Melting of Fibers

Specially difficult systems are represented by fibers of macromolecules. All the topics treated in the prior sections must be considered under the additional aspect of the presence of crystal deformations caused by the drawing process (see Sect. 5.2.6 and 5.3.6) and possible strain retained in the amorphous areas (see Sect. 6.3.3), as well as the existence of sttain-induced mesophases (see Figs. 5.69-72 and 5.113-115). 

Standard D3C Traces of Drawn and Undrawn Polyethylene with and Without being Held at Constant Length 

On gel-crystaUization from solution, the UHMMPE obtains a lamellar crystal morphology with 13-nm-thick crystals. Stretching such gel-crystallized samples below the melting temperature leads to big changes in the sample morphology. 

Details on calorimetry coupled with information on morphology, full-pattern X-ray analysis (see Fig. 6.4), solid-state NMR (see Figs. 5.157 and 5.158), mechanic properties, and quantitative TMDSC were derived for a number of commercial gel-spun UHMMPE fibers. The structure of an UHMMPE gel-spun fiber is rather complicated and changes on heating with constraint, as seen in Fig. 6.103. It consists of a metastable system of four interconnected, recognizably different phases. At low 

X-Ray Diffraction Data on UHMMPE Gel-spun Fibers on Heating with and without Constraint 

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Dissolving of fiber bales Cleaning and mixing/blending of fiber flakes Melting of fibers... 

Properties. As prepared, the polymer is not soluble in any known solvents below 200°C and has limited solubiUty in selected aromatics, halogenated aromatics, and heterocycHc Hquids above this temperature. The properties of Ryton staple fibers are in the range of most textile fibers and not in the range of the high tenacity or high modulus fibers such as the aramids. The density of the fiber is 1.37 g/cm which is about the same as polyester. However, its melting temperature of 285°C is intermediate between most common melt spun fibers (230—260°C) and Vectran thermotropic fiber (330°C). PPS fibers have a 7 of 83°C and a crystallinity of about 60%. 

HoUow-fiber fabrication methods can be divided into two classes (61). The most common is solution spinning, in which a 20—30% polymer solution is extmded and precipitated into a bath of a nonsolvent, generally water. Solution spinning allows fibers with the asymmetric Loeb-Soufirajan stmcture to be made. An alternative technique is melt spinning, in which a hot polymer melt is extmded from an appropriate die and is then cooled and sohdified in air or a quench tank. Melt-spun fibers are usually relatively dense and have lower fluxes than solution-spun fibers, but because the fiber can be stretched after it leaves the die, very fine fibers can be made. Melt spinning can also be used with polymers such as poly(trimethylpentene), which are not soluble in convenient solvents and are difficult to form by wet spinning. 

Most off-quahty or scrap polypropylene fibers may be repeUetized and blended in small percentages with virgin polymer to produce first-grade spunbonded fabrics. The economics are of great importance in a process where high yields are required in order to be competitive. Some manufacturing equipment direcdy recycles edge-trim back into the extmder where it is blended back into the polymer melt (see Fibers, olefin). 

Melt-spun fiber is produced from PMP at 280°C and is drawn around three times in air at 95°C its fiber strength is 0.18—0.26 N/tex (2—3 g/den), its elongation is around 30%. Melt-spun hoUow fibers are also manufactured. PMP has one of the highest permeabiHties for gases, and many of its appHcations capitalize on this property. 

Delusterants reduce the transparency, iacrease the whiteness, and alter the fiber s reflectance of light. Ti02 is the delusterant of choice for aU melt-spun fiber types because of its high cover, whiteness, and chemical and thermal stabUity. Nylon is translucent and requires ia most textile and... 

Miscibility or compatibility provided by the compatibilizer or TLCP itself can affect the dimensional stability of in situ composites. The feature of ultra-high modulus and low viscosity melt of a nematic liquid crystalline polymer is suitable to induce greater dimensional stability in the composites. For drawn amorphous polymers, if the formed articles are exposed to sufficiently high temperatures, the extended chains are retracted by the entropic driving force of the stretched backbone, similar to the contraction of the stretched rubber network [61,62]. The presence of filler in the extruded articles significantly reduces the total extent of recoil. This can be attributed to the orientation of the fibers in the direction of drawing, which may act as a constraint for a certain amount of polymeric material surrounding them. 

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