Natural Polymer Natural polymer:Polymer that exists naturally and is not made by manfrom chemical reaction ExamplesNatural polymer MonomerNatural rubber IsopreneStarch GlucoseCellulose GlucoseProtein Amino acid
Natural rubber Obtained from the latex (rubber tree) White milk-like fluid Poly(isoprene) Monomer: 2-methylbutan-1,3-dieneC CHCH3CH3CH3n C CHCH3CH2CH2n
Properties of Natural rubberPropertiesof naturalrubberEasilyoxidizedby airElasticitydecreasesSoftSensitiveto heat
Use of natural rubber Rubber tubes Shoe soles & door stoppers Rubber bands Rubber hoses, rubber caps
Coagulation processLatex is milk-like liquid obtained from tapped rubbertree Latex is a colloid (group) which contains suspensionof rubber particles in water. Each group of particles of rubber is surrounded by alayer of protein membrane. This membrane is negatively charged and willrepel with each other. So, natural rubber remain in liquid form.
The coagulation processHas 2 ways the rubber can be coagulated Added with acid (fast reaction) Action of bacteria (slow reaction)
1 234Long-chainrubber molecules
(a) Added with acid Acid (methanoic acid or ethanoic acid) whichconsists of H+ ion is added into the latex. The H+ ion from acids neutralize the negativelycharged protein membrane. The rubber particles collide with each other. Protein membrane of the rubberparticle breaks. Rubber molecules clump together. Latexcoagulates.
(b) Action with bacteria Bacteria from the air enter the latex. The growth of bacteria produces lactic acid. Lactic acid causes coagulation of latex.
Prevention of coagulation By adding ammonia, NH3 solution. Ammonia solution consists of OH- ion thatneutralize the acid produced by the bacteria. The rubber particles remain negatively charged. Coagulation of latex is prevented.
Vulcanization of rubber Natural rubber can be vulcanized by the followingmanner;(a) Heating natural rubber with sulphur and a little bitof zinc oxide (catalyst)or(a) Immersing strips of natural rubber in sulphurmonochloride, SCl or disulphide dichloride, S2Cl2in methylbenzene
Vulcanization making rubber: Tough materials More elastica. The sulphur atoms form cross linkages betweenthe long rubber moleculesb. This reduces the ability of the rubber polymers toslide over each otherc. The rubber molecules return to their originalpositions after being stretched More heat resistance Less soluble in organic solvent
Vulcanized VS Unvulcanized rubberProperties Natural rubberVulcanizedrubberElasticityHardnessStrengthMelting pointResistance totemperatureResistance tooxidation