Rubber composition for studless tires

Abstract

To provide a rubber composition for studless tires which improves on-ice performance and wear resistance.SOLUTION: The rubber composition for studless tires contains 10-30 pts.mass of thermally expandable microcapsules based on 100 pts.mass of a diene rubber. An outer shell constituting the thermally expandable microcapsules comprises a thermoplastic resin being a polymer of a polymerizable component containing a monofunctional monomer (A) and a polyfunctional monomer (B). The monofunctional monomer (A) contains a nitrile-based monomer and a carboxyl group-containing monomer. The polyfunctional monomer (B) has two or more (meth)acryloyl groups and a reactive carbon-carbon double bond other than the (meth)acryloyl groups, is represented by general formula: R1-O-R2-O-R3 (where R1 and R3 are each a (meth)acryloyl group; and R2 is a structure containing a polymer chain having a reactive carbon-carbon double bond), and has a weight average molecular weight of 500-50,000.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a rubber composition for studless tires that improves ice performance and wear resistance. [Background technology]

[0002] To improve the ice performance of studless tires, it is known that increasing the surface roughness (undulation) of the tread rubber is beneficial. It is thought that increasing surface roughness has the effect of increasing the contact area with the ice surface compared to tread rubber with a smooth surface, as the recesses trap the water film present on the ice surface and the protrusions come into contact with the ice surface. For this reason, there are methods to increase the surface roughness of the rubber by incorporating foaming agents or thermally expandable microcapsules into the tread rubber (see, for example, Patent Document 1).

[0003] However, incorporating conventional foaming agents or thermally expandable microcapsules raises concerns that the tread rubber's tensile strength may decrease, leading to reduced wear resistance. In particular, if the shell material of the expanded thermally expandable microcapsules detaches from the tread rubber, or if the dispersion of thermally expandable microcapsules within the tread rubber is poor, it can lead to a decrease in tensile strength and further deterioration of wear resistance.

[0004] In recent years, the demands for improved ice performance and wear resistance in studless tires have become more stringent, and further improvements in the balance between ice performance and wear resistance are required. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2018-123209 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The object of the present invention is to provide a rubber composition for studless tires that improves ice performance and wear resistance to a level beyond that of conventional tires. [Means for solving the problem]

[0007] The present invention provides a rubber composition for studless tires that achieves the above objective, comprising 100 parts by mass of diene rubber and 0.3 to 30 parts by mass of thermally expandable microcapsules, wherein the outer shell constituting the thermally expandable microcapsules is made of a thermoplastic resin which is a polymer of a polymerizable component containing a monofunctional monomer (A) and a polyfunctional monomer (B), the monofunctional monomer (A) includes a nitrile monomer and a carboxyl group-containing monomer, and the polyfunctional monomer (B) has at least two (meth)acryloyl groups and reactive carbon-carbon double bonds other than the (meth)acryloyl groups, is represented by the following general formula (1), and has a weight-average molecular weight of 500 to 50000. R 1 -OR 2 -OR 3 (1) (In the formula, R 1 and R 3 is a (meth)acryloyl group, R 2 This structure includes a polymer chain having the reactive carbon-carbon double bond. Furthermore, the polymer chain contains a diene as a constituent unit, and the diene is butadiene and / or isoprene. .) [Effects of the Invention]

[0008] The present invention provides a rubber composition for studless tires in which the shell material of the heat-expandable microcapsules is a copolymer of a monofunctional monomer (A) containing a nitrile monomer and a carboxyl group-containing monomer, and a polyfunctional monomer (B) having a (meth)acryloyl group and a reactive carbon-carbon double bond. This improves the dispersibility of the heat-expandable microcapsules in the diene rubber and makes them less likely to fall out of the rubber, thereby suppressing the decrease in rubber strength and improving ice performance and abrasion resistance to levels above conventional levels.

[0009] The polymer chain contains a diene as a constitutional unit, and the diene may be butadiene and / or isoprene. Also, in a total of 100% by mass of the monofunctional monomer (A) and the polyfunctional monomer (B), the polyfunctional monomer (B) may be 0.1 to 10.0% by mass. The average particle diameter of the thermally expandable microcapsules may be 20 to 30 μm.

[0010] The rubber composition for studless tires preferably has a rubber hardness of 60 or less according to JIS K6253, type A durometer, at 23°C, and / or a glass transition temperature of -60°C or less, and can have more excellent ice performance. It is preferable that the diene rubber contains a modified butadiene rubber, and it is more preferable that the rubber composition for studless tires further contains an aromatic-modified terpene resin.

[0011] A studless tire formed by molding a tread portion with the rubber composition for studless tires can improve ice performance and wear resistance to levels higher than the conventional levels.

Embodiments for Carrying Out the Invention

[0012] The diene rubber constituting the rubber composition for studless tires of the present invention preferably includes polybutadiene and natural rubber. By including polybutadiene, flexibility at low temperatures can be ensured. Polybutadiene is preferably contained at 30% by mass or more, more preferably 30 to 70% by mass, and even more preferably 35 to 65% by mass in 100% by mass of the diene rubber. When polybutadiene is 30% by mass or more, flexibility at low temperatures is ensured, which is preferable. Polybutadiene is at least one selected from unmodified polybutadiene and modified polybutadiene (modified butadiene rubber), and when both unmodified polybutadiene and modified butadiene rubber are included, the total of the two may be 30% by mass or more.

[0013] The modified butadiene rubber is not particularly limited as long as it has an affinity with the shell material of the thermally expandable microcapsules. Examples of the modifying group possessed by the modified butadiene rubber include a hydroxyl group, an amino group, a carboxyl group, an epoxy group, an alkoxy group, a silyl group, an amide group, an organosiloxane group, and the like. Among them, an organosiloxane group and a carboxyl group are preferable.

[0014] The natural rubber is not particularly limited and can usually contain those used in the rubber composition for studless tires. The natural rubber is preferably contained in an amount of 30 to 70% by mass, more preferably 35 to 65% by mass, in 100% by mass of the diene rubber. By containing 30% by mass or more of the natural rubber, excellent abrasion resistance is preferable. Also, by containing 70% by mass or less of the natural rubber, excellent flexibility at low temperatures is preferable.

[0015] The diene rubber can contain other diene rubbers other than polybutadiene and natural rubber. Examples of the other diene rubbers include isoprene rubber, styrene-butadiene rubber, styrene-isoprene rubber, styrene-isoprene-butadiene rubber, acrylonitrile-butadiene rubber, and the like. Among them, styrene-butadiene rubber is preferable. These other diene rubbers may be unmodified, or may be modified diene rubbers in which the terminals and / or side chains of their molecular chains are modified with an epoxy group, a carboxyl group, an amino group, a hydroxy group, an alkoxy group, a silyl group, an amide group, or the like.

[0016] <~! The rubber composition for studless tires preferably contains a white filler. The compounding amount of the white filler is preferably 30 parts by mass or more, more preferably 30 to 100 parts by mass, still more preferably 40 to 90 parts by mass, and particularly preferably 45 to 80 parts by mass with respect to 100 parts by mass of the diene rubber. By setting the compounding amount of the white filler to 30 parts by mass or more, the mechanical properties of the rubber composition for studless tires can be improved and the abrasion resistance can be enhanced, which is preferable. Also, by setting the compounding amount of the white filler to 100 parts by mass or less, the flexibility of the rubber composition for studless tires can be maintained and the ice performance can be ensured, which is preferable. Further, an increase in weight can be suppressed when making studless tires.

[0017] Examples of the white filler include silica, calcium carbonate, magnesium carbonate, talc, clay, alumina, aluminum hydroxide, titanium oxide, and calcium sulfate. These may be used alone or in combination of two or more. Among them, silica is preferable and can make the ice performance more excellent.

[0018] Examples of silica include wet silica (hydrated silicic acid), dry silica (anhydrous silicic acid), calcium silicate, aluminum silicate, etc. These may be used alone or in combination of two or more. The CTAB adsorption specific surface area of silica is not particularly limited, but is preferably 80 to 260 m 2 / g, more preferably 140 to 200 m 2 / g. By setting the CTAB adsorption specific surface area of silica to 80 m 2 / g or more, the abrasion resistance of the rubber composition for studless tires can be ensured. Also, by setting the CTAB adsorption specific surface area of silica to 200 m 2 / g or less, the wet performance and low rolling resistance can be improved. In this specification, the CTAB specific surface area of silica is the value measured according to ISO 5794.

[0019] In this invention, it is preferable to incorporate a silane coupling agent together with silica. By incorporating a silane coupling agent, the dispersibility of silica in diene-based rubber can be improved, and a better balance between wear resistance and ice performance can be achieved.

[0020] The type of silane coupling agent is not particularly limited as long as it can be used in silica-containing rubber compositions for studless tires. Examples include sulfur-containing silane coupling agents such as bis-(3-triethoxysilylpropyl)tetrasulfide, bis(3-triethoxysilylpropyl)disulfide, 3-trimethoxysilylpropylbenzothiazoletetrasulfide, γ-mercaptopropyltriethoxysilane, and 3-octanoylthiopropyltriethoxysilane.

[0021] The amount of silane coupling agent added is preferably 3 to 15% by mass relative to the weight of silica, and more preferably 5 to 10% by mass. If the amount of silane coupling agent added is less than 3% by mass of the silica content, there is a risk that the dispersion of silica cannot be sufficiently improved. If the amount of silane coupling agent added exceeds 15% by mass of the silica content, the silane coupling agents will condense with each other, making it impossible to obtain the desired hardness and strength in the rubber composition for studless tires.

[0022] The rubber composition for studless tires preferably contains carbon black. When carbon black is added, the total amount of white filler and carbon black is preferably more than 30 parts by mass and 100 parts by mass, more preferably 40 to 90 parts by mass, and even more preferably 45 to 80 parts by mass, per 100 parts by mass of diene rubber. Examples of carbon black include furnace carbon black such as SAF, ISAF, HAF, FEF, GPF, HMF, and SRF, which may be used individually or in combination of two or more. The nitrogen adsorption specific surface area of ​​carbon black is not particularly limited, but is preferably 70 to 240 m². 2 / g, more preferably 90-200m 2It is desirable that the nitrogen adsorption specific surface area of ​​carbon black be 70 m². 2 By increasing the nitrogen adsorption specific surface area of ​​carbon black to 240 m² or more, the mechanical properties and wear resistance of the rubber composition for studless tires can be ensured. 2 By reducing the nitrogen adsorption specific surface area to less than / g, ice performance can be improved. In this specification, the nitrogen adsorption specific surface area of ​​carbon black shall be measured in accordance with JIS K6217-2.

[0023] The rubber composition for studless tires can improve ice performance and wear resistance compared to conventional compositions by incorporating heat-expandable microcapsules having a specific shell material. The outer shell constituting the heat-expandable microcapsule is made of a thermoplastic resin which is a polymer of polymerizable components containing a monofunctional monomer (A) and a polyfunctional monomer (B). The monofunctional monomer (A) includes a nitrile monomer and a carboxyl group-containing monomer, and the polyfunctional monomer (B) has at least two (meth)acryloyl groups and reactive carbon-carbon double bonds other than (meth)acryloyl groups, is represented by the following general formula (1), and has a weight-average molecular weight of 500 to 50000. R 1 -OR 2 -OR 3 (1) (In the formula, R 1 and R 3 is a (meth)acryloyl group, R 2 (This structure includes a polymer chain having the aforementioned reactive carbon-carbon double bond.)

[0024] The outer shell constituting the thermally expandable microcapsule is made of a thermoplastic resin, which is a polymer of a polymerizable component containing a monofunctional monomer (A) containing a nitrile monomer and a carboxyl group-containing monomer, and a polyfunctional monomer (B). A polymerizable component means a monomer having at least one polymerizable group in its molecule, and is a component that becomes a thermoplastic resin that forms the outer shell of the thermally expandable microcapsule when polymerized. The polymerizable component includes a monofunctional monomer having one reactive carbon-carbon double bond and a polyfunctional monomer having two or more reactive carbon-carbon double bonds. A cross-linking structure can be introduced into the polymer by the polyfunctional monomer. The reactive carbon-carbon double bond referred to here means a carbon-carbon double bond that exhibits radical reactivity, and is not a carbon-carbon double bond in an aromatic ring, but rather a carbon-carbon double bond contained in vinyl groups, (meth)acryloyl groups, allyl groups, vinylene groups, etc.

[0025] The monofunctional monomer (A) includes nitrile monomers and carboxyl group-containing monomers. The inclusion of such monofunctional monomers (A) in the polymerizable components enhances the gas barrier properties of the thermoplastic resin constituting the outer shell. This reduces leakage when the encapsulated foaming agent vaporizes, allowing for efficient expansion. Furthermore, the increased strength of the thermoplastic resin suppresses crushing and deformation during the creation of the rubber composition for studless tires.

[0026] Examples of nitrile monomers include acrylonitrile, methacrylonitrile, and fumaronitrile. The nitrile monomer is preferably acrylonitrile and / or methacrylonitrile.

[0027] Examples of carboxyl group-containing monomers include acrylic acid, methacrylic acid (MAA), itaconic acid, maleic acid, fumaric acid, and citraconic acid.

[0028] In monofunctional monomer (A), the nitrile monomer is preferably 40 to 95% by mass, more preferably 50 to 90% by mass, out of a total of 100% by mass of nitrile monomers and carboxyl group-containing monomers. By increasing the nitrile monomer content to 40% by mass or more, ice performance can be improved, and by decreasing it to 95% by mass or less, the strength of the thermally expandable microcapsules can be improved.

[0029] The monofunctional monomer (A) may include monomers other than nitrile monomers and carboxyl group-containing monomers. Examples of such monomers include (meth)acrylic acid ester monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, stearyl (meth)acrylate, phenyl (meth)acrylate, isobornyl (meth)acrylate, cyclohexyl (meth)acrylate, benzyl (meth)acrylate, and 2-hydroxyethyl (meth)acrylate; vinylidene chloride; vinyl acetate; styrene monomers such as styrene, α-methylstyrene, and chlorostyrene; and (meth)acrylamide monomers such as (meth)acrylamide and substituted (meth)acrylamide.

[0030] The polyfunctional monomer (B) has a weight-average molecular weight of 500 to 50,000, preferably 600 to 35,000, more preferably 1,000 to 30,000, and even more preferably 1,500 to 25,000. A weight-average molecular weight of 500 or more for the polyfunctional monomer (B) improves its compatibility with thermally expandable microcapsules in the diene rubber. A weight-average molecular weight of 50,000 or less allows for uniform dispersion in the diene rubber, enabling uniform expansion of the thermally expandable microcapsules.

[0031] The polyfunctional monomer (B) has at least two (meth)acryloyl groups and reactive carbon-carbon double bonds other than the (meth)acryloyl groups, and is represented by the following general formula (1). R 1 -OR2 -OR 3 (1) (In the formula, R 1 and R 3 is a (meth)acryloyl group, R 2 This structure includes polymer chains with reactive carbon-carbon double bonds.

[0032] The polyfunctional monomer (B) has at least two (meth)acryloyl groups, and these two or more (meth)acryloyl groups may be identical or different. The (meth)acryloyl group has a structure in which it has a reactive carbon-carbon double bond and a polar carbon-oxygen double bond, and therefore has very high radical reactivity. For this reason, the (meth)acryloyl group mainly contributes to the bridging structure of the polymer, and furthermore, the polymer has a reactive carbon-carbon double bond in its molecule, improving the compatibility between the diene rubber and the heat-expandable microcapsules.

[0033] R 1 and R 3 This is a (meth)acryloyl group. R 2 This structure includes polymer chains having reactive carbon-carbon double bonds. 2 The polymer chain may have a reactive carbon-carbon double bond in a structural part other than the polymer chain. 2 These can take the form of a linear or branched structure.

[0034] R 2The polymer may consist solely of polymer chains, or it may be a structure in which polymer chains are bonded to organic and / or inorganic groups other than polymer chains. Organic groups are defined as functional groups containing carbon. There are no particular limitations on organic groups, but examples include alkyl groups, alkylene groups, alkenyl groups, alkynyl groups, alkoxy groups, oxyalkylene groups, carboxyl groups, anhydrous carboxyl groups, ester groups, carbonyl groups, amide groups, urethane groups, phenyl groups, phenylene groups, (meth)acryloyl groups and allyl groups having reactive carbon-carbon double bonds, etc. One organic group may be bonded to the polymer chain, or two or more organic groups may be bonded to the polymer chain. Inorganic groups are defined as functional groups that do not contain carbon. There are no particular limitations on inorganic groups, but examples include hydroxyl groups, ether groups, amino groups, sulfo groups, halogen groups such as fluoro and chloro groups, and silanol groups. Similar to organic groups, one inorganic group may be bonded to the polymer chain, or two or more inorganic groups may be bonded to the polymer chain.

[0035] When the polymer chain contains a diene as a constituent unit, the number of reactive carbon-carbon double bonds in the polyfunctional monomer (B) increases, which can impart elasticity to the outer thermoplastic resin, and is therefore preferable. Examples of dienes include 1,3-butadiene (hereinafter also simply referred to as butadiene); 1,3-pentadiene; 1,3-hexadiene; 2,4-hexadiene; 1,3-heptadiene; 1,3-octadiene; isoprene; chloroprene; 2-methyl-1,3-butadiene, 2-ethyl-1,3-butadiene, 2-propyl-1,3-butadiene, 2-butyl-1,3-butadiene, 2-pentyl-1,3-butadiene, 2-hexyl-1,3-butadiene, 2-heptyl- 2-alkyl-1,3-butadiene such as 1,3-butadiene, 2-octyl-1,3-butadiene, 2-neopentyl-1,3-butadiene; 2,3-dialkyl-1,3-butadiene such as 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1,3-butadiene; 1-phenyl-1,3-butadiene; aryl-1, such as 2-phenyl-1,3-butadiene Conjugated dienes such as 3-butadiene, 1-phenyl-2,4-pentadiene, 2-chloro-1,3-butadiene, 2-cyano-1,3-butadiene, 3-methyl-1,3-pentadiene, 1,4-hexadiene, 3-methyl-1,4-hexadiene, 4-methyl-1,4-hexadiene, 5-methyl-1,4-hexadiene, 4,5-dimethyl-1,4-hexadiene, 7-methyl-1,6-octadiene, 8-methyl-4-ethylidene-1,7-nonadiene, and 4-ethylidene Examples of non-conjugated dienes include den-1,7-undecadiene; methyltetrahydroindene; 5-ethylidene-2-norbornene; 5-methylene-2-norbornene; 5-isopropylidene-2-norbornene; 5-vinylidene-2-norbornene; 6-chloromethyl-5-isopropenyl-2-norbornene; 5-vinyl-2-norbornene; 5-isopropenyl-2-norbornene; 5-isobutenyl-2-norbornene; cyclopentadiene; norbornadiene, etc.

[0036] Among the dienes, butadiene, 1,3-pentadiene, isoprene, chloroprene, 1,3-pentadiene, and 1,3-hexadiene are preferred in terms of more effectively achieving the effects of the present invention, and butadiene and isoprene are even more preferred. The polymer chain may contain one or more types of dienes. When the polymer chain contains two or more types of dienes as constituent units, the polymer chain may be a polymer in which the constituent units of each diene are polymerized randomly, such as a random copolymer, or a polymer in which the constituent units of each diene are polymerized in a cohesive manner, such as a block copolymer.

[0037] The polymer chain may contain components other than dienes as constituent units, to the extent that they do not impair the effects of the present invention. Examples of constituent units other than dienes include polyene-based constituent units having three or more reactive carbon-carbon double bonds in a constituent unit such as 1,3,5-hexatriene; nitrile-based constituent units such as acrylonitrile and methacrylonitrile; aromatic vinyl-based constituent units such as styrene, p-methylstyrene, α-methylstyrene, vinylethylbenzene, vinylxylene, vinylnaphthalene, and diphenylethylene; and olefin-based constituent units such as ethylene, polypropylene, and isobutylene. The polymer chain may contain one or more types of constituent units other than dienes. When the polymer chain contains constituent units other than dienes, the polymer chain may be a polymer in which dienes and other constituent units are randomly polymerized, such as a random copolymer, or a polymer in which dienes and other constituent units are polymerized in a cohesive manner, such as a block copolymer.

[0038] The polyfunctional monomer (B) is preferably 0.1 to 10.0% by mass, more preferably 0.2 to 7.0% by mass, and even more preferably 0.3 to 5.0% by mass, of the total 100% by mass of monofunctional monomer (A) and polyfunctional monomer (B). A polyfunctional monomer (B) of 0.1% by mass or more is preferable because it improves the compatibility between the diene rubber and the thermally expandable microcapsules. Furthermore, a polyfunctional monomer (B) of 10.0% by mass or less is preferable because it improves the expansion performance of the thermally expandable microcapsules.

[0039] Thermally expandable microcapsules acquire thermal expandability (the property of the entire microcapsule expanding when heated) by encapsulating a blowing agent, which is a component that vaporizes when heated, within an outer shell made of thermoplastic resin. The blowing agent is not particularly limited, but examples include hydrocarbons with 3 to 13 carbon atoms such as methane, ethane, propane, (iso)butane, (iso)pentane, (iso)hexane, (iso)heptane, (iso)octane, (iso)nonane, (iso)decane, (iso)undecane, (iso)dodecane, and (iso)tridecane; hydrocarbons with more than 13 carbon atoms and 20 or less such as (iso)hexadecane and (iso)eicosane; pseudocumene, petroleum ether, and normal paraffins and isoparaffins with an initial boiling point of 150 to 260°C and / or a distillation range of 70 to 360°C. Examples include hydrocarbons such as petroleum fractions; halides of hydrocarbons having 1 to 12 carbon atoms, such as methyl chloride, methylene chloride, chloroform, and carbon tetrachloride; fluorine-containing compounds such as hydrofluoroethers; silanes having alkyl groups with 1 to 5 carbon atoms, such as tetramethylsilane, trimethylethylsilane, trimethylisopropylsilane, and trimethyl-n-propylsilane; and compounds that generate gas through thermal decomposition upon heating, such as azodicarbonamide, N,N'-dinitrosopentamethylenetetramine, and 4,4'-oxybis(benzenesulfonylhydrazide). The blowing agent may consist of one compound or a mixture of two or more compounds. The blowing agent may be linear, branched, or alicyclic, and is preferably aliphatic.

[0040] The maximum expansion temperature (Tmax) of the thermally expandable microcapsules is preferably within ±20°C of the heat molding temperature of the studless tire rubber composition. By setting the maximum expansion temperature (Tmax) within ±20°C of the heat molding temperature of the studless tire rubber composition, it is possible to make the shape of the pores introduced into the studless tire uniform, which is preferable.

[0041] The volume-average particle diameter (D50) of the thermally expandable microcapsules (hereinafter sometimes referred to as "average particle diameter") is not particularly limited, but is preferably 20 to 30 μm. Having an average particle diameter within the aforementioned range of thermally expandable microcapsules is preferable because it allows for the introduction of sufficiently sized pores into the studless tire.

[0042] The maximum expansion ratio of the thermally expandable microcapsules is not particularly limited, but is preferably 10 to 200 times. Having the maximum expansion ratio of the thermally expandable microcapsules within the aforementioned range is preferable because it allows for the introduction of sufficiently sized pores into the studless tire.

[0043] The thermoplastic resin that constitutes the shell material of a thermally expandable microcapsule can be manufactured using conventionally known methods. For example, by suspension polymerization, the shell material of a thermally expandable microcapsule can be easily manufactured by dispersing a monomer mixture in an aqueous dispersion medium and polymerizing it. During suspension polymerization, polymerization initiators, electrolytes, dispersants, surfactants, etc., are used as needed.

[0044] Preferred polymerization initiators are oil-soluble peroxides or azo compounds. Specifically, examples include oil-soluble peroxides such as diisopropyl peroxydicarbonate, di-sec-butyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and dilauroyl peroxide, and oil-soluble azo compounds such as 2,2'-azobisisobutyronitrile and 2,2'-azobis(2,4-dimethylvaleronitrile). Examples of electrolytes include sodium chloride, magnesium chloride, and calcium chloride. Examples of dispersants include poorly water-soluble inorganic compounds such as colloidal silica, calcium phosphate, magnesium hydroxide, and magnesium pyrophosphate, and water-soluble polymers such as polyvinyl alcohol, methylcellulose, and polyvinylpyrrolidone. Examples of surfactants include anionic surfactants such as sodium lauryl sulfate and sodium dodecylbenzenesulfonate; nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, sorbitan fatty acid esters, polyoxyethylene alkylamines, and adipic acid-diethanolamine condensates; and amphoteric surfactants such as lauryldimethylamine oxide and lauryldimethylaminoacetic acid betaine. The polymerization initiators, electrolytes, dispersants, and surfactants mentioned above can be used individually or in combination of two or more.

[0045] The polymerization reaction is carried out by dispersing the monomer mixture in an aqueous dispersion medium so that spherical oil droplets of a predetermined particle size are prepared, and then raising the temperature while stirring. Methods for dispersing the monomer mixture in the aqueous dispersion medium include, for example, using an emulsifying disperser such as a homomixer or static mixer, using an ultrasonic disperser, or using a membrane emulsification method. The polymerization temperature is preferably 40 to 100°C, and the polymerization time is preferably 1 to 20 hours.

[0046] The thermally expandable microcapsules are blended in an amount of 0.3 to 30 parts by mass, preferably 1 to 20 parts by mass, and more preferably 3 to 15 parts by mass, per 100 parts by mass of diene rubber. If the amount of thermally expandable microcapsules is less than 0.3 parts by mass, the ice performance cannot be sufficiently improved. Conversely, if the amount of thermally expandable microcapsules exceeds 30 parts by mass, the abrasion resistance decreases.

[0047] The rubber composition for studless tires improves wear resistance by improving the dispersibility of the heat-expandable microcapsule shell material in the diene-based rubber, by blending it with a liquid rubber having a weight-average molecular weight of 1,000 to 100,000 together with the shell material of the heat-expandable microcapsules. The weight-average molecular weight of the liquid rubber is 1,000 to 100,000, preferably 2,000 to 90,000, and more preferably 3,000 to 80,000. If the weight-average molecular weight of the liquid rubber is less than 1,000, migration to other components is more likely to occur. If the weight-average molecular weight of the liquid rubber exceeds 100,000, the dispersion effect of the resin decreases. The weight-average molecular weight of the liquid rubber is the standard polystyrene equivalent value obtained by gel permeation chromatography (GPC) measurement using a radioisotope detector with tetrahydrofuran as the solvent.

[0048] The liquid rubber is not particularly limited, but examples include liquid isoprene rubber, liquid butadiene rubber, liquid styrene-butadiene rubber, liquid ethylene-propylene rubber, liquid nitrile rubber, liquid chloroprene rubber, liquid acrylic rubber, liquid epichlorohydrin rubber, etc. Liquid isoprene rubber, liquid butadiene rubber, and liquid styrene-butadiene rubber are preferred.

[0049] The liquid rubber is added in an amount of 10 parts by mass or more, preferably 12 to 50 parts by mass, and more preferably 14 to 40 parts by mass, per 100 parts by mass of diene rubber. If the amount of liquid rubber is less than 10 parts by mass, it is not possible to improve the dispersibility of the shell material of the thermally expandable microcapsules and improve the abrasion resistance. Furthermore, the mass ratio of the amount of liquid rubber to the amount of shell material of the thermally expandable microcapsules (amount of liquid rubber / amount of shell material of thermally expandable microcapsules) is preferably 1 to 100, more preferably 2 to 50. If the mass ratio (amount of liquid rubber / amount of shell material of thermally expandable microcapsules) is less than 1, it is not possible to improve the dispersibility of the shell material of the thermally expandable microcapsules and improve the abrasion resistance. Furthermore, if the mass ratio (amount of liquid rubber / amount of shell material of thermally expandable microcapsules) exceeds 50, migration to other components is more likely to occur.

[0050] The rubber composition for studless tires can be improved in terms of abrasion resistance by incorporating aromatically modified terpene resin, thereby improving the dispersibility of the shell material of the thermally expandable microcapsules. The aromatically modified terpene resin is preferably added in an amount of 1 to 20 parts by mass, more preferably 2 to 15 parts by mass, per 100 parts by mass of diene rubber. If the amount of aromatically modified terpene resin is less than 1 part by mass, the effect of improving the dispersibility of the shell material of the thermally expandable microcapsules and improving abrasion resistance will not be sufficiently obtained. If the amount of aromatically modified terpene resin exceeds 20 parts by mass, the abrasion resistance will deteriorate.

[0051] Aromatically modified terpene resins are obtained by polymerizing terpenes with aromatic compounds. Examples of terpenes include α-pinene, β-pinene, dipentene, and limonene. Examples of aromatic compounds include styrene, α-methylstyrene, vinyltoluene, and indene. Among these, styrene-modified terpene resins are preferred as aromatically modified terpene resins.

[0052] The rubber composition for studless tires should preferably have a rubber hardness of 60 or less, more preferably 40 to 57, according to JIS K6253, Durometer Type A, at 23°C. By setting the rubber hardness to 60 or less, high friction force on ice can be obtained.

[0053] Furthermore, the rubber composition for studless tires preferably has a glass transition temperature of -60°C or lower, more preferably -100 to -62°C. By setting the glass transition temperature of the rubber composition for studless tires to -60°C or lower, the flexibility of the rubber compound at low temperatures is maintained and the adhesion force to the ice surface is increased, making it suitable for use in the tread portion of studless tires. The glass transition temperature is determined by measuring a thermogram using differential scanning calorimetry (DSC) at a heating rate of 20°C / min, and taking the temperature at the midpoint of the transition region.

[0054] The rubber composition for studless tires of the present invention may contain various additives commonly used in rubber compositions for studless tires, such as vulcanizing or crosslinking agents, vulcanization accelerators, antioxidants, plasticizers, processing aids, and thermosetting resins, within limits that do not hinder the objectives of the present invention. These additives can also be mixed in a conventional manner to form a rubber composition for studless tires, which can then be used for vulcanization or crosslinking. The amounts of these additives can be conventional amounts, as long as they do not contradict the objectives of the present invention. The rubber composition for studless tires can be manufactured by mixing the above components using a conventional rubber mixing machine, such as a Banbury mixer, kneader, or roll.

[0055] The rubber composition for studless tires of the present invention is suitable for forming the tread portion of a studless tire. A studless tire in which the tread rubber is made with the rubber composition for studless tires of the present invention can improve ice performance and wear resistance to levels above conventional levels.

[0056] The present invention will be further described below with reference to examples, but the scope of the present invention is not limited to these examples. [Examples]

[0057] To prepare the studless tire rubber compositions (Examples 1-7, Standard Example, Comparative Examples 1-4) listed in Table 1, which have the common composition listed in Table 2, the components excluding sulfur, vulcanization accelerator, and heat-expandable microcapsules were kneaded in a 1.7 L Banbury mixer for 5 minutes, and released when the temperature reached 145°C to obtain a masterbatch. Sulfur, vulcanization accelerator, and heat-expandable microcapsules were added to the obtained masterbatch and kneaded in an open roll at 70°C to obtain 11 types of studless tire rubber compositions. Note that the blending amounts for the standard formulations listed in Table 2 are expressed as parts by mass per 100 parts by mass of the diene-based rubber listed in Table 1.

[0058] The obtained studless tire rubber composition was vulcanized at 170°C for 10 minutes using a mold of a predetermined shape (internal dimensions: length 150 mm, width 150 mm, thickness 2 mm) to prepare vulcanized rubber test specimens. Using the obtained vulcanized rubber test specimens, the tensile breaking strength and ice friction performance were measured using the test methods shown below.

[0059] Tensile breaking strength The obtained vulcanized rubber test specimens were cut into JIS No. 3 dumbbell-shaped specimens in accordance with JIS K6251. The tensile breaking strength was measured in accordance with JIS K6251, and the results obtained are shown in the "Breaking Strength" column of Table 1 as an index with the standard example value set to 100. A higher index indicates better abrasion resistance.

[0060] Ice friction performance The obtained vulcanized rubber test specimens were attached to a flattened cylindrical rubber base, and tested using an inside drum type ice friction tester at a temperature of -1.5°C and a load of 5.5 kg / cm². 2 The coefficient of friction on ice was measured under the condition of a drum rotation speed of 25 km / h. The obtained coefficient of friction on ice was converted into an index with the standard example value set to 100, and is shown in the "Ice Performance" column. A larger index value indicates a larger coefficient of friction on ice and superior ice performance.

[0061] The properties of thermally expandable microcapsules were determined by the following method. Measurement of the average particle size of thermally expandable microcapsules As the measuring device, a Microtrac particle size distribution analyzer (model 9320-HRA) manufactured by Nikkiso Co., Ltd. was used, and the D50 value obtained by volume-based measurement was defined as the average particle diameter.

[0062] [Table 1]

[0063] The types of raw materials used in Table 1 are listed below. • Natural rubber: RSS#3 • Polybutadiene: Polybutadiene rubber Nipol BR1220 manufactured by Nippon Zeon Co., Ltd. • Modified butadiene rubber: JSR BR54 • Carbon Black: Carbon Black Seast 6 manufactured by Tokai Carbon Co., Ltd., with a specific surface area of ​​115 m² for nitrogen adsorption. 2 / g • Silica: Nipsil AQ manufactured by Nippon Silica Industry Co., Ltd. • Coupling agent: Sulfur-containing silane coupling agent, DEXA Si69 • Thermally expandable microcapsules - A to C: Obtained by the following manufacturing methods. • Liquid rubber: Liquid butadiene rubber with a weight-average molecular weight of 30,000, manufactured by Nippon Zeon Corporation. • Modified terpene resin: Aromatic modified terpene resin, manufactured by Yasuhara Chemical Co., Ltd. (TO-125)

[0064] Method for producing thermally expandable microcapsules-A An aqueous dispersion medium was prepared by dissolving 126 parts by mass of sodium chloride in 500 parts by mass of deionized water, adding 0.25 parts by mass of polyvinylpyrrolidone, 0.1 parts by mass of carboxymethylated polyethyleneimine sodium salt, and 68 parts by mass of colloidal silica containing 20% ​​by mass of silica as an active ingredient, and adjusting the pH to 3.0. On the other hand, 100 parts by mass of acrylonitrile, 35 parts by mass of methacrylonitrile, 96 parts by mass of methacrylic acid, 9 parts by mass of methyl methacrylate, 0.5 parts by mass of polybutadiene diacrylate with a weight-average molecular weight of 10,000, 3 parts by mass of di(2-ethylhexyl)peroxydicarbonate, 30 parts by mass of isobutane, and 10 parts by mass of isooctane were mixed and dissolved to obtain an oily mixture. An aqueous dispersion medium and an oily mixture were mixed, and the resulting mixture was dispersed in a homomixer (TK Homomixer, manufactured by Plamix Corporation) at a rotation speed of 10,000 rpm for 1 minute to prepare a suspension. This suspension was transferred to a 1.5-liter pressurized reaction vessel, purged with nitrogen, and the initial reaction pressure was set to 0.35 MPa. Polymerization was carried out at a polymerization temperature of 60°C for 20 hours while stirring at 80 rpm. After polymerization, the product was filtered and dried to obtain thermally expandable microcapsules-A. The average particle size of the obtained thermally expandable microcapsules-A was 22 μm.

[0065] Method for producing thermally expandable microcapsules-B A thermally expandable microcapsule-B was obtained in the same manner as the production method for thermally expandable microcapsule-A, except that 45 parts by mass of colloidal silica containing 20% ​​by mass of silica as the active ingredient was replaced with 2 parts by mass of polybutadiene diacrylate having a weight-average molecular weight of 10,000. The average particle size of the obtained thermally expandable microcapsule-B was 29 μm.

[0066] Method for producing thermally expandable microcapsules-C A thermally expandable microcapsule-C was obtained by adjusting the preparation in the same manner as for thermally expandable microcapsule A, except that the weight-average molecular weight was changed to 0 parts by mass of polybutadiene diacrylate, which has a weight-average molecular weight of 10,000. The average particle size of the obtained thermally expandable microcapsule-C was 21 μm.

[0067] [Table 2]

[0068] The types of raw materials used in Table 2 are listed below. • Zinc oxide: Three types of zinc oxide manufactured by Seido Chemical Industry Co., Ltd. • Stearic acid: NOF bead stearic acid • Anti-aging agent: Flexis 6PPD • Aroma oil: Aroma oil manufactured by Fuji Kogyo Co., Ltd. • Wax: Sunnock manufactured by Ouchi Shinko Chemical Industry Co., Ltd. • Sulfur: Finely powdered sulfur containing Kinka oil, manufactured by Tsurumi Chemical Industry Co., Ltd. • Vulcanization accelerator: Noxellar CZ-G manufactured by Ouchi Shinko Chemical Industry Co., Ltd.

[0069] As is clear from Table 1, the rubber compositions for studless tires in Examples 1 to 6 were confirmed to improve tensile breaking strength and ice performance to levels beyond those of conventional tires.

[0070] The rubber compositions for studless tires in Comparative Examples 1 and 2 do not contain a polyfunctional monomer (B), and therefore cannot improve abrasion resistance (tensile breaking strength). The rubber composition for studless tires in Comparative Example 3 contains less than 0.3 parts by mass of thermally expandable microcapsules, and therefore cannot improve tensile breaking strength and ice performance beyond conventional levels. The rubber composition for studless tires in Comparative Example 4 has more than 30 parts by mass of thermally expandable microcapsules, resulting in inferior abrasion resistance (tensile breaking strength).

Claims

1. A rubber composition for studless tires comprising 100 parts by mass of diene rubber and 0.3 to 30 parts by mass of thermally expandable microcapsules, The outer shell constituting the thermally expandable microcapsule is made of a thermoplastic resin which is a polymer of polymerizable components containing a monofunctional monomer (A) and a polyfunctional monomer (B). The monofunctional monomer (A) includes a nitrile monomer and a carboxyl group-containing monomer. The polyfunctional monomer (B) is characterized in that it has at least two (meth)acryloyl groups and reactive carbon-carbon double bonds other than the (meth)acryloyl groups, is represented by the following general formula (1), and has a weight-average molecular weight of 500 to 50000. Rubber composition for studless tires. R １ -O-R ２ -O-R ３ (1) (In the formula, R １ and R ３ is a (meth)acryloyl group, R ２ (The structure includes a polymer chain having the reactive carbon-carbon double bond, wherein the polymer chain contains a diene as a constituent unit, and the diene is butadiene and / or isoprene.)

2. The rubber composition for studless tires according to claim 1, characterized in that the polyfunctional monomer (B) is present in an amount of 0.1 to 10.0% by mass of the total of the monofunctional monomer (A) and polyfunctional monomer (B) as described above.

3. The rubber composition for studless tires according to claim 1 or 2, characterized in that the average particle size of the thermally expandable microcapsules is 20 to 30 μm.

4. A rubber composition for studless tires according to any one of claims 1 to 3, characterized in that the rubber hardness at 23°C is 60 or less, according to JIS K6253, durometer type A.

5. A rubber composition for studless tires according to any one of claims 1 to 4, characterized in that the glass transition temperature is -60°C or lower.

6. The rubber composition for studless tires according to any one of claims 1 to 5, characterized in that the diene-based rubber includes modified butadiene rubber.

7. The rubber composition for studless tires according to any one of claims 1 to 6, further characterized by containing an aromatically modified terpene resin.

8. A studless tire characterized by having a tread portion molded with the rubber composition for studless tires described in any one of claims 1 to 7.

Images