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f-Schulmanart_26-29_Masters 12/30/13 2:06 PM Page 27 Tablets & Capsules January 2014 27 validating the granulation endpoint, a task that is especially difficult today given the global nature of the industry. Today, raw materials can come from many different sites, which can lead to slight variations in particle morphology. Those variations, in turn, can alter the process. But even when a wet granulation process runs properly, the mixer is often covered with a sticky goo, and the oven is very difficult clean to an acceptable industry standard of no more 10 parts per million of carry-over. Plus, all that cleaning takes time. If you have many products to densify, consider dry granulation by roller compaction. And dry it is: Roller compaction uses no water. As a result, there is no possibility of water-induced degradation and no need for drying. That reduces the energy the process requires and eliminates the expense of piping in water. Plus, cleanup is easier, especially since most roller compactors disassemble into easy-to-handle components that you can place into a washing machine. Table 1 lists Table 1 Advantages and disadvantages of roller compaction over wet granulation Advantages Disadvantages Faster operation, with the possibility of continuous operation Excess fines can result in tablet weight fluctuation No liquids or drying required, allowing you to process moistureand heat-sensitive products Over-compaction may lengthen dissolution times Simpler and less expensive to operate than wet processes; less space and energy required Powder blends must be compressible and must accept lubricant Elimination of API and/or color migration Structure of compounds may change Possible to shorten disintegration times with use of proper binder Figure 1 A variable-speed screw forces the powder into the gap between counter-rotating rollers. some advantages and disadvantages of roller compaction compared to wet granulation. Roller compaction also has advantages over tabletting powder blends: • Uniformity of material (bulk density, particle hardness, porosity, and particle size range) • Minimal fines and dust • Better flow properties • Faster tabletting with less weight variation • Faster tablet dissolution and better disintegration • Minimal or no explosion-proofing measures required. How it works Roller compactors use counter-rotating rollers to densify dry powder—usually a blend of powders—into sticks or sheets. In operation, the blended powder is fed from an intermediate bulk container (IBC) into the roller compactor's hopper. As the material flows from the hopper, a variable-speed screw forces it between the two rollers (Figure 1). One roller is usually fixed, while the other moves, enabling an operator or PLC to maintain the size of the gap between them and to apply the desired force. By maintaining a uniform gap and uniform force, the compacts themselves are uniform. Upon discharge, the compacts pass through a built-in comminutor or they are collected and subsequently milled. Because the API and excipients are locked into the matrix of the granules, there is little risk of segregation during tabletting or capsule filling. So long as fines are not excessive, the granulation preserves the desired ratio of API-to-excipients. Roller design and configuration The shape of the compacts—be they sheets, sticks, briquettes, or wafers—depends on the surface profile of the rollers (photo, page 26). The rollers themselves are arranged horizontally, vertically, or at an angle (Figure 2). As the powder enters the slip region—so named because of the sliding motion observed between the rollers—it is deaerated before it is forced into the gap between the rollers. This area is known as the nip region, and is typically offset 60 degrees from horizontal. See Figure 3. The size, speed, and design of the rollers determine how much powder they pull into the gap. Smooth rollers exert a great deal of force but minimize throughput. Figure 2 Courtesy of Freund-Vector, Marion, IA Common roller configurations Vertical Horizontal Inclined