FUNDAMENTAL PROPERTIES OF CARBON BLACK

A carbon black’s application performance is determined by its fundamental properties and the level of dispersion achieved. The most important physical and chemical properties include particle size, porosity, structure, and surface chemistry. The level of dispersion in any given matrix is strongly influenced by the mixing equipment, formulation (including dispersant selection), and physical form.

PARTICLE SIZE is the primary influence on color properties. Particle size is measured by electron microscopy(EM). Mean particle size is certified via statistical thickness surface area (STSA) correlation according to ASTM D3849-14. 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.

STRUCTURE is a measure of the three-dimensional fusion of carbon black particles to form aggregates. Highly structured carbon blacks provide higher viscosity, greater electrical conductivity and easier dispersion. Measures of aggregate structure may be obtained from shape distributions from EM analysis, oil absorption (OAN) or void volume analysis.

Porosity is indicated by comparing a carbon black’s external surface area predicted by STSA to the total surface area value obtained with the BET NSA method. Conductive carbon blacks tend to have a high degree of porosity.

SURFACE CHEMISTRY of carbon blacks generally refers to the oxygen-containing groups present on a carbon’s surface. 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.

PHYSICAL FORM is important in matching a carbon black to the equipment by which it is to be dispersed. 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.