ABSTRACT

FIGURE 17.3 Schematic die land insert design for screening materials of construction. (From Ref. 18.)

5 10 50 100 500 "Yn (s-')

FIGURE 17.4 The effect of gap spacing on the flow curve of HOPE. (From Ref. 20.)

where Vis the velocity of the moving plate and h is the gap spacing. Rearranging Equation I:

V . 2u, - = y +- h h

So a straight line should result if Vlh. or the nominal shear rate y,. is plotted against the reciprocal of h while keeping the shear stress unchanged. This is because a constant value of the shear stress implies a constant value of the true shear rate. The slope of this straight-line plot should equal 2u,. Hatzik:iriakos and Dealy found that HDPE slip velocity determined in this manner was a unique function of the wall-shear stress but depended on the temperature of measurement. Their results for the slip velocity are displayed in Fig. 17.5. and these reveal a power-law relationship between slip velocity and the wall-shear stress. These and other authors have also proposed models to relate the slip velocity, under both steady-and transient-stress conditions, to temperature, pressure, flow geometry, and bulk rheology of the polymers examined [21-24].