Uniform polymer beads by membrane emulsification-assisted suspension polymerisation

Abstract

This work focuses on a two-stage polymerisation process for the production of uniform polymer beads. Highly uniform droplets were firstly produced by a stirred-vessel membrane emulsification device.Methyl methacrylate (MMA) and a specific grade of polyvinyl alcohol (PVA) were used as monomer and stabiliser, respectively. The effects of various process parameters affecting the droplet size and uniformity including feeding policy, agitation speed, stabiliser concentration, and flowrate were investigated. The evolution of droplet size and its coefficient of variation (CV) were monitored over the course of emulsification. A new start-up policy, validated by monitoring droplet formation at the membrane surface, was introduced that eliminated the non-uniformity in the size of droplets formed early during emulsification. The mechanisms contributing to droplet size distribution broadening at the membrane surface during formation were decoupled from those acting in the emulsification vessel during circulation. The high CV obtained at low PVA concentration and high agitation speed was attributed to drop breakup and coalescence occurring in the emulsification vessel, respectively, after droplets formed. The emulsification was followed by a shear-controlled suspension polymerisation to convert the discrete droplets of monomer to polymer beads. A wide range of reactor impeller speeds and PVA concentrations was studied to find the conditions under which the droplets formed via membrane emulsification would not undergo further break-up or coalesce during polymerisations and a one-to-one copy of the initial droplets with the same CV can be achieved.

Particle Size Control in Miniemulsion Polymerization via Membrane Emulsification

“…Vladisavljević et al, Oh et al, and Yuyama et al investigated the effects of process parameters such as the membrane type, the average pore size and porosity, shear stress, temperature, transmembrane pressure, and emulsifier/surfactant. Other reports have focused on specific aspects related to the process, such as stirred vessel membrane emulsification and premix membrane emulsification . Agner et al compared the two-stage premix membrane emulsification and other emulsification devices for preparing submicron-sized droplets of methyl methacrylate (MMA).…”

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

“…This methodology, when optimized, delivered polymer beads with the same uniformity as the initial monomer drops. 23 To theoretically verify this concept, we considered a stabilizerfree emulsification system at the stirring speeds of 500 and 700 rpm, as defined in the caption of Figure 2a, and used exactly the same modeling parameters explained elsewhere. 14 Figure 2a shows how the Sauter-mean diameter of the drops responds to an alteration in the stirring speed.…”

“…An increase in temperature from room temperature (22.0 °C) to the reaction temperature 75.0 °C, would decrease the interfacial tension by 40.0% and theoretically decrease D 32 by 18.8%. 23 The net effect is that the decrease in σ at the onset of second stage, due to the increase in temperature, is easily compensated by a larger decrease in the shear stress associated with the reduced stirring speed, suggesting that drops remain robust against breakup. This conclusion is backed up by the results presented in the next section.…”

Water in oil emulsions from hydrophobized metal membranes and characterization of dynamic interfacial tension in membrane emulsification

“…The polymer dissolved in the aqueous phase was poly(vinyl alcohol). This is a surface-active polymer commonly used as the stabilizer in oil in water (o/w) emulsions [20][21][22]. Thus, using this formulation, there will be two surface agents adsorbing to the surface resulting in a rapid decrease of the interfacial tension while drops are being formed.…”