Fine fibers

Fibrillated Fibers. Instead of extmding cellulose acetate into a continuous fiber, discrete, pulp-like agglomerates of fine, individual fibrils, called fibrets or fibrids, can be produced by rapid precipitation with an attenuating coagulation fluid. The individual fibers have diameters of 0.5 to 5.0 ]lni and lengths of 20 to 200 )Jm (Fig. 10). The surface area of the fibrillated fibers are about 20 m /g, about 60—80 times that of standard textile fibers. These materials are very hydrophilic an 85% moisture content has the appearance of a dry soHd (72). One appHcation is in a paper stmcture where their fine fiber size and branched stmcture allows mechanical entrapment of small particles. The fibers can also be loaded with particles to enhance some desired performance such as enhanced opacity for papers. When filled with metal particles it was suggested they be used as a radar screen in aerial warfare (73). 

Fig. 13. Du Pont hoUow fine fiber Permasep permeator.

In the second step, a papermaking method is also used for the fine fibers, less than 0.1 tex (1 den). This process is usually followed by a high pressure water jet process instead of the third step. In the fourth step, to obtain the required properties in specific appHcations, a polyurethane is selected out of the segmented polyurethanes, which comprises a polymer diol, a diisocyanate, and a chain extender (see Urethane polymers). A DMF—water bath for coagulation is also controlled to create the adequate pore stmcture in combination with fibers. 

Because membranes appHcable to diverse separation problems are often made by the same general techniques, classification by end use appHcation or preparation method is difficult. The first part of this section is, therefore, organized by membrane stmcture preparation methods are described for symmetrical membranes, asymmetric membranes, ceramic and metal membranes, and Hquid membranes. The production of hollow-fine fiber membranes and membrane modules is then covered. Symmetrical membranes have a uniform stmcture throughout such membranes can be either dense films or microporous. 

HoUow-fiber fabrication methods can be divided into two classes (61). The most common is solution spinning, in which a 20—30% polymer solution is extmded and precipitated into a bath of a nonsolvent, generally water. Solution spinning allows fibers with the asymmetric Loeb-Soufirajan stmcture to be made. An alternative technique is melt spinning, in which a hot polymer melt is extmded from an appropriate die and is then cooled and sohdified in air or a quench tank. Melt-spun fibers are usually relatively dense and have lower fluxes than solution-spun 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. 

The second type of hoUow-fiber module is the bore-side feed type illustrated in Figure 23b. The fibers in this type of unit are open at both ends, and the feed fluid is usually circulated through the bore of the fibers. To minimize pressure drops inside the fibers, the fibers often have larger diameters than the very fine fibers used in the shell-side feed system and are generally made by solution spinning. These so-called capillary fibers are used in ultrafiltration, pervaporation, and in some low to medium pressure gas appHcations. Feed pressures are usually limited to less than 1 MPa (150 psig) in this type of module. 

Property Hollow-fine fibers Caphlary fibers Spiral-wound Plate and frame Tubular... 

A second factor determining module selection is resistance to fouling. Membrane fouling is a particularly important problem in Hquid separations such as reverse osmosis and ultrafiltration. In gas separation appHcations, fouling is more easily controlled. Hollow-fine fibers are notoriously prone to fouling and can only be used in reverse osmosis appHcations if extensive, costiy feed-solution pretreatment is used to remove ah. particulates. These fibers caimot be used in ultrafiltration appHcations at ah. 

A third factor is the ease with which various membrane materials can be fabricated into a particular module design. Almost ah membranes can be formed into plate-and-frame, spiral, and tubular modules, but many membrane materials caimot be fabricated into hollow-fine fibers or capihary fibers. Finahy, the suitabiHty of the module design for high pressure operation and the relative magnitude of pressure drops on the feed and permeate sides of the membrane can sometimes be important considerations. 

In reverse osmosis, most modules are of the hollow-fine fiber or spiral-wound design plate-and-frame and tubular modules are limited to a few appHcations in which membrane fouling is particularly severe, for example, food appHcations or processing of heavily contaminated industrial wastewater. 

Hollow-fiber designs are being displaced by spiral-wound modules, which are inherently more fouling resistant, and require less feed pretreatment. Also, thin-film interfacial composite membranes, the best reverse osmosis membranes available, have not been fabricated in the form of hoUow-fine fibers. 

For ultrafiltration appHcations, hollow-fine fibers have never been seriously considered because of their susceptibiUty to fouling. If the feed solution is extremely fouling, tubular or plate-and-frame systems ate still used. Recentiy, however, spiral-wound modules with improved resistance to fouling have been developed, and these modules are increasingly displacing the more expensive plate-and-frame and tubular systems. Capillary systems are also used in some ultrafiltration appHcations. 

Spiral-wound modules are much more commonly used in low pressure or vacuum gas separation appHcations, such as the production of oxygen-enriched air, or the separation of organic vapors from air. In these appHcations, the feed gas is at close to ambient pressure, and a vacuum is drawn on the permeate side of the membrane. Parasitic pressure drops on the permeate side of the membrane and the difficulty in making high performance hollow-fine fiber membranes from the mbbery polymers used to make these membranes both work against hollow-fine fiber modules for this appHcation. 

Four configurations for membranes are plate, hoUow fine fiber, spiral wound, and tubular (32). With a variety of shapes, sizes, and materials many options exist for meeting the various needs in the dairy industry. 

The great quantity of very fine fibers in a meltblown web creates several unique properties such as large surface areas and small (<1 fiva) pore sizes. These have been used in creating new stmctures for hospital gowns, sterile wrap, incontinence devices, oil spill absorbers, battery separators, and special requirement filters. It is expected that much innovation will continue in the design of composite stmctures containing meltblown webs. 

Spunbonded fabrics are effective filters in that they are layered stmctures of relatively fine fibers, the three-dimensional stmcture of which creates a torturous path. Even relatively thin spunbonded fabrics (eg, 0.2—0.25 mm) present a significant challenge to the passage of soil fines and are suitable for use in some filtration appHcations. The porosity of geotextile fabrics is classified by means of several procedures such as flux (volume flow/area per time) and equivalent opening size (EOS), which is a measure of the apparent pore size of the openings in the fabric. The flux measures the porosity to Hquid water, and the EOS measures the porosity to soHd particles of a known diameter. Literature is available on limitations of particular styles of fabrics within an apphcation (63). 

The most important properties of refractory fibers are thermal conductivity, resistance to thermal and physical degradation at high temperatures, tensile strength, and elastic modulus. Thermal conductivity is affected by the material s bulk density, its fiber diameter, the amount of unfiberized material in the product, and the mean temperature of the insulation. Products fabricated from fine fibers with few unfiberized additions have the lowest thermal conductivities at high temperatures. A plot of thermal conductivity versus mean temperature for three oxide fibers having equal bulk densities is shown in Figure 2. 

Bacterial Cellulose. Development of a new strain of Acetobacter may lead to economical production of another novel ceUulose. CeUulon fiber has a very fine fiber diameter and therefore a much larger surface area, which makes it physicaUy distinct from wood ceUulose. Its physical properties mote closely resemble those of the microcrystalline ceUuloses thus it feels smooth ia the mouth, has a high water-binding capacity, and provides viscous aqueous dispersions at low concentration. It iateracts synergisticaUy with xanthan and CMC for enhanced viscosity and stabUity. 

The precipitate of sodium chloride obtained is very sludgy and filters poorly through fine-fibered papers. 

The hollow fine fiber configuration (refer to Figure 51) consists of a bundle of porous hollow fine fibers. These fibers are externally coated with the actual membrane and form the support structure for it. Both ends of each fiber are set in a single epoxy tube sheet, which includes an 0-ring seal to match the inside diameter of the pressure vessel. 

ZMserchen, n. fine fiber, fibril, filament, zaser, a. fibrous, filamentous. 

For some critical applications (such as polymer melt, beverage, or pharmaceutical filtration), it may be important to avoid cartridges that have a nap or fuzz on the fiber used, because these extremely fine fibers tend to break off and drift through the cartridge and go out with the finished product, thereby creating a isual acceptance problem, if not outi ight contamination. 

Analysis of the above formulas leads one to conclusion that ultimate attainable strength of material increases with the increasing scatter of the strength of fine fibers characterized by good adhesion to the matrix and is lower for fibers with larger diameters and poorer adhesion. 

Asbestos It is not the name of a distinct mineral species but is a commercial term applied to fibrous varieties of several silicate minerals such as amosite and crocido-lite. These extremely fine fibers are useful as fillers and/or reinforcements in plastics. Property performances include withstanding wear and high temperatures, chemical resistance, and strengths with high modulus of elasticity. When not properly handled or used, like other fibrous materials, they can be hazardous. 

---