Role of Compaction Studies
Tablet is one of the most common dosage forms because of obvious advantages of ease of administration, easy commercial manufacturing and stability. Manufacturing of tablets involves a step of compaction wherein the bulk powder or granules are converted to a compact mass, under compaction pressure in a tablet press. Hence it is desirable that the drug substance has acceptable compaction profile. This becomes even more important in the case of high dose drugs where drug substance contributes major bulk in the final dosage form.
Stages of Compaction during Tableting
Compaction involves two distinct stages – (i) compression and (ii) consolidation. Compression involves initial particle re-arrangement under relatively lower compaction pressure. Consolidation stage occurs at relatively higher compaction pressure and involves material deformation and formation of inter-particulate bonds leading to formation of compact mass.
Deformation behavior during Compaction
Material deformation may be of three types – plastic, elastic and brittle fracture. Plastic materials deform under pressure and do not regain their original state after removal of compaction pressure. In contrast, elastic materials regain their original form after removal of compaction pressure. Brittle materials break down to smaller particles under compaction pressure. Pharmaceutical powders may show predominantly a particular type of deformation behavior. For example micro-crystalline cellulose is a plastic material, whereas ibuprofen and paracetamol undergo elastic deformation and brittle fracture.
Relationship of Solid State Properties with Compaction
Mechanistic understanding of powder compaction can be obtained using BA-BS (Bonding area- bonding strength) theory. It states that compaction is governed by bonding area available for consolidation and bonding strength per unit area. Solid state properties at molecular, particle and bulk level can affect the compaction behavior by affecting either bonding area or bonding strength. For example, polymorphic forms differ in their true density and thermodynamic properties, which can affect the bonding strength during consolidation. Similarly particle size has been shown to affect compaction behavior by its effect on bonding area.
Experimental determination of Compaction Behavior
Compaction behavior of drug substances can be studied using compaction simulator or fully instrumented tablet press having punches fitted with strain gauges. Study of compaction behavior allows selection of excipients for compacted solid dosage form and identifying process related problems. A material understanding of compaction behavior is essential for robust processing and from a ‘quality by design’ perspective.
Dr. Arvind Bansal
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