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Chemical Engineering 160/

Polymer Science and Engineering

Lecture 13:

Polymerization Techniques -

Dispersed Systems

Objectives

!^!

To outline polymerization techniques and describeTo outline polymerization techniques and describeapproaches to reducing viscosity and improving thermalapproaches to reducing viscosity and improving thermalcontrol that involve dispersion of the monomer in water.control that involve dispersion of the monomer in water. !^!

To show how dispersed systems may be stabilized fromTo show how dispersed systems may be stabilized fromaggregation by modification of the hydrocarbon/wateraggregation by modification of the hydrocarbon/waterinterface.interface.

Bulk (Mass) Polymerization

•^

Carried out to high conversion

-^

Free radical kinetics apply

-^

Used for ethylene, styrene, methylmethacrylate

Advantages:

•^

Minimum contamination of product

General Description:

MonomerInitiator

Bulk (Mass) Polymerization

Disadvantages:

•^

R

decreases at high conversion due to the Trommsdorft^

effect, making the reaction hard to control. •^

R

p^

and

ν

increase as conversion increases, broadening

the molecular weight distribution. •^

Viscosity increases as conversion increases, making heat removal and processing more difficult. Process schemes:

•^

Keep to low conversion; separate and recycle unreactedmonomer •^

Stagewise polymerization; achieve low conversion in largereactor, then prepare slabs or films

Solution Polymerization

Advantages:

•^

Solvent acts as a diluent and aids in removal of heat of polymerization. •^

Solvent reduces viscosity, making processing easier.

-^

Thermal control is easier than in the bulk.

Disadvantages:

•^

Chain transfer to solvent occurs, leading to low molecular weights. •^

Difficult to remove solvent from final form, causing degradation of bulk properties. •^

Environmental pollution due to solvent release.

Suspension (Bead or Pearl) PolymerizationGeneral Description

Monomer

Inorganic StabilizerOil-soluble Initiator

Water

•^

Must have very low monomer solubility in water or polymerwill form in aqueous phase.

•^

Used for styrene, methyl methacrylate, vinyl chloride, vinyl acetate

•^

Kinetics

-^ Droplets are0.001-1 cm indiameter. are the sameas in the bulk.

Emulsion Polymerization: Overview

General Description:

MonomerSurfactant

Water-soluble Initiator

Water

•^

Surfactant is aggregated in micelles. •^

Monomer is stabilized by surfactant and dispersed inwater. •^

Predominant process for vinyl acetate, chloroprene,butadiene/styrene/acrylonitrilecopolymers, various acrylates. •^

Used somewhat for methyl methacrylate, vinyl chloride,vinylidene chloride, styrene.

Typical Emulsion Composition

Component

Parts by weight

Styrene

Butadiene

Water

Emulsifier (surfactant

N-Dodecyl mercaptan

NaOH

Cumene hydroperoxide

FeSO

4

Na

P 4

O 2

-10 H 7

O 2

Fructose

Data from G. Odian, Principles of Polymerization, 3rd Ed., 1991, p 336.

Emulsion Components: Surfactant

•^

Surfactant (emulsifier) molecules have polar or ionic head groups and hydrocarbon tails of 10-20 carbons;dodecyl sulfate ions are typical. •^

Surfactants aggregate at the hydrocarbon/water interface and are in equilibrium with free surfactant.

HydrocarbonWater

Micelle Formation

•^

Above the

critical micelle

concentration (CMC),

there

is an equilibrium involvingclusters of surfactants, whichare typically spherical (20 to100 Å in diameter) and whichcontain 50 to 150 molecules. •^

The hydrocarbon core of the micelle will become swollenby monomer molecules.

Emulsion Components:Site of Polymerization

•^

Radicals are produced in the aqueous phase at approximately 10

13

radicals/cm

3 sec.

•^

Polymerization in the aqueous phase is insignificant due to the low monomer concentration. •^

Polymerization mainly occurs in the micelle interiors due to:

•^

high monomer concentration

-^

high surface/volume ratio

-^

presence of interface for organic monomer andwater-soluble initiator

•^

During polymerization, monomer is replenished by diffusion from droplets through the solution to micelles. Surfactant isthen redistributed, with more adsorbed on swollen polymerparticles and less on monomer droplets

Emulsion Components: Particles

•^

Three types of particles exist:

•^

monomer droplets

-^

inactive micelles (no polymerization occurring)

-^

active micelles (swollen polymer particles inwhich polymerization is occurring)