Rubber composition for sidewalls, and sidewall

Abstract

The present invention provides: a rubber composition for sidewalls, the rubber composition being capable of producing a sidewall that has excellent flex fatigue resistance and is capable of improving low fuel consumption performance; and a sidewall which uses this rubber composition for sidewalls in at least a part thereof.　This rubber composition for sidewalls contains, with respect to 100 parts by mass of a solid rubber (A) that contains natural rubber and / or an isoprene rubber (A1) and a butadiene rubber (A2), 0.1-30 parts by mass of a modified liquid diene rubber (B) that has a functional group derived from a silane compound satisfying a specific condition, and 5-80 parts by mass of a filler (C) that contains a specific silica (C1).

Description

Rubber composition for sidewall and sidewall

[0001] The present invention relates to a rubber composition for a sidewall and a sidewall at least partially using the same.

[0002] Conventionally, rubber compositions containing solid rubber, liquid diene-based rubber, and fillers have been considered as materials for use in various tire components, including treads and sidewalls, because they can maintain or improve various properties while improving processability. In recent years, improvements in fuel economy performance have also been required in tires from the perspective of reducing environmental impact. From the perspective of improving fuel economy performance, rubber compositions containing solid rubber and silica have been considered as materials for use in tire treads. For example, rubber compositions containing solid rubber, a specific modified liquid diene-based rubber, and fillers have been considered as rubber compositions suitable for use in treads of heavy-duty tires for buses, trucks, and other vehicles that are subjected to high loads (see Patent Document 1).

[0003] A tire is composed of various components in addition to the tread described above, and characteristics are desired for each component. For example, the sidewall, which is an example of a tire component, is required to have improved characteristics different from those of the tread, such as weather resistance and flex fatigue resistance. For example, in order to obtain a sidewall that combines weather resistance and appearance, a rubber composition containing a diene rubber component, a reinforcing filler, and a specific liquid diene rubber has been studied (see Patent Document 2).

[0004] International Publication No. 2019 / 044892 Japanese Patent Application Laid-Open No. 2008-120895

[0005] With regard to improving the fuel economy performance of tires, the improvement of the fuel economy performance of the tread has been mainly studied. However, a tire is composed of various components in addition to the tread. Therefore, to further improve the fuel economy performance of a tire, it is desirable to improve the fuel economy performance of tire components other than the tread, such as the sidewall. Generally, rubber compositions that can improve the fuel economy performance of a tire include rubber compositions containing solid rubber and a filler (typically silica). In particular, when such rubber compositions are used for sidewalls, reducing friction between filler particles, which is the main cause of energy loss, is considered to suppress energy loss. To reduce friction, a rubber composition containing a large particle size filler (typically silica) and solid rubber, which has a small surface area where friction occurs, is considered to be effective. However, simply adding a large particle size filler (typically silica) to solid rubber does not necessarily result in sufficient affinity between the solid rubber and the filler, and therefore, the sidewall obtained using such a rubber composition does not necessarily have sufficient flex fatigue resistance. Therefore, there has been a need for a rubber composition that can improve fuel economy performance while maintaining the properties required for sidewalls.

[0006] The present invention has been made in view of the above circumstances, and provides a rubber composition for sidewalls that can be used to produce sidewalls that have excellent flexural fatigue resistance and can also improve fuel economy, and a sidewall that uses the same at least in part.

[0007] As a result of intensive research, the inventors have discovered that by producing a sidewall using at least a part of a rubber composition containing a specific solid rubber, a specific modified liquid diene-based rubber, and a filler, the sidewall obtained from the rubber composition has excellent flexural fatigue resistance and improved fuel economy, and have thus completed the present invention.

[0008] That is, the present invention relates to the following [1] to [6]. [1] A rubber composition for a sidewall, comprising 100 parts by mass of a solid rubber (A) comprising a natural rubber and / or an isoprene rubber (A1) and a butadiene rubber (A2), 0.1 to 30 parts by mass of a modified liquid diene rubber (B) having a functional group derived from a silane compound represented by the following formula (1), and 5 to 80 parts by mass of a filler (C), wherein the filler (C) comprises silica (C1), and the BET specific surface area of ​​the silica (C1) is 170 (m ２ / g), and the modified liquid diene rubber (B) has the following (i) to (iii): (i) a weight average molecular weight (Mw) of 3,000 or more and 120,000 or less, (ii) a vinyl content of 70 mol% or less, and (iii) an average number of functional groups derived from the silane compound per molecule of the modified liquid diene rubber (B) is 1 to 20.

[0009] (In formula (1), R １ is a divalent alkylene group having 1 to 6 carbon atoms, and R ２ , R ３ and R ４ each independently represents a methoxy group, an ethoxy group, a phenoxy group, a methyl group, an ethyl group, or a phenyl group, provided that R ２ , R ３ and R ４ At least one of is a methoxy group, an ethoxy group, or a phenoxy group.

[0010] [2] The rubber composition for a sidewall according to [1], wherein the modified liquid diene rubber (B) has a melt viscosity at 38°C of 0.1 to 4,000 Pa·s. [3] The rubber composition for a sidewall according to [1] or [2], wherein the filler (C) further contains carbon black (C2). [4] The rubber composition for a sidewall according to any one of [1] to [3], wherein the mass ratio (A1) / (A2) of the natural rubber and / or isoprene rubber (A1) to the butadiene rubber (A2) contained in the solid rubber (A) is 20 / 80 or more and 60 / 40 or less. [5] A crosslinked product obtained by crosslinking the rubber composition for a sidewall according to any one of [1] to [4]. [6] A sidewall at least partially using the rubber composition for a sidewall according to any one of [1] to [5].

[0011] According to the present invention, it is possible to provide a rubber composition for sidewalls that can be used to produce sidewalls that have excellent flex fatigue resistance and can also improve fuel economy, and a sidewall that uses the same at least in part.

[0012] [Solid Rubber (A)] The solid rubber (A) used in the rubber composition for a sidewall of the present invention refers to a rubber that can be handled in a solid state at 20°C. The Mooney viscosity ML of the solid rubber (A) at 100°C １＋４ is usually in the range of 20 to 200, and is usually selected from at least one of synthetic rubber and natural rubber. The solid rubber (A) contained in the rubber composition for a sidewall of the present invention contains natural rubber and / or isoprene rubber (A1) and butadiene rubber (A2).

[0013] [Natural Rubber and / or Isoprene Rubber (A1)] Examples of natural rubbers that can be the natural rubber and / or isoprene rubber (A1) (hereinafter also referred to as component (A1)) include unmodified natural rubbers such as TSRs (Technically Specified Rubbers) such as SMR (Malaysian TSR), SIR (Indonesian TSR), and STR (Thai TSR) and natural rubbers commonly used in the tire industry, such as RSS (Ribbed Smoked Sheet), and high-purity natural rubber; and modified natural rubbers such as epoxidized natural rubber, hydroxylated natural rubber, hydrogenated natural rubber, and grafted natural rubber. Of these, unmodified natural rubber is preferred from the standpoint of processability, and SMR20, STR20, and RSS#3 are more preferred from the standpoints of less variation in quality and ease of availability. These natural rubbers may be used alone or in combination of two or more.

[0014] Examples of isoprene rubbers that can be used as component (A1) include Ziegler catalysts such as titanium tetrahalide-trialkylaluminum, diethylaluminum chloride-cobalt, trialkylaluminum-boron trifluoride-nickel, and diethylaluminum chloride-nickel; lanthanoid-based rare earth metal catalysts such as triethylaluminum-organic acid neodymium-Lewis acid; and commercially available isoprene rubbers polymerized using organic alkali metal compounds, similar to S-SBR (solution-polymerized styrene-butadiene rubber). Among these, isoprene rubbers polymerized using Ziegler-based catalysts are preferred because of their high cis content. Furthermore, isoprene rubbers with an ultra-high cis content obtained using lanthanoid-based rare earth metal catalysts may also be used.

[0015] The vinyl content of the isoprene rubber is preferably 50 mol% or less, more preferably 40 mol% or less, and even more preferably 30 mol% or less. If the vinyl content exceeds 50 mol%, rolling resistance performance tends to deteriorate. There is no particular limit on the lower limit of the vinyl content. In addition, the glass transition temperature varies depending on the vinyl content, but is preferably -20°C or less, and more preferably -30°C or less. The vinyl content means the proportion (mol%) of the total of structural units derived from 1,2-bonds or 3,4-bonds (structural units other than 1,4-bonds) out of a total of 100 mol% of structural units derived from isoprene contained in the isoprene rubber.

[0016] The weight average molecular weight (Mw) of the isoprene rubber is preferably 90,000 to 2,000,000, and more preferably 150,000 to 1,500,000. When the Mw is within the above range, the processability and mechanical strength are improved.

[0017] The isoprene rubber may have a branched structure or polar functional groups by using a polyfunctional modifier, such as tin tetrachloride, silicon tetrachloride, an alkoxysilane having an epoxy group in the molecule, or an amino group-containing alkoxysilane, as long as the effects of the present invention are not impaired. From the viewpoint of processability, it is preferable that the isoprene rubber does not have polar functional groups. These isoprene rubbers may be used alone or in combination of two or more.

[0018] Component (A1) may be natural rubber alone, isoprene rubber alone, or a mixture of natural rubber and isoprene rubber. From the viewpoint of mechanical strength, natural rubber is preferred as component (A1).

[0019] [Butadiene Rubber (A2)] Examples of the butadiene rubber (A2) include commercially available butadiene rubbers polymerized using Ziegler catalysts such as titanium tetrahalide-trialkylaluminum catalysts, diethylaluminum chloride-cobalt catalysts, trialkylaluminum-boron trifluoride-nickel catalysts, and diethylaluminum chloride-nickel catalysts; lanthanoid rare earth metal catalysts such as triethylaluminum-organic acid neodymium-Lewis acid catalysts; or organic alkali metal compounds similar to S-SBR. Among these, butadiene rubbers polymerized using Ziegler catalysts are preferred because of their high cis content. Alternatively, butadiene rubbers with an ultra-high cis content (e.g., cis content of 95% or more) obtained using lanthanoid rare earth metal catalysts may be used as the butadiene rubber (A2).

[0020] The vinyl content of the butadiene rubber (A2) is preferably 50 mol% or less, more preferably 40 mol% or less, and even more preferably 30 mol% or less. If the vinyl content exceeds 50 mol%, rolling resistance performance (fuel economy performance) tends to deteriorate. There is no particular limitation on the lower limit of the vinyl content. In addition, the glass transition temperature varies depending on the vinyl content, but is preferably -40°C or less, and more preferably -50°C or less. The vinyl content means the proportion (mol%) of the total of 1,2-bond structural units (structural units other than 1,4-bonds) in 100 mol% of the total of butadiene-derived structural units contained in the butadiene rubber.

[0021] The weight average molecular weight (Mw) of the butadiene rubber (A2) is preferably 90,000 to 2,000,000, and more preferably 150,000 to 1,500,000. When the Mw is within the above range, the processability of the rubber composition for a sidewall is improved, and the flex fatigue resistance of a sidewall partially using the rubber composition for a sidewall is also improved.

[0022] The butadiene rubber (A2) may have a branched structure or a polar functional group formed by using a polyfunctional modifier, such as tin tetrachloride, silicon tetrachloride, an alkoxysilane having an epoxy group in the molecule, or an amino group-containing alkoxysilane, as long as the effects of the present invention are not impaired. From the viewpoint of processability, it is preferable that the butadiene rubber (A2) does not have a polar functional group. These butadiene rubbers (A2) may be used alone or in combination of two or more.

[0023] In order to produce a sidewall having excellent flexural fatigue resistance and improved fuel economy, the total content of the component (A1) and the butadiene rubber (A2) in 100% by mass of the solid rubber (A) is preferably 80% by mass or more, and more preferably 90% by mass or more. From the same viewpoint, it is preferable that the total content of the component (A1) and the butadiene rubber (A2) in 100% by mass of the solid rubber (A) is 100% by mass, i.e., the solid rubber (A) consists only of the component (A1) and the butadiene rubber (A2).

[0024] In order to produce a sidewall having better flexural fatigue resistance and improved fuel economy, the mass ratio (A1) / (A2) of the component (A1) to the butadiene rubber (A2) contained in the solid rubber (A) is preferably 20 / 80 or more and 60 / 40 or less, more preferably 25 / 75 or more and 55 / 45 or less, even more preferably 30 / 70 or more and 50 / 50 or less, and even more preferably 40 / 60 or more and 50 / 50 or less.

[0025] The solid rubber (A) may contain solid rubbers other than the component (A1) and the butadiene rubber (A2) as long as the effects of the present invention are not impaired. Examples of solid rubbers other than the component (A1) and the butadiene rubber (A2) include styrene-butadiene rubber (hereinafter also referred to as "SBR"), butyl rubber, halogenated butyl rubber, ethylene-propylene-diene rubber, butadiene-acrylonitrile copolymer rubber, chloroprene rubber, acrylic rubber, fluororubber, and urethane rubber. When the solid rubber (A) contains solid rubbers other than the component (A1) and the butadiene rubber (A2), the content of the solid rubbers other than the component (A1) and the butadiene rubber (A2) in 100% by mass of the solid rubber (A) is preferably 20% by mass or less, and more preferably 10% by mass or less.

[0026] [Modified Liquid Diene Rubber (B)] The modified liquid diene rubber (B) used in the rubber composition for sidewalls of the present invention is a liquid polymer having functional groups derived from a silane compound represented by the formula (1) described below (hereinafter also referred to as silane compound (1)). The modified liquid diene rubber (B) has a weight-average molecular weight (Mw) of 3,000 to 120,000 (requirement (i)), a vinyl content of 70 mol% or less (requirement (ii)), and an average number of functional groups derived from the silane compound (1) per molecule of the modified liquid diene rubber (B) of 1 to 20 (requirement (iii)). The inclusion of the modified liquid diene rubber (B) in the rubber composition for sidewalls of the present invention improves the dispersibility of the filler (C) in the rubber composition and the interaction between the filler (C) and the solid rubber (A). Furthermore, a sidewall using at least a portion of the rubber composition exhibits good flexural fatigue resistance and improved fuel economy.

[0027] The unmodified liquid diene rubber (B') used as a raw material for the modified liquid diene rubber (B) contains a conjugated diene unit as a monomer unit constituting the polymer. Examples of the conjugated diene include butadiene, isoprene, and conjugated dienes (b1) other than butadiene and isoprene, such as 2,3-dimethylbutadiene, 2-phenylbutadiene, 1,3-pentadiene, 2-methyl-1,3-pentadiene, 1,3-hexadiene, 1,3-octadiene, 1,3-cyclohexadiene, 2-methyl-1,3-octadiene, 1,3,7-octatriene, myrcene, farnesene (α-farnesene, β-farnesene), and chloroprene. The conjugated diene units contained in the unmodified liquid diene rubber (B') preferably contain at least one selected from the group consisting of butadiene units, isoprene units and β-farnesene units, more preferably contain at least one selected from the group consisting of butadiene units and isoprene units, more preferably contain only at least one selected from the group consisting of butadiene units and isoprene units, and even more preferably contain only butadiene units.

[0028] The unmodified liquid diene rubber (B') used as the raw material for the modified liquid diene rubber (B) preferably has a conjugated diene unit content of 50% by mass or more, more preferably 60 to 100% by mass, and even more preferably 70 to 100% by mass, of 100% by mass of all monomer units constituting the polymer. In one preferred embodiment, the unmodified liquid diene rubber (B') contains only conjugated diene units (100% by mass of all monomer units).

[0029] Furthermore, the unmodified liquid diene rubber (B') used as the raw material for the modified liquid diene rubber (B) preferably contains, out of 100% by mass of all monomer units constituting the polymer, 50% by mass or more of monomer units of at least one conjugated diene (b1) selected from the group consisting of butadiene and isoprene. The content of the monomer units of at least one conjugated diene (b1) selected from the group consisting of butadiene and isoprene is preferably 60 to 100% by mass, more preferably 70 to 100% by mass, of all monomer units in the unmodified liquid diene rubber (B').

[0030] Examples of monomer units other than the conjugated diene units that may be contained in the unmodified liquid diene rubber (B') include aromatic vinyl compound (b2) units.

[0031] Examples of the aromatic vinyl compound (b2) include styrene, α-methylstyrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, 4-propylstyrene, 4-t-butylstyrene, 4-cyclohexylstyrene, 4-dodecylstyrene, 2,4-dimethylstyrene, 2,4-diisopropylstyrene, 2,4,6-trimethylstyrene, 2-ethyl-4-benzylstyrene, 4-(phenylbutyl)styrene, 1-vinylnaphthalene, 2-vinylnaphthalene, vinylanthracene, N,N-diethyl-4-aminoethylstyrene, vinylpyridine, 4-methoxystyrene, monochlorostyrene, dichlorostyrene, and divinylbenzene. Among these aromatic vinyl compounds, at least one selected from styrene, α-methylstyrene, and 4-methylstyrene is preferred.

[0032] When the unmodified liquid diene rubber (B') contains two or more types of monomer units, the bonding mode thereof may be a random copolymer or a block copolymer. When the unmodified liquid diene rubber (B') is a block copolymer, the block copolymer preferably contains a polymer block consisting of only butadiene units and a polymer block consisting of only isoprene units. In the case of this block copolymer, the content of butadiene units in 100% by mass of all monomer units contained in the unmodified liquid diene rubber (B') is preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less.

[0033] The content of monomer units other than conjugated dienes, such as aromatic vinyl compound (b2), in the unmodified liquid diene rubber (B') is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or less, based on 100% by mass of all monomer units contained in the unmodified liquid diene rubber (B'). For example, when the aromatic vinyl compound (b2) units are within the above range or less, the processability of the rubber composition tends to be improved.

[0034] The unmodified liquid diene rubber (B') is preferably a polymer obtained by polymerizing a conjugated diene and, if necessary, other monomers other than the conjugated diene, for example, by emulsion polymerization or solution polymerization.

[0035] As the emulsion polymerization method, known methods or methods equivalent to known methods can be applied. For example, a predetermined amount of a monomer containing a conjugated diene is emulsified and dispersed in the presence of an emulsifier, and emulsion polymerized using a radical polymerization initiator.

[0036] Examples of emulsifiers include salts of long-chain fatty acids having 10 or more carbon atoms, rosinate salts, etc. Examples of long-chain fatty acid salts include potassium salts or sodium salts of fatty acids such as capric acid, lauric acid, myristic acid, palmitic acid, oleic acid, and stearic acid.

[0037] As the dispersion medium, water is usually used, and a water-soluble organic solvent such as methanol or ethanol may be contained within a range that does not impair stability during polymerization. Examples of the radical polymerization initiator include persulfates such as ammonium persulfate and potassium persulfate, organic peroxides, and hydrogen peroxide.

[0038] A chain transfer agent may be used to adjust the molecular weight of the resulting unmodified liquid diene rubber (B'). Examples of the chain transfer agent include mercaptans such as t-dodecyl mercaptan and n-dodecyl mercaptan; carbon tetrachloride, thioglycolic acid, diterpenes, terpinolene, γ-terpinene, and α-methylstyrene dimer.

[0039] The temperature of the emulsion polymerization can be appropriately set depending on the type of radical polymerization initiator used, but is usually in the range of 0 to 100° C., preferably 0 to 60° C. The polymerization method may be either continuous polymerization or batch polymerization.

[0040] The polymerization reaction can be terminated by adding a polymerization terminator, such as an amine compound such as isopropylhydroxylamine, diethylhydroxylamine, or hydroxylamine, a quinone compound such as hydroquinone or benzoquinone, or sodium nitrite.

[0041] After the polymerization reaction is terminated, an antioxidant may be added as needed. After the polymerization reaction is terminated, unreacted monomers are removed from the obtained latex as needed. Next, the unmodified liquid diene rubber (B') is coagulated using a salt such as sodium chloride, calcium chloride, or potassium chloride as a coagulant, and an acid such as nitric acid or sulfuric acid is added as needed to adjust the pH of the coagulation system to a predetermined value. The dispersion medium is then separated to recover the polymer. The unmodified liquid diene rubber (B') is then obtained by washing with water, dehydrating, and drying. Note that, during the coagulation, the latex and an extender oil previously prepared as an emulsified dispersion may be mixed as needed, and the oil-extended unmodified liquid diene rubber (B') may be recovered.

[0042] As the solution polymerization method, a known method or a method equivalent to a known method can be applied. For example, a monomer containing a conjugated diene is polymerized in a solvent using a Ziegler catalyst, a metallocene catalyst, or an anionically polymerizable active metal or active metal compound, optionally in the presence of a polar compound.

[0043] Examples of the solvent include aliphatic hydrocarbons such as n-butane, n-pentane, isopentane, n-hexane, n-heptane, and isooctane; alicyclic hydrocarbons such as cyclopentane, cyclohexane, and methylcyclopentane; and aromatic hydrocarbons such as benzene, toluene, and xylene.

[0044] Examples of the anionically polymerizable active metal include alkali metals such as lithium, sodium, and potassium; alkaline earth metals such as beryllium, magnesium, calcium, strontium, and barium; and lanthanoid rare earth metals such as lanthanum and neodymium. Among the anionically polymerizable active metals, alkali metals and alkaline earth metals are preferred, and alkali metals are more preferred.

[0045] The anionically polymerizable active metal compound is preferably an organic alkali metal compound. Examples of the organic alkali metal compound include organic monolithium compounds such as methyllithium, ethyllithium, n-butyllithium, sec-butyllithium, t-butyllithium, hexyllithium, phenyllithium, and stilbenelithium; polyfunctional organic lithium compounds such as dilithiomethane, dilithionaphthalene, 1,4-dilithiobutane, 1,4-dilithio-2-ethylcyclohexane, and 1,3,5-trilithiobenzene; sodium naphthalene, potassium naphthalene, and the like. Among these organic alkali metal compounds, organic lithium compounds are preferred, and organic monolithium compounds are more preferred.

[0046] The amount of the organic alkali metal compound used can be appropriately determined depending on the melt viscosity, molecular weight, etc. of the unmodified liquid diene rubber (B') and the modified liquid diene rubber (B). For example, the organic alkali metal compound is usually used in an amount of 0.01 to 3 parts by mass per 100 parts by mass of all monomers including the conjugated diene.

[0047] The above organic alkali metal compounds can also be reacted with a secondary amine such as dibutylamine, dihexylamine, or dibenzylamine to form organic alkali metal amides.

[0048] In anionic polymerization, polar compounds are usually used to adjust the microstructure of the conjugated diene units (e.g., vinyl content) without deactivating the reaction. Examples of polar compounds include ether compounds such as dibutyl ether, tetrahydrofuran, and ethylene glycol diethyl ether; tertiary amines such as N,N,N',N'-tetramethylethylenediamine and trimethylamine; alkali metal alkoxides; and phosphine compounds. The polar compound is usually used in an amount of 0.01 to 1,000 moles per mole of the organic alkali metal compound.

[0049] The temperature for solution polymerization is usually in the range of −80 to 150° C., preferably in the range of 0 to 100° C., and more preferably in the range of 10 to 90° C. The polymerization method may be either continuous polymerization or batch polymerization.

[0050] The polymerization reaction can be terminated by adding a polymerization terminator. Examples of the polymerization terminator include alcohols such as methanol and isopropanol. The unmodified liquid diene rubber (B') can be isolated by pouring the resulting polymerization reaction solution into a poor solvent such as methanol to precipitate the unmodified liquid diene rubber (B'), or by washing the polymerization reaction solution with water, separating it, and then drying it. Of the above methods, the solution polymerization method is preferred as a method for producing the unmodified liquid diene rubber (B').

[0051] The unmodified liquid diene rubber (B') thus obtained is excellent in flexural fatigue resistance and, in order to facilitate the production of sidewalls that can improve fuel economy, it is desirable to modify it as is (without hydrogenation) with a functional group derived from a silane compound represented by formula (1) described below.

[0052] Furthermore, in order to more favorably exhibit the properties of the functional group derived from the silane compound represented by formula (1) described below, it is preferable that the unmodified liquid diene rubber (B') is not modified with a functional group (e.g., a hydroxyl group) other than the functional group derived from the silane compound represented by formula (1) described below. Since the unmodified liquid diene rubber (B') is not modified with other functional groups, the stability of the resulting modified liquid diene rubber (B) tends to be superior. Furthermore, the interaction (e.g., reactivity) of the functional group derived from the silane compound represented by formula (1) of the resulting modified liquid diene rubber (B) with the filler (C) (e.g., silica) tends to be superior.

[0053] The unmodified liquid diene rubber (B') is modified with a functional group derived from a silane compound represented by the following formula (1) (hereinafter also referred to as silane compound (1)) and used as a modified liquid diene rubber (B). That is, the modified liquid diene rubber (B) has a functional group derived from a silane compound represented by the following formula (1) (silane compound (1)).

[0054] In the above formula (1), R １ is a divalent alkylene group having 1 to 6 carbon atoms. Examples of the divalent alkylene group having 1 to 6 carbon atoms include a methylene group, an ethylene group, a propylene group, a butylene group, a pentylene group, and a hexylene group. R ２ , R ３ and R ４ each independently represents a methoxy group, an ethoxy group, a phenoxy group, a methyl group, an ethyl group, or a phenyl group, provided that R ２ , R ３ and R ４ At least one of the groups is a methoxy group, an ethoxy group, or a phenoxy group.

[0055] Examples of the silane compound (1) include mercaptomethylenemethyldiethoxysilane, mercaptomethylenetriethoxysilane, 2-mercaptoethyltrimethoxysilane, 2-mercaptoethyltriethoxysilane, 2-mercaptoethylmethoxydimethylsilane, 2-mercaptoethylethoxydimethylsilane, 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 3-mercaptopropyldimethoxymethylsilane, 3-mercaptopropyldiethoxymethylsilane, 3-mercaptopropyldimethoxyethylsilane, 3-mercaptopropyldiethoxyethylsilane, 3-mercaptopropylmethoxydimethylsilane, 3-mercaptopropylethoxydimethylsilane, etc. These silane compounds may be used alone or in combination of two or more.

[0056] The mercapto group (—SH) of the silane compound (1) undergoes a radical addition reaction with a carbon-carbon unsaturated bond contained in the unmodified liquid diene rubber (B′), thereby obtaining a modified liquid diene rubber (B) having a functional group derived from the silane compound (1), specifically, a partial structure represented by the following formula (2), as a functional group:

[0057] R in the above formula (2) １ , R ２ , R ３ and R ４ The definition and specific examples of R in formula (1) １ , R ２ , R ３ and R ４ The definition and specific examples are the same as those of the above.

[0058] The average number of functional groups per molecule of the modified liquid diene rubber (B) having functional groups derived from the silane compound (1) is 1 to 20 (requirement (iii)). If the average number of functional groups is less than 1, the affinity with the filler (C) is low, the filler dispersibility in the rubber composition cannot be improved, and it is difficult to produce a sidewall that is excellent in flexural fatigue resistance and that can also improve fuel economy. On the other hand, if the average number of functional groups exceeds 20, it is difficult to produce a sidewall that is excellent in flexural fatigue resistance using the rubber composition. Since the resulting sidewall tends to have better flexural fatigue resistance and improved fuel economy, the average number of functional groups per molecule of the modified liquid diene rubber (B) having functional groups derived from the silane compound (1) is preferably 1 to 15, more preferably 1 to 12, even more preferably 1 to 9, even more preferably 1 to 5, particularly preferably 1 to 4, and more particularly preferably 2 to 4.

[0059] The average number of functional groups per molecule of the modified liquid diene rubber (B) can be calculated using the equivalent weight (g / eq) of the functional groups of the modified liquid diene rubber (B) and the number average molecular weight Mn in terms of styrene by the following formula: (Average number of functional groups per molecule) = [(Number average molecular weight Mn) ÷ (Molecular weight of styrene units) × (Average molecular weight of conjugated diene and other monomer units other than conjugated diene contained as needed)] / (Equivalent weight of functional groups)

[0060] The equivalent weight of the functional group of the modified liquid diene rubber (B) means the mass of butadiene bonded to one functional group and the mass of other monomers other than butadiene that may be contained as needed. １ H-NMR or １３ Using C-NMR, it can be calculated from the area ratio of the peak derived from the functional group of the modified liquid diene rubber (B) to the peak derived from the main chain of the modified liquid diene rubber (B). The peak derived from the functional group refers to the peak derived from the alkoxy group contained in the group derived from the silane compound (1) contained in the modified liquid diene rubber (B).

[0061] The amount of silane compound (1) added in the modified liquid diene rubber (B) is preferably 1 to 60 parts by mass, more preferably 1 to 50 parts by mass, and even more preferably 1 to 40 parts by mass, per 100 parts by mass of the unmodified liquid diene rubber (B'). If the amount added is more than 60 parts by mass, the effect of improving the flex fatigue resistance of the resulting sidewall tends to be impaired. If the amount added is less than 1 part by mass, the dispersion effect of the filler (C) is poor, and the flex fatigue resistance of the resulting sidewall tends to be inferior. The amount of silane compound (1) added in the modified liquid diene rubber (B) can be determined using various analytical instruments such as NMR.

[0062] The method for adding the silane compound (1) to the unmodified liquid diene rubber (B') is not particularly limited. For example, a method can be employed in which the silane compound (1) and, if necessary, a radical generator are added to the unmodified liquid diene rubber (B'), and the mixture is heated in the presence or absence of an organic solvent. There are no particular limitations on the radical generator used, and commercially available organic peroxides, azo compounds, hydrogen peroxide, etc. can be used. It is not desirable to carry out the reaction of adding the silane compound (1) to the unmodified liquid diene rubber (B') by heating alone without using a radical generator. For example, if the heating temperature is too low, the addition reaction may not proceed sufficiently, and the average number of functional groups per molecule may not be within the desired range. Furthermore, if the heating temperature is increased, the addition reaction may proceed, but the generation of radicals on the polymer main chain may also simultaneously proceed with the polymer polymerization reaction. Therefore, if the Mw of the modified liquid diene rubber is not within the desired range, the viscosity of the modified liquid diene rubber may not be within the desired range. In these cases where the temperature during the addition reaction is high, the handleability of the modified liquid diene rubber may deteriorate, and the physical properties of the resulting rubber composition for sidewalls may be adversely affected. On the other hand, when the addition reaction is carried out by adding a radical generator and heating, the addition reaction proceeds sufficiently while side reactions such as polymerization reactions are sufficiently suppressed even at a relatively low heating temperature.

[0063] When the maximum peak molecular weight in terms of polystyrene obtained from GPC measurement of the modified liquid diene rubber (B) is Mt, and the total area derived from the modified liquid diene rubber (B) in the GPC chromatogram obtained by GPC measurement of the modified liquid diene rubber (B) is 100%, the proportion of the modified liquid diene rubber (B) in the region of molecular weight Mt × 1.45 or more is preferably in the range of 0 to 30%, more preferably in the range of 0 to 20%, even more preferably in the range of 0 to 18%, even more preferably in the range of 0 to 15%, particularly preferably in the range of 0 to 10%, and even more particularly preferably in the range of 0 to 8%. By compounding such modified liquid diene rubber (B) in a rubber composition, the processability of the rubber composition is improved, and the interaction with the filler (C) described below in the obtained rubber composition is improved, so the dispersion state of the filler (C) in the rubber composition is excellent, and it is estimated that this is ideal for improving the physical properties of the obtained sidewall (e.g., improved fuel economy performance, improved flex fatigue resistance).

[0064] Examples of the organic peroxides include methyl ethyl ketone peroxide, cyclohexanone peroxide, 3,3,5-trimethylcyclohexanone peroxide, methylcyclohexanone peroxide, acetylacetone peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)-cyclohexane, 1,1-bis(t-hexylperoxy)cyclohexane, 2,2-bis(t-butylperoxy)butane, t-butyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, paramenthane hydroperoxide, 2,5-dimethylhexane 2,5-dihydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, di-t-butyl peroxide, t-butylcumyl peroxide, dicumyl peroxide, bis(t-butylperoxyisopropyl ) benzene, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 2,5-hexanoyl peroxide, lauroyl peroxide, succinic peroxide, benzoyl peroxide and its substituted derivatives, 2,4-dichlorobenzoyl peroxide, meta-toluoyl peroxide, diisopropyl peroxydicarbonate, t-butyl-2-ethylhexanoate, di-2-ethylhexyl peroxydicarbonate, dimethoxyisopropyl peroxycarbonate Examples of peroxyalkylene compounds include tert-butyl peroxycarbonate, di(3-methyl-3-methoxybutyl)peroxydicarbonate, t-butyl peroxyacetate, t-butyl peroxypivalate, t-butyl peroxyneodecanoate, t-butyl peroxyoctanoate, t-butyl peroxy3,3,5-trimethylhexanoate, t-butyl peroxylaurate, t-butyl peroxycarbonate, t-butyl peroxybenzoate, and t-butyl peroxyisobutyrate.

[0065] Examples of the azo compounds include 2,2'-azobisisobutyronitrile, 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethyl-4-methoxyvaleronitrile), 2,2'-azobis(2-(2-imidazolin-2-yl)propane), 2,2'- Examples include azobis(2,4,4-trimethylpentane), 2,2'-azobis(2-methylpropane), 2,2'-azobis(2-hydroxymethylpropionitrile), 4,4'-azobis(4-cyanovaleric acid), dimethyl 2,2'-azobis(2-methylpropionate), 2-cyano-2-propylazoformamide, and 2-phenylazo-4-methoxy-2,4-dimethylvaleronitrile.

[0066] The organic solvent used in the above method generally includes hydrocarbon solvents and halogenated hydrocarbon solvents, and among these, hydrocarbon solvents such as n-butane, n-hexane, n-heptane, cyclohexane, benzene, toluene, and xylene are preferred.

[0067] Furthermore, when carrying out the addition reaction of the silane compound (1) by the above method, an antioxidant may be added from the viewpoint of suppressing side reactions, etc. Preferred antioxidants for use in this case include, for example, 2,6-di-t-butyl-4-methylphenol (BHT), 2,2'-methylenebis(4-methyl-6-t-butylphenol), 4,4'-thiobis(3-methyl-6-t-butylphenol), 4,4'-butylidenebis(3-methyl-6-t-butylphenol) (AO-40), 3,9-bis[1,1-dimethyl-2-[3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionyl] ... pionyloxy]ethyl]-2,4,8,10-tetraoxaspiro[5.5]undecane (AO-80), 2,4-bis[(octylthio)methyl]-6-methylphenol (Irganox 1520L), 2,4-bis[(dodecylthio)methyl]-6-methylphenol (Irganox 1726), 2-[1-(2-hydroxy-3,5-di-t-pentylphenyl)ethyl]-4,6-di-t-pentylphenyl acrylate (Sumilizer G S), 2-t-butyl-6-(3-t-butyl-2-hydroxy-5-methylbenzyl)-4-methylphenyl acrylate (Sumilizer GM), 6-t-butyl-4-[3-(2,4,8,10-tetra-t-butyldibenzo[d,f][1,3,2]dioxaphosphepin-6-yloxy)propyl]-2-methylphenol (Sumilizer GP), tris(2,4-di-t-butylphenyl) phosphite (Irgafos 168), di Examples of the antioxidant include octadecyl 3,3'-dithiobispropionate, hydroquinone, p-methoxyphenol, N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine (Nocrac 6C), bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate (LA-77Y), N,N-dioctadecylhydroxylamine (Irgastab FS042), bis(4-t-octylphenyl)amine (Irganox 5057), etc. The antioxidants may be used alone or in combination of two or more.

[0068] The amount of antioxidant added is preferably 0 to 10 parts by mass, more preferably 0 to 5 parts by mass, per 100 parts by mass of the unmodified liquid diene rubber (B'). In this modified liquid diene rubber (B), the position of the functional group to be introduced may be the polymerization terminal or a side chain of the polymer chain. From the viewpoint of easily introducing multiple functional groups, a side chain of the polymer chain is preferable. Furthermore, the functional group may be contained alone or in combination with two or more types. Therefore, the modified liquid diene rubber (B) may be modified with one type of silane compound (1), or may be modified with two or more types of compounds.

[0069] The mixing ratio of the unmodified liquid diene rubber (B') and the silane compound (1) may be appropriately set so that the average number of functional groups per molecule of the modified liquid diene rubber (B) is a desired value. For example, the unmodified liquid diene rubber (B') and the silane compound (1) may be mixed so that the mass ratio (B') / (1) of the unmodified liquid diene rubber (B') to the silane compound (1) is 0.3 to 300.

[0070] An effective method for producing a modified liquid diene rubber (B) having specific properties is to carry out a radical addition reaction of the silane compound (1) at an appropriate reaction temperature for a sufficient reaction time. For example, the temperature in the reaction of adding the silane compound (1) to the unmodified liquid diene rubber (B') is preferably 10°C to 200°C, more preferably 50°C to 180°C, and even more preferably 50°C to 140°C. The reaction time is preferably 1 to 200 hours, more preferably 1 to 100 hours, even more preferably 1 to 50 hours, and even more preferably 1 to 25 hours.

[0071] The weight average molecular weight (Mw) of the modified liquid diene rubber (B) is 3,000 or more and 120,000 or less (requirement (i)). In the present invention, the Mw of the modified liquid diene rubber (B) is the weight average molecular weight in terms of polystyrene determined by measurement using gel permeation chromatography (GPC). When the Mw of the modified liquid diene rubber (B) is within the above range, the processability during production is excellent and the economic efficiency is good. Furthermore, the processability of the rubber composition of the present invention is good, the dispersibility of the filler (C) is excellent, and the physical properties of the resulting sidewall (for example, improved fuel economy and improved flex fatigue resistance) are possible.

[0072] From the viewpoint of improving the fuel economy performance and flex fatigue resistance of the resulting sidewall, the Mw of the modified liquid diene rubber (B) is preferably 4,000 to 80,000, more preferably 4,500 to 60,000, even more preferably 5,000 to 40,000, even more preferably 5,500 to 35,000, even more preferably 5,500 to 12,000, and particularly preferably 5,500 to 9,000. The Mw of the modified liquid diene rubber (B) is determined by GPC (gel permeation chromatography) in terms of standard polystyrene equivalent molecular weight. Specifically, it can be determined by the method described in the examples.

[0073] The molecular weight distribution (Mw / Mn) of the modified liquid diene rubber (B) is preferably 1.0 to 20.0, more preferably 1.0 to 15.0, even more preferably 1.0 to 10.0, even more preferably 1.0 to 5.0, and particularly preferably 1.0 to 2.0. When Mw / Mn is within the above range, the viscosity variation of the obtained modified liquid diene rubber (B) is small, which is more preferable. The molecular weight distribution (Mw / Mn) means the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) in terms of standard polystyrene, determined by GPC measurement.

[0074] The vinyl content of the modified liquid diene rubber (B) is 70 mol% or less (requirement (ii)). If the vinyl content exceeds 70 mol%, fuel economy tends to deteriorate. From the viewpoint of fuel economy, the vinyl content of the modified liquid diene rubber (B) is preferably 65 mol% or less, more preferably 60 mol% or less, even more preferably 50 mol% or less, even more preferably 45 mol% or less, particularly preferably 40 mol% or less, and more particularly preferably 30 mol% or less. The vinyl content of the modified liquid diene rubber (B) is preferably 3 mol% or more, more preferably 5 mol% or more, even more preferably 7 mol% or more, and even more preferably 10 mol% or more. The preferred numerical range of the vinyl content of the modified liquid diene rubber (B) can be set by appropriately combining the above-mentioned upper and lower limits. In the present invention, the "vinyl content" of the modified liquid diene rubber (B) means the total mol% of conjugated diene units bonded via 1,2-bonds, 3,4-bonds (in the case of other than farnesene), and 3,13-bonds (in the case of farnesene) (i.e., conjugated diene units bonded via bonds other than 1,4-bonds (in the case of other than farnesene) and 1,13-bonds (in the case of farnesene)) relative to the total 100 mol% of conjugated diene units contained in the modified liquid diene rubber (B). The vinyl content is determined by the ratio of the total mol% of the conjugated diene units bonded via 1,2-bonds, 3,4-bonds (in the case of farnesene), and 3,13-bonds (in the case of farnesene) to the total mol% of the conjugated diene units contained in the modified liquid diene rubber (B). １ Analysis is performed using H-NMR, and the ratio is calculated from the area ratio of the peaks derived from conjugated diene units bonded via 1,2-bonds, 3,4-bonds (in the case of compounds other than farnesene), and 3,13-bonds (in the case of farnesene) to the area ratio of the peaks derived from conjugated diene units bonded via 1,4-bonds (in the case of compounds other than farnesene) and 1,13-bonds (in the case of farnesene).

[0075] The vinyl content of the modified liquid diene rubber (B) can be adjusted to a desired value by controlling, for example, the type of solvent used in producing the unmodified liquid diene rubber (B'), the polar compound used as needed, the polymerization temperature, etc.

[0076] The melt viscosity of the modified liquid diene rubber (B) measured at 38°C is preferably 0.1 to 4,000 Pa·s, more preferably 0.1 to 2,000 Pa·s, even more preferably 0.1 to 1,000 Pa·s, even more preferably 0.1 to 500 Pa·s, and particularly preferably 0.1 to 200 Pa·s. When the melt viscosity of the modified liquid diene rubber (B) is within the above range, the flexibility of the resulting rubber composition is improved, thereby improving processability. In the present invention, the melt viscosity of the liquid diene rubber (B) is a value measured at 38°C using a Brookfield viscometer.

[0077] The glass transition temperature (Tg) of the modified liquid diene rubber (B) may vary depending on the vinyl content of the conjugated diene units, the type of conjugated diene, the content of units derived from monomers other than conjugated dienes, etc., but is preferably −150 to 50° C., more preferably −130 to 50° C., even more preferably −130 to 30° C., and even more preferably −100 to 0° C. When the Tg is within the above range, for example, the flex fatigue resistance of the sidewall obtained from the rubber composition is improved. In addition, an increase in viscosity can be suppressed, making handling easier.

[0078] The modified liquid diene rubber (B) may be used alone or in combination of two or more. The modified liquid diene rubber (B) preferably has a catalyst residue amount derived from the polymerization catalyst used in its production in the range of 0 to 200 ppm, calculated as metal. For example, when an organic alkali metal such as an organolithium compound is used as a polymerization catalyst to produce the unmodified liquid diene rubber (B'), which is the raw material for the modified liquid diene rubber (B), the metal used as the basis for the catalyst residue amount is an alkali metal such as lithium. Having the catalyst residue amount within the above range prevents a decrease in tackiness during processing, and improves the heat resistance and tire rolling resistance performance of the cross-linked product obtained from the rubber composition of the present invention. The catalyst residue amount derived from the polymerization catalyst used in the production of the modified liquid diene rubber (B) is preferably 0 to 150 ppm, and even more preferably 0 to 100 ppm, calculated as metal. The catalyst residue amount can be measured, for example, using a polarized Zeeman atomic absorption spectrophotometer.

[0079] Examples of methods for adjusting the amount of catalyst residue in the liquid diene rubber to such a specific amount include purifying the modified liquid diene rubber (B) or the unmodified liquid diene rubber (B') used as a raw material and thoroughly removing the catalyst residue. As a purification method, washing with water or hot water, or an organic solvent such as methanol or acetone, or supercritical fluid carbon dioxide is preferred. From an economical standpoint, the number of washes is preferably 1 to 20 times, more preferably 1 to 10 times. Furthermore, the washing temperature is preferably 20 to 100°C, more preferably 40 to 90°C. Furthermore, by removing impurities that inhibit polymerization using distillation or an adsorbent before the polymerization reaction and then increasing the purity of the monomer and solvent, the amount of polymerization catalyst required can be reduced, thereby reducing the amount of catalyst residue. From the same viewpoint as above, the amount of catalyst residue in the rubber composition for a sidewall of the present invention containing the solid rubber (A), the modified liquid diene rubber (B), and the filler (C) is preferably 0 to 200 ppm, more preferably 0 to 150 ppm, and even more preferably 0 to 100 ppm, calculated as metal. The catalyst residue in this case may be catalyst residue derived from a polymerization catalyst used in producing one or more of the solid rubber (A), the modified liquid diene rubber (B), and the other optional components contained in the rubber composition for a sidewall of the present invention.

[0080] In the rubber composition of the present invention, the content of the modified liquid diene rubber (B) per 100 parts by mass of the solid rubber (A) is 0.1 to 30 parts by mass, preferably 0.5 to 25 parts by mass, more preferably 1 to 25 parts by mass, even more preferably 2 to 25 parts by mass, still more preferably 5 to 20 parts by mass, and even more preferably 5 to 15 parts by mass. When the content of the modified liquid diene rubber (B) is within the above range, the dispersibility of the filler (C) in the rubber composition is improved, and the resulting sidewall has good flex fatigue resistance and improved fuel economy.

[0081] [Filler (C)] The filler (C) used in the rubber composition for a sidewall of the present invention can be any filler commonly used in rubber compositions for a sidewall without any particular limitation, but from the viewpoint of improving the fuel economy performance of the sidewall, the filler (C) contains silica (C1).

[0082] Examples of the silica (C1) include wet silica (hydrated silicic acid), dry silica (anhydrous silicic acid), calcium silicate, aluminum silicate, etc. Among these silicas, wet silica is preferred from the viewpoint of further improving processability and fuel economy of the resulting sidewall.

[0083] From the viewpoint of resistance to bending fatigue as a sidewall, the BET specific surface area of ​​the silica (C1) is 170 (m ２ The BET specific surface area of ​​the silica (C1) is less than 130 (m ２ / g), and preferably less than 110 (m ２ The BET specific surface area of ​​silica (C1) is more preferably less than 50 (m ２ / g) or more, and ２ / g) or more, and ２ The BET specific surface area is a value obtained by the BET method in accordance with ASTM D3037-81.

[0084] From the viewpoint of improving the processability of the rubber composition for sidewalls and the fuel economy performance of the sidewalls, the average particle size of the silica (C1) is preferably 10 nm or more, more preferably 15 nm or more, even more preferably 20 nm or more, and even more preferably 25 nm or more. It is preferably 100 nm or less, more preferably 80 nm or less, even more preferably 60 nm or less, and even more preferably 40 nm or less. The average particle size of the silica can be determined by measuring the diameter of each particle in a field of view observed with a transmission electron microscope and calculating the average value. These silicas (C1) may be used alone or in combination of two or more.

[0085] From the viewpoint of improving physical properties such as mechanical strength and weather resistance of the sidewall, the filler (C) used in the rubber composition for a sidewall of the present invention preferably contains carbon black (C2). That is, the filler (C) used in the rubber composition for a sidewall of the present invention preferably contains silica (C1) and carbon black (C2).

[0086] Examples of the carbon black (C2) include furnace black, channel black, thermal black, acetylene black, and ketjen black. Among these carbon blacks, furnace black is preferred from the viewpoints of improving the crosslinking rate and improving the mechanical strength of the resulting sidewall.

[0087] From the viewpoint of improving the mechanical strength and fuel economy of a sidewall at least partially using the rubber composition for a sidewall, the average particle size of the carbon black (C2) is preferably 5 nm or more, more preferably 10 nm or more, and even more preferably 15 nm or more. The average particle size is preferably 100 nm or less, more preferably 80 nm or less, even more preferably 70 nm or less, and even more preferably 60 nm or less. The average particle size of the carbon black can be determined by measuring the diameter of each particle in a field of view observed with a transmission electron microscope and calculating the average value.

[0088] Commercially available furnace black products include, for example, "Diablack" manufactured by Mitsubishi Chemical Corporation and "Seast" manufactured by Tokai Carbon Co., Ltd. Commercially available acetylene black products include, for example, "Denka Black" manufactured by Denki Kagaku Kogyo Co., Ltd. Commercially available ketjen black products include, for example, "ECP600JD" manufactured by Lion Corporation.

[0089] From the viewpoint of improving wettability and dispersibility in the solid rubber (A), the carbon black (C2) may be subjected to an acid treatment with nitric acid, sulfuric acid, hydrochloric acid, or a mixture thereof, or a surface oxidation treatment by heat treatment in the presence of air. Furthermore, from the viewpoint of improving the mechanical strength of the rubber composition for a sidewall of the present invention and the sidewall obtained from this composition, the carbon black (C2) may be subjected to a heat treatment at 2,000 to 3,000°C in the presence of a graphitization catalyst. Examples of the graphitization catalyst include boron, boron oxide (e.g., B ２ O ２ , B ２ O ３ , B ４ O ３ , B ４ O ５ etc.), boron oxoacids (e.g., orthoboric acid, metaboric acid, tetraboric acid, etc.) and their salts, boron carbides (e.g., B ４ C, B ６ C, etc.), boron nitride (BN), and other boron compounds are preferably used.

[0090] The carbon black (C2) can also be used after adjusting its particle size by pulverization or the like. For pulverization of carbon black, a high-speed rotary pulverizer (hammer mill, pin mill, cage mill), various ball mills (tumbling mill, vibration mill, planetary mill), stirring mill (bead mill, attritor, flow-tube mill, annular mill), or the like can be used. These carbon blacks (C2) may be used alone or in combination of two or more.

[0091] In the present invention, the rubber composition for a sidewall may contain, as the filler (C), a filler other than silica (C1) and carbon black (C2), for the purposes of improving the properties of the resulting sidewall, such as increasing the mechanical strength, and improving the production cost by compounding the filler as an extender.

[0092] Examples of fillers that can be used other than silica (C1) and carbon black (C2) include organic fillers and inorganic fillers such as clay, talc, mica, calcium carbonate, magnesium hydroxide, aluminum hydroxide, barium sulfate, titanium oxide, glass fiber, fibrous fillers, and glass balloons. These fillers may be used alone or in combination of two or more.

[0093] The amount of filler (C) per 100 parts by mass of solid rubber (A) is 5 to 80 parts by mass, preferably 20 to 60 parts by mass. When the amount of filler (C) is within this range, the flex fatigue resistance and fuel economy of the sidewall obtained from the rubber composition for a sidewall of the present invention are improved.

[0094] Furthermore, when silica (C1) is used as the filler (C), the amount of silica (C1) per 100 parts by mass of the solid rubber (A) is, from the viewpoint of improving the fuel economy of the sidewall obtained from the rubber composition for a sidewall, preferably 10 parts by mass or more, more preferably 15 parts by mass or more, even more preferably 20 parts by mass or more, still more preferably 25 parts by mass or more, and particularly preferably 30 parts by mass or more. It is also preferably 60 parts by mass or less, more preferably 50 parts by mass or less, and even more preferably 40 parts by mass or less. From the above, the amount of silica (C1) per 100 parts by mass of the solid rubber (A) is preferably 10 parts by mass or more and 60 parts by mass or less, more preferably 15 parts by mass or more and 50 parts by mass or less, even more preferably 20 parts by mass or more and 40 parts by mass or less, and even more preferably 25 parts by mass or more and 40 parts by mass or less.

[0095] Furthermore, when carbon black (C2) is used as the filler (C), the amount of carbon black (C2) per 100 parts by mass of the solid rubber (A) is preferably 1 part by mass or more, more preferably 1.5 parts by mass or more, and even more preferably 2 parts by mass or more, from the viewpoint of improving the mechanical strength and weather resistance of the sidewall obtained from the rubber composition for a sidewall of the present invention, and is preferably 20 parts by mass or less, more preferably 15 parts by mass or less, and even more preferably 10 parts by mass or less.

[0096] When silica (C1) and carbon black (C2) are used in combination as the filler (C), the ratio of silica (C1) to carbon black (C2) (mass ratio = (C1) / (C2)) is preferably 1 / 99 to 99 / 1, more preferably 10 / 90 to 97 / 3, even more preferably 30 / 70 to 95 / 5, still more preferably 50 / 50 to 95 / 5, and particularly preferably 70 / 30 to 95 / 5, from the viewpoint of improving the flex fatigue resistance and further improving the fuel economy of the sidewall obtained from the rubber composition for a sidewall of the present invention.

[0097] [Other Components] Since the rubber composition for a sidewall of the present invention contains silica (C1) as the filler (C), it is preferable to contain a silane coupling agent. Examples of the silane coupling agent include sulfide compounds, mercapto compounds, vinyl compounds, amino compounds, glycidoxy compounds, nitro compounds, and chloro compounds.

[0098] Examples of sulfide compounds include bis(3-triethoxysilylpropyl)tetrasulfide, bis(2-triethoxysilylethyl)tetrasulfide, bis(3-trimethoxysilylpropyl)tetrasulfide, bis(2-trimethoxysilylethyl)tetrasulfide, bis(3-triethoxysilylpropyl)trisulfide, bis(3-trimethoxysilylpropyl)trisulfide, bis(3-triethoxysilylpropyl)disulfide, bis(3-trimethoxysilylpropyl)disulfide, and 3-trimethoxysilylpropyl-N,N-dimethylsilyl. thiocarbamoyl tetrasulfide, 3-triethoxysilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide, 2-trimethoxysilylethyl-N,N-dimethylthiocarbamoyl tetrasulfide, 3-trimethoxysilylpropyl benzothiazole tetrasulfide, 3-triethoxysilylpropyl benzothiazole tetrasulfide, 3-triethoxysilylpropyl methacrylate monosulfide, 3-trimethoxysilylpropyl methacrylate monosulfide, 3-octanoylthio-1-propyltriethoxysilane, and the like.

[0099] Examples of mercapto compounds include 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 2-mercaptoethyltrimethoxysilane, and 2-mercaptoethyltriethoxysilane.

[0100] Examples of vinyl compounds include vinyltriethoxysilane, vinyltrimethoxysilane, etc. Examples of amino compounds include 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-(2-aminoethyl)aminopropyltriethoxysilane, 3-(2-aminoethyl)aminopropyltrimethoxysilane, etc.

[0101] Examples of the glycidoxy compounds include γ-glycidoxypropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, and γ-glycidoxypropylmethyldimethoxysilane.

[0102] Examples of nitro compounds include 3-nitropropyltrimethoxysilane, 3-nitropropyltriethoxysilane, etc. Examples of chloro compounds include 3-chloropropyltrimethoxysilane, 3-chloropropyltriethoxysilane, 2-chloroethyltrimethoxysilane, 2-chloroethyltriethoxysilane, etc.

[0103] Other compounds include, for example, octyltriethoxysilane, methyltriethoxysilane, methyltrimethoxysilane, and hexadecyltrimethoxysilane.

[0104] These silane coupling agents may be used alone or in combination of two or more. Among these silane coupling agents, sulfur-containing silane coupling agents such as sulfide compounds and mercapto compounds are preferred from the viewpoint of a large reinforcing effect, and bis(3-triethoxysilylpropyl)disulfide, bis(3-triethoxysilylpropyl)tetrasulfide, and 3-mercaptopropyltriethoxysilane are more preferred.

[0105] The silane coupling agent is preferably contained in an amount of 0.1 to 30 parts by mass, more preferably 0.5 to 20 parts by mass, and even more preferably 1 to 15 parts by mass, per 100 parts by mass of silica (C1). When the content of the silane coupling agent is within the above range, the dispersibility of the filler, the coupling effect, the reinforcing properties, the flexural fatigue resistance, and the like are improved.

[0106] The rubber composition for a sidewall of the present invention may further contain a vulcanizing agent (D) to crosslink the rubber. Examples of the vulcanizing agent (D) include sulfur and sulfur compounds. Examples of sulfur compounds include morpholine disulfide and alkylphenol disulfide. These vulcanizing agents (D) may be used alone or in combination of two or more. From the viewpoint of the mechanical properties of the resulting crosslinked product, the vulcanizing agent (D) is typically contained in an amount of 0.1 to 10 parts by mass, preferably 0.5 to 10 parts by mass, and more preferably 0.8 to 5 parts by mass per 100 parts by mass of the solid rubber (A).

[0107] The rubber composition for a sidewall of the present invention, for example, when containing a vulcanizing agent (D) for crosslinking (vulcanizing) the rubber, may further contain a vulcanization accelerator (E). Examples of the vulcanization accelerator (E) include guanidine-based compounds, sulfenamide-based compounds, thiazole-based compounds, thiuram-based compounds, thiourea-based compounds, dithiocarbamic acid-based compounds, aldehyde-amine-based compounds, aldehyde-ammonia-based compounds, imidazoline-based compounds, and xanthate-based compounds. These vulcanization accelerators (E) may be used alone or in combination of two or more. The vulcanization accelerator (E) is typically contained in an amount of 0.1 to 15 parts by mass, preferably 0.1 to 10 parts by mass, per 100 parts by mass of the solid rubber (A).

[0108] The rubber composition for sidewalls of the present invention may further contain a vulcanization aid (F) when, for example, sulfur or a sulfur compound is contained as the vulcanizing agent (D) for crosslinking (vulcanizing) the rubber. Examples of the vulcanization aid (F) include fatty acids such as stearic acid, metal oxides such as zinc oxide, and fatty acid metal salts such as zinc stearate. These vulcanization aids (F) may be used alone or in combination of two or more. The vulcanization aid (F) is typically contained in an amount of 0.1 to 15 parts by mass, preferably 1 to 10 parts by mass, per 100 parts by mass of the solid rubber (A).

[0109] The rubber composition for sidewalls may contain a crosslinking agent in addition to the vulcanizing agent (D). Examples of such crosslinking agents include oxygen, organic peroxides, phenolic resins, amino resins, quinone and quinone dioxime derivatives, halogen compounds, aldehyde compounds, alcohol compounds, epoxy compounds, metal halides, organometallic halides, and silane compounds. These may be used alone or in combination of two or more. The amount of crosslinking agent is preferably 0.1 to 10 parts by mass per 100 parts by mass of the solid rubber (A).

[0110] The rubber composition for a sidewall of the present invention may contain, as needed, softeners such as silicone oil, aromatic oil, treated distilled aromatic extracts (TDAE), mild extracted solvates (MES), residual aromatic extracts (RAE), process oils such as paraffin oil and naphthenic oil, resin components such as aliphatic hydrocarbon resins, alicyclic hydrocarbon resins, C9 resins, rosin resins, coumarone-indene resins and phenolic resins, and liquid polymers such as low molecular weight polybutadiene, low molecular weight polyisoprene, low molecular weight styrene-butadiene copolymers and low molecular weight styrene-isoprene copolymers, for the purpose of improving processability, fluidity and the like, within a range that does not impair the effects of the present invention. When the rubber composition for a sidewall of the present invention contains the process oil, resin, or liquid polymer as a softener, the content thereof is preferably 50 parts by mass or less, more preferably 30 parts by mass or less, and even more preferably 15 parts by mass or less, per 100 parts by mass of the solid rubber (A), from the viewpoint of bleeding resistance.

[0111] The rubber composition for a sidewall of the present invention may contain additives such as antioxidants, antioxidants, waxes, lubricants, light stabilizers, scorch inhibitors, processing aids, colorants such as pigments and coloring agents, flame retardants, antistatic agents, matting agents, antiblocking agents, ultraviolet absorbers, release agents, foaming agents, antibacterial agents, antifungal agents, and fragrances, as needed, for the purpose of improving weather resistance, heat resistance, oxidation resistance, and the like, within a range that does not impair the effects of the present invention.

[0112] Examples of antioxidants include hindered phenol compounds, phosphorus compounds, lactone compounds, and hydroxyl compounds. Examples of antioxidants include amine-ketone compounds, imidazole compounds, amine compounds, phenol compounds, sulfur compounds, and phosphorus compounds. These additives may be used alone or in combination of two or more.

[0113] [Method for Producing Rubber Composition for Sidewall] The method for producing the rubber composition for sidewall of the present invention is not particularly limited as long as it can uniformly mix the above-mentioned components. Examples of equipment used for producing the rubber composition for sidewall include tangential or intermeshing internal mixers such as kneader-ruders, Brabenders, Banbury mixers, and internal mixers, single-screw extruders, twin-screw extruders, mixing rolls, and rollers. The rubber composition can usually be produced at a temperature ranging from 70 to 270°C.

[0114] The rubber composition for a sidewall of the present invention is preferably used as a crosslinked product (vulcanized rubber) by crosslinking. Although there are no particular restrictions on the conditions and method of vulcanization, it is preferable to carry out the vulcanization using a vulcanization mold under conditions of a vulcanization temperature of 120 to 200°C and a vulcanization pressure of 0.5 to 20 MPa.

[0115] The extraction rate of the modified liquid diene rubber (B) from the crosslinked product is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less. The extraction rate can be calculated by immersing 2 g of the crosslinked product in 400 mL of toluene at 23° C. for 48 hours and then calculating the amount of the modified liquid diene rubber (B) extracted into the toluene.

[0116] [Sidewall] The sidewall of the present invention is at least partially formed using the rubber composition for sidewall, and exhibits excellent flex fatigue resistance and improved fuel economy. Furthermore, since the rubber composition for sidewall has excellent flex fatigue resistance, the sidewall can be made thinner (lighter), thereby reducing the tire weight and achieving a tire with excellent fuel economy. The sidewall of the present invention can be obtained by molding a sidewall rubber having a predetermined cross-sectional shape from the rubber composition for sidewall obtained as described above using an extruder or the like, and then using this sidewall rubber to produce various tires (pneumatic tires) such as passenger car tires, large truck and bus tires, and motorcycle tires using a conventional method (generally including a crosslinking step), thereby obtaining a tire including the sidewall of the present invention. The structure of the sidewall of the present invention is not particularly limited and may be a single-layer structure or a multi-layer structure. However, when a multi-layer structure is used, it is preferable to use the rubber composition for sidewall as the outermost layer.

[0117] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The components used in these examples and comparative examples are as follows. <Solid rubber (A)> Natural rubber: STR20 (natural rubber produced in Thailand) Butadiene rubber: BR01 (manufactured by ENEOS Materials Corporation, Mw: 550,000, cis content: 95% by mass) <Modified liquid diene rubber (B)> Modified liquid diene rubber obtained in Production Examples 1-2 to 4-2 described below In the comparative examples, unmodified liquid diene rubber obtained in Production Examples 1-1 to 4-1 <Filler (C)> Silica 1: NipsilER-R (manufactured by Tosoh Silica Corporation, wet silica, BET specific surface area: 92 m ２ / g) The BET specific surface area of ​​the silica is a value obtained by the BET method in accordance with ASTM D3037-81. Carbon black: Diablack H (manufactured by Mitsubishi Chemical Corporation, average particle size 31 nm, specific surface area 79 m ２ / g) The specific surface area of ​​the carbon black is a value obtained in accordance with JIS K 6217-2:2001. <Vulcanizing agent (D)> Sulfur: Myucron OT-20 (manufactured by Shikoku Chemical Industry Co., Ltd., insoluble sulfur, oil content 18 to 22%) <Vulcanization accelerator (E)> Vulcanization accelerator (1): Suncera NS-G (manufactured by Sanshin Chemical Industry Co., Ltd.) Vulcanization accelerator (2): Noccela CZ-G (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) Vulcanization accelerator (3): Noccela D (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) <Vulcanization aid (F)> Stearic acid: Lunac S-20 (manufactured by Kao Corporation) Zinc oxide: Zinc oxide (manufactured by Sakai Chemical Industry Co., Ltd.) <Optional components> TDAE: VivaTec 500 (manufactured by H&R) Silane coupling agent: Si-69 (manufactured by Evonik Degussa Japan) Antioxidant: Nocrac 6C (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.)

[0118] Production Example 1-1: Production of Unmodified Liquid Diene Rubber (B'-1) A thoroughly dried 5 L autoclave was purged with nitrogen, and 1,280 g of cyclohexane and 66 g of sec-butyllithium (10.5 mass % cyclohexane solution) were charged. The temperature was raised to 50°C, and then 1,350 g of butadiene was gradually added under stirring conditions while controlling the polymerization temperature to 50°C, and polymerization was carried out for 1 hour. Methanol was then added to terminate the polymerization reaction, yielding a polymer solution. Water was added to the obtained polymer solution and stirred, and the polymer solution was washed with water. After stirring was terminated, it was confirmed that the polymer solution phase and the aqueous phase had separated, and then the water was separated. The polymer solution after washing was vacuum dried at 70°C for 24 hours to yield an unmodified liquid diene rubber (B'-1).

[0119] Production Example 1-2: Production of modified liquid diene rubber (B-1) 700 g of the unmodified liquid diene rubber (B'-1) obtained in Production Example 1-1 was charged into a 1 L autoclave and degassed with nitrogen while stirring for 3 hours at 60°C. 1.0 g of 1,1-bis(t-hexylperoxy)cyclohexane and 50 g of (3-mercaptopropyl)triethoxysilane were added, and the mixture was reacted at 105°C for 8 hours to obtain modified liquid diene rubber (B-1).

[0120] Production Example 2-1: Production of Unmodified Liquid Diene Rubber (B'-2) A thoroughly dried 5 L autoclave was purged with nitrogen, and 1,150 g of hexane and 154 g of n-butyllithium (17 mass % hexane solution) were charged. The autoclave was heated to 50°C, and 10 g of N,N,N',N'-tetramethylethylenediamine was added under stirring conditions. Then, 1,250 g of butadiene was gradually added while controlling the polymerization temperature to 50°C, and polymerization was carried out for 1 hour. Methanol was then added to terminate the polymerization reaction, yielding a polymer solution. Water was added to the obtained polymer solution, and the mixture was stirred. The polymer solution was washed with water. After stirring was terminated, it was confirmed that the polymer solution phase and the aqueous phase had separated, and the water was then separated. The polymer solution after washing was vacuum dried at 70°C for 24 hours to yield an unmodified liquid diene rubber (B'-2).

[0121] Production Example 2-2: Production of Modified Liquid Diene Rubber (B-2) 700 g of the unmodified liquid diene rubber (B'-2) obtained in Production Example 2-1 was charged into a 1 L autoclave and degassed with nitrogen while stirring for 3 hours at 60°C. 0.1 g of 1,1-bis(t-butylperoxy)cyclohexane and 119 g of (3-mercaptopropyl)triethoxysilane were added, and the mixture was reacted at 120°C for 3 hours to obtain a modified liquid diene rubber (B-2).

[0122] Production Example 3-1: Production of Unmodified Liquid Diene Rubber (B'-3) A thoroughly dried 5 L autoclave was purged with nitrogen, and 1,580 g of cyclohexane and 336 g of s-butyllithium (0.99 mol / L, cyclohexane solution) were charged. The temperature was raised to 50°C, and then 23.4 g of tetrahydrofuran was added under stirring conditions. Then, 218 g of butadiene, 776 g of isoprene, and 234 g of butadiene were sequentially added to polymerize while controlling the polymerization temperature to 50°C. Methanol was then added to terminate the polymerization reaction, yielding a polymer solution. Water was added to the obtained polymer solution, and the mixture was stirred. The polymer solution was washed with water. After stirring was terminated and it was confirmed that the polymer solution phase and the aqueous phase had separated, the water was separated. The polymer solution after washing was dried at 140°C for 3 hours to yield an unmodified liquid diene rubber (B'-3), which was a triblock copolymer consisting of a linear butadiene homopolymer block-isoprene homopolymer block-butadiene homopolymer block.

[0123] Production Example 3-2: Production of Modified Liquid Diene Rubber (B-3) 481 g of the unmodified liquid diene rubber (B'-3) obtained in Production Example 3-1 was charged into a 1 L autoclave, and the mixture was degassed with nitrogen while stirring for 3 hours at 60°C. 4.9 g of 1,1-bis(t-hexylperoxy)cyclohexane and 79 g of (3-mercaptopropyl)triethoxysilane were added, and the mixture was reacted at 105°C for 8 hours to obtain a modified liquid diene rubber (B-3), which was a triblock copolymer consisting of a linear butadiene homopolymer block-isoprene homopolymer block-butadiene homopolymer block modified with a functional group derived from the silane compound (1).

[0124] Production Example 4-1: Production of Unmodified Liquid Diene Rubber (B'-4) A thoroughly dried 5 L autoclave was purged with nitrogen, and 1,100 g of hexane and 204 g of n-butyllithium (17 mass % hexane solution) were charged. The autoclave was heated to 50°C, and then 1,300 g of butadiene was gradually added while stirring to maintain the polymerization temperature at 50°C. Polymerization was carried out for 1 hour. Methanol was then added to terminate the polymerization reaction, yielding a polymer solution. Water was added to the resulting polymer solution, followed by stirring, and the polymer solution was washed with water. After stirring was terminated, it was confirmed that the polymer solution phase and the aqueous phase had separated, and the water was then separated. The polymer solution after washing was vacuum dried at 70°C for 24 hours to yield an unmodified liquid diene rubber (B'-4).

[0125] Production Example 4-2: Production of modified liquid diene rubber (B-4) 700 g of the unmodified liquid diene rubber (B'-4) obtained was charged into a 1 L autoclave and degassed with nitrogen while stirring for 3 hours at 60°C. 2.0 g of 1,1-bis(t-butylperoxy)cyclohexane and 119 g of (3-mercaptopropyl)triethoxysilane were added, and the mixture was reacted at 120°C for 3 hours to obtain modified liquid diene rubber (B-4).

[0126] The methods for measuring and calculating the various physical properties of the modified liquid diene rubbers obtained in the Production Examples are as follows.

[0127] (Method for measuring weight average molecular weight) The Mw of the modified liquid diene rubber (B) was determined by GPC (gel permeation chromatography) in terms of standard polystyrene. The measurement device and conditions are as follows. Device: GPC device "GPC8020" manufactured by Tosoh Corporation Separation column: "TSKgel G4000HXL" manufactured by Tosoh Corporation Detector: "RI-8020" manufactured by Tosoh Corporation Eluent: tetrahydrofuran Eluent flow rate: 1.0 mL / min Sample concentration: 5 mg / 10 mL Column temperature: 40°C

[0128] (Vinyl Content) The vinyl content of the modified liquid diene rubber (B) was measured using an NMR apparatus, AVANCEIII 400 NanoBay manufactured by Bruker Japan Co., Ltd., at a sample / d2 chloroform concentration of 50 mg / 1 mL and an accumulation count of 1024. The vinyl content was calculated from the area ratio of the peaks derived from conjugated diene units bonded via 1,2-bonds and 3,4-bonds to the peaks derived from conjugated diene units bonded via 1,4-bonds in the obtained spectrum.

[0129] (Glass Transition Temperature) 10 mg of the modified liquid diene rubber (B) was placed in an aluminum pan, and a thermogram was measured by differential scanning calorimetry (DSC) at a temperature rise rate of 10°C / min, and the peak top value of the DDSC (first derivative curve of the DSC curve) was taken as the glass transition temperature (Tg).

[0130] (Method for measuring melt viscosity at 38° C.) The melt viscosity of the modified liquid diene rubber (B) at 38° C. was measured using a Brookfield viscometer (manufactured by BROOKFIELD ENGINEERING LABS. INC.).

[0131] (Average Number of Functional Groups per Molecule of Modified Liquid Diene Rubber (B)) The average number of functional groups per molecule of the modified liquid diene rubber (B) was calculated using the functional group equivalent (g / eq) of the modified liquid diene rubber (B) and the styrene-equivalent number average molecular weight Mn according to the following formula: (Average Number of Functional Groups per Molecule) = [(Number Average Molecular Weight Mn) ÷ (Molecular Weight of Styrene Unit) × (Average Molecular Weight of Conjugated Diene and Other Monomer Units Other Than Conjugated Diene, if Optionally Included)] / (Equivalent of Functional Group)

[0132] The equivalent weight of the functional group of the modified liquid diene rubber (B) means the mass of butadiene bonded to one functional group and the mass of other monomers other than butadiene that may be contained as needed. １ H-NMR or １３ The carbon monoxide content was calculated from the area ratio of the peak derived from the functional group to the peak derived from the main chain of the modified liquid diene rubber (B) using C-NMR. Note that the peak derived from the functional group was the peak derived from the alkoxy group.

[0133] The physical properties of the unmodified liquid diene rubbers and modified liquid diene rubbers obtained in Production Examples 1-1 to 4-2 are summarized in Table 1 below.

[0134]

[0135] Examples 1 to 11 and Comparative Examples 1 to 7: In accordance with the compounding ratios (parts by mass) shown in Table 2 (Examples) and Table 3 (Comparative Examples), the components other than the vulcanizing agent (sulfur) and vulcanization accelerator (in the Examples, solid rubber (A), modified liquid diene-based rubber (B), filler (C), TDAE, silane coupling agent, zinc oxide, stearic acid, and antioxidant) were each charged into an internal Banbury mixer, and the mixture was kneaded for 4 minutes at a starting temperature of 60°C and a resin temperature of 155-160°C, after which the mixture was removed from the mixer and cooled to room temperature. Next, this mixture was placed back into the Banbury mixer, and the vulcanizing agent (sulfur) and vulcanization accelerator were added. The mixture was then kneaded for 75 seconds at a starting temperature of 50°C and a final temperature of 100°C to obtain a rubber composition.

[0136] The obtained rubber composition was press-molded (150°C, 20 to 40 minutes) to prepare vulcanized rubber sheets (thickness 2 mm) and test pieces for flex crack growth tests. Hardness, fuel economy performance, and flex fatigue resistance were evaluated according to the following methods. The results are shown in Tables 2 and 3. The measurement methods for each evaluation were as follows.

[0137] (Hardness) Using vulcanized rubber sheets obtained by press-molding the rubber compositions prepared in the Examples and Comparative Examples, the hardness was measured with a Type A hardness tester in accordance with JIS K6253:2023.

[0138] (Fuel Efficiency Performance) Test pieces measuring 40 mm long x 5 mm wide were cut out from the sheets of the rubber compositions prepared in the Examples and Comparative Examples, and tan δ was measured using a dynamic viscoelasticity measuring device manufactured by GABO under conditions of a measurement temperature of 60°C, a frequency of 10 Hz, a static strain of 10%, and a dynamic strain of 2%, and used as an index of fuel efficiency. The values ​​in each Example and Comparative Example are relative values ​​when the value of Comparative Example 1 in Table 2 is set to 100. The smaller the value, the better the fuel efficiency of the rubber composition.

[0139] (Flexural fatigue resistance) A flexural crack growth test was conducted in accordance with JIS K 6260:2017. Using the above-mentioned test specimen, the number of flexions required for the crack length to grow to the growth median value ((L + 2) mm to (L + 6) mm) was determined from the crack length for each number of flexions. The values ​​for each example and comparative example in Table 2 are relative values ​​when the value for Comparative Example 1 is set to 100. The larger the value, the better the flexural fatigue resistance.

[0140]

[0141]

[0142] Even when the rubber composition for a sidewall of the present invention contains a filler (silica) with a relatively large particle size, the rubber composition can be used to produce a sidewall that has excellent flex fatigue resistance and improved fuel economy performance, making the rubber composition for a sidewall of the present invention useful.

Claims

1. A rubber composition for sidewalls, comprising 100 parts by mass of solid rubber (A) containing natural rubber and / or isoprene rubber (A1) and butadiene rubber (A2), 0.1 to 30 parts by mass of modified liquid diene rubber (B) having a functional group derived from a silane compound represented by the following formula (1), and 5 to 80 parts by mass of a filler (C), wherein the filler (C) contains silica (C1), and the BET specific surface area of ​​the silica (C1) is 170 (m ２ / g), and the modified liquid diene rubber (B) has the following (i) to (iii): (i) a weight average molecular weight (Mw) of 3,000 or more and 120,000 or less, (ii) a vinyl content of 70 mol % or less, and (iii) an average number of functional groups derived from the silane compound per molecule of the modified liquid diene rubber (B) is 1 to 20. (In formula (1), R １ is a divalent alkylene group having 1 to 6 carbon atoms; R ２ , R ３ and R ４ each independently represents a methoxy group, an ethoxy group, a phenoxy group, a methyl group, an ethyl group, or a phenyl group. ２ , R ３ and R ４ At least one of is a methoxy group, an ethoxy group, or a phenoxy group.

2. The rubber composition for a sidewall according to claim 1, wherein the modified liquid diene rubber (B) has a melt viscosity at 38° C. of 0.1 to 4,000 Pa·s.

3. The rubber composition for a sidewall according to claim 1 or 2, wherein the filler (C) further contains carbon black (C2).

4. A rubber composition for a sidewall according to any one of claims 1 to 3, wherein the mass ratio (A1) / (A2) of the natural rubber and / or isoprene rubber (A1) to the butadiene rubber (A2) contained in the solid rubber (A) is 20 / 80 or more and 60 / 40 or less.

5. A crosslinked product obtained by crosslinking the rubber composition for sidewalls according to any one of claims 1 to 4.

6. A sidewall, at least a part of which uses the rubber composition for sidewalls according to any one of claims 1 to 5.

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