Study On Microcellular Polypropylene Foamed By Physical Blow Agent

This is a part of study work of "Bead foaming technology of uncross-linked polypropylene (PP)", a project of the "Scientific Research Project of Guangdong Province". This work is focused on the study of microcellular foaming of PP through physical foaming technology using supercritical CO₂ or/and butane as the blowing agents.The microcellular plastics have some properties such as their cell size is smaller than the traditional foamed plastics, it's just 1/10 to 1/100 of the later's, and their cell density is much bigger, so that they have many outstanding properties. PP has some excellent properties such as good elasticity, high intensity, degradable while exposed to the ultraviolet radiation. Therefore, the microcellular PP is destined to be a new kind of materials in the future. However, we are facing the facts of the low melt strength and crystallization of PP. So we should choose the high melt strength PP or improve the melt strength of PP in order to make PP foamed. The diffusivity and solubility of CO₂ in PP is also studied in this work. The main work is to discover the relationship between the cellular morphology of PP and foaming condition. The relationship between the crystalline structure and foaming property of PP is also studied by comparing the crystalline property of foamed PP with unfoamed PP.1. The relationship between the cellular morphology of PP and foaming condition is studied. The cell structure of microcellular PP is characterized with SEM. The density of microcelular PP is measured. The result reveals that the change of foaming temperature and saturation pressure can control the cell density and size. With CO₂, it is difficult to make PP foamed while the temperature is lower than 90 "C or the saturation pressure is lower than 6.0MPa. With the foaming temperature elevated to 108℃, the shape of cells becomes pentagon and the cell size increases. With the higher saturation pressure, the higher cell density and smaller cell size are got. The cell density is 2.0×10⁸¹0⁹ and 2.0×10⁵¹0⁷cells/cm³, and the cell diameter is 20-50 μ m and 100-500 μ m for thesample respectively blown by supercritical CO2 and butane. The cell size distribution appears in double peak while using the mixture gas of CO2 and butane. The cell density and diameter is 7.0xl06¹.6xl09 cells/cm3 and 55-300 u m respectively while adding carbon black as nucleation agent, the cell size is bigger and the cell density is lower than that without carbon black, which is due to the different nucleation mechanism. The cell structure of "skin-core" is got in most of the microcellular PP without nucleation agent.2. Through the study of the foaming morphology of PP with different crystalline structure and the study of crystalline properties of foamed PP, the rationship between the crystalline and foaming morphology is invetigated. It reveals that the crystalline melting temperature is a key in the foaming procedure of PP, but it's not the unique factor. The melt strength and the cross-linking property are also the important factors in the foaming procedure. The crystalline state of PP has no effect on the foaming morphology. The crystalline structure of foamed PP is the same as unfoamed one.3. The diffusivity and solubility of CO2 in PP are studied. After the PP/ CO2 system is saturated at specified pressure and temperature, the saturated PP pellets are taken out of the batch and then their weight of PP is followed up. It is found that the weight of PP pellets decreases with CO2 diffusing out, and the velocity of CO2 diffusing from PP decreased gradually. The solubility of CO2 in PP increases with the pressure increased but decreases with the temperature elevated. After saturated with CO2, the samples are quickly placed in a preheated oil bath to be foamed. The skin-core cellular structure is obtained and some of the cells appear to be ruptured. The cell size decreases with the saturated pressure increased. The cell size is also dependent on the foaming temperature.