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Description  |
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FIELD OF THE INVENTION
This invention relates to azeotrope-like mixtures of
1,1-dichloro-1-fluoroethane and methanol. These mixtures are useful in a
variety of vapor degreasing, cold cleaning and solvent cleaning
applications including defluxing.
CROSS-REFERENCE TO RELATED APPLICATION
Co-pending, commonly assigned application Ser. No. 189,915, filed May 3,
1988, discloses azeotrope-like mixtures of 1,1-dichloro-1-fluoroethane,
methanol and nitromethane and their use as solvents.
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 solids 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 hardware, 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 exhibit a minimum or maximum boiling
point and 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 (FC-141b), 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.
We are aware of only one disclosure of an azeotropic composition including
1,1-dichloro-1-fluoroethane, namely Anon., Research Disclosures, Vol. 162,
p. 70 (1977) in which it is stated that n-pentane and iso-pentane form
binary azeotropes with 1,1-dichloro-1-fluoroethane.
U.S. Pat. No. 3,936,387 discloses the azeotropic composition of methanol
with 1,2-dichloro-1-fluoroethane, FC-141, which is an isomer of FC-141b.
Similarly, U.S. Pat. No. 4,035,258 discloses the azeotropic composition of
ethanol with 1,2-dichloro-1-fluoroethane. This information did not lead us
to the azeotropic composition of the invention since, as is well known in
this art, there is no published, reliable basis on which to predict
azeotropy. Moreover, as is equally well known, the existence of an
azeotropic composition does not enable one skilled in the art to predict
azeotropy between or among related components. For example, U.S. Pat. No.
3,936,387 discloses that FC-141 and isopropanol form an azeotropic
composition, whereas FC-141b and isopropanol do not form an azeotrope.
It is accordingly an object of this invention to provide novel
azeotrope-like compositions based on FC-141b 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.
DESCRIPTION OF THE INVENTION
In accordance with the invention, novel azeotrope-like compositions have
been discovered comprising FC-141b and methanol.
In a preferred embodiment of the invention, the azeotrope-like compositions
comprise from about 93 to about 98 weight percent of FC-141b and from
about 7 to about 2 weight percent methanol.
In a still preferred embodiment of the invention, the azeotrope-like
compositions comprise from about 95.0 to about 97.0 weight percent FC-141b
and from about 5 to about 3 weight percent methanol.
Our best estimate of the true azeotrope and our most preferred embodiment
is about 96.2 weight percent FC-141b and about 3.8 weight percent methanol
which exhibits a boiling point of about 29.8.degree. C. at 765 mm Hg.
All compositions within the above-identified ranges, as well as certain
compositions outside the indicated ranges, are azeotrope-like, as defined
more particularly below.
The precise or true azeotrope composition has not been determined but has
been ascertained to be within the indicated ranges. Regardless of where
the true azeotrope lies, 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. The vapor phase, however, does
exhibit a narrow range of flame limits (9.9-15.2 volume percent in air at
ambient conditions).
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
characteristic 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.
Thus, 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 pressures, 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 .+-.0.05.degree. C. (at about 765 mm Hg) of the
29.8.degree. C. boiling point of the most preferred composition disclosed
herein. 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 FC-141b and methanol components of the novel solvent azeotrope-like
compositions of the invention are known materials and are 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.
EXAMPLE 1
This example shows that a minimum in the boiling point versus composition
curve occurs in the region of 96.2 weight percent
1,1-dichloro-1-fluoroethane and 3.8 weight percent methanol, indicating
that an azeotrope forms in the neighborhood of this composition.
The temperature of the boiling liquid mixtures was measured using
comparative ebulliometry in essentially the same manner as described by W.
Swietoslawski in "Ebulliometric Measurements", p. 14, Reinhold Publishing
Corp., (1945). Two ebulliometers, each charged with measured quantities of
1,1-dichloro-1-fluoroethane, were used in the present example. The
ebulliometers were interconnected via a large ballast volume, in which the
pressure was maintained to within .+-.0.05 mm Hg using a supply of dry air
controlled with a solenoid valve and an electronic pressure transducer.
Precise pressure control is necessary for accurate boiling point
determinations.
Each ebulliometer consisted of an electrically heated sump in which the
1,1-dichloro-1-fluoroethane was 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 contained a calibrated thermistor used for
precise temperature measurements. After bringing the
1,1-dichloro-1-fluoroethane to boil under controlled pressure, measured
amounts of methanol were titrated into one of the ebulliometers. The
change in boiling point was measured with reference to the other
ebulliometer, which still contained only 1,1-dichloro-1-fluoroethane.
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
765 mm Hg, for various mixtures of 1,1-dichloro-1-fluoroethane and
methanol. Interpolation of these data shows that a minimum boiling point
occurs in the region of about 3 to 5 weight percent methanol. The best
estimate of the precise minimum is 3.8 weight percent methanol, although
the mixtures are constant boiling, to within 0.01.degree. C., in the
region of about 3 to 5 weight percent methanol. A minimum boiling
azeotrope is thus shown to exist in this composition region.
TABLE I
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LIQUID MIXTURE
Boiling Point
Parts By Weight
Parts By Weight
(.degree. C.) at
1.1-dichloro-1-fluoroethane
Methanol 765 mm Hg
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100. 0. 32.226
99.55 0.45 30.873
99.11 0.89 30.251
98.81 1.19 30.078
98.52 1.48 29.976
98.23 1.77 29.910
97.94 2.06 29.866
97.66 2.34 29.839
97.37 2.63 29.823
97.09 2.91 29.813
96.80 3.20 29.805
96.52 3.48 29.802
96.24 3.76 29.801
95.97 4.03 29.801
95.55 4.45 29.805
95.14 4.86 29.808
94.74 5.26 29.814
94.07 5.93 29.822
93.41 6.59 29.826
92.79 7.21 29.830
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EXAMPLE 2
This example further confirms the existence of the azeotrope between
1,1-dichloro-1-fluoroethane and methanol via the method of distillation.
A 5-plate Oldershaw distillation column with a cold water condensed
automatic liquid dividing head was used for this example. The distillation
column was charged with approximately 340 grams of a 3.7 weight percent
methanol and 96.3 weight percent, 1,1-dichloro-1-fluoroethane mixture
which was heated under total reflux for about an hour to ensure
equilibration. A reflux ratio of 10:1 was employed for this particular
distillation. Approximately 40 percent of the original charge was
collected in five similar-sized overhead fractions. The compositions of
these fractions, in addition to the composition of the liquid residue,
were analyzed using gas chromatography. Table II shows that the
compositions of the starting material, the five distillate fractions and
the liquid residue are identical, within the uncertainty associated with
determining the compositions, indicating that the mixture is an azeotrope.
TABLE II
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Mixture Composition
Parts by Weight
1.1-dichloro-1-
Party by Weight
Mixture fluoroethane Methanol
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Original Charge
96.3 3.7
Distillate Fractlon #1
96.3 3.7
Distillate Fraction #2
96.3 3.7
Distillate Fraction #3
96.3 3.7
Distlllate Fraction #4
96.3 3.7
Distillate Fraction #5
96.3 3.7
Liquid Residue
96.5 3.5
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Barometric pressure was 741 mm Hg
The compositions of the invention are useful as 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 cycloolefins, alkylene oxides,
sulfoxides, sulfones, nitrites and nitriles, and acetylenic alcohols or
ethers. It is contemplated that such stabilizers may be combined with the
azeotrope-like compositions of this invention.
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