Method for decomposing crosslinked rubber

Abstract

The present invention addresses the problem of providing a method for decomposing crosslinked rubber with which it is possible to obtain diene-based monomers at a high yield. The solution is a method for decomposing crosslinked rubber characterized by comprising: a first decomposition step in which crosslinked rubber containing diene rubber is decomposed in a solvent; and a second decomposition step in which the decomposition product obtained in the first decomposition step is thermally decomposed at 600-950°C in an inert gas atmosphere and in the absence of a catalyst. The solvent in the first decomposition step is preferably at least one selected from the group consisting of toluene, xylene, cyclohexane, and butyl acetate.

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. However, from the viewpoint of further enhancing the recyclability of crosslinked rubber, it is important to increase the yield of monomers obtained by decomposition, particularly diene monomers. However, as described above, when crosslinked rubber is thermally decomposed at high temperatures, the decomposition products are likely to gasify or aromatize, and not only does it not sufficiently increase the yield of diene monomers, but the carbonization reaction also progresses, forming a carbonized layer as char on the surface of the carbon black, thereby reducing the reinforcing properties of the carbon black during recycling. 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 diene monomers is low.

[0005] Therefore, an object of the present invention is to provide a method for decomposing crosslinked rubber that can obtain diene monomers in high yield while maintaining the skeleton of the constituent monomers of the diene rubber.

[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 method for decomposing a crosslinked rubber, comprising: a first decomposition step of decomposing a crosslinked rubber containing a diene rubber in a solvent; and a second decomposition step of thermally decomposing the decomposition product obtained in the first decomposition step at 600 to 950°C in an inert gas atmosphere in the absence of a catalyst.

[0008] [2] The method for decomposing crosslinked rubber according to [1], wherein the solvent in the first decomposition step is at least one selected from the group consisting of toluene, xylene, cyclohexane, and butyl acetate.

[0009] [3] The method for decomposing a crosslinked rubber according to [1] or [2], wherein the decomposition temperature in the first decomposition step is 150 to 300°C.

[0010] [4] The method for decomposing a crosslinked rubber according to any one of [1] to [3], wherein the decomposition time in the first decomposition step is 1 hour to 48 hours.

[0011] According to the present invention, it is possible to provide a method for decomposing a crosslinked rubber that can obtain a diene-based monomer in a high yield while maintaining the skeleton of the constituent monomer of the diene-based rubber contained in the crosslinked rubber.

[0012] The method for decomposing crosslinked rubber of the present invention will be described in detail below by way of example based on embodiments thereof.

[0013] [Definitions] The compounds described herein may be derived in part or in whole from fossil resources, biological resources such as plant resources, recycled resources such as used tires, or a mixture of two or more of fossil, biological, and recycled resources.

[0014] [Method for Decomposing Crosslinked Rubber] The method for decomposing crosslinked rubber of the present embodiment is characterized by comprising: a first decomposition step of decomposing a crosslinked rubber containing a diene rubber in a solvent; and a second decomposition step of thermally decomposing the decomposition product obtained in the first decomposition step at 600 to 950°C in an inert gas atmosphere in the absence of a catalyst.

[0015] In the method for decomposing crosslinked rubber of this embodiment, in the first decomposition step, a crosslinked rubber containing a diene rubber is decomposed in a solvent. The crosslinked rubber swells in the solvent, and by decomposing the crosslinked rubber under solvent swelling, gasification and aromatization of the decomposition products can be suppressed. Furthermore, compared to ordinary high-temperature pyrolysis, a decomposition product (intermediate decomposition product: liquid polymer) can be obtained that retains a higher percentage of the skeletons of the constituent monomers of the diene rubber (isoprene skeleton, butadiene skeleton, etc.). Furthermore, in the method for decomposing crosslinked rubber of this embodiment, in the second decomposition step, the decomposition product obtained in the first decomposition step is thermally decomposed at 600 to 950°C in an inert gas atmosphere and in the absence of a catalyst. This suppresses hydrogenation of the double bonds in the monomer skeletons and oxidation of the intermediate decomposition product (liquid polymer), while decomposing the intermediate decomposition product (liquid polymer) into diene monomers. Therefore, according to the method for decomposing crosslinked rubber of this embodiment, the skeletons of the constituent monomers of the diene rubber contained in the crosslinked rubber can be maintained, and the yield of diene monomers finally obtained from the crosslinked rubber can be improved.

[0016] <First Decomposition Step> The method for decomposing a crosslinked rubber according to the present embodiment includes a first decomposition step of decomposing a crosslinked rubber containing a diene rubber in a solvent.

[0017] (Crosslinked Rubber) The crosslinked rubber to be decomposed by the decomposition method of the present embodiment contains a diene rubber, and may further contain carbon black or the like.

[0018] The crosslinked rubber to be used for decomposition may be grouped according to the type of diene rubber previously compounded, and then the decomposition step may be carried out for each group. Alternatively, the crosslinked rubber may be grouped according to the type of filler previously compounded (e.g., type of carbon black, type of silica, mixing ratio of carbon black and silica, etc.), and then the decomposition step may be carried out for each group. Furthermore, the crosslinked rubber may be grouped according to both the type of diene rubber and the type of filler, and then the decomposition step may be carried out for each group. When the decomposition step is carried out for each group in this way, liquid polymers, recycled carbon black, regenerated silica, etc. having more uniform physical properties can be obtained, and therefore, when compounded again into a rubber composition, a rubber composition with better performance can be obtained.

[0019] Furthermore, when the crosslinked rubber used for degradation is derived from tires, the tires may be grouped in advance by type (e.g., for passenger cars, for trucks and buses, for large vehicles such as off-road vehicles, for aircraft, for agricultural vehicles, etc.), and then the decomposition process may be carried out for each group. Alternatively, the tires may be grouped in advance by tire component (e.g., 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. Furthermore, the tires may be grouped both by type and by tire component, and then the decomposition process may be carried out for each group. When the decomposition process is carried out for each group in this way, a liquid polymer or recycled carbon black having more uniform physical properties can be obtained, and therefore, when blended again into a rubber composition, a rubber composition with better performance can be obtained.

[0020] 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 process may include multiple steps, such as a preliminary pulverization step and a fine pulverization step. Furthermore, the particle size of the powdered rubber used may be adjusted by a classification step after the pulverization step. The crosslinked rubber may be recycled waste rubber, used rubber products, etc. Waste rubber refers not only to rubber generated from rubber products but also to all discarded rubber, including unnecessary scraps generated during the production or repair of rubber products. Examples of scraps include buffing powder and peeled rubber. Buffing powder is fine rubber generated, for example, during the buffing process of scraping the tread portion remaining on the base tire during tire retreading. Peeled rubber is a long piece of rubber, for example, 1 to 2 cm wide, peeled from the surface of a rubber product such as a tire. Peeled rubber is generated by scraping the surface of a rubber product such as a tire using a U- or V-shaped knife like a peeler. Rubber products include, for example, final products such as tires and rubber hoses, as well as rubber parts or components in the manufacturing stage of final products. Used tires may be, for example, tires to be retreaded, or may be tires discarded for some reason, such as waste tires generated when tires are replaced or scrapped, or ELTs (End-of-Life Tires) that have reached the end of their life as tires.

[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. Note that, although the diene-based rubber is usually present in a crosslinked state in the crosslinked rubber, a portion of it may not be crosslinked. Specific examples of diene-based monomers (diene-based compounds) include 1,3-butadiene, isoprene, 1,3-pentadiene, 2,3-dimethyl-1,3-butadiene, etc., and among these, 1,3-butadiene and isoprene are preferred, with isoprene being particularly preferred. On the other hand, examples of the copolymerizable comonomer include aromatic vinyl compounds, etc. Specific examples of the aromatic vinyl compound include styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, o,p-dimethylstyrene, o-ethylstyrene, m-ethylstyrene, and p-ethylstyrene.

[0022] Examples of the diene rubber include isoprene-skeleton rubber, styrene-butadiene rubber (SBR), butadiene rubber (BR), and chloroprene rubber (CR). Here, 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 includes at least one selected from the group consisting of isoprene-skeleton rubber, styrene-butadiene rubber, and butadiene rubber, with isoprene-skeleton rubber being particularly preferred. When the diene rubber includes at least one selected from the group consisting of isoprene-skeleton rubber, styrene-butadiene rubber, and butadiene rubber, decomposition of the crosslinked rubber produces a liquid polymer containing easily recyclable diene monomers, such as isoprene or butadiene, in its skeleton, along with recycled carbon black.

[0023] Furthermore, when the crosslinked rubber contains an isoprene skeleton rubber and a butadiene rubber as diene-based rubbers, the proportion of the butadiene rubber in the total amount of the isoprene skeleton rubber and the butadiene rubber is preferably 50% by mass or less. When the proportion of the butadiene rubber in the total amount of the isoprene skeleton rubber and the 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 retention rate of the isoprene skeleton can be obtained. Note that when the crosslinked rubber contains a butadiene rubber as a diene-based rubber, particularly when the proportion of the butadiene rubber in the diene-based rubber exceeds 50% by mass, it is preferable to prevent re-crosslinking by using an antioxidant, a decomposition accelerator, or the like.

[0024] The content of the diene rubber in the crosslinked rubber is not particularly limited, and is preferably, for example, in the range of 10 to 100 mass %, and more preferably in the range of 30 to 100 mass % from the viewpoint of further improving the yield of a liquid polymer containing a diene monomer in the skeleton, and of butadiene and isoprene.

[0025] -Carbon Black- The crosslinked rubber may further contain carbon black. The carbon black is not particularly limited, and examples of carbon black grades include N134, N110, N220, N234, N219, N339, N330, N326, N351, N550, and N762. In the method for decomposing a crosslinked rubber according to this embodiment, in the first decomposition step, the diene rubber in the crosslinked rubber is decomposed, and the molecular weight can be reduced, for example, to a liquid polymer. Therefore, even if the crosslinked rubber contains carbon black, the carbon black can be easily separated and recovered after the first decomposition step by, for example, centrifugation, filtration, or the like. By recovering the carbon black before the thermal decomposition in the second decomposition step, deterioration of the carbon black can be avoided, and the carbon black can be reused as high-quality carbon black. 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 30 to 120 parts by mass, per 100 parts by mass of the diene rubber. The carbon black content in the crosslinked rubber is preferably 20% by mass or more, more preferably 30% by mass or more, and is preferably 40% by mass or less, more preferably 35% by mass or less. When the carbon black content in the crosslinked rubber is 20% by mass or more, a larger amount of carbon black can be recovered.

[0026] -Other Components- In addition to the diene rubber and carbon black described above, the crosslinked rubber may contain 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 (sulfur, peroxides, etc.), etc.

[0027] (Reaction Conditions, etc.) The first decomposition step is carried out in a solvent. When the first decomposition step is carried out in a solvent, the crosslinked rubber swells due to the solvent. By decomposing the crosslinked rubber under solvent swelling, gasification and aromatization of the decomposition product can be suppressed, and a decomposition product (intermediate decomposition product: liquid polymer) can be obtained that has a higher retention rate of the skeletons (isoprene skeleton, butadiene skeleton, etc.) of the constituent monomers of the diene rubber compared to normal high-temperature thermal decomposition.

[0028] As the solvent for the first decomposition step, various solvents having the effect of swelling the crosslinked rubber can be used, for example, aromatic solvents, aliphatic solvents, alicyclic solvents, ethers, esters, etc. can be used. Aromatic solvents include benzene, toluene, xylene, etc., aliphatic solvents include pentane, hexane, heptane, etc., alicyclic solvents include cyclopentane, cyclohexane, etc., ethers include tetrahydrofuran (THF), etc., and esters include ethyl acetate, butyl acetate, etc. Among these, the solvent for the first decomposition step is preferably at least one selected from the group consisting of toluene, xylene, cyclohexane, and butyl acetate, and toluene is particularly preferred. When toluene, xylene, cyclohexane, and butyl acetate are used as the solvent for the first decomposition step, a decomposition product (intermediate decomposition product: liquid polymer) having a high retention rate of the skeletons of the constituent monomers of the diene rubber (isoprene skeleton, butadiene skeleton, etc.) can be obtained.

[0029] The amount of the solvent used is preferably 1 mL or more, more preferably 5 mL or more, and is preferably 500 mL or less, more preferably 200 mL or less, per gram of the crosslinked rubber. If the amount of the solvent used is 1 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.

[0030] The temperature of the first decomposition step is preferably 150 to 300°C. By carrying out the first decomposition step at 150°C or higher, the rate of decomposition reaction of the diene rubber in the crosslinked rubber is improved. Furthermore, by carrying out the first decomposition step at 300°C or lower, gasification and aromatization of the decomposition products can be suppressed, and after decomposition, the retention rate (selectivity) of the structural monomer skeleton of the diene rubber in the crosslinked rubber is improved. Furthermore, for example, when the crosslinked rubber contains carbon black, it can be recovered and reused as high-quality carbon black. From the viewpoint of improving the decomposition reaction rate of the crosslinked rubber, the temperature of the first decomposition step is preferably 160°C or higher, more preferably 180°C or higher. Furthermore, from the viewpoint of improving the selectivity of products that maintain the monomer skeleton, it is preferably 280°C or lower, more preferably 250°C or lower.

[0031] The decomposition time of the first decomposition step is not particularly limited. As an example, the decomposition time of the first decomposition step is preferably 1 hour to 48 hours, and more preferably 3 hours to 18 hours. By carrying out the first decomposition step for 1 hour or more, the decomposition rate of the crosslinked rubber is improved. On the other hand, if the decomposition time is too long, the skeleton (double bond, etc.) of the constituent monomer of the diene rubber is decomposed, and the decomposition products are gasified or aromatized. However, by carrying out the first decomposition step for 48 hours or less, the decomposition of the monomer skeleton and the gasification or aromatization of the decomposition products can be suppressed, and the retention rate (selectivity) of the skeleton of the constituent monomer of the diene rubber in the crosslinked rubber after decomposition is improved.

[0032] The first decomposition step is preferably carried out under an inert gas atmosphere. By carrying out the first decomposition step under an inert gas atmosphere, the active species (oxygen, hydrogen, etc.) dissolved in the solvent can be reduced, and oxidation or reduction of the decomposition products can be suppressed, and in particular, hydrogenation of double bonds in oligomers and monomers in the decomposition products can be suppressed. Furthermore, when the crosslinked rubber contains carbon black, oxidation of the carbon black can also be suppressed. Here, examples of the inert gas include nitrogen, carbon dioxide, argon, and helium.

[0033] To carry out the first 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 first decomposition step, the generated hydrogen is not taken into consideration when determining the atmosphere for the first decomposition step.

[0034] The first decomposition step can be carried out at any pressure, including reduced pressure, normal pressure, and elevated pressure. For example, the reaction pressure is preferably 1 kPa to 10 MPa, more preferably 10 kPa to 5 MPa, and even more preferably 50 kPa to 2 MPa.

[0035] The first decomposition step may or may not use a catalyst, but it is preferable not to use a catalyst. Not using a catalyst in the first decomposition step can reduce costs. When a catalyst is used, any catalyst that has the effect of accelerating the decomposition reaction of the crosslinked rubber can be used.

[0036] The first decomposition step may or may not use a decomposition aid, but it is preferable not to use a decomposition aid. Not using a decomposition aid in the first decomposition step can reduce costs. When a decomposition aid is used, any decomposition aid that has the effect of accelerating the decomposition reaction of the crosslinked rubber can be used.

[0037] (Decomposition product (intermediate decomposition product: liquid polymer)) In the decomposition method for crosslinked rubber of this embodiment, a liquid polymer is obtained as a decomposition product by the first decomposition step. Here, in this specification, a liquid polymer refers to one that is liquid at room temperature (23°C) and contains two or more diene monomer units.

[0038] The liquid polymer preferably has a weight-average molecular weight (Mw) of 500 to 300,000, more preferably 1,000 to 250,000. When the weight-average molecular weight (Mw) is 500 or more, decomposition has not progressed too far, making it easy to achieve a diene monomer skeleton retention rate of 30% by mass or more. On the other hand, when the weight-average molecular weight (Mw) is 300,000 or less, decomposition has progressed sufficiently, making it difficult for side reactions to proceed when obtaining a diene monomer from the liquid polymer due to further decomposition, and making it easy to improve the yield of the diene monomer. In this specification, the weight-average molecular weight (Mw) can be measured by gel permeation chromatography (GPC).

[0039] The liquid polymer preferably has a diene monomer skeleton retention rate of 30% by mass or more, more preferably 50% by mass or more, and even more preferably 70% by mass or more, relative to the mass of the diene monomer skeleton of the diene rubber in the crosslinked rubber before decomposition. When the diene monomer skeleton retention rate is 70% by mass or more, diene monomers can be obtained from the liquid polymer in high yield by further decomposition. In this specification, the retention rate of the diene monomer skeleton (isoprene skeleton, butadiene skeleton, etc.) is １ Measured by H-NMR.

[0040] The diene monomer skeleton of the diene rubber is preferably at least one selected from the group consisting of an isoprene skeleton and a butadiene skeleton. From the liquid polymer having a high retention rate of the isoprene skeleton and / or butadiene skeleton, easily recyclable diene monomers such as isoprene and butadiene can be obtained in high yields by further decomposition.

[0041] When the crosslinked rubber contains a diene rubber containing an isoprene skeleton, the retention rate of the isoprene skeleton is preferably 30 mass% or more relative to the mass of the isoprene skeleton of the diene rubber in the crosslinked rubber before 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.

[0042] When the crosslinked rubber contains a diene rubber containing a butadiene skeleton, the retention rate of the butadiene skeleton is preferably 30 mass% or more relative to the mass of the butadiene skeleton of the diene rubber in the crosslinked rubber before decomposition. From the 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.

[0043] Furthermore, when the crosslinked rubber contains carbon black together with the diene rubber, a mixture of the liquid polymer and recycled carbon black can be obtained by the first decomposition step. The recycled carbon black in the mixture can be recovered and reused.

[0044] <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 further thermally decomposed at 600 to 950°C in an inert gas atmosphere in the absence of a catalyst.

[0045] (Reaction Conditions, etc.) By carrying out the second decomposition step at 600°C or higher, the decomposition reaction rate of the decomposition products obtained in the first decomposition step is significantly increased, and the monomer yield is significantly improved. Furthermore, by carrying out the second decomposition step at 950°C or lower, gasification and aromatization of the decomposition products can be suppressed, and the selectivity of products that maintain the monomer skeleton is improved. From the viewpoint of improving the decomposition reaction rate and improving the monomer yield, the second decomposition step is preferably 700°C or higher. Furthermore, by carrying out the second decomposition step at 700 to 900°C, the decomposition reaction rate of the decomposition products obtained in the first decomposition step can be increased, while the selectivity of products that maintain the monomer skeleton can be improved.

[0046] The reaction time of the second decomposition step is not particularly limited. For example, the reaction time of the second decomposition step is preferably 0.001 seconds to 1000 seconds, and more preferably 0.01 seconds to 1000 seconds.

[0047] In a preferred embodiment of the second decomposition step, the decomposition product obtained in the first decomposition step is heated from 100° C. to 450° C. at a heating rate of 800° C. / min or more in an inert gas atmosphere, and then thermally decomposed at 600 to 950° C. When the heating rate from 100° C. to 450° C. is 800° C. / min or more, the generation of decomposition products such as oligomers having higher molecular weights than the diene monomers is suppressed, and after the decomposition, the retention rate (selectivity) of the skeletons of the constituent monomers of the decomposition product (liquid polymer) obtained in the first decomposition step is further improved, thereby further improving the yield of the diene monomer.

[0048] In another preferred embodiment of the second decomposition step, the decomposition product obtained in the first decomposition step is heated from 100° C. to 450° C. at a heating rate of 2000° C. / min or more in an inert gas atmosphere, and then thermally decomposed at 600 to 950° C. When the heating rate from 100° C. to 450° C. is 2000° C. / min or more, the generation of decomposition products such as oligomers having higher molecular weights than the diene monomers is suppressed, and after the decomposition, the retention rate (selectivity) of the skeletons of the constituent monomers of the decomposition product (liquid polymer) obtained in the first decomposition step is further improved, and the yield of the diene monomer is further improved.

[0049] In another preferred embodiment of the second decomposition step, the decomposition product obtained in the first decomposition step is heated from 100° C. to 600° C. at a heating rate of 800° C. / min or more in an inert gas atmosphere, and then thermally decomposed at 650 to 950° C. When the heating rate from 100° C. to 600° C. is 800° C. / min or more, the generation of decomposition products such as oligomers having higher molecular weights than the diene monomers is suppressed, and after the decomposition, the retention rate (selectivity) of the skeletons of the constituent monomers of the decomposition product (liquid polymer) obtained in the first decomposition step is further improved, and the yield of the diene monomer is further improved.

[0050] In another preferred embodiment of the second decomposition step, the decomposition product obtained in the first decomposition step is heated from 100° C. to 600° C. at a heating rate of 2000° C. / min or more in an inert gas atmosphere, and then thermally decomposed at 650 to 950° C. When the heating rate from 100° C. to 600° C. is 2000° C. / min or more, the generation of decomposition products such as oligomers having higher molecular weights than the diene monomers is suppressed, and after the decomposition, the retention rate (selectivity) of the skeletons of the constituent monomers of the decomposition product (liquid polymer) obtained in the first decomposition step is further improved, and the yield of the diene monomer is further improved.

[0051] In another preferred embodiment of the second decomposition step, the decomposition product obtained in the first decomposition step is mixed with a medium selected from fatty acid esters and hydrocarbons, and then thermally decomposed in an inert gas atmosphere in the absence of a catalyst at 600 to 950° C. By mixing the decomposition product obtained in the first decomposition step with a medium and then thermally decomposing it, the decomposition of the decomposition product (liquid polymer) obtained in the first decomposition step into diene monomers is promoted, and the diene monomers can be obtained in a high yield.

[0052] 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.

[0053] 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.

[0054] 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 65 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.

[0055] 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.

[0056] (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.

[0057] 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. The hydrocarbon compounds having 5 or less carbon atoms as decomposition products vary depending on the type of diene rubber in the crosslinked rubber to be decomposed, but examples include 1,3-butadiene, isoprene, 1,3-pentadiene, 2,3-dimethyl-1,3-butadiene, etc., 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 in the second decomposition step, such as the reaction temperature and reaction time.

[0058] <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.

[0059] The method for decomposing a 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, centrifugation, or the like, or recovered by precipitation using a poor solvent, and can be reused. The monomer (particularly, a diene monomer) finally obtained from the crosslinked rubber can be reused as a raw material for a diene rubber (polymer).

[0060] 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.

[0061] (1) Method for Analyzing Retention Rate of Monomer Skeleton in Diene Rubber The obtained decomposition products (liquid polymers) were analyzed using an NMR apparatus under the trade name "Nuclear Magnetic Resonance Spectrometer JNM-ECA-500" manufactured by JEOL Ltd., and the retention rate of the isoprene skeleton and / or butadiene skeleton derived from the natural rubber and / or butadiene rubber contained in each crosslinked rubber sample was calculated.

[0062] (2) Method for Analyzing Weight-Average Molecular Weight (Mw) of Liquid Polymer The polystyrene-equivalent weight-average molecular weight (Mw) of the degradation product (liquid polymer) was determined using 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; detector: differential refractometer RID-10A manufactured by Shimadzu Corporation; analysis system: LabSolutions manufactured by Shimadzu Corporation; eluent: tetrahydrofuran) with monodisperse standard polystyrene as the standard. The measurement temperature was 40°C.

[0063] (Preparation of Crosslinked Rubber Sample A) A rubber composition was prepared by blending 50 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 parts by mass of natural rubber, and the rubber composition was then heat-crosslinked to prepare a crosslinked rubber. The resulting crosslinked rubber was cut into a shape of 3 mm x 3 mm x 2 mm to prepare crosslinked rubber sample A.

[0064] (Preparation of Crosslinked Rubber Sample B) Crosslinked rubber sample B was prepared in the same manner as crosslinked rubber sample A, except that 60 parts by mass of natural rubber and 40 parts by mass of butadiene rubber (manufactured by UBE Elastomers Co., Ltd., product name "UBEPOL BR150L") were compounded instead of 100 parts by mass of natural rubber.

[0065] (Preparation of Crosslinked Rubber Sample C) Crosslinked rubber sample C was prepared in the same manner as crosslinked rubber sample A, except that 40 parts by mass of natural rubber and 60 parts by mass of butadiene rubber (manufactured by UBE Elastomers Co., Ltd., product name "UBEPOL BR150L") were compounded instead of 100 parts by mass of natural rubber.

[0066] <Examples of the first decomposition step> (Examples 1-1 to 1-7) 0.4 g of crosslinked rubber sample A and 4 mL of toluene were added to a reactor, and the mixture was stirred under an Ar atmosphere at 200°C or 240°C for 3, 6, 9 or 18 hours to carry out a decomposition reaction. After the reaction, the mixture was separated into a solvent (toluene) soluble component and a solvent insoluble component. The solvent soluble component was mainly composed of a liquid polymer, while the solvent insoluble component was mainly composed of carbon black. The obtained solvent soluble component (liquid polymer) was subjected to the following steps: １ H-NMR analysis was performed to calculate the retention rate of the diene monomer skeleton. The expected diene monomer yield was calculated from the product of the liquid polymer yield and the retention rate of the diene monomer skeleton. Furthermore, the weight average molecular weight (Mw) of the liquid polymer was analyzed by GPC analysis.

[0067] (Examples 2-1 to 2-7) A decomposition reaction was carried out in the same manner as in Examples 1-1 to 1-7, except that crosslinked rubber sample B was used as the decomposition target instead of crosslinked rubber sample A. １ The retention rate of the diene monomer skeleton in the product was calculated by H-NMR analysis, and the expected diene monomer yield was also calculated. Furthermore, the weight average molecular weight (Mw) of the liquid polymer was analyzed by GPC analysis.

[0068] (Examples 3-1 to 3-8) A decomposition reaction was carried out in the same manner as in Examples 1-1 to 1-7, except that crosslinked rubber sample C was used as the decomposition target instead of crosslinked rubber sample A. １ The retention rate of the diene monomer skeleton in the product was calculated by H-NMR analysis, and the expected diene monomer yield was also calculated. Furthermore, the weight average molecular weight (Mw) of the liquid polymer was analyzed by GPC analysis.

[0069] (Examples 4-1 to 4-7) A decomposition reaction was carried out in the same manner as in Examples 1-1 to 1-7, except that cyclohexane was used as the solvent instead of toluene. １ The retention rate of the diene monomer skeleton in the product was calculated by H-NMR analysis, and the expected diene monomer yield was also calculated. Furthermore, the weight average molecular weight (Mw) of the liquid polymer was analyzed by GPC analysis.

[0070] (Examples 5-1 to 5-7) A decomposition reaction was carried out in the same manner as in Examples 1-1 to 1-7, except that xylene was used as the solvent instead of toluene. １ The retention rate of the diene monomer skeleton in the product was calculated by H-NMR analysis, and the expected diene monomer yield was also calculated. Furthermore, the weight average molecular weight (Mw) of the liquid polymer was analyzed by GPC analysis.

[0071] (Example 6-1 to Example 6-2) A decomposition reaction was carried out in the same manner as in Example 1-1 to Example 1-7, except that butyl acetate was used as the solvent instead of toluene. １The retention rate of the diene monomer skeleton in the product was calculated by H-NMR analysis, and the expected diene monomer yield was also calculated. Furthermore, the weight average molecular weight (Mw) of the liquid polymer was analyzed by GPC analysis. The results of Examples 1-1 to 6-2 are shown in Tables 1 to 6, respectively.

[0072]

[0073]

[0074]

[0075]

[0076]

[0077]

[0078] Example 7: A 3.0 mg portion of the reaction mixture obtained in Example 1-1 (first decomposition step) (a reaction mixture obtained by concentrating the filtrate obtained by filtering the reaction solution of Example 1-1 under reduced pressure) was dissolved in 0.60 mL of tetrahydrofuran. A 5.0 μL portion of the solution was placed in a stainless steel container, and the solvent was distilled off under reduced pressure. 0.025 mg of the reaction mixture from the first decomposition step obtained after solvent distillation was heated to 700°C using a pyrolyzer (Frontier Labs PY-3030D), and the resulting pyrolysis product was analyzed using a gas chromatograph mass spectrometer and a gas chromatograph analyzer (Shimadzu Corporation GCMS-QP2010Plus and Shimadzu Corporation GC-2014, respectively, both equipped with Frontier Labs UltraALLOY UA-1 capillary columns). The yields were estimated to be 34.8% for isoprene, 1.3% for limonene, 17.3% for hydrocarbons with 1 to 4 carbon atoms (including butadiene), and 38.8% for hydrocarbons with 5 to 10 carbon atoms (other than isoprene and limonene).

[0079] (Examples 8 to 11) The pyrolysis reaction was carried out in the same manner as in Example 7, except that the decomposition target and heating temperature were changed as shown in Table 7, and the obtained pyrolysis products were analyzed using a gas chromatograph mass spectrometer and a gas chromatograph analyzer, and the results were estimated.

[0080]

[0081] *1 Yield of hydrocarbons with 1 to 4 carbon atoms (other than butadiene) *2 Yield of hydrocarbons with 5 to 10 carbon atoms (other than isoprene and limonene)

[0082] From Table 7, it is predicted that diene monomers can be obtained in high yields according to the method for decomposing crosslinked rubber of the present invention.

Claims

1. A method for decomposing crosslinked rubber, comprising: a first decomposition step of decomposing a crosslinked rubber containing a diene rubber in a solvent; and a second decomposition step of thermally decomposing the decomposition product obtained in the first decomposition step at 600 to 950°C in an inert gas atmosphere and in the absence of a catalyst.

2. The method for decomposing crosslinked rubber according to claim 1, wherein the solvent in the first decomposition step is at least one selected from the group consisting of toluene, xylene, cyclohexane and butyl acetate.

3. The method for decomposing crosslinked rubber according to claim 1, wherein the decomposition temperature in the first decomposition step is 150 to 300°C.

4. The method for decomposing crosslinked rubber according to claim 1, wherein the decomposition time in the first decomposition step is 1 hour to 48 hours.

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