EP0000687A1 - Process for the preparation of a microporous membrane to be used in filtration plants - Google Patents

Abstract

In the process for producing a microporous membrane for filtration systems, fine-particle, insoluble particles are mixed in a plastic and released after reaching the final position. The particles are introduced into the pores of a coarse-pored membrane and aligned perpendicular to the membrane surface, the alignment being carried out by flow etching or in a magnetic or electrostatic field.

Description

The invention relates to a method for producing a microporous membrane for filtration systems, fine particles which are insoluble therein are mixed in and aligned in a plastic or plastic pre-product and released after reaching the final position. Such membranes can be used for ultrafiltration of aqueous media, for reverse osmosis and for dialysis.

Ultrafiltration is generally understood to mean the removal of colloidal particles under moderate excess pressure, while reverse osmosis is understood to mean the task of separating or concentrating significantly smaller, namely really dissolved, particles from the solvent under high pressure.

• 1) Sie soll bei mässigem Aufgabedruck eine möglichst hohe Filterleistung erbringen (Definition: cm³ Filtrat/cm²/bar/Std., Temperatur);
• 2) sie soll eine möglichst gleichförmige Porenweite haben mit scharfer Abgrenzung nach oben und unten. Zu enge Poren beeinträchtigen die Filterleistung, zu weite führen zum Durchbruch von unerwünschten Partikeln;
• 3) die Poren sollen möglichst glatt sein (Kapillarstruktur) und scharfkantig zur Filterfläche ausmünden.
  Solche Membranen weisen geringen Druckverlust auf und sind wenig verstopfungsanfällig.
• 4) Die Membran soll in einem weiten pH-Bereich beständig sein. Sie soll nicht dem mikrobiellen Abbau unterliegen, soll inert sein gegen eine möglichst hohe Anzahl von Chemikalien, unempfindlich gegen erhöhte Arbeitstemperatur, erhöhte Drucke und Vibration.
• 5) Die Membran soll nach Möglichkeit trocken gelagert werden können, ohne dass die Filterleistung nachlässt.
• 6) Da Wasseraufnahme, Polarität, Benetzungswinkel des Membranpolymers die Trennselektivität und den Durchgangswiderstand beeinflussen, sollten diese frei wählbar für den jeweiligen Verwendungszweck sein;
• 7) sie. soll nach einem Verfahren herstellbar sein, welches auch im Produktionsmassstab gut beherrschbar ist und eine enge Klassifizierung mit geringer Ausschussquote zulässt.

The membrane is the heart of a filtration system. Their properties help determine whether it is sufficiently powerful and competitive. A good membrane should have the following properties:

• 1) It should provide the highest possible filter performance at a moderate feed pressure (definition: cm ³ filtrate / cm ² / bar / hour, temperature);
• 2) it should have the most uniform pore size possible with a clear demarcation upwards and downwards. Too narrow pores affect the filter performance, too large lead to the breakthrough of unwanted particles;
• 3) The pores should be as smooth as possible (capillary structure) and open out with sharp edges to the filter surface.
  Such membranes have little pressure loss and are not prone to clogging.
• 4) The membrane should be stable over a wide pH range. It should not be subject to microbial degradation, should be inert to the greatest possible number of chemicals, insensitive to increased working temperature, increased pressure and vibration.
• 5) The membrane should, if possible, be stored dry without the filter performance deteriorating.
• 6) Since water absorption, polarity, wetting angle of the membrane polymer influence the selectivity and volume resistance, these should be freely selectable for the respective purpose;
• 7) them. should be producible according to a process that is well manageable on the production scale and allows a narrow classification with a low reject rate.

Previously known high-performance membranes consist predominantly of an asymmetrically constructed, porous layer of plastic, such as cellulose acetate, polyamite, polyacrylonitrile, etc. They are produced by pouring out complex plastic solutions into a layer, and by evaporation or precipitation, a smooth, narrow-pored "active" top is achieved and forms the layer immediately below it by coagulation with suitable media to form a relatively large-pored support layer. Such membranes currently have a high level of development.

• Art und Konzentration des Polymers,
• Art und Konzentration der Quellmittel,
• Art und Konzentration der Lösungsmittel,
• Art und Konzentration des Fällmittels,
• Reifungsgrad der Lösung,
• Schichtdicke, Temperatur, Luftfeuchtigkeit, Luftgeschwindigkeit und Anlasstemperatur

genannt sein sollen.Your disadvantages: pore size, pore size distribution, thickness of the active layer are subject to a large number of influences sizes, of which only

• Type and concentration of the polymer,
• Type and concentration of swelling agents,
• Type and concentration of solvents,
• Type and concentration of the precipitant,
• Degree of maturation of the solution,
• Layer thickness, temperature, air humidity, air speed and tempering temperature

should be mentioned.

In addition, the number of polymers that are suitable for producing asymmetric membranes is limited. The manufacturer is therefore not necessarily able to provide a membrane substance that can be the desired chemical resistance, wettability and mechanical properties would best suit the intended purpose.

In addition, filter layers are known which are produced by limited sintering (firing) of metal-ceramic, carbon or plastic powders. Often, the side facing the filter material is also provided with a fine-pored sintered or precoat layer (so-called composite membranes).

These membranes also do not optimally meet the aforementioned requirements. The flow line of an imaginary liquid particle through the separation layer is highly branched, which creates a high volume resistance.

DE-OS 2 133 848 discloses a process for producing a porous polytetrafluoroethylene tape, in which metal or glass fibers are mixed with a plastic and formed into an ingot by pressure, as a result of which the fiber is aligned perpendicular to the direction of pressure, that is to say radially. Peeling produces a thin film in which the fibers are essentially perpendicular to the film surface are aligned, which are then rinsed out. However, the peeling phase in particular is quite difficult to carry out and is too expensive for industrial production of the microporous membrane mentioned at the beginning.

In contrast, it is an object of the present invention to produce a microporous membrane which fulfills the conditions mentioned at the outset and does not have the disadvantages described for the previously known membranes.

The method that solves this problem is characterized in that the particles which are insoluble in the plastic or plastic product are aligned perpendicular to the membrane surface when introduced into the pores of a coarse-porous support membrane in the liquid state.

• die Figuren 1 und 2 zwei Verfahrensschritte, zur Herstellung einer Umkehr-Sinterschicht,
• die Figuren 3 und 4 zwei Verfahrensschritte gemäss der Erfindung anhand von Schnitten einer Membran,
• die Figuren 5 bis 7 drei Verfahrensschritte gemäss der Erfindung, anhand von Schnitten einer Membran, und
• Figur 8 einen Schnitt einer nach einem weiteren erfindungsgemässen Verfahren hergestellten Membran.

The invention will now be explained in more detail using an exemplary drawing and exemplary embodiments. It shows, purely schematically,

• FIGS. 1 and 2 show two process steps for producing a reverse sintered layer,
• FIGS. 3 and 4 show two method steps according to the invention on the basis of sections of a membrane,
• Figures 5 to 7 three process steps according to the invention, using sections of a membrane, and
• 8 shows a section of a membrane produced by a further method according to the invention.

The intended plastic is dry, by extruder, mixing mill, or wet, by stirring in plastic solutions or in low molecular plastic precursors, fine powdery solid particles in high concentration puts. You will then be prompted to form the structure as long as the plastic part is still plastic or flowable. The plastic is then brought into its final shape, hardened and the particle content removed by etching or dissolving.

• 1) Ihre Konzentration muss nahe der von einem Teil Bindemittel zu einem Teil Füllstoff liegen: sie sollen sich im Polymer berühren;
• 2) sie müssen feinteilig sein, schmale Korngrössenverteilung aufweisen und annähernd runde oder stäbchenförmige Struktur besitzen;
• 3) sie müssen im Kunststoff oder dem verwendeten Lösungsmittel unlöslich sein;
• 4) sie sollen durch die nachgenannten Methoden im Bindemittel orientierbar sein;
• 5) sie sollen durch Wasser, Säuren oder andere Agenzien extrahierbar sein.

The particles must meet the following requirements:

• 1) Their concentration must be close to that of one part binder to one part filler: they should touch each other in the polymer;
• 2) they must be finely divided, have a narrow grain size distribution and have an approximately round or rod-like structure;
• 3) they must be insoluble in the plastic or the solvent used;
• 4) they should be orientable in the binder by the methods mentioned below;
• 5) They should be extractable by water, acids or other agents.

After the extraction process, cavities filled with air or water remain at the original location of the particles, which are connected to one another by spherical caps and, due to the orientation process, pass through the membrane in a channel-like manner. They are more or less perpendicular to the surface of the membrane.

If orientation processes are omitted during membrane production, a film remains after the extraction with a structure that is due to the hexagonally sealed spherical packing is shaped and resembles an open-celled foam foam film under the microscope. This film can be called a reverse sintered layer because the structure is similar to that of a sintered plate, with the difference that instead of the solid particles present there are uniform cavities, as can be seen from FIGS. 1 and 2. In principle, the particles can be finely ground, water-soluble salts. However, these are usually too soft and therefore have a too wide particle size and thus pore width spectrum. As a result, the resulting membranes are insufficient for the aforementioned applications.

Instead, it is more advantageous to use pyrogenically obtained silicon dioxide, aluminum oxide or titanium dioxide. These substances are finely dispersed with approximately spherical particles, narrow grain size distribution, available in defined grain sizes and extractable by hydrofluoric acid. However, other particles (fillers) produced by precipitation or grinding can also be used. Ferromagnetic fillers such as iron oxide II / III, iron powder, nickel powder, chromium II / III oxide are of particular importance.

The filter performance of the so-called reverse sintered layer still leaves something to be desired. Because their structure is more like a microfoam than a capillary layer, their flow rate - in relation to the pore size - can be described as average. In order to increase the performance, it is necessary to orient the particles in the substrate before solidification so that they form capillary or flow structures perpendicular to the membrane surface and finally expose them by etching.

The structure formation is possible in different ways, for example ':

a) Flow line formation through targeted coating processes

If a binder particle mass characterized by pronounced pseudoplastic flow behavior is pressed onto a medium-porous support or carrier layer, the desired capillary structures are formed by laminar flow processes in the relatively large-pored cavities, which are later etched out. The structurally viscous (pseudoplastic) behavior of the coating material is important so that the pearl chain structure that forms when pressed in is retained even during the closing drying process. Subsequent etching with another agent makes it possible to expand, smooth and bring the diameter of the capillaries to a desired size, FIGS. 5 to 7.

b) Flow etching of reverse sintered layers (Fig. 3 and 4)

An agent is flushed under pressure through the membrane, which is capable of eroding the plastic. A certain flow velocity during the etching process is essential. The protruding, sharp-edged, thin-walled bladder edges are preferably removed, and laminar flow structures are formed. Better flow performance with only a slightly enlarged pore diameter.

c) Magnetic line of force

If a plastic mass containing ferro- or paramagnetic particles is subjected to a magnetic field in such a way that its lines of force are perpendicular to the membrane surface, the particles orient themselves under one lacing to capillary structures. If the magnetic field is maintained during the hardening process, these structures remain.

With iron or nickel wire particles, relatively large-pored, but particularly smooth-walled structures are formed (FIG. 8).

d) Electrostatic field line layer

If high voltage is applied to a thin layer of a conductive plastic solution, oppositely charged glass fiber particles can be shot in (flocked) and later etched out after the plastic has hardened. This process leads to relatively coarse, but almost equally large and smooth capillary layers. Fine-pored, uniform and smooth-walled capillary layers are obtained if hollow fiber flock is used instead of glass fiber flock. In this case, it is even unnecessary to uncover the capillaries by etching. The prerequisite is that high molecular weight polymers are used as binders which, due to their size, are unable to penetrate the hollow fiber.

Some of the methods for structure formation described here can also be combined, for example flow etching and coating flow line formation.

example 1

The paste is spread with a metal squeegee on a Teflon base to a 0.3 mm thick layer and allowed to dry. In this way, a 0.07 mm thick, flexible, silky, opaque film is obtained, which can be easily removed from the base. It is then extracted with 40% hydrofluoric acid for two hours and rinsed with distilled water. The finished membrane is crystal clear, transparent and tough and flexible when wet. In the air, it immediately becomes milky and opaque. To determine the filtration performance, it is clamped in a commercially available filtration device by a sintered metal support plate. It provides water passage at 20 ⁰ 3.0 cm3 / cm ² / hour / bar. Crimson red gold sol (particle size = 20 - 24 nm) is filtered off quantitatively. Due to its intensive coloring, its defined particle size and the spherical particles, this sol is well suited for checking the finished membrane. For comparison, this sol passes through a commercially available so-called ultrafilter based on collodion and the pore size = 100 nm almost unhindered. A 0.01% methylene blue solution with a molecular weight of 500 is initially almost completely retained, later the dye breaks through.

Example 2

The membrane produced according to Example 1 is rinsed for one hour at 20 ° with 10% aqueous chromic acid. It is then washed with distilled water and the filtration performance is determined. 5.2 - 5.5 cm ³ / cm ² / hour / bar at 20 ° will now pass through the membrane. The behavior towards red gold sol and methylene blue solution remains unchanged compared to example 1.

Example 3 Flow line formation

The PVC paste produced according to Example 1 is applied to a commercially available polyethylene sintered plate with a pore size of 0.04 mm and completely scraped off with a metal doctor blade. The cavities adjacent to the surface are completely filled with the paste. The carrier plate is then dried and the doctoring process repeated three times. To check for leaks, the carrier plate is checked with methylene blue solution in the filtering device before etching. In order to be able to check the depth of penetration of the paste better, it is advisable to rub it beforehand with a little pigment (e.g. copper phthalocyanine blue). Then, as described above, the pores are exposed by etching with 40% hydrofluoric acid for two hours. The ready-to-use support layer now consists of, for example, a 2 mm thick support layer made of porous polyethylene and a one-sided, firmly anchored fine filtration layer of 0.04 - 0.07 mm thickness. The surface of the fine filtration layer consists of 50 - 60% of dense polyethylene particles and 50 - 40% of the actual filter mass. Their filtration performance against distilled water at 20 ⁰ 6.3 to 6.7 cm ³ / cm ² /Std./bar.

The crimson gold sol is completely filtered off.

Example 4 Magnetic Force Line Layer

Commercial nickel powder is suspended in toluene and a fraction of 3-4 microns particle size is separated by sedimentation. This is dried and used for the subsequent experiment.

12 g of phenoxy resin, Molgew are dissolved. = 20,000 in 52 g N, N'Dimethylformamide, disperses therein 36 g of the above nickel powder, applies a layer of 0.4 mm with a doctor blade to a Teflon plate and immediately places this on the face of a permanent bar magnet with a diameter of 40 mm and about 1.3 Tesla.

There is allowed to the layer 5 hours at 50 ⁰ dry, it pulls from the support and removes the nickel particles from the membrane by 4stündiges etching with 20% hydrofluoric acid containing about 10% of concentrated hydrogen peroxide. A 0.08 mm thick milky-cloudy white film is obtained. This reveals channels opening out under the microscope in the glassy matrix perpendicular to the surface. The filtration capacity of this membrane is 60 - 70 cm ³ / cm ² / hour / bar. Goldsol passes this membrane completely. ^-

1% poly-vinyl acetate dispersion with a particle size of 0.5 - 2 micrometers is completely retained, so-called "bare" filtrate

Example 5 Capillary Force Line Layer

Commercially available nickel wire with a thickness of 40 micrometers is processed into a fibrous powder with an average stack length of 0.3 mm.

• 1 g Phenoxiharz, Molgew. = 20.000 in
• 5 g N,N' Dimethylformamid,
  dispergiert darin
• 1 g obiger Nickeldrahtpartikel

und zieht von dieser Masse mit dem Rakel auf einer Teflonplatte eine 0,4 mm starke Schicht auf.You solve

• 1 g phenoxy resin, Molgew. = 20,000 in
• 5 g N, N 'dimethylformamide,
  dispersed in it
• 1 g of nickel wire particles above

and draws up a 0.4 mm thick layer of this mass with the squeegee on a Teflon plate.

Immediately afterwards, the layer is placed on the face of the above-mentioned bar magnet (with the particles standing upright) and left to dry at 50 ° for several hours. The velvet-like film obtained in this way is removed from the base and is first freed from the outer solid polymer layer by pickling with 20% chromic acid. Then, as described above, the particle fraction is removed with hydrofluoric acid and hydrogen peroxide. The throughput was approximately 50,000 cm ³ water / cm ² / hour / bar.

Example 6 Electrostatically formed capillary layer

Commercial 5 micron thick quartz fibers are converted into a fibrous form with a staple length of about 0.5 mm. A 20% solution of polyphenylsulfone in N, N'Dimethylformamide is applied in a 0.4 mm thick layer to a hard chrome-plated metal disc with a diameter of 50 mm. Using a suitable device, the particles are introduced into the polymer solution on the hard chrome disc at a potential difference of 30,000 volts.

Then it is dried with an infrared device.

The velvety layer can be easily separated from the metal plate by placing it in water containing wetting agent. The pores are completely exposed after exposure to 40% hydrofluoric acid for two hours. Under the microscope, the membrane shows completely uniform, equally large pores with a meniscus-shaped collar. A flow rate of approximately 1,200 cm ³ / cm ² / hour / bar was obtained.

Example 7 Electrostatically formed hollow fiber layer

The starting material for the hollow fibers is a borosilicate glass tube of 7 mm outside and 0.4 mm inside diameter used for the production of so-called full glass thermometers. It is inserted vertically into a ceramic tube heated to 1,400 ° with an inner diameter of 20 mm and a length of 150 mm and drawn off downwards as an endless hollow fiber over rubber squeeze rollers. Fibers with a constant cross-section of 25 micrometers and a light width of approximately 1.5 micrometers can be achieved relatively easily. They are transferred into hollow fiber meal of about 0.5 mm stack length. This flour is stored isothermally in a thin layer in a desiccator at 80 ° above high-boiling kerosene fraction. The result is that the capillaries fill with kerosene and are not blocked by the polymer solution during the subsequent flocking process.

The hollow fiber flour is introduced as described in Example 6 in 25% phenoxy resin solution in dimethylformamide (0.4 mm thick layer), dried, annealed at 90 ° for several hours and then the capillary orifices are exposed by pickling with 20% chromic acid. A velvety glossy film of approximately 0.15 mm thick and with approximately 0.5 mm long capillaries embedded upright is obtained. The flow rate is about 350 cm ³ / cm2 / hour / bar.

The membrane can be hot sterilized at 150 ° without changing its flow behavior.

Suitable particles are pyrogenic silicon dioxide as well as aluminum dioxide, titanium dioxide, zinc oxide and water-precipitated particles of aluminum hydroxide, beryllium hydroxide and zirconium hydroxide, with a grain size maximum of 7 nm - 50 nm.

Usable magnetizable particles can be made of magnetite, iron sulfide, iron oxide, chromite and iron-nickel-cobalt metal or from Heusler's alloys.

Precursors of epoxy resins, acrylic resins, phenol formaldehyde resins, silicone resins, polyester resins and the polymers PVC, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), PAN mixed polymers, polyamides, phenoxy resins and polyphenyl sulfone can be used as binders. The membrane produced by the above method can have any shape, that is to say it can be in the form of a flat layer or tubular or pot-shaped.

Claims (12)

1. Process for the production of a microporous membrane for filtration systems, fine particles which are insoluble in them being mixed in and aligned in a plastic or plastic pre-product and released after reaching the final position,
characterized,
that the particles which are insoluble in the plastic or plastic pre-product are aligned perpendicular to the membrane surface when introduced into the pores of a coarse-porous support membrane in the liquid state. 2. The method according to claim 1,
characterized, that the plastic or the plastic precursor with the insoluble, mixed-in particles is applied to the supporting membrane by means of a doctor blade and the particles are detached and that the layer thus obtained is subjected to flow etching. 3. The method according to claim 1,
characterized by
that the mixed particles are characterized by a magnetic field be judged. 4. The method according to claim 1,
characterized,
that fibrous, ferromagnetic particles are aligned in a magnetic field. 5. The method according to claim 1,
characterized,
that the particles are aligned in an electrostatic field. 6. The method according to claim 5,
characterized,
that hollow fibers made of glass are aligned in the electrostatic field. 7. The method according to claim 1,
characterized,
that particles of pyrogenically obtained silicon dioxide, aluminum oxide, titanium dioxide and zinc oxide are used. 8. The method according to claim 1,
characterized,
that water-precipitated particles of aluminum hydroxide, beryllium hydroxide and zirconium hydroxide are used. 9. The method according to claim 7 or 8,
characterized,
that particles with a grain size of 7 nm to 50 nm are used. 10. The method according to claim 4,
characterized, that magnetizable particles made of magnetite, iron sulfide, iron oxide, chromite, iron-nickel-cobalt metal and Heusler's alloy are used. 11. The method according to claim 1,
characterized,
that binders are used. 12. The method according to claim 11,
characterized,
that precursors of epoxy resins, acrylic resins, phenol formaldehyde resins, silicone resins and polyester resins and the polymers polyvinyl chloride, polyvinylidene fluoride, polyacrylonitrile, polyacrylonitrile mixed polymers, polyamides, phenoxy resins and polyphenyl sulfone are used as binders.

Images