Patent Publication Number: US-2007095420-A1

Title: Perfluorocarbon gas insufflation

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      The present application claims the priority benefit of U.S. Provisional Patent Application Ser. No. 60/596,878 filed Oct. 27, 2005. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to operating fluids for pneumatic and hydraulic devices and inflatable articles such as vehicle tires.  
      2. Brief Description of the Prior Art  
      Inflatable vehicle tires are a critical component of transportation in the modern economy. Invented in the United Kingdom in the late 19th Century, inflatable pneumatic tires are conventionally filled with compressed air, although other gases have been used in special circumstances. Similarly, commercial and industrial equipment typically use compressed air as an operating fluid due to its low cost.  
      Correct inflation is highly significant when considering tire life, safety, and performance. It is not always possible to look at a tire and detect under-inflation. However, under-inflation can cause many tire-related problems. As inflation pressure largely determines a tire&#39;s load capacity, under-inflation results in an overloaded tire. An under-inflated tire operates at high deflection resulting in decreased fuel economy, sluggish handling and may result in excessive mechanical flexing and heat build up leading to catastrophic tire failure.  
      More recently, nitrogen, a dry, inert gas has been used to inflate airplane tires, off-road truck tires, military vehicle tires, and race car tires for improved performance. The rationale for the use of nitrogen is related to the fact that oxygen in compressed air permeates through the wall of the tire, thus reducing the tire&#39;s inflation pressure. During this process and its journey through the tire wall, oxygen oxidizes the rubber compounds in the tire, causing under-inflation and deterioration of the rubber. In addition, the deterioration of the rubber further increases the tire permeability thus resulting in further under-inflation of the tire. Dry nitrogen can maintain proper inflation pressure and prevent auto-ignition, will not corrode rims, and will help the tire to run cooler. The result is increased safety and reduced operating cost.  
      Portable tire sealing and inflation devices are well known for emergency use by motorists to temporarily seal and inflate punctured vehicles tires. Typical compositions for use in such devices include a sealant and a pressurized gas to blow the sealant into the punctured tire and to inflate the tire. The pressurized gas can be generated in situ from a suitable compound and an activating material, or prepackaged with the sealant. Examples of gases that have been used or considered for use in tire sealing and other tire inflation devices include various hydrocarbons and chlorofluorocarbons, carbon dioxide, nitrous oxide, nitrogen and air.  
      However, various disadvantages have been disclosed in respect to many of these gases. For example, use of carbon dioxide, nitrous oxide, nitrogen and air is not practical, because the U.S. Department of Transportation has set a limit of 180 psig at 130 degrees F. for interstate shipping of products in pressurized aerosol cans. Under these conditions many materials that remain gases at room temperature under elevated pressure require too large a volume at the 180 psig pressure limit to be practical.  
      Even materials that condense under pressure, and thus, in the liquid phase, require much smaller volumes, may have significant disadvantages, such a flammability (hydrocarbons) or ozone-depleting qualities (chlorofluorocarbons with high ozone depletion potentials, such as “Freon 11,” trichlorofluoromethane, and “Freon 12,” dichlorodifluoromethane). Other materials may not be suitable as a tire inflatant for other reasons. For example, U.S. Pat. No. 6,605,654 discloses that 1,1,2,2-tetrafluorethane (HFC-134a) has a very high vapor pressure at 130 degrees F. (199.2 psig) such that vapor pressure depressants such as butyl cellosolve, acetone, methyl ethyl ketone, or acetonitrile must be mixed with 1,1,2,2-tetrfluoroethane when it is employed as a propellant.  
      Another consideration is the rate of diffusion of the inflatant material in the tire rubber. The greater the rate of diffusion through tire rubber, the faster the loss of the target inflation pressure, the more frequently tires must be reinflated. Compared with air, nitrogen has a lower rate of diffusion in tire rubber, and has been promoted as a desirable substitute. Filling tires with nitrogen can reduce the extent of underinflation. Tire underinflation is associated with shortened tire life, reduced fuel mileage, and poorer road handling.  
      There is a continuing need to reduce vehicle fuel consumption, reduce tire underinflation, extend tire life, and improve motor vehicle safety.  
     SUMMARY OF THE INVENTION  
      The present invention provides for the use of inert perfluorocarbon gas for inflating and operating pneumatic devices and equipment. Specific embodiments relate to tires, pneumatic equipment, and devices that require operating gases greater than atmospheric pressure. The perfluorocarbon gas&#39; larger molecules and anti-oxidative properties reduce the rate at which the working fluid is lost from tires, equipment, or devices in comparison with air or other gases commonly used gases, such as carbon dioxide or nitrogen, while retarding deterioration of the inflated component due to oxidation.  
      The present invention provides inflatable articles, such as vehicle tires, inflated with a fluid comprising at least one perfluorocarbon compound. Preferably, the fluid comprises at least one perfluorocarbon gas selected from the group consisting of perfluoromethane, perfluoroethane, perfluoropropane, and perfluorobutane, each of which has a boiling point less than about zero degrees C. In one embodiment, the fluid preferably further comprises at least one perfluorocarbon having a boiling point greater than zero degrees C., such as perfluoro-n-pentane or perfluoro-n-hexane. In another embodiment, the fluid further comprises at least one non-perfluorocarbon gas selected from the group comprising gaseous hydrofluorocarbons, gaseous chlorflurohydrocarbons, nitrogen, carbon dioxide, nitrous oxide, helium, argon and xenon. Preferably, the fluid comprises less than about one percent oxygen by volume. In addition, it is preferred that the fluid further comprises at least one anti-oxidant.  
      In another aspect, the present invention provides a process for inflating an inflatable article such as a vehicle tire. The process comprises inflating the inflatable article with a fluid comprising at least one perfluorocarbon compound, and then sealing the article to prevent escape of the fluid. Preferably, in this process, the fluid comprises at least one perfluorocarbon gas selected from the group consisting of perfluoromethane, perfluoroethane, perfluoropropane, and perfluorobutane. In one embodiment of this process, the fluid preferably further comprises at least one perfluorocarbon having a boiling point greater than zero degrees C. In another embodiment of this process, the fluid further comprises at least one non-perfluorocarbon gas selected from the group comprising gaseous hydrofluorocarbons, gaseous chlorflurohydrocarbons, nitrogen, carbon dioxide, nitrous oxide, helium, argon and xenon. Preferably, the fluid comprises less than about one percent oxygen by volume. Preferably, the fluid further comprises at least one anti-oxidant.  
      In yet another aspect, the present invention provides a process for operating a pneumatic device. This process comprises supplying a gas comprising at least one perfluorocarbon compound to the pneumatic device as a working fluid, and then applying pressure to the working fluid. Preferably, in this process the working fluid comprises at least one perfluorocarbon gas selected from the group consisting of perfluoromethane, perfluoroethane, perfluoropropane, and perfluorobutane. In one embodiment of this process, the working fluid further comprises at least one perfluorocarbon having a boiling point greater than zero degrees C. In another embodiment of this process, the fluid further comprises at least one non-perfluorocarbon gas selected from the group comprising gaseous hydrofluorocarbons, gaseous chlorflurohydrocarbons, nitrogen, carbon dioxide, nitrous oxide, helium, argon and xenon.  
      In a further aspect, the present invention provides a process for operating a hydraulic device. This process comprises supplying a liquid comprising at least one perfluorocarbon compound to the hydraulic device as a working fluid, and then applying pressure to the working fluid. In one aspect, in the present process the liquid comprises a mixture of at least one perfluorocarbon liquid and at least one non-perfluorocarbon liquid selected from the group comprising liquid hydrofluorocarbons and liquid chlorflurohydrocarbons. Preferably, in the present process, the at least one perfluorocarbon compound is selected from the group consisting of perfluoro-n-hexane, perfluoromethylcylcopentane, perfluoro-1,3-dimethylcyclohexane, perfluorodecalin, perfluoromethyldecalin, and perfluoroperhydrobenzyltetralin. 
    
    
     DETAILED DESCRIPTION  
      Inflating vehicle tires with perfluorocarbon gas according to the present invention, including pure perfluorocarbon gases, predetermined mixtures of perfluorocarbon gases, and predetermined mixtures of one or more perfluorocarbon gases in combination with other inert gases such as nitrogen, allows the tires to retain correct pressure longer while providing improved performance and economy.  
      The present invention provides for the use of perfluorocarbon gases as for inflation. The process of the present invention provides significant advantages far beyond nitrogen. These inert, high molecular weight, slower diffusion gases have anti-oxidative properties. Perfluorocarbon gases or predetermined mixtures of perfluorocarbon gases with other inert gases such as nitrogen, carbon dioxide, other naturally occurring gases as well as other synthetic gases and additives that promote anti-oxidative processes and limit diffusion, provide more reliable inflation pressure, less leakage, and better thermal properties, thus providing for better tire performance, durability, and economics.  
      As compared to compressed air and pure nitrogen, perfluorocarbon gas and mixtures of perfluorocarbon gas with other gases help to extend tire life by reducing premature failure of the tire. The causes of premature tire failure which are favorably affected by use perfluorocarbon gases to inflate tires include rubber deterioration by oxidation, rim corrosion, under-inflation, overheating, pressure increase due to heat build up, and uneven wear due to improper inflation.  
      Perfluorocarbon gases can be combined in specific proportions based on their physical properties to produce optimal economic versus high performance, such as, for example, for the inflation of tires for competitive race cars and motorcycles, or for use as a working fluid in specialized pneumatic equipment. In this regard the gas combination can be altered for thermal properties, vapor pressure, diffusivity, density, viscosity, and other physical and chemical properties, depending on the specific application. Additionally, because perfluorocarbons have high solubility coefficients, chemical additives can easily be dissolved in perfluorocarbons to augment their performance, including, but not limited, to chemical anti-oxidants and coating/adhesive molecules to help seal cracks and leaks that do occur or pre-date the use of the perfluorocarbon, reduce the possibility of blowouts, and reduce the effects of blowouts by limiting the acute rate of pressure loss in such situations.  
      Some relevant physical properties or some perfluorocarbons useful in the processes and articles of the present invention are provided in Table A below.  
                                   TABLE A                                   mp or       vapor                   pour       pressure                   point   bp   @ 21.1       Perfluorocarbon   C   F   (deg. C.)   (deg. C.)   degrees C.                                                            perfluoromethane   1   4   −184   −128               perfluoroethane   2   6   −94   −78.2   30.7   bar       perfluoropropane   3   8   −183   −37   7.92   bar       perfluorobutane   4   10   −128.2   −2   2.9   bar       perfluoro-n-pentane   5   12   −120   29       perfluoro-n-hexane   6   14   −90   58   0.283   bar       perfluoromethyl-   6   12   −70   48   0.368   bar       cyclopentane       perfluoro-1,3-   8   16   −70   102   48   mbar       dimethylcyclohexane       perfluorodecalin   10   18   −8   142   8.8   mbar       perfluoromethyldecalin   11   20   −70   155   2.9   mbar       perfluoroperhydro-   17   30   −10   260   &lt;1   mbar       benzyltetralin                  
 
      Literature studies show that oxidative aging is one of the primary causes of limited tire life. Oxidative aging is caused by the diffusion of air, which is 21% oxygen, from the pressurized air cavity of the tire through the tire wall to the outside atmosphere. Tests have shown that if tires are inflated with nitrogen, there is a significant reduction in tire failure. Thus, the use of a perfluorocarbon gas, with no oxygen present, and having specific anti-oxidative properties, provides for an increased reduction in tire failure and loss of inflation pressure in tires and other inflated articles, as well as reduced loss of operating fluid in pneumatically operated devices.  
      Tires are a significant cost in operating a fleet of vehicles, as well as, with respect to running a competitive race car team. Tire costs include procurement, maintenance and the cost of blowouts. A typical truck tire with two retreads costs $480.00 and lasts approximately 270,000 miles. Inflating tires with perfluorocarbon gases helps to prevent premature casing failure and allow tires to be retread multiple times, with confidence and reliability. Inflating tires with perfluorocarbon to eliminate and prevent oxidative aging may extend tire life by up to 50%. Increasing tire life to 500,000 miles would thus save $300 per truck tire. A fleet with 50 trucks and 900 wheel positions would save over $500,000 in tire cost by inflating with perfluorocarbon gas. The primary cost of maintaining tires is the cost of labor to check tire pressures and top off tires with compressed air on a periodic basis. Tire pressure must be checked and the tires topped off due to the diffusion of air through the tire, a condition that would not exist with perfluorocarbon gas inflation. In addition, tires filled with perfluorocarbon maintain pressure for a much longer period of time than tires filled with air. If a truck fleet conducts preventative maintenance on 5 trucks per day and presently spends 30 minutes per truck topping off tires, they could realize savings of $60,000 per year based on a labor rate of $50 per hour and 250 work days per year, by inflating tires with perfluorocarbon gases.  
      An additional advantage of filling tires with perfluorocarbon gases on automobiles and trucks is enhanced safety. Under-inflation of tires due to the diffusion of air through the tire has been identified as a significant cause of tire failures. In fact, the TREAD Act recently passed by the U.S. Congress requires the National Highway and Transportation Safety Agency (NHTSA) to develop an on board warning of low tire pressure in all automobiles.  
      With respect to car competition, the thermal properties as well as reliable inflation characteristics of perfluorocarbon gases provide a significant competitive advantage with respect to Formula 1 and NASCAR racing.  
      The process of using inert higher molecular weight perfluorocarbon gas and/or mixtures thereof can potentially extend tire life by up to 40%, improves fuel efficiency by up to 8%, provides more consistent tire pressure, eliminates rim corrosion, and should result in tires operating at lower temperatures. Perfluorocarbon mixtures can be prepared and optimized for maximum tire performance for competitive racing and specialized pneumatic operations.  
      Bridgestone states that air inflated tires lost an average of 2.7 psi per month and nitrogen inflated tires lost an average of 0.7 psi per month. Due to the increased molecular weight, lower diffusivity, and anti-oxidative properties of perfluorocarbon gases, use of perfluorocarbon gas is estimated to reduce loss to less than 0.1 psi per month. In a Michelin Technical Bulletin, November 2003, Goodyear stated that 15% under-inflation resulted in 8% less tread mileage and 2.5% decrease in fuel economy. The use of perfluorocarbon gas greatly improves these statistics with respect to tread mileage and fuel economy.  
      The use of perfluorocarbon gas insufflation has commercial application in automobiles, trucks, race cars, motorcycles, military vehicles, airplanes, off road construction vehicles, inflatable boats, and specialized pneumatic equipment, including essentially any and all pneumatic and/or inflatable devices that require consistent pressure, compressibility characteristics and/or specific thermal ranges as a function of their performance.  
      Thus, the present invention provides an inflatable article such as a vehicle tire inflated with a fluid comprising at least one perfluorocarbon compound. Vehicle tires according to the present invention include tires for automobiles, truck and aircraft. Preferably, the fluid comprises at least one perfluorocarbon gas selected from the group consisting of perfluoromethane, perfluoroethane, perfluoropropane, and perfluorobutane, all of which are gases above zero degrees C. The specific perfluorocarbon selected depends upon the environment in which the inflated article is to be employed. Thus, if the article is to be employed under low temperature conditions, perfluorocarbons having a relatively low boiling point, such as perfluoromethane, perfluoroethane, and perfluoropropane, are preferred so that there will be no condensation of the perfluorocarbon under the low temperature conditions. Thus, in some embodiments of the present invention, it is preferred that the at least one perfluorocarbon have a boiling point greater than zero degrees C.  
      In other embodiments of the present invention, it is preferred that the fluid comprise at least one non-perfluorocarbon gas selected from the group comprising gaseous hydrofluorocarbons, gaseous chlorflurohydrocarbons, nitrogen, carbon dioxide, nitrous oxide, helium, argon and xenon. Examples of hydrofluorocarbons that can be employed in the fluid of the present invention include highly fluorinated fluorocarbons such as trifluoromethane (HFC-23), pentafluoroethane (HFC-125), trifluoroethane (HFC-143a), and hexafluoropropane (HFC-236a).  
      In applications in which the article is oxidatively degradable, such as in natural or synthetic rubber vehicle tires, it is preferred that the fluid comprises less than about five percent oxygen by volume, more preferably less than about one percent oxygen by volume, and still more preferably less than about one-tenth of one percent oxygen by volume. In such applications, it is also preferred that the fluid comprise at least one anti-oxidant.  
      In some applications, such as emergency tire repair applications, it is preferred that the fluid further comprising at least one sealant compound. Examples of sealant compounds that can be employed in the process and articles of the present invention include polymeric acrylate resins, polyvinyl acetates, petroleum residual resins, propylene-ethylene copolymer resins, phenolic resins, glycerol esters of hydrogenated rosins, deproteinated natural rubber latex, terpene resins, and hydrocarbon resins.  
      In another aspect, the present invention provides a process for inflating an inflatable article such as a vehicle tire. This process comprises inflating the inflatable article with a fluid comprising at least one perfluorocarbon compound, and then sealing the article to prevent escape of the fluid. Preferably, in this process, the fluid comprises at least one perfluorocarbon gas selected from the group consisting of perfluoromethane, perfluoroethane, perfluoropropane, and perfluorobutane, which are all in the gaseous state at temperatures above zero degrees C. Depending on the specific application, the fluid further comprises at least one perfluorocarbon having a boiling point greater than zero degrees C. In addition, or in the alternative, the fluid can further comprise at least one non-perfluorocarbon gas selected from the group comprising gaseous hydrofluorocarbons, gaseous chlorflurohydrocarbons, nitrogen, carbon dioxide, nitrous oxide, helium, argon and xenon. Preferably, the fluid comprises little oxygen, preferably less than about one percent oxygen by volume, and includes at least one anti-oxidant.  
      The present invention also provides a process for operating a pneumatic device. This process comprises supplying a gas comprising at least one perfluorocarbon compound to the pneumatic device as a working fluid, and then applying pressure to the working fluid. Preferably, in this process the working fluid comprises at least one perfluorocarbon gas selected from the group consisting of perfluoromethane, perfluoroethane, perfluoropropane, and perfluorobutane, which are all gases at temperatures above zero degrees C. Depending on the environment in which the process is being carried out, perfluorocarbons having higher boiling points, such as perfluoro-n-pentane, can also be used. In one embodiment of this process, the working fluid further comprises at least one perfluorocarbon having a boiling point greater than zero degrees C., such as perfluoro-n-pentane. In another embodiment of this process, the fluid further comprises at least one non-perfluorocarbon gas selected from the group comprising gaseous hydrofluorocarbons, gaseous chlorflurohydrocarbons, nitrogen, carbon dioxide, nitrous oxide, helium, argon and xenon.  
      In a further aspect, the present invention provides a process for operating a hydraulic device. This process comprises supplying a liquid comprising at least one perfluorocarbon compound to the hydraulic device as a working fluid, and then applying pressure to the working fluid. In one aspect, in the present process the liquid comprises a mixture of at least one perfluorocarbon liquid and at least one non-perfluorocarbon liquid selected from the group comprising liquid hydrofluorocarbons and liquid chlorflurohydrocarbons. Preferably, in the present process, the at least one perfluorocarbon compound is selected from the group consisting of perfluoro-n-hexane, perfluoromethylcylcopentane, perfluoro-1,3-dimethylcyclohexane, perfluorodecalin, perfluoromethyldecalin, and perfluoroperhydrobenzyltetralin, all of which are liquids under moderate temperature conditions.  
      The perfluorocarbon gases presently preferred for use in the articles and processes of the present invention are perfluoropentane (C 5 F 12 )(? bp is 20 degrees C.); perfluoropropane (C 3 F 8 ); and perfluorobutane (C 4 F 10 ). In the articles and processes of the present invention, these gases can be mixed in combination with each other, as well as with other inert gases such as nitrogen and carbon dioxide.  
      It is know that in general the diffusion rate of gases varies with the square root of the reciprocal of the molecular weight of the gas. Air has an average molecular weight of 30, perfluoropentane an average molecular weight of 288; perfluorobutane an average molecular weight of 238, and perfluoropropane an average molecular weight of 188. Assuming that the diffusion pathway remains constant, the relative diffusion rate of these perfluorocarbon compounds compared to air is as follows: The diffusion of perfluoropentane would be 3.1 times slower than air; perfluorobutane would be 2.8 times slower than air; and perfluoropropane 2.5 times slower than air. Considering that at least a 50% reduction in oxidation can be expected assuming that since no oxygen is present in these inert gases, there would potentially be at least a six-fold reduction in leak rate at atmospheric conditions. In addition, this reduction in diffusion rate would be more favorable at elevated pressure and temperature conditions that most applications require. Finally, these inert gases all have better thermal conductivity, heat capacity, compressibility and viscosity properties relative to air.  
      Combinations of perfluorocarbons with each other in order to optimize performance like biomedical mixing of perfluorocarbon liquids for physiological benefits has been previously reported in the literature: Jeng et al.,  Ped. Res.  53(1) 81-88, 2003. Thus, perfluorocarbon gases can be combined with less expensive air, nitrogen, carbon dioxide, etc. for economy, functional performance and environmental issues. For example, in one embodiment it may be advantageous to mix perfluorocarbons in the following proportion: 10% air; 50% pentane; 20% butane; and 20% propane. This combination could optimize performance while at the same time not utilize too much of any one gas.  
      This edge for performance in commercial, industrial and consumer equipment far exceeds the cost differential in gas. For example, although it may cost only $0.75 to fill the tires of a conventional auto with air, even if the cost of filling the tires of an expensive race care is $75 dollars, the increase performance may well justify the additional expense.  
      Various modifications can be made in the details of the various embodiments of the processes and articles of the present invention, all within the scope and spirit of the invention and defined by the appended claims.