Method for fractioning oxidic nanoparticles with cross-stream membrane filtration

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DESCRIPTION

Procedure for fractionation of oxide nanoparticles by cross-current filtration through membranes

The present invention relates to a process for the fractionation of nanoparticle dispersions of oxides by filtration through membranes. He also relates to nanoparticle oxide dispersions, which are obtained by the process according to the invention.

In recent years, interest has constantly grown in obtaining particles with a size on the nanometer scale, therefore particles with a diameter of less than 1 pm, from an academic as well as an industrial point of view, since nanoparticles, because of their properties, have great potential for certain uses, for example in electronics, optics and chemicals. Particles are of particular interest in this case, the diameter of which is in the region below 100 nm. Here, so-called nano-effects usually appear, for example quantum effects, which, among other things, can be attributed to the influence of the large surface area of ​​the particles. Furthermore, in the case of these particles, the scattering of light decreases to such an extent that an increasing transparency of nano-composite materials (from the English “nano-composites) can be observed, in which the described particles are embedded in a matrix. , frequently based on synthetic materials or varnishes, in order to improve their properties.

However, for the use of nanoparticles in composite materials (composites) it is important that the spherical particles, on the one hand, do not agglomerate and, on the other hand, are present in a narrow size distribution. Even small proportions of coarser particles or agglomerates can adversely affect the properties of composite materials. In particular, this is valid for transparency. Frequently, the nanoparticles are modified to adapt to the respective matrix, which should establish a better dispersion and consequently an avoidance of the formation of agglomerates.

The synthesis of nanoparticles can be carried out according to various procedures. Along with the synthesis in the gas phase, it is possible to work in solution, finding the use of some templates or molds (in English templates). Another possibility is to grind the coarser particles. This approach is distinguished by being cheaper than synthesis from molecular precursor compounds.

Both in the case of the synthesis of the particles from molecular precursor compounds as well as in the case of grinding, a product is always obtained, which has a certain size distribution. Whereas, in the case of particles with a size in the micrometer region, a separation of the coarser particles can be achieved through sedimentation, centrifugation or sieve filtration, these methods can only be used by a restricted mode in the case of nanoparticles. If the nanoparticles are present in a dispersion, the separation of the coarser particles can possibly also be effected by sedimentation or centrifugation, in this case, however, the extreme occupation of surface and time, as well as the realization discontinuous process so disadvantageous that these processes can hardly be important. Also, certain methods such as Size Exclusion Chromatography (SEC) or gel electrophoresis are not suitable for larger amounts.

For many uses, nanoparticles, which are composed of metal oxides, are interesting, for example, for the production of polymeric composite materials protected against UV (ultraviolet) rays or fluorescent materials (Journal of Nanoscience and Nanotechnology, 2006, 6, 409 -413). For industrial practice, it would therefore be useful to have a process that works in a continuous manner and that can be carried out in a simple way for the fractionation of oxide nanoparticles. In various ways, it has been proposed to employ a membrane filtration process for this purpose. In this case, it should be noted that the separation in a membrane is influenced by the specific interaction between the particles and the membrane.

In order to generally separate particles from suspensions according to pre-established criteria, such as, for example, the diameter of the particles, certain filtration techniques are frequently employed. In this context, dead end filtrations are generally used as a discontinuous process (batch), or cross flow filtrations. as a dynamic process. In the case of dead end filtration, the entire volume to be filtered is conducted directly through the filter medium, the deposited particles generally being able to constitute a cake which, in turn, has a decisive influence on the result of the filtration. The result of the filtration is therefore determined in this case, in addition to the properties of the filter medium, above all by the filter cake that has formed and which is modified in the course of the process. Therefore, the filtration that forms a cake cannot be used for the classification of particle dispersions. Only depth filtration, which works according to the dead end procedure, is in a position, within certain limits, to filter by classifying, by means of which the particles to be separated penetrate into the structure of the filtration medium and are separated because of the adhesion of the

ES 2 402 165 T3 particles to the inner surface of the filter medium. The restrictions of this procedure lie in the fact that only very dilute dispersions can be processed and the classifying effect has a high natural imprecision, so that also significant amounts of the target product in the filter medium remain adhered and consequently , are lost.

In the case of cross-flow filtration, the medium to be filtered is conveyed tangentially next to the filtration medium. The pore size of the filter medium determines the exclusion limit in this case. Important uses are microfiltration, ultrafiltration as well as nanofiltration.

Cross-current filtration attempts to avoid the disadvantages of dead-end filtration processes by recourse that in its case, contrary to this conventional filtration, it is circulated tangentially next to the filtration medium. The inflow stream is divided into a filter stream through the filter medium and a flood stream, which flows parallel to the filter medium. In the technique using a membrane, the current flowing through the membrane is designated as permeate. The material, which is left behind on the membrane, is designated as retained material. By this way of conducting the currents, a return transport of the retentate from the surface of the filter medium to the retentate stream is made possible. Consequently, the formation of deposits and cover layers on the filter medium is counteracted.

In the bibliographic citation Advanced Materials 2005, 17 (5), 532-535 it is described how the technique of filtration with transverse current through membranes can be used for the fractionation of metal nanoparticles. For this, a special membrane is produced, which contains channels with a size on the scale of nanometers. The tests have been carried out on the smallest laboratory scale and offer no mention of a transfer to the technical scale. Regarding the membranes required for the fractionation of oxide nanoparticles, no statement is made regarding their pore radius or composition. In addition to this, it is stated that it is not possible to classify nanoparticles with common membranes.

Cross-current filtration through membranes of a dispersion of silicon dioxide nanoparticles is described in the journal citation Journal of Membrane Science 2006, 284, 361-372. In this case, no data is provided about the possibility of performing a fractionation, but rather, in this case it is an investigation about the formation of the dynamic covering layer, formed from nanoparticles, which makes it impossible perform a fractionation.

The Bokela entity from Karlsruhe distributes (sells) a sieve filtration system (Dynofilter), through which the descending coarse grain portions can be separated from the dispersion of particles down to 10 pm with a dynamic sieve filtration. Nothing is known about the possibility of using membranes in this system.

Langmuir 1997, 13, 1820-1826 describes research on the filtration through membranes of polymeric particles with a permanent surface electrical charge. In this case, nanoparticle retention is enhanced by deliberate introduction of surface electrical charges. Nothing is declared about classification effects.

In the bibliographic citation Anal. Chem. 2006, 78, 8105-8112 describes how organic colloids can be separated from an aqueous solution by cross-current ultrafiltration. In this case, the nanoparticles are retained by more than 99% on the membrane.

The US patent application document US 2004/0067485 A1 describes the synthesis of semiconductors with a size on the scale of nanometers, constituted on the basis of zinc and cadmium, combined with the elements S, Se and Te, using a protein as a template. It is stated that the resulting complex, consisting of eg. Zinc sulfide and apoferritin, is fractionable by filtration through membranes with a dead end, the diameter of the pores of the membrane used being clearly higher than the diameter of the particles. A wide range of selection of membrane materials is mentioned, but no example is given for the filtration procedure. No sorting filtration of nanoparticles can be carried out on a technical scale with this procedure, since a filter cake would be formed. Consequently, the proposed procedure only remains feasible on the laboratory scale, where it is possible to carry out in a simple way the necessary frequent change of the filters.

International patent application document WO 2006/116798 A1 describes the production of radioactive nanoparticles made on the basis of metallic technetium, which go through a filtration process through membranes according to the dead end method. In this case, a hydrophilic membrane is used. Therefore, here too, the dead end method is used, which leads, even in the case of a small agglomeration of the nanoparticles, to an almost complete deposition of all the particles near the membrane. The procedure proposed here for the fractionation of nanoparticles is, therefore, feasible only in the case of very small concentrations of nanoparticles and on a laboratory scale.

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The German patent application document DE 199 50 496 A1 discloses a process for the production of nanoparticle concentrates, in which a dispersion of nanoparticles is subjected to ultrafiltration and in this case is separated into a permeate material and a material retained, the latter containing at least 90% of the nanoparticles used. The target product (ie desired) is in this case the retentate; and it is intended that with the permeate material the least possible amount of the nanoparticles is lost. Ultrafiltration can be operated in the cross-current mode.

In WO97 / 19745 a typical representative of a transverse rotation filtration apparatus with static filter plates and stirring discs arranged between them is described. However, it is not disclosed there that dispersions containing nanoparticles can be used in this installation, and even less that in this way a permeate containing nanoparticles can be obtained.

According to the state of the art, no method is known with the use of membranes that can be used on a technical scale, in which the fine material can be separated by classification from a dispersion of particles, which predominantly contains nanoparticles but also coarser particles. All the processes according to the state of the art form a covering layer on the surface of a membrane, in such a way that it is certainly possible to carry out a separation, but not a classification of nanoparticles from a dispersion.

The mission of the present invention consisted in providing a process for the fractionation of a dispersion of metal oxide nanoparticles, which overcomes the disadvantages of the state of the art, in particular the constitution of a covering layer on a membrane, and which therefore make possible a classification of nanoparticles also on the large technical scale.

The problem posed by this mission is solved by means of a process for the fractionation of a dispersion of nanoparticles of oxides, which is characterized in that at least one process step is a filtration step with transverse current through membranes, producing a flood of the membranes with the dispersion by means of rotating parts propelled, according to the principle of transverse rotation, and obtaining a dispersion as permeated material, wherein at least 50% of the nanoparticles have a particle diameter less than or equal to 80 nm, preferably less than or equal to 50 nm, and especially preferably less than or equal to 30 nm, and performing that the filtration stage follows at least one process stage in which grinding is carried out, and the material retained from the filtration stage is returned to the grinding stage.

Surprisingly, it was found that by means of the process according to the invention, the formation of a cover layer on the membranes is avoided and the fractionation of a dispersion of oxide nanoparticles is made possible.

The process according to the invention is carried out with a filter unit, which works according to the principle of transverse rotation (CR, acronym for cross-rotation). These are filtration machines, in which shear forces and circulating currents are generated parallel to the filtration medium by means of mobile internal constructions. These can also be the filter medium itself (eg rotating membranes). A possible embodiment of such a filtration machine is the transverse rotation filter. In contrast to cross-flow filtration, in the case of cross-rotation filtration using additional rotors, flooding is decoupled from feed and pressure build-up. In the case of a typical CR filter, the filter media and rotors are stacked on top of each other in the form of a sandwich. A shaft rotates in the center of the stack, which propels the rotors, thus achieving average flood speeds, i.e. spatially averaged over the entire filter medium, which are clearly above 6 m / s. The fed medium is stepped through the plate stack. In the sections of the plate stack, there is a stepwise increase in the concentration of the medium. By decoupling between the feed (by means of a pump) and the flooding of the membranes (by means of rotors), high specific currents of filtered materials are achieved together with low pressures in the system. These systems with rotating internal constructions have hitherto been used where very small particles or sticky substances have to be separated and increased in concentration to a high degree.

Microfiltration, ultrafiltration as well as nanofiltration are also among the processes that use membranes, driven by pressure. The term "membranes" generally refers to flattened structures, which form a barrier between two fluid phases and make possible a selective exchange of material between both sides. Accordingly, a membrane is a filtration medium, which has a defined separation limit or, respectively, which retains particles of a certain size in the presence of a propelling force (pressure). The type of the propelling force as well as the relations between the pressure and the circulation next to the membranes in conjunction with the quality of this decide on the result of the separation. The classification of synthetic membranes into different classes is carried out with the help of their structures, their states of aggregation and

ES 2 402 165 T3 electrochemical behavior. A membrane in the sense used here has pores with a pore diameter of up to 10 pm. In the case of larger pore diameters, we speak of a sieve.

As such a membrane, any commercially available membrane can be used, for example based on a polyethylene, a poly (tetrafluoroethylene), a polysulfone or a cellulose. Symmetrical membranes are preferred, that is, membranes having a constant pore diameter throughout the entire cross section. The membranes used according to the invention have a pore diameter of up to 10 pm, preferably between 0.01 and 5 pm, particularly preferably between 0.1 and 1 pm.

The average flood velocities generated in the method according to the invention are between 5 and 25 m / s, preferably at least 8 m / s, and particularly preferably at least 10 m / s. These conditions are carried out as a relative velocity between the medium to be filtered and a membrane, by means of a rotation of internal constructions (eg stirrers). The mean flood velocity is determined by a measurement of the velocity over the entire filter medium and by subsequent formation of a mean value. In this case, it is particularly advantageous that this flooding velocity is decoupled from a pressure build-up, so that the above-mentioned high cross-flow velocities are achieved in the case of small transmembrane pressures. (<1 bar). Under such conditions, a constitution of a cover layer can be almost totally prevented. In a complementary way, the formation of a covering layer can be counteracted by periodically rinsing a membrane back with a permeate or a liquid (eg water) or with a mixture of a liquid and a gas (eg . water and air under pressure).

As nanoparticles within the meaning of the process according to the invention, it is possible to use particles of any metal oxides. Metal oxides can be produced for example by flame pyrolysis, a precipitation process or through sol and gel processes. Metal oxides can also be mixed oxides that are derived from two or more different metals. Nanoparticles based on titanium oxide, cerium oxide, aluminum oxide, silicon dioxide, zirconium dioxide, zinc oxide, indium tin oxide, antimony tin oxide, barium titanate or some oxides are preferred. mixed containing these components. Furthermore, several different metal oxides can also be present in the dispersion in the form of mixtures. The production of metal oxide nanoparticle dispersions by means of a liquid phase milling can be carried out, for example, as described in DE 10204470 A1. The principle of shooting jets of liquid on top of each other under high pressure, causing them to collide, and consequently crushing and dispersing the ground material contained therein, is also referred to as wet jet milling. ). Such dispersions have, in a production-driven way, an asymmetric particle size distribution. Other methods for the production of dispersions are, for example, the optionally combined use of jet mills, stirred ball mills, ultrasonic dispersing arrangements, rotor and stator machines, an ultraTurrax, planetary kneaders / mixers or homogenizers. high pressure. The dispersions resulting here can also have symmetrical particle size distributions and are also suitable for the process according to the invention.

The dispersion used within the framework of the described process can be obtained, for example, directly from a metal oxide originating from the synthesis or by redispersing the portion of solid materials from a previously dried dispersion of the same metal oxide. . Before, or also during, the dispersing process, for example dispersing aids can be added, the pH value can be adjusted or a chemical modification of the metal oxide surface can be carried out (for example , by using reactive silane compounds or compounds that are electrostatically fixed to the particles). This also applies to the case where a metal oxide is redispersed. As liquid components of the dispersion it is possible to use water, liquids based on water, organic liquids or ionic liquids or mixtures of several representatives of one of the aforementioned sets of substances or mixtures of representatives of different sets of substances . In the case of liquids, it can also be solutions.

In addition, components dissolved or dispersed in a sufficiently fine manner may still be present in the liquid component of the dispersion, for example stabilizing agents, inhibiting agents, anti-aging agents, biocidal agents, colorants, antistatic agents, salts, surfactants. or protective agents against corrosion.

In the process according to the invention, the cross-flow filtration step through membranes is coupled with a grinding process carried out earlier, in such a way that the separated coarse material (the retained material) is returned to the grinding process, and the fine material (permeate 1) is removed from the process. The permeate 1 is then brought to a higher solids content, optionally by means of a separation or concentration step, eg. by ultrafiltration, while the permeate material 2, obtained by carrying out the concentration, and which is essentially free of nanoparticles, and which is hereinafter referred to as transparent eluted material, or a condensed material corresponding to the permeate material 2 is returned as a wash water to the process, that is, to the

ES 2 402 165 T3 preceding filtration step. However, a wash water can also be added from the outside. In a preferred embodiment, the retentate from the filtration step is washed by means of diafiltration. In this case, the retained material is continuously supplied with a new wash water, until the old wash water has been completely exchanged through a membrane, and the remaining nanoparticles, present in the retained material, have moved through. of a membrane in the permeate. In a preferred embodiment, the entire process is carried out continuously.

Figure 1 shows the basic flow diagram of a possible embodiment of the process.

In order to more widely separate the fine portion from a dispersion, it may be necessary to use an additional amount of washing water optionally in conjunction with dispersing agents. Instead of the concept of water or washing water, the concept of "a liquid" can also be used in a more general way, since, in addition to water and water-based liquids, some compounds also come into question. liquid organic or ionic liquids. The liquids can also be mixtures of several substances taken from the same class of substances or mixtures of representatives of the aforementioned sets of liquids, which can constitute both the basis for the dispersion of nanoparticles and also be used as a washing water.

The present invention is described in more detail by means of the following embodiments, without its scope of protection having to be restricted in this way.

Examples

The following tests were carried out on a 30% dispersion of titanium dioxide in water, such as that which can be produced by a process described in DE 10204470 A1. The dispersion used has a D50 value of approximately 0.09 pm as well as a D90 value of approximately 0.2 pm, that is to say that 50% of the particles have a particle diameter of less than or equal to 0.9 pm, and 90% of the particles have a particle diameter of less than or equal to 0.2 pm.

The following commercially available membranes from Millipore were used:

Test 1: a membrane based on a poly (tetrafluoroethylene) with a pore diameter of 1 pm

Test 2: a membrane based on a poly (tetrafluoroethylene) with a pore diameter of 0.45 pm

Test 3: a membrane based on a poly (tetrafluoroethylene) with a pore diameter of 0.2 pm

The number of revolutions of the stirrer was in each case 1,264 min^-1. This corresponds to an average flood velocity of approximately 9 m / s.

The particle size distribution was measured in each case before (series of measurements with polygonal measuring points) and after fractionation (series of measurements with round measuring points) (see Fig. 2).

Before the dispersion of test No. 3, photographs were additionally taken with a scanning electron microscope at two different resolutions (UHR detector, with a high voltage of 10 or 20 kV respectively) (see Fig. 3).

The results of tests 1 to 3 expressly show that by the process according to the invention the particle size distribution of the dispersion used can be shifted towards smaller particle sizes in such a way that, after fractionation has been carried out , no particle with a particle diameter of 100 nm or one above it is contained in the dispersion. This finding is also confirmed in a purely qualitative way by scanning electron microscope photographs.