Sulfur-crosslinked rubber desulfurization method

Abstract

To perform desulfurization of a sulfur-crosslinked rubber in a manner that suppresses reduction of physical properties and suppresses odor generated during and after desulfurization.SOLUTION: A desulfurization method for sulfur-crosslinked rubber, the method comprising adding to the sulfur-crosslinked rubber a radical precursor that acts on sulfur bonds in the sulfur-crosslinked rubber and generates a radical active species that cleaves the sulfur bonds, and a radical initiator that generates a radical for converting the radical precursor into the radical active species, and performing heating without adding a solvent, wherein a 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 devulcanization method for sulfur crosslinked rubber, comprising adding to the sulfur crosslinked rubber a radical precursor that acts on a sulfur bond in the sulfur crosslinked rubber and generates a radical active species that cleaves the sulfur bond, and a radical initiator that generates radicals to convert the radical precursor into the radical active species (i.e., to drive a radical reaction), and heating the mixture without adding a solvent.

[0010] [2] In the above [1], 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, because these have affinity with sulfur atoms, produce little or no odor, and are easily available.

[0011] [3] In the above [1] or [2], the radical initiator is preferably at least one selected from the group consisting of azo compounds and peroxide compounds, because these compounds have little or no odor and are easily available.

[0012] [4] In the above [1], [2] or [3], the heating is preferably carried out while kneading in a kneading extruder.

[0013] [Effect] The mechanism of the desulfurization reaction in the present invention 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).

[0014] 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 bonds near the center of AIBN are broken, 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).

[0015] <About the presence or absence of solvent> It has been found that the desulfurization reaction 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 radical precursor, forming a reaction field. (b) When no solvent is added, the radical precursor liquefies in the heated reaction system and functions as a reaction field (substitute for the solvent). Therefore, it is preferable to add the radical precursor 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, while if the amount added is too large (high concentration), the reaction system is diluted. The appropriate amount of radical precursor added when no solvent is added varies depending on the type of rubber and 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.

[0016] 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, in the present invention, in order to obtain these effects, no solvent is added.

[0017] 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. [Effects of the Invention]

[0018] 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]

[0019] [Figure 1] FIG. 1 is a graph plotting the amount of radical precursor added and the degree of swelling in Examples 1, 2, and 4 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] <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.

[0021] <2> Radical Precursors Of the above examples, phosphorus compounds, germanium, tin, arsenic, antimony, selenium, and tellurium compounds are particularly preferred because they do not emit odors. Examples of phosphorus compounds include diphenylphosphine oxide (DPPO), di-p-tolylphosphine oxide, bis-3,5-dimethylphenylphosphine oxide, dicyclohexylphosphine oxide, and di-4-methoxyphenylphosphine oxide, which are shown in the following Chemical Formula 2. [ka]

[0022] The amount of radical precursor to be added is not particularly limited, as the appropriate amount varies depending on the type of rubber, heating temperature, heating time, etc., but as described above, an example is 1 to 20 equivalents per 1 g of the rubber, preferably 5 to 20 equivalents, and more preferably 8 to 15 equivalents.

[0023] <3> Radical Initiators Examples of the 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, the heating temperature and heating time described below, etc., but an example of the amount is 0.5 to 16 equivalents per 1 g of the rubber, and preferably 1 to 8 equivalents.

[0024] <4> 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 add a solvent and heat the mixture. 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.

[0025] <5> 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)

[0026] 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]

[0027] 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.

[0028] [Experiment 1] As devulcanization experiments of sulfur-crosslinked EPDM rubber, Comparative Examples 1-3 and Examples 1-6 shown in Table 1 were carried out.

[0029] [Table 1]

[0030] Comparative Example 1 is a sulfur-vulcanized EPDM rubber that was not 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.

[0031] 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 reaction vessel, and 12 mL of ortho-dichlorobenzene (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 radical precursor 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 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.

[0032] In Comparative Example 3, the amount of solvent added was changed in (1) above, and the amount of radical precursor and radical initiator added was changed in (2) above.

[0033] In Example 1-4, no solvent was added in (1) above, and therefore the mixture was not left standing at room temperature for one day, and the amounts of radical precursor and radical initiator added were changed in (2) above. In Examples 5 and 6, no solvent was added in (1) above, and therefore the mixture was not left to stand at room temperature for one day. In (2) above, the type and amount of radical precursor added were changed, and the amount of radical initiator added was changed.

[0034] In Comparative Examples 2 and 3 and Examples 1 to 6, the 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.

[0035] After the reaction, <5> The swelling ratios of Comparative Examples 1-3 and Examples 1-6 were determined by the method explained in the section "Desulfurization Index." The increase ratios of the swelling ratios of Comparative Examples 2 and 3 and Examples 1-6 relative to the swelling ratio of Comparative Example 1 were also calculated. The results are shown in Table 1.

[0036] In Comparative Examples 2 and 3 and Examples 1 to 6, the increase in swelling ratio relative to Comparative Example 1 was 1.10 or more, and devulcanization was observed in the EPDM rubber. In a comparison between Comparative Examples 2 and 3, increasing the amount of radical precursor and radical initiator added promoted desulfurization, but increasing the amount of solvent did not promote desulfurization. A comparison between Comparative Example 2 (swelling ratio 421%) and Examples 1-4 revealed that desulfurization proceeded even when no solvent was added. 1, the swelling ratio was highest in Example 2 (10 equivalents of radical precursor), and was lower in Example 1 (20 equivalents of radical precursor) and Example 4 (5 equivalents of radical precursor). This indicates that, as mentioned above, when no solvent is added, the radical precursor liquefies in the heated reaction system and functions as a reaction field, whereas the addition of 10 equivalents (including 8 to 15 equivalents) of radical precursor in Example 2 was an appropriate amount (concentration) suitable for forming a reaction field. Furthermore, as in Examples 5 and 6, desulfurization proceeded even when the type of radical precursor was changed and no solvent was added.

[0037] [Experiment 2] Next, as devulcanization experiments of sulfur-crosslinked natural rubber, Comparative Examples 4 to 7 and Examples 7 and 8 shown in Table 2 below were carried out.

[0038] [Table 2]

[0039] Comparative Example 4 is a sulfur-vulcanized 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 the resulting product was not desulfurized.

[0040] In Example 7, the same sulfur-crosslinked natural rubber as in Comparative Example 4 was simply heat-treated by the following methods (i) to (iii). (i) Using the personal organic synthesis apparatus, 1 g (1 mm square) of sulfur-crosslinked natural rubber was placed in a reaction vessel. (ii) 15% of DPPO as a radical precursor and 7.5 equivalents of AIBN as a radical initiator were added to the reaction vessel, and the mixture was heated at 70°C for 6 hours. During heating, the mixture was stirred at 1000 rpm using the stirrer of the same equipment. The heating temperature was lower than in Experiment 1 because natural rubber has low heat resistance due to the presence of carbon-carbon double bonds in the 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.

[0041] In Example 8, the heating temperature was changed in (ii) above. In Comparative Examples 5-7, 6 mL of dimethylacetamide (DMA) was added as a solvent in (i) and allowed to stand at room temperature for one day, and in (ii) no radical precursor or radical initiator was added, and the heating temperature and heating time were changed. In other words, this was a simple heat treatment, not desulfurization using a desulfurizing agent.

[0042] In Examples 7 and 8, the odor generated during the reaction was small and within an acceptable range, and the residual odor of the natural rubber after the reaction was also small and within an acceptable range.

[0043] After the heat treatment or reaction, <5> The swelling ratios of Comparative Examples 4 to 7 and Examples 7 and 8 were determined by the method described in the section "Desulfurization Index." The increase ratios of the swelling ratios of Comparative Examples 5 to 7 and Examples 7 and 8 relative to the swelling ratio of Comparative Example 4 were also calculated. The results are shown in Table 2.

[0044] In Examples 7 and 8, the increase in swelling ratio relative to Comparative Example 4 was 1.60 or more, and devulcanization of natural rubber was confirmed. The swelling ratio also increased in Comparative Example 5-7 (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.

[0045] From Experiments 1 and 2, it is clear that the progress of desulfurization can be easily controlled to obtain the desired swelling ratio by adjusting the amount of radical precursor added, the amount of radical initiator added, the heating temperature, and the heating time. It was also confirmed that the present invention selectively cleaves the sulfur bonds of sulfur-crosslinked rubber, regardless of the type of rubber, and promotes devulcanization, and that the main chain of the rubber is not cleaved, thereby suppressing deterioration in physical properties.

[0046] The devulcanization method of the present invention can be carried out industrially by heating in a large reactor, 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, the method can be carried out efficiently by heating the rubber while kneading it in a kneading extruder.

[0047] 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 to the sulfur-crosslinked rubber a radical precursor that acts on a sulfur bond in the sulfur-crosslinked rubber and generates a radical active species that cleaves the sulfur bond, and a radical initiator that generates radicals to convert the radical precursor into the radical active species, and heating the mixture without adding a solvent.

2. 2. The method for devulcanizing a sulfur-crosslinked rubber according to claim 1, wherein the radical precursor is at least one selected from the group consisting of phosphorus compounds, germanium, tin, arsenic, antimony, selenium, tellurium compounds, silicon compounds, and boron compounds.

3. 2. The method for devulcanizing a sulfur-crosslinked rubber according to claim 1, wherein the radical initiator is at least one selected from the group consisting of an azo compound and a peroxide compound.

4. The method for devulcanizing a sulfur-crosslinked rubber according to any one of claims 1 to 3, wherein the heating is carried out while kneading in a kneading extruder.

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