Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
1999-02-12
2003-03-18
Gupta, Yogendra N. (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C252S500000, C252S510000, C252S506000, C252S512000, C252S513000, C252S514000, C252S518100, C524S449000, C428S208000
Reexamination Certificate
active
06533963
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to electronically conductive compositions which are flexible, materials for producing same which are readily formable, and methods for making same. The invention also relates to articles of manufacture formed of electrically conductive compositions, e.g., shaped articles and injection-molded articles, such as gaskets.
Several publications are cited to describe more fully the state of the art to which the invention pertains and are incorporated herein by reference.
BACKGROUND OF THE INVENTION
With continued advances in industrial technology and the general standard of living, the uses of electromagnetic energy have increased, and the number of sources for generating such energy has concomitantly multiplied. The leakage of electromagnetic radiation into the environment has become a significant problem.
For instance, technology requiring the channeling of electromagnetic energy in a pre-determined well-defined path, e.g., in wave-guide applications, is increasingly an important component of modern communications and other technologies. These wave-guide applications require quantitative confinement of electromagnetic radiation for the effective implementation of the selected technology. Leakage of the electromagnetic radiation can substantially interfere with achievement of the desired results.
Further, leakage of electromagnetic radiation is intrinsically damaging to life forms and other equipment occupying the same environment. Beyond having adverse influences on the human body, leaked radiation can also cause erratic behavior of and/or damage to integrated circuits and electronic equipment. For instance, electromagnetic waves radiated from electronic computers in various office equipment can interfere with the performance of television sets and audio equipment.
It has long been known that various electroconductive materials can be used to absorb or reflect electromagnetic waves, and thus to contain electromagnetic radiation or shield other systems or equipment from it. Examples are metals which can absorb or reflect electromagnetic waves, and plastics with such metals deposited thereon.
However, compositions which incorporate metallic conductive fillers in amounts sufficiently large to insure good electroconductivity can be subject to other difficulties. Both cost and specific gravity tend to be higher than desirable. The amount of filler needed to secure electroconductivity can also impinge upon flexibility of the compositions and the formability of precursor materials from which the compositions are made. (e.g., flowability such as moldability of a paste from which the composition is formed). It is known to replace metallic particles by particles comprising a non-metallic core or substrate coated by an electroconductive material. While this expedient can drive down costs and specific gravity, it is not trouble-free. This is because of the persistence of a fundamentally problematic dichotomy which has afflicted the introduction of conductive fillers into flexible matrix materials, namely: when the conductive filler is added in smaller amounts to preserve flexibility and formability, the conductive effect is undesirably diminished; however, when the filler is added in larger amounts for high conductivity, flexibility and formability (e.g., mechanical strength, moldability) are compromised.
By way of example, attempts at rendering polymeric materials electrically conductive have met with less than uniform success. Molding polymers in which conductive fibers are incorporated has been tried, but in order to achieve acceptable electrical conductivity so much fiber must be added that there is a marked decrease in certain other desirable polymeric properties (such as those pertaining to flexibility and/or formability). Silver or other metal flakes and metal-coated glass spheres have also been added to polymers but, again, very high loading levels are needed to achieve electrical conductivity, which becomes cost prohibitive for most applications and can interfere with the desired attainment of all properties.
More specific examples of the foregoing are as follows:
U.S. Pat. No. 3,140,342 describes a method used in making conductive plastic articles having radio frequency shielding capabilities. Metallic particles are mixed with the uncured phase of a compressible resin and the mass is then cured. Particle-to-particle contact provides numerous conductive paths through the cured article, with a resulting high conductivity. However, the cost of the conductive plastic is rather high because of the high concentration of metal therein, particularly when an expensive metal such as silver is used. Moreover, with a high metallic concentration, many of the desirable physical properties of the plastic are greatly attenuated. Thus, the finished article may not have as much tensile strength as desired, and its compressibility is significantly diminished by the large number of interconnecting metal particles.
U.S. Pat. No. 3,194,860 describes a method of preparing metal-filled conductive plastic gaskets wherein the flat gasket is die-cut from a sheet of plastic elastomer loaded with a conductive metal powder. The powder particles can be solid noble metal particles or non-noble metal particles such as iron or copper coated with noble metal coating such as silver or gold. Although using non-noble metal gaskets coated with silver or gold provides a less costly conductive plastic, high loadings which are frequently required to render the plastic conductive diminish other desirable physical properties of the plastic such as compressibility and tensile strength.
U.S. Pat. No. 4,500,447; U.S. Pat. No. 4,557,859; U.S. Pat. No. 4,642,202; U.S. Pat. No. 4,765,930; U.S. Pat. No. 4,822,089 and U.S. Pat. No. 5,430,085 describe electrically conductive compositions in which carbon is the electroconductive material. The carbon cited in these references can be carbon black conventionally used in electrically conductive silicon rubbers or as part of a system which includes other metals. The metals can be Al, Zn, Fe, Ni, Sn, Pb and Ag as spheres, platelets or whiskers. The carbon can be used as fibers or filaments, uncoated or coated with metal by electrodeposition. However, a highly electroconductive composition having a low volume resistivity is not readily achieved with such techniques. However, when loading polymers with carbon fibers in order to achieve acceptable electrical conductivity, so much carbon fiber must be added that there is a marked decrease in other desirable qualities such as polymer flexibility and precursor formability. Moreover, because of the large difference in resistivity between carbon and metals such as silver, polymers loaded with carbon fibers cannot provide the resistivity required for EMI shielding. Similarly, when carbon fibers coated with electrically conductive metal are used the resulting electrical conductivity is not uniform and continuous. This is caused in part by poor adhesion of the metal to the carbon fibers.
U.S. Pat. No. 5,214,091 describes silver coated glass beads, which are an example of conductive inorganic fillers modified for the purpose of simultaneously achieving acceptable conductivity at lower cost than solid noble metal particles and also avoiding the disadvantages associated with the use of carbon. However, in order to provide the desired conductivity, it is necessary to add an overly large quantity of conductive inorganic filler.
U.S. Pat. No. 5,672,297 describes the use of gel particles in a swollen state to form expandable and contractible matrices. The conductive composite articles made from these matrices can be used as “on-off” electrical switches. The matrix disclosed in this reference contains conductive filler particles of regular or irregular shape, such as bead, fiber or flake forms. A combination of two or more conductive filler particles described. Examples of filler particles include copper powder, silver coated nickel flakes, silver coated glass bubbles, solid glass beads, mica flakes and carbon powde
Abuelhawa Mohammad S.
Schleifstein Robert A.
Zimmerman Bruce C.
Gupta Yogendra N.
Hamlin D. G.
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