Wet Mass Rheology

Deformability and Wet Mass Rheology The static yield stress of wet compacts has previously been reported in Fig. 21-113. However, the dependence of interparticle forces on shear rate clearly impacts wet mass rheology and therefore deformabihty. Figure 21-117 illustrates the dynamic stress-strain response of compacts, demonstrating that the peak flow or yield stress increases proportionally with compression velocity [Iveson et al., Powder Technol., 127, 149 (2002)]. Peak flow stress of wet unsaturated compacts (initially pendular state) can be seen to also increase with Ca as follows (Fig. 21-118) ... 

Wet mass rheology heavily impacts performance through controlling both developed pressures and extrusion through the die, as in the case of paste extrusion (see Paste Extrusion and Screw and Other Paste Extruders subsections). In addition, the developed pressure in the roller nips behaves in a similar fashion to roll presses (see Roll Presses subsection). 

A torque rheometer is a device that provides an off-line measurement of torque required to rotate the blades of the device and this torque can be used to assess rheological properties of the granulation. It has been extensively used for end-point determination.The torque values thus obtained were termed as measure of wet mass consistency. ° ... 

At the same end-point, no matter how defined, fhe rheological and dimensional properfies of fhe granules are similar. As we will see from fhe examples described below, that means that the density and dynamic viscosity of the wet mass are constant, and the only variables that are left are the process variables, namely batch mass, impeller diameter and speed, and the geometry of the vessel. 

Kleinebudde (140) suggested that with MCC the process of extrusion induced a microcrystallite-ger state in the MCC/water system, which aided the process. Chopra et al. (15) found that for certain formulations, good quality pellets could be prepared without extrusion of the wet mass. Kleinebudde (140) offered no direct experimental evidence that the extrusion in any way changed the crystallinity of the MCC, nor that the system had different rheological properties, nor, for that matter, it was a gel. Thus the whole concept is seriously flawed and, even in a later paper, it was concluded that the use of Fourier transform Raman spectroscopy did not confirm the existence of a microcrystallite-gel (141). The water holding capacity is not the sole property involved, the consistency of the wet mass must also be important, as discussed previously. 

Luukkonen P, Schaefer T. Hellen L. et al. Rheological characterization of microcrystalline cellulose and silicified microcrystalline cellulose wet masses using a mixer torque rheometer. Im J Pharm 1999 188 181-92. 

Within limits a particulate electrorheological fluid (ERF) can have its properties fixed to suit a particular task [102]. This could be done by, say, adjusting the water content within the solid phase of a wet fluid or its volume/mass fraction in relation to the base liquid or by adjusting the particle size distribution thus the rheological behaviour of the mixture will be affected. The same applies to dry (no added water) fluids, which may be polymeric by nature. Surfactants can change performance beneficially. 

In order to explain certain capillary properties of wetted soil, Versluys had cal ailated how small amounts of water are distributed in the interstices between masses of spherical particles. Using this calcubtion, Kruyt and Van Selms could obtain a semi-quantitative explanation of the influence of the third phase on the rheological properties of suspensions. 

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