polymer manufacturing.pdf

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Polymerization techniques

Bulk polymerization Solution polymerization Suspension (pearl) polymerization Emulsion polymerization

Bulk polymerization

Bulk addition polymerization is a homogeneous process which uses an organic initiator Two possibilities:

polymer soluble in monomer (example: polystyrene) polymer insoluble in monomer (example: vinylidene chloride)

polyvinylidene chloride precipitates out of the solution of vinylidene chloride monomerwhen it reaches a certain molecular weight

polystyrene is soluble in styrene to infinite molecular weight If the polymer is soluble in the monomer, then some physical changes occur with

increasing molecular weight (e.g., viscosity, etc.)

The higher the temperature, the lower the molecular weight of the polymer produced. Athigher temperatures, the initiator decomposes to form radicals at a faster rate, then for a

given amount of monomer with more radicals present more polymer chains will be

started (initiated), and the resulting polymers will have a lower molecular weight

Example: If you have 2000 monomer molecules, and 5 chains are initiated, then theaverage molecular weight will be 400, but if 10 chains are initiated, then the average

molecular weight will be 200

We can have continuous polymerization at very low temperatures if we use light toconvert the initiator molecules to radicals (which will start the polymerization)

Bulk polymerization drawbacks

The thermal conductivity of monomers and polymers is low, and as the viscosity buildsup, the ability for heat transfer via convection is substantially diminished. If the heat

energy cannot be dissipated, temperature rises, and at higher temperatures the reaction is

going to go faster, so this is a positive feedback loop with disastrous consequences.


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Removal of unreacted monomer can be a problem.Solution polymerization

The main advantage of a diluent (either water or an organic solvent) is to take up the heatof polymerization. For solution polymerizations, there are two possibilities:

Both the monomer and polymer are soluble in the diluent example: polystyrene in toluene

monomer is soluble and the polymer is insoluble in the diluent example: acrylonitrile in chloroform

The advantage of solution polymerization over bulk polymerization is better heat control;the disadvantage solution polymerization is the removal of the diluent from the polymer.

This requires a distillation, and that costs an appreciable amount of money.

Suspension polymerization

It is used only in free radical type processes The monomer is mechanically dispersed in a media, usually water. There are cases where

an organic media is used in which neither the polymer nor the monomer are soluble in the

organic media.

The initiator used can be water soluble or organic soluble [benzoyl peroxide, AIBN, or(NH4)2(SxO4)y]. Usually the initiator is organic soluble.

There are two separate phases throughout the whole process. The droplets must be kept far apart. This requires agitation: consistent, efficient, and

controlled. A suspending agent can be used. Poly(vinyl alcohol) dissolved in the aqueous

phase is a typical suspending agent.

The rate of suspension polymerization is similar to the rate of bulk polymerization, butthe heat transfer is much better.

Examples include the polymerization of MMA, and vinyl chloride. The medium to monomer ratio is 10:1 Particle size is affect by the following four factors:

stirring rate


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ratio of reactants suspension agent temperature

If the particle size gets to large, the particle will absorb too much heat. Particle size may be 0.01 to 0.5 cm, or as low as 1 micron. Asuspension agent is a material that gives a surface activation that keeps droplets from

become larger (droplets coming together to form larger droplets is calledcoalescence)

Suspension polymerization is similar to bulk polymerization, and it could be considered"bulk polymerization within a droplet."

Limitations of suspension polymerization

It only applies to free radical process Ionic catalysts don't work because they compete with water Agitation is critical because as the viscosity within the bead rises, the reaction rate

increases suddenly (Tromsdorff effect.) This leads to a surge in heat generation which

does not usually occur in solution or emulsion polymerization

Emulsion polymerization

The definition for emulsion is: "A stable colloidal suspension as milk, consisting of animmiscible liquid dispersed and held in another liquid by a substance called an

emulsifier".

Things to consider monomer media (water) soap (also referred to as the emulsifier, or the surfactant) minor amount of mechanical stirring initiator stabilizer (suspension agent)

The basic difference between the suspension process and the emulsion process is thesurfactant


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What is a Colloid?

What stops colloidal particles joining together and settling out?


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Brownian Motion Surfactants Brownian Motion stops particles settling

can see under a microscopemotion the result of:random (thermal) motions of water moleculescollisions of molecules with the larger particle

Surfactants (soaps) stop particles joining together like dissolves like

water-hating tails go into water-hating polymer water-loving heads go into water

form a stabilised latex e.g. with SDS, latex surface covered in negatively charged

surfactant head groups

charge repulsion of ionic head groups stops coalescence Making a latex (e.g. polyvinyl acetate):

chemicals requireddispersion medium water monomer vinyl acetate initiator potassium persulfate soap SDS

preparation (emulsion polymerisation) mix all chemicals warm in water bath (60 0C) stir for several hours (overnight)


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Ingredients Function during orafter

polymerization

Types in current use Effect on polymer

Emulsifier

system

Catalyst

system

(redox

type)

Modifier

Short-stop

system

Stabilizer

Coagulant

system

To keep monomers

and polymer in

emulsion

Initiator and

activator

Regulation of chain

length; prevention

of branching

To stop the reaction

at the required levelof conversion

Antioxidant

Flocculation of the

latex

Fatty acid

Organic hydroperoxide-

FeSO4

Tertiary dodecyl

mercaptan

Soluble

dithiocarbamates,sodium polysulfide, or

polyamine H

Staining or non-staining

1. salt-acid2. Acid-alum3. Glue-acid4. Glue-alum

No tack, fast curing,

non-staining

Broad mol.wt distribution improves

processing

Cure time is affected

due to residual short-stop

Stabilize the polymer; affects cure

1. Polymer slightly hygroscopic

2-4. improved electrical properties


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For an emulsion polysurfactant.

The initiator can be eibetween emulsion and

Initially, a small amou The hydrophilic (pola

placed on the inside.

erization process the small droplets are stabi

her in the aqueous or the organic phase. This

suspension polymerization.

nt of agitation is needed. The surfactant is in

) end groups lineup on the outside and the h

lized by the

is a difference

the form of micelles.

drophobic ends are


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