Types of Powders

Silica powders can be classified according to the way in which they have been made, which usually confers special characteristics. 

Pulverized gels. Made by grinding or micronizing xerogels. Ultimate gel structures remain unchanged. 

Spheroidal gels. Made by subdividing the precursor silicic acid or colloidal sol into fine droplets before gelling. Gel structure is similar to massive gels formed under the same conditions. The droplets may be suspended in air and dried or may be sus- 

Aerosils or pyrogenic silica. Powders made by condensing silica from the vapor phase at elevated temperature. The silica vapor is produced by a) direct volatilization of SiOj, (6) reduction of SiOj to volatile SiO which is reoxidized, (c) oxidation of volatile silicon compounds such as chloride or esters, or (d) vapor phase hydrolysis of SiF.  

Phyllosilicas (leaflike) or lepidoidal (scalelike) silicas. Less well known and ill-defined, but generally consisting of particles with a laminary or sheetlike shape. These are of several types, depending on how they are made, as described in Chapter 1. 

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The paint is then cured in an oven. The curing time will depend upon the thickness of coat, shape of the workpiece and type of oven and its effectiveness. Generally, a coat of up to 60-70 microns at a stoving temperature of 180-200 C (depending upon the type of powder) should take around 10-12 minutes to cure. Contact the manufacturers for exact details. 

Plate-out is observed with various hardeners if P.V.23 is incorporated in most types of powder coatings, for instance in epoxy systems. This phenomenon, however, is of no consequence for the applicability of the pigment only minor amounts of P.V.23 are required in these media to lend a blue shade to white enamels. 

The powder is supplied to the gun by a powder pump at a uniform and consisfent rate. There are two types of powder pumps ejector type, using the venturi principle, and auger pumps, which use positive displacement. A corona charging system is most widely used to electrostatically charge the powder. The electrostatic spray gun has several functions ... 

During the last hours of polishing the side door in the drum is opened so that some of the moisture escapes. The overall polishing process lasts for 4-24 hr, depending on the type of powder. The finished grains are more slippery, pour more easily and fill space better than the unpolished product. 

Type of powder Way of loading Density Rate of detonation m/sec... 

Type of powder Charge Pressure kg/cm2 Calculated temperature °C Erosion mm3/g... 

German double base powders are also marked with figures or letters indicating their calorific values. Nitrocellulose powders had no such marks, as their calorific value is approximately the same in all types of powder. 

The standard specifications permit the moisture content to range within certain limits. This enables the producer to regulate the moisture content to obtain the required ballistic properties. If the powder is too vivacious it should be moistened to the upper limit, while if it is less vivacious, the lower limit is preferable. In determining the moisture content the following practical rule should be observed. In rifle powder an increase of the moisture content by 0.01% lowers the muzzle velocity v0 by 4-5 m/sec and the pressure by 50-70 kg/cm2. In cannon powder the variations of ballistic properties are smaller and depend on the type of powder and on the calibre of the gun. 

Waste materials from soaked powder are used chiefly for preliminary ballistic tests. They can also be converted into less valuable types of powder, earmarked for rapid consumption (practice or sporting powders). Such powders are treated with solvent in the same way as waste products (4), although they require a longer time to dissolve. Thus, during the World Wars, when powders were consumed rapidly, soaked waste products were utilized for this purpose. 

In the literature, one can find other empirical or semi-empirical equations representing the kinetics of powder reactions. One can certainly take into account grain size distribution, contact probability, deviations from the spherical shape, etc. in a better way than Carter has done. Even more important are parameters such as evaporation rate, gas transport, surface diffusion, and interface transport in this context. As long as these parameters are neglected in quantitative work, the kinetic equations are inadequate. Nevertheless, considering its technological relevance, a particular type of powder reaction will be discussed in the next section. 

A BkPdr used in safety fuses must consist of fine, uniformly grained particles free from dust. This type of powder must be of special quality and constant compn, contg, in some cases, various additives for controlling the rate of burning... 

Powder pressing is a very flexible process in that materials that are difficult to melt or deform may be produced in a variety of shapes and sizes. Adding to the flexibility of the process may be the fact that the starting powder does not need to be uniform. Different size powders, or more commonly, different types of powders may be used to produce composite microstructures. These different powders may be distributed uniformly throughout the fabricated component, or isolated to form a functionally graded material. Powder pressing is commonly used to... 

The procedure for the preparation of alumina spheres is as follows. Pseudo-boehmite powder (AIOOH.2H2O) is dispersed in an aqueous solution of urea and a monovalent inorganic acid, e.g. HNO3. The type of powder or powder mixture may exert considerable influence on the properties of the sol and the end product [12]. The type of acid which is used for the peptization of the pseudoboehmite powder is not very important [12], so nitric acid is used, being the most suitable one. 

Certain types of powder-form explosives contain moisture repelling additives such as stearates in paraffinated cartridges they can be applied even under wet conditions. Non-cartridged powder form explosives must be free-flowing (-> ANFO). 

For different values of n ing(i ) = i2", other kinetic expressions can be developed. Figure 8.10 [18] shows the type of powder produced on spray diydng a solution that consists of metal salts of barium and iron in the ratio 1 12 (i.e., barium ferrite). Here we see the remains of the spherical droplets with a surface that consists of the metal salt precipitates, which form a narrow size distribution of platelet crystals (see Figure 8.10(a) and (b)). This narrow crystal size distribution is predicted by the population balance model if nudeation takes place over a short period of time. When these particles are spray roasted (in a plasma gun), the particles are highly sintered into spherical particles (see Figure 8.10(c)). 

Manufacturing techniques employed in producing instant products include spray-coating dry powders with fluid lecithin products, cospray drying powders with more hydrophihc lecithins such as the oil-free forms, or hydroxylated lecithin, and agglomeration of the powder with an aqueous dispersion of a hydrophihc lecithin. Some types of powders, for example, starches, gums, and chocolate drink mixes, require agglomeration with an aqueous dispersion of lecithin to achieve optimum wettabihty and dispersibility. 

The demand for mixed oxide chromatographic-quality stationary phase materials is small and only a few methods that detail their preparation are available in the hterature. There are, however, numerous examples of the preparation of zircon powders and Ref. [4] is an example of such a process. In general, these types of powders are unsuitable for chromatographic stationary phases because their particle sizes are in the submicron range with very broad distributions. The synthesis of mixed oxide supports for chromatographic apphcations can essentially be divided into two types coprecipitation methods and coating methods. 

Massol-Chaudeur S, Berthiaux H, Doggs J. The development and use of a static segregation test to evaluate the robustness of various types of powder mixtures. Trans IchemE June 2003 81 (Parte) 106-18. 

Property Mill-ground Type of powdered rubber Shredder-cut ... 

The basic element from which this type of powder particles is constructed was obtained by a treatment of the copper powder... 

Although the powder particles obtained with evolved hydrogen below and above the critical value for a change in the hydrodynamic conditions were very different at the macro level, their similarity at the micro level is very clear. Both types of powder particles consisted... 

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