Emulsion polymerization

The emulsion polymerization is a method of radical polymerization of monomers in an aqueous phase. The components necessary for emulsion polymerization are water, a water-soluble initiator and the monomers, which must have a low solubility in water. The result is a polymer dispersion ; H. a dispersion of the polymer particles formed from the monomer in water. In practice, surfactants and / or protective colloids are often added to the aqueous phase, which ensure colloidal stability of the dispersion formed, but also influence the particle diameter and the number of polymer particles formed by acting on the particle formation processes. In contrast to suspension polymerization , emulsion polymerization involves particle formation in the aqueous phase. There are several theories for particle formation, the best known are micellar nucleation according to Harkins, Smith and Ewart and homogeneous nucleation according to Fitch and Tsai.

Reaction principle

Reaction principle (see text)

The emulsifier is important for solubilizing the hydrophobic monomer . With the help of such amphiphilic compounds, the stabilization of the monomer in water is made possible. A common emulsifier is e.g. B. SDS ( English sodium dodecyl sulfate , sodium dodecyl sulfate ' ). Furthermore, a water-soluble initiator is used in free-radical emulsion polymerization.

First you dissolve the emulsifier in water. At a certain concentration ( critical micelle concentration ) of the emulsifier, micelles form in which the polymer chains can later grow. The monomer is then added and, at the end, reactive radicals are generated . This can be done by adding a thermally decomposing radical generator (usually peroxides or azo compounds ) and heating the mixture above its decomposition temperature. Alternatively, radicals can also be generated without thermal excitation by photochemical decomposition (e.g. AIBN ) or suitable redox reactions (e.g. between ammonium peroxodisulfate and ascorbic acid ).

First, small polymer radicals are formed in the water phase, which after the addition of some of the poorly water-soluble monomers develop an affinity for the hydrophobic micelle interior and diffuse there , so that the reaction then proceeds there. An initially generated polymer particle can also attach emulsifier and thus form a micelle around it. The micelle wall made of emulsifier basically acts like a membrane, so that further monomers can diffuse into the micelle, whereby the polymerization continues. Further emulsifier molecules are added and allow the micelle wall to grow with it. Provided that there is an excess of initiator, the polymerization does not come to a standstill until the monomer has been completely consumed. Although it is theoretically conceivable that polymerisation also takes place in the monomer droplets in the reaction mixture, their total surface, and thus the probability of the radicals occurring, is several orders of magnitude smaller than that of the micelles. This reaction is very unlikely, since contact with a polymer radical or initiator radical is rare.

kinetics

The kinetics of such processes are complex.

For the gross reaction speed, the following applies approximately to sales below 60%:

${\ displaystyle v_ {gross} = k_ {w} \ cdot 0.5 \ cdot {\ frac {N} {N_ {A}}} \ cdot \ lbrack M \ rbrack}$

With

{\ displaystyle N}

: Number of micelles,

{\ displaystyle N_ {A}}

: Avogadro's constant ,

{\ displaystyle k_ {w}}

: Rate constant of the growth reaction and

{\ displaystyle \ lbrack M \ rbrack}

: Monomer concentration.

The rate of reaction therefore depends on the number of micelles and thus on the emulsifier concentration.

Degree of polymerization

The degree of polymerization can be approximated to . With as the speed of all termination reactions. ${\ displaystyle {\ overline {P}} _ {n} = {\ frac {v_ {gross}} {v_ {st}}}}$ ${\ displaystyle v_ {st}}$

The degree of polymerization can thus be controlled via the number of latex particles and thus via the emulsifier concentration.

Advantages and disadvantages

The advantages of emulsion polymerization are:

The process is carried out in water, organic solvents are not required.
The heat of reaction can be controlled very well by the water phase (lower gel effect )
The viscosity is very low, stirring is made easier.
The dispersion obtained is usually already ready for use.

On the other hand, there are the following disadvantages :

Auxiliaries (e.g. emulsifiers) contaminate the product.

use

This polymerization process is of great industrial importance. For example, PVC , polystyrene , polyacrylate and polyvinyl acetate copolymers as well as fluoropolymers such as PTFE and PFA are produced in this way. PVC is isolated from this by precipitation, while the other polymers are usually used directly in the form of the dispersion obtained, e.g. B. as a binder for aqueous emulsion paints.

literature

Article in ChemgaPedia
Burkart Philipp, Gerhard Reinisch: Fundamentals of macromolecular chemistry . 2nd edited edition. Vieweg, Braunschweig 1976, ISBN 3-528-06811-6 , ( Science series ).

Erich Fitzer, Werner Fritz, Gerhard Emig : Technical chemistry. Introduction to chemical reaction engineering. With 34 calculation examples . 4th completely revised and expanded edition. Springer, Berlin a. a. 1995, ISBN 3-540-59311-X , ( Springer textbook ).
Erik Kissa: Fluorinated Surfactants and Repellents . 2nd Edition. CRC Press, 2001, ISBN 978-0-8247-0472-8 ( Google Books ).

Patent WO2004067588 : Emulsion polymerization of fluorinated monomers. Filed December 5, 2003 , published August 12, 2004 , applicant: 3M Innovative Properties Company, inventor: Lian S. Tan, Richard S. Buckanin. ‌