Volume Resistivity -

What is the volume resistivity of a polymer?

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.

What is the formula of volume resistivity?

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

ρ = (V/I) x (A/L)

ρ = R x (A/L)
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 lower the resistivity the higher the conductivity

What is the volume resistivity range for different materials?

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:

Thermoplastics with Volume Resistivity – View All Products
TPEs and TPVs with Volume Resistivity – View All Products
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.

What is the difference between volume and surface resistance?

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

Volume Resistance	Surface Resistance
Volume resistance is the resistance to leakage if the electric current passes through the body of the material. It depends largely on the nature of the material.	Surface resistance is the resistance to leakage along the surface of a material. It is a function of surface finish and cleanliness. Surface resistance is reduced by:oil or moisture on the surface andsurface roughness *Note*: Smooth or polished surface gives greater surface resistance

What factors affect insulation resistance?

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.

What are the applications of 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.

Which instrument measures volume resistivity?

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

What test methods measure volume resistivity?

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

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