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.
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.
The Manufacturing Process
Acrylic plastic polymers are formed by reacting a monomer, such as methyl methacrylate, with a catalyst. A typical catalyst would be an organic peroxide. The catalyst starts the reaction and enters into it to keep it going, but does not become part of the resulting polymer.
Acrylic plastics are available in three forms: flat sheets, elongated shapes (rods and tubes), and molding powder. Molding powders are sometimes made by a process known as suspension polymerization in which the reaction takes place between tiny droplets of the monomer suspended in a solution of water and catalyst. This results in grains of polymer with tightly controlled molecular weight suitable for molding or extrusion.
Acrylic plastic sheets are formed by a process known as bulk polymerization. In this process, the monomer and catalyst are poured into a mold where the reaction takes place. Two methods of bulk polymerization may be used: batch cell or continuous. Batch cell is the most common because it is simple and is easily adapted for making acrylic sheets in thicknesses from 0.06 to 6.0 inches (0.16-15 cm) and widths from 3 feet (0.9 m) up to several hundred feet. The batch cell method may also be used to form rods and tubes. The continuous method is quicker and involves less labor. It is used to make sheets of thinner thicknesses and smaller widths than those produced by the batch cell method.
The storage, handling, and processing of the chemicals that make acrylic plastics are done under controlled environmental conditions to prevent contamination of the material or unsafe chemical reactions. The control of temperature is especially critical to the polymerization process. Even the initial temperatures of the monomer and catalyst are controlled before they are introduced into the mold. During the entire process, the temperature of the reacting material is monitored and controlled to ensure the heating and cooling cycles are the proper temperature and duration.
Samples of finished acrylic materials are also given periodic laboratory analysis to confirm physical, optical, and chemical properties.
Toxic Materials, Safety Considerations, and Recycling
Acrylic plastics manufacturing involves highly toxic substances which require careful storage, handling, and disposal. The polymerization process can result in an explosion if not monitored properly. It also produces toxic fumes. Recent legislation requires that the polymerization process be carried out in a closed environment and that the fumes be cleaned, captured, or otherwise neutralized before discharge to the atmosphere.
Acrylic plastic is not easily recycled. It is considered a group 7 plastic among recycled plastics and is not collected for recycling in most communities. Large pieces can be reformed into other useful objects if they have not suffered too much stress, crazing, or cracking, but this accounts for only a very small portion of the acrylic plastic waste. In a landfill, acrylic plastics, like many other plastics, are not readily biodegradable. Some acrylic plastics are highly flammable and must be protected from sources of combustion.
The average annual increase in the rate of consumption of acrylic plastics has been about 10%. A future annual growth rate of about 5% is predicted. Despite the fact that acrylic plastics are one of the oldest plastic materials in use today, they still hold the same advantages of optical clarity and resistance to the outdoor environment that make them the material of choice for many applications.
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