Other Packing Properties

Compressibility can be defined as the compaction of a powder by tapping or mechanical compression. These processes can occur either unintentionally as a result of handling or transporting, or intentionally as when producing tablets or agglomerates. In the materials processing industry, unintentional compression is normally undesirable, while operations aimed at obtaining defined shapes are usually required in some processes. The former will be 

Sone (1972) reported the following compressibility relationship, by tapping, for food powders  

The constant a in Equations 1.13 and 1.14 represents the asymptotic level of the volume change or, in other words, the level obtained after a large number of tappings or a long time in vibration. The constant b is representative of the rate at which this compaction is achieved, that is, 1/b is the number of vibrations necessary to reach half of the asymptotic change. In general, this form of data presentation is very convenient for systems comparisons since it only involves two constants. 

A very common undesirable aspect of compressibility is its negative influence on flowing capacity. Compression tests have been used widely in pharmaceutics, ceramics, metallurgy, civil engineering, and in the food powder field, as a simple and convenient technique to powder compressibility and flowability. 

The pressure-density for powders in a compression test at low-pressure range can be described by the following equation (Barbosa-Canovas et al., 1987)  

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Titanium has two allotropic modifications (1) alpha form and (2) beta modification. The alpha form has a close-packed hexagonal crystal structure density 4.54 g/cm3 at 20°C and stable up to 882°C. It converts very slowly to a body-centered cubic beta form at 882°C. The density of the beta form is 4.40 g/cm3 at 900°C (estimated). The other physical properties are as follows ... 

The closed-belt (zipper) conveyor of Figure 5.13 is a carrier that is not limited by fineness or packing properties or abrasiveness. Of course, it goes in any direction. It is made in a nominal 4-in. size, with a capacity rating by the manufacturer of 0.07 cuft/ft of travel. The power requirement compares favorably with that of open belt conveyors, so that it is appreciably less than that of other types. The formula is... 

Figure 13.6 shows a schematic for IGC operation. Inverse, in this instance, refers to the observation that the powder is the unknown material, and the vapor that is injected into the column is known, which is inverse to the conditions that exist in traditional gas chromatography. After the initial injection of the known gas probe, the retention time and volume of the probe are measured as it passes through the packed powder bed. The gas probes range from a series of alkanes, which are nonpolar in nature, to polar probes such as chloroform and water. Using these different probes, the acid-base nature of the compound, specific surface energies of adsorption, and other thermodynamic properties are calculated. The governing equations for these calculations are based upon fundamental thermodynamic principles, and reveal a great deal of information about the surface of powder with a relatively simple experimental setup (Fig. 13.6). This technique has been applied to a number of different applications. IGC has been used to detect the following scenarios ...

Polymorphs are typically differentiated by SS-NMR based on their chemical shift differences, but other NMR properties (such as relaxation) can be used as well. Chemical shifts are very sensitive to molecular conformations and crystal packing. In order for the polymorphs to be distinguished by SS-NMR, they must show differences in either conformations or crystal packing. 

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