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.,