Electrical Properties

The electrical properties of elastomers most commonly measured are as follows:
  1. Resistivity
  2. Dielectric Strength
  3. Dielectric Constant
  4. Power Factor

Materials suitable for electrical insulators have high dielectric strength and resistivity and low dielectric constant and power factor. Such a combination is rarely obtained since companion physical and chemical properties are also required. There is no single best overall material, but urethane offers a favorable combination of properties.

Direct Current Resistivity

Direct current resistivity, or volume resistivity, is similar to insulation resistance in that both indicate the magnitude of electrical current expected to pass through the sample located between two electrodes. This test may be used to check the suitability of an elastomeric composition for electrical insulation.

Resistivity or specific resistance to electric current is usually considered to be a DC property and is measured as such. It can be defined as the resistance between opposite faces of a unit cube where precautions have been taken to allow no current flow along the other four faces of the cube.

Conductivity

Elastomeric compositions are considered electrically conductive when they possess a direct current resistivity of less than 10^5 to 10^7 ohm-cm. Conductive compounds find their principal application where the dissipation of static electricity is desired.

Dielectric Strength

The dielectric strength of an elastomer is the voltage required to puncture, a sample of known thickness and is expressed as volts per mil of thickness. The rate of voltage application, the geometry of the electrodes, and the shape of the test specimen influence the results obtained.

Dielectric Constant and Power Factor

The dielectric constant, or specific inductive capacity (SIC), is a measure of an insulator's ability to store electrical energy. The dielectric constant is the ratio of the electrical capacity of a condenser using the elastomer under test as the dielectric, to the capacity of a similar condenser using air as the dielectric. One of the factors which influences the dielectric constant is frequency, and tests may be run from a low of 25 cycles per second to many megacycles per second.

The power factor of an electrical insulation material indicates its tendency to generate heat in service. If a capacitor using an elastomer as the dielectric is charged by a direct current and then immediately discharged, there is an energy loss in the form of heat. If this capacitor is repeatedly charged and discharged by an alternating current, the electrical loss results in heating. The ratio of this loss to the energy required to charge the capacitor is known as power factor. It is expressed as a decimal fraction or a percent of the charging energy.

Urethane has been used in potting and insulation compositions at frequencies up to 100 kilohertz at temperatures at 212F. Its natural translucency permits easy inspection of the encapsulated assemblies and the flexibility and strength of urethane helps protect the assemblies from damage. The electrical properties of vulcanizates of urethane measured at different temperatures and frequencies are shown in Table I.

TABLE I
Electrical Properties of 90A urethane with Epoxy Resin Vulcanizates
Effect of Temperature   DC Volume Resistivity, ohm-cm Dielectric Strength
Volts/mil
Dielectric Constant at 1 khz Power Factor at 1 khz, %
Tested at 75F 5x10^15 932 3.5 1.1
  167F 3x10^13 -- 5.5 5.1
  212F 1x10^12 -- 6.5 5.6
  250F 3x10^11 -- 6.8 3.3
Effect of Aging          
Orginal, tested 75F 5x10^15 932 3.5 1.1
Heat Aged, 7 days 250F 5x10^15 818 1.9 0.6
Aged in Water, 7 days 158F 1x10^14 551 5.8 2.8
Aged in ASTM Oil No 1, 7 days 158F 5x10^15 784 4.5 1.6

The basic polymers may be modified with epoxy resins for improved electrical properties. This is shown in Table II.

TABLE II
Physical Properties    
Durometer Hardness   55A 90A 50D
Electrical Properties   0.1 khz 100 khz 0.1 khz 100 khz 0.1 khz 100 khz
Power Fact, %
ASTM D150
75F 2.15 9.5 4.7 5.92 7.25 4.35
  158F 9.05 6.5 9.45 4.15 6.65 4.75
  212F 20.07 6.95 12.6 3.9 8.75 4.00
Dielectric Constant (SIC)
ASTM D150
75F 11.10 9.41 9.37 7.78 9.25 7.58
  158F 11.83 10.83 11.05 9.62 11.65 9.74
  212F 12.57 10.80 11.48 9.87 12.19 9.98
DC Volume Resistivity ohm-cm
ASTM D257
75F 4.3x10^11 4.8x10^11 3.7x10^12
158F 3.9x10^10 3.8x10^10 2.0x10^11
  212F 2.9x10^10 2.3x10^10 1.1x10^11
Dielectric Strength, Volts/mil
ASTM D-149
75F 450-500 650-700

1 Hertz (HZ) = 1 cycle/second
1 khz = 1000 cycles/second
kc = kilocycle = 1000 cycles/second