Techniques of Polymerization

Fundamentals of Polymer Engineering
Polymer Engineering

1. Fundamentals of Polymer Engineering Techniques of Polymerization 3/13/2019 1 Course Code: PE-3107 Credits : 3-0-3 Teacher: Engr. Asra Tariq Email Id: [email protected]

2. Techniques of Polymerization 3/13/2019 2 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 3/13/2019 3 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 3/13/2019 4 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 3/13/2019 5 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 3/13/2019 6  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 3/13/2019 7 Polymers that polymerise from bulk polymerization are: Polystyrene (PS) Polyvinyl chloride (PVC) Polymethyl methacrylate (PMMA) Polyester Low Density Polyethylene (LDPE)

8. Solution Polymerization 3/13/2019 8 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 3/13/2019 9 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 3/13/2019 10 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 3/13/2019 11 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 3/13/2019 12 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 3/13/2019 13 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 3/13/2019 14 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 3/13/2019 15 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 3/13/2019 16 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 3/13/2019 17 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 3/13/2019 18 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 3/13/2019 19 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 3/13/2019 20 Common emulsifiers are anionic and nonionic surfactants. Typical anionic emulsifiers are sodium, potassium, or ammonium salts of fatty acids and C 12  - C 16 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 3/13/2019 21 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 3/13/2019 22 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 3/13/2019 23 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 3/13/2019 24 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 3/13/2019 25 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 3/13/2019 26 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 3/13/2019 27 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.