Fiber stretching

The elongation of a stretched fiber is best described as a combination of instantaneous extension and a time-dependent extension or creep. This viscoelastic behavior is common to many textile fibers, including acetate. Conversely, recovery of viscoelastic fibers is typically described as a combination of immediate elastic recovery, delayed recovery, and permanent set or secondary creep. The permanent set is the residual extension that is not recoverable. These three components of recovery for acetate are given in Table 1 (4). The elastic recovery of acetate fibers alone and in blends has also been reported (5). In textile processing strains of more than 10% are avoided in order to produce a fabric of acceptable dimensional or shape stabiUty. 

Prices of spandex fibers are highly dependent on thread size selling price generally increases as fiber tex decreases. Factors that contribute to the relatively high cost of spandex fibers include (/) the relatively high cost of raw materials, (2) the small size of the spandex market compared to that of hard fibers which limits scale and thus efficiency of production units, and (J) the technical problems associated with stretch fibers that limit productivity rates and conversion efficiencies. 

The quantitative proportion of the mesomorphous phase in PET fibers is strongly diversified and depends on the conditions of fiber manufacture. According to Lindner fl7], it is estimated to be in the range 0.21-0.36. For poorly stretched fibers (below 300% of draw ratio), the mesophase constitutes almost exclusively an ordered part of the polymer in the fiber. With an increase in the draw ratio of the fiber, the proportion of the mesophase in the whole fiber mass increases, mainly at the cost of the amorphous phase. It is assumed that the mesophase constitutes about 0.5 of the amorphous phase mass for poorly stretched fibers and about 1.2 of this mass for highly stretched fibers. 

In this sense it should be mentioned that (Ala)n is dissolved by hexafluorisopropanol (HFIP) without conformation change and that stretched fibers of (Leu) shrink in HFIP (20 °C) at about 60 % only by disorientation of (Leu)n molecules, because no conformation change occurs according to X-ray measurements 122,123). An interaction of the OH-group of the alcohol with the back-bone-CO-NH-groups is not very likely because, in this case, conformation changes should be observed. 

A stretched fiber containing many poly-L-alanine molecules is suspended vertically and exposed to a collimated monochromatic beam of CuKa x-rays, as shown in figure 1(a). Only a small percentage of the x-ray beam is diffracted most of the beam travels through the specimen with no change in direction. A photographic film is held in back of the specimen. A hole in the center of the film allows the incident undiffracted beam to pass through. 

Thus, macroradicals have been obtained by stretching fibers (20), deforming plastics by compression (37), ball mill grinding (11), freezing and grinding of polymer solutions (10), ultrasonic irradiation (I), mastication (19), dispersion in a microblender (25), and other mechanical techniques (36). Many reviews on the formation of macroradicals by degradative processes have also been published (5, 12,13,16, 33). 

Figure 3.52 The fiber spinning process with detail of a stretching fiber during the cooling process.

If we neglect the effect inertia has on the stretching fiber and drop the inertial term in the above equation, it can be solved as... 

Figure 11.2. (a) PVA/carbon nanotube fibers collected on a winder, produced by wet-spinning (b) Scanning Electron Micrograph of a stretched fiber. The white arrow indicates the fiber axis. 

This is generally termed cohesive set. When steamed or boiled in water and released in cold water stretched fibers retain much of the extension, but on immersion in boiling water this decreases rapidly at first and then more slowly over a prolonged period. The set which is lost in 1 hr is arbitrarily designated temporary set and the residual set permanent set, It is probable that several reactions occur simultaneously and the so-called permanent set continues to decrease with continued boiling of the slack fibers for several hours (Blackburn and Bindley, 1948),... 

Spandex stretch fiber, based on polyurethanes, was developed by DuPont and appeared in 1962. From this time, polyurethanes would account for the greater part of demand for anilines. Aniline production alone had more than doubled, to over 100 million lbs. per year, between 1939 and 1957, in part to satisfy demand in products other than dyes. Half the US output was consumed in the production of rubber additives, mainly diphenylamine and cyclohexylamine, the latter used as a chain stopper in manufacture of polyurethanes (also as a boiler water additive and, in the US until banned in 1970, in the manufacture of cyclamate sweeteners). Other polymers, such as epoxy resins, relied on the bulk availability of various aromatic amines (Chapter 14). 

However, it was later discovered12 that, when aged, stretched fibers of sulfur were extracted with carbon disulfide, some of the reflections no longer appeared in the diffraction pattern. This result was explained13 on the basis that the original diffraction pattern was the superposition of two patterns, that of true fibrous sulfur, plus oriented crystals of one of the allotropes of S8. The earlier unit cell is thus clearly invalidated. 

The x-ray structure of oriented fibers from the carrageenan fractions, k and X, was examined by Bayley over a decade ago. The fractions differ in the sulfate substitution. The level of order in his stretched fibers appears to have been of the paracrystalline type, but an ordered conformation along the backbone was clearly present. He drew the important conclusion that the x-ray pattern from the whole carrageenan does not represent the sum of those from the separate k and X components, and that, therefore, the two components must exist in a unique structural relationship with respect to one another. Only continued, intensive efforts on the native and pure components will permit a full appreciation of this complex structure. The same comment applies to all of the acidic and ester polysaccharide studies mentioned in this Section. A review of chemical work on carrageenans mentions that Bayley interpreted his diagrams in terms of incorrect structures for both components. 

Figure 9. Wide-angle x-ray diffraction pattern of a stretched fiber of a 57% 4GT/PTMEG-T copolyester...

Adhesives and sealants, cast elastomers, microcdWar products, polyurethane stretch fibers (Spandex fibers i, surface coatings, thermoplastic elastomers and Billable gums—... 

By contrast, the ECP must have conjugated rigid-rod macromolecules. Several such polymers show high electrical conductivity (usually after doping), viz. polyacetylene (PAc), polyaniline (PANI), polypyrrole (PPy), polyparaphenylenes (PPP), or poly-3-octyl thiophene (POT). The resins are expensive, difficult to process, brittle and affected by ambient moisture, thus blending is desirable. For uniaxially stretched fibers the percolation threshold is 1.8 vol%, hence low concentration of ECP (usually 5-6 vol%) provides sufficient phase co-continuity to ascertain conductivity similar to that of copper wires (see Table 1.79). 

Fig. 4. Redevelopment of tension in presence of ATP, and elastic after-effect in the absence of ATP, following release. Curve 1 stretched fiber without ATP Curve 2 unstretched fiber with ATP Curve 3 fiber after washing out ATP (from A. Weber, 1951). Curve la stretched actomyosin thread without ATP curve 2a the same, unstretched, with ATP (from Portzehl, 1950b).

The most perfect reversible a—>0 transformation has been obtained by stretching in saturated ammonium sulfate at 90-100°C. Fibers can be stretched quite easily in this solution and show little tendency to contract when they break. The ammonium sulfate hydrates and lubricates air-dried fibers to just the right extent to facilitate transformation. There is no tendency to produce disoriented 0, but a difficulty arises during the washing away of ammonium sulfate. With the extended fibers the internal structure relaxes and 0 returns to a, so that washed but stretched fibers show a mixture of a and 0 patterns. This has been overcome to a considerable extent as follows After stretching in hot ammonium sulfate and cooling, the specimens are transferred to cold saturated solution containing about 5% formaldehyde. Left in this solution for 36-48 hours, the ammonium sulfate can be washed away,... 

Amidine derivatives such as phenyldiguanide or a-naphthylethyliso-thiourea also cause in cats a transient arrest of breathing in the position of expiration, sometimes with an increased expiratory volume. In rabbits they have an analogous but even more complicated action. None of these substances excited the pulmonary stretch fibers. The receptors sensitive... 

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