Fundamentals of Polymer Engineering Polymer Engineering

1. Fundamentals of Polymer Engineering
Techniques of Polymerization
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Course Code: PE-3107
Credits: 3-0-3
Teacher: Engr. Asra Tariq
Email Id: asra.tariq@ntu.edu.pk

2. Techniques of Polymerization
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 Polymerization reaction may take place in any of the three states of matter-
solid, liquid, gas. The solid state reactions are usually slow, they are not
considered practical for this reason. The gas-phase reactions do take place
but normally they require maintenance of high temperatures, a condition in
which almost all high molecular weight polymers become unstable. Hence
almost all commercial process of polymerization are liquid phase reactions
LIQUID-PHASE REACTIONS:
Homogenous type
Bulk Polymerization/Mass Polymerization
Solution Polymerization
Heterogeneous type
Emulsion Polymerization
Suspension Polymerization/Pearl/Bead Polymerization

3. Techniques of Polymerization
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 The homogenous or heterogeneous terms refers only to initial conditions of
the system and are not to the subsequent conditions. For example,
Polymerization of vinyl chloride by mass polymerization method is
considered a homogenous type of reaction, because initially the system
is homogenous. But the polymer being incompatible to the monomer phase,
the system subsequently becomes heterogeneous. Hence the concept of
homogeneity or heterogeneity is based on only the initial conditions of the
system.

4. Bulk Polymerization
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 Bulk polymerization is the simplest and involves only the monomer
and a monomer-soluble initiator. The high concentration of
monomer gives rise to high rates of polymerization and high degrees
of polymerization. However, the viscosity of the reaction medium
increases rapidly with conversion (Le. as polymer forms), making it
difficult to remove the heat evolved upon polymerization because of
the inefficient stirring, and leading to auto acceleration. These
problems can be avoided by restricting the reaction to low
conversions, though on an industrial scale the process economics
necessitate recovery and recycling of unreacted monomer.

5. Bulk Polymerization
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 A different complication arises when the polymer is insoluble in its
monomer, (e.g. acrylonitrile, vinyl chloride) since the polymer
precipitates as it forms and the usual kinetics do not apply.
 The principal advantage of bulk polymerization is that it produces
high molar mass polymer of high purity. For example, it is used to
prepare transparent sheets of poly(methyl methacrylate) in a two-
stage process. The monomer is first partially polymerized to yield a
viscous solution which then is poured into a sheet mould where
polymerization is completed. This method reduces the problems of
heat transfer and contraction in volume upon polymerization.

6. Bulk Polymerization
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 Bulk polymerization has several advantages over other methods, these advantages are:
 The system is simple.
 The polymer obtained is pure.
 Large castings may be prepared directly.
 The product obtained has high optical clarity.
 Disadvantages are:
 Heat transfer and mixing become difficult as the viscosity of reaction mass increases.
 The problem of heat transfer is compounded by the highly exothermic nature of free radical
addition polymerization.
 The polymerization is obtained with a broad molecular weight distribution due to the
high viscosity and lack of good heat transfer.
 Gel effect.

7. Bulk Polymerization
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 Polymers that polymerise from bulk polymerization are:
 Polystyrene (PS)
 Polyvinyl chloride (PVC)
 Polymethyl methacrylate (PMMA)
 Polyester
 Low Density Polyethylene (LDPE)

8. Solution Polymerization
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 Many of the difficulties associated with bulk systems can be
overcome if the monomer is polymerized in solution. The solvent
lowers the viscosity of the reaction medium, thus assisting heat
transfer and reducing the likelihood of auto acceleration. However,
the presence of solvent leads to other complications.

9. Solution Polymerization
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 The reduced monomer concentration gives rise to proportionate decreases
in the rate and degree of polymerization. Furthermore, if the solvent is not
chosen with care chain transfer to solvent may
be appreciable and can result in a major reduction in the degree of
polymerization.
 Finally, isolation of the polymer requires either evaporation of the solvent
or precipitation of the polymer by adding the solution to an excess of a
non-solvent. For these reasons, commercial use of solution
polymerization tends to be restricted to the preparation of polymers for
applications which require the polymer to be used in solution.

10. Solution Polymerization
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 Advantages
• The solvent acts as a diluent & helps in facilitating continuous
transfer of heat of polymerization. Therefore temperature control is
easy.
• The solvent allows easy stirring as it decreases the viscosity of
reaction mixture.
• Solvent also facilitates the ease of removal of polymer from the
reactor.
• Viscosity build up is negligible.

11. Solution Polymerization
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 Disadvantages
• To get pure polymer, evaporation of solvent is required additional
technology, so it is essential to separate & recover the solvent.
• The method is costly since it uses costly solvents.
• Polymers of high molecular weight polymers cannot be formed as the
solvent molecules may act as chain terminators.
• The technique gives a smaller yield of polymer per reactor volume, as the
solvent waste the reactor space.
• The purity of product is also not as high as that of bulk polymerization.
Removal of last traces of solvent is difficult.

12. Solution Polymerization
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 Applications
 Widely used for coating and adhesives.
 Industrial production of PAN by free radical polymerization.
 Polyisobutylene by cationic polymerization use this
technique.

13. Suspension Polymerization
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 A better way of avoiding the problems of heat transfer associated with
bulk polymerization on an industrial scale is to use suspension
polymerization. This is essentially a bulk polymerization in which the
reaction mixture is suspended as droplets in an inert medium.

14. Suspension Polymerization
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 The initiator, monomer and polymer must be insoluble in the
suspension medium which usually is water. A solution of initiator in
monomer is prepared and then added to the pre-heated aqueous
suspension medium. Droplets of the organic phase are formed and
maintained in suspension by the use of (i) vigorous agitation
throughout the reaction and (ii) dispersion stabilizers dissolved in the
aqueous phase (e.g. surfactants and/or low molar mass polymers such
as poly(vinyl alcohol) or hydroxymethylcellulose).

15. Suspension Polymerization
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 The low viscosity of the aqueous continuous phase and the high
surface area of the dispersed droplets provide for good heat transfer.
Each droplet acts as a small bulk polymerization reactor for which
the normal kinetics apply and polymer is produced in the form of
beads (typically 0.1-2 mm diameter) which are easily isolated by
filtration provided that they are rigid and not tacky. Thus the reaction
must be taken to complete conversion and normally is not used to
prepare polymers that have low glass transition temperatures. At
high conversions autoacceleration can occur but is better controlled
than in bulk polymerization due to the greatly improved heat
dissipation.
 Suspension polymerization is widely used on an industrial scale (e.g.
for styrene, methyl methacrylate and vinyl chloride), though care has
to be taken to remove the dispersion stabilizers by thorough washing
of the beads.

16. Suspension Polymerization
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 Advantages
 No chain transfer problem
 No mass transfer problem
 No heat transfer problem
 Stirring is easy
 The product obtained is in granular form which is convenient to
handle, isolation is easy

17. Suspension Polymerization
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 Disadvantages
 Only water insoluble monomers are used
 Stabilizers for suspension are used
 Purity is less when compared to polymer that obtained by mass
polymerization
 Yield is low when compared to mass polymerization
 Requires long time for very high conversions

18. Suspension Polymerization
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 Applications
 Expandable polystyrene beads, styrene-divinylbenzene copolymer
beads used in preparation of ion exchange resins are produced.
 PVA beads are produced by this technique using free radical
initiators.

19. Emulsion Polymerization
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 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. Water-soluble polymers, such as
certain polyvinyl alcohols or
hydroxyethyl celluloses, can also be used to
act as emulsifiers/stabilizers.

20. Emulsion Polymerization
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 Common emulsifiers are anionic and nonionic surfactants. Typical
anionic emulsifiers are sodium, potassium, or ammonium salts of fatty
acids and C12 - C16 alkyl sulfates. Typical nonionic surfactants are
poly(ethylene oxide), poly(vinyl alcohol) and hydroxyethyl cellulose. A
combination of both anionic and nonionic surfactants will often
improve the stability of the dispersed droplets.
 Water is normally the continuous phase in which the various
components are dispersed by the emulsifiers. The monomers are only
slightly soluble in water. They form droplets that are suspended and
stabilized by the emulsifiers, that is, the emulsifier molecules associate
and form micelles that surround small amounts of monomer. The
remaining monomer is dispersed in small droplets.

21. Emulsion Polymerization
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 Advantages:
 High molecular weight polymers can be made at fast polymerization
rates. By contrast, in bulk and solution free-radical polymerization, there
is a trade off between molecular weight and polymerization rate.
 The continuous water phase is an excellent conductor of heat, enabling
fast polymerization rates without loss of temperature control.
 Since polymer molecules are contained within the particles,
the viscosity of the reaction medium remains close to that of water and is
not dependent on molecular weight.

22. Emulsion Polymerization
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 Disadvantages:
 Surfactants remain in the polymer or are difficult to remove
 For dry (isolated) polymers, water removal is an energy-intensive
process
 Emulsion polymerizations are usually designed to operate at high
conversion of monomer to polymer. This can result in significant chain
transfer to polymer.
 Can not be used for condensation, ionic, or Ziegler-Natta
polymerization.

23. Emulsion Polymerization
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 Applications:
Emulsion polymerization is one of the most important methods for the
polymerization of a large number of monomers, like vinyl acetate,
vinyl chloride, chloroprene, acrylamide, acrylates, and methacrylates.
It is also used for the production of various copolymers, like
acrylonitrile-butadiene-styrene (ABS).

24. Gas Phase Polymerization
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 Vapour phase polymerization, also
called gas phase or gas fluidized bed
polymerization, is a widely used
polymerization technique for gaseous
monomers such as ethylene (LDPE,
HDPE), tetrafluoroethylene (PTFE), and
vinyl chloride (PVC).
 A highly purified (olefinic) monomer
gas is continuously fed into a fluidized
bed reactor and combined with a dry-
powder catalyst. Polymerization occurs
at the interface between the fluidized
catalyst and the monomer.

25. Gas Phase Polymerization
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 The growing polymer particles sink
downwards and are continuously
removed at the bottom of the reactor and
separated from residual monomer and
pre-polymer which is fed back into the
reactor. The monomer (mixed with
gaseous diluent) is also continuously fed
into the reactor and passes upward in the
reactor through a series of vertical
fluidized bed reaction zones. At the top
of the reactor gas is removed,
compressed and cooled and fed back
into the reactor to control the
temperature.

26. Gas Phase Polymerization
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 Advantages:
 The method has the advantage that it does not require any diluent and
that no residual catalyst remains in the resin granulate or powder which is
continuously removed from the reactor.
 The system does not involve any liquid phase in the polymerization zone
 Disadvantages:
 The reactor operating temperature must be lower than the melting point
of the polymer produced.
 The productivity of catalyst is also limited.
 As polymerization progresses, fine particles will deposit on heat-transfer
surfaces, compressor blades, and sloped walls of the reactor.

27. Gas Phase Polymerization
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 Applications:
 Common thermoplastic polymers can be made by this method. This
includes high volume resins such as propylene (PP), methyl methacrylate
(PMMA), methyl acrylate (PMA), vinyl acetate (PVA), ethylene vinyl
acetate (PEVA) and many other polymers that are stable in the gaseous
phase. On an industrial scale, however, vapour phase polymerization is
mainly used to produce HDPE, LDPE, and PEVA.