Polymer - Fiber Spinning

7/28/2019 Polymer - Fiber Spinning

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6.10 FIBER FORMATION AND STRUCTURE

The synthetic fibers of today, the polyamides, polyesters, rayons, and so on, aremanufactured by a process called spinning. Spinning involves extrusionthrough fine holes known as spinnerets. Immediately after the spinningprocess, the polymer is oriented by stretching or drawing. This both increasespolymer chain orientation and degree of crystallinity. As a result the modulusand tensile strength of the fiber are increased.

Fiber manufacture is subdivided into three basic methods, melt spinning,dry spinning, and wet spinning; see Table 6.10 (159). Melt spinning is the sim-plest but requires that the polymer be stable above its melting temperature.

Polyamide 66 is a typical example. Basically, the polymer is melted and forcedthrough spinnerets, which may have from 50 to 500 holes. The fiber diameterimmediately after the hole and before attenuation begins is larger than thehole diameter. This is called die swell, which is due to a relaxation of theviscoelastic stress-induced orientation in the hole; see Section 5.4 andFigure 10.20.

During the cooling process the fiber is subjected to a draw-down force,which introduces the orientation. Additional orientation may be introducedlater by stretching the fiber to a higher draw ratio.

In dry spinning, the polymer is dissolved in a solvent.A typical example ispolyacrylonitrile dissolved in dimethylformamide to 30% concentration. Thepolymer solution is extruded through the spinnerets, after which the solventis rapidly evaporated (Figure 6.41) (159). After the solvent is evaporated, the

fiber is drawn as before.In wet spinning, the polymer solution is spun into a coagulant bath. An

example is a 7% aqueous solution of sodium cellulose xanthate (viscose),which is spun into a dilute sulfuric acid bath, also containing sodium sulfateand zinc sulfate (160). The zinc ions form temporary ionic cross-links betweenthe xanthate groups,holding the chains together while the sulfuric acid, in turn,

6.10 FIBER FORMATION AND STRUCTURE 307

Table 6.10 Spinning processes (159)

Solution Spinning

Wet Spinning

Melt Spinning Dry Spinning Coagulation RegenerationPolyamide Cellulose acetate Viscose rayonPolyester Cellulose triacetate CuproPolyethylene Acrylic Acrylic

Modacrylic ModacrylicPolypropylene Aramid AramidPVDC Elastane Elastane

PVC PVCVinylal


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removes the xanthate groups, thus precipitating the polymer. After orienta-tion, and so on, the final product is known as rayon.

6.10.1 X-Ray Fiber Diagrams

For the purpose of X-ray analyses, the samples should be as highly orientedand crystalline as possible. Since these are also the conditions required forstrong, high-modulus fibers, basic characterization and engineering require-ments are almost identical.

Figure 6.42 (27,161,162) illustrates a typical X-ray fiber diagram, for polyal-lene, . The actual fiber orientation is vertical. The mostintense diffractions are on the equitorial plane; note the 110 and 200 reflec-tions. Note the rather intense amorphous halo, appearing inside the 011reflection.

nCH2 CH2C

308 THE CRYSTALLINE STATE

Figure 6.41 In typical dry-spinning operations, hot gas is used to evaporate the solvent in the

spinning cabinet. The fibers are simultaneously oriented (159).


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Because of the imperfect orientation of the polymer in the fiber, arcs areseen, rather than spots.The variation in the intensity over the arcs can be used,however, to calculate the average orientation.

A further complication in the interpretation of the fiber diagram arisesbecause it actually is a full rotation photograph. In these ways fiber diagramsdiffer from those of single crystals. Vibrational analyses via infrared andRaman spectroscopy studies play an important role in the selection of the mol-

ecular model, as described above.

6.10.2 Natural Fibers

Natural fibers were used long before the discovery of the synthetics in thetwentieth century. Natural fibers are usually composed of either cellulose orprotein, as shown in Table 6.11. Animal hair fibers belong to a class of pro-teins known as keratin, which serve as the protective outer layer of the highervertebrates. The silks are partly crystalline protein fibers. The crystalline por-

6.10 FIBER FORMATION AND STRUCTURE 309

Figure 6.42 X-ray fiber diagram of polyallene (a), and its indexing (b) (27).


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tions of these macromolecules are arranged in antiparallel pleated sheets, aform of the folded-chain lamellae (163).

The morphology of the natural fibers is often quite complex; see Figure 6.43(164). The cellulose making up these trachieds is a polysaccharide; see Table1.4. The crystalline portion of the cellulose making up the trachieds is highlyoriented, following the various patterns indicated in Figure 6.43. The windingangles of the cellulose form the basis for a natural composite of great strengthand resilience. A similar morphology exists in cotton cellulose.

Thefibrous proteins (keratin) are likewise highly organized; see Figure 6.44

(165). Proteins are actually polyamide derivatives, a copolymeric form ofpolyamide 2, where the mers are amino acids. For example, the structure ofthe amino acid phenylalanine in a protein may be written

(6.57)C

O

C

H

CH2

N

H

310 THE CRYSTALLINE STATE

Table 6.11 Chemical nature of natural fibers

Cellulose Protein

Cotton WoolTracheid (wood) HairFlax SilkHemp Spider websCoirRamieJute

Figure 6.43 The cell walls of a tracheid or wood fiber have several layers, each with a differ-

ent orientation of microfibrils (164). ML, middle lamella, composed of lignin; P, primary wall; S1,S2, S3, layers of the secondary wall; W, warty layer. The lumen in the interior of the warty layer

is used to transport water.


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There are some 20 amino acids in nature, see Figure 14.29. These are orga-nized into an a-helix in the fibrous proteins, which in turn are combined toform protofibrils as shown. In addition to being crystalline, the fibrous pro-teins are cross-linked though disulfide bonds contained in the cystine aminoacid mer, which is especially high in keratin.Animal tendons,composed of col-lagen, another fibrous protein, have also been shown to have a surprisinglycomplex hierarchical structure (166).

6.11 THE HIERARCHICAL STRUCTURE OF POLYMERIC MATERIALS

While all polymers are composed of long-chain molecules, it is the organiza-

tion of such materials at higher and higher levels that progressively determinestheir properties and ultimately determines their applications. Table 6.12 sum-marizes the hierarchical structure of the several classes of polymers accordingto size range. The largest synthetic crystalline polymer structures are thespherulites, while the largest structures of the natural polymers are the wallsof the whole (living) cell.

Of course, there are many kinds of natural polymers. Starch and bread arediscussed in Section 14.3, and silk fibers in Section 14.4; both are semicrys-talline materials.The hierarchical structure of polymers has been reviewed by

6.11 THE HIERARCHICAL STRUCTURE OF POLYMERIC MATERIALS 311

Figure 6.44 A wool-fiber cortical cell is complex structure, being composed ultimately of pro-

teins (165)

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Polymer - Fiber Spinning