Azeotropic composition of 1,1,1,2-tetrafluoroethane and dimethyl ether

Abstract: The use of constant boiling mixtures of 1,1,1,2-tetrafluoroethane and dimethylether as blowing agents is disclosed for manufacture of polymer foams.

Description: Background of the Invention This invention relates to mixtures of 1,1,1,2-tetrafluoroethane (HFC-134a) and dimethyl ether (DME). Such mixtures are useful as foam expansion agents. These mixtures are potentially environmentally safe substitutes for dichlorodifluoromethane (CFC-12), which is a large volume commercial refrigerant. Closed-cell polyurethane foams are widely used for insulation purposes in building construction and in the manufacture of energy efficient electrical appliances. In the construction industry, polyurethane (polyisocyanurate) board stock is used in roofing and siding for its insulation and load-carrying capabilities. Poured and sprayed polyurethane foams are also used in construction. Sprayed polyurethane foams are widely used for insulating large structures such as storage tanks, etc. Pour-in-place polyurethane foams are used, for example, in appliances such as refrigerators and freezers plus they are used in making refrigerated trucks and railcars. All of these various types of polyurethane foams require expansion agents (blowing agents) for their manufacture. Insulating foams depend on the use of halocarbon blowing agents, not only to foam the polymer, but primarily for their low vapor thermal conductivity, a very important characteristic for insulation value. Historically, polyurethane foams are made with CFC-11 (CFCl 3 ) as the primary blowing agent. A second important type of insulating foam is phenolic foam. These foams, which have very attractive flammability characteristics, are generally made with CFC-11 and CFC-113 (1,1,2-trichloro-1,2,2-trifluoroethane) blowing agents. A third type of insulating foam is thermoplastic foam, primarily polystyrene foam. Polyolefin foams (polyethylene and polypropylene) are widely used in packaging. These thermoplastic foams are generally made with CFC-12. US-A-4 771 080 describes an expandable vinylidene chloride composition, a foam therefrom, and a process therefor, wherein the volatile organic blowing agent is selected from those agents having a lower boiling point than the T G of the copolymer composition. In the early 1970's, concern began to be expressed that the stratospheric ozone layer (which provides protection against penetration of the earth's atmosphere by ultraviolet radiation) was being depleted by chlorine atoms introduced to the atmosphere from the release of fully halogenated chlorofluorocarbons. These chlorofluorocarbons are used as propellants in aerosols, as blowing agents for foams, as refrigerants and as cleaning/drying solvent systems. Because of the great chemical stability of fully halogenated chlorofluorocarbons, according to the ozone depletion theory, these compounds do not decompose in the earth's atmosphere but reach the stratosphere where they slowly degrade liberating chlorine atoms which in turn react with the ozone. Concern reached such a level that in 1978 the U.S. Environmental Protection Agency (EPA) placed a ban on nonessential uses of fully halogenated chlorofluorocarbons as aerosol propellants. This ban resulted in a dramatic shift in the U.S. away from chlorofluorocarbon propellants (except for exempted uses) to primarily hydrocarbon propellants. However, since the rest of the world did not join the U.S. in this aerosol ban, the net result has been to shift the uses of chlorofluorocarbons in aerosols out of the U.S., but not to permanently reduce the world-wide total chlorofluorocarbon production, as sought. In fact, in the last few years the total amount of chlorofluorocarbons manufactured worldwide has exceeded the level produced in 1978 (before the U.S. ban). During the period of 1978-1987, much research was conducted to study the ozone depletion theory. Because of the complexity of atmospheric chemistry, many questions relating to this theory remained unanswered. However, assuming the theory to be valid, the health risks which would result from depletion of the ozone layer are significant. This, coupled with the fact that world-wide production of chlorofluorocarbons has increased, has resulted in international efforts to reduce chlorofluorocarbon use. Particularly, in September, 1987, the United Nations through its Environment Programme (UNEP) issued a tentative proposal calling for a 50 percent reduction in world-wide production of fully halogenated chlorofluorocarbons by the year 1998. This proposal was ratified January 1, 1989 and became effective on July 1, 1989. Because of this proposed reduction in availability of fully halogenated chlorofluorocarbons such as CFC-11, CFC-12 and CFC-113, alternative, more environmentally acceptable, products are urgently needed. As early as the 1970's with the initial emergence of the ozone depletion theory, it was known that the introduction of hydrogen into previously fully halogenated chlorofluorocarbons markedly reduced the chemical stability of these compounds. Hence, these now destabilized compounds would be expected to degrade in the atmosphere and not reach the stratosphere and the ozone layer. The accompanying Table lists the ozone depletion potential for a variety of fully and partially halogenated halocarbons. Halocarbon Global Warming Potential data (potential for reflecting infrared radiation (heat) back to earth and thereby raising the earth's surface temperature) are also shown. OZONE DEPLETION AND HALOCARBON GLOBAL WARMING POTENTIALS OZONE DEPLETION AND HALOCARBON GLOBAL WARMING POTENTIALS Blowing Agent Ozone Depletion Potential Halocarbon Global Warming Potential CFC-11 (CFCl 3 ) 1.0 1.0 CFC-12 (CF 2 Cl 2 ) 1.0 2.8 HCFC-22 (CHF 2 Cl) 0.05 0.3 HCFC-123 (CF 3 CHCl 2 ) 0.02 0.02 HCFC-124 (CF 3 CHFCl) 0.02 0.09 HFC-134a (CF 3 CH 2 F) 0 0.3 HCFC-141b (CFCl 2 CH 3 ) 0.1 0.09 HCFC-142b (CF 2 ClCH 3 ) 0.06 0.3 HFC-152a (CHF 2 CH 3 ) 0 0.03 CFC-113 (CF 2 Cl-CFCl 2 ) 0.8-0.9 1.4 Halocarbons such as HFC-134a have zero ozone depletion potential. Dimethyl ether, having no halogen content, is also a zero czone depleter. Although 1,1,1,2-tetrafluoroethane has utility as a refrigerant, aerosol propellant or foam blowing agent, an azeotrope offers the possibility of producing more economical nonfractionating systems with improved properties such as polymer and refrigerant oil solubility. Unfortunately, as recognized in the art, it is not possible to predict the formation of azeotropes. This fact obviously complicates the search for new azeotropes which have application in the field. Nevertheless, there is a constant effort in the art to discover new azeotropic compositions, which have desirable characteristics. Summary of the Invention In accordance with the present invention, azeotropic and other mixtures have been discovered, which consist essentially of 1,1,1,2-tetrafluoroethane (HFC-134a) and dimethyl ether (DME). Included among the mixtures is an azeotrope with the composition 50 weight percent HFC-134a and 50 weight percent DME at 22°C (± 5 weight percent). Therefore, one azeotropic composition to be used in the invention consists essentially of about 45-55 weight percent of 1,1,1,2-tetrafluoroethane and 55-45 weight percent of DME, at 22°C. For the purpose of this discussion, azeotropic or constant boiling is intended to mean also essentially azeotropic or essentially constant boiling. In other words, included within the meaning of these terms are not only the true azeotrope described above, but also other compositions containing the same components in different proportions, which are true azeotropes at other temperatures and pressures, as well as those equivalent compositions which are part of the same azeotropic system and are azeotrope-like in their properties. As is well recognized in this art, there is a range of compositions which contain the same components as the azeotrope, which not only will exhibit essentially equivalent properties for refrigeration and other applications, but which will also exhibit essentially equivalent properties to the true azeotropic composition in terms of constant boiling characteristics or tendency not to separate or fractionate on boiling. The azeotropic compositions described in the invention are useful as expansion agents among other applications. They have zero ozone depletion potential opposite CFC-12, which is rated at 1.0. Mixtures of HFC-134a and DME may be formulated such that they are nonflammable. Therefore, also included among the compositions used in this invention are nonflammable compositions containing 0.5 to 9.0 weight percent DME and 91.0 to 99.5 weight percent HFC-134a. A preferred composition used in the invention consists of 40.0-99.5 wt. % HFC-134a and 0.5-60.0 wt. % DME. Preferably the nonflammable compositions will contain at least about 91.0 weight percent HFC-134a. The azeotropes are particularly attractive as they do not separate or fractionate when used. The HFC-134a/DME azeotropic and constant boiling azeotrope-like compositions are excellent polymer blowing agents. The dimethyl ether component unexpectedly solubilizes the poorly soluble HFC-134a in polyurethane, phenolic and polystyrene foam, resulting in excellent insulating foams. Once solubilized in the foam, the HFC-134a insulating gas does not diffuse out of the foam. The novel azeotropic composition used in the invention was discovered during a phase study wherein the compositions were varied and vapor pressures measured. The azeotropic composition occurred at the minimum point of the vapor pressure-concentration plot, being at 50 weight percent HFC-134a and 50 weight percent DME at 22°C (± 5 weight percent). The azeotrope has an atmospheric pressure boiling point of -22.7°C., compared with -26.5°C for HFC-134a and -24.6°C for DME. The language "an azeotropic composition consisting essentially of..." is intended to include mixtures which contain all the components of the azeotrope of this invention (in any amounts) and which, when fractionally distilled, would produce an azeotrope containing all the components in at least one fraction, alone or in combination with another compound, e.g., one which distills at substantially the same temperature as said fraction. The nonflammable mixtures of this invention consist essentially of effective amounts of HFC-134a and DME within the range of about 91.0 to 99.5 weight percent of HFC-134a and 0.5 to 9.0 weight percent of DME. More usually, for purposes of the invention, the mixtures will contain from about 40.0 to 99.5 weight percent of HFC-134a, the remainder being DME. The compositions of this invention have vapor pressures values near that of CFC-12. They also show zero ozone depletion potential and the amount of dimethyl ether can be chosen such that the composition is nonflammable. By vapor pressure substantially equal to the vapor pressure of dichloro-difluoromethane (CFC-12) is meant a vapor pressure which is within twenty percent of the vapor pressure of CFC-12 at the same temperature. The vapor pressure of CFC-12 is described in "Handbook of Chemistry and Physics", 50th Edition, page D-163. As stated above, the preferred binary 1,1,1,2-tetrafluoroethane/DME compositions are essentially nonflammable. By nonflammable is meant a gas mixture which in air will not burn when subjected to a spark igniter as described in "Limits of Flammability of Gases and Vapors", Bulletin 503, H. F. Coward, et.al., Washington, U.S. Bureau of Mines, 1952. The HFC-134a/dimethyl ether azeotrope of the invention has a vapor pressure at 70°F (21°C) of about 58 psig (400 kPa). The HFC-134a/dimethyl ether azeotrope has been determined to be a good solvent for polystyrene. Thus the azeotrope and, more particularly, the nonflammable mixtures of HFC-134a and dimethyl ether are excellent blowing agents for polystyrene and will make it possible to solubilize HFC-134a in polystyrene. Additionally, the HFC-134a/dimethyl ether azeotrope is soluble in polyurethane polyols; whereas, HFC-134a alone has quite poor solubility. The compositions of the azeotropic mixtures used in the instant invention can be prepared by any convenient method including mixing or combining, by other suitable methods, the desired amounts of the components, using techniques well-known to the art. Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. In the foregoing and in the following examples, all temperatures are set forth uncorrected in degrees Celsius and unless otherwise indicated, all parts and percentages are by weight. EXAMPLES Example 1 A phase study is made on 1,1,1,2-tetrafluoroethane and dimethyl ether wherein the composition is varied and the vapor pressures measured, at a temperature of 22°C. An azeotropic composition is obtained as evidenced by the minimum vapor pressure observed and is identified as 50.0 weight percent 1,1,1,2-tetrafluoroethane and 50.0 weight percent dimethyl ether. (± 5 weight percent). Example 2 An evaluation of the refrigeration properties of the mixtures of the invention versus dichlorodifluoromethane (CFC-12) and 1,1,1,2-tetrafluoroethane (HFC-134a), respectively, are shown in the following Table. The data are generated on a one ton basis, that is to say, on the removal of heat from a space at the rate of 12,000 Btu/hr (12.66 x 10 6 J/hr). The data are based on the ideal refrigeration cycle. COMPARISON OF REFRIGERATION PERFORMANCES Weight Percentages CFC-12 HFC-134a 46% HFC-134a 54% DME 90% HFC-134a 10% DME Evaporator Temp., °F 35.0 35.0 35.0 35.0 (°C) (1.67) (1.67) (1.67) (1.67) Evaporator Press., psig 47.2 45.1 39.7 42.3 (kPa) (325) (311) (274) (292) Condenser Temp., °F 150.0 150.0 150.0 150.0 (°C) (65.6) (65.6) (65.6) (65.6) Condenser Press., psia 249.5 274.9 239.6 261.4 (kPa) (1717) (1895) (1652) (1802) Superheat, °F. with Vapor-Liq. Heat Exchange 30.0 30.0 30.0 30.0 (°C) (- 1.1) (- 1.1) (- 1.1) (- 1.1) Coefficient of Performance 2.96 2.81 3.12 2.91 Compressor Displacement, Ft. 3 /min/ton 4.44 4.55 4.56 4.61 Coefficient of Performance (COP) is a measure of refrigerant energy efficiency. For a refrigeration cycle typified by the above conditions, the COP of the (46/54) weight percent HFC-134a/DME composition represents a somewhat better performance than both dichlorodifluoromethane (CFC-12), the industry refrigerant standard and HFC-134a (1,1,1,2-tetrafluoroethane) alone. This azeotropic mixture is flammable. The 90/10 weight percent HFC-134a/DME mixture has a COP of 2.91, which is near that of CFC-12 and is better than HFC-134a alone. Additives such as lubricants, corrosion inhibitors, stabilizers, dyes and other appropriate materials may be added to the novel compositions for a variety of purposes provided they do not have an adverse influence on the composition, for their intended applications. Example 3 An aerosol room freshener was prepared with the HFC-134a/dimethyl ether azeotrope. The formulation and vapor pressure are shown in Table II. AEROSOL ROOM FRESHENER FORMULATION Ingredient Wt. % Perfume 2.0 HFC-134a/Dimethyl ether (46/54) 98.0 Vapor Pressure at 70°F, psig (at 21°C, kPa) 60.5 (417) Example 4 The solubility of the HFC-134a/dimethyl ether azeotrope is determined in a polyurethane polyol. The azeotrope is readily soluble at 30.0 weight percent; whereas, HFC-134a is insoluble. The solubility data are summarized in Table III. SOLUBILITY OF HFC-134a/DIMETHYL ETHER AZEOTROPE IN POLYOL Blowing Agent Wt. % in Polyol Appearance HFC-134a 30.0 Insoluble, two phases HFC-134a/Dimethyl ether (46/54) 30.0 Soluble, single phase Example 5 The solubility of the HFC-134a/dimethyl ether azeotrope in polystyrene is determined by combining a piece of polystyrene (about 2.5 cm long, 0.5 cm wide and 0.5 cm thick) with about 50g. azeotrope. Whereas HFC-134a has essentially no solvency in polystyrene, the HFC-134a/dimethyl ether azeotrope softens and deforms the polystyrene. The data are summarized in Table IV. SOLUBILITY OF HFC-134a/DIMETHYL ETHER AZEOTROPE IN POLYSTYRENE Blowing Agent Appearance of Polystyrene HFC-134a No effect HFC-134a/Dimethyl ether (46/54) Polystyrene softened and deformed Example 6 The quantity of HFC-134a required in the vapor space above blends of HFC-134a and dimethyl ether for nonflammability is determined. The test conditions and results are summarized in Table V. NONFLAMMABLE BLEND OF HFC-134a/DIMETHYL ETHER Ignition Source: 3-mil copper exploding wire (110 Volts) Temperature: 80°C 150°C Quantity of HFC-134a required for nonflammability greater than 91 wt. % greater than 98 wt. % The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

Claims: A process for preparing a polymer foam which comprises introducing a substantially constant boiling mixture of 1,1,1,2-tetrafluoroethane (HFC-134a) and dimethyl ether (DME) into a foamable polymer composition in an amount of from 1 to 30 weight percent of the polymer composition. A process as claimed in claim 1 wherein the constant-boiling mixture comprises 40 to 55 weight percent of HFC-134a and 55 to 45 weight percent of DME. A process as claimed in claim 1 or 2 wherein the polymer foam is a polyurethane foam, a phenolic foam or a polystyrene foam. A foamable polymer composition which comprises a blowing mixture comprising a substantially constant boiling mixture of HFC-134a and DME and a foamable polyurethane, phenolic or polystyrene formulation. A composition as claimed in claim 4 wherein the blowing agent is present in the foamable formulation in an amount of 1 to 30 percent by weight. A composition as claimed in claim 4 or 5 wherein the blowing agent comprises 40 to 55 weight percent of HFC-134a and 55 to 45 weight percent of DME. A polyurethane, polystyrene or phenolic foam which has been foamed using a blowing agent comprising a constant boiling mixture of HFC-134a and DME.

Inventors: BARTLETT PHILIP LEE, BIVENS DONALD BERNARD, YOKOZEKI AKIMICHI, BARTLETT, PHILIP LEE, BIVENS, DONALD BERNARD, YOKOZEKI, AKIMICHI
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1991
Main classification: C08J9
IPC codes: C09K5, C08J9, C09K3