Polycarbonates

OVERVIEW

Polycarbonate (pol-ee-KAR-bun-ate) is a term used both for a specific compound and for a class of compounds with similar chemical structures. The members of this family are made by reacting phosgene (COCl₂) with any compound having two phenol structures. Phenol is hydroxybenzene, C₆H₅OH. The most common polycarbonate is made in the reaction between phosgene and bisphenol A (C₆H₅OHC(CH₃)₂C₆H₅OH). Polycarbonates are sold under a number of trade names, including Cyrolon^®, Lexan^®, Markrolon^®, Merlon^®, Tuffak^®, and Zelux^®.

Polycarbonates are strong, lightweight plastics that are resistant to heat, light, chemicals, and physical shock. They are used in a number of commercial and industrial products ranging from consumer electronics to sporting goods to storage containers.

KEY FACTS

FORMULA:

Varies

ELEMENTS:

Carbon, hydrogen, oxygen

COMPOUND TYPE:

Organic polymer

STATE:

Solid

MOLECULAR WEIGHT:

Very large; varies

MELTING POINT:

Varies

BOILING POINT:

Not applicable; decomposes above melting point

SOLUBILITY:

Virtually insoluble in water

The original polycarbonate was discovered in 1953 by the German chemist Hermann Schnell (1916–1999), an employee at the Bayer AG chemical company. The product was discovered almost simultaneously in the United States by chemist Daniel W. Fox (1923–1989) at the General Electric Company. Commercial use of the product began in the late 1950s, with the first large-scale production plant beginning operation in 1960. At first, polycarbonate was used primarily for electrical devices such as fuse boxes. In 1982, compact discs (CDs) were introduced and were made almost entirely of polycarbonate. Fifteen years later, polycarbonate digital videodiscs (DVDs) entered the market. In the mid-1980s, polycarbonate bottles began to replace the more cumbersome and breakable glass bottles. Today, polycarbonates are used for dozens of commercial applications.

HOW IT IS MADE

Polycarbonates are made in the reaction between phosgene and any compound containing two phenol groups, such as bisphenol A . One chlorine atom from the phosgene combines with a hydrogen atom from the hydroxyl group (-OH) in each phenol to make hydrogen chloride (HCl). As the hydrogen chloride is removed from the reaction, the phosgene remnant links with the phenol remnant. As the reaction proceeds, the phosgene-phenol grouping grows longer and longer, eventually forming a large, straight-chain polymer of polycarbonate.

Other types of polycarbonates have also been made using a very different approach from that involving bisphenol A and related compounds. For example, the reaction between phosgene and allyl alcohol (CH₂=CHCH₂OH) produces a monomer with carbon-carbon double bonds at both ends of the molecule that can be used for polymerization. Interestingly enough, the polycarbonate produced by this process has very different physical properties from the traditional bisphenol A polymer. The allyl polymer is a clear, transparent, flexible plastic whose primary use is in the production of eyeglass lenses.

Interesting Facts

One of polycarbonate plastics most interesting uses is in the manufacture of bulletproof windows.

COMMON USES AND POTENTIAL HAZARDS

Polycarbonates are strong, heat resistant, and lightweight, making them ideal for applications in construction, electronics, automobiles, and appliances. Many polycarbonates are substituted for glass in safety and athletic goggles, building components, and car instrument panels because they are transparent, yet shatterproof. They are also more resistant to ultraviolet radiation than is glass. Some of the specific products made with polycarbonates include:

• Consumer electronic devices, such as cell phones, computers, pagers, and fax machines;
• Data storage devices, such as CDs and DVDs;
• Automobile parts, including tail lights, turn signals, fog lights, and headlamps;
• Appliances such as refrigerators, food mixers, hair dryers, and electric shavers;
• Safety devices, including helmets, goggles, and bulletproof windows;

• Healthcare devices, such as incubators, kidney dialysis machines, and eyeglasses.

The U.S. Food and Drug Administration has accepted polycarbonates as safe for use with foods. There is little evidence that the compounds have any harmful health effects on humans. Minimal evidence exists to suggest that bisphenol A may have some harmful effects on experimental animals. But there is no evidence that similar effects occur in humans, and the amount of free bisphenol A in polycarbonates is so small as to be negligible.

Words to Know

POLYMER

A compound consisting of very large molecules made of one or two small repeated units called monomers.

FOR FURTHER INFORMATION

Lazear, N. R. "Polycarbonate: High-Performance Resin." Advanced Materials & Processes (February 1995): 43-45.

"Polycarbonate." Association of Plastics Manufacturers in Europe. http://www.plasticseurope.org/content/default.asp?PageID=43 (accessed on October 24, 2005).

"Polycarbonate (Makrolon®, Lexan®, Zelux®)." San Diego Plastics. http://www.sdplastics.com/polycarb.html (accessed on October 24, 2005).

"Polycarbonate Plastics." Bisphenol A. http://www.bisphenol-a.org/human/polyplastics.html (accessed on October 24, 2005).

"Polycarbonates." Polymer Science Learning Center. http://www.pslc.ws/mactest/pc.htm (accessed on October 24, 2005).