WO1984003645A1

WO1984003645A1 - Microporous metal-plated polytetrafluoroethylene articles and method of manufacture

Info

Publication number: WO1984003645A1
Application number: PCT/US1984/000397
Authority: WO
Inventors: James L Manniso
Original assignee: Gore & Ass
Priority date: 1983-03-18
Filing date: 1984-03-16
Publication date: 1984-09-27
Also published as: US4557957A; CA1224092A; EP0142518B1; JPH0464305B2; AU2587784A; JPS60500905A; EP0142518A1; AU576490B2; EP0142518A4; JPH05269903A; JPH0659699B2; JPH0659700B2; ZA842018B; DE3471263D1; JPH05269904A

Abstract

Microporous polytetrafluoroethylene (PTFE) articles (12, 14, 16), having a microstructure of nodes interconnected by fibrils also having continuous interporous metal coating which encapsulate the nodes and fibrils of the PTFE while maintaining substantial porosity, together with a method of producing temporary liquid filled hydrophilic microporous article resulting in an improved metal plating manufacturing process.

Description

ICROPOROUS METAL-PLATED POLYTETRAFLUOROETHYLENE ARTICLES AND METHOD OF MANUFACTURE BACKGROUND OF THE INVENTION FIELD OF THE INVENTION; The present invention relates to microporous metal-coated articles, preferably polytetrafluoroethylene (PTFE) articles, and a method for manufacturing the articles.

BACKGROUND OF THE INVENTION; There has long been a need for a method of obtaining a high quality, adherent conductive metal coating on articles made from polytetrafluoroethylene (PTFE) polymer. However, the same qual¬ ities that have provided an impetus to use PTFE, namely, the relative inertness to a wide range of corrosive chemicals and low dielectric constant (good insulator) also have rendered this material extremely difficult to "metallize" using conventional metal plating techniques, and/or can result in products having poor and non-uniform metal coatings. Representatives of the prior art concerning metal plating techniques for PTFE are the U.S. Patent Nos. 4,150,171 to Feldstein and 4,021,314 to Dafter, Jr.

In addition to the problem of creating an adherent coating for PTFE, it is especially difficult to metallize the inner sur¬ face area of any porous structure. This characteristic is fur¬ ther affected by a reduction in the size of the porous openings, particularly with miniscule size pores in the order of 10 microns and even as small as 0.1 micron. Representative of prior art dealing with metal plating of porous electroplatable conductive bodies is U.S. Patent No. 4,204,918 to McIntyre et al.

A recent invention (U.S. Patent No. 3,953,566) provides a process for manufacturing microporous, high strength, shaped PTFE articles. These articles are characterized by a micro- structure of nodes of substantially solid PTFE interconnected by fibrils. Products produced by this process should find wide ac¬ ceptance in the industrial, medical, and electrical arts, for example, in industrial filter bag filtration art. "

SUBSTITUTE SHEET SUMMARY OF THE INVENTION

In accordance with the present invention, as embodied and broadly described herein, the metal-coated microporous article of manufacture preferably comprises a microporous polytetra¬ fluoroethylene membrane having a microstructure characterized by nodes, interconnected by fibrils, and a continuous interporous coating of metal, the metal coating substantially encapsulating at least some of the individual nodes and fibrils, wherein at least one surface of the article is metal-coated and wherein at least a portion of volume of the article beneath the one surface is metal-coated.

Preferably, the article has an average pore size less than about 100 urn and a porosity of greater than 25% by volume. It is also desireable for the article to be flexible, electrically conductive, hydrophilic, and to have high fluid permeability, an effective surface area greater than 1 m /g , and a metal coating that tenaciously encapsulates both nodes and fibrils of the microporous PTFE membrane while maintaining substantial porosity. By "effective surface area" is meant the surface area which is available for contact throughout the volume by a mate¬ rial as a result of the porous structure. By "tenaciously encapsulate" is meant adhering or tending to hold strongly to the surfaces of the microporous structure, while substantially enclosing the PTFE structure.

It is also preferred that the article have a bulk density of less than about 10.0 gm/cc, that the conductive metal be nickel, and that the thickness of the nickel coating surrounding o the nodes and fibrils be about 10 to 1000 A.

Further in accordance with the present invention, as embodied and broadly described herein, it is preferable that the articles of manufacture comprise a microporous first layer, the first layer also being flexible and electrically conductive along at least one surface layer and also from the one surface along a path perpendicular to the one surface through at least a portion of the first layer thickness, the first layer also having a bulk density less than about 10.0 gm/cc; and a second layer contacting the first layer at the surface opposite the one surface.

_SUBSTITU_TE SHEET ( W°^»IPO^* Preferably, the first layer is a metal-coated thin micro¬ porous PTFE membrane having a microstructure characterized by nodes interconnected by fibrils and being conductive through its total thickness and having a thickness of about 0.5 mils, and said second layer is a porous, fabric backing with a thickness of about 10.0 mils, and wherein the fabric backing also is coated with a conductive metal, the fabric backing layer being conductive along its surface opposite the membrane.

It is also preferred that the metal coating on the fiber backing surround the individual fibers, and the fiber backing is conductive through the entire second layer thickness so that a microporous article of construction is formed, which is com¬ prised of a metal-coated polytetrafluoroethylene microporous membrane, supported by a porous metal-coated fabric backing. Therefore, the layered construction of the articles of manufacture contemplates a structure being microporous (15-0.01 um pores), highly porous (98-50%), flexible, electrically con¬ ductive along its exterior surfaces and through its volume, hydrophilic, and highly permeable. The article of the invention has an effective metal-coated surface area greater than 1 m /gm, and metal coating which tenaciously encapsulates both exterior and interior surfaces without substantially reducing porosity. Table 1 provides additional characterization of metal-coated ar¬ ticles.

Still further in accordance with the present invention, as embodied and broadly described herein, there is included the process for coating microporous polytetrafluoroethylene membrane with conductive metal, the membrane having a microstructure of nodes interconnected with fibrils, and the conductive metal coating substantially surrounding at least some of the fibrils, the coated membrane being conductive along at least one surface and through at least a portion of the membrane volume beneath the one surface, the coated membrane retaining a certain degree of porosity, the process including the sequential steps of cleaning the microporous PTFE substrate through immersion in a liquid solution; displacing, coating, and filling the membrane with an aqueous surfactant; displacing, concentrating, and

SUBSTITUTE SHEET depositing the membrane by immersion in an aqueous palladium/tin activator solution; immersing the activated membrane in an ac¬ celerator to substantially displace the tin portion of the pal¬ ladium/tin activator; immersing the membrane in a water wash to displace at least a portion of the accelerator; and a final step of electroless plating of the microporous PTFE membrane.

The PTFE microstructure, which is clean and filled with aqueous surfactant or aqueous soluble solvent becomes a tempo¬ rary liquid-filled hydrophilic membrane. The saturated pore spaces now can serve to facilitate later pore space filling by liquids capable of displacing the initial liquid medium. There¬ fore, this saturation step enables subsequent displacement by activator and accelerator solutions before the process of coating the membrane with conductive metal. This method of aqueous surfactant or soluble solvent filling of PTFE micro¬ porous spaces and subsequent displacement by selected and com¬ patible water soluble liquids has been found to be a most surprising, effective, and useful method for the production of metal-coated microporous PTFE articles.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings which are incorporated in and constitute a part of this specification, illustrate one embodi¬ ment of the invention and, together with the description, serve to explain the principles of the invention.

Fig. 1A is a photomicrograph at 5000x magnification, showing the surface of a nickel-plated 0.02 um pore size poly¬ tetrafluoroethylene (PTFE) membrane sheet, designated sample A, made in accordance with the present invention.

Fig. IB is a cross-section of sample A with a superimposed energy dispersive X-ray analysis in line profile mode indicating relative amounts of.nickel penetration.

Fig. 2 is a photomicrograph at 500x magnification showing the surface of a nickel-plated 15 um pore size (PTFE) membrane sheet, designated sample B, made in accordance with the present invention.

Fig. 3 is a photomicrograph at 150X.65 magnification showing a cross-section of a nickel-plated 2.0 um pore size PTFE

SUBSTITUTE SHEET tube with superimposed energy dispersive X-ray analysis in line profile mode indicating relative amounts of nickel penetration.

Fig. 4 is a schematic cross-section of a layered article including a metalized PTFE layer and a microporous spunbonded polyester non-woven substrate, designated sample C, made in ac¬ cordance with the present invention.

Figs. 5A and 5B are photomicrographs at 5000X magnification showing the reduction in pore size after metallizing.

Table 1 represents coated membrane articles characteriza¬ tion data, comparing metallized membranes as shown in Fig. 1 and Fig. 2 and Fig. 4 with the same non-metallized membranes and layered articles.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to Fig. 1, and in accordance with the present invention, the conductive microporous article of manufacture of the present invention includes a metal-plated microporous polytetrafluoroethylene (PTFE) membrane wherein the microstructure of the membrane consists of a matrix of PTFE nodes interconnected by PTFE fibrils. Fig. 1 shows the membrane coated with nickel, wherein the basic node and fibril structure of the underlying microporous PTFE membrane can be discerned. U.S. 4,283,448 shows electromicroscopic photographs of uncoated expanded PTFE.

The preferred microporous PTFE membrane material is that disclosed in U.S. 3,953,566 and is produced by rapidly expanding unsintered PTFE. The material used in the article shown in Fig. 1 includes a PTFE membrane approximately 0.003 inches in thick¬ ness, and having an average pore size of 0.02 um, a porosity of 50%, and an average bulk density of 1.1 gm/cc. Expanded PTFE material in various forms and with various densities and porosi¬ ties is available from W.L. Gore & Associates, Inc., P.O. Box 1220, Elkton, MD 21921, and is sold under the trademark G0RE-TEX®i/ Membrane. The particular microporous membrane mate¬ rial used for the article in Fig. 1 is sold under the designation of 3B1.1.

1/ ^®G0RE-TEX is a trademark of W.L. Gore & Associates, Inc.

SUBSTITUTE SHEET The scope of the present invention is not restricted to flat sheets. Microporous polytetrafluoroethylene shapes such as rods, tubes, spheres, etc. can be effectively metal-coated under the invention. Additionally, a number of other plating mate¬ rials covering a range of porosities have been found to yield metallized articles with some or all of the beneficial charac¬ teristics to be discussed hereinafter (see Table 1 and related discussions, infra) . Therefore, the scope of the present inven¬ tion is not restricted to nickel. Metal coatings made of copper, cobalt, silver, gold, platinum, rhodium, etc. can be ef¬ fectively utilized. Moreover, electroplating in conjunction with electroless plating tinder the principles of the invention has been found to yield metallized PTFE articles with some or all of the beneficial characteristics contained in the disclo¬ sure. Metallized PTFE membranes can also be used in conjunction with a variety of substrates as layered constructions.

Further in accordance with the present invention, the microporous PTFE metal-coated membrane has an interporous metal coating, with the metal coating completely surrounding at least some portion of" the individual PTFE fibrils. By interporous is meant that the metal coating in the articles of the present in¬ vention is not restricted to the membrane surface but extends into the volume below the surface to a significant extent. This is a surprising and unexpected feature of the present invention, as it renders the article metal-coated not only along the sub¬ strate surface, but also along at least a path normal to the surface extending into the article volume, while retaining a significant fraction of the porosity of the original uncoated membrane.

The novelty of this present invention in causing interporous metal coatings to encapsulate both interior and ex¬ terior surfaces while maintaining microporosity is contrasted with the teachings in U.S. Patent No. 4,204,918 where it is taught that "standard electroplating techniques tend to cause a buildup of plating deposit on the exterior surfaces of the porous bodies, especially around and about pore egress sites. This is often serious enough to engender pore blocking."

SUBSTITUTE SHfc-fcf As embodied herein, the article in Fig. 1A shown generally as 10 shows the microporous PTFE membrane coated with nickel by an improved electroless plating process to be described herein¬ after. Fig. 1 shows nickel-coated nodes 12 and fibrils 14, with interfibril spaces being designated by the numeral 16. A further surprising aspect of the present invention, as can be seen in Fig. 1, is that the coating does not take the form of metal lodged in the PTFE matrix spaces, but a true microscopic encapsulation that surrounds and covers the individual nodes 12 and fibrils 14 was formed. The nickel coating thickness in the Fig. 1 microporous membrane article ranges from about 10 to 1000 o

A, and the coated membrane average bulk density was about 2.0 gm/cc. The average pore size of the coated membrane shown in Fig. 1 was less than about 0.02 um as determined by an electron scanning microscope (ESM) .

A cross-section shown in Fig. IB reveals that the metal coating is not just on the exterior surface but has encapsulated fibrils and nodes and has penetrated into the depth but not through to the opposite surface of the microporous PTFE mem¬ brane. The superimposed energy dispersive X-ray analysis indi¬ cates relative amounts of nickel penetration. The article of Fig. 1 has high surface electrical conductivity but low volumet¬ ric conductivity. The extent of electrical conductivity will be more thoroughly addressed throughout the following discussion. Quite apart from the foregoing, the article in Fig. 1 was shown to be water wettable on the metal-coated surface (hydrophilic) and non-water wettable (hydrophobic) on the nonmetal-coated sur¬ face.

The coated membrane article in Fig. 2 is "plated through"; that is, it exhibits an electrically conductive path both along the surface and through the membrane thickness. As uncoated microporous expanded PTFE is an excellent insulator, the nickel coating was shown to be both surface-wise and depth-wise continuous.

The article shown in Fig. 2 reveals that the metal coating is not just on the exterior surface but has encapsulated fibrils and nodes and has penetrated into the depth and through to the

SUBSTITUTE SHEET opposite exterior surface of the microporous PTFE membrane. The article identified in Fig. 2 had both high surface and volumet¬ ric electrical conductivity. In addition, the article of Fig. 2 was shown to be water wettable on both surfaces and throughout the volume of the membrane, thus maintaining a high degree of fluid permeability.

The material used in the article shown in Fig. 2 is a PTFE membrane approximately 0.0005 inches in thickness, and having an average pore size of 15 um, porosity of 98%, and an average bulk density of 0.8 gm/cc. Expanded PTFE material in various forms and with various densities and porosities is available from W.L. Gore & Associates and is sold under the trademark GORE-TEX. The particular microporous membrane material used for the article in Fig. 2 is sold under the designation lC-1 ( .8/40F) .

The coated membrane article in Fig. 3 is an electroless nickel-plated microporous expanded PTFE tube. The article in Fig. 3 was made from a 0.4724 inch inner diameter by 0.0394 wall thickness microporous expanded PTFE tube having a nominal pore size of 2.0 um. The particular microporous PTFE tube used for the article in Fig. 3 is sold under the designation TA012. A cross-section shown in Fig. 3 reveals that the metal coating is not just on the exterior of the outer tubular surface, but has encapsulated fibrils and nodes allowing the coating to penetrate into the depth but not through to the inside surface. With respect to the inside surface, the metal coating is not just on the exterior of the inside surface but has encapsulated fibrils and nodes and has penetrated into the depth but not through to the outside surface. The superimposed energy dispersive x-ray analysis in Fig. 3 indicates relative amounts of nickel penetra¬ tion. As a result, the article is electrically conductive along its inner and outer surfaces but is not electrically conductive through its volume. The article of Fig. 3 is also water wettable on both the inner and outer metal-coated surfaces (hydrophilic) .

For various physical, electrical, and/or chemical reasons, - it may also be advantageous to provide a layered article includ¬ ing a conductive microporous layer. For instance, the coated

SUBSTITUTE SHEET (, __O PI and expanded PTFE article may be a relatively thin membrane which may require a backing layer for support, and such an al¬ ternative preferred embodiment is shown schematically in Fig. 4 and designated by the numeral 20.

In accordance with the present invention, the layered arti¬ cle includes a microporous first layer which is flexible and electrically conductive both along one layer surface and through at least a portion of the layer thickness perpendicular to the surface, and has an average bulk density of about 0.8 gm/cc. As embodied herein, the layered article 20 in Fig. 4 includes first layer 22 which is formed of a nickel-coated expanded PTFE mem¬ brane material substrate similar to those described previously in relation to Fig. 2. A representative sample (corresponding to item B in Table 1) having a pore size of 15 um, was conduc¬ tive not only from free surface 26 but also through the entire membrane layer thickness.

Further in accordance with the present invention, the lay¬ ered article includes a second layer in contact witn the first layer. Thus, as embodied herein, layered article 20 also in¬ cludes second layer 24 contacting first layer 22 at surface 28. Second layer 22 can be a thin sheet of non-woven polyester scrim fabric which is also coated with nickel, and which also is flex¬ ible and electrically conductive both along the sheet surface, such as surface 30, and through the sheet thickness.

The presently preferred method of fabricating layered arti¬ cle 20 is to use a laminate of prebonded microporous PTFE mem¬ brane and the non-woven fabric sheet, and then to nickel-coat the laminate using the improved coating method to be described hereinafter. Such uncoated laminates are available commercially from W.L. Gore & Associates, Inc. in a variety of thicknesses and PTFE membrane porosities.

However, for certain applications, it may be desirable to have the second layer uncoated or coated in a pattern different from first layer 22, in which case it may be preferable to fab- ricate layers 22 and 24 separately and then assemble them into contacting relation. In such cases, one of ordinary skill in the art would appreciate that various means would be available

SUBSTITUTE SHEET f _ QMPI for assembling the layers, including mechanical fasteners and chemical bonding agents, depending upon the nature of the sur¬ faces to be contacted. Thus, the scope of the present invention is not intended to be limited by the means or method used to maintain contact between the separate layers.

The coated membrane article in Fig. 4 is layered article comprising a nickel-coated microporous PTFE membrane and a nickel-coated microporous spunbonded polyester non-woven sub¬ strate.. The material used in the article shown in Fig. 4 in¬ cludes a PTFE membrane approximately 0.0005 inches in thickness and having an average pore size of 15 um, a porosity of 98% and an average bulk density of 0.8 gm/cc. The particular micro¬ porous membrane material used for the article in Fig. 4 is sold by W.L. Gore & Associates, Inc. under the designation lC-1 (.8/40F). The substrate backing material used in Fig. 4 is a spunbond non-woven polyester material approximately 0.007 inches in thickness, and having an average pore size of 200 um, a porosity of 80% and an average density of 0.16 gm/cc. Spunbond polyester non-woven materials in various densities and porosities are available from E.I. DuPont Company, Inc., Wilmington, DE 19898, and are sold under the trademark βReemay. The particular non-woven material used for the article in Fig. 4 is designated ®Reemay 2011.

The descriptions of preferred embodiments as described by Figs. 1, 2, 3, 4, and 5 and references to pore size, % porosity, surface area, resistivity, etc. are illustrative of the inven¬ tion as a whole and are not meant to limit the broadest scope of the invention. While in many applications of this invention, it might be necessary to minimize the reduction in pore size due to the effects of the metallized coatings which act to narrow some¬ what the pores in the PTFE microstructure, in other cases regu¬ lation of coating deposition thickness can be an effective and further unique means of reducing pore size in PTFE microporous o membranes from some 0.02 um to some 0.0002 um (2A) or less. Fig. 5A is a photomicrograph showing a non-metallized PTFE 0.02 um pore size membrane. Fig. 5B is a metallized nickel-plated o membrane having 0.002 um (20A) effective pore size made from the

0.02 um pore size PTFE membrane.

SUBSTITUTE SHEE

-li¬

lt is well known in the art that plating disposition is non-uniform with respect to thickness. The coating tends to concentrate on projections and edges, depositing on recessed surfaces to a much lesser extent. It has been found that this plating phenomenon will tend to form pores in the PTFE micro¬ structure which are narrower at the outermost surfaces and which tend to widen in the volume below the surface. An anisotropic microporous structure is quite useful in applications such as ultrafiltration where a key element is the availability of mem¬ branes having useful trans-membrane flux and separation charac¬ teristics. Useful ultra-filtration membranes having adequate flux characteristics at reasonable pressure drops are anisotropic.

The nickel coating on the article shown in Figs. 1, 2, 3, 4, and 5 was found to be relatively durable as it did not flake or peel off when rubbed with a rubber eraser. The eraser test consists of a rubber eraser weighted down by a 1 kg gram weight..?/ Metallized vacuum deposited samples were used for control. The object of the test was to determine the extent of wear due to the contact of the eraser with the metal-plated sur¬ faces. The samples were placed under the weighted eraser and drawn through the eraser's pinch point. It was found that after only 1 pull the control sample showed wear, while the nickel- plated sample was pulled through 100 times and showed no signs of wear. The metal coating also passed a crease test and a fil¬ ter bag blaster test. The crease test consisted of folding sam¬ ples in a sharp crease, and microscopically viewing the crease line for signs of wear. Metallized vacuum deposited samples were used for control. The vacuum deposited metallized samples showed wear after only 1 crease, the metallized-plated samples of the invention were creased 100 times without showing signs of wear.

The filter bag blaster test is an abrasion test which uti¬ lizes abrasive dust to blast the sample surface. Non-metallized samples of the same construction as metallized plated samples

2/ A 5 gram weight was used for sample 2

SUBSTITUTE SHEET were used for comparative testing. The abrasion durability on the metallized plated samples were found to be equal to or better than the non-metallized samples.

Also, a 72-hour soak of the coated membranes in a 47% KOH solution revealed no visible flaking or leaching of nickel or organic deterioration.

Importantly, it was found that the nickel coating rendered the microporous PTFE substrate hydrophilic, a surprising feature in view of the strongly hydrophobic character of the uncoated PTFE material. This feature allows the coated membrane shown in Fig. 2 to be easily saturated with plain water, without the need for aqueous surfactants and/or excessive pressures. Table 1 shows testing details.

SUBSTITUTE SHEET TABLE I

The values in Table 1 are to be considered illustrative and as not circumscribing the invention in its broadest scope.

SAMPLE DESIGNATION Al Bl Cl

Notesi A Gurley Seconds/ASTM D726-.58 Method A

B Ethanol Bubble Point/ASTM

C Eraser Test: Abrasive test compares durability to a metal vacuum deposited sample by rubbing with eraser an equal number of times at equal pressure and observing wear.

D Blaster Test: Samples are pneumatically blasted with an abrasive dust at equal impinge¬ ment velocities and frequency samples are visually inspected for abrasion and compared against metal vacuum deposited samples.

* ^βReemay ** Tested at 20.36 in. Hg.

Table 1 also represents data on a layered article made in accordance with the present invention, namely item no. C. The manufacturer's designations for the samples are included.

For coated laminate sample of item no. C, the porosities of the final coated article, as a fraction of the porosities of the uncoated laminate were greater than 0.5 indicating retention of a significant portion of the microporosity of the uncoated lam¬ inates. The ratio of the resistivity per unit length measured along the coated PTFE surface divided by the resistivity per unit length through the coated laminate thickness averaged greater than about 20 for sample C, indicating good subsurface interporous coatings. Average coated PTFE layer pore size for sample C was less than about 15 um.

The method of tenaciously encapsulating the microporous PTFE membrane with continuous interporous metal coatings in¬ volves a series of sequential steps including cleaning the microporous PTFE membrane through immersion in a liquid solution; displacing, coating, and filling the membrane with an aqueous surfactant; displacing, concentrating, and depositing the membrane by immersion in an aqueous palladium/tin activator solution; immersing the activated membrane in an accelerator to substantially displace the tin portion of the palladium/tin activator; immersing the membrane in a water wash to displace at least a portion of the accelerator; and a final step of electroless plating of the microporous PTFE membrane. In the process of the present invention following the initial cleaning step, the microstructure of nodes and interconnected fibrils is coated with a water-surfactant solution or a suitable water- soluble organic solvent (e.g., acetone, isopropanol, etc.) which fills the interior microporous spaces. The time required to saturate the PTFE microstructure is largely dependent on the size of the pores. Pressurized flow could be used to ensure maximum pore space filling and coating of the various nodes and fibrils and shorten saturation times.

After imbibing the microporous material, the aqueous sur¬ factant is replaced by another solution, always keeping the mem¬ brane completely filled. This displacement affected by the

SUBSTITUTE SHEET °^MPI later solution may result in some circumstances in a total substitution of the original aqueous liquid. Varying the amount of time, concentration, and pressure controls the degree of dis¬ placement. Following the activation step, to be described here¬ inafter, the aqueous solution covering the PTFE membrane is replaced with an electroless plating solution. The resultant coating will surround the interior and exterior surfaces of the substrate with a durable coating while retaining a substantial fraction of the original porosity. The liquid-liquid displace¬ ment has been found to be particularly effective in allowing the electroless plating solution to fill the voids in the micro¬ porous material.

Therefore, in accordance with the present invention, the process for forming interporous coatings of a conductive metal on a microporous PTFE substrate includes as a first step the cleaning of the substrate. As embodied herein, the 4" by 4" samples listed in Table 1 were cut and soaked 1.5 to 10 minutes in anhydrous acetone.

Further in accordance with the present invention, the cleaned samples were saturated with an aqueous surfactant solu¬ tion. As embodied herein, the cleaned samples were soaked for 5-10 minutes in a solution of Macuplex Preactivator PA-3 (a surfactant activator manufactured and sold by McDermid Inc., Brookside Road, Waterbury, CT) , methanol, and distilled water. The samples were then given a final water rinse. Care was taken not to contaminate the cleaned and saturated samples.

In accordance with the invention, the next step is activat¬ ing the substrate. As embodied herein, the activation step in¬ cludes a series of substeps. The substrate was soaked in Shipley Cuposit Catalyst 9F (tin, palladium salt and hydrochlo¬ ric acid) for 5-20 minutes followed by a water rinse. Thereaf¬ ter, and as the final activation substep the substrates were soaked in Shipley Cuposit Accelerator 19 (hydrochloric acid) for 10-40 minutes. The final substep acts to remove substantially all of the tin from the tin-palladium catalyst within the micro¬ porous substrate. Ammonium hydroxide could also be used in this substep.

SUBSTITUTE SHEET In accordance with the invention, at least a portion of the accelerator remaining in the activated substrate is displaced using an aqueous solution. As embodied herein, a water rinse is use .

In accordance with the invention, the next step in the coating process includes plating the activated substrates using an electroless plating solution. As embodied herein, the acti¬ vated substrates were freely floated in a solution of Shipley Niposit 468 (electroless nickel plating system manufactured and sold by the Shipley Company, 2300 Washington Street, Newton, Massachusetts 02162) contained in a glass beaker. The plating solution was maintained at 150°F and adjusted to a pH of 7.2 using ammonium hydroxide. The substrates were stirred with a stirring bar magnet but were not allowed to contact the stirring bar. Under these conditions, plating commenced immediately and was concluded in all cases within 5 minutes.. The thickness of o the plating surrounding the nodes and fibrils is about 10-lOOOA. Following plating, the substrates were rinsed in water and allowed to dry before making the recorded porosity and conduc¬ tivity measurements.

Experimental plating has included plating electroless nickel and copper on GORE-TEX Membrane materials from 50 um pore size to 0.02 um pore size. The thickness of plated materials ranges from a fraction of a mil to 0.125 greater thicknesses. Plating on just one side, both sides, and through plating has been achieved. Microporous PTFE sheets, tubes, rods, and PTFE insulated electrical wire and cables have been successfully plated.

Certain possible applications of the metal-plated PTFE microstructure are;

(1) As a cleanable precipitator collector plate, where the material's conductivity would allow an electrical gra¬ dient to be established and consequent collection of particulate on the microporous surface without parti¬ cle penetration. The material's permeability and flexibility would allow it to be cleaned;

SUBSTITUTE SHEET

(2) As an electrically conductive grounding and shielding material which is flexible and uses a minimum of metal. (This would be important if precious metals are needed.) Also, the combination of the dielectric properties of PTFE Membranes, in conjunction with the plating, may have surprising electrical properties. (Flexible and formable printer circuit boards, etc.);

(3) For battery and fuel cell applications;

(4) As a catalytic material (poison gas suit, filter bags for No and So removal, etc.) ;

(5) As flow through electrode and electrolite systems;

(6) As a water wettable membrane filtration material;

(7) In conductive garments (protective clothing, etc.);

(8) Conductive filter bags and other conductive filtration materials;

(9) Instrumentation applications (particle counter, ane¬ mometer, etc.);

(10) Bacteriological and viral sterilization through chemi¬ cal arid electrocution means;

(11) In electrofiltration applications; and

(12) Microwave applications, antenna systems, strip lines, etc.

It will be apparent to those skilled in the art that various modifications and variations could be made to the metal- plated PTFE articles and method of manufacture without departing from the scope or spirit of the invention.

SUBSTITUTE SHEET

Claims

VL8-WHAT IS CLAIMED IS;

1. A- etal-coated microporous article of manufacture comprising;

(a) a microporous polytetrafluoroethylene substrate having a microstructure of nodes interconnected by fibrils, and

(b) a continuous interporous coating of conductive metal, said metal coating substantially encapsulating at least some of the individual nodes and fibrils, wherein at least one surface of the article is electrically conductive and wherein at least a portion of volume of the article beneath said one sur¬ face is electrically conductive.

2. The article as in claim 1 wherein the article is a sheet and is conductive along both sheet surfaces and through the entire sheet thickness.

3. The sheet as in claim 2 wherein the ratio of the re¬ sistivity per unit length along a sheet surface divided by the resistivity per unit length through the sheet thickness is greater than about 0.1.

4. The sheet as in claim 1 or 2 wherein the ratio of the permeability of the article divided by the permeability of the uncoated substrate is greater than about 0.1.

5. The article as in claim 1 or 2 wherein the average pore size is less than about 100 um.

6. The article as in claim 1 or 2 which is flexible.

7. The article as in claim 1 or 2 which has high abrasion resistance.

8. The article as in claim 1 or 2 wherein the metal- coated volume portion is hydrophilic.

9. The article as in claim 1 wherein the metal-coated volume portion is hydrophilic and any nonmetal-coated volume portion is hydrophobic.

10. The article as in claim 1 or 2 having an average bulk density of less than about 10 gm/cc.

11. The sheet as in claim 1 or 2 wherein the permeability of the uncoated substrate is between about less than 0.1 fn and 100 fn, permeability being measured on the fraziometer.

SUBSTITUTE SHEET

12. The article as in claim 1 or 2 wherein the metal is selected from a group consisting of nickel, copper, silver, gold, platinum, or rhodium and the thickness of the metal o coating surrounding the nodes and fibrils is about 10 to 1000 A.

13. The article as in claim 1 or 2 wherein the effective surface area of said coated volume is greater than 1 m2/gram.

14. A layered article of manufacture comprising:

(a) a microporous first layer, said first layer also being flexible and electrically conductive along at least one layer surface and also from said one surface along a path per¬ pendicular to said one surface through at least a portion of the first layer thickness, said first layer also having a bulk den¬ sity less than about 10 gm/cc; and

(b) a second layer contacting the first layer at the surface opposite said one surface.

15. The article as in claim 14 wherein said first layer comprises:

(i) a microporous polytetrafluorethylene substrate having a microstructure of nodes interconnected by fibrils, and

(ii) a continuous interporous coating of conductive metal, said metal coating substantially surrounding at least some of the individual nodes and fibrils.

16. The article as in claim 14 or 15 wherein said second layer is flexible.

17. The article as in claim 14 or 15 wherein said second layer is conductive along its surface opposite said first layer.

18. The article as in claim 14 or 15 wherein said second layer is conductive through the second layer thickness.

19. The article as in claim 14 or 15 wherein said second layer is permeable.

20. The article as in claim 14 or 15 wherein said second layer is hydrophilic.

21. The article as in claim 14 or 15 wherein said second layer is hydrophobic.

22. The article as in claim 14 or 15 wherein the average pore size of the coated first layer is less than about 50 um.

rORE "

SUB^βTI^TUTE «HE*T -wff-

23. The article as in claim 15 wherein said first layer is a thin membrane conductive through its total thickness and having a thickness of about .5 mils, and said second layer is a porous, fabric backing with a thickness of about 5 mils, and wherein said fabric backing is coated with a conductive metal, said fabric backing layer being conductive along a second layer surface.

24. The article as in claim 23 wherein the metal coating on said fabric backing surrounds the individual fibers and the fabric backing is conductive through the entire second layer thickness.

25. The article as in claim 24 wherein the conductive metal coating on said membrane and said fabric backing is selected from a group consisting of nickel, copper, silver, gold, platinum, or rhodium.

26. The article as in claim 23 wherein the ratio of the resistivity per unit length along the first layer surface divid¬ ed by^" the resistivity per unit length through the combined first and second layer thicknesses is greater than about 0.1.

27. The article as in claim 23 or 24 wherein the ratio of the permeability of the uncoated article divided by the permea¬ bility of the coating article is greater than about 0.1 fn.

28. A metal-coated article of manufacture wherein said ar¬ ticle is a microporous polytetrafluoroethylene tube having a microstructure of nodes interconnected by fibrils, the tube having inner and outer surfaces, said tube also being flexible and electrically conductive through at least a portion of the microporous structure.

29. The article as in claim 28 wherein the metal coating has encapsulated the fibrils and nodes of a portion of said inner surface, penetrating into the depth but not through to the outside surface.

30. The article as in claim 28 wherein the metal coating has encapsulated the fibrils and nodes of a portion of said outer surface, penetrating into the depth but not through to the- inside surface.

SUBSTITUTE SHEET

31. The article as in claim 28 wherein the metal coating has encapsulated the fibrils and nodes of portions of both of said inner and outer surfaces of the tubular membrane but has not penetrated throughout its volume.

32. The article as in claim 28 wherein the metal coating has encapsulated the fibrils and nodes of portions of both of said inner and outer surfaces of the tubular membrane and has penetrated through its volume.

33. The article as in claim 28 wherein the metal-coated surface area is microporous and hydrophilic and any nonmetal- coated volume portion is hydrophobic.

34. The article as in claim 1, 15 or 28 wherein the coated metal thickness is non-uniform throughout the microporous struc¬ ture, said micropores having a smaller cross-sectional flow area at said conductive surface compared to the cross-sectional flow area in said volume beneath said conductive surfaces.

35. A process for coating a microporous polytetrafluoro¬ ethylene substrate with a conductive metal, the substrate having a microstructure of nodes interconnected with fibrils, and the conductive metal coating substantially surrounding at least some of the nodes and fibrils, the coated substrate being conductive along at least one surface and through at least a portion of the substrate volume beneath the one surface, the coated substrate retaining at least some of its porosity, the process comprising the steps of:

1) cleaning the substrate;

2) saturating the membrane with an aqueous surfactant solution;

3) activating the surfaces of said substrate to be plated using a palladium/tin activator solution wherein the activating step includes as a final activating substep displacing the tin portion of said activator solution with an accelerator solution;

4) displacing at least a portion of the accelerator using an aqueous medium; and —

5) plating the substrate using an electroless plat¬ ing solution.

SUBSTITUTE SHEET f OMPI TIO

36. The process as in claim 35 wherein the accelerator is selected from a group consisting of hydrochloric acid and ammonium hydroxide.

37. The process as in claim 35 wherein said cleaning step includes immersion of the substrate in anhydrous acetone.

38. The process as in claim 35 wherein said saturating step includes immersing the cleaned substrate in a solution comprising a surfactant, methanol and distilled water; and rinsing the substrate in water.

39. The process as in claim 35 wherein the activating step includes the further substeps of

(i) immersing said substrate in said palladium/tin activator solution; and

(ii) rinsing said substrate in water; said further substeps being carried out immediately before said final acti¬ vating substep.

40. The process as in claim 35 wherein the plating step includes the step of displacing the liquid in said activated substrate resulting from step 4 using the electroless plating solution.

41. The process as in claim 35 wherein the plating step includes the step of adjusting the pH of the plating solution to a preselected value using ammonium hydroxide.

42. The process as in claim 35 wherein the plating solution is maintained at a temperature of about 150°F and a pH of about 7.2.

43. The process as in claim 35 further including the steps of removing the plating substrate from the plating solution, rinsing the plated substrate in water, and drying the rinsed plated substrate.

44. A process for controllably reducing the effective pore size in a microporous polytetrafluoroethylene article, the poly¬ tetrafluoroethylene having a microstructure of nodes intercon¬ nected by fibrils, the process comprising the step of encapsu¬ lating the individual nodes and fibrils with a metal coating of a desired thickness to reduce the effective cross-sectional flow area of said micropores.

"gf EX

SUBSTITUTE SHEET >-ray-

45. The process as in claim 44 wherein the metal coating is applied by plating.

46. The process as in claim 44 wherein the average micro¬ pore cross-sectional flow area at a coated surface of said arti¬ cle is smaller than the average micropore flow area in the volume beneath said coated surface.

SUBSTITUTE SHEET