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Study On The Computer Simulation Of Polymer Mixing, Branch Reaction And Extrusion Processing Process

Posted on:2011-02-13Degree:DoctorType:Dissertation
Country:ChinaCandidate:J ZhouFull Text:PDF
GTID:1101360305456702Subject:Polymer Chemistry and Physics
Abstract/Summary:PDF Full Text Request
Polymer (reactive) processing process is made up of transportation, melting, mixing, chemical reaction (integrate with processing and molding), desorption and devolatilization and molding etc. On undergoing (reactive) processing process, raw material will be converted to products having certain internal structure and appearance. And during this process, physical mixing, reactive mixing and molding process are the key steps that lead to the final property of polymer products. Due to the shortage of necessary experimental technologies, developing the studies of computer simulation on these steps will help people understand the real polymer (reactive) processing process in depth and reveal why processing process can influence the performance of products.This work mainly carried out studies on the flow behavior in three kinds of transportation processes of polymer in internal mixer (mixing of binary immiscible polymers, homogeneous reactive mixing of polymer and mixing process of one polymer) and three important extrusion molding processes (fiber spinning, blow molding and co-extrusion). The transportation phenomena of complex polymer system in complex flow fields were focused profoundly. The chemical reaction mechanism of polymer and its transportation pattern under real processing flow were investigated. The theoretical model and simulation method of polymer reactive processing process under complex chemical and physical conditions were set up. Also the effects of processing parameters in extrusion molding on production process and performance of products were generalized.Most prior attempts at modeling research on incompatible polyblends were only focused on single (or finite number of) drop(s) dispersed in the other continuous matrix under simple and homogeneous flow conditions. Only the microscopic scale was emphasized. These studies had not incorporated the uneven nature of real processing flow field and were far from the practical case. This work carried out the numerical studies on immiscible mixing process of polymers under real processing flow field and both of dispersive and distribute mixing were included. The mixing rule was summed up and the factors influencing mixing efficiency were studied. Besides, the determination of viscosity of polymer material mixed in torque rheometer merely through the torque data was considered to be objective for scientific researchers. A method aided by three-dimensional numerical simulation on mixing of polymer in torque rheometer was adopted for inverse prediction of viscosity. On the other hand, during the polymer processing process incorporating chemical reaction, the reaction mechanism and kinetics of entangled polymer were different from that of small molecules, in other words, the external flow can influence the reaction process of entangled polymer. The prior studies on reactive kinetics of macromolecules under external flow fields were mainly focused on the simple and homogeneous flow conditions. No quantitative results about the effects of flow on reaction kinetics of macromolecules were obtained yet. Besides, the nature of complex flow field and rheological system in practical polymer reactive processing process also restricted the development of simulation work on polymer reactive processing. The previous simulation work on polymer reactive processing merely pointed to the quasi-steady, simple and even flow field and the rheokinetic behavior of long chain polymer has not been incorporated. This work studied the reactive kinetics of macromolecules under practical processing conditions and found out the quantitative relationship of flow effect with respect to reactive kinetic constant. Furthermore, the theoretical model and computer simulation on real polymer processing process were carried out. Melt spinning of fiber, blow molding and co-extrusion are three kinds of extrusion molding technologies for manufacturing polymer products. It was revealed by experimental and numerical studies on the melt spinning of shaped fiber that surface tension in spinning process had an important action on the change of cross sectional shape of shaped fiber. Due to the transient elongation characteristics of melt in fiber spinning, the surface tension in fiber spinning was dynamic parameter in nature and not the static one. The dynamic one was not able to be tested under present experimental conditions. If the dynamic surface tension can be determined, it will have great significance on the researches of rheological behavior of material in non-equilibrium state and can lead to more accurate prediction in melt spinning of profiled fiber. In this work, an inverse method for determining surface tension of shaped fiber through computer simulation was used and the integrated method was suggested and proved to be a good solution for the design and processing of profiled fibers. Besides, a simulation analysis of spinning melt flowing in spin-pack was carried out and the best layout of spinning holes for acquiring the fibers with the same diameter was designed. A simulation method for continuous stretch-blow molding process of plastic bottle was successfully established and a method aided by computer simulation was suggested for developing a simple device which can evaluate the elongational viscosity of material used for blow molding. Prior numerical studies on co-extrusion process of compound film mainly focused on the theoretical development and little works emphasized the effects of technological parameters on the performance of film products. In this work, the factors that influenced film property were studied and a quantitative formulation between the shift of processing conditions and change of film property was finally built.The main research work and conclusions were introduced as follows:(1) With the help of finite volume method, the binary immiscible polymer (HDPE/PS) mixing process in internal batch mixer was numerically studied. Some contours of flow parameters were predicted. The mixing mechanism and rule of HDPE/PS was revealed. A common binary immiscible mixing polymer system was studied by the suggested simulation method above. And the effects of interface, viscosity ratio and flow intensity on immiscible mixing process were studied. On the other hand, a computer simulation aided method for determining the viscosity of mixing polymer in torque rheometer only through the data of torque rheometer was presented and it should lead to more accurate prediction. The mixing experiments of HDPE and PS in torque rheometer demonstrated the reliability of suggested method. A code which could invoke finite element algorithm (model the three-dimensional mixing process of polymer in internal mixer and compute the torque) repeatedly was programmed for inversely predicting viscosity of material.(2) The branch reaction kinetics of polyethylene d initiated by dicumyl peroxide in real processing flow field was studied through rheokinetic method. Based on the branch mechanism and batch viscosity dependent mixer torque, a new reaction conversion was defined and it was closer to the real reaction conversion compared with the traditional rheological conversion. According to the static simulation of reaction kinetics and flow analysis inside mixer, a quantitative conclusion of flow affected macromolecular reaction kinetics was drawn from experiments for the first time. This conclusion was similar to Fredrickson et al's theoretical prediction and it also testified their theory to be reasonable in certain extent. Besides, the two-dimensional transient simulation on practical polymer reactive processing process was established, where the effects of temperature, molecular diffusion and flow convection on the reaction kinetics were considered simultaneously.(3) Two extension-dominant processing processes, i.e. fiber spinning and blow-molding and co-extrusion were simulated by finite element method. Firstly, shaped fiber was chosen as the studied system. Two-dimensional fiber model was used for computation of temperature profile along spinline and three-dimensional model was used for capturing cross section development. The influence of temperature on surface tension of fiber was incorporated in simulation and a surface tension scale factor was introduced. An inverse method was utilized to determine the surface tension of shaped fiber under different spinning conditions by changing surface tension scale factor till computed cross section of fiber was close to experimental one. It was found the surface tension of fiber was related with draw ratio of fiber. An orthogonal numerical experiment and statistics analysis were carried out to find out the optimum spinning conditions and significance level of processing condition to fiber shape. The order of significance levels about the effects of spinning factors on cross sectional shape of profiled fiber was: spinning temperature, take-up velocity, volumetric flow rate and cooling mode. The effects of porous media in spin-pack on the commutating and homogenization of tributary melts were numerically studied and a best layout of spinning holes was designed. Secondly, the numerical simulation on continuous stretch-blow molding process of plastic bottle was developed. And experimental counterpart proved the simulation can predict the evolution of blown bottle thickness well. On the basis of dynamic analysis of the free inflation of bubble, a simple method for evaluating the double axial elongational viscosity of material was suggested. This method indicated that on the premise of certain blow pressure and ignoring the interfacial tension between bubble and air, the extension property of material was only correlated with the radius of blowing bubble. Thirdly, in view of the importance of even film thickness during production of compound film, the finite element simulation of single-layer melt flowing in parallel channel and co-extrusion flow of seven-layer melts was respectively developed. The flow imbalance in parallel channel and its superposition effects when conflux occurred was studied. The influence of changing feeding rate of each layer melt on the thickness of compound film was investigated quantitatively.The main innovations of this thesis are listed as follows:1. Prior numerical studies on immiscible polymer mixing were mainly focused on microscopic scale under simple, even flow conditions and they were far away from the real polymer processing conditions. Here, the mixing process of an immiscible polymer melt system (PE/PS) in internal batch mixer was numerically investigated considering the space-time characteristic of practical polymer processing flow field. The simulation method suggested here was testified to be well according to mixing experiments. This work was considered to provide valuable theoretical basis for purposefully preparing mixing material and designing flow field inside mixer.2. For the first time, the quantitative effect of flow field on polymer reaction kinetics was determined experimentally through rheokinetic method and there was a scaling relation between strength of flow field and reaction rate parameter of polymer. It provided strong basis for evaluating Fredrickson et al's rheokinetic theory and optimizing polymer reactive processing process. A new reaction conversion rate was defined and it was closer to chemical conversion rate comparing with rheological conversion rate. The two-dimensional transient simulation on real polymer reactive processing process was set up, in which the complex coupled system of convection, reaction and mass transfer was successfully solved. The simulation can describe real polymer reactive processing process more precisely and play an important role in deeply recognizing polymer reactive processing process and optimizing processing conditions.3. An inverse method (compare simulated and experimental cross sectional shape of profiled fiber) which combined two- and three-dimensional simulation on fiber spinning was adopted to find out the surface tension in melt spinning. Two–dimensional model was used for computation of temperature profile along spinline and three-dimensional model was used for capturing cross section development. And the influence of temperature change on the surface tension of shaped fiber was firstly considered and introduced into simulation. It was found the surface tension of fiber was not only dependent on temperature but also on the draw ratio of stretched spinning melt. And the effects of temperature and draw ratio on the surface tension in melt spinning of shaped fiber can be separated. It was proved that the integrated method in this work, including the numerical method, the inverse method to determining the surface tension and the orthogonal analysis, can be a good solution for the design and processing of profiled fibers.
Keywords/Search Tags:Computer simulation, Rheology, Flow field, Mixing, Branching, Extrusion molding
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