Decomposition method for liquid polymer and decomposition method for crosslinked rubber

Abstract

The present invention addresses the problem of providing a decomposition method for a liquid polymer and a decomposition method for a crosslinked rubber, whereby it is possible to obtain a diene-based monomer at high yield. Said problem is solved by: a decomposition method for a liquid polymer, the method being characterized in that a liquid polymer and at least one medium selected from fatty acid esters and hydrocarbons are mixed and thermally decomposed at 300-950°C in an inert gas atmosphere; and a decomposition method for a crosslinked rubber, the method being characterized by including a first decomposition step in which a crosslinked rubber containing a diene-based rubber is decomposed to obtain a liquid polymer and a second decomposition step in which the obtained liquid polymer is decomposed by said decomposition method for a liquid polymer.

Description

Method for decomposing liquid polymers and method for decomposing crosslinked rubber

[0001] The present invention relates to a method for decomposing a liquid polymer and a method for decomposing a 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 improving 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 tend to gasify or aromatize, and the yield of diene monomers cannot be sufficiently increased. 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] In response to this, the present inventors have found that by decomposing a crosslinked rubber containing a diene rubber at a temperature lower than that of conventional thermal decomposition, a liquid polymer derived from the diene rubber (i.e., a partial decomposition product of the diene rubber) can be obtained, and by further thermally decomposing the liquid polymer, a diene monomer can be obtained. However, even with this method, there is still room for improvement in terms of the yield of the diene monomer in the thermal decomposition of the liquid polymer.

[0006] Therefore, an object of the present invention is to provide a method for decomposing a liquid polymer, which can obtain a diene monomer in a high yield.A further object of the present invention is to provide a method for decomposing a crosslinked rubber, which can obtain a diene monomer in a high yield by applying the method for decomposing a liquid polymer.

[0007] The method for decomposing a liquid polymer and a method for decomposing a crosslinked rubber according to the present invention, which solve the above problems, are outlined as follows.

[0008] [1] A method for decomposing a liquid polymer, comprising mixing a liquid polymer with at least one medium selected from fatty acid esters and hydrocarbons, and thermally decomposing the mixture at 300°C to 950°C in an inert gas atmosphere.

[0009] [2] The method for decomposing a liquid polymer according to [1], wherein the medium has a weight average molecular weight (Mw) of 150 to 1,200.

[0010] [3] The method for decomposing a liquid polymer according to [1] or [2], wherein the medium is a hydrocarbon.

[0011] [4] The method for decomposing a liquid polymer according to any one of [1] to [3], wherein the thermal decomposition is carried out at 600°C to 800°C.

[0012] [5] A method for decomposing a crosslinked rubber, comprising: a first decomposition step of decomposing a crosslinked rubber containing a diene rubber to obtain a liquid polymer; and a second decomposition step of decomposing the liquid polymer by the method for decomposing a liquid polymer according to any one of [1] to [4].

[0013] According to the present invention, it is possible to provide a method for decomposing a liquid polymer, which is capable of obtaining a diene monomer in a high yield. Also, according to the present invention, it is possible to provide a method for decomposing a crosslinked rubber, which is capable of obtaining a diene monomer in a high yield.

[0014] The method for decomposing a liquid polymer and the method for decomposing a crosslinked rubber according to the present invention will be described in detail below by way of example based on embodiments thereof.

[0015] [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.

[0016] [Method for Decomposing Liquid Polymers] The method for decomposing liquid polymers according to the present embodiment is characterized in that the liquid polymer is mixed with at least one medium selected from fatty acid esters and hydrocarbons, and the mixture is thermally decomposed at 300°C to 950°C in an inert gas atmosphere.

[0017] In the method for decomposing a liquid polymer of this embodiment, the liquid polymer is mixed with a medium selected from a fatty acid ester and a hydrocarbon, and the mixture is thermally decomposed at 300°C to 950°C in an inert gas atmosphere, thereby making it possible to obtain a diene monomer from the liquid polymer in high yield while suppressing hydrogenation of double bonds in the skeletons (isoprene skeleton, butadiene skeleton, etc.) of the constituent monomers of the liquid polymer and oxidation of the decomposition products. Without wishing to be bound by theory, it is believed that in the method for decomposing a liquid polymer of this embodiment, the medium selected from a fatty acid ester and a hydrocarbon provides hydrogen to the liquid polymer during decomposition, thereby accelerating the decomposition of the liquid polymer into a diene monomer, and thereby allowing the diene monomer to be obtained in high yield.

[0018] <Liquid Polymer> In the liquid polymer decomposition method of this embodiment, the liquid polymer to be decomposed is liquid at room temperature (23°C) and contains units derived from diene monomers (diene units), and can also be called a liquid diene polymer. Examples of diene monomers (diene compounds) constituting the diene monomer units include 1,3-butadiene, isoprene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. Among these, 1,3-butadiene and isoprene are preferred, with isoprene being particularly preferred. The liquid polymer is easily mixed with the medium described below, and, because it is liquid, is easier to handle than the crosslinked rubber described below. The liquid polymer and medium do not need to be mixed at room temperature (23°C). Any medium that becomes liquid at a temperature lower than the thermal decomposition temperature can be mixed sufficiently well with the liquid polymer.

[0019] The method for producing the liquid polymer is not particularly limited, and the liquid polymer can be obtained by, for example, (i) a method of decomposing a crosslinked rubber using a metathesis catalyst (first decomposition method), (ii) a method of thermally decomposing a crosslinked rubber at 150°C or higher and 400°C or lower (second decomposition method), (iii) a method of decomposing a crosslinked rubber in a solvent (third decomposition method), or (iv) a method of radically decomposing a crosslinked rubber (fourth decomposition method). These first, second, third, and fourth decomposition methods will be described in detail in the section "First decomposition step" of "Method for decomposing crosslinked rubber" below.

[0020] In the method for decomposing a liquid polymer according to the present embodiment, the liquid polymer is mixed with a medium, and the medium is at least one selected from the group consisting of a fatty acid ester and a hydrocarbon. By mixing the liquid polymer with the medium as a preliminary step to the thermal decomposition, the mobility of the polymer chains in the liquid polymer can be increased, leading to a state where the liquid polymer is more susceptible to thermal decomposition, and as a result, the yield of the diene monomer can be increased.

[0021] Examples of the fatty acid ester include esters of fatty acids and alcohols such as glycerin or aliphatic alcohols. Examples of the fatty acids include myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, and erucic acid. The fatty alcohol preferably has 12 to 24 carbon atoms. Specific examples of the fatty acid ester include rapeseed oil, soybean oil, olive oil, sunflower oil, safflower oil, corn oil, castor oil, jojoba oil, and palm oil. These may be hydrogenated (hydrogenated), and examples of hydrogenated fatty acid esters include hydrogenated rapeseed oil and hydrogenated jojoba oil. These fatty acid esters may be recycled or discarded, such as waste cooking oil. When the alcohol is glycerin, the fatty acid ester may be a triglyceride, as well as a monoglyceride or a diglyceride. The fatty acid moiety of the fatty acid ester preferably has 8 or more carbon atoms, more preferably 12 or more carbon atoms, and preferably 24 or less carbon atoms, more preferably 18 or less carbon atoms. When the fatty acid moiety of the fatty acid ester has 8 or more carbon atoms, the fatty acid ester is present in the reaction field during thermal decomposition, allowing it to fully exert its effect. In addition, the amount of hydrogen generated when the fatty acid ester is decomposed increases, providing more hydrogen to the liquid polymer and accelerating the decomposition of the liquid polymer. Furthermore, when the fatty acid moiety of the fatty acid ester has 24 or less carbon atoms, it is easily miscible with the liquid polymer.

[0022] Examples of the hydrocarbon include liquid paraffin, microcrystalline wax, asphalt-mixed naphthenic oil, polyethylene, polypropylene, etc., of which liquid paraffin and microcrystalline wax are preferred, and microcrystalline wax is more preferred. The hydrocarbon preferably has 12 or more carbon atoms, more preferably 20 or more carbon atoms, and preferably 80 or less carbon atoms, more preferably 60 or less carbon atoms. When the hydrocarbon has 12 or more carbon atoms, it is present in the reaction field during thermal decomposition, allowing it to fully exert its effects. In addition, the amount of hydrogen generated during the decomposition of the hydrocarbon increases, providing more hydrogen to the liquid polymer and promoting the decomposition of the liquid polymer. Furthermore, when the hydrocarbon has 80 or less carbon atoms, it is easily miscible with the liquid polymer.

[0023] In the method for decomposing a liquid polymer according to the present embodiment, the medium is preferably the hydrocarbon described above. By using a hydrocarbon as the medium, the yield of diene monomers can be further improved.

[0024] The medium has an SP value of 7.5 to 11 (cal / cm ３ ) １／２ It is preferable that the viscosity is 7.8 to 10 (cal / cm ３ ) １／２ It is more preferable that the SP value of the medium is 7.5 (cal / cm ３ ) １／２ If the SP value of the medium is 11 (cal / cm ３ ) １／２ When the SP value is less than 1 / 2, the yield of the diene monomer can be further improved. In this specification, the SP value (solubility parameter) is measured by the Fedors method, and specifically, it can be calculated by the Fedors method described in Polymer Engineering Science, 14, 147 (1974). In addition, a medium whose SP value is known in advance can also be used.

[0025] Furthermore, the endothermic peak temperature of the medium due to decomposition and volatilization measured by thermogravimetry (TGA) is preferably 120°C to 500°C, more preferably 180°C to 450°C. When the endothermic peak temperature of the medium is 120°C or higher, the medium is present in the reaction field during thermal decomposition and can fully exert its effects. When the endothermic peak temperature of the medium is 500°C or lower, the medium is easily miscible with the liquid polymer. Note that, in this specification, the endothermic peak temperature measured by thermogravimetry (TGA) refers to the endothermic peak temperature when the heating rate is 10°C / min or higher, preferably 10 to 20°C / min. For the TGA measurement, the endothermic peak temperature can be obtained using a differential thermobalance (e.g., "Thermo Mass Photo" manufactured by Rigaku Corporation). Alternatively, a medium whose endothermic peak temperature is known in advance can be used.

[0026] Furthermore, the medium is preferably liquid at 150°C. When mixed with the liquid polymer, the medium remains liquid without vaporizing before the temperature reaches the thermal decomposition temperature, thereby increasing the mobility of the polymer chains in the liquid polymer and bringing it into a state where it is more susceptible to thermal decomposition. Furthermore, the medium that is liquid at 150°C can be distinguished from crosslinked rubbers and resin materials that are solid at 150°C.

[0027] The weight-average molecular weight (Mw) of the medium is preferably 150 to 1200, and more preferably 170 to 1000. When the weight-average molecular weight (Mw) of the medium is 150 or more, the medium is present in the reaction field during thermal decomposition, allowing the medium to fully exert its effects. Furthermore, the amount of hydrogen generated during decomposition of the medium increases, providing more hydrogen to the liquid polymer, thereby accelerating the decomposition of the liquid polymer. Furthermore, when the weight-average molecular weight (Mw) of the medium is 1200 or less, the medium is easily miscible with the liquid polymer. In this specification, the weight-average molecular weight (Mw) of the medium can be calculated, for example, using a general composition ratio according to the formula Mw = ΣCi * Mi / ΣCi, or can be measured by gel permeation chromatography (GPC).

[0028] The amount of the medium used is preferably 50 parts by mass or more, more preferably 100 parts by mass or more, and even more preferably 150 parts by mass or more, and is preferably 500 parts by mass or less, more preferably 400 parts by mass or less, and even more preferably 300 parts by mass or less, relative to 100 parts by mass of the liquid polymer. When the amount of the medium used is 50 parts by mass or more relative to 100 parts by mass of the liquid polymer, decomposition of the liquid polymer is sufficiently promoted, and the yield of the diene monomer is further improved. When the amount of the medium used is 500 parts by mass or more relative to 100 parts by mass of the liquid polymer, the amount of diene monomer produced per unit volume can be sufficiently increased.

[0029] <Reaction Conditions> In the method for decomposing a liquid polymer according to this embodiment, the liquid polymer is mixed with a medium and then thermally decomposed at 300°C to 950°C in an inert gas atmosphere. By carrying out the thermal decomposition at 300°C or higher, the rate of the decomposition reaction of the liquid polymer is increased, thereby improving the yield of the diene monomer. Furthermore, by carrying out the thermal decomposition at 950°C or lower, gasification and aromatization of the decomposition product can be suppressed. Furthermore, after the decomposition, the retention rate (selectivity) of the structural monomers of the liquid polymer is improved, thereby improving the yield of the final diene monomer. The thermal decomposition temperature is preferably 450°C or higher, more preferably 600°C or higher, from the viewpoint of improving the decomposition reaction rate of the liquid polymer. Furthermore, the thermal decomposition temperature is preferably 800°C or lower, from the viewpoint of improving the selectivity of the product that maintains the monomer skeleton. Therefore, by carrying out the thermal decomposition at 600°C to 800°C, the yield of the diene monomer obtained from the liquid polymer is further improved.

[0030] The time for the pyrolysis is not particularly limited. As an example, the time for the pyrolysis is preferably 0.1 to 300 seconds, and more preferably 1 to 150 seconds. By carrying out the pyrolysis for 0.1 seconds or more, the decomposition rate of the liquid polymer is improved. On the other hand, if the decomposition time is too long, the skeleton (double bond, etc.) of the constituent monomers of the liquid polymer is decomposed, and the decomposition products are gasified or aromatized. However, by carrying out the pyrolysis for 300 seconds 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 monomers of the liquid polymer after decomposition is improved.

[0031] The pyrolysis is carried out under an inert gas atmosphere. By carrying out the pyrolysis under an inert gas atmosphere, oxidation and reduction of the liquid polymer and decomposition products can be suppressed, and in particular, hydrogenation of double bonds in the diene monomers produced can be suppressed. Examples of the inert gas include nitrogen, carbon dioxide, argon, and helium.

[0032] To carry out the pyrolysis 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 pyrolysis, the generated hydrogen is not taken into consideration when determining the pyrolysis atmosphere.

[0033] In a preferred example of the method for decomposing a liquid polymer according to the present embodiment, the liquid polymer is mixed with a medium, and then heated in an inert gas atmosphere from 100° C. to 450° C. at a heating rate of 800° C. / min or more, and thermally decomposed at 450 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 that have a higher molecular weight than diene monomers is suppressed, and the retention rate (selectivity) of the skeleton of the constituent monomers of the liquid polymer after decomposition is further improved.

[0034] In another preferred example of the method for decomposing a liquid polymer according to the present embodiment, the liquid polymer is mixed with a medium, and then the mixture is heated from 100° C. to 450° C. at a heating rate of 2000° C. / min or more in an inert gas atmosphere, and thermally decomposed at 450 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 that have a higher molecular weight than diene monomers is suppressed, and the retention rate (selectivity) of the skeleton of the constituent monomers of the liquid polymer after decomposition is further improved.

[0035] In another preferred example of the method for decomposing a liquid polymer according to the present embodiment, the liquid polymer is mixed with a medium, and then the mixture is heated from 100° C. to 600° C. at a heating rate of 800° C. / min or more in an inert gas atmosphere, and 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 that have a higher molecular weight than diene monomers is suppressed, and the retention rate (selectivity) of the structural monomers of the liquid polymer after decomposition is further improved.

[0036] In another preferred example of the method for decomposing a liquid polymer according to the present embodiment, the liquid polymer is mixed with a medium, and then the mixture is heated from 100° C. to 600° C. at a heating rate of 2000° C. / min or more in an inert gas atmosphere, and 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 that have a higher molecular weight than diene monomers is suppressed, and the retention rate (selectivity) of the skeleton of the constituent monomers of the liquid polymer after decomposition is further improved.

[0037] The thermal decomposition 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. For example, the pressure for the thermal decomposition is preferably 1000 kPa to 65 kPa. By carrying out the thermal decomposition under reduced pressure or normal pressure, polymerization (repolymerization) of the diene monomer, which is a decomposition product, can be suppressed.

[0038] The thermal decomposition may or may not involve the use of a catalyst, but it is preferable not to use a catalyst. Not using a catalyst in the thermal decomposition can reduce costs. When a catalyst is used, any catalyst that has the effect of accelerating the decomposition reaction of the liquid polymer can be used.

[0039] <Decomposition Products> In the method for decomposing a liquid polymer according to the present embodiment, diene monomers and the like are obtained as decomposition products by the thermal decomposition.

[0040] In the liquid polymer decomposition method of this embodiment, the decomposition product obtained by the thermal decomposition preferably contains 30% by mass or more of diene monomers, more preferably 50% by mass or more, and even more preferably 60% by mass or more. By improving the yield and amount of diene monomers, the economic and environmental value of the decomposition method is further improved. Furthermore, the diene monomers used as decomposition products vary depending on the type of constituent monomer of the liquid polymer 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 diene monomers in the decomposition product can be controlled by, for example, the type of medium and reaction conditions such as the reaction temperature and reaction time of the thermal decomposition.

[0041] <Others> The liquid polymer decomposition method of this embodiment can be carried out in either a batch reactor or a flow reactor. Furthermore, the liquid polymer decomposition method of this embodiment may further include another step in addition to the thermal decomposition described above. Such a step may include a separation and purification step of the decomposition product. For example, by distilling the decomposition product, the target diene monomer can be separated from other by-products, and the target diene monomer can be reused as a raw material for synthesizing a diene rubber (diene polymer).

[0042] [Method for Decomposing Crosslinked Rubber] The method for decomposing crosslinked rubber of this embodiment is characterized by comprising: a first decomposition step of decomposing a crosslinked rubber containing a diene rubber to obtain a liquid polymer; and a second decomposition step of decomposing the liquid polymer obtained in the first decomposition step by the above-mentioned method for decomposing a liquid polymer. In the second decomposition step, the method for decomposing crosslinked rubber of this embodiment decomposes the liquid polymer by the above-mentioned method for decomposing a liquid polymer of this embodiment, so that diene monomers can be obtained in a high yield.

[0043] <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 to obtain a liquid polymer.

[0044] (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.

[0045] 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.

[0046] 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.

[0047] 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 scrap tires. The pulverization step may include multiple steps such as a preliminary 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.

[0048] 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 during the buffing process of scraping off the tread portion remaining on the base tire, for example, during tire retreading. Peeled rubber is a long piece of rubber, for example, 1 to 2 cm wide, peeled off 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, rubber hoses, and rubber conveyor belts, as well as rubber parts or components used in the manufacturing process 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 as a result of tire replacement or scrapping, or ELTs (End-of-Life Tires) that have reached the end of their life as tires.

[0049] -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.

[0050] 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 contains 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 contains 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 an easily recyclable diene monomer, such as isoprene or butadiene, in its skeleton.

[0051] 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.

[0052] 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, after the first decomposition step, the carbon black can be easily separated and recovered, for example, by centrifugation, filtration, or the like, and the recovered carbon black can be reused. 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. 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 large amount of carbon black can be recovered.

[0053] -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.

[0054] (Decomposition Method) The first decomposition step is not particularly limited, and can be carried out by, for example, (i) a method of decomposing a crosslinked rubber using a metathesis catalyst (first decomposition method), (ii) a method of thermally decomposing a crosslinked rubber at 150°C or higher and 400°C or lower (second decomposition method), (iii) a method of decomposing a crosslinked rubber in a solvent (third decomposition method), or (iv) a method of radically decomposing a crosslinked rubber (fourth decomposition method).

[0055] -First Decomposition Method (Metathesis Decomposition)- In the first decomposition method, the crosslinked rubber is decomposed using a metathesis catalyst. Here, the metathesis catalyst is a compound represented by the following general formula (1), (2), or (3): The catalysts represented by the general formula (1), (2) or (3) are excellent in promoting metathesis decomposition, and can decompose diene rubbers simply (under mild conditions) and quickly.

[0056] 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.

[0057] 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).

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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:

[0066] The amount of the catalyst (metathesis 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.

[0067] The decomposition by the metathesis catalyst (hereinafter sometimes simply referred to as "metathesis decomposition") is preferably carried out at a temperature of 20°C or higher and 200°C or lower. By carrying out the metathesis decomposition 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 metathesis decomposition at 200°C or lower, the decomposition of the catalyst (metathesis catalyst) represented by the above general formula (1), (2), or (3) can be suppressed, and the retention rate (selectivity) of the monomer skeleton of the diene rubber in the crosslinked rubber after the decomposition is improved. From the viewpoint of improving the decomposition reaction rate of the diene rubber, the metathesis decomposition temperature is more preferably 25°C or higher, and from the viewpoint of suppressing the decomposition of the metathesis catalyst and improving the selectivity of the product retaining the monomer skeleton, it is more preferably 100°C or lower.

[0068] The metathesis decomposition 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 1 MPa, and even more preferably 50 kPa to 500 kPa.

[0069] In the metathesis decomposition, the catalyst represented by the above-mentioned general formula (1), (2), or (3) may be dissolved in a solvent, or the crosslinked rubber may be immersed in the solvent. By allowing the catalyst to act on the crosslinked rubber in a solvent, the decomposition reaction of the diene rubber in the crosslinked rubber can be 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 can be facilitated. The amount of the solvent used is preferably 10 mL or more, more preferably 50 mL or more, and preferably 500 mL or less, more preferably 200 mL or less, per 1 g of the crosslinked rubber. If the amount of the solvent used is 10 mL or more per 1 g 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 1 g of the crosslinked rubber, it is preferable from the standpoint of cost.

[0070] The metathesis degradation 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.

[0071] - Second Decomposition Method (Low-Temperature Pyrolysis) - In the second decomposition method, the crosslinked rubber is thermally decomposed at a temperature of 150°C or higher and 400°C or lower. By carrying out the thermal decomposition at 150°C or higher, the rate of the decomposition reaction of the diene rubber in the crosslinked rubber is improved, and by carrying out the thermal decomposition at 400°C or lower, gasification and aromatization of the decomposition products can be suppressed, and after the 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 thermal decomposition temperature is preferably 175°C or higher, and more preferably 190°C or higher. Furthermore, from the viewpoint of improving the selectivity of products that maintain the monomer skeleton, the thermal decomposition temperature is preferably 350°C or lower, and more preferably 300°C or lower.

[0072] The second decomposition method is preferably carried out under an inert gas atmosphere. By carrying out the thermal decomposition under an inert gas atmosphere, oxidation and 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, oxidation of the recycled carbon black can also be suppressed. Examples of inert gases include nitrogen, carbon dioxide, argon, and helium.

[0073] To carry out the pyrolysis 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 pyrolysis, the generated hydrogen is not taken into consideration when determining the pyrolysis atmosphere.

[0074] The thermal decomposition 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. For example, the reaction pressure for the thermal decomposition is preferably 1000 kPa to 65 kPa. By carrying out the thermal decomposition under reduced pressure or normal pressure, polymerization (repolymerization) of oligomers and monomers in the decomposition products can be suppressed.

[0075] The reaction time of the thermal decomposition is not particularly limited. For example, the reaction time of the thermal decomposition is preferably 1 minute to 180 minutes, more preferably 3 minutes to 60 minutes, and even more preferably 5 minutes to 30 minutes.

[0076] The thermal decomposition may or may not use a catalyst, but it is preferable not to use a catalyst. Not using a catalyst for thermal decomposition 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.

[0077] -Third Decomposition Method (Solvent Decomposition)- In the third decomposition method, the crosslinked rubber is decomposed in a solvent. When the decomposition of the crosslinked rubber 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 products can be suppressed, and a 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.) than in normal high-temperature thermal decomposition.

[0078] Various solvents capable of swelling the crosslinked rubber can be used as the solvent. For example, aromatic solvents, aliphatic solvents, alicyclic solvents, ester solvents, etc. can be used. Aromatic solvents include benzene, toluene, xylene, etc.; aliphatic solvents include pentane, hexane, heptane, etc.; alicyclic solvents include cyclopentane, cyclohexane, etc.; and ester solvents include ethyl acetate, propyl acetate, butyl acetate, etc. Among these, at least one solvent selected from the group consisting of toluene, xylene, and cyclohexane is preferred, with toluene being particularly preferred. Using toluene, xylene, or cyclohexane as the solvent allows for the production of a decomposition product (liquid polymer) with a high retention rate of the skeleton (isoprene skeleton, butadiene skeleton, etc.) of the constituent monomers of the diene rubber. The amount of the solvent used is preferably 1 mL or more, more preferably 5 mL or more, per gram of the crosslinked rubber, and is preferably 500 mL or less, more preferably 200 mL or less. If the amount of the solvent used is 10 mL or more per 1 g 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 1 g of the crosslinked rubber, it is preferable from the standpoint of cost.

[0079] The temperature for decomposition in the solvent is preferably 150 to 300°C. By carrying out the decomposition in the solvent at 150°C or higher, the rate of the decomposition reaction of the diene rubber in the crosslinked rubber is improved, and by carrying out the decomposition in the solvent at 300°C or lower, gasification and aromatization of the decomposition products can be suppressed, and after the decomposition, the retention rate (selectivity) of the skeleton of the constituent monomer of the diene rubber in the crosslinked rubber is improved. From the viewpoint of improving the decomposition reaction rate of the crosslinked rubber, the decomposition temperature in the solvent is preferably 160°C or higher, more preferably 180°C or higher. From the viewpoint of improving the selectivity of the product that maintains the monomer skeleton, the temperature is preferably 280°C or lower, more preferably 250°C or lower.

[0080] The decomposition time in the solvent is not particularly limited. As an example, the decomposition time in the solvent is preferably 1 hour to 48 hours, and more preferably 3 hours to 18 hours. By carrying out the decomposition in the solvent for 1 hour or more, the decomposition rate of the crosslinked rubber is improved. On the other hand, if the reaction 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 decomposition in the solvent 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 the decomposition is improved.

[0081] The decomposition in the solvent is preferably carried out under an inert gas atmosphere. By carrying out the decomposition in the solvent under an inert gas atmosphere, the active species (oxygen, hydrogen, etc.) dissolved in the solvent can be reduced, and the oxidation or reduction of the decomposition product can be suppressed, and in particular, hydrogenation of double bonds in the oligomers and monomers in the decomposition product can be suppressed. Here, examples of the inert gas include nitrogen, carbon dioxide, argon, and helium.

[0082] To carry out the decomposition in the solvent 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 decomposition in the solvent, the generated hydrogen is not taken into consideration when determining the atmosphere for the decomposition in the solvent.

[0083] The decomposition in the solvent can be carried out at any pressure, such as reduced pressure, normal pressure, or increased 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.

[0084] The decomposition in the solvent may or may not involve the use of a catalyst, but it is preferable not to use a catalyst. Not using a catalyst in the decomposition in the solvent 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.

[0085] -Fourth Decomposition Method (Radical Decomposition)- In the fourth decomposition method, a radical initiator is reacted with a crosslinked rubber in an atmosphere containing oxygen gas. The radical initiator is thought to act mainly on crosslinking sites caused by sulfur, severing the crosslinked bonds. Therefore, in the case of diene rubber, it is thought that its main chain structure is basically maintained even after the decomposition reaction.

[0086] As the radical initiator, various known compounds can be used, but from the viewpoint of reaction efficiency, peroxides and azo compounds are preferred, and peroxides are more preferred. Examples of peroxides include acyl peroxide, percarboxylic acid, dialkyl peroxide, and alkyl hydroperoxide. Among these, from the viewpoint of reaction rate, acyl peroxide and percarboxylic acid are preferred, and acyl peroxide is more preferred. Specific examples of acyl peroxides include benzoyl peroxide, toluoyl peroxide, chlorobenzoyl peroxide, dichlorobenzoyl peroxide, methoxybenzoyl peroxide, phthaloyl peroxide, acetyl peroxide, propanoyl peroxide, caprylyl peroxide, decanoyl peroxide, lauroyl peroxide, myristoyl peroxide, and stearoyl peroxide. Among acyl peroxides, benzoyl peroxide, lauroyl peroxide, and acetyl peroxide are preferred from the viewpoints of reaction rate, cost, and availability. From the viewpoints of reaction rate and peroxide stability, lauroyl peroxide is preferred over benzoyl peroxide. Examples of percarboxylic acids include perbenzoic acid, chloroperbenzoic acid, peracetic acid, and perpropionic acid. Examples of dialkyl peroxides include di-tert-butyl peroxide, dicumyl peroxide, and tert-butylcumyl peroxide. Examples of alkyl hydroperoxides include tert-butyl hydroperoxide and cumene hydroperoxide. Examples of azo compounds include azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, azobiscyclohexanecarbonitrile, and methyl azobisisobutyrate. These radical initiators may be in the form of a hydrate from the viewpoints of safety, availability, and the like. By using a suitable initiator, crosslinked rubber can be efficiently decomposed under mild conditions.

[0087] The amount of the radical initiator used is preferably 0.1 part by mass or more, more preferably 1 part by mass or more, and is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, per 100 parts by mass of the crosslinked rubber. If the amount of the radical initiator used is 0.1 part by mass or more per 100 parts by mass of the crosslinked rubber, the decomposition reaction of the crosslinked rubber proceeds more quickly, and if the amount of the radical initiator used is 30 parts by mass or less per 100 parts by mass of the crosslinked rubber, it is preferable from the standpoint of cost.

[0088] The fourth decomposition method is carried out in an atmosphere containing oxygen gas. The concentration (volume %) of oxygen gas in the reaction atmosphere is typically 1 to 100%, preferably 1 to 80%, and more preferably 5 to 50%, including the atmospheric oxygen concentration of approximately 20%. When the oxygen gas concentration is not 100%, an inert gas such as nitrogen gas can be used as the remaining gas component. The decomposition reaction in the fourth decomposition method is generally more advantageous as the oxygen concentration increases, but because undesirable side reactions are more likely to occur, it is preferable to appropriately control the reaction time, reaction temperature, etc. when the oxygen concentration is high. On the other hand, using atmospheric air with an oxygen concentration of approximately 20% eliminates the need for separate oxygen gas, which is advantageous in terms of equipment and economy. In a reaction under an air atmosphere, when the amount of crosslinked rubber is small, the reaction can proceed even in a sealed container with a sufficiently large internal volume. In this case, the internal volume of the reaction container is generally 50 parts by volume or more, more preferably 100 parts by volume or more, per 1 part by volume of crosslinked rubber. When the fourth decomposition method is carried out in a closed system, a large amount of crosslinked rubber requires a reaction vessel with a large internal volume, which is not desirable from the standpoint of equipment. Therefore, from the standpoint of equipment, it is preferable to carry out the reaction in a non-closed system that allows the flow of air, etc. In order to make the reaction system a non-closed system, methods such as opening a portion of the reaction vessel, installing a membrane that allows the flow of air, etc., or allowing air, etc., to overflow can be considered. Another advantageous embodiment is a method in which the reaction is carried out while continuously or intermittently blowing air, etc. into the solution. In this case, the blowing rate is not particularly limited, but is usually 1 to 1000 mL / min, preferably 2 to 500 mL / min, and more preferably 3 to 200 mL / min per gram of crosslinked rubber.

[0089] In the fourth decomposition method, the radical initiator may be dissolved in a solvent, or the crosslinked rubber may be immersed in a solvent to perform the decomposition. By allowing the radical initiator to act on the crosslinked rubber in a solvent, the decomposition of the crosslinked rubber can be facilitated. Any solvent that does not inhibit the decomposition reaction can be used as the solvent, and examples thereof include aromatic hydrocarbons (aromatic solvents), chlorinated hydrocarbons, aliphatic hydrocarbons, alicyclic hydrocarbons, ethers, etc. Specific examples of aromatic hydrocarbons include toluene, xylene, ethylbenzene, trimethylbenzene, and propylbenzene. Chlorinated hydrocarbons include chloroform, dichloromethane, dichloroethane, trichloroethane, tetrachloroethane, tetrachloroethene, chlorobenzene, and dichlorobenzene. Aliphatic hydrocarbons include pentane, hexane, and octane. Alicyclic hydrocarbons include cyclopentane, cyclohexane, and cyclooctane. Ethers include tetrahydrofuran, methyltetrahydrofuran, tetrahydropyran, and dioxane. Among these, aromatic hydrocarbons and chlorinated hydrocarbons are preferred from the viewpoints of reaction rate, availability, etc. Specific examples thereof include toluene, xylene, ethylbenzene, chloroform, chlorobenzene, dichlorobenzene, etc. Among aromatic hydrocarbons, ethylbenzene is preferred over toluene from the viewpoint of reaction rate. The solvent suitable for the decomposition reaction also depends on the type of crosslinked rubber. For example, when the crosslinked rubber is an NR-based, IR-based, or BR-based crosslinked rubber, aromatic hydrocarbons and chlorinated hydrocarbons are preferred from the viewpoints of reaction rate, solubility of the product, etc., specific examples thereof include toluene, ethylbenzene, chloroform, chlorobenzene, etc. On the other hand, when the crosslinked rubber is an SBR-based crosslinked rubber, chlorinated hydrocarbons are more preferred from the viewpoint of reaction rate, etc., specific examples thereof include chloroform, chlorobenzene, etc. Use of a preferred solvent facilitates the decomposition of the crosslinked rubber, allowing for more efficient production of a liquid polymer. The amount of the solvent used is preferably 2 mL or more, more preferably 10 mL or more, per 1 g of the crosslinked rubber, and is preferably 100 mL or less, more preferably 50 mL or less.If the amount of solvent used is 2 mL per 1 g of crosslinked rubber, the decomposition reaction of the crosslinked rubber is promoted, and if the amount of solvent used is 50 mL or less per 1 g of crosslinked rubber, this is preferable from the standpoint of cost.

[0090] The fourth decomposition method is preferably carried out at a temperature of 0°C or higher and 200°C or lower. By carrying out the fourth decomposition method at a temperature of 0°C or higher, the rate of decomposition reaction of the crosslinked rubber is improved, and by carrying out the fourth decomposition method at a temperature of 200°C or lower, the retention rate (selectivity) of the monomer skeleton in the crosslinked rubber is improved. In terms of improving the decomposition reaction rate of the crosslinked rubber, the fourth decomposition method is more preferably carried out at a temperature of 10°C or higher, and more preferably at a temperature of 150°C or lower, from the viewpoint of improving the selectivity of the product that retains the monomer skeleton. Furthermore, when a peroxide-based radical initiator is used, the decomposition efficiency is high, so the reaction proceeds at a low temperature of 60°C or lower. Another advantage of using a peroxide-based radical initiator is that the reaction proceeds smoothly even at around room temperature (20 to 30°C), so no special equipment for heating or cooling is required.

[0091] The fourth decomposition method can be carried out under any pressure, including reduced pressure, normal pressure, and increased pressure. Specifically, 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.

[0092] In the fourth decomposition method, it is important to efficiently stir the raw materials, namely, the crosslinked rubber, the radical initiator, and the solvent. As a stirring method, various conventionally known methods can be used, such as a method using a magnetic stirrer, a method using a mechanical stirrer, or a method using a shaker. Furthermore, to promote the reaction, ultrasonic irradiation, microwave irradiation, light irradiation, or the like may be used in combination. These methods can also be used in combination.

[0093] Other Decomposition Methods The method for decomposing crosslinked rubber to obtain recycled carbon black of this embodiment is not limited to the first, second, third, and fourth decomposition methods described above, and any decomposition method that preserves 30% or more of the basic polymer structure is sufficient. Furthermore, a decomposition method that preserves 50% or more of the polymer structure is more preferred, and a decomposition method that preserves 70% or more of the polymer structure is even more preferred.

[0094] (Decomposition Products (Intermediate Decomposition Products)) In the method for decomposing a crosslinked rubber of this embodiment, a liquid polymer or the like is obtained as a decomposition product by the first decomposition step described above. Furthermore, when the crosslinked rubber contains carbon black together with a diene rubber, by passing through the first decomposition step, the carbon black can be recovered together with the liquid polymer, and the carbon black can be reused.

[0095] The method for decomposing a crosslinked rubber according to the present embodiment includes a second decomposition step of decomposing the liquid polymer by the method for decomposing a liquid polymer described above. The second decomposition step can be carried out according to the method described in the section "Method for decomposing a liquid polymer" above.

[0096] (Decomposition Products) In the method for decomposing a crosslinked rubber according to the present embodiment, diene monomers and the like are obtained as decomposition products by the second decomposition step. The decomposition products are as described above in the section "Method for decomposing a liquid polymer."

[0097] <Others> The method for decomposing a crosslinked rubber according to this embodiment can be carried out using either a batch reactor or a flow reactor. Furthermore, the method for decomposing a crosslinked rubber according to this embodiment may further include other steps in addition to the first and second decomposition steps described above. Examples of such steps include a pretreatment step for the crosslinked rubber (e.g., a cutting step or a pulverization step) and a separation and purification step for the decomposition product. Furthermore, when the crosslinked rubber contains carbon black, it is preferable to include a step for recovering the carbon black from the decomposition product (intermediate decomposition product) after the first decomposition step. Furthermore, for example, by distilling the decomposition product of the second decomposition step, the target diene monomer can be separated from other by-products, and the target diene monomer can be reused as a raw material for synthesizing a diene rubber (diene polymer).

[0098] 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.

[0099] <Measurement of molecular weight> The weight average molecular weight (Mw) of media other than liquid paraffin was calculated using the formula Mw = ΣCi * Mi / ΣCi using a general composition ratio, where Ci is the composition ratio and Mi is the molecular weight. The weight average molecular weight of liquid paraffin was listed in the manufacturer's catalog.

[0100] <Preparation of Crosslinked Rubber Sample> 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 a crosslinked rubber sample.

[0101] <Preparation of Liquid Polymer (First Decomposition Step)> 10 mL of tetrahydrofuran (THF) was added to 0.125 g of the crosslinked rubber sample and stirred at 25°C for 24 hours to swell the rubber. Next, 24 mg of the Grubbs second-generation catalyst represented by the structural formula (1-2) above 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, 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 mixture containing a liquid polymer and carbon black. The resulting mixture was filtered to remove the carbon black, yielding a liquid polymer. The weight-average molecular weight (Mw) of the resulting liquid polymer was analyzed and found to be 610.

[0102] <Decomposition of Liquid Polymer (Second Decomposition Step) and Evaluation> (Examples 1-1 to 1-5 and Examples 2-1 to 2-5) 20 mg of the liquid polymer obtained as described above was mixed with 40 mg of a medium shown in Tables 1 and 2 (liquid polymer:medium=1:2 (mass ratio)), and the mixture was further dissolved in 3 mL of an n-hexane / chloroform mixed solvent (n-hexane / chloroform=1 / 2 (volume ratio)). A 30 μL portion of the solution was placed in a measurement eco-cup and dried to distill off the solvent. 0.6 mg of the mixture of liquid polymer and medium obtained after distilling off the solvent was heated to 500°C (Examples 1-1 to 1-5) or 700°C (Examples 2-1 to 2-5) using a pyrolyzer (manufactured by Frontier Labs, Inc., trade name "EGA / PY-3030D", carrier gas: helium), and the resulting pyrolysis products were analyzed using a gas chromatograph analyzer (manufactured by Agilent Technologies, Inc., trade name "8890GC", carrier gas: helium, detector: FID) to determine the yields of isoprene, limonene (dimer), and isoprene tetramer.

[0103] (Comparative Example 1-1 and Comparative Example 2-1) For comparison, the liquid polymer was pyrolyzed at 500°C (Comparative Example 1-1) or 700°C (Comparative Example 2-1) in the same manner as in the Examples, except that no medium was mixed, and the yields of isoprene, limonene (dimer), and isoprene tetramer were determined.

[0104] (Evaluation) In Table 1, the results are expressed as indexes, with the yield of isoprene, the yield of limonene (dimer), and the yield of isoprene tetramer in Comparative Example 1-1 each set to 100, and in Table 2, the results are expressed as indexes, with the yield of isoprene, the yield of limonene (dimer), and the yield of isoprene tetramer in Comparative Example 2-1 each set to 100. A larger index value indicates a higher yield.

[0105]

[0106]

[0107] * 1 Rapeseed oil: Nisshin Oillio Group Co., Ltd., product name "Nissin Canola Oil", SP value = 8.94 (cal / cm ３ ) １／２ *2 Hydrogenated rapeseed oil: Yamakei Sangyo Co., Ltd., product name "rapeseed extremely hardened oil", SP value = 8.95 (cal / cm ３ ) １／２ , weight average molecular weight (Mw) = 888 *3 Liquid paraffin: hydrocarbon having 16 to 32 carbon atoms, manufactured by MORESCO Corporation, trade name "Moresco White P-100", SP value = 8.06 (cal / cm ３ ) １／２ , weight average molecular weight (Mw) = 365 *4 Microcrystalline wax: hydrocarbon having 20 to 60 carbon atoms, SP value = 7.84 (cal / cm ３ ) １／２ *5 Hydrogenated jojoba oil: a fatty acid ester mainly consisting of a component having 40 or 42 carbon atoms and one ester bond in the molecular chain, manufactured by Yamakei Sangyo Co., Ltd., trade name "Hydrogenated Jojoba Oil", SP value = 8.59 (cal / cm ３ ) １／２ , weight average molecular weight (Mw) = 606

[0108] It is clear from Tables 1 and 2 that the method for decomposing a liquid polymer of the present invention makes it possible to obtain isoprene (diene monomer) in high yield.

Claims

1. A method for decomposing a liquid polymer, comprising mixing the liquid polymer with at least one medium selected from fatty acid esters and hydrocarbons, and thermally decomposing the mixture at 300°C to 950°C in an inert gas atmosphere.

2. The method for decomposing a liquid polymer according to claim 1, wherein the medium has a weight average molecular weight (Mw) of 150 to 1,200.

3. The method for decomposing a liquid polymer according to claim 1, wherein the medium is a hydrocarbon.

4. The method for decomposing a liquid polymer according to claim 1, wherein the thermal decomposition is carried out at 600°C to 800°C.

5. A method for decomposing crosslinked rubber, comprising: a first decomposition step of decomposing a crosslinked rubber containing a diene rubber to obtain a liquid polymer; and a second decomposition step of decomposing the liquid polymer by the method for decomposing a liquid polymer according to any one of claims 1 to 4.

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