Influence of Contact Strength between Particles on Constitutive Law for Powder Compaction and the Strength of Powder Compacts

Powder compaction is used in a wide range of industrial applications ranging from powder metallurgy, food, pharmaceuticals, consumer goods, catalysts, fuels briquettes, etc. Powder compaction can be defined in terms of 1) compactibility, which represents the ability to form strong compacts, which is quantified by compact strength and 2) compressibility, which represents the ability of the powder mass to form dense compacts and is described by the constitutive law. The aim of this work is to establish relationships between particle properties, (including mechanical properties of particles and interactions between particles, which are included in the contact law), and bulk powder properties (including compactibility and compressibility). Excipients used in pharmaceutical tablet formulations, such as microcrystalline cellulose, calcium phosphate and mannitol (a sugar) and their mixtures were characterised. Compacts of different densities were manufactured and their compressive and tensile strength was measured. The break force of curved faced tablet made of these materials and mixtures was also measured under an extensive range of conditions. A predictive model was developed and validated to estimate the break force of curved faced tablets using diametrical and uniaxial compression tests only; this has a significant practical importance in the pharmaceutical industry. The mechanical properties of individual particles were characterised using nano-indentation. The data for pharmaceutical excipient were augmented with results for a model powder material consisting of spherical aluminium particles. The now classic micromechanical model of Fleck was used as a theoretical framework to relate particle properties and constitutive laws for compaction. A parameter describing the strength of contact between particles was estimated using the strength of compacts. The contact strength parameter was then related to adhesive contact laws between particles. This work represents the first complete framework for relating contact law (which includes the elastic and plastic properties that determine the deformation of particles in contact and the friction and adhesion between particles) to compact strength.