Numerical Simulation Of Reactive Extrusion Processes For Anionic Polymerization

The reactive extrusion process for polymerization is an integrated process that combines polymerization reaction and extrusion processing. It is more complex than traditional polymer synthesis processes because there are complex interactions among such variables as the fluid flow, heat transfer and chemical reaction. During the reactive extrusion process, the chemical reaction and the fluid flow are processed in a closed state and under the conditions of high temperature, high pressure and high shear rate, and the viscosity may change abruptly, so the process is very complex.There are various theoretical investigations and numerical simulations focused on the mechanism and kinetic of polyreaction, the fluid flow during the molding process, the conveying and mixing of fluid and the melting and mixing of solid in screw extruders, however, only few studies have been carried out on the fluid flow coupling chemical reaction in screw extruders. Therefore, it is very necessary to simulate this process by means of the computer technique. On the basis of the numerical simulation, the effects of the reaction on the material structures and physicochemical properties can be quantitatively studied under the condition of the complicated external fields, and then the material compositions, the reactive processing conditions and the extrusion reactors can be optimized.In this paper, such disciplines as polymer chemistry, polymer physics, statistical theory of polymer reaction, computational fluid dynamics, mechanical science and software engineering were used together to analyze the reactive extrusion process for the anionic polymerization and copolymerization. The conservation equation of momentum, continuity equation, and the numerical calculation equations of monomer conversion, the average molecular weight and viscosity were numerically solved by means of the finite volume method and the uncoupled semi-implicit iterative algorithm. And then, the evolutions of such variables as flow velocity, shear rate, pressure, released heat of reaction, reactive calorific intensity, polymerization rate, monomer concentration, monomer conversion, average molecular weight, fluid viscosity were obtained quantitatively under the given initial and boundary conditions.The main contents of this paper were as follows:The configuration of the closely intermeshing co-rotating twin screw extruder is very complex, which makes it very difficult to perform the numerical simulation of reactive extrusion processes based on the real reactor model. According to the geometry of closely intermeshing co-rotating twin screws, an equivalent reactor model was established in this study. On the basis of the equivalent model, the physical model and the constitutive equation describing the chemorheological behaviors in closely intermeshing co-rotating twin screw extruders were built, and then the numerical computation expressions of such variables as fluid flow velocity, pressure and shear rate were deduced.The kinetics and mechanism of reactions under the conditions of the complicated external fields are different from that under the usual conditions. The kinetic model of the anionic polymerization under the conditions of the complicated external fields was constructed, and the effects of the complicated external fields on the polymerization rate were studied, and then the numerical computation expressions of such variables as polymerization rate, monomer concentration and monomer conversion were deduced. According to the statistical theory of polymer reaction, the expressions of average molecular weights related to monomer concentration and initiator concentration were built, and then the numerical computation expressions of such variables as number-average molecular weight and weight-average molecular weight were constructed.The rheological behaviors of polymer with chemical reaction have their own characters. According to polymer physics, the expressions of rheological properties of materials with chemical reaction related to processing conditions and material structures under the conditions of the complicated external fields were built, and then the numerical computation expressions of such variables as zero shear viscosity and apparent viscosity were constructed.The energy transfer and balance of materials during the reactive extrusion processes are different from that in usual chemical production. The phenomenon of the chemical calorific effect during the reactive extrusion process for polymerization was analyzed, and the numerical computation expressions of released heat of reaction and reactive calorific intensity were deduced, and then the relationship between released heat of reaction and reactive calorific intensity, monomer concentration, initiator concentration and temperature was obtained. And the phenomenon of the viscous dissipated heat during the reactive extrusion process for polymerization was analyzed, and the numerical computation expression of viscous dissipated heat was deduced.On the basis of the construction of above models, the software on the numerical simulation of the reactive extrusion process for anionic polymerization was developed by means of Fortran program language, the finite volume method and the uncoupled semi-implicit iterative algorithm. The processes of anionic polymerization of styrene and PA6 in the closely intermeshing co-rotating twin screw extruders were numerically simulated. The evolutions of key variables were obtained and the influences of material compositions and key processing parameters on the reactive extrusion process were discussed, the simulated results were compared with the experimental results to verify the reliabilities of the models constructed and the program, and the laws controlling the material structures and rheological properties in the reactive extrusion process for polymerization were specified.On the basis of the numerical simulation of the reactive extrusion process for the anionic homopolymerization, a further study on the reactive extrusion process for the anionic copolymerization was carried out. The anionic copolymerization process of styrene-butadiene (S/B) block copolymer in a closely intermeshing co-rotating twin screw extruder with butyl-lithium initiator was simulated. According to the anionic copolymerization mechanism and the reactive extrusion characteristics, the mathematical models of monomer conversion, average molecular weight and fluid viscosity during the anionic copolymerization of S/B were constructed, and then the reactive extrusion process was simulated. Finally, the influences of the material compositions and key processing parameters on conversion were discussed. The simulated results were compared with the experimental results to verify the applicability of the program.