Carbon Black
Authors: Dannenberg Eli M. , Paquin Lyn , Gwinnell Harry 
Published: August 2004

Carbon black is a generic term for an important family of products used principally for the reinforcement of rubber, as a black pigment, and for its electrically conductive properties. It is a fluffy powder of extreme fineness and high surface area, composed essentially of elemental carbon. Plants for the manufacture of carbon black are strategically located worldwide in order to supply the rubber tire industry, which consumes 70% of production. A number of processes have been used to produce carbon black. Since over 95% of the total output of carbon black is produced by the oil‐furnace process, emphasis is on this process. The arrangement of carbon atoms in carbon black has been well‐established by x‐ray diffraction methods, which show diffuse rings at the same positions as diffraction rings from pure graphite. The diffuse reflections sharpen, but the pattern never achieves that of true graphite. Whereas graphite has three‐dimensional order, carbon black has two‐dimensional order. The x‐ray data indicate that carbon black consists of well‐developed graphite platelets stacked roughly parallel to one another. Three terms are used to describe structures of increasing scale and complexity.particles(nodules) are the primary structure element;aggregatesare the primary dispersable elements of carbon black in all but thermal blacks; andagglomeratesare undispersed clusters of aggregates held together by van der Waals forces or by binders. The term structure is used to describe both the extent and the complexity with which the particles are interconnected in aggregates. Size and shape of the aggregates in composite systems are the principal features that determine the performance of carbon black as a reinforcing agent and as a pigment. The term structure is widely used in the carbon black and rubber industries. It is used to describe the relative void volume characteristics of grades of black of the same surface area. Structure comparisons of grades with different surface areas cannot be made. It is now known that the properties associated with structure are associated principally with the bulkiness of individual aggregates. High structure blacks in unvulcanized rubber give higher Mooney viscosities, lower die swell, faster extrusion rates, and better and more rapid dispersion after incorporation. In vulcanized rubber higher modulus is obtained. Oil‐furnace blacks used by the rubber industry contain over 97% elemental carbon. Thermal and acetylene black consist of over 99% carbon. Mechanisms of formation or carbon black must account for its unique morphology and microstructure. These features include the presence of nodules, or particles, multiple growth center within some nodules, the fusion of nodules into large aggregates, and the paracrystalline or concentric layer plane structure of the aggregates. The oil‐furnace process, based on the partial combustion of liquid aromatic residual hydrocarbons, channel (impingement) and gas‐furnace processes, gave improved yields over channel (impingement) and gas‐furnace processes, and better product qualities. It was also independent of the geographical source of raw materials, making possible the worldwide location of manufacturing closer to tire customers. Environmentally it favored elimination of particulate air pollution. The basic process consists of atomizing the feedstock into the combustion zone of the reactor where the combustion of natural gas and preheated excess air create a high temperature environment of 1200 to 1900°C that almost instantly vaporizes the feedstock and decomposes most of it to carbon black and hydrogen. Water is added to transform the product into wet granules. The wet pellets are then dried in a rotary dryer. Feedstocks are viscous aromatic hydrocarbons consisting of branched polynuclear aromatics with smaller quantities of paraffins and unsaturates. The principal sources of feedstocks in the United States are the decant oils from petroleum refining operations. Carbon blacks differ in particle or nodule size, surface area, aggregate size, and aggregate morphology. Aggregate size distribution and morphology determine such properties as surface area, dibutyl phthalate absorption (DBPA), and testing strength. The principal rubber grades are designated as N110, N121, N220, N299, N234, N339, N347, N375, N330, N326, N550, N650, N660, and N762. The consumption of the various carbon black grades can be divided into tread grades for tire reinforcement and nontread grades for nontread tire use and other rubber applications. Most of the special black grades are manufactured by methods to meet specific product specifications required for their end uses. They sell for a higher average price than the rubber grades. Of increasing importance in recent years have been applications in plastics to improve weathering resistance and to impart antistatic and electrically conductive properties. The manufacture of carbon results in trace amounts of organic and inorganic impurities suspected of causing potential health problems. Of particular concern have been the salts of toxic metals and adsorbed polynuclear aromatic hydrocarbons (PNAs). A few of the polyaromatic hydrocarbons are known to be mutagens and/or animal carcinogens. There is no evidence of increased cancer risk from exposure to industrial carbon blacks. The carbon black industry takes extreme efforts to confine product during all stages of manufacture. Highly efficient bag filters are used to collect the product. After collection the carbon black is pelletized to minimize dusting problems during shipping and use. Process gas is used as a fuel for in‐plant heat needs, and it may be burned to generate steam or power.