Pneumatic tires

Abstract: A pneumatic tire has a tread comprising a rubber composition produced by blending a rubber component with a styrene based thermoplastic elastomer, the rubber component being composed mainly of a diene based rubber, and the elastomer being so preliminary internally crosslinked that tensile stress at 100% elongation is increased to 10 to 1,000% of that of the styrene based thermoplastic elastomer not internally crosslinked. A pneumatic tire is also disclosed, having a tread portion comprising a rubber composition blended with a vinyl aromatic hydrocarbon thermoplastic elastomer in an amount of 5-80 parts by weight relative to 100 parts by weight of the rubber component, the vinyl aromatic hydrocarbon thermoplastic elastomer satisfying the following requirements: (1) the elastomer is a vinyl aromatic hydrocarbon-conjugated diene based block type thermoplastic elastomer; (2) the elastomer has a weight ratio between a vinyl aromatic hydrocarbon and a conjugated diene as constituent monomer units thereof of 40/60 to 95/5; (3) the content of the vinyl aromatic hydrocarbon in vinyl aromatic hydrocarbon based segments in the elastomer is not less than 50% by weight; and (4) the weight average molecular weight of the elastomer is not less than 1 x 10⁵.

Description: The present invention relates to pneumatic tires. More particularly, the invention relates to pneumatic tires which greatly improve driving performance, braking performance and cornering stability on ice and snow roads without adversely affecting cornering stability, durability and wear resistance in the summer. In order to ensure driving performance, braking performance and cornering stability in running on ice and snow roads, spike tires in which spike pins are provided into a surface portion of a tread have been frequently formerly used. However, such tires give rise to serious problems in that dust is produced due to wearing of the spike pins themselves and wearing or damage of the roads. To try to overcome such problems, the projecting amount of the spike pins and the number of the spike pins per unit area have been controlled and the material of the spike pins has been examined. However, no fundamental solution of such problems has been realized. On the other hand, studless tires exhibiting driving performance, braking performance and cornering stability on ice and snow roads without using spikes or chains have been recently examined and have become widely used, but it cannot necessarily be said that fully satisfactory performances on ice and snow roads are exhibited as compared with the spike tires. With respect to the rubber for the studless tires, a rubber composition using a polymer having a low glass transition point is used to ensure elasticity of the rubber at low temperature. However, depending upon the hysteresis characteristic of such a polymer, there remains a problem in that although the tire performances are exhibited to some extent in an ice and snow temperature range, the braking performance and cornering stability are not sufficient on wet roads or dry roads. Further, as disclosed in Japanese patent application Laid-open Nos. 55-135,149, 58-199,203 and 60-137,945, it is known that when a rubber composition compounded with a large amount of a softener or a plasticizer is used as a tread rubber, the performances at low temperatures are similarly improved. However, these techniques have serious problems in that for the rate at which the performances on ice and snow roads are improved, reduction in wear resistance and separation of the tread rubber occur in running on general roads. Although each of the above techniques affords excellent performances on ice and snow roads in a relatively low temperature range (a so-called dry-on-ice state) not more than -5°C, a sufficient coefficient of friction cannot be obtained on ice and snow in a wet state (a so-called wet-on-ice state) near 0°C. Therefore, it cannot be said that driving performance, braking performance and cornering stability are improved over the entire range. It is an aim of the present invention to provide pneumatic tires having improved driving performance, braking performance and cornering stability on ice and snow roads without adversely affecting cornering stability and durability in the summer season. Particularly, the invention aims to provide pneumatic tires which have improved driving performance, braking performance and cornering stability on ice and snow roads in the above-mentioned wet state without adversely affecting wear resistance. The present inventors have discovered that when a styrene based thermoplastic elastomer having an internal crosslinked structure preliminarily formed is incorporated into a rubber of a tread, the elastomer forms domains without being fused and flown, so that the coefficient of friction on ice and snow of the rubber of the tread is increased. The inventors have further confirmed that since the styrene based thermoplastic elastomer is co-crosslinked with the rubber as a matrix, wear resistance of the rubber composition of the tread is equally maintained irrespective of the presence or absence of the thermoplastic elastomer. The present invention has been accomplished based on the discovery and the confirmation mentioned above. The present invention provides a pneumatic tire having a tread including or comprising a rubber composition produced by blending a rubber component with a styrene based thermoplastic elastomer, said rubber component being composed of 50 to 100% of a diene based rubber, and said elastomer being internally crosslinked at such an internally crosslinked rate that tensile stress at 100% elongation is increased to 10 to 1,000% of that of the styrene based thermoplastic elastomer not internally crosslinked, and wherein an average domain diameter of the styrene based thermoplastic elastomer is in a range of 1 to 1,000 µm. As the internally crosslinked styrene based thermosplastic elastomer to be used in the present invention, any of variously crosslinked types, such as a sulfur-crosslinked type, a peroxide-crosslinked type, and a radiation crosslinked type, which have been formerly employed for the styrene based thermoplastic elastomers, may be used. The internally crosslinked styrene based thermoplastic elastomer is not particularly limited to a specific crosslinking way or a specific crosslinked structure. The styrene based thermoplastic elastomer to be crosslinked contains at least one, preferably one or two, non-rubbery polymer block segments each consisting mainly of styrene (hereinafter referred to as "polystyrene block") and at least one rubbery polymer block segment (hereinafter referred to as "rubber block"). Such a styrene based thermoplastic elastomer has, for example, a structure that polystyrene blocks exist at opposite ends and a rubber block is located at an intermediate portion, a structure that a polystyrene block and a rubber block are located on opposite sides, a structure that at least three polystyrene end portions are connected together by a rubber block, or a structure that polystyrene blocks and rubber blocks are alternately introduced into each of the above structures. As the rubber block, a polyolefine based elastomer block such as a polybutadiene block or a polyisoprene block may be used. However, the present invention is not limited to a specific type. Among them, a styrene-butadiene-styrene (S-B-S) type block copolymer elastomer or a styrene-butadiene (S-B) type block copolymer elastomer obtained with a catalyst of an organic lithium compound is preferred. Processes for producing these styrene based thermoplastic elastomers are well known. For example, Japanese patent application Laid-open No. 60-243,109 and Japanese patent publication No. 48-2,423 describe the production of S-B-S type block copolymer elastomers with organic Li compounds as catalysts, and Japanese patent publication No. 36-19286 describes the production of S-B type block copolymer elastomers or S-B-S type block copolymer elastomers with similar catalysts. The styrene based thermoplastic elastomer needs to be internally crosslinked to such an extent that the tensile stress at the elongation of 100% as measured by ASTM D 638 is increased in a range of 10 to 1,000% by the internal crosslinking. When the styrene based thermoplastic elastomer is internally crosslinked to satisfy the tensile stress increased in this range of 10 to 1,000%, the internally crosslinked styrene based thermoplastic elastomer as incorporated into the tread rubber can be dispersed, in the matrix, in the form of particles having the average domain diameter of not less than 1 µm. If this average domain diameter of the particles is less than 1 µm, the above performances of the tire on ice and snow are insufficient. However, when the internal crosslinking reaction is excessively effected, the average domain diameter exceeds 1,000 µm, so that wear resistance of the tread is unfavorably deteriorated. The internally crosslinked styrene based thermoplastic elastomer is preferably blended into the rubber component in an amount of 1-50 parts by weight, more preferably 5-40 parts by weight, relative to 100 parts by weight of the rubber component. If less than 1 part by weight of the styrene based thermoplastic elastomer is compounded, effects of improving the driving performance, braking performance and cornering stability on ice and snow roads are small. On the other hand, if more than 50 parts of the crosslinked styrene based thermoplastic elastomer is blended, the kneading efficiency of the rubber composition is deteriorated. A main portion of the rubber component may be composed of natural rubber, polyisoprene rubber, polybutadiene rubber, styrene-butadiene copolymer rubber, styrene-isoprene-butadiene terpolymer rubber, styrene-isoprene copolymer rubber, isoprene-butadiene copolymer rubber, etc. The main portion of the rubber composition means that at least one kind of these rubbers occupies 50 to 100% by weight of the entire rubber component. Besides the diene based rubber and the internally crosslinked styrene based thermoplastic elastomer, other rubber, and a compounding agent ordinarily used in the tread rubber may be incorporated into the rubber composition for the tread of the pneumatic tire according to the present invention. The compounding agent may include filler, antioxidant, vulcanizer, vulcanization accelerator, etc. The kinds and amounts of other rubber and the compounding agents are selected to fall in ordinarily employed ranges, and are not particularly limited. Now, the present invention will be explained below in more detail with reference to examples and comparative examples. Properties of tread rubbers and tire performances of test tires were tested by the following methods. (1) Average domain diameter of internally crosslinked styrene based thermoplastic elastomer in tread rubber: Ten lot samples were selected among a test piece, and diameters of twenty domains in the internally crosslinked styrene based thermoplastic elastomer inside a visual view of an optical microscope were measured. The average domain diameter was calculated with respect to each of the lots, and then the average domain diameter was calculated with respect to the ten lots. The diameter of a domain was determined by measuring the maximum diameter and the minimum diameter thereof, and dividing the sum of these maximum and minimum diameters by 2. (2) Coefficient of friction on ice: The coefficient of friction of the rubber on ice, particularly that on ice near 0°C in a wet state, was measured by using a dynamic-static friction meter in the state that a surface of a sample (dimension: 10 mm in length, 10 mm in width, and 5 mm in thickness) was contacted with that of ice having a surface temperature of -0.5°C. Measurement conditions were that a load was 2 kg/cm², a sliding speed was 10 mm/sec, an atmospheric temperature was -2°C, and the surface state was made near a mirror surface. (3) Braking performance on ice: A passenger car radial tire PSR (165 SR 13) was prepared as a test tire, and subjected to ordinary running over 50 km as idling. Then, the tire was tested (the following wearing test was similarly effected). In the test, four test tires were fitted to a passenger car having a displacement of 1,500 cc, and a braked distance was measured on ice at an atmospheric temperature of -5°C. Results were indicated by an index by taking that of Comparative Example 1 as 100. The greater the value, the better is the braking performance. (4) Two test tires were fitted to a driving shaft of the passenger car having the displacement of 1,500 cc, and run on a concrete road of a test course at a given speed. Changes in depths of grooves were measured, and results were indicated by an index by taking that of Comparative Example 1 as 100. The greater the value, the better is the result. Examples 1-5 and Comparative Examples 1-4: S-B-S type and S-B type styrene based thermoplastic elastomers were internally crosslinked at a compounding recipe or by an irradiating amount of radioactive rays as shown in Table 1 to attain a sulfur crosslinking or radiative crosslinking structure. Samples A through D were each prepared by kneading starting materials at a compounding recipe shown in Table 1, and introducing sulfur-crosslinked structure into the elastomer by vulcanization at 145°C for 40 minutes under pressure. Rubber compositions each containing the thus internally crosslinked styrene based thermoplastic elastomer and those containing no such elastomer were prepared as examples and comparative examples at compounding recipes shown in Table 2, respectively, and coefficients of friction on ice and the average domain diameters of vulcanizates were measured. Braking performance and wearing performance of tires in which the thus obtained rubber composition was arranged in the tread were measured. The results obtained are shown in Table 2. As is clear from Table 2, the rubber composition in Examples 1 through 5 in which the average domain diameter of the styrene based thermoplastic elastomer was adjusted to a range of 1 to 1,000 µm by internally crosslinking had improved coefficients of friction on ice as compared with the ordinary tread rubber composition given in Comparative Example 1. Thus, the tires using the rubber compositions in Examples 1 through 5 for the treads, respectively, had equal or better wearing performance and conspicuously improved braking performance on ice as compared with the tire using the composition in Comparative Example 1. To the contrary, in the case of Comparative Examples 2 and 3 in which the internal crosslinking of the styrene based thermoplastic elastomer was not sufficient and the average domain diameter in the rubber composition was less than 1 µm, it is seen that the coefficient of friction on ice or braking performance on ice was almost not improved. In Comparative Example 4, it is seen that since the styrene based thermoplastic elastomer was internally crosslinked to an excessive extent, the average domain diameter in the rubber composition exceeded 1,000 µm, so that the wear resistance was greatly lowered. As is clear from the above-mentioned Examples and Comparative Examples, a pneumatic tire wherein there is arranged, in the tread, a rubber composition in which a thermoplastic elastomer internally crosslinked to exhibit an increase of 10% in tensile stress at 100% elongation is blended into a rubber component composed mainly of diene based rubber has improved driving performance, braking performance and cornering stability on ice and snow roads in a dry state or in a wet state without lowering the wear resistance.

Claims: A pneumatic tire having a tread including or comprising a rubber composition produced by blending a rubber component with a styrene based thermoplastic elastomer, said rubber component being composed of 50 to 100% of a diene based rubber, and said elastomer being internally crosslinked at such an internally crosslinked rate that tensile stress at 100% elongation is increased to 10 to 1,000% of that of the styrene based thermoplastic elastomer not internally crosslinked, and wherein an average domain diameter of the styrene based thermoplastic elastomer is in a range of 1 to 1,000 µm. A pneumatic tire as claimed in claim 1, characterized in that the styrene based thermoplastic elastomer is blended in an amount of 1-50 parts by weight, preferably 5-40 parts by weight, relative to 100 parts by weight of the rubber component.

Inventors: ARAKI MITSURU, MORIMOTO YOSHIYUKI, ARAKI, MITSURU, MORIMOTO, YOSHIYUKI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1991
Main classification: C08L21
IPC codes: C08L21, B60C1