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

Seed and Feed Stage Agitations of BMA and NMA 371
Figure 2.
Table 2.
Particle size distribution in seed and feed latexes for the experiments in
the particle diameter in the seed was smaller when the agitation speed was
higher. The figure shows a fairly good reproducibility of the particle size
distributions. The final latexes show peaks at higher or lower particle
diameters, corresponding to the particle sizes in the seed latexes. The
diameter in the final latex is not affected by the feed stage agitation.
The decrease in the particle diameter with an increase in the agitation
is a recurring feature for particle nucleation in systems with surfactant
concentrations near the critical micelle concentration. Arai et al. (1981)
have observed an increase in the number of particles with the increase in
the agitation speed during surfactant free emulsion polymerizations of
methyl methacrylate using KPS initiator at 65°C. Varela de la Rosa (1991)
has reported the heat evolution rates during emulsion polymerizations of
styrene using KPS initiator at 70°C at an SDS concentration of 10 mmol
dm 3 for agitator speeds of 300 rpm and 500 rpm. The particle diameter
was smaller and the reaction rate higher, when the agitator speed was 500
rpm. In the absence of any oxygen impurity in the reactor, this effect can
be explained by the interfacial nucleation mechanism recently proposed by
Ni et al. (2001). According to this mechanism, under the influence of
shear, minidroplets are formed at the interface of the monomer droplets
and water. These minidroplets result in the nucleation of the polymer
particles. The greater the agitation, the higher the number of these minidroplets,
and the resulting polymer particles. Trace amounts (ppm levels) of
oxygen impurity in the reactor headspace and the agitation dependent mass