Method for decomposing crosslinked rubber and system for decomposing crosslinked rubber

Abstract

The present invention provides a method for decomposing crosslinked rubber, said method comprising: a thermal decomposition step for obtaining a decomposition product containing a liquid polymer and carbon black by heating crosslinked rubber containing diene-based rubber to 150-300°C in a first solvent containing a solvent component X so that the crosslinked rubber is thermally decomposed; a solid-liquid separation step for separating the decomposition product into a liquid phase containing the liquid polymer and the solvent component X and a solid phase containing the carbon black by solid-liquid separation after diluting the decomposition product with a second solvent containing the solvent component X; and a solvent separation step for separating the liquid phase into a product containing the liquid polymer and a recovery solvent containing the solvent component X, wherein the recovery solvent is used as at least one of the first solvent and the second solvent.　

Description

Method for decomposing crosslinked rubber and system for decomposing crosslinked rubber

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

[0002] As a method for recycling rubber products made of crosslinked rubber, there is a thermal recycling method in which used rubber products are combusted, and the thermal energy generated during combustion is recovered and utilized. However, with the growing interest in environmental issues in recent years, there is a demand for the development of material recycling methods and chemical recycling methods that do not burn rubber products as fuel, but reuse the materials obtained by decomposing the rubber products.

[0003] As a method for decomposing crosslinked rubber, for example, Patent Document 1 discloses a method for thermally decomposing used tires.

[0004] Japanese Patent Publication No. 2023-542420

[0005] The decomposition method disclosed in Patent Document 1 recovers and utilizes the solvent used for the thermal decomposition of tires. In the process of material recycling and chemical recycling, more sustainable recycling methods with reduced energy consumption and environmental impact are required. The decomposition method described in Patent Document 1 has sufficient room for improvement from the perspective of reducing the number of facilities and energy consumption in the entire process.

[0006] An object of the present invention is to provide a method for decomposing crosslinked rubber and a system for decomposing crosslinked rubber that can reduce equipment investment and suppress energy consumption.

[0007] To solve the above problems, one aspect of the present invention includes the following aspects: [1] A method for decomposing crosslinked rubber, comprising: a thermal decomposition step of heating a crosslinked rubber containing a diene rubber in a first solvent containing a solvent component X to 150°C or more and 300°C or less to thermal decompose it and obtain a decomposition product containing a liquid polymer and carbon black; a solid-liquid separation step of diluting the decomposition product with a second solvent containing the solvent component X and then performing solid-liquid separation to separate it into a liquid phase containing the liquid polymer and the solvent component X and a solid phase containing carbon black; and a solvent separation step of separating the liquid phase into a product containing the liquid polymer and a recovered solvent containing the solvent component X, wherein the recovered solvent is used as at least one or both of the first solvent and the second solvent. [2] The method for decomposing crosslinked rubber according to [1], wherein the solvent component X is at least one selected from the group consisting of toluene, ethylbenzene, chloroform, chlorobenzene, chloroform, and chlorobenzene. [3] A method for decomposing crosslinked rubber according to [1] or [2], further comprising a washing step of washing the solid phase and recovering the carbon black and the liquid polymer remaining in the solid phase. [4] A method for decomposing crosslinked rubber according to [3], wherein the washing solution used in the washing step is a third solvent containing the solvent component X. [5] A method for decomposing crosslinked rubber according to any one of [1] to [4], wherein the solvent separation step is performed in a distillation column, and the liquid phase is separated into a high-purity solvent obtained from the top of the distillation column, a low-purity solvent obtained from the middle of the distillation column, and a product containing the liquid polymer, wherein the high-purity solvent is used as the second solvent and the low-purity solvent is used as the first solvent. [6] A method for decomposing crosslinked rubber according to [5], wherein the temperature of the middle is below the reaction temperature of the thermal decomposition reaction in the thermal decomposition step.[7] A cross-linked rubber decomposition system comprising: a reactor for thermally decomposing a cross-linked rubber containing a diene rubber in the presence of a first solvent containing solvent component X to obtain a decomposition product containing a liquid polymer and carbon black; a solid-liquid separator for separating the decomposition product, diluted with a second solvent containing solvent component X, into a liquid phase containing solvent component X and the liquid polymer, and a solid phase containing carbon black; a solvent separator for separating the liquid phase into a product containing the liquid polymer and a recovered solvent containing solvent component X; and a circulation means for circulating at least a portion of the recovered solvent as either the first solvent or the second solvent, or both. [8] The cross-linked rubber decomposition system according to [7], further comprising a washing means for washing the solid phase containing carbon black and recovering the carbon black and the liquid polymer remaining in the solid phase. [9] The cross-linked rubber decomposition system according to [8], further comprising a third solvent supply means for supplying a third solvent containing solvent component X as a washing solution used in the washing means.

[10] The solvent separation apparatus is a stepped distillation column, and the system for decomposing crosslinked rubber according to any one of [7] to [9], comprising a high-purity solvent supply means for supplying a high-purity solvent withdrawn from the top of the stepped distillation column as the second solvent, and a low-purity solvent supply means for supplying a low-purity solvent withdrawn from an intermediate stage of the stepped distillation column as the first solvent.

[0008] According to the present invention, it is possible to provide a method and system for decomposing cross-linked rubber that can reduce capital investment and energy consumption.

[0009] This is an explanatory diagram illustrating a method for decomposing crosslinked rubber and a system for decomposing crosslinked rubber according to one aspect of the present invention.

[0010] <Method and System for Decomposing Crosslinked Rubber> According to the method for decomposing crosslinked rubber of this embodiment, by decomposing crosslinked rubber containing diene-based rubber in a solvent, a liquid polymer that can be easily decomposed into low molecular weight monomers such as isoprene and butadiene can be obtained. By further decomposing the liquid polymer, isoprene and butadiene, which are easy to reuse, can be recycled as raw materials for crosslinked rubber. Patent Document 1, mentioned above, discloses a method for obtaining carbon black by converting used tires. The present invention differs from Patent Document 1, which aims to obtain carbon black, in that it aims to obtain isoprene and butadiene, which are easy to reuse.

[0011] The cross-linked rubber decomposition method of this embodiment comprises a thermal decomposition step, a solid-liquid separation step, and a solvent separation step. The cross-linked rubber decomposition system and each step will be described below with reference to the drawings.

[0012] ≪First Embodiment≫ [Thermal Decomposition Process] Figure 1 is an explanatory diagram illustrating a method for decomposing crosslinked rubber according to the first embodiment. The thermal decomposition process involves heating crosslinked rubber containing diene rubber in a first solvent containing solvent component X to a temperature of 150°C to 300°C to thermally decompose it and obtain a decomposition product containing a liquid polymer and carbon black.

[0013] The crosslinked rubber decomposition system 1 shown in Figure 1 comprises a rubber chip pit 20 for containing crosslinked rubber G, a supply means 31 for supplying crosslinked rubber G to a reactor 22, a solvent tank 40 for containing a first solvent L1, and a supply means 32 for supplying the first solvent L1 from the solvent tank 40 to the reactor 22.

[0014] One embodiment of the method for decomposing crosslinked rubber involves first supplying crosslinked rubber G containing diene rubber and a first solvent L1 containing solvent component X into a reactor 22. When the thermal decomposition step is carried out in the first solvent L1, the crosslinked rubber G swells due to the first solvent L1. Then, decomposition products are obtained by thermally decomposing the crosslinked rubber G under solvent swelling at a temperature of 150°C to 300°C. The decomposition products include the first solvent L1, a liquid polymer, and carbon black.

[0015] The solvent component X contained in the first solvent L1 can be various solvents that have the effect of swelling the crosslinked rubber G. For example, organic solvents such as aromatic hydrocarbons (aromatic solvents), chlorinated hydrocarbons, aliphatic hydrocarbons, alicyclic hydrocarbons, esters, ethers, and nitriles can be used.

[0016] Examples of aromatic hydrocarbons include benzene, toluene, xylene, benzonitrile, diethylbenzene, o-dichlorobenzene, p-cymene, and 1,2,4-trimethylbenzene. Examples of chlorinated hydrocarbons include chloroform, dichloromethane, dichloroethane, trichloroethane, tetrachloroethane, tetrachloroethene, chlorobenzene, and dichlorobenzene. Examples of aliphatic hydrocarbons include pentane, hexane, and heptane. Examples of alicyclic hydrocarbons include cyclopentane and cyclohexane. Examples of esters include ethyl acetate, butyl acetate, and methyl benzoate. Examples of ethers include tetrahydrofuran (THF), cyclopentyl methyl ether, and 4-methyltetrahydropyran. Examples of nitriles include benzonitrile and acetonitrile. From the viewpoint of reaction rate and availability, aromatic hydrocarbons and chlorinated hydrocarbons are preferred as solvents.

[0017] The type of solvent used in the first decomposition step may be selected according to the type of rubber component contained in the object. For example, if the object contains NR, IR, or BR as a rubber component, aromatic hydrocarbons and chlorinated hydrocarbons are preferred as solvents, specifically toluene, ethylbenzene, chloroform, chlorobenzene, etc. For example, if the object contains SBR as a rubber component, chlorinated hydrocarbons are preferred as solvents, specifically chloroform, chlorobenzene, etc.

[0018] In the thermal decomposition process, the amount of the first solvent L1 used is preferably 1 mL or more, more preferably 5 mL or more, more preferably 20 mL or less, and more preferably 15 mL or less, per 1 g of crosslinked rubber G.

[0019] If the amount of the first solvent L1 used per gram of crosslinked rubber G is greater than or equal to the lower limit mentioned above, the decomposition reaction of the diene rubber in the crosslinked rubber G proceeds easily. Conversely, if the amount of the first solvent L1 used per gram of crosslinked rubber G is less than or equal to the upper limit mentioned above, it is preferable because the amount of solvent used can be reduced and the size of the reactor can be reduced. It is also preferable because the energy required to heat the solvent can be reduced.

[0020] In reactor 22, the crosslinked rubber G is thermally decomposed at a temperature between 150°C and 300°C. Performing the thermal decomposition process at a temperature above 150°C improves the rate of the decomposition reaction of the diene rubber in the crosslinked rubber G. Performing the thermal decomposition process at a temperature below 300°C makes the decomposition products less likely to gasify and less likely to become aromatized. As a result, decomposition products with a high retention rate of the monomer skeleton constituting the diene rubber can be obtained.

[0021] Furthermore, by performing the thermal decomposition process at temperatures below 300°C, for example, the carbon black contained in the cross-linked rubber G can be recovered as high-grade carbon black and reused.

[0022] From the viewpoint of improving the decomposition reaction rate of the crosslinked rubber G, the reaction temperature of the thermal decomposition step is preferably 160°C or higher, and more preferably 180°C or higher. Furthermore, from the viewpoint of obtaining a decomposition product with a high retention rate of the monomer skeleton constituting the diene rubber, the reaction temperature of the thermal decomposition step is preferably 280°C or lower, and more preferably 260°C or lower.

[0023] The decomposition time in the thermal decomposition process is not particularly limited. For example, the decomposition time in the thermal decomposition process is preferably 1 to 48 hours, and more preferably 3 to 18 hours. Performing the thermal decomposition process for 1 hour or more improves the decomposition rate of the crosslinked rubber G. On the other hand, if the decomposition time is too long, double bonds and other components in the monomer skeleton constituting the diene rubber are more likely to decompose, and the decomposition products are more likely to gasify and aromatize. Therefore, performing the thermal decomposition process for 48 hours or less makes the decomposition products less likely to gasify and less likely to aromatize. As a result, it is possible to obtain decomposition products with a high retention rate of the monomer skeleton constituting the diene rubber.

[0024] The thermal decomposition process may be carried out under an inert gas atmosphere or an active gas atmosphere. From the viewpoint of avoiding unwanted side reactions, it is preferable to carry it out under an inert gas atmosphere. By carrying out the thermal decomposition process under an inert gas atmosphere, the number of undesirable active species dissolved in the first solvent L1 can be reduced, so that the decomposition products are less likely to be oxidized and less likely to be reduced.

[0025] In particular, carrying out the thermal decomposition process under an inert gas atmosphere makes it less likely for the double bonds in the oligomers and monomers in the decomposition products to be hydrogenated. Also, if the crosslinked rubber G contains carbon black, the carbon black becomes less likely to oxidize. Examples of inert gases include nitrogen, carbon dioxide, argon, and helium.

[0026] To carry out the pyrolysis process under an inert gas atmosphere, for example, if a batch reactor is used, the atmosphere charged into the reactor should be an inert gas, and if a flow-through reactor is used, the atmosphere circulating through the reactor should be an inert gas.

[0027] The pyrolysis process can be carried out at any pressure, including at atmospheric pressure or under pressurized pressure. For example, the reaction pressure is preferably 1 kPa to 10 MPa, more preferably 10 kPa to 5 MPa, and particularly preferably 50 kPa to 2 MPa. Note that the pressures mentioned herein refer to absolute pressures.

[0028] The pyrolysis process may or may not use a catalyst, but it is preferable not to use a catalyst. By not using a catalyst in the pyrolysis process, costs can be reduced. If a catalyst is used, any catalyst that promotes the decomposition reaction of the cross-linked rubber G can be used.

[0029] The thermal decomposition process may or may not use a decomposition aid, but it is preferable not to use one. By not using a decomposition aid in the thermal decomposition process, costs can be reduced. If a decomposition aid is used, any decomposition aid that promotes the decomposition reaction of the cross-linked rubber G can be used.

[0030] (Decomposition Products) In the decomposition method for crosslinked rubber of this embodiment, the decomposition products obtained by the thermal decomposition step include a liquid polymer. Hereinafter, in this specification, a liquid polymer refers to a polymer that is liquid at room temperature (23°C) and contains two or more diene monomer units.

[0031] The weight-average molecular weight (Mw) of the liquid polymer is preferably, for example, 500 to 300,000, and more preferably 1,000 to 250,000. When the weight-average molecular weight (Mw) is above the lower limit, it is not excessively decomposed, and it is easy to maintain a retention rate of 30% by mass or more of the monomer skeleton constituting the diene rubber.

[0032] On the other hand, when the weight-average molecular weight (Mw) is below the above upper limit, the decomposition has progressed sufficiently, and therefore, when obtaining diene monomers from the liquid polymer through further decomposition, side reactions are less likely to occur, and the yield of diene monomers tends to improve.

[0033] In this specification, the weight-average molecular weight (Mw) can be measured by gel permeation chromatography (GPC).

[0034] The liquid polymer preferably has a retention rate of 30% by mass or more of the monomer skeleton constituting the diene rubber in the crosslinked rubber before decomposition, more preferably 50% by mass or more, and even more preferably 70% by mass or more. When the retention rate of the diene monomer skeleton is 70% by mass or more, diene monomers can be obtained from the liquid polymer in high yield by further decomposition.

[0035] In this specification, the retention rate of diene monomer skeletons (isoprene skeleton, butadiene skeleton, etc.) is measured by 1H-NMR.

[0036] When the crosslinked rubber G contains a diene rubber containing an isoprene skeleton, it is preferable that the retention rate of the isoprene skeleton is 30% by mass or more relative to the mass of the isoprene skeleton of the diene rubber in the crosslinked rubber G before thermal decomposition. From a liquid polymer with a high retention rate of the isoprene skeleton, isoprene that can be easily reused as a monomer can be obtained in high yield by further decomposition.

[0037] When the crosslinked rubber G contains a diene rubber containing a butadiene skeleton, it is preferable that the retention rate of the butadiene skeleton is 30% by mass or more relative to the mass of the butadiene skeleton of the diene rubber in the crosslinked rubber G before thermal decomposition. From a liquid polymer with a high retention rate of the butadiene skeleton, butadiene that can be easily reused as a monomer can be obtained in high yield by further decomposition.

[0038] [Solid-Liquid Separation Process] The solid-liquid separation process involves diluting the decomposition product obtained in the thermal decomposition process with a second solvent L2 containing solvent component X, and then separating the solid-liquid product into a liquid phase containing liquid polymer and solvent component X, and a solid phase containing carbon black. In one embodiment of the present invention, the solid-liquid separation process is preferably carried out in a range of 20°C to 150°C, and more preferably in a range of 30°C to 100°C. If the solid-liquid separation process is carried out at a temperature above the lower limit, the viscosity of the decomposition product decreases, and the amount of diluting solvent used can be reduced. If the solid-liquid separation process is carried out at a temperature below the upper limit, the materials used in the solid-liquid separation apparatus do not require high resistance, and relatively inexpensive apparatus can be used.

[0039] In the decomposition system 1 shown in Figure 1, the decomposition products are supplied from the reactor 22 to the solid-liquid separator 23 by the supply means 33. Furthermore, the second solvent L2 stored in the solvent tank 41 is supplied to the solid-liquid separator 23 by the supply means 34, and the decomposition products are diluted with the second solvent L2. The decomposition products may be diluted with the second solvent L2 before being supplied to the solid-liquid separator 23.

[0040] Because the decomposition products obtained in the thermal decomposition process are highly viscous, they are diluted with a second solvent L2 in order to perform solid-liquid separation.

[0041] The solvent component X contained in the second solvent L2 is preferably the same as the solvent component X contained in the first solvent L1 described above, and is preferably at least one selected from the group consisting of toluene, xylene, cyclohexane, and butyl acetate, with toluene being particularly preferred.

[0042] In the solid-liquid separation step, the decomposition product obtained in the aforementioned pyrolysis step is separated into a liquid phase and a solid phase. The liquid phase contains at least solvent component X and a liquid polymer, and the solid phase contains at least carbon black.

[0043] The solid-liquid separation device 23 is, for example, a centrifugal separator, a filtration device, or a filter press device. The solid phase contained in the decomposition product can be easily separated and recovered by the solid-liquid separation device 23. Since the solid phase contains at least carbon black, the carbon black can be recovered and reused.

[0044] The liquid phase obtained in the solid-liquid separation step is supplied from the solid-liquid separation device 23 to the solvent separation device 24 via the supply means 36.

[0045] [Washing Step] The solid phase obtained in the solid-liquid separation step is preferably supplied to the washing means 50 by the supply means 35 for washing. Liquid polymer may remain in the carbon black contained in the solid phase. The carbon black contained in the solid phase is washed by the washing means 50, the remaining liquid polymer LP can be recovered, and carbon black CB containing no liquid polymer can be recovered. Further, after the washing step, a mixture containing the solvent that is the washing liquid and the remaining liquid polymer LP is obtained. This mixture may be supplied to the solvent separation device 24 or may be supplied to the reactor 22.

[0046] The washing liquid used for washing the solid phase is preferably the third solvent L3 containing the solvent component X. In this case, the third solvent L3 is supplied to the washing means 50. The third solvent is a newly supplied solvent as needed. Washing efficiency is improved by using a fresh solvent as the washing liquid for carbon black.

[0047] [Solvent Separation Step] The solvent separation step is a step of separating the liquid phase obtained in the solid-liquid separation step into a product P containing a liquid polymer and a recovered solvent M containing a solvent component X. By further decomposing the product P containing the liquid polymer in a downstream step, isoprene and butadiene, which are easy to reuse, can be recycled as raw materials for crosslinked rubber. The recovered solvent M is a solvent recovered by solvent separation from the liquid phase obtained in the solid-liquid separation step, and may be a single solvent composed of one type of solvent, or may be a solvent containing a plurality of types of solvents.

[0048] The recovered solvent M obtained in the solvent separation step is used as at least one or both of the first solvent L1 and the second solvent L2. That is, the recovered solvent M may be circulated to the solvent tank 40 for cyclic use as the first solvent L1, may be circulated to the solvent tank 41 for cyclic use as the second solvent L2, or may be circulated to the solvent tank 40 and the solvent tank 41 for cyclic use as the first solvent L1 and the second solvent L2.

[0049] The crosslinked rubber decomposition system 1 can supply the required amount of solvent for each of the first solvent L1 used in the thermal decomposition step and the second solvent L2 used in the solid-liquid separation step. Therefore, facilities such as reactors and solid-liquid separation devices can be adjusted to the optimal size and number, reducing capital investment. Specifically, by supplying the solvent required for dilution in a step after the thermal decomposition step, the amount of solvent used in the thermal decomposition step can be reduced, so that a smaller-sized reactor can also be used. Furthermore, by reducing the amount of solvent used, the energy required to raise the temperature of the solvent in the thermal decomposition step can be reduced. In addition, since both the first solvent L1 and the second solvent L2 are solvents containing the solvent component X, there is no need to separate the solvents from each other, and a solvent separation device with a simple configuration can be used, so capital investment can be reduced.

[0050] In the method for decomposing crosslinked rubber of the present embodiment, a third solvent L3 containing the solvent component X may be additionally supplied as necessary. The third solvent L3 may be, for example, supplied to the solvent tank 40 and additionally supplied as the first solvent L1, may be supplied to the solvent tank 41 and additionally supplied as the second solvent L2, or may be supplied to the cleaning means 50 as a cleaning liquid as described above.

[0051] The solvent component X contained in the third solvent L3 is preferably the same as the solvent component X contained in the first solvent L1 described above, and is preferably at least one selected from the group consisting of toluene, xylene, cyclohexane, and butyl acetate, with toluene being particularly preferred.

[0052] In one embodiment of the present invention, the first solvent L1, the second solvent L2, and the third solvent L3 all contain solvent component X. Preferably, the solvent component X contained in the first solvent L1, the second solvent L2, and the third solvent L3 is the same. Preferably, the difference in the content of solvent component X contained in the first solvent L1, the second solvent L2, and the third solvent L3 is 0% by mass or more and 20% by mass or less.

[0053] The decomposition method for crosslinked rubber in this embodiment may be a batch reactor, a flow reactor, or a reactor capable of alternating reactions. Although two reactors 22 are shown in Figures 1 and 2, the number of reactors is not limited to these and can be increased as appropriate.

[0054] ≪Second Embodiment≫ In the second embodiment of the method for decomposing cross-linked rubber, the solvent separation step is carried out in a distillation column. The decomposition system 2 shown in Figure 2 includes a distillation column 25. Explanations of Figure 2 that overlap with those of Figure 1 are omitted.

[0055] The liquid phase obtained in the solid-liquid separation process is supplied from the solid-liquid separation apparatus 23 to the distillation column 25 by the supply means 36, where it is separated into a high-purity solvent S1, a low-purity solvent S2, and a product P containing a liquid polymer. The high-purity solvent S1 is obtained from the top of the distillation column 25, and the low-purity solvent S2 is obtained from the middle of the distillation column 25.

[0056] In the distillation column 25, obtaining a large amount of high-purity solvent from the top of the column results in high energy consumption. On the other hand, by extracting low-purity solvent S2, which has a lower purity of solvent component X, from the middle section, the energy consumption of the distillation column 25 can be reduced.

[0057] In the second embodiment, the high-purity solvent S1 is recycled as the second solvent L2, and the low-purity solvent S2 is recycled as the first solvent L1. The decomposition system 2 shown in Figure 2 includes a high-purity solvent supply means 37 that supplies the high-purity solvent S1 as the second solvent, and a low-purity solvent supply means 38 that supplies the low-purity solvent S2 as the first solvent.

[0058] Since high-purity solvent S1 tends to have lower viscosity than low-purity solvent S2, it is preferable to recycle high-purity solvent S1 as the second solvent L2 from the viewpoint of reducing the amount of diluent used. On the other hand, since the solvent supplied to reactor 22 can be of high viscosity, low-purity solvent S2 may be supplied to reactor 22 by being recycled as the first solvent L1. By recirculating low-purity solvent S2, which may contain liquid polymer, as the first solvent L1, the liquid polymer is further decomposed inside reactor 22, and decomposition products are obtained that tend to improve the yield when obtaining diene monomer.

[0059] The temperature at the intermediate stage where the low-purity solvent S2 is extracted is preferably below the reaction temperature of the thermal decomposition reaction in the thermal decomposition process. By extracting the low-purity solvent S2, which is to be circulated as the first solvent L1, at a temperature above the internal temperature of the distillation column 25 but below the reaction temperature of the thermal decomposition reaction, the energy required to heat the solvent in the reactor 22 can be reduced.

[0060] ≪Third Embodiment≫ In the third embodiment, the high-purity solvent S1 is recycled as the first solvent L1, and the low-purity solvent S2 is recycled as the second solvent L2.

[0061] By circulating the high-purity solvent S1 as the first solvent L1, the thermal decomposition process in the reactor 22 is accelerated, and a decomposition product with a high retention rate of the diene monomer skeleton is obtained. In this case, it is preferable to recycle the low-purity solvent S2 as the second solvent L2.

[0062] (Crosslinked Rubber) The crosslinked rubber G that is the target of decomposition in the decomposition method of this embodiment includes diene rubber and may further contain carbon black or the like. The crosslinked rubber G used for decomposition may be grouped in advance by the type of diene rubber it contains, and then the decomposition process may be carried out for each group. Alternatively, it may be grouped in advance by the type of filler it contains (for example, the type of carbon black, the type of silica, the mixing ratio of carbon black and silica, etc.), and then the decomposition process may be carried out for each group. Furthermore, it may be possible to group the rubber by the type of diene rubber and by the type of filler together, and then carry out the decomposition process for each group. When the decomposition process is carried out for each group in this way, a liquid polymer with more uniform physical properties can be obtained.

[0063] Furthermore, if the cross-linked rubber G used for decomposition is derived from tires, it may be grouped beforehand by tire type (for example, for passenger cars, trucks and buses, heavy vehicles such as off-road vehicles, aircraft, agricultural vehicles, etc.) and then the decomposition process may be carried out for each group. Alternatively, it may be grouped beforehand by tire component (for example, tread rubber, sidewall rubber, bead rubber, steel cord coated rubber, organic fiber coated rubber, pad rubber, cushion rubber, etc.) and then the decomposition process may be carried out for each group. Moreover, it may be possible to group by both tire type and tire component and then carry out the decomposition process for each group. When the decomposition process is carried out for each group in this way, a liquid polymer with more uniform physical properties can be obtained.

[0064] The form of the crosslinked rubber G is not particularly limited and may be, for example, powdered rubber. This powdered rubber can be obtained by cutting and crushing used rubber products such as waste tires. The crushing process may include multiple steps such as a preliminary crushing step and a fine crushing step, and the particle size of the powdered rubber to be used may be adjusted by a classification step after the crushing step.

[0065] Cross-linked rubber G may be recycled waste rubber, used rubber products, etc. Waste rubber refers to all discarded rubber, including not only rubber generated from rubber products but also unwanted scraps generated during the production or repair of rubber products. Examples of scraps include buffing powder and peeling rubber. Buffing powder is fine rubber generated in the buffing process, for example, in tire retreading, where the tread portion remaining on the base tire is scraped off. Peeling rubber is long pieces of rubber, for example, 1 to 2 cm wide, that are peeled off from the surface of rubber products such as tires. Peeling rubber is generated by scraping the surface of rubber products such as tires using a U-shaped or V-shaped knife like a peeler. Rubber products include, for example, final products such as tires and rubber hoses, and rubber parts or components at the manufacturing stage of final products. Used tires may be retreaded, or they may be discarded for any reason, such as tires resulting from tire replacement, vehicle scrapping, or ELT (End-of-Life Tire) tires that have reached the end of their lifespan.

[0066] (Diene-based rubber) Diene-based rubber is a rubber containing units derived from diene monomers (diene units), and may further contain units derived from copolymerizable comonomers. The units derived from the diene monomers enable crosslinking (vulcanization) of the diene-based rubber and can exhibit rubber-like elongation and strength. In crosslinked rubber, the diene-based rubber usually exists in a crosslinked state, but some parts may not be crosslinked. Specific examples of diene monomers (diene compounds) include 1,3-butadiene, isoprene, 1,3-pentadiene, 2,3-dimethyl-1,3-butadiene, and among these, 1,3-butadiene and isoprene are preferred, with isoprene being particularly preferred.

[0067] On the other hand, examples of copolymerizable comonomers include aromatic vinyl compounds. Specifically, examples of aromatic vinyl compounds include styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, o,p-dimethylstyrene, o-ethylstyrene, m-ethylstyrene, and p-ethylstyrene.

[0068] Examples of diene rubbers include isoprene-backed rubber, styrene-butadiene rubber (SBR), butadiene rubber (BR), and chloroprene rubber (CR). Here, isoprene-backed rubber is a rubber whose main backbone is isoprene units, and specifically includes natural rubber (NR) and synthetic isoprene rubber (IR). Among these, the diene rubber preferably contains at least one selected from the group consisting of isoprene-backed rubber, styrene-butadiene rubber, and butadiene rubber, with isoprene-backed rubber being particularly preferred. When the diene rubber contains at least one selected from the group consisting of isoprene-backed rubber, styrene-butadiene rubber, and butadiene rubber, a liquid polymer containing easily recyclable diene monomers such as isoprene and butadiene in its backbone is obtained by decomposition of the crosslinked rubber, along with recycled carbon black.

[0069] Furthermore, when the crosslinked rubber contains isoprene-backed rubber and butadiene rubber as diene-based rubber, the proportion of butadiene rubber in the total amount of isoprene-backed rubber and butadiene rubber is preferably 50% by mass or less. When the proportion of butadiene rubber in the total amount of isoprene-backed rubber and butadiene rubber is 50% by mass or less, gelation can be suppressed in the first decomposition step, and a decomposition product (liquid polymer) with a high isoprene backbone retention rate can be obtained.

[0070] Furthermore, if the crosslinked rubber contains butadiene rubber as a diene-based rubber, and especially if the proportion of butadiene rubber in the diene-based rubber exceeds 50% by mass, it is preferable to prevent re-crosslinking by using antioxidants, decomposition accelerators, etc.

[0071] The content of the diene-based rubber in the crosslinked rubber is not particularly limited, but is preferably in the range of 10 to 100% by mass, and more preferably in the range of 30 to 100% by mass from the viewpoint of further improving the yield of liquid polymers containing diene-based monomers in the backbone, as well as butadiene and isoprene.

[0072] (Carbon Black) The carbon black contained in the cross-linked rubber is not particularly limited, and examples of carbon black grades include N134, N110, N220, N234, N219, N339, N330, N326, N351, N550, N762, etc.

[0073] The carbon black content in the crosslinked rubber is not particularly limited, and is, for example, in the range of 10 to 150 parts by mass, preferably in the range of 30 to 120 parts by mass, per 100 parts by mass of the diene-based rubber. The carbon black content in the crosslinked rubber is preferably 20% by mass or more, more preferably 30% by mass or more, preferably 40% by mass or less, and even more preferably 35% by mass or less. When the carbon black content in the crosslinked rubber is 20% by mass or more, the amount of carbon black that can be recovered increases.

[0074] In addition to the diene-based rubber and carbon black mentioned above, crosslinked rubber may also contain various components commonly used in the rubber industry, such as rubber components other than diene-based rubber, 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 oxide), vulcanization accelerators, crosslinking agents (sulfur, peroxide, etc.), etc.

[0075] 1, 2: Cross-linked rubber decomposition system, 20: Rubber chip pit, 22: Reactor, G: Cross-linked rubber, 40, 41: Solvent tank, L1: First solvent, 23: Solid-liquid separation device, L2: Second solvent, 24: Solvent separation device, 50: Washing means, LP: Liquid polymer, CB: Carbon black, P: Product containing liquid polymer, 25: Distillation column, L3: Third solvent, 31, 32, 33, 34, 35, 36: Supply means, M: Recovery solvent, S1: High-purity solvent, S2: Low-purity solvent, 37: High-purity solvent supply means, 38: Low-purity solvent supply means

Claims

1. A method for decomposing crosslinked rubber, comprising: a thermal decomposition step of heating crosslinked rubber containing diene rubber in a first solvent containing solvent component X to 150°C or more and 300°C or less to thermally decompose it and obtain a decomposition product containing a liquid polymer and carbon black; a solid-liquid separation step of diluting the decomposition product with a second solvent containing solvent component X and then performing solid-liquid separation to separate it into a liquid phase containing the liquid polymer and solvent component X and a solid phase containing carbon black; and a solvent separation step of separating the liquid phase into a product containing the liquid polymer and a recovered solvent containing solvent component X, wherein the recovered solvent is used as at least one or both of the first solvent and the second solvent.

2. The method for decomposing crosslinked rubber according to claim 1, wherein the solvent component X is at least one selected from the group consisting of toluene, ethylbenzene, chloroform, chlorobenzene, chloroform, and chlorobenzene.

3. A method for decomposing crosslinked rubber according to claim 1 or 2, comprising a washing step of washing the solid phase and recovering the carbon black and the liquid polymer remaining in the solid phase.

4. The method for decomposing crosslinked rubber according to claim 3, wherein the cleaning solution used in the cleaning step is a third solvent containing the solvent component X.

5. The method for decomposing crosslinked rubber according to claim 1 or 2, wherein the solvent separation step is carried out in a distillation column, the liquid phase is separated into a high-purity solvent obtained from the top of the distillation column, a low-purity solvent obtained from the middle of the distillation column, and a product containing the liquid polymer, the high-purity solvent is used as the second solvent, and the low-purity solvent is used as the first solvent.

6. The method for decomposing crosslinked rubber according to claim 5, wherein the temperature of the intermediate stage is less than or equal to the reaction temperature of the thermal decomposition reaction in the thermal decomposition step.

7. A system for decomposing crosslinked rubber, comprising: a reactor for thermally decomposing crosslinked rubber containing diene rubber in the presence of a first solvent containing solvent component X to obtain a decomposition product containing a liquid polymer and carbon black; a solid-liquid separation device for separating the decomposition product, diluted with a second solvent containing solvent component X, into a liquid phase containing solvent component X and the liquid polymer, and a solid phase containing carbon black; a solvent separation device for separating the liquid phase into a product containing the liquid polymer and a recovered solvent containing solvent component X; and a recycling means for recycling at least a portion of the recovered solvent as either the first solvent or the second solvent, or both.

8. The crosslinked rubber decomposition system according to claim 7, comprising a washing means for washing the solid phase containing the carbon black and recovering the carbon black and the liquid polymer remaining in the solid phase.

9. The crosslinked rubber decomposition system according to claim 8, further comprising a third solvent supply means for supplying a third solvent containing the solvent component X as a cleaning solution used in the cleaning means.

10. The system for decomposing crosslinked rubber according to claim 7 or 8, wherein the solvent separation apparatus is a stepped distillation column, and comprises a high-purity solvent supply means for supplying a high-purity solvent withdrawn from the top of the stepped distillation column as the second solvent, and a low-purity solvent supply means for supplying a low-purity solvent withdrawn from an intermediate stage of the stepped distillation column as the first solvent.

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