1. Natural rubber comes from the latex of plants like the rubber tree. It is a polymer called polyisoprene that is elastic due to its coiled structure. 2. Latex is processed into rubber by coagulation with acids and passing through rollers to form sheets. It is then dried and smoked. 3. Guayule and gutta percha are other sources of natural rubber that are extracted and processed into sheets using solvents and mechanical methods. Natural rubber has properties like elasticity but also drawbacks like a limited temperature range and weakness that make it vulnerable to chemicals and permanent deformation.

1. ELASTOMERS
BY
CH. SAI SRI RAMA CHANDRA
MURTHY

2. Natural Rubber and Vulcanisation:
Those polymers which possess the property of elasticity is called rubber.
Temporary deformation in structure on applying some stress is more than 600 e.u.
When stress is applied polymer chain gets partially aligned with respect to another
there by causing crystallisation, which makes the polymer stiff. On releasing the
stress, the chain gets reverted back to their original coiled state. The elasticity of the
rubber is due to its coiled helix structure,

3. Natural Rubber:
Natural rubber is processed from the cell sap (latex) of three main types
tress plants, Hevea Brasiliensis, Guayule, Dichopsis percha, Palagum gutta found in
tropical and semi-tropical countries. The main composition of natural rubber is
polyisoprene, which is in the form of long coiled chains, responsible for the
elasticity of natural rubber is shown below.

5. Latex (form) Hevea braciliensis
Diluted to 20%
FilteredFed into rectangular tanks Treated with
CH3COOH / HCOOH
Coagulation
Treated with CH3COOH /
HCOOH and stirred
Insert vertical partition
and allow for 16 hours
Rubber slabs
Passed between series of
rollers
Final roller giving
ribbed- pattern sheet
Hanged in smoked
chambers for 4days
Smoked rubber
Filtered
Coagulant
Washed with water
Passed between
two rollers
Rubber sheetDried in sunCrepe rubber

6. Constituents Percentages
Water 55-70
Rubber 30-40
Resins 1.5-2.0
Proteins 1.5-3.0
Sugar 1-2
Ash 0.5-1.0

7. The latex is diluted to 15-20% and filtered to remove suspended impurities
like pieces of bark, leaves etc. Then it is taken into tank and treated with acetic or
formic acid. The rubber coagulates (precipitates) as soft white mass (coagulum)
which is filtered and washed with water. The coagulum is passed between two rollers
about 3mm apart and 50cm wide and extruded in the form of a sheet which resemble
the crepe paper, which possess rough surface. The rubber processed by this method
is called crepe rubber. The crepe rubber sheet is dried in sun.
In the second type of processing, the filtered latex is fed into long rectangular
tanks of 1m wide and 30cm deep and treated with acetic acid or formic acid along
with constant stirring. Vertical partition plates are inserted and left undisturbed for 16
hours. The coagulum sets in the form of slabs is passed between a series of rollers
with decreasing clearance in between them while spraying water on these rollers.
The final roller gives ribbed pattern to the rubber sheet which help in quick drying
and preventing the adherence of rubber sheet to one another. The sheet are hanged
for about 4 days in smoked cahmber at 40-50°C. This is called Smoked rubber.

8. Guayule Rubber:
Guayule rubber is processed from guayule shrub, which is a source of natural
rubber in North America. This rubber resembles Hevea rubber as its composition is
cis-polyisoprene. Latex is enclosed in the cell of the shrub. Guayule shrub is cut into
pieces of very small size after removing the leaves and milled in pebble mill with
water into a smooth paste. The paste is sent to floatation tanks, where rubber floats
on the top is collected. The rubber is extracted with CCl4 solvent and extruded in the
from of sheet after removal of solvent.
Processing of Gutta Percha:
Gutta Percha is processed from the mature leaves of Dichopsis gutta and
Palagum gutta by solvent extraction process. The mature leaves are ground carefully,
heated with water at about 70°C for half an hour time and poured into cold water.
The gutta percha rubber floats on the surface, is removed and with CCl4.

9. The solvent is evaporated and the rubber is extruded in the form of sheet by passing
between two moving rollers. Gutta percha has transpolyisoprene in its composition.

10. Properties:
At room temperature it is horny and tough, but it softens and becomes tacky
at about 100oC. It is soluble in aliphatic hydrocarbons, but insoluble in aromatic and
chlorinated hydrocarbons. Structurally it is trans-polyisoprene, whereas have a
rubber is cis-isomer.
Uses:
In the manufacture of golf ball covers, submarine cables, adhesives and
tissues for surgical purposes.

11. Drawbacks of raw rubber:
1. It is soft at high temperature and brittle at low temperature like plastic. So its
working temperature range is limited to 10oC to 60oC.
2. It is weak and low tensile strength of 200 Kg/cm2.
3. It has large water–absorption capacity.
4. Non–resistant to non-polar solvents like vegetables and mineral oils, gasoline,
benzene and CCl4.
5. It is attacked by oxidizing agents like nitric acid, sulphuric acid, chromic acid,
sodium hypochlorite, chlorine–dioxide etc.
6. It swells in organic solvents and gradually disintegrates.
7. It undergoes permanent deformation when stretched to a great extent.