Relative Importance of the Effects of Seed and Feed Stage ...

376 Krishnan et al.
CONCLUSIONS
Agitation has a strong effect on the properties of the latex prepared by
emulsion polymerization. More particles are nucleated under a higher
agitation speed. This observation is consistent with the mechanism of
interfacial particle nucleation (Varela de la Rosa, 1991), wherein the shear
stress generated by the agitation results in the formation of minidroplets
near the surface of the monomer droplets, and polymer particles are formed
by the nucleation of these minidroplets. Although the reactions were carried
out under a constant flow of nitrogen through the reactor, trace amounts of
oxygen impurity in the reactor headspace can also result in a higher particle
concentration at a higher agitation speed (Krishnan, 2002).
The main factor influencing the amount of water-soluble solids in
the latex serum is the number of particles produced at the end of the seeding
stage. The greater the number of particles, the lower was the amount of watersoluble
polymer. More NMA monomer copolymerizes with BMA in the
polymer particles when the number of particles is higher. The agitation
during the feed stage did not have a significant effect unless the
emulsification of the added BMA monomer was poor and pooling occurred
(as with the 4 cm Rushton turbine). The low viscosity of the latex, and the
good mixing with the 8 cm agitator even at 150 rpm, were the reasons for no
effect of the feed stage agitation on the water-soluble polymer formation.
As expected, the amount of coagulum increased with the agitation
intensity. Almost all coagulum formed during the reaction was on the
agitator blades, baffle, etc. If the trailing vortices behind the agitator blades
are the sites of coagulum formation, then the amount of coagulum is
expected to scale with the Reynolds number.
The viscosity of the final latex was more sensitive to the number of
polymer particles than the amount of water-soluble polymer. The latex
viscosity was higher when the latexes were prepared using a higher agitation
speed during the seed stage and thereby contained more polymer particles.
ACKNOWLEDGMENTS
The help of Mr. William Anderson with NMR data acquisition and
interpretation, and the financial support from the Emulsion Polymers
Liaison Program is greatly appreciated.
REFERENCES
Ali, S. I., Zollars, R. L. (1987). Changes in coagulum configuration resulting
from shear-induced coagulation. J. Colloid Interface Sci. 117(2):425–430.