Processing of Fine Powders

Fine powder resins are extremely sensitive to shear, and the sheared polymer cannot be processed. Because of that they have to be handled with great care dnring transport and processing. 

Most commonly, fine powder resins are processed in the form of a paste. Snch a paste is prepared by mixing the powder with 15% to 25% hydrocarbon Inbricant, such as kerosene, white oil, or naphtha, with the resultant blend appearing mnch like the powder alone [7], 

Lubricants enable PTFE 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. 

FIGURE 4.10 Twin shell blender. (Conrtesy of Patterson-Kelley Co.) 

The extrusion step is performed at temperatures above 19°C (66°F), the first transition point of the resin, where it is highly deformable and can be extruded smoothly. The resin preform is placed in the extrusion cylinder, which is kept at a temperature of 38°C (100°F) for several minutes to heat up to the higher temperature. 

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Another application of PTFE dispersions is the preparation of a variety of compositions with other materials, such as mineral fillers, other polymers in powdered form by co-coagulation. The dispersion of the other component is blended with the PTFE dispersion and the blend is then coagulated. The resulting composition can be processed by extrusion with lubricants (see processing of fine powders) or by compression molding.16... 

Primary amides for slip/antiblock agents in LDPE, LLDPE, PP films. Lubricants can tie up catalyst residues, usually calcium stearate stearates and ethylene bis-stearamide waxes are sometimes used in processing of fine powdered polyolefins erucamides preferred for films and mouldings fluoropolymer alloys give better use of machinery methacrylate-reactive silicones... 

Dec RT. Problems with processing of fine powders in roll press. Proceedings of the 24th Institute for Briquetting and Agglomeration. 1995 24 199-210. 

As far as processing of fine powders resulting from coagulation of the suspensions is concerned, addition of a lubricant and various additives improves the flow of the mixture through a die and leads to profiles of small section. Then the lubricant is vaporized and the extrudate is sintered as previously described for granular PTFE. 

An improved yield is obtained by the following process. Add a mixture of 75 g. (70-5 ml.) of propionyl chloride and 90 g. (103 ml.) of sodium-dried A.R. benzene to a vigorously stirred suspension of 75 g. of finely-powdered anhydrous aluminium chloride in 100 ml, of dry carbon disulphide, Then introduce more of the aluminium chloride (about 15 g.) until no further evolution of hydrogen chloride occurs. The yield of propiophenone, b.p. 123°/25 mm., is about 90 g. 

Fine Powder Resins. Fine powder PTFE resins are extremely sensitive to shear. They must be handled gendy to avoid shear, which prevents processing. However, fine powder is suitable for the manufacture of tubing and wire insulation for which compression molding is not suitable. A paste-extmsion process may be appHed to the fabrication of tubes with diameters from fractions of a millimeter to about a meter, walls from thicknesses of 100—400 )J.m, thin rods with up to 50-mm diameters, and cable sheathing. Calendering unsintered extmded soHd rods produces thread-sealant tape and gaskets. 

In this process, the fine powder of lithium phosphate used as catalyst is dispersed, and propylene oxide is fed at 300°C to the reactor, and the product, ahyl alcohol, together with unreacted propylene oxide is removed by distihation (25). By-products such as acetone and propionaldehyde, which are isomers of propylene oxide, are formed, but the conversion of propylene oxide is 40% and the selectivity to ahyl alcohol reaches more than 90% (25). However, ahyl alcohol obtained by this process contains approximately 0.6% of propanol. Until 1984, ah ahyl alcohol manufacturers were using this process. Since 1985 Showa Denko K.K. has produced ahyl alcohol industriahy by a new process which they developed (6,7). This process, which was developed partiy for the purpose of producing epichlorohydrin via ahyl alcohol as the intermediate, has the potential to be the main process for production of ahyl alcohol. The reaction scheme is as fohows ... 

Production of fine powder during auxiliary processing. Possibility of a dust or dus hybrid explosion. 

At the present time, doped ICPs are not normally capable of being processed like normal thermoplastics. Processes usually Involve high-pressure moulding of finely powdered polymers under vacuum or an inert gas. However, modification of some ICPs with, for example, alkyl or alkoxy side groups may produce soluble, and thus more tractable, polymers. 

A mixture of 300 kilos of toluene and 700 kilos of 65 per cent, sulphuric acid is thoroughly agitated, and 90 kilos of finely powdered manganese dioxide added little by little. The temperature is maintained about 40°. The process is a very slow one, and finally a mixture of unchanged toluene and benzaldehyde is obtained, and from this the sulphite compound is prepared, purified, and the pure benzald,ehyde isolated as usual. 

Fluidised bed This process is used for powder coating Basically, the equipment consists of a dip tank with a perforated shelf near the bottom. The powder is placed on this shelf and low pressure air is fed under the perforated shelf, resulting in a cloud of fine powder in the body of the dip tank. The article is heated to a little above the melting point of the powder and is then dipped into the fluidised bed for a short period. It is then withdrawn... 

The most intensive development of the nanoparticle area concerns the synthesis of metal particles for applications in physics or in micro/nano-electronics generally. Besides the use of physical techniques such as atom evaporation, synthetic techniques based on salt reduction or compound precipitation (oxides, sulfides, selenides, etc.) have been developed, and associated, in general, to a kinetic control of the reaction using high temperatures, slow addition of reactants, or use of micelles as nanoreactors [15-20]. Organometallic compounds have also previously been used as material precursors in high temperature decomposition processes, for example in chemical vapor deposition [21]. Metal carbonyls have been widely used as precursors of metals either in the gas phase (OMCVD for the deposition of films or nanoparticles) or in solution for the synthesis after thermal treatment [22], UV irradiation or sonolysis [23,24] of fine powders or metal nanoparticles. 

Laboratory reactor for studying three-phase processes can be divided in reactors with mobile and immobile catalyst particles. Bubble (suspension) column reactors, mechanically stirred tank reactors, ebullated-bed reactors and gas-lift reactors belong the class of reactors with mobile catalyst particles. Fixed-bed reactors with cocurrent (trickle-bed reactor and bubble columns, see Figs. 5.4-7 and 5.4-8 in Section 5.4.1) or countercurrent (packed column, see Fig. 5.4-8) flow of phases are reactors with immobile catalyst particles. A mobile catalyst is usually of the form of finely powdered particles, while coarser catalysts are studied when placing them in a fixed place (possibly moving as in mechanically agitated basket-type reactors). 

For glazing a small quantity of finely powdered graphite is added during finishing process... 

In the first stage of the investigation the catalyst can be considered in the form of powder in order to derive intrinsic transient kinetics of all the relevant reactive processes. To this purpose, dynamic reactive experiments can be performed in a simple tubular fixed-bed microreactor over small quantities (50-200 mg) of finely powdered catalyst in principle, this guarantees negligible transport limitations and more controlled conditions (e.g. isothermal catalyst bed), hence enabling a direct estimation of intrinsic rate parameters by kinetic fit. Internal diffusion limitations are particularly relevant to the case of bulk (extruded) monolith catalysts, such as vanadium-based systems for NH3/urea SCR however, they... 

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