Abstract:
A radial tire for use in construction vehicles comprises a tread of a two layer structure composed of tread cap rubber and tread base rubber. In this case, the tread base rubber is divided into crown center portion and shoulder portion, and these rubbers satisfy particular relations of loss factor at 50° C., storage modulus at 50° C. and 300% modulus at room temperature.

Description:
This is a continuation of application Ser. No. 07/232,749, filed Aug. 16, 1988, and abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to radial tires for construction vehicles which largely suppress the crack growth from tread cut failure produced in a belt end to considerably improve the durable life without damaging cut resistance and heat generating durability. 
     2. Related Art Statement 
     In general, tires for construction vehicles mainly running on rough roads have a problem that cracks propagate up to a belt end portion due to the increase of strain under an authorized payload also, a heat build-up problem is based on the thickness profile of tire itself and tread cut failure at stone pit and consequently separation failure is caused from the belt end to largely degrade the durable life. 
     As a technique for solving the above problem, Japanese Patent laid open No. 49-80,703 proposes that two or more different rubber materials having a low heat build-up are arranged side by side in the widthwise direction of the tire so as to match with a deformation system of crown portion and shoulder portion of base rubber during the running for reducing internal heat generation. 
     Furthermore, Japanese Patent laid open No. 61-287,802 proposes a radial tire having a two layer structure of a tread composed of cap rubber and base rubber, wherein a rubber composition having a good impact cut resistance is used in the center portion of the crown of the base rubber and a rubber composition having a low heat build-up is used in the shoulder portion of the base rubber to thereby differ rubber properties as a tread rubber in the cap rubber and the base rubber at its crown and shoulder portions. 
     In the technique of Japanese Patent laid open No. 49-80,703, however, the considerable effect of suppressing the heat build-up of the tire is observed by rendering the base rubber into the above moving shape, but since the rubber used is substantially low in heat build-up, the cut resistance is poor so that when cut failure is caused at the tread, there is caused a problem that cracks propagate up to the belt end to finally cause separation failure at belt end. 
     On the other hand, when the technique of Japanese Patent laid open No. 61-287,802 is applied to the tire for a construction vehicle aiming at the invention, the heat generation level in the central portion of the tread base rubber is high, which comes into problem in view of the heat generating durability, and also since the shoulder portion of the tread base rubber is substantially a low heat build-up rubber, the resistance to crack growth is poor and the cracks are apt to propagate up to the belt end when cut failure is caused in the tread and finally the separation failure at belt end is caused to largely reduce the durable life. 
     As seen from the above, the aforementioned problems in the tire for construction vehicle can not sufficiently be solved by the above conventional techniques. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the invention to provide a technique of considerably improving the durable life of the tire for a construction vehicle by largely suppressing the crack growth from the belt end due to the cut failure of the tread without damaging the cut resistance and heat generating durability. 
     The inventors have made studies in order to solve the technical problems of the conventional technique and found that the object of the invention can be achieved by adopting a two layer structure in the tread of the tire for a construction vehicle, dividing the base rubber into a shoulder portion and a crown center portion and optimizing rubber properties of base rubber of these portions and tread cap rubber in connection with problem inherent to the construction tire, and as a result the invention has been accomplished. 
     According to the invention, there is the provision of a radial tire for construction vehicles comprising a casing reinforcement consisting of a carcass of radial structure and a belt surrounding an outer periphery of a crown portion of said carcass and having a tread of a two layer structure composed of a tread base rubber covering an outer portion in the radial direction of said belt and both side portions in widthwise direction of the tire and a tread cap rubber (A) arranged outside said tread base rubber in the radial direction, characterized in that said tread base rubber consists of a tread base rubber (B) at crown center portion arranged outside said belt in the radial direction and a tread base rubber (C) at shoulder portion arranged in both sides of said crown center portion in the widthwise direction of the tire and covering at least a belt end, and said tread base and cap rubbers satisfy the following relations: 
     
         tanδ(B)&lt;tanδ(C)&lt;tanδ(A)                  (1a) 
    
     
         tanδ(C)-tanδ(B)≧0.02                    (1b) 
    
     (where tanδ (A), tanδ (B) and tanδ (C) are loss factors at 50° C. of the rubbers (A), (B) and (C), respectively); 
     
         E&#39;(A)&gt;E&#39;(C)&gt;E&#39;(B)                                          (2a) 
    
     
         E&#34;(C)-E&#39;(B)≧5.0×10.sup.6 (dyn/cm.sup.2)       (2b) 
    
     (where E&#39;(A), E&#39;(B) and E&#39;(C) are storage moduli of elasticity at 50° C. of the rubbers (A), (B) and (C), respectively); and 
     
         15×10.sup.-7 M.sub.300 (B)/E&#39;(B)-M.sub.300 (C)/E&#39;(C)≧2×10.sup.-7 (kg/dyn)               (3) 
    
     (where M 300  (B) and M 300  (C) are 300% moduli at room temperature of the rubbers (B) and (C), respectively). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with reference to the accompanying drawings, wherein: 
     FIG. 1 is a partially sectional view of an embodiment of the radial tire for construction vehicles according to the invention; and 
     FIG. 2 is a partially sectional view of the belt cord used for illustrating an outline of a test for resistance to crack growth at belt end. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the invention, it is required to arrange the tread base rubber so as to cover the belt end portion with the tread base rubber (C) for the shoulder portion as shown in FIG. 1 because when the rubber (C) is not arranged in the above arrangement but the rubber (B) is arranged in this arrangement, if the cut failure is caused from the tread, since the outermost end of the belt is not covered with rubber having good cut resistance and resistance to crack growth, cracks invades into the belt to undesirably cause the belt troubles. 
     As illustrated in FIG. 1 the tread cap rubber (A) extends in the tire widthwise direction to radially cover both of the base rubbers (B) and (C). The tread cap rubber (A) overlies and extends laterally beyond a laterally outer end of the tread base rubber (C). FIG. 1 also, illustrates a belt structure with a plurality of belt layers with portions of the belt layers laterally outside the base rubber (B). The tread cap (A) and the tread base rubber (C) taper radially inward toward a shoulder portion of the tire and the tread cap rubber (A) overlies and extends laterally beyond a laterally outer end of the base rubber (C) in the shoulder portion. 
     Furthermore, according to the invention, it is required that the each tan δ value at 50° C. in the cap rubber (A) and the base rubbers (B) and (C) satisfy the relations of the above formulae (1a) and (1b). Because, when these relations are not satisfied, the function allotment of the rubber (B) making much of heat generation resistance and the rubber (C) making much of cut resistance is not sufficiently conducted and the object of the invention can not be achieved. 
     Similarly, when each of storage modulus of elasticity E&#39; at 50° C. and 300% modulus M 300  at room temperature in the rubbers (A), (B) and (C) does not satisfy the relations of the above formulae (2a), (2b) and (3), the balance among the cut resistance of the rubber (C) and other performances is not desirably maintained. Particularly, when the value of M 300  /E&#39; is not within the given range as a difference between the rubber (B) and the rubber (C), the object of the invention can not be achieved. That is, when this difference is smaller than 2×10 -7  kg/dyn, the effect of establishing the resistance to crack growth and the cut resistance is not obtained, while when it exceeds 15×10 -7  kg/dyn, the cut resistance and the resistance to crack growth of the rubber (B) are degraded, which is considerably disadvantageous in view of the durability of the tire. 
     As mentioned above, according to the invention, there are noticed troubles inherent to the construction tire, and the rubber properties in the tread cap rubber and tread base rubbers for the crown center portion and shoulder portion are related to each other, whereby the object of the invention is first achieved. 
     The following examples are given in illustration of the invention and are not intended as limitations thereof. 
     Various radial tires for construction vehicles having a tire size of ORR24.00 R49 were manufactured by applying a rubber composition having a compounding recipe (parts by weight) shown in the following Table 1 to a tread cap and applying a rubber composition having a compounding recipe (parts by weight) shown in the following Table 2 to crown center portion and shoulder portion of tread base. 
     Moreover, the rubber composition at the crown center portion in Example 1 and Comparative Example 5 of Table 2 was same, and also the rubber composition at the shoulder portion in Example 2 was the same as at the crown center portion in Comparative Example 4. 
     
                       TABLE 1______________________________________Tread cap rubber composition             Compounding recipeCompounding ingredient             (parts by weight)______________________________________Styrene-butadiene rubber             100Carbon black ISAF *1             60ZnO               2.5Vulcanization accelerator *2             0.4Vulcanization accelerator *3             0.8Antioxidant *4    1.0Sulfur            1.5______________________________________ *1 DBP absorption: 116 ml/100 g, N.sub.2 SA value: 117 m.sup.2 /g, IA value: 121 mg/g *2 Soxinol CZ (trade name of Ncyclohexyl-2-benzothiazyl sulfeneamide, Sumitomo Chemicals Co., Ltd.) *3 Soxinol D (trade name of diphenylguanidine, Sumitomo Chemicals Co., Ltd.) *4 Santoflex 13 (trade name of N(1,3-dimethylbutyl)-Nphenyl-p-phenylenediamine, Mitsubishi Monsanto Chemicals Company) 
    
     
                                           TABLE 2__________________________________________________________________________Tread base rubber composition                            Compara-                                  Compara-                                        Compara-                                              Compara-                                                    Compara-                            tive  tive  tive  tive  tive          Example 1                Example 2                      Example 3                            Example 1                                  Example 2                                        Example 3                                              Example                                                    Example__________________________________________________________________________                                                    5Rubber in crown centerportionnatural rubber 100   100   100   100   100   100   100   100carbon black Li-HAF *1          40    40    40    42    38    32    37 *7 40ZnO            3.5   3.5   4.0   3.5   3.5   3.5   3.5   3.5vulcanization accelerator *2          1.0   1.0   1.0   1.0   1.5   1.2   1.0   1.1antioxidant *3 1.0   1.0   1.0   1.0   1.0   2.0   0.2 *8                                                    1.0sulfur         2.0   2.0   2.0   1.8   3.5   3.5   2.0   2.0Rubber in shoulder portionnatural rubber 100   100   100   100   100   100   100   100carbon black *4          40    37    32    32    32    32    40 *9 40 *9ZnO            3.5   3.5   3.5   3.5   3.5   3.5   3.5   3.5vulcanization accelerator *5          1.0   1.0   1.2   1.2   1.2   1.2   1.1   1.1antioxidant *6 0.8   0.8   0.8   0.8   0.8   0.8   1.0 *10                                                    1.0 *10sulfur         1.5   2.0   2.7   2.7   2.7   2.7   2.0   2.0__________________________________________________________________________ *1 DBP absorption: 104 ml/100 g, N.sub.2 SA value: 71 m.sup.2 /g, IA value: 68 mg/g *2 Soxinol CZ (trade name, Sumitomo Chemicals Co., Ltd.) *3 Nocrac 810NA (trade name of Nphenyl-Nisopropyl-p-phenylenediamine, Ohuchi Shinko Kagaku K.K.) *4 DBP absorption: 93 ml/100 g, N.sub.2 SA value: 131 m.sup.2 /g, IA value: 132 mg/g *5 Soxinol CZ (trade name, Sumitomo Chemicals Co., Ltd.) *6 Antigen RD (trade name of polymerized 2,2,4trimethyl-1,2-dihydroquinoline, Sumitomo Chemicals Co., Ltd.) *7 use of carbon black *4 *8 use of Antigen RD as an antioxidant *9 use of carbon black LiHAF *10 use of Nocrac 810NA as an antioxidant 
    
     As regards the above test tires, the measurement of the properties as shown in Table 3 and the tests of tire performances were carried out by the following methods. 
     The thus obtained results are shown in the following Table 3. 
     Storage modulus E&#39; and loss factor tan δ 
     E&#39; and tan δ were measured by using a viscoelastic spectrometer made by Iwamoto Seisakusho under conditions that the frequency was 50 Hz, the temperature was 50° C. and the gravity strain was 1%. 
     Modulus M 300   
     The modulus at 300% elongation was measured at room temperature according to a tensile test method of JIS K6301. 
     Heat generating durability 
     The durability was evaluated according to a drum test method of JIS D4230 and represented according to the following equation by an index on the basis that the running distance of the control was 100: ##EQU1## The larger the index value, the better the property. 
     Cut resistance 
     After the test tire was run an a rough road for about 2 weeks, the tread was peeled off to expose the belt and the number of cuts arriving at the belt was measured, from which the cut resistance was evaluated according to the following standards: 
     
         ______________________________________Cut number of not less than 30                        1Cut number of not less than 20 but less than 30                        2Cut number of not less than 10 but less than 20                        3Cut number of not less than 2 but less than 10                        4Cut number of less than 2    5______________________________________ 
    
     The larger the numerical value, the better the property. 
     Resistance to crack growth at belt end 
     In the above test tire evaluating the cut resistance, after the cut arrived at the tread base shoulder portion, the state of growing cracks up to the belt end was evaluated between the adjoining belt layers (L Bi  -L Bi+1 ) shown in FIG. 2 as follows. That is, when a distance of L Bi+1  -L Bi  in the radial direction of the tire is D, the length of crack from either L Bi+1  or L Bi  was quantitatively evaluated at the following five stages: 
     
         ______________________________________length of not less than 50%                      1length within a range of 25˜50%                      2length within a range of 10˜25%                      3length within a range of 0˜10%                      4zero                       5______________________________________ 
    
     The larger the value, the better the property. 
     Moreover, the stage 1 means that the durable life of the tire is substantially completed. 
     
                                           TABLE 3__________________________________________________________________________                                 Compar-                                      Compar-                                           Compar-                                                Compar-                                                     Compar-                                 ative                                      ative                                           ative                                                ative                                                     ative                  Example                       Example                            Example                                 example                                      example                                           example                                                example                                                     example                  1    2    3    1    2    3    4    5__________________________________________________________________________tread cap rubber (A)tan δ(A)         0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18E&#39;(A) (× 10.sup.7 dyn/cm.sup.2)                  12.0 12.0 12.0 12.0 12.0 12.0 12.0 12.0tread base rubberrubber (B) in crown center portiontan δ(B)         0.07 0.07 0.07 0.110                                      0.065                                           0.055                                                0.110                                                     0.07E&#39;(B) (× 10.sup.7 dyn/cm.sup.2)                  6.0  6.0  6.2  6.0  8.2  4.0  8.0  6.0M.sub.300 (B)/E&#39;(B) (× 10.sup.-7 kg/dyn)                  31.0 30.0 29.0 30.0 31.0 50.0 20.0 30.0rubber (C) in shoulder portiontan δ(C)         0.15 0.11 0.095                                 0.095                                      0.095                                           0.095                                                0.07 0.07E&#39;(C) (× 10.sup.7 dyn/cm.sup.2)                  9.5  8.0  7.2  7.2  7.2  7.2  6.0  6.0M.sub.300 (C)/E&#39;(C) (× 10.sup.-7 kg/dyn)                  16.0 20.0 24.0 24.0 24.0 24.0 30.0 30.0tanδ(C)-tan δ(B)                  0.08 0.04 0.025                                 -0.015                                      0.03 0.04 -0.04                                                     0E&#39;(C)-E&#39;(B) (× 10.sup.7 dyn/cm.sup.2)                  3.5  2.0  1.0  1.2  -1.0 3.2  -2.0 0M.sub.300 (B)/E&#39;(B)-M.sub.300 (C)/E&#39;(C) (× 10.sup.-7                  15.0yn)                       10.0 5.0  6.0  7.0  26.0 -10.0                                                     0Cut resistance         5    4    3˜4                                 3˜4                                      3    2    4    3Heat generating durability                  95   100  100  75   115  120  80   100Resistance to crack growth at belt end                  5    5    4    3    3˜4                                           2    1    3__________________________________________________________________________ 
    
     As seen from Table 3, when the relations among the rubber properties in the tread cap rubber, tread base rubber in the shoulder portion and tread base rubber in the crown center portion satisfy the conditions defined in the invention, the resistance to crack growth at belt end in the radial tire for construction vehicle is improved without damaging the cut resistance and the heat generating durability, and consequently the durable life of the tire is largely increased.