SAR - Review
The report reviews the ways to characterise the morphology of free and embedded particles.
For such a purpose quantitative image analysis is the principal and the most adequate method for fine characterisation. The different steps involved are described subsequently.
The main imaging techniques used for particle visualisation are recalled: they are selected according to the particle size range, desired level of details and eventual supplementary information that the imaging device can produce (transparency, chemical composition, etc.).
Images being 2D projection on a plane of 3D reality, the basic image treatment aims at the segmentation to retrieve particle silhouettes. There is no fixed procedure, each being case-dependent.
Sizing principles are presented: they enable to determine the projected equivalent diameter, the length and the breath of the particle silhouette and its perimeter.
The 2D shape descriptors are based on both contour and silhouette, on silhouette only or on contour only. The different types are reviewed. Tools to produce some information on the 3D-morphology of particles are then presented.
When particles are embedded, further morphological information can be obtained, related to the anisotropy of the sample and/or orientation of particles in the sample.
Statistical elements, in terms of the number of particles to analyse as well as ways to treat the large sets of parameters necessary to quantify the shape are outlined.
Introduction
The surface chemical and surface energetic nature of materials used in the formulation of commercial products or used in the manufacture of these products is often important to the final quality of the product. Despite the importance of surface chemistry to the ultimate performance of the product, not as much recognition as is deserved has been given to the characterization of surface chemistry and the effects of its variation on product performance. Difficulties with both theoretical descriptions of interfaces and the measurement of surface chemical characteristics make the incorporation of material surface chemical specifications for the manufacture of many products challenging. Both theoretical developments related to surface characterization and experimental methods used to characterize materials are discussed in this paper.
Surface chemistry is important to processes involving spreading, wetting, liquid penetration and adhesion. Such processes might include, printing, drug formulation, painting, and gluing. The formulation of composite materials used for construction materials, tires, gaskets and pharmaceutical capsules also rely on the surface chemical nature of component materials. The relation of processes such as spreading, wetting, liquid penetration and adhesion to surface chemical properties is also discussed.
There are a number of applications in which particles are added to rheologically ”complex” fluids. The latter include polymer melts and solutions, surfactant systems, associative poly- mers and gels. In order to process these materials in a rational manner, controlling the the rheological behavior is a prerequisite. In addition, the end-use properties may also require certain rheological features or a specific suspension microstructure, such as e.g. particle orientation.
When particles are dispersed in visco-elastic media, there will be changes in both the thermodynamic and the hydrodynamic interactions as compared to particles dispersed in low-molecular weight Newtonian fluids. These changes are reflected in the rheological behavior, for which some general trends emerge from the available literature on polymeric solutions and melts.
- The flow curves can be reduced using an internal shear rate concept. This has, however, limited predictive value as the limits of applicability have not yet been established.
- Adding particles will weaken the impact of elasticity, which has a pronounced effect on processing operations (e.g. reduction of the die swell, reduction of the strain hardening, reduction of flow instabilities).
- The addition of fibers or non-spherical (nano-)particles allows control of the elonga- tional viscosities, whithout increasing the elasticity of the medium (inkjet printing, coatings, sprays.)
A vast amount of data can be found in literature. However, few truly systematic data sets, in which there is a systematic variation of the medium rheology, particle size and interactions have been reported. Therefore no quantitative, predictive scalings, theories or simulations exist.
The changed hydrodynamic interactions also lead to very specific microstructural fea- tures. The formation of chains of spherical particles, as well as segregation and migration effects are qualitative understood as being associated with normal stress effects. Less at- tention has been paid as to how these specific microstructural features can be exploited technologically. Also rational control over structure development in complex, processing flows or mixing operations is lacking. The effects of medium elasticity on flow induced ori- entation of non-spherical particles, such as fibers or some nanofillers, are also non-trivial. Overall, for the rational formulation or intelligent processing of dispersions in viscoelastic media, the key features seem to be understanding how particles interact (hydrodynam- ically) and how the structural changes that occur during flow relate to the rheology of the medium. This calls for combined experiments on the rheology, flow-induced structure and dynamics, of suspensions containing particles with known interparticle interactions, dispersed in fluids with a well defined rheological behaviour. This should enable one to establish predictive scaling laws and provide a solid base for theoretical work.
The properties of powders and disperse systems containing solid particles depend greatly on the geometry of the solid phase. There is as yet no universal method to describe the shape of powder particles. This, therefore, results in problems in associating the function of powders during processing and handling and their shape.
Microscopy has become very efficient with respect to the characterisation of particle size and shape since the provision of modern image analysis techniques. The shape factors available are based on the dimensions of two-dimensional particle outlines i.e. the shadows projected from the microscope onto the screen. Four characteristic features should be distinguished here: particle size, shape, roundness and “roughness”. All four features show a distribution function in the powder bulk. Hence they should be assessed as distributions rather than single values.
The simplest two-dimensional shape factors are derived from particle dimensions such as length and breadth only. Here, care has to be taken, because the individual image analysis systems define these dimensions differently, and hence, shape factors based on these dimensions are not always comparable. The most popular of these shape factors are Aspect Ratio and Elongation. These shape factors are unable to distinguish between different geometric figures such as circles and squares, or rectangles and ellipses. This is also valid for shape factors involving the projected area or perimeter of a particle outline, for example, Roundness and Circularity Shape Factor. These shape factors should only be used to evaluate the deviation from being round, and it is an ongoing misuse that these shape factors are employed to assess general particle shape. Shape factors based on inscribing and circumscribing circles are also not suitable for general particle shape analysis. However, there are a few shape factors, which combine particle dimensions, projected area and perimeter in a more sophisticated way (shape factor NS, surface and volume shape factors, One Plane Critical Stability). These appear useful to define the geometric shape of powder particles, and they can detect small variability in particle shape. Fractal dimensions define the degree of irregularity of the particle perimeter, but they do not describe surface roughness in a physical sense. Also, they are extremely insensitive to changes in particle shape, and hence their general use in particle shape analysis is questionable. Fourier analysis can provide a particle or a powder signature, but to process such data requires more mathematical effort, and the comparison between sets of data can only be performed using multivariate statistical methods such as Principal Component or Cluster Analysis, or the use of Neural Networks.
Other methods to detect and describe particle shape are based on laser light scattering. In many industries, particle size analysis is performed with laser light scattering techniques, and hence, it would be very useful and time saving, if the assessment of particle shape and shape distribution could be performed simultaneous with the particle size analysis. However, here more fundamental research is required, and the collaboration with equipment manufacturers is needed.
The choice of a suitable shape factor is crucial, when the influence of particle shape on powder behaviour, processing and handling has to be described. For example, the use of pellet shaped particles relies on their sphericity, and a shape factor characterising the roundness of the pellets is required. In composites, fibrous particles are used to enhance the mechanical strength and a shape factor describing roundness would be misplaced. Instead, the shape factor should quantify the elongation of the particles. When predicting the re-suspension properties of powder inhalations, a distinction between spherical, regular shaped or fibrous particles is required. Particles in a powder bulk or a powder mixture may comprise various particle shapes, and the distribution of shape influences, for example, powder flow, packing and compaction. Hence, the shape factor should be able to distinguish between geometric figures, and it should quantify small variability in particle shape. Particles conveyed on chutes should be characterised with ‘One Plane Critical Stability’ to identify conveyer problems related to particle shape.
There is controversy in the literature data relating powder handling properties to particle shape. However, here dubious results often arise due to other uncontrolled influence factors, first of all particle size and size distributions, but also other particulate properties such as true surface roughness, surface free energy, hardness or elasticity. Also, process parameters are often randomly varied in one study, and there is generally no consistency in test criteria and process parameters between individual studies.
The main areas, in which relationships between particle shape and powder properties should be sought, are fracture mechanics, powder compaction and compact properties, particle re-suspension and powder mixing. Packing and powder flow have been studied rather extensively in the past. There are inconsistencies in the relationships reported, but it seems unlikely that a more concise concept can be developed in the near future. This is mainly due to a lack of good, reproducible and controlled methods to study powder flow, and the knowledge about the role of adhesion and friction forces during powder packing appears to have got lost, leading to general use of inappropriate test equipment.
One major problem in testing the influence of particle shape on powder properties is the separation of different particle shapes in one powder bulk. While shape sorting machines are readily available for larger particles, there is no suitable shape sorting method known to date, that can sort particle shapes in powders of particle size below about 60 μm.
Executive Summary
A review of the literature relating to flow aids, their application and their testing has been undertaken with the objective of improving both the speed and efficiency of the selection procedure.
The selection of a suitable flow aid is comparable to arranging a marriage and potentially just as difficult. Finding a legal and lasting coupling is always hard but in the case of flow aids it is traditionally complicated by being a last minute arrangement when the host powder can no longer flow on its own.
A dossier of commercially available aids and their properties has been compiled and successful instances of both historical and current liaisons delivered. Such a record is useful but cannot provide a guide to all future relationships.
A more fundamental selection procedure requires a careful knowledge of the tendency of the partners to form a stable bond together while at the same time moving freely within a powder society. The complex interplay of the forces between the flow aid and the bulk powder has been documented and needs to be understood if a correct choice of aid is to be made.
Current developments put an additional demand on the flow aid partner. When added to a multi-component mixture the aid has the potential to displace other active ingredients and damage mixture quality. Pre-mixing or sequential addition may then be necessary. 'Add-on' qualities are also in demand with the aid being asked not only to promote flow but also to add some colour, a perfume or some additional quality to the bulk mixture. Such demands add complexity to the selection procedure but give added value to the flow aids having the required flexibility of application.
The effectiveness of a flow aid addition to a process will almost certainly require practical assessment and suitable methods are reviewed as are methods of conditioning the bulk powder.
In the interests of better marriage guidance a logic structure is proposed which will sequentially pose questions about the host powder, the flow aid and the method of testing.
Executive Summary
Non-invasivness is an ideal trait for any measurement technique. Unfortunately, very few materials characterization methods are truly non-invasive. For the purpose of this review, a non- invasive method is one which does not change the sample under study in any way. Techniques which require the insertion of a physical probe have been excluded and, while some methods which are widely used for on-line measurement have been included, on-line applications have not been emphasized. In the context of particle characterization, non-invasive measurements take on added significance because some of the most important and interesting samples occur in environments or at concentrations which are difficult to study without sample removal, dilution, drying, coating or other conditioning, all of which can alter critical inter-particle relationships.
This review examines a group of techniques which meet the criteria for non-invasiveness and have experienced significant development and/or commercialization within the last 5-10 years. Therefore, a few well-known techniques, such as classical static light scattering, which is quite non- invasive and very widely used in dilute suspensions, have been left out. The goal, rather, has been to examine newer techniques which are only recently being applied to particle characterization. These include several optical methods including some conventional and holographic imaging techniques, confocal microscopy and dynamic light scattering (with and without fiber optics). Tomographic techniques have been limited to optical and optical coherence tomography and x-ray tomography. Also included are environmental scanning electron microscopy, stray-field nuclear magnetic resonance imaging and ultrasonic spectroscopy.
These methods are bringing a whole new level of information about particulate systems, especially those that provide complete, microscopic 3-dimensional imaging. The ability to follow the detailed physical behavior of an ensemble of particles with single particle resolution opens up remarkable opportunities for understanding inter-particle behavior and the dynamics of heterogeneous particle systems. Where possible, examples of recent applications and instrumental developments are given.
To talk of Particle Technology in Latin America represents nowadays almost an eccentricity in both industrial and academic circles, although it makes really sense for most of the key industries within the region.
Latino America is still a continent of raw materials extraction and processing, coexisting with ancestral production systems for agricultural goods. Huge differences appear among the different Latin American countries depending on the amount of under-earth resources. The countries considered as riches ones are generally in such a position due to oil and mineral resources, Argentina being almost the only exception. Once the scope of this report is restricted to five of the more industrially active economies, some similarities and differences can be drawn.
In general, all these economies grew under the shadow of oil exploitation. In the first part of the 20th century, this exploitation was made by foreign oil companies, which extracted these resources producing buoyant economies in Europe and USA. The typical social structure based on a more dispersed population spread was then substituted by big people concentration around big cities and production sites: Mexico, Sao Paulo and Caracas are bigger than many European cities, but surrounded by “misery belts”. Similar situation but in another size scale is repeated near production sites, caused by people that left their traditional lifestyle attracted by the money river besides the extraction activities. In the middle of the last century, most governments (especially Mexico, Brazil and Venezuela), nationalized their oil industries and established import substitution policies and drove for the development of the manufacture, promoting the down-stream petrochemical activities. Like carbon-paper copies, the three countries grounded state owned companies with quite similar names: Petroleos Mexicanos (PEMEX), Petroleos de Venezuela (PDVSA) and Petroleo Brasileiro (Petrobras). However, some important different exists, Venezuela was promoter and founder of the OPEC, while Mexico has remain excluded from it and Brazil was for many years a net oil importer, until the appearance of the Campos Basin. The three countries built a petrochemical subsidiary (Pemex-Petroquimica, Pequiven and Petroquisa) to promote the chemical industry. By the end of the century the three countries had liberalized the state control and private petrochemical companies that had already certain operations began to gain their own space and leadership. But again some differences appear: while Venezuela, that enter late into this policy had relatively low success, Mexico and Brazil petrochemical industries are well developed, with chemical industries very active and with most of the global chemical companies having subsidiaries and production sites for the whole region in both countries.
For the mineral sector each country has its own history, while Mexico cement, steel and cooper industries were the ones exhibiting better growth, Brazil has important development in almost all minerals (iron and steel, aluminium, but also niobium and tantalum), Venezuela has also iron ore and aluminium, but Chile had formerly only inorganic salt mines and nowadays is world leader in cooper. While most European countries saw their mineral reserves to decline, it is far to be the Latin American case and it could be safely expected that 21st century remains signed by the mineral explotation. This situation conduct to a reality that already is present but will be increased: the world knowledge and experience in mineral processing will be in the southern hemisphere (Latin America, South Africa and Australia).
The already explained facts can maybe suggest why the “nanoparticles wave” has not that height in Latin America: there are still enough problems with the former called fine particles for looking additional ones with smaller particles. It does not mean in any case that global trends in chemicals are not valid in Latin America, but they are usually taken by the global chemical companies because of their global thinking way and not due to regional needs. The same arguments can be set for phama and biotechnology, the only exception being the importance of bio-leaching in ore treatment, which has been promoted by Latin American companies, like Codelco.
Food and beverage sector is also a potential consumer for particle technology, but two aspects make it a non-priority item: the inherent dispersion of the industry and the apparent inexistence of big challenges to be solved. Although, some giant companies exist, the small ones can survive with certains limits, this situation being impossible in oil & gas and in most minerals.
Having the oil companies so large influence into Latin American economies (even in other countries like Ecuador, Colombia and even Argentina and Chile), it seems to be addressed that a broader concept for particle technology must be used, letting bubbles and droplets and the interfacial phenomena involved in these multiphase systems to be also included. Oil and mining are activities occurring in huge tonnages, therefore the waste problems will remain as relevant ones for a while (tailing handling and slop oil treatment), being a predictable development the improvement of actual production patterns for “clean technologies”, i.e. not just treat the waste but minimize its production through new production processes.
Accepting that the global chemical companies are not a Latin American targets, because thea are attracted to Particle Technology from their headquarters, the companies to be monitored should be the different regional oil companies, that means not the global energy holdings, but especially PEMEX, PDVSA and Petrobras, with their associated petrochemical subsidiaries; the chemical divisions of Mexican groups like Alfa and Desc; the chemical Odebretch company, Braskem; the Brazilian CVRD, the Chilean Codelco, the Venezuelan CVG, the cement giant Cemex and Grupo Minero Mexicano, among others.
In the academic sector, the opportunity to permeate the Particle Technology concept is greater. However academic research centers are mostly “product oriented” instead of “technology oriented”. The interdisciplinary work in Particle technology has an outstanding example, the FIRP Laboratory in Merida, Venezuela, where people from five faculties deal with interfacial phenomena and product formulation for achieving desired (designed) properties of the dispersions, besides the several solid-liquid separations groups in Brazil, Chile and Venezuela. However, there is no doubt that the more successful example in a Particle Technology approach is already running since 30 years ago, the Brazilian Congress of Particulate Systems, known as ENEMP, which has put together different sectors of the Brazilean industry.
Almost all industrial research centers are tied to companies (Instituto Mexicano del Petroleo, PDVSA-Intevep, Cenpes-Petrobras, IM2-Codelco, CVRD-research center) but they normally has certain independence degree and could be also individually approached.
Most curricula are still separated in the traditional fashion but Particle Technology could represent an option for advanced in-company training, as well as in an eventual regional Master degree program.