Handbook of Industrial Drying

COMMON POLYMERIZATION PROCESSES

The selection of dryers for polymer drying depends to a large extent on the upstream operations, e.g., poly- merization, since the behavior of a polymerization reaction and the properties of the resulting polymer can vary greatly according to the nature of the physical system in which the polymerization reaction is carried out. Several processes are used commercially to pre- pare polymers. Each process has its advantages, usu- ally depending on the type and final use of the polymer. The following types of polymerization processes based on physical systems are considered briefly in this sec- tion: (1) bulk; (2) solution; (3) suspension; (4) emul-
sion; and (5) gas-phase polymerization [3].

1. Bulk Polymerization

The polymerization of the pure monomer without diluent is called bulk polymerization or mass polymer- ization. The monomer (e.g., styrene, vinyl chloride (VC), vinyl acetate (VA), acrylic esters, butadiene, or acrylonitrile) is first purified to remove oxygen or other inhibitors (by bubbling nitrogen through it, by distillation, or by evacuation) and then the polymer- ization is started through heating, ultraviolet (UV) radiation, or the addition of an initiator (e.g., perox- ides, azo compounds, and others). Usually, after a short period of heating, the reaction mixture con- tinues to heat by itself, and therefore it is necessary to remove the heat by cooling. With increasing con- version, because of the rapidly increasing viscosity of the polymer–monomer mixture, this becomes more and more difficult. With large amounts of monomer, bulk polymerization often takes very turbulent and even explosive form as a result of the rapidly increas- ing temperature. The violence of the reaction is even further increased by the increase in the radical con- centration that occurs with increasing viscosity.

Because of the difficulty in heat removal, bulk poly- merization is only carried out in a few cases. However, in the cases in which it is used, it is done on a very large- scale, e.g., the bulk polymerization of styrene or ethylene (high-pressure process). Since in this type of polymerization the possibility of chain transfer is rela- tively small and because of self-acceleration, other poly- mers with high molecular weights are found. One of the characteristics of this process that is always a technical advantage is the great purity of the polymer resulting from the lack of additives during polymerization.

2. 
   SOLUTION POLYMERIZATION
   

For solution polymerization, a solvent inert to the monomer is used to control the polymerization. High exothermicity is limited by dilution, causing the reac- tion rate to be slowed owing to solvent addition. The solvent is recycled after cooling and is sent back to the polymerization reactor. The concentration of the solv- ent is chosen in such a way that the polymerization mixture can still be stirred after complete conversion.
Solution polymerization has been employed almost exclusively in cases in which the polymer is then used in the form of solutions (50 to 60%) for lacquers, adhe- sives, impregnation materials, and other products. Obtaining the pure polymer by distilling off the solvent is complicated because the hard polymer cannot be taken out of the vessel after evaporation of the solvent. Through construction of extruders with vacuum distil- lation zones and by using other special evaporators, it is possible to separate the polymer from the solvent.
In this process the choice of the solvent’s chain
transfer constant is very important because this influ- ences the molecular weight to a considerable extent. Because of chain transfer with the solvent and be- cause of the lower monomer concentration, the mo- lecular weight of polymers prepared by solution polymerization is usually lower than that of the cor- responding bulk polymers.
Commercially, solution polymerizations are not carried out to high conversions (near 100%) but continuously at a constant monomer concentration. The unreacted and evaporated monomer is recycled together with the solvent. This type of production process has two advantages. The reactor always works in a range of high polymerization rates, and the molecular weight distribution curve is not so broad as it is with polymers produced in a discontinu- ous process with high conversions.

3. 
   SUSPENSION POLYMERIZATION
   

In the suspension polymerization process, water is used to control heat generation. A catalyst is dis-
solved in the monomer, which is dispersed in water. A dispersing agent is incorporated to stabilize the suspension formed. For any nonpolar monomers, this method offers a method of eliminating many of the problems encountered in bulk and in solution polymerization, especially the heat dissipation prob- lem in the former and solvent reactivity and removal of the latter.
Another attractive feature of large batch prepar- ations is that the polymeric products obtained from a suspension polymerization, if correctly carried out, are in the form of finely granulated beads that are easily filtered and dried.
On a technical scale, suspension polymerization is used in the production of polyvinylchloride, polystyr- ene, polymethyl methacrylate, and others. For the production of rubbery, sticky, polymers (e.g., the polyacrylates), this is less suitable.

4.