EFFECTS OF MEMBRANE-MAKING CONDITIONS AND SHRINKAGE TREATMENT ON MORPHOLOGY AND PERFORMANCE OF CELLULOSE-ACETATE BUTYRATE MEMBRANES

The functions of additives in cellulose acetate butyrate (CAB) membran e casting solution, effect of thermal shrinkage treatment on porous CA B membranes, and the changes of CAB membrane surface morphology during the solvent evaporation step have been investigated. Additives (glyce rol and lactic acid) in CAB membrane casting solution function only as pore number promoting agents when used at low concentration and funct ion both as pore number and pore size promoting agents when used at hi gher concentrations. Triethyl phosphate in CAB membrane casting soluti on functions both as a pore number promoting agent and as a secondary solvent for CAB. Three distinct phases can be observed in the solvent evaporation step in making CAB membranes. With the increase in solvent evaporation time, the number of pores in the first pore size distribu tion increases in the initial small pore-forming phase and decreases i n the large pore-forming phase, and the number of pores in the second pore size distribution always increases with solvent evaporation time. These changes in pore numbers, pore sizes, and pore number ratio in t wo pore size distributions as well as the membrane skin layer thicknes s together govern the ultimate membrane performance and result in a ma ximum solute separation which, in the case of CAB/acetone membranes, f alls at 60 seconds of solvent evaporation time. Significant improvemen t of the performance of a porous CAB membrane can be achieved by therm al shrinkage treatment. Equally high CAB membrane performance can also be achieved by using a lower concentration of additives in the membra ne casting solution.