Tread rubber compositions for heavy duty pneumatic tire

Abstract: A tread rubber composition for a heavy duty pneumatic tire comprises 30-100 parts by weight of a particular carbon black satisfying CTAB of 130-160 m²/g, DBP of 90-120 mℓ/100g and Dm of not more than 210 Å, based on 100 parts by weight of at least one rubber component.

Description: This invention relates to a tread rubber composition for pneumatic tires, and more particularly to a rubber composition having good wear resistance and fatigue resistance and low heat build-up and for use in treads for heavy duty pneumatic tires running on bad roads. Heretofore, carbon blacks of ISAF grade have been used as fillers for tread rubbers having a good reinforcing property from the viewpoint of the wear resistance, fatigue resistance and low heat build-up. Furthermore, carbon blacks of SAF grade have been used in order to improve the wear resistance. Moreover, it has been known that carbon blacks having a fine particle size and a high structure are generally suitable for improving the wear resistance of tire treads. However, as the particle size of carbon black is made finer, the low heat build-up is considerably degraded to lower the durability of the tire, while as the structure of carbon black becomes high, the wear resistance is good, but the fatigue resistance is undesirably degraded. Furthermore, when such a carbon black is compounded with a rubber composition, the viscosity of the compounded rubber undesirably rises and hence the processing of rubber becomes difficult. It is, therefore, an aim of the present invention to provide a tread rubber composition for a pneumatic tire capable of improving the wear resistance and fatigue resistance and low heat build-up without causing the aforementioned problems. The inventors have made various studies with respect to colloidal properties of carbon black in order to solve the above problems and found that these problems can be solved by compounding a particular carbon black with a rubber composition, and as a result the present invention has been accomplished. According to the invention, there is provided a tread rubber composition for a pneumatic tire comprising 30-100 parts by weight of carbon black satisfying the following conditions: (1) cetyl trimethyl ammonium bromide adsorption specific surface area (CTAB) is within a range of 130-160 m 2 /g; (2) dibutylphthalate absorption (DBP) is within a range of 90-120 mℓ/100 g; and (3) average void size of aggregate (Dm) is not more than 210 Å, based on 100 parts by weight of at least one rubber component selected from natural rubber and diene series rubbers. As the diene series synthetic rubber used in the invention, mention may be made of isoprene rubber, butadiene rubber, butyl rubber, styrene-butadiene copolymer rubber and the like, but the invention is not limited to these rubbers. Of course, additives usually used in the rubber industry such as sulfur, vulcanization accelerator, accelerator activator, antioxidant and the like may be compounded with the rubber composition according to the invention. Among the characteristics of carbon black according to the invention, CTAB is a value measured according to a method of ASTM D 3765-89, and DBP is a value measured according to a method of JIS K 6221-1982. Moreover, Dm is an average void size giving a maximum differential void volume when mercury is pressed into a carbon black sample from 16 psi to 30000 psi through a mercury porosimeter made by Micrometrix Co., Ltd. (trade name: Pore Sizer 9310) to measure a void size of aggregate and then a curve of void volume distribution is plotted from the measured void sizes of aggregates. The carbon black according to the invention can be produced by using an oil furnace for hard series carbon black and controlling combustion conditions in the furnace and residence time of resulting carbon black flow inside the furnace. The ranges of the characteristics of the carbon black used in the invention are defined as follows. When CTAB is less than 130 m 2 /g, the wear resistance considerably lowers, while when it exceeds 160 m 2 /g, the low heat build-up is largely degraded, so that CTAB is required to be within a range of 130-160 m 2 /g. When DBP is less than 90 mℓ/100g, the wear resistance considerably lowers and sufficient dispersibility of carbon black is not obtained, while when it exceeds 120 mℓ/100g, the cut resistance and fatigue resistance are degraded. Therefore, DBP is required to be within a range of 90-120 mℓ/100g. Preferably, DBP is within a range of 90-100 mℓ/100g. When Dm is more than 210 Å, the fatigue resistance is considerably poor, and even if the value of DBP is within the range defined in the invention, large degradation of the fatigue resistance is caused. Therefore, Dm is required to be not more than 210 Å. Preferably, Dm is not more than 200 Å. When the amount of carbon black is less than 30 parts by weight, sufficient wear resistance is not obtained, while when it exceeds 100 parts by weight, the viscosity of the rubber composition undesirably rises and the operability is considerably degraded. The following Examples are given in illustration of the invention and are not intended as limitations thereof. The following Table 1 shows values of characteristics of carbon black used in the Examples. Kind of carbon black CTAB (m 2 /g) DBP (mℓ/100g) Dm (Å) Remarks A 133 92 191 Invention products B 145 120 209 C 158 114 206 D 122 91 240 E 168 126 209 Comparative products F 144 121 234 G 154 65 180 H 142 95 218 The carbon blacks A-C shown in Table 1 are those according to the invention, and the carbon blacks D-H are those used for the comparison. The comparative carbon blacks have the following characteristics. The carbon black D has a small CTAB value and a large Dm value as compared with those of the invention. The carbon black E has large CTAB and DBP values as compared with those of the invention. The carbon black F has large DBP and Dm values as compared with those of the invention. The carbon black G has a small DBP value as compared with that of the invention. The carbon black H has a large Dm value as compared with that of the invention. Each of the above carbon blacks was used according to a compounding recipe (parts by weight) as shown in the following Table 2 to prepare rubber compositions. Each thus obtained rubber composition was used as a tread rubber to manufacture a tire having a tire size of 1000R 20 14PR. After the tire was run around a stone pit and dam site scattering stones thereon over a distance of 4000-6000 km, the wear resistance and fatigue resistance (rubber cut number) of the tire were evaluated. These evaluations were represented by an index on the basis that the tire of Comparative Example 3 was 100. The larger the index value, the better the property. The heat build-up was represented by an index on the basis that the tire of Comparative Example 3 was 100 on results of rebound resilience tested according to BS 903: Part A8: 1963 with respect to a part of a tread taken out from a new tire. The larger the index value, the better the property. Moreover, Example-4 was compared with Comparative Example 6. As seen from Table 2, in the tread rubber compositions for heavy duty tires according to the invention, the wear resistance and fatigue resistance can considerably be improved without damaging the low heat build-up by compounding a given amount of specified carbon black as compared with the rubber composition containing conventional carbon black.

Claims: A tread rubber composition for a pneumatic tire comprising 30-100 parts by weight of carbon black satisfying the following conditions: (1) cetyl trimethyl ammonium bromide adsorption specific surface area (CTAB) is within a range of 130-160 m 2 /g; (2) dibutylphthalate absorption (DBP) is within a range of 90-120 mℓ/100g; and (3) average void size of aggregate (Dm) is not more than 210 Å, based on 100 parts by weight of at least one rubber component selected from natural rubber and diene series rubbers. A tread rubber composition as claimed in claim 1, characterized in that said diene series synthetic rubber is selected from isoprene rubber, butadiene rubber, butyl rubber and styrene-butadiene copolymer rubber. A tread rubber composition as claimed in claim 1 or 2, characterized in that said DBP is within a range of 90-100 mℓ/100g. A tread rubber composition as claimed in any of claims 1 to 3, characterized in that said Dm is not more than 200 Å. A pneumatic tire the tread of which comprises a rubber composition as claimed in any of claims 1 to 4.

Inventors: OHASHI MASAYUKI, YAMADA HIROSHI, OHASHI, MASAYUKI, YAMADA, HIROSHI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1991
Main classification: C08K3
IPC codes: C08K7, C08K3, C09C1, C08L21