Page 456 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Inorganic Polymers 419

• With good vision in only one eye, and
• With refractive surgery

Plastic lenses are best when light weight and clear vision are important. They offer the widest
range of bifocal designs.

Glass lenses are best for people

• Who work in a dusty environment
• Leave glasses in the car
• Wear sunglasses on the beach
• Want the best photochromic performance, and
• Who often take off and on their glasses

Reading glasses must pass a number of standard tests outlined in LCO993. Several of
these are described following. The impact resistance test is described by the Federal Regulation
on Impact Resistant Lenses (21 CFR 80.410) where a 5/8-in. diameter steel ball weighing
0.56 oz is dropped from a height of 50 in. onto the horizontal upper, convex surface of the
lens. To pass, the lens must not fracture. The flammability test is passed when no evidence
o
of ignition is found when the glasses assembly is placed in a preheated oven set to 200 C for
15 minutes.
The most common material for decent safety glasses is polycarbonate. Those glasses referred
to as high impact must pass the ANSI test for high velocity impact. The requirement are that the
lenses withstand an impact from a 6.35 mm (1/4 in.) diameter steel ball traveling at a velocity of
45.7 mps (150 fps). From a sample size of 20, none may shatter or the lenses fail the test.

Glass lenses can be made finished from a mold or the general shape made in a mold and latter
worked providing the fi nished lens.
In the late 1960s, PPG introduced a new lens material that today is simply known as “plas-
tic lenses.” In comparison to crown glass, this material was lighter, produced thinner lenses,

offered greater impact resistance, and was more flexible. While the new material is referred to
by the sales people at eye glasses outlets as simply “plastic,” its name is CR-39. The “CR” stands
for Columbia Resin and the 39 was the batch or formula made by the Columbia Laboratories
in Ohio. CR-39 is made from allyl diglycol carbonate (or diethylene glycol bis(allyl carbonate))
monomer. On heating, the two vinyl groups open up forming a cross-linked thermoset plas-
tic that cannot be resoften on heating. By comparison, polycarbonate is a linear plastic that
is a thermoplastic that can be resoften, and recycled, on addition of heat and pressure. The
lenses are formed by melting polycarbonate pellets and injecting them into a mold. Lenses from
CR-39 are made by casting the monomer into appropriate molds followed by polymerization

creating the lens either as a finished product or further working the solid lens material creating
the fi nished lens.
Initially, there was a problem casting lenses from CR-39 because there is typically shrinkage,
with the polymer denser than the liquid monomer, when vinyl monomers are polymerized. In this
case, there was 14% shrinkage. This was not a problem when casting flat lenses since the resulting

lens dimensions were simply a little less. Further, the shrinkage was different creating optical dis-
tortions. Robert Graham, working with others for the Armorlite Company, overcame this problem

by casting thick blanks where the back curve matched the finished front curve. The lens are then
ground and polished to the required thickness and curve. CR-39 Monomer (top; 12.15) polymerized
forming CR-39 Polymer (bottom; 12.16).

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