Poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA) is a man-made polymer made from methyl methacrylate. This clear thermoplastic serves as an engineering plastic and is frequently produced in sheets, offering a lightweight and shatter-resistant option compared to glass. Commonly called acrylic or acrylic glass, it is sold under brand names like Plexiglas, Perspex, and Lucite. Because it is a non-crystalline vitreous substance, it qualifies technically as a type of glass, which explains the nickname acrylic glass.

History

The first acrylic acid was created in 1843, and methacrylic acid—derived from it—was formulated in 1865. The ester methyl methacrylate comes from a reaction between methacrylic acid and methanol. Many chemists, including William R. Conn, Otto Röhm, and Walter Bauer, worked on its development in 1928 across several labs. The German company Röhm & Haas AG (now part of Evonik Industries as of January 2019) and its former U.S. affiliate Rohm and Haas Company first marketed it in 1933 under the trademark Plexiglas.

British chemists Rowland Hill and John Crawford at Imperial Chemical Industries (ICI) in the UK discovered PMMA in the early 1930s, registering it as Perspex. Around the same time, Otto Röhm in Germany tried making safety glass by polymerizing methyl methacrylate between two glass layers; the polymer separated as a clear sheet, which he trademarked Plexiglas in 1933. Both Perspex and Plexiglas hit the market in the late 1930s. In the U.S., E.I. du Pont de Nemours & Company introduced its own version as Lucite.

ICI Acrylics began the first commercially viable production of acrylic safety glass in 1936. During World War II, both Allied and Axis forces used acrylic glass for submarine periscopes, aircraft windscreens, canopies, and gun turrets. Scraps were fashioned into clear pistol grips for the M1911A1 pistol or handles for M1 bayonets and theater knives, allowing soldiers to insert small photos of loved ones or pin-up girls; these were called "Sweetheart Grips" or "Pin-up Grips." Others became handles for theater knives made from scrap. Civilian uses followed after the war.

Names

Common spellings include polymethyl methacrylate and polymethylmethacrylate. The full IUPAC name is poly(methyl 2-methylpropenoate), though people often mistakenly use "an" instead of "en." While PMMA is frequently called simply "acrylic," that term 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 (used in Omega watches), Lucite, Optix, Oroglas, PerClax, Perspex, Plexiglas, R-Cast, and Sumipex.

Properties

PMMA is strong, tough, and lightweight, with a density of 1.17–1.20 g/cm³—roughly half that of glass (2.2–2.53 g/cm³, depending on composition). It offers good impact strength, higher than glass and polystyrene but much lower than polycarbonate and some engineered polymers. It ignites at 460 °C (860 °F) and burns, producing carbon dioxide, water, carbon monoxide, and low-molecular-weight compounds like formaldehyde. PMMA is an economical alternative to polycarbonate when tensile strength, flexural strength, transparency, polishability, and UV tolerance matter more than impact strength, chemical resistance, and heat resistance.

It also lacks the potentially harmful bisphenol-A subunits found in polycarbonate and is less prone to combustion and discoloration during laser cutting. Its moderate properties, easy handling and processing, and low cost make it a popular choice. Unmodified PMMA behaves brittlely under load, especially on impact, and scratches more easily than conventional glass, but modified versions can achieve high scratch and impact resistance.

It transmits up to 92% of visible light (at 3 mm thickness) and reflects about 4% from each surface due to its refractive index of 1.4905 at 589.3 nm. It filters UV light below about 300 nm, similar to ordinary window glass. Some manufacturers add coatings or additives to boost absorption in the 300–400 nm range. PMMA passes infrared light up to 2,800 nm and blocks IR of longer wavelengths up to 25,000 nm.

Colored varieties allow specific IR wavelengths through while blocking visible light, useful for remote controls or heat sensors. PMMA swells and dissolves in many organic solvents and has poor resistance to many chemicals due to its easily hydrolyzed ester groups. However, its environmental stability surpasses most other plastics like polystyrene and polyethylene, making it a common choice for outdoor use.

Its maximum water absorption ratio is 0.3–0.4% by weight, and tensile strength drops with increased water absorption. Its coefficient of thermal expansion is relatively high at (5–10)×10⁻⁵ °C⁻¹. PMMA can be joined using cyanoacrylate cement (superglue), heat welding, or chlorinated solvents like dichloromethane or chloroform, which dissolve the plastic at the joint, fusing it into an almost invisible weld. Scratches can be easily removed by polishing.