Viscosity biaxial

Biaxially oriented polystyrene film, 23 408 Biaxial viscosity, 21 718 Bibenzyl solution, VDC polymer degradation in, 25 717 Bibliographic patent information, 18 212 Bibutyltin dichloride, 24 820 Bicarbonates, 12 66... 

Compression of a weakly structured food between parallel plates may achieve squeezing flow (Steffe, 1996). When lubricated parallel plates are used, the result is a form of biaxial extension. Biaxial extension may be used to measure biaxial viscosity, which is a reflection of resistance to radial stretching flow in a plane. Lubricated squeezing flow of a semi-solid... 

For an unvulcanized polydimethylsiloxane, the biaxial viscosity was approximately six times the shear viscosity over the biaxial extensional rates from 0.003 to 1.0 s (Chatraei et al., 1981), a result expected for Newtonian fluids, that is, the relationship between the limiting value of biaxial extensional viscosity (j, ) at zero strain rate and the steady zero-shear viscosity (i o) of a non-Newtonian food is ... 

To perform constant rate squeezing rather than constant stress requires programming the gap to close at an exponentially decreasing rate, eq. 7.2.4. Soskey and Winter (1985) have done this. They were able to get the linear viscoelastic limit, but for e > 1 they found it difficult to determine whether they had strain hardening or simply loss of lubrication. Isayev and Azari (1986) did the simpler constant velocity squeezing experiments. They calculated a biaxial viscosity from their force versus time curves using a differential constitutive model and found behavior very similar to Figure 7.3.5 for a polybutadiene gum (r <, 10 Pa-s). 

Figure 7.4.4 shows data from the rotating clamp device for the transient equibiaxial viscosity at three different extension rates. For comparison, the linear viscoelastic viscosity and the uniaxial viscosity are shown. Results for the biaxial viscosity compare well to those measured in lubricated compression on the same polyisobutylene sample as in Figure 7.4.4 (Chatraei et al., 1981). So far, only results with the rotating clamp method have been reported for this sample. Maximum strains were 2.5 in the biaxial and multiax-ial tests and k < 0.1 s. Friction on the talcum powder may limit the total strain and the detectable stress values. Much larger, more homogeneous samples are required than were used in the lubricated squeezing experiments. However, because the rotating clamps can...

Inflation of a sheet clamped over a circular hole suffers the same difficulty. More work has been done with this method (see Dealy, 1982). Inflation has been done with both liquid and gas. The stress and deformation equations are treated in Chapter 1, Exercise 1.10.8, andby Dealy (1982). The results can give the biaxial viscosity function and e up to 2 has been achieved (Rhi-Sausi and Dealy, 1981 Yang and Dealy, 1987). But results so far have been limited to relatively low extension rates. The method requires photography and a somewhat complex apparatus. Inflation tests, however, are similar to the vacuum forming process for making shaped plastic items (Schmidt and Carley, 1975 DeVries and Bonnebat, 1976). Thus there is motivation to continue this work. 

The elongation viscosity defined by Equation (1.19) represents a uni-axial extension. Elongational flows based on biaxial extensions can also be considered. In an equi-biaxial extension the rate of deformation tensor is defined as... 

Unlike shear viscosity, extensional viscosity has no meaning unless the type of deformation is specified. The three types of extensional viscosity identified and 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 ... 

Extensional Viscosity. AH three types of extensional viscosity can be measured (101,103) uniaxial, biaxial, and pure shear. Only a few commercial instmments are available, however, and most measurements are made with improvised equipment. Extensional viscosity of polymer melts can be estimated from converging flow (entrance pressure) or from a melt strength drawdown test (208). 

A method for measuring the uniaxial extensional viscosity of polymer soHds and melts uses a tensile tester in a Hquid oil bath to remove effects of gravity and provide temperature control cylindrical rods are used as specimens (218,219). The rod extmder may be part of the apparatus and may be combined with a device for clamping the extmded material (220). However, most of the mote recent versions use prepared rods, which are placed in the apparatus and heated to soften or melt the polymer (103,111,221—223). A constant stress or a constant strain rate is appHed, and the resultant extensional strain rate or stress, respectively, is measured. Similar techniques are used to study biaxial extension (101). 

Extensional viscosity that results purely from shear deformation seems to be of less interest, but has been measured (108). The theology of several different polymer melts in terms of shear viscosity and uniaxial and biaxial extensional viscosity has been compared (231). Additional information on the measurement of extensional viscosity are also available (105,238—240). 

Analyze lubricated squeezing flow to determine biaxial extensional viscosity (T)R), which is calculated from biaxial stress (cB) and biaxial extensional strain rate (eB). 

The approach to analysis of biaxial extension of melts in the simulation of the sleeve inflation process was developed by Pirson and Petrie in 1966-1970 with the use of ideas of the thin shell theory which allows to substitute sleeve film by flat film in analysis. The problem was formulated more accurately and completely and solved in works by Han et al. The author made several conclusion the velocity of material extension changes in the main direction of sleeve motion while effective longitudinal viscosity may increase, decrease, or remain constant depending on the nature of material and the range of strain velocities under consideration longitudinal viscosity of the material at fixed process parameters decreases with temperature rise (the behavior of longitudinal velocity is described more strictly above, in Sect, 2.2.6). 

As mentioned in Sect. 2.1, we consider a shear induced smectic C like situation (but with a small tilt angle, i.e., a weak biaxiality). We neglect this weak biaxiality in the viscosity tensor and use it in the uniaxial formulation given above (with the director h as the preferred direction). This assumption is justified by the fact that the results presented in this chapter do not change significantly if we use p instead of h in the viscosity tensor. 

An elongational or extensional viscosity (%) develops as a result of a conformational transition when disperse systems are forced through constrictions, or compressed or stretched (Kulicke and Haas, 1984 Rinaudo, 1988 Barnes et al., 1989 Odell et al., 1989 Clark, 1992). The intuitive logic is that the random coils resist the initial distortion. % is believed to elicit the human sensation of stringiness (Clark, 1995). If shear viscosity is denoted iq, rheologists define a Trouton ratio as %/ti, wherein % > T) by a factor approximating 3 for uniaxial extension and 6 for biaxial extension. Alternatively stated, the Newtonian ly calculates to one-third to one-sixth % (Steffe, 1992). 

The sample is subjected to compression by moving the crosshead downwards at a constant speed. The sample is extruded from between the two discs, undergoing elongational or biaxial flow the sample is stretched radially and azimuthally as it flows outwards between the approaching discs. Lubrication ensures that shear flow cannot occur. Elongational viscosity is calculated directly from the measured force-distance data, disc radius and crosshead speed no rheological model is required (Campanella and Peleg, 2002). 

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