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The chemistry of_polymers.
This document discusses the chemistry of polymers, including: - The process of polymerization where monomers react to form long chain molecules. Common types are addition polymerization and condensation polymerization. - Sources of monomers from petrochemical and other raw materials. Examples of polymers produced from common monomers like ethylene, propylene, formaldehyde. - Mechanisms of polymerization including free radical, ionic, coordinative, and step-wise condensation polymerization. Factors that control the polymerization process and properties of the resulting polymers. - Common plastics and fibers produced including polyethylene, polypropylene, nylon, polyester, phenol formaldehyde, and their applications.
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The chemistry of_polymers.
- 1. 11-Mar-14 1 The Chemistry ofPolymers • Where does every thing start? – Raw materials for the polymer industry Monomers ( how monomers are produced). – Polymer synthesis Polymerization Polymerization Reactor Monomer Pellets/powder Controlling parameters •Ratio of reactants •Special additives •Temperature •Pressure Separation and recovery • Different grades of polymers can be produced The process by which monomers react and form long chain molecules is called Polymerisation. Monomer • Any material that is able to be polymerized, possesses a unique chemical structure , which is termed polyfunctionality. • Having double bond. • Paraffins can not be polymerized. • Cyclic monomers
- 2. 11-Mar-14 2 Monomer • Small moleculeshaving bifunctional groups can be polymerized (an organic acid and an alcohol neutralize each other but if having one functional group can not further react to form long chain). • Dicarboyxlic acid and glycol to give ester that have two opposing groups at the ends, which enables more and more reactions, thus building a large ester molecule- a polyester. Formation of polyethylene terephthalate (PET) Raw Materials • Petrochemical Industry Crude oil Coal Anaerobic Cracking Acetylene, methanol, phenol Distillation Gasoline and Kerosene Ethylene, propylene and butylene Natural Gas Methane Acetylene from coal Vinyl chloride from Acetylene Ethylene from Acetlyene
- 3. 11-Mar-14 3 • Methanol fromCoke Polymers derived from Formaldehyde Polyacetal, phenol formaldehyde, urea-formaldehyde Melamine-formaldeyhde Natural gas to methanol Methane to accetylene Monomers from Ethylene Vinyl chloride Styrene Vinyl Acetate PVC PVA PS Emulsified vinyl acetate is used as glue, paste for adhesion. Cellulose acetate yarn + rayon fibres Combination fabrics simulate of silk and linen Monomers from Propylene Acrylonitrile Methyl methacrylate Epichlorohydrin Polyol (glycerol) PAN PMMA epoxy
- 4. 11-Mar-14 4 Monomers for thermosets Ureaformaldehyde Melamine formaldehyde Epoxy Phenol formaldehyde Polymerization • Chain (Addition polymerization) • Stepwise polymerization (Condensation polymerization) Polymerization Chain (Addition polymerization) • Long chains appear at the early stage. • Monomers are added to long chains, and steadily disappear during the process. • No elimination of small molecules. • Quite high molecular weights may be obtained (105 to 2 x 106 g/mol) • Extension of polymerization only increases conversion not the chain length. Polymerization Examples of polymers produced via Addition mechanism PE PP PS PVC PTFE PMMA PAN PIB PVA
- 5. 11-Mar-14 5 Polymerization Stepwise polymerization (condensationpolymerization) • At the early stages dimers, trimers, and tetramer are formed. • Monomers disappear in the early stage. • Elimination of small molecules. • Molecular weights are low to medium (<50,000 g/mol) • Extension of the polymerization increases both conversion and molecular weights. Polymerization Examples of polymers produced via Condensation mechanism Phenol formaldehyde, Nylon 6,6 (polyamide), urea-formaldehyde, Melamine formaledhyde, polyester (PET), polyurethane, polycarbonate, etc. Chain polymerization Free radical Anion cation Ionic polymerization Free radical polymerization Chemical nature of the monomer (characteristics of the substituent groups) dictates the preferred mechanism Free radical Polymerization • Mechanism Initiation Free radical is not a catalyst Common Intiator Benzoyl peroxide 85-95 ºC Reaction between free Radical and monomer Azobiisobutylronitrile (AZBN) 50-70 ºC Will remain with growing chain
- 6. 11-Mar-14 6 Propagation Termination Occur frequently And leadsto longer chain Monomer reaches the chain radical by diffusion and is attached to growing chain. Reactions proceed in fractions of a second and lead to a giant chain, with a free radical at the end. Kinetics of polymerization Propagation step determines Rate of the polymerization Chain length Rate of propagation= Rate of polymerization rp depends on the concentration of reactants and rate constant Kp Free radicals reach a constant concentration ( steady-state assumption) Rate of intiation= Rate of termination Rate of polymerization α Concentration of the monomer and square root of the concentration of the initiator Arrhenius Relationship T-1 lnk Slope (–E/R) indicates the sensitivity to the effects of temperature The rate increases roughly two fold to three fold with a 10 ºC rise in temperature. Chain addition mechanism leads to formation of linear polymers
- 7. 11-Mar-14 7 Chain Transfer Reaction Freeradical passes from the growing chain to another molecule (usually a small one like solvent, the monomer itself, or even a polymeric “dead” chain) leading to the termination of the one chain together with the starting of a new one that is capable of growing. Branches are formed Shortening of the chain LDPE Branched chain polymers Ionic polymerization Uses specific catalysts to serve as electron withdrawing agents (Lewis acid) or electron donor ( amines) Cationic catalyst Anionic catalyst Propagation step is in principle resembles that of free radical polymerization But termination is different and difficult Two growing same ion chains repel, for termination it is required that growing ion has to react with a counterion or in a transfer reaction carbanion Ionic polymerization Cationic Polymerisation • Small activation energy • Polymerisation occur at low temperature Ionic polymerization Anionic Polymerisation • Monomers with electron withdrawing groups • Living polymer chains • Deliberate termination
- 8. 11-Mar-14 8 Coordinative (stereospecific) Polymerization Based onCatalysts Ziegler NattaSpecific catalysts orient the mers in the chain into a highly ordered configuration HDPE Isotactic, syndiotactic, atactic Control of properties Crystallinity, mechanical properties,etc. Organo-metallic compounds (trialkyl aluminium and titanium tetrachloride in solvents) Tacticity/Stereoisomerism Step wise ( condensation ) Polymerization Polyfunctionality = 2 Linear chain Polyester Thermoplastics Step wise ( condensation ) Polymerization Polyfunctionality = 2 Linear chain Polyamide (nylon 6,6) Thermoplastics Gears (no lubricant needed), bearing, electrical mountings, wire insulation, filaments Step wise (Condensation ) Polymerization Polyfunctionality = 3 Reactive branches, 3D structures, Crosslinked, vulcanization Thermosets Melamine formaldehyde Phenol formaldehyde Resistant to heat and moisture (Dinnerware and decorative table tops (formica)
- 9. 11-Mar-14 9 Step wise (Condensation) Polymerization Polyfunctionality = 3 Reactive branches, 3D structures, Crosslinked, vulcanization Thermosets Phenol formaldehyde Step wise (Condensation ) Polymerization Polyfunctionality = 3 Reactive branches, 3D structures, Crosslinked, vulcanization Thermosets Step wise (Condensation ) Polymerization Polyfunctionality = 3 Reactive branches, 3D structures, Crosslinked, vulcanization Thermosets Melamine formaldehyde Phenol formaldehyde Step wise (Condensation ) Polymerization Polyfunctionality = 3 Reactive branches, 3D structures, Crosslinked, vulcanization Thermosets Melamine formaldehyde
- 10. 11-Mar-14 10 •Phenol formaldehyde. •Three reactionsites available resulting in thermoset. • Low cost materials • Resoles and Novolacs • Resoles (excess of formaldehyde used in alkali solution) are uncured liquid linear (non-cross linked) polymers •Novolacs (insufficient formaldehyde used in acid solution) are powders. •Novolacs require curing agent (hexa, hexamethylene tetramine) and heat to form Bakelite. •Molded phenolic parts are made from Novolacs. These have high shrinkage and are brittle in nature. •Novolacs are blended with fillers to improve their strength and toughness (phenolic molding powders) •Common filler is wood flour Phenol formaldehyde (Phenolics) •Phenol formaldehyde has excessive OH groups which impart adhesive ability to it which is used to produce phenolic adhesives used for plywood, printed circuit boards, foundry shells and cores, sandpaper, brake linings and grinding wheels. • Mold releases are required to avoid problem of stickiness. •Phenolics are nonflammable (char rather than melt or burn) and due to the ability to form char they are used as coating for rocket nozzles. •Low thermal conductivity (pan handles, bases for toasters, knobs for appliances and motor housings) •To improve resistance to UV dark pigment is usually added in phenolics. •High electrical resistance (electrical switches, circuit breakers, connectors, automotive electrical parts). •Solvent sensitivity is good for organic solvents but is poor for acids and bases. •Compression molding is the technique for making products. Phenol formaldehyde (Phenolics) Unsaturated polyester Linear polyester+ Ethylene glycol Maleic acid MEKP (peroxide curing agent) is used for room temperature curing, Cobalt naphthanate is used as activator Differences between Addition and Condensation Polymerisation • Polymer growth mechanism • Dependence on previous step • Initiator needed • Type of monomer • Number of active sites (functional groups per monomer) • Number of different types of monomers needed to form polymer • By-product formed • Basis (polymer repeat unit representation) • Polymer chain characteristics • Branching
- 11. 11-Mar-14 11 Two or moremonomers polymerize in situ Process controlled by tendency to homopolymerize and cross polymerize Polymers formed by condensation polymerization are not copolymers Copolymerization Kinetic constants Homopolymerization Cross polymerization Copolymers Alternating copolymer Random Chain copolymer Block copolymer Graft copolymer Examples PVC-PVA, SBR, ABS, etc. Example Problem A copolymer consists of 15 wt.% polyvinyl acetate (PVA) and 85 wt.% polyvinyl chloride (PVC). Determine the mole fraction of each component. Basis: 100 g Small amount of PVA in PVC reduces rigidity of PVC . Example Problem Determine the mole fractions of vinyl chloride and vinyl acetate in a copolymer having a molecular weight of 10,250 g/mol and a degree of polymerisation (DP) of 160.
- 12. 11-Mar-14 12 Techniques of Polymerization Mass/BulkPolymerization Autoclave or reactor Thermometer Monomer and initiator •Entire batch is single phase. •Styrene to polystyrene •Stirring required as reaction mass becomes viscous •Batch process •Process is used extensively for condensation polymerisation (due to low heat of reaction) Water is circulated ( Ist steam then cooling water) Pros Cons Purest products Heat removal problem for exothermic reactions is difficult Gives largest possible yield per reaction volume As concentration of reaction mass increases , viscosity also increases, agitation becomes difficult No subsequent separation, purification and recovery steps Unreacted monomer traps Mass Polymerization Solution Polymerization •Monomer is dissolved in an inert solvent together with initiator. •Can be batch or continuous •Also called solvent polymerization Heat removal and control easier Tracking reaction easier because rate of reaction are lower A readymade solution of polymer is directly available from the reactor. It is useful in applications such as lacquers and adhesives Rate of polymerization is low, and it lowers the average chain length of the polymer Solvents toxicity and flammability increase cost Recovery of used solvent required addition technology increasing cost of production Yield per reactor volume is less than that in mass polymerization Suspension Polymerization •Monomer is mixed with a catalyst and then dispersed as a suspension in water. •Heat released by the reaction is absorbed by the water. •After polymerisation, the product is separated and dried. •Production of vinyl type polymers. 10-500 µm Emulsion Polymerization What is an emulsion? A stable colloidal solution, e.g, Milk, consist of an immiscible liquid dispersed and held in another liquid by a substance called emulsifier Soaps and surfactants have two ends of different solubility
- 13. 11-Mar-14 13 Monomer Monomer takes position inthe micelle and emulsion form. • The initiator is water soluble and free radicals and the monomers diffuse into the hydrophobic interior of micelles while water is attracted to hydrophilic exterior zone. •Emulsion polymerization is suitable for Addition reaction systems R = 0.2-10 nm Micelles serves as core for the growing polymer chain Good thermal control, high production rate, high molecular weight, narrow MWD