Alternative solvents for cellulose derivatives:: Miscibility and density of cellulosic polymers in carbon dioxide+acetone and carbon dioxide+ethanol binary fluid mixtures

Abstract

Identification of alternative solvents for processing of cellulosic polymers is of interest in a number of industrial applications, ranging from fibers to films. In this study, miscibility of cellulose acetate, cellulose triacetate, cellulose acetate butyrate, cellulose propionate and ethyl cellulose in `carbon dioxide+acetone' and `carbon dioxide+ethanol' binary fluid mixtures have been investigated. Data are reported at 3% by mass polymer concentration in two binary solvent fluid mixtures containing 30% and 70% by mass carbon dioxide. Demixing pressures are reported in the temperature range from 60 to 175°C. Complete miscibility of the polymers was achieved at all pressures in carbon dioxide+ethanol mixtures containing 30% carbon dioxide, while in mixture containing 70% carbon dioxide, miscibility required pressures greater than 20 MPa. Higher pressures were required for miscibility in carbon dioxide+acetone mixtures with pressures being greater than 5 MPa and greater than 35 MPa for mixtures containing 30 and 70% carbon dioxide, respectively. The solutions all displayed LCST (lower critical solution temperature) behavior, with demixing pressures increasing with temperature. The relative ease of miscibility of the polymers followed essentially the same order in either the ethanol or acetone fluid mixtures, with demixing pressures increasing as ethyl cellulose<cellulose propionate<cellulose acetate butyrate<cellulose triacetate<cellulose acetate, except that in acetone mixture, cellulose acetate butyrate displayed higher demixing pressures than cellulose triacetate. For solutions of cellulose acetate butyrate and cellulose propionate in 70/30 carbon dioxide/ethanol fluid mixture, pressure–density isotherms were determined both in the one-phase and in the two-phase regions and the demixing densities were noted. The solution density at the demixing pressure gives a measure of the volume expansion needed to bring about phase separation at a given temperature.