matrix in an amount of between about 3-15 volume percent, and silicone elastomer particles disposed within said matrix present in an amount between 1 and 75 percent of the total weight of composite 2 The article of Claim 1 , wherein the article is formed into a sheet having a thickness of about 10 mils to about 100 mils

3 The sheet of Claim 4, wherein the sheet is resilient in both the radial and longitudinal axis The article of Claim 1 wherein the energy expandable hollow particles have a shell selected from a group consisting of polyacrylonitrile and polymethacrylonitπle A composite article comprising a matrix of expanded porous polytetrafluoroethylene, energy expanded hollow particles embedded within said matrix in an amount of between about 3 to 15 volume percent, and silicone elastomer particles disposed within said matrix present in an amount between 1 and 75 percent of the total weight of composite Process for preparing the article of Claim 5 wherein polytetrafluoroethylene is mixed with expandable hollow particles and silicone elastomer, and the mixture heated until the expandable hollow particles expand Process for preparing the article of Claim 5 wherein expanded porous polytetrafluoroethylene having expanded hollow particles on the PTFE matrix is treated with an elastomer silicone composition such that the silicone composition is disposed in the pores of the expanded porous polytetrafluoroethylene