[ATM] Plastic Lenses (POLYCARBONITE)

[Mark Holm]

Let's straighten out one thing.  So far as I know, there is no material
known as polycarbonite except as a typo, or perhaps the sort of
neologism marketing people make up when they don't want to use the real
name of what they are selling.

There is polycarbonate.  Polycarbonate refers to a family of related
polymers.

Quoting from "Z94.3.1-02 Protective Eyewear: A User's Guide" by Canadian
Standards Association, 2002

What is the difference between plastic and polycarbonate?

The "standard plastic" lenses in safety glasses are often called "Hard
Resin", "CR-39 plastic", or just "plastic" lenses. CR-39® is actually a
PPG Industries registered trade name for a DADC (diallyl diglycol
carbonate) polymer that was introduced in 1941. The "CR" stands for
Columbia Resin and CR-39 was the 39th batch or formula made by Columbia
Laboratories in Ohio. This polymer is a polycarbonate but its starting
materials are different from the resins used in safety glasses with
"polycarbonate" lenses. The "hard resin" or "CR-39 plastic" is a
thermoset plastic meaning it cannot be molded or bent when heated.

On the other hand, the polycarbonate polymers (e.g., Lexan®, a GE trade
name) are thermoplastic which means that the lenses can be formed by
melting polycarbonate pellets and injecting them into a mold.



Although the quote refers to safety glasses, I understand that the same
resins are used for ordinary eyeglasses and sunglasses.  What I have
read, is that thermoplastic polycarbonate has the highest impact
resistance, but suffers in scratch resistance.  In comparison, CR-39 is
somewhat less impact resistant, more scratch resistant, has a bit lower
refractive index, is a bit denser and costs more than thermoplastic
polycarbonate.

There are a number of other plastic lens materials.  I refer you to the
Wikipedia article http://en.wikipedia.org/wiki/Corrective_lens  (just
search Wikipedia on Corrective Lens).  One of the important points made
in this article is the importance of high Abbe number (low chromatic
dispersion) for lens performance, particularly is stronger
prescriptions.  The article covers a number of other corrective lens
issues.  Clearly the subject is more sophisticated than "polycarbonite
is junk".

-- 
Mark Holm
mdholm at telerama.com