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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 azeotropic 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
azeotropic, 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 application. For example: U.S. Pat. No. 3,903,009
discloses the ternary azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane
with ethanol and nitromethane; U.S. Pat. No. 2,999,815 discloses the
binary azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane and acetone;
U.S. Pat. No. 2,999,817 discloses the binary azeotrope of
1,1,2-trichloro-1,2,2-trifluoroethane 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
chlorofluorocarbons 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, an azeotrope has been discovered
comprising admixtures of effective amounts of
2,3-dichloro-1,1,1,3,3-pentafluoropropane with methanol. More
specifically, the azeotrope consists essentially of an admixture of about
92-98 weight percent 2,3-dichloro-1,1,1,3,3-pentafluoropropane and about
2-8 weight percent methanol.
The present invention provides nonflammable azeotropic compositions which
are well suited for solvent cleaning, aerosal propellant, blowing agent
and refrigerant applications.
DETAILED DESCRIPTION OF THE INVENTION
The compositions of the instant invention comprise admixtures of effective
amounts of 2,3-dichloro-1,1,1,3,3-pentafluoropropane (CF.sub.3
--CHCl--CClF.sub.2, boiling point=50.4.degree. C.) and methanol (boiling
point=64.6.degree. C.) to form an azeotropic mixture. The aforementioned
halocarbon is known as HCFC-225da, in nomenclature conventional to the
halocarbon field.
By azeotropic composition is meant, a constant boiling liquid admixture of
two 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 azeotropic, exhibit either a
maximum or minimum boiling point, as compared with that of the
nonazeotropic mixtures of the same substances.
For purposes of this invention, "consisting essentially of" is defined as
the amount of each component of the instant invention admixture which,
when combined, results in the formation of the azeotropes of instant
invention. This definition includes the amounts of each component, which
amounts may vary depending upon the pressure applied to the composition,
which will cause a mixture to be formed which exhibits azeotropic
characteristics, albeit over varying pressures and boiling points.
Therefore, "consisting essentially of" includes the weight percentages of
each component of the composition of the present invention, which form
azeotropes at pressures other than atmosphere pressure. "Consisting
essentially of" is not intended to exclude the presence of other materials
which do not significantly affect the azeotropic nature of the azeotrope.
It is possible to characterize, 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 and B, since the very
term "azeotrope" is at once both definitive and limitative, and requires
that effective amounts of A and B 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 and B
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 and B, 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 and B
actually exist for a given azeotrope, varied by the influence of pressure.
Azeotrope A and B 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.
Binary mixtures of 92-98 weight percent
2,3-dichloro-1,1,1,3,3-pentafluoropropane and 2-8 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 binary azeotrope. The binary composition
consisting of about 95.5 weight percent
2,3-dichloro-1,1,1,3,3-pentafluoropropane and 4.5 weight percent methanol
has been established, within the accuracy of the fractional distillation
method, as a true binary azeotrope, boiling at about 45.2.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 instant invention permits easy recovery and reuse of
the solvent from vapor defluxing and degreasing operations because of its
azeotropic nature. In addition, the azeotrope of the present invention is
useful as an aerosol propellant, refrigerant and as a blowing agent for
forming polymeric foams. 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 instant 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
An ebullioscope was used to determine the composition versus boiling point
temperature characteristics for the minimum boiling azeotrope, as follows:
2,3-dichloro-1,1,1,3,3-pentafluoropropane was placed in the distillation
flask and brought to boiling at atmospheric pressure and the boiling
points (vapor and liquid) were recorded. Small quantities of the
individual binary component (methanol) were added to the distillation
apparatus. The distillation was allowed to to reequilibrate for 30 minutes
or less and the boiling points (vapor and liquid) were noted for that
particular mixture composition.
When the mixture temperature reached its lowest boiling point for the given
composition (temperature lower than the boiling points of either pure
component), the temperature recorded was that of the azeotrope, at the
azeotrope composition.
EXAMPLE 2
In order to verify the exact azeotropic composition and temperatures, two
mixtures of 2,3-dichloro-1,1,1,3,3-pentafluoropropane and the individual
binary component (methanol) were prepared with component contents slightly
higher and slightly lower than the azeotropic composition. The mixtures
were distilled in a twenty-five plate oldershaw column, at total reflux.
Minimum boiling azeotropes were achieved with both mixture distillates.
Head temperatures were corrected to 760 mm Hg pressure Azeotropic
compositions were determined by gas chromatography.
A statistical analysis of the distillation data indicates that the true
binary azeotrope of 2,3-dichloro-1,1,1,3,3-pentafluoropropane and methanol
has the following characteristics at atmospheric pressure (99 percent
confidence limits):
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1,3-dichloro-1,1,1,3,3-
=95.5 .+-. 0.4 wt. %
pentafluoropropane
Methanol =4.5 .+-. 0.4 wt. %
Boiling point, .degree.C.
=45.2 .+-. 2.8
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EXAMPLE 3
Several single sided circuit boards were coated with activated rosin flux
and soldering by passing the boards over a preheater, to obtain top side
board temperatures of approximately 200.degree. F. (93.3.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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