Method for desulfurization of sulfur-crosslinked rubber

Abstract

To provide a method for desulfurization of sulfur-crosslinked rubber which improves desulfurization efficiency while decreasing rubber tackiness so as to enhance workability during post-treatment.SOLUTION: A method for desulfurization of sulfur-crosslinked rubber, comprising adding, to the sulfur-crosslinked rubber, a desulfurizing agent that acts on sulfur bonds in the sulfur-crosslinked rubber and cleaves the sulfur bonds, a nonpolar solvent, and a polar solvent, followed by heating. As the sulfur-crosslinked rubber, a used sulfur-crosslinked rubber can be suitably used.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, chemical desulfurization using these desulfurization agents has problems in that it has low desulfurization efficiency and takes a long time (for example, it takes 2 hours in the example of Patent Document 3). For the desulfurization reaction to proceed, the desulfurization agent needs to penetrate deep into the rubber, but there are problems such as it being difficult to penetrate or taking a long time to penetrate. Furthermore, the inventors have also recognized the problem that the rubber becomes sticky and tends to stick to the reaction vessel, making it difficult to remove the rubber from the reaction vessel during post-treatment.

[0007] Furthermore, all of these desulfurizing agents are compounds that emit a distinctive odor, which causes physical strain on workers due to the odor generated during desulfurization. Furthermore, the reclaimed rubber after desulfurization also has an odor, making it difficult to use as a recycled material. [Prior art documents] [Patent documents]

[0008] [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 (Patent No. 4633988) Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, an object of the present invention is to increase the devulcanization efficiency of sulfur-crosslinked rubber and to reduce the stickiness of the rubber to improve the workability in post-treatment. A further object of the present invention is to suppress odors during and after desulfurization. [Means for solving the problem]

[0010] [1] A method for devulcanizing sulfur-crosslinked rubber, comprising adding a devulcanizing agent that acts on and cuts sulfur bonds in the sulfur-crosslinked rubber, a non-polar solvent, and a polar solvent to the sulfur-crosslinked rubber, and then heating the mixture.

[0011] (action) 1. Highly efficient desulfurization When only a non-polar solvent is added as the solvent, the sulfur-crosslinked rubber tends to swell due to the non-polar solvent, so the devulcanizing agent penetrates deep into the rubber, facilitating the devulcanization reaction and increasing the devulcanization efficiency, as shown in the center of Table 4 below. However, the sulfur-crosslinked rubber becomes difficult to stir due to swelling, which reduces the stirring efficiency and may counteract the effect of increasing the devulcanization efficiency. When only a polar solvent is added as the solvent, as shown in the right part of Table 4 below, the sulfur cross-linked rubber is difficult to swell with the polar solvent, and therefore the devulcanizing agent does not easily penetrate into the rubber, making it difficult for the devulcanization reaction to proceed, and therefore the devulcanization efficiency does not increase. In the present invention, by using a non-polar solvent and a polar solvent in combination as the solvent (hereinafter, both solvents may be collectively referred to as a "mixed solvent"), as shown in the left part of Table 4 below, the non-polar solvent swells the sulfur-crosslinked rubber, allowing the desulfurizing agent to penetrate deep into the rubber, while the polar solvent acts as a reaction field to increase the stirring efficiency, which is reduced by the swelling, and therefore the desulfurization efficiency can be reliably increased.

[0012] 2. Improved workability in post-processing If only a non-polar solvent is added, the rubber becomes sticky due to swelling and tends to stick to the reaction vessel, making it difficult to remove the rubber from the reaction vessel during post-treatment. In the present invention, the addition of the mixed solvent reduces the stickiness of the rubber, making it easier to remove in post-treatment.

[0013] [2] The method for devulcanizing a sulfur-crosslinked rubber according to [1] above, wherein the volume ratio of the non-polar solvent to the polar solvent is 0.7:0.3 to 0.3:0.7.

[0014] The volume ratio of the non-polar solvent to the polar solvent is not particularly limited, but is preferably within the range of 0.7:0.3 to 0.3:0.7, since this provides a good balance between the swelling effect of the non-polar solvent and the improvement in stirring efficiency and suppression of stickiness of the polar solvent, and more preferably within the range of 0.6:0.4 to 0.4:0.6.

[0015] [3] The method for devulcanizing sulfur-crosslinked rubber according to [1] or [2], wherein the desulfurizing agent is a combination of a radical precursor that generates a radical active species that cleaves sulfur bonds, and a radical initiator that generates radicals to convert the radical precursor into the radical active species (i.e., to drive a radical reaction).

[0016] (action) The mechanism of the desulfurization reaction in this embodiment is presumed to be as follows. 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).

[0017] To explain this in more detail using the example of diphenylphosphine oxide (DPPO) and 2,2'-azobis(isobutyronitrile) (AIBN), it is estimated as shown in the following chemical formula 1. [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).

[0018] The radical active species generated by the radical precursor selectively react with sulfur bonds in the rubber and sever the sulfur bonds, so that even if devulcanization progresses, severing of the main chain of the rubber is unlikely to occur, and deterioration of physical properties can be suppressed. Compared with the desulfurizing agents listed in the Background Art section above, radical precursors and radical initiators generate almost no or only a small amount of odor, so odor generated during devulcanization is suppressed and odor is less likely to remain in the rubber after devulcanization.

[0019] [4] The method for devulcanizing a sulfur-crosslinked rubber according to [1] or [2], wherein the desulfurizing agent is at least one selected from the group consisting of primary and secondary phosphine oxides and their analogues, primary and secondary phosphines and their analogues which become oxides when oxidized, sulfenic acid, and sulfinic acid.

[0020] (action) The mechanism of the desulfurization reaction in this embodiment is presumed to be as follows. 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).

[0021] To explain this in more detail using the example of diphenylphosphine oxide (DPPO), it is estimated as shown in the following chemical formula 2. [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.

[0022] The devulcanizing agent in this embodiment selectively reacts with sulfur bonds in the rubber to cleave the sulfur bonds, so that even if devulcanization proceeds, cleavage of the main chain of the rubber is unlikely to occur, and deterioration of physical properties can be suppressed. The desulfurizing agent of this embodiment generates almost no or little 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.

[0023] [5] The method for devulcanizing a sulfur-crosslinked rubber according to any one of [1] to [4], wherein the heating is carried out with stirring.

[0024] (action) By heating with stirring, the desulfurization efficiency is improved. [Effects of the Invention]

[0025] According to the present invention, it is possible to increase the devulcanization efficiency of sulfur-crosslinked rubber, and also to reduce the stickiness of the rubber, thereby improving the workability in post-treatment. Furthermore, by using the desulfurizing agent described in the above [3] or [4] as the desulfurizing agent, odors during and after desulfurization can be suppressed. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is a schematic diagram showing the experimental method for the examples of the present invention and the comparative examples. DETAILED DESCRIPTION OF THE INVENTION

[0027] <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.

[0028] <2> Desulfurization Agent The desulfurizing agent is not particularly limited, but examples thereof include the desulfurizing agents described in Patent Documents 1 to 3, the desulfurizing agent described in [3] above, and the desulfurizing agent described in [4] above.

[0029] [Desulfurizing agent described in [3] above] The radical precursor is preferably at least one selected from the group consisting of phosphorus compounds, germanium, tin, arsenic, antimony, selenium, tellurium compounds, silicon compounds, and boron compounds. These compounds have affinity with sulfur atoms, emit little or no odor, and are easily available. Among these, phosphorus compounds, germanium, tin, arsenic, antimony, selenium, and tellurium compounds are particularly preferred because they emit no odor. Examples of phosphorus compounds include diphenylphosphine oxide, di-p-tolylphosphine oxide, and bis-3,5-dimethylphenylphosphine oxide.

[0030] The amount of radical precursor to be added is not particularly limited, as the appropriate amount varies depending on the type of rubber, the heating temperature and heating time described below, etc., but can be exemplified as 0.5 to 16 equivalents per 1 g of the rubber, and preferably 1 to 8 equivalents.

[0031] 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).

[0032] 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.

[0033] [Desulfurizing agent described in [4] above] Examples of the desulfurizing agent 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]

[0034] The amount of the 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 an example of the amount is 0.5 to 25 equivalents per 1 g of the rubber, and preferably 1 to 20 equivalents.

[0035] <3> Polar solvents The polar solvent is not particularly limited, but examples thereof include dimethylacetamide (DMA), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), 1-pentanol, and ethanol (EtOH).

[0036] <4> Nonpolar solvents The non-polar solvent is not particularly limited, but examples thereof include benzene, toluene, xylene, ortho-dichlorobenzene (o-DCB), chlorobenzene, tetrachloroethane, and 1,4-dioxane.

[0037] <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.

[0038] <6> stirring The stirring method is not particularly limited, but examples include stirring with a stirrer and kneading with a kneading device.

[0039] <7> 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)

[0040] 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]

[0041] 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.

[0042] [Experiment 1] As devulcanization experiments of sulfur-crosslinked EPDM rubber, Comparative Examples 1-5 and Examples 1-4 shown in Table 1, and Comparative Examples 6-8 and Examples 5-7 shown in Table 2 were carried out.

[0043] [Table 1]

[0044] [Table 2]

[0045] Comparative Example 1 is a sulfur-crosslinked 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.

[0046] In Comparative Example 2, the same sulfur-crosslinked EPDM rubber as in Comparative Example 1 was reacted by 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 recovery flask (capacity: 100 ml) as a reaction vessel, as shown in Figure 1(a), and 5 mL of the nonpolar solvent o-DCB was added, followed by leaving the mixture at room temperature for one day. (2) 5 equivalents of DPPO as a desulfurizing agent and 2.5 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 stirred at a rotation speed of 1000 rpm using the stirrer (a stirrer tip with a total length of approximately 1 cm) 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.

[0047] In Comparative Example 3, the radical initiator was not added, and the heating temperature was changed, as compared with Comparative Example 2. In Comparative Example 4, the solvent was changed to DMA, which is a polar solvent, and the heating temperature was changed in comparison with Comparative Example 2. In Comparative Example 5, the solvent was changed to 1-pentanol, a polar solvent, and the heating temperature was changed in comparison with Comparative Example 2.

[0048] In Example 1, the solvent was changed to a mixed solvent of o-DCB, a non-polar solvent, and DMA, a polar solvent (volume ratio 0.6:0.4), the total amount added was 6 mL, and the heating temperature was changed compared to Comparative Example 2. In Example 2, the volume ratio of o-DCB to DMA was changed to 0.5:0.5 compared to Example 1. Example 3 is the same as Example 2 except that no radical initiator was added. Example 4 is the same as Example 3 except that the solvent was changed to a mixed solvent of o-DCB and 1-pentanol.

[0049] In Comparative Example 6, the same sulfur-crosslinked EPDM rubber as in Comparative Example 1 was reacted by the following methods (1) to (3). (1) Using the personal organic synthesis apparatus, 1 g of sulfur-crosslinked EPDM rubber (ground material) was placed in a test tube (diameter 24 mm) as a reaction vessel, as shown in Figure 1(b), and 6 mL of o-DCB as a nonpolar solvent was added, followed by leaving the mixture at room temperature for one day. (2) Two equivalents of DPPO as a desulfurizing agent were further added to the reaction vessel, and the mixture was heated at 150°C for 1 hour. During heating, the mixture was stirred at 1000 rpm using the stirrer (a stirrer tip with a total length of approximately 1 cm) 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.

[0050] In Comparative Example 7, the heating time was changed from Comparative Example 6 to 3 hours. In Comparative Example 8, the heating time was changed from Comparative Example 6 to 5 hours.

[0051] Example 5 is the same as Comparative Example 6 except that the solvent was changed to a mixed solvent of o-DCB and DMA (volume ratio 0.5:0.5). In Example 6, the heating time was changed from Example 5 to 3 hours. In Example 7, the heating time was changed from Example 5 to 5 hours.

[0052] In all of the Comparative Examples and Examples, the amount of odor generated during the reaction was small and within an acceptable range, and the residual odor of the EPDM rubber after the reaction was also small and within an acceptable range.

[0053] After the reaction, <7> The swelling ratios of the comparative examples and examples were determined by the method explained in the section "Desulfurization index." In addition, the increase ratio of the swelling ratio of the other comparative examples and examples relative to the swelling ratio of Comparative Example 1 (hereinafter simply referred to as "increase ratio of swelling ratio") was calculated. Furthermore, based on the swelling rate, the ease of penetration of the desulfurizing agent (into the rubber) was evaluated on a three-point scale: ○ (good), △ (not good), × (bad). In addition, the stirring efficiency was evaluated on a two-point scale of ○ (good) and × (bad) based on the stirring state of the stirrer during the reaction. The reactivity was evaluated based on the ease of penetration of the desulfurizing agent and the stirring efficiency, using a four-level scale: ◎ (best), ○ (good), △ (unacceptable), × (poor). Furthermore, the presence or absence of stickiness of the rubber after the reaction was checked, and the workability when removing the rubber from the reaction vessel was evaluated on a two-level scale of ○ (good) or × (bad). These results are shown in Tables 1 and 2.

[0054] In Comparative Example 4, the increase in swelling ratio was less than 1.10, and devulcanization of the EPDM rubber was not observed. This is thought to be because the EPDM rubber hardly swells with DMA, and the devulcanizing agent hardly penetrates into the rubber. In Comparative Example 5, the increase in swelling ratio was less than 1.50, and devulcanization of the EPDM rubber was observed but was not sufficient. This is thought to be because EPDM rubber is difficult to swell with 1-pentanol, making it difficult for the devulcanizing agent to penetrate into the rubber. In contrast, in Comparative Examples 2, 3, 6-8 and Examples 1-7, the increase in swelling ratio was 1.50 or more, and devulcanization of the EPDM rubber was confirmed to a certain extent. This is thought to be because EPDM rubber is easily swollen by o-DCB, making it easy for the devulcanizing agent to penetrate into the rubber. Compared to Comparative Example 3, which used only o-DCB, Example 3, which used an o-DCB / DMA mixed solvent, showed a similar swelling ratio despite the lower o-DCB content. The polar solvent acts as a reaction field to enhance the stirring efficiency, which is reduced by swelling due to o-DCB, and this is thought to have resulted in the higher desulfurization efficiency. The same can be said for Example 4, which used an o-DCB / 1-pentanol mixed solvent. In comparison between Example 1 and Example 2, the swelling ratio was higher when the amounts of o-DCB and DMA were closer to the same. Comparing Example 2 and Example 3, the swelling ratio was higher when a radical initiator was added. In Comparative Examples 6-8, the swelling ratio was still low at a reaction time of 1 hour, but increased over time to 3 and 5 hours. This is thought to indicate that the desulfurization efficiency decreased due to a decrease in stirring efficiency caused by swelling, and desulfurization took longer. In contrast, in Example 5-7, the swelling ratio was already higher than that of Comparative Example 8 after 1 hour of reaction time, and was even higher after 3 hours, but was not significantly different after 5 hours. This is thought to indicate that the stirring efficiency, which decreases due to swelling, is increased by the polar solvent acting as a reaction field, resulting in higher desulfurization efficiency.

[0055] In addition, in Comparative Examples 2, 3, and 6-8, in which only o-DCB was used, the rubber became sticky due to swelling and stuck to the reaction vessel, making it difficult to remove the rubber from the reaction vessel. In contrast, in Example 1-7, in which a mixed solvent was used, the rubber was less sticky and the rubber could be easily removed from the reaction vessel.

[0056] [Experiment 2] Next, as devulcanization experiments of sulfur-crosslinked natural rubber, Comparative Examples 9-11 and Examples 8 and 9 shown in Table 3 were carried out.

[0057] [Table 3]

[0058] Comparative Example 9 is a sulfur-crosslinked natural rubber (sulfur component: 0.99 mmol) that was obtained by kneading 100 phr of natural rubber (SVR-CV60 manufactured by Zau Tien 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, and this natural rubber was not desulfurized.

[0059] In Comparative Example 10, the same sulfur-crosslinked natural rubber as in Comparative Example 9 was simply heat-treated by the following methods (i) to (iii). (i) Using the personal organic synthesis apparatus, 1 g of sulfur-crosslinked natural rubber (1 mm square) was placed in a recovery flask (100 ml capacity) as a reaction vessel, as shown in FIG. 1(a), and 10 mL of o-DCB 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 desulfurizing 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. This treatment was carried out only once.

[0060] In Comparative Example 11, the solvent in Comparative Example 10 was changed to DMA.

[0061] Example 8 is the same as Comparative Example 10 except that the solvent was changed to a mixture of o-DCB and DMA (volume ratio 0.5:0.5). In Example 9, the heating time was changed to 3 hours.

[0062] In all of the Comparative Examples and Examples, the amount of odor generated during the reaction was small and within an acceptable range, and the residual odor of natural rubber after the reaction was also small and within an acceptable range.

[0063] In Experiment 2, as in Experiment 1, the swelling ratio and the swelling ratio increase ratio were determined for the comparative example and the example, and the ease of penetration of the desulfurizing agent, stirring efficiency, reactivity, presence or absence of stickiness, and workability were evaluated. These results are shown in Table 3.

[0064] In Comparative Example 11, the increase in swelling ratio was less than 2.50, and devulcanization of the natural rubber was observed but was not sufficient. This is thought to be because natural rubber is difficult to swell with DMA, making it difficult for the devulcanizing agent to penetrate into the rubber. In contrast, in Comparative Example 10 and Examples 8 and 9, the increase in swelling ratio was 2.50 or more, indicating that the natural rubber had been devulcanized to a certain extent. This is thought to be because natural rubber is easily swollen by o-DCB, allowing the devulcanizing agent to easily penetrate into the rubber. Compared to Comparative Example 10, which used only o-DCB, Examples 8 and 9, which used an o-DCB / DMA mixed solvent, showed high swelling ratios despite the low o-DCB content. Although swelling due to o-DCB was reduced, it is thought that the DMA suppressed the decrease in stirring efficiency due to swelling, resulting in higher desulfurization efficiency.

[0065] The main points of the evaluation results of Experiments 1 and 2 are summarized in Table 4, along with a schematic diagram showing the reaction during the reaction.

[0066] [Table 4]

[0067] 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 devulcanization method for sulfur-crosslinked rubber, comprising adding a devulcanizing agent that acts on and cuts sulfur bonds in the sulfur-crosslinked rubber, a non-polar solvent, and a polar solvent to the sulfur-crosslinked rubber, and then heating the mixture.

2. 2. The method for devulcanizing sulfur-crosslinked rubber according to claim 1, wherein the volume ratio of the nonpolar solvent to the polar solvent is 0.7:0.3 to 0.3:0.

7.

3. 2. The method for devulcanizing a sulfur-crosslinked rubber according to claim 1, wherein the devulcanizing agent is a combination of a radical precursor that generates a radical active species that cleaves sulfur bonds, and a radical initiator that generates radicals to convert the radical precursor into the radical active species (i.e., to drive a radical reaction).

4. 2. The method for devulcanizing a sulfur-crosslinked rubber according to claim 1, wherein the desulfurizing agent is at least one selected from the group consisting of primary and secondary phosphine oxides and their analogues, primary and secondary phosphines and their analogues which become oxides when oxidized, sulfenic acid, and sulfinic acid.

5. The method for devulcanizing a sulfur-crosslinked rubber according to any one of claims 1 to 4, wherein the heating is carried out while stirring.

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