JP7757802B2 - pressure vessel - Google Patents

Description

The present invention relates to a pressure vessel with excellent pressure resistance, which is formed from a resin liner for pressure vessels that can be directly blow molded and has excellent pinch-off adhesion, and an outer shell in which the outer surface of the resin liner is covered with a reinforcing layer made of a cured fiber-reinforced composite material.

Conventionally, pressure vessels (Figure 1) used for fuel tanks in automobiles and for storing and transporting natural gas and hydrogen gas have been constructed with a thermoplastic resin container body (liner) reinforced by an outer shell made of a fiber-reinforced resin material, due to their light weight and excellent durability (high toughness) under pressure. Glass fiber and carbon fiber are the main reinforcing fibers used for the outer shell. Carbon fiber, with its high specific strength, is used officially as a tank or pressure accumulator for transporting natural gas and hydrogen, as it allows for a design that improves the strength and rigidity of pressure vessels while reducing their weight.

A commonly used pressure vessel comprises, for example, a resin liner (Figure 2) with a cylindrical barrel and hemispherical domes at both ends of the barrel, and an outer shell formed around the liner body. The outer shell is made by winding a fiber-reinforced resin material, consisting of long reinforcing fiber bundles impregnated with a matrix resin, around the liner body using the filament winding method (hereinafter sometimes abbreviated as FW), and then heating and curing the material to form the pressure vessel. In recent years, particular attention has been focused on pressure vessel applications intended to fill fuels for natural gas and fuel cell vehicles. With the potential for large-scale market demand emerging in the future, manufacturers are exploring ways to reduce the cost of pressure vessels.

Currently, pressure vessel liners are mainly made using injection molding (half-split) followed by welding (laser or heat), as shown in Figure 3. However, this has led to issues such as an increase in liner quality control items and higher costs due to low productivity, and methods such as direct blow molding and rotational molding, which do not require post-processing, have been considered as cost-cutting methods.

Recently, many manufacturers have been considering using direct blow molding to produce liner prototypes. Direct blow molding generally involves pinching a portion of a cylindrical hot parison made from thermoplastic resin between a pair of molds, creating a pinch-off section (Figure 4) in the form of a stripe that extends in the axial direction of the liner. Because part of the hot parison is constrained by the mold in the pinch-off section, this creates a section with a different thickness from the rest of the parison, resulting in a thinner pinch-off section that extends in the axial direction, which can easily weaken the strength of the liner itself. Therefore, various manufacturers are actively investigating ways to resolve the issue of pinch-off occurring during direct blow molding.

For example, Patent Document 1 (WO 2018/207771) is known as a resin liner with a pinch-off shape that improves pressure resistance. Patent Document 1 discloses a pressure vessel using a resin liner that has a gentler valley shape in the pinch-off section that occurs during direct blow molding.

Patent Document 2 (WO 2013/172226) is also known as a blow-molded product made from a resin material with excellent pinch-off adhesion. Patent Document 2 discloses a blow-molded product made from a resin composition in which an olefin-based resin is blended with an EVOH resin, which has excellent blow-moldability.

Patent Document 3 (WO 2006/112252) is known as a method for manufacturing a pressure-resistant container liner using a liquid crystal polyamide resin with excellent pinch-off adhesion. Patent Document 3 discloses a pressure-resistant container liner that is direct blow molded using a liquid crystal polyamide resin, with a parison extrusion rate of 0.3 kg/min to 5 kg/min, a parison temperature set in the range of the melting point + 40°C, and a tensile elongation of 1% or more in the pinch-off area.

Patent Document 4 (JP Patent Publication No. 7-32460) is known as a blow-molded product that suppresses oxidative degradation of resin materials. Patent Document 4 discloses a blow-molded product obtained by sandwiching a parison between molds, pinching it off, and then blowing the parison with an inert gas.

International Publication No. 2018/207771 International Publication No. 2013/172226 WO 2006/112252 Japanese Patent Application Publication No. 7-32460

However, while the inventions described in the above Patent Documents 1 to 4 improve pinch-off adhesion during direct blow molding, the "hot parison extrusion rate, hot parison formation temperature," and "oxygen concentration of the inert gas blown into the hot parison," which are essential for improving pinch-off adhesion, are not sufficiently optimized, and therefore are not at a satisfactory level for practical use. Furthermore, there is no mention of a thermoplastic resin liner in which the pinch-off portion of the molded product of the present invention has a tensile strength of 25 MPa or more and a tensile elongation at break of 200% or more.

Therefore, the present invention aims to provide a thermoplastic resin liner that can be directly blow molded and has excellent pinch-off adhesion.

In order to solve the above problems, the pressure vessel and the method for manufacturing the pressure vessel according to the present invention have any of the following configurations:
A pressure vessel comprising a cylindrical straight body portion and dome portions provided at both ends of the straight body portion and tapering away from the straight body portion, the straight body portion and the dome portions being formed by a liner body made of a thermoplastic resin and an outer shell in which the outer surface of the liner body is covered with a reinforcing layer made of a cured product of a reinforced fiber composite material, wherein the liner body is manufactured by direct blow molding, and the pinch-off portion of the molded product has a tensile strength of 25 MPa or more and a tensile elongation at break of 200% or more, and the peak intensity ratio (peak intensity of the direct blow molded product/peak intensity of the resin material before direct blow molding) measured on the inner surface of the liner body by FT-IR at a measurement wavelength range of ^1,700 cm -1 ^to 1,750 cm -1 is 0.005 or less.

Thermoplastic resins used in the liner body of the present invention include, for example, polyamide resins, polyester resins, polyethylene resins, and fluororesins, and these may be used alone or in combination. In other words, it is preferable that the thermoplastic resin used in the liner body of the present invention is at least one resin selected from the group consisting of polyamide resins, polyester resins, polyethylene resins, and fluororesins.

It is preferable that the thickness of the pinch-off portion formed on the dome portion of the present invention is 0.90 to 1.05 times the thickness of the dome portion.

According to the present invention, by using a direct blow-molded thermoplastic resin liner with improved pinch-off adhesion, it is possible to obtain a pressure vessel with significantly improved pressure resistance and cost competitiveness.

FIG. 2 is a schematic diagram showing the component configuration of a pressure vessel. FIG. 2 is a schematic diagram showing the shape of a resin liner. 1 is a schematic diagram of an injection molded and welded resin liner. This is a molded product showing the pinch-off shape of a resin liner processed by direct blow molding. FIG. 1 is a schematic diagram showing a direct blow molding device. 1 is a graph showing the results of FT-IR analysis of the inner surface of a resin liner. This is a molded product showing the degree of pinch-off depression. FIG. 2 is a schematic diagram showing the shape of a resin liner directly blow-molded in the examples. 1 is a schematic diagram of an apparatus for carrying out a pressure test on a pressure vessel.

Embodiments of the present invention are described in detail below.

The pressure vessel of the present invention comprises a cylindrical straight barrel portion and dome portions provided at both ends of the straight barrel portion and tapering away from the straight barrel portion. The straight barrel portion and dome portions are formed from a liner body made of a thermoplastic resin and an outer shell in which the outer surface of the liner body is covered with a reinforcing layer made of a cured fiber-reinforced composite material.

<Thermoplastic resin>
There are no particular restrictions on the thermoplastic resin used in the liner body of the present invention, but polyamide resin, polyester resin, HDPE resin, and fluororesin, which have excellent blow molding processability, are preferred.

The polyamide resins used in the present invention include nylons synthesized using amino acids, lactams, or diamines and dicarboxylic acids as the main raw materials.

Typical examples of raw materials include amino acids such as 6-aminocaproic acid, 11-aminoundecanoic acid, 12-aminododecanoic acid, and para-aminomethylbenzoic acid, lactams such as ε-caprolactam and ω-laurolactam, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, 2-methylpentamethylenediamine, undecamethylenediamine, dodecamethylenediamine, 2,2,4-/2,4,4-trimethylhexamethylenediamine, 5-methylnonamethylenediamine, metaxylylenediamine, paraxylylenediamine, 1,3-bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)cyclohexane, and 1-amino-3-amino Examples include aliphatic, alicyclic, and aromatic diamines such as methyl-3,5,5-trimethylcyclohexane, bis(4-aminocyclohexyl)methane, bis(3-methyl-4-aminocyclohexyl)methane, 2,2-bis(4-aminocyclohexyl)propane, bis(aminopropyl)piperazine, and aminoethylpiperazine, as well as aliphatic, alicyclic, and aromatic dicarboxylic acids such as adipic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, terephthalic acid, isophthalic acid, 2-chloroterephthalic acid, 2-methylterephthalic acid, 5-methylisophthalic acid, 5-sodium sulfoisophthalic acid, hexahydroterephthalic acid, and hexahydroisophthalic acid.

Preferred polyamide resins include polycaproamide (nylon 6), polyhexamethylene adipamide (nylon 66), polytetramethylene adipamide (nylon 46), polypentamethylene adipamide (nylon 56), polyhexamethylene sebacamide (nylon 610), polyhexamethylene dodecamide (nylon 612), polyhexamethylene adipamide/polyhexamethylene terephthalamide copolymer (nylon 66/6T), polyhexamethylene adipamide/polyhexamethylene isophthalamide copolymer (nylon 66/6I), polyhexamethylene adipamide/polyhexamethylene terephthalamide/polyhexamethylene isophthalamide copolymer (nylon 66/6T/6I), polyxylylene adipamide (nylon XD6), and mixtures or copolymers thereof. Particularly preferred examples include nylon 6, nylon 66, nylon 610, nylon 6/66 copolymer, and nylon 6/12 copolymer.

The polyester resin used in the present invention can be obtained by a condensation reaction using dicarboxylic acid (or its ester-forming derivative) and diol (or its ester-forming derivative) and/or hydroxycarboxylic acid (or its ester-forming derivative) as the main raw materials.

Examples of the dicarboxylic acid component include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, bis(p-carboxyphenyl)methane, anthracenedicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 4,4'-biphenyldicarboxylic acid, and 5-sodiumsulfoisophthalic acid; aliphatic dicarboxylic acids such as adipic acid, sebacic acid, azelaic acid, and dodecanedioic acid; alicyclic dicarboxylic acids such as 1,3-cyclohexanedicarboxylic acid; and ester-forming derivatives of these.

Diol components include aliphatic glycols having 2 to 20 carbon atoms, such as ethylene glycol, propylene glycol, 1,4-butanediol, neopentyl glycol, 1,5-pentanediol, 1,6-hexanediol, decamethylene glycol, cyclohexanedimethanol, and cyclohexanediol; long-chain glycols having a molecular weight of 400 to 6,000, such as polyethylene glycol, poly-1,3-propylene glycol, and polytetramethylene glycol; and ester-forming derivatives of these. Examples of these polymers or copolymers include polybutylene terephthalate, polybutylene (terephthalate/isophthalate), polybutylene (terephthalate/adipate), polypropylene terephthalate, polypropylene (terephthalate/isophthalate), polyethylene terephthalate, polyethylene (terephthalate/isophthalate), polyethylene (terephthalate/adipate), bisphenol A (terephthalate/isophthalate), polybutylene naphthalate, polybutylene (terephthalate/isophthalate), polypropylene naphthalate, polyethylene naphthalate, polycyclohexanedimethylene terephthalate, polycyclohexanedimethylene (terephthalate/isophthalate), poly(cyclohexanedimethylene/ethylene) terephthalate, and poly(cyclohexanedimethylene/ethylene) (terephthalate/isophthalate).

In addition, thermoplastic polyester resins exhibiting thermotropic liquid crystallinity and consisting of structural units selected from aromatic oxycarbonyl units, aromatic dioxy units, aromatic dicarbonyl units, ethylenedioxy units, etc. can also be used.

The aromatic oxycarbonyl units referred to here include structural units generated from p-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid, and 4'-hydroxydiphenyl-4-carboxylic acid; the aromatic dioxy units include structural units generated from 4,4'-dihydroxybiphenyl, hydroquinone, and t-butylhydroquinone; the aromatic dicarbonyl units include structural units generated from terephthalic acid, isophthalic acid, and 2,6-naphthalenedicarboxylic acid; and the aromatic iminoxy units include, for example, structural units generated from 4-aminophenol. Specific examples include thermotropic liquid crystalline polyesters such as p-oxybenzoic acid/polyethylene terephthalate and p-oxybenzoic acid/6-oxy-2-naphthoic acid.

Preferred polyester resins are polyesters selected from polyalkylene terephthalates such as polyethylene terephthalate, polybutylene terephthalate, and polypropylene terephthalate; polyalkylene naphthalates such as polyethylene-2,6-naphthalate and polybutylene-2,6-naphthalate; polyesters exhibiting thermotropic liquid crystallinity; and polylactic acid; and more preferred are polyesters selected from polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyesters exhibiting thermotropic liquid crystallinity, and polylactic acid.

The polyethylene resins used in the present invention mainly include low-density polyethylene (hereinafter abbreviated as LDPE), high-density polyethylene (hereinafter abbreviated as HDPE), and linear low-density polyethylene. LDPE is produced by polymerizing ethylene under high pressure with a radical polymerization catalyst, has low crystallinity, and has a density of 0.92 g/ ^cm3 . HDPE is produced by polymerizing ethylene with a Ziegler catalyst, has high crystallinity, and has a density of 0.95 g/ ^cm3 . Linear low-density polyethylene is a type of low-density polyethylene, and because of its branched molecular structure, it has higher toughness and strength than LDPE. Since high heat resistance is preferable for the liner of the present invention, HDPE is preferably used.

The fluororesin used in the present invention is not particularly limited, but a preferred example is one into which a reactive functional group has been introduced. The reactive functional group is not particularly limited, but specific examples include vinyl groups, epoxy groups, carboxyl groups, acid anhydride groups, ester groups, aldehyde groups, carbonyldioxy groups, haloformyl groups, alkoxycarbonyl groups, amino groups, hydroxyl groups, styryl groups, methacryl groups, acrylic groups, ureido groups, mercapto groups, sulfide groups, isocyanate groups, and hydrolyzable silyl groups. Of these, epoxy groups, carboxyl groups, acid anhydride groups, amino groups, and hydroxyl groups are preferred, with carboxyl groups and acid anhydride groups being even more preferred. Two or more of these reactive functional groups may be included.

Methods for introducing reactive functional groups into fluororesin include blending a compound or resin compatible with the fluororesin and containing the functional group; copolymerizing the fluororesin with a polymerizable monomer containing the functional group or a functional group convertible to the functional group; polymerizing the fluororesin using an initiator containing the functional group or a functional group convertible to the functional group; reacting the fluororesin with a polymerizable monomer containing the functional group or a functional group convertible to the functional group in the presence of a radical generator; and modifying the fluororesin by oxidation, thermal decomposition, or other techniques. Of these, the methods of introducing functional groups into the main chain or side chain of the fluororesin by copolymerization and reacting the fluororesin with a polymerizable monomer containing a functional group in the presence of a radical generator are preferred from the standpoints of quality, cost, and control of the amount introduced.

The polymerizable monomer containing the functional group is not particularly limited, but examples include acrylic acid, methacrylic acid, maleic acid, itaconic acid, citraconic acid, crotonic acid, himic acid, their acid anhydrides, glycidyl acrylate, glycidyl methacrylate, glycidyl ethylacrylate, glycidyl itaconate, vinyl acetate, vinyl propionate, vinyltrimethoxysilane, vinyltriethoxysilane, and γ-methacryloxypropyltrimethoxysilane.

When the fluororesin containing reactive functional groups is alloyed with other resins, the amount of functional groups contained in the fluororesin containing reactive functional groups is preferably 0.01 mol% or more, more preferably 0.05 mol% or more, and even more preferably 0.1 mol% or more, per gram of the fluororesin containing reactive functional groups. The upper limit of the amount of functional groups is not particularly limited as long as the inherent properties of the fluororesin are not impaired, and taking into consideration deterioration of flowability, for example, a range of 10 mol% or less can be given as an example.

The structure of the fluororesin used in the present invention is not particularly limited, but it is desirable that it be composed of at least one fluoroolefin. Examples include homopolymers such as tetrafluoroethylene or chlorotrifluoroethylene, copolymers with hexafluoropropylene, perfluoro(alkyl vinyl ether), vinylidene fluoride, and vinyl fluoride, as well as copolymers with non-fluorine-containing ethylenic monomers such as ethylene, propylene, butene, and alkyl vinyl ethers. More specific examples include polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene-tetrafluoroethylene-hexafluoropyrene copolymer (EFEP), polyvinylidene fluoride (PVDF), and polychlorotrifluoroethylene (PCTFE). Of these, from the viewpoints of high heat resistance and ease of melt molding, ethylene-tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymer (PFA), and tetrafluoroethylene-hexafluoropropylene copolymer (FEP) are preferred, with ETFE being more preferred.

<Reinforced fiber composite material>
The reinforced fiber composite material of the reinforcing layer covering the outer surface of the liner body of the present invention is a cured product obtained by impregnating a reinforcing fiber bundle with a thermosetting resin and then heating and curing it.

The thermosetting resin used in the present invention is not particularly limited as long as it is liquid, but examples include epoxy resins, unsaturated polyester resins, phenolic resins, urea resins, and melamine resins. In particular, epoxy resins whose precursors are compounds such as phenols, amines, carboxylic acids, and intramolecularly unsaturated carbons are preferred due to their high adhesive strength.

Glycidyl ether epoxy resins that use phenols as precursors include bisphenol A epoxy resins, bisphenol F epoxy resins, bisphenol S epoxy resins, epoxy resins with a biphenyl skeleton, phenol novolac epoxy resins, cresol novolac epoxy resins, resorcinol epoxy resins, epoxy resins with a naphthalene skeleton, trisphenylmethane epoxy resins, phenol aralkyl epoxy resins, dicyclopentadiene epoxy resins, diphenylfluorene epoxy resins, and their various isomers, alkyl, and halogen-substituted derivatives. Also included in this type are compounds in which phenolic epoxy resins are modified with urethane or isocyanate.

Glycidylamine-type epoxy resins that use amines as precursors include tetraglycidyldiaminodiphenylmethane, glycidyl compounds of xylenediamine, triglycidylaminophenol, and their respective positional isomers and alkyl or halogen-substituted derivatives of glycidylaniline.

Epoxy resins that use carboxylic acids as precursors include glycidyl compounds of phthalic acid, and various isomers of glycidyl compounds of hexahydrophthalic acid and dimer acid.

Examples of epoxy resins that use intramolecularly unsaturated carbon as a precursor include alicyclic epoxy resins.

The curing agent used to heat-cure the thermosetting resin of the present invention is not particularly limited as long as it cures the thermosetting resin. It may be an addition-reaction curing agent such as an amine or an acid anhydride, or a curing catalyst that induces addition polymerization such as cationic polymerization or anionic polymerization. Two or more curing agents may be used in combination. Compounds containing an amino group, an acid anhydride group, or an azide group are preferred as curing agents. Examples include dicyandiamide, alicyclic amines, aliphatic amines, aromatic amines, aminobenzoic acid esters, various acid anhydrides, phenol novolac resins, cresol novolac resins, imidazole derivatives, phenolic compounds such as t-butylcatechol, and Lewis acid complexes such as boron trifluoride complexes and boron trichloride complexes.

The type of reinforcing fiber that constitutes the reinforcing fiber bundles used in the reinforced fiber composite material of the present invention is not particularly limited, and examples include carbon fiber, metal fiber, organic fiber, and inorganic fiber. Two or more types of these may be used.

Examples of carbon fibers include polyacrylonitrile (PAN)-based carbon fibers made from PAN fibers, pitch-based carbon fibers made from petroleum tar or petroleum pitch, cellulose-based carbon fibers made from viscose rayon or cellulose acetate, vapor-grown carbon fibers made from hydrocarbons, and graphitized fibers of these. Of these carbon fibers, PAN-based carbon fibers are preferred due to their excellent balance of strength and elastic modulus.

Examples of metal fibers include fibers made from metals such as iron, gold, silver, copper, aluminum, brass, and stainless steel.

Examples of organic fibers include fibers made from organic materials such as aramid, polybenzoxazole (PBO), polyphenylene sulfide, polyester, polyamide, and polyethylene. Examples of aramid fibers include para-aramid fibers, which have excellent strength and elastic modulus, and meta-aramid fibers, which have excellent flame retardancy and long-term heat resistance. Examples of para-aramid fibers include polyparaphenylene terephthalamide fibers and copolyparaphenylene-3,4'-oxydiphenylene terephthalamide fibers, while examples of meta-aramid fibers include polymetaphenylene isophthalamide fibers. Para-aramid fibers, which have a higher elastic modulus than meta-aramid fibers, are preferred.

Examples of inorganic fibers include fibers made from inorganic materials such as glass, basalt, silicon carbide, and silicon nitride. Examples of glass fibers include E-glass fiber (for electrical use), C-glass fiber (for corrosion resistance), S-glass fiber, and T-glass fiber (high strength, high elastic modulus). Basalt fiber is a fiber made from the mineral basalt, and is a fiber with extremely high heat resistance. Basalt generally contains 9 to 25 mass% of FeO or _FeO2 , which are iron compounds, and 1 to 6 mass% of TiO or _TiO2 , which are titanium compounds. However, it is also possible to increase the amounts of these components in the molten state and fiberize the material.

The fiber-reinforced resin substrate in the first and second embodiments of the present invention is often expected to function as a reinforcing material, so it is desirable for it to exhibit high mechanical properties. In order to exhibit high mechanical properties, it is preferable for the reinforcing fibers to contain carbon fibers.

In the fiber-reinforced resin substrates of the first and second embodiments of the present invention, the reinforcing fibers are typically composed of one or more reinforcing fiber bundles, each bundle consisting of a large number of single fibers. When one or more reinforcing fiber bundles are arranged, the total number of filaments (single fibers) in the reinforcing fiber is preferably 1,000 to 2,000,000.

From the viewpoint of productivity, the total number of filaments in the reinforcing fibers is more preferably 1,000 to 1,000,000, even more preferably 1,000 to 600,000, and particularly preferably 1,000 to 300,000. The upper limit on the total number of filaments in the reinforcing fibers should be set while taking into consideration the balance between dispersibility and handleability, as long as good productivity, dispersibility, and handleability are maintained.

The pinch-off portion formed on the dome portion of the liner body of the present invention is described below.

During direct blow molding, a portion of a cylindrical hot parison made of thermoplastic resin is sandwiched between a pair of molds, forming a pinch-off portion (Figure 4) in the dome portion, extending axially in the liner. Because part of the parison resin in the pinch-off portion is constrained by the mold, a portion with a different thickness from the other portions is created. This thins the axially extending pinch-off line portion, making the pinch-off portion prone to strength reduction. Therefore, to achieve both adhesion and strength in the pinch-off portion, the thickness of the pinch-off portion is preferably at least 0.90 times the dome portion thickness, with an upper limit of 1.05 times being preferred. If the pinch-off portion thickness is less than 0.90 times the dome portion thickness, cracks are more likely to propagate from the pinch-off portion during a pressure test. On the other hand, if the pinch-off portion thickness is more than 1.05 times the dome portion thickness, the pinch-off portion is too thick and does not adhere well when clamped in the mold.

If the pinch-off portion formed on the dome portion of the present invention is not firmly bonded, the pressure resistance of the pressure vessel will be significantly reduced, so the tensile strength at the pinch-off portion must be 25 MPa or more and the tensile breaking elongation must be 200% or more. Preferably, the tensile strength is 35 MPa or more and the tensile breaking elongation is 300% or more, and more preferably, the tensile strength is 50 MPa or more and the tensile breaking elongation is 400% or more.

The thermoplastic resin used in the liner body of the present invention does not contain reinforcing fibers, so the upper limit of tensile strength is essentially 50 MPa, and the upper limit of tensile breaking elongation is 400% of the measurement limit.

<Pressure vessel manufacturing method>
The method for manufacturing a pressure vessel of the present invention is a method for manufacturing a pressure vessel comprising a cylindrical straight body portion and dome portions provided at both ends of the straight body portion and tapering away from the straight body portion, the straight body portion and the dome portions being formed by a liner body made of thermoplastic resin and an outer shell having the outer surface of the liner body covered with a reinforcing layer made of a cured product of a reinforced fiber composite material, the liner body being formed by extruding a thermoplastic resin at an extrusion rate of 0.10 kg/sec to 0.50 kg/sec to form a hot parison, fixing the upper and lower ends of the hot parison in a mold, and molding the liner body by direct blow molding under molding conditions of at least 10°C above the melting point of the thermoplastic resin, and wrapping a reinforced fiber composite material around the liner body and curing it to form the outer shell.

<Method of manufacturing a resin liner for a pressure vessel>
Of these, direct blow molding (Figure 5) is an important method for manufacturing the liner body that constitutes the pressure vessel. Direct blow molding is a hot parison method in which air is directly blown into the molten extruded hot parison before it cools, and is also known as extrusion blow molding. Resin heated and molten in an extruder is extruded from a die head into a tubular shape (hot parison), the molten parison is sandwiched between molds, air is blown into the interior, and after cooling, the molds are opened and the molded product is removed. In the present invention, air is usually blown from above the mold, but it can also be blown from below or from the side of the mold.

In the direct blow molding process, a depression known as pinch-off (Figure 4) occurs during the process of clamping the hot parison between molds. Therefore, when pressure is applied from inside the molded product, cracks begin to propagate from the pinch-off, making it prone to fracture. Therefore, in this invention, to improve depressions at the pinch-off area, it is necessary to control the hot parison extrusion speed to within the range of 0.10 kg/sec to 0.5 kg/sec. A hot parison extrusion speed below the lower limit of 0.10 kg/sec is undesirable because it lengthens the molten residence time in the extruder, causing thermal degradation of the resin and a decrease in strength. A hot parison extrusion speed above the upper limit of 0.50 kg/sec is undesirable because the hot parison discharge speed is too fast and may cause resin burning due to shear forces.

In addition, in the present invention, to strengthen the adhesion of the pinch-off portion, the hot parison formation temperature must be at least 10°C above the melting point of the thermoplastic resin. However, if the hot parison formation temperature is higher than the melting point + 100°C, the resin viscosity will decrease due to the progress of thermal decomposition of the resin, causing the hot parison to draw down and become unmoldable, which is undesirable. In addition, in the present invention, to improve strength by improving oxidation degradation of the resin liner, it is preferable to use an inert gas (such as nitrogen, helium, or argon) with an oxygen concentration of 10% by volume or less in the hot parison blowing process during direct blow molding. Using air in the hot parison blowing process is undesirable, as it may cause oxidation degradation of the resin liner and make it brittle.

<Method of manufacturing outer shell>
The present invention relates to a method for manufacturing a pressure vessel comprising a cylindrical straight body portion and dome portions provided at both ends of the straight body portion and tapering away from the straight body portion, wherein the straight body portion and the dome portions are formed from a liner body made of a thermoplastic resin and an outer shell in which the outer surface of the liner body is covered with a reinforcing layer made of a cured fiber-reinforced composite material. The method for forming the outer shell in which the outer surface of the liner body is covered with a reinforcing layer made of a cured fiber-reinforced composite material (called a filament winding method) is described below.
This manufacturing method comprises the steps of: (a) preparing a molded intermediate formed with reinforcing layers composed of a plurality of reinforcing fiber composites by wrapping a reinforcing fiber composite impregnated with a liquid thermosetting resin composition around a liner; (b) maintaining the molded intermediate at room temperature to allow the thermosetting resin composition impregnated into the reinforcing fiber composite to flow; and (c) heating the molded intermediate after step (b) to obtain a cured product of the reinforcing fiber composite impregnated with the thermosetting resin. In the present invention, room temperature refers to a temperature in the range of 5°C to 35°C.

In process (a) of preparing a molded intermediate, the reinforced fiber composite is drawn out, impregnated with a thermosetting resin composition, and then wound onto a liner. The liner can be freely selected depending on the application of the filament winding molded product. For example, in the production of hollow pipe components, cylindrical liners that can be de-cored after the molded product has hardened, or liners of various shapes that can be de-cored by melting them with heat, etc., can be used. In the production of pressure vessels, metal or resin liners that ensure sealing of the specified contents can be used. From the perspective of moldability and the mechanical properties of the molded product, the method of winding the reinforced fiber composite impregnated with the thermosetting resin composition onto the liner preferably involves supplying the reinforced fiber composite from a head that can be freely moved relative to the liner, and positioning the reinforced fiber composite so as to meet the required performance of the filament winding molded product.

In step (b), the molded article intermediate is maintained at room temperature. By maintaining the molded article intermediate at room temperature, the thermosetting resin composition impregnated into the reinforced fiber composite maintains its fluidity for at least part of the process. When the thermosetting resin composition is in a flowable state, air bubbles that have entered the reinforcing layer of the reinforced fiber composite obtained in step (a) emerge at the surface of the reinforcing layer of the reinforced fiber composite due to factors such as buoyancy acting on the air bubbles and tight winding caused by tension remaining in the fibers when they are wound around the winding core. Therefore, by maintaining the molded article intermediate at room temperature, at least some of the air bubbles in the reinforcing layer of the reinforced fiber composite can be removed, reducing the amount of voids remaining in the filament-wound molded article. The molded article intermediate can be maintained while rotating around the liner of the molded article intermediate. This prevents the fluid thermosetting resin composition from dripping or falling off due to gravity. Falling off of the thermosetting resin composition increases the fiber volume content (Vf:%) of the filament winding molded product, which can degrade product performance. Furthermore, the fallen thermosetting resin composition is often discarded, resulting in a decrease in product yield. By rotating the intermediate molded product while it is being held, the effects of falling off of the thermosetting resin composition can be eliminated.

When the intermediate molded article is maintained, the maintenance temperature is room temperature, but preferably within the range of room temperature, and can be any temperature ±5°C determined depending on the type of thermosetting resin composition and the conditions of use. If this range of ±5°C exceeds room temperature, the excess is to be discarded. Depending on the type and conditions of use of the thermosetting resin composition, high temperatures can cause the thermosetting resin composition to gel too quickly, potentially preventing sufficient resin flow time. Furthermore, low temperatures can reduce the viscosity of the thermosetting resin composition, potentially requiring longer times to achieve the same degree of void reduction compared to high temperatures. Therefore, the maintenance temperature is not particularly limited as long as it is room temperature, but it is preferable for it to be in a range determined depending on the type of thermosetting resin composition used and the conditions of use.

In step (c) of obtaining a cured product of the reinforced fiber composite material impregnated with the thermosetting resin, the molded intermediate product after being held at room temperature is heated to thermally cure the thermosetting resin composition. The method for this is not limited, and any method, such as a heater or induction heating coil, can be used. During heating, the molded intermediate product can be held while rotating. By holding the molded intermediate product while rotating, it is possible to prevent the thermosetting resin composition from falling off.

The locations where step (a) of preparing a molded article intermediate, step (b) of maintaining the molded article intermediate at room temperature, and step (c) of obtaining a cured product of the reinforced fiber composite material impregnated with a thermosetting resin can be performed are not limited. That is, the molded article intermediate may be moved between steps (a) and (b), or the steps can be performed consecutively without moving. The molded article intermediate may also be moved between steps (b) and (c), or the steps can be performed consecutively without moving. Furthermore, if the molded article intermediate is moved between steps (b) and (c), the location where it is moved can be the same as the location where step (a) was performed.

The filament winding molded products obtained by the present invention can be widely used for aerospace, leisure, and general industrial applications, including pressure vessels, rolls, propeller shafts, flywheels, fishing rods, and golf club shafts. They are particularly suitable for applications such as pressure vessels where strength is required. Pressure vessels manufactured by the present invention are not limited to hydrogen gas vehicles and natural gas vehicles, but are also suitable for ships and aircraft, as well as stationary vessels fixed to the ground and air respirators used in hospitals and firefighters. Furthermore, substances stored in these pressure vessels may be gases such as nitrogen, oxygen, argon, liquefied petroleum gas, and hydrogen, or liquefied versions of these substances.

<Method for analyzing oxidation degradation of resin liners>
To improve the strength and toughness of the resin liner of the present invention during pinch-off adhesion, the peak intensity ratio (peak intensity of the direct blow-molded product/peak intensity of the resin material before direct blow molding) due to oxidative degradation of the material in the range of 1,700 cm ^-1 to 1,750 cm ^-1 during FT-IR measurement must be 0.005 or less, preferably 0.002 or less. Specifically, the FT-IR measurement method of the present invention involved scraping off approximately 1 g of the surface of the molded product at three locations, and measuring the FT-IR using an FT-IR device (TENSOR II manufactured by Bruker) using the ATR method (attenuated total reflection method: DLaTGS detector, 45° incident angle, Ge prism, 4 cm ^-1 resolution). The degree of oxidative degradation was quantitatively evaluated based on the magnitude of the absorption peak intensity in the wavelength range of 1,700 cm ^-1 to 1,750 cm ^-1 , which is easily observed due to thermal decomposition of the resin caused by oxidative degradation. Figure 6 shows the results of FT-IR analysis of resin liners obtained under blow molding conditions that caused oxidative degradation and those that suppressed oxidative degradation. As can be seen from this graph, there is a difference in the absorption peak intensity in the wavelength range of 1,700 cm ^-1 to 1,750 cm ^-1 , and it can be seen that this absorption peak is hardly observed under blow molding conditions that suppressed oxidative degradation.

<Tensile test at pinch-off part of resin liner>
The pinch-off portion of the dome portion of the direct blow molded resin liner (with a wall thickness of 3 mm) was cut into a strip of 15 mm width x 125 mm length, and a tensile test (n=5 for each) was carried out in accordance with ASTM D3039 to measure the tensile strength and tensile elongation at break.

The present invention will be explained in more detail below using examples, but the present invention is not limited to the descriptions in these examples. Physical properties in each example and comparative example were evaluated according to the following methods.

[Manufacturing method of resin liner]
Using a Tahara accumulator-type extrusion blow molding machine, the resin liner shape shown in Figure 8 (wall thickness: 3 mm ± 0.5 mm, thread portion: inner diameter φ13 mm, outer diameter φ24 mm) was obtained under the blow molding conditions of each example and comparative example.

<Extrusion blow molding machine/equipment specifications>
Extruder screw diameter: φ80 mm
・Extruder screw arrangement: Deep groove full flight specification ・Accumulator pump capacity: 4,000cc
Extruder screw rotation speed: 35 rpm
・Die shape: Die purge <Molding conditions>
・Hot parison length: 70 mm
・Hot parison mass: 2,000g
・Hot parison blowing gas: Air or inert gas (nitrogen gas is used).

[Method for measuring hot parison extrusion speed]
Under the blow molding conditions of each Example and Comparative Example, the time required for extrusion of the hot parison to a length of 70 mm (hot parison mass 2,000 g) was measured with a stopwatch (N=3), and the extrusion rate (kg/sec) was calculated.

[Method for measuring hot parison forming temperature]
The surface temperature of a hot parison (70 mm long) extruded under the blow molding conditions of each Example and Comparative Example was measured using a non-contact infrared thermograph at two locations: the extrusion side and the opposite side to the extrusion direction (hereinafter referred to as the counter-extrusion side).

[Method for measuring thickness of pinch-off part]
The resin liner obtained by the resin liner manufacturing method was cut into 3 cm x 3 cm pieces for the pinch-off portion and the dome portion, and the thickness of each was measured with a vernier caliper. The ratio of the pinch-off portion thickness to the dome portion thickness (pinch-off portion thickness/dome portion thickness) was then calculated.

[FT-IR measurement of resin liner (degree of oxidative degradation of resin material)]
Approximately 1 g of the inner surface of the resin liner obtained above was scraped off in three places, and the results were measured by the ATR method using an FT-IR device (TENSOR II, manufactured by Bruker). The degree of oxidative degradation was quantitatively evaluated based on the absorption peak intensity ratio (peak intensity of the resin liner molded product/peak intensity of the resin raw material before blow molding) in the wavelength range of 1,700 cm ^-1 to 1,750 cm ^-1 , which is easily observed when the resin is thermally decomposed due to oxidative degradation. The smaller this value, the less oxidative degradation there was in the resin liner.

<Measurement conditions>
Light source: Globar (SiC)
・Detector: DLaTGS
・Resolution: 4cm ^-1
Number of measurements: 128 times. Accessory equipment: Thunderdome, single-reflection ATR, incident angle 45°, Ge prism used.

[Resin liner/pinch-off part tensile test (pinch-off adhesion)]
The pinch-off portion of the dome portion of the resin liner obtained above was cut into strips of 15 mm width x 125 mm length from two locations, one on the parison extrusion direction side and one on the parison counter-extrusion direction side, and tensile strength and tensile elongation at break were measured in accordance with ASTM D3039 (n=5 for each). Note that the larger these values, the better the resin liner's pinch-off adhesion.

[Pressure vessel manufacturing method using the filament winding method and pressure resistance test (pressure resistance)]
The liner obtained above was placed in a filament winding molding machine, and a resin containing a liquid thermosetting resin composition (epoxy base:curing agent = 100:32 (mass ratio) homogeneously mixed at room temperature of 25°C) was impregnated into a single bundle of Toray Industries, Inc.'s carbon fiber "TORAYCA" (registered trademark) T700SC-24K. The fiber was wound over a 60 mm wide area at a winding angle of ±83° relative to the axial direction of the winding core, resulting in a 1 mm thick laminate, preparing a molded intermediate. After fiber winding, the intermediate was rotated at a speed of 7 rpm and held at 20°C for 15 minutes. At the start of the hold, the viscosity of the resin was 1,100 mPa s.

After this holding period, the molded intermediate was heated at 80°C for 2 hours and then at 110°C for 4 hours to harden the resin and obtain a pressure vessel for pressure testing. The resulting pressure vessel was then placed in the pressure vessel hydraulic burst test device shown in Figure 9, and water was pumped in and pressurized using a hydraulic pump. The burst pressure at the time the vessel burst was measured (N=3), and the pressure resistance was evaluated using the following criteria.

<Pressure resistance performance/criteria>
◯: Burst pressure 80 MPa or more △: Burst pressure greater than 30 MPa but less than 80 MPa ×: Burst pressure 30 MPa or less.

[Raw materials]
In the examples and comparative examples, the following raw materials were used.

Reference Example 1 Thermoplastic resin used in direct blow molding Nylon 6 resin: CM1056 (manufactured by Toray Industries, Inc., high viscosity, high impact blow molding grade, "Amilan" (registered trademark))
Polyethylene resin: HB-216R (Japan Polyethylene Co., Ltd., high-density polyethylene for large-scale hollow blow molding, "Novatec" (registered trademark))
Fluorine resin: P-66P (manufactured by AGC, tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymer for blow molding, "Fluon" (registered trademark))
Reference Example 2 Liquid Thermosetting Resin Composition Epoxy Base: Bisphenol A-type liquid epoxy resin ("jER" (registered trademark) 828 (manufactured by Mitsubishi Chemical Corporation))
Curing agent: a mixture of poly(propylene glycol) diamine, isophorone diamine, cyclohexylamine, and polypropylene glycol ("ARADUR" (registered trademark) 3486 (manufactured by Huntsman Japan Co., Ltd.))

As described above, by comparing the Examples and Comparative Examples, it can be seen that the resin liner and pressure vessel of the present invention have improved pinch-off depressions and strong adhesion at the pinch-off portion, as shown in Figure 7, thereby achieving a dramatic improvement in pinch-off adhesion and pressure resistance.

101 Pressure vessel 102 Reinforced fiber composite material 103 Liner container 104 Mouthpiece portion 201 Cylindrical straight body portion 202 Hemispherical dome portions provided on both ends of the straight body portion 301 Injection molded liner (half part)
302 Injection molded liner (half parts welded together)
401 Direct blow molding liner 402 Pinch-off depression area 501 Single screw extruder 502 Direct blow molding mold 503 Hot parison 701 Direct blow molding liner with large pinch-off depression 702 Direct blow molding liner with small pinch-off depression

Claims (3)

a pressure vessel comprising a cylindrical straight body portion and dome portions provided at both ends of the straight body portion and tapering away from the straight body portion, the straight body portion and the dome portions being formed by a liner body made of a thermoplastic resin and an outer shell in which the outer surface of the liner body is covered with a reinforcing layer made of a cured product of a reinforced fiber composite material, the liner body being manufactured by direct blow molding, the pinch-off portion formed in the dome portion having a tensile strength of 25 MPa or ^more and a tensile elongation at break of 200 ^% or more , and the peak intensity ratio (peak intensity of the direct blow molded product/peak intensity of the resin material before direct blow molding) measured on the inner surface of the liner body by FT-IR in the measurement wavelength range of 1,700 cm -1 to 1,750 cm -1 is 0.005 or less . The pressure vessel described in claim 1, characterized in that the thermoplastic resin used for the liner body is at least one resin selected from the group consisting of polyamide resin, polyester resin, polyethylene resin, and fluororesin. 3. The pressure vessel according to claim 1, wherein the thickness of the pinch-off portion formed on the dome portion is 0.90 to 1.05 times the thickness of the dome portion .