Semi-Batch Emulsion Copolymerization of Vinyl Acetate and Butyl ...

Semi-Batch Emulsion Copolymerization of Vinyl Acetate ... 2621
Figure 10. The effect of initial initiator and electrolyte concentrations
at constant ionic strength on the average particle size
(surfactant S2: 1 wt.-%; 0.25 wt.-% initiator continuously fed).
was found that even at constant ionic strength, differences
in the average particle size were observed (Figure 10)
implying a possible interaction mechanism between
initiator and surfactant, which influences the latex particle
stabilization or/and the radical entry rate. The most
stable case was observed when using a low initiator concentration
and a high electrolyte concentration (e.g., C I =
0.013 mol/L and C E = 0.023 mol/L electrolyte). Notice
that increased initiator concentrations eventually lead to
larger particle sizes.
Effect of Monomer Addition Rate
The existence of “pseudo-steady states” in semi-batch
emulsion copolymerization has been investigated and
reported in the literature for various polymerization systems.
[5, 6, 17–19] According to these reports, there is a critical
monomer addition rate above which the monomer concentration
in the latex particles becomes saturated. Thus,
above this critical value, the polymerization rate remains
constant irrespectively of the monomer addition rate provided
that the monomer diffusion rate through the aqueous
phase is larger than the polymerization rate. On the
other hand, for monomer addition rates below the critical
addition rate, the polymerization rate will be equal to the
monomer addition rate. The existence of the critical
monomer addition rate has been experimentally tested
and confirmed with monomers having different water
solubilities such as styrene and vinyl acetate.
In the present study, the effect of monomer addition
rate was examined using surfactant S3. It was observed
that the polymerization rate was initially increased (e.g.,
in the first 30–60 min) and then reached a constant value
which was equal to the respective monomer addition rate
(Figure 11a). The small overshoot observed in the polymerization
rate at about 90 min was attributed to the
polymerization of the excess monomer accumulated in
the aqueous phase during the non-starved period (Figure
Figure 11. The effect of monomer addition rate on (a) the
polymerization rate and (b) the average particle size. Dashed
lines indicate the monomer addition rates (surfactant S3: 2
wt.-%; initiator addition policy 0.25–0.25 wt.-%).
11a). Furthermore, it was observed that an increase in the
monomer addition rate resulted in an increase in the average
particle size (Figure 11b). Notice that larger monomer
addition rates result in higher monomer concentrations
in the latex particles and, thus, in larger particle
growth rates. The above results for the polymerization
rate (Figure 11a) and the average particle size (Figure
11b) are in agreement with the results of Dimitratos et
al. [5, 6] obtained for the semi-batch emulsion copolymerization
of VAc/BuA (80:20) at 608C.
Conclusions
For all the investigated alkyl polyglucoside surfactants, it
was found that the latex particle stability and the rate of
polymerization increased with an increase in the surfactant
concentration up to the standard recipe value. However,
excessive surfactant concentrations resulted in particle
destabilization. As a result, the final average particle
size exhibited a “U-shape” behavior with respect to the
surfactant concentration. A possible cause for the
observed particle destabilization at high surfactant con-