Uniform fibers

In RPs, insufficient compaction and consolidation before plastic solidification or cure will result in air pockets, incomplete wet-out and encapsulation of the fibers, and/or insufficient fiber or uniform fiber content. These deficiencies lead to loss of strength and stiffness and susceptibility to deterioration by water and aggressive agents. 

With the demise of the uniform fiber model in 1974, it became necessary to devise other models to account for the early electron micrographs of chromatin fibers and the X-ray diffraction studies (see Ref. [1], Chapter 1). Two models appeared in 1976, and were the major contenders for consideration in 1978. The superbead model of Franke et al. [36] envisioned the chromatin fiber as a compaction of multi-nucleosome superbeads . The solenoid model of Finch and Klug [37] postulated a regular helical array of nucleosomes, with approximately six nucleosomes per turn and a pitch of 10 nm. Although a number of competing helical models appeared in the 1980s (see Ref. [1], Chapter 7) the solenoid model remains a serious contender to this day. Structural details of this model, such as the precise disposition of linker DNA, are still lacking. 

Bundle Preparation. Packages of multifileiment yarns are backwound to prepare bundles necessary for the manufacture of a reverse osmosis module. A proprietary winder for this operation has been designed and constructed at Albany International Research Co. This device is capable of helically winding multifilament yarns into bundles around a mandrel. This is done in a manner such that the resulting bundle has uniform cylindrical dimensions and uniform fiber density. This minimizes channeling and optimizes exposure of membrane surface area. 

R.O. Crowder and E.L. Cussler, Mass Transfer in Hollow-fiber Modules with Non-uniform Fibers, 7. Membr. Sci. 134, 235 (1997). 

A comparison of the diameter of the electrospun fibers with the electrical conductivity of the solutions was shown in Figure 6. There was a significant drop in the diameter of the electrospun polymer fibers when the electrical conductivity of the solution increased. Beads were also observed due to low eonductivity of the solution, which results in insufficient elongation of a jet by eleetrieal foree to produce uniform fiber. 

The capability to mass-produce uniform, small diameter (40-100 p) hollow fibers has been another important element in the commercial development of gas separators. Apparently, hollow fibers were first produced in the 1920 s however, mass production of uniform fiber contours dates from the 1960 s. Even with the... 

Liqui-Cel Extra-Flow (Figure 19.12) is one of the well studied hollow fiber modules for concentration-driven mass transfer. This module consists of several microporous polypropylene fibers, which are woven into a fabric and wrapped around a central tube feeder that supplies the shell side fluid. The woven fabric allows more uniform fiber spacing, which in turn leads to higher mass transfer coefficients than those obtained with individual fibers. The fiber lumen diameter and wall thickness are 240 and 30 pm, respectively [93,130]. The smallest Liqui-Cel modules are 2.5 in. in diameter and contain 1.4 m of contact area, while the largest are 10 in. in diameter and offer 130 m of contact area by virtue of 225,000 fibers. The large modules can handle liquid flow rates of several 1000 L/min. 

Next, the material must be handled by conventional construction equipment following conventional procedures, including mixing, pouring, and finishing. The fibers must be metered and added to the concrete mix according to proportion specifications. An automated fiber dosing system is preferred. Uniform fiber dispersion in concrete is essential. The workability of fresh concrete is reduced when fibers are added. Superplasticizer is often added to maintain an acceptable level of workability. 

In order to relieve the stresses of compression at higher densities by thermoplastic processes instead of with wire or twine, several conditions must occur. A balance between the pressure, and retention time must be found so that the material is exposed to temperatures of 200° to 300°F (93-150°C), and pressures of 1000 psi to 1500 psi (6.9-10.3 MPa) at moisture levels of 12 to 25 (9, 10). Moisture and agglutinant substances such as soluble sugars, starches, extractives, phenolic acids and lignins which will plasticize at these conditions must be evenly distributed throughout the material. The geometry of the material must allow a uniform fiber matrix and intimate contact between adhesive surfaces during compression. 

Although they are specifically treated in Chapter 17, activated carbon fibers (ACFs) and derived cloths and felts deserve special mention here due to their uniform pore size distribution (PSD) and small and uniform fiber diameter, which confer on them, respectively, both size selectivity and rapid... 

The applications of SFRC take advantage of the static and dynamic tensile strength, energy absorbing characteristics, toughness, and fatigue endurance of the composite [7], Uniform fiber dispersion provides isotropic strength properties. The applications include... 

Despite the uniform fiber material, a certain density profile is created due to the action of... 

The deaerated dope is metered by positive displacement pumps to hundreds of individual yam extrusion positions from the manifold. It is necessary to have a separate pump for each spinneret position to ensure uniform fiber formation and denier. Good control and maintenance of the metering pumps are necessary to produce uniform, high-quality yarn products. Moelter and Steele [35] commented that, although the principal of extrusion is simple in conception, the precision of the operation reflects the ingenuity of its engineering. 

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