Chemistry Reference
In-Depth Information
Bulk polymerization is the main process for making high-impact polystyrene
(HIPS). Polybutadiene is dissolved in styrene at 3
10% (w/w) concentration
and the styrene is polymerized with careful agitation. Phase separation occurs
with polybutadiene-g-polystyrene separating out. The final product is a dispersion
of polybutadiene particles, which themselves contain occluded polystyrene.
Polymerization conditions are adjusted to control the size and volume of these
particles, which range respectively from 0.1 to 6.0
μ
m and 0.1 to 0.4 volume frac-
tion of the material.
A variation of the HIPS process uses diblock polybutadiene-polystyrene rub-
bers to produce core-shell rubber particles with polystyrene cores and thin poly-
butadiene shells. The small particle size of 0.1 to 0.4
μ
m is less than optimum for
toughening but provides a high-gloss material.
12.4.2.2
Heterogeneous Solution Polymerizations
Solution systems are heterogeneous when the monomer is soluble but the polymer
is not. This is typical of many coordination polymerizations of polyolefins
(Section 11.5). The process, which is commonly termed a slurry process, consists
basically of these steps:
1.
A catalyst preparation step. The catalysts, which are generally solids, are
produced with the careful exclusion of water and oxygen.
2.
Polymerization occurs at pressures usually less than 50 atm and at temperatures
below 110
C (to avoid dissolving the polymer) to form a slurry of about 20%
polymer in an aliphatic liquid diluent. The diluent can be liquid propylene itself
in the manufacture of polypropylene.
3.
Polymer recovery is done by stripping of the diluent, washing to remove
residual catalyst, and extraction of undesirable polymer components, if
necessary.
4.
A compounding step is used to mix various stabilizers and additives into the
polymer melt, which is finally chilled and pelletized.
Catalyst removal steps can be eliminated in very efficient processes
(Section 9.5) in which the residual catalyst concentration is negligible.
Conversion levels are generally higher than in the free-radical, high-pressure
polymerization process, and less monomer recycle is therefore required. The reac-
tion temperature in typical slurry processes is controlled by refluxing the solvent.
12.4.2.3
Suspension Systems
Suspension polymerization is also known as pearl or bead polymerization.
Kinetically, suspension polymerizations are water-cooled bulk reactions.
Monomer droplets with dissolved initiator are dispersed in water. As the polymer-
ization proceeds the droplets become transformed into sticky, viscous monomer-
swollen particles. Eventually, they become rigid particles with diameters in the
range of about (50
500) 10
2
4
cm. The final reaction mixture typically contains
25
50% of polymer dispersed in water. The viscosity of the system remains
