DE102014011635A1 - Process for the preparation of nanostructurable microparticles from fluid media by spray-drying - Google Patents

Abstract

The invention relates to a process for the preparation of nanostructurable microparticles from fluid media by spray drying, wherein low viscosity solutions, dispersions or suspensions of a high pressure unit via at least one nozzle transferred to a low pressure drying unit and the droplet collective formed dried and particles are deposited. According to the invention, high atomizing pressures in the pressure range P ≥ 3 × 10 6 Pa and a high media throughput with a low mass ratio of fluid media as the spraying medium to the gas stream are selected such that a pressure jump from the high-pressure unit to the low-pressure-controlled drying unit adjoins the nozzle by substantially five orders of magnitude, wherein the particle formation can be influenced by the choice of the high and low pressure ranges.

Description

The invention relates to a process for producing nanostructurable microparticles from fluid media by spray drying, wherein low-viscosity solutions, dispersions or suspensions are transferred from a high-pressure unit via at least one nozzle into a low-pressure drying unit and the drop collectives formed are dried and deposited, according to the preamble of claim 1.

Furthermore, the invention relates to various uses according to the method produced particles.

From the DE 10 2008 006 607 B4 For example, a method for producing finely divided particles in a pulsating hot gas stream is previously known. The particles are formed by supplying a mixture of raw materials in the pulsating hot gas stream and separated in a downstream of a combustor filter system. The hot gas flow is generated with variably adjustable pressure amplitudes and oscillation frequency by a burner flame with periodic pressure oscillation of a burner. Furthermore, the propagation space and / or the degree of turbulence of the hot gas stream is variable by the arrangement of the position of the burner in the combustion chamber. With a pulsating hot gas flow an increased heat transfer due to high flow turbulence should be present, which is crucial for the phase reaction process in the material. The disadvantage, however, is that the hot gas flow itself has an undesirable influence on the particle formation and thus the particle properties.

In the process for producing a nanoparticle-polymer composite according to DE 10 2005 044 529 A1 the composite is obtained as a product of spray drying. For this purpose, initially an optically transparent polymer is selected which is soluble in a liquid solvent. Thereafter, the nanoscale filler is dispersed in the solution. The dispersion obtained is spray-dried, as a result of which the degree of dispersion of the nanoscale filler in the composite is virtually frozen.

In the process for the production of nanoparticles according to DE 602 25 242 T2 the nanoparticles should be suitable for the preparation of therapeutically active agents, in particular for oral, pulmonary or transdermal administration. Accordingly, free uncharged, spherical, and crystalline nanoparticles should be obtained from active agents having a narrow aerodynamic particle size distribution using an aerosol flow reactor method. As active agents preferably therapeutic, cosmetic or diagnostic agents are used. In terms of the process, therefore, a liquid initial supply is initially produced which comprises an active agent or the combination of two or more active agents. The liquid starting material is comminuted to produce droplets and the droplets are suspended in a carrier gas. Subsequently, the carrier gas and the suspended droplets are treated by a heated tube flow reactor at a given residence time and temperature to recover the nanoparticles.

Regardless of the prior art, there is still a need for highly pure, redispersible silica-based particles which have a targeted, adjustable morphology and textural properties.

It is therefore an object of the invention to provide a further developed method for the production of nanostructurable microparticles from fluid media by spray drying, wherein low viscosity solutions, dispersions or suspensions of a high pressure unit via at least one nozzle transferred to a low pressure drying unit and the droplet collective formed are dried and deposited. The process should make it possible to obtain nano-scaled powders and particle size distributions from fluid media, while ensuring direct adjustment of product properties using the process parameters. Specifically, targeted production of submicron-scale hollow body geometries using concentrated fluids, for example sols, from inorganic or organic starting materials by means of spray drying is to be made possible. The disadvantages of existing processes, such as low use concentrations of the starting materials, broad particle size distributions of the generated products, a high number of particle defects and unselective particle shapes should be avoided. One focus of the invention is the formation of hollow spheres to minimize the formation of ineffective bulk material.

The object of the invention is achieved by the teaching according to claim 1, wherein the dependent claims represent at least expedient refinements and developments.

In addition, according to the invention, the use of the particles produced according to the definition of claims 13 and 14, wherein the stated applications and excited uses are only exemplary and not to be understood as exhaustive.

It is therefore assumed that a process for the preparation of nanostructurable microparticles from fluid media by spray drying, wherein the starting materials are low-viscosity solutions, dispersions or suspensions. These are fed to a high-pressure unit via at least one nozzle and then transferred to a low-pressure-controlled drying unit. The drop collectives formed are dried and the resulting particles are deposited and fed to further use.

According to the invention, high atomizing pressures in the pressure range P ≥ 3 × 10 ⁶ and a high media throughput at a low mass ratio of fluid media as the spraying medium to the gas stream are selected such that a pressure jump from the high pressure unit to the low pressure controlled drying unit downstream of the nozzle substantially increases ., sets at least five orders of magnitude, whereby particle formation can be actively influenced by the choice of high and low pressure ranges.

Ausgestaltend the size of the particles or the diameter and the particle size distribution on the pressure-related defined droplet size distributions can be adjusted at the nozzle outlet.

In turn, the fluid medium, a high-pressure inert gas can be added to compensate for pressure differences in the spray droplets formed, so that the particles formed have a reduced number of defects and / or a modified particle size distribution.

It is also possible to supply reactive gases to the spray solution in order to influence the rheological properties. As an exemplary inert gas, nitrogen can be mixed with the starting material under pressure.

To form hollow spherical particles, rapid relaxation and drying takes place in the low-pressure-guided drying unit, the resulting dust granules being deposited via a filter arrangement or similar means.

To ensure a targeted spray inlet temperature, the fluid medium can be thermostated.

By way of example, the cross section of the at least one spray nozzle is selected at ≦ 10 ^-3 m.

According to the invention, the use of colloidal silicic acids or silica sols can be carried out as a medium to be sprayed, in order to generate nanoparticle powders which have morphological properties of the starting material or take over in the sense of an action which is referred to hereafter as inheritance.

The solutions, suspensions or dispersions used advantageously have a solids content m _s > 5% (m: m, ie based on mass). Basically, a solids content of 0.0001 to 80% (m: m) is selectable.

The particle drying can be supported according to the invention by targeted supply of radiation in order to promote drying of the particles, in particular the hollow spherical particles.

Particle drying can also be assisted by a pre-fed gas which dissolves during drying.

The process also serves to produce redispersible agglomerates.

According to the invention, the use of the particles produced by the process z. B. as an additive and / or filler for paints and / or plastics.

Likewise according to the invention, the use of the particles according to the invention as a means for encapsulating agents z. B. in the medical or cosmetic field.

Optical properties of materials are mainly characterized by their composition, surface morphology and layer thickness. In particular, find hollow sphere pigments as a result of targeted linking of these properties, increased use in thin layers or complex applications.

The advantage of these structures is a combination of optical and mechanical effects in combination with economic benefits. Hollow bodies can be easily adapted in their optical properties by a specific selection of the diameter in combination with the layer thickness, so that special, innovative color effects (eg nano-gold) can be emphasized. Furthermore, the morphology of the surface, as an essential, color-describing property plays a role. This is used in particular in coating systems (for example coating lacquers). If these structures are realized on the nanometer scale, additional mechanical material properties can be modified. Thus, in such additives, the amount of binder in paints is demonstrably reduced locally around the pigment, as a result of which this modification called "cross-linking space" brings about an increase in the elasticity-the mechanical strength of these systems. Economical advantages over bulk materials result from the use of thin-layered hollow bodies. In this context, only a small part of the most expensive materials in the production is necessary, which speaks for the application of the method according to the invention.

The pressure swing technology according to the invention allows targeted access to such forms. With your help it is possible to present hollow body structures with specific layer thicknesses in a wide range of diameters (5 · 10 ^-9 m ≤ d ≤ 10 ^-3 m). By selecting the precursor material, this technology also makes it possible to represent surface structures appropriately, in order to enhance or reduce optical effects. A particular advantage of the pressure swing technology is the direct representation of such structures from fluid media, regardless of their phys.-chem. Construction (dispersions, solutions, suspensions). The use of templates as well as their subsequent elimination is not required as a further, often cost-intensive process step. Active Pharmaceutical Ingredients (API) are elemental components of all medications. The knowledge of aerodynamic properties in the course of application and during respiration (pulmonary uptake) is a characteristic feature of particular in the case of inhaled products. In this context, the defined micronization of the API particles in combination with a targeted chemical activation / chemistry is a fundamental application criterion.

Often, APIs or their mixtures are characterized by a chemical structure (eg, chirality) and composition, which in the course of phys.-chem. Influences are easily modifiable. Consequently, the selection of the micronization method, the targeted adjustment of a particle size distribution (PSD), has a quality-relevant role. Thus mechanical reduction methods such as, for example, milling often lead to high energy inputs into the product as a result of which the amorphous portion of the structures or their crystal structure (eg salbutamol sulphate) can be changed. Furthermore, this mostly generates irregular shapes, which have unfavorable aerodynamic properties. In contrast, wet-chemical methods for the controlled presentation of PSD distributions of active ingredients often lead to impurities (racemate formation, co-crystallization, accumulation of impurities), which severely restricts their applicability. Sensitive methods such as spray-drying methods for the preparation of powders preferably generate particles in regular-spherical form without modifying their chemical properties. Disadvantage of known methods, however, are the representable diameters of the particles. Pulmonary particles (d ₉₀ (V) <5 × 10 ^-6 m) can not be selectively produced by known methods. Likewise, necessary drying temperatures for transferring the active ingredients into the powder form for temperature-sensitive APIs are often an insurmountable obstacle.

For example, in current asthma only about 10 to 20% of the active ingredient (delievered dose) can be transferred to the alveoli of the lungs, in which you develop their full effect. The remaining portion of the API is due to its unfavorable aerodynamic properties on the one hand exhaled immediately (d <5 × 10 ^-7 m) or the second, this already sets in the mouth and throat (d> 5 · 10 ^-6 m) , The pressure swing technology according to the invention allows gentle access to structures with such phys.-chem. Properties. In this case, aerodynamic properties and particle size distributions can be defined by targeted selection of the educts and technical parameters. This would significantly increase the safety of the patient as the largest pharmacological criterion, since the proportion of active agent in defective regions of the human target milieu could be significantly reduced. Furthermore, the structure as a release property (aggregate vs. agglomerate) is variably adjustable as a function of the drying properties. By the additional admission with a gas a process with lower drying conditions (eg temperature) can be realized, so that APIs which had previously been refused the transfer by means of spray drying due to negative drying influences, are accessible.

Ceramic building blocks are an integral part of today's everyday life. They are used in many areas, such as catalyst carriers, building bases and heat protection systems.

As the requirements for the nature and properties of these systems increase, the challenges associated with material selection have increased significantly. In order to optimize ceramics in the high-performance range, it has been found that especially components in the nanometer range (10 ^{-8.ltoreq.d.ltoreq.10 -6} m) are missing within the packing. Especially silica as a necessary silicate germ as the basis of many chemical reactions finds special attention here.

The pressure swing technology according to the invention offers the possibility of generating extremely small particles of silica. In particular, the combination of large-scale agglomerates and their decomposition into particles in the nanometer range under mechanical conditions offers the possibility of access to highly reactive silicate nuclei. The active control of reactivity can be selectively defined by the choice of starting material.

Griechische Symbole

Dimensionslose Kennzahlen Kennzahl Bezeichnung Bo Bond-Zahl Bo_R relative Bond-Zahl Lp Laplace-Zahl Ma Mach-Zahl Pe Péclet-Zahl Re Reynolds-Zahl Re_P Druck-Reynolds-Zahl We Weber-Zahl The invention will be explained in more detail with reference to the following embodiment. List of abbreviations General abbreviations used abbreviation description BET BROWN AUER-EMMETT-TELLER BJH BERET-JOYNER-Halenda DIN German industrial standard glycol 1,2-ethanediol KP compressor methyl orange Sodium 4-dimethylamino-azobenzene-4'-sulfonic acid MFC Mass Flow Controller m: m The mixture was based on mass. N standard NMR Nuclear Magnetic Resonance PA 6 Polyamide 6 (brand name ^Perlon® ) PdI Polydisperse index PEG polyethylene glycol PMMA Polymethylmethacrylate (brand name Plexiglas ^® ) rel. relative REM scanning electron Microscopy sub under TEM transmission electron Microscopy TEOS Tetraethyl-ortho-silicate van der Waals VAN DER WAALS v: v The mixture was volume-wise at 298 K. Symbols used Latin symbols

Greek symbols

Dimensionless metrics identification number description Bo Bond number Bo _R relative bond number Lp Laplace number Ma Mach number Pe Peclet number re Reynolds number Re _P Pressure Reynolds number We Weber number

1 Introduction and objective

• i. Sekundärer Tropfenzerfall: Die hydrodynamische Instabilität der gebildeten Lamelle an der Düse sowie die folgende Oszillation der Tropfen wird durch entlösendes Gas verstärkt. Als Folge steigt die Wahrscheinlichkeit kritischer Randbedingungen, endend in einem weiteren, sekundären Tropfenzerfall.
• ii. Reduzierung von Partikeldefekten: Als Folge einer herabgesetzten Schallgeschwindigkeit bei Gemischen von Gasen in Flüssigkeiten können Druckausgleichsprozesse signifikant schneller ablaufen als in den reinen Medien. Dies kann grundlegenden Mechanismen der Bildung von Partikeldefekten entgegenwirken.
• iii. Optimierung der Trocknung: Im gebildeten Tropfen wird durch zusätzlich gebildete Phasengrenzflächen des sich entlösenden Gases der Übergang von Flüssigkeitsmolekülen in die Gasphase optimiert.

Humanity recognized early on in its evolution the importance of readily available metal oxide compounds such as silica sand and feldspar. By combining toolmaking and architectural expertise, true masterpieces could be created long before our time. In the course of the industrial and digital revolution, the demands on these materials increased significantly. The driving force of these developments was the necessary accessibility of ever more innovative, tailor-made properties, as a result of which development increasingly penetrated into the field of nanotechnology (Greek nano-dwarf). Nanoscale materials and compounds are characterized by a unique combination of surface and interface effects. As a result, the established areas of building materials technology and ceramics were supplemented by a large number of application areas. While nanoscalar titanium dioxide is used for UV absorption in sunscreens, colloidal silica is used for chemical mechanical planarization (CMP), silver nanoparticles in packaging protect against microbiological contamination. The ratio of surface to bulk cathars significantly differentiates nanoscale materials from substances in the microscale cosmos. The generation of such particles can be carried out by means of a "bottom-up" or "top-down" method. Well-known examples of the "bottom-up" modeling process, the construction of subnanometer-based structures, is the synthesis of colloidal silicas from water glasses or the self-assembly (self-assembly) of dendrimers. Reduction mechanisms using mechanical forces pursue the "top-down" approach. These include grinding, impact and friction techniques. Raw materials and compounds on the microscale level are usually used as raw materials. Targeted product engineering is also a central focus in medical technology. Here, the topics over-, erroneous and point dosing get an increasingly significant influence. From this point of view, investigations are taking place to minimize the concentration of drugs and to outsource essential transport and release tasks to carrier substances. When a biocompatible carrier material has become established amorphous silica, also called silica. Advantages of increased pH and temperature stability are specifically combined with an adjustable surface morphology. Priorities of previous work are in particular on the targeted inclusion or storage of active ingredients in labile hollow body geometries. Physical limitations, such as oral uptake (<10 ^-5 m; peroral) and pulmonary (<10 ^-6 m; pulmonary), are precluded by physical limitations in releasing the drug. The required combination of particle diameter, decay rate and flowability of physically usable particles (d _G ~ 2 to 4 · 10 ^-6 m) is currently the focus of many scientific studies. The production of bioaccumulative metal oxide particles (titanium dioxide (TiO ₂ ), silicon dioxide (SiO ₂ ), aluminum oxide (Al ₂ O ₃ )) in the sub-micron range (<10 ^-6 m) is usually subject to restrictions of a physical-chemical nature. The spray-drying process, with its many possibilities of influence, finds in this connection a sustainable, scientific interest in the synthesis of such particle diameters. Spray-dried amorphous silicon dioxide particles in the nanometer range have so far only been produced at very low educt concentrations and high mass ratios of air flow to the spray medium. The (sub-) micrometer-sized particles formed in this way are usually aggregates whose diameter, but not their surface morphology, can be controlled. The focus of this work is the question to what extent a targeted selection of the fluid media to be sprayed generates novel powders whose basic macroscopic and microscopic properties are inherited from the starting material. To simplify the description of structures and properties, the retrieval of structural elements as well as physicochemical characteristics in the further course of this work is referred to as inheritance. The basis for the preparation of these powders are dispersions of colloidal silica in water, the so-called silica sols used. The diameter of these dispersed particulates ranges from 5 to 50 x 10 ^-9 m. As nanoscale particles rely on surface characteristics, the focus of scientific interest is on the formation of hollow spheres (d _P <10 ^-5 m) to reduce the formation of ineffective bulk material. Selection criteria of the dispersions chosen here are the particle diameter, the stabilization form and additives. Subsequently, the obtained amorphous powders are compared with each other. In addition, it will be investigated whether the process of drying is controllable, so that agglomerates are formed instead of aggregates known from the literature. Here, the nanoscale, colloidal monomer unit should be recognizable as a basic building block and ideally be regenerated again during dispersion. Textural investigations represent a further area of characterization of the particles formed. For these investigations, a new type of spray drying, the pressure swing technology, is to be applied. This technique is characterized by markedly high spraying pressures and throughputs as well as a low mass ratio of air flow to spray medium. In order to realize this technique, a corresponding apparatus is planned and built in the context of this work. Based on this, technical process parameters should be considered in this work as well as the investigation of whether the process of particle formation in this process technology can be controlled using exceptionally high solids contents (≥ 10% (m: m)). Subsequently, conclusions about process parameter influences are to be made on the basis of the powders obtained. The aqueous chemistry of colloidal silica poses immense challenges to a spray process. In the second part of this work, the base material silica sol is to be charged with a gas. After reaching the target parameters, the silica gas, which is ideally saturated with gas, is released very quickly in a heated chamber. The anticipated high Péclet numbers (Pe) in the drying process should promote the formation of hollow spheres. The scientific background of the loading of the silica sol with gas under high pressure and subsequent transfer to low pressures is the theoretical derivation of the following possible effects:

• i. Secondary droplet decay: The hydrodynamic instability of the formed lamella at the nozzle as well as the following oscillation of the droplets is amplified by dissolving gas. As a result, the likelihood of critical boundary conditions increases, ending in a further, secondary drop in decay.
• ii. Reduction of Particle Defects: As a consequence of a reduced speed of sound in mixtures of gases in liquids, pressure equalization processes can proceed significantly faster than in pure media. This can counteract basic mechanisms of particle defect formation.
• iii. Optimization of drying: The transition of liquid molecules into the gas phase is optimized in the formed droplets by additionally formed phase interfaces of the releasing gas.

Based on the obtained results of the spray experiments a model of particle formation has to be proposed and discussed. Necessary parameters of the colloidal starting materials such as the solubility of the gas used as well as concentration-dependent diffusion coefficients should also be recorded. For this purpose, an apparatus for volumetric measurements is set up.

Literature Review

Targeted product engineering [1] nanoscale technologies and materials [2-7] is currently an essential basis for the development of new application areas in medicine [8-38], building materials engineering [39-41] or ceramic synthesis / molding [42, 43] , Currently, three main methods (Table 2.1) are dominated by the production of metal oxide powders in the lower micrometer range (<5 · 10 ^-5 m) used in these fields.

Spraypyrolysis [45] and spray drying [46, 47] follow the "bottom-up" approach, while various reduction techniques (milling, cleats, rubbing) implement the "top-down" process. Advantages of spray pyrolysis are very narrow particle size distributions as well as controllable particle formation within limits. Through selective selection of precursors and process parameters, full bodies (volume precipitation) or hollow forms (surface precipitation) can be produced [45]. As a consequence of the demanding (laminar) flow technique, the applicability of only low concentrations of the usually expensive starting materials as well as the intensive pyrolysis temperatures, which often cause sintering effects of the generated powders, are negative [48]. For higher use concentrations, depending on the starting material and product requirement, the spray drying and grinding processes are used. Disadvantages of these processes are broad particle size distributions and an increased proportion of irregular particles. Furthermore, sintering and breakage stress effects as well as contamination, which are not observed during the more gentle spray-drying process, can occur during grinding. Additional advantages of spray drying are high reproducibility, optimized production, easy automation and the elimination of an additional drying step. In this work, the process of spray drying in its modified form, the newly developed pressure swing technology, is thematically in the foreground. The basic procedures, terms and technologies are described below and the equipment used is presented [49].

2.1 Method of spray drying

First publications on the method [50] and the underlying principle of spray drying (PERCY: "... a process of atomizing and desiccating fluid and solid substances ..." [51]) were carried out in the second part of the 19th century. While in the beginning food [50, 52-55] and things of daily use were in the foreground [56, 57], in modern times medical products [36, 37] as well as ceramic precursors and intermediates [49, 58-60] added. Industrial convection spray dryers convert liquids, sludges, slurries and pastes into powdery substances. These continuous processes break up and spread the medium into droplets with droplet diameters of ~ 2 to 500 · 10 ^-6 m, in special cases up to 10 ^-3 m, to a suspended particle stream, which is then brought into contact with a drying gas stream. Targeted heat supply and controlled flow properties ensure the drying and targeted separation of the particles. In technical terminology, the term atomization is used for dividing a fluid medium with conversion into a solid particle. In the context of this thesis, this formulation will be extended to standardize to pure liquids, even if the term nebulization would be appropriate in this context. Although there is much information on the drying mechanism and on the formation of particles in the spray-drying process [47, 61], detailed understanding is still lacking in some areas in order to fully understand the relationships between application, evaporation [62] and particle formation [63]. Important influencing factors and physical-chemical limits in spray-drying are discussed in the following sections.

2.1.1 Atomization techniques and drop formation processes

• i. Oberflächenspannung Das Bestreben des Fluids, seine Oberfläche zu minimieren. Sie sorgt für einen Zusammenhalt der Flüssigkeitsmoleküle.
• ii. Trägheitskräfte Zwei kollidierende Strömungen differenzieller Geschwindigkeit sorgen für eine Beschleunigung, Abtrennung (Druckkräfte) oder Ablenkung (Schiebekräfte) der Medien.
• iii. Viskositätskräfte Die Kraft, welche benötigt wird um angrenzende Gase und Flüssigkeiten gegeneinander zu verschieben.

The atomization pursues the goal of dividing fluid media into single drops (spray). This multitude of drops, also called drop collectives, should ideally consist of drops of the same diameter. In general, however, these drops are present as droplet size distributions. When atomizing a medium, mechanical energy (work) must be applied to overcome the forces listed below [64-66]:

• i. Surface tension The tendency of the fluid to minimize its surface area. It ensures a cohesion of the liquid molecules.
• ii. inertial forces Two colliding flows of differential speed provide for acceleration, separation (compressive forces) or deflection (pushing forces) of the media.
• iii. Viscosity forces The force needed to move adjacent gases and liquids against each other.

A₀ = nπd_Tr ² (2.3) These forces, which are important for atomization, are essential fluid-specific properties. An example of the atomization of a liquid with a volume of 10 ^-3 m ³ and spherical shape (A ₀ = 0.048 m ² ) to a spray with an average drop diameter of 5 × 10 ^-5 m is intended to illustrate this fact. A ₀ = πd ² (2.1)

A ₀ = nπd _Tr ² (2.3)

The combination of Equations 2.1 to 2.3 illustrates the effects of atomization of this fluid. In this process, a collective of 1.5 × 10 ¹⁰ drops is generated, which has a summed surface area of 120 m ² . As a result, the surface area of the sputtering process has increased 2500 times. W = σ (A _drop - A ₀ ) (2.4)

According to Equation 2.4, using the example of water (ethanol) [67] at 293 K, a mechanical work of 8.7 J (2.7 J) is carried out solely for atomization. Calculations by WOZNIAK [64] show that such a water droplet requires about 4 s for evaporation under standard conditions (298 K, 101325 Pa) (in the 373 K warm air stream ~ 0.15 s) [24]. In the antagonism of spray pyrolysis, in which laminar flows are in the foreground, in spray drying, turbulent flows (Re> 2300) are used to optimize the momentum exchange (mass and heat transfer). The turbulent transverse motions are superimposed on molecular diffusion [68]. In the subsequent atomization, kinetic energy is subsequently obtained from pressure energy. By means of a tapering of the cross-section at the nozzle, the intended increase in the average speed occurs. For an applied continuous mass flow ṁ = dm / dt = ūAρ (2.5) the law of mass maintenance applies, so that assuming an incompressible flow for different areas of the system the following condition is fulfilled: ū ₁ A ₁ = ū ₂ A ₂ (2.6)

The current thread theory provides the one-dimensional BERNOULLI equation for lossless, stationary, and incompressible flows [69]:

By combining equation 2.6 and 2.7 the average nozzle exit velocity can be calculated:

As can be seen from Equation 2.8, there is a relationship between the pressure difference of the system (ΔP) and the mean velocity ū of the fluid at the nozzle exit orifice. frequently Methods used for transferring fluids in droplet form are single-fluid nozzles, pneumatic atomizers (dual-fluid nozzles) [70, 71] as well as rotary [72] and ultrasonic atomizers [44, 72, 73]. Related descriptions of the influence of nozzle geometry and particle morphology have been described several times in the literature [74-76]. In this work, a hollow cone nozzle is used for the atomization. The field of application of this nozzle form is the atomization of low-viscosity media into the smallest possible drops. In this context, attention is focused on the shear rate as a spatial function of the fluid velocity. The load on the fluid increases from the area in front of the nozzle to the time after the nozzle by 100 to 1000 times, then to minimize in the wider area of the drying chamber (~ 0 s ^-1 ). NONNEWTONonic liquids and mixtures may have interesting shear stress properties and associated viscosity [46, 66, 77]. This point is discussed in detail in Section 2.1.2 (Page 10). Drop size distributions are an essential component of many technical and natural processes. Drop collectives arise, for example, in waterfalls and in the formation of clouds. Individual drops can ideally be considered as liquid spheres. According to Lappace's equation [78] (Equation 2.9), the relation between the internal pressure of the droplets p; and the pressure on its surface p _a :

The stability of a drop is therefore ensured as long as the internal pressure of the drop is greater than the pressure on the outer surface. As a result of this pressure increase on the inside of a curved surface, according to KELVIN's equation [78] (equation 2.10) an increase in the vapor pressure p _{Tr occurs}

From this it follows that with decreasing drop radius r _TR, the boiling temperature of the liquid phase decreases and therefore drops evaporate more easily. High-energy fluid molecules can thus pass into the gas phase at lower temperatures. As stated in this chapter, mechanical work must be used to generate drop collectives. First works on the subject of decay mechanisms come from PLATEAU [79] and RAYLEIGH [80] at the end of the 19th century. They were able to show that hydrodynamic instability is the cause of the decay of a liquid jet [64, 81]. For spray drying essential decay mechanisms in the spray formation are the liquid jet decay, the lamellar decay and the atomization of liquids by gases. LEFEBVRE [82] subdivided the evolution of a spray into the area of primary atomization, in which the liquid jet breaks up into droplets as a result of disturbances and vibrations, and into a secondary decay in which critical-sized droplets (see Equation 2.9) undergo further decay.

• i. Rayleigh-Bereich
• ii. erster windinduzierter Bereich
• iii. zweiter windinduzierter Bereich
• iv. (turbulenter) Zerstäubungsbereich

The exact mechanisms of disintegration are currently still the focus of much research [83]. LIN and REITZ [84] discuss some points of view in their work. An important factor influencing the formation of droplet collectives is the flow velocity of the fluid at the nozzle, in which four areas are discussed [84, 85]:

• i. Rayleigh range
• ii. first wind-induced area
• iii. second wind-induced area
• iv. (turbulent) sputtering area

The pressure swing technology used in this work, which includes high Reynolds numbers for the fluid flow state, is mainly focused on iv. Range of drop formation. Hollow cone nozzles use the lamella-forming decay principle in this context. In this case, even low pressures lead to the disintegration of lamellae emerging from the nozzle with the formation of very small droplet radii. In the course of a slat tear (wave) it comes to the formation of irregular drop forms. As a consequence, the system oscillates until the formation of an energetically stable, mostly approximately spherical form [86]. Significant changes in the spray pattern were shown by HOOPER [87] observations in the variation of ambient and spray pressures. The higher the respective pressure is selected, the shorter the lamella formed and more intense the disintegration of the individual lamellae. In this context, WEBER [88] was able to show that the length of the induced wave depends on the dimensionless Reynolds number (Re) - the ratio of momentum convection to momentum diffusion in the system. The higher the value of this index, the shorter the wave formed and the smaller the calculated droplet size distribution. A characterizing quantity for this type of nozzles is the pressure Reynolds number

It identifies all relevant influencing variables necessary for liquid atomization in hollow cone nozzles. With their help, various authors attempted to record flow characteristics and flow patterns of this nozzle design using systems of equations [89-92]. However, this model as well as other theories on radiation decay according to RAYLEIGH [93] and STERLING [81] did not provide any reliable predictions on effects in droplet formation in experiments and existing contexts. Particularly in the case of turbulent radiation decay, science discusses diverging principles in the interaction between decay mechanisms and radiation decay, the results of which differ significantly [94, 95]. The principal statement of all authors is that the further the mean droplet diameter decreases, the width of the corresponding distribution of the droplet collective must increase. A consideration of the formed radii of the drop collective is an important starting point for the subsequent drying process. Smaller drops have the highest heat and mass transfer coefficient (Biot number) and the lowest diffusion path. As a result, lower humidities and faster drying can be achieved [47].

2.1.2 drying and particle formation mechanisms

• i. Stoffgrößen des zerstäubten Fluids,
• ii. Stoffgrößen des umströmenden Gases,
• iii. Relativgeschwindigkeit zwischen Tropfen und umströmenden Gas,
• iv. Parameter des Tropfens zum Zeitpunkt seiner Bildung.

The drying section of a spray is technologically subdivided into two main areas - depending on the flow of the feed and air flow [46]. In addition to the frequently used DC technology, the countercurrent technology is used in practice. Novel developments in mixed-flow operation are only used for special issues. Particles shown by countercurrent principle always have higher packing densities than granules produced in co-current under analogous conditions. The reason for this is that in the direct current at the beginning of the particles contact with drying gas at maximum temperature ( ∂T / dz ) and minimum humidity ( ∂c / dz ) to have. This contact with the maximum drying conditions as a starting point causes a temporal focusing and fixation of the shaping and is used, for example, in temperature-sensitive materials [47]. In antagonism, the particles are in countercurrent contact first with process gas of the minimum drying temperature and maximum humidity and then move in warmer and drier regions. This gives the particles more time to align themselves spatially in the system [46]. Fundamental studies on the principles and mechanisms of spray drying have been compiled by MASTERS [86]. For example, he explains the relationship of illustrated powders with increasing residual moisture as the difference between inlet and outlet temperature and / or the amount of drying air increases [96]. This effect can be enhanced by increasing the throughput [97]. Another fundamental scientific-technical point of view of a spray application is the lifetime of the droplets produced - hence the time required for the droplet to evaporate completely [98]. In the course of an atomization, the interface gas-liquid is increased, as a result of which material and heat transfer processes are optimized and new viewing angles become necessary. The following parameters determine the lifetime of a drop after atomization:

• i. Fabric sizes of the atomized fluid,
• ii. Substance sizes of the flowing gas,
• iii. Relative velocity between drops and flowing gas,
• iv. Parameters of the drop at the time of its formation.

The desired evaporation of droplets is the result of a combination of mass loss and heat transfer. High-energy molecules of the droplet overcome the liquid-gas interface and enter the gas phase. As a result, the liquid phase loses mass and energy, causing it to cool. The heat flow through evaporation can be described as follows:

On the other hand, the body is in contact with the gas flowing around, which transfers part of its heat to the cooler drops. This heat transfer takes place only in a thin boundary layer at the drop surface [99, 100] Q · = α (t) A _Tr (t) (T _G - T _Tr (t)) (2.13)

The energy transfer is convective depending on the heat transfer coefficient and the temperature gradient at the drop surface. In special applications, the system heat radiation (infrared, microwave) for materialless heat transfer can be supplied. The resulting absorption is described by the following equation: Q · = α (t) k _B A _Tr (t) (T _Tr (t)) ⁴ (2.14)

According to FICK's first law of diffusion [78], the transport processes described (Equations 2.12 and 2.13) are due to their concentration ( ∂c / dz ) and temperature gradients ( ∂T / dz ) speeds up. RANZ and MARSHALL [101] used different series of experiments to generate equations for the calculation of the heat and mass transfer coefficients, which, assuming saturation vapor pressure at the surface of the droplet and constant humidity of the gas flowing around, can be used to determine the specific molar rate (N _Tr ). It indicates the rate of loss of fluid molecules through the surface layer into the gas phase. Later investigations showed their unlimited applicability only to flows with Reynolds numbers less than 200.

In order to describe the fluidic interactions of spherical particles / volumes in gases, the aerodynamic diameter d _A has established itself as an essential descriptive factor of the particle properties in fluid mechanics.

In particular, in medical technology, the knowledge of these properties is essential in considering the inclusion of particles in the human organism. The spray-drying of silica dispersions (silica sol) may represent a solution approach to realize a successful, necessary interaction of micrometer-sized particles (d _G > 5 × 10 ^-6 m) with low effective densities (ρ _eff , hollow body) [23, 23]. 24]. Another important factor for the drying of a spray is the loading μ, which lists the ratio of the mass flows of the drying gas and the sprayed medium.

The smaller the value of the load, the higher the difference between the initial and final conditions of the circulating drying gas. As a consequence, impulse exchange at the droplet surface is slowed down [99] and particles with larger radii are generated [86]. As the loading value increases, more gas is available to receive fluid from the drop surface. Consequently, the products shown under analogous conditions are characterized by lower residual moisture and smaller diameters. Further studies on low mass flow ratios of liquid gas in combination with the process parameters of low viscosity and surface tension led to significantly smaller distributions of the droplet collective obtained [70, 102, 103]. When considering particle formation, various assumptions can be deduced [104, 105]. After the atomization of a fluid, interaction forces between droplets and gas flowing around cause a slowing down of the droplets. The mean velocity of the drop collective decreases. Furthermore, equilibria between vapor, gas and liquid are formed at the phase interfaces. Representations of MESSING et al. [45] show that the characteristic time for the diffusion of the fluid vapor at the surface of the droplet is significantly smaller than the time for the shrinkage of the associated droplet. As a result, the droplet surface is in equilibrium with the process gas at virtually any point in the droplet size change. The analog applies to the bulk temperature of the drop. After reaching equilibrium, the drop temperature T _TR can be equated to that of the drying gas T _G. T _G = T _Tr (2.18)

• i. Einstellung des Gleichgewichtes auf der Tropfenoberfläche,
• ii. Trocknung bei einer konstanten Stoffübergangsgeschwindigkeit.

Based on the adjustment of these equilibria, the drying takes place at a constant rate as long as the surface layer of the droplets is saturated with steam [86]. In summary, drying takes place in two essential steps [46].

• i. Adjustment of the balance on the surface of the drop,
• ii. Drying at a constant mass transfer rate.

When using solutions, dispersions and suspensions, however, additional (solid) effects must be considered. From a system-dependent concentration, the rate of gas saturation decreases as a result of the forming additional interface gas-solid or liquid-solid. This is called a critical point. In the course of the formation of porous systems, this is stated as a wet core [106]. For aqueous (ceramic) systems, this critical point is usually reached in the formation of solid particles [86]. While in solutions these points ((over) saturation concentration) are defined, such concentration limits for suspensions and dispersions can be determined much more difficult. Here already solid particle contacts and interactions take place before the actual particle formation. As a result of this interfacial formation, permeability and mass transfer rate decrease significantly. As a result of this inhibited mass transfer, vaporized liquid in the first step can no longer unhindered overcome the liquid-gas interface. There is an increase in pressure inside the shell formed, as a result of which the temperature rises locally. Accordingly, unlike pure liquids (Equation 2.18), the temperature inside the droplet T _{TR; Bulk may be} greater than at the surface T _{TR thereof} . T _Tr <T _{Tr; Bulk} (2.19)

If this internal temperature (T _{Tr; bulk} ) rises above the boiling point of the liquid, depending on the nature of the surface, the growth of the droplets (puffing) or breakage of the shell formed may occur [63, 86, 107, 108]. Such defects in particle formation will be discussed in the second part of this section. When spraying droplets, uniform, spherical particles are preferred. However, by fusing drops (coagulation) during the spraying and drying process, irregular particles or clusters may form [109, 110]. Assuming pure BROWN motion [111, 112] as the cause of the coagulation can be defined:

welche ein Teilchen für die Diffusion vom Tropfenrand in das Tropfeninnere benötigt. Das Verhältnis dieser zwei charakteristischen Zeiten bestimmt die dimensionslose, stofftransportbeschreibende Péclet-Nummer [23, 118, 119]

Equation 2.20 shows that the probability of effective collisions N (t) of droplets increases with their number and diameter, as direct influencing variables on τ _K. Technically, this means that with a reduction in droplet size distribution, the number of droplets and, consequently, the probability of particle defects in the coagulation of these droplets increases [45]. An additional challenge of the drying process is the oscillation of the newly formed droplets. As shown in chapter 2.1.1 (page 5), the system requires a certain amount of energy minimization time by forming spherical shapes. If solid structures are formed during this period, irregular particles (circular, needle-like [113], platelets) are generated as a consequence [107, 108, 114]. Although attempts to find correlations between granule form and surface tension show similarities, their thermodynamic interpretation is not without doubt understood [96, 113-117]. Two major mechanisms and their influence on particle formation in the spraying process are discussed in the literature. On the one hand, the required period of time (t) to dry the resulting drop and, secondly, the time

which requires a particle for the diffusion from the edge of the drop into the drop inside. The ratio of these two characteristic times determines the dimensionless, transport-describing Péclet number [23, 118, 119]

• i. Ballooning/Aufblähung [47, 121]
• ii. Fehlstellen bei der Hüllenbildung [60, 63, 121, 125]
• iii. Luftfehlstellen [86, 107, 108]
• iv. Luftvermischungen [86, 107, 108]

This figure describes the ratio of the mass transport of diffusion and convection. Small values (Pe << 1) describe a very slow drying of the spray drop in which the particles have sufficient time to diffuse into the interior of the drop in order to minimize surface energy. This process continues as long as the gas-solid surface energy is greater than the gas-liquid surface energy. As a result, dense particles (solid bodies) are formed. Mathematical calculations by LENGGORO [120] on the relationship between droplet size and the formation of solid spheres prove this dependence of the particle morphology on the droplet radius. According to LUKASIEWICZ [121], formation of these types of particles (granules) is achieved with low mass transfer rates, high solids contents and low temperatures. Due to the increased solids content, the constant mass transfer rate of the drying can be shortened and more water can be evaporated [63]. Solid bodies may show porous or self-contained properties [47]. Defects caused by gas mixtures in the atomization process characterize the difficult-to-reproduce porosity of this type of particle. A very high speed of the drying process, however, illustrate high Péclet numbers (Pe >> 1). The solid particles do not have sufficient time to diffuse into the interior of the droplet under such conditions. Due to the receding gas-liquid interface, energy is minimized at the droplet surface by solid phase formation - resulting from the relatively large gas-liquid surface energy, which significantly outweighs the fraction of the difference between gas-solid and liquid-solid surface energy [122 ]. Characteristic shaped hollow body of sheath and hollow core are the result. Often, a low porosity and permeability characterizes the properties of this formed shell, which can then act as a barrier to mass transfer in the further drying process. Hollow bodies generated under the same conditions always have higher packing densities than their analogous solid bodies [123, 124]. According to Equation 2.21, it can be seen that parameters that promote the formation of this type of bodies are increased drying temperatures, low solids content, and high binder concentrations [45, 109]. Several authors report that preferential shaping of hollow and solid granules also shows dependency on the dispersion level [123]. Using silicon carbide dispersion, LEE [123] demonstrated that the formation of hollow bodies is favored at low dispersion levels. Particle formation mechanisms are often considered idealized. Variable influencing factors show an increased susceptibility to internal defects, especially in the formation of (spherical) hollow bodies. Four major mechanisms are discussed here:

• i. Ballooning / Inflation [47, 121]
• ii. Defects in shell formation [60, 63, 121, 125]
• iii. Airborne defects [86, 107, 108]
• iv. Air blending [86, 107, 108]

In ballooning, it is believed that, like inflating a balloon, the internal pressure created by vaporising fluid increases (inflates) due to the inhibited mass transport to the solidifying surface. Most thin-walled or eggshell-like fragments are a result of this mechanism [121, 125]. In the approach of defects in the sheath formation is based on a local solidification of the drop surface. The evaporating liquid can only escape at certain points. As a result of the formation of currents (suction), an irregular mass transfer occurs inside the drop. Based on this, various authors discuss the formation of an internal, partial vacuum as an intermediate of a defect [63, 125]. The resulting increased convection inside the drop again leads to increased mass transport to the surface of the drop. In the course of the resulting accumulation phenomena, menisci form between the individual particles. Resulting capillary forces then cause a deformation of the granules such as the so-called ring formation. Repulsive forces counteract this principle. Finally, VAN DER WAALS forces (vdW) fix the formed particles in their "assigned" space [126-129]. SHAW [125] recommends an increase in the solids content of the spray and lower drying temperatures to minimize these defects [121]. Since the concentration is a significant factor influencing the particle diameter [47, 130], the advantage of a reduction in the defect density combines the disadvantage of the formation of larger-radius particles or granules with a higher packing density [86, 131]. (Page 60) shows examples of possible particle defects.

If the pressure in the interior of a solidifying droplet surface (shell formation) rises above a critical value due to evaporating fluid and inhibited mass transfer, deep craters ("blowholes" [109]) arise as a result of the yielding surface [63, 121, 132]. This deformation can occur in the direction of the interior of the particle [63, 121] or particle exterior [109, 132] - depending on the time of pressure release and the possibility of forming an internal, partial vacuum. Discussions that a formation of craters can also be the result of an internal collapse (mechanism ii.) Due to lack of bulk solids in bulk, are also conducted. An essential element here is the formation of a structurally weakened (thinner) inner particle shell, which is exposed to the superheated gas inside the droplet (Equation 2.19) in the further drying process [125]. If the rate of diffusion of the solid particles into the interior of the droplet is slower than the evaporation of the fluid in bulk, vapor bubbles (air imperfections) are formed in the droplet, which additionally inhibit the further transport of material in the droplet and can produce defects. Often nozzles are used with additional air streams of variable speeds for better atomization. In this context, GREENWALD and KING [133] discuss a mixing of air and fluid (mechanism iv.) As a result of which defects in the phase interface gas-liquid on the inside of the shell arise in the drying process. Depending on the collision gas, additional effects such as, for example, chemical reactions or influencing of fluid properties may also occur. The mobility of particles as the sum of their repulsive and attractive interaction forces is a crucial factor in particle formation. WALKER [46] was able to show that increasing the repulsive forces increases the packing density of the resulting granules. The increased mobility of the particles increases the likelihood of alignment during the drying process. Converse effects show up in the shear stress of the systems. Here, the packing density decreases with increasing load (voltage). WALKER [46, 77] described the yield strength or shear-thinning effects in polymer dispersions to have a strong influence on the formation of granules. Correlations between the position of these yield points and the mass transfer inside the drop are discussed here. A model developed by WALKER [46] on ceramic suspensions (46) ) for particle formation will be briefly explained here since essential properties can be transferred to the silica dispersions (silica sols) used in this work. Emphasis of the ideas of this model is the rheology of the medium. When atomizing a dispersion, the shear rate after the nozzle drops to near zero in a timely manner. The elasticity of the medium dominates and increases with increasing temperature. From a system-dependent solids content, local gel formation occurs. With evaporating fluid, this elasticity continues to increase to a critical point. From this moment the drop size is constant (fixed) [63]. Calculations by WALKER [46] have shown that all comparable systems have the same density at this point. If the viscoelasticity is below a certain value, which depends on the respective system, formation and growth of a shell on the outer surface of the drop (hollow body) occurs. Resulting imperfections due to excessive drying (hardening) lead to a high pressure difference between the inside and the outside of the drop as well as to the (partial) collapse of the shell or to the formation of particle defects. If the viscoelasticity exceeds the system-dependent value at the critical point, partial solid forms are formed (bulk gelation - solid spheres). Then, cohesive forces decide whether pores form (weak) or the droplet continues to shrink (strong).

2.1.3 Separation of particles

Minimal sinking velocities in cyclones, aggregation phenomena on filter surfaces and unfavorable flow properties of the powders represented represent decisive technical limitations for the separation of particles in the (sub-) micrometer range. HUANG [134] and GIMBUN [74] have shown that at high flow velocities in combination with high Reynold numbers (Re) particles with diameters <10 ^-6 m can be deposited in cyclones. In practice, this type of deposition is limited to small cyclonic radii. WALTON [47] characterizes flowability in the separation of powders as a characterization possibility for surface chemistry and morphology. It describes the interaction forces of the particles with each other, the structure and shape of the individual particles as well as further physical-chemical powder properties. Knowing this property can be a key decision aid in obtaining these materials. With respect to agglomeration phenomena, attractive interactions (VAN DER WAALS forces, electrostatic forces) with small particle diameters (<5 × 10 ^-6 m) attract particular attention with regard to particle mass. A separation by means of the low weight is usually not feasible due to the strong-acting attractive forces. As a result, cohesive flow behavior is usually observed at these particle radii.

2.2 Spray Drying to Microscale Silica

The presentation of silica-based nanocomposites for the optimization of mechanical parameters is a widely diversified field of research [135]. The focus of interest is often in the field of optimization of mechanical parameters of a variety of materials. The production of silica particles in the (sub-) micrometer range (d ₅₀ (V) <5 × 10 ^-6 m) has already been described by various authors [136-145]. NAMIKI [138] used suspensions of fumed silicas to convert this type of particulate matter into powder. The resulting granules represent complex, agglomerated structures. Nanostructured spherical particles with silica sols as starting material ("Sol Spraying" [146]) are the subject of work by KAMIYA [141], IWAMOTO, [142] and ISKANDAR [140]. The production of this type of particles However, it was only possible with low throughputs and solids contents of the reactants (<2% (m: m)) and high drying temperatures and high loadings (μ> 200). While KAMIYA [141] and IWAMOTO [142] reported high levels of particle defects, ISKANDAR [140] was able to reduce defects by a combination of spray drying and spray pyrolysis (Re <300; laminar) and the effects of concentration, droplet diameter, and educt discuss particle morphology [139, 147]. According to the content of his work, colloidal educts (silica sols) form porous solid bodies whose surfaces have a roughness depending on the colloidal radius (monomer) of the starting material. Furthermore, their porosity increases with monomer size. An exact interpretation of the effects of the primary particles on the morphology could not be given by ISKANDAR [139]. On the other hand, it characterizes particles produced from organic precursors (TEOS) by predominantly smooth surfaces as well as the degree of hydrolysis of the alkoxy compound [45, 148]. Syntheses of large-volume, porous nanomaterials have been studied by TSAPIS, [23] among others. He found that the formation of such particles at low concentrations is more subject to process parameters and independent of the chemical nature of the materials. CROSBY and MARSHALL [63] described that the particle density obtained by spray-drying granules seldom exceeds 50% of the pure substance density ρ _true (packing density ≤ 53%) [149]. Industrially generated products are often characterized by densities of ~ 5 x 10 ² g L ^-1 . Attempts to theoretically predict the packing density by particle radius could not be substantiated by WALTON's [47] experimental experience. The reasons for this are the particle shape, the size distribution and the particle structure of the powders. For low-concentration suspensions LUKASIEWICZ [121] showed that only the capillary-generated liquid stream takes over the mass transfer to the droplet surface. In highly concentrated mixtures delineation of the drying steps presents an immense challenge. To simplify modeling, it is generally accepted that the particle diameter is constant (fixed) at the end of the constant mass transfer rate of a drop [46, 121, 150]. For small droplet size distributions of a silica dispersion, LANG et al. [151] a uniform temperature profile in the drop (BIOT number <1). Based on this, they described that with decreasing initial particle radius and increasing drop diameter, the tendency to form hollow spheres must increase. The reason for this is explained by the permeability (Equation 2.22), which depends directly on the primary particle diameter.

The concentration gradient between the drop surface and the bulk phase is significantly increased by the slower mass transport with smaller particle diameters d _p compared to larger colloid diameters. As a result of this reduced diffusion, such fluids on the surface of the drop are more likely to crust over time. The generation of particles with larger radii is observed [151]. For silicates and their analogues, the formation of hard structures during the drying process is usually described. Their breaking behavior under load is characterized by a breaking of a particle into a few large parts. To optimize the mechanical strength of these systems often binders (eg., Polyvinyl alcohols, glycerol) are added as a structural enhancer during spray drying. In addition, the structures described in the literature show a high proportion of particle defects (craters) [47]. Dependencies of the degree of milling of aqueous, concentrated suspensions of pyrogenic silicic acids on their "Hollowness" - the proportion of inner cavity in the particle - could be demonstrated by ECKHARD [152]. The more finely ground the educts were sprayed, the higher the proportion of free bulk phase. However, the resulting spherical particles are characterized by diameters beyond 5 × 10 ^-5 m. Silicon dioxide hollow spheres using a mixture of ethanol and silica sols to lower the surface tension are described by HADINOTO [24]. However, reproducible results of small particles (d <10 ^-5 m) could only be achieved with trizma (2-amino-2-hydroxymethyl-1,3-propanediol) buffered systems at very low reactant solids (<1.8% (m: m )) and high loadings (μ> 500) can be realized. Nevertheless, only a fraction of the particles formed are hollow bodies ( ), the proportion of which additionally decreased with increasing educt concentration. In addition, these types of particles are characterized by a very high probability of defects and uneven particle surfaces. The reason was sought in the case of the drying process formed, not yet solidified shell, which deforms due to the selected drying conditions and the resulting capillary pressure between the accumulation points ("buckling") [153, 154]. TSAPIS et al. [155] confirmed the established hypotheses based on model calculations for polymer dispersions. Dependencies of the particle morphology on the drying temperature could not be determined, but an increase of the calculated shell width with a decrease of the particle diameter was described [24].

In a subsequent work based on aqueous dispersions, HADINOTO concluded that silica nanoparticles are only accessible at a low pH (<7) and low solids contents (<1% (m: m)) [118]. BAHADUR [156] was able to show an influence on particle formation in the spray drying of diluted colloidal silica dispersions. By adding polyethylene glycols (PEG 400 or PEG 1450), the particle formation mechanism could be actively influenced to form hollow body structures. All the results presented are based on atomization, the transfer of fluid media into powdery materials, by means of pneumatic atomizers, rotary or ultrasonic atomizers at low throughputs (<< 1 L h ^-1 ). The aim of these atomization variants is the generation of drop collectives, consisting of small drops and narrow size distributions. No work has been found in the literature on sub-microscale silica particles using single-fluid nozzles.

2.3 Influence of gases in spray drying

The proposed BERNOULLI approach of stream-thread theory (Equation 2.7) applies to continuous, incompressible flows. Gases, on the other hand, show significant volume changes upon modification of pressure and / or temperature. These effects are described by gas dynamics [157], whose simplified energy equation reads:

This equation states that a change in the velocity of a gas stream leads to a conflicting behavior of the temperature of the gas. When applying a gas from a nozzle, pressure energy is converted into kinetic energy. The gas is accelerated, as a result of which its temperature decreases [64]. This process is subject to temporal and local changes. With the help of the speed of sound a, the propagation speed of these pressure fluctuations can be quantified assuming ideal gas behavior. a = √ κRT (2.24)

In a mixture of gases in liquids, a plurality of phase distribution states may occur ( ), whose transitions are fluid, and most of which are simultaneous [158, 159]. This poses immense challenges to the spraying process. With regard to equation (2.24), gas-liquid mixtures have values for sound velocities far below those of the respective individual components. As a result, pressure fluctuations in these systems can be compensated for much faster in terms of time and friction losses reduced. The derivation of this situation can be described assuming a pressure wave in the x-direction p (x, t), which acts on a fluid element with the volume .DELTA.V. p (x, t) = p _max · sin (2π (ft-x / λ) (2.25)

Through integration over time, the speed can be detected.

The velocity results from the integral of the velocity difference before v (x, t) and after the fluid element v (x + dx, t):

The maximum compression is therefore:

The speed of sound of the fluid thus results in:

It turns out that the high compressibility of the gas portion of a gas-liquid mixture has a significant influence on the reduction of the speed of sound. The exact derivation of the listed equations can be found in the literature [64]. For atomization of fluid media with the aid of gases in practice two-fluid nozzles are used. These gain the necessary energy to form droplet collectives by the incoming gas and find use in fluids with high viscosity. The interaction forces in the atomization with the assistance of a gas represent a hydrodynamic problem. Under equilibrium conditions, a formed droplet: p _w + p _σ = constant (2.30)

F_σ = πr_Trσ (2.32) This drop reaches its critical state when the aerodynamic drag F _w equals the force caused by the surface tension (equations 2.31 and 2.32).

F _σ = πr _Tr σ (2.32)

For the description of the processes in pneumatic atomizers various indicators have been proposed. In this work SHRAIBER [160] gives an overview of the large number of common influencing variables and droplet decay modes (Equation 2.33).

WOZNIAK [64] was able to show that in the antagonism to pneumatic atomizers (two-substance nozzles), the gas density on the atomization of fluids with hollow cone nozzles (single-fluid nozzles) has no significant influence. Accordingly, this aspect may be neglected with respect to the formation of a drop collective in this work.

2.4 Solubilities of gases

The solubility of gases under a variety of physico-chemical conditions has already been discussed by several authors. In the case of the binary system water-nitrogen, numerous experimental and theoretical studies on pressure-dependence have been carried out [161-168]. In particular, this is a significant factor as nitrogen is often a major component in the use of fossil resources (eg natural gas) [169]. In addition, nitrogen is a commercially available product of known reactivity.

The indication of the solubility of a gas component in a fluid (m _FL ) is given in the volume which would be the same amount under standard conditions (273.15 K; 101325 Pa) (V _G ).

Further studies on the solubility of gases in saline solutions showed direct dependencies on the salt concentration and the salting-out coefficient [167, 170, 171]. The higher the concentration an aqueous saline solution, the lower the proportion of dissolved gas under the same conditions. In this work, emphasis will be placed on the gas nitrogen. Studies on the solubility of this gas in aqueous, colloidal silica dispersions are not yet known.

2.5 Properties of silica sols

Colloidal systems and their interactions are receiving much interest in science [172, 173]. Advantages of such dispersions are the accessibility of very small particles (10 ^-9 m <r <10 ^-7 m) in a controllable distribution, controllable reactivities and high concentrations usually, in other orders of magnitude, sparingly soluble components. Through specific selection of the dispersing conditions, special physical-chemical characteristics of the dispersions can be adjusted. Colloidal dispersions of silica, called silica sols, are characterized by a wide variety of properties and modifications. In addition to a large variance in monomer sizes (5 to 200 × 10 ^-9 m), high purities (up to 99.9% (m: m) silicon dioxide based on the solids content of the dispersion) and solids contents (up to 60% (m: m) ) also surface modifications of the individual colloids are known. At low concentrations, silica sols behave analogously to their water dispersant like NEWTON's liquids [174-177]. The deformation rate is linearly dependent on the shear rate. Outside the solids-related, viscous boundaries, colloidal silicas have dilatant behavior. OKUBO's [178] measurements showed that the surface tension of silica sols shows only small deviations from the reference system water. Colloidal stabilities of silica sols to a variety of parameters have been widely studied [179]. Results of the group um BLUTE et al. show that at pH values of 4 to 5 as a result of the lowered surface charge of the colloidal silica particles aggregations arise, which form a less hydrophilic, but much more active surface. As a consequence, dense and stable gel systems are formed in the present sol-gel process. Studies on the diffusion of colloidal silica particles and the determination of the corresponding coefficients yielded values in the range of 10 ^-12 m ² s ^-1 for large-scale silica sols [24]. Accordingly, technically high Péclet numbers (Pe> 10), hence the preferred shaping of hollow bodies, are to be expected in combination of pressure swing technology and Equation 2.21.

3 Experimental part

Generation of the xerogels produced in this work requires the knowledge of physico-chemical properties of the selected or modified reactants. The chemicals and equipment used for this purpose are presented in sections 3.1 and 3.2. Pressure swing technology used for the subsequent transfer of these materials to powder is discussed in Section 3.3. The methods used to characterize the dispersions and powders are given in sections 3.4 (dispersions) and sections 3.5 (xerogels).

3.1 Chemicals used

An overview of the chemicals and additives used in this work is given in Table 3.1. This is followed by a list (Table 3.2) of all commercial silica-based commercial products used as starting material, intermediate or comparative medium. Table 3.3 shows the gases and gas mixtures used under points 3.2 to 3.5.

3.2 Preparation and analysis of silica sols

The method used in this work to produce unstabilized silica sols is based on a variety of ion exchange steps of the colloidal, alkaline starting materials. As unstabilized or demineralized silica sols in this work colloidal silica dispersions are referred to, which have been removed by physical and chemical means ions. For this purpose, various resins are used with their wide range of applications for targeted depletion of ions. The key point is concentrations of the alkali ions of <50 ppm. The method of manometry was used to determine the solubility of various gases in the prepared silica sols. For this purpose, a newly constructed, modular apparatus was used. The solubility L of the gas can be calculated from the mass balance of the gas phases before and after the expansion experiment as a function of the absorbance mass. In the following, the equipment used in terms of their structure, their properties and their performance will be presented. In addition, an overview of the components and peripherals used is given.

3.2.1 Apparatus for the preparation of demineralized silica dispersions

The experimental depletion steps were carried out on ion exchange columns. The starting material was the alkaline stabilized Köstrosol ^® 1530 from Chemiewerk Bad Köstritz GmbH. After the targeted substitution of the ions was diluted with deionized water to the desired use concentration. The deionization of the dispersions was carried out in three stages. After each portion of this procedure, the silica sol was conditioned at room temperature for 24 hours to ensure diffusion of ions within the colloidal particles into the dispersing medium. The scheme of the applied apparatus is in to see. The volumes of the ion exchangers used (Table 3.4) were chosen so that a maximum of 50% of their maximum capacity was exhausted. Characteristic of the first substitution step was the reduction of the content of cations by means of a strongly acidic ion exchanger. The resulting, cation-exchanged sol then has a pH of about 2. In the following anion exchange step, the concentrations of the production-induced added chloride and sulfate ions are reduced in the system. The third stage of the process was the reintroduction of the deionized silica sol via a strongly acidic ion exchanger. This served to reduce heavy metal ions as well as the cations diffused from the bulk of the colloidal particles. Unstabilized (fully desalted) silica sols prepared in such a way are characterized by a pH of 3.3 ± 0.2. Their storability is due to missing, gelation promoting, ions several months [179]. From experiences of the manufacturer with such exchange processes a temperature of 293 K as well as normal pressure for the ion exchange steps was used.

3.2.2 Determination of solubilities of gases

For the measurement of the gas solubility of ternary mixtures, a new apparatus was built on a volumetric basis. The principle of this system is the expansion of a gas from a defined original volume into a sample chamber which is filled with absorbent component. After introducing a fluid into the test chamber and setting constant pressure and constant temperature in the reservoir, the valve is opened to the sample chamber. Based on this, a pressure drop in the course of isothermal expansion as well as absorption effects in the selected fluid is observed. With the help of a mass balance, the absorbed mass can be determined from the pressure difference and the volume of the gas phases. For the construction of the apparatus was in shown schema taken as a template. Due to the exclusive use of stainless steel components, measurements with corrosive gases are possible. The system is designed by its design of components for pressures up to 1.4 · 10 ⁷ Pa (Table 3.5). As an indispensable basis for the measurement of isothermal expansions, the apparatus for thermostating was integrated into a water bath. An additional circulation pump guarantees a homogeneous material and temperature distribution in the closed interior. Sensors for temperature and pressure control were used to record the measurement results and could be read out with the help of a computer. To ensure high-quality data, different pressure sensors with specific measuring ranges were used for the different pressure ranges. To validate the newly designed apparatus, the required system volumes were determined in a first phase. This was done by expansion experiments with nitrogen at a temperature of 298 K to pressure levels of 1.2 · 10 ⁷ Pa. The calculated investment volumes are summarized in Table 3.6. A final verification of the plant was carried out with the binary system nitrogen-water and compared with known, experimental data from the literature [162]. The good correlation of the solubility data for nitrogen in water can be found in Table 7.1 (see Appendix).

Nitrogen solubility studies were conducted at 298K. In the first step, the silica sol was placed in the working room, vacuumed and tempered. Subsequently, the loading of the storage vessel was carried out with nitrogen. After tempering the complete system, the data points were recorded every five seconds. Balance was defined as the values obtained deviated less than 0.5% of the final value within ten minutes. In the following expansion experiment, a minimum opening of the valve to the sample space was carried out. With the aid of the installed circulation pump, the gas was then carefully circulated for optimum gas and temperature distribution. The previously mentioned equilibrium conditions applied. From the mass balance of the gas phases of the expansion experiment, the solubility of the respective gas as a function of the absorbing colloidal silica dispersion can be calculated. The gas densities required for the calculation of the adsorbed substance quantities were calculated by means of equations of state. In this work the Fluidcal program of the Ruhr-Universität Bochum was used [181]. Part of the program are equations of state of pure gases with the aid of which different properties of the fluids can be calculated. On the development and Derivation of these equations is referred to the known literature [182-186]. The relative error of solubility is ~ 0.2%.

3.2.3 Measurement accuracy

For a determination of absorption equilibria from volumetric experiments the knowledge of the absorbed mass is necessary. As measured data, however, only pressure, temperature and mass of the absorbent can be detected directly. The absorbed mass is only indirectly accessible by means of a mass balance. Required bases of such calculations are the plant volumes used and the volume of absorbate. An estimation of a maximum error for all measured variables listed is given in Table 3.7 taking into account the device overview presented in Table 3.5.

3.3 Transfer of silica sols into xerogels

The method used in this work to convert the aqueous silica sols into xerogels is based on a modified process control of spray drying, pressure swing technology ( ), on. Unusually high spraying pressures (> 10 ⁷ Pa) and throughputs as well as a low mass ratio of air flow to spray medium characterize this technique. The main feature of this process is a pressure jump from the high pressure part before the low pressure part after the nozzle by at least five orders of magnitude. This kind of spray drying is built up by a flexible high-pressure part, a heated spraying chamber and a modular filter system. These are presented in detail in the following sections. In the second part of this work, the loading of the colloidal starting material with a gas took place. After reaching the target parameters, the silica gas, which was ideally saturated with gas, was released very rapidly in a heated volume. The expected high Péclet numbers (Pe> 10) in the drying process should promote the formation of hollow spheres. Subsequently, the dust granules are deposited on a filter system and removed regularly.

3.3.1 High pressure unit

This component of the pressure swing technology includes the construction of pressures up to 4 · 10 ⁷ Pa and temperatures up to 423 K. Since silica sols are very abrasive media, all components are held in stainless steel, ceramic or composite materials. The core of the system is a modifiable high-pressure pump, which allows variable volume and frequency control and a reduction of pressure fluctuations. The feed of the educts is made possible from two storage containers. clarifies the general structure of the apparatus. In a further process step, the high-pressure colloidal silica dispersion can be mixed with a gas in a targeted manner. With the help of a compressor (KP), the selected gas is compressed and pre-set with pressure regulators a constant pressure. Subsequently, gas is added to the sol in the mixing chamber by means of a mass flow controller (MFC) under continuous flow. To ensure the desired spray inlet temperature, the fluid then flows through a thermostatically controlled piping system. In order to reduce pressure flow fluctuations, a maximum of 2 × 10 ^-4 m was used as the cross section of the nozzle outlet opening. With larger diameters at the nozzle exit there were strong pulsations, interruptions of the fluid flow by gas phases, the spray.

A loading of the silica sol with gas took place in dependence of the drying temperature in the spray chamber at 0.5 (363 K) and 0.7 NL min ^-1 (398 K). The resulting temperature-related discussion of the mass transfer rate (drying rate) can be found in Section 4.2.2.2. The throughputs were pressure dependent at 5.5 to 8.5 L h ^-1 . As discussed in Section 4.2 (page 50), the saturation of the silica sol with nitrogen at 298 K using such flow rates is different. At pressures below 1.5 · 10 ⁷ Pa, a formation of gas phases can be observed.

3.3.2 Drying system

To achieve the desired, elevated Péclet numbers (Pe> 10), the drying process step needed to be made as effective as possible. In this context, the drying gas flowing tangentially into the spraying chamber and controllable by means of a mass flow controller (MFC) was supported by additional heat radiation. The infrared radiators used as heat radiation source are variably adjustable for uniform and targeted heat dissipation. For the parameters of pressure swing technology used in this work, hollow cone nozzles (single-fluid nozzles) are used to form a spray. They allow the atomization of the medium under very high pressure at corresponding throughputs in combination with the realization of small droplet size distributions (d ₅₀ (V) <5 x 10 ^-5 m). When using full cone nozzles, the probabilities of coagulation phenomena of the formed drop collective due to collapsing lamellae are significantly higher. Due to the resulting length of the nozzle exit jet (> 2 m, ν _Tr > 60 ms ^-1 ), only one drying process was carried out in direct current for discharging the product. As in shown, the determination of the temperature at the inlet and outlet to the spraying chamber. In order to discuss the influence of physisorbed water on the silica surface, two drying profiles were used in this context. Table 3.7 indicates the selected physical-chemical parameters. For a timely removal of moisture and for optimizing the drying conditions, the volume flow of the drying gas was set to 330 m ³ h ^-1 in the reactor by means of a mass flow controller (MFC). By means of a further blower on the filter, a vacuum of <2 · 10 ² Pa could be realized in the spraying chamber.

3.3.3 Filtration

For the realization of the filtration and ensuring the negative pressure in the spraying chamber so-called filter stockings were used to obtain the particles formed. shows on the example of dedusting devices of the SFK series of the company MAHLE Filtersysteme GmbH the nature of the filter system built up in the context of this work [187]. Advantages of this filter design are the provision of large filter surfaces and the operation of such systems under volume flows of> 100 m ³ h ^-1 . The particle-laden crude gas stream enters the filter tangentially for uniform flow distribution and pre-separation of coarser dust particles (d _P > 10 ^-3 m). During the subsequent passage of the filter elements, the particulate matter contained is deposited on the element surface. In certain, depending on the dust load and the filter surface load, time intervals, the filter cake is cleaned by means of compressed air. The control of the diaphragm valves via an electronic control attached laterally on the filter housing [187]. The detached dust falls into a, located in the lower area, sampling vessel. To reduce the particulate matter pollution and to protect the obtained xerogels against humidity, two switchable control flaps were inserted between this container and the filter. The cleaned gas then flows into the upper filter area and is sucked off by means of a blower. A condense of humidity on the filter system could be avoided as a result of this arrangement by a pressure in the exhaust air system of <10 ² Pa.

3.3.4 Used devices

Table 3.8 lists all equipment and measuring equipment used to perform the experiments, based on the equipment presented in Sections 3.3.1 to 3.3.3.

The volume flows of the samples to be sprayed were determined on the basis of samples of the containers used. For this special volumes were marked and a regression of the obtained values was carried out. Each volume flow determination of the samples used was carried out on at least five variable markings and times.

3.3.5 Measurement accuracy

Basic considerations of spray drying to convert the aqueous systems into powders are the input and output conditions of the drying step. An exact temperature and pressure detection are accordingly also the basis of such analyzes, as the indirect detection of the volume flow. The estimation of a maximum error for all measured variables listed is given in Table 3.10 taking into account the device overview presented in Tables 3.9.

3.4 Characterization

3.4.1 Characterization of the dispersions

Determinations of the surfaces of the colloidal systems were carried out by means of a titration with potassium hydroxide solution of pH 4 to pH 9. This characterization is based on the published by SEARS [188] 1956 operation for the correlation of the surface of colloidal silica dispersions with the BET method. Modifications concern the reagents used as well as the conversion of the silica sols to their initial value (pH value) of the titration. For this purpose, the colloidal silicic acids using a strongly acidic ion exchanger (Lewatit ^® S100, Table 3.4) was acidified in a preliminary stage. Subsequently, the titration was carried out in two independent steps. As a titrator TitroLine ^® alpha plus TA 20 Schott Instruments Analytics was used. The titration was carried out with potassium hydroxide solution (0.1 N) using the Einstabmesskette Schott Instruments H61 at 298 K. Analyzes of the pH were carried out at 298 K by an EGC 151 electrode from Sensortechnik Meinsberg GmbH. All applied electrodes were calibrated daily. Viscosity measurements were made using a falling ball viscometer Hoppler ^® KF 3.2 Company RHEOTEST instruments Medingen GmbH. Its thermostatization at 293 K was carried out using a Julabo ED from Julabo Labortechnik GmbH. Provisions of the diffusion speeds as the basis of the hydrodynamic diameter were made with the Zetasizer ^® Nano ZS from Malvern Instruments GmbH. This device works on the basis of the dynamic light scattering according to the backscatter principle at an angle of 173 °. Unless otherwise stated, the reference measurements of the samples were made at a dilution of 1: 2500 (v: v) at a pH of 9.2 using ammonia-adjusted ultrapure water. The MIE theory [189, 190] served as the basis for an evaluation of the analysis results. The examination of the diffusion rates of concentrated sols was carried out directly in the respective colloidal dispersion. In this case, no viscosity corrections of the monomodal evaluation algorithm of the partially metastable samples were carried out. Redispersed samples were transferred to the measuring cells for the required reduction of suspension effects with the aid of a syringe filter (8 × 10 ^-7 m, PE). Analyzes of droplet size distributions were carried out using the device Spraytec ^® from Malvern Instruments GmbH. This method uses a static laser diffraction principle with temporal focusing and local fixation. Responsible for the provision of the spray pattern was the high-pressure section constructed under point 3.3.1. The analysis of the droplet collective was carried out at 298 K and normal pressure due to the length of the nozzle exit jet. Associated measuring points were arranged 5 × 10 ^-2 m and 0.24 m behind the nozzle outlet opening.

To concentrate the colloidal starting materials used, the Laborotta 4011 rotary evaporator from Heidolph Instruments GmbH & Co. KG was used. The evaporation of the fluid was routinely carried out at 333 K using the vacuum pump 113043 Ilmvac GmbH. By titration of the silica sol samples with hydrochloric acid against methyl orange at 298 K, the alkalinity of the starting materials was determined. The indication of the analysis value takes place as mass percent sodium oxide. The density of liquid samples was determined using the DMA 4500 instrument from Anton Paar GmbH. For this purpose, the samples were applied by means of a syringe into the instrument, tempered and measured by means of bending oscillator. The solids content of silica dispersions was analyzed in an oven L 24/11 M P320 from Nabertherm GmbH. Here, the starting material was dried at 1223 K for two hours. The dry matter content was then calculated from the difference between the masses of the sample before and after the temperature control.

3.4.2 Characterization of the xerogels

durchgeführt. Untersuchungen zur Oberflächenmorphologie der Xerogele erfolgten anhand der Geräte Nova^® 3000e (Stickstoff-Sorption) und Poremaster^® 60 GT (Quecksilberporosimetrie) der Firma Quantachrome GmbH & Co. KG. Vor den jeweiligen Analysen wurden die ausgewählten Xerogele eineinhalb (Quecksilberporosimetrie) bzw. drei Stunden (Stickstoff-Sorption) bei 573 K unter Hochvakuum getrocknet. Die Messung und Auswertung der Daten der Stickstoff-Sorption erfolgten mittels der Software NovaWin^® (Version 1.12). Eine mechanische Instabilität der Partikel bei den Quecksilberintrusionsversuchen konnte nicht beobachtet werden. Die Bestimmung der Schüttdichte erfolgte anhand der Auswaage eines Gefäßes mit genormtem Volumen. Hierfür wurde die Probe möglichst luftfrei in den Behälter eingefüllt und ausgewogen. Aus der Massendifferenz zwischen gefülltem und leerem Gefäß ist das Verhältnis Masse zu Volumen, die Schüttdichte, zugänglich. Angegebene Werte in dieser Arbeit sind das arithmetische Mittel aus mindestens drei Analysen. Eine Analyse des Siliziumdioxid-Gehaltes wurde nach dem vollständigen Trocknen der Proben bei 1223 K durch Zugabe von Fluorwasserstoffsäure (40% (m:m)) bei analoger Temperatur durchgeführt. Aus der Differenz der beiden Massen, bezogen auf das Ausgangsgewicht, ergibt sich der Gehalt an Siliziumdioxid. Feuchtigkeitsgehaltsuntersuchungen erhaltener Xerogele erfolgten am Gerät MA 35M der Firma Sartorius AG. In diesem Zusammenhang wurden die Proben bei 298 K eingewogen und bei 433 K bis zur Massekonstanz getrocknet. Mit Hilfe einer Massenbilanz ist die Feuchtigkeit zugänglich. Zur Messung des pH-Wertes der Xerogele wurden 10%ige (m:m) Suspensionen der erhaltenen Pulver in Reinstwasser hergestellt. Anschließend wurden diese unter Rühren bei 298 K mit einer Elektrode EGC-151 der Firma Sensortechnik Meinsberg GmbH vermessen. Eine Bestimmung des Kohlenstoffgehaltes der modifizierten Xerogele erfolgte am Gerät multi EA 2000-CS von Analytik Jena AG. In diesem Zusammenhang wurde die Probe auf ein Schiffchen aufgebracht und im Sauerstoffstrom bei 1473 K verbrannt. Angewendete Zerkleinerungstechniken an den erhaltenen Pulvern waren das Mörsern der Proben mit Hilfe eines Achat-Mörsers sowie die Vermahlung mittels einer Labormühle A 11 B von IKA Labortechnik GmbH. Die Betriebszeit betrug hierbei 5 × 30 s. Röntgendiffraktogramme wurden mit einem Gerät des Typs D4 „ENDEAVOR” der Firma Bruker AXS GmbH mit einer Röntgenröhre für CuK_α-Strahlung aufgenommen. Die Messungen erfolgten mit einer Schrittweite von 0,02° 2 Theta (1 s pro Schritt) in einem Winkelbereich zwischen 5 und 85° 2 Theta.The tests carried out in this work laser diffraction experiments were carried out at the Mastersizer ^® 2000 from Malvern Instruments GmbH. The instrument-specific attachment Scirocco ^® 2000 was used for the analyzes with the aid of compressed air as the dispersing medium. Vibrating troughs were used to supply the sample to the measuring cell. In this connection, the dispersion pressure for mechanical stability tests was gradually increased in 10 ⁵ Pa steps from 10 ⁵ Pa to 4 × 10 ⁵ Pa. After an analysis time of one minute each, the evaluations were based on the MIE theory [189, 190]. As a control an also produced via sol-gel chemistry xerogel (Köstrosolid ^® 0403) was the chemical Bad Köstritz GmbH used. The measurements of the particle distributions in the aqueous subsoil were carried out by means of the article Hydra ^® 2000S. In this regard, the samples were suspended in ultrapure water, treated under ultrasound for two minutes and then pumped to the appropriate measuring cell. The evaluation of the results was carried out under the principles of FRAUNHOFER diffraction. Calculations of the span value (relative) of the particle size distributions were made according to the equation

carried out. Studies on the surface morphology of the xerogels made using the device Nova ^® 3000e (nitrogen sorption) and Poremaster ^® 60 GT (mercury) from Quantachrome GmbH & Co. KG. Prior to the respective analyzes, the selected xerogels were dried one and a half (mercury porosimetry) or three hours (nitrogen sorption) at 573 K under high vacuum. The measurement and evaluation of the nitrogen sorption data were carried out using the NovaWin ^® software (version 1.12). Mechanical instability of the particles in the mercury intrusion experiments could not be observed. The determination of the bulk density was carried out on the basis of the weight of a vessel with a standardized volume. For this purpose, the sample was filled into the container as free of air as possible and balanced. From the mass difference between filled and empty vessel, the mass to volume ratio, the bulk density, is accessible. Stated values in this work are the arithmetic mean of at least three analyzes. An analysis of the silica content was carried out after complete drying of the samples at 1223 K by adding hydrofluoric acid (40% (m: m)) at an analogue temperature. From the difference between the two masses, based on the initial weight, the content of silicon dioxide results. Moisture content studies of preserved xerogels were carried out on the MA 35M device from Sartorius AG. In this context, the samples were weighed at 298 K and dried at 433 K to constant mass. With the help of a mass balance, the moisture is accessible. To measure the pH of the xerogels, 10% (m: m) suspensions of the resulting powders were prepared in ultrapure water. These were then measured with stirring at 298 K with an electrode EGC-151 from Sensortechnik Meinsberg GmbH. The carbon content of the modified xerogels was determined on the Multi EA 2000-CS device from Analytik Jena AG. In this context, the sample was applied to a boat and burned in an oxygen stream at 1473 K. Applied crushing techniques on the obtained powders were the mortars of the samples using an agate mortar and the grinding by means of a laboratory mill A 11 B IKA Laboratory GmbH. The operating time was 5 × 30 s. X-ray diffractograms were recorded using a device of the type D4 "Endeavor" Bruker AXS GmbH with an X-ray tube for CuK _α radiation. The measurements were taken with a step size of 0.02 ° 2 theta (1 s per step) in an angle range between 5 and 85 ° 2 theta.

3.4.3 Electron micrographs

Characteristics of the method of laser diffraction are the limitations in the course of their physical limits and the evaluation algorithms used. In this context, some datasets provided information about particles with diameters of> 10 ^-4 m. Often such results are a consequence of fragmentation phenomena during a measurement. Solution algorithms of the software then try to convert them into an evaluable, spherical form. However, as a result of the determined droplet size distributions, such particle diameters can be considered extremely unlikely. Even coagulation of various drops should not generate such particle dimensions. Another challenge is the low resolution of the static laser diffraction at particle radii in the range of <2.5 · 10 ^-7 m. Mathematical evaluations of the received data packets accordingly take into account only a fraction of the particles in the nanoscale size range. By contrast, scanning electron micrographs show the presence of a significant proportion of particles in the range <5 × 10 ^-7 m. Furthermore, they refute particle diameters of> 3 · 10 ^-5 m. In this context, the author would like to thank Dr. Ing. Melzer (TU Freiberg), Ms. Gerlinde Habler (TU WIEN), Ms. Andrea Prager (IOM Leipzig) as well as Wacker Chemie AG (Burghausen) for the electron microscopic measurements. In the following chapters, the sources of the respective images are identified by their analysis location.

3.5 Application test

As an application test for redispersibility studies, the incorporation into various polymer matrices (PMMA, polyamide 6) and reference coating systems was selected. In an upstream step, deagglomeration in deionized water and 2-propanol was experimentally considered. In order to avoid influences of components used for the stabilization as well as impurities, xerogels with a high content of silicon dioxide were used for the investigations for the redispersion. All investigated xerogels were generated from educts with nitrogen loading to reduce the effects of defect-related phenomena. For dispersing in water, the disperser T 25 from IKA Labortechnik GmbH was used. In this case, the dispersing medium (100 g) was introduced and the xerogel (2 g) was mixed. The mixture was then stirred for 0.5 h at 100% power. Optional subsequent ultrasound treatments were performed with the Sonifier 450 from Branson Ultrasonics Co. For this purpose, the sample was treated with stirring four times 15 min under 80% power in the 80% cycle of action. The incorporation of the powders produced into different polymer matrices was carried out by Grafe Color Batch GmbH. Using a twin-screw extruder from Leistritz AG, the displayed xerogels with a target content of 10% (m: m) were incorporated into the various matrices and then transferred into injection molds using an Allrounder 221 K from Arburg GmbH. The incorporation of the particles obtained in paint systems was carried out by Worlée Chemie GmbH. For this purpose reference paint formulations were used and their properties determined before and after complete or partial substitution of currently used fillers.

4 results and discussion

The spray-drying process, with its wide range of influence, has a sustained scientific interest in the synthesis of particles. In this context, the knowledge of technologically important physico-chemical properties of the starting materials (Section 4.1) is the basis for scientific considerations. Results of the spraying experiments and the discussion of a model suggestion of particle formation are discussed in Section 4.2. In the following section results of the redispersing experiments are listed in different matrices. With reference to the results of the previous sections, Section 4.4 illustrates the consequences of incorporating the obtained powders (xerogels) on the physicochemical properties of the products by means of practical examples.

4.1 Properties of colloidal silicas

One focus of this work is the question to what extent a targeted selection of the colloidal silicas (silica sols) to be sprayed generates novel powders whose essential morphological properties are inherited from the starting material. The boundary conditions of this process are the use of suspensions with high solids contents (m _s > 5% (m: m)) and the formation of hollow body forms. For this purpose, the characteristics in the concentration process of silica sols are to be determined. Section 4.1.1 included the determination of the diffusion coefficient as a direct influence on the Péclet number (Equation 2.21). In the second part of this work, the loading of the colloidal starting material should be carried out with a gas. For the model to be discussed in this context of a dissolved gas-induced secondary dripping, as a result of a hydrodynamic instability of the lamella formed at the nozzle [64, 81, 93] and the subsequent oscillation of the droplets [86], knowledge of the solubilities in this work selected gases in the silica sols needed. The discussion of solubility results can be found in sections 4.1.2 and 4.1.3.

4.1.1 Diffusion rate of colloidal silica particles

The values for the diffusion coefficients of colloidal silica solid particles in the selected dispersions are in the range of 1 to 4.5 x 10 ^-11 m ² s ^-1 . These results are in good agreement with the literature values of TAN et al. [24]. (Page 46) illustrates the differential behavior of differently stabilized Krosrosols ^® with an identical particle diameter (1.5 · 10 ^-8 m). In highly dilute dispersions, the BROWN motion dominates the rate of diffusion due to non-directional collisions of the solvent molecules with the sol particles. The shift to higher values of the diffusion coefficient in the alkaline stabilized variant (D = 2.9 × 10 ^-11 m ² s ^-1 at 1% (m: m); + 60%) in such concentration ranges is a consequence of the pH value. conditional higher surface charge (Coulomb repulsion) of the silica particles. Higher solids contents (> 5% (m: m)) cause an additional acceleration of the colloidal particles by increased electrostatic interactions (anionic surface charge). The surrounding hydrate shell of the sol particle is thereby deformed or sheared off. At very high concentrations, the velocity of the particles is further increased, but their mean free path is severely limited due to the small particle-to-particle distance. The diffusion coefficients of the silica particles of differently stabilized silica sols are virtually identical over a wide concentration range ( ; No. I). Both variants are characterized by a rheological transition, a viscoelastic region, in which the colloidal sol particles form smaller agglomerate forms (<5 x 10 ^-9 m). The diffusion coefficient of the colloids decreases rapidly (> -50%, D <1.6 · 10 ^-11 m ² s ^-1 ). This point is characterized by a rapid increase in viscosity and represents the beginning of gelation. Significant differences, however, can be observed in the concentration and nature of this transition from the viscous to the elastic embossed part of a silica sol. In the absence of stabilization, this transition is shifted to lower solids levels (~25% (m: m)), but remains at an intermediate level ( ; No. II) at hydrodynamic diameters of r _H = 1.6 × 10 ^-8 m. A continuing investigation of the considered system was not possible due to the rapidly increasing viscosity at such high solids contents of unstabilized silica sols (> 40% (m: m)). For an interpretation of the results, for spherical particles ( ; right side) the Stokes-Einstein relation (Equation 4.1) can be used to a good approximation:

It establishes the relationship between the diffusion coefficient D and the hydrodynamic radius r _H of sol particles. Unstabilized silica sols pass through in their viscoelastic region Intermediate to form smaller, local agglomerates. These forms inhibit gel formation due to their reduced mobility (-50%) in the further concentration process (m _s = 25-50% (m: m)). Only at solids contents> 50% (m: m) can a final formation of a gel be visually observed. In contrast, alkaline silica sols show a decrease in the diffusion coefficient to near 0 m ² s ^-1 in their (25 to 48% (m: m)) viscoelastic range shifted to higher solids ^contents ( ; No. III). This leads to an abrupt formation of larger agglomerates (r _H > 1 · 10 ^-7 m at 52% (m: m)) which by the previously stabilizing effect, alkali ions (100 times higher diffusion coefficient) [78] on electrostatic Base is promoted. This gel formation takes place throughout the bulk. On the basis of these findings, denser particles are expected to be present in alkaline-stabilized silica sols as a consequence of the higher solids content at the gel point than in the case of the analogously unstabilized variants. Deformations, on the other hand, should have a lower share in powders of unstabilized silica sols due to the slower gelling speed and the associated higher selectivity. The transport of substances in a drop, as explained in Chapter 2, is an essential descriptive feature for the formation of faults and defects during the drying process. Investigations on the dependence of the diffusion in the water-relevant temperature range from 278 to 363 K in 10% dispersions (m: m) showed only a slight increase in the diffusion coefficient by a factor of 2.5 to D <5.9 × 10 ^-11 m ² s ^-1 (363K). Since a kinetic inhibition of gel formation is observed (Table 7.9), supplementary factors such as, for example, an increasing solid-liquid phase interface are not considered separately in this work. Furthermore, in a wide range of pH values (2 to 11), a constant diffusion coefficient as a function of the mass transport is determined independently of the selected particle diameter [191]. Accordingly, the pH shift associated with a concentration of alkali ions in alkaline silica sols can not have any direct influence on the particle formation in the drying process.

4.1.2 Solubility of nitrogen

For a validation of the newly constructed apparatus for the volumetric determinations, a system extensively described in the literature was chosen. Based on the binary mixture nitrogen-water a comparability of the obtained measured data with literature values is guaranteed. HIEBE's [162] published data on the solubility of nitrogen in water show a direct dependence on system pressure and an indirect relation to temperature. The amount of dissolved gas decreases steadily with increasing temperature. Assuming ideal behavior, the volume of gases V _{G is} proportional to the ratio of pressures and temperatures:

und Drucksprung

nach der Düse bewirkt eine signifikante Volumenänderung des gelösten Gases. In sind die Messwerte für die Löslichkeit von Stickstoff (L_N2) für das binäre Gemisch Stickstoff-Wasser sowie das ternäre System kolloidale Kieselsäure-Stickstoff-Wasser gegen den Gasdruck graphisch aufgetragen. Im gerätetechnischen Bereich der Versuchsanlage ist eine gute Übereinstimmung der Messwerte mit den publizierten Werten [162] erkennbar. Der relative Fehler im Bereich von 2 bis 11·10⁶ Pa liegt bei < 5%. Folgerichtig kann von einer erfolgreichen Validierung der Ergebnisse ausgegangen werden. Für Kieselsäure-Dispersionen werden in weiten Druckbereichen ähnliche Werte für die Löslichkeit von Stickstoff gefunden, wie im binären Vergleichssystem. In höheren Druckbereichen (P_N2 > 3·10⁶ Pa) zeigen kolloidale Kieselsäuren, analog SUNs [167] Erfahrungen mit Natriumchlorid-Lösungen, eine Erniedrigung der Löslichkeit von Stickstoff. Tendenziell werden im betrachten ternären Gemisch ca. 15% geringere Löslichkeiten als im binären Vergleichssystem Wasser-Stickstoff gemessen. Ursachen hierfür sind der geringere Lösungsmittelanteil pro Gramm fluiden Mediums (9:10 (m:m)) sowie das teilweise gebundene Lösungsmittel in der Hydrathülle der Ionen bzw. Kolloide. Die Diskrepanz der Daten ist bei Gasdrücken P_N2 < 2·10⁶ Pa geringer ausgeprägt (< –8%; ~2·10^–6 NL g^–1). Die Begründung liegt in den aktiven Oberflächen der kolloidalen Teilchen. Hier dominieren Grenzflächeneffekte. Stickstoffmoleküle werden hierbei in den Ebenen über und unter den Silanolgruppen stabilisiert [192]. Lösungsmittelbezogene Interpretationen der Löslichkeitswerte von Stickstoff in Köstrosol^® 1510 im Vergleich zum bekannten binären System Stickstoff-Wasser offenbaren in höheren Druckbereichen (P_N2 > 10⁷ Pa) vergleichbare Steigungen. Der relative Fehler dieser Methode liegt im Bereich von maximal 7,5%. Dementsprechend kann das Gemisch Stickstoff-Wasser in guter Näherung als Modell für die Löslichkeit von Stickstoff in Kieselsolen betrachtet werden. Aus der Synthese von Brandschutzgelen, einer Mischung von Kieselsolen mit Kaliumhydroxid, ist bekannt, dass die Freigabe von Gasen an der Oberfläche von nanoskalaren Kieselsäurepartikeln kinetisch gehemmt abläuft [193]. Ursachen für diese verlangsamte Entlösung sind zusätzliche Phasengrenzflächen Feststoff-Gas bzw. Feststoff-Flüssig, Wechselwirkungen mit den oberflächenaktiven Silanolgruppen sowie die reduzierte Diffusionsgeschwindigkeit in der Hydrathülle der dispergierten Teilchen.In this context, a temperature of 298 K was chosen as the starting point of the considerations to dissolve a sufficient amount of gas in the fluid. Another advantage of a low inlet temperature of the spray (T _nozzle ), according to LUKASIEWICZ [121], is a reduction in the probability of particle defects. The temperature carried out by pressure swing technology

and pressure jump

after the nozzle causes a significant volume change of the dissolved gas. In For example, the solubility values of nitrogen (L _N2 ) for the binary mixture nitrogen-water and the ternary system colloidal silica-nitrogen-water plotted against the gas pressure. In the instrumentation area of the test facility, a good agreement of the measured values with the published values [162] can be seen. The relative error in the range of 2 to 11 · 10 ⁶ Pa is <5%. Consequently, a successful validation of the results can be assumed. For silica dispersions similar values for the solubility of nitrogen are found in wide pressure ranges, as in the binary comparison system. In higher pressure ranges (P _N2 > 3 · 10 ⁶ Pa), colloidal silicas, analogously to Sun's [167] experience with sodium chloride solutions, show a reduction in the solubility of nitrogen. In the ternary mixture the solubility is about 15% lower than in the binary comparison system water-nitrogen measured. The reasons for this are the lower solvent content per gram of fluid medium (9:10 (m: m)) and the partially bound solvent in the hydrate shell of the ions or colloids. The discrepancy of the data is less pronounced for gas pressures P _N2 <2 · 10 ⁶ Pa (<-8%, ~ 2 · 10 ^-6 NL g ^-1 ). The reason lies in the active surfaces of the colloidal particles. Here interface effects dominate. Nitrogen molecules are stabilized in the planes above and below the silanol groups [192]. Solvent-related interpretations of the solubility values of nitrogen in Köstrosol ^® 1510 compared to the known binary system nitrogen-water reveal comparable slopes in higher pressure ranges (P _N2 > 10 ⁷ Pa). The relative error of this method is in the range of a maximum of 7.5%. Accordingly, the nitrogen-water mixture can be approximated as a model of the solubility of nitrogen in silica sols. From the synthesis of fire protection gels, a mixture of silica sols with potassium hydroxide, it is known that the release of gases on the surface of nanoscalar silica particles is kinetically inhibited [193]. The reasons for this slowed-down separation are additional solid-gas or solid-liquid interfaces, interactions with the surface-active silanol groups and the reduced diffusion rate in the hydrated shell of the dispersed particles.

An analysis of the hydrodynamic diameter of Köstrosol ^® 1510 before and after contact with nitrogen at varying pressure (0.1 ≤ P ≤ _N2 10 ⁷ Pa)) showed no structural changes in the sol particles. The data for this value ranged from (43.6 ± 0.81) x 10 ^-9 m (Table 7.6, Appendix). Consequently, it can be assumed that the colloidal system is inert to this gas. Promoted formation of agglomerates in the dispersions is not observable. Long-term tests (> 180 d) confirm these statements.

4.1.3 Solubility of carbon dioxide

The background to the investigation of the solubility of carbon dioxide in silica sols was its use as an (in situ) agglomerating agent. The metering of this gas should lead to a targeted punctual agglomeration as a result of a local pH shift in the drop through the equilibrium CO ₂ + H ₂ O ⇄HCO ₃ ^- + H ⁺ (pK _s = 6.1) [78] to lead. Subsequently, these local agglomerates should act as nuclei for targeted particle formation in the drying process. An analysis of the experiments on loading Köstrosol ^® 1510 with carbon dioxide in the selected amount of dissolved gas (L _CO2 > 10 ^-3 NL g ^-1 ) was not effective when considering a spray application. Regardless of the stabilization of the colloidal media, gel formation did not produce a reproducible result of atomization. The cause is the formation of bicarbonate ions in the silica sol by redissolution of carbon dioxide. In this context, not only a pH shift in unfavorable stability for silica sols ( ), but also an additional ion enrichment. These formed bicarbonate ions interact with the silanol groups on the particle surface and the Helmholtz (electrochemical double layer) layer around the colloidal particle. Promoted destabilization and agglomeration of the system are the consequences of such enrichment at the interface.

4.2 Spray drying of silica sols

In the following sections of this thesis the results of the spray drying experiments are presented and discussed. In a first step, the reasons for the selection of the heat radiation source and the determined droplet size distributions, as a discussion approach for the secondary drip decay, are described. In the following sections the differences of a spray application without (Section 4.2.1) and with the application of nitrogen (Section 4.2.2) are presented. Based on this, physicochemical parameters are discussed as influencing factors in subitems. The results of the surface morphology of the particles formed are described in Section 4.2.3. Section 4.2.4 contains proof of the amorphous character. Based on the results of the preceding points, a model of particle formation is proposed under point 4.2.5. Spray drying is a widely used method of generating particles. A technological challenge in this regard is the combination of high throughputs and low moisture contents. In order to fulfill these boundary conditions, the use of a heat radiation source to optimize the drying was determined in this work. This should serve the support of the warm gas flow as well as a reduction of the cooling of the drying gas. In a preliminary experiment, the question was asked whether microwave radiation can fulfill this requirement [194]. The advantage of this form of energy, a drying of the drop from the inside to the outside while reducing the temperature gradient in the bulk, could not be realized. An analysis of the results revealed that the size of the drops does not provide enough volume to provide enough energy for evaporation couple. Due to flow conditions, the challenge of a targeted placement of the shaft maximum while minimizing the reflection could not be realized. According to this result, the infrared region was used as a heat source for supplying the necessary energy. In this case, the heating was carried out from the drop exterior to the core.

Characteristic of the pressure swing technology, as a sub-type of spray drying is the transfer of fluid media under high pressure (P _nozzle > 10 ⁷ Pa) in the drying process (P _VK ≤ 10 ² Pa). The resulting pressure jump is usually at least five orders of magnitude. In this context, the admixture of dissolved gas quantities L> 10 ^-3 NL g ^{-1 is} of great scientific interest. As a result of the hydrodynamic instability of the lamella formed at the nozzle outlet opening [64, 81, 93], the droplets oscillate until they reach their most energetically favorable shape [86]. These effects could be amplified by releasing gas. As a result, the likelihood of critical boundary conditions would increase, ending in a further, secondary drop in precipitation [82]. The droplet size distribution would shift to low values. An analysis of the associated experimental results ( and Appendix Tables 7.10 to 7.12) refutes this postulated mechanism of dissolved gas-induced secondary dropping. It can be determined in the context of the measurement inaccuracy no change in the droplet size distribution when exposed to nitrogen. The reason for this lies in the relatively short residence time of the droplets formed in the spray chamber (τ _Tr <0.15 s) in comparison to the rate of removal of the nitrogen (> 1 s) from the fluid droplet. Gas-liquid mixtures are characterized by a multiplicity of phase distribution states ( ; Page 24). Examination of the droplet size distributions under gas injection showed that a spray pressure of at least 1.5 × 10 ⁷ Pa must be introduced in order to maintain a constant medium flow without phase change. At lower spray pressures, due to segregation phenomena as a result of supersaturation of the solvent, the fluid flows at the nozzle are disrupted by gas phases.

The potential energy introduced by the system determines the size distribution of the droplet collective formed [195]. shows the presence of a local minimum of the droplet size distribution between 2.2 and 2.4 x 10 ⁷ Pa at the experimental conditions listed. The reason for the increase in the d ₉₀ (V) value at higher nozzle exit pressures (P _nozzle > 2.4 x 10 ⁷ Pa) is the coagulation of droplets as a result of the increased throughput. The surrounding drying gas slows down the drops formed, which significantly increases the likelihood of these events. Within the following 0.2 m in the spray, the measured droplet diameters increase by a factor of 2.5 (d ₉₀ (V)). Reasons for this are the evaporation of smaller drops and, in the event of a lack of fine distribution caused by the drying gas flow, deceleration of the drops in the sequence coagulation occurs.

The test arrangement makes it possible to obtain droplet diameters of d ₉₀ (V) <2.5 × 10 ^-5 m. Accordingly, only one-eighth of the volume of HADINOTO's [24] calculated drops (5 · 10 ^-5 m, page 7) must be evaporated with the additional use of thermal radiation. Accordingly, a time-reduced evaporation of the fluid drop can be assumed. When the results obtained are the droplet diameter (d ₉₀ (V) <2.5 × 10 ^-5 m), the diffusion coefficient (D = 4 × 10 ^-11 m ² s ^-1 ) and the time required for complete evaporation of the formed Drop (τ _Tr ~ 0.1 s) is necessary (model according to HIND [35], see appendix), combined using Equation 2.21, Péclet numbers of Pe> 40 are present. Consequently, the formation of particles with hollow body geometry can be assumed (Section 2.1.2, page 10).

4.2.1 Spray drying without loading with nitrogen

As a result of the very high Péclet numbers determined in the preceding section, spray-dried, unstabilized silica sols characterize as a hollow body of predominantly spherical shape. In addition, in the structure of the shell formed and on the surface of the particles, the monomeric, nanoscale colloidal basic building blocks are easily recognizable ( ). A formation of pores or channels can not be observed by means of SEM images. The surfaces measured by nitrogen sorption (BET, N ₂ , 77 K) correspond approximately to the surfaces of the colloidal starting materials determined by the SEARS method [188] (Table 4.1). Consequently, by a specific choice of the starting material, the BET (N ₂ , 77 K) surface of the powders obtained can be defined. A successful conversion of the aqueous silica sols to xerogels is characterized by the low moisture content of the spray-dried powders produced. These have moisture contents of <2.5% (m: m). The diameter of the particles obtained in the generated powders is in the range of 0.2 to 20 × 10 ^-6 m. shows the comparison of the particle diameter between the data of the laser diffraction and the electron micrographs. Variances are especially in the range d _p <5 · 10 ^-7 m. While in Scanning electron microscopic analyzes of such particles are detectable, prevent the physical and device technical limitations of the laser diffraction analytically usable statements. Moreover, contrary to the observations of WALTON [47], the resulting xerogels are characterized by good pouring and flow behavior compared to reference materials (Köstrosolid ^® 0403). Köstrosolid ^® 0403 is also via sol-gel produced, commercially available product of amorphous silica. The cause of the improved casting and flow behavior lies in the surface morphology, which is discussed in detail in Section 4.2.3, as well as in the sphericity of the particles obtained.

Characteristic of the powders produced by pressure swing technology are the unusually high bulk densities (ρ _s > 4.10 ² g L ^-1 ) for amorphous silicas. From HADINOTO's [118] considerations on the ratio of the shell width S to the particle radius R (equation 4.3) results in a proportion of the shell width at the hollow body radius of a maximum of 10%.

beobachtet.Both mentioned factors ( S / R ; ρ _s ) increase with increasing atomizing pressure (Table 7.14 and 7.17, Appendix) and solids content of the educts. In the course of a print review, this can be interpreted as a consequence of a decrease in the average droplet diameter and the resulting faster evaporation (see Section 2.1.1, page 5) [86]. Analogous behavior also applies to the density ratio of the effective to the pure substance density

observed.

Values independent of spray pressure are given by the results for the aerodynamic diameter (d _A , Equation 2.16), which is approximately 4.5 · 10 ^-6 m in the target range of this work. Higher values of the physicochemical parameters tend to be found when using alkaline stabilized silica sols as starting materials. Table 4.1 gives an overview. In addition, this listing (Table 4.1) can be seen that inheritance of the pH of the spray-dried powder also results from a specific choice of the starting material. In the course of the concentration of the alkali ions in the drying process, this is slightly increased in the analyzed suspension (10% (m: m)) (~ 0.2 pH units). Laser diffraction experiments on the obtained xerogels with varying dispersing pressure give an outlook on the mechanical stability of the hollow bodies. Increasing the dispersion pressure four times to 4 × 10 ⁵ Pa causes a significant shift of the statistical distribution values to smaller values (-30% for the d ₉₀ (V) value). For comparative measurements the Köstrosolid ^® 0403 already presented in this work was used. In terms of production, after gelation from a silica sol in this product, the resulting hydrogel is adjusted to its particle size distribution by means of drying and milling techniques. Accordingly, this industrial product is an irregular porous solid which, under the low mechanical stress (dispersion pressure P _Disp max 4 · 10 ⁵ Pa), is subject to no measurable decomposition in the experiment. shows a comparison of the two microscale products.

Powder of unstabilized starting materials disintegrate uniformly with increasing mechanical load - constant chip (Köstrosol ^® 1510 1.8 at _nozzle P = 2.3 x 10 ⁷ Pa). These sols lack the alkali ions necessary to accelerate the agglomeration. Alkali-stabilized variants, on the other hand, exhibit a steady increase in the span value as a consequence of the lower decreasing d ₉₀ (V) value with increasing dispersion pressure. For example. increases the tension to alkaline gel Köstrosol ^® 1510 1.83 _(Disp P = 10 ⁵ Pa) to 2.21 (4 x 10 ⁵ Pa). The cause of this effect is the concentration of material in the drop. Drops of the larger diameter spray contain abundant colloidal material and have more time to reach the critical concentration by evaporating fluid at the surface, which is smaller in percentage compared to smaller drop diameters. As a result, there is a denser packing of the shell, resulting in a gain in stability derived. By contrast, smaller droplet radii contain significantly less material and abruptly reach their critical concentration. This rapid sequence of events leads to a rapid increase in viscosity in combination with an inhibited mass transport in drops. The result is an unselective arrangement of the monomeric particles. Labile compounds of colloids and agglomerates with each other - Defects - are the result. Accordingly, larger diameter alkaline particles have higher resistance to mechanical stress than their smaller counterparts.

4.2.2 Spray drying under nitrogen loading

Nitrogen loading of the starting colloidal materials allows access to smaller particle diameters for analog (unstabilized starting materials, Span = 1.7) or narrower (alkaline stabilized starting materials; Span = 1.7, -6.6%) particle size distribution widths. Without exception, both stabilization variants show a shift towards lower distribution ratios (Table 4.2 or Table 7.18 in the appendix). In a solution of nitrogen in the sols, the tendency observed from Section 4.2.1 to confirm that xerogels made from unstabilized silica sols have smaller particle size distribution values than alkaline gel types prepared under analogous conditions. In particular, the d ₉₀ (V) data shift significantly to the lower micrometer range (d ₉₀ (V) <10 ^-5 m and Table 7.18, respectively, compared with sols applied without dissolved gas). shows the dependence of the particle size distribution on the nozzle outlet pressure at a dispersion pressure of 10 ⁵ Pa. The higher the pressure used at the nozzle outlet opening, the more significantly shifts the distribution in the Submikrometerbereich. This effect can be enhanced by adding nitrogen. Analogous behavior between alkaline powders and unstabilized silica sols with respect to their geometrical parameters is determined by mechanical stress on the basis of a dispersion pressure increase (Table 7.18, Appendix). Here, too, the present alkali ions promote the formation of the first rigid structures of the particles through an optimized crosslinking of monomeric building blocks, as a result of which the calculated span increases. For powders generated from Alkaline Köstrosol ^® 1510, the span or d ₅₀ (V) value (P _Disp = 4 · 10 ⁵ ) for nitrogen-loaded starting materials drops by 10% (Span = 2.0) and 13% (d ₅₀ (V) = 8.6 × 10 ^-6 m) compared to the unloaded variant. BET (N ₂ ; 77 K) surfaces, measured by nitrogen sorption, are nearly equivalent in both variants to surfaces of the colloidal starting products determined by SEARS method [188].

• ii. Minimierung von Partikeldefekten: Bei Gemischen von Gasen in Flüssigkeiten ist die Schallgeschwindigkeit herabgesetzt. Dementsprechend sollten Druckausgleichsprozesse signifikant schneller als in den reinen Medien ablaufen. Hierfür gibt es zwei prinzipielle Möglichkeiten im Partikelbildungsprozess. Wie in Abschnitt 2.3 (Seite 22) vorgestellt, resultiert aus der Gasdynamik beim Entspannen von Gasen eine Abkühlung des Systems. Bei einer sich während des ersten Trocknungsschrittes verfestigenden Tropfenoberfläche könnte dieser Effekt einer Überhitzung des Tropfeninneren (Abschnitt 2.1.2; Seite 10) entgegenwirken. Darüber hinaus sind Gase kompressibel, was den, mit der Überhitzung assoziierenden Druckanstieg im Tropfeninneren, reduzieren wür de. Resultierend daraus sollte der Partikeldefekt eines Aufblähens („Ballooning”) [47, 121] bei der Synthese unwahrscheinlicher werden. Im zweiten Teil der Partikelbildung könnte das sich aus dem Fluid entlösende Gas der Formung eines partiellen inneren Vakuums als Gegendruck entgegenwirken. Fehlerbehaftete Stofftransportvorgänge an der noch nicht verfestigten Hülle könnten hierdurch reduziert werden.
• iii. Optimierung der Trocknung: Der gebildeten Tropfen wird zusätzlich zum angelegten Trocknungsgasstrom vom sich entlösenden Gas getrocknet. Zusätzliche Phasengrenzflächen durch Gasbläschen optimieren den Übergang der Flüssigkeitsmoleküle in die Gasphase.

In this part of the work, the raw material silica sol was charged with a gas. In addition to the discussion of a secondary drip decay (mechanism i, see Section 4.2), the theoretical background of this loading of the silica sol with gas under high pressure and subsequent transfer to low pressures was the theoretical derivation of the following two other possible effects:

• ii. Minimization of Particle Defects: For mixtures of gases in liquids, the speed of sound is reduced. Accordingly, pressure equalization processes should proceed significantly faster than in the pure media. There are two principal possibilities in the particle formation process for this. As described in Section 2.3 (page 22), gas dynamics result in a cooling of the system when gases are released. In the case of a drop surface solidifying during the first drying step, this effect could counteract overheating of the drop inside (Section 2.1.2, page 10). In addition, gases are compressible, which would reduce the pressure increase inside the drop associated with the overheating. As a result, the particle defect of "ballooning" [47, 121] in synthesis should become less likely. In the second part of the particle formation, the gas releasing from the fluid could counteract the formation of a partial internal vacuum as backpressure. Faulty mass transfer operations on the not yet solidified shell could be reduced thereby.
• iii. Optimization of the drying: The formed drop is dried in addition to the applied drying gas stream of the releasing gas. Additional phase interfaces through gas bubbles optimize the transition of the liquid molecules into the gas phase.

With the help of the significantly higher proportion of spherical, without defects present, hollow bodies in the application of nitrogen-loaded silica sols can be illustrated. The proportion of particles with defects is ~ 50% for gels of unbeaten silica sols and ~ 21% in the gels under exposure to nitrogen. The main focus is on the contribution of ring-forming [126-129] and blowhole defects [63, 109, 121, 132] ( ). In this context, the postulated mechanisms II and III - the reduction of particle defects by a modified drying process and an improved balance of pressure fluctuations - can be regarded as explanations of the listed observations. Contradictory behavior is observed at the bulk density ρ _s . Contrary to the possibility of optimizing the packing density by means of a smaller particle size distribution, the bulk density values of the nitrogen-sprayed, unstabilized silica sols are below their counterparts without being exposed to gas. For both stabilization variants, the bulk density increases with increasing nozzle outlet pressure ( ). However, this increase is much less pronounced with nitrogen-loaded, unstabilized silica sol. While in unstressed sprayed Kostrosol ^® 1510, the bulk density of the resulting gel in the range of P _nozzle = 5 to 23.5 x 10 ⁶ Pa to 149 g L ^-1 (+ 35%) increases, changes the bulk density for the gels from the applied variant only by 44 g L ^-1 (+ 10%). One reason for this opposite effect lies in the particle geometry itself. As a result of a lowering of the effective density ρ _{eff of} the powder (Table 4.2), the "Hollowness" - the proportion of the internal hollow volume in the particle - of individual particles must increase. Consequently, by loading with nitrogen, the proportion of the shell width at the particle radius decreases in the direct comparison of the powders produced with and without nitrogen. This effect significantly reduces the theoretical increase in bulk density caused by the much smaller particle size distribution. A reduction of the aerodynamic diameter as a combination of these facts (Equation 2.16) is also observable (Table 4.2).

als beschreibender Faktor der „Hollowness”, signifikant von 0,26 auf 0,22 erniedrigt. Diese Varianz in den physikalischchemischen Eigenschaften, der aus unstabilisierten Kieselsolen gewonnenen Xerogele kann nur mit einem stickstoffmanipulierten Partikelbildungs- und Trocknungsmechanismus erklärt werden. Derartige Kieselsole charakterisieren sich durch eine Hemmung der Gelbildung (Geliergeschwindigkeit) im viskoelastischen Bereich (Abschnitt 4.1.1). Des Weiteren ist dieser Bereich deutlich unschärfer gezeichnet (Feststoffgehalt von 25 bis 50% (m:m)) als ihre alkalisch stabilisierten Analoga. Als Folge sind hier die Einflüsse des sich entlösenden Stickstoffes wesentlich prägnanter. Vorstellbar ist in diesem Zusammenhang ein Modell mit kleinen lokalen Erstarrungspunkten zu Beginn des viskoelastischen Bereiches. Diese liegen im weiteren Trocknungsprozess vorerst isoliert vor. Ab einem kritischen Punkt können diese verfestigten Stellen anschließend koagulieren oder voneinander abgegrenzt kleine, einzelne Partikel bilden. Dies bestätigt ebenfalls den postulierten Mechanismus iii (Seite 58) in Bezug auf die Steuerung von Defektwahrscheinlichkeiten durch einen veränderten Trocknungsprozess.Comparisons of the droplet diameter values obtained with the X-ray diffraction device Spraytec with the size distribution values obtained for the xerogels (d ₉₀ (V)) indicate a direct influence of the gas loading. In the case of unstabilized educts, the ratio of the particle diameter to the determined drop diameter (d ₉₀ (V)) at a nozzle outlet pressure of 2.3 · 10 ⁷ Pa decreases from 54% by application of nitrogen to below 39%. Furthermore, the value

as a descriptive factor of "Hollowness", decreased significantly from 0.26 to 0.22. This variance in the physicochemical properties of xerogels obtained from unstabilized silica sols can only be explained by a nitrogen manipulated particle formation and drying mechanism. Such silica sols are characterized by an inhibition of gelation (gelation rate) in the viscoelastic region (Section 4.1.1). Furthermore, this area is markedly blurred (solids content of 25 to 50% (m: m)) as their alkaline-stabilized analogues. As a result, the influences of the releasing nitrogen are much more prominent here. In this context, a model with small local solidification points at the beginning of the viscoelastic region is conceivable. These are initially isolated in the further drying process. From a critical point, these solidified sites can then coagulate or, separated from one another, form small, single particles. This also confirms the postulated mechanism iii (page 58) for controlling defect probabilities by changing the drying process.

erhalten (Tabelle 4.2). Auf Grund der kleineren Partikelgrößenverteilung durch die zusätzlichen Gaseffekte ist eine optimalere Packung der Festkörperteilchen alkalischer Gele möglich, wodurch die Schüttdichte ρ_s von 580 auf 593 g L^–1 (+2,2%) steigt. Tabelle 4.2 zeigt eine Übersicht der erhaltenen physikalisch-chemischen Daten von hergestellten Pulvern. Im Zusammenhang mit alkalischen Gelen kann dem beaufschlagten Gas nur eine sehr geringe Modifikation des Agglomerationsverhaltens im Tropfen zugrunde gelegt werden. Da dies im Widerspruch zu dem Verhalten von Pulvern aus unstabilisierten (vollentsalzten) Solen steht, muss bei derartigen Gelen der erhöhte Alkaligehalt der wesentliche Einflussfaktor auf die Bildung von Partikeln darstellen.In the case of the produced alkaline xerogels, when loaded with nitrogen, essential properties remain, such as geometric diameter d _g , proportion of the shell width at the particle radius in comparison to atomization without gas S / R and the ratio of effective to pure substance density

obtained (Table 4.2). Owing to the smaller particle size distribution due to the additional gas effects, a more optimal packing of the solid particles of alkaline gels is possible, whereby the bulk density ρ _{s increases} from 580 to 593 g L ^-1 (+ 2.2%). Table 4.2 shows an overview of the obtained physicochemical data of prepared powders. In connection with alkaline gels, the acted upon gas can only be based on a very slight modification of the agglomeration behavior in the droplet. Since this is in contradiction to the behavior of powders of unstabilized (demineralized) sols, in such gels the increased alkali content must be the main factor influencing the formation of particles.

The analogous comparisons of the droplet size distribution values obtained by the Spraytec with the values obtained for the alkaline powders (d ₉₀ (V)) illustrate this effect. In the case of alkaline-stabilized educts, the particle diameter is already fixed at about 62% of the initial droplet diameter (53% with nitrogen). As a result, the alkaline sols are converted into gels with shorter time to reach their critical concentration point. The reason for this is an increased gelling speed as a result of the increased electrostatic interactions with the alkali ions. When passing through this, compared to the unstabilized sols on the concentration related smaller viscoelastic range, only a few possibilities of a spatial and temporal modification of the particle structure can be realized.

The decomposition of alkaline-produced xerogels with increasing dispersion pressure in the laser diffraction experiment increases slightly compared to the unbounded version (+ 7%). One explanation for this reproducible variance is that the releasing nitrogen in the droplet impairs the crosslinking of monomeric repeat units during the formation of the particle shell. Additional phase interfaces in combination with an associated increased drying effect cause a time-delayed mass transfer in the drop (Mechanism iii). As a result, the particle distribution is shifted to lower values with the same macroscopic particle properties. Analogous to the observations on the flow and casting behavior of the obtained xerogels Section 4.2.1, the powders produced in this section have analogous properties. Cohesive effects are observed in scanning electron micrographs only at very small particle diameters (<5 × 10 ^-7 m) as a consequence of their surface chemistry. At the same time an inheritance of the pH of the starting material is detectable.

4.2.2.1 Influence factor solids content

In order to ensure the reproducibility of the results with the aim of reducing particle defects, only silica sols subjected to nitrogen (V × _{N 2} = 0.7 NL min ^-1 ) were used for the further considerations of this section. The drying profile (see section 3.3.2) was selected as operating mode 2 (T _E = 398 K, T _A = 393 K). The parameter solids content of the starting material has a significant influence on the particle shape and size distribution. MAAs et al. [131] as well as MASTER's [86] considerations on a direct dependence of educt concentration and particle radius can be confirmed by laser diffraction experiments on the obtained powders in this work ( ). As the solids content of the silica sol increases, the average particle diameter and the packing density of the powders produced increase. describes an increase of the d ₉₀ (V) value of the particle size distribution (P _Disp = 10 ⁵ Pa) from 8.5 × 10 ^-6 m (m _s = 0.65% (m: m)) to 11.4 × 10 10 ⁶ m (m _s = 11.89% (m: m), + 33.8%). Furthermore, according to SHAWs [125] explanations, the probability of defects of the resulting solids decreases with increasing solids content. SEM images of xerogels with different solids contents of the starting materials support these statements ( ).

LUKASIEWICZ [121] observed that with increasing solids content, the proportion of the hollow body decreases ("Hollowness") and that of the solid structures (solid body) increase. As a cause he characterized the shortening of the constant mass transfer rate of the droplet [63]. HADINOTO [118] used for this type of description the ratio of the effective (ρ _eff ) to the pure substance density (ρ _true ) as a representative factor. The basis of the pure substance density for amorphous silicon dioxide (2.2 · 10 ³ kg m ^-3 ) in this work was the literature value according to ILER [179]. For the pressure swing technology, however, the statement of an increase in the full body content with increasing starting material concentration applies only to xerogels of alkaline-stabilized educts. As a result of the droplet size distribution in combination with the concentration of the alkali ions, which demonstrably promote gel formation [179], the stated reduction of the hollow body fraction occurs. The ratio of the shell width to the particle radius S / R rising steadily. This is illustrated by the example of Kostrosol ^® 1510, in which the ratio S / R from 9.2% (m _s = 5.1% (m: m)) to 10.6% (m _s = 19.9% (m: m)). Bulk gelation can not be detected in the considered ranges of the solids content (up to m _s = 30.3% (m: m)).

in Abhängigkeit zum Feststoffgehalt bei den Ergebnissen in dieser Arbeit ist eine Folge der Tröpfchengrößenlimitierung. Diese beschreibt den maximal erreichbaren Durchmesser der herstellbaren Granulate. Aus unstabilisierten Kieselsolen generierte Partikel können nur bis zu jener vorgegebenen Größe, dem prozessbedingten Tropfendurchmesser, in Abhängigkeit zu ihrem Feststoffgehalt aufwachsen (Abbildung 4.12; Bereich I). Anschließend muss analog zu den alkalischen Varianten der Hohlkörperanteil („Hollowness”) abnehmen (Bereich II). Bei einem Feststoffgehalt der kolloidalen Ausgangsdispersion von 0,65% (m:m) liegt der d₉₀(V)-Wert der Verteilung bei 37% (v:v) des Tropfendurchmessers (d₉₀(V); P_Disp = 2,3·10⁷ Pa). Wird dieser Feststoffgehalt um das Achtzehnfache (11,98% (m:m)) bei analogen Sprühparametern erhöht, sind bereits 64% des maximalen Tropfendurchmessers (Tabelle 7.10 bis 7.12) erreicht. Im direkten Vergleich sind die Radien (d₉₀(V)) der hergestellten Partikel um 34% von 4,2·10^–6 m (m_s = 0,65% (m:m)) auf 5,7·10^–6 m (m_s = 11,89% (m:m) angewachsen. ist zu entnehmen, dass im Widerspruch zu HADINOTOs [118] Ergebnissen, keine signifikante Zunahme des Verhältnisses

bei geringen Erhöhungen des Feststoffgehaltes festgestellt werden kann (Tabelle 7.22). Die erhaltenen Daten bestätigen, als Folge des geringen Anstieges der Regressionsgeraden im betrachteten Feststoffgehaltsbereich, das aufgeführte Prinzip der Tröpfchengrößenlimitierung der vorausgehenden Absätze. HADINOTO [118] nutzte für seine Untersuchungen Ultraschallzerstäuber zur Generierung geringer Tropfendurchmesser (~4·10^–6 m), hohe Beladungen (μ > 500) und mehrstufige Trocknungstechnik. Eine Tröpfchengrößenbeeinflussung wurde, obgleich der Darstellung kleinerer Kollektive, für die verwendeten Feststoffgehalte (m_s < 2% (m:m)) nicht beschrieben. Lineare Abhängigkeiten vom Feststoffgehalt zeigen ebenfalls die Daten für den geometrischen und aerodynamischen Durchmesser ( ; rechtes Diagramm). Diese Ergebnisse bestätigen die allgemeinen Aussagen von MASTER [86] und MAA et al. [131] zum Einfluss der Konzentration auf die mittleren Partikeldurchmesser, stehen aber wiederum im Widerspruch zu HADINOTOs [118] Ergebnissen zu Sprühtrocknungsversuchen mit kolloidalen Kieselsäuren als Ausgangsmaterialien. Generell liegen die ermittelten Partikeldurchmesser mit d₅₀(V) < 5,7·10^–6 m im Zielbereich dieser Arbeit. Tabelle 4.3 kann entnommen werden, dass erwartungsgemäß der Anteil von Partikeln mit Durchmessern im Submikrometerbereich (d_P < 10^–6 m) mit steigendem Feststoffgehalt der Edukte abnimmt.Unstabilized starting materials (silica sols) lack this kind of "acceleration", so that they are subject to the interaction of fast drying and slow gelation. When spraying such sols it is confirmed that the tendency to generate particles is determined by the type of concentration and the position of the viscoelastic region. A special feature of these gel types is the constancy of the shell width at the particle radius ( S / R ~ 8%) between solids contents of 0.65% and 11.89% (m: m). Consequently, an increase in the solids content of the starting materials leads to an adaptation of the particle radii of the respective gels instead of a lowering of the "hollowness" known from the literature (Tables 7.22 and 7.23, Appendix) [121]. This characteristic peculiarity of pressure swing technology has not yet been described in this high concentration range without template use for spray drying. shows the previous literature developments of silica-based spray applications as a function of the solids content of the educt [24, 118]. The observed slight increase in the straight line for the ratio

depending on the solids content in the results in this work is a consequence of droplet size limitation. This describes the maximum achievable diameter of the producible granules. Particles generated from unstabilized silica sols can only grow up to the specified size, the process-related droplet diameter, depending on their solids content (Figure 4.12, Area I). Subsequently, similar to the alkaline variants, the hollow body portion ("Hollowness") must decrease (range II). At a solids content of the starting colloidal dispersion of 0.65% (m: m), the d ₉₀ (V) value of the distribution is 37% (v: v) of the droplet diameter (d ₉₀ (V); P _Disp = 2.3 · 10 ⁷ Pa). Will this Increased solids content by 18 times (11.98% (m: m)) with analog spray parameters, already 64% of the maximum drop diameter (Table 7.10 to 7.12) are reached. In direct comparison, the radii (d ₉₀ (V)) of the produced particles are 34% of 4.2 × 10 ^-6 m (m _s = 0.65% (m: m)) to 5.7 × 10 ^-6 m (m _s = 11.89% (m: m) grown. it can be seen that, in contradiction to HADINOTO's [118] results, no significant increase in the ratio

can be detected at low increases in the solids content (Table 7.22). The data obtained confirm, as a result of the small increase in the regression line in the considered solids content range, the principle of droplet size limitation of the preceding paragraphs. HADINOTO [118] used ultrasonic atomizers for the generation of small droplet diameters (~ 4 · 10 ^-6 m), high loadings (μ> 500) and multi-stage drying technology. Droplet size control, although representing smaller collectives, has not been reported for the solids used (m _s <2% (m: m)). Linear dependencies of the solids content also show the data for the geometric and aerodynamic diameter ( ; right diagram). These results confirm the general statements of MASTER [86] and MAA et al. [131] on the influence of the concentration on the average particle diameter, in turn, contradict HADINOTO's [118] results on spray-drying experiments with colloidal silicas as starting materials. In general, the determined particle diameters d ₅₀ (V) <5.7 · 10 ^-6 m are within the target range of this work. Table 4.3 shows that, as expected, the proportion of particles with diameters in the submicrometer range (d _P <10 ^-6 m) decreases with increasing solids content of the educts.

für alkalische Xerogele nimmt in diesem Zusammenhang ebenfalls um 14% von 0,25 auf 0,28 zu (Tabelle 7.24; Anhang). Eine, die Gelbildung fördernde, Wirkung von Alkali-Ionen als Ursache dieser kolloidalen Zusammenlagerungen wurde in dieser Arbeit bereits ausführlich diskutiert.Powders of alkaline stabilized silica sols also exhibit droplet size limitation. If the solids content of a dispersion (m _s ~ 5% (m: m); d _K = 1.5 x 10 ^-8 m) is doubled by a factor of two, the ratio of the shell width to the particle radius increases S / R in the produced powder by 8.4% from 9.17 to 9.94%. In a quadrupling of the concentration of the d ₉₀ (V) value increases the distribution of around 33% from 11.3 · 10 ^-6 m to 15.1 x 10 ^-6 m, whereas the ratio S / R increases by 15% to 10.56%. The relationship

for alkaline xerogels in this context also increases by 14% from 0.25 to 0.28 (Table 7.24, Appendix). An effect of alkali ions, which promotes the formation of gel, as the cause of these colloidal assemblages has already been discussed in detail in this work.

Geometric diameters of d _g <7 x 10 ^-6 m have not been described in the literature for comparable products in combination with such concentrations and short drying times (τ _Tr ~ 0.1 s; Pe> 40). Also unknown is the reduction in the likelihood of particle defects as a combination of the factors concentration, atomizing pressure and nitrogen loading. Confirmation of the increase in larger and smaller particle diameter decreases with increasing solids content of the starting material is provided in the overview in Table 4.4.

Both types of stabilization are characterized by an increase in bulk density in their prepared powders with increasing educt concentration, with alkaline starting materials tending to have higher values (Tables 7.22 and 7.24, Appendix). Contrary to Walton's [47] statement, bulk densities of more than 6 × 10 ² g L ^-1 can be achieved with alkaline xerogels even at solids contents of> 15% (m: m). However, as the particle diameter increases, the bulk density decreases as a result of the decreasing packing density (Section 4.2.2.3). While powders of starting ^{colloid sizes} of d _K = 1.5 × 10 ^-8 m (m _s = 10% (m: m); P _nozzle = 2.3 × 10 Pa) have bulk densities of ρ _s = 593 g L ^-1 , are the values of powders with larger sol particles (d _K = 4.5 · 10 ^-8 m) in the range of ρ _s = 550 g L ^-1 . The cause of this effect is the shift to a bimodal distribution of the educts, which is unavoidable as a consequence of the production conditions. Accordingly, different particle diameters compete in the formation of the particle shells. A modified viscoelastic region and altered gel formation are the result. The determined BET (N ₂ ; 77 K) surfaces correspond to those of the colloidal starting materials (Sears surface [188]) and are independent of the pH of the starting material. No evaluable pore formation is observed. Analogous to the observations of the previous chapter, the particle scattering of the particles with increasing dispersion pressure can be observed in the laser diffraction experiment. Since the xerogels obtained from unstabilized silica sols have the ratio of shell width to particle radius S / R is constant in the considered concentration range, it is surprising not that the decomposition rates (Span, d ₉₀ (V)) of these gels show comparable values for all used solids contents of the starting materials. Alkaline silica sols, on the other hand, show an increase in the ratio S / R with increasing concentration. This leads contrary to the theoretical expectations to a weakening of the particle shell. The cause of this effect may be the concentration gradient in the spray drop. The higher the initial concentration, the lower the concentration gradient ( ∂c / dz ) between the droplet core and its surface in the viscoelastic region and at the critical point. This leads to a rapid increase in viscosity in the entire bulk, which fundamentally inhibits the required mass transport to the solidification points. Accordingly, in the ensuing formation of the shell, the colloidal particles have less time and movement to spatially align. Defects in the structure (defects) are the result. Overall, the particles produced in this way disintegrate better with increasing concentration (> 70% when the dispersing pressure is increased from 1 to 4 × 10 ⁵ Pa), compared to their initial value (d ₉₀ (V)) than gels obtained from unstabilized sols (~ 60%). , Furthermore, the mechanical decay, based on the d ₉₀ (V) value, also increases with increasing initial colloid diameter (Table 7.25). The industrially indispensable, polymodal particle size distribution of the silica sols as the cause of the structure of predetermined breaking points (structural defects) in the particle shell is in this respect an arguable motive. An evaluation of the results of the laser diffraction experiment with respect to the wet measurement of the obtained xerogels was not expedient. Due to the manifold effects of aqueous silica chemistry, no general statements are to be made. Obviously, contact with aqueous media appears to activate the surface in which agglomeration, deagglomeration, and aggregation compete [179]. The same result is provided by measurements in 2-propanol as dispersing medium. Discussions on redispersibility studies are presented in Section 4.3 of this paper.

4.2.2.2 Influence factor temperature

Amorphous silicon dioxide (silica) is characterized by a complex surface morphology whose composition consists of a large number of different silanol group types ( ) as well as their interaction with differentiable water levels can exist. For a comprehensive overview, see the review article by ZHURAVLEV [192].

While bulk water can be vaporized even at low temperatures and pressure differences, the water molecules bound to the surface of the solid surface by attractive interactions with the silanol groups can only be removed at about 393 K and low pressures [197-203]. The crosslinking of the internal silanol groups with elimination of water and the transfer of the residual water in the interior of the silica phase in the gas phase take place depending on the degree of crosslinking at temperatures between 420 to 500 K [204-206]. However, a transfer of the last silanol groups in the silica system with elimination of water takes place only at about 720 K [207-209].

mit anteilsmäßig breiteren Hüllen ( S / R ) der hohlen Partikel formen. In diesem Zusammenhang nimmt die Schüttdichte, als makroskopische Beschreibung der aufgeführten Faktoren, ebenfalls zu. Tabelle 4.5 verdeutlicht am Beispiel von aus unstabilen Kieselsolen gewonnenen Pulvern, dass die Verhältnisse ρ_eff/ρ_true und S / R als Indikatoren des Hohlkörperanteils direkt mit der Temperatur verknüpft sind.In this section of the chapter the influence factor temperature on the powder properties is discussed [86]. The first part presents the results of the spray chamber temperatures and the drying gas temperatures (T _A and T _E ). Subsequently, the consequences of a modified spray inlet temperature (T _nozzle ) on the formation of particles are discussed. In connection with the temperature of the drying gas, LUKASIEWICZ's [121] statement could be confirmed that with increasing drying temperature the proportion of hollow particles in the obtained particles increases. At low temperatures, a controlled (slower) drying, resulting in characteristically more compact body

with proportionally wider envelopes ( S / R ) of the hollow particles. In this context, the bulk density, as a macroscopic description of the listed factors, also increases. Using the example of powders obtained from unstable silica sols, Table 4.5 illustrates that the ratios ρ _eff / ρ _true and S / R are linked as indicators of the hollow body portion directly to the temperature.

It turns out that drying temperatures, which remove water molecules bound to the surface (T> 393 K), in combination with low pressures in the drying chamber, promote the formation of spherical particles. Contrary to SHAW's [125] Recommendations on the probability of particle defects (defect density), the proportion of regular particles decreases with the drying temperature from number-related ~ 90% (mode 2, 398 K) to ~ 70% (mode 1, 363 K) ( ). One cause of this may be sought in the intermediate stage of viscoelastic agglomeration in the concentration of unstabilized starting materials. In the course of a slower drying it comes to a greater proportion of localized solidification, higher bulk concentrations and concomitant increased viscosity. As a result, there is a decrease in the diffusion (diffusion rate) in the context of mass transport is inhibited in the drop. The further increasing viscosity in the ongoing drying process slows down this mass transport progressively. Defects (particle defects) are the logical consequence. The particle agglomerates formed at low drying temperatures (363 K, mode 1) show smaller size distribution values due to their smaller temperature gradient from drop surface to core and the associated longer time of constant mass transfer rate (Table 7.26, Appendix). The system has more time to selectively dispose its colloidal constituents at higher material densities. As a result of this optimized structure formation and more pronounced particle shell, a mechanical stability gain is detectable. In the dispersion experiment of the laser diffraction experiment at different air pressures, it is found that the disintegration of the agglomerates is much lower than that of powders produced at higher temperatures. Thus, the reduced d ₉₀ (V) value of the powder produced from unstabilized Köstrosol ^® 1510 with an increase in Dispergierdruckes of 1 to 4 x 10 ⁵ Pa at a drying at 363 K of 40.6% to 7.2 x 10 ^{- 6} m, while when dried at 398 K a reduction of 50.1% (6.9 x 10 ⁶ m) is observable. High operating temperatures in the drying process lead to a rapid removal of the physisorbed water on the colloid surface. Accordingly, the same is no longer available as an intermediate for the formation of covalent bonds between the sol particles (interparticle). This prevents sufficient crosslinking of the colloidal particles by means of the silanol groups. There are accumulations of individual particles and only a few, isolated, stabilizing siloxane bridges between the monomers and agglomerates.

While the geometric diameter as a result of these observations increases with increasing drying temperature from 5.6 × 10 ^-6 m to 6.2 × 10 ^-6 m, the aerodynamic diameter of the particles (d _A = 4.3 × 10 ^-6 m) constant under the considered conditions (Table 4.5). This is a significant medical science effect. Thus, with the same "flight behavior", differentiated properties such as, for example, particle radius, bulk density and decay behavior of the encapsulated active substance can be adapted to the properties of the target milieu. Scanning electron micrographs of the prepared silica gels with respect to the temperature of the starting material (T _Disp = 298 K and 323 K) show a clear picture. The temperature of the dispersion is a significant factor influencing the formation of particles [86]. With an increase in the input temperature of the dispersion by 25 K to 323 K, irregular particles are invariably formed from predominantly spherical particles. illustrates these statements. The solubility of nitrogen in the silica sols is lower at 323 K compared to the dispersion temperatures [162]. As a result of the reduced temperature difference between the drying gas and the surface of the droplet, the removal of the nitrogen proceeds kinetically favored (gas dynamics). In this context, the releasing nitrogen seems to provide a less favorable, capillary-promoting, mass transport. In combination with the increased bulb temperature of the droplet, more fluid molecules can be transferred into the gas phase at fixed times. Accordingly, other phase distribution and flow conditions are present in the droplet, so that the formation of menisci between the solidification points on the droplet surface is additionally favored. The defect of ring formation [126-129] as a particle form increases significantly.

As can be seen from Table 4.6, with increasing the spray inlet temperature, further negative effects of particle formation dominate. The particle diameter (+ 10.9%), the size distribution width (+ 4.7%), the bulk density (+ 15.1%) and the ratio of shell width to particle radius (+ 16.6%) resp more effective to pure substance density (+ 15.1%). In the course of this increase in the geometric data, the aerodynamic diameter of 3.3 × 10 ^-6 m (T _Disp = 298 K) also increases to 4.1 × 10 ^-6 m (T _Disp = 323 K). Redispersion in laser diffraction experiment of these granules is also less favorable than the selected reference material (Köstrosol ^® 1510 T _Disp = 298 K).

4.2.2.3 Variation of the starting material

Colloidal silicas can be produced in variable particle diameters. Depending on the selected synthesis route and technical parameters, monomodal or polymodal distributions can be generated. Starting from the raw material sodium water glass, bimodal distributions are usually unavoidable for the nanoscalar particles obtained from a diameter of about 25 × 10 ^-9 m. In this work, alkaline stabilized starting materials (silica sols) with particle diameters of 7, 15 and 45 · 10 ^-9 m were investigated and compared. With the aim of reducing particle defects, all spray drying experiments were carried out under nitrogen (V × _{N 2} = 0.7 NL min ^-1 ). An evaluation of the physico-chemical parameters of the silica gels with each other and with the educts showed that the choice of the starting material could give (inherited) essential surface parameters. it can be seen that silica sols with small colloid diameters generate high BET (N ₂ ; 77 K) surfaces, the opposite finding applies to large colloid diameters. An analogous behavior is observed for the SEARS surfaces [188] of the silica sol relative to the BET (N ₂ ; 77 K) surfaces of the xerogels. Inheritance of properties of the starting material to the geometric (d _a ; d _g , d ₅₀ (V)) and other physico-chemical characteristics (R) of the powders is also observed. An overview is given in Table 4.7. Background of the rising, geometric values with decreasing colloid diameter is the colloidal stability and activity of the educts in the concentration process. In the drying process, there is a steady increase in the solids content of the silica sol, with small-particle silica sols passing through their viscoelastic region at lower concentrations than comparable dispersions with comparatively larger colloid diameters [179]. For example. Alkaline silica sols with colloid diameters of d _K = 7 · 10 ^-9 m undergo their viscoelastic range at solids contents of m _s ~ 33% (m: m). Larger colloids (d _K = 45 · 10 ^-9 m), however, reach this culmination point only at solids contents of m _s > 55% (m: m). Accordingly, the solidification point on the drop surface, which fixes the particle diameter, is achieved at larger drop radii. Consequently, the proportion of particles d _P decreases 10 ^-6 m.

A more prolonged, more uniform transfer of the dispersion medium into the gas phase (mass transfer rate) is a fundamental influencing factor on the particle size distribution of a powder. As a result, the xerogels prepared from small-particle colloids have a broader particle size distribution than their analogs with larger colloid diameters. This situation allows a reduction of the intermediate grain volume in the powder, as a result of which the packing density and bulk density increase. This effect is independent of the initial concentration of the colloidal particles. The xerogels produced have no evaluable difference in the proportion of irregular particle shapes or defect frequencies. Scanning electron micrographs in emphasize these statements. Clearly recognizable is the decreasing particle size distribution with increasing initial radius of the colloids.

In combination with the findings of the previous sections, it can be said that the particle formation has a direct relation to the stabilization, the concentration of alkali ions and the associated pH value of the dispersions. The colloid diameter of the starting materials plays only a minor role in this process. It mainly influences the properties of the particle size distribution. Through targeted selection of the reactant, the pressure swing technology can directly influence desired product properties such as defect density, particle distribution and the surface under variable boundary conditions. By varying the concentration, the geometric characteristics of these xerogels can be defined. An additional application of nitrogen reduces the maxima of the particle size distributions, the probability of defects and the mechanical strength of the granules obtained. Variations of the drying and spray inlet temperature directly affect the particle shape. Thus, specific modifications of these parameters may favor particle shapes or reduce particle defects.

4.2.2.4 Influence of additives

In all investigations carried out an unstabilized Köstrosol ^® 1510 served as starting material. With this pure reference gel prepared therefrom, all modification measures were compared. All samples were at the same nozzle exit pressure P _nozzle = 2.3 x 10 ⁷ Pa, analogous drying temperatures (mode 2, T _A = 398 K, T _E = 393 K) and nitrogen (V x _N 2 = 0.7 NL min ^-1 ) applied. In order to optimize properties of the starting materials and spray-drying evaporation processes, it is possible to introduce modifications to the colloids themselves or to add additives for influencing the surface and interfacial chemistry. In addition to pure solvents, these additives can also be wetting agents and surfactants. In the following section, the influence of an admixture of monoethylene glycol (σ = 47.7 · 10 ^-3 N m ^-1 , p _Fl = 5.3 · 10 ³ Pa at 293 K) [210] was investigated. This was intended to modify the polarity of the dispersing medium and to vary the surface tension in a targeted manner. The addition of monoethylene glycol (glycol) actively influenced the particle formation of the resulting xerogels. As the organic solvent content increases, the particle size distribution numbers decrease significantly (Table 4.8). Thus, by a 10% admixture of (m: m) 1510 d ₉₀ (V) value of the distribution of the powders produced are reduced by ~ 36% to 6.9 x 10 ^-6 m of mono ethylene glycol to a Köstrosol ^®. However, similar to CHU's [96] results, no correlation between organic solvent addition and bulk density can be found. This is due to the varying particle size distributions of the products as well as an increase in defects and defects. Characteristic differences exist in the structure of the particle shell of the produced xerogels. As clarified, are on the outside hardly detectable defects, while the inside is drawn by gas bubbles. The reason for this is the reduced surface tension of the fluid mixture. This leads to a faster evaporation of the solvent mixture (higher mass transfer rate) on the surface of the drop, as a result, the concentration gradient drops surface to bulk ( ∂c / dz ) is shifted to higher values. Building on the basis of the model of WALKER [46] (page 17), in which comparable systems have the analog density at the critical point, this critical value is due to the increased density of the mixture by monoethylene glycol (ρ = 1,11 · 10 ³ kg m ^-3 ) [211] shifted to lower solids concentrations. There is less material available for solid phase formation in the same volume. Upon reaching the viscoelastic range in the drying process, the viscosity increases rapidly in the boundary region of liquid gas. Due to the optimized evaporation due to the lowering of the surface tension and the increase in the concentration gradient, the (local) solid phase formation (solidification of the shell) is accelerated on the surface of the drop, whereupon the permeability decreases strongly depending on location (drop surface). In the viscous bulk of the dissolving gas and evaporating fluid can now no longer trouble-free from the boundary layer formed, a precursor of the subsequent particle shell, penetrate as a result of hindered material flows. Due to this hindered diffusion of the gas phases during the solidification of the shell, gas bubbles are formed on the still soft inside of the particle shell. As the drying process continues, solidification of this boundary layer and incorporation of these phases into the shell structure occurs. In addition, due to the higher boiling point, the organic component concentrates in the drop. Formation of local centers with a high proportion of organic solvent (islands) is the result. This island formation in the drying process creates isolated areas, which additionally hinder the penetration of vapor and dissolving gas. Table 4.8 shows that when admixing 5% (m: m) of monoethylene glycol (glycol) to the reference sol, particles having a high ratio of shell width to particle radius (US Pat. S / R = 9.1%) can be produced. The presented results of this work on the influence of an increased ratio S / R (Section 4.2.2.1) are also confirmed in the introduction of additives. These particles have, analogously to the gels prepared from alkaline sols, a reduced mechanical load-bearing capacity compared to the selected reference material in the laser diffraction experiment (Table 7.29, Appendix).

von 0,22 auf 0,13 verdeutlichen dies. Analog dem Zusatz von Monoethylenglykol werden im inneren Aufbau der Partikelhülle Fehlstellen – Gasblasen – beobachtet ( rechts). Diese Tatsache bestätigt den beim Zusatz von Monoethylenglykol aufgeführten Mechanismus der sinkenden Permeabilität durch einen begünstigten Stoffübergang auf der Tropfenoberfläche in Kombination mit einer teilverfestigten Partikelhülle. Diese Schwächung der Hüllenstruktur bewirkt eine messbare mechanische Destabilisierung. Die auf diese Art entstehenden Partikel kennzeichnen sich durch geringe Wandstärken aus. Unter den angewendeten Trocknungsparametern neigen diese anfangs noch labilen Hüllen in Folge von Energie- und Stofftransportprozessen zur Deformation. Insbesondere der Einfluss von Kapillarkräften im Trocknungsprozess ist diesbezüglich zu diskutieren („Buckling”) [153,154]. Der Mechanismus dieses morphologischen Phänomens bei Zuführung von Wärme ist noch nicht eindeutig geklärt [47]. TSAPIS et al. [155] begründet diese Gestalt eines Partikels mit einem starken Einfluss von Kapillarkräften im viskoelastischen Bereich eines Fluids. Typische derartige Erscheinungen beim Sprühtrocknen wurden von MARSHALL et al. [108] für filmbildende Mittel wie zum Beispiel Natriumwasserglas beobachtet. Weitere mögliche Ursachen dieser in Kernrichtung fokussierten Abweichungen von der sphärischen Form können Sogwirkungen auf Grund der Formung eines lokalen internen Vakuums sein. Aber auch Störungen der Wechselwirkungen an den Phasengrenzflächen werden in der Literatur diskutiert. Die so entstehenden plastischen Verformungen destabilisieren die anteilig dünneren Hüllen zusätzlich [45]. Folglich sind diese äußerst labil und werden bereits bei geringen Scherbelastungen gebrochen. Der d₉₀(V)-Wert der Verteilung fällt bei Erhöhung des Dispergierdruckes im Laserbeugungsexperiment von 1 auf 4·10⁵ Pa um 54% auf 5,3·10⁶ m (Referenzgel: 37%; 6,8·10⁶m) ab. Die Sorption-Isotherme (N₂; 77 K) zeigt keine texturellen Modifikationen der Oberfläche. Lediglich ein geringer Anstieg des adsorbierten Volumens (+6·10^–5 L g^–1) in Folge der beobachteten Fehlstellen in den Partikelhüllen ist zu verzeichnen. In diesem Zusammenhang wirken die durch Gasblasen gebildeten „Hohlkammern” als eine Art Pore.Nitrogen sorption measurements on the resulting glycol-modified xerogels provide the analogous results as the unstabilized reference material. The evaluation of the textural data further show that at low concentrations of the additive monoethylene glycol (≤ 5% (m: m)), no changes take place. Accordingly, the inheritance of surface features (A _BET vs. A _SEARS ) from the colloidal starting materials can also be confirmed for these materials. On this basis, the effects of a low additive concentration can only be observed on the evaporation of the fluid and the associated particle size distribution [96]. If these additive concentrations are increased (> 5% (m: m)), a comparison and an evaluation of the nitrogen sorption isotherms is no longer feasible. The obtained pore structure textural data are not in agreement with the morphological data of the scanning electron studies. One cause in this context may be the described defects in the sheath structure. As a further additive in this work, the wetting agent Dynol ™ 607 (2,5,8,11-tetramethyl-6-dodecyne-5,8-diol ethoxylate) was used. The focus of the investigation in combination with this additive was the modification of the interface effects on the silanol groups. The resulting interactions should fundamentally influence the process of drying. Such surface-active substances promote the transition of water molecules at the boundary layer solid-liquid (silanol groups) into the bulk phase by lowering the phase transition energies. As a result, the difference in the velocities between diffusion of the particles in the bulk and supply of fluid molecules from the bulk and removal of these from the surface (Péclet number) is substantially increased. It follows that the gradient of concentration ( ∂c / dz ) increases between the droplet core and the surface of the droplets, as a result of which, even at larger droplet radii, the concentration thresholds for forming the viscoelastic region on the droplet surface are locally reached. Analysis of the spray-dried, Dynol ™ 607-modified (0.1% (m: m)) silica gel showed that, compared to the reference gel listed, much larger particle radii (d ₉₀ (V) + 7.9%; 11.6 x 10) ^-6 m at P _Disp = 10 ⁵ Pa). The powders produced with Dynol TM 607 additive are characterized by a very low ratio of shell width to particle radius ( S / R ~ 4.5%) as well as strong deviations from a spherical particle shape ("buckling"[153-155]; ). Accordingly, optimal, macroscopic packing of the powders is impossible. A low bulk density (-38%, ρ _s = 284 g L ^-1 ) compared to the reference gel and a decrease in the ratio of the effective to the pure substance density

from 0.22 to 0.13 illustrate this. Similar to the addition of monoethylene glycol, voids - gas bubbles - are observed in the internal structure of the particle shell ( right). This fact confirms the decreasing permeability mechanism of monoethylene glycol addition favored mass transfer on the drop surface in combination with a partially solidified particle shell. This weakening of the shell structure causes a measurable mechanical destabilization. The resulting particles in this way are characterized by low wall thicknesses. Among the drying parameters used, these initially unstable sheaths tend to deform as a result of energy and mass transfer processes. In particular, the influence of capillary forces in the drying process is to be discussed in this regard ("Buckling") [153,154]. The mechanism of this morphological phenomenon when heat is applied has not yet been clearly established [47]. TSAPIS et al. [155] explains this shape of a particle with a strong influence of capillary forces in the viscoelastic region of a fluid. Typical such spray drying phenomena have been described by MARSHALL et al. [108] observed for film-forming agents such as sodium water glass. Other possible causes of these deviations from the spherical shape focused in the core direction may be suction effects due to the formation of a local internal vacuum. But also disturbances of the interactions at the phase interfaces are discussed in the literature. The resulting plastic deformations additionally destabilize the proportionally thinner sheaths [45]. Consequently, these are extremely unstable and are already broken at low shear loads. The d ₉₀ (V) value of the distribution drops by 54% to 5.3 × 10 ⁶ m when the dispersing pressure in the laser diffraction experiment is increased from 1 to 4 × 10 ⁵ Pa (reference gel: 37%, 6.8 × 10 ⁶ m). from. The sorption isotherm (N ₂ , 77 K) shows no textural modifications of the surface. Only a small increase in the adsorbed volume (+ 6 · 10 ^-5 L g ^-1 ) due to the observed defects in the particle shells can be observed. In this context, the "hollow chambers" formed by gas bubbles act as a kind of pore.

sowie die Schüttdichte ρ_s bleiben auf dem Niveau des Referenzpulvers. Folglich muss der Partikelbildungsmechanismus analog dem des Referenzsoles ablaufen. Modifikationen gibt es jedoch im viskoelastischen Bereich. Dieser ist auf Grund der verringerten Reaktivität (Anzahl der Silanolgruppen auf der Oberfläche) der Solteilchen zu höheren Feststoffgehalten verschoben. Es kann mehr Fluid verdampft werden in dessen Folge sich die Daten der Partikelgrößenverteilung zu kleineren Werten verschieben. Die BET(N₂; 77 K)-Oberfläche steigt leicht (+13%) an.Another possibility of influencing the colloid chemistry is a modification of the surface of the sol particles. By means of a targeted coating with sodium aluminate liquor, the number of silanol groups on the surface of the colloidal particles can be substantially reduced ( ). As a result, there are significantly fewer sites of linkage between the sol particles to form siloxane bridges, which would cause gelation, reducing interactions and increasing the stability of the dispersion [179]. Spray drying of dispersions with such modified particles provides smaller particle size distribution _values (d ₉₀ (V) - 23%; 8.2 x 10 ⁶ m at P _disp = 10 ⁵ Pa) compared to the reference _gel . Scanning electron micrographs show no altered shape of the particles formed compared to the reference material. Other characteristic physico-chemical parameters such as S / R .

and the bulk density ρ _s remain at the level of the reference powder. Consequently, the particle formation mechanism must proceed analogously to that of the reference salt. However, modifications exist in the viscoelastic field. This is shifted to higher solids contents due to the reduced reactivity (number of silanol groups on the surface) of the sol particles. More fluid can be vaporized, as a result of which the particle size distribution data shifts to smaller values. The BET (N ₂ ; 77 K) surface increases slightly (+ 13%).

As As can be seen, all modifications of the starting colloidal products lead to a change in the geometric and aerodynamic properties. Depending on the type of reactivity influencing the colloidal starting material, specific microscopic and macroscopic properties are influenced. A change in the polarity of the solvent in combination with a reduction in the reactivity of the particles leads to the positive effect of smaller particle size distribution values of the resulting xerogels. The d ₉₀ (V) values decrease by up to 36% to 6.9 · 10 ^-6 m (P _Disp = 10 ⁵ Pa). Comparable observations can also be made for surface-modified silica sols. These actively influence the reactivity of the individual sol particles (colloids). A variation of the solid-liquid interface effects, on the other hand, significantly influences the structure of the particles. In addition to spherically divergent particle shapes of the shell structure of the hollow body is significantly disturbed. This has positive effects on the desired decay rate under mechanical stress in the laser diffraction experiment. Common feature of all presented modifications is a reduction of the aerodynamic diameter. While the surface-modified types and the partially solvent-substituted variants realize this by reducing the particle diameter, the Dynol TM 607-added variant uses the influence of the reduced bulk density on this parameter. Consequently, by selective addition of additives in the pressure swing technology, the particle size distribution, the decay as a function of the mechanical load and the bulk density can be actively controlled.

4.2.3 Surface morphology

As described in the previous sections, the BET (N ₂ ; 77 K) surfaces measured by nitrogen sorption reveal dependencies on the starting material. These data are independent of the concentration used or the technical spraying parameters selected (P _nozzle , T _VK , T _E , T _A , T _Disp ). Furthermore, scanning electronical images ( ) no evidence of channels or a pore structure. The measured nitrogen isotherms do not provide any definite statements regarding the porosity (BJH model). During the evaluation of the sorption isotherm for the xerogel from Köstrosol ^® can expect 4510 void volume, the corresponding isotherms suggest for under analogous conditions from Köstrosol ^® 1510 powder obtained in the presence of mesopores. All adsorbed volumes are between 1.8 and 2.7 · 10 ^-4 L g ^-4 . In addition, loading with nitrogen is crucial for the macroscopic particle shape, but has no influence on the fine structure of the particle surface. As can be seen, the monomeric, nanoscale building blocks characteristic as a structural feature of the particle shell. The resulting, defined surface roughness is an essential descriptive feature of the respective particle and powder. This is the basis and explanation of the relationship between the SEARS surface [188] of the silica sol and the BET (N ₂ ; 77 K) surface of the powders made from these dispersions.

The proof of a dependence of the particle morphological properties of the spray-dried xerogels on the starting particle diameter of the colloidal systems can be carried out by means of a pore size analysis. Starting from a densest packing of these monomers on the surface of the formed particles, the resulting volume between the individual colloidal particles (monomer) of the starting material ( _{interparticle} space V _ZPR ) is a structuring feature. Resulting octahedral (V _O ) and tetrahedral (V _T ) in this structure are therefore directly dependent on the diameter of the pack-forming building blocks [42]. V _ZPR = n (2V _T + V _O ) (4.4)

According to Equation 4.4, this interparticle space is linearly dependent on the number (n) and the radius of the colloidal monomers (V _O , V _T ). With the aid of nitrogen sorption (BJH model) and mercury porosimetry, the pore size obtained (pore radius) was plotted against the colloid diameter of the starting materials ( ). Clearly recognizable is the linear relationship between the determined pore radii and the diameter of the colloidal sol particles. For infinitely small initial colloid diameters, the straight line of both methods of determination goes through the origin of the coordinates within the scope of the measurement inaccuracy. This is the necessary confirmation of the theses listed. By means of mercury porosimetry, the differentiation of interparticle space ( ; No. I) and intergrain volume (No. II) are easily possible. The cause of the observed discrepancy to the computational data is due to the particle size distribution of the considered silica sols. Not only is there exactly one defined particle diameter. In addition, with regard to nitrogen sorption, the BJH model is mainly applicable to the mesopore range (2 to 50 x 10 ^-9 m). The relative error in the range <2 · 10 ^-9 m is increased accordingly.

illustrates a further relation of the determined pore radius and the formation of interparticle volume. It shows the required independence of the textural properties of the obtained xerogels from the solids content of the colloidal starting materials as well as from the mechanical reduction state (Table 4.9). As already mentioned in this chapter, it is possible to generate a defined interparticle space (V _ZPR ) by carefully selecting the starting material. This is drawn due to the relatively uniform monomer diameter in a very narrow range. In this context, it is also possible by mixing different xerogels to adjust interparticle volume profiles and to combine BET (N ₂ ; 77 K) surfaces (Table 4.10). This is illustrated in the example of a mixture of from Kostrosol ^® 1510 and Köstrosol ^® 4510 produced xerogels.

The fact that they are interparticle volumes also illustrates that the resulting powder morphologies are independent of the drying conditions. Analogous statements can also be given for the spray temperature of the starting material. For both parameters, BET (N ₂ , 77 K) surface and adsorbed volume remain nearly identical (Table 7.31).

Alkaline xerogels tend to have lower values in their adsorbed volumes during nitrogen sorption than the unstabilized silica gels prepared under analogous conditions. The volume in the interparticle space of the obtained powders of different stabilization forms is a function of the colloidal diameter of the starting materials. As the unstabilized brine from the alkaline brines can be prepared, the diameter of the particles is thus equivalent, the volume formed from this can be equated. Consequently, only a modification of the inter-grain volume can explain this tendency. In this context, the varying particle size distribution of the gels between the stabilization forms is the explanation of this finding. However, different interparticle maximum maxima (pore radius maxima, BJH model) are observable for the xerogels prepared from aluminate-modified silica sols. Here, due to the reduced reactivity and different colloid diameter of the modified particles, a decrease in the interparticle volume ( ). Silica sols with addition of monoethylene glycol are characterized by a widening of their "pore radii" distributions (BJH model). This extension depends on the amount of added additive ( ). This is due to the observed gas pockets inside the shell (Section 4.2.2.4). These generate additional scattering of the pore distribution.

4.2.4 Proof of the amorphous character

In the context of a medical and occupational health and safety law, crystalline silica in nanoscalar form is a health risk that should not be underestimated. It is known that this form of silica is classified as carcinogenic [196]. Accordingly, the detection of an amorphous structure for powders having pulmonary (pulmonary; d _p <10 ^-6 m) is of significant scientific and technical interest. shows by X-ray powder diffractograms the amorphous character of the powder produced. Crystalline components can not be detected.

4.2.5 Model of particle formation

The particle formation model presented in this work ( ; Page 90) builds on the foundations of WALKER [46], who sees the rheological properties as a key consideration in particle formation. Furthermore, MESSING's [45] observations in the spray pyrolytic particle formation process play an important role. Modifications are included in the field of colloidal chemistry as well as the properties of colloidal silica. After the formation of the spray at the nozzle outlet opening, the droplet collective formed comes into contact with the flows of the drying gas. At time zero, the colloidal particles are statistically distributed over the drop volume ( ; Number: I). With increasing evaporation of the fluid, the concentration and temperature gradient from the surface of the drop to the inside of the drop continues to increase. Furthermore, the viscosity in the drop volume increases steadily. From a critical point of concentration, the viscoelastic region, local agglomeration (d _H <10 ^-7 m) occurs on the surface of the droplet (number II). From this point on the further drying process depends on the type of stabilization of the silica sol. In the case of alkaline colloidal silicic acids (route A), the particle diameter is fixed at this moment and the formation of a particle shell begins. In this context, the alkali ions present in high concentrations promote the further agglomeration process of the colloidal particles into a gel. As the drying process progresses, this shell is strengthened and reinforced. Silica sols without stabilizers lack such ions (alkali ions). Agglomeration of the sol particles at the viscoelastic point causes a rapid increase in viscosity. This inhibits the mass transport, whereby insufficient colloidal particles reach the surface of the drop to solidify them. At this point, the spray droplet can continue through the shrinkage process as a result of the fluid mass transfer into the drying gas (path B) or split into smaller droplets in the interaction of capillary and shear forces with the drying gas (path C). The reason for this splitting are the forming menisci of the liquid-gas and liquid-solid interfaces between the agglomerates. As a result, the drops formed by this decomposition process are smaller than their starting drop. Accordingly, the surface available for the mass transfer in the drying process is increased. Consequently, the drying is accelerated and the concentration gradient drop surface to drop inside increases steadily. The time until fixation of the particle diameter by forming a particle shell is shortened. Due to the earlier embodiment of the shell, the ratio of the shell width to the particle radius decreases significantly. An increase in the number of drops as well as the required deformation of drops in a split and split promote the probability of defects. Please refer in particular to section 2.1.2 (page 10) of this thesis. Path B represents an imaginable way of hollow sphere formation, in particular for very small droplet diameters. Due to the shrinkage of these droplets in combination with a temporally prolonged constant mass transfer rate, the concentration of the colloidal materials increases continuously with subsequent formation and solidification of a particle shell. Larger drops can go through both variants. The tendency to smaller particle diameter from the spray drying of unstabilized brines can be understood accordingly by shrunken drops or by a plurality of droplet splits. The pressure swing technology is characterized by the possibility of loading the brine with gases. Nitrogen was used in this work. In the absence of gas loading, there is a high level of irregular particles ( ; Page 60). In this context, a mechanism is discussed, which starts from first solidification points on the drop surface. As a result of the slowly solidifying compared to the evaporation structures in the further drying step more water is discharged inside the droplet through the still soft or vacant points on the surface of the drop into the gas phase. As a result, a local vacuum is formed. Capillary forces force the progress of solidification into the interior of the particle. It comes to deformation. Furthermore, already formed solid phases can have a very low permeability. The mass transport through these phases into the gas phase is inhibited and there is an increase in pressure and temperature inside the envelope. Experiments on the rate of release of nitrogen have shown that its escape immediately after the nozzle does not abruptly. Consequently, there is a constant release by the evaporating in the course of drying and concentration solvent. This solubility and the speed of the removal process of nitrogen in silica sols are pressure and temperature dependent.

Cause of the process-technically possible defect mechanisms - formation of a partial vacuum as well as overpressure in the drop inside by reduced permeability - in the drying process are pressure fluctuations in the drop by mass transfer processes. The combination of dissolved gas and fluid reduces the speed of sound compared to pure media. This allows pressure fluctuations in the system to be compensated faster. The drying process is "smoother". During the formation of a partial vacuum, more dissolved gas is released from the fluid into the gas phase due to pressure. The resulting pressure build-up counteracts the resulting pressure loss. In connection with the overheating of the drop interior, the resulting cooling effect on release of the gas as a result of the gas dynamics causes a time delay or weakening of a pressure build-up. These controllable drying conditions during drying lead to the formation of particles with a lower probability of defects. As a further consequence of the steady release of gas from the fluid, there is a reduced ratio of shell width to particle radius (shell thickness). The transfer of the fluid molecules into the gas phase occurs at locations where no rigid material is present. This mass transfer inside the drop is promoted by the release and flow of the nitrogen. As a result, faster evaporation and targeted solidification of high permeability sites occur - sites with less solids and higher probability of defects.

4.3 Dispersion experiments

The dispersion of powder is an essential basis in many areas in order to optimize the processing and processing of products and product groups. For reasons of applicability, especially dusty powders are usually converted into less burdensome, bulky forms. Often, however, the reconversion of such systems into dispersions is difficult to achieve. In the previous chapters it could be shown that the nanoscalar, colloidal monomer unit of the prepared amorphous powders is recognizable as the basic building block of the particles. This field of work will investigate, if with the help of the pressure swing technology agglomerates were formed instead of known aggregates. For this purpose, the redispersibility of these powders should be investigated. Section 4.3.1 covers the results of redispersibility tests based on aqueous media. In addition to pure aqueous systems, investigations with bases and xerogels from additized starting materials are also explained. In the following section the redispersibility studies in 2-propanol are presented.

4.3.1 Dispersion in an aqueous medium

Using selected xerogels, redispersion is possible even at low shear forces. Already a weak mechanical energy input, such as, for example, stirring the xerogels with a magnetic stirrer in water leads to a comminution of the powders produced by spray-drying. illustrates that this deagglomeration can be additionally accelerated by means of an ultrasonic treatment. The resulting colloids are identical in size and shape to the monomeric, spherical basic building blocks of the starting materials (silica sol particles). This can be understood as a proof of the composition of agglomerates. Such a transfer of technically known silicon dioxide products with these selected energies into an aqueous system is currently not known in the literature. Currently, colloid mills or high-frequency agitators are used to reduce the size of silicon dioxide products. The particles formed in this case usually have an irregular shape. Furthermore, these dispersions or suspensions are usually not stable. The reference material used is a xerogel produced from unstabilized silica sol (d _K = 1.5 × 10 ^-8 m, m _s = 1.35% (m: m)). With this From this reference gel produced all experiments or comparisons are performed. The observed reduction or decay process takes place in two stages ( and ). In the first step, the smaller particles are deagglomerated with their relatively high content of surface building blocks. In a second stage, the gradual size reduction of the larger particle diameter ( ; right side).

The dispersion of additized xerogels by means of magnetic stirrer in deionized water led to a more irregular disintegration of the agglomerates. shows the comparison of dispersing a Referenzgels of an unstabilized Köstrosol ^® 1510 a, produced under analogous conditions with Dynol TM 607 (0.1% (m: m)) treated powder. After dispersion with ultrasound, smaller agglomerates ( ; No. II) to the starting monomers (# I), while larger particle diameters tend to suspend. The gels obtained from silica sols with the additive Dynol ™ 607 show another special feature. The defects of the shell described in Section 4.2.2.4 (Page 76) provide better redispersion of the agglomerates to their colloidal monomers analogously to the data from the laser diffraction experiment of the powders produced under mechanical stress.

Studies on the redispersion of xerogels as a function of the solids content of the educts show a further influencing factor. The higher the concentration of the unstabilized starting material in the spraying process, the less energy input is required for the conversion of the resulting gels into a dispersion. shows this with the example of xerogels from 1.4% and 12% (m: m) spray-dried educts. Causes are the, with increasing solids content increasing, sphericity and particle diameter as well as the reduction of the defect density. It is not necessary to overcome the usual, for defects, energetic barriers in the redispersion process.

An addition of bases additionally promotes the redispersibility of the generated powders. Leaches favor the cleavage of the crosslinking siloxane bridges analogously to the dissolution of silicates [179]. Furthermore, the literature mentions the pH-dependent solubility of silicon dioxide. As the pH increases, this increases significantly [179]. Parallel to the introduction into water, a gradual disintegration of the agglomerates formed takes place. In this case, a disintegration of the agglomerates is also observed in their monomeric components. The stability of the resulting sols depends on the concentration of the alkalis used (potassium hydroxide solution; ). Low concentrations of the base used and the treatment with ultrasound lead to stable, colloidal dispersions. At higher concentrations, in a first step, dissolution processes lead to a water glass-like intermediate. Subsequently, in a subsequent step, the formation of higher agglomerates and aggregates is observed by the increased electrolyte content in the fluid ( ; Page 49).

Based on these experiences, these powders offer themselves as silicate nuclei for the hydration of concretes. It has been shown that grain sizes ranging from 2 to 6 · 10 ^-7 m are missing in their microstructure in order to achieve ideal packing densities [39]. Added as a microscale component, these powders can also decompose under the mechanical stress of a mixture into nanoscale reactive building blocks - silicate nuclei. The highly alkaline pH of these formulations further promotes the process of bond cleavage (siloxane bridges). As a result, an optimization of the packing and cross-linking density of the bed is conceivable. An additional advantage of these specialized silicate seeds is that they are already increasing their insensitivity to acidic and abrasive media already practiced with microsilica [39]. A disadvantage of microsilica is the unfavorable particle size distribution of the silicon dioxide constituents (2 × 10 ⁶ m ≤ d _P ≤ 2.5 × 10 ^-4 m) for ultrahigh-strength concrete systems.

4.3.2 Dispersion in 2-propanol

Attempts to incorporate the obtained xerogels in organic solvents (2-propanol) also led to a redispersion of the agglomerates produced. In this context, a decay up to the monomeric constituents is observable. As in In analogy to a dispersion in water (Section 4.3.1), in organic fluids the gradual decomposition of the formed particles occurs. Attempts to energetically support deagglomeration by ultrasound did not improve the conversion to the colloids (silica sols), contrary to observations in aqueous systems. Comparative studies on the decomposition of the agglomerates in water and isopropanol indicated a lower rate of decomposition in the organic medium. The reason for this may be the lower reactivity of the 2-propanol molecules to split siloxane bridges. Another reason lies in the chemistry of these compounds. These interact by means of hydrogen bonds with the silanol groups of the silica surface and settle on these (colloidal) particles like a kind of inert film. The goal of this work, the generation of redispersible powders which have inherited properties of the starting colloidal materials can thus be considered to have been achieved. In contrast to commercially available, conventionally dried products (eg. Köstrosolid ^® 0403), the powder produced in this work of agglomerates whose redispersibility to the monomers, the starting colloid diameter is possible exist. Köstrosolid ^® 0403 has no decay within the measurement inaccuracy in, applied in this work conditions. Transmission electron micrographs and the laser diffraction data confirm this finding. Pyrogenic silicas can break down into smaller aggregates under similar mechanical loads [152], but their diameter, shape and stability are not regular. In the case of redispersibility studies of such powders, broad particle size distributions, pronounced deviations from the spherical shape and strong agglomeration phenomena are usually observed. A redispersibility of pure fumed silicic acids up to the monomeric basic unit has been postulated by various authors, but this possibility has not yet been proven beyond doubt.

4.4 Evaluation of gels in the application test

This chapter presents the results of the application tests. The basis of these experiments are the results of the investigations in the preceding chapters. As already described in this work, no significant cohesive interactions could be observed in the prepared powders. In comparison with reference materials (Köstrosolid ^® 0403), they have good flow and casting behavior. Furthermore, geometrical, mechanical and textural properties can be adjusted in a targeted manner by suitably selecting the educt and the spray parameters. Furthermore, a redispersion up to the colloidal basic building blocks can be achieved under trivial conditions, so that, for example, optical influences of larger particle diameters can be eliminated. Ideally, disintegration occurs up to the colloidal monomer to ensure complete retention of transparency. Studies have shown that with particle diameters up to d _P <4 · 10 ^-8 m, this property is maintained, independent of the solids content of the system [212, 213]. Silica-based fillers usually consist of irregular particle shapes in the microscale size range. In this context, the powders produced in this work, with their high proportion of spherical particles in the sub-micron range (d _P <5 x 10 ^-6 m), can show new properties in application tests. The following sections discuss the results of incorporation into various fields of application (matrices). Section 4.4.1 introduces in this connection the influence of the obtained powders on the physicochemical properties of reference coating systems. Next, the results of the incorporation into, for silica as filler, unusual polymers are considered (Section 4.4.2).

4.4.1 Introduction into reference coating systems

The xerogels produced in this work allow the setting of specific product parameters in reference varnish formulations [214-216]. Depending on the selection of the property of the powder produced, for example the BET (N ₂ ; 77 K) surface, a number of macroscopic parameters can be varied and adjusted. The spherical shape of the granules and their distribution make it possible to exert direct influence on the viscosity and hardness of a coating. An optimized scratch resistance and increased flame retardance are the consequence [217]. Furthermore, gloss and matting properties can be variably adjusted [214-216]. As an example, by substituting a microsilica made by milling, the viscosity may be typically (-15%) lowered at the same or, ideally, increased feed level (Table 4.11).

Due to their low effective density, hollow body components bring a further, essential advantage in a paint formulation. By means of a targeted (partial) exchange of previously used fillers, properties such as the density of the system used can be specifically defined (Table 4.12). By adapting this system density, it is possible to avoid intermixing or penetrating multi-level coating systems. Furthermore, sedimentation or floating of fillers can be avoided or delayed. In addition, these types of particles can diffuse to the surface of a paint film. This causes a reinforced matting effect (anti-fingerprint). As a result, the necessary amount of other matting agents may possibly be reduced. Other mechanical properties such as elasticity can also be actively influenced [214, 215]. Most fillers such as silica are used to increase the hardness of a paint. But this has negative consequences for the elasticity of these systems. By using the powders produced in these matrices, it is possible, due to an increase in hardness and elasticity, otherwise to develop unusual applications for paints. The explanation for both effects lies in the decay of the particles used (agglomerates) as well as the embedding of the resulting nanoparticles in the paint system. The ideally randomized particles have in their immediate environment a space with reduced binder concentration and high elasticity. This space is called "Cross-Linking Space" [212]. When a force is applied, an elastic deformation of this space occurs. After termination of the force, this space can ideally regress, while retaining the macroscopic properties of the paint. For microscale fillers, the ratio of cross-linking space [212] to particle number and diameter is much less favorable. Consequently, these effects are not observable here.

Another requirement for the use of the generated powders as fillers are often low specific surface areas in order to avoid that unnecessary paint material is not adsorbed. As a result, large silica sols with their low specific surface areas (A _BET , A _Sears ) prove to be ideal starting materials for the production of powders for use in primers and paints.

4.4.2 incorporation into selected polymers

Fillers are widely used in a variety of plastic applications. They serve to increase the breaking strength and elasticity, protection against abrasion and the reduction of interactions. However, the application of these fillers is for transparent polymers such as polymethyl methacrylate (PMMA; Plexiglas ^® brand name) or polyamide 6 (PA 6; Brand Name Perlon ^®) limited. In the course of current silica-based fillers, the optical properties of the plastics obtained are greatly impaired due to the available particle diameters (d _P ≥ 2 × 10 ^-6 m). A chemical integration of amorphous silica particles in these systems is carried out by reaction of the surface-active silanol groups with methyl methacrylate (PMMA) or ε-caprolactam (polyamide 6). In all investigations carried out an unstabilized Köstrosol ^® 1510 served as starting material. All incorporations were carried out with this pure reference gel (mode 2, T _A = 398 K, T _E = 393 K) prepared under nitrogen (V · _N 2 = 0.7 NL min ^-1 ). In order to ensure the preservation of the transparency of the polymers considered in this work, a deagglomeration of the resulting xerogels to conglomerates of less than 3 · 10 ^-7 m must take place. In the area of silicon dioxide there is no such material. The results of the shear stresses during extrusion (simple extrusion) of the plastic melts lead to a disintegration of the powder particles produced. Even at these low mechanical stresses, only silicon dioxide particles in the nanoscale range (d _K < ^{3.times.10.sup.-7} m) can be detected in the polymer matrix by scanning electron microscopy. makes it clear that PMMA focuses on local polymers of colloidal particles (fragments). Likewise, individual monomers are observable. An optimization of the incorporation in terms of a more even (statistical) distribution of the colloids may be the content of further research. In the context of this work, only attempts to redispersibility were made. Notably, there is no significant loss of transmission of the filler-loaded plastics. An incorporation in polyamide 6 could be realized by the low residual moisture of the produced xerogels under pressure reduction ( ). In this polymer, the disintegration of the spray-dried particles by shear forces is also displayed. The fragments are present as a loose juxtaposition of small balls. These "loose" agglomerations have diameters in the range of < ^{2x10 -7} m. In this context, in addition to the results from PMMA, an optimization of the incorporation in terms of a more uniform (statistical) distribution of the colloids is proposed.

In summary, it can be explained that with the help of the pressure swing technology novel powders can be regenerated, which currently still open up for the use of silica unusable plastics (PMMA, PA 6). Incorporation of these xerogels causes the particles to decay into nanoscale agglomerates (d _K <3 × 10 ^-7 m). Similar to the experience of incorporating silica particles (d _K > 2 · 10 ^-6 m) in polyethylene or polypropylene, property improvements such as increased mechanical strength and an extension of the elastic limits ("cross-linking space" [212], see section) 4.4.1) within the possibilities. Promoted flame retardance [217] while maintaining transparency is also a highly interesting criterion for further research. In both plastics optimization potential in form and type of incorporation for a targeted dispersion is present. A comparable disintegration of such particles to the monomeric building blocks has not yet been described in the literature for the plastics used.

5 conclusion and outlook

The process of spray drying is used to convert a wide range of fluid media into powder. A direct adjustment of product properties can be made using a variety of process parameters. In the present work, novel, deagglomerable submicrometer particles (d _K <5 · 10 ^-6 m) based on silicon dioxide were synthesized by means of a first - time modified Process control of spray drying, the pressure swing technology to be produced. The powders obtained in this way are characterized by a specific shaping of the particles, their structural and textural structure as well as physicochemical properties. By dividing this technology into two pressure ranges, it is possible to directly influence the process of particle formation. In the first part, a high-pressure unit, the low-viscosity solutions, dispersions or suspensions under high pressure (P ≥ 10 ⁷ Pa) in a second part, a low-pressure-controlled drying unit (P ≤ 10 ² Pa) transferred. The transition between the two components takes place by means of a nozzle. The size distribution of the droplet collective (spray) formed in this case is a function of the nozzle outlet pressure and type. These defined droplet size distributions are the basis of limiting the particle diameter. Regardless of the sprayed fluid, the particles formed can only have this maximum size. After the formation of the drop collective, the rheological properties of the system determine the further particle formation process. Drying times (τ _Tr ≤ 0.1 s) and droplet radii (r _Tr <5 · 10 ^-5 m) defined by the system set the focus of the particle formation process on the diffusion rate of the particles responsible for solid phase formation within the so-called Péclet number (Pe) as well as the viscosity development of the considered drop in the concentration process. These system-specific properties determine whether solid bodies or hollow bodies are formed. In particular, the boundary conditions of the viscoelastic region, as a point with the significant change between viscous and elastic rheological properties of a fluid, are a significant factor influencing the particle formation process.

Within the scope of this work it could be shown with the example of colloidal silica dispersions (silica sols) that modifications in the area of stabilization have a direct influence on the characteristics of solid phase formation (gel formation) in the spray drying process. Depending on the type of stabilization, the viscoelastic region of the system could be supplemented with an intermediate stage of agglomeration formation (unstabilized) or shifted to greater concentration (alkaline). This was followed by an abrupt solid phase formation in the alkaline variant. This modification of the rheology of a system has a direct influence on the particle shape, the physical-chemical powder properties and the probability of particle defects. In this context, by means of a targeted modification of the viscosity properties of a system, intended particle properties in the drying process can be preset by intervention in the particle formation process. An additional advantage of the applied spray-drying technology is the alternative solution of gases under high pressure into the medium to be sprayed. Here, a distinction is made between inert and interacting with the system gases. With the release of inert gases under high pressure into the fluid gas-dynamic effects occur in the following spray formation, which significantly influence the process of particle formation. Using the example of silica dispersion, the macroscopic consequences of exposure to nitrogen include a reduction in particle defects and a change in the particle size distribution of the resulting powders. On the basis of a reduced speed of sound in gas-liquid mixtures (with respect to the pure media) as a result of the gas dynamics, defect-causing pressure differences in the droplet can be compensated faster. This counteracts the basic mechanisms of particle defect formation. Shifts of the particle size distributions to smaller values can be understood by an optimized drying process of the drop collective. Additional formed phase interfaces of the releasing gas promote the transition of liquid molecules into the gas phase. Both effects, reduction of particle defects and a change in the particle size distribution of the powders obtained, can be discussed in inert gases as independent of the sprayed fluid. Only the solubility of these gases in these media is characterized as a process engineering factor. A redemption of reactive gases, however, represents a substance-specific parameter, since this actively changes the rheological properties of the medium. Accordingly, they must be considered individually for each fluid. Solvent-induced, secondary dropper decay could not be observed. Based on the Péclet numbers (Pe> 40) used in this work, the produced xerogels (silica gels) consist of spherical particles with hollow body geometry. Descriptive properties of the obtained xerogels are their low humidities (<2.5% (m: m)) as confirmation of successful drying, unusually high bulk densities (> 5 · 10 ² g L ^-1 ) and good flow and casting behavior in comparison with Reference materials based on silica. The particle size distributions of these powders are a function of solids content and colloid diameter of the starting materials, spray pressure as a description of droplet diameter and drying temperature. These parameters are most likely to be considered to be material independent for similar fluid systems in terms of pressure swing technology. A peculiarity of the silica gels produced in this work is the transfer of structural features by the starting materials used. Those used educts allow the targeted adjustment of textural (interparticle space, BET (N ₂ , 77K) surface) and physico-chemical properties (pH, particle size distribution). Surface morphological analyzes show that the nanoscale colloidal building blocks (sol particles) are characteristic as a structural feature of the particle shell and define their properties. The surface roughness can thus be considered as a function of colloid diameter. This parameter is independent of the technical influencing factors considered in this work. For future manufactured powders, with comparable particle geometry of spray dried colloidal sols in combination with pressure swing technology, analogous results in terms of textural and physicochemical properties are expected. With the help of the use of additives, a direct manipulation of the drying and particle formation process is possible. These may relate to geometric and aerodynamic properties of the powders as well as strong deviations from the spherical particle shape. Modifications of surface tension and interfacial effects cause a change in the phase transition of fluid molecules into the gas phase. This led in the products of this work to particles with hollow body geometry whose shell structure is significantly disturbed (eg gas inclusions). A reduction in mechanical stability is observable in this context. The pressure swing technology is characterized by low drying times in which the particles in the sprayed fluid have little time for spatial alignment and formations of (covalent or interparticular) bonds. This unselective structure of the particle structure from the monomers was demonstrated by redispersibility studies. A decay of the particles due to mechanical stress can be achieved even at low pressures. Complete deagglomeration up to the nanoscalar colloidal building blocks is possible with ultrasound in different fluid matrices. Application tests confirm the tendencies of the particles to deagglomerate as well as the macroscopic effects on the physico-chemical properties resulting from the powders.

As the results of this work show, a transfer of attributes of the colloidal starting materials to the xerogels to be generated is achievable. By means of a suitable selection of the educt, the pressure swing technology can directly influence desired product properties such as bulk density, particle formation and surface morphology under variable boundary conditions. By varying the concentration, the geometric parameters of these xerogels can be specifically defined. Additional nitrogen loading reduces the likelihood of particle defects and particle size distribution. In subsequent investigations, the transfer of the results of this work to a wide range of fluid systems may be the focus of scientific considerations. Especially in areas where accessibility to submicroscale particles (d _P <5 · 10 ^-5 m) could not be achieved or where only powders from aggregated forms are known. Because nanoscale particle-dependent properties are surface-dependent, the focus of scientific interest is on the formation of hollow spheres to minimize the formation of ineffective bulk material. In the medical sector, studies can be made to use this cavity of the bioaccumulative carrier system to encapsulate sensitive substances (drugs). In conjunction with the choice of the drying temperature parameters such as the geometric diameter and the density ratio can be varied while maintaining the aerodynamic diameter. This is a significant medical science effect. Thus, with the same aerodynamic behavior, differentiated properties, such as, for example, particle diameter and decay behavior of the encapsulated active substance, can be adapted to the properties of the human target milieu. Also of interest for such hollow body geometries are materials whose essential effects are characterized by the surface properties, but these are not yet accessible in the submicrometer range. As an example, the syntheses of hollow sphere pigments and core shell particles are mentioned here. Based on the experience gained from the dispersing experiments, the powders produced are suitable as functional fillers for hydration (calcium silicate hydrate phases) of high-strength concretes. Added as a microscale component, scientific investigations of these powders can discuss whether nanoscalar reactive building blocks - silicate nuclei - are formed and act under the mechanical stresses of the mixture. Another field of application can be developed with the area of paint and plastic applications. The demonstrated adjustability of the micro- and macroscopic properties as well as the accessibility of the nanoscale building blocks offers an outstanding scientific challenge for the optimization of these materials.

Those innovative powders presented in this work are the basis for the development of new scientific fields of work and questions. They provide starting points for combining the nanocosm with the micro- and macroscale space.

bibliography

• [1] M. Kind, Chem. Eng. Proc. Of. 1999, 38, 405-410.
• [2] E. Matijevic, Chem. Mater. 1993, 5 (4), 412-426.
• [3] Committee on Materials with Submicron-Sized Microstructures, Commission on Engineering and Technical Systems, National Research Council, National Academy of Science, Research Opportunities for Materials With Ultrafine Microstructures, 1st Ed., National Academies Press, Washington, D.C., 1989.
• [4] CNR Rao, SRC Vivekchand, K. Biswasa, A. Govindaraj, Dalton Trans. 2007, 34, 3728-3749.
• [5] J.-H. Smått, FM Sayler, AJ Grano, MG Bakker, Advanced Engineering Materials 2012, 14 (12), 1059-1073.
• [6] MR Israel, RD dos Anjos, R. Salomao, VC Pandolfelli, Refractories Applications and News 2006, 11 (4), 16-20.
• [7] U. Kühner, O. Graalmann, GAK - Rubber Fibers Plastics 2008, 61 (9), 557-560.
• [8th] M. Schedel, production and characterization of antibacterial bone substitute material based on sol-gel, diploma thesis, Nuremberg 2008.
• [9] I. Gonda, Physico-chemical Principles in Aerosol Delivery, In Topics in Pharmaceutical Sciences 1991, Crommelin, DJ and KK Midha, Eds., Medpharm Scientific Publishers, Stuttgart, 1992, 95-117.
• [10] JH Bell, PS Hartley, JSG Cox, J. Pharm. Sci. 1971, 60 (10), 1559-1564.
• [11] P. Begat, DAV Morton, J. Shur, P. Kippax, JN Staniforth, R. Price, J. Pharm. Sci. 2009, 98 (8), 2770-2783.
• [12] M. Vidgren, A. Kärkkäinen, P. Karjalainen, P. Paronen, J. Nuutinen, Int. J. Pharm. 1988, 42 (1-3), 211-216.
• [13] P. Chattopadhyay, RB Gupta, Ind. Eng. Chem. Res. 2001, 40 (16), 3530-3539.
• [14] KS Finnie, DA Jacques, MJ McGann, MG Blackford, CJ Barbe, J. Mat. Chem. 2006, 16, 4494-4498.
• [15] J. Choi, AA Burns, RM Williams, Z. Zhou, A. Flesken-Nikitin, WR Zipfel, U. Wiesner, AY Nikitin, J. Biomed. Opt. 2007, 12 (6), 064007- (1-11).
• [16] JS Patton, CS Fishburn, JG Weeres, Proc Am Thorac Soc 2004, 1 (4), 338-344.
• [17] AR Clark, Aero. Sci. Techn. 1995, 22 (4), 374-391.
• [18] KP Lee, DP Kelly, Fund. App. Tox. 1992, 19 (3), 399-410.
• [19] DJ Belton, O. Deschaume, CC Perry, FEBS Journal 2012, 279 (10), 1710-1720.
• [20] X. Li, L. Zhang, X. Dong, J. Liang, J. Shi, Microporous and Mesoporous Mat. 2007, 102 (1-3), 151-158.
• [21] C. Roux, F. Chai, N. Ollivier, A. Ochsenbein, S. Winter, O. Melnyk, H. Hildebrand, Biomol. Closely. 2007, 24 (5), 549-554.
• [22] C. Barbé, J. Bartlett, L. Kong, K. Finnie, HQ Lin, M. Larkin, S. Calleja, A. Bush, Adv. Mat. 2004, 16 (21), 1959-1966.
• [23] Tsapis, D. Bennett, B. Jackson, DA Weitz, DA Edwards, Proc. Natl. Acad. Sci. 2002, 99 (19), 12001-12005.
• [24] K. Hadinoto, P. Phanapavudhikul, Z. Kewu, RBH Tan, Ind. Eng. Chem. Res. 2006, 45 (10), 3697-3706.
• [25] DA Edwards, AlChE Journal. 2002, 48 (1), 2-6.
• [26] LA Dailey, E. Kleemann, M. Wittmar, T. Gessler, T. Schmehl, C. Roberts, W. Seeger, T. Kissel, Pharm. Res. 2003, 20 (12), 2011-2020.
• [27] M. Brown, Drug Dis. Yes. 2003, 8 (17), 767-768.
• [28] D. Bechet, P. Couleaud, C. Frochot, M.-L. Viriot, F. Guillemin, M. Barberi-Heyob Trends in Biotechnology 2008, 26 (11), 612-621.
• [29] I. Roy, TY Ohulchanskyy, HE Pudavar, EJ Bergey, AR Oseroff, J. Morgan "TJ Dougherty, PN Prasad, J. Am. Chem. Soc. 2003, 125 (26), 7860-7865.
• [30] A. Grenha, B. Seijo, C. Remunan-Lopez, Eur. J. Pharm. Sci. 2005, 25 (4-5), 427-437.
• [31] AL Adjei, PK Gupta, (Eds.), Inhalation delivery of therapeutic peptides and proteins, Marcel Dekker Inc., New York, 1997.
• [32] JOH Sham, Y. Zhang, WH Finlay, WH Roa, R. Lobenberg, Int. J. Pharm. 2004, 269 (2), 457-467.
• [33] DA Edwards, J. Hanes, G. Caponetti, J. Hrkach, A. Ben-Jebria, ML Eskew, J. Mintzes, D. Deaver, N. Lotan, R. Langer, Science 1997, 276 (5320), 1868- 1,871th
• [34] J. Heyder, J. Gebhart, G. Rudolf, CF Schiller, W. Stahlhofen, J. Aerosol Sci. 1986, 17 (5), 811-825.
• [35] WC Hinds, Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles: Properties, Behavior and Measurement of Airborne Particles, 2nd ed., John Wiley & Sons Inc., New York, 1999.
• [36] K. Enk, Pharmaceutical Newspaper. 2004, 149 (27), 16-21.
• [37] S. White, DB Bennett, S. Cheu, PW Conley, DB Guzek, S. Gray, J. Howard, R. Malcolmson, JM Parker, P. Roberts, N. Sadrzadeh, JD Schumacher S. Seshadri, GW Sluggett, CL Stevenson, NJ Harper, Diabet. Techn. Ther. 2005, 7 (6), 896-906.
• [38] S. Zimontkowski, Spray Drying of Proteins - Investigations of Factors Affecting Particle Morphology and Improvement of Dispersibility by Particle Design, Dissertation, Bonn, 2006.
• [39] M. Schmidt, E. Fehling, C. Glotzbach, S. Frohlich, S. Piotrowski, Ultra High Performance Concrete and Nanotechnology in Construction, 1st ed., Kassel University Press, Kassel, 2012.
• [40] B. Friede, P. Fidjestol, Chemistry News 2011, 59 (10), 956-958.
• [41] DM Smith, A. Maskara, U. Boes, J. Non-Crystalline Solids. 1998, 225 (1), 254-259.
• [42] E. Riedel, C. Janiak, TM Klapotke, H.-J. Meyer, Modern Inorganic Chemistry, 2nd ed., Walter de Gruyter GmbH, Berlin, 2003.
• [43] L. Böttcher, cfi / Ber. DKG 2009, 86 (6), 37-40.
• [44] ML Nielsen, PM Hamilton and RJ Walsh, Ultrafine Metal Oxides by Decomposition of Salts in a Flame, In Ultrafine Particles, H. Lamprey, WE Kuhn, C. Sheer, VCH Wiley Publishing, London, 1964.
• [45] GL Brass, S.-C. Zhang, GV Jayanthi, J. Am. Ceram. Soc. 1993, 76 (11), 2707-2726.
• [46] WJ Walker, JS Reed, J. Am. Chem. Soc. 1999, 82 (7), 1711-1719
• [47] DE Walton, CJ Mumford, Trans. Int. Chem. E. 1999, 77 (A), 21-38.
• [48] Y. Xiong, TT Kodas, J. Aerosol Sci. 1993, 24 (7), 893-908.
• [49] S. Gablenz, spray drying and spray hydrolysis - new methods for the preparation of metal oxides and for the modification of BaTiO3, Dissertation, Halle-Wittenberg, 2001.
• [50] CA La. Mont, Preserving Eggs, patent US 51263 , 28.11.1865.
• [51] SR Percy, Improvement in Drying and Concentrating Liquid Substances by Atomizing, Patent US 125406 , 09.04.1872.
• [52] R. Stauf, Method of desiccating Blood, Patent D 666711, 29.01.1901.
• [53] LC Menting, B. Hoogstad, J. Food Sci. 1967, 32 (1), 87-90.
• [54] CJ King, Proc. Techn. Proceedings 1987, 5, 5-6.
• [55] AC Patsvas, Chem. Eng. Progress 1963, 59 (4), 65-70.
• [56] K. Masters, Spray Drying Handbook, 4th ed., Halsted Press, New York, 1985.
• [57] WR Marshall, Ind. Eng. Chem. 1953, 45 (1), 47-54.
• [58] H. Kamiya, M. Naito, T. Hotta, K. Isomura, J. Tzubaki, K. Nematsu, J. Am. Ceram. Soc. Bull. 1997, 76 (10), 79-82.
• [59] G. Pravdic, MSJ Gani, J. Mater. Sci. 1996, 31 (13), 3487-3495.
• [60] SJ Lukasiewicz, Eng. Mater. Handbook 1991, 4, 100-108.
• [61] HAC Thijssen, Torrebulletfies Leurs Deriv. 1970, 4, 108-117.
• [62] DH Charlesworth, WR Marshall Jr., AlChE Journal. 1960, 6 (1), 9-23.
• [63] EJ Crosby, WR Marshall, Chem. Eng. Prog. 1958, 54 (7), 56-63.
• [64] G. Wozniak, Atomization Technology: Principles, Procedures, Devices, 1st ed., Axel Springer-Verlag, Heidelberg, 2003.
• [65] W. Wang, KH Ngan, J. Gong, P. Angeli, Colloids and Surfaces A: Physiochem. Closely. Aspects 2009, 334 (1-3), 197-202.
• [66] OE Yildririm, OA Basaran, J. Non-Newtonian Fluids Mech. 2006, 136, 17-37.
• [67] VDI-Gesellschaft Process Engineering and Chemical Engineering, VDI-Wärmeatlas, 10th ed., Axel Springer-Verlag, Berlin Heidelberg, 2006.
• [68] K. Gersten, H. Herwig, Fluid Mechanics: Principles of Momentum, Heat and Mass Transfer from an Asymptotic Perspective, 1st ed., Vieweg Verlagsgesellschaft, Braunschweig, 1992.
• [69] J. Zierep, K. Büchler. Fundamentals of Fluid Mechanics: Fundamentals, Statics and Dynamics of Fluids, 1st ed., Vieweg + Teubner Verlag, Berlin, 2010.
• [70] SH Song, SY Lee, Atomization and Sprays. 1996, 6, 193-209.
• [71] A. Glathe, G. Wozniak, T. Richter, Atomization and Sprays. 2001, 11 (1), 21-33.
• [72] P. Walzel, Chem. Ing. Tech. 1990, 62 (12), 983-994.
• [73] W. Eisenmenger, Acoustica 1959, 9 (2), 327-340.
• [74] J. Gimbun, TG Chuah, TSY Choong, A. Fakhru'l-Razi, Aerosol Sci. 2005, 36 (8), 1056-1065.
• [75] JE Elversson, A. Millqvist-Fureby, G. Alderborn, U. Elofsson, J. Pharm. Sci. 2003, 92 (4), 900-910.
• [76] K. Bechtold-Peters, Production of Finest Particles for Biopharmaceutical Agents for the Pulmonary Administration of Medicinal Products, Dissertation, Heidelberg, 2001.
• [77] WJ Walter, JS Reed, SK Verma, Ceram. Closely. Sci. Proc. 1993, 14 (11-12), 58-79.
• [78] PW Atkins, Physical Chemistry, 2nd ed., VCH Verlagsgesellschaft mbH, Weinheim, 1990.
• [79] J. Plateau, Statique expérimentale et théorique des liquides soumis aux seules forces moléculaires. University Library Ghent. 1,873th
• [80] L. Rayleigh, Proc. London Math. Soc. 1878, 10, 4-13.
• [81] AM Sterling, CA Sleicher, J. Fluid Mech. 1975, 68 (3), 477-495.
• [82] AH Lefebvre, Atomization and Sprays, 1st ed., Hemisphere Publishing Corporation, New York, 1989.
• [83] H. Kim, Physica B 2007, 389, 377-379.
• [84] SP Lin, RD Reitz, Ann. Rev. Fluid Mech. 1998, 30 (1), 85-105.
• [85] S. Leroux, C. Dumonchel, M. Ledoux, Atomization and Sprays 1996, 6, 479-500.
• [86] K. Masters, Spray Drying Handbook, 5th Ed., VCH Wiley Publishing, New York, 1991.
• [87] N. Dombrowski, PC Hooper, Chem. Eng. Sci. 1962, 17 (4), 291-305.
• [88] CZ Weber, Math Mech. 1931, 11, 136-154.
• [89] M. Doumas, R. Laster, Chem. Eng. Prog. 1953, 49 (9), 518-526.
• [90] N. Dombrowski, DL Wolfsohn, Trans. Instn. Chem. Engrs. 1972, 50, 259-269.
• [91] N. Dombrowski, MA Tahir, J. Inst. Fuel. 1977, 1, 63-81.
• [92] P. Walzel, Ger. Chem. Eng. 1984, 7, 1-12.
• [93] L. Rayleigh, Phil. Mag. Ser. 1882, 5 (14), 184-186.
• [94] RD Reitz, FV Bracco, Phys Fluids. 1982, 25 (2), 1730-1741.
• [95] RP Grant, S. Middlemann, AlChE Journal. 1966, 12 (4), 669-678.
• [96] JC Chu, EL Stout, RM Busche, Chem. Eng. Prog. 1951, 47 (1), 29-38.
• [97] KB Prinn, HR Costantino, M. Tracy, AAPS Pharm. Sci. Tech. 2002, 3 (1), E4.
• [98] I. Zbicinski, C. Strumillo, A. Delag, Drying Technology 2002, 20 (9), 1751-1768.
• [99] H. Schlichting, K. Gersten, Grenzschicht-Theorie, 10th ed., Axel Springer-Verlag, Berlin, 2006.
• [100] W. Nusselt, Journal of Applied Mathematics and Mechanics 1930, 10 (2), 105-206.
• [101] WE Ranz, WR Marshall, Chem. Eng. Progr. 1952, 48, Part 16-141; Part II 80-173.
• [102] S. Nukiyama, Y. Tanasawa, Trans. Soc. Mech. Eng. Jpn. 1939, 5, 68-229.
• [103] GE Lorenzetto, AH Lefebvre, AIAA Journal 1977, 15 (7), 1006-1010.
• [104] KH Leong, J. Aerosol Sci. 1987, 18 (5), 511-524.
• [105] GV, Jayanthi, SC Zhang, GL Brass, Aerosol Sci. Techn. 1993, 19 (4), 478-490.
• [106] N. Abuaf, FW Staub, Drying 1986, 86 (1), 277-284.
• [107] JA Duffie, WR Marshall, Chem. Eng. Prog. 1953, 49 (8), 417-423.
• [108] JA Duffie, WR Marshall, Chem. Eng. Prog. 1953, 49 (9), 480-486.
• [109] JS Reed, Introduction to the Principles of Ceramic Processing, 2nd Ed., Wiley-Interscience, New York, 1995.
• [110] SK Friedlander, Smoke, Dust, and Haze: Fundamentals of Aerosol Dynamics, 2nd ed., Oxford University Press, Oxford, 2000.
• [111] A. Einstein, Annalen der Physik 1905, 322 (8), 549-560
• [112] M. Smulochski, Annalen der Physik 1906, 326 (14), 756-780
• [113] KJ Konsztowicz, E. Vargha-Butler, T. Maksym, HW King, Ceram. Trans. 1991, 22, 363-369.
• [114] KJ Konsztowicz, G. Maksym, T. Makysm, HW King, WF Caley, E. Vargha-Butler, Ceramic Transactions 1992, 26, 46-53.
• [115] KJ Konsztowicz, W.F. Caley, E. Vargha-Butler, Ceram. Trans. 1991, 22, 235-240.
• [116] J. Dlouhy, WH Gauvin, AlChE J. 1960, 6 (1), 29-34.
• [117] JA Buckham, RW Moulton, Chem. Eng. Prog. 1955, 51 (3), 126-133.
• [118] WS Cheow, S. Li, K. Hadinoto, Chem. Eng. Res. Des. 2010, 88 (5-6), 673-685.
• [119] AD Stroock, SKW Dertinger, A. Ajdari, I. Mezic, HA Stone, GM Whitesides, Science 2002, 295 (5555), 647-651.
• [120] IW Lenggoro, T. Hata, F. Iskandar, MM Lunden, K. Okuyama, J. Mater. Res. 2000, 15 (3), 733-743.
• [121] SJ Lukasiewicz, J. Am. Ceram. Soc. 1989, 72 (4), 617-624.
• [122] P. Pieranski, Phys. Rev. Lett. 1980, 45, 569-572.
• [123] H.-W. Lee, H. Song, I.-S. Suk, S.-R. Oh, S.-G. Choi, Ceram. Trans. 1995, 54, 41-50.
• [124] H. Takahashi, N. Shinohara, M. Okumiya, K. Uematsu, T. Junichiro, Y. Iwamoto, H. Kamiya, J. Am. Ceram. Soc. 1995, 78 (4), 903-908.
• [125] FS Shaw, Am. Chem. Soc. Bull. 1990, 69 (9), 1484-1489.
• [126] OD Velev, K. Furusawa, K. Nagayama, Langmuir. 1996, 12 (10), 2374-2384.
• [127] OD Velev, K. Furusawa, K. Nagayama, Langmuir. 1996, 12 (10), 2385-2391.
• [128] OD Velev, K. Furusawa, K. Nagayama, Langmuir. 1997, 13 (6), 1856-1859.
• [129] N. Tsapis, ER Dufresne, SS Sinha, CS Riera, JW Hutchinson, L. Mahadevan, DA Weitz, Physical review letters 2005, 94 (1), Art.-No. 018,302th
• [130] WR Marshall, E. Seltzer, Chem. Eng. Prog. 1950, 46 (10), 501-508 and 46 (11), 575-584.
• [131] YF Maa, HR Costantino, P.-A. Nguyen, CC Hsu, Pharm. Dev. Technol. 1997, 2 (3), 213-223.
• [132] HH Ali, CJ Mumford, GV Jeffreys, GS Bains, eds., Drying '89, Proceedings from 6th International Drying Symposium, Versailles, 1988, 378-385.
• [133] CG Greenwald, CJ King, AlCh.E. Symp. Ser. 1982, 78 (218), 101-110.
• [134] L. Huang, K. Kumar, AS Mujumdar, Drying Techn. 2003, 21 (3), 385-412.
• [135] JA Balmer, A. Schmid, SP Armes, J. Mater. Chem. 2008, 18, 5722-5730.
• [136] T. Ishimori, M. Senna, J. Mater. Sci. 1995, 30 (2), 488-495.
• [137] A. Maskara, DM Smith, J. Am. Ceram. Soc. 1997, 80 (7), 1715-1722.
• [138] N. Namiki, Y. Otani, CM Yun, H. Emi, T. Yamada, Kagaku Kogaku Ronbunshu 2000, 26, 100-104.
• [139] F. Iskandar, IW Lenggoro, TO Kim, N. Nakao, M. Shimada, K. Okuyama, J. Chem. Eng. Japan. 2001, 34 (10), 1285-1292.
• [140] F. Iskandar, IW Lenggoro, B. Xia, K. Okuyama, J. Nanoparticle Res. 2001, 3 (4), 263-270
• [141] H. Kamiya, H. Suzuki, D. Kato, G. Jimbo, J. Am. Ceram. Soc. 1993, 76 (1), 54-64.
• [142] Y. Iwamoto, H. Nomura, I. Sugiura, J. Tsubaki, H. Takahashi, K. Ishikawa, N. Shinohara, M. Okumiya, T. Yamada, H. Kamiya, K. Uematsu, J. Mater. Sci. 1994, 9 (5), 1208-1213.
• [143] A. Hemati, R. Cherif, K. Saleh, V. Pont, Powder Technol. 2003, 130 (1-3), 18-34.
• [144] K. Saleh, R. Cherif, M. Hemati, Adv. Powder Technol. 1999, 10 (3), 255-277.
• [145] N. Baccile, D. Grosso, C. Sanchez, J. Mater. Chem. 2003, 13, 3011-3016.
• [146] N. Balagopal, KGK Warrier, AD Damodaran, KS Krishnan, J. Eur. Ceram. Soc. 1993, 12 (6), 449-453.
• [147] L. Gradoń, S. Janeczko, M. Abdullah, F. Iskandar, K. Okuyama, AlChE. J. 2004, 50 (10), 2583-2593.
• [148] K. Nogami, O. Sakurai, N. Mizutani, M. Kato, Yogyo Kyokaishi. 1987, 95 (7), 682-685.
• [149] R. Vanbever, JD Mintzes, J. Wang, J. Nice, D. Chef, R. Batycky, R. Langer, DA Edwards, Pharm. Res. 1999, 16 (11), 1735-1742.
• [150] GW Scherer, J. Am. Ceram. Soc. 1990, 73 (1), 3-14.
• [151] H. Lang, K. Shinohara, H. Minoshima, K. Matsushima, Chem. Eng. Sci. 2001, 56 (6), 2205-2213.
• [152] S. Eckhard, M. Fries, K. Lenzner, M. Nebelung, Chemie Ingenieur Technik 2012, 84 (3), 335-342.
• [153] L. Pauchard, C. Allain, Europhys. Lett. 2004, 66, 667-673
• [154] L. Pauchard, Y. Couder, Europhys. Lett. 2003, 62, 897-903
• [155] N. Tsapis, ER Dufresne, SS Sinha, CS Riera, JW Hutchinson, L. Mahadevan, DA Weitz, Phys. Review. Lett. 2005, 94 (1), 018302
• [156] J. Bahadur, D. Sen, S. Mazumder, B. Paul, H. Bhatt, SG Sing, Langmuir 2012, 28 (3), 1914-1923.
• [157] JA Owczarek, Fundamentals of Gas Dynamics, 3rd ed., McGraw-Hill, New York, 1971.
• [158] Brauer, H. Basics of single-phase and multi-phase flows. 1st edition, Sauerländer Verlag, Frankfurt am Main, 1971.
• [159] C. Crowe, Y. Tsuji, M. Sommerfeld, Multiphase Flows with Droplets and Particles, 1st Ed., CRC Press, 1998.
• [160] AA Shraiber, Atomization and Sprays 1996, 6, 667-692.
• [161] R. Wiebe, VL Gaddy, C. Heins Jr., J. Am. Chem. Soc. 1933, 55 (3), 975-978.
• [162] R. Wiebe, VL Gaddy, C. Heins Jr., Ind. Eng. Chem. 1932, 24 (8), 927-933.
• [163] RP Pollack, GL Kennan, J. Chem. Phys. 1990, 93 (4), 2724-2735.
• [164] HA Pray, CE Schweickert, BH Minnich, Ind. Eng. Chem. 1952, 44 (5), 1146-1151.
• [165] TR Radich, R. Battino, E. Wilhelm, J. Solution Chem. 1984, 13 (5), 335-348.
• [166] Y.-K. Li, LX Nghiem, The Canadian J. Chem. Eng. 1986, 64 (3), 486-496.
• [167] R. Sun, W. Hu, Z. Duan, J. Solution Chem. 2001, 30 (6), 561-573.
• [168] A. Chapoy, AH Mohammadi, B. Tohidi, D. Richon, J. Chem. Eng. Data 2004, 49 (4), 1110-1115.
• [169] P. Harting, The influence of pressure, temperature and salinity on the solubility isotope effects of methane and nitrogen in water, Dissertation, Leipzig, 1977.
• [170] TD O'Sullivan, NO Smith, J. Phys. Chem. 1970, 74 (7), 1460-1466.
• [171] S. Mao, Z. A Duan, Fluid Phase Equilibria 2006, 248 (2), 103-114.
• [172] Y. Lang, N. Hilal, P. Langston, V. Starov, Adv. Colloid and Interface Sci. 2007, 134-135, 151-166.
• [173] XJ Cao, HZ Cummins, JF Morris, J. Colloid and Interface Sci. 2012, 368 (1), 86-96.
• [174] KR Harris, LA Woolf, J. Chem. Eng. Data. 2004, 49, 1064-1069.
• [175] Test Report, Rheological Characterization of Silica Samples, Anton Paar Germany GmbH, Ostfildern, 2012.
• [176] P. Gallagher, C. Conlee, K. Rollins, J. Geotechn. Geoenvironm. Closely. 2007, 133 (2), 186-196.
• [177] FJ Rubin-Hernández, MF Ayúcar-Rubin, JF Velázquez-Navarro, FJ Galindo-Rosales, J. Colloid and Interface Sci. 2006, 298 (2), 967-972.
• [178] T. Okubo, J. Colloid and Interface Sci. 1995, 171 (1), 55-62.
• [179] RK Iler, The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica, 1st ed., John Wiley & Sons Inc., Hoboken, 1979.
• [180] I. Blood, RJ Pugh, J. v. Pas, I. Callaghan, Colloids and Surfaces A: Physicochem. and Eng. Aspects. 2009, 337 (1-3), 127-135.
• [181] http://www.ruhr-uni-bochum.de/thermo/Software/Seiten/Fluidcal.htm, retrieved on 15.09.2010
• [182] C. Tegeler, R. Span, W. Wagner, J. Phys. Chem. Ref. Data 1999, 28, 779-850.
• [183] R. Span, EW Lemmon, RT Jacobsen, W. Wagner, A. Yokozeki, J. Phys. Chem. Ref. Data 2000, 29, 1361-1433.
• [184] R. Span, W. Wagner, J. Phys. Chem. Ref. Data 1996, 25, 1509-1596.
• [185] RD McCarty, VD Arp, Adv. Cryo. Closely. 1990, 35, 1465-1475.
• [186] W. Wagner, A. Pruss, J. Phys. Chem. Ref. Data 2002, 31, 387-535.
• [187] Instruction Manual 70351165.02 / 2009, Design and Operation Dust Collector SFK-02/03/11 FL 4, MAHLE Filtersysteme GmbH, Öhringen, 2009.
• [188] GW Sears, Am. Chem. Soc. 1956, 28 (12), 1981-1983.
• [189] G. Mie, Annalen der Physik, Fourth Series 1908, 25 (3), 377-445.
• [190] HC vd Hulst, Light scattering by small particles, 1st ed., Dover Publications, New York, 1981.
• [191] A. Braun, K. Franks, V. Kestens, G. Roebben, A. Lamberty, T. Linsinger, Certification of Equivalent Spherical Diameters of Silica Nanoparticles in Water, European Commission, EUR 24620 EN - 2011, Geel, 2011.
• [192] LT Zhuravlev, Colloids and Surfaces A: Physiochem. Closely. Aspects 2000, 173 (1-3), 1-38.
• [193] W. Egli, H. Seidel, S. Frommelt, C. Giesbrecht, Translucent Heat Protection Element, Patent WO 94/04355 , 03.03.1994.
• [194] Behnisch, protocol for microwave experiments at Fricke and Mallah in Peine, Bad Köstritz, 2009.
• [195] Technical Information 4/01, Schlick Circular Fog Nozzles Model Series 121-123, Düsen-Schlick GmbH, Untersiemau / Coburg, 2001.
• [196] Committee for Hazardous Substances, Federal Institute for Occupational Safety and Health, Federal Worksheet 11/2002.
• [197] JH d. Boer, MEA Hermans, JM Vleeskens, Proc. Koninkl. Nederl. Akad. Wetensch. 1957, B60 (45), 54.
• [198] JH d. Boer, JM Vleeskens, Proc. Koninkl. Nederl. Akad. Wetensch. 1958, B61 (2), 85.
• [199] JH d. Boer, Angew. Chem. 1958, 70 (13), 383-389.
• [200] C. Okkerse, Physical and Chemical Aspects of Adsorbents and Catalysis, BG Lens (Ed.), Academic Press, London, 1970.
• [201] VM Bermudez, J. Phys. Chem. 1971, 75 (21), 3249-3257.
• [202] JAG Taylor, JA Hockey, J. Phys. Chem. 1966, 70 (7), 2169-2172.
• [203] JAG Taylor, JA Hockey, BA Pethica, Proc. Brit. Ceram. Soc. 1965, 5, 133-141.
• [204] GJ Young, J. Colloid Sci. 1958, 13 (1), 67-85.
• [205] G. J Young, TP Bursh. J. Colloid Sci. 1960, 15 (4), 361-369.
• [206] M. Baverez, J. Bastick, Bull Soc. Chim. Fr. 1964, 60, 3226.
• [207] JJ Fripiat, J. Uytterhoeven, J. Phys. Chem. 1962, 66 (5), 800-805.
• [208] MM Egorov, WI Kvlividze, VF Kiselev, KG Krassilnikov, ZZ Polym. 1966, 212, 126-138.
• [209] LA Ignatyeva, VI Kvlividze, VF Kiselev, In Bound Water in Dispersed Systems, 1st Ed., Moscow State University Press, Moscow, 1970.
• [210] JA Dean, Lange's Handbook of Chemistry; 14th ed., McGraw Hill, New York, 1992.
• [211] Entry to CAS no. 107-21-1, [GESTIS Substance Database], Institute for Occupational Safety of the German Social Accident Insurance, available on 04.01.2013.
• [212] N. Hantzsch, Breaking new ground: novel additives based on highly functionalized nanoparticles, European Coatings Congress, Nuremberg 2013.
• [213] J. Haigh, Hollow sphere pigments - Beyond decorative paint, European Coatings Congress, Nuremberg 2013.
• [214] M. Heitmann, Experimental Protocol: Determination of Properties Precursor Redispersible Nanopowders with Limited Surface Area by CWK, 2012, No. 6236/203505, Worlee Chemie GmbH.
• [215] M. Heitmann, Experimental Protocol: Testing Nanogel "small BET" in exchange for filler Heladol 30, 2012, No. 6298/203505, Worlee Chemie GmbH.
• [216] M. Heitmann, Experimental Protocol: Testing Nanogel "small BET" in a rust protection primer (1.5938-06) based WorléeKyd MH 38 against Microdol 1, 2012, No. 6299/203505, Worlee Chemie GmbH.
• [217] A. Beard, D. Eisenhauer, Flame Retardants Fundamentals, European Coatings Congress, Nuremberg 2013.
• [218] DIN German Institute for Standardization e. V., DIN EN ISO 2813: 2012, Determination of reflectometer value of coatings (except metallic coatings) below 20 °, 60 ° and 85 °, Beuth Verlag GmbH, Berlin, 2012.
• [219] G. Kalinna, J. for Surface Engineering 2012, 6, 64-66.
• [220] DIN German Institute for Standardization e. V., ISO 7724, Paints and varnishes; Color measurement, Beuth Verlag GmbH, Berlin, 1984.
• [221] Association of the German Paint Industry e. V., VDL-RL 10, Guideline Permissible color tolerances for unicolour powder coatings in architectural applications, Frankfurt am Main, 2003.
• [222] DIN German Institute for Standardization e. V., ISO 1522, Coating Materials - Pendulum damping test, Beuth Verlag GmbH, Berlin, 2006.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102008006607 B4 [0003]
• DE 102005044529 A1 [0004]
• DE 60225242 T2 [0005]

Cited non-patent literature

• BLUTE et al. [0084]

Claims (15)

Process for the preparation of nanostructurable microparticles from fluid media by spray drying, wherein low viscosity solutions, dispersions or suspensions transferred from a high pressure unit via at least one nozzle in a low pressure drying unit and the droplet collective formed are dried and deposited, characterized in that high spraying pressures in the pressure range P ≥ 3 × 10 ⁶ Pa and a high medium throughput with a low mass ratio of fluid medium as the spray medium to the gas flow are selected such that a pressure jump from the high pressure unit to the low pressure controlled drying unit adjusts to the nozzle by ≥ five orders of magnitude, whereby the choice of high and low pressure areas the particle formation can be influenced. A method according to claim 1, characterized in that the size of the particles or their diameter on the pressure-related defined droplet size distributions at the nozzle outlet is adjustable. A method according to claim 1 or 2, characterized in that the medium under high pressure inert gases are added to compensate for pressure differences in the spray droplets formed, so that the particles formed have a reduced number of defects. A method according to claim 1 or 2, characterized in that the medium reactive gases are supplied to influence the rheological properties. A method according to claim 3, characterized in that the starting material under pressure nitrogen is supplied. Method according to one of the preceding claims, characterized in that for the formation of hollow spherical particles a rapid relaxation and drying in the low-pressure-controlled drying unit takes place, wherein the particles obtained are deposited via a filter arrangement. Method according to one of the preceding claims, characterized in that to ensure a targeted spray inlet temperature, the fluid medium is thermostated. Method according to one of the preceding claims, characterized in that the cross section of the at least one spray nozzle with ≤ 2 x 10 ^-3 m is selected. Method according to one of the preceding claims, characterized in that colloidal silica or silica sol is used as the fluid to be sprayed to generate nanoparticle powder, which have morphological, texturelle and physico-chemical properties of the starting material or take over. Method according to one of the preceding claims, characterized in that the solutions used suspensions or dispersions have a solids content m _s in the range of 0.0001 to 80% (m: m, ie, based on mass). Method according to one of the preceding claims, characterized in that the particle drying is supported by targeted supply of, for example, radiation. Method according to one of the preceding claims, characterized in that the particle drying is assisted by a pre-supplied, during the drying entlösendes gas. Method according to one of the preceding claims for the preparation of redispersible agglomerates. Use of particles produced by a process according to one of the preceding claims as an additive and / or filler for paints and / or plastics. Use of particles in a hollow sphere, produced by a process according to any one of claims 1 to 13 as an agent for the encapsulation of active ingredients, in particular in the medical, therapeutic and / or cosmetic field.

Images