Determination of Dielectric Strength of Silicone-Epoxy Layered Materials

Dielectric strength testing determines the maximum electric field that silicone-epoxy layered materials can withstand before electrical breakdown occurs, making it one of the most critical parameters for qualifying electrical insulation materials. Accurate measurement of breakdown voltage and dielectric strength ensures the safety, reliability, and long-term performance of insulating components used in power electronics, electrical equipment, and new energy systems. This article provides a comprehensive overview of the sample requirements, test items, methodologies, and applicable standards for the determination of voltage withstand strength in silicone-epoxy layered composite materials.

Technical Overview

Silicone-epoxy layered materials are hybrid thermosetting composites that combine the flexibility, thermal stability, and arc resistance of organosilicon (polysiloxane) chemistry with the strong adhesion, mechanical integrity, and processing convenience of epoxy resins. These materials are typically constructed as multilayer structures, often incorporating glass fabric, mica paper, polymer films, or mineral fillers between resin layers to achieve a balanced profile of electrical, thermal, and mechanical properties. They are widely used as insulation barriers, layer insulation in transformers and motors, substrate laminates, and encapsulation structures in high-voltage applications.

Dielectric strength, also referred to as voltage withstand strength, is defined as the maximum electric field gradient (expressed in kV/mm) that a material can sustain without experiencing electrical breakdown. During testing, an increasing voltage is applied across the thickness of the material until dielectric breakdown occurs, creating a conductive failure path. The breakdown voltage divided by the sample thickness yields the dielectric strength. For silicone-epoxy layered materials, this parameter is particularly important because the interfaces between layers can act as preferential breakdown paths, and manufacturing defects such as voids, incomplete curing, or delamination directly reduce withstand capability.

Testing is typically performed by accredited third-party laboratories or in-house quality control departments in accordance with internationally recognized standards. The results support material qualification, incoming inspection, product design margin calculation, and compliance demonstration for electrical safety regulations.

Testing Samples

Laminate Sheets and Plates

Flat, rigid silicone-epoxy laminate sheets are the most common sample form. Specimens should be sufficiently large to prevent surface flashover between electrodes, typically at least 100 mm × 100 mm, with a representative thickness as specified by the product standard. Thickness must be uniform and measured at multiple points, since dielectric strength values are strongly thickness-dependent.

Molded and Pressed Layered Components

Finished or prototype components such as insulating bushings, barrier boards, gaskets, and molded multi-layer parts can be tested either as sectioned coupons or as complete parts, provided that the electrode contact area and test thickness are clearly defined.

Flexible Laminated Films and Tapes

Thin, flexible silicone-epoxy layered films and insulation tapes are tested using small-diameter sphere or cylindrical electrodes and are frequently immersed in insulating oil during testing to suppress flashover and surface discharges.

Coated and Encapsulated Structures

Conductive substrates coated with silicone-epoxy layered insulating coatings can be evaluated by applying one electrode to the substrate and the counter electrode to the coating surface, simulating real service conditions.

Testing Items

The determination of voltage withstand strength for silicone-epoxy layered materials generally includes the following measurable and verifiable items:

• AC breakdown voltage at power frequency (48–62 Hz), expressed in kV
• Dielectric strength (breakdown voltage divided by specimen thickness), expressed in kV/mm
• DC breakdown voltage and DC dielectric strength for direct-voltage applications
• Impulse breakdown voltage under standard lightning impulse waveforms (1.2/50 μs)
• Withstand voltage verification, confirming that a specified voltage level is held for a specified duration (for example, 1 minute) without breakdown
• Statistical analysis of breakdown data, including median values, minimum values, and Weibull distribution parameters for reliability evaluation
• Ancillary measurements such as specimen thickness, visual examination of breakdown channels, and identification of interlayer failure paths

Testing Methods

Applicable Standards

The primary reference standards for dielectric strength determination include IEC 60243-1 (electric strength of insulating materials, tests at power frequencies), IEC 60243-2 (additional requirements for tests using direct voltage), IEC 60243-3 (impulse tests), ASTM D149 (dielectric breakdown voltage and dielectric strength of solid electrical insulating materials at commercial power frequencies), ASTM D3755 (DC dielectric breakdown), and the corresponding Chinese national standards such as GB/T 1408. Product-level specifications for industrial laminates, including IEC 60893 and related material standards, define the specific test conditions and acceptance values for layered composite grades.

Specimen Conditioning and Preparation

Specimens are conditioned prior to testing, typically at 23 ± 2 °C and 50 ± 5% relative humidity for at least 24 hours, unless the material specification requires drying or other pretreatment. Thickness is measured at the electrode contact area using a calibrated micrometer or equivalent instrument. Specimen surfaces must be clean, dry, and free of defects that are not representative of the material itself. For layered materials, the orientation of the layers relative to the electrodes must be recorded, because through-thickness (perpendicular to the layers) testing is the standard configuration, while some specifications also require evaluation along the layer direction.

Electrode Configurations

Standard electrode systems defined in IEC 60243-1 and ASTM D149 include symmetrical cylindrical electrodes of 25 mm diameter with rounded edges, sphere electrodes of 12.5 mm to 20 mm diameter for thin specimens, and asymmetric sphere-to-plane configurations for specific geometries. Electrodes must be smooth, clean, and free of pitting caused by previous breakdown events. To prevent premature surface flashover, particularly for thin specimens, testing is frequently conducted with the specimen and electrodes immersed in a clean insulating liquid such as transformer oil conforming to IEC 60296.

Voltage Application Procedures

Three voltage application methods are recognized. The short-time (rapid rise) method increases the voltage from zero at a uniform rate, commonly 0.5 to 2 kV/s or a rate appropriate to the expected breakdown level, until breakdown occurs. The step-by-step method applies an initial voltage equal to 50% of the expected breakdown voltage, followed by successive equal increments, typically held for a defined dwell time at each step. The slow rate-of-rise method combines an initial step with a slower continuous increase. The method selection depends on the material specification and the intended comparison basis; all tests must use a consistent method because different procedures yield systematically different results.

Breakdown Detection and Termination

Breakdown is defined as the point at which the specimen loses its insulating capability, detected by a rapid increase in current that trips the protective circuit of the test apparatus, typically at a current level specified in the standard. If breakdown does not occur at the specimen (for example, flashover across the surface), the result is discarded and the test is repeated. Each material condition should be tested on a statistically adequate number of specimens, generally no fewer than five, with the median breakdown value reported together with individual values.

Data Evaluation and Reporting

The dielectric strength is calculated by dividing the breakdown voltage by the thickness measured at the breakdown point. The test report should identify the material and its layered construction, electrode configuration, surrounding medium, voltage application method and rate of rise, ambient test conditions, specimen thickness, individual and median breakdown voltages, calculated dielectric strength, and the location and appearance of breakdown channels, noting whether failure followed an interlayer interface, which is a characteristic weak point in layered silicone-epoxy systems.

Factors Influencing Results

• Specimen thickness: thinner specimens generally exhibit higher apparent dielectric strength in kV/mm
• Temperature and humidity: elevated temperature and moisture absorption significantly reduce withstand performance
• Voltage application rate and method: faster rise rates tend to produce higher breakdown values
• Electrode geometry and contact pressure: field concentration at electrode edges influences the failure path
• Interlayer interface quality: voids, delamination, and incomplete resin impregnation act as defect sites that lower breakdown voltage
• Cure state and post-curing: residual volatiles and under-cured resin degrade electrical performance

Applications

Determination of the voltage withstand strength of silicone-epoxy layered materials is essential across multiple industries and product categories:

• Transformer and reactor manufacturing: layer insulation, barrier insulation, and end-fill components in dry-type and oil-immersed equipment
• Motor and generator insulation: slot liners, phase insulation, and laminated stator components requiring high thermal class and arc resistance
• Power electronics: insulation substrates, busbar coatings, and encapsulation layers for IGBT modules and converter assemblies
• Printed circuit and substrate laminates: electrical qualification of layered composite boards for high-voltage circuit designs
• New energy vehicles and charging infrastructure: battery pack insulation barriers, busbar sleeves, and high-voltage connector components
• Rail transit and aerospace: lightweight layered insulation structures subjected to demanding thermal cycling and altitude conditions
• Lighting and semiconductor packaging: encapsulation and protective layered coatings for LED and power device assemblies
• Quality control and certification: incoming material inspection, supplier qualification, and compliance verification against product safety requirements referenced by IEC, UL, and EN standards frameworks

FAQ

What is a typical dielectric strength value for silicone-epoxy layered materials?

Depending on formulation, filler content, layer construction, and specimen thickness, silicone-epoxy layered materials typically exhibit dielectric strength values in the range of approximately 15 kV/mm to 30 kV/mm. Because results are strongly influenced by thickness and test conditions, comparison is only valid when the same standard, electrode system, and specimen geometry are used.

Which standard should be selected for testing?

For power-frequency AC testing, IEC 60243-1 or ASTM D149 is the most widely applied basis, with GB/T 1408 as the corresponding Chinese national standard. DC applications should reference IEC 60243-2 or ASTM D3755, while impulse withstand capability is evaluated according to IEC 60243-3. The product specification for the specific laminate grade should always be consulted to confirm the required method and acceptance criteria.

Why is breakdown voltage lower along the layer direction than through the thickness?

Interlayer interfaces, resin-rich zones, and possible micro-voids between layers provide preferential tracking and breakdown paths parallel to the layer structure. This anisotropy is characteristic of layered composites and is the reason through-thickness testing is the standardized default configuration.

How does absorbed moisture affect the test result?

Water absorption increases the loss factor and conductivity of silicone-epoxy systems, accelerating thermal and electrochemical breakdown mechanisms and measurably reducing withstand strength. Standard conditioning at 23 °C and 50% relative humidity prior to testing is therefore essential for reproducible results.

Can finished parts be tested instead of laboratory coupons?

Yes. Routine withstand voltage verification at a specified level for a specified duration, for example one minute, is commonly performed on finished layered components as a 100% or sampling quality gate, whereas destructive breakdown testing to determine the actual dielectric strength is carried out on representative coupons.

How many specimens are required for a reliable determination?

A minimum of five valid breakdown results per material condition is normally required, and larger sample sets are recommended when statistical evaluation, such as Weibull analysis, is needed to characterize the scatter of breakdown behavior in layered structures.