Rubber composition for a winter tire and winter tires

Abstract

Rubber composition for the manufacture of a tread for a winter tire, which contains: a rubber component, (a) a zinc salt of an aliphatic C4-C 12 -carboxylic acid or (b) an aliphatic C4-C 12 -carboxylic acid and zinc oxide and an oil or a plasticizer, wherein the rubber component contains 45 mass% or more of butadiene rubber based on 100 mass% of the rubber component, wherein the amount of the zinc salt (a) and the amount of the mixture (b) are 0.2 parts by mass or more and 10 parts by mass or less, respectively, based on 100 parts by mass of the rubber component, and where in case (a), in the case of a zinc salt of an aliphatic C4-C 12 -carboxylic acid, the aliphatic C4-C 12-Carboxylic acid is a saturated fatty acid selected from the group consisting of heptanoic acid, nonanoic acid, undecanoic acid, isobutanoic acid, isopentanoic acid, pivalic acid, isohexanoic acid, isoheptic acid, isooctanoic acid, dimethyloctanoic acid, isononanoic acid, isodecanoic acid, isoundecanoic acid, isododecanoic acid, 2-ethylbutanoic acid, 2-ethylhexanoic acid, 2-butyloctanoic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid and sebacic acid or an unsaturated C4-C 12 -fatty acid and where in case (b), with an aliphatic C4-C 12 -carboxylic acid and zinc oxide, the aliphatic C4-C 12-Carboxylic acid is a saturated fatty acid selected from the group consisting of butanoic acid, pentanoic acid, heptanoic acid, nonanoic acid, undecanoic acid, isobutanoic acid, isopentanoic acid, pivalic acid, isohexanoic acid, isoheptic acid, isooctanoic acid, dimethyloctanoic acid, isononanoic acid, isodecanoic acid, isoundecanoic acid, isododecanoic acid, 2-ethylbutanoic acid, 2-ethylhexanoic acid, 2-butyloctanoic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid and sebacic acid or an unsaturated C4-C 12 -fatty acid.

Description

TECHNICAL FIELD

[0001] The present invention relates to a rubber composition for a winter tire (a studless tire) and a winter tire. STATE OF THE ART

[0002] The use of studded tires has been banned by law in Japan to prevent powder dust contamination resulting from the use of studded tires. As a result, winter tires are now being used in cold regions instead of studded tires. Winter tires, by having a lower elastic modulus at low temperatures, can provide, for example, better grip performance on ice and snow and higher traction. In particular, braking force on ice is largely influenced by the effective contact area between the rubber and ice. Therefore, there is a need for a vulcanized rubber composition that is flexible at low temperatures and can provide a more effective contact area.

[0003] Reducing only the hardness of a vulcanized rubber composition by, for example, increasing the amount of oil disadvantageously causes poor steering stability.

[0004] Generally, the tread rubber of winter tires is often made of natural rubber or butadiene rubber as the main component (see, for example, Patent Document 1). This is because these rubbers have a low glass transition temperature and flexibility, as well as high strength. However, natural rubber and butadiene rubber undergo reversion during sulfur vulcanization. This phenomenon causes the decomposition of the rubber or deterioration of the crosslinking state of the rubber, thereby reducing the elastic modulus of the rubber at low temperatures. Furthermore, the present inventors found through their investigations that this phenomenon causes an excessive reduction in the hardness of the rubber, resulting in poor steering stability and poor abrasion resistance.Furthermore, reversion may excessively increase tan δ at high temperatures, which may cause poor fuel economy.

[0005] Post-published Patent Document 2 describes a rubber composition for winter tires containing a mixture of a zinc salt of an aliphatic carboxylic acid and a zinc salt of an aromatic carboxylic acid. This rubber composition further contains an oil or a plasticizer, and a rubber component containing 40 mass% or more of a butadiene rubber based on 100 mass% of the total amount of the rubber component.

[0006] In some cases, tires, not limited to winter tires, are vulcanized at higher temperatures to achieve higher productivity. In such cases, the aforementioned phenomenon occurs on a larger scale. Accordingly, there is another problem of a reduction in abrasion resistance caused by reversion.

[0007] Conventionally, for example, by containing a vulcanization accelerator in a larger amount than that of sulfur (a vulcanizing agent) or by containing a thiuram vulcanization accelerator as a vulcanization accelerator, vulcanizable rubber compositions used in rubber products such as tires are prevented from undergoing reversion and are allowed to exhibit better heat resistance. Furthermore, it is known that by containing a crosslinking agent capable of forming a long-chain crosslinking structure represented by formulas such as -(CH2)6-S-, rubber compositions can be prevented from undergoing reversion.Examples of crosslinking agents include PERKALINK 900 and DURALINK HTS (each manufactured by Flexsys), as well as Vulcuren KA9188 (manufactured by Bayer AG). These techniques effectively inhibit the reversion of natural rubber or isoprene rubber; however, these techniques disadvantageously exhibit less or no inhibition effect on the reversion of butadiene rubber. Patent Document 1: JP 2007-169,500 A Patent document 2: DE 10 2009 033 610 A1 SUMMARY OF THE INVENTION

[0008] The object of the present invention is to provide a rubber composition for a winter tire and a high-performance winter tire made therefrom, which solve the aforementioned problems and provide good braking force and good steering stability on ice and snow. Another object of the present invention is to produce a rubber composition and a winter tire with higher productivity and offer them to consumers at lower prices.

[0009] The present invention relates to a rubber composition according to claim 1.

[0010] The rubber composition further preferably contains 10 parts by mass or more of silica based on 100 parts by mass of the rubber component.

[0011] The present invention also relates to a winter tire with a tread made from the rubber composition according to claim 3.

[0012] The present invention relates to a rubber composition for a winter tire, which contains: a rubber component; a zinc salt of an aliphatic C4-C 12 -carboxylic acid or an aliphatic C4-C 12 -carboxylic acid and zinc oxide; and an oil or a plasticizer, wherein the rubber component contains 40 mass% or more of butadiene rubber based on 100 mass% of the rubber component. Consequently, a winter tire made therefrom exhibits good braking power and good steering stability on ice and snow. BEST MODES FOR CARRYING OUT THE PRESENT INVENTION

[0013] The rubber composition for the production of a tread for a winter tire according to the present invention contains a rubber component; (a) a zinc salt of an aliphatic C4-C 12 -carboxylic acid or (b) an aliphatic C4-C 12-carboxylic acid and zinc oxide; and an oil or a plasticizer. The rubber component contains 45 mass% or more of butadiene rubber based on 100 mass% of the rubber component. The amount of the zinc salt (a) and the amount of the mixture (b), based on 100 parts by mass of the rubber component, is 0.2 parts by mass or more and 10 parts by mass or less, respectively, and in the case of (a), with a zinc salt of an aliphatic C4-C 12 -carboxylic acid, the aliphatic C4-C 12 -Carboxylic acid is a saturated fatty acid selected from the group consisting of heptanoic acid, nonanoic acid, undecanoic acid, isobutanoic acid, isopentanoic acid, pivalic acid, isohexanoic acid, isoheptic acid, isooctanoic acid, dimethyloctanoic acid, isononanoic acid, isodecanoic acid, isoundecanoic acid, isododecanoic acid, 2-ethylbutanoic acid, 2-ethylhexanoic acid, 2-butyloctanoic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid and sebacic acid or an unsaturated C4-C 12-fatty acid and where in case (b), in an aliphatic C4-C 12 -carboxylic acid and zinc oxide, the aliphatic C4-C 12 -Carboxylic acid is a saturated fatty acid selected from the group consisting of butanoic acid, pentanoic acid, heptanoic acid, nonanoic acid, undecanoic acid, isobutanoic acid, isopentanoic acid, pivalic acid, isohexanoic acid, isoheptic acid, isooctanoic acid, dimethyloctanoic acid, isononanoic acid, isodecanoic acid, isoundecanoic acid, isododecanoic acid, 2-ethylbutanoic acid, 2-ethylhexanoic acid, 2-butyloctanoic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid and sebacic acid or an unsaturated C4-C 12 -fatty acid.

[0014] In the rubber composition, the rubber component contains 45 mass% or more of butadiene rubber (BR) based on 100 mass% of the rubber component. A winter tire containing BR exhibits better braking power and better steering stability on ice. The lower limit of the BR content is 45 mass%, more preferably 55 mass%, and further preferably 60 mass%. The amount of BR is preferably as high as possible for achieving better braking power and better steering stability on ice and snow. The amount of BR is preferably 80 mass% or more, and most preferably 100 mass%. With an amount of BR of less than 40 mass%, it is less likely to achieve a low glass transition temperature, thereby causing poor braking power on ice and snow.On the other hand, if BR is contained in too large an amount, there is a tendency to obtain poor mechanical strength and poor abrasion resistance although good performance on ice and snow is obtained. In this case, the amount of BR can be set to preferably 85 mass % or less, more preferably 75 mass % or less, and further preferably 65 mass % or less. In the present invention, the rubber composition with a BR content as high as possible can have better abrasion resistance and better performance on ice and snow.

[0015] The BR to be used may have a cis content of 95 mass% or more and, in the form of a 5% toluene solution at 25°C, may have a viscosity of 80 cps or higher. When the rubber composition contains such a BR, the rubber composition can be more easily processed and can have better abrasion resistance. The viscosity of the BR solution in toluene is preferably 200 cps or less. A BR with a viscosity in a 5% toluene solution higher than 200 cps may be too viscous, and thus the BR tends to have poor processability and cannot be easily mixed with other rubbers. The viscosity is particularly preferably 110 cps or more and 150 cps or less.

[0016] By including a BR with a molecular weight distribution (Mw / Mn) of 3.0 or less, the rubber composition can exhibit more suitable viscosity, better processability, and better abrasion resistance. Furthermore, a BR with an Mw / Mn of 3.0 to 3.4 can be used. Such a BR can exhibit better processability and better abrasion resistance.

[0017] In the case of using a blend of BR and other rubbers as the rubber component, examples of the other rubbers include, but are not limited to, natural rubber (NR), epoxidized natural rubber (ENR), styrene-butadiene rubber (SBR), isoprene rubber (IR), ethylene-propylene-diene rubber (EPDM), chloroprene rubber (CR), acrylonitrile-butadiene rubber (NBR), butyl rubber (IIR), and halogenated butyl rubber (X-IIR). In particular, the rubber component preferably contains NR and / or ENR. This is because a rubber component containing these rubbers can be environmentally friendly, will be less affected by a future reduction in oil supply, and can exhibit better abrasion resistance.

[0018] The rubber component may have at least one functional group selected from the group consisting of an alkoxy group, an alkoxysilyl group, an epoxy group, a glycidyl group, a carbonyl group, an ester group, a hydroxy group, an amino group, and a silanol group (such a functional group is hereinafter referred to as a "functional group"). Such a functional group-containing rubber component may be a commercially available one or may be a suitably modified one.

[0019] In the case of using BR in blending with at least one rubber selected from the group consisting of NR, ENR, and IR, the total amount of these rubbers in the rubber component is preferably 70 mass% or more. The total amount of 70 mass% or more can provide good performance on ice and snow and better abrasion resistance. Even in such a case, the rubber composition according to the present invention can exhibit sufficiently high reversion resistance. The total amount of these rubbers is preferably 80 mass% or more, further preferably 90 mass% or more, and most preferably 100 mass%.

[0020] The rubber composition according to the present invention contains (a) a zinc salt of an aliphatic C4-C 12 -carboxylic acid or (b) an aliphatic C4-C 12-carboxylic acid and zinc oxide. These components (a) and (b) can each serve as an anti-reversion agent. The use of the zinc salt (a) or the mixture (b) enables the BR to have better reversion resistance and makes the composition containing silica easier to process. Furthermore, the rubber composition containing silica can be more effectively prevented from undergoing reversion. The rubber composition according to the present invention can contain both the zinc salt (a) and the mixture (b).

[0021] The aliphatic carboxylic acid in each of the zinc salt (a) and the mixture (b) may be a straight chain or a branched chain, or may have a cyclic structure such as a cycloalkyl group. Further, the aliphatic carboxylic acid may be either a saturated fatty acid or an unsaturated fatty acid. Further, the aliphatic carboxylic acid may be an aliphatic polycarboxylic acid such as an aliphatic dicarboxylic acid or an aliphatic tricarboxylic acid. Components (a) and (b) may be in liquid form, and therefore, the handling ability of the components may be poor. In such a case, components (a) and (b) may be supported by silica or the like.

[0022] The aliphatic carboxylic acid in each of components (a) and (b) has four or more carbon atoms, preferably six or more carbon atoms, and more preferably seven or more carbon atoms. An aliphatic carboxylic acid with fewer than four carbon atoms tends to be poorly dispersed. The aliphatic carboxylic acid has twelve or fewer carbon atoms, preferably ten or fewer carbon atoms, and more preferably nine or fewer carbon atoms. An aliphatic carboxylic acid with more than twelve carbon atoms may have insufficient reversion resistance.

[0023] The aliphatic carboxylic acids in component (a) are: saturated fatty acids selected from the group consisting of heptanoic acid, nonanoic acid, undecanoic acid, isobutanoic acid, isopentanoic acid, pivalic acid, isohexanoic acid, isoheptic acid, isooctanoic acid, dimethyloctanoic acid, isononanoic acid, isodecanoic acid, isoundecanoic acid, isododecanoic acid, 2-ethylbutanoic acid, 2-ethylhexanoic acid, 2-butyloctanoic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid and sebacic acid; or unsaturated C4-C 12-fatty acids, such as butenoic acid, pentenoic acid, hexic acid, heptenoic acid, octenoic acid, nonenoic acid, decenoic acid, undecenoic acid, and dodecenoic acid. Of these aliphatic carboxylic acids, 2-ethylhexanoic acid is particularly preferred. This is because 2-ethylhexanoic acid strongly inhibits reversion, is industrially abundant, and is inexpensive. These aliphatic carboxylic acids can be used alone, or two or more of them can be used in a mixture. In component (b), with an aliphatic C4-C 12 -carboxylic acid and zinc oxide, is the aliphatic C4-C 12-Carboxylic acid a saturated fatty acid selected from the group consisting of butanoic acid, pentanoic acid, heptanoic acid, nonanoic acid, undecanoic acid, isobutanoic acid, isopentanoic acid, pivalic acid, isohexanoic acid, isoheptic acid, isooctanoic acid, dimethyloctanoic acid, isononanoic acid, isodecanoic acid, isoundecanoic acid, isododecanoic acid, 2-ethylbutanoic acid, 2-ethylhexanoic acid, 2-butyloctanoic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid and sebacic acid or an unsaturated C4-C 12 -Fatty acid.

[0024] Examples of the zinc oxide in component (b) include those commonly used in the rubber industry. Specific examples include zinc oxide #1 and #2 (manufactured by Mitsui Mining & Smelting Co., Ltd.).

[0025] The amount of zinc in the zinc salt (a) and the amount of zinc in the total amount of the mixture (b) are each preferably 3 mass% or more, and more preferably 5 mass% or more. A zinc amount of less than 3 mass% tends to provide insufficient reversion resistance. The amounts of zinc therein are each preferably 95 mass% or less, more preferably 90 mass% or less, further preferably 80 mass% or less, and most preferably 30 mass% or less. A zinc amount exceeding 95 mass% tends to cause poor processability and may result in unnecessarily high costs.

[0026] The amount of the zinc salt (a) and the amount of the mixture (b) (the total amount of the aliphatic carboxylic acid and the zinc oxide) are each 0.2 parts by mass or more, preferably 0.5 parts by mass or more, further preferably 1 part by mass or more, and most preferably 1.4 parts by mass or more, based on 100 parts by mass of the rubber component. An amount of less than 0.2 parts by mass may provide insufficient reversion resistance and is less likely to provide better control stability. The amounts thereof are each 10 parts by mass or less, preferably 7 parts by mass or less, and more preferably 5 parts by mass or less, based on 100 parts by mass of the rubber component. An amount exceeding 10 parts by mass may cause bleeding or fading and may result in excessively low viscosity and higher adhesion, which may lead to poor processability.Furthermore, the effects may not increase with increasing quantity and there is a tendency for the costs to be unnecessarily high.

[0027] In the case where the rubber composition contains both the zinc salt (a) and the mixture (b), the aforementioned "zinc amount" represents the amount of zinc based on 100 mass % of the components (a) and (b) in total, and the aforementioned "amount" means the total amount of the components (a) and (b). In the case where the rubber composition contains zinc oxide whiskers, the aforementioned "zinc amount" and "amount" do not include the amount of zinc in the zinc oxide whiskers or the amount of zinc oxide whiskers, respectively. In the case where the rubber composition contains the zinc oxide whiskers, the amount of the other zinc oxides or the amount of zinc in the components (a) and (b) may be reduced, or alternatively, no zinc oxide may be blended into the rubber composition.

[0028] The rubber composition contains an oil or a plasticizer. The rubber composition containing the oil or plasticizer is enabled to have a suitably low hardness and exhibit good braking performance on ice. Examples of the oil and the plasticizer include paraffinic process oils, aromatic process oils, and naphthenic process oils. In particular, paraffinic process oils are suitably used because they provide good low-temperature properties and excellent performance on ice. Specific examples of the paraffinic process oils may include PW-32, PW-90, PW-150, and PS-32 (manufactured by Idemitsu Kosan Co., Ltd.). Specific examples of the aromatic process oils may include AC-12, AC-460, AH-16, AH-24, and AH-58 (manufactured by Idemitsu Kosan Co., Ltd.).

[0029] The amount of the oil or plasticizer is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and further preferably 15 parts by mass or more, based on 100 parts by mass of the rubber component. An oil or plasticizer in an amount of less than 5 parts by mass is less likely to provide sufficient on-ice performance. The amount of the oil or plasticizer is preferably 60 parts by mass or less, more preferably 40 parts by mass or less, and further preferably 30 parts by mass or less, based on 100 parts by mass of the rubber component. An oil or plasticizer in an excessive amount may cause poor abrasion resistance and may also cause poor reversion resistance.Furthermore, even an aromatic oil or an alternative aromatic oil, each of which results in a comparatively small reduction in abrasion resistance, may cause poor low-temperature properties, resulting in poor performance on ice and snow. Furthermore, such an aromatic oil or an alternative aromatic oil may exhibit a high tan δ at high temperatures, resulting in a deterioration in rolling resistance performance. In the case where the rubber composition contains both the oil and the plasticizer, the previously defined "amount" here means the total amount of the oil and the plasticizer.

[0030] More preferably, the rubber composition further contains silica. The silica-containing rubber composition is enabled to exhibit better braking performance on ice and better steering stability on ice and snow, which are important properties for winter tires. In particular, due to the zinc salt (a) or the mixture (b), the silica-containing rubber composition can be processed more easily and can be more effectively prevented from undergoing reversion. Examples of the silica include, but are not limited to, silica produced by a wet process and silica produced by a dry process.

[0031] The specific surface area (N2SA) of the silica measured by nitrogen adsorption is preferably 40 m 2 / g or more, particularly preferably 50 m 2 / g or more, further preferably 100 m 2 / g or more and particularly preferably 130 m 2 / g or more. A silica with an N2SA of less than 40 m 2 / g may have insufficient reinforcement effects. The N2SA of the silica is preferably 450 m 2 / g or less, particularly preferably 400 m 2 / g or less, furthermore preferably 300 m 2 / g or less and particularly preferably 200 m 2 / g or less. Disadvantageously, a silica with an N2SA of more than 450 m 2 / g may be less dispersed and cause the rubber composition to have a higher heat build-up.

[0032] The amount of silica is preferably 10 parts by mass or more, more preferably 15 parts by mass or more, further preferably 20 parts by mass or more, and most preferably 35 parts by mass or more, based on 100 parts by mass of the rubber component. A rubber composition having a silica amount of less than 10 parts by mass is less likely to provide better braking performance on ice and better steering stability on ice and snow. The amount of silica is preferably 150 parts by mass or less, more preferably 120 parts by mass or less, further preferably 100 parts by mass or less, and most preferably 50 parts by mass or less, based on 100 parts by mass of the rubber component. Unfortunately, a rubber composition having a silica amount of more than 150 parts by mass may cause poor processability and poor workability.

[0033] The rubber composition preferably contains a silane coupling agent.

[0034] The silane coupling agent can be any silane coupling agent that is conventionally used with silica in the rubber industry. Examples include: sulfide-type silane coupling agents such as bis(3-triethoxysilylpropyl)tetrasulfide, bis(2-triethoxysilylethyl)tetrasulfide, bis(4-triethoxysilylbutyl)tetrasulfide, bis(3-trimethoxysilylpropyl)tetrasulfide, bis(2-trimethoxysilylethyl)tetrasulfide, bis(4-trimethoxysilylbutyl)tetrasulfide, bis(3-triethoxysilylpropyl)trisulfide, bis(2-triethoxysilylethyl)trisulfide, bis(4-triethoxysilylbutyl)trisulfide, bis(3-trimethoxysilylpropyl)trisulfide, bis(2-trimethoxysilylethyl)trisulfide, bis(4-trimethoxysilylbutyl)trisulfide, bis(3-trimethoxysilylpropyl)trisulfide, bis(2-trimethoxysilylethyl)trisulfide, bis(4-trimethoxysilylbutyl)trisulfide, bis(3-triethoxysilylpropyl)disulfide, bis(2-triethoxysilylethyl)disulfide, Bis(4-triethoxysilylbutyl) disulfide, Bis(3-trimethoxysilylpropyl) disulfide, Bis(2-trimethoxysilylethyl) disulfide, Bis(4-trimethoxysilylbutyl) disulfide, 3-trimethoxysilylpropyl-N,N-di-methylthiocarbamoyl tetrasulfide,3-Triethoxysilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide, 2-triethoxysilylethyl-N,N-dimethylthiocarbamoyl tetrasulfide, 2-trimethoxysilylethyl-N,N-dimethylthiocarbamoyl tetrasulfide, 3-trimethoxysilylpropylbenzothiazolyl tetrasulfide, 3-triethoxysilylpropylbenzothiazole tetrasulfide, 3-triethoxysilylpropyl methacrylate monosulfide, and 3-trimethoxysilylpropyl methacrylate monosulfide; mercapto-type silane coupling agents such as 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 2-mercaptoethyltrimethoxysilane, and 2-mercaptoethyltriethoxysilane; vinyl-type silane coupling agents such as vinyltriethoxysilane and vinyltrimethoxysilane; Amino-type silane coupling agents such as 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-(2-aminoethyl)aminopropyltriethoxysilane and 3-(2-aminoethyl)aminopropyltrimethoxysilane; glycidoxy-type silane coupling agents such as γ-glycidoxypropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane,γ-Glycidoxypropylmethyldiethoxysilane and γ-glycidoxypropylmethyldimethoxysilane; nitro-type silane coupling agents such as 3-nitropropyltrimethoxysilane and 3-nitropropyltriethoxysilane; and chlorine-type silane coupling agents such as 3-chloropropyltrimethoxysilane, 3-chloropropyltriethoxysilane, 2-chloroethyltrimethoxysilane, and 2-chloroethyltriethoxysilane. Each of these silane coupling agents can be used alone, or two or more of them can be used in admixture.

[0035] The amount of the silane coupling agent is preferably 1 part by mass or more, and more preferably 2 parts by mass or more, based on 100 parts by mass of the silica. A silane coupling agent in an amount of less than 1 part by mass may insufficiently exert its effects. The amount of the silane coupling agent is preferably 20 parts by mass or less, and more preferably 15 parts by mass or less, based on 100 parts by mass of the silica. A silane coupling agent in an amount of more than 20 parts by mass may disadvantageously fail to provide a higher coupling effect commensurate with the increased cost and may result in poor reinforcement and poor abrasion resistance.

[0036] In addition to the aforementioned ingredients, the rubber composition may contain other blending ingredients commonly used in the rubber industry. Examples of these blending ingredients include stearic acid, fillers such as carbon black and eggshell powder, antioxidants, antiozonants, anti-aging agents, zinc oxide, which serves as a vulcanization accelerator aid, peroxides, vulcanizing agents such as sulfur and sulfur-containing compounds, and vulcanization accelerators.

[0037] The carbon black preferably has an average particle size of 30 nm or less and / or a DBP oil absorption of 100 ml / 100 g or more. Such carbon black can provide the required reinforcement to winter tire treads and can ensure sufficient block rigidity, steering stability, partial abrasion resistance, and abrasion resistance. A rubber composition containing carbon black is likely to have a higher viscosity and therefore cause poor processability; however, in the case of containing the zinc salt (a) and / or the blend (b), the unvulcanized rubber composition is allowed to have a lower viscosity and therefore exhibit better processability.

[0038] The carbon black content is preferably 2 parts by mass or more, more preferably 4 parts by mass or more, further preferably 8 parts by mass or more, and most preferably 20 parts by mass or more, based on 100 parts by mass of the rubber component. A rubber composition with a carbon black content of less than 2 parts by mass may result in insufficient reinforcement and may be less likely to provide the required block rigidity, steering stability, partial abrasion resistance, and abrasion resistance. The amount of carbon black is preferably 120 parts by mass or less, more preferably 80 parts by mass or less, and further preferably 40 parts by mass or less, based on 100 parts by mass of the rubber component. A rubber composition with a carbon black amount of more than 120 parts by mass may have poor processability and excessive hardness.

[0039] The total amount of the silica and the carbon black is preferably 10 parts by mass or more, more preferably 30 parts by mass or more, further preferably 40 parts by mass or more, and particularly preferably 45 parts by mass or more, based on 100 parts by mass of the rubber component. The total amount thereof is preferably 150 parts by mass or less, more preferably 100 parts by mass or less, further preferably 80 parts by mass or less, and particularly preferably 65 parts by mass or less, based on 100 parts by mass of the rubber component. Together with the aforementioned components, the use of the silica and the carbon black in a predetermined total amount can ensure excellent steering stability and abrasion resistance, as well as good performance on ice and snow.

[0040] The zinc oxide serving as a vulcanization accelerator aid may be the same zinc oxide as in the aforementioned component (b). The amount of zinc oxide as a vulcanization accelerator aid is preferably 1 part by mass or more, and more preferably 2 parts by mass or more, based on 100 parts by mass of the rubber component. If the amount of zinc oxide is less than 1 part by mass, its effects cannot be achieved. The amount thereof is preferably 10 parts by mass or less, and more preferably 7 parts by mass or less, based on 100 parts by mass of the rubber component. If the amount of zinc oxide exceeds 10 parts by mass, the increased effects corresponding to the increased amount cannot be achieved, and the cost tends to be high.

[0041] The rubber composition may contain zinc oxide whiskers. The zinc oxide whiskers can provide significantly better grip on ice. Advantageously, the combined use of the zinc oxide whiskers with the zinc salt (a) and / or with the mixture (b) can significantly inhibit reversion.

[0042] The needle fiber length of the zinc oxide whiskers is preferably 1 µm or longer, and more preferably 10 µm or longer. The needle fiber length thereof is preferably 5000 µm or less, and more preferably 1000 µm or less. Zinc oxide whiskers with a needle fiber length shorter than 1 µm are less likely to improve grip on ice. There is a tendency for zinc oxide whiskers with a needle fiber length longer than 5000 µm to provide significantly inferior abrasion resistance.

[0043] The needle fiber diameter (average value) of the zinc oxide whiskers is preferably 0.2 µm or more, and more preferably 0.5 µm or more. The needle fiber diameter thereof is preferably 2000 µm or less, and more preferably 200 µm or less. Zinc oxide whiskers with a needle fiber diameter of less than 0.2 µm are less likely to improve grip on ice. Zinc oxide whiskers with a needle fiber diameter greater than 2000 µm tend to have remarkably poor abrasion resistance.

[0044] The amount of zinc oxide whiskers is preferably 0.3 parts by mass or more, more preferably 1.3 parts by mass or more, and further preferably 2.0 parts by mass or more, based on 100 parts by mass of the rubber component. The amount of zinc oxide whiskers is preferably 30 parts by mass or less, and more preferably 15 parts by mass or less, based on 100 parts by mass of the rubber component. Zinc oxide whiskers in an amount of less than 0.3 parts by mass are unlikely to improve crosslinking efficiency and ice grip. There is a tendency that zinc oxide whiskers in an amount of more than 30 parts by mass may provide poor abrasion resistance and result in unnecessarily high costs.

[0045] The aforementioned tread has a JIS-A hardness of preferably 50 degrees or less, more preferably 48 degrees or less, and further preferably 46 degrees or less. A tread with a JIS-A hardness of 50 degrees or less can be flexible and can exhibit more excellent performance on ice and snow. Meanwhile, the JIS-A hardness is preferably 40 degrees or more. A tread with a JIS-A hardness of less than 40 degrees may be accompanied by poor processability of the unvulcanized rubber composition and is unlikely to exhibit a suitable hardness that ensures good steering stability.

[0046] The rubber composition according to the present invention can be applied to tires of motor vehicles, such as trucks and buses. In particular, the rubber composition according to the present invention can be preferably used for winter tires for passenger cars, which require high steering stability on ice and snow. Furthermore, the rubber composition according to the present invention can be suitably used for treads of winter tires.

[0047] The rubber composition according to the present invention can be used to manufacture a winter tire by a conventional method. That is, the winter tire can be manufactured by forming a tire tread using the rubber composition, assembling the tread with the other components, and heating the assembled components under pressure on a tire molding machine. EXAMPLES

[0048] The present invention will be described below specifically with reference to, but not limited to, the examples.

[0049] The respective chemical agents used in the examples and comparative examples are listed below: No.: RSS #3 BR 1: BR 150B (amount of cis-1,4 bonds: 97%, ML 1+4(100 °C): 40, viscosity of a 5% solution in toluene at 25 °C: 48 cps, Mw / Mn: 3.3) manufactured by Ube Industries, Ltd. BR2: BR 360L (amount of cis-1,4 bonds: 98%, ML 1+4 (100 °C): 51, viscosity of a 5% solution in toluene at 25 °C: 124 cps, Mw / Mn: 2.4) manufactured by Ube Industries, Ltd. BR 3: BR A (a test product, amount of cis-1,4 bonds: 98%, ML 1+4 (100 °C): 47, viscosity of a 5% solution in toluene at 25 °C: 122 cps, Mw / Mn: 3.3) manufactured by Ube Industries, Ltd. Carbon black: DIABLACK I (ISAF carbon black, average particle size: 23 nm, DPB oil absorption: 114 ml / 100 g) manufactured by Mitsubishi Chemical Corporation) Silica: Ultrasil VN3 (N2SA: 175 m 2 / g) manufactured by Degussa AG Silane coupling agent: Si-69 manufactured by Degussa AG Mineral oil: PS-32 (paraffinic process oil) manufactured by Idemitsu Kosan Co., Ltd. Stearic acid: KIRI manufactured by NOF Corporation Aliphatic carboxylic acid: 2-Ethylhexanoic acid manufactured by Wako Pure Chemical Industries, Ltd. Antireversion agent 1: Struktol ZEH (zinc(II) 2-ethylhexanoate, number of carbon atoms: 8, zinc content: 23% by mass) manufactured by Schill + Seilacher Struktol AG Antireversion agent 2: PERKALINK 900 (1,3-bis(citraconimidomethyl)benzene) manufactured by Flexsys Anti-reversion agent 3: Zinc(II) butyrate (number of carbon atoms: 4, zinc amount: 27 mass%) manufactured by Mitsuwa Chemicals Co., Ltd. Anti-reversion agent 4: Zinc(II) octanoate (number of carbon atoms: 8, zinc amount: 19 mass%) manufactured by Mitsuwa Chemicals Co., Ltd. Anti-reversion agent 5: Zinc laurate (zinc(II) dodecanoate, number of carbon atoms: 12, zinc amount: 14.1 mass%) manufactured by Wako Pure Chemical Industries, Ltd. Zinc myristate (aliphatic zinc carboxylate): Zinc myristate (number of carbon atoms: 14, zinc amount: 12.6 mass%) manufactured by Wako Pure Chemical Industries, Ltd. Zinc oxide: Zinc oxide #2 manufactured by Mitsui Mining & Smelting Co., Ltd. Zinc oxide whisker: PANATETRA WZ-0501 (the number of protrusions is: 4, needle fiber length: 2 to 50 µm, needle fiber diameter (average value): 0.2 to 3.0 µm) manufactured by AMTEC Co., Ltd. Antiaging agent: NOCRAC 6C (N-(1,3-Dimetylbutyl)-N'-phenyl-p-phenylenediamine) manufactured by OUCHI SHINKO CHEMICAL INDUSTRIAL CO., LTD. Wax: OZOACE wax manufactured by Nippon Seiro Co., Ltd. Sulfur: sulfur powder manufactured by Tsurumi Chemical Industry Co., Ltd. Vulcanization accelerator BBS: NOCCELER NS (N-tert-butyl-2-benzothiazolylsulfenamide) manufactured by OUCHI SHINKO CHEMICAL INDUSTRIAL Co., LTD. Vulcanization accelerator DPG: NOCCELER D (N,N'-Diphenylguanidine) manufactured by OUCHI SHINKO CHEMICAL INDUSTRIAL Co., LTD. Examples 1 to 23 and Comparative Examples 1 to 13 (Examples 1 and 20 to 23 are not according to the invention)

[0050] The chemical agents were charged into a Banbury mixer in the amounts shown in Process 1 in Tables 1 and 2. They were then mixed and kneaded for five minutes to raise the outlet temperature to approximately 150°C. Subsequently, sulfur and vulcanization accelerators were added to the mixture obtained in Process 1 in the amounts shown in Process 2. These were mixed and ground for three minutes at approximately 80°C using an open-roll mill. This yielded an unvulcanized rubber composition. This unvulcanized rubber composition was molded into a tread shape, assembled with the other tire parts, and then vulcanized for 15 minutes at 170°C. Thus, winter tires were manufactured.

[0051] The respective samples were examined by the following methods. (Reversion resistance)

[0052] The vulcanization curve of the unvulcanized rubber composition at 170 °C was determined using a curing elastometer. The maximum torque increase (MH-ML) was considered to be 100, and the torque increase obtained 15 minutes after the start of vulcanization was expressed as a relative value. Then, a value obtained by subtracting the relative value from 100 was considered the reversion ratio. A lower reversion ratio indicates that reversion is more inhibited and there is better reversion resistance. (Hardness)

[0053] The hardness of each of the vulcanized rubber composition samples in the examples and comparative examples was determined using a Type A hardness tester in accordance with JIS K 6253 “Rubber, vulcanized or thermoplastic - Determination of hardness”. (Performance on ice and snow)

[0054] The winter tires of the examples and the comparative examples were tested for their performance on a motor vehicle on ice and snow under the following conditions. Winter tires for a passenger car were manufactured, which had a size of 195 / 65 R15 and a DS-2 pattern, and the tires were mounted on a Japanese-made front-wheel drive motor vehicle with a 2000 cm 3 displacement. The test was conducted at a test course operated by Sumitomo Rubber Industries, Ltd. in Nayoro, Hokkaido, Japan. The temperature on ice was between -6°C and -1°C, and the temperature on snow was between -10°C and -2°C.

[0055] - Steering stability (feeling evaluation): The starting, acceleration, and stopping of the aforementioned vehicle were evaluated by feeling. For the evaluation, the tire in the comparison sample was considered the standard, with its performance rated at 100. Grading was then performed such that the tires were rated at 120 if the test driver considered their performance to be clearly improved, and the tires were rated at 140 if the test driver considered their performance to be as high as ever.

[0056] - Braking performance (distance on ice to stop after braking): The braking distance on ice was measured, which is the distance required for a vehicle traveling at 30 km / h to stop after the brakes are fully applied. The braking distance in Comparative Example 1 was used as a reference, and the braking performance index was calculated using the following equation: (Braking performance index)(Braking distance in Comparative Example 1) / (Braking distance)×100.

[0057] A larger index indicates better braking performance. (Abrasion resistance)

[0058] The winter tires were manufactured to a size of 195 / 65 R15 and mounted on a front-wheel drive vehicle manufactured in Japan. The groove depths in the tread portion were measured after the vehicle had been driven for 8,000 km. The running distance at which the tire groove depth decreased by 1 mm was calculated, and the abrasion resistance was expressed as an index calculated using the following formula: (Abrasion resistance index) = (running distance causing a 1 mm reduction in tire groove depth) / (running distance causing a 1 mm reduction in tire groove depth in Comparative Example 1) × 100.

[0059] A larger index indicates better abrasion resistance.

[0060] Tables 1 and 2 show the evaluation results of the respective studies. The samples in the examples exhibited a low reversion ratio and good reversion resistance. Furthermore, the samples in the examples each exhibited suitable hardness and demonstrated good steering stability on snow and good braking performance on ice. In particular, the samples in the examples with a higher BR content or with a higher silica content demonstrated significantly better braking performance on ice and good steering stability on snow.

[0061] In Examples 7 to 9 and 11 to 14, the rubber compositions containing BR with a cis amount of 95% or more, with a viscosity of a 5% solution in toluene at 25°C of 80 to 200 cps (further 110 to 150 cps), and with an Mw / Mn of 3.0 or less or 3.0 to 3.4, exhibited better processability and better abrasion resistance. In particular, the better processability resulted in a better dispersion state of the silica, and this consequently allowed the rubber composition to contain a larger amount of the silica to provide better abrasion resistance. Furthermore, the combined use of the zinc salt of an aliphatic C4-C 12-carboxylic acid with BR exhibited much better reversion resistance. Consequently, especially with a higher BR content, excellent steering and braking performance on ice were achieved while maintaining sufficiently high abrasion resistance.

[0062] In Comparative Examples 1, 3, 5, and 7, the rubber compositions containing BR and not containing an anti-reversion agent exhibited a high reversion ratio and inferior reversion resistance, and exhibited poor control stability. In Comparative Examples 2, 4, 6, and 8, the rubber compositions containing BR and anti-reversion agent 2 exhibited poor control stability and slightly inferior reversion resistance.

[0063] The rubber composition in Comparative Example 9, which contained only NR as the rubber component and did not contain an anti-reversion agent, and the rubber composition in Comparative Example 11, which contained only NR as the rubber component and further contained Anti-reversion Agent 2, each exhibited poor reversion resistance. Furthermore, these rubber compositions provided poor steering stability and poor braking performance on ice and snow. The rubber composition in Comparative Example 10, which contained only NR as the rubber component and further contained Anti-reversion Agent 1, exhibited good reversion resistance but exhibited poor steering stability on snow and poor braking performance on ice.Furthermore, the rubber composition in Comparative Example 12, which contained the BR in an amount of as little as 30 mass% and further contained the anti-reversion agent 1, had good reversion resistance, but showed poor steering stability on snow and poor braking performance on ice.

[0064] In Examples 17 and 18, the rubber compositions contained a mixture of 2-ethylhexanoic acid and zinc oxide, and these exhibited improved properties, particularly excellent reversion resistance, excellent steering stability, and excellent abrasion resistance, compared to the rubber composition in Comparative Example 5, which did not contain a reversion agent, and compared to the rubber composition in Comparative Example 6, which contained antireversion agent 2. The rubber compositions in Examples 19, 22, and 23, which contained antireversion agents 3 or 5, exhibited better reversion resistance and steering stability on snow and also had good abrasion resistance. In particular, the rubber compositions in Examples 19 and 23, which contained zinc oxide whiskers, exhibited better braking performance on ice.The rubber compositions in Examples 20 and 21 containing Antireversion Agent 4 exhibited better reversion resistance, better steering stability on snow, and better braking performance on ice, and also exhibited good abrasion resistance, although these were slightly inferior compared to the rubber compositions in Examples 5 and 15 containing Antireversion Agent 1, which is a zinc salt of a branched fatty acid having the same number of carbon atoms as that of Antireversion Agent 4. On the other hand, the rubber composition in Comparative Example 13 containing zinc myristate having a number of carbon atoms outside the range of the present patent application exhibited poor reversion resistance and particularly poor abrasion resistance, and also exhibited a poor balance between the individual performances.

Claims

[1] Rubber composition for the manufacture of a tread for a winter tire, which contains: a rubber component, (a) a zinc salt of an aliphatic C4-C 12 -carboxylic acid or (b) an aliphatic C4-C 12 -carboxylic acid and zinc oxide and an oil or a plasticizer, wherein the rubber component contains 45 mass% or more of butadiene rubber based on 100 mass% of the rubber component, wherein the amount of the zinc salt (a) and the amount of the mixture (b) are 0.2 parts by mass or more and 10 parts by mass or less, respectively, based on 100 parts by mass of the rubber component, and where in case (a), in the case of a zinc salt of an aliphatic C4-C 12 -carboxylic acid, the aliphatic C4-C 12-Carboxylic acid is a saturated fatty acid selected from the group consisting of heptanoic acid, nonanoic acid, undecanoic acid, isobutanoic acid, isopentanoic acid, pivalic acid, isohexanoic acid, isoheptic acid, isooctanoic acid, dimethyloctanoic acid, isononanoic acid, isodecanoic acid, isoundecanoic acid, isododecanoic acid, 2-ethylbutanoic acid, 2-ethylhexanoic acid, 2-butyloctanoic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid and sebacic acid or an unsaturated C4-C 12 -fatty acid and where in case (b), with an aliphatic C4-C 12 -carboxylic acid and zinc oxide, the aliphatic C4-C 12-Carboxylic acid is a saturated fatty acid selected from the group consisting of butanoic acid, pentanoic acid, heptanoic acid, nonanoic acid, undecanoic acid, isobutanoic acid, isopentanoic acid, pivalic acid, isohexanoic acid, isoheptic acid, isooctanoic acid, dimethyloctanoic acid, isononanoic acid, isodecanoic acid, isoundecanoic acid, isododecanoic acid, 2-ethylbutanoic acid, 2-ethylhexanoic acid, 2-butyloctanoic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid and sebacic acid or an unsaturated C4-C 12 -fatty acid. [2] A rubber composition for producing a tread for a winter tire according to claim 1, which further contains 10 parts by mass or more of silica based on 100 parts by mass of the rubber component. [3] Winter tires comprising a tread made from a rubber composition according to claim 1.