Development of asymmetric hollow fibers from polyimides for air separation

Abstract

We have developed 6FDA/6FDAM polyimide hollow fibers with a selectivity of 6.7 for O 2/N 2 and a permeance of 8.3×10 −6 cc(STP)/cm 2-sec-cmHg. The dense layer thickness was about 1000−1900 Å. These hollow fibers were dry-jet wet-spun from a 42% low IV (intrinsic viscosity)6FDA/6FDAM polyimide/N-methyl-pyrrolidone (NMP)/propionic acid (PA) (57/43) dope using methanol as a coagulant and 80/20 NMP/H 2O as a bore fluid. The hollow fiber must be coated with a silicone elastomer. The fiber diameter was about 215 μm with a wall thickness about 25−30 μm. SEM observation showed that the dense layer was located at the outer surface, while the region near the inner surface had an interconnceted open-cell structure. The total void percentage in this fiber was about 60%. The calculated Hildebrand parameter for this study was about 2.3 to 2.99 (cal/cm 3) 0.5 between the NMP:PA solvent complex and the polyimide. The first T g for our 6FDA/6FDAM polyimide hollow fiber was much lower than the T g of its bulk, or second T g.