ASTM D2304-23

Standard Test Method for Thermal Endurance of Rigid Electrical Insulating Materials

SIGNIFICANCE AND USE
6.1 Thermal degradation is often a major factor affecting the life of insulating materials and the equipment in which they are used. The temperature index provides a means for comparing the thermal capability of different materials in respect to the degradation of a selected property (the aging criterion). This property needs to directly or indirectly represent functional needs in application. For example, it is possible that a change in dielectric strength will be of direct, functional importance. However, more often it is possible that a decrease in dielectric strength will indirectly indicate the development of undesirable cracking (embrittlement). A decrease in flexural strength has the potential to be of direct importance in some applications, but also has the potential to indirectly indicate a susceptibility to failure in vibration. Often, it is necessary that two or more criteria of failure be used; for example, dielectric strength and flexural strength.  
6.2 Other factors, such as vibration, moisture and contaminants, have the potential to cause failure after thermal degradation takes place. In this test method, water absorption provides one means to evaluate such considerations.  
6.3 For some applications, the aging criteria in this test method will not be the most suitable. Other criteria, such as elongation at tensile or flexural failure, or resistivity after exposure to high humidity or weight loss, have the potential to serve better. The procedures in this test method have the potential to be used with such aging criteria. It is important to consider both the nature of the material and its application. For example, it is possible that tensile strength will be a poor choice for glass-fiber reinforced laminates, because it is possible that the glass fiber will maintain the tensile strength even when the associated resin is badly deteriorated. In this case, flexural strength is a better criterion of thermal aging.  
6.4 When dictated by the needs of t...
SCOPE
1.1 This test method2 provides procedures for evaluating the thermal endurance of rigid electrical insulating materials. Dielectric strength, flexural strength, or water absorption are determined at room temperature after aging for increasing periods of time in air at selected-elevated temperatures. A thermal-endurance graph is plotted using a selected end point at each aging temperature. A means is described for determining a temperature index by extrapolation of the thermal endurance graph to a selected time.  
1.2 This test method is most applicable to rigid electrical insulation such as supports, spacers, voltage barriers, coil forms, terminal boards, circuit boards and enclosures for many types of application where retention of the selected property after heat aging is important.  
1.3 When dielectric strength is used as the aging criterion, it is also acceptable to use this test method for some thin sheet (flexible) materials, which become rigid with thermal aging, but is not intended to replace Test Method D1830 for those materials which must retain a degree of flexibility in use.  
1.4 This test method is not applicable to ceramics, glass, or similar inorganic materials.  
1.5 The values stated in metric units are to be regarded as standard. Other units (in parentheses) are provided for information.  
1.6 When determining the thermal endurance of rigid EIM, the basic concepts in this standard follow IEEE 1, IEEE 98, and IEEE 101.  
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. A specific warning statement is given in 11.3.4.  
1.8 This international standard was developed in accordance with internationally recognized principles on standardization establis...

General Information

• Status: Published
• Publication Date: 30-Apr-2023
• ICS: 29.035.01 - Insulating materials in general
• Technical Committee: D09 - Electrical and Electronic Insulating Materials
• Drafting Committee: D09.07 - Electrical Insulating Materials

Relations

• Refers: ASTM D1830-17(2024) - Standard Test Method for Thermal Endurance of Flexible Sheet Materials Used for Electrical Insulation by the Curved Electrode Method
• Effective Date: 01-Mar-2024
• Refers: ASTM D149-20 - Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies
• Effective Date: 01-Jan-2020
• Refers: ASTM D229-19 - Standard Test Methods for Rigid Sheet and Plate Materials Used for Electrical Insulation
• Effective Date: 01-Mar-2019
• Refers: ASTM D1830-17 - Standard Test Method for Thermal Endurance of Flexible Sheet Materials Used for Electrical Insulation by the Curved Electrode Method
• Effective Date: 01-Nov-2017
• Refers: ASTM D790-17 - Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
• Effective Date: 01-Jul-2017
• Refers: ASTM D790-15e1 - Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
• Effective Date: 01-Dec-2015
• Refers: ASTM D790-15 - Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
• Effective Date: 01-Dec-2015
• Refers: ASTM D1830-99(2012) - Standard Test Method for Thermal Endurance of Flexible Sheet Materials Used for Electrical Insulation by the Curved Electrode Method
• Effective Date: 01-Jan-2012
• Refers: ASTM D229-09b - Standard Test Methods for Rigid Sheet and Plate Materials Used for Electrical Insulation
• Effective Date: 01-Oct-2009
• Refers: ASTM D229-09a - Standard Test Methods for Rigid Sheet and Plate Materials Used for Electrical Insulation
• Effective Date: 01-Jun-2009
• Refers: ASTM D229-09 - Standard Test Methods for Rigid Sheet and Plate Materials Used for Electrical Insulation
• Effective Date: 15-Jan-2009
• Refers: ASTM D790-07e1 - Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
• Effective Date: 01-Sep-2007
• Refers: ASTM D790-07 - Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
• Effective Date: 01-Sep-2007
• Refers: ASTM D570-98(2005) - Standard Test Method for Water Absorption of Plastics
• Effective Date: 01-Nov-2005
• Refers: ASTM D5423-93(2005) - Standard Specification for Forced-Convection Laboratory Ovens for Evaluation of Electrical Insulation (Withdrawn 2014)
• Effective Date: 01-Sep-2005

Overview

ASTM D2304-23, "Standard Test Method for Thermal Endurance of Rigid Electrical Insulating Materials," is an internationally recognized standard developed by ASTM International Committee D09. This test method outlines procedures for evaluating the thermal endurance of rigid electrical insulating materials (EIM). By systematically aging specimens at elevated temperatures and measuring changes in key properties such as dielectric strength, flexural strength, and water absorption, this method establishes a temperature index to compare the long-term thermal capabilities and durability of different insulating materials.

Thermal degradation significantly affects the performance and lifespan of electrical insulation used in a range of industrial and commercial equipment. ASTM D2304-23 provides critical guidance for selecting materials suitable for demanding applications by quantifying how thermal aging impacts their insulating properties.

Key Topics

• Thermal Endurance Assessment: The test method describes procedures for aging rigid insulating materials at various elevated temperatures and measuring their properties post-exposure to create a thermal endurance profile.
• Temperature Index Determination: The temperature index is calculated by extrapolating data from thermal endurance graphs, allowing direct comparison of materials based on expected long-term thermal performance.
• Aging Criteria: Common properties monitored include dielectric strength, flexural strength, and water absorption. Selection of criteria depends on the material type and specific application requirements.
• Test Conditions: Recommendations are provided for specimen preparation, oven aging conditions, test intervals, and safe handling practices during high-voltage measurements.
• Applicability: The standard is specifically intended for rigid electrical insulation products, such as circuit boards, terminal boards, voltage barriers, and similar components, with limitations noted regarding inorganic materials like ceramics or glass.

Applications

ASTM D2304-23 is widely applied across industries to ensure the reliability and longevity of electrical insulation in products where heat resistance is critical. Its applications include:

• Product Qualification: Manufacturers of rigid electrical insulating materials use this method to validate and certify products for use in high-temperature environments.
• Material Comparison: Engineers utilize the temperature index to compare different materials' thermal endurance, aiding in the selection of the most suitable insulation for a given application.
• Quality Control: Routine testing of production batches can identify variability and ensure materials meet performance standards over their anticipated service life.
• Equipment Safety: Helps assess and mitigate risks related to thermal aging in electrical assemblies such as transformers, switchgear, control panels, and motor windings.
• Research and Development: Supports the development of improved insulating materials by providing a standardized approach to quantifying thermal aging and durability.

Related Standards

ASTM D2304-23 references and aligns with several related standards and industry guides for testing electrical insulating materials:

• ASTM D149: Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials.
• ASTM D229: Test Methods for Rigid Sheet and Plate Materials Used for Electrical Insulation.
• ASTM D570: Test Method for Water Absorption of Plastics.
• ASTM D790: Test Methods for Flexural Properties of Plastics and Electrical Insulating Materials.
• ASTM D1830: Test Method for Thermal Endurance of Flexible Sheet Materials.
• ASTM D5423: Specification for Forced-Convection Laboratory Ovens for Electrical Insulation Evaluation.
• IEEE No. 1, No. 98, No. 101: Covering temperature rating and statistical analysis of test data for electrical insulation.
• IEC 216: Guide for Determination of Thermal Endurance Properties of Electrical Insulating Materials.
• UL 746B: Polymeric Materials-Long Term Properties Evaluations.

Practical Value

Implementing ASTM D2304-23 ensures that rigid electrical insulating materials can withstand long-term use in elevated temperature conditions, maintain critical properties, and support the safety, efficiency, and reliability of electrical and electronic equipment. This standard enables material specifiers, manufacturers, and quality assurance professionals to confidently assess insulation performance and make informed decisions that enhance product durability and compliance.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D2304 − 23
Standard Test Method for
Thermal Endurance of Rigid Electrical Insulating Materials
This standard is issued under the fixed designation D2304; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope* priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use. A
1.1 This test method provides procedures for evaluating the
specific warning statement is given in 11.3.4.
thermal endurance of rigid electrical insulating materials.
1.8 This international standard was developed in accor-
Dielectric strength, flexural strength, or water absorption are
dance with internationally recognized principles on standard-
determined at room temperature after aging for increasing
ization established in the Decision on Principles for the
periods of time in air at selected-elevated temperatures. A
Development of International Standards, Guides and Recom-
thermal-endurance graph is plotted using a selected end point
mendations issued by the World Trade Organization Technical
at each aging temperature. A means is described for determin-
Barriers to Trade (TBT) Committee.
ing a temperature index by extrapolation of the thermal
endurance graph to a selected time.
2. Referenced Documents
1.2 This test method is most applicable to rigid electrical
2.1 ASTM Standards:
insulation such as supports, spacers, voltage barriers, coil
D149 Test Method for Dielectric Breakdown Voltage and
forms, terminal boards, circuit boards and enclosures for many
Dielectric Strength of Solid Electrical Insulating Materials
types of application where retention of the selected property
at Commercial Power Frequencies
after heat aging is important.
D229 Test Methods for Rigid Sheet and Plate Materials
1.3 When dielectric strength is used as the aging criterion, it
Used for Electrical Insulation
is also acceptable to use this test method for some thin sheet
D570 Test Method for Water Absorption of Plastics
(flexible) materials, which become rigid with thermal aging,
D790 Test Methods for Flexural Properties of Unreinforced
but is not intended to replace Test Method D1830 for those
and Reinforced Plastics and Electrical Insulating Materi-
materials which must retain a degree of flexibility in use.
als
D1830 Test Method for Thermal Endurance of Flexible
1.4 This test method is not applicable to ceramics, glass, or
Sheet Materials Used for Electrical Insulation by the
similar inorganic materials.
Curved Electrode Method
1.5 The values stated in metric units are to be regarded as
D5423 Specification for Forced-Convection Laboratory Ov-
standard. Other units (in parentheses) are provided for infor-
ens for Evaluation of Electrical Insulation
mation.
2.2 IEEE:
1.6 When determining the thermal endurance of rigid EIM,
No. 1 General Principles Upon Which Temperature Limits
the basic concepts in this standard follow IEEE 1, IEEE 98, and
Are Based in the Rating of Electric Equipment
IEEE 101.
No. 98 Guide for the Preparation of Test Procedures for the
1.7 This standard does not purport to address all of the Thermal Evaluation of Electrical Insulating Materials
safety concerns, if any, associated with its use. It is the No. 101 Guide for the Statistical Analysis of Test Data
responsibility of the user of this standard to establish appro-
3. Terminology
3.1 Definitions:
This test method is under the jurisdiction of ASTM Committee D09 on
3.1.1 Arrhenius plot, n—a graph of the logarithm of thermal
Electrical and Electronic Insulating Materials and is the direct responsibility of
life as a function of the reciprocal of absolute temperature.
Subcommittee D09.07 on Electrical Insulating Materials.
Current edition approved May 1, 2023. Published June 2023. Originally issued
as D2304 – 64 T. Last previous edition approved in 2018 as D2304 – 18. DOI:
10.1520/D2304-23. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
This test method is a revision of a procedure written by the Working Group on contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Rigid Electrical Insulating Materials of the Subcommittee on Thermal Evaluation, Standards volume information, refer to the standard’s Document Summary page on
IEEE Electrical Insulation Committee, which was presented as CP 59-113 at the the ASTM website.
IEEE Winter General Meeting Feb. 1–6, 1959. See references at end of this test Available from the Institute of Electrical and Electronics Engineers, 445 Hoes
method. Ln., P.O. Box 1331, Piscataway, NJ 08854-1331.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D2304 − 23
3.1.1.1 Discussion—This is normally depicted as the best aging temperature. A thermal-life curve is plotted for each
straight line fit, determined by least squares, of end points aging temperature. The values of thermal life determined from
obtained at aging temperatures. It is important that the slope, the thermal-life curve are used to plot the thermal-endurance
which is the activation energy of the degradation reaction, be graph. A temperature index is determined from the thermal-
approximately constant within the selected temperature range endurance graph for each aging criterion used. (It is possible to
to ensure a valid extrapolation. obtain different values for the thermal index of a material with
different aging criteria.)
3.1.2 temperature index, n—a number which permits com-
parison of the temperature/time characteristics of an electrical
6. Significance and Use
insulating material, or a simple combination of materials, based
6.1 Thermal degradation is often a major factor affecting the
on the temperature in degrees Celsius which is obtained by
extrapolating the Arrhenius plot of life versus temperature to a life of insulating materials and the equipment in which they are
used. The temperature index provides a means for comparing
specified time, usually 20 000 h.
the thermal capability of different materials in respect to the
3.1.3 thermal life, n—the time necessary for a specific
degradation of a selected property (the aging criterion). This
property of a material, or a simple combination of materials, to
property needs to directly or indirectly represent functional
degrade to a defined end point when aged at a specified
needs in application. For example, it is possible that a change
temperature.
in dielectric strength will be of direct, functional importance.
3.1.4 thermal life curve, n—a graphical representation of
However, more often it is possible that a decrease in dielectric
thermal life at a specified aging temperature in which the value
strength will indirectly indicate the development of undesirable
of a property of a material, or a simple combination of
cracking (embrittlement). A decrease in flexural strength has
materials, is measured at room temperature and the values
the potential to be of direct importance in some applications,
plotted as a function of time.
but also has the potential to indirectly indicate a susceptibility
3.2 Definitions of Terms Specific to This Standard:
to failure in vibration. Often, it is necessary that two or more
3.2.1 rigid electrical insulating material, n—an electrical
criteria of failure be used; for example, dielectric strength and
insulating material having a minimum flexural modulus of
flexural strength.
690 MPa and minimum use thickness of 0.5 mm (0.02 in.). It is
6.2 Other factors, such as vibration, moisture and
generally used as terminal boards, spacers, coil forms, voltage
contaminants, have the potential to cause failure after thermal
barriers, and circuit boards.
degradation takes place. In this test method, water absorption
provides one means to evaluate such considerations.
4. Hazards
6.3 For some applications, the aging criteria in this test
4.1 High Voltage:
method will not be the most suitable. Other criteria, such as
4.1.1 Lethal voltages are a potential hazard during the
elongation at tensile or flexural failure, or resistivity after
performance of this test. It is essential that the test apparatus,
exposure to high humidity or weight loss, have the potential to
and all associated equipment electrically connected to it, be
serve better. The procedures in this test method have the
properly designed and installed for safe operation.
potential to be used with such aging criteria. It is important to
4.1.2 Solidly ground all electrically conductive parts which
consider both the nature of the material and its application. For
it is possible for a person to contact during the test.
example, it is possible that tensile strength will be a poor
4.1.3 Provide means for use at the completion of any test to
choice for glass-fiber reinforced laminates, because it is pos-
ground any parts which were at high voltage during the test or
sible that the glass fiber will maintain the tensile strength even
have the potential for acquiring an induced charge during the
when the associated resin is badly deteriorated. In this case,
test or retaining a charge even after disconnection of the
flexural strength is a better criterion of thermal aging.
voltage source.
6.4 When dictated by the needs of the application, it is
4.1.4 Thoroughly instruct all operators as to the correct
possible that an aging atmosphere other than air will be needed
procedures for performing tests safely.
and used. For example, thermal aging can be conducted in an
4.1.5 When making high voltage tests, particularly in com-
oxygen-free, nitrogen atmosphere.
pressed gas or in oil, it is possible for the energy released at
breakdown to be sufficient to result in fire, explosion, or
7. End Point
rupture of the test chamber. Design test equipment, test
chambers , and test specimens so as to minimize the possibility 7.1 An expression of the thermal life of a material, even for
of such occurrences and to eliminate the possibility of personal comparative purposes only, inevitably involves the choice of
injury. If the potential for fire exists, have fire suppression an end point. The end point is one of the following 4 criteria:
equipment available. See 11.3.4. a fixed magnitude of the property criterion, a percentage
reduction from its initial magnitude, the minimum magnitude
5. Summary of Test Method obtainable with time (that is, when change with time ceases), or
a fixed degrading change rate (that is, a fixed value for the
5.1 Test specimens are aged in air at three or preferably four
negative derivative of property with respect to time).
temperatures above the expected use temperature. The aging
temperatures are selected so that the thermal life is at least 7.2 Experience has shown that the choice of an end point
100 h at the highest aging temperature and 5000 h at the lowest can affect the comparative thermal life. A choice of end points
D2304 − 23
is guided by the limiting requirements imposed on the insula- thermal degradation of two or more materials unless informa-
tion by the manner and conditions of use in the complete tion relating degradation to thickness is available that indicates
system. End points are not specified in this test method. The the contrary. This test method specifies the specimen size,
first concern is to determine the values of the chosen properties including thickness, for each property selected.
as a function of time of thermal exposure at specified tempera-
PROCEDURES
tures. The properties are determined at various intervals of time
until a practical minimum or maximum magnitude, whichever
10. Oven Aging (Thermal Exposure)
is applicable, is reached. The data that result are thus universal,
that is, usable for any subsequently chosen end point as
10.1 Factors such as moisture, chemical contamination, and
determined by the specific application of the rigid electrical
mechanical stress or vibration usually do not in themselves
insulation.
cause failure, but are factors that have the potential to result in
failure only after the material has been weakened by thermal
7.3 The specification for each material needs to state the end
deterioration. For this reason, exposure to elevated tempera-
point to be used.
tures is the primary deteriorating influence considered in this
8. Aging Ovens test method.
8.1 The accuracy of the test results will depend on the 10.2 Table 1 is intended as a guide for the selection of
accuracy with which the exposure temperature of the test thermal exposure. Select times and temperatures from those
specimens is known. Experience has shown, as indicated in given in this table. The exposure times given are approximately
Table 1, that the thermal life is approximately halved for a equal to the average estimated life at each exposure tempera-
10 °C increase in exposure temperature. ture based on thermal aging data obtained on insulating
materials and systems. The potential exists that this table will
8.2 Use aging ovens that conform to the requirements of
be revised as a result of experience. The potential that either the
Type I of Specification D5423.
time or the temperature will be adjusted to make the best use
of available oven facilities.
9. Test Specimen
10.3 Age at a minimum of three and preferably four
9.1 The accuracy of the test results depends significantly
temperatures. Choose the lowest temperature to be less than
upon the number of specimens exposed at each temperature
25 °C above the hottest-spot temperature expected in use so
and the dispersion of the test results. The larger the individual
that the thermal life is at least 5000 h. Select the highest
deviations from the mean, the greater is the number of test
temperature so that the thermal life is at least 100 h. If possible,
specimens needed to achieve satisfactory accuracy. Experience
for best results, the aging temperatures need to differ from each
has shown that a minimum of five test specimens needs to be
other by at least 20 °C.
used at each exposure temperature. A separate group of test
specimens is required for each exposure period.
10.4 For an unknown material, the selection of the appro-
priate aging temperatures will require a short exploratory test
9.2 The rate of deterioration will be significantly influenced
performed at the highest likely aging temperature. Results from
by specimen thickness. Consequently it is important to test
thermal aging tests for a material with similar composition
specimens of the same nominal thickness when comparing the
have the potential to provide clues for an appropriate selection
of the first exploratory temperature. The chemical composition
TABLE 1 Temperature and Exposure Time in Days
of the material to be tested, if known, has the potential to also
Exposure Estimated Hottest-Spot Temperature Range, °C
provide a means for estimating the first aging temperature to be
Temperature,
100 to 125 to 150 to 175 to 200 to
°C used. Additional tests can then be made at lower or higher
120 145 170 195 240
temperatures as indicated by the first exploratory test. (See
300 . . . . 10
290 . . . . 20 Table 1 and 10.3.)
280 . . . . 40
10.5 Place a sufficient number of specimens to conduct the
270 . . . . 70
260 . . . . 140
tests used for the selected aging criterion in each aging oven.
Remove all of the test specimens after a selected interval of
250 . . . 10 280
time. (See 10.6.) Select the test specimens needed for the test
240 . . . 20 490
230 . . . 40 .
at random. Return the remaining samples to the aging oven and
220 . . 10 70 .
repeat the process after each succeeding time interval (aging
210 . . 20 140 .
period).
200 . 10 40 280 .
10.6 Suggested total exposure times with associated test
190 . 20 70 490 .
180 10 40 140 . . temperatures are given in Table 1. Initially, at least seven,
170 20 70 280 . .
evenly-spaced, test intervals at each test temperature are
160 40 140 490 . .
usually needed to provide sufficient data for the thermal life
150 70 280 . . . curves. (It is wise to provide sufficient specimens for ten
140 140 490 . . .
intervals.) It is most important to adequately define the later
130 280 . . . .
portion
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D2304 − 18 D2304 − 23
Standard Test Method for
Thermal Endurance of Rigid Electrical Insulating Materials
This standard is issued under the fixed designation D2304; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 This test method provides procedures for evaluating the thermal endurance of rigid electrical insulating materials. Dielectric
strength, flexural strength, or water absorption are determined at room temperature after aging for increasing periods of time in
air at selected-elevated temperatures. A thermal-endurance graph is plotted using a selected end point at each aging temperature.
A means is described for determining a temperature index by extrapolation of the thermal endurance graph to a selected time.
1.2 This test method is most applicable to rigid electrical insulation such as supports, spacers, voltage barriers, coil forms, terminal
boards, circuit boards and enclosures for many types of application where retention of the selected property after heat aging is
important.
1.3 When dielectric strength is used as the aging criterion, it is also acceptable to use this test method for some thin sheet (flexible)
materials, which become rigid with thermal aging, but is not intended to replace Test Method D1830 for those materials which must
retain a degree of flexibility in use.
1.4 This test method is not applicable to ceramics, glass, or similar inorganic materials.
1.5 The values stated in metric units are to be regarded as standard. Other units (in parentheses) are provided for information.
1.6 When determining the thermal endurance of rigid EIM, the basic concepts in this standard follow IEEE 1, IEEE 98, and IEEE
101.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use. A specific warning statement is given in 11.3.4.
1.8 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
This test method is under the jurisdiction of ASTM Committee D09 on Electrical and Electronic Insulating Materials and is the direct responsibility of Subcommittee
D09.07 on Electrical Insulating Materials.
Current edition approved May 1, 2018May 1, 2023. Published May 2018June 2023. Originally issued as D2304 – 64 T. Last previous edition approved in 20102018 as
D2304 – 10.D2304 – 18. DOI: 10.1520/D2304-18.10.1520/D2304-23.
This test method is a revision of a procedure written by the Working Group on Rigid Electrical Insulating Materials of the Subcommittee on Thermal Evaluation, IEEE
Electrical Insulation Committee, which was presented as CP 59-113 at the IEEE Winter General Meeting Feb. 1–6, 1959. See references at end of this test method.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D2304 − 23
2. Referenced Documents
2.1 ASTM Standards:
D149 Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at
Commercial Power Frequencies
D229 Test Methods for Rigid Sheet and Plate Materials Used for Electrical Insulation
D570 Test Method for Water Absorption of Plastics
D790 Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
D1830 Test Method for Thermal Endurance of Flexible Sheet Materials Used for Electrical Insulation by the Curved Electrode
Method
D5423 Specification for Forced-Convection Laboratory Ovens for Evaluation of Electrical Insulation
2.2 IEEE:
No. 1 General Principles Upon Which Temperature Limits Are Based in the Rating of Electric Equipment
No. 98 Guide for the Preparation of Test Procedures for the Thermal Evaluation of Electrical Insulating Materials
No. 101 Guide for the Statistical Analysis of Test Data
3. Terminology
3.1 Definitions:
3.1.1 Arrhenius plot, n—a graph of the logarithm of thermal life as a function of the reciprocal of absolute temperature.
3.1.1.1 Discussion—
This is normally depicted as the best straight line fit, determined by least squares, of end points obtained at aging temperatures.
It is important that the slope, which is the activation energy of the degradation reaction, be approximately constant within the
selected temperature range to ensure a valid extrapolation.
3.1.2 temperature index, n—a number which permits comparison of the temperature/time characteristics of an electrical insulating
material, or a simple combination of materials, based on the temperature in degrees Celsius which is obtained by extrapolating the
Arrhenius plot of life versus temperature to a specified time, usually 20 000 h.
3.1.3 thermal life, n—the time necessary for a specific property of a material, or a simple combination of materials, to degrade
to a defined end point when aged at a specified temperature.
3.1.4 thermal life curve, n—a graphical representation of thermal life at a specified aging temperature in which the value of a
property of a material, or a simple combination of materials, is measured at room temperature and the values plotted as a function
of time.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 rigid electrical insulating material, n—an electrical insulating material having a minimum flexural modulus of 690 MPa
690 MPa and minimum use thickness of 0.5 mm (0.02 in.). 0.5 mm (0.02 in.). It is generally used as terminal boards, spacers, coil
forms, voltage barriers, and circuit boards.
4. Hazards
4.1 High Voltage:
4.1.1 Lethal voltages are a potential hazard during the performance of this test. It is essential that the test apparatus, and all
associated equipment electrically connected to it, be properly designed and installed for safe operation.
4.1.2 Solidly ground all electrically conductive parts which it is possible for a person to contact during the test.
4.1.3 Provide means for use at the completion of any test to ground any parts which were at high voltage during the test or have
the potential for acquiring an induced charge during the test or retaining a charge even after disconnection of the voltage source.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from the Institute of Electrical and Electronics Engineers, 445 Hoes Ln., P.O. Box 1331, Piscataway, NJ 08854-1331.
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4.1.4 Thoroughly instruct all operators as to the correct procedures for performing tests safely.
4.1.5 When making high voltage tests, particularly in compressed gas or in oil, it is possible for the energy released at breakdown
to be sufficient to result in fire, explosion, or rupture of the test chamber. Design test equipment, test chambers , and test specimens
so as to minimize the possibility of such occurrences and to eliminate the possibility of personal injury. If the potential for fire
exists, have fire suppression equipment available. See 11.3.4.
5. Summary of Test Method
5.1 Test specimens are aged in air at three or preferably four temperatures above the expected use temperature. The aging
temperatures are selected so that the thermal life is at least 100 h 100 h at the highest aging temperature and 5000 h 5000 h at the
lowest aging temperature. A thermal-life curve is plotted for each aging temperature. The values of thermal life determined from
the thermal-life curve are used to plot the thermal-endurance graph. A temperature index is determined from the thermal-endurance
graph for each aging criterion used. (It is possible to obtain different values for the thermal index of a material with different aging
criteria.)
6. Significance and Use
6.1 Thermal degradation is often a major factor affecting the life of insulating materials and the equipment in which they are used.
The temperature index provides a means for comparing the thermal capability of different materials in respect to the degradation
of a selected property (the aging criterion). This property needs to directly or indirectly represent functional needs in application.
For example, it is possible that a change in dielectric strength will be of direct, functional importance. However, more often it is
possible that a decrease in dielectric strength will indirectly indicate the development of undesirable cracking (embrittlement). A
decrease in flexural strength has the potential to be of direct importance in some applications, but also has the potential to indirectly
indicate a susceptibility to failure in vibration. Often, it is necessary that two or more criteria of failure be used; for example,
dielectric strength and flexural strength.
6.2 Other factors, such as vibration, moisture and contaminants, have the potential to cause failure after thermal degradation takes
place. In this test method, water absorption provides one means to evaluate such considerations.
6.3 For some applications, the aging criteria in this test method will not be the most suitable. Other criteria, such as elongation
at tensile or flexural failure, or resistivity after exposure to high humidity or weight loss, have the potential to serve better. The
procedures in this test method have the potential to be used with such aging criteria. It is important to consider both the nature
of the material and its application. For example, it is possible that tensile strength will be a poor choice for glass-fiber reinforced
laminates, because it is possible that the glass fiber will maintain the tensile strength even when the associated resin is badly
deteriorated. In this case, flexural strength is a better criterion of thermal aging.
6.4 When dictated by the needs of the application, it is possible that an aging atmosphere other than air will be needed and used.
For example, thermal aging can be conducted in an oxygen-free, nitrogen atmosphere.
7. End Point
7.1 An expression of the thermal life of a material, even for comparative purposes only, inevitably involves the choice of an end
point. The end point is one of the following 4 criteria: a fixed magnitude of the property criterion, a percentage reduction from
its initial magnitude, the minimum magnitude obtainable with time (that is, when change with time ceases), or a fixed degrading
change rate (that is, a fixed value for the negative derivative of property with respect to time).
7.2 Experience has shown that the choice of an end point can affect the comparative thermal life. A choice of end points is guided
by the limiting requirements imposed on the insulation by the manner and conditions of use in the complete system. End points
are not specified in this test method. The first concern is to determine the values of the chosen properties as a function of time of
thermal exposure at specified temperatures. The properties are determined at various intervals of time until a practical minimum
or maximum magnitude, whichever is applicable, is reached. The data that result are thus universal, that is, usable for any
subsequently chosen end point as determined by the specific application of the rigid electrical insulation.
7.3 The specification for each material needs to state the end point to be used.
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8. Aging Ovens
8.1 The accuracy of the test results will depend on the accuracy with which the exposure temperature of the test specimens is
known. Experience has shown, as indicated in Table 1, that the thermal life is approximately halved for a 10°C10 °C increase in
exposure temperature.
8.2 Use aging ovens that conform to the requirements of Type I of Specification D5423.
9. Test Specimen
9.1 The accuracy of the test results depends significantly upon the number of specimens exposed at each temperature and the
dispersion of the test results. The larger the individual deviations from the mean, the greater is the number of test specimens needed
to achieve satisfactory accuracy. Experience has shown that a minimum of five test specimens needs to be used at each exposure
temperature. A separate group of test specimens is required for each exposure period.
9.2 The rate of deterioration will be significantly influenced by specimen thickness. Consequently it is important to test specimens
of the same nominal thickness when comparing the thermal degradation of two or more materials unless information relating
degradation to thickness is available that indicates the contrary. This test method specifies the specimen size, including thickness,
for each property selected.
PROCEDURES
10. Oven Aging (Thermal Exposure)
10.1 Factors such as moisture, chemical contamination, and mechanical stress or vibration usually do not in themselves cause
failure, but are factors that have the potential to result in failure only after the material has been weakened by thermal deterioration.
For this reason, exposure to elevated temperatures is the primary deteriorating influence considered in this test method.
10.2 Table 1 is intended as a guide for the selection of thermal exposure. Select times and temperatures from those given in this
table. The exposure times given are approximately equal to the average estimated life at each exposure temperature based on
thermal aging data obtained on insulating materials and systems. The potential exists that this table will be revised as a result of
experience. The potential that either the time or the temperature will be adjusted to make the best use of available oven facilities.
TABLE 1 Temperature and Exposure Time in Days
Exposure Estimated Hottest-Spot Temperature Range, °C
Temperature,
100 to 125 to 150 to 175 to 200 to
°C
120 145 170 195 240
300 . . . . 10
290 . . . . 20
280 . . . . 40
270 . . . . 70
260 . . . . 140
250 . . . 10 280
240 . . . 20 490
230 . . . 40 .
220 . . 10 70 .
210 . . 20 140 .
200 . 10 40 280 .
190 . 20 70 490 .
180 10 40 140 . .
170 20 70 280 . .
160 40 140 490 . .
150 70 280 . . .
140 140 490 . . .
130 280 . . . .
120 490 . . . .
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10.3 Age at a minimum of three and preferably four temperatures. Choose the lowest temperature to be less than 25°C25 °C above
the hottest-spot temperature expected in use so that the thermal life is at least 5000 h. Select the highest temperature so that the
thermal life is at least 100 h. If possible, for best results, the aging temperatures need to differ from each other by at least
20°C.20 °C.
10.4 For an unknown material, the selection of the appropriate aging temperatures will require a short exploratory test performed
at the highest likely aging temperature. Results from thermal aging tests for a material with similar composition have the potential
to provide clues for an appropriate selection of the first exploratory temperature. The chemical composition of the material to be
tested, if known, has the potential to also provide a means for estimating the first aging temperature to be used. Additional tests
can then be made at lower or higher temperatures as indicated by the first exploratory test. (See Table 1 and 10.3.)
10.5 Place a sufficient number of specimens to conduct the tests used for the selected aging criterion in each aging oven. Remove
all of the test specimens after a selected interval of time. (See 10.6.) Select the test specimens needed for the test at random. Return
the remaining samples to the aging oven and repeat the process after each succeeding time interval (aging period).
10.6 Suggested total exposure times with associated test temperatures are given in Table 1. Initially, at least seven, evenly-spaced,
test intervals at each test temperature are usually needed to provide sufficient data for the thermal life curves. (It is wise to provide
sufficient specimens for ten intervals.) It is most important to adequately define the later portion of the thermal life curve. With
experience, it is possible that fewer test specimens and time intervals will be needed. At the start, place only about half of th
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