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Description  |
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BACKGROUND OF THE INVENTION
Fluorocarbon solvents, such as trichlorotrifluoroethane, are widely used as
degreasing agents due to their excellent solvent power for greases and
some emulsion-type lubricants. Trichlorotrifluoroethane also finds wide
use in removing solder fluxes from printed circuit boards due to its
selective solvency and non-flammability. Since trichlorotrifluoroethane is
non-polar, however, it does not remove polar contaminates. Thus, to
overcome this inability, trichlorotrifluoroethane has, in the past, been
mixed with polar components, such as aliphatic alcohols.
The art has looked towards azeotropic compositions including desired
fluorocarbon components, such as trichlorotrifluoroethane, which include
the desired polar components, and other components which contribute
desired characteristics, such as stabilizers. Azeotropic compositions are
desired because they exhibit a minimum boiling point and do not
fractionate upon boiling. This is desirable because in vapor degreasing
equipment, in 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. This is also important in the use of the solvent
compositions to remove solder fluxes from printed circuit boards.
Preferential evaporation of the more volatile components of the solvent
mixtures, which would be the case if they were not azeotropic, or
azeotropic-like, would result in mixtures with changed compositions which
may have less desirable properties, such as lower solvency for rosin
fluxes, less inertness toward the electrical components and increased
flammability.
A number of trichlorotrifluoroethane based azeotropic compositions have
been discovered which have been tested and in some cases employed as
solvents for miscellaneous vapor degreasing applications and for the
removal of solder fluxes from printed circuit boards. For example, U.S.
Pat. No. 2,999,815 discloses the azeotrope of
1,1,2-trichloro-1,2,2-trifluoroethane with acetone; U.S. Pat. No.
3,903,009 discloses a ternary azeotrope of
1,1,2-trichloro-1,2,2-trifluoroethane with nitromethane and ethanol; U.S.
Pat. No. 3,573,213 discloses the binary azeotrope of
1,1,2-trichloro-1,2,2-trifluoroethane with nitromethane; U.S. Pat. No.
3,789,006 discloses the ternary azeotrope of
1,1,2-trichloro-1,2,2-trifluoroethane with nitromethane and isopropanol;
and U.S. Pat. No. 3,728,268 discloses the ternary azeotrope of
1,1,2-trichloro-1,2,2-trifluoroethane with acetone and ethanol.
Unfortunately, as it is recognized in the art, it is not possible to
predict the formation of azeotropes and this obviously complicates the
search for new azeotropic systems which have application in this field.
Nevertheless, there is a constant effort in the art to discover new
azeotropic or azeotrope-like systems which have desirable solvency
characteristics and particularly a greater versatility of solvency power.
It is accordingly an object of this invention to provide novel azeotropic
or 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 applications and for the
removal of solder fluxes from printed circuit boards.
Other objects and advantages of the invention will be apparent from the
following description.
DESCRIPTION OF THE INVENTION
In accordance with the invention, novel azeotrope-like compositions have
been discovered which consist essentially of about 90.25 weight percent of
1,1,2-trichloro-1,2,2-trifluoroethane, about 0.33 weight percent of
nitromethane and about 9.42 weight percent of acetone. Such compositions
have a minimum boiling point at 760mm Hg of about 44.5.degree. C.
It has been found that these azeotrope-like compositions are stable,
non-flammable, inert to electronic components of printed circuit boards
and exhibit excellent solvency power which make such compositions
particularly effective in vapor degreasing applications and for the
removal of solder fluxes from printed circuit boards.
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-azeotropic
or non-azeotrope-like compositions in which during boiling or evaporation,
the liquid composition changes to a substantial degree. The continued
presence of nitromethane in the composition, even after repeated
distillation, prevents any metal-attack caused by the presence of
1,1,2-trichloro-1,2,2-trifluoroethane and acetone that would occur in the
absence of nitromethane.
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.
The 1,1,2-trichloro-1,2,2-trifluoroethane, nitromethane and acetone
components of the novel solvent compositions of the invention 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. A
suitable grade of 1,1,2-trichloro-1,2,2-trifluoroethane, for example, is
sold by Allied Chemical Corporation under the trade name "Genesolv" D.
The novel azeotrope-like compositions of the invention may be purified and
reclaimed for use after saturation with dissolved materials by simple
flash distillation.
The novel azeotrope-like compositions of this invention may be used to
clean a variety of materials such as synthetic organic polymers, plastics,
resins, resin laminates, resin-bonded paperboard, bakelite, fiber glass
and like materials. The novel solvents of the invention are particularly
well suited for the removal of rosin-based fluxes which are used in the
preparation of printed circuit boards. Vapor degreasers are generally used
to apply the solvent to the boards. In a conventional operation of a vapor
degreaser, the board is passed into a sump of boiling solvent, which
removes the bulk of the resin, and thereafter through a sump containing
freshly distilled solvent near room temperature, and finally through
solvent vapors over the boiling sump which provides a final rinse with
clean pure solvent which condenses on the circuit board. In addition, the
board can also be sprayed with distilled solvent before final rinsing.
From the above description it can be appreciated that a preferred process
embodiment of the invention involves cleaning a solid surface comprising
contacting said surface with a novel azeotrope-like composition in
accordance with this invention.
A still preferred process embodiment of the invention involves so cleaning
a solid surface which is a printed circuit board contaminated with
soldering flux.
The novel solvent mixtures of the invention find other applications such as
for removing gases and oils from a variety of industrial items, for the
cleaning of photographic films and prints, for the removal of buffing
compounds such as rouge, for the extraction of high quality natural and
synthetic products such as vitamins, enzymes, hormones, spices, and
essential oils, as heat exchange media, as electrical transfer media, as
chemical reaction media and as hydraulic fluids.
It will be apparent to those skilled in the art that for specialized
purposes, various additives could be incorporated with the novel solvent
mixtures of the invention, for example, lubricants, detergents and the
like. These additives are chosen so as not to adversely affect the
essential properties of the mixtures for a given application.
EXAMPLE 1
A distillation pot was charged with about 11 pounds of a mixture which
comprised about 89.5 weight percent of
1,1,2-trichloro-1,2,2-trifluoroethane, about 10 weight percent acetone and
about 0.5 weight percent of nitromethane and the mixture was heated. The
barometric pressure was measured at 754.4mm Hg. A constant boiling
fraction at 44.0.degree. C. was collected and found to contain all three
components. Hence this fraction was determined to be azeotrope-like.
Analysis by gas chromatography determined that the weight percentages of
the components of this constant boiling fraction were:
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1,1,2-trichloro-1,2,2-trifluoroethane
90.25 wt.%
nitromethane 0.33 wt.%
acetone 9.42 wt.%
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The boiling point of this constant boiling mixture at 760mm Hg is
44.5.degree. C.
This azeotrope-like composition was tested for flammability by the open cup
flask point test (ASTM D1310-63) and was classified as non-flammable.
EXAMPLE 2
A standard measure of solvency for certain classes of solvents is the
Kauri-Butanol value. This test (ASTM 1163-61) was made on the preferred
azeotrope-like composition in accordance with this invention. The
established value is then compared with those of some related binary
azeotropic systems and other common solvents. The results are given in
Table I.
TABLE I
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K-B
Solvents Value*
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1. 1,1,2-trichloro-1,2,2-trifluoroethane
32
2. nitromethane 10
3. acetone 86
4. binary azeotrope of 1,1,2-trichloro-
1,2,2-trifluoroethane (97.5 wt.%),
nitromethane (2.5 wt.%) 43
5. binary azeotrope of 1,1,2-trichloro-
1,2,2-trifluoroethane (87.5 wt.%),
acetone (12.5 wt.%) 58
6. azeotrope-like compositions of Example 1, of
1,1,2-trichloro-1,2,2-trifluoroethane (90.25
wt%), nitromethane (0.33 wt.%), acetone
(9.42 wt.%) 48
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*These values may vary from analyst to analyst because of the nature of
the test.
The above data show that the K-B value for the azeotrope-like composition
of the invention (Solvent No. 6) is substantially higher than that of the
1,1,2-trichloro-1,2,2-trifluoroethane (Solvent No. 1) or nitromethane
(Solvent No. 2) components alone, and higher than that of the binary
azeotrope of Solvent No. 4. Although the K-B value of the novel
azeotrope-like composition is lower than that of acetone (Solvent No. 3)
and the acetone containing binary azeotrope (Solvent No. 5), the
azeotrope-like composition of the invention is more inert towards the
electronic components of printed circuit boards than is the said binary
azeotrope in which acetone is present in a higher concentration, and thus
combines the desirable properties of high solvency power with inertness
towards circuit board components.
EXAMPLE 3
To further indicate the solvency power of the azeotrope-like compositions
of the invention, the following test was conducted. A few drops of Alpha
Milros Flux 611 (Mil 14256 Type A), a product of Alpha Metals Inc., was
placed on copper clad printed circuit boards, then baked for 1 minute at
550.degree. F. over a hot plate. The boards were immersed in room
temperature solvents and timed until the flux residue dissolved under
constant stirring. The test was repeated for all solvents tested. The
results are given in Table II.
TABLE II
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Solvent Condition
Remarks
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1,1,2-trichloro-1,2,2-trifluoroethane
3 minutes
not dissolved
acetone 25 seconds
slight residue
binary azeotrope of 1,1,2-trichloro- -
1,2,2-trifluoroethane (97.5 wt.%),
nitromethane (2.5 wt.%)
30 seconds
no residue
binary azeotrope of 1,1,2-trichloro-
1,2,2-trifluoroethane (87.5 wt.%),
acetone (12.5 wt.%) 15 seconds
no residue
ternary azeotrope of 1,1,2-
trichloro-1,2,2-trifluoroethane
(90.25 wt.%),
nitromethane (0.33 wt.%),
acetone (9.42 wt.%) 11 seconds
no residue
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The above data indicate the excellent solvency power of the azeotrope-like
compositions of the invention for a typical solder flux used on printed
circuit boards.
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