1、Designation: D2304 10Standard Test Method forThermal Endurance of Rigid Electrical Insulating Materials1This standard is issued under the fixed designation D2304; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last rev
2、ision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method2provides procedures for evaluating thethermal endurance of rigid electrical insulating materials.Dielectric s
3、trength, flexural strength, or water absorption aredetermined at room temperature after aging for increasingperiods of time in air at selected-elevated temperatures. Athermal-endurance graph is plotted using a selected end pointat each aging temperature. A means is described for determin-ing a tempe
4、rature index by extrapolation of the thermalendurance graph to a selected time.1.2 This test method is most applicable to rigid electricalinsulation such as supports, spacers, voltage barriers, coilforms, terminal boards, circuit boards and enclosures for manytypes of application where retention of
5、the selected propertyafter heat aging is important.1.3 When dielectric strength is used as the aging criterion, itis 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 thosemateri
6、als which must retain a degree of flexibility in use.1.4 This test method is not applicable to ceramics, glass, orsimilar inorganic materials.1.5 The values stated in metric units are to be regarded asstandard. Other units (in parentheses) are provided for infor-mation.1.6 This standard does not pur
7、port to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. A specific warningstatement is given in 10.3
8、.4.2. Referenced Documents2.1 ASTM Standards:3D149 Test Method for Dielectric Breakdown Voltage andDielectric Strength of Solid Electrical Insulating Materialsat Commercial Power FrequenciesD229 Test Methods for Rigid Sheet and Plate MaterialsUsed for Electrical InsulationD570 Test Method for Water
9、Absorption of PlasticsD790 Test Methods for Flexural Properties of Unreinforcedand Reinforced Plastics and Electrical Insulating Materi-alsD1830 Test Method for Thermal Endurance of FlexibleSheet Materials Used for Electrical Insulation by theCurved Electrode MethodD5423 Specification for Forced-Con
10、vection Laboratory Ov-ens for Evaluation of Electrical Insulation2.2 IEEE:4No. 1 General Principles Upon Which Temperature LimitsAre Based in the Rating of Electric EquipmentNo. 98 Guide for the Preparation of Test Procedures for theThermal Evaluation of Electrical Insulating MaterialsNo. 101 Guide
11、for the Statistical Analysis of Test Data3. Terminology3.1 Definitions:3.1.1 Arrhenius plot, na graph of the logarithm of thermallife as a function of the reciprocal of absolute temperature.3.1.1.1 DiscussionThis is normally depicted as the beststraight line fit, determined by least squares, of end
12、pointsobtained at aging temperatures. It is important that the slope,which is the activation energy of the degradation reaction, beapproximately constant within the selected temperature rangeto ensure a valid extrapolation.3.1.2 temperature index, na number which permits com-parison of the temperatu
13、re/time characteristics of an electricalinsulating material, or a simple combination of materials, basedon the temperature in degrees Celsius which is obtained by1This test method is under the jurisdiction of ASTM Committee D09 onElectrical and Electronic Insulating Materials and is the direct respo
14、nsibility ofSubcommittee D09.07 on Flexible and Rigid Insulating Materials.Current edition approved Oct. 1, 2010. Published October 2010. Originallyissued as D2304 64 T. Last previous edition approved in 2002 as D2304 97R02.DOI: 10.1520/D2304-10.2This test method is a revision of a procedure written
15、 by the Working Group onRigid Electrical Insulating Materials of the Subcommittee on Thermal Evaluation,IEEE Electrical Insulation Committee, which was presented as CP 59-113 at theIEEE Winter General Meeting Feb. 16, 1959. See references at end of this testmethod.3For referenced ASTM standards, vis
16、it the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4Available from the Institute of Electrical and Electronics Engineers, 445 HoesLn., P.O. Box 133
17、1, Piscataway, NJ 08854-1331.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1extrapolating the Arrhenius plot of life versus temperature to aspecified time, usually 20 000 h.3.1.3 thermal life, nthe time necessary for a specificproper
18、ty of a material, or a simple combination of materials, todegrade to a defined end point when aged at a specifiedtemperature.3.1.4 thermal life curve, na graphical representation ofthermal life at a specified aging temperature in which the valueof a property of a material, or a simple combination of
19、materials, is measured at room temperature and the valuesplotted as a function of time.3.2 Definitions of Terms Specific to This Standard:3.2.1 rigid electrical insulating material, nan electricalinsulating material having a minimum flexural modulus of 690MPa and minimum use thickness of 0.5 mm (0.0
20、2 in.). It isgenerally used as terminal boards, spacers, coil forms, voltagebarriers, and circuit boards.4. Summary of Test Method4.1 Test specimens are aged in air at three or preferably fourtemperatures above the expected use temperature. The agingtemperatures are selected so that the thermal life
21、 is at least 100h at the highest aging temperature and 5000 h at the lowestaging temperature. A thermal-life curve is plotted for eachaging temperature. The values of thermal life determined fromthe thermal-life curve are used to plot the thermal-endurancegraph. A temperature index is determined fro
22、m the thermal-endurance graph for each aging criterion used. (It is possible toobtain different values for the thermal index of a material withdifferent aging criteria.)5. Significance and Use5.1 Thermal degradation is often a major factor affecting thelife of insulating materials and the equipment
23、in which they areused. The temperature index provides a means for comparingthe thermal capability of different materials in respect to thedegradation of a selected property (the aging criterion). Thisproperty needs to directly or indirectly represent functionalneeds in application. For example, it i
24、s possible that a changein dielectric strength will be of direct, functional importance.However, more often it is possible that a decrease in dielectricstrength will indirectly indicate the development of undesirablecracking (embrittlement). A decrease in flexural strength hasthe potential to be of
25、direct importance in some applications,but also has the potential to indirectly indicate a susceptibilityto failure in vibration. Often, it is necessary that two or morecriteria of failure be used; for example, dielectric strength andflexural strength.5.2 Other factors, such as vibration, moisture a
26、ndcontaminants, have the potential to cause failure after thermaldegradation takes place. In this test method, water absorptionprovides one means to evaluate such considerations.5.3 For some applications, the aging criteria in this testmethod will not be the most suitable. Other criteria, such aselo
27、ngation at tensile or flexural failure, or resistivity afterexposure to high humidity or weight loss, have the potential toserve better. The procedures in this test method have thepotential to be used with such aging criteria. It is important toconsider both the nature of the material and its applic
28、ation. Forexample, it is possible that tensile strength will be a poorchoice for glass-fiber reinforced laminates, because it is pos-sible that the glass fiber will maintain the tensile strength evenwhen the associated resin is badly deteriorated. In this case,flexural strength is a better criterion
29、 of thermal aging.5.4 When dictated by the needs of the application, it ispossible that an aging atmosphere other than air will be neededand used. For example, thermal aging can be conducted in anoxygen-free, nitrogen atmosphere.6. End Point6.1 An expression of the thermal life of a material, even f
30、orcomparative purposes only, inevitably involves the choice ofan end point. The end point could be a fixed magnitude of theproperty criterion, a percentage reduction from its initialmagnitude, the minimum magnitude obtainable with time (thatis, when change with time ceases), or a fixed degrading cha
31、ngerate (that is, a fixed value for the negative derivative ofproperty with respect to time).6.2 Experience has shown that the choice of an end pointcan affect the comparative thermal life. A choice of end pointsneeds to, therefore, be guided by the limiting requirementsimposed on the insulation by
32、the manner and conditions of usein the complete system. End points are not specified in this testmethod. The first concern is to determine the values of thechosen properties as a function of time of thermal exposure atspecified temperatures. The properties are determined at vari-ous intervals of tim
33、e until a practical minimum or maximummagnitude, whichever is applicable, is reached. The data thatresult are thus universal, that is, usable for any subsequentlychosen end point as determined by the specific application ofthe rigid electrical insulation.6.3 The specification for each material needs
34、 to state the endpoint to be used.7. Aging Ovens7.1 The accuracy of the test results will depend on theaccuracy with which the exposure temperature of the testspecimens is known. Experience has shown, as indicated inTable 1, that the thermal life is approximately halved for a10C increase in exposure
35、 temperature.7.2 Use aging ovens that conform to the requirements ofType I of Specification D5423.8. Test Specimen8.1 The accuracy of the test results depends significantlyupon the number of specimens exposed at each temperatureand the dispersion of the test results. The larger the individualdeviati
36、ons from the mean, the greater is the number of testspecimens needed to achieve satisfactory accuracy. Experiencehas shown that a minimum of five test specimens needs to beused at each exposure temperature. A separate group of testspecimens is required for each exposure period.8.2 It is possible tha
37、t the rate of deterioration will besignificantly influenced by specimen thickness. ConsequentlyD2304 102it is important to test specimens of the same nominal thicknesswhen comparing the thermal degradation of two or morematerials unless information relating degradation to thicknessis available that
38、indicates the contrary. This test methodspecifies the specimen size, including thickness, for eachproperty selected.PROCEDURES9. Oven Aging (Thermal Exposure)9.1 Factors such as moisture, chemical contamination, andmechanical stress or vibration usually do not in themselvescause failure, but are fac
39、tors that have the potential to result infailure only after the material has been weakened by thermaldeterioration. For this reason, exposure to elevated tempera-tures is the primary deteriorating influence considered in thistest method.9.2 Table 1 is intended as a guide for the selection ofthermal
40、exposure. Select times and temperatures from thosegiven in this table. The exposure times given are approximatelyequal to the average estimated life at each exposure tempera-ture based on thermal aging data obtained on insulatingmaterials and systems. It is recognized that the potential existsthat t
41、his table will be revised as a result of experience. Thepotential that either the time or the temperature will be adjustedto make the best use of available oven facilities.9.3 Age at a minimum of three and preferably four tempera-tures. Choose the lowest temperature to be less than 25Cabove the hott
42、est-spot temperature expected in use so that thethermal life is at least 5000 h. Select the highest temperature sothat the thermal life is at least 100 h. If possible, the agingtemperatures needs to differ from each other by at least 20C.9.4 It is possible that the selection of the appropriate aging
43、temperatures for an unknown material will require a shortexploratory test performed at the highest likely aging tempera-ture. Results from thermal aging tests for a material withsimilar composition have the potential to provide clues for anappropriate selection of the first exploratory temperature.
44、Thechemical composition of the material to be tested, if known,has the potential to also provide a means for estimating the firstaging temperature to be used. Additional tests can then bemade at lower or higher temperatures as indicated by the firstexploratory test. (See Table 1 and 9.3.)9.5 Place a
45、 sufficient number of specimens to conduct thetests used for the selected aging criterion in each aging oven.Remove all of the test specimens after a selected interval oftime. (See 9.6.) Select the test specimens needed for the test atrandom. Return the remaining samples to the aging oven andrepeat
46、the process after each succeeding time interval (agingperiod).9.6 Suggested total exposure times with associated testtemperatures are given in Table 1. Initially, at least seven,evenly-spaced, test intervals at each test temperature areusually needed to provide sufficient data for the thermal lifecu
47、rves. (It is wise to provide sufficient specimens for tenintervals.) It is most important to adequately define the laterportion of the thermal life curve. With experience, it is possiblethat fewer test specimens and time intervals will be needed. Atthe start, place only about half of the test specim
48、ens in theaging oven. Then use a relatively long, initial aging period. Thetest results after this initial aging period can provide guidancefor subsequent time intervals for the remaining specimens inthe oven. Then place the so-far, unaged specimens in the ovenor withhold for an even longer period a
49、s suggested by the testresults.10. Dielectric Strength10.1 Apparatus:10.1.1 A testing device shall be employed whereby the testspecimen is clamped under pressure between elastomericgaskets to prevent flashover during the measurement. A suit-able apparatus and details of the electrode assembly used inthis apparatus are illustrated in Fig. 1.10.1.2 The test assembly shall consist of an upper electrodeholder, 2, which is stationary, and a movable lower electrodeholder, 6. Each holder shall contain a 19-mm (34-in.) diameterelectrode, 11, with edges rounded to a radius of 3.
