Electrical insulation properties of plastics

Plastics are among the most important electrically insulating materials in electrical engineering and electronics. As electrical insulators, they prevent unwanted current flows between conductive components and are used, among other things, for insulating bodies, connectors, housings, cable components and switchgear.

However, whether a plastic is suitable for electrical insulation cannot be recognised solely from the fact that it conducts current poorly. Important characteristic values are above all the dielectric strength, the tracking resistance or CTI value, the surface and volume resistance as well as the dielectric properties. In addition, temperature, moisture, contamination and ageing influence the long-term insulating effect.

The material selection must therefore always match the electrical, thermal and mechanical load of the respective application.

Fundamentals of electrical insulation

Electrical insulation is understood as the ability of a material to prevent or strongly limit the flow of electric current. Plastics are among the classic insulators, as they have a very high specific resistance.

Their use is advantageous in many respects: they are light, flexibly formable, chemically resistant and, compared with ceramic or glass insulators, usually offer significantly lower production costs. These properties make plastics an ideal material for safely separating electrical conductors from one another and ensuring defined current paths.

Electrical insulators made of plastic

Due to their high electrical resistance, plastics are particularly well suited as electrical insulators. They prevent or limit the flow of current between conductive components and are therefore used, for example, for insulating bodies, spacers, housings, connectors and components in switchgear. Compared with ceramic or glass insulating materials, plastics offer additional advantages such as low weight, good machinability and great freedom of design. Which type of plastic is suitable depends, among other things, on voltage, temperature, moisture as well as the required dielectric and tracking resistance.

Electrical properties of plastics: important characteristic values

In order for plastics to be used in a targeted manner, certain characteristic values are of decisive importance. Dielectric strength describes the maximum voltage that a material withstands before it becomes conductive. The higher this value, the better a plastic is suited for high-voltage applications.

Tracking resistance, also referred to as CTI (Comparative Tracking Index), is a measure of the resistance to conductive tracks that arise from moisture or contamination. High CTI values are particularly important in harsh environments. The surface and volume resistance indicate how strongly a plastic blocks the flow of current across the surface or through the material.

These values are essential for the dimensioning of insulators. The dielectric constant, also called permittivity, determines the behaviour of a material in electric fields and is of particular importance in capacitors or high-frequency applications. In addition, the dielectric losses provide information about energy losses in an alternating field, which should be as low as possible in areas such as high-frequency technology or communications electronics.

Factors influencing the insulation properties

The electrical properties of plastics are not only material-related but also strongly dependent on external factors.

Temperature

Temperature is one of the most critical factors for electrical insulation. As temperature rises, the mobility of the molecules in the polymer increases, which leads to a decrease in electrical resistance. In many standard plastics, this reduces the dielectric strength and the risk of flashovers increases.

Particularly in power electronics, where components are exposed to high continuous loads, this can lead to failure. High-performance plastics such as PEEK, PTFE or polyimide were specially developed to retain their insulating capability even at temperatures of 200 °C or more. In areas such as the automotive industry, aviation or in switchgear, these materials are therefore indispensable.

Moisture

Moisture affects plastics in several respects at once. On the one hand, water penetrates hydrophilic polymers such as polyamide (PA) and changes their electrical conductivity. On the other hand, a thin conductive film forms on the surface, which promotes creep currents. This is a challenge particularly in humid or tropical climates.

Therefore, water-repellent materials such as polypropylene (PP) or fluorinated plastics (e.g. PTFE) are frequently used in outdoor applications. Coatings or special additives can also reduce water absorption. In the cable industry, moisture resistance is additionally improved by multilayer sheathing.

Contamination

Surface contamination such as dust, oil or conductive particles can severely impair the insulation performance. The deposits create non-uniform electric fields that can lead to creep currents and ultimately to insulation failure.

In industrial plants where dust or oil mist occur frequently, this is a well-known problem. Smooth surfaces are therefore preferred, as they are easy to clean and particles adhere less strongly to them. In addition, special coatings can be used in high-voltage applications to prevent the formation of conductive deposits.

Ageing

The ageing of plastics is a creeping process that leads to considerable losses in the insulation properties in the long term. Causes are UV radiation, oxygen, ozone or aggressive chemicals. These influences attack the polymer chains, weaken the molecular structure and lead to embrittlement or microcracks.

As a result, the dielectric strength decreases and the risk of failures increases. To counteract this, additives such as UV stabilisers, antioxidants or flame retardants are used. Coatings or multilayer systems can also significantly extend the service life. In critical applications, for example in medical technology or aviation, such protective measures are absolutely necessary.

Mechanical loads

In addition to environmental influences, mechanical stress also plays a major role. Alternating bending loads, pressure or vibrations can cause microdefects in the material. These microcracks form entry points for moisture and promote electrical breakdowns. This is an important criterion particularly for flexible leads and cables that are constantly moved in machines.

Plastics with high toughness and elasticity are therefore used here, such as special silicone compounds or thermoplastic elastomers. Reinforcements with glass fibres or protective braids can also considerably increase the service life of the insulation.

Typical plastics with outstanding insulation properties

• Polyethylene (PE): Excellent dielectric strength, widely used in cable insulation and high-voltage engineering.
• Polypropylene (PP): Very low water absorption, therefore ideal for capacitors and insulating films.
• Polytetrafluoroethylene (PTFE): Almost universal chemical resistance, very high temperature range, ideal insulation even in aggressive environments.
• Polycarbonate (PC): Combination of high mechanical stability and good dielectric strength, popular for switchgear and housings.
• Epoxy resins: As thermosets, particularly resistant to heat and chemicals, standard material for circuit boards and potting compounds.
• Polyimide (PI): Outstanding at high temperatures, used in aerospace as well as in microelectronics.
• Silicone elastomers: Flexible, temperature-resistant and with good insulation properties – often in high-voltage insulators for outdoor use.

Test methods for assessing the insulating capability

The reliability of insulating materials is ensured by a series of standardised test methods. In the breakdown voltage test, the maximum voltage that a material withstands before it becomes conductive is determined. The CTI test in accordance with IEC 60112 determines the tracking resistance under defined conditions and provides important data for use in humid or contaminated environments.

The surface and volume resistance is determined, for example, in accordance with ASTM D257 and provides information about the ability of the material to block the flow of current. For applications in high-frequency technology, the measurement of the dielectric constant and the loss factor in accordance with ASTM D150 is decisive.

In addition, ageing tests under the influence of UV radiation, heat or chemicals simulate the load on the material over long periods of time and show how stable the insulation values are in practical operation.

These tests provide data that are indispensable for selecting the right material in critical applications.

Significance in practice

The electrical insulation properties of plastics are of far-reaching significance in many industries:

• Power supply: Cables, insulators and switchgear depend directly on the performance capability of the plastics used.
• Electromobility: Battery modules, power electronics and high-voltage lines require plastics with the highest reliability.
• Medical technology: Devices such as defibrillators or imaging systems require safe insulating materials.
• Aerospace: Here, materials are required that insulate reliably at extreme temperatures and radiation conditions.
• Communications technology: High-frequency applications require plastics with low dielectric power loss.

Future and material innovations

The requirements for plastics with an insulating function are rising continuously. Nanocomposites, which combine improved electrical and mechanical properties and thus open up new fields of application, are particularly promising.

Recyclable insulating materials are also gaining in importance, as they help to implement sustainability and the circular economy in electrical engineering. In addition, smart plastics are emerging that adapt their properties depending on the load and can indicate changes in the material condition at an early stage. Such developments open up prospects for the next generation of high-performance insulators and help to further improve the safety, efficiency and durability of electrical systems.

In conclusion

Electrical insulation properties are among the most important material properties of plastics in electrical engineering. They are a central link between safety, efficiency and innovation and play a key role in almost every electrical or electronic system. From the selection of the suitable material through the influence of environmental conditions to testing and further development: only a deep understanding of these factors makes it possible to design safe and durable systems.

With new material developments, consistent research and targeted engineering, the rising requirements of the future can be met. As a result, plastics can not only improve existing applications but also open up completely new technological possibilities – whether in energy, mobility, medicine or high-tech electronics.

FAQ

Which plastics have the best insulation properties? PTFE, polyimide, epoxy resins as well as polyethylene and polypropylene show particularly good values. They combine high dielectric strength with resistance to temperature and moisture.

How does moisture affect the insulation? Moisture can penetrate into the material or form conductive films on the surface. This leads to a decrease in tracking resistance and can cause failures in the long term.

Why is temperature so decisive? High temperatures reduce the electrical resistance and can greatly reduce the dielectric strength. High-performance plastics are therefore indispensable in power electronics or aviation.

What role does the ageing of materials play? Ageing caused by UV radiation, oxygen or chemicals weakens the polymer structure and leads to cracks or embrittlement. Protective measures such as additives or coatings are essential here.

How are the insulation properties tested? Via standardised tests such as the measurement of breakdown voltage, CTI tests, resistance measurements or ageing tests in accordance with standards such as IEC 60112 or ASTM D257.