Typical sources of error in the kinetic analysis of models with pre-equilibria: DSC investigations of epoxide-amine curing reactions

Reversible steps are included in many reaction models. If the intermediate reaches an equilibrium with the reactants, the kinetic evaluation may be simplified by using the corresponding equilibrium constant. Then, the adequate modelling of the reaction is based on the equilibrium concentrations of all participants of such a pre-equilibrium. Using DSC data for the investigation of kinetic problems, the concentrations are frequently replaced by the degrees of conversion (reaction). The formal use of degrees of conversion instead of concentrations results in incorrect kinetic equations. A typical example is the overall equation for the description of the epoxide-amine curing reaction. Another serious source of error is the unknown degree of conversion together with the unknown heat-flow rate at the apparent ending of the reaction. The resulting difficulties can partially be overcome, if the parameters of the complete set of differential equations are determined by multivariate nonlinear regression. (C) 1997 Elsevier Science B.V.