Dielectric strength — the voltage per unit thickness that an insulating material can withstand before electrical breakdown — is the property that determines whether a potting compound, encapsulant, or insulating adhesive provides electrical isolation adequate for its circuit voltage environment. For high-temperature epoxy used in power electronics, high-voltage equipment, and process instrumentation, dielectric strength must remain above the minimum required throughout the service temperature range and over the life of the assembly. Dielectric strength degrades with temperature, moisture absorption, and thermal aging, and understanding the mechanisms of each degradation pathway, and the formulation and design choices that slow them, is essential for specifying high-temperature epoxy in electrically critical applications.
What Dielectric Strength Is and How It Is Measured
Dielectric strength is measured by applying an alternating or direct voltage across a defined-thickness specimen of the material and increasing the voltage until electrical breakdown occurs. The breakdown voltage divided by the specimen thickness gives the dielectric strength in volts per millimeter (V/mm) or kilovolts per millimeter (kV/mm).
The electrical breakdown of an epoxy specimen occurs when the electric field is strong enough to ionize the polymer material and create a conducting channel through it. In the initial breakdown event, a narrow channel of carbonized material bridges the high and low voltage surfaces; once formed, this conductive path allows sustained current flow and the breakdown is typically irreversible.
Cured high-temperature epoxy has dielectric strength values in the range of 15 to 25 kV/mm at room temperature and low humidity, depending on formulation and cure state. These values represent the intrinsic electrical breakdown resistance of the polymer network in its ideal dry state.
Temperature Effects on Dielectric Strength
As temperature increases toward and above the glass transition temperature, dielectric strength decreases for reasons related to both the physical state of the polymer and its mobility.
Below Tg, the polymer network is rigid and the electrical polarizability of the chain segments is limited by the network constraint. The dielectric constant increases modestly with temperature in this range; dielectric strength decreases modestly. For a well-formulated high-temperature epoxy with Tg of 180°C, the dielectric strength at 150°C may be 12 to 18 kV/mm — still adequate for most high-voltage applications but reduced from the room-temperature value.
Above Tg, the polymer transitions to a rubbery state with greatly increased chain mobility and higher polarizability. Both the dielectric constant and dielectric loss increase substantially, and dielectric strength decreases more steeply. Operating a high-temperature epoxy encapsulant above its Tg — which occurs when the service temperature exceeds the Tg — results in significantly degraded electrical insulation performance.
This is the electrical engineering rationale for the same requirement that structural engineers cite: the Tg must be above the maximum service temperature, not just at it. For electrical applications, the Tg margin of at least 30°C to 50°C above maximum service temperature maintains the polymer in its high-performance glassy state throughout the operating range and ensures that neither structural nor electrical properties are degraded by operation above Tg.
Moisture Effects on Dielectric Strength
Moisture is the most significant environmental factor degrading dielectric strength in high-temperature epoxy encapsulants and insulating adhesives. Water absorbed by the polymer increases the dielectric constant, introduces ionic conduction through dissolved electrolytes from surface contamination, and reduces the dielectric breakdown threshold by providing a polar medium that polarizes more readily than the dry polymer.
Equilibrium moisture content of a high-temperature epoxy at 100 percent relative humidity is typically 2 to 5 percent by mass. This absorbed water reduces dielectric strength from the dry baseline by 30 to 50 percent or more at high humidity levels. At elevated service temperatures, the rate of moisture diffusion into the polymer is higher, and the equilibrium state is reached faster — a moist operating environment at 80°C equilibrates the adhesive to its full moisture content faster than the same exposure at 20°C.
Formulation choices that reduce moisture absorption improve dielectric performance under humid conditions. Cresyl glycidyl ether and other bulky ring-containing epoxy monomers reduce water uptake compared to bisphenol A systems. Anhydride hardeners produce networks with lower moisture uptake than amine-cured systems because the ester linkages are less polar than the amine-hydroxyl groups in amine cures. Hydrophobic filler additions — silica treated with hydrophobic coupling agents — reduce the overall moisture uptake of filled formulations.
For applications requiring dielectric strength data at specific temperature and humidity conditions for design verification, Email Us — Incure can provide dielectric strength as a function of temperature and moisture content for relevant formulations.
Thermal Aging Effects on Dielectric Properties
Long-term exposure at elevated temperature progressively changes the dielectric properties of high-temperature epoxy through oxidative degradation, additional post-cure, and gradual structural changes in the polymer network.
Additional post-cure — the continuation of the curing reaction at service temperature — typically increases Tg and crosslink density, which can initially improve dielectric properties. As this additional cure approaches completion, properties stabilize. In the first weeks to months of service at elevated temperature, this post-cure effect may produce slightly improved dielectric performance compared to the initial cured state.
Oxidative degradation — progressive chain scission and network disruption from high-temperature oxidation — eventually reduces crosslink density and introduces polar oxidation products into the network. These changes increase the dielectric constant and decrease dielectric strength. The timeline for this degradation depends on temperature, atmosphere oxygen content, and the antioxidant systems incorporated in the formulation.
At very high temperatures — above 200°C — thermal aging produces char formation and carbonization of the polymer network. Carbonized regions are conductive rather than insulating; their formation degrades the electrical isolation properties of the encapsulant progressively as the carbon content increases. This mechanism is the ultimate limit to long-term dielectric integrity at extreme service temperatures.
Design Factors That Maintain Dielectric Performance
Beyond formulation selection, several design and process factors maintain the dielectric strength of high-temperature epoxy encapsulants throughout the service life of the assembly.
Void-free encapsulation is critical because voids — air-filled bubbles — concentrate electric field at their surfaces and initiate partial discharge at voltages well below the bulk dielectric breakdown threshold. Partial discharge — small, localized ionization events within voids — progressively erodes the surrounding polymer surface and eventually creates a conducting path. Vacuum degassing of mixed adhesive before application, or vacuum impregnation during potting, eliminates voids from the encapsulated assembly.
Minimum bondline thickness at high-voltage interfaces determines the voltage withstand capability of the insulating path. The design must provide at least the minimum thickness calculated from the applied voltage divided by the required dielectric strength margin, with additional margin for manufacturing variation in bondline thickness.
Grading the encapsulant at high-field edges — where the field concentration from electrode geometry would otherwise produce local breakdown before bulk breakdown — uses field-grading filler materials or geometry design to distribute the field more uniformly. This is particularly relevant for potted high-voltage connections where the electrode edges create field concentrations.
Contact Our Team to discuss dielectric strength requirements, formulation selection, and application design for high-temperature epoxy electrical insulation in power electronics, process instrumentation, and high-voltage equipment.
Visit www.incurelab.com for more information.