Application and development of inorganic powder materials in the rubber industry
Rubber is widely used in transportation, machinery, electronics, defense, and other sectors of the national economy. However, rubber also has its own significant drawbacks, such as weak intermolecular forces, large free volume, and poor self-crystallization ability, resulting in low strength and modulus, and poor wear resistance in rubber materials. Therefore, it is necessary to add inorganic non-metallic fillers to meet the requirements of these applications.
Generally speaking, inorganic non-metallic fillers in rubber mainly serve the following functions: reinforcement, filling (increasing volume) and cost reduction, improving processing performance, regulating vulcanization characteristics, and imparting special functions.
Commonly Used Inorganic Non-metallic Mineral Fillers in Rubber
(1) Silica
Silica is currently the second most widely used reinforcing agent in the rubber industry after carbon black. The chemical formula of silica is SiO2·nH2O. Its particle structure contains many voids. When these voids are in the range of 2nm-60nm, they easily combine with other polymers, which is the main reason why silica is used as a reinforcing agent. As a reinforcing agent, silica can greatly improve the wear resistance and tear resistance of materials. It can also significantly improve the mechanical properties of tires and is widely used in vehicles, instruments, aerospace, and other fields.
(2) Light Calcium Carbonate
Light calcium carbonate is one of the earliest and most widely used fillers in the rubber industry. Large amounts of light calcium carbonate added to rubber can increase the volume of the product, thereby saving expensive natural rubber and reducing costs. Light calcium carbonate filling rubber can achieve higher tensile strength, wear resistance, and tear strength than pure rubber vulcanizates. It has a significant reinforcing effect in both natural and synthetic rubber, and can also adjust consistency. In the cable industry, it can provide a certain degree of insulation. (3) Kaolin
Kaolinite is a hydrous aluminosilicate, a common clay mineral. Its practical application in rubber enhances the rubber’s elasticity, barrier properties, elongation, and flexural strength. Adding modified kaolinite to styrene-butadiene rubber (SBR) significantly improves the rubber’s elongation, tear strength, and Shore hardness, while also extending its service life.
(4) Clay
Clay can be added during tire manufacturing, depending on the production process requirements. Clay is used as a filler to reduce costs. However, it must be activated clay to facilitate bonding with rubber. Activated or modified clay can partially replace carbon black in the formulation.
Studies show that as the amount of clay increases, the hardness, 300% tensile stress, and tensile strength of the rubber compound decrease slightly, but this can be compensated for by adjusting the vulcanization system. When used in tread formulations, after system optimization, it can also reduce rolling resistance.
(5) Barium Sulfate
It can effectively enhance the anti-aging and weather resistance of rubber products such as tire rubber and belts. In addition, it can improve the surface smoothness of rubber products. As a powdered rubber filler, it can not only improve the powder application rate, but also has obvious advantages in terms of economic cost.
(6) Talc
Talc powder is usually divided into general industrial talc powder and ultrafine talc powder. The former, as a rubber filler, does not play a reinforcing role and has a negligible effect on improving the physical properties of rubber. Therefore, general industrial talc powder is often used as a separating agent. Ultrafine talc powder, on the other hand, has a good reinforcing effect. If it is used as a rubber filler, the tensile strength of the rubber itself is equal to the effect produced by silica.
(7) Graphite
Graphite belongs to the lamellar silicate non-metallic minerals and has good thermal conductivity, electrical conductivity, and lubricity. Using graphite as a rubber filler involves a similar process to that used for montmorillonite, where graphite is broken down into nano-sized particles using a special technique. When these nanoparticles combine with the rubber matrix, various functional properties of the rubber are improved. For example, electrical conductivity, thermal conductivity, airtightness, and mechanical properties are all significantly enhanced.