ARR - Annual Report

The aim of this study is to understand the behavior of mineral particles in concentrated electrolyte solutions using surface force techniques. To this end there are two significant challenges.

1. The first challenge relates to the type of surface forces that dominate at high electrolyte concentration. They are very short in range and poorly understood theoretically, but it is known that they are related to the solvation of the surface layer of a material or ions adsorbed to that layer, hence they are called solvation or in aqueous solutions hydration forces.

2. The second challenge is to prepare surfaces that are suitable for investigation by surface force measurement techniques and is intimately related to the first challenge as the very short range over which hydration forces operate requires that surface roughness is controlled at a level comparable to or less than the range of the hydration forces.

Therefore much effort has concentrated on acquiring or producing suitable surfaces for investigation. With silica surfaces this has not posed any significant challenge but other surfaces have not met with success. To this end we have purchased an Atomic Layer Deposition system that will enable us to deposit materials that mimic the mineral surface of interest onto silica surfaces. This instrument has now arrived and been installed in our laboratory. However there have been considerable delays in the arrival of the precursor chemicals, therefore we have not been able to use the instrument for the preparation of mineral like surfaces to date. These chemicals have now just arrived and we can now proceed. We have continued to develop the photon pressure technique but we do not present any new data relevant to this study here as measurements using the photon pressure technique require ultra-smooth surfaces and we have already presented data on silica surfaces the only surfaces that have been sufficiently smooth for this use up to this time. These surfaces will now be available to us with the arrival of the precursor chemicals for the ALD system.

The overall aim of the project is to elucidate the effect of feed material properties, mill dynamics and prevailing environment on milling of organic materials. This requires the use of a multi-scale approach covering molecular scale, to single particle scale and the bulk scale.

The recently concluded IFPRI programme (IFPRI FRR 52-03) addressed the effect of material properties on single particle breakage and bulk milling, which enabled establishment of a relationship between the single particle properties and the bulk milling. The main aims of this follow-up programme are to understand single particle breakage behaviour from the molecular scale, and to bridge the gap between the properties and behaviour at the single particle scale and those at the molecular scale. The principle methodology used in the follow-up programme is to investigate the single particle breakage behaviour as a function of temperature, humidity and strain rate. The work over the past 12 months has led to the following results:

• Effect of temperature was investigated at a relative humidity of ~30%. The results show that the extent of impact breakage of aspirin particles increases with increasing temperature, whereas little effect of temperature is seen for sucrose.

• Effect of humidity was studied at the ambient temperature (20oC). The results show no clear influence of the relative humidity for sucrose and aspirin particles on their single particle breakage behaviour.

• Effect of repeated impacts on single particle breakage (fatigue tests) was investigated using sucrose at the ambient temperature and ~30% relative humidity. The results show that the extent of breakage increases first with increasing number of impacts, reaches a maximum after a certain number of impact, and then followed by a slow decrease with a further increase in the number of impacts. The impact velocity has a great effect on the maximum value of the cumulative extent of breakage; a higher impact velocity gives a higher peak breakage extent; however, the number of impacts needed to reach the peak breakage extent also increases with increasing impact velocity.

We have employed Atomic Layer Deposition (ALD) to produce mineral like surfaces that are extremely smooth for use in surface force studies. Our investigations to date have focused on alumina and titania surfaces.

Alumina Surfaces

Alumina surfaces were found to be unstable in aqueous solution – they slowly dissolved. This prevented surface force investigations in simple aqueous solutions. However the surface could be passivated against dissolution through the adsorption of short chain carboxylic acids. These acids are of industrial interest as they have significant effects on the rheology of alumina dispersions. Examination of the surface forces between alumina surfaces in solutions of muconic acids revealed DLVO type forces under some conditions Non‐DLVO forces where a strong attraction was evident between the surfaces and is significantly stronger than van der Waals attraction. This attraction was attributed to the formation of a capillary consisting of an oil‐like muconic acid phase forming between the surfaces. This phase change is induced by the close proximity of the surfaces and is possible because the muconic acid is present at concentrations that approach the solubility limit in these solutions. The presence of a capillary between the surfaces results in a strong attraction. Attempts were made to form stable alumina surfaces that would enable surface force measurements to be conducted in water and electrolyte solutions. This included looking at much thicker layers and using different binding layers (such as titania). To date none have been successful. We are still pursuing this though it may be possible that all alumina surfaces – not just ALD surfaces – have this property. A slow rate of dissolution would not be revealed in many studies and therefore may have previously gone unnoticed. Evidence from Optical Reflectometer (OR) shows that the surface dissolves at a rate of ~8 nm per hour. So indeed the rate of dissolution is slow, but sufficient to prevent surface force or optical reflectometry measurements.

Titania Surfaces

In contrast, titania surfaces are stable and this has allowed us to perform a range of surface force studies at both low and high salt concentrations. At low salt concentrations a long range, pH dependent electrostatic force was observed. This data could be fit using the DLVO theory, which enabled the surface potential to be determined. This showed that the isoelectric point was between pH 5 and pH 6. At short range a repulsive interaction dominated the attractive van der Waals force. This is attributed to hydration forces. At high salt concentrations adhesion was seen that was dependent on both the pH and specific salt present. We find that this trend does not follow the Hofmeister series.

In many suspensions, slurries and complex fluid formulations of industrial interest, the colloidal scale interparticle forces are attractive. These attractions set up a complex microstructure with slow dynamics that determine properties such as yield stress, stability and shear-rate dependent viscosity. In particular industrial situations, these properties may be more or less desirable; however, in every case, prediction of these rheological and stability properties from underlying microstructure, especially their variation as function of time, is of paramount interest in process and product development. Because industrial materials are comprised of colloids with heterogeneous interparticle forces, polydisperse size distribution and non-uniform shape, the effect of these parameters on the microstructural origin of yield stress and stability should be assessed. Current capabilities for prediction of rheology from underlying microstructure is largely limited to linear properties in systems with well-characterized interparticle forces and monodisperse particle sizes.

In this project we are developing tools for characterization of the microstructural evolution in gelled colloidal systems that can be applied to non-linear rheological and stability phenomena, such as stress-induced yielding and gravitationally-induced collapse. These microstructural characterization tools can be applied to develop a quantitative link between microstructure and bulk suspension properties, such as the gel modulus, yield stress and delayed sedimentation. This project will expand our current knowledge of attractive systems by explicitly considering the effects of size polydispersity on the gel microstructure and bulk rheology, thus enabling us to bridge the results of model systems to relevant industrial materials. Microstructure will be quantified by direct visualization with confocal and optical microscopy during flow and sedimentation. Concomitant measurements of non-linear rheological properties such as the yield stress on identical systems will also be conducted. Interactions and forces will be probed by laser tweezers microrheology. Systematic exploration of polydispersity effects will be accomplished by mixing fractions of monodisperse colloids in known amounts to achieve a suspension with well-characterized polydispersity.

The ARR53-03 report presented some further results obtained from both laser triangulations and multiple projection techniques. It contains new developments about 3D image processing of particles with special reference to the use of a Euclidean distance function as a tool to address different shape properties (dissolution; wear; etc.). A case study concerning the 3D analysis of a Zn powder is presented. The size distribution curve is discussed and presented using different parameters to compare with laser diffraction data. The geometrical interpretation of both 2D and 3D image analysis data is very coherent and demonstrates if needed, the sensitivity of laser diffraction to shape. Because of randomized particle orientation and non spherical shapes the laser diffraction results show a wide size range and an overestimation of large sizes whereas image analysis reveals a rather well calibrated distribution.

Despite considerable efforts that have been made in the past with regards to milling, no single model has the capability to predict the milling behaviour of materials. The objective here is to further understand the role of material properties in breakage, under the influence of temperature. The methodology is to characterise the mechanical properties of materials at the single particle level using quasi-static nano-indentation and dynamic impact tests. The effect of temperature on the breakage behaviour of these materials is investigated under dynamic impact conditions and the effect of temperature on mechanical properties indirectly inferred. Furthermore, dynamic impact breakage is compared to bulk milling behaviour, under the influence of temperature. A widely used pharmaceutical active ingredient, Aspirin, and two excipients sucrose and -lactose monohydrate (-lm) are chosen as model materials. All three are regarded as semi-brittle materials with different mechanical properties.

Granulation is a complex process in which many input variables influence many product properties. As Iveson et al. describe in a review paper (2001), the understanding of the fundamental processes that control granulation behavior and product properties have increased in recent years. This knowledge can be used during process design, in choosing the right formulation and operating conditions, and it can also be used to improve process control. Although many variables are set constant during process design, variations during production in input variables occur due to the variable nature of the powder feed. Even if all granule properties, except for size, are ignored for process control, a one dimensional granule size distribution can be constructed by multiple discrete output variables, in order to represent the shape of the distribution (these can be mean sizes (with coefficients of variation), percentile sizes, moments 4 or size bins). Model Predictive Control (MPC) is an effective method to control such multiple input, multiple output processes (García, et al., 1989). More details motivating the choice of MPC for granulation processes along with examples from the literature can be found in previous reports (IFPRI# ARR51-02 and IFPRI# ARR51-03).

Partially saturated powders consist of a bulk solid whose cavities are to a certain extent lled with binder liquid. The presence of air signicantly aects the ow behavior of such systems due to interparticle frictional forces showing a strong normal force dependency. The role of the binder liquid is ambiguous as it increases normal forces between particles by capillary suction pressures and surface tensions at the contact line between liquid and solid but on the other hand reduces the coecient of internal friction between the particles by acting as a lubricant on the particle surface. Additionally interlocking eects between particles can signicantly hinder shear ow when partially saturated powders are in a densely packed state. Thus the extrusion of wet powders with minimised binder liquid fractions is often impeded by blocking of the material in the die inlet region. However, under certain process and geometrical conditions shear and pressure forces in the die inlet region can transform the 3-phase-system (3P) into a state where increased saturation as well as an orientation of the particles in shear layers reduce the resistance against ow. The reduction of the entrapped air cells ideally results in a local saturation of the system making it a 2-phase-suspension (2S) with viscous friction instead of Coulomb solid friction dominating its ow behavior. Small-scale ram extrusion combined with visual observations can be used in order to quantify critical stresses which have to be surmounted before reaching steady-state ow. Critical stresses are in uenced by particle properties (size/size distribution, shape and surface), binder liquid viscosity and particle/binder liquid interfacial properties (contact angle, interfacial tensions). An understanding of the mechanisms of wet powder ow and the interdependencies of process and material parameters during extrusion can be used for the design of product specic tailor-made processes and allow for processing of powder/binder systems with minimized binder fraction, thus reducing potential drying costs.

Project Report

We report here on work performed on the project in its third year ending October, 2009. Two previous reports have been submitted to IFPRI in 2006 and 2008, respectively. The present research is focused on the study of Powder Mechanics and the ultimate goal is to develop a quantitative description of active flows of a wide variety of powders. The study is centered on the slow, frictional and the dense, “intermediate” regimes of flow where both frictional and inertial effects are important. The novelty of the project is the study of a large range of materials and flow geometries to gain meaningful insight.

Materials and Methods

We report on a series of materials from simple (round beds) to complex (fine, odd-shaped and elastic), used in an axial-flow Couette, high shear mixer, in a centripetal geometry characteristic of a “spheronizer and two hopper flows with a funnel and a flat bottom, respectively to measure stresses and their fluctuations as a function of geometry and shear rate.

Main Novelty

The main novelty during this year is the measurement of porosity (void fraction) and porosity distribution in the flowing material using a capacitance probe in addition to stress measurements.

Executive Summary

The aim of this study is to understand the behavior of mineral particles in concentrated electrolyte solutions using surface force techniques. To this end there are two significant challenges.

1. The first challenge relates to the type of surface forces that dominate at high electrolyte concentration. They are very short in range and poorly understood theoretically, but it is known that they are related to the solvation of the surface layer of a material or ions adsorbed to that layer, hence they are called salvation forces, or in aqueous solutions, hydration forces.
2. The second challenge is to prepare surfaces that are suitable for investigation by surface force measurement techniques and is intimately related to the first challenge as the very short range over which hydration forces operate requires that surface roughness is controlled at a level comparable to or less than the range of the hydration forces.

At this point in the project we have successfully produced titania and alumina surfaces that are ideally smooth. The alumina surfaces have proved to be unstable in electrolyte solutions, though it was possible to measure the forces between alumina surfaces bearing adsorbed molecules that passivate the surface to dissolution. We have some evidence that stable Alumina surfaces can be formed by annealing the Ald surfaces and this is related to an increase in crystallinity. Investigations using Titania surfaces are continuing both in salt solutions and in the presence of both cationic and anionic surfactants and in the presence of a range of organic acids.