BLENDING OF SOLID PARTICLES

In the mixing of solid particles, the following three mechanisms may be involved  

These mechanisms operate to varying extents in different kinds of mixers and with different kinds of particles. A trough mixer with a ribbon spiral involves almost pure convective mixing, and a simple barrel-mixer involves mainly a form of diffusion mixing. 

The mixing of pastes is discussed in the section on Non-Newtonian Technology in Volume 1, Chapter 7. 

This equation illustrates the importance of the size of the sample in relation to the size of the particles. In an incompletely randomised material, s2 will be greater, and in a completely unmixed system, indicated by the suffix 0, it may be shown that  

with convective mixing, 1 — b depends on the size of the sample. 

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Alternatives to compounding in the melt are solution mixing or powder blending of solid particles. Mixing with the aid of solvents can be performed at lower temperatures with minimal shear. However, difficulties in removal of the solvent results in plasticization of tJie polymer matrix and altered erosion/drug release performance in addition to residual solvent toxicity concerns. Powder blending at room temperature minimizes thermal/shear stresses, but achieving intimate mixtures is difficult. 

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