0.8 LO 1.2 Trichlorodiphenyl
4.7.6 Rubber
Rubber is a natural or synthetic vulcanizable high polymer having high elastic properties. Electrical properties of rubber depend on the degree of compounding and vulcanizing. General impurities, chemical changes due to ageing, moisture content and variations in temperature and frequency have substantial effects on the electrical properties of rubber. Some important electrical properties and applications of dif- ferent types of rubber are given in Table 4.8.
Table 4.8 Properties and Applications of Rubber Type of
rubber
Natural rubber
Polylsar kryflex rubber and Styrene butadine rubber Butyl rubber
and Polysar butyl rubber
Water absorp- tion (per cent) (M.8
(M.5
0-2
tr
(50 Hz)
2.9-6.6
3.8-6.2
2.2-3.2
tanS (50 Hz)
0.02-0,1
0.02-0.09
0.003-0.03 a.c.
break- down voltage
kV/cm 100-390
80-380
80-200
Application and limitations
Inexpensive, flexible, gpod elec- trical properties, resistant to co- rona, maximum operating tem- perature 6O0C.
Used for low voltage (<11 kV) cables. Not gpod under weather- ing, water, oils, etc.
Used in aerial, submarine and underground cables. Good electrical properties at low tempe- ratures. Excellent resistance to tear, abrasion, acids, alkalies and chemicals.
(Contd.)
Type of rubber
Silicone rubber
Water absorp-
tion (per cent)
0-3
Sr
(50 Hz)
2.6-3.4
tanS (50 Hz)
0.006-0.02 a.c.
break- down voltage
kV/cm 90-390
Application and limitations
Used in shipping and aircraft cables, transformers, lightning equipment, etc. High service temperature (15O0C). Easily attacked by acids, alkalies and chlorinated compounds.
4.7.7 Plastics
Plastics are very widely used as insulating materials because of their excellent dielectric properties. Many new developments in electrical engineering and electronics would not have been possible without the development of plastics. Plastics are made by combining large numbers of small molecules into a few big ones. When small molecules link to form the bigger molecules of the plastics, many different types of structures result Most thermoplastic resins approximate to a structure in which several thousand atoms are tied together in one direction. The thermosetting resins on the other hand, form a three-dimensional network. In view of the large number of plastics available, it will not be possible to deal with all of them, and only materials which are commonly used for insulation purposes are described.
Polyethylene
Polyethylene is a thermoplastic material which combines unusual electrical properties, high resistance to moisture and chemicals, easy processability, and low cost. It has got high resistivity and good dielectric properties at high frequencies, and therefore, is widely used for power and coaxial cables, telephone cables, multi-conductor control cables, TV lead-in wires, etc.
Table 4.9 Electrical Properties of Polyethylene Property
Volume resistivity (ohm-cm) Dielectric strength
(kV/cm) Dielectric constant
(50 Hz-IO6 Hz) tan
(50 Hz-IO6 Hz) Arc resistance
Low density polyethylene
>1016 170-280
2.3 0.0002
melts
Medium density polyethylene
>1016 200-280
2.3 0.0002
melts
High density polyethylene
> 1016 180-240
235 0.0002
melts
Irradiated polyethylene
>1016
720-1000 2.3 0.0005
melts
By varying the methods of manufacture different types of polyethylene are made with specific properties for different applications. They may have low density, medium density, high density or may be irradiated types. The dielectric prbperties of these are summarized in Table 4.9.
Fluorocarbon Plastics
Polytetrafluoroethylene (P.T.F.E.), polychlorotrifluoroethylene (P.C.T.F.E.) and polyvinylidene (P-VJ2) plastics come under this category. P.T.F.E. is the most thermally stable and chemically resistant of all the three. It is considered as one of the best plastics used for insulation because of its excellent electrical and mechanical properties. It can be used without decomposition up to temperatures of 3270C. It is widely used in almost all applications. P.C.T.F.E. has higher dielectric constant and loss factor than P.T.F.E., but melts at 19O0C. P.V.F2 can be worked in the temperature range -30 0C to 15O0C. It is used as thin wall insulation, as jacketing for computer wires and special control wires, and for tubing and sleeving for capacitors, resistors, terminal junctions, and solder sleeves. The electrical properties of fluorocarbons are tabulated in Table 4.10.
Table 4.10 Properties of Fluorocarbon Plastics
Property P.T.F.E P.C.T.F.E. P.V.F2
Volume resistivity > jQ18 1.2 x 101^ 2 x 1014 (ohm-cm)
Dielectric strength 200 210 104-512 (kV/cm)
Dielectric constant 2.0 2.3-2.8 8.4-6.49 (50 Hz-IO6Hz)
tan 8 < 0.0002 0.0012-0.0036 0.0491-0.15 (50 Hz-IO6 Hz)
Nylon
Nylon is a thermoplastic which possesses high impact, tensile, and flexural strengths over a wide range of temperature (O to 30O0C). It also has high dielectric strength and good surface and volume resistivities even after lengthy exposure to high humidity.
It is also resistant to chemical action, and can be easily moulded, extruded and machined. It is generally recommended for high frequency low loss applications. In electrical engineering, nylon mouldings are used to make coil forms, fasteners, connectors, washers, cable clamps, switch housings, etc.
There are three different types of nylon commonly used. They are nylon 6/6, nylon 6 and nylon 6/10. The dielectric properties of these three types are given in Table 4.11.
Table 4.11 Dielectric Properties of Nylon
Property Nylon 6/6 Nylon 6 Nylon 6/10 Volume resistivity 4.5 x io13 1013 x 1015 4. x 1014
(ohm-cm)
Dielectric strength 154 176-204 190 (kV/cm)
Dielectric constant
50 Hz 4.1 5.0-14.0 4.6
IO3 Hz 4-° 4.9-10.1 4.5
!O6Hz 3.4 4.0-4.7 3.5
tan 6
50 Hz 0.014 0.06-0.10 0.04 1()3 Hz 0.02 0.06-0.10 0.04
IO6 Hz O-04 0.04-0.13 0.03
Polyvinyl Chloride
Polyvinyl chloride or P. V.C. is used commercially in various forms. It is available as an unplasticized, tough, and rigid sheet material and can be easily shaped to any required form. It is chemically resistant to strong acids and alkalis and is insoluble in water, alcohol and organic solvents like benzene. The upper temperature limit of operation is about 6O0C. The dielectric strength, volume resistivity and surface resistivity are relatively high. The dielectric constant and loss tangent are 3.0-3.3 and 0.015-0.02 respectively, at all frequencies up to 1 MHz.
P.V.C. is also available as a highly plasticized flexible material, which is used extensively for wire covering, insulated sleeving, and cable sheathing in preference to natural rubber because of its resistance to the action of sunlight, water and oxygen.
Polyesters
Polyesters have excellent dielectric properties and superior surface hardness, and are highly resistant to most chemicals. They represent a whole family of thermosetting plastics produced by the condensation of dicaiboxylic acids and dihydric alcohols, and are classified as either saturated or unsaturated types. Unsaturated polyesters are used in glass laminates and glass fibre reinforced mouldings, both of which are widely used for making small electrical components to very large structures. Saturated polyesters are used in producing fibres and film. Polyester fibre is used to make paper, mat and cloth for electrical applications. The film is used for insulating wires and cables in motors, capacitors and transformers. The dielectric properties of polyester compounds are given in Table 4.12.
Table 4.12 Dielectric Properties of Polyesters
Property Glass reinforced type Cast resins Premixed Preformed Rigid Flexible Volume resistivity 1012-1015 IO14 1012-1014 —
(ohm-cm) Dielectric constant
50 Hz 5.3-73 3.8-6.0 3.3-4.3 4.4-8.1 IO3 Hz 4-68 4-G-6-0 3.2-4.3 4.5-7.1 IO6 Hz 5.6-6A 3.5-5.5 3.2-4.3 4.1-5.7 tan S
50 Hz 0.01-0.04 0.01-0.04 0.006-0.05 0.026-0.031 IO3 Hz — 0.01-0.05 0.006-0.04 0.016-0.050 IO6 Hz 0.008-0.022 0.01-0.03 0.017-0.019 0.020-0.060 Mylor polyester film is being largely used in preference to paper insulation. At power frequencies, its dissipation factor is very low, and it decreases as the tempera- ture increases. It has got a dielectric strength of 2000 kV/cmf and its volume resistivity is better than IO15 ohm-cm at 10O0C. Its high softening point enables it to be used at temperatures above the operating limit of paper insulation. It has got high resistance to weathering and can be buried under the soil also. Therefore, this can be used for motor and transformer insulation at power frequencies and also for high frequency applications which are subjected to varying weather conditions.
Polystyrenes
Polystyrenes are obtained when styrene is polymerized with itself or with other polymers or monomers producing a variety of thermoplastic materials with varying properties in different colours. Electrical grade polystyrenes have a dielectric strength comparable to that of mica, and have low dielectric losses which are independent of the frequency. Their volume resistivity is about IO19 ohm-cm and the dielectric strength is 200-350 kV/cm. The dielectric constant at 2O0C is 2.55, and the loss tangent is 0.0002 at all frequencies up to 10,000 MHz.
Polystyrene films are extensively used in the manufacture of low loss capacitors, which will have a very stable capacitance and extremely high insulation resistance.
Films and drawn threads of polystyrene are also used for high frequency and cable insulations.