By: Muhammad Haseeb Iqbal National Textile University Week # 06 Polymerization Techniques Polymer Science and Engineering (TM-2052) By: Muhammad Haseeb Iqbal National Textile University

Concepts to be covered Pros and cons of each technique Techniques used for polymerization of polymers like Bulk polymerization Solution polymerization Suspension Polymerization Emulsion Polymerization Precipitation Polymerization Pros and cons of each technique

Sometimes for one monomer several techniques of polymerizing are available. Choice of a specific technique depends on a number of factors Kinetic / mechanistic factors related to chain length, chain composition Technological factors e.g. heat removal, reaction rate, viscosity of the reaction mixture, morphology of the product Economic factors; production costs, environmental aspects, purification steps etc. 3

Polymerization in Homogeneous systems Monomers may be polymerized by the following methods Polymerization in homogeneous systems Polymerization in heterogeneous systems Polymerization in Homogeneous systems The homogeneous polymerization techniques involve pure monomer or homogeneous solutions of monomer and polymer in a solvent. These techniques can be divided into two methods: Bulk polymerization Solution polymerizations

Bulk polymerization Bulk polymerization is the simplest technique and produces the highest-purity polymers Only monomer, a monomer-soluble initiator are used This method helps easy polymer recovery and minimum contamination of product The viscosity of the mixture is low initially to allow ready mixing heat transfer, and bubble elimination, this method is used for the preparation of polyethene, polystyrene, etc.

Y FOR FREE RADICAL MOSTLY? These are highly exothermic so increasing temperature causes additional heat to dissipate and increase in rate of polymerization. At the end of polymerization heat removal becomes particularly difficult because viscosity of polymer becomes very high. This is because high viscosity limits the diffusion of long chain radicals as requires for the termination. It means radical conc. will increase and hence rate of polymerization also increases. Since diffusion of small monomer molecules to propagation sites is less restricted so propagation rate will increase while termination will decrease. This also increases heat production. This auto acceleration is called as Trommsdorff effect or gel effect or auto- acceleration.

TROMMSDORFF EFFECT In radical polymerization we speak about: 1) low conversion, i.e. polymer chains are in dilute solution (no contact among chains) 2) “intermediate” conversion, i.e. the area in between low and high conversion high conversion, i.e. chains are getting highly entangled. Somewhere in the “intermediate” conversion regime: * polymer chains loose mobility. * Termination rate decreases * Radical concentration increases * Rate of polymerization increases * Molar mass increases This effect is called: gel effect, Trommsdorff effect, or auto-acceleration In the polymerization of MMA this occurs at relatively low conversion.

Advantages Advantage of the high concentration of monomer result in 1. High rates of polymerization 2. High degree of polymerization 3. High purity of product 4. High molar mass polymer are produce

Disadvantages Reaction medium becomes increasingly viscous as reaction goes to higher conversion, making stirring, heat removal and processing more difficult It leads to uneven polymerization and loss of monomer, Free-radical polymerizations are typically highly exothermic Near the end of polymerization, the viscosity is very high and difficult to control the rate as the heat is “trapped” inside It leads to the auto acceleration process in which the propagation rate is very higher than that of termination rate, this method is seldom used in commercial manufacture

Solution polymerization This method is used to solve the problems associated with the bulk polymerization because the solvent is employed to lower the viscosity of the reaction, thus help in the heat transfer and reduce auto acceleration It requires the correct selection of the solvents. Both the initiator and monomer be soluble in each other and that the solvent are suitable for boiling points, regarding the solvent-removal steps It is often used to produce copolymers, this method is used for the preparation of polyvinyl acetate, poly (acrylic acid), and polyacrylamide

CHOICE OF SOLVENT Both initiator and monomer be soluble in it Solvent should have suitable melting and boiling points for the conditions of polymerization and subsequent removal. Flash point, cost and toxicity may influence the choice of solvent. Aliphatic and aromatic hydrocarbons, esters, ethers, and alcohols.

Advantages Solvent has low viscosity, reaction mixture can be stirred Solvent acts as a diluent and aids in removal of heat of polymerization Solvent reduces viscosity, making processing easier Thermal control is easier than in the bulk and Cheap materials for the reactors (stainless steel or glass lined)

Disadvantages Reduce monomer concentration which results in decreasing the rate of the reaction and the degree of polymerization Solvent may terminate the growing polymer chain, leading to low molecular weight polymers Difficult to remove solvent from final form, causing degradation of bulk properties Clean up the product with a non solvent or evaporation of solvent Small production per reactor volume Not suitable for dry polymers

Polymerization in heterogeneous systems Polymerization occurs in disperse phase as large particles in water or occasionally in another non-solvent (suspension polymerisation), or dispersed as fine particles The last-named process is usually known as emulsion polymerisation

Suspension (Bead or Pearl) polymerisation Monomer, initiator (must soluble in monomer) and polymer must be insoluble in the suspension media such as water i.e., the reaction mixture is suspended as droplets in an inert medium Suspension polymerization consists of an aqueous system with monomer as a dispersed phase and results in polymer as a dispersed solid phase This method is used for the preparation of polystyrene, polyvinyl chloride, polyvinyl acetate, PMMA etc.

A reactor fitted with a mechanical agitator is charged with a water insoluble monomer and initiator Droplets of monomer (containing the initiator) are formed, as the polymerization proceeds, the viscosity of dispersed phase increases and they become sticky Aggregation of these sticky droplets is prevented by the addition of a dispersing agent (protective colloid, e.g., water-soluble colloid such as gum acacia) Near the end of polymerization, the particles are hardened, are the bead or pearl shaped polymers recovered by filtration, and followed by washing step

Reaction Conditions The reaction mixture consists of two phases, a liquid matrix and monomer droplets. The monomer and initiator are insoluble in the liquid phase, so they form drops within the liquid matrix. A suspension agent is usually added to stabilize the monomer droplets and hinder monomer drops from coming together. The reaction mixture usually has a volume ratio of monomer to liquid phase of 0.1 to 0.5. The liquid phase acts as a heat transfer agent, enabling high rates of polymerization with little change in the temperature of the polymerizing solution.

Stirring Importance The reactions are usually done in a stirred tank reactor that continuously mixes the solution using turbulent pressure or viscous shear forces. The stirring action helps to keep the monomer droplets separated and creates a more uniform suspension, which leads to a more narrow size distribution of the final polymer beads.

Droplet Size The size of the monomer droplet formed depends upon the Monomer to water ratio The type and concentration of stabilizing agent Type and speed of agitation employed Product is obtained as spherical beads or pearls Coagulation of these sticky droplets is prevented by the addition of protective colloids

Initiator (monomer soluble) Water Protective colloid + Initiator Protective Colloid

Advantages Polymerisation to high conversion Low viscosity due to the suspension Easy heat removal due to the high heat capacity of water Excellent heat transfer because of the presence of the solvent Solvent cost and recovery operation are cheap Polymerization yields finely divided, stable latexes and dispersions to be used directly in coatings, paints, and adhesives

Disadvantages Contamination by the presence of suspension and other additives low polymer purity Must separate and purify polymer, or accept contaminated product Reactor cost may higher than the solution cost

Emulsion polymerisation An emulsion polymerization consists of Water (solvent or as the heat-transfer agent) Monomer Initiator (is soluble in water and insoluble in the monomer) A surfactant or emulsifier (such as sodium salt of long-chain fatty acid) This method is used for the preparation of polyvinyl acetate, polychloroprene, butadiene/styrene/acrylonitrile copolymers, etc.

Critical micelle concentration(CMC) The highest concentration of surfactants where in all the molecules are in the dispersed state, or the concentration beyond which only micelle formation is possible is known as critical micelle concentration Introduction to polymer science and engineering

Above a certain surfactant concentration, the critical micelle concentration, residual surfactant molecules can align to form micelles( aggregate of molecule ), which are small rod like structures that contain b/w 50 to 100 surfactant molecules. Initiator must be soluble in water (difference b/w emulsion and suspension)

Emulsifier or surfactants are made up of two parts a long non polar hydrocarbon chain to which is attached a polar group such as –COONa,-SO3NA,- NH2HC In micelle formation, the emulsifier molecule aggregate in such a way that the polar end of the molecule align themselves outward and the hydrocarbon ends come close to each other at the interior

Monomer(s) - Water insoluble Initiator(s) - Water soluble Surfactant(s) Water

When a small amount of soap is added to water, the soap ionizes and the ions move around freely, the soap anion consists of a long oil-soluble portion (R) terminated at one end by the water-soluble portion, so emulsifier molecules arrange themselves into colloidal particles called micelles In water containing a insoluble monomer molecule, the soap anion molecules orient themselves at the water–monomer interfaces with the hydrophilic ends facing the water, while the hydrophobic ends face the monomer phase

When the water-soluble initiator undergoes thermal decomposition to form the water-soluble radicals react with monomer dissolved in interior of the micelle Emulsion polymerization takes place almost exclusively in the micelles As polymerization proceeds, the active micelles consume the monomers within the micelle, monomer depletion within the micelle is replenished first from the aqueous phase and subsequently from the monomer droplets The active micelles grow in size with polymer formation, to preserve their stability; these growing polymer particles absorb the soap of the parent micelles

Advantages Disadvantages Overcomes many environmental problems: “solvent” is water If final desired product is polymer is washed with water to remove the soap phase by coagulation Latex may be directly used 100% conversion with high molecular weight at high rates and small particle size can be obtained Disadvantages Emulsifier, surfactants and coagulants must be removed High residual impurity may affect certain polymer properties, high cost, washing and drying.

Surfactant / Emulsifier Its actually the surface active agents The surfactants can be anionic, cationic and non-ionic Surfactant serve the purpose of lowering the surface tension at the monomer water interface and facilitate emulsification of the monomer in water

A micelle forming emulsifier is used. Initiator is water soluble. The formed latex particles are much smaller than suspension particles (0.05-2 µm). Kinetics differ considerable from other techniques. Polymer is formed within the micelles and not in the monomer droplets.

Precipitation Polymerization The monomer and initiator are soluble in solvent but upon initiation the formed polymer is insoluble and thus precipitates. The precipitated polymer can be separated in the form of a gel or powder by centrifugation or simple filtration. There is no problem of heat dissipation High degree of polymerization can be achieved Polyethylene, polyvinyl esters, polyacrylic esters are obtained commercially using hydrocarbon as solvent.

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