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Poly(methyl methacrylate) (PMMA) or poly(methyl 2-methylpropenoate) is the synthetic polymer of methyl methacrylate. This thermoplastic and transparent plastic is sold by the tradenames Plexiglas, Limacryl, R-Cast, Perspex, Plazcryl, Acrylex, Acrylite, Acrylplast, Altuglas, Polycast and Lucite and is commonly called acrylic glass or simply acrylic. Acrylic, or acrylic fiber, can also refer to polymers or copolymers containing polyacrylonitrile. The material was developed in 1928 in various laboratories and was brought to market in 1933 by Rohm and Haas Company.
PMMA is often used as an alternative to glass, and in competition with polycarbonate (PC). It is often preferred on account of its moderate properties, easy handling and processing, and low cost, but behaves in a brittle manner when loaded, especially under an impact force. To produce 1 kg of PMMA, about 2 kg of petroleum is needed. In the presence of oxygen,[clarify:what?] PMMA ignites at 460 °C and burns completely to form only carbon dioxide and water.[citation needed]
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.
Processing
Thermoplastic PMMA is typically processed at 240-250 °C. 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 moulding grades owing to its extremely high molecular mass. Rubber toughening has been used to increase the strength of PMMA owing to its brittle behaviour in response to applied loads.
PMMA can be joined using cyanoacrylate cement (so-called "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.
Laser cutting may be used to form intricate designs from PMMA sheets. PMMA vaporises 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 polyethylene and polycarbonate, which require higher laser powers and give more messy and charred laser cuts.
Properties
- PMMA has a density of 1150-1190 kg/m3. This is less than half the density of glass, and similar to that of other plastics.
- PMMA has a good impact strength higher than that of glass or polystyrene, but significantly lower than that of polycarbonate or engineering polymers. It does not shatter but instead breaks into large dull pieces.
- PMMA is softer and more easily scratched than glass. Scratch-resistant coatings (which may also have other functions) are often added to PMMA sheets.
- PMMA transmits up to 98% of visible light (per metre)[1], and gives a ~4% reflection from each of its surfaces on account of its refractive index of 1.4893 to 1.4899.
- PMMA filters ultraviolet (UV) light at wavelengths below ~300 nm. Some manufacturers[2] add coatings or additives to PMMA to improve absorption in the 300-400 nm range.
- PMMA allows infrared light of up to 2800 nm wavelength to pass. IR of longer wavelengths, up to 25,000 nm, are essentially blocked. Special formulations of colored PMMA exist to allow specific IR wavelengths to pass while blocking visible light (for remote control or heat sensor applications, for example).
- PMMA has excellent environmental stability compared to other plastics such as polycarbonate, and is therefore often the material of choice for outdoors applications.
- PMMA has poor resistance to solvents, as it swells and dissolves easily. It also has poor resistance to many other chemicals on account of its easily hydrolyzed ester groups.
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.
Related polymer 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.[3] 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 or Acrylic is a versatile material and has been used in a wide range of fields and applications.
Impact resistant substitute for glass
- PMMA Acrylic glass is commonly used for constructing residential and commercial aquariums.
- PMMA is used in the lenses of exterior lights of automobiles. [4]
- The spectator protection in ice hockey stadiums is made of PMMA.
- Motorcycle helmet visors
- Police vehicles for riot control often have the regular glass replaced with acrylic to protect the occupants from thrown objects.
- 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. It is used to form/make many things.
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. Hard contact lenses are 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 orthopaedics, 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. [5] 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. [6]
- Dentures are often made of PMMA, and can be colour-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.
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, giving otherwise ordinary objects a transparent or futuristic look.
- Perspex has been used as a surface to paint on, for example by Salvador Dalí.
- Occasionally used as a glass substitute in picture framing, due to its relatively inexpensive cost, light weight, and shatter-resistant nature, as well as the fact that it can be ordered in larger sizes than standard picture-framing glass. Conventional glass may or may not be preferred.
- 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.
Other uses
- PMMA was used in laserdisc optical media, and has been developed for use in CDs and DVDs.[7], although the more expensive polycarbonate is usually used because of its higher strength. PMMA-based optical media is also under development for the "TeraDisc" next-generation 3D optical data storage solution by Mempile.[8]
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