Patent Publication Number: US-8524030-B2

Title: Fuel tank and method for reducing vapor permeation through a fuel tank

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a division of copending U.S. patent application Ser. No. 11/134,486, filed May 20, 2005. Applicants claim priority of application Ser. No. 11/134,486 and provisional patent application Ser. No. 60/576,977, filed Jun. 4, 2004. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to fuel tanks and, more particularly, to a method of reducing vapor permeation through a multi-layer fuel tank. 
     BACKGROUND OF THE INVENTION 
     Multi-layer polymeric fuel tanks are often used in the automotive industry because they are lighter in weight, have greater flexibility and are cheaper to manufacture than fuel tanks made of metal. Multi-layer polymeric fuel tanks typically include a tank wall composed of at least two layers of a high density polyethylene (HDPE) structural material and an ethylene vinyl alcohol copolymer (EVOH) hydrocarbon vapor barrier layer disposed between them. The fuel tanks may be manufactured by, for example, co-extruding the multiple layers into two sheets and vacuum forming each sheet to form two complementary sections or halves of the fuel tank, blow molding coextruded parisons into two shell sections of the fuel tank, blow-molding a coextruded cylindrical parison, or other fuel tank manufacturing processes known in the art. Component parts of a fuel system, such as a spout, a fuel pump, a vent valve, a weld cap and/or a fuel level sensor may be disposed in or on one or both sections of the tank, or may be disposed in or on one or more openings in the parison. The molded halves of the fuel tank or the openings in the parison may then be welded together and/or pinched closed. The welded junctures and the pinched regions form seams in the tank wall with a discontinuous EVOH barrier layer in those regions, thereby creating permeation windows through which fuel vapors pass more easily through the tank wall and to the atmosphere. 
     SUMMARY OF THE INVENTION 
     A method of reducing fuel and fuel vapor permeation through a multi-layer tank comprises providing a multi-layer overlay on a polymeric structural layer of the multi-layer tank and over at least a portion of a vapor permeable seam formed in the tank. The multi-layer overlay comprises at least one barrier layer composed of a vapor barrier material that is resistant to vapor permeation therethrough. The multi-layer overlay further comprises at least one structural layer composed of a polymeric structural material that is compatible with one or more of the polymeric structural layer(s) of the multi-layer tank. The multi-layer overlay may be secured to the polymeric structural layer of the multi-layer tank by applying pressure and heat thereto or by molding the overlay to the tank surface during a tank forming process. 
     One presently preferred embodiment of a multi-layer fuel tank includes a multi-layer wall comprising at least one polymeric structural layer, at least one seam formed in the multi-layer wall and a multi-layer overlay carried by the multi-layer wall and disposed over at least a portion of the seam. The multi-layer overlay comprises at least one barrier layer composed of a barrier material that is resistant to vapor permeation therethrough. The multi-layer overlay further comprises at least one structural layer composed of a polymeric structural material that is compatible with an adjacent polymeric structural layer of the multi-layer tank. 
     Objects, features and advantages of this invention include providing a fuel tank that has improved resistance to fuel and fuel vapor permeation therethrough, is rugged, durable, of relatively simple design and economical manufacture, and a method of reducing fuel and fuel vapor permeation through a fuel tank that is relatively simple, inexpensive and efficient for the manufacture and production of fuel tanks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of this invention will be apparent from the following detailed description of the preferred embodiments and best mode, appended claims and accompanying drawings in which: 
         FIG. 1  is a perspective view of a portion of an assembled multi-layer fuel tank showing a component weld seam, a pinch seam and a surface of the fuel tank body partially sealed by portions of a multi-layer overlay; 
         FIG. 2  is a fragmentary cross-sectional view taken generally along line  2 - 2  of  FIG. 1 ; 
         FIG. 3  is a fragmentary cross-sectional view taken generally along line  3 - 3  in  FIG. 1 ; 
         FIG. 4  is a somewhat schematic view of the multi-layer overlay being secured to the fuel tank using a single compression roller and a high temperature roller; 
         FIG. 5  is a somewhat schematic view of the multi-layer overlay being secured to the fuel tank using multiple compression rollers and high temperature rollers; 
         FIG. 6  is a somewhat schematic view of the multi-layer overlay being secured to the fuel tank using compression rollers and an induction coil; 
         FIG. 7  is a somewhat schematic view of the multi-layer overlay being secured to the fuel tank using a heated platen; 
         FIG. 8  is a somewhat schematic view of the multi-layer overlay being secured to the fuel tank using a hot drop manifold and an injection molding apparatus; 
         FIG. 9  is a perspective view of a mold for molding a fuel tank shell or half, where the mold includes a cavity in which a die-cut piece of a flat overlay is placed within for molding the overlay to the surface of the fuel tank shell body during the fuel tank molding or forming process; 
         FIG. 10  is a perspective view of the formed fuel tank shell or half having the multi-layer overlay of  FIG. 9  molded to a surface thereof; 
         FIG. 11  is a perspective view of an alternate embodiment multi-layer overlay die-cut to correspond or conform to a surface or surfaces of the fuel tank shell body during the fuel tank molding or forming process; 
         FIG. 12  is a perspective view of the multi-layer overlay of  FIG. 11  molded or formed onto the surface of the fuel tank shell; 
         FIG. 13  is a fragmentary cross-sectional view taken generally along line  13 - 13  in  FIG. 1  of a spout of the multi-layer polymeric fuel tank; 
         FIG. 14  is a fragmentary perspective view of a corner of the multi-layer fuel tank showing a portion of a convoluted pinch seam covered by a multi-layer overlay. 
     
    
    
     DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS 
     Referring in more detail to the drawings,  FIG. 1-3  illustrate a fuel tank assembly  10  comprising a fuel tank body  12  having a tank wall  26  with two weld seams  14 ,  16 , where vapor permeation through the tank wall  26  is reduced by covering at least a portion of the fuel tank wall and the weld seams  14 ,  16  with a nonpermeable multi-layer overlay  18 . The multi-layer overlay  18  includes a barrier material that prevents or substantially inhibits hydrocarbon fuel and fuel vapor permeation through the fuel tank wall, particularly through the seams  14 ,  16 . The multi-layer overlay  18  may be provided in the form of a tape, strip, sheet or any other suitable form, and can be cut or shaped into any desired size and/or shape for covering any hydrocarbon permeable seam or surface area formed in an area or region of the fuel tank assembly  10  or for covering any desired surface area of the tank wall. The overlay  18  may be secured to the tank body  12  by applying pressure and heat thereto or by molding the overlay  18  to the tank body  12  during the fuel tank molding or forming process. 
     As shown in  FIG. 1 , the overlay  18  may be cut into a strip for sealing the seam  14  which is a pinch seam fainted when two molded shells or halves  20 ,  22  of the fuel tank body  12  were welded together. Also shown in  FIG. 1 , the overlay may be die-cut into a disc  18 ′ for sealing the seam  16 , which is a circular component weld seam formed when a component part, such as a spout  24 , is welded to the tank body  12 . Also shown in  FIG. 1 , the overlay  18 ′ may be die-cut to cover a desired a surface area of the fuel tank wall to provide an additional barrier to vapor permeation through the tank wall. Of course, other vapor permeable areas may be present in a fuel tank, including by way of example and without limitation, pinch seams formed when blow molding a fuel tank from a hollow, cylindrical, multi-layer plastic parison. 
     The fuel tank body  12  comprises a tank wall  26  that preferably is made of three or more co-extruded layers. As shown in  FIG. 2 , the fuel tank wall  26  is composed of five layers including a hydrocarbon barrier layer  28  sandwiched between two structural layers; an outer-most structural layer  30  and an inner-most structural layer  32 . The barrier layer  28  has an outer surface  34  that is adhered to an inner surface  36  of the outer-most structural layer  30  by a first bonding layer  38  composed of an adhesive material. The barrier layer  28  further has an inner surface  40  that is adhered to an outer surface  42  of the inner-most structural layer  32  by a second bonding layer  44 , also composed of an adhesive material. Use of the five-layer construction of the tank wall  26  is merely for illustrative purposes in describing the method of the present invention, other multi-layer tank wall constructions and configurations may also be used in, by way of example, three, four or six-layer or other tank wall configurations. 
     The structural layers  30 ,  32  of the tank wall  26  are composed of a suitable polymeric material to provide structural integrity to the fuel tank assembly  10 . Typically, the tank wall of multi-layer fuel tanks includes at least one structural layer, but more commonly, comprises two or more structural layers. As shown in  FIG. 2 , the fuel tank wall  26  comprises two structural layers  30 ,  32 , one or both of which may be formed from treated recycled polymeric material consisting essentially of a mixture of waste and/or scrap material left over from the manufacture of previous fuel tanks. Such polymeric material is often referred to as re-grind material. One or both structural layers  30 ,  32  can also be formed from virgin high density polyethylene (HDPE). In some fuel tanks, an intermediate layer of re-grind or recycled material may be used, for example, between an adhesive layer and one of the inner or outer structural layers providing a six-layer fuel tank construction. 
     Although the HDPE-containing structural layers  30 ,  32  are structurally suitable for fuel tanks, they have very poor hydrocarbon permeation or vapor barrier properties. Thus, the barrier layer  28  is incorporated into the multi-layer structure of the tank wall  26  to prevent or greatly inhibit hydrocarbon vapors from permeating through the tank wall  26 . Since the preferred barrier material for the barrier layer  28  of the multi-layer fuel tank wall  26  is a relatively expensive ethylene vinyl alcohol copolymer (EVOH), a very thin layer of this material is incorporated between the structural layers  30 ,  32  and is adhered to the structural layers  30 ,  32  using an adhesive material that makes up the bonding layers  38 ,  44 . Suitable adhesives for use in fuel tank construction include modified polyethylene materials or other adhesives known in the art. 
     As previously mentioned in conjunction with  FIG. 1 , the fuel tank body  12  may be manufactured by welding together two pre-molded fuel tank shells or halves  20 ,  22 . The fuel tank halves  20 ,  22  are each separately manufactured by first co-extruding the multiple layers of the fuel tank wall  26 . Then each half  20 ,  22  is formed using a blow molding, a vacuum forming, or another suitable tank-forming method. Additional component parts to complete the fuel tank assembly  10 , such as the spout  24 , a fuel pump (not shown), a vent valve, a weld cap and/or a fuel level sensor (all of which are not shown in  FIG. 1 ), are then welded to or otherwise assembled or disposed in the appropriate areas either inside or outside the fuel tank body  12 . Then the two halves  20 ,  22  of the tank body  12  are connected together by joining peripheral edges of each half  20 ,  22  and thermal welding them together. One or more welding cycles of heat is preferably applied to the entire periphery of the joined edges of the halves  20 ,  22  to thereby form the seam  14 . 
     As shown in  FIG. 3 , the welded pinch seam  14  joins the outer-most structural layer  30 ′ of the tank wall  26 ′ in the first half  20  of the tank body  12  to the outer-most structural layer  30 ″ of the tank wall  26 ″ in the second half  22  of the tank body  12  to thereby form a single, continuous structural layer that becomes, as shown in  FIG. 1 , the outer-most structural layer  30  of the fuel tank wall  26 . In reference again to  FIG. 3 , the barrier layers  28 ′,  28 ″ each located underneath their respective structural layers  30 ′,  30 ″ of the two halves  20 ,  22  typically do not completely join together after welding the two halves  20 ,  22  together, thereby leaving a gap or permeation window  48  along at least a portion of the pinch seam  14 . The size of the permeation window  48  is exaggerated in  FIG. 3 , as are the thicknesses of the multiple layers of the tank wall  26  and the overlay  18  for descriptive purposes. Typically, the permeation window  48  is quite small, but large enough to permit hydrocarbon vapors to permeate through the pinch seam  14  at a significant rate. This is true, at least in part, for blow molded tanks formed from co-extrusion of a cylindrical parison. It should be noted, however, that for vacuum-formed tanks or tanks formed by joining two or more sections together, the permeation window  48  may be much larger because the region between the barrier layers is larger. A method, as will be described, is suitable for covering and/or sealing weld or pinch seams formed in the manufacture of fuel tanks, as well as covering and/or sealing other portions of the fuel tank, including without limitation, fuel tank areas having a gap in a barrier layer, or a thin barrier layer, or otherwise as desired. 
     One presently preferred embodiment of a method of controlling vapor permeation in a fuel tank includes applying a piece of the multi-layer overlay  18  to the fuel tank body  12  along and over a vapor permeable seam, such as the pinch seam  14  or the component weld seam  16 , as shown in  FIG. 1 , to thereby form a fuel and fuel vapor permeation seal. As shown in  FIG. 3 , a piece of the multi-layer overlay  18  may include five layers including a vapor barrier layer  50  sandwiched between two structural layers; an outer-most structural layer  52  and an inner-most structural layer  54 . This configuration is substantially similar to the five-layer arrangement of the fuel tank wall  26  shown in  FIG. 2 . As shown in  FIG. 3 , the barrier layer  50  includes an outer surface  56  that is adhered to an inner surface  58  of the outer-most structural layer  52  by a first bonding layer  60  composed of an adhesive material. The barrier layer  50  further includes an inner surface  62  that is adhered to an outer surface  64  of the inner-most structural layer  54  by a second bonding layer  66 , also composed of an adhesive material. 
     The barrier layer  50  of the multi-layer overlay  18  is preferably composed of EVOH or any other suitable hydrocarbon vapor barrier material. The structural layers  52 ,  54 , may be composed of any number of suitable structural materials known in the art. The inner-most structural layer  54  of the overlay  18  is preferably composed of a structural material that is compatible (e.g., weldable or bondable) with the outer-most structural layer  30  of the fuel tank wall  26 . Since the structural material of the outer-most structural layer of most multi-layer fuel tanks includes HDPE, which is chemically not adhesive to most, if not all, other materials, the inner-most structural layer  54  of the overlay  18  is preferably composed of HDPE as well. This promotes adhesion or bonding between the overlay  18  and the fuel tank body  12  in the presence of heat treatment or other adhesion promoting processes. The outer-most structural layer  52  of the overlay  18  may include any suitable structural material. As an example, in the overlay  18  provided in  FIG. 3 , the outer layer  52  is made of polyethylene including a foaming or blowing agent. This material composition provides the overlay  18  with some contact resistance to prevent tearing of the overlay  18  and further environmentally protects and structurally reinforces the seam  14  to which the overlay  18  is applied. 
     Other representative examples of available multi-layer overlays that can suitably be used in the method of the present invention include, without limitation, three or five-layer configurations. Not inclusively, the polymeric barrier layer  50  may be replaced by a metallic barrier strip or layer made of, for example, aluminum. A suitable multi-layer overlay according to one presently preferred implementation includes a polyester-polyethylene-aluminum-polyester-polyethylene five-layer overlay commercially available from Unipac Corp. Other suitable multi-layer overlays include a polyethylene-adhesive-EVOH-adhesive-polyethylene five-layer overlay, a polyethylene-adhesive-metal-adhesive-polyethylene five-layer overlay or other overlays of similar construction. Such multi-layer overlays are available as either a thin roll or as a wide sheet that can be cut into the desired size and shape. 
     The choice between a polymeric barrier layer and a metallic barrier layer can depend at least in part upon the method by which the overlay  18  will be adhered to the fuel tank body  12 . Overlays including a polymeric barrier layer may be adhered to the tank wall  26  by physical heating methods, such as hot rolling, whereas overlays including a metallic barrier layer may be adhered to the tank body  12  by electrical heating methods, such as induction. These methods will later be discussed in more detail. 
     Prior to sealing a weld or pinch seam formed in a multi-layer fuel tank body, the seam can be assessed for rough edges that may have been formed during a pinching or welding process. For example, a flash trim is often formed when forming a pinch seam between two halves of a fuel tank body or when pinching closed an end of a cylindrical extruded parison. The flash trim can be a relatively sharp protrusion of polymeric material that follows all or at least one or more sections of the periphery of the pinch seam. This trim flash is undesirable because it may tear or slice the overlay when applied thereto, or inhibit close attachment of the overlay to the tank, thereby hindering and weakening the vapor seal. 
     To remove the trim flash and smooth the pinch seam  14 , a pre-treatment step may be performed. One potential pre-treatment step includes applying a flame from, for example, a flame torch to the flash trim along the periphery of the pinch seam  14  until the flash trim is removed and the seam  14  is smooth. The flame may be applied continuously until the flash trim is removed or may be applied in cycles. Other non-smooth surfaces, for example, formed from component weld seams may also be treated with the flame. Although optional, the advantage of using the pre-treatment process, besides smoothing the surfaces of the weld seams for better vapor sealing effect, includes further ionizing the HDPE surface thereby making it slightly more adhesive. Thus, the HDPE inner-most structural layer  54  of the overlay  14  can be better attached to the fuel tank wall  26  for better adhesion of the overlay  18  thereto. 
     Application of pressure and heat to the overlay  18  to achieve a good vapor seal over a welded seam of a fuel tank, in accordance with one presently preferred aspect of the present invention, can be achieved by a variety of different processes, some of which are shown in  FIGS. 4-9 . It should be appreciated that these processes are not inclusive and other processes may be employed. The processes shown in  FIGS. 4-8  will be described in sealing the pinch seam  14  whereas the process shown in  FIG. 9  will be described in sealing an arbitrarily selected surface area of the fuel tank body  12 . Any one of the processes shown in  FIGS. 4-9 , however, may be used to seal any seam or any selected surface area on the fuel tank body  12 . 
     In applying the overlay to the fuel tank, the multi-layer overlay  18  is prepared by cutting a piece that will conform to the shape of the pinch seam  14  and is large enough to form an adequate hydrocarbon vapor seal. As shown in  FIG. 4-6 , the multi-layer overlay  18  may be applied to the tank wall  26  by press rolling the overlay  18  on the outer structural layer  30  of the tank wall  26  in the presence of a heating device. Generally, for all of the disclosed processes and apparatus, the amount of heat to be administered to the overlay  18  to form a suitable seal is about 200° C. to about 270° C. In reference to  FIG. 3  in conjunction with  FIGS. 4-6 , an adhesive bond is formed between the structural layer  54  of the overlay  18  and the structural layer  30  of the tank wall  26  and in all of the disclosed processes and apparatus is strengthened by applying a pressure of at least 5 psi to the heated area to urge the overlay into engagement with the structural layer. 
     As shown in  FIG. 4 , the overlay  18  preferably is adhered to the tank wall  26  by simultaneously applying heat and pressure to the top surface  73  of the overlay  18  along the length of the pinch seam (not shown) using a high temperature roller  68 . The high temperature roller  68  supplies enough heat that will travel through the multiple layers of the overlay  18  and effectively heat the inner-most structural layer (not shown) of the overlay  18  and the mating layer  30 ′,  30 ″ of the tank wall to a temperature in the range of about 200° C. to 270° C. A pressure roller  70  then follows the high temperature roller  68  to strengthen the adhesive bond between the overlay  18  and the outer structural layer  30  of the tank wall  26 . Alternatively, an adhesive bond may be formed between the overlay  18  and the tank wall  26  using two high temperature rollers  68 , where one roller heats the outer-most structural layer  30  of the tank wall  26  and the other roller heats the inner-most structural layer (not shown) of the overlay  18 , which is shown in  FIG. 5 , or the pre-treatment flame process as described above may supply enough heat to bond the overlay to the tank wall and only pressure need then be applied (not shown). For multi-layer overlays including a metallic barrier layer, heat may be applied using induction heating and thus the heating device may be an induction coil  72 , as generally shown in  FIG. 6 . 
     Alternative bonding methods may be used in adhering the overlay  18  to the tank wall  26  along a welded or pinched seam, examples of which are shown in  FIGS. 7-8 . As shown in  FIG. 7 , a pre-cut piece of overlay  18  may be placed over the welded seam and a heated platen  74 , which is a heated sheet of pre-formed geometry mimicking the shape of tank wall  26 , may then be placed thereover. The heated platen  74  may operate as both a heating device and a pressure device for properly sealing the overlay  18  along the weld seam  14 . 
     As shown in  FIG. 8 , a pre-formed hotdrop manifold or a hot tip runner  77  having a geometry mimicking the shape of the tank wall  26  may be placed over the overlay  18 , preferably leaving a space  76  of a couple millimeters between them. Relatively low pressure of about 50 psi from a plastic injection molding press  78  is applied to the overlay to initially bond the overlay  18  to the outer-most structural layer  30  of the overlay wall  26 . Then a molten plastic, preferably a polyethylene based resin having a high melt flow property, is injected into the space, which heats up the overlay and allows it to bond to the surface of the tank wall  26 . The plastic layer remains on the surface of the overlay  18 , thereby providing a protective barrier or shield over the overlay  18 . 
     The multi-layer overlay  18  may also be adhered to the surface of the fuel tank wall  26  by molding, such as insert molding the overlay  18  to the fuel tank during the fuel tank molding or forming process, where the overlay  18  will act as an additional barrier layer to vapor permeation through the tank wall  26 . As shown in  FIG. 9 , a sheet or piece  18 ′ of the multi-layer overlay is die-cut to a size that is just slightly smaller than the top surface of the fuel tank body  12 . The multi-layer overlay  18 ′ is placed within a cavity  90  of a mold  92  against a pre-determined desired surface, such as a bottom surface  94  of the mold cavity  90 , and may be held in place, for example, by vacuum (not shown) or by any other suitable means. As shown in  FIG. 9 , the overlay  18  is cut to a desired size so that the overlay  18  covers the desired surface area of the tank body  12 , as shown in  FIG. 10 , or in the cut-away portion of the overlay  18 ′ disposed on the top surface of the tank body  12 , as shown in  FIG. 1 . 
     Alternatively, as shown in  FIGS. 11 and 12 , an overlay  18 ″ may be concave or otherwise shaped to facilitate application of the overlay  18 ″ to the corresponding portion of the fuel tank. The overlay  18 ,  18 ′,  18 ″ may extend around or over one or more corners where the fuel tank barrier layer may be further stretched as the tank is formed and hence, may be thinner than adjacent areas. The overlay  18 ,  18 ′,  18 ″ which may include perforations to allow the release of air that may become trapped between the overlay and tank, may be molded to the tank body  12  during the vacuum forming, injection molding, blow molding, or other known fuel tank molding or fuel tank forming process. 
     The overlay  18 ,  18 ′,  18 ″ may be formed with detailed or contoured surface features to match the fuel tank shell and may further include holes or cut-outs (not shown) for welding other fuel tank components to the fuel tank, such as a filler pipe or other fuel module interface components. Also, the size and shape of the overlay  18 ,  18 ′,  18 ″ whether flat or cup-shaped, may be varied to cover a variable amount of surface area of the fuel tank shell. The molded overlay  18  permits the coverage of larger surface areas of the tank body  12  with the same type of permeation reduction as would be experienced with pinch or component weld seams, such as seams  14 ,  16  as shown in  FIG. 1 . 
     Component weld seams may also be sealed with a multi-layer overlay, as shown in  FIG. 13 . In reference to  FIG. 13 , the spout  24  comprises a neck  82  and a flange  84 , both of which are composed of polyethylene material that, in general, has low resistance to hydrocarbon vapor permeation. A component weld seam  16  exists between a bottom surface  86  of the flange  84  and outer-most structural layer  30  of the tank wall  26 . As better shown in  FIG. 1 , the multi-layer overlay  18  is die-cut into an annular disc having a hole  87  (shown in  FIG. 13 ) formed in its center that is sized to receive the neck portion  82  of the spout  24 . The disc comprises a body  88  that is radially large enough to encircle the flange  84  of the spout  24 . Using the same methods as described above, depending of course upon the barrier material used in the multi-layer overlay, the overlay  18  can be bonded or otherwise attached to the fuel tank to seal the component weld seam  16 . The overlay may also be used with other components mounted on or carried by the fuel tank, including, for example, vent valves and fuel pump modules which may have flanges that overlie and are coupled to the fuel tank. The overlay may also cover plugs or caps that close holes in the fuel tank, as well as anywhere else desirable, including on or over tubes, pipes or other fittings in a fuel system or associated with a fuel tank. 
     As previously mentioned, the multi-layer overlay  18  may be provided as a roll or as a sheet of material, as examples. For sealing straight-lined pinch seams, like pinch seam  14  shown in  FIG. 1 , the roll of overlay provides relatively thin or flat strips of overlay material that will suitably cover the pinch seam  14  and create a suitable hydrocarbon vapor seal. Sometimes, however, the fuel tank has a somewhat convoluted shape and thus also has a convolutedly-shaped pinch seam  80 , such as shown in  FIG. 14 . Application of the straight-edge multi-layer overlay provided in the roll to the convolutedly-shaped pinch seam  80  will pose some difficulties. In such cases, the overlay  14  may be obtained as a sheet rather than as a roll and the sheet may then be die-cut to the desired shape of the seam  80  for proper coverage of the area to be sealed. This is also true for sealing component weld seams, which are commonly circular in shape, such as the component weld seam  16  formed by welding the spout  24  to the tank body  12  shown in  FIGS. 1 and 13 . This also is true for sealing surface areas of the tank body  12  using either a relatively flat overlay  18 ′ as shown in  FIG. 9  or a shaped or contoured overlay  18 ″ as shown in  FIG. 11 . 
     Accordingly, permeation windows often formed in pinch or weld seams formed in multi-layer polymeric fuel tanks can be covered by a multi-layer overlay or substrate that spans any gaps in adjacent barrier layers in the area of the seams to prevent or reduce hydrocarbon vapor permeation through the fuel tank. Also, the fuel tank body  12  may be covered with a large piece of the overlay  18  to provide an additional barrier to vapor permeation through the tank wall  26 . The method may be performed with commercially available materials and is relatively inexpensive to carry out. 
     While the forms of the invention herein disclosed constitute presently preferred embodiments, many others are possible. It is not intended herein to mention all the possible equivalent forms or ramifications of the invention. For example, the term “overlay” is not intended to be limited to an overlay or sheet and furthermore is not intended to be limited to disposal on the exterior surfaces of the tank wall but may also be disposed on interior surfaces of the tank wall as well. It is understood that terms used herein are merely descriptive, rather than limiting, and that various changes may be made without departing from the spirit and scope of the invention as defined by the following claims.