Method for decomposing crosslinked rubber

Abstract

The present invention addresses the problem of providing a method for decomposing a crosslinked rubber, with which monomer yield can be improved. A means for solving this problem is a method for decomposing a crosslinked rubber, the method being characterized by including: a first decomposition step for carrying out decomposition using a catalyst represented by general formulae (1) to (3): [In the formulae, M is ruthenium, molybdenum, or the like, X1 and X2 and L1, L2 and L3 each independently denote a ligand, R1, R2 and R3 each independently denote hydrogen, an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aryl group, or the like (here, these groups may be substituted with one or more alkyl groups, halogens, alkoxy groups, or the like), and L1 and L2, R1 and R2 or L1 and R1 may bond to each other to form a ring]; and a second decomposition step in which the decomposition product obtained in the first decomposition step is thermally decomposed at 300-450°C in an inert gas atmosphere in the absence of a catalyst.

Description

Method for decomposing crosslinked rubber

[0001] The present invention relates to a method for decomposing crosslinked rubber.

[0002] Conventionally, rubber products primarily made of cross-linked rubber such as vulcanized rubber have been difficult to reuse, and after the end of their product life, they have often been reused as fuel, particularly in cement factories. However, with the recent rise in environmental concerns, there has been a demand for the development of methods for recycling materials obtained by decomposing rubber products, rather than burning them as fuel. There are various methods for decomposing cross-linked rubber, and for example, a technique for thermally decomposing cross-linked rubber at high temperatures is known. Furthermore, Patent Document 1 listed below discloses a method for decomposing polyisoprene-based rubber using microorganisms.

[0003] JP 2009-247241 A

[0004] As described above, there are various methods for decomposing crosslinked rubber, but from the viewpoint of further enhancing the recyclability of crosslinked rubber, it is important to increase the yield of monomers obtained by decomposition. However, as described above, when crosslinked rubber is thermally decomposed at high temperatures, the decomposition products are gasified or aromaticized, resulting in a decrease in the yield of monomers. Furthermore, when crosslinked rubber is decomposed using microorganisms, as in the technology disclosed in Patent Document 1, there are problems in that the decomposition takes a long time and the yield of monomers is low.

[0005] Therefore, an object of the present invention is to provide a method for decomposing crosslinked rubber that can solve the above-mentioned problems of the conventional technology and improve the yield of monomers.

[0006] The gist of the method for decomposing crosslinked rubber of the present invention, which solves the above problems, is as follows.

[0007] [1] A crosslinked rubber containing a diene rubber represented by the following general formula (1), (2), or (3): wherein M is ruthenium, titanium, molybdenum, or tungsten; １ and X ２ each independently represents a ligand; １ , L ２ and L３ each independently represents a ligand; R １ , R ２ and R ３ each independently represent a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aryl group, a carboxylate group, an alkoxy group, an alkenyloxy group, an alkynyloxy group, an aryloxy group, an alkoxycarbonyl group, an alkylamino group, an alkylthio group, an arylthio group, an alkylsulfonyl group, or an alkylsulfinyl group (wherein these groups are optionally substituted with one or more alkyl groups, halogens, alkoxy groups, aryl groups, or heteroaryl groups); L １ and L ２ may be bonded to each other to form a ring, R １ and R ２ may be bonded to each other to form a ring, １ and R １ and a second decomposition step of thermally decomposing the decomposition product obtained in the first decomposition step at 300°C or higher and 450°C or lower in an inert gas atmosphere in the absence of a catalyst.

[0008] [2] The method for decomposing a crosslinked rubber according to [1], wherein the rubber component of the crosslinked rubber contains isoprene units and / or butadiene units, the total proportion of the isoprene units and butadiene units in the rubber component is 40% by mass or more, and the total mass (A) of the isoprene units and butadiene units in the rubber component and the mass (B) of aromatic compounds derived from the isoprene units and / or butadiene units in the rubber component, in the decomposition products obtained in the first decomposition step, satisfy the relationship of the following formula (1): B / A×100≦50 (mass%) (1) [wherein A is the total mass of isoprene units and butadiene units in the rubber component of the crosslinked rubber, and B is the mass of aromatic compounds derived from the isoprene units and / or butadiene units in the rubber component of the crosslinked rubber, in the decomposition products obtained in the first decomposition step].

[0009] [3] The method for decomposing crosslinked rubber according to [1] or [2], wherein the liquid component of the decomposition product obtained in the first decomposition step is an oligomer having a weight-average molecular weight of 100 to 50,000.

[0010] [4] The method for decomposing crosslinked rubber according to any one of [1] to [3], wherein the decomposition product obtained in the second decomposition step contains 15 mass % or more of hydrocarbon compounds having 5 or less carbon atoms and limonene.

[0011] [5] The method for decomposing a crosslinked rubber according to any one of [1] to [4], wherein the first decomposition step is carried out at a temperature of 20°C or higher and 200°C or lower.

[0012] [6] The method for decomposing crosslinked rubber according to any one of [1] to [5], wherein the diene rubber comprises at least one selected from the group consisting of isoprene skeleton rubber, styrene-butadiene rubber, and butadiene rubber.

[0013] [7] The method for decomposing a crosslinked rubber according to any one of [1] to [6], wherein the crosslinked rubber further contains carbon black.

[0014] [8] The method for decomposing a crosslinked rubber according to any one of [1] to [7], wherein the crosslinked rubber further contains sulfur.

[0015] According to the present invention, it is possible to provide a method for decomposing a crosslinked rubber that can improve the yield of monomers.

[0016] The method for decomposing crosslinked rubber of the present invention will be described in detail below by way of example based on embodiments thereof.

[0017] The method for decomposing a crosslinked rubber of the present invention comprises treating a crosslinked rubber containing a diene rubber with a compound represented by the following general formula (1), (2) or (3): wherein M is ruthenium, titanium, molybdenum, or tungsten; １ and X ２ each independently represents a ligand; １ , L ２ and L ３ each independently represents a ligand; R １ , R ２ and R ３each independently represent a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aryl group, a carboxylate group, an alkoxy group, an alkenyloxy group, an alkynyloxy group, an aryloxy group, an alkoxycarbonyl group, an alkylamino group, an alkylthio group, an arylthio group, an alkylsulfonyl group, or an alkylsulfinyl group (wherein these groups are optionally substituted with one or more alkyl groups, halogens, alkoxy groups, aryl groups, or heteroaryl groups); L １ and L ２ may be bonded to each other to form a ring, R １ and R ２ may be bonded to each other to form a ring, １ and R １ may be bonded to each other to form a ring]; and a second decomposition step of thermally decomposing the decomposition product obtained in the first decomposition step at 300°C or higher and 450°C or lower in an inert gas atmosphere in the absence of a catalyst.

[0018] The catalyst represented by the general formula (1), (2), or (3) used in the first decomposition step of the method for decomposing a crosslinked rubber of the present invention can promote metathesis decomposition, causing metathesis decomposition at unsaturated bonds in the diene rubber, thereby decomposing the diene rubber. Unlike conventional thermal decomposition, metathesis decomposition does not require high temperatures, allowing the diene rubber to be decomposed under mild conditions. Furthermore, metathesis decomposition can suppress aromatization of the decomposition products, thereby improving the retention rate of the monomer skeleton (isoprene skeleton, butadiene skeleton, etc.) in the diene rubber compared to conventional thermal decomposition. Furthermore, in the second decomposition step of the method for decomposing a crosslinked rubber of the present invention, the decomposition products obtained in the first decomposition step are thermally decomposed in an inert gas atmosphere in the absence of a catalyst at a temperature of 300° C. to 450° C., thereby decomposing the decomposition products (intermediate decomposition products) into monomers (particularly diene monomers) while suppressing cyclization and aromatization of double bonds in the monomer skeleton and oxidation of the decomposition products. Therefore, according to the method for decomposing a crosslinked rubber of the present invention, the yield of the monomer finally obtained from the crosslinked rubber can be improved.

[0019] <First Decomposition Step> The method for decomposing a crosslinked rubber of the present embodiment includes a first decomposition step of decomposing a crosslinked rubber containing a diene rubber using a catalyst represented by the above general formula (1), (2), or (3).

[0020] (Crosslinked Rubber) The crosslinked rubber to be decomposed by the decomposition method of this embodiment contains a diene rubber and may further contain carbon black, sulfur, etc. The form of the crosslinked rubber is not particularly limited and may be, for example, powdered rubber. The powdered rubber can be obtained by cutting and pulverizing used rubber products such as waste tires. The pulverization step may include multiple steps such as a pre-pulverization step and a fine pulverization step, and the particle size of the powdered rubber to be used may be adjusted by a classification step after the pulverization step.

[0021] --Diene-Based Rubber-- The diene-based rubber is a rubber containing units derived from a diene-based monomer (diene-based units) and may further contain units derived from a copolymerizable comonomer. The units derived from the diene-based monomer enable the diene-based rubber to be crosslinked (vulcanized) and to exhibit rubber-like elongation and strength. While the diene-based rubber in the crosslinked rubber is typically present in a crosslinked state, it may be partially uncrosslinked. Specific examples of diene-based monomers (diene-based compounds) include 1,3-butadiene, isoprene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. On the other hand, examples of the copolymerizable comonomer include aromatic vinyl compounds. Specific examples of the aromatic vinyl compounds include styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, o,p-dimethylstyrene, o-ethylstyrene, m-ethylstyrene, and p-ethylstyrene. Examples of the diene rubber include isoprene-skeleton rubber, styrene-butadiene rubber (SBR), butadiene rubber (BR), and chloroprene rubber (CR). The isoprene-skeleton rubber is a rubber whose main skeleton is an isoprene unit, and specific examples include natural rubber (NR) and synthetic isoprene rubber (IR). Among these, the diene rubber preferably comprises at least one selected from the group consisting of isoprene-skeleton rubber, styrene-butadiene rubber, and butadiene rubber. When the diene rubber comprises at least one selected from the group consisting of isoprene-skeleton rubber, styrene-butadiene rubber, and butadiene rubber, easily recyclable diene monomers such as isoprene and butadiene can be obtained. The content of the diene rubber in the crosslinked rubber is not particularly limited, and is preferably in the range of 10 to 100% by mass, for example. From the viewpoint of further improving the yield of butadiene and isoprene, a range of 30 to 100% by mass is more preferable.

[0022] --Carbon Black-- The crosslinked rubber may further contain carbon black. In the method for decomposing a crosslinked rubber of the present invention, in the first decomposition step, metathesis decomposition is performed using a catalyst represented by the above general formula (1), (2), or (3), thereby decomposing the diene rubber and reducing its molecular weight, for example, to a liquid polymer or liquid oligomer. Therefore, even if the crosslinked rubber contains carbon black, the carbon black can be easily separated and recovered after the first decomposition step, for example, by solid-liquid separation. By recovering the carbon black before the thermal decomposition in the second decomposition step, it can be reused as high-quality carbon black. Furthermore, even if the crosslinked rubber contains carbon black, the crosslinked rubber can be efficiently decomposed. The content of carbon black in the crosslinked rubber is not particularly limited, and is, for example, in the range of 10 to 150 parts by mass, preferably 30 to 120 parts by mass, per 100 parts by mass of the diene rubber.

[0023] --Sulfur-- The crosslinked rubber may further contain sulfur. Note that, in the crosslinked rubber, sulfur is usually present in a state where the diene rubber is crosslinked (as a crosslink for the diene rubber), but some of it may be free. In the method for decomposing a crosslinked rubber of the present invention, in the first decomposition step, metathesis decomposition is performed using a catalyst represented by the above general formula (1), (2), or (3), thereby decomposing the diene rubber and reducing its molecular weight, for example, to a liquid polymer or liquid oligomer. Therefore, even if the crosslinked rubber contains sulfur, the sulfur can be easily recovered after the first decomposition step, for example, by solid-liquid separation. By recovering the sulfur before the thermal decomposition in the second decomposition step, the sulfur can be reused. Furthermore, even if the crosslinked rubber contains sulfur, the crosslinked rubber can be efficiently decomposed. The sulfur content in the crosslinked rubber is not particularly limited, and is, for example, in the range of 0.1 to 10 parts by mass, preferably 1 to 5 parts by mass, per 100 parts by mass of the diene rubber.

[0024] --Other Components--The crosslinked rubber may contain, in addition to the diene rubber, carbon black, and sulfur described above, various components commonly used in the rubber industry, such as rubber components other than diene rubbers, fillers other than carbon black (silica, calcium carbonate, etc.), silane coupling agents, antioxidants, softeners, processing aids, resins, surfactants, organic acids (stearic acid, etc.), zinc oxide (zinc white), vulcanization accelerators, crosslinking agents other than sulfur (peroxides, etc.), and the like.

[0025] (Catalyst) In the method for decomposing a crosslinked rubber according to the present embodiment, in the first decomposition step, the crosslinked rubber is decomposed by a catalyst represented by the following general formula (1), (2) or (3): The catalyst represented by the general formula (1), (2), or (3) can promote metathesis decomposition, and can decompose diene rubber simply (under mild conditions) and quickly.

[0026] In the above general formulas (1), (2), and (3), M is ruthenium (Ru), titanium (Ti), molybdenum (Mo), or tungsten (W). Among these, ruthenium is preferred as M from the viewpoint of promoting the decomposition reaction of the diene rubber in the crosslinked rubber.

[0027] In the above general formulas (1) and (2), X １ and X ２ Each independently represents a ligand, and is preferably an anionic ligand. １ and X ２ Examples of the X include hydrogen, halogen, pseudohalogen, a linear or branched alkyl group having 1 to 30 carbon atoms, an aryl group having 6 to 24 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, an aryloxy group having 6 to 24 carbon atoms, an alkyldiketonate having 3 to 20 carbon atoms, an aryldiketonate having 6 to 24 carbon atoms, a carboxylate having 1 to 20 carbon atoms, an alkylsulfonate having 1 to 20 carbon atoms, an arylsulfonate having 6 to 24 carbon atoms, an alkylthiol group having 1 to 20 carbon atoms, an arylthiol group having 6 to 24 carbon atoms, an alkylsulfonyl group having 1 to 20 carbon atoms, and an alkylsulfinyl group having 1 to 20 carbon atoms. １ and X ２may be substituted with one or more further groups, for example, a halogen (preferably fluorine), an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, or an aryl group having 6 to 24 carbon atoms, wherein these groups may also be further substituted with one or more substituents selected from the group consisting of a halogen (preferably fluorine), an alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, and a phenyl group. １ and X ２ are the same or different, and each is a halogen (particularly fluorine, chlorine, bromine or iodine), a benzoate, a carboxylate having 1 to 5 carbon atoms, an alkyl group having 1 to 5 carbon atoms, a phenoxy group, an alkoxy group having 1 to 5 carbon atoms, an alkylthiol group having 1 to 5 carbon atoms, an arylthiol group having 6 to 24 carbon atoms, an aryl group having 6 to 24 carbon atoms, or an alkylsulfonate having 1 to 5 carbon atoms. １ and X ２ are identical and each represents a halogen (especially chlorine), CF ３ COO, CH ３ COO, CFH ２ COO, (CH ３ ) ３ CO, (CF ３ ) ２ (CH ３ ) CO, (CF ３ ) (CH ３ ) ２ CO, PhO (phenoxy), MeO (methoxy), EtO (ethoxy), tosylate (p-CH ３ -C ６ H ４ -SO ３ ), mesylate (2,4,6-trimethylphenyl) or CF ３ SO ３ (trifluoromethanesulfonate).

[0028] In the above general formulas (1), (2) and (3), L １ , L ２ and L ３ each independently represents a ligand, preferably a neutral (uncharged) electron donor (also called an "electron-donating neutral ligand"). １ , L２ and L ３ can be, for example, independently of one another, a phosphine, sulfonated phosphine, phosphate, phosphinite, phosphonit, arsine, stibine, ether, amine, amide, aryloxy, sulfonate, sulfoxide, carboxyl, nitrosyl, pyridine, thioether or imidazolidine ligand. １ , L ２ and L ３ are each independently an arylphosphine ligand having 6 to 24 carbon atoms, an alkylphosphine ligand having 1 to 10 carbon atoms or a cycloalkylphosphine ligand having 3 to 20 carbon atoms, a sulfonated arylphosphine ligand having 6 to 24 carbon atoms or a sulfonated alkylphosphine ligand having 1 to 10 carbon atoms, an aryl phosphinite ligand having 6 to 24 carbon atoms or an alkyl phosphinite ligand having 1 to 10 carbon atoms, an arylphosphonite ligand having 6 to 24 carbon atoms or an alkylphosphonite ligand having 1 to 10 carbon atoms, an aryl phosphite ligand having 6 to 24 carbon atoms or an alkyl phosphite ligand having 1 to 10 carbon atoms, an aryl arsine ligand having 6 to 24 carbon atoms or an alkyl phosph ... is preferably an alkylarsine ligand having 1 to 10 carbon atoms, an arylamine ligand having 6 to 24 carbon atoms or an alkylamine ligand having 1 to 10 carbon atoms, a pyridine ligand, an arylsulfoxide ligand having 6 to 24 carbon atoms or an alkylsulfoxide ligand having 1 to 10 carbon atoms, an arylether ligand having 6 to 24 carbon atoms or an alkylether ligand having 1 to 10 carbon atoms, or an arylamide ligand having 6 to 24 carbon atoms or an alkylamide ligand having 1 to 10 carbon atoms, each of which may be substituted with a phenyl group, and the phenyl group may also be optionally further substituted with a halogen, an alkyl group having 1 to 5 carbon atoms or an alkoxy group having 1 to 5 carbon atoms. The term "phosphine" includes, for example, PPh ３ , P(p-Tol) ３ , P(o-Tol) ３ , PPh(CH ３ ) ２ , P(CF ３ ) ３ , P(p-FC ６ H ４ ) ３ , P(p-CF ３ C ６ H４ ) ３ , P(C ６ H ４ -SO ３ Na) ３ , P(CH ２ C ６ H ４ -SO ３ Na) ３ , P(isopropyl) ３ , P(CHCH ３ (CH ２ CH ３ )) ３ , P(cyclopentyl) ３ , P (cyclohexyl) ３ , P (neopentyl) ３ and P(neophenyl) ３ The term "phosphinite" includes, for example, triphenyl phosphinite, tricyclohexyl phosphinite, triisopropyl phosphinite, and methyldiphenyl phosphite. The term "phosphite ester" includes, for example, triphenyl phosphite, tricyclohexyl phosphite, tri-tert-butyl phosphite, triisopropyl phosphite, and methyldiphenyl phosphite. The term "stibine" includes, for example, triphenylstibine, tricyclohexylstibine, and trimethylstibine. The term "aryloxy" includes, for example, 2-tert-butyl-4,5-dimethylphenyloxy. The term "sulfonate" includes, for example, trifluoromethanesulfonate, tosylate, and mesylate. The term "sulfoxide" includes, for example, (CH ３ ) ２ S(=O) and (C ６ H ５ ) ２ The term "thioether" includes, for example, CH ３ SCH ３ , C ６ H ５ SCH ３ , C.H. ３ OCH ２ CH ２ SCH ３and tetrahydrothiophene. The term "pyridine" includes, for example, pyridine, picolines (α-, β-, and γ-picoline), lutidines (2,3-, 2,4-, 2,5-, 2,6-, 3,4-, and 3,5-lutidine), collidine (2,4,6-trimethylpyridine), trifluoromethylpyridine, phenylpyridine, 4-(dimethylamino)pyridine, chloropyridines, bromopyridines, nitropyridines, quinoline, pyrimidine, pyrrole, imidazole, and phenylimidazole. In the imidazolidine ligand, hydrogen atoms bonded to carbon atoms or nitrogen atoms constituting the imidazolidine ring may be substituted with a linear or branched alkyl group having 1 to 30 carbon atoms, a cycloalkyl group having 3 to 20 carbon atoms, an alkenyl group having 2 to 20 carbon atoms, an alkynyl group having 2 to 20 carbon atoms, an aryl group having 6 to 24 carbon atoms, a carboxylate having 1 to 20 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, an alkenyloxy group having 2 to 20 carbon atoms, an alkynyloxy group having 2 to 20 carbon atoms, an aryloxy group having 6 to 20 carbon atoms, an alkoxycarbonyl group having 2 to 20 carbon atoms, an alkylthio group having 1 to 20 carbon atoms, an arylthio group having 6 to 20 carbon atoms, an alkylsulfonyl group having 1 to 20 carbon atoms, an alkylsulfonate having 1 to 20 carbon atoms, an arylsulfonate having 6 to 20 carbon atoms, or an alkylsulfinyl group having 1 to 20 carbon atoms.

[0029] In the above general formulas (1), (2) and (3), R １ , R ２ and R ３each independently represent a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aryl group, an aralkyl group, a carboxylate group, an alkoxy group, an alkenyloxy group, an alkynyloxy group, an aryloxy group, an alkoxycarbonyl group, an alkylamino group, an alkylthio group, an arylthio group, an alkylsulfonyl group, or an alkylsulfinyl group, wherein these groups are optionally substituted with one or more alkyl groups, halogens, alkoxy groups, aryl groups, or heteroaryl groups. Furthermore, the alkyl group is preferably an alkyl group having 1 to 30 carbon atoms, the cycloalkyl group is preferably a cycloalkyl group having 3 to 20 carbon atoms, the alkenyl group is preferably an alkenyl group having 2 to 20 carbon atoms, the alkynyl group is preferably an alkynyl group having 2 to 20 carbon atoms, the aryl group is preferably an aryl group having 6 to 24 carbon atoms, the aralkyl group is preferably an aralkyl group having 7 to 24 carbon atoms, the carboxylate group is preferably a carboxylate group having 1 to 20 carbon atoms, the alkoxy group is preferably an alkoxy group having 1 to 20 carbon atoms, and the alkenyloxy group is preferably an alkenyloxy group having 2 to 20 carbon atoms. As the alkynyloxy group, an alkynyloxy group having 2 to 20 carbon atoms is preferred, as the aryloxy group, an aryloxy group having 6 to 24 carbon atoms is preferred, as the alkoxycarbonyl group, an alkoxycarbonyl group having 2 to 20 carbon atoms is preferred, as the alkylamino group, an alkylamino group having 1 to 30 carbon atoms is preferred, as the alkylthio group, an alkylthio group having 1 to 30 carbon atoms is preferred, as the arylthio group, an arylthio group having 6 to 24 carbon atoms is preferred, as the alkylsulfonyl group, an alkylsulfonyl group having 1 to 20 carbon atoms is preferred, and as the alkylsulfinyl group, an alkylsulfinyl group having 1 to 20 carbon atoms is preferred. In one embodiment, R １ and R ２one of the groups is hydrogen, and the other is alkyl having 1 to 20 carbon atoms, cycloalkyl having 3 to 10 carbon atoms, alkenyl having 2 to 20 carbon atoms, alkynyl having 2 to 20 carbon atoms, aryl having 6 to 24 carbon atoms, aralkyl having 7 to 24 carbon atoms, carboxylate having 1 to 20 carbon atoms, alkoxy having 1 to 20 carbon atoms, alkenyloxy having 2 to 20 carbon atoms, alkynyloxy having 2 to 20 carbon atoms, aryloxy having 6 to 24 carbon atoms, alkoxycarbonyl having 2 to 20 carbon atoms, alkylamino having 1 to 30 carbon atoms, alkylthio having 1 to 30 carbon atoms, arylthio having 6 to 24 carbon atoms, alkylsulfonyl having 1 to 20 carbon atoms, or alkylsulfinyl having 1 to 20 carbon atoms, each of which is optionally substituted with one or more alkyl groups, halogens, alkoxy groups, aryl groups, or heteroaryl groups.

[0030] In the above general formulas (1), (2) and (3), L １ and L ２ may be bonded to each other to form a ring. １ and L ２ The ring formed by the combination of may be aliphatic or aromatic and may be optionally substituted and contain one or more heteroatoms, such as oxygen, sulfur, nitrogen, phosphorus, etc.

[0031] In the above general formulas (1), (2) and (3), R １ and R ２ may be bonded to each other to form a ring. １ and R ２ However, these R １ and R ２ The ring formed together with the common carbon atom to which it is attached may be aliphatic or aromatic and may be optionally substituted and contain one or more heteroatoms.

[0032] In the above general formulas (1), (2) and (3), L １ and R １ may be bonded to each other to form a ring. １ and R １The ring formed by the combination of may be aliphatic or aromatic and may be optionally substituted and contain one or more heteroatoms, such as oxygen, sulfur, nitrogen, phosphorus, etc.

[0033] The catalyst represented by the general formula (1) above includes those represented by the following structural formulas (1-1) to (1-3): [In the formula, Cy represents a cyclohexyl group, and Mes represents a mesityl group (also called a "2,4,6-trimethylphenyl group")]. The catalyst represented by structural formula (1-1) is called a Grubbs first-generation catalyst, the catalyst represented by structural formula (1-2) is called a Grubbs second-generation catalyst, and the catalyst represented by structural formula (1-3) is called a Grubbs-Hoveyda second-generation catalyst. When a catalyst represented by any of structural formulas (1-1) to (1-3) is used, the decomposition reaction (metathesis decomposition) of the diene rubber in the crosslinked rubber proceeds more rapidly.

[0034] The catalyst represented by the general formula (2) above is represented by the following structural formula (2-1): The catalyst represented by structural formula (2-1) is called a Grubbs third generation catalyst. When the catalyst represented by structural formula (2-1) is used, the decomposition reaction (metathesis decomposition) of the diene rubber in the crosslinked rubber proceeds more rapidly.

[0035] The catalyst represented by the general formula (3) above is represented by the following structural formula (3-1): Examples of the catalyst include those represented by the following formula:

[0036] The amount of the catalyst used is preferably 0.1 part by mass or more, more preferably 1 part by mass or more, and is preferably 10 parts by mass or less, and more preferably 8 parts by mass or less, per 100 parts by mass of the diene rubber. If the amount of the catalyst used is 0.1 part by mass or more per 100 parts by mass of the diene rubber, the decomposition reaction of the diene rubber will further proceed, and if the amount of the catalyst used is 10 parts by mass or less per 100 parts by mass of the diene rubber, it is preferable from the standpoint of cost.

[0037] (Reaction Conditions, etc.) The first decomposition step is preferably carried out at a temperature of 20° C. or higher and 200° C. or lower. By carrying out the first decomposition step at 20° C. or higher, the rate of the decomposition reaction of the diene rubber in the crosslinked rubber is improved, and by carrying out the first decomposition step at 200° C. or lower, decomposition of the catalyst (metathesis catalyst) represented by the above general formula (1), (2), or (3) can be suppressed, and after decomposition, the retention rate (selectivity) of the monomer skeleton of the diene rubber in the crosslinked rubber is improved. From the viewpoint of improving the decomposition reaction rate of the diene rubber, the first decomposition step is more preferably 25° C. or higher, and from the viewpoint of suppressing decomposition of the metathesis catalyst and improving the selectivity of the product maintaining the monomer skeleton, it is more preferably 100° C. or lower.

[0038] The first decomposition step can be carried out at any pressure, including reduced pressure, normal pressure, and increased pressure. For example, the reaction pressure is preferably 1 kPa to 10 MPa, more preferably 10 kPa to 1 MPa, and even more preferably 50 kPa to 500 kPa.

[0039] In the method for decomposing a crosslinked rubber according to this embodiment, the catalyst represented by the general formula (1), (2), or (3) may be dissolved in a solvent, and the crosslinked rubber may be immersed in the solvent to carry out the first decomposition step. By allowing the catalyst to act on the crosslinked rubber in the solvent, the decomposition reaction of the diene rubber in the crosslinked rubber is facilitated. Any solvent that does not inhibit the decomposition reaction can be used as the solvent, and examples thereof include ethers, aliphatic hydrocarbons, alicyclic hydrocarbons, and aromatic hydrocarbons (aromatic solvents). More specifically, the solvent is preferably tetrahydrofuran (THF), hexane, cyclohexane, pentane, cyclopentane, toluene, or xylene, and more preferably toluene or tetrahydrofuran. When the solvent is selected from tetrahydrofuran, hexane, cyclohexane, pentane, cyclopentane, toluene, or xylene, the decomposition reaction of the diene rubber in the crosslinked rubber is facilitated. The amount of the solvent used is preferably 10 mL or more, more preferably 50 mL or more, and is preferably 500 mL or less, and more preferably 200 mL or less, per gram of the crosslinked rubber. If the amount of the solvent used is 10 mL or more per gram of the crosslinked rubber, the decomposition reaction of the diene rubber in the crosslinked rubber will further proceed, and if the amount of the solvent used is 500 mL or less per gram of the crosslinked rubber, this is preferred from the standpoint of cost.

[0040] The first decomposition step may be carried out in the presence of a chain transfer agent (CTA). Examples of the chain transfer agent include cis-1,4-diacetoxy-2-butene and cis-1,4-dibenzyloxy-2-butene. The amount of the chain transfer agent used is preferably in the range of 1 to 100 mol per 1 mol of the catalyst.

[0041] (Decomposition Products (Intermediate Decomposition Products)) In the method for decomposing a crosslinked rubber according to this embodiment, a liquid oligomer, a liquid polymer, or the like is obtained as a decomposition product in the first decomposition step.

[0042] In the method for decomposing crosslinked rubber of this embodiment, it is preferable that the rubber component of the crosslinked rubber contains isoprene units and / or butadiene units, the total proportion of the isoprene units and butadiene units in the rubber component is 40% by mass or more, and the total mass (A) of the isoprene units and butadiene units in the rubber component and the mass (B) of aromatic compounds derived from the isoprene units and / or butadiene units in the rubber component in the decomposition products obtained in the first decomposition step satisfy the relationship of the following formula (1): B / A×100≦50 (mass%) (1) [wherein A is the total mass of the isoprene units and butadiene units in the rubber component of the crosslinked rubber, and B is the mass of aromatic compounds derived from the isoprene units and / or butadiene units in the rubber component of the crosslinked rubber in the decomposition products obtained in the first decomposition step.] In the method for decomposing crosslinked rubber according to this embodiment, the first decomposition step is carried out using a catalyst represented by the general formula (1), (2), or (3), thereby suppressing the aromatization of the decomposition products. Furthermore, the ratio (B / A x 100) of the mass (B) of aromatic compounds derived from isoprene units and / or butadiene units in the rubber component of the crosslinked rubber in the decomposition products obtained in the first decomposition step to the total mass (A) of isoprene units and butadiene units in the rubber component of the crosslinked rubber is 50% by mass or less, which means that the aromatization of the decomposition products is suppressed, and in this case, the yield of easily recyclable diene monomers such as isoprene and butadiene is improved. From the viewpoint of suppressing the aromatization of the decomposition products, it is more preferable that the ratio (B / A x 100) is 40% by mass or less. Here, when the rubber component of the crosslinked rubber contains aromatic compound units (such as styrene units), the mass of aromatic compounds derived from the aromatic compound units in the rubber component of the crosslinked rubber is subtracted from the mass of aromatic compounds in the decomposition products obtained in the first decomposition step in calculating the left side of formula (1). The ratio (B / A x 100) can be controlled by, for example, the reaction conditions in the first decomposition step, such as the reaction temperature and reaction time.

[0043] In the method for decomposing crosslinked rubber according to this embodiment, the liquid component of the decomposition product obtained in the first decomposition step is an oligomer having a weight-average molecular weight of preferably 100 to 50,000, more preferably 400 to 12,000, even more preferably 400 to 5,000, and even more preferably 400 to 1,500. By decomposing the crosslinked rubber (the rubber component in the crosslinked rubber) in the first decomposition step into an oligomer having a weight-average molecular weight of preferably 100 to 50,000, more preferably 400 to 12,000, even more preferably 400 to 5,000, and even more preferably 400 to 1,500, the yield of the monomer can be further improved in the second decomposition step described below.

[0044] In the method for decomposing a crosslinked rubber according to this embodiment, it is preferable that 80% by mass or more of the diene rubber in the crosslinked rubber is decomposed into diene oligomers having a weight-average molecular weight of 100 to 50,000 through the first decomposition step, and more preferably into diene oligomers having a weight-average molecular weight of 300 to 10,000. By decomposing 80% by mass or more of the diene rubber in the crosslinked rubber into diene oligomers having a weight-average molecular weight of 100 to 50,000 through the first decomposition step, the yield of diene monomers can be further improved through the second decomposition step described below. From the viewpoint of the yield of diene monomers after the second decomposition step, it is more preferable that 85% by mass or more of the diene rubber in the crosslinked rubber is decomposed into diene oligomers having a weight-average molecular weight of 100 to 50,000 through the first decomposition step. In this specification, the term "oligomer" refers to an oligomer having a weight-average molecular weight of 50,000 or less and containing two or more monomer units. The term "diene-based oligomer" refers to an oligomer containing two or more units derived from a diene monomer (diene-based units). The weight-average molecular weight (Mw) can be measured by gel permeation chromatography (GPC).

[0045] <Second Decomposition Step> The method for decomposing a crosslinked rubber of the present embodiment includes a second decomposition step in which the decomposition product obtained in the first decomposition step is thermally decomposed at 300°C or higher and 450°C or lower in an inert gas atmosphere in the absence of a catalyst.

[0046] (Reaction Conditions, etc.) By carrying out the second decomposition step at 300° C. or higher, the rate of the decomposition reaction of the decomposition products obtained in the first decomposition step is increased, and the yield of monomers is improved, while by carrying out the second decomposition step at 450° C. or lower, gasification and aromatization of the decomposition products can be significantly suppressed, and the selectivity of products that maintain the monomer skeleton is significantly improved. The second decomposition step is preferably carried out at 320° C. or higher from the viewpoints of increasing the decomposition reaction rate and improving the monomer yield, and preferably at 380° C. or lower from the viewpoint of suppressing gasification and aromatization of the decomposition products.

[0047] The second decomposition step is carried out under an inert gas atmosphere. By carrying out the second decomposition step under an inert gas atmosphere, oxidation or reduction of the decomposition product can be suppressed, and in particular, hydrogenation of double bonds in the monomers in the decomposition product can be suppressed. Examples of the inert gas include nitrogen, carbon dioxide, argon, and helium.

[0048] To carry out the second decomposition step under an inert gas atmosphere, for example, when a batch reactor is used, the atmosphere charged into the reactor may be an inert gas, and when a flow reactor is used, the atmosphere circulated through the reactor may be an inert gas. Note that although hydrogen may be generated during the second decomposition step, the generated hydrogen is not taken into consideration when determining the atmosphere for the second decomposition step.

[0049] The second decomposition step can be carried out at any pressure, whether under reduced pressure, normal pressure, or increased pressure, but is preferably carried out under reduced pressure or normal pressure. As an example, the reaction pressure in the second decomposition step is preferably 1000 kPa to 67 kPa. By carrying out the second decomposition step under reduced pressure or normal pressure, polymerization (repolymerization) of monomers in the decomposition products can be suppressed.

[0050] The reaction time of the second decomposition step is not particularly limited. For example, the reaction time of the second decomposition step is preferably 3 to 60 minutes, more preferably 5 to 30 minutes.

[0051] The second decomposition step is carried out in the absence of a catalyst (i.e., no catalyst is used). Not using a catalyst in the second decomposition step can reduce costs. Here, "in the absence of a catalyst" means that no catalyst that has the effect of promoting the decomposition reaction is present in the reaction system of the second decomposition step.

[0052] (Decomposition Products) In the method for decomposing a crosslinked rubber according to the present embodiment, monomers (particularly, diene-based monomers) and the like are obtained as decomposition products in the second decomposition step.

[0053] In the method for decomposing crosslinked rubber according to this embodiment, the decomposition products obtained in the second decomposition step preferably contain 15% by mass or more of hydrocarbon compounds having 5 or less carbon atoms and limonene (particularly, hydrocarbon compounds having 2 to 4 carbon atoms, isoprene, and limonene), more preferably 20% by mass or more, and even more preferably 25% by mass or more. By improving the yield of hydrocarbon compounds having 5 or less carbon atoms and limonene (particularly, hydrocarbon compounds having 2 to 4 carbon atoms, isoprene, and limonene), the yield of reusable monomers is improved, further improving the economic and environmental value of the decomposition method. From the viewpoint of the yield of reusable monomers, it is more preferable that the decomposition products obtained in the second decomposition step contain 40% by mass or more of hydrocarbon compounds having 5 or less carbon atoms. The total amount of hydrocarbon compounds having 5 or less carbon atoms and limonene (particularly, hydrocarbon compounds having 2 to 4 carbon atoms, isoprene, and limonene) in the decomposition products obtained in the second decomposition step may depend on the reaction conditions described above as well as the reaction apparatus used. In this embodiment, the preferred range of the total amount of hydrocarbon compounds having 5 or less carbon atoms and limonene is the preferred range when using a reaction apparatus described in the Examples below. When a reaction apparatus more suitable for the present invention is used, the total amount of hydrocarbon compounds having 5 or less carbon atoms and limonene may be even greater. The hydrocarbon compounds having 5 or less carbon atoms as decomposition products vary depending on the type of rubber component in the crosslinked rubber to be decomposed. Examples include 1,3-butadiene, isoprene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene, with 1,3-butadiene and isoprene being preferred. The proportion of hydrocarbon compounds having 5 or less carbon atoms in the decomposition products can be controlled by, for example, the reaction conditions, such as the reaction temperature and reaction time, of the second decomposition step.

[0054] <Others> The method for decomposing a crosslinked rubber according to this embodiment may further include another step in addition to the first and second decomposition steps described above. Such steps include a pretreatment step for the crosslinked rubber (for example, a cutting step or a pulverization step). Furthermore, when the crosslinked rubber contains carbon black, it is preferable to include a step of recovering the carbon black from the decomposition product (intermediate decomposition product) between the first and second decomposition steps.

[0055] The decomposition method for crosslinked rubber according to this embodiment can be carried out in either a batch reactor or a flow reactor. The decomposition product after the decomposition reaction can be separated and recovered by filtration, distillation, or the like, or recovered by precipitation using a poor solvent, and can be reused. The diene monomer finally obtained from the crosslinked rubber can be reused as a raw material for diene rubber (polymer).

[0056] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0057] <Measurement of Molecular Weight> The polystyrene-equivalent weight average molecular weight (Mw), number average molecular weight (Mn), and molecular weight distribution (D=Mw / Mn) of the degradation products (intermediate degradation products) were determined using monodisperse standard polystyrene as a standard by gel permeation chromatography (hereinafter sometimes referred to as GPC analysis; solution delivery unit: LC-20AB manufactured by Shimadzu Corporation; column: a combination of KF-803 and KF-804 manufactured by Showa Denko K.K. or a combination of G2000HXL and G4000HXL manufactured by Tosoh Corporation; analysis system: LabSolutions manufactured by Shimadzu Corporation; eluent: tetrahydrofuran). The measurement temperature was 40°C.

[0058] <Decomposition rate in the first decomposition step> The decomposition rate in the first decomposition step was calculated based on the following formula, in which the unreacted solid rubber after the reaction was recovered and weighed, and the ratio to the weight of the rubber before the reaction was used. The decomposition rate was set to 100% when no solid rubber remained after the reaction. Decomposition rate in the first decomposition step (%) = {1 - (weight of unreacted solid rubber after the reaction / weight of rubber before the reaction)} x 100

[0059] <Structural Formula of Catalyst Used> The structural formula of the catalyst used in the first decomposition step is as follows.

[0060] <Preparation of Crosslinked Rubber Samples> (Crosslinked Rubber Sample A) A rubber composition was prepared by blending 50.0 parts by mass of carbon black, 1.0 part by mass of antioxidant 6PPD [N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine], 2.0 parts by mass of stearic acid, 2.5 parts by mass of zinc oxide, 1.5 parts by mass of vulcanization accelerator (N-cyclohexyl-2-benzothiazolylsulfenamide), and 4.5 parts by mass of sulfur with 100.1 parts by mass of natural rubber. The rubber composition was then heated and crosslinked to prepare a crosslinked rubber. The resulting crosslinked rubber was cut to approximately 2 to 6 mm on a side to prepare a crosslinked rubber sample. Furthermore, crosslinked rubber samples (approximately 1 mm on a side or less) were also prepared by further freeze-pulverization after cutting.

[0061] (Crosslinked Rubber Sample B) A rubber composition was prepared by blending 50.0 parts by mass of carbon black, 2.0 parts by mass of antioxidant 6PPD [N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine], 1.0 part by mass of antioxidant trimethyldihydroquinoline polymer, 2.0 parts by mass of stearic acid, 2.25 parts by mass of zinc oxide, 0.6 parts by mass of vulcanization accelerator (N-cyclohexyl-2-benzothiazolylsulfenamide), and 1.8 parts by mass of sulfur with 100.0 parts by mass of natural rubber, and the rubber composition was heated and crosslinked to prepare a crosslinked rubber. The obtained crosslinked rubber was cut and further freeze-pulverized to prepare crosslinked rubber samples (approximately 1 mm on a side).

[0062] <Examples of the First Decomposition Step> (Example 1) 10 mL of tetrahydrofuran (THF) was added to 0.125 g of the crosslinked rubber sample A (a sample that had only been cut) and stirred at 25°C for 24 hours to swell the rubber. 24 mg of a second-generation Grubbs catalyst (G2) was added, and the mixture was stirred at 25°C for an additional 24 hours to carry out a metathesis decomposition reaction. After the reaction, 10 mL of methanol and 2.5 mL of ethyl vinyl ether were added to terminate the reaction. Since no rubber sample remained after the reaction, the decomposition rate of the rubber sample was calculated to be 100%. Subsequently, the solvent was distilled off under reduced pressure to obtain a liquid polymer and carbon black. A portion of the mixture was taken, tetrahydrofuran was added, and the carbon black was removed by filtration using a syringe filter. The molecular weight of the liquid polymer was measured by GPC analysis. The results are shown in Table 1.

[0063] (Examples 2 to 10) The reaction conditions (type and amount of catalyst, type and amount of solvent, reaction scale) were changed, and the reaction in the first decomposition step was carried out in the same manner as in Example 1. The results are shown in Table 1.

[0064] Example 11: 10 mL of tetrahydrofuran and 24 mg of the second-generation Grubbs catalyst (G2) were added to 0.125 g of the crosslinked rubber sample A (a sample that was chopped and then freeze-pulverized), and the mixture was stirred at 25°C for 24 hours to carry out a metathesis decomposition reaction. After the reaction, the reaction was stopped by adding 10 mL of methanol and 2.5 mL of ethyl vinyl ether. Since no rubber sample remained after the reaction, the decomposition rate of the rubber sample was calculated to be 100%. Subsequently, the solvent was distilled off under reduced pressure to obtain a liquid polymer and carbon black. A portion of the mixture was taken, and tetrahydrofuran was added. The carbon black was removed by filtration using a syringe filter, and the molecular weight of the liquid polymer was measured by GPC analysis. The results are shown in Table 1.

[0065] Example 12: 10 mL of tetrahydrofuran was added to 0.125 g of the crosslinked rubber sample A (a sample that had only been cut), and the mixture was stirred at room temperature for 24 hours to swell the rubber. Subsequently, 40 mg of cis-1,4-diacetoxy-2-butene was added as a chain transfer agent (CTA), and 24 mg of a second-generation Grubbs catalyst (G2) was added, and metathesis degradation was carried out for 24 hours. After the reaction, the reaction was terminated by adding 10 mL of methanol and 2.5 mL of ethyl vinyl ether. Since no rubber sample remained after the reaction, the decomposition rate of the rubber sample was calculated to be 100%. Subsequently, the solvent was distilled off under reduced pressure to obtain a liquid polymer and carbon black. A portion of the reaction mixture was sampled, tetrahydrofuran was added, and the carbon black was removed by filtration using a syringe filter. The molecular weight of the liquid polymer was measured by GPC analysis. The results are shown in Table 1.

[0066] Example 13: 10 mL of tetrahydrofuran and 8 mg of second-generation Grubbs catalyst (G2) were added to 0.125 g of the crosslinked rubber sample B (a sample that was freeze-pulverized after cutting), and the mixture was stirred at 25°C for 24 hours to carry out a metathesis decomposition reaction. Since no rubber sample remained after the reaction, the decomposition rate of the rubber sample was calculated to be 100%. The reaction solution was diluted with tetrahydrofuran, and the carbon black was separated by centrifugation. The carbon black was washed with tetrahydrofuran. The initial supernatant and the washings were combined and concentrated under reduced pressure to obtain a liquid polymer. A portion was sampled and the molecular weight of the liquid polymer was measured by GPC analysis. The results are shown in Table 1.

[0067] (Examples 14 to 22) The reaction conditions (amount of catalyst, type and amount of solvent, and reaction scale) were changed, and the reaction of the first decomposition step was carried out in the same manner as in Example 13. The results are shown in Table 1.

[0068]

[0069] (Notes for Table 1) 1) In Examples 1 to 10 and 12, samples were used in which the crosslinked rubber was only cut, while in Examples 11 and 13 to 22, samples were used in which the crosslinked rubber was cut and then freeze-pulverized. When freeze-pulverized samples were used, the step of swelling the rubber was omitted. 2) GPC was not measured.

[0070] <Second Decomposition Step> (Example 23) 0.1 g of the oligomer obtained in Example 1 was filled into the reaction section of the reaction tube, and quartz wool was filled upstream and downstream of the oligomer. A helium inlet line was installed upstream of the reaction tube, and a cold trap filled with chloroform (60% ethylene glycol aqueous solution, cooled to -20°C) was installed downstream. Gas passing through the trap was collected using a gas bag. The reaction section of the reaction tube was heated at 400°C for 30 minutes under a helium flow rate of 50 mL / min to perform the second decomposition step. After the reaction was completed, the reaction tube was cooled, and the products precipitated in the reaction tube and trap were collected. Furthermore, the yields of aromatic and aliphatic components in the product, as well as the yields of hydrocarbon compounds with 2 to 4 carbon atoms, isoprene, and limonene in the product, were measured by gas chromatography (GC), and the total yield was 21% by mass. Of this total amount, the mass of hydrocarbon compounds having 5 or less carbon atoms is 70%.

Claims

1. The crosslinked rubber containing a diene rubber is a compound represented by the following general formula (1), (2), or (3): 【Chemistry 1】 wherein M is ruthenium, titanium, molybdenum, or tungsten; X １ and X ２ each independently represents a ligand, L １ , L ２ and L ３ each independently represents a ligand, R １ , R ２ and R ３ each independently represent a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aryl group, a carboxylate group, an alkoxy group, an alkenyloxy group, an alkynyloxy group, an aryloxy group, an alkoxycarbonyl group, an alkylamino group, an alkylthio group, an arylthio group, an alkylsulfonyl group, or an alkylsulfinyl group (wherein these groups are optionally substituted with one or more alkyl groups, halogens, alkoxy groups, aryl groups, or heteroaryl groups); L １ and L ２ may be bonded to each other to form a ring, R １ and R ２ may be bonded to each other to form a ring, L １ and R １ and a second decomposition step in which the decomposition product obtained in the first decomposition step is thermally decomposed at 300°C or higher and 450°C or lower in an inert gas atmosphere and in the absence of a catalyst; A method for decomposing crosslinked rubber, comprising:

2. the rubber component of the crosslinked rubber contains isoprene units and / or butadiene units, The total proportion of the isoprene units and butadiene units in the rubber component is 40% by mass or more, The total mass (A) of the isoprene units and butadiene units in the rubber component and the mass (B) of aromatic compounds derived from the isoprene units and / or butadiene units in the rubber component, in the decomposition product obtained in the first decomposition step, satisfy the following formula (1): B / A×100≦50 (mass%) ... (1) The method for decomposing crosslinked rubber according to claim 1, wherein the following relationship is satisfied: [wherein A is the total mass of isoprene units and butadiene units in the rubber component of the crosslinked rubber, and B is the mass of aromatic compounds derived from isoprene units and / or butadiene units in the rubber component of the crosslinked rubber, in the decomposition product obtained in the first decomposition step.]

3. 2. The method for decomposing crosslinked rubber according to claim 1, wherein the liquid component of the decomposition product obtained in the first decomposition step is an oligomer having a weight average molecular weight of 100 to 50,000.

4. 2. The method for decomposing crosslinked rubber according to claim 1, wherein the decomposition product obtained in the second decomposition step contains 15% by mass or more of hydrocarbon compounds having 5 or less carbon atoms and limonene.

5. The method for decomposing a crosslinked rubber according to claim 1, wherein the first decomposition step is carried out at a temperature of 20°C or higher and 200°C or lower.

6. 2. The method for decomposing crosslinked rubber according to claim 1, wherein the diene rubber comprises at least one rubber selected from the group consisting of isoprene skeleton rubber, styrene-butadiene rubber, and butadiene rubber.

7. The method for decomposing a crosslinked rubber according to claim 1 , wherein the crosslinked rubber further contains carbon black.

8. The method for decomposing a crosslinked rubber according to claim 1 , wherein the crosslinked rubber further contains sulfur.

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