Coal tar

Coal tar

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Coal tar is a brown or black liquid of extremely high viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

, which smells of naphthalene
Naphthalene
Naphthalene is an organic compound with formula . It is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings...

 and aromatic hydrocarbons. Coal tar is among the by-products when coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

 is
carbonized
Carbonization
Carbonization or carbonisation is the term for the conversion of an organic substance into carbon or a carbon-containing residue through pyrolysis or destructive distillation. It is often used in organic chemistry with reference to the generation of coal gas and coal tar from raw coal...

 to make coke
Coke (fuel)
Coke is the solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal. Cokes from coal are grey, hard, and porous. While coke can be formed naturally, the commonly used form is man-made.- History :...

 or gasified
Coal gasification
Coal gasification is the process of producing coal gas, a type of syngas–a mixture of carbon monoxide , hydrogen , carbon dioxide and water vapour –from coal...

 to make coal gas
Coal gas
Coal gas is a flammable gaseous fuel made by the destructive distillation of coal containing a variety of calorific gases including hydrogen, carbon monoxide, methane and volatile hydrocarbons together with small quantities of non-calorific gases such as carbon dioxide and nitrogen...

. Coal tars are complex and variable mixtures of phenols
Phenols
In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of a hydroxyl group bonded directly to an aromatic hydrocarbon group...

, polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbons , also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH...

s (PAHs), and heterocyclic compounds, about 200 substances in all.

Industrial


Being flammable, coal tar is sometimes used for heating or to fire boilers. Like most heavy oils, it must be heated before it will flow easily.

Tar was a vital component of the first sealed, or "tarmac
Tarmac
Tarmac is a type of road surface. Tarmac refers to a material patented by Edgar Purnell Hooley in 1901...

", roads. Coal tar was formerly used as one of the primary ingredients of Tarmac
Tarmac
Tarmac is a type of road surface. Tarmac refers to a material patented by Edgar Purnell Hooley in 1901...

adam pavements, when mixed with ironworks slag
Slag
Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form...

. Today, petroleum derived binders and sealers are more commonly used. These sealers are used to extend the life and lower maintenance cost associated with asphalt
Asphalt
Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...

 pavements, primarily in asphalt road paving, car parks and walkways.

A large part of the binders used in the graphite industry for making "Green Blocks" are Coke Oven Volatiles or COV. A considerable portion of these COV used as binders is Coal Tar. During the heating or baking process of the green blocks as a part of commercial graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

 production, most part of the coal tar binders gets vaporised and is generally burned in an incinerator to prevent release in the atmosphere as the COV and Coal Tar can be injurious to health.

Coal tar is also used to manufacture paints, synthetic dyes, and photographic materials.

Medical


Also known as liquor carbonis detergens (LCD), and liquor picis carbonis (Latin: coal tar solution) (LPC), it can be used in medicated shampoo
Shampoo
Shampoo is a hair care product used for the removal of oils, dirt, skin particles, dandruff, environmental pollutants and other contaminant particles that gradually build up in hair...

, soap
Wrights Coal Tar Soap
Created by William Valentine Wright in 1860, Wright's Traditional Soap, or Wright's Coal Tar Soap, is a popular brand of antiseptic soap that is designed to thoroughly cleanse the skin. It is an orange colour....

 and ointment, as a treatment for dandruff
Dandruff
Dandruff is the shedding of dead skin cells from the scalp . Dandruff is sometimes caused by frequent exposure to extreme heat and cold. As it is normal for skin cells to die and flake off, a small amount of flaking is normal and common; about 487,000 cells/cm2 get released normally after...

 and psoriasis
Psoriasis
Psoriasis is an autoimmune disease that appears on the skin. It occurs when the immune system mistakes the skin cells as a pathogen, and sends out faulty signals that speed up the growth cycle of skin cells. Psoriasis is not contagious. However, psoriasis has been linked to an increased risk of...

, as well as being used to kill and repel head lice. When used as a medication in the U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, coal tar preparations are considered an OTC (over-the-counter drug
Over-the-counter drug
Over-the-counter drugs are medicines that may be sold directly to a consumer without a prescription from a healthcare professional, as compared to prescription drugs, which may be sold only to consumers possessing a valid prescription...

) pharmaceutical and are subject to regulation by the United States Food and Drug Administration. Name brands include Denorex
Denorex
Denorex Shampoo is used for treating and preventing itching, flaking, and scaling of the scalp caused by dandruff or seborrhea ....

, Balnetar, Psoriasin, Tegrin, T/Gel, and Neutar. When used in the extemporaneous preparation of topical medications, it is supplied in the form of Coal Tar Topical Solution USP
United States Pharmacopeia
The United States Pharmacopeia is the official pharmacopeia of the United States, published dually with the National Formulary as the USP-NF. The United States Pharmacopeial Convention is the nonprofit organization that owns the trademark and copyright to the USP-NF and publishes it every year...

, which consists of a 20% w/v
Percentage solution
"Percentage solution" is an ambiguous term which is used to describe a solution with the unit "%". It may refer to:* Mass fraction if mass/mass is meant...

 solution of coal tar in alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

, with an additional 5% w/v of polysorbate 80
Polysorbate 80
Polysorbate 80 is a nonionic surfactant and emulsifier derived from polyethoxylated sorbitan and oleic acid, and is often used in foods. Polysorbate 80 is a viscous, water-soluble yellow liquid...

; this must then be diluted in an ointment base such as petrolatum.

(Pine tar
Pine tar
Pine tar is a sticky material produced by the high temperature carbonization of pine wood in anoxic conditions . The wood is rapidly decomposed by applying heat and pressure in a closed container; the primary resulting products are charcoal and pine tar.Pine tar consists primarily of aromatic...

 has historically also been used for this purpose, but has been banned as a medical product by the FDA, since no evidence was submitted proving it is effective.)

Safety


According to the International Agency for Research on Cancer
International Agency for Research on Cancer
The International Agency for Research on Cancer is an intergovernmental agency forming part of the World Health Organisation of the United Nations....

, preparations that include more than five percent of crude coal tar are Group 1 carcinogen
Carcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...

.

According to the National Psoriasis Foundation
National Psoriasis Foundation
The National Psoriasis Foundation is the United States’ leading patient-driven, nonprofit advocacy organization that serves the estimated 7.5 million Americans affected by psoriasis and psoriatic arthritis....

 and the FDA, coal tar is a valuable, safe and inexpensive treatment option for millions of people with psoriasis
Psoriasis
Psoriasis is an autoimmune disease that appears on the skin. It occurs when the immune system mistakes the skin cells as a pathogen, and sends out faulty signals that speed up the growth cycle of skin cells. Psoriasis is not contagious. However, psoriasis has been linked to an increased risk of...

 and other scalp or skin conditions. Coal tar concentrations between 0.5% and 5% are safe and effective for psoriasis, and no scientific evidence suggests that the coal tar in the concentrations seen in non-prescription treatments is carcinogenic. The NPF states that coal tar contains approximately 10,000 chemicals, of which only about 50% have been identified, and the composition of coal tar varies with its origin and type of coal (for example,: lignite
Lignite
Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad,is a soft brown fuel with characteristics that put it somewhere between coal and peat...

, bituminous
Bituminous coal
Bituminous coal or black coal is a relatively soft coal containing a tarlike substance called bitumen. It is of higher quality than lignite coal but of poorer quality than Anthracite...

 or anthracite) used to make it.

Coal tar causes increased sensitivity to sunlight, so skin treated with topical coal tar preparations should be protected from sunlight.

The residue from the distillation
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....

 of high-temperature coal tar, primarily a complex mixture of three or more membered condensed ring aromatic hydrocarbons, was listed on 28 October 2008 as a substance of very high concern
Substance of very high concern
A substance of very high concern is a chemical substance for which it has been proposed that the use within the European Union be subject to authorisation under the REACH Regulation...

 by the European Chemicals Agency
European Chemicals Agency
The European Chemicals Agency is an agency of the European Union which manages the technical, scientific and administrative aspects of the Registration, Evaluation, Authorisation and Restriction of Chemicals system...

.

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