Microfibers

A PET microfiber is loosely defined as one with a decitex (see Section 5 earlier) per filament less than one. This translates to a fiber diameter of 10 xm or less. In fabric form, such fibers provide a very soft hand and a non-shiny appearance. They can also make moisture-repellant fabrics without sacrificing comfort or air porosity, ideal for sportswear. The larger fiber surface area also can be useful for filtration applications. 

The islands-in-the-sea approach uses bico technology to extrude filaments that contain a multiplicity of small fibrils encased in a soluble matrix. After fiber processing and fabric formation, the matrix is dissolved away to leave behind the microfibers. Fibers with sub-micron diameters can be produced. The process is expensive, but luxurious fabrics and nonwoven materials such as Ultrasuede are made in this way. 

Bico technology can also be used to form composite fibers that can be broken apart, by using polymers with poor mutual adhesion (e.g. polyolefin and PET). A fiber made with a dozen or more segments, alternating between two polymer 

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Typical textile fibers have linear densities in the range of 0.33—1.66 tex (3 to 15 den). Fibers in the 0.33—0.66 tex (3—6 den) range are generally used in nonwoven materials as well as in woven and knitted fabrics for use in apparel. Coarser fibers are generally used in carpets, upholstery, and certain industrial textiles. A recent development in fiber technology is the category of microfibers, with linear densities <0.11 tex (1 den) and as low as 0.01 tex. These fibers, when properly spun into yams and subsequendy woven into fabrics, can produce textile fabrics that have excellent drape and softness properties as well as improved color clarity (16). 

The tendency of the strong, highly crystalline fibers to fibnUate, ie, to develop a hairy surface on wet-abrasion has, for the textile appUcations, been minimized by process changes both in fiber production and fabric manufacture. However, for nonwoven or speciaUty paper appUcations, this property can aUow potential users to develop ceUulosic microfibers during processing. 

During the third quarter of the twentieth century, with improved nonwoven fabrics, man-made leathers finally succeeded in simulating leather to such an extent that they are nearly identical in appearance, physical properties, and stmcture. These leathers have enjoyed success in all leather-use areas. With the technology of microfibers, they continue to evolve both in quaUty and quantity. 

Significant improvement in the fiber stmctuie of leather is finally achieved by using microfibers as fine as 0.001—0.0001 tex (0.01—0.001 den). With this microfiber, a man-made grain leather Sofrina (Kuraray Co., Ltd.) with a thin surface layer (Fig. 7), and a man-made suede Suedemark (Kuraray Co., Ltd.) with a fine nap (Fig. 8) were first developed for clothing, and have expanded their uses. Ultrasuede (Toray Industries, Inc.) also uses microfibers with a rather thick fineness of 0.01 tex (0.1 den). Contemporary (1995) man-made leathers employ microfibers of not mote than 0.03 tex (0.3 den) to obtain excellent properties and appearance resembling leather. 

Man-Made Leathers. These materials contain a nonwoven fabric which is impregnated with a polyurethane to improve fiexibiHty, processibiHty, and conformabiHty (Fig. 9). Advanced man-made leathers contain microfibers as fine as 0.03 tex (0.3 den) or less to imitate coUagen fiber bundles, thereby attaining the soft feel and appearance essential for soft leather use. Polyurethane in the substrate is usually provided with porous stmcture by poromeric technology. The coating layer is also porous in the two-layer type man-made leathers (5—10). 

Fig. 10. Formation of fibers used in Kuraray man-made leather (a) porous fiber, and (b) a bundle of microfibers.

Fig. 11. (a) Cross-sectional view of substrate with porous fibers and polyurethane sponge, (b) Cross-sectional view of substrate with bundle of microfibers... 

I. M. Asher and P. P. McGrath, eds.. Symposium on Electron Microscopy of Microfibers, Proceedings of the First FDA Office of Science Symposium, Stock No. 017-012-00244-7, Superintendent of Documents, U.S. Government Printing Office, Washington, D.C., 1976. 

This dynamic increase in production was accompanied by the qualitative development of PET fibers, which manifested itself in the widening of assortment of the fibers being produced (e.g., staple microfibers and filament yarns of the POY, MOY, FOY, and HOY type) and in the manufacture of second-generation fibers on... 

Polyethylene separators Phenol- formaldehyde- resorcinol separators PVC sepa- rators Rubber separators Microfiber glass mat separators Total... 

For the separation of such batteries, gel construction and microfiber glass fleece separators again compete because of the deep discharge cycles, the gel construction with its lower tendency to acid stratification and to penetration shorts has advantages for the required power peaks, microfiber glass fleece construction would be the preferred solution. The work on reduction of premature capacity loss with lead-calcium alloys has shown that considerable pressure (e.g., 1 bar) on the positive electrode is able to achieve a significantly better cycle life [31-36], Pressure on the electrodes produces counter pressure on the separators, which is not unproblematic for both separation systems. New separator developments have been presented with... 

Microfiber glass fleece mats are typically produced from a blend of 20 - 30 percent glass microfibers <1 //m in diameter, with the balance of the glass fibers thicker (3 - 10 //m) and longer (cf. Fig. 1), on a specialized paper machine (Fou-drinier), since this is the only way of achieving the desired tensile strength without binder. The material is supplied in roll form, even though it is normally not processed into pockets, which are not required due to the absence of free electrolyte. The classification here as a leaf separator should be seen in this sense. 

The microfiber glass separators have to fill the space between the electrodes completely the backweb thickness, is thus identical to the total thickness. Due to the high compressibility of such porous glass mats, a standard measuring pressure of 2 kPa or 10 kPa (BCI method) is generally used during assembly they are compressed... 

The range of microfiber glass mat separators offered by the leading producers are presented in Sec. 9.2.3.3 with typical data in connection with their predominant application in sealed stationary batteries. 

Because of the increased shedding with these alloys, pure leaf separation is hardly suitable. Separations with supporting glass mats or fleeces as well as microfiber glass mats provide technical advantages, but are expensive and can be justified only in special cases. Also under these conditions of use the microporous polyethylene pocket offers the preferred solution [40]. Lower electrical properties at higher temperatures, especially decreased cold crank duration, are battery-related the choice of suitable alloys and expanders gains increased importance. 

These generally defined requirements are met quite comprehensively by microfiber glass fleeces. These are blends of C-glass fibers of various diameter, which are processed in the usual way on a Foudrinier paper machine into a voluminous glass mat. The blending ratio gains special importance since cost aspects have to be balanced against technical properties. The... 

Table 13 compares the specification data of microfiber glass fleeces from various manufacturers. 

Absorptive microfiber glassmat separators (100% glass fibers ) ... 

Glass fiber boards are manufactured from glass microfibers with a diameter of less than 1pm, by pressing into plates [44]. The heat treatment at 500 °C after the pressing ensures the stability of the shape. The boards have a low compressibility (about 5 percent). This will be obtained by adjusting the density of the boards to 0.3gem-3. ... 

The decrease in the fiber diameter of fabric resulted in a decrease in porosity and pore size, but an increase in fiber density and mechanical strength. The microfiber fabric made of PCLA (1 1 mole ratio) was elastomeric with a low Young s modulus and an almost linear stress-strain relationship under the maximal stain (500%) in this measurement. 

Kwon K, Kidoaki S, and Matsuda T. Electrospun nano- to microfiber fabrics made of biodegradable copolyesters Structural characteristics, mechanical properties and cell adhesion potential. Biomaterials, 2005, 26, 3929-3939. 

Fig. 4.14 SEM micrograph of CVD nickel-coated carbon microfibers (INCOEIBER 12K20) before (a) and after (b) the cathodic electrosynthesis of ZnSe on their surfaces (the scale bar is 8 and 10 p.m, respectively). Such low-dimensional substrates find apphcation in new-generation photovoltaic solar cells, chemical/biological sensors, and light-emitting devices. (Reprinted from [127], Copyright 2009, with permission from Elsevier)...

The most exciting challenge is probably the preparation of BN nanostructures, including nanofibers and nanotubules, using the template-assisted PDCs route. Such an approach could allow us to control the morphology and size of the nanostructured BN materials to be incorporated into the BN matrix. This should significantly enhance the mechanical performance of the resulting composites compared to composites reinforced by BN microfibers. 

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