Patent Application
20080003127
The fluoroolefin compositions of the present invention are generally effective as cover gases to impede the oxidation of molten reactive metals when the surface of the metal is exposed to source of oxygen, such as air. As used herein, the term “nonferrous reactive metal” means a metal or alloy which is sensitive to destructive, vigorous oxidation when exposed to air, such as magnesium, aluminum, or lithium, or an alloy comprising at least one of these metals. For convenience, the following description of illustrative embodiments of the invention shall refer to magnesium. It is understood, however, that the present invention can also be used with aluminum, lithium, or other nonferrous reactive metal, or an alloy containing at least one of these metals.
Without necessarily being bound by theory, it is believed that by impeding oxidation, the cover gas composition of the present invention is capable of protecting the molten metal from ignition. As is the case with known fluorine-containing cover gases, it is believed that the fluoroolefin compositions of the present invention can react with the molten metal surface to create a thin passivation layer or film that can function as a barrier between the metal and an oxygen source. In contrast to conventional fluorine compounds that are used in cover gases, the fluoroolefins of the present invention are particular advantageous in that they have a relatively low GWP and a relatively low atmospheric lifetime, while also being non-toxic, effective at low concentrations, and have a low boiling point.
In certain preferred embodiments, the compositions of the present invention comprise fluoroolefins consisting of carbon, fluorine, and optionally hydrogen atoms. In certain preferred embodiments, the fluoroolefins are selected from a C2-C4 perflorinated olefin. However, more preferred are C2-C4 fluoroolefins having at least one hydrogen atom. Examples of preferred fluoroolefins include, but are not limited to, trans-HFO-1234ze, cis-HFO-1225ye, HFC-1233xf, cis-HFCO-1233zd, and trans-HFCO-1233zd.
Fluoroolefin compositions of the present invention may include a mixture of at least one fluoroolefin and, optionally, a carrier gas. Preferred carrier gases include, but are not limited to, nitrogen, carbon dioxide, air, and/or noble gas such as argon. Preferably, the composition comprises a minor amount of at least one fluoroolefin and a major amount of a carrier gas. In certain preferred embodiments, the composition comprises from about 0.01 to about 2 weight percent of at least one fluoroolefin and from about 99.99 to about 98 weight percent of a carrier gas.
As used herein, “GWP” is a relative measure of the warming potential of a compound based on the structure of the compound. The concept of GWP was developed to compare the ability of each greenhouse gas to trap heat in the atmosphere relative to another gas. Generally, the GWP for a particular greenhouse gas is the ratio of heat trapped by one unit mass of the greenhouse gas to that of one unit mass of CO2 over a specified time period. More specifically, the GWP of a compound, as defined by the Intergovernmental Panel on Climate Change (IPCC) in 1990 and updated in Scientific Assessment of Ozone Depletion: 1998 (World Meteorological Organization, Scientific Assessment of Ozone Depletion: 1998, Global Ozone Research and Monitoring Project—Report No. 44, Geneva, 1999), is calculated as the warming due to the release of 1 kilogram of a compound relative to the warming due to the release of 1 kilogram of CO2 over a specified integration time horizon (ITH):
where F is the radiative forcing per unit mass of a compound (the change in the flux of radiation through the atmosphere due to the IR absorbance of that compound), C is the atmospheric concentration of a compound, τ is the atmospheric lifetime of a compound, t is time, and x is the compound of interest.
The commonly accepted ITH is 100 years representing a compromise between short-term effects (20 years) and longer-term effects (500 years or longer). The concentration of an organic compound, x, in the atmosphere is assumed to follow pseudo first order kinetics (i.e., exponential decay). The concentration of CO2 over that same time interval incorporates a more complex model for the exchange and removal of CO2 from the atmosphere (the Bern carbon cycle model).
The cover gas compositions of the present invention preferably include those compositions wherein the fluoroolefin compounds included therein have a GWP of less than about 1000, more preferably less that about 150 and even more preferably of less than about 100. In certain preferred embodiments, each component present in the composition in a substantial amount has a GWP of less than about 1000, more preferably less that about 150 and even more preferably of less than about 100. In certain highly preferred embodiments, each component of the composition which is present in more than an insubstantial amount has a GWP of less than about 10, and even more preferably less than about 5. For comparison, the GWP of CO2, certain conventional cover gases, and certain cover gases according to the present invention are shown in Table A.
Preferably, the cover gas compositions of the present invention include those compositions wherein each fluoroolefin component has a atmospheric lifetime of less than about 20 (years), preferably less than about 10 (years), and even more preferably less than about 1 (year). As used herein, the term “atmospheric lifetime” is the approximate amount of time it would take for the concentration of the compound to fall to e−1 of its initial value as a result of either being converted into another chemical compound (wherein e is the base of natural logarithms). Atmospheric lifetime is closely related to GWP since relatively short lifetimes limit the duration that a reactant can participate in a reaction.
In addition, preferred cover gas compositions of the present invention comprise what are more compounds wherein each compound present in more than an insubstantial amount has a boiling point of less than about 25° C., and even more preferably less than about 0° C. Cover gases that have boiling points close to or above room temperature (i.e. which are liquids at room temperature) typically require additional metering equipment to disperse the cover gas material in a controlled fashion onto the surface of the molten metal.
Preferably, fluoroolefins used in the present compositions have low or no toxicity. In this regard, it is preferred that fluoroolefin components that a present in the compositions in more than an insubstantial amount have a LC-50 value of at least about 100,000 ppm, and more preferably at least about 200,000 ppm. As used herein, the term “LC-50 value” means the concentration of the fluoroolefin in air that will kill 50% of test subject (e.g. mice) when administered as a single exposure (e.g. 4 hours). For example, HFC-1234ze has been found to have a 4-hour LC-50 of at least 100,000, and HFC-1234yf has been found to have a 4-hour LC-50 of at least about 200,000. For comparison, C2F5C(O)CF(CF3)2 (a fluoroketone cover gas marketed by Minnesota Mining and Manufacturing Co. of St. Paul, Minn., under the tradename Novec™) has a 4-hour LC-50 of about 100,000. Other compounds, such as sulfuryl fluoride, nitrosyl fluoride, and nitrogen trifluoride are known to be toxic and/or hazardous materials.
Another measure of toxicity is a compound's No Observed Adverse Effect Level (NOAEL). As used herein, the term NOAEL refers to the greatest concentration or amount of a substance, found by experiment or observation, which causes no detectable adverse alteration of morphology, functional capacity, growth, development, or life span of the target organism under defined conditions of exposure. For cardiac sensitization tests, the NOAEL for HFO-1234yf and HFO-1234ze are greater than 12 vol. %. By comparison, the NOAEL for C2F5C(O)CF(CF3)2 is only 10 vol. %.
Applicants have found that different isomeric forms of certain fluoroolefins do not possess the same advantageous characteristics for cover gas applications. For example, among isomers of HFO-1225, the HFO-1225zc isomer is much more toxic, and thus less preferred, than the HFO-1225ye or HFO-1225yc isomer. In certain preferred embodiments, the cover gas consists essentially of only a single isomer of fluoroolefin. For example, in certain embodiments the trans-isomer of HFO-1234ze can be utilized in the present invention with much greater success than the related cis-isomer or than mixtures of the cis- and trans-isomers. In particular, the trans-isomer is more preferred not only because is less toxic than the cis-isomer, but also because it has a lower normal boiling point (−18.4° C. vs. 9° C. for trans- and cis-isomers, respectively). This low boiling point correlates to a higher vapor pressure of the gas which is advantageous in that the gas is more easily metered as it is applied to a molten metal. Isomeric mixtures of the cis- and trans-isomers can be problematic because the isomers do not have the same vapor pressure, and thus are not evenly dispensed from a container. That is, dispersement of the isomeric mixture from a container will initially result in a cover gas having a higher concentration of the lower boiling isomer and will eventually result in a cover gas having a higher concentration of the higher boiling isomer. Such a mixture makes it more difficult to maintain a steady flow and composition.
Certain aspects of the present invention are further illustrated, but is not limited by, the following examples.
Examples 1-5 demonstrate the efficacy of a fluoroolefin as a Mg cover gas according to the present invention.
A quartz tube having a well was equipped with a metered source of cover gas and a thermocouple which was placed in the well. The well was filled with about 0.2 to 0.3 g of solid magnesium pieces. The cover gas was a mixture of air (a carrier gas) and trans-HFO-1234ze. The air and the trans-HFO-1234ze were provided from separate cylinders and the relative amounts of each entering the mixture were controlled to give composition of about 4.5% trans-HFO-1234ze by volume.
The tube containing the magnesium was placed in an oven. A flow of cover gas through the tube and over the well containing the magnesium was then established at about 1 liter/minute. The oven was then heated to about 700° C. The flow of cover gas proceeded until a surface film was formed on the magnesium or the magnesium ignited.
After the test was complete, the magnesium was removed from the oven and visually inspected to determine the quality of the cover gas.
The magnesium contained a white coating (presumably MgO or MgF2) indicating that the magnesium was well protected.
The experiment of Example 1 was repeated, except that the cover gas contained about 1.5% trans-HFO-1234ze by volume.
The magnesium contained a white coating and the pieces were not stuck together indicating that the magnesium was well protected.
The experiment of Example 1 was repeated, except that the cover gas contained about 0.5% trans-HFO-1234ze by volume.
The magnesium contained a white coating and the pieces were not stuck together indicating that the magnesium was well protected.
The experiment of Example 1 was repeated, except that the cover gas contained about 0.2% trans-HFO-1234ze by volume.
The magnesium contained a white coating with some dark spots and the pieces were not stuck together indicating that the magnesium was well protected.
The experiment of Example 1 was repeated, except that the cover gas contained about 0.1% trans-HFO-1234ze by volume.
The magnesium contained a white coating with a few brown specks indicating that the magnesium was protected in general.
The experiments of Examples 1-5 were repeated, except that the cover gas contained about either SF6 or HFC-134a.
The results of the comparative examples are provided in Table B. In general, trans-HFO-1234ze, SF6, and HFC-134a performed well as cover gases at concentrations at or above about 1.5% by volume. However, performance of the different cover gases began to vary at about 0.5% by volume, with HFC-134a performing better than SF6, and trans-HFO-1234ze performing better than HFC-134a. It is believed that the ability of the cover gas to protect the magnesium, and particularly to keep the magnesium from igniting, corresponds to the amount of fluorine it provides to create a protective barrier. Thus, cover gases that are more reactive, such as trans-HFO-1234ze, are better suited to protect magnesium compared to more stable gases, such as SF6.
Having thus described a few particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements, as are made obvious by this disclosure, are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.
This application claims the priority of U.S. Provisional Application 60/818,416, filed on Jul. 3, 2006. The contents of this provisional application are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60818416 | Jul 2006 | US |
