Chemistry Reference
In-Depth Information
rate controlling step, because the basic chemical reactions must be very fast to be
useful in interfacial reactions.
Properties of fibers can be altered by carrying out interfacial polymerizations
on their surfaces. Thus the shrink resistance of wool can be improved by immers-
ing the fiber first in a solution containing one component of a condensation poly-
mer and then immersing it in another solution containing the other component.
Polyamides, polyurethanes, polyureas, and other polymers and copolymers may
be grafted on wool in this manner.
12.5
Batch, Semibatch, and Continuous Processes
Polymerization reactions can be further classified into batch and continuous pro-
cesses. Continuous operation is feasible for the production of large quantities of
polymers with uniform properties. Frequent product changes are not economical,
because off-grade polymer is made during start-ups until the reaction conditions
are stabilized and because there may be long lags between changes in operating
variables and subsequent steady production of uniform product. Continuous pro-
cessing will sometimes permit production of polymers with narrower property
ranges than can be obtained in batch processes.
Semibatch operation involves the continuous or intermittent addition of mono-
mer or other ingredients during polymerization. It is often employed in copoly-
merizations (Section 9.5) to minimize the drift of copolymer composition when
the reactivities differ greatly.
Periodic addition of monomer to an operating batch polymerization assists in
controlling the reaction temperature because of its cooling effect.
In emulsion polymerizations semibatch operation provides better control of
the particle size of the product. The properties of the product polymers can be
modified also by continuous or intermittent changes in the composition of the
monomer feed in emulsion copolymerizations, where a given monomer can be
preferentially concentrated in the interior or on the surface of the final particles,
as described in Chapter 10.
12.6
Polymerization Reactors
Reactors are conveniently considered in three idealized categories: batch, tubular,
and continuous stirred tank reactors (CSTR). The operations of real reactors may
be modeled on the basis of one of these types or combination thereof.
The detailed course of a polymerization is determined by the nature of the
particular reaction as well as by the characteristics of the reactor which is used.
The design and control of the operation are greatly aided by mathematical model-
ing of the process. Such models may be based on empirical relations between
the independent and dependent operating variables. This is not as satisfactory,
