JP7630763B2 - Pneumatic tire and manufacturing method thereof - Google Patents

Description

The present invention relates to a pneumatic tire that exhibits sufficient road grip performance immediately after being mounted on a vehicle, and a method for manufacturing the same.

Pneumatic tires (hereafter simply referred to as "tires") are required to have sufficient road grip performance from the perspective of driving stability.

In recent years, studless tires have come into widespread use as tires for driving on snowy and icy roads. Because these studless tires are used on snowy and icy roads, which have greater surface irregularities than general road surfaces, they are required to have even stricter grip performance (grip performance on ice) than regular tires.

As a result, a technique for improving grip performance on ice has been proposed, for example, to form a tread using a rubber composition that contains a water-soluble inorganic filler (Patent Documents 1 and 2). The water-soluble inorganic filler mixed into the tread dissolves when it comes into contact with moisture on the icy road surface, creating unevenness on the tire surface, thereby improving grip performance on ice.

It has also been proposed to modify the mold to form fine irregularities on the tread surface during tire vulcanization (Patent Document 3). These fine irregularities on the tire surface come into contact with the icy road surface, improving grip performance on ice.

Patent No. 5592885 Patent No. 5834086 Patent No. 5456363

However, in the case of the technology shown in Patent Documents 1 and 2, even though water-soluble inorganic filler is mixed into the tread to give it sufficient grip performance on ice, sufficient grip performance on ice is not exhibited initially after installation, and sufficient grip performance on ice is not exhibited until about 100 km has been driven on asphalt roads.

And this problem of the tread's inherent grip performance not being realized unless the vehicle is fitted with the studless tires and driven a certain distance is not limited to the studless tires mentioned above, but is the same for regular tires.

On the other hand, while the technology shown in Patent Document 3 can provide excellent grip performance on ice from the initial installation, it is not easy to process fine irregularities in a mold, and precise cleaning techniques are required to maintain the irregular shape after processing, resulting in problems in terms of productivity and cost.

As such, the existing technologies are still not sufficient, and further improvements are required. In particular, in the case of studless tires, which are used on snowy and icy roads, the lack of grip performance on ice when first installed is a major problem.

The present invention aims to provide a manufacturing technique for pneumatic tires that can provide excellent grip performance, particularly grip performance on ice, from the initial installation stage without the need for finely textured processing on the mold.

The inventors conducted extensive research into solving the above problems and discovered that the invention described below can solve the above problems, leading to the completion of the present invention.

The invention described in claim 1 is
The tread portion is formed by a tread main body portion made of a tread rubber composition and a surface layer portion made of a rubber composition having a higher polymer concentration than the tread rubber composition of the tread main body portion,
The thickness of the surface layer is several tens to 100 μm,
The pneumatic tire is characterized in that the surface layer contains 10 to 80 mass % of a water-soluble inorganic filler.

The invention described in claim 2 is
The pneumatic tire according to claim 1, characterized in that the water-soluble inorganic filler in the surface layer portion of the tread portion is one or a mixture of two or more of a metal sulfate, a metal chloride , a metal hydroxide, a carbonate, and a phosphate.

The invention described in claim 3 is
3. The pneumatic tire according to claim 2 , wherein the water-soluble inorganic filler in the surface layer of the tread portion is a metal sulfate.

The invention described in claim 4 is
4. The pneumatic tire according to claim 3 , wherein the metal sulfate in the surface layer of the tread portion is magnesium sulfate.

The invention described in claim 5 is
5. The pneumatic tire according to claim 4 , wherein the magnesium sulfate is any one of anhydrous magnesium sulfate, magnesium sulfate dihydrate, and magnesium sulfate trihydrate, or a mixture of two or more of them.

The invention described in claim 6 is
The pneumatic tire according to any one of claims 1 to 5 , characterized in that a water-insoluble inorganic filler is further added to a surface layer portion of the tread portion in addition to the water-soluble inorganic filler.

The invention described in claim 7 is
The pneumatic tire according to claim 6 , characterized in that a content ratio of the water-insoluble inorganic filler to the water-soluble inorganic filler is 3 to 30 mass %.

The invention described in claim 8 is
The pneumatic tire according to claim 6 or 7, characterized in that the water-insoluble inorganic filler is one or a mixture of two or more of egg shells, zinc oxide needle crystals, rice husks , talc, magnesium oxide, and iron powder .

The invention described in claim 9 is
9. The pneumatic tire according to claim 1 , wherein the tread portion is a studless tire formed from a rubber composition containing a water-soluble inorganic filler.

The invention described in claim 10 is
10. The pneumatic tire according to claim 9 , wherein the studless tire is a studless tire for passenger cars.

The invention described in claim 11 is
The method for manufacturing a pneumatic tire is characterized in that, between a molding process for molding a raw tire and a vulcanization process for vulcanizing the molded raw tire, an inorganic filler spraying process is provided in which a water-soluble inorganic filler is sprayed onto the surface of a tread portion of the molded raw tire.

The invention described in claim 12 is
The method for manufacturing a pneumatic tire according to claim 11 , characterized in that the inorganic filler spraying step is a step of spraying the water-soluble inorganic filler by applying an aqueous solution of the water-soluble inorganic filler to the surface of the tread portion.

The invention described in claim 13 is
The method for manufacturing a pneumatic tire according to claim 11 , characterized in that the inorganic filler spraying step is a step of spraying the water-soluble inorganic filler by applying an organic solvent dispersion of the water-soluble inorganic filler to the surface of the tread portion.

The invention described in claim 14 is
The method for producing a pneumatic tire according to claim 13 , characterized in that the organic solvent in the organic solvent dispersion is any one of a hydrocarbon organic solvent, anhydrous ethanol, and hexane, or a mixture of two or more of them.

The invention described in claim 15 is
The method for manufacturing a pneumatic tire according to any one of claims 12 to ¹⁴ , characterized in that the inorganic filler spraying step is a step of applying and spraying an aqueous solution of the water-soluble inorganic filler or an organic solvent dispersion of the water-soluble inorganic filler while adjusting the amount of the water-soluble filler sprayed on the surface of the tread portion after drying to be 0.005 to 0.04 g/ cm2 .

The invention described in claim 16 is
The method for manufacturing a pneumatic tire according to any one of claims 12 to 15 , characterized in that a water-insoluble inorganic filler is added to the aqueous solution of the water-soluble inorganic filler or the organic solvent dispersion of the water-soluble inorganic filler.

The present invention provides a manufacturing technique for pneumatic tires that can provide excellent grip performance, particularly grip performance on ice, from the initial installation stage without the need for finely textured processing on the mold.

2 is an optical microscope photograph showing the observation result of the tread surface of a tire in one embodiment of the present invention.

The present invention will be specifically described below based on the embodiment. Note that the following description will be based on studless tires, which are particularly effective when used on snowy and icy roads, but the same can be said for general tires.

1. Pneumatic tire according to the present embodiment A pneumatic tire according to the present embodiment has a tread portion (hereinafter also referred to as "tread") formed of a tread main body portion made of a tread rubber composition and a surface layer portion made of a rubber composition having a higher polymer concentration than the tread rubber composition of the tread main body portion, and is characterized in that the surface layer portion contains a water-soluble inorganic filler (hereinafter also referred to as "water-soluble filler").

The inventors have investigated why conventional tires do not exhibit the inherent grip performance of the tread unless the tire is driven a certain distance after being fitted.

As a result, it was found that the tread of a vulcanized tire is not formed uniformly over the entirety of the tread rubber composition, but rather that a polymer-rich layer (hereinafter also referred to as the "skin layer") with a higher polymer concentration than the inner layer of the tread rubber composition is formed on the outermost surface of the tread with a thickness of about several tens to 100 μm, and that the inherent grip performance of the tread is not exhibited until this skin layer is removed and the tread is exposed.

For example, when studless tires are first installed, the skin layer, which is rich in polymer and has few surface irregularities, comes into contact with the icy road surface, and so the tires do not exhibit sufficient grip performance on ice as studless tires do. It was found that only after driving about 100 km on an asphalt road surface and removing the skin layer can the tires exhibit sufficient grip performance on ice.

This phenomenon also occurs in regular tires, not just studless tires, and it was found that the presence of this skin layer inhibits the tires' grip performance in the initial period after installation.

The inventors therefore came up with the idea of mixing a water-soluble filler into this skin layer. Specifically, the water-soluble filler is sprayed onto the surface of the tread of a raw tire before vulcanization, and then the tire is vulcanized. This allows the water-soluble filler to be incorporated into the skin layer, and a skin layer containing the water-soluble filler is formed on the surface of the tread, allowing the tire to exhibit sufficient grip performance even in the early stages of installation.

In other words, when a skin layer rich in water-soluble filler is formed, the tire is left with some of the water-soluble filler exposed on the tread surface. When the tread of this tire comes into contact with water, the exposed water-soluble filler dissolves in the water, and voids are formed in the skin layer on the surface of the tread after the water-soluble filler has dissolved, forming fine irregularities. When these fine irregularities come into contact with the road surface, water on the road surface is removed via the fine irregularities, allowing the tire to exhibit sufficient grip performance even immediately after installation.

The skin layer is gradually worn away and the original tread layer is revealed, and the formation of these voids and the removal of water are repeated to maintain grip performance, after which the grip performance of the tread layer itself is fully demonstrated. Since the skin layer rich in water-soluble fillers has lower abrasion resistance than conventional skin layers, the skin layer wears away quickly and the tread layer appears in a short time. As a result, it is possible to achieve stable and sufficient grip performance from the initial installation. However, this decrease in abrasion resistance has little effect on the wear performance of the entire tire, since the skin layer is only 100 μm thick.

This effect is particularly evident in studless tires that run on snowy and icy roads where grip is difficult, as water on the snowy and icy road surface is removed via the minute irregularities, allowing the tires to immediately provide sufficient grip on ice even immediately after installation.

2. Water-Soluble Inorganic Filler Next, the water-soluble inorganic filler that is characteristically used in this embodiment will be described.

In this embodiment, the water-soluble filler can be any inorganic filler (preferably porous) that is soluble in water, without any particular limitations. For example, a material with a solubility in water at room temperature (20°C) of 1 g/100 g water or more can be used.

In general, the smaller the apparent specific gravity of the filler, the more fine or porous it is. Here, the nitrogen adsorption specific surface area ( _N2SA ) of the filler indicates the pore surface area of the filler, and generally, when the particle has a smooth surface that is not porous, there is a mathematical relationship in which the median particle size (median diameter, D50) is 0.7 μm and the _N2SA is 7 ^m2 /g, the _D50 is 0.5 μm and the N2SA is ₁₅ ^m2 /g, and the D50 is 0.3 μm and the N2SA is 34 ^m2 /g. Magnesium sulfate used as a water-soluble filler in the examples described later has a D50 of 7.3 μm and an _N2SA of 9.0 ^m2 /g, which shows that the _N2SA is large compared to the particle size and is porous.

In this embodiment, the median particle size of the water-soluble filler is 100 μm or less, and preferably 90 μm or less. It is more preferably 80 μm or less, and even more preferably 70 μm or less. It is particularly preferably 20 μm or less, and most preferably 10 μm or less. On the other hand, it is preferably 0.4 μm or more, more preferably 0.6 μm or more, and even more preferably 1.0 μm or more. Here, the median particle size (D50) can be measured by a laser diffraction method, and means the particle size at 50% of the integrated value in the mass-based particle size distribution curve obtained by the laser diffraction scattering method.

To specifically measure the median particle size (D50), a SALD-2000J model manufactured by Shimadzu Corporation is used to disperse the water-soluble filler in a mixed solution of a dispersion solvent (toluene) and a dispersant (a 10% by mass solution of di-2-ethylhexyl sodium sulfosuccinate in toluene) at room temperature, and the resulting dispersion is stirred for 5 minutes while irradiating it with ultrasonic waves to obtain a test liquid, which is then transferred to a batch cell and measured after 1 minute (refractive index: 1.70-0.20i).

From the viewpoint of the balance between grip performance and abrasion resistance, the apparent specific gravity of the water-soluble filler is 0.70 g/ml or less, and preferably 0.65 g/ml or less. It is more preferably 0.62 g/ml or less. On the other hand, it is preferably 0.20 g/ml or more, more preferably 0.30 g/ml or more, even more preferably 0.40 g/ml or more, and particularly preferably 0.55 g/ml or more. Note that the apparent specific gravity here is a value calculated from the mass when 30 ml of apparent volume is measured out into a 50 ml measuring cylinder.

In addition, from the viewpoints of particle stability and deliquescence resistance, as well as the balance between grip performance and abrasion resistance, it is preferable that the moisture content of the water-soluble filler is low. Specifically, it is preferably 1.0 mass% or less, more preferably 0.8 mass% or less, even more preferably 0.3 mass% or less, and particularly preferably 0.1 mass% or less. Here, the moisture content is a value determined from the weight loss rate when left to stand in a thermostatic chamber at 250°C for 1 hour.

In addition, from the viewpoint of the balance between grip performance and abrasion resistance, the nitrogen adsorption specific surface area (N ₂ SA) of the water-soluble filler is preferably 1.3 m ² /g or more, more preferably 3.0 m ² /g or more, and even more preferably 4.0 m ² /g or more. It is particularly preferable that it is ^{5.0 m 2} /g or more, and most preferably 8.0 m ² /g or more. On the other hand, it is preferably 40 m ² /g or less, more preferably 30 m ² /g or less, and even more preferably 20 m ² /g or less. Here, N ₂ SA is a value measured by the BET method according to ASTM D3037-81, and the larger the N ₂ SA, the more porous it is, and the more suitable it is for absorbing moisture.

In the present embodiment, the water-soluble filler may be, in consideration of not causing corrosion or damage to the vulcanization mold and of the impact on the environment, including the human body, metal sulfates such as magnesium sulfate, sodium sulfate, potassium sulfate, etc.; metal chlorides such as potassium chloride, sodium chloride, calcium chloride, magnesium chloride, etc.; metal hydroxides such as potassium hydroxide, sodium hydroxide, etc.; carbonates such as potassium carbonate, sodium carbonate, calcium hydrogen carbonate, etc.; phosphates such as sodium hydrogen phosphate, sodium dihydrogen phosphate, etc. These may be used alone or in combination of two or more kinds.

Among the water-soluble fillers described above, metal sulfates are more preferred, and magnesium sulfate is even more preferred. Among magnesium sulfates, anhydrous magnesium sulfate, magnesium sulfate dihydrate, and magnesium sulfate trihydrate are preferred, and anhydrous magnesium sulfate is more preferred. Sodium sulfate is also preferred, and anhydrous sodium sulfate is more preferred. These may be used alone or in combination of two or more.

In this embodiment, the amount of the water-soluble filler scattered on the tread surface of the molded raw tire is preferably 0.005 to 0.04 g/cm ² , more preferably 0.01 to 0.03 g/cm ² , and even more preferably 0.012 to 0.017 g/cm ^2. By using such a scattered amount, the content of the water-soluble filler in the skin layer can be set to 10 to 80 mass %, and appropriate surface grip performance can be exhibited from the initial stage of mounting.

In this embodiment, a water-insoluble inorganic filler (hereinafter, also referred to as "water-insoluble filler") may be further added. Unlike water-soluble fillers, this water-insoluble filler does not dissolve when it comes into contact with water, but remains in the skin layer and scratches the road surface, which increases the frictional resistance between the fine irregularities formed and the road surface (such as a snowy or icy road surface), and further improves grip performance (especially grip performance on ice).

Specific examples of water-insoluble fillers include egg shells, tetrapod-type zinc oxide crystals, rice husks, talc, magnesium oxide, and iron powder. The addition rate relative to the water-soluble filler is preferably 3 to 30% by mass, more preferably 5 to 20% by mass, and even more preferably 10 to 15% by mass. These may be used alone or in combination of two or more kinds.

The water-soluble filler described above can also be used in the tread rubber composition of studless tires. In this case, the content of the water-soluble filler is preferably 1 part by mass or more, more preferably 2 parts by mass or more, and even more preferably 3 parts by mass or more, per 100 parts by mass of the rubber component. On the other hand, it is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, and even more preferably 30 parts by mass or less. It is particularly preferably 20 parts by mass or less.

3. Tread rubber composition As described above, during vulcanization, a polymer-rich skin layer is inevitably formed on the tread surface of the tire, and the inherent grip performance of the tread cannot be exhibited immediately after installation. However, after a certain amount of running and the skin layer is removed, the inherent grip performance of the tread needs to be exhibited. Therefore, the following describes each material other than the water-soluble filler used in the tread rubber composition in order to exhibit the inherent grip performance of the tread.

In this embodiment, the tread rubber composition includes a rubber component containing isoprene-based rubber, butadiene rubber, and styrene-butadiene rubber, water-soluble fine particles, and a liquid plasticizer, and has a specified styrene-butadiene rubber and liquid plasticizer.

(1) Rubber Component In the present embodiment, examples of the rubber component of the tread rubber composition include diene rubbers such as isoprene rubber, butadiene rubber (BR), styrene butadiene rubber (SBR), styrene isoprene butadiene rubber (SIBR), acrylonitrile butadiene rubber (NBR), chloroprene rubber (CR), and butyl rubber (IIR). The rubber component may be used alone or in combination of two or more. Among them, SBR, BR, and isoprene rubber are preferred, and SBR and BR are more preferred.

There are no particular limitations on the SBR, and for example, emulsion-polymerized SBR (E-SBR), solution-polymerized SBR (S-SBR), and other SBRs commonly used in the tire industry can be used. These may be used alone or in combination of two or more types.

Here, the rubber component preferably has a weight average molecular weight (Mw) of 150,000 or more, and more preferably 350,000 or more. On the other hand, it is preferably 4 million or less, and more preferably 3 million or less. Here, the weight average molecular weight (Mw) is a value calculated by converting it into standard polystyrene based on the measured value obtained by gel permeation chromatography (GPC) (GPC-8000 series manufactured by Tosoh Corporation, detector: differential refractometer, column: TSKGEL SUPERMALTPORE HZ-M manufactured by Tosoh Corporation).

The styrene content of SBR is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more. On the other hand, it is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less. Here, the styrene content of SBR is a value calculated by ^1H -NMR measurement.

The vinyl content of SBR (amount of 1,2-bonded butadiene units) is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 15% by mass or more. On the other hand, it is preferably 50% by mass or less, and more preferably 40% by mass or less. Here, the vinyl content of SBR refers to the vinyl content of the butadiene portion (the number of vinyl group units in the butadiene structure), and is a value calculated by ¹ H-NMR measurement.

The SBR may be unmodified or modified. The modified SBR may be any SBR having a functional group that interacts with a filler such as silica. For example, a terminal-modified SBR (terminal-modified SBR having the above-mentioned functional group at the terminal) in which at least one terminal of the SBR is modified with a compound (modifier) having the above-mentioned functional group, a main-chain-modified SBR having the above-mentioned functional group in the main chain, a main-chain-terminal-modified SBR having the above-mentioned functional group in the main chain and at least one terminal modified with the above-mentioned modifier, or a terminal-modified SBR modified (coupled) with a polyfunctional compound having two or more epoxy groups in the molecule and having a hydroxyl group or an epoxy group introduced therein may be used. These may be used alone or in combination of two or more.

Examples of the functional groups include amino groups, amide groups, silyl groups, alkoxysilyl groups, isocyanate groups, imino groups, imidazole groups, urea groups, ether groups, carbonyl groups, oxycarbonyl groups, mercapto groups, sulfide groups, disulfide groups, sulfonyl groups, sulfinyl groups, thiocarbonyl groups, ammonium groups, imide groups, hydrazo groups, azo groups, diazo groups, carboxyl groups, nitrile groups, pyridyl groups, alkoxy groups, hydroxyl groups, oxy groups, and epoxy groups. These functional groups may have a substituent. Among them, amino groups (preferably amino groups in which the hydrogen atoms of the amino groups are substituted with alkyl groups having 1 to 6 carbon atoms), alkoxy groups (preferably alkoxy groups having 1 to 6 carbon atoms), alkoxysilyl groups (preferably alkoxysilyl groups having 1 to 6 carbon atoms), and amide groups are preferred.

As the SBR, either oil-extended or non-oil-extended SBR can be used. When using oil-extended SBR, the amount of oil extension of the SBR, i.e., the amount of oil extension oil contained in the SBR, is preferably 10 to 50 parts by mass per 100 parts by mass of rubber solids of the SBR.

The glass transition temperature (Tg) of SBR is preferably -90°C or higher, and more preferably -50°C or higher. On the other hand, it is preferably 0°C or lower, and more preferably 10°C or lower. The glass transition temperature is a value obtained by measuring in accordance with JIS-K7121 using a differential scanning calorimeter (Q200) manufactured by TA Instruments Japan Ltd. at a heating rate of 10°C/min.

As SBR, for example, SBR manufactured and sold by Sumitomo Chemical Co., Ltd., JSR Corporation, Asahi Kasei Corporation, Nippon Zeon Co., Ltd., etc. can be used.

The content of SBR in 100% by mass of the rubber component is preferably 40% by mass or more, more preferably 50% by mass or more, and even more preferably 70% by mass or more. On the other hand, it is preferably 95% by mass or less, and more preferably 90% by mass or less.

The BR is not particularly limited, and may be any of those commonly used in the tire industry, such as BR with a high cis content, BR containing 1,2-syndiotactic polybutadiene crystals (SPB-containing BR), butadiene rubber synthesized using a rare earth catalyst (rare earth-based BR), and tin-modified butadiene rubber modified with a tin compound (tin-modified BR). These may be used alone or in combination of two or more. Of these, rare earth-based BR is preferred.

Rare earth BR is a butadiene rubber synthesized using a rare earth element catalyst, and is characterized by a high cis content and a low vinyl content. General-purpose products used in tire manufacturing can be used as rare earth BR.

As the rare earth element catalyst, known catalysts can be used, such as lanthanum series rare earth element compounds, organoaluminum compounds, aluminoxanes, halogen-containing compounds, and catalysts containing Lewis bases as necessary. Among these, Nd-based catalysts using neodymium (Nd)-containing compounds as the lanthanum series rare earth element compounds are particularly preferred.

The cis content of BR (amount of cis-1,4-bonded butadiene units) is preferably 90% by mass or more, more preferably 93% by mass or more, and even more preferably 95% by mass or more. Note that the cis content here is a value calculated by infrared absorption spectrum analysis.

The vinyl content of BR (amount of 1,2-bonded butadiene units) is preferably 1.8% by mass or less, more preferably 1.0% by mass or less, and even more preferably 0.5% by mass or less. It is particularly preferably 0.3% by mass or less.

Either unmodified or modified BR can be used. Modified BR includes modified BR with the same functional groups as the modified SBR described above.

For example, products from Ube Industries, Ltd., JSR Corporation, Asahi Kasei Corporation, Zeon Corporation, etc. can be used as BR.

The BR content in 100% by mass of the rubber component is preferably 5% by mass or more, and more preferably 10% by mass or more. On the other hand, it is preferably 60% by mass or less, more preferably 50% by mass or less, and even more preferably 30% by mass or less.

Examples of isoprene-based rubber include natural rubber (NR), isoprene rubber (IR), modified NR, modified NR, and modified IR. Examples of NR include SIR20, RSS#3, TSR20, and other rubbers commonly used in the tire industry. Examples of IR include IR2200 and other rubbers commonly used in the tire industry. Examples of modified NR include deproteinized natural rubber (DPNR), high-purity natural rubber (UPNR), and other rubbers. Examples of modified NR include epoxidized natural rubber (ENR), hydrogenated natural rubber (HNR), and grafted natural rubber. Examples of modified IR include epoxidized isoprene rubber, hydrogenated isoprene rubber, and grafted isoprene rubber. These may be used alone or in combination of two or more. Of these, natural rubber is preferred.

The total content of SBR and BR in 100% by mass of the rubber component is preferably 30% by mass or more, more preferably 50% by mass or more, and even more preferably 70% by mass or more. It is particularly preferably 90% by mass or more, and most preferably 100% by mass.

(2) Wax In this embodiment, the tread rubber composition preferably contains a wax. The wax is not particularly limited, and examples thereof include petroleum waxes such as paraffin wax and microcrystalline wax; natural waxes such as vegetable waxes such as candelilla wax, carnauba wax, Japan wax, rice wax, and jojoba wax, and animal waxes such as beeswax, lanolin, and spermaceti; mineral waxes such as ozokerite, ceresin, and petrolactam; and synthetic waxes such as polymers of ethylene and propylene. These may be used alone or in combination of two or more. Among them, petroleum wax is preferred, and paraffin wax and microcrystalline wax are more preferred. In addition, it is preferred that the content of branched alkanes in 100% by mass of microcrisslin wax is 50% by mass or more.

As wax, for example, products from Ouchi Shinko Chemical Industry Co., Ltd., Nippon Seiro Co., Ltd., Seiko Chemical Co., Ltd., etc. can be used.

The content of paraffin wax per 100 parts by mass of the rubber component is preferably 2.3 parts by mass or less, more preferably 2.0 parts by mass or less, and even more preferably 1.5 parts by mass or less. It is particularly preferable that it is 1.3 parts by mass or less. On the other hand, it is preferably 0.3 parts by mass or more, more preferably 0.5 parts by mass or more, and even more preferably 0.8 parts by mass or more. It is particularly preferable that it is 1.0 part by mass or more.

The content of microcrystalline wax per 100 parts by mass of the rubber component is preferably 0.3 parts by mass or more, more preferably 0.5 parts by mass or more, and even more preferably 0.8 parts by mass or more. It is particularly preferable that it is 1.0 part by mass or more. On the other hand, it is preferably 3 parts by mass or less, more preferably 2 parts by mass or less, and even more preferably 1.5 parts by mass or less.

(3) Antiaging Agent In the present embodiment, the tread rubber composition preferably contains an antiaging agent. The antiaging agent is not particularly limited, but examples thereof include naphthylamine-based antiaging agents such as phenyl-α-naphthylamine, diphenylamine-based antiaging agents such as octylated diphenylamine and 4,4'-bis(α,α'-dimethylbenzyl)diphenylamine, N-isopropyl-N'-phenyl-p-phenylenediamine, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, N,N'-di-2-naphthyl-p-phenylenediamine, and the like. Examples of the antioxidants include p-phenylenediamine antioxidants such as N-phenylenediamine, quinoline antioxidants such as polymers of 2,2,4-trimethyl-1,2-dihydroquinoline, monophenol antioxidants such as 2,6-di-t-butyl-4-methylphenol and styrenated phenol, and bis-, tris-, and polyphenol-based antioxidants such as tetrakis-[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane. These antioxidants may be used alone or in combination of two or more. Among them, phenylenediamine antioxidants are preferred because they have a high migration rate to the surface layer and an excellent bleeding function, and N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine and N,N'-bis-(1,4-dimethylpentyl)-p-phenylenediamine are more preferred. A quinoline-based antioxidant may be used in combination with the phenylenediamine-based antioxidant; however, the quinoline-based antioxidant has a slower migration rate than the phenylenediamine-based antioxidant, and tends to have a lower softening effect on the surface layer.

As anti-aging agents, for example, products from Seiko Chemical Co., Ltd., Sumitomo Chemical Co., Ltd., Ouchi Shinko Chemical Co., Ltd., Flexis, etc. can be used.

The content of the phenylenediamine-based antiaging agent per 100 parts by mass of the rubber component is preferably 2.1 parts by mass or more, and more preferably 2.4 parts by mass or more. On the other hand, it is preferably 20 parts by mass or less, more preferably 15 parts by mass or less, and even more preferably 7 parts by mass or less.

The total content of the antioxidants per 100 parts by mass of the rubber component is preferably 2 parts by mass or more, and more preferably 2.4 parts by mass or more. On the other hand, it is preferably 20 parts by mass or less, more preferably 15 parts by mass or less, and even more preferably 8 parts by mass or less.

(4) Liquid Plasticizer In the present embodiment, the tread rubber composition preferably contains a liquid plasticizer. The liquid plasticizer is not particularly limited as long as it is a plasticizer that is in a liquid state at 20°C, and examples thereof include oil, liquid resin, liquid polymer, etc. These may be used alone or in combination of two or more kinds. Among them, oil and liquid resin are preferred.

The content of the liquid plasticizer per 100 parts by mass of the rubber component is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, and even more preferably 25 parts by mass or more. It is particularly preferable that it is 35 parts by mass or more. On the other hand, it is preferably 100 parts by mass or less, more preferably 80 parts by mass or less, and even more preferably 60 parts by mass or less. It is particularly preferable that it is 50 parts by mass or less. In this specification, the content of the liquid plasticizer also includes the amount of oil contained in the oil-extended rubber.

Examples of the oil include process oil, vegetable oil, and mixtures thereof. Examples of the process oil include paraffin-based process oil, aromatic process oil, naphthenic process oil, mild extraction solvates (MES), and treated distillate aromatic extracts (TDAE). Examples of the vegetable oil include castor oil, cottonseed oil, linseed oil, rapeseed oil, soybean oil, palm oil, coconut oil, peanut oil, rosin, pine oil, pine tar, tall oil, corn oil, rice oil, safflower oil, sesame oil, olive oil, sunflower oil, palm kernel oil, camellia oil, jojoba oil, macadamia nut oil, tung oil, and oleic acid-containing oil. These may be used alone or in combination of two or more. Among these, process oils are preferred, with aromatic process oils and oleic acid-containing oils being more preferred.

As oils, products from, for example, Idemitsu Kosan Co., Ltd., Sankyo Yuka Kogyo Co., Ltd., Japan Energy Co., Ltd., Orisoi Co., Ltd., H&R Corporation, Toyokuni Oil Mills Co., Ltd., Showa Shell Sekiyu K.K., Fuji Kosan Co., Ltd., etc. can be used.

Examples of liquid resins include terpene resins (including terpene phenol resins and aromatic modified terpene resins) that are liquid at 20°C, styrene resins, C5 resins, C9 resins, C5/C9 resins, dicyclopentadiene (DCPD) resins, coumarone indene resins (including coumarone and indene simple resins), phenol resins, olefin resins, polyurethane resins, and acrylic resins. These may be used alone or in combination of two or more. Of these, coumarone indene resins are preferred.

Examples of liquid polymers (liquid diene-based polymers) include liquid styrene butadiene copolymers (liquid SBR), liquid butadiene polymers (liquid BR), liquid isoprene polymers (liquid IR), liquid styrene isoprene copolymers (liquid SIR), liquid styrene butadiene styrene block copolymers (liquid SBS block polymers), liquid styrene isoprene styrene block copolymers (liquid SIS block polymers), liquid farnesene polymers, and liquid farnesene butadiene copolymers that are liquid at 20°C. The ends or main chains of these may be modified with polar groups. These may be used alone or in combination of two or more.

(5) Resin In this embodiment, the tread rubber composition preferably contains a resin (solid resin: a resin that is in a solid state at room temperature (25° C.)).

The softening point of the resin is preferably 60°C or higher, more preferably 65°C or higher, and even more preferably 80°C or higher. On the other hand, it is preferably 160°C or lower, and more preferably 135°C or lower. The softening point of the resin is the temperature at which the ball drops when the softening point specified in JIS K 6220-1:2001 is measured using a ring and ball softening point measuring device.

The resin (solid resin) is not particularly limited, but examples include aromatic vinyl polymers, coumarone-indene resins, terpene resins, dicyclopentadiene resins (DCPD resins), C5 petroleum resins, C9 petroleum resins, C5/C9 petroleum resins, p-t-butylphenol acetylene resins, acrylic resins, and the like. These may be used alone or in combination of two or more. The resins may also be hydrogenated. Among these, aromatic vinyl polymers and terpene resins are preferred.

Aromatic vinyl polymers are resins obtained by polymerizing α-methylstyrene and/or styrene, and examples include homopolymers of styrene (styrene resins), homopolymers of α-methylstyrene (α-methylstyrene resins), copolymers of α-methylstyrene and styrene, and copolymers of styrene and other monomers.

Coumarone-indene resin is a resin that contains coumarone and indene as the main monomer components that make up the resin skeleton (main chain). Other monomer components contained in the skeleton besides coumarone and indene include styrene, α-methylstyrene, methylindene, vinyltoluene, etc.

The terpene resin is not particularly limited as long as it has units derived from a terpene compound, and examples thereof include polyterpene (a resin obtained by polymerizing a terpene compound), terpene aromatic resin (a resin obtained by copolymerizing a terpene compound and an aromatic compound), and aromatic modified terpene resin (a resin obtained by modifying a terpene resin with an aromatic compound). These may be used alone or in combination of two or more kinds. Among them, terpene aromatic resin and aromatic modified terpene resin are preferred, and aromatic modified terpene resin is more preferred.

The terpene compounds are hydrocarbons represented by the formula (C ₅ H ₈ ) _n and their oxygen-containing derivatives, and are compounds having a basic skeleton of a terpene classified into monoterpene (C ₁₀ H ₁₆ ), sesquiterpene (C ₁₅ H ₂₄ ), diterpene (C ₂₀ H ₃₂ ), etc. Examples of the terpene compounds include α-pinene, β-pinene, dipentene, limonene, myrcene, alloocimene, ocimene, α-phellandrene, α-terpinene, γ-terpinene, terpinolene, 1,8-cineole, 1,4-cineole, α-terpineol, β-terpineol, and γ-terpineol.

The above terpene compounds also include resin acids (rosin acids) such as abietic acid, neoabietic acid, palustric acid, levopimaric acid, pimaric acid, and isopimaric acid. In other words, the above terpene resins also include rosin resins whose main component is rosin acid obtained by processing pine resin. Examples of rosin resins include naturally occurring rosin resins (polymerized rosins) such as gum rosin, wood rosin, and tall oil rosin, as well as modified rosin resins such as maleic acid-modified rosin resins and rosin-modified phenolic resins, rosin esters such as rosin glycerin esters, and disproportionated rosin resins obtained by disproportionating rosin resins.

The aromatic compound is not particularly limited as long as it has an aromatic ring, but examples thereof include phenolic compounds such as phenol, alkylphenol, alkoxyphenol, and phenol containing an unsaturated hydrocarbon group; naphthol compounds such as naphthol, alkylnaphthol, alkoxynaphthol, and naphthol containing an unsaturated hydrocarbon group; and styrene derivatives such as styrene, alkylstyrene, alkoxystyrene, and styrene containing an unsaturated hydrocarbon group. Of these, styrene is preferred.

Examples of the resins and liquid resins that can be used include products from Maruzen Petrochemical Co., Ltd., Sumitomo Bakelite Co., Ltd., Yasuhara Chemical Co., Ltd., Tosoh Corporation, Rutgers Chemicals, BASF, Arizona Chemical Company, Nitto Chemical Co., Ltd., Nippon Shokubai Co., Ltd., JX Nippon Energy Corporation, Arakawa Chemical Industries Co., Ltd., and Taoka Chemical Co., Ltd.

The resin content is preferably 3 parts by mass or more, more preferably 4 parts by mass or more, and even more preferably 5 parts by mass or more, per 100 parts by mass of the rubber component. On the other hand, it is preferably 60 parts by mass or less, more preferably 50 parts by mass or less, and even more preferably 40 parts by mass or less. It is particularly preferably 30 parts by mass or less.

(6) Other Materials In this embodiment, the tread rubber composition preferably contains an amide compound and/or a nonionic surfactant having an SP value of 9.0 or more.

The amide compound is not particularly limited, but examples thereof include fatty acid amides and fatty acid amide esters. These may be used alone or in combination of two or more. Among these, fatty acid amides are preferred, and a mixture of fatty acid amides and fatty acid amide esters is more preferred.

The amide compound may be a mixture with a fatty acid metal salt, and it is preferable that the amide compound is a mixture with a fatty acid metal salt, because this more effectively achieves the desired effect.

Metals constituting fatty acid metal salts include potassium, sodium, magnesium, calcium, barium, zinc, nickel, molybdenum, etc. These may be used alone or in combination of two or more. Among these, alkaline earth metals such as calcium and zinc are preferred, and calcium is more preferred.

The fatty acid constituting the fatty acid metal salt may be a saturated fatty acid or an unsaturated fatty acid. Examples of saturated fatty acids include decanoic acid, dodecanoic acid, and stearic acid, while examples of unsaturated fatty acids include oleic acid and elaidic acid. These may be used alone or in combination of two or more. Among these, saturated fatty acids are preferred, and stearic acid is more preferred. Furthermore, oleic acid is preferred as an unsaturated fatty acid.

The fatty acid amide may be a saturated fatty acid amide or an unsaturated fatty acid amide. Examples of the saturated fatty acid amide include stearic acid amide and behenic acid amide, and examples of the unsaturated fatty acid amide include oleic acid amide and erucic acid amide. These may be used alone or in combination of two or more. Among these, unsaturated fatty acid amides are preferred, and oleic acid amide is more preferred.

The fatty acid amide ester may be a saturated fatty acid amide ester or an unsaturated fatty acid amide ester. Examples of the saturated fatty acid amide ester include stearic acid amide ester and behenic acid amide ester, and examples of the unsaturated fatty acid amide ester include oleic acid amide ester and erucic acid amide ester. These may be used alone or in combination of two or more. Among them, unsaturated fatty acid amide ester is preferred, and oleic acid amide ester is more preferred.

As amide compounds, for example, products from NOF Corp., Struktol, LANXESS, etc. can be used.

The content of the amide compound relative to 100 parts by mass of the rubber component is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, and even more preferably 0.8 parts by mass or more. On the other hand, it is preferably 5 parts by mass or less, more preferably 4 parts by mass or less, and even more preferably 3 parts by mass or less. It is particularly preferably 2 parts by mass or less. Note that when the amide compound is a mixture with a fatty acid metal salt, the content of the amide compound also includes the amount of fatty acid metal salt contained in the amide compound.

Nonionic surfactants having an SP value of 9.0 or more are not particularly limited, and examples thereof include nonionic surfactants represented by the following formula (1) and/or the following formula (2); Pluronic type nonionic surfactants; sorbitan fatty acid esters such as polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan monooleate, polyoxyethylene sorbitan monopalmitate, polyoxyethylene sorbitan trioleate, polyoxyethylene sorbitan tristearate, and polyoxyethylene sorbitan tripalmitate; polyoxyethylene dodecyl ether, polyoxyethylene lauryl ether, polyoxyethylene 2-ethylhexyl ether, polyoxyethylene oleyl ether, ethylene glycol dibutyl ether, ethylene glycol dilauryl ether, ethylene glycol di-2-ethylhexyl ether, and ethylene glycol dioleyl ether. These nonionic surfactants may be used alone or in combination of two or more. Among them, Pluronic type nonionic surfactants are more preferable.

（式（１）中、Ｒ^ｌは、炭素数６～２６の炭化水素基を表す。ｄは整数を表す。）

(In formula (1), ^R1 represents a hydrocarbon group having 6 to 26 carbon atoms, and d represents an integer.)

（式（２）中、Ｒ^２及びＲ^３は、同一若しくは異なって、炭素数６～２６の炭化水素基を表す。ｅは整数を表す。）

(In formula (2), R ² and R ³ are the same or different and represent a hydrocarbon group having 6 to 26 carbon atoms, and e represents an integer.)

Examples of nonionic surfactants represented by the above formula (1) include ethylene glycol monooleate, ethylene glycol monopalmiate, ethylene glycol monopalmitate, ethylene glycol monopaccenate, ethylene glycol monolinoleate, ethylene glycol monolinolenate, ethylene glycol monoarachidonate, ethylene glycol monostearate, ethylene glycol monocetylate, and ethylene glycol monolaurate.

Examples of nonionic surfactants represented by the above formula (2) include ethylene glycol dioleate, ethylene glycol dipalmiate, ethylene glycol dipalmitate, ethylene glycol dipaccenate, ethylene glycol dilinoleate, ethylene glycol dilinolenate, ethylene glycol diarachidonate, ethylene glycol distearate, ethylene glycol dicetylate, and ethylene glycol dilaurate.

The above Pluronic type nonionic surfactants are also called polyoxyethylene polyoxypropylene glycol, polyoxyethylene polyoxypropylene block polymers, and polypropylene glycol ethylene oxide adducts, and are generally nonionic surfactants represented by the following formula (I). As represented by the following formula (I), Pluronic type nonionic surfactants have hydrophilic groups composed of ethylene oxide structures on both sides, and have hydrophobic groups composed of propylene oxide structures sandwiched between the hydrophilic groups.

（式（Ｉ）中、ａ、ｂ、ｃは整数を表す。）

(In formula (I), a, b, and c represent integers.)

The degree of polymerization of the polypropylene oxide block of the Pluronic nonionic surfactant (b in formula (I) above) and the amount of polyethylene oxide added (a+c in formula (I) above) are not particularly limited and can be appropriately selected according to the conditions of use and purpose. The higher the proportion of the polypropylene oxide block, the higher the affinity with rubber and the slower the rate of migration to the rubber surface. The degree of polymerization of the polypropylene oxide block is preferably 100 or less, more preferably 10 to 70, and even more preferably 10 to 60. It is particularly preferably 20 to 60, and most preferably 20 to 45. Similarly, the amount of polyethylene oxide added is preferably 100 or less, more preferably 3 to 65, and even more preferably 5 to 55. It is particularly preferably 5 to 40, and most preferably 10 to 40.

Examples of Pluronic-type nonionic surfactants include the Pluronic series manufactured by BASF Japan Ltd., the Newpol PE series manufactured by Sanyo Chemical Industries Ltd., the Adeka Pluronic L or F series manufactured by Asahi Denka Kogyo Co., Ltd., the Epan series manufactured by Daiichi Kogyo Seiyaku Co., Ltd., the Pronon series or Unilube manufactured by NOF Corporation, etc. These may be used alone or in combination of two or more types.

It is preferable that the SP value of the nonionic surfactant has a larger difference from the SP value of the rubber component. Specifically, it is preferable that the difference is 9.0 to 12, more preferably 9.1 to 11, and even more preferably 9.2 to 10.5.

The SP value means the solubility parameter calculated by the Hoy method based on the structure of the compound. The Hoy method is a calculation method described, for example, in K. L. Hoy "Table of Solubility Parameters", Solvent and Coatings Materials Research and Development Department, Union Carbites Corp. (1985).

The content of the nonionic surfactant with an SP value of 9.0 or more per 100 parts by mass of the rubber component is preferably 0.1 to 5 parts by mass, more preferably 0.5 to 3 parts by mass, and even more preferably 0.8 to 2 parts by mass.

The rubber composition may contain reinforcing fillers such as silica and carbon black. The inclusion of these fillers imparts an appropriate degree of hardness to the tread rubber composition.

Examples of silica include dry-process silica (anhydrous silicic acid) and wet-process silica (hydrated silicic acid). Wet-process silica is preferred because it has a large number of silanol groups. These may be used alone or in combination of two or more types.

The nitrogen adsorption specific surface area (N ₂ SA) of silica is preferably 80 m ² /g or more, more preferably 110 m ² /g or more, and even more preferably 150 m ² /g or more. It becomes more preferable as it becomes 180 m ² /g or more, 200 m ² /g or more, 220 m ² /g or more, and most preferably 230 m ² /g or more. On the other hand, it is preferably 300 m ² /g or less, more preferably 280 m ² /g or less, and even more preferably 250 m ² /g or less. The N ₂ SA of silica is a value measured by the BET method in accordance with ASTM D3037-81.

As silica, for example, products from Degussa, Rhodia, Tosoh Silica, Solvay Japan, Tokuyama, etc. can be used.

The content of silica having an average surface area of 220 ^m2 /g or more is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 15 parts by mass or more, based on 100 parts by mass of the rubber component, while it is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, and even more preferably 30 parts by mass or less.

The total silica content is preferably 30 parts by mass or more, more preferably 50 parts by mass or more, and even more preferably 60 parts by mass or more, relative to 100 parts by mass of the rubber component. It is particularly preferable that it is 80 parts by mass or more. On the other hand, it is preferably 200 parts by mass or less, more preferably 160 parts by mass or less, and even more preferably 150 parts by mass or less, and it is particularly preferable that it is 130 parts by mass or less.

In the tread rubber composition of this embodiment, the content of silica in 100% by mass of the filler (reinforcing filler) is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more. It is particularly preferably 90% by mass or more. There is no particular upper limit, and it may be 100% by mass.

When the tread rubber composition of this embodiment contains silica, it is preferable that it further contains a silane coupling agent. The silane coupling agent is not particularly limited, and examples thereof include bis(3-triethoxysilylpropyl)tetrasulfide, bis(2-triethoxysilylethyl)tetrasulfide, bis(4-triethoxysilylbutyl)tetrasulfide, bis(3-trimethoxysilylpropyl)tetrasulfide, bis(2-trimethoxysilylethyl)tetrasulfide, bis(2-triethoxysilylethyl)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, bis(3-trimethoxysilylpropyl)disulfide, bis(2-trimethoxysilylethyl)disulfide, bis(4-trimethoxysilylbutyl)disulfide, 3-trimethoxysilylpropyl-N , N-dimethylthiocarbamoyl tetrasulfide, 2-triethoxysilylethyl-N,N-dimethylthiocarbamoyl tetrasulfide, 3-triethoxysilylpropyl methacrylate monosulfide and other sulfide-based compounds; mercapto-based compounds such as 3-mercaptopropyltrimethoxysilane and 2-mercaptoethyltriethoxysilane; vinyl-based compounds such as vinyltriethoxysilane and vinyltrimethoxysilane; amino-based compounds such as 3-aminopropyltriethoxysilane and 3-aminopropyltrimethoxysilane; glycidoxy-based compounds such as γ-glycidoxypropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane; nitro-based compounds such as 3-nitropropyltrimethoxysilane and 3-nitropropyltriethoxysilane; and chloro-based compounds such as 3-chloropropyltrimethoxysilane and 3-chloropropyltriethoxysilane. Commercially available products include those from Degussa, Momentive, Shin-Etsu Silicone, Tokyo Chemical Industry, Azumax, and Dow Corning Toray. These may be used alone or in combination of two or more.

The content of the silane coupling agent is preferably 3 parts by mass or more, and more preferably 5 parts by mass or more, per 100 parts by mass of silica. On the other hand, it is preferably 20 parts by mass or less, and more preferably 15 parts by mass or less.

Carbon black is not particularly limited, and examples thereof include N134, N110, N220, N234, N219, N339, N330, N326, N351, N550, and N762. These may be used alone or in combination of two or more types.

The nitrogen adsorption specific surface area (N ₂ SA) of carbon black is preferably 80 m ² /g or more, and more preferably 100 m ² /g or more. On the other hand, it is preferably 400 m ² /g or less, more preferably 300 m ² /g or less, and even more preferably 150 m ² /g or less. It is particularly preferably 130 ^{m 2} /g or less. The N ₂ SA of carbon black is a value determined according to JIS K6217-2:2001.

Carbon black products that can be used include, for example, those manufactured by Asahi Carbon Co., Ltd., Cabot Japan Co., Ltd., Tokai Carbon Co., Ltd., Mitsubishi Chemical Corporation, Lion Corporation, Shin-Nichika Carbon Co., Ltd., Columbia Carbon Co., Ltd., etc.

The carbon black content is preferably 3 parts by mass or more, and more preferably 5 parts by mass or more, per 100 parts by mass of the rubber component. On the other hand, it is preferably 60 parts by mass or less, more preferably 50 parts by mass or less, and even more preferably 40 parts by mass or less. It is particularly preferably 20 parts by mass or less, and most preferably 10 parts by mass or less.

The tread rubber composition of this embodiment may contain aluminum hydroxide, alumina, magnesium oxide, talc, and diatomaceous earth as reinforcing fillers.

The nitrogen adsorption specific surface area (N ₂ SA) of the aluminum hydroxide is preferably from 3 to 60 m ² /g, more preferably from 6 to 50 m ² /g, and even more preferably from 10 to 40 m ² /g.

Aluminum hydroxide is poorly water-soluble, has little coagulation in a rubber composition, and usually exists as a single particle in the rubber composition. Therefore, it does not impair the reinforcing properties of the rubber. The N ₂ SA of aluminum hydroxide is a value measured by the BET method in accordance with ASTM D3037-81.

The content of aluminum hydroxide is preferably 3 parts by mass or more, and more preferably 4 parts by mass or more, per 100 parts by mass of the rubber component. On the other hand, it is preferably 60 parts by mass or less, more preferably 50 parts by mass or less, and even more preferably 40 parts by mass or less. It is particularly preferably 20 parts by mass or less, and most preferably 15 parts by mass or less.

The tread rubber composition of this embodiment may contain a fatty acid, preferably stearic acid. As the stearic acid, a conventionally known product may be used, for example, a product from NOF Corporation, NOF Corporation, Kao Corporation, Fujifilm Wako Pure Chemical Industries, Ltd., Chiba Fatty Acid Co., Ltd., etc. may be used.

The content of the fatty acid (preferably stearic acid) is preferably 0.5 parts by mass or more, and more preferably 1 part by mass or more, per 100 parts by mass of the rubber component. On the other hand, it is preferably 10 parts by mass or less, and more preferably 5 parts by mass or less.

The tread rubber composition of this embodiment may contain zinc oxide. As the zinc oxide, any conventionally known product may be used, such as products from Mitsui Mining & Smelting Co., Ltd., Toho Zinc Co., Ltd., Hakusui Tech Co., Ltd., Seido Chemical Industry Co., Ltd., Sakai Chemical Industry Co., Ltd., etc.

The amount of zinc oxide is preferably 0.5 parts by mass or more, and more preferably 1 part by mass or more, per 100 parts by mass of the rubber component. On the other hand, it is preferably 10 parts by mass or less, and more preferably 5 parts by mass or less.

The tread rubber composition of the present embodiment may contain sulfur. Examples of sulfur include powdered sulfur, precipitated sulfur, colloidal sulfur, insoluble sulfur, highly dispersible sulfur, and soluble sulfur, which are commonly used in the rubber industry. These may be used alone or in combination of two or more types.

As sulfur, for example, products from Tsurumi Chemical Industry Co., Ltd., Karuizawa Sulfur Co., Ltd., Shikoku Chemical Industry Co., Ltd., Flexis Corporation, Nippon Kanzuri Kogyo Co., Ltd., Hosoi Chemical Industry Co., Ltd., etc. can be used.

The sulfur content is preferably 0.1 parts by mass or more, and more preferably 0.5 parts by mass or more, per 100 parts by mass of the rubber component. On the other hand, it is preferably 5 parts by mass or less, and more preferably 3 parts by mass or less.

The tread rubber composition of the present embodiment may contain a vulcanization accelerator. Examples of the vulcanization accelerator include thiazole-based vulcanization accelerators such as 2-mercaptobenzothiazole, di-2-benzothiazolyl disulfide, and N-cyclohexyl-2-benzothiaryl sulfenamide; thiuram-based vulcanization accelerators such as tetramethylthiuram disulfide (TMTD), tetrabenzylthiuram disulfide (TBzTD), and tetrakis(2-ethylhexyl)thiuram disulfide (TOT-N); sulfenamide-based vulcanization accelerators such as N-cyclohexyl-2-benzothiazole sulfenamide, N-t-butyl-2-benzothiazole sulfenamide, N-oxyethylene-2-benzothiazole sulfenamide, and N-oxyethylene-2-benzothiazole sulfenamide; and guanidine-based vulcanization accelerators such as diphenyl guanidine, di-orthotolyl guanidine, and orthotolyl biguanidine. These may be used alone or in combination of two or more. Among them, sulfenamide-based vulcanization accelerators and guanidine-based vulcanization accelerators are preferred, and it is preferable to use a sulfenamide-based vulcanization accelerator and a guanidine-based vulcanization accelerator in combination.

As vulcanization accelerators, for example, products manufactured by Kawaguchi Chemical Co., Ltd. and Ouchi Shinko Chemical Co., Ltd. can be used.

The content of the vulcanization accelerator is preferably 1 part by mass or more, and more preferably 2 parts by mass or more, per 100 parts by mass of the rubber component. On the other hand, it is preferably 10 parts by mass or less, and more preferably 7 parts by mass or less.

The tread rubber composition of this embodiment may further contain additives commonly used in the tire industry, such as organic peroxides, fillers such as calcium carbonate, clay, and mica, in addition to the above-mentioned materials. The content of these additives is preferably 0.1 to 200 parts by mass or more per 100 parts by mass of the rubber component.

In this embodiment, the tread rubber composition can be produced by a known method, for example, by kneading the above-mentioned materials using a rubber kneading device such as an open roll or a Banbury mixer, and then extrusion processing is performed according to the shape of the tread to form the tread.

As for the kneading conditions, in the base kneading process where additives other than the vulcanizing agent and vulcanization accelerator are kneaded, the kneading temperature is usually 100 to 180°C, preferably 120 to 170°C, and the kneading time is usually 30 seconds to 30 minutes, preferably 1 minute to 30 minutes. In the finish kneading process where the vulcanizing agent and vulcanization accelerator are kneaded, the kneading temperature is usually 120°C or lower, preferably room temperature 80 to 110°C. Furthermore, the composition kneaded with the vulcanizing agent and vulcanization accelerator is usually subjected to a vulcanization process such as press vulcanization. In the case of passenger car tires (PC tires), the vulcanization temperature is usually 140 to 190°C, and the vulcanization time is usually 5 to 15 minutes.

4. Manufacturing of Tire According to the Present Invention Next, manufacturing of the tire according to the present invention will be described.

(1) Green Tire Molding Process First, a green tire is molded by a normal method on a tire building machine using a tread molded from the above tread rubber composition and other tire components prepared separately.

(2) Water-soluble filler spraying step Next, a prepared aqueous solution or organic solvent dispersion of the water-soluble filler (hereinafter also collectively referred to as "water-soluble filler liquid") is applied to the tread surface of the molded green tire. At this time, the amount of application is adjusted so that the amount of water-soluble filler sprayed on the tread surface after drying is 0.005 to 0.04 g/ ^cm2 . When the applied water-soluble filler liquid dries, the water-soluble filler particles are precipitated on the tread surface, and a green tire can be obtained in a state in which the above-mentioned amount of water-soluble filler is sprayed on the tread surface.

At this time, the concentration of the water-soluble filler in the water-soluble filler liquid may be set taking into consideration the ease of application to the tread surface, but it is usually preferable to adjust the concentration to 10 to 30 mass %.

When preparing the water-soluble filler liquid as an organic solvent dispersion, it is preferable to use a hydrocarbon organic solvent, anhydrous ethanol, hexane, etc. as the organic solvent, and these may also be used as a mixed solvent.

As mentioned above, a water-insoluble filler may be added in an amount of 5 to 15% by mass relative to the water-soluble filler.

Specific methods for applying the agent to the tread surface can be known methods such as brush application or spray application.

(3) Vulcanization Step Next, the raw tire with the water-soluble filler scattered on the tread surface is set in a predetermined vulcanization mold and heated and pressurized using a vulcanization device.

At this time, as described above, a skin layer is formed on the surface of the tread, but in this embodiment, the water-soluble filler is dispersed on the tread surface, so the water-soluble filler is mixed into the skin layer, resulting in a tire with some of the water-soluble filler exposed on the tread surface.

When the tread of such a tire comes into contact with water, the exposed water-soluble filler dissolves from the skin layer, forming voids with an average diameter of 0.1 to 100 μm on the tread surface, forming fine irregularities, allowing the tire to immediately exhibit sufficient grip performance on ice even immediately after mounting. Note that while it is preferable for the voids to have an average diameter of 0.1 to 100 μm, from the standpoint of steering stability and grip performance on ice, it is more preferable for the voids to have an average diameter of 1 to 80 μm, and even more preferable for them to be 10 to 70 μm.

The average diameter of the voids can be measured by observation with a scanning electron microscope (SEM). Specifically, a photograph is taken with the scanning electron microscope, and the diameter is determined as the diameter of the sphere if the voids are spherical, the minor axis if they are needle-like or rod-like, and the average diameter from the center if they are irregular. The average diameter can be calculated by averaging the diameters of 100 voids.

By manufacturing the tire according to the present embodiment based on each of the above steps, it is possible to provide a tire that exhibits sufficient grip performance (especially grip performance on ice) even in the initial stage of installation, and it is possible to provide a tire that is particularly suitable as a passenger car tire (PC tire) or a studless tire (winter tire).

The present invention will be explained in more detail below based on examples. In these examples, raw tires were made using the same tread rubber composition, and the difference in grip performance on ice depending on whether or not water-soluble filler was sprayed on the tread surface was measured.

1. Preparation of tires for evaluation (Examples and Comparative Examples) First, tires for evaluation (Examples and Comparative Examples) were prepared.

The various chemicals used in the examples and comparative examples will be described below.
<SBR1>: N9548 manufactured by Zeon Co., Ltd. (styrene content: 35% by mass, vinyl content: 18% by mass, contains 37.5 parts by mass of oil per 100 parts by mass of rubber solids)
<SBR2>: NS612 manufactured by Zeon Co., Ltd. (styrene content: 15% by mass, vinyl content: 30% by mass)
<BR>: BUNA-CB25 manufactured by LANXESS AG (rare earth-based BR synthesized using a Nd-based catalyst, vinyl content: 0.7% by mass, cis content: 97% by mass, SP value: 8.2)
<Carbon black>: Show Black N220 (N ₂ SA: 114 m ² /g) manufactured by Capot Japan Co., Ltd.
<Silica 1>: Ultrasil VN3 (N ₂ SA: 175 m ² /g) manufactured by Evonik Degussa
<Silica 2>: Ultrasil U9000Gr ( _N2SA : ^235m2 /g) manufactured by Evonik Degussa
<Magnesium sulfate>: prototype manufactured by Kyowa Chemical Industry Co., Ltd. (anhydrous magnesium sulfate, median particle size (median diameter): 7.3 μm, apparent specific gravity: 0.58 g/ml, solubility in water at room temperature (20° C.): 255 g/100 g water, moisture content: 0.1 mass %, N ₂ SA: 9.0 m ² /g, equivalent to a water-soluble filler with a median particle size of 100 μm or less and an apparent specific gravity of 0.70 g/ml or less)
<Silane coupling agent>: Si75 (bis(3-triethoxysilylpropyl disulfide) manufactured by Evonik Degussa
<α-Methylstyrene resin>: SYLVARESSA 85 manufactured by Arizona Chemical Co., Ltd. (copolymer of α-methylstyrene and styrene, softening point: 85° C.)
<Terpene-based resin>: YS Resin TO125 (aromatic modified terpene resin, softening point: 125°C) manufactured by Yasuhara Chemical Co., Ltd.
<Aroma oil>: AH-24 (aromatic process oil) manufactured by Idemitsu Kosan Co., Ltd.
<WB16>: WB16 manufactured by Struktol (mixture of fatty acid calcium, fatty acid amide and fatty acid amide ester, ash content 4.5%)
<Stearic acid>: Beaded stearic acid "Tsubaki" manufactured by NOF Corporation
<Anti-aging agent 1>: Antigen 6C (6PPD, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine) manufactured by Sumitomo Chemical Co., Ltd. <Anti-aging agent 2>: Nocrac 224 (TMQ, 2,2,4-trimethyl-1,2-dihydroquinoline polymer) manufactured by Ouchi Shinko Chemical Industry Co., Ltd.
<Paraffin wax>: Ozoace 0355 manufactured by Nippon Seiro Co., Ltd.
<Zinc oxide>: Zinc oxide type 2 manufactured by Hakusui Tech Co., Ltd. <Sulfur>: HK200-5 manufactured by Hosoi Chemical Industries Co., Ltd. (powdered sulfur containing 5% oil)
<Vulcanization accelerator 1>: Noccela-NS (N-tert-butyl-2-benzothiazolyl sulfenamide) manufactured by Ouchi Shinko Chemical Industry Co., Ltd.
<Vulcanization accelerator 2>: Noccelaer D (diphenyl guanidine) manufactured by Ouchi Shinko Chemical Industry Co., Ltd.

According to the compounding recipe shown in Table 1, chemicals other than sulfur and vulcanization accelerator were mixed for 5 minutes at 150°C using a 1.7L Banbury mixer manufactured by Kobe Steel, Ltd. to obtain a kneaded product. Next, sulfur and vulcanization accelerator were added to the resulting kneaded product, and the mixture was kneaded for 5 minutes at 80°C using an open roll to obtain an unvulcanized rubber composition.

The resulting unvulcanized rubber composition was molded into a cap tread shape and bonded together with other tire components to produce 16 raw tires.

Half of the raw tire was press-vulcanized at 170°C for 10 minutes to obtain a comparative tire (size: 225/50R17).

For the remaining raw tires, a 20% aqueous solution of magnesium sulfate (the same magnesium sulfate as above) was applied to the tread surface so that the amount of magnesium sulfate dispersed after drying was 0.01 g/cm ^2. After drying, the tires were press-vulcanized at 170° C. for 10 minutes, as in the comparative example, to obtain tires of the example (size: 225/50R17).

2. Evaluation The following items were evaluated using each of the obtained tires. The evaluation was performed twice: immediately after mounting the tires and starting to drive, and after driving 100 km.

(1) Grip performance on ice at low temperature Using each tire, actual vehicle performance on ice was evaluated under the following conditions. The test was conducted at Sumitomo Rubber Industries, Ltd.'s Nayoro test course in Hokkaido, and the temperature was -10 to -6°C. Each tire was mounted on a domestically produced 2000cc front-engine, front-wheel drive vehicle, and the stopping distance on ice required to apply the lock brakes and stop the vehicle at 30 km/h was measured. Using the measured value of the comparative tire after driving 100 km as a reference, the results were expressed as an index according to the following formula. The higher the index, the better the grip performance on ice at low temperatures.
(Grip performance on ice at low temperature)=(braking stopping distance of comparative tire after 100 km travel)/(stopping distance of each tire)×100

(2) Grip performance on ice at high temperatures Using each tire, actual vehicle performance on ice was evaluated under the following conditions. The test was conducted at Sumitomo Rubber Industries, Ltd.'s Nayoro test course in Hokkaido, and the temperature was 0 to -5°C. The test tires were mounted on a domestically produced 2000cc front-engine, front-wheel drive vehicle, and the stopping distance on ice required to apply the lock brakes and stop the vehicle at 30 km/h was measured. Using the measured value of the comparative tire after driving 100 km as a reference, the value was expressed as an index according to the following formula. The higher the index, the better the grip performance on ice at high temperatures.
(Grip performance on ice at high temperatures)=(braking stopping distance of comparative tire after 100 km travel)/(stopping distance of each tire)×100

The evaluation results are shown in Table 2. Figure 1 shows an optical microscope photograph of the outermost surface of the tread immediately after driving, observed with a digital microscope (Keyence Corporation: VHX-6000) (observation conditions: ring illumination, magnification 500x).

As shown in Table 2, the comparative tire does not demonstrate sufficient performance immediately after mounting and running on ice at low temperatures and high temperatures.

The reason for this result is that immediately after installation, a skin layer containing almost no water-soluble filler was formed on the tread surface of the tire, which prevented the tire from providing sufficient grip on ice.

In contrast, it can be seen that the tires of the examples demonstrate sufficient grip performance on ice at low temperatures and on ice at high temperatures immediately after installation.

The reason for this difference is that, as shown in Figure 1, in the comparative tire immediately after installation, a skin layer containing almost no water-soluble filler was formed on the tread surface, and there were almost no irregularities, so sufficient grip performance on ice was not demonstrated, whereas in the example tire immediately after installation, voids caused by the dissolved water-soluble filler formed irregularities on the tread surface, so sufficient grip performance on ice was demonstrated.

The present invention has been described above based on an embodiment, but the present invention is not limited to the above embodiment. Various modifications can be made to the above embodiment within the same or equivalent scope as the present invention.

Claims (16)

The tread portion is formed by a tread main body portion made of a tread rubber composition and a surface layer portion made of a rubber composition having a higher polymer concentration than the tread rubber composition of the tread main body portion,
The thickness of the surface layer is several tens to 100 μm,
The pneumatic tire is characterized in that the surface layer portion contains 10 to 80 mass % of a water-soluble inorganic filler. The pneumatic tire according to claim 1, characterized in that the water-soluble inorganic filler in the surface layer of the tread portion is one of metal sulfates, metal chlorides, metal hydroxides, carbonates, and phosphates, or a mixture of two or more of them. The pneumatic tire according to claim 2, characterized in that the water-soluble inorganic filler in the surface layer of the tread portion is a metal sulfate. The pneumatic tire according to claim 3, characterized in that the metal sulfate in the surface layer of the tread portion is magnesium sulfate. The pneumatic tire according to claim 4, characterized in that the magnesium sulfate is one of anhydrous magnesium sulfate, magnesium sulfate dihydrate, and magnesium sulfate trihydrate, or a mixture of two or more of them. The pneumatic tire according to any one of claims 1 to 5, characterized in that in addition to the water-soluble inorganic filler, a water-insoluble inorganic filler is further added to the surface layer of the tread portion. The pneumatic tire according to claim 6, characterized in that the content of the water-insoluble inorganic filler relative to the water-soluble inorganic filler is 3 to 30 mass %. The pneumatic tire according to claim 6 or 7, characterized in that the water-insoluble inorganic filler is one of egg shells, zinc oxide needle crystals, rice husks, talc, magnesium oxide, and iron powder, or a mixture of two or more of them. The pneumatic tire according to any one of claims 1 to 8, characterized in that the tread portion is a studless tire formed from a rubber composition containing a water-soluble inorganic filler. The pneumatic tire according to claim 9, characterized in that the studless tire is a studless tire for passenger cars. A method for manufacturing pneumatic tires, characterized in that an inorganic filler spraying process is provided between a molding process for molding a raw tire and a vulcanization process for vulcanizing the molded raw tire, in which a water-soluble inorganic filler is sprayed onto the surface of the tread portion of the molded raw tire. The method for manufacturing a pneumatic tire according to claim 11, characterized in that the inorganic filler spraying process is a process for spraying the water-soluble inorganic filler by applying an aqueous solution of the water-soluble inorganic filler to the surface of the tread portion. The method for manufacturing a pneumatic tire according to claim 11, characterized in that the inorganic filler spraying step is a step of spraying the water-soluble inorganic filler by applying an organic solvent dispersion of the water-soluble inorganic filler to the surface of the tread portion. The method for producing a pneumatic tire according to claim 13, characterized in that the organic solvent in the organic solvent dispersion is one of a hydrocarbon organic solvent, anhydrous ethanol, and hexane, or a mixture of two or more of them. 15. The method for manufacturing a pneumatic tire according to claim ¹² , wherein the inorganic filler spraying step is a step of applying and spraying an aqueous solution of the water-soluble inorganic filler or an organic solvent dispersion of the water-soluble inorganic filler while adjusting the amount of the water-soluble filler sprayed on the surface of the tread portion after drying to be 0.005 to 0.04 g/cm2. The method for manufacturing a pneumatic tire according to any one of claims 12 to 15, characterized in that a water-insoluble inorganic filler is added to the aqueous solution of the water-soluble inorganic filler or the organic solvent dispersion of the water-soluble inorganic filler.