Carbon blacks are typical fillers for reinforcing rubber

Carbon blacks are synthetic materials which essentially contain carbon as the main element. The structure of carbon black is similar to graphite (hexagonal rings of carbon forming large sheets), but its structure is tridimensional and less ordered. The layers of carbon blacks are parallel to each other but not arranged in order, usually forming concentric inner layers (turbostratic structure). Some typical properties are: density 1.7-1.9 g/cm3; pH of water suspension 2–8; primary particle size 14–250 nm; oil absorption 50–300 g/100 g; specific surface area 7–560 m2/g.

Carbon blacks are obtained by pyrolysis or combustion of hydrocarbon-containing materials (hydrocarbon gases, viscous residual aromatic hydrocarbons). Thermal and furnace carbon blacks are used in rubber base adhesives. In the thermal decomposition process, natural gas is fed in absence of air into a generator at a temperature of 1300°C, where it undergoes cracking. The thermal carbon blacks are obtained from the stream of products gases. The oil-furnace process is the most prevalent method of carbon black production. A reactor is fed by liquid hydrocarbon feedstock which is mixed with preheated air and natural gas. The furnace carbon back is obtained from the combustion gases. In general, the furnace carbon blacks have lower particle size and higher specific surface area than thermal carbon blacks.

Particle and aggregate size are the most important factors in carbon blacks. During the manufacture of carbon blacks, primary particles are obtained in the gas or liquid states. As the formation of primary particles at high temperature is produced, they tend to adhere to each other and partial fusion may occur producing aggregates. The aggregates can also be joined to produce agglomerates. The aggregates determine the properties of the carbon blacks because they cannot break even under very abrasive processing methods. On the other hand, depending on the manufacturing process, different active groups are exposed on the surface of carbon black; these groups affect wetting, dispersion, adsorption of moisture and reinforcing of polymers by carbon blacks.