Research On The Crystallization And Foaming Behavior Of Polypropylene/supercritical CO₂ System

In this thesis, we use supercritical CO₂ as foaming agent to prepare isotactic polypropylene（i-PP） and β nucleating agent modified polypropylene（β-PP） foams via a batch process. Then the crystallization behavior and foaming properties of polypropylene/supercritical CO₂ system were researched under the different saturation temperatures, saturation pressures, saturation time and depressurization rate.The results showed that a large amount of β-crystals could be obtained in i-PP foams after the supercritical CO₂ foaming process. And with the augment of depressurization rate, the content of β-crystal in i-PP foams increased linearly. While under the same conditions of crystallization temperature, β-crystal could not be formed in i-PP samples crystallized under ambient air and CO₂ atmosphere. This showed that the supercritical CO₂ foaming process had an important influence on the i-PP crystallization behavior. After contrasting and analyzing we know that the tensile force field caused by bubble expanding was the main reason for the formation of β-crystals. Although the presence of supercritical CO₂ could not induce the change of crystal type in i-PP, it had an important influence on the crystal morphology of i-PP. I-PP could generate large spherulitic structure when crystallized at atmospheric pressure, but spherulitic structure crystal would not be formed in i-PP and the crystal size decreased obviously when crystallized under CO₂ atmosphere.In the preparing process of i-PP foams using supercritical CO₂ as foam agent, the saturation temperature, saturation pressure and depressurization rate were found to have large influence on the cell morphology and foaming ratio, while the saturation time had less effect on the crystallization behavior and foaming properties. When the saturation temperature was increased from 140 ℃ to 160 ℃, the cell average diameter of i-PP foams was increased and the cell density decreased much, while the influence of saturation pressure on the cell average diameter and density was just reversed. It was investigated that the cell average diameter and density was decreased significantly with the decrease of depressurization rate. When the depressurization rate was 16 MPa/s, the cells had polygon structures and thin inner wall with the density of 1.52×106 cell/cm3 and the expansion ratio of 16.2; while after the depressurization rate was decreased to 1.3 MPa/s, the cells had circular structures and thick inner wall with the density of 1.3×105 cell/cm3 and the expansion ratio of 3.3.After adding 0.3wt% β nucleating agent into i-PP for the preparing of β-PP, the foaming properties and the cell morphology of β-PP were significantly improved. Noticeably, the cell density was increased about two orders of magnitude after adding 0.3wt% β nucleating agent into i-PP. When the saturation temperature, saturation pressure, saturation time and depressurization rate were changed, the changes of the cell morphology and expansion ration in β-PP foams were consistent with i-PP. However, the influence of the supercritical CO₂ foaming process on the formation of β-crystal was different in polypropylene after adding β nucleating agent. The tensile force field in the foaming process could induce i-PP to generate β-crystal, while for β-PP the tensile force field caused by bubble expanding could inhibit the formation of β-crystal. With the increase of depressurization rate, the β-crystal content in β-PP foams was decreased, which was attributed to the different β-crystal nucleation mechanism in β-PP and i-PP.