Melt-spinning conditions

Under actual melt-spinning conditions, the situation is far more complicated as the velocity always increases (D decreases), while the fluid generally does not show Newtonian behaviour. Moreover, the temperature decreases, so that the physical properties change in the course of the process. 

Rheology—stable under melt-spinning conditions... 

Bashir [317] studied the interaction of water and PAN under conditions similar to those created by the melt-spinning conditions described above. PAN powder that had been blended with water could be compression molded at 210°C like a thermoplastic material. The water-plasticized films were flexible and could be uniaxially drawn. A comparison of the x-ray diffraction patterns before and after addition of the water showed the formation of a new polymorph attributed to hydrogen bonding of the water with the nitrile groups. On drawing the plasticized film, the water was mostly expelled from the film and the x-ray patterns reverted to the original hexagonal structure. 

A solution applied to typical melt-spinning conditions, indicates that the ambient air temperature would be reached not far from the spinneret face. Of course, in actuality, the requirement of matching the polymer s heat of fusion will greatly delay the fiber s reaching ambient temperature. 

Optimization of melt spinning conditions involves not only quenching—it is just as important to ensure that the auxiliaries (i.e., the supply air conditions, room air conditions, geometry of the building) have to be optimized, as well [142]. 

FIGURE 9.12 Space of aU possible melt-spinning conditions, including regions of hydrodynamic stability H, cohesive fracture F, capillary breakup C, spinnability S, and hydrodynamic instability x-H. (Reprinted by permission of the publisher from Ziabicki, 1976.)... 

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. 

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. 

Bourbigot et al.85 at Lille have used poly(vinylsilsesquioxane) (POSS) in PP (110 wt%) to melt spin filaments, which were then knitted into fabrics. POSS was thermally stable and no degradation was detected in the processing conditions. They have tested the flammability of the fabrics using cone calorimetry. POSS presence had minimal effect on peak heat and total heat release values of PP fabric, but delayed the TTI. This behavior of POSS is opposite to that of layered silicates, which have minimal effect on TTI, but reduce PHRR. Authors claim that POSS does not act as a FR but only as a heat stabilizer via a decrease of the ignitability. 

Fibers emerging from the spinneret are cooled under controlled conditions, passing over guides and rollers to a take-up spool or bobbin. Often a finish is applied before windup to control static electricity and friction (Stevens 1993). Large-scale production machinery produces fiber at a rate of thousands of feet a minute. A schematic diagram of a melt-spinning apparatus is drawn in Figure 8-10. 

In practice, many fabrication processes take place under non-isothermal, non-quiescent and high-pressure conditions. Mechanical deformation and pressure can enhance the crystallisation as well as the crystal morphology, by aligning the polymer chains. This leads to pressure-induced crystallisation and to flow-induced or stress-induced crystallisation, which in fact is the basis for fibre melt-spinning (see Sect. 19.4.1)... 

The stability of melt spinning under non-isothermal conditions and for non-Newtonian fluids has been discussed by Pearson and Shah (1972, 1974). Their results cannot be summarised in a few words, but again the quantity vq/y plays an important part. 

PLA Fambri et al. [27] compiled a list of present literature data on PLA fibers, which were spun under quite different technological conditions, dry spinning, solution spinning, melt spinning, and spin drawing. 

Relatively little catalytic work has been carried out so far under conditions where the surface of the metal alloys can be regarded as unreconstructed, i.e., where the chemical composition and structure of the surface can be assumed to be in the state characteristic for the freshly quenched material. In principle, such investigations can only be performed at temperatures far below the crystallization temperature of the alloy and require special precaution to eliminate possible contamination of the alloy during its transfer from the fabrication (melt spinning) to the catalytic reactor. 

The effect of oxygen exposure during the quenching process has nicely been demonstrated by Guczi and coworkers [4.17,18], who studied the structural and chemical properties of Fe-B alloys prepared by melt spinning under atmospheric conditions. They found drastic differences in the chemical and structural properties between the dull (in contact with copper wheel) and shiny side (exposed to air) of as-prepared ribbons. Such a behavior is likely to occur with alloys containing constituents with largely different heat of formation. 

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