Abstract:
A new copper dispersion coating composition particularly useful as a copper shielding coating for plastic enclosures to protect electronic equipment from electromagnetic interference (EMI). The coating maintains high electrical conductivity even after exposure to heat, humidity or salt spray. The coating composition may also be used as a one-coat air dry system that is spray applied and needs no protective overcoat.

Description:
BACKGROUND OF THE INVENTION 
     This invention broadly relates to a new coating composition and to a method of applying said coating composition to various enclosures to protect electronic equipment from electromagnetic interference (EMI). More particularly this invention relates to a new copper dispersion coating composition usable as a copper shielding coating that maintains high electrical conductivity even after exposure to heat, humidity and/or salt spray; and thereby, it eliminates many of the shortcomings of other copper coatings that have been attempted for this purpose. 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 or the like. 
     The state of the art is indicated by the following: U.S. Pat. Nos. 2,750,303; 2,980,719; 3,074,818; 3,142,814; 3,382,203; 3,998,993; 4,123,562; 2,750,307; 3,501,353; 3,532,528; 3,653,931; 3,867,738; 3,867,739; 3,716,427; 3,060,062; German Pat. No. 1,941,328; and Acheson Colloids Co. product data sheets on Electrodag 112, 414, 424, and 433. 
     It has been known that copper particles dispersed in a binder resin material or solution could be used to make an electrically conductive coating. However all of the prior coatings of this type have had the major deficiency that during storage or during usage, the copper oxidizes and the electrical properties changed such that there was a detrimental effect. Under many usage conditions this electrical property change is of such magnitude that the formerly conductive coating becomes an insulating coating. Attempts to overcome this change of electrical properties have called for replacement of the copper conductive particles with the more expensive noble-metals such as silver, gold and the platinum group, all of which greatly and significantly increase the cost of the electrically conductive coating. Thus a copper dispersion coating has long been sought after, which when applied in film form to a substrate, would provide good electrical conductance properties even after exposure to elevated temperatures for substantial time periods. 
     Another proposed solution which has been investigated in the past is the treatment of the copper particle to prevent its oxidation and subsequent change in electrical properties. Some of the solutions that have been suggested are treatment of the copper with chemicals such as nitrosobenzene, benzotriazole, chromium salts, silicates, and the like. Other treatments suggested have included high molecular weight alcohols and stearates. These attempted solutions have been ineffective or temporary at best. In addition many of these surface treatments resulted in the copper particle becoming an electrical insulator rather than the desired electrical conductor. 
     Accordingly a main object of this invention is to provide a new coating composition containing finely particulated copper particles and which composition is suitable for use in forming applied coatings having very good electrical conductance properties. 
     Another important object of this invention is to provide a novel method of forming EMI shielding coatings on enclosures for electronic equipment. 
     Another object of the invention is to provide a new copper coating composition which when applied as a coating maintains high electrical conductivity even after exposure to heat, humidity or salt spray. 
     Another object of the invention is to provide an environmentally stable air dry copper coating. 
     Another object of the invention is to provide a new copper coating composition which provides controlled electrical properties. 
     Another object of the invention is to provide a novel method of protecting electronic equipment from electromagnetic interference (EMI) by applying a copper shielding coating to enclosures for the equipment. 
     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 a method of making an electrically conductive coating composition containing copper particles which coating composition has a long storage life and yet the coating still retains highly useful electrical conductance properties after exposure to elevated temperatures for significant periods of time. By elevated temperatures it is meant temperatures in the range of 160° F., and/or temperatures up to as high as approximately 200° F. or higher in some instances. The uses for these new compositions include electromagnetic interference shielding; production of printed circuits by silk screening, and similar uses where a highly conductive film coating is needed. 
     Numerous prior art deficiencies have been overcome through the discovery that if finely divided copper particles are co-mixed with special organic titanates as described herein, the copper particles do not appear to degrade and the conductivity of the deposited film remains stable over a long period of time even at the elevated temperatures encountered in many specialized uses. Although the discovery of this invention has been used primarily to preserve the desirable electrical properties of the copper in such applied coatings or films, it has also been discovered that an improved decorating effect is also obtained by means of the non-oxidizing copper coatings which are disclosed herein. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     While it is not fully understood as to why the invention operates to provide such significantly useful electrically conductive copper coatings, particularly in the area of EMI shielding coatings, the following preferred embodiments and preferred aspects of the invention will now be described. 
     The new compositions discovered and disclosed herein have the desirable features of ease in manufacturing, long shelf life, ease of application, and most importantly, have acceptable electrical properties even when used at elevated temperatures. 
     The pigment material used in the coating composition is substantially of copper. For example the pigment material is substantially pure electrical grade copper and/or it may be such as an electrical grade copper alloy. Normally the copper particles used herein are of 95% purity or greater and preferably they are of 99% purity or greater. 
     The pigment particle size broadly stated should be under 200 microns in average particle size and preferably it is under 50 microns average particle size. 
     The binder resin used in the coating composition can be any one of a number of different materials. The binder resin is preferably a thermoplastic resin material which is compatible with the copper particles and with the titanate material used in the coating composition. Thermosetting resin materials may also be used as the binder resin herein. The binder resin is selected from at least one of the group consisting of thermoplastic acrylic, vinyl, urethane, alkyd, polyester, hydrocarbon, fluoroelastomer and celluosic resins; and, thermosetting acrylic, polyester, epoxy, urethane, and alkyd resins. The binder resin material chosen should generally be one which is easily used for spray coating applications, and also it should be non-reactive with the copper and non-reactive with the titanate. 
     The pigment to binder ratio by weight in the coating composition of the invention should broadly be within the range between about 20 to 1 and about 2 to 1, and preferably it should be maintained in the range between about 10 to 1 and about 4to 1. 
     The organic titanate material used in the coating composition is one which provides good heat stability to the coating as applied on a substrate, and it enables the coating to maintain good electrical conductivity during sustained exposure to elevated temperatures. The organic titanate used in the composition should be present within the broad range of about 1/2% to about 18% by weight of the pigment material in the composition. Preferably it should be within the range of about 2% to about 12% by weight of the pigment material, and best results are obtained within the range of about 3% to about 10%. The organic titanate should preferably be a pyrophosphate type, with best results being obtained with an organic titanate which is of the pyrophosphate type selected from at least one of the group consisting of monoalkoxy titanates or titanium chelates. As described herein the organic titanate material enables the coating as applied to maintain an electrical conductivity of under 10 ohms per square at one mil applied film thickness after exposure to an elevated temperature of about 160° F. for substantial time periods. Preferred results can be obtained, using the coatings as described herein, of under 5 ohms per square. For some applications, conductivity values up to 150 ohms per square are satisfactory, but for most electrical uses, resistance values between about 10 ohms per square are desirable. 
     Particularly useful organic titanates for use in the coating composition of this invention are the following: 
     (1) isopropyl tri(dioctylpyrophosphato) titanate 
     (2) titanium di(dioctylpyrophosphate) oxyacetate 
     (3) tri (butyl, octyl pyrophosphato) isopropyl titanate mono (dioctyl, hydrogen phosphite) 
     (4) titanium di(butyl, octyl pyrophosphate) di (dioctyl, hydrogen phosphite) oxyacetate 
     (5) di(butyl, methyl pyrophosphato), isopropyl titanate mono(dioctyl, hydrogen) phosphite 
     (6) di(butyl, methyl pyrophosphato) ethylene titanate mono(dioctyl, hydrogen phosphate) 
     The organic solvent carrier used with the coatings are conventional organic solvents or solvent blends useful for dissolving or dispersing the binder resin. Because of the known tendency of the organic titanates to react with water, the solvent carrier should be of very low water content or substantially water free. 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 used should be one which is not only compatible with the resin and copper particles but also 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. For ease of application, it is generally desirable to use the coating composition at a low total solids level. Many of the conventional solvents such as ketones, alcohols, acetates, etc. can be used as diluents. Generally suitable solvents are the ketones, aromatics, alcohols, aliphatics or blends of same. 
     Other materials which may also optionally be present in the coating composition are for example various thixotropic agents selected from at least one of the group consisting of finely divided silicas or hydrated silicates. The thixotropic agent when used may be present in the amount of about 0.1% up to about 7% by weight of the total solids and preferably within the range of about 0.1% to about 5% by weight. Particular materials for this purpose are the Bentone clays and fumed colloidal silicas such as Cab-O-Sil. 
     The percent total solids in the coating composition should broadly be within the range of about 20% up to about 85% by weight, and preferably within the range of about 40% to about 80% by weight. 
     In the following examples the formulating process was maintained essentially the same for comparative purposes. In addition to the small size shot mills, larger equipment such as ball mills, pebble mills, attritors (continuous or batch processing types), high shear mixers and the like can be used. 
    
    
     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* 
    
     
         ______________________________________Thermoplastic Methyl/Butyl MethacrylateCopolymer Resin(Acryloid B-66)            10.0Titanium Di(Dioctylpyrophosphate) OxyacetateKR-138S Organic Titanate   5.4Toluol                     30.0Copper powderRL500 (Copper Pigment)     60.0Denatured Ethyl Alcohol(Jaysol)                   10.0Formulating Procedure: (1)            Predisperse titanate and resin            in solvents.(2)              Load all ingredients in 8 oz.            shot mill and mix for 15 - minutes.Results for 1 mil thick applied coating:Initial Resistance.412 ohms per square at 1 mil.One hour at 160° F..520 ohms per square at 1 mil.24 hours at 160° F..700 ohms per square at 1 mil.______________________________________ 
    
     Test coatings made by spraying on clean glass substrate and air drying 24 hours for initial reading and afterdrying for stated intervals. 
     EXAMPLE NO. 2 
     
         ______________________________________Ethyl Methacrylate ResinAcryloid B-72              10.0Copper PowderMD 750 Copper Pigment      40.0Toluol                     20.0Methyl Ethyl Ketone        20.0Di(Butyl, Methyl Pyrophosphato), IsopropylTitanate Mono(Dioctyl, Hydrogen) PhosphiteKR 62ES (Ken-Rich)         5.0Fumed Colloidal Silica(Cabosil M-5)              0.5Formulating Procedure same as Example 1.Results for 1 mil thick applied coating:Initial Resistance9.93 ohms per square at 1 mil.One hour at 160°0 F.13.4 ohms per square at 1 mil.24 hours at 160° F.34.5 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 3 
     
         ______________________________________Methyl Methacrylate ResinAcryloid A-11              7.0Copper ParticlesRL500                      70.0Methyl Ethyl Ketone        20.0Methyl Isobutyl Ketone     20.0Isopropyl Tri(Dioctylpyrophosphato) TitanateKR38S Organic Titanate (Ken-Rich)                      1.4Bentonite ClayBentone 27                 2.0Formulating Procedure same as Example 1.Results for 1 mil thick applied coating:Initial Resistance.616 ohms per square at 1 mil.One hour at 160° F..616 ohms per square at 1 mil.24 hours at 160° F..716 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 4 
     
         ______________________________________Thermoplastic Vinyl ResinUnion Carbide Corp. - VAGH 10.0Copper ParticlesRL500                      120.0Butyl Acetate              60.0Di(Butyl, Methyl Pyrophosphato), IsopropylTitanate Mono(Dioctyl, Hydrogen) Phosphite                      8.0Formulating Procedure same as Example 1.Results for 1 mil thick applied coating:Initial Resistance2.92 ohms per square at 1 mil.One hour at 160° F.2.82 ohms per square at 1 mil.24 hours at 160° F.4.00 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 5 
     
         ______________________________________Thermoplastic Vinyl ResinUnion Carbide Corp. - VYND 5.0Copper ParticlesRL500                      50.0Methyl Ethyl Ketone        40.0Titanium Di(Butyl, Octyl Pyrophosphato)Di(Dioctyl, Hydrogen Phosphite) OxyacetateKR158FS                    5.0Formulating Procedure same as Example 1.Results for 1 mil thick applied coating:Initial Resistance10.24 ohms per square at 1 mil.One hour at 160° F.11.56 ohms per square at 1 mil.24 hours at 160° F.14.88 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 6 
     
         ______________________________________Rosin-Ester Coating ResinCellolyn 102               5.4Ethyl Cellulose            5.4Butanol                    3.2Xylol                      2.2Methyl Ethyl Ketone        16.7Butyl Acetate              11.0Copper PowderRL500                      50.0Di(Butyl, Methyl Pyrophosphato), IsopropylTitanate Mono(Dioctyl, Hydrogen) PhosphiteKR62ES                     9.0Fumed Colloidal SilicaCab-O-Sil M-5              1.0Formulating Procedure same as Example 1.Results for 1 mil thick applied coating:Initial resistance.563 ohms per square at 1 mil.One hour at 160°F..605 ohms per square at 1 mil.24 hours at 160° F..735 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 7 
     
         ______________________________________Methyl Methacrylate ResinAcryloid A-11              5.0Toluol                     30.0Di(Butyl, Methyl Pyrophosphato), IsopropylTitanate Mono(Dioctyl, Hydrogen) PhosphiteKR62ES                     8.0Bentonite ClayBentone 34                 1.5Copper PowderRL500                      60.0Formulating Procedure same as Example 1.Results for 1 mil thick applied coating:Initial Resistance.184 ohms per square at 1 mil.One hour at 160° F..196 ohms per square at 1 mil.24 hours at 160° F..208 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 8 
     
         ______________________________________Ethyl Cellulose            8.0Xylol                      49.0Butanol                    3.0Denatured Ethyl Alcohol    10.0Tri(Butyl, Octyl Pyrophosphato) IsopropylTitanate Mono(Dioctyl, Hydrogen Phosphite)KR58FS                     6.0Copper PowderRL500                      40.0Formulating Procedure same as Example 1.Results for 1 mil thick applied coating:Initial Resistance.584 ohms per square at 1 mil.One hour at 160° F..604 ohms per square at 1 mil.24 hour at 160° F..660 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 9 
     
         ______________________________________Nitrocellulose             4.2Toluol                     3.2Ethanol                    12.0Copper PowderRL500                      36.0Titanium Di(Butyl, Octyl Pyrophosphate)Di(Dioctyl, Hydrogen Phosphite) OxyacetateKR158FS                    1.8Formulating Procedure same as Example 1.Results for 1 mil thick applied coating:Initial Resistance3.2 ohms per square at 1 mil.One hour at 160° F.3.2 ohms per square at 1 mil.24 hours at 160° F.3.2 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 10 
     
         ______________________________________Methyl Methacrylate ResinElvacite 2008              10.0Cellulose Acetate ButyrateCAB 381-20                 0.5Copper PowderRL500                      50.0Toluol                     20.0Methyl Ethyl Ketone        24.5Titanium Di(Butyl, Octyl Pyrophosphate)Di(Dioctyl, Hydrogen Phosphite) OxyacetateKR158FS                    8.0Formulating Procedure same as Example 1.Results for 1 mil thick applied coatings:Initial Resistance1.63 ohms per square at 1 mil.One hour at 160° F.1.71 ohms per square at 1 mil.24 hours at 160° F.1.93 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 11 
     
         ______________________________________Thermoset Acrylic ResinAcryloid AT-50             20.0Toluol                     10.0Copper PowderRL500                      69.0Isopropyl Tri(Dioctylpyrophosphato) TitanateKR-38S                     6.0Formulating Procedure same as Example 1.Cured 20 min. at 300° F.Results for 1 mil thick applied coating:Initial Resistance.124 ohms per square at 1 mil.One hour at 160° F..128 ohms per square at 1 mil.24 hours at 160° F..128 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 12 
     
         ______________________________________Cellulose Acetate ButyrateCAB 381-20                 5.0Copper PowderRL500                      40.0Methyl Ethyl Ketone        49.0Di(Butyl, Methyl Pyrophosphate), IsopropylTitanate Mono(Dioctyl, Hydrogen) Phosphite                      3.0KR62ESFormulating Procedure same as Example 1.Results for 1 mil thick applied coatings:Initial Resistance.728 ohms per square at 1 mil.One hour at 160° F..796 ohms per square at 1 mil.24 hours at 160° F..928 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 13 
     
         ______________________________________Methyl Methacrylate ResinAcryloid B-82              5.0Copper PowderRL500                      90.0Di(Butyl, Methyl Pyrophosphato), IsopropylTitanate Mono(Dioctyl, Hydrogen) PhosphiteKR62ES                     7.0Toluol                     40.0Isopropanol                10.0Colloidal Fumed SilicaCabosil M-5                1.5Butanol                    5.0Formulating Procedure same as Example 1.Results for 1 mil thick applied coating:Initial Resistance1.28 ohms per square at 1 mil.One hour at 160° F.1.38 ohms per square at 1 mil.24 hours at 160° F.2.15 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 14 
     
         ______________________________________Polyester Desmophen 1300   10.0Copper PowderRL500                      50.0Tri(Butyl, Octyl Pyrophosphato) IsopropylTitanate Mono(Dioctyl, Hydrogen) Phosphite                      6.5KR58FSMethyl Ethyl Ketone        10.0Butyl Acetate              20.0PolyisocyanateMondur CB-75               18.0Formulating Procedure: (1)            Disperse KR58FS in solvents and            load with RL500 and Des. 1300            in 8 oz. shot mill for 15            minutes.(2)              Add Mondur CB-75 and mix            thoroughly.Results for 1 mil thick applied coating:Initial Resistance24.9 ohms per square at 1 mil.One hour at 160° F.24.5 ohms per square at 1 mil.24 hours at 160° F.29.4 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 15 
     
         ______________________________________Vinyl ResinUnion Carbide Corp. VAGH   8.0Copper PowderRL500                      60.0Methyl Isobutyl Ketone     40.0Titanium Di(Octylpyrophosphate) OxyacetateKR138S                     8.0Formulating Procedure same as Example 1.Results for 1 mil thick applied coating:Initial Resistance.616 ohms per square at 1 mil.One hour at 160° F..736 ohms per square at 1 mil.24 hours at 160° F..952 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 16 
     
         ______________________________________Polyester Coating ResinINOLEX 5171-200            10.0Copper PowderRL500                      80.0Toluol                     30.0Di(Butyl, Methyl Pyrophosphato), IsopropylTitanate Mono(Dioctyl, Hydrogen) PhosphiteKR62ES                     9.0PolyisocyanateDesmodur N-75              9.0Formulating Procedure:(1)            Predisperse titanate in solvents.(2)              Load blend with RL500 and            polyester for 15 minutes in            8 oz. shot mill.(3)              Combine with polyisocyanate.Results for 1 mil thick applied coating:Initial Resistance2.4  ohms per square at 1 mil.One hour at 160° F.2.54 ohms per square at 1 mil.24 hours at 160° F.3.02 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 17 
     
         ______________________________________Thermosetting Epoxy ResinEpon 1001                  7.0Urea ResinUformite F-492             3.0Toluol                     20.0Methyl Ethyl Ketone        20.0Copper PowderRL500                      70.0Di(Butyl, Methyl Pyrophosphato) EthyleneTitanate Mono(Dioctyl, Hydrogen Phosphate)KR262ES                    5.0BentoniteBentone 27                 1.0Formulating Procedure same as Example 1.Cured 15&#39; at 300° F.Results for 1 mil thick applied coating:Initial Resistance 8.4 ohms per square at 1 mil.One hour at 160° F. 8.4 ohms per square at 1 mil.24 hours at 160° F.11.2 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 18 
     
         ______________________________________Thermosetting Polyester ResinCyplex 1600                18.0Urea Curative ResinBeetle 80                  3.0para-toluenesulfonic acidPTSA                       0.2Copper PowderRL500                      140.0Di(Butyl, Methyl Pyrophosphato) EthyleneTitanate Mono(Dioctyl, Hydrogen Phosphate)KR262ES                    9.0Toluol                     10.0Butyl Acetate              10.0Ethylene Clycol Monoethyl EtherCellosolve                 10.0Formulating Procedure same as Example 1.Cured 60&#39; at 300° F.Results for 1 mil thick applied coating:Initial Resistance137.6 ohms per square at 1 mil.One hour at 160° F.136.0 ohms per square at 1 mil.24 hours at 160° F.136.0 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 19 
     
         ______________________________________Methyl/Butyl Methacrylate CopolymerAcryloid B-66              13.6Copper PowderRL500 Copper Pigment       54.3Toluol                     18.3EthanolJaysol                     9.2Titanium Di(Dioctylpyrophosphate) OxyacetateKR138S                     4.5Formulating Procedure same as Example 1.Results for 1 mil thick applied coating:Initial Resistance.417 ohms per square at 1 mil.One hour 15 160° F..447 ohms per square at 1 mil.24 hours at 160° F..498 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 20 
     
         ______________________________________Methyl/Butyl Methacrylate CopolymerAcryloid B-66              13.6Copper PowderRL500 Pigment              55.0Toluol                     18.3EthanolJaysol                     5.0Ethylene Glycol Monoethyl EtherCellosolve                 4.3BentoniteBentone 34                 1.5Tri(Butyl, Octyl Pyrophosphato) IsopropylTitanate Mono(Dioctyl, Hydrogen Phosphite)KR-58FS                    2.3Formulating Procedure same as Example 1.Results for 1 mil thick applied coating:Initial Resistance.129 ohms per square at 1 mil.One hour at 160° F..144 ohms per square at 1 mil.24 hours at 160° F..159 ohms per square at 1 mil.______________________________________ 
    
     EXAMPLE NO. 21 
     
         ______________________________________Thermoplastic Fluoroelastomer ResinViton                      10.0Butyl Acetate              50.0Titanium Di(Butyl, Octyl Pyrophosphate)Di(Dioctyl, Hydrogen Phosphite) OxyacetateKR158FS                    5.0Copper PowderRL500 Copper Pigment       80.0Methyl Ethyl Ketone        20.0Formulating Procedure same as Example 1.Results for 1 mil thick applied coatings:Initial Resistance13.6 ohms per square at 1 mil.One hour at 160° F.18.4 ohms per square at 1 mil.24 hours at 160° F.26.4 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.