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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, methanol and nitromethane. 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. 189,932, filed May 3,
1988, discloses azeotrope-like mixtures of 1,1-dichloro-1-fluoroethane and
methanol 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 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 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, tractionation 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 ot 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. 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.
Nitromethane is a known stabilizer for preventing metal attack by
chlorofluorocarbon mixtures containing alcohols. For example, U.S. Pat.
No. 3,573,213 discloses an azeotropic mixture of
1,1,2-trichloro-1,2,2-trifluoroethane with nitromethane in which mixture
nitromethane is stated to perform such stabilizing function.
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, methanol and nitromethane.
In a preferred embodiment of the invention, the azeotrope-like compositions
comprise from about 93 to about 98 weight percent of FC-141b, from about 7
to about 2 weight percent methanol and from about 0.1 to about 0.01 weight
percent nitromethane.
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, from about 5 to about 3 weight percent methanol, and from about
0.1 to about 0.01 weight percent nitromethane.
Our best estimate of the true azeotrope and our most preferred embodiment
is about 96.0 weight percent FC-141b, about 3.9 weight percent methanol
and about 0.01 weight percent nitromethane, which exhibits a boiling point
of about 29.4.degree. C..+-.0.1.degree. C. at 760 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).
It is believed that the azeotrope-like compositions of the invention are
also more stable in the presence of moisture than the azeotrope-like
compositions of co-pending, commonly assigned application Ser. No.
189,932.
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 ot 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 .+-.0.1.degree. C. (at about 765 mm Hg) of the
29.4.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, methanol and nitromethane 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 ot the system.
EXAMPLE
This example confirms the existence of the azeotrope between
1,1-dichloro-1-fluoroethane, methanol and nitromethane.
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 a three component blend consisting of 3.8 weight
percent methanol, 96.0 weight percent 1,1-dichloro-1-fluoroethane and 0.2
weight percent nitromethane which was heated under total reflux for about
an hour to ensure equilibration. A reflux ratio of 5:1 was employed.
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. The Table 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
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Nitro-
Methanol
FC-141b methane
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Starting Material (wt. %)
3.8 96.0 0.2
Constant Boiling Fraction
3.9 96.0 0.01
(wt. %)
Vapor Temperature (.degree.C.)
28.6
Barometric Pressure (mm Hg)
740.9
Vapor Temperature (.degree.C.)
29.4
(corrected to 760 mm Hg)
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The compositions of the invention are useful as solvents in a variety ot
vapor degreasing, cold cleaning and solvent cleaning applications
including defluxing.
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