1. NATURAL RUBBER Natural Polymer Polymer Monomer Natural Rubber
(Polyisoprene)
Isoprene
Starch
Glucose
Cellulose
Protein
Amino acid
Faridah Hamat Saser 2010

2. Natural Rubber Monomer : Isoprene or 2-methylbuta-1,3-diene
Structure Formula:
Faridah Hamat Saser 2010

3. Coagulation process
Rubber molecule - - -
Rubber particle - -
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
Faridah Hamat Saser 2010

4. The rubber particles can now close together
When acid is added;
The hydrogen ions, H+ neutralize the negative-charges 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
Faridah Hamat Saser 2010

5. Lactic acid contains hydrogen ions, H+
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
Faridah Hamat Saser 2010

6. 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
Faridah Hamat Saser 2010

7. Properties of natural rubber
Elastic
Unstable to heat
When heated above 50oC, its softens and becomes sticky
Stretched
released
Faridah Hamat Saser 2010

8. - Cannot dissolve in water - Soluble in organic solvents
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
Faridah Hamat Saser 2010

9. 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
Faridah Hamat Saser 2010

10. 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 cross-linkages pull the chains back to their original arrangement.
This improves the elasticity and strength of the rubber
Faridah Hamat Saser 2010