Suspension Polymerization

Suspension polymerization is a heterogeneous radical polymerization process that uses mechanical agitation to mix a monomer or mixture of monomers in a liquid phase, such as water, while the monomers polymerize, forming spheres of polymer. The monomer droplets (size of the order 10-1000 μm) are suspended in the liquid phase. The individual monomer droplets can be considered as undergoing bulk polymerization. The liquid phase outside these droplets help in better conduction of heat and thus tempering the increase in temperature.

While choosing a liquid phase for suspension polymerization, low viscosity, high thermal conductivity and low temperature variation of viscosity are generally preferred. The primary advantage of suspension polymerization over other types of polymerization is that a higher degree of polymerization can be achieved without monomer boil-off. During this process, there is often a possibility of these monomer droplets to stick to each other and cause creaming in the solution. To prevent this, the mixture is carefully stirred or a protective colloid is often added. One of the most common suspending agents is polyvinyl alcohol (PVA). Usually, the monomer conversion is completed unlike in bulk polymerization, and the initiator used in this is monomer-soluble.

This process is used in the production of many commercial resins, including polyvinyl chloride (PVC), a widely used plastic, styrene resins including polystyrene, expanded polystyrene, and high-impact polystyrene, as well as poly(styrene-acrylonitrile) and poly(methyl methacrylate).

Particle properties

Suspension polymerization is divided into two main types, depending on the morphology of the particles that result. In bead polymerization, the polymer is soluble in its monomer and the result is a smooth, translucent bead. In powder polymerization, the polymer is not soluble in its monomer and the resultant bead will be porous and irregular. The morphology of the polymer can be changed by adding a monomer diluent, an inert liquid that is insoluble with the liquid matrix. The diluent changes the solubility of the polymer in the monomer and gives a measure of control over the porosity of the resulting polymer.

The polymer beads that result can range in size from 100 nm to 5 mm. The size is controlled by the stirring speed, the volume fraction of monomer, the concentration and identity of the stabilizers used, and the viscosities of the different components. The following equation derived empirically summarizes some of these interactions:
:$\bar = k$

d is the average particle size, k includes parameters related to the reaction vessel design, D_v is the reaction vessel diameter, D_s is the diameter of the stirrer, R is the volume ratio of the monomer to the liquid matrix, N is the stirring speed, ν_m and ν_l are the viscosity of the monomer phase and liquid matrix respectively, ε is the interfacial tension of the two phases, and C_s is the concentration of stabilizer. The most common way to control the particle size is to change the stirring speed.

See also

* Radical polymerization
* Polymer
* Polymerization
* Step-growth polymerization
*Emulsion polymerization
*Superabsorbent polymer

References

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Polymerization reactions

fr:Procédé de polymérisation#Polymérisation en suspension