Abstract:
The invention is related to a coating delivery system that includes at least one pressure vessel having an inner surface, a flexible bladder disposed in a first pressure vessel and having an open condition and a closed condition, an internal region disposed between the inner surface and the bladder, and a deliverable substance including a coating component interspersed with at least one of liquefied carbon dioxide and supercritical carbon dioxide. The deliverable substance is disposed in one of the flexible bladder and the internal region, and the pressure-conveying fluid is received in the other to exert pressure on the deliverable substance and thereby permit transport thereof when the flexible bladder is in the open condition.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of the U.S. National Stage designation of co-pending International Patent Application PCT/US03/05275 filed Feb. 21, 2003, which claims the benefit of U.S. Provisional Application No. 60/358,393 filed Feb. 22, 2002, the entire contents of which are expressly incorporated herein by reference thereto. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention is related to an apparatus and method for dispensing coatings. In particular, the invention is related to a delivery system that includes at least one pressure vessel within which is stored a deliverable substance having a coating component interspersed with a fluid component.  
       BACKGROUND OF THE INVENTION  
       [0003]     The use of volatile organic compounds as carriers for the delivery of coatings is well-known. However, increasingly there is a need for environmentally friendly carriers which minimize the use of organic carriers such as organic solvents. Supercritical fluids have emerged as such a viable carrier in coating applications, particularly in applications requiring the delivery of a substance in spray form. While supercritical fluids are known to have solvating powers similar to organic solvents, they also present advantages over organic solvents because of their higher diffusivities, lower viscosities, and lower surface tensions.  
         [0004]     A supercritical carrier may be considered any compound at a temperature and pressure above certain critical values of temperature and pressure. The critical temperature of a compound is the temperature above which the pure compound in gaseous state cannot be converted to a liquid, while a compound&#39;s critical pressure is the vapor pressure of the pure compound in gaseous state at the critical temperature. The critical point of the compound occurs at the temperature and pressure at which the gas and liquid phases are no longer separately defined, but instead a fluid exists in a state that is considered neither liquid nor gas. In the supercritical state, a fluid confers the carrier properties expected from a liquid while at the same time providing transport characteristics expected from gases.  
         [0005]     Various compounds are known to exist as supercritical fluids, including ethylene, carbon dioxide, ethane, nitrous oxide, propane, and even methanol and water. The low cost and ready availability of supercritical carbon dioxide have made it a popular choice for a variety of applications. Also, with its critical temperature of 31.1° C., critical pressure of about 73 atm, and critical density of about 470 kg/m 3 , supercritical carbon dioxide has properties amenable to applications using standard pressure vessel technology.  
         [0006]     Various applications have been explored for supercritical carriers, including use in the delivery of protective coatings to various commercial building substrates such as marble, stone, cast stone, architectural terra cotta, concrete, and concrete block. The degradation of such materials due to pollution, acid rain, and other destructive forces can be substantially decreased if a relatively thin protective coating is applied.  
         [0007]     Several supercritical fluid technologies have been disclosed by investigators. For example, U.S. Pat. No. 4,923,720 to Lee et al. is directed to the use of supercritical fluids as diluents in the liquid spray application of coatings. A process and apparatus for coating substrates is provided in which a supercritical fluid, such as supercritical carbon dioxide fluid, is used as a viscosity reduction diluent for coating formulations.  
         [0008]     However, prior art methods and devices for applying coatings using supercritical fluids suffer from complexity and concomitant bulky equipment, rendering the technology inconvenient to use and inaccessible to many potential customers. Commercial and laboratory equipment for applying coatings using supercritical fluids generally fall into two classes, batch and continuous. Typically, the main storage element of prior art batch systems is a floating piston accumulator. The coating material and supercritical fluid are held captive on one side of the piston, while the pressurization fluid is stored on the other. In such systems, the coating material and CO 2  are added at a pressure typically above 1000 psi so that the CO 2  remains in a liquid state. Such an arrangement requires high-pressure pumps. After the desired amounts of coating material and CO 2  have been added, the two components must be mixed. Mixing usually is effected by circulating material in and out of the piston accumulator. The pressurizing fluid, disposed on the other side of the piston accumulator, is used to effect transport of the deliverable substance through a hose to a spray nozzle. Such batch systems are heavy due to the weight of the piston accumulator, high-pressure pumps, and associated controls. The weight of commercial units ranges between about 3000 lbs and about 1500 lbs. for equipment capable of delivering 6 kgs per batch, not including the CO 2  supply bottle.  
         [0009]     Continuous systems typically require two or three high-pressure pumps, along with complex flow meters and controls for accurately metering and mixing the coating material and supercritical fluid components. Multiple control loops and a programmable logic controller may be required. Such systems are less common, due to the required level of sophistication of controls. Further, although the commercial, continuous systems are capable of supplying about 100 grams to about 300 grams per minute of deliverable product, they are heavy, typically weighing between about 180 lbs. and 1000 lbs.  
         [0010]     The above-described batch and continuous systems are heavy, bulky, require multiple high-pressure pumps, and require heavy CO 2  cylinders with high stored energies. These systems also require significant equipment maintenance, as well as an additional energy source to power pumps and controls.  
         [0011]     Thus, there exists a need for an improved apparatus and an improved method for dispensing coatings using supercritical fluids. There also exists a need for an apparatus with simplicity in design, compactness, and portability so that the device may be manually transported. Moreover, there exists a need for methods and devices that can deliver coatings with controllable composition and thickness.  
       SUMMARY OF THE INVENTION  
       [0012]     The invention is related to a coating delivery system including a first pressure vessel having an inner surface, a flexible bladder disposed in the first pressure vessel and having an open condition and a closed condition, and an internal region disposed between the inner surface and the bladder. The coating delivery system also includes a deliverable substance having a coating component interspersed with at least one of liquefied carbon dioxide and supercritical carbon dioxide. A pressure-conveying fluid is provided (1) at a pressure greater than the vapor pressure of carbon dioxide if the deliverable substance comprises liquefied carbon dioxide, or (2) at a pressure greater than the critical pressure of carbon dioxide if the deliverable substance is supercritical carbon dioxide. The deliverable substance is disposed in one of the flexible bladder and the internal region, and the pressure-conveying fluid is received in the other to exert pressure on the deliverable substance and thereby permit transport thereof when the flexible bladder is in the open condition. The flexible bladder may be formed of an elastomeric material, while the first pressure vessel may be formed of carbon fiber.  
         [0013]     The coating delivery system may further include a second pressure vessel, with the pressure-conveying fluid being stored in the second pressure vessel in communication with one of the internal region of the first pressure vessel and the flexible bladder. A regulator may be provided for regulating the transport of pressure-conveying fluid from the second pressure vessel to the first pressure vessel. In some embodiments, the pressure-conveying fluid may be a gas, while in other embodiments the pressure-conveying fluid may be a liquid. Also, the coating component may be an enamel, an alkylsilicone resin, or a fluorinated resin.  
         [0014]     The invention also is related to a method of applying a coating to a substrate including: separating a deliverable substance from a pressurizing fluid with a flexible membrane disposed in a first pressure vessel, the deliverable substance comprising a coating component interspersed with at least one of liquefied carbon dioxide and supercritical carbon dioxide; allowing the pressurizing fluid to apply pressure to the deliverable substance (1) at a pressure at least the vapor pressure of carbon dioxide if the deliverable substance comprises liquefied carbon dioxide, or (2) at a pressure at least the critical pressure of carbon dioxide if the deliverable substance comprises supercritical carbon dioxide; delivering the deliverable substance to the substrate.  
         [0015]     In some embodiments, the pressurizing fluid may be provided in a second pressure vessel that communicates with the first pressure vessel. The deliverable substance may be provided in a bladder in the first pressure vessel, or the pressurizing fluid may be provided in a bladder in the first pressure vessel. The method may additionally include heating the deliverable substance prior to spray discharge, pumping the deliverable substance, agitating the deliverable substance, and/or recirculating a portion of the deliverable substance. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     Preferred features of the present invention are disclosed in the accompanying drawings, wherein similar reference characters denote similar elements throughout the several views, and wherein:  
         [0017]      FIG. 1  shows a partial cross-sectional view of an embodiment of a delivery system according to the present invention with a single outer pressure vessel and an inner bladder; and  
         [0018]      FIG. 2  shows a partial cross-sectional view of another embodiment of a delivery system according to the present invention with two pressure vessels, one of which includes an inner bladder. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]     Referring to  FIG. 1 , there is shown an exemplary delivery system according to a first embodiment of the present invention. Delivery system  200  includes a pressure vessel  202  with a cylinder fitting  204  and a flexible bladder  206 . A quantity of a deliverable substance  208  is preferably stored in bladder  206 , while a pressurizing gas is stored in region  210  between vessel  202  and bladder  206 . Deliverable substance  208  preferably includes a desired coating component interspersed with a fluid component. The coating component preferably is chosen to suit a desired application, and in the preferred embodiment is a paint or resin suitable for application to commercial building substrates. Among the coating components contemplated are acrylics, alkylsilicone resins, and fluorinated resins, however the present invention also may apply to other organics, inorganics, hydrocarbons, and silicones. Exemplar coating components include such substances as Sherwin Williams Industrial Enamel HS #B54TZ404, a high performance all-purpose solvent reducible water repellent for mineral substrates such as product designation BS 290 (an alkylsilicone resin with alkoxy groups) supplied by Wacker Silicones Corporation (Adrian, MI), and a formulated composition whose main ingredient is a fluorinated resin with molecular weight less than 15,000. In general, both monomers and polymers may be used. The fluid component preferably is chosen from compounds suitable for use as supercritical solvents, including CO 2 , C 2 H 4 , N 2 O, NH 3 , n-Cs, n-C 4 , CCl 2 F 2 , and CHF 3 , and most preferably is CO 2 . A delivery line  212  communicates with, and is sealed to, an opening in bladder  206 , and terminates at an isolation valve  214 . Preferably, isolation valve  214  is connected to a regulator (not shown), through which deliverable substance  208  flows.  
         [0020]     A fluid delivery line  215  communicates with region  210  in vessel  202 , so that region  210  may be filled with fluid, preferably a pressurizing gas. A fluid delivery valve  216  may be connected to a source of fluid for filling region  210 . Any of a wide range of pressurization gases may be used, for example air or nitrogen which are relatively inexpensive. In an alternate embodiment, region  210  is filled with a liquid. High pressure rated rigid cylinders are preferably used, permitting safe storage of fluids such as liquid carbon dioxide at ambient temperatures. Such cylinders should preferably be rated for use over a pressure range of about 200 psi to about 4500 psi. To prevent cylinder rupture due to over-pressurization, a frangible disk, relief valve, or other safety mechanism may be employed.  
         [0021]     Gas cylinders with a head space (i.e., without an internal bladder) contain both liquid and gas when full. For example, at 70° F., a full carbon dioxide gas cylinder has carbon dioxide in both liquid and gaseous states. The liquid carbon dioxide may fill about two-thirds of the space in the cylinder, while the remainder of the cylinder has carbon dioxide gas. It is known that some exchange occurs between the liquid and gas. This is also true for cylinders that are provided with several types of gases having different densities. Helium-headspace (HHS) carbon dioxide, otherwise known as helium head pressure (HHP) carbon dioxide, may be used, with carbon dioxide in the deliverable substance and helium present in the head space. In such a case, some exchange can be expected between the headspace filler gas and the carbon dioxide. In particular, some researchers have shown that some of the helium may dissolve in the liquid carbon dioxide. Because of the exchange, HHS carbon dioxide contains helium as an impurity in the liquid carbon dioxide. The presence of the impurity can have a measurable impact in some sensitive applications such as supercritical fluid extraction and supercritical fluid chromatography.  
         [0022]     The prior art suggests that carbon dioxide would be expected to be found in the head space. From such indication, it would be expected that the composition of the deliverable substance would vary as the deliverable substance is emptied from the pressure vessel, due to the transport of some carbon dioxide from the deliverable substance to the head space. Further, in an experiment using a non-bladder delivery system and an exemplar formulated composition and CO 2  deliverable substance, a 4.5% by weight change in composition was found between 20° C. and 40° C. For the test, the coating component was a formulated composition whose main ingredient is a fluorinated resin, as described previously. Advantageously, the bladder-based embodiments of the present invention permit a constant composition of the deliverable substance to be delivered, for example by spray, independent of temperature and pressure.  
         [0023]     In the preferred embodiment, flexible bladder  206  is generally impermeable to deliverable substance  208  and to the pressurizing gas in region  210 , thereby providing a physical barrier. Thus, the construction of delivery system  200  advantageously permits deliverable substance  208  to be delivered in a “pure” state, i.e. without the presence of dissolved pressurizing gas as may occur with other systems disclosed herein. As a result, a consistent composition of deliverable substance  208  may be dispensed. In addition, higher pressures may be applied (indirectly) to deliverable substance  208  during dispensing, because there is no concern about reaching the saturation pressure of the pressurizing gas with respect to deliverable substance  208 .  
         [0024]     In some embodiments, flexible bladder  206  is formed of a metalisized polymer. Preferably, bladder  206  is formed of an elastomeric material. Bladder  206  is sized as desired to fit within pressure vessel  202 , and may initially fill the inner space of vessel  202  so that a region  210  is not initially present. As pressurizing gas is delivered to vessel  202  to fill a region  210 , pressure is applied to bladder  206 . Consequently, bladder  206  collapses to a smaller volume, and accordingly deliverable substance  208  is expelled in an amount about the same as the decrease in volume. Among the additional advantages realized with delivery system  200  include the potential complete expulsion of the stored deliverable substance  208  from bladder  206  when it has completely collapsed, so that waste is eliminated. Also, while dense gas and/or liquid might remain at the bottom of a pressure vessel when a dip tube is used, such excess is avoided. In addition, delivery system  200  may be used in any orientation, using standard commercially available cylinders. Finally, because of the design of delivery system  200 , deliverable substance  208  may be a supercritical fluid. Preferably, when deliverable substance  208  includes a supercritical fluid, the pressure in region  210  is above the critical pressure of the supercritical fluid.  
         [0025]     Delivery system  200  may be supplied “turnkey,” so that a user need only unpack the system and attach a suitable spray hose with an orifice to valve  214 . The system may be provided with suitable pressurizing gas in region  210 , and a bladder  206  filled with a deliverable substance  208 . Such a system may initially have a high pressure gas, i.e., 4000 psi in region  210 . As deliverable substance is expelled from bladder  206 , the pressure in region  210  decreases, along with the corresponding expulsion pressure of deliverable substance  208 .  
         [0026]     In an alternate embodiment, deliverable substance  208  may instead be stored in region  210 , while a pressurizing gas is stored in bladder  206 . Bladder  206  is thus filled, like a balloon, so that pressure is exerted against deliverable substance  208  to expel it from pressure vessel  202 . Geometrically, however, a bladder of a shape that would conform to the inner walls of vessel  202  may be more difficult to produce. Furthermore, regardless of whether pressurizing gas is stored in region  210  or bladder  206 , the pressurizing gas storage location serves as an accumulator, compensating for volume changes occurring, for example, due to changes in temperature.  
         [0027]     Turning to  FIG. 2 , an exemplary preferred embodiment of a delivery system according to the present invention is shown. Delivery system  220  includes a pressure vessel  222  with a cylinder fitting  224  and a flexible bladder  226 . A quantity of a deliverable substance  228  is preferably stored in bladder  226 , while a pressurizing gas is stored in region  230  between vessel  222  and bladder  226 . A delivery line  232  communicates with, and is sealed to, an opening in bladder  226 , and terminates at an isolation valve  234 . A fluid delivery line  235  communicates with region  230  in vessel  222 , so that region  230  may be filled with fluid, preferably a pressurizing gas. A fluid delivery valve  236  is connected to a source of fluid for filling region  230 . In the preferred embodiment, a pressure vessel  238  with a cylinder fitting  240  is provided. Cylinders  222  and  238  communicate through fluid delivery line  235 . In particular, fluid delivery valve  236 , regulator  242 , and pressurizing cylinder valve  244  are connected between cylinders  222  and  238 . Bladder  226  stores a quantity of a deliverable substance  228 , as described above, while constant pressure is maintained in region  230  by regulator  242 . In one non-limiting exemplary arrangement, cylinder  238  may be filled to an initial pressure of 3000 psi with nitrogen gas, while region  230  of cylinder  222  may be pressurized with the nitrogen gas at a generally constant pressure of  1500  psi using regulator  242 .  
         [0028]     Advantageously, use of delivery system  220  with two pressure vessels  222  and  238  permits a constant spray pressure to be achieved for deliverable substance exiting isolation valve  234 . Furthermore, when a user stops spraying deliverable substance  228  and some remains in the delivery hose, the remaining deliverable substance is permitted to return to its initial source (i.e., bladder  226  which re-expands to accommodate the fluid and coating component), thereby avoiding excessive pressure increase in the delivery hose.  
         [0029]     In preferred embodiments of the present invention, a heated hose is connected to the delivery system. The heated hose, for example, permits a desired coating component intermixed with liquid/supercritical carbon dioxide, to be delivered with desired spray characteristics such as droplet size. In addition, heating may permit the deliverable substance to be discharged without solidification proximate the nozzle, and thus plugging of the delivery system can be averted. In one preferred embodiment, a substance enters the final delivery hose at a temperature of 20° C. and is heated to a temperature of 50° C. at the exit of the hose proximate the nozzle.  
         [0030]     Preferably, pressure vessels  202 ,  222 ,  238  are provided as cylinders or other suitably rigid tanks. Although carbon fiber cylinders are preferred, other cylinders such as fiberglass, aramid, aluminum or steel cylinders may be used.  
         [0031]     In addition, pressure vessels and or bladders disclosed in the present invention may be provided with means for producing agitation, such as magnetic stirrers, one or more mixing balls, or other agitation arrangements. Such agitation is advantageous because when pressurizing gas is applied to the head space (or exterior of a bladder), it causes some of the gaseous CO 2  to be liquefied due to the increase in pressure. The CO 2  is less dense than the coating component, and thus stratifies above the coating component/CO 2  mixture creating a non-homogenous mixture. Further, homogenous systems also may develop density gradients, over time, due to the vastly different densities of the mixture components (coating component and CO 2 ). Preferably, mixing is undertaken after pressurization.  
         [0032]     In one exemplary, preferred embodiment of the present invention, the delivery system is provided in compact, lightweight form to permit transport in a midsize automobile and single-person handling. Such an embodiment may, for example, have two pressure vessels, an overall size of 26″×12″×48″, and an overall weight of about 70 lbs. Cylinders are preferably pre-filled, requiring minimal preparation by users, and the delivery system may be used in a batch process. In addition, due to the size and weight, and concomitantly the nature of the materials that are used, the delivery systems may be shipped by common carrier.  
         [0033]     Advantageously, the embodiments of the present invention may be operated without the used of external energy sources, which for example, are typically required with prior art delivery systems which employ one or more pumps and control systems. Pumps, in particular, require significant energy. Moreover, the embodiments of the present invention preferably only require an energy source for the heated discharge hose. Such an energy source may be provided in a small battery pack, which may be directly attached to the delivery system or connected to the heated discharge hose yet carried, for example, on the waist belt of a workman using the system.  
         [0034]     While various descriptions of the present invention are described above, it should be understood that the various features can be used singly or in any combination thereof. Therefore, this invention is not to be limited to only the specifically preferred embodiments depicted herein.  
         [0035]     Further, it should be understood that variations and modifications within the spirit and scope of the invention may occur to those skilled in the art to which the invention pertains. For example, the bladder-based delivery systems of the present invention may be used in supercritical fluid extraction and supercritical fluid chromatography, in order to minimize the presence of impurities in the delivered supercritical fluid. Furthemore, the bladder-based delivery systems of the present invention may be used in industrial painting applications, such as automotive painting. In addition, each of the delivery systems may be configured to be portable, for example, as a back-pack unit. Also, other embodiments may include more than two pressure vessels. For example, a coating delivery system may include two pressure vessels for storing and selectively delivering different deliverable substances, while a third pressure vessel may be included for storing pressure conveying fluid. Alternatively, a coating delivery system may include several pressure vessels of a standardized size which contain the same deliverable substance, thereby in the aggregate providing greater volume of available deliverable substance when a coating is being applied. Accordingly, all expedient modifications readily attainable by one versed in the art from the disclosure set forth herein that are within the scope and spirit of the present invention are to be included as further embodiments of the present invention. The scope of the present invention is accordingly defined as set forth in the appended claims.