Extruders, ‘spinning

A conventional spinning plant essentially consists of melting and filtration systems, melt distribution pipes for static mixtures, melt dosing pumps, a side color melt extruder, spinning packs, a quenching system, a takeup system, spin-finish baths, and a draw and heat system [29, 30]. 

A variation on the wet-spinning technique involves extruding into a heated gas environment. In this dry-spinning process, the temperature and composition of the gas control the extraction process. 

The melt-spinning process used to convert mesophase pitch into fiber form is similar to that employed for many thermoplastic polymers. Normally, an extruder melts the pitch and pumps it into the spin pack. Typically, the molten pitch is filtered before being extruded through a multi-holed spinnerette. The pitch is subjected to high extensional and shear stresses as it approaches and flows through the spinnerette capillaries. The associated torques tend to orient the liquid crystalline pitch in a regular transverse pattern. Upon emerging from the... 

In an extension of this work, pellets of a blend of PCL and hy-droxypropylcellulose containing fluridone were prepared by grinding, blending, and then melt-spinning the mixture with a Berstorff twin screw extruder (78). The extruded rod was subsequently water-quenched and pelletized. Pellets were also prepared by coating bundles of extruded rods with the water-soluble excipients PEG 3350 and PEG 600 (95 5). In vitro release rate measurements were conducted in the simulant medium of 50% aqueous ethanol or hardened water. 

Fig. 25. The effect of metal ion size on porphyrin ruffling. Very small metal ions [P(V) with an ideal P-N bond length of 1.84 A and low-spin Ni(II) with an ideal Ni-N length of 1.90 A in (a) and (b)) cause extensive S4 ruffling. Metal ions close to the right size (M-N = 2.035 A) give planar structures [Zn(II) in (c)]. Metal ions that are too large [Pb(II) at (d) with ideal Pb-N of 2.39 A] are extruded from the plane of the porphyrin and cause it to dome. For clarity, substituents on the porphyrins such as phenyl or ethyl groups have been omitted. Modified after Ref. (77).

Poly[5-(alkylamino)borazines] 7-10 exhibited suitable viscoelastic and thermal stabilities to be extruded in the molten state through the monohole spinneret of a lab-scale melt-spinning apparatus. Thus, an extruded filament (diameter, 200 gm) was drawn with a windup unit, that is, a graphite spool. This provided green fibers with a wide range of diameters (16 =s (f> =s 50 gm Table 2), depending on polymer architecture. 

The technique of spin dyeing chemical fibers may be located somewhere between the textiles and the plastics area. In contrast to textile coloration, the material which is to be extruded is colored before the fiber is made. The requirements to be met by pigments are therefore similar to those which apply to the coloration of... 

Elastomeric fibres based upon both polyester-urethane and polyether-urethane structures followed. The early work by Bayer led to the use of highly polar solvents such as dimethyl formamide. Formation of fibres by reactive spinning, where the isocyanate-ended polymer is extruded into an aqueous solution of a chain-coupling agent, was described in 1949 [75] and by dry-spinning a solution of the chain-coupled polymer in 1951 [76], However, Bayer did not immediately use their technology to produce commercial fibres [77],... 

PTT polymer pellets must be dried to a moisture level of <30 ppm, preferably in a close-loop hot air dryer, to avoid hydrolytic degradation during melt processing. Drying is carried out with 130 °C hot air with a dew point of < -40 °C for at least 4 h. Because of the faster crystallization rate, PTT pellets are already semicrystalline after pelletizing, and do not require pre-crystallization prior to drying as with PET. The dried polymer is extruded at 250-270 °C into bulk continuous filaments (BCFs), partially oriented yam (POY), spin-draw yam (SDY) and staple fiber. 

Bico fibers are a new class of fibers, rather than a sub-set of PET fibers. Such fibers are formed from two different polymers, which are melted separately, and then combined into a single fiber at the last moment before extrusion. In some cases, the fibers are actually extruded separately, and then combined beneath the spinneret while they are still molten, so that they fuse together after spinning. 

Traditional melt spinning is not the best way to make microfibers. Technical problems occur with very low hole throughputs, and economics suffer. By employing bico technology, it is possible to extrude larger shapes that can be separated into smaller components after extrusion (Figure 12.18). 

Like nylon, when it is made PET can be extruded, cooled, cut into chips, and stored for later melt spinning. It also can go directly to the spinneret. The downstream operations of the spinneret are much the same as with nylon. 

---