Industrial Polymerization andPolymersINTRODUCTION: The word “polymer” is derived from two Greek words, polys= many, and meros = parts or units. A polymer is a large moleculewhich is formed by repeated linking of small molecules called“monomers” and the reactions by which monomers are joined togetherto form polymers are called polymerization reactions. For example,the polyethylene is formed by repeated linkages of simple ethylenemolecules which are the monomers;

The number of repeating units in the chain so formed is calledthe degree of polymerization. Polymers with a high degree ofpolymerization are called “high polymers” and those with low degreeof polymerization are called “oligopolymers”. High polymers havevery high molecular weights (104 to 106) and hence are calledmacromolecules.

The macromolecule may consist of monomers of identical ordifferent chemical structure and accordingly they are calledhomopolymers and co-polymers or mixed polymers respectively. Themonomeric units may combine with each other into a macromolecule toform polymers of linear, branched or cross-linked (threedimensional) structures.

If the atom of the same species is present in the main chain,they are called “homochain polymers”. If the chain is made up ofdifferent atoms than they are called “heterochain polymers”. Theorientation of monomeric units in a macromolecule can take anorderly or disorderly fashion with respect to the chain. If themonomers have entered the chain in a random fashion, it is calledan “atactic” polymer. If all the side groups lie on the same sideof the chain (cis-arrangement), it is called “isotatic” polymers.If the arrangement of side group is in alternating fashion (trans-arrangement), it is called a “syndiotactic” polymer. Polypropylenechain arrangement is an example of atactic polymer, natural rubberis isotactic and gutta-percha is syndiotactic. CLASSIFICATION OF POLYMERS: The capacity of an element to form polymeric compounds dependson its position in the periodic table. The elements of 1st group andother univalent elements are entirely incapable of forming

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polymers, because, to form a chain an element must have at leasttwo valences. All other elements are capable of forming homochainor heterochain polymeric compounds, the stability of which dependson the interatomic bond strength. On the basis of Chemical Composition:

Polymers may be broadly classified as follows.1. ORGANIC POLYMERS:

These include compounds containing, apart from carbon atoms,hydrogen, oxygen, nitrogen, sulfur and halogen atoms, even if theoxygen, nitrogen, or sulfur is in the back-bone (or main) chain.Examples;

2. INORGANIC POLYMERS:These are polymers containing no carbon atoms chain. The

chains of these polymers are composed of different atoms joined bychemical bonds, while weaker inter-molecular forces act between thechains. Examples;

On the basis of Synthetic Mode:The polymers may be classified based on the mode of synthesis

of polymers into two types 2

1) Addition polymers2) Condensation polymers

1) Addition or Chain Polymerizations:In addition-polymerization, the polymer is formed from the

monomer, without the loss of any material and the product is anexact multiple of the original monomeric molecule. The monomerunits are generally unsaturated compounds, usually derivatives ofalkenes. Application of energy in the form of heat, light,pressure, ionization radiation or the presence of catalyst, isusually necessary for initiating the chain polymerization. Ingeneral, addition polymerization proceeds by the initial formationof some respective species, such as free radicals or ions and bythe addition of the reactive species to another molecule, with theregeneration of the relative feature.

2) Condensation Polymerization:In condensation polymerization, the chain growth is

accompanied by elimination of small molecules, such as H2O, CH3OH,etc. Condensation of phenol and formaldehyde to form Bakelite.Example;

Polymers may be classified in more than one manner. Theconventional classification is denoted in the following chart;

THERMOSETTING:3

Definition:IUPAC defines a thermosetting resin as a petrochemical in a

soft solid or viscous state that changes irreversibly into aninfusible, insoluble polymer network by curing. Curing can beinduced by the action of heat or suitable radiation, or both. Acured thermosetting resin is called a thermoset.Process:

Thermosetting materials are those plastics which require heatand pressure to mould them into shape. When heat is applied, theyfirst become soft and plastic and on further heating they undergochemical change and set hard. The process is called thermosettingor thermo hardening. When material is thermoset, it is permanentlyset and does not soften to any appreciable extent when againheated. However, intense heating will bring about the breakdown ofthe material by burning. This implies that thermosets cannot berecycled, except as filler material.

The curing process transforms the resin into a plastic orrubber by a cross-linking process. Energy and /or catalysts areadded that cause the molecular chains to react at chemically activesites (unsaturated or epoxy sites), linking into a rigid, Threedimensional structure. The cross-linking process forms a moleculewith a larger molecular weight, resulting in a material with ahigher melting point. During the reaction, the molecular weight hasincreased to a point so that the melting point is higher than thesurrounding ambient temperature, the material forms into a solidmaterial.Properties:

Thermoset materials are generally stronger than thermoplasticmaterials due to the three-dimensional network of bonds (cross-linking), and are also better suited to high-temperatureapplications up to the decomposition temperature. However, they aremore brittle. Since their shape is permanent, they tend not to berecyclable as a source for newly made plastic.Examples:

Polyester fibreglass systems: sheet molding compounds and bulkmolding compounds

Polyurethanes : insulating foams, mattresses, coatings,adhesives, car parts, print rollers, shoe soles, flooring,synthetic fibers, etc. Polyurethane polymers are formed bycombining twoor higher functional monomers/oligomers.

Vulcanized rubber Bakelite , a phenol-formaldehyde resin used in electrical

insulators and plasticware

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Duroplast , light but strong material, similar to bakelite usedfor making car parts

Urea-formaldehyde used in plywood, particleboard and medium-density fiberboard

Melamine resin used on worktop surfaces[3]

Epoxy resin used as the matrix component in many fiberreinforced plastics such as glass-reinforced plastic andgraphite-reinforced plastic)

Polyimides used in printed circuit boards and in body parts ofmodern aircraft

Cyanate esters or polycyanurates for electronics applicationswith need for dielectric properties and high glass temperaturerequirements in composites

Mold or mold runners (the black plastic part in integrated

circuits or semiconductors)

THERMOPLASTIC:Thermoplastic is a polymer that turns to liquid when heated

and freezes to glassy state when left to cool sufficiently.Thermoplastics are elastic and flexible over glass transitiontemperature and can go through melting/freezing cycles repeatedly.Source:

This is a material that has the quality of softening or fusingwhen it is heated and of hardening and becoming rigid when it iscooled. As the hardening in thermoplastic materials is not due toany chemical action, so the shaped articles from thermoplasticmaterials will resoften on heating.A thermoplastic, also known as a thermosoftening plastic, it isused for Chairs, boxes, sterilizeable equipment, bottles, helmlets,gasoline deposits and gloves.Thermoplastic is a pavement marking that has been used in the U.S.since 1958 with continuing good results. A mixture of glass beads,binder, pigment and filler materials, thermoplastic, as its namesuggests, becomes liquid when heated. A thermoplastic material hasthe property of softening or fusing when heated and of hardeningand becoming rigid again when cooled; thermoplastic materials canbe re-melted and cooled time after time without undergoing anyappreciable chemical change.Examples of thermoplastic materials include polyethene andpolypropylene.Properties:

Thermoplastic is a type of plastic made from polymer resinsthat becomes a homogenized liquid when heated and hard when cooled.

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The major characteristic of this plastic material is that it isreversible. It can be reheated, reshaped, and frozen repeatedly.

Thermoplastics can be moulded into shapes after heating them.Thermoplastic polymers are found in many household items. Athermoplastic polymer is made up of long, unlinked polymermolecules, generally with a high molecular weight. Mostthermoplastics have a high molecular weight. The polymer chainsassociate through intermolecular forces, which permitthermoplastics to be remolded because the intermolecularinteractions increase upon cooling and restore the bulk properties.In this way, thermoplastics differ from thermosetting polymers,which form irreversible chemical bonds during the curing process.Thermosets often do not melt, but break down and do not reform uponcooling.Acrylic:

Acrylic, a polymer called poly (methyl methacrylate) (PMMA),is also known by trade names such as Lucite, Perspex and Plexiglas.It serves as a sturdy substitute for glass for such items asaquariums, motorcycle helmet visors, aircraft windows, viewingports of submersibles, and lenses of exterior lights ofautomobiles. It is extensively used to make signs, includinglettering and logos. In medicine, it is used in bone cement and toreplace eye lenses. Acrylic paint consists of PMMA particlessuspended in water. Most schools use it when teaching aboutthermoplastics.Nylon:

Nylon, belonging to a class of polymers called polyamides, hasserved as a substitute for silk in products such as parachutes,flak vests and women's stockings. Its fibers are useful in makingfabrics, rope, carpets and musical strings, whereas in bulk form,nylon is used for mechanical parts including machine screws, gearwheels and power tool casings. In addition, nylon is used in themanufacture of heat-resistant composite materials.Polybenzimidazole:

Polybenzimidazole (or Celazole, PBI) is regarded as thehighest performance engineering thermoplastic available. It offersthe highest heat resistance and mechanical property retention over400° F of any unfilled plastic Parts molded from PBI are used insemiconductor and flat paneldisplay manufacture, photovoltaicproduction, oil and gas recovery, and industrial applications. PBIparts are commonly used as chamber seals, wafer transportationdevises, electrical insulating parts, glass handling, plasmacutting torch insulators, valve seats, seals, bearings, bushingsand thrust washers.

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Polyethylene:Polyethylene (or polyethene, polythene, PE) is a family of

similar materials categorized according to their density andmolecular structure. For example, ultra-high molecular weightpolyethylene (UHMWPE) is tough and resistant to chemicals, and itis used to manufacture moving machine parts, bearings, gears,artificial joints and some bulletproof vests. High-densitypolyethylene (HDPE), recyclable plastic is commonly used as milkjugs, liquid laundry detergent bottles, outdoor furniture,margarine tubs, portable gasoline cans, water drainage pipes, andgrocery bags. Medium-density polyethylene (MDPE) is used forpackaging film, sacks and gas pipes and fittings. Low-densitypolyethylene (LDPE) is softer and flexible and is used in themanufacture of squeeze bottles, milk jug caps, retail store bags. Polypropylene:

Polypropylene (PP) is useful for such diverse products asreusable plastic food containers "microwave and dishwasher safe"plastic containers, ropes, carpets, plastic moldings, pipingsystems, car batteries, insulation for electrical cables andfilters for gases and liquids. In medicine, it is used in herniatreatment and to make heat-resistant medical equipment.Polypropylene sheets are used for stationery folders and packagingand clear storage bins. Although relatively inert, it is vulnerableto ultraviolet radiation and can degrade considerably in directsunlight. Polystyrene:

Polystyrene is manufactured in various forms that havedifferent applications. Extruded polystyrene (PS) is used in themanufacture of disposable cutlery, CD and DVD cases, plastic modelsof cars and boats, and smoke detector housings. Expandedpolystyrene (EPS) is used in making insulation and packagingmaterials, such as the "peanuts" and molded foam used to cushionfragile products. Extruded polystyrene foam (XPS), known by thetrade name Styrofoam, is used to make architectural models anddrinking cups for heated beverages. Polystyrene copolymers are usedin the manufacture of toys and product casings.Polyvinyl chloride:

Polyvinyl chloride (PVC) is a tough, lightweight material thatis resistant to acids and bases. Much of it is used by theconstruction industry, such as for vinyl siding, drainpipes,gutters and roofing sheets. It is also converted to flexible formswith the addition of plasticizers, thereby making it useful foritems such as hoses, tubing, electrical insulation, coats, jackets

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and upholstery. Flexible PVC is also used in inflatable products,such as water beds and pool toys.Teflon:

Teflon is the a polymer called polytetrafluoroethylene (PTFE),which belongs to a class of thermoplastics known as fluoropolymers.It is famous as a coating for non-stick cookware. Being chemicallyinert, it is used in making containers and pipes that come incontact with reactive chemicals. It is also used as a lubricant toreduce wear from friction between sliding parts, such as gears,bearings and bushings.INORGANIC POLYMERS: The polymers whose backbone chains are made of carbonatoms, is called organic polymers. But there are some other kindsof polymers whose back bone is not made of carbon atoms but consistof atoms other than carbon. These are called inorganic polymers.Silicones: Silicones are the most common inorganic polymers. Thedevelopment of these commercial polymeric compounds give itsbeginning mostly to:a) synthesis of tetraethyl silicon (tetraethyl silane) around 1865.b) The fundamental contributions of F.S. Kipping concerning the

analogous characteristics of the silicon and carbon compounds.The silicone products posses certain unusual but very useful

properties such as remarkable stability over wide range oftemperature from 700 to 2500C, good water repellency, chemical andphysiological inertness, good resistance to effects weathering, lowvapour pressure and desirable low temperature characteristics.These properties of siloxanes are due to (a) their molecular size(b) the number and type of organic groups attached to slicon, and(c) the configuration of the molecule. The varied properties,combined with the many forms, in which the silicones can beformulated (e.g., liquids, greases, resins and rubbers) lead totheir widespread application in a broad range of processes andindustries. Such as;

Silicones are synthetic polymers made from the productsof nature. Although “silicone” is often used as a generic term fornearly all substances that contain a silicon atom, it is more

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properly described as an entirely synthetic polymer containing aSi-O backbone. To this backbone, organic groups are frequentlyattached to the silicon atoms via a Si-C bond. This general description defines the broad class ofpolymers known as silicones. The most common example is poly(dimethylsiloxane) or PDMS. This polymer has a repeating (CH3)2SiOunit. These materials are the basic building blocks of the siliconeindustry. Depending upon the number of repeat units in the polymerchain and the degree of cross-linking at least six classes ofcommercially important products such as fluids, emulsions,compounds, lubricants and resins can be produced. History: F.S.Kipping’s systematic study of the organosiliconcompounds gave the way for the development of the siliconeindustry. He synthesized dialkyldichlorosilanes from SiCl4 andGrignard reagents, which could be rapidly hydrolyzed to thecorresponding dihydroxy derivatives.

Gem-diols, >C(OH)2, being unstable can decomposeeliminating a molecule of water and give ketones. Kipping expecteda similar behaviour from these compounds and hence named themsilicones. However, instead of the expected ketones, the reactionresulted in only oily liquids and resins, which were recognised aslong-chain polymers arising from intermolecular condensations.

These are still called silicones although they arereally polyorganosiloxanes. In other words dimethylsiliconhydroxides polymerise to give silicones. The silicones rubbers canbe vulcanized by organic peroxides to form cross-links between twoneighbouring chains.

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These silicones may be linear, cyclic or cross-linked as shownbelow.

Preparation:The preparation of the silicone polymers involves two

main steps: (a) preparation of intermediates, and (b)polymerization of the intermediates. The intermediates required arethe various organic chlorosilanes, of which methyl, ethyl or phenylchlorosilanes are of commercial significance. Sometimes, certainother organic groups are also incorporated to develop specificproperties.

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There are three commercial methods for the manufacture oforganochlorosilanes. (1). In the direct method, which is generally used to preparemethyl chlorosilanes, methyl chloride is passed through a containerconsisting of powdered silicon and powdered copper (acting ascatalyst) heated to about 230-290 0C.

(2). In the Grignard method, a solution of silicon tetrachloride indry ether is allowed to react with the Grignard reagent which isslowly sent into the reactor:

(3). In the third method, an olefin hydrocarbon is added to achlorosilane in a bomb Calorimeter under pressure and at atemperature of 400 0C without a catalyst or at a temperature of 450C using a peroxide catalyst:

The lower silicones like dimethyldichlorosilane onhydrolysis yield colourless oil, octa-methylcyclotetrasiloxane(Cyclic tetramer) which on heating above 100C0 with a trace of acidor base polymerizes to form a high viscous liquid or a gum. Theyare remarkably stable towards heat and chemical reagents. They arenot wetted by water and non-toxic and chemically inert. Silicone rubber is made from the gum by cross linkingthe chains by free radical type process.Mechanism : Silioxane polymerization follows an ionic process. Basiccatalyst such as alkali metal hydroxide or alkoxide, cleave Si–Oskeletal bonds to yield linear species which can function as chainpropagation sites. Cyclic molecules are formed due to -O-K+ ionicbond.

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A sketch of the reactor in which methyl chloride and silicon werecontacted to prepare silicone is shown in Fig.7.11. A "superheater"heats the methyl chloride before it reaches the reactor.  Thesilicon is fed into a reactor that is stirred or "fluidized" bypassage of hot gas through the bed of copper-containing silicon.  A"heat transfer coil" removes excess heat by transferring it to acooler, circulating fluid.  The engineers stop silicon powder frombeing carried through the reactor by collecting it in two differenttypes of filters.  A "condenser" cools the gases from the reactorto a liquid that contains all the products. But a chemical reactoris not sufficient equipment to isolate a usable chemical product inthe purified state from all other substances. A complex distillingplant is used to isolate the products formed. Dimethylsiloxane structure forms the basic basis ofmost silicone polymers; other substituents groups have also beenintroduced. These include vinyl, ethyl, trifluoropropyl, p-cynoethyl, and phenyl and biphenyl groups. The introduction ofspecific group improves the oil resistance strength and toughness,flame resistance or compatibility of the polymers.

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Properties and uses of silicone polymers: The silicone compounds, most of which are grease-like inappearance and feel, are so named to distinguish them from thesilicon greases which are exclusively used as lubricants. Thesilicone compounds are formulated from dimethyl fluids by additionof suitable quantities of very finely divided silica. Theirconsistency varies from free-flowing liquids to heavy ointment-likematerials. They retain substantially all the properties of dimethylsilicones such as heat resistance, low freezing point, lowviscosity and good electrical properties. The grease-like compoundshave a remarkable difference from the fluids in that they have aperfectly flat temperature-viscosity curve.

Silicone compounds are widely used as moisture and waterrepellent and high dielectric strength sealing compounds foraviation type spark plugs. As moisture-proof seals or corrosioninhibitors, they are used for electrical connectors, aircraft radioantennae, x-ray equipment, ignition systems of marine engines, andbattery cases. They are also used in a myriad of industrialapplications such as for mould release, as a dumping medium inphonograph pick-ups, in water proofing and preservation of leatherand rubber diaphragms, and as lubricants for rubber brushings and

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valves handling corrosive liquids.are highly valued materialsbecause they have a combination of physical properties not found inother polymers. They have outstanding heat stability and can beused in applications where organic materials would melt ordecompose. Many silicones seem to be impervious to the effects ofaging, weather, sunlight, moisture, heat, cold, and some chemicalassaults. Some silicones are used to stick, bond, or couple thingstogether as glue.

However, unique surface properties make silicones reallydifferent from other materials. The low surface tension of siliconefluids make them ideal for applications such as: paper releaseagents, fiber lubricants, textile hand modifiers, mold releaseagents, antifouling materials and water repellents. In fact,silicones have been used as foam-control agents, anticaking aids,corrosion inhibitors, emulsifiers, lubricants, conditioners, andgloss enhancers–all because of their unique surface properties.Some applications of individual silicones are given as under.Chloromethylsilanes: Chloromethylsilanes are the basic building blocks of allof our silicon-based materials. They are used in the basicsynthesis of silanes and siloxanes and as protecting agents forintermediates in pharmaceutical syntheses.Chlorosilanes: Chlorosilanes are essential raw materials in theelectronics and telecommunications industries and are used for theproduction of optical fibers, silicon wafers and chips, as well asthe starting material for fumed silicas.Organofunctional silanes: The basic structure of organofunctional silanes is:RnSi(OR)4-n (with "R" being an alkyl, aryl, or organofunctional groupand with "OR" being methoxy, ethoxy, or acetoxy). Industries thatuse organofunctional silanes include adhesives and sealants,electronics, foundry resins, glass fibers/fabrics, mineral fillers,paints and coatings, pharmaceuticals, pigments, silicones,textiles, thermosets, wire and cable. The organofunctionality andprimary applications are as follows. Amino:

Adhesion promoter, coupling agent, and resin additive Improves chemical bonding of resins to inorganic fillers and

reinforcing materials Used for epoxies, phenolics, melamines, nylons, PVC, acrylics,

poly(olefins), poly(urethanes), and nitrile rubbers Surface pretreatment of fillers and reinforcers

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Vinyl: Used for free-radical, cross-linked polyester, rubber,

poly(olefins), styrenics, and acrylics Used to couple fiberglass to resins Used to copolymerize with ethylene Used to graft to poly(ethylene) for moisture cure

Epoxy: Adhesion promoter for epoxies, urethanes, and acrylics Surface treatment for fillers and reinforcers.

Methacryl: Adhesion promoter and coupling agent Used for free-radical, cross-linked polyester, rubber,

poly(olefins), styrenics, and acrylics Used to couple fillers or fiberglass to resins Moisture cross-linking of acrylics .

Sulfur: Used as coupling agent for inorganic fillers in sulfur-

vulcanized rubber mixtures.Alkyl:

Hydrophobic surface treatment of fillers and inorganicsurfaces

Silicone synthesisPhenyl:

Synthesis of silanes and siloxanes Hydrophobic surface treatment Hydrophobic additive to other silane coupling agents Thermal stability additive to other silanes.

Greases: The greases are prepared from the silicone fluids using filler

such as silica, lithium soap and carbon black. The silica-carryinggreases are used in lines carrying solvents and corrosive chemicalsand also for lubricating places where rubber must move againststeel. Resins:

The preparations of silicone resins is by hydrolyzing andcondensing or polymerizing mixture of bifunctional or trifunctionalalkyl chlorosilanes under carefully controlled conditions. Theproperties of the resins can be varied by controlling the relativeamounts of bifunctional and trifunctional units, the type oforganic units attached to silicon, the polymer size orconfiguration.The silicone resins can be broadly classified intosix categories, although there is overlapping because some can beapplied in more than one way. (1) coating resins (2) laminating

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resins (3) release resins (4) water repellent resins (5)electrical resins (6) foamed resins.In Pharmacy and Medicine:

The silicones have been largly used in pharmacy and medicine.Hand lotions containing silicone fluids are reported to be usefulfor treating skin affections where contact with water in to beavoided. A dental polyantibiotic silicone paste containingpenicillin, bacotracin and streptomycin has been developed for rootcanal therapy. Silicone antifoam has been reported to be useful intreating extreme cases of pneumonia.Ceramics: Ceramics are inorganic, non-metallic, solid materials andcan be crystalline or non-crystalline. Ceramics possess a covalentnetwork structure or ion bonding or a combination of the two.Ceramics are hard and brittle but stable to high temperatures. Theimportant products of ceramic industries are building bricks andtiles, sewer pipes, drain tiles, all kinds of refractory bricks,electrical and chemical porcelain and stoneware, white ware, chinafloor and wall tiles, enamels and abrasives and insulators in sparkplugs.Engineering ceramics: The ceramic materials which might be used in place ofother engineering materials such as metals, wools or plastics aretermed as engineering ceramics. These materials withstand hightemperatures, resist greater pressures, have superior mechanicalproperties and can protect against corrosive chemicals.Basic raw materials: The three main raw materials used for ceramicindustry are clay, feldspar and sand. Clay:

It is hydrated aluminum silicate. Some common claysare:

(i) Kaolinite (Al2O3.2SiO2.2H2O ) (ii) Montmorillonite (Mg, (Ca ) O, Al2O3.5SiO2.nH2O)(iii) Illite (K2O, MgO, Al2O3, SiO2,H2O all in variable

amounts).Feldspar:

There are three common types of feldspar,:(iv) Potash feldspar. (K2O. Al2O3. 6SiO2)(v) Soda feldspar. (Na2O. Al2O3. 6SiO2)(vi) Lime feldspar. (CaO. Al2O3. 6SiO2).

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Sand: Sand or flint (SiO2) is the main constituent ofceramic industry. It is used as an opener for reducing theshrinkage of the clay. In addition to the principal raw materials, a widevariety of other minerals, salts and oxides are used as fluxingagents and special refractory ingredients. Some common fluxingagents which lower temperatures are borax, boric acid, soda ash,sodium nitrate, pearl ash, apatite, cryolite, iron, lead oxideetc.

Manufacture: All ceramic products are made by combining various amounts

of the raw materials such as clay, potter’s flint and feldspar etc.All these are mixed with water to form a paste called “slip orcomposite”. The slip is passed through the filter press to removealmost all water and moist cake is obtained. The shape of thearticle is designed by an artist, by potter wheel or by diepressing.The composite consists of ceramic matrix containing embedded fibersof a ceramic material, which may or may not be the same chemicalcomposition as the matrix. Fibers typically have great strengthwith respect to loads applied along the long axis. For example, theformation of silicon carbide (SiC) of ceramic fibers is as follow;The first step in the production of SiC fibers is the synthesis ofa polymer polydimethylsilane:

When this polymer is heated to about 400 0C, it converts to amaterial that has alternating carbon and Si atoms along the chain

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Fibers formed from this polymer are heated slowly to about 1200 oCin a nitrogen atmosphere to derive off all the hydrogen and allcarbon atoms other than those that directly link the silicon atoms.The final product is a ceramic material of composition SiC in theform of fibers ranging in diameter from 10 to 15 μm. But similarprocedures, beginning with an appropriate organic polymer, ceramicfibers of others compositions such as boron nitride can befabricate.

The articles are now dried in air or chambers through warm aircirculation. The dried articles are placed in clay boxes called“suggers” Which are placed in the furnaces and firing kiln, calledhovel oven so that the articles may not come in direct contact ofthe flame. The temperature of the first firing is as low as 700 0Cand then as high as 20000C.The objects obtained from the kiln insolid form is called “biscuits”. In order to make the earth wareimpervious to liquid, glazes are applied on it. Various methods areapplied for glazing such as dipping, pouring, spraying, dusting,volatization etc. the biscuit is again fired in a furnace at atemperature 700-900C0. During the firing, the articles becomesmooth and shining in appearance. Colouring of articles:

The colouring agents such as cobalt oxide (blue), copperoxide (green or red), iron oxide (yellow, orange or red), MnO2,(violet) etc. are mixed in paste.Chemical conversions:

All ceramic products are made from low temperature tohigh temperature range. Such temperatures cause a number ofreactions for the chemical conversions.

1. Dehydration at 150 to 650C0. 2. Calcinations at 600 to 900C0. 3. Oxidation of ferrous iron and organic matter at 350 To 900C0. 4. Silicate formation at 900C0 and higher.

Some of the initial chemical changes are relatively simplesuch as calcinations of CaCO3 and the dehydrations anddecompositions of kaolinite. Other reactions such as silicateformations are quite complex and change with temperature. Thecommon element of all ceramic products is clay and therefore thechemical reactions which occur on heating clay are quite important.The first effect of the heat is to drive off the water ofhydration; this occurs at about 400 to 700C0 and the hard porousmass, which does not soften with water, is obtained.

When heated further to 900 to 10000C, the hardness and porosityincreases. The silica and alumina combine to form mullite

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(3Al2O3.2SiO2).At a still higher temperature, the remaining silicais converted into crystalline cristobalite. At 1400 0C to 1800 0C,it melts to give a glassy mass. The basic reaction in the heatingof clay is as follows:

The plasticity of clays can be increased by ageing, thatis, by storing it under wet condition or by weathering. This is dueto hyration accompanied by gelation and also due to furthersubdivision of the particles by bacterial action.

The impurities associated with clays also influence theirproperties to some extent. For instance, presence of iron oxideimparts red colour to the burnt material. Presence of silicaincreases its refractory nature and porosity and reduces the extentof air-shrinkage. Presence of CaO. MgO and Fe2O3 act as flux andlower the fusion point of the clay. Lime reduces the vitrificationrange. Clays containing high percentage of silica and very lowpercentage of Fe2O3 are known as fire-clays which have importantindustrial applications due to their high fusion point. Clays, ingeneral, are very important raw materials for the manufacture ofpottery, earthenware, bricks, tiles, terracotta, conduits forelectrical cables and drain pipes.

To avoid the cracking and fractures, scientists frequentlyproduce very uniform particles of the ceramic material that areless than a μm (10-6m) in diameter. These are then heated at hightemperature, so that the individual particles bond together to formthe desired object.

The sol-gel process is an important method of formingextremely fine particles of uniform size. A typical sol-gelprocedure begins with a metal alkoxide. To illustrate this process,the titanium as the metal and ethanol as the alcohol is used.

The alkoxide product, Ti(OCH2CH3)4, is dissolved in appropriatealcohol solvent, water is then added, and it reacts with thealkoxide to form Ti-OH groups and to regenerate ethanol;

The titanium oxide and titanium hydroxide produce a sol, suspensionof extremely small particles. The acidity or basicity of the sol isadjusted to cause water to be split out from between two of the Ti-OH bonds;

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This is a condensation reaction. The condensation also occursat some of the other OH groups bonded to the central Ti-atom,producing a three dimensional network. The resultant material,called a gel, is a suspension of extremely small particles with theconsistency of gelatin. When this material is heated carefully at200 0C to 500 0C, all the liquid is removed, and the gel isconverted to a finely divided metal oxide powder with particles ofamorphous silica, SiO2, are formed by precipitation from a methanolsolution of Si(OCH3)4 upon addition of water and ammonia. Theaverage diameter is 550 nm.Applications :

1. When ceramics objects are mixed with other materials, theresulting material shows greater toughness. Such a mixture ofceramic and other materials called Ceramic Composite. Thetemperature limits of such bonded materials are exceedinglyhigh; therefore, they are used for aerospace hardware such asheat shields and rocket nozzles.

2. A mixture of ceramic and metallic components is known as“cermets” which is very hard and greater weights and is usedin lining for brakes and clutches and also non lubricatingbearings in the temperature range from 370 to 815C0.

3. Ceramic fibers are used for thermal insulation, for zonecurtains in annealing furnaces and for high temperature, lowvoltage wiring circuits of space and aircraft equipment.

4. Ceramic fibers reinforced metals, metal impregnated ceramics,ceramic- reinforced plastics and metal-ceramic laminates havebeen developed and to withstand high temperatures.

5. Ferromagnetic ceramic materials are used in electrical devicessuch as television sets, computers, magnetic switches,transformers, recorders and memory devices.

6. Ceramic composites are widely used in cutting tool equipment.For example, cast iron and nickel alloys can be cut withcomposite of alumina and silicon.

ORGANIC POLYMERS:A polymer is a large molecule built up by successive

repetition of small chemical unit, which in turn, is derived fromsimple compound(s) called monomer(s). The number of monomerspresent in a polymer molecule is called of degree ofpolymerization.Classification of Polymers:

High polymers are usually classified as;1. Classification on basis of the outline of the principal chain of

atoms.

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2. Classification on the basis of number of monomermolecule.

3. Classification on the basis of origin of a polymer.4. Classification on the basis of reaction involved in their

synthesis.Classifies on basis of the outline of the principal chain of atoms:Linear Polymers:

When the polymers contain isomers joined together in the formof a rope or chain, it is called linear. Such as linearpolyisoprene;

Branch Polymer:When the linear structure bears side chains composed of the

same linear structure as the main chain, it is called branchedpolymer. Such as;

Graft Polymer:If the branches are of a different structure than the linear

polymer chain, it is called a graft polymer. There are severalother types of branched polymers; such as cruci form, cross linkedform, block form, semi-ladder, ladder form etc.Classification on the basis of number of monomer molecule:

These are of three types;1. Homo-polymer, in which repeating structural units are of

the same kind such as polyethylene.2. co-polymer, in which more than type of structural unit or

when two different type of monomers polymerizes, such aspolyamide.

3. Ter-polymer, when three different types of monomerspolymerize three dimentionally.

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Classification on the basis of origin of a polymer: Polymers may be classified as

Natural:Such as cellulose, wool, nucleic acid, natural rubber, etc.

Synthetic:Such as nylon, backlites, styrene, butadiene, rubber etc.

Structures of some important polymers are given as under:

Classification on the basis of reaction involved in their synthesis:These polymers may be classified based on the mode of

synthesis and they are divided into three types:(1) Addition polymers. (2) Condensation polymers.(3) Ring opening reaction polymers.Addition polymers (Chain Groth Polymers):

These are the products formed when the monomer units arerepeatedly added to form long chain without the elimination of anyby-product molecules. In these polymers following mechanisms areinvolved;Free-Radical Polymerization;

Free radical polymerization offers a convenient approachtoward the design and synthesis of special polymers. In a freeradical addition polymerization, the growing chain end bears anunpaired electron. A free radical is usually formed by thedecomposition of a relatively unstable material called initiator.The free radical is capable of reacting to open the double bond ofa vinyl monomer and add to it, with an electron remaining unpaired.The energy of activation for the propagation is 2 to 5 kcal/mol,that indicates extremely fast reaction (for condensation reactionthis is 30 to 60 kcal/mol). Thus in a very short time many moremonomers add to the growing chain. At each addition step, theunpaired electron from the free radicals pairs with one of the πelectrons of the alkene monomer to form a σ bond. One end of the

22

chain retains an unpaired electron and can add another monomermolecule. The reaction is terminated either by combination when twofree radical chains combine or by disproportionation. Free Radical Initiators:

Most monomers require some kind of initiator. A large numberof free radical initiators are available; they may be classifiedinto the following four major types;1. Organic peroxides (ROOR) and hydroperoxides (ROOH): Thesecompounds are thermally unstable and decompose thermally bycleavage of the oxygen bond to yield RO· and HO·radicals. Example;

2. Azo Compounds: RN=NR isobutyronitrile

3.Redox initiators: This is produced by one electron transferreaction. Example is;

4. Photo-initiators: These are compounds that dissociate under theinfluence of light to form radicals. Peroxides and azo-compoundsdissociate photolytically as well as thermally. Such as;

Mechanism of the reaction:Initiation of reaction:

In case of free radical mechanism of polymerixation organicand inorganic peroxides, benzoyl peroxide, acetyl peroxide, H2O2

etc. initiate to generate free radicals, then polymerizationproceeds by chain reaction; A typical example of the free radical polymerization is thepolymerization of ethene in the presence of peroxide under a veryhigh pressure (about 1000 atm) and at a temperature of more than100 0C, to give polyethene. Initiation step is

23

Propagation reaction:Chain propagation reaction involves the addition of a free

radical to the double bond (RM·) to which more monomer molecules (M)add successively. This step is bimolecular reaction.

Termination reaction:Free radical chains can be terminated by any reaction that

destroys the active chain centers. This can happen to a smallextent through the following reactions;1. With initiator radicals such as;

2. By combination of the two growing free radicals;

3. Disproportionation, where an atom is transferred from onepolymer radical to another.

4. By addition of a foreign molecules in small amount beforepolymerization;

The average molecular mass of the polymer depends on thenumber of monomer units which combine before termination, the so-called chain length. Inhibitors are also used to inhabitation orkilling the chain growth by combining with the active free-radicalsand forming either stable products or inactive free radicals, suchas p-Benzoquinone, nitrobenzene, dinitrobenzene and benzothiazine

24

are some of the inhibitors customarily used in the polymerindustry.Condensation reaction polymers:

In condensation polymerization, the chain growth is producedby elimination of small molecules such as H2O, CH2OH, etc. Forexample; The reaction between diamine and dicarboxylic acid is

Condesation of phenol and formaldehyde to form Bakelite.

Ring Opening Polymers:This type of polymerization takes place by the compounds

containing ring with at least one hetero-atom, such as cyclicsulphides, cyclic ethers, cyclic imines, cyclic propanes, lactanes,lactones etc. Normally, the polymerization of the ring openingtakes place by the use of anionic or cationic initiators.

Nomenclature of Polymers:There are three types of procedures for the naming of

polymers;1. Simple method based on source.2. Type of structure of polymers.3. Trade name.

Simple Method:In this case the prefix “Poly” is followed by the name of a

monomer. For example; Ethylene CH2=CH2 Polyethylene CH2-CH2

Styrene Polystyrene Vinyl Chloride Polyvinylchloride

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Type of structure of Polymer:This type is concerned with the functional group present in

the polymer. For example;

Trade Name:It has no systematic rule or system. Different manufacturers

market their products with different names. For example;1. Poly(methyl methacrylate). Its trade name is PMMA. It is also

soled in the market with the name of as Lucite, perspix,acrylite, plexiglass etc.

2. Poly(arylonitrile). –(CH2-CH-CN)n- also called as Orlan,Creston, Acrilan.

3. Poly(ethylene, terphthalate):

also called as Decaran, Terylene, Fortach, Mylas, Arnite.4. Poly(terafluro ethylene):

also called as, PTFE, Teflon5. PVC is also called Benvic, Carina.6. Polyamide is also called as Nylon.7. Poly(hexamethylene Sebbacamide) is also called Nylon 6-10.8. Hexamethylenediamine + adipic acid also called as Nylon6/6 or

Nylon 6,6 Nylon 66 or 66 Nylon, 6/6 Nylon or 6,6 Nylon, different

manufacturer named it according to their product or their ownchoice.

Polyethylene:Introduction:

Polyethylene is high molecular weight paraffin. It is thethermoplastic material which is soften on heating and becomes firmon cooling. It is a polymerization product of ethylene. Theconversion of the alkene into high molecular weight compounds semicrystalline (Polymers) is one of their most industrial reactions.It was first time prepared in 1934; Commercial preparation was

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carried out in 1939. A Noble Prize in 1963 was awarded to K.Ziegler and G. Natta for their contribution in alkenepolymerization.

In this polymerization long chain polymers with recurringunits are obtained. Depending on its manufacturing process, itsdegree of polymerization will vary from about 500 to 5,00,000. Thenet result of such a polymerization is opening up of the C-C doublebond and the formation of a joint molecule. Several mechanisms mayoperate during such polymerization of which four basic kinds ofpolymerization of vinyl monomers i.e. radicals, cationic, anionic,and co-ordination. The polymers obtained as a result ofpolymerization of alkenes includes the familiar names like orlon,polyethylene etc. The synthesis of these polymers gives greatrevolution of chemical industry.Raw Material:

Ethylene is prepared from natural gas or from thermal crackingof hydrocarbons. The main source of hydrocarbon is petroleumrefining. Hydrocarbon vapors are diluted with steam and heated atabout 700-900°C in the presence of catalyst like alumina or silica.The product is liquefied and then distilled.Preparation:Laboratory Methods:Ethylene is prepared in the laboratory by the dehydration of ethylalcohol at 160°C with conc. H2SO4. First ethyl hydrogen sulphate isformed which then undergoes decomposition.

Commercial Method:There are three methods which are commercially used for thepreparation of polyethyleneHigh Pressure Method:

In this method, polymerization is carried out at pressureof 1000 to 3000 atmosphere and at a temperature range from 150 to250°C. Polymerization takes place by a free radical mechanisminitiated by molecular oxygen or peroxides such as benzoylperoxide. The high pressure increases the concentration ofreactants and the probability that the growing chain will encountera monomer before termination can occur. The chain transfer reactionoccurs when a hydrogen atom is picked off the backbone of a growingintermediate by another free radical center. Chain transfer

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reaction reduces molecular weight and causes branching. The polymermade by this way is highly branched. Due to the branching the closepacking of the molecules is prevented and the crystallization isnot difficult. At this pressure the crystallinity produced about 20to 55% as compared to 60 to 90% for unbranched polyethylene.Different grades of polymers are formed by adding chain transferagents to the monomer feed.Procedure:

Ethylene is carried out in two stages, at first stage thepressure is 300 atm. and the oxygen is supplied which act as aninitiator. In the second stage pressure is 2000 atm. andtemperature is 250°C. This material is then carried out to theautoclave which is fitted with steam. At this stage polymerizationoccur as the monomer passes down the reactor. The monomers and thepolymers are both miscible and form the concentrated solution inwhich 10 to 30% conversion occurs. The process may be batch type orcontinuous process. In continuous process a long tube about 100feet is used and the temperature is maintained at about 200°C.Miscible solution is then sent to the separator where unreactedethylene is separated, which sent back by recycle. Polymer isallowed to pass through cooler and then fed to the extruder. Flow Sheet Diagram:

Zeigler Process:In the low pressure process, catalysts are used called

Zeigler-Natta catalysts for the polymerization of ethylene. These

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catalysts were developed in the early 1950’s through thediscoveries of Karl Zeigler in Germany and Giulio Natta in Italyand their importance brought Zeigler and Natta the Noble Prize inchemistry in 1963. Both heterogeneous and soluble Zeigler-Nattacatalysts are used. They are formed by combining compounds ofelements from group IV to VII with organometallics from metals forgroup I to III. Typically they are combinations of Aluminum alkylsand Aluminum alkyl halides with Ti(III) and Ti(IV) halides. Theycan be adsorbed on finely divided mineral supports to provide alarge surface area for heterogeneous processes. The function ofthese catalysts is that monomers from pi-complexes with thetransition metal component and that additional monomers areinserted one after another into the polarized metal carbon bond.Polymerization with these catalysts takes place at temperaturesbelow 100°C and pressures in the neighborhood of 2 to 4 atmosphere.Polymers are obtained in the form of suspension. Some alcohol isadded to stop reaction. Transition metal catalysts producepolyethylene series with less branching than polyethylene made byhigh pressure processes. The material is taken to the washing unit,which is performed by adding dilute HCl followed by some alkali toneutralized the excess of acid. Then water is added in excess towash it thoroughly. The polymer is separated by centrifuge machineand then sent to the blending tank where it is pulverized andfiller or colours are added. It is then sent to the extruder and tostorage.Flow Sheet Diagram:

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Phillips Process:In this process the conditions are maintained

intermediate between that of High Pressure method and that ofZeigler method. The process is carried out in the presence ofcatalyst chromium or molybdenum oxide on finely divided silica orsilica-alumina supports. Polymerization occurs between 70 and 200°Cand at pressure in the range of 30 to 40 atm. The solvent used iscyclohexane. The suspension material is held to the reactor and theethylene from the storage is taken to the reactor. The reactor isfitted with cooling arrangement in order to control the temperaturein desired range. The polymer is soluble in solvent and slurry andslurry is formed. The slurry is passed to a separator tank in orderto remove unreacted ethylene, which sent back for reprocessing. Theprecipitates of the polymer are separated by centrifuge machine andwashed with water. After washing, the polymer was dried and takento the extruder. The polymer produce is linear and of high density.Flow Sheet Diagram:

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Polystyrene:The polystyrene is a linear homo-polymers of monomer styrene.

It is thermoplastic, it soften on application of heat. It hasattracted attention because of excellent insulation properties,used in thermal insulation. It is clear transparent polymer. It cantransmit 90% of light and refractive index of 1.59. it is a verygood insulator, maintain its insulating properties in humidconditions. It is a cheap material and can be molded easily. It issoluble in wide range of solvents such as CCl4, CHCl3, benzene,toluene, MEK (methyl ethyl ketone), ethyl acetate, aryl acetate,pine oil etc. Strong oxidizing agents attack it.Draw backs:

It becomes yellowish on prolong exposure to sunlight becauseof degradation of polymer. Its mechanical properties are affectedby the addition of certain additives i.g., O-hydroxy benzo-phenone.It is readily attack by a number of solvents including dry-cleaningagents. It has a poor stability for outdoor weathering. Its twomajor mechanical defects are its brittleness and low heatdistortion temperature (softening temperature) which is only about82-100 0C. Hence polystyrene articles cannot be sterilized. Uses of Polystyrene:

It is used for injection moulding of articles such as combs,buttons, toys, buckles, high-frequency electric insulators, lenses,radio, T.V. and refrigerator parts, indoor lighting panels, etc.High impact strength polystyrene made by compounding polystyrene

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with styrene butadiene synthetic rubber is used as an injectionmoulding material for producing all the above mentioned articles. Main Supplier:

Dow chemical is the word’s largest producer with a totalcapacity of 1.8 million metric tones in the USA, Canada, andEurope.Raw Materials:

Polystyrene is prepared by the addition polymerization ofstyrene monomer unit usually in presence of a peroxide (e.g.,benzol peroxide) initiator. The monomer, styrene is prepared frombenzene and ethylene under pressure at 90 0C in presence of acatalyst AlCl3. The resulting ethyl benzene is dehydrogenated tostyrene by passing over an iron oxide or magnesium oxide oraluminium oxide catalyst at about 600 0C. The styrene is thenrefined by distillation. C6H6 + CH2 = CH2 AlCl3 C6H5-CH2-CH3 Fe2O3

C6H5-CH2=CH2 + H2

Benzene ethene 80 0C ethyl benzene600 0C StyreneIndustrial Preparation:

The different methods available for styrene polymerization are;1. Bulk polymerization.2. Solution polymerization.3. Emulsion polymerization.4. Suspension polymerization.

Bulk polymerization: This is the simplest and involves the use of monomer and

initiator only as the major components. It may be carried outeither by batch process or by continuous process. Batch operation is mostly employed when it is desired to produce anumber of small formulations in small quantities in the sameequipment. Only about 70% of the cycle time is used for carryingout the reaction and hence this process is uneconomic for large-scale preparation. But this process is advantageous to produce thepolymer in the desired form and shape from the monomer directlylike perpex sheets, cast phenolic resins, etc.n a batch reactor themonomer and catalyst are added simultaneously. From time to timelittle amount of fresh monomer is to be added. The polymer whichforms a particular shape is taken out after proper residence timein the reactor. In continuous process the conversion of monomer to polymer may becarried out to certain percent and thereafter the concentration ofmonomer-polymer is maintained and fed into another reactor for

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better heat-transfer. This process is generally carried out in twostages;The pre-polymerization (first stage) is carried out in O2

atmosphere in the reaction vessels, which on decomposition givefree radical, that initiate polymerization. It is heated hot jacketor hot water coils at about 80 0C and require 10-30 hours. Themonomer is added continuously with stirring at constant rate intothe vessel. The monomer is polymerized to about 35%.In second stage, the polymerization mixture, which is highlyviscous and in a clear state, is fed into the upper end of avertical reactor, which in turn, is further subdivided into anumber of zones, for post-polymerization operation. Reactor zonesare kept at desired temperatures by means of superheated steam,except the lowest one, which is electrically heated to get maximumheat necessary for complete conversion of monomer. As, the materialgoes down, the viscosity go on increasing because the remainingportion is converting to polymer. The highest possible conversionof monomer is carried out at the lowest zone and the resultingmolten polymer is discharged into the extruder unit. Polymer isconveyed through conveyor belts over an air or water coolingsystems, followed by chopping and grinding of the solid polymer.Schematic Flowsheet-Bulk Polymerization.

Solution Polymerization:

Solution polymerization is commonly referred to as masspolymerization in the industry. The vast majority of allpolystyrene produced today is produced via this technology.

In this method, the styrene monomer is dissolved in a suitablesolvent like ethyl benzene and initiator such as boron trifluoride,is then added to start the polymerization reaction. There are twotypes of processes, one is batch process and the other iscontinuous process.In batch process 80% conversion is completed.But today, continuous process is most widely used. The solution is

33

continuously prepared in a holding vessel and will then be injectedinto the reactor system. The polymer is precipitated by adding anon-solvent to the mixture. Sometimes the polymer itself isinsoluble in the polymerization. The solid polymer is then filteredoff, washed free of any adhered monomer or solvent and dried beforeuse. The solvent-unreacted monomer-non-solvent mixture is separatedby distillation or any other suitable methods and reused. Thispolymerization is accomplished in a chain of three reactors withheating and cooling arrangement. The polymer is added into thefirst reactor along with the initiator. The solvent is used in thefirst two reactors. The polymerization reaction gives off heat thatis carried away from the reactors by jacketing them with a heattransfer fluid. The temperature of the reactors should not very bymore than 15 0C throughout the reactor series. The reactiontemperature range is 40-70 0C. There are certain systems in whichwater can be used as solvent for polymerization like N-vinylpyrolidone, acrylic acid, acrylonitrile, etc. This provides acheap solvent and also permitting the use of inexpensive inorganicredox initiator. Monomer conversions of up to 85% polystyrene areobtained in these reactors.

The disadvantages of this polymerization technique are bulkhandling, tiresome mechanical separation processes, purification ofthe solvents for reuse and difficulty to remove the solvent fromthe monomer completely. However, the most serious problem is due tochain transfer to solvent restricting the polymer molecular weightvalue and reducing the rate of polymerization. Schematic Flowsheet-Solution Polymerization.

Emulsion polymerization: This method is applicable to almost all vinyl compounds and

dienes except those which have a good water solubility likeacrylonitrile and vinyl acetate. In this method, the monomers aredispersed as fine droplets in a large amount of water and then

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emulsified with common soap or special emulsifying agent. Tostabilize the emulsion, protective colloids like casein, gum,gelatin or dextrine are used. The initiator is usually a redox typewhich may be either water-soluble or organo-soluble. The rate ofpolymerization is dependent on the number of carbon atoms in thesoap upto C12 and then becomes independent of it. Industry normallyuses a soap containing 16-18 carbon atoms. To conduct thepolymerization, the pH of the medium must be controlled and innormal case should be slightly alkaline. To achieve this, bufferslike sodium acetate or sodium phosphate are often used. Catalystsused are highly chlorinated aliphatics like carbon tetrachloride,inorganic cyanides, organic sulphur compounds, etc. they regulatethe molecular weight and its distribution and suppress branchingand cross-linking formation tendencies. The rate of polymerizationincreases with increase in temperature. But higher temperatureleads to poor quality products as regard to properties likeabrasion resistance, flux resistance, tensile strength, modulus,etc.Process:

Polymerization is accomplished in 3 to 5 m3 enameled reactorsfitted with a water jacket stirrer and reflux condenser. Themonomer is suspended in aqueous phase using a stabiliser. Sodiumsulphate is added to control pH. A through agitation keeps themonomer suspended in medium. The aqueous phase emulsified and mixedwith monomer. The emulsion is sent into reactor which is heated andkept at 60 0C. There may be number of reactors in series,polymerization is carried out in nitrogen atmosphere. Catalysts andchain transfer agents are added into the reactor. The reactiontakes place for 3 to 6 hours, after which it is sent intocoagulator. The polymer from the latex is separated bycentrifugation. The polymer is wased and sent to dryer. Suspension polymerization:This method was developed with a view to improve and simplify theemulsion polymerization method. The principle is to add noemulsifying agent but to keep relatively large drops (particle size0.1-1 mm) of the monomer dispersed in the non-solvent by mechanicalagitation. This method is also called pearl or bead polymerization.There are many different ways of making polystyrene usingsuspension process. Most producers use a batch process In thesuspension process a number of small styrene drops 0.15-0.50mm indiameter are suspended in water. Detailed process of suspension polymerization:The requirements of polymerization are:

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a) Initiator. b) Suspending agent. c) Stabilizing agent.d) CatalystInitiators:

The initiators generally used are benzoyl peroxide and t-butylhydroperoxide.Suspending agent:

To aid in the formation of the proper size drops a suspendingagent is added. Some typical suspending agents are methylcellulose,ethyl cellulose and polyacrylic acids. Their concentration in thesuspension is between 0.01-0.5% of monomer charged.Stabilizing agent:

To keep the drops at proper size, a stabilizing agent isadded. The stabilizing agents are often insoluble inorganic such ascalcium carbonate, calcium phosphates or bentonite clay. They arepresent in small amount than the suspending agents.Catalyst:

A catalyst is used to control the reaction rate. The catalystsare usually peroxides. The most common ones are benzoyl, diacetyl,lauroyl, caproyl and tert-butyl. Their concentration varies from0.1-0.5% of the monomer charged. The ratio of monomer to dispersingmedium is between 10 and 40%.Polymerization temperature:

Polymerization of styrene occurs at temperature range of 90-950C.Process description:

The suspension method is carried out in large reactorsequipped with agitators, the styrene monomer being maintained inthe aqueous phase as droplets with a diameter varying between 0.4-1mm by use of a dispersing agent such as partially hydrolyzedpolyvinyl acetate, inorganic phosphates or magnesium silicates. Toreduce the cycle time of the reactors, the entering water andstyrene will be preheated. The temperatures of the input streamswill be sent so as to obtain the desired reaction temperature. Thewater entering the reactor will be heated to 95 0C. The bulk of thestyrene is to be heated to 85 0C before being charged. This is donein a vertical double pipe heat exchanger, which is directly abovethe reactor. The catalyst, rubber stabilizer, and suspending agentare premixed in styrene and discharged by gravity into the reactor.This mixture will not be preheated, since it might polymerize.Typical water to monomer ratios is 1:1 to 3:1. A combination of twoor more initiators is used with a programmed reaction temperatureto reduce the polymerization time to a minimum for a given amountof residual styrene.

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Purification steps and Extrusion:If the water can be removed using physical separation

processes, then the styrene and the other impurities dissolved init will also be discharged. A centrifuge with a washing step willbe used to do this. The material leaving the centrifuge has 1-5%water. The final purification step is drying. The polystyreneleaving this unit must meet the specifications set. (0.03% water).Then it is passed through a devolatisation extruder to remove thevolatile residues and to convert the polymer into pellets. It wasassumed that 3% of polystyrene would be removed from the process inairveying, drying, centrifuging, transferring, or as bad as badproduct. At least 95% of that which is lost in processing must beintercepted before it leaves the plant. Most of it can be removedand sold as off-grade material. This waste is split among thevarious streams leaving the processing area.Polyurethane:

There are polymers containing the group typically formed byreaction between di-iso-cyanates; R(CNO2)2 e.g. ethylenediisocyanate.

Polyalcohol e.g. ethylene glycolSome other groups such as urea, ester also may be included

such a diversity of structure makes a very wide range of polymerproperties possible. Manufacture:There are three stages in their preparation

1. Production of a polyester by condensing adipic acid with aglycol until a molecular weight approximately 5000 isreached. Several other glycols can be used.

Hard waxy product, melting point 60 to 70°C with molecularweight 2000 to 5000.

2. To prepare a prepolymer by reaction with diisocyanate. Excessisocyanate over that calculated is used to ensure that endgroups are isocyanate.

R(CHO)2 may be

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Urethane linkage

Product has molecular weight 70,000 to 80,000.3. For further linking together, the molecules by the action of

chain extender a glycol, diamine or water in the terminal 180cyanate groups.

Reaction of disocyanate and diols usually lead to linearpolymers and polyisocayanates and polydiols produce branchedcopolymers. The diol usually used is a low molecular masspolyester( derived from adipic acid and ethylene glycol) withterminal hydroxyl groups.

Vulcanization or conversion of brought into threedimensional group in one chain reacting with amino group of a ureagroup in another chain to give a methane bridge.

Properties:Outstanding properties of this class of polymers are;

1. High abrasion resistance and tear strength even without carbonblack.

2. Resistance to aliphatic solvents3. Non inflammable

Application:1. They are widely used in foams. Addition of small amount of

water during polymerization hydrolysis some of the isocyanateto give a light mass spongy material called polyurethane foam.

2. It is used for building insulation, coating and padding and inpillows.

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3. For making thread for elastic garments and sports food, toplift for babies heal soles and also engineering components ofresistant type.

Phenol Formaldehyde: Adolph Von Baeyer was the first chemist (in the latterpart of the nineteenth century) to discover the reaction betweenphenol and formaldehyde in the presence of aqueous sodiumhydroxide. Later on Leo Bacheland has studied on this reaction andwas able to show that it had a very high molecular weight.Preparation: There are the most versatile materials in the entirefamily of polymers. Their versatility is due partly to the low costof raw materials and partly because of the many forms in whichthese can be made to fit specific requirements. Its raw materialsare phenol and formaldehyde. Phenol formaldehyde resins are theoldest of the commercially important polymers were patented in 1909by L.H. Backland and are thus known as Bakellite. Phenol isobtained as a byproduct from the reduction of bituminous coal.Cresols and xylenols are sometime used in place of or with phenolto develop special resin formulations. Formaldehyde, normally agas, is produced from methyl alcohol by several methods. But it isused in the form of formalin which is nothing but a 37 to 40% watersolution of formaldehyde. Phenol reacts with aldehyde to givecondensation products if there are free positions on the benzenering at ortho and para to the hydroxyl group. Phenol-formaldehydeplastic or Backelite is a high molecular weight substance whereinseveral phenol rings are linked together by methylene (-CH2-)groups. This is obtained by polymerization of a monomer obtained bycondensation of one molecule of phenol and one molecule offormaldehyde using basic or acidic catalysts. The followingsequence of reaction may be considered for this polymerization.Chemical reaction:1) Reaction of phenol with formaldehyde to yield monomer, o- or p-hydroxymethyl phenol. This reaction will take place at the ortho orpara position and is known as methylol derivatives.

39

These products, which may be considered the monomers forsubsequent polymerization, are formed in neutral or alkalinecondition. These two conditions are described as follows:Novolac formation. The condensed and bridge formation of polymer iscalled novalac formation. In case of phenol formaldehyde methylolderivatives condense with phenol to form dihydroxydiphenylmethane.Acid catalysts 1. 3,5-Xylenol2. m-cresol 3. p-cresol4. o-cresol The reaction will proceed at a minimum at a pH of 4-5.Hydroxymethylphenol with a second molecule of phenol with the lossof water to yield a compound where two rings are joined via a –CH2-group.

As three positions in each phenol ring (i.e. two orthoand one para) are further susceptible to attack, the sequence ofreaction continues further to afford phenol-formaldehyde resinsknown as bakelites which are cross-linked three dimensionalpolymers. These are called novolacs with the structure, where orthoand para links occur at random.

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By controlling the degree of polymerization phenol-formaldehyde plastic can be obtained in different forms. However, apure phenolic resin is rarely moulded without modification toovercome limitations imposed by the moulding process and end-userequirements. Phenolic resins are brittle, and have high shrinkagecaused by polymerization and elimination of water, dimensions aredifficult to control. To overcome these difficulties resinmanufacturers resort to use of additives such as fillers, binders,hardening agents, plasticizers etc. to produce a desired compound.Each filler results in a different compound. Molecular weights mayrange as high as 1000, corresponding to about ten phenyl residues. Base catalyst: Resole formation: The polymers of phenol and formaldehyde containingalcohol groups are called resoles. The methyl phenols in thepresence of alkaline catalysts and with more formaldehyde condenseeither through methylene linkages or through ether linkages. At theend of the reaction subsequent loss of formaldehyde may occur withmethylene bridge formation. The four major reactions in phenolicresin are as follows:

(a) Addition to give methylol phenols;(b) Condensation of a methylol phenol and a phenol to give a

methylene bridge;(c) Condensation of two methylol groups to give an ether

bridge; and(d) Decomposition of ether bridges to methylene bridges and

formaldehyde, the latter reacting again by the first mod.Basic catalysts

1. Monoethylamine2. Diethylamine3. Triethylamine4. Sodium hydroxide 5. Tetraethyl ammonium hydroxideUsually, the amount of basic catalyst used is 0.05 moles per kgof reactants.

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Production of this type of resin is soluble and fusible butcontaining alcohol groups are called resoles. The formation ofresoles and novolacs, respectively, leads to the production ofphenolic resins by one stage and two-stage processes.

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AApplications

1. A number of industrial applications are based on the excellentadhesive properties and bonding strength of the phenolies.

2. Phenolies are widely used in the production of ion exchangeresins, with amine, sulfonic acid, hydroxyl or phosphoric acidfunctional group.

3. Phenolic resins are widely used in varnishes, electricalinsulation and other protective coating.

4. The ability of phenolic resins to withstand at very hightemperatures is important for their use in messile nosc cones.

5. Phenolic products are outstanding heat resistance, dimensionalstability and resistance to cold flow. They are widely usedfor their good dielectric properties in electrical,automotive, radio and television and appliance parts.

Amino resins:43

Useful resins are formed by the polymerization ofpolyfunctional amines and amides with formaldehyde. Thus, twoimportant classes of amino resins are the condensation products ofurea and of melamine with formaldehyde. They are consideredtogether because of the similarity in their production andapplications. In general the melamine resins have somewhat betterproperties but are higher in price.Urea formaldehyde resins:Chemistry and production: Urea and formaldehyde undergo polymerization to afford apolymer with a common trade name “Beetle”. This polymer isconsiderably cross linked. The first step in the preparation of urea formaldehyderesin is the reaction of urea with formaldehyde in aqueous solutionto form methylol compound.

The reaction is carried out under slightly alkaline conditions andwill yield crystalline derivatives of dimethylol urea containing upto six methylol groups.

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Properties 1. These resins are clear and colourless, so that objects of

light or pasted colour can be produced.2. Urea formaldehyde resins are used as fillers. 3. Urea formaldehyde resins are more beneficial because of their

colour ability, solvent and grease resistance, surfacehardness, mar resistance, the urea resins are widely used forcosmetic container closures, appliance housings and storehardware.

4. Urea formaldehyde resins are widely used for adhesives,largely for play wood and furniture

5. The urea based enamels are used refrigerator and kitchenappliances.

6. Urea formaldehyde resins are preferable on phenolic resins dueto following reasons. Three theories are put forward,

(i) The urea molecules, somewhat smaller in size thanphenolic and is capable of penetrating into thecellulose structure through pore spaces.

(ii) The water soluble urea may be carried through the cellwall by capillary action.

(iii) A direct chemical attraction occurs betweensubstituents.

Melamine foraldehyde resins: Chemistry and production: Melamine with formaldehyde is another class of aminoresins. It is of better properties and is higher in price. Thispolymer is also known as Melmack and is industrially manufacturedfrom calcium Cyanamid. Cyanamid is unstable and rearranges intodicyandiamide. Which reacts with ammonia and methanol under heatand pressure to yield melamine .The chemical reaction is as follow;

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Properties and Applications1. Melamine resins have better hardness heat resistance and

moisture resistance than urea resins but less thanphenolic resins.

2. Melamine resins are widely used for the production ofdecorative laminates. These laminated sheets are used forcounter, cabinet and table tops.

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3. The melamine resins are widely used for adhesives, largelyfor plywood and furniture.

4. Melamine resins modify textiles such as cotton and rayon byimparting crease resistance, stiffness, shrinkage control,fire retardace and water repellency.

5. Melamine is capable of combining with a wide variety offillers to extend the range of moulded products. α-Celluloseis used as general purpose filler, cotton fabric or fibrousglass is used to increase the impact strength, asbestos isadded to make them heat resistant and mineral fillers forelectrical properties.

6. Melamine is the best choice for impregnated paper used as thetop sheet for decorative panels.

7. Melamine is also used as an adhesive. Although more expensivethan urea glues, melamine adhesives have better waterresistance.

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