Page 258 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GNH/GRI P2: GTV Final pages
Encyclopedia of Physical Science and Technology EN012B-596 July 27, 2001 18:18

Polymers, Synthesis 767

TABLE IX Comparison of Homogeneous Polymerization than in one bulk batch-type process. However, one proba-
Processes bly would still have to isolate the polymer by a relatively
Type Advantages Disadvantages high vacuum devolatilization step. This could be done and
is done commercially, but nevertheless it might be con-
Bulk (batch type) Low impurity Thermal control difficult sidered a disadvantage. In addition, a series of reactors
level
requires one to design the engineering so as to be able to
Casting possible
recycle monomer back to the first reaction for additional
Bulk (continuous) Improved thermal Isolation difficult polymerization.
control Requires devolatization
Conducting a solution process with an inert solvent can
Solution Improved thermal Difficult to remove solvent
control solve several of these problems. This enables one to im-
Solvent recovery costly
prove the thermal control and decrease the viscosity, but
Chain Transfer may limit
molecular weight it is still difficult to remove quantitatively the solvent at
the end of this process. Moreover, the solvent may serve
as a chain transfer agent, which will limit the molecu-
four important processes. First, one can have a bulk reac- lar weight to lower values than may be desired. In the
tion in which basically only the monomer and possibly the suspension processes, an agent such as polyvinyl alcohol
intiator are used. Alternatively, a solution reaction process or perhaps an inorganic oxide, such as zinc oxide, sta-
may be used which has a monomer, initiator, and a sol- bilizes the growing polymer particles and prevents them
vent present. Bulk and solution reactions are sometimes from coalescing together. The generation of particles then
referred to as homogeneous processes. The other two pro- essentially produces a relatively low viscosity particle sys-
cesses are often termed heterogeneous processes, and they tem, which is desirable. If one converts all the monomer to
are known as suspension and emulsion reactions. Several polymer, then isolation becomes a fairly simple process of
advantages and disadvantages of the various polymeriza- filtering off the finished polymer particles from the aque-
tion processes are given in Tables IX and X. ous media. Many important “glassy” polymers are made
The bulk process has the advantage of allowing for via suspension processes; polystyrene and polyvinyl chlo-
a relatively low impurity level. Furthermore, it is pos- ride are made in this manner. On the negative side, it is
sible under some conditions to perform the polymeriza- necessary to agitate very carefully, especially early in the
tions discussed earlier. For example, some gasoline station reaction. Otherwise, the growing polymer particles might
signs are produced from cast polymethylmethacryalate aggregate rather than stay in discrete small particles. At
(PMMA). One can imagine, however, that it would be the end of the reaction, one has to accept the fact that
very difficult, if not impossible, to control the temperature the polymer will contain a small residual amount of the
of such a reaction, recalling the heat of polymerization dis- suspending agent.
cussed earlier. Moreover, the viscosity becomes enormous In emulsion reactions, as in the suspension case, wa-
rather quickly, and one has to find a way to dissipate the ter is the heat transfer medium and hence it is easy to
heat of polymerization. The advantage of continuous pro- obtain good thermal control. Very small particle sizes of
cesses is that in a series of reactions, for example, it may the order of a few hundred to a few thousand angstroms
be possible to achieve greater control of the temperature can be obtained. In emulsion polymerization, surfactants,

TABLE X Comparison of Heterogeneous Polymerization Processes
Type Advantages Disadvantages

Suspension Low viscosity Highly sensitive to agitation rate
Simple polymer isolation Particle size difficult to control
Easy thermal control Possible contamination by suspending agent
May be of direct usable particle size Washing, drying, and compaction necessary
Emulsion Low viscosity Emulsifier, surfactants, and coagulants must be removed
Good thermal control High residual impurity level may degrade certain polymer properties
Latex may be directly usable High cost
100% conversion may be achieved Washing, drying, and compacting may be necessary
High MW possible at high rates
Small particle size product can be obtained
Operable with soft or tacky polymers