Articles Having Electroconductive Components of Highly Electroconductive Resinous Compositions

The invention relates to highly conductive resinous compositions containing a resinous material and a finely divided conductive solid and to electroconductive articles employing such compositions.

Conductive resinous compositions, i.e., compositions formulated from a resinous binder material which has been filled with particulates of conductive materials such as carbon black, metals such as copper and silver, and the like have been widely used as electrical cable jacketings, electrical resistors, in heating elements and printed circuits as electrodes for capacitors, as conductive adhesives and so forth. See, for example, U.S. Pat. Nos. 3,412,358, 3,056,750, 3,359,145, and 2,165,738 and 3,185,907.

Unfortunately, in order to achieve even moderate degrees of conductivity, i.e., resistances less than one ohm-cm, required in many applications, it has been necessary to incorporate as much as 75 weight percent of the conductive particulate based on the resinous binder. At such levels of the conductive filler, the ease of fabrication and the overall strength of the conductive composition are often reduced to the point that they are either not acceptable or are marginally so for the intended use. Some high degrees of conductivity, i.e., less than 0.1 ohm-cm, cannot be practically achieved by conventional incorporation of a conductive particulate into a resinous binder.

Therefore, it would be highly desirable to provide a resinous composition having moderate or high degrees of conductivity at levels of conductive particulate much lower than those required in prior art conductive compositions.

The present invention is, in one aspect, a resinous composition comprising non-conductive or semi-conductive resinous material as the matrix of the composition having dispersed therein a finely divided, copper metal-containing solid and a salt as specified hereinafter, said salt and copper metal-containing solid being present in amounts sufficient to render the composition electroconductive. In another aspect, the invention is an article containing the resinous composition as an electroconductive component. An exemplary article is a capacitor having as essential components at least two electrodes comprising the resinous composition insulated from each other by a dielectric material and electrical leads connecting to the electrodes. Also included are articles such as electrical storage batteries; inductors for electrical motors, transformers and coils; printed circuit boards for electrical systems such as radio, television, telephone and teletype systems, and computers; conductive or resistive coatings or encapsulated layers for electrical blankets, heating pads, heated walls and floors, electrical precipitators and faraday cages; electrical wiring such as television lead-in wiring, communication cables, and resistive wiring for spark plugs; molded terminal sockets for light fixtures, electrical outlets, fuse panels and integrated circuit sockets; terminals with fuse properties; and antistatic films and fibers wherein the resinous composition of the present invention serves as the electroconductive component or components.

FIG. 1 is a cross-sectional view of a capacitor having at least two electrodes 1 comprising the electroconductive resinous composition wherein the electrodes are insulated from each other by a layer 2 of dielectric material as defined hereinafter. Electrical leads 3 are connected to the electrodes 1.

FIG. 2 is a cross-sectional view of an electrical wire having the electroconductive resinous composition and electrical component 1 covered by a dielectric material 2.

FIGS. 3 and 3a are respectively a top view and a side view in cross-section of a printed circuit board wherein shaded segments 1 comprise the electroconductive resinous composition, non-shaded segments 2 comprise a dielectric material and sockets 5 hold leads for connecting the circuit board to an electrical circuit.

FIG. 4 is a cross-sectional view of a multilayer structure comprising at least two electroconductive layers 1 of the electroconductive resinous composition that are insulated from each other and covered by layers 2 of a dielectric material.

FIG. 5 is a cross-sectional view of a coated substrate comprising an electroconductive layer 1 applied to a non-conductive substrate 4.

The resinous matrix of the present invention is suitably any poorly conductive resinous material capable of serving as a binder for finely divided metals. For the purposes of this invention, the term "resinous materials" means solid or semi-solid materials derived from natural products (so-called natural resins) and those produced by polymerization (so-called synthetic resins). By "poorly conductive" is meant that the resinous materials may be non-conductive materials having volume resistivities in the order of 10.sup.14 ohm-cm as those commonly employed as dielectric materials in many electrical applications or they may be semi-conductive such as those materials commonly employed as electroconductive paper coatings and the like having volume resistivities in the range of 10.sup.6 -10.sup.9 ohm-cm. Preferably, the resinous material is a normally solid or semi-solid, thermoplastic polymer, especially one which can be readily fabricated by normal extrusion and molding methods. It is understood, however, that thermosetting polymers are also suitable.

Exemplary preferred polymers include the organic addition polymers of the following monomers: aliphatic .alpha.-monoolefins such as ethylene, propylene, butene-1, and isobutene; vinyl halides such as vinyl chloride, vinyl bromide and vinylidene chloride; esters of .alpha.,.beta.-ethylenically unsaturated carboxylic acids such as ethyl acrylate, methyl methacrylate, hydroxyethyl acrylate and diethyl maleate; .alpha.,.beta.-ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic anhydride, itaconic acid and fumaric acid; monovinylidene aromatic carbocyclic monomers such as styrene, .alpha.-methyl styrene, ar-chlorostyrene, ar-(t-butyl)-styrene and vinyl benzyl quaternary ammonium compounds; conjugated dienes such as butadiene and isoprene; ethylenically unsaturated nitriles, amines, ethers, ketones and other ethylenically unsaturated compounds such as acrylonitrile, vinyl pyridine, ethyl vinyl ether and methyl vinyl ketone. Also suitable are the cellulosic polymers such as methyl cellulose and ethyl cellulose, polyamines such as polyethyleneimine, polyamides such as nylon, polyesters such as poly(ethylene terephthalate), polycarbonates and the like. Also, non-organic polymers as the silicone rubbers are suitable resinous materials. Especially preferred resinous materials are the ethylene polymers such as polyethylene, ethylene/vinyl acetate copolymer, ethylene/acrylic acid copolymer and ethylene/butene-1 copolymers, halogen-containing polymers such as polyvinyl chloride, polyvinylidene chloride, vinylidene chloride copolymers including such other monomers as vinyl chloride and acrylonitrile, and chlorinated polyethylene.

In addition to the foregoing polymers, the natural resins such as rosin, pine tar and the like also may be suitably employed as resinous materials.

The metal component employed in the practice of this invention is suitably any finely divided, copper metal-containing solid, preferably one having an average particle size in the range from about 0.01 to about 150 microns, especially from about 1 to about 15 microns. In addition to metal particles composed entirely of copper, metal alloys containing copper and other metals such as zinc, aluminum bismuth, silver, iron, nickel, and gold are also suitable. When the distribution of copper is uniform throughout the metal particles as in an alloy, the copper should constitute at least about 50 volume percent, preferably at least about 70 volume percent of the metal particles. Especially preferred are copper and metal alloys containing at least 90 volume percent copper and up to about 10 volume percent of zinc such as brass and bronze. The proportion of copper in the metal particulate can be reduced to a minimum of 10 volume percent of the particulate by plating copper on the surface of a particulate of another conductive metal such as iron, silver, gold and the like, e.g., as shown in U.S. Pat. No. 3,476,530 to Ehrreich et al. or by depositing the copper on a particulate of a non-conductive resinous material by conventional metallizing techniques. Resinous materials suitable for this purpose are described hereinbefore. In such cases including plating or metallization, the copper should form an essentially continuous layer covering the particle surfaces of the particulate.

The salt employed in the conductive polymeric composition is suitably any chloride, bromide, bicarbonate, tetraborate or carboxylate of metals such as, the alkali metals, calcium, zinc, silver, copper and lead. Also suitable are the fluorides of sodium and potassium. Exemplary salts include sodium chloride, sodium fluoride, potassium bromide, sodium bromide, potassium chloride, sodium tetraborate, lithium chloride, lithium bromide, sodium bicarbonate, sodium acetate, sodium benzoate, zinc chloride, zinc bromide, zinc acetate, zinc stearate, calcium chloride, lead (II) chloride, silver chloride, cupric chloride and the like. Generally the carboxylates have from 1 to 20, preferably from 2 to 12, carbon atoms and include aliphatic and aromatic varieties. Exemplary carboxylates include the acetates, benzoates, laurates and stearates of the aforementioned metals. Preferably, the salt is zinc chloride. The salt is advantageously employed at particle size less than about 150 microns, preferably less than 5 microns.

In the conductive composition, the resinous material is used in sufficient amount to act as a continuous or semi-continuous phase thereby serving as a binder for the metal component and the salt. The metal component and salt are used in amounts sufficient to render the composition electroconductive. For the purposes of this invention, a composition is considered to be electroconductive if the volume resistivity of the composition is less than 10.sup.4 ohm-cm. Preferably, in order to obtain an easily fabricated, economical material, the copper-containing solid constitutes from about 4 to about 50, especially from about 7 to about 15, volume percent of the composition, and the salt constitutes from about 0.03 to about 5, especially from about 0.1 to about 3 volume percent of the composition.

The conductive compositions are suitably prepared by incorporating the metal particulate and salt into the polymer by conventional polymer blending or mixing techniques such as roll milling, extrusion, kneading, dry blending or tumbling and the like wherein sufficient heat is employed to achieve the desired conductivity. Temperatures in the range from about 300.degree. to about 600.degree.F, preferably from about 350.degree. to about 475.degree.F, and residence times in the range from a few seconds to about an hour, preferably from about 1 to about 10 minutes, are advantageously employed. Alternatively, mixing or blending techniques may be carried out at lower temperatures in which case the compositions are subsequently activated by applying sufficient heat during or after fabrication.

In the fabrication of the conductive composition into thin layers, particularly into thin layered capacitors or printed circuits, the conductive material is coextruded with dielectric materials using a coextrusion apparatus as disclosed in U.S. Pat. No. 3,557,265 of Chisholm et al to form a multilayer structure wherein two or more conductive layers are insulated from each other by layers of dielectric material. The thicknesses of layers that can be achieved by this technique range from about 0.004 to about 50 mils, especially from about 0.1 to about 30 mils. Exemplary dielectric materials which are suitably employed are non-conductive, resinous materials as described hereinbefore including normally solid, organic polymers and blends of such polymers and inorganic dielectric materials such as barium titanate, strontium zirconate, titanium dioxide, calcium titanate, strontium titanate, barium zirconate, magnesium zirconate, calcium zirconate and other known dielectric materials. In the manufacture of capacitors, it is desirable to modify the coextrusion apparatus so that consecutive layers of the electroconductive composition are offset. In the resulting multilayer layer capacitor, alternating layers of the electroconductive composition can be connected to an electrical lead wire which is insulated from the next adjacent layers of the electroconductive composition.

Fabrication of the conductive resinous materials into other articles as described hereinbefore is carried out using conventional apparatus and techniques for fabricating the particular resinous material in the non-conductive state. Exemplary fabrication methods include molding such as injection and compression molding, extrusion, die casting and the like.

The following example is given to illustrate the invention and should not be construed as limiting its scope. All parts and percentages are by weight unless otherwise indicated.

Several blends are prepared using a blend of polyethylene and ethylene/vinyl acetate copolymer as the resinous material and metals and salts as identified in Table I by mixing the foregoing ingredients on a roll compounding apparatus at 266.degree.F until a uniform mixture of the metal powder and salt in the polymer is achieved. The blends are molded into test tabs (4 inches .times. 4 inches .times. 0.02 inches) by pressing the samples of the blends between two plates and heating at 475.degree.F for 5 minutes. The volume resistivities of the resultant test tabs are measured and the results are recorded in Table I.

For the purposes of comparison, two control samples (C.sub.1 and C.sub.2) are prepared by the foregoing procedure except that no salt is added. The volume resistivities of the control samples are measured, and the results are recorded in Table I.

Results similar to the ones shown in Table I are obtained when high density polyethylene, polypropylene, polystyrene and other resinous materials as described hereinbefore are substituted for the polymer blend employed in the foregoing Example. Also it is found that compositions containing different amounts of the copper-containing solid and salt within the ranges of about 4 to about 50 volume percent and about 0.03 to about 5 volume percent, respectively, have conductivities suitable for the purposes of this invention.