Research On The Microcelluar Foaming And Crystallization Behavior Of Polypropylene Modified By β Nucleating Agent Foamed By Supercritical CO₂

In this thesis, we use three kinds of β nucleating agents to modify isotactic polypropylene（i-PP） and β nucleated polypropylene（β-PP） was obtained. Then take supercritical CO₂ gas as foaming agent to prepare β-PP foams by a high pressure vessel via a batch process. The microcellular foaming properties and crystallization behaviors of β-PP /supercritical CO₂ system were investigated under different saturation temperatures, pressures, time and depressurization rates. In order to research the influence of structure matching between β nucleator and PP, tensile force field caused by bubble expanding on the fromation of β-crystals during the foaming process, the relative content of β-crystals of unfoamed β-PP samples crystallized under under CO₂ atmosphere and ambient air pressure were investigated.The results showed that the cell morphology of β-PP foams were significantly improved when three kinds of β nucleating agent（0.3wt%） were added. This was because β nucleating agent could serve as heterogeneous nucleation agent, then the nucleation rate was accelerated and a large number of cell nuclei were generated in β-PP melt. During the supercritical CO₂ foaming process of β-PP foams, it was found that the cell density and foam expansion ratio were influenced greatly by the saturation temperature, pressure and depressurization rate, while the saturation time had less effect on the cell morphology, expansion ratio and cell nuclei density. When the saturation pressure was increased from 16 MPa to 22 MPa, the cell density of β-PP foams increased obviously and the cell size decreased significantly, while the saturation temperature had a reversed effect on the cell density and cell size. In addition, the cell density and foam expansion ratio were decreased significantly with the decrease of depressurization rate. When the depressurization rate was 16MPa/s, the cell denity and foam expansion ratio of β-PP（WBG） foams were 1.15×108 cell/cm3 and 7.8-fold respectly, and the cells were almost polygon structures with thin inner wall. When the depressurization rate was decreased to 1.8MPa/s, the cells had circular structures and thick inner wall with the density of 1.1×107 cell/cm3 and the expansion ratio of 3.3-fold.After adding three kinds of β nucleating agent（0.1wt%, 0.3wt%） into i-PP to prepare β-PP foams, the content of β-crystals was improved greatly when β nucleating agents（TMB-5, WBG-Ⅱ） were added, while the NT-C β nucleating agent had less effect on the content of β-crystals. In addition, the supercritical CO₂ foaming process could also influence the formation of β-crystals in three kinds of β-PP foams, especially the saturation pressure and depressurization rate. When the saturation pressure was increased from 16 MPa to 22 MPa, the content of β-crystals of β-PP（TMB-5, 0.3wt%） foams was decreased from 89% to 62%. When the depressurization rate was increased from 1.8MPa/s to 16MPa/s, the content of β-crystals of β-PP（WBG, 0.3wt%） foams was decreased from 90.2% to 84%. First,only when the absorption effect and crystallization model effect from β nucleating agent were good enough, the formation of β-crystals in PP was available, but large amounts of CO₂ gas would permeate into the interface between PP and β nucleating agent during cell nucleation process, then absorption effect and crystallization model effect from β nucleating agent were weakened, which could inhibit the formation of β-crystals. Second, during the foaming process, bubble expanding would cause the tensile force field, which also could inhibit the formation of β-crystals. With the increase of saturation pressure, the tensile force field was increased, causing the content of β-crystals decreased.In order to verify the content of β-crystals in β-PP foams was influended by the foaming process, but not the temperature or supercritical CO₂ pressure, β-PP samples crystallized under ambient air pressure and supercritical CO₂ atmosphere were prepared. WAXD results showed that the content of β-crystals of unfoamed samples was higher than β-PP foams and kept almost consistent when the saturation pressure increased from 16 MPa to 22 MPa. This showed that the saturation temperature and supercritical CO₂ pressure could hardly change the content of β-crystals in β-PP foams. After the analysis and comparison, it was found that the decreased crystallization nucleation ability of β nucleating agent, the tensile force field caused by bubble expanding were two main reason to decrease the content of β-crystals during the supercritical CO₂ foaming process.