Unit II

POLYMER SCIENCE
POLYMERS - Introduction
Polymers are large molecules composed of repeated chemical units. The smallest
repeating unit is called monomer (mono [Single] + mer [part]). The word polymer
is derived from the Greek word ‘poly’ = many; mers = parts. It is generally described
in terms of single repeated units such as

CH2 = CH2 → (CH2 – CH2)n

Ethylene Polyethylene

Definition: Polymer is a macromolecule built-up by linking together of a large

number of smaller molecules called monomers.

Eg: Polyethylene is a polymer formed by linking together of a large number of

ethylene molecules.

The process by which the simple molecules (monomers) are converted into
polymers is called “polymerization”.

Degree of polymerization

The number of repeating units (n) in a polymer chain is called degree of

polymerization (DP).

Eg: If 100 molecules of ethylene polymerize to give the polymer chain, the DP
of ethylene is 100.

Functionality

“The number of bonding sites (or) reactive sites or functional groups present in a
monomer molecule is called Functionality”. For a substance to act as a monomer,
it must have at least two reactive sites or bonding sites.
For example, the double bond in vinyl monomers (CH2 = CHX) can be considered
as a site for two free valencies. When the double bond is broken, two single bonds
become available for combination.
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 H2C=CHX → –CH2–CHX–

Monomer Functionality
Vinyl compounds (C=C) 2
Glycerol (CH2OH-CHOH-CH2OH) 3
Adipic acid (COOH-(CH2)4-COOH) 2
Phenol (C6H5OH) 3

Classification of Polymers
Polymers can be classified in several ways.

1. Classification based on the origin or based on source:

a) Natural polymers – They are polymers that occur in nature.

Eg: Starch, cellulose, proteins, nucleic acids, natural rubber, etc.

b) Synthetic polymers – They are polymers that are prepared artificially in the
laboratory..

Eg: Polyethylene (PE), Polystyrene (PS), PVC, nylon, terylene, bakelite, etc.

2. Classification based on type of monomers used:

a) Homopolymer – If a polymer consists of identical monomers, it is termed as

homopolymer.

—A—A—A—A—A—A—A—
Homo polymer

Eg:

Monomers Polymers
Vinyl chloride Polyvinyl chloride (PVC)
Ethylene Polyethylene (PE)
Styrene Polystyrene (PS)
b) Copolymer – If a polymer is made up of more than one type of monomers it is
called a copolymer.
—A—B—A—B—A—B—A—B—
Copolymer
Eg:
Monomeric units Polymer
Styrene – butadiene Styrene Butadiene rubber (SBR)
Styrene isoprene Styrene Isoprene rubber (SIS)
Depending on the arrangement of the monomeric units, the copolymers may
be further classified as –

i) Random copolymers – The monomeric units are randomly arranged.

—A—B—B—A—B—A—A—B—B—A—B—

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 ii) Alternating copolymers – The monomeric units are arranged in an
alternating manner.
—A—B—A—B—A—B—

iii) Block copolymers – The monomeric units are arranged in blocks.

A—A—A—A—B—B—B—B

iv) Graft copolymers – They are branched copolymers in which the backbone
is formed of one type of monomer and the branches are formed of the other
types of monomers.
—A—A—A—A—A—A—A—A—

3. Classification based on structure:

a) Linear polymers – in these monomeric units are joined in the form of long
straight chains.
Eg: High density polyethylene (HDPE), nylon, polyester, etc.

—A-A-A-A-A-A-A— —B-A-B-A-B-A-B—
Linear Homo polymer Linear Copolymer

They possess high M.P., density and tensile strength due to close packing of
polymer chain.

b) Branched chain polymers – (two-dimensional). These are also linear in

nature, but possess some branches along the main chain.

Eg: Low density polyethylene (LDPE), glycogen, etc.

Branched chain homo polymer Branched chain copolymer

They possess low M.P., density and tensile strength due to poor packing of
polymer chains in presence of branches.

c) Cross-linked polymers – contain monomer molecules connected to each

other by only covalent bonds. These are giant molecules in which movement
of individual monomeric units is prevented by strong cross-links.

Eg: Bakelite, vulcanized rubber, urea-formaldehyde, etc.

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 Cross-linked homo polymer

Due to presence of cross link, they are hard, rigid, brittle, and do not melt,
but burn on strong heating.

4. Classification based on chain composition:

a) Homochain polymer – Polymers having all carbon atoms along their
backbone are called homochain polymers.
Eg: polyethene, PVC, rubber, etc.
b) Heterochain polymer – If the polymeric chain contains a heteroatom, then
they are called heterochain polymers.
Eg: nylon (polymeric chain contains nitrogen atom) and terylene (polymeric
chain contains oxygen atom)
5. Classification based on mechanism of polymerization:
a) Addition polymers: In these the monomeric molecules bond to each other
without the loss of any other atoms. Alkene monomers are the biggest groups
of polymers in this class.
Eg: PE, PP, PS, PVC, etc.
b) Condensation polymers: In these usually two different monomers combine
with the loss of a small molecule (H2O, HCl, etc.)
Eg: Polyesters, polyamides (nylon), etc.
6. Classification based on molecular forces:
a) Thermoplastics or thermoplastic polymers: These are linear, long chain
polymers which get softened when heated and hardened when cooled. The
cycle can be carried out many times without affecting their chemical
properties.
Eg: Polyethylene, PP, PVC, teflon, Plexiglass, etc.
b) Thermosets or thermosetting polymers: The polymers which on heating
get hardened and once they have solidified, they cannot be softened, i.e., they
are permanent setting polymers. Such polymers during heating acquire three-
dimensional cross-linked structure with predominantly strong covalent
bonds. Thus, a thermosetting polymer once molded cannot be reprocessed.
Eg: Phenol-formaldehyde resin (Bakelite), urea-formaldehyde resin, epoxy
resin (araldite), etc.

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 c) Elastomers or Synthetic rubber: The polymers which undergo a very large
elongation when pulled but returns to the original length on release of force
are called elastomers. The polymer chains of elastomers are long, coiled and
entangled. There are no intermolecular forces except weak Vander Waal’s
forces.
Eg: Natural rubber, Buna-S, Neoprene rubber, Silicone rubber, etc.
d) Fibres: Fibres are long, thin, and thread-like polymer chains which do not
undergo stretching or deformation like elastomers. These long chain
molecules are lined up and held together by hydrogen bonding. They have
high tensile strength and less elasticity. Fibre forming materials may be
synthetic or natural.
Eg: Natural: Wool, Silk, Cotton, Jute, etc.
Synthetic: Nylon 6, Nylon 6,6, Terylene, etc.
7. Classification based on Tacticity:
The orientation of monomeric units in a polymer molecule can take place in
an orderly or disorderly fashion with respect to the main chain. The stereo
chemical arrangement of functional groups on carbon backbone of the polymer
is called tacticity of the polymer. This affects the physical properties like
crystallinity, rigidity of the polymer. Therefore, tacticity helps to understand at
what temperature the polymer melts, solubility in a solvent and its mechanical
properties. Depending on the tacticity there are three different types of polymers.
a) Isotactic polymers: Those polymers in which the functional groups are
arranged on the same side are called isotactic polymers.
Eg: Polystyrene, PVC

Polystyrene

In this the groups or substituents are arranged in stereochemical fashion. This

results in high crystallinity, high M.P. and mechanically strong due to
presence of weak interchain forces.
b) Atactic polymers: When there is no regular arrangement of functional groups
on the backbone of the polymer chain, those polymers are called atactic
polymers.
Eg: PVC, Polypropylene

These possess low crystallinity, low M.P. and mechanically weak.

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 c) Syndiotactic polymers: The polymers with alternate arrangement of
functional groups are called syndiotactic polymers.
Eg: Gutta Percha, PVC

8. Classification based on Degree of Polymerization:

a) Oligopolymers – Polymers whose degree of polymerization is less than 600
are called Oligopolymers. These polymers do not possess the engineering
properties.
b) High Polymers or Macromolecules – Polymers whose degree of
polymerization is more than 600 are called high polymers or macromolecules.
These possess the desired engineering properties and widely used.
9. Classification based on Chemical composition:
a) Organic polymers – A polymer whose backbone chain is made mainly of
carbon atoms and the side chains consist of oxygen, nitrogen, sulphur, etc.
Eg: polythene, PVC, nylon, etc.
b) Inorganic polymers – These polymers backbone chain is made of elements
other than carbon atom.
Eg: silicone rubbers, phosphazene, etc.

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Types of polymerizations
The conversion of a monomer into a polymer is an exothermic process and if
heat is not dissipated or properly controlled, explosions may take place. The
polymerization reactions are broadly classified into three types.

1. Addition polymerization or Chain polymerization

2. Condensation polymerization or Step growth polymerization and
3. Copolymerization.
• Addition or Chain polymerization: The polymerization that takes place by self-
addition of the monomer molecules to each other through a chain reaction is
called addition polymerization.
• The functionality of monomer is a double bond, and it is bifunctional.
• Polymerization takes place by self-addition of the monomer molecules to each
other.
• No by-products like H2O, CH3OH etc. are produced.
• The polymer has the same chemical composition as that of monomer.
• The molecular weight of the polymer is the exact multiple of the monomers.
• An initiator is required to start the polymerization reaction.
• The mechanism is carried out in three steps, i.e., initiation, propagation and
termination.
• The mechanism is rapid.
• The conversion of a  bond to a  bond takes place during the polymerization,
liberating 20k.cal/mole of energy. (Exothermic reaction).
Eg: PE, PVC, PS, etc.

• Condensation or Step polymerization: Step polymerization takes place by

condensation reactions of functional groups of the monomers and elimination
of small molecules like water, HCl, etc.
• The monomers contain functional groups like -OH, -COOH, -NH2, halides,
etc.
• The polymer is built up by a slow stepwise condensation of the functional
groups of the monomer.
• The polymers produced are living polymers containing functional groups at
the end of the chain.
• The reactions are catalysed by catalysts.
• The functionality of the monomer must be two or more than two. The
monomers must be dibasic acids, diols, diamines or triols, etc.
• The polymerization reaction is accompanied by the elimination of by-
products like HCl, CH3OH, H2O, etc.
• The reactions are not exothermic.
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 • The molecular weight of the polymer is not the sum of the molecular weights
of the monomers.
• It is not a three-step mechanism of initiation, propagation and termination.
Eg: Nylon, Polyester, Bakelite, etc.

Polymerization of a diamine with diacid gives nylon 6,6 (polyamide)

• Copolymerization: It is the joint polymerization of two or more species. High

molecular weight compounds obtained by copolymerization are called
copolymers.
Eg: Butadiene and styrene copolymerize to yield Styrene-butadiene rubber.

Differences between Addition and Condensation polymerization

S.No. Addition Polymerization Condensation Polymerization
1. It is also known as chain growth It is also known as step growth
polymerization polymerization
2. It takes place only in monomers It takes place in monomers having
having multiple bonds reactive functional groups
3. No by-products like H2O, CH3OH By-products like HCl, CH3OH, H2O,
etc. are produced etc. are produced
4. The product obtained by this The product obtained by this
polymerization is thermoplastic method is either thermoplastic or
thermosetting
5. It takes place without elimination It takes place with the elimination
of simple molecules of simple molecules like H2O, HCl,
NH3, etc.
6. The mechanism is rapid The mechanism is slow

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 7. An initiator is required to start the A catalyst is required for the
reaction reaction
8. The mechanism is exothermic The mechanism is not exothermic
9. The mechanism is carried out in 3 The mechanism is carried out by
steps – initiation, propagation, slow step wise condensation
termination
10. The molecular weight of polymers The molecular weight of polymers is
is sum of molecular weights of not the sum of molecular weights of
monomers monomers
11. Ex: PE, PS, PVC, etc. Ex: Nylon, Polyester, Bakelite

Properties of Polymers
Physical state of polymers
Relative arrangement of polymeric chains with respect to each other may result in
an amorphous or crystalline state of a polymer.

An amorphous state [Fig.(a)] is characterized by a completely random arrangement

of molecules; while a crystalline state [Fig.(b)] consists of definite regions of
crystallinity (called as crystallites) embedded in an amorphous matrix.

The crystallization tendency of a polymer depends on the ease with which the
chains can be aligned in an orderly arrangement. Polymers with a long repeating
unit or with a low degree of symmetry do not crystallize easily and therefore
generally form amorphous structures. For example, polystyrene, polyvinyl acetate
and polymethyl methacrylate (all having bulky side groups attached at random to
the main carbon chain) are typically amorphous.

Crystallinity
The degree to which the molecules of a polymer are arranged in an ordered pattern
with respect to each other is a measure of its crystallinity. In a crystalline solid, the
atoms or molecules are arranged in a regular, periodic manner. It is the indication
of the amount of crystalline region in the polymer with respect to amorphous
content.
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Crystallinity has a vital role in determining the hardness, permeability, density,
tensile strength, impact resistance, heat capacity and solubility of a polymer. In
fact no polymer is 100% crystalline.
Factors effecting Crystallinity

1. Molecular weight: With increase in molecular weight of a polymer, %

crystallinity decreases. (Due to large number of entanglement of chains)
2. Tacticity: Geometrical regularity is desired in a polymer to show crystallinity.
The irregularity prevents the chains from packing closely to each other.
Therefore, Atactic polymers are poorly crystalline, while syndiotactic and
isotactic polymers are more crystalline in nature.
3. Chain Branching: Linear polymers have higher crystallinity than branched
polymers. Because linear chains can pack well than branched polymer chains.
And also, when the number and density of branches are increased, crystallinity
will be decreased.
4. Presence of side groups: When the size of the side of the group is increased, it
becomes more difficult for that particular polymer to pack tightly and results in
the more amorphous polymer.
5. Intermolecular forces: The presence of polar side groups helps to form strong
intermolecular interactions. So, polymer chains can come closer and pack
tightly. Dispersion forces, dipole-dipole interactions and hydrogen bonds are
some examples for intermolecular forces in polymers.
6. Chain composition: Homopolymers are highly crystalline whereas the
properties of the copolymers depend on the monomers and their configuration.
Alternating copolymers are more crystalline than Block, Random and Graft
Copolymers.
7. Rate of cooling: When the cooling rate is high, polymer molecules have no time
to organize orderly. So, crystallinity becomes low. Slower cooling promotes
crystal formation and growth.
8. Plasticizers: Plasticizers are the low molecular weight additives which are used
for keeping polymer chains separated from each other. So, adding a plasticizer
reduces crystallinity.
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Crystallinity affects the following properties of the polymer:
With increase in % of crystallinity,

➢ Strength and stiffness of polymer increases, but brittleness also increases,

➢ Solubility and permeability of polymer decreases,

➢ Density and melting point of polymer increases,

➢ Opacity of the polymer also increases.

Thermal behaviour of polymers

Glass Transition Temperature
Glass transition temperature (Tg) is the temperature at which amorphous polymers
undergo a transition from the glassy state to the rubbery state. It is represented by
Tg.

Glassy state is hard and brittle state of material which consists of short-range
vibrational and rotational motion of atoms in polymer chain, while Rubbery state
is soft and flexible state of material which is a long-range rotational motion of
polymer chain segments.

Some polymers are used above their glass transition temperature, and some are
used below.

➢ Hard plastics like polystyrene and poly methyl methacrylate are used below
their glass transition temperature; that is in their glassy state. Their Tg’s are
well above room temperature.
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 ➢ Elastomers like polyisoprene and polyisobutylene are used above their Tg’s, that
is in the rubbery state, where they are soft & flexible.
Factors effecting Glass transition temperature (Tg)

1. Chain flexibility: As Tg depends on the ability of a chain to undergo internal

rotations, chain flexibility to be associated with low glass transitions.
E.g., Polymers that contain −CH2−CH2− sequences and ether linkages in the
main chain have relatively easy internal rotations and therefore low Tg values.
2. Steric effects: The presence of bulky side groups hinders rotation of the
backbone atoms due to steric hindrance, and therefore results in an increase in
Tg. The magnitude of this effect depends on the size of the side groups.
3. Effect of Intermolecular Forces: The presence of polar side groups leads to
strong intermolecular attractive interactions between chains which hinders
molecular motion thus causing an increase in Glass transition temperature.
4. Copolymerization: It is possible to alter the glass transition of a homo polymer
by copolymerisation with a second monomer.
If the two homo polymers prepared from the monomers have different Tgs, then
it is reasonable to expect that their random copolymer should have a glass
transition which is intermediate between the Tgs of the homo polymers.
5. Cross-linking & Crystallinity: Both cross-linking and crystallinity cause an
increase of the glass transition temperature.
The presence of covalent bonding between chains reduces molecular freedom
and thus the free volume.
Similarly, the presence of crystalline regions in a semicrystalline material
restricts the mobility of the disordered amorphous regions.
6. Plasticizer: By the addition of plasticizers to the polymer, it becomes flexible. So
Tg is reduced.
PLASTICS – Intoduction
The word plastic is derived from the Greek word plastikos (= capable of being
shaped or moulded). In a broad sense, a plastic refers to a material which exhibits
plasticity, i.e. the ability to get deformed or to undergo change of shape under
pressure. However, the term plastic generally refers to organic materials of high
molecular weight that can be moulded to any desired shape when subjected to
pressure and temperature in the presence of a catalyst.

Types of plastics
From engineering point of view plastics can be classified into two types. They are

1. Thermoplastics
2. Thermosettings
1. Thermoplastics: The polymers which become soft on heating and hard on
cooling are called thermoplastics. The cycle can be carried out many times
without affecting their chemical properties. These have either linear or branched
structure and can be amorphous or semi-crystalline materials. Neighbouring
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 polymeric chains are held together by weak Vander Waal’s forces and hence do
not have any cross-links.
Eg: Polyethylene, Polystyrene, Polypropylene, nylon, Teflon, PVC, polyester, etc.
2. Thermosettings: The polymers which on heating get hardened and once they
have solidified, they cannot be softened, i.e., they are permanent setting
polymers. Such polymers during heating acquire three-dimensional cross-linked
structure with predominantly strong covalent bonds. Thus, a thermosetting
polymer once moulded cannot be reprocessed.
Eg: Bakelite, urea-formaldehyde resin, epoxy resin, vulcanized rubber, etc.
Differences between thermoplastics and thermosettings
S.No. Thermoplastics Thermosettings
1. They are the products of addition These are the products of
polymerizations. condensation polymerization.
2. They have either linear or They have three-dimensional cross-
branched structures. linked network structures.
3. They soften on heating and stiffen They do not soften on heating.
on cooling.
4. Adjacent polymer chains are held Adjacent polymer chains are held
by either Vander Waal’s forces or together by strong covalent bonds
by dipole-dipole forces or H- called cross-links.
bonding.
5. Soluble in organic solvents. Insoluble in organic solvents.
6. They are soft, weak and less They are hard, strong and more
brittle. brittle.
7. They can be remoulded, reshaped They cannot be remoulded and
and reused. hence cannot be reused.
8. They can be reclaimed from waste They cannot be reclaimed from waste
i.e., they can be recycled. and hence cannot be recycled.
9. Eg: PE, PS, PVC, etc. Eg: Bakelite, Urea-formaldehyde
resin, vulcanized rubber, etc.

Individual Polymers

1. Teflon or Poly tetra fluoro ethylene (PTFE) or Fluon:

Preparation: Teflon is obtained by the chain polymerization of tetra fluoro
ethylene in presence of benzoyl peroxide as an initiator.

Properties:
Due to the presence of highly electronegative fluorine atoms, TEFLON has got
• High melting point.
• Exceptionally high chemical resistance.
• High density 2.1 – 2.3 gm/cc.
• It is very strong, hard polymer that can be machined to drilling, punching, etc.
• Teflon is a very good electrical insulator.
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 • It possesses very good abrasion resistance.
Applications:
• It is used as non-stick coating for pans and other cookware.
• Teflon is a very good insulating material for motors, transformers, cables,
wires, fittings, etc.
• It is used for making gaskets, pump parts, tank linings, tubing, etc.
• Due to its extreme chemical resistance, it is used for making chemical carry
pipes.
• Non-lubricating bearings and non – stick stop cock for burettes are made
from teflon.
• It is also used for coating as impregnating glass fibre, asbestos fibres.
2. Polycarbonates (PC) (Lexan, Merlon):
Polycarbonates are commonly known by the trademarked name Lexan. They
received their name because they are polymers containing carbonate groups (-
O-(C=O)-O-). Most polycarbonates of commercial interest are derived from rigid
monomers.
Preparation: Polycarbonbates are prepared by the interaction of diphenyl
carbonate with bisphenol-A.

Properties:
• They are characterized by high impact and tensile strength over a wide range
of temperature.
• They are soluble in organic solvents and alkalis.
• They have high transparency and stiffness.
• Good dimensional stability.
• Flame retardancy and excellent fire performance
• Biologically inert and readily recyclable
Applications:
• For preparing moulded domestic ware,
• housing for apparatus, and
• electrical insulator in electronics and electrical industries.
• used in cameras (lens holders, shutter assembly etc)
• CD’s, DVD’s, Cell phones, laptops

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 • Automobile headlights
3. Poly methyl methacrylate (PMMA) or Plexiglass or Lucite
Preparation: It is prepared by polymerization of methyl methacrylate in the
presence of acetyl peroxide or hydrogen peroxide as catalyst.

Properties:
• Amorphous, colourless, transparent thermoplastic with high optical
transparency.
• Presence of methyl groups restrict the chain flexibility. So, it is hard and has
higher Tg.
• Polar compound, hence, does not have electrical insulation properties
comparable with PE.
• Excellent weather ability
• Compared to glass, PMMA weighs only one-third, can be readily moulded to
desired shape.
• Low chemical resistance to hot acids and alkalis and low scratch resistance.
• Scratches on it can be easily removed by rubbing it with a cloth moistened
with acetone.
Applications:
• Display signs both illuminated and non-illuminated for internal and external
use.
• Light fittings for streetlamp housing, ceiling lighting for school rooms, railway
stations, factories.
• Familiar bubble body of many helicopters.
• Motor-cycle windscreen.
• Wash basins.
• Dome-shaped covers of solar collectors (solar heaters)
• Optical fibres.
• For making lenses, artificial eyes, dentures, etc.
4. Polyethylene Terephthalate (PET) or Terylene or Dacron:
PET is a thermoplastic polyester resin. It is mostly used to create clothing labels
under the name “polyester”, and synthetic fibres and plastic bottles.
Preparation: PET resins are produced commercially from ethylene glycol (EG)
and either dimethyl Terephthalate (DMT) or terephthalic acid (TPA) by
condensation polymerization with loss of methanol or water molecule.

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 Properties:

• Because of the symmetrical structure and presence of numerous polar groups,

the polyester is a good fibre forming material.
• It's Glass transition temperature and melting point are 80°C and 265°C
• PET fibres have high stretch-resistance, high crease and wrinkle-resistant.
• It is insoluble in most organic solvents.
• It is resistant to mineral and organic acids but is less resistant to alkalis.
Applications

• Used for making synthetic fibres like Terylene, Dacron etc.,

• It is mainly used in making plastic bottles, sheets, ropes, nets and sails.
• To make transparencies for overhead projectors.
• Used for preparation of magnetic recording tapes.
• For making electrical insulation materials.
• For making storage containers for carbonated beverages.
• Materials for microwaves and conventional ovens
5. Bakelite or Phenol-formaldehyde resin:
Bakelite is an important thermoset resin (World’s first synthetic resin) named
after the scientist Bakeland, who synthesized this resin in the year 1909.
Preparation: It is prepared by the step polymerization of phenol with
formaldehyde in the presence of an acid or alkali as a catalyst. The
polymerization takes place in three stages.
I Stage: Phenol is made to react with formaldehyde in presence of acid/alkali to
produce non–polymeric mono, di and tri methylol phenols depending on the
phenol formaldehyde ratio (P/F ratio).

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II Stage: The mono, di and tri methylol phenols are heated to produce two types
of straight chain resins by condensation of the methylol group with hydrogen
atom of benzene ring or another methylol group.

III Stage: This stage of preparation includes heating of ‘A’ stage resin and ‘B’
stage resin together, which develops cross linkages and Bakelite plastic resin is
produced.

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 Bakelite
Properties:

• Bakelite plastic resin is hard, rigid, and strong.

• It is a scratch resistant and water-resistant polymer.
• It has good chemical resistance, resistant to acids, salts and many organic
solvents, but it is attacked by alkalis due to the presence of –OH group.
• It is a good anion exchanging resin, exchanges –OH group with any other anion.
• Bakelite is an excellent electrical insulator.
• It is a very good adhesive.
• It has very good corrosion resistance, resistant to atmospheric conditions like
O2, CO2, moisture, light, UV radiation, etc.
Applications:
Bakelite is used widely–

• For making electrical insulator parts like switches, switch boards, heater
handles, etc.
• For making handles for cookers and saucepans.
• For making moulded articles like telephone parts, cabinets for radio and
television.
• For making tarpaulins, wood laminates and glass laminates.
• As an anion exchanger in water purification by ion exchange method in boilers.
• As an adhesive for grinding wheels, etc.
• In paints and varnishes.
• For making bearings used in propellers shafts, paper industry and rolling mills.
Moulding or Fabrication of plastics
Giving desired shape to plastic material with the help of mould at desired
temperature and pressure is called moulding or fabrication. Depending on the type
of resin whether thermoplastic or thermoset, there are different methods of
fabrication.
• Compression
• Extrusion
• Blowing
• Thermoforming
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• Injection
• Transfer
Compression moulding
It is the oldest mass production process for polymer products. It is used in
fabrication of both thermoplastics and thermosets.
Process:
In this process, initially, a synthetic plastic material with added fillers and
ingredients is placed between the mould and heated (130-180°C) under pressure
(100-500Kg/cm2). The plastic material is converted in to fluidized plastic in the
mould and gets moulded into the required shape after curing. Curing is done by
cooling in thermoplastics whereas heating in thermosetting plastics. Hence the
required moulded article is taken from the opening of the moulded parts.

Advantages:

• The initial setup cost is low.

• It gives good surface finish.
• The material loss is less.
• Low mould maintenance.
• Product has low residual stress.
Disadvantages:

• It is not economical for making a small number of parts.

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• Sometimes secondary processing of product is required.
• Greater waste
• It has slower processing times.
Applications: It is used in fabrication of –

• Thermoset products like electrical plugs and sockets, switches, etc.

• Thermoplastic products like buttons, gramophone records, etc.
• Rubber products like O-ring seals, springs, anti-vibration mounting pad, etc.
Extrusion moulding
It is used for moulding of thermoplastic materials into articles of uniform cross-
section like tubes, rods, sheets, wires, cables, etc.
Process:
The thermoplastic ingredients are fed into the barrel with the help of a hopper. The
feed is heated with the help of heaters to plastic state and then pushed by means
of a screw conveyor into a die, having the required outer shape of the article to be
fabricated. The extruded article gets cooled due to atmospheric exposure or
artificially by air jets. A long conveyor carries away the cooled product continuously.

Advantages:

• Products can be produced quickly and at a high volume, ensuring cost-efficiency

and speed.
• Low-cost relative to other moulding processes.
• Better flexibility in manufacturing products.
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• Many types of raw materials can be used.
• Continuous operation
• Good mechanical properties obtained in cold extrusion.
• Post-extrusion manipulations (as plastic remains hot when it leaves the
extruder) are possible.
• Surface finish obtained is good.
Disadvantages:

• It limits on the kinds of products it can manufacture (products of uniform cross-

section only manufactured).
• It can be used only for linear polymers.
• When hot plastic exits the extruder, it frequently expands.
Applications:

• It is used in manufacturing of tubes, electric cables, optical fibres, pipes etc., by

hot extrusion.
• Collapsible tubes, gear blanks, aluminium cans, cylinders are some of the items
produced by cold extrusion.
• PVC window frames, curtain rails, window screens, doors, flat and corrugated
sheeting (e.g., roofing), etc., are prepared by profile extrusion.
• It is also used for preparing sheets for thermoforming.
Blow moulding
It is used for producing hollow articles like bottles and hollow toys. Thermoplastic
articles such as PE, polycarbonate, PVC, nylon, and styrene are blow-moulded.
Process:
In this process a hot, softened thermoplastic tube called ‘parison’ is properly placed
inside the two-piece hollow mould. The split mould is then closed, sealing the
bottom. This joint is usually seen at the bottom of the plastic bottles. Air is then
blown in the hot parison. It is inflated and acquires the shape of the mould. The
mould is allowed to cool, and the rigid thermoplastic article formed is removed by
opening the mould.

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Types of Blow moulding
In general, there are three main types of blow moulding:
• Extrusion blow moulding
• Injection blow moulding
• Stretch blow moulding.
All the types have one primary difference that is making of parison by extrusion,
injection and stretching.
Extrusion blow moulding: In the extrusion blow moulding process, plastic is
melted and extruded into a hollow tube (a plastic parison). The extruded parison is
cut off and moved to the mould and clamped there. The mould closes and parison
is blown to shape. The blown bottle is cooled and then ejected.

Injection blow moulding: In the injection blow moulding process, the molten
plastic is injected into a mould to form a plastic tube called parison. The parison
is moved to the mould, blown into shape, cooled, and then ejected.

Stretch blow moulding: Stretch blow moulding is the common method for
producing soda bottles. The process begins with an injection moulded preform. The
preform is typically pre-heated then stretched in the axial direction and blown into
its final shape by a stretch blow holding machine. The blown article is cooled and
ejected.

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Advantages:
• Low tooling costs.
• Fast production rates.
• Ability to mould complex parts.
• Little scrap generated.
• Large hollow shape can be produced.
• Produced parts can be recycled.
Disadvantages:
• Limited to hollow parts.
• Thick parts cannot be manufactured.
• Higher scrap rate.
• Limited wall thickness control.
• It is highly dependent on petroleum.
• It creates a huge impact on the environment.
Applications:
• Thermoplastic materials like Low Density Polyethylene (LDPE), High Density
Polyethylene (HDPE), Polyethylene Terephtalate (PET), Polypropylene (PP),
Polyvinyl Chloride (PVC) etc., can be processed by this method.
• Hollow products like bottles, containers, jars, jerry cans and automotive fuel
tanks, dust bins, drums, oil storage containers, petrol tanks for cars, hollow
spheres, toys, etc.

Thermoforming Process
It is the combination of extrusion with compression moulding. Materials like PS,
PVC, PMMA, PC, HDPE, PP, etc., can be processed by this method.
Process:
Thermoplastic resin is extruded in the form of a sheet and heated to its softening
temperature. The warm flexible sheet is compressed between the two moulds. On
cooling, the shaped article, becomes rigid. It is then removed from the mould.
Compression of the moulding can be done by applying anyone of the following -
vacuum (vacuum thermoforming), pressure (pressure thermoforming) and
mechanical forces (mechanical thermoforming).
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Advantages:
• Design flexibility
• Low initial setup costs
• Low production cost
• Less thermal stresses
• Good dimensional stability
Disadvantages:
• Poor surface finish.
• Parts may have non-uniform thickness.
• All parts need to be trimmed.
• Limited materials can be made.
Applications:
• It is useful for the fabricating 3D articles, Automotive parts, Submarine hulls,
Food packaging items, Disposable cups, glasses, ice cream cups, plastic trays
for cookies and candy, Aircraft windscreens, Vehicle doors, etc.

Fibre-Reinforced Plastics (FRPs)

The combination of polymeric substance with solid fillers is called reinforced plastic.
The filler acts as reinforcing material, while the polymer links the filler particles.
Polymer composites consist of two or more phases, one of which is a dispersed
phase (fibres, particles, flakes, etc.) in a continuous matrix phase (polymer/plastic).
Fibre reinforced plastics are produced by reinforcing a plastic matrix with a high
strength fibre material such as glass, graphite, alumina, carbon, boron, beryllium,
and aromatic polyamide (aramid). Natural fibres such as sisal, asbestos are also
used for reinforcement. Depending on the desired properties of the final reinforced

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composite, the nature of the fibre used is decided. FRPs find extensive use in space
crafts, aeroplanes, boat-hulls, acid-storage tanks, motor cars and even buildings.
The main advantages of FRPs:

• Light weight- easy to handle and transport.

• High strength to weight ratio.
• Corrosion resistant-will not corrode.
• Better toughness, impact and thermal shock-resistance.
• Lower electrical conductivity.
• Better creep and fatigue strength.
• Higher specific stiffness.
• Cheaply and easily fabricable.
• Lower thermal expansion.
Types of Fibre-Reinforced Plastics
FRP’s are classified into three types based on the type of reinforced material used.
They are:
1. Glass fibre reinforced plastics
2. Carbon fibre reinforced plastics
3. Aramid fibre reinforced plastics
1. Glass fibre-reinforced plastics (GFRP’s)
Glass fibre is the most extensively used reinforced fibre because of durability,
acid proof, waterproof and fireproof nature of glass. Glass fibres are basically
made by mixing silica, sand, limestone, folic acid, and other minor ingredients.
The mix is heated until it melts at about 1260°C. The molten glass is then
allowed to flow through fine holes in a platinum plate. The glass strands are
cooled, gathered, and wound.
Glass is drawn into threads or fibres in the form of filaments fine than cotton or
silk thread. Then the filaments are woven in the form of mats. The fibre material
is suitably bonded with plastic materials to be reinforced. The common plastic
resins used are polyesters, epoxies, silicones, melamine, vinyl derivatives and
polyamides.
Properties:
• Lower densities.
• High tensile strengths.
• High impact resistance.
• Excellent chemical resistance and
• Corrosion resistance.
Disadvantages:

• Limited-service temperatures because most polymeric matrices begin to

deteriorate or flow at high temperatures.
• These materials do not possess the desired stiffness and rigidity, particularly
in their applications as structural components.

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 Applications: Some typical applications of these materials are in-

• Automotive parts
• Boat hulls
• Storage tanks
• Industrial flooring
• Plastic pipes
• Transportation industries to reduce vehicle weight and boost fuel efficiency.
2. Carbon fibre-reinforced plastics (CFRP’s)
Carbon fibres are used as reinforcement in a polymer matrix. Carbon fibres do
not absorb water and are resistant to many chemical solutions. They withstand
fatigue excellently; do not stress corrode and do not show any creep or
relaxation.
Properties:
• Alkali resistance
• Resistance to corrosion
• Low thermal conductivity
• High mechanical strength
• High fatigue resistance
Disadvantages:
Carbon fibre is electrically conductive and therefore might give galvanic
corrosion in direct contact with steel.
Applications: Some typical applications of these materials are in-

• Space vehicles, satellites

• Sports equipment
• High speed reciprocating parts for industrial machinery
• High temperature machinery
• Structural components requiring high specific stiffness and strength.
3. Aramid fibre-reinforced plastics (AFRP’s)
Aramid is the short form for aromatic polyamide. Aramid fibres are usually used
as fibre reinforcement for polymer matrix composites.
E.g., Kevlar and Nomex are extremely tough and resistant materials, which find
use in bullet-proof vests, fire-resistant clothing and puncture resistant bicycle
tyres.
These can be further classified into following two categories:
a.) Short Aramid fibre reinforced plastics
b.) Long Aramid fibre reinforced plastics
a.) Short Aramid fibre reinforced plastics:
Properties: They give effective reinforcement due to their-
• High surface area
• Strength
• Heat stability
• Inherent toughness and
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 • High wear resistance
Applications:

• Used in Automotive brakes and clutches.

b.) Long Aramid fiber reinforced plastics:
Properties:
• Metal-like ductile
• High compressive strength as they are capable of absorbing energy.
• High Temperature range (-200 to 200°C)
Applications:

• Used in helicopter industry in making rotor blades, motor housing, etc.

Conducting polymers
A polymer which can conduct electricity is termed as conducting polymer. Most
polymeric materials are poor conductors of electricity, because of the non-
availability of free electrons in the conduction process.
Within the past several years, polymeric materials have been synthesized which
possess electrical conductivities similar to that of metallic conductors. Such
polymers are called conducting polymers.
Eg: Polyacetylene, polypyrrole, polyaniline, polynaphthalene, polyquinoline, etc.
Conducting polymers can be classified into following types:

1. Intrinsically conducting polymers (ICP)

These polymers have extensive conjugation in the backbone, which is
responsible for conductance. These can be further classified as two classes. They
are as follows:
a.) Conducting polymers having conjugated π electrons in the backbone:
These polymers contain conjugated π electrons in the backbone, which
increases their conductivity to a large extent. This is because, overlapping of
conjugated π electrons over the entire backbone results in the formation of
valence as well as conduction bands.
The electrical conductance could occur only after thermal or photolytic
activation.
E.g., Polyacetylene, Polypyrrole, Polyaniline, etc.
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b.) Doped conducting polymers: It is obtained by exposing a polymer to a
charged transfer agent in either gas phase or in solution. ICP possess low
conductivity, but these possess low ionization potential and high electron
affinities, so these can be easily oxidized or reduced. Consequently, the
conductivity of ICP can be increased by creating either +ve or –ve charges on
the polymer backbone by oxidation or reduction. This technique is called
doping which is of two types:
p-doping: It is done by oxidation process. Conducting polymers having
conjugation is treated with Lewis acids or with iodine vapour or iodine
molecule.

Oxidation process leads to the formation of delocalized radical ion called

polaron. p-doping of polyacetylene gives a Polaron, a bipolaron and a soliton
pair. This can be explained on the basis of the following equations:

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 n-doping: It is done by reduction process. For this conductance, polymers
having conjugation is treated with Lewis base like sodium naphthalide.

The formed radicals yield to carry negative charge for conductance. This type
of doping also gives a polaron, bipolaron and a soliton pair.

2. Extrinsically conducting polymers (ECP)

These polymers possess their conductivity due to the presence of externally
added ingredients in them. These can be of the following types:
a.) Conductive element filled polymers: The polymer acts as the binder to
hold the conducting element (such as carbon black, metallic fibres, metallic
oxides, etc.) together in the solid entity.
Minimum concentration of conductive filler, which should be added so that
polymer starts conducting, is known as percolation threshold. Because at this
concentration of filler, a conducting path is formed in polymeric material.
Generally, Carbon-black is used as a filler which has very high surface area,
more porosity and more of filamentous properties.
b.) Blended conducting polymers: These polymers can be obtained by blending
a conventional polymer with a conducting polymer. Such polymers possess
better physical, chemical, electrical, and mechanical properties and they can
be easily processed.
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 E.g., up to 40% of polypyrrole will have little effect on tensile strength and also
give a much higher impact strength than obtained with carbon-black filled
compound.
Applications:
There are several utilities of conducting polymer due to their better physical,
chemical, mechanical properties, light weight and easy to process. Some of them
are-

• To make electromagnetic screening materials.

• In electronic devices such as transistors and diodes.
• Making of solar cells.
• Drug delivery system for human body.
• In the manufacture of non-linear optical materials, optical filters and electro
chromic displays.
• Used in molecular wires and molecular switches.
• To make rechargeable light weight batteries.
• In making of analytical sensors for pH, O2, NOx, SO2, NH3 and glucose.
• Making of ion-exchangers.
Shape memory polymers (SMPs)
SMPs are considered as smart materials as they have ability to return from a
deformed state (temporary shape) to original (permanent shape) through external
stimuli (activation mode).
Ex: PTFE, PLA, EVA, PEG.

Features/properties of SMPs

• Rubbery in nature, composed of long, entangled-polymer chains and these

chains gets stretched under tension to accommodate the deformation.
• High elastic deformation (strain up to 200% in most of the materials).
• Low cost, low density, biocompatibility, and biodegradability.
• SMPs are capable of recovering fully under low deformation level.
• If these are cooled below their glass transition temperature and deformed by
external forces.

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• Once the external forces are removed it will return to its previous unreformed
state.
• When the temperature is greater than “Glass transition temperature” (Tg), the
material enters the soft rubber phase and becomes easily definable.
Working Principle of SMPs

• In shape memory polymers, typical shape recovery process begins at a

temperature above Tg.
• At this temperature, the polymer is in a rubbery elastic state.
• The polymer is then deformed with an applied stress which creates a strain in
the polymer.
• When cooled below Tg, the material autonomously returns to the original shape
(shape recovery characteristic).
• This property of the material which repeatedly returns to its original shape is
called “shape memory”
Semicrystalline polymers possess SME as they occur in both states (crystalline &
amorphous) at the same time within specific temperature, usually room temp.
It has 3 stages of working
Programmable – By applying heat and deformation stress the polymer will be
converted to deformed state and this deformed state is fixed by removing stress at
cooling.
Storage – temporary shape of the polymer
Recovery – By applying external stimuli, they are deformed to permanent shape
called as recovery state.

Classification of Shape Memory based on their mode of activation

SMPs are classified based on their mode of activation as thermo-responsive, light
responsive, electrically, magnetically induced and water induced).

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Most common is thermo-responsive.
Thermo-responsive shape memory polymers
The shape memory polymers which change in shape with the change of
temperature are called thermo-responsive shape memory polymers. By far these
are the most common shape memory polymers.
The following is a list of some of the consumables for Thermoresponsive polymers:
• Poly (N-alkyl acrylamide);
• Poly (N-vinyl caprolactam) [PVC];
• Poly (N-ethyl oxazolone) [PET Ox];
• Poly (methyl vinyl ether) [PMVE];
• Poly (acrylic acid-co-acrylamide);
• Elastin-like oligo- and polypeptides.
Electrical heating induced shape memory polymers
The shape memory polymers are generally nonconductive. So, they are made
conductive by blending with carbon nano powders. The electric current is converted
into heat. They recover the original shape when an electric current is passed
through the shape memory polymers.
Light induced shape memory polymers
The shape memory polymers which are to be activated by light should have some
photosensitive groups which act as molecular switches. The shape memory
polymers are stretched and illuminated by a light of wavelength greater than a fixed
wavelength and the photosensitive groups form cross links.
Magnetically induced shape memory polymers
Non-contact triggering of shape changes in polymers has been realized by
incorporating magnetic nanoparticles in shape memory polymers and inductive
heating of these compounds in alternating magnetic fields.
Water activated shape memory polymers
The activation of the polymer can be achieved by immersion in water. A shape
memory polymer which has the permanent shape of a straight rod is programmed
into a Z shape. The left part of the polymer is dipped into water and the right part
is not dipped. There is a reduction of the glass transition temperature for the left
part and it gets activated i.e., gets back to its original form with the help of the
room temperature water itself.

Applications
Textiles
• Smart breathable garments that can regulate heat & moisture to the wearer’s
body.
• Wrinkle free, anti-shrinkable, crease retention fabrics.
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• Wearable electronic devices

Industry
• Automotive seat belts – absorb kinetic energy increasing safety.
• Self-tightening tubes – to avoid leakage
• Packaging – perfect to the size & shape of product
• Damping elements
• Temperature sensors, MEMS (Micro-electronic mechanical systems)
Biomedical
• Implantable biomedical devices
• Orthodontics – metal wires replaced with SMP’s.
• Bandages – adapt perfectly to patient skin.
• Sutures – Self shrinking.
• Intravenous needles
• Drug delivery
Packaging
• Perfect size & shape of the product
Rewritable digital storage devices

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