BRANCHED STYRENE-BUTADIENE COPOLYMERS AND PNEUMATIC TIRES USING THE SAME

Abstract: A branched styrene-butadiene copolymer is produced by coupling an active styrene-butadiene copolymer anion, which is obtained by polymerizing styrene and butadiene in a hydrocarbon solvent in the presence of ether or a tertiary amine and an initiator comprising an organolithium com­ pound, with a tin halide compound. In the copolymer, the ratio of branched polymer connected with tin-butadienyl bonds is at least 20% by weight, and the content of bound styrene is not less than 3% by weight but less than 25% by weight, and the content of vinyl bonds in the butadiene portion is not less than 30% but less than 50%. There is also disclosed a pneumatic tire having a tread base rubber which contains at lest 20 parts by weight of a branched styrene-butadiene copolymer rubber produced by coupling a styrene-butadiene copolymer anion with a tin halide.

Description: The present invention relates to branched styrene-butadiene copolymers and to pneumatic tyres using the same. More particularly, the invention relates to a branched styrene-butadiene copolymer having a low hysteresis loss and improved fracture properties, and to a pneumatic tyre having improved high-speed durability and rolling resistance by using a branched styrene-butadiene copolymer as a tread base rubber in tyres comprising a tread portion of a cap/base structure. Lately, it has been strongly demanded to use rubber materials having a low hysteresis loss as a rubber for tyres in order to obtain a reduction of fuel consumption in automobiles. As the rubber material having a low hysteresis loss, there have generally been used rubber blends of natural rubber or synthetic cis-1,4 polyisoprene with low or high cis-1,4 polybutadiene. However, the above-mentioned polybutadiene does not always exhibit sufficient hysteresis loss and fracture properties and causes a reversion phenomenon and other disadvantages when blended with natural rubber or synthetic polyisoprene. Heretofore, it has been known that the problem of reversion phenomenon may be overcome by using polybutadiene with a middle to high vinyl content, but this polybutadiene still has insufficient hysteresis loss and fracture properties. It is, therefore, a first aim of the present invention to provide branched styrene-butadiene copolymer rubbers having a low hysteresis loss and improved fracture properties without causing the reversion phenomenon. It is a second aim of the present invention considerably improve the rolling resistance and high-speed durability of pneumatic tyres by using a branched styrene-butadienen copolymer rubber as a base rubber for the tread portion of a cap/base structure. According to a first aspect of the present invention, there is provided a branched and substantially random styrene-butadiene copolymer obtained by coupling a tin halide with an active styrene-butadiene copolymer anion, which is obtained by polymerizing styrene and butadiene in a hydrocarbon solvent in the presence of an ether or a tertiary amine and an initiator comprising an organolithium compound, wherein: (I) the ratio of branched polymer connected with tin-butadienyl bonds in the copolymer is at least 20% by weight; (II) the content of bound styrene in the copolymer is not less than 3% by weight but not more than 15% by weight; and (III) the content of vinyl bonds in the butadiene portion of the copolymer is not less than 30% but less than 50%. According to a second aspect of the present invention, there is provided a pneumatic tyre comprising a toroidal carcass reinforcement, and a tread portion superimposed about a crown region of the carcass and having a cap/base structure comprising a tread base rubber and tread cap rubber, wherein the tread base rubber is a rubber composition containing at least 20 parts by weight of a branched styrene-butadiene copolymer rubber produced by coupling a styrene-butadiene copolymer anion with a tin halide compound, based on 100 parts by weight of total rubber content, and wherein the said branched styrene-butadiene copolymer rubber is obtained from a copolymer according to the first aspect of the invention. The present invention will now be described in greater detail below. In order to achieve a reduction of fuel consumption and an improvement of high-speed durability in tyres as described above, it has been attempted to reduce heat build-up of the tyre, or hysteresis loss of the tyre, especially the tyre tread portion. As a method for the reduction of hysteresis loss, it is usual to use materials having a low glass transition temperature such as high cis-1,4 polybutadiene or materials having a high rebound resilience such as natural rubber. However, the use of these rubber materials considerably degrades wet braking performance, cornering stability on a wet road, handling performance during high-speed running, and running stability such as braking performance, so that it is very difficult to simultaneously accomphish these running stabilities together with low rolling resistance and high-speed durability. Furthermore, it is known that an attempt has been made to improve the low rolling resistance and the wet skid resistance by optimizing the molecular structure of a polymer to be used in a single tread portion, as disclosed in U.S. Patent No. 4,334,567, but the improvement obtained by this means cannot be said to be satisfactory. This is because a styrene-butadiene copolymer rubber produced by solution polymerization using an organolithium compound is used alone, so that the breaking strength and elongation at break are low and many difficulties are caused in the running of the tyre or in the manufacture of the tyre. That is, in large pneumatic tyres to be mounted on trucks or buses, tread damage such as rib tear (rib breaking) are produced during running under high load and high deformation, while when the tyre is taken out from a mould after vulcanization, moulding defects are caused by damaging the tread with the pattern of the mould. The latter effect is particularly notable in tyres for passenger cars having a complicated tread pattern. Therefore, it is difficult to satisfy all performances required for the tread portion with a single rubber composition in either large tyres for trucks and buses or small tyres for passenger cars. In order to overcome these drawbacks, a so-called cap/base structure has been applied to not only large tyres but also low fuel consumption tyres for passenger cars as described, for example, in Japanese Patent laid-open No. 55―99,403. In the tread portion of such a cap/base structure, the running stability and resistance to tread damage are sufficiently satisfied when the same rubber composition as used in the conventional single tread portion is used as a tread cap rubber. That is, a rubber composition consisting mainly of styrene-butadiene copolymer rubber is used as a tread cap rubber in tyres for passenger cars, while natural rubber and/or polyisoprene rubber or a rubber blend of styrene-butadiene copolymer rubber therewith is used as a tread cap rubber in large tyres for trucks and buses. The present inventors have made various studies with the aim of improving the low rolling resistance and high-speed durability of pneumatic tyres by effectively utilizing a cap/base structure without adversely affecting the running stability and resistance to tread damage such as rib tear, and have found that the low rolling resistance and high-speed durability of the tyre are considerably improved by using a rubber composition containing styrene-butadiene copolymer rubber of particular molecular structure, natural rubber and/or polyisoprene rubber as a tread base rubber of the tread portion. In the styrene-butadiene copolymer according to the present invention, the content of bound styrene is not less than 3% by weight but not more than 15% by weight, more preferably 5 to 15% by weight, while the content of vinyl bonds in the butadiene portion is not less than 30% but less than 50%. When the content of bound styrene is less than 3% by weight, fracture properties are poor, while when the content of bound styrene is not less than 25% by weight, the hysteresis loss is undesirably high. In the application of the copolymer to the tread base rubber of a pneumatic tyre, the content of bound styrene is not more than 15% by weight. When the content of vinyl bonds is less than 30%, not only is the hysteresis loss degraded due to the formation of block polystyrene, but also modulus and tensile properties lower in over vulcanization when blending with natural rubber, i.e. a so-called reversion phenomenon occurs. On the other hand, when the content of vinyl bonds is not less than 50%, not only are the fracture properties and wear resistance lower, but also the glass transition point rises, so that the heat build-up is poor. The copolymer according to the present invention is characterized by containing at least 20% by weight of branched polymer connected with tin-butadienyl bonds. In the case of copolymers containing branched polymer connected with silicon-butadienyl bonds or carbon-carbon bonds other than tin-butadienyl bonds, the improvement of hysteresis loss is not expected. Furthermore, the fracture properties and hysteresis loss of the resulting copolymer vulcanizate are improved when tin-carbon bonds in the branching of the copolymer are tin-butadienyl bonds rather than tin-styryl bonds. When the ratio of branched polymer connected with tin-butadienyl bonds is less than 20% by weight, the hysteresis loss is substantially equal to that of the well-known low cis-1,4 polybutadiene or high cis-1,4 polybutadiene, so that the aim of the present invention cannot be achieved and also the processability is poor. The copolymers according to the present invention are usually produced by polymerizing styrene and butadiene in the presence of ether or a tertiary amine and an initiator of an organolithium compound in a hydrocarbon solvent, adding 1 to 15 mole of 1,3-butadiene per 1 gram atom of lithium in the initiator to the resulting polymer and then coupling it with a tin halide compound. The preferred copolymers according to the present invention are obtained by polymerizing styrene and butadiene under temperature conditions that the polymerization initiation temperature is 0―50°C, the maximum access temperature is not more than 120°C, and the increase of polymerization temperature is at least 30°C as a difference from the polymerization initiation temperature, adding a small amount of butadiene to change the terminal of the resulting polymer into butadienyl-lithium, and then coupling with a tin halide compound. In the thus obtained copolymers, the content of vinyl bonds in the butadiene portion gradually reduces toward the terminal of the polymer and the butadiene portion at the branched connection is tin-1,4-butadienyl bond (Sn―C―C=C―C―), so that such copolymers have low viscosity during kneading and excellent processability, and also the vulcanizate therefrom has excellent hysteresis loss. The content of vinyl bonds in the molecular chain of the styrene-butadiene copolymer is determined by the amount of ether or tertiary amine in the polymerization system and the polymerization temperature. Since the amount of ether or tertiary amine in the polymerization system is usually constant, the distribution of the content of vinyl bonds in the molecular chain of the copolymer is changed by the hysteresis of the polymerization temperature. Moreover, the styrene-butadiene copolymer according to the present invention is a substantially random copolymer and suitably the content of block polystyrene in bound styrene is not more than 10% as measured by a method described by I.M. Kolthoff et al in J. Polymer Sci. Vol. 1, 429 (1946). The Mooney viscosity of the copolymer is not particularly critical, but its ML₁+₄ 100°C is suitably within a range of 20―150, preferably 40―80. As the hydrocarbon solvent to be used in the production of the styrene-butadiene copolymer, use may be made of hexane, heptane, cyclohexane, benzene, xylene and mixtures thereof. The organolithium compound may be, for example, an alkyl lithium such as n-butyl-lithium, sec-butyl-lithium, or 1,4-dilithiobutane, or an alkylene dilithium, which is used in an amount of 0.02―0.2 part by weight per 100 parts by weight of the monomer. The ether or tertiary amine are used as a randomization agent for styrene and butadiene as well as an adjusting agent for the microstructure of the butadiene portion; typical examples of suitable ethers and tertiary amines are dimethoxybenzene, tetrahydrofuran, dimethoxyethane, diethylene glycol dibutyl ether, diethylene glycol dimethyl ether, triethylamine, pyridine, N-methylmorpholine, N,N,N′,N′-tetramethylethylene diamine, and 1,2-diperidino ethane. The coupling reaction is suitably performed at a temperature of 50 to 120°C. The tin halide compound is suitably used in an amount of 0.2 to 3 equivalent of halogen atom per 1 equivalent of lithium atom existent in the terminal of the polymer. Suitable tin halide compounds are tin tetrachloride, tin tetrabromide, tin methyl trichloride, tin butyl trichloride, and bis(trichlorostannyl) ethane. The styrene-butadiene copolymer according to the present invention may be used alone or in a blend with at least one rubber selected from natural rubber, synthetic cis-1,4 polyisoprene, emulsion polymerized styrene-butadiene copolymer, high cis-1,4 polybutadiene, low cis-1,4 polybutadiene, and ethylenepropylene-diene terpolymer, for use in tyres as well as rubber springs, belts, hoses and other industrial goods. In this case, the copolymer or its rubber blend is extended with oil, added with additives usually used for vulcanizates and then vulcanized, if necessary. When the branched styrene-butadiene copolymer according to the present invention is used in a tread base rubber in a pneumatic tyre, it is preferable that the tread base rubber contains at least 20 parts by weight of the branched copolymer having a content of bound styrene of not less than 3% by weight but not more than 15% by weight and a content of vinyl bonds in the butadiene portion of not less than 30% but less than 50%, and not less than 30 parts by weight of natural rubber and/or polyisoprene rubber, based on 100 parts by weight of total rubber content. The second aspect of the present invention lies in that a rubber composition containing a branched styrene-butadiene copolymer connected with tin-butadienyl bonds, which is obtained by coupling a styrene-butadiene copolymer anion with a tin halide compound, is used as a tread base rubber. This results from the fact that such a branched copolymer containing composition considerably improves not only the heat build-up but also fracture properties, particularly breaking strength at high temperature. In the branched copolymer according to the present invention, the crosslinking with tin is chemically stable and is hardly subjected to oxidative degradation even at high temperature as compared with the crosslinking formed by vulcanization usually employed in the rubber industry. Furthermore, since molecular chains of copolymers are connected to each other by the coupling reaction to form a starlike polymer, molecular weight between the crosslink points is determined only by the molecular weight of the copolymer before the coupling reaction, and the number of copolymer molecules passing the crosslink point is determined by the functionality of the tin halide compound, whereby a regular network is formed in the branched copolymer. In other words, the conventional rubber composition containing no branched copolymer has a broad distribution in the molecular weight between crosslink points due to the presence of only irregular crosslink points by the vulcanizing agent such as sulfur, while in the rubber composition containing the branched copolymer the ratio of the molecular weight between particular crosslink points becomes relatively large. These facts are a cause of improving the fracture properties. That is, when external force is applied to the conventional rubber composition containing no branched copolymer, the molecular chain having a low molecular weight between crosslink points is tensioned and strain is concentrated thereon to selectively produce the chain breaking, which results in lowering of the breaking strength for the composition. This phenomenon is less in the rubber composition containing the branched copolymer, which results in improvement of the fatigue properties. This is considered to be a case of improving the fatigue properties even at high temperature because the reinforcing action of carbon black is reduced and the crosslink point of sulfur becomes chemically unstable at such high temperature. In general, when strain is applied to the rubber composition, there are generated enthalpy stress resulting from the reinforcing action of carbon black and entropy stress resulting from the crosslink points. The latter is a stress generating mechanism usually referred to as rubber elasticity and does not substantially follow the energy loss. Therefore, in order to improve the low heat build-up or low rolling resistance, it is sufficient to make uniform the molecular weight between crosslink points and the number of molecules passing the crosslink point or to reduce the number of free terminal chains (a molecular chain terminal having no crosslink point), which can be achieved by using a branched copolymer having a crosslinking structure as described above. In the conventional rubber composition, the vulcanization is usually carried out at high temperature in a short time in order to increase mass production and productivity, during which a so-called reversion produced by the breaking of the crosslinked network becomes evident and considerably adversely affects the heat build-up of the composition controlling the low rolling resistance and high-speed durability. On the other hand, according to the present invention, the deterioration of the heat build-up is improved by the use of a rubber composition containing not less than 20 parts by weight of the branched copolymer per 100 parts by weight of total rubber content. When the amount of the branched copolymer is less than 20 parts by weight, the effect of suppressing the reversion cannot be expected. The effect of suppressing the reversion is excellent heat build-up and fracture properties and is further developed when the branched copolymer is used together with natural rubber and polyisoprene rubber exhibiting remarkable reversion. In particular, the deterioration of the heat build-up due to the reversion is considerably improved when a rubber composition for a tread base rubber contains not less than 20 parts by weight of the branched copolymer according to the present invention and not less than 30 parts by weight of at least one of natural rubber and polyisoprene rubber, based on 100 parts by weight of total rubber content. When the amount of at least one of natural rubber and polyisoprene rubber is less than 30 parts by weight, the effect of improving the heat build-up by the synergistic action with the branched copolymer cannot be expected. Moreover, the rubber composition to be used as the tread base rubber of a tyre according to the present invention may contain additives usually used in the rubber industry such as a vulcanization accelerator, vulcanizing agent, supplement accelerator, carbon black, softener, and antioxidant. The rubber composition according to the present invention as described above may be advantageously applied to any tyre having a tread portion of cap/base structure. In particular, they may be suitably used in heavy duty radial tyres for trucks, buses and construction vehicles as well as in radial tyres for passenger cars. The invention will be further described with reference to the following illustrative Examples. The various measurements referred to below are made by the following methods. The tensile properties are measured according to the method of JIS K-6301. The rebound resilience at 70°C (resilience measured by Dunlop tripsometer) is used as an index for hysteresis loss. The wear is measured by means of a Pico type abrasion machine. The microstructure is determined by infrared spectrophotometry (Morero's method). Furthermore, the content of bound styrene is determined from a calibration curve based on absorption of phenyl group at 699 cm⁻¹ by infrared spectrophotometry. Also, the ratio of branched polymer connected with tin-carbon bonds is determined by gel permeation chromatography (GPC). Examples 1―4, Comparative Examples 1―7 Into a reaction vessel of 5 l capacity were charged cyclohexane, 1,3-butadiene, styrene and tetrahydrofuran in predetermined amounts as shown in the following Table 1 under nitrogen atmosphere. After the temperature of the resulting mass was adjusted to the predetermined initiation temperature, n-butyllithium was added to perform polymerization while raising temperature under heat insulation. After 40 minutes, the conversion rate for polymerization reached 100%. Then, a small amount of 1,3-butadiene was added to form butadienyllithium in the terminal of the polymer and thereafter tin tetrachloride was added to perform coupling reaction for 30 minutes. In Example 4, however, the coupling reaction was performed after isothermal polymerization was carried out at 60°C for 1 hour and a small amount of 1,3-butadiene was added. In Comparative Example 7, the coupling reaction was performed such that the terminal of the polymer is styryl anion without adding additional 1,3-butadiene after the isothermal polymerization. The resulting polymer solution was added with 2,6-di-tert-butyl p-cresol, subjected to steam stripping to perform the removal of solvent, and dried on a roll heated at 110°C to obtain a copolymer. Moreover, Comparative Example 5 used high cis-1,4 polybutadiene (trade name: "BROI") made by Japan Synthetic Rubber Co., Ltd. The copolymer was mixed with other ingredients according to the compounding recipe shown in the following Table 2 by means of brabender and roll and then vulcanized at 145°C for 35 minutes. The properties of the copolymer and vulcanizate are shown in the following Table 3. As apparent from Table 3, the copolymers of Examples 1―4 have superior tensile strength (fracture property) to the copolymer of Comparative Example 1 and have excellent elongation as compared with Comparative Example 3. Further, they have excellent rebound resilience at 70°C as compared with Comparative Examples 2―7. The copolymers of Examples 1―4 have superior wear resistance to those of Comparative Examples 1, 3, 4 and 6. Also, they have low Mooney viscosity of composition as compared with that of Comparative Example 6 and have excellent processability. Example 5 The copolymer of each of Example 1 and Comparative Example 5 was mixed with other ingredients according to the compounding recipe shown in the following Table 4 and then vulcanized, during which optimum vulcanizing times at 145°C and 165°C were determined by means of a curelastometer. Then, loss percentages in tensile stress and tensile strength between the vulcanizate after the optimum vulcanizing time and the vulcanizate after a vulcanizing time corresponding to 3 times the optimum vulcanizing time were measured as a reversion to obtain results as shown in the following Table 5. From the data of Table 5, it is apparent that no reversion occurs in Example 1. Example 7 In this example, there were prepared and used the following copolymers A―F. The copolymers A and B corresponded to Examples 1 and 2, respectively. The copolymer C was obtained by the same polymerization and coupling reaction as described in Example 1 except for the use of 420 g of 1,3-butadiene, 75 g of styrene, 4.0 g of tetrahydrofuran and 0.32 g of n-butyllithium. The copolymer D corresponded to Comparative Example 1. The copolymer E was obtained by the same polymerization as described in Example 2 without performing the coupling reaction for butadienyllithium in the terminal of the polymer. The copolymer F was obtained by the same polymerization as described in Comparative Example 7. The content of bound styrene, content of vinyl bonds in butadiene portion and ratio of polymer containing tin-butadienyl bonds were measured with respect to these copolymers to obtain results as shown in the following Table 6. According to the present invention, it is necessary that the ratio of polymer connected with tin-butadienyl bonds in the branched copolymer is at least 20% by weight, preferably not less than 40% by weight. In this connection, all of the copolymers according to the present invention shown in Table 6 had a ratio of not less than 50% by weight and exhibited a good coupling efficiency. Then, a rubber composition containing each of the copolymers A―F was prepared according to the compounding recipe shown in the following Table 7 and used as a tread base rubber to manufacture a radial tyre having a tyre size of 165 SR 13. The rolling resistance and high-speed durability were measured with respect to such a tyre to obtain results as shown in the following Table 8. Moreover, the evaluations of these properties were made as follows: Rolling resistance The test tyre subjected to an internal pressure of 1.7 kg/cm² was trained on a steel drum with a diameter of 1,707.6 mm and width of 350 mm, which was rotated by the driving of a motor, at a speed of 80 km/hr under JIS 100% load (385 kg) for 30 minutes and thereafter the rotating speed of the drum was raised to 100 km/hr. Then, the driving of the motor was stopped to run the drum by inertia, during which the rolling resistance of the tyre to the drum was measured on a basis of deceleration speed of drum and time change. The rolling resistance was indicated by an index according to the following equation on the basis that the Tyre No. 11 is 100 (which corresponds to a rolling resistance of 5.0 kg). The smaller the index value, the better the property. Rolling resistance of test tyre 5.0 kg × 100 High-speed durability The measurement was performed according to a method of FMVSS No. 109, wherein the running speed of the test tyre was stepped up in the order of 140 km/hr, 150 km/hr, 160 km/hr, 170 km/hr, 180 km/hr, 185 km/hr, 190 km/hr, 195 km/hr, 200 km/hr and 205 km/hr every 30 minutes. The high-speed durability was indicated by a step speed in the breaking of the test tyre and a lapse time at this step speed. As apparent from the data of Table 8, the rolling resistance and high-speed durablilty are considerably improved in the pneumatic tyre according to the present invention.

Claims: 1. A branched and substantially random styrene-butadiene copolymer obtained by coupling a tin halide with an active styrene-butadiene copolymer anion, which is obtained by polymerizing styrene and butadiene in a hydrocarbon solvent in the presence of an ether or a tertiary amine and an initiator comprising an organolithium compound, characterized in that: (I) the ratio of branched polymer connected with tin-butadienyl bonds in the said copolymer is at least 20% by weight; (II) the content of bound styrene in the said copolymer is not less than 3% by weight but not more than 15% by weight; and (III) the content of vinyl bonds in the butadiene portion of the said copolymer is not less than 30% but less than 50%. 2. A branched styrene-butadiene copolymer as claimed in claim 1, characterized in that the said content of bound styrene is 5 to 15% by weight. 3. A branched styrene-butadiene copolymer as claimed in claim 1 or 2, characterized in that a content of block polystyrene in the said bound styrene is not more than 10%. 4. A branched styrene-butadiene copolymer as claimed in any of claims 1 to 3, characterized in that the said active styrene-butadiene copolymer anion is obtained by polymerization of styrene and butadiene under temperature conditions that the polymerization initiation temperature is 0―50°C, the maximum access temperature is not more than 120°C, and the increase of polymerization temperature is at least 30°C from the polymerization initiation temperature. 5. A pneumatic tyre comprising a toroidal carcass reinforcement, and a tread portion superimposed about a crown region of the said carcass and having a cap/base structure comprising a tread base rubber and a thread cap rubber, characterized in that the said tread base rubber is a rubber composition containing at least 20 parts by weight of a branched styrene-butadiene copolymer rubber produced by coupling a styrene-butadiene copolymer anion with a tin halide, based on 100 parts by weight of total rubber content, and in that the said branched styrene-butadiene copolymer rubber is obtained from a copolymer as claimed in any of claims 1 to 4. 6. A pneumatic tyre as claimed in claim 5, characterized in that the said rubber composition contains at least 20 parts by weight of the said branched styrene-butadiene copolymer rubber having a content of bound styrene of 3 to 15% by weight and a content of vinyl bonds in the butadiene portion of not less than 30% but less than 50%, and not less than 30 parts by weight of at least one of natural rubber and polyisoprene rubber, based on 100 parts by weight of total rubber content.

Inventors: FUJIMAKI, TATSUO, OHSIMA, NOBORU, SHIMIZU, ISAMU, TOMITA, SEISUKE, YOSHIMURA, YOSHITO
Applicants: BRIDGESTONE TIRE COMPANY LIMITED, JAPAN SYNTHETIC RUBBER CO., LTD.
Application year: 1983
Main classification: C08C19
IPC codes: B60C1, C08C19