Powder and Bulk Engineering

PBE0720

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July 2020 / 35 MAINTAINING POWDER BLEND CONTENT HOMOGENEITY USING DRY GRANULATION This article discusses how dry granulation can help ensure a homogenous powder blend and flow for a tablet press. The article also discusses supplier-conducted tests on how recycling screened-out granules back into the raw powder affects that blend. Matthew Walczer and Albin Friedrich, Alexanderwerk react to the compression force exerted on them. The "slip" region is the first phase where the powder parti- cles move with respect to each other. The "nip" region is the second phase in which the particles cease to move with the same speed and position as phase one and move relative to the rollers. Process phases There are four process phases that compress and transform powder into a cohesive, densified ribbon or "flake." These phases are rearrangement, deformation, fragmentation, and bonding. Particle rearrangement. Particle rearrangement occurs as the powder begins to move, with particles filling the interparticle spaces. Air also begins to move out of the spaces between the particles. Particle shape and size are key factors in the effectiveness of this step. Particle deformation. Particle deformation occurs as the pressure forces exerted by compaction between the rollers increases the points of contact between particles. This causes plastic deformation, which is a permanent change to the shape of a solid body without fracture. Particle fragmentation. Particle fragmentation happens as the compression forces increase and cause particles to fracture into smaller particles. This creates multiple new contact points and surface and bonding sites. Particle bonding. Particle bonding occurs at the compression pressure setpoint when plastic deforma- tion and fragmentation occur, allowing Van der Waals forces to create a stable bond between particles. The powder, having been compressed and densified in this way, exits the rollers as a ribbon or "flake." One or more size-reduction techniques are then employed to change that densified flake into granules. This is typically accomplished by using a rotating bar to press the densified material through one or more screens or wire screen mesh of certain sizes. When done suc- M aking tablets from powders is a mature technology, and some of the same powder handling problems that impacted the industry in its early days continue to challenge scientists and process engineers today. For instance, efforts to get a homogeneously blended powder mix to flow well and resist segregation is a concern that can be managed but not eliminated. That's because during handling, the physics of powder movement pushes particles to seg- regate based on size and mass. A segregated powder going to a tablet press, for instance, will produce tablets that vary in weight and content. This is unacceptable. Dry granulation by use of a roller compactor can help avoid this less-than-desirable state and shepherd a homogenous powder blend through the manufacturing process to a tablet press. Basically, dry granulation involves using compaction and comminution to agglomerate and then break these poorly flowing powders into granules. (In wet granulation, a liquid or alcohol binder is added to the process along with a downstream drying step.) Compaction During roller compaction, powders are compressed and densified between two counter-rotating rollers and then compression-milled to a granule of a certain size. This yields a powder that's physically different but chemically unaltered and has flow characteristics that are more predictable and reliable than the original raw material feed. In an ideal world, the process begins with a per- fectly blended powder. This material is delivered to a feed hopper and conveyed by a feed screw to the set of counter-rotating rollers. These come in various configurations and sizes depending on the equip- ment manufacturer. The powder then reaches what are known as the "slip" and "nip" regions of the process where particles

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