Effect of recrystallization on the oxidative stability of poly(butylene terephthalate)
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Effect of recrystallization on the oxidative stability of poly(butylene terephthalate) Antonio Motori, Gian Carlo Montanari, and Andrea Saccani University of Bologna, Bologna, Italy
Steve Giannoni Underwriters Laboratories, New York (Received 2 March 1999; accepted 12 October 1999)
The oxidative stability of poly(butylene terephthalate) was investigated over the range 175 to 220 °C by isothermal differential calorimetry. It is shown that this property is affected by a fast recrystallization process, which takes place between 200 and 220 °C, with a maximum around 208 °C. In fact, recrystallization produces an increase of the crystallinity degree of the polymer, which enhances its resistance to oxidation, as proven by the longer oxidation times detected.
I. INTRODUCTION
Oxidative stability measurements were successfully applied for diagnosis of aging, as well as for short-term thermal endurance characterization of polyolefins, like cross-linked polyethylene (XLPE) and ethylenepropylene rubber (EPR), which are used as insulating materials for medium and high voltage cables.1 For these materials, oxidation is recognized to be the prevailing degradation mechanism in the life tests and design temperature range.2–5 Therefore, it seemed interesting to check the validity of this technique for materials different from polyolefins; to this aim, poly(butylene terephthalate) (PBT), a polyester used for low-medium voltage applications, was chosen. The oxidative stability method consists in performing isothermal differential calorimetry tests in pure oxygen at three or more temperatures until a characteristic time (the oxidation induction time or the oxidation maximum time) is obtained; thus, the activation energy of the oxidation process is derived, according to the Arrhenius law: tox = A exp共E Ⲑ kT兲 ,
,
(2)
where tL is the time-to-failure at the temperature T. The slope of the endurance line can be assumed as directly proportional to the activation energy of the prevailing degradation mechanism [e.g., E of Eq. (1) is referred to J. Mater. Res., Vol. 15, No. 1, Jan 2000
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a = log tLC − b Ⲑ TC
.
(3)
Once the thermal endurance line is drawn, the endurance indices, i.e., the temperature index and the halving interval, which are the parameters for thermal endurance characterization of insulating materials and insulating systems design,4 are finally obtained. The results of preliminary oxidative stability tests on PBT showed a lack of fitting of the Arrhenius law close to 200 °C.6 Both scanning and isothermal differential calorimetry of PBT were deeply investigated, in order to understand this behavior and ascertain the existence of other phenomena, like recrystallization, which may superimpose the oxidation process. In this paper, the results of these tests are presented and discussed. Correlations between the recrystallization process and the resistance to oxidation of PBT are finally made.
(1)
where tox is the characteristic time provided by the
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