CR-39 is made by polymerization of diethyleneglycol bis allylcarbonate in presence of diisopropyl peroxydicarbonate initiator. The presence of the allyl groups allows the polymer to form cross-links; thus, it is a thermoset resin. The monomer structure is The polymerization schedule of ADC monomers using IPP is generally 20 hours long with a maximum temperature of 95 °C. The elevated temperatures can be supplied using a water bath or a forced air oven. Benzoyl peroxide is an alternative organic peroxide that may be used to polymerize ADC. Pure benzoyl peroxide is crystalline and less volatile than diisopropyl peroxydicarbonate. Using BPO results in a polymer that has a higher yellowness index, and the peroxide takes longer to dissolve into ADC at room temperature than IPP.
Applications
CR-39 is transparent in the 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 with an index of refraction only slightly lower than that of crown glass, and its high Abbe number yields low chromatic aberration, altogether making it an advantageous material for eyeglasses and sunglasses. 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 solvents and other chemicals, gamma radiation, aging, and to material fatigue. It can withstand the small hot sparks from welding, something glass cannot do. It can be used continuously in temperatures up to 100 °C and up to one hour at 130 °C." In the radiation detection application, CR-39 is used as a solid-state nuclear track detector to detect the presence of ionising radiation. Energetic particles colliding with the polymer structure leave a trail of broken chemical bonds within the CR-39. When immersed in a concentrated alkali solution hydroxide ions attack and break the polymer structure, etching away the bulk of the plastic at a nominally fixed rate. However, along the paths of damage left by charged particle interaction the concentration of radiation damage allows the chemical agent to attack the polymer more rapidly than it does in the bulk, revealing the paths of the charged particle ion tracks. The resulting etched plastic therefore contains a permanent record of not only the location of the radiation on the plastic but also gives spectroscopic information about the source. Principally used for the detection of alpha radiation emitting radionuclides, the radiation-sensitivity properties of CR-39 are also used for proton and neutron dosimetry and historically cosmic ray investigations. The ability of CR-39 to record the location of a radiation source, even at extremely low concentrations is exploited in autoradiography studies with alpha particles, and for detection of alpha-emitters like uranium. Typically, a thin section of a biological material is fixed against CR-39 and kept frozen for a timescale of months to years in an environment that is shielded as much as possible from possible radiological contaminants. Before etching, photographs are taken of the biological sample with the affixed CR-39 detector, with care taken to ensure that prescribed location marks on the detector are noted. After the etching process, automated or manual 'scanning' of the CR-39 is used to physically locate the ionising radiation recorded, which can then be mapped to the position of the radionuclide within the biological sample. There is no other non-destructive method for accurately identifying the location of trace quantities of radionuclides in biological samples at such low emission levels.
