What is carbon black? A vital component in making many of the products we use every day stronger, deeper in color and longer lasting, carbon black in its pure form is a fine black powder, essentially composed of elemental carbon. It is produced by partial burning and pyrolysis of low-value oil residues at high temperatures under controlled process conditions.
Carbon black is mainly used to strengthen rubber in tires, but can also act as a pigment, UV stabilizer, and conductive or insulating agent in a variety of rubber, plastic, ink and coating applications. Apart from tires, other everyday uses of carbon black include hoses, conveyor belts, plastics, printing inks and automotive coatings.
The fundamental properties of carbon black determine application performance. These include:
- Particle Size
- Surface Chemistry or Surface Activity
- Physical Form
Measured by electron microscopy, this is the fundamental property that has a significant effect on rubber properties, as well as color properties for specialty carbon blacks.
For specialty carbon blacks, smaller particle diameter gives rise to higher surface area and tinting strength. High surface area is usually associated with greater jetness, higher conductivity, improved weatherability, and higher viscosity, but requires increased dispersion energy.
For rubber, finer particles lead to increased reinforcement, increased abrasion resistance, and improved tensile strength. To disperse finer particles size, however, requires increased mixing time and energy. Typical particle sizes range from around 8 nanometers to 100 nanometers for furnace blacks. Surface area is utilized in the industry as an indicator of the fineness level of the carbon black and, therefore, of the particle size.
This is a measure of the three-dimensional fusion of carbon black particles to form aggregates, which may contain a large number of particles. The shape and degree of branching of the aggregates is referred to as structure.
Highly structured carbon blacks provide higher viscosity, greater electrical conductivity and easier dispersion for specialty carbon blacks. Measures of aggregate structure may be obtained from shape distributions from EM analysis, oil absorption (OAN) or void volume analysis.
The structure level of a carbon black ultimately determines its effects on several important in-rubber properties. Increasing carbon black structure increases modulus, hardness, electrical conductivity, and improves dispersibility of carbon black, but increases compound viscosity.
This is a fundamental property of carbon black that can be controlled during the production process. It can affect the measurement of surface area providing a total surface area (NSA) larger than the external value (STSA).
Conductive specialty carbon blacks tend to have a high degree of porosity, while an increase in porosity also allows a rubber compounder to increase carbon black loading while maintaining compound specific gravity. This leads to an increase in compound modulus and electrical conductivity for a fixed loading.
SURFACE CHEMISTRY OR SURFACE ACTIVITY
This is a function of the manufacturing process and the heat history of a carbon black and generally refers to the oxygen-containing groups present on a carbon black’s surface.
For specialty carbon blacks, oxidized surfaces improve pigment wetting, dispersion, rheology, and overall performance in selected systems. In other cases, oxidation increases electrical resistivity and makes carbon blacks more hydrophilic. The extent of surface oxidation is measured by determining the quantity of the “volatile” component on the carbon black. High volatile levels are associated with low pH.
While difficult to measure directly for rubber, surface chemistry manifests itself through its effects on such in-rubber properties as abrasion resistance, tensile strength, hysteresis, and modulus. The effect of surface activity on cure characteristics will depend strongly on the cure system in use.
This is important in matching a carbon black to the equipment by which it is to be dispersed. The physical form (beads or powder) can affect the handling and mixing characteristics.
The ultimate degree of dispersion is also a function of the mixing procedures and equipment used. Powdered carbon blacks are recommended in low-shear dispersers and on three-roll mills. Beaded carbon blacks are recommended for shot mills, ball mills, and other high energy equipment. Beading provides lower dusting, bulk handling capabilities, and higher bulk densities, while powdered carbon blacks offer improved dispersibility.