Fine powders, typical properties

However, a pelletized or extruded catalyst prepared by compacting fine powder typically exhibits a bimodal (macro-micro) pore-size distribution, in which case the mean pore radius is an inappropriate representation of the micropores. There are several analytical approaches and models in the literature which represent pelletized catalysts, but they involve complicated diffusion equations and may require the knowledge of diffusion coefficients and void fractions for both micro- and macro-pores [31]. An easier and more pragmatic approach is to consider the dimensional properties of the fine particles constituting the pellet and use the average pore size of only the micropore system because diffusional resistances will be significantly higher in the micropores than in the macropores. This conservative approach will also tend to underestimate Detr values and provide an upper limit for the W-P criterion. 

Gravimetric hoppers feed a mold with a prescribed weight of polymer. The feed stock is either a finely divided powder or a liquid plastisol. A plastisol is a suspension of a resin powder, typically polyvinyl chloride, in a plasticizer, used to manufacture. Rotational molders use liquids and powders in their process since both flow freely. This property permits the easy addition of the materials to the mold. More importantly, they flow smoothly around the interior of the mold as it rotates. In doing so, they coat the entire surface. 

Lubricants enable P ILE fine powders to be processed on commercial equipment. Liquids with a viscosity between 0.5 and 5 cP are preferred, although more viscous liquids are used occasionally. When selecting a lubricant, its ability to be incorporated easily into the blend and to vaporize completely and rapidly in a later processing step without leaving residues that would discolor the product or adversely affect its properties is important. The amounts of lubricant added are typically 16 to 19% of the total weight of the mix. 

Typical properties of thermoplastic hinders are summarized in Table 3.8. Application is mainly by fluidized bed but includes application by flocking. Electrostatic spraying is also used for finely ground powders. The main uses are in the interior sector. The PVC powders are the most widely used thermoplastic binders for exterior use. 

A common point of discontent with size analysis using sieves is that the process requires quite a bit of preparatory work, weighing, and subsequent washing. Yet, a typical analysis would yield only seven to eight points on the size distribution and this may not be sufficiently discriminating. Nevertheless, sieving is a straightforward and robust technique suitable for a wide variety of fine to very coarse powders. Material properties such as density, optical property, water solubility, or conductivity are not required for computation of the particle size. 

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