EMI (ELECTROMAGNETIC ENERGY) gaskets such as shown in U.S. Patent 3,140,342, are now conventionally used in radar, communications equipment and guidance system applications to prevent the escape of or exclude the entry of electromagnetic energy. While gaskets of the type shown in Patent 3,140,342, in use to date, essentially prevent corrosion of enclosures in environments such as salt spray, and rain, the military and manufacturers for the military have for a long time desired an EMI gasket construction which would provide even greater assurance that in long term use corrosion of enclosures caused by exposed metal particles in the gasket coming into contact with salt water and the enclosure would be prevented.
The present invention provides a solution to the long felt need of the military and manufacturers for the military in that water is essentially prevented from coming into contact with the metal particles of the gasket.
The invention provides a form stable shielding gasket for use in preventing the escape of electromagnetic energy and preventing corrosion of exposed surfaces in contact therewith comprising a core of compressible plastic matrix binder having electrically conductive metal particles dis-persed therethrough in an amount sufficient to provide electromagnetic energy shielding and an insulator polymer shell completely encapsulating the entire core whereby no metal particles are exposed to the atmosphere absent sufficient compression of the gasket, said shell being penetratable by a sufficient number of particles in the direction of compression to cause the shell portion being compressed to become a conductor and act in the same manner as the core in providing electromagnetic energy shielding.
Brief Description of the Drawings Figure 1 illustrates in a sectional view an O
ring gasket of this invention in place to provide EMI shielding;
Figure 2 is a top plain view of an O ring gasket of this invention;
Figure 3 is a sectional view taken along lines 3-3 in Figure 2;
Figure 4 is the gasket of Figure 1-3 shown compressed;
Figure 5 is an enlarged view of a portion of the gasket in section showing the constructive particles penetrating the shell andmaking electrical contact with the flange;
Figure 6 is a plan view of a washer gasket o~ this invention;
Figure 7 is a sectional view taken along line 707; and Figure 8 is a plan view of a rectangular gaske~
of this invention.
Thus with this invention greater assurance of corrosion resis-tance is now possible. In addition to the foregoing, the present invention provides a gasket that can be constructed to withstand the deleterious effects caused by gasoline or other solvents which attach the polymer bind-er matrix portion of the gasket.
Detailed Description of the Invention This invention is directed to an EMI shielding gasket having a core of plastic matrix binder with electrically conductive metal particles homogeneously dispersed therethrough in an amount sufficient to provide EMI
shielding, and an outer electrically insulator shell of the thickness of a film encapsulating the core, said shell being penetrable by said particles, and extending to the outer surface thereof when the gasket is squeezed be-tween two surfaces e.g., the metal flanges of a waveguide to make electri-cal contact with said flanges.
The remainder of the shell which is not being compressed seals out water and other materials e.g., oils, gasoline, etc., that may deleter-iously effect the binder or metal particles.
Thus with this invention water, salt or other materials are essen-tially kept away from the metal in the gasket and corrosion is thus pre-vented. In addition, by suitably selecting the polymer used for forming the shell, materials such as gasoline and other solvents which may attack the plastic matrix binder or the particles themselves and are yet essentially sealed out.
With this invention the shell conforms to the shape of the gasket and may be formed by dipping the core one or more times in a liquid solu-tion containing the shell forming material and a solvent therefore. There-after the solvent is permitted to evaporate at room temperature.
5~1 When compressed, the particles penetrate the shell to provide a contact surface and yet are still surrounded in the hori~ontal plane by the shell. In this manner the particles are further protected in this invention.
As used herein "metal particles" include the noble metals such as silver, gold, as well as nickel, copper, silver coated copper, and other metals well known in the art, such as disclosed in U.S. Patent 3,140,342, 3,202,488, and 3,194,860. As used herein the phrase "metal particles" are also intended to include particles such as glass, silica, polymer or other non-conductive materials having an outer layer or coating of metal e.g., silver, gold or other noble metals or coated with an outer layer of non-noble metal such as nickel, copper, etc., or particles of conductive polymer (i.e., polymer having electrically conductive metal e.g., such as particles dispersed therethrough) all of which, for the purpose of this invention are also referred to as metal particles. The art sometimes refers to electrically conductive metal particles as conductive powder and it is intended that particles used herein include conductive powders.
In general the core of this invention will contain about 10 to 80 volume per cent of electrically conductive particles although the amount of particles will depend upon the particular application and the amount of other fillers e.g., foam rubber, silica, etc., as well known in the art (see U.S.
Patents 3,140,342 and 3,583,930).
Preferably the loading of particles for a good general purpose EMI
gasket will be this amount, about 25 to 40% of silver coated copper being most preferred.
Since this invention does not revolve around the precise volume percent of the particles present and is a matter of choice depending upon the circum-stances, it is intended that this invention can cover all EMI gaskets as long as they contain sufficient electrically conductive metal particles to make the binder in which they are homogeneously dispersed sufficiently elec-trically conductive to provide suitable EMI shielding.
In general to provide good EMI shielding, i.e., the central binder core with particles should have a volume resistivity of less than 10 ohm centimeters, and preferably less than 1 ohm centimeter.
In this invention the particles are irregularly shaped, or are of other shapes, such as flakes or plates and generally spherically shaped par-ticles. Suitable average maximum dimensional sizes for the particles may be between 100 mils to 0.05 mils as set forth in United States Patent 3,140,342, and most preferably have an average dimension of 1 to 3 mils.
The insulator film forming the shell of this invention is prefer-ably of a thickness of about .1 to 2 mil with .2 to .5 mils being most pre-ferred. The films forming the shell of this invention may preferably com-prise insulator plastics of a combination of plastic such as silicone poly-mers, e.g., silicone rubber, fluorosilicone rubber, urethane polymers, vinyl polymers, e.g., vinyl chloride polymers, acrylate polymers, ethylene poly-mers, propylene polymers, neoprene, ethyl propylene rubber and a host of others, sufficiently soft in comparison with the hardness of the particles to be penetrable thereby.
Other suitable plastics may be selected from those set forth in the book entitled Plastics In The Modern World authored by E. G. Couyens and V. E. Yarsley, published by Penguin Books9 Inc. Baltimore, Maryland and copyright 1968 by the authors. In particular, certain polymers such as the thermo-softening (thermoplastic) plastics disclosed on pages 76 to 95 may be used or thermohardening ~thermosetting) plastics such QS the polyurethanes and epoxides may also conveniently be used.
In addition, because of their high temperature properties sili-cone polymers are most preferred for military applications and flurosilicone polymers are most preferred for use in environments in which gasoline or other solvents are liable to be present.
The matrix binder used in the gasket of this invention may be any of those conventionally used in the art, e.g., as shown in Patent No.
3,140,342. The plastic binder is preferably resilient and compressible and is most preferably an elastomer.
The matrix binder is also preferably of a hardness greater than the shell so that when the gasket is compressed the particles will penetrate through the thin shell.
The matrix binders which may be used are well known in the art, see United States Patent 3,140,342 with silicone polymers, e.g., rubber being the most preferred for its high temperature and elastomeric properties. How-ever, this invention does not preclude the use of thermoplastic polymers such as polyvinylchloride, polyethylene, polypropylene, polyamides or thermo-hardening plastics.
Accordingly, the term "plastic" as used herein is intended to in-clude resins and elastomers (rubber) besides the conventionally accepted plastics such as polyethylene and the epoxides. The plastic matrix and the shell of this invention can thus be thermosetting or thermoplastic, depend-ing upon the use to which the gasket is to be put, e.g., asphalts, poly-urethane, polyesters, acrylates, polyamides and natural rubber.
The term "matrix binder" means something holding or capable of holding embedded within another object to which it gives shape or form.
The gasket herein has a stable form or shape (termed form stable~
i.e., it substantially retains its shape until compressed when in use unlike an adhesive that is spreadable when taken out of a can or tube.
Reference should now be had to Figures 1 to 5 which illustrate an 0 ring gasket at 20 in place in a waveguide joint consisting of wave guide 16~i~5~
sections 30 and 31 ending in two flanges 30-1 and 31-1 held together in a manner to compress the gasket as shown by closure bolts 32.
Figure 3 illustrates the gasket section showing the core of binder and metal particles 20-1 having an outer shell 20-2. Figures 1 and 4 illus-trate the shape of this gasket after squeezing with the portions 20-2a thereof serving as the electrical contacts to these flanges.
Figure 5 illustrates a view of the 20-2a portion showing particles 22 in both the core binder 20-1 and penetrating into the shell portion 20-2a to electrically contact flange 31-1 when the gasket is compressed. Figures 6, 7 and 8 show other gasket configurations and Figure 7 shows in section the central core 35 surrounded by ~he non-conductive shell 36.
The gaskets when uncompressed exhibit a resistance greater than 1000000 ohms between opposite points, e.g., 37 and 38 in Figure 2. When the gasket is compressed as in Figure 4 points 39 and 40 would be less than 10 ohms and in most cases less than .3 ohms if the ID of the 0 ring was 2"
and the diameter of the 0 ring section was 125 mils.
The core, e.g., 20-1 of this invention has a volume resistivity less than 10 ohms centimeters and preferably less than 1 ohm centimeter.
The following examples illustrate the invention.
EXAMPLE l A molded electrically conductive core in the shape of an "0" ring with a 1/8" diameter cross sectional rim and a 2" internal diameter was made using 80% by weight of silver coated copper powder prepared as disclosed in Example 1 of United States Patent No. 3,140,342 and 20% by weight of Dow Corning 440 silicone rubber. The material was molded as is taught in United States Patent No. 3,140,342 (Figure 2) and cured to form an "0" ring. A
shell forming mixture comprising 100 grams Dow Corning 9605 (1 part RTV*
silicone rubber) and 75 grams toluene was prepared. The molded "0" ring was washed with toluene and dried at room temperature (68 - 72F). The washed "0" ring core was then immersed in the shell forming mixture for 15 seconds *room temperature vulcanizing _ 7 -and then removed and left suspended so that an excessive coating would flow to the lowest point of the gasket. After coating the flow stopped and a smooth and even coating remained on the gasket surface. The shell formed on the core was permitted to cure for 16 hours at room temperature.
The conductive core was prepared as in Example I and then washed as in Example I. A shell forming mixture was prepared from Dow Corning A
4000 (Part A and B) silicone rubber as follows:
Part A 100 grams Part B 4.5 grams toluene solvent 90 grams Thereafter the core was immersed in the mixture for 15 seconds and then sus-pended to dry as in Example I.
The conductive core was prepared as in Example I and then washed as in Example I. A shell forming mixture was prepared from 100 grams of flurosilcone rubber RTV Dow Corning 94034 and 160 grams of methylisobutyl ketone (MIBK). Thereafter the core was immersed in the mixture for 15 sec-onds and then suspended to dry as in Example I.
No. 1250 extruded 0 ring strips, round strips (15 feet with 62 mil diameter) sold by Chomerics, Inc., Woburn, Massachusetts, was singly coated with the shell forming mixture of Example I as taught therein after being washed with toluene. No. 1250 O ring is silicone rubber filled with silver powder.
No. 1250 stock as in Example IV was double coated with the shell forming mixtures of Examples II and III by immersing and then drying and then repeating the process of immersing and drying a second time.
EXAMPLE VI - VII
A CHO-SEAL (R) rectangular gas~et #1215 silicone rubber and silver coated copper sold by Chomerics, Inc. was single coated with the shell form-ing mixtures of Examples I, II and III as set forth therein.
While particular embodiments of the present invention have been described~ they are intended to be exemplary only, with the true scope and spirit of the invention being indicated in the following claims.