Volume Resistivity of polymers material measures how strongly a plastic material opposes the flow of electric current through a volume of cubic specimen. The lower the resistivity the higher the conductivity (electric charges meet a weak resistance to circulation).
It is also known as electrical resistivity, bulk resistivity, specific electrical resistance, specific volume resistance, or simply resistivity.
Volume Resistivity is measured in units: ohm – meter (Ohm-m or Ohm-cm).
» Below 105 Ohm.cm the material is considered as conductive.
» Above 109 Ohm.cm the material is considered as an electrical insulator.
Check out more on Volume Resistivity:
» Volume Resistivity Values of Several Plastics
» Importance of Volume Resistivity
» How to Measure Volume Resistivity?
» Volume Resistivity Vs Surface Resistivity
» Factors Affecting Insulation Resistance
Importance of Volume Resistivity
Volumes resistivity can be used as an aid in designing an insulator for a specific application. The change in resistivity with temperature and humidity may be great and must be known when designing for operating conditions.
Volumes resistivity determinations are often used in checking the uniformity of an insulating material, either with regards to:
» Processing, or
» Detect the conductive impurities that affect the quality of the material
Volume resistivities above 1021 Ω·cm (1019 Ω·m), calculated from data obtained on specimens tested under usual laboratory conditions, are of doubtful validity, considering the limitations of commonly used measuring equipment.
Applications include:
» Design of an insulator for a specific application
» Screening of conductive pastes
» Define applications for conducting composites
How to Measure Volume Resistivity?
The most usual test methods to determine plastics volume resistivity are ASTM D257, ASTM D4496-04, ASTM D991-89(2005) or IEC 60093 (of course there exists several other methods as well!)
In the usual test, a standard size specimen is placed between two electrodes. For sixty seconds, a voltage is applied and the resistance is measured. Volumetric resistivity is, then, calculated and the apparent value for a 60 second electrification time is given. A 4 inch disk is preferable as the specimen size for the test.
Volume Resistivity Vs Surface Resistivity
The resistance offered by an insulating material to the electric current is the composite effect of volume and surface resistances, which always act in parallel.
-
- Volume resistance is the resistance to leakage if the electric current through the body of the material.
- It depends largely on the nature of the material
- Volume resistance is the resistance to leakage if the electric current through the body of the material.
- On the other hand, surface resistance, which is the resistance to leakage along the surface of a material, is largely a function of surface finish and cleanliness
- Surface resistance is reduced by oil or moisture on the surface and by surface roughness
- And, very smooth or polished surface gives greater surface resistance
The insulation resistance of a dielectric is represented by its “Volume Resistivity” and “Surface Resistivity”.
The range of volume resistivities of different materials is shown below in ‘The Resistivity Spectrum’
Values for plastics typically range from 1010 ohm-cm for Cellulose Acetate to about 1019 ohm-cm for a high-performance polystyrene.
Even PS, which has very high insulation resistance at room temperature, becomes generally unsatisfactory above 80°C (176°F). Under these conditions, polymers like PTFE and PCTFE are more suitable.
Plastics that have high water resistance are relatively less affected by high humidities.
The longer the voltage is applied (longer electrification times) the higher the volume resistivity that is measured.
The presence of fillers in the polymer will affect the volume resistivity. The type and amount of filler changes the volume resistivity.
Find commercial grades matching your target using “Property Search – Volume Resistivity” filter in Omnexus Plastics Database:
