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DEFINITION:
The word polymer is derived from the two greek words
poly and mers
Polymers are macro molecules formed by linking smallermolecules repeatedly, called monomers.
parts or unitsmany
C C C C C CHHHHHH
HHHHHH
Polyethylene (PE)
mer
ClCl Cl
C C C C C CHHH
HHHHHH
Polyvinyl chloride (PVC)
mer
Polypropylene (PP)
CH3
C C C C C CHHH
HHHHHH
CH3 CH3
mer
e.g.
HIGH POLYMERS
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1
Examples:
Polyethylene is formed by linking a large number ofethylene molecules
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 CH
Hn Polymerisation
nC C
H
H
H
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2
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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3
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 polymers and these have very high molecular weights(104 to 106).
Polymers with low degree of polymerization are called oligomers.
e.g.,
D.P.
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4
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 modifiednatural polymers
polymers can be classified as
e.g., cellulose acetate, cellulose nitrate, halogenated rubbers etc.,
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5
Based on the molecular structure
polymers can be classified as
Linear Branched
Cross-linked
the monomeric units combine linearly with each other In 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 polymers are formed by self-addition of monomers The molecular mass of a polymer is an integral multiple of the 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 softening
Thermosetting
soften on heating and can be converted into any shapeand can retain its shape on cooling
thermosoftening or thermoplastics
under go chemical change on heating and convert themselves into an infusible mass
thermosetting polymers
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9
Based on the properties or applications
Plastics
Elastomers
Fibers
Resins
Plastics The 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 forcebut readily regain their original shape when the force is withdrawn
e.g., natural rubber, synthetic rubbers, silicone rubbers etc.,
Fibers In these polymers, the molecular chains are arranged parallel to each other in a spiral or helical pattern andthe 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 impartdifferent properties to plastics
e.g., polysulphide sealants, epoxy adhesives, etc.,
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Functionality
the number of reactive 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 Polymerisation
Polymerisation occurs basically in two different modes.• addition (chain growth) polymerization• condensation (step growth) polymerization
• Additiono monomers react through stages of initiation,
propagation, and terminationo initiators such as free radicals, cations, anions
opens the double bond of the monomer o monomer becomes active and bonds with other
such monomerso rapid chain reaction propagateso reaction is terminated by another free radical or
another polymerwww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
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- two monomers react to establish a covalent bond
- a small molecule, such as water, HCl, methanol or CO2 is released.
- the reaction continues until one type of reactant is used up
• condensation
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21
DISTINGUISHING FEATURES OF ADDITION AND CONDENSATION POLYMERISATION
ADDITION CONDENSATION
Monomers undergo self addition to each other without loss of by products
Monomers undergo intermolecular condensation with continuous eliminationof by products such as H2O, NH3, HCl, etc.,
It follows chain mechanism It follows step mechanism
Unsaturated vinyl compounds undergoaddition polymeristion
Monomers containing the functional groups (-OH, -COOH, -NH2, ….) undergothis polymerization
Monomers are linked together through C – C covalent linkages
Covalent linkages are throughtheir functional groups
High polymers are formed fast The reaction is slow and the polymermolecular weight increases steadily throughout 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., www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
22
• Random, Alternating, Blocked, and Grafted
CoPolymers
• Synthetic rubbers are often copolymers.
e.g., automobile tires (SBR)
Styrene-Butadiene Rubber random polymer
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Addition polymerization can be explained on the basis of free radical mechanism
It involves three stages
viz., (i) Initiation
(ii) Propagation
(iii) termination
oru.v.light
I(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 m th 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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At some stage this chain propagation is terminated when the free radicals combine either by coupling (combining) of the two radicals or by disproportionation
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
H X
H
RC
H
H
C
X
H
m-1
C
H
H
C
X
H
saturated highpolymer (dead polymer)
coupling
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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
saturated oligomerunsaturated oligomer(dead polymer) (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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Disadvantages:
as the reaction proceeds stirring become difficult as the product becomes more and more viscous
Uncontrolled temperature rise may lead to discoloration
thermal degradation
branching cross linking
and some times explosion also
Advantages:
The method is simple
It needs simple equipments
The percentage of conversion is high
Product obtained is pure with high optical clarity
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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 www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
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Suspension Polymerization
the monomer is suspended in water as droplets of colloidal 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 droplet
the 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 water
Product 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 Polymerisation
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 an emulsifier will be added
Soaps and detergents are examples for emulsifiers
Emulsifier contains a hydrophilic (water loving) polar end group (head) and a 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, the emulsifier molecules form aggregates, called miscelles
The monomer molecules dissolve in the hydrocarbon centre of the miscelles
water soluble initiator will be added and the system is kept agitated at the required temperature.
The initiator molecules diffuse into the centre of miscelles through its polar head
Reaction takes place at the centre of the miscelles
The polymer is formed and the miscelles begins to swell
The monomer consumed inside the miscelles is replenished by diffusion from aqueous phase
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Advantages:
Disadvantages:
Rate of polymerization is high
polymers with higher molar masses are formed
thermal control is easy
control over the polymer molar mass is possible
no viscosity build up and hence agitation is easy
the polymer formed may contain impurities such as the emulsifiers and coagulants
It needs further purification by other chemical techniques
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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 in this 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 transition temperature can 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 brittle plastic) (soft 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 Tg segmental motion becomes possible but molecular mobility is disallowed. Hence flexible
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The glassy state and the glass The glassy state and the glass
transitiontransition
• In general for ordinary compounds of low molar mass:
• crystalline solid
• liquid
• increase in volume at Tm;
• slopes of FC and BA: expansion coefficients of crystalline phase and liquid, respectively.
V
TTm
A
B
C
F
melting
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Non-crystallisable materials Non-crystallisable materials
V
TTg Tm
A
B
C
F
Some materials CANNOTcrystallize, e.g. ordinary glass
Why?Molecular structure is too irregular
liquid amorphous or glassy phase
rubber
•Cooling of liquid via AB continues until D •The area BD has elastomeric properties and is the rubbery state•D is called the glass-rubber transition, Tg = glass transition temperature•DE has the same slope as CF
D
E
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Crystalline vs. Crystalline vs. AmorphousAmorphous
Phase transitions for long-chain polymers.
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Factors influencing the glass transition temperature
Glass transition temperature of a polymer depends on parameters such as
• chain geometry• chain flexibility• molecular aggregates• hydrogen bond between polymer chains• presence of plasticizers and • presence of substrates in the polymer chains
A polymer having regular chain geometry show high glass transition temperature
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HIGH-DENSITY POLYMERSHIGH-DENSITY POLYMERS
LOW-DENSITY POLYMERSLOW-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 Tg s than amorphous polymers
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PolypropyleneTg =
R
PolystyreneTg = 100 0C
the bulky groups on chain, increases the Tg of the polymer
PolyethyleneTg = -110 0C
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• e.g., the Tg of nylon 6,6 (Tg = 50 0C) is higher than PE (Tg = -110 0C)
N|
H
H
|
C
|
H
6
N|
H
O
||
C
H
|
C
|
H
4
N|
H
O
||
C
N|
H
H
|
C
|
H
6
N|
H
O
||
C
H
|
C
|
H
4
N|
H
O
||
C
+++
+++
H
C
H
H
C
H
H
C
H
H
C
H
+++
+++
nylon 6,6 polyethylene
Hydrogen bondsVan der Waals bonds
With H-bonds vs vdW bonds, nylon is expected to have (and does) higher Tg.
The presence of H-bonds between the polymer molecules increases the Tg
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The presence of a plasticizer reduces the Tg of a polymer
The plasticizers are 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 chainsfrom one another. It acts as a lubricant which reducesthe 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 is around 20000
With increase in molecular mass, the Tg increases
PE (low Mw)
PE (high Mw)
-110 0C
- 90 0C
e.g.,
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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
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Callister, Fig. 16.9
T
Molecular weight
Tg
Tmmobile liquid
viscous liquid
rubber
tough plastic
partially crystalline solid
crystalline solid
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 lowerswww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
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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 mass
polarity
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
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Tensile Strength
This can be discussed based on
the forces of attraction and
slipping power
Based on forces of attraction:
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 www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
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In straight chain and branched chain polymers, the individual chains are held together by weak intermolecular force of attraction
strength increases with increase in chain length (in turn increase in molecular weight) as the longer chains are entangled (anchored) better
In cross-linked polymers, monomeric units are held together only by means of covalent forces
Increase in Strengthwww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
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Linear Polymers: Polyethylene, polyvinyl chloride (PVC), polystyrene, polymethyl methacrylate (plexiglass), nylon, fluorocarbons (teflon)
Branched Polymers: Many elastomers or polymeric rubbersCross-linked Polymers: Many elastomers or polymeric rubbers are cross-linked (vulcanization process); most thermosetting polymers
Network Polymers: Epoxies, phenol-formaldehydes.
Examples:
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Based on slipping power:
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 strength than PE
Cl Cl Cl Cl
Cl Cl Cl Cl
Cl Cl Cl Cl
Cl Cl Cl Cl
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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 in
shape 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 of plastic deformation, under pressure
at high pressure and temperature, the weak Vander waal’s forces between molecules becomemore and more weak
Such type of materials are called thermoplastic materials www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
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No slippage occurs in case of cross-linked molecules,because of only primary covalent bonds are presentthroughout the entire structure
i.e., plasticity does not increase with rise in temperature or pressure or both
Such type of polymers are known as thermosetting polymers
However, when considerable external forceor temperature exceeding the stability of material is applied,it will result in total destruction
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Crystallinity
Polymers are part crystalline and part amorphous
An amorphous state is characterized by completerandom arrangement of molecules
crystalline form by regular arrangement of molecules
crystalline region
amorphous region
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• A linear polymer will have a high degree of crystallinity, and be stronger, denser and more rigid.
• The more “lumpy” and branched the polymer, the less dense and less crystalline.
• The more crosslinking the stiffer the polymer. And, networked polymers are like heavily crosslinked ones.
• Isotactic and syndiotactic polymers are stronger andstiffer due to their regular packing arrangement.
• Polymers with a long repeating unit or with low degree of symmetry 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 tranparent 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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Polystyrene (PS) possess greater strength when compared toPE and PVC because of the presence of bulky phenyl group.
R
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Which polymer more likely to crystallize? Can it be decided?
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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Which polymer more likely to crystallize? Can it be decided?
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.
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Chemical Resistance
Chemical resistance of the polymer depends upon the • chemical nature of monomers and• their molecular arrangement
As a general rule of dissolution,
‘like materials attract’ and‘unlike materials repel’
Thus a polymer is more soluble in structurally similar solvent
polymers containing polar groups like – OH, - COOH etc., usually dissolve in polar solvents like water, ketone, alcohol etc.,
e.g.,
but these are chemically resistant to non-polar solvents www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
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Similarly non-polar groups such as methyl, phenol dissolve only in non-polar solvents like benzene, toluene, etc.,
polymers of more aliphatic character are more soluble inaliphatic solvents, hence chemical resistance is less in aliphatic solvents and more in aromatic solvents
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 containing –NHCO– group can undergo easily the hydrolysis by strong acid or alkali
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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 molecular weight
Linear polymers have lower resistivity than branched chain and cross linked polymers
Permeability of the solvents in the polymers also dependson crystallinity
crystalline polymers exhibits higher chemical resistance than less crystalline polymers because of denser packing
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69
Elasticity
Elasticity of the polymer is mainly because of the uncoiling andrecoiling of the molecular chains on the application of force
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
So the factors which allows the slippage of the moleculesshould be avoided to exhibit an elasticity
The slippage can be avoided by• introducing cross-linking at suitable molecular positions • introducing bulky side groups such as aromatic and
cyclic groups on repeating units • introducing non-polar groups on the chains www.bookspar.com | Website
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a polymer to show elasticity, the structure should be amorphous
By introducing a plasticizer the elasticity of polymer can enhance
to get an elastic property, any factor that introduces crystallinity should be avoided
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71
Molecular Weight of Molecular Weight of
PolymersPolymers
A simple compound has a fixed molecular weight
e.g., acetone has mol. Wt. of 58 (regardless of how it is made)
in any given sample of acetone, each molecule has the same molecular weight
This is true for all low molecular weight compounds
In contrast, a polymer comprises molecules of different molecular weights
hence, its molecular weight is expressed in terms of an ‘average’ value
e.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
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72
upon polymerization, it forms polyethylene and we encounter an indefinite chemical structure of --(-CH2 – CH2 -)n—
where ‘n’ can change its value from one polyethylene molecule to another present in the same polymer sample
When ethylene is polymerized to form polyethylene,
a number of polymer chains start growing at any instant, but all of them do not get terminated after growing to the same size
The chain termination is a random process
hence, each polymer molecule formed can have a different number of monomer units and thus different molecular weights
So, a sample polymer can be thought of as a mixture of molecules of the same chemical type, but of different molecular weights www.bookspar.com | Website
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In this situation, the molecular weight of the polymer can only be viewed statistically and expressed as some average of the Mol. Wt.s contributed by the individual molecules that make the sample
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74
the molecular weight of a polymer can be expressed by two most and experimentally verifiable methods of averaging
(i) Number – average
(ii) weight – average
Number average molecular mass of a polymer can be definedas the total mass of all the molecules in a polymer sample divided by the total number of molecules present
The molecular mass of a polymer can use either number fractions or the weight fractions of themolecules present in the polymer
In computing the number average molecular mass of a polymer, we consider the number fractions
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75
Assume that there are n number of molecules in a polymer sample
n1 of them have M1 molecular weight (each) n2 of them have M2 molecular weight
ni of 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
nn 1fraction offraction Number
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76
i
11
n
Mn 1fraction by on contributiweight Molecular
i
ii
i
44
i
33
i
22
i
11n
n
Mn............................
n
Mn
n
Mn
n
Mn
n
Mn M
Similarly,
Molecular weight contribution by other fractions are
;n
Mn;
n
Mn;
n
Mn
i
33
i
22
i
11
i
ii
n
Mn
Number average molecular mass of the whole polymer is given by
i
iin
n
M n M
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77
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 = ni Mi
Weight of fraction 1 = W1= n1M1
ii
1111
Mn
Mn
W
Mn 1fraction offraction weight
1ii
11M
Mn
Mn 1fraction by on contributiweight Molecular
ii
211
Mn
Mnwww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
78
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
wMn
Mn.................
Mn
Mn
Mn
Mn
Mn
Mn
Mn
Mn M
ii
2ii
wMn
M n M
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79
Polymers: Molecular Polymers: Molecular WeightWeight
• 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 Mw to Mn is known as the polydispersity index (PI)
o a measure of the breadth of the molecular weight
o PI = 1 indicates Mw = Mn, i.e. all molecules have equal length (monodisperse)
o PI = 1 is possible for natural proteins whereas synthetic polymers have 1.5 < PI < 5
o At best PI = 1.1 can be attained with special techniques
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81
The number-average molecular mass (Mn)is determined by the measurement of colligative properties such as
depression in freezing point
elevation in boiling point
osmotic pressure
lowering of vapour pressure
The weight-average molecular mass (Mw) is determined by
light scattering and
ultra-centrifugal techniques
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82
i Ni Mi NiMi NiMi2
1 50 500 25000 12500000
2 100 1000 100000 1E+08
3 300 1500 450000 6.75E+08
4 400 2000 800000 1.6E+09
5 600 4000 2400000 9.6E+09
6 400 5000 2000000 1E+10
7 300 10000 3000000 3E+10
8 100 15000 1500000 2.25E+10
9 50 30000 1500000 4.5E+10
SUM 2300 69000 11775000 1.19E+11
Mn= 5119.565
Mw= 10147.56
PDI= 1.982113 www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
83
Polymers: Molecular Polymers: Molecular WeightWeight
• Biomedical applications: 25,000<Mn<100,000 and 50,000<Mw<300,000
• Increasing molecular weight increases physical properties; however, decreases processibility
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84
TEFLON or FLUON or Polytetrafluoroethylene (PTFE):
Preparation
FC 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
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85
• It does not dissolve in any of the strong acids including 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 chainsgives the extreme toughness, high softening point, exceptionally high chemical resistance
• It has high density 2.1 to 2.3 gm/cm3
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86
• 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 required
• It is used in non-lubricated bearings
• It is used in non-sticking stop-cocks like burettes etc.,
• It is used for products where resistance to acid and alkalies are needed
• It is used for coating and impregnating, glass fibers, asbestos fibers (to form belts), filter cloth etc.,
• It is used as catheters, artificial vascular grafts etc.,www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
87
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 the diamine 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 of nylon 6
Polyamides are prepared by the melt poly condensation
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88
Preparation
Heat- 2n H2O
+n n
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89
Properties
• It has a good tensile strength, abrasion resistance and toughness upto 150 oC
• It offers resistance to many solvents. However, it dissolves in formic acid, cresols and phenols
• They are translucent, wheatish, horny, high melting polymers (160 – 264 oC)
• They possess high thermal stability
• Self lubricating properties
• They possess high degree of crystallinity
• The interchain hydrogen bonds provide superior mechanical strength (Kevlar fibers stronger than metals)
• Its Hardness is similar to tinwww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
90
• Nylon 6,6 used as sutures
• It is used as a plastic as well as fiber
Uses
• This is used to produce tyre cord
• It is used to make mono filaments and roaps
• Nylon 6,6 is used to manufacture articles like brushes and bristles
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91
P – F Resins
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
• Resoleswww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
92
Both novolacs and resoles are linear, low molecular weight, soluble and fusible prepolymers
During moulding operations, these two undergo extensive 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 ortho or para methylol phenols
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93
CH2OH
OH
CH2OH
OH
and
o-methylol phenolp-methylol phenol
Phenol (excess)
OH
formaldehyde
C = O
H
H
+
H+
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These methylol phenols condense rapidly to form Novolacs
CH2OH
OH
CH2OH
OH
or
o-methylol phenolp-methylol phenolOH
OHH2
CH2
COH H2
COH
OH
H2
C
HO
Novolacswww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
95
These novolacs are linear and low mol. wt. polymers
About 5 – 6 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
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96
OHH2
CH2
COH H2
COH
OH
H2
C
HO
Novolacs (prepolymer) Curing with Formaldehyde or hexamine
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97
Resoles
When the molar ratio of P/F is < 1 and the catalyst used is a base, the polymer formed are called Resoles
The first step in the reaction is the formation of mono, di and trimethylol phenols.
They undergo condensation to form resoles
Phenol
OH
Formaldehyde(excess)
C = O
H
H
+
OH--
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98
CH2OH
OH
o-methylol phenol
CH2OH
OH
p-methylol phenol
CH2OH
OH
CH2OH
CH2OH
OH
CH2OH
HOH2C
di methylol phenol
tri methylol phenol
+ ++
Curing
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99
The resoles in which phenols are linked through methylene bridges are soluble and fusible
Since they contain alcoholic groups, further reaction during curing leads to cross linking, resulting in a network, infusible and insoluble product
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100
Properties
• 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 copperwww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
101
• These are usable up to 400 °F (204°C)
• 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
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Uses
• 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
• For impregnating fabrics, wood and paper
• 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
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103
Epoxy resins
Preparation
Cl CH2CH2CH
On
CH3
C OHHO
CH3
+
n
CH2 CH2CH
OH
C OO
CH3
CH3
epichlorhydrin bis phenol Alkaline catalyst
60 OC-n HCl
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In epoxy resins, n ranges from 0 to 20
The molecular weight of the epoxy resin depends upon the relative proportions of the reactants
The 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 polymers by curing with some chemicals like diethylene triamine, triethylene tetramine and meta-phenylene diamine
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Properties
• Epoxy resins have ability of getting cured, without application of heat
• They have good resistance to chemicals
• They have less shrinkage during curing process
• They may be used in solid or liquid form
• 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
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• No size-change upon cross-linking/hardening
This means they make ideal adhesivesShrinkage causes adhesive failuresAdhesives require no dimensional change
• Resins can be changed to modify epoxy properties
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 • Epoxy resins are used as potting compounds for
electrical equipment www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
107
• They are used as stabilizers for PVC resins
• Epoxy resins are used for skit-resistant surfaces, for highways rendering a number of advantages
• Delayed wearing of road surfaces in hot and cold climates
• Excellent resistance to freezing conditions, de-icing salts, solvents and water
• Non-porosity which protects the original pavements from scaling and spalling
• Permanent high traction even under wet or oily conditions
• 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 bleached fabrics to impart crease resistance and shrinkage control
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108
ELASTOMERS
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
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109
Elastomers are high polymers, which have elastic properties in excess of 300 %
The elastic deformation in an elastomer arises due tothe fact that the molecule is not a straight chained in 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 www.bookspar.com | Website
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Natural rubber is an addition polymer formed from the 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
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111
As each isoprene unit contains C = C bond, polyisoprene exists in two isomeric forms
viz., cis and trans
Cis-polyisoprene trans-polyisoprenewhere R= CH3
Natural rubber contains the cis isomer while the gutta percha contains the trans isomer
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112
Natural rubber consists of basic material latex, which is a dispersion of isoprene
During the treatment, these isoprene molecules polymerize to form long-coiled chains of cis-polyisoprene
The mol. wt. of raw rubber is about 100,000 – 150,000
Natural rubber is made from the saps of a wide range of plants like havea brasillians and 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 www.bookspar.com | Website
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Gutta Percha is trans-polyisoprene and is obtained from the mature leaves of dichopsis gutta and palagum gutta trees (belonging to sapetaceae family)
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
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114
Deficiencies of natural rubber
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 sticky
• It 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 water
• Its chemical resistivity is low and is attacked by acids, alkalies, oxidizing and reducing agents
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• Its tensile strength, abrasion resistance wear and tear resistances are low
• It possesses marked tackiness i.e., when two fresh raw rubber surfaces are pressed together, they coalesce to form a single piece
• It has little durability
• When stretched to a great extent, it suffers permanent deformation, 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 more conducive for their technical uses
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116
A comparative account of the properties of natural and synthetic rubbers
Property Natural rubber Synthetic rubber
Tensilestrength
Low (only 200 kg/cm2) High
Chemicalresistivity
Low – gets oxidizedeven in air
High – not oxidized in air
Action of heat
Cold condition it is hardand brittle, at higher
temp.s soft andsticky
Withstand effect ofheat over a range
of temperature.
With organicsolvents
Swells and dissolves Do not swell and dissolve
Ageing Undergoes quickly Resists ageing
Elasticity On increased stress undergoes permanent
deformation.
Has high elasticity.
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117
Vulcanization of rubber
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 compounded with 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 100 – 140 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 intermolecular movement of rubber springs
The extent of stiffness of vulcanized rubber depends on the amount of sulphur added www.bookspar.com | Website
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e.g., a tyre rubber may contain 3 to 5% sulphur, but a battery case rubber may contain as much as 30% sulphur
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 +
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119
H
HH
H
H
H
H
H
H
HH
H
H
H
H
H
H
HH
H
H
H
H
H
C C
C
C CC C
C
C C CC 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
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120
Advantages of vulcanization
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 resiliencei.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 as compared to raw rubber
• is better a electrical insulator, although it tends to absorb small amounts of water
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121
• is resistant to organic solvents (such as petrol, 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 only slight tackiness
• has low elasticity and is depending on the extent of vulcanization
e.g., vulcanite (32% Sulphur) has practically no elasticity
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122
Compounding of rubber
Compounding is mixing of the raw rubber (synthetic or natural) with other substances so as to impart the specific properties to the product, which are suitablefor a particular job
Besides rubber, the following materials may be incorporated
• Softners and plasticizers
These are added to give the rubber greater tenacity and adhesion. Important materials are vegetable oils, waxes, stearic acid, rosin, etc.
•Vulcanizing agents
The main substance added is sulphur
Depending on the nature of the product required, the % of sulphur added varies between 0.15 and 32.0% www.bookspar.com | Website
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Many other vulcanizing agents are now-a-days added to rubber, among them are sulphur monochloride, hydrogen sulphide, benzoyl chloride, trinitrobenzene and alkylphenol sulphides
• Accelerators
These materials drastically shorten the time required for vulcanization
The most used accelerators are 2-mercaptol, benzothiozole and zinc alkyl zanthate
•Antioxidants
Natural rubber has a tendency to perish, due to oxidation
For this reason, anti oxidation materials, such as complex amines like phenyl naphthylamine and phosphates are added www.bookspar.com | Website
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•Reinforcing fillers
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
•Colouring 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 www.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
125
Styrene rubber (GR-S or Buna-S or SBR)
Preparation
This is produced by copolymerization of butadiene (about 75% by wt.) and styrene (about 25% by wt.)
H2C CH CH CH2
xH2C CH
n
H2C CH CH CH2n x
H2C CH
n+
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126
Styrene-butadiene Styrene-butadiene copolymercopolymer
Styrene domains act as anchors or junctions
Butadienes provide flexible linkages
The desire to maximize the ways you can arrange the flexible links is what causes rubbers to return to given shapeswww.bookspar.com | Website for Students | VTU NOTES | QUESTION PAPERS
127
Properties
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 vulcanization
Styrene rubber resembles natural rubber in processing characteristics as well as the quality of the finished products
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Uses
It is used for the manufacture of
• floor tiles
• motor tyres
• shoe soles
• gaskets
• wire and cable insulations
• carpet backing
• adhesives
• tank-lining etc.,
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Silicone rubber
Silicone resins contain alternate silicone – oxygen 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
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Dimethyl silicone dichloride is bifunctional and can yield very long chain polymer
CH3
CH3
OSi
n
CH3
CH3
Cl ClSin
CH3
CH3
HO OHSin
unstable
Hydrolysis
- HCl
H2O
polymerization
CH3
CH3
OSi( )
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Vulcanized silicone rubbers are obtained by mixing 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
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O +
CH3
OSi
CH2H
CH3
CH3
OSi
CH3
CH3
OSi
CH3
CH3
OSi
CH3
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
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Properties
They possess exceptional resistance to
• prolonged exposure to sun light • weathering • most of the common oils
• boiling water • dilute acids and alkalies
They remain flexible in the temp. range of 90 – 250 OC hence, find use in making tyres of fighter aircrafts, since they prevent damage on landing. Ordinary rubbertyre becomes brittle and hence disintegrates
silicone rubber at very high temp. s (as in case of fibers) decomposes; leaving behind the non-conducting silica (SiO2), instead of carbon tar
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Uses
• as a sealing material in search-lights and ain aircraft engines
• 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 Armstrong used silicone rubber boots when he walked on the moon
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Reclaimed rubber
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 8 – 15 hours in steam-jacketed autoclave
By this process, the fibers are hydrolyzed
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Sulphur gets removed as sodium sulphide and rubber becomes 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.,
After the removal of fibers, reclaiming agents like petroleum and coal-tar based oils and softeners are added
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Properties
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
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Uses
for manufacturing tyres
tubes
automobile floor mats
belts
hoses
battery containers
mountings
shoes, heals etc.,
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