Abstract:
A run-flat pneumatic radial tire having an aspect ratio of up to 50%, which includes a reinforcing liner layer mode of a rubber having a complex modulus at 20° C. E* 20 , a ratio of a complex modulus at 100° C. to the complex modulus at 20° C., a 100% modulus and a loss tangent (tan δ) at 100° C. which satisfy certain specified numeric values. The liner has a crescent sectional shape and is disposed in sidewall portions of a carcass layer with its end portions overlapping the end portions of a belt layer and a bead filler having a specified hardness and height h from a wheel rim base. The carcass layer consists of an inner carcass layer turned up around the bead core from inside to outside the tire so that its end portion is positioned at a position higher than the height h of the bead filler and an outer carcass layer having an end portion disposed axially outward of the bead core.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a pneumatic radial tire having improved run-flat durability. 
     A run-flat pneumatic tire is the tire which can keep running for a predetermined distance even when an internal pressure of the tire drops drastically due to puncture and the like. To provide such run-flat performance, conventional run-flat tires are primarily directed to reduce heat generation at sidewall portions by increasing mainly rigidity of the sidewall portions as much as possible and making it more difficult for the tires to undergo flexibility. For example, Japanese patent application Kokai publication No. 64-30809 proposes a flat run-flat tire having a reinforcing liner layer which has a crescent sectional shape, is made of a rubber having a JIS--A hardness of 50 to 65 and is disposed on the inner surface of the sidewall portions of a tire having an aspect ratio of up to 55% so that its upper end overlaps with a belt layer and its lower end overlaps with a bead filler, from the aspect that the lower the aspect ratio of the tire, the higher can be made rigidity and more difficult does it become for the tire to undergo flexibility. 
     However, since these tires are directed to increase rigidity of the sidewall portions as much as possible so as to reduce their deflection, general excellent running performance inherent to the radial tires are reduced unavoidably. If rigidity of the sidewall portions is extremely increased in radial tires having a low aspect ratio, local strain increases particularly at the portion ranging from near the upper end of a rim flange to a belt edge of a ground contact portion because the height of the sidewall portions is small, so that run-flat durability drops. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a run-flat pneumatic radial tire having run-flat durability without deteriorating markedly ordinary running characteristics of the radial tires while making the most of the features of a tire having a low aspect ratio in the aspect of rigidity. 
     In a run-flat pneumatic radial tire having an aspect ratio of up to 50%, the object of the present invention described above can be accomplished by a run-flat tire characterized in that a reinforcing liner layer made of a rubber having a complex modulus at 20° C., E* 20 , of at least 16 MPa and preferably up to 60 MPa, a ratio of a dynamic elastic modulus at 100° C., E* 100 , to the dynamic elastic modulus at 20° C., E* 20 , that is, E* 100  /E* 20 , of at least 0.80 and preferably up to 1.00, a 100% modulus of at least 60 kg/cm 2  and preferably up to 120 kg/cm 2 , and a loss tangent (tan δ) at 100° C. of up to 0.35 and preferably at least 0.05, and having a crescent sectional shape, is disposed inside a carcass layer at sidewall portions in such a manner that one of its end portions overlaps with the end portion of a belt layer of a tread portion and the other end portion overlaps with a bead filler of the bead portion; 
     the bead filler of the bead portion is made of a rubber having a JIS--A hardness of from 60 to 80 and has a height h of up to 35 mm from its rim base in a direction vertical to the axis of rotation of the tire; 
     The carcass layer consists of inner and outer two layers, the inner carcass layer is turned up from inside to outside the tire around the bead core so that its end is positioned at a position higher than the height h of the bead filler and is clamped between the inner carcass layer and the outer carcass layer; and 
     the outer carcass layer is wound down without being turned up around the bead core in such a manner as to position its end near the bead core, or both of the two carcass layers are turned up around the bead core from inside to outside the tire so that the end of the carcass layer on the side of the bead core is disposed near the bead core and the end of the other carcass layer is disposed beyond the upper end of the bead filler. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are semi-sectional views, showing an example of the tire of the present invention; and 
     FIGS. 3 and 4 are schematic sectional views, showing the structure of bead portions of tires which are not the tires of the present invention, respectively. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, the tire of the present invention is a flat tire which consists of bead portions 1, a pair of right and left sidewall portions 3 continuing from the bead portions 1 and a tread portion 4 extending in the tire circumferential direction between these sidewall portions 3, and which has an aspect ratio of below 50% expressed by the ratio of the tire sectional height to the tire maximum sectional width. When the aspect ratio is set to be below 50%, rigidity of the sidewall portions of the present tire can be made higher and the tire can be made more difficult to undergo flexibility than a high aspect ratio tire having the sidewall portions made of the same rubber. 
     A carcass layer 5 consisting of cords crossing substantially at 90° to the tire circumferential direction has a two-layered structure consisting of an inner carcass layer 5-1 which is disposed between the bead portions 1 and whose both ends are turned up from inside to outside of the tire around the bead cores 2 disposed at the bead portions 1 of the tire and an outer carcass layer 5-2 which extends down to a position axially outside the bead cores 2. Each end of the inner carcass layer 5-1 turned up around the bead core 2 is clamped between a portion of the carcass layer 5-1 before the inner carcass layer is turned up and the outer carcass layer 5-2. Two belt layers 6 which have a cord angle of 10°˜30° relative to the tire circumferential direction and cross one another are disposed on the carcass layer 5 at the tread portion 4 and a cover layer 7 is disposed further on this belt layer 6. 
     A reinforcing liner layer 8 whose upper end in the radial direction overlaps with the lower end of the belt layer 6 and whose lower end in the radial direction overlaps with a bead filler 9 and which has a crescent sectional shape is disposed inside the carcass layer 5 of each sidewall portion 3. The bead filler 9 is disposed above the bead core 2 and wrapped in by the main body of the inner carcass layer 5-1 and the inner carcass layer 5-1 which is turned up from inside to outside the tire around the bead core 2. 
     In the present invention, the reinforcing liner layer to be disposed inside each sidewall portion must have a crescent sectional shape so that the thickness is the greatest at the center and decreases gradually towards the upper and lower portions in the radial direction of the tire, respectively. Moreover, this crescent sectional shape must be such that one of its ends overlaps with the end portion of the belt layer and the other end overlaps with the bead filler of each bead portion. When the reinforcing liner layer has such a sectional shape, the sidewall portion undergoes smooth deformation, prevents the occurrence of local strain and can limit the drop of general running performance of a radial tire because suitable flexibility is permitted. 
     The rubber constituting the crescent reinforcing liner layer described above must have a complex modulus E* 20  at 20° C. of at least 16 MPa. For, if the complex modulus E* 20  is lower than 16 MPa under the condition of the aspect ratio of below 50%, the local strain of the reinforcing liner layer is increased. The rubber described above must keep this standard not only at 20° C. but also when the temperature rises due to running. Therefore, the ratio of the complex modulus E* 100  at 100° C. to E* 20 , that is, E 100  /E* 20 , of this rubber must be at least 0.80. Furthermore, this rubber must have a 100% modulus of at least 60 kg/cm 2 . If the 100% modulus is less than 60 kg/cm 2 , distortion of the tire sidewall portions becomes great as a whole at the time of run-flat running and destruction of the tire is promoted. 
     Moreover, tan δ of this rubber at 100° C. must be below 0.35. If any rubber having tan δ exceeding this value is used, heat generation at the reinforcing liner layer becomes so great at the time of bending that deflection of the side portions becomes great and run-flat durability drops. 
     In the present invention, the complex modulus at 20° C. E* 20 , the complex modulus at 100° C. E* 100  and tan δ at 100° C. are measured under the condition of a frequency of 20 Hz, an initial strain of 5% and a dynamic strain 1% elongation by use of a viscoelastic spectromer &#34;Rheograph Solid&#34; produced by Toyo Seiki K. K., respectively. The 100% modulus is measured by the measurement method stipulated in JIS K 6301. 
     If the thickness of the reinforcing liner layer having the crescent sectional shape is small, the effect of improving run-flat performance cannot be obtained. Therefore, this thickness is generally made greater with an increasing weight of a vehicle. If this thickness is too great, rigidity becomes too great, and general running characteristics as a radial tire will drop. It is therefore advisable that the maximum thickness of this reinforcing liner layer be from 3 mm to 8 mm. 
     In the tire of the present invention, the bead filler is made of a rubber having a JIS--A hardness of 60 to 80 which is lower than the hardness of the reinforcing liner layer, and its height h from a rim base in a direction vertical to the axis of rotation of the tire must be smaller than 35 mm. If the hardness of the bead filler is greater than JIS--A hardness of 80 or if the height h is greater than 35 mm, the local strain of the sidewall portions is increased and smooth deformation becomes difficult to occur. If the JIS--A hardness of the bead filler is lower than 60, general running performance of the radial tire such as maneuvering stability will drop. In the present invention, the JIS--A hardness described above is the value measured by the measurement method stipulated in JIS K 6301. 
     In the tire of the present invention, the structure of the carcass layer around the beads becomes another important factor in order to insure the smooth deformation to reduce the local strain while imparting suitable rigidity to the sidewall portions and permitting small flexibility. As shown in FIG. 1, in the two-layered carcass layer, the inner carcass layer 5-1 is turned up around the bead core 2 from inside to outside the tire in such a manner as to position its end to a position higher than the height h of the bead filler 9 and to be clamped by the outer carcass layer 5-2 extending down to a position axially outside the bead core 2. Alternatively, as shown in FIG. 2, two carcass layers 5-1a and 5-1b are turned up around the bead core 2 from inside to outside the tire, the end of one of the carcass layers 51-b is disposed near the bead core 2 and the end of the other carcass layer 5-1a is turned up beyond the upper end of the bead filler 9. Among these structures, the tire having the carcass layer of the structure shown in FIG. 1 is superior from the aspect of run-flat durability. These carcass layer structures exhibit the synergistic effects in improving flexibility resistance and limiting the local strain when combined with the reinforcing liner layer and make smooth deformation possible, so that run-flat durability can be improved remarkably. 
     In the tire of the present invention, known tire retaining mechanisms, such as the bead shape and the rim structure are naturally employed in order to prevent rolling-off of the tire from the rim even when the air pressure of the tire drops. 
     Hereinafter, effectiveness of each of the constituent requirements of the present invention described above will be explained with reference to Examples. 
     In each of the following Examples, the term &#34;run-flat durability&#34; represents the value obtained by measuring the distance until the tires get out of order when the test tires are fitted to a vehicle receiving a load of 500 kg per tire and so as to prevent the test tires from rolling off from the rims at an air pressure of 0 kg/cm 2 . 
     In each Example, run-flat durability is represented by an index and the greater this index value, the higher run-flat durability. 
     EXAMPLE 1 
     Seven kinds of tires A, B, C, D, E, F and G are produced and their run-flat durability is evaluated. Each tire is a radial tire having the structure shown in FIG. 1, the following tire specification and a rubber blend composition (Table 1) for the bead filler in common with the others but having different rubber characteristics only for the reinforcing liner layer, as tabulated in Table 2. The size of each tire is 255/40R17. 
     Incidentally, run-flat durability is an index using the evaluation result of the tire B as 100. 
     belt cover layer=nylon cord having cord angle of 0° with respect to the tire circumferential direction 
     belt layer=steel belt having a crossing angle of 24° 
     carcass layer=rayon cords 
     JIS--A hardness of bead filler=75, height h=33 mm 
     maximum thickness of reinforcing liner layer=5 mm 
     aspect ratio=40% 
     
                       TABLE 1______________________________________           Blend amountblend component (parts per hundred rubber)______________________________________natural rubber (SIR 20)           70.00SBR &#34;Nipol&#34; 1502*           30.00zinc oxide      5.00stearic acid    2.00anti-oxidant    1.00carbon black HAF           70.00aromatic oil    7.00accelerator     1.00sulfur          3.00______________________________________ *A tradename for a low temperature polymerization styrenebutadiene rubber (SBR) manufactured by Nippon Zeon K.K., a corporation of Japan. 
    
     
                                           TABLE 2__________________________________________________________________________      Tire A          Tire B              Tire C                  Tire D                      Tire E                          Tire F                              Tire G      Comp.          Inven-              Inven-                  Comp.                      Comp.                          Comp.                              Inven-      Ex. tion              tion                  Ex. Ex. Ex. tion__________________________________________________________________________BlendCompositionnatural rubber      40.00          40.00              40.00                  100.00                      40.00                          40.00                              40.00TSR*.sup.1UBEPOL VCR 412*.sup.2      --  --  10.00                  --  --  --  30.00&#34;Nipol&#34; BR 1220*.sup.3      60.00          60.00              50.00                  --  60.00                          60.00                              30.00Zinc oxide 5.00          5.00              5.00                  5.00                      5.00                          5.00                              5.00stearic acid      1.50          1.50              1.50                  1.50                      1.50                          1.50                              1.50anti-oxidant 6C      1.00          1.00              1.00                  1.00                      1.00                          1.00                              1.00resin*.sup.4      1.00          8.00              8.00                  1.00                      5.00                          8.00                              8.00KOMOREX 300*.sup.5      2.00          --  --  1.00                      --  --  --carbon black      60.00          65.00              65.00                  60.00                      60.00                          --  65.00LS-HAFcarbon black ISAF      --  --  --  --  --  65.00                              --insoluble sulfur       8.40          7.50              7.50                  7.50                      8.40                          8.40                              8.40accelerator*.sup.6      1.20          1.20              2.50                  1.50                      1.20                          1.20                              2.50PropertiesE*.sub.20  12.5          17.4              23.5                  9.6 18.0                          20.2                              30.2E*.sub.100 10.3          14.7              21.2                  6.9 16.0                          18.9                              28.3E*.sub.100 /E*.sub.20      0.82          0.86              0.90                  0.71                      0.89                          0.94                              0.94100% modulus      56  65  80  59  56  78  92tan δ      0.10          0.28              0.33                  0.15                      0.16                          0.38                              0.30run-flat durability      62  100 108 56  85  93  115__________________________________________________________________________ *.sup.1 Technically Specified Rubber; *.sup.2 a tradename for syndiotactic polybutadiene manufactured by UBE Kosan K.K., a corporation of Japan; *.sup.3 &#34;Nipol&#34; BR 1220 is a tradename for polybutadiene rubber manufactured by Nippon Zeon K.K., a corporation of Japan; *.sup.4 methacresolformaldehyde resin; *.sup.5 aromatic oil extender 20% manufactured by Nippon Oil Co., Ltd., a corporation of Japan; and *.sup.6 Noxydiethylene-2-benzothiazolyl sulfenamide (OBS). 
    
     It can be understood from Table 2 that run-flat durability drops if any one of E* 20 , ratio E* 100  /E* 20 , 100% modulus and tan δ of the reinforcing liner layer is out of the range stipulated in the present invention. 
     EXAMPLE 2 
     Run-flat durability is evaluated for eight kinds of tires B and H through N, shown in Table 3, produced by changing the bead filler height h (mm) and its JIS--A hardness of the tire B used in Example 1. In the table, run-flat durability is expressed by an index using the evaluation result of the tire E as 100. 
     
                       TABLE 3______________________________________            Bead filler            Height                  JIS-A    Run-flat            (h)   hardness durability______________________________________Tire B (This Invention)              33 mm   75       211Tire H (Comparative Example)              43 mm   90       100Tire I (Comparative Example)              43 mm   75       102Tire J (Comparative Example)              38 mm   90       103Tire K (Comparative Example)              38 mm   75       106Tire L (Comparative Example)              33 mm   90       132Tire M (Comparative Example)              28 mm   90       157Tire N (This Invention)              28 mm   75       208______________________________________ 
    
     It can be understood from Table 3 that if the height h of the bead filler from the rim base is greater than 35 mm, run-flat durability is not improved (tires H through K). In contrast, run-flat hardness can be improved in all of the Tires B, L, M and N wherein the height h is below 35 mm. However, the degree of improvement is low in the tires having a high JIS--A hardness and run-flat durability can be further improved by reducing this hardness (tires B and N). 
     The reason is believed as follows: In addition to the disposition of the reinforcing liner layer having the crescent sectional shape described already, the local strain of the sidewall portions, particularly near the portions from the upper end of the rim flange to the belt edge of the ground contact portion, can be reduced and the smooth deformation can take place by reducing the height of the bead filler and decreasing its hardness. 
     EXAMPLE 3 
     Run-flat durability is evaluated for three kinds of tires O, P and Q produced by changing only the structure near the beads of the carcass layer of the tire B of Example 1, as shown in FIGS. 2, 3 and 4, and for the tire B. The results are tabulated in Table 4. In this table, run-flat durability is expressed by an index using the evaluation result of the tire B as 100. 
     In the bead portion shown in FIG. 3, both of the two carcass layers 5-1a and 5-1b are turned up around the bead core 2 and the end portions of these carcass layers 5-1a and 5-1b are disposed at positions higher than the height h of the bead filler 9. In the structure shown in FIG. 3, the bead portion shown in FIG. 4 has the structure wherein the end portions of the two carcass layers 5-1a and 5-1b are disposed at positions lower than the height h of the bead filler 9. Furthermore, in the bead portion shown in FIG. 2, both the two carcass layers 5-1a and 5-1b are turned up around the bead core 2, the end portion of one 5-1a of the carcass layers is disposed near the bead core 2 and the end portion of the other carcass layer 5-1b is disposed at a position higher than the height h of the bead filler 9. 
     
                       TABLE 4______________________________________Tire B       Tire O      Tire P     Tire QThis         Comparative Comparative                               ThisInvention    Example     Example    Invention______________________________________Structure   FIG. 1   FIG. 3      FIG. 4   FIG. 2aroundbeadRun-flat   100      63          54       89durability______________________________________ 
    
     As can be understood from the table, Tires 0 and P have remarkably low run-flat durability. Though run-flat durability of the Tire Q is lower than that of the tire B, it is not lower than that of the tires 0 and P and the Tire Q has practically usable run-flat durability.