Jeff Israel

March 24, 1999

Materials Science

Webster's Dictionary gives the definition for acrylic as acrylic resin, acrylic fiber, or a paint containing acrylic resin. This, however, is a very broad definition. Acrylics are synthetic plastic materials which contain at least one derivative of acrylic acid. The most common acrylic plastic is polymethyl methacrylate (PMMA) which is a tough yet highly transparent material. It is highly resistant to ultraviolet radiation and weathering and is very durable. They are easily formed and can be manufactured in sheets, rods and tubes or they can be injected or compression molded as powder, cast, or sheets.

As a result of the high durability of acrylics, there are many applications for its use. They include laminated glass, protective coatings, costume jewelry and other molded products, dental materials (especially dentures), windows and shields in aircraft acrylic fingernails, skylights, automobile taillights, and outdoor signs. Lucite and Plexiglas, two brands of acrylic plastics, are used in solution form to bond other acrylics to other substances. Ethyl acrylic, also a solution, is used as a pressure sensitive adhesive: it is flexible and remains slightly tacky. One interesting application of acrylic plastic worth noting is that the ceiling of the Houston Astrodome, a sports arena in Texas, is constructed of hundreds of panels of PMMA which are double insulated to help with temperature regulation and to prevent precipitation from leaking into the arena.

Acrylic resins are colorless, highly transparent, and will not discolor or fade when exposed to light. They also have excellent ultraviolet and white light transmission. Acrylic resins are resistant to oxidizing agents and have good resistance to water. They are also fairly strong and flexible with a tensile strength of 8,000 pounds per square inch and compressive and flexure strength of about 15,000 pounds per square inch. However, these plastics do have low abrasion resistance and are subject to crazing when used as glass unless they are specially treated.

Acrylic plastics are polymers, as all plastics are. Acrylic acid, the substance which distinguishes acrylics from other plastics, was first prepared in 1843. Following this discovery was the creation of methacrylic acid, a derivative of acrylic acid, in 1865. In 1936, the polymerization process, which was originally discovered in 1877, was used to commercially produce sheets of acrylic safety glass.

The basic monomer that polymethyl methacrylate and other acrylic plastics are formed is methyl methacrylate. The chemical formula for this monomer is C₅H₈O₂, however it should be noted that there is a double bond between the central carbon atoms. During polymerization, , one part of the carbon double bond breaks away and bonds with the middle carbon of another methyl methacrylate molecule. This starts a chain link process which repeats itself until the final polymer is formed.

There are several different ways in which methyl methacrylate can be formed. The most common method is to react acetone [CH₃COCH₃] with sodium cyanide [NaCn]. The product of this reaction, acetone cyanhydrin is reacted with methanol alcohol to yield methyl methacrylate. Different acrylic plastics can be formed with other, similar monomers. Methyl acrylate [CH₂=CHOOCH₃] can be reacted with acrylonitrile [CH₂CHCN] and methyl methacrylate to form the copolymers which are the basis of some acrylic plastics. By using different methods and controlling the proportions the elasticity and other properties of the acrylic can be altered.

Acrylic plastic polymers are formed when a monomer, such as methacrylate is reacted with a catalyst, such as organic peroxide. Three different forms of acrylics are available: flats sheets, elongated shapes, and molding powers. The molding powders, which are tightly packed grains of polymer used for molding or extrusion, are made through suspension polymerization. This reaction takes place between tiny droplets of the monomer which is suspended in a solution of water and a catalyst. Acrylics plastic sheets are formed through bulk polymerization. In this process, a mold is where the reaction between the monomer and catalyst takes place. There are two different kinds of bulk polymerization. They are batch cell, which is the most common because it is a simple, adaptive process, and continuous, which is quicker and involves less labor. In the bulk cell process, the thickness and width of an acrylic sheet can vary from 0.06 to 6.0 inches and 3 to several hundred feet, respectively. The continuous method produces sheets of less thickness and small widths than those of the batch cell method.

In order to prevent contamination of materials or unsafe chemical reactions, the storage, handling, and processing of the chemicals used to produce acrylic plastics is done under controlled environmental conditions. Temperature control is especially crucial because the heating and cooling cycles of the formation process are vital to produce a quality finished product. In fact, the temperatures of the monomer and catalyst are controlled before they are even introduced into the mold and are monitored throughout the duration of the polymerization process.

The manufacturing of acrylic plastics involves highly toxic substances which require careful handling, storage, and disposal. Explosion can result from the polymerization process if it is not monitored properly. There are also the toxic fumes which are generated during the process. These fumes are required by government legislation, to be cleaned, captured, or otherwise neutralized before they are allowed to enter the atmosphere.

Acrylic plastics are also not easily recycles. There are classified as a group 7 plastic and are not collected for recycling in most communities. The only re-use of these plastics occurs when some large pieces, provided that they have not suffered too much stress and are not crazing or cracking, can be reformed into other useful object. However, this accounts for only a small amount of all acrylic plastics. Most acrylic plastics are not readily biodegradable in landfills and some are highly flammable and must be kept away from all sources of combustion.

Acrylic plastics are some of the oldest plastic materials still in use today. However, they still have the advantages of optical clarity and resistance to the outdoor environment that them the choice for many applications. In fact, the average annual growth rate of acrylics plastics has been around 10 per cent and the future forecast is that the gains will continue, at approximately 5 per cent annually.