Abstract:
Disclosed is a vessel and a method of forming the vessel including an inner shell, an outer shell, and a diffusion layer disposed therebetween, wherein the diffusion layer facilitates the venting of a fluid from between the inner shell and the outer shell.

Description:
FIELD OF THE INVENTION 
     The invention relates to a hollow vessel and more particularly to a method and apparatus for forming a hollow pressure vessel having an inner shell, an outer shell, and a diffusion layer disposed therebetween, wherein the diffusion layer facilitates a venting of a fluid that has penetrated the inner shell. 
     BACKGROUND OF THE INVENTION 
     Fuel cells have been proposed as a power source for electric vehicles and other applications. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied as a fuel to an anode of the fuel cell and oxygen is supplied as an oxidant to a cathode of the fuel cell. A plurality of fuel cells is stacked together in fuel cell stacks to form a fuel cell system. The fuel and oxidant are typically stored in large pressurized hollow vessels, such as fuel tanks, disposed on an undercarriage of the vehicle. 
     The pressurized vessels may be multi-layered and include at least an inner shell and an outer shell. Some inner shells are manufactured using a rotational molding method. The inner shell is formed utilizing the rotational molding method by disposing bosses in a die cavity with a polymer resin, heating the mold while it is rotated causing the resin to melt and coat walls of the die cavity, cooling the die, and removing the molded inner shell. To form the outer shell, the molded inner shell may undergo a filament winding process. The filament winding process often results in the creation of a space between the outer shell and the inner shell of the fuel tank. Gas that diffuses through the inner shell creates gas pockets between the inner shell and the outer shell. 
     It would be desirable to develop a hollow pressure vessel and method of forming the same having an inner shell, an outer shell, and a diffusion layer disposed therebetween, wherein the diffusion layer facilitates a venting of a fluid that has diffused through the inner shell. 
     SUMMARY OF THE INVENTION 
     Concordant and congruous with the present invention, a hollow pressure vessel and method of forming the same having an inner shell, an outer shell, and a diffusion layer disposed therebetween, wherein the diffusion layer facilitates a venting of a fluid that has diffused through the inner shell, has surprising been discovered. 
     In one embodiment, the method of forming a vessel comprises the steps of providing a boss; forming a hollow inner shell from a moldable material, wherein the inner shell is formed in contact with the boss; forming a diffusion layer on at least a portion of the inner shell; and forming an outer shell over the diffusion layer and the inner shell. 
     In another embodiment, the vessel comprises a hollow inner shell formed from a moldable material and adapted to store a fluid; a boss adhered to the moldable material of said inner shell and forming a substantially fluid tight seal therebetween; a diffusion layer formed around at least a portion of said inner shell, wherein said diffusion layer facilitates the flow of a fluid from the diffusion layer to an exterior of said vessel; and an outer shell formed around said diffusion layer and said inner shell. 
     In another embodiment, the vessel comprises a hollow inner shell formed from a moldable material and adapted to store a fluid; a boss adhered to the moldable material of said inner shell and forming a substantially fluid tight seal therebetween, wherein said boss includes a flow channel formed therein in fluid communication with an exterior of said vessel and the diffusion layer; a diffusion layer formed around at least part of said inner shell, wherein said diffusion layer facilitates the flow of a fluid from the diffusion layer, through said boss, and to the exterior of said vessel; and an outer shell formed around said diffusion layer and said inner shell. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view of a vessel according to an embodiment of the invention; 
         FIG. 2  is a cross-sectional view of a vessel according to another embodiment of the invention; 
         FIG. 3  is a cross-sectional view of a vessel according to another embodiment of the invention; and 
         FIG. 4  is a cross-sectional view of a vessel according to another embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following detailed description and appended drawings describe and illustrate various exemplary embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. In respect of the methods disclosed, the steps presented are exemplary in nature, and thus, the order of the steps is not necessary or critical. 
       FIG. 1  illustrates a hollow pressure vessel  10  having an inner shell  12 , a diffusion layer  14 , and an outer shell  16 . The vessel  10  has a substantially cylindrical shape and is adapted to hold a pressurized fluid (not shown). It is understood that the vessel  10  may have any shape as desired. The pressurized fluid may be any fluid such as a gas such as hydrogen gas and oxygen gas, a liquid, and both a liquid and a gas, for example. 
     The vessel  10  includes a boss  18  disposed on a first end  20  thereof. The boss  18  is a separately produced finish that forms an opening into an interior of the vessel  10 , and is typically shaped to accommodate a specific closure. The vessel  10  may include any number of bosses, as desired. The boss  18  includes an annular groove  22  formed on an inner surface  28  and a flow channel  24  formed therein. The groove  22  is adapted to receive a portion of a hose, nozzle, conduit, or other means for fluid communication (not shown) with the boss  18  and the interior of the vessel  10 . Rather than the groove  22 , the inner surface  28  of the boss  18  may be threaded to receive the various means for fluid communication. The flow channel  24  is formed in a sidewall of the boss  18 . An opening  24   a  of the flow channel  24  is in fluid communication with the diffusion layer  14 , and an opening  24   b  of the flow channel  24  is in fluid communication with an exterior of the vessel  10 . The opening  24   b  may also be in fluid communication with the atmosphere or a ventilation system, as desired. It is also understood that the boss  18  may be formed from any conventional material such as a plastic, steel, a steel alloy, or aluminum, for example. As shown in  FIG. 4 , the vessel  10  may include a second boss  19  substantially identical to the boss  18  disposed on a second end  21  thereof, as desired. 
     The inner shell  12  of the vessel  10  is a hollow container adapted to store the pressurized fluid. As shown, the inner shell  12  has a substantially cylindrical shape. However, the inner shell  12  may have any shape, as desired. A first end of the inner shell  12  is received in an aperture formed by the inner surface  28  of the boss  18  at the first end  20  of the vessel  10 . The inner shell  12  may also be received on an outer surface  30  of the boss  18 , as desired. As shown, the inner shell  12  is formed from a plastic such as polyethylene, PET, ethylene vinyl alcohol, or an ethylene vinyl acetate terpolymer, for example. The inner shell  12  may also be formed from any moldable material such as a metal, a glass, and the like, as desired. 
     The diffusion layer  14  is disposed between the inner shell  12  and the outer shell  16  of the vessel  10 . The diffusion layer  14  is formed around the inner shell  12  of the vessel  10  and is in communication with the opening  24   a  of the flow channel  24  of the boss  18 . As shown in  FIG. 1 , the diffusion layer  14  is formed from carbon fibers filament wound around the inner shell  12 . It is understood that that the diffusion layer  14  may be formed from any fluid permeable material adapted to facilitate the diffusion of a fluid such as a foam, a carbon paper, a resin coated carbon fiber, a glass fiber, and the like, for example. 
     The outer shell  16  of the vessel  10  is disposed on the diffusion layer  14  and has a substantially cylindrical shape. As shown, the outer shell  16  substantially abuts the diffusion layer  14 . The outer shell  16  is disposed on the boss  18  at the first end  20  of the vessel  10 . The outer shell  16  may be formed from any moldable material such as a metal and a plastic, for example, or the outer shell  16  may be formed with a filament winding process. If the outer shell  16  is formed by a filament winding process, the outer shell  16  may be formed from a carbon fiber, a glass fiber, a composite fiber, and a fiber having a resin coating, as desired. It is understood that the material used to form the outer shell  16  may be selected based on the process used to affix the outer shell  16  to the diffusion layer  14  and the inner shell  12 , the use of the vessel  10 , and the properties of the fluid to be stored in the vessel  10 . 
     To form the vessel  10 , the inner shell  12  is typically formed using a blow molding process. The boss  18  is disposed in a die (not shown) and the die is then closed. The boss  18  may be heated prior to being disposed in the die to facilitate adhesion to the inner shell  12  as it is formed. Melted pellets or flakes of plastic are then extruded into the die in the form of a parison (not shown). Because the parison is continuously extruded into the die, the parison is hollow. A fluid (not shown) is then caused to flow through the parison in the die causing the parison to expand and contact the walls of the die, thereby taking the shape of the die cavity. The inner shell  12  may be formed from other conventional processes such as rotational molding, for example, as desired. It is understood that the fluid may be any conventional fluid such as air, nitrogen, hydrogen, or oxygen, as desired. As the parison is expanded by the fluid, a portion of the parison is caused to contact, adhere to, and form a substantially fluid tight seal with the boss  18 . 
     As shown in  FIG. 1 , a neck portion  26  of the material forming the parison is blow molded into the inner surface  28  of the boss  18 . Material may be blown into the groove  22  and on the inner surface  28 , and may be cut away or otherwise machined from the boss  18 , as desired. It is understood that the surfaces of the boss  18  that contact the moldable material during the blow molding process may be etched, coated with a primer, or coated with an adhesive prior to the blow molding process to facilitate adhesion of the boss  18  to the moldable material, as desired. It is also understood that the boss  18  may include grooves, cavities, channels, or protuberances adapted to receive a portion of the material to mechanically attach the material to the boss  18 . Once the blow molded material has cooled sufficiently, the die is opened and the inner shell  12  is removed. 
     The diffusion layer  14  is typically formed around the inner shell  12  with a filament winding process. The inner shell  12  may be disposed over a mandrel (not shown) or disposed in an automated filament winding apparatus (not shown) and rotated as the diffusion layer  14  is applied to the inner shell  12  and a portion of the boss  18 . A first layer of the carbon fibers or other material used to form the diffusion layer  14  is wound around the inner shell  12 . The first layer of the carbon fibers is coated with a minimal amount of resin to fix the carbon fibers to the inner shell  12 . Another layer of carbon fibers is typically applied perpendicularly over the first layer of carbon fibers to complete the diffusion layer  14 . Depending on a rate of diffusion of the pressurized fluid through the inner shell  12 , the diffusion layer  14  may be comprised of numerous layers of carbon fibers or as little as a single layer of carbon fiber. Other methods can be used to form the diffusion layer  14  as desired. 
     Carbon fibers heavily impregnated with the resin are typically filament wound around the diffusion layer  14  to form the outer shell  16 . The carbon fibers of the outer shell  16  heavily impregnated with resin are applied to form a substantially fluid tight resin seal over the diffusion layer  14  and the inner shell  12 . To militate against the penetration of the resin from the heavily impregnated carbon fibers into the diffusion layer  14  a protective layer  15  may be placed onto the diffusion layer  14 . As shown, the protective layer  15  is a foil, however, the protective layer  15  may be a plastic, a cloth, or another material, as desired. It is understood that the outer shell  16  may be applied by a dipping process in a molten polymer or metal, by spraying a coating, or by sewing a leather or fabric material onto the diffusion layer  14  and inner shell  12 . Once the outer shell  16  is applied, the vessel  10  may be placed in an autoclave (not shown) to allow the resin of the outer shell  16  to cure. Once the resin of the outer shell  16  is cured, the vessel  10  is complete. Due to the resin tight seal of the outer shell  16  over the diffusion layer  14  and because of the winding pattern of the diffusion layer  14 , flow paths (not shown) in fluid communication with the opening  24   a  of the boss  18  are formed in the diffusion layer  14 . 
     During use of the vessel  10 , fluid contained in the vessel  10  diffuses through the inner shell  12  and into the diffusion layer  14 . The fluid then flows through the flow paths formed in the diffusion layer  14 , to the opening  24   a  of the flow path  24  of the boss  18 , through the boss  18 , and out into the atmosphere, thereby militating against a pressure and parasitic fluid cushion build-up between the inner shell  12  and outer shell  16  and extending a useful life of the vessel  10 . 
       FIG. 2  shows a hollow pressure vessel  10 ′ according to another embodiment of the invention. The embodiment of  FIG. 2  is similar to the vessel  10  of  FIG. 1 , except as described below. Similar to the structure of  FIG. 1 ,  FIG. 2  includes the same reference numerals accompanied by a prime (′) to denote similar structure. 
       FIG. 2  illustrates the hollow pressure vessel  10 ′ having an inner shell  12 ′, a diffusion layer  14 ′, and an outer shell  16 ′. The vessel  10 ′ has a substantially cylindrical shape and is adapted to hold a pressurized fluid (not shown). It is understood that the vessel  10 ′ may have any shape as desired. The pressurized fluid may be any fluid such as a gas such as hydrogen gas and oxygen gas, a liquid, and both a liquid and a gas, for example. 
     The vessel  10 ′ includes a boss  18 ′ disposed on a first end  20 ′ thereof. The boss  18 ′ is a separately produced finish that forms an opening into an interior of the vessel  10 ′, and is typically shaped to accommodate a specific closure. The vessel  10 ′ may include any number of bosses, as desired. The boss  18 ′ includes an annular groove  22 ′ formed on an inner surface  28 ′ therein. The groove  22 ′ is adapted to receive a portion of a hose, nozzle, conduit, or other means for fluid communication (not shown) with the boss  18 ′ and the interior of the vessel  10 ′. Rather than the groove  22 ′, the inner surface  28 ′ of the boss  18 ′ may be threaded to receive the various means for fluid communication. It is understood that the boss  18 ′ may be formed from any conventional material such as a plastic, steel, a steel alloy, or aluminum, for example. 
     The inner shell  12 ′ of the vessel  10 ′ is adapted to store the pressurized fluid. As shown, the inner shell  12 ′ has a substantially cylindrical shape. However, the inner shell  12 ′ may have any shape, as desired. A first end of the inner shell  12 ′ is received in an aperture formed by the inner surface  28 ′ of the boss  18 ′ at the first end  20 ′ of the vessel  10 ′. The inner shell  12 ′ may also be received on an outer surface  30 ′ of the boss  18 ′, as desired. As shown, the inner shell  12 ′ is formed from a plastic such as polyethylene, PET, ethylene vinyl alcohol, or an ethylene vinyl acetate terpolymer, for example. The inner shell  12 ′ may also be formed from any moldable material such as a metal, a glass, and the like, as desired. 
     The diffusion layer  14 ′ is disposed between the inner shell  12 ′ and the outer shell  16 ′ of the vessel  10 ′. The diffusion layer  14 ′ is formed around the inner shell  12 ′ of the vessel  10 ′ and is in communication with the outer surface  30 ′ of the boss  18 ′. As shown in  FIG. 2 , the diffusion layer  14 ′ is formed from carbon fibers filament wound around the inner shell  12 ′. It is understood that that the diffusion layer  14 ′ may be formed from any fluid permeable material adapted to facilitate the diffusion of a fluid such as a foam, a carbon paper, a glass fiber, and the like for example. 
     The outer shell  16 ′ of the vessel  10 ′ is disposed on the diffusion layer  14 ′ and has a substantially cylindrical shape. As shown, the outer shell  16 ′ substantially abuts the diffusion layer  14 ′. The outer shell  16 ′ is disposed on the boss  18 ′ at the first end  20 ′ of the vessel  10 ′. The outer shell  16 ′ may be formed from any moldable material such as a metal, and a plastic, for example, or the outer shell  16 ′ may be formed with a filament winding process. If the outer shell  16 ′ is formed by a filament winding process, the outer shell  16 ′ may be formed from a carbon fiber, a glass fiber, a composite fiber, and a fiber having a resin coating, as desired. It is understood that the material used to form the outer shell  16 ′ may be selected based on the process used to affix the outer shell  16 ′ to the diffusion layer  14 ′ and the inner shell  12 ′, the use of the vessel  10 ′, and the properties of the fluid to be stored in the vessel  10 ′. 
     To form the vessel  10 ′, the inner shell  12 ′ is typically formed using a blow molding process. The boss  18 ′ is disposed in a die (not shown) and the die is then closed. The boss  18 ′ may be heated prior to being disposed in the die to facilitate adhesion to the inner shell  12 ′ as it is formed. Melted pellets or flakes of plastic are then extruded into the die in the form of a parison (not shown). Because the parison is continuously extruded into the die, the parison is hollow. A fluid (not shown) is then caused to flow through the parison in the die causing the parison to expand and contact the walls of the die, thereby taking the shape of the die cavity. The inner shell  12 ′ may be formed from other conventional processes such as rotational molding, for example, as desired. It is understood that the fluid may be any conventional fluid such as air, nitrogen, hydrogen, or oxygen, as desired. As the parison is expanded by the fluid, a neck portion  26 ′ of the parison is caused to contact, adhere to, and form a substantially fluid tight seal with the boss  18 ′. 
     As shown in  FIG. 2 , the neck portion  26 ′ of the material forming the parison is blow molded into the inner surface  28 ′ of the boss  18 ′. Material may be blown into the groove  22 ′ and on the inner surface  28 ′, and may be cut away or otherwise machined from the boss  18 ′, as desired. It is understood that the surfaces of the boss  18 ′ that contact the moldable material during the blow molding process may be etched, coated with a primer, or coated with an adhesive prior to the blow molding process to facilitate adhesion of the boss  18 ′ to the moldable material, as desired. It is also understood that the boss  18 ′ may include grooves, cavities, channels, or protuberances adapted to receive a portion of the material to mechanically attach the material to the boss  18 ′. Once the blow molded material has cooled sufficiently, the die is opened and the inner shell  12 ′ is removed. 
     The diffusion layer  14 ′ is typically formed around the outside of the inner shell  12 ′ and the outer surface  30 ′ of the boss  18 ′ with a filament winding process. The inner shell  12 ′ may be disposed over a mandrel (not shown) or disposed in an automated filament winding apparatus (not shown) and rotated as the diffusion layer  14 ′ is applied to the inner shell  12 ′ and a portion of the boss  18 ′. A first layer of the carbon fibers or other material used to form the diffusion layer  14 ′ is wound around the inner shell  12 ′. The first layer of the carbon fibers is coated with a minimal amount of resin to fix the carbon fibers to the inner shell  12 ′. Another layer of carbon fibers is typically applied perpendicularly over the first layer of carbon fibers to complete the diffusion layer  14 ′. Depending on the rate of diffusion of the pressurized fluid through the inner shell  12 ′, the diffusion layer  14 ′ may be comprised of numerous layers of carbon fibers or as little as a single layer of carbon fiber. Other methods can be used to form the diffusion later  14 ′ as desired. 
     Carbon fibers heavily impregnated with the resin are filament wound around the diffusion layer  14 ′ to form the outer shell  16 ′. A portion  32 ′ of the diffusion layer  14 ′ is not contacted by carbon fibers of the outer shell  16 ′. The carbon fibers of the outer shell  16 ′ heavily impregnated with resin are applied to form a substantially fluid tight resin seal over the diffusion layer  14 ′ and the inner shell  12 ′. To militate against the penetration of the resin from the heavily impregnated carbon fibers into the diffusion layer  14 ′, a protective layer  15 ′ may be placed onto the diffusion layer  14 ′. As shown, the protective layer  15 ′ is a foil, however, the protective layer  15 ′ may be a plastic, a cloth, or another material, as desired. It is understood that the outer shell  16 ′ may be applied by a dipping process in a molten polymer or metal, by spraying a coating, or by sewing a leather or fabric material onto the diffusion layer  14 ′ and inner shell  12 ′. Once the outer shell  16  is applied, the vessel  10 ′ may be placed in an autoclave (not shown) to allow the resin of the outer shell  16 ′ to cure. Once the resin of the outer shell  16 ′ is cured, the vessel  10 ′ is complete. Due to the resin tight seal of the outer shell  16 ′ over the diffusion layer  14 ′ and because of the winding pattern of the diffusion layer  14 ′, flow paths (not shown) in fluid communication with the exterior of the vessel  10 ′ are formed in the diffusion layer  14 ′. 
     During use of the vessel  10 ′, fluid contained in the vessel  10 ′ diffuses through the inner shell  12 ′ and into the diffusion layer  14 ′. The fluid then flows through the flow paths formed in the diffusion layer  14 ′ and out into the atmosphere, thereby militating against a pressure and parasitic fluid cushion build-up between the inner shell  12 ′ and outer shell  16 ′ and extending a useful life of the vessel  10 ′. 
       FIG. 3  shows a hollow pressure vessel  10 ″ according to another embodiment of the invention. The embodiment of  FIG. 3  is similar to the vessel  10  of  FIG. 1 , except as described below. Similar to the structure of  FIG. 1 ,  FIG. 3  includes the same reference numerals accompanied by a double-prime (″) to denote similar structure. 
       FIG. 3  illustrates the hollow pressure vessel  10 ″ having an inner shell  12 ″, a diffusion layer  14 ″, and an outer shell  16 ″. The vessel  10 ″ has a substantially cylindrical shape and is adapted to hold a pressurized fluid (not shown). It is understood that the vessel  10 ″ may have any shape as desired. The pressurized fluid may be any fluid such as a gas such as hydrogen gas and oxygen gas, a liquid, and both a liquid and a gas, for example. 
     The vessel  10 ″ includes a boss  18 ″ disposed on a first end  20 ″ the vessel  10 ″ and a blind boss  34  disposed on a second end  36  of the vessel  10 ″. The boss  18 ″ is a separately produced finish that forms an opening into an interior of the vessel  10 ′″, and is shaped to accommodate a specific closure. The vessel  10 ′″ may include any number of bosses, as desired. The boss  18 ″ includes an annular groove  22 ″. The groove  22 ″ is adapted to receive a portion of a hose, nozzle, conduit, or other means for fluid communication (not shown) with the boss  18 ″ and the interior of the vessel  10 ″. Rather than the groove  22 ″, the inner surface  28 ″ of the boss  18 ″ may be threaded to receive the various means for fluid communication. The blind boss  34  is a separately produced and outwardly projecting closed hole fixed to the vessel  10 ′″. A blind boss typically includes threads adapted to receive the threaded portion of another vessel, pump, or motor to anchor another vessel, pump, or motor to the vessel. The blind boss  34  includes a flow channel  38  formed therein. The flow channel  38  is formed in a sidewall of the blind boss  34 . An opening  38   a  of the fluid channel  38  is in fluid communication with the diffusion layer  14 ″, and an opening  38   b  of the flow channel  38  is in fluid communication with an exterior of the vessel  10 ′″. The opening  38   b  may also be in fluid communication to the atmosphere or a ventilation system, as desired. It is understood that the blind boss  34  may formed from any conventional material such as a plastic, steel, a steel alloy, or aluminum, for example. 
     The inner shell  12 ″ of the vessel  10 ″ is a hollow container adapted to store the pressurized fluid. As shown, the inner shell  12 ″ has a substantially cylindrical shape. However, the inner shell  12 ″ may have any shape, as desired. A first end of the inner shell  12 ″ is received in an aperture formed by the inner surface  28 ″ of the boss  18 ″ at the first end  20 ″ of the vessel  100 ″. The inner shell  12 ″ may also be received on an outer surface  30 ″ of the boss  18 ″, as desired. As shown, the inner shell  12 ″ is formed from a plastic such as polyethylene, PET, ethylene vinyl alcohol, or an ethylene vinyl acetate terpolymer, for example. The inner shell  12 ″ may be formed from any moldable material such as a metal, a glass, and the like, as desired. 
     The diffusion layer  14 ″ is disposed between the inner shell  12 ″ and the outer shell  16 ″ of the vessel  10 ″. The diffusion layer  14 ″ is formed around the inner shell  12 ″ of the vessel  10 ″ and is in communication with the opening  38   a  of the flow channel  38  of the boss  18 ″. As shown in  FIG. 3 , the diffusion layer  14 ″ is formed from carbon fibers filament wound around the inner shell  12 ″. It is understood that that the diffusion layer  14 ″ may be formed from any fluid permeable material adapted to facilitate the diffusion of a fluid, such as a foam, a carbon paper, a glass fiber, and the like, for example. 
     The outer shell  16 ″ of the vessel  10 ″ is disposed on the diffusion layer  14 ″ and has a substantially cylindrical shape. As shown, the outer shell  16 ″ substantially abuts the diffusion layer  14 ″. The outer shell  16 ″ is disposed on the boss  18 ″ at the first end  20 ″ of the vessel  10 ″. The outer shell  16 ″ may be formed from any moldable material such as a metal and a plastic, for example, or the outer shell  16 ″ may be formed with a filament winding process. If the outer shell  16 ″ is formed by a filament winding process, the outer shell  16  may be formed from a carbon fiber, a glass fiber, a composite fiber, and a fiber having a resin coating, as desired. It is understood that the material used to form the outer shell  16 ″ may be selected based on the process used to affix the outer shell  16 ″ to the diffusion layer  14 ″ and the inner shell  12 ″, the use of the vessel  10 ″, and the properties of the fluid to be stored in the vessel  10 ″. 
     To form the vessel  10 ″, the inner shell  12 ″ is typically formed using a blow molding process. The boss  18 ″ and blind boss  34  are disposed in a die (not shown) and the die is closed. The boss  18 ″ and blind boss  34  may be heated prior to being disposed in the die to facilitate adhesion to the inner shell  12 ″ as it is formed. Melted pellets or flakes of plastic are then extruded into the die in the form of a parison (not shown). Because the parison is continuously extruded into the die, the parison is hollow. A fluid (not shown) is then caused to flow through the parison in the die causing the parison to expand and contact the walls of the die, thereby taking the shape of the die cavity. The inner shell  12 ″ may be formed from other conventional processes such as rotational molding, for example, as desired. It is understood that the fluid may be any conventional fluid such as air, nitrogen, hydrogen, or oxygen, as desired. As the parison is expanded by the fluid, a neck portion  26 ″ of the parison is caused to contact, adhere to, and form a substantially fluid tight seal with the boss  18 ″. 
     As shown in  FIG. 3 , the neck portion  26 ″ of the material forming the parison is blow molded into the inner surface  28 ″ of the boss  18 ″ and against the blind boss  34 . Material may be blown into the groove  22 ″ and on the inner surface  28 ″, and may be cut away or otherwise machined from the boss  18 ″, as desired. It is understood that the surfaces of the boss  18 ″ that contact the moldable material during the blow molding process may be etched, coated with a primer, or coated with an adhesive prior to the blow molding process to facilitate adhesion of the boss  18 ″ to the moldable material, as desired. It is also understood that the boss  18 ″ may include grooves, cavities, channels, or protuberances adapted to receive a portion of the material to mechanically attach the material to the boss  18 ″. Once the blow molded material has cooled sufficiently, the die is opened and the inner shell  12 ″ is removed. 
     The diffusion layer  14 ″ is typically formed around the inner shell  12 ″ with a filament winding process. The inner shell  12 ″ may be disposed over a mandrel (not shown) or disposed in an automated filament winding apparatus (not shown) and rotated as the diffusion layer  14 ″ is applied to the inner shell  12 ″ and a portion of the boss  18 ″. A first layer of the carbon fibers or other material used to form the diffusion layer  14 ″ is wound around the inner shell  12 ″. The first layer of the carbon fibers is coated with a minimal amount of resin to fix the carbon fibers to the inner shell  12 ″. Another layer of carbon fibers is typically applied perpendicularly over the first layer of carbon fibers to complete the diffusion layer  14 ″. Depending on the rate of diffusion of the pressurized fluid through the inner shell  12 ″, the diffusion layer  14 ″ may be comprised of numerous layers of carbon fibers or as little as a single layer of carbon fiber. Other methods can be used to form the diffusion later  14 ″ as desired. 
     Carbon fibers heavily impregnated with the resin are filament wound around the diffusion layer  14 ″ to form the outer shell  16 ″. The carbon fibers of the outer shell  16 ″ heavily impregnated with resin are applied to form a substantially fluid tight resin seal over the diffusion layer  14 ″ and the inner shell  12 ″. To militate against the penetration of the resin from the heavily impregnated carbon fibers penetrates into the diffusion layer  14 ″ a protective layer  15 ″ may be placed onto the diffusion layer  14 ″. As shown, the protective layer  15 ″ is a foil, however, the protective layer  15 ″ may be a plastic, a cloth, or another material, as desired. It is understood that the outer shell  16 ″ may be applied by a dipping process in a molten polymer or metal, by spraying a coating, or by sewing a leather or fabric material onto the diffusion layer  14 ″ and inner shell  12 ″. Once the outer shell  16 ″ is applied, the vessel  10 ″ may be placed in an autoclave (not shown) to allow the resin of the outer shell  16  to cure. Once the resin of the outer shell  16 ″ is cured, the vessel  10 ″ is complete. Due to the resin tight seal of the outer shell  16 ″ over the diffusion layer  14 ″ and because of the winding pattern of the diffusion layer  14 ″, flow paths (not shown) in fluid communication with the opening  38   a  of the blind boss  34  are formed in the diffusion layer  14 ″. 
     During use of the vessel  10 ″, fluid contained in the vessel  10 ″ diffuses through the inner shell  12 ″ and into the diffusion layer  14 ″. The fluid then flows through the flow paths formed in the diffusion layer  14 ″, to the opening  38   a  of the flow path  38  of the boss  18 ″, through the boss  18 ″, and out into the atmosphere, thereby militating against a pressure and parasitic fluid cushion build-up between the inner shell  12 ″ and outer shell  16 ″ and extending a useful life of the vessel  10 ″. 
     From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications to the invention to adapt it to various usages and conditions.