Poly(methyl methacrylate) (PMMA) is the
synthetic polymer derived from
methyl methacrylate. It is used as an
engineering plastic, and it is a
transparentthermoplastic. PMMA is also known as acrylic, acrylic glass, as well as by the trade names and brands Crylux, Hesalite, Plexiglas, Acrylite, Lucite, and Perspex, among several others (
see below). 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 historic 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[4] 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 German Röhm & Haas AG (as of January 2019, part of
Evonik Industries) and its partner and former U.S. affiliate
Rohm and Haas Company under the
trademark Plexiglas.[5]
Polymethyl methacrylate was discovered in the early 1930s by British chemists Rowland Hill and John Crawford at
Imperial Chemical Industries (ICI) in the United Kingdom. [citation needed] 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.[6] Both Perspex and Plexiglas were commercialized in the late 1930s. In the United States, E.I. du Pont de Nemours & Company (now DuPont Company) subsequently introduced its own product under the trademark Lucite. In 1936 ICI Acrylics (now Lucite International) 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.[7] Civilian applications followed after the war.[8]
Names
Common orthographic stylings include polymethyl methacrylate[9][10] and polymethylmethacrylate. The full IUPAC chemical name is poly(methyl 2-methylpropenoate). (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 Acrylite,[11] Altuglas,[12] Astariglas,[13] Cho Chen,[14] Crystallite, Cyrolite,[15] Hesalite (when used in
Omega watches), Lucite,[16] Optix,[15] Oroglas,[17] PerClax, Perspex,[15] Plexiglas,[15][18] R-Cast,[19] and Sumipex.
PMMA is an economical alternative to
polycarbonate (PC) when
tensile strength,
flexural strength,
transparency, polishability, and UV tolerance are more important than
impact strength, chemical resistance, and heat resistance.[20] Additionally, PMMA does not contain the potentially harmful
bisphenol-A subunits found in polycarbonate and is a far better choice for laser cutting.[21] 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.
Properties
PMMA is a strong, tough, and lightweight material. It has a
density of 1.17–1.20 g/cm3,[1][22] which is less than half that of glass.[1] It also has good impact strength, higher than both glass and polystyrene, but significantly lower than polycarbonate and some engineered polymers. PMMA ignites at 460 °C (860 °F) and
burns, forming
carbon dioxide,
water,
carbon monoxide, and low-molecular-weight compounds, including
formaldehyde.[23]
PMMA
transmits up to 92% of
visible light (3 mm thickness), and gives a reflection of about 4% from each of its surfaces due to its
refractive index (1.4905 at 589.3 nm).[3] It filters
ultraviolet (UV) light at wavelengths below about 300
nm (similar to ordinary window glass). Some manufacturers[24] add coatings or additives to PMMA to improve 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 (for
remote control or heat sensor applications, for example).
PMMA swells and dissolves in many organic
solvents; it also has poor resistance to many other chemicals due to its easily
hydrolyzedester 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.[25]
PMMA has a maximum water absorption ratio of 0.3–0.4% by weight.[22] Tensile strength decreases with increased water absorption.[26] Its
coefficient of thermal expansion is relatively high at (5–10)×10−5 °C−1.[27]
The
Futuro house was made of fibreglass-reinforced polyester plastic, polyester-polyurethane, and poly(methylmethacrylate); one of them was found to be degrading by
cyanobacteria and
Archaea.[28][29]
PMMA can be joined using
cyanoacrylate cement (commonly known as
superglue), with heat (welding), or by using chlorinated solvents such as
dichloromethane or
trichloromethane[30] (chloroform) 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 (including its monomers) 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.[31] 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, it 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 (as well as possible other functions).
Pure poly(methyl methacrylate) 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 ("unzipping") 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 [32]
Dyes may be added to give color for decorative applications, or to protect against (or filter) UV light.
The
glass transition temperature (Tg) of
atactic PMMA is 105 °C (221 °F). The Tg values of commercial grades of PMMA range from 85 to 165 °C (185 to 329 °F); 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 Tg. The forming temperature starts at the glass transition temperature and goes up from there.[34] 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.
Being transparent and durable, PMMA is a versatile material and has been used in a wide range of fields and applications such as rear-lights and instrument clusters for vehicles, appliances, and lenses for glasses. PMMA in the form of sheets affords to shatter resistant panels for building windows, skylights, bulletproof security barriers, signs & displays, sanitary ware (bathtubs), LCD screens, furniture and many other applications. It is also used for coating polymers based on MMA provides outstanding stability against environmental conditions with reduced emission of VOC. Methacrylate polymers are used extensively in medical and dental applications where purity and stability are critical to performance.[33]
Glass substitute
PMMA is commonly used for constructing residential and commercial
aquariums. Designers started building large aquariums when poly(methyl methacrylate) could be used. It is less often used in other building types due to incidents such as the
Summerland disaster.
PMMA is used for viewing ports and even complete pressure hulls of submersibles, such as the
Alicia submarine's viewing sphere and the window of the
bathyscaphe Trieste.
PMMA is used in the lenses of exterior lights of automobiles.[35]
Spectator protection in
ice hockey rinks is made from PMMA.
Historically, PMMA was an important improvement in the design of aircraft windows, making possible such designs as the bombardier's transparent nose compartment in the
Boeing B-17 Flying Fortress. Modern aircraft transparencies often use stretched acrylic plies.
Police vehicles for
riot control often have the regular glass replaced with PMMA to protect the occupants from thrown objects.
PMMA is an important material in the making of certain lighthouse lenses.[36]
PMMA was used for the roofing of the compound in the
Olympic Park for the
1972 Summer Olympics in Munich. It enabled a light and translucent construction of the structure.[37]
PMMA (under the brand name "Lucite") was used for the ceiling of the
Houston Astrodome.
Laser cut acrylic panels have been used to redirect sunlight into a
light pipe or tubular skylight and, from there, to spread it into a room.[38] Their developers Veronica Garcia Hansen,
Ken Yeang, and Ian Edmonds were awarded the
Far East Economic Review Innovation Award in bronze for this technology in 2003.[39][40]
Attenuation being quite strong for distances over one meter (more than 90% intensity loss for a 3000 K source[41]), acrylic broadband light guides are then dedicated mostly to decorative uses.
Pairs of acrylic sheets with a layer of microreplicated prisms between the sheets can have reflective and refractive properties that let them redirect part of incoming sunlight in dependence on its
angle of incidence. Such panels act as miniature
light shelves. Such panels have been commercialized for purposes of
daylighting, to be used as a
window or a
canopy such that sunlight descending from the sky is directed to the ceiling or into the room rather than to the floor. This can lead to a higher illumination of the back part of a room, in particular when combined with a white ceiling, while having a slight impact on the view to the outside compared to normal glazing.[42][43]
Medicine
PMMA has a good degree of compatibility with human
tissue, and it is used in the manufacture of rigid
intraocular lenses which are implanted in the
eye when the original lens has been removed in the treatment of
cataracts. This compatibility was discovered by the English ophthalmologist
Harold Ridley in WWII RAF pilots, whose eyes had been riddled with PMMA splinters coming from the side windows of their Supermarine
Spitfire fighters – the plastic scarcely caused any rejection, compared to glass splinters coming from aircraft such as the
Hawker Hurricane.[44] Ridley had a lens manufactured by the Rayner company (Brighton & Hove, East Sussex) made from Perspex polymerised by ICI. On 29 November 1949 at St Thomas' Hospital, London, Ridley implanted the first intraocular lens at St Thomas's Hospital in London.[45]
In particular, acrylic-type lenses are useful for cataract surgery in patients that have recurrent ocular inflammation (uveitis), as acrylic material induces less inflammation.
Historically, hard
contact lenses were frequently made of this material. Soft contact lenses are often made of a related polymer, where acrylate monomers containing one or more
hydroxyl groups make them
hydrophilic.
In
orthopedic surgery, PMMA
bone cement is used to affix implants and to remodel lost bone.[46] It is supplied as a powder with liquid methyl methacrylate (MMA). Although PMMA is biologically compatible, MMA is considered to be an irritant and a possible
carcinogen. PMMA has also been linked to
cardiopulmonary events in the operating room due to
hypotension.[47] Bone cement acts like a
grout and not so much like a glue in
arthroplasty. Although sticky, it does not bond to either the bone or the implant; rather, it primarily fills the spaces between the prosthesis and the bone preventing motion. A disadvantage of this bone cement is that it heats up to 82.5 °C (180.5 °F) while setting that may cause thermal necrosis of neighboring tissue. A careful balance of initiators and monomers is needed to reduce the rate of polymerization, and thus the heat generated.
In
cosmetic surgery, tiny PMMA microspheres suspended in some biological fluid are injected as a soft-tissue filler under the skin to reduce wrinkles or scars permanently.[48] PMMA as a soft-tissue filler was widely used in the beginning of the century to restore volume in patients with HIV-related facial wasting. PMMA is used illegally to shape muscles by some
bodybuilders.
Plombage is an outdated treatment of
tuberculosis where the
pleural space around an infected
lung was filled with PMMA balls, in order to compress and collapse the affected lung.
Emerging biotechnology and
biomedical research use PMMA to create
microfluidiclab-on-a-chip devices, which require 100 micrometre-wide geometries for routing liquids. These small geometries are amenable to using PMMA in a
biochip fabrication process and offers moderate
biocompatibility.
Bioprocesschromatography columns use cast acrylic tubes as an alternative to glass and stainless steel. These are pressure rated and satisfy stringent requirements of materials for
biocompatibility, toxicity, and extractables.
Dentistry
Due to its aforementioned biocompatibility, poly(methyl methacrylate) is a commonly used material in modern dentistry, particularly in the fabrication of dental prosthetics, artificial teeth, and orthodontic appliances.
Acrylic prosthetic construction
Pre-polymerized, powdered PMMA spheres are mixed with a Methyl Methacrylate liquid monomer, Benzoyl Peroxide (initiator), and NN-Dimethyl-P-Toluidine (accelerator), and placed under heat and pressure to produce a hardened polymerized PMMA structure. Through the use of injection molding techniques, wax based designs with artificial teeth set in predetermined positions built on gypsum stone models of patients' mouths can be converted into functional prosthetics used to replace missing dentition. PMMA polymer and methyl methacrylate monomer mix is then injected into a flask containing a gypsum mold of the previously designed prosthesis, and placed under heat to initiate polymerization process. Pressure is used during the curing process to minimize polymerization shrinkage, ensuring an accurate fit of the prosthesis. Though other methods of polymerizing PMMA for prosthetic fabrication exist, such as chemical and microwave resin activation, the previously described heat-activated resin polymerization technique is the most commonly used due to its cost effectiveness and minimal polymerization shrinkage.
Artificial teeth
While denture teeth can be made of several different materials, PMMA is a material of choice for the manufacturing of artificial teeth used in dental prosthetics. Mechanical properties of the material allow for heightened control of aesthetics, easy surface adjustments, decreased risk of fracture when in function in the oral cavity, and minimal wear against opposing teeth. Additionally, since the bases of dental prosthetics are often constructed using PMMA, adherence of PMMA denture teeth to PMMA denture bases is unparalleled, leading to the construction of a strong and durable prosthetic.[49]
Art and aesthetics
Acrylic paint essentially consists of PMMA suspended in water; however since PMMA is
hydrophobic, a substance with both hydrophobic and hydrophilic groups needs to be added to facilitate the
suspension.
Modern
furniture makers, especially in the 1960s and 1970s, seeking to give their products a space age aesthetic, incorporated Lucite and other PMMA products into their designs, especially office chairs. Many other products (for example, guitars) are sometimes made with acrylic glass to make the commonly opaque objects translucent.
Perspex has been used as a surface to paint on, for example by
Salvador Dalí.
Diasec is a process which uses acrylic glass as a substitute for normal glass in
picture frames. This is done for its relatively low cost, light weight, shatter-resistance, aesthetics and because it can be ordered in larger sizes than standard
picture framing glass.
As early as 1939, Los Angeles-based Dutch sculptor
Jan De Swart experimented with samples of Lucite sent to him by DuPont; De Swart created tools to work the Lucite for sculpture and mixed chemicals to bring about certain effects of color and refraction.[50]
From approximately the 1960s onward, sculptors and glass artists such as
Jan Kubíček,
Leroy Lamis, and
Frederick Hart began using acrylics, especially taking advantage of the material's flexibility, light weight, cost and its capacity to refract and filter light.
In the 1950s and 1960s, Lucite was an extremely popular material for jewelry, with several companies specialized in creating high-quality pieces from this material. Lucite beads and ornaments are still sold by jewelry suppliers.
Acrylic Sheets are produced in dozens of standard colors,[51] most commonly sold using color numbers developed by Rohm & Haas in the 1950s.
Methyl methacrylate "
synthetic resin" for casting (simply the bulk liquid chemical) may be used in conjunction with a polymerization catalyst such as
methyl ethyl ketone peroxide (MEKP), to produce hardened transparent PMMA in any shape, from a mold. Objects like insects or coins, or even dangerous chemicals in breakable quartz ampules, may be embedded in such "cast" blocks, for display and safe handling.
Other uses
PMMA, in the commercial form Technovit 7200 is used vastly in the medical field. It is used for plastic histology, electron microscopy, as well as many more uses.
PMMA has been used to create ultra-white opaque membranes that are flexible and switch appearance to transparent when wet.[52]
Acrylic is used in tanning beds as the transparent surface that separates the occupant from the tanning bulbs while tanning. The type of acrylic used in tanning beds is most often formulated from a special type of polymethyl methacrylate, a compound that allows the passage of ultraviolet rays.
Sheets of PMMA are commonly used in the sign industry to make flat cut out letters in thicknesses typically varying from 3 to 25 millimeters (0.1 to 1.0 in). These letters may be used alone to represent a company's name and/or logo, or they may be a component of illuminated channel letters. Acrylic is also used extensively throughout the sign industry as a component of wall signs where it may be a backplate, painted on the surface or the backside, a faceplate with additional raised lettering or even photographic images printed directly to it, or a spacer to separate sign components.
PMMA was used in
Laserdisc optical media.[53] (
CDs and
DVDs use both acrylic and polycarbonate for impact resistance).
It is used as a light guide for the backlights in
TFT-LCDs.[54]
Plastic optical fiber used for short-distance communication is made from PMMA, and perfluorinated PMMA, clad with fluorinated PMMA, in situations where its flexibility and cheaper installation costs outweigh its poor heat tolerance and higher attenuation versus glass fiber.
In
semiconductor research and industry, PMMA aids as a
resist in the
electron beam lithography process. A solution consisting of the polymer in a solvent is used to
spin coatsilicon and other semiconducting and semi-insulating wafers with a thin film. Patterns on this can be made by an electron beam (using an
electron microscope), deep UV light (shorter wavelength than the standard
photolithography process), or
X-rays. Exposure to these creates chain scission or (de-
cross-linking) within the PMMA, allowing for the selective removal of exposed areas by a chemical developer, making it a positive photoresist. PMMA's advantage is that it allows for extremely high resolution patterns to be made. Smooth PMMA surface can be easily nanostructured by treatment in oxygen
radio-frequency plasma[56] and nanostructured PMMA surface can be easily smoothed by
vacuum ultraviolet (VUV) irradiation.[56]
PMMA is used as a shield to stop beta radiation emitted from radioisotopes.
Small strips of PMMA are used as
dosimeter devices during the
Gamma Irradiation process. The optical properties of PMMA change as the gamma dose increases, and can be measured with a
spectrophotometer.
In the 1960s,
luthierDan Armstrong developed a line of electric guitars and basses whose bodies were made completely of acrylic. These instruments were marketed under the
Ampeg brand.
Ibanez[58] and
B.C. Rich have also made acrylic guitars.
Some modern briar, and occasionally meerschaum, tobacco pipes sport stems made of Lucite.
PMMA technology is utilized in roofing and waterproofing applications. By incorporating a polyester fleece sandwiched between two layers of catalyst-activated PMMA resin, a fully reinforced liquid membrane is created in situ.
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