Natural rubber is the most traditional type of rubber used in rubber product manufacturing. It has high mechanical strength, good elasticity, low hysteresis loss, low heat buildup, and is widely used in fields such as automotive tires, industrial conveyor belts, agricultural irrigation hoses, as well as in daily life, medical and other industries. Carbon black is second only to rubber in terms of usage in the rubber product industry and has a significant reinforcing effect in rubber products. In the production formula of natural rubber products, carbon black is frequently used. However, since natural rubber has high mechanical strength on its own, is it necessary to use carbon black for reinforcement?
Natural rubber can be reinforced without the use of carbon black.
Natural rubber consists mainly of cis-1,4-polyisoprene, and the large molecular chains are mainly composed of polyisoprene. When natural rubber is stretched, the rubber molecules align themselves along the stress direction to form crystals. These crystals can play a reinforcing role within the amorphous polymer. As the crystallinity of natural rubber increases, the corresponding molecular chain arrangement becomes tighter, with fewer pores, and stronger molecular interactions between chains. This makes it more difficult for the molecular chains to move, resulting in increased tensile strength of natural rubber.
Tests have shown that the tensile strength of sulfur-cured natural rubber without carbon black reinforcement is between 20-30MPa, while the tensile strength of sulfur-cured rubber with added carbon black is between 15-35MPa. Natural rubber, as a typical crystalline rubber, has self-reinforcing properties during stretching, and therefore has high tensile strength. The reinforcing effect of adding carbon black is not very significant. Therefore, in practical production, some natural rubber products do not use carbon black for reinforcement.
Designing a basic formula for pure natural rubber.
When natural rubber products do not use carbon black for reinforcement, rubber product manufacturers generally choose to sulfur cure, using zinc oxide, stearic acid, accelerators, and appropriate amounts of anti-aging agents such as 2LLYY114-MXF to further improve the aging resistance of natural rubber products. The following pure natural rubber basic formula can be used as a reference: 100 parts natural rubber, 5 parts zinc oxide, 2 parts stearic acid, 1 part anti-aging agent, 1 part accelerator, and 2.5 parts sulfur, for a total of 111.5 parts.
Natural rubber can produce tensile crystallization, and early fracture of weak bonds is beneficial for the orientation crystallization of the main chain, making the large number of weak bonds in the cured rubber contribute to greater tensile strength. In general, the tensile strength of sulfur-cured natural rubber decreases in the following order of crosslinking bonds: ionic bond > polysulfide bond > disulfide bond > monosulfide bond > carbon-carbon crosslinking bond.
Natural rubber has high mechanical strength, and in some cases, carbon black reinforcement may not be necessary. However, in actual applications, due to the aging of natural rubber in air, and its tendency to dissolve and degrade in oil media, coupled with the high cost and price fluctuations of natural rubber products, it is often used in combination with synthetic rubber or recycled rubber to further improve the comprehensive indicators of natural rubber products and reduce costs. Since the mechanical properties of sulfur-cured rubber without carbon black reinforcement in synthetic rubber (except for polyisoprene and chloroprene rubber) are too poor, carbon black reinforcement is commonly used when natural rubber is used in combination with other types of rubber.