Abstract:
A new nickel dispersion coating composition containing a special additional pigment material useful as a shielding coating to protect electronic equipment or the like from electromagnetic interference (EMI). The new coating composition is more economical to produce than prior coating compositions without significantly effecting performance characteristics.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a new nickel coating composition to protect electronic equipment such as computers, video games or the like as well as cathode ray tubes, oscilloscope tubes or the like from electromagnetic interference (EMI). More particularly, this invention relates to a new nickel dispersion coating composition useable as a shielding coating 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 as a one-coat air dry system that is spray applied and needs no protective overcoat. It also, of course, may be applied by other means such as brushing, dipping, roller coating, or the like, and may also be overcoated with a decorative finish. 
     The state of its art is indicated by the following U.S. Pat. Nos. 3,968,056; 3,390,104; 3,920,452; 3,412,043; 2,280,135; 4,305,847; and 4,382,981. 
     It has generally been known that metal particles, such as copper, silver, gold platinum and the like, dispersed in a binder resin material or solution can be used to make an electrically conductive coating. 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 electrical 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, greater product capabilities and, perhaps more importantly, lower processing costs have been sought. 
     Accordingly, a main object of this invention is to provide a new coating composition containing finely divided nickel 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 object of this invention is to provide a new nickel 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 nickel coating composition that includes a special additional pigment component. 
     Another object of this invention is to provide an environmentally stable coating composition that is relatively economical to produce without significantly sacrificing electrical conductance properties of the coating composition. 
     A still further object of this invention is to provide a new coating composition capable of forming EMI shielding coatings on enclosures for electronic equipment. 
     An additional object of this invention is to provide a new composition with desirable magnetic properties. 
     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 nickel 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 similar performance characteristics. These new compositions retain highly useful electrical conductance properties after exposure to elevated temperatures for significant periods of time. By elevated temperatures is meant temperatures in the range of 200° F. and higher. In addition, the new coating composition exhibits generally excellent conductivity after exposure to significant variations in temperature. By significant variations in temperature is meant: 30 minutes at 160° F.; five minutes at room temperature; 30 minutes at -40° F.; and five minutes at room temperature. 
     The uses for these new coating compositions includes electromagnetic interference shielding; production of printed circuits by silk screening, and similar uses where a highly conductive film coating is needed. 
     These new coating compositions are typically applied to the interior and/or exterior surfaces of enclosures for electronic equipment to protect the contents of such enclosures from EMI. The coating compositions may also be applied as an EMI shielding coating directly onto the glass surface of cathode ray tubes, oscilloscope tubes or the like. 
     Numerous prior art deficiencies have been overcome through the discovery that if finely divided nickel 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, while the coating itself can be manufactured at a significantly reduced cost over prior coating compositions. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While it is not fully understood as to why the invention operates to provide such significantly useful electrically conductive nickel coatings, particularly in the area of EMI shielding coatings, the following preferred embodiments and preferred aspects of the invention will now be described. 
     The nickel pigment material used in the coating composition is substantially of nickel. The nickel pigment material broadly stated is finely divided nickel powder having an average particle size of less than about 10 microns and over about 99% purity. Preferably, the nickel powder used herein should be formed by the carbonyl process and have an average particle size of between about 2.0 microns and about 7.0 microns and 99+% in purity. Best results are obtained where the average particle size is between about 2.9 microns and about 3.6 microns. Such nickel pigment material broadly stated should have a surface area of approximately 0.25-0.75 m 2  /gram and preferably of approximately 0.34-0.68 m 2  /gram. Best results are obtained where the surface area of the nickel powder is approximately 0.58 m 2  /gram. Small amounts of other conductive metals such as silver, copper and the like may also be present in amounts of about 2-3% or less by weight without substantially effecting performance characteristics. Nickel-plated glass beads, nickel-plated copper particles, nickel-plated ceramic particles and the like may also be used in this invention. 
     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 ferro 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 ferro 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% 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 the 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 this 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 10% to about 50% by weight of the pigment material, and best results are obtained within the range of about 15% to about 35%. 
     The solvent carrier used with the coatings can be the 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, nickel particles, and ferro alloy particles. Also, because one use of the coating composition is to obtain an electromagnetic interference shield on the interior surfaces of plastic enclosed electronic devices, the solvent blend should be one which is compatible with the plastic containers and one which will not degrade the plastic materials. For example, with many solvent sensitive plastics, a blend of isopropanol and toluol has been found desirable. Broadly stated, solvents such as glycols, glycol ethers, esters, ketones, alcohols, acetates, etc., or water can be used as diluents. Generally suitable solvents are water, ketones, aromatics, alcohols, aliphatics or blends of same. 
     The binder resin used in the coating composition may be of any of a number of different materials. The binder resin may be a thermoplastic resin material which is compatible with the nickel particles and with the ferro alloy materials used in the coating composition. Thermosetting resin materials and silicates 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 resins. Vinyls, polyesters, and acrylics are preferred. The particular binder resin material chosen is dictated by the desired application method and which is also 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 3 to 1. 
     Other materials which may also optionally be present in the coating composition are for example various thixotropic agents (anti-settling agents) selected from at least one of the group consisting of finely divided silicas, castor oil derivatives or organo-clays. Particularly suitable materials for this purpose are the bentonite clays. 
     When used, the thixotropic agent may be present in an amount of between about 0.1% and about 8% by weight of the total solids. Preferably, it should be within the range of about 0.1% to about 4% by weight of total solids and best results are obtained between about 1% and about 2%. 
     The percent total solids in the coating composition of the present invention should broadly be within the range of about 20% up to about 100% by weight. Preferably, it should be within the range of about 30% 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 0.2 to about 1000 ohms per square at 1 mil. On a preferred basis, the conductivity is within the range of about 0.5 to about 5 ohms per square at 1 mil, and particularly preferred embodiments exhibit conductivity of about 0.5 to about 3 ohms per square at 1 mil. 
     It has been found that the new coating compositions disclosed herein exhibit generally excellent abrasion resistance, adhesion, humidity resistance and heat resistance properties. 
    
    
     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* 
    
     
         ______________________________________Methyl methacrylate polymer (binder resin)                      9.8Castor oil derivative (anti-settling agent)                      2.0Ethylene glycol monoethyl ether (solvent)                      7.9Denatured ethyl alcohol (solvent)                      1.6Toluol                     6.8n-Butyl Acetate            13.0Hooker Corp. Ferrophos ® 2131                      5.9Di-Iron phosphideNickel powder              53.0Formulating procedure:           (1)   Predissolve resin in mixed                 solvents           (2)   Add nickel powder and anti-                 settling agent           (3)   Disperse in a ball mill                 for seven hours           (4)   Add Di-Iron phosphide and                 disperse for 15 minutes           (5)   UnloadStandardized Application           (1)   Sprayed on Lexan ® plasticAnd Evaluation Procedure                 sheets           (2)   Air dryed for 24 hours           (3)   Resistance measured and                 standardized to 1 mil                 thicknessResults of 1 mil thick applied coating:Electrical resistance of 1.28 ohms per square at1 mil.______________________________________ 
    
     Test coatings of examples 1 to 12 made by spraying on Lexan® (registered trademark of General Electric Co.) plastic sheets and air drying for 24 hours. 
     EXAMPLE NO. 2 
     
         ______________________________________Methyl methacrylate polymer (binder resin)                    9.8Castor oil derivative (anti-settling agent)                    2.0Ethylene glycol monoethyl ether (solvent)                    7.9Denatured ethyl alcohol (solvent)                    1.6Toluol                   6.8n-Butyl Acetate          13.0Hooker Corp. Ferrophos ® 2131                    11.8Di-Iron phosphideNickel powder            47.1Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of 1.77 ohms per squareat 1 mil.______________________________________ 
    
     EXAMPLE NO. 3 
     
         ______________________________________Methyl methacrylate polymer (binder resin)                      9.8Castor oil derivative (anti-settling agent)                      2.0Ethylene glycol monoethyl ether (solvent)                      7.9Denatured ethyl alcohol (solvent)                      1.6Toluol                     6.8n-Butyl Acetate            13.0Hooker Corp. Ferrophos ® 2131                      17.7Di-Iron phosphideNickel powder              41.2Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of 1.72 ohms per square at1 mil.______________________________________ 
    
     EXAMPLE NO. 4 
     
         ______________________________________Methyl methacrylate polymer (binder resin)                      9.8Castor oil derivative (anti-settling agent)                      2.0Ethylene glycol monoethyl ether (solvent)                      7.9Denatured alcohol (solvent)                      1.6Toluol                     6.8n-Butyl Acetate            13.0Hooker Corp. Ferrophos ® 2131                      23.6Di-Iron phosphideNickel powder              35.3Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of 2.96 ohms per square at1 mil.______________________________________ 
    
     EXAMPLE NO. 5 
     
         ______________________________________Methyl methacrylate polymer (binder resin)                      9.8Castor oil derivative (anti-settling agent)                      2.0Ethylene glycol monoethyl ether (solvent)                      7.9Denatured ethyl alcohol (solvent)                      1.6Toluol                     6.8n-Butyl Acetate            13.0Hooker Corp. Ferrophos ® 2131                      29.5Di-Iron PhosphideNickel powder              29.5Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of 4.42 ohms per square at1 mil.______________________________________ 
    
     EXAMPLE NO. 6 
     
         ______________________________________Methyl methacrylate polymer (binder resin)                      9.8Castor oil derivative (anti-settling agent)                      2.0Ethylene glycol monoethyl ether (solvent)                      7.9Denatured ethyl alcohol (solvent)                      1.6Toluol                     6.8n-Butyl Acetate            13.0Nickel powder              44.2Iron Carbide               14.7Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of 1.42 ohms per square at1 mil.______________________________________ 
    
     EXAMPLE NO. 7 
     
         ______________________________________Methyl methacrylate polymer (binder resin)                      9.8Castor oil derivative (anti-settling agent)                      2.0Ethylene glycol monoethyl ether (solvent)                      7.9Denatured ethyl alcohol (solvent)                      1.6Toluol                     6.8n-Butyl Acetate            13.0Nickel powder              44.2Ferrotitanium              14.7Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of 1.17 ohms per square at1 mil.______________________________________ 
    
     EXAMPLE NO. 8 
     
         ______________________________________Methyl methacrylate polymer (binder resin)                      9.8Castor oil derivative (anti-settling agent)                      2.0Ethylene glycol monoethyl ether (solvent)                      7.9Denatured ethyl alcohol (solvent)                      1.6Toluol                     6.8n-Butyl Acetate            13.0Hooker Corp. Ferrophos ® 2131                      14.7Di-Iron phosphideNickel powder              44.2Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of 1.19 ohms per square at1 mil.______________________________________ 
    
     EXAMPLE NO. 9 
     
         ______________________________________Methyl methacrylate polymer (binder resin)                      9.8Castor oil derivative (anti-settling agent)                      2.0Ethylene glycol monoethyl ether (solvent)                      7.9Denatured ethyl alcohol (solvent)                      1.6Toluol                     6.8n-Butyl Acetate            13.0Nickel powder              44.2Ferroboron                 14.7Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of 1.34 ohms per square at1 mil.______________________________________ 
    
     EXAMPLE NO. 10 
     
         ______________________________________Methyl methacrylate polymer (binder resin)                    9.8Castor oil derivative (anti-settling agent)                    2.0Ethylene glycol monoethyl ether (solvent)                    7.9Denatured ethyl alcohol (solvent)                    1.6Toluol                   6.8n-Butyl Acetate          13.0Nickel powder            44.2Ferrovanadium            14.7Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of 1.51 ohms per squareat 1 mil.______________________________________ 
    
     EXAMPLE NO. 11 
     
         ______________________________________Methyl methacrylate polymer (binder resin)                    9.8Castor oil derivative (anti-settling agent)                    2.0Ethylene glycol monoethyl ether (solvent)                    7.9Denatured ethyl alcohol (solvent)                    1.6Toluol                   6.8n-Butyl Acetate          13.0Nickel powder            44.2Ferromanganese           14.7Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of 1.11 ohms per squareat 1 mil.______________________________________ 
    
     EXAMPLE NO. 12 
     
         ______________________________________Methyl methacrylate polymer (binder resin)                    9.8Castor oil derivative (anti-settling agent)                    2.0Ethylene glycol monoethyl ether (solvent)                    7.9Denatured ethyl alcohol (solvent)                    1.6Toluol                   6.8n-Butyl Acetate          13.0Nickel powder            44.2Ferrotungsten            14.7Formulating procedure same as Example 1.Results for 1 mil thick applied coating:Electrical resistance of .942 ohms per squareat 1 mil.______________________________________ 
    
     EXAMPLE NO. 13 
     
         ______________________________________Methyl butyl methacrylate copolymer (binder resin)                        5.5Bentonite Clay               0.6Bentone ® 27Nickel powder                35.4Ferromanganese               8.8n-Butanol                    4.2n-Propyl Acetate             32.4Isopropanol                  1.4Toluol                       11.7Formulating procedure:          (1)    Predissolve binder resin                 in mixed solvents          (2)    Add dissolved resin, nickel                 powder and clay          (3)    Load all ingredients into                 shot mill and mix for 15                 minutesResults for 1 mil thick applied coating:Electrical resistance of 1.32 ohms per square atmil.______________________________________ 
    
     Test coatings for Examples 13, 14, 16, 17, 19, 21 and 22 made by spraying on Lexan® sheets and air drying for 72 hours. 
     EXAMPLE NO. 14 
     
         ______________________________________Union Carbide Corp. VROH   6.9Vinyl Terpolymer Resinn-Butanol                  5.1n-Propyl Acetate           40.3Isopropanol                1.8Toluol                     14.6Fumed Colloidal Silica     0.4(Cabosil M-5)Nickel powder              24.0Iron Carbide               6.9Formulating procedure as in Example 13.Results for 1 mil thick applied coating:Electrical resistance of 1.97 ohms per square at1 mil.______________________________________ 
    
     EXAMPLE NO. 15 
     
         ______________________________________Rohm and Haas Co.           11.8Acryloid ® AT-50Thermosetting Acrylic polymer (binder resin)Ethylene glycol monoethyl ether acetate (solvent)                       11.7Methyl Ethyl Ketone (solvent)                       35.3Nickel powder               29.4Ferrotitanium               11.8Formulating procedure as in Example 13.Results for 1 mil thick applied coating:Electrical resistance of 20.8 ohms per squareat 1 mil.______________________________________ 
    
     Test coating for Examples 15 and 20 made by spraying on glass plate and curing for 30 minutes at 325° F. 
     EXAMPLE NO. 16 
     
         ______________________________________Shell Chemical Co.           7.1EPON ® 1007 epoxy resinMethyl Ethyl Ketone          28.5Toluol                       28.5Nickel powder                21.3Ferrovanadium                14.2Formulating procedure as in Example 13.Before coating, add Shell Chemical Co. EPON ® H-3                        0.4curing agentResults for 1 mil thick applied coating: -Electrical resistance of 5.59ohms per square at1 mil.______________________________________ 
    
     EXAMPLE NO. 17 
     
         ______________________________________Mobay Chemical Corp.          11.4MULTRON R-221-75 saturated polyester resinEthylene glycol monoethyl ether acetate                         22.7Butyl Acetate                 11.3Nickel powder                 31.8Ferroboron                    11.4Formulating procedure as in Example 13.Before coating, add Mobay Chemical Corp. DESMODUR                         11.4IL polyisocyanateResults for 1 mil thick applied coating:Electrical resistance of 3.04 ohms per square at1 mil.______________________________________ 
    
     EXAMPLE NO. 18 
     
         ______________________________________KASIL #1 (Aqueous Potassium Silicate solution)                        34.8Nickel powder                35.6Hooker Corp. Ferrophos ® 2131                        23.8Di-Iron PhosphideDeionized water               5.8Formulating procedure as in Example 13.Test coating after spraying on glass plates and curingfor 30 minutes at 200° F.Results for 1 mil thick applied coating:Electrical resistance of 6.64 ohms per square at1 mil.Test coating after spraying on glass plates and curingfor 15 minutes at 1000° F.Results for 1 mil thick applied coating:Electrical resistance of 2.72 ohms per square at1 mil.______________________________________ 
    
     EXAMPLE NO. 19 
     
         ______________________________________Mobay Chemical Corp. MULTRON R-221-75                         11.4Saturated Polyester ResinNickel powder                 31.8Hooker Corp. Ferrophos ® 2131                         11.4Di-Iron PhosphideEthylene Glycol Monoethyl Ether Acetate                         22.7Butyl Acetate                 11.3Formulating procedure same as in Example 13.Before coating, add Mobay Chemical Corp. DESMODUR                         11.4IL PolyisocyanateResults for 1 mil thick applied coating:Electrical resistance of 2.06 ohms per square at1 mil.______________________________________ 
    
     EXAMPLE NO. 20 
     
         ______________________________________Rohm and Haas Co.          11.8Acryloid ® AT-50Thermosetting Acrylic PolymerEthylene Glycol Monoethyl Ether Acetate                      11.7Methyl Ethyl Ketone        35.3Nickel Powder              30.0Hooker Corp. Ferrophos ® 2131                      15.0Di-Iron PhosphideFormulating procedure same as in Example 13.Results for 1 mil thick applied coating:Electrical resistance of 20.4 ohms per square at1 mil.______________________________________ 
    
     EXAMPLE NO. 21 
     
         ______________________________________Methyl Butyl Methacrylate Copolymer                      5.5n-Butanol                  4.2Toluol                     11.7n-Propyl Acetate           32.4Isopropanol                1.4Bentonite Clay             0.6Bentone ® 27Nickel Powder              36.0Hooker Corp. Ferrophos ® 2131                      9.0Di-Iron PhosphideFormulating procedure same as in Example 13.Results for 1 mil thick applied coating:Electrical resistance of 1.5 ohms per square at1 mil.______________________________________ 
    
     EXAMPLE NO. 22 
     
         ______________________________________Union Carbide Corp. VROH   6.9Vinyl Terpolymer Resinn-Butanol                  5.1Toluol                     14.6n-Propyl Acetate           40.3Isopropanol                1.8Fumed Colloidal Silica     0.5(Cabosil M-5)Nickel powder              25.0Hooker Corp. Ferrophos ® 2131                      7.0Di-Iron PhosphideFormulating procedure same as in Example 13.Results for 1 mil thick applied coating:Electrical resistance of 1.55 ohms per square at1 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.