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Friday, 4 October 2019

Classification of Rubber Materials

Classification of Rubber :

Rubber is classified into two main groups

(a) Natural Rubber
(b) Synthetic Rubber

Natural rubber is obtained from the latex of source trees. It is soft and plastic but its insulation resistance fails rapidly with increase in temperature. Its mechanical properties also deteriorate with temperature. Unless treated properly it is of not used as insulator in Electrical field. Synthetic rubber is obtained by Copolymerization of isobutylene and isoprene and is tough but its electrical properties are inferior to natural rubber. The Classification of Rubber Materials are explained below.


Natural Rubber Materials:

It is extracted from milky sap collected from special trees and so it is of botanical origin. Water is then evaporated and then further masticated by passing it through warm rollers repeatedly. Additives like sulphur, oxidation inhibitors like auromatic amino compound, softeners like vegetable oils and fillers like carbon black and zinc oxide or magnesium carbonate are mixed in it. The mixture is then ready for vulcanisation when sulphur becomes 4 to 45% depending on the requirements. The main disadvantages are :

(a) Low resistance to petroleum oils.
(b) Cannot be exposed to sunlight.
(c) Cannot be used for high voltage insulation.
(d) Low operating temperatures as it becomes brittle and cracks are developed at a temperature of 60°C.
(e) Sulphur in rubber reacts with copper hence copper wires are to be tinned.

If sulphur is added to an extent of 30% vulcanite or ebonite is obtained which is a hand material with excellent properties. The operating temperature is about 50°C. Pure rubber is hygroscopic in nature and melts at about 250°C. It gets softened if heated beyond 160°C.

Hard Rubber:

Increased sulphur content and prolonged vulcanisation yields a rigid rubber product called hard rubber or ebonite. It possesses good electrical and mechanical properties. It is used for battery containers, A- boards, bushings, ebonite tubers and various other parts.

Synthetic Rubber :

Copolymerization of isobutylene and isoprene gives synthetic rubbers. There are two types of rubbers available. Styrene Butadiene and Nitrile Butadiene. These rubbers possess the following properties.

(a) They are tough and solvent resistant in mineral insulating oils.
(b) Its electrical properties are inferior to natural rubber.
(c) They offer large resistance to oxidation. The outstanding features of Butyl rubber is superiority to natural rubber in resistance to ageing and in chemical resistance. Its tensile strength is poorer than that of natural rubber and has relatively low permeability for gases. It possesses high resistance to ozone and thermally stable. It is easily attacked by petroleum oils and greases, but offers excellent resistance to vegetable oils and alcoholic solvents. It consists of a major proportion of isobutylene and minor proportion of isoprene butadiene. Styrene butadiene and Nitrile Butadiene are also of this category.

Chloroprene Rubber (Neoprene):

It is prepared by the emulsion polymerisation of chloroprene with or without the present of a copolymer. As compared to natural rubber, they are more flame resistant and have better heat conductivity.

They exhibit better adhesion to metals. Though they possess better resistance to sunlight, oxidation, gas diffusion and thermal ageing, they have inferior mechanical properties like tear, tensile strength and abrasion resistance. When attacked by solvents they possess better resistance. They are used as insulation for wires and conductors.

Silicon Rubber :

Silicon rubbers are polysiloxanes. They have flexibility at low temperatures. Their electrical properties and thermal conductivity is good but they have low tensile strength. Exceptional resistance to ozone, water, oil, oxidation and severe atmospheric condition are exhibited. Their operating temperatures range between 60°C to 150°C. Silicon rubber is used as insulation for electric wires and cables, in the manufacture of moulded parts as an insulating tape and coating material. Table 8.1 gives the properties of different types of rubber products and Table 8.2 gives the general properties and applications.

Important Properties of Rubber Materials

Natural Rubber
Hard Rubber
Butyl Rubber
Nitrite Butadience
1. Max service Temperature (°C)
80 - 90
175 - 200
2. Water absorption
1 - 2
0.2 - 1.0
0.3 - 0.5
3 - 4
0.5 - 2.4
3. Specific gravity
0.93 - 1.6
1.1 - 1.4
0.9 -1.5
1.09 - 1.6
1.2 - 1.5
4. Tensile strength (kg / cm2 x 10-3)
5. Dielectric constant (at 106 Hz)
2.7 - 5
2 .1 - 4.0
7.5 - 14
3.9 - 10 2
2.8 - 7 0
6. Dielectric strength (kV/mm)
18 - 24
12 - 28
16 - 32
16 - 20
12 - 28
7. Power factor (at 106 Hz)
0.05 – 2.0 x 10-3
0.5 - 2.0 x 10-2

0.3 - 8.0
1.0 – 6.0 x 10-2
3 – 5 x 10-2
0.1 - 1.0 10-2
8. Volume resistance (Ohm-cm)
9. Surface resistance (ohm)

Properties and Applications of Different Rubber Materials :

1. Nitrile Butadiene Rubber
Good resilence wear resistance, flexibility at low temperature, resistance to ageing, oxidation, lower tensile strength, higher thermal conductivity, lower hygroscopicity.
2. Butyl Rubber
It is attacked by Petroleum oils and gases and alcoholic solvents. It has thermal and oxidation stability. High resistance to Ozone.
Used as insulation in hot and wet conductions, used as tapes in repair work.
3. Hypalon Rubber
Resistance to deterioration when exposed to sunlight and temperature (upto 150°C).
Used in jacketing of electric wires and cable.
4. Neoprene Rubber
Better resistance to ageing, oxidation and gas diffusion, better thermal conductivity and flame resistance, poor mechanical properties.
Used for wire insulation and cable sheathing.
5. Silicon Rubber
High operating temperature (200°C), flexibility, moisture and corrosion resistance, to oxidation, ozone, arcing good insulating properties and thermal conductivity even in the form of ash it is a good insulator.
Insulation for cables of power and control wires of blast furnace, coke ovens, steel mills and nuclear power stations for high frequency generators. boiler, airport, lighting cranes.

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