The effects of spinning conditions on asymmetric 6FDA/6FDAM polyimide hollow fibers for air separation

Abstract

: We have found that the chemistries of inner and external coagulants, gelation bath temperature, and air gap distance have profound effects on 6FDA-polyimde hollow-fiber morphology and performance. This 6FDA-polyimide is made of 50 mol % 2,2-bis(3,4-dicarboxyphenyl) hexafluoro propane dianhydride (6FDA) and 50 mol % 2,2-bis(3-aminophenyl) hexafluoro propane (4,4 6F-diamine) (6FDAM). An increase in air gap distance tends to induce three-dimensional open-cell pore formation. Raising bath temperature has a similar effect. Multilayer finger-void structure can be completely eliminated if one properly chooses a bore-fluid flow rate and a gelation bath temperature. Experimental data demonstrate that the location of the dense layer can be shifted from the inner skin to the outer skin based on the chemistry (solubility parameter) of coagulants. The location of finger voids is also dependent on the chemistry of coagulants, and the dense layer location may shift from the inner surface to the external surface or appear in both surfaces dependent on the differences in solubility and coagulation rate. A defect-free 6FDA/6FDAM polyimide fiber with a selectivity of 4.73 and a permeance of 38.1 GPU is produced.

Preparation and characterization of thermally stable copoly(phthalazinone biphenyl ether sulfone) hollow fiber ultrafiltration membranes

“…The reduction of loose finger-like macro-voids may increase the resistance for water and solute to transport through UF membrane. As a result, the water flux decreased substantially and the separation 16 18 rejection of the PPBES membrane increased. However, the hollow fiber membrane is a self-supporting structure.…”

“…11. It was showed that the water flux of the UF membrane increased from 83 to 124 L m −2 h −1 , whereas the rejection of PEG decreased slightly as the coagulation bath temperature was elevated from 10 to 30 • C. The reason may be that the higher coagulation bath temperature could promote the solvent and non-solvent exchange rate and improve the connectivity of the membrane pores [16]. In addition, the humidity of air gaps with coagulation bath temperature increases, which can accelerate the delay phase demixing [31].…”

“…known that the morphologies and performance of UF membranes prepared by phase inversion are often controlled by numerous variables in the polymer dope solution formulation and fiber spinning conditions. Although PPBES flat-sheet UF membranes had been successfully fabricated, the formation mechanism and controlling factors for hollow fiber membranes via phase inversion are more complicated than those for flat-sheet membranes [16]. Therefore, it is very difficult to directly adopt flat membrane process conditions for the preparation of hollow fiber ultrafiltration membrane.…”

Fabrication of Hollow‐Fiber Membranes by Phase Inversion

“…Most of their works have been summarized elsewhere (Mulder, 1996;Chung and Kafchinski, 1997;Freeman and Pinnau, 1999;Sharpe et al, 1999;Clausi and Koros, 2000;Nunes and Peinemann, 2001;Carruthers et al, 2003;Ho, 2003;Baker, 2004). Here we only summarize key progress in the last 10 years.…”

“…Several researchers believe it originates from thermodynamic aspects of chemical potential gradient (Broens et al, 1980;Reuvers et al, 1987;Yao et al, 1988;Smolders et al, 1992). Others consider it starts from local surface instability and material and stress imbalance, which induce solvent intrusion and capillary flow (Matz, 1972;Strathmann et al, 1975;Strathmann and Kock, 1977;Chung and Kafchinski, 1997;Wang et al, 2004a;Widjojo and Chung, 2006). Other mechanisms such as Marangoni effects (Levich and Krylov, 1969;Shojaie et al, 1994), osmosis pressure (McKelvey and Koros, 1996), and moisture effects (Menut et al, 2002;Tsai et al, 2005;Khare et al, 2005) have also been proposed.…”