ASTM D1673
Understanding ASTM D1673 Relative Permittivity And Dissipation Factor of Expanded Cellular Polymers Used For Electrical Insulation
The ASTM D1673 standard outlines detailed methods for determining the relative permittivity and dissipation factor of expanded cellular polymers used for electrical insulation. These properties are crucial for understanding how these materials will perform when used to insulate electrical components, cables, and other devices. Given the unique nature of expanded cellular polymers, conventional testing methods for solid insulating materials do not apply directly. Therefore, specific procedures and adaptations are necessary.
1. Scope
The standard covers the determination of relative permittivities and dissipation factors for both rigid and flexible expanded cellular polymers, from 60 Hz to 100 MHz. It accommodates specimens up to 50 mm in thickness, recommending that those greater than 25 mm be tested at frequencies up to 1 MHz maximum.
2. Referenced Documents
Key references include ASTM D150 for AC loss characteristics and permittivity of solid electrical insulating materials, ASTM D374 for thickness measurements, ASTM D1056 for specifications of flexible cellular materials, and ASTM D1711 for terminology related to electrical insulating materials.
3. Terminology
Definitions relevant to the tests include terms like relative permittivity, dissipation factor, and loss index, detailed in ASTM D150 and ASTM D1711.
4. Significance and Use
Relative Permittivity: Highlights how expanded cellular polymers, due to their structure, have lower permittivities compared to solid resins, affecting capacitance in encapsulated components.
Dissipation Factor and Loss Index: These measure the AC power loss in a material, providing insights into efficiency, thermal performance, and quality control aspects.
5. Apparatus
Measurements require electrical apparatus capable of performing tests across the specified frequency range, adhering to characteristics outlined in ASTM D150.
6. Electrodes
Due to the surface properties of expanded cellular polymers, conventional electrodes are unsuitable, necessitating the use of prefabricated rigid metal plate electrodes. These can be direct-contact or non-contacting, with various systems like the Hartshorn-Ward type for different specimen sizes and measurement conditions.
Test Method
The test method involves several critical steps, each designed to ensure accurate, repeatable results:
Preparation of Specimens: This involves cutting the polymer into appropriate sizes, considering the recommendations for thickness and ensuring minimal surface imperfections.
Measurement Set-up: Depending on the specimen's properties (thickness, electrical characteristics), select the appropriate electrode system. For thinner specimens, direct-contact methods like the Hartshorn-Ward type may be suitable. For thicker or larger specimens, non-contacting methods or sandwich electrode configurations might be necessary.
Testing Procedure:
For direct-contact systems, ensure specimens are lightly clamped between electrodes without compressing the material.
For non-contacting systems, an air gap is maintained, requiring calibration to account for this setup.
Measurements are taken across the specified frequency range, noting any frequency-specific behaviors or anomalies.
Data Analysis: Calculate the relative permittivity and dissipation factor using the recorded measurements, applying the correct formulas and corrections for electrode configurations and frequencies.
Reporting: Results should include the material's relative permittivity and dissipation factor at each frequency tested, along with any relevant conditions (temperature, humidity) or deviations from the standard procedure.
Safety and Precautions: Emphasize safety throughout the test, particularly regarding electrical hazards and material handling. Ensure all practices comply with regulatory and safety standards.
Conclusion
ASTM D1673 provides a comprehensive framework for accurately determining the electrical insulating properties of expanded cellular polymers. Through careful preparation, precise measurement, and thorough analysis, this standard ensures these materials meet the necessary criteria for their intended electrical insulation applications.