Transcript NATURAL RUBBER

NATURAL RUBBER
Natural Polymer
Polymer
Monomer
Natural Rubber
(Polyisoprene)
Isoprene
Starch
Glucose
Cellulose
Glucose
Protein
Amino acid
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Natural Rubber
Monomer : Isoprene or
2-methylbuta-1,3-diene
Structure Formula:
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Coagulation process
Rubber molecule
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Rubber particle
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Negatively charged protein membrane
The repulsion between the negative-charged particles
prevents the rubber particles from coming close to each
other. Hence, latex could not coagulate
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When acid is added;
•The hydrogen ions, H+ neutralize the negativecharges on the protein membrane
•The rubber particles can now close together
• This enables them to collide with one another
• Resulting in the breakage of the protein
membrane
•The rubber molecules combine with one another
and entangle
• Causing the latex to coagulate
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Latex will coagulate if left after 2 or 3 hours (without
acid is added) because;
• Activities (growth) of Bacteria on the protein
membrane produce lactic acid
•Lactic acid contains hydrogen ions, H+
•Hydrogen ion neutralize the negative charges on the
protein membrane
• Allow the coagulation to occur
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Prevent Coagulation Process
When Alkali (NH3 (aq) ) is added;
•Ammonia aqueous contains hydroxide ion, OH•Hydroxide ion can neutralize the acid produced
by the bacteria
• Hence, the rubber particles remain negatively
charged and the coagulation is prevented
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Properties of natural rubber
• Elastic
Stretched
released
• Unstable to heat
• When heated above 50oC, its softens and
becomes sticky
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Unstable to oxidation
– Because the presence of double bonds
between carbon atoms
– Oxygen from air with UV radiation breaks up
the polymers chains
Insoluble in water
- Cannot dissolve in water
- Soluble in organic solvents
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Vulcanisation of Rubber
1 – 3 % by weight of sulphur is added to
raw rubber and the mixture is carefully
heated
In Laboratory;
Strip of rubber sheet is dipped into a
solution of disulphur dichloride for about 5
minutes
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Sulphur atoms form cross-links between
adjacent chains of rubber polymer at the
carbon-carbon double bonds
Vulcanization
Cross-linkage of sulphur
•When vulcanized rubber is stretched and released, the crosslinkages pull the chains back to their original arrangement.
•This improves the elasticity and strength of the rubber
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