Method for fractioning oxidic nanoparticles with cross-stream membrane filtration

p0001The present invention relates to a method for the fractionation of dispersions of oxide nanoparticles by membrane filtration. It further relates to dispersions of oxide nanoparticles which are obtained by the inventive process.

p0002In recent years, interest in nanoscale particles, ie particles with a diameter less than 1 .mu.m, grown both academically and also manufacture of industrial site ever since nanoparticles due to their properties have great potential in terms of applications, for example in electronics which is attributed and optics in chemical products. Of particular interest are particles whose diameter is in the range below 100 nm. Here usually occur the so-called "nano-effects", for example, quantum effects, which can be partly attributed to the influence of large particle surface. In addition, these particles increases the scattering of light from such an extent that an increasing transparency of "nanocomposites" can be observed, in which the particles described are in a matrix, often made of plastics or paints embedded, to improve their properties.

p0003Important for the application of nanoparticles in composites, however, is that the spherical particles do not agglomerate on the one hand, on the other hand are present in a narrow size distribution. Already by small proportions of coarser particles or agglomerates, the properties of the composites can be negatively affected. This particularly applies to the transparency. Often nanoparticles be modified to suit the particular matrix, which should result in better dispersion and thus prevent agglomeration.

p0004The synthesis of nanoparticles can be made by various methods. In addition to the gas-phase synthesis can be carried out in solution, which can also use template. A further possibility consists in the grinding coarser particles. This approach is characterized in that it is more cost effective than the synthesis of molecular precursors.

p0005Both in the synthesis of the particles from molecular precursors as well as during the grinding always a product is obtained that has a size distribution. While particles in the micrometer range, separation of coarser particles through sedimentation, centrifugation or sieve filtration can be achieved, these methods are limited in nanoparticles usable. If the nanoparticles in a dispersion before, the separation of coarser particles can possibly also be done via sedimentation or centrifugation, but in this case the extreme surface and time required and the discontinuous process management are so disadvantageous that these methods practically may be relevant barely. Likewise, methods such as size exclusion chromatography (SEC) or gel electrophoresis are not suitable for larger quantities.

p0006Nanoparticles are of interest, which consist of metal oxides, for example for the production of UV-retardant polymer composites or of fluorescent materials (for many applications<nplcit id="ncit0001" npl-type="s"><text>Journal of Nanoscience and Nanotechnology, 2006, 6, 409-413</text></nplcit>). For industrial practice, therefore, it would be helpful to have a continuously operating, easily implemented process for fractionation of oxide nanoparticles. It has been variously proposed to use for this purpose, membrane filtration processes. It should be noted that the separation is influenced on a membrane by the specific interaction between the particles and membrane.

p0007To generally separate particles according to predetermined criteria such as the particle diameter of suspensions, filtration techniques are often used. Find generally "dead-end filtration" as "batch" process or "cross-flow filtration" as a dynamic process using. In the dead-end filtration, the entire volume to be filtered is passed directly through the filtration medium, wherein the separated particles usually can build a cake, in turn the result of the filtration a decisive influence. The filtration result therefore is determined in addition to the properties of the filtration media, especially through the filter cake formed and changing the process time here. Accordingly, the cake-forming filtration can not be used for classifying particle dispersions. Only the working according to the dead-end method depth filtration is by a classifying to filter within limits in a position that the particles to be separated to penetrate into the structure of the filter medium and are separated at the inner surface of the filter medium due to particle adhesion. The limitations of this method are that only very dilute dispersions can be treated and the classifying effect has a high natural blur, making significant amounts of the target product remain stuck in the filter medium and be lost.

p0008In cross-flow filtration, the medium to be filtered is introduced tangentially past the filtration medium. The pore size of the filtration medium determines the cutoff. Important applications include microfiltration, ultrafiltration or nanofiltration.

p0009The disadvantages of the dead-end filtration method tried the cross-flow filtration (cross-flow filtration) thereby circumvent that in contrast to this conventional filtration, the filter medium is flowing tangentially. The feed stream is divided into a filtration flow through the filter medium and the transfer flow parallel to the filter medium. In membrane technology, the current that flows through the membrane as permeate is referred to. The residual on the membrane material is referred to as retentate. Due to this flow, a return transport of the retained component from the surface of the filtration medium to the retentate stream is made possible. Thus, the formation of deposits and top layers is counteracted on the filtration medium.

p0010In <nplcit id="ncit0002" npl-type="s"><text>Advanced Materials 2005, 17 (5), 532-535</text></nplcit> is described, such as for fractionation of metallic nanoparticles, the technique of cross-flow filtration membrane is to be used. For this purpose, a special membrane is prepared which contains nanoscale channels. The experiments have been carried out in the smallest laboratory scale and provide no evidence for a transfer to the industrial scale. is not yet made in view of its pore radius in terms of their composition, a statement about the need for fractionation of oxide nanoparticles membranes. Moreover, it is found that a classification of nanoparticles with conventional membranes is not possible.

p0011In <nplcit id="ncit0003" npl-type="s"><text>Journal of Membrane Science 2006, 284, 361-372</text></nplcit> the membrane-cross fabric filtration of a dispersion of silica nanoparticles is described. Here, however, no information on the possibility of fractionation be made, but here is a study of the formation of the dynamic outer layer formed from nanoparticles, which makes fractionation impossible.

p0012The company Bokela, Karlsruhe distributes a Siebfiltrationssystem (Dynofilter), can be separated from the dynamic sieve filtration coarse particles down to 10 microns from a particle dispersion. About the possibility of using membranes in this system, nothing is known.

p0013<nplcit id="ncit0004" npl-type="s"><text>Langmuir 1997, 13, 1820-1826</text></nplcit> describes studies of membrane filtration of polymer particles with a permanent surface charge. Here the retention of nanoparticles is improved by targeted introduction of surface charges. About classifying effects, nothing is said.

p0014In <nplcit id="ncit0005" npl-type="s"><text>Anal. Chem. 2006, 78, 8105-8112</text></nplcit> is described as organic colloids are separated by crossflow ultrafiltration of aqueous solution. In this case, nanoparticles are retained more than 99% across the membrane.

p0015<patcit id="pcit0001" dnum="US20040067485A1"><text>US 2004/0067485 A1</text></patcit> describes the synthesis of nanoscale semiconductors on the basis of zinc and cadmium, in combination with the elements S, Se and Te, wherein a protein is used as a template. It is stated that the resulting complex of z. B. zinc sulphide / apoferritin by dead-end membrane filtration was fraktionierbar, wherein the diameter of the pores of the membrane used is considerably higher than the diameter of the particles. A wide range of membrane materials is called, but no examples of the filtration process are listed. This procedure can not be classified nanoparticle filtration on an industrial scale are carried out as a filter cake would make. Thus, the proposed method is only feasible on a laboratory scale, where necessary, frequent filter replacement is easily possible.

p0016<patcit id="pcit0002" dnum="WO2006116798A1"><text>WO 2006/116798 A1</text></patcit> describes the preparation of radioactive nanoparticles based on metallic technetium, which go through a membrane filtration process in the dead-end method. In this case, a hydrophilic membrane is used. Also here is thus worked according to the dead-end method, which leads even at low agglomeration of the nanoparticles to an almost complete separation of all the particles to the membrane. The method proposed here for Nanopartikelfraktionierung is therefore only feasible with very small nanoparticles concentrations and at laboratory scale.

p0017According to the prior art does not usable on an industrial scale membrane process is known in which particles from a dispersion which contains predominantly but nanoparticles also coarser particles, the fine material can be removed classified. All methods according to the prior art form a top layer on the membrane surface, so that although a separation, but no classification of nano particles from a dispersion is made possible.

p0018The object of the present invention was to provide a process for fractionating a dispersion of metal-oxide nanoparticles, having the disadvantages of the prior art, especially the structure of a top layer on the membrane overcomes, and thus allows a classification of nanoparticles also on an industrial scale.

p0019This object is achieved by a method for fractionation of a dispersion of oxide nanoparticles, which is characterized in that at least one process step is a membrane cross-flow filtration step, wherein a flow over the membrane with the dispersion is produced by powered rotating parts.

p0020Surprisingly, it was found that the inventive method prevents the development of a covering layer on the membrane and the fractionation of a dispersion of oxidic nanoparticles is made possible.

p0021The inventive process is with a filtration unit out by working according to the cross-rotation (CR) principle. There are filtration machines in which shear forces and currents are generated across the filter medium through moving internals. This may include the filter medium itself (eg rotating membranes). One possible embodiment of such a filter machine is the cross-rotation filter. In contrast to cross-flow filtration the overflow is carried out at the cross-rotation-filtration by the use of additional rotors decoupled from the feed and the pressure build-up. In a typical CR filter filtration means and rotors are stacked sandwiched. In the middle of the stack rotates a shaft which drives the rotors, thereby medium, ie, spatially s can be achieved over the entire filter medium averaged overflow speeds of well over 6 m / s. The fed medium is layered through the stack of plates. In these sections of the disk pack gradually, a concentration of the medium takes place. By decoupling the feed (using a pump) and the flow over the membrane (by rotors) high specific filtrate flows are achieved at low system pressures. These systems with rotating internals are previously used where very small particles or sticky substances have to be separated and very highly concentrated.

p0022Microfiltration, ultrafiltration and nanofiltration are also counted among the pressure-driven membrane processes. As membranes generally planar structures are referred to, which form a barrier between two fluid phases and allow selective mass transfer between the two sides. Accordingly, the membrane is a filter medium, which has or particles of a certain size in the presence of a driving force (pressure) retains a defined cutoff. The nature of the driving force and the pressure and flow conditions at the membrane in conjunction with the nature of this decision on the separation result. The classification of synthetic membranes in different tiers relies on their structures, states of matter and the electrochemical behavior. A membrane in the sense used here has pores with a pore diameter of up to 10 microns. With larger pore diameters is spoken by a sieve.

p0023As membrane any commercially available membrane can be used, for example, polyethylene, polytetrafluoroethylene, polysulfone or cellulose. Preferably symmetrical membranes, that is membranes having over the entire cross-section a constant pore diameter is. The membranes used in the invention have a pore diameter of up to 10 .mu.m, preferably between 0.01 and 5 .mu.m, more preferably between 0.1 and 1 microns.

p0024The produced in the inventive process medium crossflow be between 5 and 25 m / s, preferably at least 8 m / s, and particularly preferably at least 10 m / s. are realized such conditions as relative velocity between medium to be filtered and the membrane by rotation of fixtures (eg. as a stirrer). The mean flow velocity is determined by measuring the speed of the entire filtration media and subsequent averaging. It is especially advantageous that this overflow rate is decoupled from the pressure build-up, which will reach the high crossflow velocities above at low transmembrane pressures (&lt;1 bar). Under such conditions, a surface layer structure can be almost completely prevented. In addition the surface layer structure can be countered by periodic backwashing of the membrane with the permeate or a liquid (eg water) or a liquid-gas mixture (eg, water and compressed air).

p0025Nanoparticles in the sense of the inventive particles of any metal oxides may be used. The metal oxides can be produced for example the flame pyrolysis, precipitation process or sol-gel processes. The metal oxides may be mixed oxides, which are derived from two or more different metals. Preferably, the nanoparticles of titanium oxide, cerium oxide, aluminum oxide, silicon dioxide, zirconium dioxide, zinc oxide, indium tin oxide, antimony tin oxide, barium titanate, or of mixed oxides containing these components. Further, may also be present in the dispersion more different metal oxides as mixtures. The preparation of dispersions metal-oxide nanoparticles by means of a grinding in liquid phase, for example, as in<patcit id="pcit0003" dnum="DE10204470A1"><text>DE 10204470 A1</text></patcit> described occur. The principle of liquid jets to shoot under high pressure on each other to collide and consequently is to crush contained regrind and disperse is also referred to as wet-jet-milling. Such dispersions have production reasons in an asymmetric particle size distribution. Other methods for producing dispersions are, for example, the optionally combined use of jet mills, attrition mills, ultrasonic dispersers, rotor-stator machines, Ultra-Turrax, Planetenknetern / -mixern or high-pressure homogenizers. The resulting dispersions here may also symmetrical particle size distributions and are also suitable for the novel process.

p0026The dispersion used in the process described, for example, be obtained directly from a metal oxide from the synthesis or by redispersion of the solids content of a previously dried dispersion thereof metal oxide. Before or during the dispersion, for example, dispersing aids may be added, the pH adjusted, or a chemical surface modification of the metal oxide (for example, by using reactive silane compounds, or compounds that bind electrostatically to the particles) are performed. This also applies to the case that a re-dispersion of a metal oxide is carried out. As the liquid component of the dispersion water, water-based liquids, organic liquids or ionic liquids can be used, or mixtures of several representatives of the mentioned groups of substances or mixtures of representatives of different groups of substances. The liquids may also be solutions.

p0027Additionally dissolved or sufficiently finely dispersed constituents may be present in the liquid component of the dispersion, for example, stabilizers, inhibitors, antioxidants, biocides, dyes, antistatic agents, salts, surfactants or corrosion inhibitors.

p0028In a particular embodiment of the method of the membrane cross-flow filtration step is coupled with an upstream milling process such that separated coarse material (retentate) returned to the grinding process and the fine material (permeate 1) is discharged from the process. The permeate 1 is then optionally placed through a separation or concentration step z. B. by ultrafiltration to a higher solids content, while the permeate obtained in the concentration 2, which is free of nanoparticles substantially and is hereinafter referred to as a clear effluent, or a permeate 2 corresponding condensate as "wash water" in the process, that is returned to the previous filtration step. But it can also be added from the outside "wash water". In a preferred embodiment, the retentate of the filtration step is washed with a diafiltration. Here the retentate is continuously new "wash water" supplied to the old "wash water" is replaced completely over the membrane and remaining present in the retentate nanoparticles have migrated through the membrane into the permeate. Another possibility would be the return metering in the grinding. In a preferred embodiment, the entire process takes place continuously.

p0029<figref idrefs="f0001">figure 1</figref> shows the basic flow of a possible litigation.

p0030To separate out the fine fraction from a dispersion going on, it may be necessary, if necessary, use an additional "wash water" in conjunction with dispersants. Instead of the concept of water or "wash water" can also generally be liquid used the term, because in addition to water and water-based liquids and liquid organic compounds or ionic liquids in question. The liquids may also be mixtures of several substances of the same class of substances or mixtures of representatives of the aforementioned groups of liquids which can both form the basis of the nanoparticle dispersion, as can be used as a "wash water".

p0031The present invention is further described by the following embodiments, without their scope should be limited thereby.

<u>Examples</u>

p0032The following experiments were performed on a 30% dispersion of titanium dioxide in water, as for example, by in <patcit id="pcit0004" dnum="DE10204470A1"><text>DE 10204470 A1</text></patcit> described method can be produced. The dispersion used has a D₅₀Value of about 0.09 microns and a D₉₀Value of about 0.2 microns, ie, 50% of the particles have a particle diameter less than equal to 0.9 microns, and 90% have a particle diameter less than or equal to 0.2 microns.

p0033. The following commercially available membranes from Millipore were used:<dl id="dl0001"><dt><b>Experiment 1:</b></dt><dd>Membrane of polytetrafluoroethylene having a pore diameter of 1 .mu.m </dd><dt><b>Experiment 2:</b></dt><dd>Membrane of polytetrafluoroethylene having a pore diameter of 0.45 microns</dd><dt><b>Experiment 3:</b></dt><dd>Membrane of polytetrafluoroethylene having a pore diameter of 0.2 .mu.m</dd></dl>

p0034The stirrer speed was 1264 rpm respectively^-1, This corresponds to an average flow velocity of about 9 m / s.

p0035the particle size distribution was measured both before (measurement series with angular measurement points) and after fractionation (measurement series with round measuring points) (<figref idrefs="f0002">FIG. 2</figref>).

p0036additional scanning electron micrographs (detector WATCH, high voltage 10 and 20 kV) from the dispersion of the experiment no. 3 were made in two different resolutions (<figref idrefs="f0003">Fig. 3</figref>).

p0037The results of experiments 1 to 3 make abundantly clear that the particle size distribution of the dispersion used is shifted in such a way to smaller particle sizes by the method according to the invention that after fractionation no particles contained with a particle diameter of 100 nm or more in the dispersion. This finding is also supported by the purely qualitative scanning electron micrographs.