natural rubber
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TRANSCRIPT
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I. NATURAL RUBBER
Chapter 2: Carbon Compounds
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Natural Polymer
Natural polymer:Polymer that exists naturally and is not made by man from chemical reaction
Examples
Natural polymer Monomer Natural rubber Isoprene
Starch Glucose
Cellulose Glucose
Protein Amino acid
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Natural rubber
Obtained from the latex (rubber tree)White milk-like fluidPoly(isoprene)Monomer: 2-methylbut-1,3-diene
C CH
CH3
CH3CH3n C CH
CH3
CH2CH2
n
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Properties of Natural rubber
Properties of natural
rubber
Easily oxidized by
air
Elasticity decreasesSoft
Sensitive to heat
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Use of natural rubber
Rubber tubesShoe soles & door stoppersRubber bandsRubber hoses, rubber caps
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Coagulation process
Latex is milk-like liquid obtained from tapped rubber treeLatex is a colloid (group) which contains
suspension of rubber particles in water.Each group of particles of rubber is
surrounded by a layer of protein membrane.This membrane is negatively charged and
will repel with each other.So, natural rubber remain in liquid form.
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The coagulation process
Has 2 ways the rubber can be coagulatedAdded with acid (fast reaction)Action of bacteria (slow reaction)
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1
• The negatively charged protein membrane surrounding the rubber molecules causes the rubber molecules to repel each other. • This condition causes natural rubber to remain in a liquid form and could not coagulate.
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2
• When an acid is added, the hydrogen ions, H+ neutralize the negative charges on the protein membrane.
**Latex can coagulate naturally if left for a few hours due to bacterial activity. Bacterial activity produces acids which contains hydrogen ion which can neutralize the negative charges on the protein membrane.
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• As a result the rubber molecules can come closer and collide with each other resulting the breaking of protein membranes.
3
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4
• The rubber molecules combine with one another and entangle, thus causing the latex to coagulate.
**Latex can be prevented from coagulating by adding ammonia, NH3 solution. Ammonia solution contains hydroxide ions, OH- that did not neutralize the negative charged on the protein membrane.
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(a) Added with acid
Acid (methanoic acid or ethanoic acid) which consists of H+ ion is added into the latex.
The H+ ion from acids neutralize the negatively charged protein membrane.
The rubber particles collide with each other.
Protein membrane of the rubber particle breaks.
Rubber molecules clump together. Latex coagulates.
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(b) Action with bacteria
Bacteria from the air enter the latex.The growth of bacteria produces lactic
acid.Lactic acid causes coagulation of latex.
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Prevention of coagulation
By adding ammonia, NH3 solution.Ammonia solution consists of OH- ion that
neutralize the acid produced by the bacteria.The rubber particles remain negatively
charged.Coagulation of latex is prevented.
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Vulcanization of rubber
Natural rubber can be vulcanized by the following manner;
(a) Heating natural rubber with sulphur and a little bit of zinc oxide (catalyst)
or(b) Immersing strips of natural rubber in
sulphur monochloride, SCl or disulphide dichloride, S2Cl2 in methylbenzene
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C=C C=
CC=C C=
CC=C
C C C
C CC
S
S
S
S
S
S
vulcanization
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Vulcanization making rubber:
Tough materialsMore elastic
a. The sulphur atoms form cross linkages between the long rubber molecules
b. This reduces the ability of the rubber polymers to slide over each other
c. The rubber molecules return to their original positions after being stretched
More heat resistanceLess soluble in organic solvent
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Vulcanized VS Unvulcanized rubber
Properties Natural rubber
Vulcanized rubber
ElasticityHardnessStrengthMelting pointResistance to temperatureResistance to oxidation