Fiber variants

In a previous section, data and plots were given showing the rapid rise in consumption and production of manufactured fibers at the expense of natural fibers. The principal reason for this has been the wide range of manufactured fiber variants that can be produced from a single fiber-forming polymer. The wide range of polymers available, each with its particular properties, adds yet another dimension. This is not to say that there is only one type of cotton, wool, silk, or asbestos fiber there are many varieties of natural fibers, but their supply is limited by natural factors such as climate and genetics. The relative availabilities of manufactured fiber types can be altered by controlled chemical-process... 

For the purposes of this discussion, fiber variants will be divided into two types chemical and physical. Chemical variants will be those involving a small but significant change in composition, whereas physical changes will be those involving a change in either the dimensions of the fiber or its stress/strain or stability features. The definitions of the two variants also could be based on modification of either esthetics or functionality. 

Fig. 7. Cross sections of stems at internodes of a low THC fiber variant (left) and of a high THC variant (right). Fiber varieties have hollower stems at the internodes, allowing more energy to be directed into the production of fibers in the phloem. (Reprinted from Small, Plant Science Bulletin, vol. 35, 1975.)...

The figure shown here has been reversed in printing and the fiber variant is found on the right. Emboden [J Psychoactive Drugs 13,15, (81)1 has forcefully stated the case for polyspecific nature of the genus Cannabis. I agree completely. 

Greater success was achieved by DuPont who copolymerized, the sodium salt of 5-sulfoisophthalic acid into PET to render the polymer dyeable with cationic (basic) dyes. Basic dyeable PET was successfully launched as Dacron 64 in the form of a low-pill staple product [64]. The presence of the sulfonate groups in the polymer chain also acts as an ionic dipolar cross-link and increases the melt viscosity of the polymer quite markedly. Thus, it is possible to melt-spin polymer with IV 0.56 under normal conditions, giving a low-pill fiber variant. The fiber also has a greater affinity for disperse dyes due to the disruption of the PET structure. Continuing this theme, there are deep dye variant PET fibers, often used in PET carpet yarns, which are copolymers of PET with chain-disrupting copolymer units like polyethylene adipate. They have less crystallinity and a lower Tg. therefore, they may be dyed at the boil without the use of pressure equipment or carrier at the cost of some loss of fiber physical properties. 

Although regular rayon, with water retention as much as 100% of its weight, is the most absorbent of the human-made fibers, variants have been produced that far exceed this capacity [291]. The demand for superabsorbent fibers arises from the growing use of rayon in surgical and medical supplies, sanitary napkins, disposable diapers, and other nonwovens [292]. The rayon industry has responded with several versions of modified fibers. 

In a previous section, data and plots were given showing the rapid rise in consumption and production of manufactured fibers at the expense of natural fibers. The principal reason for this has been the wide range of manufactured fiber variants that can be produced from a single fiber-forming polymer. The wide range of polymers available, each with its particular... 

High Tena.city Sta.ple Fibers. When stronger staple fibers became marketable, the tire yam processes were adapted to suit the high productivity staple fiber processes. Improved staple fibers use a variant of the mixed modifier approach to reach 0.26 N /tex (3 gf/den). The full 0.4 N /tex (4.5 gf/den) potential of the chemistry is uimecessary for the target end uses and difficult to achieve on the regular staple production systems. 

Both fiber producers and fabric mills have realized that many of the performance variants that are difficult to iacorporate iato fiber melt spinning can be accompHshed by post-treating yams or fabrics. Mills ia the 1990s can apply flame retardants, softeners, dye-fade inhibitors, and stain- and soil-resisting agents as part of the finishing of a fabric. 

The eccentric rotation of filler in virtue of the variant shear rates along the extrudate cross-section has been proposed as an alternative to the plug flow mechanism to explain this orientation pattern [355]. In [357] it was noted that the cross-wise orientation of fibers at the core of molded specimens was established only after the entire mold had been filled the authors assumed this orientation to be due to the quasi-static stresses arising in the material under pressure. 

Centronuclear myopathy with type 1 fiber hypotrophy is sometimes regarded as a separate entity because many cases show central nuclei only in the hypotrophic type 1 fibers, while the type 2 fibers are morphologically normal. Affected type 1 fibers are even more myotubelike than in other variants of the disorder, with the exception of the severe X-linked form, due to the persistence of a mitochondria-rich core within a peripheral ring of myofibrils. These features are clearly demonstrable using histochemical methods for the localization of SDH activity and myofibrillar ATPase, respectively. 

In this chapter we have introduced the basic principles underlying polymer extrusion. As we have seen, extruders have two primary purposes they convert polymer granules into a homogeneous molten stream that is pumped to a forming device. The forming devices can take many forms to make a wide variety of continuous products. Among the continuous products made by extrusion are films, pipes, and fibers, which are used in all manner of products that make modern life more convenient and comfortable. Each of the processes that we have described has multiple variants, both major and minor. In each of the sections... 

PET fibers are produced in a variety of forms, broadly classified as staple fibers, textile filament and industrial filament. End-uses for these variants have different requirements in terms of fiber properties and physical geometry, and so different fiber manufacturing processes have evolved according to the special needs of each market segment. 

The manufacturing processes for textile filament, staple and industrial filament yams have become so specialized that it is not possible to make one such class of fibers on the others equipment. Within these classes, there are production machines specialized for certain types of fibers for specific types of consumer products. Large machines designed to produce high volumes of commodity products (e.g. staple for cotton blending) at high efficiency and low cost are not well suited to the efficient production of specialty staple variants (e.g. fibers with special dyeing properties) and vice-versa. 

The C3 family of materials [11-13] exhibits this chemical stability due to a highly functionalized fraction of sp2 carbon [14-17], but in addition contains a carbon fiber backbone in its second bulk component. Carbon fibers [18-21] are the ordered variant of interlaced ribbons in fibers the anisotropic sp2 basic structural units are oriented in one direction [20] by various mechanisms during synthesis. The result is a high... 

Fig. 9.1 Top view on two variants of C3 materials. The carbon fibers (a) themselves exhibit a complex inner microstructure that needs carful optimization for strength and stability. The isotropic filler phase (b) should be free of pores and other weak points caused by uneven distribution in the composite body. The ordered graphitic BSU (c) can provide a very strong but still flexible anchoring of the fibers in the isotropic matrix.

The similarity of the various histone fibers is probably correlated with the similarity in the distribution of the amino acids in the sequences of the four core histones and reflects their function as the skeleton or backbone of chromatin. However, from the presence of a specific pattern of interactions of the core histones and the existence of histone variants and histone postsynthetic modifications, one can anticipate modulations in the basic general pattern of histone structure. In Section V, a possible mechanism for histone microheterogeneity influencing chromatin structure is suggested. Analogous to other assembly systems, small subunit modifications may be amplified to produce major changes in the assembled superstructure. 

A fiber-optic device has been described that can monitor chlorinated hydrocarbons in water (Gobel et al. 1994). The sensor is based on the diffusion of chlorinated hydrocarbons into a polymeric layer surrounding a silver halide optical fiber through which is passed broad-band mid-infrared radiation. The chlorinated compounds concentrated in the polymer absorb some of the radiation that escapes the liber (evanescent wave) this technique is a variant of attenuated total reflection (ATR) spectroscopy. A LOD for chloroform was stated to be 5 mg/L (5 ppm). This sensor does not have a high degree of selectivity for chloroform over other chlorinated aliphatic hydrocarbons, but appears to be useful for continuous monitoring purposes. 

In summary what have we learned in 25 years In some areas, surprisingly little— for example, we cannot say that we really understand the condensed chromatin fiber structure much better than we did in 1978. Although the significance of the great majority of histone variants remains unknown, replacement histones appear now to be involved in major chromosomal functions. There are areas in which we have accrued incredible amounts of detailed information yet still do not quite know what to do with it. Histone acetylation is a prime example. Allfrey et al. [56] could predict its role in a general sense in 1964. We now know a whole rogue s gallery of acetylases and deacetylases plus the specific histone sites for many. Nevertheless, authorities in the field must still write in 2000, The mechanisms by which histone acetylation affects chromatin structure and transcription is not yet clear [58]. 

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