Carbon black is the name of a chemical family with its C.A.S. NO. 1333-86-4, it is virtually pure elemental carbon in the form of colloidal particles that are produced by incomplete combustion or thermal decomposition of gaseous or liquid hydrocarbons under controlled conditions. Its physical appearance is that of a black, finely divided pellet or powder. Approximately 90% of carbon black is used in rubber applications, 10% as a pigment, that's we called Pigment Carbon Black with C.I. Pigment Black 7. It is special grades of carbon black are used as pigment, to enhance conductivity, static charge control, to provide UV protection and to enhance mechanical properties in products such as plastics, leather coloring, cable, printing inks, coatings, and textile paste.
Two carbon black manufacturing processes (furnace black and thermal black) produce nearly all of the world's carbon blacks, with the furnace black process being the most common. The furnace black process uses heavy aromatic oils as feedstock. The production furnace uses a closed reactor to atomize the feedstock oil under carefully controlled conditions (primarily temperature and pressure). The primary feedstock is introduced into a hot gas stream (achieved by burning a secondary feedstock, e.g., natural gas or oil) where it vaporizes and then pyrolyzes in the vapor phase to form microscopic carbon particles. In most furnace reactors, the reaction rate is controlled by steam or water sprays. The carbon black produced is conveyed through the reactor, cooled, and collected in bag filters in a continuous process. Residual gas, or tail gas, from a furnace reactor includes a variety of gases such as carbon monoxide and hydrogen. Most furnace black plants use a portion of this residual gas to produce heat, steam, or electric power.
The thermal black process uses natural gas, consisting primarily of methane or heavy aromatic oils, as feedstock material. The process uses a pair of furnaces that alternate approximately every five minutes between preheating and carbon black production. The natural gas is injected into the hot refractory lined furnace, and, in the absence of air, the heat from the refractory material decomposes the natural gas into carbon black and hydrogen. The aerosol material stream is quenched with water sprays and filtered in a bag house. The exiting carbon black may be further processed to remove impurities, pelletized, screened, and then packaged for shipment. The hydrogen off-gas is burned in air to preheat the second furnace.
The final performance of carbon black is determined by its physical and chemical properties. Three most important properties are particle size/surface area, structure and surface chemistry.
Particle size & surface area
Particle size, which is measured by electron microscopy, is a primary determinant of color properties. It's inversely related to surface area in that the smaller the particle size the larger the surface area per unit weight.
A finer particle size will increase blackness, tint and UV protection, electrical conductivity, vehicle demand and viscosity, while lowering the dispersibility.
Structure
Structure is a measure of the three dimensional fusion of carbon black particles to create aggregates. A carbon black characterized by aggregates composed of many primary particles with considerable branching and chaining is referred to as a high structur black. The level of structure is determined by the DBP absorption value.
High structure carbon blacks provide higher viscosity, electrical conductivity and easier dispersion, but tinting strength will be reduced as structure increases.
Surface Chemistry
Surface Chemistry refers to the quantity of oxygen complexs chemisorbed on the surface of the carbon black. These complexes include carboxylic, quinonic, lactonic and phenolics groups. Surface chemistry is a function of the conditions of manufacture and after treatment p, rocesses for applications. In particular, high surface activity groups improve the vehicle wetting of the carbon black, because the chemisorbed oxygen complexes act like surfactant in wetting process. Higher surface activity will also reduce viscosity and conductivity.
Specialty Carbon Blacks applications
There are three main markets for specialty blacks, plastics, coatings and inks. Carbon black is useful in these markets because of its intensely black pigmentation, UV resistance and electrical/thermal conductivity to products in which it is incorporated.
In plastics, carbon black is used as a colorant and to improve properties such as UV resistance and electrical conductivity. Major plastics related applications including film, fiver, blow molding, injection molding, wire and cable, conductive plastics and engineering plastics.
In inks, carbon black will affect the jetness, undertone, gloss, dispersibility and rheological properties of ink. The Smaller Particle Size (Higher Surface Area), blackness and tint strength will increase; mean while the result in poor dispersibility and higher viscosity.
The Higher Structure, blackness and tint strength will decrease; mean while the result in better dispersibility and higher viscosity.
The Higher surface activity (Volatile Content), the better dispersibility and gloss of Pigment Carbon Black, also will be a lower viscosity in liquid systems.
Pigment Carbon blacks are also widely used in coating industry, including Auto OEM &refinish, general industrial coatings, architectural coatings, powder coatings and universal colorants. In this industry, carbon blacks are categorized as high color, medium color and regular color based on the blackness performance.
High color carbon black has smallest primary particle size around 9nm to 15nm. Because of the very large surface area, most of these grades are treated to improve its dispersibility. There are mostly used in auto paint and other applications with high visual performance demands.
Primary particle size of medium color carbon black is around 16nm to 25nm, they are mostly used for general purpose.
Regular color carbon black has a primary particle size larger than 25nm, some regular grade with high structure are widely used for tinting which showed bluish undertone.
The selection of a specific carbon black depends on the end product requirements, it's affected by properties of carbon blacks, as well as processing conditions.