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f-Schulmanart_26-29_Masters 12/31/13 9:55 AM Page 28 Tablets & Capsules 28 January 2014 Figure 3 Slip region and nip angle between rollers Two common methods of sealing rollers Standard rollers DP rollers Gap Slip region Nip region Rim Nip angle Axially grooved rollers pull large amounts of powder through, but if the powder is not cohesive enough, a weak compact may result. Knurled rollers are the best default option. Their grooves pull in the material, while their smooth sections exert great force. The result is an evenly compacted material across the entire surface of the wafer. Some materials, such as potassium chloride, need high pressure, while others, such as penicillin and amoxicillin, require low pressure to avoid discoloration. (Adding sugar to penicillin prevents it from over-compacting and turning yellow.) In fact, sticky powders and those with a low melting point are difficult to process and may require dilution in a suitable excipient, as discussed below. But the vast majority of dry powders can be economically and consistently dry-granulated. Bonding mechanisms include particle rearrangement, deformation, fragmentation, van der Waals' forces, valence forces, partial melting and solidification, and interlocking granules. Simply stated, dry granulation is agglomeration by the removal of interstitial air between particles. All compacts, however, undergo elastic recovery (expansion) after discharge. How much they expand is a function of the physical characteristics of the materials, the roller diameters, and the speed at which they turn. Basic trial and error will help you identify the set points. Processing aids If a powder isn't compressible, it can be made to compress by adding binders, such as microcrystalline cellulose (MCC), methyl cellulose, and hydroxypropyl cellulose (HPC). Sometimes, fillers—lactose and/or starches—are added either before or after compaction to aid compression. Glidants—starches, modified starches, and colloidal silicon dioxide—may be added to improve the flow properties of uncompacted blends. Courtesy of Freund-Vector, Marion, IA Side plate Sealing method used by Freund-Vector roller compactors In most cases, disintegrants (starches and modified starches) and super-disintegrants (croscarmelose sodium and crosspovidone) are added twice: before and after compaction. Added before compaction, they speed disintegration and dissolution. Added after compaction, they surround the granules and promote disintegration of the final dosage when wetted. Lubricants—usually magnesium stearate or another metallic stearate—are also added in two parts. Before compaction, they're essential to prevent the blend from sticking to the rollers. But be careful not to overdo it. Over-lubrication can lead to soft compacts and retard dissolution, since metallic stearates are hydrophobic. The second portion is added to the final granulation so it doesn't stick to the punch tips or dies walls of the tablet press. Table 2 lists some effects of overly soft and hard granules.