The temperature at which “tar” melts is complex because the term is used generically for various black, viscous organic materials. These substances are not simple crystalline solids that transition sharply from solid to liquid at a single melting point. Instead, they are amorphous, meaning they soften gradually over a wide range of temperatures. Therefore, the concept of a true “melting point” does not apply, and engineers rely on a different measurement to define their thermal performance.
Understanding “Tar”: Bitumen, Coal Tar, and Pitch
The material most people think of as “tar” on roads is actually bitumen, known as asphalt in North America. Bitumen is a semi-solid, sticky residue derived from the fractional distillation of crude petroleum. Chemically, it is composed mainly of complex hydrocarbon molecules. It serves primarily as the binding agent in asphalt concrete for road construction and is used for waterproofing.
In contrast, true coal tar is derived from the destructive distillation of coal, which involves heating coal in the absence of air. This origin gives coal tar a distinct chemical profile rich in aromatic hydrocarbons, which differ structurally from petroleum-derived bitumen. The term “tar” can also refer to residue from wood or peat distillation, though coal tar is the most common industrial form.
Pitch is defined as the residue left after the further distillation or processing of either coal tar or petroleum. For example, coal-tar pitch is the heavy, non-volatile residue remaining after crude coal tar is distilled. Due to its origin as a refined residue, pitch often has a higher carbon content and a denser consistency compared to the original tar or bitumen. The specific origin dictates the properties.
The Viscoelastic Transition: Softening Point vs. Melting Point
Since bitumen and true tars are amorphous, they lack the ordered crystalline structure necessary for a distinct melting point. When heated, they undergo a viscoelastic transition, gradually moving from a glassy, brittle state to an elastic, rubbery state, and finally to a viscous liquid state. This continuous change in consistency is a function of both temperature and the duration of the applied force.
To standardize the point at which these materials are considered workable, the industry uses a measurement called the softening point. This value does not represent a true thermodynamic melting temperature but rather an equi-viscous temperature, indicating when the material reaches a specific level of softness. This measurement is achieved using the Ring and Ball Softening Point Test.
The Ring and Ball Softening Point Test provides a reproducible metric for the material’s high-temperature performance.
- A disc of the material is held in a brass ring while supporting a steel ball on its surface.
- The apparatus is submerged in a liquid bath, typically water or glycerin.
- The bath is heated at a strictly controlled rate of \(5^\circ\text{C}\) per minute.
- The softening point is recorded as the temperature at which the material has softened enough for the ball to descend \(25\text{ mm}\) to a base plate below.
Key Temperature Ranges and Real-World Relevance
The softening point indicates a material’s temperature susceptibility and its resistance to deformation, particularly in hot climates. Standard road-paving bitumen grades typically have softening points ranging from \(40^\circ\text{C}\) to \(70^\circ\text{C}\). For instance, a common road grade like Bitumen 60/70 has a softening point between \(48^\circ\text{C}\) and \(52^\circ\text{C}\).
Materials requiring greater heat resistance, such as high-grade roofing pitch or polymer-modified bitumen, are engineered to have higher softening points. Specialized mastic asphalts and polymer-modified binders can range from \(85^\circ\text{C}\) to over \(105^\circ\text{C}\). This higher value indicates better resistance to flow or rutting in high-temperature service conditions, such as on a sun-baked rooftop or heavily trafficked roadway.
These softening points are far below the temperatures required for paving. Hot-mix asphalt concrete, which uses bitumen as a binder, is typically delivered and paved at temperatures between \(150^\circ\text{C}\) and \(180^\circ\text{C}\) (\(300^\circ\text{F}\) to \(350^\circ\text{F}\)). These high temperatures reduce the material’s viscosity sufficiently to coat the aggregate and allow for proper compaction. If the mix cools below approximately \(85^\circ\text{C}\), it becomes too stiff to be compacted correctly, leading to inadequate density and reduced durability.