Page 454 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Inorganic Polymers 417

glass in automobile accidents. He remembered the flask that did not splinter into small pieces when

broken because of the cellulose nitrate coating. He experimented with placing cellulose nitrate
between sheets of glass and applying pressure to help adhere the glass with the cellulose nitrate. By
1909, Benedictus had patented the material.
Before its use in windshields, safety glass found its initial major application as the lenses for gas
masks during World War I. Manufacturers found it easy to work with so that the technology and
ability to manufacture safety glass windshields came easily.
As automobiles became more common, so did the hazards of mud, rocks, and so on, so that by

1904 windshields were introduced. These first windshields could be folded or moved if they became
blocked by excessive mud. While the usefulness of the windshield was abundantly obvious, drivers
found that they were dangerous during a wreck cutting passengers, drivers, and passer-byes alike.
Because the drivers were believed to be primarily responsible for automobile safety, most manu-
factures were slow to adopt safety glass. In 1919, Henry Ford addressed the windshield problem by
having safety glass windshields on his automobiles.
This safety glass turned yellow after several years of exposure to light. The bonding layer was
replaced in 1933 by cellulose acetate, made from the reaction of cotton with acetic acid. By 1939,
this was replaced by poly(vinyl butyral) (PVB) still in use today as the adhesive placed between
sheets of glass to produce laminated safety glass. This is one of a very few modern-use materials
that has retained the same basic materials for more than 60 years.

R R
R R

OH OH O O
+

O CH 3 (12.14)

CH 3
Poly(vinyl alcohol)/butyraldehyde Poly(vinyl butyral)

Poly(vinyl butyral) is made from poly(vinyl alcohol) which itself is made from poly(vinyl acetate)
because the monomer vinyl alcohol does not exist (Equation 12.14).
Today, safety glass is divided into three general categories, laminated safety glass, tempered
safety glass, and armed glass. Tempered safety glass is made by heating the glass to its melting
o
point, about 700 C, and then cooling it rapidly by blowing cold air onto its surfaces. The effect is
similar to the production of stressed concrete where the concrete is allowed to harden under stress
giving a stronger concrete. In the case of glass, when it is rapidly cooled, a structure is locked in
that produces extra stress on the glass structure making it stronger. As the glass is cooled, the

surfaces harden first locking in the overall glass volume. As the center cools, it forces the surfaces
and edges into compression. With appropriate rapid cooling, the glass is not only stronger, but
when shattered, produces granulates rather than sharp cutting shards. The typical force necessary
to break tempered glass is about four times that required to shatter ordinary glass of the same
thickness.
While the front “windshield” is made of safety glass, the remainder of the automotive glass win-
dows are generally made from tempered glass. Tempered glass is also used for commercial build-
ing doors and windows, sidelights, patio-door assemblies, storm doors, shower and tub enclosures,
refrigerator, oven, and stove shelves, and fi replace screens.

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