Compressible powders, particle

The simplest way of introducing Che pore size distribution into the model is to permit just two possible sizes--Tnlcropores and macropotes--and this simple pore size distribution is not wholly unrealistic, since pelleted materials are prepared by compressing powder particles which are themselves porous on a much smaller scale. The small pores within the powder grains are then the micropores, while the interstices between adjacent grains form the macropores. An early and well known model due to Wakao and Smith [32] represents such a material by the Idealized structure shown in Figure 8,2,... 

Because of the large number of powders studied and their generally refractory character, it appears that it can safely be concluded that substantial particle comminution does not occur in shock-compressed powders. Rather,... 

It is particularly significant that no evidence is found for localized melting at particle interfaces in the inorganic materials studied. Apparently, effects commonly observed in dynamic compaction of low shock viscosity metals are not obtained in the less viscous materials of the present study. To successfully predict the occurrence of localized melting, it appears necessary to develop a more realistic physical model of energy localization in shock-compressed powders. 

The sample format used for XRF measurement is typically compressed fine particle size powder (2-10 mm thick, 20-50 diameter), moulded film or... 

Fluidization The process of suspending powder particles using compressed air, creating a fluid mixture of air and powder. 

The equipment feed design consisted of a funnel-shaped powder hopper (with no vacuum deaeration system) that was located directly above the rolls. The powder hopper was retrofitted with vacuum deaeration capability. A high compression feed screw, fitted inside the funnel hopper, fed the powder directly into knurled rolls. The compaction trials were conducted with and without vacuum deaeration. The compact was carefully collected directly on a 10-mesh screen. Powder particles, that were not compacted (i.e., those particles which were not attached to the compact, for example, fines bypassing roll compaction and the non-adhering compacted powder particles) were weighed and separated. The compact was not milled. The parameters are noted in Table 4. 

Large-pore glasses, wide-pore Xerogels and compressed powders made from nonporous particles ( -100A in size and specific surface areas <300 m2/g). 

For ideal mixtures there is a simple relationship between the measurable ultrasonic parameters and the concentration of the component phases. Thus ultrasound can be used to determine their composition once the properties of the component phases are known. Mixtures of triglyceride oils behave approximately as ideal mixtures and their ultrasonic properties can be modeled by the above equations [19]. Emulsions and suspensions where scattering is not appreciable can also be described using this approach [20]. In these systems the adiabatic compressibility of particles suspended in a liquid can be determined by measuring the ultrasonic velocity and the density. This is particularly useful for materials where it is difficult to determine the adiabatic compressibility directly, e.g., powders, biopolymer or granular materials. Deviations from equations 11 - 13 in non-ideal mixtures can be used to provide information about the non-ideality of a system. 

When manufacturing a tablet formulation by direct compression, the particle size and size distribution of excipients have a significant impact on blending homogeneity, powder segregation, and flowability. This can result in unacceptable content uniformity and high tablet weight variation. In such situations, control of excipients can be critical to product quality. 

Second, movement of both punches between the compression rollers compresses powders in the die. The distance between the punch tips decreases under compression force and progressively reduces the porosity of the powder bed in the die. Once the particles are close enough together, interparticulate forces lead to bond formation and the individual particles aggregate, forming a tablet. 

Tablet binder Substance used to cause adhesion of powder particles in tablet granulations Acacia, alginic acid, carboxymethylcellulose sodium compressible sugar, ethylcellulose gelatin, liquid glucose, metylcellulose povidone, pregelatinized starch...

With this correlation we can describe the most important group of catalysts, namely all metal and metal oxide catalysts (which are produced by compressing powders at different pressures). In most cases, the primary particles are already porous themselves, so bimodal pore-size distributions are obtained. The authors recommend a value of 1.05 when m cannot be determined experimentally. For a large series containing widely varying data, values of m between 0.70 and 1.65 have been observed. 

Considering a mass of ceramic powder about to be molded or pressed into shape, the forces necessary and the speeds possible are determined by mechanical properties of the diy powder, paste, or suspension. For any material, the elastic moduli for tension (Young s modulus), shear, and bulk compression are the mechanical properties of interest. These mechanical properties are schematically shown in Figure 12.1 with their defining equations. These moduli are mechanical characteristics of elastic materials in general and are applicable at relatively low applied forces for ceramic powders. At higher applied forces, nonlinear behavior results, comprising the flow of the ceramic powder particles over one another, plastic deformation of the particles, and rupture of... 

Precompression is often used to tamp or apply a small compression force prior to the main compact compression cycle. Rotary tablet presses are often equipped with a separate precompression station, which is positioned between the die-filling feed frame and the main compression station. Typically, precompression is used to improve the quality of tableted products, where it increases the strength of the compact and/or decreases the incidences of capping and lamination. The compact strength is enhanced through the increase in the effective contact time in which the powder particles are in contact under an applied force. During this extended contact time, stronger interparticulate bonds form and stress relaxation occurs. 

The first method, wettability, can be evaluated from the contact angle of a drop of liquid deposited on the flat surface of the solid. This method hardly applies to powders like silicas because special care must be taken to control the surface porosity of a silica disk made from compressed silica particles. For a chromatographic silica, Kessaissia et al. (1) determined a Ys value close to 100 mJ/m2 whereas the polar component of the surface energy was found to be 46 mJ/m. Hence, the silica exhibits a large surface energy. 

Properties White powder particle diameter approximately 1 micron. Mp 3000C (sublimes). Graphite-like, hexagonal plate structure. High electrical resistance. Compressed at 106 psi, it becomes hard as diamond. Excellent heat-shock resistance, low mechanical strength, hygroscopic. Noncombustible. 

Major components of the pressed powder blusher include the powder phase and binder (or oil) phase. Components of the powder phase include mineral powder fillers such as talc (a magnesium silicate), mica (a magnesium ahuninum silicate), sericite (a form of hydrated mica), and kaolin (known as China clay). Talc is the most popular of all fillers used, as it tends to be virtually transparent depending on the particle size, and it is very soft to the touch. Although mica is also used extensively as a result of its transparency and smooth texture, it often exhibits a shiny appearance with skin application and has poor compression characteristics when used in a pressed powder. Modern non-oil control formulations rarely contain kaolin, as it tends to exhibit course texture, an extremely matte appearance, and excessive oil-absorption capabilities. Dry binders are also used to allow tlie compressed powder to retain form these include metallic soaps such as zinc stearate and magnesium stearate and... 

Flow problems are mainly dependent on interparticle/intraparticle forces, powder particle size and shape, and moisture and fat content. Conditioners (or anticaking agents) enhance powder flow by reducing interparticle force cohesiveness and compressibility while increasing bulk density (Peleg and Manheim, 1973). Peleg et al. (1973) showed that as concentrations of stearate or silicate (added to sucrose) were increased from 1% to 3%, there was no reduction in cohesiveness at agent concentrations of 1-2%, but cohesiveness decreased as more flow conditioner was added. 

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