Elastomer commonly known as rubber is a linear polymer which possesses elasticity and good resistance to corrosive fluids.
The properties of rubber depend upon the type of fillers and adhesives used.
Properties/Characteristics of Rubber:
Rubber finds a wide field of application because of the following properties:
1. It is elastic; because of this property a rubber band can be stretched to 9 or 10 times its original length and when the load is removed it regains its original length.
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2. It is strong and tough. Because of this property it can be put to use even under abnormal conditions.
3. It is highly impermeable to both water and air and therefore, it can be used to retain water as in rubber bottles, hoses etc.
4. It exhibits a great resistance to abrasion, tearing and cutting over a wide range of temperature—7 to 115°C.
5. It is a bad conductor of heat.
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6. It can contain liquids and gases.
7. The synthetic rubber offers great resistance to acids, petroleum products etc.
9. Its properties such as hardness, strength, abrasion, resistance etc., can be modified to the desired extent by compounding techniques.
9. When two fresh surfaces of milled rubber are pressed together, they coalesce to form a single piece. This property (known as tackiness) of rubber makes the manufacturing of composite articles such as a tyre, from a separate piece very simple.
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10. The plasticity of rubber makes it pliable and amenable to all manufacturing processes.
11. By vulcanizing the rubber, its mechanical properties can be considerably improved.
12. Rubber insulation and other rubber products require shaping prior to vulcanisation because the vulcanised rubber cannot be shaped by mechanical pressing once it has acquired its characteristic plasticity. By vulcanising rubber in moulds, products of rather intricate shape can be obtained.
13. The electrical insulating properties of pure rubber are inferior to those of vulcanised rubbers.
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14. Ordinary electrical insulating rubbers, have the following electrical characteristics under normal conditions –
Volume resistivity- 1 x 1014 to 1 x 1015 ohm cm.
Loss tangent- 0.01 to 0.03 at 50 Hz.
Dielectric Constant: 2.5 to 5.
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15. The electric strength of organic rubbers strongly depends on the kind of current involved, the degree of stretch, and the time during which the voltage remains applied. When left unstretched and subjected to a short-time 50 Hz test voltage, rubber will have an electric strength within the following limits, -depending on the pure rubber constant.
For a 20 to 25% rubber content: 20 to 30 kV/mm
For 30 to 35% rubber content: 30 to 45 kV/mm
The D.C. electric strength of rubber is 2 to 2.5 times the electric strength at 50 Hz.
16. Although rubber is practically water and gas tight its electrical characteristics are affected by moisture, especially for rubbers, compounded with considerable quantities of the substances which increase the sensitivity to moisture. Only specially compounded rubbers can maintain their electrical characteristics nearly unchanged when kept continuously in contact with moisture.
17. For normal rubbers the maximum operating temperature is usually considered to be 55°C, for rubbers of great heat resistance it is equal to 65°C. For butyl rubbers the working temperature can be as high as 90°C. 9.2.3.
Uses of Rubber:
The following are some of the uses of rubber:
1. Thermal insulation
2. Lining of reservoir
3. Rain water and flexible tubing
4. Belting of all types
5. Tyres and tubes
6. Adhesive
7. Hose pipes, printing roller etc.
9. Mounting material
9. As a buffer
10. Gasketina material.
Types of Rubber:
There are generally two types of rubber:
1. Natural rubber
1. Natural Rubber:
Natural rubber is an elastic material present in the latex of certain plants. More than 95 per cent of this rubber is obtained from the latex of rubber trees. Hevea prasiliensis, grows on plantations, mainly in Sri Lanka and Malaya Peninsula. Small quantities of rubber are produced in Brazil from uncultivated trees and from the guagle shrub in Mexico and south western United States.
Latex is a milky colloidal fluid containing from 30 to 40 percent of the rubber, the remaining being mainly water and a small amount of protein and resinous material. It oozes from superficial cuts in rubber trees and is collected in containers. A plantation grown tree continues to yield for about 40 years and give enough latex to make from 1.5 kg to 3 kg of rubber each year.
Latex is treated in two ways to obtain rubber goods. The crude rubber is either co-agulated from it by acids of heat and then processed; or the latex itself is mixed with appropriate compounding materials and then precipitated directly with appropriate compounding materials and then precipitated directly from solution in the shape to be used, such as rubber glove.
The recovery of rubber from guayule is carried out in different manner. The entire plant, which may contain as much as 20 percent by weight of rubber, is harvested after 4 years growth, ground up and soaked in water. The latex comes at the surface and is skinned off; the woody material becomes water logged and sink to the bottom. The resin content of guayule rubber is 18 to 20 percent compared to about 4 percent for hevea rubber. This resin may be extracted by means of solvents and can be recovered and utilized.
Natural rubber is a polymerised from of isoprene (2-methyl-l, 3-butadiene). The polyhydrocarbon chain consists of 2000-3000 monomer links. The polymerisation occurs by a biochemical reaction in which a particular type of enzyme acts as a catalytic agent.
It is usually obtained from latex by coagulation with organic acids, washing and coagulum with water as it passes between rolls, and finally drying the washed sheet as it comes from the rolls.
i. If the rough rolls rotating at high speeds are used and the rubber is then hung in the air to dry, the product is known as pale “crepe rubber”.
ii. If smooth rolls rotating at the same speed are used and the rubber is then dried in a smoke house, the product is known as smoke sheets. It is tougher than pale crepe.
Crude rubber is tough, strong, elastic substance made up of 92% or more of a hydrocarbon chain polymer (C5H8).
Crude rubber does not possess the properties that cause the familiar manufactured articles to be considered so valuable. At the temperature of very hot summer weather, pure rubber becomes soft and sticky, during very cold weather it becomes hard and brittle. In either of these conditions, it is useless for the purposes to which it is commonly put. However its properties are greatly improved by addition of other materials followed by suitable heat or other treatment.
Physical Properties of Crude Natural Rubber:
The physical properties of crude natural rubber are indicated below:
1. At low temperature rubber becomes stiff and when it frozen, it attains fibrous structure.
2. Raw rubber when heated to 130°C becomes soft and plastic. The plasticity can be varied within certain ranges by chemicals.
3. Co-efficient of cubical expansion is 670 x 10-8. When rubber is extended heat is produced. This is called Joule’s effect. When rubber is stretched to 82% it generates 680 calories/gm of heat.
4. Heat of combustion of raw rubber is 10547 calories/gm.
5. Specific gravity of raw rubber at 0°C is 0.95 and 0.934 at 20°C.
6. Specific heat of raw rubber at room temperature is 0.502.
2. Synthetic Rubber:
Rubber produced by artificial chemical process is known as “synthetic rubber”.
Synthetic rubbers, or “elastomes” are derived from such raw materials as coke, limestone, petroleum, natural gas, salt, alcohol, sulphur, ammonia and coal tar.
Elastomers are not strictly speaking synthetic rubber, for rubber has never been synthesized. They are rubber like materials which have many of the characteristic properties of rubber and some of which resemble rubber in their chemical nature.
The processing of synthetic rubbers involves approximately the same steps and equipments as that of crude rubber. Their properties while similar to those of rubber, are capable of wider variation. Some elastomers are more resistant that rubber to sunlight, others have greater solvent resistance; and some have greater elasticity.
Forms of Rubber:
The important forms of rubber are described below:
1. Gutta Percha Rubber:
It is a variety of natural rubber, prepared from the leaves of tress known as the dichopsis gutta and palaginum gutta (mostly grown in Malaya peninsula).
It becomes soft and sticky at a temperature of about 100°C.
As compared to other varieties of rubber, it absorbs less water.
Uses:
It is extensively used for preparing ropes of submarine and as an insulating material in electrical works.
2. Foam Rubber:
It is prepared by adding the chemically producing gases in the liquid latex and stirring the mixture till foam is formed. It is then converted into solid foam and is given the desired shape.
Uses:
It is wisely used for packing pads, pillows etc.
3. Sponge Rubber:
It is prepared by adding sodium bicarbonate during the process of vulcanisation. The evaporation of moisture leaves pores which result in sponge rubber.
Uses:
It is used as a heat and sound insulating material.
4. Guayle Rubber:
It is a variety of natural rubber available in North America.
It is prepared from the branches of guayule,
It contains 70 percent of hydrocarbon, 20 percent of resin, 10 percent of insoluble materials, cellulose, liquid etc.
5. Smoked Rubber:
It is a variety of crude rubber.
The rubber pieces after coagulation are dried in room filled with smoke at a temperature of about 40°C to 50°C.
It is so named as drying is carried out in a smoke room.
6. Polybutadiene Rubber:
It is a variety of synthetic rubber (produced by The Indian Petrochemicals Corporation Ltd., near Baroda); commercially known as Cisrub.
It has high abrasion resistance and strength.
Uses:
It is nicely used in producing beltings, floor tiles, automotive moulded goods, hoses, tyres, seals, gaskets etc.
Plasticization:
In order to make crude rubber workable, it is kneaded to a plastic mass by passing it through heated rollers which travel towards each other at different speeds. By this process the rubber is subjected to both compression and shearing stresses which cause the rubber to twist, crack or tear and become plastic. Other ingredients can also be added into the rubber mass during the process.
Compounding:
The rubber available in plastic state is compounded with other ingredients to modify its properties.
A number of different substances which can be mixed with rubber to give it definite properties are classified as follows:
(i) Vulcanates:
These are the substances (e.g., sulphur etc.) which are added to crude rubber to reduce its plasticity while maintaining its elasticity.
These are essentially required for carrying out the process of vulcanization.
(ii) Plasticizers:
These are the materials (e.g., vegetable oils, waxes, stearic acid, etc.) which are used to soften the crude rubber.
(iii) Accelerators:
These are the substances (e.g., lime, magnesia, litharge, white lead etc.) which are used to reduce the time required for the completion of the process of vulcanization.
With their use, the quantity of sulphur required for vulcanization is also reduced.
(iv) Antioxidants:
These are the materials (e.g., phenols, amines, waxes etc.) which when added in small quantities to rubber decrease the rate of its deterioration by agencies like light, heat and by the presence of copper or manganese, salts or soaps etc.
(v) Fillers, Reinforcing Agents and Colouring Agents:
The addition of fillers is made to modify the properties of rubber and to lower the cost of rubber product.
“A reinforcing agent “is one which when added to rubber increases its tensile strength. Materials like zinc oxide and carbon black serve both as fillers and reinforcing agents. Some other reinforcing agents are magnesium carbonate, calcium carbonate and barium sulphate.
“Colouring agents” are added to obtain the rubber in desired shade.
Example:
Ferric oxide—red colour; “Lithophone”—white colour; “Lead chromate”—yellow colour etc.
Calendering:
A calendering machine is used to obtain rubber in the form of sheets of definite thickness. The rubber mass is made to pass between large steel rollers of the machine, at carefully controlled temperature and pressure. The rollers press the rubber mass into sheets of uniform thickness and desired finish.
Vulcanisation:
The crosslinking process in elastomers is called vulcanization, which is achieved by a non-reversible chemical reaction, ordinarily carried out at an elevated temperature.
In most vulcanization reactions, sulphur compounds are added to the heated elastomer; sulphur atoms bond with adjacent chains and crosslinks them.
Useful rubbers result when about 1 to 5 parts (by weight) of sulphur is added to 100 parts of rubber. Increasing the sulphur content further hardness the rubber and also reduces its extensibility.
Unvulcanized rubber is soft and tacky, and has poor resistance to abrasion. Vulcanization enhances modulus of elasticity, tensile strength and resistance to gradation. The magnitude of modulus of elasticity is directly proportional to the density of the crosslinks.
Vulcanization normally results in increased elasticity and strength, increased durability and increased resistance to adverse effect of weather and chemical agents.
Vulcanized rubber has low electrical and thermal conductivity and is resistant to abrasion and action of chemical reagents and weather.
The process of vulcanization, in the manufacture of rubber goods, is carried out after calendering and extrusion. When the rubber goods are to be manufactured by the process of moulding, vulcanization is performed simultaneously with moulding.
Fabricating operations and lathe cutting are performed after the vulcanization process.
Extruding:
By this process rubber goods of constant cross-section like tubes, insulation on wire etc. can be manufactured.
In this process, this rubber compound is fed into the extrusion die by means of a screw fed and’ the extruded products are either coiled or laid in lengths in large flat trays and vulcanized in open stem heaters.
Moulding:
In this process the rubber is forced into the mould (of heavy metallic construction) under high pressure (rubber is not melted and poured like metal).
For getting non-porous uniform product, the process of moulding is performed in heated moulds in which the rubber compound is forced under high pressure.
Reclaimed Rubber:
Reclaimed rubber is the rubber recovered from worn-out rubber articles and rubber wastes from factories and then treated suitably for re-use.
For reclaiming rubber, the metal and fabric contents of the worn-out rubber are first removed and then it is heated in a closed iron vessel which contains an alkali solution. This treatment makes the rubber free from the remaining fabric contents and free sulphur. The treated rubber is washed and dried.
1. It is durable.
2. It is cheap.
3. Can be prepared speedily.
4. Has a uniform composition.
1. Elasticity is low.
2. Low resistance to friction.
3. Low tensile strength.
Uses:
It may be used for making the articles like shoe soles and heels, water-proof footwear, electrical goods etc.