Spinning Fiber

The fiber-spinning process is a prime example of uniaxial extension. The process consists of two regions (i) the first is the extrudate swell region, where normal forces accumulated during extrusion suddenly relax to cause swelling (ii) the second is the draw-down region, where the fiber diameter decreases according to the velocity 

Numerical simulation of the fiber-spinning process began with the early work of Matovich and Pearson [92], who analyzed the spinning of a Newtonian liquid and arrived at an analytical solution. Attempts were then made to analyze the process with differential constitutive models. Early work by Denn et al. [93] considered the upper converted Maxwell model, including nonisothermal effects [94]. Later, Gagon and Denn [95] used the PTT model and included nonisothermal effects to simulate 

More recently, nanofibers (particularly carbon nanotubes) have found their way into fibrous polymer composites, offering even greater strength per mass when compared to composites made using the larger fibers above. 

Inert gas - solution vapor Cell gas typical temperature distribution 280-320 C 

Heated cell wall typical tenr rature 24Q-290 C 

FIGURE 17.10 Dry spinning of polymer fibers [24]. Reprinted with permission of John Wiley and Sons, Inc. 

Wief spinning is similar to dry spinning in that a polymer solution is forced through the spinnerette. Here, however, the solution strands pass directly into a liquid bath. The liquid might be a nonsolvent for the polymer, precipitating it from solution as the solvent diffuses outward into the nonsolvent. The bath might also contain a substance that precipitates a 

The production of man-made fibers usually includes the following processes (Ziabicki, 1976)  

Preparation of the spinning fluid (polymer melt or solution). 

Spinning (extrusion, solidification, and deformation of the spinning line or filament). 

Drawing (due to higher linear speed at the take-up roll than that at the die drawing is used to increase the degree of molecular orientation and improve the tensile strength, modulus of elasticity, and elongation of the fibers). 

Process 3 can be achieved mainly by three procedures melt spinning, solution dry spinning, and solution wet 

The process of fiber spinning, described in Chapter 3 and schematically represented in Fig. 6.18, will be modeled in this section using first a Newtonian model followed by a shear thinning model. To simplify the analysis, it is customary to set the origin of the coordinate system at the location of largest diameter of the extrudate. Since the distance from the spinnerette to the point of largest swell is very small, only a few die diameters, this simplification will not introduce large problems in the solution. 

If we take the schematic of a differential fiber element presented in Fig. 6.19, we can define the fiber geometry by the function R(x) and the unit normal vector n. The continuity equation tells us that the volumetric flow rate through any cross-section along the rr-direction must be Q 

We further assume that surface tension is negligible and that in steady state the surface will only move tangentially, which means that 

The components of the normal vector are described by the geometry of the fiber as 

Due to the negligible effects of surface tension, we can assume that the stress boundary condition is 

Until the 20th century mankind was limited to natural fibers such as wool, cotton, linen, and for the rich, silk. The first man-made fiber was artificial silk rayon (1910), which was based on cellulose. The big jump came with the invention of nylon by Wallace Carothers, with commercial production starting in 1939, followed in the 1950s by acrylics (which, when mixed with cotton, produced the wash-and wear textiles), polyesters, and many others. 

Principles of Polymer Processing, Second Edition, by Zehev Tadmor and Costas G. Gogos. Copyright 2006 John Wiley Sons, Inc. 

The final properties of the fiber, such as tenacity,1 modulus, luster, and flex loss, are determined by the spinning process. This is because, as the molten filament moves from 

Tenacity equals the breaking strength (grams) divided by denier. Denier is the weight in grams of 9000 meters of filament. 

Crystallization during melt spinning of linear polyethylene 

We use the spinning process to make polymer fibers and filaments that can be converted into fabrics and cordage. During fiber spinning, molten polymer is pumped fiirough holes in a plate to form a multiplicity of strands that are rapidly stretched and cooled. The finished product comprising oriented fibers is either wound up on spools or converted directly into a non-woven fabric. 

We control fiber properties by changing the relative speeds of different stages of the process. Orientation is increased and fiber thickness decreased by increasing the final take-up speed relative to the rate at which the molten polymer strands leave the spinneret. To produce high modulus fibers we generaUy adopt conditions that maximize orientation. Fiber diameters 

We can manufacture fibers from a wde range of polymers. Polyamides, polyesters, and polypropylene can be woven or knitted into fabrics, ranging from those as coarse and strong as those used in back packs, luggage, and sails, to soft and highly flexible fabrics used in s veaters, shirts, and other apparel. Polymer filaments and yarns can be twisted or woven to make string, t vine, cords, and ropes. 

The presence of this secondary phase within the polyamide matrix comprising the fiber leads to roughness on the surface, which affects the look and feel of the fiber. The fiber-to-fiber frictional behavior is altered and the appearance is delustered and less shiny. These attributes make these blend fibers more similar to natural fibers, such as cotton, making them desirable for applications such as carpet fiber and apparel. 

Wessel, T., Kolb, I J. Syndiotactic polystyrene A new polymer for high performance automotive applications. Proceedings from the SEE International Congress and Exposition, Detroit, MI, March 1993. 

Cieslinski, R. C., Dineen, M. T., Wood, C. J. Dow polymer compendium. Dow Internal Chemical Report, ML-AL-94-301287, July 1994. 

Powers, J. R., Spalding, M. A., Hughes, K. R, Meyette, G. Extrusion of syndiotactic polystyrene. Dow Internal Report, November 1999. 

Wagner, J. R., Mount, E. M. Extrusion The Definitive Processing Guide and Handbook, William Andrew Publishing, Norwich, NY, 2005. 

Melt spinning is the least complex of the methods. The polymer from which the fiber is made is melted and then forced through a spinneret and into air to cause solidification and fiber formation. 

Emul sion spinning is used only for those fiber-forming polymers that are insoluble. Polymer is mixed with a surface-active agent (detergent) and possibly a solvent and then mixed at high speeds with water to form an emulsion of the polymer. The polymer is passed through the spinneret and into a coagulating bath to form the fiber. 

In suspension spinning, the polymer is swollen and suspended in a swelling solvent. The swollen suspended polymer is forced through the spinneret into dry hot air to drive off solvent or into a wet non-solvent bath to cause the fiber to form through coagulation. 

The spinning process can be divided into three steps  

Melt spinning is basically an extrusion process. The polymer is plasticized by melting and pumped through the spinnerette. The fibers are usually solidified by 

In dry spinning, a solution of the polymer is forced through the spinnerette. As the fibers proceed downward to the drawing rolls, a countercurrent stream of warm air evaporates the solvent. In this process, the cross section of the fiber is determined not only by the shape of the spinnerette holes, but also by the complex nature of the diffusion-controlled solvent evaporation process, because there is considerable shrinkage as the solvent evaporates. The acrylic fibers (Orion , Creslan, Acrilan , etc.), mainly polyacrylonitrile, are produced by dry spinning. 

Example 3. Suggest processing techniques for the manufacture of the following  

Polymers are almost always used in combination with other ingredients. These ingredients are discussed in subsequent chapters, but they must be combined with the polymer in a compounding operation. 

Because of their extremely high melt viscosities, specialized equipment is usually needed to compound ingredients with high molecular weight thermoplastics, however. In general, high shear rates and large power inputs per unit volume of material are required to achieve a uniform and intimate dispersion of 

It has also been reported that polypropylene modified with nanosized silicon oxide can distinctively improve the processability of polypropylene while simultaneously increasing its strength. On the basis of the four property indices, specific electrical resistance, water absorption, flex stiffness, and rigidity, the modified fiber has reached or surpassed the indices of polyamide 6. 

The processes we have considered thus far - extrusion, wire coating, and injection and compression molding - are dominated by shear between confined surfaces. By contrast, in fiber and film formation the melt is stretched without confining surfaces. It is still possible to gain considerable insight from very elementary flow and heat transfer models, but we must first parallel Section 2.2 and develop some basic concepts of extensional flow. The remainder of the chapter is then devoted to an analysis of fiber formation by melt spinning. 

Our analysis of fiber spinning in this chapter will be based on an inelastic rheological model of the stresses. This rheological description appears to be adequate for polyesters and nylons, which comprise the bulk of commercial spinning applications, and our spinning model is essentially the one used in industrial computer codes. This is a process in which melt viscoelasticity can sometimes play an important role, however, and we will revisit the process in Chapter 10. 

One s first reaction is to wonder how such an experiment can be carried out. If suffices here to say that clever experimental designs for highly viscous materials have been implemented and even commercialized, although the experiment is a difficult one to do well. In fact, the first reported measurements, by Trouton, were done one hundred years ago, together with an analysis parahehng the one given here. 

The primary kinematical assumption is that the axial component of velocity, v, is independent of r. Thus, layers of fluid at different distances from the axis do not move past one another, in which case no shear stresses are generated. We thus see the fundamental difference between this flow and the ones we have encountered previously, will, however, depend on z this is obvious since we have assumed that the end at z = 0 is flxed = 0 at z = 0), while the end at z = T must have a finite velocity if the rod is to be extended. 

let us first apply the continuity equation. We assume axisymmetry (9/90 = 0) and no circumferential flow (vg = 0). In cylindrical coordinates we thus have, from Table 2.1, 

The question of how to spin a lyotropic liquid crystalline polymer which has high thermal stability and degrades before it melts presented a major challenge. Fortunately for the makers of PBO this question had already been answered during the development of Kevlar. PBO is spun directly from the PPA solution used 

In Sections 7.2-7.4, we saw that it is possible to test high viscosity samples, particularly molten polymers, in extension. The major problems are sample preparation, clamping, and buoyancy. Exten-sional rheometry is more difficult, and the upper limits of strain rates and strains are much lower than for shear. Nonetheless, accurate and reproducible data can be obtained, particularly in unieixial extension. 

Comparison of shear and ex-tensional properties of two polymer solutions with similar shear viscosities 2% polyacrylamide and 3% xan-than, both in water at room temperature. Apparent uniaxial extensional viscosity by fiber spinning. Replotted from Jones et al. (1987). 

The extension rate can be determined from measurements of fiber diameter and flow rate. If we assume that the only component of velocity is Vx and that it is uniform across the radius, then the flow rate is 

For constant flow rate using the definition of extension rate, we have 

Differentiating the R x) profile gives the extension rate down the fiber. The practice of taking derivatives of experimental data is prone to errors. Fitting the data first with a spline function can improve accuracy of dR/dx (Secor, 1988). 

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Because of the high melt viscosity of polyolefins, normal spinning melt temperatures are 240—310°C, which is 80—150°C above the crystalline melting point. Because of the high melt temperatures used for polyolefin fiber spinning, thermal stabilizers such as substituted hindered phenols are added. In the presence of pigments, the melt temperature must be carefully controlled to prevent color degradation and to obtain uniform color dispersion. 

A. Ziabicki and H. Kawai, eds.. High Speed Fiber Spinning ohm. Wiley Sons, Inc., New York, 1985. 

Both American Enka (87) and Courtaulds set up pilot-plant work in the eady 1980s with the objectives of developing fiber spinning and solvent... 

Initially, fluorescent whitening agents (FWAs) were used exclusively in textile finishing the detergent and paper industries followed thereafter. These products are also used in fiber spinning masses, plastics, and paints. 

Because of the capacity to tailor select polymer properties by varying the ratio of two or more components, copolymers have found significant commercial appHcation in several product areas. In fiber-spinning, ie, with copolymers such as nylon-6 in nylon-6,6 or the reverse, where the second component is present in low (<10%) concentration, as well as in other comonomers with nylon-6,6 or nylon-6, the copolymers are often used to control the effect of sphemUtes by decreasing their number and probably their size and the rate of crystallization (190). At higher ratios, the semicrystalline polyamides become optically clear, amorphous polymers which find appHcations in packaging and barrier resins markets (191). 

Tensile Properties. Tensile properties of nylon-6 and nylon-6,6 yams shown in Table 1 are a function of polymer molecular weight, fiber spinning speed, quenching rate, and draw ratio. The degree of crystallinity and crystal and amorphous orientation obtained by modifying elements of the melt-spinning process have been related to the tenacity of nylon fiber (23,27). 

Depending on the final polymerization conditions, an equilibrium concentration of monomers (ca 8%) and short-chain oligomers (ca 2%) remains (72). Prior to fiber spinning, most of the residual monomer is removed. In the conventional process, the molten polymer is extmded as a strand, solidified, cut into chip, washed to remove residual monomer, and dried. In some newer continuous processes, the excess monomer is removed from the molten polymer by vacuum stripping. 

These solvents include tetrahydrofuran (THF), 1,4-dioxane, chloroform, dichioromethane, and chloroben2ene. The relatively broad solubiHty characteristics of PSF have been key in the development of solution-based hoUow-fiber spinning processes in the manufacture of polysulfone asymmetric membranes (see Hollow-fibermembranes). The solvent Hst for PES and PPSF is short because of the propensity of these polymers to undergo solvent-induced crysta11i2ation in many solvents. When the PES stmcture contains a small proportion of a second bisphenol comonomer, as in the case of RADEL A (Amoco Corp.) polyethersulfone, solution stabiHtyis much improved over that of PES homopolymer. 

Polymer Solvent. Sulfolane is a solvent for a variety of polymers, including polyacrylonitrile (PAN), poly(vinyhdene cyanide), poly(vinyl chloride) (PVC), poly(vinyl fluoride), and polysulfones (124—129). Sulfolane solutions of PAN, poly(vinyhdene cyanide), and PVC have been patented for fiber-spinning processes, in which the relatively low solution viscosity, good thermal stabiUty, and comparatively low solvent toxicity of sulfolane are advantageous. Powdered perfluorocarbon copolymers bearing sulfo or carboxy groups have been prepared by precipitation from sulfolane solution with toluene at temperatures below 300°C. Particle sizes of 0.5—100 p.m result. 

The primary driving forces behind investigation of new solvents include environmental concerns and the abiUty to form Hquid crystals in the new solvent systems. By analogy with Kevlar, a synthetic aromatic polyamide fiber, spinning from a Hquid crystalline solution should yield cellulose fibers with improved strength, as has been demonstrated in laboratory experiments. 

Cyanohydrins are used primarily as intermediates in the production of other chemicals. Manufacture of methyl methacrylate, used to make acrylic mol ding resins and clear sheet, eg, Plexiglas acrylic sheet, from acetone cyanohydrin is the most economically important cyanohydrin process (see Methacrylic polymers). Cyanohydrins are also used as solvents in appHcations including fiber-spinning and metals refining. Cyanohydrins and derivatives reportedly act as antiknock agents in fuel oil and motor fuels and serve as electrolytes in electrolytic capacitors. 

Sulfolane is a water-soluble biodegradable and highly polar compound valued for its solvent properties. Approximately 20 million pounds of sulfolane are consumed annually in applications that include delignification of wood, polymerization and fiber spinning, and electroplating bathes.It is a solvent for selectively extracting aromatics from reformates and coke oven products. 

Open-cast molding Fiber spinning Blow molding Injection molding Extrusion/pultrusion Reaction injection molding... 

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