Size Reduction

The present annual report composes of three parts; (1) mechanochemical (MC) syntheses of rare earth oxyhalides (ROX, R=La, Sm, Nd, Pr, X=F, Cl, Br) to form functional materials from inorganic systems, (2) MC syntheses of solid state solutions such as LaOCl1-xBrx from mixtures of rare earth oxides (R2O3) and rare earth halides (RX3, X=F, Cl and Br) from the inorganic systems, and (3) MC synthesis of LaOF from an inorganic-organic system (La2O3 and polyvinylidene fluorine (PVDF)). Regarding the first part, the grinding the constituent components (R2O3 and RF3) enables us to form ROF monophase in the product, and the reaction proceeds with an increase in grinding time, and the crystallite size of the product formed is about 15~20nm. As for the second part, the MC reaction proceeds as grinding progresses, forming LaOCl1-xBrx. Unit cell dimensions, a, c and lattice volume of the solutions, evolve linearly with an increase in x in the LaOCl1-xBrx series. Comparing unit cell dimensions of LaOX synthesized by MC reaction to those of LaOX synthesized by solid state reaction at high temperature, there is not any significant difference in the length of c, while a is shortened slightly. This may be attributed to the complex cation layer of (LaO)nn+ with a close relationship to a of the cell dimensions, being affected by the intensive grinding. As for the final part, the reaction through the substitution of F- by O2- results in defluorination of PVDF forming LaOF. This reaction proceeds with an increase in grinding time and is almost completed by about 240min. Mean size of the synthesized LaOF particles is sub-micron order, and the particles look like agglomerates. By prolonged grinding, agglomerated fine particles, consisting of LaOF and resultant organic materials, are transformed to the composites of rupture-like shape and these particles are covered with residual fine particles, progressively. Bonds such as C-O-H and C=C, are formed in the resultant organic phase in the ground mixture. The reaction yield reaches about 98 % at 240min. From these investigations, they have found a new route as well as something that leads perhaps to high possibility of application of this MC reaction to pharmaceutical field.

The future work will be focused on the grinding a mixture of a pharmaceutical raw material like talc with a binder such as cellulose to investigate MC reaction and molecular design between the two materials. In addition, the MC reaction and effect for food materials such as lactose will be investigated. In this study, co-grinding of lactose with cellulose with or without additive such as paracetamol will be made.

Our basic concept in the project was as follows: for a fundamental discussion on the phenomena of electrostatic charging of powder, it is essential to study the charge generation due to a single impact/contact on a single particle. To realize the concept we proposed two directions:

Approach 1: Impact Charging Experiment:

To apply the previous version of “impact charging experiment,” in which rather bigger particles were used as samples, to smaller particle sizes, the sensitivity of the charge measurement and trajectory control of the accelerated particles were required to be improved. In the first year project, we carried our the improvements, and new rig, which is available for hundreds micro- meter particles, was manufactured successfully. In the second year, actual experiments were performed, but we encountered with data scattering, while the order of magnitude of the data agreed well with that predicted by “charge relaxation model” which we have proposed as a scheme determining the amount of the impact charge.

In the third year, we tried to improve the rig with some points which could cause the scattering. In a point, particle trajectory control, was improved successfully, and the data accumulation was improved. However, the data scattering did not disappear: a question to be answered here was if there was a reasonable cause of this data scattering. Some discussions were done, and as its result, a localization of the initial charge was supposed. A top and a rear side localization against the contact point, gave two limitations. The scattering data were indeed held within the range. As a conclusion, the charge relaxation model works in the range of the particle size from 100 to 300 micro-meter. For the future study, a successive impact experiment for electrostatic charging of particles would be recommended.

Approach 2: Development of the Method of Electrostatic Adhesive Force Measurement:

This is the completely new approach to determine the amount of charge transferred onto a par- ticle due to a contact from a measurement of adhesive force curve at approaching and separating particle against a metal target.

In the third year project, we have actually launched AFM study. Now the experiments are still pilot status, but a force curve due to electrostatic charge was measured successfully for a 35 micro-meter particle. The amount of the electrostatic charge was estimated in the order of 0.1fC, and at this moment the sensitivity is enough for the order of magnitude of the amount of charge, which corresponds 1000 electrons (elementary charges). In the next year project, actual and detailed discussion will be available.

Present annual report-2002 described results on the second year’s work after renewal of the first period (1998-2000) in the theme of “Mechanochemistry of Materials” approved by the IFPRI organization. The work focuses on structural change of cellulose by grinding and its dissolution in aqueous NaOH solution. Under this concept, the contractor (F. Saito) and his group have extensively investigated the mechanochemical (MC) work from both viewpoints of experimental and computer chemistry. Several characterizations such X-ray diffraction (XRD), TG-DTA and FT-IR analyses have been conducted for the ground sample, and regarding the XRD analysis, the grinding enables us to transform the structure of cellulose into disordered system like an amorphous state in the prolonged grinding. The TG-DTA and FT-IR spectra are almost the same as those in the initial stage, the dissolution of cellulose in the aqueous NaOH solution is however improved. Cellulose has donor and accepter composing hydrogen bonds in the molecule, and the bond is strong relatively in comparison with other similar structured materials such as glucose and lactose. In spite of this, improvement in the dissolution of cellulose in the solution may be due to the disturbance of internal molecules of hydrogen bonds and their rupturing. This is followed by the investigation on the computer chemistry work, which has shown the weakest part of the chain structure of cellulose in the all, so that the rupture may be initiated to break at C-O bonds in the molecule. Of course, other parts of the structure are also subjected to damage by the grinding, however, the unit cell like cellubiose may remain unaltered in the prolonged grinding. This may be confirmed from the results of TG-DTA and FT-IR analyses. The substance re-crystallized from the solution may be cellulose, of which structure may have slightly different from the initial one. Grinding the cellulose results in the agglomeration of particles, of which sizes are not significantly changed. These facts suggest the suitable application of the ground cellulose to food and pharmaceutical fields.

This work has been presented at the IFPRI AGM 2002 held at Sendai, Japan in 15-18, July 2002. At this moment, it is still unknown, but “mechanochemistry of cellulose” has a potential to extend to food and pharmaceutical fields, because the ground sample of cellulose is, in fact, still maintaining the same unit cell even in the prolonged grinding stage, changing its part of the structure, correspondingly its physico-chemical properties such as solubility. During the course of this investigation, the evidence may be initiation to start an application of mechanochemistry to food and drugs field by modification of morphology and structure of such products. This is also a great expectation of this series of work to come.

The increasing inter-particle interactions with increasing fineness are a serious problem during wet grinding in stirred media mills. Because these inter-particle interactions have a big influence on the stability against agglomeration and on the rheology of the suspension. As shown in the literature [1] and in this report, a lower boundary of possible particle size is typically reached at around 1 μm. Furthermore the agglomerate size can even increase with further grinding, building larger particles than the original ones, that are strong enough to withstand the comminution process.

In this work we postulate that colloidal stability must be considered in wet grind- ing to understand there results and surmount there limitations on the production of nano- particles.

To study the grinding limits of particle sizes below 1 μm a detailed understanding of the agglomeration process and its mechanism is needed. Therefore the agglomeration process of fine particles is discussed in this report first under Brownian and later on under turbulent shear motion similar to the motion in stirred media mills. The agglomeration kinetics as function of added salt was measured using dynamic light scattering (DLS). Further information on the agglomeration process and the structure of the agglomerates give small angle neutron scattering (SANS) experiments.

Theoretical predictions on the stability ratio and the critical coagulation con- centration based on the DLVO-theory were calculated using measured values of the ζ-potential.

Furthermore an initial investigation to describe the equilibrium between break- age, agglomeration and deagglomeration in stirred media mills has been conducted. The problem is described by the population balance model, which will be solved using the moment method.

Executive summary

The contractor (Fumio Saito) has conducted his group to investigate his work on mechanochemistry of materials supported financially by IFPRI Inc. for six-years since 1998 to 2003. The first year’s work has been devoted to study the effect of dry grinding of a CaO-SiO2 mixture on synthesis of para-wollastonite by heating, followed by synthesis of tricalcium aluminate hydrate (3CaO·Al2O3·6H2O (C3AH6)) by dry grinding a mixture of calcium hydroxide and boehmite. He has also dealt with mechanochemical (MC) direct synthesis of CaTiO3 from a CaO-TiO2 mixture and its high-resolution transmission electron microscope (HR-TEM) observation. These were the work done in 1998, and, the main results are as below:

The contractor has attempted to synthesize CaTiO3 from a mixture of TiO2 and CaO. It has been known that there are three polymorphs in TiO2, i.e., anatase (TiO2, tetragonal), rutile (TiO2, tetragonal) and brookite (TiO2, orthorhombic). The use of anatase leads to MC synthesis of crystalline CaTiO3 from the mixture with CaO easier than from the CaO-rutile system. Grinding the CaO-anatase mixture for 2 hours or more enables us to produce very fine particles of about 20nm in the first order mean size. The amount of CaTiO3 in the ground product increases with improving its crystallinity as the grinding progresses. Many CaTiO3 crystal grains of about 5nm are formed in the mixture ground for 2 hours, and they grow up in the prolonged grinding. The grain size of the CaTiO3 crystals reaches about 20nm by about 5 hours of grinding, and the grain boundary and lattice fringe become clear as the grinding progresses.

The second material is 3CaO·Al2O3·6H2O (C3AH6), which has been synthesized mechanochemically from the mixture composed of calcium hydroxide (Ca(OH)2) and pseudo-boehmite ( -AlO(OH)) powders by room temperature grinding using a planetary ball mill. Use of -AlO(OH) sample with inferior crystallinity is more favorable for the mechanochemical synthesis rather than that with well crystalline one. The time required to form C3AH6 from the Ca(OH)2 - -AlO(OH) mixture is much longer than that from the Ca(OH)2-gibbsite (Al(OH)3) one. Adsorbed water from air during grinding plays a significant role in the formation of C3AH6 from the former mixture. After water addition to the Ca(OH)2 - -AlO(OH) mixtures ground for various times, excess hydrated calcium aluminates such as C2AH8, C3AH8-12 and C2A0.5H6.5 are formed in the starting and the short time ground mixtures, while a few amount of these compounds is formed in these hydrated mixtures after prolonged grinding. Formation of these excess hydrated compounds, which belong to layered structural materials, is enhanced in the presence of free Ca, Al compounds and water. A mixture of CaO and silica-gel (SiO2) was subjected to grinding using a planetary ball mill, followed by heating to investigate the temperature for synthesizing para-wollastonite (CaO·SiO2). The MC treatment of the mixture brings about amorphous aggregates with almost homogeneous chemical composition. 2-hours MC treatment enables us to synthesize para-wollastonite by heating for 2-hours at 1273K, which is significantly lower by about 130K as usual. Heating the 5-hour ground mixture at 923K gives us to form a precursor of wollastonite, leading to its easy crystallization at higher temperature than about 1273K. Thus, it is found that the MC treatment for the mixture before heating is quite effective for synthesizing para-wollastonite.

The suggestion from the IFPRI members has come to the contractor for the second year’s work in 1999 on MC interaction between organic and inorganic materials. This is the initiation to start the solid-state reaction between polyvinyl chloride (PVC, [CH2CHCl]n) and CaO and/or Ca(OH)2 powders. The mixture was subjected to grinding using a planetary ball mill under different conditions, to investigate their MC reactions. The grinding causes dehydrochlorinating reaction, forming CaOHCl and [CH=CH]x. The reactivity against PVC of CaO is superior to that of Ca(OH)2, but all the same, the reaction yield is advanced as the grinding progresses. Furthermore, the yield and rate of the reaction are improved with an increase in the molar ratio of (CaO/PVC) as well as the rotational speed of the mill. Impact energy of balls would be also an important operational parameter governing the MC reaction.

In the third year (2000), the contractor has made the work for estimating the yield of MC processes by the use of ball mill simulation work based on the Discrete Element Method (DEM). This work enables us to find out the optimum condition of the MC process as well as the scale-up role of a MC reactor (a mill). The present study has been composed of three examples:

1. MC treatment of EP dust, forming soluble vanadium (V) compound in water,
2. MC treatment of fluorescent powder, accelerating its structure change, and
3. dechlorination of polymers with halogen by its MC treatment with inorganic material such as CaO.

Regarding the first example, the yield of vanadium extracted by water leaching is well correlated with impact energy of balls in the mill calculated from the result obtained by. As for the second example, the dry grinding the EP dust enables us to form a water soluble vanadium compound. The well correlation between the V-yield and the impact energy of balls is obtained, suggesting that the ball impact energy plays a significant role to control the formation of vanadium compound. The third one is dechlorination of polymers such as PVC (poly-vinyl chloride), PVDF (poly-vinylidene fluoride) and PTFE (poly-tetra fluoro-ethane) by their MC treatment with inorganic material such as CaO. The report described only the dechlorination of PVC and its correlation with the impact energy of balls in the mill calculated from the result simulated. All the same, the impact energy of balls in a mill is a significant key to control MC effect and reaction. In such sense, the computer simulation regarding the ball motion during milling is a quite useful tool for determining the optimum operational parameters, mill design with scaling-up.

The paper shows the possibility to produce alumina nanoparticles in a stirred media mill by an appropriate adjustment of the suspension properties and the milling parameters. Besides a high electrostatic suspension stability that can be realised for metal oxides by means of pH value adjustment small grinding beads favour the production of alumina particles with a median particle size of around 10 nm. In addition to size reduction mechanochemical changes and the formation of alumina hydroxid are detected during wet grinding of alumina. This is analysed by means of X-ray diffraction analysis, (XRD), thermogravimetry (TG) and dynamic scanning calorimetry (DSC) measurement and a quantitatively good agreement between the three methods could be obtained. Further, it is proved that the hydroxide produced dissolves at pH values lower than 5 thus influencing the grinding process under these conditions.

The increasing industrial demand for nanoparticles challanges the application of stirred media mills to grinding in the sub-micron size range. It was shown recently [1] that the grinding behavior of particles in the sub-micron size range in stirred media mills and the minimum achievable particle size is strongly influenced by the suspension stability and thus the agglomeration behavior of the suspension. Therefore, an appropriate modeling of the process must include a superposition of the two opposing processes in the mill i.e. breakage and agglomeration which can be done by means of population balance models. Modeling must now include the influence of colloidal surface forces and hydrodynamic forces on particle aggregation and breakup.

In this report the modeling of the sub-micron grinding is done by a superposition of the population balance models for agglomeration and grinding with the appropriate kernels. This leads to a system of partial differential equations, which can be solved in various ways numerically. Here a modified h-p Galerkin algorithm which is implemented in the commercially available software package PARSIVAL developed by CiT (CiT GmbH, Rastede, Germany) and the moment methodology according to Diemer [2], [3] are used and compared. This includes a comparison of the derived particle size distributions, moments and its accuracy depending on the starting particle size distribution and the used agglomeration and breakage kernels. Finally, the computational effort of both methods in comparison to the prior mentioned parameters is evaluated in terms of practical application.

Furthermore the fundamental work on the agglomeration process and its mechanism described in the IFPRI report 2002 was continued and improved. Experiments are performed on a well-characterized, model system of monodisperse primary nanoparticles that are destabilized and aggregated under various milling conditions. Conditions spanning Brownian to turbulent collision aggregation in model stirred media mills are explored to study the effects of colloidal stability on the aggregation process. The agglomeration kinetics are measured using dynamic light scattering (DLS) as a function of particle and electrolyte concentrations. Further information on the agglomeration process and the structure of the agglomerates are also obtained from small angle neutron scattering (SANS) and rheo-optical light scattering experiments (ROA). Theoretical predictions based on independently measured particle and solution properties as well as mill characteristics are compared against the experimental results to demonstrate that particle aggregation kinetics in a stirred media mill can be controlled through the colloidal interactions and the milling conditions. This research provides a theoretical basis for understanding stirred media milling of nanoparticle slurries and as such, is a step towards a predictive model of sub-micron stirred media milling.

Finally the paper reports about mechanochemical changes during wet grinding of alumina in stirred media mills. The results show the formation of alumina hydroxide that can be analysed by means of X-ray diffraction analysis, thermogravimetry and dynamic scanning calorimetry. The formation of hydroxide is strongly dependent on pH value which influences the grinding mechanism in the nanometer size range.

The investigations of the last five years have shown that a production of stable suspensions of hard ceramic materials with a median particle size smaller than 10 nm is possible by wet grinding in a stirred media mill. Nevertheless, no absolute limit of the grindability was yet reached.

By means of experimental results the influence of the electrostatic stabilization on different pH-values during the comminution of fused corundum on the grinding progress and the grinding media wear is discussed.

Results for comminution of fused corundum with grinding media of different diameters between 100 μm and 1300 μm and stirrer tip speeds between 6 m/s and 15 m/s are presented and discussed with regard to the grinding progress and the grinding media wear.

The particles in stirred media mills are stressed and ground between two grinding beads, one grinding bead and a wall or between a grinding bead and an agitator element in zones of high energy density [1, 2, 3]. Only for a desagglomeration or a desintegration of microorganisms shear forces are a further grinding mechanism.

During the approaching process of two grinding beads or a grinding bead and a wall the fluid between the collision partners will flow out of the gap. The smaller the product particles are, the better they will follow the fluid out of the active volume being the volume in which particles can be captured and stressed [4, 5].

With decreasing product particle sizes two opposite effects can be observed:

• While two grinding media approach each other, the fluid between them will be displaced. Wereby particles will follow the fluid flow out of the gap more easily the smaller the particles are. This may lead to media contacts without any grinding.

• On the other hand, with decreasing product particle size the absolute number of particles increases with the reduction factor to the power of three. Thus, a capture and stressing of more than one particle or a particle bed is possible.

The loss of kinetic energy during the collision is partly used for comminution. It has to be asked, how the energy is transferred to the feed particles. For that reason it is interesting to know how many particles are captured. According to the number of captured particles three cases can be distinguished [6]:

a) Only one particle is captured, which is stressed with the total energy (single particle stressing).

b) More than one particle is captured between two beads, all particles have contact to both beads during the stress event and all particles are stressed independent of each other. In this case at first that particle is captured, which has the largest size and/or which has the smallest distance to the connection line of both bead centers. This particle is stressed with the maximum energy. The particles, which are captured between the two beads after the first particle, are stressed with a considerably reduced energy. At the end of the stress event diverse single particle stressings with different intensities occur.

c) A particle bed is captured and stressed between two grinding beads.

Up to now it is not investigated how many particles are captured and stressed.

For a better understanding of the stress events in the sub micron particle size range, the particle motion in the displacement flow between a grinding chamber wall and an approaching grinding media was observed. Due to the small sizes of grinding beads and especially of product particles the product particle motion cannot be investigated optically in existing (real sized) stirred media mills. For that reason a model was developed. Parameters as volume concentration, approaching velocity and angle, model grinding bead diameter and material were varied.

Nanotechnology applications in the pharmaceutical, materials, and chemical industries has renewed interest in the use of wet grinding in stirred media mills for the production of nanoparticles. However, challenges arise in the production of sub-micron particles that are, in part, due to colloidal surface forces influencing slurry stability and rheology. As often observed in the literature, a grinding limit in the range of 0.5 μm is reached despite high energy inputs and aggressive milling conditions. Furthermore, the product size can even increase with increased energy input, a seemingly counterintuitive result that may be attributed to aggregation of fine particles during the comminution process. In this work we postulate that colloidal stability and rheology must be considered in wet grinding to understand these results and to surmount limitations on the production of nano-sized particles. Experiments are performed on a well-characterized, model system of monodisperse primary nanoparticles that are destabilized and aggregated under various milling conditions. Conditions spanning Brownian to turbulent collision aggregation in a model stirred media mill are explored to study the effects of colloidal stability on the aggregation process.

The agglomeration kinetics are measured using dynamic light scattering (DLS) as a function of particle and electrolyte concentrations. Further information on the agglomeration process and the structure of the agglomerates are also obtained from small angle neutron scattering (SANS) experiments both at rest and under flow. Theoretical predictions based on independently measured particle and solution properties as well as mill characteristics are compared against the experimental results to demonstrate that particle aggregation kinetics in a stirred media mill can be controlled by tailoring colloidal interactions and the milling conditions. Furthermore it is shown that the concept of electrostatic stabilization during wet grinding of nanoparticles can also be applied to the system of tin oxide. It is shown that in contrast to alumina no mechanochemical changes occur for the system of tin oxide during the wet grinding process. Thus the obtained median particle sizes are the result of pure mechanical grinding. In addition the suspension rheology in a stirred media mill as function of grinding time and inter particle interactions is studied.

This research provides a theoretical basis for understanding stirred media milling of nanoparticle slurries and as such, is a step towards a predictive model of nanogrinding in stirred media milling.

The aim of the project is to establish a relationship between the product properties and feed material and the mill functions for milling of organic solids. The specific objectives are:

1. to characterise the physical, mechanical, and thermal properties of model organic (feed) materials (material function) at the single particle level, and to examine the effects of temperature and humidity on these properties
2. to investigate the breakage behaviour of single organic particles under various conditions
3. to characterise the properties of milled products, and to correlate the product properties to material and mill functions Models materials that are planned and approved for use in the project by the TC of IFPRI include aspirin (low hardness, relatively high fracture toughness, ductile), á-lactose monohydrate (áLM, high hardness, high fracture toughness, semi-brittle), sucrose or sorbitol (high hardness, relatively low fracture toughness, regarded as brittle in the literature), starch (ductile and mechanical properties greatly affected by the strain rate), and microcrystalline cellulose (MCC, medium hardness, high fracture toughness, semi-brittle). These materials cover a fairly wide range of physical, mechanical and thermal properties, hence ensuring generality of the results to be achieved. This report summarises the work done over the first year on five model organic materials, á-lactose monohydrate (áLM), sucrose, sorbitol, starch and microcrystalline cellulose (MCC). The work includes extensive experimental investigation into the behaviour of both the single particle impact breakage and the bulking milling under various conditions, and preliminary mathematical modelling based on the distinct element method (DEM) and the population balance method. Also included in the report are some results of the milling of MCC and áLM obtained by the investigators before the project was started. The single particle impact experiments provide data of the breakage extent of single particles as a function of impact velocity, which is then used to infer the physical and mechanical properties of the tested particles by utilising the Ghadiri-Zhang model for semi-brittle materials. The results show that the modes of failure of the five tested materials agree well with that reported in the literature. Sucrose and sorbitol, which are generally regarded as brittle materials, demonstrate the highest extent of breakage. On ii the other hand, starch and MCC, two materials regarded as more difficult to mill, show the lowest extent of breakage. Sub-ambient impact experiments are conducted on áLM, sorbitol and MCC. The results suggest that the breakage propensity, thus the mechanical properties, of áLM and MCC are little affected by the temperature, but a lower temperature is seen to reduce the breakage extent of sorbitol particles. The bulk milling experiments are conducted in a single ball mill and the results are quantified by an analogy to the first-order rate process. The results show that milling of both MCC and áLM is little affected by the temperature, in agreement with the single particle testing results.
4. Attempts are made to relate the single particle impact breakage behaviour to the bulk milling behaviour. For MCC, starch and áLM, the milling behaviour expressed by the milling rate constant relates linearly to the single particle parameter containing the physical and mechanical properties of the particles. However, the relationships for sucrose and sorbitol are non-linear, indicating alternative methods should be explored for these types of materials. The relationships between the single particle breakage and the bulking milling behaviour are different for different materials. Preliminary efforts are made to unifying these relationships. This is achieved by quantifying the mill function with a term called ‘milling power’ predicted by DEM simulations of motions of both the milling media and the feed particles. The new bulk milling parameter is shown to relate to the single particle behaviour well and a unified relationship is obtained for áLM, starch and MCC. Preliminary modelling work using the population balance method has been carried out on the milling of sucrose in collaboration with Dr R Bertrum Diemer of DuPont. The results will be reported in the future.