1、Designation: D 2304 97 (Reapproved 2002)An American National StandardStandard Test Method forThermal Endurance of Rigid Electrical Insulating Materials1This standard is issued under the fixed designation D 2304; the number immediately following the designation indicates the year oforiginal adoption
2、or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method2provides procedures for evaluating thethermal endurance of
3、rigid electrical insulating materials.Dielectric strength, 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 temperat
4、ure. A means is described for determin-ing a temperature 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 enclosure
5、s for manytypes of application where retention of the selected propertyafter 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 notintended to rep
6、lace Test Method D 1830 for those materialswhich 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 info
7、r-mation.1.6 This standard does not purport 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 spe
8、cific warningstatement is given in 10.3.4.2. Referenced Documents2.1 ASTM Standards:D 149 Test Method for Dielectric Breakdown Voltage andDielectric Strength of Solid Electrical Insulating Materialsat Commercial Power Frequencies3D 229 Test Methods for Rigid Sheet and Plate MaterialsUsed for Electri
9、cal Insulation3D 570 Test Method for Water Absorption of Plastics4D 790 Test Methods for Flexural Properties of Unreinforcedand Reinforced Plastics and Electrical Insulating Materi-als4D 1830 Test Method for Thermal Endurance of FlexibleSheet Materials Used for Electrical Insulation by theCurved Ele
10、ctrode Method3D 5423 Specification for Forced-Convection LaboratoryOvens for Evaluation of Electrical Insulation52.2 IEEE:6No. 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
11、of Electrical Insulating MaterialsNo. 101 Guide 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 beststraigh
12、t line fit, determined by least squares, of end 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 n
13、umber which permits com-parison of the temperature/time characteristics of an electricalinsulating material, or a simple combination of materials, basedon the temperature in degrees Celsius which is obtained byextrapolating the Arrhenius plot of life versus temperature to aspecified time, usually 20
14、 000 h.1This test method is under the jurisdiction of ASTM Committee D09 onElectrical and Electronic Insulating Materials and is the direct responsibility ofSubcommittee D09.07 on Flexible and Rigid Insulating Materials.Current edition approved Sept. 10, 1997. Published November 1997. Originallyissu
15、ed as D 2304 64 T. Last previous edition D 2304 96.2This test method is a revision of a procedure written 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 Ge
16、neral Meeting Feb. 16, 1959. See references at end of this testmethod.3Annual Book of ASTM Standards, Vol 10.01.4Annual Book of ASTM Standards, Vol 08.01.5Annual Book of ASTM Standards, Vol 10.02.6Available from the Institute of Electrical and Electronics Engineers, 445 HoesLn., P.O. Box 1331, Pisca
17、taway, NJ 08854-1331.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.3 thermal life, nthe time necessary for a specificproperty of a material, or a simple combination of materials, todegrade to a defined end point when aged at a
18、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 ofmaterials, is measured at room temperature and the valuesplotted as a function of time.3.2 Definitions o
19、f 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 (0.02 in.). It isgenerally used as terminal boards, spacers, coil forms, voltagebarriers, and circuit boards
20、.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 is at least 100h at the highest aging temperature and 5000 h at the lowestaging temperature. A thermal-
21、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 from the thermal-endurance graph for each aging criterion used. (Differentvalues for the thermal index of a
22、 material may be obtained withdifferent aging criteria.)5. Significance and Use5.1 Thermal degradation is often a major factor affecting thelife of insulating materials and the equipment in which they areused. The temperature index provides a means for comparingthe thermal capability of different ma
23、terials in respect to thedegradation of a selected property (the aging criterion). Thisproperty should directly or indirectly represent functionalneeds in application. For example, a change in dielectricstrength may be of direct, functional importance. However,more often a decrease in dielectric str
24、ength may indirectlyindicate the development of undesirable cracking (embrittle-ment). A decrease in flexural strength may be of directimportance in some applications, but may also indirectlyindicate a susceptibility to failure in vibration. Often two ormore criteria of failure should be used; for e
25、xample, dielectricstrength and flexural strength.5.2 Other factors, such as vibration, moisture and contami-nants, may cause failure after thermal degradation takes place.In this test method, water absorption provides one means toevaluate such considerations.5.3 For some applications, the aging crit
26、eria in this testmethod may not be the most suitable. Other criteria, such aselongation at tensile or flexural failure, or resistivity afterexposure to high humidity or weight loss, may serve better. Theprocedures in this test method may be used with such agingcriteria. It is important to consider b
27、oth the nature of thematerial and its application. For example, tensile strength maybe a poor choice for glass-fiber reinforced laminates, becausethe glass fiber may maintain the tensile strength even when theassociated resin is badly deteriorated. In this case, flexuralstrength is a better criterio
28、n of thermal aging.5.4 When dictated by the needs of the application, an agingatmosphere other than air may be needed and used. Forexample, thermal aging can be conducted in an oxygen-free,nitrogen atmosphere.6. End Point6.1 An expression of the thermal life of a material, even forcomparative purpos
29、es 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 changerate (that is, a
30、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 pointsshould, therefore, be guided by the limiting requirementsimposed on the insulation by the manner and conditi
31、ons 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 time until a practical mi
32、nimum 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 should state the endpoint t
33、o 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 temperature.7.2 Use agi
34、ng ovens that conform to the requirements ofType I of Specification D 5423.8. 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 individualdeviations from the mean, the
35、greater is the number of testspecimens needed to achieve satisfactory accuracy. Experiencehas shown that a minimum of five test specimens should beused at each exposure temperature. A separate group of testspecimens is required for each exposure period.8.2 The rate of deterioration may be significan
36、tly influencedby specimen thickness. Consequently it is important to testspecimens of the same nominal thickness when comparing thethermal degradation of two or more materials unless informa-tion relating degradation to thickness is available that indicatesthe contrary. This test method specifies th
37、e specimen size,including thickness, for each property selected.D 2304 97 (2002)2PROCEDURES9. Oven Aging (Thermal Exposure)9.1 Factors such as moisture, chemical contamination, andmechanical stress or vibration usually do not in themselvescause failure, but are factors that may result in failure onl
38、y afterthe material has been weakened by thermal deterioration. Forthis reason, exposure to elevated temperatures is the primarydeteriorating influence considered in this test method.9.2 Table 1 is intended as a guide for the selection ofthermal exposure. Select times and temperatures from thosegive
39、n 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 this table may berevised as a result of experience. Either the time or thetemper
40、ature may be adjusted to make the best use of availableoven facilities.9.3 Age at a minimum of three and preferably four tempera-tures. Choose the lowest temperature to be less than 25Cabove the hottest-spot temperature expected in use so that thethermal life is at least 5000 h. Select the highest t
41、emperature sothat the thermal life is at least 100 h. If possible, the agingtemperatures should differ from each other by at least 20C.9.4 The selection of the appropriate aging temperatures foran unknown material may require a short exploratory testperformed at the highest likely aging temperature.
42、 Results fromthermal aging tests for a material with similar composition mayprovide clues for an appropriate selection of the first explor-atory temperature. The chemical composition of the material tobe tested, if known, may also provide a means for estimatingthe first aging temperature to be used.
43、 Additional tests can thenbe made at lower or higher temperatures as indicated by thefirst exploratory test. (See Table 1 and 9.3.)9.5 Place a 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
44、 interval oftime. (See 9.6.) Select the test specimens needed for the test atrandom. Return the remaining samples to the aging oven andrepeat the process after each succeeding time interval (agingperiod).9.6 Suggested total exposure times with associated testtemperatures are given in Table 1. Initia
45、lly, at least seven,evenly-spaced, test intervals at each test temperature areusually needed to provide sufficient data for the thermal lifecurves. (It is wise to provide sufficient specimens for tenintervals.) It is most important to adequately define the laterportion of the thermal life curve. Wit
46、h experience, fewer testspecimens and time intervals may be needed. At the start, placeonly about half of the test specimens in the aging oven. Thenuse a relatively long, initial aging period. The test results afterthis initial aging period can provide guidance for subsequenttime intervals for the r
47、emaining specimens in the oven. Thenplace the so-far, unaged specimens in the oven or withhold foran even longer period as suggested by the test results.10. Dielectric Strength10.1 Apparatus:10.1.1 A testing device shall be employed whereby the testspecimen is clamped under pressure between elastome
48、ricgaskets 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
49、 holder shall contain a 19-mm (34-in.) diameterelectrode, 11, with edges rounded to a radius of 3.18 mm (18in.). An elastomeric gasket, 12, shall surround each electrode,allowing approximately 1.59-mm (116-in.) circumferentialclearance between the gasket and the electrode. The specimen,5, shall be placed between the electrodes, which shall bespring-loaded, 10, to provide 2.22-N (12-lbf) electrode pres-sure. Application of compressed air, controlled by a regulator,9, to the air cylinder, 8, causes the lower electrode assembly tomove upward against the specimen. The speci
