Filament density

B. A. Zeitlin, A. Petrovich, J. R. Hughes, and M. S. Walker, Fabrication of a High Filament Density Bronze and Niobium Composite for Multifilament NbsSn Conductors, paper presented at the Manufacture of Superconducting Materials Conference, Port Chester, New York, November 1976. 

Denier—A term used in the textile industry to designate the weight per unit length of a filament. Density— The ratio of weight (mass) to volume of any substance, usually expressed as grams per... 

Compare the left and right columns of Figure 3-27, where the purely convective flow is compared with the reactive/diffusive case. The hot zones, where the reactive interface is, closely agree with the locations of highest filament density. The landscape of the nomeactive mixture (left side of Figure 3-27) looks almost identical to the landscape of the reactive flow (right side of Figure 3-27). 

The rate of evaporation of ions from a heated surface is given by Equation 7.3, in which Q, is the energy of adsorption of ions on the filament surface (usually about 2-3 eV) and Cj is the surface density of ions on the surface (a complete monolayer of ions on a filament surface would have a surface density of about 10 ions/cm" ). 

As ions and neutrals evaporate from a heated filament surface, the amount of sample decreases and the surface densities (C, Cq) must decrease. Therefore, Equation 7.1 covers two effects. The first was discussed above and concerns the changing value for the ratio n+/n° as the temperature of the filament is varied, and the other concerns the change in the total number of ions desorbing as the sample is used up. The two separate effects are shown in Figure 7.8a,b. Combining the two effects (Figure 7.8c) reveals that if the temperature is increased to maintain the flow of ions, which drops naturally as the sample is used up (time), then eventually the flow of ions and neutrals becomes zero whatever the temperature of the filament because the sample has disappeared from the filament surface. 

Schematic illustrations of the effect of temperature and surface density (time) on the ratio of two isotopes, (a) shows that, generally, there is a fractionation of the two isotopes as time and temperature change the ratio of the two isotopes changes throughout the experiment and makes difficult an assessment of their precise ratio in the original sample, (b) illustrates the effect of gradually changing the temperature of the filament to keep the ratio of ion yields linear, which simplifies the task of estimating the ratio in the original sample. The best method is one in which the rate of evaporation is low enough that the ratio of the isotopes is virtually constant this ratio then relates exactly to the ratio in the original sample.

Visual and Manual Tests. Synthetic fibers are generally mixed with other fibers to achieve a balance of properties. Acryhc staple may be blended with wool, cotton, polyester, rayon, and other synthetic fibers. Therefore, as a preliminary step, the yam or fabric must be separated into its constituent fibers. This immediately estabUshes whether the fiber is a continuous filament or staple product. Staple length, brightness, and breaking strength wet and dry are all usehil tests that can be done in a cursory examination. A more critical identification can be made by a set of simple manual procedures based on burning, staining, solubiUty, density deterrnination, and microscopical examination. 

Slit-Film Fiber. A substantial volume of olefin fiber is produced by slit-film or film-to-fiber technology (29). For producing filaments with high linear density, above 0.7 tex (6.6 den), the production economics ate more favorable than monofilament spinning (29). The fibers are used primarily for carpet backing and rope or cordage appHcations. The processes used to make slit-film fibers are versatile and economical. 

Fiber Density, kg/m Strength, GPa Modulus, GPa Elongation to break, % Filament diameter, mm... 

The air jet textured yam process is based on overfeeding a yam into a turbulent air jet so that the excess length forms into loops that are trapped in the yam stmcture. The air flow is unheated, turbulent, and asymmetrically impinges the yam. The process includes a heat stabilization zone. Key process variables include texturing speed, air pressure, percentage overfeed, filament linear density, air flow, spin finish, and fiber modulus (100). The loops create visual and tactile aesthetics similar to false twist textured and staple spun yams. 

Along with cotton blends, polyester blends with rayon or wool are also important. Wool—polyester blends are widely used in men s suiting materials. For these fabrics, PET staple or tow can be used with a linear density typically about 0.16—0.45 tex per filament (1.5—4 dpf) and a staple length of 50—75 mm (2—3 in.). 

Filament. Eully drawn flat yams and partially oriented (POY) continuous filament yams are available in yam sizes ranging from about 3.3—33.0 tex (30—300 den) with individual filament linear densities of about 0.055 to 0.55 tex per filament (0.5—5 dpf). The fully drawn hard yams are used directly in fabric manufacturing operations, whereas POY yams are primarily used as feedstock for draw texturing. In the draw texturing process, fibers are drawn and bulked by heat-setting twisted yam or by entangling filaments with an air jet. Both textured and hard yams are used in apparel, sleepwear, outerwear, sportswear, draperies and curtains, and automotive upholstery. 

Process. Any standard precursor material can be used, but the preferred material is wet spun Courtaulds special acrylic fiber (SAF), oxidized by RK Carbon Fibers Co. to form 6K Panox B oxidized polyacrylonitrile (PAN) fiber (OPF). This OPF is treated ia a nitrogen atmosphere at 450—750°C, preferably 525—595°C, to give fibers having between 69—70% C, 19% N density less than 2.5 g/mL and a specific resistivity under 10 ° ohm-cm. If crimp is desired, the fibers are first knit iato a sock before heat treating and then de-knit. Controlled carbonization of precursor filaments results ia a linear Dow fiber (LDF), whereas controlled carbonization of knit precursor fibers results ia a curly carbonaceous fiber (EDF). At higher carbonizing temperatures of 1000—1400°C the fibers become electrically conductive (22). 

Nonwoven bonding processes iatedock webs or layers of fibers, filaments, or yams by mechanical, chemical, or thermal means. The extent of bonding is a significant factor ia determining fabric strength, dexibiUty, porosity, density, loft, and thickness. Bonding is normally a sequential operation performed ia tandem with web formation, but it is also carried out as a separate and distinct operation. 

By comparison, the multiple spinnerette per bank process requites additional effort prior to laydown in order to compensate for the gaps between the individual spinnerettes. Failure to present a filament array to the laydown screen, which is not uniformly distributed, can result in spot-to-spot variations in fiber density and a web that has the appearance of blotch. 

It is also necessary to reduce the intensity of the radiation admitted into the pyrometer, because pyrometer lamp filaments should not be subjected to temperatures exceeding 1250°C. The reduction is accomplished by a screen or screens in manually operated secondary pyrometers they are usually neutral-density filters. 

Textile fibers must be flexible to be useful. The flexural rigidity or stiffness of a fiber is defined as the couple required to bend the fiber to unit curvature (3). The stiffness of an ideal cylindrical rod is proportional to the square of the linear density. Because the linear density is proportional to the square of the diameter, stiffness increases in proportion to the fourth power of the filament diameter. In addition, the shape of the filament cross-section must be considered also. For textile purposes and when flexibiUty is requisite, shear and torsional stresses are relatively minor factors compared to tensile stresses. Techniques for measuring flexural rigidity of fibers have been given in the Hterature (67—73). 

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