Melt-spun polymers

Summary The heterogeneous catalytic redistribution reaction of methylchlorodisilanes provides spinnable poly(methylchlorosilanes/-carbosilanes). Especially copolymers like poly(methylchlorosilanes-co-styrenes) are suitable polymers for melt spinning. The high reactivity caused by Si-Cl bonds enables oxygen free curing methods of the melt spun polymer filaments with ammonia. The synthesis is achieved without the employment of highly reactive metals and any solvents. The thus produced SiC fibers exhibit oxygen contents lower than 1 wt. %. 

Orthodontics is an area in which polymers are desirable for both their esthetic appeal and shape-memory effect. In 2007, Eliades published an opinion paper on projected future materials for orthodontics and discussed research into polymer-based archwires [67]. In the following year, Jung and Cho demonstrated the use of shape-memory polyurethanes for arch wires [68]. An in vitro dental model was used to test the correction of misaligned teeth and can be seen in Fig. 10. The melt spun polymer, synthesized from 4,4 -methylene bis(phenylisocyanate) and PCL-diol, was stretched to the length required to realign the teeth and attached to stainless... 

The most important melt-spun polymers are polyester (PET), polyamide (PA6 and PA66) and polypropylene (PP). A scheme for a melt-spinning machine is given in Figure 17.6. We will follow the process scheme, from extruder to winder. 

Catastrophic strain hardening, resulting in breaking filaments, is rare for melt-spun polymers. Buildup of fairly high orientation in the spin-line has become common, however, in modem high-speed spinning processes. 

Finally, the relative importance of the various forces in Eq.9. 58 is shown in Figure 9.7 (from Ziabicki and Kawai, 1985) as a function of the take-up speed. The melt-spun polymer is a polyester, the spinneret radius is 125 [xm, the filament radius at take-up is 9.25 fim, the viscosity is 300 Pa s, the cooling air is stationary, and the mass flow rate is proportional to the spinning speed. It is clear fi-om this figure that for low take-up speeds the rheological and... 

One of the principal problems in early commercialization of acryUc fibers was the lack of a suitable spinning method. The polymer caimot be melt spun, except possibly at high pressure in the presence of water. Solution spinning was the only feasible commercial route. However, hydrogen bonding between... 

More recentiy, melt-spun biconstituent sheath—core elastic fibers have been commercialized. They normally consist of a hard fiber sheath (polyamide or polyester) along with a segmented polyurethane core polymer (11,12). Kanebo Ltd. in Japan currentiy produces a biconstituent fiber for hosiery end uses called Sideria. 

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. 

Polyethylene. Traditional melt spun methods have not utilized polyethylene as the base polymer because the physical properties obtained have been lower compared to those obtained with polypropylene. Advances in polyethylene technology may result in the commercialization of new spunbonded stmctures having characteristics not attainable with polypropylene. Although fiber-grade polyethylene resin was announced in late 1986 (11,12), it has seen limited acceptance because of higher costs and continuing improvements in polypropylene resin technology (see Olefin POLYMERS, POLYETHYLENE). 

The polymer may be prepared by running the isocyanate into the glycol while the temperature is raised slowly to near 200°C. The reaction is exothermic and carried out under a blanket of nitrogen. The polymers produced have a molecular weight of 10000-15 000 and after filtration may be melt spun into fibres. 

Fabrication of drug-containing fibers is a natural progression when one considers the extensive history of lactide/glycolides in suture applications. The lactide/glycolide polymers are easily melt-spun into mono- or multifilament products at relatively low temperatures. 

In UK, in 1941, Whinfield and Dickson discovered the polyester fibre called terylene chemically it is polyethylene terephthalate. Teryene polymer can be melt spun into a fibre which is widely used in textile industry. 

Nitrogen is pumped in and molten polymer is extruded and cooled on a drum to form ribbons. Nylon 6, 10 is formed in similar way at a lower temperature. Nylon 6, 6 can also be prepared in a continuous process in three separate tubes wherein reaction is started, steam removed and polymerisation completed. The product is directly melt spun into fibres. 

Polymers were melt-spun by using a flow tester apparatus having a capillary with a diameter of either 0.3 or 0.5 mm. 

Many nylon and polyester assemblies are configured so that there is a continuous progression from the melt formation of the polymer, and without hardening, the melted polymer is melt-spun into fibers. 

Fibers produced by dry spinning have lower void concentrations in comparison to melt-spun fibers because the presence of solvent molecules causes voids that are often remembered by the polymer. This is reflected by greater densities and lower dyeability for the dry spun fibers. 

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