US 3446906 A
Description (OCR text may contain errors)
May 27, 1969 D. ZULAUF 3,446,906
RESILIENT CONDUCTIVE COATED FOAM MEMBER AND ELECTROMAGNETIC SHIELD EMPLOYING SAME Filed May 17, 1967 FIG. I FIG. 2
DIETER ZULAUF INVENTOR BY BUCKHO/PN, BLORE, KLAROU/ST 8 SPAR/(MN ATTORNEYS United States Patent US. Cl. 17435 8 Claims ABSTRACT OF THE DISCLOSURE The subject matter of the present invention relates generally to a porous, resilient, electrically conductive member formed of a body of open-celled plastic foam having a coating of electrically conductive material provided throughout such foam body on the surface of the foam cells. This resilient, conductive foam member is employed as an electromagnetic shield to prevent electrical signal radiation from passing between two metal plates between which such conductive member is compressed. A number of different methods are employed to form the present resilient, conductive member including vacuurn vapor deposition, spraying, dipping or electroplating. A polyurethane plastic foam having a plurality of interconnected open cells is employed to provide a foam body which is suificiently porous to enable fluid to pass through such body so that all of the surfaces of the cells throughout the body are coated with nickel or other metal.
Background 0 the invention The resilient electrically conductive coated foam member of the present invention is especially useful as an electromagnetic shield to prevent electrical signal radiation from passing therethrough. The electromagnetic shield may be provided by a gasket of such conductive coated foam compressed between two metal members. In addition, the resilient conductive foam member can be mounted over an opening in the housing of an electronic instrument, such as a cathode ray oscilloscope, through which air is circulated by a fan mounted within the housing for cooling purposes, in order to employ such foam member as a dust filter and as an electromagnetic shield to prevent external radio frequency interference from entering the housing through such opening. The resilient conductive foam member can be used merely as an electrical connector between two metal plates between which it is compressed. It can also be employed in an electrostatic precipitator as an air filter to which an electrical potential is applied to remove dust and other charged particles from the air. The conductive foam electromagnetic shield can also be employed in many other different places on an oscilloscope such as around the knob openings on the front panel through which the knob shafts extend, and over the fan opening at the rear of the instrument.
Previously, plastic foam material has been employed in electromagnetic shields with a conducting layer or plate provided only on the outside of such shield, as shown in US. Patent 2,870,439 of H. E. Stinehel-fer and US. Patent 3,147,336 of H. G. Mathews. These prior art shields have the disadvantage that they are not resilient and are not sufficiently conductive throughout to enable them to be employed as gaskets or other members which are compressed between two metal plates and still provide proper shielding. These disadvantages are overcome by the resilient, conductive coated foam member of the present invention which is sufliciently porous to enable fluid to be transmitted therethrough so that the conductive coating is provided on the surface of all of the foam cells throughout the member.
Summary of the invention It is therefore one object of the present invention to provide a resilient, electrically conductive coated plastic foam member which is suificiently porous to enable the passage of fluid therethrough.
Another object of the invention is to provide an electromagnetic shield of electrically conductive coated plastic foam material which is porous, resilient and conductive throughout which is capable of high attenuation of electrical signal radiation.
A further object is to provide an improved gasket of resilient electrically conductive plastic foam.
An additional object of the present invention is to provide a porous, electrically conductive member which includes a body of plastic foam having .a plurality of interconnected open cells which provide a sufliciently porous structure to enable fluid to pass through such body, and a coating of conductive material on the surface of the cells of the foam throughout the body to provide an inexpensive, porous, lightweight member of high conductivity.
Brief description of drawings Additional objects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments thereof, and from the attached drawings, in which:
FIG. 1 is an elevation view of an electromagnetic shield gasket made of a porous, resilient, conductive coated plastic foam material in accordance with the present invention;
FIG. 2 is a vertical section view taken along the line 2-2 of FIG. 1;
FIG. 3 is an enlarged view of a portion of the conductive coated plastic foam member of the present invention;
FIG. 4 is an enlarged section view taken along the line 44 of FIG. 3 of one element of the conductive coated plastic foam;
FIG. 5 is a section view showing the use of the gasket of FIG. 1 as a shield surrounding an opening in a metal housing and compressed between a closure plate and such housing to prevent electrical signal radiation from entering the housing through such opening; and
FIG. 6 is a section view showing the present conductive coated foam member employed as an electromagnetic shield and an air filter over the opening in a metal housing through which air is circulated by a fan within such housing.
Detailed description of preferred embodiments As shown in FIGS. 1 and 2 the resilient, conductive coated plastic foam member of the present invention can be made in the form of a gasket 10* having a plurality of bolt openings 12 provided through such gasket at positions placed around its periphery and having a large central opening 14. As shown in FIG. 2 the gasket 10 is a thin, flexible sheet of plastic foam material. The plastic foam is an open-celled, resilient polyurethane, which is coated throughout with a conducting material, such as gold, silver, aluminum or nickel in any suitable manner, such as by vacuum vapor deposition, liquid spraying or dipping, or electroplating.
As shown in FIGS. 3 and 4, the plastic foam has a plurality of interconnected open cells 16 in order to provide a body which is suificiently porous to enable the transmission of fluid, including liquid or gas, therethrough. This is necessary for coating all of the surfaces of the foam cells through the body with conductive material. As shown in FIG. 4, each of the plastic elements 18 of the foam cells is provided with a coating 20 of conducting material on the surface thereof. It should be noted that while a single-layer conductive coating 20 is shown, multilayer coatings can also be employed as is necessary with electroplating since the first layer must be provided by dipping in order to provide a conductive surface on the insulating foam which forms the cathode on which the electroplating takes place. For example, the coating 20 may be formed by an inner layer of copper provided by dipping and an outer layer of nickel electroplated onto such copper layer.
While the size of the open cells or pores in the plastic foam may vary between approximately 30 to 90 pores per inch, it has been found that a polyurethane foam having 45 pores per inch enables sufficient drainage to remove excess copper dip solution while retaining enough conductive material to provide a uniform conductivity throughout the foam body. It should be noted that one of the problems with spraying a metal coating is that the metal sometimes oxidizes, providing an insulating rather than a conductive coating. Therefore, the spraying may have to be carried out in an inert atmosphere. Also some metal coatings may be too brittle to provide the necessary resiliency or too fragile so they are reduced to powder by compression of the plastic foam during use.
Vacuum vapor deposition can be employed to provide a conducting coating of gold, silver, copper or aluminum by providing the metal in a layer on a tungsten heater filament next to which is placed the plastic foam in a vacuum chamber which may be evacuated to approximately 2x10 torr. The tungsten filament is then heated to the evaporation temperature of the metal to cause the metal vapor to coat the plastic foam. With thick pieces of foam it may be necessary to vacuum evaporate from both sides of the sheet from several different places in order to cause the metal vapor to penetrate the foam sufficiently to provide a substantially uniform conductive characteristic throughout the foam. Therefore, for sheets of large area or thickness, it is preferable to use an electroplating technique mentioned above.
A sulfamate nickel electroplating technique which has been employed to coat a sheet of polyurethane foam approximately 1.25 square feet in area is hereafter described. First, the polyurethane foam sheet is cleaned in a neutral detergent and rinsed with cold water. Then the foam sheet is dipped in a sulfuric acid solution and rinsed in cold water. After this it is then dipped in a solution of hydrochloric acid. Next the foam sheet is dipped in a catalyst solution of stannous chloride, hydrochloric acid and palladium chloride. The stannous chloride reacts with the palladium chloride to form stannic chloride and palladium which is deposited as a thin film of palladium on the surface of the cells in the polyurethane foam. This palladium film is necessary to enable the intermediate layer of copper to adhere to the polyurethane in a manner hereafter described. The polyurethane foam sheet is then rinsed in cold water and dipped in a bath of conventional accelerator material which increases the speed with which the copper deposits out of electroless copper olution onto the surface of the plastic foam. After rinsing in cold water, the plastic foam sheet is next dipped into the electroless copper solution which includes copper sulfate, and remains in such solution for approximately five minutes to provide a copper coating of approximately 90 microinches thick. During this time the palladium replaces the copper in the copper sulfate to form palladium sulfate and a layer of copper on the plastic foam because palladium is lower in the electromotive series than copper.
After this copper coating, the coated foam sheet is rinsed in cold water and dried by blasts of air emitted from an air gun at approximately 100 lbs. per square inch pressure which removes any excess copper solution re maining in the pores of the foam sheet after drainage. The copper coated foam sheet is then mounted in a frame 4 or rack and dipped into a solution of sulfuric acid and rinsed with cold water. Next the copper coated foam sheet is placed in a bath of sulfamate nickel plating solution and electroplated with nickel over the copper layer which serves as the cathode. The plating operation takes place for approximately one minute at a current density of 40 amperes per square foot or a total amperage of 50' amps in order to produce a layer of nickel approximately 30 microinches thick on the surface of the copper layer. After plating, the nickel coated foam member is then rinsed in cold: water and air-dried in an oven at Fahrenheit. Finally, the nickel coated foam member is removed from the support frame or rack and cut to the desired configuration such as the gasket of FIG. 1.
As shown in FIG. 5, the porous, resilient, conductive coated foam gasket 10 of FIG. 1 can be employed as an electromagnetic shield around an opening 22 in an aluminum housing 24 of a cathode ray oscilloscope or other instrument. A plurality of threaded studs or bolts 26 are provided on the housing 24 to enable a cover plate 28 of aluminum to be fastened to the housing by nuts 30 threaded onto studs 26 extending through apertures 12 in the gasket 10 and similar apertures in the cover plate. As a result of the tightening of the nuts 30, the gasket 10 of the present invention is compressed between the cover plate 28 and the housing 24 to effectively fill the space between such cover plate and housing which is grounded. As a result the gasket 10 provides an electromagnetic shield which prevents electrical signal radiation from entering the housing through opening 22 by way of the space previously existing between the cover plate and housing.
While the gasket embodiment of FIG. 5 is suitable when the opening 22 is merely used as an access opening for repair of the instrument, it may also be necessary to provide the conductive foam member of the present invention as an integral sheet 32 extending across the opening 22 when it is desirable to have air circulate through such opening. Thus a fan 34 may be provided within the instrument for cooling purposes, as shown in FIG. 6. In this case, the cover plate 28 is replaced by a metal frame member 36 surrounding only the periphery of the opening 22. The frame 36 is fastened to the housing 24 by nuts 30 threaded on to the bolts 26 which extend through openings in the frame 36 and the conductive plastic foam sheet 32 in order to compress such sheet only in the area around its outer edge. Since air must pass through the conductive foam sheet 32 in the region over the opening 22, it acts as a filter to remove dust while at the same time functioning as an electromagnetic shield to prevent electrical signal radiation from entering the housing 24. The shield 32 is grounded to the housing 24, which may be the housing of a cathode ray oscilloscope containing electrical circuits which are sensitive to stray electrical fields.
An electromagnetic shield made in accordance with the above described nickel plating method is especially useful between two aluminum plates, because it does not chemically react with aluminum. One such shield was operated successfully as an electromagnetic shield against electrical signal radiation having a frequency of 2000 megacycles per second which it attenuated by 28.8 decibels. Thus, the electrical signal radiation transmitted through the shield was 28.8 decibels less than the radiation transmitted between the test panels when no shield was employed.
It will be obvious to those having ordinary skill in the art that many changes may be made in the details of the above described preferred embodiment of the present invention without departing from the spirit of the invention. For example, other plastic foam materials can be employed than polyurethane and other conductive coating materials can be used than nickel.
1. A porous, resilient, electrically conductive member comprising:
a body of resilient plastic foam material having a plurality of interconnected open cells and being sulficiently porous to allow fluid to pass through said body; and
a coating of electrical conductive material provided throughout said body on the surfaces of the plastic elements of said body which form the cells of the foam material while maintaining the cells open so that said conductive material is positioned inside the body as Well as on its outer surface.
2. A member in accordance with claim 1 in which the plastic foam material is polyurethane.
3. A member in accordance with claim 2 in which the conductive material is a metal taken from the group consisting of gold, silver, copper, aluminum and nickel.
4. A member in accordance with claim 1 in which the body is a thin, flexible sheet of plastic foam having approximately 45 pores per linear inch.
5. An electromagnetic shield including a resilient conductive member in accordance with claim 1 and means for grounding said member to attenuate electrical signal radiation tending to be transmitted through said member.
6. A shield in accordance with claim 5 in which the resilient conductive member is compressed between two metal plates to attenuate electrical signal radiation tending to be transmitted through the space between said plates.
7. A shield in accordance with claim 5 in which the resilient conductive member is supported over a hole extending through a metal plate to prevent electrical signal radiation from passing through such hole.
8. An electromagnetic shield including the resilient conductive member of claim 1 formed as as a gasket having centrally located opening therethrough much larger than the pores of the plastic foam.
References Cited UNITED STATES PATENTS 2,674,644 4/ 1954 Goodloe.