Key Additives in Tire Manufacturing: Enhancing Performance and Safety
Key Additives in Tire Manufacturing: Enhancing Performance and Safety
We all know that the primary material for tires is natural rubber, sourced from rubber trees. Unprocessed rubber typically exists in forms such as blocks, latex, liquid, or powder. Clearly, these forms fall short of meeting the numerous performance requirements of tires, such as durability, grip, and puncture resistance. Therefore, during the manufacturing process, various additives must be incorporated in specific quantities to achieve the desired design and functional properties.
1. Coumarone Resin: The Versatile Tackifier and Reinforcer
Also known as: Coumarone-indene resin, benzofuran-indene resin, coumarone resin, indene resin, coal tar resin.
Softening point: 75–135°C
Properties: Viscous liquid or solid.
Characteristics: Coumarone resin resembles rosin in appearance. Neither liquid coumarone resin nor rubber alone possesses adhesive properties, but when combined, they impart excellent adhesiveness to rubber, including pressure-sensitive or hot-melt characteristics.
Application in Rubber Manufacturing: Coumarone resin exhibits good compatibility with rubber and acts as a solvent-based tackifier, plasticizer, and softener. The liquid form serves as an effective tackifier with slightly lower reinforcing properties, while the solid form, especially products with a high softening point, functions as a reinforcing agent that enhances the mechanical properties and aging resistance of rubber compounds. However, its tackifying effect is inferior to that of liquid coumarone resin.
Usage: 3–6 parts per hundred parts of rubber. It dissolves sulfur, aiding in the dispersion of sulfur and carbon black, and prevents scorching. Coumarone resin enhances tackiness, reinforcement, cut resistance, tensile strength, abrasion resistance, toughness, and hardness, meeting the demands of tire manufacturing. Notably, coumarone resin is also used in road marking paints.
2. Accelerators: Boosting Vulcanization Efficiency
Product name: Accelerator/Promoter
Properties: White powder.
Characteristics: Can be ignited with an open flame at room temperature; poorly soluble in ether, aromatic hydrocarbons, etc.
Application in Rubber Manufacturing: Primarily used in radial tires. Firstly, it acts as a curing agent for reinforcing resins, increasing the hardness of rubber products. Secondly, along with resorcinol and other additives, it forms an adhesion system that plays a critical role in bonding rubber to fibers. Vulcanization accelerators promote the vulcanization process, shortening curing time, reducing vulcanization temperature, decreasing the amount of vulcanizing agent required, and improving the physical and mechanical properties of rubber.
3. Zinc Oxide: Enhancing Heat Dissipation and Durability
Also known as: Zinc oxide, zinc white.
Properties: White or light yellow powder or hexagonal crystals; odorless with a bitter taste.
Under normal conditions, tires are the only medium between a vehicle and the road. During regular driving, tire rolling generates heat. When driving with insufficient tire pressure, the reduced pressure increases friction between the tire and the road, raising the tire temperature further. Prolonged driving can cause tire temperatures to reach up to 122°C, and continued temperature increases may lead to blowouts. This necessitates effective heat dissipation in tires.
Zinc oxide is one of the raw materials in rubber manufacturing. The combination of zinc oxide and stearic acid enhances the hardening degree of rubber. In automotive tire production, zinc oxide significantly improves the thermal conductivity of rubber, thereby aiding tire heat dissipation and ensuring driving safety. Additionally, zinc oxide additives protect rubber from microbial mold and UV degradation.
4. Carbon Black: The Classic Reinforcing Agent
Product name: Carbon black
As early as 1912, carbon black was discovered to have reinforcing effects on rubber. Since then, it has become an indispensable raw material in the rubber industry. Carbon black consumption typically accounts for 40%–50% of rubber usage. Simply put, for every two parts of rubber used in a formulation, approximately one part of carbon black is added.
In tire manufacturing, finer carbon black particles offer superior reinforcing properties. Higher-structure carbon black provides greater modulus and stress at definite elongation. Fine-particle reinforcing varieties are primarily used in tire treads to impart excellent wear resistance. Other parts of the tire, such as the sidewall, carcass ply, belt buffer layer, and inner liner, require rubber compounds with resistance to flex cracking, ozone oxidation, good elasticity, and low heat generation. These areas generally use larger-particle semi-reinforcing furnace blacks (with a surface area below 40 m²/g).
5. White Carbon Black: Revolutionizing Tire Performance
Product name: White carbon black/Silica
White carbon black has been produced since 1948 and used as a filler in the rubber industry. When used alongside silane coupling agents in tire tread compounds, it reduces rolling resistance while improving wear resistance and wet grip. Rubber compounds incorporating white carbon black exhibit enhanced tensile strength, tear resistance, and abrasion resistance. It is particularly advantageous in transparent and colored rubber products.
In the 19th century, rubber tires varied in color due to different additives. By 1915, the addition of carbon black turned rubber into a pure, ink-like black. In the 1990s, Michelin collaborated with a French chemical group to replace carbon with silica, achieving remarkable results. Since silica is inherently white, it can be combined with any dye, allowing tires to be produced in various colors. In recent years, the development of new rubber anti-aging agents, along with white and light-colored reinforcing agents, has further advanced tire color possibilities.
6. Aromatic Oil: Improving Elasticity and Processing
Aromatic oil, also known as aromatic hydrocarbons or arenes, refers to hydrocarbons containing benzene ring structures. It is a fundamental product and raw material in the petrochemical industry, primarily including benzene, toluene, xylene, ethylbenzene, etc.
Historically, early-discovered compounds of this type often had aromatic odors, hence the name "aromatic hydrocarbons." Later, even hydrocarbons without aromatic odors were classified under this term.
Aromatic oil exhibits good compatibility with black rubber and improves elasticity and toughness. It has strong affinity with natural rubber and butadiene rubber, promotes dispersibility of compounding agents, and facilitates calendering and extrusion processes.
However, one type of aromatic oil used in tire manufacturing, called polycyclic aromatic hydrocarbons (PAHs), has been identified as carcinogenic. Clinical studies indicate that long-term exposure to high concentrations of PAH mixtures can lead to skin cancer, lung cancer, stomach cancer, and liver cancer. PAHs can damage genetic material, trigger cancer cell growth, and increase cancer incidence rates.