Azeotrope-Like Compositions of 1,1-Dichloro-1-Fluoroethane, Dichlorotrifluoroethane and Methanol

This invention relates to azeotropic-like mixtures of 1,1-dichloro-1-fluoroethane, dichlorotrifluoroethane and methanol. These mixtures are useful in a variety of vapor degreasing, cold cleaning and solvent cleaning applications including defluxing.

Co-pending, commonly assigned application Serial No. 189,932, filed 05/03/88, discloses azeotrope-like mixtures of 1,1-dichloro-1-fluoroethane and methanol.

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.

Cold cleaning is another application where a number of solvents are used. In most cold cleaning applications the soiled part is either immersed in the fluid or wiped with rags or similar objects soaked in solvents and allowed to air dry.

Fluorocarbon solvents, such as trichlorotrifluoroethane, have attained widespread use in recent years as effective, nontoxic, and nonflammable agents useful in degreasing applications and other solvent cleaning applications. Trichlorotrifluoroethane 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 hardwater, aluminum parts and the like.

The art has looked towards azeotropic compositions including the desired fluorocarbon components such as trichlorotrifluoroethane which include components which contribute additionally desired characteristics, such as polar functionality, increased solvency power, and stabilizers. Azeotropic compositions are desired because they do not fractionate upon boiling. This behavior 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 an 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.

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. Currently, of particular interest, are such azeotrope-like mixtures which are based on fluorocarbons which are considered to be stratospherically safe substitutes for presently used fully halogenated chlorofluorocarbons. The latter are suspected of causing environmental problems in connection with the earth's protective ozone layer. Mathematical models have substantiated that hydrochlorofluorocarbons, such as 1,1-dichloro-1-fluoroethane (HCFC-141b) and dichlorofluoroethane (HCFC-123 or HCFC-123a), will not adversely affect atmospheric chemistry, being negligible contributors to ozone depletion and to green-house global warming in comparison to the fully halogenated species.

U.S. Pat. No. 3,936,387 discloses the azeotropic composition of methanol with 1,2-dichloro-1-fluoroethane (HCFC-141) U.S. Pat. No. 4,035,258 discloses the azeotropic composition of ethanol with HCFC-141.

It is an object of this invention to provide novel azeotrope-like compositions based on HCFC-141b and dichlorotrifluoroethane which are liquid at room temperature and which will not fractionate under the process of distillation or evaporation, which are useful as solvents for use in vapor degreasing and other solvent cleaning applications including defluxing applications.

Another object of the invention is to provide novel environmentally acceptable solvents for use in the aforementioned applications.

Other objects and advantages of the invention will become apparent from the following description.

FIG. 1 is a plot of the data shown in Table I which demonstrate that a minimum boiling ternary azeotropic composition is formed. The indicated HCFC-141b/HCFC-123 ratio is by weight percent.

FIG. 2 is a plot of the data shown in Table II which demonstrate that the boiling point of the tertiary azeotrope goes through a maxima. The indicated HCFC-141b/methanol ratio is by weight percent.

FIG. 3 is a contour plot of the boiling points of the ternary saddle azeotrope which data are shown in Table III. The compositions within the area defined by the ellipse-shaped curve boil within 0.05.degree. C. of 29.58.degree. C. the boiling point of the saddle azeotrope, an estimate of which is denoted by the triangular shaped area on the plot. At room temperature, the area defined by the ellipse is anticipated to be somewhat greater than at the boiling point.

In accordance with the invention, novel azeotrope-like compositions have been discovered comprising HCFC-141b, dichlorotrifluoroethane and methanol. The dichlorotrifluoroethane component can be either of its isomers 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) or 1,2-dichloro-1,2,2-trifluoroethane (HCFC-123a), or mixtures thereof. The preferred isomer is HCFC-123.

Dichlorotrifluoroethane and HCFC-141b do not form binary azeotrope systems. HCFC-141, dichlorotrifluoroethane and methanol do not form a ternary azeotropic system.

The azeotrope-like compositions of the invention comprise from about 60 to about 94 weight percent of HCFC-141b, from about 5 to about 35.5 weight percent of dichlorotrifluoroethane and from about 1 to about 4.7 weight percent of methanol.

In a preferred embodiment of the invention, the azeotrope-like compositions of the invention comprise from about 70 to about 94 weight percent of HCFC-141b, from about 5 to about 26.0 weight percent of dichlorotrifluoroethane and from about 1 to about 4.0 weight percent of methanol.

In a still more preferred embodiment of the invention, the azeotrope-like compositions of the invention comprise from about 75 to about 90 weight percent of HCFC-141b, from about 8.0 to about 21.0 weight percent of dichlorotrifluoroethane and from about 2.0 to about 3.8 weight percent of methanol.

Our best estimate of the true azeotrope containing HCFC-123 is about 86.2 weight percent HCFC-141b, about 10 weight percent HCFC-123 and about 3.8 weight percent methanol which exhibits a boiling point of about 29.58.degree. C. at 760 mm Hg.

Our best estimate of the true azeotrope containing HCFC-123a is about 86.0 weight percent HCFC-141b, about 10.2 weight percent HCFC-123a and about 3.8 weight percent methanol which exhibits a boiling point of about 29.7.degree. C. at 760 mm Hg.

The azeotrope-like compositions of the invention containing a mixture of HCFC-123 and HCFC-123a behave as an azeotrope-like composition because the separate ternary azeotropic compositions with HCFC-123 and HCFC-123a have boiling points so close to one another as to be indistinguishable for practical purposes. Such mixtures exhibit a boiling point of 29.64.degree. C. .+-.3.degree. C. at 760 mm Hg.

The precise or true azeotrope compositions have not been determined but have been ascertained to be within the indicated ranges. Regardless of where the true azeotropes lie, 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 on the whole nonflammable liquids, i.e. exhibit no flash point when tested by the Tag Open Cup test method--ASTM D 1310-86.

From fundamental principles, the thermodynamic state of a fluid is defined by four variables: pressure, temperature, liquid composition and vapor composition, or P-T-X-Y, respectively. An azeotrope is a unique characteristics of a system of two or more components where X and Y are equal at the stated P and T. In practice, this means that the components of a mixture cannot be separated during distillation, and therefore in vapor phase solvent cleaning as described above.

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 with non-azeotrope-like compositions in which during boiling or evaporation, the liquid composition changes to a substantial degree.

This, one way to determine whether a candidate mixture is "azeotrope-like" within the meaning of this invention, is to distill a sample thereof under conditions (i.e. resolution--number of plates) which would be expected to separate the mixture into its separate components. If the mixture is non-azeotropic or non-azeotrope-like, the mixture will fractionate, i.e. separate into its various components with the lowest boiling component distilling off first, and so on. If the mixture is azeotrope-like, some finite amount of a first distillation cut will be obtained which contains all of the mixture components and which is constant boiling or behaves as a single substance. This phenomenon cannot occur if the mixture is not azeotrope-like i.e., it is not part of an azeotropic system. If the degree of fractionation of the candidate mixture is unduly great, then a composition closer to the true azeotrope must be selected to minimize fractionation. Of course, upon distillation of an azeotrope-like composition such as in a vapor degreaser, the true azeotrope will form and tend to concentrate.

It follows from the above that 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 pressure, the composition of a given azeotrope will vary at least slightly as does the boiling point of the composition. Thus, an azeotrope of A and B represents a unique type of relationship but with a variable composition depending on temperature and/or pressure. Accordingly, another way of defining azeotrope-like within the meaning of this invention is to state that such mixtures boil within about .+-.3.degree. C. (at about 760 mm Hg) of the boiling point of the most preferred compositions disclosed herein, i.e. 29.58.degree. C. at 760 mm Hg in the case of HCFC-123 and 29.70.degree. C. at 760 mm Hg in the case of HCFC-123a. As is readily understood by persons skilled in the art, the boiling point of the azeotrope will vary with the pressure.

In the process embodiment of the invention, the azeotrope-like compositions of the invention may be used to clean solid surfaces by treating said surfaces with said compositions in any manner well known to the art such as by dipping or spraying or use of conventional degreasing apparatus.

The HCFC-141b, dichlorotrifluoroethane and methanol components of the novel solvent azeotrope-like compositions of the invention are known materials. 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.

Examples 1-3 show that a novel saddle (positive-negative) azeotrope is formed with the HCFC-141b. dichlorotrifluoroethane and methanol systems. Saddle types of azeotropes are extremely rare in this art. The nature and advantages of a saddle azeotrope is described below.

Of the possible binary combinations of the three components which form the saddle azeotropes of this invention, only two form azeotropes: HCFC-123 and methanol (27.49.degree. C. boiling point at 760 mm Hg) and HCFC-141b and methanol (29.63.degree. C. boiling point at 760 mm Hg), both of which are minimum boiling azeotropes. HCFC-141b and HCFC-123 do not form a binary azeotrope together. If the ternary mixture did form a minimum boiling azeotrope, which is the most common type in this art, then it would boil below the lowest boiling binary azeotrope constituent, i.e., its boiling point would be less than 27.49.degree. C. However, because the ternary mixture forms a saddle azeotrope, its boiling point is not depressed below that of the minimum boiling constituent binary azeotrope. Indeed, the higher boiling point of the saddle azeotrope. 29.58.degree. C. at 760 mm Hg, is advantageous in that the higher boiling point will decrease solvent losses from a machine such as vapor degreasing or defluxing machines.

The advantages of the ternary systems over the two binary azeotropes, HCFC-123/methanol and HCFC-141b/methanol, are: (a) decreased vapor flammability in comparison to HCFC-141b/methanol, and (b) higher boiling point than the HCFC-123/methanol blend.

These examples were carried out in an ebulliometer. The ebulliometer consisted of an electrically heated sump in which various binary blends were brought to boil. A condenser was connected to this sump and the system was operated under total reflux. Slugs of boiling liquid and vapor were pumped from the sump, via a Cottrell pump, over a thermowell, which contains a calibrated thermistor used for precise temperature measurements. After bringing the two component blends to boil under controlled pressure, measured amounts of the third component were titrated into one of the ebulliometers. The change in boiling point of the resulting mixture was measured.

In some of the measurements, blends containing three of the components in various proportions were added and the boiling points of the resulting mixtures were measured. The boiling point contour was then plotted and the composition of the azeotrope was thus determined. As demonstrated by the data presented herein, it was found that when HCFC-123 was added to a binary azeotropic mixture of HCFC-141b and methanol, the boiling point increased and a maximum boiling ternary mixture formed. However, in the case where methanol was added to a mixture of HCFC-141b and HCFC-123, the azeotropic blend formed was a minimum boiling type. This proved the existence of a unique saddle azeotrope of the subject three component system comprised of HCFC-141b, dichlorotrifluoroethane and methanol.

Temperature and pressure measurements, as well as the measured titration, were all performed automatically with the aid of a computerized data acquisition system. Boiling point measurements were performed at two pressures, generally in the region of 760 mm Hg and 765 mm Hg, for each composition. These measurements were corrected to exactly 760 mm Hg and 765 mm Hg by applying a small, measured, linear correction. Such boiling point measurements are believed accurate to .+-.0.002.degree. C.

The following Table I shows the boiling point measurements, corrected to 760 mm Hg, for the various mixtures obtained when methanol was added to a mixture of HCFC-141b and HCFC-123. These data are plotted in FIG. 1 which show a minimum boiling ternary azeotrope composition.

The following Table II shows the boiling point measurements, corrected to 760 mm Hg for various mixtures of HCFC-123, HCFC-141b and methanol. In this experiment the composition of methanol was kept constant at its azeotropic composition with HCFC-141b. The proportion of HCFC-141b and HCFC-123 in the blend was varied and boiling points were measured in the ebulliometer. The boiling point goes through a maxima. The data are plotted in FIG. 2.

In order to construct a composition-temperature contour diagram of the saddle azeotrope, ternary mixtures were prepared and their boiling points measured at 760 mm Hg using the ebulliometer apparatus. These data are listed in Table III. These data long with the data in Table II are plotted in the ternary diagram depicted in FIG. 3. This plot depicts the region of compositions where the saddle point occurs.

These examples further confirm the existence of the azeotropes between 1,1-dichloro-1-fluoroethane, methanol and HCFC-123 or HCFC-123a via the method of distillation. It also illustrates that these mixtures do not fractionate during distillation.

A 5-plate Oldershaw distillation column with a cold water condensed automatic liquid dividing head was used for these examples. For Example 4 the distillation column was charged with approximately 310 grams of 86.62 weight percent HCFC-141b 9.70 weight percent HCFC-123 and 3.67 weight percent methanol mixtures which were heated under total reflux for about an hour to ensure equilibration. A reflux ratio of 2:1 was employed for this particular distillation. Approximately 50 percent of the original charges were collected in four similar-sized overhead fractions. The compositions of these fractions were analyzed using gas chromatography. Table IV shows the compositions of the starting materials. The averages of the distillate fractions and the overhead temperatures are quite constant within the uncertainty associated with determining the compositions, indicating that the mixtures are azeotropic.

Another distillation (Example 5) was performed in an analogous fashion, this time using a different starting composition. In this particular example a small amount, 0.23 weight percent of nitromethane was added to the starting material to ascertain whether or not this commonly used stabilizer would azeotrope with the ternary blend. Example 5 shows that the nitromethane segregates from the starting material and does not form an azeotrope-like mixture with HCFC-141b/HCFC-123/methanol. The HCFC-123/HCFC-141b/methanol composition does remain essentially constant indicating that this particular composition is azeotrope-like. Examples 6-7 illustrate that HCFC-141b/HCFC-123a and methanol likewise form constant boiling azeotrope-like mixtures.

The compositions of the invention are useful in solvents in a variety of vapor degreasing, cold cleaning and solvent cleaning applications including defluxing.

It is known in the art that the use of more active solvents, such as lower alkanols in combination with certain halocarbons such as trichlorotrifluoroethane, may have the undesirable result of attacking reactive metals such as zinc and aluminum, as well as certain aluminum alloys and chromate coatings such as are commonly employed in circuit board assemblies. The art has recognized that certain stabilizers, such as nitromethane, are effective in preventing metal attack by chlorofluorocarbon mixtures with such alkanols. Other candidate stabilizers for this purpose, such as disclosed in the literature, are secondary and tertiary amines, olefins and cylcoolefins, alkylene oxides, sulfoxides, sulfones, nitrites and nitriles, and acytelenic alcohols or ethers. It is contemplated that such stabilizers as well as other additives may be combined with the azeotrope-like compositions of this invention.