Spinning process conditions

Table 8.49 is a smnmary of spinning process conditions for producing single filaments of PEA resins. The fiber was spun into ambient air under the above conditions. The properties of the fibers have been summarized in Table 8.50. Tables 8.51 and 8.52 present spinning process conditions and fiber properties for FEP resins. Figure 8.40 compares fiber tenacity vs melting point of the fiber for Heffher, et al. s, and Vita sf " ] works.

Process conditions that favor chemical crimp formation are similar to those used for improved tenacity staple (2inc/modifier route). However, spin bath temperature should be as high as possible (ca 60°C) and the spin-bath acid as low as possible (ca 7%). Attempts have been made to overcome some of the leanness of high strength rayons by increasing the crimp levels. ITT Rayonier developed the Prim a crimped HWM fiber (36) and made the process available to their customers. Avtex developed Avdl 111. Neither remain in production. 

Synthetic Fiber and Plastics Industries. In the synthetic fibers and plastics industries, the substrate itself serves as the solvent, and the whitener is not appHed from solutions as in textiles. Table 6 Hsts the types of FWAs used in the synthetic fibers and plastic industries. In the case of synthetic fibers, such as polyamide and polyester produced by the melt-spinning process, FWAs can be added at the start or during the course of polymerization or polycondensation. However, FWAs can also be powdered onto the polymer chips prior to spinning. The above types of appHcation place severe thermal and chemical demands on FWAs. They must not interfere with the polymerization reaction and must remain stable under spinning conditions. 

Because of the rotation of the N—N bond, X-500 is considerably more flexible than the polyamides discussed above. A higher polymer volume fraction is required for an anisotropic phase to appear. In solution, the X-500 polymer is not anisotropic at rest but becomes so when sheared. The characteristic viscosity anomaly which occurs at the onset of Hquid crystal formation appears only at higher shear rates for X-500. The critical volume fraction ( ) shifts to lower polymer concentrations under conditions of greater shear (32). The mechanical orientation that is necessary for Hquid crystal formation must occur during the spinning process which enhances the alignment of the macromolecules. 

Spinnerette Process. The basic spinning process is similar to the production of continuous filament yams and utilizes similar extmder conditions for a given polymer (17). Fibers are formed as the molten polymer exits the >100 tiny holes (ca 0.2 mm) of each spinnerette where it is quenched by chilled air. Because a key objective of the process is to produce a relatively wide (eg, 3 m) web, individual spinnerettes are placed side by side in order that sufficient fibers be generated across the width. This entire grouping of spinnerettes is often called a block or bank, and in commercial production it is common for two or more blocks to be used in tandem in order to increase the coverage and uniformity of laydown of the fibers in the web. 

For a long period of time, too litde attention has been paid to the content and the role of oligomers in the spinning process. Due to the equilibrium conditions in the reaction mixture, PET contains about 1-2% of oligomers. In certain conditions, this amount can be reduced to values below 1 % by solid-state polycondensation (SSP) processes. Figure 13.8 shows the variation of the oligomer content as a function of temperature and time during SSP processes. 

The dyeing of polypropylene fibers, being an item of research for decades, is successfully accomplished with partially stearate-modified hyperbranched polyesteramides. The long alkyl chains ensure compatibility with the polypropylene matrix. The mixing-in of hyperbranched polyesteramides via extrusion affected neither the melt spinning process nor the final polypropylene fiber properties. The modified fibers are dyeable under standard conditions as are, e.g., polyesters or cotton. They can even be used for printing for example a picture pattern on a polypropylene carpet. 

Fibres are, as a result of the spinning process, molecularly oriented, and they have, therefore, a 2 to 3 times higher stiffness than the non-oriented polymer (e.g. polyamide and polyester textile fibres). With the highest attainable orientation, such as in aromatic polyamides (Twaron and Kevlar), and in the PE-fibre (Dyneema) the stiffness can be a hundred times higher than the one in the unoriented condition ... 

Meh Spinning. This process is used to produce a broad range of polypropylene fibers ranging from fine, dtex (one denier) staple coarse continuous filaments. Hoiuopolyiners are almost exclusive used to produce fibers, although copolymer blends are used in some special applications. Processing conditions and polymer melt flow vary with the desired fiber type. 

The mathematical formulation of the fiber-spinning process is meant to simulate and predict the hydrodynamics of the process and the relationship between spinning conditions and fiber structure. It involves rapid extensional deformation, heat transfer to the surrounding quenching environment, air drag on the filament surface, crystallization under rapid axial-orientation, and nonisothermal conditions. 

Fig. 14.11 Schematic representation of fiber spinning process simulation scheme showing the multiple scale simulation analysis down to the molecular level. This is the goal of the Clemson University-MIT NSF Engineering Research Center for Advanced Engineering Fibers and Films (CAEFF) collaboration. CAEFF researchers are addressing fiber and film forming and structuring by creating a multiscale model that can be used to predict optimal combinations of materials and manufacturing conditions, for these and other processes.

In 4.3 we have already seen that polymers, in the rubber or fluid condition, crystallize much more rapidly when their chains are oriented. Therefore a stretched rubber, if stereospecific in its molecular structure, is able to crystallize at a temperature considerably above its equilibrium thermodynamic melting point. Also a thermoplast such as polyethylene, when in the molten state or in solution, can crystallize spontaneously when the chains are being orientated in elongational flow. The latter case is utilized when polyethylene is spun from a diluted solution (gel spinning process), resulting in fibres of super-high strength and stiffness ( Dyneema fibres). 

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