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

Surfactants 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.

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 O O H 2 N (CH 2 ) 6 NH 2 + Cl C (CH 2 ) 8 C Cl hexamethylene diamine sebacoyl chloride -(2n-1) HCl O O H NH (CH 2 ) 6 NH C (CH 2 ) 8 C Cl Nylon 6,10 n

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)

Polymer processing operations polymerization and forming Monomer Polymer Compound Processing operations Polymerization Compounding Main operations Extrusion Injection molding Compression molding Transfer molding Blow molding Rotational molding Thermoforming Calendering Film blowing Fiber spinning, Final product

Soften and Form Resin Heat Melt Tool Part Thermoplastics are heated until they flow then forced by pressure to form a finished 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

Extruder Design Parameters Design parameters: Screw diameter Helix angle Length (L / D) Compression ratio Channel depth Single screw extruder Twin screw extruder

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.

Injection molding 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. Molded products

* Schematic of thermoplastic Injection molding machine

Injection molding cycle Thermoplastics: cooling in the mold Thermosets: curing, crosslinking in hot mold

Injection Cycle

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

Injection Blow Molding

Thermoforming 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

Compression molding 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

Transfer 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