Tire tread rubber composition and tire tread

Abstract

Provided are: a tire tread rubber composition capable of achieving both an improvement in low fuel consumption performance and an improvement in wear resistance without impairing a wet grip performance; and a tire tread using the tire tread rubber composition in at least a part thereof.　The tire tread rubber composition contains, relative to 100 parts by mass of a specific solid rubber (A), 0.1-50 parts by mass of a modified liquid diene rubber (B), which has a functional group derived from a specific silane compound, and 20-200 parts by mass of a filler (C).

Description

Rubber composition for tire tread and tire tread

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

[0002] Rubber compositions containing a rubber component such as a diene rubber and a filler such as carbon black or silica have been investigated for use in tire components, such as tire treads, which are required to have excellent braking performance on wet roads (wet grip performance) in order to improve automobile safety.

[0003] Furthermore, in recent years, from the viewpoint of reducing environmental impact, rubber compositions used in tire treads have been required to have low fuel consumption performance. For example, rubber compositions containing solid rubber, a specific modified liquid diene-based rubber, and a filler have been considered as rubber compositions suitable for use in tire treads with the aim of improving fuel efficiency (see Patent Documents 1 and 2). From the viewpoint of similarly reducing environmental impact, automobiles, including passenger cars, are being considered to be moving away from automobiles powered solely by internal combustion engines and toward automobiles using electric motors as at least part of their power source (electric vehicles). In these electric vehicles, the state of transmission of driving force from the power source to the tires is different. Furthermore, in order to stably drive the electric motor for a long period of time, a battery is often required to be installed on the vehicle. Therefore, compared to conventional automobiles powered solely by internal combustion engines, electric vehicles initially experience greater forces on their tires, and the weight of automobiles is also on the rise. Therefore, the rubber compositions used in tire treads for electric vehicles are required to have high wear resistance.

[0004] JP 2013-249359 A International Publication No. 2019 / 044892

[0005] As described above, in electric vehicles, compared to conventional vehicles, the state of transmission of driving force from the power source to the tires is different. The force applied to the tires, particularly initially, is relatively large. Furthermore, electric vehicles are heavier than conventional vehicles. Therefore, tire treads used in electric vehicles are required to emphasize greater wear resistance while also improving fuel economy without sacrificing wet grip performance. However, it is difficult to achieve both improved wear resistance and wet grip performance in a tire. It is also difficult to achieve both improved fuel economy and wet grip performance. For these reasons, a tire tread that excels in a well-balanced manner in all three physical properties has been desired. The present invention has been made in consideration of the above-mentioned circumstances, and provides a rubber composition for a tire tread, and a tire tread at least partially using the same, which can achieve both improved fuel economy and improved wear resistance without sacrificing wet grip performance.

[0006] As a result of intensive research, the present inventors have found that a tire tread at least partially using a rubber composition containing a specific solid rubber, a specific modified liquid diene-based rubber, and a filler can achieve both improved fuel economy and improved wear resistance without impairing wet grip performance, and have thus completed the present invention.

[0007] That is, the present invention relates to the following [1] to [9]: [1] A rubber composition for a tire tread, comprising 100 parts by mass of solid rubber (A) comprising natural rubber (A1), butadiene rubber (A2), and styrene-butadiene rubber (A3), 0.1 to 50 parts by mass of modified liquid diene rubber (B) having functional groups derived from a silane compound represented by the following formula (1), and 20 to 200 parts by mass of a filler (C), wherein 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.

[0008] (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.

[0009] [2] The rubber composition for tire treads 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 tire treads according to [1] or [2], wherein the content of natural rubber (A1) in 100 mass% of solid rubber (A) is 40 to 80 mass%. [4] The rubber composition for tire treads according to any one of [1] to [3], wherein the mass ratio (A2) / (A3) of the butadiene rubber (A2) to the styrene-butadiene rubber (A3) contained in the solid rubber (A) is 0.12 or more and 3 or less. [5] The rubber composition for tire treads according to any one of [1] to [4], wherein the filler (C) contains silica. [6] The rubber composition for tire treads according to [5], further containing carbon black as the filler (C). [7] A crosslinked product obtained by crosslinking the rubber composition for a tire tread according to any one of [1] to [6]. [8] A tire tread at least partially using the rubber composition for a tire tread according to any one of [1] to [7]. [9] A pneumatic tire for an electric vehicle, comprising the tire tread according to [8].

[0010] According to the present invention, it is possible to provide a rubber composition for a tire tread, which can achieve both improved fuel economy and improved abrasion resistance without impairing wet grip performance, and a tire tread at least partly using the same.

[0011] [Solid Rubber (A)] The solid rubber (A) used in the rubber composition for a tire tread 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. The solid rubber (A) contained in the rubber composition for a tire tread of the present invention includes natural rubber (A1), butadiene rubber (A2) and styrene-butadiene rubber (A3).

[0012] [Natural Rubber (A1)] Examples of the natural rubber (A1) include unmodified natural rubbers such as TSR (Technically Specified Rubber) and RSS (Ribbed Smoked Sheet) commonly used in the tire industry, such as SMR (Malaysian TSR), SIR (Indonesian TSR), and STR (Thai TSR), and unmodified natural rubbers such as 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. The glass transition temperature of the natural rubber (A1) varies depending on the amount of modification in modified natural rubbers such as epoxidized natural rubber, but is preferably −40°C or lower, more preferably −50°C or lower. These natural rubbers (A1) may be used alone or in combination of two or more.

[0013] [Butadiene Rubber (A2)] Examples of the butadiene rubber (A2) include Ziegler catalysts such as titanium tetrahalide-trialkylaluminum catalysts, diethylaluminum chloride-cobalt catalysts, trialkylaluminum-boron trifluoride-nickel catalysts, and diethylaluminum chloride-nickel catalysts; lanthanoid-based rare earth metal catalysts such as triethylaluminum-organic acid neodymium-Lewis acid catalysts; and commercially available butadiene rubbers polymerized using organic alkali metal compounds (e.g., n-butyllithium, sec-butyllithium, etc.) similar to the solution-polymerized styrene-butadiene rubber described below. Among these, butadiene rubbers polymerized using Ziegler-based 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 a lanthanoid-based rare earth metal catalyst may be used as the butadiene rubber (A2).

[0014] 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 of the butadiene rubber (A2) varies depending on the vinyl content, but is preferably -40°C or less, 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.

[0015] The weight average molecular weight (Mw) of the butadiene rubber (A2) is preferably 90,000 to 2,000,000, more preferably 150,000 to 1,500,000. When the Mw is within the above range, the processability of the rubber composition for a tire tread is improved, and the abrasion resistance of the tire tread at least partly using the rubber composition for a tire tread is also improved.

[0016] 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.

[0017] [Styrene-butadiene rubber (A3)] As the styrene-butadiene rubber (A3) (hereinafter, "styrene-butadiene rubber" may also be referred to as "SBR"), a rubber generally used for tires can be used. Specifically, the SBR (A3) preferably has a styrene content of 0.1 to 70 mass%, more preferably 5 to 60 mass%, and even more preferably 5 to 50 mass%. Furthermore, the SBR (A3) preferably has a vinyl content of 0.1 to 80 mol%, more preferably 5 to 70 mol%.

[0018] In this specification, the vinyl content of SBR 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 all butadiene-derived structural units contained in SBR.

[0019] The weight average molecular weight (Mw) of the SBR (A3) is preferably 100,000 to 2,500,000, more preferably 150,000 to 2,000,000, and even more preferably 150,000 to 1,500,000. When the weight average molecular weight (Mw) of the SBR is within the above range, the processability of the rubber composition for a tire tread is improved, and the wet grip performance of a tire having a tire tread obtained from the rubber composition for a tire tread is improved, and further, the mechanical strength, abrasion resistance, and steering stability are also improved. Note that the weight average molecular weight in this specification is the weight average molecular weight in terms of polystyrene determined by measurement using gel permeation chromatography (GPC).

[0020] The glass transition temperature (Tg) of SBR (A3) determined by differential thermal analysis is preferably −95 to 0° C., more preferably −90 to −5° C., even more preferably −85 to −10° C., still more preferably −80 to −15° C., and particularly preferably −70 to −20° C. When the glass transition temperature is within the above range, an increase in viscosity of the rubber composition for tire treads can be suppressed, making it easier to handle and also improving wet grip performance.

[0021] The SBR (A3) that can be used in the present invention is obtained by copolymerizing styrene and butadiene. There are no particular limitations on the method for producing the SBR (A3), and any of emulsion polymerization, solution polymerization, gas phase polymerization, and bulk polymerization can be used. Among these production methods, emulsion polymerization and solution polymerization are preferred.

[0022] Emulsion-polymerized styrene-butadiene rubber (hereinafter also referred to as E-SBR) can be produced by a known or similar ordinary emulsion polymerization method, for example, by emulsifying and dispersing predetermined amounts of styrene and butadiene monomers in the presence of an emulsifier, and then emulsion-polymerizing the resulting mixture with a radical polymerization initiator.

[0023] As the emulsifier, for example, a salt of a long-chain fatty acid having 10 or more carbon atoms or a rosinate may be used. Specific examples include potassium salts or sodium salts of fatty acids such as capric acid, lauric acid, myristic acid, palmitic acid, oleic acid, and stearic acid.

[0024] 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.

[0025] A chain transfer agent can also be used to adjust the molecular weight of the resulting E-SBR. 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.

[0026] The temperature of the emulsion polymerization can be appropriately selected depending on the type of radical polymerization initiator used, but is usually preferably 0 to 100°C, more preferably 0 to 60°C. The polymerization method may be either continuous polymerization or batch polymerization. The polymerization reaction can be terminated by adding a polymerization terminator.

[0027] Examples of the polymerization terminator include amine compounds such as isopropylhydroxylamine, diethylhydroxylamine, and hydroxylamine; quinone compounds such as hydroquinone and benzoquinone; and sodium nitrite.

[0028] 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 polymer 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, and the dispersion medium is then separated to recover the polymer as crumbs. The crumbs are washed with water, dehydrated, and dried using a band dryer or the like to obtain E-SBR. During coagulation, if necessary, the latex may be mixed with extender oil previously emulsified and dispersed, and recovered as oil-extended rubber. In the composition of the rubber composition for tire treads in this specification, extender oil is not included in the solid rubber (A).

[0029] Commercially available E-SBR products include oil-extended styrene-butadiene rubber "ESBR1723" manufactured by ENEOS Materials Corporation.

[0030] Solution-polymerized styrene-butadiene rubber (hereinafter also referred to as S-SBR) can be produced by a conventional solution polymerization method, for example, by polymerizing styrene and butadiene in a solvent using an active metal capable of anion polymerization, optionally in the presence of a polar compound.

[0031] 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 and toluene. These solvents are usually preferably used in such a manner that the monomer concentration becomes 1 to 50% by mass.

[0032] Examples of the active metal capable of anion polymerization 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 these active metals, alkali metals and alkaline earth metals are preferred, and alkali metals are more preferred. Furthermore, among alkali metals, organic alkali metal compounds are more preferably used.

[0033] Examples of organic alkali metal compounds include organic monolithium compounds such as n-butyllithium, sec-butyllithium, t-butyllithium, hexyllithium, phenyllithium, and stilbenelithium; polyfunctional organic lithium compounds such as dilithiomethane, 1,4-dilithiobutane, 1,4-dilithio-2-ethylcyclohexane, and 1,3,5-trilithiobenzene; sodium naphthalene, and potassium naphthalene. Among these, organic lithium compounds are preferred, and organic monolithium compounds are more preferred. The amount of the organic alkali metal compound used is determined appropriately depending on the molecular weight of the required S-SBR. The organic alkali metal compound can also be reacted with a secondary amine such as dibutylamine, dihexylamine, or dibenzylamine to be used as an organic alkali metal amide.

[0034] The polar compound is not particularly limited as long as it is a compound that is commonly used in anionic polymerization to adjust the microstructure of butadiene units and the distribution of styrene in the copolymer chain without deactivating the reaction, and examples thereof include ether compounds such as dibutyl ether, tetrahydrofuran, and ethylene glycol diethyl ether; tertiary amines such as tetramethylethylenediamine and trimethylamine; alkali metal alkoxides; and phosphine compounds.

[0035] The temperature of the polymerization reaction is usually in the range of −80 to 150° C., preferably 0 to 100° C., and more preferably 30 to 90° C. The polymerization may be carried out batchwise or continuously. In order to improve the random copolymerization of styrene and butadiene, it is preferable to continuously or intermittently supply styrene and butadiene to the reaction liquid so that the composition ratio of styrene and butadiene in the polymerization system falls within a specific range.

[0036] The polymerization reaction can be terminated by adding an alcohol such as methanol or isopropanol as a polymerization terminator. After the polymerization reaction has been terminated, the solvent can be separated from the polymer solution by direct drying or steam stripping, and the target S-SBR can be recovered. Before removing the solvent, the polymer solution and extender oil may be mixed in advance to recover the oil-extended rubber.

[0037] The SBR (A3) may be a modified SBR in which a functional group has been introduced into the SBR, as long as the effects of the present invention are not impaired. Examples of the functional group include an amino group, an alkoxysilyl group, a hydroxyl group, an epoxy group, and a carboxyl group.

[0038] Examples of methods for producing modified SBR include a method in which, before adding a polymerization terminator, a coupling agent capable of reacting with the active polymerization terminals is added, such as tin tetrachloride, tetrachlorosilane, dimethyldichlorosilane, dimethyldiethoxysilane, tetramethoxysilane, tetraethoxysilane, 3-aminopropyltriethoxysilane, tetraglycidyl-1,3-bisaminomethylcyclohexane, or 2,4-tolylenediisocyanate; a polymerization terminal modifier such as 4,4'-bis(diethylamino)benzophenone or N-vinylpyrrolidone; or other modifiers described in JP 2011-132298 A. In this modified SBR, the position in the polymer where the functional group is introduced may be the polymerization terminal or a side chain of the polymer chain.

[0039] The content of the natural rubber (A1) in 100% by mass of the solid rubber (A) is preferably 40 to 80% by mass, more preferably 45 to 75% by mass, and even more preferably 50 to 70% by mass. When the content of the natural rubber (A1) in the solid rubber (A) is within the above range, abrasion resistance is improved.

[0040] The content of the butadiene rubber (A2) in 100% by mass of the solid rubber (A) is preferably 3 to 45% by mass, more preferably 3 to 40% by mass, and even more preferably 5 to 35% by mass. When the content of the butadiene rubber (A2) in the solid rubber (A) is within the above range, abrasion resistance is improved.

[0041] The content of the styrene-butadiene rubber (A3) in 100% by mass of the solid rubber (A) is preferably 5 to 50% by mass, more preferably 10 to 45% by mass, and even more preferably 10 to 40% by mass. When the content of the styrene-butadiene rubber (A3) in the solid rubber (A) is within the above range, fuel economy performance is improved.

[0042] In the solid rubber (A), the mass ratio (A2) / (A3) of the butadiene rubber (A2) to the styrene-butadiene rubber (A3) is preferably 0.12 to 3, more preferably 0.13 to 2.5, more preferably 0.14 to 2.0, even more preferably 0.14 to 1.8, and even more preferably 0.2 to 1.2. When the mass ratio (A2) / (A3) in the solid rubber (A) is within the above range, the balance between wet grip performance, fuel economy, and abrasion resistance is improved.

[0043] From the viewpoint of enabling the production of a tire tread having further improved fuel economy and wear resistance, the total content of the natural rubber (A1), butadiene rubber (A2), and styrene-butadiene rubber (A3) in 100% by mass of the solid rubber (A) is preferably 80% by mass or more, more preferably 90% by mass or more. From the same viewpoint, it is preferable that the total content of the natural rubber (A1), butadiene rubber (A2), and styrene-butadiene rubber (A3) in 100% by mass of the solid rubber (A) is 100% by mass, i.e., the solid rubber (A) consists only of the natural rubber (A1), butadiene rubber (A2), and styrene-butadiene rubber (A3).

[0044] The solid rubber (A) may contain solid rubbers other than the natural rubber (A1), butadiene rubber (A2), and styrene-butadiene rubber (A3) as long as the effects of the present invention are not impaired. Examples of solid rubbers other than the natural rubber (A1), butadiene rubber (A2), and styrene-butadiene rubber (A3) include isoprene rubber, 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 natural rubber (A1), butadiene rubber (A2), and styrene-butadiene rubber (A3), the content of the solid rubbers other than the natural rubber (A1), butadiene rubber (A2), and styrene-butadiene rubber (A3) in 100% by mass of the solid rubber (A) is preferably 20% by mass or less, and more preferably 10% by mass or less.

[0045] [Modified Liquid Diene Rubber (B)] The modified liquid diene rubber (B) used in the rubber composition for tire treads of the present invention is a liquid polymer having functional groups derived from a silane compound represented by formula (1) described below (hereinafter also referred to as silane compound (1)), and having 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 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 tire treads 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), and a tire tread using at least a portion of this rubber composition can achieve both improved fuel economy and improved wear resistance.

[0046] 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, further 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.

[0047] 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).

[0048] The unmodified liquid diene rubber (B') used as the raw material for the modified liquid diene rubber (B) preferably has 50% by mass or more of monomer units of at least one conjugated diene (b1) selected from the group consisting of butadiene and isoprene out of 100% by mass of all monomer units constituting the polymer. 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').

[0049] 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.

[0050] 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.

[0051] In the unmodified liquid diene rubber (B'), the content of monomer units other than conjugated dienes, such as aromatic vinyl compound (b2), 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. For example, when the aromatic vinyl compound (b2) unit is within the above range or less, the processability of the rubber composition tends to be improved.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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').

[0070] In order to facilitate the production of tire treads that achieve both improved fuel economy and improved wear resistance without impairing wet grip performance, it is desirable that the unmodified liquid diene rubber (B') thus obtained be modified as is (without hydrogenation) with a functional group derived from a silane compound represented by formula (1) described below.

[0071] 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.

[0072] 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)).

[0073] 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.

[0074] 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.

[0075] 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:

[0076] 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.

[0077] 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 tire tread that also achieves improved fuel economy. On the other hand, if the average number of functional groups exceeds 20, it is difficult to obtain an improved wear resistance effect in a tire tread obtained from that rubber composition. Since the resulting tire tread tends to have further improved fuel economy and wear resistance, 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.

[0078] 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)

[0079] 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).

[0080] 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 dispersion effect of the filler (C) is poor, and the abrasion resistance of the resulting tire tread tends to decrease. If the amount added is less than 1 part by mass, the dispersion effect of the filler (C) is poor, and the improvement in fuel economy and abrasion resistance of the resulting tire tread tends to be insufficient. The amount of silane compound (1) added in the modified liquid diene rubber (B) can be determined using various analytical instruments such as NMR.

[0081] 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 tires 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 even at a relatively low temperature while sufficiently suppressing side reactions such as polymerization reactions.

[0082] When the maximum peak molecular weight in terms of polystyrene obtained from GPC measurement of the modified liquid diene rubber (B) is defined as 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 defined as 100%, the proportion of polymers 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 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 improves, and the interaction with the filler (C) in the resulting rubber composition, which will be described later, is improved, making it more likely to be present in the vicinity of the filler (C) when the rubber composition is produced. As a result, the filler (C) and other components are well dispersed in the rubber composition, which is presumably ideal for improving the physical properties of the resulting tire tread (for example, improving fuel economy and abrasion resistance).

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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, they 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.

[0089] 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 to 200°C, more preferably 50 to 180°C, and even more preferably 50 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.

[0090] 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 calculated in terms of polystyrene as determined by 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 economic efficiency is improved. Furthermore, the processability of the rubber composition of the present invention is improved, and the dispersibility of the filler (C) is excellent, enabling the physical properties of the resulting tire tread to be improved (improved fuel economy and wear resistance).

[0091] From the viewpoint of improving the fuel economy and abrasion resistance of the resulting tire tread, 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, particularly preferably 5,500 to 12,000, and more 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.

[0092] 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 even more 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.

[0093] 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 performance tends to deteriorate. From the viewpoint of fuel economy performance, 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" refers to 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) (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 other than farnesene), and 3,13-bonds (in the case of farnesene) to the total 100 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).

[0094] 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.

[0095] 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, particularly preferably 0.1 to 500 Pa·s, and even more 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.

[0096] 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 −120 to 30° C., even more preferably −100 to 10° C., and even more preferably −100 to 0° C. When the Tg is within the above range, for example, the fuel economy and abrasion resistance of a tire tread obtained from the rubber composition are likely to be improved. In addition, an increase in the viscosity of the modified liquid diene rubber (B) can be prevented, making it easier to handle.

[0097] 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 tack during processing, and improves the performance of tire treads 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.

[0098] 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 tire treads containing the solid rubber (A), the modified liquid diene rubber (B), and the filler (C) of the present invention is preferably 0 to 200 ppm, more preferably 0 to 150 ppm, and even more preferably 0 to 100 ppm, calculated as metal. In this case, the amount of catalyst residue may be the amount of catalyst residue derived from the polymerization catalyst used in producing any 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 tire treads of the present invention.

[0099] 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 50 parts by mass, preferably 1 to 45 parts by mass, more preferably 1 to 40 parts by mass, even more preferably 2 to 40 parts by mass, still more preferably 5 to 40 parts by mass, still more preferably 5 to 35 parts by mass, more particularly preferably 5 to 30 parts by mass, more particularly preferably 5 to 25 parts by mass, and more particularly preferably 10 to 25 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 fuel economy and wear resistance of the resulting tire tread are improved.

[0100] [Filler (C)] The filler (C) used in the rubber composition for a tire tread of the present invention can be any filler that is generally used in rubber compositions for tire treads, without any particular limitation. From the viewpoint of improving the fuel economy performance of the tire tread, however, it is preferable that the filler (C) contains silica (C1).

[0101] 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, mechanical strength and abrasion resistance of the resulting tire tread, and further improving fuel economy performance. These silicas (C1) may be used alone or in combination of two or more.

[0102] From the viewpoints of processability of the rubber composition for a tire tread, improvement in fuel economy performance of a tire tread at least partly using the rubber composition for a tire tread, and improvement in abrasion resistance, the average particle size of the silica (C1) is preferably 0.5 nm or more, more preferably 2 nm or more, even more preferably 5 nm or more, still more preferably 8 nm or more, and particularly preferably 10 nm or more. The average particle size is preferably 200 nm or less, more preferably 150 nm or less, even more preferably 100 nm or less, still more preferably 50 nm or less, particularly preferably 30 nm or less, and most preferably 20 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.

[0103] The filler (C) used in the rubber composition for a tire tread of the present invention preferably contains carbon black (C2) from the viewpoint of improving the physical properties of the tire tread, such as improving the mechanical strength and abrasion resistance, etc. In other words, the filler (C) used in the rubber composition for a tire tread of the present invention preferably contains silica (C1) and carbon black (C2).

[0104] 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, improving the mechanical strength of the resulting tire tread, improving abrasion resistance, and improving fuel economy. These carbon blacks may be used alone or in combination of two or more.

[0105] 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, from the viewpoint of improving the mechanical strength, abrasion resistance, and fuel economy of a tire tread at least partly using the rubber composition for a tire tread. The average particle size is preferably 100 nm or less, more preferably 80 nm or less, even more preferably 60 nm or less, and still more preferably 40 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.

[0106] 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.

[0107] From the viewpoint of improving wettability and dispersibility in the solid rubber (A), the carbon black (C2) may be subjected to an acid treatment using nitric acid, sulfuric acid, hydrochloric acid, or a mixed acid 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 tire treads of the present invention and tire treads 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. Suitable graphitization catalysts include boron, boron oxides (e.g., BO, BO, BO, etc.), boron oxoacids (e.g., orthoboric acid, metaboric acid, tetraboric acid, etc.) and salts thereof, boron carbides (e.g., BC, BC), boron nitride (BN), and other boron compounds.

[0108] The carbon black (C2) can also be used after adjusting the particle size by pulverization, etc. 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), etc. can be used.

[0109] The rubber composition for a tire tread of the present invention may contain a filler other than silica (C1) and carbon black (C2) for the purpose of improving properties such as increasing the mechanical strength of the resulting tire tread, improving production costs by blending a filler as an extender, and the like.

[0110] 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.

[0111] The amount of filler (C) per 100 parts by mass of the solid rubber (A) is 20 to 200 parts by mass, preferably 20 to 150 parts by mass. When the amount of filler (C) is within this range, it is possible to achieve both improved fuel economy and improved wear resistance in a tire tread at least partially using the rubber composition for a tire tread. From the above-mentioned viewpoints, the amount of filler (C) per 100 parts by mass of the solid rubber (A) is more preferably 30 parts by mass or more, even more preferably 50 parts by mass or more, still more preferably 70 parts by mass or more, and preferably 140 parts by mass or less, more preferably 130 parts by mass or less, even more preferably 120 parts by mass or less, and still more preferably 110 parts by mass or less.

[0112] When silica (C1) is contained as the filler (C), the amount of silica (C1) per 100 parts by mass of the solid rubber (A) is preferably 20 parts by mass or more, more preferably 30 parts by mass or more, even more preferably 40 parts by mass or more, still more preferably 50 parts by mass or more, and particularly preferably 60 parts by mass or more, from the viewpoint of improving the wet grip performance, fuel economy, and abrasion resistance of a tire tread at least partly using the rubber composition for a tire tread. The amount of silica (C1) is preferably 145 parts by mass or less, more preferably 135 parts by mass or less, and even more preferably 105 parts by mass or less.

[0113] Furthermore, when carbon black (C2) is contained as the filler (C), the amount of carbon black (C2) per 100 parts by mass of the solid rubber (A) is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, even more preferably 20 parts by mass or more, and particularly preferably 30 parts by mass or more, from the viewpoint of improving the abrasion resistance and fuel economy of a tire tread at least partly using the rubber composition for a tire tread. The amount of carbon black (C2) is preferably 80 parts by mass or less, more preferably 60 parts by mass or less, and even more preferably 40 parts by mass or less.

[0114] When silica (C1) and carbon black (C2) are contained 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 95 / 5, and even more preferably 30 / 70 to 90 / 10, from the viewpoint of making it easier to improve the fuel economy performance and wear resistance of a tire tread obtained from the rubber composition for a tire tread of the present invention. Also, from the same viewpoint, it is preferably 1 / 99 to 99 / 1, more preferably 10 / 90 to 95 / 5, even more preferably 50 / 50 to 95 / 5, and even more preferably 70 / 30 to 95 / 5.

[0115] [Other Components] In the rubber composition for tire treads of the present invention, when silica (C1) or the like is contained as the filler (C), it is a preferred embodiment that the rubber composition contains 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.

[0116] 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.

[0117] Examples of mercapto compounds include 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 2-mercaptoethyltrimethoxysilane, and 2-mercaptoethyltriethoxysilane.

[0118] 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.

[0119] Examples of the glycidoxy compounds include γ-glycidoxypropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, and γ-glycidoxypropylmethyldimethoxysilane.

[0120] Examples of nitro compounds include 3-nitropropyltrimethoxysilane, 3-nitropropyltriethoxysilane, etc. Examples of chloro compounds include 3-chloropropyltrimethoxysilane, 3-chloropropyltriethoxysilane, 2-chloroethyltrimethoxysilane, 2-chloroethyltriethoxysilane, etc.

[0121] Other compounds include, for example, octyltriethoxysilane, methyltriethoxysilane, methyltrimethoxysilane, and hexadecyltrimethoxysilane.

[0122] 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.

[0123] 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, dispersibility, coupling effect, reinforcement, and abrasion resistance are improved.

[0124] The rubber composition for a tire tread 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 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).

[0125] The rubber composition for a tire tread 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).

[0126] The rubber composition for a tire tread of the present invention may further contain a vulcanization aid (F) when, for example, sulfur or a sulfur compound is contained as a 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).

[0127] The rubber composition for a tire tread 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).

[0128] The rubber composition for a tire tread of the present invention may contain, as needed, a softener such as silicone oil, aromatic oil, treated distilled aromatic extracts (TDAE), mild extracted solvates (MES), residual aromatic extracts (RAE), process oil such as paraffin oil or naphthenic oil, a resin component such as an aliphatic hydrocarbon resin, an alicyclic hydrocarbon resin, a C9 resin, a rosin resin, a coumarone-indene resin or a phenolic resin, or a liquid polymer such as low-molecular-weight polybutadiene, low-molecular-weight polyisoprene, a low-molecular-weight styrene-butadiene copolymer or a low-molecular-weight styrene-isoprene copolymer, 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 tire tread 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.

[0129] The rubber composition for a tire tread of the present invention may contain additives such as antioxidants, antioxidants, waxes, lubricants, light stabilizers, scorch inhibitors, processing aids, colorants such as pigments and dyes, 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.

[0130] 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.

[0131] [Method for producing rubber composition for tire tread] The method for producing the rubber composition for tire tread 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 tire tread 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.

[0132] The rubber composition for tire treads 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 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.

[0133] 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.

[0134] [Tire Tread] The tire tread of the present invention uses, at least in part, the rubber composition for a tire tread. This tire tread can achieve both improved fuel economy and improved wear resistance without impairing wet grip performance. It is particularly suitable as a tire tread for use in tires for electric vehicles, which are relatively heavy and characterized by electric motor drive characteristics (e.g., high initial torque). An electric vehicle is a vehicle that uses an electric motor as at least part of its power source (e.g., an electric vehicle, a hybrid vehicle, or a fuel cell vehicle). The tire tread of the present invention can be obtained by molding a tire tread rubber having a predetermined cross-sectional shape from the rubber composition for a tire tread obtained as described above using an extruder or the like, and then using this tire tread rubber to produce tires for various applications, such as passenger car tires (e.g., pneumatic tires, solid tires, and airless tires), by a conventional method (generally including a crosslinking step). A tire containing the tire tread of the present invention (e.g., a pneumatic tire for an electric vehicle) can be obtained.

[0135] 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 (A1): STR20 (natural rubber produced in Thailand) Butadiene rubber (A2): BR01 (manufactured by ENEOS Material Corporation, Mw: 550,000, cis content: 95% by mass) Styrene-butadiene rubber (A3): solution-polymerized styrene-butadiene copolymer HPR850 (manufactured by ENEOS Material Corporation, styrene content: 27.5% by mass, vinyl content: 59% by mass) Modified Liquid Diene Rubber (B) Modified liquid diene rubber obtained in Production Examples 1 to 3 described below Filler (C) Silica: ULTRASIL9100GR (manufactured by Evonik Degussa Japan, wet silica) Carbon black: Diablack I (manufactured by Mitsubishi Chemical Corporation, average particle size: 23 nm) Vulcanizing Agent (D) Sulfur: Myucron OT-20 (manufactured by Shikoku Chemicals Corporation, insoluble sulfur, oil content: 18 to 22%) <Vulcanization accelerator (E)> Vulcanization accelerator (1): Noccela CZ-G (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) Vulcanization accelerator (2): Noccela D (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) <Vulcanization aid (F)> Stearic acid: Lunac S-20 (manufactured by Kao Corporation) Zinc white: Zinc oxide (manufactured by Sakai Chemical Industry Co., Ltd.) <Optional components> TDAE: VivaTec 500 (manufactured by H&R) Silane coupling agent: Si-75 (manufactured by Evonik Degussa Japan) Antioxidant: Nocrac 6C (manufactured by Ouchi Shinko Chemical Industry Co., Ltd.) Wax: Suntite S (manufactured by Seiko Chemical Co., Ltd.)

[0136] Production Example 1: Production of Modified Liquid Diene Rubber (B-1) 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'1). Subsequently, 700 g of the unmodified liquid diene rubber (B'1) obtained was placed in 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-1).

[0137] Production Example 2: Production of Modified Liquid Diene Rubber (B-2) A thoroughly dried 5 L autoclave was purged with nitrogen, and 1,100 g of hexane and 204 g of n-butyllithium (17% by mass hexane solution) were charged. The temperature was raised to 50°C, and then 1,300 g of butadiene was added sequentially while stirring to maintain the polymerization temperature at 50°C. Polymerization was then 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, separation of the polymer solution phase and the aqueous phase was confirmed, followed by separation of the water. The washed polymer solution was vacuum-dried at 70°C for 24 hours to yield an unmodified liquid diene rubber (B'2). Subsequently, 700 g of the resulting unmodified liquid diene rubber (B'2) was charged into a 1 L autoclave, and the mixture was degassed with nitrogen while stirring at 60°C for 3 hours. 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 a modified liquid diene rubber (B-2).

[0138] Production Example 3: Production of Modified Liquid Diene Rubber (B-3) 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, under stirring conditions, 1,350 g of butadiene was added sequentially 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, separation of the polymer solution phase and the aqueous phase was confirmed, and the water was then separated. The polymer solution after washing was vacuum dried at 70 ° C. for 24 hours to obtain an unmodified liquid diene rubber (B'3). Subsequently, 700 g of the obtained unmodified liquid diene rubber (B'3) was charged into a 1 L autoclave, and the mixture was degassed with nitrogen while stirring at 60 ° C. for 3 hours. 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 a modified liquid diene rubber (B-3).

[0139] The methods for measuring and calculating the various physical properties of the modified liquid diene rubbers obtained in the Production Examples are as follows.

[0140] (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

[0141] (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 concentration of modified liquid diene rubber (B) / d2 chloroform = 50 mg / 1 mL, and with an accumulation number 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.

[0142] (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).

[0143] (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.).

[0144] (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 determined from 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. (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 Weight of Functional Group)

[0145] 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 It was calculated from the area ratio of the peak derived from the functional group to the peak derived from the polymer main chain obtained by C-NMR. The peak derived from the functional group refers to the peak derived from the alkoxy group.

[0146] The physical properties of the modified liquid diene rubbers (B-1) to (B-3) obtained in Production Examples 1 to 3 are summarized in Table 1 below.

[0147]

[0148] Examples 1 to 10 and Comparative Examples 1 to 6: In accordance with the compounding ratios (parts by mass) listed in Tables 2 and 3, the components other than the vulcanizing agent (sulfur) and vulcanization accelerator were each charged into an internal Banbury mixer, and the mixture was kneaded for 4 minutes while controlling the starting temperature to 60°C and the resin temperature to 155-160°C. The mixture was then removed from the mixer and cooled to room temperature. The mixture was then recharged into the Banbury mixer, and the starting temperature was 60°C and the resin temperature was 155-160°C. The mixture was kneaded for 3 minutes while controlling the starting temperature to 60°C and the resin temperature to 155-160°C. The mixture was then removed from the mixer and cooled to room temperature. The mixture was then recharged into the Banbury mixer, and the vulcanizing agent (sulfur) and vulcanization accelerator were added. The mixture was then kneaded for 75 seconds at an initial temperature of 50°C and an ultimate temperature of 100°C to obtain a rubber composition.

[0149] The obtained rubber composition was press-molded (160°C, 20 to 30 minutes) to prepare a vulcanized rubber sheet (2 mm thick), and the fuel economy performance, abrasion resistance, and wet grip performance 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.

[0150] (Fuel Efficiency Performance) Test pieces measuring 40 mm long x 5 mm wide were cut from the sheets of the rubber compositions prepared in the Examples and Comparative Examples. 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 numerical values ​​for each Example and Comparative Example in Table 2 are relative values ​​when the value for Comparative Example 1 is set to 100. Note that a relative value of 100±5 (95 to 105) to Comparative Example 1 is equivalent to Comparative Example 1, a relative value of less than 95 means that the fuel efficiency performance of the rubber composition is better than Comparative Example 1, and a relative value of more than 105 means that the fuel efficiency performance of the rubber composition is worse than Comparative Example 1. In addition, rubber compositions containing resin were examined in Table 3. The numerical values ​​for each Example and Comparative Example in Table 3 are relative values ​​when the value for Comparative Example 2 is set to 100. The performance was evaluated using the same criteria as in Table 2, with Comparative Example 2 as the standard.

[0151] (Wear Resistance, FPS Wear Tester) Test specimens with holes the size of dedicated mandrels, 50 mm in diameter and 10 mm thick, were prepared from the rubber compositions prepared in the Examples and Comparative Examples. Using an FPS wear tester manufactured by Ueshima Seisakusho, the wear amount was measured under the following conditions: METABRIT (grain size 240, abrasive grain A) manufactured by Noritake Coated Abrasives Co., Ltd., as the wear surface; a sample speed of 80 m / min; a load of 40 N; a talc feeder speed of 0.4 rpm; a set temperature of 35°C; and a slip ratio of 15%. The numerical values ​​for each Example and Comparative Example in Table 2 are relative values, with the value for Comparative Example 1 set to 100. A relative value of 100±5 (95-105) to Comparative Example 1 indicates the same as Comparative Example 1; a relative value of less than 95 indicates a small amount of wear and better wear resistance than Comparative Example 1; and a relative value of more than 105 indicates a large amount of wear and worse wear resistance than Comparative Example 1. Furthermore, rubber compositions containing resin were examined in Table 3. The values ​​of each Example and Comparative Example in Table 3 are relative values ​​when the value of Comparative Example 2 is set to 100. The performance was evaluated using Comparative Example 2 as the standard and the same criteria as in Table 2.

[0152] (Wet Grip Performance) The wet road friction coefficient (μ) was evaluated as an index of the wet grip performance of the rubber composition. The wet road friction coefficient was measured using cylindrical friction coefficient measurement test pieces prepared in the Examples and Comparative Examples. The measurement equipment and conditions are as follows. The friction coefficient was measured over a slip ratio range of 0 to 40% between the tire and the road surface, and the maximum obtained friction coefficient was taken as the wet road friction coefficient (μ). The numerical values ​​for each Example and Comparative Example in Table 2 are relative values ​​when the value for Comparative Example 1 is set to 100. A relative value of 100±5 (95 to 105) to Comparative Example 1 indicates the same wet grip performance as Comparative Example 1, a relative value exceeding 105 indicates better wet grip performance than Comparative Example 1, and a relative value less than 95 indicates worse wet grip performance than Comparative Example 1. Furthermore, rubber compositions containing resin were examined in Table 3. The numerical values ​​for each Example and Comparative Example in Table 3 are relative values ​​when the value for Comparative Example 2 is set to 100. The performance was evaluated based on the same criteria as in Table 2, with Comparative Example 2 as the standard. [Measurement device and measurement conditions] Device: RTM friction tester manufactured by Ueshima Seisakusho Co., Ltd. Measurement temperature: 20°C Road surface: METABRIT manufactured by Noritake Coated Abrasives Co., Ltd., grain size 120, abrasive grain A Amount of water supplied to road surface: 1.0 L / min Temperature of water supplied to road surface: 20°C Speed: 30 km / hrs Load: 50 N Slip ratio: 0 to 40%

[0153]

[0154]

[0155] Compared with tire treads obtained from conventional rubber compositions, tire treads obtained from the rubber composition for tire treads of the present invention have improved fuel economy performance while maintaining the same or improved wet grip performance. Furthermore, they have improved abrasion resistance while maintaining the same or improved wet grip performance. Therefore, they achieve a balanced improvement in the three performances of fuel economy performance, abrasion resistance, and wet grip performance, which was previously difficult. Therefore, they are useful as tire treads, and are particularly useful as tire treads for automobiles that are heavier than conventional ones, such as electrically powered automobiles (electric automobiles, hybrid automobiles, and fuel cell automobiles).

Claims

1. A rubber composition for tire treads, comprising 100 parts by mass of solid rubber (A) comprising natural rubber (A1), butadiene rubber (A2), and styrene-butadiene rubber (A3), 0.1 to 50 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 20 to 200 parts by mass of a filler (C), wherein 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) being 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 tire treads 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 tire treads according to claim 1 or 2, wherein the content of the natural rubber (A1) in 100% by mass of the solid rubber (A) is 40 to 80% by mass.

4. A rubber composition for tire treads according to any one of claims 1 to 3, wherein the mass ratio (A2) / (A3) of the butadiene rubber (A2) to the styrene-butadiene rubber (A3) contained in the solid rubber (A) is 0.12 or more and 3 or less.

5. A rubber composition for a tire tread according to any one of claims 1 to 4, which contains silica as the filler (C).

6. The rubber composition for tire treads according to claim 5, further comprising carbon black as the filler (C).

7. A crosslinked product obtained by crosslinking the rubber composition for tire treads according to any one of claims 1 to 6.

8. A tire tread, at least in part, using the rubber composition for tire treads according to any one of claims 1 to 7.

9. A pneumatic tire for an electric motor vehicle comprising the tire tread of claim 8.

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