Bonding Surface area and Bonding Mechanism-Two Important Factors fir the Understanding of Powder Comparability

Two factors could be regarded as primary factors for the compactability of powders: the dominating bond mechanism and the surface area over which these bonds are active. Owing to considerable experimental difficulties, these factors have not been evaluated in any detail for pharmaceutical materials. Instead, more indirect, secondary factors are normally studied and used for correlations with tablet strength. Such secondary factors are particle size, shape and surface texture. Also the importance of volume reduction mechanisms, i.e. elastic deformation, plastic deformation and particle fragmentation have been studied in detail.

For the investigation of dominating bond mechanisms and estimation of the magnitude of the surface area of the solids involved in interparticulate attraction in compacts several pharmaceutical excipients representing both plastically deforming materials (sodium chloride, Avicel® PH 101, Sta-Rx 1500®, and sodium bicarbonate) and fragmenting materials (lactose, sucrose, paracetamol and Emcompress®) have been used in a series of publications from our laboratory.

The bonding mechanisms discussed have been solid bridges, representing continous solid bridges between tablet particles, intermolecular forces, representing weaker attraction forces active over distances and mechanical interlocking, representing a bond type dependent on hooking and twisting of irregularly shaped particles. To characterize the dominating bond mechanisms, measurements of compact strength has been performed in media known to reduce bonding with intermolecular forces. The media used were liquids with different dielectric constants and films of magnesium stearate. The results establish that the intermolecular forces constitute the dominating bond mechanism for pharmaceutical materials. Bonding with solid bridges contribute to the compact strength only for coarse plastically deforming materials that can melt during compaction. Only for sodium chloride, of the materials tested, is there substantial evidence for the existence of solid bridges. Bonding with mechanical interlocking is a bonding mechanism of minor importance for most of the investigated materials with the possible exception of Avicel® PH 101.

The results indicate that the surface area utilized for bonding with solid bridges for sodium chloride as measured with gas adsorption is small in relation to the total surface area of the compact. For all the materials bonding with intermolecular forces, only a proportional relation between compact surface area and bonding surface area could be possible. By using permeametry surface area data, the surface specific compact strength was characterized and found similar for all materials bonding primarily with intermolecular forces. For such materials a large bonding surface area will thus be obtained if the surface area of the particles in the tablet is large. This could either be achieved by the use of materials that undergo extensive fragmentation or by the use of very fine paniculate materials or qualities with pronounced surface roughness. It is suggested that most of the so called plastically deforming pharmaceutical materials often possess inadequate plasticity for the development of large zones that could take part in the interparticulate attraction by intermolecular forces.