Dielectric Strength of Polymers
An important electrical property of insulators is their dielectric strength. If a voltage is applied to an insulator and steadily increased, a point will be eventually reached where the electrical properties break down. The breakdown is usually observed as an electrical arc accross the electrodes which causes a catastrophic decrease in resistance.
The dielectric strength will depend on the type and shape of the plastic and electrodes, the rate with which the field is increased, and the medium that surrounds the insulator. Most (pure) plastics have a dielectric strength in the range of 100 to 300 kV/cm with an average around 200 kV/cm and some chlorinated polymers show values up to 500 kV/cm and Teflon up to 700 kV/cm.
The breakdown current in dielectric polymers does not obey Ohm’s law. Before breakdown, the current density increases almost exponentially with the electric field, and at breakdown, it jumps to very high values at which the dielectric is destroyed (punctured and burned by the arc). The two known kinds of electric breakdown are thermal and electrical breakdown. Thermal breakdown is the result of excessive heating of the insulator by the electric current which causes, at a certain voltage, the polymer to melt or burn. In this case, the dielectric strength is proportional to the square root of the ratio of thermal and electrical conductivity of the plastic. Electrical breakdown is usually caused by impact ionization but it can also be the result of tunneling of charge carriers from the electrode and from the valence band to the conducting band. The tunnel effect happens mainly in thin layers (p-n junctions).
The table below shows the dielectric constant and dielectric strength of some common polymers with comparison to some common inorganic materials.
|Compound||Dielectric Constant, ε
|Polyacrylonitrile||4.0 - 4.2|
|Poly(vinyl chloride)||2.9 - 3.1||140 - 200|
|Poly(methyl methacrylate)||2.8 -2.9||100 - 300|
|Poly(ethylene terephthalate)||3.0 -3.5||150 - 200|
|Polycarbonate||2.8 - 3.0||150 -340|
|Polyethylene||2.2 - 2.3||200 - 300|
|Polypropylene||2.2 - 2.3||230 - 250|
|Polytetrafluoroethylene (Teflon)||2.0 - 2.1||600 - 700|
|Air||1.0||15 - 30|
|Distilled Water||80||65 - 70|
|Fused Silica||250 - 400|