Pneumatic tires

Abstract: A pneumatic tire has a tread portion comprised of a rubber composition obtained by compounding 40-80 parts by weight of a particular defined carbon black with 100 parts by weight of a rubber blend of 15-50 parts by weight of a high styrene rubber ingredient composed of an emulsion-polymerized styrene-butadiene copolymer having a bound styrene content of 30-50% by weight and the balance of a low styrene rubber ingredient composed of at least one conjugated diene polymer other than the above emulsion-polymerized styrene-butadiene copolymer, at least one of which polymers being styrene-butadiene copolymer. Such a tire has specified rubber properties as a vulcanizate, and has improved all-weather type running performance and excellent wear resistance.

Description: This invention relates to pneumatic tires, and more particularly to a pneumatic tire having high all-weather type running performances and excellent wear resistance, or a pneumatic tire applicable as a passenger car bias tire, a passenger car radial tire or a motorcycle tire and having satisfactory running performances for providing sufficient road gripping force on dry road surface in fine weather, wet road surface in rainy weather and so-called ice snow road surface covered with ice and/or snow as well as excellent wear resistance. Heretofore, there have been known various techniques for high running performance tires. For example, each of Japanese Patent laid open Nos. 55-60539 and 55-86827 discloses a tread rubber composition having an improved road gripping force on wet road surface by compounding a polymer having a high glass transition temperature for giving a large loss around room temperature, for example styrene-butadiene rubber having a high bound styrene content, or compounding large amounts of carbon black and oil in order to ensure a high road gripping force. Further, there are known tires running on ice and snow roads or so-called snow tires and studless tires. For example, Japanese Patent laid open No. 59-206209 discloses a pneumatic tire having an improved road gripping force on ice and snow road surfaces by using a tread rubber composition consisting essentially of a polymer having a low glass transition temperature, i.e. a rubber selected from polybutadiene rubber, polyisoprene rubber and natural rubber. Moreover, Japanese Patent laid open No. 62-62840 discloses a tire having high all-weather running performances by using a tread rubber composition compounded with an ester series low temperature softening agent. The tread rubber disclosed in the above Japanese Patent laid open Nos. 55-60539 and 55-86827 cures at a low temperature, so that when the tire is run on ice-snow road surface, sufficient friction force can not be given to the tire between the ground contact portion of the tire and the ground surface and hence the running is not easy and also the wear resistance is insufficient. On the other hand, the tire disclosed in the above Japanese Patent laid open No. 59-206209 exhibits satisfactory performances on ice-snow road surface, but has drawbacks that the wear resistance is poor and also sufficiently high running performances are not obtained because the hardness of the tread rubber around room temperature is too low and the friction coefficient of the tread rubber itself is low. Moreover, the tire disclosed in the above Japanese Patent laid open No. 62-62840 is a tire having high all-weather running performances, but the wear resistance is not still sufficient. Attention is also drawn to the disclosure of US-A-4,398, 582, which relates to a pneumatic tire having improved low rolling resistance and high wet skid resistance by using a particular rubber composition in the ground-contact area of its tread. It is, therefore, an object of the present invention to solve the aforementioned problems of the conventional techniques and to provide a pneumatic tire having high all-weather type running performances and an improved wear resistance. The inventors have made studies for solving the above problems and found that the above object can be achieved by compounding a particular reinforcing carbon black with a rubber blend of high styrene rubber ingredient of emulsion-polymerized styrene-butadiene copolymer having a high bound styrene content and low styrene rubber ingredient of at least one conjugated diene polymer other than the above, and as a result the present invention has been accomplished. According to the invention, there is provided a pneumatic tire comprising a tread portion, a sidewall portion and a bead portion, characterized in that said tread portion is comprised of a rubber composition obtained by compounding 40∼80 parts by weight of carbon black having a specific surface area of nitrogen adsorption (N₂SA) of 146∼159 m²/g and satisfying the following relation: S ≦ 0.375 Dn + 0.69 (1) (wherein S is a distribution width of particle size as a unit of mµ and Dn is an arithmetic mean particle size as a unit of mµ measured by means of an electron microscope) with 100 parts by weight of a rubber blend of 15∼50 parts by weight of a high styrene rubber ingredient composed of an emulsion-polymerized styrene-butadiene copolymer having a bound styrene content of 30∼50% by weight and the balance of a low styrene rubber ingredient composed of at least one conjugated diene polymer other than the above emulsion-polymerized styrene-butadiene copolymer, at least one of which polymers being styrene-butadiene copolymer, and having an average bound styrene content of not less than 15% by weight but less than 30% by weight in a total of said polymers constituting said low styrene rubber ingredient, and said rubber composition has a modulus at -20°C and 100% elongation of not more than 40 kg/cm² and a loss factor (tan δ) at 30°C of not less than 0.3 as a vulcanizate. On the following description, reference will be made to the accompanying drawing, wherein: Fig. 1 is a graph showing a relation between agglomerate size and relative frequency in carbon black. In the rubber composition according to the invention, the high styrene rubber ingredient composed of the emulsion-polymerized styrene-butadiene copolymer having a bound styrene content of 30∼50% by weight (hereinafter referred to as high styrene SB emulsion copolymer) is compounded at an amount of 15∼50 parts by weight. The high styrene SB emulsion copolymer having the bound styrene content of the above range has an appropriately high glass transition temperature and makes the loss of tread rubber around room temperature large and provides a high gripping road force. However, when the compounding amount exceeds 50 parts by weight, sufficient wear resistance is not obtained and the ice-snow performances are insufficient because the SB emulsion copolymer is cured at a low temperature, while when the compounding amount is less than 15 parts by weight, the desired running performances on dry and wet road surfaces can not be obtained. The remaining low styrene rubber ingredient constituting the rubber blend is comprised of at least one conjugated polymer other than the above high styrene SB emulsion copolymer. The conjugated diene polymer constituting the low styrene rubber ingredient may be polymers obtained through emulsion polymerization process or polymers obtained through solution polymerization process. However, at least one of the conjugated diene polymers should be a polymer containing bound styrene, i.e. styrene-butadiene copolymer. In any case, the low styrene rubber ingredient must have an average bound styrene content of not less than 15% by weight but less than 30% by weight as a whole. The styrene-butadiene copolymer as the low styrene rubber ingredient includes a uniform copolymer as well as a styrene-butadiene copolymer having a distribution width of bound styrene content of not less than 30% when the bound styrene is partly existent in the molecule of the polymer. Moreover, the distribution width of bound styrene content is measured by cross fractionation using cyclohexane/iso-octane mixed solvent and benzene/methyl ethyl ketone according to a Teramachi's method [J. Macromol Sci. Chem., A4(8), page 1785 (1970)]. The total styrene-butadiene copolymer as the low styrene rubber ingredient is preferably compounded at an amount of 30∼85 parts by weight based on 100 parts by weight of the rubber blend. When the low styrene rubber ingredient is comprised of two or more conjugated polymers, at least one conjugated diene homopolymer such as natural rubber (NR), synthetic polyisoprene rubber (IR) or polybutadiene rubber (BR) may be included at an amount of not more than 20 parts by weight in total. When the average bound styrene content of the low styrene rubber ingredient is less than 15% by weight, the desired high running performances are not obtained, while when it is not less than 30% by weight, the wear resistance and ice-snow performances are unsatisfactory. The low styrene rubber ingredient has a lower glass transition temperature because the average bound styrene content is low, and improves the curing resistance at low temperature of the rubber composition as a whole and hence the ice-snow performances, and further can attain the improvement of the balance with the other properties. Therefore, the compounding ratio of the high styrene rubber ingredient to the low styrene rubber ingredient defined in the rubber blend according to the invention is required to balance the high all-weather type running performances such as road gripping force, hardness, resistance to heat build-up and the like. If such a compounding ratio is outside the range defined in the invention, the balance of the above performances is destroyed and the object of the invention can not be achieved. In the carbon black used in the invention, the reason why the specific surface area of nitrogen adsorption (N₂SA) is limited to a range of 146∼159 m²/g is due to the fact that when it is less than 146 m²/g, it is difficult to obtain the desired wear resistance, while when it exceeds 159 m²/g, the dispersion of carbon black in the rubber composition is poor and the wear resistance becomes undesirably degraded. Further, the reason why the distribution width (S) of carbon black as measured by means of an electron microscope is limited to a range satisfying the following relation: S ≦ 0.375 Dn + 0.69 (1) is due to the fact that a large particle size portion in the above particle size distribution is cut off to remove a low reinforced portion and make the reinforcibility of the carbon black high. That is, the wear resistance is improved and further the loss factor (tan δ) around room temperature is increased to more enhance the running performances. According to the invention, the reason why the compounding amount of the reinforcing carbon black having the above properties is limited to a range of 40∼80 parts by weight based on 100 parts by weight of the rubber blend is due to the fact that when the amount is less than 40 parts by weight, the loss factor (tan δ) around room temperature becomes too small and it is difficult to obtain the desired high running performances, while when such amount exceeds 80 parts by weight, the modulus becomes too high and it is difficult to obtain the given modulus at low temperature. Further, when the carbon black according to the invention satisfies the following relation: ΔD₅₀ ≦ Dst - 18 (wherein Dst is a mode size (mµ) of agglomerate in carbon black through a centrifugal settling process and ΔD₅₀ is a half width (mµ) of the mode size), the performances on snow road and the processability can be further improved. In this case, large size portions of the agglomerate are cut off, so that the degradation of performances on snow due to the increase of the modulus as well as the degradation of processability due to the increase of viscosity in the unvulcanized rubber can be prevented. Moreover, the carbon black used in the invention preferably has a colouring force (tinting value) of 124∼141. When the tinting value is less than 124, the effect of more improving the wear resistance and running performances may not be obtained, while when the tinting value exceeds 141, the dispersibility into rubber may be insufficient and the processability and wear resistance tend to lower. The rubber composition according to the invention is required to have a modulus at -20°C and 100% elongation of not more than 3.923 N/mm² (40 kg/cm²) and a loss factor (tan δ) at 30°C of not less than 0.3 as a vulcanizate. When the modulus at -20°C and 100% elongation is more than 3.923 N/mm² (40 kg/cm²), it is difficult to obtain desired performances on severer ice and snow roads. Preferably, the modulus is not more than 35 kg/cm². Further, when tan δ at 30°C is less than 0.3, it is difficult to obtain desired high running performances. Preferably, tan δ is not less than 0.35. In the rubber composition according to the invention, it can generally be said that the high all-weather type running performances are achieved by the rubber blend being a combination of the high styrene rubber ingredient and the low styrene rubber ingredient, while the high wear resistance is achieved by the particular carbon black. These rubber blend and carbon black are closely related to each other, so that the above performances can be balanced and simultaneously be established by properly selecting the kind and amount of the rubber and carbon black used without adversely affecting each of these performances. If necessary, the rubber composition according to the invention may be compounded with proper amounts of additives usually used in rubber compositions such as sulfur as a vulcanizing agent, vulcanization accelerator, accelerator activator, antioxidant, softening agent or filler. The following examples are given in illustration of the invention Examples 1∼5, Comparative Examples 1∼4 Among carbon blacks used in these examples, carbon S was an experimentally prepared carbon black, which could be produced in an exclusive furnace for the production of hard carbon black through a furnace process for usual rubber. In this case, the production conditions were mainly changed as follows: 1) The starting oil was uniformly sprayed to disperse as uniform oil droplets. 2) These oil droplets were maintained so as not to be exposed to an excessively high temperature over a long period of time in order to prevent the recirculation of suspended carbon agglomerates in the burnt product, whereby the size distribution of the agglomerate was narrowed to remove the large size portion of the agglomerate. 3) The linear velocity (flowing rate) at the reaction zone was raised to change the quenching position with water for effective quenching. 4) The furnace for hard carbon black was used as a reaction furnace. The measurement for the properties of carbon black was as follows: a) Specific surface area of nitrogen adsorption (N₂SA) It was measured according to ASTM D-3037. b) Arithmetic mean particle size (Dn) through an electron microscope and particle size distribution width S (standard deviation) The carbon black sample was dispersed in chloroform by an ultrasonic washing process (dispersing conditions: frequency 28 kHz, 30 minutes). Then, the dispersion sample was fixed to a carbon black supporting membrane and inspected by means of an electron microscope at a direct magnification of 20000 times and total magnification of 80000∼100000 times. Thereafter, the diameter of 1000 carbon black particles was measured from the thus obtained microphotograph to form a histogram of 3 mµ section, from which the arithmetic mean particle size (Dn) was determined as mµ. c) Mode size (Dst) of agglomerate and half width (ΔD₅₀) The distribution of carbon black agglomerates through a centrifugal settling process was measured by the following method using a disc centrifuge made by Joice Leble Corporation. That is, carbon black was weighed and added to an aqueous solution of 20% ethanol to adjust the concentration of carbon black to 0.01% by weight, which was dispersed by ultrasonic wave (e.g. 10 minutes) to form a sample solution. After the sample solution (0.25∼1.00 mℓ) was poured into 30 mℓ of a spinning solution (aqueous solution of 2% glycerine) through a syringe, the centrifugal settling was started by setting a revolution speed of the disc centrifuge to 6000 rpm, during which an agglomerate distribution curve was prepared by a photoelectric settling process. The half width ΔD₅₀ (Dst) and most frequent value Dst (mode) were measured from Fig. 1. Moreover, portions shown by broken lines are the same length, and the half width is the length of a portion shown by a solid line. d) Colouring force (tinting value) of carbon black It was measured according to JIS K-6221. The properties of carbon blacks used in Examples and Comparative Examples are shown in the following Table 1. Carbon black N 339 N 220 N 110 Carbon S Specific surface area of nitrogen adsorption (m²/g) 92 114 141 155 Arithmetic mean particle size Dn (mµ) 26.3 24.1 18.0 17.3 Distribution width of particle size S (mµ) 11.06 9.69 6.9 6.91 Value of 0.375 Dn+0.69 (mµ) 10.55 9.73 7.44 7.18 Mode size of agglomerate Dst (mµ) 103 94 70.5 68 Half width of agglomerate ΔD₅₀ (mµ) 91 67 55 47 Value of Dst-18 (mµ) 85 76 52.5 50 Colouring force (tinting value) 109 113 129 132 In these Examples and Comparative Examples, the modulus at -20°C and 100% elongation and the loss factor (tan δ) at 30°C were measured with respect to rubber compositions (after vulcanization) having a compounding recipe shown in the following Table 2. Then, these rubber compositions were used as a tread rubber to manufacture nine tires having a tire size of 185/70 SR14, and the brakability on wet road surface, steering stability on dry road surface, brakability on snow and wear resistance were evaluated with respect to these tires. The measured results are also shown in Table 2. In this table, Comparative Example 1 corresponds to a conventional high running performance tire using a high running performance tread rubber for a summer tire, which is used as a control tire for comparison with the other tires as an index. Comparative Example 2 corresponds to a conventional snow tire. Moreover, the measurement of the above properties and the evaluation of the above performances were as follows. (a) Modulus at -20°C and 100% elongation It was measured in a thermostatic chamber of -20°C (preliminary cooling of 10 minutes) according to a tensile test method of JIS K-6301. Moreover, the sample was vulcanized for a time longer by 1.2 times than a time showing a maximum value of torque through a rheometer. (b) Loss factor: tan δ It was measured by means of a viscoelastic spectrometer made by Iwamoto Seisakusho under conditions that frequency was 50 hertz (50 cps), static strain was 5% and dynamic strain was 1%. Moreover, the sample was a strip-like form having a chack length of 20 mm, a width of 5 mm and a thickness of 2 mm. The vulcanization conditions were the same as in the above item (a). (c) Brakability on wet road surface The running distance until the vehicle was completely stopped was measured by braking from a speed of 40 km/hr, 70 km/hr or 100 km/hr on a wet asphalted road. The measured value was represented by an index on the basis that Comparative Example 1 was 100. The larger the numerical value, the better the property. (d) Steering stability on dry road surface The lap time was measured by running on a circuit consisting of straight course, curved course, bank and the like on a fine day. The time was represented by an index on the basis that Comparative Example 1 was 100. The larger the numerical value, the better the property. (e) Brakability on snow The stopping distance on snow was measured by braking from a speed of 20 km/hr, 30 km/hr or 40 km/hr during the running on a snow road. The distance was represented by an index on the basis that Comparative Example 1 was 100. The larger the numerical value, the better the property. (f) Wear resistance After the tire was actually run on a general road over a distance of 40000 km, the running distance D of the non-controlled tire per 1 mm of worn amount was calculated from the remaining groove depth and also the running distance D₀ of Comparative Example 1 (control tire) was calculated, from which the wear resistance was represented by an index according to the following equation: Wear resistance = (D/D₀) × 100 The larger the numerical value, the better the property. In the rubber blends of Table 2, the emulsion-polymerized SBR A, emulsion-polymerized SBR B and solution-polymerized SBR were a styrene-butadiene copolymer capable of constituting the low styrene rubber ingredient according to the invention or a part thereof, and NR and BR were a conjugated diene polymer capable of constituting a part of the low styrene rubber ingredient, while the emulsion-polymerized SBR C was a high styrene SB emulsion copolymer capable of constituting the high styrene rubber ingredient. The compounding ratio of high styrene rubber ingredient to low styrene rubber ingredient in the rubber blend defined by the invention were satisfied by Comparative Examples 3 and 4 and Examples 1∼5, while Comparative Example 1 was outside the above range and Comparative Example 2 did not include the high styrene rubber ingredient. In Table 2, only carbon S used in Examples 1∼5 satisfied both of specific surface area of nitrogen adsorption and the relation of the formula (1) defined in the invention. Further, only the rubber compositions used in Examples 1∼5 satisfied the modulus at -20°C and 100% elongation and the tan δ at 30°C as the rubber property defined in the invention. The tires of Examples 1∼5 are equal to or better than the tire of Comparative Example 1 as the conventional high running performance tire in respect of the brakability on wet road surface and the steering stability on dry road surface, and are notably improved in respect of the brakability on snow and the wear resistance. As seen from these Examples and Comparative Examples, the pneumatic tires according to the invention have a tread portion made from a rubber composition consisting of a particular rubber blend and a carbon black having particular properties and having particular rubber properties, so that they possess higher all-weather type running performances as compared with the conventional high running performance tire as well as better wear resistance.

Claims: A pneumatic tire comprising a tread portion, a sidewall portion and a bead portion, characterized in that said tread portion is comprised of a rubber composition obtained by compounding 40-80 parts by weight of carbon black having a specific surface area of nitrogen adsorption (N₂SA) of 146-159 m²/g and satisfying the following relation: S ≦ 0.375 Dn + 0.69 (1) (wherein S is a distribution width of particle size as a unit of mµ, and Dn is an arithmetic mean particle size as a unit of mµ measured by means of an electron microscope) with 100 part s by weight of a rubber blend of 15-50 parts by weight of a high styrene rubber ingredient composed of an emulsion-polymerized styrene-butadiene copolymer having a bound styrene content of 30-50% by weight and the balance of a low styrene rubber ingredient composed of at least one conjugated diene polymer other than the above emulsion-polymerized styrene-butadiene copolymer, at least one of which polymers being styrene-butadiene copolymer, and having an average bound styrene content of not less than 15% by weight but less than 30% by weight in a total of said polymers constituting said low styrene rubber ingredient, and said rubber composition has a modulus at -20°C and 100% elongation of not more than 40 kg/cm² and a loss factor (tan δ) at 30°C of not less than 0.3 as a vulcanizate. A pneumatic tire as claimed in claim 1, characterized in that said low styrene rubber ingredient further contains not more than 20 parts by weight in total of at least one of natural rubber synthetic polyisoprene rubber and polybutadiene rubber. A pneumatic tire as claimed in claim 1 or 2, characterized in that said carbon black further satisfies a relation between a mode size (Dst, mµ) of carbon black agglomerate through centrifugal settling process and a half value (ΔD₅₀, mµ) thereof shown by the following equation: ΔD₅₀ ≦ Dst - 18 A pneumatic tire as claimed in any of claims 1 to 3, characterized in that said carbon black further has a colouring force of 124-141.

Inventors: YUTO KAZUAKI, YUTO, KAZUAKI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1989
Main classification: B60C1
IPC codes: C08K3, C08L9, C08L21, B60C11, B60C1