Electricity: conductors and insulators – Anti-inductive structures – Conductor transposition
Reexamination Certificate
2001-04-26
2003-11-11
Ngo, Hung V. (Department: 2831)
Electricity: conductors and insulators
Anti-inductive structures
Conductor transposition
C174S034000, C277S920000
Reexamination Certificate
active
06646199
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electrically conductive, flexible, low-density polyurethane foams useful for conductive gaskets and seals.
BACKGROUND OF THE INVENTION
There is a need for gaskets, seals, and other products that can seal enclosure cabinets and other electrical and electronic housings to prevent the transmission of electromagnetic & radio frequency noise into and out of the enclosed areas they are sealing. They must also provide protection against moisture and particulates such as dust. Such gaskets are placed in covers and between frames, panels, and doors of electronic equipment, cabinets, and housings. These gaskets must be soft with low compression deflection values. Lower compression deflection values generate lower closing forces. This will allow the enclosures and housings to be made from thinner, less rigid materials. They must be capable of being compressed at least 50% for long periods of time without taking a compression set. They must also maintain their conductive and compression recovery properties after many compression-relaxation cycles.
A number of products have been developed to address the need for conductive gaskets. One method uses a two-layer approach. The outer layer contains a conductive metal. The inner core or layer is intended to provide the desirable properties that an effective gasket should have. Such a product is described in co-pending U.S. patent application Ser. No. 09/421,559, which is hereby incorporated by reference in its entirety.
Another product is a high frequency EMI/RFI shielding gasket made by wrapping a strip of knit mesh material or wire mesh around the exterior of a resilient core. Such a mesh-covered core is described in U.S. Pat. No. 4,652,695. The core can be made from any highly compressible material but is usually a flexible, non-conductive polyurethane or polyethylene foam. In U.S. Pat. No. 4,857,668, a fabric-like sheath wrap is used. This wrap is tough and imparts good cut and abrasion resistance. The method of applying the wrap to the core is very efficient and less expensive than other available technology. Good shielding values are obtained; however, the wrap is stiff, causing high compression deflection values. The stiff wrap also makes it very difficult to bend the gasket and thus the gasket is usually applied in straight sections whereby pieces are butted up against each other to form a continuous gasket. It is also very difficult to make a waterproof seal since leaks may form wherever the gasket sections are joined. If a waterproof seal is required, a second, conventional gasket must be used. Moreover, even though the polyurethane foam may have good compression recovery, the wrap has poor memory. This results in a gasket with poor compression recovery. Furthermore, the wrap usually contains large quantities of nickel or silver. This makes the wrap very expensive. In addition, these gaskets cannot be formed-in-place but must be prefabricated. Installation of these gaskets is very labor intensive causing the installation costs and therefore the final gasket costs to be high.
Another method of manufacturing conductive gaskets is to encapsulate conductive fillers inside a plastic matrix. Flexible elastomers, such as silicone and neoprene, are commonly used. U.S. Pat. No. 4,011,360 is an example of many such patents in this area. Flexible elastomers are particularly desirable for gasket materials because of their good performance characteristics and ease of manufacture. Such elastomers have low water absorption and good resistance to cutting. However, due to their high concentrations of conductive metals, the cost of these gaskets is high, particularly when a conductive metal such as silver is used. Most conductive fillers are hard and because these fillers must be used in high concentrations, the elastomers tend to become hard, stiff, and brittle compared to elastomers that do not contain conductive fillers. Currently available conductive elastomeric gaskets are much harder than non-conductive gaskets and it is very difficult, if not impossible, to make them flexible. These gaskets also have very high compression deflection values and poor compression recovery.
Attempts to produce conductive foams have had limited success. U.S. Pat. No. 4,378,322 describes impregnating a prefabricated foam with conductive materials. U.S. Pat. No. 4,931,479 describes producing a high density, high hardness, conductive polyurethane foam for gap filling applications. However, neither of these approaches is suitable for (foam-in-place) gasketing applications.
There is a need for foam in place gaskets without the disadvantages associated with the prior art.
SUMMARY OF THE INVENTION
The invention is directed to electrically conductive flexible polyurethane foams that are suitable for use as gaskets and seals. The gaskets and seals can be used between two conductive surfaces to provide EMI/RFI shielding. The gaskets and seals can be foamed-in-place, prefabricated, or molded, at both room temperature and elevated temperatures. The foams are soft, flexible, and have low compression deflection. Moreover, the foams have good compression recovery, are cut resistant, and have low water absorption.
The invention is directed to an electrically conductive flexible polyurethane foam comprising a polyurethane and at least one conductive filler dispersed therein in an amount effective to provide EMI/RFI shielding, wherein the polyurethane comprises an isocyanate component to active hydrogen component in a ratio of at most about 0.20:1, and the foam has compression deflection value at 50% compression of at most about 25 psi.
The invention is further directed to an electrically conductive flexible polyurethane foam comprising a polyurethane and at least one conductive filler dispersed therein in an amount effective to provide EMI/RFI shielding, wherein the polyurethane comprises an isocyanate component to active hydrogen component in a ratio of at most about 0.20:1, and the foam density is at most about 0.95 g/cm
3
.
The invention is further directed to an electrically conductive flexible polyurethane foam comprising a polyurethane and at least one conductive filler dispersed therein in an amount effective to provide EMI/RFI shielding, wherein the polyurethane comprises an isocyanate component to active hydrogen component in a ratio of at most about 0.20:1, and wherein the foam has a noise shielding effectiveness of the foam at a frequency of about 20 to about 1000 MHz of at least about 10 dB.
The invention is further directed to a gasket or seal comprising an electrically conductive flexible polyurethane foam comprising a polyurethane and at least one conductive filler dispersed therein in an amount effective to provide EMI/RFI shielding, wherein the polyurethane comprises an isocyanate component to active hydrogen component in a ratio of at most about 0.20:1, wherein the foam has a density of at most about 3 g/cm
3
, wherein the foam has a noise shielding effectiveness at about 20 to about 1000 MHz of at least about 10 dB and a compression deflection value at 50% compression of less than about 25 psi, and wherein the conductive filler concentration is about 140 phr to about 900 phr.
DETAILED DESCRIPTION OF THE INVENTION
The invention is directed to flexible, conductive polyurethane foam gaskets and seals that are suitable for EMI/RFI shielding. The foams are easy to apply, whether prefabricated or foamed in place, and cured at room temperature and/or at elevated temperatures. The foams are soft, flexible, and have low compression deflection values. They are resilient, have a low compression set, have low water absorption, and have good cut resistance. They also retain their conductivity after many compression-relaxation cycles.
The chemistry of polyurethane foams is well known by those skilled in the art. For years, excellent non-conductive flexible foam gaskets have been made from polyurethane resins. They are primarily thermosetting systems. The foams are mechanically or chemically blown or a combination of both. They are
Chemque, Inc.
Ngo Hung V.
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