Study On The Relationship Between The Mixing Property Of The Kneading-block Mixing Zone And The Grafting Reactive Extrusion In Co-rotating Twin-screw Extruder

Grafting reactive extrusion is a new method in modern polymer processing, and compared with other processing technologies, there are lots of advantages. As for the process equipment, co-rotating twin-screw extruder depending on its excellent performance in transportation and mixing has become the main equipment used in grafting reactive extrusion (GRE). By now, many experts had done a number of researches on grafting recipes and processing conditions such as rotation speed of the screws, throughput, temperature of the barrel and feeding mode. There is no systematical research about screw element and screw configuration. However, the mixing among the monomer, initiator and polymer melt is very critical in the manufacturing of high grafting products, and the screw, a core component of an extruder, would have significant impacts on the GRE process.Focused on the kneading-block mixing element (KBME) in co-rotating twin-screw extruder, the effect upon GRE was researched in the thesis, and the relationship between the mixing property of the kneading-block mixing zone (KBMZ) and the GRE was investigated. The main research work consists of: (1) Seven kneading blocks and one normal screw element were simulated by using the CFD package, POLYFLOW, and the mixing property of each screw element was analyzed; (2) Based on the microscopic rheology, combined with the kinetics of grafting reaction, the process of GRE in the KBMZ was simulated and analyzed; (3) The experiment of the GRE for LDPE-g-MAH was performed to investigate the influence of KBME; and (4) The experiments of filling ratio, residence time distribution (RTD) and immiscible polymer blends were carried out to explore the relationship between the KBME and the GRE.According to the simulation of the mixing property for KBME, the axial distributive mixing property and the shear property of reversed kneading blocks are better than those of neutral and forward ones, and those of normal screw element are worst, whereas the kneading disc width has few impacts on these two properties. In addition, based on the simulation of strain distribution, some conclusions were drawn: the mixing property of the normal screw element is the worst owing to the smaller mean strain and the wider strain distribution; the mean strains of the kneading block KB120 and KB150 are better, and the KB120 has the best mixing property due to its narrower strain distribution, but the KB150 is not favorable to the uniformity of the material because of its wider strain distribution; the strain distribution of the neutral element KB90 is much narrower, which would benefit the uniformity of the material, while its mean strain is smaller, which would result in the bad mixing of the material; as to the forward kneading block, the mixing ability of KB60 is worse than that of KB30. Furthermore, with the increase of the kneading disc width, the mean strain increase little, but the strain distribution becomes wider.Based on the simulation of the mixing property for the screw elements and combined the microscopic rheology with the kinetics of grafting reaction, the model of particulate reactive extrusion was put forward in this thesis. According to the analysis of the GRE in KBMZ, a conclusion was drawn that the screw element which could provide large mean strain but narrow strain distribution could improve the grafting efficiency.The result of the GRE for LDPE-g-MAH showed that as to the kneading blocks with the same kneading disc width, the grafting degree of the element KB120 is the largest, while that of KB60 is the smallest. With the increase of the kneading disc width, the grafting degrees of the specimens from each sampling point would decline. Moreover, as the kneading disc is wide enough, the grafting degree for the kneading block is even less than that for the normal screw element. In addition, the trend of the simulation is good agreement with that of the experiment in the GRE. There is some deviation for the forward element, which results from the difference of the filling ratio between the simulation and the experiment.In order to explore the relationship between the mixing property of KBME and GRE, the experiments of filling ratio, residence time distribution (RTD) and immiscible polymer blends were performed in this thesis. Seen from the results of these experiments, the ability of building pressure for the screw element would affect the GRE process, such as the initial position of the reaction and the flow state of the material. The RTD and dispersive mixing property for each screw configuration have direct impact on the grafting degree. And the dispersive mixing property is related to the shear stress and the shear rate. So the grafting degree of the extrudate depends on the residence time and the shear rate. Moreover, the strain equals to the multiple of the residence time with the shear rate. Therefore, the GRE would correlate to the strain course experienced by the material.Generally, the screw configuration suited to the GRE would provide large strain but narrow strain distribution. And the narrow strain distribution would favor the uniformity of the material. As for the KBME investigated in this work, the kneading block with the narrow kneading disc and the staggering angle 120°, that is KB120, is most suitable for the GRE.