WO1993023226A1

WO1993023226A1 - Injection co-molded emi/rfi shielding gasket

Info

Publication number: WO1993023226A1
Application number: PCT/US1993/004534
Authority: WO
Inventors: Vincent Squitieri; Stanley W. Tozlowski; Michael Bunyan
Original assignee: Chomerics, Inc.
Priority date: 1992-05-13
Filing date: 1993-05-11
Publication date: 1993-11-25
Also published as: AU4247893A

Abstract

Method for making a conductive gasket (1), and a gasket (1) formed thereby, wherein a first conductive thermoset elastomeric material (2) is applied to the surfaces of a gasket mold defining a cavity and a second non-conductive thermoset elastomeric material (3) is injected into the mold cavity, heated at or near the curing temperature of the second material (3), and the gasket material is cured. Preferably the first conductive elastomeric material (2) is applied by spraying the material onto a mold at a temperature sufficient to cure the material. The gasket (1) provides excellent EMI shielding and low closure force, while being easy and inexpensive to manufacture compared to previously used methods.

Description

INFECTION CO-MOLDED EMI/RFI SHIELDING CASKET

FIELD OF THE INVENTION

The present invention relate to EMI/RFI shielding gaskets and more particularly to two-layer gaskets of this type made by a novel injection co- molding process.

BACKGROUND OF THE INVENTION

The operation of conventional electronic equipment is typically accompanied by the generation of radio frequency and/or electromagnetic radiation in the electronic circuitry of the electronic system. If not properly shielded, such radiation can cause considerable interference with unrelated equipment. Accordingly, it is necessary to effectively shield all sources of radio frequency and electromagnetic radiation from entering or exiting the electronic system. In instances where the radiation-generating equipment is permanently housed in a container, effective shielding may be accomplished easily through proper construction of the enclosure. This, however, is not always possible; when an equipment housing is provided with a readily openable access panel or door, effective radio frequency interference (RFI) or electromagnetic interference (EMI) shielding presents more of a problem.

Many electronic installations such as computer rooms, communication switch boxes, etc. need to be fitted with RFI/EMI shielding devices around these access openings. To this end, it has been known to provide EMI/RFI shielding gaskets of all manners of construction. One type of gasket ma erial used comprises knitted wire mesh or knitted wire mesh over an elastomeric core. Although these kinds of gasket materials are known to provide excellent shielding, the resistance offered by the tightly knit mesh makes it difficult to seal the door. Also, since these kinds of gaskets tend to be heavy, they are disfavored for use in applications where weight is a factor. Another type of shielding gasket generally comprises an outer layer of conductive elastomer superimposed over a non-conductive core. This type of gasket is usually made by extrusion of the conductive outer layer over a non- conductive elastomeric core, or by coating a conductive layer onto a non- conductive elastomeric core. The resulting gasket has a desirably low durometer and is thus highly useful in shielding applications as above because less closure force is needed for a good seal.

While useful and relatively inexpensive in relatively straight pieces, a drawback to using this type of gasket material however, is its cost, mainly 5 due to its manufacture when it is formed into non-linear configurations. For example, when a multi-component shield is made, several lengths of material need to be spliced together physically and electrically. Present manufacturing techniques are laborious and difficult because they involve hand splicing the pieces together. As such, the maniifacturing procedure l o takes more time than is desirable, and requires skilled labor.

Thus it is an object of the present invention to provide EMI-shielding gaskets which are inexpensive, lightweight, allow a low closure force, and which may be made in complex geometries.

It is another object of this invention to provide an improved method for 15 making a low durometer conductive gasket material with greater ease and at a lower cost.

Other objects of the invention will be apparent to those skilled in the art upon reading this specification.

20 SUMMARY OF THE INVENTION

The present invention relates to a method of making a gasket comprising the steps of applying a first thermoset elastomer to the inside surfaces of a mold defining a mold cavity, and injecting a second thermoset elastomeric composition into the mold cavity, wherein the mold cavity is 5 heated at or near the temperature at which the second thermoset composition cures, wherein the first elastomer is cured before the second elastomer is injected into the mold cavity, and one of the elastomers is conductive.

The invention further relates to a gasket formed by the method of the invention. 0

DESCRIPTION OF THE DRAWINGS Figure 1 depicts, in cross-section, a conductive gasket of the invention having a substantially linear, cylindrical shape.

Figure 2 depicts, in cross-section, an alternative embodiment of a 5 conductive gasket of the invention having a substantially linear, cylindrical shape, comprising a foam core. Figure 3 illustrates, in top view, another embodiment of a conductive gasket of the invention comprising a more complex physical structure comprising elongated ribs.

Figure 4 illustrates, in cross-section, detail of one of the elongated ribs shown in Figure 3.

PESCRIΓΠON OF THE INVENTION

The objects of the invention have been substantially accomplished through a co-molding process which, in a preferred embodiment, comprises applying a first, preferably conductive, thermoset elastomeric material to the inside surface(s) of a mold, curing the conductive elastomer, assembling said mold if necessary, then injecting a molten or fluid second, preferably non- conductive, thermoset elastomeric material into the center of the mold, and curing the gasket material. The first elastomeric material may be applied to the mold cavity by any procedure appropriate for the particular elastomer used, e.g., it may be brushed in, a sheet of the material may be laid in, or the material, if prepared in a suitable form, may be sprayed into the mold cavity. The latter technique is desirable from the standpoint of ease of application and precision with which the material may be applied. In a preferred embodiment, the first elastomer is sprayed onto the surfaces of a hot mold cavity, so as to cure it and thus eliminate a separate curing step. It is also advantageous to surface treat the mold surface with a release agent to insure that the first elastomer does not stick tenaciously to the mold when the completed gasket is removed. Such release agents will be apparent to those skilled in the art, but fluoropolymer coatings such as Teflon®(E. I. DuPont), a titanium nitride plating, or a simple soap composition are exemplary.

In a preferred embodiment wherein the first elastomeric material is conductive, the first elastomeric material must be at least of a sufficient thickness to allow electrical contact between the conductive filler particles contained therein. However, if the layer is too thick, it may crack at higher degrees of compression; also, more conductive elastomer will be needed in a thicker layer (which is generally unnecessary for shielding purposes), making the gasket unnecessarily more expensive to make. As such, the inventors have found that the first elastomeric material is advantageously thin, e.g., it is preferably applied in a manner to form a cured layer of from about 0.007 to about 0.075mm, preferably from about 0.02 to about 0.04mm. The term "first" elastomeric material is defined to mean that material which is applied to the mold cavity before the injection of the second elastomeric material. The word "elastomeric" should be given its usual meaning given the purpose for which the invention is intended. In a preferred embodiment the inventors intend for this first elastomer to be conductive, whereas the "second" elastomer, as defined below, is preferably non-conductive. The second elastomer may also be conductive, with the first elastomer being conductive or non-conductive, as the case may be. However, it is contemplated that the second elastomer will usually be non-conductive; such a gasket would be less expensive to make because the non-conductive materials costless than the conductive counterparts and because preferably so much more of the second elastomer is used in the gasket compared to the first. Also, a wider selection of non-conductive elastomers affording a low durometer, e.g., about 30 to 80, is available relative to the conductive elastomers; many of these non-conductive elastomeric materials are also of lower viscosity when being pumped into the mold (as will be described below), allowing for more complete loading of the non-conductive elastomer into the mold cavity.

The curing of the first elastomeric material before the subsequent injection of the second elastomeric material is necessary to maintain the integrity of the first layer during the injection process. Preferably, after the first elastomeric material has been applied to the mold surfaces it is cured according to the characteristics of the particular elastomeric material. (As mentioned above, if the first elastomeric material is applied in the preferred manner by spraying the material onto a hot mold surface, that is, heated to the elastomer's curing temperature, no separate curing step will be required.) The mold is then assembled if necessary. The assembled mold will have an orifice formed in it to allow the injection of the second elastomeric material into the mold cavity. The second elastomer may be delivered into the mold cavity by a pumping means such as an extruder like those commonly used for injection molding. The orifice may be advantageously designed to couple to the screw tip of the extruder, from which the fluid second elastomer is delivered. When such a delivery method is used it is advantageous that the second elastomeric material desirably be of a low enough viscosity when it is being pumped to make it easier for the pumping means to more completely fill the mold cavity with the second elastomeric material. This is particularly important when the mold pattern is complex or intricate. When multiple orifices are used a manifold connecting the pumping means to each orifice may be employed. Also, sprue holes or vents may be incorporated into the mold design to release any trapped air.

After the mold cavity has been filled with the second elastomeric material, the injection device is stopped; the operator will generally allow a little time for the material inside the mold cavity to gel or partially cure before disconnecting the pumping means from the mold. This allows for the complete filling of the mold cavity with the second elastomer so as to avoid any cavitation beneath the first elastomeric layer when the gasket is cured. Thereafter the material inside the mold is cured under the appropriate conditions of temperature and pressure for the elastomeric composition being used. Such conditions will be apparent to those skilled in the art.

Typically the same elastomer base will be used for the first and second elastomeric materials, with the distinction that one of the elastomers is impregnated with a filler that makes it conductive. Certain particular applications for which the gaskets are intended, however, may dictate that different elastomeric compositions be used. The elastomer bases used in the invention are thermosetting resins; these resins begin to cross-link and subsequently cure at a critical temperature. Any flexible thermosetting elastomer bases are suitable for use in the invention, such as EPDM copolymers, silicone rubbers, fluorosilicone rubbers, nitrile rubbers, butyl rubbers, and mixtures thereof. Omission of other thermosetting elastomers is not meant to specifically exclude their use in the invention.

The fillers that are used to impregnate elastomers to make them electrically conductive are well-known in the art. Examples of these fillers include but are not limited to electrically conductive noble metal-based fillers such as pure silver, noble metal-plated noble metals such as silver plated gold, and noble metal-plated non-noble metals such as silver plated copper, nickel or aluminum, for example, silver plated aluminum core particles, noble metal-plated glass, plastic or ceramics such as silver plated glass r.- rospheres, noble metal-plated alumina or noble metal-plated plastic ~: , crospheres, noble metal-plated mica, and other such noble metal conductive fillers. Non-noble metal-based materials are also suitable, like non-noble metal-plated non-noble metals such as copper-coated iron particles; non-noble metals, e.g., copper, aluminum, nickel, cobalt; and non- metal materials such as carbon black and graphite.

The shape and size of the electrically conductive fillers is not critical to the present invention. The fillers may be of any shape that is generally used in the manufacture of conductive materials, including spherical, flake, platelet, irregular or fibrous (such as chopped fibers). In making gaskets in a preferred embodiment of the invention where the first elastomer is conductive it is preferred that the particle shape be irregular, such as in flake or platelet form. The particle size of the electrically conductive fillers can be within the range normally used for fillers in conductive materials. Generally the particle size of the one or more fillers is from about 0.25μ to about 75Uy preferably from about 0.25μ to about 50μ, and more preferably from 0.25μ to about 15μ. As the thickness of the conductive layer decreases, the particle size advantageously decreases also.

The amount of the one or more electrically conductive fillers in the conductive elastomeric material used in the present invention can vary over a wide range, as long as it is present in an amount sufficient to provide EMJ/RFI shielding properties. Generally the fillers comprise from about 10 to about 98 percent of the total volume of conductive elastomer. Preferably the fillers comprise from about 60 to about 90 percent of the total volume of conductive elastomer. More preferably the fillers comprise about 80-86 percent of the total volume of conductive elastomer.

Other fillers may also be added to the elastomer base if desired. Such fillers include microwave absorbing materials, thermally conductive fillers, inert or reinforcement fillers and pigmentation fillers. Also, curing agents, cross-linking agents, solvents, diluents, or dispersion aids, etc, may be added as is well known in the art to form the desired conductive elastomeric material. In addition to the compounds described above, the elastomers used in the gasket and method of the invention may additionally comprise any other compounds, fillers, or agents that impart desirable properties to the cured gasket. For example, a foaming agent could be added to the second elastomer to allow for a gasket having a conductive outer layer and a non- conductive foam center. A gasket with a foam core would be more compressible than one with, e.g., a solid silicone rubber core. If a foaming agent, for example, blowing agents or plastic expandable microspheres which are well-known to those skilled in the art, were used, the second elastomer containing the agent could be pumped into the mold to 'underfill' the mold to allow for expansion of the second elastomeric material, which would then form a foam during the curing process. In any case such techniques for making a foamed core will be apparent to those skilled in the art. Installed in a shielded enclosure, this gasket would have a lower closure force than those without the foamed center.

The description above, and that which follows, is only meant to describe one particularly advantageous embodiment of the present invention and as 5 such is only meant to illustrate, not limit it.

Referring now to the drawings, FIG. 1 depicts in cross-section a gasket 1 made in accordance with the invention, having a substantially linear, cylindrical shape which comprises a first thin conductive elastomeric layer 2 and a second non-conductive elastomeric center 2. Such a gasket provides l o excellent EMI shielding while being easy and inexpensive to manufacture. An alternative embodiment of the gasket of FIG.1 is shown in cross-section in FIG.2, wherein a gasket 4 having a substantially linear, cylindrical shape comprises a first thin conductive elastomeric layer 5 and a second foamed non-conductive elastomeric center £. As described above, such a gasket has

15 the shielding characteristics of the gasket of FIG. 1 while being more compressible and having a smaller closure force.

FIG.3 depicts another alternative embodiment of the invention having a more complex geometry wherein a conductive gasket Z comprises elongated ribs S defining openings 5- A cross-section of rib 5 is shown more clearly in 0 FIG.4, wherein rib S has convex sides Ifi and H. Such a gasket may be used to provide a conductive seal for isolational separation of electrical components in an electronic device such as a cellular telephone. Such devices comprise electronic components which interfere with the operation of the each other, e.g., transmitting and receiving components. If the components 5 are not isolated from each other and from external radiation sources such as radar, television and radio signals, the electromagnetic energy will interfere with the operation of the device; thus effective shielding like that provided by a gasket of the invention is needed. Also, since gaskets made in accordance with the invention may be made of a low durometer, the seal between the 0 components, the gasket and any surrounding parts is more easily made, simplifying design and assembly of the device. The making of such a gasket can be accomplished simply by making a mold in the shape of the complex gasket material, then carrying out the method of the invention, which makes the hand splicing methods of the current art unnecessary. 5

EXAMPLE

A conductive flexible gasket made in accordance with the invention was made as follows. A conventional compression mold was prepared for use by separating the halves of the mold and heating the mold halves to 176^* C Thereaf er the mold surfaces were sprayed with a .025-.050mm thin layer of a first conductive elastomer. The conductive elastomer composition was prepared by combining 130g of a room temperature vulcanizable (RTV) silicone, 651g of silver filler (a powder/flake mixture in an 80/20 proportion of powder to flake), and 75g of a 50/50 (v/v) mixture of ethanol and toluene. Thereafter 400g of the above mixture was combined with 6g of a curing catalyst and I50g of toluene. The mold halves were assembled and a second, non-conductive elastomeric material comprising the RTV silicone and a curing catalyst used to prepare the conductive elastomer was injected into the mold cavity through an orifice located at one end of the cavity. The gasket was then cured at 176^* C for 15 minutes. The gasket material produced had good EMI-shielding properties and was of a low durometer.

Other complex or even simple gasket designs may of course be used in the invention, such as wave guide flanges, circular, square, cylindrical, square, and various other shapes.

It should be noted that the above example and description of embodiments of the invention are intended to illustrate the invention and are not meant as a limitation on it. It is intended that modifications, variations and changes to the invention may be made within the scope of the appended claims without departing from the spirit and scope of the present invention.

Claims

What Is Claimed Is: 1. A method of making a conducti e gasket comprising the steps of: (a) applying a first thermoset elastomer to the inside surfaces of a mold, said mold defining a mold cavity; and (b) injecting a second thermoset elastomeric composition into said mold cavity, said mold cavity heated at or near the temperature at which said second thermoset composition cures; wherein said first elastomer is cured before said second elastomer is injected into said mold cavity, and one of said elastomers is conductive.

2. The method of claim 1 wherein said first thermoset elastomer is, after curing, of a thickness in the range of from about 0.007 to about 0.075mm.

3. The method of claim 1 wherein said first thermoset elastomer is, after curing, of a thickness in the range of from about 0.02 to about 0.04mm.

4. The method of claim 1 wherein said first thermoset elastomer is selected from the group consisting of EPDM copolymers, silicone rubbers, fluorosilicone rubbers, nitrile rubbers, urethane rubbers, butyl rubbers, and mixtures thereof .

5. The method of claim 1 wherein said second thermoset elastomer is selected from the group consisting of EPDM copolymers, silicone rubbers, fluorosilicone rubbers, nitrile rubbers, urethane rubbers, butyl rubbers, and mixtures thereof.

6. The method of claim 1 wherein sa id first thermoset elastomer is a conductive elastomer comprising a polymeric matrix and conductive filler particles selected from the group consisting of noble metal-based fillers, noble metal-plated noble metals, and noble metal-plated non-noble metals, noble metal-plated glass, plastic or ceramics, noble metal-plated alumina, noble metal- plated plastic microspheres, noble metal-plated mica, non-noble metals, non-noble metal-plated non-noble metals, carbon black, and graphite.

7. The method of daim 6 wherein the shape of said conductive filler is selected from the group consisting of spherical, flake, platelet, irregular and fibrous shapes.

8. The method of claim 6 wherein the size of said partides is in the range of from about 0.25μ to about 75μ.

9. The method of claim 6 wherein the size of said particles is in the range of from about 0.25μ to about 50μ.

10. The method of claim 6 wherein the size of said partides is in the range of from about 0.25μ to about 15μ.

11. The method of daim 6 wherein the amount of conductive filler in said conductive thermoset elastomer comprises from about 10 to about 98 percent of the total volume of said conductive elastomer.

12. The method of daim 6 wherein the amotmt of conductive filler in said conductive thermoset dastomer comprises from about 60 to about 90 percent of the total volume of said conductive elastomer.

13. The method of claim 6 wherein said step of applying of first thermoset elastomer is accomplished by spraying and wherein said inside mold surfaces are heated to a temperature suffident to cure said first thermoset elastomer. 14. A gasket formed by the method of: (a) applying a first thermoset elastomer to the inside surfaces of a mold, said mold defining a mold cavity; and (b) injecting a second thermoset elastomeric composition into said mold cavity, said mold cavity heated at or near the temperature at which said second thermoset composition cures; wherein said first thermoset elastomer is cured before said second thermoset elastomer is injected into said mold cavity, and one of said dastomers is conductive.

15. The gasket of claim 14 wherein said first thermoset elastomer is, after curing, of a thickness in the range of from about 0.007 to about 0.075mm.

16. The gasket of claim 14 wherein said first thermoset elastomer is, after curing, of a thickness in the range of from about 0.02 to about 0.04mm.

17. The gasket of daim 14 wherein said first thermoset elastomer is selected from the group consisting of EPDM copolymers, silicone rubbers, fluorosilicone rubbers, nitrile rubbers, urethane rubbers, butyl rubbers, and mixtures thereof.

18. The gasket of claim 14 wherein said second thermoset elastomer is selected from the group consisting of EPDM copolymers, silicone rubbers, fluorosilicone rubbers, nitrile rubbers, urethane rubbers, butyl rubbers, and mixtures thereof.

19. The gasket of daim 14 wherein said first thermoset elastomer is a conductive elastomer comprising a polymeric matrix and conductive filler partides selected from the group consisting of noble metal-based fillers, noble metal-plated noble metals, and noble metal-plated non-noble metals, noble metal-plated glass, plastic or ceramics, noble metal-plated alumina, noble metal- plated plastic microspheres, noble metal-plated mica, non-noble metals, and non-noble metal-plated non-noble metals, carbon black, and graphite. 20. The gasket of claim 19 wherein the shape of said conductive filler is selected from the group consisting of spherical, flake, platelet, irregular and fibrous shapes.

21. The gasket of claim 19 wherein the size of said particles is in the range of from about 0.25μ to about 75μ.

22. The gasket of daim 19 wherein the size of said partides is in the range of from about 0.25μ to about 50μ.

23. The gasket of daim 19 wherein the size of said partides is in the range of from about 0.25μ to about 15μ.

24. The gasket of claim 19 wherein the amount of conductive filfer in said conductive elastomer comprises from about 10 to about 98 percent of the total volume of said conductive elastomer.

25. The gasket of daim 19 wherein the amount of conductive filler in said conductive elastomer comprises from about 60 to about 90 percent of the total volume of said conductive elastomer.

AMENDED CLAIMS

[received by the International Bureau on 4 October 1993 (04.10.93); original claims 1, 13 and 14 amended; remaining claims unchanged (4 pages)] 1. A method of making a conductive gasket comprising the steps of: (a) applying a first thermoset elastomer to the inside surfaces of a mold, said mold defining a mold cavity, wherein said inside mold surfaces are heated to a temperature sufficient to cure said first thermoset elastomer as said first thermoset elastomer contacts said inside surfaces of said mold: and (b) injecting a second thermoset elastomeric composition into said mold cavity, said mold cavity heated at or near the temperature at which said second thermoset composition cures; wherein [said first elastomer is cured before said second elastomer is injected into said mold cavity, and ]one of said elastomers is conductive.

2. The method of daim 1 wherein said first thermoset elastomer is, after curing, of a thickness in the range of from about 0.007 to about 0.075mm.

3. The method of daim 1 wherein said first thermoset elastomer is, after curing, of a thickness in the range of from about 0.02 to about 0.04mm.

4. The method of daim 1 wherein said first thermoset elastomer is selected from the group consisting of EPDM copolymers, silicone rubbers, fluorosilicone rubbers, nitrile rubbers, urethane rubbers, butyl rubbers, and mixtures thereof.

5. The method of daim 1 wherein said second thermoset elastomer is selected from the group consisting of EPDM copolymers, silicone rubbers, fluorosilicone rubbers, nitrile rubbers, urethane rubbers, butyl rubbers, and mixtures thereof. 1 6. The method of daim 1 wherein said first thermoset elastomer is a

2 conductive elastomer comprising a polymeric matrix and

3 conductive filler partides selected from the group consisting of

4 noble metal-based fillers, noble metal-plated noble metals, and

5 noble metal-plated non-noble metals, noble metal-plated glass,

6 plastic or ceramics, noble metal-plated alumina, noble metal-

7 plated plastic microspheres, noble metal-plated mica, non-noble

8 metals, non-noble metal-plated non-noble metals, carbon black,

9 and graphite.

1 7. The method of daim 6 wherein the shape of said conductive filler

2 is selected from the group consisting of spherical, flake, platelet,

3 irregular and fibrous shapes.

1 8. The method of claim 6 wherein the size of said partides is in the

2 range of from about 0.25μ to about 75μ.

1 9. The method of claim 6 wherein the size of said partides is in the

2 range of from about 0.25μ to about 50μ.

i

10. The method of daim 6 wherein the size of said partides is in the 2 range of from about 0.25μ to about 15μ.

1 11. The method of daim 6 wherein the amount of conductive filler in

2 said conductive thermoset elastomer comprises from about 10 to

3 about 98 percent of the total volume of said conductive elastomer.

1 12. The method of daim 6 wherein the amount of conductive filler in

2 said conductive thermoset elastomer comprises from about 60 to

3 about 90 percent of the total volume of said conductive elastomer.

i

13. The method of claim 6 wherein said step of applying of first

2 thermoset elastomer is accomplished by spraying[ and wherein

3 said inside mold surfaces are heated to a temperature suffident to cure said first thermoset elastomer].

14. A gasket formed by the method of: (a) applying a first thermoset elastomer to the inside surfaces of a mold, said mold defining a mold cavity, wherein said inside mold surfaces are heated to a temperature sufficient to cure said first thermoset elastomer as said first thermoset elastomer contacts said inside surfaces of said mold; and (b) injecting a second thermoset elastomeric composition into said mold cavity, said mold cavity heated at or near the temperature at which said second thermoset composition cures; wherein[ said first thermoset elastomer is cured before said second thermoset elastomer is injected into said mold cavity, and ]one of said elastomers is conductive.

15. The gasket of claim 14 wherein said first thermoset elastomer is, after curing, of a thicknes? . the range of from about 0.007 to about 0.075mm.

16. The gasket of daim 14 wherein said first thermoset elastomer is, after curing, of a thickness in the range of from about 0.02 to about 0.04mm.

19. The gasket of daim 14 wherein said first thermoset elastomer is a conductive elastomer comprising a polymeric matrix and conductive filler partides selected from the group consisting of noble metal-based fillers, noble metal-plated noble metals, and noble metal-plated non-noble metals, noble metal-plated glass, plastic or ceramics, noble metal-plated alumina, noble metal- plated plastic microspheres, noble metal-plated mica, non-noble metals, and non-noble metal-plated non-noble metals, carbon black, and graphite.

20. The gasket of daim 19 wherein the shape of said conductive filler is selected from the group consisting of spherical, flake, platelet, irregular and fibrous shapes.

21. The gasket of daim 19 wherein the size of said partides is in the range of from about 0.25μ to about 75μ.

24. The gasket of daim 19 wherein the amount of conductive filler in said conductive elastomer comprises from about 10 to about 98 percent of the total volume of said conductive elastomer.