Crystalline materials milling

When dealing with partially crystalline materials, such as those produced by milling, the effect of water uptake is intensified. The amorphous component likely absorbs greater quantities of water than its crystalline counterpart, leading to reduced Tg, increased molecular mobility, and both physical and chemical instability. 

The technique generally used to minimize the degree of change in crystallinity of the milled product is to eliminate the water or other solvents from the product, usually by packaging the material within a suitable barrier (for example, aluminum foil laminate). Other techniques include the production of a 100% crystalline material, which may eliminate the effects of moisture. This technique, however, may require a secondary production stage of armealing or a period to allow the product to equilibrate under controlled storage conditions. 

High-energy processes (milling, lyophilisation, granulating, drying) can introduce certain amounts of amorphicity into otherwise highly crystalline material [20]. As has been previously indicated, enhanced levels of amorphicity lead to increased local levels of moismre, and increased chemical reactivity in these areas. Hancock and Zografi [49] reported on the impact of a roller-compaction process on the water vapour sorption of a sample of aspirin. They speculated that... 

Boric acid crystals are usually separated from aqueous slurries by centrifugation and dried in rotary driers heated indirecdy by warm air. To avoid overdrying, the product temperature should not exceed 50°C. Powdered and impalpable bone acid are produced by milling the crystalline material. 

Dea lomeration can take place by two methods comminution and ultrasonification. Comminution is the subject of Chapter 4, so it will not be discussed in detail here. In the comminution of aggregates, the popvdation balance can be used to predict the size distribution as a function of time in a batch mill or as a function of mean residence time in a continuous mill. Aggregates have the same type of birth and death fvmctions for particle breakage as polycrystalline particles but the rate constants are much hi er and the size selectivities for aggregates are different than those for the comminution of crystalline materials. The... 

Milling of crystalline materials introduces or increases amorphous character as the result of the significant mechanical activation that takes place during the process, including friction, deformation, attrition, and agglomeration. The extent of disorder, or... 

Particle size distribution the particle size distribution of xylitol depends upon the grade selected. Normal crystalline material typically has a mean particle size of 0.4-0.6 mm. Milled grades are commercially available that offer mean particle sizes as low as 50 pm. Individual suppliers literature... 

These results, i.c., the reduction in intensity of the elemental X-ray diffraction peaks and the formation of an ultralinc layered composite during the early stages of milling, and the coexistence of the quasicrystallinc phase with crystalline material for some period, are quite similar to what is known from amorphous phase formation by mechanical alloying (Sect. 3.3.2). The local... 

An exponent a = 0.1 in Eq. (5.23) means that a size reduction by a factor of 2 requires an energy increase by a factor of approx. 1000. Note that for the dispersion of other materials, e.g., floes of micrometre-sized PS latex (Kobayashi et al. 2004, laminar elongation), micrometre-sized dense agglomerates of crystalline Si02 (Wengeler et al. 2004, rotor-stator and HP systems) and for the grinding of crystalline materials in stirred media mills (e.g. Kwade and Schwedes 2002 Mende et al. 2004), much higher values of a are obtained. 

In this process, the particles themselves can have a size distribution of less than 100 nm, and then be called nanoparticles. Sometimes, as is common in materials milled using mechanical attrition, the product is highly crystalline, and the crystallite size is between 1 and 10 mn in diameter, and the system is then called nanocrystalline materials. 

Although in some cases heterogeneous seeding can be applied, the seeds in this chapter are understood as crystalline material of the solute to be crystallized. These are usually in milled form, both with respect to chemical purity and with respect to the solid-state form (polymorphism or solvate form). 

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