Polymers and Elastomers for Engineers

8/13/2019 Polymers and Elastomers for Engineers

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DEFINITIONThe wordpolymeris derived from the two greek words

poly and mers

Polymers are macro molecules formed by linking smaller

molecules repeatedly, called monomers.

partsor unitsmany

C C C C C C

HHHHHH

HHHHHH

Polyethylene (PE)

mer

ClCl Cl

C C C C C C

HHH

HHHHHH

Polyvinyl chloride (PVC)

mer

Polypropylene (PP)

CH3

C C C C C C

HHH

HHHHHH

CH3 CH3

mer

e.g.


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Examples:

Polyethylene is formed by linking a large number ofethylenemolecules

nC C

H

H H

H

C CH

H H

Hn Polymerisation

Ethylene polyethylene

polystyrene is formed by linking styrene molecules

H

styrene polystyrene

C CHHn Polymerisation

nC C

H

H

H


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The number of repeating units (n) in the chain is

known as the degree of polymerization.

Polymers with high degree of polymerization are called

high polymersand these have very high molecular weights

(104to 106).

Polymers with low degree of polymerization are called

oligomers.

e.g.,

D.P.


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Classification of Polymers

Polymers can be classified in several ways, based on

origin

structure

methods of formation

response to heat

properties (or applications)


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Based on the origin

Natural polymers

synthetic polymers

Natural polymers are those which are obtained naturally

e.g., Cellulose, Silk, Starch, RNA, DNA, Proteins etc.,

Synthetic polymers are those which are made by man

e.g., polyethylene, polystyrene, PVC, polyester, etc.,

semi-synthetic polymers which are chemically modified

natural polymers

polymers can be classified as

e.g., cellulose acetate, cellulose nitrate, halogenated

rubbers etc.,


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Based on the molecular structure

polymers can be classified as

Linear

Branched

Cross-linked

the monomeric units combine linearly with each otherIn linear polymers,

secondary bonding


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Branch polymers

Cross linked polymers


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Based on the method of formation

Addition polymers

Condensation polymers

Addition polymersare formed by self-addition of monomers

The molecular mass of a polymer is an integral multiple ofthe molecular mass of a monomer

Condensation polymers are formed by condensation reaction

i.e., reaction between two or more monomer molecules

with the elimination of simple molecules like water,

ammonia, HCl etc.,


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Based on the response to heat

Thermo softeningThermosetting

soften on heating and can be converted into any shape

and can retain its shapeon cooling

thermosoftening or thermoplastics

under go chemical change on heating and convert

themselves into an infusible mass

thermosetting polymers


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Based on the properties or applications

PlasticsElastomers

Fibers

Resins

PlasticsThe polymers, which are soft enough at some temperature

to be moulded into a desired shape and hardened on coolingso that they can retain that shape.

e.g., polystyrene, polyvinylchloride, polymethylmethacrylate etc.,


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Elastomers

The polymers in which the structural units are either zig zag or

in helical chains.

They undergo elastic changes when subjected to an external force

but readily regain their original shape when the force is withdrawn

e.g., natural rubber, synthetic rubbers, silicone rubbers etc.,

FibersIn these polymers, the molecular chains are arranged parallel to

each other in a spiral or helical pattern and

the molecular length is at least 100 times its diameter

e.g., nylons, terylene, etc.,


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Resins

These polymers have a glossy appearance

These constitutes the major essential part of the plastics

These suffers the polymerization reactions and impart

different properties to plastics

e.g., polysulphide sealants, epoxy adhesives, etc.,


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Functionality

the number ofreactive sites or bonding sites

Alcohols Methyl alcohol

Ethers Dimethyl Ether

Acids Acetic acid

Some mono functional hydrocarbons


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Aldehydes Formaldehyde

Aromatic

hydrocarbons Phenol

Some bi functional hydrocarbons

adipic acid (hexanedioic acid)


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1,6-hexanediamine

Terephthalic acid

ethylene glycol


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Stereo regular polymers (or) Tacticity of Polymers

Isotactic

On one side

Syndiotactic

Alternating sides

Atactic

Randomly placed

- Conversion from one stereoisomerism to another is not possible by simple

rotation about single chain bond; bonds must be severed first, then reformed!


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Types of Polymerization

Polymerization occurs basically in two different modes. addition (chain growth) polymerization

condensation (step growth) polymerization

Addition

monomers react through stages of initiation, propagation,and termination

initiators such as free radicals, cations, anions opens thedouble bond of the monomer

monomer becomes active and bonds with other suchmonomers

rapid chain reaction propagates

reaction is terminated by another free radical or anotherpolymer


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- two monomers react to establish a covalent bond

- a small molecule, such as water, HCl, methanol or CO2is

released.

- the reaction continues until one type of reactant is used up

condensation


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DISTINGUISHING FEATURES OF

ADDITION AND CONDENSATION POLYMERISATIONADDITION CONDENSATION

Monomers undergo self addition to

each other without loss of by products

Monomers undergo intermolecular

condensation with continuous elimination

of by products such as H2O, NH3, HCl, etc.,

It follows chain mechanism It follows step mechanism

Unsaturated vinyl compounds undergo

addition polymerization

Monomers containing the functional

groups (-OH, -COOH, -NH2, .) undergothis polymerization

Monomers are linked together

through CC covalent linkages

Covalent linkages are through

their functional groups

High polymers are formed fast The reaction is slow and the polymer

molecular weight increases steadilythroughout the reaction

Linear polymers are produced

with or without branching

Linear or cross linked polymers

are produced

e.g., polystryrene, plexiglass,

PVC, etc.,

e.g., nylons, terylene, PF resins, etc.,


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Co-polymers

Random

Alternating

Block

Graft


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Random

random poly(styreneethylene) copolymer


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Alternating

alternating poly(styreneethylene) copolymer


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Block

block poly(styreneethylene) copolymer


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Graft


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Addition polymerization can be explained on the basis off ree radical mechanism

It involves three stages

viz., (i) Initiation

(ii) Propagation

(iii) termination

D or

u.v.lightI

(Initiator)

R*(Free radical)

Initiation


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C C

H

X H

H

+R*

(Free radical)

Vinyl monomer

C C *

H

H X

H

R

(new free radical)

The new free radicals attack monomer molecules further in quick

succession leading to chain propagation

Vinyl monomer

C C

H

X H

H

C* +C

H

H X

H

R

(Free radical)

C C

H

H X

H

R C C*

H

H X

H

(new free radical)

Propagation


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Vinyl monomer

C C

H

X H

H

+

(new free radical)

C C

H

H X

H

R C C*

H

H X

H

(another new free radical)

C*C

H

H X

H

C C

H

H X

H

R C C

H

H X

H

at mth stage,

C C

H

X H

H

+C

H

H

R C

X

H

C

H

H

C

X

H

m-2

C*C

H

H X

H

C

H

H

R C

X

H

C

H

H

C

X

H

m-1

C*C

H

H X

H


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R C

H

H

C

X

H

m-1

C*C

H

H X

H

+ RC

H

H

C

X

H

m-1

C*

X

H

C

H

H

+R C

H

H

C

X

H

m-1

CC

H X

H

H RC

H

H

C

X

H

m-1

C

H

H

C

X

H

unsaturated oligomer

(dead polymer)

saturated oligomer(dead polymer)

disproportionation


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TECHNIQUES OF POLYMERISATION

Addition polymerization is brought about using four different

techniques

Bulk or Mass polymerization

Solution polymerization

Suspension polymerization

Emulsion polymerization


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Bulk or Mass Polymerization

only the monomer and the initiator are involved

monomer is taken in the liquid state

the initiator should dissolve in the monomer

Initiation can be done either by heating or by exposing

to radiation

the reaction is exothermic

As the reaction proceeds, the reaction mixture becomes

viscous

the polymer molecules with wide range of molecular

masses will be obtained


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Solution Polymerization

the medium chosen is an inert solvent

the monomer, the initiator and a chain transfer agent

should dissolve in an inert solvent

The solution is heated with constant agitation

After the reaction is over,

the polymer formed may dissolve in the solvent

along with the monomer or may be precipitated

Advantages:

Solvent will reduce the viscosity of the reactant mixture

heat transfer will be better

Disadvantages:

the polymer will not be pure and has to be isolated

by chemical techniques

high molecular mass polymers will not be obtained


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Suspension Polymerization

the monomer is suspended in water as droplets, ofcolloidal size

Water is used as a solvent

Initiators used are soluble in monomer droplets

protective colloids are added to suppress the coagulation

of the monomer molecules

The reaction mixture is heated or exposed to radiation

with constant stirring.

Polymerisation takes place inside the dropletthe polymer formed being insoluble in water, produce

spherical pearls or beads


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Advantages:

Disadvantages:

Process is cheap since it uses water as a solvent

instead of costly solvents

Viscosity increase is negligible

Agitation and thermal control are easy

Product isolation is easy since the product is

insoluble in waterProduct formed is pure

the method can be used only for water insoluble monomers

it is difficult to control polymer size


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Emulsion Polymerization

This method is used for water insoluble monomers

Emulsion is the colloidal dispersion of a liquid in

another immiscible liquid

Emulsion of water and the monomer is allowed to form

To maintain the system stable, a small amount of anemulsifier will be added

Soaps and detergents are examples for emulsifiers

Emulsifier contains

ahydrophilic (water loving) polar end group (head) anda hydrophobic (water hating) non polar end group (tail)

At very low concentration, the soap or detergent

(emulsifier) dissolves completely in water


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at slightly higher concentrations (above CMC), the emulsifier

molecules form aggregates, called micelles

The monomer molecules dissolve in the hydrocarbon centre

of the micelles

water soluble initiator will be added and the system is kept

agitated at the required temperature.

The initiator molecules diffuse into the centre of micelles

through its polar head

Reaction takes place at the centre of the micelles

The polymer is formed and the micelles begins to swell

The monomer consumed inside the micelles is replenishedby diffusion from aqueous phase

This continues till the size of the polymer is big enough

to come out of the micelles


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The glassy state and the glass transition

V

TTm

A

B

C

F

melting

In general for ord inary com pounds o f low molar mass:

crystalline solid

liquid

increase in volume at Tm

slopes of FC and BA:

expansion coefficients of crystalline phase and liquid,

respectively


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Non-crystallisable materials

Some materials CANNOT

crystallize, e.g.ordinary glass

Why?

Molecular structure is too

irregular

liquid

Cooling of liquid via AB continues until D

The area BD haselastomericproperties and is the rubberystate

D is called the glass-rubber transition,

Tg= glass transition temperature

DE has the same slope as CF

V

TTg Tm

A

B

CF

D

E

amorphous or glassy phase

rubber


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GLASS TRANSITION TEMPERATURE (Tg)

Amorphous polymers do not have sharp melting points

They possess softening point

At low temperature, polymers exist as glassy substances

Since the molecular chains can not move at all easily inthis state, the solid tends to shatter, if it is hit

If the solid polymer is heated, eventually it softens and

becomes flexible

This softness and flexibility is obtained at the

glass transition temperature


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So the glass trans it ion temperaturecan be defined as

the temperature below which an amorphous polymer is

brittle, hard and glassy and above the temperature

it becomes flexible, soft and rubbery

Glassy state rubber state

(Hard and brittle plastic) (soft and flexible)

In the glassy state of the polymer, there is

neither molecular motion nor segmental motion

When all chain motions are not possible, the rigid solid results

On heating beyond Tgsegmental motion becomes possible

but molecular mobility is disallowed. Hence flexible


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HIGH-DENSITY POLYMERS

LOW-DENSITY POLYMERS

Linear polymers with chains that can pack closelytogether. These polymers are often quite rigid.

Branched-chain polymers that cannot pack together as

closely. There is often a degree of cross-linking.

These polymers are often more flexible than high-density polymers.

crystalline polymers have higher Tgs than amorphous polymers


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PolypropyleneTg = - 18 0C

R

Polystyrene

Tg = 100 0C

the bulky groups on chain, increases the Tg of the polymer

PolyethyleneTg = - 110 0C


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The presence of a plasticizerreduces the Tg of a polymer

The plasticizersare usually dialkyl phthalate esters,such as dibutyl phthalate, a high boiling liquid.

C

C

O

O

O

O

CH2CH2CH2CH3

CH2CH2CH2CH3

dibutyl phthalate

The plasticizer separates the individual polymer chains

from one another. It acts as a lubricant which reduces

the attractions between the polymer chains.


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The Tg of a polymer is influenced by its molecular weight

However, it is not significantly affected if molecular weight isaround 20000

With increase in molecular mass, the Tg increases

e.g., PE (low Mw)

PE (high Mw)

- 110 0C

- 900

C


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The glass transition temperature helps in choosing the right

processing temperature

It also gives the idea of

thermal expansion

heat capacity

electrical and mechanical properties

T


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Callister,Fig. 16.9

T

Molecular weight

Tg

Tm

mobile

liquid

viscousliquid

rubber

toughplastic

partially

crystallinesolid

crystallinesolid

Tm:melting over wide range of T depends upon history of sample,a consequence of lamellar structure

thicker the lamellae, higher the Tm.

Tg:from rubbery to rigid as T lowers

STRUCTURE PROPERTY RELATIONSHIP OF POLYMERS


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STRUCTURE PROPERTY RELATIONSHIP OF POLYMERS

Macromolecules show a wide range of properties which are

quite different from those of respective monomers

They may be

elastic or rigid

hard or soft

transparent or opaque

have strength of steel but can have very light weight

soften on heating or

can set to a hard mass on cooling the melt

These properties may vary from one type of polymer to

another and even among the same type


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The fundamental parameters which influence the

structure - property relationship are

molecular masspolarity

crystallinity

molecular cohesion

stereochemistry of the molecules

the nature of polymeric chains and

The properties like tensile strength, crystallinity, elasticity,

resistance to chemicals, wear and tear depend mostly on the

polymer structure

Tensile strength:

Th i t f t i l t f t di t t it t


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This can be discussed based on

1.the forces of attraction and

2.slipping power

Based on forces of att ract ion:

Strength of the polymer is mainly determined by

the magnitude and distribution of attraction forces

between the polymer chains

These attractive forces are of two different types

primary or covalent bond

secondary or intermolecular forces

The resistance of a material to a force tending to tear it apart,

measured as the maximum tension the material can withstand

without tearing.


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Linear Polymers: Polyethylene, polyvinyl chloride (PVC),

polystyrene, polymethyl methacrylate (plexiglass), nylon,fluorocarbons (Teflon)

Branched Polymers: Many elastomers or polymeric rubbers

Cross-linked Polymers: Many elastomers or polymeric rubbers arecross-linked (vulcanization process); most thermosetting polymers

Network Polymers: Epoxies, phenol-formaldehydes.

Examples:


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Based on sl ipp ing p ower:

It is defined as the movement of molecules one over the other

It depends on the shape of the molecule

E.g., polyethylene molecule is simple and uniform

the movement of molecules one over other is easyi.e., slipping power is high

Hence it has less strength.


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in case of poly vinyl chloride (PVC), the bulky chlorine atoms are

present along the chain length hence movement is restricted

i.e., slipping power is less

Hence PVC has higher tensile strength than PE

Cl Cl Cl Cl

Cl Cl Cl Cl

Cl Cl Cl Cl

Cl Cl Cl Cl

P l t (PS) t t th h d t


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Polystyrene (PS) possess greater strength when compared to

PE and PVC because of the presence of bulky phenyl group.

R

In cross linked polymers, all structural units are connected by

strong covalent forces and so the movement of the intermolecular

chains is totally restricted.Hence they are

most strong and tough


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Polymers exhibit mechanical strength, heat resistance and

tougher only when chain length is greater than 150 - 200 atoms in

a line.

Polymers of low mol. wt. are soft and gummy. Thus by controlling

the chain length or mol. wt., it is possible to vary the physical

properties of the polymer from soft and flexible to hard.


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Plastic deformation

When a polymer is subjected to some stress in the form

of heat or pressure or both, permanent deformation inshape takes place, which is known as plastic deformation

Slippage is more in case of linear molecules than branched

and cross-linked, because of the presence of only the weak

intermolecular forces

Hence linear molecules show greatest degree ofplastic deformation, under pressure

at high pressure and temperature, the weak Vander waals

forces between molecules become more and more weak

Such type of materials are called thermoplastic materials

U d l d f i ll t hi h


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No slippage occurs in case of cross-linked molecules,

because of only primary covalent bonds are present

throughout the entire structure

i.e., plasticity does not increase with rise in temperatureor pressure or both in cross-linked molecules.

Such type of polymers are known as thermosetting polymers

However, when considerable external force or temperature

exceeding the stability of material is applied, it will result in

total destruction

Under pressure polymers deform especially at high

temperature because the Vander Waals forces acting

between different molecules become more and more

Weak and hence are easily overcome.

Crystallinity Crystallinity:refers to the degree of structural order in a solid. In a crystal, theatoms or molecules are arranged in a regular, periodic manner. The degree


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Polymers are part crystallineand part amorphous

An amorphous state is characterized by completerandom arrangement of molecules

crystalline form by regular arrangement of molecules

Crystalline

region

Amorphousregion

of crystallinity has a big influence on hardness, density, transparency and

diffusion. ...

A linear polymer will have a high degree of crystallinity


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A linear polymer will have a high degree of crystallinity

The more lumpy and branched the polymer, the less dense

and less crystalline.

More the crosslinking, more stifferthe polymer.

And, networkedpolymers are like heavily crosslinked ones.

Isotactic and syndiotactic polymers are stronger and

stiffer due to their regular packing arrangement.

Polymers with a long repeating unit or with low degree ofsymmetry do not crystallize easily

Optical properties: crystalline -> scatter light (Bragg)

amorphous -> transparent.

Most covalent molecules absorb light outside visible spectrum, e.g.

PMMA (Lucite) is a high clarity transparent materials.


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Which polymer more likely to crystallize? Can it be decided?

Linear and highly crosslink

cis-isoprene

Not possible to decide which might crystallize. Both not likely to do so.

Networked and highly crosslinked structures are near impossible to

reorient to favorable alignment.

H+

+ H20

Networked

Phenol-Formaldehyde(Bakelite)


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alternating

Poly(styrene - ethylene)

Copolymer

Alternating co-polymer more likely to crystallize than random ones, as they are

always more easily crystallized as the chains can align more easily.

random

poly(styrene - ethylene) copolymer


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Linear syndiotactic polyvinyl chloride Linear isotactic polystyrene

Linear and syndiotactic polyvinyl chloride is more likely to crystallize.

The phenyl side-group for PS is bulkier than the Cl side-group for PVC.

Generally, syndiotactic and isotactic isomers are equally likely to crystallize.

For linear polymers, crystallization is more easily accomplished as chain

alignment is not prevented.

Crystallization is not favored for polymers that are composed of

chemically complex mer structures, e.g.polyisoprene.

Ch i l R i t

the ability to withstand contact with specified chemicalswithout a significant change in properties.


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Chemical Resistance

depends upon the

chemical nature of monomers

&

their molecular arrangement

As a general rule of dissolution,

like materials attract&unlike materials repel

polymers containing polar groups likeOH, - COOHetc.,usually dissolve in polar solvents like water, ketone, alcohol etc.,

e.g.,

but these are chemically resistantto non-polar solvents


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Similarly non-polar groups such as methyl, phenyl dissolve

only in non-polar solvents like benzene, toluene, etc.,

polymers of more aliphatic characterare more soluble in

aliphatic solvents, hence chemical resistance is less

polymers with more aromatic groups dissolve more in

aromatic solvents, hence chemical resistance is less in

aromatic solvents and more in aliphatic solvents

Polymers containing ester groups (e.g., polyesters) undergo

Hydrolysis with strong alkalis at high temperature

Implies less chemical resistance in alkalies

Polyamides like nylon containingNHCOgroup can undergo

easily the hydrolysis by strong acid or alkali

Polymers containing residual unsaturation e g rubbers


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Polymers containing residual unsaturation e.g., rubbers

(natural and some synthetic) easily undergo degradative

oxidation in air in presence of light or ozone

Because of the dissolution of polymers in suitable solvents,there occurs softening, swelling and loss of strength of

polymer material

The tendency of swelling and solubility of polymers in a

particular solvent decreases with increase in molecularweight

Linear polymers have lower resistivity than cross linked

polymers

crystalline polymers exhibits higher chemical resistance than

less crystalline polymers because of denser packing

Elasticitythe tendency of a body to return to its original shape after it has

been stretched or compressed;


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When a polymer is stretched the snarls begin todisentangle and

straighten out

Elasticity of the polymer is mainly because of the uncoiling and

recoiling of the molecular chains on the application of force

Elasticity

i.e., the orientation of the chains occurs which in turn

enhances the forces of attraction between the chains andthereby causing the stiffness of the materials

a polymer to show elasticity the individual chains should not

break on prolonged stretching

Breaking takes place when the chains slip over the other

and get separated

been stretched or compressed;

So the factors which allows the slippage of the molecules


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a polymer to show elasticity, the structure should be

amorphous

By introducing a plasticizer the elasticity of polymercan enhance

to get an elastic property, any factor that introduces

crystallinity should be avoided

So the factors which allows the slippage of the molecules

should be avoided to exhibit an elasticity

The slippage can be avoided by

introducing bulky side groups such as aromatic and

cyclic groups on repeating units

introducing non-polar groups on the chains

introducing cross-linking at suitable molecular positions

Molecular Weight of Polymers


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Molecular Weight of Polymers

A simple compound has a fixed molecular weight

e.g., acetone has mol. wt. of 58.

in any given sample of acetone, each molecule has the

same molecular weight

This is true for all low molecular weight compoundse.g., ethylene gas, which is a low mol. wt. compound

each of its molecules have the same chemical structure and

hence, a fixed molecular weight of 28

In contrast, a polymer comprises molecules of different

molecular weights


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In this situation, the molecular weight of the polymer can

only be viewed statistically and expressed as some averageof the Mol. Wt.s contributed by the individual molecules that

make the sample


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the molecular weight of a polymer can be expressed by

two most and experimentally verifiable methods of averaging

(i) Numberaverage

(ii) Weightaverage

In computing the number average molecular mass of a polymer, we

consider the number fractions

In computing the weight average molecular mass of a polymer, we

consider the weight fractions

So, a polymer sample can be thought of a mixture of

molecules of the same chemical type, but of different

molecular weights

A th t th b f l l i

(i) Numberaverage


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Assume that there are nnumber of molecules in a

polymer sample

n1of them have M1molecular weight (each)

n2of them have M2molecular weight

niof them have Mi molecular weight

Total no. of molecules (n) is given by

n =n1+n2+n3+n4+n5+n6++ni

No. of molecules in fraction 1 = n1

i

1

nn1fractionoffractionNumber

11Mn1f tibt ib tii htM l l


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in1fractionbyoncontributiweightMolecular

Similarly,

Molecular weight contribution by other fractions are

;n

Mn;

n

Mn;

n

Mn

i

33

i

22

i

11

iii

n

Mn

Number average molecular mass of the whole polymer

is given by

i

ii

i

44

i

33

i

22

i

11n

n

Mn............................

n

Mn

n

Mn

n

Mn

n

MnM

i

ii

n

n

MnM

In computing the weight average molecular mass of aWeightaverage


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In computing the weight average molecular mass of a

polymer, we consider the weight fractions

Total weight of the polymer (W) is given by

W = S ni Mi

Weight of fraction 1 = W1= n1M1

ii

1111

Mn

Mn

W

Mn1fractionoffractionweight

1

ii

11

MMn

Mn1fractionbyoncontributiweightMolecular

ii

211

Mn

Mn

Molec lar eight contrib tion b other fractions are


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Molecular weight contribution by other fractions are

;

Mn

Mn;

Mn

Mn;

Mn

Mn

ii

233

ii

222

ii

211

ii

2ii

Mn

Mn

Weight average molecular mass of the whole polymer

is given by

ii

2ii

ii

244

ii

233

ii

222

ii

211

w

Mn

Mn.................

Mn

Mn

Mn

Mn

Mn

Mn

Mn

MnM

ii

2ii

w

Mn

Mn

M


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Polymers: Molecular Weight

number average, Mn

weight average, Mw

Ni: no. of molecules with degree of polymerization of i

Mi: molecular weight of i


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Ratio of Mwto Mnis known as the polydispersity index (PI)

a measure of the breadth of the molecular weight

PI = 1 indicates Mw= Mn, i.e. all molecules have equal length(monodisperse)

PI = 1 is possible for natural proteins whereas syntheticpolymers have 1.5 < PI < 5

At best PI = 1.1 can be attained with special techniques


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Polymers: Molecular Weight

Biomedical applications: 25,000 < Mn < 100,000and 50,000 < Mw < 300,000

Increasing molecular weight increases physical

properties; however, decreases processibility

Problem: A Polymer sample contains 1 2 3 and 4 molecules


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Problem: A Polymer sample contains 1,2,3 and 4 molecules

having molecular weights 105, 2 x 105, 3 x 105and 4 x 105,

respectively. Calculate the number average and weight

average molecular weight of the polymer.

Solution

Mn=

=

i

ii

n

Mn

SS

4321

)104(4)103(3)102(2)101( 5555

xxxxxxx

10

10)16941( 5

10

1030 5x =3.0x105=

Mw =ii

MnS2


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Mw =

iiMnS

=

)103(4)103(3)102(2101)104(4)103(3)102(2)101(

5555

25252525

xxxx

xxxx

= 5

10

)16941(

10642781

=5

10

1030

10100

x

x

= 3.3 x 105

A polymer of polypropylene is found to have the following


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p y p yp py g

composition calculate the number average and weight

average molecular masses of the polymer. (atomic mass of

C=12, H=1, neglect the molecular mass of R)

[ R CH2

CH

CH3

]400R 20%(a)

R]

3CH

CH2

CH[ R500

(b) 30%

[ R CH2

CH

CH3

] R600

(c) 50%

Molecular mass of (a) = [(12x3)+(6x1)]x400=16800


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Molecular mass of (a) = [(12x3)+(6x1)]x400=16800Molecular mass of (b) =[(12x3)+(6x1)]x500=21000

Molecular mass of (c) =[(12x3)+(6x1)]x600=25200

Mn=

n1= 20, n2= 30, n3= 50,n1M1= 20 x 16800; n2M2= 30 x 21000; n3M3= 50 x 25200;

= 22200

Mw= = 22715

321

332211

nnn

MnMnMn

iiMnMnMn

MnMnMn

2211

2

33

2

22

2

11

If polymer sample has population as:


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If polymer sample has population as:

10 molecules of molecular mass each = 5,000

20 molecules of molecular mass each = 7,50020 molecules of molecular mass each = 10,000

25 molecules of molecular mass each = 15,000

20 molecules of molecular mass each = 20,0005 molecules of molecular mass each = 25,000

Calculate its number-average and weight-average

molecule mass of polymer.


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Solution. Number-average molecular-mass of

polymers

Mn=

=

=

= 13000

52025202010

)250005200002015000251000020750020500010[

xxxxxx

100

12500040000037500020000015000050000

100

103.1 6x

M


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Mw=

= 15480

1300000

])25000(5)20000(20)15000(25)10000(20)7500(20)5000(10[ 222222 xxxxxx


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Problems(1)A protein sample consists of an equimolar mixture of

Haemoglobin (M=15.5 Kg mol-1), Ribonuclease (M=13.7 Kg

mol-1) & Myoglobin (M=17.2 Kg mol-1). Calculate Mn& Mw

(2) A polypropylene [-CH2CH(CH3)-] sample contains the

following composition.Degree of polymerization 400 800 600

% of composition 25 35 40

Calculate Mn& Mwof polypropylene sample by neglecting

the end groups. Given that atomic masses of C = 12 and H =

1 amu.

TEFLON or FLUON or

P l t t fl th l (PTFE)


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Polytetrafluoroethylene (PTFE):

Preparation

F

C C

FF

Fn

Water emulsion

polymerization

peroxide

F

C C

FF

F

n

Properties

a highly regular and linear polymer without branching

a highly crystalline polymer with a melting point of

around 330 oC

Its mechanical strength remains unchanged over a wide

temperature range from -100 oC to 350 oC

It does not dissolve in any of the strong acids including

h t f i it i id


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hot fuming nitric acid

It is resistant to corrosive alkalies and known organic solvents

It reacts with only molten alkali metals (to any significant

extent) probably, this is because fluorine atoms from the

polymer chain get removed by the alkali metals

It has very low dielectric constant

The conventional techniques used for the processing of

other polymers can not be applied to Teflon because

of its low melt flow rates

The strong attractive forces between the polymer chains

gives the extreme toughness, high softening point,exceptionally high chemical resistance

It has high density 2.1 to 2.3 gm/cm3

It has low coefficient of friction (low interfacial forces


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It has low coefficient of friction (low interfacial forces

between its surface and another material)

It has very low surface energy

Uses

It is used in making articles such as pump valves and

pipes where chemical resistance is requiredIt is used in non-lubricated bearings

It is used in non-sticking stop-cocks like burettes etc.,

It is used for coating and impregnating, glass fibers,asbestos fibers (to form belts), filter cloth etc.,

NYLON 6 6


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NYLON 6, 6

The aliphatic polyamides are generally known as nylons

The nylons are usually indicated by a numbering system

The nylons obtained from dibasic acids and diamines

are usually represented by two numbers

the first one indicating the number of C atoms in thediamine and the second that in the dicarboxylic acid

Nylons made by the self condensation of an amino acid

or by the ring opening polymerization of lactams are

represented only by a single number as in the case ofnylon 6

Polyamides are prepared by the melt poly condensation

Preparation


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Heat- 2nH2O

+n n

Properties


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It has a good tensile strength, abrasion resistance and

toughness upto 150 oC

It offers resistance to many solvents. However, itdissolves in formic acid, cresols and phenols

They are translucent, wheatish, horny, high melting

polymers (160264 oC)

They possess high thermal stability

Self lubricating properties

They possess high degree of crystallinity

The interchain hydrogen bonds provide superiormechanical strength

(Kevlar fibers stronger than metals)

Its Hardness is similar to tin

Uses


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Nylon 6,6 used as sutures

It is used as a plastic as well as fiber

This is used to produce tyre cord

It is used to make mono filaments and ropes

Nylon 6,6 is used to manufacture articles like brushes

and bristles

P F Resins


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These are formed by condensation polymerization and

are thermosetting polymers

The phenol ring has three potential reactive sites

while the formaldehyde has two reactive sites

The polycondensation reaction between these two

are catalyzed by either acids or alkalies

The nature of the product formed depends largely

on the molar ratio of phenol to formaldehyde and also

on the nature of the catalyst

There are two important commercial PF resins

Novolacs

Resoles

Both novolacs and resoles are linear low molecular


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Both novolacsand resolesare linear, low molecular

weight, soluble and fusible prepolymers

During moulding operations, these two undergoextensive branching leading to the formation of highly

cross linked, insoluble, hard, rigid and infusible products

Novolacs

When P/F molar ratio is > 1 and the catalyst used is an acid,

low mol. wt. polymers formed are called Novolacs

The first step in the reaction is the addition of

formaldehyde to phenol to form orthoorparamethylolphenols

OH


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CH2OH

OH

CH2OH

OH

and

o-methylol phenol

p-methylol phenol

Phenol (excess) formaldehyde

C = O

H

H

+

H+

These methylol phenols condense rapidly to form Novolacs


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y p p y

CH2OH

OH

CH2OH

OH

or

o-methylol phenolp-methylol phenol

OH

OHH2

C

H2

C

OH H2

C

OH

OH

H2

C

HO

Novolacs

These novolacs are linear and low mol. wt. polymers


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About 56 phenol rings per molecule are linked through

methylene bridges

They are soluble and fusible

Since they contain no active methylol groups, they

themselves do not undergo cross linking

However, when heated with formaldehyde or hexamine, they

undergo extensive cross linking, resulting in the formation

of infusible, insoluble, hard and rigid thermosetting product

OHH2H2OH H2OHH2


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C2

COH 2

C

OH

2

C

HO

Novolacs(prepolymer) Curing withFormaldehyde or

hexamine


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The resoles in which phenols are linked through


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methylene bridges are soluble and fusible

Since they contain alcoholic groups, further reactionduring curing leads to cross linking, resulting in a

network, infusible and insoluble product

Properties


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These are (bakelite) set to rigid and hard

They are scratch-resistant

They are infusible

They are water-resistant

They are insoluble solids

They are resistant to non-oxidizing acids, salts and

many organic solvents

but are attacked by alkalis, because of the presence

of free hydroxyl group in their structures

They possess excellent electrical insulating character

Their Hardness is similar to copper

These are usable up to 400 F (204C)


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The properties can be modified by fillers& reinforcements

These have the highest compressive strength

These are machinable

Phenolics are the resin in plywood

These tends to be brittle

Uses


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For making electric insulator parts like switches, plugs,

switch-boards, heater-handles etc.,

For making moulded articles like telephone parts,

cabinets for radio and television

As adhesives (e.g., binder) for grinding wheels

In paints and varnishes

As hydrogen-exchanger resins in water softening

For making bearings, used in propeller shafts for

paper industry and rolling mills

Epoxy resins


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Preparation

Cl CH2 CH2CH

On

CH3

C OHHOCH3

+

n

CH2 CH2CH

OH

C OO

CH3

CH3

epichlorhydrin

bis phenolAlkaline catalyst60 OC-n HCl

In epoxy resins, nranges from 1 to 20


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The molecular weight of the epoxy resin depends upon

the relative proportions of the reactantsThe epichlorhydrin acting as a chain stopper

Molecular weight ranges from 350 to 8000

It is a mobile and easy flowing liquid at a mol. Wt. of 350

It is a solid at higher mol. wt. with a melting range

of 145 oC - 155 oC

Linear epoxy resins are converted into 3D polymersby curing with some chemicals like diethylene triamine,

triethylene tetramine and meta-phenylene diamine

Properties


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Epoxy resins have ability of getting cured, without

application of heatThey have good resistance to chemicals

They have less shrinkage during curing process

Their properties can be modified by adding compounds

like unsaturated fatty acids or amines and

some of the solvents

They possess excellent electrical resistance

Epoxy resins stick well to a number of substances

including metal and glass

Uses


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Uses

epoxy resins are mainly used as adhesives

They are used for surface coatings

Moulds are made with epoxy resins, which are used for

the production of metallic components of aircrafts

and automobiles

They are used as laminating and casting materials

ELASTOMERS

Elastomer is defined as a long chain polymer which


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Elastomer is defined as a long chain polymer which

under stress undergoes elongation by several times and

regains its original shape when the stress is fully released

Stretched

Returned to

randomization

Elastomers are high polymers, which have elastic

properties in excess of 300 %


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The elastic deformation in an elastomer arises due to

the fact that the molecule is not a straight chainedin the unstressed condition and is in the form of a coil

Hence, it can be stretched like a spring

So, the unstretched rubber is in an amorphous state

As stretching is done, the macromolecules get partially

aligned with respect to another, thereby causing

crystallization

Consequently, stiffening of material (due to increased

attractive forces between these molecules) taking place

On releasing the deforming stress, the chains get reverted

back to their original coiled state and the material again

becomes amorphous

Natural rubber is an addition polymer formed from the


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monomer called isoprene i.e., 2-methyl-1,3-butadiene

The average D.P. (n) of rubber is around 5000

Addition between molecules of isoprene takes place by

1,4 addition and one double bond shifts between 2nd and

3rd positions

As each isoprene unit contains C = Cbond,


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p

polyisoprene exists in two isomeric forms

viz., cisand trans

Cis-polyisoprene trans-polyisoprene

where R= CH3

Natural rubber contains the cisisomer while the

gutta percha contains the transisomer

Natural rubber consists of basic material latex, which is

a dispersion of isoprene


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p p

During the treatment, these isoprene molecules polymerize

to form long-coiled chains of cis-polyisopreneThe mol. wt. of raw rubber is about 100,000150,000

Natural rubber is made from the saps of a wide range of

plants like havea brasilliansand guayule, found in

tropical countries (such as Indonesia, Malaysia, Thailand,Ceylon, India, South America, etc.,)

The rubber latex (or milky liquid rubber ) is obtained by

making incisions in the bark of the rubber trees and

allowing the saps to flow out into small vessels

Tapping is, usually done at intervals of about six months

The latex is emptied into buckets and transferred to a

factory for treatment

Gutta Percha is trans-polyisoprene and is obtained from


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Gutta Percha is trans-polyisoprene and is obtained from

the mature leaves of dichopsis guttaandpalagum gutta

trees (belonging to sapetaceaefamily)

These trees are grown mostly in Broneo, Malaya and Sumatra

Gutta percha may be recovered by solvent extraction

Alternatively, the mature leaves are ground carefully;

treat with water at about 70 oC for half an hour and

poured into cold water, then the gutta percha floats on

water surface and can be easily removed

Deficiencies of natural rubber


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Natural rubber is addition product of isoprene units

and still contains a large number of double bonded

carbon atoms

Hence it exhibits a large number of deficiencies

At low temp. it is hard and brittle but as the temp.

rises it becomes soft and stickyIt gets oxidized easily in air and produces bad smell even

if kept as such for a few days

It is soluble in many organic solvents

It absorbs large quantities of waterIts chemical resistivity is low and is attacked by acids,

alkalies, oxidizing and reducing agents

Its tensile strength, abrasion resistance wear and tear

resistances are low


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resistances are low

It possesses marked tackiness

i.e., when two fresh raw rubber surfaces are pressedtogether, they coalesce to form a single piece

It has little durability

When stretched to a great extent, it suffers permanentdeformation, because of the sliding or slippage of

some molecular chains over each other

Synthetic rubbers have slightly modified structures from

natural rubber they exhibit properties that are moreconducive for their technical uses

A comparative account of the properties ofnatural and synthetic rubbers

P t N t l bb S th ti bb


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Property Natural rubber Synthetic rubberTensile

strength

Low (only 200 kg/cm2) High

Chemical

resistivity

Lowgets oxidized

even in air

Highnot oxidized in

air

Action of heat Cold condition it is hard

and brittle, at highertemp.s soft and

sticky

Withstand effect of

heat over a range oftemperature.

With organic

solvents

Swells and dissolves Do not swell and

dissolve

Ageing Undergoes quickly Resists ageing

Elasticity On increased stress

undergoes permanent

deformation.

Has high elasticity.

Vulcanization of rubber

This process was discovered accidentally by Goodyear


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This process was discovered accidentally by Goodyear

when he dropped rubber and sulfur on a hot stove

To improve the properties of rubber, it is compoundedwith some chemicals like sulphur, hydrogen sulphide,

benzoyl chloride etc., It is known as vulcanisation of rubber

The process consists of heating the raw rubber with

sulphur at 100140 oC

The added sulphur combines chemically at the double

bonds of different rubber springs

Thus this process serves to stiffen the material by a sort of

anchoring and consequently, preventing the intermolecularmovement of rubber springs

The extent of stiffness of vulcanized rubber depends on

the amount of sulphur added

e.g., a tyre rubber may contain 3 to 5% sulphur,

but a battery case rubber may contain as much as 30% sulphur


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H

HH H

H

H

H

H

H

HH H

H

H

H

H

H

HH H

H

H

H

H

C C

C

CC

C C

C

C C C

C C

C

C

C C

C

C

H

H

H

C

H

H

H

H

H

C C

C

C

H

H

H

C

H

H

H

H

H

C C

C

C

H

H

H

C

H

H

H

H

H

+ S +

HHH


131/153

H

H

HH

H

H

H

H

H

H

HH

H

H

H

H

H

H

HH

H

H

H

H

C C

C

C C

C C

C

C C C

C C

C

C

C C

C

C

H

H

H

C

H

H

H

H

H

C C

C

C

H

H

H

C

H

H

H

H

H

C C

C

C

H

H

H

C

H

H

H

H

H

S S

Advantages of vulcanization


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Vulcanized rubber

has good tensile strength and extensibility, when a

tensile force is applied, can bear a load of 2000 kg/cm2

before it breaks

has excellent resilience

i.e., article made from it returns to the original shape,

when the deforming load is removed

possesses low water-absorption tendency

has higher resistance to oxidation and to abrasion

has much higher resistance to wear and tear ascompared to raw rubber

is a better electrical insulator, although it tends to

absorb small amounts of water

is resistant to organic solvents (such as petrol,

benzene and carbon tetrachloride) fats and


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benzene, and carbon tetrachloride), fats and

oils. However, it swells in these liquids

is very easy to manipulate the vulcanized rubber to

produce the desired shape articles

has useful temperature range of - 40 to 100 oC

has low elasticity and is depending on the extent of

vulcanization

e.g., vulcanite (32% Sulphur) has practically no elasticity


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Reinfo rcing f i l lers

Th dd d t i t th d i idit t th


136/153

These are added to give strength and rigidity to the

rubber products

Common reinforcing fillers are carbon black, zinc oxide,

calcium carbonate and magnesium carbonate

Colour ing matter

These are added to give the desired colour to therubber product

for white colour titanium dioxide

Green chromium oxide

red ferric oxide

Crimson antimony sulphide

yellow lead chromate

---- pigments are added

Styrene rubber (GR-S or Buna-S or SBR)


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Preparation

This is produced by copolymerization of butadiene(about 75% by wt.) and styrene (about 25% by wt.)

H2C CH CH CH2

x

H2C CH

n

H2C CH CH CH2n x

H2C CH

n+

Styrene-butadiene copolymer


138/153

Styrene domains act as

anchors or junctions

Butadienes provideflexible linkages

The desire to maximize the ways you can arrange the flexible

links is what causes rubbers to return to given shapes

Properties


139/153

It possess high abrasion-resistance

It possess high load-bearing capacity and resilience

It gets readily oxidized, especially in presence of

traces of ozone present in the atmosphere

It swells in oils and solvents

It can be vulcanized in the same way as natural

rubber either by sulphur or sulphur monochloride

However, it requires less sulphur, but more

accelerators for vulcanizationStyrene rubber resembles natural rubber in

processing characteristics as well as the quality

of the finished products

Uses


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It is used for the manufacture of

floor tiles

motor tyres

shoe soles

gaskets

wire and cable insulations

carpet backing

adhesives

tank-lining etc.,

Silicone rubber

Sili i t i lt t ili


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Silicone resins contain alternate siliconeoxygen

structure, which has organic radicals attached to

silicone atoms

SiO

C

C

H

HH

H

H

H

SiO

C

C

H

HH

H

H

H

O

Dimethyl silicone dichloride is bifunctional and

can yield very long chain polymer


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y y g p y

CH3

CH3

OSi

n

CH3

CH3

Cl ClSin

CH3

CH3

HO OHSin

unstable

Hydrolysis

- 2 HCl

H2O

polymerization

CH3

CH3

OSi( )

unstable

Vulcanized silicone rubbers are obtained by mixing

high molecular weight linear dimethyl silicone polymers


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high molecular weight linear dimethyl silicone polymers

with filler

The fillers are either a finely divided silicon dioxide

or a peroxide

It may also contain the curing agents

Peroxide causes the formation of dimethyl bridge

(cross link) between methyl groups of adjacent chains

CH3 CH3 CH3 CH3CH3


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O +

OSi

CH2H CH3

OSi

CH3

OSi

CH3

OSi

CH3

OSi

CH3

OSi

CH2H CH3

CH3

OSi

CH3

CH3

OSi

CH3

CH3

OSi

CH3

CH3

OSi

H2O


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CH3

OSi

CH2

CH3

CH3

OSi

CH3

CH3

OSi

CH3

CH3

OSi

CH3

CH3

OSi

CH3

OSi

CH2 CH3

CH3

OSi

CH3

CH3

OSi

CH3

CH3

OSi

CH3

CH3

OSi


146/153

Uses

as a sealing material in search-lights and in aircraft engines


147/153

g g g

for manufacture of tyres for fighter aircrafts

for insulating the electrical wiring in ships

In making lubricants, paints and protective coatings for

fabric finishing and water proofing

as adhesive in electronics industry

For making insulation for washing machines and electric

blankets for iron board covers

For making artificial heart valves, transfusion tubing and

padding for plastic surgery

For making boots for use at very low temp., since they are

less affected by temperature variation

e.g., Neil Armstrongused silicone rubber boots when he

walked on the moon

Reclaimed rubber

Reclaimed rubber is rubber obtained from waste rubber


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Reclaimed rubber is rubber obtained from waste rubber

articles

like worn out tyres, tubes, gaskets, hoses, foot-wears etc.,

The waste is cut to small pieces and powdered by using a

cracker, which exerts powerful grinding and tearing action

The ferrous impurities, if any, are removed by the

electro-magnetic separator

The purified waste powdered rubber is then digested with

caustic soda solution at about 200 oC under pressure for

815 hours in steam-jacketed autoclave

By this process, the fibers are hydrolyzed

After the removal of fibers, reclaiming agents like petroleum

and coal-tar based oils and softeners are added


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Sulphur gets removed as sodium sulphide and rubberbecomes devulcanized

The rubber is then thoroughly washed with water sprays

and dried in hot air driers

Finally, the reclaimed rubber is mixed with small

proportion of reinforcing agents like clay, carbon black etc.,

and coal-tar based oils and softeners are added

Properties


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The reclaimed rubber has

has lower elasticity

less tensile strength

possesses lesser wear-resistance than

natural rubber

it is much cheaper, uniform in composition

and has better ageing properties

it is quite easy for fabrication

Uses

f f t i t


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for manufacturing tyres

tubes

automobile floor mats

belts

hoses

battery containers

mountings

shoes, heals etc.,


152/153

They are used as stabilizers for PVC resins

Epoxy resins are used for skit-resistant surfaces, for

Uses of epoxy resins


153/153

highways rendering a number of advantages

Excellent resistance to freezing conditions,

de-icing salts, solvents and water

Non-porosity which protects the original pavements

from scaling and spalling

Fast curing, causing minimum interruption to the flow of

traffic .

Light weight, especially useful for surfacing bridges Epoxy resins are applied over cotton rayon and

Delayed wearing of road surfaces in hot and cold climates

Documents

Polymers and Elastomers for Engineers