Rubber Carbon Black

Carbon black is the predominant reinforcing filler used in rubber compounds.  Through the addition of carbon black to the rubber compound, durability and strength are significantly improved.  Improvement in rubber properties is a function of the physical and chemical characteristics of carbon black.  Carbon black’s most important fundamental characteristics are aggregate size and shape (structure), particle size, surface activity, and porosity. 

Fundamental Characteristics

Structure / Aggregate Size
Carbon blacks do not exist as primary particles. During the manufacture of carbon black the primary particles fuse to form aggregates. The shape and degree of aggregate branching is referred to as structure. Increasing structure typically increases modulus, hardness, electrical conductivity and compound viscosity and improves dispersability of carbon black. Oil absorption is the measure of structure with a high number representing higher structure.

Particle Size
Typical carbon black primary particle size ranges from 8 nanometers for furnace blacks to 300 nanometers for thermal blacks. Finer particles increase reinforcement, abrasion resistance, and improve tensile strength. Surface area (STSA m2 per gram) is used by the industry to define carbon black fineness level – the higher the number the finer the black.

Surface Activity/ Chemistry
Carbon black surface chemistry is a function of the manufacturing process and the heat history of the carbon black.  Difficult to directly measure and quantify, surface chemistry can effect abrasion resistance, tensile strength, hysteresis and modulus.  The impact on cure characteristics is dependent upon the cure system used by the compounder.

Porosity
Increasing primary particle porosity ultimately leads to decreasing aggregate density.  Higher porosity allows compounders to increase carbon black loading, while maintaining compound specific gravity.  This leads to increases in modulus and electrical conductivity for a fixed loading of carbon black.

Physical Form
Rubber carbon black is typically sold as a beaded product (pellets).  Columbian also produces powdered and oil-beaded carbon black.