Cleaning composition based on 1,1-dichloro-1-fluoroethane, methyl formate and methanol
FIELD OF THE INVENTION 
     The present invention relates to the area of chlorofluorinated hydrocarbons and more particularly to a novel composition exhibiting an azeotrope which can be used as a cleaning and degreasing agent for solid surfaces, in particular for removing flux and low-temperature cleaning of printed circuits. 
     BACKGROUND OF THE INVENTION 
     1,1,2-Trichloro-1,2,2-trifluoroethane (known in the art under the name F113) is widely used in industry for cleaning and degreasing solid surfaces. Apart from its application in electronics for cleaning soldering fluxes to remove any flux still adhering to the printed circuits, its application in degreasing heavy metal parts and for cleaning high-quality, high-precision mechanical parts, such as, for example, gyroscopes and military or aerospace equipment may be mentioned. In its various applications, F113 is most often combined with other organic solvents (for example methanol), preferably in the form of azeotropic or pseudoazeotropic mixtures which do not separate and when being refluxed have essentially the same composition in the vapor phase as in the liquid phase. 
     However, F113 is one of the completely halogenated chlorofluorocarbons which are currently suspected of attacking and decomposing stratospheric ozone. 
     DESCRIPTION OF THE INVENTION 
     As a contribution to solving this problem, the present invention proposes to replace the compositions based on F113 by a novel composition based on methyl formate, methanol and 1,1-dichloro-I-fluoroethane. The latter compound, known in the art under the name F141b, is virtually devoid of any destructive effect with respect to ozone. 
     The composition to be used according to the invention comprises 55 to 79.5% by weight of F141b, 20 to 40% of methyl formate and 0.5 to 5% of methanol. This range gives rise to an azeotrope whose boiling temperature is 28.3° C. at standard atmospheric pressure (1.013 bar), while the composition according to the invention has pseudoazeotropic behavior, i.e. the composition of the vapor phase and liquid phase is essentially the same, which is particularly advantageous for the intended applications. Preferably, the F141b content is chosen from between 65 and 75% by weight, that of methyl formate from between 24 and 31.5% by weight, and that of methanol from between 1 and 3.5% by weight. 
     The F141b/methyl formate/methanol azeotrope is a positive azeotrope, since its boiling point (28.3° C.) is below that of the constituents (F141b : 32° C.; methyl formate : 31.7° C.; methanol : 65° C.). 
     Similarly to the known compositions based on F113, the composition according to the invention can be advantageously stabilized against hydrolysis and/or attack by free radicals, which are likely to occur during the cleaning process, by adding a conventional stabilizer, such as, for example, nitromethane, propylene oxide or a mixture of the compounds, the proportion of the stabilizer ranging from 0.01 to 5%, relative to the total weight of F141b +methyl formate + methanol. 
     The composition according to the invention can be used for the same applications and using the same techniques as the former compositions based on F113. 
    
    
     EXAMPLES The examples which follow illustrate the invention without limiting it. 
     EXAMPLE 1 : DETECTION OF THE AZEOTROPE 
     90g of methyl formate, 60g of methanol and 150g of F141b are introduced in the bottom of a distillation column (30 plates). The mixture is then refluxed for one hour to bring the system to equilibrium. After reaching a steady temperature (28.3° C.), a fraction (about 50g) is removed and analyzed by gas-phase chromatography. 
     The test results shown in the table below indicate the presence of an F141b/methyl formate/methanol azeotrope. 
     
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         COMPOSITION (% by weight)                                        
         F141b     HCOOCH.sub.3                                           
                             Methanol                                     
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Initial mixture                                                           
           50          30        20                                       
Fraction removed                                                          
           69.8        28.1      2.1                                      
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     EXAMPLE 2 : VERIFICATION OF THE AZEOTROPIC COMPOSITION 
     200g of a mixture comprising 70% by weight of F141b, 7.5% by weight of methyl formate and 2.5% by weight of methanol are introduced into the boiler of an adiabatic distillation column (30 plates). The mixture is then refluxed for one hour to bring the system to equilibrium, and a fraction of about 50g is then removed and it is then analyzed by gas-phase chromatography. The results listed in the table below show the presence of a positive azeotrope, since its boiling point is below that of the pure constituents: F141b, methyl formate, and methanol. 
     
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         COMPOSITION (% by weight)                                        
         F141b     HCOOCH.sub.3                                           
                             Methanol                                     
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Initial mixture                                                           
           70          27.5      2.5                                      
Fraction collected                                                        
           69.8        28.1      2.1                                      
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 Boiling temperature corrected for 1.013 bar: 28.3° C.             
 
    
     When employed for cleaning soldering flux or degreasing mechanical parts, this azeotrope gives results which are as good as those of the compositions based on F113 and methanol. 
     EXAMPLE 3: COMPOSITION STABILIZED WITH NITROMETHANE 
     150g of a mixture containing 69.7% by weight of F141b, of methyl formate, 2.1% of methanol and 0.1% of nitromethane as stabilizer is introduced into an ultrasound cleaning bath. After the system has been refluxed for one hour, one aliquot of the vapor phase is removed. Its analysis by gas-phase chromatography shows the presence of nitromethane, which indicates that the mixture is stabilized in the vapor phase. 
     
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         COMPOSITION (% by weight)                                        
         F141b HCOOCH.sub.3                                               
                          Methanol CH.sub.3 NO.sub.2                      
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Initial mixture                                                           
           69.7    28.1       2.1    0.1                                  
Vapor phase                                                               
           69.79   28.1       2.1    0.01                                 
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     EXAMPLE 4: COMPOSITION STABILIZED WITH PROPYLENE OXIDE 
     Example 3 is repeated, replacing the nitromethane by propylene oxide, to give the following results: 
     
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         COMPOSITION (% by weight)                                        
         F141b HCOOCH.sub.3                                               
                          Methanol C.sub.3 H.sub.6 O                      
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Initial mixture                                                           
           69.7    28.1       2.1    0.1                                  
Vapor phase                                                               
           69.73   28.1       2.1    0.07                                 
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     EXAMPLE 5: BISTABILIZED COMPOSITION 
     Example 3 is repeated, using 0.1% of nitromethane and 0.1% of propylene oxide, to give the following results: 
     
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       COMPOSITION (% by weight)                                          
       F141b HCOOCH.sub.3                                                 
                       Methanol CH.sub.3 NO.sub.2                         
                                       C.sub.3 H.sub.6 O                  
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Initial  69.7    28        2.1    0.1    0.1                              
mixture                                                                   
Vapor phase                                                               
         69.73   28.1      2.1    0.01   0.06                             
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     EXAMPLE 6: CLEANING-OFF OF SOLDERING FLUX 
     200g of the azeotropic F141b/methyl formate/methanol composition are introduced into an Annemasse ultrasonic bath, and the mixture is then brought to the boiling temperature. 
     Glass plates which are coated with soldering flux and have been heated in an oven at 220° C. for 30 seconds are immersed in the boiling ultrasonic liquid for 3 minutes and then rinsed in the vapor phase for 3 minutes. 
     After drying in air, inspection using low-angle illumination reveals a complete absence of any residual soldering flux. Thus, the same result was obtained as when using an F113/methanol (93.7%/6.3%) composition. 
     Although the invention has been described in conjunction with specific embodiments, it is evident that many alternatives and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, the invention is intended to embrace all of the alternatives and variations that fall within the spirit and scope of the appended claims.