PNEUMATIC RADIAL TIRE

Abstract: A pneumatic radial tire wherein a carcass comprising plies of cords extending in a toroidal fashion between a pair of bead portions and radially arranged constitutes a framework of said tire, wherein a belt is disposed radially outwardly of said carcass, and wherein a belt reinforcing layer comprising organic fiber cords is provided radially outwardly of said belt, said pneumatic tire being characterized in that said organic fiber cords constituting said belt reinforcing layer are of folding-twist construction and have a twisting multiplier N t set between 0.06 and 0.30 to thereby improve its high speed durability, said twist coefficient being defined by the expression (1): N t = T x (0.139 x D/ρ) 1/2 x 10⁻³ , where T is the number of twists (per 10 cm), D is the total denier, ρ is the specific gravity (g/cm³).

Description: This invention relates to pneumatic radial tires for use in passenger cars or trucks and buses, and more particularly aims to improve high-speed durability by improving a belt reinforcing layer. In this type of pneumatic radial tire, it is common that at least two belt layers each being a composite of steel cords arranged at a given inclination angle with respect to an equatorial direction of the tire and coated with rubber are superimposed on a carcass to form a structure providing steering performance, wear resistance and the like in accordance with vehicle performance requirements. Recently, the running speed of vehicles has become faster with development of expressway networks as well as higher output and performances of the vehicles, and it is strongly demanded to improve the high-speed durability of tires. For this end, there have been proposed various attempts for improving the high-speed durability of tires. That is, the pushing-out of the tread or shoulder in the tire through centrifugal force acting to the tread portion of the tire during high-speed running, and standing waves are serious factors affecting tires. In order to suppress such problems, it is known to add a belt reinforcing layer such as a so-called cap, layer or the like to the outside of the belt layer in the radial direction of the tire. However, it is difficult to completely compensate a lack of rigidity by the simple addition of the belt reinforcing layer, and consequently the expansion of the tire in the radial direction during high-speed running can not be sufficiently suppressed. For this end, it has been attempted to improve the rigidity by increasing the number of the belt reinforcing layers. However, the increase of the belt reinforcing layer number increases the rubber thickness to cause heat build-up and to increase the tire weight, so that satisfactory high-speed durability can not be obtained. Furthermore, when using plural belt reinforcing layers, the dimensional stability of the tire during heat build-up is degraded and also the heat build-up becomes large, so that there is caused a problem of reduced steering stability during running for a long time. On the other hand, it has been attempted to improve the rigidity by increasing an end count of cords embedded in rubber for the belt reinforcing layer. However, as the end count increases, the number of cords occupied per unit width in the radial section of the tire or a cord occupying ratio becomes large and a ratio or occupied rubber becomes relatively small, so that the adhesion property of the cord lowers to induce separation failure and hence there may be caused a premature failure of the tire. As compared with these attempts, it is effective to enhance the modulus of elasticity of the cord constituting the belt reinforcing layer. Specifically, it is known to use a high modulus material such as aromatic polyamide (aramid) fiber, polyethylene terephthalate (PET) fiber or the like. As another means, JP-B-59-1601 proposes a method wherein polyamide fibers, polyester fibers or rayon fibers, particularly polyamide fibers, are used to form a cord of single twisting structure having a twisting number of 10-40 turns/10 cm and then the resulting cords are used at an end count of 60-300 cords/10 cm in the belt reinforcing layer. However, when cords of aramid fiber or the like having a low elongation are used as the cord for the belt reinforcing layer, there may be caused tire deformation or cord breakage due to the expansion of the tire during the tire build-up. On the other hand, when the tire expansion during the tire build-up is made small in order to prevent such disadvantages, since the thermal shrinkage of the aramid fiber cord is substantially zero, the cords undesirably move in a zigzag direction during the vulcanization of the tire. When using polyester fibers such as PET fiber or the like, since the modulus of elasticity is higher than that of the polyamide fiber, the size growth of the tire is suppressed, while if used under high-speed running, the heat-resistant adhesion property to other parts of the tire is poor, and as a result separation failure is easily produced. Moreover, when a single twisted cord is used, since there is a residual torque in the cord itself, the operability is poor in the weaving step and the dipping step with an adhesive, which causes problems during the tire production. Furthermore, the high-speed durability of the tire can not be said to be satisfactory even if the twisting number and end count of the cord are defined to given values. Attention is also drawn to the disclosure of EP-A-0331501, which corresponds to the preamble of claim 1. It is an object of the present invention to overcome the aforementioned problems of the conventional pneumatic radial tires and to provide a pneumatic radial tire having an excellent high-speed durability without adversely affecting other characteristics thereof. The present inventor has made various studies with respect to the high-speed durability of the tire in order to attain various performances required in accordance with the high speed and high performance requirements of the vehicle to which the tire is to be applied, by making the twisting coefficient of the cord in the belt reinforcing layer very small. The present invention provides a pneumatic radial tire comprising a carcass made of a ply of radially arranged cords toroidally extending between a pair of bead portions, a belt arranged outside the carcass in a radial direction of the tire, and a belt reinforcing layer of organic fiber cords arranged outside the belt in the radial direction of the tire, wherein the organic fiber cord constituting the belt reinforcing layer has a strand construction and a twisting coefficient N t of 0.06-0.30 defined according to the following equation: N t = T x (0.139 x (D/2)/ρ) 1/2 x 10 -3 where T: twisting number (turns/10 cm), D: total denier, ρ: specific gravity (g/cm 3 ); characterized in that the cord has a residual tension F w per unit width of the belt reinforcing layer of 80-400 kgf defined according to the following equation: F w = F x E x number of members where F: residual tension per one cord in a belt edge covering layer immediately after taking out from the tire, E: end count of cords in the belt edge covering layer per 5 cm in a widthwise direction of the tire. Also, the belt reinforcing layer preferably has a cord occupying ratio of not more than 65%, and the organic fiber cord preferably has a total denier of 1500-4000. Moreover, the cord occupying ratio is defined by the following equation (3) when the number of cords per unit width (5 cm) or end count is C (cords/5 cm) and the diameter of the cord is D (mm) in the belt reinforcing layer cut in a direction perpendicular to an axial line of the cord embedded in the tire: Cord occupying ratio (%) = [(D x C)/50] x 100 The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a left-side half section view of a pneumatic radial tire according to the invention in which a belt reinforcing layer comprises a single layer; Fig. 2 is a left-side half section view of a pneumatic radial tire according to the invention in which a belt reinforcing layer comprises two layers; Fig. 3 is a diagrammatic section view of another pneumatic radial tire according to the invention; Fig. 4 comprises schematic views of other belt reinforcing layers of tires according to the invention; Fig. 5 is a schematic section view of a tire showing a portion for taking a sample for the measurement of residual tension of cord in the belt reinforcing layer; Fig. 6 is an enlarged view of the sample taking portion in Fig. 5; and Fig. 7 is a schematic view showing a state of setting the sample immediately after the taking from the tire to an autograph for the measurement of residual tension per cord in the belt reinforcing layer. The belt reinforcing layer of the tire according to the invention serves to reinforce the belt layer and may take various arrangements and structures in accordance with the performances required in the tire. For example, there is known an arrangement shown in Fig. 1. That is, a pneumatic radial tire 1 shown in Fig. 1 is mainly composed of a tread portion 2, a bead portion 3, a carcass ply 4, a belt layer 5 of steel cords of two-layer structure, and a belt reinforcing layer 6 of single layer structure. The belt reinforcing layer 6 is arranged on the belt layer 5 outward in the radial direction of the tire and at a width corresponding to, for example, 10-40% of the width of the belt layer 5 so that a side edge portion of the belt reinforcing layer extends to a position of 0-20 mm ranging from the side edge portion of the belt layer 5 toward the tire shoulder. Moreover, the belt reinforcing layer is formed by spirally winding a narrow-width rubberized strip containing a plurality of organic fiber cords therein on the belt layer at least one or more times so that these cords are substantially in parallel with the circumferential direction of the tire. Although the belt reinforcing layer of the illustrated embodiment comprises a single layer, it is possible to have a two-layer structure as shown in Fig. 2. Furthermore, as shown in Fig. 3, the rigidity of the tread portion can be more enhanced by arranging a belt reinforcing layer 7 so as to cover the full width of the belt layer 5 in addition to the belt reinforcing layer 6 covering the side edge of the belt layer 5. Moreover, other arrangements of the belt reinforcing layer are shown in Figs. 4(a)-(c). As the organic fiber cord constituting the belt reinforcing layer, use may be made of organic fibers of aliphatic polyamide, aromatic polyamide, polyethylene terephthalate, polyvinyl alcohol and the like. Among them, the aliphatic polyamide fibers are advantageously used. As the aliphatic polyamide fiber, mention may be made of fibers of 6-nylon, 6T-nylon, 6,6-nylon, 4,6-nylon, a copolymer obtained by combining units thereof, and a mixture thereof. Among them, the aliphatic polyamide fibers having a ratio of 6,6-nylon unit of 4,6-nylon unit of not less than 80% by weight are preferable. Furthermore, these polyamide fibers can usually be used by compounding with a stabilizing agent composed of copper salt and antioxidant in order to provide the durability to heat, light, and oxygen. When such preferable polyamide fibers are used as the cord for the belt reinforcing layer while satisfying the requirements according to the invention, there are not caused tire deformation, cord breakage, and zigzag movement as previously mentioned, which is entirely different from the conventional fiber cords, and also the heat-resistant adhesion property to the other parts of the tire becomes good. Further, the organic fiber cords preferably have a total denier (D) of 1500-4000, particularly 1500-2000. When the total denier exceeds 4000, the cord may become too thick and the heat build-up becomes large, and the amount of rubber between the cords increases to increase the rubber thickness and weight of the belt reinforcing layer. When the total denier is less than 1500, even if the number of the cords is increased as far as possible, the total modulus of elasticity in the circumferential direction of the tire becomes small and hence sufficient rigidity may not be obtained. There are used various twisting structures in the organic fiber cord. The twisting structure according to the invention is preferably a strand twisting structure in which cable twist and ply twist are opposite to each other, i.e. cable twist Z/ply twist S or cable twist S/ply twist Z. On the other hand, the single twisting structure is not preferable because there is a residual torque and the operability in the weaving and adhesive dipping steps is poor. An important feature for improving the high-speed durability of the tire according to the invention is the twisting coefficient of the organic fiber cord for the belt reinforcing layer. The twisting coefficient (Nt) is defined by the aforementioned equation (1) and is within a range of 0.06≤N t ≦0.30, preferably 0.15≦N t ≦0.28. This twisting coefficient enhances the modulus of elasticity of the cord. When it exceeds 0.30, sufficiently high modulus of elasticity is not obtained, while when it is less than 0.06, the fatigue resistance is undesirably degraded to easily cause cord breakage. As mentioned above, the twisting coefficient is a very small value of not more than 0.30 to make the modulus of elasticity of the organic fiber cord high and hence the belt reinforcing layer Sufficiently develops an effect of suppressing the expansion of the tire in the radial direction during high-speed running or a so-called hoop effect, whereby the durability during high-speed running and further the steering stability can largely be improved. In the invention, it is found that the residual tension in the organic fiber cord for the belt reinforcing layer in a new tire per unit width of the belt reinforcing layer has a large influence upon tire deformation in the tire production, and upon the high-speed durability of the tire. That is, the residual tension F w of the organic fiber cord in the belt reinforcing layer per unit width of the belt reinforcing layer is defined by the aforementioned equation (2). As the residual tension F w per cord, when the belt reinforcing layer is a two-layer structure, a measured value in an outer layer in the radial direction of the tire, a measured value in an inner layer in the radial direction of the tire, or an average value of these values is used. In the invention, the residual tension per unit width F w is 80 kgf≦F w ≦400 kgf, preferably 100 kgf≦F w ≦300 kgf. The residual tension per unit width is calculated from parameters shown in the equation (2), but the residual tension of 80-400 kgf is achieved by selecting the properties or the cord, twisting number, condition of higher tension at adhesive dipping treatment as compared with the conventional technique, and higher expansion ratio of the belt reinforcing layer before and after the tire vulcanization as compared with the conventional technique. When the residual tension per unit width is within the above range, the size growth of the tire during high-speed running is small and the generating speed of standing waves is suppressed, which gives a good effect to the high-speed durability of the tire. When the value exceeds 400 kgf, tire deformation or deformation of the belt layer is apt to be caused in the tire production and also tire uniformity is poor and flat spots may occur. When the value is less than 80 kgf, the size growth of the tire becomes larger and satisfactory high-speed durability is not obtained. The cord occupying ratio defined by the above equation (3) is limited to not more than 65%, which contributes to the improvement of the high-speed durability. That is, when the cord occupying ratio exceeds 65%, the number of cords per unit width may be too large and hence the amount of rubber for ensuring the adhesion property of the cord or between the belt reinforcing layers becomes relatively less, so that separation failure may be caused and it is difficult to ensure the adhesion property. The invention will be further described with reference to the following examples. Example I Tires manufactured according to Table 1 are subjected to a test for high-speed durability. In Table 1 are shown tire size, structure of belt reinforcing layer and various parameters of cord in the belt reinforcing layer of the pneumatic radial tire of each example. Moreover, the twisting structure of the cord is a strand twisting structure having a twisting direction shown in Table 1. The twisting coefficient N t is calculated according to the above equation (1): N t = T x (0.139 x (D/2)/ρ) 1/2 x 10 -3 wherein T: twisting number (turns/10 cm), D: total denier and ρ: specific gravity (g/cm 3 ). Furthermore, the residual tension F w per unit width is calculated according to the above equation (2): F w = F x E x number of members wherein F: residual tension per one cord in a belt edge covering layer immediately after taking out from the tire E: end count of cords in the belt edge covering layer per 5 cm in a widthwise direction of the tire. The residual tension F w per cord in the belt reinforcing layer is measured as follows. First, rubber in a shoulder portion of an unused new tire is peeled off so as to expose the cords in the belt reinforcing layer (circled portion shown by dotted lines in Fig. 5 and a shadowed portion 8 in Fig. 6 showing an enlarged view of the above vicinity). The tire to be tested is a round tire containing no cut pattern on its circumference, because the residual tension changes in the cut tire or tire section. In order to conduct the accurate measurement, the peeling operation should be conducted so as not to leave rubber on the cords in the belt reinforcing layer as far as possible and not to damage the cords. As to 10 cords in total (cords 9 blackened in Fig. 6), a length of 300 mm is accurately measured on the tire and both ends corresponding to such a length are marked. Then, the cords are cut at a position of not less than 200 mm apart from each marked point. Thereafter, each of the cords is taken out to measure the residual tension. In this case, the taken cord rapidly contracts due to the release of residual tension, or if it is left to stand as it is, the cord further contracts due to the influence of humidity in air, so that each sample 10 of the cords immediately after the taking-out from the tire (e.g. within 5 minutes) is set onto chucks 12 having a chuck distance of 450 mm in an autograph (S-500) made by Shimazu Seisakusho as shown in Fig. 7 and showing a setting state of a sample immediately after the taking-out from the tire. Then, the contracted cord sample 10 is pulled at a rate of 10 mm/min until the distance between marked points 11 returns to an original length of 300 mm, during which a tension is read. An average value of the measured tensions on 10 cords is rendered into residual tension F w per cord in the belt reinforcing layer. For comparison, tires manufactured according to Table 2 are also evaluated in the same manner as described above. The tire size, structure of belt reinforcing layer and various parameters of the cord of the belt reinforcing layer in the tires of comparative examples are shown in Table 2. Moreover, the twisting structure of the cord is a strand twisting structure having a twisting direction shown in Table 2. The calculation of twisting coefficient N t and residual tension per unit width F w is carried out in the same manner as in the examples. Moreover, the value of residual tension per cord F w is measured in the outer layer in the radial direction of the tire when the belt reinforcing layer is a two-layer structure (Example 4, Comparative Examples 2, 4 and 5). An evaluation test of the high-speed durability is conducted with respect to the tire of each example shown in Table 1 and the tire of each comparative example shown in Table 2. The evaluation for the high-speed durability of the tire is carried out by a step speed system according to a test method of United States standard FMVSS No. 109. That is, the evaluation is carried out by increasing the speed every 30 minutes until the occurrence of damage, during which the speed (km/h) at which the damage occurs and the time (minutes) elapsed up to such speed are measured to obtain results shown in Tables 1 and 2. As seen from Table 1 showing the examples and Table 2 showing the comparative examples, the high-speed durability is excellent in the pneumatic radial tires according to the invention. As shown in the results of the comparative examples in Table 2, when the values of the twisting number and end count of the cords in the belt reinforcing layer are the same as in the examples and well-known, if the cord does not satisfy the twisting coefficient and/or residual tension per unit width as defined in the invention, the tire after the vulcanization is deformed and can not be put into practical use, or the high-speed durability of the tire is poor. Example II A pneumatic radial tire used in Example II has a tubeless structure having a tire size of 205/60R15, in which the structures of the carcass and the belt layer are as follows: Carcass: The material of the carcass is polyethylene terephthalate (PET), and the structure and number of the cord are 1000 denier/2 and 2 plies, respectively. Belt layer: two separated layer structure of steel cords and added with a belt reinforcing layer having a given structure. The test results of examples and a comparative example obtained by using the above pneumatic radial tire are shown in Tables 3 and 4. Moreover, symbol in parenthesis in column of "Structure of belt reinforcing layer" in Tables 3 and 4 corresponds to the structure of the belt reinforcing layer shown in Fig. 4 showing an outline view of the belt reinforcing layer. Example 1 Example 2 Example 3 Example 4 Structure of belt reinforcing layer (a) (b) (c) (b) cap number 1 1 1 1 layer number 2 1 none 1 Material of belt reinforcing layer 66-nylon 66-nylon 66-nylon PET cord structure 840d/2 1260d/2 1890d/2 1500d/2 total denier of cord 1680 2520 3780 3000 Twisting coefficient (N t ) 0.07 0.23 0.25 0.18 Cord occupying ratio (%) 60 50 48 27 Steering Index 115 120 110 110 High-speed durability (km/h) 270 270 260 260 Damage form blowout of shoulder portion blowout of shoulder portion blowout of shoulder portion blowout of shoulder portion Comparative Example 1 Structure of belt reinforcing layer (b) cap number 1 layer number 1 Material of belt reinforcing layer 66-nylon cord structure 1260d/2 total denier of cord 2520 Twisting coefficient (N t ) 0.68 Cord occupying ratio (%) 50 Steering Index 100 High-speed durability (km/h) 240 Damage form separation at shoulder portion The test methods for the steering index and high-speed durability described in Tables 3 and 4 are as follows. The steering stability is evaluated by a feeling test and represented by an index on the basis that a control tire is 100. The larger the index value, the better the steering stability. The test for the high-speed durability is carried out by increasing the speed from 120 km/h at a rate of 10 km/h every 30 minutes until the occurrence of damage. In Tables 3 and 4 are shown the speed at which the damage occurred and the form of damage. In all the examples and the comparative example, the belt structure of the belt portion is comprised of two belt layers 5 as shown in Figs. 3 and 4, and "cap number" in the column of "Structure of belt reinforcing layer" of Tables 3 and 4 is a layer number of the belt reinforcing layer (cap) 7 covering substantially the full width of the belt layer 5 as shown in Figs. 3 and 4, and "layer number" is a layer number of belt reinforcing layers (layer) 6 arranged in the vicinity of both end portions of the belt layer 5 as shown in Figs. 3 and 4 (the layer number of the layer arranged on both end portions of the belt layer 5 is same and shows a layer number of the layer at one end portion but does not show the total layer number at both end portions). As shown in Table 3, the tires of Examples 1-4 have a large steering index as compared with that of Comparative Examples 1 shown in Table 4, which shows a considerable improvement of the steering stability. Furthermore, the high-speed durability is largely improved in the tires of Examples 1-4 and also the damage to the tire is not a dangerous separation but is a blowout of the shoulder portion, which is entirely different from that of Comparative Examples 1. Particularly, Example 2 and Comparative Example 1 are the same other than in respect of the twisting coefficient N t . In Comparative Example 1, the twisting coefficient N t is 0.68 which is outside the range defined in the invention, so that the steering stability and high-speed durability are considerably degraded as compared with those of Example 2. The invention can effectively control the expansion of the tire in the radial direction during high-speed running and no problems such as poor processability of cord, tire deformation, cord breakage, zigzag movement and poor heat-resistant adhesion property occur, so that the high-speed durability can be improved without sacrificing various performances of the tire.

Claims: A pneumatic radial tire (1) comprising a carcass (4) made of a ply of radially arranged cords toroidally extending between a pair of bead portions (3), a belt (5) arranged outside the carcass in a radial direction of the tire, and a belt reinforcing layer (6) of organic fiber cords arranged outside the belt in the radial direction of the tire, wherein the organic fiber cord constituting the belt reinforcing layer (6) has a strand construction and a twisting coefficient N t of 0.06-0.30 defined according to the following equation: N t = T x (0.139 x (D/2)/ρ) 1/2 x 10 -3 where T: twisting number (turns/10 cm), D: total denier, ρ: specific gravity (g/cm 3 ); characterized in that the cord has a residual tension F w per unit width of the belt reinforcing layer of 80-400 kgf defined according to the following equation: F w = F x E x number of members where F: residual tension per one cord in a belt edge covering layer immediately after taking out from the tire, E: end count of cords in the belt edge covering layer per 5 cm in a widthwise direction of the tire. A pneumatic radial tire as claimed in claim 1, characterized in that the belt reinforcing layer (6) has a cord occupying ratio of not more than 65%. A pneumatic radial tire as claimed in claim 1 or 2, characterized in that the organic fiber cord has a total denier of 1500-4000. A pneumatic radial tire as claimed in any of claims 1 to 3, characterized in that 0.15≦N t ≦0.28. A pneumatic radial tire as claimed in any of claims 1 to 4, characterized in that 100 kgf≦F w ≦300 kgf.

Inventors: KODAMA TADASHI, KODAMA, TADASHI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1993
Main classification: B60C9
IPC codes: B60C9