Investigation Of Solubility And Diffusivity Of CO₂in Polypropylene Composites And Their Application In The Simulation Of CO₂Foaming Injection Molding Process

Both the melt strength improvement effect and the heterogeneous nucleation effect of fillers in polypropylene (PP) composites benefit the CO2foaming process of PP composites. The dissolution and diffusion behavior of CO2in PP composites governs the formation of composites/CO2solution, bubble nucleation and bubble growth during the CO2foaming process. Therefore CO2solubility and diffusivity in PP composites is important data for both optimization of foaming process and regulation of bubble morphology of foamed polymer.In this work, the measurement method for directly and accurately determining the swelling, solubility and diffusion data of isotactic polypropylene (iPP)/CO2mixture were firstly established and validated. Then the solubility and diffusion coefficient of CO2in iPP/CaCO3, iPP/carbon fiber and iPP/MMT composites were measured to investigate the effects of polymer/filler interface bonding, filler size, filler shape, the polymer state and the adsorption feature, respectively. Different CO2solubility and diffusion coefficient models were proposed based on different effects of fillers accordingly. Finally, the solubility and diffusivity data was applied in the Moldex3D simulation of the CO2foaming injection molding of iPP and iPP composites, both the bubble size and bubble density were well predicted. The detail works are as follows:The apparent solubility and diffusion coefficient of CO2in two iPP melts (iPP1and iPP2) with different molecular weight distributions were measured by a magnetic suspension balance (MSB), which were corrected by the swelling volume of the iPP/CO2solutions that was experimentally measured under the same conditions by using a high-pressure view cell with directly visual observation. The solubility of CO2in the iPP melts was also corrected using swelling ratio predicted by Sanchez-Lacombe equation of state (S-L EOS) for comparison, which showed a good agreement with experimental solubility under15MPa. However it showed a large deviation to the experimental solubility at high CO2pressures due to the overestimation of swelling ratio predicted by Sanchez-Lacombe (S-L) equation of state. Lower swelling degree, CO2solubility and diffusion coefficient were found in iPP with a wider molecular weight distribution due to its smaller free volume.The effect of interface bonding condition between the fillers and polymer matrix was investigated by determining the solubility and diffusion coefficient of CO2in iPP2/Micro-calcium carbonate (MicroCaC03) composites with and without interface compatibilizer. It was found that the solubility of CO2in the iPP2/MicroCaCO3composites without the interface compatibilizer increased with increasing filler content due to the interface gap, while the CO2solubility remained almost unchanged in iPP2composites with compatibilizer. The diffusion coefficient of CO2in both iPP composites was found to decrease with increasing the filler content. The Henry’s law and a modified Henry’s law were used to well correlate the solubility of CO2in the iPP2composites with and without the interface compatibilizer, respectively. The diffusion coefficients of CO2in pure iPP2and iPP2/MicroCaCO3composites were well correlated by Kulkarni and Stern free volume model and its modified version with a parameter accounting for the barrier effect of the filler. The average relative deviation (ARD) between the predictions and experiments of CO2solubility at200℃and220℃were2.14%and3.56%, respectively:The relative deviation between the predictions and experiments of CO2diffusivity coefficient was less than10%.The effect of filler size of spherical filler was investigated by determining the solubility and diffusion coefficient of CO2in iPPl/NanoCaCO3and iPPl/MicroCaCO3composites. As the NanoCaCO3and MicroCaCO3fillers placed opposite effects on the free volume of polymer composites, the swelling degree, CO2solubility and CO2diffusion coefficient also showed opposite dependence on filler concent in iPPl/NanoCaCO3and iPP1/MicroCaCO3composites respectively. The Maeda free volume model modified with lubricant factor B and tortuosity t factor were applied to correlate the diffusion coefficient of CO2in iPP1/NanoCaCO3and iPP1/MicroCaCO3, respectively. The (ARD) between the predictions and experiments of the CO2diffusivity coefficient in iPPl/MicroCaCO3composites at200℃and220℃were2.29%and1.74%, respectively; and those in iPP1//NanoCaCO3composites at200℃and220℃were1.52%andl.47%, respectivelyThe effects of nano filler shape and the polymer state on the CO2solubility and diffusivity was investigated. The results showed that different to the lubricant effect of spherical filler nanoCaCO3, the addition of platelet-like NanoMMT reduced the free volume of polymer matrix, therefore reduce the swelling degree, CO2solubility and diffusion coefficient in iPP1/NanoMMT composites. And as the free volume of iPP1composites in the solid state is far lower than that in its melton state, the sorption and diffusion data is also much smaller in solid iPPl composites. The free volume model corrected with tortuosity t for platelet-like filler, and that corrected with combined tortuosity t for spherulites and platelet-like filler were applied to correlate CO2diffusion coefficient in melton and solid PP1/NanoMMT, respectively, with the (ARD) of1.04%and4.16%for composites in solid and melton state, respectively.The effect of absorptive filler was investigated by determining the solubility and diffusivity of CO2in iPPl/carbon Nano fiber (CNF) composites. The results showed that CO2solubility and diffusition coefficient in iPP1/CNF increased with increasing CNF loading due to the gas absorption nature of CNF. Both the Langmuir absorption modified Henry law and the absorption driving factor modified free volume model showed good correlation on the CO2solubility and diffusion coefficient in iPPl/CNF composites. The ARD between the predictions and experiments for CO2solubility at200℃and220℃were2.59%and3.72%, respectively; and those for the CO2diffusivity coefficient werel.24%and1.23%, respectively.Based on the experimental study on the effects of injection speed, mold temperature, and CO2dosage concentration, as well as the filler on the final bubble size and density of CO2foaming injection molded iPP and iPP/NanoCaCO3composites, the Moldex3D software was used to simulate the foaming injection molding process, with the employment of the measured CO2solubility and diffusion data in iPP and iPP/NanoCaCO3composites. The predicted bubble size and density distribution in the center section of tensile bars showed a good agreement with the experimental values.