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Plastic lenses made from CR-39 allyl diglycol carbonate monomer combine the
optics of glass with the excellent mechanical, thermal and chemical resistance
properties of a thermoset material. CR-39 monomer is used for casting plastic
lenses for prescription eyewear, sunglasses and other plastic products requiring
high abrasion resistance and high quality optical properties. Plastic lenses
from CR-39 monomer give less chromatic aberration compared to polycarbonate
lenses, as measured by the Abbe number.
CR-39 is a trade marked product of PPG Industries, originally developed by Columbia Chemical Co Inc which evolved through acquisition into the Chemical Division of PPG Industries. An alternative use includes a purified version that is used to measure neutron radiation, a type of ionizing radiation, in neutron dosimetry. The CR stands for Columbia Resin. During WWII uncured CR-39 Resin was used to coat aircraft fuel tanks. The CR-39 would "self heal" the tanks when struck by flak.
Synthesis
CR-39 is made by polymerization of diethyleneglycol bis allylcarbonate (ADC) in presence of diisopropyl peroxydicarbonate (IPP) catalyst. The presence of the allyl groups allows the polymer to form cross-links; As a result, it is a thermoset resin. The monomer structure is
CH2=CH-CH2-O-CO-O-CH2CH2-O-CH2CH2-O-CO-O-CH2-CH=CH2
Applications
CR-39 is transparent in visible spectrum and is almost completely opaque in the
ultraviolet range. It has high abrasion resistance, in fact the highest abrasion/scratch resistance of any uncoated optical plastic. CR-39 is about half the weight of glass and index of refraction only slightly lower than that of crown glass, making it an advantageous material for eyeglasses, sunglasses lenses
and camera filters. A wide range of colors can be achieved by dyeing of the surface or the bulk of the material. CR-39 is also resistant to most of solvents and other chemicals, to gamma radiation, to aging, and to material fatigue. It can withstand the small hot sparks from welding. It can be used continuously in temperatures up to 100 °C and up to one hour in 130 °C.
In the radiation detection application, raw CR-39 material is exposed to proton recoils caused by incident neutrons. The proton recoils cause tracks, which are enlarged by an etching process in a caustic solution of sodium hydroxide. The enlarged tracks are counted under a microscope (commonly 200x), and the number of tracks is proportional to the amount of incident neutron radiation.
It is used in some photographic filters, such as the
Cokin filter system.
See also
- Glass
- Polymethyl methacrylate
- Polycarbonate
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