GTBL042-14 GTBL042-Callister-v2 August 29, 2007 8:59
14.11 Polymerization • 603components are removed by evaporation; this green product is a flexible tape that
may be cut or into which holes may be punched prior to a firing operation. Tape
thicknesses normally range between 0.1 and 2 mm (0.004 to 0.08 in.). Tape casting
is widely used in the production of ceramic substrates that are used for integrated
circuits and for multilayered capacitors.Cementation is also considered to be a ceramic fabrication process (Figure 14.15).
The cement material, when mixed with water, forms a paste that, after being fash-
ioned into a desired shape, subsequently hardens as a result of complex chemical
reactions. Cements and the cementation process were discussed briefly in Section
13.9.Synthesis and Fabrication
of Polymers
The large macromolecules of the commercially useful polymers must be synthe-
sized from smaller molecules in a process termed polymerization. Furthermore, the
properties of a polymer may be modified and enhanced by the inclusion of additive
materials. Finally, a finished piece having a desired shape must be fashioned during
a forming operation. This section treats polymerization processes and the various
forms of additives, as well as specific forming procedures.14.11 POLYMERIZATION
The synthesis of these large molecules (polymers) is termedpolymerization;it is
simply the process by which monomers are linked together to generate long chains
composed of repeat units. Most generally, the raw materials for synthetic polymers
are derived from coal, natural gas, and petroleum products. The reactions by which
polymerization occur are grouped into two general classifications—addition and
condensation—according to the reaction mechanism, as discussed below.Addition Polymerization
addition Addition polymerization(sometimes calledchain reaction polymerization) is a pro-
polymerization cess by which monomer units are attached one at a time in chainlike fashion to form
a linear macromolecule. The composition of the resultant product molecule is an
exact multiple for that of the original reactant monomer.
Three distinct stages—initiation, propagation, and termination—are involved
in addition polymerization. During the initiation step, an active center capable of
propagation is formed by a reaction between an initiator (or catalyst) species and
the monomer unit. This process has already been demonstrated for polyethylene
(Equation 4.1), which is repeated as follows:(14.1)
R·C
H
H
C
H
H
R C
H
H
C·
H
H
Again, R·represents the active initiator, and·is an unpaired electron.