Emulsion polymerization

The emulsion polymerization is a method of radical polymerization of monomers in an aqueous phase. The need for an emulsion polymerization components are water, a water-soluble initiator and monomers must have a low water solubility. The result is a polymer dispersion that is a dispersion of polymer particles formed from the monomer in water. In the practice of the aqueous phase still often surfactants and / or protective colloids may be added which provide colloidal stability of the dispersion formed, but also by acting on the Teilchenbildungsprozesse affect the particle diameter and the number of polymer particles formed. In contrast to the suspension polymerization takes place in the emulsion particle formation in the aqueous phase instead. For the particle several theories that are known to exist, the micellar nucleation Harkins, Smith and Ewart and the homogeneous nucleation according to Fitch and Tsai.

General

Important for the solubilization of the hydrophobic monomer is the emulsifier. With the help of such amphiphilic compounds, the stabilization of the monomer is made possible in water only. A common emulsifier, for example, SDS ( sodium dodecyl sulfate engl. = sodium dodecyl sulfate). Further be used in the free radical emulsion polymerization, a water-soluble initiator. This polymerization is industrially very important. For example, PVC, polystyrene, polyacrylate and polyvinyl acetate copolymers are produced in this way. The former is from this isolated by precipitation, while the other polymers are usually used directly in the form of the dispersion obtained, for example, as binders for aqueous emulsion paints.

Reaction Principle

First you cut the emulsifier in water. At a certain concentration (CMC critical micelle concentration ) of the emulsifier form of micelles, where later the polymer chains can grow. You are still the monomer to the end and generates reactive radicals. This can be done in that a thermally decomposing radical generator (usually peroxides or azo compounds ) is added and the mixture heated over its decomposition temperature. Alternatively, one can create (e.g., between ammonium peroxodisulfate and ascorbic acid ) radicals or without thermal excitation, by photochemical decomposition (for example, azobisisobutyronitrile, AIBN) or suitable redox reactions. Initially, small polymer radicals form in the aqueous phase, which develop an affinity for the hydrophobic Mizellinneren after the attachment of some of the poorly water- soluble monomers and then diffuse, so that the reaction then progresses there. Also can attach and so around them form a micelle, an initially produced polymer emulsifier. The Mizellenwand of emulsifier works in principle like a membrane, so that additional monomers can diffuse into the micelle, so that the polymerization progresses. More emulsifier draw near and let the Mizellenwand grow as it were. With the proviso that the initiator is present in excess, polymerization comes to a standstill only when the monomer is completely consumed. While it is theoretically conceivable that in the monomer droplets in the reaction mixture undergo polymerization, however, the concentration of which is in contrast to the micelles by a few orders of magnitude smaller. Thus, this reaction is very unlikely since there is rarely a contact of a polymer radical or radical initiator.

Kinetics

The kinetics of such processes is complicated. The following approximation is valid at conversions below 60%: The overall rate of reaction is given by:

.

By: Micellenzahl, : Avogadro's number, : rate constant of the propagation reaction, : monomer concentration.

Thus, the reaction rate depends on the Mizellenzahl and thus of the emulsifier.

Degree of polymerization

Approached the degree of polymerization can be specified. With than the speed of all termination reactions.

Thus, the polymerization degree of the number of latex particles and thus of the emulsifier is controlled.

Pros and Cons

The advantages of emulsion polymerization are:

• The litigation takes place in water, organic solvents are not needed.
• The heat of reaction can be very well controlled by the water phase (lower gel effect )
• The viscosity is very low, stirring is facilitated.
• The resulting emulsion is usually ready for use.

This contrasts with the following disadvantages:

• Excipients ( eg emulsifiers) contaminate the product.