Acrylic Plastic

Acrylic Plastic

Acrylic Plastic


Acrylic plastic refers to a family of synthetic, or man-made, plastic materials containing one or more derivatives of acrylic acid. The most common acrylic plastic is polymethyl methacrylate (PMMA), which is sold under the brand names of Plexiglas, Lucite, Perspex, and Crystallite. PMMA is a tough, highly transparent material with excellent resistance to ultraviolet radiation and weathering. It can be colored, molded, cut, drilled, and formed. These properties make it ideal for many applications including airplane windshields, skylights, automobile taillights, and outdoor signs. One notable application is the ceiling of the Houston Astrodome which is composed of hundreds of double-insulating panels of PMMA acrylic plastic.

Like all plastics, acrylic plastics are polymers. The word polymer comes from the Greek words poly, meaning many, and meros, meaning a part. A polymer, therefore, is a material made up of many molecules, or parts, linked together like a chain. Polymers may have hundreds, or even thousands, of molecules linked together. More importantly, a polymer is a material that has properties entirely different than its component parts. The process of making a polymer, known as polymerization, has been likened to shoveling scrap glass, copper, and other materials into a box, shaking the box, and coming back in an hour to find a working color television set. The glass, copper, and other component parts are still there, but they have been reassembled into something that looks and functions entirely differently.

The first plastic polymer, celluloid, a combination of cellulose nitrate and camphor, was developed in 1869. It was based on the natural polymer cellulose, which is present in plants. Celluloid was used to make many items including photographic film, combs, and men's shirt collars.

In 1909, Leo Baekeland developed the first commercially successful synthetic plastic polymer when he patented phenol formalde-hyde resin, which he named Bakelite. Bakelite was an immediate success. It could be machined and molded. It was an excellent electrical insulator and was resistant to heat, acids, and weather. It could also be colored and dyed for use in decorative objects. Bakelite plastic was used in radio, telephone, and electrical equipment, as well as counter tops, buttons, and knife handles.

Acrylic acid was first prepared in 1843. Methacrylic acid, which is a derivative of acrylic acid, was formulated in 1865. When methacrylic acid is reacted with methyl alcohol, it results in an ester known as methyl methacrylate. The polymerization process to turn methyl methacrylate into polymethyl methacrylate was discovered by the German chemists Fittig and Paul in 1877, but it wasn't until 1936 that the process was used to produce sheets of acrylic safety glass commercially. During World War II, acrylic glass was used for periscope ports on submarines and for windshields, canopies, and gun turrets on airplanes.

Raw Materials
Methyl methacrylate is the basic molecule, or monomer, from which polymethyl methacrylate and many other acrylic plastic polymers are formed. The chemical notation for this material is CH 2 =C(CH 3 ).
COOCH 3 . It is written in this format, rather than the more common chemical notation C 5 H 8 O 2 , to show the double bond (=) between the two carbon atoms in the middle. During polymerization, one leg of this double bond breaks and links up with the middle carbon atom of another methyl methacrylate molecule to start a chain. This process repeats itself until the final polymer is formed. (See Figure 1)
Methyl methacrylate may be formed in several ways. One common way is to react acetone [CH 3 COCH 3 ] with sodium cyanide [NaCN] to produce acetone cyanhydrin [(CH 3 ) 2 C(OH)CN]. This in turn is reacted with methyl alcohol [CH 3 OH] to produce methyl methacrylate.

Other similar monomers such as methyl acrylate [CH 2 =CHCOOCH,] and acrylonitrile [CH 2 =CHCN] can be joined with methyl methacrylate to form different acrylic plastics. (See Figure 2) When two or more monomers are joined together, the result is known as a copolymer. Just as with methyl methacrylate, both of these monomers have a double bond on the middle carbon atoms that splits during polymerization to link with the carbon atoms of other molecules. Controlling the proportion of these other monomers produces changes in elasticity and other properties in the resulting plastic.

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