| Thanks to its excellent comprehensive properties, polypropylene (PP) has been widely used in large quantities of applications. However, low notched impact strength of PP in low temperature limits its more applications. One of the most common ways to improve the impact strength of PP is to blend PP with impact modifiers, such as rubbers or elastomers, and inorganic nanoparticles. So, it is very critical to uniformly disperse these dispersed phases in PP. Shearing flow and elongation flow can improve the dispersion of dispersed phases. Compared with the shearing flow, the elongation flow with the same strength is more effective, but it is very difficult to generate elongation flow in traditional extruders. Many studies have found that when the bubble nucleats and expands in CO2-induced foaming of polymers, the rapid bubble inflation will result in strong stretching flow field inside polymer melts surrounding bubbles.In order to increase the notched impact strength of PP, a two-stage extruder, consisting of a co-rotating intermeshing twin-screw extruder and a single-screw extruder, which was equipped with a SC-CO2 injection system, was designed. A series of PP multi-phase systems, such as PP/ethylene-octene copolymer (POE) blends, PP/nano-CaCO3 composites, PP/POE/nano-CaCO3 ternary composites, and PP/nano-clay composites, were prepared by sc-CO2 assisted extrusion with the two-stage extruder. The dispersion of the impact modifiers in PP was improved by altering two important parameters, the injection concentration of sc-CO2 and the pressure differential (AP) of melt between the entrance and exit of the die. Structure and properties of PP multi-phase systems were analyzed by various characterization techniques. Dispersion mechanism of the bubble explosion in polymers with the processing aid of sc-CO2 was put forward. Our studies provide a new way and theoretical basis for the fabrication of PP multi-phase systems with high touhgness. The main research work and conclusions were shown as following.Effect of three types of twin-screw configurations (i.e., conveying screws, shearing screws and mixing screws) on the dispersion of different dispersed phases in PP was studied. When the mixing screw was applied to the twin-screw extruder, PP multi-phase systems with uniform dispersion of dispersed phase were prepared in the two-stage extruder.There will be an optimal injection concentration of sc-CO2 for every PP multi-phase system with different dipersed phases. The most homogeneous dispersion of dispersed phases, the smallest crystals with the largest amount, and the highest notched impact strength of systems can be found when the optimal injection loading is injected into the barrel.AP has important influence on the bubble nucleats of CO2. It is found that the nucleation rate and the function of biaxial stretching are enhanced, and the toughness of the systems is improved with the increasing ?P.The different disperse phases can be used as effective nucleating agents and facilitate the crystallization of PP. Nevertheless, the injection loading of sc-CO2 and ?P have little impact on the crystallization behaviors of PP. ?-crystals of PP can not be induced by altering the injection loading of SC-CO2. However, a very few ?-crystals of PP, which has little influence on the notched impact strength of systems, can be induced when AP is low and the injection loading of SC-CO2 is optimal. The main reason for the improvement of notched impact strength of PP multi-phase systems is that different dispersed phases can be dispersed uniformly in PP matrix.The notched impact stength of PP multi-phase systems prepared by SC-CO2 assisted extrusion can be improved significantly while the tensile strength remains almost unchanged. The purpose of the toughening of PP is achieved. |
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