A low cobalt content rubber composition and its use in tire belts

Abstract

This invention discloses a low-cobalt-content rubber composition and its application in tire belt layers. The raw materials of the rubber composition are composed of the following weight ratios: 100 parts natural rubber, 45-65 parts carbon black, 5-10 parts insoluble sulfur, 0.3-0.8 parts sulfenamide accelerator, 0.5-1.5 parts vulcanization stabilizer, 5-15 parts zinc oxide, 0.1-0.5 parts stearic acid, and 1-3 parts antioxidant. The total cobalt content in the cobalt salt composite system is 0.12-0.18 parts based on elemental cobalt. This invention uses only 0.12-0.18 parts of cobalt in the cobalt salt composite system, lower than the 0.20-0.30 parts of cobalt in traditional formulations. While maintaining the initial adhesion performance between natural rubber and brass-plated steel cord, it effectively reduces the catalytic aging effect of excessive cobalt on the rubber matrix, improving the adhesion retention rate of the belt layer rubber after thermo-oxidative aging, damp-heat aging, and dynamic fatigue aging.

Description

Technical Field

[0001] This invention relates to the field of rubber polymer technology, and in particular to a low-cobalt-content rubber composition and its application in tire belt layers. Background Technology

[0002] The belt layer of a radial tire is a critical load-bearing component, requiring excellent initial adhesion and adhesion retention after aging between the rubber and the brass-plated steel cord. Traditional formulations typically use cobalt neodecanoate as an adhesion promoter, at a concentration of 0.20–0.30 parts by weight of total cobalt. However, excessive cobalt can catalyze rubber aging and reduce durability. Therefore, this paper proposes a low-cobalt-content rubber composition and its application in the tire belt layer to address the aforementioned problems. Summary of the Invention

[0003] The purpose of this invention is to provide a low-cobalt-content rubber composition and its application in tire belt layers, in order to solve the problems mentioned in the background art.

[0004] To achieve the above objectives, the present invention provides the following technical solution: a rubber composition with low cobalt content, wherein the raw materials of the rubber composition are composed of the following parts by weight ratio:

[0005] The composition includes 100 parts natural rubber, 45-65 parts carbon black, 5-10 parts insoluble sulfur, 0.3-0.8 parts sulfenamide accelerator, 0.5-1.5 parts vulcanization stabilizer, 5-15 parts zinc oxide, 0.1-0.5 parts stearic acid, 1-3 parts antioxidant, and a cobalt salt composite system. The total cobalt content in the cobalt salt composite system is 0.12-0.18 parts based on elemental cobalt.

[0006] Preferably, the cobalt salt composite system is composed of cobalt neodecanoate and cobalt imidazolium, wherein the amount of cobalt neodecanoate is 0.10-0.15 parts and the amount of cobalt imidazolium is 0.020-0.035 parts.

[0007] Preferably, the cobalt salt composite system is prepared by first weighing cobalt neodecanoate and cobalt imidazole separately, with 0.10-0.15 parts by weight of cobalt neodecanoate and 0.020-0.035 parts by weight of cobalt based on elemental cobalt. The two weighed cobalt salts are then premixed in a container to form a powder mixture.

[0008] Preferably, the antioxidant is a mixture of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine and 2,2,4-trimethyl-1,2-dihydroquinoline polymer, wherein the amount of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine is 1.5-2.5 parts and the amount of 2,2,4-trimethyl-1,2-dihydroquinoline polymer is 0.5-1.5 parts.

[0009] Preferably, the imidazole cobalt is selected from one or more of 2-methylimidazole cobalt, 2-ethylimidazole cobalt, benzimidazole cobalt, and 2-undecylimidazole cobalt.

[0010] Preferably, the amount of insoluble sulfur is 5.5-6.5 parts, and the sulfenamide accelerator is accelerator DZ, with an amount of 0.5-0.7 parts.

[0011] Preferably, the cobalt neodecanoate can also be borylated cobalt neodecanoate, cobalt stearate, borylated cobalt, or cobalt naphthenate, with cobalt neodecanoate being the most preferred.

[0012] Preferably, the rubber composition is prepared by first feeding natural rubber, carbon black, zinc oxide, stearic acid, antioxidant, and cobalt salt composite system into an internal mixer and mixing them at 120°C-150°C until homogeneous. The rubber is then discharged to obtain a first-stage compound. After cooling the first-stage compound, it is fed into an internal mixer with insoluble sulfur, sulfenamide accelerator, and vulcanization stabilizer and mixed at below 100°C to obtain the rubber composition.

[0013] The application of a low-cobalt-content rubber composition in a tire belt layer, as described above, wherein the belt layer comprises brass-plated steel cords, and the rubber composition is directly bonded to the steel cords.

[0014] The technical effects and advantages of this invention are as follows: In this invention, the total amount of cobalt in the cobalt salt composite system is only 0.12-0.18 parts, which is lower than the 0.20-0.30 parts of cobalt in the traditional formulation. In addition, adhesive resin is added. While maintaining the initial adhesion performance between natural rubber and brass-plated steel wire cord, the catalytic aging effect of excessive cobalt on the rubber matrix is ​​effectively reduced, and the adhesion retention rate of the belt layer rubber after thermo-oxidative aging, humid heat aging and dynamic fatigue aging is improved. Attached Figure Description

[0015] Figure 1 This is a flowchart illustrating the preparation process of the rubber composition of the present invention. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] This invention provides, for example Figure 1 The rubber composition shown is a low cobalt content rubber composition, wherein the raw materials of the rubber composition are composed of the following parts by weight: The composition includes 100 parts natural rubber, 45-65 parts carbon black, 5-10 parts insoluble sulfur, 0.3-0.8 parts sulfenamide accelerator, 0.5-1.5 parts vulcanization stabilizer, 5-15 parts zinc oxide, 0.1-0.5 parts stearic acid, 1-3 parts antioxidant, and a cobalt salt composite system. The total cobalt content in the cobalt salt composite system is 0.12-0.18 parts based on elemental cobalt.

[0018] Natural rubber is the basic material for constructing the mechanical skeleton of tire belt layer rubber. Its molecular chain structure endows the rubber with excellent high elasticity, high strength and excellent dynamic fatigue durability. Its unique strain-induced crystallization characteristics enable it to spontaneously strengthen under stress, providing tear resistance and crack growth resistance that synthetic rubber cannot match. This is the core of withstanding the complex stress state of tires and the basic material for constructing the mechanical skeleton of tire belt layer rubber. Carbon black, as a core reinforcing filler, forms a strong physical adsorption and chemical bond with rubber molecular chains through its nano-sized particles, which is key to improving the mechanical properties of rubber compounds. It can improve tensile strength, tensile stress, abrasion resistance, and tear resistance. Its structure, particle size, and surface activity determine the reinforcing effect. High-structure carbon black can form a more developed filler network, effectively disperse stress and inhibit crack propagation. At the same time, its excellent thermal conductivity helps to reduce dynamic heat generation, which is crucial for tire durability. As a highly efficient vulcanizing agent, insoluble sulfur maintains physical and chemical stability during rubber compounding, storage, and heat processing temperatures, avoiding the risks of blooming and scorching associated with ordinary sulfur. This ensures processing safety and component uniformity. Upon reaching the vulcanization temperature, it rapidly isomerizes into highly reactive soluble sulfur, uniformly participating in the cross-linking reaction between rubber molecular chains to form a stable three-dimensional network structure, thereby endowing the final product with shape stability, elasticity, and comprehensive physical properties. Sulphamide accelerators, as delayed-acting and highly efficient accelerators, are relatively stable at processing temperatures, providing a safe scorch time window for the rubber compound. However, they decompose rapidly after reaching a specific vulcanization temperature, releasing highly active accelerator substances that accelerate the crosslinking reaction between sulfur and rubber. This characteristic of being safe in the early stage and highly efficient in the later stage enables them to improve vulcanization efficiency, optimize the quality of crosslinking bonds, and help form a more uniform vulcanization network, which is key to balancing processability and performance. As an anti-reversion agent, vulcanization stabilizer participates in and stabilizes the vulcanization network through its unique chemical structure. It can effectively inhibit the thermal-oxidative breakage and structural rearrangement of polysulfide crosslinks in the over-sulfur state or high-temperature use environment, i.e., reversion, thereby maintaining the long-term stability of physical properties such as modulus, hardness, elasticity and key adhesion of the rubber compound, and greatly extending the service life of the product under harsh conditions. Zinc oxide is an indispensable activator for vulcanization. It reacts with stearic acid to form soluble zinc salts. This complex can effectively activate organic accelerators, improve the efficiency of the vulcanization reaction and the crosslinking density. In addition, it also has certain thermal conductivity, which helps heat transfer during the vulcanization process. Its alkalinity can also neutralize acidic substances in the vulcanization byproducts and delay aging. Modern formulations tend to use highly active or nano zinc oxide to achieve reduced dosage and increased efficiency. Stearic acid functions as both a processing aid and a vulcanization activator. As a processing aid, it acts as an internal lubricant and softener, improving the mixing and molding flowability of rubber compounds. More importantly, it reacts with zinc oxide to form soluble zinc salts, a key component of an efficient vulcanization activation system, directly affecting the activity of accelerators and crosslinking efficiency. Its dosage needs precise control; excessive use may lead to blooming. Antioxidants, as anti-degradation agents, effectively delay the aging and degradation of rubber caused by factors such as heat, oxygen, ozone, and dynamic fatigue through various mechanisms such as capturing free radicals, decomposing hydrogen peroxide, or passivating metal ions. In belt layer rubber compounds, antioxidants are often compounded to produce a synergistic effect, comprehensively protect the rubber compound, and prevent hardening, embrittlement, cracking, and adhesion decline. The cobalt salt composite system serves as a core adhesion promoter specifically designed for brass-plated steel wire cords. This composite system is composed of cobalt neodecanoate and imidazole cobalt in a specific ratio. Cobalt neodecanoate provides a stable release of cobalt ions, which react with the brass layer to generate the crucial cuprous sulfide adhesive layer. Imidazole cobalt, as a highly efficient organic cobalt ligand, plays a superior catalytic and directional role at the adhesive interface, optimizing the interface reaction efficiency and product stability. The synergistic effect of the two aims to achieve high initial adhesion and excellent heat and damp heat aging adhesion retention with the lowest total cobalt usage, combining performance and environmental considerations.

[0019] Furthermore, the cobalt salt composite system is composed of cobalt neodecanoate and imidazole cobalt in a specific ratio. The amount of cobalt neodecanoate is 0.10-0.15 parts, and the amount of imidazole cobalt is 0.020-0.035 parts, based on elemental cobalt content. This ensures that the total cobalt content is precisely controlled at a low level of 0.12-0.18 parts. Compared to traditional formulations that typically use 0.20-0.30 parts of cobalt, this invention reduces the total cobalt content by more than 40% through the synergistic effect of cobalt neodecanoate and a small amount of highly efficient imidazole cobalt, thus reducing the introduction of cobalt at the source. The core advantages of this design are: firstly, the synergistic effect of the low-volume cobalt salt composite is sufficient to ensure excellent initial adhesion between the rubber compound and the brass-plated steel wire cord; secondly, the reduced cobalt content effectively weakens the catalytic oxidative aging effect of excessive cobalt ions on the rubber matrix. Actual test data shows that, while maintaining the aging performance of the rubber body to meet the usage requirements, the adhesive durability of this composition is comprehensively improved compared with the traditional reference formulation using 0.20-0.30 parts of cobalt: the pull-out force of the steel cord after thermo-oxidative aging is increased by 8-10%, the pull-out force after salt spray aging is increased by 3-5%, and under harsh humid heat aging conditions, its adhesive retention rate is comparable to that of the traditional high-cobalt formulation. This proves that the present invention has successfully achieved better overall adhesive durability while reducing environmental impact and raw material costs.

[0020] Specifically, the cobalt salt composite system consists of cobalt neodecanoate and cobalt imidazolium, with the amount of cobalt neodecanoate being 0.10-0.15 parts and the amount of cobalt imidazolium being 0.020-0.035 parts. The cobalt salt composite system is prepared by first weighing cobalt neodecanoate and cobalt imidazolium separately, with 0.10-0.15 parts by weight of cobalt neodecanoate and 0.020-0.035 parts by weight of cobalt element. Then, the weighed cobalt salts are premixed in a container to form a powder mixture.

[0021] It should be noted that in the cobalt salt composite system, the amount of each component is precisely controlled based on the cobalt content: the amount of cobalt neodecanoate is 0.125 parts by weight, mainly responsible for providing a stable cobalt ion source to react with the brass layer to form initial bonds; the amount of imidazole cobalt is 0.025 parts by weight, and the total amount of cobalt is 0.15 parts by weight. As a highly efficient catalyst and interface modifier, it can more effectively optimize the formation and structure of the copper sulfide layer at the bonding interface. The preparation of this composite system follows a clear process step: First, cobalt neodecanoate and imidazole cobalt are precisely weighed according to the above formula ratio; then, the weighed cobalt salts are placed together in a mixing container and fully premixed by mechanical stirring or rotary mixing.

[0022] Specifically, the antioxidant is a mixture of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine and 2,2,4-trimethyl-1,2-dihydroquinoline polymer, wherein the amount of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine is 1.5-2.5 parts and the amount of 2,2,4-trimethyl-1,2-dihydroquinoline polymer is 0.5-1.5 parts.

[0023] It should be noted that N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine is a highly efficient p-phenylenediamine antioxidant. Its core function is to provide excellent protection against ozone aging and dynamic fatigue. The amine groups in its molecular structure can quickly capture and terminate the free radicals of rubber molecular chains generated by ozone attack or mechanical stress, effectively preventing the spread of chain oxidation reaction, thereby preventing cracking of the rubber surface due to ozone action and delaying the initiation and growth of cracks under repeated deformation. For tire belt layer rubber compounds exposed outdoors and subjected to complex stress, it is an indispensable key component for resisting environmental aging, maintaining the integrity of the product surface, and preventing the destruction of the adhesive interface and the final overall failure due to micro-cracks. The main function of 2,2,4-trimethyl-1,2-dihydroquinoline polymer is to provide excellent and long-lasting protection against thermo-oxidative aging. It decomposes the hydroperoxides generated by rubber under the action of heat and oxygen and converts them into relatively stable non-free radical products, thereby interrupting the autocatalytic oxidation cycle and effectively inhibiting the hardening, embrittlement and decline in physical and mechanical properties of rubber caused by long-term heating.

[0024] Specifically, imidazole cobalt is selected from one or more of 2-methylimidazolium cobalt, 2-ethylimidazolium cobalt, benzimidazole cobalt, and 2-undecylimidazolium cobalt.

[0025] In the cobalt salt composite system of the rubber composition, imidazole cobalt is a key component, and its specific chemical structure can be selected within a certain range to finely control its solubility, migration and interfacial reactivity in the rubber matrix, thereby optimizing the adhesive effect. The imidazole cobalt can be selected from one of 2-methylimidazole cobalt, 2-ethylimidazole cobalt, benzimidazole cobalt, and 2-undecylimidazole cobalt, or it can be a composite of two or more of these substances mixed in any proportion. The design of this selection range is based on the influence of different substituents on the overall performance of imidazole cobalt: 2-methylimidazole cobalt is the preferred option with balanced comprehensive performance. Its methyl group provides moderate steric hindrance and hydrophobicity, which can ensure a certain level of reactivity and help it to exist stably in the rubber compound; 2-ethylimidazole cobalt has a slightly longer ethyl chain, which may give it better organic compatibility and migration regulation ability; benzimidazole cobalt, due to its benzene ring structure, may exhibit higher thermal stability and different coordination electron effects, which may further optimize the structure of the interfacial products; while 2-undecylimidazole cobalt has a long carbon chain, and its strong hydrophobicity and large steric hindrance can change its dispersion and migration kinetics in the rubber phase. It is particularly suitable for occasions where it is necessary to delay the release of active cobalt species to match a specific vulcanization process. By selecting or compounding from the above series, the release rate, distribution and catalytic behavior of cobalt ions at the bonding interface can be fine-tuned for different processing conditions, vulcanization systems or aging environments. This selectivity makes the cobalt salt composite system of the present invention highly flexible and adaptable in formulation.

[0026] Specifically, the amount of insoluble sulfur used is 5.5-6.5 parts, and the amount of sulfenamide accelerator, accelerator DZ, is 0.5-0.7 parts.

[0027] Specifically, the cobalt neodecanoate can also be borylated cobalt neodecanoate, cobalt stearate, borylated cobalt, or cobalt naphthenate, with cobalt neodecanoate being preferred.

[0028] It should be noted that in the rubber composition, the cobalt carboxylate component in the cobalt salt composite system is not limited to cobalt neodecanoate, but can be extended to its modified derivatives or similar substitutes. Specifically, borylated cobalt neodecanoate, cobalt stearate, borylated cobalt, and cobalt naphthenate can be selected. These substances can all act as carriers of cobalt ions, migrating to the rubber-brass interface during vulcanization, catalyzing and participating in the formation of the key cuprous sulfide adhesive layer, which is the basis for achieving high-strength adhesion between rubber and brass-plated steel wire cord. Among them, cobalt neodecanoate is identified as the preferred option due to its excellent thermal stability, good dispersibility in rubber, moderate reactivity, and balanced cost-effectiveness brought about by the branched structure of the neodecanoate group. It can remain stable during processing and effectively release cobalt ions during vulcanization, thereby achieving high initial adhesion and good aging-resistant adhesion retention rate with low dosage. It is the representative choice with the best comprehensive performance.

[0029] Specifically, the preparation of the rubber composition involves first putting natural rubber, carbon black, zinc oxide, stearic acid, antioxidant, and cobalt salt composite system into an internal mixer and mixing them at 120℃-150℃ until homogeneous. The rubber is then discharged to obtain a first-stage compound. After cooling the first-stage compound, it is then added into an internal mixer with insoluble sulfur, sulfenamide accelerator, and vulcanization stabilizer and mixed homogeneously at below 100℃. The rubber composition is then discharged.

[0030] Step 1: Prepare a first-stage compound. The main polymer, natural rubber, is fed into an internal mixer for short-term plasticizing to reduce Mooney viscosity and increase its filler-accommodating capacity. Subsequently, reinforcing filler carbon black, activator zinc oxide, processing aid stearic acid, antioxidant, and a cobalt salt composite system as the bonding core are added sequentially or in batches. In a closed mixing chamber, the high shear and kneading action of the rotor ensures thorough mixing within a temperature range of 120℃–150℃. The energy input and thermal process during this stage allow the powder to be completely impregnated and coated by the rubber, forming a strong bond. Simultaneously, it ensures the initial dispersion of functional additives such as cobalt salt and antioxidant. Once the power curve stabilizes and the temperature reaches the set range, the first-stage compound is discharged, pressed into sheets, and forcibly cooled to below room temperature to prevent early vulcanization (scorching) caused by heat accumulation and to promote relaxation of rubber molecular chain stress. The second step involves preparing the rubber composition. The cooled compound is then returned to the internal mixer, and insoluble sulfur, sulfenamide accelerators, and vulcanization stabilizers are added at a temperature below 100°C. During this stage, the mixing temperature must be strictly controlled below 100°C. The core purpose is to prevent the insoluble sulfur from prematurely converting to soluble sulfur and migrating, and to avoid the accelerators triggering cross-linking prematurely during mixing. This ensures the rubber compound has sufficient processing safety time. Under low-temperature, short-time processing conditions, all additives are dispersed in the rubber compound. Once evenly mixed, the rubber is immediately discharged, resulting in the final rubber composition ready for subsequent molding and vulcanization operations.

[0031] The application of a low-cobalt-content rubber composition in a tire belt layer, as described above, is an example. The belt layer comprises brass-plated steel cords, and the rubber composition is directly bonded to the steel cords.

[0032] It should be noted that the primary and preferred industrial application of low-cobalt content rubber compositions is as belt layer adhesive compounds in radial tires for passenger cars or trucks. In this application, the rubber composition is compounded with brass-plated steel cords, which serve as the skeleton reinforcement material, through a calendering process to form belt layer components. During the subsequent tire vulcanization process, the cobalt salt composite system within the rubber compound plays a crucial role: cobalt ions migrate to the interface and chemically react with the brass plating on the surface of the steel cords, generating a robust transition layer in situ. Simultaneously, the adhesive resin and other components in the rubber compound work synergistically to form a stable chemical crosslink with the rubber matrix. This creates a strong and durable composite adhesive interface between the rubber and the metal. This direct adhesion ensures that the belt layer structure and the rubber matrix do not delaminate or shift relative to each other when the tire is subjected to the enormous centrifugal force during high-speed driving, the impact of complex road surfaces, and long-term flexural fatigue, thereby guaranteeing the tire's high-speed performance, handling stability, safety, and service life.

[0033] Example 1: Prepare a rubber composition according to the following parts by weight (total cobalt content 0.15 phr): The composition includes 100 parts natural rubber, 55 parts carbon black (N330), 11.625 parts zinc oxide, 0.2 parts stearic acid, 1.8 parts antioxidant 4020, 0.5 parts antioxidant RD, 6 parts insoluble sulfur, 0.5 parts accelerator DZ, 1 part vulcanization stabilizer, 0.125 parts cobalt neodecanoate (calculated as cobalt), 0.025 parts 2-methylimidazolium cobalt (calculated as cobalt), and 0.15 parts scorching inhibitor CTP. Comparative Example 1, the traditional formula: Cobalt neodecanoate 0.25 phr, without imidazole cobalt, otherwise the same as in Example 1.

[0034] Comparative Example 2, representing the reduction in cobalt neodecanoate: Cobalt neodecanoate 0.175 phr, without imidazole cobalt, otherwise the same as in Example 1.

[0035] Comparative Example 3, with an excess of imidazole cobalt: Cobalt neodecanoate 0.125 phr + 2-methylimidazolium cobalt 0.05 phr, the rest is the same as in Example 1.

[0036] Pull-out force test was performed using vulcanization conditions of 150℃ for 30 minutes, with 3+9 steel wire cords. 0.22+0.15NT, data on extraction force testing and thermo-oxidative aging (105℃ for 3 days), brine aging (5% NaCl solution for 7 days), and damp heat aging (93℃, 95% relative humidity for 7 days); The results of the extraction force and adhesive residue test are shown in Table 1 below: Table 1. Results of Pull-out Force and Adhesive Retention Rate Tests

[0037] Mechanical property test and vulcanization property test data were obtained. The vulcanization conditions for rubber sample preparation were: 150℃ for 30 min and rheological temperature of 150℃ for 60 min.

[0038] The mechanical property test and vulcanization performance test results are shown in Table 2 below: Table 2 Mechanical property test and vulcanization performance test table

[0039] Note: In Example 1 of this invention, the strength of the rubber body decreases after aging, but it still meets the requirements for use in the belt layer, typically ≥10MPa. To further improve the aging performance of the rubber, 0.5-1.0 parts of vulcanization stabilizer can be added.

[0040] Comparative Example 2 showed a similar adhesive rate to Example 1 after wet heat aging, but the pull-out force decreased significantly. The adhesive rate after thermo-oxidative aging was 90%, which was lower than that of Example 1 and Comparative Example 3.

[0041] Comparative Example 3 showed a pull-out force of 661 N after wet heat aging and a glue adhesion rate of 80% after salt spray aging, which was slightly lower. This indicates that the amount of imidazole cobalt should not exceed 0.05 phr, and preferably 0.020~0.035 phr.

[0042] The rubber composition provided by this invention can be directly applied to the production of belt layers in all-steel radial tires without modifying existing mixing and vulcanization processes, reducing cobalt salt costs while improving tire durability in hot and oxygen environments.

[0043] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A rubber composition with low cobalt content, characterized in that, The raw materials of the rubber composition are composed of the following parts by weight: The composition includes 100 parts natural rubber, 45-65 parts carbon black, 5-10 parts insoluble sulfur, 0.3-0.8 parts sulfenamide accelerator, 0.5-1.5 parts vulcanization stabilizer, 5-15 parts zinc oxide, 0.1-0.5 parts stearic acid, 1-3 parts antioxidant, and a cobalt salt composite system. The total cobalt content in the cobalt salt composite system is 0.12-0.18 parts based on elemental cobalt.

2. The low-cobalt-content rubber composition according to claim 1, characterized in that, The cobalt salt composite system consists of cobalt neodecanoate and cobalt imidazolium, wherein the amount of cobalt neodecanoate is 0.10-0.15 parts and the amount of cobalt imidazolium is 0.020-0.035 parts.

3. The low-cobalt-content rubber composition according to claim 2, characterized in that, The cobalt salt composite system is prepared by first weighing cobalt neodecanoate and cobalt imidazolium separately, with 0.10-0.15 parts by weight of cobalt neodecanoate and 0.020-0.035 parts by weight of cobalt based on elemental cobalt. The two weighed cobalt salts are then premixed in a container to form a powder mixture.

4. The low-cobalt-content rubber composition according to claim 1, characterized in that, The antioxidant is a mixture of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine and 2,2,4-trimethyl-1,2-dihydroquinoline polymer, wherein the amount of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine is 1.5-2.5 parts and the amount of 2,2,4-trimethyl-1,2-dihydroquinoline polymer is 0.5-1.5 parts.

5. A low-cobalt-content rubber composition according to claim 1, characterized in that, The imidazole cobalt is selected from one or more of 2-methylimidazolium cobalt, 2-ethylimidazolium cobalt, benzimidazole cobalt, and 2-undecylimidazolium cobalt.

6. The low-cobalt-content rubber composition according to claim 1, characterized in that, The amount of insoluble sulfur used is 5.5-6.5 parts, and the amount of sulfenamide accelerator is accelerator DZ, which is 0.5-0.7 parts.

7. A low-cobalt-content rubber composition according to claim 1, characterized in that, The cobalt neodecanoate can also be borylated cobalt neodecanoate, cobalt stearate, borylated cobalt, cobalt naphthenate, etc., with cobalt neodecanoate being preferred.

8. A low-cobalt-content rubber composition according to claim 1, characterized in that, The rubber composition is prepared by first feeding natural rubber, carbon black, zinc oxide, stearic acid, antioxidant, and cobalt salt composite system into an internal mixer and mixing them at 120℃-150℃ until homogeneous. The rubber is then discharged to obtain a first-stage compound. After cooling the first-stage compound, it is fed into an internal mixer with insoluble sulfur, sulfenamide accelerator, and vulcanization stabilizer and mixed at below 100℃ to obtain the rubber composition.

9. The application of a low-cobalt-content rubber composition as described in claims 1 to 8 in a tire belt layer, the belt layer comprising brass-plated steel cord, the rubber composition being directly bonded to the steel cord.

Images