Morphology Development And Control Of PC/PE Blend During Processing And The Morphology, Structure And Properties Of The Morphology Controlled Blend

Performance enhancement of general-purpose plastics (mainly polyethylene (PE) and polypropylene (PP)) is one of the most important topics in the field of polymer materials science and engineering at present and in the future. Blending modification with engineering plastics is a major route to enhance the performance of general-purpose plastics. Polycarbonate (PC) was used to modify the high density polyethylene in this thesis. Controlling of phase morphology and compatibilization are both the important problems that should be settled to realize the performance enhancement of polyethylene blend. Phase morphology of PC/PE blend is an important factor to influence the properties of the blend. Therefore, the morphology development of PC/PE during the twin-screw extrusion and injection molding was investigated in this thesis. The interface interaction is weak in PC/PE blend, which induced the decreasing of the mechanical properties. In this thesis, the macromolecular reaction between PC and ethylene-acrylic acid (EAA) was used to in-situ compatibilize the PC/PE blend. Morphology of PC/PE blend was controlled by adjusting the technical parameter of blending, injection molding and the reactive compatibilization. A novel skin-core structure was obtained in the processing, which could be the reason that the enhancement of the impact property of PC/PE blend. The main results are: 1) Morphology development of PC/PE blend during compounding in a twin-screw extruderThe morphology of PC/PE blends at different positions along the screw axis was studied and the model of morphology development of the dispersed phase in the initial stage during twin-screw extrusion was proposed. The polycarbonate pellets partially deformed to sheets and ribbons during the melt softening step. Due to the effect of interfacial tension and flow characteristics, those sheets or ribbons became unstable and holes were formed. The holes rapidly grew in size and in concentration until the ribbons were changed to fibers.The effects of blending temperature, viscosity ratio (the ratio of the viscosity of the dispersed phase to that of the matrix), screw speed and the screw configuration on the morphology of the PC/PE blend during the extrusion were discussed in detail. It was found that the morphology of the dispersed particles and the interfacial adhesion between the dispersed phase and matrix were both influenced by the extrusion temperature. The dispersed phase exhibits a spheroidal shape and a small size during high temperature processing, and an irregular shape and a large size when it was processed at low temperature. The PC phase with a lower viscosity was easier to. disperse and also to coalesce. Therefore, the deformation of the low-viscosity dispersed phase during the processing was more intense than that of the high-viscosity dispersed phase. It was found that both of the shape and size of the dispersed phase in the uncompatibilized PC/PE blend are influenced by the screw speed. The evolution of dispersed phase morphology can be affected by increasing screw speed, and the dispersed particle size decreases with the increasing of the screw speed. By comparing the effect of the screw configuration on the morphology development of the PC/PE blend, it was found that the melting and breaking up of the dispersed phase were mainly affected in the initial blending stages by the number of the kneading blocks. When a kneading block with a 90 degree staggering angle was used, the size of the dispersed particles decreased and the long fibers were shortened, the large particles were drawn by the additional kneading zone. Finally, all of these structures were completely changed to the short fibers. 2) Macromolecular reaction between PC and EAA and the effect of reactive compatibilization on the morphology and properties of PC/PE blend A grafted copolymer PC-graft-ethylene-co-acrylic acid (PC-g-EAA) was generated as a compatibilizer in situ during processing operation by ester and acid reaction between PC and EAA in the presence of the catalyst of dibutyl tin oxide (DBTO). The effects of the blend composition, catalyst content and mixing time on the reaction between PC and EAA were discussed. The influence of this copolymer formation at the interface between PC and EAA on the rheological properties and crystallization behavior for EAA/PC binary blends were studied. The equilibrium torque increased with the DBTO content increasing in EAA/PC blends on Haake torque rheometer, indicating the in situ formation of the graft copolymer, which enhanced the viscosity of PC/EAA blend.DSC studies suggested that the heat of fusion of the EAA phase in PC/EAA blends with or without DBTO reduced with the formation of copolymer compared with pure EAA. This indicated that the generation of PC-g-EAA enhanced the interaction between PC and EAA molecule, which decreases the mobility of EAA chain. Therefore, the crystallization of EAA chain was hindered and the degree of crystallinity of EAA phase decreased. Study on the crystallization of PC/PE/EAA blend indicated that PE phase crystallized still at its bulk crystallization temperature. The degree of crystallinity of PE phase in PC/PE/EAA blends was also reduced with the addition of EAA and DBTO compared to the uncompatibilized PC/PE blend, indicating that the graft copolymer PC-graft-EAA improves the interaction between PC and PE phase and hindered the movement of PE chain and arrangement into the lattices. It was found by dynamic mechanical analysis that the temperature of relaxationαandγof PE both shifted to the lower temperature due to the decreasing of the crystallinity of PE phase.Then morphology of the uncompatibilized and compatibilized blends of PC/PE was studied with different contents of EAA and DBTO. Morphological observations in PC/PE blends also revealed that the number of microvoids was reduced and the interface was imprived by increasing EAA content or adding catalyst DBTO as compared to the uncompatibilized PC/PE blends. This implied the interface tension of compatibilized PC/PE blend became lower owing to the addition of EAA and the generation of PC-g-EAA copolymer. It was found by dynamic rheological analysis that the elastic modulus, viscous modulus and complex viscosity were all increased along with the generation of PC-g-EAA copolymer, which indicated that the reactive compatibilizaion with EAA could improve the interface interaction of PC/PE blend.3) Morphology control of PC/PE/EAA blend during blending and injection molding The morphology of dispersed phase was tailored by optimizing of the combination of the share rate and the resident time during twin-screw extrusion. It was found that the increasing of screw speed provided higher shear rate and hence resulted in the less resident time. At a medium speed, the morphology of dispersed phase was modified, and the size of dispersed particles decreased and the diameter distribution narrowed. The size of dispersed phase was controlled by changing the screw configuration, and the number average diameter of PC particles was reduced to 0.50 um and the volume average diameter was 1.24 um.The interface tension of the different PC/PE/EAA blends was calculated using Palierne model. The interface tension of PC/PE blend was reduced due to the reactive compatibilization. The interface tension of uncompatibilized PC/PE blend, 19.3mN/m, was dropped to 7.3mM/m of the compatibilized blend. The resident time was variable with the varieties of the screw configuration, which influenced the generation of PC-g-EAA copolymer. Therefore, the interface tension of PC/PE/EAA blend was obviously influenced by the screw configuration. In this thesis, the PC/PE/EAA blend with a lower interface tension of 2.5mN/m was obtained by changing the screw configuration. The morphology and interface interaction were both improved by the controlling of the technical parameter during twin-screw extrusion.In the injection molding, the dispersed phase in the compatibilized PC/PE blend is easy to deform by the shear. At a high injection rate, the dispersed particles at different zone (skin, sub-skin, intermediate and core) all deformed to the fiberious structure. But in the cooling and solidification of the polymer melt, the dispersed fibers, in the intermediate and core, would relaxed and necked, and broke up to the oriented particles. On the other hand, at a low injection rate, the deformation of dispersed phase was smaller than that of high injection rate due to the decreasing of shear rate. Therefore, the dispersed fibers in the core zone with a small length/diameter ratio did not break up to particles, and the fiber structure was held, which could be a novel skin-core structure differing from the former proposed model.4) Morphology and impact strength property of PC/PE/EAA blend The impact properties of PC/PE/EAA blends injected at different injection rate were studied in this thesis. The notched impact strength of the gate side of PC/PE/EAA blend injected at an injection rate of 3.6cm~3/sec was 52.1kJ/m~2, and 24.5kJ/m~2 of the non-gate side. The notched impact strength of PC/PE blend at the same positions was 11.64 and 8.36kJ/m~2 respectively. This indicated that the reactive compatibilization improved the toughness of the blend. The impact properties of the gate side and non-gate side of uncompatibilized blend were comparative, but the difference at gate and non-gate side of compatibilized blend was obvious. The impact property of compatibilized PC/PE blend depended on the injection rate and the impact strength decreased along with the increasing of the injection rate.The novel skin-core structure in the injection bar with a low injection rate improved obviously the impact strength because the dispersed fiber with a small length/diameter ratio and a large diameter could absorb the sizeable energy during the impacting. It was found from the SEM micrograph of the impact fracture surface of the blend injected at the low injection rate that the dispersed fiber at the gate side was pulled out and the dispersed fibers stuck on the matrix. This indicated that the interface interaction in the PC/PE/EAA blend was too intense to be destroyed, which induced that the matrix was broken during the impacting, and the improved interface bonding enhanced the impact strength of the blend. At the non-gate side, the interface contact decreased, which induced the invalidation of the interface bonding during the impact experiment. Therefore, the impact strength at non-gate side of compatibilized blend was lower than that at gate side. The dispersed particles in the core zone of the blend injected at the high rate absorbed only few of the energy during the impacting, which induced that the impact property of the blend at high injection rate was much lower than that at low injection rate.