Optimising Powder Processing – The Benefits of Cohesion
Tim Freeman, Jamie Clayton and Rajeev Dattani
Powders are often described as free-flowing or cohesive, with a typical perception that cohesive powders present processing challenges whereas free-flowing powders are relatively problem free. However, simple flow categories do not describe the breadth of behaviour that powders can exhibit, nor identify how this changes depending on the process environment. Furthermore, perspectives vary on how to quantify ‘cohesion’.
Shear cell testing, for example, generates a flow function coefficient which categorises powders into discrete groups and primarily assesses the ability of powders to discharge from hoppers and silos, where higher cohesion will typically inhibit flow. However, hopper discharge is often only the first stage of a manufacturing process, with subsequent unit operations applying a range of stress conditions and flow regimes where less cohesive powders may not always deliver the best results. In certain applications, such as spray coating and dense phase conveying, a degree of inter-particular bonding and friction is necessary for optimal process performance.
In order to optimise productivity and achieve a high quality final product, powders must be selected on the basis of their compatibility with a process, rather than specifying materials using simple parameters that may have little or no relevance to the process.
This poster illustrates how powder rheology can be used to evaluate flow properties under a range of stress and strain conditions, and demonstrates how this data can be applied to better understand and predict performance in a number of unit operations. The studies presented will also demonstrate how cohesion can be a positive attribute.
Evaluation of a New Dispersion Technique for Assessing Triboelectric Charging of Powders
U. Zafar, F. Alfano, and M. Ghadiri*
School of Chemical and Process Engineering, University of Leeds, Leeds LS2 9JT, UK
In a number of applications, especially in pharmaceutical drug development, there is often a very small powder quantity available for evaluating the manufacturability of new drugs. However, it is highly desirable to be able to quickly evaluate processing issues, and where possible using the smallest powder quantity. In the present work, a proprietary commercial powder dispersion device (the disperser of Malvern© Morphologi G3) is adapted to evaluate the triboelectric charging tendency. A very small powder quantity (as small as 0.1 mg) is dispersed by a pressure pulse of compressed gas such as air or nitrogen. This causes the particles to become air borne and collide with the containing walls, resulting in dispersion and leading to triboelectric charge transfer between the particles and the walls. In this work, the charging propensity of a number of materials is evaluated and the effect of particle surface functional groups on the tribo-electric charge transfer is analysed. Model materials with a well-defined shape (glass ballotini) but with different silane groups deposited on their surfaces as well as a number of organic crystalline particles (such as aspirin, α-lactose monohydrate and paracetamol) are tested. Following dispersion the particles move immediately to a Faraday cup placed directly underneath the disperser. Therefore, particle charge is measured with no decay. The method can differentiate charging of different polymorphs of the same material, different silane groups on the surfaces of glass ballotini and different crystal morphologies obtained from crystallisation from various solvents.
Keywords: Triboelectric charging, Malvern Dispersion unit, Pharmaceutical powders, Surface properties.
 On Erasmus Exchange Programme from Universita della Calabria, Arcavacata di Rende (CS), 87030, Italy.
THE ROLE OF GLIDANTS IN INTERPARTICLE FRICTION
Sadegh Nadimi1, Mingwen Bai2, Beverley Inkson2, Anne Neville1& Mojtaba Ghadiri1
1University of Leeds, Leeds, UK
2University of Sheffield, Sheffield, UK
Manipulating friction in particle engineering is of great interest in a wide range of manufacturing processes, but its characterisation is very challenging due to measurement difficulties. In bulk shear deformation, the interparticle interactions are directly affected by surface characteristics, including shape, roughness and adhesion. It is common practice to add a glidant to reduce friction in order to improve the powder flow and compaction. For example in pharmaceutical industry, this improves the drug delivery, in particular for the pulmonary route1, but also in tabletting. However, the role of glidant as shape or roughness modifier and adhesion reducer is not well understood. In this study, the carrier with known shape in roughness and adhesion is coated with magnesium stearate (MgSt) using a high shear mixer. The spatial distribution of the glidant coating on the particle surfaces is quantified using energy-dispersive X-ray spectroscopy (EDX) under scanning electron microscopy (SEM) / electron probe micro-analyzer (EPMA) andtime of flight secondary ion mass spectrometry (TOF-SIMS). The adhesion and friction of the coated particles are quantified using the Drop-Test Method2and Nano-Tribometer. The insights into the role of glidant are fundamental for better understanding and control of particulate solids processes in many manufacturing operations.
Keywords: Friction, Glidant, Magnesium Stearate, Powder Flow.
1Preedy, E.C. and Prokopovich, P., 2013. Novel coatings and biotechnology trends in inhaler devices. In Inhaler Devices, edited by Polina Prokopovich, Woodhead Publishing, pp. 37-50.
2Zafar, U., Hare, C., Hassanpour, A. and Ghadiri, M., 2014. Drop test: A new method to measure the particle adhesion force. Powder Technology, 264, pp.236-241.
Impact of Milling on Flowability of Polyols in Pneumatic Conveyers
Pari Rao(_Pari Rao), Jean-Yves Mugnier*
Scientist II, Reading Scientific Services Ltd.
* Senior Scientist II, Reading Scientific Services Ltd.
Milling is often used to reduce the particle size of powders to meet formulation and texture requirements during product development. However, it can impact the physical properties of materials and result in problems during production such as caking, ratholing, cohesive arching and seggregation. It is therefore crucial to understand the structure-function relationship of powders from both product development and manufacturing perspectives.
The objective of this study was to investigate the root cause of poor flow and caking observed in polyols milled from two suppliers during flow through dilute phase pneumatic conveyers. Physical properties such as particle size, crystallinity, morphology, moisture uptake, permeability and compressibility of the two powders were compared to correlate them to flowability and caking observed in manufacturing.
The results of particle size measurements indicated clear differences in size distribution; the powder that flowed well had a larger d90. The SEM images indicated clear differences in surface morphology with indication of zone melting. The SEM images also illustrated blocking of interstitial voids between the larger particles by the fines in the poor performing sample. This phenomenon could potentially restrict the passage of air and reduce permeability. These findings correlated with the FT4 permeability tests that indicated a higher pressure drop and compressibility for the poor performing sample, indicating poor permeability to air.
The removal of 50% of particles <25µm in the polyol with poor flow performance, reduced the pressured drop by 50% and compressibility by 30%; subsequently increasing permeability. The DVS analysis on samples before and after milling indicated a significant difference in moisture uptake that can impact caking.It was hypothesized that a combination of higher amount of fines and a potential amorphous layer generated could lead to formation of liquid bridges that could recrystallize to irreversible solid bridges, subsequently leading to caking. Work is still in progress to identify the caking mechanisms in these polyols as this could not be confirmed by DSC and XRD.
Cohesive Powder Flow of Faceted Particles in Screw Feeders
Alejandro López, Vincenzino Vivacqua, Mojtaba Ghadiri, Robert Hammond
School of Chemical and Process Engineering, University of Leeds, Leeds, UK
Powder flow in screw feeders is of great interest to a wide range of industries. However, analysis of flow of cohesive powders with sharp corners and edges presents a great challenge and is not yet well understood. In the present work, an in depth analysis of cohesive powder flow of faceted particles in screw feeders is carried out. The influence of fundamental parameters (physical properties) and phenomena such as cohesive arching in the hopper and screw feeder pitches are studied and their influence on the outlet mass flow rate is evaluated. Cohesive arching only takes place when the surface energy of the particles goes above certain values and its onset is also affected by the particle geometry.
Parameters for the simulations are carefully calibrated through different test methods such as the Drop Test Method for surface energy and High speed camera footage for the coefficient of restitution.
Cohesive arching and flow irregularities are studied and the parameters leading to this conditions are analysed. Different contact models for cohesive particles are implemented and tested both in Rocky DEM and EDEM software packages and the results obtained with faceted particles are compared with the clumped-spheres method. Differences arise between both methodologies due to, mainly, the sharp corners of the faceted particles. The computational results are compared with experiments in the FT4 rheometer showing a good correlation between both.