Azeotropic Composition of 1,1-Difluoro-2,2-Dichloroethane and Acetone

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.

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.

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.

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.

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.

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.