Hot stretching

Stretching during graphitization produces fibers which differ significantly from unstretched fibers in their mechanical properties. 

Hot stretching of cellulosic fibers was undertaken by Union Carbide and reported by Bacon and Schalamon [120] who found that 300% stretching at 2700°C increased the modulus from 70 GPa to 770 GPa. However, the strength did not increase pro rata but did increase from 0.7 to 3.5 GPa, with the extension dropping from 1.0 to 0.5%. 

It is not surprising that similar work was undertaken with PAN fiber and Johnson, Marjaram and Rose [121] at Rolls Royce, were able to stretch PAN fiber 30% to give a modulus of 620 GPa and, although the fiber did increase in strength, the extension decreased to 0.5%. Johnson [122] at RAE, obtained similar results using a 29% stretch at 2800°C, which increased the modulus from 403 GPa to 656 GPa with an extension of 0.5%. 

The increase in preferred orientation on stretching [121] is accompanied by an increase in crystal size in the direction of the c-axis, measured by the parameter L, was obtained by measuring the line broadening of the graphite (002) diffraction arc. The results for different stretch ratios and temperatures are given in Table 5.14. 

The YM was directly proportional to the induced tension, with some evidence that smaller diameter fibers showed greater improvements [126]. Tensile strength was particularly sensitive to the fiber diameter and extrapolation to 5 pm diameter suggested a tentative tensile strength of 8 GPa might be achieved. 

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Filament is prepared by extrusion followed by hot stretching. It may be used for brush bristles or for decorative purposes such as in the manufacture of woven lampshades. 

The most common examples of uniaxially oriented materials include fibers, films, and sheets hot-stretched in one direction and composites containing fibers all aligned in one direction. Some injection-molded objects are also primarily uniaxially oriented, but most injection-molded objects have a complex anisotropy that varies from point to point and is a combination of uniaxial and biaxial orientation. 

The polymer was extruded at 240 C to produce 28-strand multifilament yarn. The yam was hot-stretched and fiber tensile strengths in the range 3-6 g/den (1-2 dpf) could be obtained. The yam was placed on braider bobbins on a 12-carrier machine with 7-ply core. The braid was made with 51 picks per inch and hot-stretched 25% at215 F. 

Acrylic fibers possess a property that made it possible for them in the late 1950s and early 1960s to find immediate, even spectacular, acceptance in the knitted sweater field, until then dominated by wool. When acrylic fibers, normally in the form of a heavy tow, are hot-stretched (e.g., by being drawn over a hot plate and then cooled under tension), they are converted to a labile state. Upon immersion in hot water, such fibers will contract considerably, but not to their prior unstretched length. In practice, this characteristic is used to... 

Figure 2.9. (a) and (b) Tensile modulus and strength of PAN nanocomposites. Curves a and c represent theoretical values of hot stretched and original electrospun composites, whereas curves b and d represent the experimental values of these composites. Reproduced from reference 47 with permission from American Chemical Society. 

Hot-stretched wet-spun fibers HipCo SWNT, MWNT, (SDS) 30 wt% 195000 6.5 Miaudet 05... 

Figure 11.7. (a) Stress-strain curve of a raw wet-spun fiber (the inset focuses on the elastic regime), (b) stress-strain curves of wet-spun fibers that have been hot-stretched at 180°C, at various draw-ratios, from 0 (raw) to 800%. 

This behavior seems to be related to the already discussed strong interactions between the nanotubes and the PVA. Indeed, as shown in (39), hot-stretching induces the orientation of the PVA and... 

Table 11.2. Mechanical properties of wet-spun fibers, hot-stretched at different draw-ratios. The fibers are drawn at 180°C and contain a fraction of 25wt% nanotubes. The PVA is 195000 g/mol and 99% hydrolized...

Figure 10. Effect of hot-stretching on subsequent room temperature Instron stress strain curves of one BPFC-DMS block polymer. 27% silicone of DPn = 36.

The next major development takes up the idea of the gel layer, and combines this with the tensile drawing of fibres at high temperature, hopefully to give a preparation route which is more acceptable in terms of production rates. We therefore now have a two stage process, in which a fibre of suitable initial structure is first produced, followed by a hot stretching process, and historically two parallel accounts have been given more or less contemporaneously. Smith and Lemstra describe the... 

Two categories of pitch-based fiber exist isotropic carbon fiber produced from an isotropic pitch precursor, and an oriented, anisotropic fiber produced from a mesophase pitch precursor. Isotropic fibers were developed from low melting point isotropic pitches The precursor was melt-spun into fibers, which were oxidized to render them infusible, and then carbonized. Their low strengths and moduli make these fibers unsuitable for use in advanced composites. Orientation was accomplished by a hot-stretching process (>2200°C), but it is accompanied by the same processing difficulties encountered in the rayon precursor process. A different approach was suggested by the discovery of carbonaceous mesophase. ... 

The spinneret was heated to 162°C and kept 1/2" above the spin bath liquid. After passage under a guide at the bottom of the spin bath, the fiber was passed over a water wash roll, a neutralizing roll rotating in 4% aqueous sodium carbonate solution, through a water bath at 95°C, over another wash roll, a steam-heated drying roll and was finally hot-stretched at 357°C over a hot shoe. The fiber obtained at 1010 fpm had a tenacity of 14.4 gpd, an elongation of 7.2% and an initial modulus of 303 gpd. 

The crystalline phase birefringence can be divided by the refractive index difference n — n for the crystal, to give 2. However, there is also a contribution to the birefringence from the molecular orientation of the amorphous phase. Figure 3.32 shows the contributions to the overall birefringence of polypropylene films, hot stretched at 110°C by different amounts. The increase in the orientation with strain is non-linear and it differs between the phases. 

The hot-stretching was performed at the temperature where the tensile storage modulus at 10 Hz was 10 MPa [212°C]. The stress-strain curve at a strain rate of 0.05 s is shown in Fig. 9.21. The curve is a typical one for a viscoelastic body in a rubbery region. 

The refractive indices at 588 nm along three principle axes for films with various draw ratios are illustrated in Fig. 9.22. In this experiment, the hot-stretching was performed in the x direction. 

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