Numerical Simulation Study Of PA6T/66 Reactive Extrusion Process

Poly(hexamethylene terephthalamide)/Poly(hexamethylene adipamide)(PA6T/66)is a semi-aromatic copolyamide,which has excellent high temperature resistance,low water absorption,dimensional stability,high strength and stiffness,and excellent chemical resistance.The PA6T/66 continuous production process is divided into three stages:salt formation,pre-polymerization and post condensation.PA6T/66 prepolymer was pumped into twin screw extruder(TSE)for reactive extrusion(REX),and the prepolymer continued to polycondensate to increase molecular weight.It is difficult to investigate the mechanism of PA6T/66 reactive extrusion process by experiment method because of the complex flow field and numerous operating parameters coupling in the extruder.In this thesis,computational fluid dynamics(CFD)was used to simulate the PA6T/66 reactive extrusion process in twin screw extruder.Based on the interaction among rheology,reaction and mixing in extruder,the influence of screw configuration and operating parameters on the reactive extrusion process of PA6T/66 was explored,in order to provide theoretical guidance for the reactive extrusion process of PA6T/66.Firstly,the relationship between PA6T/66 rheological properties and shear rate,shear temperature and terminal amino content was explored,and the rheological model of PA6T/66 was established.The structure of PA6T/66 was characterized by NMR and IR,the melting temperature and galss transition temperature of PA6T/66 were 303℃ and 130℃ by DSC.The initial decomposition temperature and maximum decomposition temperature of PA6T/66 were 392.84℃ and 457.69℃ by TGA.The rheological equation of PA6T/66 was established by high pressure capillary rheometer,the relationship of PA6T/66 viscosity with shear rate was studied and the experimental data were fitted.The results showed that PA6T/66 has a shear thinning phenomenon,non newtonian index n=0.2504,Bird-Carraeu model can obtain a higher fitting accuracy under the condition of fewer parameters.The Arrhenius model was used to fit the experimental data of PA6T/66 viscosity changing with temperature,the results show that activation energy of viscous flow decreases with the increase of shear rate.The effect of terminal amino content on the rheological properties of PA6T/66 was investigated,the results show that with the decrease of terminal amino content,the shear viscosity curve moves upward,and the zero shear viscosity obtained by fitting equation is also bigger.In this thesis,a three dimensional numerical simulation method is used to simulate the post condensation process of PA6T/66 prepolymer in a corotating twin-screw extruder,aiming at the critical procedure of PA6T/66 continuous production process,which is the reaction extrusion process coupled with flow,heat transfer,mass transfer and chemical reaction.The process of PA6T/66 reaction extrusion and the water removal were modeled by subsection simulation method,the extruder temperature,shear rate and viscosity distribution were obtained by coupling PA6T/66 polycondensation reaction kinetics and rheological model.The influences of screw speed,barrel temperature,feeding rate and prepolymer feeding state on the polycondensation reaction process were studied,and the evolution of each component along the extrusion process was calculated.The results showed that the reaction degree of PA6T/66 decreased slightly with the increase of screw speed.The rate of polycondensation can be increased and the reaction process can be promoted by increasing the barrel temperature.Increasing the feeding rate will shorten the residence time and delay the reaction process.Under the condition of multi-component competitive reaction,the molecular weight and composition of prepolymers significantly affect the polycondensation rate and the reaction process.The mixing condition in extruder has a great influence on the reaction extrusion process.In this thesis,a three-dimensional mixing model of the twin screw extruder was constructed by using computational fluid dynamics based on finite element method.The velocity and shear rate distribution in the extruder were obtained by using mesh superposition technique(MST)to solve the changing computational domain.Particle tracing method was used to calculate the mixing index and mixing efficiency under different screw revolution,feed rates and screw configurations.The simulation results show that the kneading block and the reversing element have the largest leakage flow.The stress of the conveying element decreases with the increase of the screw lead.The results of dispersive mixing show that with the increase of screw speed or feed rate,the average mixing index decreases,the maximum mixing index increases correspondingly,which indicates that PA6T/66 dispersive mixing effect in extruder is worser,but some particles experience stronger maximum stretching effect.The results of the distributive mixing show that PA6T/66 particles experience the stretching and compression effect in the extruder,and the instantaneous mixing efficiency and distributive mixing performance of kneading block elements are the highest.The results of residence time evolution show that the dead time increases along the extrusion direction,the residence time distribution becomes wider,the average residence time increases linearly,the dimensionless variance increases and the axial mixing effect becomes stronger.