Preparation of semi-permeable composite membranes.

The invention relates to the production of semipermeable membranes and their use in pressure filtration and reverse osmosis of water-containing mixtures or solutions, for example for seawater desalination.

Pressure filtration, such as reverse osmosis and ultrafiltration, are separation processes. In these separation processes, the solution to be separated is passed under pressure over the surface of a semi-permeable membrane, the solvent and possibly some of the solutes penetrating through the membrane, while the remaining components of the solution are retained on the surface of the membrane and in the solution be enriched.

Separations of dissolved substances from the solvent by pressure filtration, for example the separation of salts from sea and brackish water for the production of fresh water or the separation of certain ingredients from process water and production processes or the removal of undesirable substances from waste water, are processes that are becoming increasingly important.

So far, membranes made of cellulose esters, in particular cellulose acetate, or made of polyamides and polysulfones have mainly gained technical importance for these processes. Cellulose acetate membranes are used for technical reasons because of their good flow rate and their high separability, although they have a number of disadvantageous properties which limit their general applicability. These are once inadequate chemical resistance, especially the sensitivity to hydrolysis at high or low pH and the susceptibility to degradation by microorganisms. This leads to a deterioration in the membrane properties over time.

Furthermore, the cellulose acetate membranes are restricted in their use due to their low heat resistance. Polyamide membranes have a higher resistance to alkalis or acids and solvents, but also generally have lower flow rates than cellulose acetate membranes. Only permeable membranes can be produced from polysulfones, which are not suitable for the separation of substances with a small particle size or with a low molecular weight, such as salts.

In general, the membranes described above are asymmetrical in their structure, in which - as a rule by phase inversion methods - a porous underlayer and then a selective surface layer - the effective separation layer - is produced from the same material.

However, many polymers that are potentially suitable for membranes cannot be processed into membranes using the phase-migration method. Such materials can be used as membrane separation layers by producing so-called composite or composite membranes. These have a different structure: a very thin polymer layer made of the same or a different material is applied as a selective separation layer on a stable base, which usually consists of a microposous membrane.

Composite membranes are obtained, for example, by bringing microporous support membranes into contact with solutions of the reaction components which are to form the separating layer.

A known method for producing composite membranes is the so-called interfacial polycondensation. For this purpose, for example, a thin, crosslinkable prepolymer film is produced on the support membrane by bringing the support membrane into contact with a solution of the prepolymer. The composite membrane or its thin, selective separating layer is then produced by reacting the prepolymer film with a suitable crosslinking agent. For this purpose, the support membrane coated with the solution of a prepolymer is brought into contact with a solution of the crosslinking agent in an immiscible solvent; the easiest way to do this is by immersing it in the crosslinking agent solution.

The solvent used for the crosslinking agent must neither dissolve the prepolymer used nor damage or dissolve the support membrane. To complete the crosslinking reaction, the membrane is removed from the solution and left for a certain time at room temperature or at an elevated temperature. The boundary polycondensation is usually carried out so that a water-containing support membrane, for example an ultrafiltration membrane, coated with an aqueous oligo- or polyamine solution. This can be done according to known methods, such as brushing, brushing, pouring and spraying (only asymmetric support membranes treat one side of the active separation layer). The coating is carried out in a simple manner by immersion. Subsequently, to produce the thin selective separating layer, the amine which has been drawn up on the supporting membrane is treated with a solution of the crosslinking agent in an immiscible solvent; methods such as pouring on, spraying, but best again immersion, are recommended here. Crosslinking agents are, for example, oligoisocyanates or oligocarboxylic acid chlorides. Aprotic, organic solvents such as hexane or heptane are required as solvents. The contact time of the 2 liquid phases is in the range of 5-300 s, the subsequent tempering takes place at 60-120 ° C. The polyurea or polyamide composite membranes thus obtained are among the most effective to date in terms of retention and flow when working up aqueous solutions by pressure filtration or reverse osmosis.

Since, in the interfacial polycondensation process, the two reaction components forming the selective separation layer are dissolved in immiscible solvents, polycondensation only occurs at the interface of the two solutions and extremely thin polymer films are formed. These films must be practically free of defects in order to be effective as a separating layer. However, difficulties arise with this procedure in practice:<ul id="ul0001" list-style="none"><li>The oligocarboxylic acid chlorides or oligocyanates used as crosslinking agents can hydrolyze by contact with the aqueous phase before the reaction with amine, this can lead to holes and defects in the selective separation layer due to adsorption of the hydrolysates and insufficient crosslinking.</li></ul>

JE Cadotte et al. (J. Macromol. Sci. Chem. A 15 (5) pp. 727-755 (1981)) described that amines partially diffuse into the organic phase, which leads to the formation of porous polymer layers and thus to poor retention values. It is also noted that the method is very sensitive to the quality of the microporous support layer.

Elsewhere (JE Cadotte, RJ Petersen, Am. Chem. Soc. Symposium Series Vol, 153, Synthetic Membranes Vol. 1 Desalination pp. 305-326 (1981)) describes that the production of reverse osmosis membranes after interfacial polycondensation with certain amines and isocyanates only with difficulty, sometimes even not at all.

In interfacial polycondensation for the production of composite membranes, mechanical turbulence occurs at the interface of the two liquid phases, ie partial mixing of the reaction components, which can lead to the formation of porous spots on the selective separation layer. This leads to a deterioration in the retention values ​​and thus to difficulties in membrane production.

It has now been found that semipermeable composite membranes which have a selective membrane layer based on polymers containing urea structures, by using oligo- or polyamines and isocyanates capped with bisulfite, without two liquid phases, ie without a liquid-liquid Phase interface can be made.

The invention relates to a method for producing semipermeable composite membranes from at least one porous support and at least one semipermeable layer formed on the porous support, which is characterized in that an at least bifunctional amine with primary or secondary amino groups and at least one at the surface of the porous support bifunctional bisulfite-capped isocyanate is reacted, wherein the amine and the capped isocyanate are applied separately in dissolved form to the porous support and water or aqueous mixtures are used as solvents for the amine and the capped isocyanate, the solvents for the amine and the capped isocyanate must be miscible with one another.

Bisulfite-capped isocyanates are soluble and stable in water. They form water-insoluble polyureas at room temperature with oligo- or polyamine in an aqueous medium.

Bisulfite-capped, water-soluble isocyanates are easily obtained by reacting, for example, sodium bisulfite and isocyanates. The production of these so-called isocyanate bisulfite adducts is possible in methods of organic chemistry (Houben-_Weyl), Volume XIV / 2, p. 63.

The process for producing the composite membranes is described in more detail below.

A thin, selective separation layer is created on a microporous carrier material. You can treat a microporous support as follows:<ul id="ul0002" list-style="none"><li>Procedure A</li></ul>

<ul id="ul0003" list-style="none"><li>1. Coating with an aqueous solution containing amine.</li><li>2nd Coating the support membrane pretreated in this way with an aqueous solution containing isocyanate bisulfite adduct.</li><li>3rd The trigger material coated in this way is left to air to crosslink the components located on the supporting membrane or is subjected to a heat treatment. (Generally 1 to 30 minutes, at 50 to 150 ° C.</li><li>4th Another layer can be applied to the selective separation layer, for example to protect it from damage.</li></ul>

Procedure B

<ul id="ul0004" list-style="none"><li>1. Coating the support membrane with an aqueous solution containing isocyanine bisulfite adduct.</li><li>2nd Coating the pretreated support membrane with an aqueous solution containing amine.</li></ul>

Steps 3 and 4 are the same as for method A.

The type of process, A or B, can depend on the reaction components amine or isocyanate adduct. The more suitable method can be determined by preliminary tests.

Otherwise, the coating processes 1 and 2 can be repeated several times in the corresponding order, so that a permselective separating layer consisting of a plurality of successively produced layers is produced.

The first coating process can be carried out by known methods such as brushing, brushing, pouring, spraying, but preferably by immersing the support. The second coating process and all subsequent coating processes take place by pouring on, spraying, but preferably also by immersion. Between coating processes 1 and 2, the excess reagent solution should be removed from coating process 1, for example by draining, dabbing, unrolling, etc.

The microporous supports are known per se. Any suitable carrier material can be used for the membrane production process according to the invention. Preferred microporous supports are those made from polysulfones, sulfonated polysulfone, polyvinyl chloride, polyphenyl oxide, polyamides, polyimides, polyamideimides, (co) polymers from aromatic heterocycles, cyclic polyureas such as, for example Polyparabanic acids and polyhydantoins, which are obtained according to DOS 1 494 443, 1 570 552, 1 720 744, 2 003 398 and 1 770 146. Polysulfones and polyhydantoins have proven to be particularly useful carrier materials for the membranes according to the invention. The backing of the carrier used can be reinforced with a fleece, paper or woven fabric. Such reinforcing materials can be made of polyethylene, polypropylene, polyester or polyamide. The production of microporous supports is described, for example, in Office of Saline Water Research and Development Development Report No. 359, October 1968. However, any ultrafiltration membranes on the market are also suitable.

Suitable isocyanates for the preparation of the bisulfite adducts according to the invention are all diisocyanates and higher-functional isocyanates which can be capped with bisulfite and which in this form are soluble in water or aqueous solvent mixtures. For example:<ul id="ul0005" list-style="none"><li>- Aliphatic isocyanates such as hexamethylene diisocyanate and the higher molecular weight polyisocyanates made therefrom such as Desmodur N® from BAYER AG.</li><li>- Cycloaliphatic isocyanates such as isophorone diisocyanate (Desmodur Z®, BAYER AG) or bis-84-isocyanato-cyclohexyl) methane (Desmodur M®, BAYER AG).</li><li>- Aromatic isocyanates such as tolylene diisocyanate or xylylene diisocyanate.</li><li>- Isocyanate-terminated heterocyclic compounds such as tris (6-isocyanatohexyl) isocyanuric acid</li><li>- Isocyanate-terminated polyethers or polythioethers</li><li>- Isocyanate-terminated polyesters and polyurethanes.</li></ul>

It is also possible to use mixtures of isocyanate bisulfite adducts in the process according to the invention.

The isocyanate-containing coating solution is preferably prepared at room temperature, the solvent being water or mixtures of water with other solvents. If necessary, other additives, such as surfactants, which are favorable for membrane production can be added to the coating solution.

In principle, suitable amine components are oligo- or polyamines which are soluble in water or aqueous solvent mixtures and have a content of at least 2.0 meq / g, preferably at least 5.0 meq / g, of primary or secondary amino groups, based on the dry weight, contain.

Examples of suitable amine components are:<ul id="ul0006" list-style="none"><li>- Polyamines of the polyvinylamine type, or their substitution products according to. EP-A 174 045</li><li>- polyallylamine</li><li>- Homopolymers and copolymers of the aminostyrene type or of the aminoalkylstyrene type</li><li>- Amine-modified polyepihalohydrins, for example according to US Pat. No. 4,005,012</li><li>- Amine-modified polychloroethyl vinyl ether, for example according to EP-A 10 425</li><li>- Hyrazine-modified (eg EP-A 8345) or amine-modified poly (meth) acrylates</li><li>- Amine-modified polyepoxy compounds, for example according to DE-OS 2 822 784 or EP-A 10 425</li><li>- polyethyleneimines, oligoethyleneimines</li><li>- Amine functionalized polyethers</li><li>- Amidamines from oligoamines and dicarboxylic acids, for example according to EP 14 054 or EP-15 149</li><li>- Polymers with substituted piperidine rings according to GB-PS 2 027 614</li><li>- Aromatic oligo- or polyamines.</li></ul>

It is possible, in some cases even preferred, to use a mixture of 2 or more different amine components for the production of membranes according to the invention. If appropriate, further additives, such as surfactants, which are favorable for membrane production, can be added to the amine-containing coating solution.

The concentration of the isocyanate bisulfite adducts or the amines in the coating solutions can vary according to the properties of the desired membrane. In general, the concentrations of the amine components are between 0.05 and 10% by weight, preferably between 0.1 and 5% by weight, in particular the range from 0.3 to 3% by weight is preferred. The isocyanate bisulfite adducts are present in the corresponding coating solution in concentrations of 0.05 to 15% by weight, preferably 0.1 to 10% by weight, in particular 0.5 to 7.5% by weight.

The contact time for the application of the first reaction component to the porous support material is completely uncritical, it can be from 1 second to a few hours. Times from 10 seconds to one hour, in particular from 30 seconds to 30 minutes, are preferred. The contact time with the second reaction component can be from 15 seconds to 60 minutes, times from 30 seconds to 30 minutes, in particular from 1, are preferred Min. To 15 min.

The membranes produced by the process according to the invention are suitable for the separation and concentration of substances by reverse osmosis and pressure filtration. the membranes can be used, for example, for the desalination of sea water or brackish water for the treatment of drinking water, for the treatment of industrial process water and production processes and for the separation of organic materials.

In order to determine the membrane properties, which are given in the following examples, the finished membrane was applied to a porous sintered plate made of metal and the test solutions indicated were applied to the specified test solutions under the specified pressures in stirred high-pressure cells (type GH 100-400, from Berghof). The test was carried out at room temperature. The membrane discs tested had a diameter of 7.5 cm.

The solvent of the test solutions was water in all cases. Concentration in eluates was carried out for salt solutions by conductivity measurement, for solutions of organic components by the refractive index, depending on the previous calibration.

The filtration capacity of the membrane is in liters / m² Day specified. The percentage reluctance is usually given as follows:<maths id="math0001"><img file="EP0243876A2_D0001.tif" /></maths>

Examples

A. Preparation of the microporous support

a) Polysulfone carrier

A casting solution with 15% by weight of a polysulfone (Udel P 3500® Union Carbide) in dimethylformamide was prepared. The solution was applied in a layer thickness of 0.25 mm at room temperature to a polyester fleece (basis weight: 180 g / m²) applied. The polysulfone layer was then coagulated in room temperature water containing 0.5% by weight of sodium dodecyl sulfate.

b) polyhydantoin carrier

It became a casting solution with 20% polyhydantoin of the formula<chemistry id="chem0001" num="0001"><img file="EP0243876A2_D0002.tif" /></chemistry>and an average molecular weight of 80,000 in N-methyl-pyrrolidone. The solution was applied in a layer thickness of 0.15 mm at room temperature to a polyester fleece (basis weight: 100 g / m). The polyhydantoin layer was then coagulated in water at room temperature.

In both cases, microporous membranes were obtained which were used for the composite membranes in the following examples.

B. Preparation of the Isocyanate Bisulfite Adducts

Approx. 1% Mersolat K 30® (BAYER AG, alkyl sulfonate) as a dispersing agent and then an equimolar amount of isocyanate are added to a 30% sodium bisulfite solution. Optionally, up to 30% of the reaction mixture can be tert as a further solvent. Butanol can be added. The components are stirred at room temperature, the reaction mixture possibly warming up slightly and a precipitate occurring which is redissolved by further addition of solvent. After about 20 The reaction is complete for hours, the reaction mixture is filtered and the blocked isocyanate is precipitated:<ul id="ul0007" list-style="none"><li>a) by adding potassium chloride</li><li>b) by adding alcohols such as methanol, ethanol, isopropanol</li><li>c) by concentrating the reaction mixture on a rotary evaporator.</li></ul>

The capped isocyanates are suctioned off and washed on the frit. The degree of purity or the degree of implementation was determined by determining the sulfur content. The following isocyanates were reacted:<tables id="tabl0001" num="0001"><img file="EP0243876A2_D0003.tif" /></tables><chemistry id="chem0002" num="0002"><img file="EP0243876A2_D0004.tif" /></chemistry>

These bisulfite-capped isocyanates were used in the examples below.

C. Manufacturing and Examination of the Membranes

example 1

Examples of chloride retention

The membranes were produced by vertically immersing the microporous support membranes in the solutions of the reactants forming the separating layer. Between the first and the second immersion, the air is left in a vertical position in order to allow excess solution to drip off; if necessary, dab with a soft paper. After the second immersion, leave it in the vertical position in the air or treat it at a higher temperature in a forced-air drying cabinet. After the crosslinking process, the membranes were kept in water until the test. The details are given in Table I below.

Example 2

Examples of nitrate retention

The membrane was produced analogously to Example 1.<tables id="tabl0002" num="0002"><img file="EP0243876A2_D0005.tif" /></tables>

Example 3

Examples of sulfate retention

The membrane was produced as in Example 1. For details, see Table II.

Example 4

Examples of the retention of water-soluble organic compounds.

The membrane was produced analogously to Example 1.<tables id="tabl0003" num="0003"><img file="EP0243876A2_D0006.tif" /></tables><tables id="tabl0004" num="0004"><img file="EP0243876A2_D0007.tif" /></tables><tables id="tabl0005" num="0005"><img file="EP0243876A2_D0008.tif" /></tables><tables id="tabl0006" num="0006"><img file="EP0243876A2_D0009.tif" /></tables>

Example 5

Application of the method according to the invention for the production of composite membranes using commercially available ultrafiltration membranes.

The Bayperm HIE 779-24 APE 1 membrane from BAYER AG was used as the support membrane. The membrane has a nominal molecular weight separation limit of approx. 25,000 g / mol. Before coating, the membrane was stored in water for 24 hours, then coated like membrane 3c. The following results were obtained:<tables id="tabl0007" num="0007"><img file="EP0243876A2_D0010.tif" /></tables>

Example 6

This example shows the effect of repeated diving.

6a: The polyhydantoin support membrane is treated as follows:<ul id="ul0008" list-style="none"><li>1. Immersion: 10 min. Hexamethylwndiiwocyanat-bitulfitaddukt 3.2% in water</li><li>2nd Diving: 5 min. Polymin P 2% in water.</li></ul>

Part of the support membrane treated in this way is treated once for 15 minutes at 80 ° C: membrane I. The other part is stored in air for 15 minutes, after which the dips are repeated and also treated for 15 minutes at 80 ° C: membrane II The test with 3.5% NaCl gave the following result:<tables id="tabl0008" num="0008"><img file="EP0243876A2_D0011.tif" /></tables>6b: The polysulfone support membrane is treated analogously:<ul id="ul0009" list-style="none"><li>1. Immersion: 10 min. 3.2% hexamethylene diisocyanate in water</li><li>2nd Diving: 5 min. Polymin P 2% in water.</li></ul>

One part is treated for 15 minutes at 80 ° C: membrane I. The other part is stored in air for 15 minutes, after which the dips are repeated and also treated for 15 minutes at 80 ° C: membrane II.

The test with 1% KNO₃ gave the following result:<tables id="tabl0009" num="0009"><img file="EP0243876A2_D0012.tif" /></tables>