As-spun hollow fibers

A comparison between Pluronic and PEG as additives confirmed the importance of the presence of poly(propylene oxide) chain in Pluronic in the formation of high performance membranes. When Pluronic F127 concentration was 10%, the as-spun hollow fiber exhibited the highest water permeation of 113.8 lm h bar and the lowest molecular weight cut-off of 9,000 Dalton (77). 

Heat treatment has been considered as an effective method to (1) control pores size, (2) reduce pore sizes, and (3) remove membrane defects of as-spun hollow fibers since the birth of RO (Sourirajan and Matsuura, 1985). Its effects on microporous (Chung, 1996), gas separation (Chung et al., 2003), and pervaporafion (Chung et al., 2006 Liu et al., 2005 Qiao and Chung, 2006) membranes have been summarized elsewhere. Basically, heat treatment induces molecule relaxation and microscopically repackages the polymeric chains, which tend to perfect and density the selective skins and minimize the surface defects. 

Post-Treatment of Hollow Fibers. End use of ihe hollow-fiber membrane dictates the ty pe of post-tre.iiment. if any There are three main categories fibers thal are spun, fibers that vv ill be chemically or physically modified, and fibers that will serve as a porous matrix lor support ol another lactivc) polymer deposited (nr entrapped) upon (or within) its walls. There is no theoretical impediment to the inclusion of all conventional treatments in the spinning line photochemical cross-linking, llmirinniion, and anliplaslici/ers have been successful. 

Samples examined were 1) Cellulose triacetate hollow fibers, CTAHF, as spun. In distilled water 2) CTAHF stretched for eight days In water with a 3.57 gram weight (22 psi stress). In distilled water 3) CTAHF air dried for two weeks. In an unsealed capillary ... 

Figure 8 Dlffractogram of as-spun and water-stored CTA hollow fiber (for reverse osmosis).

If an asymmetric hollow fiber with the skin on the outside is to be produced, the precipitant in the inner bore is replaced by an inert gas and the fiber is spun into the precipitation bath. Between the precipitation bath and the spinneret there is an air gap as indicated in Figure 1.36 (b) where the fiber may be drawn to obtain the desired dimensions before precipitation. Hollow fibers have, therefore, often significantly smaller diameters than the nozzle. 

Disadvantages of the known porous polymeric membrane preparation processes are that they involve additional process steps after the formation of the fiber to come to a final product. It is therefore desirable to have a more efficient preparation process. A new method to prepare structures of any geometry (Figure 6.13c through f) and large variety of functionality was recently proposed [61]. The authors proposed to incorporate the functionality by dispersion of particles in a polymeric porous structure formed by phase inversion. A slurry of dissolved polymer and particulate material can be cast as a flat film or spun into a fiber and then solidified by a phase inversion process. This concept is nowadays commercialized by Mosaic Systems. The adsorber membranes prepared via this route contain particles tightly held together within a polymeric matrix of different shapes, which can be operated either in stack of microporous flat membranes or as a bundle of solid or hollow-fiber membranes. 

Flat-sheet asymmetric-skinned membranes made from synthetic polymers (also copolymers and blends), track-etched polymer membranes, inorganic membranes with inorganic porous supports and inorganic colloids such as Zr02 or alumina with appropriate binders, and melt-spun thermal inversion membranes (e.g., hollow-fiber membranes) are in current use. The great majority of analytically important UF membranes belong to the first type. They are usually made of polycarbonate, cellulose (esters), polyamide, polysulfone, poly(ethylene terephtha-late), etc. 

Hollow-fiber fabrication methods can be divided into two classes (62,63). The most common is solution spinning, in which a 20-30% polymer solution is extruded and precipitated into a bath of a nonsolvent, generally water. Solution spinning allows fibers with the asymmetric Loeb-Sourirajan structure to be made. An alternative technique is melt spinning, in which a hot polymer melt is extruded from an appropriate die and is then cooled and solidified in air or a quench tank. Melt-spun fibers are usually relatively dense and have lower fluxes than solution-spim fibers, but, because the fiber can be stretched after it leaves the die, very fine fibers can be made. Melt spinning can also be used with polymers such as poly(trimethylpentene), which are not soluble in convenient solvents and are difficult to form by wet spinning. 

Fig 5 The Performance of Asymmetric Polysulfone Hollow Fiber Membranes spun from N-methylpyrrolidone/Formamide as Function of Oxygen Plasma Ablation Time... 

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