When a polymer is stretched the snarls begin to disentangle and straighten out

1
the tendency of a body to return to its original
shape after it has been stretched or compressed
Elasticity
Elasticity of the polymer is mainly because of
the uncoiling and recoiling of the molecular
chains on the application of force
When a polymer is stretched the snarls begin to
disentangle and straighten out
i.e., the orientation of the chains occurs which
in turn enhances the forces of attraction between
the chains and thereby causing the stiffness of
the materials
However when the strain is released snarls return
to their original arrangement
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
2
So the factors which allows the slippage of the
molecules should 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

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
3
Molecular Weight of Polymers
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
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
In contrast, a polymer comprises molecules of
different molecular weights
4
upon polymerization, ethylene 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 polymer sample can be thought of a mixture
of molecules of the same chemical type, but of
different molecular weights
5
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
6
the molecular weight of a polymer can be
expressed by two most and experimentally
verifiable methods of averaging
(i) Number average
(ii) Weight average
The molecular mass of a polymer can use either
number fractions or the weight fractions of the
molecules present in the polymer
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
7
i Ni Mi NiMi NiMi2
1 50 500 25000 12500000
2 100 1000 100000 1E08
3 300 1500 450000 6.75E08
4 400 2000 800000 1.6E09
5 600 4000 2400000 9.6E09
6 400 5000 2000000 1E10
7 300 10000 3000000 3E10
8 100 15000 1500000 2.25E10
9 50 30000 1500000 4.5E10
SUM 2300 69000 11775000 1.19E11
         
Mn 5119.565      
Mw 10147.56      
PDI 1.982113      
8
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
9
Similarly,
Molecular weight contribution by other fractions
are
Number average molecular mass of the whole
polymer is given by
10
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
11
Molecular weight contribution by other fractions
are
Weight average molecular mass of the whole
polymer is given by
12
Polymers Molecular Weight
Ni no. of molecules with degree of
polymerization of i Mi molecular weight of i

• number average, Mn

• weight average, Mw

13

• Ratio of Mw to Mn is 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
  synthetic polymers have 1.5 lt PI lt 5
• At best PI 1.1 can be attained with special
  techniques

14
The number-average molecular mass (Mn) is
determined by the measurement of colligative
properties such as
lowering of vapour pressure
osmotic pressure
depression in freezing point
elevation in boiling point
The weight-average molecular mass (Mw) is
determined by
and
light scattering
ultra-centrifugal techniques
15
Polymers Molecular Weight

• Biomedical applications 25,000 lt Mn lt 100,000
  and 50,000 lt Mw lt 300,000
• Increasing molecular weight increases physical
  properties however, decreases processibility

16

• A protein sample consists of an equimolar mixture
  of Haemoglobin (M15.5 Kg mol-1), Ribonuclease
  (M13.7 Kg mol-1) Myoglobin (M17.2 Kg mol-1).
  Calculate Mn Mw
• A polypropylene -CH2CH(CH3)- sample contains
  the following composition.
• Degree of polymerization 400 800 600
• of composition 25 35
  40
• Calculate Mn Mw of polypropylene sample by
  neglecting the end groups. Given that atomic
  masses of C 12 and H 1 amu.

17
TEFLON or FLUON or Polytetrafluoroethylene
(PTFE)
Preparation
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

18

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

• It has high density 2.1 to 2.3 gm/cm3

19

• 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 coating and impregnating, glass
  fibers,
• asbestos fibers (to form belts), filter cloth
  etc.,

• It is used for products where resistance to acid
  and
• alkalies are needed

• It is used as catheters, artificial vascular
  grafts etc.,

20
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
21
Preparation
22
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 tin

23
Uses

• 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

• Nylon 6,6 used as sutures

24
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

• Resoles

25
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 gt 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
26
(No Transcript)
27
These methylol phenols condense rapidly to form
Novolacs
or
o-methylol phenol
p-methylol phenol
Novolacs
28
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
29
(No Transcript)
30
Resoles
When the molar ratio of P/F is lt 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
31
(No Transcript)
32
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
33
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 copper

34

• These are usable up to 400 F (204C)

• These tends to be brittle

• The properties can be modified by fillers
• reinforcements

• These have the highest compressive strength

• These are machinable

• Phenolics are the resin in plywood

35
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

36
Epoxy resins
Preparation
37
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
38
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

• They possess excellent electrical resistance

• Epoxy resins stick well to a number of
  substances
• including metal and glass

• Their properties can be modified by adding
  compounds
• like unsaturated fatty acids or amines and
• some of the solvents

39

• No size-change upon cross-linking/hardening

This means they make ideal adhesives
Shrinkage causes adhesive failures
Adhesives 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

40

• 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

41
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
42
Elastomers are high polymers, which have elastic
properties in excess of 300
The elastic deformation in an elastomer arises
due to the 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
43
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
44
As each isoprene unit contains C C bond,
polyisoprene exists in two isomeric forms
viz., cis and trans
where R CH3
Natural rubber contains the cis isomer while the
gutta percha contains the trans isomer
45
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
46
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
47
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

48

• 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
49
A comparative account of the properties of
natural and synthetic rubbers
Property Natural rubber Synthetic rubber
Tensile strength Low (only 200 kg/cm2) High
Chemical resistivity Low gets oxidized even in air High not oxidized in air
Action of heat Cold condition it is hard and brittle, at higher temp.s soft and sticky Withstand effect of heat over a range of temperature.
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.
50
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
51
e.g., a tyre rubber may contain 3 to 5 sulphur,
but a battery case rubber may contain as much as
30 sulphur
52
(No Transcript)
53
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 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 as
• compared to raw rubber

• is a better electrical insulator, although it
  tends to
• absorb small amounts of water

54

• 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
55
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 suitable for 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
56
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
57

• 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
the rubber product
for white colour titanium dioxide
Green chromium oxide
red ferric oxide
Crimson antimony sulphide
yellow lead chromate
---- pigments are added
58
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.)
59
Styrene-butadiene copolymer
60
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

61
Uses
It is used for the manufacture of

• motor tyres

• floor tiles

• shoe soles

• gaskets

• wire and cable insulations

• carpet backing

• adhesives

• tank-lining

etc.,
62
Silicone rubber
Silicone resins contain alternate silicone
oxygen structure, which has organic radicals
attached to silicone atoms
63
(No Transcript)
64
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
65
(No Transcript)
66
(No Transcript)
67
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 rubber tyre 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
68
Uses

• as a sealing material in search-lights and in
  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

69
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
70
After the removal of fibers, reclaiming agents
like petroleum and coal-tar based oils and
softeners are added
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.,
71
Properties
The reclaimed rubber has

• less tensile strength

• has lower elasticity

• 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

72
Uses

• for manufacturing tyres

• tubes

• automobile floor mats

• belts

• hoses

• battery containers

• mountings

• shoes, heals etc.,