Poly(methyl methacrylate)

Structure of PMMA: (C₅O₂H₈)_n

File:Methyl methacrylate.png

Structure of methyl methacrylate

Polymethyl methacrylate (PMMA) or polymethyl-2-methylpropanoate is the synthetic polymer of methyl methacrylate. This thermoplastic and transparent plastic is sold by the names Plexiglas, Perspex, Acrylite, Acrylplast and Lucite and is commonly called Acrylic glass or simply Acrylic. The material was developed in 1928 in various laboratories and was brought to market in 1933.

Properties

The material is often used in place of glass. Differences of the two materials include:

• PMMA is lighter: its density is about half that of glass
• PMMA does not shatter
• The softness of PMMA leads to it being easily scratched compared with glass.
• PMMA can be easily formed, by heating it to 100 degrees Celsius.
• PMMA transmits more light than glass
• Unlike glass, PMMA does not filter UV (ultraviolet) light. Some manufacturers coat their PMMA with UV films to add this property.
• Up to wavelengths of 2800 nm, PMMA allows most IR (infrared) light to pass. Longer wavelengths of IR energy, 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 can be joined using cyanoacrylate cement (so-called "Superglue"), or by using liquid dichloromethane to dissolve the plastic at the join which then fuses and sets, forming an almost invisible weld. PMMA can also be easily polished, by which method cut edges (which turn opaque) can be returned to transparency.

To produce 1 kg of PMMA, about 2 kg of petroleum is needed. In the presence of air, PMMA ignites at 460 degrees Celsius; it burns completely to carbon dioxide and water.

If in the structure of PMMA the methyl groups (CH₃) attached to the C atoms are replaced by single hydrogen atoms, we obtain polymethyl acrylate, a white soft rubbery material. It is softer than PMMA because its long polymer chains are thinner and smoother and can more easily slide past each other.

Uses

PMMA is used for instance in the rear lights of cars. The spectator protection in ice hockey stadiums is made of PMMA, as are the very largest windows and fish tanks in the world. The material is used to produce laserdiscs, and sometimes also for DVDs, but the more expensive polycarbonate (that's also used for CDs) has better properties when exposed to moisture.

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.

PMMA has a good degree of compatibilty 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, in which acrylate monomers are used that contain one or more hydroxyl groups to make them hydrophilic.

In orthopedics, PMMA bone cement is used to affix implants and to remodel lost bone. It is supplied as a powder and a liquid; when mixed, they yield a dough which then hardens. Dentures are often made of PMMA. In cosmetic surgery, tiny PMMA microspheres suspended in some biological fluid are injected under the skin to permanently reduce wrinkles or scars.

See also

• Other transparent plastics: polystyrene, polycarbonate

External link

• Optical & Transmission Characteristics of Plexiglas (manufacturer's data)