This application claims benefit of the filing date of PCT/JP2011/065118, filed Jun. 30, 2011, the entire contents of which is incorporated herein by reference for all purposes.

BACKGROUND

1. Field

The present invention relates to a pneumatic tire tread, and in further detail, to a pneumatic tire tread with which flexible fences are formed in the primary groove of the tread and it is thereby possible to reduce air column resonance and maintain water drainage performance while ensuring that the flexible fences and tread portion will be abraded to a similar extent.

2. Description of Related Art

The air column resonance of a primary groove formed in the tread of a tire is generated by resonance inside a tube (air column) formed between the primary groove and the road surface, and this resonance frequency depends on the length of the air column of the primary groove formed with the road surface.

This air column resonance is manifested in the form of noise inside and outside the vehicle and often peaks at approximately 1 kHz, which is audible to the human ear. Technology for reducing the air column resonance of the primary groove includes disposing so-called flexible fences, that is, projections, which extend from the groove walls or groove base of the primary groove and block all or the majority of the primary groove such that the air column resonance is reduced by blocking the flow of air in the direction in which the primary groove is formed. Nevertheless, when the tire runs on a wet road surface, the water that has entered the primary groove is prevented from flowing through the primary groove because air is prevented from flowing in the direction in which the primary groove is formed. There is therefore a reduction in drainage of water between the pneumatic tire and the road surface and as a result, there is a reduction in operating safety on wet road surfaces.

FIG. 4 of patent document 1 discloses technology that serves to reduce air column resonance and impart water drainage wherein three group fences 3 (flexible fences) that extend from opposing walls on the inside of the groove and the groove base, respectively, are disposed such that they are each separated by a space.

Moreover, FIG. 3 of patent document 2 discloses technology that serves to reduce air column resonance and improve water drainage by disposing a diaphragm 30 (flexible fence) that extends from the groove base inside the primary groove such that the majority area from the opening in the primary groove does not contact the groove walls.

Patent document 1 JP-A H11-105511

Patent document 2 JP-A 2006-341655

SUMMARY

Problems to be Solved

Nevertheless, according to the technology disclosed in patent documents 1 and 2, in order to function as a flexible fence, the flexible fence is thin in the direction of tire circumference and therefore can easily change shape. As a result, depending on the material that is used, the flexible fence that easily changes shape will be abraded less than the tread portion with repeated running and there is therefore a difference in the amount of abrasion between the flexible fence and the tread portion. This difference in the amount of abrasion is a problem in that the flexible fence projects from the surface of the tread portion (the surface of the tread that forms the tread contact patch) and the flexible fence that projects from the tread surface hits the road surface when running and generates noise. This generation of noise is contrary to the original role of the flexible fence, which is reduction of air column resonance and noise mitigation. Moreover, there is a problem in that projection of the flexible fence from the tread portion causes unintentional grounding with the road surface and leads to early abrasion and loss of the flexible fence.

Therefore, the present disclosure is intended to solve the problems of the prior art, and the purpose thereof is to provide a pneumatic tire tread with which it is possible to maintain water drainage performance while preventing projection of the flexible fence from the tread portion and preventing a reduction in the air column resonance-reducing effect that is attributed to early abrasion of the flexible fence.

Means for Solving the Problems

In order to accomplish the above-mentioned purpose, the present disclosure relates to a pneumatic tire tread that is formed from a rubber composition and that contacts the road surface when the tire rolls, the pneumatic tire tread being characterized by having a tread portion, in which is formed at least one primary groove of depth D and width W having a base surface and two opposing walls, and multiple flexible fences of thickness E, which extend inside the primary groove such that at least 70% of the cross-sectional area of the primary groove is blocked and which are disposed at intervals such that at least one of the multiple flexible fences is in the primary groove in the tread contact patch when the tire is rolling, wherein the multiple flexible fences are formed from at least a rubber composition for fences that is different from the rubber composition for the tread portion; the rubber composition for fences of the flexible fences has an abrasion index that is more than or equal to twice the abrasion index of the rubber composition of the tread portion; and the rubber composition for fences comprises a filler composition that is based on (a) a diene elastomer, (b) a reinforcing filler selected from the group consisting of carbon black, inorganic fillers, organic fillers, and mixtures of said fillers, which has a BET specific surface area of less than 100 m²/g and is in an amount of between 20 phr and 120 phr, preferably between 30 phr and 100 phr; (c) a plasticizing system selected from the group consisting of liquid plasticizer, hydrocarbon resins and mixtures thereof; and (d) a vulcanizing agent.

According to embodiments of the present invention configured in this way, flexible fences are formed inside the primary groove such that they block at least 70% of the cross-sectional area of the primary groove, and are disposed at intervals such that at least one is present inside the primary groove in the tread contact patch when the tire is rolling. As a result, the length of the air column of the primary groove formed with the road surface is different from when there is no flexible fence and the peak air column resonance is easily outside the frequency band audible to the human ear. As a result, the noise due to air column resonance is mitigated.

Furthermore, according to embodiments of the present invention, the rubber composition for the fences has a higher abrasion index than the rubber composition that forms the tread portion and includes a reinforcing filler having a BET specific surface area of less than 100 m²/g in an amount of 20 phr to 120 phr, and therefore ensures that the flexible fences will be abraded to a similar extent to the tread portion. As this type of reinforcing filler, a filler having reinforcing properties for a rubber composition used for manufacturing a general tire tread, for example, an organic filler such as carbon black, an inorganic reinforcing filler such as silica, or a mixture of these fillers, particularly a filler mixture of carbon black and silica may be used. According to embodiments of the present invention, the amount of the reinforcing filler of the rubber composition is preferably between 30 phr and 100 phr, more preferably between 30 phr and 80 phr, and even more preferably between 30 phr and 60 phr.

Herein, the reinforcing agent used in the rubber composition forming the tread portion preferably has a greater BET specific surface area than the reinforcing agent used in the fence rubber composition, specifically, the BET specific surface area is more than or equal to 120 m²/g, more preferably between 140 m²/g and 220 m²/g.

Examples of carbon black press are any type of carbon black, but HAF, ISAF, or SAF blacks normally used in tires (tire-grade blacks) are particularly ideal. Of such blacks, series 100, 200, or 300 reinforcing carbon blacks (ASTM grades) are examples, including blacks N115, N134, N234, N326, N330, N339, N347, and N375. Also, carbon black may be preliminarily incorporated in an isoprene elastomer, for example, in the form of a masterbatch (for example, see International Publication Nos. WO 97/36724 and WO 99/16600). Examples of an organic filler other than carbon black may include functional polyvinyl aromatic organic fillers as mentioned in International Publication Nos. WO 2006/069792 and WO 2006/069793. The term “reinforcing inorganic filler” in the present application means an arbitrary inorganic or mineral filler regardless of colour or source (natural or synthetic), and such fillers are, in contrast to carbon black, known as “white fillers,” “coloured fillers,” or “non-black fillers”. Moreover, these fillers themselves can be used to reinforce the rubber composition used to produce the tire without using intermediate coupling agents and other means. That is, these fillers can be substituted for tire-grade carbon black and used for their reinforcing performance. As is well known, such fillers are characterized by having hydroxyl (—OH) groups at their surface. As the inorganic reinforcing filler, its physical status may be optional, that is, powder, microsphere, granules, beads or other appropriate dense forms may be used. The inorganic reinforcing filler, of course, includes various inorganic reinforcing fillers, particularly, mixtures of high dispersible silica and/or alumina fillers described below. In particular, silica type mineral filler, particularly silica (SiO2) and alumina type mineral fillers, particularly alumina (Al2O3) are useful as reinforcing inorganic fillers. Any reinforcing silica known to a person skilled in the art can be used as the silica, and in particular, any precipitated or calcined silica having a BET surface area and CTAB specific surface area of less than 450 m²/g, particularly within a range of 30 to 400 m²/g can be used. High dispersing precipitated silica (“HDS”) can be used. Examples are:

Ultrasil 7000 and Ultrasil 7005 produced by Degussa, Zeosil 1165MP, 1135MP, and 1115MP produced by Rhodia, Hi-Sil EZ150G produced by PPG, Zeopol 8715, 8745, and 8755 produced by Huber, and silica having high specific surface area as described in International Patent WO 03/16837. Examples of high dispersible precipitated alumina include the following:

• • alumina Baikalox CR 125 manufactured by Baikowski; and AKP-G015 manufactured by Sumitomo Chemicals.

In the present specification, the BET specific surface area is measured according to a known method, that is, by gas adsorption using the Brunauer-Emmett-Teller method described in “The Journal of the American Chemical Society”, Vol. 60, page 309, February 1938, and more specifically, according to French standard NF ISO 9277 of December 1996 (multipoint volumetric method (5 points); where gas: nitrogen, degassing: 1 hour at 160° C., relative pressure range p/po: 0.05 to 0.17).

According to embodiments of the present invention, a reinforcing filler preferably includes nanoparticles having the average grain size (by weight average) of 500 nm, especially in the case of silica or carbon black. The average grain size (by weight average) of the nanoparticles is preferably between 20 nm and 200 nm, more preferably between 20 nm and 150 nm.

The average grain size (by weight average) of the nanoparticles as represented by d_w is determined as usual after dispersing the filler to be analyzed in water or an aqueous solution containing a surfactant and exposing the product to ultrasound disaggregation.

Determination of an inorganic filler such as silica is by the following procedural method using an XDC (X-ray disc centrifuge) marketed by Brookhaven Instruments. That is, a suspension formed from 3.2 g of the inorganic filler specimen to be analyzed in 40 ml of water is prepared by performing the procedure for 8 minutes with the output of the 1500 W ultrasound probe (1.91 cm (¾ inch) Vibracell Sonicator marketed by Bioblock) set at 60% (that is, 60% from the maximum “output control” position). After ultrasound treatment, 15 ml of the suspension is introduced to a rotating disc. After precipitation for 120 minutes, the grain size mass distribution and particle mass-average grain size d_w are calculated by XDC precipitation speed gauge software (d_w=Σ(n_id_i⁵)/Σ(n_id_i⁴); in the formula n_i is the target number of the grain size or diameter group dⁱ).

This procedure is performed for carbon black using an aqueous solution containing 15 vol % of ethanol and 0.05 vol % of a nonionic surfactant. Determination is performed by a DCP centrifugal photo-precipitation speed gauge (disc centrifugal photo-precipitation speed gauge marketed by Brookhaven Instruments). A suspension containing 10 mg of carbon black is pre-prepared by performing the procedure for 10 minutes in 40 ml of an aqueous solution containing 15 vol % of ethanol and 0.05 vol % of nonionic surfactant using a 600 W ultrasound probe (1.27 cm (½ inch) Vibracell Sonicator marketed by Bioblock) set at an output of 60% (that is, at 60% of the maximum position of the “tip amplitude”). During ultrasound treatment, 15 ml of water (containing 0.05% of nonionic surfactant) and a concentration gradient formed from 1 ml of ethanol are injected into the precipitation speed gauge disc turning at 8,000 rpm to form a “stepped gradient.” Next, 0.3 ml of carbon black suspension is injected onto the surface of the gradient. 120 minutes after continuous precipitation, the grain size mass distribution and mass-average grain size d_w are calculated by the precipitation speed gauge software as described above.

Here, when the flexible fences contact the road surface directly as they pass through the tread contact patch while the tire is rolling, grounding pressure and slipping are produced between the road surface and the flexible fences due to the counterforce that is received when the tip of the flexible fences contacts the road surface. According to embodiments of the present invention, the abrasion index of the rubber composition for the fences is greater than the abrasion index of the rubber composition of the tread portion and therefore, even if there is little grounding pressure and slipping due to deformation of the flexible fences, abrasion occurs and as a result, the flexible fences are abraded to a similar extent to the tread portion. In other words, when the abrasion index of the rubber composition for the fences is less than twice the abrasion index of the rubber composition of the tread portion, it is impossible to ensure that the flexible fences will be abraded to a similar extent to the tread portion because the flexible fences will not be abraded under low grounding pressure and slipping that are generated when the flexible fences contact the road. An abrasion index that is more than or equal to twice is appropriate according to the inventors' evaluation (particularly in terms of water drainage and flexible fence abrasion performance) of a pneumatic tire tread with which air column resonance is reduced and water drainage is maintained while ensuring that the flexible fences will be abraded to a similar extent to the tread portion. The inventors analytically and experimentally identified that the above-mentioned water drainage and abrasion performance are effectively realized when this index is within the above-mentioned range (more than or equal to twice).

An example of a method for producing a tire tread having flexible fences is described in a 2009 patent application filed by the inventors (WO2010/146180 published on Oct. 23, 2010).

The term “groove” here refers to a space having width and depth that is formed by using another surface (base surface) to join two opposing surfaces (walls) that do not normally contact with one another.

Moreover, the term “primary groove” refers to a groove that is primarily responsible for drainage of fluid and is relatively wide among the various grooves formed in the tread. The primary groove is often the groove that extends in the direction of tire circumference linearly, in zigzag fashion, or in undulating form, but can also be a relatively wide groove that extends at an angle in the direction of tire rotation and is primarily responsible for drainage of fluid.

Moreover, the “tread contact patch” is the surface region of the tread that contacts the road surface when the tire is mounted on an applicable rim as specified by the following industrial standards, the tire is filled with air to the rated internal pressure, and the tire carries the rated load.

The term “standards” here refers to industrial standards that are in effect where the tire is produced or used. For instance, in Europe the industrial standards are the “Standards Manual” of ETRTO (The European Tire and Rim Technical Organization); in the US they are the “Yearbook” of the TRA (The Tire and Rim Association, Inc.); and in Japan they are the “JATMA Yearbook” of JATMA (the Japan Automobile Tire Manufacturers Association). Moreover, the “applicable rim” is a rim that is specified in these standards in accordance with tire size; the “rated internal pressure” is the air pressure specified in accordance with the load capacity in these standards; and the “rated load” is the maximum allowable weight that the tire can carry.

Moreover, “abrasion of the rubber composition” is defined by JIS K6264-1 and JIS K6264-2. It means the volume of abrasion of the rubber composition as obtained by the Acron abrasion test. The volume of abrasion of a rubber composition is determined by finding the difference in mass before and after subjecting a vulcanized rubber test piece to testing using an Acron abrasion tester on the basis of the above-mentioned regulations. The number of rotations of the grinding wheel is set at 500 rotations for pre-operation and 3,000 rotations for the test itself and there is a 15 degree angle of inclination between the test piece and the grinding wheel. The test piece is pressed to the grinding wheel under added force of 27.0 N and the test piece rotates at a speed of 100 rotations/minute.

Moreover, the “abrasion index” is the index of the above-mentioned “abrasion of the rubber composition”, and a larger index indicates that the abrasion volume is high, that is, the abrasion speed is fast (tendency towards abrasion). This “abrasion index” is used in order to compare the abrasion of multiple rubber compositions and according to the results of the above-mentioned abrasion index, the numerical abrasion of a specific rubber composition is converted to 1.00, for instance, as a reference point and the numerical abrasion of a rubber composition that is the subject of comparison is obtained by calculation using the same conversion rate. A numerical abrasion index that is twice indicates that the rubber composition that is the subject of comparison will be abraded twice as fast as the rubber composition that serves as the reference point.

The amount of each structural component contained in the rubber composition is represented by phr (parts by mass per 100 parts by mass of elastomer or rubber). Furthermore, unless otherwise specified, the percent (%) is wt %, a range represented as “between a and b” means greater than a and less than b (does not include the upper and lower limiting numbers), and a range represented as “from a to b” represents more than or equal to a and less than or equal to b (includes the upper and lower limiting numbers).

Moreover, as is well known, the “‘diene’ elastomer or rubber” is an elastomer (or several types of elastomers), that is a homopolymer or copolymer, derived at least in part from a diene monomer (conjugate monomer or monomer that is not conjugate and has two carbon-carbon double bonds).

The diene elastomer in the present invention is preferably selected from the group consisting of polybutadienes (BR), polyisoprenes (IR), natural rubbers (NR), butadiene copolymers, isoprene copolymers, and mixtures of these elastomers. The diene elastomer is further preferably selected from a group consisting of natural rubber, polyisoprenes, polybutadienes wherein the cis-1,4 unit content exceeds 90%, butadiene/styrene copolymers and mixtures of these elastomers.

According to embodiments of the present invention, the BET specific surface area of the reinforcing filler is preferably between 5 m²/g and 80 m²/g, more preferably from 10 m²/g to 70 m²/g.

According to embodiments of the present invention, preferably, the primary filler in the reinforcing filler is carbon black, and the amount thereof is between 40 phr and 140 phr.

According to embodiments of the present invention, preferably, the primary filler in the reinforcing filler is an inorganic filler, and the amount thereof is between 50 phr and 120 phr.

According to embodiments of the present invention, preferably, the primary filler in the reinforcing filler is an organic filler, and the amount thereof is between 50 phr and 120 phr.

According to embodiments of the present invention, preferably, the liquid plasticizer is selected from the group consisting of naphthene oils, paraffin-based oils, MES oils, TDAE oils, mineral oils, vegetable oils, ether plasticizers, ester plasticizers, phosphate plasticizers, sulphonic acid ester plasticizers and mixtures thereof. According to the present invention, the amount of liquid plasticizer is preferably between 2 phr and 60 phr, preferably 3 phr and 40 phr, more preferably 5 phr and 20 phr.

Moreover, according to a preferred embodiment of the present invention, the composition of the present invention can contain as a solid (at 23° C.) plasticizer a hydrocarbon-based resin having a Tg that is higher than +20° C., preferably higher than +30° C., such as described in International Patent WO 2005/087859, International Patent WO 2006/061064, or International Patent WO 2007/017060. The hydrocarbon-based resin is a polymer known to a person skilled in the art, and when additionally described as being “plasticizing,” is miscible in the diene elastomer composition. The hydrocarbon-based resin can be an aliphatic, aromatic, or aliphatic/aromatic resin. That is, an aliphatic and/or aromatic monomer can serve as the base. The hydrocarbon-based resin can be natural or synthetic. According to embodiments of the present invention, preferably, the plasticizing hydrocarbon-based resin is selected from the group consisting of cyclopentadiene (CPD) homopolymer or copolymer resins, dicyclopentadiene (DCPD) homopolymer or copolymer resins, terpene-based homopolymer or copolymer resins, C₅ fraction homopolymer or copolymer resins, C₉ fraction homopolymer or copolymer resins, and mixtures of these resins.

According to embodiments of the present invention, the plasticizing system is preferably a hydrocarbon-based resin, and the amount thereof is between 2 phr and 60 phr, more preferably between 3 phr and 40 phr, and even more preferably between 5 phr and 20 phr.

According to embodiments of the present invention, the total amount of the plasticizing system, for example, the total amount of a mixture including a hydrocarbon-based resin and liquid plasticizer is between 40 phr and 100 phr, more preferably 50 phr to 80 phr.

According to embodiments of the present invention, the thickness E of the flexible fences is preferably more than or equal to 0.3 mm and less than or equal to 1.0 mm.

According to embodiments of the present invention configured as such, it is possible to guarantee water drainage performance and to ensure that the flexible fences will be abraded to a similar extent to the tread portion while mitigating air column resonance noise. That is, when the flexible fences are thinner than 0.3 mm, there is a chance that the flexible fences will be pushed down by air pressure and there will be a reduction in the air column resonance noise-mitigating effect. Moreover, when the fences are knocked down, they will become difficult to contact with the ground surface at the tread contact patch and therefore, there is a chance that there will be a marked reduction in the rate of abrasion of the flexible fences when compared to the rate of abrasion of the tread portion and the flexible fences will project from the tread portion. On the other hand, when the flexible fences are thicker than 1.0 mm, the percentage of open primary groove cross section will be smaller when the flexible fences fall down into the primary groove and there will be a reduction in drainage performance.

By means of the pneumatic tire tread of the present invention, it is possible to reduce air column resonance and maintain water drainage performance while ensuring that the flexible fences are abraded to a similar extent to the tread portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of the pneumatic tire tread that is an embodiment of the present invention.

FIG. 2 is an enlarged cross section of the pneumatic tire tread as seen along II-II of FIG. 1.

FIG. 3 is an enlarged cross section of the pneumatic tire tread as seen along III-III of FIG. 1.

FIG. 4 is an enlarged cross section of the pneumatic tire tread as seen along II-II in FIG. 1 that is also shown in FIG. 2, and shows the state where the tire is running on a wet road surface.

FIG. 5 is an enlarged cross section of the pneumatic tire tread as seen along III-III in FIG. 1 that is also shown in FIG. 3, and shows the state where the tire is running on a wet road surface.

FIG. 6 is an enlarged cross section of the pneumatic tire tread as seen along III-III in FIG. 1 that is also shown in FIG. 3, and is a schematic representation of the state where the groove depth of the tread portion has been abraded by approximately 30%.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Preferred embodiments of the present invention will now be described while referring to the drawings.

First, the pneumatic tire of one embodiment of the present invention will be described using FIGS. 1 to 3.

FIG. 1 is a schematic drawing of the tread portion of a pneumatic tire of one embodiment of the present invention; FIG. 2 is an enlarged cross section of the tread portion of a pneumatic tire as seen along II-II in FIG. 1; and FIG. 3 is an enlarged cross section of the tread portion of a pneumatic tire as seen along III-III in FIG. 1.

First, as shown in FIG. 1, 1 is a pneumatic tire tread 1 of the present embodiment, and the pneumatic tire tread 1 has a tread portion 2 and flexible fences 4 that are described later. Two primary grooves 3 of width W extending in the tire circumferential direction shown by XX′ are formed in tread portion 2. Primary groove 3 has three surfaces, that is, opposing walls 31 and 32 and base surface (base part) 33. It should be noted that the tire size in this example is 225/55R16 and the direction of tire rotation of tread portion 2 is not specified. According to the present embodiment, opposing walls 31 and 32 each extend perpendicularly to the tire radius, groove width W is 14.5 mm, and groove width D is 8.0 mm.

This drawing shows tread contact patch 5 when the tire is filled to the rated air pressure and is carrying the rated load and the tread contact patch length L at this time. It should be noted that according to the “ETRTO Standard Manual 2011,” the appropriate rim for this size is 7 J, the rated internal pressure is 250 kPa, and the rated load is 690 kg. According to the present embodiment, tread contact patch length L is 143 mm.

Each primary groove 3 that passes through tread contact patch 5 as the tire is turning forms an air column with the road surface and the resonance frequency of the primary columns 3 is dependent on the length of the air columns formed in this way. According to the present embodiment, flexible fences 4 are disposed in primary groove 3 as shown in FIGS. 1 to 3 in order to vary the length of the air columns and thereby vary the frequency of the air column resonance. As shown in FIG. 1, each flexible fence 4 formed in primary groove 3 that goes around the tire once is disposed at an interval P, which is an interval shorter than the tread contact patch length L such that at least one flexible fence is always in the tread contact patch 5 of each primary groove 3.

Next, as shown in FIGS. 2 and 3, the base parts 41 of these flexible fences 4 are connected as illustrated to the groove base part 33 of the primary grooves 3 and, as shown in FIG. 3, the flexible fences 4 extend in the direction of the tire radius (perpendicular to the tire axis of rotation). Moreover, as shown in FIG. 2, side parts 42 on both sides of flexible fences 4 are disposed such that with the exception of the above-mentioned connected parts (33), a specific space is formed with opposing walls 31 and 32 of primary groove 3.

As shown in FIG. 1, flexible fences 4 are formed such as to extend perpendicular to the direction in which the primary grooves 3 extend. Flexible fences 4 have a rectangular cross section, and this rectangular cross section has width l (refer to FIG. 2) and thickness E (refer to FIG. 3).

Moreover, as shown in FIG. 2, the flexible fences 4 are formed into a rectangle as seen from the lengthwise direction of primary groove 3 (from the front), and as shown in FIGS. 2 and 3, the flexible fences have a height h that is somewhat lower than the depth D of the primary grooves.

The flexible fences 4 are formed such as to block at least 70% of the cross section of the primary groove 3, and are formed such as to collapse under air pressure from a liquid such as the water that flows through the primary groove 3. According to the present embodiment, thickness E of the flexible fences 4 is 0.6 mm. Moreover, according to the present embodiment, as shown in FIG. 2, the height h and width l of the flexible fences are a height h of 7.0 mm and width l of 13.5 mm versus a primary groove 3 depth D of 8.0 mm and groove width W of 14.5 mm such that approximately 87% of the cross section of the primary groove 3 is blocked.

It should be noted that, for instance, according to the tire in the present embodiment, the flexible fences 4 can also be rectangular having a height h of approximately more than or equal to 5.6 mm such that at least 70% of the cross section of the primary grooves is blocked. It should be noted that the tire is not restricted to this example of the embodiment. The groove width W and groove depth D of the tire primary groove 3 can be changed and the width l and height h of the flexible fences 4 can be changed accordingly as long as at least 70% of the cross section of primary groove 3 is blocked.

According to the present embodiment, flexible fences 4 are formed from a rubber composition for fences that is different from that for tread 2. The rubber composition for fences contains, in order to produce a faster abrasion rate than at the tread 2, a reinforcing filler having a BET specific surface area of less than 100 m²/g, in an amount of between 20 phr and 120 phr, and therefore the abrasion index of the rubber composition for fences is more than or equal to twice as high as the abrasion index of the rubber composition forming the tread 2.

Next, the state when the pneumatic tire tread of one embodiment of the present invention is running on a wet road surface will be described using FIGS. 4 and 5.

FIG. 4 is an enlarged cross section showing the pneumatic tire tread as seen along II-II in FIG. 1 when the tire is running on a wet road surface, and FIG. 5 is an enlarged cross section showing the pneumatic tire tread as seen along III-III in FIG. 1 when it is running on a wet road surface.

As shown in FIGS. 4 and 5, flexible fences 4 formed from a rubber composition for fences that is different from tread 2 collapse or bend under water pressure generated by a liquid such as water passing through the inside of primary groove 2 when the tire is running on a wet road surface. As a result, height decreases to h* and this reduction in height opens the necessary part of primary groove 3 to guarantee water drainage.

Next, the state after the tread has been abraded in the pneumatic tire tread according to an embodiment of the present invention is shown in FIG. 6.

FIG. 6 is an enlarged cross section showing the pneumatic tire tread as seen along III-III in FIG. 1 that has been abraded to a groove depth of approximately 30%.

FIG. 6 shows the state where after running a specific distance, the groove depth D of the primary groove 3 has been reduced to D′, which is shallower than initial height h of flexible fences 4 due to abrasion. Once the tire has run for a specific distance, the flexible fences 4 formed by rubber composition for fences that is different from tread 2 are obliquely abraded at the tip that contacts the road surface such that the height of the fences is reduced from the initial height h to h′ on the high side and h″ on the low side, and the height h″ on the low side is the same as the depth D′ of the primary groove.

It should be noted that as long as a modified version of the rubber composition for fences has the above-mentioned operation, it can be used for a part (such as the connecting part (33) (refer to FIGS. 2 and 3)) or all (not illustrated) of the base part 33 of the primary groove 3 where the flexible fences 4 are connected. In such a case, the production process can be simplified because the flexible fences and the base part 33 of the primary groove can both be formed from the rubber composition for fences.

As long as the operation is the same as described above, in other modifications of the present embodiment, the flexible fences 4 formed from the rubber composition from fences can be partitioned multiple times, the flexible fences can be formed such that they are connected to one or both of opposing walls 31 and 32 of primary groove 3 and extend in the direction of groove width, or the flexible fences connected in this way to walls 31 and 32 can be used in combination with the flexible fences 4 joined to the base 33 of the primary groove 3 (not illustrated).

The rubber composition for the fences and the rubber composition forming the tread portion are produced in an appropriate mixer, preferably using two linked production steps known to a person skilled in the art, that is, a first production step (called the “non-production” step) for thermomechanical working or mixing at a high temperature wherein the maximum temperature is between 110 and 190° C., preferably 130 and 180° C., and a second mechanical working step (“production” step) wherein the temperature is lower, typically lower than 110° C., for instance, between 40 and 100° C. A crosslinking system is introduced to the finishing step (production step).

The method for producing a rubber composition for fences comprises at least the following processes:

• • a process wherein, according to the first (“non-production”) step at least one reinforcing filler (where the particles are nanoparticles having an average grain diameter (by weight average) smaller than 500 nm) and more than 50 phr of a non-reinforcing filler (wherein the particles are microparticles having a median grain size (by weight average) greater than 1 μm) are mixed in the diene elastomer, and after the mixture is obtained or while the mixture is being obtained, the product is subjected one or more times to thermomechanical mixing up to a maximum temperature between 110 and 190° C.;
  • a process wherein the mixture is cooled to a temperature lower than 100° C.;
  • a process wherein, according to the second (“production”) step, the crosslinking system is introduced; and
  • a process wherein the mixture is mixed to a maximum temperature that is lower than 110° C.

In one example, the non-production step is performed during one thermomechanical process. During this process, all of the essential base components (diene elastomer, reinforcing filler and coupling agent as needed, non-reinforcing filler, and plasticizing system) are first introduced to an appropriate mixer such as a standard sealed mixer and then second, after mixing for example one to two minutes, the other additives except the crosslinking system, and coating agent or auxiliary finishing agent as optional components are introduced. The total mixing time during this non-production step is preferably between 1 and 15 minutes.

The mixture obtained in this way is cooled and then the crosslinking system is introduced to an open mixer, such as an open mill maintained at a low temperature (for instance, between 40 and 100° C.). This mixture is then mixed for several, for instance, 2 to 15, minutes (production step).

The crosslinking system is preferably a vulcanization system based on sulphur and an accelerator. An arbitrary compound that can react as a vulcanization accelerator of a diene elastomer in the presence of sulphur can be added. This compound is selected in particular from the group consisting of 2-mercaptobenzothiazyldisulphide (abbreviated “MBTS”), N,N-dicyclohexyl-2-benzothiazylsulphenamide (abbreviated “DCBS”), N-tert-butyl-2-benzothiazylsulphenamide (abbreviated “TBSS”), N-tert-butyl-2-benzothiazylsulphenimide (abbreviated “TBSI”) and mixtures of these compounds. Preferably a sulphonamide primary accelerator is used.

Various known secondary vulcanizers or vulcanization activators, such as zinc oxide, stearic acid, and guanidine derivatives (particularly diphenylguanidine) can be added to the first non-production step and/or production step. The sulphur content is between, for instance, 0.5 and 3.0 phr, and the primary accelerator content is between, for instance, 0.5 and 5.0 phr.

The final product obtained in this way is, for instance, calendered into a sheet or a plaque or extruded into a rubber element that can be used as a tire tread, for instance.

The above-mentioned has described particularly preferred embodiments of the present invention, but the present invention is not limited to the illustrated embodiments and various modifications are possible.

Key

• 1 Pneumatic tire tread
• 2 Tread portion
• 3 Primary groove
• 31 Opposing wall of primary groove
• 32 Opposing wall of primary groove
• 33 Base part of primary groove
• 4 Flexible fence
• 41 Base part of flexible fence 4 (connecting part with groove base part 33 of primary groove 3)
• 42 Side part of both sides of flexible fence 4
• 5 Tread contact patch