When looked at casually, carbon black seems a lot like soot, and, in a sense, that is what it is. Soot is also made up mostly of carbon, resulting from partially burning carbon-based matter. In its purest form, carbon black exists as a fine powder almost exclusively made up of carbon. Carbon black is a vital component in many everyday products, produced by igniting low-grade petroleum, natural gas, or other types of carbon-based residue, then heating it at high temperatures under a controlled process.
What is the History of Carbon Black?
Ancient people in China and Egypt mixed soot into tar, vegetable oils, or resins to create dyes and inks. The ancient Greeks, and later the Romans at a larger scale, used furnaces lined with bricks to make large quantities of carbon black, which they used to decorate walls. This was the precursor to the lamp black process for making carbon black.
With the invention in the mid-1400s of the printing press in Germany, carbon black became an ingredient for printing ink. Using pine resin from the Black Forest, the area became the epicenter of a thriving carbon black production industry. The demand for carbon black increased further with the invention of the letterpress, as its consistency worked better for the new technology than more viscous inks.
During the industrial revolution, the easy availability of coal tar made it possible to mass-produce carbon black. However, techniques were still virtually the same as those used in ancient Rome. In the mid-1800s, carbon black fabrication began to use filter systems, making the process more environmentally friendly. At the same time, in North America, a method that used natural gas as a feedstock was developed. Firing plants in the United States were designed to take advantage of the abundant supplies of natural gas produced through oil extraction, with firing plants becoming mobile operations that moved on to another well once gas from one was depleted.
Carbon black made this way is known as channel black. The market dramatically grew once the industry discovered that it could reinforce rubber, while the nascent auto industry found finer particles from this new process increased longevity in tires. Meanwhile, the lack of natural gas in Europe led instead to the use of coal tar, known as the “gas black” method. Since that time, manufacturers have developed various grades of carbon black, each with slightly different properties.
What is Carbon Black Used for Now?
Still used mostly in rubber products, and particularly in tires, the discovery of carbon black’s reinforcing properties happened by accident. Previously, manufacturers used zinc oxide to eliminate stickiness that occurred naturally in rubber. Not only did carbon black reduce rubber’s adhesive tendencies, but it also helped strengthen it, resulting in longer-lasting tires. By the early 1920s, carbon black became the primary means by which car tires were reinforced.
It also became increasingly clear that processing techniques could alter the properties of materials with which carbon black was combined. This resulted in new carbon black classifications, such as MPC (medium processing channel) and HAF (high abrasion furnace) blacks. The different carbon black iterations led to tires with reduced rolling resistance, which minimized friction and increased fuel efficiency.
Properties: What is Carbon Black?
Manufacturers use highly controlled processes to produce carbon black that is engineered to have certain properties. How these carbon aggregates join together determines what physical, optical, and electrical properties the material has.
Applications for carbon black depend upon the following fundamental properties:
- Particle size impacts both coloration and properties of rubber, with smaller particles offering higher conductivity and viscosity, improved weathering capabilities, stronger reinforcement, increased tensile strength and improved abrasion resistance.
- Permeability can be controlled during production, with carbon blacks that are very porous, increasing resistance and electrical conductivity in the material with which it’s combined.
- The physical form of carbon black – either beaded or powdered – affects how it’s handled and mixed, with beaded blacks requiring less dusting, higher bulk densities, and bulk handling capabilities.
- Structure refers to the shape and how carbon particles branch out when forming into aggregates. Higher viscosity, easier dispersion, and better electrical conductivity result from material mixed with highly structured carbon particles.
- Surface activity or chemistry involves its exposure to heat. This generally relates to oxidized surfaces, which improve rheology, wetting, dispersion, and overall material performance. It also affects rubber’s abrasion resistance, modulus, tensile strength, and other physical characteristics.
What is Carbon Black’s Main Physical Property?
The rubber industry uses the vast majority of carbon black today to increase performance. Carbon black is used with natural rubber and synthetic elastomers; carbon black acts as a reinforcer, strengthening the material. This involves mixing carbon black with elastomers and sulfur, along with multiple processing oils and chemicals. These are then heated to produce a wide variety of products.
It provides the following benefits for both natural and synthetic elastomers:
- Economical reinforcing agent
- Improves material’s resilience
- Keeps material from tearing
- Promotes conductivity
- Reinforces material
Because of its importance within the rubber industry, it’s often referred to as rubber carbon black.
What is Carbon Black’s Electrical Conductivity Property?
Electrical conductivity is usually only measured for the compounds that contain carbon black. These polymers and other binding agents have increased electrical conductivity when blended with carbon black. Conductivity depends upon how concentrated and dispersed carbon black is within the mixture, along with the type of materials with which it’s combined. Carbon black also controls static charges, so polymers used to insulate cabling and wiring often use specialty carbon blacks.
What are Carbon Black’s Optical Properties?
Though used primarily for rubber reinforcement, carbon black has several visual properties, making it essential as a coating material and pigment. It is also still used in the printing industry and other specialized applications. Fine carbon black particles are used in paints, including as a key ingredient in jet-black colors for coloring polymers.
Compared to black organic dyes, specialty carbon blacks have the following advantages:
- Keeps colors stable
- Obscures surfaces upon which it is applied
- Resistant to solvents, acids, and alkaline substances
- Stable when exposed to heat
Many black pigments and paints with bluish undertones use carbon blacks made with finer particulates for their visual appeal within the automotive industry. Coarser carbon blacks are commonly used for paints with brownish undertones, providing a grayish hue.
What is Carbon Black’s UV Blocking Capability?
One other major benefit of carbon black is its ability to absorb ultraviolet light (UV) from the sun, which it then converts to heat. Used with polymers like polyethylene or polypropylene, it keeps them from degrading when exposed to UV radiation in sunlight.
What is Carbon Black Used For?
The rubber industry uses over 90 percent of the world’s carbon black production. Though used mainly to strengthen tires, its properties make it an important ingredient in a wide variety of materials as a conductive or insulating agent, pigment, and UV stabilizer. Carbon black’s fundamental properties within elastomers, particularly when it comes to tires, are determined according to particle size, surface activity, and how they form into aggregates.
Carbon black is used in:
- Conveyor belts
- Industrial rubber goods
- Insulation for cabling and wiring
- Mechanical rubber goods
- Membrane roofing
- Motor vehicle paints
- Pigments for the printing industry
- Printing inks
- Seals, belts, gaskets, and hoses within internal combustion engines
- Tire components such as treads, sidewalls, and inner lining
- Vehicle tires
When used for industrial and mechanical components, modern carbon blacks are valued for their ability to disperse efficiently, reliability during processing, and consistent quality. Its electrical insulation properties make it an important ingredient for insulating wiring in the electrical and construction industries, including enhancing insulation properties for polystyrene.
Post-treating binding agents with carbon black also improves ink’s properties, and it continues to serve as an important ingredient for non-impact printing. Carbon blacks are also used for various other coatings, construction, fiber, paint, paper, plastics, and printing industries.
Processing Carbon Black
The prime piece of equipment used for processing carbon black is the reactor in which it’s fired, with adjustments to conditions producing different grades. Temperatures, flow rates, time in the reactor, and reactor design all affect the physical characteristics of the end product.
A typical carbon black process follows this path:
- Preheated air and feedstock – natural gas, oil, or other carbon-based fuel – are fed into the reactor in closely regulated amounts.
- The feedstock and air react until hydrocarbons are partially combusted, decomposed by heat, nucleated, and aggregated, a process lasting about one-hundredth of a second.
- Water is then injected to reduce the temperature to halt the “smoke” reactions as it leaves the reactor.
- This stream of “smoke” then goes to a baghouse, where the carbon black gets separated from the reactor’s tail gas, with filters capturing the carbon black and sending it to an accumulator tank.
- Pellets are then formed by mixing a binding agent with water to the powdered carbon black.
- These pellets are then dried in a rotary drier, increasing bulk density and making them easier to transport.
- The carbon black pellets are then packaged and sent on to end-users.
What is Carbon Black’s Range in Marketable Particulate Sizes?
The carbon particles can vary according to surface chemistry, particle, and aggregate size, shape, and permeability with commercially-produced carbon black. Typically, carbon black comprises more than 95 percent carbon, with traces of hydrogen, nitrogen, and oxygen. During the fabrication process, particles of carbon black form in sizes ranging from 10-500 nanometers. These particles then join into chains of aggregates, with the structure of these determining the carbon black grade, with the end product either powdered or granulated.
Using Air Classification in Carbon Black Production
During the manufacturing process, impurities known as “grit” form within the usable carbon black. These impurities are several times the size of even the aggregated carbon black. Regarding tire manufacturing, if these impurities are not removed from usable carbon black, it will adversely affect the quality of the final product.
Because of this factor, high-speed grinding mills and air classifiers have succeeded in achieving lower grit levels. When using air classification to remove grit, the construction of the classifier is essential, as carbon black has low bulk density, so it cannot use standard bottom-feeding configurations. Instead, it’s necessary to feed materials from the top of the classifier and, once classified and separated, the resulting fine powders can be sold as a higher quality carbon black, while the coarser material can either be reground or sold as a lower grade.