I. NATURAL RUBBER

Chapter 2: Carbon Compounds

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

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

Properties of Natural rubber

Properties of natural

rubber

Easily oxidized by

air

Elasticity decreasesSoft

Sensitive to heat

Use of natural rubber

Rubber tubesShoe soles & door stoppersRubber bandsRubber hoses, rubber caps

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.

The coagulation process

Has 2 ways the rubber can be coagulatedAdded with acid (fast reaction)Action of bacteria (slow reaction)

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.

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.

• As a result the rubber molecules can come closer and collide with each other resulting the breaking of protein membranes.

3

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.

(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.

(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 that

neutralize 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 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

C=C C=

CC=C C=

CC=C

C C C

C CC

S

S

S

S

S

S

vulcanization

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

Vulcanized VS Unvulcanized rubber

Properties Natural rubber

Vulcanized rubber

ElasticityHardnessStrengthMelting pointResistance to temperatureResistance to oxidation