Effect of Porosity on Strength Distribution of Microcrystalline Cellulose

Dynamic compaction of cohesive granular materials: scaling behavior and bonding structures

The compaction of cohesive granular materials is a common operation in powder-based manufacture of many products. However, the influence of particle-scale parameters such as bond strength on the packing structure and the general scaling of the compaction process are still poorly understood. We use particle dynamics simulations to analyze jammed configurations obtained by dynamic compaction of sticky particles under a fixed compressive pressure for a broad range of system parameter values. We show that relative porosity, representing the relative importance of porosity with respect to its minimum and maximum values, is a unique function of a modified cohesion number that combines adhesion force, confining pressure, and particle size, as well as contact stiffness, which is often assumed to be ineffective but is shown here to play an essential role in compaction. An asymmetric sigmoidal form based on two power laws provides an excellent fit to the data. The statistical properties of the bond network reveal self-balanced force structures and an exponential fall-off of the number of both tensile and compressive forces. Remarkably, the properties of the bond network depend on the cohesion number rather than the modified cohesion number, implying that similar bond network characteristics are compatible with a broad range of porosities mainly due to the effect of contact stiffness. We also discuss the origins of data points escaping the general scaling of porosity and show that they reflect either finite system size or rigid confining walls.

Flow and Tableting Behaviors of Some Egyptian Kaolin Powders as Potential Pharmaceutical Excipients

The present work aimed at assessing the pharmaceutical tableting properties of some Egyptian kaolin samples belong to the Abu Zenima kaolin deposits (estimated at 120 million tons). Four representative samples were selected based on kaolinite richness and their structural order-disorder degree, and after purification, they were dried at 70 °C and heated from room temperature up to 400 °C (10 °C/min). Mineralogy, micromorphology, microtexture, granulometry, porosimetry, moisture content, bulk and tapped density, direct and indirect flowability, and tableting characteristics are studied. Results indicated that purified kaolin samples were made up of 95–99% kaolinite, <3% illite, 1% quartz and 1% anatase. The powder showed mesoporous character (pore diameters from 2 to 38 nm and total pore volume from 0.064 to 0.136 cm3/g) with dominance of fine nanosized particles (<1 μm–10 nm). The powder flow characteristics of both the ordered (Hinckley Index HI > 0.7, crystallite size D001 > 30 nm) and disordered (HI < 0.7, D001 < 30 nm) kaolinite-rich samples have been improved (Hausner ratio between 1.24 and 1.09) as their densities were influenced by thermal treatment (with some observed changes in the kaolinite XRD reflection profiles) and by moisture content (variable between 2.98% and 5.82%). The obtained tablets exhibited hardness between 33 and 44 N only from the dehydrated powders at 400 °C, with elastic recovery (ER) between 21.74% and 25.61%, ejection stress (ES) between 7.85 and 11.45 MPa and tensile fracture stress (TFS) between 1.85 and 2.32 MPa, which are strongly correlated with crystallinity (HI) and flowability (HR) parameters. These findings on quality indicators showed the promising pharmaceutical tabletability of the studied Egyptian kaolin powders and the optimization factors for their manufacturability and compactability.

Stress Transmission and Failure in Disordered Porous Media

By means of extensive lattice-element simulations, we investigate stress transmission and its relation with failure properties in increasingly disordered porous systems. We observe a non-Gaussian broadening of stress probability density functions under tensile loading with increasing porosity and disorder, revealing a gradual transition from a state governed by single-pore stress concentration to a state controlled by multipore interactions and metric disorder. This effect is captured by the excess kurtosis of stress distributions and shown to be nicely correlated with the second moment of local porosity fluctuations, which appears thus as a (dis)order parameter for the system. By generating statistical ensembles of porous textures with varying porosity and disorder, we derive a general expression for the fracture stress as a decreasing function of porosity and disorder. Focusing on critical sites where the local stress is above the global fracture threshold, we also analyze the transition to failure in terms of a coarse-graining length. These findings provide a general framework which can also be more generally applied to multiphase and structural heterogeneous materials.