Coating powder
BACKGROUND OF THE INVENTION 
     At the present time, form-wound stator coils for elevated temperature use are made from served wire which is formed into coils, then wrapped with mica tape. The coils are impregnated with a resin, typically by a vacuum-pressure impregnation (VPI) technique. This method, while producing excellent coils, is nevertheless slow, labor intensive, and expensive. Great benefits could be realized if a technique and a material could be developed that would enable coils of various sizes and configurations to be coated in an automatic or semi-automatic fashion with insulation that had properties adequate to meet the stringent electrical and mechanical requirements for stator coils. 
     SUMMARY OF THE INVENTION 
     We have discovered a method of making a coating powder for insulating electrical apparatus. The coating powder of this invention is prepared from a composition which employs an epoxy rubber that is the reaction product of a carboxy terminated acrylonitrile butadiene copolymer and a solid epoxy resin. The resulting coating, when cured, has superior electric properties, including electric strength, voltage endurance, and dissipation factor. It also has good physical properties, including humidity/moisture resistance and windability (flexibility). Finally, the coating is superior to other powder coatings in its thermal endurance, and its thermal cycling/thermal shock resistance. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 is an isometric view partially in section showing a certain presently preferred embodiment of a coil of served wire insulated according to this invention. 
     FIG. 2 is a graph which presents experimental results discussed in Example 2. 
    
    
     In FIG. 1, conductors 1 are insulated with wire insulation 2 and formed into coil 3. Coil 3 is then coated with insulation 4 according to the method of this invention. 
     In the first step of the process of making a coating powder according to this invention, an epoxy rubber is prepared by reacting about 5 to about 50% (all percentages herein are by weight) of a carboxy terminated butadiene-acrylonitrile copolymer (CTBN) with about 50 to about 95% of a solid epoxy resin, based on total epoxy rubber weight. If less than 5% of the CTBN is used it has no beneficial affect on the resulting insulation, and if more than 40% CTBN is used the epoxy rubber product is too sticky to be ground and worked with. The best properties are obtained with about 25 to about 35% of the CTBN and about 65 to about 75% of the solid epoxy resin. 
     Only acrylonitrile and butadiene are suitable in this invention for use in preparing the epoxy rubber used in this invention. The butadiene-acrylonitrile copolymer should be about 5 to about 30% acrylonitrile and about 70 to about 95% butadiene, and is preferably about 10 to about 26% acrylonitrile--and about 74 to about 90% butadiene. The acrylonitrile butadiene copolymer must be carboxy terminated in order to react with the epoxy resin. The carboxy functionality of the resin has a theoretical maximum of 2.0. Functionalities of 1.1 to 2.0 are acceptable, with values greater than 1.8 being preferred. Carboxy values less than 1.1 will result in incomplete reaction of the copolymer with the epoxy resin. The CTBN is a commercially available product. 
     The epoxy resin used in preparing the epoxy rubber must be a solid at room temperature, but any type of epoxy resin can be used including cycloaliphatics, diglycidyl ethers of bisphenol A or bisphenol F, novalacs, and hydantoins. Diglycidyl ethers of bisphenol A (DGEBA) are preferred as they produce the best properties. 
     The reaction of the CTBN with the epoxy resin can occur at a temperature of about 80° to about 100° C. for about 2 to about 5 hours; higher temperatures requires shorter reaction times but may result in charring. An epoxycarboxyl catalyst such as triphenylphosphite, a tertiary amine, stannous octoate, or a polyhydric phenol is required for this reaction. The preferred epoxy-carboxyl catalyst is triphenylphosphite as it has been found to work well. 
     The resulting epoxy rubber, when cooled, is a solid friable reaction product. It is then crushed and ground, preferably to a diameter of a 1/4 inch or less. It should have an epoxy equivalent weight (E.E.W.) of about 1000 to about 3000, with a preferred E.E.W. of about 1500 to about 2500, an acid value between 0 and 0.5, and a softening point of about 75° to about 100° C. No solvent must be present in any of the reactants or in the product, as once solvent is present it is extremely difficult to entirely remove, and residual solvent adversely affects the properties of the insulation. 
     In the next step of the process of this invention, a coating powder is prepared of about 30 to about 70% of a second solid epoxy resin, about 5 to about 35% of the epoxy rubber, about 5 to about 25% of an anhydride curing agent, about 5 to about 40% mica filler, and about 10 to about 50% glass fibers. The second solid epoxy resin used in this composition need not be the same epoxy resin that was used in preparing the epoxy rubber, although preferably it is the same. Any anhydride curing agent can be used, although preferably the anhydride curing agent is a cyclic non-aromatic monoanhydride as they produce a more flexible product. Suitable monoanhydrides include hexahydrophthalic anhydride, tetrahydrophthalic anhydride, and phthalic anhydride. Non-monoanhydrides, such as trimellitic anhydride or polyazelaic polyanhydride, can also be used. The preferred anhydride is tetrahydrophthalic anhydride as it results in a more flexible product. An optional ingredient is about 0.01 to about 5% of an accelerator for the anhydride curing agent. The preferred amount of accelerator is about 0.1 to about 1%, and the preferred accelerator is a quaternary phosphonium salt. Other accelerators that can be used include stannous octoate, tertiary amines such as benzyldimethylamine, quaternary phosphonium salts, and fluoroborates. 
     The mica filler must be present if the resulting insulation is to have adequate voltage endurance. It may be used as a flake or as a powder and may be either natural or synthetic, with or without surface treatment such as silanes. Mica particles up to 1/4 inch in diameter may be used, although the preferred size is finer than 325 mesh. The preferred amount of the mica particles is in the range of about 10 to about 30%. 
     The glass fibers are preferably continuous glass filaments hammer milled into lengths between 1/16 inch and 1/4 inch with a nominal fiber diameter of about 0.0051 inches. The amount of glass fiber is preferably in the range of about 10 to about 30%. 
     In addition, the coating powder may contain certain optional components such as about 1 to about 10% of a pigment, about 0.2 to about 5% of a flow control agent such as a polyacrylate or silicon, about 0.5 to about 10% benzoin, which acts as a leveling agent, and about 0.2 to about 5% by weight of an antioxidant such as octadecyl 3-(3&#39;,5&#39;-di-tert-butyl-4&#39;hydroxyphenyl)propionate or tetrakis[methylene3-(3&#39;,5&#39;-di-tert-butyl-4&#39;-hydroxyphenyl)propionate]methane. 
     After the components of the coating powder are mixed, they are crushed and ground to a particle size of preferably less than about 200 microns. The application of the coating powder to a coil of insulated wire or other electrical apparatus can be accomplished in several ways. The preferred method of application is to heat the coil or electrical apparatus above the melting point of the coating powder and dip the heated part into a fluidized bed of the coating powder. The temperature required to melt and flow the coating powder will depend on the particular components used in preparing the coating powder, but this temperature is, generally speaking, between 100° and 200° C., and is usually between about 125° and 190° C. When the hot coil or electrical apparatus contacts the coating powder in the fluidized bed, the coating powder will melt and flow to cover the electrical apparatus to a thickness which depends upon the thermal capacity of the coil or apparatus, its weight, and the length of time that it remains in the fluidized bed. The coil or apparatus is then removed and heated to cure the melted coating on it. The cure time and temperature will again depend upon the particular composition used. The coating powder can also be applied in other ways, such as by electrostatic spray gun. 
     The following examples further illustrate this invention. 
     EXAMPLE 1 
     Preparation of Epoxy Rubber 
     The following composition was prepared: 
     50 parts by weight pbw solid DGEBA resin having an E.E.W. of 575-700 sold by Shell Chemical Company under the trade designation &#34;Epon 662,&#34; 
     22 pbw solid DGEBA resin having an E.E.W. of 875 to 1025 sold by Shell Chemical Company under the trade designation &#34;Epon 1004,&#34; 
     31 pbw CTBN having a molecular weight of 3500, an acrylonitrile content of 17%, a Brookfield viscosity of 125,000 cps at 27° C., and a specific gravity of 0.9 for a carboxyl functionality of 1.85, sold by B. F. Goodrich Company under the trade designation &#34;Hycar CTBN (1300 X8),&#34; and 
     0.06 pbw triphenylphosphite catalyst. 
     The composition was reacted for 6 hours at 100° C. and the product was cooled, crushed, and chipped. 
     EXAMPLE 2 
     Five coating powders were prepared which had the following composition: 
     
         ______________________________________                                    
                Formulation                                               
Ingredients       1      2      3    4    5                               
______________________________________                                    
Solid DGEBA resin having an                                               
                  44.4                                                    
E.E.W. of 575 to 700, sold                                                
by Dow Chemical Co. as                                                    
&#34;DER 662.&#34;                                                                
Solid DGEBA resin having an                                               
                  22.2                                                    
E.E.W. of 875 to 1025, sold                                               
by Shell Chemical Co. as                                                  
&#34;Epon 1004.&#34;                                                              
Solid DGEBA resin having an                                               
                         48.1   48.1 53.9 45.7                            
E.E.W. of 2000 to 2500, sold                                              
by Shell Chemical Co. as                                                  
&#34;Epon 1007.&#34;                                                              
Epoxy rubber from Example 1                                               
                         5.8    5.8       5.5                             
Trimellitic anhydride    10.8   10.8 10.8                                 
Tetrahydrophthalic anhydride              8.0                             
Dicyandiamide     4.7                                                     
Tributyl tin acetate     0.05   0.05 0.05 0.02                            
Catechol                 0.5    0.5  0.5                                  
Proprietary accelerator sold                                              
                         0.25   0.25 0.25                                 
by Cordoba Chemicals, and                                                 
believed to be a mixture                                                  
of tertiary amines                                                        
Mica (325 mesh)          32.5   10.8 10.8 15.4                            
Glass fibers      26.7          21.7 21.7 20.5                            
Yellow pigment    2.0    1.9    1.9  1.9  1.8                             
______________________________________                                    
 
    
     Of the above compositions, Formulations 1, 2, and 4 are outside the scope of this invention due to the absence of mica, anhydride curing agent, glass fibers, and epoxy rubber, respectively. The compositions were ground to a particle size of less than 200 microns. The following table gives the some of the properties of the coating powder compositions, and gives a comparison with a commercial coating powder. 
     
         __________________________________________________________________________
                          Dissipation                                     
                                  Voltage     Thermal                     
         Cure  Breakdown  Factor  Endurance   Cycling                     
Formulation                                                               
         (°C./Min)                                                 
               (kV)  Porosity                                             
                          (1 kV,155° C.)                           
                                  (hr)  Flexibility                       
                                              Resistance                  
__________________________________________________________________________
1        180/60                                                           
               --    Small                                                
                          10      63,77,98,207                            
                                        Good  Good                        
2        180/30                                                           
               40    High --      --    Poor  Poor                        
3        180/30                                                           
               38    Small                                                
                          0.74    --    Good  Good                        
4        180/30                                                           
               --    Small                                                
                          0.53    --    Poor  Good                        
5        180/30                                                           
               --    Small                                                
                          1.27    1323,1427                               
                                        Good  Good                        
                                  1460,2469                               
Commercial                                                                
         180/30                                                           
               --    Small                                                
                          2.90    784,785                                 
                                        Good  Poor                        
Coating Powder                    767,753                                 
__________________________________________________________________________
 
    
     The above table shows that the compositions of this invention, Formulations 3 and 5, were superior to the other compositions in most respects. The commercial powder, Formulations 1 and 5, and a mica tape VPI system insulation were tested for voltage endurance at 23° C. and 240 V/mil stress. FIG. 2 gives the results and shows that coating powder formulation No. 5 was superior to both coating powder formulation No. 1 and to the commercial coating powder formulation, and was as good as the VPI insulation at a 60% failure level. These results are especially significant as the abscissa in FIG. 2 is on a logarithmic scale. 
     EXAMPLE 3 
     Moisture Resistance 
     Various powders were coated onto 18 inch long U-shaped rectangular copper bars 0.125 inches by 0.75 inches. Five bars were used to test each powder. Four coils of glass served wire wrapped with mica-polyethylene terephthalate tape, impregnated with a polyester resin using a vacuum-pressure impregnation technique (VPI), were included in the moisture resistance test for comparison. 
     The samples were immersed to 80% of their lengths in a 5% aqueous saline solution and 1330 volts AC were applied across the insulation of the sample. The following table gives the hours until the insulation failed. 
     
         ______________________________________                                    
HOURS TO FAILURE                                                          
Formulation 5                                                             
         Commercial    Formulation 1                                      
(Example 2)                                                               
         Powder        (Example 2)                                        
                                  VPI Coil                                
______________________________________                                    
1217     2148          1270        83                                     
7034     2148          1105        12                                     
7368     1299          1037       899                                     
3327     1466          1490       899                                     
6415      730          --         --                                      
______________________________________                                    
 
    
     The above table shows that the coating powder of this invention, Formulation 5, was clearly superior in moisture resistance.