Patent Publication Number: US-2021190266-A1

Title: Method of manufacturing high-pressure tank

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the priority based on Japanese Patent Application No. 2017-040168 filed on Mar. 3, 2017, the disclosure of which is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     Field 
     The technique disclosed herein relates to a high-pressure tank. 
     Related Art 
     A known high-pressure tank filled with a fluid such as hydrogen gas under high pressure includes a liner having a gas barrier property and a carbon fiber-reinforced resin layer (outer shell) formed on a surface of the liner (for example, refer to JP-A-1996-285189). 
     In such a high-pressure tank, cracking can occur in the outer shell due to, for example, a change in the volume in the high-pressure tank as a result of repeated filling and release of the fluid. A technique for suppressing occurrence of cracking in the outer shell of the high-pressure tank is hereinafter disclosed. 
     The technique disclosed herein has been developed in order to address at least part of the aforementioned problem, and can be achieved in the following aspects. 
     SUMMARY 
     In an aspect of the technique disclosed herein, there is provided a high-pressure tank. The high-pressure tank comprises: a liner; and a fiber-reinforced epoxy resin layer formed on an outer side of the liner. The fiber-reinforced epoxy resin layer may include an epoxy resin and fibers. The epoxy resin has a contact angle on polytetrafluoroethylene ((C 2 F 4 ) n ) of 70° or less in an uncured state. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a schematic configuration of a high-pressure tank according to one embodiment of the technique of the present disclosure. 
         FIG. 2  is a diagram illustrating components of epoxy resin included in a reinforcement layer. 
         FIG. 3  is a flowchart illustrating a method for manufacturing the high-pressure tank. 
         FIG. 4  is a diagram schematically illustrating the configuration of a reinforcement layer according to a second embodiment. 
         FIG. 5  is a diagram illustrating relationship between the contact angle of the epoxy resin and the durability of the high-pressure tank. 
     
    
    
     DETAILED DESCRIPTION 
     A1. First Embodiment 
       FIG. 1  is a cross-sectional view illustrating a schematic configuration of a high-pressure tank  100  according to one embodiment of the technique of the present disclosure.  FIG. 1  is a cross-sectional view taken along a line passing through the center axis of the high-pressure tank  100 . For example, the high-pressure tank  100  according to the present embodiment is filled with compressed hydrogen. For example, the high-pressure tank  100  is installed in a fuel cell vehicle and supplies hydrogen to a fuel cell. The high-pressure tank  100  may be installed in a vehicle other than fuel cell vehicles, such as an electric vehicle or a hybrid vehicle, and may be installed in other moving bodies such as a vessel, an aircraft, and a robot. The high-pressure tank  100  may be stationary equipment installed in a residence, a building, or the like. 
     The high-pressure tank  100  is a hollow container including: a cylindrical portion  102  having a substantially cylindrical shape; and dome portions  104  that have a semispherical shape, provided to both ends of the cylindrical portion  102 , and are integrally formed with the cylindrical portion  102 . In  FIG. 1 , the boundaries between the cylindrical portion  102  and the dome portions  104  are illustrated in broken lines. The high-pressure tank  100  has the center axis matching that of the cylindrical portion  102 . 
     The high-pressure tank  100  includes a liner  10 , a reinforcement layer  20 , a protection layer  25 , a mouthpiece  30 , and a mouthpiece  40 . The liner  10  to which the mouthpiece  30  and the mouthpiece  40  are attached is hereinafter also referred to as “tank main body”. 
     The liner  10  is made of nylon resin, and has a property (what is known as a gas barrier property) for sealing hydrogen or the like, filled in an internal space, so that the hydrogen or the like does not leak out. The liner  10  may also be made of another synthetic resin having the gas barrier property such as polyethylene-based resin, or may be made of metal such as aluminum or stainless steel. 
     The reinforcement layer  20  is formed to cover the outer surface of the tank main body. Specifically, the reinforcement layer  20  is formed to entirely cover the outer surface of the liner  10  and partially cover the mouthpieces  30  and  40 . The reinforcement layer  20  is made of Carbon Fiber-Reinforced Plastics (CFRP), which is a composite material of epoxy resin and carbon fibers, and has pressure resistance. A physical property of epoxy resin is described later. The reinforcement layer  20  according to the present embodiment is also referred to as “fiber-reinforced epoxy resin layer”. 
     The protection layer  25  is formed on the reinforcement layer  20 . The protection layer  25  is made of Glass Fiber-Reinforced Plastics (GFRP), which is a composite material of thermoset resin and glass fibers, and has higher impact resistance than the reinforcement layer  20 . In the present embodiment, the impact resistance is evaluated with the Charpy impact test (ISO 179-1). In the present embodiment, the epoxy resin that is same as that in the reinforcement layer  20  is used as the thermoset resin. The thermoset resin included in the protection layer  25  may be epoxy resin with a physical property different from that of the epoxy resin included in the reinforcement layer  20 , or may be another thermoset resin such as unsaturated polyester resin. The epoxy resin with a physical property different from that of the epoxy resin included in the reinforcement layer  20  is obtained by adjusting the type or amount of hardening agent and hardening accelerator and the molecular weight of the epoxy resin to be different from those of the epoxy resin included in the reinforcement layer  20 . The protection layer  25  according to the present embodiment is also referred to as “fiber-reinforced resin layer”. 
     The mouthpieces  30  and  40  are each attached to a corresponding one of two opening ends of the liner  10 . The mouthpiece  30  functions as an opening of the high-pressure tank  100 , and also functions as an attachment portion for attaching a pipe or a valve to the tank main body. The mouthpieces  30  and  40  also function as attachment portions for attaching the tank main body to a filament winding device (hereinafter, referred to as “FW device”), when the reinforcement layer  20  and the protection layer  25  are formed. 
       FIG. 2  is a diagram illustrating components of the epoxy resin included in the reinforcement layer  20 . The epoxy resin included in the reinforcement layer  20  includes 50 to 70% by weight of bisphenol A type epoxy resin serving as a main agent, 30 to 50% by weight of phthalic anhydride serving as a hardening agent, 1.0% by weight or less of amines serving as a hardening accelerator, and 0.2% by weight or less of a silicone-based surfactant serving as a surfactant. The epoxy resin included in the reinforcement layer  20  has such a composition that the main agent and the hardening agent fall within the range in  FIG. 2  and the total of these accounts for 100% by weight. 
     The epoxy resin included in the reinforcement layer  20  has a contact angle on a polytetrafluoroethylene ((C 2 F 4 ) n ) plate of 70° or less in an uncured state. In the following description, the contact angle on polytetrafluoroethylene ((C 2 F 4 ) n ) is also simply referred to as “contact angle”. The contact angle is measured by the following method. 
     &lt;Method for Measuring Contact Angle&gt; 
     Measuring device: Kyowa Interface Science Co., Ltd., CA-X150
 
Polytetrafluoroethylene ((C 2 F 4 ) n ) plate: The Nilaco Corporation, Teflon plate, model number 965653 (Teflon is a registered trademark)
 
Measuring method: Measuring the contact angle at seven points 10 seconds after resin droplets hit on the surface of the Teflon plate, and averaging the five points excluding the maximum and minimum values.
 
     The contact angle is an index of wettability, and a smaller contact angle indicates better wettability. Polytetrafluoroethylene ((C 2 F 4 ) n ), which has a water-repellent surface, enables stable measuring of the contact angle of the epoxy resin. For this reason, the contact angle on polytetrafluoroethylene ((C 2 F 4 ) n ) plate is used herein as an index of wettability. The epoxy resin included in the reinforcement layer  20  according to the present embodiment has a contact angle of 70° or less, which indicates good wettability. 
       FIG. 3  is a flowchart illustrating a method for manufacturing the high-pressure tank  100 . In the present embodiment, the high-pressure tank  100  ( FIG. 1 ) is formed by a filament winding method (FW method). In step S 12 , the liner  10  and resin-impregnated fibers (resin-impregnated carbon fibers and resin-impregnated glass fibers) are prepared. More specifically, the tank main body including the liner  10  to which the mouthpiece  30  and the mouthpiece  40  are attached is set to a FW device (not illustrated) as a mandrel. The resin-impregnated carbon fibers wound around a bobbin are set to predetermined positions of the FW device. The glass fibers wound around a bobbin are set to predetermined positions of the FW device. In the present embodiment, tow (bundle) carbon fibers and glass fibers are used. In addition, the above-described epoxy resin is used as the resin. As described above, the epoxy resin used in the present embodiment has good wettability in an uncured state and thus sufficiently penetrates between the fibers. 
     In step S 14 , resin-impregnated carbon fibers are wound around the outer surface of the tank main body. More specifically, as the FW device starts operating to cause the tank main body to rotate, the resin-impregnated carbon fibers are fed from the bobbin, and the resin-impregnated carbon fibers are wound around the outer surface of the tank main body. In this process, hoop winding, helical winding, and other types of winding are combined as appropriate, whereby the resin-impregnated carbon fibers are wound around the outer surface of the tank main body. Hereinafter, the tank main body with the resin-impregnated carbon fibers wound around its outer surface is also referred to as “carbon fiber-wound tank main body”. After the resin-impregnated carbon fibers are wound for a predetermined number of times to form a resin-impregnated carbon fiber layer, the resin-impregnated carbon fibers are cut and their winding finish end (terminal end) is bonded under pressure (e.g., thermal compression bonding) to a winding start end (start end) of resin-impregnated glass fibers. 
     In step S 16 , on the resin-impregnated carbon fiber layer of the carbon fiber-wound tank main body formed in step S 14 , the resin-impregnated glass fibers are wound in the same manner as in step S 14  to form a resin-impregnated glass fiber layer. 
     In step S 18 , the fiber-wound tank main body including the liner  10  the outer periphery of which is provided with the resin-impregnated carbon fiber layer and the resin-impregnated glass fiber layer formed through steps S 14  and  16  is placed in a furnace. The fiber-wound tank main body is heated, while being rotated, so that the epoxy resin in the resin-impregnated carbon fiber layer and the resin-impregnated glass fiber layer reaches its curing temperature (for example, about 160° C.). For example, the fiber-wound tank main body is heated at a set temperature of 180° C. of the furnace for 50 minutes, and then the fiber-wound tank main body is heated at a set temperature of 160° C. for 20 minutes. 
     As the epoxy resin is cured in step S 18 , the reinforcement layer  20  and the protection layer  25  are formed. Subsequently, the set temperature of the furnace is lowered, and the high-pressure tank  100  is taken out therefrom. In this manner, since the high-pressure tank  100  is formed by the FW method, the reinforcement layer  20  and the protection layer  25  each have a plurality of layers corresponding to the number of winding of the resin-impregnated carbon fibers and the resin-impregnated glass fibers. For example, the reinforcement layer  20  may include 30 layers, while the protection layer  25  may include 2 layers. 
     As described above, in the high-pressure tank  100  according to the present embodiment, the epoxy resin included in the reinforcement layer  20  has a contact angle of 70° or less and good wettability in an uncured state, and thus easily penetrates between tow carbon fibers when the fibers are impregnated with the resin. Therefore, the epoxy resin when cured fills the space between the fibers, whereby formation of voids (minute cavities) in the reinforcement layer  20  is suppressed. As a result, concentration of stress in the reinforcement layer  20  is alleviated, whereby occurrence of cracking in the reinforcement layer  20  because of a change in the volume as a result of the filling and release of hydrogen gas in the high-pressure tank  100  can be suppressed. 
     A2. Second Embodiment 
     A high-pressure tank according to a second embodiment includes a reinforcement layer  20 A, instead of the reinforcement layer  20  in the high-pressure tank  100  according to the first embodiment. The high-pressure tank according to the second embodiment have the configuration same as that of the high-pressure tank  100  according to the first embodiment, except for the reinforcement layer  20 A, and the same components are denoted with the same reference numerals and their descriptions will be omitted. 
       FIG. 4  is a diagram schematically illustrating the configuration of the reinforcement layer  20 A according to the second embodiment.  FIG. 4  is an enlarged view of a part of the configuration of the high-pressure tank  100  along a cut surface passing through the center axis of the high-pressure tank. The reinforcement layer  20 A includes a first reinforcement layer  21  and a second reinforcement layer  22 . The first reinforcement layer  21  is formed on the liner  10  (that is, in contact with the liner  10 ), and the second reinforcement layer  22  is formed on the first reinforcement layer  21  (that is, in contact with the first reinforcement layer  21 ). The protection layer  25  is formed on the second reinforcement layer  22  (that is, in contact with the second reinforcement layer  22 ). The epoxy resin included in the first reinforcement layer  21  is the same as the epoxy resin included in the reinforcement layer  20  according to the first embodiment and has a contact angle of 70° or less. The epoxy resin included in the second reinforcement layer  22  has a contact angle larger than 70°. The first reinforcement layer  21  in the present embodiment is also referred to as “fiber-reinforced epoxy resin layer”. 
     Like the reinforcement layer  20  according to the first embodiment, the reinforcement layer  20 A according to the present embodiment include a plurality of layers each including carbon fibers. For example, the first reinforcement layer  21  may include 5 layers, while the second reinforcement layer  22  may include 25 layers. 
     In the high-pressure tank, the innermost layer of the reinforcement layer  20 A including a plurality of layers bears stress due to the inner pressure, and thus cracking is likely to occur in the innermost layer (layer formed in contact with the liner  10 ) of the reinforcement layer  20 A first. Cracking formed in the innermost layer will develop toward the outer side. In the high-pressure tank according to the present embodiment, since the epoxy resin included in the first reinforcement layer  21  formed on the liner  10  has a contact angle of 70° or less, occurrence of cracking in the innermost layer of the outer shell can be suppressed. As a result, occurrence of cracking in the outer shell can be suppressed. In addition, selecting different types of resins for the first reinforcement layer  21  and the second reinforcement layer  22  in the reinforcement layer  20 A including the carbon fiber-reinforced epoxy resin can achieve the use of types of epoxy resin suited for required performance, for example, to make the first reinforcement layer  21  have a high cracking suppression effect and the second reinforcement layer  22  work well with the protection layer  25 . 
     A3. Experimental Results 
       FIG. 5  is a diagram illustrating relationship between the contact angle of the epoxy resin and the durability of the high-pressure tank. High-pressure tanks in Examples 1 to 3 and Comparative Example each have configurations similar to the configuration of the high-pressure tank  100  according to the first embodiment described above. However, more specifically, the high-pressure tanks in Examples 1 to 3 and Comparative Example differ from each other in the contact angle of epoxy resin included in the reinforcement layer  20  and the protection layer  25 . The contact angle of epoxy resin is 58° in Example 1, 68° in Example 2, 70° in Example 3, and 79° in Comparative Example. The epoxy resin included in the high-pressure tank in Comparative Example has a main agent that is a bisphenol A type epoxy resin with a molecular weight that differs from that of the main agent of the epoxy resin included in the high-pressure tanks in Examples 1 to 3 and has an olefin-based surfactant. The high-pressure tanks in Examples 1 to 3 and Comparative Example are adjusted such that they have a satisfactory fracture toughness value, tensile elastic modulus, and breaking elongation as a high-pressure tank. More specifically, they are adjusted to achieve a fracture toughness value K1C equal to or more than 1.6 [MPa·m 1/2 ], a tensile elastic modulus equal to or more than 1500 [MPa], and a breaking elongation equal to or more than 4.5[%]. 
     Examples 1 to 3 resulted in good durability, while Comparative Example resulted in poor durability. In  FIG. 5 , results with good durability are indicated by ◯, and results with poor durability are indicated by X. Evaluation of durability was made such that a room-temperature pressure cycle test was carried out 22,000 times after a pressure test, and no leakage of the fluid from the high-pressure tank led to good durability and leakage observed led to poor durability. From the high-pressure tank in Comparative Example, leakage was observed after the pressure cycle test was carried out 10,100 times and 15,400 times. The following lists details of the pressure test and the room-temperature pressure cycle test. 
     &lt;Pressure Test&gt; 
     Complying with the expansion measurement test (expansion volume not measured) in the KHKS0128(2010) set test, which is a technical standard for containers in 70-MPa compressed hydrogen automobile fuel devices. 
     Initial pressure: 3 MPa Retention time: 60 (+30/−0) sec 
     Pressure rise rate: 0.2 MPa/sec or less 
     Final pressure: 105 MPa Retention time: 30 (+15/−0) sec 
     &lt;Room-Temperature Pressure Cycle Test (Hydraulic Pressure)&gt; 
     Complying with the Global Technical Regulations No. 13 (GTR No. 13), 5.1.1.2. and 6.2.2.2. 
     Pressure medium: Tap water 
     Environmental temperature and tank surface temperature: Room temperature+/−5° C. 
     Cycle: 3 times/min or less (20 sec/time or more) 
     Pressure: Max 87.5 (+4/−0) MPa, Min  2  (+0/−2) MPa 
     Number of times the cycle test was repeated: 22,000 cycles 
     As can be apparent from the experimental results ( FIG. 5 ), if the epoxy resin has a contact angle of 70° or less, occurrence of cracking in the outer shell of the high-pressure tank was able to be suppressed satisfactorily. The epoxy resin with a contact angle of 70° or less has good wettability in an uncured state and thus easily penetrates between tow carbon fibers when the fibers are impregnated with the resin. Therefore, the epoxy resin when cured fills the space between the fibers, whereby formation of voids (minute cavities) in the reinforcement layer  20  is suppressed. As a result, concentration of stress in the reinforcement layer  20  is alleviated, whereby occurrence of cracking in the reinforcement layer  20  because of a change in the volume as a result of the filling and release of hydrogen gas in the high-pressure tank  100  can be suppressed. 
     B. Modifications 
     (1) The fluid in the high-pressure tank  100  is not limited to compressed hydrogen described above, as long as it is a high-pressure fluid such as compressed nitrogen. 
     (2) Examples of the fibers included in the reinforcement layers  20 ,  20 A and the protection layer  25  may include various types of fibers that can serve as fiber-reinforced resin, such as carbon fibers, glass fibers, aramid fibers, Dyneema fibers, Zylon fibers, and boron fibers. The types of fibers are preferably selected such that the reinforcement layers  20 ,  20 A can withstand high pressure and the protection layer  25  has higher impact resistance than the reinforcement layer  20 . Preferably, carbon fibers are used for the reinforcement layers  20 ,  20 A and glass fibers or aramid fibers are used for the protection layer  25 , so that the reinforcement layers  20 ,  20 A can withstand high pressure and the protection layer  25  has higher impact resistance than the reinforcement layers  20 ,  20 A. 
     (3) The protection layer  25  may be formed using only thermoset resin. In other words, the protection layer  25  may include no fibers. In this case, thermoset resin that has higher desired impact resistance than the reinforcement layer  20  is preferably selected for the protection layer  25 . To form the protection layer  25  using only thermoset resin, the thermoset resin is sprayed by a known method, such as spray coating, and then heated to form the protection layer  25 . For example, to form the protection layer  25  using only thermoset resin, carbon fibers impregnated with epoxy resin are wound around the liner  10 , the thermoset resin is sprayed by a known method, such as spray coating, and then heated to cure the epoxy resin and the thermoset resin, whereby the reinforcement layer  20  and the protection layer  25  are formed. 
     (4) The above-described embodiments illustrate configurations in which the reinforcement layer  20  and the protection layer  25  are disposed on the liner  10 , but this is not construed in a limiting sense. The outer side of the liner  10  is provided at least with a fiber-reinforced epoxy resin layer including epoxy resin with a contact angle of 70° or less. That is, in the above-described embodiments, the protection layer  25  may be omitted. Furthermore, an additional layer may be provided at least one of the following: between the liner  10  and the reinforcement layer  20 , between the reinforcement layer  20  and the protection layer  25 , and to the outer side of the protection layer  25 . 
     (5) The above-described embodiments illustrate examples in which the fiber-reinforced epoxy resin layer including the epoxy resin with a contact angle of 70° or less is formed in contact with the liner  10 ; however, the fiber-reinforced epoxy resin layer including the epoxy resin with a contact angle of 70° or less is not necessarily formed in contact with the liner  10  as long as it is formed on the outer side of the liner  10 . In other words, the innermost layer of the outer shell is not necessarily such a fiber-reinforced epoxy resin layer that includes epoxy resin with a contact angle of 70° or less. For example, a layer including epoxy resin with a contact angle larger than 70° may be formed between the fiber-reinforced epoxy resin layer including the epoxy resin with a contact angle of 70° or less and the liner  10 . Also in this configuration, the fiber-reinforced epoxy resin layer including the epoxy resin with a contact angle of 70° or less can suppress occurrence and development of cracking, and thus leakage of the fluid filled in the high-pressure tank can be prevented. 
     (6) The method for manufacturing the high-pressure tank  100  is not limited to the above-described embodiments. The heating temperature and heating time can be changed as appropriate depending on the type of resin used, the shape of the tank, and the like. The high-pressure tank may also be manufactured by, for example, a sheet winding method in which a sheet of fiber-reinforced resin is bonded, a Resin Transfer Molding (RTM) method in which a sheet of fibers is bonded and then impregnated with resin, and the like. 
     The technique disclosed herein is not limited to the embodiments or modifications described above but may be implemented by a diversity of other configurations without departing from the scope of the technique. For example, the technical features of any of the above embodiments, examples, and modifications corresponding to the technical features of each of the aspects described in Summary may be replaced or combined as appropriate, in order to solve part or all of the problems described above or in order to achieve part or all of the advantageous effects described above. Any of the technical features may be omitted as appropriate unless the technical feature is described as essential in the description hereof. 
     The disclosure is not limited to any of the embodiment and its modifications described above but may be implemented by a diversity of configurations without departing from the scope of the disclosure. For example, the technical features of any of the above embodiments and their modifications may be replaced or combined appropriately, in order to solve part or all of the problems described above or in order to achieve part or all of the advantageous effects described above. Any of the technical features may be omitted appropriately unless the technical feature is described as essential in the description hereof. The present disclosure may be implemented by aspects described below. 
     (1) In an aspect of the technique disclosed herein, there is provided a high-pressure tank. The high-pressure tank comprises: a liner; and a fiber-reinforced epoxy resin layer formed on an outer side of the liner. The fiber-reinforced epoxy resin layer may include an epoxy resin and fibers. The epoxy resin has a contact angle on polytetrafluoroethylene ((C 2 F 4 ) n ) of 70° or less in an uncured state. 
     In the high-pressure tank according to the present aspect, since the uncured epoxy resin has a contact angle on polytetrafluoroethylene ((C 2 F 4 ) n ) of 70° or less, the uncured epoxy resin has good wettability and easily penetrates between the fibers. Therefore, the epoxy resin when cured fills the space between the fibers, whereby formation of voids (minute cavities) in the fiber-reinforced epoxy resin layer is suppressed. As a result, concentration of stress in the fiber-reinforced epoxy resin layer is alleviated, whereby occurrence of cracking in the fiber-reinforced epoxy layer because of a change in the volume as a result of the filling and release of a fluid in the high-pressure tank can be suppressed. 
     (2) In the high-pressure tank according to the above-described aspect, the fiber-reinforced epoxy resin layer may be formed in contact with the liner. Among layers formed on the outer side of the liner (hereinafter, also referred to as “outer shell”), cracking is most likely to occur in the layer formed in contact with the liner (innermost layer of the outer shell). In the high-pressure tank according to the present embodiment, the fiber-reinforced epoxy resin layer, in which cracking is less likely to occur, is formed in contact with the liner, whereby occurrence of cracking in the outer shell can be further suppressed. 
     (3) In the high-pressure tank according to the above-described aspect, the fibers may be carbon fibers, and the high-pressure tank may further comprise a fiber-reinforced resin layer formed on an outer side of the fiber-reinforced epoxy resin layer. The fiber-reinforced resin layer may include; fibers having higher impact resistance than the carbon fibers; and a thermoset resin. This configuration enhances impact resistance and thus can achieve a high-pressure tank with higher strength. 
     (4) In the high-pressure tank according to the above-described aspect, the fibers having higher impact resistance than the carbon fibers may be glass fibers or aramid fibers. This configuration enables easy manufacturing of a high-pressure tank with higher strength. 
     The technique disclosed herein may be implemented by any of various aspects. For example, the technique may be implemented in such aspects as a fuel cell system including a high-pressure tank, a moving body equipped with the fuel cell system, a method for manufacturing a high-pressure tank, and the like.