BIAS IN ISOTHERMAL TIME-TO-EVENT STUDIES DUE TO APPROACH TO TEST TEMPERATURE

Oxidative induction time (OIT), constant temperature stability (CTS) a nd isothermal crystallization are examples of isothermal time-to-event (TTE) measurements obtained using differential scanning calorimetry, ln TTE experiments, a test specimen is heated/cooled at a constant rat e from the setup temperature to an isothermal test temperature. Once t he test temperature is achieved, a clock is started and the time to th e thermal event (e.g., onset to oxidation; thermal decomposition or cr ystallization exotherm peak) is measured. Such TTE values may be used to rank stability of the material at the test temperature. Some portio n of the reaction of interest, however, takes place during the pre-iso thermal period as the test specimen approaches the test temperature. T his amount of reaction is unmeasured and represents a bias in the resu ltant TTE value. An equation has been derived and numerically integrat ed to estimate this bias. This approach shows that the bias is depende nt upon the activation energy of the test reaction, the heating/coolin g rate used and the temperature range between the melting temperature and the test temperature. For commonly used heating rates, the bias fo r OIT and CTS tests is small. Further, the myth that isothermal crysta llization kinetics determinations required high cooling rates is dispe lled with the bias of less than 0.9 min resulting from heating rates a s low as 10 degrees C min(-1). Knowledge of magnitude of this bias per mits the selection of experimental conditions without the expense of h igh heating/cooling rate apparatus or extra cost cooling accessories.