Volume Resistivity

The volume resistivity of a polymer material is its ability to oppose the flow of electric current through a volume of the cubic specimen. The SI unit of volume resistivity is ohm-meter (Ohm-m).

Volume resistivity is also known as:
 

• electrical resistivity,
• bulk resistivity,
• specific electrical resistance, or
• specific volume resistance.

Volume resistivity is denoted by the Greek alphabet, rho (ρ). The formula to calculate it is:

 

where,

• ρ = volume resistivity (in ohm-m or ohm-cm)
• V = voltage (in volts)
• I = current (in ampere)
• R = electrical resistance (in ohms)
• A = cross-sectional area of the material (in square meters)
• L = length of the material (in meters)
   

The volume resistivities for plastics range from 10¹⁰ ohm-cm for cellulose acetate to 10¹⁹ ohm-cm for polystyrene. 

Some materials that show volume resistivity:
 

1. Thermoplastics with Volume Resistivity - View all Products
2. TPEs and TPVs with Volume Resistivity - View all Products
3. Rubbers with Volume Resistivity - View all Products
    

The range of volume resistivities of different materials is shown in 'The Resistivity Spectrum' below.
 

Source: Plastics Technology Handbook, Fifth Edition

Note:
 

• Below 10⁵ Ohm-cm the material is considered as conductive.
• Above 10⁹ Ohm-cm the material is considered as an electrical insulator.
• Volume resistivities above 10²¹ Ω-cm (10¹⁹ Ω-m) are of doubtful validity. This is due to the limitations of measuring equipment.

The insulation resistance of a dielectric is represented by its 'volume resistivity' and 'surface resistivity'. This always acts in parallel.
 

• Temperature: The insulation resistance falls off with an increase in temperature. For example, PS has high insulation resistance. It becomes unsatisfactory above 80°C (176°F). Under these conditions, polymers like PTFE and PCTFE are more suitable.

• Relative humidity: The insulation resistance falls off with an increase in humidity. Plastics with high water resistance are less affected by high humidities.

• Voltage: The longer the application of voltage the higher the volume resistivity.

• Presence of fillers: The presence of fillers in the polymer affects the volume resistivity. The type and amount of filler change the volume resistivity.

 The applications of volume resistivity include:
 

• Designing an insulator for a specific application.
• Screening of conductive pastes.
• Defining applications for conducting composites.
• It is used to check the uniformity of insulating materials. This is with respect to:
  • processing, or
  • detection of the conductive impurities that affect the quality of the material.

Resistivity meter
 

• It determines the volume resistivity of a material.
• It measures the resulting current by applying a known voltage.
• This allows the calculation of the material's resistivity.

Four point probe
 

• It determines the resistivity of materials, especially that of thin films.
• The four-point probe configuration helps minimize errors caused by contact resistance.
• This makes it a precise and accurate method for resistivity measurements.
   

Source: ScienceDirect

The most usual test methods to determine plastics volume resistivity are:

 

• ASTM D257 — It is used to measure the DC resistance of insulating materials. It also measures conductance.

• ASTM D4496-04 — It measures the DC resistance of moderately conductive materials. It also measures conductance.

• ASTM D991-89(2020) — It determines the volume resistivity of conductive and antistatic products.

• IEC 60093:1980 — It measures the volume resistivity of solid electrical insulating materials. It also measures the surface resistivity. This method is partially replaced by:
  • IEC 62631-3-2:2015: Part 3-2 — It measures surface resistance and surface resistivity.
  • IEC 62631-3-1:2016: Part 3-1 — It measures volume resistance and volume resistivity.
    
     

Procedure
 

1. In the usual test, a standard-size specimen is placed between two electrodes.
2. For 60 seconds, a voltage is applied and the resistance is measured.
3. Volumetric resistivity is then calculated. The apparent value for a 60-second electrification time is given.
4. A 4-inch disk is preferable as the specimen size for the test.