Rubber composition and method for producing rubber composition

Abstract

Problems addressed by this invention include providing a rubber composition having low haze, and a method for producing such a rubber composition. This rubber composition contains a crosslinked product of a fluorine-containing copolymer. In a dynamic viscoelasticity measurement at a measurement temperature of 23-200°C and a measurement frequency of 1 Hz, a temperature T indicating the minimum value of the storage elastic modulus E' is 55°C or higher.

Description

Rubber composition and method for producing the rubber composition

[0001] The present invention relates to a rubber composition and a method for producing the rubber composition.

[0002] Rubber compositions containing crosslinked products of fluorine-containing copolymers are widely used as sealing materials (e.g., O-rings, packings, oil seals, gaskets) and cushioning materials in the fields of vehicles, ships, aircraft, semiconductor devices, medical devices, general machinery, construction, etc. As crosslinkable compositions used to obtain such rubber compositions, Patent Document 1 discloses a crosslinkable fluorine-containing elastomer composition containing a fluorine-containing elastomer and an aromatic compound having two or more crosslinkable unsaturated double bonds, and a crosslinked product obtained by crosslinking the crosslinkable fluorine-containing elastomer composition.

[0003] Patent No. 6304253

[0004] In recent years, there has been a demand in various fields for improved performance of rubber compositions, and specifically, in particular for rubber compositions used in components such as sealing materials in semiconductor devices, there has been a demand for reduced haze to make it easier to detect foreign matter. In response to this demand, the present inventors have studied rubber compositions containing cross-linked products of fluoroelastomer compositions with reference to Patent Document 1, and have found that there is room for improvement in the haze of rubber compositions.

[0005] In view of the above circumstances, an object of the present invention is to provide a rubber composition having low haze, and a method for producing the rubber composition.

[0006] As a result of intensive investigations into the above-mentioned problems, the present inventors have found that the haze of a rubber composition can be reduced if the rubber composition contains a cross-linked product of a fluorine-containing copolymer, and the temperature T at which the storage modulus E' has a minimum value in dynamic viscoelasticity measurement at a measurement temperature of 23 to 200°C and a measurement frequency of 1 Hz is equal to or higher than a predetermined value, thereby completing the present invention.

[0007] That is, the present inventors have found that the above problems can be solved by the following configurations. [1] A rubber composition containing a crosslinked product of a fluorine-containing copolymer, wherein the temperature T showing the minimum value of storage modulus E' is 55°C or higher in dynamic viscoelasticity measurement at a measurement temperature of 23 to 200°C and a measurement frequency of 1 Hz. [2] The rubber composition according to [1], which does not contain a black filler. [3] The rubber composition according to [1] or [2], wherein the fluorine-containing copolymer contains units based on tetrafluoroethylene and units based on perfluoromethyl vinyl ether. [4] The rubber composition according to [3], wherein in the fluorine-containing copolymer, the ratio of the content of units based on tetrafluoroethylene to the content of units based on perfluoromethyl vinyl ether is 73 / 27 to 65 / 35 in molar ratio. [5] The rubber composition according to any of [1] to [4], wherein the crosslinked product is a crosslinked product of the fluorine-containing copolymer and a compound having a plurality of double bonds. [6] A method for producing a rubber composition, comprising: subjecting a crosslinkable composition containing a fluorine-containing copolymer and a crosslinking agent to a pressure treatment to obtain a precursor composition; and subjecting the precursor composition obtained by the pressure treatment to a heat treatment, wherein the pressure treatment applies to the crosslinkable composition a pressure such that a surface pressure is 6.0 MPa or less at a temperature of 150°C or higher but less than 155°C, or a pressure such that a surface pressure is 10 MPa or less at a temperature of 155°C or higher.

[0008] According to the present invention, it is possible to provide a rubber composition having low haze and a method for producing the rubber composition.

[0009] FIG. 2 is a schematic cross-sectional view showing an example of the configuration of a press device used in producing a precursor composition of a rubber composition.

[0010] The meanings of terms used in the present invention are as follows. A numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. A "unit" is a collective term for an atomic group derived from one molecule of the monomer that is formed directly by polymerizing the monomer, and an atomic group obtained by chemically converting a part of the atomic group. A "unit based on a monomer" will hereinafter also be simply referred to as a "unit." "Rubber" means a rubber that exhibits properties defined by JIS K 6200 (2008), and is distinguished from "resin."

[0011] [Rubber composition] The rubber composition of the present invention (hereinafter also referred to as "the present rubber composition") is a rubber composition which contains a crosslinked product of a fluorine-containing copolymer and has a temperature T of 55°C or higher at which the storage modulus E' has a minimum value in dynamic viscoelasticity measurement at a measurement temperature of 23 to 200°C and a measurement frequency of 1 Hz. The reason why a rubber composition which contains a crosslinked product of a fluorine-containing copolymer and has a temperature T of 55°C or higher at which the storage modulus E' has a minimum value is low in haze has not been clarified in detail, but is presumed to be due to the following reason.

[0012] The present inventors have used the minimum value of the storage modulus E' (hereinafter referred to as "minimum storage modulus E'") measured by dynamic viscoelasticity measurement at a measurement frequency of 1 Hz within a predetermined temperature range as an index of a rubber composition containing a crosslinked product of a fluorine-containing copolymer. ｍｉｎ The temperature T at which the minimum storage modulus E' is reached is also called "minimum storage modulus E'". ｍｉｎ It can be said that the higher the temperature T at which the temperature T becomes higher, the more uniformly the crosslinking points (crosslinking structures) of the fluorine-containing copolymer constituting the crosslinked product are dispersed in the rubber composition. In a rubber composition containing a crosslinked product in which the crosslinking points are uniformly dispersed, phase separation of uncrosslinked components and the like is unlikely to occur, and it is therefore presumed that the haze of the rubber composition is low. It is presumed that for this reason, a rubber composition with low haze was obtained.

[0013] The composition of the rubber composition will be described below. The rubber composition contains a cross-linked product of a fluorine-containing copolymer. The rubber composition may also contain other components in addition to the cross-linked product of a fluorine-containing copolymer.

[0014] [Cross-linked product of fluorocopolymer] The cross-linked product of fluorocopolymer contained in the present rubber composition is a product obtained by cross-linking the fluorocopolymer, and is synthesized, for example, by a cross-linking reaction between the fluorocopolymer and a cross-linking agent. Examples of the method for cross-linking the fluorocopolymer (method for producing the cross-linked product of the fluorocopolymer) include a method of subjecting a cross-linkable composition containing the fluorocopolymer and a cross-linking agent to pressure treatment.

[0015] <Crosslinkable Composition> The crosslinkable composition containing a fluorine-containing copolymer and a crosslinking agent will be described in detail below. The crosslinkable composition may contain components such as a crosslinking aid, which will be described later.

[0016] (Fluorine-containing copolymer) The fluorine-containing copolymer is not particularly limited as long as it is a polymer that contains fluorine atoms and exhibits rubber properties through crosslinking, but it preferably has units based on a monomer containing fluorine atoms (hereinafter referred to as "fluorine-containing monomer"), and a perfluoropolymer is preferred in terms of reducing the haze of the rubber composition. Here, "perfluoropolymer" refers to a polymer that does not substantially contain hydrogen atoms bonded to carbon atoms, has fluorine atoms instead of those hydrogen atoms, and has a main chain consisting of a chain of carbon atoms. The side chain of the perfluoropolymer may have a polyvalent atom other than carbon atoms, and the polyvalent atom is preferably an oxygen atom. Here, "substantially does not contain hydrogen atoms" means that the content of hydrogen atoms in the perfluoropolymer is 0.5% by mass or less, preferably 0.1% by mass or less, more preferably 0.07% by mass or less, and even more preferably 0.05% by mass or less. The lower limit is 0% by mass. When the content of hydrogen atoms is within the above range, good heat resistance or chemical resistance is likely to be obtained.

[0017] Specific examples of the fluorine-containing monomer include tetrafluoroethylene (hereinafter also referred to as "TFE"), perfluoro(alkyl vinyl ether) (hereinafter also referred to as "PAVE"), vinylidene fluoride (hereinafter also referred to as "VdF"), hexafluoropropylene (hereinafter also referred to as "HFP"), and chlorotrifluoroethylene (hereinafter also referred to as "CTFE").

[0018] The PAVE unit is a unit based on perfluoro(alkyl vinyl ether). From the viewpoint of excellent polymerization reactivity and rubber physical properties, the PAVE is preferably a monomer represented by formula (1): CF ２ =CF-O-R ｆ１ (1) In formula (1), R ｆ１ represents a perfluoroalkyl group having 1 to 10 carbon atoms. ｆ１ From the viewpoint of better polymerization reactivity, the number of carbon atoms in the perfluoroalkyl group is preferably 1 to 8, more preferably 1 to 6, still more preferably 1 to 5, and particularly preferably 1 to 3. The perfluoroalkyl group may be linear or branched.

[0019] Examples of PAVE include perfluoromethyl vinyl ether (hereinafter also referred to as "PMVE"), perfluoroethyl vinyl ether (hereinafter also referred to as "PEVE"), and perfluoropropyl vinyl ether (hereinafter also referred to as "PPVE"), and among these, PMVE and PPVE are preferred.

[0020] The fluorine-containing copolymer may have units based on monomers other than those mentioned above (hereinafter also referred to as "other monomers"). Examples of other monomers include a monomer having two or more polymerizable unsaturated bonds (hereinafter also referred to as "DVE"), a monomer represented by the following formula (5), ethylene, and propylene (hereinafter also referred to as "P"). Further examples include monomers having a halogen atom other than the above-mentioned fluorine-containing monomers, DVE, and the monomer represented by formula (5) (e.g., bromotrifluoroethylene, iodotrifluoroethylene).

[0021] The DVE unit is a unit based on a monomer having two or more polymerizable unsaturated bonds. Examples of the polymerizable unsaturated bond include a carbon-carbon double bond (C=C) and a carbon-carbon triple bond (C≡C). The number of polymerizable unsaturated bonds in the DVE is preferably 2 to 6, more preferably 2 or 3, and even more preferably 2, which provides better polymerization reactivity. The DVE preferably further contains a fluorine atom.

[0022] The DVE is preferably a monomer represented by formula (2): (CR２１ R ２２ =CR ２３ ) ａ２ R ２４ (2) In formula (2), R ２１ , R ２２ and R ２３ each independently represents a hydrogen atom, a fluorine atom, a methyl group, or a trifluoromethyl group; a2 represents an integer of 2 to 6; R ２４ represents a divalent perfluorohydrocarbon group having 1 to 10 carbon atoms or a group having an etheric oxygen atom at the end or between the carbon-carbon bonds of the perfluorohydrocarbon group. ２１ , multiple R ２２ and multiple R ２３ a2 is preferably 2 or 3, and more preferably 2.

[0023] Since the polymerization reactivity of DVE is superior, R ２１ , R ２２ , R ２３ is preferably a fluorine atom or a hydrogen atom, and R ２１ , R ２２ , R ２３ It is more preferable that all of R are fluorine atoms or all of R are hydrogen atoms, and from the viewpoint of heat resistance and chemical resistance of the rubber composition, ２１ , R ２２ , R ２３ It is more preferable that all of R are fluorine atoms. ２４ R may be linear, branched, or cyclic, preferably linear or branched, more preferably linear. ２４ The number of carbon atoms in R is preferably 2 to 8, more preferably 3 to 7, still more preferably 3 to 6, and particularly preferably 3 to 5. ２４ Although R may or may not have an etheric oxygen atom, it is preferable that R has an etheric oxygen atom in view of better crosslinking reactivity and rubber physical properties. ２４ The number of etheric oxygen atoms in R is preferably 1 to 6, more preferably 1 to 3, and even more preferably 1 or 2. ２４ The etheric oxygen atom in R ２４It is preferred that the nucleotide sequence is located at the end of the nucleotide sequence.

[0024] Of the monomers represented by formula (2), preferred examples include a monomer represented by formula (3) and a monomer represented by formula (4).

[0025] (CF ２ =CF) ２ R ３１ (3) In formula (3), R ３１ represents a divalent perfluorohydrocarbon group having 1 to 10 carbon atoms or a group having an etheric oxygen atom at the end or between the carbon-carbon bonds of the perfluorohydrocarbon group.

[0026] Examples of the monomer represented by formula (3) include CF ２ = CFO (CF ２ ) ２ OCF = CF ２ , C.F. ２ = CFO (CF ２ ) ３ OCF = CF ２ , C.F. ２ = CFO (CF ２ ) ４ OCF = CF ２ , C.F. ２ = CFO (CF ２ ) ６ OCF = CF ２、 CF ２ = CFO (CF ２ ) ８ OCF = CF ２ , C.F. ２ = CFO (CF ２ ) ２ OCF (CF ３ )CF ２ OCF = CF ２ , C.F. ２ = CFO (CF ２ ) ２ O(CF(CF ３ )CF ２ O) ２ CF = CF ２ , C.F. ２ = CFOCF ２ O (CF ２ CF ２ O) ２ CF = CF ２, C.F. ２ = CFO (CF ２ O) ３ O(CF(CF ３ )CF ２ O) ２ CF = CF ２ , C.F. ２ = CFOCF ２ CF (CF ３ ) O(CF ２ ) ２ OCF (CF ３ )CF ２ OCF = CF ２ , C.F. ２ = CFOCF ２ CF ２ O (CF ２ O) ２ CF ２ CF ２ OCF = CF ２ Among the monomers represented by formula (3), more preferred monomers include, for example, CF ２ = CFO (CF ２ ) ３ OCF = CF ２ (hereinafter referred to as "C3DVE"), CF ２ = CFO (CF ２ ) ４ OCF = CF ２ (hereinafter also referred to as "C4DVE").

[0027] (CH ２ =CH) ２ R ４１ (4) In formula (4), R ４１ represents a divalent perfluorohydrocarbon group having 1 to 10 carbon atoms or a group having an etheric oxygen atom at the end or between the carbon-carbon bonds of the perfluorohydrocarbon group.

[0028] Examples of the monomer represented by formula (4) include CH ２ =CH(CF ２ ) ２ CH=CH ２ , C.H. ２ =CH(CF ２ ) ４ CH=CH ２ , C.H. ２ =CH(CF ２) ６ CH=CH ２ Among the monomers represented by formula (4), more preferred monomers include, for example, CH ２ =CH(CF ２ ) ６ CH=CH ２ (hereinafter also referred to as "C6DVE").

[0029] When DVE is copolymerized, the polymerizable double bonds at the terminals of DVE react during polymerization to give a fluorine-containing copolymer having a branched chain.

[0030] Equation (5) is as follows: CF ２ =CF-O-R ｆ２ (5) In formula (5), R ｆ２ represents a perfluoroalkyl group having 1 to 8 carbon atoms and containing 1 to 5 etheric oxygen atoms. ｆ２ The number of carbon atoms is preferably 1 to 6, and more preferably 1 to 5.

[0031] Examples of the monomer represented by formula (5) include perfluoro(3,6-dioxa-1-heptene), perfluoro(3,6-dioxa-1-octene), and perfluoro(5-methyl-3,6-dioxa-1-nonene).

[0032] When the fluorine-containing copolymer contains TFE units, the content of the TFE units is preferably 50 to 80 mol%, more preferably 55 to 75 mol%, even more preferably 65 to 73 mol%, and especially preferably 67 to 71 mol%, based on all units of the fluorine-containing copolymer. When the fluorine-containing copolymer contains PAVE units, the content of the PAVE units is preferably 20 to 50 mol%, more preferably 25 to 45 mol%, even more preferably 27 to 35 mol%, and especially preferably 29 to 33 mol%, based on all units of the fluorine-containing copolymer. When the fluorine-containing copolymer contains DVE units, the content of the DVE units is preferably 0.01 to 1 mol%, more preferably 0.03 to 0.5 mol%, even more preferably 0.05 to 0.3 mol%, and especially preferably 0.1 to 0.15 mol%, based on all units of the fluorine-containing copolymer. When the fluorine-containing copolymer contains VdF units, the content of VdF units is preferably 50 to 78 mol%, more preferably 55 to 77 mol%, based on all units of the fluorine-containing copolymer. When the fluorine-containing copolymer contains HFP units, the content of HFP units is preferably 22 to 50 mol%, more preferably 23 to 45 mol%, based on all units of the fluorine-containing copolymer. When the fluorine-containing copolymer contains propylene units, the content of propylene units is preferably 20 to 80 mol%, more preferably 40 to 50 mol%, based on all units of the fluorine-containing copolymer.

[0033] The fluorine-containing copolymer preferably contains TFE units and PAVE units, more preferably TFE units and PMVE units, from the viewpoint of further reducing the haze of the rubber composition. When the fluorine-containing copolymer contains TFE units and PAVE units, the ratio of the content of TFE units to the content of PAVE units (more preferably PMVE units) contained in the fluorine-containing copolymer (TFE units / PAVE units) is preferably 80 / 20 to 50 / 50, more preferably 75 / 25 to 55 / 45, still more preferably 73 / 27 to 65 / 35, and particularly preferably 67 / 33 to 71 / 29, in molar ratio.

[0034] Preferred combinations of units contained in the fluorine-containing copolymer are shown below: Combination 1: A combination of TFE units and PAVE units Combination 2: A combination of TFE units, PAVE units and DVE units Combination 3: A combination of VdF units and HFP units Combination 4: A combination of TFE units and propylene units Among these, from the viewpoints of the heat resistance and chemical resistance of the rubber composition, Combination 1 or Combination 2 is preferred, and Combination 2 is more preferred.

[0035] The copolymerization composition of the fluorine-containing copolymer in Combinations 1 to 4 is preferably the following, in view of excellent heat resistance and chemical resistance of the rubber composition: Combinations 1 and 2: TFE units / PAVE units=80 / 20 to 50 / 50 (molar ratio), and, when DVE units are contained, (total of TFE units and PAVE units) / DVE units=100 / 0.01 to 100 / 1 (molar ratio) Combination 3: VdF units / HFP units=50 / 50 to 78 / 22 (molar ratio) Combination 4: TFE units / propylene units=60 / 40 to 50 / 50 (molar ratio) In the above Combinations 1 and 2, the PAVE units are preferably PMVE units.

[0036] The amount of each unit relative to the total units in the fluorine-containing copolymer was determined by nuclear magnetic resonance (NMR) analysis ( １９ F-NMR, １ It can be measured by known methods such as H-NMR analysis.

[0037] The fluorine-containing copolymer may contain iodine atoms. In this case, it is preferable that the iodine atoms be present at the terminals of the fluorine-containing copolymer (polymer chain). Examples of the iodine atoms include iodine atoms derived from an iodine compound that functions as a chain transfer agent, as described below, and iodine atoms in units based on a monomer having an iodine atom among other halogen-containing monomers such as iodotrifluoroethylene, as described above. An iodine atom derived from an iodine compound that functions as a chain transfer agent is preferred. When the fluorine-containing copolymer contains iodine atoms, the content of the iodine atoms is preferably 0.01 to 5.0 mass%, more preferably 0.05 to 2.0 mass%, relative to the total mass of the fluorine-containing copolymer. When the iodine atom content is within the above range, the crosslinking reactivity of the fluorine-containing copolymer is improved, and the mechanical properties of the rubber composition are improved.

[0038] (Method for Producing Fluorine-Containing Copolymer) An example of a method for producing a fluorine-containing copolymer is a method in which the above-mentioned monomers are copolymerized in the presence of a radical polymerization initiator. Examples of polymerization methods include emulsion polymerization, solution polymerization, and suspension polymerization, with emulsion polymerization being preferred from the viewpoints of ease of adjusting the molecular weight and copolymerization composition and excellent productivity. When the fluorine-containing copolymer is produced by emulsion polymerization, it can be carried out, for example, by heating the above-mentioned monomers in the presence of an aqueous medium, an emulsifier, and a radical polymerization initiator. The aqueous medium is preferably water or a mixed solvent of water and a water-soluble organic solvent. Examples of water-soluble organic solvents include tert-butanol, propylene glycol, dipropylene glycol, dipropylene glycol monomethyl ether, and tripropylene glycol. Tert-butanol and dipropylene glycol monomethyl ether are particularly preferred. When the aqueous medium contains a water-soluble organic solvent, the content of the water-soluble organic solvent is preferably 1 to 40 parts by mass, more preferably 3 to 30 parts by mass, per 100 parts by mass of water.

[0039] The amount of the aqueous medium used is preferably 150 to 400 parts by mass, more preferably 250 to 350 parts by mass, per 100 parts by mass of the monomer.

[0040] The radical polymerization initiator is preferably a water-soluble polymerization initiator or a redox polymerization initiator. Examples of water-soluble polymerization initiators include persulfates such as ammonium persulfate, sodium persulfate, and potassium persulfate, and organic polymerization initiators such as disuccinic acid peroxide and azobisisobutylamidine dihydrochloride. Among these, persulfates are preferred, and ammonium persulfate is more preferred. Examples of redox polymerization initiators include polymerization initiators combining persulfates with a reducing agent. Among these, polymerization initiators capable of polymerizing each monomer at a polymerization temperature in the range of 0 to 80°C are preferred. Examples of persulfates constituting the redox polymerization initiator include alkali metal salts of persulfate such as ammonium persulfate, sodium persulfate, and potassium persulfate, with ammonium persulfate being preferred. Examples of reducing agents to be combined with persulfates include thiosulfates, sulfites, hydrogen sulfites, pyrosulfites, and hydroxymethanesulfinates. Hydroxymethanesulfinates are preferred, and sodium hydroxymethanesulfinate is more preferred.

[0041] In the method for producing the fluorine-containing copolymer, the above-mentioned monomers may be copolymerized together with a radical polymerization initiator in the presence of a chain transfer agent. The chain transfer agent is preferably an iodine compound represented by the formula RI ２ In the above formula, R represents an alkylene group or a perfluoroalkylene group having 3 or more carbon atoms (preferably 3 to 8 carbon atoms). ２ Specific examples of the iodo compound represented by the formula (I) include 1,3-diiodopropane, 1,4-diiodobutane, 1,6-diiodohexane, 1,8-diiodooctane, 1,3-diiodoperfluoropropane, 1,4-diiodoperfluorobutane, 1,6-diiodoperfluorohexane, and 1,8-diiodoperfluorooctane. As the iodo compound, an iodo compound having a perfluoroalkylene group is preferred, and 1,4-diiodoperfluorobutane is particularly preferred. When the above-mentioned monomers are copolymerized in the presence of these iodine compounds, iodine atoms are introduced into the fluorine-containing copolymer.

[0042] When the fluorocopolymer is produced by emulsion polymerization, the latex obtained by emulsion polymerization may be coagulated to isolate the fluorocopolymer. Coagulation methods include the addition of a metal salt and the addition of an inorganic acid such as hydrochloric acid, sulfuric acid or nitric acid, and the addition of an inorganic acid is preferred because the resulting fluorocopolymer contains less metal and can be suitably used for components (e.g., O-rings) in semiconductor manufacturing equipment.

[0043] For details of the components other than those mentioned above used in producing the fluorine-containing copolymer and the production method, reference can be made to the method described in paragraphs 0019 to 0034 of WO 2010 / 082633.

[0044] The content of the fluorine-containing copolymer in the crosslinkable composition is preferably from 60 to 99 mass%, more preferably from 70 to 99 mass%, and even more preferably from 80 to 99 mass%, based on the total mass of the crosslinkable composition.

[0045] (Crosslinking Agent) Examples of the crosslinking agent contained in the crosslinkable composition include organic peroxides and compounds having two or more amino groups (hereinafter also referred to as "polyamine compounds"), with organic peroxides being preferred.

[0046] Examples of organic peroxides include dialkyl peroxides, α,α'-bis(tert-butylperoxy)-p-diisopropylbenzene, α,α'-bis(tert-butylperoxy)-m-diisopropylbenzene, benzoyl peroxide, tert-butylperoxybenzene, 2,5-dimethyl-2,5-bis(benzoylperoxy)hexane, tert-butylcumyl peroxide, and dicumyl peroxide. Examples of dialkyl peroxides include 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethylhexane-2,5-dihydroxyperoxide, 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane, 2,5-dimethyl-2,5-bis(tert-butylperoxy)-3-hexyne, tert-butylperoxymaleic acid, and tert-butylperoxyisopropyl carbonate. Among these, 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane or 2,5-dimethyl-2,5-bis(tert-butylperoxy)-3-hexyne is preferred from the viewpoint of availability.

[0047] Examples of polyamine compounds include hexamethylenediamine, hexamethylenediamine carbamate, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis(3-amino-4-hydroxyphenyl)propane, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (hereinafter also referred to as "BOAP", also known as bisaminophenol AF), 2,2-bis(3,4-diaminophenyl)propane, 2,2-bis(3,4-diaminophenyl)hexafluoropropane, 2,2-bis(3-amino-4-(N-phenylamino)phenyl)hexafluoropropane, 4,4'-methylenedianiline, m-phenylenediamine, adipic acid dihydrazide, and the compound represented by formula (XII) of Japanese Patent No. 5,833,657.

[0048] The content of the crosslinking agent in the crosslinkable composition is preferably less than 1.0 part by mass, more preferably 0.8 part by mass or less, and even more preferably 0.6 part by mass or less, per 100 parts by mass of the fluorocopolymer. The content of the crosslinking agent may be, for example, 0.01 part by mass or more per 100 parts by mass of the fluorocopolymer.

[0049] (Crosslinking Auxiliary Agent) The crosslinkable composition preferably further contains a crosslinking auxiliary agent from the viewpoint of improving the crosslinking reactivity of the fluorine-containing copolymer. In particular, from the viewpoint of further reducing the haze of the rubber composition, it is more preferable to further contain a compound having a plurality of polymerizable unsaturated bonds as the crosslinking auxiliary agent. Examples of the polymerizable unsaturated bond include a carbon atom-carbon atom double bond (C=C) and a carbon atom-carbon atom triple bond (C≡C), and from the viewpoint of more excellent crosslinking reactivity, a carbon atom-carbon atom double bond (C=C) is preferred. The number of polymerizable unsaturated bonds in the crosslinking auxiliary is preferably 2 to 6, more preferably 2 to 4, even more preferably 2 or 3, and a compound having a plurality of double bonds is even more preferable.

[0050] Examples of the compound having a plurality of double bonds include triallyl cyanurate, triallyl isocyanurate, trimethallyl isocyanurate, triacryl formal, triallyl trimellitate, N,N'-m-phenylene bismaleimide, dipropargyl terephthalate, diallyl phthalate, tetraallyl terephthalate amide, triallyl phosphate, bismaleimide, fluorinated triallyl isocyanurate (1,3,5-tris(2,3,3-trifluoro-2-propenyl)-1,3,5-triazine-2, 4,6-trione), tris(diallylamine)-S-triazine, triallyl phosphite, N,N-diallylacrylamide, N,N,N',N'-tetraallylphthalamide, trivinyl isocyanurate, 2,4,6-trivinylmethyltrisiloxane, tri(5-norbornene-2-methylene)cyanurate, triallyl phosphite, 1,4-divinyl(perfluoro)butane, or 1,6-divinyl(perfluorohexane) is preferred, and triallyl isocyanurate is more preferred from the viewpoint of availability. The content of the crosslinking aid is preferably from 0.03 to 5 parts by mass, more preferably from 0.1 to 4 parts by mass, and even more preferably from 0.3 to 3 parts by mass, per 100 parts by mass of the fluorine-containing copolymer.

[0051] The crosslinked product of the fluorine-containing copolymer contained in the present rubber composition is preferably a cured product of a fluorine-containing copolymer and a compound having a plurality of polymerizable unsaturated bonds, and more preferably a cured product of a fluorine-containing copolymer and a compound having a plurality of double bonds, from the viewpoint of further reducing the haze of the rubber composition. The cured product can be obtained, for example, by pressure-treating a crosslinkable composition containing the fluorine-containing copolymer, the crosslinking aid which is a compound having a plurality of polymerizable unsaturated bonds, and the crosslinking agent, to crosslink the fluorine-containing copolymer and the crosslinking aid.

[0052] (Other Components) The crosslinkable composition may contain other components in addition to those described above, provided that the effects of the present invention are not impaired. Examples of such other components include acid acceptors (e.g., fatty acid esters, fatty acid metal salts, and oxides of divalent metals (magnesium oxide, calcium oxide, zinc oxide, lead oxide, and the like)), fillers functioning as bulking or reinforcing materials (e.g., barium sulfate, calcium metasilicate, calcium carbonate, titanium oxide, silicon dioxide, polytetrafluoroethylene (PTFE), clay, and talc) (excluding black fillers), scorch retarders (e.g., phenolic hydroxyl group-containing compounds such as bisphenol A, quinones such as hydroquinone, and α-methylstyrene dimers such as 2,4-di(3-isopropylphenyl)-4-methyl-1-pentene), crown ethers (e.g., 18-crown-6), and release agents (e.g., sodium stearate).

[0053] When the crosslinkable composition contains other components than the fluorine-containing copolymer, the crosslinking agent, the crosslinking aid, and the black filler, the total content of the other components is preferably more than 0.1 part by mass and not more than 30 parts by mass, more preferably from 1 to 25 parts by mass, and even more preferably from 5 to 15 parts by mass, per 100 parts by mass of the fluorine-containing copolymer.

[0054] The crosslinkable composition preferably does not contain a black filler such as carbon black, as this increases the transparency of the rubber composition and makes it easier to detect foreign matter when the rubber composition is used in a component (e.g., an O-ring) of a semiconductor manufacturing apparatus. Furthermore, from the above viewpoint and in terms of dispersibility, the crosslinkable composition preferably does not contain a substance selected from the group consisting of carbon black and carbon nanotubes. Here, the crosslinkable composition "does not contain" a black filler or the like means that the content of the black filler or the like relative to the total mass of the crosslinkable composition is 4.7% by mass or less. The content of the black filler or the like may be 0% by mass relative to the total mass of the crosslinkable composition.

[0055] The crosslinkable composition can be prepared by mixing the above-mentioned components using a rubber mixing device such as a roll, a kneader, a Banbury mixer, or an extruder.

[0056] The pressure treatment for producing a crosslinked product of the fluorine-containing copolymer contained in the present rubber composition will be described in detail in the method for producing the present rubber composition below.

[0057] The content of the crosslinked fluorine-containing copolymer in the rubber composition is preferably 49% by mass or more, more preferably 94% by mass or more, based on the total mass of the rubber composition. The content of the crosslinked fluorine-containing copolymer may be 100% by mass, based on the total mass of the rubber composition, and when the rubber composition contains components other than the crosslinked product, the content is preferably 99% by mass or less, based on the total mass of the rubber composition.

[0058] <Other Components Other Than Cross-Linked Product of Fluorine-Containing Copolymer> The present rubber composition may contain other components other than the cross-linked product of the fluorine-containing copolymer. Examples of the other components include components contained in the above-mentioned cross-linkable composition excluding the fluorine-containing copolymer and the cross-linking aid, as well as residues derived from these components. When the rubber composition contains the above-mentioned other components, the content thereof is preferably 0.1 to 50 mass%, more preferably 1 to 10 mass%, and even more preferably 1 to 5 mass%, relative to the total mass of the rubber composition.

[0059] Furthermore, it is preferable that the rubber composition does not contain a black filler (for example, carbon black, etc.). A rubber composition that does not contain a black filler has high transparency, and when used in a component (for example, an O-ring) of a semiconductor manufacturing device, adhering foreign matter can be more easily detected. Here, "the rubber composition does not contain a black filler" means that the content of the black filler relative to the total mass of the rubber composition is 4.7% by mass or less. The content of the black filler relative to the total mass of the rubber composition may be 0% by mass.

[0060] The shape and size of the rubber composition are not limited, and it can be used as a molded article having a shape and size appropriate for the intended use.

[0061] [Properties of Rubber Composition] <Minimum storage modulus E' ｍｉｎ The present rubber composition has a minimum storage modulus E' measured by dynamic viscoelasticity measurement at a measurement temperature of 23 to 200°C and a measurement frequency of 1 Hz. ｍｉｎThe temperature T is preferably 65°C or higher, more preferably 70°C or higher, from the viewpoint of further reducing the haze of the rubber composition. Moreover, the temperature T is preferably 150°C or lower, more preferably 120°C or lower, and even more preferably 100°C or lower, from the viewpoint of more excellent processability.

[0062] The temperature T is determined by measuring the storage modulus E' of the rubber composition by performing dynamic viscoelasticity measurement at a measurement frequency of 1 Hz within a temperature range of 23 to 200°C, and determining the minimum value (minimum storage modulus E') of the measured storage moduli E'. ｍｉｎ Therefore, the temperature T obtained by the above measurement method is any temperature within the range of 23 to 200°C. In the present invention, when there are multiple minimum values ​​in the storage modulus E' measured by the dynamic viscoelasticity measurement, the lowest temperature among the multiple minimum values ​​is referred to as the minimum storage modulus E'. ｍｉｎ However, if there are multiple minimum values, and some are minimum values ​​that are local minimums and some are not local minimums, the minimum value that is local minimum is regarded as the minimum storage modulus E' ｍｉｎ The storage modulus E' of the rubber composition is a value measured in accordance with ASTM D5289 and ASTM D6204 using a viscoelasticity measuring device (e.g., "DMA7100" manufactured by Hitachi High-Technologies Corporation). More detailed measurement conditions for the storage modulus E' will be described in the examples below.

[0063] Minimum storage modulus E' of rubber composition ｍｉｎ can be adjusted by, for example, the composition of the fluorine-containing copolymer, the type and content of each component contained in the crosslinkable composition, and the production conditions of the rubber composition described below (particularly, the pressure conditions and temperature conditions in the pressure treatment).

[0064] The minimum storage modulus E' of the rubber composition measured by the dynamic viscoelasticity measurement ｍｉｎ is preferably 2.4 to 3.1 MPa, more preferably 2.5 to 3.0 MPa, in view of a better balance between haze and mechanical properties.

[0065] [Method for producing rubber composition] Examples of a method for producing the present rubber composition include a method in which a crosslinkable composition containing a fluorine-containing copolymer and a crosslinking agent is subjected to a pressure treatment under specific conditions to obtain a precursor composition, and the precursor composition obtained by the pressure treatment is then subjected to a heat treatment. Hereinafter, with reference to the drawings, a method for producing the present rubber composition by subjecting a crosslinkable composition to a pressure treatment and a heat treatment will be specifically described. Note that the following method is an example of a method for producing the present rubber composition using a crosslinkable composition, and the method for producing the present rubber composition is not limited to the following method.

[0066] <Pressure Treatment> FIG. 1 is a schematic cross-sectional view showing an example of the configuration of a press apparatus used to produce a precursor composition of the present rubber composition. The illustrated press apparatus 10 is an apparatus for performing pressure treatment and includes a cylinder 1, a slide 2, an upper mold 3, a lower mold 4, a bolster 5, and a pressure gauge 6. The cylinder 1 has a function of moving a slide 2 connected to the cylinder 1 in the vertical direction indicated by the arrow on the page by a driving device (not shown). The upper surface of the upper mold 3, which is one side of a mold, is connected to the lower surface of the slide 2, and the upper mold 3 moves in one direction together with the slide 2. The lower mold 4, which is the other side of the mold, is disposed below the upper mold 3. The lower surface of this lower mold 4 is connected to the upper surface of a bolster 5, which is fixed so as not to move in the vertical direction. A convex core 3a is provided on the lower surface of the upper mold 3, and a concave cavity 4a is provided on the upper surface of the lower mold 4. The core 3a and the cavity 4a are disposed at positions facing each other in the vertical direction on the page (the direction in which the mold is opened and closed) and have a shape corresponding to the shape of the desired rubber composition. The pressure gauge 6 measures the pressure applied to the cylinder 1 during the pressurization process.

[0067] The pressure treatment of the crosslinkable composition is carried out using a press apparatus 10 shown in FIG. 1 as follows. First, composition X (crosslinkable composition) is placed in the cavity 4a of the lower mold 4. Before placing composition X in the cavity 4a, the upper mold 3 and the lower mold 4 may be preheated. Next, the slide 2 and upper mold 3 are moved downward by the cylinder 1, and the upper mold 3 and the lower mold 4 are closed. This carries out a pressure treatment in which a predetermined pressure is applied to composition X from the core 3a of the upper mold 3 and the cavity 4a of the lower mold 4 while heating it at a predetermined temperature. After a predetermined time has elapsed, the slide 2 and upper mold 3 are moved upward by the cylinder 1, and the upper mold 3 and the lower mold 4 are separated, thereby removing the pressure on composition X and completing the pressure treatment. By carrying out such a pressure treatment, the fluorine-containing copolymer and the crosslinking agent are reacted (crosslinked) to obtain a precursor composition.

[0068] The shape and size of the precursor composition obtained by pressure treatment are appropriately selected depending on the application of the rubber composition. After pressure treatment, the precursor composition may be trimmed by cutting or the like, if necessary.

[0069] Here, the present inventors have investigated the relationship between the magnitude of the pressure applied to the crosslinkable composition in the pressure treatment and the properties of the rubber composition obtained through the pressure treatment and the heat treatment described below. As a result, it has been unexpectedly found that when a pressure that reduces the surface pressure is applied to the crosslinkable composition in the pressure treatment, the haze of the resulting rubber composition tends to be lower. More specifically, the pressure applied to the crosslinkable composition in the pressure treatment is preferably a pressure that reduces the surface pressure to 6.0 MPa or less, more preferably a pressure that reduces the surface pressure to 5.0 MPa or less, and even more preferably a pressure that reduces the surface pressure to 4.8 MPa or less, under temperature conditions of 150°C or higher and lower than 155°C, in order to reduce the haze of the resulting rubber composition. Furthermore, under temperature conditions of 155°C or higher, a pressure that reduces the surface pressure to 10 MPa or less is preferred, a pressure that reduces the surface pressure to 8.0 MPa or less is more preferred, and even more preferably a pressure that reduces the surface pressure to 7.3 MPa or less is applied. Furthermore, in terms of achieving better dimensional stability and releasability, the pressure applied to the crosslinkable composition in the pressure treatment is preferably a pressure that results in a surface pressure of 1.0 MPa or more, more preferably a pressure that results in a surface pressure of 2.0 MPa or more, and even more preferably a pressure that results in a surface pressure of 4.0 MPa or more, under temperature conditions of 150° C. or more and less than 155° C. Furthermore, under temperature conditions of 155° C. or more, a pressure that results in a surface pressure of 1.0 MPa or more is preferred, more preferably a pressure that results in a surface pressure of 3.0 MPa or more, and even more preferably a pressure that results in a surface pressure of 5.0 MPa or more.

[0070] Here, "surface pressure" refers to the magnitude of pressure actually applied to the crosslinkable composition by the pressurization treatment. The press apparatus 10 shown in FIG. 1 is equipped with a pressure gauge 6 as a pressure gauge for measuring the magnitude of pressure applied to the mold (upper mold 3 and lower mold 4). However, pressure gauges such as the pressure gauge 6 connected to the cylinder 1 usually measure the pressure applied to the cylinder during pressurization, and do not measure the pressure applied to the object contained in the mold cavity 4a. Taking the press apparatus 10 shown in FIG. 1 as an example, the surface pressure Y of the pressure applied to the composition X is calculated by the following formula (P) from the contact area A where the cylinder 1 contacts the slide 2, the area B of the area obtained by orthogonally projecting the space occupied by the composition X contained in the cavity 4a (the molding area within the mold) in the opening / closing direction of the mold (the movement direction of the slide 2), and the pressure C with which the cylinder 1 presses the slide 2 during the pressurization treatment: Y=(C×A) / B(P)

[0071] The contact area A where the cylinder 1 comes into contact with the slide 2 is also called the ram area. The diameter of the bottom surface of the cylinder, which is cylindrical and moves in the axial direction, is also called the "ram diameter." The contact area A is calculated from the ram diameter R as follows: A = π × R ２ In the illustrated press device 10, the pressure C is the pressure measured by the pressure gauge 6.

[0072] The temperature conditions for the pressure treatment are preferably 150° C. or higher, more preferably 155° C. or higher, and even more preferably 160° C. or higher, in order to further reduce the haze of the rubber composition. In particular, the conditions for the pressure treatment are preferably a surface pressure of 8.0 MPa or lower and a temperature of 155° C. or higher, and more preferably a surface pressure of 8.0 MPa or lower and a temperature of 160° C. or higher, in order to further reduce the haze of the rubber composition.

[0073] The temperature condition for the pressure treatment is preferably 200° C. or lower, more preferably 170° C. or lower, since this allows further reduction in the amount of energy used. The time for the pressure treatment is appropriately selected depending on the pressure and temperature, but is preferably 1 to 30 minutes, more preferably 5 to 20 minutes.

[0074] <Heat Treatment> The precursor composition obtained by the above heat treatment is further subjected to heat treatment to obtain the present rubber composition.

[0075] The heat treatment can be carried out using known heating means such as an oven using electricity, hot air, steam, or the like as a heat source. The heat treatment temperature is preferably 200 to 350°C, more preferably 230 to 310°C. The heat treatment time is selected appropriately depending on the temperature, but is preferably 0.1 to 24 hours, more preferably 1 to 5 hours. By heat treating the precursor composition within the above range, the mechanical properties and rubber physical properties of the rubber composition are improved, and low-molecular-weight impurities such as peroxides can be decomposed, volatilized, and reduced.

[0076] [Uses] The rubber composition is suitable for use as a material for O-rings, sheets, gaskets, oil seals, diaphragms, V-rings, and the like. The present invention can also be applied to heat-resistant and chemical-resistant sealing materials, heat-resistant and oil-resistant sealing materials, wire coating materials, sealing materials for semiconductor devices, sealing materials for liquid crystal display panel manufacturing equipment, sealing materials for light-emitting diode manufacturing equipment, corrosion-resistant rubber coating materials, sealing materials for urea-resistant greases, and the like, rubber coating materials, adhesive rubbers, hoses, tubes, calendered sheets (rolls), sponges, rubber rolls, oil drilling components, heat-dissipating sheets, solution-crosslinked products, rubber sponges, bearing seals (urea-resistant grease, etc.), linings (chemical-resistant), insulating sheets for automobiles, insulating sheets for electronic devices, rubber bands for watches, endoscope packings (amine-resistant), bellows hoses (processed from calendered sheets), water heater packings / valves, fenders (offshore civil engineering, ships), fibers and nonwoven fabrics (protective clothing, etc.), board sealing materials, rubber gloves, stators for uniaxial eccentric screw pumps, parts for urea SCR systems, vibration-proofing agents, vibration-damping agents, sealants, additives for other materials, and toys. In particular, the present rubber composition has low haze and high transparency, making it particularly suitable for use as a member for semiconductor manufacturing equipment, since it allows for easier detection of attached foreign matter, etc. If the haze of the present rubber composition is 5.0% or less, it becomes easy to check for deterioration (e.g., cracks) of the present rubber composition, but if it exceeds 5.1%, it becomes difficult to check for deterioration.

[0077] The present invention will be described in detail below with reference to examples. Examples 1, 5, 6, 10, 12, 13, 15, 17, 18, and 20 are working examples, and Examples 2 to 4, 7 to 9, 11, 14, 16, 19, and 21 are comparative examples. However, the present invention is not limited to these examples. The content of each component in the tables below is based on mass unless otherwise specified.

[0078] [Production of Fluorocopolymers] Fluorocopolymers 1 to 5 were produced by the following methods.

[0079] (Fluorocopolymer 1) A stainless steel pressure reactor having an internal volume of 20 L and equipped with an anchor blade was degassed, and then 8.2 L of ultrapure water and C ２ F ５ OCF ２ CF ２ OCF ２ COONH ４ 733 g of a 30% by mass solution of 10.0 g of C3DVE, and 15.9 g of a 5% by mass aqueous solution of disodium hydrogen phosphate dodecahydrate were charged, and the gas phase was replaced with nitrogen. While stirring at a speed of 375 rpm using an anchor blade, 198 g of TFE and 454 g of PMVE were pressure-charged into the vessel after the internal temperature reached 80°C. The pressure inside the reactor was 0.90 MPa [gauge]. 40 mL of a 1% by mass aqueous solution of ammonium persulfate was added to initiate polymerization. The molar ratio of the monomers (hereinafter also referred to as "initial monomers") pressure-charged before the start of polymerization was TFE:PMVE:C3DVE = 41.74:57.64:0.61.

[0080] As the polymerization progressed, when the pressure inside the reactor dropped to 0.89 MPa [gauge], TFE was injected, and the pressure inside the reactor was increased to 0.90 MPa [gauge]. This was repeated, and 62 g of PMVE was also injected every time 80 g of TFE was injected. In addition, 7.0 g of 1,4-diiodoperfluorobutane was injected into the reactor together with 50 mL of ultrapure water from the ampoule tube when 60 g of TFE was injected. When the total added mass of TFE reached 1,200 g, the addition of the monomer injected after the start of polymerization (hereinafter also referred to as "post-added monomer") was stopped, and the internal temperature of the reactor was cooled to 10°C to terminate the polymerization reaction, thereby obtaining a latex containing a fluorine-containing copolymer. The polymerization time was 360 minutes. The total mass of the post-added monomers was 1,200 g for TFE and 868 g for PMVE, which was converted into a molar ratio of TFE:PMVE=68:32.

[0081] Nitric acid (Kanto Chemical Co., Ltd., special grade) was dissolved in ultrapure water to prepare a 3% by mass aqueous solution of nitric acid. The latex was added to an aqueous nitric acid solution in a TFE / perfluoro(alkyl vinyl ether) copolymer (PFA) container to coagulate the fluorine-containing copolymer. The amount of the aqueous nitric acid solution was 150 parts by mass per 100 parts by mass of the fluorine-containing copolymer in the latex. The coagulated fluorine-containing copolymer was recovered by filtration, poured into ultrapure water in a PFA container, and washed by stirring at 200 rpm for 30 minutes. The amount of ultrapure water was 100 parts by mass per 100 parts by mass of the fluorine-containing copolymer. The above washing was repeated 10 times. The washed fluorine-containing copolymer was recovered by filtration and dried at 50°C and 10 kPa under reduced pressure to obtain fluorine-containing copolymer 1. The molar ratio of each unit in fluorine-containing copolymer 1 was TFE unit: PMVE unit: C3DVE unit = 71.40:28.43:0.17. The iodine atom content in the fluorine-containing copolymer was calculated using an apparatus combining an automatic sample combustion apparatus, a pretreatment device for ion chromatography (manufactured by Mitsubishi Chemical Analytech Co., Ltd., Model AQF-100), and an ion chromatograph.

[0082] (Fluorocopolymer 2) A stainless steel pressure reactor having an internal volume of 2,100 mL and equipped with an anchor blade was degassed, and then 804 g of ultrapure water, C２ F ５ OCF ２ CF ２ OCF ２ COONH ４ 80.1 g of a 30% by weight solution of 1,4-diiodoperfluorobutane, 0.72 g of C3DVE, 1.8 g of a 5% by weight aqueous solution of disodium hydrogen phosphate dodecahydrate, and 0.87 g of 1,4-diiodoperfluorobutane were charged, and the gas phase was purged with nitrogen. While stirring at a speed of 600 rpm using an anchor blade, 13 g of TFE and 65 g of PMVE were pressure-charged into the vessel once the internal temperature reached 80°C. The internal pressure of the reactor was 0.90 MPa [gauge]. 20 mL of a 1% by weight aqueous solution of ammonium persulfate was added to initiate polymerization. The molar ratio of the initial monomers added was TFE:PMVE:C3DVE = 25:75:0.19.

[0083] As the polymerization progressed, when the reactor internal pressure dropped to 0.89 MPa [gauge], TFE was injected, and the reactor internal pressure was increased to 0.90 MPa [gauge]. This was repeated, and 7 g of PMVE was also injected every time 8 g of TFE was injected. When the total added mass of TFE reached 80 g, the addition of the post-added monomer was stopped, the reactor internal temperature was cooled to 10 ° C, and the polymerization reaction was stopped, thereby obtaining a latex containing a fluorine-containing copolymer. The polymerization time was 185 minutes. In addition, the total added mass of the post-added monomers was 80 g of TFE and 63 g of PMVE, which was converted into a molar ratio of TFE:PMVE = 65:35.

[0084] Nitric acid (Kanto Chemical Co., Inc., special grade) was dissolved in ultrapure water to prepare a 3% by mass aqueous solution of nitric acid. The latex was added to the nitric acid aqueous solution in a PFA container to coagulate the fluorine-containing copolymer. The amount of the nitric acid aqueous solution was 150 parts by mass per 100 parts by mass of the fluorine-containing copolymer in the latex. The coagulated fluorine-containing copolymer was recovered by filtration, poured into ultrapure water in a PFA container, and washed by stirring at 200 rpm for 30 minutes. The amount of ultrapure water was 100 parts by mass per 100 parts by mass of the fluorine-containing copolymer. The above washing was repeated 10 times. The washed fluorine-containing copolymer was recovered by filtration and dried under reduced pressure at 50°C and 10 kPa to obtain a white fluorine-containing copolymer 2. The molar ratio of each unit in fluorine-containing copolymer 2 was TFE unit:PMVE unit:C3DVE unit = 65.9:34.0:0.1. The iodine atom content was 0.15% by mass. The method for calculating the content of iodine atoms in the fluorine-containing copolymer is the same as in Fluorine-containing copolymer 1.

[0085] (Fluorine-containing copolymer 3) The monomer used is changed from C3DVE to CF ２ = CFOCF ２ CF (CF ３ ) OCF ２ CF ２ Fluorocopolymer 3 was obtained in the same manner as in the above Fluorocopolymer 1, except that the amount of TFE unit was changed to TFE, the amount of PMVE unit, the amount of CF3 unit was changed to CF4, the amount of ultrapure water, the amount of solution, the amount of aqueous solution and the amount of monomers used was changed, and the polymerization temperature and stirring speed were changed. ２ = CFOCF ２ CF (CF ３ ) OCF ２ CF ２ The CN units were 59.4:40.1:0.5.

[0086] (Fluorocopolymer 4) Fluorocopolymer 4 was obtained with reference to the method of Example 5 of WO 2017 / 057512. The molar ratio of each unit in fluorine-containing copolymer 4 was TFE unit:P unit:C3DVE unit = 56.0:43.8:0.2. The iodine content in fluorine-containing copolymer 4 was 0.37% by mass.

[0087] (Fluorocopolymer 5) Fluorocopolymer 5 was obtained with reference to the method of Example 14 of WO2021 / 010443. The fluorine content in fluorine-containing copolymer 5 was 73% by mass.

[0088] Example 1 (Production of Crosslinkable Composition) 100 parts by mass of Fluorine-containing copolymer 1, 0.5 parts by mass of TAIC (trade name, manufactured by Mitsubishi Chemical Corporation, triallyl isocyanurate, crosslinking aid), and 0.5 parts by mass of Perhexa 25B (trade name, manufactured by NOF Corporation, 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane, organic peroxide) were blended and kneaded with a twin roll at room temperature for 10 minutes to obtain a mixed crosslinkable composition 1.

[0089] (Production of Rubber Composition Containing Crosslinked Product of Fluorine-Containing Copolymer) A 50T hydraulic press (model: SA-301 50T type, manufactured by Tester Sangyo Co., Ltd., ram diameter: 180 mm) was prepared as press apparatus A. The cavity of the mold equipped with press apparatus A was rectangular, measuring 150 mm in length and 80 mm in width. The crosslinkable composition 1 was filled into the cavity of the mold and subjected to a pressure treatment for 10 minutes under the conditions shown in Table 1 described below, to obtain a precursor composition 1 in the form of a plate measuring 150 mm in length, 80 mm in width, and 2 mm in thickness. Next, the precursor composition 1 was placed in an oven ("Fine Oven DH612", manufactured by Yamato Scientific Co., Ltd.) and heat-treated at 250°C for 4 hours, to obtain a rubber composition 1 in the form of a plate measuring 150 mm in length, 80 mm in width, and 2 mm in thickness, containing a crosslinked product of the fluorine-containing copolymer 1.

[0090] [Examples 2 to 4] Rubber compositions 2 to 4 containing a crosslinked product of fluorine-containing copolymer 1 were obtained in accordance with the method described in Example 1, except that the rubber compositions were pressure-treated under the conditions shown in Table 1 below when producing the rubber compositions.

[0091] [Examples 5 to 9] A 70T hydraulic press (model: H305A2, manufactured by Daishin Machinery Co., Ltd., ram diameter: 225 mm) was prepared as press apparatus B. Rubber compositions 5 to 9 containing a crosslinked product of fluorine-containing copolymer 1 were obtained in accordance with the method described in Example 1, except that press apparatus B was used instead of press apparatus A when producing the rubber composition and that pressure treatment was carried out under the conditions shown in Table 1 described below.

[0092] [Example 10] Crosslinkable composition 2 was prepared according to the method described in [Production of crosslinkable composition] of Example 1, except that fluorocopolymer 2 was used instead of fluorocopolymer 1. Then, rubber composition 10 containing a crosslinked product of fluorocopolymer 2 was obtained according to the method described in Example 1, except that crosslinkable composition 2 was used instead of crosslinkable composition 1 when producing the rubber composition, and pressure treatment was carried out under the conditions shown in Table 1 below.

[0093] [Example 11] Rubber composition 11 containing a cross-linked product of fluorine-containing copolymer 2 was obtained in accordance with the method described in Example 5, except that cross-linkable composition 2 was used in place of cross-linkable composition 1 when producing the rubber composition, and that pressure treatment was carried out under the conditions shown in Table 1 described later.

[0094] [Example 12] (Production of crosslinkable composition) 100 parts by mass of Fluorine-containing copolymer 3, silicon nitride (Si ３ N ４ ) (trade name, SN-A00, crosslinking agent, manufactured by Ube Industries, Ltd.) was mixed and kneaded for 10 minutes with a two-roll mill at room temperature to obtain a mixed crosslinkable composition 12.

[0095] (Production of Rubber Composition Containing Crosslinked Product of Fluorine-Containing Copolymer) A 50T hydraulic press (model: SA-301 50T type, manufactured by Tester Sangyo Co., Ltd., ram diameter: 180 mm) was prepared as press apparatus A. The cavity of the mold equipped with press apparatus A was rectangular, measuring 150 mm in length and 80 mm in width. The crosslinkable composition 12 was filled into the cavity of the mold and subjected to a pressure treatment for 10 minutes under the conditions shown in Table 2 described below, thereby obtaining a precursor composition 12 in the form of a flat plate measuring 150 mm in length, 80 mm in width, and 2 mm in thickness, containing a crosslinked product of fluorine-containing copolymer 3. Next, the precursor composition 12 was placed in an oven ("Fine Oven DH612", manufactured by Yamato Scientific Co., Ltd.) and heat-treated at 340°C for 48 hours in an air atmosphere, thereby obtaining a rubber composition 12 in the form of a flat plate measuring 150 mm in length, 80 mm in width, and 2 mm in thickness, containing a crosslinked product of fluorine-containing copolymer 3.

[0096] Examples 13 to 14 Rubber compositions 13 and 14 containing a crosslinked product of fluorocopolymer 3 were obtained according to the method described in Example 12, except that when producing the rubber compositions, press apparatus C described later was used and pressure treatment was performed under the conditions shown in Table 2. A 500T hydraulic press (model: PV-500 500T type, manufactured by PAN STONE PRECISION INDUSTRIES CO., LTD., ram diameter: 599 mm) was prepared as press apparatus C. The mold cavity included in press apparatus C was rectangular, measuring 150 mm in length and 80 mm in width. The crosslinkable composition was filled into the mold cavity and pressure treatment was performed for 10 minutes under the conditions shown in Table 2 described later, to obtain a precursor composition containing a crosslinked product of the fluorocopolymer and having a plate shape measuring 150 mm in length, 80 mm in width, and 2 mm in thickness.

[0097] Example 15 (Production of Crosslinkable Composition) 100 parts by mass of fluorine-containing copolymer 4, 1.6 parts by mass of TAIC (trade name, manufactured by Mitsubishi Chemical Corporation, triallyl isocyanurate, crosslinking aid), 0.6 parts by mass of Perhexa 25B (trade name, manufactured by NOF Corporation, 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane, organic peroxide), and 0.1 parts by mass of calcium stearate were blended and kneaded with a twin roll at room temperature for 10 minutes, to obtain a mixed crosslinkable composition 15.

[0098] (Production of Rubber Composition Containing Crosslinked Product of Fluorine-Containing Copolymer) A 50T hydraulic press (model: SA-301 50T type, manufactured by Tester Sangyo Co., Ltd., ram diameter: 180 mm) was prepared as press apparatus A. The cavity of the mold equipped with press apparatus A was rectangular, measuring 150 mm in length and 80 mm in width. The crosslinkable composition 15 was filled into the cavity of the mold and subjected to a pressure treatment for 10 minutes under the conditions shown in Table 2 described below, thereby obtaining a precursor composition 15 in the form of a flat plate measuring 150 mm in length, 80 mm in width, and 2 mm in thickness, containing a crosslinked product of fluorine-containing copolymer 4. Next, the precursor composition 15 was placed in an oven ("Fine Oven DH612", manufactured by Yamato Scientific Co., Ltd.) and heat-treated at 250°C for 24 hours in an air atmosphere, thereby obtaining a rubber composition 15 in the form of a flat plate measuring 150 mm in length, 80 mm in width, and 2 mm in thickness, containing a crosslinked product of fluorine-containing copolymer 4.

[0099] [Examples 16 to 17] Rubber compositions 16 to 17 containing a crosslinked product of fluorine-containing copolymer 4 were obtained according to the method described in Example 15, except that the rubber compositions were pressurized using press B or press C under the conditions shown in Table 2.

[0100] Example 18 Production of Crosslinkable Composition 100 parts by mass of Fluorocopolymer 5, 2.0 parts by mass of TAIC (trade name, manufactured by Mitsubishi Chemical Corporation, triallyl isocyanurate, crosslinking aid), 1.0 part by mass of Perhexa 25B (trade name, manufactured by NOF Corporation, 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane, organic peroxide), and 0.1 part by mass of calcium stearate were blended and kneaded with a twin roll at room temperature for 10 minutes, to obtain a mixed crosslinkable composition 18.

[0101] (Production of Rubber Composition Containing Crosslinked Product of Fluorine-Containing Copolymer) A 50T hydraulic press (model: SA-301 50T type, manufactured by Tester Sangyo Co., Ltd., ram diameter: 180 mm) was prepared as press apparatus A. The cavity of the mold equipped with press apparatus A was rectangular, measuring 150 mm in length and 80 mm in width. The crosslinkable composition 18 was filled into the cavity of the mold and subjected to a pressure treatment for 10 minutes under the conditions shown in Table 2 described below, thereby obtaining a precursor composition 18 in the form of a flat plate measuring 150 mm in length, 80 mm in width, and 2 mm in thickness, containing a crosslinked product of fluorine-containing copolymer 5. Next, the precursor composition 18 was placed in an oven ("Fine Oven DH612", manufactured by Yamato Scientific Co., Ltd.) and heat-treated at 180°C for 4 hours in an air atmosphere, thereby obtaining a rubber composition 18 in the form of a flat plate measuring 150 mm in length, 80 mm in width, and 2 mm in thickness, containing a crosslinked product of fluorine-containing copolymer 5.

[0102] [Examples 19 to 21] Rubber compositions 19 to 21 containing a crosslinked product of fluorine-containing copolymer 5 were obtained according to the method described in Example 18, except that the rubber composition was produced by using press A or press C under the conditions shown in Table 2.

[0103] [Measurements] The molded rubber compositions 1 to 21 obtained in Examples 1 to 21 were subjected to the following measurements.

[0104] <Dynamic Viscoelasticity Measurement> Test specimens measuring 85 mm in length and 45 mm in width were prepared by punching out the flat-plate-shaped rubber compositions 1 to 21 obtained in each example using a lever-type sample cutter (model: SDL-100, manufactured by Dumbbell Co., Ltd.). When preparing the test specimens, the flat-plate-shaped rubber composition and the test specimen were set so that their longitudinal directions were parallel and their centers were aligned, and then punched out. The obtained test specimens were attached to a viscoelasticity measuring device ("DMA7100", manufactured by Hitachi High-Tech Corporation), and dynamic viscoelasticity measurements were carried out under the following measurement conditions. From the measurement results, the minimum value E' of the storage modulus E' in the range of 23 to 200°C was ｍｉｎ (unit: Pa), and minimum value E' ｍｉｎ The temperature T (unit: ° C.) at which the storage modulus was reached was determined. The exact dimensions of the test piece were measured before measurement with the viscoelasticity measuring device, and these dimensions were reflected in the calculation of the storage modulus.

[0105] (Temperature conditions) Lamp mode temperature program {start temperature: 23°C, end temperature: 200°C, temperature rise rate: 3°C / min, sampling time: 3 seconds}

[0106] (Setting conditions) Measurement mode: tension DMA frequency: 1 Hz Frequency mode: sine wave Strain amplitude: 10 μm Minimum tension: 100 mN Tension gain: 1.5 Initial force amplitude: 100 mN Gas: Air Before each measurement, an automatic offset adjustment in the measurement program was performed.

[0107] <Haze> The haze (unit: %) of rubber compositions 1 to 21 obtained in each example was measured using a haze meter (product name "NDA500H", manufactured by Nippon Denshoku Industries Co., Ltd.) according to a method conforming to JIS K7105. More specifically, measurement light from the haze meter was incident on the main surface of each flat-plate-shaped rubber composition that faced the mold cavity during the pressure treatment for producing the precursor composition, and the haze was calculated from the measured diffuse transmittance (Td) and total light transmittance (Tt) using the following formula: Haze (%) = Td / Tt × 100 The haze measurement was performed four times, replacing the rubber composition each time, and the arithmetic mean value of the obtained measurements was taken as the haze of the rubber composition of each example.

[0108] Table 1 shows the fluorine-containing copolymer used in the production of the rubber composition, the press device and pressure treatment conditions used in the pressure treatment to produce the precursor composition, and the minimum value E' of the storage modulus E' in each example. ｍｉｎ the temperature T at which the storage elastic modulus is at a minimum value E' ｍｉｎ、 The table also shows the measurement results of the haze of the rubber composition. In the table, "gauge pressure [MPa]" indicates the pressure applied to the cylinder of each press device during pressure treatment. Furthermore, "surface pressure [MPa]" is the pressure applied to the crosslinkable composition filled in the cavity of the mold, and indicates the surface pressure Y calculated using the above formula (P) from the contact area A where the cylinder contacts the slide, the area B of the region obtained by orthogonally projecting the space occupied by the crosslinkable composition filled in the cavity of the mold in the opening and closing direction of the mold, and the gauge pressure C. The contact area A is calculated from the ram diameter of each press device.

[0109]

[0110]

[0111] As shown in Tables 1 and 2, the crosslinked product of the fluorine-containing copolymer was included, and in the dynamic viscoelasticity measurement at a measurement temperature of 23 to 200°C and a measurement frequency of 1 Hz, the minimum value of the storage modulus E' (E' ｍｉｎ It was confirmed that a rubber composition having a temperature T of 55°C or higher exhibits lower haze than a rubber composition having a temperature T of less than 55°C. The entire contents of the specification, claims, drawings and abstract of Japanese Patent Application No. 2023-172422 filed on October 4, 2023 are incorporated herein by reference as part of the disclosure of the present invention.

[0112] REFERENCE SIGNS LIST 1 Cylinder 2 Slide 3 Upper die 3a Core 4 Lower die 4a Cavity 5 Bolster 6 Pressure gauge 10 Press device

Claims

1. A rubber composition comprising a cross-linked product of a fluorine-containing copolymer, wherein the temperature T at which the storage modulus E' has a minimum value is 55°C or higher in dynamic viscoelasticity measurement at a measurement temperature of 23 to 200°C and a measurement frequency of 1 Hz.

2. The rubber composition according to claim 1, which does not contain a black filler.

3. The rubber composition according to claim 1 or 2, wherein the fluorine-containing copolymer contains units based on tetrafluoroethylene and units based on perfluoromethylvinyl ether.

4. The rubber composition according to claim 3, wherein in said fluorine-containing copolymer, the ratio of the content of units based on said tetrafluoroethylene to the content of units based on said perfluoromethylvinyl ether is 73 / 27 to 65 / 35 in terms of molar ratio.

5. The rubber composition according to claim 1 or 2, wherein the crosslinked product is a crosslinked product of the fluorine-containing copolymer and a compound having a plurality of double bonds.

6. A method for producing a rubber composition, comprising: subjecting a crosslinkable composition containing a fluorine-containing copolymer and a crosslinking agent to a pressure treatment to obtain a precursor composition; and subjecting the precursor composition obtained by the pressure treatment to a heat treatment. The pressure treatment applies to the crosslinkable composition a pressure such that the surface pressure is 6.0 MPa or less at a temperature of 150°C or higher and less than 155°C, or a pressure such that the surface pressure is 10 MPa or less at a temperature of 155°C or higher.