Blocked isocyanate composition for forming T hermoparticulating coating
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is related to application Ser. No. 426,391 filed Dec. 19, 1973 by Emil M. Fort, Thomas D. Karzmarek, and David Colin Phillips titled &#34;Sampling System for Power Generators.&#34; 
     This application is also related to application Ser. No. 568,221, filed of even date, by J. D. B. Smith and D. C. Phillips titled &#34;Metal Acetyl Acetonate Composition For Forming Thermoparticulating Coating.&#34; 
     This application is related to application Ser. No. 568,219 filed of even date by J. D. B. Smith and D. C. Phillips titled &#34;Malonic Acid Derivative Composition For Forming Thermoparticulating Coating.&#34; 
     This application is related to application Ser. No. 568,224 filed of even date by J. D. B. Smith and D. C. Phillips titled &#34;Diazonium Salt Composition For Forming Thermoparticulating Coating.&#34; 
     This application is related to application Ser. No. 568,223 filed of even date by J. D. B. Smith and D. C. Phillips and K. W. Grossett titled &#34;Grease Thermoparticulating Coating.&#34; 
     This application is related to application Ser. No. 568,218 filed of even date by D. C. Phillips, W. M. Hickam, and J. D. B. Smith titled &#34;Multiple Signal Thermoparticulating Coating.&#34; 
     PRIOR ART 
     A book titled &#34;Polyurethanes: Chemistry and Technology, I. Chemistry&#34; by J. H. Saunders and K. C. Frisch, Interscience Publishers, 1962, on pages 118 and 121 describes the formation, splitting, and decomposition products of blocked isocyanates. 
     U.S. Pat. No. 3,705,119 describes the splitting of blocked isocyanates. 
     BACKGROUND OF THE INVENTION 
     Electrical apparatus, such as motors and turbine generators, occasionally overheat due to shorts or other malfunctions. The longer the overheating continues the more damage is done to the apparatus. A malfunction detected immediately may mean only a quick repair but if the overheating continues, the entire machine may be damaged. 
     Large rotational electrical apparatus is usually cooled with a hydrogen gas stream. The organic compounds in the apparatus are first to be affected by the overheating and they decompose to form particles which enter the gas stream. Monitors then detect particles in the gas stream and sound a warning or shut down the apparatus when too many particles are detected. 
     Descriptions of such monitors and how they function may be found in U.S. Pat. No. 3,427,880 titled &#34;Overheating Detector For Gas Cooled Electrical Machine&#34; and in U.S. Pat. No. 3,573,460 titled &#34;Ion Chamber For Submicron Particles.&#34; Another monitor, &#34;The Condensation Nuclei Detector,&#34; is described by F. W. VanLuik, Jr. and R. E. Rippere, in an article titled &#34;Condensation Nuclei, A New Technique For Gas Analysis,&#34; in Analytical Chemistry 34, 1617 (1962) and by G. F. Skala, in an article titled &#34;A New Instrument For The Continuous Detection of Condensation Nuclei,&#34; in Analytical Chemistry 35, 702 (1963). 
     As U.S. Pat. Nos. 3,427,880 and 3,807,218 suggest, special coatings may be applied to the apparatus which decompose to form detectable particles at a lower temperature than the usual organic compounds found in the apparatus. For example, that patent mentions polyalphamethylstyrene, polystyrene, polymethyl methacrylate, and cellulose propionate which decompose to form particles at 230° to 340° C. Unfortunately, since these machines normally operate at about 50° to 100° C, they may be severely damaged by the time the temperature reaches 230° to 340° C. 
     Efforts to identify materials which will decompose to form detectable particles (i.e., thermoparticulate) at temperatures closer to the operating temperature of the machine have met with several difficulties. Many compounds, such as succinic acid, maleic acid, fumaric acid, and polyacrylic acid, do not decompose below 190° C. Others such as acetic acid, are liquids which boil and therefore are unsuitable. Some compounds, such as oxalic acid, decompose at a low temperature but the decomposition products to not include detectable particles. Compounds such as 1, 2-diformylhydrazine have some of the desirable properties but cannot withstand several years operation at 60° to 100° C. A few compounds contain toxic or corrosive substances in their decomposition products which may render them unsuitable. 
     SUMMARY OF THE INVENTION 
     We have found that blocked isocyanates can be used in a composition to form a thermoparticulating coating which produces detectable particles. The coating can be made compatible with the other organic compounds in the apparatus. The coating is very stable and can withstand several years operation at 60° C without decomposing, yet still produce detectable particles when the temperature reaches about 125° to 180° C (depending on the particular blocked isocyanate used). Also, when the coating is heated to about the thermoparticulating temperature of the coating it blisters and becomes a very dark brown color which is a considerable aid in locating the malfunction. 
     DESCRIPTION OF THE INVENTION 
     A composition is prepared of a blocked isocyanate in a solution of a resinous carrier. The blocked isocyanate may be dispersed if it is insoluble in the solvent (e.g., toluene) or it may be in solution if it is soluble in the solvent (e.g., ethyl alcohol or diethyl ether). Dispersions are preferred as they produce much more particulation than do solutions. A particle size of the dispersed blocked isocyanate of about 25 to about 1000 microns is suitable. 
     The composition also preferably contains about 0.1 to about 3 phr of a drier when the resinous carrier is an epoxy resin or similar resin, to promote its room temperature cure. Lead naphthenate or cobalt naphthenate is preferred although stannous octoate, zinc stearate, etc. could also be used. Resins such as polyesters may also require the presence of an organic peroxide as is known in the art. Mixtures of various resins, solvents, or driers are also contemplated. 
     The composition may be prepared by simply mixing the ingredients, but it is preferable to mix the drier, resinous carrier, and solvent first and then add the blocked isocyanate to prevent the occlusion of the drier in the blocked isocyanate and thereby to obtain a more homogeneous dispersion of the blocked isocyanate. 
     A suitable composition is a resinous carrier, about 20 to about 250 phr (parts by weight per hundred parts of resinous carrier not including solvent) of a blocked isocyanate, and about 25 to about 75% (by weight based on the resinous carrier) of a solvent for the resinous carrier. If the amount of blocked isocyanate is less than about 20 phr, the quantity of particles given off during decomposition may be too low to be detected by presently-existing detectors. However, the construction of more sensitive detectors would permit a lower amount of blocked isocyanate. If the amount of blocked isocyanate exceeds about 250 phr, the composition is thick, difficult to apply, and does not bond well. The preferred amount of blocked isocyanate, which generally gives the best results, is about 40 to about 60 phr. If the amount of solvent is less than about 25%, the composition is generally too viscous to apply easily and if the amount of solvent is greater than about 75%, the composition is unnecessarily dilute and the coating may be too thin to produce an adequate number of particles during decomposition, at least while the malfunction is highly localized. Best results are usually obtained with about 45 to about 55% solvent. 
     The thermoparticulating compounds of this invention are blocked isocyanates. The blocked isocyanate must decompose between about 60 and about 200° C and produce particles larger than about 25A in order for the particles to be detected with presently-existing monitors. They are preferably solids at 60° C. 
     Blocked isocyanates may be regarded as the reaction product of an isocyanate and a Lewis base according to the equation 
     
         R.sub.1 - NCO + HR.sub.2 → R.sub.1 - NH - CO - R.sub.2 
    
     in the above equation R 1  - NCO is any isocyanate including aliphatics, aryls, cycloaliphatics, hetrocyclics, alkylaryls, and isocyanate substitutions of any of these. Also included are isocyanate-terminated pre-polymers such as the isocyanate formed when polyoxymethylene glycol is reacted with a diisocyanate such as toluene diisocyanate or hexamethylene diisocyanate. Specific examples of suitable isocyanates include: 
     4,4&#39;-diisocyanato diphenyl 2,2-propane 
     4,4&#39;-diisocyanato diphenyl methane 
     4,4&#39;-diisocyanate benzidine 
     4,4&#39;-diisocyanato diphenyl sulfur 
     4,4&#39;-diisocyanato diphenyl sulfone 
     4,4&#39;-diisocyanato diphenyl ether 
     4,4&#39;-diisocyanato diphenyl 1,1-cyclohexane 
     oxides of methyl-and of bis (meta-isocyanato-phenyl)phosphine 
     diisocyanato 1,5-naphthalene 
     meta-phenylene diisocyanate 
     toluylene diisocyanate 
     dimethyl 3,3&#39;-diisocyanato 4,4&#39;-diphenylene 
     dimethoxy 3,3&#39;-diisocyanato 4,4&#39;-diphenylene 
     meta-xylylene diisocyanate 
     para-xylylene diisocyanate 
     diisocyanato 4,4&#39;-dicyclohexyl methane 
     hexamethylene diisocyanate 
     dodeca-methylene diisocyanate 
     diisocyanato 2,11-dodecane 
     bis (para-phenylene isocyanate oxadiazole-1,3,4) paraphenylene 
     bis (para-phenylene isocyanate) oxadiazole-1,3,4 
     bis (meta-phenylene isocyanate) oxadiazole-1,3,4 
     bis (meta-phenylene isocyanate) 4-phenyl triazole-1,2,4 
     bis (4-paraphenylene isocyanate thiazole 2-yl) metaphenylene 
     (2-phenylene) benzimidazole 5,4&#39;-diisocyanate 
     (2-phenylene) benzoxazole 5,4&#39;-diisocyanate 
     (2-phenylene) benzothiazole 6,4&#39;-diisocyanate 
     bis (2-phenylene isocyanate benzimidazole 6-yl) 2,5-oxadiazole-1,3,4 
     bis (para-phenylene isocyanate 2-benzimidazole 6-yl) 
     bis (para-phenylene isocyanate 2-benzoxazole 6-yl) 
     The preferred isocyanates are phenyl isocyanate, butyl isocyanate, toluene diisocyanate, and hexamethylene diisocyanate because these compounds are readily available and blocked isocyanates can be easily made from them. 
     In the same equation HR 2  is a Lewis base, where &#34;Lewis base&#34; means an electron donor. Oximes, secondary amines, mercaptans, alcohols, esters, carboxyesters, phenols, thiophenols, and glycidyl ethers are suitable types of Lewis bases. Specific examples of suitable Lewis bases include: 
     dimethylamine 
     diethylamine 
     mercaptobenzothiazole 
     phenol 
     nitrosophenol 
     thiophenol 
     4,4&#39;-thiophenol 
     4,4&#39;-sulfonyl diphenol 
     phenol glycidyl ether 
     butyl glycidyl ether 
     α-pyrrolidine 
     1,3-diethylol-5,5-dimethyl hydantoin 
     styrene oxide 
     dicyclophentenyl alcohol 
     dimethyl ketoxime 
     ethylmethyl ketoxime 
     diethyl ketoxime 
     diphenyl ketoxime 
     N(2-hydroxyethyl) piperazine 
     The preferred Lewis bases are the secondary amines, specifically diethylamine and dimethylamine, and the oximes, preferably ethylmethyl ketoxime and diethyl ketoxime, because they react easily with the isocyanate and give good yields. 
     The preferred blocked isocyanates are hexamethylene diisocyanate blocked with dimethylamine, toluene diisocyanate blocked with mercaptobenzothiazole, and toluene diisocyanate blocked with diethylamine, because these blocked isocyanates are stable at 80° C on aging, and thermoparticulate at low temperatures with strong signals. Mixtures of blocked isocyanates are also contemplated. 
     The resinous carrier performs the function of bonding the blocked isocyanate to the apparatus since a coating of blocked isocyanate by itself does not adhere well. The resinous carrier should be compatible with the other resins used in the apparatus and therefore it is usually advantageous to use the same resin used elsewhere. The resinous carrier is curable below 60° C and is preferably air-dryable since it cannot be easily cured in place with heat. Also, it should be stable when cured for several years at 60° C. The resin must be unreactive with the blocked isocyanate for otherwise suitable thermoparticulation will not occur. The blocked isocyanate and the resin form a mixture and the blocked isocyanate does not catalyze the cure of the resin. Epoxy resins are preferred as they are usually used elsewhere in the apparatus, but polyester, silicone rubber, styrene, etc. could also be used. 
     The solvent for the resinous carrier depends on the particular resinous carrier used. Toluene, xylene, benzene, methyl ethyl ketone, ethyl alcohol, diethyl ether, acetone, cellosolve, etc., are common solvents that may be used. Toluene is preferred as it is inexpensive and dissolves most resins. 
     The composition is applied to portions of the electrical apparatus which are exposed to the gas stream. The coating formed does not function as insulation and is usually applied on top of insulation, but it can also be applied to conductors. The application may be made by painting, spraying, dipping, grease gun, or other techniques. A suitable coating thickness (after drying) is about 1/16 to about 1/2 inch. The dispersed particles of blocked isocyanate should not be covered with excessive resinous carrier as that may prevent the decomposition particles from escaping into the gas stream. After evaporation of the solvent and room temperature cure of the resinous carrier, if necessary, the apparatus is ready to be operated. 
     When thermoparticulation and the resulting alarm occur, a sample of the gas stream can be collected and analyzed. Since different thermoparticulating compounds can be used in different areas of the apparatus and their thermoparticulation products are different, analysis of the sample can pinpoint the location of the overheating. 
    
    
     The following examples further illustrate this invention. 
     EXAMPLE 1 
     The following composition was prepared using various blocked isocyanates: 
     
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                        Parts                                             
                        by                                                
                        Weight                                            
______________________________________                                    
blocked isocyanate        100                                             
epoxy resin (50% solids in toluene) made                                  
                          100                                             
 from 200 phr (parts by weight) linseed                                   
 fatty acids, 200 phr styrene, and 300                                    
 phr diglycidyl ether of Bisphenol A,                                     
 sold by Westinghouse Electric                                            
 Corporation as &#34;B-276&#34; Varnish                                           
 (See Example I of U.S. Patent                                            
 2,909,497 for detailed descrip-                                          
 tion)                                                                    
6% solution in low boiling hydro-                                         
 carbons of cobalt naphthenate                                            
24% solution in line boiling                                              
 hydrocarbons of lead naphthenate                                         
                          0.25                                            
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     The cobalt and lead naphthenate solutions were added to the epoxy resin prior to the addition of the blocked isocyanate. 
     Samples were prepared by brushing the above composition onto 3 inch by 1 inch aluminum and copper sheets 1/16 to 1/4 inches thick. The samples were dried to form coatings about 1/4 inches thick, then placed in an oven at 60° or 80° C for various periods to determine if they were stable and would function after aging. 
     The samples were placed one at a time in a stainless steel boat within a 1 inch stainless steel tube. Hydrogen was passed over the samples at a flow rate of 7 l/min. A phase-controlled temperature regulator and programmer controlled the temperature in the boat. The temperature in the boat was measured by mounting a hot junction chromel-alumel thermocouple within a small hole in the boat. The output of the thermocouple and the detector were monitored on a two-pen potentiostatic recorder. A 5° C/min. heating rate was maintained in each experiment after the insertion of the sample in the boat. The threshold temperature at which considerable particulation occurred was taken from the chart produced by the recorder. The &#34;alarm&#34; temperature at which considerable particulation occurred corresponded to a 50% decrease in the initial ion current of the detector (usually 0.8 to 0.4 mA). The occurrence of particulation was detected using a Generator Condition Monitor or a Condensation Nuclei Monitor. Both instruments are sold by Environment One Corporation. 
     The following table gives the results: 
     
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BLOCKED ISOCYANATE                                                        
Isocyanate Lewis Base                                                     
                    Melting                                               
                          Aging    Thermoparticulation                    
Moiety     Moiety   Point (° C)                                    
                          Conditions                                      
                                   Temperature Range (° C)         
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Hexamethylene Diisocyanate                                                
           Dimethylamine                                                  
                    166-170                                               
                           ##STR1##                                       
                                    ##STR2##                              
Toluene Diisocyanate                                                      
           Mercaptobenzo- thiazole                                        
                    153-161                                               
                           ##STR3##                                       
                                    ##STR4##                              
Toleuen Diisocyanate                                                      
           Diethylamine                                                   
                    90-95                                                 
                           ##STR5##                                       
                                    ##STR6##                              
Phenyl     1,3-diethylol-5,5-                                             
                    Visious                                               
                          8 days at 60 20 C                               
                                   189-195                                
Isocyanate dimethyl hydantoin.sub.(1)                                     
                    Liquid                                                
P,P&#39;-Diphenylmethane                                                      
           Nitrosophenol                                                  
                    179   8 days at 60°C                           
                                   &gt;190°C                          
Diisocyanate                                                              
&#34;Mondur S&#34;.sup.(2)                                                        
           Phenol   125-130                                               
                          7 days at 60°C                           
                                   190-196                                
&#34;Mondur SH&#34;.sup. (3)                                                      
           Phenol   161-166                                               
                          7 days at 60°C                           
                                   &gt;190                                   
Hexamethylene                                                             
           α-Pyrrolidone                                            
                    94-96 7 days at 60°C                           
                                   &gt;190                                   
Diisocyanate                                                              
Hexamethylene                                                             
           Phenol   128-131                                               
                          42 days at 60°C                          
                                   179-189                                
Diisocyanate                                                              
Toluene    Phenol   153-156                                               
                          42 days at 60°C                          
                                   170-177                                
Diisocyanate                                                              
P, P&#39;-Diphenylmethane                                                     
           Phenol   192-194                                               
                          42 days at 60°C                          
                                   190-194                                
Diisocyanate                                                              
Phenyl     Phenyl glycidyl                                                
                          overnight at 60°C                        
                                   167                                    
Isocyanate ether                                                          
Phenyl     Styrene oxide  overnight at 60°C                        
                                   172                                    
Isocyanate                                                                
Phenyl     Butyl glycidyl overnight at 60°C                        
                                   172                                    
Isocyanate ether                                                          
Hexamethylene                                                             
           Thiophenol     overnight at 60°C                        
                                   143                                    
Diisocyanate                                                              
Phenyl     Ethylmethyl    overnight at 60°C                        
                                   169-173                                
Isocyanate ketoxime                                                       
Phenyl     N(2-hydroxyethyl)                                              
                          overnight at 60°C                        
                                   180-185 (4)                            
Isocyanate piperazine                                                     
Phenyl     Dicyclopentenyl                                                
                          overnight at 60°C                        
                                   168                                    
Isocyanate Alcohol                                                        
Butyl      4,4&#39;-thiophenyl                                                
                          overnight at 60°C                        
                                   175                                    
Isocyanate                                                                
Butyl      4,4&#39;-sulfonyl  overnight at 60°C                        
                                   181                                    
Isocyanate diphenol                                                       
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(1)                                                                       
   Sold by Glyco Chemicals, Inc. under the trademark &#34;Dantocol DHE&#34;       
(2)                                                                       
   &#34;Mondur S&#34; is sold by the Mobay Chemical Co. and has the structure     
    ##STR7##                                                              
(3)                                                                       
   &#34;Mondur SH&#34; is sold by the Mobay Chemical Co. and has the structure    
    ##STR8##                                                              
(4)                                                                       
   Signal very strong                                                     
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