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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 cleaning of the boards after soldering
becomes more important. Current industrial processes for soldering
electronic components to circuit boards involve coating the entire circuit
side of the board with a flux and thereafter passing this coated side of
the board over preheaters and through molten solder. The flux cleans the
conductive metal parts and promotes adhesion of the solder. Commonly used
fluxes consist, for the most part, of rosin 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 and
flux residues are often removed from the board with an organic solvent.
The requirements of such a solvent are stringent: a solvent should have a
low boiling point, have low toxicity and exhibit high solvent power so
that flux and flux residues can be removed without damage to the substrate
being cleaned.
While boiling, flammability and solvent power characteritics can often be
adjusted by preparing mixtures of solvents, these mixtures are often
unsatisfactory because they fractionate to an undesirable degree during
evaporation or boiling. Such mixtures also fractionate during recovery,
making it difficult to reuse a solvent mixture with the original
composition.
On the other hand, azeotropic mixtures, with their constant boiling and
constant composition characteristics, have been found to very useful.
Azeotropic mixtures exhibit either a maximum or minimum boiling point and
do not fractionate upon boiling. These characteristics are also important
in the use of the solvent compositions to remove solder fluxes and flux
residues from printed circuit boards. Preferential evaporation of the more
volatile components of the solvent mixtures, which would be the case if
they were not azeotropes or azeotrope-like, would result in mixtures with
changed compositions having less desirable properties, such as lower
solvency for rosin fluxes and less inertness toward the electrical
components. Unchanging composition during use is also desirable in vapor
degreasing operations where redistilled material is generally used for
final rinse-cleaning. Thus, the vapor defluxing and degreasing systems act
as a still. Unless the solvent composition exhibits a constant boiling
point, i.e., is a pure component, an azeotrope or azeotrope-like,
fractionation will occur and undesirable solvent distribution may act to
upset the safety and effectiveness of the cleaning operation.
A number of chlorofluorocarbon-based azeotropic compositions have been
discovered and, in some cases, used as solvents for the removal of solder
fluxes and flux residues from printed circuit boards and for miscellaneous
vapor degreasing applications. Some of these chlorofluorocarbons currently
being used for cleaning and other applications have been theoretically
linked to the depletion of the ozone layer. As early as the 1970's, with
the initial emergence of the ozone theory, it was known that the
introduction of the hydrogen moiety into previously fully halogenated
chlorofluorocarbons reduced the chemical stability of these compounds.
Hence, these now destabilized hydrogen-containing compounds would be
expected to degrade in the lower atmosphere and not reach the
stratospheric ozone layer. What is also needed, therefore, are substitute
chlorofluorocarbons which have low theoretical ozone depletion potential.
Unfortunately, as is recognized in the art, it is not possible to predict
the formation of azeotropes. This 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 or
azeotrope-like composition which have desirable solvency characteristics
and particularly a greater range of solvency power.
SUMMARY OF THE INVENTION
According to the present invention, an azeotrope or azeotrope-like
composition has been discovered comprising an admixture of effective
amounts of 1,1-difluoro-2,2-dichloroethane and acetone, which comprises an
admixture of about 69-75 weight percent 1,1-difluoro-2,2-dichloroethane
and about 31-25 weight percent acetone.
The present invention provides an azeotropic composition which is well
suited for solvent cleaning and other applications.
DETAILED DESCRIPTION OF THE INVENTION
The compositions of the instant invention comprise admixtures of effective
amounts of 1,1-difluoro-2,2-dichloroethane (CHF.sub.2 CHCl.sub.2, normal
boiling point about 60.0.degree. C.), and acetone to form an azeotrope or
azeotrope-like composition. The fluorinated material is also known as
HCFC-132a in the nomenclature conventional to fluorinated aliphatic
compounds.
By azeotrope or azeotrope-like composition is meant constant boiling liquid
admixtures of two or more substances. These admixtures behave like a
single substance in that the vapor produced by partial evaporation or
distillation has the same, or substantially the same, composition as does
the liquid, i.e., the admixtures distill without a substantial change in
composition. Constant boiling compositions characterized as azeotropes or
azeotrope-like exhibit either a maximum or minimum boiling point as
compared with that of nonazeotropic mixtures of the same substances.
By effective amount is meant the amount of each component of the admixture
of the instant invention, which when combined results in the formation of
the azeotrope or azeotrope-like composition of the instant invention.
It is possible to characterize a constant boiling admixture, which may
appear under varying guises depending on the conditions chosen, by any of
several criteria:
The composition can be defined by an azeotrope of A and B, since the very
term "azeotrope" is at once both definitive and limitative, requiring 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 who are skilled in the art that, at differing
pressures, the composition of a given azeotrope will vary, at least to
some degree. Changes in pressure 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 depending upon
temperature and/or pressure. Therefore, compositional ranges, rather than
fixed compositions, are often used to define azeotropes.
Or, the composition can be defined as a particular weight percent
relationship or mol percent relationship of A and B, while recognizing
that such specific values describe only one particular such relationship
and that, in actuality, a series of such relationships represented by A
and B actually exists for a given azeotrope, varying with changes in
pressure.
Or, recognizing that the azeotrope A and B does represent just such a
series of relationships, the azeotropic series represented by A and B can
be characterized by defining the azeotrope as a composition characterized
by a boiling point at a given pressure. Thus, identifying characteristics,
are given 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.
A binary mixture of 69-75 weight percent HCFC-132a and 31-25 weight percent
acetone is characterized as an azeotrope or azeotrope-like composition in
that the 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 or boiling. 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 71.6
weight percent HCFC-132a and 28.4 weight percent acetone has been
established, within the accuracy of the fractional distillation method, as
a true binary azeotrope, boiling at about 66.0.degree. C. at substantially
atmospheric pressure. It is the preferred azeotropic composition of the
instant invention.
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 is described in U.S. Pat. No.
3,881,949, which is incorporated herein by reference.
Another important advantage of the azeotrope or azeotrope-like composition
of the instant invention is that the hydrochlorofluorocarbon component,
HCFC-132a, has a low ozone depletion potential of about 0.05 relative to
fluorotrichloromethane (CFC-11). It may be useful as a substitute for
chlorofluorocarbons currently being used for cleaning and other
applications which have higher ozone depletion potentials.
The azeotropes of the instant invention can be prepared by any convenient
method including mixing or combining the desired amounts of the
components. A preferred method is to weigh the desired amounts of each
component, combine them in an appropriate container and mix them
thoroughly.
EXAMPLE 1
An apparatus consisting of a flask and a total reflux condenser was used to
determine the composition versus boiling temperature characteristics for
the azeotrope. Pure HCFC-132a was placed in the flask and brought to
boiling at atmospheric pressure, and the temperature of the boiling liquid
and the vapor above the boiling liquid were recorded. Small quantities of
acetone were then added to the flask. Boiling under reflux was allowed to
re-equilibrate after each addition for 10-30 minutes, and the temperatures
of the boiling liquid and the vapor above the boiling liquid were noted
for each particular mixture composition. The mixture reached a temperature
higher than the normal boiling point of each pure component, confirming a
maximum boiling azeotrope. The maximum temperature recorded was that of
the azeotrope at the azeotropic composition.
EXAMPLE 2
In order to verify the exact azeotropic composition and temperature, two
mixtures of HCFC-132a and acetone were prepared with the acetone contents
slightly higher and slightly lower than the azeotropic composition. The
mixtures were distilled separately in a distillation apparatus using a
packed column which contained approximately 24 theoretical plates at total
reflux. After removing the low boiling component (acetone) in foreshots, a
maximum boiling azeotrope distilled over in several heart cuts. The
azeotropic composition was determined by gas chromatography to be 71.6
weight percent 1,1-difluoro-2,2-dichloroethane and 28.4 weight percent
acetone.
EXAMPLE 3
A single-sided circuit board was coated with activated rosin flux, and
soldered by passing the board over a preheater to obtain a top side board
temperature of approximately 200.degree. F. and then through 500.degree.
F. molten solder. The soldered board was defluxed in an azeotropic mixture
of 71.6 weight percent HCFC-132a and 28.4 weight percent acetone by
suspending it, first, for three minutes in the boiling sump, then one
minute in the rinse sump and, thereafter, for one minute in the solvent
vapor above the boiling sump. The board thus cleaned had no visible
residue remaining on it.
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