Chemistry Reference
In-Depth Information
fairly constant during the reaction and is determined mainly by the continuous
water phase.
Note that suspension polymerization is only superficially related to emulsion
polymerization, which was outlined in Chapter 10. In suspension processes the
coagulation of the dispersion is controlled by agitation plus the action of a water-
soluble polymer and/or a fine particle size inorganic powder. The role of water is
to act primarily as a heat transfer medium. In vinyl chloride suspension polymeri-
zation the specific heat of the monomer and polymer are about equal and are one-
quarter that of water, on an equal weight basis. Thus, at the typical 1.5
/
1 water/
vinyl chloride mass ratio the heat capacity of the aqueous phase is about six times
that of the organic phase. Another use of water is, of course, to keep the viscosity
of the reaction medium at a useful level. Water/monomer ratios of 1.5
/
1 to 1.75
/
1
provide a good compromise between suspension concentration and viscosity.
Since emulsion polymerization is initiated in the aqueous phase, the undesir-
able formation of latex polymer can be minimized in suspension systems by using
water-soluble inhibitors, like sodium nitrite.
Prevention of the coalescence of the sticky, partially polymerized particles is a
major problem in suspension polymerizations, and proper selection of stabilizing
agents is important. Two kinds of additives are used to hinder coalescence of par-
ticles in suspension polymerizations. These are platelet-like mineral particles that
concentrate at the organic-water interface, like Ca
3
(PO
4
)
2
, and/or macromolecular
species that are soluble in water and insoluble in the particular organic phase.
Poly(vinyl alcohol) and starch products are examples of the latter type.
The normal sequence of operations is as follows:
1.
Premix the initiator and monomer(s).
2.
Add mix from step 1 to water, up to about 35% (v/v) with agitation to
produce the desired droplet sizes.
3.
Add inorganic or organic stabilizers, which should concentrate at the droplet/
H
2
O interfaces.
4.
Reduce stirrer speed so as to minimize coalescence and prevent separation of
the droplets and water, because of their density differences.
5.
Raise reaction temperature and hold at the desired polymerization level until
the softening temperature of the particles reaches or exceeds the reactor
temperature (this occurs with depletion of the monomer, which normally
plasticizes the polymer).
6.
Increase the temperature to complete monomer conversion.
7.
Dump the reactor contents, cool the reaction mixture, and separate the
polymer.
Formation of a scale of polymer on the reactor walls is normally less than in
corresponding bulk or solution polymerizations. Scale formation is troublesome in
PVC suspension polymerizations, however, because the polymer is not soluble in
its monomer, and a deposit formed on the wall will not be washed off by fresh
monomer. This build-up has to be removed in order to maintain satisfactory heat
