Fine powder flow is a topic of great interest to industry, in particular for the pharmaceutical industry; a major concern being their poor flow behavior due to high cohesion. In this study, cohesion reduction, produced via surface modification, at the particle scale as well as bulk scale is addressed. The adhesion force model of Derjaguin–Muller–Toporov (DMT) was utilized to quantify the inter-particle adhesion force of both pure and surface modified fine aluminum powders (∼8 μm in size). Inverse Gas Chromatography (IGC) was utilized for the determination of surface energy of the samples, and Atomic Force Microscopy (AFM) was utilized to evaluate surface roughness of the powders. Surface modification of the original aluminum powders was done for the purpose of reduction in cohesiveness and improvement in flowability, employing either silane surface treatment or dry mechanical coating of nano-particles on the surface of original powders. For selected samples, the AFM was utilized for direct evaluation of the particle pull-off force. The results indicated that surface modification reduced the surface energy and altered the surface nano-roughness, resulting in drastic reduction of the inter-particle adhesion force. The particle bond number values were computed based on either the inter-particle adhesion force from the DMT model or the inter-particle pull-off force obtained from direct AFM measurements. Surface modification resulted in two to three fold reductions in the Bond number. In order to examine the influence of the particle scale property such as the Bond number on the bulk-scale flow characterization, Angle of Repose (AOR) measurements were done and showed good qualitative agreements with the Bond number and acid/base surface characteristics of the powders. The results indicate a promising method that may be used to predict flow behavior of original (cohesive) and surface modified (previously cohesive) powders utilizing very small samples, and that the surface modification can drastically improve the powder flow for industrially relevant materials.

Simultaneous micronization and surface modification of drug particles is considered in order to mitigate disadvantages of micronization, e.g., agglomeration, poor flowability, marginal increase in surface area and low bulk density. Particles of ibuprofen (102 μm), a model drug, pre-blended with hydrophilic nano-silica, are micronized down to 10 and 5 μm in a continuous fluid energy mill (FEM) to obtain fine surface modified particles. The solid feeding rate and the grinding pressure are shown as critical parameters for achieving the desired particle size and size distribution. The powder properties were characterized via SEM, laser scattering, powder rheometer with shear-cell, and dissolution test. Significant improvement in flow properties and dissolution rate was observed when micronization accompanied surface modification. Additionally, co-grinding with water-soluble polymer during micronization led to further increase in bulk density and more enhanced dissolution rate improvement, which is attributed to improved wettability. XRD, DSC and Raman were used to examine crystallinity, indicating minimal detectable physical transformation with FEM processed ibuprofen. The surface modified, micronized powders also showed improved dispersion, higher bulk densities (>0.4 g/ml), reduced electrostatic, and higher flowability (FFC ≥ 6) compared to just micronized powder (0.19 g/ml, FFC=1.0), indicating they may be used in high drug loaded formulations amenable to direct compression.

In this work, characterization of cohesion reduction at the particle as well as bulk scales is addressed, including the investigation of analytical models to relate the properties at two scales. Two techniques, surface silanization and dry particle coating, are utilized to reduce the cohesiveness and improve the flowability of fine cohesive aluminum powders. Cohesiveness of these surface modified powders is evaluated at the particle scale through the particle properties such as surface roughness and surface energy, and at the bulk scale in both consolidated and aerated state using a Sevilla Powder Tester through unconfined tensile strength, solid fraction, settling and bubbling velocity. In addition, bulk-scale characterization using standard Angle of Repose (AoR) method is carried out. An analytical model is proposed, which allows for calculating interparticle pull-off force where the representative surface morphology is randomly generated and the plastic deformation of asperities in contact is taken into account. Experimental results indicate that surface silanization and dry particle coating can dramatically improve the flowability, and make otherwise unfluidizable powder fluidizable. The reduction of cohesiveness is largely ascribed to the reduction of surface free energy, which becomes less than 1/4 of the original value. Bond number is computed for all samples from the interparticle pull-off force and measured particle size. Surface modification drastically reduces the Bond number by approximately 2 orders of magnitude indicating the significant improvement of flowability. Representation of particle cohesiveness by Bond number has good qualitative agreement with the bulk-scale cohesiveness characterized by the Angle of Repose as well as the bubbling velocity which is measured using the Sevilla Powder Tester.

Fluidized beds are widely used for particle coating and granulation in the pharmaceutical and chemical industries because of their excellent mixing properties, high heat and mass transfer rates. and ease of scale-up. However, a conventional fluidized bed cannot be used for handling fine cohesive particles (less than 40 mu m) due to their poor fluidizability. In this paper a proprietary technique, which involves the use of nanosized particles to pre-coat the fine particles to be granulated, is developed for improving the fluidization behavior of cohesive powders. Utilizing this technique, experiments on granulation of fine particles (15 mu m cornstarch) are performed using a Mini-Glatt fluidized bed unit. After precoating the cohesive cornstarch particles can be stably fluidized in the Mini-Glatt device. By adding a top spray binder solution of PVP in water, granules having desired properties (average size and size distribution, flowability, density, etc.) can be obtained through this novel approach. The effect of various operating variables to control the properties of the final granules is also investigated. (C) 2008 Elsevier B.V. All rights reserved.

In this paper, a three-dimensional model taking into account the contact deformation and surface area coverage (SAC) of nanoadditives is proposed to predict the force required to separate two contacting particles (the pull-off force) under consolidation stress up to 10 KPa, for cornstarch, a Geldart group C powder, sparsely and densely dry-coated with nanosilica. The experimental pull-off force measurement is conducted in a Seville powder tester. Comparison of the predicted results with the experimental results indicates (1) that the pull-off force of sparsely coated cornstarch is larger than that of densely coated cornstarch due to the greater hardness and small particle radius of fumed silica; (2) there is not a continuous variation in the pull-off force with the coverage of silica; on the contrary, values of the pull-off force of sparsely coated samples are grouped in similar range, while the values of the pull-off force of densely coated samples are grouped in another range of lower values. (3) Within a range, the SAC does not have a big effect on the pull-off force for sparsely coated samples and only a slight effect for densely coated samples (4) the pull-off force increases with increasing consolidation force due to larger deformation in the contact area; (5) under consolidation stresses up to 10 KPa, the deformation of the cornstarch particles is not large enough to fully embed the nanosized silica.

The difficulty of coating cohesive Geldart group C powders in a conventional fluidized bed is attributed to strong inter-particle force between fine particles leading to poor fluidization behavior. Dry coating approach involving deposition of nanosize particles on the surface of group C powders is considered to reduce the interparticle force and improve the fluidization behavior of fine powders. Polymer film coating at an individual particle level is achieved on these pre-coated fine powders in a commercially available spouting fluidized bed (MiniGlatt). The effect of operating conditions such as inlet air temperature, polymer concentration, polymer weight ratio, water percentage in solvent and spray rate of coating solution on the quality of film coating are investigated. Experimental results demonstrate that the quality of film coating goes down as polymer concentration in coating solution goes higher. whereas the lower inlet air temperature is found to enhance polymer film generation and coating quality. It is also observed that the higher polymer weight ratio promotes agglomeration without affecting the coating quality to a great extent. An optimum water ratio in acetone-water solvent as well as spray rate can be optimized to achieve superior coating quality with acceptable agglomeration ratio. (C) 2008 Elsevier B.V. All rights reserved.

While it is well known that flow aids such as fumed silica can be added to improve flowability and fluidizability of cohesive powders, the improvements observed depend on how well the flow aids are blended together with the cohesive powders. In this work, dry particle coating is used to deposit a very small amount of nano-sized particles (as low as 0.01 wt %) with a high degree of precision onto the surface of cohesive, Geldart Group C powders to make them fluidize like Group A powders. A model taking into account the effect of the size of the guest and host particles as well as surface area coverage (SAC) of the coated nano-sized particles is developed to predict the effect of coating on the adhesion reduction of cohesive powders. A series of experiments are performed to investigate the improvement in the fluidizability of dry particle coated Group C powders (e.g., cornstarch and aluminum), and the effect of various parameters such as SAC, guest particle size and host particle size are systematically investigated. The results clearly show the effect of each of these parameters on the fluidization behavior of cohesive powders, and also validate the model. The study also indicates that a critical SAC is required to make the coated cornstarch fluidize, which is about 5%; the smaller the guest size, the better its effect on improved fluidizability, although the improvement is reduced if the guest size is smaller than about 10 nm; and if the conditions regarding the SAC and guest size are satisfied, dry particle coating will significantly improve the fluidization of cohesive particles even as small as 5–10 μm. © 2007 American Institute of Chemical Engineers AIChE J, 2008