Rubber composition

Abstract: The present invention provides a rubber composition comprising: (i) a rubber component comprising 70 to 90 % by weight of a styrene-butadiene copolymer rubber (A) containing 15 to 40 % by weight of a styrene content and 30 to 60 % by weight of a vinyl content in a butadiene portion, which is obtained by solution polymerization, and 10 to 30 % by weight of a polybutadiene rubber (B) (ii) a vulcanizing agent in an amount of 0.75 to 1.75 parts by weight based on 100 parts by weight of the rubber component (i) and (iii) a plasticizer; 20 to 26 % by weight of said rubber composition being extracted by acetone when a vulcanized rubber sample obtained from the rubber composition is subjected to a Soxhlet extractor for 24 hours, and at least 80 % by weight of the acetone extract being the plasticizer.

Description: The present invention relates to a rubber composition. More particularly, it relates to a rubber composition for a tire tread. Passengers cars now have very high performances, for example, the power output of engines is enhanced and the body weight is lightened. Thereby, the starting, accelerating and turning properties are significantly improved. The improvement of these moving properties requires a high steering stability to a tire which transmits the power of the car to the road surface. Also, the recent environmental protection and energy-saving requirements require a reduction of the rolling resistance of the tire (low heat build-up) and a high wear resistance. In order to ensure steering stability, it is necessary to enhance the grip force at the tread part of the tire which shows high energy loss properties, for which various improvements have been proposed. An example of a conventional method for enhancing the grip force of a rubber as a tread material, comprises formulating a large amount of high reinforcing carbon black in a rubber composition for tire treads and controlling the hardness and modulus by means of a plasticizer. However, the self heat build-up becomes high and the stiffness of the tread also deteriorates due to rolling resistance or hardness change on driving. Furthermore, when an excessive amount of the plasticizer is added, the wear resistance is extremely deteriorated. A method is also known of enhancing the molecular weight of a rubber as the tread material to improve heat build-up and wear resistance. However, the workability becomes inferior and the enhancement of the molecular weight is limited. As described above, the grip force, heat build-up and wear resistance of the tire tread material are contradictory to each other even if a conventional material is used. Therefore it has been considered to be impossible to improve the above properties, simultaneously. We have now found that, by formulating a styrene-butadiene copolymer rubber (A) and a polybutadiene rubber (B) in specific proportions and specifying an amount of an acetone extract, the above properties can be improved, simultaneously. One object of the present invention is to provide a rubber composition having a high grip force, low heat build-up and high wear resistance. This object as well as other objects and advantages will be apparent to those skilled in the art from the following description. According to the present invention, there is provided a rubber composition which comprises: (i) a rubber component comprising 70 to 90 % by weight of a styrene-butadiene copolymer rubber (A) containing 15 to 40 % by weight of a styrene content and 30 to 60 % by weight of a vinyl content in the butadiene portion, which is obtained by solution polymerization, and 10 to 30 % by weight of a polybutadiene rubber (B) (ii) a vulcanizing agent in an amount of 0.75 to 1.75 parts by weight based on 100 parts by weight of the rubber component (i) and (iii) a plasticizer; 20 to 26 % by weight of the rubber composition being extracted by acetone when a vulcanized rubber sample obtained from the rubber composition is subjected to a Soxhlet extractor for 24 hours, and at least 80 % by weight of the acetone extract being the plasticizer. The present invention also provides a tire tread obtained by molding the rubber composition. According to the present invention, a rubber composition having suitable physical properties (e.g. hardness, modulus, etc.) as well as a high grip force, low heat build-up and high wear resistance can be obtained by the combination of the above components. The copolymer rubber (A) used in the present invention is produced by solution-polymerizing styrene and butadiene. The resulting styrenebutadiene copolymer has a binding styrene content of 15 to 40 % by weight, preferably 25 to 35 % by weight, and a vinyl content in the butadiene portion of 30 to 60 % by weight, preferably 40 to 60 % by weight. When the binding styrene content is less than 15 % by weight, there is a high hysteresis loss (grip force) and the practical strength becomes insufficient. When it exceeds 40 % by weight, the glass transition point becomes too large, which results in a high temperature dependence. Furthermore, the heat build-up and rolling resistance increase. When the vinyl content in the butadiene portion is less than 30 % by weight, the high hysteresis loss is insufficient. It is an advantage when the vinyl content in the butadiene portion is large in comparison with the binding styrene content. However, when the vinyl content exceeds 60 % by weight, its strength is poor and the wear resistance and tread-groove cracking resistance are inferior. In the present invention, in order to improve the wear resistance and low temperature properties, the polybutadiene rubber (B) is admixed in an amount of 10 to 30 % by weight with 70 to 90 % by weight of the styrene-butadiene copolymer rubber (A) to form a rubber component. When the mixing proportion of the component (B) is less than 10 % by weight, the improvement in the wear resistance and low-temperature properties is insufficient. When it exceeds 30 % by weight, the high hysteresis loss becomes insufficient and the grip force on wet road surfaces deteriorates. A plasticizer is formulated in the rubber composition to improve the workability and stability of the vulcanized rubber. The plasticizer is formulated such that an amount of an acetone extract obtained by extracting a vulcanized rubber obtained from the rubber composition with a Soxhlet apparatus in an acetone solvent for 24 hours comprises 20 to 26 % by weight of the whole rubber composition. In the present invention, at least 80 % by weight of the acetone extract is the plasticizer or softener. Any plasticizer which is known in the art can be used. Typical examples of the plasticizers are aromatic oils, mineral oils and paraffinic oils. Examples of formulating components other than the plasticizer are stearic acid, wax and vulcanization accelerators. When the amount of the acetone extract is less than 20 % by weight, it is necessary to decrease the amount of a filler in order to obtain a suitable hardness. As a result, hysteresis loss becomes insufficient. On the other hand, when the amount of the acetone extract exceeds 26 % by weight, the wear resistance rapidly decreases in spite of an increase of high hysteresis loss accompanied by the formulation of the filler, and it is of no practical use. A vulcanizing agent (e.g. sulfur, etc.) and optionally a vulcanization accelerator may be added to the rubber composition of the present invention. The amount of the vulcanizing agent is from 0.75 to 1.75 parts by weight based on 100 parts by weight of the rubber component (i). It has been found that a balance between high hysteresis loss and wear resistance is excellent in the above range. When the amount of the vulcanizing agent is less than 0.75 parts by weight, a sufficient hardness can not be obtained. Furthermore, since SBR having a high vinyl content is used as the main component, the number of double bonds of the main chain is small and the vulcanization rate becomes extremely low, and it is of no practical use. When the amount exceeds 1.75 parts by weight, the wear resistance and mechanical fatigue resistance deteriorate. Examples of the vulcanization accelerator include thiazole based accelerators (e.g. accelerator M, etc.) and sulfenamide based accelerators (e.g. accelerator CZ, accelerator NS, etc.). The vulcanization accelerator is formulated in an amount of from 1.00 to 4.00 parts by weight, preferably 1.00 to 3.00 parts by weight based on 100 parts by weight of the rubber component. When the amount is less than 1.00 parts by weight, the vulcanization rate is low and sufficient hardness can not be obtained, and it is of no practical use. When it exceeds 4.00 parts by weight, the crosslink density becomes large and, furthermore, the flex fatigue and wear resistance become inferior. A filler (e.g. carbon black, zinc white, stearic acid, etc.) or other additive (e.g. oil, antioxidant, wax, etc.) may be added to the rubber composition of the present invention. The carbon black which may be used has an iodine adsorption of at least 80 mg/g, for example, HAF, ISAF or SAF which are commercially available from Showa Cabot Co., Mitsubishi Chemical Industries Co. or Tokai Carbon Co. When the iodine adsorption is less than 80 mg/g, no wear resistance is obtained because of insufficient reinforcement and, furthermore, sufficient grip performances can not be obtained. The amount of the carbon black is from 50 to 150 parts by weight (based on 100 parts by weight of the rubber). When the amount is less than 50 parts by weight, the wear resistance and grip performances become insufficient because of insufficient reinforcement. When it exceeds 150 part by weight, the heat build-up of the rubber becomes large and the rolling resistance also becomes large. Therefore, the amount is preferably from 70 to 120 parts by weight. As the oil, an aromatic oil is preferred. The amount is from 20 to 100 parts by weight, preferably from 40 to 70 parts by weight, based on 100 parts by weight of the rubber component. When the amount is less than 20 parts by weight, the hardness of the vulcanized rubber becomes too large and it is not suitable as the tread. When it exceeds 100 parts by weight, the viscosity of the rubber before vulcanizing becomes too small and, therefore, a problem in workability arises. The rubber composition thus obtained is kneaded with an extruder and formed into a predetermined form of the rubber, which is vulcanized together with other constituent rubber components of a tire to obtain a tire. The vulcanization conditions are known. As described above, a rubber composition having suitable physical properties (e.g. hardness, modulus, etc,) as well as a high grip force, low heat build-up and high wear resistance can be obtained according to the present invention. EXAMPLES The following Examples and Comparative Examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof. In the Examples and Comparative Examples, all "parts" and "percents" are by weight unless otherwise stated. Examples 1 to 3 and Comparative Examples 1 to 8 By using styrene-butadiene copolymer rubbers shown in Table 1 and components shown in Table 2, a rubber composition was prepared by a normal method, respectively. The rubber composition was extruded and molded to obtain a tire (tire size: 225/50R16). The following items were evaluated using the tires obtained above. All tests were conducted at a tire inside pressure of 2.5 Kgf/cm2. The results are shown in Table 3. An evaluation of the test sample was conducted as follows. (1) Turning properties: A person rode on a 3,000 cc passenger car and drove with turning an asphalt surface having a diameter of 50 m. After driving, feeling of turning was evaluated with 5 grades. The larger the number, the better the feeling. (2) Brake index: A speed of 3,000 cc passenger car was reduced from 40 Km/h to 20 Km/h to obtain a retardation. Numbers in Table 3 are indexes which are calculated as the tires of Example 2 being 100. The larger the number, the better. The roads for the tests (1) and (2) were an asphart road (wetted state) of Skid No. about 50. (3) Rolling resistance: A rolling resistance was determined using a rolling resistance tester at a speed of 80 Km/h and a load of 35 Kgf. Numbers in Table 3 are indexes which are calculated as the tire of Example 2 being 100. The smaller the number, the better the rolling resistance. (4) Wear resistance: A person rode on a 3,000 cc passenger car and drove on a wear test road. After covering 10,000 Km, a wear loss was measured. Number in Table 3 are indexes which are calculated as the tires of Example 2 being 100. The larger the number, the better. As is apparent from Table 3, Examples 1 to 3 are superior in grip performances, rolling resistance and wear resistance in comparison with Comparative Examples 1 to 8. A tire of Comparative Example 1 wherein an emulsion-polymerized SBR having a large bound styrene content and a small vinyl content in butadiene portion is used is inferior in rolling resistance. Further, Comparative Example 2 wherein the bound styrene content is small is inferior in rolling resistance and wear resistance and Comparative Example 3 wherein the vinyl content is large is inferior in wear resistance. Comparative Example 4 wherein the bound styrene content is small and Comparative Example 5 wherein the vinyl content is small are inferior in grip performances. Comparative Examples 6 and 7 wherein the amount of polybutadiene (PHR) is small are inferior in wear resistance in comparison with Example 2. Comparative Example 8 wherein the amount of PHR is large is superior in wear resistance, but grip on the wetted road surface is extremely deteriorated. Examples 4 to 6 and Comparative Examples 9 and 10 By using a styrene-butadiene copolymer rubber (g) shown in Table 1 and components shown in Table 4, a test sample shown in Table 5 were prepared, respectively. Then, tire performances in the case that the amount of the acetone extract is changed were evaluated. The amount of the acetone extract shown in Table 5 indicates an amount of the acetone extract obtained by extracting the rubber composition with a Soxhlet apparatus in an acetone solvent, based on the total rubber composition. According to the same manner as that described in the above Examples, a tire was prepared using the resulting rubber composition, respectively. Then, the following test was conducted, in addition to the tests (1) to (4). Critical G test: A person rode on a 3,000 cc passenger car and drove with turning an dried asphart surface having a diameter of 50 m to measure maximum turning transverse G. Component Amount (parts) SBR (g) 85 Polybutadiene rubber 15 ISAF Aroma oil Wax 2 Antioxidant 2 Stearic acid 1 Zinc white 3 Sulfur 1 Vulcanization accelerator 2 As is apparent from Table 5, Comparative Example 9 wherein the amount of the acetone extract is smaller than 20 % by weight is inferior in turning properties and brake index and lacks in absolute grip. In Examples 4 to 6, grip performances increases as the amount of the acetone extract increases, and rolling resistance and wear resistance are liable to be deteriorated. However, even when the acetone extract exceeds 26 % as Comparative Example 10, remarkable improvement of grip performances are not accomplished and deterioration of wear resistance is rather large, and it is not preferred. Example 7 and Comparative Examples 11 to 13 By using a styrene-butadiene copolymer rubber (g) shown in Table 1 and components shown in Table 6, a test sample shown in Table 7 were prepared, respectively. Then, tire performances in the case that the amount of sulfur is changed were evaluated. The hardness (Hs) shown in Table 7 indicates a hardness of the vulcanized rubber measured at room temperature, using a JIS-A hardness tester. According to the same manner as that described in the above Examples, a tire was prepared using the resulting rubber composition, respectively. Then, the tests (1) to (4) were conducted. Component Amount (parts) SBR (g) 85 Polybutadiene rubber 15 ISAF 100 Aroma oil 60 Wax 2 Antioxidant 2 Stearic acid 1 Zinc white 3 Sulfur Vulcanization accelerator Regarding Comparative Example 11 wherein the amount of sulfur is smaller than 0.75 phr, a proper hardness can not be obtained, which results in inferior grip performances and wear resistance. Regarding Comparative Examples 12 and 13 wherein the amount of sulfur is larger than 1.75 phr, wear resistance is deteriorated. Regarding Comparative Examples 11 and 13, grip performances are also deteriorated. Examples 7 and 8 showed excellent balance between grip performances, rolling resistance and wear resistance.

Claims: A rubber composition comprising: (i) a rubber component comprising 70 to 90 % by weight of a styrene-butadiene copolymer rubber (A) containing 15 to 40 % by weight of a styrene content and 30 to 60 % by weight of a vinyl content in the butadiene portion, which is obtained by solution polymerization, and 10 to 30 % by weight of a polybutadiene rubber (B) (ii) a vulcanizing agent in an amount of 0.75 to 1.75 parts by weight based on 100 parts by weight of the rubber component (i) and (iii) a plasticizer; 20 to 26 % by weight of said rubber composition being extracted by acetone when a vulcanized rubber sample obtained from the rubber composition is subjected to a Soxhlet extractor for 24 hours, and at least 80 % by weight of the acetone extract being the plasticizer. The rubber composition as claimed in claim 1, which further comprises carbon black having an iodine adsorption of at least 80 mg/g in an amount of from 50 to 150 parts by weight based on 100 parts by weight of the rubber component (i). A tire tread obtained by molding the rubber composition as claimed in claim 1 or claim 2.

Inventors: KIKUCHI NAOHIKO, KIKUCHI, NAOHIKO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1993
Main classification: C08L9
IPC codes: C08K3, C08L9, C08L21, B60C1