Polymer Reaction Engineering

Polymerization Techniques

Bulk

Solution

Suspension

Emulsion

Interfacial Polymerization

Solid-State Gas-Phase Plasma Polymerization in Supercritical Fluids

Bulk Polymerization

Monomer, Monomer-soluble initiator, optional chain transfer agent (rxn control)

Advantages Simplest polymerization technique

High yield and highest purity

Easy polymer recovery

Direct processing (casting) possibility

Limitations Difficulty in removing residual monomer

Heat Dissipation esp. towards the end of rxn.

Used for free radical polymerization (PS, PMMA) and some step-growth (condensation) polymerizations

1.Styrene is polymerized at 80 °C to %30-

35% monomer conversion in a stirred

reactor known as prepolymerizer.

2. The resulting reaction mass — a viscous

solution or syrup of polymer in monomer —

subsequently passes down a tower with

increasing temperature. The increasing

temperature helps to keep the viscosity

at manageable levels and also enhances

conversion, which reaches at least 95% at

the exit of the tower.

3. By removal of the heat of polymerization

at the top of the tower and proper

temperature control of the finished polymer

at the bottom of the tower, an optimum

molecular weight may be achieved

and channeling of the polymer may be

minimized.

Gas Phase Polymerization

Unipol Process gaseous ethylene or propylene, Fluidized bed reactor with solid catalyst. PE, PP and PP copolymers

Solution Polymerization Organic Solvent (or Water) Solvent soluble Monomer and Initiator. (solvent choice is

important )

Advantages Better heat removal The efficiency of the catalyst is sustained and the removal of catalyst residues from the

polymer is simplified. Mixing is facilitated as solvent reduces the rate of increase of the reaction medium

viscosity as the reaction progresses.

Limitations Solvent recovery, solvent handling and separation of polymer and unreacted monomers Limited solubility of polymers, particularly at higher molecular weights. Lower yield per reactor volume, reduced reaction rate and average chain length Necessity of selecting an inert solvent ( to eliminate the possibility of chain transfer)

Used mainly for ionic polymerization and coordination polymerization.(high density polyethyelene (HDPE), polybutadiene, butyl rubber)

Limited commercial utility in free radical polymerization

Suspension Polymerization

Water, Insoluble/Suspended Monomer, Initiator, Optional Chain transfer agent, Agitator.

Advantages

Good heat removal

Easier solvent recovery

Low molecular weight distribution

Limitations

Polymer purity is low (suspending and stab. additives)

More expensive reactor

Used mainly for free radical polymerisation of styrenic-ion exchange resins (PVC, SAN)

Suspension Polymerization

1. Water insoluble monomer.

2. Water insoluble initiator.

3. Suspending agent (optional).

4. Droplets are 50-200 m diameter.

"Mini reactors.“

Emulsion Polymerization

Monomer(s), dispersing medium (water), water-soluble initiator, surfactant (such as sodium salt of a fatty acid) and possibly, a transfer agent.

Advantages

Good heat removal

High polymerization rate

High moleculer weight polymer

Polymer/Latex can be used directly or can be recovered by coagulation with acids

Disadvantages.

Polymer purity is low (emulsion surfactant)

More expensive reactor

Used mainly for paints

Above a critical concentration of emulsifying agent known as the critical micelle

concentration (CMC), only a small fraction of the emulsifying agent is dissolved in

the water.

Surfactants

Emulsion Polymerization

1. Water insoluble monomer.

2. Water soluble initiator.

3. Surfactant (except in special

cases).

4. Complicated mechanism.

Representation of Ideal Stages of Emulsion Polymerization

(a) prior to initiation;

(b) polymerization stage 1; shortly after initiation;

(c) polymerization stage 2; all emulsifier micelles consumed;

(d) Polymerization stage 3; monomer droplets disappear; and

(e) end polymerization

(-O) emulsifier molecule; (M) monomer molecule; (P) Polymer molecule; and (R·) a free radical

Stages in an Emulsion Polymerization

Interfacial Polymerization

H2N (CH2)6 NH2+ Cl C (CH2)8

O

C Cl

O

hexamethylene diamine

-(2n-1) HCl

H NH (CH2)6 NH C (CH2)8

O

C Cl

O

n

Nylon 6,10

sebacoyl chloride

Solid State, Plasma and Supercritical Fluid Polmerization

Solid State Polymerization

Polymerization by heating and irradiation (visible, X-ray, UV or - rays) of monomers in their crystalline

phase (PET, POM)

Plasma Polymerization

Polymerization by low pressure glow discharge of positively and negatively charged species, electrons,

excited and neutral species, and electromagnetic deposition (Graft copolymers or thin polymer film

deposition on a metal or other substrate)

Polymerization in Supercritical Fluids

SCF is a fluid bv. critical pressure and temperature=>liquid and gas properties such as high diffusivity, low viscosity and liquid-like densities

Rate of FRP increases with increasing pressure, therefore high molecular weight polymers can be obtained by SCFs.

Example: FRP of styrene, vinyl acetate, acrylonitrile, methyl methacrylate

Fluoropolymers)

Monomer

Polymer

Processing

operations

Final product

Polymerization

polymerization

and forming

Main operations

Extrusion

Injection molding

Compression molding

Transfer molding

Blow molding

Rotational molding

Thermoforming

Calendering

Film blowing

Fiber spinning, …

Compound

Compounding

Polymer processing operations

Soften and Form

Thermoplastics are heated until they flow then forced by pressure to form a finished part.

Resin Heat Melt Tool Part

Conversion Processes

Extrusion Blow Molding Injection Molding

Extrusion

Extrusion is a processing technique for converting thermoplastic materials in

Powdered or granular form into a continuous uniform melt, which is shaped into

Items of uniform cross-sectional area by forcing it through a die.

Extruder

D

Design parameters:

Screw diameter

Helix angle

Length (L / D)

Compression ratio

Channel depth

Single screw extruder Twin screw extruder

Extruder Design Parameters

Uniform Profiles and Indefinite Length

The plastic flow is extruded through a die using mechanical back pressure to form a part with a uniform profile that can be cut to any

desired length.

Molded products

Injection molding is one of the processing techniques for converting

thermoplastics, and recently, thermosetting materials, from the pellet or

powder form into a variety of useful products.

The melt is then injected into and held in a cooled mold under pressure until

the material solidifies.

The mold opens and the product is ejected.

The injection molding machine must, therefore, perform essentially three

functions:

1. Melt the plastic so that it can flow under pressure.

2. Inject the molten material into the mold.

3. Hold the melt in the cold mold while it solidifies and then eject the solid

plastic.

Injection molding

*

Schematic of thermoplastic Injection molding machine

Injection molding cycle

Thermoplastics: cooling in the mold

Thermosets: curing, crosslinking in hot mold

Injection Cycle

A parison is a hollow tube of softened plastic. Its shape is determined by screw speed and/or the die opening.

Hollow Parison

Extrusion blow molding

Injection Blow Molding

Thermoforming is a process for forming moderately complex shaped parts that cannot

be injection molded because the part is either very large and too expensive or has very

thin walls.

It consists essentially of two stages: elevation of the temperature of a thermoplastic

sheet material until it is soft and pliable and forming the material into the desired shape

using one of several techniques.

a) Vacuum forming b) Mechanical forming c)Air blowing process

Vacuum forming

Thermoforming

Thermoforming

Thermosets

Hot mold walls,

polymerization and

crosslinking

Simpler molds with

geometric limitations

Slower process

From small objects (a few grams) to

Vehicle body parts, giant tires

Compression molding

Extension of compression molding, intermediate stage to injection molding

Charge is melted in a separate pot and transferred into the cavity by a ram

Multi-impression molds

Transfer molding