Suspension polymerization, sometimes called bead,
pearl or granular polymerization, is one of the most widely used polymerization techniques.
It is essentially a water or solvent cooled bulk polymerization,
though water/solvent soluble initiators may be present that could
alter the reaction kinetic.
Practically all common themoplastic polymers can be made by this method. This includes all high volume resins such as methyl methacrylate, vinyl chloride, vinyl acetate, stryrene and some gaseous monomers such as ethylene, propylene and formaldehyde.
Suspension polymerization has several advantages over other polymerization techniques; since water is usually the continuous phase, it acts as a very effective heat-tranfer medium which is very ecconomical and more environmental friendly than the solvents employed in solution polymerization. Also temperature and viscosity control is fairly easy. Compared to emulsion polymerization, purification and processing of the polymer is much easier since very little catalyst is used and the final product is a 100% solid resin.
In general, a suspension polymerization system consists of a dispersing medium, monomer(s), stabilizing agents and a monomer soluble initiator. Water1 is almost in all cases the continuous phase. For the monomer to be dispersed in the water it has to be fairly insoluble. If the monomer is not sufficient insoluble, prepolymers or partially polymerized monomers (oligomers) can be used that are either insoluble or have a much lower solubility in the water than the monomers. This will also increase the particle size. The initiators employed for suspension polymerization are mostly peroxides, and in some cases, azo compounds (AIBN). Typical initiators are benzoyl, t-butyl, diacetyl and lauroyl peroxide. Typical stabilizers are surfactants such as sodium, potassium, or ammonium salts of fatty acids that lower the surface tension, and dispersing agents such as polyelectrolytes and some inorganic salts that stabilize the particles by providing a surface charge (electro-static interaction). Common polyelectrolytes are alkali salts of poly(methyl acrylate) and poly(methyl methacrylate) and common salts are magnesium and calcium carbonate, calcium phosphate, and aluminum chloride. In some cases, water soluble polymers such as methyl and ethyl cellulose, poly(vinyl alcohol), gums, alginates, casein, gelatins, and starches and/or (nano)particles such as talc, silicates, clays and bentonites are added. The surfactants and electrolytes reduce the viscosity and stabilize the particles, whereas the water soluble polymers and dispersed (nano)particles increase the viscosity and act as a protective coating. This prevents coalescence or agglomeration of the sticky polymer particles. Usually, very small amounts of stabilizing agents are sufficient to stabilize the monomer particles. In the case of surfactants, the concentration is usually not more than 0.01 % and for dispersing agent, it is around 0.1 % based on the water. Then the monomer or monomers blend is added so that the overall concentration is not more than 25 – 40 %.
The polymerization is carried out in the small droplets of liquid monomer. It can be considered as a form of mass polymerization on a much smaller scale, that is, the reaction steps and the kinetics are the same as those in bulk polymerization and consist of initiation, propagation and termination. During the polymerization, the immiscible droplets slowly convert from a liquid to a sticky, viscose material and, when reaching a sufficient high molecular weight, form solid, rigid particles.
During the polymerization, the suspension has to be stirred and sufficiently stabilized or the particles agglomerate and form one big mass. The final product is usually not sticky if the glass transition temperature or melting point is above the granulate temperature. The final polymer particles have a size in the range 0.1 to 5 mm in diameter and are at least ten times larger than those produced during emulsion polymerisation.
- It is very important that the water is demineralized (salt-free), because electrolytes can negatively affect the stability of the emulsion.