Biaxial squeeze-flow

THE BIAXIAL SQUEEZE-FLOW CHARACTERISTICS OF ATACTIC POLYSTYRENE ABOVE THE GLASS TRANSITION TEMPERATURE... 

Biaxial squeeze-flow techniques are relatively new methods for the rheological characterization of polymer melts. With the method employed in this study, a plastic specimen is uniaxially compressed between two flat steel compression plates. Uniaxial compression is theologically equivalent to biaxial extension for incompressible materials. To minimize the effects of friction and adhesion at the metal-polymer interfaces, a lubricated surface was interposed between the polymer and the compression plates. Tliis technique was developed some years ago as a tool to help understand pressure-forming fabrication processes being researched by The Dow Chemical Company. 

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

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... 

The lubricated squeezing flow technique was applied to melting American cheese where an Universal Testing Machine with lubricated platens was employed (Campanella et al., 1987) to obtain experimental data. This technique is particularly well suited for highly viscous semisolid foods prone to either slip or require considerable effort in pressure-driven methods, such as melted cheese, cheese spreads, com massa, tomato pastes, mayonnaise, and peanut butter, and provides data at low values of strain rate of the order of about 0.01 s Conversely, it is not easy to obtain high strain rates with Universal Testing Machines. In addition, it should be ensured that the lubricant is not depleted and is in place during a biaxial strain experiment. 

If the radius increases exponentially with time, the experiment is start-up of biaxial extension. However, a simpler technique is lubricated squeeze flow in which the thickness of a sample d decreases with time, and the biaxial strain is given by ... 

Step strain experiments have been carried out using lubricated squeeze flow to determine the damping function for biaxial extension h e ). The Doi-Edwards tube model (without lA assumption) prediction of this function is as follows [24] ... 

To generate equibiaxial extension, sheet inflation, lubricated squeeze flow, and the rotary clamp technique have all been used [9, p. 261]. The most rehable of these is the one based on the rotary clamp technique, and the latest version of this instrument has been described by Hachman and Meissner [160]. This impressive instrument is presently the only rheometer capable of generating homogeneous biaxial extension, but it is somewhat complex and difficult to use. Lubricated squeeze flow is much simpler, but there are important limitations on its capabilities due the difficulty of maintaining lubrication [220].Isakief a/. [161] used lubricated squeeze flow to carry out stress relaxation experiments for determination of the damping... 

B.3 Squeezing Flow Between Lubricated Disks. Biaxial stretching flow can be generated in lubricated squeezing flow which is shown in Figure 3.36. A thin layer of lubricant applied to the upper and lower disks prevents the fluid from sticking to the plates. 

The effect of temperature on biaxially squeezed ar-PS is seen more completely in Figure 6 where the apparent true stress at = 2.2 is plotted as a function of temperature for several initial strain rates. As previously discussed (Fig. 5), the values of stress fall sharply in the temperature range up to MO C. However, the stress values do not decrease in a monotonic fashion but appear to show a region of instability around 157-165 C for most of the strain rates used. The temperamre regime over which this effect occurs appears to be relatively insensitive to the rate of deformation but is most noticeable with an initial strain rate of 0.67 sec l. The standard deviation in measured loads was less than 10% for the temperature interval from 120°C to 155 C, but in some cases at 160 C the standard deviation was as great as 20%. Evidence of instability was quite reproducible at temperatures around 160°C. The observed behavior does not seem to be reconcilable within the scope of simple theories of viscoelasticity or with the more recent theories of rheological flow developed by Doi and Edwards, and will be discussed further later. 

Lubricated squeezing is also attractive because of its simple operation and sample preparation. The flow is homogeneous, but loss of lubricant at relatively low strains is discouraging. The fact that biaxial extension is not as strong a flow as uniaxial reduces the interest in making many biMial experiments. Results to date show no surprises in biaxial response. Current constitutive equations can predict biaxial behavior from shear and uniaxial measures. 

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