Dry granulation is a technique to compress powders through external forces in granulation. The forces include force of solid bridge, adhesion force of low-viscosity liquid, surface force and cohesion. Successful agglomeration of granules depends on effective use and transmission of external forces, as well as physical properties of granules. There exist many problems in wet granulation:
Dry granulation/Roller compaction
1.Too hard granules
During dry granulation, if roller pressure is too large, granules will be too hard, which affects granule compressibility. In the following tablet press, larger pressure will be needed. Otherwise, tablet might be not that hard. Besides, if some granules are too hard, the final tablets will have spots. Last, too hard dry granules are difficult to dissolve in water. Their disintegration will be affected.
2.Low roundness of granules
Granules roundness will directly influence their fluidity. Roundness of granules made by dry granules will be worse than that of granules made by wet granulation. But it can be controlled by adjusting tablet thickness and machine structure.
3.Too many powders after granulation
1)Materials are of poor compressibility. They can be changed to microcrystalline cellulose (MCC), pre-gelatinized starch and some other materials of good compressibility.
2)Adhesive is used in small dosage. Its dosage can be increased.
3)Small pressure of roller produces easily breakable tablets. The pressure or material feeding speed can be increased.
4.Material sticking to roller
a)Lubricant in materials is not enough and can be increased. But the total dosage of lubricant should be controlled. On the basis of solving this problem, dosage of lubricant should be as small as possible.
b)Humidity-absorption materials will also be the reason of this problem.
5.Discontinuous material feeding leading to discontinuous medicine sheet
1)The mixture of medicines and auxiliary materials is of poor fluidity. Material of better fluidity can be changed or flow aid can be added into materials.
2)Multiple granulation can be applied (some materials such as MCC cannot be processed repeatedly). A part of materials can be process first to improve material fluidity.
Feature comparison of these three techniques
| Items | Wet granulation | Dry granulation |
| Fluid bed granulation | High shear granulation |
| Proper medicines | Medicines are sensitive to heat and highly viscous | Medicines are not sensitive to temperature and humidity | Medicines are sensitive to temperature and humidity |
| Granule properties | Small hardness, small bulk density and good fluidity | Hardness larger than that of granules in wet granulation, large bulk density, relatively good fluidity | Fluidity worse than that of granules in wet granulation, large bulk density |
| Adding adhesive or not | Y | Y | Y/N |
| Adhesive dosage | Relatively high | Lower than that of fluid bed granulation | The lowest |
| Application | Relatively strait | The widest | Relatively strait |
| Output | Relatively low | Relatively high | Relatively low |
| Operating time | Relatively long | Relatively short | Relatively long |
| Granulating repeatedly or not | N | N | Y |
These three techniques, especially fluid bed granulation and high shear granulation, are interchangeable. But before changing, it should be noted that different granulation techniques have their own requirements on material, adhesive dosage and granule properties.
