Basics of Batch Emulsion Polymerization

In emulsion polymerization, a dispersion of polymer in aqueous phase (latex) is obtained. The solids content is usually between 40-65 %w. These polymers are broadly used in paints, adhesives, rubbers, binders and additives for the paper and textile industry. The development of reliable mathematical models for emulsion polymerization requires a high level of understanding of the phenomena taking place in the reactor. In this article, we offer a brief overview of the basic steps in batch emulsion polymerization and we discuss the kinetics of the process. Common monomers employed in this polymerization are styrene, methyl methacrylate or vinyl acetate.

The monomer feed is dispersed using water emulsifiers or surfactants. These are typically molecules like Sodium Lauryl Sulfate with a non-polar tail and a water soluble end which stabilize the monomer droplets in the aqueous phase. The excess of surfactant forms micelles which are swollen with monomer, as it is described in the following figure:

Polymerization is started by means of a water soluble initiator (tert-butyl hydroperoxide, potassium persulfate). Thus radicals are formed in the aqueous phase from the small amount of monomer dissolved. When the oligoradical chain has grown to a certain extent, it becomes hydrophobic and enters one of the micelles (the entry to the monomer droplets is unlikely because of the low total surface area that these represent). This is called heterogeneous nucleation and as a result a polymer particle is formed. If the oligoradical does not enter a micelle, it will continue growing until it precipitates absorbing emulsifier (homogeneous nucleation). This situation is described in the following figure:

In this stage there is a coexistence of monomer droplets, monomer swollen micelles and polymer particles in the reactor. Monomer is consumed inside the polymer particles according to the free radical polymerization mechanisms. The monomer consumed is replaced by monomer that diffuses from the droplets. The micelles in the system decrease because they are converted to polymer particles or they are destroyed to provide enough surfactant to the growing polymer particles. When all the micelles disappear (5-10% of monomer conversion), the nucleation interval has come to an end:

At this stage, the system consists of polymer particles and monomer droplets. Given that the mass transfer rate for monomers with higher water solubility than styrene is higher than the polymerization rate, the monomer concentration in the polymer particles reaches a maximum and constant value. When solubility is too low, this stage can be diffusionally limited.

As monomer diffuses from droplets to particles, these grow until the monomer droplets disappear. At this point the system is exclusively composed of polymer particles. The monomer conversion at which this takes place highly depends on the capacity of the particles to be swollen with monomer (40% for styrene, 15% for vinyl acetate). In this new interval the monomer concentration is the particles decreases until polymerization is completed. Finally, polymer particles in a latex of size varying from 80-300nm are obtained:

These stages are summarized in the following conversion-time figure:

The rate of polymerization in the polymer particles is similar to the bulk polymerizations, where k_p is the propagation constant [m³ mol^-1 s^-1], [M]_p is the monomer concentration in the polymer particles, n is the average number of radicals per particle, N_p the number of polymer particles and N_A the Avogadro’s number:

Since in emulsion polymerization the radicals are distributed among the particles, the overall radical concentration is much higher than in bulk polymerization, resulting in a higher polymerization rate. Radicals have a longer life time leading to higher molecular weights, and are not terminated until another radical enters the particle, which is less probable if there are more polymer particles in the system. Therefore, the polymerization rate and the molecular weight can be increased simultaneously by increasing the number of particles. This is an advantageous characteristic of emulsion polymerization that could not be attained in bulk or solution polymerization.

Carlos Arnaiz del Pozo