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UNIT IV HIGH POLYMERS 2015-16

Engineering Chemistry Page 79

HIGH POLYMERS

Syllabus: Types of polymerization – Stereo specificity of polymers –properties of polymers – Plastics –

Thermoplastics and thermo setting plastics – Compounding and Fabrication of plastics – Preparation and

properties of Polyethylene, PVC and Bakelite – Elastomers – Rubber and Vulcanization – Synthetic

rubbers – Styrene butadiene rubber – Thiokol – applications. Fibre reinforced plastics – Biodegradable

polymers – Conducting polymers and their applications.

Objectives: Plastics are widely used engineering materials and understanding their properties helps in

selecting suitable materials for various purposes. Engineers should also be aware some of the advanced

polymer materials that have specific significance.

Outcomes: Students gain the knowledge on structure, synthesis properties and applications of polymers,

additives to be mixed with polymers to obtain desired plastics and moulding techniques, advanced topics

on plastics like conducting polymers and biodegradable polymers, fibre reinforced plastics and bullet

proof plastics, synthetic plastics that are essential to latest technology.

OUTLINES

Introduction

Methods of polymerization

Stereo specific polymers

Properties of polymers, PE, PVC and Bakelite

Plastics

Compounding of a plastic

Selected individual polymers

Rubbers or elastomers

Vulcanization

Synthetic rubbers

Fabrication of plastic articles

Biodegradable polymers

Conducting 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 -mer.

For example, polythene is a polymer formed by linking together of a large number of ethene

(C2H4) molecules. Similarly polystyrene is formed by the linking of styrene monomer molecules.

The number of repeating units (n) in chain formed 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.

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:

C

H

H

C

H

n C

H

H

C

H

C

H

H

C

H

C

H

H

C

H

Mer

Styrene(Monomer)

Polystyrene, PS(Polymer)



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.

Ex: 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 as illustrated below:

-M1 – M1 – M1 – M1 – M1 – M1 – M1 – M1 – M1– M1 – M1 – M1-

Linear homopolymer

M M M M M

M

M

Backbone

M1 M2 M1 M2 M1 M2

Branching

Branching

Branched Chain homopolymer Branched Chain Heteropolymer

M M1

M2

M1

M2



M M M M M

M

M

M

M

M

M M M M M M

CrossLinkage

M1 M2 M1 M2 M1

M1

M2

M2

M2

M1

M2 M1 M2 M1 M2 M1

CrossLinkage

Crosslinked Homopolymer Crosslinked Heteropolymer

2. Methods of polymerization

The process of polymerization reaction involves union of two or more small, same or different

monomer molecules to form a single large macro-molecule, called polymer. The conversion of a

monomer into a polymer is an exothermic process and if heat is not dissipated or properly controlled,

explosion may result. This is basically due to the difference in the mechanisms of the two different types

of polymerization processes i) Addition or chain polymerization and ii) Step or condensation

polymerization.

2.1 Addition or chain polymerization: The reaction that yields a product, which is an exact

multiple of the original monomeric molecule. Such a monomeric molecule, contains one or more

double bonds, which by intermolecular rearrangement, may make the molecule bi-functional. The

monomer molecules simply add themselves at the double bonds (π bond) by self addition and form a

chain of a macro-molecule, leaving the ends open for further addition, if any. Since the process takes

place by a chain reaction, it is also termed as chain polymerization. The length of the chain is controlled

by external factors. The addition polymerization reaction must be instigated by the application of heat,

light, pressure or a catalyst for breaking down the double bonds of monomers. For this, unsaturation in

the monomer units is a necessary factor. The polymer will have the same chemical composition as the

monomer. Addition polymers will have their molecular weights as integral multiple of their monomer

unit.

i.e., M = n. m

where M and m are the molecular weights of the polymer and the monomer respectively and n is

the degree of polymerization.



2.2 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.

C

H

H

NN

H

H

H

H

+ C

H

H

CC

OH

O

OH

O

N

H

C

H

H

N

H

C C

O

H

H

C

O

6 4

n

Hexamethylenediamine

Adipic acid

Condensation Polymerization

- 2n H2O

6 4 n

Polyhexamtheylene Adipate (Nylon 6,6)(Polyamide)

n

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.

Copolymerization: Copolymerization is the joint polymerization of two or more different monomer

species. High molecular weight compounds obtained by copolymerization are called copolymers. For

example, butadiene and styrene copolymerize to yield SBR (Styrene – butadiene rubber).



2.3 Differences between addition and condensation polymerization processes:

Addition polymerization Condensation polymerization

1. The functionality of a monomer is the 1. The functionality of a monomer 2, 3 or

Π bond which is bifunctional any

2. The polymerization is by self addition 2. The polymerization is by condensation

And is by chain mechanism 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 4. The molecular weight of the polymer is less

sum of molecular weights of monomer. 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.

3 Stereo-specific polymers

3.1. 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. Based on 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

R

R

R

R

R

H

H

H

H

H

R R R R R

ISOTACTIC

or

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

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

R

R

R

R

R

H

H

H

H

H

R R RR R R

SYNDIOTACTIC

or

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.



R

R

R

R

R

H

H

H

H

H

R R R R R

ATACTIC

or

3.2. Co-ordination polymerization (Ziegler-Natta catalysts):

Ziegler (1953) and Natta (1955) suggested that in the presence of a combination of a transition

metal halide (like TiCl4 or TiCl3, ZrBr3, TiCl2, halides of V, Zr, Cr, Mo and W) with organo metallic

compounds like triethyl aluminium or trimethyl aluminium, stereospecific polymerization can be carried

out. A combination of such metal halides and organo-metallic compounds is called as Zeigler- Natta

catalysts.

Mechanism of co-ordination polymerization can be illustrated as :

Initiation:

Cat-R' + CH2 = CHR Cat-CH2CH(R)R'

Complex catalyst Monomer

Propagation :

Cat-CH2-CH-R' + nCH2 = CHR'

R

Cat-CH2-CH-CH2-CH-R'

RR n

Termination (with acive hydrogen compound) :

Cat-CH2-C-CH2-CH-R'

RR n

+ HX CH3-CH-CH2-CH-R'

RR n

Cat-X +

Ziegler- Natta polymerization is useful in the preparation of polypropylene, poly ethylene, etc. The

importance of this method lies in the fact that stereospecific polymers of desired configuration are

obtained. For example, during the polymerization of propylene, using conventional catalysts, normally

random or atactic polymer is obtained. But by using suitable Zeigler-Natta catalyst, solvent and

temperature, it is possible to make a desired type (atactic or isotactic or syndiotactic) of polypropelyne.

4. Properties of polymers for engineering applications

4.1. Structure and chemical properties

a) Chemical Reactivity: The polymer is prepared by linking small monomeric units. So their

properties depend upon number and chemical nature of chemical groups present in the monomers.

The thermal stability and mechanical strength of different polymers are related to difference in

bonding and structure of the monomer. Polymers containing high electronegative atoms in their


Engineering Chemistry 6

backbone chain undergo hydrolysis. E.g. Nylon and polyester. Polymers containing double bonds

undergo ozonalysis. E.g., Rubbers like isoprene, neoprene.

b) Solubility and swelling nature: Polar polymers such as PVA, PVC, and polyamide are soluble

in polar solvent like water, alcohol, phenol etc,, while non-polar polymers like PE, PP, PS can be

dissolved in non-polar solvents like benzene, toluene, xylene, n - hexane etc. Polymers of

aliphatic character are more soluble in aliphatic solvents, whereas aromatic polymers are soluble

in aromatic solvents. Polymers dissolve in solvents and swell in size.

c) Ageing and weathering: The reason for the stability of the polymer is bond strength between the

atoms in the polymer chain. The stability of polymer can be enhanced by increasing bond

strengths. Heat, ultraviolet light, high energy radiation, atmospheric effect and chemical

environments are the main agencies to affect the properties of polymers. PTFE, PE and PVC

have good stability towards light and heat due to the fact that the bond energies of these are

greater than light energy. The heat stability of these polymers is in the order PTFE > PVC > PE

d) Permeability and diffusion: Diffusion occurs in polymers through vacant gaps between adjacent

polymer molecules. Crystalline polymers resist in diffusion because of greater degree of

molecular packing. Amorphous polymers above Tg have appreciable permeability The crystalline

polymers have high resistance to permeability than amorphous polymers.

4.2. Physical Properties

a) 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 higher softening 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.

b) 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.

4.3. Mechanical properties:

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

Rubbery polymers

Polymer fibres

Rigid high Impact thermoplastics

Hard, Brittle Polymers

Strain

Stre

ss



d) Structure and electric properties: Most of the polymers are electrical insulators and the insulating

property can be removed by application of a strong field. The electronic polarization is responsible

for dielectric constant in non-polar polymers. Water has high dielectric constant and conducting

property so the absorbed water molecules enhance the conductivity of a polar polymer.

5. PLASTICS

Plastics are polymers which can be moulded into any desired shape or form, when subjected to heat

and pressure in the presence of a catalyst. They undergo permanent deformation under stress termed

as plasticity. The term plastic and resin are synonymous. 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 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 reasonable

temperature and pressure.

Types of plastics

5.1.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.

5.2.Thermosets

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.



Thermoplastics Vs Thermosets

Thermoplastics Thermosets

1. They soften on heating readily 1. They do not soften on heating. On

prolonged heating, they get charred.

2. They consist of long – chain linear

molecules.

2. Their set molecules have three-

dimensional network structure, joined by

strong covalent bonds

3. They are mostly formed by addition

polymerization

3. They are formed by condensation

polymerization

4. They can be softened, reshaped and reused

by heating. (recycled)

4. They cannot be softened, reshaped and

reused.

5. They are usually soft, weak and less brittle 5. They are usually hard, strong and brittle.

6. They can be reclaimed from wastes. 6. They cannot be reclaimed from wastes.

7. They are usually soluble in some organic

solvents

7. They are insoluble in almost all organic

solvents, because of their structures.

6. 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) Lubricants; (5) Catalysts or accelerators; (6)

Stabilizers.

6.1. 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. The binders 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.

6.2. 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 greater freedom 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)

6.3. 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.

6.4. 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.

6.5. Catalyst or promoters: These are added to thermosetting plastic, during moulding operation, to

accelerate the polymerization of fusible resin, into cross-linked infusible form.

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.

6.6. 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.

6.7. 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, anthra quinones (yellow), azodyes (yellow, orange, red) , phthalocyanins (green)

7. Fabrication of plastics into articles

The fabrication of plastic into commercial goods is done by five common methods

1. Casting; 2. Blowing; 3. Extrusion; 4. Lamination; 5. Moulding

7.1. Casting: This method of moulding is used to mould both thermoplastic and thermosetting resins.

Here, the molten resin is poured into a suitable mould and heated up to 70 C for several hours at

atmospheric pressure. The products formed are free from internal stress.

7.2. Blowing: In this process, the softened thermoplastic resin is blown by air or steam into a close

mould.



7.3. Extrusion: In this method, the material of the required composition is forced by a screw conveyer

into a heated chamber, where it softens and then is forced through a die, having the desired shape.

The finished product that extrudes out is cooled by atmospheric exposure or by blowing air or by

spraying water. This method is only used for thermoplastic. It is used for the manufacture of articles

like sheets, tubes, rods etc.

7.4. Lamination: Sheets of cloth, wood or paper are impregnated with a resin solvent solution. These

are then piled up one over the other until the desired thickness is obtained and heated to remove the

excess of the solvent , pressed together between two highly polished steel surfaces to get the

laminated product. Phenolic and urea type resins are commonly used. Laminated plastic have high

tensile strengths and impact resistance.

7.5. 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

A. Compression moulding

B. Injection moulding

C. Transfer moulding

D. Extrusion moulding

7.5.1. 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.



7.5.2. 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.

7.5.3. 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.



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..

7.5.4. 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 .

8. Some individual polymers

8.1. 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 80-125 0C low density polythene (LDPE)

with density of 0.92g/ccis produced, while by using an ionic catalyst like tri ethyl aluminium,, high-

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

8.1.1. Preparation:

C

H

H

C

H

n C

H

H

C

H

C

H

H

C

H

C

H

H

C

H

MerEthylene

(Monomer)Polyethylene, PE

(Polymer)

HH HH n-3



8.1.2. Properties:

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

crystallization varies from40-95% depending on the number of branching in the polymeric chain.

4. Commercially polyethylenes are sub divide in to three groups based on its density.

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.

8.1.3. 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.

8.2. 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.

C C

H

H

Cl

H

nC C

H

H

Cl

H n

benzoyl peroxide

Vinyl chloride Poly Vinyl Chloride

HC CH + HCl H2C CHCl

Vinyl chloride

8.2.1. Properties:

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

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.



8.2.3. 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.

8.3. 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.

OH

+ HCHO

OH

CH2OH

OH

CH2OHPhenol

p-Hydroxy methyl phenol

o-Hydroxy methyl phenol

and

OHOH

CH2-OH + H H

HO

+ HO-H2C

HO

+ HO-H2CH

Monomethyl phenol

Monomethyl phenol

Monomethyl phenol

Phenol

H2C

OH OHH2C

H2C

H2C

OH OH

Novolac



H2C

OH OH

H2C

OH

CH2

OH OH

CH2

OH

CH2CH2

Cross-linked polymer bakelite

H2C

CH2

8.3.1. Properties:

Phenolic resins ( like bakelite) set to rigid, hard, strong, scratch-resistant, infusible, water-resistant,

insoluble solids, which are resistant to non-oxidizing acids, salts and many organic solvents. But these are

attacked by alkalis, because of the presence of free hydroxyl group in their structures. They possess

excellent electrical insulating character. They are good anion exchange resins capable of replacing anions

with –OH groups. They are good adhesives, corrosion resistant and resistant to atmospheric gases,

moisture and UV light.

8.3.2. Engineering applications:

The phenol-formaldehyde resins are extensively used

1. for making electric insulator parts like switches, plugs, switch-boards, heater handles, etc.

2. for making moulded articles like telephone accessories, cabinets for radio and 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 softening

7. for making bearings, used in propeller shafts for paper industry and rolling mills.

9. Rubbers or 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.

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 polymerize 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.

C C

CC

H H

C HH

H H

H H

C C

CC

H H

C HH

H H

H H

n

n

IsopreneCis-polyisoprene (Natural rubber)

9.1.1. Processing of latex: Latex obtained from tapping of the tree is diluted to contain between 15 to

20% of rubber and filtered to eliminate any impurity like bark or leaves present in it. Then natural rubber

is coagulated to soft white mass by addition of water /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 50 oC. Theses 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 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 40-50 oC. The rubber thus

obtained is amber in coloured and translucent.

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.

Gutta percha

C

C

C

C

C

C

C

C

C

C

C

CH H

H

CH H

H

CH H

H

H

H

H H H H H

C

H

H

H

H

H

H

H H

Structure of gutta percha

Properties

a. Gutta-percha is tough and horny at room temperature but turns soft at about 100 oC.

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

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 poor tensile 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 like benzene.

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

5) It undergoes permanent deformation when stretched.

These properties make rubber limited in use and compounding of rubber solves the problems.

10. Vulcanization

Rubber is compounded with some chemicals like sulphur, hydrogen sulphide, benzoyl chloride, zinc

oxide etc., to improve the properties of rubber. The process is called vulcanization, which makes rubber



stable and more useful. The most important vulcanizer is sulphur. When rubber is heated with sulphur

and lead oxide 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 natural rubber. 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 contain 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 reaction is:

CH2 C

CH3

CHH2C

H2C C

CH3

CH

CH2

CH2 C CHH2C

H2C C C

HCH2

CH3 CH3

Vulcanization

( + Sulfur)

+

CH2 C

CH3

HC

H2C

H2C C

CH3

HC C

H2

CH2 C CH

H2C

H2C C C

HCH2

CH3 CH3

S S S S

Raw or unvulcanized rubber springs

Sulphur cross link

Vulcanization of raw rubber with sulphur as vulcanizing agent

The temperature used is 100 to 140 oC. The curing time may vary and over curing temperature

decreases stretch and tensile strength, under the curing makes it too soft. So proper curing is required.

The amount of sulphur used for ordinary soft rubber is 1 to 5% where as for hard rubber it is 40 to 45% of

the rubber. The other vulcanizing agents used include Se, Te, benzoyl chloride, tri nitro benzene, alkyl

phenol sulphides, H2S, MgO, benzoyl peroxide etc.

10.1. Advantages of vulcanization

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

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

oC.

2. The tensile strength increases. (2000kg/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.

10.2. The other ingredients in the 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 and toughness 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, rosin etc.

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

TiO2, lithophane - White Ferric oxide - Red

Lead chromate - Yellow Carbon black - Black

Chromium trioxide - Green Ultra marine - Blue

6) Miscellaneous agents: These include baking soda for sponge rubber, abrasives (eg: silica and

pumice),

10.3. 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 rubber

The 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 as styrene 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.

C C

H

C C

H H

H

H H

n + nC C

H

H

H

C C

H

C C

H H

H

H H

C C

H

H

H

x

Copolymeri

zation

Butadiene Styrenen

Properties

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

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

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 for Butadiene, N-

stands for acrylonitrile.

C C

H

C C

H H

H

H H

m + n C C

H

H

H

CN

C C

H

C C

H H

H

H H

C C

H

H

H

CN

Polymerization

Butadiene Acrylonitrile

m n

Polybutadine-co-acrylonitrile (Nitrile 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.

11.3. Polyurethane foam (PUF)

Preparation: Polyurethanes is produced by the reaction of polyalcohols with di-isocyanates.

C

H

H

OHHO

2

C

H

H

NN

2

CC

OO

+ PolymerizationC

H

H

OO

2

C

H

H

NN

2

CC

OO

n

HH

Ethylene glycol Ethylene diisocyante Polyurethane rubber (or isocyanate rubber)

n

Properties

1. It has high strength, good resistance to ozone and aromatic hydrocarbons and weather proof.

2. It is highly resistant to oxidation, because of the saturated character. It have good resistance to

many organic solvents.

Engineering Applications

1. It used for surface coatings, manufacture of foams & spandex fibres.

2. PU flexible foams are employed as furniture material, insulation & crash pads.

3. It is used for insulating wires, the PU coated wires can be soldered directly.

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.

C

H

H

ClCl

2

1,2 Dichloroethane

n

Na S S Na

Sodium polysulphide

C

H

H

S

2

S

Thiokol rubber ( or thiokol)

S S

S S



Properties

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

atoms. It possess 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.

12. FIBRE RIENFORCED PLASTICS

Fibre – Reinforced plastic (FRP) is one of a composite material. An FRP composite is defined as

a polymer that is reinforced with a fibre. A composite is an artificially prepared multiphase material. . The

primary function of fibre – reinforcement is to carry load along the length of the fibre and to provide the

strength and stiffness. Composite materials consists of two phases, one is called the matrix which is

continuous and surrounds the other phase called the dispersed phase (reinforcement). FRP is produced by

reinforcing a plastic matrix with a high strength fibre material such as glass, graphite, alumina, carbon,

boron, beryllium and aromatic polyamides. Glass fibre is most widely used reinforced fibre, because of its

durability, acid/water/fire proof nature of glass. The polymer is usually an epoxy, vinyl ester or polyester

thermosetting plastic. The composite materials are prepared by binding two or more homogeneous

materials with different material properties to derive a final product with certain desired material and

mechanical properties.

12.1. Composite Components

i) Fibres: The composite’s properties are mainly influenced by the choice of fibers. These have

generally higher stress capacity and linearly elastic until failure. In civil engineering materials,

three types of fibres viz. carbon, glass and aramid fibres are used. They have different properties.

ii) Matrix: Matrix should transfer the forces between the fibers and protect the fibers from the

environment. Commonly used matrices are thermo-sets viz. vinyl ester or epoxy. Epoxys are

employed mostly as they have good strength, bond, creep properties, chemical resistances and low

cost.

12.2. Methods for producing FRP: The Fibre – reinforced plastic are produced by suitably bonding a

fiber material with a resin matrix and curing the same under pressure and heat. The common resin



matrices used in FRP are polyesters, epoxy, phenolics, silicones, melamine, vinyl derivatives and

polyamides. The following common methods are employed as processing techniques for producing

FRP.

a) Matched metal die moulding: This is the most efficient and economical method for mass production

of high strength parts. The parts are press moulded in matched male and female moulds at a pressure

of 200 -300 psi and at a temperature of 235 – 260 oC. The upper mould containing the resin and

reinforced fibres is pressed on to the lower mould.

b) Injection moulding : A mix of short fibres and resin is forced by a screw or plunger through an

orifice into the heated cavity of a closed matched metal mould and allowed to curve. This is suitable

for reinforced thermoplastics.

c) Hand – lay- up: In this method, the reinforcing mat or fabric is cut to fit, laid in the female mould

and saturated with resin by hand, using a brush, roller or a spray gun. Layers are built up to their

desired thickness and then the laminate is cured to render it hard, generally at room temperature. This

is the simplest method for thermosetting composites.

d) Spray – up: This method is well suited for complex thermosetting moulds and its portable equipment

is amenable for onsite fabrication and repair. Short lengths of reinforcement and resin are projected by

a specially designed spray gun so that they are deposited simultaneously on the surface of the mould.

Curing is done with a catalyst in the resin at room temperature.

e) Continuous lamination: This is the most economical method of producing flat and corrugated panels,

glazings etc. In this method, reinforcing mats or fabrics are impregnated with resin, run through and

resin content. They are then cured in a heating chamber.

f) Centrifugal casting: In this method, chopped fibres and resins are placed inside a mandrel and are

uniformly distributed as the mandrel is rotated inside an oven. This method is suitable for providing

round, oval, tapered or rectangular parts.

g) Pultrusion: In this method, continuous fibre strands combined with mat or woven fibres for cross-

strength, are impregnated with resin and pulled through long heated steel die. The die shapes the

product and controls the resin content. This method is suitable for providing shapes with high uni-

directional strength.

h) Filament windings: In this method, continuous fibre strands are wound on a suitably shaped mandrel

or core and positioned in a predetermined pattern. The strands may be pre-impregnated or the resin

may be applied during or after winding. Final curing is done by heating.

12.3. Types of Fiber Reinforced Plastics

(a) Glass Fiber Reinforced Plastics (GFRP): Glass fibres are basically made by mixing silica sand,

limestone, folic acid and other minor ingredients. The mix is heated up to 1260 oC and allowed



through fine holes in a platinum plate. The glass strands are cooled, gathered and wound. For example,

aluminium lime borosilicate glass fibres have high electrical insulating properties, low susceptibility to

moisture and high mechanical properties. Glass fibres have excellent characteristics equal or better to

steel in certain forms.

Properties

They have high ratio of surface area to weight. As a result, they are more useful and also

they are vulnerable for chemical attacks.

Blocks of fibres trap the air, which makes them good thermal insulators. Their thermal

resistance is 0.04W/mk.

The fresh and thin fibres are the strongest. If the surface of the FRP is more, its strength

is less. Moisture is easily adsorbed and worsen the microscopic cracks, surface defects

etc.

It can undergo more elongation before it breaks.

The viscosity of molten glass is very important. If the viscosity is more, the FRP breaks

during drawing or if the viscosity is too low, the glass will form droplets.

Uses: Fibre glass possess low density, high tensile strength, high impact resistance and excellent

chemical and corrosion resistance and are stiff and rigid. They are used in making automotive parts,

storage tanks, plastic pipes, industrial floorings, transport vehicles to boost fuel efficiency.

(b) Carbon Fiber Reinforced Plastics (CFRP) : Carbon fibres have high modulus of elasticity and

low elongation coefficient. These are highly stiff, do not absorb water and are resistant to chemical

attack. They do not undergo stress corrosion, do not show any creep and withstand fatigue. They are

electrically conductive and might be corrected in a galvanic way, when they are in contact with steel.

Properties

They are alkali resistant and are resistant to corrosion. Hence, they are used for corrosion

control and rehabilitation of concrete structure.

They have low thermal conductivity.

These have high strength to weight ratio, which eliminates the requirement of heavy

construction equipment and supporting structures.

These are available in rolls of very long length. Therefore, they need very few joints

avoiding laps and their transportation is easy.

CFRP has a short curing time. So, they reduce the project duration and downtime of the

structure to a great extent.

CFRP is a bad conductor of electricity and is non-magnetic.



(c) Aramid Fiber Reinforced Polymers (Bullet Proof Plastic): Aramids (Aromatic polyamides)

belong to family of nylons (for example, Nomex, Kevlar) which are being used to make bullet proof

vests and puncture resistant tyres. They are also used in the manufacture of helmets. They are

sensitive to elevated temperatures, moisture and ultraviolet radiation. But they have problems with

relaxation and stress corrosion.

i) Kevlar: It is a poly p-phenylene tere-phthalamide .It has recurring units joined by

amide links, which have a carbonyl group and an amine group. The repeating units

consist of benzene rings as shown:

NH2

H2N

Cl

Cl

O

O

+-2n HCl O

N

H

N

H

H

OH

O

n

n

ii) Nomex: This is produced by a condensation reaction between m-phenylenediamine and iso-

phthaloyl chloride. It has meta-phenylene groups which are attached to phenyl ring at 1 and

3 positions as shown below:

H2N NH2

O

Cl

O

Cl

+

-2nHCln

NH

O

H2N

ONomex

n

Properties

They appear yellow in colour, have low density and high strength. These have good impact

resistance, abrasion and chemical resistance and resistance to thermal degradation. However

some grades of aramids undergo degradation when exposed to UV-light.

Applications

They are used in manufacture of protective apparels such as gloves, bullet proof vests, motorcycle

protective clothing etc. Belts and hosing for industrial and automotive parts, aircraft body parts,

fiber optic and electro mechanical cables are also made with them.



12.4 General advantages of FRP

They can provide maximum stiffness to density ratio 3.5 to 5 times that of aluminium or steel.

They can absorb impact stress and have high fatigue endurance limits.

They strengthen the material properties and reduce corrosion potential.

General properties of FRP

Their strength is more than 5 times that of steel.

They are resistant to chemical attack and do not undergo rusting.

They withstand the temperatures in the range from -196 _ 400

oC and are used in making

bullet proof vests

13. Conducting Polymers

Most polymeric materials are poor conductors of electricity because of the non-availability of

large number of free electrons for the conduction process. Thus most of the polymers are used as

insulators. However some polymers have electrical conductivity and can be used in place of metals due to

their light weight and low cost,

Polymeric materials which possess electrical conductivities on par with the metallic conductors are called

conducting polymers. Special polymers with conductivities as high as 1.5 x 107 ohm

-1 m

-1 have been made

Their conductivity may be due to unsaturation or due to the presence of externally added ingredients to

polymers..

Polymers Electrical conductivity (Ohm-1

m-1

)

Phenol formaldehyde 10-9

– 10-10

Poly methyl methacrylate < 10-12

Nylon 6,6 10-12

– 10-13

Polystyrene < 10-14

Polyethylene 10-15

– 10-17

Poly tetra fluoro ethylene < 10-17

13.1. Types of conducting polymers:

a) Intrinsically Conducting Polymers (ICP)

These are characterized by intensive conjugation of bonds in their structure. This is a polymer whose

back bones or associated groups consisting of delocalized electron pair or residual charge, which

increases their conductivity to a large extent. The conduction process is due to the overlapping of orbitals

containing conjugated -electrons, resulting in the formation of valence bands as well as conduction

bands separated by significant Fermi energy gap. The electrical conductivity is due to thermal or



photolytic activation of the electrons, which gives them sufficient energy to cross the Fermi gap and cause

conduction.

Commercially produced conducting polymers

Poly acetylene polymers e.g., poly – p-phenylene, polyquinoline etc.

Polymers with condensed aromatic rings e.g. poly aniline, pol anthryleneetc

Polymers with aromatic, hetero aromatic and conjugated aliphatic units e.g. poly pyrrole, poly

thiophene, poly butadiene etc

b) Doped conducting polymers:

These are obtained by exposing a polymer to a charge transfer agent either in gas phase or in solution.

These possess low ionization potential and high electron affinities, as they can be easily oxidized and

reduced. The conductivity of such an ICP can be increased by creating either positive or negative charges

on the polymer backbone by oxidation or reduction. This technique is called doping, which is of two

types.

p-doping: It involves treating an ICP with a lewis acid, there by the positive charges are created on

polymer back bone by an oxidation process. This is also called oxidative doping. PA is an example.

Some of the common p-dopants are I2, Br2, A2F5, PF6, Naphthyl amine etc.

(C2H2)n + (C2H2)n FeCl4

2 (C2H2)n + 2 (C2H2)n I3

2 FeCl3

3I2

N

H

N

H

N N N

H

N

H

N N

H H

Cl ClHCl (Lewis acid)

Oxidation

Emeroldine Base Emeroldine Hydrochloride

n-doping polymers: This involves treating an intrinsically conducting polymer with a lewis base, thereby

reduction process takes place and negative charge on the polymer backbone is created. Some of the

common n-dopants used are Li, Na, Ca, FeCl3, tetrabutyl ammonium etc.

CH

CH

CH

CH

+ BReduction

CH

CH

C CH

BPolyacetyleneLewis Base

c) Extrinsically conducting polymers:

These are the polymers whose conductivity is due to the presence of “externally” added ingredient to

them. They are of two types.



i) Conductive element filled polymers: This is a resin or polymer filled with conducting elements

such as carbon black, metallic fibres, metal oxides etc. The polymer acts as a binder to hold the

conducting element together in the solid entity. They have reasonably good conductivity. These

are low cost polymers having light weight, mechanical durability and have design compatibility.

They are extensively used in medical field. The disadvantage with them is that addition of 10%

carbon black will reduce the tensile, impact and elongation strengths of the polymer.

ii) Blended conducting polymers: They are obtained by blending a conventional polymer with a

conducting polymer either by physical or chemical change to improve physical, chemical and

mechanical properties of the polymer. They are used in electromagnetic shielding. They are also

used in making rechargeable batteries, analytical sensors, ion exchangers, electronic displays,

optical fibres and photovoltaic devices.

d) Coordination conducting polymers (Inorganic polymers):

This is a charge transfer complex containing polymer obtained by combining a metal atom with a

polydentate ligand. The degree of polymerization in such polymers is small (18).

13.2. Applications of conducting polymers: The conjugation length of a polymer chain, doping level,

temperature of operation, frequency of current are some important factors which influence the

conductivity of a conducting polymer. They have wide applications due to their low weight, easy process

of manufacture and have good mechanical properties. They possess good conductivity and store charge.

They are transparent to X rays. They can be easily processed with product stability and efficient

recycling. Some of their important applications are:

1. In rechargeable light weight batteries based on perchlorate doped poly acetylene –

lithium system, which is 10 times lighter than lead storage batteries.

2. In optical display devices based on poly thiophene.

3. In wiring systems in aircrafts and aerospace components.

4. In telecommunication systems.

5. In antistatic coatings for clothing.

6. In electromagnetic screening materials.

7. In electronic devices such as transistors and diodes.

8. In solar cells and drug delivery system for human body.

9. In molecular wires and molecular switches.

However their conductivities are inferior to metal and hence have limited applications compared to

metals.



14. Biodegradable polymers

The polymers which undergo degradation when exposed to moisture heat, oxygen, ozone and

microorganisms etc. are called Biodegradable polymers. These external agents lead to the breakdown of

chemical structure and this in turn changes the properties of polymers.

Biodegradable polymers are defined as degradable polymer in which degradation results from the

action of naturally occurring microorganisms such as bacteria, fungi and algae.

Biodegradable plastics are two types. They are hydro biodegradable plastics (HBP), which

undergo chemical degradation by hydrolysis and Oxo-biodegradable plastics (OBP), which undergo

chemical degradation by oxidation.

HBP, OBP on degradation forms the same products; generally both are converted into carbon

dioxide (CO2), water (H2O) and biomass.

HBP tend to degrade more quickly than OBP. HBP emits methane in anaerobic degradation

conditions, but OBP does not emit methane.

HBP are naturally occurring biodegradable polymers mostly from agricultural resources such as

corn, wheat, sugar cane etc. naturally occurring biodegradable. HBP are classified into four groups.

1. Polysaccharides E.g.: Starch & Cellulose 2. Proteins E.g.: Gelatin, Silk, Wool.

3. Polyesters 4. Others E.g.: lignin, Shellac, Natural Rubber.

OBP can be made from by products of oil or natural gases. There are many polymers produced

from petrochemicals or biological sources are synthesized biodegradable polymers.

Polyalkylene esters, polylactic acid and its co-polymers, Poly amide esters polyvinyl esters,

polyvinyl alcohol, polyanhydrides are biodegradable synthetic resins.

Properties:

Biodegradable polymers are non-toxic.

They are able to maintain good mechanical integrity until degraded.

They are capable of controlling rates of degradation.

Applications:

They are used in sutures, suture is a medical device used to hold body tissues together after an

injury or surgery and tissue engineering.

Used in drug delivery systems.

Used to coat a stent and release drugs in a controlled way.

Used in dental devices and orthopedic fixation devices.

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Exercise Questions

1. Explain how polymers are classified on the basis of their thermal behaviour and method of

polymerization. Give one example for each case.

2. How will you synthesis Nylon 6,6 from 1,3 buta diene? Describe a method of preparation of

polyester and mention its properties and uses.

3. What is the repeating unit of Natural rubber and Teflon

4. What is Buna-N rubber? How is it manufactured? Give its properties and uses?

5. Write four moulding constituents of plastics and their function with examples.

6. Explain the injection moulding process with a neat diagram? Mention its advantages.

7. How HDPE is prepared? Give its properties and uses.

8. Why silicones are called inorganic polymers? Discuss the synthesis of linear chain silicones.

9. Why Bakelite can’t be remoulded and write its repeating unit

10. Describe condensation polymerization with example.

11. Describe glass reinforced, fiber reinforced and bullet proof plastics. Discuss various properties

and Uses of these materials?

12. Write short notes on Bulletproof plastics

13. Explain in brief the preparation and structure of Kevlar and Nomex. Discuss their engineering

applications

14. Write in brief about conducting and biodegradable polymers.

15. Define polymer with an example. Explain how polymers are classified on the basis of their

method of polymerization. Give one example for each case.

16. Explain how polymers are classified on the basis of their structure. Give one example for each

case.

17. What is meant by degree of polymerization with example and differentiate homo and hetero

polymers.

18. Define polymerization. Differentiate between addition and condensation polymerization.

19. Explain condensation polymerization with example.

20. Give any three properties of polymers.

21. What are plastics? Write the various properties of plastics.

22. Distinguish between thermo plastics and thermosettings.

23. Discuss in detail about the methods of mouldings of plastics in articles.

24. Write the preparation, properties and uses of the following

a. Polyethylene b. PVC

25. Write the preparation, properties and uses of the following

a. Bakelite b. PVC

26. Write about the preparation of natural rubber from latex of heveabrasiliensis.

27. Write a short note on vulcanization of rubber.



28. Give the preparation, properties and uses of the following

Buna – N b. Buna – S c. Thiokol Rubber

29. What are the commercial uses of plastics and rubbers?

30. Describe glass reinforced, fiber reinforced and bullet proof plastics. Discuss various properties

and Uses of these materials?

31. Explain the injection moulding process with a neat diagram? Mention its advantages.

32. Describe condensation polymerization with example.

20. Define tacticity. Write the different types of stereo specific polymers with examples.

21. Write aboutmoulding constituents of plastic and their functions with examples.

22. Define elastomer. Write the characteristics of elastomers.

23. What are the different types of rubbers? What is the basic chemical unit present in the natural

rubber?

24. What is gutta – percha rubber? Give the structure with configuration of chemical unit present in

this rubber.

25. What are the draw backs of natural rubber?

26. What is vulcanization? How it is carried out?

27. Write short notes on Bulletproof plastics

28. Explain in brief the preparation and structure of Kevlar and Nomex. Discuss their engineering

applications.

29. Write in brief about conducting and biodegradable polymers.

30. Write the structure of recurring unit of natural rubber.

31. Why natural rubbers need vulcanization?

32. How crepe rubber obtained from latex.

33. Explain with examples about conducting polymers.

34. Write about the polymers used in medicine and surgery.

35. Why Bakelite can’t be remoulded and write its repeating unit.

36. Discuss the disadvantages of natural rubber over the synthetic rubber.

unit iv high polymers 5-16 - · pdf fileunit iv high polymers 201 5-16 ... styrene butadiene...

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