Two‐stage stabilizer addition protocol as a means to reduce the size and improve the uniformity of polymer beads in suspension polymerization

Abstract

A two-stage stabilizer addition protocol is suggested for reducing the size and improving the uniformity of polymer beads resulting from conventional suspension polymerization. The stabilizer load was divided into an initial charge and a secondary addition. The use of a low concentration of stabilizer in the initial charge served to assist drop rupture while avoiding significant reduction in drop size and production of too many satellite droplets. The secondary addition time of stabilizer occurred just before the onset of the growth stage when drops were vulnerable to coalescence but robust against break up due to their high viscosity. The secondary addition of stabilizer served to provide stability to monomer drops during the growth stage and as a result the drops underwent limited coalescence. This resulted in the formation of smaller and more uniform polymer beads in comparison to beads obtained by conventional suspension polymerization at the same overall concentration of stabilizer. V C 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45671.

Suppressing Coalescence and Improving Uniformity of Polymer Beads in Suspension Polymerization Using a Two-Stage Stirring Protocol

“…By the same token, the reason for the small average particle size and PSD narrowing in the (isothermal) two-stage protocol lies in the fact that drops were not exposed to high intensity shear for long, and therefore did not significantly reduce in size during the transition stage. The longer was t 2nd , within the safe range of transition time >10 min, the smaller and more vulnerable (against coalescence) were the drops at the onset of the growth stage, due to a lower stabilizer surface coverage . This can explain why beads became larger and their size distribution wider with increasing t 2nd .…”

“…In this section, we show how stabilizer partitioning can be used along with the two-stage stirring protocol to further suppress coalescence/flocculation and improve the uniformity of beads. In a recent paper, it has been demonstrated that if a part of a stabilizer is added to the reactor prior to the onset of the growth stage, beyond which polymerizing monomer drops are viscous and cannot be ruptured, polymer beads with a smaller size and more uniform distribution may be produced …”

“…The reason for this seems to be due to the lack of stability of the drops at a very low concentration of PVA. In general, the hybrid process was less vulnerable to coalescence due to a smaller interfacial area developed by the reduced amount of PVA used in the initial charge and a better surfactant coverage during the polymerization stage . As a result, the hybrid protocol produced more uniform beads.…”

“…The longer was t 2nd , within the safe range of transition time >10 min, the smaller and more vulnerable (against coalescence) were the drops at the onset of the growth stage, due to a lower stabilizer surface coverage. 28 This can explain why beads became larger and their size distribution wider with increasing t 2nd . Nevertheless, the results clearly show that the two-stage protocol was more efficient when the emulsification stage was carried out at low temperature (i.e., in the presence of PVA).…”

“…In a recent paper, it has been demonstrated that if a part of a stabilizer is added to the reactor prior to the onset of the growth stage, beyond which polymerizing monomer drops are viscous and cannot be ruptured, polymer beads with a smaller size and more uniform distribution may be produced. 28 Following the same line of thought, we explored if the emulsification stage could be conducted using only a portion of PVA from the main recipe, followed by polymerization at a reduced stirring speed using the remainder of PVA, in order to produce more uniform beads.…”