Bulk Density and Compaction

To illustrate the compaction process that occurs in an extruder, a Maddock solidification [1] experiment (described in detail in Section 10.3.1) was performed using a 63.5 mm diameter machine [2]. The extruder was operated at a screw speed of 60 rpm with a poly(vinylidene chloride) copolymer (PVDC) powder. After the extruder reached a steady-state operation, screw rotation was stopped and full cooling was applied to the extruder. After several hours of cooling, the screw and PVDC resin were removed from the extruder and the density of the bed was measured using Archimedes s principle. The compaction phenomenon in the extruder is shown by the density measurements of the solid bed in Fig. 4.1. As shown in this figure, the density of the solid bed increased from the feedstock bulk density of 0.73 g/cm to nearly the solid density of 1.7 g/cmT 

The bulk density of the feedstock at ambient temperature and pressure should be measured prior to the design of a new screw, especially if it contains in-plant recycle resin. The measurement method is extremely simple and requires only a calibrated cell and a scale. A calibrated measuring cell with a volume of 500 cm can easily be constructed by welding a thin-walled metal pipe to a flat sheet of metal, as shown in Fig. 4.2. The bulk density is measured by filling the cell with feedstock, leveling the top with a steel ruler, and then weighing the cell contents. A more formal measurement technique was developed by ASTM as standard method D1895. 

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In this chapter the physical properties of resins related to processing will be described. The chapter and Appendix A4 include physical properties for many resins. These properties include bulk density and compaction, lateral stress ratio, stress at a sliding interface, melting flux, heat capacity, thermal conductivity, and melt density. Some of these properties are easy to measure by many laboratories while others such as the melting flux and stress at a sliding interface can be measured in only a few places using highly specialized equipment. 

Zuurman K, Riepma KA, Bolhuis GK, et al. The relationship between bulk density and compactibility of lactose granulations. Int ] Pharm 1994 102 1-9. 

The roller compactor s full axial rolls produced compacts in the form of sticks. The unmilled compacts from both machines had the same density, 1.3g/cm. The milled roller compacts produced comparative granules except for bulk density and correspondingly the Carr index values ... 

The behaviour of powders is often quite different from the behaviour of liquids and gases. Engineers and scientists are used to dealing with liquids and gases whose properties can be readily measured, tabulated and even calculated. With particle systems the picture is quite different. The flow properties of certain powders may depend not only on the particle size, size distribution and shape, but also on surface properties, on the humidity of the atmosphere and the state of compaction of the powder. These variables are not easy to characterise and so their influence on the flow properties is difficult to predict. In the case of particle systems it is almost always necessary to perform appropriate measurements on the actual powder in question rather than to rely on tabulated data. The measurements made are generally measurements of bulk properties, such as bulk density and shear stress. 

Based on the definition of density, two new terms are defined. Porosity is defined as the proportion of a powder bed or compact that is occupied by pores and is a measure of the packing efficiency of a powder and relative density is the ratio of the measured bulk density and the true density ... 

As with bulk density, permeability is a function ofpacking voidage and its uniformity, and in practice, it is best measured. It can vary substantially with previous compaction of the sample. An example is the change in bulk density—and therefore interstitial voidage—that occurs with a material as it moves through a hopper. By applying a load to the upper surface of the bed, permeability may be also determined as a function of solids consolidation pressure (see Bulk Flow Properties ). Permeability is a decreasing function of applied solids pressure, and bulk density is often written in log form, or... 

The postmining land-use objective must be selected with the characteristics of the topsoil in mind. Chemical soil properties are more easily amended than physical properties such as texture. Thus, when adequate topsoil is not available, the substitute material must be selected with special attention to texture, coarse-fragment content, and mineral content. Bulk density and soil strength must be considered during soil handling. Methods and equipment used for redistribution of topsoil must be selected to minimize compaction, which can inhibit root penetration and movement of air and water. 

Bulk density (Dj,) is defined as the mass (weight) of a rmit volume of diy soil. This volume includes both solid and pores. The values for clay, clay loam and silt loam surface soils varies from 1.00 - 1.60 g/cm, for sand and sandy loams from 1.20 - 1.80 g/cm. Fine - texture soils tend to have lower bulk densities and therefore higher porosities in comparison to the coarse textured soils due to loose packing of the clay particles. Bulk density measurement for soils is important since it determines the degree of compactness as a measure for soil structure and is used for calculating pore space of soils. 

In general terms, compressibility refers to the variation in bulk density with respect to consolidating confined pressure acting on a powder bed. Bulk density (in terms of apparent, compact, or tap density) and normal stress have been associated in empirical logarithmic or semilogarithmic relationships, from which a constant slope value is defined as mechanical compressibility. Simultaneous decrease in a powder s loose bulk density and increase in compressibility indicate greater attractive and cohesive interactions among powders. 

However, bulk density does not measure the particle compaction which occurs when the particles rearrange themselves or are deformed. Thus, a second measure, compressibility, is the percent difference between the loosely packed bulk density and the packed bulk density. When the compressibility is less than 20 percent, particles are free flowing, whereas at values from 20 to 40 percent, the particles are prone to packing during storage. Compressibilities in excess of 40 percent indicate that the material compacts easily and will probably not flow from the hopper without assistance. In addi-... 

Compaction, that is, redaction of voids and increase in bulk density and bond formation... 

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