Types of fibers used

Production of viscose rayon and cuprammonium rayon will be described as they are among the oldest wet-spun man-made fibers. 

Viscose rayon fiber production benefits from the wet spinning process, as it necessitates spinning of filament from a liquid of regenerated cellulose in a spinning bath. The viscose process consists of the following steps (Fig. 9.3)  

Steeping cellulose pulps in large white sheets are immersed in vats containing 17-20% aqueous sodium hydroxide (NaOH) at a temperature 

Pressing the swollen alkali cellulose sheets are pressed so that a press weight ratio of 2.7-3.0 is obtained. This is the ratio of the weight of pressed alkali cellulose sheet to air-dried pulp.  

Shredding the sheets of alkali cellulose are mechanically shredded in the form of crumbs, which are hnely divided particles. This process provides increased surface area to increase the reaction ability of alkali cellulose. 

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Requirements for space suits are more complex and frequently involve garments that can circulate water and/or air through the fibrous assembly. Laminated and/or coated garments with specific requirements to pressure, radiation, temperature, and humidity are more stmcturaHy complex as a textile product relative to the types of fibers used in this aerospace fabrication. 

As mentioned earlier PMR polyimide thermosts are used as matrix resins for glass- and carbon fiber composites, mainly in aeroengine applications. At this point it has to be mentioned that the thermal oxidative stability of a PMR composite is dependent on the type of fiber used (113) and the cure conditions (time/temperature/atmosphere) employed for molding. Very interesting is the observed higher thermal oxidative stability of PMR-II composites when cured/-... 

The commercially available fibers include polydi-methylsiloxane (PDMS 100, 30, and 7 pm), PDMS-divinylbenzene (PDMS-DVB 65 pm), polyacrylate (PA 85 pm), carboxen-PDMS (CAR-PDMS 75 and 85 pm), Carbowax-DVB (CW-DVB 65 and 75 pm), Carbowax-templated resin (CW-TPR 50 pm), and DVB-CAR-PDMS (50/30 pm). The type of fiber used affects the selectivity of extraction. In general, polar fibers are used for polar analytes, and nonpolar types are used for nonpolar analytes. Selectivity toward target analytes and interferences can be enhanced by surfaces common to affinity chromatography. Fibers can be reused up to 100 analyses or more depending on the sample matrix, on the care of the analyst, and on the applications for which used. 

In Fig. 15, it is seen that the sheet made of cotton linters is more conductive than the other sheets. The sheet made with "Seagull W" hardwood pulp is the least conductive while the three other pulps have intermediate conductivity values. This behaviour seems to be related to the size of the different types of fibers used to make the sheets. As mentioned previously, hardwood fibers are shorter than softwood fibers, and consequently, there are more interfiber contacts in the sheets made of hardwood fiber. The surface area of a contact is very important to the conductivity. There may be a constrictive effect due to smaller surface at these contacts which results in a restricted flow of ions through the paper, and therefore a lower conductivity. Cotton linters are chemically different and longer than wood fibers and the sheet made of these fibers is more conductive. The difference between the three softwood pulps is probably related to a different chemical composition of its fibers. 

No attempt has been made in this chapter to measure wicking and to correlate it directly with microviscosities, because the kinetics of sorption may be complicated by difiusion of liquid into the fibers rather than capillary movement between fibers (13). In a real printing operation, the fixation conditions and rate of drying of the print, the type of fabric used and its thickness, the twist of the yams, and the type of fibers used have a strong influence on wicking behavior (11). 

Many fibers are used in laminates and reinforced plastics. The type of fiber used will depend on the cost, the properties required, and the nature of the polymeric system. Although glass fiber is the most common reinforcement, many others are used. Fiber reinforcements can also come in many forms such as discontinuous fibers, continuous fibers, mat and fabric. Fiber content is the amount of fiber present in reinforced plastics and composites, usually expressed as a percentage volume fraction or weight fraction. 

Contact between fabrics can lead to shedding and transfer of individual strands of fiber. These strands are typically 2-5 mm in length and a wide range of dyes or dye mixtures are used to color the different types of fibers used. A fiber will contain about 10 ng of dye and any extraction method will be required to work with very small amounts. Additionally, although some dyes are easily removed from the fiber others are covalently bonded to it and are extremely difficult to extract. The use of the in situ SERRS method is again successful [13]. 

Pectin also increase bile acid excretion in rats[22j. Kelley et al.[23] observed that rats fed 15% cellulose as part of a semi-purified diet excreted 2.52 mg of bile acids per gm of faces and the ratio of fecal cholic-chenodeoxycholic acid (C/CD) was 1.04. When cellulose was replaced by alfalfa the fecal bile acid concentration rose to 3.75 mg/gm and the C/CD ratio became 0.64. Substitution of whole ground oats for cellulose increased fecal bile acid concentration to 6.16 mg/gm and the C/CD ratio was 0.67. A similar effect of fiber on bile acid spectrum was seen in studies in which baboons were fed commercial or semipurified rations[24]. The effect of fiber on bile acid metabolism is a function of the type of fiber used. Binding may be one mechanism by which fiber increases bile acid excretion. 

Similar to those of sampling headspace with gastight syringes, the results obtained by SPME were highly dependent on absorption conditions as well as type of fiber used. Because of the large variety of fibers available (e.g., PDMS, DVB, Carboxen, including fiber combinations) as well as the development of new and improved techniques [e.g., headspace sorptive extraction (HSSE) [8]], a detailed description would be too extensive for this chapter. However, an excellent review [12] as well as theoretical aspects [4] and applications of SPME in food, flavor, and fragrance have been published [5-7]. 

Type of fiber The principal types of fibers used to produce geotextiles for soil reinforcement are monofilament, multifilament, slit-fihn monofilament, and slit-film multifilament (see chapter Manufacturing process of geotextiles). 

It is important to note that the studies discussed above were conducted between 1994 and 1996. PDMS SPME fibers were the predominant type of fiber used prior to 1996. Since then, numerous additional fiber types have been developed and are significantly superior to PDMS for extracting polar organics and highly volatile compounds. 

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