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Description  |
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DESCRIPTION
Field of the Invention
This invention relates to azeotrope-like mixtures of
trichlorotrifluoroethane, methanol, acetone, nitromethane and hexane.
These mixtures are useful as vapor degreasing agents and as solvents to
remove rosin fluxes from printed circuit boards.
BACKGROUND OF THE INVENTION
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. Trichlorotrifluoroethane also finds wide use in
removing solder fluxes from printed wiring boards and printed wiring
assemblies in the electronics industry. Such circuit boards normally
consist of a glass fiber reinforced plate of electrically resistant
plastic having electrical circuit traces on one or both sides thereof. The
circuit traces are thin flat strips of conductive metal, usually copper,
which serve to interconnect the electronic components attached to the
printed wiring board. The electrical integrity of the contacts between the
circuit traces and the components is assured by soldering.
Current industrial processes of soldering circuit boards involve coating
the entire circuit side of the board with a flux and thereafter passing
the coated side of the board through molten solder. The flux cleans the
conductive metal parts and promotes a reliable intermetallic bond between
component leads and circuit traces and lands on the printed wiring board.
The preferred fluxes consist, for the most part, of rosin used alone or
with activating additives such as dimethylamine hydrochloride,
trimethylamine hydrochloride, or an oxalic acid derivative.
After soldering, which thermally degrades part of the rosin, the flux is
removed from the board by means of an organic solvent.
Trichlorotrifluoroethane, being non-polar, adequately cleans rosin fluxes;
however, it does not easily remove polar contaminants such as the
activating additives.
To overcome this deficiency, trichlorotrifluoroethane has been mixed with
polar components such as aliphatic alcohols or chlorocarbons such as
methylene chloride. As example, U.S. Pat. No. 2,999,816 discloses the use
of mixtures of 1,1,2-trichloro-1,2,2-trifluoroethane and methanol as
defluxing solvents.
The art has looked, in particular, 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 stability. 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 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 for rosin fluxes, less inertness towards the electrical
components soldered on the printed circuit board, and increased
flammability.
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 and defluxing 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.
2,999,816 discloses an azeotropic composition of
1,1,2-trichloro-1,2,2-trifluoroethane and methyl alcohol; U.S. Pat. No.
3,960,746 discloses azeotrope-like compositions of
1,1,2-trichloro-1,2,2-trifluoroethane, methanol, and nitromethane;
Japanese Pat. Nos. 81-34,798 and 81-34,799 disclose azeotropes of
1,1,2-trichloro-1,2,2-trifluoroethane, ethanol, nitromethane and
2,2-dimethylbutane or 2,3-dimethylbutane or 3-methylpentane; and Japanese
Pat. No. 81,109,298 discloses an azeotrope of
1,1,2-trichloro-1,2,2-trifluoroethane, ethanol, n-hexane and nitromethane;
U.S. Pat. No. 4,045,366 discloses the ternary azeotrope of
1,1,2-trichloro-1,2,2-trifluoroethane, nitromethane and acetone; Japanese
Pat. No. 73-7,333,878 discloses the ternary azeotrope of
1,1,2-trichloro-1,2,2-trifluoroethane, methanol and acetone; U.S. Pat. No.
4,279,664 discloses the ternary azeotrope of
1,1,2-trichloro-1,2,2-trifluoroethane, acetone and hexane, and U.S. Pat.
No. 4,476,306 discloses the azeotrope of
1,1,2-trichloro-1,2,2-trifluoroethane, acetone, 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 applications and for the removal of solder fluxes from printed
circuit boards.
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, methanol, acetone,
nitromethane and hexane, 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 86.5 to about 93.5 weight percent of
1,1,2-trichloro-1,2,2-trifluoroethane, from about 5.0 to about 6.2 weight
percent of methanol, from about 0.03 to about 0.6 weight percent of
nitromethane, from about 0.3 to about 6.0 weight percent of hexane and
from about 0.6 to 4.5 weight percent acetone.
In another preferred embodiment of the invention, the azeotrope-like
compositions comprise from about 91.0 to about 91.6 weight percent of
1,1,2-trichloro-1,2,2-trifluoroethane, from about 5.6 to about 6.1 weight
percent of methanol, from about 0.05 to about 0.3 weight percent of
nitromethane, from about 0.3 to about 4.1 weight percent of hexane and
from about 0.6 to about 4.2 weight percent acetone.
The most preferred embodiment of the invention comprises from about 90.2 to
about 91.6 weight percent of 1,1,2-trichloro-1,2,2-trifluoroethane, from
about 5.7 to about 6.0 weight percent of methanol, from about 0.05 to
about 0.2 weight percent of nitromethane, from about 1.6 to about 2.1
weight percent of hexane and from about 0.6 to 2.1 weight percent acetone.
Such compositions possess constant or essentially constant boiling points
of about 39.8.degree. C. at 760 mm Hg.
All compositions within the above-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 D 1310) and exhibit excellent solvency power. These
compositions have been found to be particularly effective when employed in
conventional degreasing units for the dissolution of rosin fluxes and the
cleaning of such 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
nonazeotrope-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, methanol, nitromethane, acetone,
and hexane components of the novel solvent azeotrope-like compositions of
the invention are all commercially available. A suitable grade of
1,1,2-trichloro-1,2,2-trifluoroethane, for example, is sold by Allied
Corporation under the trade name "GENESOLV.RTM. D".
The term "hexane" is used herein as to mean any C.sub.6 paraffin
hydrocarbon (C.sub.6 H.sub.14) (see Hackh's Chemical Dictionary, 3rd Ed.,
McGraw Hill Book Co. (1944) p. 408). Thus, the term "hexane" includes
n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane,
2,3-dimethylbutane and any and all mixtures thereof. 2-Methylpentane is
commonly referred to as isohexane. Specifically included is "commercial
isohexane" which is a mixture of isohexane with other hexane isomers,
typically containing at least about 35 weight percent isohexane and
usually from about 40-45 weight percent isohexane. It has been found that
each hexane isomer, separately and in combination with other hexane
isomers, form azeotrope-like compositions with
1,1,2-trichloro-1,2,2-trifluoroethane, methanol, and nitromethane in
accordance with the invention.
EXAMPLES 1-2
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, methanol, acetone,
nitromethane, and hexane. 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 5:1 reflux ratio at a boil-up rate of 250-300 grams per
hr. Approximately 150 cc of product were distilled and 5 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.
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 barometic correction factor of about 26 mm Hg/.degree. C., to
the observed values. However, it is to be noted that this corrected
boiling point is generally accurate up to .+-.0.4.degree. 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 39.9.degree..+-.0.2.degree. C. at 760 mm Hg was formed
for compositions comprising about 81.7 to about 91.0 weight percent
1,1,2-trichloro-1,2,2-trifluoroethane (FC-113), about 6.1 to about 5.9
weight percent methanol (Me0H), about 0.03 to about 0.3 weight percent
nitromethane, about 2.2 to about 2.6 weight percent 2-methylpentane (2-MP)
and about 0.8 to 4.5 weight percent acetone. Supporting distillation data
for the mixtures studied are shown in Table I.
TABLE I
______________________________________
Starting Material (wt. %)
Example
(Distil-
lation)
FC-113 MeOH Nitromethane
Acetone
2-MP
______________________________________
5-Plate
1 81.7 5.8 0.3 9.8 2.4
2 90.2 5.9 0.15 1.2 2.5
______________________________________
Constant Boiling Fraction (wt. %)
Example
FC-113 MeoH Nitromethane
Acetone
2-MP
______________________________________
1 87.8 5.1 0.03 4.5 2.6
2 91.0 5.9 0.1 0.8 2.2
______________________________________
Vapor Barometic Corrected B.P.
Example Temp (.degree.C.)
Pressure (mm Hg)
to 760 mm
______________________________________
1 39.6 747.3 40.1
2 39.2 747.8 39.7
Average 39.9 .+-. 0.2
______________________________________
EXAMPLES 3-4
To explore the constant-boiling composition range of mixtures comprised of
1,1,2-trichloro-1,2,2-trifluoroethane, methanol, nitromethane, hexane
isomers and acetone, a 5-plate distillation apparatus and procedure were
utilized as previously described in Examples 1 and 2. Into the
distillation pot was charged a mixture of
1,1,2-trichloro-1,2,2-trifluoroethane (FC-113), methanol, nitromethane,
hexane and acetone.
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, methanol, nitromethane and acetone
and that each hexane isomer and mixtures thereof form azeotrope-like
constant boiling mixtures at about 39.8.degree..+-.0.3.degree. 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
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Hexane Isomer Normal Boiling Point
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2,2-dimethylbutane
49.75
2,3-dimethylbutane
58.1
2-methylpentane (isohexane)
60.13
3-methylpentane 64
n-hexane 68.74
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A number of distillations were performed. Isomeric ratios and
concentrations of the other mixture components were varied in the
distillation starting material. Isomers were used either in their pure
state as mixtures proportional to their concentration found in inexpensive
commercial grade material, or were synthesized by blending isomers in
various proportions. Commercial grade isohexane as sold by Phillips
Petroleum Company (46% isohexane) was analyzed by gas chromatography and
found to typically contain:
______________________________________
wt. %
______________________________________
2-methylpentane 46.5
3-methylpentane 23.5
2,3-dimethylbutane
14.4
2,2-dimethylbutane
13.5
n-hexane 0.9
isopentane 0.2
n-pentane 0.1
Unknown lights 0.9
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Distillation overhead fractions were collected and analyzed by gas
chromatography, and the vapor temperature and barometic pressure were
recorded. Normalizing the observed boiling points to 760 mm of mercury
pressure as described previously, it was discovered that constant-boiling
mixtures exhibiting a boiling point of approximately
39.8.degree..+-.0.3.degree. C. were found to be formed comprising about
86.5 to about 91.6 weight percent 1,1,2-trichloro-1,2,2-trifluoroethane,
about 5.8 to about 6.0 weight percent methanol, about 0.05 to about 0.1
weight percent nitromethane, about 3.8 to about 5.2 weight percent hexane
isomer at random isomeric ratios and concentrations and about 0.6 to 2.3
weight percent acetone. Supporting distillation data for the mixtures
studied are shown in the following Table III. The results from Examples
1-2 are also included. The results show that the mixtures studied are
constant boiling or essentially constant boiling in the same context as
described in connection with Examples 1-5. The weight percentages shown in
the Table have been rounded to the nearest significant digit and,
therefore, may not necessarily total 100%. Figures shown as--XX--bridging
two columns mean that the figures represent the sum of the compositions in
both columns.
TABLE III
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Nitro- 2,3-
2,2- Total
Examples
FC-113
MeOH
Acetone
methane
2-MP
3-MP
DMB DMB n-hex
Hexane
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Starting Material Compositions (wt %)
(5-plate
distillations)
1-2 81.7-90.2
5.8-5.9
9.8-1.2
0.3-0.15
2.4-2.5
-- -- -- -- 2.4-2.5
3 84.3 5.0 4.1 0.6 3.0
-- 3.0 -- -- 6.0
4 91.0 6.0 0.8 -3.5
-- 0.3 0.3 0.02
4.1
Constant Boiling Distillation Fraction (wt. %)
1-2 87.8-91.0
5.1-5.9
4.5-0.8
0.03-0.1
2.6-2.2
-- -- -- -- 2.6-2.2
3 86.5 5.9 2.3 0.1 2.4
-- 2.8 -- -- 5.2
4 91.6 5.9 0.6 0.05 -3.0
-- 0.3 0.4
0.01 3.8
__________________________________________________________________________
Examples
Vapor Temp (.degree.C.)
Barometric Pressure (mm Hg)
Corrrected B.P. to 760
__________________________________________________________________________
mm
1-2 39.2-39.6
747.3-747.8 39.9
3 39.5 745.1 40.0
4 38.9 745.5 39.5
Average 39.8 .+-. 0.3.degree. 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, methanol, acetone, nitromethane and
hexane, nor the outer limits of its compositional ranges which are
constant boiling or essentially constant boiling. As indicated, anyone or
ordinary skill in the art can readily ascertain other constant boiling or
essentially constant boiling mixtures, it being kept in mind that
"constant boiling" or "essentially constant boiling" 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 "azeotrope-like" within the
meaning of this invention.
EXAMPLE 5
To illustrate the azeotrope-like nature of the mixtures of this invention
under conditions of actual use in vapor phase degreasing operation, a
vapor phase degreasing machine was charged with a preferred azeotrope-like
mixture in accordance with the invention comprising about 91.1 weight
percent 1,1,2-trichloro-1,2,2-trifluoroethane (FC-113), about 5.8 weight
percent methanol, about 1.0 weight percent acetone, about 2.0 weight
percent commercial grade isohexane and about 0.1 weight percent
nitromethane. The mixture was evaluated for its constant boiling or
non-segregating characteristics. Solvents were tested in a Branson B-400
refrigeration cooled 2-sump VPD. The solvent charge was brought to reflux
and the individual sump compositions were determined with a Hewlett
Packard 5890 Gas Chromatograph. Refluxing was continued for 63 hours 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. 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).
EXAMPLE 6
This example illustrates the use of the preferred azeotrope-like
composition of the invention to clean (deflux) printed wiring boards and
printed wiring assemblies.
Two commercial rosin-based fluxes were used in this test. The fluxes were
Kester 1585-MIL (manufactured by Kester Solder) and Kenco 885
(manufactured by Kenco Industries Inc.). Predesigned printed wiring boards
were fluxed in a Hollis 10-inch TDL wave solder machine. For Kester
1585-MIL flux, altogether twelve such test boards were prepared for
defluxing. Of these, six contained some electronic components soldered to
the board and the other six did not have any components on the board. For
Kenco 885, eight boards were run; four with components and the other four
without any components.
The printed wiring assemblies with electronic components (used in this
test) were high density boards each having a one sided surface area of
18.97 square inches and containing two 36 pin dual in line packages (DIP),
two 24 pin DIP's, five 16 pin DIP's and forty-one discrete components
(resistors and capacitors).
Prior to fluxing and soldering, all specimens were pre-cleaned following a
vigorous pre-cleaning schedule to ensure very low levels of contamination
before fluxing. In our experiments, the determination of the ionic
contaminants on printed wiring board surfaces was made with a Kenco.RTM.
Omega-meter, which is a standard industry test method for cleanliness. The
Kenco Omega-meter employs a 75/25 volume % mixture of isopropyl
alcohol/water to rinse the printed wiring boards, and the changes in
specific resistivity of the solution are monitored up to 30 minutes. Three
resistivity readings were taken for each run: (i) the inital resistivity
at time zero, (ii) the resistivity after 15 minutes, and (iii) the
resistivity at 30 minutes. The raw data were converted to micrograms (mg)
per square inch of ionic contaminants, which is expressed in the standard
way in terms of equivalents of sodium chloride (NaCl).
Utilizing this technique, it was determined that all specimens used for our
experiments would be precleaned to 0.05 mg or less of sodium chloride
equivalent per square inch.
Cleaning (defluxing) was performed in a Branson B400R two-sump vapor
degreaser. The first sump is used as the working sump and holds boiling
solvent, and the second sump is used as the rinse sump. Refrigerated
cooling coils line the upper wall of the apparatus to maintain a vapor
blanket.
The cleaning schedule employed to demonstrate the usefulness of this
invention was as follows: (i) two (2) minute exposure to the vapors over
the boil sump, (ii) half a minute full immersion in the cold sump, (iii)
half a minute re-exposure to the vapors over the boil sump.
After defluxing two replicate analyses of boards with no components and two
replicate analyses of boards with components were made in the Kenco
Omega-meter. In the case of Kester 1585-MIL, each replicate analysis
consisted of testing three boards together at the same time in the Omega
meter test tank and in the case of Kenco 885 each replicate analysis
consisted of testing two boards together at the same time in the Omega
meter test tank.
The azeotrope-like composition used to illustrate the usefulness of the
invention to deflux printed wiring boards was comprised of about 90.9
weight percent of 1,1,2-trichloro-1,2,2-trifluoroethane, about 5.9 weight
percent of methanol, about 2.1 weight percent of pure (99%) isohexane,
about 0.1 weight percent of nitromethane and about 1.0 weight percent
acetone.
The cleaning performance of this invention was also compared to that of two
commercial defluxing solvents, Genesolv.RTM. DMS and Freon.RTM. TMS, where
both commercial solvents consist of azeotrope-like compositions of
trichlorotrifluoroethane, primary alcohol(s), and nitromethane.
Genesolv.RTM. DMS is a blend of 92.0 weight percent
trichlorotrifluoroethane, 4.0 weight percent of methanol, 2.0 weight
percent of ethanol, 1.0 weight percent of isopropyl alcohol, and 1.0
weight percent of nitromethane. Freon.RTM. TMS is a blend of 94.05 weight
percent of trichlorotrifluoroethane, 5.7 weight percent of methanol, and
0.25 weight percent of nitromethane. The following table summarizes the
residual ionic contamination left on fluxed printed circuit boards cleaned
by the above composition of this invention, Genesolv.RTM. DMS and
Freon.RTM. TMS.
TABLE IV
______________________________________
Performance Testing
Residual Ionic Contamination
(average of all runs) (mg NaCl/in.sup.2)
Boards with Boards with
Azeotrope-Like
Solder No Components
Components
Solvent Flux 15 min. 30 min.
15 min.
30 min.
______________________________________
This invention
Kester 4.39 4.90 11.06 12.45
1585-MIL
DMS Kester 5.96 6.92 12.38 14.29
1585-MIL
TMS Kester 8.64 9.75 19.38 21.37
1585-MIL
This invention
Kenco 885 11.46 13.31 19.73 23.00
DMS Kenco 885 14.95 17.61 30.93 35.95
TMS Kenco 885 9.67 11.24 27.72 31.51
______________________________________
As stated earlier, the industry has recognized that admixtures of
trichlorotrifluoroethane with polar components such as aliphatic alcohols
greatly enhance the ability of trichlorotrifluoroethane alone to clean
rosin fluxes from printed wiring boards. Unexpectedly, we found that
adding the nonpolar hydrocarbon component hexane with acetone to a mixture
of trichlorotrifluoroethane, alcohol, and nitromethane produces an
apparent synergistic effect which improves the cleaning ability of the
blend. As the above example shows, in the case of boards fluxed with
components on them with highly activated rosin fluxes such as Kester
1585-MIL and Kenco 885, there is a statistically significant improvement
in cleaning ability for the solvent of this invention over the two
commercial defluxing solvents.
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