Fiber diameters

It is difficult to determine the cross-sectional area of a fiber. Direct observation and measurement of a cross section under a microscope is the most accurate method (15). This is a destmctive test that does not allow subsequent study of fiber mechanical properties, and is slow and tedious. Also, it does not take into account any variations in the cross-sectional area along the fiber length. Measurement of fiber diameters from microscopic observations of longitudinal views is somewhat easier, but the eUipticity of the cross section in certain fibers can lead to serious errors. 

A selection of fiber property data is given ia Table 2 as an illustration of the range of fiber properties available commercially for use ia manufacturiag nonwoven fabrics. In general, fiber diameters range from 5 to >40 p.m for natural fibers, and from less than 10 p.m (microdenier) to as high as needed for manufactured fibers. 

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. 

Woolen yams caimot be spun as fine as worsted yams, even when using the same fiber diameter. The fabric weights are greater and they have a harsher feel. For these reasons woolen products have not been able to foUow the modem trend to smoother lightweight clothing as easily as their worsted counterparts. 

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. 

FIG. 22-75 Air fractionation by membrane. O2 in retentate as a function of feed fraction passed tbrougb tbe membrane (stage cut) showing tbe different result with changing process paths. Process has shell-side feed at 690 kPa (abs) and 298 K. Module comprised of hollow fibers, diameter 370 im od X 145 im id X 1500 mm long. Membrane properties (X = 5.7 (O2/N2), permeance for O2 = 3.75 X 10 Barrer/cm. Coutiesy Innovative Membrane Systems/ Fraxair)... 

SEM examination of the steam activated PAN fiber monoliths showed the fiber diameter to be significantly reduced during the activation process, suggesting the fibers are consumed radially by a gasification process of the external surface [28]. 

This formula has proven accurate for fiber diameters ranging from 1.6 to 80 /x, and for filter porosities ranging from 0.700 to 0.994. 

Analytical determination of the hydraulic resistance of the medium is difficult. However, for the simplest filter medium structures, certain empirical relationships are available to estimate hydraulic resistance. The relationship of hydraulic resistance of a cloth of monofilament fiber versus fiber diameter and cloth porosity can be based on a fixed-bed model. 

Disposable The opposite of a cleanable filter, which after collecting a certain dust burden is thrown away. Dry cell panel A dry filter mounted in a rigid frame. In the past these were manufactured from woven fabrics and felts however, synthetic fibers are replacing these. They have fiber diameters of 20 pm with average spacing of 300 pm and allow air velocity in the 2 m s-1 range. 

Experimental Results for Fiber Buckle Wavelength versus Fiber Diameter (After Dow and Rosen [3-28])... 

Because the buckle wavelength is L/m, the second term in Equation (3.134) is small when the buckle wavelength is large relative to the fiber diameter, h. Thus, the fiber buckling stress is approximately... 

Fibrous fillers are now gaining more importance over particulate fillers due to the high performance in mechanical properties. The influence of fiber diameter on the tensile behavior of short glass fiber on polyimide was reported [95], At higher concentrations thick fibers seem to be more advantageous probably because of the... 

Developments to produce such absorptive mats totally from organic fibers even go one step further. Only recently success came in achieving a suitable Fiber diameter and permanent hydrophilization [100J. Such materials are not yet commercially available, however, and field experience has not been reported as yet. 

Only very recently the production of melt-blown polypropylene fleeces with considerably thinner fiber diameter became possible [100], thus making it possible — a low-cost hydrophilization provided — to achieve attractive properties with regard to small pore size and excellent tensile performance for use in highly automated assembly processes. 

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