Sources, Plantation Economy Coagulation & Processing of Coagulate Structure & Composition Properties of raw NR Compounding, Processing of NR Properties of NR vulcanizates Uses of NR Competitive products of NR

1. Natural Rubber

2. Natural Rubber
•Sources, Plantation Economy
•Coagulation & Processing of Coagulate
•Structure & Composition
•Properties of raw NR
•Compounding, Processing of NR
•Properties of NR vulcanizates
•Uses of NR
•Competitive products of NR
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3. Sources
•Many plants produce a milky sap, referred to
as latex, which is a caoutchouc dispersion in
an aqueous medium
•Latex producing plants are predominantly
found in tropical climates
•Not all caoutchouc producing plants are
harvested for industrial purposes
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4. Plantation Economy
•Early plantation economies used Ficus elastica,
Funtumia, de Castilloa and Manihot plants
•But were spoon displaced by Hevea
Brasiliensis, due to greater yield
•Yield was improved by the stimulation of latex
flow.
Several chemicals, such as 2-amino-3,5,6-trichloropicolinic acid
or 2-chloroethyl phosphoric acid were used, which after
application, penetrates the bark and produce ethylene within the plant
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5. Tapping
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6. Coagulation
•Preferred method is Acid Coagulation (Formic
Acid/acetic Acid)
•Latex from different sources are blended in
huge tanks in order to achieve good uniformity
•Latex is diluted with water to a solids content
up to 12 to 18%
•Iso-electric point is reached at a pH of 5.1 to
4.8, which coagulates the latex
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7. Processing of Coagulate
•Coagulum is processed immediately to prevent changes in
properties under the influence of bacteria when exposed
to air
•When kept immersed in water or in its own
serum, impurities in the caoutchouc are decomposed by
bacteria into gases, such as CO2, CH4 & Nitrogen containing
compounds
Two Standard ways of Processing the coagulum:
1. Drying by exposure to wood smoke (Smoked Sheet)
2. Air drying (Pale Creep)
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8. Structure & Composition
•Rubber Hydrocarbons + Impurities + Others
•Average composition measured for latex crepe: 89-92%
Rubber HC, 2.5-3% acetone extractables, 2.5-3.5%
protein, 2.5-3.5% moisture, 0.15-0.5% ash
•Rubber HC content of NR consists of 99.9% cis-1,4-
polysioprene. Presence of even small amount of trans
results in greatly different properties
•Average molecular weight ranges from 2,00,000 –
4,00,000
•Has a broad molecular weight distribution (results in great
process ability)
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9. Structure
Chemical structure of
cis-polyisoprene, the main
constituent of natural
rubber.
Synthetic cis-polyisoprene
and natural
cis-polyisoprene are
derived from different
precursors.
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10. Chemical Properties of raw NR
•If heated above 300 ° C destructive distillation occurs
•Double bond in the isoprene unit and the α-methylene
groups are reactive for sulphur vulcanization
•May also lead to age degradation due to the presence
of double bond(reacts with O2 or O3)
•Other reactions with hydrogen leads to formation of
Hydrogenated rubber
•Reacts with Chlorine to form chlorinated rubber; reacts
with HCl to form hydro-chlorinated rubber; cyclization
reactions also may occur
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11. Cont..
•Since NR contains some amount of
antioxidants, it is stable for longer storage
periods in air at room temperature. BUT On
storage at high temperatures and on exposure to
light, NR oxidizes by forming hydroperoxides
through oxygen radicals
•Unsaturation facilitates reaction with oxidizing
agents like peroxides, peroxy acids, potassium
permagnate, ozone, chlorine etc..
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12. Physical Properties of raw NR
•Specific gravity: 0.934 @ 20°C
•Specific heat: 0.502 @ 20°C
•Purified NR after extraction with acetone has
refractive index of 1.5215 to 1.5238
•Electrical properties of NR are determined by its water
soluble impurities. For instance the specific resistivity
of sheets is 10^15 and that of crepe is 2*10^15
•When NR is stretched more than 80% of its original
length, crystallization occurs
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13. Cont..
•Un-crosslinked NR can be extended to 800 to 1000% of its
original length without breaking
•As the temperature increases forces required to deform the
material becomes larger
•At high rates of strain, deformations are completely
recoverable, whereas at low strain rates some deformation
may remain. This deformation will however recover on
heating or after some time. Because of this incomplete
recovery the extension and retraction do not coincide. This
hysteresis leads to a heat build up, and is more pronounced
in the first heat cycle and less in the subsequent cycles.
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14. Cont..
Behaviour in Solvents:
•When in contact with organic liquids like
benzene, gasoline, vegetable oils, mineral
oils, carbon tetrachloride, swell raw rubber to a
considerable amount to form a highly viscous
solution or gel.
•Physical bonds are broken in this process
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15. Compounding of NR
Blends with NR:
•NR is non-polar
•Therefore NR can be readily blended with other
non-polar rubbers
•NR has to be masticated so that it has the same
viscosity as the blend partner to achieve intimate
blending of the polymers
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16. Cont..
Vulcanization Chemicals:
•Sulphur & Accelerators: Suphur is predominantly used as
crosslinking agent eventhough peroxides/High energy radiation
can also be used to crosslink
For lower S concentrations, large amount of Accelerators are used
and viceversa
High concentrations of Sulphur may produce Ebonite
•Metal Oxides: Used in a compound to develop the full potential
of accelerators
Eg: Zinc oxide, Lead oxide, Factice, MgO in the presence of acidic
compounding ingredients
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17. Factice
•Added to improve processability ( extrusion
and calendaring operations)
•Prevent deformations while curing
•Improves the appearance of vulcanizates
•Used in large conc. in compounding for
preparation of rubber erasers
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18. Cont..
•Activators: Many accelerator systems require
additional activators like fatty acids or salts of fatty
acids namely stearic acid, zinc soaps or amine
stearates
•Vulcanization Inhibitors: Used to prevent scorch
Pthalamide sulphinamide based Inhibitors are used
when accelerators or acidic compounding
ingredients doesn't provide sufficient scorch life
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19. Cont..
Protective Agents:
•Since unsaturation in NR causes the material to degrade
due to ageing, it has to be compounded with protective
agents.
•Most effective ones are aromatic amines such as p-
phenylene diamine derivatives which also doubles up as an
antidegradant from Ozone and heat & also against dynamic
fatigue
•PAN/PBN impart good fatigue resistance to NR
•More effective PAs discolour light coloured vulcanizates so
less effective PAs such as phenols, bisphenols, MBI are used
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20. Cont..
Fillers:
•Fillers are not necessarily used in NR to obtain high tensile
strength but to reach a level and range of properties that are
required for technical reasons( such as density, colour, price)
•Reinforcing fillers enhance the already high tensile properties
of NR, and the improve the abrasion and tear resistance+
improves the processability, affects the hardness and rebound
elasticity of NR vulcanizates
•Less reinforcing fillers: n770, n990 ; Light inactive fillers:
kaolin, ZnO, MgCO3,CaCO3, Kaolin
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21. Cont..
Softeners:
•Most important mineral oils including paraffinic to
aromatic
•Animal and Vegetable Oils are also used
•Blooming potential must be taken into consideration
while selecting softeners for NR
Resins:
•Added for improving building tack, so that they can be
fabricated
•Rosin, tar, pitch
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22. Cont..
Process Aids:
•Important in facilitating the dispersion of fillers
•Ensure smooth processing
•In pure white or pastel coloured articles, calcium soaps of
unsaturated fatty acids are preferred
•Stearic acids, zinc and calcium soaps and residues of fatty
acids
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23. Processing of NR
•Good processing properties
•Viscosity stabilized grades of NR do not require premastication
before filler incorporation
•For non-stabilized grades mastication is a common practice
•Mastication is carried out below 80 degrees or above 120 degree
•Chemical peptizers are used to carry out mastication at lower
temperatures
•Break down behaviour is dependant on PRI values and on the
initial viscosity of the rubber
•Latex grade rubbers such as RSS1 and SMR L (high PRI values)
breakdown much faster than cuplump grades such as SMR 20 ( low
PRI values) and therefore require more mastication
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24. Cont..
•Viscosity is the most widely used measure of
processing quality
•During mixing control of viscosity is essential for
smooth operation during processing such as extrusion
and injection moulding
•In Extrusion of a fully mixed batch, the batch viscosity
is the main factor controlling the die swell and stress
developed
•A masticated rubber has better extrusion properties
than a non masticated rubber of same viscosity
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25. Properties of NR vulcanizates
Strength:
•TS of gum vulcanizates: 17 to 24 Mpa
•TS of black filled vulcanizates: 24 to 32 Mpa
•Strength/tear resistance/cut growth resistance: Excellent ( due
to the ability to undergo crystallization)
•Decreases with increase in Temperature
Abrasion & Wear:
•Excellent abrasion resist., improved by blending with polybutadiene
•Wear resistance: dependant on surface temperature
•Below 35 deg. NR shows better wear than SBR
•Above 35 deg. SBR is better
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26. Cont..
Dynamic Properties:
•High resilience with values exceeding 90% + Good flexing properties
•At large strains fatigue life of NR > SBR and vice versa for small strains
Compression Set:
•Poor in NR compared to SR (Due to presence of non rubber
substances in SR, and DPNR shows much improved properties)
Ageing:
•Heat-ageing resistance is given by choosing proper vulcanizates
systems and by use of amines or phenolic antioxidants
•Ozone resistance is improved by adding Waxes and antioxidants of
the p-phenylene diamine type
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27. Uses of NR
•Tires
•Footwear
•Engineering products
•Mechanical goods
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28. Competitive products of NR
Ethylene Propylene Diene Monomer
EPDM rubber is used in seals, glass-run channels,
radiators, garden, tubing, pond liners, washers, belts,
electrical insulation, O-rings, solar panel heat collectors,
UV protection (for tubular motors in shutters) and
speaker cone surrounds.
EPDM doesn't require maintenance with products to
protect it from ultraviolet light, like natural rubber does.
EPDM rubber offers the same colour stability and
durability in heat as natural rubber but at a lower price.
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29. Competitive products of NR
Isoprene Rubber is beneficial for the production of
gloves, medical
products, adhesives, tackifiers, paints, coatings, and
photoresistors.
They combine the key qualities of natural rubber such as
good mechanical properties and hysteresis with superior
features such as high purity, excellent clarity, good
flow, low gel content, no nitrosamines, and no natural
rubber proteins!
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30. Competitive products of NR
Styrene Butadiene Rubber is used widely in pneumatic
tires, shoe heels and soles, gaskets and even chewing
gum.
SBR is more resistant to abrasion and oxidation than
natural rubber.
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31. References
•Rubber Technology Handbook, Werner Hoffman
•Rubber Technology, Maurice Morton
•Wikipedia
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