ASTM D2304-97(2002)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
An American National Standard
Designation:D2304–97 (Reapproved 2002)
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 2. Referenced Documents
2 3
1.1 Thistestmethod providesproceduresforevaluatingthe 2.1 ASTM Standards:
thermal endurance of rigid electrical insulating materials. D149 Test Method for Dielectric Breakdown Voltage and
Dielectric strength, flexural strength, or water absorption are Dielectric Strength of Solid Electrical Insulating Materials
determined at room temperature after aging for increasing at Commercial Power Frequencies
periods of time in air at selected-elevated temperatures. A D229 Test Methods for Rigid Sheet and Plate Materials
thermal-endurance graph is plotted using a selected end point Used for Electrical Insulation
at each aging temperature. A means is described for determin- D570 Test Method for Water Absorption of Plastics
ing a temperature index by extrapolation of the thermal D790 Test Methods for Flexural Properties of Unreinforced
endurance graph to a selected time. and Reinforced Plastics and Electrical Insulating Materials
1.2 This test method is most applicable to rigid electrical D1830 Test Method for Thermal Endurance of Flexible
insulation such as supports, spacers, voltage barriers, coil Sheet Materials Used for Electrical Insulation by the
forms, terminal boards, circuit boards and enclosures for many Curved Electrode Method
types of application where retention of the selected property D5423 Specification for Forced-Convection Laboratory
after heat aging is important. Ovens for Evaluation of Electrical Insulation
1.3 When dielectric strength is used as the aging criterion, 2.2 IEEE:
this test method may also be used for some thin sheet (flexible) No. 1 General Principles Upon Which Temperature Limits
materials, which become rigid with thermal aging, but is not Are Based in the Rating of Electric Equipment
intended to replace Test Method D1830 for those materials No. 98 Guide for the Preparation of Test Procedures for the
which must retain a degree of flexibility in use. Thermal Evaluation of Electrical Insulating Materials
1.4 This test method is not applicable to ceramics, glass, or No. 101 Guide for the Statistical Analysis of Test Data
similar inorganic materials.
3. Terminology
1.5 The values stated in metric units are to be regarded as
3.1 Definitions:
standard. Other units (in parentheses) are provided for infor-
mation. 3.1.1 Arrhenius plot, n—a graph of the logarithm of thermal
life as a function of the reciprocal of absolute temperature.
1.6 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the 3.1.1.1 Discussion—This is normally depicted as the best
straight line fit, determined by least squares, of end points
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica- obtained at aging temperatures. It is important that the slope,
which is the activation energy of the degradation reaction, be
bility of regulatory limitations prior to use. Aspecific warning
statement is given in 10.3.4. approximately constant within the selected temperature range
to ensure a valid extrapolation.
3.1.2 temperature index, n—a number which permits com-
This test method is under the jurisdiction of ASTM Committee D09 on
parison of the temperature/time characteristics of an electrical
Electrical and Electronic Insulating Materials and is the direct responsibility of
Subcommittee D09.07 on Flexible and Rigid Insulating Materials.
Current edition approved Sept. 10, 1997. Published November 1997. Originally
issued as D2304 – 64 T. Last previous edition D2304 – 96. DOI: 10.1520/D2304-
97R02. 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 service@astm.org. 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.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D2304–97 (2002)
insulatingmaterial,orasimplecombinationofmaterials,based material and its application. For example, tensile strength may
on the temperature in degrees Celsius which is obtained by be a poor choice for glass-fiber reinforced laminates, because
extrapolating theArrhenius plot of life versus temperature to a the glass fiber may maintain the tensile strength even when the
specified time, usually 20 000 h. associated resin is badly deteriorated. In this case, flexural
3.1.3 thermal life, n—the time necessary for a specific strength is a better criterion of thermal aging.
property of a material, or a simple combination of materials, to 5.4 When dictated by the needs of the application, an aging
degrade to a defined end point when aged at a specified atmosphere other than air may be needed and used. For
temperature. example, thermal aging can be conducted in an oxygen-free,
3.1.4 thermal life curve, n—a graphical representation of nitrogen atmosphere.
thermal life at a specified aging temperature in which the value
of a property of a material, or a simple combination of 6. End Point
materials, is measured at room temperature and the values
6.1 An expression of the thermal life of a material, even for
plotted as a function of time.
comparative purposes only, inevitably involves the choice of
3.2 Definitions of Terms Specific to This Standard:
an end point. The end point could be a fixed magnitude of the
3.2.1 rigid electrical insulating material, n—an electrical
property criterion, a percentage reduction from its initial
insulating material having a minimum flexural modulus of 690
magnitude, the minimum magnitude obtainable with time (that
MPa and minimum use thickness of 0.5 mm (0.02 in.). It is
is, when change with time ceases), or a fixed degrading change
generally used as terminal boards, spacers, coil forms, voltage
rate (that is, a fixed value for the negative derivative of
barriers, and circuit boards.
property with respect to time).
6.2 Experience has shown that the choice of an end point
4. Summary of Test Method
can affect the comparative thermal life.Achoice of end points
4.1 Test specimens are aged in air at three or preferably four
should, therefore, be guided by the limiting requirements
temperatures above the expected use temperature. The aging
imposed on the insulation by the manner and conditions of use
temperatures are selected so that the thermal life is at least 100
in the complete system. End points are not specified in this test
h at the highest aging temperature and 5000 h at the lowest
method. The first concern is to determine the values of the
aging temperature. A thermal-life curve is plotted for each
chosen properties as a function of time of thermal exposure at
aging temperature. The values of thermal life determined from
specified temperatures. The properties are determined at vari-
the thermal-life curve are used to plot the thermal-endurance
ous intervals of time until a practical minimum or maximum
graph. A temperature index is determined from the thermal-
magnitude, whichever is applicable, is reached. The data that
endurance graph for each aging criterion used. (Different
result are thus universal, that is, usable for any subsequently
valuesforthethermalindexofamaterialmaybeobtainedwith
chosen end point as determined by the specific application of
different aging criteria.)
the rigid electrical insulation.
6.3 The specification for each material should state the end
5. Significance and Use
point to be used.
5.1 Thermaldegradationisoftenamajorfactoraffectingthe
lifeofinsulatingmaterialsandtheequipmentinwhichtheyare
7. Aging Ovens
used. The temperature index provides a means for comparing
7.1 The accuracy of the test results will depend on the
the thermal capability of different materials in respect to the
accuracy with which the exposure temperature of the test
degradation of a selected property (the aging criterion). This
specimens is known. Experience has shown, as indicated in
property should directly or indirectly represent functional
Table 1, that the thermal life is approximately halved for a
needs in application. For example, a change in dielectric
10°C increase in exposure temperature.
strength may be of direct, functional importance. However,
7.2 Use aging ovens that conform to the requirements of
more often a decrease in dielectric strength may indirectly
Type I of Specification D5423.
indicate the development of undesirable cracking (embrittle-
ment). A decrease in flexural strength may be of direct
8. Test Specimen
importance in some applications, but may also indirectly
indicate a susceptibility to failure in vibration. Often two or 8.1 The accuracy of the test results depends significantly
more criteria of failure should be used; for example, dielectric upon the number of specimens exposed at each temperature
strength and flexural strength. and the dispersion of the test results. The larger the individual
5.2 Other factors, such as vibration, moisture and contami- deviations from the mean, the greater is the number of test
nants, may cause failure after thermal degradation takes place. specimens needed to achieve satisfactory accuracy. Experience
In this test method, water absorption provides one means to has shown that a minimum of five test specimens should be
evaluate such considerations. used at each exposure temperature. A separate group of test
5.3 For some applications, the aging criteria in this test specimens is required for each exposure period.
method may not be the most suitable. Other criteria, such as 8.2 The rate of deterioration may be significantly influenced
elongation at tensile or flexural failure, or resistivity after by specimen thickness. Consequently it is important to test
exposuretohighhumidityorweightloss,mayservebetter.The specimens of the same nominal thickness when comparing the
procedures in this test method may be used with such aging thermal degradation of two or more materials unless informa-
criteria. It is important to consider both the nature of the tion relating degradation to thickness is available that indicates
D2304–97 (2002)
TABLE 1 Temperature and Exposure Time in Days
be made at lower or higher temperatures as indicated by the
Exposure Estimated Hottest-Spot Temperature Range, °C first exploratory test. (See Table 1 and 9.3.)
Temperature,
9.5 Place a sufficient number of specimens to conduct the
100 to 125 to 150 to 175 to 200 to
°C
120 145 170 195 240
tests used for the selected aging criterion in each aging oven.
300 . . . . 10 Remove all of the test specimens after a selected interval of
290 . . . . 20
time. (See 9.6.) Select the test specimens needed for the test at
280 . . . . 40
random. Return the remaining samples to the aging oven and
270 . . . . 70
260 . . . . 140 repeat the process after each succeeding time interval (aging
period).
250 . . . 10 280
9.6 Suggested total exposure times with associated test
240 . . . 20 490
230 . . . 40 . temperatures are given in Table 1. Initially, at least seven,
220 . . 10 70 .
evenly-spaced, test intervals at each test temperature are
210 . . 20 140 .
usually needed to provide sufficient data for the thermal life
200 . 10 40 280 .
curves. (It is wise to provide sufficient specimens for ten
190 . 20 70 490 .
intervals.) It is most important to adequately define the later
180 10 40 140 . .
portion of the thermal life curve. With experience, fewer test
170 20 70 280 . .
160 40 140 490 . .
specimensandtimeintervalsmaybeneeded.Atthestart,place
only about half of the test specimens in the aging oven. Then
150 70 280 . . .
use a relatively long, initial aging period. The test results after
140 140 490 . . .
130 280 . . . . this initial aging period can provide guidance for subsequent
120 490 . . . .
time intervals for the remaining specimens in the oven. Then
place the so-far, unaged specimens in the oven or withhold for
an even longer period as suggested by the test results.
the contrary. This test method specifies the specimen size,
10. Dielectric Strength
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
causefailure,butarefactorsthatmayresultinfailureonlyafter
holder, 2, which is stationary, and a movable lower electrode
the material has been weakened by thermal deterioration. For
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 test 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,
giveninthistable.Theexposuretimesgivenareapproximately
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 regul
...