Standard methods for the electrical testing of plastics and polymers

Plastic materials are commonly used in electrical applications due to their lightweight nature and insulative performance. Meeting stringent quality criteria is important when developing materials for such environments, as the loss of insulative properties can lead to device malfunction and pose serious safety hazards, including the risk of electric shock, burns, and fire.¹

To ensure that plastics are suitable for electrical applications, they should be tested against internationally recognized standards. ASTM and IEC standards are followed most often, although they typically have equivalent EN methods referenced in European product safety legislation.² These established and standardized methods provide an objective framework for comparing material properties when advancing R&D or seeking certification for new materials.

Summary of key properties & tests to evaluate them

Some of the most commonly used electrical testing methods for plastics are listed in Table 1. More information regarding the practical applications of these tests is provided in the subsequent sections.

Dielectric measurements

Dielectric strength is an evaluation of a material’s ability to withstand high voltages. The reported value indicates the voltage at which a plaque of the material is perforated by the applied voltage, i.e., the point at which it loses its insulative characteristics. The result is often reported in units of kV/mm, which is an intrinsic material property and can be scaled for thicker geometries. 

Sufficient dielectric strength is essential for materials used as insulation or as protective casings, such as plastics used for cable sheathing or as insulation in circuitry.

ASTM D149 and IEC 60243 specify test procedures where a sample is subjected to increasing voltage until failure occurs. Voltage can be applied by various protocols, such as a constant increase or step-wise. The measurements can also be conducted at a wide range of temperatures, from below 0 °C to 250 °C, as polymers’ dielectric properties often change with temperature. 

Alongside dielectric strength, the dielectric constant or relative permittivity, κ, is another intrinsic electric property. The dielectric constant is the permittivity value of the material expressed relative to the permittivity of a vacuum. It indicates how well a material can store electrical energy when exposed to an electric field. A high dielectric constant means that the material can store more energy, making it suitable for use in capacitors and other energy storage applications. 

Volume & surface resistivity

Resistivity is an evaluation of a material's ability to resist the flow of an electrical current. When measuring volume resistivity (IEC 62631-3-1), the current flowing through the bulk material is measured. For surface resistivity (IEC 62631-3-2), the current flowing along the two-dimensional surface of a sample is measured. Resistivity values are then calculated from the applied voltage and measured current. It’s commonplace for both measurements to be conducted on the same sample. 

Similar to the dielectric measurements, resistivity tests can also be conducted at a range of temperatures from cryogenic to 250°C to replicate the end-use conditions of the material. Volume resistivity measurements are often performed for materials used as electrical insulation or in power transmission, while surface resistivity can be especially useful to know for materials used as thin coatings or insulative films. Surface resistivity can also be a good metric to quantify a material’s ability to dissipate static charge. 

EMI shielding 

Electromagnetic interference (EMI) shielding is a material property described as its ability to absorb, redirect, or attenuate electromagnetic radiation. It has rapidly become a crucial parameter to test in recent years as plastics have become more commonplace in battery assemblies and electronic devices. Materials with good shielding properties are able to protect sensitive equipment from damage or malfunctions. 

While these tests are typically performed on complete electronic devices to gauge compliance with electromagnetic compatibility (EMC) standards and legislation, it is also possible to assess the EMI shielding performance of the materials themselves. Standards such as ASTM D4935 and IEEE-299 are often used to evaluate these properties. ASTM D4935 is appropriate for testing planar materials, such as polymer sheets. In contrast, IEEE-299 is intended for testing entire electromagnetic shielding enclosures, such as rooms or cabinets containing sensitive electronics. 

Comparative tracking index (CTI)

The comparative tracking index (CTI) is a measurement of a material's resistance to electrical breakdown on its surface. When a large voltage difference is applied, periodic breakdowns occur on the surface and conductive pathways are created due to the carbonization of the polymer. This phenomenon is known as conductive leakage or “tracking”. 

The test method IEC 60112 is carried out by the drop-wise application of a conductive solution to the surface of the sample at gradually increasing voltage. This provides a simulation of an environment where conductive leakage or tracking would commonly occur. Materials are rated based on the maximum voltage at which they resist tracking. The highest rating described in IEC 60112, Insulating Material Group I, can resist tracking at more than 600V when 50 drops of conductive solution are applied.

CTI is a key material property when selecting the correct insulative polymer for a given application. A material not suited for the environment could lead to the failure and damage of electrical equipment, or – in extreme cases – the combustion of the material or device. 

All electrical tests for plastics in one place

Measurlabs offers a full range of standardized electrical tests for plastics and polymers, ensuring compliance with regulatory requirements, industry standards, and customers’ quality criteria. Contact our experts using the form below and describe your testing needs – we will get back to you within one business day.