1、BRITISH STANDARD BS4618-4.6: 1974 Recommendations for The presentation of plastics design data Part 4: Environmental and chemical effects Section 4.6: The thermal endurance of plastics IMPORTANT NOTE. Before reading this section it is essential to read the General Introduction to this series of Reco
2、mmendations issued separately. UDC001.8:678.5/.8:620.193.94:536.495BS4618-4.6:1974 BSI11-1999 The following BSI references relate to work on this Recommendation: Committee reference PLC/36/5 Draft for comment 72/54305 ISBN 0 580 08467 1 A British Standard does not purport to include all the necessar
3、y provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pagesi andii, pages1to11
4、and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. Amendments issued since publication Amd. No. Date CommentsBS4618-4.6:1974 BSI 11-1999 i Contents Page 0 Introductio
5、n 1 1 Principle 1 2 General 1 3 Special considerations for thermoplastics 2 4 Procedure 2 5 Form of presentation 3 Appendix A Example of test procedure 5 Appendix B Notes on ageing ovens 5 Appendix C Method for determining line of “best fit” (least squaresmethod) 6 Appendix D Tables of test methods
6、8 Figure 1 Example of life temperature characteristics for materials,basedonflexural strength 10 Figure 2 Example of test results from flexural strength testsforspecimensaged at 170 C 11 Table 1 Suggested ageing intervals (days) 4 Table 2 Model data and calculation 7 Table 3 Suggested test methods (
7、mainly from BS 2782) 9ii blankBS4618-4.6:1974 BSI 11-1999 1 0 Introduction This section gives a basis for assessing the thermal endurance of plastics in air. A procedure for heat ageing is given, together with suggested tests for assessing changes in properties which may be affected. The section dea
8、ls with a test for one property but the procedure may be repeated, if required, using tests for other properties (seeAppendix D andTable 3). The changes in properties after ageing at various temperatures are plotted and extrapolated to show the temperature at which a life of20000h can be expected. T
9、his temperature is known in the electrical industry as the “temperature index” of the material and the procedure given in this section originates from that industry. This index is a good basis for the comparison of materials but the life times actually achieved in service may be affected by other fa
10、ctors not included in the tests. The measurements of changes in properties are usually made at room temperature. Where the material has to remain serviceable at an elevated temperature, it is necessary to estimate the permissible change in a property at room temperature which corresponds to the limi
11、t of useful life at the operating temperature. Where appropriate, changes in properties at the service temperature may be measured at that temperature. Special effects due to contact between different materials are not considered. The interpretation of results can often be assisted if parallel tests
12、 are carried out on a reference material whose thermal endurance in practical applications is well known. The technique was originally developed for thermosetting materials, but its application to thermoplastics is also considered. It should also be noted that using the same experimental data, more
13、complete information than is given by the “temperature index” may be provided by adding to this the temperature corresponding to a measured life of5000h and the temperature corresponding to the lower95% confidence limit on this. This form of presentation of results is called the “thermal endurance p
14、rofile” and is written, for example,139/159(154). The procedure for obtaining the temperature corresponding to the lower confidence limit is given in IEC Publication 1)“Guide for determination of thermal endurance properties of electrical insulating materials”: PartI“General procedure for determinat
15、ion of thermal endurance properties, temperature indices and thermal endurance profiles”, and PartII “List of materials and available tests”. 1 Principle In this procedure, of which an example is given inAppendix A, a material is thermally aged at three, or preferably more, temperatures. The measure
16、d life of the material,i.e.the point at which the measured property has changed by a stated amount which is taken to indicate the limit of serviceability for the material, is obtained at each of these temperatures so that the results can be extrapolated to the operating temperature (seeFigure 1). It
17、 is customary to quote as the “temperature index”, the temperature corresponding to an extrapolated life of20000h. 2 General 2.1 The effects of ageing can be characterized by the change in any suitable property or properties; mechanical and electrical measurements are often convenient. It should be
18、noted, however, that even for materials used for electrical applications, the end of useful life is often due to mechanical failure which may, of course, lead to an electrical failure. 1) At present in the form of a draft.BS4618-4.6:1974 2 BSI 11-1999 2.2 The following safeguards are necessary to av
19、oid misleading results. 2.2.1 The extrapolation to the temperature corresponding to20000h should not be by more than25 C. 2.2.2 The lowest ageing temperature should give an average measured life of over5000h. 2.2.3 The highest ageing temperature should give an average measured life of over100h. 2.2.
20、4 At least three ageing temperatures should be used at suitable temperature intervals. 3 Special considerations for thermoplastics The safeguards given in2.2 present little difficulty with thermosetting materials, but there may be difficulty with thermoplastics for the following reasons. a) The oper
21、ating temperatures for thermoplastics are normally close to the temperatures at which appreciable softening occurs over small temperature intervals, with corresponding changes in most other physical properties. In these circumstances the ageing temperature intervals may be reduced, but much closer t
22、emperature control is necessary than that specified in the normal procedure. Extrapolation, as shown inFigure 1, from above to below the glass transition temperature cannot generally be assumed valid. b) The morphological structure of thermoplastics, and hence their physical properties, can be affec
23、ted by their complete thermal ageing history,i.e.,by the rates of heating and cooling as well as by the ageing temperatures and duration of ageing. If partially crystalline thermoplastics are aged at temperatures which cause crystallite melting, then recrystallization on cooling can give a structure
24、 different from the original structure and the associated changes in the physical properties of the cooled test pieces will be superimposed upon the changes due to ageing. The test pieces may have “frozen-in” strains. Thermal ageing will cause molecular re-orientation, with a consequent reduction in
25、 the anisotropy of the physical properties. The associated change in the level of the measured physical properties will be superimposed upon changes due to ageing of the material. Care must be taken that additional strains are not introduced during the test. For example, specimens should not be susp
26、ended in the oven in a manner which would allow them to deform under their own weight; cooling after tests should be done under stress-free conditions and not too rapidly. These difficulties may be minimized if ageing can be carried out at temperatures below the glass transition temperature. 4 Proce
27、dure 4.1 Selection of ageing temperatures. The temperatures at which the material should be aged are selected according to the following criteria. 4.1.1 The lowest temperature should be sufficiently close to the anticipated operating temperature corresponding to an extrapolated life of20000h to ensu
28、re that extrapolation to this temperature is valid. 4.1.2 The temperatures selected should be sufficiently far apart to allow a reliable extrapolation to be made and to make unavoidable variations in oven temperatures unimportant. 4.1.3 The number of ageing temperatures should be sufficient to justi
29、fy the extrapolation of the results by means of a straight line, or to indicate a chemical change point above which results may be ignored. In the latter case it may be necessary to carry out further tests at lower temperatures. 4.1.4 In accordance with these criteria, test specimens should be aged
30、at a minimum of three temperatures,20 C apart, with the lowest not more than25 C and preferably not more than20 C above the anticipated operating temperature. If five or more ageing temperatures are selected instead of the minimum three, then it is permissible to use temperatures10 C apart. The lowe
31、st temperature should be chosen to give a measured life which exceeds5000h (approximately208days) and the highest ageing temperature should be chosen to give a measured life which exceeds100h (approximately4days). 4.2 Selection of ageing periods. The periods of ageing and the ageing temperatures giv
32、en in Table 1 have been selected so that specimens subjected to the temperatures appropriate to the expected thermal endurance of the material concerned may be expected to produce the specified degradation in10periods of ageing. 4.3 Selection of test methods. Appropriate test methods should be selec
33、ted in accordance withAppendix D.BS4618-4.6:1974 BSI 11-1999 3 4.4 Number of test specimens. The number of test specimens is dependent on various factors and is given by the expression: number of test specimens = (a b c)+d In the example given inAppendix A it was found necessary to prepare a minimum
34、 of170specimens. 4.5 Establishment of the initial value. As the end point is taken as a percentage degradation of the initial value, it is important that this initial value should be established as accurately as possible. The use of initial values after a short heating period, rather than values in
35、the “as received” condition, is intended to eliminate the influence of transient initial effects of heating, such as the drying out of the specimens or stress relaxation, which are not connected with thermal endurance. The following procedure should be used. Select the required number of specimens a
36、t random from the total number required for test. The number is dependent on the test method chosen to establish the end point and is governed by the factors b and d of4.4. The initial value is established on specimens which have been heated for48h at the lowest ageing temperature. 4.6 Ageing proced
37、ure. By random selection divide the specimens to be aged into groups, one for each ageing temperature selected. Before insertion in an oven already containing specimens being aged, new specimens should be heated at the ageing temperature in another oven to remove volatile products, for a period equa
38、l to the duration of the first exposure period or96h, whichever is the shorter. This heating time is included in the total ageing time for the specimen. (Notes on ageing ovens are given inAppendix B.) Put each group into an oven at the required temperature, selected fromTable 1, ensuring that the sp
39、ecimens are kept separate in the oven to allow free circulation of air around them. It is preferable to heat only one type of material in an oven but where this is not practicable the first few hours of heating should be done separately from other specimens. At the end of the first ageing interval f
40、or each group, selected in accordance withTable 1, remove at random the number of specimens previously decided as being necessary to establish one point on the curve (seeb of4.4) and test according to the method chosen for establishing the end point. At the end of the second ageing interval, remove
41、the same number of specimens also selected at random. Test as before. Repeat the process until the appropriate percentage degradation for each selected temperature is reached. The number of specimens in each group allows a total of ten ageing intervals before the specimens are exhausted. If, after t
42、hree exposure intervals, it becomes apparent that the requisite degradation will not be attained before the completion of10ageing intervals, increase the duration of individual ageing intervals by an amount judged to be necessary,e.g.the next ageing interval given inTable 1. It is essential that at
43、least one value is obtained beyond the agreed end point. 5 Form of presentation 5.1 Calculate the average of the test values obtained after each ageing interval and plot the results against ageing time for each ageing temperature. Draw a smooth curve for each temperature and from the curves thus obt
44、ained read off the ageing times corresponding to the measured life. 5.2 Plot a new graph of the ageing times so obtained on a logarithmic scale, against measured average temperatures on a scale using the reciprocal of the absolute temperature (seeFigure 1 which uses a scale for linear reciprocal tem
45、peratures). The line of “best fit” may be calculated by the method of least squares, given inAppendix C. This line is extended to give the temperature of operation for an extrapolated life of20000h. where a is the number of ageing temperatures (minimum three, see4.1). b is the number of specimens re
46、quired to establish each point on the curve and is dependent on the variability of the measurement chosen for establishing the end point. Sufficient tests are required to establish a reliable average value. If the chosen method is liable to give a large scatter in results, more test specimens will b
47、e required than if the chosen method is known to give consistent results. c is the number of points on the curve to establish the measured life at each ageing temperature. The essential requirement is that there should be at least one average value beyond the agreed failure value. Experience has sho
48、wn that to make certain of achieving this and to ensure that the curve gives a satisfactory indication of the degradation in relation to ageing time, it is necessary to age10sets of specimens at each ageing temperature. d is the number of specimens required to establish the initial value (see4.5). I
49、t is customary to test twice the number of specimens deemed necessary (seeb).BS4618-4.6:1974 4 BSI 11-1999 Table 1 Suggested ageing intervals (days) 5.3 The report of the results of the life testing of a single material for a single property should contain the following information. a) Manufacturers description and identification of the material, including the type common name in accordance with BS3502 2) ; additives if known. b) Specimen shape and dimensions and method of preparation, including subsequent conditioning, test method used, temperatures of tes
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