Redispersible powders

Takeuchi, H., Saski, H., Niwa, T., Hino, T., Kawashima, Y., Uesugi, K., Kayano, M., and Miyake, Y. (1991) Preparation of a powdered redispersible vitamin E acetate emulsion by spray-dffrieiigi.. Pharm. 

Figure 5.9 Particle size distribution of (a) an acrylic powder redispersion (b) the original...

The coalescence factor is the percent silica that has to be dissolved to restore the light transmission under standard conditions of a silica powder redispersed in water (68). 

Almost the same improvements are obtained by applying the repair mortar through a dry shotcrete process. Within this process the water is mixed with the dry mortar only in the jet. After that the mixed mortar is immediately sprayed onto the surface. Despite this extremely short mixing and almost no slake time, the redispersible powder redisperses quickly and completely enough in order to improve the tensile adhesion strength and the flexural strength in almost the same magnitude compared to a conventional application by hand. 

After hardening, UF-resins consist of insoluble, more or less three-dimensional networks and cannot be melted or thermoformed again. At their application stage, UF-resins are still soluble or dispersed in water or are spray dried powders, which in most cases are redissolved and redispersed in water for application. 

Polymerization in dilute solution also allows a very straightforward isolation and purification of the microgels. After polymerization, the resulting microgels can be conveniently precipitated from the reaction solution by using suitable nonsolvents for the microgel molecules. The resulting powders can be filtered off, dried, and redispersed in suitable solvents when needed. 

The microgels could be conveniently isolated by precipitation as white powders, readily redispersable in many different organic solvents such as dialkylamides, nitriles, dichloromethane, acetone and THF. Further to this, the DMAA-based microgels exhibited a rather amphiphilic character and were also soluble in water and in alcohols such as methanol or ethanol in contrast, their counterparts based on MMA turned out to be more lipophilic and therefore insoluble in water and alcohols but soluble in organic solvents of low polarity such as toluene. 

C Bindschaedler, R Gurny, E Doelker. Process for preparing a powder of water insoluble polymer which can be redispersed in a liquid phase, the resulting powder and utilization thereof. Swiss Patent 1497/88 (1988). 

Colloidal systems were reported to be much more effective than immobilized or supported catalysts for photodegradation of ary hazardous molecule [72,73], Powdered materials exhibit an important inconvenience. The main difficulty in employing an insoluble, powdered semiconductor in aqueous dispersion is the need to remove the solids after treatment and subsequent redispersion in a second aqueous solution to be purified. However, in the case of colloids it was speculated that they may either be dispersed in the irradiated aqueous solution as a colloidal suspension, or attached to a suitable support as a fixed or mobile fluidized bed [72,73],... 

PVA is used for example as a water soluble dispersing agent for many different polymer dispersions used in the construction industry. An important property of PVA is the ability to redisperse polymer dispersions that have been dried to powders. 

Of the several types of the polymer-modified mortars and concretes used for various construction applications, latex-modified mortar and concrete are by far the most widely used materials. Latex-modified mortar and concrete are prepared by mixing a latex, either in a dispersed liquid or as a redispersible powder form with fresh cement mortar and concrete mixtures. The polymers are usually added to the mixing water just as other chemical admixtures, at a dosage of 5-20% by weight of cement. Polymer latexes are stable dispersions of very small (0.05-5 pm in diameter) polymer particles in water and are produced by emulsion polymerization. Natural rubber latex and epoxy latex are exceptions in that the former is tapped from rubber trees and the latter is produced by emulsifying an epoxy resin in water by the use of surfactants [87]. 

Latexes are usually copolymer systems of two or more monomers, and their total solids content, including polymers, emulsifiers, stabilizers etc. is 40-50% by mass. Most commercially available polymer latexes are based on elastomeric and thermoplastic polymers which form continuous polymer films when dried [88]. The major types of latexes include styrene-butadiene rubber (SBR), ethylene vinyl acetate (EVA), polyacrylic ester (PAE) and epoxy resin (EP) which are available both as emulsions and redispersible powders. They are widely used for bridge deck overlays and patching, as adhesives, and integral waterproofers. A brief description of the main types in current use is as follows [87]. 

These polymers are thermoplastic materials usually supplied as redispersible powders. They are widely used in proprietary prepackaged products such as tile grouts, self-leveling underlay and patching materials. 

Have a low air-entraining capability during mixing when a suitable antifoaming agent is used in the emulsion or redispersible powder. 

Capped semiconductor particles could be isolated as powders and redispersed in an organic solvent This permitted convenient purification. Importantly, capped semiconductor particles could be grown, like living polymers, to desired sizes by the addition of appropriate amounts of metal ions and RMSiMe3 [621]. 

Small-Volume Parenterals Color, clarity of solutions, particulate matter, pH, sterility, endotoxins. Powders for injection solutions include clarity, color, reconstitution time and water content, pH, sterility, endotoxins/pyrogens, and particulate matter. Suspensions for injection should include additional particle size distribution, redispersability, and rheological properties. Emulsion for injection should include phase separation, viscosity, mean size, and distribution of dispersed globules. 

Takeuchi H, Sasaki H, Miwa T, Hino T, Kawashima Y, Uesugi K, Ozawa H. Improvement of photostability of ubidecarenone in the formulation of a novel powdered dosage form termed redispersible dry emulsion. Int J Pharm. 1992 86 25-33. 

The polymers are usually applied either from aqueous or from organic solvent. More recently, aqueous dispersions or redispersible powders of the polymers have become available that ensure economical, fast, and environmentally safe processing of the film coatings. In some cases it might be necessary to prevent an interaction between the acidic functional groups and the drug. In these cases a subcoat (e.g., of hydroxypropylmethylcellulose [HPMC]) is recommended. 

K. Lehmann, Acrylic latices from redispersible powders for peroral and transdermal drug formulations, Drug Dev Ind Pharm 72 265-287 (1986). 

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