Patent Publication Number: US-2023134272-A1

Title: Tank

Description:
TECHNICAL FIELD 
     This disclosure relates to a tank made of a fiber reinforced plastic (FRP). 
     BACKGROUND 
     An FRP is a composite material prepared by reinforcing a resin with reinforcing fibers. FRPs are widely used since they are capable of exerting strength and rigidity that are equivalent to or higher than those of metallic materials such as iron and aluminum while having less weights compared to the metallic materials. 
     FRPs can be formed into various shapes such as flat panel shapes, H shapes, and cylindrical shapes. In particular, tanks to be used for storing a high-pressure gas or a liquid are generally produced by filament winding molding. The filament winding molding method is a molding method in which a fiber bundle wound on a bobbin is continuously unwound while applying tension thereto, and the fiber bundle is impregnated with a thermosetting resin, followed by winding the fiber bundle on a liner to form a reinforcing layer. 
     In a tank prepared by the filament winding molding method, the presence of gaps between fiber bundles during molding may lead to decreased pressure resistance. Therefore, in general, the molding is carried out in a state where adjacent fiber bundles overlap with each other such that no gap is formed between fibers. However, in that method, irregularity occurs between portions where fiber bundles are overlapping and portions where fiber bundles are not overlapping, leading to a difference in the tension applied to the fiber bundles. This produces slack in fiber bundles to which lower tension is applied, resulting in disturbance of the alignment of the fiber bundles. As a result, the pressure resistance of the tank may decrease. 
     Therefore, for example, JP 2015-209887 A proposes a method in which the mode of overlapping of fiber bundles is improved to prevent occurrence of irregularity due to overlapped fiber bundles, wherein fiber bundles having different thicknesses in the width direction are provided and molding is carried out such that thinner portions of the fiber bundles overlap with each other. Further, JP 2012-140997 A proposes a method in which flat fiber bundles are deformed into a square shape to prevent formation of gaps between bands during molding. Further, JP 2017-7104 A proposes a method of detecting gaps between bands during filament winding molding. 
     However, under circumstances requiring a high production rate, it is difficult to wind fiber bundles as in JP 2015-209887 A such that their side surfaces are joined together to prevent formation of gaps, or to deform flat fiber bundles into a square shape in the process as in JP 2012-140997 A. Further, in a method of detecting gaps between fiber bundles during filament winding molding as described in JP 2017-7104 A, a detection apparatus is necessary so that the cost of the tank may increase. 
     It could therefore be helpful to provide an inexpensive tank for which the conventional operation of accurately arranging fiber bundles, an apparatus for detecting gaps between resin-impregnated fiber bundles and the like are not required. 
     SUMMARY 
     We thus provide: 
     A tank comprising:
 
a liner which is an inner shell; and
 
a reinforcing layer covering the outer surface of the liner;
 
wherein
 
the reinforcing layer is formed by continuously winding resin-impregnated fiber bundles around the liner,
 
the reinforcing layer comprises a hoop layer placed in the liner side, and a helical layer, gaps are formed between adjacent bundles of the resin-impregnated fiber bundles wound in the hoop layer,
 
there is at least one site where the resin-impregnated fiber bundles are wound without forming a gap between adjacent bundles in the helical layer, and
 
the resin constituting the tank has a resin toughness value of not less than 1.0 MPa·m 0.5 .
 
     The gaps are preferably formed only in the hoop layer in contact with the liner. 
     The ratio of the total of the areas exposed from the gaps formed between adjacent bundles of the resin-impregnated fiber bundles wound in the hoop layer, to the barrel surface area of the liner is preferably more than 0% and less than 50%. 
     The tank preferably satisfies: 
       0≤Lmin/W&lt;0.5 and 0.01&lt;Lmax/W&lt;0.5
 
     wherein Lmin represents the minimum value (mm) of the gaps; Lmax represents the maximum value (mm) of the gaps; and W represents the average width (mm) of the resin-impregnated fiber bundles adjacent to the gaps. 
     The Lmin and W of the tank preferably satisfy: 
       0.01&lt;Lmin/W&lt;0.5. 
     The weight ratio of the resin contained in the reinforcing layer is preferably 21% to 30%. 
     The resin contained in the resin-impregnated fiber bundles preferably has a resin viscosity of 10 to 150 Pa·s at 25° C. 
     We thus provide an inexpensive tank for which the conventional operation of accurately arranging resin-impregnated fiber bundles, an apparatus for detecting gaps between resin-impregnated fiber bundles and the like are not required. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram illustrating one example of a cross-sectional view of a tank. 
         FIG.  2 ( a )  is a schematic diagram illustrating one example of a hoop layer of a tank. 
         FIG.  2 ( b )  is a schematic diagram illustrating one example of a helical layer of a tank. 
         FIG.  3    is a schematic diagram illustrating one example of a laminated structure of a tank. 
         FIG.  4    is a schematic diagram illustrating one example of a tank comprising a hoop layer in contact with a liner, wherein fiber bundle gaps are formed in the hoop layer. 
         FIG.  5    is a schematic diagram illustrating one example of a tank comprising a hoop layer in contact with a liner, wherein the hoop layer has one or more sites where no fiber bundle gap is formed. 
         FIG.  6    is a schematic diagram illustrating one example of a production process of a tank. 
     
    
    
     DESCRIPTION OF SYMBOLS 
     
         
           101 : Tank 
           102 : Liner 
           103 : Reinforcing layer 
           201 : Hoop layer 
           202 : Circumferential direction 
           203 : Helical layer 
           204 : Tank axial direction 
           401 : Tangent line 
           402 : Gap between resin-impregnated fiber bundles 
           403 : Resin-impregnated fiber bundle 
           601 : Largest gap between resin-impregnated fiber bundles 
           602 : Liner axial direction 
           603 : Smallest gap between the resin-impregnated fiber bundles along the same axial direction as in  601   
           604 : Circumferential direction of the liner 
           605 : Overlap of resin-impregnated fiber bundles 
           701 : Molding flow 
           702 : Bobbin 
           703 : Fiber bundle 
           704 : Creel roller 
           705 : Resin impregnation roller 
           706 : Resin impregnation tank 
           707 : Guide roller 
           708 : Feed Eye 
           709 : Fixed shaft 
       
    
     DETAILED DESCRIPTION 
     Examples of our tanks are described below in order. The examples are merely representative, and this disclosure is not limited to the examples. 
     Our tank  101  comprises: 
     a liner  102  which is an inner shell; and
 
a reinforcing layer  103  covering the outer surface of the liner  102 ,
 
wherein
 
the reinforcing layer  103  is formed by continuously winding resin-impregnated fiber bundles  403  around the liner  102 ,
 
the reinforcing layer  103  comprises a hoop layer  201  placed in the liner  102  side, and a helical layer  203 ,
 
gaps  402  are formed between adjacent bundles of the resin-impregnated fiber bundles wound in the hoop layer  201 ,
 
there is at least one site where the resin-impregnated fiber bundles  403  are wound without forming a gap between adjacent bundles in the helical layer  203 , and
 
the resin constituting the tank  101  has a resin toughness value of not less than 1.0 MPa·m 0.5 .
 
       FIG.  1    illustrates a cross-sectional view of the tank  101 . The liner  102  comprises: a barrel part having a cylindrical shape; and dome parts having a domed shape provided at both ends of the barrel part, wherein each dome part is connected to an aperture. For the tank  101 , it is preferred to use a material having gas permeability resistance such as aluminum, a steel, or a resin to retain a gas filled in the tank. 
     The reinforcing layer  103  is formed by continuously winding resin-impregnated fiber bundles around the liner  102 , and capable of exerting pressure resistance of the tank  101 . 
     The reinforcing layer  103  comprises a hoop layer placed in the liner  102  side, and a helical layer. By combining necessary numbers of hoop layers and helical layers at necessary angles to achieve a necessary thickness, a necessary pressure resistance can be exerted. The hoop layer and the helical layer are now described. 
       FIG.  2 ( a )  is a schematic diagram illustrating one example of the hoop layer  201 . The hoop layer  201  is formed by winding and layering resin-impregnated fiber bundles  403  around the cylindrical barrel part of the liner  102  at an angle(s) of 80° to 110° with respect to the axial direction  204  of the liner. Resin-impregnated fiber bundles are wound from one end to the other end of the barrel part at, for example, the layering angle shown in  FIG.  2 ( a )  to cover the whole barrel part. This molding pattern is defined as hoop winding, and this unit is regarded as one layer. By repeating the hoop winding, a necessary hoop layer thickness is obtained. The pressure resistance of the tank in the circumferential direction  202  is dependent on the thickness of the hoop layer  201  deposited. 
       FIG.  2 ( b )  is a schematic diagram illustrating one example of the helical layer  203 . The helical layer  203  is formed by winding and layering resin-impregnated fiber bundles  403  at an angle(s) of more than 0° and less than 80°, or more than 110° and less than 180° with respect to the axial direction  204  of the liner. The molding pattern in which the dome parts and the barrel part are entirely covered at the layering angle is defined as helical winding, and this unit is regarded as one layer. By repeating the helical winding, a necessary helical layer thickness is obtained. The pressure resistance of the tank in the axial direction  204  is dependent on the thickness of the helical layer  203  deposited. 
     Thus, by combination of the hoop layer  201  and the helical layer  203 , the tank  101  illustrated in  FIG.  3    is prepared. 
     It is important to form gaps  402  between adjacent bundles of the resin-impregnated fiber bundles wound in the hoop layer  201 , and provide at least one site where the resin-impregnated fiber bundles  403  are wound without forming a gap between adjacent bundles in the helical layer  203 . 
     To secure safety upon fracture of the tank  101 , in some instances, the fracture origin is required to be in the barrel part as described in KHKS0121 (2016). To satisfy this requirement, the reinforcing layer  103  is generally designed to have a thickness at which the pressure generated in the reinforcing layer  103  during filling with gas is higher in the circumferential direction  202  of the tank than in the axial direction  204  of the tank such that the fracture origin is in the barrel part. 
     Therefore, when the resin-impregnated fiber bundles  403  in the hoop layer  201  are not uniformly wound, in other words, when the alignment is disturbed, strength expression of the resin-impregnated fiber bundles  403  may be insufficient so that the fracture expected by the design may not necessarily occur in the barrel part. In particular, when the resin-impregnated fiber bundles  403  wound around the liner  102  overlap with each other, a difference in the tension occurs between portions where the resin-impregnated fiber bundles  403  are overlapping and portions where the resin-impregnated fiber bundles  403  are not overlapping. This leads to looseness of the resin-impregnated fiber bundles  403  having low tension, resulting in disturbance of the alignment. Therefore, it is important to form gaps  402  between adjacent bundles of the resin-impregnated fiber bundles wound in the hoop layer  201 . Without such a configuration, the overlap between the resin-impregnated fiber bundles  403  cannot be eliminated so that the orderly fiber alignment cannot be achieved due to the difference in the tension of the resin-impregnated fiber bundles  403 . 
     Further, it is important to provide at least one site where the resin-impregnated fiber bundles  403  are wound without forming a gap between adjacent bundles in the helical layer  203 . Although the providing of the gaps in the helical layer  203  is useful to achieve the orderly fiber alignment, the presence of a large amount of gaps leads to formation of connected gaps as a large defect, and such a defect acts as an origin to decrease the pressure resistance of the tank  101 . During filling with gas, the pressure that acts on the helical layer  203  is lower than the pressure that acts on the hoop layer  201 . Therefore, disturbance of the fiber alignment of the helical layer  203  does not lead to fracture of the tank  101 . Thus, it is important to provide at least one site where the resin-impregnated fiber bundles are wound without forming a gap between adjacent bundles in the helical layer  203 . Without such a configuration, the formation of connected gaps in the whole reinforcing layer  103  cannot be prevented. 
     Further, it is important that the resin constituting the tank  101  has a resin toughness value of not less than 1.0 MPa·m 0.5 . The resin toughness value is preferably not less than 1.4 MPa·m 0.5 . When the resin toughness value is less than 1.0 MPa·m 0.5 , a gap between the resin-impregnated fiber bundles  403  acts as a fracture origin so that the pressure resistance of the tank  101  decreases. The upper limit of the resin toughness value is preferably 3.0 MPa·m 0.5 , more preferably 2.6 MPa·m 0.5 . When the upper limit of the resin toughness value is within the preferred range described above, propagation of cracks generated upon application of a pressure can be suppressed and, therefore, the pressure resistance required for the tank  101  can be easily secured. 
     Further, the gaps  402  between the resin-impregnated fiber bundles are preferably formed only in the hoop layer  201  in contact with the liner  102 . The pressure generated in the tank reinforcing layer  103  upon filling with gas is higher in the inner layer than in the outer layer, and the highest pressure is applied to the layer in contact with the liner  102 . Thus, the hoop layer  201  is preferably placed on the contact surface with the liner  102 , which contact surface acts as the fracture origin. In the outer side of the helical layer  203  placed on the innermost hoop layer  201  in the reinforcing layer, gaps between resin-impregnated fiber bundles  403  do not necessarily need to be formed in either a hoop layer  201  or a helical layer  203 . This is because, since the pressure applied to the reinforcing layer is lower in the outer-side layers than in the innermost layer, the effect of disturbance of the alignment of the resin-impregnated fiber bundles  403  on the pressure resistance is low. 
     The ratio of the total of the areas exposed from the gaps formed between adjacent bundles of the resin-impregnated fiber bundles  403  wound in the hoop layer  201  (“gap area between fiber bundles”), to the barrel surface area of the liner  102  as the inner shell is preferably more than 0% and less than 50%. The ratio is more preferably more than 0.01% and less than 45%. When the ratio is within the preferred range described above, gaps can be secured in the hoop layer  201  while formation of a large defect by the gaps in the hoop layer  201  is unlikely so that a decrease in the pressure resistance can be effectively prevented. 
     Methods of measuring the gap area between the fiber bundles and the barrel area of the liner  102  as the inner shell are described using  FIG.  4   . The liner  102  comprises: a barrel part having a cylindrical shape, positioned in the inner side of tangent lines  401  which are the border lines between the straight barrel part of the tank and curved surfaces; and dome parts having a domed shape, provided at both ends of the barrel part, wherein each dome part is connected to an aperture. The barrel area of the liner  102  is the area of the barrel part having a cylindrical shape positioned in the inner side of tangent lines  401 , and the area of the gaps  402  between the resin-impregnated fiber bundles is determined by subtracting the wound area of the resin-impregnated fiber bundles  403  from the barrel area. These areas can be measured by temporarily stopping the filament winding molding machine during the winding of the resin-impregnated fiber bundles  403  around the liner  102 . 
     After acquiring an appearance image of the liner  102 , the gap area between the fiber bundles and the barrel area of the liner as the inner shell are calculated using imaging software, a measure or the like. When the areas are calculated based on an image, the calculation may be carried out by an automatic program using an algorithm such as the color difference, or by visual trimming of the image. It is not necessary to use a single image covering the whole liner  102 , and a plurality of images may be used by connecting them. When the measurement of the gaps  402  between the resin-impregnated fiber bundles is impossible during the molding, the tank after curing may be decomposed to measure the widths of the resin-impregnated fiber bundles  403  and the gaps  402  between the resin-impregnated fiber bundles. 
     More specifically, the tank  101  is cut along the axial direction  204  of the tank to separate the liner  102  from the reinforcing layer  103 , and then an image of the reinforcing layer  103  is acquired from the liner  102  side. Thereafter, the gap area between the fiber bundles and the barrel area of the liner  102  as the inner shell are calculated. As long as the shape of the reinforcing layer  103  is retained, the separation of the reinforcing layer  103  may be carried out by applying a force between layers or blowing away the resin by application of heat. 
     Our tanks preferably satisfy: 
       0≤Lmin/W&lt;0.5 and 0.01&lt;Lmax/W&lt;0.5
 
     wherein Lmin represents the minimum value (mm) of the gaps  402  between the resin-impregnated fiber bundles formed in the reinforcing layer  103 ; Lmax represents the maximum value (mm) of the gaps  402  between those resin-impregnated fiber bundles; and W represents the average width (mm) of the resin-impregnated fiber bundles  403  adjacent to the gaps  402  between those resin-impregnated fiber bundles. When Lmin/W and Lmax/W satisfy the preferred lower limits described above, sites where the resin-impregnated fiber bundles  403  overlap with each other are less likely to be formed even by changes in the fiber bundle width due to looseness of the wound resin-impregnated fiber bundles  403  or by shifts of the positions of the resin-impregnated fiber bundles  403  due to bleed out of the resin during curing. Therefore, the gaps  402  between the resin-impregnated fiber bundles can be stably formed. When Lmin/W and Lmax/W satisfy the preferred upper limits described above, large defects are less likely to be formed by the gaps  402  between the resin-impregnated fiber bundles so that a decrease in the pressure resistance can be effectively prevented. The lower limit of Lmin/W is more preferably higher than 0.01, still more preferably higher than 0.05. The lower limit of Lmax/W is more preferably higher than 0.05, still more preferably higher than 0.06. The upper limit of Lmin/W is more preferably lower than 0.45, still more preferably lower than 0.4. The upper limit of Lmax/W is more preferably lower than 0.45, still more preferably lower than 0.4. 
     The methods of measuring the gaps  402  between the resin-impregnated fiber bundles and the widths of the resin-impregnated fiber bundles  403  are described using  FIG.  5   . The gaps  402  between the resin-impregnated fiber bundles and the widths of the resin-impregnated fiber bundles  403  can be measured by temporarily stopping the filament winding molding machine during the winding of the resin-impregnated fiber bundles  403  around the liner  102 . After visually finding the largest gap  601  between the resin-impregnated fiber bundles (“largest gap”), the gap is measured. The method of the measurement is not limited as long as the gap can be measured, and examples of the method include use of a caliper of a laser sensor. Using the found largest gap as the starting point, gaps  402  between the resin-impregnated fiber bundles are observed along the axial direction  204  of the liner. The width of the largest gap  601  described above is defined as Lmax, and the width of the smallest gap  603  between the resin-impregnated fiber bundles along the same axial direction  204  as in the largest gap  601  (“smallest gap”) is defined as Lmin. Subsequently, the widths of the resin-impregnated fiber bundles  403  adjacent to the largest gap  601  are measured (at two sites), and the widths of the resin-impregnated fiber bundles  403  adjacent to the smallest gap  603  are measured by the same method. The measured values of the widths at the four sites are averaged to provide the average width W of the resin-impregnated fiber bundles  403 . Thereafter, Lmin/W and Lmax/W are calculated using the Lmin, Lmax, and W obtained. When an overlap  605  is present between the resin-impregnated fiber bundles along the axial direction  204  of the liner, Lmin is defined as 0. When Lmin is 0, the average width W of the resin-impregnated fiber bundles  403  is the average of only the widths (at two sites) of the resin-impregnated fiber bundles  403  adjacent to the largest gap. Further, when no gap is present between resin-impregnated fiber bundles  403  and the largest gap  601  of the resin-impregnated fiber bundles, the average width W of the resin-impregnated fiber bundles  403  is defined as “none”. When the average width W of the resin-impregnated fiber bundles  403  is “none”, Lmin/W and Lmax/W at the measurement site are defined as no value. Subsequently, a point rotationally shifted at an angle of about 1° in the circumferential direction  202  of the liner from the gap used as the starting point of the measurement is used as a starting point from which Lmin/W and Lmax/W are measured along the axial direction  204  of the liner by the method described above. This measurement is repeated for the entire circumference in the circumferential direction  202  of the liner, and the minimum Lmin/W and the maximum Lmax/W are determined among all obtained measured values. When no gap is present between the resin-impregnated fiber bundles  403  at any of the measurement sites, both Lmin/W and Lmax/W are defined as 0. 
     Further, the weight ratio of the resin contained in the reinforcing layer  103  (=the weight of the matrix resin in the resin-impregnated fiber bundles  403 /the whole weight of the resin-impregnated fiber bundles  403 ) is preferably 21 to 30%. The weight ratio of the resin contained in the reinforcing layer  103  is more preferably 22.5% to 28.5%. When the weight ratio of the resin contained in the reinforcing layer  103  is within the preferred range described above, the amount of the resin in the resin-impregnated fiber bundles  403  is sufficient so that the gaps formed between the resin-impregnated fiber bundles  403  are unlikely to remain as voids, and a decrease in the pressure resistance can therefore be effectively prevented. On the other hand, in such configurations, the resin-impregnated fiber bundles  403  are unlikely to expand during winding of the fiber bundles around the liner so that the gaps can be easily stably formed. 
     The resin viscosity of the resin is preferably 10 to 150 Pa·s at 25° C. When the resin viscosity is within the preferred range described above, the resin-impregnated fiber bundles  403  are unlikely to expand during winding of the fiber bundles around the liner so that the gaps can be easily stably formed. On the other hand, since resin impregnation can be easily allowed to proceed, the gaps formed between the resin-impregnated fiber bundles  403  hardly remain as voids so that a decrease in the pressure resistance can be effectively prevented. 
     The fiber bundles and the resin are described below. 
     Examples of the fibers constituting the fiber bundles include glass fibers, carbon fibers, graphite fibers, aramid fibers, boron fibers, alumina fibers, and silicon carbide fibers. A mixture of two or more kinds of these reinforcing fibers may also be used. Preferably, to obtain a molding having higher strength, carbon fibers are used as the fiber bundles. 
     Any type of carbon fibers may be used depending on the use. From the viewpoint of obtaining a molding having higher strength, carbon fibers having a tension modulus of 3 to 8 GPa in a strand tension test according to the method described in JIS R 7601 (1986) are preferably used. 
     The resin is preferably a liquid-form resin. More specifically, from the viewpoint of obtaining the heat resistance and the environmental resistance required for the tank, the resin is preferably an epoxy resin composition containing an epoxy resin and a hardener. Further, when appropriate, a curing catalyst may be added for reducing the curing time. 
     One example of the filament winding molding machine is illustrated in  FIG.  6   .  FIG.  6    is a schematic diagram illustrating the entire configuration of one example of a molding flow in the method of producing the tank. 
     The molding flow  701  mainly illustrates that: 
     a creel roller  704  that plays a role in a sending process for sending a fiber bundle  703  from a bobbin  702 ;
 
a resin impregnation section that plays a role in a resin impregnation process for impregnating the fiber bundle  703  with a resin, the section comprising a resin impregnation roller  705 , a resin impregnation tank  706 , and a guide roller  707 ;
 
a feed eye  708  that plays a role in a winding process for winding the resin-impregnated fiber bundle  703 ;
 
a liner  102 ; and
 
a fixed shaft  709  that connects a molding machine to the liner  102 ,
 
are arranged in this order.
 
     In  FIG.  6   , only one bobbin  702  is shown. However, the molding flow  701  is not limited to such a configuration, and a plurality of bobbins  702  may be arranged. 
     As the bobbin  702 , a tow prepreg preliminarily impregnated with a resin in a separate process may be used. When a tow prepreg is used, the resin impregnation process can be omitted. 
     The tank produced can be preferably used not only for hydrogen gas vehicles and natural gas vehicles, but also for ships, aircraft and the like, stationary tanks immobilized on the ground, and air respirators to be used in hospitals or by firemen. Examples of substances that may be stored in this tank include gases such as nitrogen, oxygen, argon, liquefied petroleum gas, and hydrogen; and liquefied products of these substances. 
     EXAMPLES 
     Method of Evaluating Fracture Toughness Value of Resin Used for Tank 
     The uncured resin to be used for the tank was defoamed under vacuum, and was then cured for the lengths of time and at the temperatures described in Examples and Comparative Examples in a mold whose thickness was set to 6 mm using a spacer made of “TEFLON (registered trademark)” having a thickness of 6 mm to obtain resin hardened plates having a thickness of 6 mm. Each obtained resin hardened plate was processed into the test piece shape described in ASTM D5045-99, and then an SENB test was carried out according to ASTM D5045-99. The test was carried out for 16 samples (n=16), and the resulting average was employed as the fracture toughness value. 
     Method of Evaluating Resin Viscosity of Resin Used for Tank 
     The viscosity of the uncured resin to be used for the tank was measured according to the “method of measuring the viscosity with a cone-flat plate type rotational viscometer” in JIS Z8803 (2011) using an E-type viscometer (TVE-22HT, manufactured by Toki Sangyo Co., Ltd.) equipped with a standard cone rotor (1° 34′×R24), at 25° C. at a rotational speed of 5 rotations/minute. The viscosity was measured five times, and the resulting average was employed. 
     Method of Preparing Tow Prepreg 
     Using a tow prepreg production apparatus comprising a creel, a kiss-roll, a nip-roll, and a winder, one side of a carbon fiber “TORAYCA” (registered trademark) was coated with a resin composition whose temperature was adjusted to 25° C., and then the carbon fiber was passed through the nip-roll to allow impregnation of the inside of the fiber bundle with the resin composition, to thereby obtain a tow prepreg. Regarding the tow prepreg bobbin, 2300 m of the tow prepreg was wound around a paper core with an initial tension of 600 to 1000 gf at a winding ratio of 6 to 10 such that a cylindrical shape having a winding width of 230 to 260 mm was formed. 
     Methods of Measuring Gap Area between Fiber Bundles, and Barrel Area of Liner as Inner Shell 
     After stopping the filament winding molding machine during molding, photographs of the liner on which resin-impregnated fiber bundles are wound are taken using a camera (IXY650, manufactured by Canon). This operation is carried out each time when the liner is rotated at 90°, 180°, and 270° in the circumferential direction, to take photographs. An acquired image is read using image processing software Image J. While visually identifying resin-impregnated fiber bundle portions, the area is calculated using polygon and Measure in this software. Subsequently, by the same method, the barrel area of the liner is calculated from the image. Thereafter, the area of the resin-impregnated fiber bundles is subtracted from the barrel area of the liner to calculate the gap area between the fiber bundles. By carrying out the same process, the gap area between the fiber bundles and the barrel area of the liner are calculated for each of the remaining acquired images. Finally, by averaging the calculated areas, the gap area between the fiber bundles and the barrel area of the liner were determined. 
     Calculation of Minimum Value Lmin of Gaps Between Resin-Impregnated Fiber Bundles, Maximum Value Lmax of Those Gaps, and Average Width W of Resin-Impregnated Fiber Bundles Adjacent to Those Gaps 
     After stopping the filament winding molding machine during molding, the largest site is visually found, and the width of the site is measured using a caliper (Pocket Caliper 100 mm, manufactured by Shinwa Rules Co., Ltd.). Using this gap as a starting point, gaps between the resin-impregnated fiber bundles were observed along the axial direction of the liner. The maximum value (Lmax) of the gaps between the resin-impregnated fiber bundles and the minimum value (Lmin) of the gaps between the resin-impregnated fiber bundles were measured. Subsequently, the widths of the resin-impregnated fiber bundles adjacent to the Lmax are measured (at two sites), and the widths of the resin-impregnated fiber bundles adjacent to the Lmin are measured by the same method. The measured values of the widths at the four sites are averaged to provide the average width W of the resin-impregnated fiber bundles. However, when there is no minimum value (Lmin) of the gaps between the fiber bundles at the measurement site, the gap is defined as 0, and the average width of the resin-impregnated fiber bundles is defined as the average of only the widths (at two sites) of the resin-impregnated fiber bundles adjacent to the largest gap. Further, when no gap is present between resin-impregnated fiber bundles  403  and the largest gap  601  of the resin-impregnated fiber bundles, the average width of the resin-impregnated fiber bundles is defined as “none”. When the average width of the resin-impregnated fiber bundles is “none”, Lmin/W and Lmax/W at the measurement site are defined as no value, and not included in the measurement result. 
     Subsequently, a point rotationally shifted at an angle of about 1° in the circumferential direction of the liner from the gap used as the starting point is used as a starting point from which the gaps between the resin-impregnated fiber bundles are observed along the axial direction of the liner. The maximum value of the gaps between the resin-impregnated fiber bundles is defined as Lmax, and the minimum value of the gaps between the resin-impregnated fiber bundles is defined as Lmin. Thereafter, the width W of the resin-impregnated fiber bundles is measured by the method described above, and Lmin/W and Lmax/W are calculated. This measurement is repeated for the entire circumference of the liner, and the maximum Lmax/W and the minimum Lmin/W are calculated among the obtained results. When no gap is present between the resin-impregnated fiber bundles in the entire circumference of the measurement range, both Lmin/W and Lmax/W are defined as 0. 
     Method of Calculating Strength Utilization Rate 
     By the method described in KHKS0121 (2005), a water pressure was applied in the tank to perform a bursting test, to measure the pressure at the time of bursting. According to the equation of MIL-HDBK-17-3F Volume 3 of 5 17 Jun. 2002 5.3.5.3.1(e), the strength of the tank was calculated (theoretical strength), and the strength utilization rate of the tank was calculated according to the following equation. 
       Strength utilization rate (%)=(pressure at the time of bursting/theoretical strength)×100
 
     Our tanks are described below in more detail by way of Examples. 
     Example 1 
     A resin composition was obtained by stirring and mixing 88 parts by mass of “JER (registered trademark)” 828, 12 parts by mass of “DENACOL (registered trademark)” EX821, 4 parts by mass of “HYPRO (registered trademark)” 1300λ8 as a carboxy-modified acrylic rubber, 17 parts by mass of “KANACE (registered trademark)” MX125, 7.4 parts by mass of DICY7T as dicyanodiamide, and 2.7 parts by mass of DCMU99 as an aromatic urea compound. 
     The fracture toughness value of this resin composition was evaluated according to the “Method of Evaluating Fracture Toughness Value of Resin Used for Tank”. As a result, the fracture toughness value was found to be 2.1 MPa·m 0.5 . Using this resin composition and carbon fibers “TORAYCA (registered trademark)” T910SC-36K-50C, a tow prepreg having a resin content of 24% was prepared. The strand strength of the carbon fibers used was 6200 MPa. In the filament winding molding machine, a 7.5-L aluminum liner having an outer diameter of 160 mm was placed, and the tow prepreg was wound around the whole liner. As a first layer, a hoop layer was wound to a thickness of 0.79 mm at 89°/91° with respect to the axial direction of the liner. In this process, the tow prepreg was arranged such that adjacent tow prepregs did not overlap with each other to provide periodic gaps in the first layer. At this time, Lmin/W and Lmax/W were calculated according to the “Calculation of Minimum Value Lmin of Gaps between Resin-Impregnated Fiber Bundles, Maximum Value Lmax of Those Gaps, and Average Width W of Resin-Impregnated Fiber Bundles Adjacent to Those Gaps”. As a result, Lmin/W was 0.1, and Lmax/W was 0.44. As a second layer, a helical layer was wound to a thickness of 1.07 mm at 18°/162° with respect to the axial direction of the liner. Further, as a third layer, a hoop layer was wound to a thickness of 0.52 mm at 89°/91° with respect to the axial direction of the liner to obtain an intermediate. The intermediate was cured at 150° C. for 2 hours with rotation in a curing furnace to obtain a tank. 
     The strength utilization rate of the obtained tank was calculated according to the “Method of Calculating Strength Utilization Rate”. The strength utilization rate was 98.3%; the bursting pressure was 71.5 MPa; and the theoretical strength was 72.7 MPa. 
     Example 2 
     A resin composition was obtained by stirring and mixing 21 parts by mass of “JER (registered trademark)” 828, 67 parts by mass of “JER (registered trademark)” 806, 5 parts by mass of “EPOTOTO (registered trademark)” YDF-2001, 7 parts by mass of TETRAD-X as a xylene diamine type epoxy resin, 7 parts by mass of “KANACE (registered trademark)” MX-125, 5 parts by mass of DICY7T as dicyanodiamide, and 4 parts by mass of DCMU99 as an aromatic urea compound. 
     The fracture toughness value of this resin composition was evaluated according to the “Method of Evaluating Fracture Toughness Value of Resin Used for Tank”. As a result, the fracture toughness value was found to be 1.6 MPa·m 0.5 . Using this resin composition and carbon fibers “TORAYCA (registered trademark)” T720SC-36K-50C, a tow prepreg having a resin content of 24% was prepared. The strand strength of the carbon fibers used was 5800 MPa. In the filament winding molding machine, a 7.5-L aluminum liner having an outer diameter of 160 mm was placed, and the tow prepreg was wound around the whole liner. As a first layer, a hoop layer was wound to a thickness of 0.79 mm at 89°/91° with respect to the axial direction of the liner. At this time, Lmin/W and Lmax/W were calculated according to the “Calculation of Minimum Value Lmin of Gaps between Resin-Impregnated Fiber Bundles, Maximum Value Lmax of Those Gaps, and Average Width W of Resin-Impregnated Fiber Bundles Adjacent to Those Gaps”. As a result, Lmin/W was 0.19, and Lmax/W was 0.45. As a second layer, a helical layer was wound to a thickness of 1.07 mm at 18°/162° with respect to the axial direction of the liner. Further, as a third layer, a hoop layer was wound to a thickness of 0.52 mm at 89°/91° with respect to the axial direction of the liner to obtain an intermediate. The intermediate was cured at 110° C. for 10 hours with rotation in a curing furnace to obtain a tank. 
     The strength utilization rate of the obtained tank was calculated according to the “Method of Calculating Strength Utilization Rate”. The strength utilization rate was 108.5%; the bursting pressure was 73.8 MPa; and the theoretical strength was 68.0 MPa. 
     Example 3 
     A tank was prepared by the same method as in Example 1 except that 0.5-mm gaps were formed between the resin-impregnated fiber bundles of the hoop layer in the first layer during molding. In this process, width variation of the resin-impregnated fiber bundles led to five “sites where no gap was formed” in which the fiber bundles were overlapping in the hoop layer of the first layer. At this time, Lmin/W and Lmax/W were calculated according to the “Calculation of Minimum Value Lmin of Gaps between Resin-Impregnated Fiber Bundles, Maximum Value Lmax of Those Gaps, and Average Width W of Resin-Impregnated Fiber Bundles Adjacent to Those Gaps”. As a result, Lmin/W was 0, and Lmax/W was 0.1. 
     Subsequently, the strength utilization rate of the obtained tank was calculated according to the “Method of Calculating Strength Utilization Rate”. The strength utilization rate was 102.1%; the bursting pressure was 74.2 MPa; and the theoretical strength was 72.7 MPa. 
     Comparative Example 1 
     A tank was prepared by the same method as in Example 1 except that molding was carried out such that the resin-impregnated fiber bundles in the hoop layer of the first layer overlapped with each other by 2.5 mm each. At this time, since no gap was formed in the hoop layer of the first layer, both Lmin/W and Lmax/W were regarded as 0 according to the definition in the “Calculation of Minimum Value Lmin of Gaps between Resin-Impregnated Fiber Bundles, Maximum Value Lmax of Those Gaps, and Average Width W of Resin-Impregnated Fiber Bundles Adjacent to Those Gaps”. 
     The strength utilization rate of the obtained tank was calculated according to the “Method of Calculating Strength Utilization Rate”. The strength utilization rate was 90.6%; the bursting pressure was 65.9 MPa; and the theoretical strength was 72.7 MPa. Thus, the presence of overlapping resin-impregnated fiber bundles resulted in a decrease in the strength utilization rate. 
     Comparative Example 2 
     A resin composition was obtained by stirring and mixing 75 parts by mass of “JER (registered trademark)” 828, 40 parts by mass of GAN (manufactured by Nippon Kayaku Co., Ltd.), 8 parts by mass of DICY7T (manufactured by Mitsubishi Chemical Corporation) as a hardener, and 2 parts by mass of DCMU as a curing aid. 
     The fracture toughness value of this resin composition was evaluated according to the “Method of Evaluating Fracture Toughness Value of Resin Used for Tank”. As a result, the fracture toughness value was found to be 0.72 MPa·m 0.5 . Using this resin composition and carbon fibers “TORAYCA (registered trademark)” T720SC-36K-50C, a tow prepreg having a resin content of 24% was prepared. The strand strength of the carbon fibers used was 5800 MPa. In the filament winding molding machine, a 7.5-L aluminum liner having an outer diameter of 160 mm was placed, and the tow prepreg was wound around the whole liner. As a first layer, a hoop layer was wound to a thickness of 0.79 mm at 89°/91° with respect to the axial direction of the liner. At this time, Lmin/W and Lmax/W were calculated according to the “Calculation of Minimum Value Lmin of Gaps between Resin-Impregnated Fiber Bundles, Maximum Value Lmax of Those Gaps, and Average Width W of Resin-Impregnated Fiber Bundles Adjacent to Those Gaps”. As a result, Lmin/W was 0.09, and Lmax/W was 0.32. As a second layer, a helical layer was wound to a thickness of 1.07 mm at 18°/162° with respect to the axial direction of the liner. Further, as a third layer, a hoop layer was wound to a thickness of 0.52 mm at 89°/91° with respect to the axial direction of the liner to obtain an intermediate. The intermediate was cured at 110° C. for 10 hours with rotation in a curing furnace to obtain a tank. 
     The strength utilization rate of the obtained tank was calculated according to the “Method of Calculating Strength Utilization Rate”. The strength utilization rate was 79.0%; the bursting pressure was 53.7 MPa; and the theoretical strength was 68.0 MPa. These results indicate that, when gaps are formed between the resin-impregnated fiber bundles and the fracture toughness value is low, the strength utilization rate decreases.