unit-4 high polymers

8/18/2019 Unit-4 High Polymers

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NARASARAOPETA ENGINEERING COLLEGE - NARASARAOPE ENGG.CHEM

Dr. P.V.Narayana Page

UNIT – IV

HIGH POLYMERS

1. INTRODUCTION

Polymers are ‘macromolecules’ built up by the linking together of a large

number of small molecules or units. Thus, small molecules which combine with each

other to form polymer molecules are termed monomers and the “repeat unit” in a

polymer is called as monomer.

For example, polystyrene is a polymer formed by linking together of a large

number of styrene monomer molecules. Similarly polythene is formed by the linking of

ethylene (C2H4) monomer molecules.

The number of repeating units (n) in a polymer is known as the “Degree of

polymerization” (DP). The process of formation of a polymer from its monomer units is

termed as Polymerization . Many polymers are naturally occurring like starch, cellulose

etc., and as many are synthetically made such as polystyrene, PVC, etc. The organic

polymers like starch, polyethene have carbon backbone, while the inorganic polymers

have atoms other than carbon, which have catenation property like silicon, sulphur,

phosphorous. E.g: silicates.

Oligo Polymers : Polymers with low degree of polymerisation are known as

oligo polymers, their molecular weight < 10000.

High Polymers : Polymers with high degree of polymerisation are known as

high polymers, and their molecular weight ranges from 10,000 - 2, 00,000.


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Dr. P.V.Narayana Page2

1.1. Nomenclature of polymers

Polymers consisting of identical monomer units are called homo-polymers and

monomers of different chemical unit structures are called hetero-polymers or co-

polymers.

-M-M-M-M-M-M-M-M- -M1-M2-M1-M2-M1-M2-M1-M2-

Homopolymer Hetero or copolymer

Based on the arrangement of monomeric units (structural units), copolymers can be

classified as:

Alternating copolymers: These polymers are formed by regularly altering the

two different monomeric units.

-M1 – M2 – M1 – M2 – M1 – M2 –

Statistical copolymer (Random copolymer): These are copolymers in which the

sequence of monomer units follows a statistical rule. The probability of finding a

given type of monomer unit, at a particular point in the chain is equal to the mole

fraction of that monomer unit in a chain.

-M1 – M2 – M2 – M2 – M1 – M2 – M1 – M1

Block copolymer: The copolymer consisting of two or more homopolymer

subunits linked through covalent bonds is called a block copolymer.

-M1 – M1 – M1 – M1 – M2 – M2 – M2 – M2 – M3 – M3 – M3 – M3-

1.2. Functionality: In the process of polymerization, for any molecule or unit to act as a

monomer, it must have at least two reactive sites or bonding sites for the extension of a

monomer to a dimer, trimer and ultimately a polymer. The number of such reactive

sites in the monomer is termed as its functionality.

Eg: In ethylene the double bond can be considered as site for two free valancies. Thus, ethylene is

considered to be bifunctional.

If the monomer has bifunctionality, it can only form a linear polymer. If the

functionality is more than two, the monomer has a chance to form cross linked

polymers having 2D or 3D structures. Based on functionality and the process of

polymerization, the polymer may be present in linear, branched or cross-linked (three-

dimensional) structure.


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2.STEREO-SPECIFIC POLYMERS

Tacticity: The stereo chemical placement of the asymmetric carbons in a polymer chain

is called tacticity. The differences in configuration or arrangement of functional groups

on the carbon backbone of the polymer (tacticity) affects the physical properties. Basedon the stereo chemical orientation of the atoms or groups at asymmetric carbons, the

polymers can be classified as

1. In a head-to-tail configuration, if the arrangement of functional groups are all on the

same side of the chain, it is called as an isotactic polymer. e.g., PVC

2. If the arrangement of functional groups is in an alternating fashion in the chain, it is

called syndiotactic polymer. e.g., gutta-percha.

3. If the arrangement of functional groups is at random around the main chain

without any regularity, it is called atactic polymer. e.g., polypropylene.

Fig.: Tacticity in the polymers

3.POLYMERIZATION

The chemical reaction by which the monomers are combined to form the

polymer is called polymerization.

Two different types of polymerization processes i) Addition or chain

polymerization and ii) Step or condensation polymerization.


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Addition or chain polymerization: The original monomeric molecule, usually,

contains one or more double bonds. In this addition polymerisation there is no

elimination of any byproduct. It is a reaction that yields a polymer, which is an exact

multiple of the original monomeric molecule. The polymer will have the same

chemical composition as the monomer.

Eg 1: Polyethylene is produced from ethylene.

n CH2=CH2 Heat/pressure/catalyst (CH2-CH2)n

Ethylene Polyethylene

Eg 2: PVC is produced from vinyl chloride.

n CH2=CH Heat/pressure/ catalyst (CH2-CH)n Cl Cl

Vinyl chloride Polyvinyl chloride

Condensation or step polymerization: Condensation or step-polymerization may be

defined as “a reaction occurring between monomers having simple polar functional

groups ( like -OH, COOH etc.,) forming a polymer by the elimination of small

molecules like water, HCl, ammonia etc. For example, hexamethylene diamine and

adipic acid condense to form a polymer, nylon 6:6.

The molecular weight of a condensation polymer is always less than the integral

multiple of their monomer units. Condensation polymerization is an intermolecular

combination, and it takes place through different functional groups (in the monomers)

having affinity for each other in a step-wise process. When monomers contain three

such functional groups, they may give rise to a cross-linked polymer.

Eg: Hexamethylene diamine and adipic acid condense to form a polymer, Nylon 6:6

n H2 N - (CH

2)

6 - NH

2+ n HOOC - (CH

2)

4 – COOH

Hexamethylene diamine Adipic acid

-(HN -(CH2)6-NH –C = O –(CH2)4 –C =O)- n

Nylon 6:6


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Differences between addition and condensation polymerization processes:

Addition polymerization Condensation polymerization

1. The functionality of a monomer is

the Π bond which is bifunctional

1. The functionality of a

monomer 2, 3 or any.2. The polymerization is by self

addition and is by chain

mechanism.

2. The polymerization is by

condensation which is slow

step wise.

3. No by products are produced. 3. By products of small molecules

like H2O, NH3 , CH3OH & HCl

are formed.

4.

The molecular weight of the

polymer is

sum of molecular weights of

monomer.

4.

The molecular weight of the polymer is

less than the sum of molecular weights

of monomers

5. The process is highly exothermic. 5. The process not exothermic.

6. An initiator is required for the

reaction.

6. A catalyst is required for the reaction

7. Thermoplastics are produced

Eg:polyethylene , PVC etc.

7. Thermosetting plastics are produced

Eg:Bakelite, urea formaldehyde

3.1. Mechanism of Addition Polymerisation

The mechanism of addition polymerisation can be explained by any one of the

following three types.1. Free radical mechanism

2. Ionic mechanism

3. Co-ordination mechanism

All the above mechanisms occur in three major steps namely,


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(i) Initiation

(ii) Propagation and

(iii) Termination

Free radical mechanism

(i) Initiation

It is considered to involve two reactions.

(a) First reaction involves production of free radicals by homolytic dissociation of an

initiator (or catalyst) to yield a pair of free radicals (I.).

I 2I.

Intiator Free radicals

Examples of some commonly used thermal initiators

Thermal initiator is a substance used to produce free radicals by homolytic

dissociation at high temperature.

70 --- 800 C

(i) CH3COO – OOC CH3 2(CH3COO).

Acetyl peroxide Free radicals

80---95o C

(ii) C6H5COO – OOCC6H5 2(C6H5COO).

Benzoyl peroxide Free radicals

(b) Second reaction involves addition of this free radical to the first monomer to

produce chain initiating species.

H H

I. + CH2 = C R – CH2 – C

X X

Free radical First monomer Chain initiating species


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(ii) Propagation

It involves the growth of chain initiating species by successive addition of large

number of monomers.

H H H H

R - CH2 - C. + n CH2 = C I –( CH2 – C_)n __CH2 – C

.

X X X X

Growing chain (living polymer)

The growing chain of the polymer is known as living polymer.

(iii) Termination

Termination of the growing chain of polymer may occur either by coupling

reaction or disproportionation.

a) Coupling (or) Combination

It involves coupling of free radical of one chain end to another free radical forminga macro molecule.

H H H H

R – CH2 – C. +

.C – CH2 –I I – CH2 – C – C – CH2 - I

X X X X

Dead polymer

b) Disproportionation

It involves transfer of a hydrogen atom of one radical centre to another radical

centre, forming two macromolecules, one saturated and another unsaturated.

H H H H H H H H

R – C – C. + .C – C – R R –C =C + H- C – C - R

H X X H X X H

Unsaturated saturatedmacromolecule macromolecule

The product of addition polymerisation is known as Dead polymer.


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2.Ionic Polymerisation

The addition polymerization that takes place due to ionic intermediate is called

ionic polymerization. Based on the nature of ions used for the initiation process ionic

polymerization classified into two types;

(a) Cationic polymerization and (b) Anionic polymerization

(a) Cationic polymerization : Cationic polymerization depends on the use of cationic

initiators whi ch include reagents capable of providing positive ions or H+ ions.

Typical examples are aluminum chloride with water (AlCl3+H2O) or boron

trifluoride with water (BF3+H2O). They are effective with monomers containing electron

releasing groups like methyl (−CH3) or phenyl (−C6H5) etc. They include propylene

(CH3−CH=CH2) and the styrene (C6H5−CH=CH2).

(i) Chain Initiation:

Decomposition of the initiator is shown as

BF3 + H2O → H+ + [HOBF3]

–

The proton (H+) adds to C – C double bond of alkene to form stable carbocation

[HOBF3] –

H+

+ CH2=CHX →CH3-+CHX [HOBF3]

–

(ii) Chain Propagation: Carbocation add to the C – C double bond of another monomer

molecule to from new carbocation.

CH3-+CHX [HOBF3]

–+ CH2=CHX → CH3-CHX- CH2-

+CHX [HOBF3]

–

CH3-CHX- CH2-+CHX [HOBF3]

–+ n CH2=CHX →

CH3-CHX-(CH2-CHX)n- CH2-+CHX [HOBF3]

–

Living Polymer

(iii) Chain Termination: Reaction is terminated by coupling of negative ion to

carbocation.

CH3-CHX-(CH2-CHX)n- CH2-+CHX [HOBF3]

– →

CH3-CHX-(CH2-CHX)n-CH2-CHX-OH + BF3

Dead polymer


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(b) Anionic Polymerization

Anionic polymerization depends on the use of anionic initiators which include

reagents capable of providing negative ions. Typical catalysts include sodium in liquid

ammonia, alkali metal alkyls, Grignard reagents and triphenylmethyl sodium[(C6H5)3C-Na]. They are effective with monomers containing electron withdrawing

groups like nitrile (–CN) or chloride (- Cl), etc They include acrylonitrile

[CH2=CH(CN)], vinyl chloride [CH2=CH(Cl)], methyl methacrylate

[CH2=C(CH3)COOCH3], etc.,


Decomposition of the initiator is shown as

R

-

Na

+

+ CH2=CHX→ R

-CH2-

-

CHX + Na

+

(ii) Chain Propagation: Carbocation add to the C – C double bond of another monomer

molecule to from new carbocation.

R-CH2--CHX + CH2=CHX → R- CH2-CHX-CH2-

-CHX

R- CH2-CHX-CH2--CHX + n CH2=CHX → R- CH2-CHX-(CH2-CHX)n-CH2-

-CHX

Living Polymer

(iii) Chain Termination: Reaction is terminated by addition of H+.

R- CH2-CHX-(CH2-CHX)n-CH2--CHX +H+→ R - CH2-CHX-(CH2-CHX)n-CH2-CH2X

Dead Polymer

3.Coordination Polymerization

It usually involves transition-metal catalysts. Here, the "active species" is a

coordination complex, which initiates the polymerization by adding to the monomer’s

carbon-carbon double bond. The most important catalyst for coordination

polymerization is so-called Ziegler-Natta catalyst discovered to be effective for alkene

polymerization. Ziegler-Natta catalysts combine transition-metal compounds such aschlorides of titanium with organometallic compounds [TiCl3 with Al(C2H5)3]. An

important property of these catalysts is that they yield stereoregular polymers when

higher alkenes are polymerized, e.g., Polymerization of propene produces produces

polypropene with high selectivity.


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Cat-R + CH2=CHX → Cat-CH2-CHX-R

(ii) Chain Propagation :

Cat-CH2-CHXR+ CH2=CHX → Cat- CH2-CHX-CH2-CHX-R

Cat- CH2-CHX-CH2-CHX-R +nCH2=CHX → Cat- CH2-CHX-(CH2-CHX)n-CH2-CHX-R

Living Polymer

(iii) Chain Termination: by the addition of HX

Cat- CH2-CHX-(CH2-CHX)n-CH2-CHX-R + HX

Cat-X+ CH3-CHX-(CH2-CHX)n-CH2-CHX-R

Dead Polymer

4. PHYSICAL AND MECHANICAL PROPERTIES OF POLYMERS

4.1. Physical Properties

Crystallinity: The degree of structural order arrangement of polymeric molecules is

known as crystallinity. Crystallinity favours denser packing of molecules, thereby

increasing the intermolecular forces of attractions. This accounts for a sharp and highersoftening point, greater rigidity and strength. The polymers with low degree of

symmetry and with long repeating units are partially crystalline and are amorphous in

structure. The crystalline polymer units have packing close to each other through

intermolecular forces. Completely crystalline polymers are brittle. The crystallinity

influences properties like solubility, diffusion, hardness, toughness, density and

transparency of polymers.

Amorphous state: Random arrangement of molecules, less intermolecular forces lead to amorphous nature of a polymer. So they can be moulded into a desired shape. Both

thermosetting and thermoplastic polymers can exist in amorphous state.

4.2. Mechanical Properties:

Strength: The polymer chains adjacent to each are held together by weak


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intermolecular forces. The strength of intermolecular forces can be increased by either

increasing chain length or molecular weight or the presence of polar groups (-OH, -

COOH, -OMe, -COOR, -X). The lower molecular weight polymers are quite soft and

gummy. High molecular weight polymers are tough and heat resistant. The cross linked

polymer chains are strongly linked to each other by strong covalent bonds, which cause

greater strength, toughness, brittleness and low extensibilities. The strength of the

polymer is characterized by the stress and strain curve. Strength of the polymers also

depends on the shape of the polymer.

Eg: In PVC, large size chlorine atoms are present. The strong attractive forces restrict the

movement of molecules and so PVC is tough and strong.

b) Elastic character:. Elasticity is the relaxation to original shape after removal of

applied stress. Polymers like nylon, having this stretching nature are called elastomers.

Elastomers are slightly cross linked, amorphous and rubber like polymers. In the

absence of deforming forces these polymers have peculiar chain configuration of

irregularly coiled ‘snarls’. So the polymer is amorphous due to random arrangement.

When they are stretched cross-links begin to disentangle and straighten out.

c) Plastic deformation: This is found in thermoplastics. These polymers have structure

which is deformed under heat or pressure. This property is used to process them into

desired shape. Due to weak inter molecular forces, these polymers show permanent

deformation at high temperature and pressure. The Vander wall forces are weak in a

linear polymer at high temperature and result in ‘slippage’. The plasticity of a polymer

decreases with temperature.

5. PLASTICS

Plastics are polymers which can be moulded into any desired shape by the

application of heat and pressure in the presence of a catalyst. They undergo

permanent deformation under stress termed as plasticity. The terms plastic and resin

are used synonymously. Plastics are obtained by mixing a resin with other

ingredients to impart special engineering properties. These are characterized by light

weight, good thermal and electrical insulation, corrosion resistance, chemical


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resistance, adhesive nature, low cost, high abrasion resistance, dimensional stability,

strength, toughness and impermeability to water. A plastic material should have

sufficient rigidity, dimensional stability and mechanical system at room temperature

to serve as a useful article. It may be moulded to shape by application of reasonabletemperature and pressure.

Types of Plastics

Thermoplastics

These are linear, long chain polymers, which can be softened on heating and

hardened on cooling reversibly. Their hardness is a temporary property and it

changes with the raise or fall of temperature. They can be reprocessed.

Examples: Polyethylene (PE), Polypropylene (PP), Polyvinyl chloride (PVC),polystyrene (PS), Nylons, Poly tetra fluoro ethylene (PTFE) etc.

Thermosetting Plastics

These polymers, during moulding get hardened and once they are solidified,

cannot be softened i.e, they are permanently set polymers. During moulding, these

polymers acquire three dimensional cross-linked structure, with strong covalent

bonds. Thermosets once moulded cannot be reprocessed.

Examples: Polyester (terylene), Bakelite, epoxy- resin (araldite), Melamine, urea-formaldehyde resin etc.

Difference between thermoplastic and thermosetting resins

Thermoplastic resins Thermosetting resins

1. They are formed by addition

polymerization

1. They are formed by condensation

polymerization

2. They consist of linear long chain

polymer

2.

They consist of three dimensional

network structure

3. All the polymer chains are held

together by weak vanderwaals

forces

3. All the polymer chains are held

together by strong covalent bonds


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4. They are weak, soft & less brittle4. They are strong, hard & more

brittle

5. They soften on heating & harden on

cooling5. They do not soften on heating

6. They can be remoulded 6. They cannot be remoulded

7. They have low molecular weights 7. They have high molecular weights

8. They are soluble in organic solvents8. They are insoluble in organic

solvents

5.1.Compounding of a plastic

A high polymeric material is mixed with 4 to 10 ingredients during fabrication,each of which these ingredients either discharge a useful function during moulding

or impart some useful property to the finished article. This is called a mix. Some of

the main types of compounding ingredients are:

(1) Resin or a binder; (2) Plasticizers; (3) Fillers; (4) Plasticizers / Lubricants; (5)

Catalysts or accelerators; (6) Stabilizers.

Resin or a binder: The product of polymerization is a resin, which forms the major

portion of the body of the plastic. It also holds the different constituents together. Thebinders used may be natural or synthetic resin or cellulose derivatives. Resin forms

the major part of the plastic and determines the types of treatment needed in the

moulding operations.

Plasticizers: These are the materials that are added to resins to increase their

plasticity and flexibility. Their action is considered to be the result of the

neutralization of part of the intermolecular forces of attraction between macro

molecules. They decrease the strength and chemical resistance. They impart greaterfreedom of movement between the polymeric macro molecules of resins. Most

commonly used plasticizers are vegetable oils (non-drying type),camphor, esters (of

Stearic, Oleic or phthalic acids) and some phosphates ( tricresyl phosphate, tributyl

phosphate, tetra butyl phosphate and triphenyl phosphate)


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Fillers: Fillers are added to give to the plastic better hardness, tensile strength,

opacity, finish and workability. They reduce the cost, shrinkage on setting and

brittleness. They are also added to impart special characters to the product. The

percentage of fillers is up to 50% of the total moulding mixture.

Eg: a). Carborundum and mica are added to provide extra hardness b). Barium salts are

added to make plastic impervious to X- rays. c ). Addition of asbestos provides heat and

corrosion resistance. Most commonly used fillers are wood flour, asbestos, china clay,

talc, gypsum, metallic oxides like ZnO, PbO and metal powders like Al, Cu, Pb etc.

The fillers which enhance mechanical strength are reinforcing fillers.

Eg: Addition of carbon black to natural rubber, increase its strength to 40% and also

enhances its abrasion resistance.

Plasticizers/Lubricants: Lubricants like waxes, oils, soaps are employed to make the

moulding of plastic easier. They impart a glossy finish to the products. They also

prevent the plastic material from sticking to the fabricating equipment. They make

moulding easier and impart glossy flawless finish to the product. Commonly used

lubricants are waxes, oils, stearates, oleates and soaps.

Catalyst/Accelerator: These are added to thermosetting plastic, during mouldingoperation, to accelerate the polymerization of fusible resin, into cross-linked infusibleform.

Eg: Catalysts used for compounding include H2O2, benzoyl peroxide, acetyl sulphuric

acid, metals like Ag, Cu, and Pb; metallic oxides like ZnO, NH3 and its salts.

Stabilizers: They improve the thermal stability during polymerization and further

processing. Vinyl chloride shows a tendency to undergo decomposition and

discoloration at moulding temperature. Hence, during moulding, heat stabilizers are

used. Commonly used stabilizers

a) Opaque moulding compounds like salts of lead (viz. white lead, litharge, lead

chromate, red lead etc.)b) Transparent moulding compounds like stearates of lead, Cd and Ba.

6.6. Colouring materials: Color and appeal are very important for commercial high

polymer goods. Commonly used coloring materials are organic dye stuffs and opaque

inorganic pigments.

Eg: Carbon black, anthraquinones(yellow), azodyes(yellow,red),phthalocyanins (green).


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5.2. Fabrication of Plastics into Articles

The fabrication of plastic into commercial goods is done by moulding

Moulding: Moulding is an important method of fabrication of plastic. The moulding of

the plastic is done around a metal insert so that the finished product has a metal part

firmly bonded to the plastic.

Commonly used moulding methods are

(i). Compression moulding

(ii). Injection moulding

(iii). Transfer moulding

(iv). Extrusion moulding

(i) Compression moulding:

This method is applied to both thermoplastic and thermosetting resins. A

predetermined quantity of ingredients required for the plastic are filled between the

two half- pieces of the mould. Heat and pressure are then applied as per required

standards. The cavity gets filled with fluidized plastic. The halves of the mould are

closed slowly. Final curing (the time required for the plastic to set in the shape) is done

either by heating (for thermosetting) or cooling (for thermoplastic). These moulded

articles are then taken out by opening the parts of the mould.


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(ii) Injection moulding:

This method is applicable for thermoplastic resins. The moulding plastic powder is fed

into a heated cylinder through a hopper and is injected at a controlled rate, into the

tightly locked mould, by means of a screw arrangement or by a piston plunger.

The mould is kept cold to allow the hot plastic to cure and become rigid. When

the material has been cured sufficiently, half of the mould is opened to allow the

removal of the finished article without any deformation. Heating is done by oil or

electricity.

Advantages:

This is most widely used method because of its high speed of production, low

mould cost, very low loss of material and low cost. There is a limitation of design of

articles to be moulded, because large number of cavities cannot be filled

simultaneously.

(iii) Transfer moulding: This method is useful for moulding of thermosetting plastics.

The powdered compounding material to be moulded is placed in a heated chamber,

maintained at a minimum temperature, where powder just begins to become plastic.

This is then injected through an orifice into the mould by a plunger, working at high

pressure. Due to the friction developed at the orifice, the temperature rises to the extent

that the moulding powder becomes liquid and flows quickly into the mould. This is

then heated up to curing temperature for setting. This is then heated up to curing

temperature for setting.


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Advantages: The plasticized mix flows into cavity in highly plasticized condition and

hence very delicate articles can be handled without distortion or displacement. Thick

pieces can also be cured completely and uniformly. Non attainable shapes by

compression moulding can be obtained. The article produced is free from flow marks.

Finishing cost of fabricated article is almost low and blistering of the goods is almost

eliminated..

(iv) Extrusion moulding:

This method is mainly used for continuous moulding of thermo plastics into

materials of uniform cross-section like tubes, rods, sheets, wires, cables etc.. The

thermoplastic ingredients are heated to plastic state ( a semi solid condition) and then

pushed by means of screw conveyor into a die, having the shape of the article to be

fabricated. The plastic mass gets cooled due to atmosphere exposure or artificially by

air jets or a spray of water.


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6. SOME INDIVIDUAL POLYMERS

6.1. Polyethylene or PE

Polyethylene is most commonly used polymer, produced by the polymerization

of ethylene in presence of a catalyst. By using free radical initiator (benzoyl peroxide) at

80-1250C low density polythene (LDPE) with density of 0.92g/cc is produced, while by

using an zieglar-natta catalyst like tri ethyl aluminium,, high-density polythene

(HDPE) with density 0.965g/cc is obtained.

n CH2=CH2 Heat/pressure/benzoyl peroxide (CH2-CH2)n

Ethylene Polyethylene

Preparation:Low density polythene (LDPE) is produced from ethylene by free radical addition

polymerization.


Ethylene LDPE

High density polythene (HDPE) is produced from ethylene by Zieglar Natta

addition polymerization. In the polymerization of HDPE the catalyst is usually based

on titanium tetrachloride/aluminium alkyl (e.g., diethylaluminium chloride).


Ethylene Titanium tetrachloride/Tri alkyl aluminium

HDPE

Properties:

1. Polyethylene is a rigid, waxy, white, translucent, non polar solid material with


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good electrical insulation property. It is a soft flexible polymer.

2. It exhibits chemical resistance to strong acids, alkali and salt solutions at room

temperature but attacked by oils, organic solvents especially kerosene.

3. Polyethylene crystallizes very easily due its highly symmetrical structure. Thedegree of crystallization varies from40-95% depending on the number of

branching in the polymeric chain.

4. Commercially polyethylenes are sub divided into three groups based on itsdensity.

i) Low Density Polyethylene LDPE ; ii) Medium density Polyethylene; iii) High

Density Polyethylene(HDPE)

5. It is resistant to atmospheric gases, moisture and UV light

Engineering applications: PE is used for making high frequency insulator parts, bottle

caps, packing materials, tubes, coated wires, tank linings in chemical plants and

domestic appliances.

6.2. Poly vinyl chloride or PVC

It is a thermoplastic polymer and is obtained by the free radical addition

polymerization of vinyl chloride in the presence of benzyl peroxide or hydrogen

peroxide. In PVC the mass of chlorine is 57% of the total mass of the polymer. Vinyl

chloride is obtained by treating acetylene with HCl at 60-800C in the presence of metal

oxide catalyst.

Preparation: Preparation of PVC involves the following two steps.

1 Step: Vinyl chloride is prepared by treating acetylene with hydrogen chloride at

60 - 80°C in the presence of metal chloride as catalyst.

II Step: Polyvinyl chloride is obtained by heating water emulsion of vinyl chloride

in presence of benzoyl peroxide (or) hydrogen peroxide under pressure.

Heat/pressure/ H2O2

n CH2=CH -(- CH2-CH -)-n

Cl Cl

Vinyl chloride Polyvinyl chloride


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Properties:

1. PVC is colorless, non – inflammable and chemically inert powder. It is strong butbrittle.

2. It is resistant to ordinary light, atmospheric gases, moisture, inorganic acids and

alkalis, but undergoes degradation in heat or UV light.

3. It is soluble in hot chlorinated hydrocarbons like ethyl chloride

4. Pure resin possesses a high softening point.

5.

It has greater stiffness and rigidity compared to polyethylene.

Engineering applications:

1. It is widely used as a synthetic plastic.

2. Rigid PVC is used for making sheets, light fittings, safety helmets, refrigerator

components, tyres, and cycle and motor cycle mudguards.

3.

Plasticized PVC is used in making continuous sheets viz., table cloths, raincoats,curtains etc.,

4.

Used in injection moulding of articles like toys, tool – handles, radio –

components, chemical containers, conveyor belts etc.

6.3. Bakelite

It is prepared by condensing phenol with formaldehyde in presence of

acidic/alkaline catalyst. The initial reaction results in the formation of non polymeric

mono, di and tri methylol phenols depending on the reactant ratio. These compounds in

the first stage react to form a linear polymer, Novolac. Novolac in the second stage

undergoes further reaction with these linear polymers to form cross linking and bakelite

plastic resin is produced.

All these stages in a step wise manner are shown in the reaction below,

ultimately giving the cross linking polymer, bakelite.


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Stage :1

Stage : 2

Stage : 3

Properties:

1. Phenolic resins ( like bakelite) set to rigid, hard, strong, scratch-resistant,


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infusible, water-resistant, insoluble solids, which are resistant to non-oxidizing

acids, salts and many organic solvents.

2. But these are attacked by alkalis, because of the presence of free hydroxyl

group in their structures.3. They possess excellent electrical insulating character.

4. They are good anion exchange resins capable of replacing anions with –OH

groups.

5. They are good adhesives, corrosion resistant and resistant to atmospheric

gases, moisture and UV light.

Engineering applications:

The phenol-formaldehyde resins are extensively used1. for making electric insulator parts like switches, plugs, switch-boards, heater

handles, etc.

2. for making moulded articles like telephone accessories, cabinets for radioand television.

3. for impregnating fabrics, wood and paper.

4. as adhesives (e.g., binder) for grinding wheels.

5.

in paints and varnishes.

6.

as hydroxyl group exchanger resins in water softening7. for making bearings, used in propeller shafts for paper and rolling mills.

9. Elastomers

Rubbers, also known as elastomers are high polymers, having elastic

property i.e.; the ability to regain their original shape after releasing the stress.

They have temporary deformation in their physical structure on application of

stress of more than 600 elastic units. Thus, a rubber can be stretched to 4 to 10

times its original length. The elasticity of rubber is due to its coiled structure.

Elastomers are expected to have the following characteristics.

1. They have elasticity i.e.; it can be stretched by applying stress and can regain

original shape and dimension by releasing the stress.

2. They have very low inter chain attraction forces.


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3. They have coiled structure.

4. They can absorb water.

5. They has low chemical sensitivity

6. At high temperature they become sticky.

Elastomers are classified into two types 1) Natural rubber 2) Synthetic rubber

9.1. Natural rubber

Natural rubber consists of basic material latex (cell sap) , which is a dispersion of

isoprene. During the treatment, these isoprene molecules polymerizes to form, long-

coiled chains of cis-polyisoprene.

The main source of natural rubber is the latex of the “Hevea brasiliensis”. More than

95% of the rubber is obtained from Hevea brasiliensis. Natural rubber obtained from

Hevea brasiliensis is a cis- polymer of isoprene (2-methyl. 1,3 – butadiene). The

polyisoprene in natural rubber is in long coiled chain form, responsible for its

elasticity.

nCH2= C – CH – CH2 -(-CH2 -C = CH – CH2-)-n

CH3 CH3

Isoprene Polyisoprene

Processing of Rubber : Latex obtained from tapping of the tree is diluted to contain

25 to 40% of rubber. Then filtered to eliminate any impurities like bark or leaves

present in it. Then natural rubber is coagulated to soft white mass by addition of acetic

acid or formic acid. The coagulated white mass is washed. The coagulum is treated as

below:

(a) Crepe rubber: The coagulum is allowed to drain for about 2 hours. It is then passed

through a creping machine and the spongy coagulum is converted into a sheet,

dried in air for 5 to 10 days at about 50oC. The sheets posses an uneven rough

surface resembling a crepe paper .

(b) Smoked rubber: Coagulation is carried out in long rectangular tanks fitted with

metal plates. Diluted latex is poured into these tanks to which dilute acetic acid or

formic acid is added and the mixture is stirred thoroughly. The tanks are kept


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undisturbed for about 16 hrs. After inserting the partition plates into the grooves,

the coagulum forms into tough slabs between the plates. The slabs are passed

through a series of rollers, so as to give ribbed pattern to the final rubber sheet. The

sheets are then hung for about 4 days in a smoke chamber, at a temperature

between 40-50oC.

9.2. Gutta-Percha

This is another type of natural rubber obtained from the mature leaves of

dichopsis gutta and palgum gutta trees. The mature leaves are ground carefully; treated

with water at about 70 oC for half an hour and then poured into cold water, when gutta

perch floats on water surface and is removed by extraction with CCl4. After the

evaporation of the solvent, it is extruded in a sheet form by passing between two rollers.

Properties

a. Gutta-percha is tough and horny at room temperature but turns soft at about

100oC.

b. It is soluble in chlorinated and aromatic hydrocarbons, but not in aliphatichydrocarbons.

c. Gutta percha is used in the manufacture of submarine cables, golf ball

covers, tissue for adhesive and surgical purposed.

Engineering applications

1) Dentists use it to make temporary fillings.

2) It is used in conjunction with Balata resin, in conveyor belts.

9.3. Draw backs of natural rubber

1) It is soft at high temperature, brittle at low temperatures, weak and has poortensile strength.

2) It has a high water absorption capacity, swells in water.

3)

It dissolves in mineral oils, acids, bases and non-polar organic solvents likebenzene.

4) It is attacked by oxidizing agents including atmospheric oxygen and becomessticky.

5) It undergoes permanent deformation when stretched.


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10. Vulcanization

When rubber is heated with sulphur at temperature of 100-140 oC , sulphur

combines chemically at the double bonds of the different chains of rubber and produces

three dimensional crossed linked rubber, which over comes all the drawbacks of naturalrubber. This vulcanized rubber does not melt on heating. This is the fundamental

difference between a thermoplastic and rubber. The extent of stiffness of vulcanized

rubber depends on the amount of sulphur added. For example., a tyre rubber may

contains 3-5 % of sulphur, but a battery case rubber may contain as much as 30%

sulphur. Vulcanization provides cross linking of sulphur atoms between the adjacent

chains of rubber.

The amount of sulphur used for ordinary soft rubber is 0.5 to 5% where as for

hard rubber it is 32 to 35% of the rubber. The other vulcanizing agents used include Se,Te, benzoyl chloride, tri nitro benzene, alkyl phenol sulphides, H2S, MgO, benzoyl

peroxide etc.

CH3 CH3

….-CH2 – C = CH – CH2 – C = CH – CH2-...

+

….-CH2 – C = CH – CH2 – C = CH – CH2-...

CH3 CH3

Vulcanization +S

CH3 CH3

….-CH2 – C - CH – CH2 – C - CH – CH2-...

S S S S (sulphur cross links)

….-CH2 – C - CH – CH2 – C - CH – CH2-...

CH3 CH3

Vulcanization of raw rubber with sulphur as vulcanizing agent


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Advantages of Vulcanization

Vulcanization transforms the weak, thermoplastic rubber into a strong and toughrubber.

1. The working temperature range is – 10 oC to 100 oC.

2.

The tensile strength increases. (2000 kg/cm3)

3. The water absorptivity decreases.

4. The article made from vulcanized rubber returns to the original shape when the

deforming load is removed, i.e., the resilience power is increased.

5. The vulcanized rubber becomes resistant to organic solvents like CCl4, benzene, fats

and oils, however it swells in these solvents

6. It becomes resistant to abrasion, ageing and reactivity with oxygen & ozone.

7.

It becomes better electrical insulator.

8. It can be easily manipulated into desired shape.

Compounding of rubber

1) Accelerators: These are meant for catalyzing the vulcanization process, thus

reducing the time required for vulcanization and maintain the vulcanization

temperature. The inorganic accelerators include lime, magnesia, litharge and white

lead, where as the organic accelerators are complex organic compounds such as

aldehydes and amines. Sometimes, ZnO can acts as an accelerator activator.

2) Antioxidants: These substances retard the deterioration of rubber by light and

air. These are complex organic amines like phenyl naphthyl amine, phenolic substance

and phosphates.

3) Reinforcing agents: These are usually added to give strength, rigidity andtoughness to the rubber

and may form as much as 35% of the rubber. The commonly used reinforcing agents are

carbon black, ZnO, MgCo3, BaSO4, CaCO3 and some clays.

4) Fillers: The function of the fillers is to alter the physical properties of the mix to

achieve simplification of the subsequent manufacturing operations, or to lower the cost

of the product.

5) Plasticizers (or) softeners: These are added to impart great tenacity and adhesion to

the rubber. The most commonly used plasticizers are vegetable oils, waxes, stearic acid,


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rosin etc.

6) Coloring agents: These are added to impart desired colour to the rubber.

TiO2, lithophane - hite Ferric oxide - Red

Lead chromate - ellow Carbon black - Black

Chromiumtrioxide - Green Ultra marine - Blue

Engineering applications of rubber: The major application of rubber is in making tyres

and tubes. It is also used in making belts for transport, material handling, tank inner

lining in chemical plants where corrosive materials are stored. Rubber sandwiches are

used in machine parts as gaskets to reduce vibrations. Foamed rubber is used in making

cushions, mattresses and paddings.

11. SYNTHETIC RUBBERThe natural rubber sources are not sufficient and could not supplement the needs of

automobile industry. An attempt was made to synthesize rubber, but rubber like

materials were synthesized to supplement the needs of various industries. These

materials synthesized by various processes are called elastomers. The artificially

prepared polymer, which has elastomeric property, is known as synthetic rubber. There

are several types of synthetic rubbers available and used on commercial grade.

11.1. SBR (Styrene – Butadiene Rubber) or BUNA -S

It is a copolymer of about 75% butadiene and 25% styrene. Hence it is called asstyrene rubber.

Preparation : It is produced by the copolymerization of butadiene, CH2=CH–CH=CH2

(about 25% by weight) and styrene, C6H5CH = CH2 (75% by weight), in presence of

sodium as catalyst.

nCH2 = CH – CH = CH2 + nCH2 =CH

C6H5

Butadiene styrene

Na -(-CH2 - CH = CH - CH2 - CH2 – CH-)-n

C6H5

Styrene butadiene rubber (Buna-S-Rubber)


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Properties

1) It has excellent abrasion resistance and high load bearing capacity.

2) A reinforcing filler (carbon black) is essential to achieve good physicalproperties.

3)

It is a good electrical insulator

Uses: It is used for lighter – duty tyres, hose pipes, belts, moulded goods, unvulcanized

sheet, gum, floorings, rubber shoe soles and electrical insulation cables, chemical plant

inner linings etc.

11.2. BUNA-N or Nitrile Rubber (NBR)

Nitrile rubber is the copolymer of butadiene and acrylonitrile. Bu – stands forButadiene, N-stands for acrylonitrile.

nCH2 = CH – CH = CH2 + nCH2 =CH

CN

Butadiene acrylonitrile

-(-CH2 - CH = CH - CH2 - CH2 – CH-)-n

CN

Nitrile butadiene rubber (Buna-N-Rubber)

Properties

1) Because of the presence of –CN group in the structure BUNA-N possess excellent

resistance to heat, sunlight, oils, acids and salts and less resistant to alkalis than

natural rubber.

2) It is a strong and tough polymer with light weight

3) BUNA-N is also vulcanized with sulphur

Engineering Applications

1) It is used for making conveyor belts, aircraft components.

2) BUNA-N is extensively used for fuel tanks, gasoline hoses, creamery equipment,

and automobile parts.


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3)

11.4. Polysulphide rubber (or) Thiokol rubber (GR-P)

This is synthesized by the copolymerization of sodium polysulphide (Na2S4) and

ethylene dichloride and during the reaction NaCl gets eliminated.

Properties

1. The properties of the material depend upon the length of aliphatic group and

number of sulphur atoms. It possesses strength and impermeability to gases and

low abrasion resistance.

2. Thiokol is resistant to swelling, oils, solvents and fuels.

3. Thiokol is inert to fuels, lubricating oils, gasoline and kerosene.

Uses

1.

It is used for coating fabrics, for making life rafts and jackets.2. It is used for making gaskets, diaphragms and seals in contact with solvent and

for printing rolls.

3. It is used for lining hoses for gasoline and other transport pipes

4. Liquid Thiokol can be used to make tough solvent resistant temperature liquid

compounds which are used as liners for aircraft.

ΩΩΩΩΩ

unit-4 high polymers

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