Blend flow

Flowability is important to the successful scale-up of a tableting process. The rate at which the precompacted blend flows into the hoppers of the tablet press and subsequently into the die cavity could be crucial to dose uniformity. Three measurements are most commonly used to measure flow ... 

The rate of tlie compaction process is another variable that should be considered throughout development, including scale-up. Typically, the development of a tablet formulation takes place on tablet presses that are relatively slow. The tableting rate is important to consider for several reasons. Blend flow is important to ensure bulk blend transfer into the tablet press (hopper) and consistent die fiU. Variation or difficulty in the bulk flow and die fill can contribute to tablet weight variation. As the compaction rate increases, the blended material must be able to... 

As felt above, the concept of powder blend homogeneity cannot be dissociated from the scale at which a mixture is observed. For continuous mixers, homogeneity must be defined at the level of the blend flowing out of the vessel, and falling down to a conveyor or any other transportation equipment. In the case of a batch mixer, the mixture structure can be particularly complex to define, but in practice and when the mixer is emptied, the content of the vessel is discharged through a pipe to a container or a tableting machine. Hence, in both cases and if we take into account the entire cross-section of the pipe, the mixture flow can be assimilated as a 1-D particle stream. 

The first steps toward such a theory of blend flow behavior were proposed by Helfand and Fredrickson [1989], then by Doi and Onuki [1992]. A greatly simplified constitutive equation for immiscible 1 1 mixture of two Newtonian fluids having the same viscosity and density was also derived [Doi and Ohta, 1991]. The derivation considered time evolution of the area and orientation of the interface in flow, as well as the interfacial tension effects. The relation predicted scaling behavior for the stress and the velocity gradient tensors ... 

PPE/PS mixtures are considered classical examples of miscible polymer blends. Within the accessible range of temperatures, single-phase melts have been observed with the size of homogeneity below 20 nm. Dynamic flow behavior of PPE/PS blends, with molecular weight ratio MW(PS)/MW(PPE) = 1, was studied in a wide range of temperatures and compositions (Prest and Porter 1972). The authors assumed additivity of the free volumes of the components and characterized the blend flow behavicM under iso-free volume conditions. Increasing PPE content... 

In miscible blends, flow behavior depends on free volume, entanglements, and specific interactions. From Doolittle s equation, assuming additivity of the occupied volume and non-additivity of the free volume, Steller and Zuchowska (1990) derived ... 

During the last few years, there has been significant progress in the fundamental understanding of micro-rheology and its role for polymer blending. Flow visualizations of the Newtonian and non-Newtonian systems have been carried out (Mighri et al. 1996 Picot 1997 Bourry et al. 1999). The boundary element method (BEM)... 

Bousmina, M., Palierne, J.F., and Utracki, L.A. (1998) Modeling of immiscible polymer blends flow in laminar shear field. Polymer Engng Sci., (in press). 

Apart from the influence of temperature on the thermodynamic state of a polymer blend, flow also affects the equilibrium position. Both flow-induced mixing and demixing have been observed. Nakatani et al. [31] have published results on polystyrene (PS)-poly(vinyl methyl ether) blends where the PS was... 

Sakellarides SL, McHugh AJ. Structure formation during polymer blend flows. Polym Eng Sci 1987 27 1662-74. 

Figure 7-22 Blending (simple distributive mixing) and dispersive mixing (a) blending flow streams are interleaved (b) dispersive mixing one phase is discontinuous.

They have demonstrated excellent performance characteristics in rigid PVC compounding, extruding, and wlding, where they serve to reduce friction and plate-out, ioq>rove powder-blend flow, regulate fusion rates, and enhance heat stability. 

If statements provided above are justified, than according to [155], there must always be some threshold concentration and temperature values where the flow mechanism of blended systems changes. This conclusion has been confirmed experimentally the curves of functions - T for the studied compositions have well-shown bend, where viscosity drop rate changes. The processing of these data involving peak Newtonian viscosity ( 7n), obtained by extrapolation to zero shear rate, demonstrates (Figure 3.40), that dependencies Ig n on HT can be approximated by two linear sections - 20-40°C and 50-80°C, that can be the effect of the blend flow mechanism varied near 45 °C. 

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