Experimental Measurements and Modeling of Solubility and Diffusivity of CO₂ in Polypropylene/Micro- and Nanocalcium Carbonate Composites

The effects of the filler size and concentration on the solubility and diffusivity of CO₂ in polypropylene (PP)/calcium carbonate (CaCO₃) composites were investigated in this work. The apparent solubility of CO₂ in PP and its composites containing 5 wt % and 10 wt % micro- or nano-CaCO₃ was measured by using a magnetic suspension balance (MSB) at temperatures of 200 and 220 °C and CO₂ pressures up to 22 MPa. Meanwhile, the swelling volume of the PP composites/CO₂ solutions was experimentally measured at the same conditions by using a high-pressure view cell with direct visual observation. It was then used to correct the gas buoyancy acting on the PP composites in the MSB measurement so that the real solubility of CO₂ in the PP composites was determined. Meanwhile, the diffusion coefficient of CO₂ in the PP composites was estimated from the sorption lines at gas pressures ranged from 5 to 10 MPa. It was found that the experimental solubility and diffusivity of CO₂ in PP/nano-CaCO₃ composites were higher than those in PP/micro-CaCO₃. Moreover, the solubility and diffusivity of CO₂ decrease with increasing micro-CaCO₃ concentration whereas they increase with increasing nano-CaCO₃ loading in the PP composites. Two new models based on free volume theory considering the effects of micro-CaCO₃ on the free volume and the diffusion path and the lubricant effect of nano-CaCO₃ were proposed and used to well correlate the experimental diffusion coefficient of CO₂ in PP/micro-CaCO₃ and PP/nano-CaCO₃ composites, respectively.