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 fluid

Poly(isoprene)

Monomer: 2-methylbutan-1,3-diene

C CH

CH3

CH3CH3n C CH

CH3

CH2CH2

n

Properties of Natural rubber

Properties of natural

rubber

Easily oxidized

by airElasticity decreases

Soft

Sensitive to heat

Use of natural rubber

Rubber tubes

Shoe soles & door stoppers

Rubber bands

Rubber hoses, rubber caps

Coagulation process

Latex is milk-like liquid obtained from tapped rubber tree

Latex 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 coagulated

Added with acid (fast reaction)

Action of bacteria (slow reaction)

1 2

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Long-chain rubber molecules

(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

(a) Immersing strips of natural rubber in sulphurmonochloride, SCl or disulphide dichloride, S2Cl2

in methylbenzene

C=CC=C

C=C

C=C

C CC

CC

C

S

S

S

S

S

S

vulcanization

Vulcanization making rubber:

Tough materials

More 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 resistance

Less soluble in organic solvent

Vulcanized VS Unvulcanized rubber

Properties Natural rubberVulcanized

rubber

Elasticity

Hardness

Strength

Melting point

Resistance to temperature

Resistance to oxidation