Methyl methacrylate "
synthetic resin" for casting (simply the bulk liquid chemical) may be used in conjunction with a polymerization catalyst such as
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.
Properties
Skeletal structure of methyl methacrylate, the constituent
monomer of PMMA
PMMA is a strong, tough, and lightweight material. It has a
density of 1.17–1.20 g/cm
3,
[1][18] 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.
[19]
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
[20] 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
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.
[21]
PMMA has a maximum water absorption ratio of 0.3–0.4% by weight.
[18] Tensile strength decreases with increased water absorption.
[22] Its
coefficient of thermal expansion is relatively high at (5–10)×10
−5 K
−1.
[23]
. PMID 17077862.
