Method of desulfurizing sulfur-crosslinked rubber

Abstract

To desulfurize sulfur-crosslinked rubber while reducing deterioration in physical properties and reducing odors during desulfurization and after desulfurization.SOLUTION: A method of desulfurizing sulfur-crosslinked rubber, comprises adding to the sulfur-crosslinked rubber at least one selected from the group consisting of primary and secondary phosphine oxides and analogues thereof, primary and secondary phosphines which become oxides when oxidized and analogues thereof, sulfenic acid, and sulfinic acid, as a desulfurizing agent that acts on a sulfur bond in the sulfur-crosslinked rubber and cleaves the sulfur bond, and performing heating, wherein used sulfur-crosslinked rubber can be suitably employed.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for devulcanizing sulfur-crosslinked rubber. [Background technology]

[0002] Currently, as a method for devulcanizing sulfur-crosslinked rubber (particularly a method for devulcanizing used sulfur-crosslinked rubber to produce reusable recycled devulcanized rubber), a method (shear devulcanization) in which the sulfur-crosslinked rubber is melt-kneaded to form a high-level shear flow field and physically devulcanized is generally used. Patent Document 1 describes a method in which sulfur-crosslinked rubber containing carbon black is pulverized and fed into a twin-screw extruder, heated to a temperature of 180 to 350°C, and subjected to a pressure of 10 to 150 kg / cm. 2 The document also describes a method for producing recycled devulcanized rubber in which the sulfur crosslinks are broken by a devulcanization treatment that applies a shear stress of 1000 MPa. The document also describes the use of a devulcanizing agent in combination with shear devulcanization.

[0003] However, shear devulcanization has the problem of reducing physical properties (low selectivity) due to the scission of the rubber main chain (other than SS and CS bonds), and it is currently difficult to restore recycled devulcanized rubber to the same state as new raw rubber. In addition, devulcanization is performed under high temperature conditions.

[0004] On the other hand, chemical devulcanization, a method of desulfurizing sulfur-crosslinked rubber by chemically cleaving the sulfur bonds within the rubber structure using a desulfurizing agent (regenerator), selectively cleaves the sulfur bonds, making it less likely for the main chain to break. Therefore, recycled devulcanized rubber obtained through chemical devulcanization can maintain the same molecular weight as new raw rubber, preventing deterioration of physical properties. Furthermore, devulcanization can be performed under mild conditions.

[0005] Examples of such desulfurizing agents include disulfide compounds (RSSR), thiol compounds (R-SH), dimethyl sulfoxide (DMSO), and amine compounds (NR3). The desulfurizing agents described in Patent Document 1 are diaryl disulfide, dihexyl disulfide, and thiophenol-iron oxide. Patent Document 2 describes phenyl-hydrazine-iron chloride, triphenylphosphine, thiol, and disulfide as desulfurizing agents. Patent Document 3 describes amine compounds (octylamine, hexadecylamine, dioctylamine, trioctylamine, benzylamine, or 4-piperidinopiperidine) as desulfurizing agents.

[0006] However, all of these desulfurizing agents are compounds that emit a distinctive odor, which causes a physical burden on workers due to the odor generated during desulfurization. In addition, the reclaimed rubber after desulfurization also has an odor, making it difficult to use as a recycled material. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 9-227724 [Patent Document 2] Special Publication No. 2010-535912 [Patent Document 3] Special Publication No. 2003-510437 Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, an object of the present invention is to devulcanize sulfur-crosslinked rubber while suppressing deterioration in physical properties and suppressing odors during and after devulcanization. [Means for solving the problem]

[0009] [1] A method for devulcanizing sulfur-crosslinked rubber, comprising adding to the sulfur-crosslinked rubber at least one kind of desulfurizing agent that acts on sulfur bonds in the sulfur-crosslinked rubber and cuts the sulfur bonds, selected from the group consisting of primary and secondary phosphine oxides and their analogues, primary and secondary phosphines and their analogues that become oxides when oxidized, sulfenic acid, and sulfinic acid, and heating the sulfur-crosslinked rubber.

[0010] [2] The method for devulcanizing a sulfur-crosslinked rubber according to [1] above, further comprising adding to the sulfur-crosslinked rubber a radical initiator that generates radicals for generating radical active species from the devulcanizing agent.

[0011] [3] The method for devulcanizing a sulfur-crosslinked rubber according to [1], wherein a radical initiator that generates radicals for generating radical active species from the devulcanizing agent is not added to the sulfur-crosslinked rubber.

[0012] [4] The devulcanization method for a sulfur-crosslinked rubber according to any one of [1] to [3], wherein a solvent is also added to the sulfur-crosslinked rubber.

[0013] [5] The method for devulcanizing a sulfur-crosslinked rubber according to any one of [1] to [3], wherein no solvent is added to the sulfur-crosslinked rubber.

[0014] [6] The method for devulcanizing a sulfur-crosslinked rubber according to any one of [1] to [5], wherein the heating is carried out while kneading in a kneading extruder.

[0015] <Estimation of the mechanism of the desulfurization reaction in the present invention> Secondary phosphine oxides, which can be described as R1R2HP=O (R1 and R2 are not particularly limited), are in a state of chemical equilibrium such as R1R2HP=O⇔R1R2P-OH, and in particular R1R2P-OH reacts nucleophilically with the sulfur in the sulfur-crosslinked rubber, ultimately causing desulfurization. A similar reaction mechanism occurs with primary phosphine oxides, but not with tertiary phosphine oxides. A similar reaction mechanism occurs with sulfenic acid (RSOH) and sulfinic acid (RS(O)OH).

[0016] To explain this in more detail using the example of diphenylphosphine oxide (DPPO), it is estimated as shown in the following chemical formula 1. [ka] DPPO is in chemical equilibrium with compound 1 (the bias is on the DPPO side). Compound 1 attacks the sulfur in the sulfur-crosslinked rubber, cleaving the S—S bond and forming compounds 3 and 4. Here, the negatively charged sulfur in compound 4 receives a proton from compound 3, giving compounds 5 and 6. Desulfurization proceeds through a similar process.

[0017] The devulcanizing agent in the present invention selectively reacts with sulfur bonds in the rubber to sever the sulfur bonds, so that even if devulcanization proceeds, severing of the main chain of the rubber is unlikely to occur, and deterioration of physical properties can be suppressed. Furthermore, the desulfurizing agent of the present invention generates almost no or only a small amount of odor compared to the desulfurizing agents listed in the Background Art section above, so odor generated during devulcanization is suppressed and odor is less likely to remain in the rubber after devulcanization.

[0018] <Whether or not a radical initiator is used> In the present invention, devulcanization proceeds regardless of whether a radical initiator is added, but adding a radical initiator may further promote devulcanization. This is because the devulcanizing agent used in the present invention is also a radical precursor that acts on the sulfur bonds in the sulfur-crosslinked rubber and generates radical active species that cleave the sulfur bonds, and the following presumed mechanism of the devulcanization reaction may also be involved. 1) Radicals are generated from the radical initiator by heating. 2) The generated radical reacts with a radical precursor, and the radical precursor generates an active radical species. 3) The radical active species react with the sulfur bond in the sulfur crosslinked rubber to generate a radical intermediate. 4) The radical active species generated in 2) further reacts with the radical intermediate in 3) to cleave the sulfur bond. 5) The devulcanization of sulfur crosslinked rubber progresses by repeating steps 1)-4).

[0019] To explain this in more detail using the example of DPPO and 2,2'-azobis(isobutyronitrile) (AIBN), it is estimated as shown in the following chemical formula 2. [ka] 1) Upon heating, two C—N double bonds near the center of AIBN are cleaved, generating nitrogen gas and a 2-cyano-2-propyl radical. 2) The generated propyl radical reacts with H of the phosphorus-centered radical precursor, and the radical transfers to the phosphorus atom, generating a phosphine oxide radical. 3) Phosphine oxide radicals react with sulfur bonds in sulfur-crosslinked rubber to generate radical intermediates. 4) The phosphine oxide radical generated in 2) further reacts with the radical intermediate in 3) to cleave the sulfur bond. 5) The devulcanization of sulfur crosslinked rubber progresses by repeating steps 1)-4).

[0020] However, since many radical initiators, such as AIBN, are self-reactive and must be handled with care, it is preferable not to add a radical initiator from the standpoint of ease of handling.

[0021] <About the presence or absence of solvent> It has been found that the desulfurization reaction in the present invention proceeds regardless of whether a solvent is added or not. (a) When a solvent is added, the solvent swells the sulfur crosslinked rubber and liquefies the desulfurizing agent, forming a reaction field. (b) When no solvent is added, the desulfurizing agent liquefies in the heated reaction system and functions as a reaction field (substitute for the solvent). Therefore, it is preferable to add the desulfurizing agent in an appropriate amount (concentration) suitable for forming a reaction field. If the amount added is too small (low concentration), the liquid component becomes thin, making it difficult to form a reaction field and reducing stirring efficiency. If the amount added is too large (high concentration), the reaction system will be diluted. The appropriate amount of desulfurizing agent added when no solvent is added varies depending on the type of rubber, so is not particularly limited, but an example is 1 to 20 equivalents per gram of rubber, preferably 5 to 20 equivalents, and more preferably 8 to 15 equivalents.

[0022] However, by not adding a solvent, the following effects can be obtained that are difficult to obtain when a solvent is added. (1) The heating can be carried out while kneading, for example, in a kneading extruder, which is practical. (2) Drying of the rubber after the reaction is no longer necessary, simplifying post-processing. (3) It reduces the environmental impact. Therefore, when these effects are important, it is preferable not to add a solvent. [Effects of the Invention]

[0023] According to the present invention, sulfur-crosslinked rubber can be devulcanized while suppressing deterioration in physical properties and suppressing odors during and after devulcanization. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 1 is a graph plotting the amount of desulfurizing agent added and the degree of swelling for Samples 4, 5, and 7, which correspond to examples of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0025] <1> sulfur crosslinked rubber The type of rubber used for the sulfur-crosslinked rubber is not particularly limited, but examples include ethylene propylene rubber (EPDM, EPM), natural rubber (NR), isoprene rubber (IR), butyl rubber (IIR), butadiene rubber (BR), styrene butadiene rubber (SBR), chloroprene rubber (CR), and nitrile rubber (NBR). The sulfur crosslinked rubber is preferably in the form of flakes, granules or the like which have been crushed before being devulcanized. As the sulfur-crosslinked rubber, a used one can be suitably used, and the time and conditions of use are not particularly limited. According to the present invention, the used sulfur-crosslinked rubber can be devulcanized to produce a reusable recycled devulcanized rubber.

[0026] <2> Desulfurization Agent As described above, at least one desulfurizing agent selected from the group consisting of primary and secondary phosphine oxides and their analogs (phosphites, etc.), primary and secondary phosphines and their analogs that become oxides when oxidized, sulfenic acid, and sulfinic acid is used. Specific examples include diphenylphosphine oxide (DPPO), di-p-tolylphosphine oxide, diadamantylphosphine, bis-3,5-dimethylphenylphosphine oxide, dicyclohexylphosphine oxide, di-4-methoxyphenylphosphine oxide, diphenylphosphine, and diethyl phosphite, which are shown in the following Chemical Formula 3. [ka] The amount of desulfurizing agent to be added is not particularly limited, as the appropriate amount varies depending on the type of rubber, heating temperature, heating time, etc., but can be 0.5 to 25 equivalents per 1 g of rubber, and preferably 1 to 20 equivalents. The appropriate amount of desulfurizing agent to be added when no solvent is added is as described above.

[0027] <3> Radical Initiators The radical initiator is preferably at least one selected from the group consisting of azo compounds and peroxide compounds, because these compounds emit little or no odor and are easily available. Examples of azo compounds include 2,2'-azobis(isobutyronitrile) (AIBN), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis(isobutyrate) dimethyl, and 4,4'-azobis(4-cyanovaleric acid). Examples of peroxide compounds include di-tert-butyl peroxide, cumene hydroperoxide, dicumyl peroxide, and benzoyl peroxide (BPO). The amount of radical initiator to be added is not particularly limited, as the appropriate amount varies depending on the type of rubber, heating temperature, heating time, etc., but can be 0.5 to 16 equivalents per 1 g of the rubber, and preferably 1 to 8 equivalents.

[0028] <4> solvent The solvent is not particularly limited, but examples thereof include the following: Non-polar solvents (benzene, toluene, xylene, etc.) Low polarity solvents (o-dichlorobenzene (o-DCB), 1-pentanol, chlorobenzene, etc.) Highly polar solvents (dimethylacetamide (DMA), dimethyl sulfoxide (DMSO), etc.) Halogenated solvents (tetrachloroethane, etc.) Ether solvents (1,4-dioxane, etc.) In particular, the combined use of o-DCB and DMA is preferable, as it significantly increases the desulfurization efficiency (Samples 21, 23, and 24, described below). The reason for this is currently unknown, but it is thought to be due to some effect of combining a low-polarity solvent with a specific high-polarity solvent.

[0029] <5> Heating conditions The mixture is heated to a required temperature at which substantial progress of devulcanization is observed. The required temperature is not particularly limited as it varies depending on the type of rubber, the amount of each additive, heating time, etc., but can be exemplified as 60 to 200°C, with 110°C or higher being preferred for EPDM rubber and 60°C or higher being preferred for natural rubber. The heating time is not particularly limited, as the appropriate temperature varies depending on the type of rubber, the amount of each additive, the heating temperature, etc., but can be, for example, 1 to 24 hours. It is preferable to stir or knead the mixture during heating. Whether to stir or knead the mixture can be appropriately selected depending on the state of the material.

[0030] <6> Desulfurization indicators When rubber comes into contact with a solvent, it absorbs the solvent and swells. This swelling occurs even in rubber before devulcanization, but the more devulcanized the rubber is, the higher the swelling ratio calculated by the following formula 1 becomes. This is because the solvent penetrates into the spaces where sulfur bonds have been broken. Swelling ratio (%) = (swelled weight - dry weight) / dry weight × 100 (Equation 1) Therefore, in the present invention, rubber before devulcanization (1 g dry weight) was immersed in toluene as a solvent at room temperature for 24 hours, and the swelling weight was measured to determine the swelling ratio of the rubber before devulcanization. Also, rubber after devulcanization (1 g dry weight) was immersed in the same solvent for the same time, and the swelling weight was measured to determine the swelling ratio of the rubber after devulcanization. The increase in the swelling ratio, calculated by the following formula 2, was used as an index of devulcanization. Swelling rate increase ratio = swelling rate of rubber after devulcanization / swelling rate of rubber before devulcanization (Equation 2)

[0031] However, since the ease of swelling differs depending on the type of rubber, it is difficult to uniformly evaluate the degree of devulcanization of various rubbers by the above-mentioned increase in swelling rate, and there is a preferred increase in swelling rate for each type of rubber. For example, the increase ratio of the swelling ratio in EPDM rubber is preferably 1.10 or more (this is considered to be a substantial progress of devulcanization), more preferably 1.30 or more, even more preferably 1.50 or more, and most preferably 2.00 or more. The increase in swelling ratio of natural rubber is preferably 1.60 or more (this is considered to be a substantial progress of devulcanization), more preferably 2.00 or more, even more preferably 2.50 or more, and most preferably 3.00 or more. [Example]

[0032] Next, examples of the present invention will be described. Note that the materials, conditions, structures, shapes, and dimensions in the examples are merely examples and can be changed as appropriate without departing from the spirit of the invention.

[0033] [Experiment 1] As a devulcanization experiment of sulfur-crosslinked EPDM rubber, Sample 1-26 shown in Table 1-3 was carried out.

[0034] [Table 1]

[0035] [Table 2]

[0036] [Table 3]

[0037] Sample 1 is a sulfur-vulcanized EPDM rubber that has not been devulcanized, obtained by kneading 100 phr (parts by mass) of EPDM polymer (ethylene content: 53.7% by mass, diene content: 9.4% by mass, sulfur component: 0.43 mmol), 5.0 phr of zinc oxide, 1.0 phr of stearic acid, 1.5 phr of sulfur, 1.0 phr of accelerator (TMTD), and 0.5 phr of accelerator (MBT) in an 8-inch roll, and then press-molding the mixture at 160°C for 20 minutes.

[0038] Sample 2 was prepared by reacting the same sulfur-crosslinked EPDM rubber as Sample 1 using the following methods (1)-(3). (1) Using a personal organic synthesis apparatus, model PPS-5511, manufactured by Tokyo Rikakikai (EYELA), 1 g of sulfur-crosslinked EPDM rubber (ground material) was placed in a reaction vessel, and 12 mL of o-DCB was added as a solvent. The mixture was then left to stand at room temperature for one day. (2) Four equivalents of DPPO as a desulfurizing agent and two equivalents of AIBN as a radical initiator were further added to the reaction vessel, and the mixture was heated at 160°C for 5 hours. During heating, the mixture was continuously stirred at a rotation speed of 1000 rpm using the stirrer of the same apparatus. (3) The EPDM rubber was removed from the reaction vessel, washed with acetone three times, and then vacuum dried at 40° C. The above reaction was carried out only once.

[0039] Sample 3 was obtained by changing the amount of solvent added in (1) above, and by changing the amounts of desulfurizing agent and radical initiator added in (2) above. Sample 4-7 was prepared by adding no solvent in the above (1), and therefore not leaving it to stand at room temperature for one day, and by increasing the amounts of desulfurizing agent and radical initiator added in the above (2). Samples 8-10 were prepared by adding no solvent in (1) above, and therefore not leaving the mixture at room temperature for a day, and by adding no radical initiator in (2) above, and changing the heating temperature. Samples 11 and 12 were prepared by adding no solvent in the above (1), and were not left to stand at room temperature for one day. In the above (2), the type and amount of desulfurizing agent were changed, and the amount of radical initiator added was also changed. Samples 13 and 14 were prepared by changing the amount of solvent added in (1) above, changing the amount of desulfurizing agent added in (2) above, and not adding any radical initiator. Samples 15-19 were prepared by changing the amount of solvent added in (1) above, changing the type and amount of desulfurizing agent added in (2) above, and not adding any radical initiator. In Samples 20-26, the type of solvent was changed in (1), the amount of desulfurizing agent added was changed in (2), and the amount of radical initiator added was changed or no radical initiator was added. In Samples 22-26 and 28, two types of solvents were used in combination in the volume ratios shown in Table 3.

[0040] Sample 2-26 emitted little odor during the reaction, which was within the acceptable range, and the remaining odor of the EPDM rubber after the reaction was also little, which was within the acceptable range.

[0041] After the reaction, <6> The swelling ratio of Sample 1-26 was determined by the method explained in the section "Desulfurization Index." The increase ratio of the swelling ratio of Sample 2-26 relative to that of Sample 1 was also calculated. The results are shown in Table 1-3.

[0042] In Samples 2-9 and 11-26, the increase in swelling ratio relative to Sample 1 was 1.10 or more, and devulcanization of the EPDM rubber was confirmed. Comparing samples 2 and 3, increasing the amount of desulfurizing agent and radical initiator added promoted desulfurization, but increasing the amount of solvent did not promote desulfurization. A comparison of sample 2 (swelling ratio 421%) with samples 4-7 revealed that desulfurization proceeded even without adding a solvent. Furthermore, as shown in Figure 1, sample 5 (10 equivalents of desulfurizing agent) had the highest swelling ratio, while sample 4 (20 equivalents of desulfurizing agent) and sample 7 (5 equivalents of desulfurizing agent) had lower swelling ratios. As mentioned above, when no solvent is added, the desulfurizing agent liquefies in the heated reaction system and functions as a reaction field. This indicates that the 10 equivalents of desulfurizing agent (8 to 15 equivalents inclusive) in sample 5 was an appropriate amount (concentration) to form a reaction field. Comparing samples 8 to 10, devulcanization progressed when heated to a specified temperature, even without adding a solvent. Since the swelling ratio of sample 10 was low at 0.979, it is thought that a heating temperature of 110°C or higher is preferable for devulcanization of EPDM rubber. As shown in samples 11 and 12, desulfurization proceeded even when the type of desulfurization agent was changed and no solvent was added. Comparing samples 3 and 13, the progress of desulfurization did not change significantly with or without the addition of a radical initiator. Comparing samples 13 and 14, the progress of desulfurization did not change significantly even when the amount of desulfurization agent added was increased.

[0043] As shown in samples 11, 12, 15-19, desulfurization progressed even when the type of desulfurization agent was changed. Comparing samples 20-26, desulfurization progressed even when the type of solvent was changed.

[0044] From the above, Samples 2-9 and 11-26 are considered to be examples of the present invention. Sample 10 is a reference example because the desulfurizing agent is the same as that of the present invention but the heating temperature is not the required temperature.

[0045] [Experiment 2] Next, as a devulcanization experiment of sulfur-crosslinked natural rubber, Samples 27 to 38 shown in Table 4 below were carried out.

[0046] [Table 4]

[0047] Sample 27 is a sulfur-vulcanized natural rubber (sulfur component: 0.99 mmol) that has not been desulfurized. It was obtained by kneading 100 phr of natural rubber (SVR-CV60 manufactured by Zautien Rubber Co., Ltd.), 6.0 phr of zinc oxide, 0.5 phr of stearic acid, 3.5 phr of sulfur, and 0.5 phr of an accelerator (MBT) in an 8-inch roll and then press-molding the mixture at 150°C for 30 minutes.

[0048] Sample 28 was prepared by simply heat-treating the same sulfur-crosslinked natural rubber as Sample 27 using the following methods (i)-(iii). (i) Using the personal organic synthesis apparatus, 1 g (1 mm square) of sulfur-crosslinked natural rubber was placed in a reaction vessel, and 6 mL of DMA was added as a solvent, followed by standing at room temperature for one day. (ii) One equivalent of DPPO was added to the reaction vessel as a desulfurization agent, and the mixture was heated at 70°C for 6 hours. During heating, the mixture was stirred at 1,000 rpm using the stirrer of the same equipment. The heating temperature was set lower than in Experiment 1 because natural rubber has low heat resistance due to the carbon-carbon double bond in its main chain. (iii) The natural rubber was removed from the reaction vessel, washed three times with acetone, and then vacuum dried at 40° C. The above treatment was carried out only once.

[0049] Samples 29-31 were obtained by changing the heating temperature and heating time in (ii) above. Samples 32-34 were prepared without any desulfurizing agent in (ii) above, but with different heating temperatures and heating times in (ii) above. In other words, they were simply heated, and not desulfurized by a desulfurizing agent. Samples 35 and 36 were prepared by adding no solvent in (i) above, and were not left standing at room temperature for one day, but by increasing the amount of desulfurizing agent and adding a radical initiator in (ii) above. Samples 37 and 38 were prepared by adding no solvent in the above (i), and therefore not leaving the mixture at room temperature for one day, but by increasing the amount of desulfurizing agent added in the above (ii).

[0050] Samples 28-38 emitted little odor during the reaction or heat treatment, which was within the acceptable range, and the remaining odor of natural rubber after the reaction or heat treatment was also little and within the acceptable range.

[0051] After the reaction or heat treatment, <6> The swelling ratios of Samples 27-38 were determined using the method explained in the section "Desulfurization Index." The increase in the swelling ratio of Samples 28-38 relative to that of Sample 27 was also calculated. The results are shown in Table 4.

[0052] In Samples 28-31 and 35-38, the increase in swelling ratio relative to Sample 27 was 1.60 or more, indicating that natural rubber had been devulcanized. The swelling ratio also increased in Samples 32-34 (simple heat treatment), but this is thought to be due to thermal decomposition caused by heating (due to the low heat resistance of natural rubber as mentioned above), and not due to the progress of desulfurization. Comparing samples 28-31, desulfurization progressed more when the heating time was longer, even if the heating temperature was lower. A comparison of samples 35 and 36, and a comparison of samples 37 and 38, showed that desulfurization progressed more when the heating temperature was increased if the heating time was the same. Comparing samples 35 and 37, and also samples 36 and 38, it was found that desulfurization proceeded more effectively without the addition of a radical initiator.

[0053] From the above, Samples 28-31 and 35-38 are considered to be examples of the present invention, while Samples 32-34 are comparative examples because they are not desulfurization reactions.

[0054] Experiments 1 and 2 show that the progress of desulfurization can be easily controlled to obtain a desired swelling ratio by adjusting the amount of desulfurizing agent, whether or not a radical initiator is added and the amount added, whether or not a solvent is added and the amount added, the heating temperature, and the heating time. For example, it is estimated that if the heating temperature is increased for Sample 10, the increase in the swelling ratio can easily be increased by 1.10 or more. Furthermore, from the samples corresponding to the examples, it was confirmed that the present invention selectively cleaves the sulfur bonds of sulfur-crosslinked rubber, regardless of the type of rubber, and thus advances devulcanization, and that deterioration of physical properties can be suppressed because the main chain of the rubber is not cleaved.

[0055] The devulcanization method for the samples corresponding to the examples can be carried out industrially by heating in a large reaction vessel, a kneader, or a kneading extruder (such as a twin-screw extruder), and the devulcanized rubber can be reused as a high-quality raw rubber. In particular, heating while kneading in a kneading extruder allows for efficient implementation.

[0056] The present invention is not limited to the above-described embodiments, and can be embodied by making appropriate modifications within the scope of the invention.

Claims

1. A method for devulcanizing a sulfur-crosslinked rubber, comprising adding to the sulfur-crosslinked rubber at least one selected from the group consisting of primary and secondary phosphine oxides and analogs thereof, primary and secondary phosphines and analogs thereof which become oxides when oxidized, sulfenic acid, and sulfinic acid as a desulfurizing agent which acts on sulfur bonds in the sulfur-crosslinked rubber and cuts the sulfur bonds, and heating the sulfur-crosslinked rubber.

2. 2. The method for devulcanizing a sulfur-crosslinked rubber according to claim 1, further comprising adding to the sulfur-crosslinked rubber a radical initiator that generates radicals for generating radical active species from the devulcanizing agent.

3. 2. The method for devulcanizing a sulfur-crosslinked rubber according to claim 1, wherein no radical initiator that generates radicals for generating radical active species from the devulcanizing agent is added to the sulfur-crosslinked rubber.

4. 2. The method for devulcanizing a sulfur-crosslinked rubber according to claim 1, wherein a solvent is also added to the sulfur-crosslinked rubber.

5. 2. The method for devulcanizing a sulfur-crosslinked rubber according to claim 1, wherein no solvent is added to the sulfur-crosslinked rubber.

6. The method for devulcanizing a sulfur-crosslinked rubber according to any one of claims 1 to 5, wherein the heating is carried out while kneading in a kneading extruder.

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