Understanding The Dry Granulation Process
Dry granulation is a process whereby granules are formed without the aid of any liquid solution. The process is used if the ingredients to be granulated are sensitive to moisture or heat. Compaction is used to densify the powder and form granules. The process is carried out using a slugging tool or a roller compactor/ tablet press machine. It is important to note that if a tablet press is used for the dry granulation process, the powders may not possess enough natural flow to feed the product uniformly into the die cavity, resulting in varying degrees of densification, leading to non-uniform granules when they are milled.
The original method of dry granulation employed a heavy duty tablet press to compact the dry powder. The compacting process in this case is known as slugging and the compacted material was therefore in the form of “slugs” - typically 25 mm in diameter and 10-15 mm thick. A hammer mill is ideal for breaking up the slugs to create granules. A manufacturing company may also use a conventional tablet machine or a heavy-duty rotary press for compacting the dry powder.
A gentler alternative to slugging is compaction using a roller compactor. In this device (commonly referred to as a Chilsonator) the mixed powder is squeezed through two counter-rotating rollers to form a compressed sheet. The sheets are brittle and break easily into flakes. The flakes need careful treatment to break them to granules, after which they can be milled to the desired size. Roller-compaction or dry-granulation equipment offers a wide range of pressures and roll types to attain proper densification. This equipment is loud and dusty compared with other process machinery. Material feed rates are critical for attaining the final objective. The process may require repeated compaction steps to attain the proper granular end point
End Point of the Granulation Process
Granulation is perhaps more of an art rather than science, and one of the difficulties is determining when granulation is complete. There are several measures that can be used to determine the end point of the granulation process, i.e. when the desired particle size has been reached. A simple method that an operator can use is to make a ball of granules by rolling the granules in the hand. The ball should be neither hard nor soft and should break easily after applying a little finger pressure. What remains after the ball is broken should be granular and not resemble fine powder. In the case of dry granulation, there are several measures that can be applied to the mill or shear mixer to detect the end point. These include measuring the voltage and current of the motor driving the mixer or mill (it is assumed that the force or torque exerted by the motor is related to the particle size of the granules), This method is widely used in the wet granulation process. Another option is to remove a sample of the powder and measure its basic flow energy in a powder rheometer. Basic flow energy (BFE), a dynamic powder parameter, can be a sensitive marker of the transition point from wet mass to granules and a precise tool for detecting when granules have attained their optimal properties.
Successful compaction depends on the compatibility of the products being compressed. If the material is not fully granulated, then the batch may contain too many fines if they are not removed or reprocessed. Such a situation can contribute to capping, chipping, laminating, weight, and hardness problems on the tablet press.
The need for screening large amounts of fines is common to roller compaction, and the degree to which it can be managed depends on the nature of the ingredients. Any product that is removed from the rest of the batch because of particle size must be analyzed to determine what is being removed. Roller compacting the complete formula is not usually necessary. The object is to densify powders and form granules of the products in the formula that must be compacted, mill the granules, and then blend them back in with the rest of the formula’s ingredients. Most dry-granulated products do not have problems with picking and sticking because moisture is not present.
Over granulation, on the other hand, may produce batches that have an uneven distribution of fines, leading to compressibility problem, hardness, uneven colour distribution, or otherwise compromised tablet quality.