Mathematical Modeling of Nylon 6/6,6 Copolymerization
This thesis develops a mathematical model for the hydrolytic copolymerization of caprolactam with hexamethylene diamine (HMD) and adipic acid (ADA) in a batch reactor to produce nylon 6/6,6 copolymer. The reaction mechanism includes hydrolysis of caprolactam and cyclic dimer, polycondensation, polyaddition, transamidation, and ring formation via end biting and back biting. The catalytic effect of carboxyl end groups on reaction rates is accounted for using kinetic and equilibrium parameters from the literature. This model can predict dynamic changes in concentrations of water, caprolactam monomer, comonomers, aminicaproic acid, end groups and degree of polymerization. This model is used to simulate laboratory-scale nylon 6/6,6 batch copolymerization at conditions of industrial interest. To account for the complex influence of water on nylon reaction equilibria, semi-empirical expressions are developed to describe the equilibrium behaviour for the five dominant reactions that occur during nylon 6, nylon 6,6 and nylon 6/6,6 polymerizations. These expressions are developed by considering additional side reactions involving amidine ends, cyclopentanone ends and hydrated carboxyl ends. A subset of these side reactions is also used to extend the rate expression for polycondensation to account for the influence of water on the overall polycondensation reaction. The kinetic and equilibrium parameters in the updated model are estimated using a large body of literature data values that cover a wide range of operating conditions in nylon 6 and nylon 6,6 homopolymerizations. The updated model and new parameter estimates result in good fit to all of the kinetic data. The objective function value for parameter estimation is reduced by 70% compared to its value using parameters of Arai et al. The predictions for linear monomer and end-group give the most improvement in fit. The proposed models and improved parameter values in this thesis will be helpful to guide nylon 6/6,6 experiments and to assist with improved understanding of nylon 6, nylon 6,6 and nylon 6/6,6 polymerization processes.