CR-39

Structure

CR-39, or allyl diglycol carbonate (ADC), is a plastic polymer commonly used in the manufacture of eyeglass lenses. The abbreviation stands for “Columbia Resin #39,” because it was the 39th formula of a thermosetting plastic developed by the Columbia Resins project in 1940.^[1]

The first commercial use of CR-39 monomer was to help create glass-reinforced plastic fuel tanks for the B-17 bomber aircraft in World War II, reducing weight and increasing range of the bomber. After the War, the Armorlite Lens Company in California is credited with manufacturing the first CR-39 eyeglass lenses in 1947. CR-39 plastic has an index of refraction of 1.498 and an Abbe number of 58. CR-39 is now a trade marked product of PPG Industries.^[2]

An alternative use includes a purified version that is used to measure neutron radiation, a type of ionizing radiation, in neutron dosimetry.

CR-39 should not be confused with polycarbonate, a tough homopolymer usually made from bis-phenol A.^[3]

Synthesis

CR-39 is made by polymerization of diethyleneglycol bis allylcarbonate (ADC) in presence of diisopropyl peroxydicarbonate (IPP) initiator. The presence of the allyl groups allows the polymer to form cross-links; thus, it is a thermoset resin. The monomer structure is

Monomer

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 (BPO) 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, raw CR-39 material is exposed to proton recoils caused by incident neutrons. The proton recoils cause ion tracks, which are enlarged by an etching process in a caustic solution of sodium hydroxide. The enlarged ion tracks are counted under a microscope (commonly 200x), and the number of ion tracks is proportional to the amount of incident neutron radiation.^[4]

It is used in some photographic filters, such as the Cokin filter system.

A direct equivalent is produced by Acomon AG with the brand name RAV, and another one by Danyang Yueda FineChemichal Co. Ltd in China.^{[citation needed]}

See also

• Glass
• Polymethyl methacrylate
• Polycarbonate

References

1. ^ http://corporateportal.ppg.com/NA/chemicals/Optical/CR39/cr39History.htm
2. ^ http://corporateportal.ppg.com/NA/chemicals/Optical/CR39/
3. ^ http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA091656&Location=U2&doc=GetTRDoc.pdf
4. ^ Mosier-Boss, P.A.; Szpak1, S.; Gordon, F.E.; Forsley, L.P.G. (June 2009). "Characterization of tracks in CR-39 detectors obtained as a result of Pd/D Co-deposition". The European Physical Journal Applied Physics 46 (3). Bibcode 2009EPJAP..46c0901M. doi:10.1051/epjap/2009067. 

External links

• Characteristics of CR-39