CR-39

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CR-39

CR-39, or allyl diglycol carbonate, is a plastic polymer commonly used in the manufacture of eyeglass lenses. It should not be confused with polycarbonate, a homopolymer made from bis-phenol A, a tough polymer widely used for glazing. 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; thus, 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 and sunglasses lenses. 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.