Abstract:
A new silver dispersion coating composition containing a special additional pigment material useful as an electrically conductive coating on electronic equipment or the like. The new coating composition maintains high electrical conductivity at a significant economic advantage over similar prior coating compositions.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a new coating composition, usable as a conductive coating. More particularly, this invention relates to a new silver dispersion coating composition that maintains high electrical conductivity even after exposure to elevated temperatures and humidity during operation. The new coating composition of this invention may be applied by silk screening, spraying, brushing, dipping, roller coating, or the like. 
     The state of the art is indicated by the following U.S. Pat. Nos. 4,371,459; 3,562,124; 3,099,578; 4,101,710; 3,920,452; and 3,412,043; and European Patent Application No. 0,068,168. 
     It has been generally known that noble metal particles, such as copper, silver gold and the platinum group metals, dispersed in a binder resin material or solution could be used to make electrically conductive coatings. However, all of these prior coatings have suffered from various deficiencies. Copper, for example, oxidizes during storage or use and the electrical properties of the coating are detrimentally effected. The use of silver, gold or platinum as the conductive material in sufficient quantity to insure good electric conductivity greatly increases the cost of the electrically conductive coating. 
     The demands for conductive compositions are significantly increasing with increased emphasis on performance characteristics such as durability under harsh environmental changes, ability to use these compositions at increased temperatures, greater abrasion resistance and the like. At the same time, and more importantly, lower processing costs and product costs with increased product capabilities have long been sought. 
     Accordingly, a main object of this invention is to provide a new conductive coating composition containing finely divided silver particles which is suitable for use in forming applied coatings having very good electrical conductance properties, and which composition is considerably more economical to produce than prior coatings having similar performance characteristics. 
     Another important objective of this invention is to provide a new silver coating composition which when applied as a coating maintains high electrical conductivity even after exposure to heat, humidity, or other harsh environmental conditions. 
     Another object of the invention is to provide a new silver coating composition that includes a special additional pigment component. 
     An additional object of this invention is to provide a new silver-containing composition which has a desirable magnetic property. 
     Other objects, features and advantages of the present invention will become apparent from the subsequent description and the appended claims. 
     SUMMARY OF THE INVENTION 
     A significant purpose of this invention is to describe an electrically conductive coating composition containing silver particles and a special additional pigment component which has highly useful electrical conductance properties and is considerably more economical to produce than prior coating compositions having essentially similar performance characteristics. These new compositions retain highly useful electrical conductance properties after exposure to elevated temperature for significant periods of time. Elevated temperatures include temperatures in excess of 200° F. In addition, the new coating composition exhibits generally excellent flexibility as determined by measuring conductivity before and after folding a 1 mil thick coating applied to a plastic film substrate. The uses for these new coating compositions include silk screening applications where a highly conductive patterned coating is needed, and where an electromagnetic interference shielding coating is needed. 
     Numerous prior art deficiencies have been overcome through the discovery that if finely divided silver particles are co-mixed with special refractory ferro alloys as described herein, the electrical conductance properties of the deposited film are maintained with very good characteristics (approaching that of a 100% pure silver only composition), while the coating itself can be manufactured at a significantly reduced cost over prior coating compositions. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     While is is not fully understood as to why the invention operates to provide such significantly useful electrically conductive silver coatings, the following preferred embodiments and preferred aspects of the invention will now be described. 
     The silver pigment material used in the coating composition is substantially of silver. The silver particles used herein can be of either flake or nonflake morphology. The nonflake powders can be irregularly shaped or spherical. However, flake silver powders are preferred due to their greater contribution to conductivity and dispersibility. Flake morphology should be understood to mean silver powder whose predominate shape is flake as determined by scanning electron microscopy. The silver pigment particle size broadly stated should be under about 50 microns in average particle size. Such silver flake powders typically have a surface area of approximately 0.80-1.40 m 2  /gram and over 99% purity. Small amounts of other conductive metals such as nickel, copper and the like may also be present in amounts of about 2-3% or less by weight without substantially effecting performance characteristics. However, on a preferred basis, the silver pigment used herein is 99+% in purity. Although silver particles are preferred, conductor coated materials such as silver-coated glass beads, silver-coated copper particles and the like can also be used. 
     The additional pigment material used in the composition, which has been found to give very good properties to the coating is a refractory ferro alloy. Such ferror alloys include materials which are selected from at least one of the group consisting of ferrophosphorous, ferromanganese, ferromolybdenum, ferrosilicon, ferrochrome, ferrovanadium, ferrozirconium, ferrotitanium, ferrotungsten, ferroboron and ferrocarbide or iron carbide. The ferro alloys discovered for use herein are described in U.S. Pat. No. 3,562,124, the disclosure of which is incorporated herein by reference. 
     Of the refractory ferror alloys indicated hereinabove, the preferred material is ferrophosphorous, and especially preferred is di-iron phosphide. The preferred ferrophosphorous refractory ferro alloy is an iron phosphide composition, generally containing from about 20 to 28% by weight of phosphorous and corresponding to a mixture of Fe 2  P and FeP. The principal impurities occurring in ferrophosphorous are silicon, vanadium, chromium, nickel and manganese, as well as trace amounts of other elements. Of these, silicon and manganese are the major impurities, typically being present in amounts up to about 7% by weight. Ferrophosphorous is commercially available from the Hooker Chemicals and Plastics Corp. under trademark Ferrophos® and is usually denominated di-iron phosphide. 
     The ferro alloy component of the present coating composition is a powder preferably having a random and angular particle shape. The particles of the ferro alloy constituent have an average particle size less than about 70 microns on a broad basis, and preferably less than about 30 microns. Best results are obtained where the average particle size is less than about 20 microns. 
     The ferro alloy used in the composition is present within the broad range of about 5% to about 90% by weight of the total pigment material in the composition. Preferably it is within the range of about 20% to about 80% by weight of the pigment material, and best results are obtained within the range of about 20% to about 70%. 
     The solvent carrier used with the coatings are conventional organic solvents or solvent blends useful for dissolving or dispersing the binder resin which will subsequently be described. The solvent used is one which is compatible with the binder resin, silver particles, and ferro alloy particles. Broadly stated, solvents such as ketones, acetates, ethers, aromatics, and chlorinated solvents can be used. Preferably, solvents such as ethylene glycol phenyl ether, benzyl alcohol, glycol ether acetates, and Carbitol® acetate may be used. Cellosolve® acetate, butyl Cellosolve® acetate and butyl Carbitol® acetate are especially preferred. Cellosolve® and Carbitol® are registered trademarks of Union Carbide Corporation. 
     The binder resin used in the coating composition may be any of a number of different materials. The binder resin may be a thermoplastic resin material which is compatible with the silver particles and with the ferro alloy material used in the coating composition. Thermosetting resin materials may also be used as the binder resin herein. The binder resin broadly stated is selected from at least one of the group consisting of thermoplastic acrylic, vinyl, urethane, alkyd, polyester, hydrocarbon, fluoroelastomer and cellulosic resins; and thermosetting acrylic, polyester, epoxy, phenolic, urethane, and alkyd reins. Vinyls, polyesters, and acrylics are preferred and polyester is especially preferred. The particular binder resin material chosen may be dictated by the desired application method and must also be compatible with the substrate. 
     The pigment to binder ratio by weight in the coating composition of this invention should broadly be within the range between about 20 to 1 and about 2 to 1. Preferably it should be maintained in the range between about 10 to 1 and about 4 to 1, and best results are obtained between about 8 to 1 and about 5 to 1. 
     Another material which may optionally be present in the coating composition is hydroquinone. It is believed the addition of hydroquinone helps to further lower the electrical resistance of the final cured coating. Purified hydroquinone in finely divided form is preferred. 
     When used, hydroquinone may be present in an amount of up to about 1% by weight based upon the amount of silver present. 
     The percent total solids in the coating composition of the present invention should broadly be within the range of about 35% up to about 100% by weight. Preferably, it should be within the range of about 50% to about 85% and best results are obtained at about 60% to about 80% by weight. 
     The properties of the new coating composition as referred to herein are believed to be highly advantageous and unique. Broadly stated, the conductivity of the coating is within the range of about 0.001 to about 30 ohms per square at 1 mil. On a preferred basis, the conductivity is within the range of about 0.010 to about 5 ohms per square at 1 mil, and particularly preferred embodiments exhibit conductivity of about 0.020-2 ohms per square at 1 mil. 
     It has been found that the new coating compositions disclosed herein exhibit generally excellent flexibility, abrasion resistance, adhesion, humidity resistance, and heat resistance properties; and, the coating also has a reduced tendency for silver migration to occur. 
     In order to further illustrate the invention, the following examples are provided. It is to be understood however that the examples are included for illustrative purposes and are not intended to be limiting of the scope of the invention as set forth in the subjoined claims. 
    
    
     EXAMPLE NO. 1* 
    
     
         ______________________________________Goodyear Tire &amp; Rubber Co.                    7.50Vitel ® PE-307 Polyester ResinGoodyear Tire &amp; Rubber Co.                    3.22Vitel ® PE-200 Polyester ResinCellosolve ® Acetate                   24.96Hydroquinone             0.06Hooker Corp. Ferrophos ® 2131                   12.85Di-Iron PhosphideSilver Flake            51.41Formulating Procedure:             (1)   Predissolve Vitel  ®                   PE-307, Vitel ® PE-200                   and Hydroquinone in                   Cellosolve ® Acetate                   using propeller mixer             (2)   Stir in Di-iron Phosphide                   and Silver Flake using                   propeller mixer             (3)   Pass through Three Roll                   Mill onceStandardized Application and             (1)   Two mil Drawdown ontoEvaluation Procedure:   Mylar ®** Film             (2)   Cure 30 minutes at 160° F.Results for 1 mil thick applied coating:Electrical resistance of .027 ohms per square at 1 mil.______________________________________ **Mylar ® is a registered trademark of E. I. duPont de Nemours and Company, Inc. 
    
     EXAMPLE NO. 2 
     
         ______________________________________Goodyear Tire &amp; Rubber Co.                 7.50Vitel ® PE-307 Polyester ResinGoodyear Tire &amp; Rubber Co.                 3.22Vitel ® PE-200 Polyester ResinCellosolve ® Acetate                 24.96Hydroquinone          0.06Hooker Corp. Ferrophos ® 2131                 19.28Di-Iron PhosphideSilver Flake          44.98Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of .053 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 3 
     
         ______________________________________Goodyear Tire &amp; Rubber Co.                 7.50Vitel ® PE-307 Polyester ResinGoodyear Tire &amp; Rubber Co.                 3.22Vitel ® PE-200 Polyester ResinCellosolve ® Acetate                 24.96Hydroquinone          0.06Hooker Corp. Ferrophos ® 2131                 25.70Di-Iron PhosphideSilver Flake          38.56Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of .064 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 4 
     
         ______________________________________Goodyear Tire &amp; Rubber Co.                 7.50Vitel ® PE-307 Polyester ResinGoodyear Tire &amp; Rubber Co.                 3.22Vitel ® PE-200 Polyester ResinCellosolve ® Acetate                 24.96Hydroquinone          0.06Hooker Corp. Ferrophos ® 2131                 32.13Di-Iron PhosphideSilver Flake          32.13Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of .105 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 5 
     
         ______________________________________Goodyear Tire &amp; Rubber Co.                 7.50Vitel ® PE-307 Polyester ResinGoodyear Tire &amp; Rubber Co.                 3.22Vitel ® PE-200 Polyester ResinCellosolve ® Acetate                 24.96Hydroquinone          0.06Hooker Corp. Ferrophos ® 2131                 38.56Di-Iron PhosphideSilver Flake          25.70Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of .217 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 6 
     
         ______________________________________Goodyear Tire &amp; Rubber Co.                 7.50Vitel ® PE-307 Polyester ResinGoodyear Tire &amp; Rubber Co.                 3.22Vitel ® PE-200 Polyester ResinCellosolve ® Acetate                 24.96Hydroquinone          0.06Hooker Corp. Ferrophos ® 2131                 44.98Di-Iron PhosphideSilver Flake          19.28Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of .738 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 7 
     
         ______________________________________Goodyear Tire &amp; Rubber Co.                 7.50Vitel ® PE-307 Polyester ResinGoodyear Tire &amp; Rubber Co.                 3.22Vitel ® PE-200 Polyester ResinCellosolve ® Acetate                 24.96Hydroquinone          0.06Hooker Corp. Ferrophos ® 2131                 51.41Di-Iron PhosphideSilver Flake          12.85Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of 5.06 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 8 
     
         ______________________________________Goodyear Tire &amp; Rubber Co.                 6.00Vitel ® PE-307 Polyester ResinGoodyear Tire &amp; Rubber Co.                 2.60Vitel ® PE-222 Polyester ResinHydroquinone          0.14Cellsolve ® Acetate                 9.90Butyl Cellosolve ® Acetate                 10.00Silver Flake          25.70Hooker Corp. Ferrophos ® 2131                 25.70Di-Iron PhosphideFormulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of .146 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 9 
     
         ______________________________________Goodyear Tire &amp; Rubber Co.                 6.00Vitel ® PE-307 Polyester ResinGoodyear Tire &amp; Rubber Co.                 2.60Vitel ® PE-222 Polyester ResinHydroquinone          0.14Cellosolve ® Acetate                 9.90Butyl Cellosolve ® Acetate                 10.00Silver Flake          25.70Iron Carbide          25.70Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of .131 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 10 
     
         ______________________________________Goodyear Tire &amp; Rubber Co.                 6.00Vitel ® PE-307 Polyester ResinGoodyear Tire &amp; Rubber Co.                 2.60Vitel ® PE-222 Polyester ResinHydroquinone          0.14Cellosolve ® Acetate                 9.90Butyl Cellosolve ® Acetate                 10.00Silver Flake          25.70Ferrotitanium         25.70Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of .108 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 11 
     
         ______________________________________Goodyear Tire &amp; Rubber Co.                 6.00Vitel ® PE-307 Polyester ResinGoodyear Tire &amp; Rubber Co.                 2.60Vitel ® PE-222 Polyester ResinHydroquinone          0.14Cellosolve ® Acetate                 9.90Butyl Cellosolve ® Acetate                 10.00Silver Flake          25.70Ferroboron            25.70Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of .163 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 12 
     
         ______________________________________Goodyear Tire &amp; Rubber Co.                 6.00Vitel ® PE-307 Polyester ResinGoodyear Tire &amp; Rubber Co.                 2.60Vitel ® PE-222 Polyester ResinHydroquinone          0.14Cellosolve ® Acetate                 9.90Butyl Cellosolve ® Acetate                 10.00Silver Flake          25.70Ferrovanadium         25.70Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of .122 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 13 
     
         ______________________________________Goodyear Tire &amp; Rubber Co.                 6.00Vitel ® PE-307 Polyester ResinGoodyear Tire &amp; Rubber Co.                 2.60Vitel ® PE-222 Polyester ResinHydroquinone          0.14Cellosolve ® Acetate                 9.90Butyl Cellosolve ® Acetate                 10.00Silver Flake          25.70Ferromanganese        25.70Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of .090 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 14 
     
         ______________________________________Goodyear Tire &amp; Rubber Co.                 6.00Vitel ® PE-307 Polyester ResinGoodyear Tire &amp; Rubber Co.                 2.60Vitel ® PE-222 Polyester ResinHydroquinone          0.14Cellosolve ® Acetate                 9.90Butyl Cellosolve ® Acetate                 10.00Silver Flake          25.70Ferrotungsten         25.70Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of .084 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 15 
     
         ______________________________________Union Carbide Corp. VROH                 5.00Vinyl Terpolymer ResinButyl Cellosolve ® Acetate                20.00Silver Flake         28.00Ferroboron           12.00Formulating procedure same as Example 1.(Except Drawdown with 3.0 mil. Blade)Results for 1 mil thick applied coating:Electrical resistance of .063 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 16 
     
         ______________________________________Rohm and Haas Co.     7.00Acryloid ® B48NThermoplastic Acrylic ResinCellosolve ® Acetate                16.30Silver Flake         22.40Ferrovanadium         6.00Formulating procedure same as Example 1.(Except Drawdown with 3.0 mil. Blade)Results for 1 mil thick applied coating:Electrical resistance of .332 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 17 
     
         ______________________________________Union Carbide Corp. Phenolic resin BK-5918                 10.00Propylene Glycol      10.00Silver Flake          30.00Ferromanganese        20.00Formulating procedure same as Example 1.(Except Cure 30 minutes at 325° F.)Results for 1 mil thick applied coating:Electrical resistance of 3.25 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 18 
     
         ______________________________________Spencer-Kellogg Polyol 8100-A4-80                  10.5Functional ResinCellosolve ® Acetate                   5.0Silver Flake           42.0Ferroboride            25.0Formulating procedure same as Example 1.(Except before use add Spencer-KelloggSpenkel ® PSI-Al-75 Polyurethane Prepolymerand cure 30 minutes at 200° F.)Results for 1 mil thick applied coating:Electrical resistance of 1.17 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 19 
     
         ______________________________________Spencer-Kellogg Polyol 8100-A4-80                  10.0Functional ResinCellosolve ® Acetate                   5.0Silver Flake           42.0Ferroboride            25.0Formulating procedure same as Example 1.(Except before use add Spencer-KelloggSpenkel ® P81-Al-75 Polyurethane Prepolymerand cure by air drying for 72 hours)Results for 1 mil thick applied coating:Electrical resistance of 1.04 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 20 
     
         ______________________________________Ciba-Geigy Corp. Araldite ® 506 Epoxy Resin                         5.5Ciba-Geigy Corp. Epoxide No. 7                         0.5Reactive EtherSilver Flake                  14.9Ferrovanadium                 10.0Formulating procedure same as Example 1.(Except before use add:Ciba-Geigy Araldite ® HY 956 Amine Hardener                         1.1Ciba-Geigy XU 225 Amine Hardener                         0.1Cure for 30 minutes at 200° F.)Results for 1 mil thick applied coating:Electrical resistance of 1.96 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 21 
     
         ______________________________________Union Carbide Corp. VAGD                 10.5Vinyl ResinButyl Cellosolve ® Acetate                 25.9Carbitol ® Acetate                 13.6Silver Flake          22.5Hooker Corp. Ferrophos ® 2131                 30.0Di-Iron PhosphideFormulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of .394 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 22 
     
         ______________________________________Rohm and Haas Co.      7.0Acryloid ® B48NThermoplastic Acrylic ResinCellosolve ® Acetate                 16.3Silver Flake          20.0Hooker Corp. Ferrophos ® 2131                 36.0Di-Iron PhosphideFormulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of 5.42 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 23 
     
         ______________________________________Union Carbide Corp. Phenolic                 10.00resin BK-5918Propylene Glycol      10.00Silver Flake          30.00Hooker Corp. Ferrophos ® 2131                 20.00Di-Iron PhosphideFormulating procedure same as Example 1.(Except cure 30 minutes at 325° F.)Results for 1 mil thick applied coating:Electrical resistance of .326 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 24 
     
         ______________________________________Ciba-Geigy Corp. Araldite ® 506 Epoxy Resin                         5.5Ciba-Geigy Corp. Epoxide No. 7                         0.5Reactive EtherSilver Flake                  14.9Hooker Corp. Ferrophos ® 2131                         10.0Di-Iron PhosphideFormulating procedure same as Example 1.(Except before use add:Ciba-Geigy Araldite ® HY 956 Amine Hardener                         1.1Ciba-Geigy XU 225 Amine Hardener                         0.1Cure for 30 minutes at 200° F.)Results for 1 mil thick applied coating:Electrical resistance of .734 ohms per square at 1 mil.______________________________________ 
    
     While it will be apparent that the preferred embodiments of the invention disclosed are well calculated to fulfill the objects above stated, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the subjoined claims.