Abstract:
A pressure vessel for containing materials under elevated pressures includes a metal liner and an adhesive layer, applied to the outer surface of the metal liner, where the adhesive layer is treated with a vacuum bag in order to secure the adhesive lo the outer surface of the liner. An overwrap layer is applied on top of the adhesive on the outer surface of the metal finer, where the overwrap layer is formed by winding a filamentary material around the liner, such that the filamentary material adheres to the adhesive forming an overwrap layer on the outer surface of the metal liner, forming the pressure vessel.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a divisional application of co-pending U.S. patent application Ser. No. 12/290,819, filed on Nov. 4, 2008, which is a divisional of U.S. patent application Ser. No. 11/232,463, filed on Sep. 21, 2005 (now issued as U.S. Pat. No. 7,497,919). The entirety of these related disclosures are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to containers in general. More particularly, the present invention relates to light weight vessels capable of accommodating various media at relatively high pressures. 
       BACKGROUND 
       [0003]    In many technical fields, a need exists for staring various liquid or gaseous media, such as compressed or liquefied gases, for extended periods of time and frequently at very high pressures. Many attempts have already been made in the past to satisfy this need by developing lightweight pressurized medium containers or pressure vessels that would accommodate the pressurized medium without suffering leakage losses or structural damage. 
         [0004]    For a variety of reasons, not the least important of which is the relatively high ratio of pressure that the vessel walls are able to withstand to the weight of a vessel of a given capacity, it has been found advantageous to give such walls a multilayer or composite structure, including an inner liner and an outer shell surrounding the liner and in intimate contact therewith. The liner is formed of a material usually a metallic material that is compatible with (i.e. inert with respect to) and also completely or at least highly impermeable to the medium being stored. 
         [0005]    All-metallic pressure vessels have been disclosed, for example, in U.S. Pat. Nos. 2,127,712; 2,661,113; 3,140,006; and 4,964,524, of which all but the second one are directed to vessels of multilayer construction. In this instance, one of the purposes of the liner is to form an inert protective barrier preventing the medium from reaching through gross leakage or permeation through the liner to the outer shell and possibly damaging the shell. However, due to their considerable thickness and intimate contact or engagement with the shell, the liners of all-metallic pressure vessels generally contribute significantly to the load bearing capacity of the vessel. In classical state of the art vessel fabrication, the liner represents a significant fraction of the total weight. Experience with such and similar all-metallic pressure vessel constructions has shown, on the other hand, that they are limited in applicability because they are either too heavy (a criterion that is of paramount importance for applications where weight is at a premium, such as in outer space applications), or expensive to manufacture, or prone to failure, especially due to metallic material fatigue at weakened or stress concentration regions after having been subjected to a number of pressurization and depressurization cycles. 
         [0006]    With the advent and development of high strength filaments such as glass, graphite, and synthetic plastic material fibers, and of materials, such as epoxy resins, capable of forming a matrix embedding such filaments and bonding them together into a composite structure, attempts have been made, some more successful than others, to use such composite materials for the outer shell of the pressure vessel. Of course, due to the high strength-to-weight ratio of such materials, the overall weight of the resulting vessel is significantly reduced relative to that of a comparable all-metallic vessel of the same capacity and pressure rating. Examples of vessels of this kind are disclosed, for example, in U.S. Pat. Nos. 2,744,043; 2,827,195; 3,943,010; 3,969,812, 4,040,163; and 5,653,358. 
         [0007]    For example, U.S. Pat. No. 5,653,358 among other elements, describes a tank of composite structure. A vessel is comprised principally of an inner liner (such as a metal liner) coated with a primer and an overwrap or jacket. To that end, the outer jacket is constructed, in a known manner, by superimposed and overlapping layers of impregnated filamentary material that contains glass, graphite or Kevlar™ fibers wrapped in different directions around the liner, with the interstices between the fibers or filaments being filled by impregnating material such as hardenable epoxy resin that, upon setting or hardening, forms a matrix that firmly embeds such fibers or filamentary material. 
         [0008]    Thus, after hardening, the filamentary and impregnating material together form a composite, fiber reinforced solid body that is capable of withstanding most if not all of the forces applied to the vessel during its lifetime. 
         [0009]    However, the prior art methods of applying the wrapped jacket to the metal liner of the vessel suffers from many drawbacks. In the prior art, a metal liner is first coated with a primer and then the adhesive is used to structurally couple the liner to the overlying filament wound composite. Most commonly a reticulating film adhesive is used. In such an application, pre-cut pattern shapes or gore panels are applied by gloved hands prior to the commencement of composite lay-up. In this application, fibers are impregnated with a wet winding resin before application. The bearing pressure for bondline curing, required to ensure a good bond between the adhesive and the metal liner, is developed as a by-product of the tension in the fiber tows (jacket material) that are filament wound in a pre-programmed repeating closure pattern over the liner and adhesive. 
         [0010]    Upon completion of the winding, which consists of multiple layers of the reinforcing fibers and impregnating resin, the composite structure is cured. The adhesive and wet winding resins are compatible and co-curable. 
         [0011]    This wet winding process by its nature develops variable bearing pressures on the liner as a result of fiber buildup near the polar regions (ie. boss and/or exit of the vessel) of the wind. This frequently results in roping/bridging of the fiber with a resulting loss of bearing pressure. The hearing pressure also varies due to resin rheology (time and temperature dependent viscosity response.) Resin trough behavior is recognized as resin bleedout during the early stages of wet winding cure (first stage also known as gelation). The variability in bearing pressure is a limiting factor in developing optimal adhesion of the overwrap layer to the metal liner. The critical parameter is adhesion to the metal liner, the adhesion to the co-cured overlying composite structure is readily achieved through selection of compatible and co-curable adhesives and wet winding epoxies. 
         [0012]    It is understood, that structural coupling between the inner liner layer and the outer jacket layer is critical, particularly when the pressure vessels are filled with loads under high pressure. Poor adhesion between the overwrap layer and the metal liner caused by irregular bearing pressure during the overwrap application can result in liner elastic stability/buckling failure. The thin metal liner, as a standalone structural entity, is incapable of supporting the high bearing pressure imposed at zero or low pressure by the overlying composite after vessel autofrettage. The elastic stability or buckling failure of the liner results in dramatic reductions in fatigue life, resulting through cracks in the metal liner and leakage of contents within a small number of cycles. 
       SUMMARY 
       [0013]    As such it is the object of this invention to provide a high pressure light-weight pressure vessel that overcomes the drawbacks associated with the prior art and is constructed having improved adhesion between the inner liner and outer jacket layers. Additionally, it is an object of the present invention to provide a method of consistently manufacturing such pressure vessels with improved adhesion between the inner liner and outer jacket layers. 
         [0014]    To this end, the present invention provides a method for manufacturing a high pressure light-weight pressure vessel including the steps of obtaining a metal liner, applying a first layer of adhesive to an outer surface of the metal liner, placing the metal liner into a vacuum bag and applying a vacuum to the vacuum bag, thus securing the adhesive to the outer surface of the metal liner. After curing of the adhesive, a filamentary material is wound around the liner, where the filamentary material adheres to the first layer of adhesive forming an overwrap layer on the outer surface of the metal liner, forming the pressure vessel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with features, objects, and advantages thereof may best be understood by reference to the following detailed description when read with the accompanying drawings: 
           [0016]      FIG. 1  is a cross section of a pressure vessel, according to one embodiment of the present invention; 
           [0017]      FIG. 2  is a close up the filament of the overwrap layer, in accordance with one embodiment of the present invention; 
           [0018]      FIG. 3  is a flow chart depicting the process for forming the pressure vessel from  FIG. 1 , in accordance with one embodiment of the present invention; 
           [0019]      FIG. 4  is a close up of the adhesive layer being applied to the liner, in accordance with one embodiment of the present invention; and 
           [0020]      FIG. 5  is a view of the adhesive layer being cured to the liner of the pressure vessel by a vacuum bag, in accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Referring now to the drawings, and initially to  FIG. 1 , it may be seen that the general reference numeral  30  identifies a pressure vessel embodying the present invention. Pressure vessel  10  (also referred to as vessel  10 ) has a main portion  11  that bounds an internal chamber or interior  12  of vessel  10 , and a stem or neck portion  13  that projects out of the main portion  11  along an axis and is hollow to define a passage  14  for establishing communication between the interior  12  of the vessel  10  and its exterior. 
         [0022]    As illustrated in Fig. I, the vessel  10  is of a multilayer or composite structure in that it includes an inner liner  15  and an outer jacket or overwrap  16  that surrounds the liner  15  and, more particularly, at least a main portion  11  of the liner that bounds interior  12 . Liner  15  used in the vessel  10  of the present invention is preferably very thin, such that its thickness is chosen to be just above the minimum needed to prevent permeation of the medium contained in interior  12  through liner  15  at the highest pressure differential expected to be encountered between interior  12  of vessel  10  and its exterior during the lifetime of vessel  10 , and at a level needed to prevent tearing of or other physical damage to liner  15  when exposed to the highest anticipated or intended internal pressure. 
         [0023]    Liner  15  is a thin metal element formed via seamless spinning, welding of a formed and machined part, electrodeposition or other such techniques. The diameter to thickness ratio of the liner may be 500/1 and above. The thickness of liner  15  generally is insufficient to enable liner  15  to withstand the expected internal pressures on its own or even to make more than a rather insignificant (less than 5%) contribution to the overall strength of vessel  10 . More importantly, without a high integrity coupling to a overlying composite  16  described below, the compressive stresses in the metal liner  15 , developed in reaction to the imposed bearing from the pre-stressed overlying composite at the zero pressure condition typically causes and elastic stability or buckling failure. At operating pressure the overwhelming majority of the load is borne by the outer jacket  16 . Both liner  15  and composite  16  are in a state of tension. 
         [0024]    As illustrated in  FIG. 2 , a close up of a single fiber or filamentary material  50  from overwrap layer  16  is shown. Overlay layer  16  is typically made from superimposed and overlapping layers of this impregnated filamentary material  50  that contains glass, graphite or Kevlar™ fibers  52  wrapped in different directions around liner  15 , with the interstices  54  between the fibers or filaments being filled by impregnating material such as hardenable epoxy resin that, upon setting or hardening, forms a matrix that firmly embeds such fibers or filamentary material. 
         [0025]    Turning now to the construction process of vessel  10 , a flow chart is shown in  FIG. 3 , illustrating a typical step by step process for applying overwrap  16  to liner  15 . It is understood that the following process is exemplary of the salient features of the process, but that certain steps may either be added, eliminated moved or otherwise altered, provided that the essential steps are all included. 
         [0026]    At a first step  100 , the metal liner  15  is subjected to a media blasting operation for surface roughening and/or mechanical surface activation. This step is to ensure that the adhesive discussed in detail below has improved surface area for maximum binding with the outer surface of liner  15 . Media blasting may take the form of glass bead, Al (Aluminum) Oxide or other conventional media as used to impart random roughening to a surface. 
         [0027]    Next, at step  102 , the media blasted liner  15  is treated with an acid wash for cleaning away all molecular/organic contaminants and surface resident blasting particles and to provide chemical activation of the surface. This is accomplished by different acid mixtures depending on liner material. Nitric acid is a common constitute and is widely used for aluminum liners. Fluoric or oxalic acid additions are made for nickel or titanium based alloys. The intent of media blasting, followed by acid washing is to provide a roughened surface free of detrimental oxide films and any molecular contamination that could ultimately contribute to reduced bond strength. The liner is then water rinsed to remove all acid residue. 
         [0028]    After cleaning, at step  104 , a water break test is performed to the dried liner. Distilled water is sprayed against liner  15 . If it is clean, the water should sheet off of the part. If not clean, the water tends to bead. If liner  15  does not pass the water test it may be returned to step  102  and washed again until clean. 
         [0029]    Assuming the part is clean, at step  106 , a film adhesive layer  20  is applied to liner  15 , as shown in close up illustration  FIG. 4 . Thus, rather than using a typical primer as done in the prior art, which provides inferior structural response properties to the adhesive, the present invention, binds the adhesive directly to the metallic outside of liner  15 . This adhesive layer  20  is applied to the outside of the liner  15  by the conventional method using pre-cut pattern shapes or gore panels, by gloved hands. 
         [0030]    At step  108 , peel ply cloth  21 , is applied to the uncured adhesive  20 . Removal of peel ply  21 , post cure, provides optimal surface roughness. A second purpose of peel ply  21  is to provide a direct contact breather element, allowing volatiles from the film adhesive  20  to de-gas during consolidation and cure without leaving bubble artifacts post cure as are often seen in vacuum bagged cured film adhesive. 
         [0031]    Next, at step  110 , a release film  22  is applied to the peel ply  21 , as well as a breaker cloth  24 , both shown in  FIG. 4 . The purpose of release film  22  is to allow parting or disassembly upon completion of cure. Breather cloth  24 , typically constructed of coarse weave fiberglass serves to provide a bearing distribution path from vacuum bag  30 , discussed below, onto release film  22 , further transmitting onto peel ply  21 , film adhesive  20 , and ultimately metal liner  15 , thus providing uniform bearing pressure during cure. 
         [0032]    After adhesive layer  20 , peel ply  21 , release film  22  and breaker cloths  24  are applied, at step  112 , a vacuum bag  30  is placed over the entire coated liner  15 . As with typical vacuum bag processes liner  15  is placed in vacuum bag  30 , which is then evacuated, thus by bag pressure, providing a pressurized cure environment for adhesive  20 . Vacuum bag  30  is evacuated through a fitting in the bag via a vacuum pump  32 . This vacuum bag process is most typically used to de-gas/consolidate, and subsequently cure hand layup pre-impregnated fibrous composites structures. De-gassing can also be referred to as de-bulking as it works to reduce the void fraction in the layed-up composite structure. 
         [0033]    The vacuum process may act as both a de-bulking process and a cure process. Typically, however heat is applied to cure the adhesive  20  to perform the curing operation. Positive pressure cure ovens (autoclaves), which will operate at 25 to 75 psi are commonly used. This allows release of the internal vacuum from bag  30  (venting the inside of the bag to atmospheric pressure), while maintaining a pressure differential across bag  30  by virtue of pressurization of the oven (autoclave). The venting of the inside of bag  30  is most often done to prevent rapid volatilization of impregnating resin or adhesive constituents, which will result in reduced composite properties due to high void fraction (porosity.) 
         [0034]    In the present invention, a typical heat the vacuum process may employ a ramping up and down of the curing temperature. On such example, would follow the following steps: 1) vacuum to 25 inches of Mercury or better; 2) hold for 10 minutes; 3) ramp up temperature to 250° F. at approximately 2 to 5° F./minute; 4) hold at 250° F. +/−1O° F. for 90 +/−15 minutes; ramp down temperature at approximately 2 to 5° F./minute. It is understood that this is just one example of temperatures used for curing, however other temperatures may be used as necessary for different adhesives  20 . 
         [0035]    In the present invention, the need to vent bag  30  for curing operations is eliminated by two factors. The first factor being that peel ply  21  placed in intimate contact with the thin layer of adhesive  20  serves the function of a secondary breather element. There is 100% surface contact to the thin underlying adhesive  20  to peel ply  21  giving adhesive volatiles a low resistance path for evacuation. Thus, large detrimental voids are not created. Secondly, the small voids in the thin cured adhesive film  20  will be open to the surface upon removal of peel ply  21 . The second layer of film adhesive included in impregnated filamentary material  50  upon application of the winding  16  flows into the open surface voids providing beneficial mechanical interlocking between adhesive layer  20  and filamentary material  50  of overwrap layer  16 . 
         [0036]    Once the vacuum step is complete, at step  114 , the cured adhesive layer  20  is, if required, treated by sanding or scuffing to remove any wrinkles, leaving the textured peel ply surface in the adhesive  20  as is achieved through the stripping (removal of the peel ply  21 . 
         [0037]    Next, at step  116 , once adhesive layer  20  is prepared and cured, overlap layer  16  is then applied to the coated liner  15 . As discussed above, overlapping layers of impregnated filamentary material  50  that contains glass, graphite or Kevlar™ fibers are wrapped in different directions around liner  15 , with the interstices  52  between the fibers or filaments being filled by impregnating material  54  such as hardenable epoxy resin (wet winding resin) that, upon setting or hardening, forms a matrix that firmly embeds such fibers or filamentary material  50 . The filament  50  is wound in a pre-programmed repeating closure pattern over the liner and adhesive layer  20  of liner  15 . This results in a thorough and complete binding between liner  15  and overwrap layer  16 . 
         [0038]    The above described process provides a distinct advantage over the prior art. First, consistent high strength bonds are readily achieved on epoxy compatible epoxy substrates than metal substrates. Roughened epoxy substrates, such as a peel ply surface, represent the ideal substrate. 
         [0039]    Thus, by using the vacuum process outlined above for adhesive layer  20 , a solid and complete bond between the metal liner  15  and adhesive  20  is formed. The pressure of the vacuum ensures this process significantly better than in prior art systems where the pressure to cause adhesion between the adhesive  20  and metal liner  15  was only a by-product result of the uneven and unpredictable binding pressure of the overwrap layer  16 . Bonding to the metal surface of liner  15  (substrate) is done at known, consistent and verifiable bearing pressure by the above vacuum process. 
         [0040]    Thereafter, the present invention allows for overwrap layer  16 , with its impregnated wet winding resin, to be applied to a cured adhesive layer  20 , having an excellent roughened binding surface as well as a fully connected and cured adhesion to the metal surface of liner  15 . 
         [0041]    Splitting the operation into two steps, vacuum and cure of adhesive later  20  and winding of overwrap layer  16 , dramatically enhances the adhesion of overwrap layer  16  to the metal substrate (liner  15 ). The vacuum bag de-bulking insures removal of entrapped air between metal liner  15  and adhesive  20 . A consistent bearing pressure as imposed by the vacuum bag  30  maximizes flow of adhesive  20  into the micromechanical valleys of the random roughness prepared metal substrate (liner  15 ). The consistent bearing pressure cannot be achieved in traditional processing. After cure the integrity of the adhesive to liner binding (cohesion) can be verified through both the mechanical stripping of the peel ply  21 , which provides an in-process peel test, to screen substandard bonding and through visual inspection upon removal of peel ply  21 . 
         [0042]    The overall adhesion of overwrap layer  16  to metal liner  15  is the fabrication critical element for vessel  10 . Far less bearing pressure is required to develop a high integrity bond to a compatible epoxy substrate than to a primed metal liner of the prior art. The direct and continuous processing through surface isolation of metal liner  15  by adhesive layer  20  is accomplished through the above process. The cured adhesive layer  20  serves as a protective primer, with superior performance to conventional brush on or spray on primers. 
         [0043]    The epoxy substrate (adhesive layer  20 ) from the above outlined vacuum process is rougher. At a micromechanical level there is open surface porosity, which allows a second layer of adhesive (epoxy impregnated into the filamentary material  50 ) to flow into and cure within the microvoids during step  116  when overwrap layer  16  is applied. This provides a mechanical interlocking to the substrate (adhesive layer  15 ) resulting in a high fidelity bond. 
         [0044]    Further steps may be taken to improve the overall quality of vessel  10 . For example, witness coupons for the critical bonding process can be prepared and tested and accurately reflect the peel performance to metal liner  15 . Post operation visual inspection is readily performed as well as removal and rework of substandard cure, readily achieved by nitric acid digestion. 
         [0045]    While only certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes or equivalents will now occur to those skilled in the art. It is therefore, to be understood that this application is intended to cover all such modifications and changes that fall within the true spirit of the invention.