JP5373729B2 - Studless tires for passenger cars - Google Patents

Description

The present invention relates to a studless tire for a passenger car.

In order to prevent dust pollution caused by spiked tires, the prohibition of the use of spiked tires was legalized, and studless tires were used instead of spiked tires in cold regions. In order to improve the grip performance of the studless tire on ice or snow, there is a method of improving the adhesive friction by reducing the elastic modulus at low temperature. In particular, the braking force on ice is greatly affected by the effective contact area between rubber and ice, and therefore, a flexible rubber at low temperature is required to increase the effective contact area.

On the other hand, if only the hardness of the rubber is lowered by a method such as increasing the amount of oil, there is a problem that steering stability is lowered in the studless tire for passenger cars.

In general, tread rubber for studless tires is not limited to trucks, buses and light trucks, but is also mainly used for passenger cars because of its high strength but low glass transition temperature and flexibility. As a component, it is often produced by adding about 1.5 parts by mass of sulfur as a vulcanizing agent to 100 parts by weight of a rubber component and vulcanizing (for example, see Patent Document 1). However, when natural rubber or butadiene rubber is sulfur vulcanized, reversion occurs. This phenomenon is that the rubber is deteriorated or the cross-linked state is deteriorated, and at this time, the elastic modulus at low temperature is also lowered, but the hardness is also lowered more than necessary, and the steering stability is lowered. As a result of the studies by the present inventors, it has been found.

In recent years, in order to improve the productivity of tires, tires are often produced by vulcanization at a high temperature of 170 ° C. or higher. It has been found that the phenomenon of resulfurization often occurs, leading to a decrease in steering stability. Further, when reversion occurs, the wear resistance is reduced, or tan δ at a high temperature is unnecessarily increased, resulting in a reduction in fuel consumption, which is a very important characteristic.

JP 2007-176417 A

An object of the present invention is to solve the above-mentioned problems and to provide a high-performance studless tire for a passenger car that achieves both good braking force on ice and snow and steering stability.

The present invention relates to a studless tire for a passenger car having a tread having a total sulfur amount of 0.7% by mass or less and a crosslink density of 2.0 × 10 ⁻⁵ mol / cm ³ or more.

The tread is preferably obtained from a rubber composition containing zinc oxide whiskers.
The tread is preferably obtained from a rubber composition containing a mixture of a zinc salt of an aliphatic carboxylic acid and a zinc salt of an aromatic carboxylic acid.

According to the present invention, by using a rubber composition having a total sulfur amount of 0.7% by mass or less and a crosslinking density of 2.0 × 10 ⁻⁵ mol / cm ³ or more, A high-performance studless tire for a passenger car that achieves both good braking force and steering stability can be provided.

It is the schematic which shows typically the TMA apparatus in the measurement of a crosslinking density.

The studless tire for passenger cars of the present invention has a tread having a total sulfur amount of 0.7% by mass or less and a crosslink density of 2.0 × 10 ⁻⁵ mol / cm ³ or more. Thus, by controlling the total sulfur amount of the tread below a specific amount and the crosslink density above a specific amount, unnecessary sulfur and sulfur cross-linked in an unfavorable state are reduced, and effective and stable cross-linking. Can provide points. For this reason, it is possible to give the rubber a feeling of rigidity necessary for handling stability on ice and snow. In addition, since the tread rubber is soft against small strains and has a high grip force at low temperatures, a good braking force can be obtained on ice and snow.

In the tread, the total sulfur amount is 0.7% by mass or less, preferably 0.65% by mass or less, more preferably 0.6% by mass or less, still more preferably 0.55% by mass or less, and 0.5% by mass. % Or less is most preferable. If it exceeds 0.7% by mass, unnecessary sulfur and sulfur that is crosslinked in an unfavorable state increase, so that braking force and handling stability on ice and snow deteriorate. Further, the smaller the total sulfur amount, the better. However, in order to set the crosslinking density to 2.0 × 10 ⁻⁵ mol / cm ³ or more, usually 0.2 mass% or more is necessary. The lower limit of the total sulfur amount is preferably 0.3% by mass, more preferably 0.4% by mass, and still more preferably 0.45% by mass.

In the above tread, the crosslinking density (total crosslinking density) is 2.0 × 10 ⁻⁵ mol / cm ³ or more, preferably 2.1 × 10 ⁻⁵ mol / cm ³ or more, and 2.3 × 10 ⁻⁵ mol. / Cm ³ or more is more preferable, 2.5 × 10 ⁻⁵ mol / cm ³ or more is more preferable, and 3.0 × 10 ⁻⁵ mol / cm ³ or more is most preferable. The crosslink density is preferably 5.1 × 10 ⁻⁵ mol / cm ³ or less, more preferably 4.5 × 10 ⁻⁵ mol / cm ³ or less, and 3.5 × 10 ⁻⁵ mol / cm ³ or less. Further preferred. If it is less than 2.0 × 10 ⁻⁵ mol / cm ³ , the effective cross-linking point is insufficient and the hardness of the tread rubber is lowered, the rigidity is insufficient, and steering stability on ice and snow is deteriorated. If it exceeds 5.1 × 10 ⁻⁵ mol / cm ³ , the hardness tends to be high, which makes it difficult to obtain a good braking force on ice and snow.

In the tread, the crosslink density of monosulfide bonds in the crosslinked form (hereinafter also referred to as monocrosslink density) is preferably 0.9 × 10 ⁻⁵ mol / cm ³ or less, and preferably 0.75 × 10 ⁻⁵ mol / cm ^3. The following is more preferable, 0.65 × 10 ⁻⁵ mol / cm ³ or less is further preferable, and 0.55 × 10 ⁻⁵ mol / cm ³ or less is most preferable. Also, the mono-crosslink density is as low as possible is desirable, actually 0.1 × ¹⁰ -5 mol / cm ³ or more ^{or, 0.2 × 10 -5 mol / cm} 3 or more, and further 0.4 × 10 ^{- It is} about ⁵ mol / cm ³ . When it exceeds 0.9 × 10 ⁻⁵ mol / cm ³ , the flex crack growth resistance tends to be lowered.
The total crosslink density and monocrosslink density can be measured using the method described in JP-A-2002-17898.

In the present invention, as a component in the rubber composition, a mixture of a zinc salt of an aliphatic carboxylic acid and a zinc salt of an aromatic carboxylic acid described later or a zinc oxide whisker is used, and vulcanization is performed at a low temperature. Furthermore, the tread which has the said total sulfur amount and a crosslinking density can be obtained by adjusting the compounding quantity of sulfur.

The tread is obtained using a rubber composition containing a rubber component.
The rubber component is not particularly limited. For example, natural rubber (NR), epoxidized natural rubber (ENR), styrene butadiene rubber (SBR), butadiene rubber (BR), isoprene rubber (IR), ethylene propylene diene rubber (EPDM), chloroprene rubber (CR), acrylonitrile butadiene rubber (NBR), butyl rubber (IIR), halogenated butyl rubber (X-IIR) and the like. Among them, it is preferable to include NR and / or ENR because it is environmentally friendly, can be prepared for a future decrease in the amount of petroleum supply, and can further improve wear resistance.

When the rubber component contains butadiene rubber, the content of butadiene rubber with respect to 100% by mass of the rubber component is preferably 40% by mass or more, more preferably 45% by mass or more, still more preferably 55% by mass or more, and 60% by mass or more. Is most preferred. The content is preferably 85% by mass or less, more preferably 75% by mass or less, and still more preferably 65% by mass or less. If it is less than 40% by mass, it will be difficult to lower the glass transition temperature, and the braking force on ice and snow will be reduced. If it exceeds 85% by mass, the performance on ice and snow will be good, but the mechanical strength and Abrasion resistance and kneading processability tend to be reduced.

When butadiene rubber is used by mixing with natural rubber or polyisoprene rubber, the rubber component preferably contains a total amount of these rubber components of 70% by mass or more. By setting it to 70% by mass or more, good performance on ice and snow and wear resistance can be achieved, and the effect of reversion resistance can be increased. The blending amount of these rubber components is more preferably 80% by mass or more, still more preferably 90% by mass or more, and most preferably 100% by mass.

As the butadiene rubber, those having a cis content of 95% by mass or more are preferably used. By blending such butadiene rubber, the effect of improving wear resistance is improved. The toluene solution viscosity is preferably 200 cps or less, and more preferably 150 cps or less. If it exceeds 200 cps, the viscosity tends to be too high, and the processability tends to be reduced, or it tends to be difficult to mix with other rubber components.

A butadiene rubber having a molecular weight distribution (Mw / Mn) of butadiene rubber of 3.0 to 3.4 may be used. By using such butadiene rubber, it is possible to achieve both improvement in workability and improvement in wear resistance.

The rubber component includes at least one functional group selected from the group consisting of an alkoxyl 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 (hereinafter referred to as a functional group). May be included. As the rubber having these functional groups, commercially available rubbers may be used, or they may be appropriately modified and used.

The rubber composition preferably contains zinc oxide whiskers. Zinc oxide whisker improves sulfur cross-linking efficiency, can provide a more effective cross-linking point with a small amount of sulfur, and can greatly improve the grip on ice.

The needle-like short fiber length of the zinc oxide whisker is preferably 1 μm or more, and more preferably 10 μm or more. The needle-like short fiber length is preferably 5000 μm or less, and more preferably 1000 μm or less. If the length of the acicular short fiber is less than 1 μm, the effect of improving the grip force on ice tends to be small. Moreover, when it exceeds 5000 micrometers, there exists a tendency for abrasion resistance to fall remarkably.

The needle-like short fiber diameter (average value) of the zinc oxide whisker is preferably 0.5 μm or more. The needle-like short fiber diameter is preferably 2000 μm or less, and more preferably 200 μm or less. When the needle-like short fiber diameter is less than 0.5 μm, the effect of improving the grip force on ice tends to be small. Moreover, when it exceeds 2000 micrometers, there exists a tendency for abrasion resistance to fall remarkably.

The compounding amount of the zinc oxide whisker is preferably 0.3 parts by mass or more, more preferably 1.3 parts by mass or more, and still more preferably 2.0 parts by mass or more with respect to 100 parts by mass of the rubber component. Moreover, 30 mass parts or less are preferable, as for the said compounding quantity, 15 mass parts or less are more preferable, and 7 mass parts or less are still more preferable. If the blending amount is less than 0.3 parts by mass, the effect of improving the crosslinking efficiency and the effect of improving the grip strength on ice tend not to be observed. Moreover, when it exceeds 30 mass parts, besides having a tendency for abrasion resistance to fall, cost also increases unnecessarily.

In order to improve the adhesion between zinc oxide whisker and diene rubber, polypropylene (PP), polyethylene (PE), polystyrene (PS), polyurethane (PU), polyvinyl alcohol (PVA), silane coupling agent and silylated The whisker may be surface-treated with an agent or the like.

The rubber composition preferably contains a mixture of a zinc salt of an aliphatic carboxylic acid and a zinc salt of an aromatic carboxylic acid. When the above mixture is used, improvement of the crosslinking effect and suppression of vulcanization reversion can be realized very effectively when butadiene rubber is used in addition to natural rubber and isoprene rubber as the rubber component. Therefore, by adding the mixture to the rubber component, even if the ratio of the butadiene rubber in the rubber component is increased, the crosslinking effect can be improved and the vulcanization reversion can be greatly suppressed. Obtaining the density with a small amount of total sulfur can be realized even when the amount of butadiene rubber is large. Furthermore, deterioration of wear resistance and mechanical strength due to blending of butadiene rubber can be suppressed. As a result, it is possible to achieve both a good braking force and steering stability on ice and snow at a high level.

Aliphatic carboxylic acids include palm oil, palm kernel oil, camellia oil, olive oil, almond oil, canola oil, peanut oil, rice sugar oil, cocoa butter, palm oil, soybean oil, cottonseed oil, sesame oil, linseed oil, castor oil , Aliphatic carboxylic acids derived from vegetable oils such as rapeseed oil, aliphatic carboxylic acids derived from animal oils such as beef tallow, aliphatic carboxylic acids chemically synthesized from petroleum, etc. It is also possible to prepare for a decrease in the supply amount of oil, and furthermore, since the vulcanization return can be sufficiently suppressed, an aliphatic carboxylic acid derived from vegetable oil is preferable, and an aliphatic carboxylic acid derived from palm oil, palm kernel oil or palm oil is preferred. More preferred.

The aliphatic carboxylic acid preferably has 4 or more carbon atoms, more preferably 6 or more carbon atoms. If the aliphatic carboxylic acid has less than 4 carbon atoms, the dispersibility tends to deteriorate. The carbon number of the aliphatic carboxylic acid is preferably 16 or less, more preferably 14 or less, and still more preferably 12 or less. When the carbon number of the aliphatic carboxylic acid exceeds 16, there is a tendency that the vulcanization return cannot be sufficiently suppressed.

The aliphatic group in the aliphatic carboxylic acid may be a chain structure such as an alkyl group or a cyclic structure such as a cycloalkyl group.

Examples of the aromatic carboxylic acid include benzoic acid, phthalic acid, mellitic acid, hemimellitic acid, trimellitic acid, diphenic acid, toluic acid, naphthoic acid, and the like. Of these, benzoic acid, phthalic acid, or naphthoic acid is preferable because reversion can be sufficiently suppressed.

Content ratio of zinc salt of aliphatic carboxylic acid and zinc salt of aromatic carboxylic acid in the mixture (molar ratio: zinc salt of aliphatic carboxylic acid / zinc salt of aromatic carboxylic acid, hereinafter referred to as content ratio) is 1/20 or more is preferable, 1/15 or more is more preferable, and 1/10 or more is still more preferable. If the content ratio is less than 1/20, it is not possible to consider the environment or prepare for a future reduction in the amount of oil supplied, and the dispersibility and stability of the mixture tend to deteriorate. The content ratio is preferably 20/1 or less, more preferably 15/1 or less, and still more preferably 10/1 or less. When the content ratio exceeds 20/1, there is a tendency that the vulcanization return cannot be sufficiently suppressed.

The zinc content in the mixture is preferably 3% by mass or more, and more preferably 5% by mass or more. If it is less than 3% by mass, there is a tendency that the vulcanization return cannot be sufficiently suppressed. Moreover, 30 mass% or less is preferable and, as for the zinc content rate in a mixture, 25 mass% or less is more preferable. If it exceeds 30% by mass, the workability tends to decrease and the cost increases unnecessarily.

The content of the mixture is preferably 0.2 parts by mass or more, more preferably 0.5 parts by mass or more, still more preferably 1 part by mass or more, most preferably 1.4 parts by mass with respect to 100 parts by mass of the rubber component. That's it. If the content of the mixture is less than 0.2 parts by mass, sufficient anti-vulcanization resistance cannot be ensured, and it becomes difficult to obtain effects such as steering stability. The content of the mixture is preferably 10 parts by mass or less, more preferably 7 parts by mass or less, and still more preferably 5 parts by mass or less. When the content of the mixture exceeds 10 parts by mass, the tendency to bloom increases, and the viscosity tends to decrease unnecessarily, and the workability tends to decrease due to increased tackiness. However, the improvement of the effect tends to be small, and the cost increases unnecessarily.

The rubber composition may contain stearic acid. The addition of stearic acid, and the presence of zinc oxide and zinc oxide whiskers, is not as good as a mixture of aliphatic carboxylic acid zinc salt and aromatic carboxylic acid zinc salt, but improves cross-linking efficiency and vulcanization. Return can be suppressed.

The amount of stearic acid is preferably 0.2 parts by mass or more, more preferably 0.5 parts by mass or more, with respect to 100 parts by mass of the rubber component. The content is preferably 10 parts by mass or less, more preferably 8 parts by mass or less, and still more preferably 6 parts by mass or less. If the amount is less than 0.2 parts by mass, the crosslinking improving effect and the vulcanization return suppressing effect by stearic acid tend not to be sufficiently confirmed. If it exceeds 10 parts by mass, there is a tendency that blooming occurs or the viscosity is unnecessarily lowered and the workability is lowered.

The rubber composition preferably contains oil or a plasticizer. Thereby, the hardness can be adjusted to an appropriate low level and good braking performance on ice can be obtained. As the oil, for example, paraffinic process oil, aroma based process oil, naphthenic process oil, and the like can be used. Among these, paraffinic process oil is preferably used from the viewpoint that the low-temperature characteristics are good and excellent performance on ice is obtained. Specific examples of the paraffinic process oil include PW-32, PW-90, PW-150, and PS-32 manufactured by Idemitsu Kosan Co., Ltd. Specific examples of the aroma-based process oil include AC-12, AC-460, AH-16, AH-24, and AH-58 manufactured by Idemitsu Kosan Co., Ltd.

When the rubber composition contains an oil or a plasticizer, the blending amount thereof is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and further 15 parts by mass or more with respect to 100 parts by mass of the rubber component. preferable. When the amount is less than 5 parts by mass, it is difficult to obtain a sufficient performance improvement effect on ice. On the other hand, the blending amount is preferably 60 parts by mass or less, more preferably 40 parts by mass or less, and still more preferably 30 parts by mass or less with respect to 100 parts by mass of the rubber component. If the amount of these components is too large, the wear resistance is lowered and the reversion resistance is sometimes lowered. Even with aroma oil or alternative aroma oil with relatively low wear resistance, the low-temperature characteristics may deteriorate and the performance on ice and snow may deteriorate, or the tan δ at high temperatures may increase and the rolling resistance may deteriorate. is there.

The rubber composition preferably further contains silica. By blending silica, braking performance on ice and steering stability on ice and snow, which are important as a studless tire, can be improved. In particular, a mixture of a zinc salt of an aliphatic carboxylic acid and an aromatic carboxylic acid can improve the processability of silica blending and more effectively suppress reversion with silica blending. Examples of the silica include silica produced by a wet method, silica produced by a dry method, and the like, but there is no particular limitation.

The nitrogen adsorption specific surface area (N ₂ SA) of silica is 40 m ² / g or more, preferably 50 m ² / g or more. When N ₂ SA of silica is less than 40 m ² / g, the reinforcing effect is insufficient. N ₂ SA of silica is 450 m ² / g or less, preferably 400 m ² / g or less. If N ₂ SA of silica exceeds 450 m ² / g, the dispersibility is lowered and the exothermic property of the rubber composition is increased, which is not preferable.

The content of silica is preferably 10 parts by mass or more, more preferably 15 parts by mass or more, still more preferably 20 parts by mass or more, and particularly preferably 35 parts by mass or more with respect to 100 parts by mass of the rubber component. If the silica content is less than 10 parts by mass, it tends to be difficult to improve the braking performance on ice and the handling stability on ice and snow. The silica content is preferably 150 parts by mass or less, more preferably 120 parts by mass or less, still more preferably 100 parts by mass or less, and most preferably 50 parts by mass or less. When the content of silica exceeds 150 parts by mass, workability and workability deteriorate, which is not preferable.

The rubber composition preferably contains a silane coupling agent.
As the silane coupling agent, any silane coupling agent conventionally used in combination with silica can be used in the rubber industry. For example, bis (3-triethoxysilylpropyl) tetrasulfide, bis (2-tri Ethoxysilylethyl) tetrasulfide, bis (4-triethoxysilylbutyl) tetrasulfide, bis (3-trimethoxysilylpropyl) tetrasulfide, bis (2-trimethoxysilylethyl) tetrasulfide, bis (4-trimethoxysilyl) Butyl) tetrasulfide, bis (3-triethoxysilylpropyl) trisulfide, bis (2-triethoxysilylethyl) trisulfide, bis (4-triethoxysilylbutyl) trisulfide, bis (3-trimethoxysilylpropyl) Trisulfi 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-dimethylthiocarbamoyl tetrasulfide, 3-triethoxysilylpropyl-N, N-dimethylthiocarbamoyl tetrasulfide, 2-triethoxysilylethyl-N, N-dimethylthiocarbamoyl tetrasulfide, 2-trime Xylylethyl-N, N-dimethylthiocarbamoyl tetrasulfide, 3-trimethoxysilylpropylbenzothiazolyl tetrasulfide, 3-triethoxysilylpropylbenzothiazole tetrasulfide, 3-triethoxysilylpropyl methacrylate monosulfide, 3- Sulfide type such as trimethoxysilylpropyl methacrylate monosulfide, mercapto type such as 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 2-mercaptoethyltrimethoxysilane, 2-mercaptoethyltriethoxysilane, vinyltri Vinyl-based compounds such as ethoxysilane and vinyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3- (2-amino Noethyl) aminopropyltriethoxysilane, 3- (2-aminoethyl) aminopropyltrimethoxysilane and the like amino, γ-glycidoxypropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycid Glycidoxy type such as xylpropylmethyldiethoxysilane, γ-glycidoxypropylmethyldimethoxysilane, nitro type such as 3-nitropropyltrimethoxysilane, 3-nitropropyltriethoxysilane, 3-chloropropyltrimethoxysilane, 3 Examples include chloro-based compounds such as -chloropropyltriethoxysilane, 2-chloroethyltrimethoxysilane, and 2-chloroethyltriethoxysilane. These silane coupling agents may be used alone or in combination of two or more.

The content of the silane coupling agent is 1 part by mass or more, preferably 2 parts by mass or more with respect to 100 parts by mass of silica. If content of a silane coupling agent is less than 1 mass part, the effect by containing a silane coupling agent is not enough. Moreover, content of a silane coupling agent is 20 mass parts or less with respect to 100 mass parts of silica similarly, Preferably it is 15 mass parts or less. When the content of the silane coupling agent exceeds 20 parts by mass, a coupling effect cannot be obtained for an increase in cost, and the reinforcing property and wear resistance are lowered.

The rubber composition includes the rubber component, a mixture of an aliphatic carboxylic acid zinc salt and an aromatic carboxylic acid zinc salt, stearic acid, oil, plasticizer, silica and silane coupling agent, sulfur, and a sulfur-containing compound. In addition to vulcanizing agents such as, conventional compounding agents used in the rubber industry, for example, fillers such as carbon black and eggshell powder, antioxidants, ozone degradation inhibitors, anti-aging agents, vulcanization accelerators, You may contain a zinc oxide, a peroxide, a vulcanization accelerator, etc.

As carbon black, carbon black having an average particle size of 30 nm or less and / or a DBP oil absorption of 100 ml / 100 g or more is preferable. By blending such carbon black, it is possible to provide the tread with the necessary reinforcing properties as a studless tire, and to ensure block rigidity, steering stability, uneven wear resistance, and wear resistance. In addition, the rubber composition containing carbon black tends to increase in viscosity and deteriorate in workability, but when a mixture of zinc salt of aliphatic carboxylic acid and zinc salt of aromatic carboxylic acid is used, unvulcanized The viscosity of rubber can be lowered and processability can be improved.

The content of carbon black is 2 parts by mass or more, preferably 4 parts by mass or more, more preferably 8 parts by mass or more, and most preferably 20 parts by mass or more with respect to 100 parts by mass of the rubber component. If the carbon black content is less than 2 parts by mass, the reinforcing property is insufficient, and it tends to be difficult to ensure the required block rigidity, steering stability, uneven wear resistance, and wear resistance. The carbon black content is 120 parts by mass or less, preferably 80 parts by mass or less, and more preferably 40 parts by mass or less. If the carbon black content exceeds 120 parts by mass, the processability is deteriorated and the hardness is too high.

The tread has a JIS-A hardness of preferably 50 degrees or less, more preferably 48 degrees or less, and still more preferably 46 degrees or less. By setting the hardness to 50 degrees or less, flexibility can be achieved, and more excellent performance on ice and snow can be obtained. On the other hand, the hardness is preferably 40 degrees or more. If the hardness is less than 40 degrees, the processability of the unvulcanized rubber tends to deteriorate, or it becomes difficult to obtain a preferable hardness while ensuring steering stability.

The studless tire for passenger cars of this invention has the tire tread produced from the said rubber composition. Here, the passenger car is an automobile used exclusively for human movement. This is the most common form of car.

The studless tire for passenger cars of this invention is manufactured by a normal method using the said rubber composition. That is, a tire tread can be produced using the rubber composition, bonded together with other members, and heated and pressurized on a tire molding machine.

The present invention will be specifically described based on examples, but the present invention is not limited to these examples.

Hereinafter, various chemicals used in Examples and Comparative Examples will be described together.
Natural rubber (NR): RSS # 3
BR1: BR150B manufactured by Ube Industries, Ltd. (cis 1,4 bond amount 97%, ML _{1 + 4} (100 ° C.) 40, 5% toluene solution viscosity at 25 ° C. 48 cps, Mw / Mn 3.3)
Carbon Black: Dia Black I manufactured by Mitsubishi Chemical Corporation (ISAF carbon, average particle size 23 nm, DBP oil absorption 114 ml / 100 g)
Silica: Ultrasil VN3 manufactured by Degussa (N ₂ SA: 175 m ² / g)
Silane coupling agent: Si-69 manufactured by Degussa
Mineral oil: PS-32 (paraffinic process oil) manufactured by Idemitsu Kosan Co., Ltd.
Stearic acid: anti-paulownia reversion inhibitor (mixture of zinc salt of aliphatic carboxylic acid and zinc salt of aromatic carboxylic acid) manufactured by NOF Corporation: Activator 73A ((i) aliphatic carboxyl Zinc salt of acid: Zinc salt of fatty acid derived from palm oil (carbon number: 8 to 12), (ii) Zinc salt of aromatic carboxylic acid: Zinc benzoate, Content molar ratio: 1/1, Zinc content: 17% by mass)
Zinc oxide: 2 types of zinc oxide manufactured by Mitsui Mining & Smelting Co., Ltd. Zinc oxide whisker: Panatetra WZ-0501 manufactured by Amtec Co., Ltd. (Average value) 0.2-3.0 μm)
Anti-aging agent: NOCRACK 6C (N- (1,3-dimethylbutyl) -N′-phenyl-p-phenylenediamine) manufactured by Ouchi Shinsei Chemical Industry Co., Ltd.
Wax: Ozoace wax manufactured by Nippon Seiwa Co., Ltd. Sulfur: Powder sulfur vulcanization accelerator manufactured by Tsurumi Chemical Co., Ltd. BBS: Noxeller NS (N-tert-butyl- manufactured by Ouchi Shinsei Chemical Co., Ltd.) 2-benzothiazolylsulfenamide)
Vulcanization accelerator DPG: Noxeller D (N, N'-diphenylguanidine) manufactured by Ouchi Shinsei Chemical Co., Ltd.

Examples 1-7 and Comparative Examples 1-7
In accordance with the formulation shown in Table 1, using a Banbury mixer, the amount of chemicals shown in Step 1 of Table 1 other than sulfur and vulcanization accelerator is added, and the discharge temperature is about 150 ° C. for 5 minutes. Kneaded. Then, sulfur and a vulcanization accelerator in the blending amounts shown in Step 2 are added to the mixture obtained in Step 1, and the mixture is kneaded at about 80 ° C. for 3 minutes using an open roll. I got a thing. The obtained unvulcanized rubber composition was molded into a tread shape, bonded to another tire member, and vulcanized at 170 ° C. for 15 minutes, thereby producing studless tires of the respective examples and comparative examples. In Examples 4 and 5, the vulcanization conditions were changed, and vulcanization was performed at 150 ° C. for 45 minutes.

Each sample was evaluated by the method described below.

(Reversion property)
Using a curast meter, the vulcanization curve of the unvulcanized rubber composition at 170 ° C. was measured. The maximum torque increase value (MH-ML) value was set to 100, the torque increase value 20 minutes after the start of vulcanization was shown as a relative value, and the value obtained by subtracting the relative value from 100 was taken as the reversion rate. The smaller the reversion rate, the lower the vulcanization return and the better. In Examples 4 and 5, the vulcanization curve at 150 ° C. was measured, the torque increase value 45 minutes after the vulcanization start time was indicated as a relative value, and the reversion rate was obtained in the same manner as described above.

(hardness)
According to JIS K6253 "Method for testing hardness of vulcanized rubber and thermoplastic rubber", the hardness of each vulcanized rubber specimen of each example and each comparative example was measured with a type A durometer.

(Performance on ice and snow)
Using the studless tires (winter pneumatic tires) of each example and each comparative example, the actual vehicle performance was evaluated on ice and snow under the following conditions. As winter pneumatic tires, 195 / 65R15 size DS-2 pattern studless tires for passenger cars were manufactured, and these tires were mounted on FR cars with a displacement of 2000 cc. The test place was the Hokkaido Nayoro Test Course of Sumitomo Rubber Industries, Ltd., the temperature on ice was -1 to -6 ° C, and the temperature on snow was -2 to -10 ° C.
・ Handling performance (steering stability): Using the above-mentioned vehicle, measurements were made of starting, accelerating and stopping on snow, and evaluated by feeling. Feeling evaluation is based on Comparative Example 1 as 100, with the test driver clearly judging that the performance has improved, 120, and a good level that has never been seen so far as 140. Scoring was performed.
-Braking performance (on-ice braking stop distance): The stop distance on ice required to depress and stop the lock brake at 30 km / h was measured. And it computed from the following formula by making Comparative Example 1 into a reference.
(Braking performance index) = (braking stop distance of comparative example 1) ÷ (stop distance) × 100

(Abrasion resistance)
195 / 65R15 size DS-2 pattern studless tires for passenger cars were manufactured using the tread rubber of each example and each comparative example, and these tires were mounted on a FF car having a displacement of 2000 cc.
Then, the groove depth of the tire tread portion after the traveling distance of 8000 km was measured, the traveling distance when the tire groove depth decreased by 1 mm was calculated, and indexed by the following formula.
[(Travel distance when the groove depth of 1 mm decreases) / (travel distance when the groove depth of the tire of Comparative Example 1 decreases by 1 mm)] × 100
It can be said that the larger the index, the better the wear resistance.

(Flexible crack growth resistance)
According to JIS K6260 “Testing Method for Demature Bending Crack Growth of Vulcanized Rubber and Thermoplastic Rubber”, under the conditions of a temperature of 23 ° C. and a relative humidity of 55%, For samples, measure the number of cracks after one million bend tests, or the number of bend tests until the crack growth reaches 1 mm, and assign each sample based on the obtained crack length or number of bend tests. The number of bendings until a 1 mm crack grew was expressed as a common logarithm value. Further, the obtained common logarithmic value is shown as an index with Comparative Example 1 as 100.
In addition, it bent repeatedly with the elongation rate of 50% with respect to the length of the sample of the original rubber composition. It can be said that as the index is larger, cracks are harder to grow and the resistance to flex crack growth is superior.

(Total amount of sulfur)
It calculated | required according to the method described in the fixed_quantity | quantitative_assay of the total sulfur content of 6th and 7th pages among "the rubber product analysis methods" of JISK6350 (1976).

(Crosslink density, monocrosslink density)
The calculation of the crosslinking density was performed according to the following procedure.
First, a cylindrical test piece having a diameter of 3 mm was punched from the vulcanized rubber sheet produced in each example and each comparative example. This test piece was immersed in acetone at 20 ° C. for 24 hours to extract oil and an antioxidant. The test piece after extraction was immersed in a solvent at 20 ° C. in which tetrahydrofuran (THF) and benzene were mixed at a mass ratio of 1: 1 for 24 hours to swell.
Next, the test piece was put into a TMA apparatus (FIG. 1) filled with a 20 ° C. solvent in which tetrahydrofuran (THF) and benzene were mixed at a mass ratio of 1: 1. And in this TMA apparatus, the value of ((tau) ₀ / (1 / (alpha) ^2- (alpha))) was calculated | required from the relationship between compressive stress and distortion. The numerical value thus obtained and various dimensions of the test piece were substituted into the Flory's theoretical formula shown by the following formula (I), and the total crosslink density (ν _T ) of the rubber was calculated. The test was performed on three test pieces, and the results were averaged.

The measurement for calculating the crosslink density (ν _M ) of the monosulfide bond is the same as the total crosslink density described above except that a LiAlH ₄ catalyst is added to a 1: 1 mixture of tetrahydrofuran (THF) and benzene to swell the test piece. The measurement was performed in the same manner as the measurement of ν _T ). The calculation of the crosslinking density (ν _M ) was performed according to the formula (I). At this time, (ν _T ) in Formula (I) was replaced with (ν _M ).

Table 1 shows the evaluation results of the above tests.

In the samples of Examples 1 to 7, since the total sulfur amount is as low as 0.7% by mass or less and the crosslinking density is 2.0 × 10 ⁻⁵ mol / cm ³ or more, unnecessary sulfur is present. The crosslinked state is considered to be good. As a result, both high snow handling performance and high ice braking performance were achieved. Furthermore, the wear resistance was also good.
Moreover, the reversion rate was low in all cases, and reversion was suppressed. The hardness was also appropriate. Furthermore, the mono-crosslinking density was appropriate, and the flex crack growth resistance was also good.

In Example 5 in which the silica content was increased and Examples 6 and 7 in which zinc oxide whiskers were blended, the braking performance on ice was further enhanced. The handling performance on snow was also good. Also, the reversion rate was low.

On the other hand, in Comparative Example 1, the total amount of sulfur was too large, and the handling performance on snow was not very good. The reversion rate was also slightly higher. Moreover, in Comparative Examples 3-7, the crosslinking density was too low and the handling performance on snow was not good. Also, the reversion rate was high or slightly high.
In Comparative Example 2, the amount of total sulfur was large and the crosslink density was high. Particularly, the monocrosslink density was also high, so that the flex crack growth resistance was not good.

Claims (3)

Having a tread having a total sulfur content of 0.7% by mass or less and a crosslink density of 2.0 × 10 ⁻⁵ mol / cm ³ or more;
The tread is obtained from a rubber composition containing a rubber component including butadiene rubber and natural rubber and / or isoprene rubber;
A studless tire for a passenger car, wherein a total content of the butadiene rubber, the natural rubber, and the isoprene rubber in 100% by mass of the rubber component is 70% by mass or more. The studless tire for passenger cars according to claim 1, wherein the content of the butadiene rubber in 100% by mass of the rubber component is 40% by mass or more. The studless tire for a passenger car according to claim 1 or 2, wherein a crosslinking density of monosulfide bonds in the crosslinked form of the tread is 0.9 x 10 ^-5 mol / cm ³ or less.

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