Experimental study of the control of pore sizes of porous membranes applying chemicals methods

Poly(vinylidene fluoride) (PVDF), polymethyl methacrylate (PMMA) and cellulose acetate (CA) in the presence of N-methyl pyrrolidone were blended in various combinations by using dimethylformamide (DMF) and glacial acetic acid (CH3COOH) as solvent and used to prepare flat-sheet membranes by the phase-inversion process. A squarecrossing table with four factors and three levels for the PVDF–PMMA–CA system was designed to study the influences of the casting solution composition and gelation conditions on membrane pore size; preparation conditions for the system were optimized. The results showed that the effect of the casting solution composition was stronger than that of the solid content and the content of the coagulation medium on characteristics of the prepared membrane. The suitable composition for the PVDF–PMMA–CA system was found to be a solid content of 18%, an additive content of 4%, DMF/CH3COOH = 4:1 and a Na2CO3 solution with aa concentration of 2% as the coagulation medium. In addition, it was found that carbon dioxide gas produced by the reaction of glacial acetic acid in the cast solution with sodium carbonate in a coagulation bath can be used to control the even distribution of the membrane pore size during the process of coagulation.

1. The square-crossing experiments with four factors and three levels for the PVDF–PMMA– CA system showed that the ingredients of the coagulation bath are the most important factor affecting membrane characteristics. The second most important factor was solid content, but coagulation medium content had little influence on the membranes. The optimum preparation condition was A2B1C2D1. That is to say, when the solid content was 18%, the additive content 4%, DMF/CH3COOH=4:1 and Na2CO3 solution of concentration 2% as the coagulation medium, a membrane with even pore size distribution was prepared. 2. During the membrane-making process, producing gas can improve the uniformity of membrane pore size distribution. The gas was produced by a reaction between glacial acetic acid in the casting solution and sodium carbonate as an ingredient of the coagulation bath: 2CH3 COOH + Na2CO3 ] 2CH3 COONa + CO2 8 However, the porosity of the membrane was not high, and this part of the experiment should be studied further.