Hollow fibers, fabrics cross-section

Khulbe et al. [4] conducted an AFM study of the cross section of UF poly(ether-imide) hollow fibers, fabricated by the dry-wet spinning method at various air gaps. 

Figure 6.11 shows the cross section of the wall of a hollow fiber fabricated at a 30-cm air gap. There are layers of nodules in rows, but in the middle there is a long. 

Fig. 6.11. AFM image of the cross section of a UF poly(etherimide) hollow fiber membrane (middle section) fabricated at a 30-cm air gap. The sample was prepared by cutting the hollow fiber with a sharp edge. Reprinted from [4]. Copyright 2006, with kind permission from Elsevier...

Polyacrylonitrile hollow fibers fabricated at Gulf South Research Institute were used. Their hydraulic permeability was 9x 10 cm/s atm, the wall thickness 50 /x, the inside diameter 200 microns and the wall micropore diameter about 100 A. Hollow fibers (150) assembled in bundles with a total surface area of 140 cm were washed first with water, and then with methanol and dried by passing nitrogen gas through them for one hour. They were immersed in a mixture of 4-VP and a,co-dihaloalkane (2 1 molar). The reaction was permitted to proceed for 10 days in case of dibromo ethane and 2 days in the case of dibromohexane. A cross section of a typical fiber is shown in Figure 5. 

Cross-section. The cross-section of a fiber can be observed using a microscope. It has been found that the cross-sectional shape of a fiber can have a significant effect on its thermal insulation characteristics (Varshney et al., 2011). A fiber s cross-section with more trapped air may provide higher thermal insulation than a perfectly cylindrical fiber. For example, a hollow fiber traps more air inside its structure than a solid circular fiber. This is the reason why hoUow-fiber based fabrics can provide higher thermal insulation than solid circular-fiber based fabrics. In the same fashion, a noncircular fiber, say with a trilobal or scalloped oval surface, can trap more air than a circular fiber, because of its shape. Relatively large amounts of air trapped by noncircular fibers ultimately enhance thermal insulation characteristics (Matsudaira et al., 1993 Murakami etal, 1978). 

High Absorbency Rayons. Over the past years, disposable products have become commonplace, especially in the United States and Europe. Cellulosic fibers, particularly rayons, have served the needs of the disposables industry because of their absorbent qualities. The most useful fibers for disposable/ absorbent applications are the rayons with crenulations, crimp, and hollow regions, all of which add to the absorbency of the fiber. These characteristics are achieved in varying degrees by physical and chemical alterations in the spinning process. Crenulations, or random irregularities in the shape of the cross-section, typical for most rayon fibers, are caused by the rapid formation of skin before the dehydration is complete. As the fiber interior loses solvent, it collapses in certain areas and produces the crenulated shape. Furthermore, fabricators have learned how to... 

Glass, carbon, and aramid fibers are used as unidirectional or fabric mat reinforcements, with E-glass/ polyester being the most commonly used system. The limitation of pultrusion is that only constant cross-section parts can be fabricated. However, a variety of hollow and solid profiles of any length can be manufactured. 

Fiber, hollow These plastic fibers can produce high bulk, low-density fabrics. Other fiber configurations can be produced such as trilobal cross section. Annular dies are used to produce the desired hollow cross section shape. Fiber spinning methods used are (1) wet from a plastic solution into a liquid coagulant, (2) dry firom a plastic solution in a volatile solvent with an evaporative column, and (3) conventional melt systems. 

Kesting and Fritzsche [5] studied the cross-sectional structure of polysulfone hollow fibers they fabricated using Lewis acid/base complexes as solvents for the spinning solutions. Figures 6.3 and 6.4 are the SEM pictures of the outer edge of the hollow... 

Figure 6.10 shows the AFM image (scan range 2 im) of the midsection of the wall of a hollow fiber that was fabricated at a 50-cm air gap. From Fig. 6.10 it seems there are spheres in the cross section, which are designated as nodules due to their dimensions, as suggested by Resting [15]. Some nodules are fused with each other to form nodule aggregates. In Fig. 6.10 there are dark areas, which maybe macrovoids, or paths of pores that are commonly observed by SEM. 

Spinneret Spi-no- ret (1826) n. (1) An extrusion die consisting of a plate with many tiny holes, through which a plastic melt or solution is forced, to make fine fibers and filaments. Early spirmeret holes were round and thus produced fibers of circular cross-section. Today, spinneret holes have many different shapes, even annular ones, to produce fibers of corresponding cross-sections. One purpose is to decrease the fiber-bundle density, giving added warmth, moisture permeability, and enhanced dye receptivity to the textile fabric. An important application of hollow fibers is in artificial kidneys for dialysis. Filaments emerging from the spinneret may be hardened by cooling in air or water, or by chemical action of solutions. (2) A spinneret hole. 

Cross-sectional modifications of a more extreme nature than skin-bursting, which nevertheless do not form crimp, have grown in importance since the early 1980s. These yield a permanent bulk increase, which can be translated into bulky fabrics without the need for special care. The first commercial staple fiber of this type was Courtaulds hollow Viloft, developed in the 1970s using a carbonate inflation technique (37). 

The ability to execute this method has been proven by Li et al. [37] in a study where pure anatase-titania nanotubes/polymer were obtained. The presence of a sol-gel precursor was required for the formation of a stable and coaxial jet and the formation of hollow fibers with resistant walls. The authors anphasize that the uniform circular cross-section of these fibers, their uniform size, and good spatial orientation, are particularly attractive in the fabrication of devices for fluid transport, as well as guides for optical signals. 

Generally, it is important to know how to fabricate fibers with circular cross sections at will, and the aim of electrospinning for particular applications is in general to obtain just these particular types of fibers. On the other hand, other types of structure of fibers may be beneficial for specific functionality and applications. To date, by carefully controlling the preparing conditions, a variety of specific fiber structures originating from the complex self-assembly processes intrinsic in electrospinning have been fabricated, such as ribbon fiber, helix fiber, porous fiber, necklace-like fiber, core-shell fiber, and hollow fiber, as shown in Fig. 1.13. 

FIGURE 78.1 Microporous hollow fiber membranes used in artificial lungs (a) Cross-sectional view of fibers, (b) Longitudinal view of fibers in Celgard fiber fabric, (c) Microporous outer wall surface of Celgard fiber. 

Fabrication of dual-layer membranes either in hollow-fiber or flat-sheet configuration is rather complex as many parameters are involved, which control the thermodynamic property and the phase inversion kinetics to obtain good lamination between the two layers as well as regular and uniform membrane cross-sectional morphology. Fabrication parameters can be classified into the chemistry of polymer solutions and the operating conditions. The chemistry of polymer solutions depends on the polymer concentration and type, the solvents affinity to the polymer or coagulant, and... 

FIGURE 15.26 (a) Schematic diagram of a dual-layer hollow-fiber spinning process (b) cross section of triple-orifice spinneret (Adapted from L. Setiawan et al.. Journal of Membrane Science, 423-424, 73-84, 2012.) and (c) fabrication process of a dual-layer flat-sheet membrane using a double-blade casting machine. (Adapted from S.A. Hashemifard et al. Journal of Membrane Science, 375, 258-267, 2011.)... 

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