Pneumatic radial tires

Abstract: When polyester fiber cords having elongations under loads of 0.5 g/d and 2.25 g/d within particular ranges and a heat shrinkage (177°C, 30 minutes) within a particular range are used in a carcass ply of a pneumatic radial tire, the ride comfort tire uniformity and tire appearance can simultaneously be improved.

Description: This invention relates to a pneumatic radial tire having considerably improved ride comfort, uniformity and side appearance. Heretofore, organic fiber cords such as rayon fiber cords and nylon fiber cords have favorably been used in a carcass of a pneumatic radial tire for passenger cars. In case of using the rayon fiber cord as a reinforcement of the pneumatic tire, the strength is low as compared with that of the nylon fiber cord, so that if it is intended to manufacture tires based on tire design for safe strength, the amount of the rayon fiber cords used increases to largely increase tire weight and hence the tire performances such as rolling resistance and the like are undesirably degraded. Furthermore, since a raw material for the rayon fiber cord is wood pulp, the use of the rayon fiber cord becomes limited from a viewpoint of problems on resource protection, bad odor produced in the production step, waste treatment and the like. On the other hand, the nylon fiber cords have a high strength and an excellent fatigue resistance, so that they are used with some success as a reinforcing cord for the tire. However, satisfactory results are not always obtained in pneumatic radial tires for passenger cars. Because the nylon fiber cords are low in the modulus of elasticity and large in the creep property, when these cords are used as a reinforcement for the tire, the dimensional stability of the tire is poor and the flat spot is undesirably large. Lately, polyester fiber cords having a high strength and an excellent dimensional stability are used as a reinforcement for pneumatic radial tires in passenger cars instead of the above rayon and nylon fiber cords. There are proposed many techniques for improving strength, elasticity, shrinkage, heat resistance or fatigue resistance of the polyester fiber cord. Recently, the tendency for high-grade cars, high user needs and high quality becomes severe and it is particularly required to improve ride comfortability of high-graded cars, reduce the amplitude of vibration or deflection generated through nonuniformity of the tire (breakage of true circle or the like) and improve the unevenness level of sidewall portions showing a finer tire appearance. That is, the above three requirements should be satisfied simultaneously. In case of using the conventionally known polyester fiber cords, however, it is possible to satisfy one or two of the three requirements, but it is impossible to simultaneously satisfy the three requirements. It is, therefore, an object of the invention to provide a pneumatic radial tire having improved ride comfortability, small value of radial force variation (hereinafter abbreviated as RFV) indicating a high body sensitivity as a standard of tire uniformity and good appearance without producing sidewall undulation by using the polyester fiber cords as a carcass ply for the pneumatic radial tire. In order to solve the aforementioned problems, the inventors have made various studies with respect to polyester polymer itself, primary, secondary and high-ordered structure of polyester fiber, an interreation between polyester fiber cord and adhesion treatment, as well as into relationsip of properties of the polyester fiber cord to the above three requirements, and found clearly that each of the three requirements to closely be related to certain properties of the polyester fiber cord as a tire reinforcement, and as a result the invention has been accomplished. According to the invention, there is the provision of a pneumatic radial tire comprising a tread, a belt arranged inside the tread and a radial carcass comprised of at least one carcass ply, said carcass ply comprising polyester fiber cords each characterized by having an elongation of not less than 1.2% under a load of 0.5 g/d, an elongation of not more than 6.0% under a load of 2.25 g/d and a dry heat shrinkage of not more than 2.0% under an initial load of 50 g at 177°C for 30 minutes. The invention will be described with reference to the accompanying drawing, wherein: Fig. 1 is a flow diagram showing an adhesive treatment of polyester fiber cords according to the invention. The polyester fiber cords used as a carcass ply in the pneumatic radial tire according to the invention can be produced in the following manner. The polyester used in the invention is synthesized by reacting an aromatic dicarboxylic acid such as terephthalic acid, isophthalic acid, napthalene 2,6-dicarboxylic acid, sodium 3,5-dicarboxybenzene sulfonate and the like, an aliphatic dicarboxylic acid such as adipic acid, sebacic acid and the like, or an ester thereof with a diol compound such as ethylene glycol, diethylene glycol, 1,2-propylene glycol, 1,4-butane diol, 1,6-hexane diol, neopentyl glycol, cyclohexane 1,4-dimethanol or the like and is preferably a copolymerized polyethylene terephthalate containing not less than 85 mol% of polyethylene terephthalate. Particularly, the polyester is substantially a polyethylene terephthalate. At first, raw fibers of the polyester (relative viscosity: 1.51) produced by solid phase polymerization at a yarn production step are rapidly cooled beneath a spinneret in a gas atmosphere of 10-60°C, in which a spinning rate and a draw ratio are selected from a range of 6000-7000 m/min and a range of 1.20-1.36, respectively. Then, the thus spun fibers are twisted to obtain a polyester fiber cord of a given twisted structure, which is then subjected to a treatment with an adhesive under the following conditions as shown in Fig. 1. That is, the cords 1 are passed through a tank of an adhesive 2, a drying zone 3, a heat setting zone 4, a normalizing zone 5 and an annealing zone 6 in this order, in which the drying is carried out at 170°C for 120-160 seconds, and the heat setting is carried out at 250-258°C under a cord tension of 0.30-0.80 g/d for 60-80 seconds and the normalizing is carried out at 250-258°C under a cord tension of 0.08-0.20 g/d for 60-80 seconds. After the pass through the normalizing zone, the cords have sufficient properties, but when they pass through the annealing zone, more preferable properties are obtained. In the annealing zone, the treating temperature is 140-170°C and the treating time is 150-600 seconds. Then, a pneumatic radial tire for passenger car according to the invention can be manufactured by applying the thus adhesive-treated polyester fiber cords to a carcass ply of the tire according to usual manner of tire building, vulcanization and post cure inflation (PCI). Moreover, the tire structure other than the carcass ply is the same as in the conventionally known radial tire for passenger car. As regards the ride comfort of the radial tire for passenger car, when the tire rides on protrusions or the like during the running, if the input power is transmitted to the car without mitigation, the shock is large and hence the ride comfort becomes poor. As a means for mitigating such an input power, there are proposed various known methods. In this connection, the invention aims at a contribution of the carcass ply material to the ride comfort. That is, the inventors have found that the properties, particularly modulus of elasticity of the carcass ply material largely acts to mitigate the input power and contributes to improve the ride comfort. The term "modulus of elasticity of the carcass ply material" used herein is expressed by an elongation (degree of elongation) corresponding to tension applied to the cord when the tire actually rides over the protrusion under nominal internal pressure and load (internal pressure and load defined by Japan Automobile Tire Manufactures Association), which corresponds to a value represented by elongation under a load of 0.5 g/d in case of the radial tire for passenger car. According to the invention, the elongation under a load of 0.5 g/d is not less than 1.2%, preferably not less than 1.4%. When the elongation under a load of 0.5 g/d is less than 1.2%, the mitigation of the input power can not sufficiently be attained. On the other hand, the upper limit of the elongation under a load of 0.5 g/d is 3.5% because it is technically impossible to hold the elongation under a load of 0.5 g/d at a level of more than 3.5% when the elongation under a load of 2.25 g/d is maintained at a level of not more than 6.0%, As to the above apparently conflicting phenomena , the inventors have aimed at improving the sidewall appearance (reducing undulation) and made further studies and as a result, the following knowledge was obtained. In the pneumatic radial tire, since the joint of the carcass ply is generally a lap joint in the usual tire-building method, the undulation of the outer surface of the sidewall portion may clearly be distinguished at the joint portion even from exterior. Such an amount of undulation can be reduced to a certain extent by decreasing the size of overlap of the carcass ply, but if the overlap is too small, the joint may be separated in the course from tire-building to vulcanization according to the usual manner, which results in aproblem of safety. Therefore, the overlap should be maintained to an extent of avoiding separation of the joint or and avoiding distinguishable undulation. In this connection, the inventors have made studies and found that the undulation amount in the sidewall portion is closely related to the elongation of the carcass ply material in the tire but must be achieved without damaging the ride comfort of the tire. That is, when the elongation under a load of 2.25 g/d in the carcass ply cord exceeds 6.0%, the undulation of the outer surface of the sidewall portion considerably increases, which is unfavorable in practical use. Therefore, in order to maintain the undulation of the sidewall portion at a low level, the elongation under a load of 2.25 g/d is not more than 6.0%, preferably not more than 5.8%, more particularly not more than 5.3%. As the RFV value showing the non uniformity of the tire becomes small, the better the uniformity of the tire. The increase of RFV value is mainly caused by break away from a true circle in the tire, irregularity of tire weight on the circumference of the tire and the like. If the tire uniformity is poor, vibration or deflection is caused during the running of the tire. The inventors have also made studies with respect to a relation between RFV value and a property of the carcass ply cord and found that the RFV value is largely dependent upon the heat shrinkage of the carcass ply cord in the vulcanization of the tire. That is, when the heat shrinkage of the carcass ply cord immediately after the vulcanization is large, the irregularity of shrinkage in the carcass ply cords arranged at an angle of 90° with respect to the equatorial plane of the tire increases and hence the true circle of the tire is broken to increase the RFV value. In the invention, the heat shrinkage of the cord at 177°C under an initial load of 50 g for 30 minutes is not more than 2.0% for preventing the above irregularity of shrinkage in the carcass ply cords. Preferably, the heat shrinkage is not more than 1.8%, more particularly not more than 1.1%, which can rapidly reduce the RFV value. In the conventional polyester fiber cord, there is observed a similar relation to the above relation of the properties of the carcass ply cord to each performance required in the tire of this type as defined in the invention. However, it is impossible to simultaneously satisfy the above three performances required in the tire when the conventional polyester fiber cord is used as the carcass ply cord. For instance, when the conventional polyester fiber cords are produced so as to have an elongation at 0.5 g/d of not less than 1.2% by spinning, drawing and heat-treating with an adhesive and then used as a carcass ply cord to manufacture a tire in usual manner, the elongation at 2.25 g/d as a carcass ply cord exceeds 6,0%. In other words, in the tire manufactured by using the conventional polyester fiber cords, the ride comfort is improved, but the undulation in the joint portion of the carcass ply in terms of sidewall appearance is undesirably increased. On the other hand, when the conventional polyester fiber cords are produced so as to have an elongation at 2.25 g/d of not more than 6.0%, the elongation at 0.5 g/d is less than 1.2% and also the heat shrinkage exceeds 2.0%. Further, when the conventional polyester fiber cords are produced so as to have a heat shrinkage of not more than 2.0%, it is impossible to provide the elongation at 2.25 g/d of not more than 6.0%. As mentioned above, the aforementioned three properties required as a carcass ply cord are conflicting with each other and are not simultaneously satisfied in the conventional polyester fiber cords. That is, these properties are first established in the invention. The following examples are given in illustration of the invention and are not intended as limitations thereof. In these examples, the measurement of each property is as follows. (1) Elongation of cord The elongations (%) under loads of 0.5 g/d and 2.25 g/d were measured by taking out the carcass ply cord from the tire to remove excessive rubber adhered to the cord with scissors and then stretching the cord at room temperature (25±2°C) by means of a constant-speed stretching type tension tester according to a test method of JIS L1017. Moreover, the denier of fiber before twisting was used as a denier number because the change of denier caused by the change of cord length through twisting, adhesion treatment, vulcanization and the like is to be ignored. For example, the cord of 1000 d/2 was indicated as 2000 denier. (2) Heat shrinkage of cord An initial load corresponding to 1/60 of total denier (e.g. 50 g in case of 3000 deniers) was applied to the carcass ply cord removed in the same manner as in the above item (1), which was placed in an oven previously held at 177°C and left to stand therein for 30 minutes and thereafter the length of the cord in the oven was measured. Then, the heat shrinkage was calculated according to the following equation. Heat shrinkage = {(cord length before heat shrink - cord length after heat shrink)/cord length before heat shrink} x 100 (%) (3) Ride comfort (vibration test when the tire rides over protrusion) After an iron protrusion (upper side: 19 mm, lower side: 38 mm, height: 9.5 mm) was fixed onto an outer surface of a drum having an outer diameter of 2000 mm, a test tire subjected to an internal pressure of 1.70 kgf/cm² was run at a speed of 80 km/h under a load of 400 kg for 20 minutes Then, the internal pressure was readjusted to 1.70 kgf/cm² under no load. Then, the tire was again run on the drum at a speed of 20 km/h under a load of 400 kg and the speed was increased at a rate of 5 km/h every one hour, during which an average waveform of axial load fluctuation at the fixed shaft of the tire riding over the protrusion was measured at each speed to calculate the peak-to-peak(p-p)value. The direction of the axial load fluctuation at the fixed shaft was the running direction of the car (front and back spring), and so-called longitudinal stiffness became maximum at a speed region of 30-40 km/h . Therefore, p-p value (kg) was calculated at this speed region. The ride comfortability was determined according to the following equation and represented by an index value on the basis that control tire of Comparative Example 1 was 100. Index of ride comfort = 100 + 100 x {(p-p value of control tire - p-p value of test tire)/p-p value of control tire} The larger the index value, the better the ride comfort. (4) Tire appearance (sidewall undulation) The undulation of the tire sidewall (at a position corresponding to maximum width in radial direction) was measured over a full periphery in the circumferential direction of the tire by means of a surface roughness meter. In this case, the tire was subjected to an internal pressure of 2.0 kgf/cm² in a room of 25 ± 2°C, left to stand for 24 hours and then the internal pressure was readjusted to 2.0 kgf/cm². When the measured value exceeded 0.5 mm, the sidewall undulation was visually observed, which equated to a problem of tire appearance. The test tire had a tire size of 165 SR 13 and an internal service pressure (nominal pressure) of 1.7 kgf/cm². However, the measured pressure was 2.0 kg/cm² considering the change of temperature and the like. (5) Tire uniformity (RFV value) It was measured according to a test method for uniformity of automobile tire described in JASO C607. In the examples and comparative examples, adhesive-treated polyester fiber cords were used in a carcass ply of a pneumatic radial tire for passenger car (tire size: 165 SR 13). As to such cords, the spinning and drawing conditions are shown in Table 1, and the adhesive treating conditions are shown in Table 2. Further, the result evaluated on the performances of the test tire are shown in Table 3. Condition A Condition B Condition C Condition D Spinning rate (m/min) 2000 4000 6000 6700 Draw ratio 2.40 1.80 1.36 1.24 Condition 1 Condition 2 Condition 3 Condition 4 Condition 5 Drying zone temperature (°C) 170 170 170 170 170 exposure time (second) 120 120 160 120 120 Heat setting zone temperature (°C) 250 258 258 258 258 exposure time (second) 60 60 80 60 60 Normalizing zone temperature (°C) 250 258 258 258 258 exposure time (second) 60 60 80 60 60 Annealing zone temperature (°C) - - - 150 150 exposure time (second) - - - 300 600 As seen from Table 3, the adhesive-treated polyester fiber cords satisfying all features of the invention are a combination of condition C in Table 1 with condition 2 in Table 2, a combination of condition D in Table 1 with condition 3 in Table 2, a combination of condition C in Table 1 with condition 4 in Table 2 and a combination of condition C in Table 1 with condition 5 in Table 2, respectively. That is, the conventional adhesive-treated polyester fiber cords of the comparative examples are produced by using a relatively mild spinning rate of 2000-4000 m/min as shown in conditions A and B of Table 1 as well as a low temperature of 240-250°C as shown in condition 1 of Table 2. On the other hand, the adhesive-treated polyester fiber cords according to the invention are produced by using a super-high spinning rate of 6000-6700 m/min as shown in conditions C and D of Table 1 as well as a high temperature near to the melting point of the conventional polyester, e.g. 255-260°C as shown in conditions 2 and 3 of Table 2. Since the super-high speed spun fibers are microscopically large in crystal size and high in melting point, it is technically possible that they be treated at a higher temperature for a longer time as compared with conventional fibers, which is particularly effective for improving heat shrinkage and the like. Further, as shown in conditions 4 and 5 of Table 2, the heat treatment is carried out in the annealing zone after the condition 3. In this case, the cord tension is selected within a range of 0,03-0,20 g/d. The cord tensions of conditions 4 and 5 are 0,10 g/d and 0,09 g/d, respectively. Thus, all of the properties of the cord required in the invention are more improved. When Examples 1 and 2 are compared with Comparative Examples 1-3, the elongation α of the cord under a load of 0.5 g/d is remarkably related to the ride comfort as seen from Table 3. That is, when the elongation α is less than 1,2%, the ride comfort is poor, while when it is not less than 1,2%, the ride comfort is improved. Moreover, Comparative Example 3 shows a good ride comfort, but the elongation β of the cord under a load of 2.25 g/d becomes too large and hence the sidewall undulation undesirably increases. On the other hand, in Examples 1 and 2, the sidewall undulation and RFV value are low, so that the three requirements are simultaneously improved. When Examples 3 and 4 are compared with Comparative Examples 4-6, the elongation β is remarkably related to the sidewall undulation. As previously mentioned, when the sidewall undulation exceeds 0.5 mm, the undulation is visually distinguishable, so that the sidewall undulation is practically required to be less than 0.5 mm. From these examples, it is apparent that when the elongation β exceeds 6.0%, the sidewall undulation exceeds 0.5 mm. In Comparative Example 6 using the conventional adhesive-treated polyester fiber cords, only the sidewall undulation can be suppressed, but the ride comfort and tire uniformity are degraded. In Examples 3 and 4, there is no degradation of tire performances caused as in Comparative Example 6. Examples 5-7 and Comparative Examples 7-10 show a relation between the heat shrinkage of the cord and the RFV value of the tire. In Comparative Examples 7-9 having the heat shrinkage of more than 2.0%, the RFV value as a tire uniformity exceeds 5 kg and there is danger of vibration or deflection during the running of the tire. On the other hand, when the heat shrinkage is not more than 2.0%, there is a tendency of rapidly lowering the RFV value. Particularly, Example 7 shows the RFV value of about 2 kg, which is not observed in the conventional cord, and maintains a very good level of ride comfort and sidewall undulation. When the adhesive-treated polyester fiber cords are passed through the annealing zone after the normalizing zone as shown in Examples 8 and 9, the effect of improving the tire performances is very high, and particularly it is possible to control the RFV value (tire uniformity) to 2.3-2.6 kg. As mentioned above, in the pneumatic radial tire for passenger car according to the invention, the ride comfort tire uniformity (RFV value) and tire appearance (sidewall undulation) can simultaneously be improved by using particular polyester fiber cords satisfying all requirements as a carcass ply cord. Throughout the present specification, the term "g/d" shall be understood to mean "grams per denier".

Claims: A pneumatic radial tire comprising a tread, a belt arranged inside the tread and a radial carcass comprised of at least one carcass ply, said carcass ply comprising polyester fiber cords (1) each characterized in that the cords (1) have an elongation of not less than 1,2% under a load of 0,5 g/d, an elongation of not more than 6,0% under a load of 2,25 g/d and a heat shrinkage of not more than 2,0% under an initial load of 50 g at 177°C for 30 minutes. The pneumatic radial tire according to claim 1, wherein said elongation under a load of 0,5 g/d is not less than 1,4%, and said elongation under a load of 2,25 g/d is not more than 5,8%, and said heat shrinkage is not more than 1,8%.

Inventors: TAMURA YASUYUKI, TAMURA, YASUYUKI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1992
Main classification: B60C9
IPC codes: B60C9