Poly(methyl methacrylate)

Poly(methyl methacrylate)
IUPAC name Poly(methyl 2-methylpropenoate)
Other names Poly(methyl methacrylate) (PMMA) methyl methacrylate resin
Identifiers
CAS number	9011-14-7 Y
SMILES C[C](C)C(=O)OC
Properties
Molecular formula	(C5O2H8)n
Molar mass	varies
Density	1.18 g/cm3[1]
Melting point	160 °C (320 °F)[2]
Boiling point	200.0 °C (392.0 °F)
Refractive index (nD)	1.4914 at 587.6 nm.[3]
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Poly(methyl methacrylate) (PMMA) is a transparent thermoplastic, often used as a light or shatter-resistant alternative to glass. Chemically, it is the synthetic polymer of methyl methacrylate. The material was developed in 1928 in various laboratories, and was first brought to market in 1933 by Rohm and Haas Company, under the trademark Plexiglas^[4]. It has since been sold under many different names.

PMMA is an economical alternative to polycarbonate (PC) when extreme strength is not necessary. Additionally, PMMA does not contain the potentially harmful bisphenol-A subunits found in polycarbonate. It is often preferred because of its moderate properties, easy handling and processing, and low cost, but behaves in a brittle manner when loaded, especially under an impact force, and is more prone to scratching compared to 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. The German chemists Fittig and Paul discovered in 1877 the polymerization process that turns methyl methacrylate into polymethyl methacrylate. In 1933 the German chemist Otto Röhm patented and registered the brand name PLEXIGLAS. In 1936 the first commercially viable production of acrylic safety glass began. During World War II acrylic glass was used for submarine periscopes, and windshields, canopies, and gun turrets for airplanes.^[5]

Names

PMMA has been sold under a variety of brand names and generic names. It is often generically called acrylic glass^[6], although it is chemically unrelated to glass. It is sometimes called simply acrylic^[7], although acrylic can also refer to other polymers or copolymers containing polyacrylonitrile. Other notable trade names include:

• Altuglas ^[8] (Arkema)
• Lucite ^[9][10]
  • Perspex ^[9][11]
• Optix (Plaskolite) ^[12]

Synthesis

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 1 kg (2.2 lb) of PMMA, about 2 kg (4.4 lb) of petroleum is needed.

Processing

The glass transition temperature of PMMA ranges from 85 to 165 °C (185 to 329 °F); the range is so wide because of the vast number of commercial compositions. The forming temperature starts at the glass transition temperature and goes up from there.^[13] 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 strength of PMMA owing to its brittle behavior in response to applied loads.

Handling, cutting, and joining

PMMA can be joined using cyanoacrylate cement, more commonly known as superglue, with heat (melting), or by using solvents such as di- 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 (including its monomers) upon laser cutting, so a very clean cut is made, and cutting is performed very easily. In this respect PMMA has an advantage over competing polymers such as polystyrene and polycarbonate, which require higher laser powers and give more messy and charred laser cuts.

In the majority of applications, it will not shatter. Rather, it breaks into large dull pieces. 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).

Properties

Skeletal structure of methyl methacrylate, the monomer that makes up PMMA

PMMA a is strong and lightweight material. It has a density of 1.17–1.20 g/cm³,^[1][14] which is less than half that of glass.^[1] It also has good impact strength, higher than both glass and polystyrene; however, PMMA's impact strength is still 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.^[15]

PMMA transmits up to 92% of visible light (3 mm thickness), and gives a reflection of about 4% from each of its surfaces on account of its refractive index (1.4914 at 587.6 nm).^[3] It filters ultraviolet (UV) light at wavelengths below about 300 nm (similar to ordinary window glass). Some manufacturers^[16] add coatings or additives to PMMA to improve absorption in the 300–400 nm range. PMMA passes infrared light of up to 2800 nm and blocks IR of longer wavelengths up to 25  µm. Colored PMMA varieities 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 on account of its easily hydrolyzed ester groups. Nevertheless, its environmental stability is superior to most other plastics such as polystyrene and polyethylene, and PMMA is therefore often the material of choice for outdoor applications.^[17]

PMMA as maximum water absorption ratio of 0.3–0.4% by weight.^[14] Tensile strength decreases with increased water absorption^[18]. Its coefficient of thermal expansion is relatively high as (5–10)×10⁻⁵ /K.^[19]

Modification of properties

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, or improve impact properties.
• Dyes may be added to give color for decorative applications, or to protect against (or filter) UV light.
• Fillers may be added to improve cost-effectivness.

Poly(methyl acrylate)

The polymer of methyl acrylate, PMA or poly(methyl acrylate), is similar to poly(methyl methacrylate), except for the lack of methyl groups on the backbone carbon chain.^[20] PMA is a soft white rubbery material that is softer than PMMA because its long polymer chains are thinner and smoother and can more easily slide past each other.

Uses

PMMA is a versatile material and has been used in a wide range of fields and applications.

Transparent glass substitute

• PMMA Acrylic glass is commonly used for constructing residential and commercial aquariums. Designers started building big aquariums when poly(methyl methacrylate) could be used. The spectacular size of both flat panels and tunnels in aquariums such as Monterey Bay, Tokyo Sea Life Park, Osaka, Nagoya, Georgia and Dubai Aquariums were made possible with the introduction of acrylic.
• PMMA is used in the lenses of exterior lights of automobiles.^[21]
• The spectator protection in ice hockey rinks is made from PMMA.
• Motorcycle helmet visors
• Police vehicles for riot control often have the regular glass replaced with acrylic to protect the occupants from thrown objects.
• In some Motor racing championships the glass windows in the cars are replaced with acrylic to prevent glass shattering on the driver and track during a crash. They also help to save some weight making the car lighter and faster.
• Acrylic is used for viewing ports and even complete hulls of submersibles, such as the Alicia submarine's viewing spheres and the Bathyscaphe Trieste's windows.
• Polycast acrylic sheet is the most widely used material in aircraft transparencies (windows). In applications where the aircraft is pressurized, stretched acrylic is used.
• Acrylic is an important material in the making of certain lighthouse lenses.^[22]

Daylight redirection

• Laser cut acrylic panels have been used to redirect sunlight into a light pipe and, from there, to spread it into a room.^[23] 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.^[24][25]
• Attenuation being quite strong for distances over one meter (more than 90% intensity loss for a 3000 K source^[26]), 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.^[27][28]

Medical technologies and implants

• PMMA has a good degree of compatibility with human tissue, and can be used for replacement intraocular lenses in the eye when the original lens has been removed in the treatment of cataracts. This compatibility was discovered 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. 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. It is supplied as a powder with liquid methyl methacrylate (MMA). When mixed these yield a dough-like cement that gradually hardens. Surgeons can judge the curing of the PMMA bone cement by pressing their thumb on it. 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.^[29] 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, it primarily fills the spaces between the prosthesis and the bone preventing motion. A big disadvantage to this bone cement is that it heats to quite a high temperature while setting and because of this it kills the bone in the surrounding area. It has a Young's modulus between cancellous bone and cortical bone. Thus it is a load sharing entity in the body not causing bone resorption.^[30]
• Dentures are often made of PMMA, and can be color-matched to the patient's teeth & gum tissue.
• In cosmetic surgery, tiny PMMA microspheres suspended in some biological fluid are injected under the skin to reduce wrinkles or scars permanently.
• A large majority of white Dental filling materials (i.e. composites) have PMMA as their main organic component.
• Emerging biotechnology and Biomedical research uses PMMA to create microfluidic lab-on-a-chip devices, which require 100 micron-wide geometries for routing liquids. These small geometries are amenable to using PMMA in a biochip fabrication process and offers moderate biocompatibility.
• Bioprocess chromatography 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.

Artistic and aesthetic uses

• 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 framing. 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.
• From approximately the 1960s onward, sculptors and glass artists 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 jewellery, with several companies specialized in creating high-quality pieces from this material. Lucite beads and ornaments are still sold by jewellery suppliers.

Other uses

High heel shoes made of Lucite

An electric bass guitar with its body made out of perspex

• 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 channel letters which are neon or LED illuminated. Acrylic's attractiveness, durability and resistance to warping make it an ideal interior and exterior sign material. 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. One of the most popular sheets is a non-glare, translucent which is sold in 1.6 millimeters (0.06 in) or 3 millimeters (0.12 in) in thicknesses.
• PMMA was used in laserdisc optical media. (CDs and DVDs use polycarbonate for higher impact resistance.)
• It is used as a light guide for the backlights in TFT-LCDs.
• 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 over glass fiber.
• PMMA, in a purified form, is used as the matrix in dye-doped solid-state gain media for solid state dye lasers.^[31]
• 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 coat silicon 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 (nanoscale) patterns to be made. It is an invaluable tool in nanotechnology.
• 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 density of PMMA changes as the Gamma dose increases and can be measured with a spectrophotometer.
• Recently a blacklight-reactive tattoo ink using PMMA microcapsules was developed. This ink is reportedly safe for use, and claims to be Food and Drug Administration (FDA) approved for use on wildlife that may enter the food supply.
• PMMA can be used as a dispersant for ceramic powders to stabilize colloidal suspensions in non-aqueous mediums.
• PMMA has also been used extensively as a hybrid rocket fuel.
• In the 1960s, luthier Dan 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^[32] and B.C. Rich have also made acrylic guitars.
• Ludwig-Musser makes a line of acrylic drums called Vistalites, well known as being used by Led Zeppelin drummer John Bonham.
• Artificial fingernails are made of acrylic.
• The majority of stems for modern tobacco pipes are made of Lucite.

See also

• Polystyrene
• Polycarbonate

References

Bibliography

• Ashby, Michael F. (2005). Materials Selection in Mechanical Design (3rd ed.). Elsevier. ISBN 0-7506-6168-2. 
• Smith, William F.; Hashemi, Javad (2006). Foundations of Materials Science and Engineering (4th ed.). McGraw-Hill. ISBN 0-07-295358-6. 

External links

• Cutting Plexiglass
• efunda.com - Mechanical properties of PMMA
• 'Vessels' - a sculptural book made from perspex – created by Adele Outteridge and held by the Australian Library of Art, State Library of Queensland