Experimental analysis of polymer nanocomposite foaming using carbon dioxide

Currently, the polymer foam industry is testing carbon dioxide (CO 2) for its applicability as a physical blowing agent (PBA) due to the phase-out of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) according to the Montreal Protocol[1]. CO2 is one of most promising alternatives because it is environmentally safe, non-toxic, non-flammable and inexpensive. However, CO2 has its drawbacks, such as low solubility and high diffusivity in comparison with other blowing agents. Therefore CO 2 sometimes leads to foams with higher density and/or poor surface quality, and almost always requires higher operating pressures than other agents. Currently, the concept of adding nanoparticles to a polymer is being investigated around the world as one possible way to overcome these problems. Polymer blends can be another approach.; In general, generating foams using a PBA includes saturating the polymer with the PBA at a certain pressure and temperature via thorough mixing. Then the mixture is subjected to a sudden thermodynamic change (temperature increase or pressure drop), resulting in the escape of the PBA and formation of the cellular structure. The typical foaming process includes cell nucleation, cell growth, and cell stabilization, the first two being the focus of this study. In the foaming process, several operating variables, e.g. temperature, pressure, pressure drop rate, and material related properties, e.g. solubility, diffusivity, and viscosity are crucial. In this study, primary attention was given to the effects of the material related properties on cell nucleation and cell growth. The reduction of gas solubility (or supersaturation) is the driving force of the foaming process. Diffusivity as well as viscosity affect both cell nucleation and growth.; Two equations of state (EOS), Sanchez-Lacombe (S-L) EOS and perturbed chain statistical associating fluid theory (PC-SAFT), were used to model the solubility and also phase boundaries (binodal and spinodal curves). Nucleation is a very complex phenomenon in physical foam processing and its experimental and theoretical studies cannot provide sufficient information to have a clear picture of the nucleation phenomenon. Proposed scaling functions provide a possible way to calculate the energy barrier in the nucleus formation, W. Cell nucleation rate data were extracted from the paper by Guo [2] et al. and also by our experiments. From the phase boundaries predicted, several parameters in the nucleation theory, e.g. gamma, Delta P, and W were calculated based on the experimental data. Several sets of W/Wcl and Deltamu/Deltamus were calculated. The initial slope of the possible scaling function was calculated by the diffuse interface theory. A possible scaling function (exponential decay type equation) was correlated to describe the relationship between W/ Wcl and Deltamu/Deltamus based on the calculation from experimental data. It provided insight for the connection between the phase boundary and the complex nucleation behavior.; Shear viscosity of polymers and nanocomposites were measured by parallel plate rheometry under a nitrogen blanket. Shear viscosity of polymers and nanocomposites under high pressure CO2 were studied via unique modified high pressure Couette rheometry. The effects of nanoparticles on shear viscosity of polymer w/ and w/o CO2 were compared.; The permeability coefficient, defined as the product of the solubility coefficient and diffusivity, of CO2 and water vapor in polymer nanocomposites and foams were measured near ambient temperature and pressure. The effects of nanoparticle and foam morphology on permeation were studied. The results provided valuable feedback on the design of polymer nanocomposites and foams, as well as an approximation for relative values of diffusivities in polymers under foaming conditions (higher temperatures and pressures).; To gain more insight on the early stages of the foaming process, in-situ observation of batch foamin...