Wall Slip Phenomenon

It is well known that HDPE and PVC, as well as many polymer suspensions and fllled composites, exhibit slip at the capillary wall. Considering slip at the wall, the volumetric flow rate Q through the die is given by 

The first term in Eq. (17.17) is the flow rate dne to shearing, and the second one is the flow rate dne to slip. The apparent shear rate in case of wall slip is given by the Mooney equation [5]  

a plot of y against 1/7 gives % as the intercept and 41 as the slope. This works only if the shear stress is not a function of R that is, why LID ratio of the die shonld be kept constant [6]. 

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Imposition of no-slip velocity conditions at solid walls is based on the assumption that the shear stress at these surfaces always remains below a critical value to allow a complete welting of the wall by the fluid. This iraplie.s that the fluid is constantly sticking to the wall and is moving with a velocity exactly equal to the wall velocity. It is well known that in polymer flow processes the shear stress at the domain walls frequently surpasses the critical threshold and fluid slippage at the solid surfaces occurs. Wall-slip phenomenon is described by Navier s slip condition, which is a relationship between the tangential component of the momentum flux at the wall and the local slip velocity (Sillrman and Scriven, 1980). In a two-dimensional domain this relationship is expressed as... 

An important element in melt fracture is also wall slip phenomenon [5, 49]. It is related to the so-called sharkskin, or sharkskin melt fracture, which is also called surface melt fracture. It is a low amplitude surface distortion of extruded polymer. Sharkskin is generally observed in case of linear polymers with narrow MWD, below the oscillating stick-slip transition. Sometimes (but not always), there is a change... 

Princen [57, 64, 82] and others [84] also noted the presence of wall-slip in rheological experiments on HIPEs and foams. However, instead of attempting to eliminate this phenomenon, Princen [64] employed it to examine the flow properties of the boundary layer between the bulk emulsion and the container walls, and demonstrated the existence of a wall-slip yield stress, below that of the bulk emulsion. This was attributed to roughness of the viscometer walls. Princen and Kiss [57], and others [85], have also showed that wall-slip could be eliminated, up to a certain finite stress value, by roughening the walls of the viscometer. Alternatively [82, 86], it was demonstrated that wall-slip can be corrected for and effectively removed from calculations. Thus, viscometers with smooth walls can be used. This is preferable, as the degree of roughness required to completely eradicate wall-slip is difficult to determine. 

Wall-slip is not an easy phenomenon to detect. Although in principle, the velocity profile should reveal whether or not the fluid velocity is zero at the stationary wall, in reality determining the velocity profile with sufficient resolution near the wall is very difficult. So alternate means, e.g., checking for viscosity variation with appropriate changes in the test geometry, are also widely used in practice. 

To date, most wall-slip studies have concentrated on either eliminating or quantifying slip effects in laboratory rheometry. Slip effects have been traditionally treated as flow anomalies, which make the measurement of material properties a challenging task. There is less evidence on how to utilize this flow anomaly to industry s advantage. A comprehensive understanding of the slip phenomenon as it relates to food texture and food processing is of crucial importance to the food industry, where the goal is to produce quality products at reduced cost... 

The extent to which die-wall slip is involved can be assessed by using dies of different lengths and diameters. An important characteristic that can be observed in the extrudate is its quality in terms of surface structure. Harrison, Newton, and Rowe, have shown how this can vary from a smooth, regular surface via a rough, shark-skinned extrudate. There is obvious need to prevent this phenomenon if extrudate of the correct quality is to be produced. The occurrence of the surface defects is associated with both the composition of the material and the operating conditions (e.g., die length and diameter and the rate of... 

Capillary rheometry and parallel-plate rheometry use the fact that wall slip will manifest itself as a geometry-dependent phenomenon. That is, wall slip will appear as a geometric effect on apparent rheological properties. In the capillary-rheometer technique, slip will manifest itself as an effect of capillary diameter ( )) on the shear stress (t, ). Wall slip in capillary rheology can be calculated from an analysis that involves the following ... 

In the capillary viscometer, the slip phenomena can also be observed due to inhomogenous flow (Cohen and Metzner, 1986 de Vargas et ai, 1993). Therefore, the influence of the slip phenomenon must be considered when analyzing the data. In the case of 0.2% xanthan gum solution, the slip velocity is an increasing function of the wall shear stress and also of the length to diameter ratio UD. However, the slip velocity becomes independent of UD at large UD (de Vargas et ai, 1993). 

An example for self-excited vibrations is the stick-slip phenomenon. In machining applications, stick-slip typically arises at the glides. The mechanical model can be seen in Fig. 12. The block of mass m is fixed to the moving wall by a spring of stiffness k and a dashpot of damping c. The wall is moving with velocity vq-The friction force acting on the block is... 

The occurrence of this phenomenon may be tested by comparing the viscosity ftmctions obtained using capillaries of similar length-to-radius ratios, L/R, but of different radii. Any apparent wall slip may then be corrected for and the true viscosity of the fluid determined by extrpolating the results obtained to infinite pipe diameter. In the relation developed by Mooney [1931], apparent wall shear rates obtained for constant length-to-radius ratio are plotted against L/K). 

EFFECT OF DIE DESIGN AND WALL SLIP ON DIE DROOL PHENOMENON FOR METALLOCENE BASED LLDPE THEORETICAL AND EXPERIMENTAL... 

The effect of die design and wall slip on the die drool phenomenon was investigated for metallocene based LLDPE. It has been found that die exit opening and wall slip can significantly reduce the die drool phenomenon. Moreover, theoretical research has revealed that die drool onset can be explained by the negative/non-monotonic pressure profile generated inside the die and/or at the die exit region due to melt elasticity. 

It has been found theoretically that introducing the wall slip at the end of the die leads to significant reduction of the negative pressure. In more detail, it has been revealed that during extrusion of mLLDPE Exact 0201 (1 kg/h, 90°C), the introduction of the wall slip (melt velocity at the die exit wall was equal to 0.016667 m/s) causes reduction of the negative pressure from 100% down to 60%. It means that the use of proper type of PPA in extrusion of mLLDPEs leads to significant reduction of the die drool phenomenon which is in good correspondence with the experimental reality. 

It has been revealed theoretically that wall slip reduces the negative pressnre at the die exit region which is stabilizing effect from the die drool phenomenon point of view. 

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