Page 478 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Inorganic Polymers 441

is available at a low price in large amounts. While the stable form of sulfur at room temperature is
cycloocta-sulfur, S , linear polysulfur is formed on heating. Unfortunately, the thermodynamically
8
stable form of sulfur is the cycloocta-sulfur monomer and the polymer undergoes depolymerization
after awhile (Equation 12.31).

S
S
S
S R R
S (12.31)
S
S
S
S
Methods have been studied to inhibit this reversal process. Some have involved the addition of
olefins such as limonene, myrcene, and cyclopentadiene to the ends to inhibit the depolymerization.

Such stabilized polysulfur has been incorporated into concrete and asphalt mixes to strengthen
them. Concrete blocks, posts, and parking tire restrainers containing polysulfur are now being
produced.

12.23 CERAMICS

The term “ceramics” comes from the Greek word keramos, which means “potter’s clay” or
“burnt stuff.” While traditional ceramics were often based on natural clays, today’s ceramics are
largely synthetic materials. Depending on which ceramic and which definition is to be applied,

ceramics have been described as inorganic ionic materials and as inorganic covalent (poly-
meric) materials. In truth, many ceramics contain both covalent and ionic bonds and thus can be
considered “to be or not to be” (shades of Shakespeare) polymeric materials. Many of the new
ceramics, such as the boron nitriles and the silicon carbides, are polymeric without containing
any ionic bonds.
Ceramics are typically brittle, strong; resistant to chemicals such as acids, bases, salts, and reduc-
ing agents; and they are high melting. They are largely composed of carbon, oxygen, and nitrogen
and made from silicates such as clay, feldspar, bauxite, silica. But now ceramics contain other mate-
rials such as borides, carbides, silicides, nitrides.
Ceramics are generally made by two processes—sintering and fusing. In sintering, the starting
material is reduced to a powder or granular form by a series of crushing, powdering, ball-milling,
and so on. The ground preceramic material is then sized, separated according to particle size, using
different-sized screens.
Ceramic material is generally shaped by pressing it into a form or through extruding, molding,
jiggering, or slip casting. Slip casting uses a suspension of the preceramic material in water. The

mixture must be dilute enough to allow it to be poured. Deflocculates are often added to assist in
maintaining the suspension. The “slip” is poured into a plaster of Paris mold that absorbs water,

leaving the finished shape. The preceramic material hardens next to the mold and surplus “slip”
material poured off leaving a hollow item. At this point, the molded material is referred to as a
“green body,” which has little strength. Coffee pots and vases are formed using this technique.
In jiggering, machines press the preceramic material into a rotating mold of desired shape.
Dinnerware products are often made using jiggering.
Abrasives and insulators are formed from simply pressing the preceramic material into a mold
of desired shape. In extrusion, the preceramic material is forced through an opening in a “shaping”
tool. Bricks and drainpipes are formed using extrusion.
After the product has dried, it is heated or fired in a furnace or kiln. Modern ceramics gener-

ally require certain heating schedules that include the rate and duration of heating and under what

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