Chemistry Reference
In-Depth Information
these loci decreases. The interior of the particles becomes hot because of poor
heat transfer to the surrounding water, and this causes progressive accelerations in
the propagation rate and temperature rise. If heat kicks are not subdued, the reac-
tor can go out of control. The effect can be moderated by venting monomer or by
adding pentane to absorb thermal energy as it vaporizes. Another alternative is
the addition of styrene as a “shortstop.” Styryl radicals are too stable to reinitiate
vinyl chloride polymerization, and the growth of macroradicals with styrene ends
is essentially terminated (Section 9.10.1).
Ideally, a suspension polymerization is run so that the heat of reaction just
equals the maximum rate of heat removal through the reactor walls. The total
heat transfer from the reaction medium,
H
T
,is
1
H
T
5
1
H
F
1
1
H
w
1
1
H
J
(12-14)
where
H
F
5
film coefficient on the inside of the autoclave,
H
w
5
thermal conduc-
tivity of the jacket wall, and
H
J
5
jacket side film coefficient. Good agitation and
freedom from wall fouling are necessary to keep
H
F
as high as possible.
Commercial suspension polymerizations are not strictly isothermal, since the
reactor contents must be heated to the final reaction temperature. Mixtures of
initiators are therefore used in an attempt to maintain a rate of heat generation
close to the cooling capacity of the reactor. Particular initiators are useful only
over a limited temperature range. Most initiators for suspension polymerizations
have half-lives of about 2 h in the 50
70
C range. After 6 h, then, the final initi-
ator concentration will be 10
15% of the amount charged initially to the reactor
[from Eq. (8-32)]. In PVC synthesis, it is fairly common to use one initiator with
a
t
1/2
of 1-2 h and another with a longer
t
1/2
of 4
6 h. Other factors that affect
the usefulness of initiators include:
1.
Storage stability and safety
2.
Color development in the polymer (this is a problem with some azo initiators)
3.
Water insolubility and resistance to hydrolysis; water solubility could lead to
more reaction in the aqueous phase and wall fouling. Other expedients to
reduce aqueous phase reactions include use of a water-soluble free-radical
scavenger or a chelating agent to minimize redox reactions in the aqueous
phase. (Such water-soluble chelating agents include salts of oxalic acid and
ethylene diamine tetraacetic acid.)
The kinetic features of suspension polymerizations are thought to be as
described in Chapter 8 for free-radical reactions, in general. However, the particle
sizes and structures that are produced are very important polymer properties, and
these depend on factors other than the chemistry of the polymerization. In vinyl
chloride polymerization, the particle character is related to the agitation level,
which depends on the impeller diameter and rotational speed. At low agitation
levels large monomer droplets and polymer particles are formed. The droplet and
particle sizes decrease at higher agitation levels. At even higher agitation levels,
