Mixing dissimilar particles

Particulate Composites. In addition to the geometric obstacles to mixing dissimilar shape and size particles, there are also chemical barriers that must be overcome in processing composites, requiring novel techniques. 

The axial voidage distribution resulting from mixing of dissimilar particles, as shown in Fig. 26, will now be examined in the light of the dynamics of these particles at the interfacial boundary between the properly juxtaposed phases of a binary particle mixture (Lou, 1964 Kwauk, 1973). Both the lighter particles at the top and the heavier particles at the bottom share the same tendency of invading the region occupied by the other. This behavior conforms to the concept of random walk, for which Fick s law can be adapted to describe the macroscopic diffusion flux of the smaller particles 2 ... 

A mixture is a combination of two or more pure substances in which each substance retains its own identity. Alcohol and water can be combined in a mixture. They coexist as pure substances because they do not imdergo a chemical reaction they exist as thoroughly mixed discrete molecules. This collection of dissimilar particles is the mixture. A mixture has variable composition there are an infinite number of combinations of quantities of alcohol and water that can be mixed. For example, the mixture may contain a small amount of alcohol and a large amount of water or vice versa. Each is, however, an alcohol-water mixture. 

Separation Prevention of the inter-mixing of particles from dissimilar soil layers, commonly a fine-grained soil and a granular drainage soil. 

Complex as this joint process of mixing and segregation is, the mechanism is best understood by studying the simplest case of a binary mixture of two kinds of solid particles having dissimilar properties, e.g., density and/or particle size. 

Fluidized and transport bed processes have also been developed for better management of the heat released. The former prevents the occurrence of hot spots in the catalyst bed through a more uniform temperature profile. The concentration of the -butane can also be higher, even within the explosion limits, thanks to the barrier to flame propagation constituted by the fluidized bed of particles. Selectivity is not however dissimilar to that in fixed bed operation due to considerable back-mixing of the products and longer residence times. 

To account for scale-up effects, especially using dissimilar equipment, formulators have employed a variety of approaches. Typically, trial and error adjustments, empirical correlations between known machines, and several geometric rules of thumb have been devised. Usually, trial and error approaches will alter the amount of granulation liquid used, the post-liquid addition mixing time kneading, or the liquid addition rate to arrive at an acceptable formulation upon scale-up. Although particle size distribution and bulk/tap density measurements may be used to establish equivalence in these methods, often visual judgments are made by experienced operators on the floor with a feel for proper appearance and manual consistency. 

The most common method of stabilizing latex particles is by addition of mixed anionic and nonionic surfactants. The key property of a siufactant is that it possesses chemically dissimilar groups one hydrophobic and one hydrophilic. The hydrophobic group is physically adsorbed onto the polymer latex particle while the hydrophilic portion extends into the aqueous phase. It is the hydrophilic groups that provide the stabilization. 

Polymer blends and compatibilizing agents historically have been the subject of a wide variety of studies and an extensive body of literature on these materials exists. Without specific chenucal interactions between dissimilar polymers, most polymer mixtures tend to phase separate due to the unfavorable entropy of mixing between the polymer chains. Efforts to control or retard the phase separation process have led to the research and development of compatibilizing agents for polymer blends. For a variety of systems the dispersed phase particle size has been found to decrease with increasing copolymer concentration. Above a critical concentration of copolymer, the size of the dispersed phase remains constant. 

---