Fiber spinning extensional viscosity

Extensional viscosity can, depending on process conditions, therefore affect the homogeneity of the film thickness or the evenness of the layer thickness in coating processes. Extremely high extensional speeds can occur in spinning as the fibers are drawn... 

Figure 3-25 A Spin-Line or Fiber-Spin Rheometer for Extensional Viscosity Measurement (Redrawn from Bames et al., 1989).

An increase in extensional flow viscosity in film blowing, fiber spinning, and bottle blowing, etc., is usually advantageous, but the change in viscosity will depend very much on the polymer structure [17]. 

During fiber-spinning and film-blowing, one may expect the extensional viscosity increases with the increase of dv/dr. As the local increase of dv/dx leads to the reduction of the fiber diameter, the increase of viscosity could prevent further thinning and thus makes the fiber more homogeneous in thickness and stronger. Such a performance of polymer fluids has been described as spinnability (Petrie 2006). 

The extrusion method using a lubricated die [85,86] allows the measurements of melt systems with viscosity levels as low as 10 P. Thus, it can be used for extensional viscosity determinations in the case of nylon and polyester melts which are often spun to make s)mthetic fibers. Higher extensional rates even up to 200 s are also achievable in this apparatus [85,86], thus making the information relevant for the polymer processing industries involved in fiber spinning. 

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). 

From this relation and eq. 7.5.5 we can define an apparent uniaxial extensional viscosity for the fiber spinning experiment... 

TABLE 7. 1 / Woridng Equations for Apparent Uniaxial Extensional Viscosity from Fiber Spinning... 

These working equations, along with the limitations and utility of fiber spinning measurements, are summarized in Table 7.5.1. The major problem is that typically is not constant, so the force, which is measured over the entire fiber, is an integration of stresses due to various strain rates and even the upstream shear histmy in the die. For these reasons, the fiber spinning experiment is not a true rheometer, but gives only an iq>paient uniaxial extensional viscosity. 

For polymer melts, fiber spinning results can be compared to true uniaxial extensional viscosity data measured by rod pulling. Figure 7.5.7 shows such a comparison for a low density polyethylene. We see qualitative agreement between the methods but, a very strong effect of upstream history is evident in the fiber data. Both die extrusion velocity and residence time in the die exert a big influence. 

The other important commercial design for extensional measurements on low viscosity fluids is the opposed nozzle device shown in Figure 8.5.2 (Fuller et al., 1987 Mikkelsen et al., 1988). In addition to the opposed-nozzle configuration, if the arm G is turned 90°, the device can also be operated as fiber spinning and tubeless siphon rheometers (Cai et al., 1992). 

Ramesh, N.S., Lee, S.T. Blowing agent effect on extensional viscosity calculated from fiber spinning method for foam processing , SPE Foams 99 (1999), 85-96 Werner, R, Verdejo, R., Wdllecke, F., Altstadt, V., Sandler, J.K.W., Shaffer, M.S.P. Carbon nanofibres allow foaming of semicrystalline poly(ether ether ketone) , Adv. Mater. 17(23) (2005), 2864-2869... 

Unlike shear viscosity, extensional viscosity has no meaning unless the type of deformation is specified. The three types of extensional viscosity identified aind measured are uniaxial or simple, biaxial, and pure shear. Uniaxial viscosity is the only one used to characterize fluids. It has been employed mainly in the study of polymer melts, but also for other fluids. For a Newtonian fluid, the uniaxial extensional viscosity is three times the shear viscosity ( fe)uni = 3/ . This is confirmed at very low shear rates in Figure 13, which provides a typical example of the extensional viscosity behavior of a polymer (129). The two other extensional viscosities are used to study elastomers in the form of films or sheets. Uniaxial and biaxial extensions are important in industry (118,125-128,130,132), the former for the spinning of textile fibers and roller spattering of paints, and the latter for blow molding, vacuum forming, film blowing, and foam processes. 

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