Transcript Slide 1

Elasticity
the tendency of a body to return to its original shape after it
has been stretched or compressed;
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
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
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
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
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
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
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
M n=
5119.565
M w=
10147.56
PDI=
1.982113
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
Number fraction of fraction 1  n1
 ni
Molecularweight contribution by fraction1 
n1M1
 ni
Similarly,
Molecular weight contribution by other fractions are
n1M1 n2M2 n3M3
;
;
;
 ni  ni  ni
n iM i
 ni
Number average molecular mass of the whole polymer
is given by
Mn 
Mn
n1M1 n2M2 n3M3 n4M4
niMi



 ............................
 ni  ni  ni  ni
 ni
n M


n
i
i
i
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
n1M1
n1M1
weight fractionof fraction1 

W
 niMi
 n1M1 
M1
Molecularweight contribution by fraction1  
  niMi 


n1M12

 niMi
Molecular weight contribution by other fractions are
n1M12 n2M2 2 n3M3 2
;
;
;
 niMi  niMi  niMi
niMi 2
 niMi
Weight average molecular mass of the whole polymer
is given by
n1M12
n2M2 2
n3M32
n4M4 2
niMi 2
Mw 



 .................
 niMi  niMi  niMi  niMi
 niMi
Mw 
2
n
i
M
i

nM
i
i
Polymers: Molecular Weight
Ni: no. of molecules with degree of polymerization of i
Mi: molecular weight of i
• number average, Mn
• weight average, Mw
• 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 < PI < 5
 At best PI = 1.1 can be attained with special techniques
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
light scattering
and
ultra-centrifugal techniques
Polymers: Molecular Weight
• Biomedical applications: 25,000 < Mn < 100,000
and 50,000 < Mw < 300,000
• Increasing molecular weight increases physical
properties; however, decreases processibility
(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 [-CH2–CH(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.
TEFLON or FLUON or
Polytetrafluoroethylene (PTFE):
Preparation
F
F
C
n
F
C
F
Water emulsion
polymerization
F
F
C
C
F
F
peroxide
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
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
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.,
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
Preparation
n
+n
Heat
- 2n H2O
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
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
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
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
OH
H
+
C=O
H
Phenol (excess)
H+
formaldehyde
OH
OH
CH2OH
and
o-methylol phenol
CH2OH
p-methylol phenol
These methylol phenols condense rapidly to form Novolacs
OH
OH
CH2OH
or
o-methylol phenol
OH H2
C
HO
OH
H2
C
OH H2
C
CH2OH
p-methylol phenol
H2
C
OH
OH
Novolacs
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
OH H2
C
H2
C
HO
Novolacs (prepolymer)
OH H2
C
H2
C
OH
OH
Curing with
Formaldehyde or
hexamine
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
OH
H
+
C=O
H
Phenol
OH--
Formaldehyde
(excess)
OH
CH2OH
OH
OH
OH
+
+
+
CH2OH
o-methylol phenol
CH2OH
CH2OH HOH2C
p-methylol
phenol
CH2OH
CH2OH
di methylol
phenol
tri methylol phenol
Curing
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
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
• These are usable up to 400 °F (204°C)
• 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
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
Epoxy resins
Preparation
CH3
n Cl
OH
C
CH2 + HO
CH2 CH
O
CH3
epichlorhydrin
bis phenol
Alkaline catalyst
60 OC
-n HCl
CH3
O
C
CH3
O
CH2
CH
OH
CH2
n
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
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
• 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
• 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
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
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
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
As each isoprene unit contains C = C bond,
polyisoprene exists in two isomeric forms
viz., cis and trans
Cis-polyisoprene
trans-polyisoprene
where R= CH3
Natural rubber contains the cis isomer while the
gutta percha contains the trans isomer
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
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
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
• 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
A comparative account of the properties of
natural and synthetic rubbers
Property
Tensile
strength
Natural rubber
Low (only 200 kg/cm2)
Synthetic rubber
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.
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
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
H
H
C
C
C
H
H
H
C
H
H
C
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
C
C
C
H
C
H
H
+S+
H
H
H
H
H
H
C
C
C
C
C
C
C
C
H
H
H
C
C
H
H
H
C
H
H
H
C
C
H
H
H
C
H
H
H
C
H
H
H
H
C
C
C
C
C
H
H
H
H
H
C
C
H
HH
H
H
HH
H
H
S
C
H
H
C
C
HH
H
C
C
H
H
H
C
C
H
C
S
H
H
H
H
H
H
C
C
C
C
C
C
C
C
H
H
C
H
HH
H
C
C
H
H
C
H
HH
H
C
C
C
HH
H
H
H
H
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
• 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
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%
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
•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
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
n x H2C
H2C
CH
CH
CH
CH
CH
CH2 + n
H2C
CH2
CH
x
n
Styrene-butadiene copolymer
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 shapes
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
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.,
Silicone rubber
Silicone resins contain alternate silicone – oxygen
structure, which has organic radicals attached to
silicone atoms
H
H C
O
H
H
Si
H C
O
H
Si
H C H
H C H
H
H
O
Dimethyl silicone dichloride is bifunctional and
can yield very long chain polymer
CH3
n Cl
Si
CH3
Cl
Hydrolysis
- 2 HCl
n HO
Si
OH
CH3
CH3
unstable
CH3
CH3
Si
CH3
H 2O
polymerization
O
n
( Si
O)
CH3
unstable
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
CH3
CH3
Si
H
CH2
O
+
H
CH2
Si
CH3
O
O
Si
CH3
O
Si
CH3
O
Si
CH3
O
Si
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
Si
CH3
O
Si
O
CH3
Si
CH3
H 2O
O
Si
CH3
O
O
CH3
CH3
Si
O
Si
CH3
O
Si
CH3
O
Si
CH3
O
Si
CH2
CH3
CH3
CH3
CH3
CH2
CH3
CH3
CH3
CH3
Si
CH3
O
Si
CH3
O
Si
CH3
O
Si
CH3
O
Si
CH3
O
O
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
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
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
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.,
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
Uses
 for manufacturing tyres
 tubes
 automobile floor mats
 belts
 hoses
 battery containers
 mountings
 shoes, heals etc.,