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48 October 2014 Tablets & Capsules Agglomeration is frequently ob - served when one or more ingredients in a blend are cohesive, hydro philic, or tend to acquire an electrostatic charge, and it can lead to several blend ing failure modes: • In applications where the final pro - duct is small, such as a tablet, and where the mass percentage of the agglomerated ingredient is small, agglomeration can cause a large va - riation in the ingredient's concen- tration. Acetaminophen particles of 10 to 200 microns, for ex ample, can agglomerate to form a lump that is 5 millimeters in diameter. • Extensive agglomeration of a ma jor ingredient can change the density and flow properties of the blend as a whole. In a granulation process, this is desirable, but spontaneous agglomeration of a large proportion of the particles in the blend can lead to unwanted changes in prod- uct appearance and performance. • An agglomerated ingredient forms pseudo-particles, which behave as individual, large particles. If these pseudo-particles are large enough, they can trigger other phenom- ena, such as segregation. Agglomeration before blending The most commonly ob served ag - glomeration occurs when the ingredi- ent forms lumps in its raw state, before blending. This is called caking, and it can occur in a hydrophilic ingredient exposed to moisture, which causes capillary bridges (and sometimes solid bridges due to recrystallization) to form be tween particles. A highly cohesive ingredient can also form lumps, possibly caused by intermolec- ular attractions (van der Waals forces). If an ingredient agglomerates be - fore blending, pass it though a de- lumper or mill, which is a good prac- tice for most ingredients. Avoid pre- milling ingredients that acquire an electrostatic charge when exposed to shear. Agglomeration during blending or subsequent processing Spontaneous agglomeration dur- ing blending or subsequent process- ing is less common and can arise for several reasons: A tendency to stick to metal. This usually occurs when the ingredient acquires either an electrostatic mono- pole charge or a dipole moment, which induces a mirror charge or dipole in metal parts. When that hap- pens, the ingredient can coat and form layers on metal surfaces. Adjacent par- ticles in the coating can then develop bonds and become agglomerates. Free moisture in a blend. This can cause hydrophilic ingredients to agglomerate. For example, if a blend becomes warm during processing and is allowed to cool, moisture can con- dense from the air and be absorbed by the hydrophilic particles, which form capillary bridges and agglomerate. Softening of low-melting-point ingredients. These include wax, poly- ethylene glycol (PEG), and solid alco- hol. These can soften during process- ing when they approach their melting points or when they undergo high shear. These softened particles can bind to each other and act as binders for other ingredients. A wax will often bind to a hydrophobic ingredient, while PEG will often bind to a hydro - philic one. Take a two-step approach to pre- vent agglomeration during processing: Add shear. Agglomerates often form and persist because the blending process lacks the shear to break them. Try de-lumping the blend right before final use. The additional shear will often break apart agglomerates and prevent them from re-forming. Change the process or formula- tion. For some blends, adding shear will promote agglomeration by evap- orating moisture, softening the ingre- dient, or inducing an electrostatic charge. In those cases, characterize your ingredients to determine what is causing the agglomeration. Once you understand the root mechanism behind it, you can make changes to prevent agglomeration: • If the problem is moisture uptake, reduce the blend's moisture level, reduce the blend storage time, reduce the processing temperature or shear level, or incorporate a desiccant ingredient. • If the problem is softening, find a substitute for that low-melting- point ingredient, reduce its con- centration in the blend, or change the process so the ingredient is exposed to less shear or elevated temperatures by adding the ingre- dient later in the blending or pro- cessing step. • If the problem is from powder lay- ering on metal surfaces, consider re-designing the equipment (by using glass-lined equipment, for example) or finding a substitute ingredient. T&C [Editor's note: To comment on the Back Page, visit www.tabletscapsules. com.] Fernando J. Muzzio is director of the Na - tional Science Founda- t i o n 's E n g i n e e r i n g Research Center on Structured Organic Particulate Systems, 98 Brett Road, Piscataway, NJ 08854. Tel. 732 445 3357. E-mail: fjmuzzio@ yahoo. com. A version of this article orig- inally appeared in the September 2014 issue of Powder and Bulk Engineering. b a c k p a g e Powder blending and ingredient agglomeration

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