How does the addition of nano-silica affect the performance of rubber?

Tensile Strength:

When the amount of added nano-silica is low, the tensile strength of the rubber usually increases with the increase in the amount added. This is because nano-silica can act as a reinforcing agent, forming physical or chemical adsorption with the rubber molecular chains. For example, in natural rubber, when the amount of nano-silica added increases from 0 to 5 phr (parts per hundred rubber), the tensile strength may gradually increase. The reason is that the nano-silica particles interact with the rubber molecular chains, and during stretching, they can effectively transmit and disperse stress, preventing the slippage of the rubber molecular chains, thereby increasing the tensile strength.

However, when the amount added exceeds a certain limit (e.g., exceeding 15-20 phr), because nano-silica particles are prone to agglomeration, forming stress concentration points, it will lead to a decrease in tensile strength. These agglomerated particles cannot work well with the rubber molecular chains, and are easily damaged locally during stretching, reducing the overall tensile strength of the rubber.

Tear Strength:

Adding an appropriate amount of nano-silica can improve the tear strength of rubber. When the rubber product is subjected to tearing force, the nano-silica particles can enhance the bonding force between the rubber molecular chains, making the rubber more difficult to tear. For example, in nitrile rubber, adding 3-8 phr of nano-silica can significantly improve the tear strength. This is because nano-silica particles are distributed in the rubber matrix, and when there is a tendency to tear, they can hinder the expansion of the tear, like setting many "obstacles" in the tearing path.

However, if the amount added is too high, the distance between the nano-silica particles is too close, which will make the rubber too rigid and lack toughness, thus leading to a decrease in tear strength. At the same time, excessive addition will also exacerbate the problem of particle agglomeration, affecting the overall performance of the rubber.

Hardness and Elastic Modulus:

With the increase in the amount of nano-silica added, the hardness and elastic modulus of the rubber generally increase gradually. Nano-silica particles play a filling and reinforcing role in the rubber, restricting the movement of the rubber molecular chains. For example, in silicone rubber, when the amount of nano-silica added increases from 0 to 30 phr, the hardness of the rubber will increase significantly. This is because the addition of nano-silica reduces the intermolecular distance of the rubber, enhances the interaction force between the molecular chains, making the rubber harder and the elastic modulus correspondingly higher.

However, excessive hardness and elastic modulus may not be suitable for all rubber applications. For example, in some places where soft rubber is needed, such as rubber gloves, if too much nano-silica is added, the gloves will become too hard, affecting the comfort of use.

Abrasion Resistance

Adding an appropriate amount of nano-silica can significantly improve the abrasion resistance of rubber. Nano-silica particles can form an abrasion-resistant structure in the rubber matrix. When the rubber surface is subjected to friction, these particles bear part of the frictional force, reducing the wear of the rubber matrix. For example, in tire tread rubber, adding 10-20 phr of nano-silica can effectively improve the abrasion resistance of tires. With further increase in the amount added, the abrasion resistance may continue to improve, but if the amount added is too high (e.g., exceeding 30 phr), because nano-silica particles may agglomerate, resulting in uneven abrasion resistance on the rubber surface, which will affect the overall abrasion resistance.

Processing Performance

In the mixing process of rubber, an appropriate amount of nano-silica can improve the flowability and dispersibility of the rubber compound. When the amount added is low (e.g., 1-5 phr), nano-silica can be uniformly dispersed in the rubber matrix, making the processing performance of the rubber compound more uniform. This helps the rubber to be easier to operate in subsequent forming processes, such as extrusion and vulcanization.

However, when the amount added is too high, nano-silica will increase the viscosity of the rubber compound and reduce its flowability. This is because excessive nano-silica particles interact with each other, increasing the internal friction of the rubber compound. For example, in the processing of rubber seals, if too much nano-silica is added, the rubber compound will be difficult to fill in the mold, and processing problems such as material shortage are prone to occur, and the uniformity of rubber crosslinking during vulcanization will also be affected.

Anti-aging Performance

Adding an appropriate amount of nano-silica can improve the anti-aging performance of rubber. As mentioned above, nano-silica can absorb and scatter ultraviolet rays, capture free radicals, and block oxygen and ozone. When the amount added is within a reasonable range (e.g., 3-10 phr), these anti-aging effects can be effectively exerted, delaying the aging process of rubber.