Chapter 2: Carbon Compounds
I. NATURAL RUBBER
Natural Polymer ď‚— Natural polymer:
Polymer that exists naturally and is not made by man from chemical reaction ď‚— Examples
Natural polymer
Monomer
Natural rubber Starch Cellulose Protein
Isoprene Glucose Glucose Amino acid
Natural rubber Obtained from the latex (rubber tree)
White milk-like fluid Poly(isoprene) Monomer: 2-methylbutan-1,3-diene
CH3
n
CH3
C
CH3 CH CH3
CH2
C
CH CH2
n
Properties of Natural rubber
Easily oxidized by air
Sensitive to heat
Elasticity decreases
Properties of natural rubber Soft
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
Long-chain rubber molecules
3
4
(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 ď‚— 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 sulphur monochloride, SCl or disulphide dichloride, S2Cl2 in methylbenzene
C=C
C=C
C=C
C=C
vulcanization
C
C
C
S
S
S
S
S
C
C
S C
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 Elasticity Hardness Strength Melting point Resistance to temperature Resistance to oxidation
Natural rubber
Vulcanized rubber