The primary particle size is directly related to the surface area interacting with the polymer matrix. As most of the pores, cracks, and cavities on the surface of the primaries are not accessible to the polymers, they do not contribute to the reinforcement effect. Therefore, the CTAB surface area specifically measuring the external surface area of the carbon black is more significant for the interacting surface than the BET SSA. The external surface area increases with decreasing particle size. A convenient relationship used to estimate the surface area (SA) from the particle diameter (d) of a nonporous carbon black is SA = 32,000/d. Typical carbon blacks for the reinforcement of elastomers are nonporous and have primary particle sizes between 20 and 65 nm. Porous particles tend to absorb accelerator and therefore retard the vulcanization. In principle, decreasing particle size (increasing surface area) improves the reinforcement. Tensile strength and modulus (stiffness) and tear and abrasion resistance increase, while elongation and resilience decrease. The drawbacks of small-particle-size carbon blacks are the higher heat buildup in the tire and the compound processing: reduced scorch time leads to poor process stability; the Mooney viscosity and incorporation time in the rubber increase, while the loading capacity in the rubber and dispersibility decrease.
The effect of primary particle size is further illustrated in Table 3. It can be seen that the tensile strength doubles on reducing primary particle size from 180 to 20 nm, while the abrasion resistance increases over sixfold.