Crosslinking method of elastomers

Abstract

A method for crosslinking an elastomer is provided, the method using an elastomer capable of easy crosslinking and decrosslinking. The crosslinking method for the elastomer is characterized in that the elastomer has a monoboronic acid ester functional group as represented by a prescribed general formula (4); the method includes the step of contacting the elastomer with a diboronic acid compound in the presence of a solvent; the SP value of the solvent is 10 (cal / cm³). 3 ) 1 / 2 Above and 13 (cal / cm 3 ) 1 / 2 The following, and at least includes an SP value of 8 (cal / cm²). 3 ) 1 / 2 Above and 10 (cal / cm) 3 ) 1 / 2 The following are single solvents.

Description

Technical Field

[0001] This disclosure relates to a method for crosslinking elastomers. Background Technology

[0002] In recent years, efforts have been made to promote the recycling of vulcanized rubber (crosslinked rubber) used in tires and other products, taking into account environmental concerns and resource conservation. Although desulfurization of vulcanized rubber is generally considered a recycling technology, it suffers from low energy efficiency due to the extremely high temperatures required in the desulfurization reaction.

[0003] Therefore, technologies for decomposing vulcanized rubber using methods other than desulfurization are being researched. For example, Patent Document (PTL) 1 discloses a method for decomposing and recycling vulcanized rubber, in which the vulcanized rubber is decomposed through a lipid peroxidation reaction, and then the lipids are removed in an alcohol containing an alkali, thereby recovering the rubber components. PTL 1 discloses that this method has a low decomposition reaction temperature, excellent energy efficiency, and a rapid decomposition rate, and produces decomposition products that are easy to recycle and recover.

[0004] Citation List

[0005] Patent documents

[0006] PTL 1: JP2011-153272A Summary of the Invention

[0007] The problem the invention aims to solve

[0008] However, the techniques described in PTL 1 extensively treat vulcanized rubber as the target of decomposition, without taking into account rubber (crosslinked elastomers) that are crosslinked by methods other than vulcanization (sulfur crosslinking).

[0009] Furthermore, from the perspective of the versatility and more efficient use of materials, it would be extremely useful to focus on the crosslinked elastomers themselves in order to develop crosslinked elastomers that can be easily decrosslinked and recrosslinked (i.e., crosslinked elastomers with "reversible crosslinking").

[0010] Therefore, this disclosure solves the problem of providing a method for crosslinking an elastomer that uses an elastomer that can be easily crosslinked and decrosslinked.

[0011] Solution for solving the problem

[0012] As a result of diligent research aimed at solving the aforementioned problems, the inventors discovered that by using an elastomer containing specific functional groups at the sites targeted for crosslinking, crosslinking and subsequent decrosslinking can be easily initiated at those sites under specific conditions, leading to the completion of this disclosure. Specifically, the main features of this disclosure for solving the aforementioned problems are as follows.

[0013] The crosslinking method for the elastomer according to this disclosure includes the step of contacting the elastomer with a diboronic acid compound in the presence of a solvent, said elastomer comprising a functional group containing a monoboronic ester as shown in general formula (4) below.

[0014] [Chemical Formula 1]

[0015]

[0016] In general formula (4), X 2 It is hydrogen or any monovalent group, wherein

[0017] The SP value of the solvent is not less than 10 (cal / cm³). 3 ) 1 / 2 And not greater than 13 (cal / cm) 3 ) 1 / 2 And at least includes an SP value of not less than 8 (cal / cm³). 3 ) 1 / 2 And not greater than 10 (cal / cm) 3 ) 1 / 2 A single solvent.

[0018] The effects of the invention

[0019] According to this disclosure, a method for crosslinking an elastomer can be provided, which uses an elastomer that is easily crosslinked and decrosslinked. Detailed Implementation

[0020] The following describes embodiments of this disclosure. However, the following description is intended for illustrative purposes only and is not intended to limit the scope of this disclosure in any way.

[0021] (Methods for crosslinking elastomers)

[0022] The crosslinking method for an elastomer according to one embodiment of the present disclosure (hereinafter also referred to as "the crosslinking method of the present embodiment") is a method for crosslinking an elastomer containing monoboron ester functional groups as shown in the general formula (4) below.

[0023] [Chemical Formula 2]

[0024]

[0025] In general formula (4), X 2 It is hydrogen or any monovalent group, and includes the step of contacting the elastomer with the diboronic acid compound in the presence of a solvent (diboronic acid contact step), wherein

[0026] The SP value of the solvent is not less than 10 (cal / cm³). 3 ) 1 / 2And not greater than 13 (cal / cm) 3 ) 1 / 2 And at least includes an SP value of not less than 8 (cal / cm³). 3 ) 1 / 2 And not greater than 10 (cal / cm) 3 ) 1 / 2 A single solvent.

[0027] Note that the term "diboronic acid compound" as used in this specification refers to a compound containing two boric acids (-B(OH)2) in one molecule.

[0028] When the aforementioned elastomer is contacted with a diboronic acid compound in the presence of a specific solvent, an exchange reaction occurs at the functional groups containing monoboronic esters in the elastomer, resulting in the replacement of the monoboronic esters with esters derived from the diboronic acid compound. Since the diboronic acid compound includes two boric acids as binding sites with the elastomer, this leads to the linking (crosslinking) of the elastomer. Therefore, crosslinking can be readily performed according to the crosslinking method of this embodiment.

[0029] Furthermore, by contacting the already cross-linked elastomer (cross-linked elastomer) with a monoboric acid compound under specific conditions, an exchange reaction occurs at the cross-linking sites of the cross-linked elastomer, resulting in the cross-linking via the diborate backbone units being replaced by the binding of the monoboric acid compound. Since the monoboric acid compound contains only one boric acid as a binding site with the elastomer, the elastomer's connection is canceled (i.e., the elastomer is decrosslinked). Therefore, the elastomer used in the cross-linking method of this embodiment can be easily cross-linked and decrosslinked.

[0030] Furthermore, since the elastomer that has undergone decrosslinking has a structure that is substantially the same as the elastomer used in the crosslinking method of this embodiment, it is expected that the elastomer can be easily decrosslinked and recrosslinked using the crosslinking method of this embodiment.

[0031] <Elastomers>

[0032] The elastomer used in the crosslinking method of this embodiment includes a functional group containing a monoboron ester as shown in the following general formula (4).

[0033] [Chemical Formula 3]

[0034]

[0035] (In general formula (4), X) 2 (It can be hydrogen or any monovalent group.) can form X in general formula (4). 2The monovalent group can be a hydrocarbon group, for example, including straight-chain and branched aliphatic groups (alkyl, alkenyl, and ynyl) with 1 to 10 carbon atoms; aromatic groups (phenyl, naphthyl, biphenyl, etc.) with 6 to 20 carbon atoms; and heteroaryl groups (2-furanyl, etc.). In addition, -O-, -S-, -OC(=O)-, -C(=O)-O-, -OC(=O)-O-, and -NR- are also possible. 2 -C(=O)-、-C(=O)-NR 2 -、-NR 2 - or -C(=O)- can be inserted into the above aliphatic groups and aromatic groups. Note that R 2 It refers to an alkyl group with 1 to 6 carbon atoms or hydrogen atoms.

[0036] The elastomer used in the crosslinking method of this embodiment is preferably an elastomer derived from a diene-based elastomer. In other words, the elastomer used in the crosslinking method of this embodiment is preferably an elastomer obtained by providing a diene-based elastomer having a functional group containing a monoboron ester. The diene-based elastomer is an elastomer comprising at least diene units, and can be, for example, natural rubber (NR), butadiene rubber (BR), styrene-butadiene rubber (SBR), isoprene rubber (IR), chloroprene rubber (CR), etc. The diene-based elastomer can be a single type of diene-based elastomer used alone, or it can be two or more types of diene-based elastomers used in combination. In these examples, from the viewpoint of sufficiently ensuring the mechanical strength of rubber products such as tires, butadiene rubber (BR) and styrene-butadiene rubber (SBR) are preferred as diene-based elastomers.

[0037] Furthermore, the diene-based elastomer preferably has a vinyl bond content of 30% by mass or less and a weight-average molecular weight (Mw) of 1,000 or more. In other words, the elastomer used in the crosslinking method of this embodiment is preferably derived from a diene-based elastomer with a vinyl bond content of 30% by mass or less and a weight-average molecular weight of 1,000 or more. When the vinyl bond content of the diene-based elastomer is 30% by mass or less, gelation of the elastomer during heating and molding can be effectively suppressed. In addition, when the weight-average molecular weight (Mw) of the diene-based elastomer is 1,000 or more, sufficient entanglement is generated in the elastomer after crosslinking, and sufficient mechanical strength can be exhibited when used in rubber products such as tires. From the same point of view, the vinyl bond content of the diene-based elastomer is preferably 28% by mass or less, more preferably 26% by mass or less. On the other hand, the lower limit of the vinyl bond content of the diene-based elastomer is preferably 3% by mass or more, more preferably 5% by mass or more, even more preferably 8% by mass or more, and particularly preferably 10% by mass or more.

[0038] Note that the vinyl bond content of diene elastomers mentioned in this specification refers to the mass ratio of vinyl-bonded units that are diene units throughout the entire diene elastomer. In other words, the vinyl bond content of a diene elastomer is not the proportion of vinyl-bonded units within the diene units.

[0039] It should also be noted that the weight-average molecular weight (Mw) can be determined by gel permeation chromatography (GPC) using polystyrene as a standard.

[0040] In the case where the elastomer is derived from a diene-based elastomer, the aforementioned monoboronate-containing functional group in the elastomer is preferably bonded to the carbon of the vinyl component remaining in the diene unit that is a vinyl bond in the diene-based elastomer. Furthermore, the aforementioned monoboronate-containing functional group and the diene-based elastomer can be bonded directly or via any group (e.g., an alkylene group having 1 to 3 carbon atoms).

[0041] As a result of compounding during the manufacturing process of the elastomer, fillers such as carbon black can be dispersed in the elastomer in the crosslinking method of this embodiment. When the filler is dispersed in the elastomer, the amount of the filler relative to 100 parts by mass of the elastomer can be set to 10 parts by mass or more, or 100 parts by mass or less.

[0042] The elastomer used in the crosslinking method of this embodiment can be manufactured by various methods without any particular limitation. For example, the elastomer used in the crosslinking method of this embodiment can be obtained by crosslinking a diene elastomer in step A to obtain a crosslinked elastomer, and then by returning the crosslinking sites of the crosslinked elastomer to a non-crosslinked state in step B.

[0043] <Step A in obtaining an elastomer>

[0044] Step A is the step of crosslinking diene elastomers (excluding diene elastomers with functional groups containing monoborates) with diborates of the following general formula (2) in the presence of a free radical initiator.

[0045] [Chemical Formula 4]

[0046]

[0047] (In general formula (2), X) 1 Y is a single bond or any divalent group. 1 and Y 2 Each is independently a single bond or a divalent hydrocarbon group. This produces a crosslinked elastomer containing diborate backbone units as shown in the following general formula (1) at the crosslinking sites.

[0048] [Chemical Formula 5]

[0049]

[0050] (In general formula (1), X) 1 (It can be a single bond or any divalent group.)

[0051] In step A above, more specifically, a crosslinked elastomer can be obtained by mixing a free radical initiator and a diboron ester compound with the aforementioned diene elastomer, mixing them using a mixer or the like, and then heating the mixture. The heating temperature and time are preferably adjusted appropriately, taking into account the vinyl bond content of the diene elastomer, the type of free radical initiator used, and its amount.

[0052] In addition to the diene elastomer, free radical initiator, and diboronate compound, other components may be further blended in appropriate amounts in step A above. Examples of other components include fillers, such as carbon black.

[0053] <Step B for obtaining the elastomer>

[0054] Step B is the step of contacting the crosslinked elastomer obtained in step A with the monoboronic acid compound represented by the following general formula (3) in the presence of an organic solvent.

[0055] [Chemical Formula 6]

[0056]

[0057] (In general formula (3), X) 2 (It can be hydrogen or any monovalent group.) In step B, an exchange reaction occurs at the crosslinking sites of the crosslinked elastomer, resulting in the crosslinking via the diboronate backbone unit being replaced by the binding of the monoboronate compound. In the above process, the two normally connected elastomer crosslinking sites are each capped by the functional group shown in the following general formula (4):

[0058] [Chemical Formula 7]

[0059]

[0060] This returns the system to a non-crosslinked state. In this way, the elastomer used in the crosslinking method of this embodiment can be obtained. Note that, concurrently with the functional group end-capping in step B, a diboronic acid compound derived from the diboronic acid ester backbone unit at the crosslinking site, as shown in formula (9), is typically generated:

[0061] [Chemical Formula 8]

[0062]

[0063] This diboronic acid compound can be used as the diboronic acid compound required in the crosslinking method of this embodiment.

[0064] The organic solvent used in step B preferably has a concentration of not less than 7 (cal / cm³). 3 ) 1 / 2 And not greater than 10 (cal / cm) 3 ) 1 / 2 The SP value. In this case, compatibility with crosslinked elastomers increases, and the exchange reaction can be triggered more reliably.

[0065] Note that the SP value can be calculated using the Hansen method. Also note that the organic solvent can be a single solvent or a mixture of solvents.

[0066] In step B above, the molar ratio of the monoboronic acid compound to the diboronic acid ester backbone unit (monoboronic acid compound / diboronic acid ester backbone unit) is preferably greater than 2 and not greater than 5. In this case, the linkage of the elastomer can be effectively canceled, and subsequent crosslinking (i.e., crosslinking by the crosslinking method of this embodiment) can be performed more easily.

[0067] The elastomer obtained through steps A and B above is, simply put, a decrosslinked elastomer. Furthermore, when using this decrosslinked elastomer for the crosslinking method of this embodiment, the resulting crosslinked elastomer has essentially the same structure as the crosslinked elastomer obtained in step A. In other words, repeated decrosslinking and recrosslinking can be easily performed in this case.

[0068] Solvent

[0069] The solvent used in the crosslinking method of this embodiment needs to have a concentration of not less than 10 (cal / cm). 3 ) 1 / 2 And not greater than 13 (cal / cm) 3 ) 1 / 2 The SP value, and at least includes an SP value of not less than 8 (cal / cm). 3 ) 1 / 2 And not greater than 10 (cal / cm) 3 ) 1 / 2 A single solvent. The SP value of all solvents deviates by no less than 10 (cal / cm³). 3 ) 1 / 2 And not greater than 13 (cal / cm) 3 ) 1 / 2 Within this range, compatibility with elastomers decreases, and exchange reactions at functional groups containing monoboron esters may not occur sufficiently. Furthermore, excluding SP values ​​not less than 8 (cal / cm²), 3 ) 1 / 2 And not greater than 10 (cal / cm) 3 ) 1 / 2In the case of a single solvent, compatibility with the elastomer decreases, and the exchange reaction at the functional groups containing monoboronate may not occur sufficiently. Furthermore, when the filler is dispersed within the elastomer, from the viewpoint of more reliably initiating the exchange reaction, the SP value of the entire solvent is preferably 11 (cal / cm³). 3 ) 1 / 2 above.

[0070] Note that the SP value can be calculated using the Hansen method.

[0071] The SP value is not less than 8 (cal / cm). 3 ) 1 / 2 And not greater than 10 (cal / cm) 3 ) 1 / 2 Examples of single solvents include, but are not limited to, toluene, tetrahydrofuran (THF), etc.

[0072] <Diboronic acid compounds>

[0073] The diboronic acid compound used in the crosslinking method of this embodiment can be, for example, a diboronic acid compound represented by the following general formula (9).

[0074] [Chemical Formula 9]

[0075]

[0076] (In general formula (9), X) 1 (It can be a single bond or any divalent group.) can form X in general formula (9). 1 The divalent group can be a hydrocarbon group, for example, including straight-chain and branched aliphatic groups (alkylene, alkenylene, and ynylene) with 1 to 10 carbon atoms; and aromatic groups (1,2-phenylene, 1,3-phenylene, 1,4-phenylene, 1,4-naphthylene, 1,5-naphthylene, 2,6-naphthylene, 4,4′-biphenylene, etc.) with 6 to 20 carbon atoms. In addition, -O-, -S-, -OC(=O)-, -C(=O)-O-, -OC(=O)-O-, and -NR- are also possible. 2 -C(=O)-、、-C(=O)-NR 2 -、-NR 2 - or -C(=O)- can be inserted into the above aliphatic groups and aromatic groups. Note that R 2 It refers to an alkyl group with 1 to 6 carbon atoms or hydrogen atoms.

[0077] X can form the general formula (9) 1 The divalent group is preferably equivalent to X from the aforementioned general formula (4). 2 The group that removes hydrogen from the monovalent group. As an example, X in general formula (4) 2In the case of an elastomer that is phenyl, X in general formula (9) is preferred. 1 It is a diboronic acid compound of phenylene. In this case, the exchange reaction can be initiated more effectively.

[0078] <Diboronic acid contact steps>

[0079] In the diboronic acid contact step, the elastomer is contacted with the diboronic acid compound in the presence of the specific solvent described above. This contact results in an exchange reaction at the functional groups containing monoboronic acid esters in the elastomer. When using a diboronic acid compound represented by general formula (9), for example, in the process described above, two elastomer molecules are typically cross-linked via sites comprising diboronic acid ester backbone units represented by the following general formula (1).

[0080] [Chemical Formula 10]

[0081]

[0082] At the same time, monoboronic acid compounds derived from functional groups containing monoboronic esters, as shown in the following general formula (3), are typically produced:

[0083] [Chemical Formula 11]

[0084]

[0085] In the diboric acid contact step, the amount of diboric acid compound relative to 100 parts by mass of elastomer is preferably 20 parts by mass or less. In this case, the properties of the elastomer after crosslinking can be sufficiently preserved. From the same viewpoint, the amount of diboric acid compound relative to 100 parts by mass of elastomer is more preferably 16 parts by mass or less, and even more preferably 11 parts by mass or less. On the other hand, although there is no specific limitation on the lower limit of the amount of diboric acid compound relative to 100 parts by mass of elastomer, as long as the amount is greater than 0 parts by mass, from the viewpoint of increasing the strength of the obtained crosslinked elastomer to a degree similar to that of sulfur crosslinking, the amount of diboric acid compound is preferably 1 part by mass or more, and more preferably 2 parts by mass or more.

[0086] In the diboronic acid contact step, the concentration of the elastomer (the mass ratio of the elastomer to the total elastomer and solvent) is preferably not less than 5% by mass and not more than 65% by mass. When the concentration of the elastomer is within the above range, more sufficient and effective crosslinking is likely to occur. From the same point of view, the concentration of the elastomer is more preferably 10% by mass or more. In particular, when the filler is dispersed in the elastomer, the concentration of the elastomer is preferably 20% by mass or more.

[0087] Note that lower concentrations of elastomers result in the elastomer being submerged in the solvent, while higher concentrations result in the elastomer being swollen in both the solvent and the solvent.

[0088] There are no specific limitations on the specific conditions (temperature, time, etc.) in the diboric acid contact step. However, it is preferable to finely grind the elastomer beforehand to increase the frequency of contact with the diboric acid compound. Furthermore, solid-liquid separation can be performed after the diboric acid contact step using known methods.

[0089] Example

[0090] The present disclosure is described in more detail below by way of examples. However, these examples are for illustrative purposes and are not intended to limit the scope of the present disclosure in any way.

[0091] Note that the following not only provides examples of crosslinking methods for elastomers according to this disclosure, but also provides steps A and B as a single series of the aforementioned steps for obtaining the elastomer.

[0092] (Manufacturing of cross-linked elastomers)

[0093] The elastomer compositions were manufactured according to the formulations shown in Table 1. Next, each elastomer composition was heated under the conditions shown in Table 1, and its storage modulus G′ was measured using a rubber processing analyzer (manufactured by Alpha Technologies). The results are shown in Table 1. In each embodiment, the storage modulus G′ was approximately the desired value. Therefore, crosslinking was determined to have occurred.

[0094] Note that Reference Example 1 is an example of sulfur crosslinking (vulcanization) without the use of carbon black as a filler, while Examples 1-1 to 1-4 are examples in which the conditions are appropriately adjusted to obtain a storage modulus G′ close to that in Reference Example 1 without the use of carbon black. Similarly, Reference Example 2 is an example of sulfur crosslinking (vulcanization) using carbon black as a filler, while Examples 1-5 are examples in which the conditions are appropriately adjusted to obtain a storage modulus G′ close to that in Reference Example 2 using carbon black.

[0095] [Table 1]

[0096]

[0097] *1 Diene-based elastomers: manufactured by Asahi Kasei Corporation 2000R (TUFDENE is a registered trademark in Japan, other countries, or both), styrene-butadiene rubber, vinyl bond content 10% to 26% by mass, weight average molecular weight 373,000 (to 3 significant figures).

[0098] *2BDB: The diboronate compound shown in formula (5) (synthetic product)

[0099] [Chemical Formula 12]

[0100]

[0101] *3CB: HAF grade carbon black

[0102] *4 Free radical initiator: Azobisisobutyronitrile (AIBN)

[0103] *5. Vulcanization accelerator DPG: 1,3-Diphenylguanidine

[0104] *6. Vulcanization Accelerator MBTS: Di-2-benzothiazole disulfide

[0105] *7 Vulcanization Accelerator TBBS: N-(tert-butyl)-2-benzothiazolylsulfonamide

[0106] Note that the crosslinked elastomers obtained in Examples 1-1 to 1-5 were confirmed by appropriate use of solution NMR to include the diboronate backbone unit shown in formula (6):

[0107] [Chemical Formula 13]

[0108]

[0109] Furthermore, in these crosslinked elastomers, crosslinking is formed by direct bonding of the carbon of the vinyl component remaining in the butadiene unit of the diene-based elastomer to the aforementioned diboronate backbone unit.

[0110] (Decrosslinking treatment of crosslinked elastomers)

[0111] The crosslinked elastomers obtained in Examples 1-4 were selected as the crosslinked elastomers in which carbon black was not mixed, and the crosslinked elastomers obtained in Examples 1-5 were selected as the crosslinked elastomers in which carbon black was mixed.

[0112] These crosslinked elastomers were each finely ground and then filled into tetrahydrofuran (THF) as an organic solvent; SP value: 8.95 (cal / cm³). 3 ) 1 / 2 Phenylboronic acid (PhB(OH)2) was further added as a monoboronic acid compound (BA), and the mixture was thoroughly mixed. The mixture was then allowed to stand at room temperature (approximately 25°C) for one day to precipitate. In the above, the concentration of the crosslinked elastomer (the mass ratio of the crosslinked elastomer to the total amount of crosslinked elastomer and organic solvent) and the molar ratio of the monoboronic acid compound to the diboronate backbone units in the crosslinked elastomer (BA / BDB backbone units) were set as shown in Table 2. After precipitation, the mixture was cast, the solvent was evaporated, and the treated elastomer was obtained.

[0113] <Evaluation of de-crosslinking>

[0114] The obtained treated elastomers were used for roll forming. Roll forming was considered feasible when softness was imparted due to decrosslinking (crosslinked sites returning to a non-crosslinked state). The results are shown in Table 2.

[0115] [Table 2]

[0116]

[0117] As can be seen from Table 2, in all Examples 2-1 to 2-7, the evaluation result for decrosslinking is “O”. In other words, the crosslinked elastomers obtained in Examples 1-4 and 1-5 can be easily decrosslinked by contacting the crosslinked elastomers with a monoboronic acid compound in the presence of a specific organic solvent.

[0118] It should also be noted that for the treated elastomers (decrosslinked elastomers) obtained in Examples 2-1 to 2-7, it was confirmed by appropriate use of solution NMR that the sites where the backbone shown in formula (6) was bound in the crosslinked elastomer before treatment were capped by the functional groups shown in formula (7):

[0119] [Chemical Formula 14]

[0120]

[0121] Therefore, the crosslinking is cancelled (the crosslinking sites return to the non-crosslinking state). Simultaneously, the production of a diboronic acid compound ((1,4-phenylene)diboronic acid) derived from the aforementioned skeleton, as shown in formula (8), was confirmed by appropriate solution NMR:

[0122] [Chemical Formula 15]

[0123]

[0124] The crosslinked elastomers obtained in Examples 1-1 to 1-3 also include the diboronate backbone units described above. Therefore, these crosslinked elastomers are considered to be easily decrosslinked by the same treatment as described above.

[0125] (Re-crosslinking of decrosslinked elastomers)

[0126] The decrosslinked elastomer (treated elastomer) obtained in Example 2-2 or Example 2-3 is selected as the decrosslinked elastomer without carbon black, and the decrosslinked elastomer (treated elastomer) obtained in Example 2-5 is selected as the decrosslinked elastomer with carbon black.

[0127] These decrosslinked elastomers, along with the diboronic acid compound generated during the decrosslinking process, were each loaded into the solvents shown in Table 3. A free radical initiator was further added according to the formulation shown in Table 3, and the mixture was allowed to stand overnight at room temperature (approximately 25°C) to precipitate. In the above, the concentration of the decrosslinked elastomer (the mass ratio of the decrosslinked elastomer to the total amount of decrosslinked elastomer and solvent) was set as shown in Table 3. After precipitation, the mixture was cast, the solvent was evaporated, and the treated elastomer was obtained.

[0128] Evaluation of cross-linking (re-cross-linking)

[0129] The obtained treated elastomers were heated at 120°C for 10 to 30 minutes (until the storage modulus G′ value stabilized), and their storage modulus G′ was measured using a rubber processing analyzer (manufactured by Alpha Technologies). In each embodiment, the measured storage modulus G′ was at least 20% higher than that of the crosslinked elastomer before the addition of the monoboronic acid compound. Therefore, it was determined that crosslinking had occurred in each embodiment. More specifically, it was determined that crosslinking had occurred to a more sufficient degree, and an evaluation of "O" was given when the measured storage modulus G′ was more than 50% higher than that of the crosslinked elastomer before the addition of phenylboronic acid, and an evaluation of "Δ" was given when the measured storage modulus G′ was less than 50% higher. The results are shown in Table 3.

[0130] [Table 3]

[0131]

[0132] As can be seen from Table 3, the decrosslinked elastomers obtained in Examples 2-2, 2-3 and 2-5 can be easily crosslinked (recrosslinked) by contacting the decrosslinked elastomers with diboronic acid compounds in the presence of a specific solvent.

[0133] Note that the treated elastomers (re-crosslinked elastomers) obtained in Examples 3-1 to 3-7 were confirmed by appropriate use of solution NMR to include the diboronate backbone unit shown in formula (6):

[0134] [Chemical Formula 16]

[0135]

[0136] In other words, the treated elastomers (re-crosslinked elastomers) obtained in Examples 3-1 to 3-7 have substantially the same structure as the crosslinked elastomers obtained in Examples 1-1 to 1-5.

[0137] Industrial availability

[0138] According to this disclosure, a method for crosslinking an elastomer can be provided, the method using an elastomer that is easily crosslinked and decrosslinked.

Claims

1. A method for crosslinking an elastomer, comprising the step of contacting the elastomer with a diboronic acid compound in the presence of a solvent, the elastomer comprising a functional group containing a monoboronic acid ester as shown in general formula (4) below. in, In general formula (4), X 2 It is hydrogen or any monovalent group, wherein The SP value of the solvent is not less than 10 (cal / cm³). 3 ) 1 / 2 And not greater than 13 (cal / cm) 3 ) 1 / 2 And at least includes an SP value of not less than 8 (cal / cm³). 3 ) 1 / 2 And not greater than 10 (cal / cm) 3 ) 1 / 2 A single solvent.

2. The crosslinking method according to claim 1, wherein the amount of the diboronic acid compound in the contact step is less than 20 parts by mass relative to 100 parts by mass of the elastomer.

3. The crosslinking method according to claim 1 or 2, wherein the elastomer is derived from a diene-based elastomer having a vinyl bond content of less than 30% by mass and a weight-average molecular weight of more than 1,000.

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