The main properties of coal tar relevant to carbon production include softening point, viscosity, density, and carbon residue. Coal tar is an amorphous thermoplastic material; strictly speaking, it does not have a fixed melting point. The softening point is a temperature value measured under specific measurement conditions. Determination methods include the ring and ball method, the Mettler method, the mercury method, the cube method in air, the ring-and-rod method, and the thermomechanical method.
The Mettler method has been widely adopted in Europe and the United States due to its advantages, such as uniform heating rate and high data accuracy, thanks to its automated temperature measurement and data display. The ring and ball method, on the other hand, is commonly used for on-site monitoring due to its simple instrumentation.
Viscosity is a more direct indicator of the flowability of coal tar. Due to different measurement methods, coal tar viscosity can be expressed in various ways, such as dynamic viscosity, kinematic viscosity, and Engler viscosity. Pitches with different softening points have similar temperature sensitivity within the same viscosity range, with viscosity decreasing rapidly with increasing temperature.
The carbon residue is also known as the coking value. It refers to the mass fraction of the solid residue obtained by dry distillation of coal tar pitch under specific conditions. Because methods for determining the carbon residue rate vary widely, the test conditions should be clearly stated when reporting results. The carbon residue rate of coal tar pitch is closely related to its composition, with lower carbon residue rates resulting in higher carbon residues. Carbon residue rate is a key quality indicator for binder pitches. Using pitch with a higher carbon residue rate improves the bulk density, mechanical strength, and electrical conductivity of carbon products.