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Description  |
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FIELD OF THE INVENTION
This invention relates to azeotrope-like mixtures of
trichlorotrifluoroethane, methanol, nitromethane, acetone, and methyl
acetate. 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 and other components which contribute
additionally desired characteristics, such as polar functionality,
hydrogen bonding strength, 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 azeotropic 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 methanol; U.S. Pat. No.
3,960,746 discloses azeotrope-like compositions of
1,1,2-trichloro-1,2,2-trifluoroethane, methanol, and nitromethane; U.S.
Pat. No. 4,268,407 discloses an azeotropic composition comprising of
1,1,2-trichloro-1,2,2-trifluoroethane, methanol, methyl acetate, 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, and
Japanese Pat. No. 73-33878 discloses the ternary azeotrope of
1,1,2-trichloro-1,2,2-trifluoroethane, methanol, and acetone.
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,
nitromethane, acetone and methyl acetate, with
1,1,2-trichloro-1,2,2-trifluoroethane being the trichlorotrifluoroethane
of choice.
In one embodiment of the invention, the azeotrope-like compositions
comprise from about 83.5 to about 93.8 weight percent of
1,1,2-trichloro-1,2,2-trifluoroethane, from about 5.1 to about 6.4 weight
percent of methanol, from about 0.01 to about 1.0 weight percent of
nitromethane, from about 0.3 to about 5.1 weight percent of acetone, and
from about 0.1 to about 6.0 weight percent of methyl acetate.
In a preferred embodiment of the invention, the azeotrope-like compositions
comprise from about 90.5 to about 93.5 weight percent of
1,1,2-trichloro-1,2,2-trifluoroethane, from about 5.7 to about 6.1 weight
percent of methanol, from about 0.05 to about 0.2 weight percent of
nitromethane, from about 0.4 to about 2.0 weight percent acetone, and from
about 0.2 to about 1.7 weight percent methyl acetate.
In the most preferred embodiment of the invention, the azeotrope-like
compositions comprise from about 91.4 to about 93.5 weight percent of
1,1,2-trichloro-1,2,2-trifluoroethane, from about 5.8 to about 6.0 weight
percent of methanol, from about 0.03 to about 0.1 weight percent of
nitromethane, from about 0.6 to about 1.2 weight percent acetone, and from
about 0.4 to 1.2 weight percent methyl acetate. All of the above-described
compositions possess constant or essentially constant boiling points of
about 39.7.degree. C..+-.0.2.degree. C. at 760 mm Hg pressure. The precise
azeotropic composition has not been determined but has been ascertained to
be within the above ranges.
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,
reasonably 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 D1310-80) 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
non-azeotrope-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 methyl acetate 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".
EXAMPLES 1-5
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 most vapor degreaser
systems. For this purpose a five plate Oldershaw distillation column was
used with a cold water condensed, timer controlled magnetically activated
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,
nitromethane, acetone, and methyl acetate. 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 which increases
rectification and 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 39.7.degree..+-.0.2.degree. C. at 760 mm Hg was formed
for compositions comprising about 90.5 to about 93.5 weight percent
1,1,2-trichloro-1,2,2-trifluoroethane (FC-113), about 5.8 to about 5.9
weight percent methanol (MeOH), about 0.01 to about 0.1 weight percent
nitromethane, about 0.3 to about 2.0 weight percent acetone, and about 0.2
to 1.7 weight percent methyl acetate. Supporting distillation data for the
mixtures studied are shown in Table I.
TABLE I
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Starting Material (wt. %)
Methyl
Example FC-113 MeOH MeN0.sub.2
Acetone Acetate
______________________________________
Distil-
lation
(5 plate)
1 83.5 5.1 0.3 5.1 6.0
2 91.5 6.0 0.1 1.2 1.2
3 92.8 5.9 0.3 0.6 0.4
4 92.5 5.8 0.2 0.9 0.6
5 91.3 5.9 0.3 1.8 0.6
______________________________________
Constant Boiling Fraction (wt. %)
Methyl
Example FC-113 MeoH MeNO.sub.2
Acetone Acetate
______________________________________
1 90.5 5.8 0.01 2.0 1.7
2 93.2 5.8 0.03 0.6 0.4
3 93.5 5.9 0.1 0.3 0.2
4 93.5 5.8 0.06 0.5 0.2
5 93.1 5.8 0.05 0.8 0.2
______________________________________
Barometic
Vapor Pressure Corrected Boiling
Example Temp (.degree.C.)
(mm Hg) Point to 760 mm Hg
______________________________________
1 39.5 747.3 40.0
2 39.2 747.8 39.7
3 38.8 743.5 39.5
4 39.0 743.5 39.7
5 39.3 751.0 39.7
39.7.degree. c. .+-. 0.2.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, nitromethane, acetone,
and methyl acetate, nor the outer limits of its compositional ranges which
are constant boiling or essentially constant boiling. As indicated, anyone
of 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 6
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 92.1 weight
percent 1,1,2-trichloro-1,2,2-trifluoroethane (FC-113), about 5.8 weight
percent methanol, about 0.1 weight percent nitromethane, about 1.2 weight
percent acetone, and about 0.8 weight percent methyl acetate. The mixture
was evaluated for its constant boiling or non-segregating characteristics.
The vapor phase degreasing machine utilized was a small water-cooled,
three-sump vapor phase degreaser with an attached still, which represents
a type of system configuration comparable to machine types in the field
today which would present the most rigorous test of solvent segregating
behavior. Specifically, the degreaser employed to demonstrate the
invention contains two overflowing rinse-sumps and a boil-sump. The sump
adjacent to the boil-sump is referred to as the work sump. The boil-sump
and the still are electrically heated, and each contains a low-level
shut-off switch. Solvent vapors in both the degreaser and the still are
condensed on water-cooled stainless-steel coils. The still is fed by
gravity from the boil-sump. Condensate from the still is returned to the
first rinse-sump, also by gravity. The capacity of the unit is
approximately 3.5 gallons. This degreaser is very similar to Baron
Blakeslee 2 LLV 3-sump degreasers with an attached still which are quite
commonly used in commercial establishments.
The solvent charge was brought to reflux and the compositions in the rinse
sump containing the clear condensate from the still, the work sump
containing the overflow from the rinse sump, the boil sump where the
overflow from the work sump is brought to the mixture boiling point, and
the still were determined with a Perkin Elmer Sigma 3 gas chromatograph.
The temperature of the liquid in the boil sump and still was monitored
with a thermocouple temperature sensing device accurate to .+-.0.2.degree.
C. Refluxing was continued for 48 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 .+-.2 sigma around the mean value. Sigma is
a standard deviation unit and it is our experience from many observations
of vapor degreaser performance that commercial "azeotrope-like" vapor
phase degreasing solvents exhibit less than a .+-.2 sigma variation in
composition with time and yet produce very satisfactory non-segregating
cleaning behavior.
If the mixture were not azeotrope-like, the high boiling components would
very quickly concentrate in the still and be depleted in the rinse sump.
This did not happen. Also, the concentration of each component in the
sumps stayed well within .+-.2 sigma. These results indicate that the
compositions of this invention will not segregate in any types of
large-scale commercial vapor degreasers, thereby avoiding potential
safety, performance, and handling problems. The preferred composition
tested was also found not to have a flash point according to recommended
procedures ASTM D 56-79 (Tag Closed Cup) and ASTM D 1310-80 (Tag Open
Cup).
EXAMPLE 7
This example illustrates the use of the preferred azeotrope-like
composition of the invention to clean (deflux) printed wiring boards and
printed wiring assemblies.
Three commercial rosin-based fluxes were used in this test. The fluxes were
Alpha 611F (manufactured by Alpha Metals Inc.), 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 Alpha 611F and Kester 1585-MIL
fluxes, 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 initial 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 Alpha 611F and 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 90.7
weight percent of 1,1,2-trichloro-1,2,2-trifluoroethane, about 5.7 weight
percent of methanol, about 0.1 weight percent of nitromethane, about 1.9
weight percent of acetone, and about 1.6 weight percent of methyl acetate.
The cleaning performance of the azeotrope-like composition 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 azeotrope-like composition of this invention,
Genesolv.RTM. DMS and Freon.RTM. TMS.
TABLE II
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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.
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This invention
AlPHA 1.25 1.49 2.88 3.33
611
Genesolv .RTM. DMS
ALPHA 1.68 2.07 3.79 4.40
611
Freon .RTM. TMS
ALPHA 1.76 2.15 4.20 4.91
611
This invention
Kester
1585-MIL 3.50 4.16 7.00 8.06
Genesolv .RTM. DMS
Kester
1585-MIL 5.96 6.92 12.38 14.29
Freon .RTM. TMS
Kester
1585-MIL 8.64 9.75 19.38 21.37
This invention
Kenco 885 7.26 9.02 15.28 18.27
Genesolv .RTM. DMS
Kenco 885 14.95 17.61 30.93 35.95
Freon .RTM. TMS
Kenco 885 9.67 11.24 27.72 31.51
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As stated earlier, the industry has recognized that admixtures of
trichlorotrifluoroethane with strongly hydrogen bonding components such as
aliphatic alcohols, especially methanol, greatly enhance the ability of
trichlorotrifluoroethane alone to remove the ionic activator components of
rosin fluxes from printed wiring boards. Unexpectedly, we found that
adding other solvents such as acetone and methyl acetate (which are not as
strongly hydrogen bonding as methanol) to a mixture of
trichlorotrifluoroethane, alcohol(s), and nitromethane produces an
apparent synergistic effect which improves the cleaning ability of the
blend. As the above example shows, particularly in the case of boards
fluxed 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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