In this article we will discuss about:- 1. Types of Rubber 2. Compounding of Rubber 3. Properties 4. Uses 5. Forms 6. Reclaimed Rubber.
Types of Rubber:
The rubber is of the following two types:
(1) Natural rubber;
(2) Synthetic rubber
(1) Natural Rubber:
This variety of rubber is obtained from latex or a viscous milky juice tapped from rubber trees. These trees grow in hot moist climate in a region about 12 degrees on either side of the equator and they are abundant in countries like Ceylon, Malaya, North Africa, Mexico, Singapore, etc. A rubber is a fast growing large tree.
The natural rubber has high strength, low hysteresis and good resistance to tear and flex cracking. But it is easily affected by gasolines and solvents.
The process of obtaining natural rubber is carried out in the following four stages:
(ii) Tapping latex
The rubber trees are planted. They start oozing out after a period of about 5 to 7 years and continue to ooze out for a life of about 30 years. A normal rubber tree gives about 10 N to 15 N of dry rubber per year.
(ii) Tapping Latex:
A vertical groove about 600 mm long is made in the rubber tree by means of a pointed knife at a height of about 1 metre from the ground level. The vertical groove is connected to slanting grooves inclined upwards as shown in fig. 17-6. An earthenware pot is attached at the lower end of vertical groove to collect the latex that oozes out of the tree.
The latex from the pot is taken by the tapper every day or every alternate day and at that time, the grooves are slightly widened. The care should be taken to see that the grooves do not penetrate beyond the cambium layer of the tree.
The impurities such as leaves, sand, debris, etc. which are present in fresh latex are then removed.
The latex is then converted into a massive form by the process of coagulation. The coagulant usually adopted is weak acetic acid. The addition of coagulant converts latex into curd like form. It is then passed through rollers. The water from latex is thus separated out and the natural rubber is obtained in the form of globules or sheets.
(2) Synthetic Rubber:
The term synthetic or artificial rubber was conceived through research and development as a polymer which could be used to replace the natural rubber. Even as early as 1940, there were several polymers on the market which could be used as replacements for the natural rubber in some applications. The real beginning of a synthetic rubber industry then started in U.S.A., Germany and other foreign countries.
The synthetic rubbers can be grouped in the following two categories:
(i) General Purpose Synthetic Rubbers:
These rubbers are used freely in wide ranging applications. The most important all-purpose rubber was then and still is a styrene-butadiene rubber. It was called earlier in U.S.A. and Canada as GR-S and in Germany as Buna-S.
However it is now referred to as SBR all over the world and it forms nearly 50 per cent of the world production capacity of all types of the synthetic rubbers. The other synthetic rubbers in this category are Polybutadiene Rubber (P.B.R.), Polyisoprene Rubber (P.I.R.), etc.
(ii) Special Purpose Synthetic Rubbers:
These rubbers are developed with a special end use in view and they include Butyl, Nitrile, Chloroprene, EDPM, Acrylic, Polysulphide, Silicone, Viton, etc. For instance, Butyl rubber is used for the air impermeability.
The world production of the synthetic rubber is shared by the different countries as follows:
It may be noted that India accounts for only about 0.5% of the total world production of the synthetic rubber.
The per capita consumption of the synthetic rubber for India is also very low. It is only 1 N as against 80 N to 110 N for Japan, U.S.A., Belgium, etc. and 27 N for the world as a whole. However the rubber consumption for India is growing much faster than the world average which will help to reduce the lag in per capita consumption.
The first synthetic rubber plant of India, Synthetics and Chemicals Ltd., was set up at Bareilly in U.P. in 1961. It is manufacturing S.B.R. The other two plants – The Indian Petrochemicals Corporation Ltd. and Apar Ltd. – have been subsequently installed.
To give desired properties to the rubber, certain compounds are to be added to the rubber.
The usual compounds which are used for natural rubber and synthetic rubber are as follows:
(2) Accelerator activators
(8) Vulcanizing agents.
The addition of accelerators decreases the period of preparation of rubber and the properties of rubber are also considerably improved.
(2) Accelerator Activators:
The composition of rubber is greatly improved by addition of accelerator activators such as zinc oxide, fatty acids, etc.
The addition of anti-oxidants results in the lengthening of the oxidation process of rubber. These substances thus act as the negative catalysts. The usual anti-oxidants are wax, phenols, phosphates, etc.
The fillers work as inert materials and they considerably increase the rigidity and strength of rubber. The usual inert fillers are cotton, carbon, etc.
These compounds are added to increase the tensile strength and hardness of rubber. When hardeners are added, the manufacture of rubber at high temperatures can be carried out smoothly. The usual hardeners are barium sulphate, calcium carbonate, magnesium carbonate, sealing wax, etc.
The addition of pigments helps in two ways – they act as fillers and they impart colour to the rubber. Table 17-6 shows the pigments to be used to get different colours.
These compounds are added to improve plasticity and to impart softness to the rubber. The commonly used plasticizers are wax, resin, vegetable oil, etc.
(8) Vulcanizing Agents:
The process of vulcanization considerably improves the important properties of rubber. The sulphur is the most commonly used vulcanizing agent.
The important properties of rubber are as follows:
(i) It can absorb shocks due to impact.
(ii) It can contain liquids and gases.
(iii) It creeps or extends or undergoes slow deformation in length as a result of applied force or stress.
(iv) It is a bad conductor of heat.
(v) It is plastic in nature and hence it can be moulded to the desired shapes.
(vi) It is possible to alter considerably its properties by the processes of vulcanizing and compounding.
(vii) It possesses the quality of flexibility.
(viii) It resists abrasion in a better way.
(ix) The natural rubber should be protected from sunlight and should not be allowed to come in close contact with oils, organic liquids, etc.
(x) The outstanding property of rubber is that it is capable to undergo great deformation without being structurally damaged. It is thus reasonably elastic.
(xi) The synthetic rubber offers great resistance to acids, petroleum products, etc.
The synthetic rubbers together with the natural rubber are the basic raw materials of the rubber industry. They are used in the manufacture of more than 35000 individual rubber items ranging from giant and off the road automobile tyres to the tiniest of the rubber parts like bottle caps, balloons, diodes, etc.
The important engineering uses to which rubber is put up are as follows:
(i) It is used as a gasket to make doors and windows air tight as in case of refrigerators, vehicles, air-conditioned rooms, etc.
(ii) It is used as a lining material for parts of machines subjected to heavy friction and for tanks to be used in the chemical processes.
(iii) It is used for preparing tyres of vehicles and machines.
(iv) It is used to prepare rubber threads which are useful as wires, ropes, etc.
(v) It is widely used for absorbing shocks and for reducing vibrations in machines.
(vi) The synthetic rubber, in particular, may be used for hose-pipes to carry petrol and kerosene oils, gaskets, insulation for high tension wires, etc.
On an average, the consumption of rubber for various items can be expressed as follows:
Forms of Rubber:
The important forms of rubber are as follows:
(1) Crepe rubber
(2) Foam rubber
(3) Guayule rubber
(4) Gutta percha rubber
(5) Polybutadiene rubber
(6) Smoked rubber
(7) Sponge rubber.
(1) Crepe Rubber:
This variety is one form of crude rubber. During the process of coagulation, the certain chemicals are added and the rubber is allowed to pass through many rollers. This rubber has irregular rough surfaces and hence it is known as the crepe rubber.
(2) Foam Rubber:
In the liquid latex, the chemicals producing gases are added and the mixture is well stirred till foam is formed. It is then converted into solid form and it is given the desired shape. The foam rubber is widely used for pillows, packing pads, etc.
(3) Guayule Rubber:
This type of rubber is a variety of natural rubber and it is available in North America. It is prepared from the branches of guayule. It contains 70% of hydro-carbon, 20% of resin, 10% of insoluble materials, cellulose, lignin, etc.
(4) Gutta Percha Rubber:
This type of rubber is a variety of natural rubber and it is prepared from the leaves of trees known as the dichopsis gutta and palaguium gutta. These trees mostly grow in Malaya peninsula. It becomes soft and sticky at a temperature of about 100°C. It absorbs less water as compared to other varieties of rubber. It is the best material for preparing ropes of submarine and as an insulating material in electrical works.
(5) Polybutadiene Rubber:
This is one type of synthetic rubber which is produced by The Indian Petrochemicals Corporation Ltd. (IPCL) near Baroda. The polybutadiene rubber produced by the IPCL has been given the commercial name of Cisrub.
It is used in a wide variety of applications especially those which require high abrasion resistance and strength. The main areas of application of this material are automotive moulded goods, beltings, engineering moulded goods, floor tiles, footwear, gaskets, hoses, seals, tyres, etc.
(6) Smoked Rubber:
This variety is one form of crude rubber. After coagulation, the rubber pieces are dried in room filled with smoke at a temperature of about 40°C to 50°C. As drying is carried out in a smoke-room, it is known as the smoked rubber.
(7) Sponge Rubber:
This rubber is prepared by adding sodium bicarbonate during the process of vulcanization. The evaporation of moisture leaves pores which result in sponge rubber. It is a good insulating material for heat and sound.
The important quality of rubber is that it can be reclaimed or in other words, it possesses heavy scrap value. The worn out rubber articles are cut into small pieces which are then ground into powder form. The metallic material is removed from the powder with the help of magnets. The required ingredients are then added in the powder and the crude rubber is then prepared.
Advantages of Reclaimed Rubber:
Following are the advantages of the reclaimed rubber:
(i) It can be prepared speedily.
(ii) It is cheap as cost of raw materials is low.
(iii) It is durable.
(iv) Its composition is uniform.
(v) The heat produced during the manufacture of reclaimed rubber is less than that produced in the manufacture of new rubber.
Disadvantages of Reclaimed Rubber:
Following are the disadvantages of the reclaimed rubber:
(i) It has low tensile strength.
(ii) Its elasticity is low.
(iii) Its resistance to the friction is low.
Uses of Reclaimed Rubber:
It is widely used for preparing rubber articles which are required for unimportant situations. It is used for mechanical equipment, tyres for vehicles, hose-pipes, etc.