Vulcanization of rubber

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. F inally 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 P hosphite 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 , MgCO 3 , BaSO 4 and CaCO 3 .

7. 6. Colouring agents: They impart desired colour to the rubber as follows Lithophane , TiO 2 - White Ferric oxide - Red Lead chromate - Yellow Chromium trioxide - Green Caron black - Black 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: Vulcanised rubber has good tensile strength and load bearing capacity. Vulcanised rubber has good elasticity depending on the extent of vulcanization. It possess low water absorbance tendency. Vulcanised rubber has higher resistance to oxidation and atmospheric gases like O 2 , CO 2 , moisture, light and U.V. radiation. Vulcanised rubber is a good electrical insulator and ebonite is best electrical insulator. The abrasion resistance of natural rubber increases tremendously.

10. The resistance towards organic solvents and chemicals increases. Vulcanised rubber possess good impact resistance. 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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