Poly(methyl methacrylate)


Poly is a synthetic polymer derived from methyl methacrylate. It is a transparent thermoplastic used as an engineering plastic. PMMA is also known as acrylic and acrylic glass and by the trade names and brands Crylux, Walcast, Wanjiale,'Hesalite, Plexiglas, Acrylite, Lucite, PerClax, and Perspex', among several others. This plastic is often used in sheet form as a lightweight or shatter-resistant alternative to glass. It can also be used as a casting resin, in inks and coatings, and for many other purposes.
It is often technically classified as a type of glass in that it is a non-crystalline vitreous substance, hence its occasional historical designation as acrylic glass.

History

The first acrylic acid was created in 1843. Methacrylic acid, derived from acrylic acid, was formulated in 1865. The reaction between methacrylic acid and methanol results in the ester methyl methacrylate.
It was developed in 1928 in several different laboratories by many chemists, such as William R. Conn, Otto Röhm, and Walter Bauer, and first brought to market in 1933 by the German company Röhm & Haas AG and its partner and former U.S. affiliate Rohm and Haas Company under the trademark Plexiglas.
Polymethyl methacrylate was discovered in the early 1930s by British chemists Rowland Hill and John Crawford at Imperial Chemical Industries in the United Kingdom. ICI registered the product under the trademark Perspex. About the same time, chemist and industrialist Otto Röhm of Röhm and Haas AG in Germany attempted to produce safety glass by polymerizing methyl methacrylate between two layers of glass. The polymer separated from the glass as a clear plastic sheet, which Röhm gave the trademarked name Plexiglas in 1933. Both Perspex and Plexiglas were commercialized in the late 1930s. In the United States, E.I. du Pont de Nemours & Company subsequently introduced its own product under the trademark Lucite. In 1936 ICI Acrylics began the first commercially viable production of acrylic safety glass. During World War II both Allied and Axis forces used acrylic glass for submarine periscopes and aircraft windscreen, canopies, and gun turrets. Scraps of acrylic were also used to make clear pistol grips for the M1911A1 pistol or clear handle grips for the M1 bayonet or theater knives so that soldiers could put small photos of loved ones or pin-up girls' pictures inside. They were called "Sweetheart Grips" or "Pin-up Grips". Others were used to make handles for theater knives made from scrap materials. Civilian applications followed after the war.

Names

Common orthographic stylings include polymethyl methacrylate and polymethylmethacrylate. The full IUPAC chemical name is poly, although it is a common mistake to use "an" instead of "en".
Although PMMA is often called simply "acrylic", acrylic can also refer to other polymers or copolymers containing polyacrylonitrile. Notable trade names and brands include Walcast, Wanjiale,Acrylite, Altuglas, Astariglas, Cho Chen, Crystallite, Cyrolite, Hesalite, Lucite, Optix, Oroglas, PerClax, Perspex, Plexiglas, R-Cast, and Sumipex.

Properties

PMMA is a strong, tough, and lightweight material. It has a density of 1.17–1.20 g/cm, which is approximately half that of glass, which is generally, depending on composition, 2.2–2.53 g/cm. It also has good impact strength, higher than both glass and polystyrene, but significantly lower than polycarbonate and some engineered polymers. PMMA ignites at and burns, forming carbon dioxide, water, carbon monoxide, and low-molecular-weight compounds, including formaldehyde.
PMMA is an economical alternative to polycarbonate when tensile strength, flexural strength, transparency, polishability, and UV tolerance are more important than impact strength, chemical resistance, and heat resistance. Additionally, PMMA does not contain the potentially harmful bisphenol-A subunits found in polycarbonate and is a far better choice for laser cutting. It is often preferred because of its moderate properties, easy handling and processing, and low cost. Non-modified PMMA behaves in a brittle manner when under load, especially under an impact force, and is more prone to scratching than conventional inorganic glass, but modified PMMA is sometimes able to achieve high scratch and impact resistance.
PMMA transmits up to 92% of visible light, and gives a reflection of about 4% from each of its surfaces due to its refractive index. It filters ultraviolet light at wavelengths below about 300 nm. Some manufacturers add coatings or additives to PMMA to increase absorption in the 300–400 nm range. PMMA passes infrared light of up to 2,800 nm and blocks IR of longer wavelengths up to 25,000 nm. Colored PMMA varieties allow specific IR wavelengths to pass while blocking visible light.
PMMA swells and dissolves in many organic solvents; it also has poor resistance to many other chemicals due to its easily hydrolyzed ester groups. Nevertheless, its environmental stability is superior to most other plastics such as polystyrene and polyethylene, and therefore it is often the material of choice for outdoor applications.
PMMA has a maximum water absorption ratio of 0.3–0.4% by weight. Tensile strength decreases with increased water absorption. Its coefficient of thermal expansion is relatively high at ×10 °C.
PMMA can be joined using cyanoacrylate cement, with heat, or by using chlorinated solvents such as dichloromethane or trichloromethane to dissolve the plastic at the joint, which then fuses and sets, forming an almost invisible weld. Scratches may easily be removed by polishing or by heating the surface of the material. Laser cutting may be used to form intricate designs from PMMA sheets. PMMA vaporizes to gaseous compounds upon laser cutting, so a very clean cut is made, and cutting is performed very easily. However, the pulsed lasercutting introduces high internal stresses, which on exposure to solvents produce undesirable "stress-crazing" at the cut edge and several millimetres deep. Even ammonium-based glass-cleaner and almost everything short of soap-and-water produces similar undesirable crazing, sometimes over the entire surface of the cut parts, at great distances from the stressed edge. Annealing the PMMA sheet/parts is therefore an obligatory post-processing step when intending to chemically bond lasercut parts together.
In the majority of applications, PMMA will not shatter. Rather, it breaks into large dull pieces. Since PMMA is softer and more easily scratched than glass, scratch-resistant coatings are often added to PMMA sheets to protect it.
Pure poly homopolymer is rarely sold as an end product, since it is not optimized for most applications. Rather, modified formulations with varying amounts of other comonomers, additives, and fillers are created for uses where specific properties are required. For example:
  • A small amount of acrylate comonomers are routinely used in PMMA grades destined for heat processing, since this stabilizes the polymer to depolymerization during processing.
  • Comonomers such as butyl acrylate are often added to improve impact strength.
  • Comonomers such as methacrylic acid can be added to increase the glass transition temperature of the polymer for higher temperature use such as in lighting applications.
  • Plasticizers may be added to improve processing properties, lower the glass transition temperature, improve impact properties, and improve mechanical properties such as elastic modulus
  • Dyes may be added to give color for decorative applications, or to protect against UV light.
  • Fillers may be substituted to reduce cost.

    Synthesis and processing

Production

PMMA is routinely produced by emulsion polymerization, solution polymerization, and bulk polymerization. Generally, radical initiation is used, but anionic polymerization of PMMA can also be performed.
The glass transition temperature of atactic PMMA is. The T values of commercial grades of PMMA range from ; the range is so wide because of the vast number of commercial compositions that are copolymers with co-monomers other than methyl methacrylate. PMMA is thus an organic glass at room temperature; i.e., it is below its T. The forming temperature starts at the glass transition temperature and goes up from there. All common molding processes may be used, including injection molding, compression molding, and extrusion. The highest quality PMMA sheets are produced by cell casting, but in this case, the polymerization and molding steps occur concurrently. The strength of the material is higher than molding grades owing to its extremely high molecular mass. Rubber toughening has been used to increase the toughness of PMMA to overcome its brittle behavior in response to applied loads.

Cast acrylic

Cast acrylic is a form of PMMA that is formed by casting the monomer methyl methacrylate, mixed with initiators and possibly other additives, into a form or mold. Sheet and rod stock are created by casting into static forms, while tubing is produced in rotational molds.
Cast acrylic has better thermal stability, higher resistance to crazing when exposed to solvents, and wider thermo-forming range than extruded acrylic. Cast acrylic has a better ability to be reworked hot and it is known for its superior surface finish and optical properties. Cast acrylic is also more scratch-resistant than extruded acrylic. Cast acrylic is preferred over extruded acrylic in applications that require machining, such as turning on lathes, milling, or drilling. Extruded acrylic, which has less thermal stability, tends to melt and clog cutting tools.

Recycling

Plexiglass can be broken down with pyrolysis at a temperature of at least. The recovered monomers then are purified, but the costs and complexity have prevented this from becoming the norm.
Another approach binds monomers to the ends of long polymer chains. Those monomers detach when heated, triggering the chain to disassemble, with monomer yields of up to 90%, although the presence of dyes reduce this number. However, polymers produced by this technology require special machinery and lack thermal stability.
A third approach adds a chlorinated solvent, often dichlorobenzene, to crushed Plexiglass. The mixture is heated to a modest and exposed to ultraviolet light. The light splits a chlorine radical from the solvent, which breaks the polymer into monomers, which are purified via distillation, yielding virgin-grade stock. Even in the presence of additives, yields are 94 to 98%.