Propylene (PRO-puh-leen) is a colorless gas with a slightly sweet odor that burns with a yellow, sooty flame. It ranks sixth among all chemicals produced in the United States and second (after ethylene) among all organics. Its primary use is in the manufacture of polypropylene, one of the most popular polymers produced in the world.
Propene; methylethylene; methylethene
Alkene; unsaturated hydrocarbon (organic)
Slightly soluble in water; very soluble in ethyl alcohol and ether
HOW IT IS MADE
Propylene is prepared commercially by the thermal or catalytic cracking of hydrocarbons. The term cracking refers to a process by which large hydrocarbons—organic compounds that contain only carbon and hydrogen—are broken down into smaller hydrocarbons. Cracking can be accomplished either by exposing the hydrocarbons to high temperatures, a process called thermal cracking, or by using a catalyst, a process called catalytic cracking. A catalyst is a material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure. Thermal cracking is also known as steam cracking because steam is used to produce the high temperatures need to bring about the cracking reactions. Propylene can also be prepared by the catalytic dehydrogenation of propane (C3H8). Catalytic dehydrogenation is a process by which hydrogen atoms are removed from a substance, resulting in the formation of double bonds where single bonds previously existed.
COMMON USES AND POTENTIAL HAZARDS
About 15.3 million metric tons (16.8 million short tons) of propylene were produced for commercial sale in the United States in 2004. About 39 percent of that amount was used for the production of polypropylene. Almost all of the remaining production was also used for the synthesis of chemical compounds, especially acrylonitrile (14 percent), propylene oxide (11 percent), cumene (10 percent), oxo alcohols (8 percent), isopropyl alcohol (7 percent), oligomers (5 percent) and acrylic acid (3 percent). Acrylonitrile, propylene oxide, oxo alcohols, and acrylic acids are all used primarily for the production of various types of polymers. Cumene is itself used as a raw material in the production of other organic compounds, especially acetone and phenol.
In addition to the propylene produced for commercial uses, large amounts of the gas are retained by the petroleum industry for conversion to products that are added to gasoline and other petroleum-based products. According to some estimates, between half and three-quarters of all propylene that comes from cracking reactions is retained for this purpose, leaving the remainder for commercial sale.
- Plants emit small amounts of propylene gas naturally.
- Small amounts of propylene are also produced when organic matter burns and is found in products such as cigarette smoke, automobile exhaust, and burning leaves.
In addition to the hazard it poses as a flammable gas, propylene does have some health risks. At relatively high concentrations, it can act as an asphyxiant, a substance that can produce unconsciousness, and can cause death. It can also act as a mild anesthetic.
Words to Know
- A material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure.
- A compound consisting of very large molecules made of one or two small repeated units called monomers.
FOR FURTHER INFORMATION
"Chronic Toxicity Summary: Propylene." California Office of Environmental Health Hazard Assessment. http://www.oehha.org/air/chronic_rels/pdf/115071.pdf (accessed on November 3, 2005).
Meikle, Jeffrey L. American Plastic: A Cultural History. Piscataway, NJ: Rutgers University Press, 1997.
"Propylene." Equistar Chemical Company. http://www.equistarchem.com/html/petrochemical/olefins/propylene.htm (accessed on November 3, 2005).
"Propylene." LG Petrochemical. http://www.lgpetro.com/eng/product_info/product/propylene.html (accessed on November 3, 2005).
pro·pyl·ene / ˈprōpəˌlēn/ • n. Chem. a gaseous hydrocarbon of the alkene series, C3H6, made by cracking alkanes.