A simultaneous TG-DTA study of the degradation in nitrogen of cellulose to carbon, alone and in the presence of other pharmaceutical excipients

The heat treatment of an alpha-cellulose and a microcrystalline cellulose (Avicel(R)) in nitrogen gives a one-stage degradation to carbon. This degradation suffers a temperature transposition in the presence of pharmaceutical excipients, namely dibasic calcium phosphate, stearic acid, magnesium stearate, and silicon dioxide. It is possible to produce a kinetic evaluation of these thermal analysis studies in terms of a comparative method by constructing the theoretical TG trace of the binary mixture based on a presumed no-interaction between the two components. There is a variation in behavior to be noted in the different forms of cellulose, and also different values are shown for the Arrhenius parameters. Such variations are also noted in the Avicel(R) degradation in the presence of other materials. These observations should be of use in the characterization of pharmaceutical excipients. The Arrhenius parameters for alpha-cellulose degradation using D3 (three-dimensional diffusion) mechanism at the temperature range from 580 to 630 K are 289.32 kJ mol(-1) and 1.85x10(23) for E-act and A, respectively. The Arrhenius parameters for Avicel(R) degradation using F1 (first-order) mechanism at the temperature range from 562 to 604 K are 291.35 kJ mol(-1) and 1.26x10(25) for E-act and A, respectively. (C) 1998 Elsevier Science B.V. All rights reserved.