Method for decomposing fluorine-containing polymers

Abstract

The purpose of the present disclosure is to provide a method for decomposing fluorine-containing polymers, with which it is possible to break down fluorine-containing polymers while suppressing the production of impurities after a decomposition reaction. The present disclosure is a method for breaking down fluorine-containing polymers, the method including a decomposition step in which fluorine-containing polymers are reacted in subcritical water in which the concentration of at least one basic compound selected from the group consisting of an alkali metal hydroxide and an alkaline earth metal hydroxide is 0.5 M or less, at a temperature of 180°C to less than 250°C.

Description

Method for decomposing fluorine-containing polymers

[0001] The present disclosure relates to a method for degrading a fluorine-containing polymer.

[0002] Because fluorine-containing polymers have high chemical stability and thermal durability, not only do they require high temperatures for incineration in waste disposal, but the hydrogen fluoride produced therefrom significantly deteriorates incinerator materials. Therefore, a new waste disposal method that does not involve incineration or landfilling is desired. For example, Patent Documents 1 and 2 and Non-Patent Document 1 describe methods for decomposing fluorine-containing polymers by reacting them in subcritical water containing a basic compound such as potassium hydroxide.

[0003] JP 2021-155478 A JP 2023-066742 A

[0004] Jin Hamaura et al. , Eur. Polym. J. 182 (2023), 111724

[0005] An object of the present disclosure is to provide a method for decomposing a fluorine-containing polymer, which can decompose the fluorine-containing polymer while suppressing the generation of impurities after the decomposition reaction.

[0006] The present disclosure (1) relates to a method for decomposing a fluorine-containing polymer, which includes a decomposition step of reacting the fluorine-containing polymer in subcritical water at a temperature of 180°C or higher and lower than 250°C, and containing at least one basic compound selected from the group consisting of alkali metal hydroxides and alkaline earth metal hydroxides, in a concentration of 0.5 M or lower.

[0007] The present disclosure (2) is the method for decomposing a fluorine-containing polymer according to the present disclosure (1), wherein the temperature of the subcritical water in the decomposition step is 200° C. or higher.

[0008] The present disclosure (3) is the method for decomposing a fluorine-containing polymer according to the present disclosure (1) or (2), wherein the fluorine-containing polymer is a fluororubber.

[0009] The present disclosure (4) is the method for decomposing a fluorine-containing polymer according to the present disclosure (3), wherein the temperature of the subcritical water in the decomposition step is 200°C or higher and 235°C or lower, the concentration of the basic compound in the subcritical water is 0.10 M or higher and 0.35 M or lower, and the fluororubber is a copolymer consisting of 50 to 80 mol % of vinylidene fluoride and 50 to 20 mol % of hexafluoropropylene.

[0010] The present disclosure (5) is the method for decomposing a fluorine-containing polymer according to the present disclosure (3) or (4), in which the decomposition target in the decomposition step is a crosslinked product of a composition containing the fluororubber.

[0011] The present disclosure (6) is a method for decomposing a fluorine-containing polymer in any combination with any of the present disclosures (1) to (5), which includes, prior to the decomposition step, a grinding step of grinding the fluorine-containing polymer into ground particles having a maximum linear length of 3 mm or less.

[0012] The present disclosure (7) is a method for decomposing a fluorine-containing polymer in any combination with any of the present disclosures (1) to (6), in which the basic compound is sodium hydroxide and / or potassium hydroxide.

[0013] The present disclosure (8) is a method for decomposing a fluorine-containing polymer in any combination with any of the present disclosures (1) to (7), which includes, after the decomposition step, a recovery step of adding calcium hydroxide to the aqueous solution obtained after the decomposition step and recovering the calcium fluoride produced.

[0014] According to the present disclosure, it is possible to provide a method for decomposing a fluorine-containing polymer, which can decompose a fluorine-containing polymer while suppressing the generation of impurities after the decomposition reaction.

[0015] The present disclosure will be specifically described below.

[0016] The present disclosure relates to a method for decomposing a fluorine-containing polymer, which comprises a decomposition step of reacting the fluorine-containing polymer in subcritical water at a temperature of 180° C. or higher and lower than 250° C., and containing at least one basic compound selected from the group consisting of alkali metal hydroxides and alkaline earth metal hydroxides, in a concentration of 0.5 M or lower. The method for decomposing a fluorine-containing polymer of the present disclosure has the above-mentioned configuration, and therefore can decompose the fluorine-containing polymer while suppressing the generation of impurities after the decomposition reaction.

[0017] In the present disclosure, a fluorine-containing polymer is treated in subcritical water containing a basic compound. During this treatment, it is believed that the basic compound extracts hydrogen fluoride (HF) from the polymer, causing decomposition of the polymer. According to this mechanism, it is believed that polymers that do not contain hydrogen atoms, such as polytetrafluoroethylene (PTFE), are difficult to decompose using the present disclosure. Therefore, polymers that do not contain hydrogen atoms are preferably excluded from the application of the present disclosure.

[0018] In addition, in the present disclosure, as described above, it is considered that decomposition occurs by abstracting HF from the polymer, and although fluorine atoms are removed from the polymer, the polymer is not completely decomposed to carbon dioxide. However, since the removal of fluorine atoms from the polymer is realized, in the present disclosure, even decomposition that does not completely decompose the polymer to carbon dioxide is treated as decomposition of the polymer.

[0019] Subcritical water is water in a liquid state that is pressurized to a temperature range above 100°C but lower than the critical temperature of 374°C. Subcritical water has different physical properties from water below 100°C. In particular, subcritical water in the range of 200°C to 300°C has a significantly reduced dielectric constant, exhibiting lipid solubility almost equivalent to that of methanol or acetone at room temperature, and exhibiting lipid solubility of 10% at room temperature. －１４ The ion product, which was mol / L, is 10 －１１ The temperature is on the order of mol / L, and the concentrations of hydrogen ions and hydroxide ions are 30 times higher than in water at room temperature. For this reason, it is known that subcritical water, particularly at 200°C to 300°C, exhibits a different reactivity than water at room temperature. In the decomposition method of the present disclosure, the temperature of the subcritical water in the decomposition step is less than 250°C, preferably 245°C or less, more preferably 240°C or less, and even more preferably 235°C or less, and is 180°C or higher, preferably 200°C or higher, more preferably 205°C or higher, even more preferably 210°C or higher, even more preferably 215°C or higher, particularly preferably 220°C or higher, and especially preferably 225°C or higher.

[0020] The water used to prepare subcritical water is not particularly limited, and any type of water may be used, such as tap water, ion-exchanged water, distilled water, or well water, but ion-exchanged water or distilled water is preferred from the viewpoint of suppressing side reactions due to the influence of coexisting salts, etc. The amount of water used should be enough to fully immerse the fluorine-containing polymer to be treated, but care must be taken if the amount of water introduced into the sealed container for pressurization is extremely small, as the water will all turn into steam after heating and will not become subcritical water.

[0021] In the decomposition method of the present disclosure, at least one basic compound selected from the group consisting of alkali metal hydroxides and alkaline earth metal hydroxides is used. As already mentioned, the basic compound is used to cause a reaction of abstracting HF from the fluorine-containing polymer.

[0022] Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide. Among these, sodium hydroxide and / or potassium hydroxide are preferred. Potassium hydroxide is known to have a higher activity and higher basicity than sodium hydroxide in a concentrated solution, and is expected to have high activity in the HF abstraction reaction. In fact, in the present disclosure, it has been confirmed that when potassium hydroxide is used to decompose a fluorine-containing polymer, the activity is higher than when sodium hydroxide is used. From this perspective, potassium hydroxide is said to be the most preferred.

[0023] Examples of alkaline earth metal hydroxides include calcium hydroxide and barium hydroxide.

[0024] The concentration of the basic compound in the subcritical water is 0.5 M or less, preferably 0.45 M or less, more preferably 0.40 M or less, and even more preferably 0.35 M or less, and is preferably 0.01 M or more, more preferably 0.05 M or more, even more preferably 0.10 M or more, and even more preferably 0.15 M or more. As is well known to those skilled in the art, the unit "M" means mol / L.

[0025] The pH of the subcritical water is preferably 12 or higher, more preferably 12.7 or higher, and even more preferably 13 or higher, and is preferably 13.7 or lower, more preferably 13.6 or lower, and even more preferably 13.5 or lower.

[0026] Next, a method for reacting a fluorine-containing polymer in subcritical water containing a basic compound will be described. Water, a basic compound, and the fluorine-containing polymer to be treated are placed in a pressure vessel of a size appropriate for the amount of the fluorine-containing polymer to be treated, and the pressure vessel is pressurized and sealed. To pressurize the pressure vessel, a gas may be enclosed. Examples of such gases include air, argon, and nitrogen. The degree of pressurization may be about 0.5 MPa, but is not particularly limited.

[0027] The pressure vessel that has undergone the above process is heated to initiate the decomposition reaction. The heating temperature is less than 250°C, but is preferably 245°C or less, more preferably 240°C or less, and even more preferably 235°C or less. It is also preferably 180°C or more, more preferably 200°C or more, more preferably 205°C or more, even more preferably 210°C or more, even more preferably 215°C or more, particularly preferably 220°C or more, and even more preferably 225°C or more. If the pressure vessel itself is equipped with a heating means, heating can be performed using that heating means. If the pressure vessel itself is not equipped with a heating means, the entire pressure vessel can be heated in an autoclave or oven. The reaction time can be about 6 to 24 hours.

[0028] The aqueous solution after the decomposition reaction contains fluorine atoms contained in the fluorine-containing polymer in the form of fluoride ions. The decomposition method of the present disclosure preferably includes, after the decomposition step, a recovery step of adding calcium hydroxide to the aqueous solution after the decomposition and recovering the calcium fluoride produced. The fluoride ions contained in the aqueous solution after the decomposition can be converted into calcium fluoride, which is a raw material for all fluorine compounds, by reacting with calcium ions generated from the added calcium hydroxide. The decomposition method of the present disclosure includes the recovery step, thereby enabling effective use of resources. Furthermore, as described above, the decomposition method of a fluorine-containing polymer of the present disclosure can suppress the generation of impurities after the decomposition reaction, and therefore can reduce impurities when recovering calcium fluoride.

[0029] After the decomposition reaction is complete, the aqueous solution contains not only fluoride ions but also polymer decomposition products remaining after HF extraction. As mentioned above, these decomposition products are considered to be elemental carbon with double bonds and are often black in color. Since they are solid, they can be easily separated, and since the fluorine atoms have been removed in the recovery process, they can be easily disposed of by incineration.

[0030] The decomposition method of the present disclosure preferably includes, prior to the decomposition step, a pulverization step of pulverizing the fluorine-containing polymer into pulverized particles having a maximum linear length of 3 mm or less. By including the pulverization step in the decomposition method of the present disclosure, the fluorine-containing polymer can be easily decomposed.

[0031] The pulverization method is not particularly limited, but includes freeze pulverization, disk mill, hammer mill, stone mill, jet mill, and the like.

[0032] The grinding temperature is preferably −200° C. or higher and 90° C. or lower, more preferably 40° C. or lower, and even more preferably 0° C. or lower.

[0033] The maximum linear length of the pulverized particles of the fluorine-containing polymer is preferably 3 mm or less, more preferably 2 mm or less, even more preferably 1 mm or less, and particularly preferably 500 μm or less. Also, it is preferably 10 μm or more, more preferably 30 μm or more, and even more preferably 100 μm or more. When the average particle size of the pulverized particles of the fluorine-containing polymer is within the above range, the fluorine-containing polymer can be more easily decomposed.

[0034] The maximum linear length was determined by observing 100 or more secondary particles using SEM images, measuring the major axis of each secondary particle, and determining the largest major axis among the secondary particles.

[0035] The transmittance of the aqueous solution after the decomposition reaction can be measured using an Agilent Cary 7000 UMS. The average value of %T from 380 to 780 nm was taken as the transmittance. The higher the transmittance value, the lighter the color of the solution and the fewer impurities there are.

[0036] The transmittance of the aqueous solution after the decomposition reaction is preferably 40% T or more, more preferably 50% T or more, and even more preferably 70% T or more. There is no particular upper limit, and it may be 100% T, but it is usually about 90% T. When the transmittance of the aqueous solution after the decomposition reaction is within the above range, calcium hydroxide can be added to the aqueous solution after the decomposition step, and the amount of impurities contained in the calcium fluoride produced can be reduced.

[0037] The fluorine-containing polymer to be decomposed in the present disclosure is a polymer containing a fluorine atom in the molecule, and any polymer containing even one fluorine atom in the molecule is subject to decomposition in the present disclosure. Fluorine-containing polymers are valued for their high chemical resistance, heat resistance, weather resistance, and other properties, and are used in a variety of fields, including industry and medicine. On the other hand, these polymers are not easily decomposed, as a result of their high chemical stability and high thermal durability. The present disclosure provides a method for chemically decomposing these polymers. Such fluorine-containing polymers preferably have hydrogen atoms in the molecule. Furthermore, the fluorine-containing polymer may be a fluororesin or a fluororubber, and may be a homopolymer or a copolymer.

[0038] Examples of the fluororesin include polytetrafluoroethylene [PTFE], tetrafluoroethylene [TFE] / perfluoro(alkyl vinyl ether) [PAVE] copolymer [PFA], TFE / hexafluoropropylene [HFP] copolymer [FEP], ethylene [Et] / TFE copolymer [ETFE], Et / TFE / HFP copolymer [EFEP], polychlorotrifluoroethylene [PCTFE], chlorotrifluoroethylene [CTFE] / TFE copolymer, CTF Examples of the perfluoroalkyl allyl ether include E / TFE / PAVE copolymer, Et / CTFE copolymer, polyvinyl fluoride [PVF], polyvinylidene fluoride [PVdF], vinylidene fluoride [VdF] / TFE copolymer, VdF / HFP copolymer, VdF / TFE / HFP copolymer, VdF / HFP / (meth)acrylic acid copolymer, VdF / CTFE copolymer, VdF / pentafluoropropylene copolymer, VdF / PAVE / TFE copolymer, and TFE / perfluoroalkyl allyl ether copolymer. ２ =CFCF ２ -O-Rf ４ (Rf ４ is a perfluoroalkyl group having 1 to 5 carbon atoms).

[0039] The fluororesin may be a partially fluorinated polymer or a perfluoropolymer, with the partially fluorinated polymer being preferred.

[0040] Among these, the fluororesin is preferably at least one selected from the group consisting of ETFE, EFEP, Et / CTFE copolymer, PVF, PVdF, VdF / TFE copolymer, VdF / HFP copolymer, VdF / TFE / HFP copolymer, VdF / HFP / (meth)acrylic acid copolymer, VdF / CTFE copolymer, VdF / pentafluoropropylene copolymer, and VdF / PAVE / TFE copolymer, and more preferably VdF / HFP copolymer or VdF / TFE / HFP copolymer.

[0041] The number of carbon atoms in the perfluoroalkyl group in the PAVE is preferably 1 to 10, and more preferably 1 to 5. Examples of the perfluoroalkyl group include a perfluoromethyl group, a perfluoroethyl group, a perfluoropropyl group, a perfluorobutyl group, a perfluoropentyl group, and a perfluorohexyl group, and perfluoro(propyl vinyl ether) [PPVE] in which the perfluoroalkyl group is a perfluoropropyl group is preferred.

[0042] The fluororubber may be a partially fluorinated rubber or a perfluororubber, with the partially fluorinated rubber being preferred.

[0043] Examples of the partially fluorinated rubber include vinylidene fluoride (VdF)-based fluororubbers, tetrafluoroethylene (TFE) / propylene (Pr)-based fluororubbers, tetrafluoroethylene (TFE) / propylene / vinylidene fluoride (VdF)-based fluororubbers, ethylene / hexafluoropropylene (HFP)-based fluororubbers, ethylene / hexafluoropropylene (HFP) / vinylidene fluoride (VdF)-based fluororubbers, ethylene / hexafluoropropylene (HFP) / tetrafluoroethylene (TFE)-based fluororubbers, etc. Among these, at least one selected from the group consisting of vinylidene fluoride-based fluororubbers and tetrafluoroethylene / propylene-based fluororubbers is preferred, and vinylidene fluoride-based fluororubbers are more preferred.

[0044] The vinylidene fluoride-based fluororubber is preferably a copolymer consisting of 45 to 85 mol % of vinylidene fluoride and 55 to 15 mol % of at least one other monomer copolymerizable with vinylidene fluoride, and more preferably a copolymer consisting of 50 to 80 mol % of vinylidene fluoride and 50 to 20 mol % of at least one other monomer copolymerizable with vinylidene fluoride.

[0045] In this specification, the content of each monomer constituting the fluoropolymer can be calculated by appropriately combining NMR, FT-IR, elemental analysis, and X-ray fluorescence analysis depending on the type of monomer.

[0046] Examples of at least one other monomer copolymerizable with vinylidene fluoride include tetrafluoroethylene [TFE], hexafluoropropylene [HFP], fluoroalkyl vinyl ether, trifluoroethylene, trifluoropropylene, pentafluoropropylene, trifluorobutene, tetrafluoroisobutene, hexafluoroisobutene, vinyl fluoride, perfluoroalkyl allyl ether, and a compound represented by the general formula (6): CH ２ = CFRf ６１ (wherein, Rf ６１ is a linear or branched fluoroalkyl group having 1 to 12 carbon atoms), a fluoromonomer represented by the general formula (7): CH ２ =CH-(CF ２ ) ｎ -X ２ (In the formula, X ２ is H or F, and n is an integer of 3 to 10. Examples of suitable fluoromonomers include fluoromonomers represented by the general formula (8): CF, monomers that provide crosslinking sites, and non-fluorinated monomers such as ethylene, propylene, and alkyl vinyl ethers. These may be used alone or in any combination. Among these, it is preferable to use at least one selected from the group consisting of TFE, HFP, and fluoroalkyl vinyl ethers, and it is more preferable to use HFP. Examples of suitable fluoroalkyl vinyl ethers include fluoromonomers represented by the general formula (8): CF ２ =CF-ORf ８１ (wherein, Rf ８１ represents a perfluoroalkyl group having 1 to 8 carbon atoms, a fluoromonomer represented by general formula (9): CF ２ = CFOCF ２ ORf ９１ (wherein, Rf ９１ is a linear or branched perfluoroalkyl group having 1 to 6 carbon atoms, a cyclic perfluoroalkyl group having 5 to 6 carbon atoms, or a linear or branched perfluorooxyalkyl group having 2 to 6 carbon atoms and containing 1 to 3 oxygen atoms), and a fluoromonomer represented by the general formula (10): CF ２ = CFO (CF ２ CF (Y １０ ) O) ｍ (CF ２) ｎ F (wherein, Y １０ represents a fluorine atom or a trifluoromethyl group. m is an integer of 1 to 4. n is an integer of 1 to 4.) and more preferably a fluoromonomer represented by general formula (8). The perfluoroalkyl allyl ether is preferably at least one selected from the group consisting of fluoromonomers represented by general formula (11): CF ２ =CF-CF ２ ORf １１１ (wherein, Rf １１１ represents a perfluoroalkyl group having 1 to 5 carbon atoms.

[0047] Specific examples of vinylidene fluoride-based fluororubbers include VdF / HFP-based rubbers, VdF / HFP / TFE-based rubbers, VdF / fluoromonomer-based rubbers represented by general formula (6), VdF / fluoromonomer-based rubbers represented by general formula (6) / TFE-based rubbers, VdF / perfluoro(methyl vinyl ether) [PMVE]-based rubbers, VdF / PMVE / TFE-based rubbers, and VdF / PMVE / TFE / HFP-based rubbers. Examples of VdF / fluoromonomer-based rubbers represented by general formula (6) include VdF / CH ２ =CFCF ３ The VdF / fluoromonomer represented by general formula (6) / TFE rubber is preferably VdF / TFE / CH ２ =CFCF ３ Rubbers based on ethylenediaminetetraacetic acid are preferred.

[0048] The above VdF / CH ２ =CFCF ３ The rubber is a rubber containing 40 to 99.5 mol% of VdF and CH ２ =CFCF ３ It is preferable that the copolymer is composed of 0.5 to 60 mol % of VdF, 50 to 85 mol % of CH ２ =CFCF ３ A copolymer consisting of 15 to 50 mol % is more preferred.

[0049] The tetrafluoroethylene / propylene-based fluororubber is preferably a copolymer consisting of 45 to 70 mol % of tetrafluoroethylene, 55 to 30 mol % of propylene, and 0 to 5 mol % of a fluoromonomer that provides a crosslinking site.

[0050] The monomer that provides a crosslinking site is a monomer (cure site monomer) having a crosslinkable group that provides a crosslinking site to the fluoropolymer for forming a crosslink with a crosslinking agent.

[0051] The monomer that provides the crosslinking site is represented by the general formula (12): CX ３ ２ =CX ３ -R ｆ １２１ CHR １２１ X ４ (In the formula, X ３ is a hydrogen atom, a fluorine atom or CH ３ , R ｆ １２１ represents a fluoroalkylene group, a perfluoroalkylene group, a fluoro(poly)oxyalkylene group or a perfluoro(poly)oxyalkylene group, R １２１ is a hydrogen atom or CH ３ , X ４ is an iodine atom or a bromine atom), a fluoromonomer represented by the general formula (13): CX ３ ２ =CX ３ -R ｆ １３１ X ４ (In the formula, X ３ is a hydrogen atom, a fluorine atom or CH ３ , R ｆ １３１ represents a fluoroalkylene group, a perfluoroalkylene group, a fluoropolyoxyalkylene group or a perfluoropolyoxyalkylene group, X ４ is an iodine atom or a bromine atom), a fluoromonomer represented by general formula (14): CF ２ = CFO (CF ２ CF (CF ３ ) O) ｍ (CF ２ ) ｎ -X ５(wherein m is an integer of 0 to 5, n is an integer of 1 to 3, X ５ represents a cyano group, a carboxyl group, an alkoxycarbonyl group, an iodine atom, a bromine atom, or —CH ２ I), and a fluoromonomer represented by general formula (15): CH ２ =CFCF ２ O(CF(CF ３ )CF ２ O) ｍ (CF (CF ３ )) ｎ -X ６ (wherein m is an integer of 0 to 5, n is an integer of 1 to 3, X ６ represents a cyano group, a carboxyl group, an alkoxycarbonyl group, an iodine atom, a bromine atom, or —CH ２ OH), and a fluoromonomer represented by general formula (16): CR １６２ R １６３ =CR １６４ -Z-CR １６５ =CR １６６ R １６７ (In the formula, R １６２ , R １６３ , R １６４ , R １６５ , R １６６ and R １６７ , are the same or different and each represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms. Z represents a linear or branched alkylene group having 1 to 18 carbon atoms, a cycloalkylene group having 3 to 18 carbon atoms, or an at least partially fluorinated alkylene or oxyalkylene group having 1 to 10 carbon atoms, which may have an oxygen atom; or -(Q) ｐ -CF ２ O-(CF ２ CF ２ O) ｍ (CF ２ O) ｎ -CF ２ -(Q) ｐ - (wherein Q is an alkylene group or an oxyalkylene group, p is 0 or 1, and m / n is 0.2 to 5), and is a (per)fluoropolyoxyalkylene group having a molecular weight of 500 to 10,000.

[0052] X ３ is preferably a fluorine atom. ｆ １２１ and R ｆ １３１ is preferably a perfluoroalkylene group having 1 to 5 carbon atoms. １２１ is preferably a hydrogen atom. ５ represents a cyano group, an alkoxycarbonyl group, an iodine atom, a bromine atom, or —CH ２ Preferably, X is I. ６ represents a cyano group, an alkoxycarbonyl group, an iodine atom, a bromine atom, or —CH ２ It is preferably OH.

[0053] The monomer that provides the crosslinking site is CF ２ = CFOCF ２ CF (CF ３ ) OCF ２ CF ２ C.N., C.F. ２ = CFOCF ２ CF (CF ３ ) OCF ２ CF ２ COOH, C.F. ２ = CFOCF ２ CF (CF ３ ) OCF ２ CF ２ CH ２ I, CF ２ = CFOCF ２ CF ２ CH ２ I, CH ２ =CFCF ２ OCF (CF ３ )CF ２ OCF (CF ３ ) CN, CH ２ =CFCF ２ OCF (CF ３ )CF ２ OCF (CF ３ ) COOH, CH ２ =CFCF ２ OCF (CF ３ )CF ２ OCF (CF ３ ) CH ２OH, CH ２ = CHCF ２ CF ２ I, CH ２ =CH(CF ２ ) ２ CH=CH ２ , C.H. ２ =CH(CF ２ ) ６ CH=CH ２ , and CF ２ = CFO (CF ２ ) ５ CN, and CF ２ = CFOCF ２ CF (CF ３ ) OCF ２ CF ２ CN and CF ２ = CFOCF ２ CF ２ CH ２ It is more preferable that the compound is at least one selected from the group consisting of I.

[0054] The fluororubber has a glass transition temperature of preferably −70° C. or higher, more preferably −60° C. or higher, and even more preferably −50° C. or higher, from the viewpoint of excellent compression set characteristics at high temperatures. Also, from the viewpoint of good cold resistance, the glass transition temperature is preferably 5° C. or lower, more preferably 0° C. or lower, and even more preferably −3° C. or lower.

[0055] The glass transition temperature can be determined by obtaining a DSC curve using a differential scanning calorimeter (DSC822e, manufactured by Mettler Toledo) by heating 10 mg of a sample at a rate of 10°C / min, and by determining the temperature as the midpoint between two intersections of an extension of the baseline of the DSC curve before and after the second-order transition and a tangent to the inflection point of the DSC curve.

[0056] The fluororubber has good heat resistance and a Mooney viscosity ML at 170°C. （１＋２０）is preferably 30 or more, more preferably 40 or more, and even more preferably 50 or more. In terms of good processability, it is preferably 150 or less, more preferably 120 or less, and even more preferably 110 or less.

[0057] The above-mentioned fluororubber has good heat resistance and a Mooney viscosity ML at 140°C. （１＋２０） is preferably 30 or more, more preferably 40 or more, and even more preferably 50 or more. In terms of good processability, it is preferably 180 or less, more preferably 150 or less, and even more preferably 110 or less.

[0058] The fluororubber has good heat resistance and has a Mooney viscosity of ML at 100°C. （１＋１０） is preferably 10 or more, more preferably 20 or more, and even more preferably 30 or more. In terms of good processability, it is preferably 120 or less, more preferably 100 or less, and even more preferably 80 or less.

[0059] The Mooney viscosity can be measured using a Mooney viscometer MV2000E manufactured by ALPHA TECHNOLOGIES at 170°C, 140°C, or 100°C in accordance with JIS K6300.

[0060] In the decomposition method of the present disclosure, the decomposition target of the decomposition step may be a crosslinked product of a composition containing the fluororubber. The composition preferably contains the fluororubber and a crosslinking agent.

[0061] Examples of the crosslinking agent include crosslinking agents used in peroxide crosslinking, polyol crosslinking, polyamine crosslinking, triazine crosslinking, oxazole crosslinking, imidazole crosslinking, and thiazole crosslinking. When the fluororubber is a fluororubber having a cyano group (-CN group), the crosslinking agent is preferably at least one selected from the group consisting of an oxazole crosslinking agent, an imidazole crosslinking agent, and a thiazole crosslinking agent.

[0062] The crosslinking agent used in peroxide crosslinking may be any organic peroxide that can easily generate peroxy radicals in the presence of heat or a redox system, and specifically, for example, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane can be used. In general, the type and amount of organic peroxide used are selected taking into consideration the amount of active -O-O-, decomposition temperature, etc.

[0063] In addition, the crosslinking aid that can be used in this case may be any compound that has reactivity with peroxy radicals and polymer radicals, such as -CH=CH ２ , -CH ２ CH=CH ２ , -CF=CF ２ , -C(CF ３ ) = CF ２ , -C(CH ３ ) = CF ２ , -CF=CF(CF ３ ), -CF=CF(CH ３ ), -C(C ６ H ５ ) = CF ２ , -CF=CF(C ６ H ５ ), —CH═CF ２ , -CF=CHF, -C(CF ３ )=CHF, -CF=CH(CF ３ ), -CH=CF(CF ３ ) and other polyfunctional compounds having functional groups ("C" in each formula) ６ H ５ " represents a phenyl group). Specific examples include triallyl cyanurate, triallyl isocyanurate (TAIC), triacryl formal, triallyl trimellitate, N,N'-n-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, and 1,6-divinyldodecafluorohexane.

[0064] Furthermore, examples of the crosslinking aid used together with the peroxide crosslinking agent include compounds represented by the general formula (48): (Wherein, six R ３１ are each independently H, a halogen atom, or an optionally halogenated group having 1 to 5 carbon atoms which may contain an ether bond; Z ３１ can also include compounds represented by a linear or branched alkylene group, cycloalkylene group, or (per)fluoropolyoxyalkylene group having 1 to 18 carbon atoms, which optionally contains a heteroatom and is optionally halogenated.

[0065] The compound represented by general formula (48) includes compounds represented by general formula (49): (wherein j is an integer of 2 to 10, preferably an integer of 4 to 8, and four R ３２ are each independently H, F, or an alkyl group or (per)fluoroalkyl group having 1 to 5 carbon atoms, a compound represented by general formula (50): (In the formula, Y ３１ are each independently F, Cl or H, and Y ３２ are each independently F, Cl, H or OR ３３ (where R ３３ is a branched or straight chain alkyl group which may be partially, substantially or completely fluorinated or chlorinated; Z ３３ is a divalent group having 2 to 10 carbon atoms, optionally fluorinated, optionally interrupted by ether linkages, preferably Z ３３ m is an integer of 3 to 5, -(CF ２ ) ｍ - group, and the compound represented by the general formula (50) is preferably F ２ C=CF-O-(CF ２ ) ５ -O-CF=CF ２ a compound represented by general formula (51): (In the formula, Y ３１ , Y ３２ and Z ３３ is as described above, and R ３４are each independently H, F, or an alkyl group or (per)fluoroalkyl group having 1 to 5 carbon atoms.

[0066] The crosslinking aid used together with the above crosslinking agent or peroxide crosslinking agent includes a crosslinking aid represented by the general formula (52): (In the formula, R ３５ ~R ３７ are each independently a hydrogen atom, a fluorine atom, an alkyl group, a fluorinated alkyl group, or a substituted or unsubstituted aryl group, and R ３５ ~R ３７ At least one of R is a fluorine atom or a group containing a fluorine atom, and m is an integer of 1 to 5. When m is 2 or more, m R ３５ ~R ３７ may be the same or different. A hydrogen atom on the benzene ring may be substituted. When m is 1, it is preferable that the compound has two or more of the structure.

[0067] The compound having the structure represented by general formula (52) includes compounds having the structure represented by general formula (53):

[0068] (In the formula, R ３５ ~R ３７ is as defined above, p is an integer of 0 to 2, and n is an integer of 2 to 6.), a compound represented by the general formula (54): (In the formula, R ３５ ~R ３７ is as above. ３８ is a single bond, -SO ２ -, -O-, -S-, -CO-, a heteroatom-containing group, a substituted or unsubstituted alkylene group, a substituted or unsubstituted cycloalkylene group, or a substituted or unsubstituted arylene group. m is an integer of 1 to 5. These groups may be partially or completely fluorinated.

[0069] The heteroatom-containing group is not particularly limited as long as it is a divalent group containing a heteroatom, and examples of the heteroatom include an oxygen atom, a nitrogen atom, a sulfur atom, a boron atom, and a phosphorus atom.

[0070] Examples of crosslinking agents used for polyol crosslinking include polyhydric alcohol compounds such as bisphenol A and bisphenol AF.

[0071] Examples of crosslinking agents used for polyamine crosslinking include polyamine compounds such as hexamethylenediamine carbamate, N,N'-dicinnamylidene-1,6-hexanediamine, and 4,4'-bis(aminocyclohexyl)methane carbamate.

[0072] Examples of crosslinking agents used for oxazole crosslinking, imidazole crosslinking, and thiazole crosslinking include those represented by the general formula (55):

[0073] (In the formula, R ４１ Ha-SO ２ -, -O-, -CO-, an alkylene group having 1 to 6 carbon atoms, a perfluoroalkylene group having 1 to 10 carbon atoms or a single bond, or

[0074]

[0075] and R ４２ and R ４３ One side is -NH ２ and the other is -NHR ４４ , -NH ２ , —OH or —SH, and R ４４ is a hydrogen atom, a fluorine atom or a monovalent organic group, preferably R ４２ Ga-NH ２ And R ４３ Ga-NHR ４４ Examples of the crosslinking agent include bisdiaminophenyl-based crosslinking agents, bisaminophenol-based crosslinking agents, and bisaminothiophenol-based crosslinking agents represented by the formula (I).

[0076] Preferred specific examples of the alkylene group having 1 to 6 carbon atoms include a methylene group, an ethylene group, a propylene group, a butylene group, a pentylene group, and a hexylene group. Preferred specific examples of the perfluoroalkylene group having 1 to 10 carbon atoms include: etc.

[0077] These compounds are known as examples of bisdiaminophenyl compounds in Japanese Patent Publication No. 2-59177, Japanese Patent Application Laid-Open No. 8-120146, etc.

[0078] The crosslinking agent used for the oxazole crosslinking, imidazole crosslinking, and thiazole crosslinking also includes a compound represented by the general formula (56): (In the formula, R ４１ As mentioned above, R ４５ are each independently any of the following groups: ), a bisamidrazone-based crosslinking agent represented by general formula (57): (wherein, Rf ４１ is a perfluoroalkylene group having 1 to 10 carbon atoms), an amidrazone crosslinking agent represented by the general formula (58): (wherein n is an integer of 1 to 10), a bisamidoxime crosslinking agent represented by the general formula (59): HN=CR ４５ R ４６ (In the formula, R ４５ , H, NH ２ , and N.H.R. ４７ and R ４６ is Ph, SO ２ H, N.R. ４８ R ４９ , 2-pyridine, and CH ２ CONH ２ and R ４７ Ph, NH ２ and CN, R ４８ is H, NHPh, CH ２ CONH ２ , a linear alkyl group having 1 to 8 carbon atoms, and a branched alkyl group having 1 to 8 carbon atoms; and R ４９ is Ph, COOC (CH ３ ) ３ , N.H. ２ , C.H. ２ COOH, CSNH ２ , CNHNH ３ ＋ Cl － , p-phenylCN, and COPh).

[0079] These bisaminophenol-based crosslinking agents, bisaminothiophenol-based crosslinking agents, bisdiaminophenyl-based crosslinking agents, etc. have traditionally been used in crosslinking systems in which the cyano group is the crosslinking point, but they also react with carboxyl groups and alkoxycarbonyl groups to form oxazole rings, thiazole rings, and imidazole rings, thereby giving crosslinked products.

[0080] The crosslinking agent may also be a compound represented by the general formula (60): X ４１ - (CH ２ ) ｎ -R ５０ - (CH ２ ) ｍ -X ４１ (In the formula, X ４１ each independently represents an alkyne group, a nitrile group, or Y ４１ Ｐ N ３ (Y ４１ SO, SO ２ , C ６ H ４ or CO, and p is 0 or 1, n and m are independently integers of 1 to 4, and R ５０ The crosslinking agent may be selected from the group consisting of: i) a fluoroalkylene group having 3 to 10 carbon atoms, ii) a fluoroalkoxylene group having 3 to 10 carbon atoms, iii) a substituted arylene group, iv) an oligomer containing copolymerized units of vinylidene fluoride and perfluoro(methyl vinyl ether), v) an oligomer containing copolymerized units of vinylidene fluoride and hexafluoropropylene, vi) an oligomer containing copolymerized units of tetrafluoroethylene and perfluoro(methyl vinyl ether), and vii) an oligomer containing copolymerized units of tetrafluoroethylene and a hydrocarbon olefin. This crosslinking agent is preferably used together with a fluororubber having a nitrile group, an azide group, a sulfonyl azide group, a carbonyl azide group, or an alkyne group. For example, the nitrile group of the fluororubber reacts with the azide group of the crosslinking agent to form a tetrazole ring, resulting in a crosslinked product.

[0081] Particularly preferred crosslinking agents include compounds having a plurality of 3-amino-4-hydroxyphenyl groups or 3-amino-4-mercaptophenyl groups, or compounds represented by the general formula (61):

[0082]

[0083] (In the formula, R ４１ , R ４２ and R ４３ are as described above), and specific examples thereof include 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (general name: bis(aminophenol) AF), 2,2-bis(3-amino-4-mercaptophenyl)hexafluoropropane, tetraaminobenzene, bis-3,4-diaminophenylmethane, bis-3,4-diaminophenyl ether, 2,2-bis(3,4-diaminophenyl)hexafluoropropane, 2,2-bis[3-amino-4-(N-phenylamino)phenyl] [3-amino-4-(N-methylamino)phenyl]hexafluoropropane, 2,2-bis[3-amino-4-(N-ethylamino)phenyl]hexafluoropropane, 2,2-bis[3-amino-4-(N-propylamino)phenyl]hexafluoropropane, 2,2-bis[3-amino-4-(N-perfluorophenylamino)phenyl]hexafluoropropane, 2,2-bis[3-amino-4-(N-benzylamino)phenyl]hexafluoropropane, and the like.

[0084] Among these, 2,2-bis[3-amino-4-(N-phenylamino)phenyl]hexafluoropropane is preferred as the crosslinking agent from the viewpoints of heat resistance, steam resistance, amine resistance, and good crosslinking properties.

[0085] The content of the crosslinking agent is preferably 0.01 to 10 parts by mass, more preferably 0.05 to 5 parts by mass, based on 100 parts by mass of the fluororubber.

[0086] The composition may also contain conventional fillers.

[0087] Examples of the common filler include imide-based fillers having an imide structure, such as polyimide, polyamideimide, and polyetherimide; organic fillers made of engineering plastics, such as polyarylate, polysulfone, polyethersulfone, polyphenylene sulfide, polyetheretherketone, polyetherketone, and polyoxybenzoate; metal oxide fillers, such as aluminum oxide, silicon oxide, and yttrium oxide; metal carbides, such as silicon carbide and aluminum carbide; metal nitride fillers, such as silicon nitride and aluminum nitride; and inorganic fillers, such as aluminum fluoride, carbon fluoride, and carbon black.

[0088] Among these, aluminum oxide, yttrium oxide, silicon oxide, polyimide, carbon fluoride, silicon carbide, silicon nitride, aluminum nitride, and carbon black are preferred.

[0089] The inorganic fillers and organic fillers may be used alone or in combination of two or more.

[0090] The amount of the general filler to be compounded is preferably 0.5 to 100 parts by mass, more preferably 5 to 50 parts by mass, per 100 parts by mass of the fluororubber.

[0091] In particular, in fields where high purity and non-staining properties are not required, ordinary additives which are blended into fluoropolymer compositions, such as fillers, processing aids, plasticizers and colorants, may be blended as necessary, and one or more kinds of commonly used crosslinking agents or crosslinking aids different from those mentioned above may also be blended.

[0092] The composition can be produced by kneading the fluororubber, the crosslinking agent, and, if necessary, the crosslinking accelerator and filler.

[0093] The kneading can be carried out using a conventional polymer processing machine, such as an open roll, a Banbury mixer, a kneader, or an internal mixer.

[0094] The crosslinked body can be produced by molding the composition and crosslinking the resulting molded article, or by simultaneously carrying out molding and crosslinking.

[0095] The molding method is not particularly limited, and examples thereof include compression molding, extrusion molding, transfer molding, and injection molding.

[0096] The crosslinking conditions are preferably determined depending on the type of crosslinking agent used, etc., and for example, crosslinking is preferably performed at a temperature of 140 to 300° C. for 1 minute to 24 hours. Crosslinking can also be performed under normal pressure, elevated pressure, reduced pressure, or in air.

[0097] The crosslinking method is not particularly limited, and may be steam crosslinking, pressure molding, or a conventional method in which a crosslinking reaction is initiated by heating, or may be radiation crosslinking at room temperature and normal pressure.

[0098] Only the initial crosslinking treatment (called primary crosslinking) may be performed, but a post-treatment called secondary crosslinking may also be performed after the primary crosslinking.

[0099] When primary crosslinking and secondary crosslinking are performed, the primary crosslinking is preferably performed at 150 to 250° C. for 5 to 120 minutes, and more preferably at 170 to 210° C. for 5 to 60 minutes. Any known crosslinking means may be used as the crosslinking means, and examples thereof include press crosslinking.

[0100] The secondary crosslinking is preferably carried out at 180 to 320°C for 2 to 24 hours, more preferably at 280 to 310°C for 5 to 20 hours. STEP crosslinking may also be used. Any known crosslinking means may be used, such as oven crosslinking.

[0101] Although the embodiments have been described above, it will be understood that various changes in form and details can be made without departing from the spirit and scope of the claims.

[0102] Next, the present disclosure will be described with reference to examples, but the present disclosure is not limited to these examples.

[0103] The values ​​in the examples were measured by the following methods.

[0104] (1) Color of the solution after the decomposition reaction The transmittance of the solution after the decomposition reaction was measured using an Agilent Cary 7000 UMS, and the average value of %T from 380 to 780 nm was taken as the transmittance. The larger the transmittance value, the lighter the color of the solution and the fewer impurities there were.

[0105] (Production of crosslinked product of fluororubber composition) The materials were mixed in the composition shown in Table 1 and kneaded with an open roll to obtain fluororubber compositions 1 and 2. The obtained fluororubber composition 1 was pressed at 160°C for 10 minutes and then subjected to heat treatment in an oven at 180°C for 4 hours to obtain crosslinked product 1. The fluororubber composition 2 was pressed at 170°C for 15 minutes and then subjected to heat treatment in an oven at 230°C for 24 hours to obtain crosslinked product 2. The materials shown in Table 1 are as follows. Fluororubber 1: VdF / HFP = 78 / 22 (mol %), Mooney viscosity (ML １＋１０ (100°C)) = 70 Fluorine rubber 2: VdF / HFP = 78 / 22 (mol%), Mooney viscosity (ML １＋１０ (100°C)) = 57 Carbon black: trade name "Thermax N990", manufactured by Cancarb Crosslinking agent 1: Perhexa 25B (2,5-dimethyl-2,5-di(t-butylperoxy)hexane, manufactured by NOF Corporation) Crosslinking agent 2: Bisphenol AF Crosslinking aid: TAIC (triallyl isocyanurate, manufactured by Taikaku Chemical Co., Ltd.) Crosslinking accelerator: 8-benzyl-1,8-diazabicyclo[5.4.0]-7-undecenium chloride (DBU-B) Acid acceptor 1: calcium hydroxide (trade name "CALDIC #2000", manufactured by Kawashige Co., Ltd.) Acid acceptor 2: magnesium oxide (trade name "Kyowamag" ＴＭ 150" manufactured by Kyowa Chemical Industry Co., Ltd.)

[0106] (Freeze-pulverization of sample) Freeze-pulverization was performed using a "JFC-5000" manufactured by Japan Analytical Industry Co., Ltd. 10 mg of sample and a 250 mm diameter stainless steel ball were added to a 75 mL sample container, and after cooling with liquid nitrogen for 10 minutes, the sample was pulverized at 50 Hz and a pulverization temperature of -196°C for 10 minutes. This process was repeated twice to obtain powder samples with a maximum linear length of 200 μm. Fluororubber 1 and crosslinked bodies 1 and 2 were pulverized using this procedure and used in the following examples and comparative examples.

[0107] Example 1 30 mg of Crosslinked Product 1 and 10 mL of 0.1 M potassium hydroxide aqueous solution were placed in a hydrothermal reactor, pressurized to 0.5 MPa with argon gas, and reacted at 230°C for 6 hours. The pressure during the reaction was 3.4 MPa. After completion of the reaction, the contents were cooled to room temperature, and the fluoride ions produced in the aqueous phase were quantified by ion chromatography to confirm the yield of fluoride ions. The results are shown in Table 2.

[0108] (Example 2) The reaction was carried out in the same manner as in Example 1, except that a 0.2 M potassium hydroxide aqueous solution was used instead of the 0.1 M potassium hydroxide aqueous solution in Example 1, and the pressure during the reaction was changed from 3.4 MPa to 3.5 MPa. The results are shown in Table 2. Furthermore, the aqueous solution after the reaction was centrifuged to remove the solids, and to the aqueous phase, a stoichiometric amount of calcium hydroxide was added, and then the mixture was washed with hydrochloric acid to obtain calcium fluoride.

[0109] Example 3 30 mg of Crosslinked Product 1 and 10 mL of 0.1 M potassium hydroxide aqueous solution were placed in a hydrothermal reactor, pressurized to 0.5 MPa with argon gas, and reacted at 200°C for 18 hours. The pressure during the reaction was 2.3 MPa. After completion of the reaction, the contents were cooled to room temperature, and the fluoride ions produced in the aqueous phase were quantified by ion chromatography to confirm the yield of fluoride ions. The results are shown in Table 2.

[0110] (Example 4) The reaction was carried out in the same manner as in Example 1, except that a 0.3 M aqueous potassium hydroxide solution was used instead of the 0.1 M aqueous potassium hydroxide solution in Example 3, and the reaction time was changed from 18 hours to 6 hours. The results are shown in Table 2.

[0111] Example 5 30 mg of Fluororubber 1 and 10 mL of 0.1 M potassium hydroxide aqueous solution were placed in a hot water reactor, pressurized to 0.5 MPa with argon gas, and reacted at 230°C for 6 hours. The pressure during the reaction was 3.4 MPa. After completion of the reaction, the contents were cooled to room temperature, and the fluoride ions produced in the aqueous phase were quantified by ion chromatography to confirm the yield of fluoride ions. The results are shown in Table 2.

[0112] Example 6 30 mg of Crosslinked Product 2 and 10 mL of 0.2 M potassium hydroxide aqueous solution were placed in a hydrothermal reactor, pressurized to 0.5 MPa with argon gas, and reacted at 200°C for 18 hours. The pressure during the reaction was 2.2 MPa. After completion of the reaction, the contents were cooled to room temperature, and the fluoride ions produced in the aqueous phase were quantified by ion chromatography to confirm the yield of fluoride ions. The results are shown in Table 2.

[0113] Example 7 30 mg of Fluororubber 1 and 10 mL of 0.2 M aqueous sodium hydroxide solution were placed in a hot water reactor, pressurized to 0.5 MPa with argon gas, and reacted at 200°C for 18 hours. The pressure during the reaction was 2.3 MPa. After completion of the reaction, the contents were cooled to room temperature, and the fluoride ions produced in the aqueous phase were quantified by ion chromatography to confirm the yield of fluoride ions. The results are shown in Table 2.

[0114] Example 8 30 mg of Fluororubber 1 and 10 mL of 0.2 M potassium hydroxide aqueous solution were placed in a hot water reactor, pressurized to 0.5 MPa with argon gas, and reacted at 200°C for 18 hours. The pressure during the reaction was 2.2 MPa. After completion of the reaction, the contents were cooled to room temperature, and the fluoride ions produced in the aqueous phase were quantified by ion chromatography to confirm the yield of fluoride ions. The results are shown in Table 2.

[0115] Comparative Example 1 30 mg of Crosslinked Product 1 and 10 mL of 0.3 M potassium hydroxide aqueous solution were placed in a hydrothermal reactor, pressurized to 0.5 MPa with argon gas, and reacted at 250°C for 6 hours. The pressure during the reaction was 5.7 MPa. After completion of the reaction, the contents were cooled to room temperature, and the fluoride ions produced in the aqueous phase were quantified by ion chromatography to confirm the yield of fluoride ions. The results are shown in Table 2.

[0116] Comparative Example 2 30 mg of Fluororubber 1 and 10 mL of 0.5 M potassium hydroxide aqueous solution were placed in a hot water reactor, pressurized to 0.5 MPa with argon gas, and reacted at 250°C for 6 hours. The pressure during the reaction was 4.4 MPa. After completion of the reaction, the contents were cooled to room temperature, and the fluoride ions produced in the aqueous phase were quantified by ion chromatography to confirm the yield of fluoride ions. The results are shown in Table 2.

[0117] Comparative Example 3 30 mg of Crosslinked Product 1 and 10 mL of 1.0 M potassium hydroxide aqueous solution were placed in a hydrothermal reactor, pressurized to 0.5 MPa with argon gas, and reacted at 250°C for 6 hours. The pressure during the reaction was 4.2 MPa. After completion of the reaction, the contents were cooled to room temperature, and the fluoride ions produced in the aqueous phase were quantified by ion chromatography to confirm the yield of fluoride ions. The results are shown in Table 2.

[0118]

[0119]

Claims

1. A method for decomposing a fluorine-containing polymer, comprising a decomposition step of reacting the fluorine-containing polymer in subcritical water at a temperature of 180°C or higher but lower than 250°C and containing at least one basic compound selected from the group consisting of alkali metal hydroxides and alkaline earth metal hydroxides in a concentration of 0.5 M or lower.

2. The method for decomposing a fluorine-containing polymer according to claim 1, wherein the temperature of the subcritical water in the decomposition step is 200° C. or higher.

3. The method for decomposing a fluorine-containing polymer according to claim 1 or 2, wherein the fluorine-containing polymer is a fluorine-containing rubber.

4. The method for decomposing a fluorine-containing polymer according to claim 3, wherein the temperature of the subcritical water in the decomposition step is from 200°C to 235°C, the concentration of the basic compound in the subcritical water is from 0.10M to 0.35M, and the fluororubber is a copolymer consisting of 50 to 80 mol % of vinylidene fluoride and 50 to 20 mol % of hexafluoropropylene.

5. The method for decomposing a fluorine-containing polymer according to claim 3 or 4, wherein the object of decomposition in the decomposition step is a crosslinked product of a composition containing the fluororubber.

6. The method for decomposing a fluorine-containing polymer according to any one of claims 1 to 5, further comprising, prior to said decomposition step, a grinding step of grinding said fluorine-containing polymer into ground particles having a maximum linear length of 3 mm or less.

7. The method for decomposing a fluorine-containing polymer according to any one of claims 1 to 6, wherein the basic compound is sodium hydroxide and / or potassium hydroxide.

8. The method for decomposing a fluorine-containing polymer according to any one of claims 1 to 7, further comprising, after the decomposition step, a recovery step of adding calcium hydroxide to the aqueous solution obtained after the decomposition step and recovering the calcium fluoride produced.

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