Polymethyl Methacrylate

OVERVIEW

Polymethylmethacrylate (POL-ee-meth-uhl-meth-AK-rill-ate) is a clear thermoplastic resin used to make windshields, visors, coatings for baths, advertising signs, and contact lenses. It is also widely used in dentistry and medicine. A thermoplastic resin is one that becomes soft when heated and hard when cooled. It can be converted back and forth any number of times between the solid and liquid states by further heating and cooling.

Polymethylmethacrylate (PMMA) is more transparent than glass and six to seventeen times more resistant to breakage than glass. Another advantage it has over glass is that, if it does break, it falls apart into dull-edged pieces. PMMA is resistant to water, inorganic acids and bases, but is vulnerable to many organic solvents.

KEY FACTS

OTHER NAMES:

Acrylic, PMMA

FORMULA:

-[-CH₂C(CH₃)(COOH)CH₂-]-_n

ELEMENTS:

Carbon, hydrogen, oxygen

COMPOUND TYPE:

Organic polymer

STATE:

Solid

MOLECULAR WEIGHT:

Varies: 250,000 to over 1,000,000 g/mol

MELTING POINT:

Varies: usually above 100°C (200°F)

BOILING POINT:

Not applicable

SOLUBILITY:

Insoluble in water; best solvents are mixtures of two or more organic solvents, aromatic hydrocarbons, halogenated hydrocarbons, and tetrahydrofuran

PMMA was first synthesized in 1928 in the laboratories of the German chemical firm Röhm and Haas. After five years of research, one of the firm's founders, Otto Röhm, found a way of manufacturing sheets of polymethylmethacrylate. He patented his invention and the company was soon producing the product under the trade name of Plexiglas®. At about the same time, the product was being developed independently in the United States by the Dow chemical company, who sold its product under the trade name of Lucite®. Over time, a number of other chemical companies produced their own version of PMMA under a variety of trade names, including Acrylite®, Acrypet®, Crinothene®, Degalan®, Diakon®, Elvacite®, Kallocryl®, Metaplex®, Osteobond®, Paraglas®, Perspex®, Pontalite®, Sumipex®, Superacryl®, and Vedril®.

HOW IT IS MADE

The monomer from which polymethylmethacrylate is made is methyl methacrylate, CH₂=C(CH₃)COOCH₃. Methyl methacrylate is made in the reaction between acetone cyanohydrin ((CH₃)₂COHCN) and methanol (methyl alcohol; CH₃OH) in the presence of a sulfuric acid catalyst. As with all polymers, methyl methacrylate can be polymerized by a number of agents, including heat, radiation, and certain chemicals known as free-radical initiators. Once polymerization of methyl methacrylate begins, it can be processed in a number of ways. One of the most common processing system used involves passing the liquid material between two polished stainless steel belts. The distance between the belts is set to the desired thickness for the acrylic sheet. The acrylic is cured by a process of cooling and heating after leaving the steel belt. The final product is then cut to desired lengths at the end of the production line.

Interesting Facts

• Polymethylmethacrylate exhibits a phenomenon known as total internal reflection. That term means that a light beam transmitted through a solid tube made of PMMA reflects off the inner surface of the tube. This property allows a light beam to be transmitted around corners and bends and out the end of a tube made of PMMA.
• A black-light reactive tatoo ink made from PMMA and microspheres of fluorescent dye is used on wildlife to track activities such as migration and growth patterns.
• The spectator shield in hockey arenas is made of PMMA.
• Plexiglas® was exhibited by Röhm & Haas at the World's Trade Fair in Paris in 1937. One of the exhibits was a transparent violin made from the product.

COMMON USES AND POTENTIAL HAZARDS

The most common use for PMMA is as a glass substitute. PMMA offers many benefits over glass because it is more transparent, less dense, stronger, and shatterproof. In ophthalmology (the branch of medicine that deals with diseases of the eye and their treatment), PMMA is used to make replacement lenses for the eye when the original lens has been removed for some reason, such as the growth of a cataract. It is also used to make hard contact lenses. Other medical and dental applications of the material include its use as a dental cement, for the manufacture of bases and linings for dentures (false teeth), and to make a bone cement used in the reconstruction of broken or damaged bones and to fix implants in place.

Powdered polymethylmethacrylate presents health hazards because it may cause irritation of the skin, eyes, and respiratory tract. This hazard is of concern to people who work with the material in its raw form.

The glass-like material with which most people come into contact poses no health hazard for humans.

Words to Know

CATALYST

a material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure.

MONOMER

one of the small, relatively simple molecules from which polymers are made.

FOR FURTHER INFORMATION

"Material Information Polymethylmethacrylate, PMMA, Acrylic." Goodfellow. http://www.goodfellow.com/csp/active/static/E/ME30.HTML (accessed on October 24, 2005).

Meikle, J. L. American Plastic: A Cultural History. New Brunswick, N.J.: Rutgers University Press, 1997.

"More on the Manufacturing of Acrylic Sheet." Plastics Distributor & Fabricator Magazine (November/December 2000). Available online at http://www.plasticsmag.com/features.asp?fIssue=Nov/Dec-00&aid=3053 (accessed on October 24, 2005).

"Polymethylmethacrylate." Kids' Macrogalleria. University of Southern Mississippi, The Polymer Science Learning Center. http://www.pslc.ws/macrog/kidsmac/pmma.htm (accessed on October 24, 2005).

"Plexiglas® Primer." Ridout Plastics. http://www.ridoutplastics.com/plexprim.html (accessed on October 24, 2005).