Assessment of the degradation of Aspirin by Differential Scanning Calorimetry (DSC)

Syed, A.R. and Antonijevic, Milan D. (2009) Assessment of the degradation of Aspirin by Differential Scanning Calorimetry (DSC). The AAPS Journal, 11 (Suppl. 2). ISSN 1550-7416

Abstract

Purpose. To study thermal stability of Aspirin and define thermal events that are associated with the thermal degradation of
aspirin.
Methods. Experiments were performed using a DSC 823e (Mettler Toledo, Swiss). Aspirin is prone to thermal degradation
upon exposure to high temperatures. The melting point of aspirin is 140.1±0.4ºC (DSC). Aspirin has been examined by
heating samples to 120ºC, 155ºC and 185ºC with subsequent cooling to -55ºC and a final heating to 155ºC. Although
different heating and cooling ranges have been used, only results obtained at a rate of 10ºC/min will be presented. All runs
where conducted in hermetically sealed pans.
Results. Upon heating the sample to 120ºC no significant thermal event can be detected. After cooling the sample and
reheating a glass transition can be observed at ~-8ºC, followed by the melting of aspirin at ~139ºC. By heating the sample to
155ºC melting of aspirin has been detected at ~139ºC. On cooling and subsequent heating a glass transition occurs at ~-32ºC,
together with a broad crystallisation (onset at ~38ºC and peak maximum at ~57ºC) followed by a broad melting with an
onset at 94ºC and peak maximum at ~112ºC. Finally, by heating the sample to 185ºC melting at ~ 139ºC was observed, and
upon cooling and reheating a glass transition was detected at ~-26ºC and no further events could be recorded.
Conclusions. This research demonstrates that the degradation steps of Aspirin depend on the thermal treatment. The main
degradation products of different thermal treatments are currently unknown it is clear that acetic acid, which is one of the
degradation products, acts as an antiplasticiser by lowering the glass transition temperature. In addition, due to the presence
of the degradation products in liquid form (observed by hot stage microscopy), Aspirin is still present in the sample and
recrystallises during the second heating step and melts at much lower temperatures.

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