Dispersed solids melting

As mentioned earlier, the CSM type melting is the one that has been most often observed in single screw extruders. However, in some experimental studies it was found that the solid particles are not contiguous but dispersed in a melt matrix [256-258] as illustrated in Fig. 7.47. 

In fact, in several extruders, dispersed solids melting (DS) is more likely to occur than contiguous solids melting. This is particularly true of extruders that do not feature a continuous, uninterrupted channel for material flow. Examples are twin screw extruders and pin barrel extruders. 

Rauwendaal [259] presented the first theoretical model of DSM that allows analytical description of the melting process. In the following, this theory will be described in detail. Predictions from the DSM theory will be compared to the CSM theory. 

In the DSM model it will be assumed that the solid particles are uniform, spherical, and dispersed in a melt matrix. This means a minimum volume of polymer melt has to be available to fill the space between the solid particles. For the closest packing of regular spheres the minimum polymer melt volume fraction is about 40%. For more random packing configurations it will be closer to 50%. This means that the DSM theory can only be applied after about half the solid material has already melted. 

The viscous heat generation per unit volume for a power law fluid can be written as  

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As discussed, the melting in twin screw extruders occurs by dispersed solids melting. A theoretical model of dispersed solids melting will be presented in Section 7.3.5. 

Good melting dispersed solids melting mechanism... 

Fig. 38. Dispersed solids melting in single screw extruders.

Phase changes, which convert a substance from one phase to another, have characteristic thermodynamic properties Any change from a more constrained phase to a less constrained phase increases both the enthalpy and the entropy of the substance. Recall from our description of phase changes in Chapter 11 that enthalpy increases because energy must be provided to overcome the intermolecular forces that hold the molecules in the more constrained phase. Entropy increases because the molecules are more dispersed in the less constrained phase. Thus, when a solid melts or sublimes or a liquid vaporizes, both A H and A S are positive. Figure 14-18 summarizes these features. 

Numerous processes for powder generation using supercritical fluids have been developed. The specific properties of dense gases allow obtaining fine dispersed solids, especially of substances with low melting point temperatures, high viscosities and very waxy or sticky properties. Economic evaluation of the process shows that these compounds cannot be efficiently and economically processed by conventioned mechanical processes and there is a big advantage of the use of supercritical fluids. 

In-process controls such as stratined sampling, process analytical technology (PAT) application, and blend homogeneity. Assess modincation of dissolution through optimization of API characteristics and then perform assessment of specialized technologies [hot-melt extrusion (HME), spray-dried dispersion, solid dispersion, etc.] for long-term resolution... 

It is made by dimerizing cyanamide in basic aqueous solution, and is a colorless solid melting at 208°C. Dicyandiamide is soluble in polar solvents, but at room temperature is insoluble in bisphenol A epoxy resins. It can be made into a very fine powder and milled into epoxy resins to form stable dispersions. Because the dicy is insoluble in the epoxy, the only possible reaction sites are at the particle surfaces. Although some reaction certainly occurs over a short time, the adhesives easily can have a useful shelf life of six months. On heating to about 150°C, the dicyandiamide becomes soluble in the epoxy resin, and the adhesive polymerizes rapidly. Cure can be accelerated by incorporation of tertiary aromatic amines or substituted ureas. 

Sodium metal may be dispersed by melting on various supporting solids (sodium carbonate, kieselguhr, etc.) or by high-speed stirring of a suspension of the metal in various hydrocarbon solvents held just above the melting point of the metal. Dispersions of the latter type may be poured in air, and they react with water only with effervescence. They are often used synthetically where sodium shot or lumps would react too slowly. Sodium and potassium, when dispersed on supports such as carbon, alumina, or silica are often more reactive than the metals. 

Ointments are prepared by melting together the active ingredient with a base, such as petroleum derivative or wax. The powdered drug components are added while stirring and the mixture is cooled. The product then is passed through a roller mill to achieve the particle size range desired for the dispersed solid. 

The principle of improving the solubility of poorly water-soluble drugs using solid dispersions has already been presented. One common method of preparing solid dispersions is melt-congealing. The carriers used to make solid dispersions of poorly water-soluble drugs have also been addressed. 

Hirasawa and coworkers prepared a naproxen solid dispersion by melting, followed by rapid cooling with liquid nitrogen using lactose as a carrier. The dissolution studies of naproxen indicated that the dissolution rate was markedly increased in solid dispersions compared with physical mixtures and pure drugs. 

Kennedy, J.P. Niebergall, P.J. Development and optimization of a solid dispersion hot-melt fluid bed coating method. Pharm. Dev. Technol. 1996,1 (1), 51-62. 

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