Depolymerization reactions, polymer precipitation, and chemical equilibria in supercritical fluid solvents

Supercritical fluid (SCF) solvents are flexible in the sense that the solvent properties, such as density, diffusivity, and viscosity, can be tuned easily by small isothermal changes in pressure. Also, and perhaps more importantly, these properties allow SCF and subcritical fluids to serve as solvents for reactions. In this work we have studied the depolymerization of styrene butadiene copolymer (SBR) in near- and supercritical water with and without an oxidant. The environmentally friendly solvent supercritical (SC) carbon dioxide (CO 2) was used to precipitate nylon out of a polymer solution. We have also explored the use of subcritical and SCFs as reaction solvents to address environmental issues.; In Chapter 2, we studied the conversion of waste tire using near-critical and SC water oxidation. The results show that the solid waste destruction efficiency was very high and that a variety of lower molecular weight hydrocarbons could be obtained from the waste depending on the reaction conditions. In Chapter 3, the depolymerization reactions of SBR were performed in both batch and semi-batch reactors at the conditions of supercritical and near-critical water, respectively. The destruction efficiency and liquid product distribution were strongly dependent upon the operating conditions of reaction temperature, reaction pressure, oxidant concentration, and flow rate. Two parallel reaction mechanisms of oxidative and thermal degradation are proposed. Oxidative degradation appears predominant at the lower temperatures studied while thermal degradation appears predominant at the higher temperatures in the depolymerization of SBR based upon the liquid analysis.; In Chapter 4, a new process for the separation of nylon from carpet waste using SCF antisolvent precipitation was described. Studies on the SCF antisolvent precipitation were performed and convenient recovery of the nylon material was achieved, as well as control over the morphology and particle size of the nylon powder. In Chapter 5, SC CO2 was also used to make micrometer-sized particles of nylon from pure nylon 6/6. No significant changes were observed by varying operating variables in the precipitation with a compressed antisolvent (PCA) process. A possible explanation for this unique result is that the surface tensions of the sprayed droplets diminish at very short distances from the nozzle outlet compared to the jet breakup length as Lengsfeld et al. (2000) proposed.; In Chapter 6, Fourier transform infrared (FT-IR) spectroscopy was used to determine the equilibrium constants (Kc) of dimerization between formic acid monomer and dimer in sub- and supercritical CO2. These results indicate that the dimer formation is more preferred in the low density CO 2 than within the packed solvent medium at higher densities. Additionally, using the modified lattice fluid hydrogen bond (MLFHB) model, the behavior has been successfully modeled, and ln Kc decreases almost linearly with increasing fluid (COD density. Furthermore, the effect of cosolvent (formic acid) in the system of supercritical CO2 + formic acid + nylon 6/6 was also examined. The dimerization constant Kc of 130 L/mol, obtained in the presence of the nylon films, was slightly lower than the Kc value of 155 L/mol, which was obtained in the binary system of formic acid + CO2 in the absence of a Nylon 6/6 film. This indicates that there is a specific interaction between formic acid and Nylon 6/6 in supercritical CO2.