Page 451 - Carrahers_Polymer_Chemistry,_Eighth_Edition

414 Carraher’s Polymer Chemistry

The exact structure varies according to the ingredients and actual processing conditions. As in
the case of Portland cement, glass is a three-dimensional array that offers short-range order and
long-range disorder—it is amorphous offering little or no areas of crystallinity. The structure is
based on the silicon atoms existing in a tetrahedral geometry with each silicon atom attached to four
oxygen atoms, generating a three-dimensional array of inexact tetrahedra. Thus, structural defects
occur, due in part to the presence of impurities such as Al, B, and Ca, intentionally or unintention-
ally introduced. These impurities encourage the glass to cool to an amorphous structure since the
different-sized impurity metal ions, and so on, disrupt the rigorous space requirement necessary to
allow crystal formation.
Processing includes shaping and pretreatments of the glass. Since shaping may create undue sites
of amorphous structure, most glass objects are again heated to near their melting point. This pro-
cess is called annealing. Since many materials tend to form more ordered structures when heated
and recooled slowly, the effect of annealing is to “heal” sites of major dissymmetry. It is important
to heal these sites since they represent preferential locations for chemical and physical attack such
as fracture.
Four main methods are employed for shaping glass. They are drawing, pressing, casting, and

blowing. Drawing is employed for shaping flat glass, glass tubing, and for creating fi brous glass.
Most fl at glass is shaped by drawing a sheet of molten glass (heated so it can be shaped but not so

it freely flows) onto a tank of molten tin. Since the glass literally floats on the tin, it is called “fl oat
glass.” The temperature is carefully controlled. The glass from the float bath typically has both sides

quite smooth with a brilliant finish that requires no polishing.
Glass tubing is made through drawing molten glass around a rotating cylinder of the appropriate
shape and size. Air is blown through the cylinder creating the hollow tubing. Fibrous glass is made
by forcing molten glass through tiny holes and drawing the resulting fibers helping to align the chain

on a molecular level.
Pressing is accomplished by simply dropping molten glass into a form and then applying pres-
sure to ensure the glass takes the form of the mold. Lenses, glass blocks, baking dishes, and ashtrays
are examples of press-processed glass objects.
Casting involves filling molds with molten glass in much the same manner that cement and plas-

ter of Paris molded objects are processes. Art glass objects are often made by casting.
Glass blowing is one of the oldest arts known to man. The objects are constructed or repaired by
a skilled worker who blows into a pipe intruded into the molten glass. The glass temperature must
be maintained to allow it to be moldable but not so it freely flows. Mass produced materials are

manufactured employing mechanical blowers often employing a combination of glass blowing and
molding to form the desired product.
As noted above, annealing encourages the removal of sites of stress and strain. Slow cooling
results in a glass with more crystallinity that is stronger but more brittle. Tempering is the name
given when the glass is rapidly cooled, resulting in an amorphous glass that is weaker but less brit-
tle. The correlation between crystallinity, rate of cooling, and brittleness is demonstrated by noting
that older window glass exposed to full sun for years, which is more brittle and can be more easily
shattered since the sunlight raises the temperature sufficiently to permit small molecular move-

ments (though even in full sunlight the glass is not near the temperature required for ready move-
ment) and over the years gives a glass with small regions of greater order.
Silicon-based glasses account for almost all of the glasses manufactured. Silica is fi nely ground
silica sand. Yet most sand is unsuitable for general glassmaking due to the presence of excessive
impurities. Thus, while sand is plentiful, sand that is useful for the production of glass is much less
common. In fact, the scarcity of large deposits of glass sand is one major reason for the need to
recycle glass items. The second major reason is associated with the lowered energy requirements
for glass to be made molten again for reshaping compared with a virgin glass mixture, that is, culled
glass becomes molten at temperatures lower than virgin glass.

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