Azeotrope-like compositions of trichlorotrifluoroethane, ethanol, nitromethane and 2-methylpentane or a mixture of hexanes
DESCRIPTION 
     1. Field of the Invention 
     This invention relates to azeotrope-like mixtures of trichlorotrifluoroethane, ethanol, nitromethane and 2-methylpentane or a mixture of hexanes. These mixtures are useful in a variety of vapor degreasing or solvent cleaning applications including defluxing. 
     2. Background of the Invention 
     Vapor degreasing and solvent cleaning with fluorocarbon based solvents have found widespread use in industry for the degreasing and otherwise cleaning of solid surfaces, especially intricate parts and difficult to remove soils. 
     In its simplest form, vapor degreasing or solvent cleaning consists of exposing a room-temperature object to be cleaned to the vapors of a boiling solvent. Vapors condensing on the object provide clean distilled solvent to wash away grease or other contamination. Final evaporation of solvent from the object leaves behind no residue as would be the case where the object is simply washed in liquid solvent. 
     For difficult to remove soils where elevated temperature is necessary to improve the cleaning action of the solvent, or for large volume assembly line operations where the cleaning of metal parts and assemblies must be done efficiently and quickly, the conventional operation of a vapor degreaser consists of immersing the part to be cleaned in a sump of boiling solvent which removes the bulk of the soil, thereafter immersing the part in a sump containing freshly distilled solvent near room temperature, and finally exposing the part to solvent vapors over the boiling sump which condense on the cleaned part. In addition, the part can also be sprayed with distilled solvent before final rinsing. 
     Vapor degreasers suitable in the above-described operations are well known in the art. For example, Sherliker et al. in U.S. Pat. No. 3,085,918 disclose such suitable vapor degreasers comprising a boiling sump, a clean sump, a water separator, and other ancillary equipment. 
     Fluorocarbon solvents, such as trichlorotrifluoroethane, have attained widespread use in recent years as effective, nontoxic, and nonflammable agents useful in degreasing applications. Trichlorotrifluoroethane in particular has been found to have satisfactory solvent power for greases, oils, waxes and the like. It has therefore found widespread use for cleaning electric motors, compressors, heavy metal parts, delicate precision metal parts, printed circuit boards, gyroscopes, guidance systems, aerospace and missile hardware, aluminum parts and the like. For certain solvent purposes, however, trichlorotrifluoroethane alone may have insufficient solvent power. Since trichlorotrifluoroethane is non-polar, it does not remove polar contaminants well. Thus, to overcome this deficiency, trichlorotrifluorethane has been mixed with polar components such as aliphatic alcohols or chlorocarbons such as methylene chloride. As example, U.S. Pat. No. 3,881,949 discloses the use of mixtures of 1,1,2-trichloro-1,2,2-trifluoroethane and ethanol as solvents for vapor degreasers. 
     The art has looked, in particular, towards azeotropic compositions including the desired fluorocarbon components, such as trichlorofluoroethane, which include components which contribute additionally desired characteristics, such as polar functionality, increased solvency power, and stabilizers. Azeotropic compositions are desired because they exhibit a minimum boiling point and do not fractionate upon boiling. This is desirable because in the previously described vapor degreasing equipment with which these solvents are employed, redistilled material is generated for final rinse-cleaning. Thus, the vapor degreasing system acts as a still. Unless the solvent composition exhibits a constant boiling point, i.e., is an azeotrope or is azeotrope-like, fractionation will occur and undesirable solvent distribution may act to upset the cleaning and safety of processing. Preferential evaporation of the more volatile components of the solvent mixtures, which would be the case if they were not azeotrope or azeotrope-like, would result in mixtures with changed compositions which may have less desirable properties, such as lower solvency towards soils, less inertness towards metal, plastic or elastomer components, and increased flammability and toxicity. 
     A number of trichlorotrifluoroethane based azeotrope compositions have been discovered which have been tested and in some cases employed as solvents for miscellaneous vapor degreasing applications. For example, U.S. Pat. No. 3,573,213 discloses the azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane and nitromethane; U.S. Pat. No. 3,903,009 discloses the ternary azeotrope of 1,1,2-trichloro-1,2,2-trifluorethane, nitromethane and ethanol; U.S. Pat. No. 3,789,006 discloses the ternary azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane, nitromethane and isopropanol; U.S. Pat. No. 3,728,268 discloses the ternary azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane, acetone, and ethanol; Japanese Pat. Nos. 81-34,799 and 81-34,798 disclose azeotrope-like mixtures of 1,1,2-trichloro-1,2,2-trifluoroethane, ethanol, nitromethane and 3-methylpentane or 2,2-dimethylbutane or 2,3-dimethylbutane and Japanese Pat. No. 81,109,298 discloses azeotrope-like mixtures of 1,1,2-trichloro-1,2,2-trifluoroethane, ethanol, n-hexane and nitromethane. 
     The art is continually seeking new fluorocarbon based azeotropic mixtures or azeotrope-like mixtures which offer alternatives for new and special applications for vapor degreasing and other cleaning applications. 
     It is accordingly an object of this invention to provide novel azeotrope-like compositions based on 1,1,2-trichloro-1,2,2-trifluoroethane which have good solvency power and other desirable properties for vapor degreasing and other solvent cleaning applications. 
     Another object of the invention is to provide novel constant boiling or essentially constant boiling solvents which are liquid at room temperature, will not fractionate under conditions of use and also have the foregoing advantages. 
     A further object is to provide azeotrope-like compositions which are relatively nontoxic and nonflammable both in the liquid phase and the vapor phase. 
     These and other objects and features of the invention will become more evident from the description which follows. 
     DESCRIPTION OF THE INVENTION 
     In accordance with the invention, novel azeotrope-like compositions have been discovered comprising trichlorotrifluoroethane, ethanol, nitromethane and 2-methylpentane or a mixture of hexanes, with 1,1,2-trichloro-1,2,2-trifluoroethane being the trichlorotrifluoroethane of choice. In a preferred embodiment of the invention, the azeotrope-like compositions comprise from about 87.3 to about 93.7 weight percent of 1,1,2-trichloro-1,2,2-trifluoroethane, from about 2.2 to about 4.9 weight percent of ethanol, from about 0.8 to about 1.9 weight percent of nitromethane and from about 0.2 to about 7.5 weight percent of 2-methylpentane or a mixture of hexanes. In the preferred embodiment of the invention, the azeotrope-like compositions comprise from about 89.2 to about 90.5 weight percent of 1,1,2-trichloro-1,2,2-trifluoroethane, from about 3.0 to about 3.8 weight percent of ethanol, from about 0.8 to about 1.5 weight percent of nitromethane and from about 5.0 to about 6.0 weight percent of 2-methylpentane or a mixture of hexanes. Such compositions possess constant or essentially constant boiling points of about 44.6° C. at 760 mm Hg. All compositions within the indicated ranges, as well as certain compositions outside the indicated ranges, are azeotrope-like, as defined more particularly below. 
     It has been found that these azeotrope-like compositions are stable, safe to use and that the preferred compositions of the invention are nonflammable (exhibit no flash point when tested by the Tag Open Cup test method--ASTM D1 310-16) and exhibit solvency power. These compositions have been found to be particularly effective when employed in conventional degreasing units for the dissolution of lubricating and machine cutting oils and the cleaning of such oils from solid surfaces. 
     For the purpose of this discussion, by azeotrope-like composition is intended to mean that the composition behaves like a true azeotrope in terms of its constant boiling characteristics or tendency not to fractionate upon boiling or evaporation. Such composition may or may not be a true azeotrope. Thus, in such compositions, the composition of the vapor formed during boiling or evaporation is identical or substantially identical to the original liquid composition. Hence, during boiling or evaporation, the liquid composition, if it changes at all, changes only to a minimal or negligible extent. This is to be contrasted to non-azeotrope-like compositions in which during boiling or evaporation, the liquid composition changes to a substantial degree. 
     As is well known in this art, another characteristic of azeotrope-like compositions is that there is a range of compositions containing the same components in varying proportions which are azeotrope-like. All such compositions are intended to be covered by the term azeotrope-like as used herein. As an example, it is well known that at differing pressures, the composition of a given azeotrope will vary at least slightly and changes in distillation pressures also change, at least slightly, the distillation temperatures. Thus, an azeotrope of A and B represents a unique type of relationship but with a variable composition depending on temperature and/or pressure. 
     The 1,1,2-trichloro-1,2,2-trifluoroethane, ethanol, nitromethane, and hexane components of the novel solvent azeotrope-like compositions of the invention are all commercially available. Preferably they should be used in sufficiently high purity so as to avoid the introduction of adverse influences upon the solvency properties or constant boiling properties of the system. 
     A suitable grade of 1,1,2-trichloro-1,2,2-trifluoroethane, for example, is sold by Allied Corporation under the trade name &#34;GENESOLV® D&#34;. 
     The term &#34;hexane&#34; is used herein as to mean any C 6  paraffin hydrocarbon (C 6  H 14 ) (see Hackh&#39;s Chemical Dictionary, 3 rd  Ed., McGraw Hill Book Co. (1944) p. 408). Thus, the term 37 hexane&#34; includes n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane and any and all mixtures thereof. Specifically included is &#34;commercial isohexane&#34; which typically contains at least about 35 weight percent of 2-methylpentane admixed with other hexane isomers. It has been found that 2-methylpentane and mixtures of the hexane isomers, form azeotrope-like compositions with 1,1,2-trichloro-1,2,2-trifluoroethane, ethanol, and nitromethane in accordance with the invention. 
    
    
     EXAMPLE 1 
     The azeotrope-like compositions of the invention were determined through the use of distillation techniques designed to provide higher rectification of the distillate than found in the most demanding vapor degreaser systems. For this purpose a five theoretical plate Oldershaw distillation column was used with a cold water condensed, manual liquid dividing head. Typically, approximately 350 cc of liquid were charged to the distillation pot. The liquid was a mixture comprised of various combinations of 1,1,2-trichloro-1,2,2-trifluoroethane, ethanol, nitromethane and hexanes. 
     The mixture was heated at total reflux for about one hour to ensure equilibration. For most of the runs, the distillate was obtained using a 2:1 reflux ratio at a boil-up rate of 400-500 grams per hr. Approximately 300 cc of product were distilled and 6 approximately equivalent sized overhead cuts were collected. The vapor temperature (of the distillate), pot temperature, and barometric pressure were monitored. A constant boiling fraction was collected and analyzed by gas chromatography to determine the weight percentages of its components. A mixture was then made up according to the approximate compositions of the constant boiling fraction and was redistilled at the same conditions. Compositions of distillate and residue were compared by chromatographic analysis to verify the constant-boiling nature of the mixture. The constant boiling mixture obtained according to the present invention through the above described distillation techniques is shown in Table I. 
     
                       TABLE I
______________________________________
                Approx.   Baro-
                Composi-  metric Vapor Azeotrope-
                tion      Pressure
                                 Temp  like
Ex.  Components (wt %)    (mm Hg)
                                 (°C.)
                                       Behavior
______________________________________
1    1,1,2-trichloro-
                89.7-90.2 744    44.0  Yes -
     1,2,2-trifluoro                   Constant
     ethane                            Boiling
     Ethanol    3.0-3.8
     Isohexane  2.4-2.8
     2,3-Dimethylbutane
                2.4-3.0
     Nitromethane
                1.3-1.4
______________________________________
 
    
     EXAMPLES 2-5 
     To explore the constant-boiling composition range of mixtures comprised of 1,1,2-trichloro-1,2,2-trifluoroethane, ethanol, nitromethane, and hexanes, a distillation apparatus and procedure were utilized as previously described in Example 1. Into the distillation pot was charged a mixture of 1,1,2-trichloro-1,2,2-trifluoroethane (FC-113), ethanol, nitromethane, and hexane isomers. 
     These examples demonstrate that each hexane isomer exhibits its own unique compositional identity in azeotrope-like mixtures with 1,1,2-trichloro-1,2,2-trifluoroethane, ethanol, and nitromethane and that each hexane isomer and mixtures thereof form azeotrope-like constant boiling mixtures at about 44.6°±0.5° C. with such components. This was particularly surprising in view of the significant variation in boiling point among the various hexane isomers. The hexane isomers and their boiling points are shown in the following Table II. 
     
                       TABLE II
______________________________________
Hexane Isomer     Normal Boiling Point
______________________________________
2,2-dimethylbutane
                  49.75
2,3-dimethylbutane
                  58.1
2-methylpentane (isohexane)
                  60.13
3-methylpentane   64
n-hexane          68.74
______________________________________
 
    
     A number of distillations were undertaken where the composition of the starting mixture was varied considerably, resultant constant-boiling fractions were collected and analyzed by gas chromotography, and the vapor temperature and barometric pressure were recorded. To normalize observed boiling points during different days to 760 mm of mercury pressure, the approximate normal boiling points of 1,1,2-trichloro-1,2,2-trifluoroethane rich mixtures were estimated by applying a barometric correction factor of about 26 mm Hg/°C., to the observed values. However, it is to be noted that this corrected boiling point is generally accurate up to ±0.4° C. and serves only as a rough comparison of boiling points determined on different days. By the above-described method, it was discovered that a constant boiling mixture boiling at about 44.6°±0.5° C. at 760 mm Hg was formed for compositions comprising 87.3 to 90.8 weight percent 1,1,2-trichloro-1,2,2-trifluoroethane, 3.6 to 4.0 weight percent ethanol, 0.8 to 1.6 weight percent nitromethane, and 4.4 to 7.5 weight percent hexanes. Supporting distillation data for the mixtures studied are shown in Table III. 
     
                                           TABLE III
__________________________________________________________________________
Starting Material Compositions (wt %)
              Nitro-         2,3-
                                 2,2-     Total
Example
     FC-113
          EtOH
              methane
                    2-MP
                        3-MP DMB DMB  n-hex
                                          hexane
__________________________________________________________________________
1    87.9 5.1 0.5   6.5                   6.5
2    87.4 2.8 1.0   5.3      1.2 1.2  1.1 8.8
3    90.3 2.2 1.5                5.8      5.8
4    86.7 4.0 1.0   4.0      4.1          8.1
__________________________________________________________________________
Constant Boiling Distillation Fraction (wt %)
                                        B.P. Corr
                        2,3 2,2     Total
                                        to approx
Example
     FC-113
         EtOH
             NM 2-MP
                    3-MP
                        DMB DMB n-hex
                                    hexane
                                        760 mm (°C.)
__________________________________________________________________________
1    90.8
         4.0 0.8
                4.4                 4.4 44.8
2    88.2
         3.8 1.1
                3.4     0.9 1.5 0.4 6.2 45.0
3    87.3
          3.6
             1.6            7.5     7.5 43.8
4    89.0
         3.8 1.3
                2.7     3.2         5.9 44.6
__________________________________________________________________________
                          Physical Properties
                          Vapor Temp
                                 Bar. Pressure
                                        B.P. Corr to
                     Example
                          (°C.)
                                 (mm Hg)
                                        760 mm (°C.)
__________________________________________________________________________
                     1    44.4   748.6  44.8
                     2    44.3   742.1  45.0
                     3    43.4   749.1  43.8
                     4    44.0   743.9  44.6
                                 mean   44.6
                                        ±0.5° C.
__________________________________________________________________________
 
    
     From the above examples, it is readily apparent that additional constant boiling or essentially constant boiling mixtures of the same components can readily be identified by anyone of ordinary skill in this art by the method described. No attempt was made to fully characterize and define the true azeotrope in the system comprising 1,1,2-trichloro-1,2,2-trifluoroethane, ethanol, nitromethane and hexane isomers, nor the outer limits of its compositional of ranges which are constant boiling or essentially constant boiling. As indicated, anyone of ordinary skill in the art can readily ascertain other constant boiling or essentially constant boiling mixtures, it being kept in mind that &#34;constant boiling&#34; or &#34;essentially constant boiling&#34; for the purposes of this invention means constant boiling or essentially constant boiling in the environment of a vapor degreaser system such as utilized in the art. All such mixtures in accordance with the invention which are constant boiling or essentially constant boiling are &#34;azeotrope-like&#34; within the meaning of this invention. 
     EXAMPLE 6 
     To illustrate the azeotrope-like nature of the mixtures of the invention under conditions of actual use in a vapor phase degreasing operation, a vapor phase degreasing machine was charged with a preferred azeotrope-like mixture in accordance with the invention comprising about 89.6 weight percent 1,1,2-trichloro-1,2,2-trifluoroethane (FC-113), about 3.6 weight percent ethanol, about 2.9 weight percent of 2,3-dimethylbutane, about 2.5 weight percent of 2-methylpentane and about 1.4 weight percent nitromethane. The mixture was evaluated for its constant boiling or non-segregating characteristics. Solvents were tested in a Baron Blakeslee refrigeration cooled 3 sump VPD (Series 5000 machine--Model No. MLR-216). The solvent charge was brought to reflux and the individual sump conditions were determined with a Hewlett Packard 5890 Gas Chromotograph. Refluxing was continued for 48 hrs and sump compositions were monitored throughout this time. A mixture was considered constant boiling or non-segregating if the maximum concentration difference between sumps for any mixture component was less than 0.3%. 
     If the mixture were not azeotrope-like, the high boiling components would very quickly concentrate in the boil sump and be depleted in the rinse sump. As the data in Table IV show, this did not happen. These results indicate that the compositions of this invention will not segregate in a commercial vapor degreaser, thereby avoiding potential safety, performance, and handling problems. The preferred composition tested was also found to not have a flash point according to recommended procedures ASTM D-56 (Tag Closed Cup) and ASTM D-1310 (Tag Open Cup). 
     
                       TABLE IV
______________________________________
COMPOSITION, % WEIGHT
           0.sup.(a) hr
                4 hr       44 hr  71 hr
______________________________________
Boil Sump
Ethanol      3.57   3.44       3.30 3.31
Nitromethane 1.42   1.36       1.35 1.34
FC-113       89.56  89.22      89.37
                                    89.29
2,3 Dimethyl-
             2.92   3.13       3.14 3.17
butane
Isohexane    2.54   2.67       2.86 2.89
(99%)
Work Sump
Ethanol      3.59   3.68       3.51 3.73
Nitromethane 1.42   1.44       1.42 1.43
FC-113       89.97  89.58      89.64
                                    89.48
2,3 Dimethyl-
             2.91   2.87       2.91 2.89
butane
Isohexane    2.53   2.46       2.52 2.49
(99%)
Rinse Sump
Ethanol      3.46   3.61       3.70 3.63
Nitromethane 1.42   1.45       1.45 1.44
FC-113       89.67  89.70      89.64
                                    89.72
2,3 Dimethyl-
             2.93   2.85       2.83 2.82
butane
Isohexane    2.53   2.41       2.40 2.39
(99%)
______________________________________
 .sup.(a) Analytical Standard  representative of initial composition of al
 three sumps
 
    
     EXAMPLE 7 
     This example illustrates the use of the preferred azeotrope-like composition of the invention to clean metal parts. 
     Cleaning was performed in a Branson B-400 two-sump vapor degreaser. A first sump was used as the working sump and held boiling solvent comprising about 89.7 weight percent 1,1,2-trichloro-1,2,2-trifluoroethane, about 3.7 weight percent ethanol, about 2.4 weight percent 2-methyl pentane, 2.8 weight percent of 2,3-dimethylbutane, and about 1.4 weight percent nitromethane. A second sump was used as the rinse sump. Refrigerated cooling coils lined the upper inner wall of the apparatus to maintain a vapor blanket. Soils were coated on two kinds of 3/4&#34;×3&#34; metal coupons. These were 316 stainless steel and 2024 aluminum. Soils were selected from two classes of metal working fluids as follows: 
     
         ______________________________________
Name       Manufacturer     Class
______________________________________
Hocut 711  E. F. Houghton &amp; Co.
                            Semi-synthetic
951        Van Straaten Chem. Co
                            Synthetic
______________________________________
 
    
     The metal coupons were sanded to give a totally clean, freshly exposed surface. Following a deionized water rinse, the coupons were rinsed in followed by methanol and air dried for 10 minutes. Four identical coupons were then dipped into each of the metal working fluids. Cleaning tests were run on two of these coupons shortly after dipping into the metal working fluids. The other two coupons were tested after standing for 24 hours. For cleaning, the parts were placed on racks in a stainless steel wire mesh basket. In a first step, this assembly was immersed in the work sump for two minutes, then transferred to the rinse sump for two minutes, followed by a two minute solvent distillate spray in the vapor zone. The final step was a one minute hold in the vapor zone. 
     The treated coupons were visually inspected for evidence of soil residue. A water-break test was also applied wherein the coupons were immersed in water and allowed to drain for 10 seconds. The coupon surface was examined for breaks in the water film over the 10 second draining period. A coupon was considered totally clean if no residues or breaks in the water film during the water break test were noticeable on the surface of the coupon. In the above-described manner, &#34;316&#34; stainless steel coupons were soiled with Hocut 711 metal working fluid, and &#34;2024&#34; aluminum coupons were soiled with 951 metal working fluid. All these soiled coupons were cleaned with the preferred azeotrope-like compositions of the invention and evaluated for cleanliness as described above. All the coupons were judged to be totally clean.