Rubber composition for a tyre sidewall

Abstract: To obtain a rubber composition which is superior in ozone resistance and is hard to be discoloured brown and white. A rubber composition for a tyre sidewall obtained by blending 0.5 to 2.5 parts by weight of a wax containing a component with 45 or more carbon atoms and having an average number of carbon atoms of 28 to 38 and 3.0 to 7.0 parts by weight of antioxidant containing 30 to 100% by weight of N-(1-methylheptyl)-N'-phenyl-p-phenylenediamine per 100 parts by weight of a rubber component comprising a diene rubber.

Description: The present invention relates to a rubber composition for a tyre sidewall. Hitherto, an antioxidant, a wax and the like have been contained in a rubber composition for a tyre sidewall to prevent cracking caused by ozone. As the antioxidant, an antioxidant containing N-phenyl-N'-isopropyl-p-phenylenediamine (IPPD) or N-(1,3-dimethylbutyl) -N'-phenyl-phenylenediamine (6PPD) is employed in general and, as the wax, a petroleum wax which contains a component having about 20 to 50 carbon atoms and has an average number of carbon atoms of 30 to 35 is employed in general. IPPD and 6PPD can provide a tyre sidewall with superior ozone resistance. However there is a problem that the surface of the tyre sidewall is discoloured brown, thereby causing deterioration of tyre appearance. Also, the wax used for improving ozone resistance blooms to form a film on the surface of the tyre sidewall. There is a problem, however, that the surface of the tyre sidewall can be discoloured to white when the bloom is excessive. For example, in JP-A-63-10646 and JP-A-63-162738, there is disclosed a technology to prevent such deterioration of tyre appearance. caused by brown discoloration with the use of, as an antioxidant, a compound represented by the general formula: wherein R and R' are independently hydrogen atom or an alkyl, and the sum of carbon atoms of R and R' is 8 to 19. There is not disclosed, however, a teaching to prevent the white discoloration caused by excess bloom of the wax. Considering the above-mentioned facts, an object of the present invention is to obtain a rubber composition which is superior in ozone resistance and does not discolour to brown and white. Accordingly the present invention provides a rubber composition for a tyre sidewall obtained by blending 0.5 to 2.5 parts by weight of a wax which contains a component having 45 or more carbon atoms in an amount of 3 to 10% by weight having an average number of carbon atoms of 28 to 38, and 3.0 to 7.0 parts by weight of an antioxidant containing 30 to 100% by weight of N-(1-methylheptyl) -N'-phenyl-p-phenylenediamine per 100 parts by weight of a rubber component comprising a diene rubber. In such a case, it is preferable that the rubber component contains 50 to 80% by weight of butadiene and the rubber composition is obtained by further blending 0.8 to 1.8 parts by weight of sulfur and an accelerator in an amount satisfying the equation of a weight ratio of sulfur/accelerator = 1.5 to 6.0 per 100 parts by weight of the rubber component. The present invention relates to a rubber composition for a tyre sidewall obtained by blending 0.5 to 2.5 parts by weight of a wax which contains a component having 45 or more carbon atoms and has an average number of carbon atoms of 28 to 38, and 3.0 to 7.0 parts by weight of an antioxidant (antiozonant) containing 30 to 100% by weight of N-(1-methylheptyl)-N'-phenyl-p-phenylenediemine per 100 parts by weight of a rubber component comprising a diene rubber. The diene rubber constituting the rubber component in the present invention may be a diene rubber employed conventionally in the field of tyres. As the diene rubber, there are, for example, natural rubber (NR), butadiene rubber (BR), styrenebutadiene rubber (SBR), isoprene rubber (IR) and the like. The diene rubbers may be used alone or in optional admixture of two or more. Among them, combination use of NR and BR is particularly preferable from the viewpoint of cut growth resistance. Particularly when BR is used, the amount thereof is preferably 50 to 80% by weight of the rubber component, further preferably 55 to 70% by weight of the rubber component from the viewpoint of compatibility between cut growth resistance and kneading and extruding processability. In the present invention, the wax which contains 3 to 10% by weight of a component having 45 or more carbon atoms and has an average number of carbon atoms of 28 to 38 is used from the viewpoint of compatibility between cut growth resistance and prevention of white discoloration. Further, a wax which contains 5 to 10% by weight of a component having 45 or more carbon atoms and has an average number of carbon atoms of 34 to 37 is preferably used from the viewpoint of prevention of white discoloration during storage. As a such wax, there is, for example, OZOACE (TM) 0355 (component having 45 or more carbon atoms: 9% by weight, average number of carbon atoms: 36, commercially available from Nippon Seiro Co., Ltd). Also, plural kinds of waxes may be used in optional admixture. In this case, a total amount of components having 45 or more carbon atoms and an average number of carbon atoms with respect to the whole was admixture are within the above-mentioned ranges. The antioxidant in the present invention contains 30 to 100% by weight of N-(1-methylheptyl)-N'-phenyl-p-phenylenediamine (8PPD) from the viewpoint of compatibility between ozone crack resistance and prevention of brown discoloration, and particularly preferably contains 30 to 60% by weight of 8PPD from the viewpoint of initial ozone crack resistance. Also, as the component other than 8PPD which the antioxidant can contain, there are, for example, compounds described as PPD tyre antioxidants in JP-A-63-10646 and JP-A-63-162738. As 8PPD, for example, OZONONE (TM) 35 commercially available from SEIKO CHEMICAL CO., LTD. Can be used suitably in the present invention. Plural kinds of antioxidants may be also used in optional admixture. In this case, 8PPD content in the whole antioxidant admixture may be within the above-mentioned range. The rubber composition for a tyre sidewall of the present invention is obtained by blending the above-mentioned rubber component, wax and antioxidant. As the mixing ratio thereof, 0.5 to 2.5 parts by weight. of the wax and 3.0 to 7.0 parts by weight of the antioxidant are blended per 100 parts by weight of the rubber component from the viewpoint of compatibility between ozone crack resistance and prevention of white discoloration and brown discoloration. Further preferably 0.5 to 1.5 parts by weight of the wax and 3.0 to 6.0 parts by weight of the antioxidant are blended from the viewpoints that white discoloration during storage is inhibited and also brown discoloration during usage is prevented. In the present invention, further to the above-mentioned components, sulfur and accelerator are preferably blended. Particularly when the rubber component contains 50 to 80% by weight of butadiene, sulfur is preferably blended in an amount of 0.8 to 1.8 parts by weight and accelerator is preferably blended in an amount satisfying the equation of a weight ratio of sulfur/accelerator = 1.5 to 6.0 per 100 parts by weight of the rubber component from the viewpoint of ozone crack resistance. In the case where the amount of sulfur is less than 0.8 parts by weight and the ratio of sulfur to accelerator is less than 1.5, it is not preferable because the amount of polysulfide bond which relaxes distortion of a tyre caused by external force becomes too low, and it is disadvantageous for ozone resistance. On the other hand, when the amount of sulfur is higher than 1.8 parts by weight and the ratio of sulfur to accelerator is higher than 6.0, it is not preferable because the crosslinking density becomes too high and the blooming rates of the antioxidant and the wax become extremely low. Further preferably, sulfur is blended in an amount of 0.8 to 1.3 parts by weight and accelerator is blended in an amount satisfying the equation of a weight ratio of sulfur/accelerator = 1.5 to 4.0 per 100 parts by weight of the rubber component from the viewpoint of ozone crack resistance being stable during the use of a tyre. In this case, if the amount of sulfur is not higher than 1.3 parts by weight and the ratio of sulfur to accelerator is not higher than 4.0, changes in physical properties of the tyre after its use, which is caused by an excess amount of polysulfide bond, can be prevented. The accelerator may be an accelerator which is usually used in the field of tyres. As the accelerator, there are, for example, N-tert-butyl-2-benzothiazolyl-sulfenamide (TBBS), N-cyclohexyl-2-benzothiazyl-sulfenamide (CBS), mercaptobenzothiazole (MBT), diphenylguanidine (DPG). Among them, TBBS and CBS are preferably used from the viewpoint of scorch time and heating time. In the present invention, further to the above-mentioned components, a component such as carbon black, process oil, stearic acid, petroleum resin, zinc oxide, silica, coupling agent, or vulcanisation retarder which is usually in the field of tyres can be blended in the rubber composition of the present invention in an amount within a range where the effect of the present invention is not decreased. As the blending method, a conventional manner can be employed to obtain the rubber composition of the present invention. Further, the rubber composition of the present invention obtained as above can be applied for the sidewall of a tyre in the conventional manner. Therefore, the present invention relates to a tyre having a sidewall comprising the above-mentioned rubber composition. Particularly, the tyre is suitable for a truck and bus tyre from the viewpoint that it is necessary for the truck and bus tyre, which is used under severe conditions to contain a high amount of antioxidant. The present invention is explained in detail according to Examples in the following. Preparation Examples 1 to 17 Rubber component, wax, antioxidant and accelerator in amounts shown in tables 1 and 2 were kneaded with 50 parts by weight of carbon black, 5 parts by weight of oil, 2 parts by weight of petroleum resin, 2 parts by weight of stearic acid and 4 parts by weight of zinc oxide with a Banbury mixer. Them vulcanising agent (Sulfur) in an amount shown in Tables 1 and 2 was added thereto and kneaded with a roller to obtain the Rubber Compositions 1 to 17 for a tyre sidewall. As carbon black, N330 available from Showa Cabot K.K. was used. As oil, X140 (TM) available from JAPAN ENERGY CORPORATION was used. As Wax-1, SUNNOC N (TM) (average number of carbon atoms: 34, content of a component with 45 or more carbon atoms: 0% by weight) available from OUCHISHINKO CHEMICAL INDUSTRIAL CO., LTD. was used. As Wax-2, OZOACE 0355 (TM) (average number of carbon atoms: 36; content of a component of 45 or more carbon atoms: 9% by weight) available from Nippon Seiro Co., Ltd. was used. As Wax-3, SUNTIGHT BW (TM) (average number of carbon atoms: 54, content of a component with 45 or more carbon atoms: 60% by weight) available from SEIKO CHEMICAL CO., LTD. was used. As antioxidant-1, ANTIGENE 6C (TM) (6PPD) available from Sumitomo Chemical C., Ltd. was used. As antioxidant-2, OZONONE 35 (TM) (8PPD) available from SEIKO CHEMICAL CO., LTD. was used. As accelerator, NOCCELER NS-F (TM) (TBBS) available from OUCHISHINKO CHEMICAL INDUSTRIAL CO., LTD. was used. Then, the obtained Rubber Compositions 1 to 17 for a tyre sidewall were evaluated according to the following methods. Evaluating Methods (i) Running test: From each Rubber Composition 1 to 6, rubber sheets, 1, 2, 3, 4, 5 and 6 or 3.5mm in thickness, 200mm in width and 190cm in length were obtained and patched on the surface of a tyre sidewall to form a sidewall divided to six portions, followed by vulcanising at 145°C for 40 minutes to make Testing Tyre 1 having a size of 285/75R24.5. Also, in the same manner, Testing Tyre 2 was made by using Rubber Compositions 5 and 7 to 11 and, further, Testing Tyre 3 was made by using Rubber Compositions 5 and 17. It is noted that the rubber sheet 5 was patched on all of Testing Tyres 1 to 4 as standard for comparison. Then, each Testing Tyre was used for running about one hundred thousand kilometres and, then, crack resistance thereof was evaluated through their appearances according to the following criteria. The results are shown in Tables 1 and 2. 5: No crack was observed 4: Small cracks were observed slightly (about 1mm). 3: Several middle size cracks were observed (about 2 to 3mm). 2: Many large cracks were observed (about 4mm). 1: Large cracks were combined with each other, growing to larger cracks. (ii) Outdoor exposure test: Testing Tyres 1 to 4 which were made in the same manner as in the above-mentioned running test were stood outdoors for two months and, then, degrees of brown discoloration thereof were evaluated through their appearances according to the following criteria. The results are shown in Tables 1 and 2. 5: No brown discoloration was observed. 4: Brown discoloration was observed slightly. 3: Brown discoloration was observed within a half portion of the whole surface of each rubber sheet. 2: Significant brown discoloration was observed over a half portion of the whole surface of each rubber sheet. 1: Brown discoloration was observed on the whole surface of each rubber sheet. (iii) Indoor storing test: Testing Tyres 1 to 4, which were made in the same manner as in the above-mentioned running test, were stood in a room for three months, while being not exposed to light directly and, then, degrees of white discoloration thereof were evaluated through their appearances according to the following criteria. The results are shown in Tables 1 and 2. 5: No white discoloration was observed. 4: White discoloration was observed slightly. 3: White discoloration was observed within a half portion of the whole surface of each rubber sheet. 2: Significant white discoloration was observed over a half portion of the whole surface of each rubber sheet. 1: White discoloration was observed on the whole surface of each rubber sheet. According to the present invention, a rubber composition for a tyre sidewall which is superior in ozone resistance and is hard to be discoloured to brown and white can be obtained.

Claims: A rubber composition for a tyre sidewall obtained by blending 0.5 to 2.5 parts by weight of at least one wax which contains a component having 45 or more carbon atoms in an amount of 3 to 10% by weight and has an average number of carbon atoms of 28 to 38, and 3.0 to 7.0 parts by weight of an antioxidant containing 30 to 100% by weight of N-(1-methylheptyl) -N'-phenyl-p-phenylenediamine; per 100 parts by weight of a rubber component comprising a diene rubber. A rubber composition according to claim 1, characterised in that the component of the was has an average number of carbon atoms in the range of 34 to 37. A rubber composition according to claim 1 or 2, characterised in that two or more kinds of wax are used in admixture. A rubber composition according to claim 1, 2 or 3, characterised by 30-60% by weight of N-(1-methylheptyl)-N'-phenyl-p-phenylenediamine. A rubber composition. according to claim 1, characterised in that the rubber component contains 50 to 80% by weight of butadiene and the rubber. composition is obtained by further blending 0.8 to 1.8 parts by weight of sulfur and an accelerator in an amount satisfying the equation of a weight ratio of sulfur/accelerator = 1.5 to 6.0 per 100 parts by weight of the rubber component.

Inventors: KAWASE MASATO, KOTANI MARINA, MIZUNO YOICHI, SAKAMOTO SHUICHI, TSUMORI ISAMU, WAKABAYASHI NOBORU, KAWASE, MASATO, KOTANI, MARINA, MIZUNO, YOICHI, SAKAMOTO, SHUICHI, TSUMORI, ISAMU, WAKABAYASHI, NOBORU, Tsumori, Isamu, 303 Minami-tsukaguchi Park Hills
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1998
Main classification: C08L21
IPC codes: C08L21, C08K5