Coal tar

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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[edit]

Pavement sealcoat[edit]

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[edit]

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[edit]

Also known as liquor carbonis detergens (LCD),[11] and liquor picis carbonis [a] (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[edit]

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[edit]

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]

See also[edit]

Notes[edit]

  1. ^ Latin: coal tar solution

References[edit]

  1. ^ "Toxicological profile for wood creosote, coal tar creosote, coal tar, coal tar pitch, and coal tar pitch volatiles". U.S. Department of Health and Human Services. September 2002. Retrieved 8 March 2013. 
  2. ^ a b "The Truth about Coaltar". 2006. Retrieved 8 March 2013. 
  3. ^ a b Mahler BJ; Van Metre PC (2 February 2011). "Coal-Tar-Based Pavement Sealcoat, Polycyclic Aromatic Hydrocarbons (PAHs), and Environmental Health". U.S. Geological Survey Fact Sheet. Retrieved 8 March 2013. 
  4. ^ "Coal Tars and Coal‑Tar Pitches". Report on Carcinogens (12th ed ed.). National Toxicology Program, Department of Health and Human Services. 2011. Retrieved 8 March 2013. 
  5. ^ Van Metre PC; Mahler BJ (15 December 2010). "Contribution of PAHs from coal-tar pavement sealcoat and other sources to 40 U.S. lakes". U.S. Geological Survey. doi:10.1016/j.scitotenv.2010.08.014. PMID 21112613. Retrieved 8 March 2013. 
  6. ^ "City of Austin Ordinance 20051117-070". 17 November 2005. Retrieved 8 March 2013. 
  7. ^ "District Bans Coal-Tar Pavement Products". 26 June 2009. Retrieved 8 March 2013. 
  8. ^ "Ordinance 80 : Establishing Regulations on Coal Tar Sealcoat Products Application and Sale". Dane County Office of Lakes and Watersheds. 1 July 2007. Retrieved 8 March 2013. 
  9. ^ "Coal Tar Free America – Bans". Retrieved 8 March 2013. 
  10. ^ Barbara J Mahler (14 April 2011). "Causes of Increasing Concentrations of Polycyclic Aromatic Hydrocarbons (PAHs) in U.S. Lakes". PAHs Increasing in Urban U.S. Lakes. Environmental and Energy Study Institute. Retrieved 8 March 2013. 
  11. ^ Paghdal KV; Schwartz RA (31 January 2009). "Topical tar: back to the future". PMID 19185953. Retrieved 8 March 2013. 
  12. ^ Berenblum I (25 September 1948). "Liquor Picis Carbonis". British Medical Journal. PMC 2091540. 
  13. ^ "Clean-Up of Ineffective Ingredients in OTC Drug Products". Food and Drug Administration. 7 November 1990. Retrieved 8 March 2013. 
  14. ^ "Pain relief: from coal tar to paracetamol". Royal Society of Chemistry. July 2005. Retrieved 8 March 2013. 
  15. ^ "The battle to save coal tar in California". 3 December 2001. Archived from the original on 19 February 2012. Retrieved 8 March 2013. 
  16. ^ Heinz-Gerhard Franck (May 1963). "THE CHALLENGE IN COAL TAR CHEMICALS". Ind. Eng. Chem., 1963, 55 (5), pp 38–44. American Chemical Society. doi:10.1021/ie50641a006. Retrieved 8 March 2013. 
  17. ^ "Sun-Sensitive Drugs (Photosensitivity to Drugs)". MedicineNet. WebMD. 2008-08-22. p. 5. Retrieved 8 March 2013. 
  18. ^ Mike Smith. "GANSG - Coal Tar Distillers". Igg.org.uk. Retrieved 8 March 2013. 

External links[edit]