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NATURAL RUBBER Natural Polymer Polymer Monomer Natural Rubber (Polyisoprene) Isoprene Starch Glucose Cellulose Glucose Protein Amino acid Faridah Hamat Saser 2010 1 Natural Rubber Monomer : Isoprene or 2-methylbuta-1,3-diene Structure Formula: Faridah Hamat Saser 2010 2 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 3 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 Faridah Hamat Saser 2010 4 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 5 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 6 Properties of natural rubber • Elastic Stretched released • Unstable to heat • When heated above 50oC, its softens and becomes sticky Faridah Hamat Saser 2010 7 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 8 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 9 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 Faridah Hamat Saser 2010 10