Emulsion polymerization
|
|
Emulsion polymerization is a type of polymerization that takes place in an emulsion typically 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 polyvinylalcohols or hydroxyethyl celluloses, can also be used to act as emulsfiers/stabilisers.
Some of the popular emulsions available in the market include Polyvinyl acetate homopolymers and copolymers, styrene butadiene latex, and acrylic emulsions. 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 VOC(Volatile Organic Compounds) in them.
Advantages of emulsion polymerization include:
- 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.
Most emulsion polymerizations use a free-radical polymerization method. Emulsion poymerization can be carried out as a batch reaction, but in many cases is performed as a starve-fed reaction to insure a good distribution of monomers into the polymer backbone chain.
The leading theory for the mechanism of starve-fed, free-radical emulsion polymerization is summarized by the following:
- 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.
- Initiator (water-soluble and introduced into the water phase) reacts with monomer in the micelles
- The micelles in total, comprise a much larger surface area in the system than the fewer, larger monomer droplets, which is why the initiator typically reacts with the micelle and not the monomer droplet.
- Monomer in the micelle quickly polymerizes and the growing chain terminates.
- More monomer from the droplets diffuses to the growing micelle/particle, where more initiators will eventually react.
- Monomer droplets and initiator are continuously, and slowly added to maintain their levels in the system as the particles grow.
- When the monomer droplets have been completely consumed, the initiator is typically added in for a little while longer to consume any residual monomer.
- The final product is a 'disperion' of polymer particles in water, it can also be known as a polymer colloid, a latex, or commonly and inaccurately as an 'emulsion'. Inaccurate because the final form is no longer an oil-phase solubilised in a continuous phase, but rather finely dispersed discrete solid polymer particles.