What is the Difference Between Coal Tar Pitch and Asphalt?

Coal tar pitch and asphalt are commonly used in industrial applications, primarily for paving roads, waterproofing, and sealing. Though they share similarities in their appearance and some uses, they differ significantly in their origins, chemical compositions, and specific applications. Understanding these differences is essential for selecting the appropriate material for a project. In this article, we’ll explore what coal tar pitch and asphalt are, their key differences, and where each material is typically used.

What is a Coal Tar Pitch?

Coal tar pitch is a black, viscous substance derived from the distillation of coal tar, a byproduct of the coke production process. When coal is heated in the absence of air, it undergoes a chemical transformation, resulting in coal tar—a mixture of hundreds of chemicals, including hydrocarbons and other organic compounds. Coal tar pitch is produced by further distilling coal tar at high temperatures to remove volatile components, leaving behind a thick, sticky residue. This residue, which consists mainly of aromatic hydrocarbons, is the coal tar pitch.

Coal tar pitch has a relatively high melting point and is typically used as a binder or adhesive in industrial applications. It has excellent water-resistant properties and is highly effective as a sealant. However, due to its chemical makeup, coal tar pitch contains several potentially harmful substances, including polycyclic aromatic hydrocarbons (PAHs), which are known to be toxic and carcinogenic.

What is Asphalt?

Asphalt, also known as bitumen, is a naturally occurring substance or a refined product derived from crude oil. It is composed of a complex mixture of hydrocarbons, including both aliphatic and aromatic compounds. Asphalt is obtained from the heavy residues left over after crude oil is distilled during the refining process. It can also be found in natural deposits, such as tar sands or asphalt lakes, where it exists in a more concentrated form.

Asphalt is a versatile and flexible material with a wide range of uses, particularly in road construction. It is commonly mixed with aggregate materials (like sand, gravel, or crushed stone) to form asphalt concrete, which is used in the paving of highways, parking lots, and driveways. Asphalt is prized for its durability, waterproofing capabilities, and ease of application. It is also considered safer to handle compared to coal tar pitch due to its lower toxicity.

Key Differences Between Coal Tar Pitch and Asphalt

1. Origin:
   • Coal Tar Pitch: Derived from the distillation of coal tar, which is a byproduct of the coke production process (a method of heating coal in the absence of air).
   • Asphalt: Obtained from the distillation of crude oil or mined directly from natural deposits such as tar sands or asphalt lakes.
2. Chemical Composition:
   • Coal Tar Pitch is composed of a complex mix of aromatic hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs), which are considered carcinogenic and toxic.
   • Asphalt: Contains a variety of hydrocarbons, including both aliphatic and aromatic compounds. It is generally considered safer to handle than coal tar pitch, as it contains lower levels of harmful chemicals.
3. Physical Properties:
   • Coal Tar Pitch: A thicker, stickier substance with a higher melting point compared to asphalt. It is less flexible and more rigid, which can limit its applications.
   • Asphalt: Generally more flexible and pliable, with a lower melting point. This makes it easier to work with, especially when mixing it with aggregates to form asphalt concrete for paving.
4. Health and Environmental Concerns:
   • Coal Tar Pitch: Coal tar pitch contain high levels of toxic PAHs, which are harmful to human health. Exposure to coal tar pitch can lead to skin irritation, respiratory problems, and long-term risks, including cancer.
   • Asphalt: While asphalt is not without its own health and environmental risks, it is generally considered safer to handle than coal tar pitch. However, the fumes produced during the heating and application of asphalt can cause respiratory issues, and long-term exposure to asphalt fumes may also pose health risks.
5. Durability and Performance:
   • Coal Tar Pitch: Known for its excellent water-resistant properties, coal tar pitch is commonly used in applications requiring a waterproofing seal, such as in roofing or as a binder in aluminum production.
   • Asphalt: Asphalt is more commonly used in road construction and paving due to its flexibility, ease of application, and ability to withstand repeated stress from traffic. It is also highly resistant to water and weathering, making it ideal for outdoor applications.

Applications of Coal Tar Pitch

1. Roofing: Coal tar pitch is often used in roofing applications, particularly in built-up roofing systems (BUR), where it acts as a waterproofing agent.
2. Paving: In some areas, coal tar pitch has been used in the past for road paving, but its use has declined due to health and environmental concerns.
3. Aluminum Production: Coal tar pitch is used as a binder in the production of aluminum, particularly in the manufacture of anode blocks for electrolytic reduction in aluminum smelting.

Applications of Asphalt

1. Road Construction: Asphalt is primarily used in the construction of highways, roads, parking lots, and driveways. It is mixed with aggregate to form asphalt concrete, which provides a durable, smooth, and flexible surface.
2. Roofing: Asphalt is used in roofing materials, particularly in modified bitumen systems, which are popular for flat or low-slope roofs.
3. Waterproofing: Asphalt is also used as a waterproofing agent for foundations, basements, and other structures exposed to moisture.

Conclusion

While both coal tar pitch and asphalt are black, sticky substances with similar applications in sealing and paving, they are distinct materials with different chemical compositions, origins, and health implications. Coal tar pitch is derived from coal tar and contains harmful polycyclic aromatic hydrocarbons, making it less desirable for certain applications due to its toxicity. Asphalt, on the other hand, is a product of crude oil distillation and is generally safer to handle. Despite the overlap in their use for paving and waterproofing, asphalt is more commonly used today, particularly in road construction, due to its greater flexibility, durability, and lower environmental impact. Understanding the differences between coal tar pitch and asphalt helps in making informed decisions about their appropriate use in various industrial and construction projects.