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When a polymer is stretched the snarls begin to disentangle and straighten out
Elasticity of the polymer is mainly because of the uncoiling andrecoiling of the molecular chains on the application of force
Elasticity
i.e., the orientation of the chains occurs which in turn enhances the forces of attraction between the chains 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
the tendency of a body to return to its original shape after it has been stretched or compressed;
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
So the factors which allows the slippage of the moleculesshould be avoided to exhibit an elasticity
The slippage can be avoided by
• introducing bulky side groups such as aromatic and cyclic groups on repeating units
• introducing non-polar groups on the chains
• introducing cross-linking at suitable molecular positions
Molecular Weight of Polymers
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
Mn= 5119.565
Mw= 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
i
1
nn 1fraction offraction Number
i
11
n
Mn 1fraction by on contributiweight Molecular
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
ii
i
44
i
33
i
22
i
11n
n
Mn............................
n
Mn
n
Mn
n
Mn
n
Mn M
i
iin
n
M n M
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
Mn
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
Polymers: Molecular Weight
• number average, Mn
• weight average, Mw
Ni: no. of molecules with degree of polymerization of i
Mi: molecular weight of i
• 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
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
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 40Calculate 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
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
• 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
• 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.,
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
Heat- 2n H2O
+n n
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
• 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 ropes
• Nylon 6,6 is used to manufacture articles like brushes and bristles
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
CH2OH
OH
CH2OH
OH
and
o-methylol phenolp-methylol phenol
Phenol (excess)
OH
formaldehyde
C = O
H
H
+
H+
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
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
OHH2
CH2
COH H2
COH
OH
H2
C
HO
Novolacs (prepolymer) 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
Phenol
OH
Formaldehyde(excess)
C = O
H
H
+
OH--
CH2OH
OH
o-methylol phenol
CH2OH
OH
p-methylol phenol
CH2OH
OH
CH2OH
CH2OH
OH
CH2OH
HOH2C
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)
• The properties can be modified by fillers& reinforcements
• These have the highest compressive strength
• These are machinable
• Phenolics are the resin in plywood
• These tends to be brittle
Uses
• 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
Cl CH2CH2CH
On
CH3
C OHHO
CH3
+
n
CH2 CH2CH
OH
C OO
CH3
CH3
epichlorhydrin bis phenol Alkaline catalyst
60 OC-n HCl
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
• 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
• 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
• 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 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
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-polyisoprenewhere 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 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.
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
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 +
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
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 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 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%
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 therubber product
for white colour titanium dioxide
Green chromium oxide
red ferric oxide
Crimson antimony sulphide
yellow lead chromate
---- pigments are added
Styrene rubber (GR-S or Buna-S or SBR)
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+
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
• floor tiles
• motor tyres
• 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
SiO
C
C
H
HH
H
H
H
SiO
C
C
H
HH
H
H
H
O
Dimethyl silicone dichloride is bifunctional and can yield very long chain polymer
CH3
CH3
OSi
n
CH3
CH3
Cl ClSin
CH3
CH3
HO OHSin
unstable
Hydrolysis
- 2 HCl
H2O
polymerization
CH3
CH3
OSi( )
unstable
Vulcanized silicone rubbers are obtained by mixing high molecular weight linear dimethyl silicone polymers 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
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
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
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
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
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
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
Uses
for manufacturing tyres
tubes
automobile floor mats
belts
hoses
battery containers
mountings
shoes, heals etc.,