Patent Application
20080000647
The cover gas 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 compositions comprising pentafluoropropane, and particularly 1,1,1,3,3-pentafluoropropane, 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. Pentafluoropropanes provide an effective and economical alternative to known fluorine-containing cover gases.
In addition to pentafluoropropane, cover gas compositions of the present invention may include 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 pentafluoropropane 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 a pentafluoropropane 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, σ 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).
In certain embodiments, cover gas composition, in addition to HFC-245fa, may further comprise one or more fluoroolefins. Preferred fluoroolefin compounds include those having 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 cover gas 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 component present in a significant amount has a atmospheric lifetime of less than about 20 (years), more preferably less than about 10 (years). 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.
Preferably, fluoroolefins used in the present compositions have low or no toxicity. In this regard, it is preferred that components present in the cover gas 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 compound 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-245fa has been found to have a 4-hour LC-50 of at least 100,000. For comparison, other known cover gas compounds, such as sulfuryl fluoride, nitrosyl fluoride, and nitrogen trifluoride are known to be toxic and/or hazardous materials.
Certain aspects of the present invention are further illustrated, but is not limited by, the following examples.
Prophetic examples 1-3 demonstrate the potential efficacy of HFC-245fa as a Mg cover gas according to the present invention.
A quartz tube having a well is equipped with a metered source of cover gas and a thermocouple which was placed in the well. The well is filled with about 0.2 to 0.3 g of solid magnesium pieces. The cover gas is a mixture of air (a carrier gas) and HFC-245fa. The air and the HFC-245fa are provided from separate cylinders and the relative amounts of each entering the mixture are controlled to give composition of about 1.0% HFC-245fa by volume.
The tube containing the magnesium is placed in an oven. A flow of cover gas through the tube and over the well containing the magnesium is then established at about 1 liter/minute. The oven is then heated to about 700° C. The flow of cover gas proceeded until a surface film is formed on the magnesium or the magnesium ignited. Deliberate rupturing of the surface film does not induce burning of the molten Mg sample.
After the test is complete, the magnesium is removed from the oven and visually inspected to determine the quality of the cover gas.
The magnesium contains a white coating (presumably MgO or MgF2) indicating that the magnesium was well protected.
The experiment of Example 1 is repeated, except that the cover gas contained about 0.5% HFC-245fa by volume.
Deliberate rupturing of the surface film does not induce burning of the molten Mg sample. The magnesium contains a white coating (presumably MgO or MgF2) indicating that the magnesium was well protected.
The experiment of Example 1 is repeated, except that the cover gas contained about 0.05% HFC-245fa by volume.
Deliberate rupturing of the surface film does not induce burning of the molten Mg sample. The magnesium contains a white coating (presumably MgO or MgF2) indicating that the magnesium was well protected.
The experiments of Examples 1-3 are repeated, except that the cover gas contained SF6.
The results of the comparative examples are provided in Table B. In general, SF6, and HFC-245fa performs well as cover gases at concentrations at or above about 1.0% by volume. However, performance of the different cover gases begins to vary at about 0.5% by volume, with HFC-245fa performing better than SF6. 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 HFC-245fa, 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 |
