PEEK polymer is often used as an electrical insulator with outstanding thermal, physical and environmental resistance.

Volume Resistance and Resistivity
Volume resistance and resistivity values are used as aids in choosing insulating materials for specific applications. The volume resistance of a material is defined as the ratio of the direct voltage field strength applied between electrodes placed on opposite faces of a specimen and the steady-state current between those electrodes. Resistivity may be defined as the volume resistance normalized to a cubical unit volume.

As with all insulating materials, the change in resistivity with temperature, humidity, component geometry and time may be significant and must be evaluated when designing for operating conditions. When a direct voltage is applied between electrodes in contact with a specimen, the current through the specimen decreases asymptotically towards a steady-state value. The change in current versus time, may be due to dielectric polarization and the sweep of mobile-ions to the electrodes. These effects are plotted in terms of volume resistivity versus electrification time in Figure 24.

The larger the volume resistivity of a material, the longer the time required to reach the steady-state current. Natural PEEK polymer has an IEC 93 value of 6.5 x 10¹⁶ W cm at ambient temperatures, measured using a steady-state current value for 1000 s applied voltage. Using the same experimental technique, the volume resistivity of 450G is plotted versus temperature in Figure 25. This shows that high values for the volume resistance of natural PEEK polymer are retained over a wide temperature range.

Surface Resistivity
The surface resistance of a material is defined as the ratio of the voltage applied between two electrodes forming a square geometry on the surface of a specimen and the current which flows between them. The value of surface resistivity for a material is independent of the area over which it is measured. The units of surface resistivity are the ohm (W), although it is common practice to quote values in units of ohm per square. A comparative bar chart of surface resistivities for some high performance engineering polymers at ambient temperatures is shown in Figure 26. This shows that natural PEEK 450G has a surface resistivity typical of high performance materials.

Relative Permittivity and Dielectric Dissipation Factor
PEEK polymer can be used to form components which support and insulate electronic devices. Often these components experience alternating potential-field strengths at various frequencies over wide temperature and environmental changes. The material response to these changes may be evaluated using IEC 250. This standard test evaluates the relative permittivity of a material and relates sinusoidal potential-field changes to a complex permittivity and a dielectric dissipation factor (tan d). The permittivity of a material (e_r) is defined as the ratio of the capacitance of a capacitor in which the space between and around is filled with that material (C_x) and the capacitance of the same electrode system in a vacuum (C_vac)

e_r = C_x / C_vac

The relative permittivity in an alternating current forms the complex relationship,

e_r* =e_r ' - je_r "

where e_r' is the storage permittivity, j is a complex number and e_r " is the imaginary loss permittivity. When such a potential difference is applied to a viscoelastic material the finite response time induced by the material means that there is a phase-lag (d) in the measured capacitance. This phase-lag may be described by the relationship,

C_x = C₀ (Sin wt + d)

where C₀ is the maximum capacitance measured. Therefore, an expression for the viscoelastic phase lag (tan d) can be derived from consideration of the storage and loss permittivities.

tan d = e_r " / e_r '

Low values of tan d are desirable for component operating conditions as this implies that the material will continuously insulate without excessive losses. The value of tan d over wide temperature and frequency ranges is shown in Figures 27 and 28 respectively.

From the data reported in Figure 27, natural PEEK polymer has a typical loss-tangent profile compared with other high performance materials over the temperature range tested.

The comparative plot shown in Figure 28 displays the excellent electrical performance of natural PEEK polymer over nine decades of applied frequency. Although many of the electrical properties of the material are described as typical of thermoplastic materials, PEEK polymers retain these excellent insulating properties over a wide range of temperature and frequency.

Information reprinted with permission from Victrex PLC