Chemistry Reference
In-Depth Information
the particles may become larger again, because there is insufficient suspending
agent for the many fine droplets that are produced. Increases in reaction tempera-
ture are generally accompanied by a modest decrease in interfacial tension and
lower droplet and subsequent particle sizes. It is important to control the bead
size distribution to avoid very small and very large granules. This is accomplished
through selection of the level and nature of suspending agents and other ingredi-
ents and an optimized stirring protocol.
Other major products of suspension processes include expandable polystyrene,
where a volatile hydrocarbon is diffused into the polymer beads, and spherical
divinylbenzene-based beads for chromatographic and ion-exchange applications.
PVC is different from most other suspension process polymers in that it is pro-
duced by precipitation polymerization, as described earlier.
Suspension polymerization is frequently employed as the second stage follow-
ing a preliminary bulk polymerization, such as in the manufacture of some HIPS
and ABS polymers. Polybutadiene or another elastomer is dissolved in liquid sty-
rene, and this monomer or a mixture of styrene and acrylonitrile is polymerized
in a batch kettle. The syrup produced at 30
35% conversion is too viscous for
effective mixing and heat transfer. It is therefore dispersed in water, and the poly-
merization is finished as a suspension reaction.
12.4.2.4
Emulsion Systems
Emulsion polymerizations were described in Chapter 10. These reactions yield
high-molecular-weight products at fast reaction rates when the corresponding sus-
pension, bul
k, o
r solution free-radical polymerizations are inefficient, because
both
R
p
and DP
n
can be changed in parallel by altering the soap concentration in
emulsion reactions. Thermal control and mixing are relatively easy, in common
with suspension polymerizations. Emulsion reactions are more convenient than
suspension polymerizations with soft or tacky polymers, because emulsion sys-
tems employ higher surfactant concentrations. It is correspondingly more difficult
to remove the soaps from the finished product, and this purification is rarely
attempted.
The major emulsion processes include the copolymerization of styrene and
butadiene to form SBR rubber, polymerization of chloroprene (Fig. 1.4) to pro-
duce neoprene rubbers, and the synthesis of latex paints and adhesives based
mainly on vinyl acetate and acrylic copolymers. The product is either used
directly in emulsion form as a paint or else the surfactants used in the polymeriza-
tion are left in the final, coagulated rubber product.
Emulsion polymerizations normally produce polymer particles with diameters
of 0.1
10
2
4
cm), although much larger particles can
be made by special techniques mentioned in Chapter 10. The polymer particles
made by suspension reactions have diameters in the range of 50
1
μ
m(1
μ
m
5
1 micron
5
μ
m. Recall
that free-radical initiation in suspension reactions is in the monomer phase,
whereas the aqueous phase is the initiation site in emulsion polymerizations.
The two processes often differ also in the types of stabilizers that are used.
500
