Emulsion polymerization

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Emulsion polymerization is a type of polymerization that usually starts with an emulsion incorporating 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[1]:

  • High molecular weight polymers 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.
  • Since polymer molecules are contained within the particles, viscosity remains 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 adjuvants remain 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.


Contents

[edit] History

Emulsion polymerization was developed in the 1920's and 1930's.

[edit] Theory

Almost all emulsion polymerizations use a radical polymerization method. Non-radical polymerization methods have been investigated, but have not shown significant benefits.

The theory of emulsion polymerization was largely developed by Smith and Ewart,[2] and Hawkins[3] in the 1940's, based on their studies of polystyrene. Smith and Ewart arbitrarily divided the mechanism of emulsion polymerization into three stages. Subsequently it has been recognized that not all monomers or systems undergo these particular three stages. Nevertheless, the Smith-Ewart description is a useful starting point to analyze emulsion polymerizations.

Schematic diagram of emulsion polymerization
Enlarge
Schematic diagram of emulsion polymerization

The Smith-Ewart-Harkins theory for the mechanism of free-radical emulsion polymerization is summarized by the following steps:

  • Surfactants emulsify the monomer in a water continuous phase.
  • 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 polymerization where an oil-soluble initiator dissolves in the monomer, followed by polymer formation in the monomer droplets themselves.) This is considered Smith-Ewart Stage 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 Stage 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 Stage 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'.

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 kinetics and particle formation kinetics, quantitative understanding of the mechanism of emulsion polymerization has required extensive computer simulation. Recent theory has been summarized.[4]

[edit] Process

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 control schemes have enabled the development of complex reaction processes, with ingedients such as initiator, monomer, and surfactant added at the beginning, during, or at the end of the reaction.

Early SBR recipes 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-fed reaction 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, acidification will destabilize a dispersion with a carboxylic acid surfactant. These techniques may be employed in cominbation 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.

[edit] Ingredients

[edit] Monomers

Typical monomers are those that undergo radical polymerization, are liquid or gaseous at reaction conditions, and are poorly soluble in 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.

Ethylene and other simple olefins must be polymerized at such a high pressure that emulsion polymerization is not economically feasible.

[edit] Comonomers

Copolymerization is common in emulsion polymerization. The same rules and comonomer pairs that exist in bulk polymerization operate in emulsion polymerization. There is a significant difference

[edit] Initiators

A typical initiator is potassium persulfate. The persulfate ion breaks up into sulfate radical ions at about 50 °C.

[edit] Surfactants

[edit] Non-surfactant stabilizers

[edit] Other ingredients

[edit] Applications

Polymers produced by emulsion polymerization can be divided into three rough categories.

[edit] Notes

  1. ^ Odian, G, Principles of Polymerization, Wiley, New York
  2. ^ Smith, W. V.; Ewart, R. H. J. Chem. Phys., (1948), 16, 592.
  3. ^ Hawkins, W. D. J. Am. Chem. Soc., (1947), 69, 1428.
  4. ^ Gilbert, R. G. Emulsion Polymerization: a Mechanistic Approach Academic Press, London, 1996.

[edit] See also

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