Powders fine cohesive

When a sample of ca 100 g has been obtained, a representative sample for use in size characterization equipment must then be taken. Some of the more modem methods of size characterization require as Htde as 1 mg of powder, thus obtaining a representative sample can be quite difficult. If the powder flows weU and does not contain too many fines, a device known as the spinning riffler (Fig. 4c) can be used. A spinning riffler consists of a series of cups that rotate under the powder supply. The time of one rotation divided into the time of total powder flow should be as large a number as possible. Although this device has been shown to be very efficient, problems can be encountered on very small (1 mg) samples, and the powder must be processed several times. Moreover, in order to avoid cross-contamination, cleanup after each of the sampling processes can be quite difficult. Furthermore, if the powder is cohesive and does not flow weU, the equipment is not easy to use. A siUca flow agent can be added to the powder to enable the powder to flow... 

Group C 20 Fine cohesive powders, difficult to fluidise and readily form channels. Flour Fine silica... 

Group C powders Very fine, cohesive powders are classified into this category, which are incapable of fluidization in the strict sense and tend to rise as a slug of solids. Their sizes are usually less than 30 pin, and they easily give rise to channeling. Examples of group C materials are talc, flour, and starch. 

In the pharmaceutical industry large, often poorly-formed, tablets or slugs are made from fine cohesive powders in large capacity tablet presses and then comminuted to form a granular feed suitable for final tableting [10]. Slugs are typically 1 or 2 in. (2.5 or 5.1 cm) in diameter and perhaps in. (1.3 cm) thick. Results comparable to those accomplished with this piston and die method are also obtained by roll compaction followed by granulation (see Section 2.2). 

Non-flowing material is composed of very fine cohesive powders, sticky material, moist material or fibrous solids. These may be stored in small containers such as drums or bags or large containers such as trucks or... 

Several of the manufacturers of Multi-angle laser light scattering instruments (MALLS) have developed dry powder feeders. The Sympatec Rodos and the Aerosizer dry powder dispenser are particularly suitable for dispersing fine cohesive powders. 

The Cohesion Tester has been used in industry, mainly as a quality control-type test. It can be useful as an aid to assessment of flow properties and of power requirements in mixers, bulk conveyors and feeders. The original developers of the tester, Warren Spring Laboratories35, have even tried to correlate the cohesion value with unconfined yield stress determined with the Jenike shear cell and found a good correlation for some powders (fc = 6 x C). The tester is, quite obviously, only useful with fine, cohesive powders because ... 

Cohesiveness and adhesion Cohesiveness of a powder is the ability of individual particles to stick together. This attraction may be caused by the presence of moisture, electrostatic charges or the fineness of the particles within a powder. Fine powders generally have a high surface area, which is usually associated with an increase in the surface energies within the powder and thus enhancement of the binding forces between particles (Stanley Wood et al. 1990). 

Adhesives. Poly(vinyl alcohol) is used as a component in a wide variety of general-purpose adhesives to bond ceUulosic materials, such as paper and paperboard, wood textiles, some metal foils, and porous ceramic surfaces, to each other. It is also an effective binder for pigments and other finely divided powders. Both fully and partially hydrolyzed grades are used. Sensitivity to water increases with decreasing degree of hydrolysis and the addition of plasticizer. Poly(vinyl alcohol) in many appHcations is employed as an additive to other polymer systems to improve the cohesive strength, film flexibiUty, moisture resistance, and other properties. It is incorporated into a wide variety of adhesives through its use as a protective coUoid in emulsion p olymerization. 

Material Characteristics InnuencingDesign. Materials ranging from fine powders to large lumpy materials, nonabrasive to very abrasive, free-flowing to cohesive, and nonfriable to friable can be handled on properly designed belt conveyors. Very sticky materials can be a problem, however, if these cannot be continuously cleaned from the belt surface. 

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