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UNIT IV HIGH POLYMERS Rev.Ed. 2014-15

Engineering Chemistry Page 80

HIGH POLYMERS

Syllabus: Types of polymerization – Stereo polymers – Physical and mechanical 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.

Objectives: Plastics are materials used very widely as engineering materials. An understanding of

properties particularly physical and mechanical properties of polymers / plastics /elastomers helps in

selecting suitable materials for different purpose.

OUTLINES

Introduction

Methods of polymerization

Stereo specific polymers

Properties of polymers, PE, PVC and Bakelite

Plastics

Compounding of a plastic

Fabrication of plastic articles

Selected individual polymers

Rubbers or elastomers

Vulcanization

Synthetic rubbers



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 7

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)

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 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.

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. 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 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 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 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. The under 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.

-oOo-



Assignment 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.

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