Chemistry Reference
In-Depth Information
[5
6]
summarized the results of early research, which focused on this and similar
systems. Current thinking is not entirely in accord with this mechanism. It is still
worthwhile to review it very briefly here, however, because it is still widely refer-
enced in the technical literature and because some aspects of the model provide
valuable insights into operating procedures.
Strictly speaking, the Harkins-Smith
Ewart model applies only to the batch
polymerization of a completely water-insoluble monomer in the presence of
micellar soap. (The terms
are used interchange-
ably in this technology.) Its predictions do in fact apply neatly to the case of sty-
rene. The polymerization reaction, after the induction period, can be classified
conveniently into three stages, as shown schematically in
Fig. 10.2
.
Interval I is a region of accelerating rate of conversion and proceeds until all
the micelles are consumed. Only 10
soap, surfactant
, and
emulsifier
15% or less of the total monomer is con-
sumed in this period. In interval II the rate of polymerization is constant. It is
assumed that this period begins when all the micelles disappear and is complete
when no monomer droplets remain. About one-third of the monomer is consumed
in this period. In interval III, the rate of polymerization begins to decrease. This
Interval II
No micellar soap
Monomer in droplets
Monomer in polymer particles
Constant number of particles
100
80
60
Interval III
No droplets
Monomer in polymer particles
Constant number of particles
40
Interval I
Monomer in micelles (diam. ~ 100 A°)
Monomer in droplets (diam. 100
5
A°)
Growing number of polymer particles
Monomer in polymer particles
20
0
Time (hr)
FIGURE 10.2
Course of a batch emulsion polymerization of a water-insoluble monomer in the presence
of micellar surfactant.
