Tablet diluents fluidity

The primary limitation on the use of direct compression is that it depends on the fluidity and compressibility of a tablet diluent. Therefore, it cannot be used for low potency, high dose active ingredients where the inclusion of sufficient diluent in the formulation to permit direct compression would lead to unacceptably large tablets. Thus, active ingredients such as paracetamol and aspirin do not lend themselves to the direct compression process. However, as stated earlier, such ingredients are often available in pregranulated form. 

Starch is a very widely used tablet excipient, but in its natural state, it does not possess the fluidity and binding characteristics needed as a tablet diluent. The major consolidation mechanism of starch is by deformation with a high elastic component.In addition, starch shows a high degree of lubricant sensitivity. 

If a major component of the formulation such as the diluent were to possess the necessary degrees of fluidity and compressibility, granulation would be unnecessary. This is the basis of the direct compression method of tablet manufacture. 

Powdered cellulose has been used as a direct compression diluent. Though it forms hard tablets, fluidity is poor and dilution potential is low. Like microcrystalline cellulose it has some self-lubricating properties, but addition of a lubricant is usually necessary, causing a marked reduction in tablet strength. ... 

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