JP7723558B2 - rubber composition - Google Patents

Description

The present invention relates to a rubber composition and a rubber molded article.

Rubber is an amorphous and soft polymeric substance, primarily composed of organic polymers such as natural rubber and synthetic rubber, and is a material (elastic rubber) with a high elastic limit and a low elastic modulus. Taking advantage of these properties of rubber, rubber compositions containing rubber are used in a variety of fields, including tires, seals, and vibration-isolating and vibration-damping materials.
Rubber compositions for tires, etc., typically contain silica as an inorganic filler to reduce energy loss associated with deformation. However, the surface of silica is highly hydrophilic, which causes a problem of low dispersibility in rubber. To take advantage of the properties of rubber and achieve better performance, the use of various additives and dispersants in rubber has been proposed.

For example, Patent Document 1 discloses a vulcanizable rubber composition made of natural rubber and/or synthetic rubber, which contains a specific polysulfide polymer (vulcanizing agent) and one or more of specific sulfenamide, mercaptobenzothiazole, phenylsulfenamide, and triazine (vulcanization accelerator).
Patent Document 2 discloses a rubber composition using a modified natural rubber obtained by adding a polar group-containing mercapto compound to natural rubber latex, adding the polar group-containing mercapto compound to natural rubber molecules in the natural rubber latex, and coagulating and drying the modified natural rubber latex.
Patent Document 3 discloses a rubber composite containing a filler having a silanol group, a polysulfide-based silane coupling agent, and triazine dithiol as a resin-rubber composite having excellent adhesion between a resin member and a rubber member that is in direct contact with the resin member. The examples describe a rubber sheet having a thickness of 2.5 mm obtained after mixing and stirring a filler having a silanol group, a polysulfide-based silane coupling agent, triazine dithiol, sulfur, and other raw materials at 110°C for 3 minutes.

Japanese Unexamined Patent Publication No. 58-167634 Japanese Patent Application Laid-Open No. 2006-152045 International Publication No. 2019/208800

However, Patent Document 1 does not use sulfur, and Patent Documents 2 and 3 contain 1.5 parts by mass or more of sulfur per 100 parts by mass of the rubber component, which does not provide sufficient dispersibility of silica, heat resistance of the rubber molded body, or fuel economy performance.
In particular, in Patent Document 3, a rubber sheet prepared by mixing a composition is attached to a resin member and vulcanized at 145 to 150° C. (1.7 to 3.4 parts by mass of sulfur is added per 100 parts by mass of rubber). This vulcanization procedure bonds the silica and rubber in the rubber sheet with a silane coupling agent, and adheres the rubber to the resin member, but the dispersion of the silica is insufficient.
An object of the present invention is to provide a rubber composition having excellent silica dispersibility and heat resistance, and a rubber molded article having excellent silica dispersibility, heat resistance, and fuel economy performance.

The inventors have discovered that the above-mentioned problems can be solved by using a specific compound having a thiol group as the conjugated heterocyclic compound in a rubber composition containing a rubber component, silica, a silane coupling agent, a conjugated heterocyclic compound, and sulfur, and by adjusting the content of sulfur and the conjugated heterocyclic compound within specific ranges.

That is, the present invention provides the following [1] and [2].
[1] A rubber composition containing a rubber component (A), silica (B), a silane coupling agent (C), a conjugated heterocyclic compound (D), and sulfur (E),
the conjugated heterocyclic compound (D) is a compound having a structure in which at least one thiol group is bonded to one or more conjugated heterocyclic rings selected from a thiazole ring, an isothiazole ring, a thiadiazole ring, an imidazole ring, a pyrazole ring, a triazole ring, a pyridine ring, a pyrimidine ring, a pyrazine ring, and a triazine ring;
the content of sulfur (E) is 1.2 parts by mass or less per 100 parts by mass of the rubber component (A),
A rubber composition, wherein the mass ratio of sulfur (E) to conjugated heterocyclic compound (D) [(E)/(D)] is 0.1 or more and 2 or less.
[2] A rubber molded article obtained by vulcanizing the rubber composition of [1] above.

The present invention makes it possible to provide a rubber composition with excellent silica dispersibility and heat resistance, and a rubber molded article with excellent silica dispersibility, heat resistance, and fuel economy performance.

[Rubber composition]
The rubber composition of the present invention is a rubber composition containing a rubber component (A), silica (B), a silane coupling agent (C), a conjugated heterocyclic compound (D), and sulfur (E),
the conjugated heterocyclic compound (D) is a compound having a structure in which at least one thiol group is bonded to one or more conjugated heterocyclic rings selected from a thiazole ring, an isothiazole ring, a thiadiazole ring, an imidazole ring, a pyrazole ring, a triazole ring, a pyridine ring, a pyrimidine ring, a pyrazine ring, and a triazine ring;
the content of sulfur (E) is 1.2 parts by mass or less per 100 parts by mass of the rubber component (A),
The mass ratio of sulfur (E) to the conjugated heterocyclic compound (D) [(E)/(D)] is 0.1 or more and 2 or less.

According to the present invention, it is possible to obtain a rubber composition having excellent silica dispersibility and heat resistance, and a rubber molded article having excellent silica dispersibility, heat resistance, and fuel economy performance. The reason for this is not entirely clear, but is thought to be as follows.
The rubber composition of the present invention contains a small amount of sulfur (E) and a specific ratio of a conjugated heterocyclic compound (D) having a thiol group. This conjugated heterocyclic compound (D) generates a thiyl radical (RS·) when the rubber composition is kneaded, and this thiyl radical accelerates the cleavage of the S—S bond contained in the silane coupling agent, which is thought to improve the dispersibility of silica.
Furthermore, the conjugated heterocyclic compound (D) having a thiol group, like sulfur, crosslinks between rubber molecules, which is thought to result in an improvement in the initial viscosity of the rubber composition and an improvement in the heat resistance of the resulting rubber molded article.
In the rubber composition of the present invention, since the silica (B) is uniformly dispersed in the rubber component (A), the affinity between the silica (B) and the rubber component (A) is increased, and it is thought that the rubber molded article obtained by vulcanizing the rubber composition has improved mechanical strength and improved fuel economy performance (reduced tan δ).

<Rubber component (A)>
The rubber component (A) used in the present invention is not particularly limited, and natural rubber and synthetic rubber can be used. Among these, one or more types selected from natural rubber and diene-based synthetic rubber are preferred from the viewpoints of abrasion resistance, availability, etc.
Examples of natural rubber include SMR, SIR, STR, and RSS, with SMR20, STR20, RSS#3, and RSS#4 being preferred.
Natural rubber can be used after being modified, and examples of modified natural rubber include epoxidized natural rubber and hydrogenated natural rubber.
Examples of diene-based synthetic rubbers include polyisoprene rubber (IR), polybutadiene rubber (BR), styrene-butadiene copolymer rubber (SBR), acrylonitrile-butadiene copolymer rubber (NBR), chloroprene rubber, and butyl rubber.

Among these, from the same viewpoint as above, one or more selected from natural rubber, modified natural rubber, IR, BR, SBR, and NBR are preferred, and one or more selected from BR, SBR, and natural rubber are more preferred. BR or SBR can also be used in combination with natural rubber.
The copolymer rubber may be a block copolymer or a random copolymer, but from the viewpoints of silica dispersibility and heat resistance, a random copolymer is preferred.
The rubber component (A) may be used alone or in combination of two or more thereof.

<Silica (B)>
The rubber composition of the present invention contains silica (B) from the viewpoint of improving heat resistance and improving the heat resistance, abrasion resistance and fuel economy of the resulting rubber molded article.
The silica (B) used in the present invention is not particularly limited, and wet silica, dry silica, and colloidal silica can be used. Among these, wet silica, which is mainly composed of hydrated silicic acid, is preferred. Wet silica includes precipitated silica, gel silica, and sol-gel silica, and precipitated silica is more preferred.
The silica (B) may be used alone or in combination of two or more kinds.
The BET specific surface area of the silica (B) (measured in accordance with ISO 5794/1) is preferably 50 m 2 /g or more, more preferably 100 m ² /g or more, even more preferably 150 m ² /g or more, from the viewpoint of dispersibility of the silica in the rubber composition and the rubber molded article and rubber reinforcement, and is preferably 350 m ² /g or less, more preferably 300 ^{m 2 /} g or less, even more preferably 250 m ² /g or less.
From the viewpoints of dispersibility of silica in the rubber composition and the rubber molded article and rubber reinforcement, the average secondary particle diameter of the silica (B) is preferably 10 μm or more, more preferably 15 μm or more, and even more preferably 18 μm or more, and is preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 50 μm or less.

Examples of commercially available silica products include Nipsil AQ (BET specific surface area: 205 m ² /g) and Nipsil KQ (BET specific surface area: 240 m ² /g) manufactured by Tosoh Silica Corporation, and Ultrasil VN3 (BET specific surface area: 175 m ² /g) manufactured by Evonik.

In the present invention, the rubber composition may further contain carbon black as an inorganic filler, if necessary. By containing carbon black, electrical resistance can be reduced and static electricity can be suppressed.
There are no particular limitations on the carbon black used, and it is possible to use high-, medium-, or low-structure carbon blacks of grades such as SAF, ISAF, IISAF, N339, HAF, FEF, GPF, and SRF, as well as carbon and silica dual-phase fillers in which silica is supported on the surface of carbon black, etc. Among these, SAF, ISAF, IISAF, N339, HAF, and FEF grade carbon blacks are preferred.
The DBP absorption of carbon black (measured in accordance with ASTM D2414-65T) is preferably 70 cm ³ /100 g or more, more preferably 80 cm ³ /100 g or more, and even more preferably 90 cm ³ /100 g or more.
The nitrogen adsorption specific surface area (N ₂ AS, measured in accordance with JIS K 6217-2:2017) of the carbon black is preferably 60 m ² /g or more, more preferably 80 m ² /g or more, and even more preferably 100 m ² /g or more.
In the present invention, alumina, calcium carbonate, clay, talc, zeolite, diatomaceous earth, etc. may be further contained as inorganic fillers, if necessary.

<Silane Coupling Agent (C)>
The rubber composition of the present invention contains a silane coupling agent (C) from the viewpoint of improving the dispersibility and heat resistance of silica and improving the dispersibility, heat resistance and fuel economy of the resulting rubber molded article.
The silane coupling agent (C) is not particularly limited, and those used in commercially available rubber compositions can be used. Examples of the silane coupling agent (C) include polysulfide-based, mercapto-based, vinyl-based, epoxy-based, styryl-based, methacrylic-based, acrylic-based, and amino-based silane coupling agents. Among these, polysulfide-based silane coupling agents are preferred from the same viewpoints as above.

The polysulfide-based silane coupling agent (C) is preferably a compound represented by the following general formula (II).
(R ³ -O) _3-p R ³ _p -Si-R ⁴ -S _n -R ⁴ -Si-R ³ _q (O-R ³ ) _3-q (II)
In formula (II), ^R3 is an alkyl group having 1 to 8 carbon atoms, ^R4 is an alkanediyl group having 1 to 8 carbon atoms, and n is an integer of 2 to 6. p and q each independently represent an integer of 0 to 3, but both p and q cannot be 3.
The number of carbon atoms in the alkyl group represented by ^R3 is 1 or more, and preferably 6 or less, more preferably 4 or less, and even more preferably 3 or less. Suitable examples of the alkyl group include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, and a tert-butyl group.
The alkanediyl group represented by ^R4 has 1 or more carbon atoms and preferably 6 or less, more preferably 4 or less, and even more preferably 3 or less.
n is an integer of 1 or more, preferably 2 or more, and preferably 5 or less, more preferably 4 or less.
Preferably, p and q are each independently 0 or 1.

Examples of the compound represented by formula (II) include bis(3-triethoxysilylpropyl)tetrasulfide, bis(3-triethoxysilylpropyl)trisulfide, bis(3-triethoxysilylpropyl)disulfide, bis(2-triethoxysilylethyl)tetrasulfide, bis(3-trimethoxysilylpropyl)tetrasulfide, bis(3-triethoxysilylethyl)tetrasulfide, bis(3-methyldimethoxysilylpropyl)tetrasulfide, bis(2-trimethoxysilylethyl)tetrasulfide, and bis(3-trimethoxysilylpropyl)disulfide.
Among these, one or more selected from bis(3-triethoxysilylpropyl)tetrasulfide, bis(3-triethoxysilylpropyl)trisulfide, and bis(3-triethoxysilylpropyl)disulfide are preferred, and one or more selected from bis(3-triethoxysilylpropyl)tetrasulfide and bis(3-triethoxysilylpropyl)disulfide are more preferred.
The polysulfide-based silane coupling agents (C) can be used alone or in combination of two or more kinds.

Commercially available examples of polysulfide-based silane coupling agents include Evonik products under the trade names Si75 and Si69, Shin-Etsu Chemical Co., Ltd. products under the trade name KBE-846, and Daiso Co., Ltd. products under the trade names Kabras 2A, Kabras 2B, and Kabras 4.

<Conjugated heterocyclic compound (D)>
The rubber composition of the present invention contains a conjugated heterocyclic compound (D) from the viewpoint of improving the dispersibility and heat resistance of silica and improving the dispersibility, heat resistance and fuel economy of the obtained rubber molded article.
A conjugated heterocyclic compound refers to a heterocyclic compound having a conjugated structure, in which electrons of two or more chemical bonds interact with each other to become delocalized, and in particular, a structure in which π electrons of multiple bonds, unpaired electrons, and non-bonding electron pairs are connected across a single bond and interact with each other to become delocalized.
The conjugated heterocyclic compound preferably has a monocyclic conjugated ring (conjugated monocyclic) structure, has 1 to 3 nitrogen atoms and 0 or 1 sulfur atom in the monocyclic ring, and has 2 to 12 carbon atoms.
Specifically, the conjugated heterocyclic compound (D) used in the present invention is a compound having a structure in which at least one thiol group is bonded to one or more conjugated heterocyclic rings selected from a thiazole ring, an isothiazole ring, a thiadiazole ring, an imidazole ring, a pyrazole ring, a triazole ring, a pyridine ring, a pyrimidine ring, a pyrazine ring, and a triazine ring.
From the same viewpoint as above, the conjugated heterocyclic compound (D) is preferably a compound having a structure in which at least one thiol group is bonded to one or more conjugated heterocyclic rings selected from a thiadiazole ring, a pyridine ring, a pyrimidine ring, and a triazine ring.

Examples of compounds having a structure in which at least one thiol group is bonded to a thiadiazole ring include 2,5-dimercapto-1,3,4-thiadiazole (MTD), 2-amino-5-mercapto-1,3,4-thiadiazole, and 2-amino-5-trifluoromethyl-1,3,4-thiadiazole. Among these, 2,5-dimercapto-1,3,4-thiadiazole is preferred from the standpoint of reactivity.

As a compound having a structure in which at least one thiol group is bonded to one or more conjugated heterocyclic rings selected from a pyridine ring, a pyrimidine ring, and a triazine ring, a compound represented by the following general formula (I) is preferred.

In formula (I), ^R1 represents a hydrogen atom, a hydrocarbon group having 1 to 8 carbon atoms, an SH group, or an amino group which may be substituted with a hydrocarbon group having 1 to 16 carbon atoms; ^R2 represents a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, or an SH group; and ^X1 and ^X2 represent a nitrogen atom or a CH group.
In formula (I), R ¹ is preferably a hydrogen atom, a hydrocarbon group having from 1 to 6 carbon atoms, an SH group, or an amino group having a hydrocarbon group having from 1 to 14 carbon atoms, more preferably a hydrogen atom, an alkyl or alkenyl group having from 1 to 4 carbon atoms, an SH group, or an amino group having an alkyl or alkenyl group having from 1 to 12 carbon atoms, even more preferably a hydrogen atom, an SH group, or an amino group having an alkyl group having from 1 to 10 carbon atoms or an alkenyl group having from 1 to 4 carbon atoms, and still more preferably a hydrogen atom, an SH group, or an alkylamino group having an alkyl group having from 1 to 8 carbon atoms.
^R2 is preferably a hydrogen atom or an SH group.

Specific examples of the compound represented by general formula (I) include 2-mercaptopyridine, 3-mercaptopyridine, 4-mercaptopyridine, 1,3,5-triazine-2,4,6-trithiol, dimethylamino-1,3,5-triazine-2,4-dithiol, 6-(dibutylamino)-1,3,5-triazine-2,4-dithiol, 6-diallylamino-1,3,5-triazine-2,4-dithiol, 6-( [0033] Examples of the thiol-substituted thiol include 6-(diisopropylamino)-1,3,5-triazine-2,4-dithiol, 6-(diisobutylamino)-1,3,5-triazine-2,4-dithiol, 6-di(2-ethylhexyl)amino-1,3,5-triazine-2,4-dithiol, 6-(allylamino)-1,3,5-triazine-2,4-dithiol, and 6-(butylamino)-1,3,5-triazine-2,4-dithiol.
Among these, one or more selected from 2-mercaptopyridine, 1,3,5-triazine-2,4,6-trithiol, 6-(dibutylamino)-1,3,5-triazine-2,4-dithiol, and 6-(diisopropylamino)-1,3,5-triazine-2,4-dithiol are preferred, and one or more selected from 2-mercaptopyridine and 6-(dibutylamino)-1,3,5-triazine-2,4-dithiol are more preferred.
The conjugated heterocyclic compound (D) can be used alone or in combination of two or more kinds.

<Sulfur (E)>
The rubber composition of the present invention contains sulfur (E) in order to vulcanize it into a rubber molded article.
Examples of sulfur (E) include powdered sulfur, precipitated sulfur, colloidal sulfur, insoluble sulfur, highly dispersible sulfur, and soluble sulfur, which are commonly used in the rubber industry.
The sulfur (E) may be used alone or in combination of two or more thereof.

<Other ingredients>
In addition to the above components, the rubber composition of the present invention may contain, if desired, various additives commonly used in the rubber industry, such as antioxidants, antiscorching agents, softeners, stearic acid, process oil, zinc oxide, vulcanizing agents, and vulcanization accelerators, within the scope of the object of the present invention.

<Content of each component in rubber composition>
The rubber composition of the present invention contains or is blended with a rubber component (A), silica (B), a silane coupling agent (C), a conjugated heterocyclic compound (D), and sulfur (E), and may further contain or be blended with the above-mentioned additives as needed.
The content of each component in the rubber composition will be described below, and the content means both the amount contained and the blended amount.
The rubber component (A) constitutes the main component of the rubber composition, and its content in the rubber composition is preferably 25% by mass or more, more preferably 30% by mass or more, even more preferably 35% by mass or more, still more preferably 40% by mass or more, and preferably 80% by mass or less, more preferably 70% by mass or less, even more preferably 60% by mass or less, and still more preferably 50% by mass or less.

The content of silica (B) is preferably 30 parts by mass or more, more preferably 40 parts by mass or more, even more preferably 50 parts by mass or more, and still more preferably 60 parts by mass or more, per 100 parts by mass of rubber component (A), from the viewpoint of improving the heat resistance, abrasion resistance, and fuel economy of the obtained rubber molded article and from the viewpoint of processability of the rubber composition. From the viewpoint of uniformly dispersing the silica in the rubber composition, the content is preferably 200 parts by mass or less, more preferably 150 parts by mass or less, even more preferably 100 parts by mass or less, and still more preferably 80 parts by mass or less.
When the rubber composition and the rubber molded product further contain carbon black, the content of carbon black is preferably 1 part by mass or more, more preferably 2 parts by mass or more, and even more preferably 5 parts by mass or more, per 100 parts by mass of the rubber component (A), from the viewpoint of improving the abrasion resistance and the like of the obtained rubber molded product, and is preferably 35 parts by mass or less, more preferably 25 parts by mass or less, and even more preferably 20 parts by mass or less, from the viewpoint of reducing the heat buildup of the obtained rubber molded product.

The content of the silane coupling agent (C), particularly the polysulfide-based silane coupling agent (C), is preferably 1 part by mass or more, more preferably 2 parts by mass or more, and even more preferably 3 parts by mass or more, per 100 parts by mass of the rubber component (A), from the viewpoint of improving the dispersibility of silica and improving the heat resistance and fuel economy of the obtained rubber molded article, and is preferably 12 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 8 parts by mass or less.
Furthermore, from the same viewpoint as above, the content of the silane coupling agent (C), in particular the polysulfide-based silane coupling agent (C), relative to 100 parts by mass of silica (B), is preferably 1 part by mass or more, more preferably 2 parts by mass or more, even more preferably 4 parts by mass or more, and is preferably 20 parts by mass or less, more preferably 16 parts by mass or less, even more preferably 12 parts by mass or less.

From the viewpoint of improving the dispersibility of silica and improving the heat resistance and fuel economy of the resulting rubber molded article, the content of the conjugated heterocyclic compound (D) is preferably at least 0.1 part by mass, more preferably at least 0.2 part by mass, even more preferably at least 0.3 part by mass, and preferably at most 5 parts by mass, more preferably at most 4 parts by mass, even more preferably at most 3 parts by mass, per 100 parts by mass of the rubber component (A).

The content of sulfur (E) is 1.2 parts by mass or less, preferably 1.1 parts by mass or less, more preferably 1.0 part by mass or less, and even more preferably 0.9 parts by mass or less, per 100 parts by mass of rubber component (A), from the viewpoint of sufficiently vulcanizing the unvulcanized rubber composition and improving the heat resistance and fuel economy of the resulting rubber molded article.

The mass ratio of the silane coupling agent (C) to the conjugated heterocyclic compound (D) [(C)/(D)] is preferably 1 or more, more preferably 1.5 or more, even more preferably 2 or more, still more preferably 2.5 or more, from the viewpoint of improving the dispersibility of silica and improving the heat resistance and fuel economy of the rubber molded article, and is preferably 30 or less, more preferably 25 or less, even more preferably 20 or less, still more preferably 15 or less.
The ratio of the number of moles of the silane coupling agent (C) to the number of moles of thiol groups in the conjugated heterocyclic compound (D) [number of moles of silane coupling agent (C)/number of moles of thiol groups in the conjugated heterocyclic compound (D)] is, from the viewpoint of improving the dispersibility of silica and improving the heat resistance and fuel economy of the rubber molded article, preferably 0.1 or more, more preferably 0.3 or more, even more preferably 0.5 or more, still more preferably 0.7 or more, and is preferably 3 or less, more preferably 2 or less, even more preferably 1.5 or less, still more preferably 1.2 or more.

The mass ratio of silica (B) to conjugated heterocyclic compound (D) [(B)/(D)] is preferably 20 or more, more preferably 30 or more, even more preferably 40 or more, from the viewpoint of improving the dispersibility of silica and improving the heat resistance and fuel economy of the rubber molded article, and is preferably 400 or less, more preferably 300 or less, even more preferably 200 or less, and even more preferably 160 or less.

From the viewpoint of improving the heat resistance and fuel economy of the resulting rubber molded article, the mass ratio of sulfur (E) to conjugated heterocyclic compound (D) [(E)/(D)] is 0.1 or more, preferably 0.2 or more, more preferably 0.3 or more, even more preferably 0.4 or more, and is 2 or less, preferably 1.9 or less, more preferably 1.8 or less, even more preferably 1.7 or less.

<Production of Rubber Composition>
The rubber composition of the present invention can be produced by kneading a composition containing silica (B), a silane coupling agent (C), a conjugated heterocyclic compound (D), and sulfur with a rubber component (A) to obtain a rubber kneaded product, and then adding and mixing sulfur into the obtained rubber kneaded product.
The preferred ranges of the contents of the components (A) to (D) are as described above.

More specifically, for example, the rubber component (A) is kneaded with silica (B), a silane coupling agent (C), a conjugated heterocyclic compound (D), and, as necessary, components such as carbon black, an antioxidant, stearic acid, and process oil using a kneader such as a Banbury mixer, a roll, or an intensive mixer at, for example, 140°C or higher to react the silane coupling agent (C) with the conjugated heterocyclic compound (D), thereby dispersing the silica (B) in the rubber composition while cleaving the -S-S- bond of the silane coupling agent (C).
The kneading temperature is preferably 143°C or higher, more preferably 146°C or higher, and even more preferably 148°C or higher, from the viewpoints of cleaving the -S-S- bond contained in component (C) with conjugated heterocyclic compound (D), bonding rubber component (A) and silica (B) with the cleaved component (C), improving the dispersibility of silica in the rubber composition, and improving the heat resistance and fuel economy of the obtained rubber molded article, and is preferably 165°C or lower, more preferably 160°C or lower, and even more preferably 158°C or lower.

After obtaining a rubber kneaded product in which the silica (B) is uniformly dispersed in the rubber composition, sulfur is added and mixed.
From the viewpoint of preventing a vulcanization reaction, the temperature at which sulfur is added and mixed is preferably less than 140° C., more preferably 130° C. or less, even more preferably 125° C. or less, and still more preferably 120° C. or less. In addition to sulfur, zinc oxide, a vulcanization accelerator (sulfenamide-based, guanidine-based, etc.), etc. can be added as needed and mixed with the kneaded rubber product to obtain an unvulcanized rubber composition.

[Rubber molded body]
The rubber molded article of the present invention is obtained by vulcanizing the unvulcanized rubber composition of the present invention.
The unvulcanized rubber composition can be molded by a known method and heated or heated and pressurized at a temperature of preferably 140°C or higher, more preferably 145°C or higher, and preferably 200°C or lower, more preferably 180°C or lower, to form a vulcanized rubber molded article.
The contents of components (A) to (D) in the rubber molded product and the preferred ranges thereof are the same as those in the rubber composition.
The obtained rubber molded article has excellent silica dispersibility, heat resistance, and fuel economy performance, and can therefore be suitably used as rubber molded articles for tire components such as tires, tire inner liners, treads, tread bases, carcasses, sidewalls, and bead portions, as well as various rubber belts, various sealing materials, vibration-isolating and vibration-damping materials, and shoe soles.

Examples 1 to 6 and Comparative Examples 1 to 7
The raw material components shown in Table 1 were prepared, and the components excluding zinc oxide, sulfur, and vulcanization accelerator were kneaded for 4 minutes at a maximum temperature of 150°C using a Banbury mixer according to the formulation shown in Table 1. Next, zinc oxide, sulfur, and vulcanization accelerator were added, and the mixture was kneaded for 2 minutes at a maximum temperature of 110°C to obtain an unvulcanized rubber composition.
The resulting unvulcanized rubber composition was heated at 160° C. for 30 minutes to obtain a sheet-like vulcanized rubber molding.

Details of each component shown in Table 1 are as follows.
[Rubber component (A)]
*1: Solution polymerized SBR manufactured by JSR Corporation, product name: HPR850, styrene content 27.5% by mass
*2: RSS #3
[Silica (B)]
*3: Manufactured by Tosoh Silica Corporation, product name: Nipsil AQ, BET specific surface area 205 m ² /g
[Carbon black]
*4: Furnace black manufactured by Beilum Carbon Chemical Co., Ltd., trade name: N234, DBP absorption: 125 cm ³ /100 g, N ₂ AS: 117 m ² /g
[Silane Coupling Agent (C)]
*5: Bis(triethoxysilylpropyl) disulfide, manufactured by Evonik, trade name: Si75 (S2)
[Conjugated heterocyclic compound (D)]
*6: 2-mercaptopyridine, manufactured by Tokyo Chemical Industry Co., Ltd. *7: 6-(dibutylamino)-1,3,5-triazine-2,4-dithiol, manufactured by Tokyo Chemical Industry Co., Ltd. *8: 2,5-dimercapto-1,3,4-thiadiazole, manufactured by Tokyo Chemical Industry Co., Ltd. *9: 2,4,6-trimethylpyridine, manufactured by Tokyo Chemical Industry Co., Ltd.

〔others〕
*10: N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, manufactured by Ouchi Shinko Chemical Industry Co., Ltd., trade name: Nocrac 6C
*11: Kao Corporation, product name: Lunac S-70V
*12: Naphthenic process oil, manufactured by Nippon Sun Oil Co., Ltd., product name: SUNTHENE 410
*13: Zinc oxide (first grade), manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.
*14: Sulfur (powder, chemical grade), manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.
*15: Sulfenamide vulcanization accelerator, N-cyclohexyl-2-benzothiazolyl sulfenamide, manufactured by Ouchi Shinko Chemical Industry Co., Ltd., trade name: Noccela CZ-G
*16: Guanidine vulcanization accelerator, 1,3-diphenylguanidine, manufactured by Ouchi Shinko Chemical Industry Co., Ltd., trade name: Noccela D

Using the vulcanized rubber molded articles obtained in the Examples and Comparative Examples, evaluation of silica dispersibility and measurement of tensile stress and loss tangent (tan δ) were carried out by the following methods. The results are shown in Table 1.
(1) Evaluation of Silica Dispersibility Using a viscoelasticity measuring device (ARES-G2, manufactured by TA Instruments), the shear modulus G' of the obtained sheet-like rubber molding was measured in a torsion mode under conditions of 50°C and a frequency of 10 Hz, within a strain range of 0.1% to 10%, and the difference ΔG' between the shear modulus G' when a 0.1% strain was applied and the shear modulus G' when a 5% strain was applied was calculated using the following formula (1), to evaluate the silica dispersibility.
ΔG' (kPa) = G' (0.1%) - G' (5%) (1)
In formula (1), G'(0.1%) is the shear modulus when a 0.1% strain is applied at 50°C, and G'(5%) is the shear modulus when a 10% strain is applied at 50°C.
In Table 1, the values are shown as relative values when the ΔG′ of the rubber molded article of Comparative Example 1 is set to 100.
If this relative value of ΔG′ is less than 100, preferably 90 or less, it indicates that the silica (filler) in the rubber molded article (rubber composition) is well dispersed.

(2) Measurement of tensile stress at 200% elongation A vulcanized rubber test piece obtained by vulcanizing the rubber composition at 160°C for 30 minutes was processed into a JIS dumbbell No. 3 test piece, and a tensile test was carried out in accordance with JIS K 6251 to measure the tensile stress (MPa) at 200% elongation at 25°C.
Furthermore, the vulcanized rubber test pieces were subjected to heat aging treatment at 100° C. for 72 hours, and then the tensile stress (MPa) at 200% elongation was measured in the same manner as above.
In Table 1, the tensile stress at 200% elongation is expressed as an index, with the tensile stress value of Comparative Example 1 before heat aging treatment set to 100. An index closer to 100 indicates less aging even after heat treatment, and an index of 130 or less indicates good gripping properties.

(3) Measurement of loss tangent (tan δ) Using a viscoelasticity measuring device (ARES-G2, manufactured by TA Instruments), the loss tangent (tan δ) of the obtained sheet-like rubber molding was measured under conditions of a temperature of 50°C, a dynamic strain of 5%, and a frequency of 10 Hz. In Table 1, the loss tangent is shown as a relative value when the tan δ of the rubber molding of Comparative Example 1 is set to 100.
If the relative value of tan δ is 95 or less, for example, when used in a tire, the tire will have low rolling resistance and low heat buildup, and when used in a vehicle tire, it will show improved fuel economy.

Table 1 shows that the rubber molded articles obtained in Examples 1 to 6 of the present invention have a difference in shear modulus (ΔG') of less than 100, excellent silica (filler) dispersibility, small changes in tensile stress due to heating, excellent heat resistance, and a small loss tangent (tan δ), resulting in excellent fuel economy performance, compared to the rubber molded articles obtained in Comparative Examples 1 to 7.

The rubber composition of the present invention has excellent silica dispersibility and heat resistance, and the rubber molded product of the present invention has excellent silica dispersibility, heat resistance, and fuel economy. Therefore, it is particularly suitable for use in various tires and tire components such as tire treads for passenger cars, small and medium-sized trucks, and large vehicles (large trucks, buses, construction vehicles, etc.), as well as various rubber belts, various sealing materials, vibration-isolating and vibration-damping materials, shoe soles, etc.

Claims (8)

A rubber composition comprising a rubber component (A), silica (B), a silane coupling agent (C), a conjugated heterocyclic compound (D), and sulfur (E),
the silane coupling agent (C) is a polysulfide-based silane coupling agent;
the conjugated heterocyclic compound (D) is a compound having a structure in which one or two thiol groups are bonded to one or more conjugated heterocyclic rings selected from a thiazole ring, an isothiazole ring, a thiadiazole ring, an imidazole ring, a pyrazole ring, a triazole ring, a pyridine ring, a pyrimidine ring, a pyrazine ring, and a triazine ring;
the content of sulfur (E) is 1.2 parts by mass or less per 100 parts by mass of the rubber component (A),
A rubber composition (excluding those in which the rubber component (A) contains an acrylic rubber) in which the mass ratio of sulfur (E) to a conjugated heterocyclic compound (D) [(E)/(D)] is 0.1 or more and 2 or less. 2. The rubber composition according to claim 1, wherein the conjugated heterocyclic compound (D) is a compound having a structure in which one or two thiol groups are bonded to one or more conjugated heterocyclic rings selected from a thiadiazole ring, a pyridine ring, a pyrimidine ring, and a triazine ring. 3. The rubber composition according to claim 1, wherein the compound having a structure in which one or two thiol groups are bonded to one or more conjugated heterocycles selected from a pyridine ring, a pyrimidine ring, and a triazine ring is a compound represented by the following general formula (I):

(In the formula, ^R1 represents a hydrogen atom, a hydrocarbon group having from 1 to 8 carbon atoms, an SH group, or an amino group which may be substituted with a hydrocarbon group having from 1 to 16 carbon atoms; ^R2 represents a hydrogen atom, an alkyl group having from 1 to 6 carbon atoms, or an SH group; and ^X1 and ^X2 represent a nitrogen atom or a CH group.) The rubber composition according to any one of claims 1 to 3, wherein the silane coupling agent (C) is a polysulfide-based silane coupling agent. The rubber composition according to any one of claims 1 to 4, wherein the content of the conjugated heterocyclic compound (D) is 0.1 parts by mass or more and 5 parts by mass or less per 100 parts by mass of the rubber component (A). The rubber composition according to any one of claims 1 to 5, wherein the ratio of the number of moles of silane coupling agent (C) to the number of moles of thiol groups in conjugated heterocyclic compound (D) [number of moles of silane coupling agent (C) / number of moles of thiol groups in conjugated heterocyclic compound (D)] is 0.1 or more and 3 or less. The rubber composition according to any one of claims 1 to 6, which is produced through a step of kneading a rubber component (A), silica (B), a silane coupling agent (C), and a conjugated heterocyclic compound (D). A rubber molded article obtained by vulcanizing the rubber composition according to any one of claims 1 to 7 .