NANOCOMPOSITE BEADS VIA SUSPENSION POLYMERIZATION
Suspension polymerization is the preferred technique for manufacturing spherical polymeric beads ranging from about 50 microns to several millimeters in diameter by starting from many monomers. Hydrocarbon droplets, containing the monomer(s), are suspended in an aqueous medium and then polymerized, with agitation (usually by stirring) to maintain the two-phase system during polymerization, resulting in spherical polymeric beads. Hence each hydrocarbon droplet is like a mini-reactor in which bulk polymerization is occurring. The formulation often also includes one or more oil-soluble initiator(s) as well as one or more additive(s) helping stabilize the suspension and controlling its rheology.
Polystyrene, high-impact polystyrene, poly(methyl methacrylate), poly(vinyl acetate), poly(vinyl chloride), copolymers of styrene and methyl methacrylate, copolymers of methyl methacrylate and vinyl acetate, and copolymers of styrene and acrylonitrile are among the thermoplastic polymers that can be produced in the shape of spherical beads on a commercial scale with favorable process economics via suspension polymerization.
Thermoset versions of the same polymers can also be produced readily in the form of spherical beads via suspension polymerization, by including one or more of many available cross-linkers (such as divinylbenzene, ethylene glycol dimethacrylate, trimethylolpropane triacrylate, 1,6-diaminohexane, p-phenylenediamine, etc.) in hydrocarbon droplets. The extent of cross-linking can be controlled by varying the concentration of cross-linker in the monomer mixture. The extent of cross-linking can also be increased further by postcuring (annealing) the as-polymerized beads for a brief period at an elevated temperature.
Vikas Mittal, Polymer Nanocomposites in Emulsion and Suspension: an Overview, Chapter 1 in Polymer Nanocomposites by Emulsion and Suspension Polymerization, edited by Vikas Mittal, Royal Society of Chemistry (Great Britain), 2011, provides an excellent review of this field. This open access book chapter is available for free download after registering on the website of the Royal Society of Chemistry.
As summarized by Mittal, “to achieve polymer nanocomposites in emulsion and suspension, the monomer is suspended in water along with various amounts of silicates in the presence of emulsifier in emulsion polymerization and suspension stabilizers in suspension polymerization. The monomer is then polymerized either in emulsion micelles or in suspension monomer droplets, leading to the generation of the nanocomposites with part of the silicate embedded inside the polymer particles and part adsorbed on the surface of the particles. In emulsion polymerization, the initiators used are generally water soluble, in contrast to suspension polymerization, where water-insoluble initiators are used. The behavior or role of the emulsifier in emulsion polymerization is different from that in suspension polymerization. The emulsifier in the emulsion polymerization, in addition to stabilizing the monomer droplets, also forms micelles once the concentration of the stabilizer exceeds the critical micelle concentration. The internal environment of a micelle is totally hydrophobic and is very favorable for monomer polymerization. The size of the polymer particles in the case of emulsion polymerization is dependent on the amount of surfactant, whereas the size of monomer droplets and hence polymer particles in suspension polymerization can be controlled by varying the ratio of monomer to the dispersion medium, i.e. water, the speed of agitation to generate droplets and the amount of stabilizing agents. The main advantage of generating polymer nanocomposites in emulsion or suspension is the that as the dispersion medium, i.e. water, surrounds every forming polymer particle, hence there is no problem with viscosity enhancement and heat can also be dissipated without problem, thus allowing suitable conditions for filler exfoliation without causing any extensive thermal damage to the polymer or the organic modification on the filler surface. Surfactant-free emulsion polymerization can also be achieved to generate nanocomposites; the mechanism of particle nucleation in this case involves homogeneous nucleation compared with micellar nucleation in the case of emulsified emulsion polymerization.”
The following images related to this technology are reproduced from Mittal.