ASTM D2304-97
(Test Method)Standard Test Method for Thermal Endurance of Rigid Electrical Insulating Materials
Standard Test Method for Thermal Endurance of Rigid Electrical Insulating Materials
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, this test method may also be used for some thin sheet (flexible) materials, which become rigid with thermal aging, but is not intended to replace Test Method D 1830 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 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 and health practices and determine the applicability of regulatory limitations prior to use. A specific warning statement is given in 10.3.4.
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Designation: D 2304 – 97 An American National Standard
Standard Test Method for
Thermal Endurance of Rigid Electrical Insulating Materials
This standard is issued under the fixed designation D 2304; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope at Commercial Power Frequencies
D 229 Test Methods for Rigid Sheet and Plate Materials
1.1 This test method provides procedures for evaluating the
Used for Electrical Insulation
thermal endurance of rigid electrical insulating materials.
D 570 Test Method for Water Absorption of Plastics
Dielectric strength, flexural strength or water absorption are
D 790 Test Methods for Flexural Properties of Unreinforced
determined at room temperature after aging for increasing
and Reinforced Plastics and Electrical Insulating Materi-
periods of time in air at selected-elevated temperatures. A
als
thermal-endurance graph is plotted using a selected end point
D 1830 Test Method for Thermal Endurance of Flexible
at each aging temperature. A means is described for determin-
Sheet Materials Used for Electrical Insulation by the
ing a temperature index by extrapolation of the thermal
Curved Electrode Method
endurance graph to a selected time.
D 5423 Specification for Forced-Convection Laboratory
1.2 This test method is most applicable to rigid electrical
Ovens for Evaluation of Electrical Insulation
insulation such as supports, spacers, voltage barriers, coil
2.2 IEEE:
forms, terminal boards, circuit boards and enclosures for many
No. 1 General Principles Upon Which Temperature Limits
types of application where retention of the selected property
Are Based in the Rating of Electric Equipment
after heat aging is important.
No. 98 Guide for the Preparation of Test Procedures for the
1.3 When dielectric strength is used as the aging criterion,
Thermal Evaluation of Electrical Insulating Materials
this test method may also be used for some thin sheet (flexible)
No. 101 Guide for the Statistical Analysis of Test Data
materials, which become rigid with thermal aging, but is not
intended to replace Test Method D 1830 for those materials
3. Terminology
which must retain a degree of flexibility in use.
3.1 Definitions:
1.4 This test method is not applicable to ceramics, glass or
3.1.1 Arrhenius plot, n—a graph of the logarithm of thermal
similar inorganic materials.
life as a function of the reciprocal of absolute temperature.
1.5 The values stated in metric units are to be regarded as
3.1.1.1 Discussion—This is normally depicted as the best
standard. Other units (in parentheses) are provided for infor-
straight line fit, determined by least squares, of end points
mation.
obtained at aging temperatures. It is important that the slope,
1.6 This standard does not purport to address all of the
which is the activation energy of the degradation reaction, be
safety concerns, if any, associated with its use. It is the
approximately constant within the selected temperature range
responsibility of the user of this standard to establish appro-
to ensure a valid extrapolation.
priate safety and health practices and determine the applica-
3.1.2 temperature index, n—a number which permits com-
bility of regulatory limitations prior to use. A specific warning
parison of the temperature/time characteristics of an electrical
statement is given in Note 3.
insulating material, or a simple combination of materials, based
2. Referenced Documents on the temperature in degrees Celsius which is obtained by
extrapolating the Arrhenius plot of life versus temperature to a
2.1 ASTM Standards:
specified time, usually 20 000 h.
D 149 Test Method for Dielectric Breakdown Voltage and
3.1.3 thermal life, n—the time necessary for a specific
Dielectric Strength of Solid Electrical Insulating Materials
property of a material, or a simple combination of materials, to
degrade to a defined end point when aged at a specified
This test method is under the jurisdiction of ASTM Committee D-9 on
temperature.
Electrical and Electronic Insulating Materials and is the direct responsibility of
3.1.4 thermal life curve, n—a graphical representation of
Subcommittee D09.07 on Flexible and Rigid Insulating Materials.
Current edition approved Sept. 10, 1997. Published November 1997. Originally
issued as D 2304 – 64 T. Last previous edition D 2304 – 96.
2 3
This test method is a revision of a procedure written by the Working Group on Annual Book of ASTM Standards, Vol 10.01.
Rigid Electrical Insulating Materials of the Subcommittee on Thermal Evaluation, Annual Book of ASTM Standards, Vol 08.01.
IEEE Electrical Insulation Committee, which was presented as CP 59-113 at the Annual Book of ASTM Standards, Vol 10.02.
IEEE Winter General Meeting Feb. 1–6, 1959. See references at end of this test Available from the Institute of Electrical and Electronics Engineers, 345 East
method. 47th St., New York, NY 10017.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
D 2304
thermal life at a specified aging temperature in which the value 6. End Point
of a property of a material, or a simple combination of
6.1 An expression of the thermal life of a material, even for
materials, is measured at room temperature and the values
comparative purposes only, inevitably involves the choice of
plotted as a function of time.
an end point. The end point could be a fixed magnitude of the
3.2 Definitions of Terms Specific to This Standard:
property criterion, a percentage reduction from its initial
3.2.1 rigid electrical insulating material, n—an electrical
magnitude, the minimum magnitude obtainable with time (that
insulating material having a minimum flexural modulus of 690
is, when change with time ceases), or a fixed degrading change
MPa and minimum use thickness of 0.5 mm (0.02 in.). It is
rate (that is, a fixed value for the negative derivative of
generally used as terminal boards, spacers, coil forms, voltage
property with respect to time).
barriers, and circuit boards.
6.2 Experience has shown that the choice of an end point
can affect the comparative thermal life. A choice of end points
4. Summary of Test Method
should, therefore, be guided by the limiting requirements
4.1 Test specimens are aged in air at three or preferably four
imposed on the insulation by the manner and conditions of use
temperatures above the expected use temperature. The aging
in the complete system. End points are not specified in this test
temperatures are selected so that the thermal life is at least 100
method. The first concern is to determine the values of the
h at the highest aging temperature and 5000 h at the lowest
chosen properties as a function of time of thermal exposure at
aging temperature. A thermal-life curve is plotted for each
specified temperatures. The properties are determined at vari-
aging temperature. The values of thermal life determined from
ous intervals of time until a practical minimum or maximum
the thermal-life curve are used to plot the thermal-endurance
magnitude, whichever is applicable, is reached. The data that
graph. A temperature index is determined from the thermal-
result are thus universal, that is, usable for any subsequently
endurance graph for each aging criterion used. (Different
chosen end point as determined by the specific application of
values for the thermal index of a material may be obtained with
the rigid electrical insulation.
different aging criteria.)
6.3 The specification for each material should state the end
point to be used.
5. Significance and Use
7. Aging Ovens
5.1 Thermal degradation is often a major factor affecting the
life of insulating materials and the equipment in which they are
7.1 The accuracy of the test results will depend on the
used. The temperature index provides a means for comparing
accuracy with which the exposure temperature of the test
the thermal capability of different materials in respect to the
specimens is known. Experience has shown, as indicated in
degradation of a selected property (the aging criterion). This
Table 1, that the thermal life is approximately halved for a
property should directly or indirectly represent functional
10°C increase in exposure temperature.
needs in application. For example, a change in dielectric
7.2 Use aging ovens that conform to the requirements of
strength may be of direct, functional importance. However,
Type I of Specification D 5423.
more often a decrease in dielectric strength may indirectly
8. Test Specimen
indicate the development of undesirable cracking (embrittle-
8.1 The accuracy of the test results depends significantly
ment). A decrease in flexural strength may be of direct
upon the number of specimens exposed at each temperature
importance in some applications, but may also indirectly
indicate a susceptibility to failure in vibration. Often two or
TABLE 1 Temperature and Exposure Time in Days
more criteria of failure should be used; for example, dielectric
Exposure Estimated Hottest-Spot Temperature Range, °C
strength and flexural strength.
Temperature,
100 to 125 to 150 to 175 to 200 to
5.2 Other factors, such as vibration, moisture and contami- °C
120 145 170 195 240
nants, may cause failure after thermal degradation takes place.
300 . . . . 10
In this test method, water absorption provides one means to
290 . . . . 20
evaluate such considerations.
280 . . . . 40
270 . . . . 70
5.3 For some applications, the aging criteria in this test
260 . . . . 140
method may not be the most suitable. Other criteria, such as
elongation at tensile or flexural failure, or resistivity after 250 . . . 10 280
240 . . . 20 490
exposure to high humidity or weight loss, may serve better. The
230 . . . 40 .
procedures in this test method may be used with such aging
220 . . 10 70 .
criteria. It is important to consider both the nature of the 210 . . 20 140 .
material and its application. For example, tensile strength may
200 . 10 40 280 .
be a poor choice for glass-fiber reinforced laminates, because
190 . 20 70 490 .
the glass fiber may maintain the tensile strength even when the
180 10 40 140 . .
170 20 70 280 . .
associated resin is badly deteriorated. In this case, flexural
160 40 140 490 . .
strength is a better criterion of thermal aging.
5.4 When dictated by the needs of the application, an aging 150 70 280 . . .
140 140 490 . . .
atmosphere other than air may be needed and used. For
130 280 . . . .
example, thermal aging can be conducted in an oxygen-free,
120 490 . . . .
nitrogen atmosphere.
D 2304
and the dispersion of the test results. The larger the individual curves. (It is wise to provide sufficient specimens for ten
deviations from the mean, the greater is the number of test intervals.) It is most important to adequately define the later
specimens needed to achieve satisfactory accuracy. Experience portion of the thermal life curve. With experience, fewer test
has shown that a minimum of five test specimens should be specimens and time intervals may be needed. At the start, place
used at each exposure temperature. A separate group of test only about half of the test specimens in the aging oven. Then
specimens is required for each exposure period. use a relatively long, initial aging period. The test results after
8.2 The rate of deterioration may be significantly influenced this initial aging period can provide guidance for subsequent
by specimen thickness. Consequently it is important to test time intervals for the remaining specimens in the oven. Then
specimens of the same nominal thickness when comparing the place the so-far, unaged specimens in the oven or withhold for
thermal degradation of two or more materials unless informa- an even longer period as suggested by the test results.
tion relating degradation to thickness is available that indicates
10. Dielectric Strength
the contrary. This test method specifies the specimen size,
including thickness, for each property selected. 10.1 Apparatus:
10.1.1 A testing device shall be employed whereby the test
PROCEDURES
specimen is clamped under pressure between elastomeric
gaskets to prevent flashover during the measurement. A suit-
9. Oven Aging (Thermal Exposure)
able apparatus and details of the electrode assembly used in
9.1 Factors such as moisture, chemical contamination, and
this apparatus are illustrated in Fig. 1.
mechanical stress or vibration usually do not in themselves
10.1.2 The test assembly shall consist of an upper electrode
cause failure, but are factors that may result in failure only after
holder, 2, which is stationary, and a movable lower electrode
the material has been weakened by thermal deterioration. For 3
holder, 6. Each holder shall contain a 19-mm ( ⁄4-in.) diameter
this reason, exposure to elevated temperatures is the primary
electrode, 11, with edges rounded to a radius of 3.18 mm ( ⁄8
deteriorating influence considered in this method.
in.). An elastomeric gasket, 12, shall surround each electrode,
9.2 Table 1 is intended as a guide for the selection of
allowing approximately 1.59-mm ( ⁄16-in.) circumferential
thermal exposure. Select times and temperatures from those
clearance between the gasket and the electrode. The specimen,
given in this table. The exposure times given are approximately
5, shall be placed between the electrodes, which shall be
equal to the average estimated life at each exposure tempera-
spring-loaded, 10, to provide 2.22-N ( ⁄2-lbf) electrode pres-
ture based on thermal aging data obtained on insulating
sure. Application of compressed air, controlled by a regulator,
materials and systems. It is recognized that this table may be
9, to the air cylinder, 8, causes the lower electrode assembly to
revised as a result of experience. Either the time or the
move upward against the specimen. The specimen is thus
temperature may be adjusted to make the best use of available
sealed between the holders by the elastomeric gaskets.
oven facilities.
10.1.3 The holders shall be constructed from suitable elec-
9.3 Age at a minimum of three and preferably four tempera-
trical insulating materials.
tures. Choose the lowest temperature to be less than 25°C
NOTE 1—Polyethylene is suggested for room-temperature tests. Ce-
above the hottest-spot temperature expected in use so that the
ramic or silicone-glass may be used for elevated-temperature tests.
thermal life is at least 5000 h. Select the highest
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