Why are polymers good electrical insulators, and what are dielectric strength, insulation resistance and surface resistivity?

In one line

Most polymers have no free electrons or ions, so they resist current flow; their insulating quality is described by dielectric strength (voltage they withstand), insulation resistance (resistance to leakage current) and surface resistivity (resistance along the surface).

Explained simply

In metals, electrons move freely like cars on an empty highway. In polymers, all electrons are locked in covalent bonds, like cars stuck in a parking lot. That is why wires are coated with PVC and switches are made of Bakelite. But even a good insulator fails if the voltage is too high or if its surface gets dirty and wet.

How it works

1. Dielectric strength: the maximum electric field (voltage per unit thickness) an insulating material can withstand without electrical breakdown (puncture). Above this, the polymer suddenly conducts, often with burning. Depends on thickness, temperature, moisture, impurities and voids.
2. Insulation resistance: the resistance (in ohms, often megaohms) offered by an insulating material to leakage current when a DC voltage is applied across it. It includes current through the volume and along the surface. It falls with moisture, temperature and contamination.
3. Surface resistivity: the resistance to current flowing along the surface of a material, between two electrodes on the same face; expressed in ohms (often "ohms per square"). Moisture, dust and ionic contamination on the surface lower it. Important for antistatic materials and outdoor insulators.
4. Volume resistivity (related): resistance through the bulk, in Ω·cm or Ω·m. Good insulating polymers have very high values (around 10¹⁴ Ω·cm or more).

Factors: non-polar polymers (PE, PTFE, PP) are better insulators, especially at high frequency, than polar ones (PVC, nylon), which absorb some moisture. Fillers, plasticizers, moisture and heat reduce insulation.

Key formula

\( \text{Dielectric strength} = \dfrac{V_{\text{breakdown}}}{t} \)

• V_breakdown = voltage at which breakdown occurs (kV)
• t = thickness of specimen (mm)
• Unit: kV/mm (or V/mil in older data)

Quick example

A 2 mm thick polymer sheet breaks down at 40 kV: dielectric strength = 40/2 = 20 kV/mm. Uses: PVC for wire insulation; PTFE and PE in cables for high-frequency signals; epoxy in PCB laminates and for encapsulating components. Exception: conducting polymers like doped polyacetylene or polyaniline conduct electricity due to conjugated chains plus doping.

Common mistakes

• Confusing dielectric strength (breakdown field) with dielectric constant (ability to store charge).
• Thinking insulation resistance is a fixed material constant; it depends on sample size, moisture and temperature.
• Saying all polymers are insulators; conducting polymers exist.

How MU asks it

"Explain the electrical properties of polymers" or "Define dielectric strength and surface resistivity" (3–5 marks). For full marks: reason polymers insulate, definition and unit of each property, factors affecting them, and applications.