Emulsion polymerization
Rather than occurring in emulsion droplets, polymerization takes place in the latex/colloid particles that form spontaneously in the first few minutes of the process.
These latex particles are typically 100 nm in size, and are made of many individual polymer chains.
They are often preferred over solvent-based products in these applications due to the absence of volatile organic compounds (VOCs) in them.
[4][5] The idea of using an emulsified monomer in an aqueous suspension or emulsion was first conceived at Bayer, before World War I, in an attempt to prepare synthetic rubber.
[6][7] The impetus for this development was the observation that natural rubber is produced at room temperature in dispersed particles stabilized by colloidal polymers, so the industrial chemists tried to duplicate these conditions.
The Bayer workers used naturally occurring polymers such as gelatin, ovalbumin, and starch to stabilize their dispersion.
[8][9] Over the next twenty years, through the end of World War II, efficient methods for production of several forms of synthetic rubber by emulsion polymerization were developed, but relatively few publications in the scientific literature appeared: most disclosures were confined to patents or were kept secret due to wartime needs.
After World War II, emulsion polymerization was extended to production of plastics.
Ironically, synthetic rubber manufacture turned more and more away from emulsion polymerization as new organometallic catalysts were developed that allowed much better control of polymer architecture.
The first successful theory to explain the distinct features of emulsion polymerization was developed by Smith and Ewart,[10] and Harkins[11] in the 1940s, based on their studies of polystyrene.
Smith and Ewart arbitrarily divided the mechanism of emulsion polymerization into three stages or intervals.
The Smith-Ewart-Harkins theory for the mechanism of free-radical emulsion polymerization is summarized by the following steps: Smith-Ewart theory does not predict the specific polymerization behavior when the monomer is somewhat water-soluble, like methyl methacrylate or vinyl acetate.
In these cases homogeneous nucleation occurs: particles are formed without the presence or need for surfactant micelles.
[12] High molecular weights are developed in emulsion polymerization because the concentration of growing chains within each polymer particle is very low.
[13] When radicals generated in the aqueous phase encounter the monomer within the micelle, they initiate polymerization.
Those micelles that did not encounter a radical during the earlier stage of conversion begin to disappear, losing their monomer and surfactant to the growing particles.
The theory predicts that after the end of this interval, the number of growing polymer particles remains constant.
The choice depends on the properties desired in the final polymer or dispersion and on the economics of the product.
Early styrene-butadiene rubber (SBR) recipes are examples of true batch processes: all ingredients added at the same time to the reactor.
This enables a starve-fed reaction to ensure a good distribution of monomers into the polymer backbone chain.
By contrast, products sold as a dispersion are designed with a high degree of colloidal stability.
[14] Typical monomers are those that undergo radical polymerization, are liquid or gaseous at reaction conditions, and are poorly soluble in water.
If monomer solubility is too high, particle formation may not occur and the reaction kinetics reduce to that of solution polymerization.
Ethene and other alkenes are used as minor comonomers in emulsion polymerization, notably in vinyl acetate copolymers.
Small amounts of acrylic acid or other ionizable monomers are sometimes used to confer colloidal stability to a dispersion.
The surfactant must enable a fast rate of polymerization, minimize coagulum or fouling in the reactor and other process equipment, prevent an unacceptably high viscosity during polymerization (which leads to poor heat transfer), and maintain or even improve properties in the final product such as tensile strength, gloss, and water absorption.
Examples of surfactants commonly used in emulsion polymerization include fatty acids, sodium lauryl sulfate, and alpha-olefin sulfonate.
Some grades of polyvinyl alcohol and other water-soluble polymers can promote emulsion polymerization even though they do not typically form micelles and do not act as surfactants (for example, they do not lower surface tension).
However, they often result in products that are very water sensitive due to the presence of the water-soluble polymer.
Preservatives are added to products sold as liquid dispersions to retard bacterial growth.
