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Description  |
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BACKGROUND OF THE INVENTION
As modern electronic circuit boards evolve toward increased circuit and
component densities, thorough board cleaning after soldering becomes a
more important criterion. Current industrial processes for soldering
electronic components to circuit boards involve coating the entire circuit
side of the board with flux and thereafter passing the flux-coated board
over preheaters and through molten solder. The flux cleans the conductive
metal parts and promotes solder fusion. Commonly used solder fluxes
generally consist of rosin, either used alone or with activating
additives, such as amine hydrochlorides or oxalic acid derivatives.
After soldering, which thermally degrades part of the rosin, the
flux-residues are often removed from the circuit boards with an organic
solvent. The requirements for such solvents are very stringent. Defluxing
solvents should have the following characteristics: a low boiling point,
be nonflammable, have low toxicity and have high solvency power, so that
flux and flux-residues can be removed without damaging the substrate being
cleaned.
While boiling point, flammability and solvent power characteristics can
often be adjusted by preparing solvent mixtures, these mixtures are often
unsatisfactory because they fractionate to an undesirable degree during
use. Such solvent mixtures also fractionate during solvent distillation,
which makes it virtually impossible to recover a solvent mixture with the
original composition.
On the other hand, azeotropic mixtures, with their constant boiling points
and constant compositions, have been found to be very useful for these
applications. Azeotropic mixtures exhibit either a maximum or minimum
boiling point and they do not fractionate on boiling. These
characteristics are also important when using solvent compositions to
remove solder fluxes and flux-residues from printed circuit boards.
Preferential evaporation of the more volatile solvent mixture components
would occur, if the mixtures were not azeotropes and would result in
mixtures with changed compositions, and with attendant less-desirable
solvency properties, such as lower rosin flux solvency and lower inertness
toward the electrical components being cleaned. The azeotropic character
is also desirable in vapor degreasing operations, where redistilled
solvent is generally employed for final rinse cleaning. In summary, vapor
defluxing and degreasing systems act as a still. Unless the solvent
composition exhibits a constant boiling point, i.e., is an azeotrope,
fractionation will occur and undesirable solvent distributions will
result, which could detrimentally affect the safety and efficacy of the
cleaning operation.
A number of chlorofluorocarbon based azeotropic compositions have been
discovered and in some cases used as solvents for solder flux and
flux-residue removal from printed circuit boards and also for
miscellaneous degreasing applications. For example: U.S. Pat. No.
3,903,009 discloses the ternary azeotrope of
1,1,2-trichlorotrifluoroethane with ethanol and nitromethane; U.S. Pat.
No. 2,999,815 discloses the binary azeotrope of
1,1,2-trichlorotrifluoroethane and acetone; U.S. Pat. No. 2,999,817
discloses the binary azeotrope of 1,1,2-trichlorotrifluoroethane and
methylene chloride.
Some of the chlorofluorocarbons which are currently used for cleaning and
other applications have been theoretically linked to depletion of the
earth's ozone layer. As early as the mid-1970's, it was known that
introduction of hydrogen into the chemical structure of previously
fully-halogenated chlorofluorocarbons reduced the chemical stability of
these compounds. Hence, these now destabilized compounds would be expected
to degrade in the lower atmosphere and not reach the stratospheric ozone
layer in-tact. What is also needed, therefore, are substitute
hydrochlorofluorocarbons which have low theoretical ozone depletion
potentials.
Unfortunately, as recognized in the art, it is not possible to predict the
formation of azeotropes. This fact obviously complicates the search for
new azeotropic compositions, which have application in the field.
Nevertheless, there is a constant effort in the art to discover new
azeotropes, which have desirable solvency characteristics and particularly
greater versatilities in solvency power.
SUMMARY OF THE INVENTION
According to the present invention, azeotropic compositions have been
discovered comprising admixtures of effective amounts of
2,3-dichloro-1,1,1,3,3-pentafluoropropane with trans-1,2-dichloroethylene
and methanol. The azeotrope is an admixture of about 43-53 weight percent
2,3-dichloro-1,1,1,3,3-pentafluoropropane and about 42-52 weight percent
trans-1,2-dichloroethylene
and about 3-9 weight percent methanol. The present invention provides
nonflammable azeotropic compositions which are well suited for solvent
cleaning applications.
DETAILED DESCRIPTION OF THE INVENTION
The composition of the instant invention comprises an admixture of
effective amounts of 2,3-dichloro-1,1,1,3,3-pentafluoropropane (CF3--CHCl
--CClF.sub.2, bOiling point = 50.4.degree. C.) and
trans-1,2-dichloroethylene (CHCl=CHCl, boiling point = 48.0.degree. C.)
and methanol (boiling point =64.6.degree. C.), to form an azeotrope or
azeotrope-like mixture. The aforementioned halocarbons are known as
HCFC-225da and trans-HCC-1130, respectively, in the nomenclature
conventional to the halocarbon field.
By consisting essentially of an azeotrope is meant, a constant boiling
liquid admixture of three or more substances, whose admixture behaves as a
single substance, in that the vapor, produced by partial evaporation or
distillation of the liquid has the same composition as the liquid, i.e.,
the admixture distills without substantial composition change. Constant
boiling compositions, which are characterized as azeotropes, exhibit
either a maximum or minimum boiling point, as compared with that of the
nonazeotropic mixtures of the same substances.
By consisting essentially of an azeotrope is meant the amount of each
component of the instant invention admixture, which when combined, results
in the formation of the azeotrope of the instant invention. The language
"consisting essentially of an azeotrope" is not meant to exclude the
presence of other materials which do not significantly alter the
azeotropic behavior of the ternary azeotropic composition of the present
invention.
It is possible to fingerprint, in effect, a constant boiling admixture,
which may appear under many guises, depending upon the conditions chosen,
by any of several criteria:
* The composition can be defined as an azeotrope of A, B and C, since the
very term "azeotrope" is at once both definitive and limitative, and
requires that effective amounts A, B and C form this unique composition of
matter, which is a constant boiling admixture.
* It is well known by those skilled in the art that at different pressures,
the composition of a given azeotrope will vary -- at least to some degree
and changes in pressure will also change -- at least to some degree -- the
boiling point temperature. Thus an azeotrope of A, B and C represents a
unique type of relationship but with a variable composition which depends
on temperature and/or pressure. Therefore compositional ranges, rather
than fixed compositions, are often used to define azeotropes.
* The composition can be defined as a particular weight percent
relationship or mole percent relationship of A, B and C, while recognizing
that such specific values point out only one particular such relationship
and that in actuality, a series of such relationships, represented by A, B
and C actually exist for a given azeotrope, varied by the influence of
pressure.
* Azeotrope A, B and C can be characterized by defining the composition as
an azeotrope characterized by a boiling point at a given pressure, thus
giving identifying characteristics without unduly limiting the scope of
the invention by a specific numerical composition, which is limited by and
is only as accurate as the analytical equipment available.
Ternary mixtures of 43-53 weight percent 35
2,3-dichloro-1,1,1,3,3-pentafluoropropane and 42-52 weight percent
trans-1,2-dichloroethylene and 3-9 weight percent methanol are
characterized as azeotropes, in that mixtures within this range exhibit a
substantially constant boiling point at constant pressure. Being
substantially constant boiling, the mixtures do not tend to fractionate to
any great extent upon evaporation. After evaporation, only a small
difference exists between the composition of the vapor and the composition
of the initial liquid phase. This difference is such that the compositions
of the vapor and liquid phases are considered substantially identical.
Accordingly, any mixture within this range exhibits properties which are
characteristic of a true ternary azeotrope. The ternary composition
consisting of about 47.7 weight percent
2,3-dichloro-1,1,1,3,3-pentafluoropropane, 46.7 weight percent
trans-1,2-dichloroethylene and 5.6 weight percent methanol has been
established, within the accuracy of the fractional distillation method, as
a true ternary azeotrope, boiling at about 41.0.degree. C., at
substantially atmospheric pressure.
The aforestated azeotrope has a low ozone-depletion potential and is
expected to decompose almost completely, prior to reaching the
stratosphere.
The azeotrope of the present invention permits easy recovery and reuse of
the solvent from vapor defluxing and degreasing operations because of its
azeotropic nature. As an example, the azeotropic mixture of this invention
can be used in cleaning processes such as described in U.S. Pat. No.
3,881,949, which is incorporated herein by reference.
The azeotrope of the present invention can be prepared by any convenient
method including mixing or combining the desired component amounts. A
preferred method is to weigh the desired component amounts and thereafter
combine them in an appropriate container.
EXAMPLE 1
A solution which contained 58.7 weight percent
2,3-dichloro-1,1,1,3,3-pentafluoropropane, 36.8 weight percent
trans-1,2-dichloroethylene and 4.5 weight percent methanol was prepared in
a suitable container and mixed thoroughly.
The solution was distilled in 25 plate Oldershaw distillation column, using
about a 5:1 reflux to take-off ratio. Head and pot temperatures were read
directly to 0.1C. All temperatures were adjusted to 760 mm pressure.
Distillate compositions were determined by gas chromatography. Results
obtained are summarized in Table 1.
TABLE 1
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Distillation of:
(58.7 + 36.8 + 4.5)
2,3-DICHLORO-1,1,1,3,3-PENTAFLUOROPROPANE
(DCPFP),
TRANS-1,2-DICHLOROETHYLENE (T-DCE)
AND METHANOL (MEOH)
Wt. %
Temperature,
Distilled
.degree.C. or
Cuts Head Recovered DCPFP T-DCE MEOH
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1 41.0 1.9 47.5 46.9 5.6
2 40.9 11.5 47.4 47.0 5.6
3 41.0 19.8 47.4 46.9 5.7
4 41.0 28.7 47.5 46.8 5.7
5 41.0 38.0 47.5 46.8 5.7
6 41.1 48.8 48.3 46.2 5.5
7 41.5 57.3 50.4 44.3 5.3
heel -- 87.9 85.9 12.2 1.9
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Analysis of the above data indicates nearly constant head temperatures and
distillate compositions, as the distillation progressed. A statistical
analysis of the data indicates that the true ternary azeotrope of
2,3-dichloro-1,1,1,3,3-pentafluoropropane, trans-1,2-dichloroethylene and
methanol has the following characteristics at atmospheric pressure (99
percent confidence limits):
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2,3-Dichloro-1,1,1,3,3-penta-
= 47.7 .+-. 1.8 wt. %
fluoropropane
trans-1,2-Dichloroethylene
= 46.7 .+-. 1.5 wt. %
Methanol = 5.6 .+-. 0.4 wt. %
Boiling Point, .degree.C.
= 41.0 .+-. 0.3
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EXAMPLE 2
Several single sided circuit boards were coated with activated rosin flux
and soldered by passing the boards over a preheater, to obtain top side
board temperatures of approximately 200.degree. F. (93.degree. C.), and
then through 500.degree. F. (260.degree. C.) molten solder. The soldered
boards were defluxed separately, with the azeotropic mixture cited in
Example 1 above, by suspending a circuit board, first, for three minutes
in the boiling sump, which contained the azeotropic mixture, then, for one
minute in the rinse sump, which contained the same azeotropic mixture, and
finally, for one minute in the solvent vapor above the boiling sump. The
boards cleaned in the azeotropic mixture had no visible residue remaining
thereon.
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