Pellet cohesion

At high discharge pressures, the pellets exit the device hot enough to form a sintered mass, as shown in Fig. 5.17(a) for PS resin. In this case, the temperature of the outside of the pellets exceeded the Tg of the resin and as the resin exited the device the pellets cohesively attached to one another. In most cases, the device was stopped and the melted masses were removed. 

The data of Table II indicate that pellet production is not sensitive to contained moisture over a wide range. Some moisture is needed for die lubrication and pellet cohesion high moisture content results in a scaly, loose-bound pellet which breaks more easily in handling than others. Considering both equipment operations and pellet properties, the preferred moisture content is judged to be in the range 15 - 20%. 

Pelletization often does not require the use of excipients and may offer an alternative to blending for high dose therapeutics. The process involves deliberate agglomeration of the fine drug material into less cohesive, larger units [23]. Pelletization is usually achieved by vibratory sieving or any tumbling process. All processes require particu-... 

In conventional feed manufacturing this by-product is regarded as a combination of bran and shorts and tends to be intermediate between the two in composition. It also may contain some wheat screenings (weed seeds or other foreign matter that is removed prior to milling). It is commonly included in commercial feeds as a source of nutrients and because of its beneficial influence on pellet quality. When middlings (or whole wheat) are included in pelleted feeds, the pellets are more cohesive and there is less breakage and fewer fines. 

The addition of fines to a powder system allows a larger top size of particle to be agglomerated due to the attendant increase in cohesive forces caused by a decrease in surface mean particle diameter and increase in agglomerate density. Thus, although eqn. (1) indicates a top size of about 150 pm for monosized particles with aqueous binders, the top size of feed for industrial disc pelletizers is usually higher at 30 to 50 mesh (300 to 600 /zm) with the provision that at least 25% should be finer than 200 mesh (75 /zm) [7]. Other liquids with surface tensions lower than that of water, or liquid/ solid systems in which the particle surface is imperfectly wetted, require finer particle sizes to make successful balling possible. 

Other applications are found in powder technology. The cohesion in pellets formed out of dry powders can be improved by treating the powder with an appropriate polymer solution. This process is used on an enormous scale to prepare iron ore in a form suitable for blast furnaces, but also for the preparation of pharmaceutical specialty products. Polymers are applied at a large scale in paper-making where they help to strengthen the network of celluloslc fibres and to trap different kinds of particles in this network. The building of a network Is also the purpose of adding carbon black to rubber, which improves its resilience and abrasive resistance. 

Avoid excessive levels of fat that interfere with making cohesive pellets (more than 4%) or restrict the desired puffing of starches and development of lamellar soy protein textures (more than 6%) in extmded feeds. 

Dry powders must be able to flow readily to leave the capsule or powder reservoir but also must generate a fine aerosol so that the patient can inhale a proper dose. These two requirements are often difficult to achieve simultaneously. Fine powders tend to be cohesive and have poor flow properties. Blending with a carrier phase, pelletization, and other approaches have been used to overcome these limitations. The use of blends and homogeneous powders is compared in Table 4 from a DPI device perspective. 

In an extrusion-spheronization process, formulation components such as fillers, lubricants, and pH modifiers play a critical role in producing pellets with the desired attributes. The granulated mass must be plastic and sufficiently cohesive and self-lubricating during extrusion. During the spheronization step, it is essential that the extrudates break at appropriate length and have sufficient surface moisture to enhance formation of uniform spherical pellets. 

Alvarez L, Concheiro A, Gomez Amoza JL, et al. Powdered cellulose as excipient for extrusion-spheronization pellets of a cohesive hydrophobic drug. Eur Pharm Biopharm 2003 55(3) 291-295. 

Material Fine/medium powder Fly ash, pulverized coal, plastic powders, alumina, granular sugar, pharma excipients Course powder Sand, salt, granules, mineral powders, glass beads Cohesive fine powder Cement, corn starch, tit anium dioxide, carbon-black powder, many pharma actives Granular Plastic pellets, wheat, large glass beads, tablets, course sand, seeds... 

New silt-clay sediment entering the Sound is processed to pellets by the benthic animals almost at once. The residence time of the silt-clay sediment in the pelletized mantle is about 10 years (see p. 98), after which it is converted into cohesive mud (or muddy sand) that is not subject to resuspension into the water column and is effectively isolated from the ambient water. 

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