What Is Acrylic?
What Is Acrylic?
Welcome to this brief introduction to, and history of, acrylic.
Around the lab we call our favored building material acrylic in a generic sense, but that term is technically incorrect because the term acrylic simply refers to any polymers or copolymers that contain polyacrylonitrile.
The acrylic material we favor is more properly called polymethyl methacrylate or PMMA for short. It takes a polymerization process to turn methyl methacrylate into polymethyl methacrylate (CH2=C[CH3]CO2H).
Actually, polymethyl methacrylate (PMMA) is an ester of meth acrylic acid and belongs to the important family of acrylic resins. To further explain, an ester is any of a class of organic compounds that react with water to produce alcohols and organic or inorganic acids.
In modern production, PMMA is obtained principally from propylene, a compound commonly refined from the lighter fractions of crude oil.
Propylene and benzene are reacted together to form cymene, or isopropyl benzene; the cymene is oxidized to cymene hydro peroxide, which is treated with acid to form acetone; the acetone is in turn converted to methyl methacrylate (CH2=C[CH3]CO2CH3) by adding hydrogen cyanide, sulfuric acid and methanol.
Then, the Methyl methacrylate, in bulk liquid form or suspended as fine droplets in water, is polymerized (a chemical reaction in which two or more molecules combine to form larger molecules that contain repeating structural units that link molecules together in large numbers) under the influence of free-radical initiators to form solid polymethyl methacrylate (PMMA).
The presence of the pendant methyl (CH3) groups prevents the polymer chains from packing closely in a crystalline fashion and from rotating freely around the carbon-carbon bonds. And, the result is PMMA – a tough, rigid and lightweight plastic that also possesses the remarkable capacity to almost perfectly transmit visible light and retain these properties over years of exposure to ultraviolet radiation and weather.
There are two basic types of acrylic: extruded and cast.
Extruded acrylic is made through a process in which the liquid plastic is pushed through rollers that press it into sheets as it cools. This is a comparatively inexpensive process, but the resulting sheets are softer than cast acrylic, can scratch easier, and may contain impurities. Extruded acrylic is still generally considered to be of good quality and is usually the more common type available on the market.
Inexpensive acrylic consumer products like dinnerware, mixing bowls, home decorating items, artistic pieces and the like are most commonly formed from extruded acrylic.
Cell cast acrylic tends of be of higher quality than extruded, but it is also more expensive. In cell casting, single sheets are made by pressing the liquid plastic between pieces of a mold, often made of glass, which is then taken through a gradual heating process. The resulting cell cast sheet is stronger than extruded acrylic. This type is often used for larger aquariums, awards, and other products that require shaping or machining of parts in order to create a final product.
Structures made with PMMA can be created with no seams. Chemical welding actually “melts” the two pieces on either side of the seam into one piece of solid material. Chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride are widely used solvent cements that actually allow PMMA pieces to be chemically welded together at the molecular level. Seams that are properly welded and polished are invisible and incredibly strong.
When properly cared for, acrylic can look brand new for several decades, regardless of age or exposure to weather and sun. It can scratch easily, but unlike glass, scratches may be buffed out and acrylic is often used as a lightweight and shatter-resistant alternative to glass.
Virtually every major public aquarium now builds display tanks out of this thermoplastic, and it is often used as safety glazing and even architecturally in many other types of buildings.
When PMMA is just over one inch thick (about 25 mm), it is bullet resistant; the presidential motorcade, the pope’s booth-vehicle, all manner of teller enclosures and drive-through window enclosures all feature bullet-resistant acrylic. It is also commonly used for airplane windows and an increasingly widening variety of other products.
A unique property of this thermoplastic is its ability to be shaped. The use of thermoplastics is by far the leader in today’s industrial society. As just one example, the packaging industry increasingly requires more and more plastics at an ever increasing rate.
In order to gain a deeper understanding of polymethyl methacrylate, perhaps we should shift our focus and explore a brief history and timeline of the development of PMMA.
In 1773, a compound called urea was discovered and isolated from the urine of mammals and other higher forms of animal life.
In 1828, urea was first produced synthetically and the foundation for phenol formaldehyde plastics was laid.
The first acrylic acid was created in 1843, but it wasn’t until 1865 that a formulation of meth acrylic acid was derived from it.
The reaction between meth acrylic acid and methanol results in the ester methyl methacrylate, and, in 1877, the German chemist Wilhelm Rudolph Fittig discovered the polymerization process that turns methyl methacrylate into polymethyl methacrylate.
In 1901, Dr. Otto Rohm published the results of his research with acrylic resinoids; in 1901, the first patent for phenol- formaldehyde plastics was secured by Dr. Leo Baekeland. He discovered that phenol and formaldehyde combined to form a resinous substance, a phenolic plastic which he called “Bakelite”.
Bakelite was the first modern plastic — it could be softened with heat and then molded into shape and set into final form by continued heating under pressure while in the mold. Baekeland’s discovery triggered the creative imagination of organic chemists and research the world over was conducted more intensely than ever before.
Acrylic glass or polymethyl methacrylate (PMMA) is a transparent thermoplastic. Chemically, it is the synthetic polymer of methyl methacrylate.
In 1928, initial formulations of PMMA were being researched, developed and studied in various laboratories by a number of chemists such as William Chalmers, Otto Röhm and Walter Bauer. Officially, polymethyl methacrylate was discovered in the early 1930s by British chemists Rowland Hill and John Crawford at Imperial Chemical Industries (ICI) in England. ICI registered the product under the trademark Perspex.
In 1933, German chemist and industrialist Otto Röhm of Rohm and Haas AG 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 1936, ICI Acrylics (now Lucite International) began the world’s first commercially viable production of acrylic safety glass.
In the United States, E.I. du Pont de Nemours & Company (now DuPont Company) subsequently introduced its own version of PMMA under the trademark Lucite.
Perspex, Plexiglas, and Lucite were commercialized in the late 1930s, but the first major application of the new plastic took place during World War II when acrylic glass sheet (PMMA) played an important role as bullet resistant glazing in our warplanes. It was found to be lightweight and very strong and it could be easily formed to fit into the structural designs of aircraft. It is interesting to note that even to this day, the cockpit canopies, windows and gun turret bubbles of those aircraft are still extremely clear and free from yellowing. The weatherability of Plexiglas is one of its most well-known traits, something no other plastic glazing can match.
Furthermore, during World War II both Allied and Axis forces used acrylic glass for submarine periscopes, windshields, electrical insulators, cockpit canopies, and gun turret bubbles. Incidentally, airplane pilots whose eyes and bodies were damaged by flying shards of PMMA fared much better than those injured by standard glass, demonstrating the much increased compatibility between human tissue and PMMA as compared to glass.
Shortly after World War II ended, myriad civilian PMMA applications burst forth and the modern era of acrylic was truly born.
To date, acrylic glass (PMMA) has been marketed under numerous brand names and over the years; Altuglas, Perspex, Optix, Plexiglas, Oroglas, Acrylite and Lucite found their way into our homes, factories and lives as safety glazing, in electrical and chemical applications, skylights, windshields, architectural elements in large aquariums and other buildings, as well as numerous other beneficial and widely-accepted products and applications.
Plastics in general, and acrylic glass specifically have come of age over the last 75 years. There are two basic kinds of plastics: thermoset and thermoplastics. Thermoset plastics are set into final form during manufacturing, and thermoplastics can be heated and reshaped.
Acrylics are a type of thermoplastic, which means acrylic glass can be formed when heated to the proper temperature. Its chemical composition is that of a clear, water-white, transparent liquid substance of monomer that can be polymerized into sheets, rods, tubes, molding pellets and additives.
Plastics have evolved from a synthetic “substitute” for other materials to a valuable new type raw material. It now occupies a permanent place in our technology and may become the basic material of which our future will be built. So think of acrylics as a new “raw” material; a secondary raw material that has become just as important as more traditional resources like wood, metal, concrete and stone.
PMMA is routinely produced by emulsion polymerization, solution polymerization, and bulk polymerization. Generally, radical initiation is used (including living polymerization methods), but anionic polymerization of PMMA can also be performed.
To produce 2.2 lb. of PMMA, about 4.4 lb. of petroleum is needed. All PMMA commercially produced by radical polymerization is atactic (a polymer exhibiting no stereo chemical regularity of structure) and completely amorphous.
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 cell cast PMMA is higher than molding grades owing to its extremely high molecular mass.
PMMA is a strong, rigid, lightweight and weather-resistant thermoplastic. It has a density and weight of less than half that of glass and only 43% of aluminum. It also features good impact strength and the remarkable ability to transmit up to 92% of visible light leaving a reflection of about 4% from each of its surfaces.
Additionally, PMMA passes infrared light of up to 2800 nm and blocks infrared of longer wavelengths up to 25,000 nm. In fact, PMMA can be formulated such that it can block destructive ultraviolet transmissions in sunlight, reflected sunlight, and from fluorescent lighting. PMMA can block 98% of degrading wavelengths that are transmitted in the 280 – 400 nanometer area. This attribute of PMMA makes it incredibly useful in remote control and heat sensor applications.
PMMA sheet is well suited for a wide variety of applications because it is dimensionally stable and resistant to breakage, features virtually distortion-free clarity resists shrinking and deterioration through long periods of use, and, when heated to a pliable state, it can be formed to almost any shape.
PMMA sheet can be successfully cut with circular saws, band saws, lasers and water-jets. It can be drilled, routed, tapped, filed and machined much like wood or brass with only very slight tool modifications. Because PMMA sheet softens quickly when exposed to excessive heat, it is necessary to keep the cutting tool and machined edge of the sheet as cool as possible. Frequent cooling of the cutting tool is highly recommended. Furthermore, tool sharpness and “trueness” are essential to prevent gumming, heat build-up and stresses in the part. Heat buildup at the machined edge could lead to subsequent stress crazing and therefore must be avoided.
PMMA sheet can be joined using common solvent cements or polymerizable cements. The most critical factor in successful joining is good edge preparation of the parts to be cemented. The edge of the sheet must be properly machined in order to achieve a square, flat surface and no stresses. Annealing of the part prior to cementing is recommended.
PMMA sheet may be annealed at 180°F with the heating and cooling times determined by the sheet thickness. An approximate guideline is annealing time in hours equals the sheet thickness in millimeters, and the cool down period is a minimum of 2 hours ending when sheet temperature falls below 140°F.
PMMA sheet is a combustible thermoplastic. Precautions should be taken to protect this material from flames and high heat sources. PMMA sheet will rapidly burn to completion if not extinguished. The products of combustion, if sufficient air is present, are carbon dioxide and water. However, in many fires sufficient air will not be available and toxic carbon monoxide and low-molecular-weight compounds, including formaldehyde, will be formed.
PMMA sheet has a number of desirable electrical properties. Its surface resistivity is higher than that of most other materials, so it makes an ideal insulator. Continuous outdoor exposure has little effect on its electrical properties.
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, and PMMA is therefore often the material of choice for outdoor applications.
PMMA has a maximum water absorption ratio of 0.3–0.4% by weight, and its tensile strength decreases slightly with increased water absorption. At relative humidity of 100%, 80%, and 60%, the dimensional changes are 0.6%, 0.3% and 0.2%, respectively.
For all of its advantages, there are two primary disadvantages of acrylic glass: It is more expensive than glass, and, if exposed to a direct flame, it will melt and eventually burn. Burning usually releases toxic fumes, so safety precautions should always be taken when it is being cut with power tools or bent using heat. When it is not cared for properly, or when inferior acrylic glass is used, it can scratch, and improperly made joints can be very visible.
Acrylic glass is often used for viewing ports and even complete pressure hulls of submersibles, such as the Alicia submarine’s 100 mm (3.9 inch) thick viewing sphere (pressure tested to 1,350 feet) and the conical, six-inch-thick, porthole of the famous bathyscaphe Trieste ( to 35,814 feet).
ACRYLITE® FF acrylic sheet is a continuously manufactured acrylic sheet. It is produced by an innovative process, resulting in a sheet offering the easy handling and processing of extruded sheet along with the high optical characteristics and low stress levels expected of cast products.
Acrylic glass or PMMA sheet will withstand exposure to blazing sun, extreme cold, sudden temperature changes, salt water spray, etc., and it will not deteriorate after many years of service because of the inherent stability of acrylic resins.
Although acrylic glass or PMMA sheet will expand and contract due to changes in temperature and humidity, it will not shrink with age, and its post-forming stability is excellent. Though it is not as rigid as glass or metals, it is more rigid than most other plastics such as acetates, polycarbonates or vinyl.
If PMMA sheet is formed into corrugated or domed shapes, rigidity is increased and deflection is minimized.
Although the tensile strength of standard PMMA sheet is 10,000 psi at room temperature (ASTM D 638), all thermoplastic materials, including PMMA sheet, will gradually lose tensile strength as the temperature approaches the maximum recommended for continuous service – 160°F. Most PMMA sheet can be used at temperatures from -30°F up to +190°F, depending on the application. It is recommended that temperatures not exceed 160°F for continuous service or 190°F for short, intermittent use.
Like most other plastics, PMMA sheet will expand and contract from three to eight times more than glass or metals.
Clear, colorless PMMA sheet allows light transmittance of 92% and can be warranted not to lose more than 3% of its light-transmitting ability in a 10-year period.
ACRYLITE® OP-3 acrylic glass sheet is formulated with ultraviolet absorbers designed to help protect pictures, photographs and posters from the damaging effects of ultraviolet light by absorbing more than 98% of the radiation in the ultraviolet range below 400 nanometers.
The thermal conductivity of a material–its ability to conduct heat–is called k-Factor. Acrylic glass sheet is a better insulator than glass. It’s U-Factor or overall coefficient of heat transfer is approximately 10% lower than that of glass of the same thickness. Plus, PMMA sheet is more resistant than glass to thermal shock and to stresses caused by substantial temperature differences between opposite surfaces of a window.
In conclusion, as a material, polymethyl methacrylate has enjoyed incredible advancements in a mere seventy five years of development. From its humble beginnings as Bakelite to seriously advancing military efficiency, to the spawning of a vast array of new consumer products to acceptance of substantial architectural and significant industrial applications, to the recent advent of applications as crucial elements of high-technology as well as extraordinary bio-medical applications, the future of PMMA is truly unimaginable.
While a Bakelite radio housing of the 1930’s and the acrylic glass cockpit canopy of a World War II fighter may seem entirely old fashioned in today’s modern world, it is important to understand that as a material PMMA has been at the cutting edge of technology since its discovery. Since this trend seems very likely to continue, it truly staggers the imagination to consider the potential PMMA applications that will be unlocked by the next seventy five years of development.