Thermal Degradation of Nylon 66: Modeling and Parameter Estimation

A fundamental dynamic model is developed to predict the influence of reactor operating conditions on the rates of undesirable reactions that can occur in the low-moisture, high-temperature finishing stage of commercial nylon 66 production. Kinetic parameters and initial values for unknown concentrations are estimated using experimental data. The proposed model is an advance over previous models because it accounts for the effect of melt-phase water concentration on thermal degradation. Estimability analysis and a mean-square error (MSE) criterion are used to determine which parameters can and should be estimated using the available data, and which parameters should remain at their nominal values. Parameters are ranked from most estimable to least estimable based on their influence on the model predictions, correlation with other parameters and uncertainty in initial guesses. Subsequently, the MSE criterion is used to determine that 48 of the 56 parameters should be estimated. The remaining 8 parameters are fixed at their initial guesses. The fitted model matches the data well, with typical errors of 1.6% for amine ends, 1.5% for carboxyl ends, 7.6% for gas evolutions rate of CO₂, 15.3% gas evolutions rate of NH₃ and 18.1% for gas evolutions rate of cyclopenatone. This model will help engineers to select temperature and moisture level setpoints that lead to high polymerization rates while ensuring good product quality.