Rubber and its products are susceptible to degradation over time due to exposure to heat, oxygen, ozone, variable metal ions, mechanical stress, light, high-energy rays, and other chemical substances and mold. This degradation, characterized by tackiness, hardening, brittleness, or cracking, leads to a decline in physical and mechanical properties, a reduction in elasticity, and ultimately, a loss of utility. To counteract this, antioxidants are incorporated into rubber and its products to enhance resistance to these destructive forces, thereby delaying or inhibiting the aging process and extending storage life and service life.
Antioxidants can be classified into five major categories based on their chemical structures:
Amine Antioxidants:
These are the most effective antioxidants with the widest variety. They offer significant protection against thermal-oxidative aging, ozone aging, catalytic oxidation by heavy metals and ultraviolet light, as well as fatigue aging. However, they are prone to staining and are not suitable for white or light-colored rubber products. Some amine antioxidants can directly form carcinogenic nitrosamines or induce their production, while others may cause severe discoloration or blooming on rubber surfaces. Additionally, they possess varying degrees of irritancy to human skin. Consequently, these antioxidants are typically used only in dark or black rubber products. Representative products include Antioxidant D (Butyl), Antioxidant A (Methyl), Antioxidant DNP, Antioxidant 4010NA/IPPD, Antioxidant 4020/6PPD, Antioxidant TMQ/RD, and Antioxidant BLE.
Phenolic Antioxidants:
These antioxidants, while less effective than amine antioxidants, excel in non-staining properties. They do not cause discoloration, staining, or blooming. However, their protective effects in rubber are generally moderate, and they are more expensive, resulting in low consumption. In Western Europe, phenolic antioxidants account for only 8.1% of total antioxidant consumption, while in China, they constitute just 6%. Phenolic antioxidants contain a phenolic group in their molecular structure and do not produce harmful substances to humans, making them non-toxic additives suitable for the food industry and rubber products in contact with animals. However, they have an odor, poor storage stability, and are prone to deterioration under light exposure. Representative products include Antioxidant 2246. Antioxidant BHT (264), Antioxidant SP, and Antioxidant BHA.
Heterocyclic Antioxidants (Sulfur-Containing):
These antioxidants exhibit good non-staining properties and resistance to thermal oxidation. They are best used in combination with other antioxidants. However, they have a bitter taste, slight staining, and can cause blooming at high dosages. They also have a retarding effect on vulcanization (except for chloroprene rubber) and are considered non-environmentally friendly products, resulting in limited use in the rubber industry. Representative products include Antioxidant MB, Antioxidant MMB, Antioxidant MBZ, and Antioxidant MMBZ.
Phosphite Antioxidants:
These antioxidants provide good stabilization and thermal-oxidative resistance, along with some plasticizing properties. However, they have certain toxicity, odor, and estrogen-like effects, as well as a tendency to bloom, which restricts their use in the rubber industry. Representative product Antioxidant TNP acts as a hydrogen peroxide decomposer and free radical scavenger in polymer systems, serving as an auxiliary antioxidant often used in conjunction with hindered phenols and rarely alone.
Other Antioxidants:
- Long-Effective Antioxidants: Developed to prevent the loss of antioxidant activity due to volatilization or extraction when rubber products are used at high temperatures and under vacuum or in prolonged contact with liquid media. Representative products include nitrosodiphenylamine (NDPA) and allyl-substituted phenol (TAP).
- Nickel Salt Antioxidants: Such as Antioxidant NDBC, Antioxidant NDIBC, and Antioxidant NDMC, are green powders with good ozone and thermal-oxidative resistance. However, they are expensive (due to limited nickel reserves on Earth and wide applications), toxic, dark in color, and prone to blooming, making them unsuitable for light-colored rubber products and resulting in limited use in the rubber industry.
- Wax Antioxidants: When the dosage in the rubber compound exceeds its solubility in rubber, it migrates to the surface after vulcanization, forming a protective film that effectively prevents static ozone cracking. Representative products include paraffin wax and microcrystalline wax.
- Thiourea Antioxidants: Originally used as vulcanization accelerators for oxygen-containing rubbers, some thiourea derivatives have been found to possess good ozone aging resistance in other rubbers. These antioxidants exhibit good ozone aging resistance under both dynamic and static conditions, as well as good thermal-oxidative resistance, significantly extending the service life of rubber products. They are non-toxic, odorless, non-staining, non-blooming, suitable for white or black products, and environmentally friendly green varieties, particularly suitable for light-colored products and rubber products in the food, medical, and sanitary industries, with significant market potential in the rubber industry. Representative products include DBTU, DETU, and tributyl urea derivatives.
- Newly Developed Antioxidants: Including lactam derivatives, non-flex (anti-flex) LAS (LAS-P), vulcanized triazine derivatives, 6-QDI, Durazone37. etc.
In summary, rubber antioxidants play a crucial role in extending the service life of rubber and its products by protecting them from various aging factors. The choice of antioxidant depends on the specific requirements of the rubber product, including color, environmental conditions, and desired performance characteristics.
