Foaming Of Polypropylene With Supercritical Carbon Dioxide And Principle Analysis

Polypropylene (PP) foams have many excellent properties such as light weight, low price, high stiffness-to-weight ratio, high thermal and acoustic insulation properties. In addition, the tip of crack in PP foams can be passivated because of the existence of bubbles, thus largely improving the mechanical properties of PP foams. Therefore, they are widely used as building materials, auto parts and package.In this paper, PP was foamed with batch foaming process using supercritical CO2. The foaming temperature range with the conventional foaming process was very narrow and it was very difficult to control cellular structures. In order to overcome these difficulties, we improved the conventional process, the low-temperature and the high-temperature impregnation foaming processes were obtained. And nucleation mechanisms of these two foaming processes were studied.PP and PP/nano calcium carbonate (PP/n-CaCO3) nanocomposites were foamed with the low-temperature impregantion foaming process, the foaming temperature range of PP with this process was5times broader than that of PP with the conventional one. Cellular structure of PP/n-CaCO3was less sensitive to variations of foaming temperature and saturation pressure, but it was more regular comparing with that of PP. Results of orthogonal designs showed that effects of four factors on the the expansion ratio of PP and PP/n-CaCO3foams decreased in the order:Saturation pressure> Impregnation temperature> Foaming temperature> Foaming time. Effects of four factors on the cell diameter were listed in the order:Foaming temperature> Saturation pressure> Impregnation temperature> Foaming time. Results of analysis of variance indicated that there were almost no interactions among those factors.Supercritical impregnation at low temperature with supercritical CO2dramatically disrupted crystals, which made cell structures of PP foams more regular. In addition, the supercritical imperagnation increased the storage modulus (G’) and complex viscosity(η*), which made cell structures stable at higher temperature. Besides, the supercritical impregnation could increase the foaming temperature range.The high-temperature imperagnation foaming process took much less time (2.5h) to loam and had much broader foaming temperature range (about55℃), whose time and foaming temperature range was3times less and14times broader than that of the conventional foaming process, respectively. The blending of n-CaCO3not only decreased the cell diameter and increased the cell density of PP foams but also induced isothermal and non-isothermal crystallization at higher temperature. In addition, some unfoamed regions were apparently observed in nanocomposite foams at low foaming temperature because n-CaCO3could accelerate crystallization in cooling and cryostat stage.An activation model was modified on the basis of the mass equilibrium, this model was combined with nucleation and S-L EOS theory to evidence the foaming behaviors, and a new activation model was obtained, which was used to simulate the cell nucleation and cell diameter of semi-crystalline polymers like PP and amorphous polymers like polystyrene (PS). A satisfactory agreement between corrections and experimental data was obtained indicating that the new activation model combining with nucleation and S-L EOS theory could be successful to decribe foaming processes of PP and PS.