Rheokinetic Studv Of4,4’/2,4’-Diphenvlmethane Diisocvanate/Polvols Bulk Polymerization

Thermoplastic polyurethane (TPU) is usually produced using twin screw extruder, in which the materials flow in a complex way. As the reaction proceeded, the viscosity would be continuously increased and affect the polymerization kinetics and the performance of final product in turn. Thus, the kinetic studies of TPU synthesis under this condition are difficult. The common methods used in TPU synthesis kinetics study cannot track high viscous system efficiently or correlate rheological parameters with measured data. The rheological parameters of TPU system are closely related with the polymer component, average molecular weight and molecular weight distribution. It is proved that gel permeation chromatography (GPC) method is an effective way to measure the average molecular weight and molecular weight distribution of polymers, and torque rheometer has good mixing and dispersing capability similar to twin screw extruder. In this thesis, the kinetic and rheology of TPU synthesis process were studied using the torque rheometer as the simulated reactor of twin screw extruder, and GPC method to track the variation of molecular weight of TPU during polymerization process.Firstly, the kinetics of4,4’/2,4’-diphenylmethane diisocyanate/polytetramethylene ether glycol/1,4-butylene glycol bulk polymerization without the catalyst and with the addition of Bi/Zn composite catalyst was studied. The molecular weight of TPU was corrected with the reaction rate constant and time. For polymerizations with and without catalyst, the expression of reaction rate constant k was as follows: k(T, Cat)=A1exp(-E/RT)+A2exp(-Ea2/RT)[Cat]α and κ(T)=A1exp(-Ea1/RT), respectively, in which A1=3.487×105, Ea1=45.1kJ/mol, A2=8.376×1013, Eα2=100.6kJ/mol, α=2.35. Using the obtained kinetics parameters, the variation of molecular weight of TPU with time was simulated and compared with the experimental data, showing well fitness between simulated and experimental results. The kinetic parameter can also offer a reference for rheo-kinetic study.Secondly, based on the obtained kinetics parameters, the termination agent was added at different reaction time to get TPU with different molecular weights. It was found that the chain growth could be effectively stopped with the addition of termination agent, and the average molecular weight of TPU was basically unchanged in further mixing or heating/cooling conditions. Considering the flow in rheometer as two-dimensional flow, and using coaxial double cylinder rheometer model to correlate torque and viscosity, the relationship between torque, rotational speed and temperature was proposed in the combination with non-Newtonian power-law and Arrhenius equation. Thus, the flow index n, viscous flow activation energy of TPU with different molecular weights was calculated. It is shown that n of melt TPU system was decreased as the molecular weight increased, but the variation of viscous flow activation energy with the molecular weight was not obvious.Finally, on basis of two-dimensional flow simplification, the correlation of torque, viscous flow activation energy, temperature and weight average molecular weight was proposed as M1=Aexp(U/RT)(?). Thus, three corrections based on the relation between shear viscosity and law molecular weight (N=3.4), kinetic fitting molecular weight as reference and fixed viscous flow activation energy (40kJ/mol) were obtained as follows, respectively:Torque is the direct measured parameter in the rheo-kinetic method. So, rheo-kinetic method is more convenient and quicker than the kinetic method to get the molecular information of TPU. It could also give real-time fitting results for the entire TPU synthesis process, but its accuracy was lower than that of the kinetics method. The simulation result also proved that the correction ⅲ was more suitable for rheo-kinetic study during TPU synthesis process.