BONDING SURFACE-AREA AND BONDING MECHANISM - 2 IMPORTANT FACTORS FOR THE UNDERSTANDING OF POWDER COMPACTIBILITY

Two factors could be regarded as primary factors for the compactabilit y of powders: the dominating bond mechanism and the surface area over which these bonds are active. Owing to considerable experimental diffi culties, these factors have not been evaluated in any detail for pharm aceutical materials. Instead, more indirect, secondary factors are nor mally studied and used for correlations with tablet strength. Such sec ondary factors are particle size, shape and surface texture. Also the importance of volume reduction mechanisms, i.e. elastic deformation, p lastic deformation and particle fragmentation have been studied in det ail. For the investigation of dominating bond mechanisms and estimatio n of the magnitude of the surface area of the solids involved in inter particulate attraction in compacts several pharmaceutical excipients r epresenting both plastically deforming materials (sodium chloride, Avi cel(R) PH 101, Sta-Rx 1500(R), and sodium bicarbonate) and fragmenting materials (lactose, sucrose, paracetamol and Emcompress(R)) have been used in a series of publications from our laboratory. The bonding mec hanisms discussed have been solid bridges, representing continous soli d bridges between tablet particles, intermolecular forces, representin g weaker attraction forces active over distances and mechanical interl ocking, representing a bond type dependent on hooking and twisting of irregularly shaped particles. To characterize the dominating bond mech anisms, measurements of compact strength has been performed in media k nown to reduce bonding with intermolecular forces. The media used were liquids with different dielectric constants and films of magnesium st earate. The results establish that the intermolecular forces constitut e 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 evi dence for the existence of solid bridges. Bonding with mechanical inte rlocking is a bonding mechanism of minor importance for most of the in vestigated materials with the possible exception of Avicel(R) PH 101. The results indicate that the surface area utilized for bonding with s olid bridges for sodium chloride as measured with gas adsorption is sm all in relation to the total surface area of the compact. For all the materials bonding with intermolecular forces, only a proportional rela tion between compact surface area and bonding surface area could be po ssible. By using permeametry surface area data, the surface specific c ompact strength was characterized and found similar for all materials bonding primarily with intermolecular forces. For such materials a lar ge bonding surface area will thus be obtained if the surface area of t he particles in the tablet is large. This could either be achieved by the use of materials that undergo extensive fragmentation or by the us e of very fine particulate materials or qualities with pronounced surf ace 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 interp articulate attraction by intermolecular forces.