Acrylic


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Acrylic - short version

Common name for poly methyl methacrylate (PMMA)



Acrylic - long version

Poly (methyl methacrylate) (PMMA) poly (methyl 2-methylpropenoate) is a transparent thermoplastic. Chemically, it is the synthetic polymer of methyl methacrylate. It is sold under many trade names, including Policril, Plexiglas, Gavrieli, Vitroflex, Limacryl, R-Cast, Per-Clax, Perspex, Plazcryl, Acrylex, Acrylite, Acrylplast, Altuglas, Polycast, Oroglass, Optix and Lucite and is commonly called acrylic glass, simply acrylic, perspex or plexiglas. 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 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. To produce 1 kg of PMMA, about 2 kg of petroleum is needed. PMMA ignites at 460 °C and burns, forming carbon dioxide, water, carbon monoxide and low molecular weight compounds, including formaldehyde.

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 methyl alcohol 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.

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 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.

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 or by heating the surface of the material.

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 polystyrene and polycarbonate, which require higher laser powers and give more messy and charred laser cuts.

Properties

Skeletal structure of methyl methacrylate, the monomer that makes up PMMAPMMA:

has a density of 1,150–1,190 kg/m3. This is less than half the density of glass, and similar to that of other plastics.
has a good impact strength higher than that of glass or polystyrene, but significantly lower than that of polycarbonate or engineering polymers. In the majority of applications, it will not shatter but instead breaks into large dull pieces.
is softer and more easily scratched than glass. Scratch-resistant coatings (which may also have other functions) are often added to PMMA sheets.
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 of 1.4893 to 1.4899.
filters ultraviolet (UV) light at wavelengths below about 300 nm. Some manufacturers[3] add coatings or additives to PMMA to improve absorption in the 300–400 nm range.
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).
has excellent environmental stability compared to other plastics such as polycarbonate, and is therefore often the material of choice for outdoors applications.
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.
[edit] 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. 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.



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