- Emulsion polymerization
Emulsion polymerization is a type of
radical polymerizationthat usually starts with an emulsionincorporating water, monomer, and surfactant. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are emulsified (with surfactants) in a continuous phase of water. Water-soluble polymers, such as certain polyvinyl alcohols or hydroxyethyl celluloses, can also be used to act as emulsifiers/stabilizers. The name "emulsion polymerization" is a misnomer that arises from a historical misconception. Rather than occurring in emulsion droplets, polymerization takes place in the latex particles that form spontaneously in the first few minutes of the process. These latex particles are typically 100 nm in size, and comprise many individual polymer chains. The particles are stopped from coagulating with each other because each particle is surrounded by the surfactant ('soap'); the charge on the surfactant repels other particles electrostatically. When water-soluble polymers are used as stabilizers instead of soap, the repulsion between particles arises because these water-soluble polymers form a 'hairy layer' around a particle that repels other particles, because pushing particles together would involve compressing these chains.
Emulsion polymerization is used to manufacture several commercially important polymers. Many of these polymers are used as solid materials and must be isolated from the aqueous dispersion after polymerization. In other cases the dispersion itself is the end product. A dispersion resulting from emulsion polymerization is often called a
latex(especially if derived from a synthetic rubber) or an emulsion (even though "emulsion" strictly speaking refers to a dispersion of a liquid in water). These emulsions find applications in adhesives, paints, paper coating and textile coatings. They are finding increasing acceptance and are preferred over solvent-based products in these applications as a result of their eco-friendly characteristics due to the absence of VOCs (Volatile Organic Compounds) in them.
Advantages of emulsion polymerization include: [Odian, G, "Principles of Polymerization", Wiley, New York]
molecular weightpolymers can be made at fast polymerization rates. By contrast, in bulk and solution free radical polymerization, there is a tradeoff between molecular weight and polymerization rate.
*The continuous water phase is an excellent conductor of heat and allows the heat to be removed from the system, allowing many reaction methods to increase their rate.
polymer moleculesare contained within the particles, viscosityremains close to that of water and is not dependent on molecular weight.
*The final product can be used as is and does not generally need to be altered or processed.
Disadvantages of emulsion polymerization include:
*Surfactants and other polymerization
adjuvantsremain in the polymer or are difficult to remove
*For dry (isolated) polymers, water removal is an energy-intensive process
*Emulsion polymerizations are usually designed to operate at high conversion of monomer to polymer. This can result in significant chain transfer to polymer.
The early history of emulsion polymerization is connected with the field of
synthetic rubber. [Whitby, G. S.; Katz, M. "Ind. Eng. Chem.", (1933), 25, 1338.] [Hohenstein, W. P.; Mark, H. "J. Polym. Chem.", (1946), 1, 127.] The idea of using an emulsified monomer in an aqueous suspension or emulsion was first conceived by workers at Bayer, before World War I, in an attempt to prepare synthetic rubber. [German patent 250690 (Sept. 12, 1909)] [U.S. Patent 1149577, filed Jan. 6, 1913.] 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 simply tried to duplicate these conditions. The Bayer workers used naturally occurring polymers such as gelatin, egg albumin, and starchto stabilize their dispersion. By today's definition these were not true emulsion polymerizations, but suspension polymerizations.
The first "true" emulsion polymerizations, which used a surface-active agent and polymerization initiator, were conducted in the 1920s to polymerize
isoprene. [ German patent 558890 (Jan. 8, 1927)] [U. S. Patent 1732795, filed Sept.13, 1927.] 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. Manufacture of dispersions to be used in latex paints and other products sold as liquid dispersions commenced. Ever more sophisticated processes were devised to prepare products that replaced
solvent-based materials. Ironically, synthetic rubber manufacture turned more and more away from emulsion polymerization as new organometalliccatalysts were developed that allowed much better control of polymer architecture.
The first successful theory to explain the distinct features of emulsion polymerization was largely developed by Smith and Ewart, [Smith, W. V.; Ewart, R. H. "J. Chem. Phys.", (1948), 16, 592.] and Hawkins [Hawkins, W. D. "J. Am. Chem. Soc.", (1947), 69, 1428.] in the 1940s, based on their studies of
polystyrene. Smith and Ewart arbitrarily divided the mechanism of emulsion polymerization into three stages or intervals. Subsequently it has been recognized that not all monomers or systems undergo these particular three intervals. Nevertheless, the Smith-Ewart description is a useful starting point to analyze emulsion polymerizations.
The Smith-Ewart-Harkins theory for the mechanism of free-radical emulsion polymerization is summarized by the following steps:
* A monomer is dispersed or
emulsifiedin a solution of surfactant and water forming relatively large droplets of monomer in water.
* Excess surfactant creates
micelles in the water.
* Small amounts of monomer diffuse through the water to the micelle.
* A water-soluble initiator is introduced into the water phase where it reacts with monomer in the micelles. (This characteristic differs from
suspension polymerizationwhere an oil-soluble initiator dissolves in the monomer, followed by polymer formation in the monomer droplets themselves.) This is considered Smith-Ewart Interval 1.
* The total surface area of the micelles is much greater than the total surface area of the fewer, larger monomer droplets; therefore the initiator typically reacts in the micelle and not the monomer droplet.
* Monomer in the micelle quickly polymerizes and the growing chain terminates. At this point the monomer-swollen micelle has turned into a polymer particle. When both monomer droplets and polymer particles are present in the system, this is considered Smith-Ewart Interval 2.
* More monomer from the droplets diffuses to the growing particle, where more initiators will eventually react.
* Eventually the free monomer droplets disappear and all remaining monomer is located in the particles. This is considered Smith-Ewart Interval 3.
* Depending on the particular product and monomer, additional monomer and initiator may be continuously and slowly added to maintain their levels in the system as the particles grow.
* The final product is a dispersion of polymer particles in water. It can also be known as a polymer
colloid, a latex, or commonly and inaccurately as an 'emulsion'.
Smith-Ewart theory does not predict the specific polymerization behavior when the monomer is somewhat water-soluble, like
methyl methacrylateor vinyl acetate. In these cases homogeneous nucleationoccurs: particles are formed without the presence or need for surfactant micelles. [Fitch, R. M. "Polymer Colloids", Plenum, NY 1971.]
High molecular weights are developed in emulsion polymerization because the concentration of growing chains within each polymer particle is very low. In conventional radical polymerization, the concentration of growing chains is higher, which leads to termination by coupling, which ultimately results in shorter polymer chains. The original Smith-Ewart-Hawkins mechanism required each particle to contain either zero or one growing chain. Improved understanding of emulsion polymerization has relaxed that criterion to include more than one growing chain per particle, however, the growing chains per particle is still considered to be very low.
Because of the complex chemistry that occurs during an emulsion polymerization, including polymerization
kineticsand particle formation kinetics, quantitative understanding of the mechanism of emulsion polymerization has required extensive computer simulation. Gilbert has summarized a recent theory. [Gilbert, R. G. "Emulsion Polymerization: a Mechanistic Approach" Academic Press, London, 1996.]
Emulsion polymerizations have been used in batch, semi-batch, and continuous processes. The choice depends on the properties desired in the final polymer or dispersion and on the economics of the product. Modern
process controlschemes have enabled the development of complex reaction processes, with ingredients such as initiator, monomer, and surfactant added at the beginning, during, or at the end of the reaction.
SBRrecipes are examples of true batch processes: all ingredients added at the same time to the reactor. Semi-batch recipes usually include a programmed feed of monomer to the reactor. This enables a starve-fedreaction to insure a good distribution of monomers into the polymer backbone chain. Continuous processes have been used to manufacture various grades of synthetic rubber.
Some polymerizations are stopped before all the monomer has been reacted. This minimizes chain transfer to polymer. In such cases the monomer must be removed or stripped from the dispersion.
Colloidal stability is a factor in design of an emulsion polymerization process. For dry or isolated products, the polymer dispersion must be isolated, or converted into solid form. This can be accomplished by simple heating of the dispersion until all water
evaporates. More commonly, the dispersion is destabilized (sometimes called "broken") by addition of a multivalent cation. Alternatively, acidificationwill destabilize a dispersion with a carboxylic acidsurfactant. These techniques may be employed in combination with application of shear to increase the rate of destabilization. After isolation of the polymer, it is usually washed, dried, and packaged.
By contrast, products sold as a dispersion are designed with a high degree of colloidal stability. Colloidal properties such as particle size, particle size distribution, and viscosity are of critical importance to the performance of these dispersions.
Living polymerizationprocesses that are carried out via emulsion polymerization such as iodine-transfer polymerization and RAFT have been developed.
monomers are those that undergo radical polymerization, are liquid or gaseous at reaction conditions, and are poorly solublein water. Solid monomers are difficult to disperse in water. If monomer solubility is too high, particle formation may not occur and the reaction kinetics reduce to that of solution polymerization. Ethyleneand other simple olefinsmust be polymerized at very high pressures (up to 800 bar).
Copolymerizationis common in emulsion polymerization. The same rules and comonomerpairs that exist in radical polymerizationoperate in emulsion polymerization. However, copolymerization kinetics are greatly influenced by the aqueous solubilityof the monomers. Monomers with greater aqueous solubility will tend to partition in the aqueous phase and not in the polymer particle. They will not get incorporated as readily in the polymer chain as monomers with lower aqueous solubility. This can be avoided by a programmed addition of monomer using a semi-batch process.
Ethylene and other olefins are used as minor comonomers in emulsion polymerization, notably in
Small amounts of
acrylic acidor other ionizablemonomers are sometimes used to confer colloidal stability to a dispersion.
thermaland redoxgeneration of free radicals have been used in emulsion polymerization. Persulfatesalts are commonly used in both initiation modes. The persulfate ion readily breaks up into sulfate radical ions above about 50°C, providing a thermal source of initiation. Redox initiation takes place when an oxidantsuch as a persulfate salt, a reducing agentsuch as glucose, Rongalite, or sulfite, and a redox catalyst such as an iron compound are all included in the polymerization recipe. Redox recipes are not limited by temperature and are used for polymerizations that take place below 50°C.
peroxidesand hydroperoxidesare used in emulsion polymerization, initiators are usually water solubleand partition into the water phase. This enables the particle generation behavior described in the theory section. In redox initiation, either the oxidant or the reducing agent (or both) must be water soluble, but one component can be water- insoluble.
Selection of the correct
surfactantis critical to the development of any emulsion polymerization process. The surfactant must enable a fast rate of polymerization, minimize coagulumor foulingin 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. Anionic, nonionic, and cationicsurfactants have been used, although anionic surfactants are by far most prevalent. Surfactants with a low critical micelle concentration(CMC) are favored; the polymerization rate shows a dramatic increase when the surfactant level is above the CMC, and minimization of the surfactant is preferred for economic reasons and the (usually) adverse effect of surfactant on the physical properties of the resulting polymer. Mixtures of surfactants are often used, including mixtures of anionic with nonionic surfactants. Mixtures of cationic and anionic surfactants form insoluble salts and are not useful.
Examples of surfactants commonly used in emulsion polymerization include
fatty acids, sodium lauryl sulfate, and alpha olefin sulfonate.
Some grades of poly(vinyl 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). It is believed that these polymers graft onto growing polymer particles and stabilize them. [A recent example: Kim, N.; Sudol, E. D.; Dimonie, V. L.; El-Aasser, M. S. "Macromolecules", (2004), 37, 2427.]
Dispersions prepared with such stabilizers typically exhibit excellent colloidal stability (for example, dry powders may be mixed into the dispersion without causing coagulation). However, they often result in products that are very water sensitive due to the presence of the water soluble polymer.
Other ingredients found in emulsion polymerization include chain transfer agents,
buffering agents, and inert salts. Preservativesare added to products sold as liquid dispersions to retard bacterial growth. These are usually added after polymerization, however.
Polymers produced by emulsion polymerization can be divided into three rough categories.
**Some grades of styrene-butadiene (SBR)
**Some grades of
**Some grades of
**Some grades of
**Some grades of
**Acrylonitrile-butadiene-styrene terpolymer (ABS)
*Dispersions (i.e. polymers sold as aqueous dispersions)
** polyvinyl acetate copolymers
** latexacrylic paint
VAE( vinyl acetate- ethylenecopolymers)
International Union of Pure and Applied Chemistry
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