A. Technical Field of the Invention
This invention comprises a method for mixing multicomponent solids to produce a structured-type of mixing and a controlled level of subdivision. It is a hierarchical mixing process, which can be used to form microcapsules for distributing components which can be useful for high temperature chemistry where enhanced mass transport and controlled sequence of reactions increase solid-state reactivity. The method can also be used to agglomerate the microcapsules into granules.
B. Background of the Invention
1. Introduction
The structure of a mixture of particles can be characterized into two broad classes. These are namely random and nonrandom structures.
Random mixtures are those in which the distribution of components in the composition of nearest neighbor particles relative to a central coordinated particle can be predicted by random population statistics. That is, based upon many samplings of a large population, this type of mixture displays a variance that can be reliably predicted by statistics.
A nonrandom mixture is one whose variance cannot be predicted in this fashion. Thus, at some scale, a poorly blended mixture is a nonrandom mixture. Another example is a perfectly structured mixture where components are arranged in uniform arrays.
There are a number of criteria that can be used to distinguish the differences between these classes of mixtures, including scale of segregation, ordering of component interfaces and packing density of components. First, scale of segregation can be used. Scale of segregation is determined by the number and size of particles of one composition separating two particles of another composition. Scale of segregation increases as the number of components increases but decreases as the particle size decreases. In a conventionally processed random mixture, the segregation scale usually corresponds to tens of particle diameters or greater. For a poorly mixed system, the scale can be many order of magnitudes greater. For the structured mixture, however, the segregation scale is perhaps only a few particle diameters. Although such a scale of mixing is difficult if not impossible to achieve, this type of structure can be approached.
When examining mixtures, at least two levels of hierarchy or levels of organization in the mixture are important, the hierarchy at the particle scale and the hierarchy at the granule scale (see definition of granule below). As to the former, if the mixture of particles is random, there is no ordered hierarchy. But if particles of one composition were to have a coating of another composition and the coated particles are not arranged into any particular array or pattern, then ordered mixing occurs on a single level of mixing hierarchy, that being the particle level. But if, instead, these coated particles are organized into spherical granules, then there are two levels of mixing hierarchy.
With these distinctions in mind, one can understand the numerous methods for making multicomponent mixtures that existed prior to the present invention, the compositions produced thereby. These mixing methods are capable of combining organic, organometallic, metal organic, and ceramic components and developing mixing scales that vary from molecular dimensions to that of many orders of magnitude greater.
For example, chemical synthesis methods provide a means for preparing compounds that have a segregation scale on the order of molecular dimensions. Specific techniques such as sol-gel and coprecipitation also offer this advantage, although the viability of these techniques depends on the constraints imposed by specific chemical system at issue. These methods are often difficult to engineer, and typically, there is a high cost for precursors required to prepare these intimate mixtures.
Dry mixing methods have usually been thought to represent the opposite end of the segregation scale spectrum. These methods, typically conducted with mixers such as pan mixers, v-blenders, and dry ball mills, rely on the force of gravity for homogenization. In part because this force competes with adhesional van der Waals forces, it has generally been thought that these methods cannot be effectively used for particle sizes less than about 44 .mu.m, which results in a relatively large scale of component segregations in the bulk mixture. Accordingly, these methods have been associated with very high segregation scales.
Conventional wet mixing methods, such as high shear intensive mixing (for pastes), wet ball mixing (for slurries) and blunging (for more dilute suspensions) all are more capable of achieving segregation scales that are smaller compared to scales of segregation achieved by dry mixing. Wet mixing can achieve better random mixtures because adhesional forces of particle agglomerates can be reduced if not overcome by suspension of particles in a liquid solvent. However, these methods are prone to other problems. As these are multi-step processes (e.g., dispersion/mixing/separation/dewatering), they are heavily dependent on optimizing colloid and rheological properties to minimize segregation and avoid chemical interactions that may alter the chemical identity of one of the components.
Constraints associated with wet mixing include the necessary selection of a solvent that will wet the surfaces of powder being dispersed without dissolving the powder. Also, solvent removal must be evaluated in order to reduce segregation of particles due to uneven settling, which is a consequence of differences in particle size and density.
A nonconventional wet mixing process is mutual flocculation, where colloidal forces are controlled as a means to yield structured mixing. However, lengthy development times are usually needed to optimize the suspension formulation. In addition, mutual flocculation (also known as selective flocculation) at best can provide ordered mixing on only one level of mixing hierarchy. Further, the organization of structures produced by this method degrades rapidly as the number of components exceeds two.
2. Microencapsulation
Microencapsulation is another nonconventional mixing process in which a particle is encased by a coating, and therefore this type of mixture is nonrandom. Microencapsulation has been used for many commercial applications such as in pharmaceuticals, cosmetics, agricultural products, and photoduplication toners. Advantages of this process include relatively short process development times ability to form a number of different microcapsule morphologies with a range of core sizes.
In addition, the porosity of the encapsulating layer can be modified to tailor dissolution properties of the core for the purpose of time-controlled release of specific compounds or to ensure passivation of toxic substances.
Wet, vapor, and dry methods are available for the synthesis of microencapsulated powders. Wet methods include precipitation coatings, made by processes in which core particles are dispersed in a solution and precipitation of an encapsulant is induced to form the surface of the core. Unfortunately, the throughput of such processes has been low, leading to poor process economy in most applications. Emulsions have also been used to encapsulate liquids, for example, by polymerizing a monomer species to stabilize an emulsified droplet. As far as is presently known, this process has not been used to prepare encapsulated solids.
Spray drying has been used to prepare encapsulated solids by taking core particles suspended in a solution, forming an aerosol of the suspension, and evaporating the solution in a heated chamber to form a coating from the solution on the core particles. Spray drying is an empirically optimized process, energy intensive, and can only be effective when conducted (for larger particles) on a large scale. Common to all processes above, wet microencapsulation methods all suffer from the complexities introduced by the need for controlled suspension stability and rheological properties. Suspension stability and rheology are properties whose control can vary considerably from chemical system to chemical system.
Vapor phase methods include dispersal of particles in a vapor stream containing the microencapsulant. Dispersal of the core powders can occur, for example, in a fluidized bed, if the core particles are large enough to be fluidized. The microencapsulant is introduced as a vapor phase precursor that can be thermally or chemically decomposed to precipitate a coating as the core particles circulate in the fluidized bed. However, this process seems to be effective only for relatively coarse particles. Alternatively, finer particles can be entrained in a gas stream to avoid some of the problems associated with a fluidized bed. Unfortunately, dilute streams of particles are employed, usually rendering the process uneconomical.
Only one dry microencapsulation method referred to as mechanofusion, is known to have been reported whereby coarse particles (i.e., &gt;30 .mu.m) are mixed with relatively fine particles in a highly compressive shearing force field to form coated particles. In all cases, one component must be significantly softer than the other; and if harder particles are assembled as a coating on a softer core, then there must be a significant size difference between core and coating particles, with the cores being at least ten times the diameter of the coating particles. Further, the method is optimized for spherical core particles; if non-spherical core materials are used that are ductile (e.g., metal powders), then such core particles are made more spherical by the mechanofusion process. Thus, organic core particles are coated by inorganic particles by an impregnation process. Metal core particles can be impregnated by ceramic particles to form a hard, dense outer coating. Alternatively, hard ceramic core particles can be coated by ductile metals or soft organic coatings to produce a controlled mixing hierarchy. However, as far as is presently known, only a single level of mixing hierarchy has been achieved.
3. Granulation
Granulation is a process whereby particles are bound together by either an aggregative or globulation process. Granulation is performed to impart flowability to the powder for facile transport, to minimize dusting by elimination of free fine particles, to impart uniform compaction properties for dry pressing forming methods, and to lead to high permeability powder beds when labile gas circulation is required.
Wet methods are primarily used to granulate powders by using either an aggregative or a globulation process. Pendant capillary forces can be used to aggregate particles using conventional mixing equipment by the gradual addition of liquid to the dry powder. In a similar manner, fluidized beds can be used to granulate via the introduction of liquid into the circulating bed of powder (the liquid can also contain dissolved species to act as a microencapsulant). However, in all of these cases, there is no means for morphology control.
Spherical agglomeration methods offer one of the few methods to achieve spherical granules. In these methods, emulsions are used to form spherical aggregates by isolating particles in an immiscible dispersed liquid phase. Globulation methods, more commonly known as spray drying or sol-gel globulation, also have morphology control limitations where the surface energy minimization of the droplet induces a spherical shape and removal of the liquid from the droplet induces agglomeration of the particles. However, these methods of obtaining dense homogeneous particles are empirical processes that usually require a great deal of trial and error in applying general idea of the process to a specific chemical System. Furthermore, control of the granule size depends on aerosol technology. In general, as the size decreases, the throughput decreases substantially.
Very rarely have dry methods been used for forming granules. A method known as hybridization has been used where fine particles entrained in a gas are impacted with coarse core particles to form a structure where fine particles adhere to the core. The morphology of the granule is dependent on the morphology of core. Viability of this process is also dependent on maintaining a coarse core particle (&gt;30 .mu.m) and a fine coating particle. Like mechanofusion, this process is dependent on combining a soft component with a hard component. Control of the hierarchic structure beyond a single level has not been reported.
Accordingly, prior to the present invention, there remained to be discovered a generalizable, hierarchically controllable mixing process for producing an ordered distribution of substances in the solid state.
4. Thermal Processing
In ceramics powder processing, thermal treatments are necessary for several objectives including the removal of organic processing aids, reaction between multicomponent powders, sintering, and vitrification. Thermal processing may be classified as intermediate (.sup..about. 200.degree.-600.degree. range) or high temperature chemistry (above 600.degree. C.). Pyrolysis reactions involving the removal (i.e., burnout) of organic components typically occur in the intermediate range. Calcination reactions involving the decomposition of metal salts (e.g., conversion of magnesium carbonate to magnesium oxide) most often occur in the intermediate range, but in some cases require higher temperatures. High temperature reaction are most often associated with enhanced mass transport phenomena in the solid state; these include solid state chemical reactions, densification and coarsening.
5. Organic Pyrolysis and Calcination
Organic binders are often used in granulation or forming processes to impart physical cohesiveness between particles. However, these binders must be eliminated in the final product piece. This is accomplished by a controlled pyrolysis of the organics at an intermediate temperature, a process known as "binder burnout."
6. Calcination
Aside from organic binders, calcination of ceramic powder mixtures is another important thermal process utilized at intermediate or high temperatures. Many times, ceramic components are utilized in forms such as carbonates, nitrates, hydroxides and acetates. During heating these salts decompose into oxides. Sometimes these reactions occur at low temperatures and other times they only proceed at high temperatures depending on the processing atmosphere, thermal history and physics and chemistry of the powder. It is well established that for a particular salt, particle size is an important variable since it controls the path and time required for mass transport or heat transfer. Mass transport and/or heat transfer processes rate-limit the decomposition of these salts (as well as the organic phases described above). In general, smaller particle sizes of the salts promote rapid reactions since they decrease the distances over which mass transport must occur.
7. Solid State Reactions
During or after the calcination stage, two classes of high temperature solid state reactions occur that are relevant to the proposed invention. Both classes are fundamentally driven by thermodynamics, resulting in a decrease of surface energy and or chemical energy. In the solid state, rates of reaction are most often limited by mass transport; mass transport is governed by diffusion which increases with increasing reaction temperature. However, there are usually processing constraints on the temperatures that can be used; therefore, other methods for increasing solid state reactivity are desirable.
The first class of solid state phenomena occur When the various phases in the powder mixture are stable. Mass transport will occur in a fashion that will facilitate the loss of surface area and possibly a reduction in porosity. A reduction of both surface area and porosity is referred to as densification, a process typically employed in the processing of metals, polymers (at low temperatures, .sup..about. 100.degree.-200.degree. C.) and ceramics. When only surface area is reduced this is referred to as coarsening, a process of use whenever interparticle bonding is not of interest (e.g., the processing of porous catalyst supports). There has been a great deal of research devoted to sintering high temperature materials such as non-oxide ceramics (e.g., silicon nitride) that are difficult to densify. It has been found that additions of small amounts of additive (i.e., yttria, magnesia, alumina) can aid in densification. There has been recent research on optimizing the distribution of sintering aids by particle coating processes. However, in the prior art, no process has been reported for incorporating sintering aids into triggerable encapsulants and distributing them in a microencapsulated form, as is provided by the present invention.
The second class of phenomena involves chemical reactions between components in the solid state mixture. This occurs when various phases in the powder mixture are thermodynamically unstable as separate species and react with one another. Like calcination, the process is rate limited by mass transport process (less likely to limited by heat transfer). Usually the rate determining mass transport process is diffusion. In all cases, the finer the level of subdivision (particle size) and homogeneity of the mixture, the more rapid are the high temperature solid state reaction kinetics. This is because both homogeneity and particle size both serve to decrease the distance over which species must diffuse in order to initiate and continue the thermodynamically driven solid state reaction. In addition, another factor promoting the reaction kinetics is the packing density of the powder mixture.
Densely packed multicomponent powder mixtures will react more rapidly than loosely packed mixtures since increasing density is a means for decreasing the distance required to transport reactant species. For this reason, multicomponent mixtures are sometimes pelletized prior to solid state reaction. However, after solid state reaction, an aggregated structure results and must be comminuted prior to any subsequent processing can commence. For many conventional mixtures, mixing homogeneity is insufficient thus requiring additional iterative pelletization, solid state reaction and comminution steps.
8. Chemical Reaction Intermediates
In many cases, the formation of metastable intermediate phases are a problem in the synthesis of multicomponent phases. In ceramics, the formation and decomposition of these phase intermediates can rate limit the formation of the desired phase instead of the characteristics of the mixture. For instance, in the synthesis of Ba.sub.4 Y.sub.2 O.sub.7, Ba.sub.2 Y.sub.2 O.sub.5 forms prior to the appearance of Ba.sub.4 Y.sub.2 O.sub.7. In other cases, intermediates form that do not necessarily have to appear prior to the formation of the desired multicomponent phase. For instance, in the synthesis of Pb(Mg.sub.1 /.sub.3 Nb.sub.2/3), highly undesirable impurity phases with other stoichiometries can also form and can be difficult or impossible to eliminate. In this case, prereaction processes are employed to preclude formation of impurity phases. For instance, in the synthesis of Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3, in order to avoid impurity lead niobate phases, magnesium and niobium oxide phases are reacted to form the magnocolombite phase before it is mixed with lead oxide. However, the introduction of prereaction steps serves to introduce many additional processing steps since two solid state reaction processes are being merged together.