Fiber chemical modification

In covalent immobilization of biomolecules onto the fibers, chemical modifications are made in electrospun polyester in order to produce reactive functional groups in its chain. Primary amine and carboxylate are chemical groups frequently used as intermediates of reaction. Through this strategy, the amino or carboxyl groups present on biomolecules are cross-linked to free carboxyl or amino groups on activated electrospun polyesters. The l-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and M-hydroxysuccinimide (NHS) are the most used intermediary reagents in activation reactions of polyesters. 

Textile dyes were, until the nineteenth century invention of aniline dyes, derived from biological sources plants or animals, eg, insects or, as in the case of the highly prized classical dyestuff Tyrian purple, a shellfish. Some of these natural dyes are so-caUed vat dyes, eg, indigo and Tyrian purple, in which a chemical modification after binding to the fiber results in the intended color. Some others are direct dyes, eg, walnut sheU and safflower, that can be apphed directly to the fiber. The majority, however, are mordant dyes a metal salt precipitated onto the fiber facUitates the binding of the dyestuff Aluminum, iron, and tin salts ate the most common historical mordants. The color of the dyed textile depends on the mordant used for example, cochineal is crimson when mordanted with aluminum, purple with iron, and scarlet with tin (see Dyes AND DYE INTERMEDIATES). 

For fabrics of thermoplastic fibers, permanent effects are obtainable if heat and pressure are appHed to soften the material. Processes dealing with carpets, nonwovens, and chemical modifications or additions that occur before the fiber is formed are not discussed herein (see Nonwoven fabrics). 

Treatments with Chemicals or Resins. Resin treatments are divided into topical or chemical modifications of the fiber itself. Most chemical treatments of synthetic fibers are topical because of the inert character of the fiber itself and the general resistance of the fiber to penetration by reagents. By contrast, ceUulosics and wool possess chemical functionality that makes them reactive with reagents containing groups designed for such purchases. Natural fibers also provide a better substrate for nonreactive topical treatments because they permit better penetration of the reagents. 

Chemical modification of the cotton fiber must be achieved within the physical framework of this rather compHcated architecture. Uniformity of reaction and distribution of reaction products are iaevitably iafiuenced by rates of diffusion, swelling and shrinking of the whole fiber, and by distension or contraction of the fiber s iadividual stmctural elements duting finishing processes. 

Chemical modification has assisted in building cotton s position in the market-place despite the advent of synthetic fibers. 

A number of other polysaccharides, such as glycogen, dextran, chitin, etc., possess interesting structures for chemical modification [103,104]. Dextran has been used as a blood plasma substitute. Although it can be converted to films and fibers, chitin s relatively small resource restricts its commercialization. 

An important chemical modification method is the chemical coupling method. This method improves the interfacial adhesion. The fiber surface is treated with a compound that forms a bridge of chemical bonds between fiber and matrix. 

When used as substitutes for asbestos fibers, plant fibers and manmade cellulose fibers show comparable characteristic values in a cement matrix, but at lower costs. As with plastic composites, these values are essentially dependent on the properties of the fiber and the adhesion between fiber and matrix. Distinctly higher values for strength and. stiffness of the composites can be achieved by a chemical modification of the fiber surface (acrylic and polystyrene treatment [74]), usually produced by the Hatschek-process 75-77J. Tests by Coutts et al. [76] and Coutts [77,78] on wood fiber cement (soft-, and hardwood fibers) show that already at a fiber content of 8-10 wt%, a maximum of strengthening is achieved (Fig. 22). 

Control of fiber friction is essential to the processing of fibers, and it is sometimes desirable to modify fiber surfaces for particular end-uses. Most fiber friction modifications are accomplished by coating the fibers with lubricants or finishes. In most cases, these are temporary treatments that are removed in final processing steps before sale of the finished good. In some cases, a more permanent treatment is desired, and chemical reactions are performed to attach different species to the fiber surface, e.g. siliconized slick finishes or rubber adhesion promoters. Polyester s lack of chemical bonding sites can be modified by surface treatments that generate free radicals, such as with corrosive chemicals (e.g. acrylic acid) or by ionic bombardment with plasma treatments. The broken molecular bonds produce more polar sites, thus providing increased surface wettability and reactivity. 

Surface Modification of Cellulose and PVA Films. Cellulose, as well as PVA,is known to be a typical non-ionic, hydrophilic polymer possessing hydroxyl groups. As this group has a high reactivity,chemical modification of these polymers is relatively easy and, in fact, has been the subject of extensive research. However, so far as we know, no work has been reported concerned with reactions occurring only at the surface of films or fibers from these polymers. 

Kumar, S. (1994). Chemical modification of wood. Wood and Fiber Science, 26(2), 270-280. 

Mahlberg, R., Paajanen, L., Nurmi, A., Kivisto, A., Koskela, K. and Rowell, R.M. (2001). Effect of chemical modification of wood on the mechanical and adhesion properties of wood fiber/polypropylene fiber and polypropylene/veneer composites. Holz als Roh- und Werkstoff, 59(5), 319-326. 

Rowell, R.M. and Ellis, W.D. (1984). Effects of moisture on the chemical modification of wood with epoxides and isocyanates. Wood and Fiber Science, 16(2), 257-267. 

Rowell, R.M., Cleary, B.A., Rowell, J.S., Clemons, C. and Young, R.A. (1993b). Results of chemical modification of lignocellulosic fibers for use in composites. In Wood Fiber/Polymer Composites Fundamental Concepts, Processes, and Material Options, Wolcott, M.P. (Ed.). Eorest Products Society, Madison, Wiseconsin, USA, pp. 121-127. 

In recent years it has become increasingly apparent that the compositional heterogeneity of the chromatin fiber through histone variants, histone post-translational modifications and chemical modifications of DNA (methylation) [9] play an important role in all these processes. The different sources of compositional heterogeneity are described in Sections 2-4 of this review. 

Also, local changes in the structural and chemical variation of DNA may have important effects on the overall extent of chromatin folding. For instance, transitions from the B to the Z form of DNA will result in nucleosome dissolution (as discussed earlier) and this could affect the folding of the fiber. As well, chemical modifications of the bases such as methylation have been shown to increase the folding of the chromatin fiber when linker histones are present [250] although the mechanism involved in this later case remains to be elucidated. 

Chemical modification takes place and carboxyl, hydroxyl and carbonyl groups are produced on the fiber surface. 

Chemical Modification of Lignocellulosic Fibers To Produce High-Performance Composites... 

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