Coal tar

Coal tar is a brown or black liquid of extremely high viscosity. Coal tar is among the by-products when coal is carbonized to make coke or gasified to make coal gas. Coal tars are complex and variable mixtures of phenols, polycyclic aromatic hydrocarbons (PAHs), and heterocyclic compounds.^[1]

Applications

Pavement sealcoat

Coal tar is incorporated into some parking-lot sealcoat products, used to protect and beautify the underlying pavement.^[2]‍^{(About Pavement Sealer)} Sealcoat products that are coal-tar based typically contain 20 to 35 percent coal-tar pitch.^[3] Substantial concerns have been raised about the safety of this application of coal tar, given that coal tar is known to cause cancer in humans ^[4] and that several PAH compounds in coal tar are toxic to aquatic life. Defenders of coal-tar use in sealcoat have pointed out that humans are exposed to PAHs through many pathways, and similarly that the urban environment has many potential sources of PAHs.^[2]‍^{(About Coal Tar and Polycyclic Aromatic Hydrocarbons (PAHs))}

The primary use of coal tar based sealcoats is regional within the US^[3] but federal research ^[5] shows it is used in states from Alaska to Florida and several areas have banned its use in sealcoat products ^[6][7][8] including: The District of Columbia; the City of Austin, Texas; Dane County, Wisconsin; Washington State; and several municipalities in Minnesota and others.^[9][10]

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.

Coal tar was a component of the first sealed roads. In its original development by Edgar Purnell Hooley, tarmac was tar covered with granite chips. Later the filler used was industrial slag. Today, petroleum derived binders and sealers are more commonly used. These sealers are used to extend the life and reduce maintenance cost associated with asphalt 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 (COV). A considerable portion of these COV used as binders is coal tar. During the baking process of the green blocks as a part of commercial graphite production, most of the coal tar binders are vaporised and are generally burned in an incinerator to prevent release into the atmosphere, as 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),^[11] and liquor picis carbonis ^{[lower-alpha 1]} (LPC) BP^[12] it can be used in medicated shampoo, soap and ointment, as a treatment for dandruff and psoriasis, as well as being used to kill and repel head lice. When used as a medication in the U.S., coal tar preparations are considered over-the-counter drug pharmaceuticals and are subject to regulation by the USFDA. Named brands include Denorex, 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, which consists of a 20% w/v solution of coal tar in alcohol, with an additional 5% w/v of polysorbate 80 USP; this must then be diluted in an ointment base such as petrolatum.

(Pine tar 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.^[13])

Various phenolic coal tar derivatives have analgesic (pain-killer) properties. These included acetanilide, phenacetin, and paracetamol (acetaminophen).^[14] Paracetamol is the only coal-tar derived analgesic still in use today, but industrial phenol is now usually synthesized from crude oil rather than coal tar.

Safety

According to the International Agency for Research on Cancer, preparations that include more than five percent of crude coal tar are Group 1 carcinogen.

According to the National Psoriasis Foundation and the FDA, coal tar is a valuable, safe and inexpensive treatment option for millions of people with psoriasis and other scalp or skin conditions.^[15] 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 (or is not) carcinogenic because there are too few studies and insufficient data to make a judgement. Coal tar contains approximately 10,000 chemicals, of which only about 50% have been identified,^[16] and the composition of coal tar varies with its origin and type of coal (for example,: lignite, bituminous or anthracite) used to make it.

Coal tar causes increased sensitivity to sunlight,^[17] so skin treated with topical coal tar preparations should be protected from sunlight.

The residue from the distillation 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 by the European Chemicals Agency.

Coal tar distillers

In the coal gas era, there were many companies in Britain whose business was to distill coal tar to separate the higher-value fractions, such as naphtha, creosote and pitch. These companies included:^[18]

• British Tar Products
• Lancashire Tar Distillers
• Midland Tar Distillers
• Newton, Chambers & Company (owners of Izal brand disinfectant)

See also

• Creosote
• Cresolene
• Preservative
• Red 40
• Tar
• Sealcoat

Notes

References

External links

Look up coal tar in Wiktionary, the free dictionary.

• "Coal Tar Pitch Volatiles". Occupational Safety & Health Administration. 22 March 2012. Retrieved 8 March 2013. 
• "NIOSH Pocket Guide to Chemical Hazards - Coal Tar Pitch Volatiles". Centers for Disease Control and Prevention. 11 April 2011. Retrieved 13 September 2013. 
• Erica Engelhaupt (19 November 2008). "Parking lots create sticky pollution problem". Environmental Science and Technology. doi:10.1021/es803118b. Retrieved 8 March 2013. 

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