Polymer Reaction Engineering
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