Abstract:
A cryogenic fuel tank includes a composite tank wall enclosing a tank interior and having a tank wall surface, at least one coating provided on the tank wall surface, a foam insulation layer provided on the at least one coating and a plurality of stiffening fibers provided in one of the at least one coating and the foam insulation layer. A method of providing a thermal strain reducer coating on a composite structure is also disclosed.

Description:
TECHNICAL FIELD 
       [0001]    The disclosure relates to coatings for composite structures. More particularly, the disclosure relates to a composite cryogenic tank having a chopped fiber and polyurethane thermal strain reducer coating. 
       BACKGROUND 
       [0002]    In some applications, it may be necessary to provide a thermal strain reducing coating between a first structure and a second structure having different coefficients of thermal expansion (CTE) to reduce thermal strain between the structures. For example, in some applications composite cryogenics may require a thermal strain reducing coating between the composite Cryogenic Tank surface and the foam insulation layer. In some applications, it may be desirable for the thermal strain reducing coating to both act as a thermal strain reducer between the foam insulation layer and the composite cryogenic tank wall and enhance adhesion of the foam insulation layer to the polyurethane coating. 
       SUMMARY 
       [0003]    The disclosure is generally directed to a cryogenic fuel tank. An illustrative embodiment of the cryogenic fuel tank includes a composite tank wall enclosing a tank interior and having a tank wall surface, at least one coating provided on the tank wall surface, a foam insulation layer provided on at least one coating and a plurality of stiffening fibers provided in one of the at least one coating and the foam insulation layer. 
         [0004]    The disclosure is further generally directed to a method of providing a thermal strain reducer coating on a composite structure. An illustrative embodiment of the method includes providing a composite structure, providing at least one coating on the composite structure, providing a foam insulation layer on the at least one coating and providing a plurality of stiffening fibers in one of the at least one coating and the foam insulation layer. 
     
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
         [0005]      FIG. 1  is a cross-sectional view of an illustrative embodiment of the composite cryogenic tank. 
           [0006]      FIG. 2  is an enlarged sectional view, taken along section line  2  in  FIG. 1 , illustrating a fiber layer interposed between a foam insulation layer and a polymeric coating provided on a tank wall surface of the composite cryogenic tank. 
           [0007]      FIG. 3  is an enlarged sectional view, also taken along section line  2  in  FIG. 1 , of an alternative illustrative embodiment of the composite cryogenic tank, with a blended fiber/foam insulation layer provided on a polymeric coating on the tank wall surface of the composite cryogenic tank. 
           [0008]      FIG. 4  is an enlarged sectional view, taken along section line  2  in  FIG. 1 , of another alternative illustrative embodiment of the composite cryogenic tank, with a polymeric fiber layer interposed between a foam insulation layer and the tank wall surface of the composite cryogenic tank. 
           [0009]      FIG. 5  is a flow diagram illustrating an illustrative embodiment of a method of providing a fiber layer as a thermal strain reducer coating on a polymeric coating provided on a surface of a composite structure. 
           [0010]      FIG. 6  is a flow diagram illustrating an illustrative embodiment of a method of providing a foam insulation layer as a thermal strain reducer coating on a polymeric coating provided on a surface of a composite structure. 
           [0011]      FIG. 7  is a flow diagram illustrating an illustrative embodiment of a method of providing a polymeric fiber layer as a thermal strain reducer coating on a composite surface. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Referring initially to  FIGS. 1 and 2 , an illustrative embodiment of the composite cryogenic tank with thermal strain reducer coating, hereinafter cryogenic tank, is generally indicated by reference numeral  1  in  FIG. 1 . The cryogenic tank  1  may include a composite tank wall  2  which encloses a tank interior  3 . The tank interior  3  may be adapted to contain a liquefied gas  6  such as liquefied natural gas or liquid hydrogen, for example and without limitation. Conduits (not shown) may communicate with the tank interior  3  to facilitate placement of the liquefied gas  6  into and removal of the liquefied gas  6  from the tank interior  3 , as is known by those skilled in the art. 
         [0013]    As shown in  FIG. 2 , the tank wall  2  of the cryogenic tank  1  has a tank wall surface  2   a  which may be an exterior surface of the tank wall  2 . A polymeric coating  10 , which may be a polyurethane coating, for example and without limitation, may be robotically sprayed on the tank wall surface  2   a . A fiber layer  12  may be provided on the polymeric coating  10 . The fiber layer  12  may include multiple chopped stiffening fibers  13  which are embedded in a polymeric matrix. The stiffening fibers  13  may be high-modulus fibers including polyurethane fibers, nomex fibers, aramid fibers, glass fibers, graphite fibers, ceramic fibers or organic fibers such as KEVLAR, for example and without limitation. An insulation layer  14  may be provided on the fiber layer  12 . The insulation layer  14  may be a spray-on foam insulation (SOFI) layer, for example and without limitation. In some applications, the fiber layer  12  may be robotically sprayed onto the polymeric coating  10  and the foam insulation layer  14  may be robotically sprayed onto the fiber layer  12 . 
         [0014]    During use of the composite cryogenic tank  1 , the polymeric coating  10  and the fiber layer  12  may act in combination as a thermal strain reducer between the foam insulation layer  14  and the tank wall  2  under cryogenic conditions. The stiffening fibers  13  in the fiber layer  12  may mitigate and/or reduce the effects of the CTE (coefficient of thermal expansion) difference between the foam insulation layer  14  and the tank wall  2  under cryogenic conditions. This may prevent delamination of the foam insulation layer  14  from the tank wall  2 . Additionally, the polymeric coating  10  and the fiber layer  12  may enhance adhesion of the foam insulation layer  14  to the tank wall surface  2   a  of the tank wall  2 . Robotic methods of applying the fiber layer  12 , polymeric coating  10  and the foam insulation layer  14  may potentially eliminate the formation of air pockets in the layers. 
         [0015]    Referring next to  FIGS. 1 and 3 , in some embodiments a blended fiber/foam insulation layer  16  may be provided on the polymeric coating  10  such as by robotic spraying, for example. The blended fiber/foam insulation layer  16  may include stiffening fibers  13  embedded in an insulating foam matrix. The combination of the stiffening fibers  13  and the polymeric coating  10  may act as a thermal strain reducer between the blended fiber/foam insulation layer  16  and the tank wall  2  under cryogenic conditions and may enhance adhesion of the blended fiber/foam insulation layer  16  to the tank wall  2 . 
         [0016]    Referring next to  FIGS. 1 and 4 , in some embodiments a polymeric fiber layer  11  may be provided on the tank wall surface  2   a  of the tank wall  2 . The polymeric fiber layer  11  may include stiffening fibers  13  embedded in a polymeric matrix such as polyurethane, for example and without limitation. In some embodiments, the polymeric fiber layer  11  may be a polyurethane tiecoat. An insulation layer  14 , which may be a spray-on foam insulation (SOFI) layer, for example and without limitation, may be provided on the polymeric fiber layer  11 . The polymeric fiber layer  11  may act as a thermal strain reducer between the foam insulation layer  14  and the tank wall  2  under cryogenic conditions and may enhance adhesion of the foam insulation layer  14  to the tank wall  2 . 
         [0017]    In an exemplary method of application, the polymeric fiber layer  11  may be robotically applied to the tank wall surface  2   a  of the tank wall  2 . To improve the adhesion and/or further reduce the CTE mismatch tension between the foam insulation layer  14  and the tank wall  2 , chopped stiffening fibers  13  may be robotically sprayed onto the partially-cured or tacky polymeric fiber layer  11 . After curing of the polymeric fiber layer  11 , the foam insulation layer  14  may be sprayed onto the polymeric fiber layer  11 . 
         [0018]    Referring next to  FIG. 5 , a flow diagram  500  illustrating an illustrative embodiment of a method of providing a fiber layer as a thermal strain reducer coating on a polymeric coating provided on a surface of a composite structure is shown. In block  502 , a composite structure is provided. In block  504 , a first coating is applied to a surface of the composite structure. In block  506 , a second coating having a fiber mixture is applied to the first coating. In block  508 , a curable foam insulation layer is applied to the second coating. 
         [0019]    Referring next to  FIG. 6 , a flow diagram  600  illustrating an illustrative embodiment of a method of providing a foam insulation layer as a thermal strain reducer coating on a polymeric coating provided on a surface of a composite structure is shown. In block  602 , a composite structure is provided. In block  604 , a coating is applied to the surface of the composite structure. In block  606 , a blended layer having a mixture of curable foam insulation and fibers is applied to the coating. 
         [0020]    Referring next to  FIG. 7 , a flow diagram  700  illustrating an illustrative embodiment of a method of providing a polymeric fiber layer as a thermal strain reducer coating on a composite surface is shown. In block  702 , a composite structure is provided. In block  704 , a coating is applied to a surface of the composite structure. In block  706 , fibers are applied to the coating. In block  708 , a foam insulation layer is applied to the coating. 
         [0021]    Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.