JP7313620B2 - Modified conjugated diene rubber - Google Patents

Description

The present invention relates to a modified conjugated diene rubber.

Conventionally, various materials have been proposed as rubber materials. For example, Examples of Patent Document 1 disclose polyisoprene (isoprene rubber) having a carboxy group.

JP 2016-27163 A

In Examples of Patent Document 1, isoprene is anionically polymerized using n-butyllithium as an initiator. In this case, most of the repeating units of the resulting isoprene rubber have a 1,4-cis structure.
When the present inventors synthesized the isoprene rubber described in Examples of Patent Document 1 and examined its characteristics, it became clear that the physical properties were insufficient. Specifically, creep properties, toughness and self-healing properties were found to be insufficient.

Therefore, in view of the above circumstances, an object of the present invention is to provide a conjugated diene rubber excellent in creep properties, toughness and self-healing properties.
In this specification, a small residual strain after stress application is also referred to as an excellent creep property. In addition, regarding the breaking strength described later, a high breaking strength is also referred to as being excellent in breaking strength. In addition, regarding the breaking elongation to be described later, a large breaking elongation is also said to be excellent in breaking elongation.

As a result of intensive studies on the above problems, the present inventors have found that the above problems can be solved by introducing a carboxylic acid metal base into at least one of the main chain and the end of a conjugated diene rubber and setting the ratio (trans ratio) of repeating units having a 1,4-trans structure among all repeating units derived from a conjugated diene to a specific range, and have arrived at the present invention.
That is, the inventors have found that the above problems can be solved by the following configuration.

(1) A modified conjugated diene rubber, which is a conjugated diene rubber having a carboxylic acid metal group on at least one of the main chain and the terminal,
A modified conjugated diene-based rubber in which repeating units having a 1,4-trans structure account for 4 to 30 mol% of all repeating units derived from a conjugated diene.
(2) a modified cis-conjugated diene-based rubber, which is a cis-conjugated diene-based rubber having a carboxylic acid metal group on at least one of the main chain and the terminal;
A mixture with a modified trans-conjugated diene-based rubber, which is a trans-conjugated diene-based rubber having a carboxylic acid metal group on at least one of the main chain and the terminal,
The content of the modified cis-type conjugated diene rubber is 50 to 99% by mass,
The modified conjugated diene-based rubber according to (1) above, wherein the content of the modified trans-type conjugated diene-based rubber is 1 to 50% by mass.
(3) The modified conjugated diene rubber according to (1) or (2) above, wherein the repeating unit having a carboxylic acid metal group accounts for 0.1 to 30 mol % of all repeating units.
(4) The modified conjugated diene rubber according to any one of (1) to (3) above, wherein the conjugated diene rubber has a weight average molecular weight of 5,000 to 10,000,000.
(5) The modified conjugated diene rubber according to any one of (1) to (4) above, wherein the conjugated diene rubber is natural rubber or isoprene rubber.

As shown below, according to the present invention, it is possible to provide a conjugated diene rubber excellent in creep properties, toughness and self-healing properties.

The modified conjugated diene rubber of the present invention is described below.
In this specification, a numerical range represented by "-" means a range including the numerical values before and after "-" as lower and upper limits.
Moreover, each component may be used individually by 1 type, or may use 2 or more types together. Here, when two or more of each component are used in combination, the content of that component refers to the total content unless otherwise specified.

[Modified conjugated diene rubber]
The modified conjugated diene rubber of the present invention is
A modified conjugated diene rubber, which is a conjugated diene rubber having a carboxylic acid metal base on at least one of the main chain and the terminal,
The modified conjugated diene rubber contains 4 to 30 mol % of repeating units having a 1,4-trans structure (trans ratio) among all repeating units derived from a conjugated diene.

Since the modified conjugated diene rubber of the present invention has such a structure, it is considered that the above effects can be obtained. Although the reason is not clear, it is presumed to be approximately as follows.

(i) Carboxylic acid “metal salt” group In the modified conjugated diene rubber of the present invention, the carboxylic acid metal bases present in at least one of the main chain and the terminal interact with each other to form a pseudo-crosslinked structure. Therefore, it exhibits excellent self-healing properties (for example, self-healing properties when left in contact with the cut surfaces of the two sample pieces after being cut with a razor into two sample pieces).
Here, when it is not a metal salt (that is, when it is a carboxyl group), it is thought that they interact with each other to form a hydrogen bond, but the bond energy of the hydrogen bond is small. In contrast, since the modified conjugated diene rubber of the present invention has carboxylic acid "metal salt" groups, they form strong ionic bonds with each other. An ionic bond has a higher bond energy than a hydrogen bond, and is considered to exhibit excellent self-healing properties as described above.

(ii) The trans ratio is within a specific range The modified conjugated diene rubber of the present invention has a trans ratio of 4 mol% or more, so it has crystallinity and is considered to be less likely to remain strain even when deformed. Moreover, since the trans ratio is 30 mol % or less, it is considered that the physical properties as a rubber material are fully exhibited.

The modified conjugated diene rubber of the present invention will be described in detail below.

As described above, the modified conjugated diene rubber of the present invention is
A modified conjugated diene rubber, which is a conjugated diene rubber having a carboxylic acid metal base on at least one of the main chain and the terminal,
It is a modified conjugated diene rubber having a trans ratio of 4 to 30 mol %.
When the modified conjugated diene-based rubber of the present invention is a mixture of two or more types of "conjugated diene-based rubbers having a carboxylic acid metal group on at least one of the main chain and the terminal", the trans ratio of the mixture as a whole may be within the above range.

[Conjugated diene rubber]
The conjugated diene rubber portion (that is, the portion other than the carboxylic acid metal base) (unmodified conjugated diene rubber) of the modified conjugated diene rubber of the present invention is not particularly limited, but specific examples include natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), acrylonitrile butadiene rubber (NBR), styrene butadiene rubber (SBR), styrene isoprene rubber (SIR), styrene isoprene butadiene rubber ( SIBR), butyl rubber (IIR), halogenated butyl rubber (Br-IIR, Cl-IIR), chloroprene rubber (CR), derivatives of these rubbers, and the like, and two or more of these may be used. Among them, natural rubber or isoprene rubber is preferable, and isoprene rubber is more preferable, because it is superior in creep property, elongation at break, strength at break, toughness, self-healing property, wear resistance, durability, resistance to oxidation deterioration, heat resistance, cold resistance, light resistance, and water resistance.
Hereinafter, "excellent in creep property, elongation at break, strength at break, toughness, self-healing property, wear resistance, durability, oxidation deterioration resistance, heat resistance, cold resistance, light resistance, and water resistance" is also referred to as "the effects of the present invention are more excellent".

A preferred aspect of the molecular weight of the conjugated diene rubber (unmodified conjugated diene rubber) is the same as a preferred aspect of the molecular weight of the modified conjugated diene rubber of the present invention, which will be described later.

In the conjugated diene-based rubber, the ratio (mol%) of all repeating units derived from a conjugated diene to all repeating units including terminals is preferably 10 mol% or more, more preferably 50 mol% or more, and even more preferably 90 mol% or more, for the reason that the effects of the present invention are more excellent. The upper limit is not particularly limited and is 100 mol %.

[Carboxylic acid metal base]
As described above, the modified conjugated diene rubber of the present invention has a carboxylic acid metal base. A carboxylic acid metal base is a salt of a carboxy group and a metal.

<Metal>
Although the metal is not particularly limited, it is preferably other than Zn, more preferably an element other than a Group 12 element, more preferably a Group 1 to 11 element or a Group 13 to 14 element, and particularly preferably a Group 1 to 11 element, for the reason that the effects of the present invention are more excellent. Among them, for the reason that the effect of the present invention is more excellent, it is preferably a Group 1-2 element or a Group 8-11 element, more preferably a Group 1 element or a Group 8 element, more preferably Li, Na or Fe, particularly preferably Li or Fe, and most preferably Li.

The carboxylate metal base is preferably a sodium carboxylate base, a lithium carboxylate base, an iron carboxylate base, or a zinc carboxylate base because the effects of the present invention are more excellent. Examples of the carboxylic acid iron base include carboxylic acid iron (II) bases, carboxylic acid iron (III) bases, etc. Among them, carboxylic acid iron (III) bases are preferable because the effects of the present invention are more excellent.

<Solubility>
The solubility of the carboxylic acid metal base in water is preferably 50 g or less, more preferably 45 g or less, even more preferably 40 g or less, and particularly preferably 35 g or less, because the effects of the present invention are more excellent. The lower limit of the solubility in water of the carboxylic acid metal base is not particularly limited, and is 0 g.
Here, the solubility of the metal carboxylate in water (hereinafter, “solubility in water” may be simply referred to as “solubility”) represents the amount dissolved as a metal acetate in 100 g of a saturated aqueous solution at 20° C. and 1 atm.
For example, the sodium carboxylate base solubility represents the solubility of sodium acetate (46.4 g), the lithium carboxylate base solubility represents the solubility of lithium acetate (40.8 g), the zinc carboxylate base solubility represents the solubility of zinc acetate (30 g), and the solubility of the iron(III) carboxylate base represents the solubility of iron(III) acetate (approximately 0 g).

[Degeneration site]
As described above, the modified conjugated diene rubber of the present invention has a carboxylic acid metal group on at least one of the main chain and the terminal. The modified conjugated diene rubber of the present invention preferably has a carboxylic acid metal group at least in the main chain and the terminal, and more preferably has a carboxylic acid metal group in both the main chain and the terminal, for the reason that the effects of the present invention are more excellent.
When the modified conjugated diene rubber of the present invention has a carboxylic acid metal group in the main chain, at least one of the repeating units other than the terminals has the carboxylic acid metal group among all the repeating units.
When the modified conjugated diene-based rubber of the present invention has a carboxylic acid metal salt at its terminal, it may have a carboxylic acid metal salt at only one of a plurality of terminals, or may have a carboxylic acid metal salt at a plurality of terminals.

[Modification rate]
Although the modification rate of the modified conjugated diene rubber of the present invention is not particularly limited, it is preferably 0.10 mol% or more, more preferably 0.50 mol% or more, further preferably 1.00 mol% or more, and particularly preferably 1.50 mol% or more because the effects of the present invention are more excellent.
Although the upper limit of the modification rate is not particularly limited, it is preferably 90 mol% or less, more preferably 50 mol% or less, even more preferably 30 mol% or less, particularly preferably 10 mol% or less, and most preferably 3.00 mol% or less, because the effects of the present invention are more excellent.
In addition, the above-mentioned modification rate represents the proportion (mol %) of repeating units having a carboxylic acid metal group among all repeating units including terminals.

When the modified conjugated diene rubber of the present invention is a mixture of two or more types of "conjugated diene rubbers having a carboxylic acid metal group on at least one of the main chain and the ends", the modification rate is the sum of the modification rate of each "conjugated diene rubber having a carboxylic acid metal group on at least one of the main chain and the ends" multiplied by the mass fraction of each "conjugated diene rubber having a carboxylic acid metal group on at least one of the main chain and the ends" (weighted average value of modification rates) (average modification rate). is. Components other than "the conjugated diene rubber having a carboxylic acid metal group on at least one of the main chain and the terminal" are not taken into consideration because they do not correspond to the modified conjugated diene rubber of the present invention.
For example, in the case of Example 3, which will be described later, the mixture is a mixture of 80 parts by mass of modified cis-isoprene rubber (Synthesis Example 1) (modification rate: 1.8 mol%) and 20 parts by mass of modified trans-isoprene rubber (Synthesis Example 2) (modification rate: 1.6 mol%). = 1.76 mol%.

[Molecular weight]

<Weight average molecular weight>
The weight average molecular weight (Mw) of the modified conjugated diene rubber of the present invention is not particularly limited, but is preferably 5,000 to 10,000,000, more preferably 100,000 to 1,000,000, because the effects of the present invention are more excellent.

<Number average molecular weight>
Although the number average molecular weight (Mn) of the modified conjugated diene rubber of the present invention is not particularly limited, it is preferably from 5,000 to 10,000,000, more preferably from 100,000 to 1,000,000, because the effects of the present invention are more excellent.

<Molecular weight distribution>
Although the molecular weight distribution (Mw/Mn) of the modified conjugated diene-based rubber of the present invention is not particularly limited, it is preferably 10 or less, more preferably 5 or less, and even more preferably 3 or less because the effects of the present invention are more excellent. Although the lower limit of the molecular weight distribution is not particularly limited, it is usually 1.0 or more.

In addition, let said Mw and Mn be a standard polystyrene conversion value obtained by the gel permeation chromatography (GPC) measurement of the following conditions.
・Solvent: Tetrahydrofuran ・Detector: RI detector

When the modified conjugated diene-based rubber of the present invention is a mixture of two or more types of "conjugated diene-based rubbers having a carboxylic acid metal group on at least one of the main chain and the terminal", the above Mn, Mw and Mw/Mn are weighted average values similar to the modification rate described above.

[Microstructure]
<Vinyl structure (vinyl rate)>
In the modified conjugated diene-based rubber of the present invention, the ratio of the vinyl structure (vinyl ratio) among the repeating units derived from the conjugated diene is preferably 70 mol% or less, more preferably 50 mol% or less, further preferably 30 mol% or less, and particularly preferably 10 mol% or less, for the reason that the effects of the present invention are more excellent. The lower limit is not particularly limited and is 0 mol %.
Here, the ratio of the vinyl structure (vinyl ratio) refers to the ratio (mol%) of repeating units having a vinyl structure (for example, 1,2-vinyl structure when the conjugated diene is 1,3-butadiene, and 1,2-vinyl structure and 3,4-vinyl structure when the conjugated diene is isoprene) among all repeating units derived from the conjugated diene.

<1,4-trans structure (trans ratio)>
In the modified conjugated diene-based rubber of the present invention, the proportion of 1,4-trans structures (trans ratio) among the repeating units derived from conjugated dienes is 4 to 30 mol %.
The trans ratio is preferably 5 to 25 mol %, more preferably 9 to 20 mol %, because the effects of the present invention are more excellent.
Here, the ratio of 1,4-trans structure (trans ratio) refers to the ratio (mol %) of repeating units having 1,4-trans structure to all repeating units derived from conjugated dienes.

<1,4-cis structure (cis ratio)>
In the modified conjugated diene-based rubber of the present invention, the ratio (cis ratio) of the 1,4-cis structure among the repeating units derived from the conjugated diene is preferably 1 to 95 mol%, more preferably 50 to 90 mol%, and even more preferably 70 to 85 mol%, for the reason that the effects of the present invention are more excellent.
Here, the ratio of 1,4-cis structure (cis ratio) refers to the ratio (mol %) of repeating units having 1,4-cis structure to all repeating units derived from conjugated dienes.

Hereinafter, among repeating units derived from a conjugated diene, "ratio of vinyl structure (mol %), proportion of 1,4-trans structure (mol %), proportion of 1,4-cis structure (mol %)" are also expressed as "vinyl/trans/cis".

When the modified conjugated diene-based rubber of the present invention is a mixture of two or more types of "conjugated diene-based rubbers having a carboxylic acid metal group on at least one of the main chain and the terminal", the above-mentioned vinyl ratio, trans ratio and cis ratio are weighted average values like the modification ratio described above.
For example, in the case of Example 3, which will be described later, the mixture is a mixture of 80 parts by mass of modified cis-isoprene rubber (Synthesis Example 1) (trans ratio: 0 mol%) and 20 parts by mass of modified trans-isoprene rubber (Synthesis Example 2) (trans ratio: 99 mol%). There is.

[Preferred embodiment]
The modified conjugated diene-based rubber of the present invention is a mixture of a modified cis-conjugated diene rubber (hereinafter also referred to as "specific modified cis-type conjugated diene rubber") which is a cis-conjugated diene-based rubber having a carboxylic acid metal base on at least one of the main chain and the terminal, and a modified trans-type conjugated diene-based rubber (hereinafter also referred to as "specific modified trans-conjugated diene-based rubber") which is a trans-type conjugated diene rubber having a carboxylic acid metal base on at least one of the main chain and the terminal. is preferred. Here, the trans ratio (average trans ratio) of the mixture is 4 to 30 mol %.

<Specific modified cis-type conjugated diene rubber>
The above-mentioned cis-type means that the ratio (cis ratio) of repeating units having a 1,4-cis structure to all repeating units derived from a conjugated diene is more than 50 mol%. That is, in the specific modified cis-type conjugated diene rubber, the ratio (cis ratio) of repeating units having a 1,4-cis structure to all repeating units derived from conjugated dienes is more than 50 mol%.
The cis ratio of the specific modified cis-type conjugated diene rubber is preferably 60 mol% or more, more preferably 70 mol% or more, even more preferably 80 mol% or more, and particularly preferably 90 mol% or more, because the effects of the present invention are more excellent. The upper limit is not particularly limited and is 100 mol %.

<Specific modified trans-type conjugated diene rubber>
The trans-type refers to a ratio (trans ratio) of repeating units having a 1,4-trans structure to more than 50 mol % of all repeating units derived from a conjugated diene. That is, in the specific modified trans-type conjugated diene rubber, the proportion of repeating units having a 1,4-trans structure in all repeating units derived from conjugated dienes is more than 50 mol %.
The cis ratio of the specific modified trans-type conjugated diene rubber is preferably 60 mol% or more, more preferably 70 mol% or more, even more preferably 80 mol% or more, and particularly preferably 90 mol% or more, because the effects of the present invention are more excellent. The upper limit is not particularly limited and is 100 mol %.

<Content>
When the modified conjugated diene-based rubber of the present invention is a mixture of a specific modified cis-conjugated diene-based rubber and a specific modified cis-conjugated diene-based rubber, the content of the specific modified cis-conjugated diene-based rubber is preferably 50 to 99% by mass for the reason that the effects of the present invention are more excellent.
Further, when the modified conjugated diene-based rubber of the present invention is a mixture of the specific modified cis-type conjugated diene-based rubber and the specific modified cis-type conjugated diene-based rubber, the content of the specific modified trans-type conjugated diene-based rubber is preferably 1 to 50% by mass for the reason that the effects of the present invention are more excellent.

[Use]
The modified conjugated diene rubber of the present invention is suitable for use as a rubber material. For example, it is suitably used for tires, conveyor belts, hoses, anti-vibration materials, rubber rolls, outer hoods of railway vehicles, and the like.

[Method for producing modified conjugated diene rubber]
The method for producing the modified conjugated diene-based rubber of the present invention is not particularly limited, but since the effect of the present invention is more excellent for the resulting modified conjugated diene-based rubber, a method of producing the above-mentioned specific modified cis-type conjugated diene-based rubber and the above-mentioned specific modified trans-type conjugated diene-based rubber and mixing them so that the trans ratio (average trans ratio) falls within the above-mentioned specific range is preferable. In addition, hereinafter, when the modified conjugated diene rubber to be obtained has superior effects of the present invention, it is also simply referred to as "excellent effects of the present invention."

[Method for producing specific modified cis-type conjugated diene rubber]
The method for producing the specific modified cis-conjugated diene-based rubber (the cis-conjugated diene-based rubber having a carboxylic acid metal base on at least one of the main chain and the terminal) is not particularly limited, but a method comprising the following steps (1) and (2) (hereinafter also referred to as "the production method of the present invention") is preferable because the effects of the present invention are more excellent.

(1) Step of adding branched alkyllithium Step of adding branched alkyllithium to cis-conjugated diene rubber (2) Step of adding carbon dioxide After that, a step of adding carbon dioxide to obtain a modified cis-conjugated diene rubber, which is a cis-conjugated diene rubber having a carboxylic acid metal base in the main chain.

Each step will be described below.

<Step of adding branched alkyllithium>
The step of adding branched alkyllithium is a step of adding branched alkyllithium to the cis-type conjugated diene rubber. As a result, an anion is generated on the main chain. As a result, a cis-conjugated diene rubber having an anion in the main chain is obtained.

(branched alkyllithium)
A branched alkyllithium is a compound in which a branched alkyl group and a lithium atom are combined.
The number of carbon atoms in the branched alkyl group is preferably 3 to 10 for the reason that the effects of the present invention are more excellent.
Specific examples of the branched alkyllithium include iso-propyllithium, sec-butyllithium, tert-butyllithium, etc. Among them, sec-butyllithium is preferable because the effects of the present invention are more excellent.

<Carbon dioxide addition step>
The carbon dioxide addition step is a step of obtaining a modified cis-conjugated diene rubber, which is a cis-conjugated diene rubber having a carboxylic acid metal group in the main chain, by adding carbon dioxide after the above-described branched alkyllithium addition step.
In the step of adding carbon dioxide, the carbon dioxide reacts with the anion of the main chain of the cis-conjugated diene rubber after the step of adding branched alkyllithium to form a lithium carboxylate base in the main chain.
In addition, the lithium carboxylate base can be converted into the desired metal carboxylate base by salt exchange.

[Method for producing specific modified trans-type conjugated diene rubber]
The method for producing the specific modified trans-type conjugated diene-based rubber (trans-type conjugated diene-based rubber having a carboxylic acid metal group on at least one of the main chain and the end) is not particularly limited, but for the reason that the effects of the present invention are more excellent, it is preferable to use a trans-type diene-based rubber instead of the cis-type conjugated diene-based rubber in the above-described production method of the present invention.

Another method for producing the modified conjugated diene-based rubber of the present invention includes a method of using a conjugated diene-based rubber having a trans ratio within the specific range described above instead of the cis-type conjugated diene-based rubber in the above-described production method of the present invention. In this case, it is not necessary to use a mixture.

EXAMPLES The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to these.

[Synthesis example]
An isoprene rubber was synthesized as follows.

<Synthesis Example 1: Main chain sodium carboxylate base-modified cis-isoprene rubber>
Cis-type isoprene rubber (Mn = 91,800, Mw = 98,200, Mw/Mn = 1.07) was dissolved in cyclohexane, sec-BuLi (sec-butyllithium) (manufactured by Kanto Kagaku: 1.0 mol/L (cyclohexane/hexane solution)) and TMEDA (tetramethylenediamine) were added and stirred, and after 2 minutes, carbon dioxide was blown in. The solution was then removed and concentrated under reduced pressure. The concentrated solution was poured into methanol to separate methanol-insoluble components. By vacuum-drying this at 60° C. for 15 hours, a cis-type isoprene rubber having a lithium carboxylate group (—COOLi) only in the main chain was obtained.
This was redissolved in THF (tetrahydrofuran), and an aqueous HCl solution (manufactured by Nacalai Tesque: 35%) was added to the THF solution. This solution was poured into methanol to separate methanol-insoluble matter. By vacuum-drying this at 60° C. for 15 hours, a cis-type isoprene rubber having carboxy groups (—COOH) only on the main chain was obtained.
This was dissolved again in THF to a concentration of 10% by mass, a methanol solution of sodium hydroxide (3.9 g/L) was added dropwise, and the mixture was vigorously stirred for 1 hour. After the reaction, the resulting solution was taken out and concentrated under reduced pressure. The concentrated solution was poured into methanol to separate methanol-insoluble components. By vacuum-drying this at 60° C. for 15 hours, a cis-isoprene rubber having sodium carboxylate groups (—COONa) only in the main chain was obtained. By IR analysis, it was estimated that vinyl/trans/cis=7/0/93 and the modification rate was 1.80 mol %.

<Synthesis Example 2: Main chain sodium carboxylate base-modified trans-isoprene rubber>
Trans-type isoprene rubber (manufactured by Kuraray Co., Ltd.) (Mn = 304,000, Mw = 1,540,000, Mw/Mn = 5.07) was dissolved in cyclohexane, sec-BuLi (sec-butyllithium) (manufactured by Kanto Kagaku: 1.0 mol/L (cyclohexane/hexane solution)) and TMEDA (tetramethylenediamine) were added and stirred, and after 2 minutes, carbon dioxide was blown in. The solution was then removed and concentrated under reduced pressure. The concentrated solution was poured into methanol to separate methanol-insoluble components. By vacuum-drying this at 60° C. for 15 hours, a trans-isoprene rubber having a lithium carboxylate group (—COOLi) only in the main chain was obtained.
This was redissolved in THF (tetrahydrofuran), and an aqueous HCl solution (manufactured by Nacalai Tesque: 35%) was added to the THF solution. This solution was poured into methanol to separate methanol-insoluble matter. By vacuum-drying this at 60° C. for 15 hours, a trans-isoprene rubber having carboxy groups (—COOH) only on the main chain was obtained.
This was dissolved again in THF to a concentration of 10% by mass, a methanol solution of sodium hydroxide (3.9 g/L) was added dropwise, and the mixture was vigorously stirred for 1 hour. After the reaction, the resulting solution was taken out and concentrated under reduced pressure. The concentrated solution was poured into methanol to separate methanol-insoluble components. By vacuum-drying this at 60° C. for 15 hours, a trans-isoprene rubber having sodium carboxylate groups (—COONa) only in the main chain was obtained. By IR analysis, it was estimated that vinyl/trans/cis=1/99/0 and the modification rate was 1.60 mol %.

<Synthesis Example 3: Main Chain Carboxy Group-Modified Cis Isoprene Rubber>
Following the same procedure as in Synthesis Example 1, a cis-isoprene rubber having carboxy groups (--COOH) only on the main chain was obtained. By IR analysis, it was estimated that vinyl/trans/cis=7/0/93 and the modification rate was 1.80 mol %.

<Synthesis Example 4: Main Chain Carboxy Group-Modified Trans-Isoprene Rubber>
Following the same procedure as in Synthesis Example 2, a trans-isoprene rubber having carboxy groups (--COOH) only on the main chain was obtained. By IR analysis, it was estimated that vinyl/trans/cis=1/99/0 and the modification rate was 1.60 mol %.

[Production of conjugated diene rubber]
Conjugated diene rubbers of Examples and Comparative Examples were obtained by blending the components shown in Table 1 below in proportions (parts by mass) shown in the same table.

Here, the conjugated diene-based rubbers of Examples 1 to 3 are all mixtures of the specific modified cis-isoprene rubber and the specific modified trans-isoprene rubber, and have an average trans ratio of 4 to 30 mol%.
On the other hand, although the conjugated diene rubber of Comparative Example 1 has a carboxylic acid metal group, it does not correspond to the modified conjugated diene rubber of the present invention described above because it has a trans ratio of less than 4 mol %.
Further, the conjugated diene rubber of Comparative Example 2 is a mixture of the conjugated diene rubber of Comparative Example 1 and the unmodified trans-type isoprene rubber, and the trans ratio is less than 4 mol%, so it does not correspond to the modified conjugated diene rubber of the present invention described above.
Further, the conjugated diene rubber of Comparative Example 3 is a mixture of a cis-isoprene rubber having a carboxy group (having no carboxylic acid metal base) and a cis-isoprene rubber having a carboxy group (having no carboxylic acid metal base), and thus does not correspond to the above-described modified conjugated diene rubber of the present invention.

〔evaluation〕
A rubber sheet was produced using the obtained conjugated diene rubber. Then, the following evaluation was performed.

<Creep characteristics>
A stress of 100 kPa was applied to the obtained rubber sheet for 10 hours, and then the residual strain (%) was measured after relaxing for 14 hours. Table 1 shows the reciprocal of residual strain. The results were expressed as an index with the reciprocal of the residual strain of Comparative Example 1 set to 100. The larger the index, the smaller the residual strain and the better the creep property.

<Toughness>
A JIS No. 3 dumbbell-shaped specimen (thickness: 2 mm) was punched out from the obtained rubber sheet according to JIS K6251:2010, and the breaking strength and breaking elongation were evaluated under the conditions of a temperature of 20°C and a tensile speed of 500 mm/min. Then, the toughness parameter was obtained from the following formula.
Toughness parameter=breaking strength×breaking elongation The results are shown in Table 1. The results were expressed as indices with Comparative Example 1 being 100. A larger index indicates better toughness.

<Self-repairability>
The resulting rubber sheet was cut with a razor to obtain two sample pieces. Then, the cut surfaces of the two sample pieces were left in contact with each other at 28° C. for 48 hours for self-healing. The self-healing rubber sheets were evaluated for breaking strength as described above. Then, the ratio of the breaking strength before cutting to the breaking strength after self-repairing (breaking strength after self-repairing/breaking strength before cutting) was determined, and self-repairability was evaluated according to the following criteria. Table 1 shows the results. From the viewpoint of self-repairing property, ◯ is preferable.
○: 0.7 or more ×: less than 0.7

Details of each component shown in Table 1 are as follows.
Modified cis-isoprene rubber (Synthesis Example 1): Main chain sodium carboxylate base-modified cis-isoprene rubber of Synthesis Example 1 synthesized as described above Unmodified trans-isoprene rubber: Unmodified trans-isoprene rubber manufactured by Kuraray Co., Ltd. (Mn = 304,000, Mw = 1,540,000, Mw/Mn = 5.07, vinyl/trans/cis = 1/99/0)
Modified trans-isoprene rubber (Synthesis Example 2): Main chain sodium carboxylate base-modified trans-isoprene rubber of Synthesis Example 2 synthesized as described above Comparative modified cis-isoprene rubber (Synthesis Example 3): Main chain carboxy group-modified cis-isoprene rubber of Synthesis Example 3 synthesized as described above Comparative modified trans-isoprene rubber (Synthesis Example 4): Main chain carboxy group-modified trans-isoprene rubber of Synthesis Example 4 synthesized as described above

In Table 1, Mn, Mw and Mw/Mn represent Mn, Mw and Mw/Mn described above, respectively. Also, in Table 1, vinyl [mol %], trans [mol %] and cis [mol %] represent the above-mentioned vinyl ratio, trans ratio and cis ratio, respectively. Moreover, in Table 1, the modification rate represents the modification rate described above.

As can be seen from Table 1, in comparison with Comparative Examples 1 and 2 having a trans ratio of less than 4 mol%, Examples 1 to 3 having a trans ratio of 4 mol% or more exhibited excellent creep properties and toughness. Moreover, in comparison with Comparative Example 3 having a carboxy group (not having a carboxylic acid metal base), Examples 1 to 3 each having a carboxylic acid metal base exhibited excellent self-healing properties.
In comparison with Examples 1-3, Examples 2-3 with a trans ratio of 8 mol % or more exhibited better creep properties. Among them, Example 3, in which the trans ratio was 15 mol % or more, exhibited even better creep properties.
Further, in comparison with Examples 1 to 3, Examples 1 to 2 having a trans ratio of 15 mol% or less exhibited superior toughness. Among them, Example 1, in which the trans ratio was 8 mol% or less, exhibited even better toughness.

Claims (5)

A modified conjugated diene rubber, which is a conjugated diene rubber having a carboxylic acid metal base on at least one of the main chain and the terminal,
A modified conjugated diene rubber in which the proportion of repeating units having a 1,4-trans structure is 4 to 30 mol% and the proportion of repeating units having a vinyl structure is 30 mol% or less among all repeating units derived from a conjugated diene. a modified cis-conjugated diene-based rubber, which is a cis-conjugated diene-based rubber having a carboxylic acid metal base on at least one of the main chain and the terminal;
A mixture with a modified trans-conjugated diene-based rubber, which is a trans-conjugated diene-based rubber having a carboxylic acid metal group on at least one of the main chain and the terminal,
The content of the modified cis-type conjugated diene rubber is 50 to 99% by mass,
The modified conjugated diene rubber according to claim 1, wherein the content of the modified trans conjugated diene rubber is 1 to 50% by mass. 3. The modified conjugated diene rubber according to claim 1, wherein the repeating unit having a carboxylic acid metal group accounts for 0.1 to 30 mol % of all repeating units. The modified conjugated diene rubber according to any one of claims 1 to 3, wherein the conjugated diene rubber has a weight average molecular weight of 5,000 to 10,000,000. The modified conjugated diene rubber according to any one of claims 1 to 4, wherein the conjugated diene rubber is natural rubber or isoprene rubber.