Tar is a dark, viscous, and sticky substance formed by heating various organic materials in the absence or limited presence of oxygen. This process, known as pyrolysis or destructive distillation, breaks down complex organic matter into simpler compounds. The term “tar” is broad, encompassing substances with diverse chemical compositions depending on their original source. Understanding these differences is important, as they influence tar’s properties.
Origins and Types of Tar
Tar originates from different organic sources through specific thermal processes. Coal tar results from heating coal to high temperatures, between 900 and 1,200°C, during coke production. Petroleum asphalt, also known as bitumen, is a heavy, viscous residue obtained during crude oil refining through distillation.
Wood tar is produced by slowly heating wood, often pine or birch, in a low-oxygen environment. Tobacco tar forms from the incomplete combustion of tobacco during smoking. Each of these distinct origins contributes to the unique chemical makeup of the resulting tar.
Common Chemical Constituents
Various types of tar share general categories of chemical compounds within their complex mixtures. Polycyclic Aromatic Hydrocarbons (PAHs) are a significant class of organic compounds found across different tars, characterized by multiple fused aromatic rings. Volatile Organic Compounds (VOCs) are also present, representing chemicals that readily evaporate at room temperature.
Heterocyclic compounds, which incorporate elements like nitrogen, sulfur, or oxygen within their ring structures, comprise another common group. Phenols and cresols are also found in tar. Some tars may contain trace amounts of metals such as nickel, vanadium, and lead.
Specific Compositions of Different Tars
The specific chemical profiles vary considerably among different types of tar, reflecting their distinct origins and formation processes.
Coal Tar
Coal tar, a byproduct of coal carbonization, is a complex mixture containing thousands of chemical compounds. It is rich in a wide range of PAHs, including naphthalene, anthracene, and phenanthrene. Larger and more complex PAHs such as benzo[a]pyrene, chrysene, and pyrene are also present. Coal tar also contains significant amounts of phenols, cresols, and various heterocyclic compounds like pyridine, quinoline, and indole.
Petroleum Asphalt
Petroleum asphalt, or bitumen, is primarily composed of four main fractions: asphaltenes, resins, aromatics, and saturates. Asphaltenes are high-molecular-weight molecules that contribute to bitumen’s dark color and viscosity. Resins enhance ductility and flexibility, while aromatics contribute to fluidity, and saturates influence hardness and temperature sensitivity. Unlike coal tar, bitumen has a lower concentration of volatile PAHs and consists largely of hydrocarbons, with smaller amounts of oxygen, nitrogen, sulfur, and trace metals.
Wood Tar
Wood tar contains a high proportion of phenolic compounds, which impart distinct aromatic properties. Specific examples include guaiacol, syringol, cresol, and dimethoxyphenol. It also contains creosote and resin acids, with the exact composition influenced by the type of wood and the pyrolysis conditions. Wood tars may also contain PAHs like naphthalenes and retene.
Tobacco Tar
Tobacco tar is a complex mixture, containing thousands of different chemicals formed during the burning of tobacco. Among its components are specific PAHs, such as benzo[a]pyrene. A notable group are the tobacco-specific nitrosamines (TSNAs), including N-nitrosonornicotine (NNN) and nicotine-derived nitrosamine ketone (NNK). Other hazardous compounds include aldehydes like formaldehyde and acetaldehyde, carbon monoxide, and various heavy metals.
Why Understanding Tar’s Components Matters
Recognizing that “tar” is not a single substance but a diverse category with widely varying chemical compositions is important. This chemical diversity directly influences the physical properties of each tar type, such as its viscosity, melting point, and adhesive qualities. The unique molecular makeup of each tar dictates how it behaves and interacts with other materials.