Abstract:
A studless tire contains tread portion made of vulcanized rubber, compounded from 100 parts by weight of rubber and 2 to 30 parts by weight of short fibers, and the tread portion has a ground connecting region with sipes, formed by pressing thin plates onto the tread rubber during vulcanizing, whereby the thin plates orient the short fibers in a radial direction. Preferably, the short fibers have a diameter of not more than 30 μm and a length of from 0.3 to 20 nm, and the thickness of the thin plates or the width of the sipes ranges from 0.2 to 0.5 mm with spacings being sipes less than 10 mm. The short fibers are circumferentially oriented before pressing the thin plates, and the total length in mm of axial component of all sipes ranges from 0.05 to 0.15 times the area in mm 2  of ground contacting region.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a pneumatic tire, more particularly to a studless tire improved in wet grip performance. 
     In recent years, studless tires or winter tires having no spike have come in to wide use. 
     DESCRIPTION OF THE RELATED ART 
     In Japanese Patent No. 2637887, a rubber compound including organic fibers which is suitably used in the studless tires as tread rubber is disclosed, wherein, in order that the fibers are not oriented in a specific direction during extruding the raw compound, the organic fibers have a diameter of 0.1 to 0.3 mm and a length of 0.5 to 3 mm and the ratio of the diameter to the length is set in the range of from 0.06 to 0.6. And the content of the fibers is set in the range of from 5 to 20 parts by weight with respect to 100 parts by weight of material rubber compound. Therefore, in the tread portion of the finished tire, the fibers are not oriented in a specific direction and the tire tread rubber do not display directional characteristics. 
     In order to improve wet grip performance, especially on the ice covered road, of the above-mentioned studless tires, the present inventors studied and found that the road grip becomes maximum when all the fibers are oriented in the tire radial direction. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a studless tire in which the tread rubber in the ground contacting region contains short fibers oriented in the radial direction of the tire. 
     According to the present invention, a studless tire comprises a tread portion, 
     the tread portion made of a vulcanized tread rubber, 
     the tread rubber compounded from 100 parts by weight of rubber component and 2 to 30 parts by weight of short fibers, 
     the tread portion provided in a ground contacting region with sipes 
     the sipes being formed by pressing thin plates onto the tread rubber during vulcanizing the tread rubber, whereby the thin plates orient the short fibers in a radial direction of the tire. 
     Preferably, the short fibers have a diameter of not more than 30 μm and a length of from 0.3 to 20 mm, and the thickness of the thin plates or the width of the sipes is in the range of from 0.2 to 0.5 mm. The spacings between the sipes are less than 10 mm. 
     Further, the short fibers are generally oriented in the circumferential direction of the tire before pressing the thin plates, and the total length in millimeter of axial component of all the sipes being in the range of from 0.05 to 0.15 times the area in square millimeter of the ground contacting region. 
     An embodiment of the present invention will now be described in detail in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of an embodiment of the present invention. 
     FIG. 2 is a developed view showing the tread pattern thereof. 
     FIG. 3 is a diagram for explaining a step of making a tread rubber strip. 
     FIG. 4 is a cross sectional view for explaining a step of making the tire. 
     FIG. 5 is a diagram for explaining how the short fibers are oriented by a plates. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the drawings, the studless tire  1  according to the present invention comprises a tread portion  2 , a pair of sidewall portions  3 , a pair of axially spaced bead portions  4  with a bead core  5  therein, a carcass  6  extending between the bead portions, and a belt  7  disposed radially outside the carcass in the tread portion. 
     The carcass  6  comprises at least one ply of cords arranged at an angle of from 75 to 90 degrees with respect to the tire equator C. 
     The belt  7  comprises at least two cross plies of parallel cords laid at angle of from 10 to 30 degrees with respect to the tire equator C. In this embodiment, the belt  7  is composed of two plies of high modulus cords such as steel cords. 
     The tread portion  2  is provided with tread grooves  10  to form a tread pattern such as a block pattern, rib-block pattern and the like including blocks  13 ,  14 . 
     In this embodiment, the tread grooves  10  include a plurality of circumferential grooves  11  extending continuously in the tire circumferential direction, and a plurality of axial grooves  12  intersecting the circumferential grooves  11 . In FIG. 2, the tread portion  2  is divided into blocks  13  and  14  in four circumferential rows, defining a block pattern. 
     Each of the blocks  13  and  14  is provided with a plurality of sipes  9  generally extending in the tire axial direction. 
     In this example, most of the sipes  9  are zigzagged partially in the middle of the blocks in the tire axial direction in order to increase the length and to provide not only an axial component but also a circumferential component to improve wet grip performance in the circumferential direction and axial direction of the tire. 
     In FIG. 2, both the ends of each sipe  9  are opened to adjacent grooves. But, it may be possible to use a sipe of which one end is opened and the other end is closed, and a sipe of which both ends are closed, either solely or in combination. 
     The sipes  9  are arranged at substantially regular intervals G in the tire circumferential direction. 
     The tread portion  2  is made of a tread rubber  15  containing 2 to 30 parts by weight, preferably not more than 20 parts by weight of short fibers F with respect to 100 parts by weight of the rubber component. 
     For the rubber component, various rubber-like elastic materials may be used. Usually, at least one kind of diene rubber selected from natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR) and the like is used. 
     Further, various additives are blended, for example: vulcanizing agent such as sulfur; coagent such as zinc oxide and stearic acid; accelerator such as thiazole accelerator, e.g. mercaptobenzothiazole (MBT) and dibenzothiazyl disulfide (MBTS) and sulfenamide accelerator, e.g. N-tert-ptyl-2-benzothiazyl sulfenamide (TBBS) and N-cyclohexyl-2-benzothiazyl sulfenamide (CBS); softener such as naphthenic oil, paraffinic oil and aromatic oil; reinforcing agent such as carbon black and silica; age resistor; wax; coupling agent; and the like. 
     The diameter of the short fibers F is not more than 30 μm, preferably not more than 20 μm, but not less than 5 μm, preferably not less than 10 μm. 
     The length of the short fibers F is not less than 0.3 mm, preferably not less than 0.4 mm, more preferably not less than 5 mm, but not more than 20 mm, preferably not more than 10 mm. 
     The tire in this invention can be manufactured using a vulcanizing mold as usual, and the tread portion  2  is formed by winding a strip  21  of raw tread rubber  15  around the carcass  6 . As explained above, the raw tread rubber  15  is compounded from the above-mentioned various materials which are mixed by a mixer such as banbury mixer, rolls or the like. 
     The tread rubber strip  21  is formed by extruding or rolling the raw tread rubber  15  as shown in FIG.  3 . By rolling or extruding the raw rubber, the short fibers therein are oriented in the longitudinal direction of the strip. In FIG. 3, calender rolls  20  are used. Thus, when the strip  21  is wound around the carcass  6 , the short fibers are oriented in the circumferential direction of the tire. 
     Next, to vulcanize the raw tire  22 , it is put into the mold  23  as shown in FIG.  4 . In this example, the mold  23  is a segmental mold comprising sectors  23 T for molding the tread portion  2 . The sectors  23 T are movable in the tire radial direction towards the tire, and the radially inside thereof is provided with thin plates  24  for forming the sipes. By moving the sectors  23 T towards the tire, the thin plates  24  are pressed onto the surface of the raw tread rubber strip and inserted in the tread rubber. As a result, the sipes  9  are formed, and at the same time, the short fibers F are oriented as shown in FIG.  5 . 
     During inserting the plates  24 , short fibers F 1  therearound are caught by the radially inner edges  24 E of the plates  24 , and pulled toward the radially inside, and short fibers F 2  positioned near the short fibers F 1  are also drawn towards the radially inside. As a result, the short fibers near the sipes  9  are almost oriented in the radial direction. 
     In order that the short fibers F are efficiently oriented, it is important that the length of the short fibers F is set in the range of from 0.3 to 20 mm, and the diameter thereof is set in the range of not more than 30 μm. If the length of the short fibers F is less than 0.3 mm, the short fibers F are hard to be caught by the radially inner edges  24 E, and thus it is difficult to orient them in the radial direction. If more than 20 mm, processing characteristics of the raw tread rubber become lowered, and a rubber flow during vulcanizing the tire becomes worse, and the above-mentioned motion of the fibers F 2  accompanying the pulled fibers F 1  is hindered. 
     It is also necessary for efficiently orienting the short fibers F 1  to set the thickness T of the plates  24  within the range of from 0.2 to 0.5 mm. If the thickness T exceeds 0.5 mm, it is difficult to orient the short fibers F. If the thickness T is less than 0.2 mm, the strength of the plates  24  is decreased, and it is difficult to make or maintain the tire vulcanizing mold  23 . 
     By using such thin plates  24 , it becomes possible to orient the short fibers existing within a range of about 5 mm from the plates or sipes. 
     Therefore, in order to orient almost all the fibers, the sipes are preferably arranged at intervals G of less than 10 mm (5 mm×2). 
     Further, it is preferable that the total length ΣXi (mm) of the axial components Xi of the respective sipes  9  is set in the range of from 0.05 to 0.15 times the gross area SS (mm 2) of the tread face  2 S. 
     If the diameter of the short fibers F is more than 30 μm, the short fibers F are hard to twine around the radially inner edges  24 E, and thus it is difficult to orient the fibers in the radial direction. Further, the tread rubber  15  becomes hard and adhesion with the road surface decreases. If the diameter is less than 5 μm, the fibers lose a minimum bending rigidity required for scratching the road surface. 
     If the amount of the short fibers F is less than 2 parts by weight, the road surface scratching effect becomes insufficient. If more than 30 parts by weight, the wear resistance of the tread rubber decreases. 
     If the ratio ΣXi/SS is less than 0.05, it becomes difficult to obtain a necessary on-the-ice performance. If more than 0.15, the tread rigidity is excessively decreased, and uneven wear resistance and maneuverability on dry roads decrease. 
     For the short fibers F, glass fiber, aluminum whisker, organic fibers, e.g. polyester, nylon, vinylon, aromatic polyamide and the like can be used. But, for the dispersion during mixing, the prevention of softening or stiffening, and the road surface scratching effect, inorganic materials having a specific gravity in the range of not less than 2.0, such as glass fiber and aluminum whisker are preferably used. From a viewpoint of orientation, it is especially preferable that the length/diameter ratio of the short fibers F is not less than 30. 
     Comparison Tests: Test tires of size 185/65R14 (Rim size: 14×5.5JJ) having the tire structure and tread pattern shown in FIGS. 1 and 2 were made according to the specifications given in Table 1 and tested for on-the-ice performance, wear resistance, uneven wear resistance and maneuverability. 
     On-the-ice performance test: In the test, a test car, 2000 cc FF passenger car provided on all the four wheels with the test tires (Inner pressure: 200 Kpa) was run on an ice-covered road at a speed of 30 km/h, and a sharp braking was made to obtain the braking distance and the average braking deceleration. The results are indicated by an index based on the reference tire 1 being 100, wherein the larger the index, the better the performance. 
     Uneven wear resistance test: After the test car run 5000 km on a dry asphalt road in a tire test course, the difference in the amount of wear was evaluated around the sipes. The results are indicated by an index based on the reference tire 1 being 100. The lager the index, the better the uneven wear resistance. 
     Maneuverability test: Running the passenger car on a dry asphalt paved 8-shaped course (25 meter radius), the lap time was measured. The results are indicated by an index based on the reference tire 1 being 100. The large the index, the better the maneuverability. 
     Wear resistance test: Using a Lambourn type wear tester, the wear resistance was measured under the following conditions. Load: 2 kg, Slip rate: 30%, Time: two minutes The amount of wear is indicated by an index based on the Example tire 1 being 100. The larger the index, the better the wear resistance. 
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Ref. 
                 Ref. 
                 Ref. 
                 Ref. 
                 Ref. 
                 Ref. 
                 Ref. 
               
               
                 Tire 
                 Ex. 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Tread rubber 
                 B 
                 A 
                 B 
                 B 
                 C 
                 D 
                 E 
                 F 
               
               
                 Σ Xi/SS 
                 0.1 
                 0.1 
                 0.04 
                 0.17 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 Ice 
                 121 
                 100 
                 95 
                 101 
                 111 
                 119 
                 113 
                 105 
               
               
                 performance 
               
               
                 Uneven wear 
                 100 
                 100 
                 100 
                 89 
                 100 
                 100 
                 100 
                 100 
               
               
                 resistance 
               
               
                 Wear resistance 
                 100 
                 — 
                 — 
                 — 
                 102 
                 80 
                 98 
                 102 
               
               
                 Maneuver- 
                 101 
                 100 
                 102 
                 95 
                 104 
                 104 
                 97 
                 101 
               
               
                 ability 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 parts by weight 
               
             
          
           
               
                 Rubber 
                 A 
                 B 
                 C 
                 D 
                 E 
                 F 
               
               
                   
               
               
                 Glass fiber 
                   
                   
                   
                   
                   
                   
               
               
                 Dia. (μm) 
                 — 
                  9 
                  9 
                  9 
                 35 
                  9 
               
               
                 Length (mm) 
                 — 
                 13 
                 25 
                 13 
                 15 
                   0.2 
               
               
                 Rubber component 
               
               
                 NR 
                 60 
                 60 
                 60 
                 60 
                 60 
                 60 
               
               
                 BR 
                 40 
                 40 
                 40 
                 40 
                 40 
                 40 
               
               
                 Carbon black (N220) 
                 30 
                 30 
                 30 
                 30 
                 30 
                 30 
               
               
                 Silica (VN3) 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
               
               
                 Paraffinic oil 
                 15 
                 15 
                 15 
                 15 
                 15 
                 15 
               
               
                 Silane coupling agents (S169) 
                  2 
                  2 
                  2 
                  2 
                  2 
                  2 
               
               
                 Paraffinic wax 
                  2 
                  2 
                  2 
                  2 
                  2 
                  2 
               
               
                 Age resistor (6PPD) 
                  2 
                  2 
                  2 
                  2 
                  2 
                  2 
               
               
                 Stearic acid 
                  2 
                  2 
                  2 
                  2 
                  2 
                  2 
               
               
                 Hydrozincite 
                  3 
                  3 
                  3 
                  3 
                  3 
                  3 
               
               
                 Sulfur 
                   1.5 
                  2 
                  2 
                  2 
                  2 
                 1.5 
               
               
                 Accelerator (CZ) 
                  1 
                  1 
                  1 
                  1 
                  1 
                  1 
               
               
                 Accelerator (DPG) 
                   0.8 
                  1 
                  1 
                  1 
                  1 
                   0.8 
               
               
                   
               
             
          
         
       
     
     In the reference tires  2  and  7 , the short fibers were hardly oriented. In the reference tire  3 , the rigidity of the tread portion became insufficient. In the reference tire  4 , the degree of orientation becomes not good due to the decreased rubber flow. In the reference tire  5 , the wear resistance was greatly decreased due to too much short fibers. In the reference tire  6 , the orientation was not good and the tread rubber became too hard to provide a good adhesion to the road surface because the short fibers were too thick.