Some Unique Properties of Particulate Solids

All the preceding particulate handling steps are affected by the unique properties of all particulates, including polymeric particulates while they may behave in a fluidlike fashion when they are dry, fluidized and above 100 pm, they also exhibit solidlike behavior, because of the solid-solid interparticle and particle-vessel friction coefficients. The simplest and most common example of the hermaphroditic solid/ fluidlike nature of particulates is the pouring of particulates out of a container (fluidlike behavior) onto a flat surface, whereupon they assume a stable-mount, solidlike behavior, shown in Fig. 4.2. This particulate mount supports shear stresses without flowing and, thus by definition, it is a solid. The angle of repose, shown below, reflects the static equilibrium between unconfined loose particulates. 

Solidlike behavior abounds when the surface-to-volume ratio is very high,1 that is, when the particulates are even mildly compacted, surface-charged, or wet all contribute to large frictional forces and to nonuniform, often unstable stress fields in both flowing and compacted particulate assemblies, as we discuss later in this chapter. We begin by discussing some of the unique properties of polymer particulates relevant to processing. Comprehensive reviews can be found in the literature f 1 —4). 

Scientific and engineering investigations into the properties and behavior of particulate solids date back to the early work of Coulomb, who in 1776 developed a theory on soil pressure and resistance, thus laying sound foundations for important engineering 

Pellets, compared to fine powders, with low surface-to-volume ratios, are readily flowable, easily fluidized and meter- or hopper-fed. These attributes justify pelletization. 

Reynolds, On Dilatancy of Media Composed of Rigid Particles in Contact. With Experimental Illustrations. Philos. Mag., Ser. 5, 20, 469 (1885). 

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