Thermal endurance evaluation of poly(butylene terephthalate) by conventional and analytical methods
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G.C. Montanari Dipartimento di Ingegneria Elettrica, University of Bologna, 40136 Bologna, Italy
A. Motori and A. Saccani Dipartimento Chimica Applicata e Scienza dei Materiali, University of Bologna, 40136 Bologna, Italy (Received 2 October 2002; accepted 27 February 2003)
The thermal endurance of poly(butylene terephthalate) was evaluated by both consolidated long-term conventional life tests and a short-term analytical procedure. The latter is based on oxidative stability measurements by isothermal differential calorimetry in a pure oxygen flow. The capability of the analytical technique to provide an activation energy for the thermo-oxidation process was verified. The results obtained by the two methods were compared and discussed. It is shown that the temperature index derived by the analytical procedure can be close to that obtained by the conventional tests, provided that a suitable selection of the test temperatures and failure criteria is made; however, benefits in term of test time shortening are lower than those previously obtained for polyolefin-based materials.
I. INTRODUCTION
Thermal degradation is one of the prevailing aging mechanism for insulators of electrical and electronic power apparatus. The evaluation of thermal endurance of electrical insulating materials can be achieved either through conventional long-term tests, according to International Electrotechnical Commission (IEC) 216 Standard,1 or resorting to short-term procedures, which are based on analytical techniques combined with conventional tests, as suggested by IEC 1026.2 The former procedure is time consuming and expensive, as at least one year is needed for completion of tests; therefore, it is not the best choice when various materials, candidates for the same application, must be compared or the effects of formulation changes on the behavior of a polymeric insulation must be investigated. A significant reduction of the test times and the number of tests to be performed can be achieved if the latter procedure is applied, but there is still concern that the analytical techniques provide results consistent to those obtained by the conventional technique. IEC 1026 simply suggests several analytical techniques that may be sensitive to thermal degradation of polymeric materials, such as thermogravimetric analysis, mass spectroscopy, gas chromatography, etc. It is, however, the task of the researchers to determine experimentally the suitability of a selected technique and the relevant procedure to be used for certain polymeric materials or for a class of them (i.e., polyolefins, polyesters, 1342
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J. Mater. Res., Vol. 18, No. 6, Jun 2003 Downloaded: 15 Mar 2015
polyamides). The analytical technique selected should be able to cause and detect in the material the degradation processes that are really representative of those taking place in service conditions. Some authors have investigated the degradation of polymeric materials by chemioluminescence,3,4 scanning thermogravimetric analysis,5 etc. Since it i
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