Pitch is a highly viscous, dark-colored material, naturally occurring or synthetically produced, characterized by its amorphous nature, meaning it lacks a defined crystalline structure. This material finds various uses across many industries due to its unique physical characteristics.
Defining Pitch
Pitch is a viscoelastic polymer, exhibiting properties of both liquids and solids. While it appears solid and can shatter upon impact, pitch flows like a liquid over extended periods. This dual behavior arises from its molecular structure, where molecules slowly rearrange. The resistance to flow in liquids is known as viscosity, and pitch demonstrates exceptionally high viscosity.
This unique characteristic means pitch deforms under stress over time, rather than instantly. The time-dependent strain is a hallmark of viscoelastic materials. The molecules within pitch experience strong intermolecular forces, which significantly slow down their movement. This results in a material that can feel rigid to the touch but will gradually yield and flow under its own weight or sustained pressure.
Diverse Origins and Types
Pitch originates from natural sources and industrial processes. Naturally occurring pitch, such as natural asphalt or bitumen, is found in deposits like the Pitch Lake in Trinidad. This natural form develops from organic marine deposits breaking down into petroleum.
Industrially, pitch is a byproduct of the destructive distillation of organic materials, a process involving heating substances in the absence of air. Common sources include coal, wood, and petroleum. Coal tar pitch, a residue from coal tar distillation, is rich in aromatic compounds and hydrocarbons.
Wood tar pitch, derived from heating wood, contains resin acids and is often obtained from pine trees. Petroleum pitch, also known as bitumen or asphalt in some contexts, is a heavy, viscous residue from crude oil refining. Each type possesses distinct characteristics and applications based on its chemical composition and origin.
Physical Properties and Characteristics
Pitch typically presents as a dark brown to black substance, often shiny, sticky, and dense. A defining physical property of pitch is its extreme viscosity, which makes it resistant to flow.
The famous Pitch Drop Experiment, initiated in 1927 at the University of Queensland, vividly illustrates this property. In this ongoing experiment, pitch placed in a funnel slowly drips, with individual drops taking approximately a decade to form and fall. This experiment has helped scientists determine that pitch can be billions of times more viscous than water.
For instance, the pitch in the University of Queensland experiment has a viscosity estimated to be around 100 billion times that of water. Even though it appears solid and can be shattered, its liquid nature is revealed by its incredibly slow flow over many years. The rate of flow can also be influenced by temperature fluctuations, with higher temperatures slightly reducing its viscosity.
Everyday Applications
Pitch has a long history of practical uses and continues to be important in modern industries. Historically, it served as a waterproofing agent, particularly for sealing the seams of wooden sailing vessels. It was also used to coat earthenware containers to preserve wine and to waterproof wooden buckets.
In contemporary applications, pitch is widely used in roofing materials, providing a waterproof barrier for buildings. It acts as a binder in road construction, notably in asphalt mixtures. Pitch also functions as a sealant, used in various construction contexts, including sealing roof penetrations.
Beyond construction, pitch finds use as a binder in carbon products, such as electrodes, and in specialized materials for polishing high-quality optical lenses and mirrors. Its diverse properties make it suitable for applications requiring durable binding, sealing, or specific flow characteristics.
Pitch Versus Related Substances
The terms pitch, tar, asphalt, and bitumen are often used interchangeably, but they refer to distinct, though related, materials. Pitch is a highly viscous, often solid or semi-solid, residue obtained from the distillation of substances like coal tar, wood tar, or petroleum. It can be considered a more refined and denser product than tar.
Tar is a dark, viscous liquid produced through the destructive distillation of organic matter such as coal, wood, or peat. It generally has a lower viscosity than pitch and serves as a precursor from which pitch can be derived through further processing. Coal tar, for example, is a specific type of tar from which coal tar pitch is obtained.
Bitumen, often synonymous with asphalt, refers to a black, petroleum-derived material. It is a residue from crude oil refining and is widely used as a binder in road construction. While bitumen is a type of pitch (petroleum pitch), asphalt typically describes the mixture of bitumen with aggregates used for paving, not the binder alone.