Method for preparing a composite semi-permeable membrane.

The invention pertains to a method for preparing a composite semi-permeable membrane for ultrafiltration, comprising the steps of : - preparing a solution of a polymeric binder in a solvent, - forming a suspension of the obtained solution and a metal oxide, -bringing said suspension in the desired shape and - removing the solvent from the suspension.

Membranes, intended for ultrafiltration are characterized by their so called "cut-off" value.This is the value of the molecular weight of the solute particles that are retained for 90 % by the membrane. This value is derived from the retention curve of the membrane. Such a retention curve is being obtained by retention measurements with different solute molecules with increasing molecular weight. When for a certain solute molecule a retention of 90 % is found, the membrane will be assigned a cut-off value, that is equal to the molecular weight of this solute. Depending on the slope of the retention curve the difference can be made between a sharp and a diffuse cut-off.

100 % polymeric utlrafiltration membranes have retention curves by which two macromolecules can be separated from each other, if their molecular weight differs at least by a factor of 5 to 10.

One has already attempted to improve the quality of membranes by adding substances to the polymeric binder.

The incorporation of inorganic, organic or metallic substances into polymeric membranes has been studied, quite extensively in the past with the aim to improve the tedious characteristics of 100 % polymeric membranes, like a bad resistance towards compaction and a low permeate flux. More recently it was found that a membrane's tendency to irreversible shrinking upon drying was prevented by the addition of inorganic substances.

Methods for preparing a membrane with the aim to improve the membrane resistance towards compaction for use in reverse osmosis and/or ultrafiltration are described in US-A-3 567 666, where particles with uniform interconnected pores are incorporated in a polymeric membrane with channels therein, and in the FR-A-2 099 286 where filler particles are incorporated in the support layer of the membrane.

Another method of preparing a membrane with the aim to improve the permeate flux of the membrane is described in NL-A-7 310 985 where the filler compound is anisotropically distributed throughout the membrane.

A method of the above mentioned type is also described in EP-A-0 077 509. In order to prevent the irreversible shrinkage of the membrane in the drying process,60 to 90 wt % of small particles of a pigment and/or a filler are mixed to the organic binder,the specific surface of the particles lying between 5 and 50 m²/g. The suspension is poured on a porous support structure and the solvent is removed.

Only examples are given of TiO₂ as substance added to the binder or of TiO₂ with a filler such as CaCl₂.

Other fillers or pigments such as soot, iron oxide, aluminium oxide, barium sulphate, zinc oxide and zinc sulfide are mentioned.

A method for manufacturing a membrane from an organic binder and thorium oxide is known per se from US-A-3 463 713. The method according to this publication is however a method for preparing an anion-exchanging membrane. The thorium oxide is added as ion-conducting material.

From US-A-4 253 936 a method for preparing a membrane is known, according to which an organic binder is dissolved in a solvent, a suspension of polyantimonic acid in the same solvent is added and mixed with the binder solution, the mixture is being cast onto a support, the solvent is removed by evaporation and the membrane is removed from the support. This known method is a method for preparing an ion-conducting membrane for electrochemical cells.

The invention has for object to provide a method for preparing a membrane with an improved selectivity for ultrafiltration with respect to the membranes obtained according to the above mentioned known methods.

For this purpose zirconium oxide is used as metal oxide , said zirconium oxide being added to the polymeric binder solution into a weight ratio zirconium oxide/polymeric binder of at least 80/20 and a small amount of a thixotropic stabilizer is added to the suspension.

Unexpectedly has been discovered that the addition, of zirconium oxide grains into the polymer solution by proceeding according to the above mentioned method results in a membrane that is very well suited for ultrafiltration. The use of zirconium oxide is not disclosed in any of the above mentioned prior art publications.

The method according to the invention results in a different way of dispersion of the inorganic compound, e.g. the zirconium oxide in the polymer binder with respect to the above mentioned known methods. It was found that the sagging stability of a suspension prepared with zirconium oxide grains by the known methods was too low to obtain the desired effect. It is known from literature that the first layer of a membrane that is formed upon quenching in the coagulation bath is the skin-layer. This skin-layer determines the mass-transfer properties of the membrane. To obtain the desired effect the zirconium oxide grains have to be present in this skin-layer. This property is obtained by rendering the suspension more stable and thixotropic by adding a suspension stabilizer.

Preferably the suspension is ground to deagglomerate the zirconium oxide grains before bringing the suspension in the desired shape.

It was found also that deagglomerating the zirconium oxide grains influences favourably the germ-­forming and cristal growth processes upon membrane formation. To do so the suspension is in situ-milled.

For the preparation of a composite semi-­permeable membrane, following the invention ,for ultrafiltration with a cut-off value below 50,000 daltons the binder solution is prepared by dissolving the polymeric binder in a mixture of a low temperature (≦ 100°C) boiling solvent and high temperature (> 100°C) boiling solvent,the low temperature boiling solvent being removed at the end by evaporation and the high temperature boiling solvent being removed by extraction.

The most suitable low temperature boiling solvents are amongst others dioxane, chloroform and methylene chloride.

For the preparation of an ultrafiltration membrane following the invention,with a cut-off value equal to or higher than 50,000 daltons the binder solution is prepared by dissolving the polymeric binder in a high temperature boiling solvent.

The most suitable high temperature boiling solvent is a solvent of the series N-methyl-2-pyr­rolidone, dimethyl formamide or dimethyl acetamide.

In an efficient form of execution of the invention the suspension is given the desired form by casting upon a glass-plate, the membrane being removed from the plate,after removing the solvent.

In a remarkable form of execution the suspension is given the form of a hollow fiber by spinning.

In another remarkable form of embodiment of the invention the suspension is given the shape of a tube by using a casting-bob.

Other features and advantages of the invention will stand out from the following description of a method for preparing a composite membrane for ultrafiltration, and of the membrane thus manufactured according to the invention; this description is only given by way of example and does not limit the invention.

For the preparation of a composite membrane for ultrafiltration, firstly a solution of the polymeric binder ,e.g. polysulfone polyvinylidene fluoride or poly­acrylonitrile, in a solvent is prepared. Zirconium oxide grains and a thixotropic suspension stabilizer are added to the solution. The suspension is then brought in the desired shape, either in the shape of a film by casting on a support like a glass-­plate or a non-woven type polyester, polyamide or polypropylene support structure, or in the shape of a hollow fiber or hollow tube by means of a spinneret or a casting-bob. Subsequently the suspension is crystallized by removal of the solvent.

Zirconium oxide is added to the polymer solution in a weight ratio zirconium oxide/polymeric binder of at least 80/20 and preferably between 85/15 and 98/2. The diameter of the zirconiumoxide grains is at most 500 µm.

A sufficient amount of a thixotropic sus­pension stabiliser has to be added (normally 1 to 2 wt % with respect to the solvent). Hereby the viscosity of the suspension is raised by a factor of 2 to 10 to prevent sagging.

Before bringing the suspension in the desired shape, the suspension is milled in order to deagglomerate the zirconium oxide grains.

When a semi-permeable membrane with a cut-off value below 50,000 daltons is wanted,the polymeric binder solution is prepared by dissolving the binder in a mixture of a low temperature (≦ 100°C) boiling solvent of the group formed by dioxane, chloroform and methylene chloride, and a high temperature (> 100°C) boiling solvent of the group formed by N-methyl-2-pyrrolidone,dimethylformamide and dimethylacetamide.

For removing the solvent from the suspension, the low boiling solvent is first removed by evaporation whereas the high boiling solvent is removed by extraction.

When a semi-permeable membrane with a cut-off value equal to or higher than 50,000 daltons has to be prepared, exclusively the above mentioned high boiling solvent is used to dissolve the polymeric binder.

According to the above described method a composite membrane is obtained, that has a sharper cut-off in comparison with the existing membranes. Moreover, this membrane is applicable in a large pH-­domain, which is an advantage for cleaning in acid as well as in alkaline media, and is thermally stable up to 125°C. Especially when the solvent is removed from the suspension, by quenching the membrane into a waterbath, the diameter of the pores in the skin-layer can be better controlled by the presence of the zirconium oxide grains.

The invention will be illustrated by means of the following examples :

Example 1

1. To prepare a membrane for ultrafiltration with a cut-off value of 150,000 daltons, 180 grams of polysulfone -(type P 1800 NT 11, supplied by Union Carbide) are dissolved in 820 grams of N-methyl-2-­pyrrolidone at 50°C. To this solution, 270 grams of zirconium oxide (type E 10, supplied by Magnesium Electron Ltd) are added. Subsequently 8.2 grams Cab-O-Sil M-5 (supplied by Cabot Inc.) are added. This mixture is wet milled in a suspension mill at room temperature in or­der to obtain the suspension. The suspension is cast onto a glass-plate into a film with a wet thickness of 200 µm. Subsequently this glass-plate is quenched in a waterbath at room temperature.

The composite membrane crystallizes and comes from the glass-plate after a few minutes. After a residence time of at least one hour in the waterbath, the membrane is boiled during one hour in a solution containing 30 wt % of glycerine and 70 wt % of water. Finally the membrane is dried in an oven at 40°C.

By this method a membrane consisting of 60 wt % of zirconiumoxide and 40 wt % of polysulfone is obtained, having a dry thickness of 100 µm and a cut-off value of 150,000 daltons for dextran molecules.

2. To prepare a second membrane with a similar cut-off value,144 grams of polysulfone (P 1800 NT 11) are dissolved in 656 grams of N-methyl-2-pyrrolidone at 50°C. To this solution 576 grams of zirconium oxide type E 10 are added. Subsequently 6.56 grams of Cab-O-Sil M-5 are added to this mixture. By proceeding in the same way as specified in example 1.1,a membrane is obtained, consisting of 80 wt % of zirconiumoxide and 20 wt % of polysulfone, having a dry thickness of 110 µm and a cut-off value of 150,000 daltons for dextran molecules.

3. To prepare a third membrane with a similar cut-off value,90 grams of the same type of polysulfone are dissolved in 410 grams of N-methyl 2-­pyrrolidone at 50°C. To this solution 810 grams of the same type of zirconiumoxide is added. To this mixture 4.1 grams of Cab-O-Sil M-5 are added.By proceeding in the same way as specified in example 1.1 a membrane is obtained, consisting of 90 wt % of zirconium oxide and 10 wt % of polysulfone, having a dry thickness of 145 µm and a cut-off value of 150,000 daltons for dextran molecules.

The membranes obtained by the above mentioned procedures have a similar cut-off value, although their retention curves have not the same shape. In table 1 the retention values for different solutes (dextran 9,000, 40,000, 74,300 and 150,000) for the three types of membranes are given. The concentration of the solutions was 1,000 ppm.The measurements were carried out in a cross-flow cell with a total geometric membrane area of 336 cm², at a transmembrane pressure of 4.7 bars and a temperature of 35°C. <tables id="tabl0001" num="0001"><img file="EP0241995A1_D0001.tif" /></tables>

It is clear from this table that the membranes with a content of respectively 80 and 90 wt % of zirconium oxide have the steepest retention curves and subsequently the sharpest cut-off. With these membranes, it becomes possible to separate two macromolecules from each other,having a molecular weight that differs only by a factor of 2, i.e. the retention for the dextran 40,000 is resp. 7 % and 8 % and for the dextran 74,300 equals 74 % respectively 87 % for the membrane with a zirconium oxide content of 80 respectively 90 wt %.

Example 2

For the preparation of a membrane with a cut-off value of 1,500 daltons 80 grams of polysulfone (type P1800 NT 11) are dissolved in a mixture of 120 grams of N-methyl-2-pyrrolidone and 1,000 grams of chloroform at 45°C under refluxing. After cooling to room temperature 320 grams of zirconiumoxide (type E10) are added to this mixture. Subsequently 22.4 grams of Cab-O-Sil M-5 are added.This mixture is wet milled during 48 hours in a suspension mill at room temperature. Subsequently the suspension is cast, with a wet thickness of 350 µm on a non-woven type support structure (supplied by Carl Freudenberg) with a thickness of 200 µm. The whole structure is quenched in a waterbath at 5°C and kept submerged for at least one hour, subsequently submerged in a bath containing a boiling mixture of 30 wt % of glycerine and 70 wt % of water. Finally the membrane is dried in an oven at 40°C. The so obtained composite membrane has a thickness of 270 µm and a cut-off value of 1,500 daltons for dextran molecules.

Example 3

For the preparation of a membrane with a cut-off value of 1,000,000 daltons, 120 grams of polyvinylidenefluoride (type Solef 1015/1001, supplied by Solvay) are dissolved in 1,080 grams of dimethyl­formamide at 35°C. To this solution 120 grams of zirconiumoxide (type SC 101) and 21.6 grains of Cab-O-Sil M-5 are added. This mixture is wet milled during 8 hours in a suspension mill at room temperature. The so obtained suspension is cast on a glass-plate with a wet thickness of 300 µm. This is quenched in a waterbath at 50°C and subsequently treated in the glycerine bath. Afterwards the membrane is dried in an oven at 40°C. The so obtained membrane has a dry thickness of 110 µm and a cut-off value of 1,000,000 daltons for dextran molecules.

Example 4

For preparing a hollow fiber module with a total geometric membrane area of 215 cm² for ultra­filtration, a suspension is prepared as described in example 1.2.

However, the suspension is not cast upon a glass-plate or a support structure, but is extruded by means of a spinneret of the "tube in orifice" design, from which the orifice has a diameter of 1.7 mm and the tube an inner/outer diameter of respectively 0.4/0.7 mm. The air-gap, that is to say the distance between the spinneret orifice and the coagulation bath, is 10 cm, whereas the operating pressure was 0.4 bars for both the suspension and the inner coa­gulation medium (here water). The nascent hollow fiber is led into the coagulation bath containing 90 wt % of water and 10 wt % of N methyl-2 pyrrolidone. By this method a hollow fiber is obtained with an inner diameter of 1.1 mm and a dry thickness of 0.2 mm. This hollow fiber is treated in the glycerine bath for one hour.

After drying at room temperature the hollow fiber is cut into equal pieces of 25 cm. A bundle of 40 of such pieces is put into a tube with an internal diameter of 19 mm. At both ends this tube is potted with an epoxy resin. Subsequently the two ends are cut from the tube to obtain a hollow fiber module with a geometric membrane area of 215 cm² and a cut-off value of 250,000 daltons for dextran molecules.

The invention is in no way limited to the above-described embodiments and within the scope of the Patent Application, many changes may be made to the described embodiments, notably as regards the shape, the composition, the arrangement and the number of the components being used to embody the invention.

The membrane should not necessarily be given one of the above-described shapes. Instead of casting on a support or shaping into a hollow fiber by spinning, the membrane may be rolled-up to form a spiral-wound module (Swiss-roll), or the membrane may be manufactured by extruding the suspension in the shape of a hollow tube (tube-like membrane).