Page 457 - Carrahers_Polymer_Chemistry,_Eighth_Edition

420 Carraher’s Polymer Chemistry

O O O O
H 2 C O

O O

(12.15)
CH 2

R
R O O O O
O
(12.16)
O O

R

R

There still remained the problem of scratching. The CR-39 lenses are less scratch resistance than
polycarbonate lenses. In the 1970s, the 3M (Minnesota Mining and Manufacturing Company) com-
pany had a research group that was expert in coatings, including lenses. One problem was the need,
for some materials, to have them particularly free from dust to achieve mar-free coated surfaces.
By the mid-1970s, they created a production facility that reduced airborne particles suffi ciently to
allow materials to be successfully coated. In 1979, 3M purchased the Armorlite Company and trans-
ferred their scratch-resistant coating technology to be used for the CR-39 lenses. American Optical
introduced photolite photochromic lenses that change color with exposure to light in 1981. PPG, in
1983, discovered a new family of photochromics, the blue pyridobenzoxanines and by 1984 formed
a joint venture with Intercast-Europe to manufacture and sell photochromic sunglasses called Attiva
sunglasses made from CR-39. Many other companies have contributed to the presence of today’s
plastic lenses. The story illustrates how companies apply their interests and strengths to achieve the
products we have today.
Today, there are new so-called high-index lenses made from a variety of polymers that are thin-
ner than CR-39 and polycarbonate lenses.

12.9 SOL-GEL
In the sol-gel process, ceramic polymer precursors are formed in solution at ambient temperature;

shaped by casting, film formation or fiber drawing; and then consolidated to furnish dense glasses

or polycrystalline ceramics. The most common sol-gel procedures involve alkoxides of silicon,
boron, titanium, and aluminum. In alcohol-water solution, the alkoxide groups are removed step-
wise by hydrolysis under acidic or basic catalysis and replaced by hydroxyl groups, that then form
–M–O–M– linkages. Branched polymer chains grow and interconnect forming a network that can
span the entire solution volume. At this point, the gel point, the viscosity, and elastic modulus rap-
idly increase.
The gel is a viscoelastic material composed of interpenetrating liquid and solid phases. The net-
work retards the escape of the liquid and prevents structural collapse. The shapes formed by cast-

ing, drawing of fibers, or film formation are locked in by the gel formation. Some gels are oriented

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