Desulfurization method of sulfur cross-linked rubber

Abstract

The purpose of the present invention is to desulfurize a sulfur-crosslinked rubber by suppressing deterioration in physical properties and suppressing odor during and after desulfurization. A desulfurization method for a sulfur-crosslinked rubber, characterized in that: in the sulfur-crosslinked rubber, a desulfurization agent that acts on a sulfur bond in the sulfur-crosslinked rubber and cuts off the sulfur bond is added to the sulfur-crosslinked rubber; at least one substance selected from the group consisting of primary and secondary phosphine oxides and analogues thereof, primary and secondary phosphines and analogues thereof that become oxides during oxidation, hyposulphonic acid, and sulfinic acid is added and heated. Used rubber can be suitably used as the sulfur-crosslinked rubber.

Description

Technical Field

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

[0002] As a method for desulfurizing sulfur-crosslinked rubber (particularly, a method for desulfurizing used sulfur-crosslinked rubber to obtain reusable regenerated desulfurized rubber), generally, a physical desulfurization method (shear desulfurization) is currently carried out in a state where the sulfur-crosslinked rubber is melt-kneaded to form a high shear flow field. Patent Document 1 describes the following method: A sulfur-crosslinked rubber containing carbon black is pulverized and introduced into a twin-screw extruder, and while heating to a temperature of 180 to 350°C, a desulfurization treatment is performed by applying a shear stress of 10 to 150 kg / cm 2 to obtain regenerated desulfurized rubber in which sulfur cross-links are cut. In addition, the use of a desulfurizing agent in shear desulfurization is also described.

[0003] However, the problem of shear desulfurization is the reduction in physical properties (low selectivity) caused by the breakage of the rubber main chain (except for S-S and C-S bonds). At present, it is difficult to restore the regenerated desulfurized rubber to the same state as the new raw material rubber. In addition, desulfurization is carried out under high-temperature conditions.

[0004] On the other hand, a method for desulfurizing by chemically cutting sulfur bonds in the structure of sulfur-crosslinked rubber using a desulfurizing agent (regenerating agent) (chemical desulfurization) selectively cuts sulfur bonds, so it is difficult for the main chain to break. Therefore, the regenerated desulfurized rubber obtained by chemical desulfurization can maintain the same molecular weight as the new raw material rubber and suppress the reduction in physical properties. In addition, desulfurization can be carried out under mild conditions.

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

[0006] However, since these desulfurizing agents are all compounds that emit unique odors, the odor generated during desulfurization places a physical burden on the operator. In addition, the regenerated rubber after desulfurization also has an odor and is difficult to use as a recycled material.

[0007] Prior Art Documents

[0008] Patent Documents

[0009] Patent Document 1: Japanese Patent Laid-Open No. 9-227724

[0010] Patent Document 2: Japanese Patent Application Laid-Open No. 2010-535912

[0011] Patent Document 3: Japanese Patent Application Laid-Open No. 2003-510437 Summary of the Invention

[0012] Problems to be Solved by the Invention

[0013] Therefore, an object of the present invention is to desulfurize a sulfur-crosslinked rubber while suppressing a decrease in physical properties and suppressing odors during and after desulfurization.

[0014] Means for Solving the Problems

[0015] [1] A method for desulfurizing a sulfur-crosslinked rubber, characterized in that, in the sulfur-crosslinked rubber, at least one selected from the group consisting of primary and secondary phosphine oxides and their analogs, primary and secondary phosphines and their analogs that become oxides upon oxidation, sulfenic acids, and sulfinic acids is added as a desulfurizing agent that acts on sulfur bonds in the sulfur-crosslinked rubber and cleaves the sulfur bonds, and heating is performed.

[0016] [2] The method for desulfurizing a sulfur-crosslinked rubber according to the above [1], wherein a radical initiator that generates radicals for generating radical active species from the desulfurizing agent is further added to the sulfur-crosslinked rubber.

[0017] [3] The method for desulfurizing a sulfur-crosslinked rubber according to the above [1], wherein a radical initiator that generates radicals for generating radical active species from the desulfurizing agent is not added to the sulfur-crosslinked rubber.

[0018] [4] The method for desulfurizing a sulfur-crosslinked rubber according to any one of the above [1] to [3], wherein a solvent is further added to the sulfur-crosslinked rubber.

[0019] [5] The method for desulfurizing a sulfur-crosslinked rubber according to any one of the above [1] to [3], wherein a solvent is not added to the sulfur-crosslinked rubber.

[0020] [6] The method for desulfurizing a sulfur-crosslinked rubber according to any one of the above [1] to [5], wherein the heating is performed while kneading in a kneading extruder.

[0021] <Speculation on the Desulfurization Reaction Mechanism of the Present Invention>

[0022] · The secondary phosphine oxide described as R¹R²HP=O (R¹ and R² are not particularly limited) is in Such a chemical equilibrium state, in particular, R1R2P-OH reacts with the sulfur of the sulfur-crosslinked rubber in a nucleophilic manner, and thus desulfurization ultimately occurs.

[0023] ·The same reaction mechanism also occurs for primary phosphine oxides. However, tertiary phosphine oxides do not undergo the same reaction.

[0024] ·The same reaction mechanism also occurs for sulfenic acids (RSOH) and sulfinic acids (RS(O)OH).

[0025] When described in more detail taking diphenylphosphine oxide (DPPO) as an example, it is speculated as shown in Chemical Formula 1 below.

[0026] [Chemical Formula 1]

[0027]

[0028] DPPO is in a chemical equilibrium state with Compound 1 (biased towards the DPPO side). Compound 1 attacks the sulfur of the sulfur-crosslinked rubber, the S-S bond is cleaved, and Compounds 3 and 4 are formed. Here, the negatively charged sulfur of Compound 4 receives a proton from Compound 3 to give Compounds 5 and 6. Desulfurization proceeds through the same process.

[0029] Since the desulfurizing agent of the present invention selectively reacts with the sulfur bonds in the rubber to cleave the sulfur bonds, even when desulfurization proceeds, it is difficult to cleave the main chain of the rubber, and a decrease in physical properties can be suppressed.

[0030] In addition, compared with the desulfurizing agents listed in the above background art items, the desulfurizing agent of the present invention hardly generates or generates very little odor, so the odor generated during desulfurization is suppressed, and the odor hardly remains in the rubber after desulfurization.

[0031] <Regarding the presence or absence of a radical initiator>

[0032] In the present invention, desulfurization proceeds regardless of the addition of a radical initiator. However, when a radical initiator is added, desulfurization sometimes proceeds further. This is because the desulfurizing agent used in the present invention is also a radical precursor that generates radical active species that act on and cleave the sulfur bonds in the sulfur-crosslinked rubber, and the following desulfurization reaction mechanism can also be speculated.

[0033] 1) Radicals are generated from the radical initiator by heating.

[0034] 2) The generated radicals react with the radical precursor, and the radical precursor generates radical active species.

[0035] 3) The radical active species react with the sulfur bonds in the sulfur-crosslinked rubber to form radical intermediates.

[0036] 4) The radical active species generated in 2) further react with the radical intermediate in 3) to cleave the sulfur bond.

[0037] 5) While repeating 1)-4), desulfurization of the sulfur-crosslinked rubber is carried out.

[0038] When described in more detail taking DPPO and 2,2′-azobis(isobutyronitrile) (AIBN) as examples, it is speculated as shown in Chemical Formula 2 below.

[0039] [Chemical Formula 2]

[0040]

[0041] 1) Two C-N double bonds near the center of AIBN are cleaved by heating to generate nitrogen gas and 2-cyano-2-propyl radicals.

[0042] 2) The generated propyl radicals react with the H of the phosphorus-centered radical precursor, and the radical is transferred to the phosphorus atom to generate a phosphine oxide radical.

[0043] 3) The phosphine oxide radical reacts with the sulfur bond in the sulfur-crosslinked rubber to generate a radical intermediate.

[0044] 4) The phosphine oxide radical generated in 2) further reacts with the radical intermediate in 3) to cleave the sulfur bond.

[0045] 5) While repeating 1)-4), desulfurization of the sulfur-crosslinked rubber is carried out.

[0046] However, radical initiators such as AIBN must mostly be handled carefully because of their self-reactivity. Therefore, from the viewpoint of operability, it is preferable not to add a radical initiator.

[0047] <Regarding the presence or absence of a solvent>

[0048] It has been clarified that the desulfurization reaction of the present invention proceeds regardless of the addition or non-addition of a solvent.

[0049] (a) In the case where a solvent is added, the solvent swells the sulfur-crosslinked rubber and liquefies the desulfurizing agent to form a reaction field.

[0050] (b) Without adding a solvent, the desulfurizing agent liquefies in the heated reaction system and functions as a reaction field (replacing the solvent). Therefore, it is preferable to set the desulfurizing agent to an appropriate addition amount (concentration) suitable for forming a reaction field. This is because if the addition amount is too small (low concentration), there is less liquid component and it is difficult to form a reaction field, resulting in poor stirring efficiency; if the addition amount is too large (high concentration), the reaction system is diluted. The appropriate addition amount of the desulfurizing agent without adding a solvent varies depending on the type of rubber and is not particularly limited. Examples can be 1 to 20 equivalents, preferably 5 to 20 equivalents, more preferably 8 to

[0051] 15 equivalents.

[0052] However, by not adding a solvent, the following effects that are difficult to obtain when a solvent is added can be achieved.

[0053] (1) The heating can be carried out while kneading in, for example, a kneading extruder, which is practical.

[0054] (2) There is no need for drying the rubber after the reaction, etc., which can simplify the post-treatment.

[0055] (3) The environmental load can be reduced.

[0056] Therefore, when these effects are emphasized, it is preferable not to add a solvent.

[0057] Effects of the Invention

[0058] According to the present invention, it is possible to suppress the reduction of physical properties and suppress the odor during and after desulfurization, and desulfurize the sulfur-crosslinked rubber. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 Figure 1 is a graph obtained by plotting the addition amounts and swelling degrees of the desulfurizing agents of Samples 4, 5, and 7 corresponding to the examples of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0060] <1>Sulfur-crosslinked Rubber

[0061] The type of rubber as the sulfur-crosslinked rubber is not particularly limited, and examples can 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), nitrile rubber (NBR), etc.

[0062] The sulfur-crosslinked rubber is preferably a crushed material such as flakes or granules crushed before desulfurization.

[0063] ​​As the sulfur-crosslinked rubber, used rubber can be appropriately used, and there are no particular limitations on its usage time and usage conditions. According to the present invention, the used sulfur-crosslinked rubber can be desulfurized to produce reusable regenerated desulfurized rubber.

[0064] <2>Desulfurizing agent

[0065] As described above, as the desulfurizing agent, at least one selected from the group consisting of primary and secondary phosphine oxides and their analogs (such as phosphites), primary and secondary phosphines and their analogs that become oxides upon oxidation, sulfenic acids, and sulfinic acids is used.

[0066] Specifically, diphenylphosphine oxide (DPPO) shown in the following Chemical Formula 3, di-p-tolylphosphine oxide, diadamantylphosphine, di-3,5-dimethylphenylphosphine oxide, dicyclohexylphosphine oxide, di-4-methoxyphenylphosphine oxide, diphenylphosphine, diethyl phosphite, etc. can be exemplified.

[0067] [Chemical Formula 3]

[0068]

[0069] As the addition amount of the desulfurizing agent, since the appropriate addition amount varies depending on the rubber type / heating temperature / heating time, etc., there is no particular limitation, and it can be exemplified as 0.5 to 25 equivalents, preferably 1 to 20 equivalents, per 1 g of the rubber. In addition, the appropriate addition amount of the desulfurizing agent without adding a solvent is as described above.

[0070] <3>Free radical initiator

[0071] The free radical initiator is preferably at least one selected from the group consisting of azo compounds and peroxide compounds. This is because almost no or very little odor is generated, and it is easily obtained.

[0072] As the azo compound, 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), dimethyl 2,2'-azobis(isobutyrate), 4,4'-azobis(4-cyanovaleric acid), etc. can be exemplified.

[0073] In addition, as the peroxide compound, di-tert-butyl peroxide, cumene hydroperoxide, dicumyl peroxide, benzoyl peroxide (BPO), etc. can be exemplified.

[0074] As the addition amount of the radical initiator, since the appropriate addition amount varies depending on the rubber type / heating temperature / heating time, etc., it is not particularly limited, and examples thereof may be 0.5 equivalents to 16 equivalents, preferably 1 equivalent to 8 equivalents, per 1 g of the rubber.

[0075] <4> Solvent

[0076] As the solvent, there is no particular limitation, and the following solvents can be exemplified.

[0077] · Non-polar solvents (benzene, toluene, xylene, etc.)

[0078] · Low-polarity solvents (o-dichlorobenzene (o-DCB), 1-pentanol, chlorobenzene, etc.)

[0079] · High-polarity solvents (dimethylacetamide (DMA), dimethyl sulfoxide (DMSO), etc.)

[0080] · Halogen-based solvents (tetrachloroethane, etc.)

[0081] · Ether-based solvents (1,4-dioxane, etc.)

[0082] The combined use of o-DCB and DMA is particularly preferred. This is because the desulfurization efficiency is significantly improved (Samples 21, 23, and 24 described later). The reason is not yet clear at present, but it is considered to be due to some effect caused by the combination of a low-polarity solvent and a specific high-polarity solvent.

[0083] <5> Heating conditions

[0084] Heat to a temperature at which the progress of desulfurization can be substantially confirmed. The required temperature varies depending on the rubber type / the above-mentioned addition amounts / heating time, etc., and thus is not particularly limited, and examples thereof may be 60 to 200 °C. In the case of EPDM rubber, it is preferably 110 °C or higher, and in the case of natural rubber, it is preferably 60 °C or higher.

[0085] As the heating time, since the appropriate temperature varies depending on the rubber type / the above-mentioned addition amounts / heating temperature, etc., it is not particularly limited, and examples thereof may be 1 to 24 hours.

[0086] Stirring or kneading is preferably carried out during heating. Stirring or kneading can be appropriately selected according to the state of the material.

[0087] <6> Index of desulfurization

[0088] When the rubber comes into contact with the solvent, it absorbs the solvent and swells. This swelling also occurs in the rubber before desulfurization, but the higher the degree of desulfurization of the rubber, the higher the swelling ratio calculated by the following formula 1. This is because the solvent enters the places where the sulfur bonds are broken.

[0089] Swelling ratio (%) = (swollen weight - dry weight) / dry weight × 100…(Equation 1)

[0090] Therefore, in the present invention, the rubber before desulfurization (dry weight 1 g) was immersed in toluene as a solvent at room temperature for 24 hours, and then the swollen weight was measured to obtain the swelling ratio of the rubber before desulfurization. In addition, the rubber after desulfurization (dry weight 1 g) was immersed in the same solvent for the same time, and then the swollen weight was measured to obtain the swelling ratio of the rubber after desulfurization. Then, the increase ratio of the swelling ratio obtained by the following Equation 2 was used as an index for desulfurization.

[0091] Increase ratio of swelling ratio = swelling ratio of rubber after desulfurization / swelling ratio of rubber before desulfurization…(Equation 2)

[0092] However, depending on the type of rubber, the ease of swelling is different. Therefore, it is difficult to uniformly evaluate the degree of desulfurization of various rubbers using the above increase ratio of swelling ratio, and there is a preferred increase ratio for each rubber type.

[0093] For example, the increase ratio of the swelling ratio of EPDM rubber is preferably 1.10 or more (it can be considered that the progress of desulfurization can be substantially confirmed), more preferably 1.30 or more, further preferably 1.50 or more, and most preferably 2.00 or more.

[0094] In addition, the increase ratio of the swelling ratio of natural rubber is preferably 1.60 or more (it can be considered that the progress of desulfurization can be substantially confirmed), more preferably 2.00 or more, further preferably 2.50 or more, and most preferably 3.00 or more.

[0095] Examples

[0096] Next, the examples of the present invention will be described. It should be noted that the materials, conditions, structures, shapes, and dimensions of the examples are exemplary and can be appropriately changed within the scope not departing from the gist of the invention.

[0097] [Experiment 1]

[0098] As a desulfurization experiment of sulfur-crosslinked EPDM rubber, Specimens 1-26 shown in Tables 1-3 were carried out.

[0099] [Table 1]

[0100]

[0101] [Table 2]

[0102]

[0103] [Table 3]

[0104]

[0105] Specimen 1 is an EPDM rubber that has not been desulfurized. It is obtained by mixing 100 phr (parts by mass) of an 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) using an 8-inch roller, followed by compression molding at 160 °C for 20 minutes and sulfur vulcanization.

[0106] Specimen 2 is obtained by reacting the sulfur-crosslinked EPDM rubber identical to Specimen 1 using the following methods (1)-(3).

[0107] (1) Using a personalized organic synthesis device PPS-5511 type manufactured by Tokyo Rika Kikai (EYELA) Co., Ltd., 1 g (crushed product) of sulfur-crosslinked EPDM rubber is placed in a reaction vessel, and further 12 mL of o-DCB as a solvent is added, and it is left standing at room temperature for 1 day.

[0108] (2) 4 equivalents of DPPO as a desulfurizing agent and 2 equivalents of AIBN as a radical initiator are further added to the reaction vessel, and it is heated at 160 °C for 5 hours. During heating, it is continuously stirred at a stirring speed of 1000 rpm using the stirrer of this device.

[0109] (3) The EPDM rubber is taken out from the reaction vessel, washed 3 times with acetone, and then vacuum dried at 40 °C. The above reaction is carried out only once.

[0110] Specimen 3 is a specimen obtained by changing the addition amount of the solvent in the above (1) and changing the addition amounts of the desulfurizing agent and the radical initiator in the above (2).

[0111] Specimens 4 - 7 are specimens obtained by not adding the solvent in the above (1) and thus not leaving it standing at room temperature for 1 day, and increasing the addition amounts of the desulfurizing agent and the radical initiator in the above (2).

[0112] Specimens 8 - 10 are specimens obtained by not adding the solvent in the above (1) and thus not leaving it standing at room temperature for 1 day, not adding the radical initiator in the above (2), and changing the heating temperature.

[0113] Specimens 11, 12 are specimens obtained by not adding the solvent in the above (1) and thus not leaving it standing at room temperature for 1 day, changing the type and addition amount of the desulfurizing agent, and changing the addition amount of the radical initiator in the above (2).

[0114] Specimens 13, 14 are specimens obtained by changing the addition amount of the solvent in the above (1), changing the addition amount of the desulfurizing agent in the above (2), and not adding the radical initiator.

[0115] Samples 15 - 19 are samples obtained by changing the addition amount of the solvent in (1) above, changing the type and addition amount of the desulfurizing agent in (2) above, and not adding a radical initiator.

[0116] Samples 20 - 26 are samples obtained by changing the type of the solvent in (1) above, changing the addition amount of the desulfurizing agent in (2) above, changing the addition amount of the radical initiator or not adding a radical initiator. In Samples 22 - 26 and 28, two solvents are used in the volume ratio shown in Table 3.

[0117] Samples 2 - 26 produce less odor during the reaction, and within the allowable range, the odor residue of the EPDM rubber after the reaction is also less, within the allowable range.

[0118] After the reaction, the swelling ratio of Samples 1 - 26 is obtained by the method described in the item of "<6 Desulfurization Index>" above. In addition, the increase ratio of the swelling ratio of Samples 2 - 26 relative to that of Sample 1 is calculated. These results are shown in Tables 1 - 3.

[0119] The increase ratios of the swelling ratios of Samples 2 - 9 and 11 - 26 relative to that of Sample 1 are all 1.10 or more, and the desulfurization of the EPDM rubber is confirmed.

[0120] In the comparison between Samples 2 and 3, desulfurization proceeds when the addition amounts of the desulfurizing agent and the radical initiator are increased; even if the solvent is increased, desulfurization does not proceed.

[0121] From the comparison between Sample 2 (swelling ratio 421%) and Samples 4 - 7, it can be seen that desulfurization proceeds even when no solvent is added.

[0122] In addition, as Figure 1 shown, the swelling ratio of Sample 5 (10 equivalents of desulfurizing agent) is the highest, and the swelling ratios of Sample 4 (20 equivalents of desulfurizing agent) and Sample 7 (5 equivalents of desulfurizing agent) are decreased. This indicates that, as described above, when no solvent is added, the desulfurizing agent liquefies in the reaction system after heating and functions as a reaction field, and at this time, 10 equivalents of the desulfurizing agent in Sample 5 (including 8 - 15 equivalents of 10 equivalents) is an appropriate addition amount (concentration) suitable for forming a reaction field.

[0123] In the comparison between Samples 8 - 10, desulfurization proceeds even when no solvent is added if heated to a specified temperature. Since the swelling ratio of Sample 10 is as low as 0.979, it is considered that the desulfurization of the EPDM rubber is preferably carried out at a heating temperature of 110°C or higher.

[0124] As shown in Samples 11 and 12, desulfurization proceeds even when the type of the desulfurizing agent is changed and no solvent is added.

[0125] In the comparison between Specimens 3 and 13, with or without the addition of a free radical initiator, the progress of desulfurization did not change significantly.

[0126] In the comparison between Specimens 13 and 14, even when the addition amount of the desulfurizing agent was increased, the progress of desulfurization did not change significantly.

[0127] As shown in Specimens 11, 12, and 15 - 19, even when the type of the desulfurizing agent was changed, desulfurization proceeded.

[0128] In the comparison between Specimens 20 - 26, even when the type of the solvent was changed, desulfurization proceeded.

[0129] Based on the above, Specimens 2 - 9 and 11 - 26 were designated as the examples of the present invention. The desulfurizing agent of Specimen 10 was the desulfurizing agent of the present invention, but the heating was not at the required temperature, so it was a reference example.

[0130] [Experiment 2]

[0131] Next, as the desulfurization experiment of sulfur - crosslinked natural rubber, Specimens 27 - 38 shown in Table 4 below were carried out.

[0132] [Table 4]

[0133]

[0134] Specimen 27 was a natural rubber (sulfur component: 0.99 mmol) that was sulfur - vulcanized after kneading 100 phr of natural rubber (SVR - CV60 manufactured by Dau Tieng Rubber Company), 6.0 phr of zinc oxide, 0.5 phr of stearic acid, 3.5 phr of sulfur, and 0.5 phr of accelerator (MBT) with an 8 - inch roll and then compression - molding at 150 °C for 30 minutes. It was not desulfurized.

[0135] Specimen 28 was a specimen obtained by subjecting the same sulfur - crosslinked natural rubber as Specimen 27 to only heat treatment using the following methods (i) - (iii).

[0136] (i) Using the above - mentioned personalized organic synthesis device, 1 g (1 mm square) of sulfur - crosslinked natural rubber was added to the reaction vessel, and further 6 mL of DMA as a solvent was added, and it was left standing at room temperature for 1 day.

[0137] (ii) 1 equivalent of DPPO as a desulfurizing agent was further added to the reaction vessel, and it was heated at 70 °C for 6 hours. During the heating process, the stirrer of the device was used to continuously stir at a stirring speed of 1000 rpm. The reason for the lower heating temperature than in Experiment 1 was that natural rubber has carbon - carbon double bonds in the main chain and thus has low heat resistance.

[0138] (iii) The natural rubber was taken out from the reaction vessel, washed three times with acetone, and then dried under vacuum at 40 °C. The above treatment was only carried out once.

[0139] Specimens 29 - 31 are specimens in which the heating temperature and heating time were changed in the above (ii).

[0140] Specimens 32 - 34 are specimens in which no desulfurizing agent was added in the above (ii) and the heating temperature and heating time were changed in the above (ii). That is, only heat treatment was carried out, not desulfurization using a desulfurizing agent.

[0141] Specimens 35 and 36 are specimens in which no solvent was added in the above (i) and thus not left standing at room temperature for 1 day, and the addition amount of the desulfurizing agent was increased and a radical initiator was added in the above (ii).

[0142] Specimens 37 and 38 are specimens in which no solvent was added in the above (i) and thus not left standing at room temperature for 1 day, and the addition amount of the desulfurizing agent was increased in the above (ii).

[0143] Specimens 28 - 38 produced little odor during the reaction or heat treatment, within the allowable range, and there was also little odor residue in the natural rubber after the reaction or heat treatment, within the allowable range.

[0144] After the reaction or heat treatment, the swelling ratio of specimens 27 - 38 was determined by the method described in the above item "<6> Desulfurization Index". In addition, the increase ratio of the swelling ratio of specimens 28 - 38 relative to that of specimen 27 was calculated. These results are shown in Table 4.

[0145] The increase ratios of the swelling ratios of specimens 28 - 31, 35 - 38 relative to that of specimen 27 were all 1.60 or more, and desulfurization of the natural rubber was confirmed.

[0146] The swelling ratio of specimens 32 - 34 (only heat treatment) also increased, but it is considered that this was caused by thermal decomposition due to heating (as described above, because the heat resistance of natural rubber is low), rather than due to the progress of desulfurization.

[0147] In the comparison of specimens 28 - 31, even when the heating temperature was lowered, desulfurization of the specimens with an extended heating time proceeded.

[0148] In the comparison of specimens 35 and 36, and also in the comparison of specimens 37 and 38, if the heating time was the same, desulfurization of the specimens with an increased heating temperature proceeded.

[0149] In the comparison of specimens 35 and 37, and also in the comparison of specimens 36 and 38, desulfurization of the specimens without adding a radical initiator proceeded.

[0150] Based on the above, specimens 28 - 31 and 35 - 38 were defined as examples of the present invention. Specimens 32 - 34 did not undergo a desulfurization reaction and were thus comparative examples.

[0151] From Experiment 1 and Experiment 2, it can be seen that by adjusting factors such as the addition amount of the desulfurizing agent, the presence or absence and addition amount of the free radical initiator, the presence or absence and addition amount of the solvent, the heating temperature, and the heating time, the progress of desulfurization can be easily controlled to obtain the desired swelling ratio. For example, it is speculated that if the heating temperature is increased in Specimen 10, the magnification of the increase in the swelling ratio is likely to be 1.10 or more.

[0152] In addition, it can be confirmed from the specimens corresponding to the examples that according to the present invention, regardless of the type of rubber, the sulfur bonds of the sulfur - crosslinked rubber are selectively cleaved for desulfurization. Moreover, since the main chain of the rubber is not cleaved, a reduction in physical properties can be suppressed.

[0153] Furthermore, the desulfurization method for the specimens corresponding to the examples can be industrially implemented by heating in a large reaction vessel, a kneader, a kneading extruder (such as a twin - screw extruder), etc. The rubber after desulfurization can be reused as high - quality raw rubber. In particular, it can be effectively implemented by heating while kneading in a kneading extruder.

[0154] It should be noted that the present invention is not limited to the above - mentioned examples and can be appropriately modified and embodied without departing from the gist of the invention.

Claims

1. A method for desulfurizing a sulfur-crosslinked rubber, characterized in that, in the sulfur-crosslinked rubber, at least one selected from the group consisting of primary and secondary phosphine oxides and their analogs, primary and secondary phosphines and their analogs that become oxides upon oxidation, sulfenic acids, and sulfinic acids is added as a desulfurizing agent that acts on and cleaves the sulfur bonds in the sulfur-crosslinked rubber, and heating is carried out.

2. The desulfurization method of the sulfur-crosslinked rubber according to claim 1, wherein, A radical initiator is further added to the sulfur-crosslinked rubber, and the radical initiator generates radicals for generating radical active species from the desulfurizing agent.

3. The desulfurization method of the sulfur-crosslinked rubber according to claim 1, wherein, No radical initiator that generates radicals for generating radical active species from the desulfurizing agent is added to the sulfur-crosslinked rubber.

4. The desulfurization method of the sulfur-crosslinked rubber according to claim 1, wherein, A solvent is further added to the sulfur-crosslinked rubber.

5. The desulfurization method of the sulfur-crosslinked rubber according to claim 1, wherein, No solvent is added to the sulfur-crosslinked rubber.

6. The desulfurization method of the 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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