Bulk (mass) is the simplest of all schemes. Only monomers and small traces of initiators and additives are charged to the reactor. As proceeds, the viscosity of the batch increases. At some point, the polymerization is terminated and the material is passed to another vessel for polymer isolation, unreacted monomer recovery, etc.
This type of polymerization is widely practiced in the manufacture of condensation polymers where the reactions are only mildly exothermic and most of the reaction occurs when the viscosity of the materials are still low enough to allow ready mixing, heat transfer, and bubble elimination. Bulk polymerization differs from other methods in that the charge to the reactor is essentially a pure monomer or mixture of monomers. The amount of polymer initiator (catalyst) and any additives are small.
Bulk polymerizations are generally used when some special properties are required such as: maximum clarity. Bulk polymerizations can either by batch or continuous and can produce polymers which are either partially or completely soluble in their producing monomers.
The disadvantages are generally:
- With strong exothermic reactions, it may be impossible to hold a constant temperature through the polymerization cycle particularly during the highest viscosity stages, which increases the molecular weight distribution
- Increasing viscosity may compound subsequent solvent removal
- Reactions may threaten to become uncontrollable if heat transfer is impossible due to viscosity increase
Sizing of the agitator is heavily dependent on the finished viscosity of the bulk polymer. A complete range of agitators are found in this service, ranging from turbines in lower viscosities to anchors and helixes in higher viscosities. Since the individual bulk polymerization schemes are so tailored to individual process requirements, further generalization is to be avoided in favor or specific recommendations on individual products. Frequently, the agitator’s power draw can be used as an index of reaction completion; this is particularly true where the polymer produced is soluble in the monomer. Often the bulk polymerization technique is combined with other methods of polymerization.
The most important commercial applications of bulk polymerizations involve those of polystyrene, methacrylates, low density polyethylene, and the two step reactions of polyester and polyethylene terephthalate.