Pitch is a highly viscous, black or dark brown residue defined as a viscoelastic polymer in material science. It is a complex hydrocarbon mixture derived from the thermal processing of organic materials. The material is typically solid and brittle at room temperature, yet its properties are complex. Pitch serves as a foundation for numerous industrial products, ranging from road construction to high-tech carbon manufacturing.
How Pitch is Derived and Defined
Commercial pitch is primarily sourced from two industrial processes: the coking of coal and the refining of crude oil. Coal-Tar Pitch is a residue left over after the distillation of crude coal tar, which is a byproduct of converting coal into coke for the steel industry. This source yields a material characterized by a high degree of aromaticity and a greater atomic ratio of carbon to hydrogen. Petroleum Pitch is a product of thermally treating heavy, aromatic fractions of petroleum-based crude oils. While both types are complex mixtures of hydrocarbons, petroleum pitch typically contains a lower percentage of highly condensed aromatic ring systems. Chemically, pitch is classified as a complex combination of three or more membered condensed ring aromatic hydrocarbons. This composition results in a material that ranges from soft and sticky to hard and brittle, depending on its source and processing temperature. Historically, pitch was also derived from the distillation of wood tar, particularly from pine trees, and was widely used in naval stores for waterproofing.
The Unique Viscoelastic Nature of Pitch
The most fascinating aspect of pitch is its viscoelastic behavior, meaning it exhibits properties of both a viscous liquid and an elastic solid. At very short timescales, such as when struck with a hammer, pitch acts like a brittle solid and will shatter into pieces. However, when left undisturbed under the influence of gravity over an extended period, it flows like an extremely thick fluid. This counterintuitive property is famously demonstrated by the Pitch Drop Experiment, initiated in 1927 at the University of Queensland in Australia. In this long-running demonstration, a sample of pitch is allowed to slowly flow and drip from a sealed funnel. Since the start of the experiment, only nine drops have fallen, each taking approximately eight to thirteen years to form and detach. Measurements from similar experiments have estimated that the viscosity of the pitch is astonishingly high. The material flows at a rate approximately 230 billion times slower than water.
Essential Uses in Modern Industry
Pitch’s dual nature as a binder and a source of carbon makes it invaluable across several modern industries. In construction and paving, it is employed in coatings, roofing materials, and waterproofing applications. Petroleum-derived pitch (bitumen or asphalt) is routinely used as a binder for aggregate in road construction. Pitch is a fundamental precursor in the manufacturing of high-performance carbon products. Its high carbon content is leveraged in producing graphite electrodes, which are consumed in large quantities by electric arc furnaces for steel manufacturing. The pitch acts as a binding agent holding the coke aggregate together during casting and graphitization. Pitch is refined into specialized impregnation pitch to fill the pores of carbon materials, increasing density and strength. This process is important for creating anodes required for aluminum smelting and for producing advanced carbon fibers. The resulting materials are used in applications requiring high thermal stability and mechanical strength, such as aerospace components and refractory bricks.
Health and Environmental Risks
The industrial use of pitch, particularly Coal-Tar Pitch, carries health and environmental risks due to its chemical makeup. Coal-Tar Pitch contains high concentrations of Polycyclic Aromatic Hydrocarbons (PAHs), compounds formed during the incomplete burning of organic matter. Many PAHs in coal-tar products are classified as known human carcinogens. Occupational exposure is a concern for workers in coking plants, roofing, and paving, requiring safety protocols like proper ventilation and personal protective equipment. Environmentally, PAHs can volatilize into the air or wash off paved surfaces into water bodies and soil. Runoff from sealed pavement contains high concentrations of PAHs, which are toxic to aquatic life. Petroleum pitch generally contains a much lower concentration of carcinogenic PAHs.