To improve the properties of rubber, Charles Good in 1839 compounded the raw rubber with some chemicals and heated to 100 - 140°C. Finally the compounded and vulcanized rubber is draw in the form of sheet by calendaring process.

1. VULCANIZATION OF
RUBBER
BY
CH. SAI SRI RAMA CHANDRA MURTHY

2. To improve the properties of rubber, Charles Good in 1839 compounded the raw
rubber with some chemicals and heated to 100 - 140°C. Finally the compounded and
vulcanized rubber is draw in the form of sheet by calendaring process.
Compounding and vulcanization: The following are materials added to natural rubber
1. Vulcanisers
2. Plasticizers
3.Accelerators
4. Antioxidants
5. Reinforcing agents
6. Colouring agents

3. 1. Vulcanisers:
Some chemicals like element sulphur, hydrogen sulphide, sulphur dichloride,
benzoyl chloride and zinc oxide are added to both natural and synthetic rubbers.
The most important vulcaniser is sulphur which combines chemically at the
double bonds of the different chains producing sulphur cross linkings, imparting
strength increases, the brittleness also increase. The percentage of sulphur added is
in the range of 0.5 to >35%. For example the rubber used for making tyres contain
3 to 5% of sulphur and for making battery case the rubber contain 30% of sulphur.
If the percentage of sulphur is more than 32, the rubber is called ebonite or
vulcanite or hard rubber for some special purposes.

4. Vulcanisers

5. 2. Plasticizers:
Plasticizers are also called as softners, which impart greater plasticity and
adhesion to the rubber. The most commonly used plasticizers are vegetable oils,
waxes, resin etc.
3. Accelerator:
They catalyze the vulcanization process by reducing the time required for
vulcanization. Most commonly used accelerators are litharge, magnesia, and white
lead. Generally 0.5 to 1% of accelerator are added. ZnO is also added which acts
as an activator to accelerator.

6. 4. Antioxidant:
They retard the deterioration of rubber by light and air. Complex organic
amines like phenylnaphthylamine, Phenolic substances and Phosphite are added as
antioxidants.
5. Reinforcing agents:
They give strength, rigidity and toughness to the rubber and may form as much
as 35% of rubber compound. Commonly used reinforcing agents are ZnO,
MgCO3, BaSO4 and CaCO3.

7. 6. Colouring agents:
They impart desired colour to the rubber as follows
1. Lithophane, TiO2 - White
2. Ferric oxide - Red
3. Lead chromate - Yellow
4. Chromium trioxide - Green
5. Caron black - Black
6. Ultra marine - Blue

8. Calendering:
The natural processed is mixed with the different compounding and
vulcanizing material after mastication (subjecting rubber to maching / grinding) in
a Bandury mix is converted to sheets (0.003 to 0.1 inch thickness) by passing
between rolls (calendering) [

9. Advantages of vulcanization:
1. Vulcanised rubber has good tensile strength and load bearing capacity.
2. Vulcanised rubber has good elasticity depending on the extent of
vulcanization.
3. It possess low water absorbance tendency.
4. Vulcanised rubber has higher resistance to oxidation and atmospheric gases
like O2, CO2, moisture, light and U.V. radiation.
5. Vulcanised rubber is a good electrical insulator and ebonite is best electrical
insulator.
6. The abrasion resistance of natural rubber increases tremendously.

10. 7. The resistance towards organic solvents and chemicals increases.
8. Vulcanised rubber possess good impact resistance.
9. Vulcanised rubber possess very good resilience i.e., the article made from it
returns to it’s original shape, when the deforming load is removed.
Applications of Rubber and Elastomers
Different varieties of rubbers, natural as well as artificial, find extensive use in the
manufacture of following items.
a. Due to remarkable resistance to electricity, it is used as insulating coating on wires
and cables, used for electrical power transmission and in electrician gloves.

11. b. Due to its elasticity, it is used for making rubber bands, mechanical rubber goods,
golf balls, printing rollers, tubes for automobiles, aeroplanes and bicycles.
c. Due to its abrasion – resistance (i.e resistance to wearing away of surface by
friction),it is used for making bicycle, automobiles and aeroplane tyres, shoe heads
and soles, conveyor belts (used for conveying grains, coal, rocks, sand, ores etc.),V-
belts (used for transmission of power), shock absorbers, mountings for heavy
machineries and automobile parts.
d. Due to its chemical – resistance, it is used for making rubber – lined metal tanks
and vessels, used in chemical industry, where corrosive chemicals are to be
processed.

12. e. Due to its chemical- resistance to petrol, mineral oils, some solvents, etc. rubber
hoses are used for their transmission. Rubber hoses are also used for water
transmission for gardening, fire – fighting, etc.
f. Hard rubber (or ebonite), due to high abrasion – resistance and good electrical –
insulation properties , is used for making storage battery cases, electrical switch –
board panels, plugs and sockets, telephone receivers, etc. Hard rubber pumps,
pipes, valves and fittings are used in chemical industry, where resistance to
chemical corrosion is needed.
g. Sponge rubber is used for shock absorption, thermal insulation and sound
insulation purposes.

13. h. Foam – rubber is used for making bed pillows, mattresses, cushions,
automotive pads and undercoat paddings, etc.
i. Rubber gaskets are used for sealing various types of equipments such as
cookers, refrigerator doors, autoclaves, oven doors, etc.
j. In medicine for making heart valves, transfusion tubings, sheathing for nerve
repairs, and padding for plastic surgery.
k. As an excellent adhesive.
l. Polysulphide rubber is used as a „solid propellant fuel‟ for rocket motors.
m. Also used for making toys, sport goods, head bands for helmets and goggles,
etc.

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