Emulsion Polymerization

EMULSION POLYMERIZA

Emulsion polymerization is a type of radical 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.

Surfactant for emulsion polymerization

Components: I I M

M M M

M M

M M

M

I M M M

M

.

M

M

R

M

M M

Micelle

Initiator

Free radical

Surfactant

Monomer droplet

M

History The first successful theory to explain the distinct features of emulsion polymerization was largely developed by Smith and Ewart, and Harkins in the 1940s, based on their studies of polystyrene. Smith and Ewart arbitrarily divided the mechanism of emulsion polymerization into three stages or intervals.

1)A monomer is dispersed or emulsified in a solution of surfactant and water forming relatively large droplets of monomer in water. Excess surfactant creates micelles in the water.

3)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.

INTERVAL 1

INTERVAL 2

INTERVAL 3

Free Monomer droplet

Monomer droplet

Surfactant Initiator Iatex particle micelle

Emulsion Polymerisation: Three intervals

Monomer Conversion

1

2

3 1. 2. 3.

time

Particle formation Particle growth at constant rate No monomer droplets

Why does polymerization happen in the particle rather than the droplet? Large polymer particles have a much lower surface area to volume ratio than the small droplets. The probability of a free radical entering a droplet versus a micelle particle is very small.

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. 2. 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. 3. Since polymer molecules are contained within the particles, viscosity remains close to that of water and is not dependent on molecular weight. 4.The final product can be used as is and does

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

application Polymers produced by emulsion polymerization can be divided into three rough categories. Synthetic rubber: Some grades of styrene-butadiene (SBR), Some grades of Polybutadiene ,Polychloroprene (Neoprene) ,Nitrile rubber ,Acrylic rubber , Fluoroelastomer (FKM) Plastic: Some grades of PVC ,Some grades of polystyrene ,Some grades of PMMA ,Acrylonitrile-butadienestyrene terpolymer (ABS) ,Polyvinylidene fluoride , PTFE

Dispersions (i.e. polymers sold as aqueous dispersions):

POLYMERS PRODUCED USING EMULSION PROCESSES