Acrylics (PAN, PMMA)
The acrylics group is dominated by two resinsone used principally for blending with other resins and as a fiber (PAN) and the other used principally for molding (PMMA). The blending and fiber resin, polyacrylonitrile (PAN), which is made from the acrylonitrile monomer, was described in Chapter 7 as part of the discussion of modified styrene. It is a blending and alloying resin for styrene and butadiene. (The acrylonitrile monomer is the "A" in ABS.) When made into a fiber, PAN is called acrylic and is characterized by softness and ease of coloring. The fiber is used to make sweaters and carpets (DuPont brand name: Orlon®). PAN is also used as a starting material in making carbon fibers for plastic reinforced composite materials. A few other acrylic resins are made, but they are less important and are used chiefly as copolymers with commodity resins. The molding resin, polymethylmethacrylate (PMMA), is the subject of the remainder of this discussion on acrylics because it is this polymer that competes with the other engineering thermoplastics. Common brand names for PMMA include Ato-Haas's Plexiglas®, Lucite International's Lucite®, and Cyro Industries' Acrylite®. The resin is polymerized by the addition polymerization method to form the polymer that is represented in Figure 8.7. The plastic is atactic and therefore amorphous.
Figure 8.7 Polymer repeating unit for polymethyl-methacrylate (PMMA), which is an acrylic.
The most important property of PMMA is its optical clarity. This plastic has as high as 92% light transmittance, the highest of any plastic material. It also has the lowest sensitivity to UV light of any plastic, very low oxidation sensitivity, and overall weather resistance, which together result in a high retention of clarity and light transmittance even after very long periods of time. PMMA has very high gloss. These excellent optical properties have led to applications such as windshields (especially for planes and helicopters), skylights, fluorescent light diffusion panels, outdoor signs, automobile taillights, headlight covers, compact discs, display cases, light fixtures, and general glass substitutes. In these applications PMMA competes with poly-carbonate. PMMA has better initial and long-term optical properties but is more brittle and not as tough and strong as polycarbonate. Impact grades of PMMA copolymer have up to 20 times the impact strength, although the optical properties are not as high as those of homopolymer PMMA. As with polycarbonate, PMMA is scratched easily, although coated grades are available that significantly reduce the scratch problem. PMMA has a significant advantage in price over polycarbonate. Some acrylic products are shown in Photo 8.7.
PMMA is sensitive to chlorinated solvents, which can result in some crazing and stress cracking but also allows the use of solvent cements for joining. The polarity of the pendant group leads to some moisture absorption, which does not seriously affect physical properties butcan lead to defects in molding, so PMMA should be dried before it is molded. In repeated hot water wash cycles, PMMA will craze, so applications in which this treatment is likely should be avoided. PMMA is easily cleaned with a mild soap, household ammonia, or dilute inorganic acids. PMMA can be dispersed or dissolved in water or solvents to form floor sealants and polishes. This property, along with the excellent ability to color and weather, has led to its use in paint.
Photo 8.7 Various acrylic applications. (a) Carbon fibers made from polyacrylonitrile. (b) Acrylic (PMMA) headlights on a car. (c) Acrylic (PMMA) display stand. (Courtesy of BYU; Courtesy of the Home Depot, Provo, Utah)
The relatively low processing temperature, low shrinkage, and good dimensional stability make PMMA easy to process in injection molding and extrusion. A major PMMA product is extruded sheet, which can be thermoformed into many of the products mentioned earlier, especially outdoor signs. PMMA is one of the easiest of plastics to machine and join, thus adding to its range of applications.
If the highest optical clarity is desired, another processing method is used to make acrylic sheet. This process is casting, which is discussed in detail in Chapter 16. The advantage of this process over extrusion is that no thermal stresses are present and optical distortions are minimal. The material cast is a solution of the PMMA polymer dissolved in the monomer (MMA) that has been initiated with a peroxide or that is initiated with UV light. This solution of PMMA in its monomer is called acrylic syrup, because of the obvious similarity in viscosities to maple syrup. Acrylic syrup could also be made by interrupting the polymerization process before the chains get very long.
Other products in addition to sheets are cast from acrylic syrup. For instance, when highly filled with calcium carbonate or alumina trihydrate, acrylic syrup can be poured into molds to form sinks, countertops, and other bathroom and kitchen fixtures that resemble marble. A brand name of these products is DuPont Corian®. Cast acrylic countertops compete against cast thermoset polyesters, which are discussed in Chapter 9, and, of course, against marble. An advantage of the acrylics in this market is their weatherability, low burning rate, low smoke emission, and ease of removal of stains and burn marks. Acrylic is cast without fillers as an encapsulating material, especially when clarity and other optical properties are required. Acrylics are among the easiest plastics to form by mechanical machining and other simple techniques. They are, therefore, widely used by hobbyists and commercially to make parts for displays and artistic works (see Photo 8.7).
Acrylic blends, alloys, and copolymers achieve selected properties that are significantly higher than homopolymer acrylic in toughness, use temperature, and ease of molding. These mixtures are inferior to the homopolymer in optical qualities, but for some applications the decrease is not significant.