DK157978B - MEMBRANE OF THE ANISOTROPIC GEL TYPE OF POLYPIPERAZINAMIDES AND PROCEDURES FOR PRODUCING THEREOF - Google Patents

Description

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DK 157978B

The invention relates to a membrane of the anisotropic gel type for reverse osmosis, and to a method of making such a membrane. The materials from which the membrane of the invention is made are polypiperazinamides and co-polypiperazinamides, hereinafter simply called "polypiperazinamides".

Membranes with selective permeability have been known for a long time, and what is also known is the principle of reverse osmosis, according to which it is possible to produce a separation of the individual components of a solution containing one or more solutes in a common solvent if this solution is pumped into a permoselective membrane under a pressure greater than the osmotic pressure of the solution.

It has also been known for a long time that reverse osmosis has been used to separate the constituents of brackish water and seawater.

For this purpose, membranes will be used which will allow passage of water and at the same time retain the salts dissolved therein.

To enable economical use of the reverse osmosis membranes, these must exhibit a large flow of water accompanied by a large capacity of retention of the dissolved salts. The membranes suitable for such purpose may have different physical structure. Indeed, the polymeric material constituting the membrane may have: 1) a dense and homogeneous structure in the form of a supported ultra-thin film, or in the form of a hollow fiber: 2) a non-homogeneous structure in the form of an "anisotropic gel" "membrane or also in the form of a" coated mem- 2

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brane ", consisting of a layer of a superficial, dense and homogeneous polymer having a thickness generally in the range of 0.1 - 0.2 µm or less, and of a porous substructure whose function is to support the thin layer.

It is known that the high permeability to water of the anisotropic gel type membranes (or coated membranes) is due to the low thickness, which is also responsible for the desalination capacity of the membrane. The term "anisotropic" means that the thin and homogeneous layer active in desalination is present only on one of the two surfaces of the membrane.

The polymeric material most commonly used in the preparation of the anisotropic membranes is cellulose acetate.

In fact, reverse osmosis is usually performed using cellulose acetate membranes.

In addition to cellulose acetate, only a few other polymeric materials are known which are capable of producing anisotropic gel-type membranes (or coated membranes), which can give rise to a large flow of water and a high salt retention.

In two prior Italian patent applications Nos. 868 524 and 868 525 (similar to U.S. Patent Nos. 3,687,842 and 3,696,031), it was disclosed that the polyamides derived from piperazine can be used with good results in the production of membranes suitable for desalination of water by reverse osmosis. The acids used in the preparation of these known polyamides are, for the first patent, fumaric acid, and for the second patent, the acids which are compiled in = column 3, lines 35-50 of U.S. Pat.

No. 3,696,031, for example. adipic acid, phthalic acid and 1,3-cyclopentanedicarboxylic acid.

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The polymers described in these patents can be converted into membranes of the non-anisotropic gel type which, on the other hand, exhibit properties which are not entirely satisfactory. These not entirely satisfactory properties are related to the fact that such polymers are not sufficiently soluble in the solvents that are the only ones useful for the preparation of membranes of the anisotropic gel type (the solvents of the m class, cf. description 10 of the "1 step" section). In addition, the non-anisotropic gel membranes either exhibit a low water flow coupled with an acceptable salt retention or an acceptable water flow coupled with a low salt retention.

The object of the invention is to provide a membrane based on polypiperazine amides capable of being readily transferable to membranes of the anisotropic gel type not afflicted with the aforementioned disadvantages, and to provide a method for preparing such membranes for reverse osmosis based on polypiperazine amides which do not suffer from the disadvantages previously stated.

The object of the invention is fulfilled with a membrane which is characterized by the characterizing part of claim 1.

It is admittedly known from Chemical Abstracts, Vol. 71, 1969, 22399e polyamides prepared from piperazine and 2,5-pyridine, 2,6-pyridine, 2,5-furan and 2,5-thiophene compounds. dicarboxylsyrechlorider. It is stated that these polyamides are white powders which are soluble in certain inorganic or organic solvents and which exhibit high thermal stability and whose X-ray spectra suggest a highly ordered structure. Although some of these polyamides represent materials,

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4 on the basis of which the anisotropic membranes of the invention can be prepared, Chemical Abstracts does not indicate that these polyamides should be used for membranes and much less for anisotropic 5 membranes.

The groups (II) may be present in the polymeric chain at various degrees of substitution; when the substituents R are present in a number greater than 1, they may be arranged in any steric position relative to the piperazine ring; formulas (I) and (II) thus comprise the pure stereoisomers (cis-trans) as well as the mixtures of the polysubstituted derivatives of piperazine.

The -K groups may all be identical and thus have the structure (III) or they may be mutually different and in this case constitute type (III) radicals and one or more organic dicarbonyl radicals derived of other dicarboxylic acids.

Since the type (III) radicals are present in amounts equal to or greater than 10 moles in relation to the total 20 dicarboxyl radicals, cf. the main requirement, polyamides having such solubility properties appear that their use in the preparation of anisotropic reverse osmosis membranes becomes highly appropriate.

The dicarbonyl radicals of the polyamides of the invention, which may be present in addition to those defined by formula (III), are as follows in the main claim:

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OC - yes

CH - CO - derived from fumaric acid

"OC - CH

II

CH 2 - C - CO - derived from mesaconic acid CO "derived from ortho- or isophthalic acid ^ or terephthalic acid j | derived from thiofurazan-3,4-di-carboxylic acid.

\ /

S

The polypiperazine amides used in the membrane of the invention may be prepared according to prior art in interface polycondensation, or in low temperature solution, by condensation between the dichlorides of dicarboxylic acids and piperazine and / or substituted piperazines.

According to a preferred method, the polycondensation is carried out using the interface method by dissolving the piperazine in a generally aqueous liquid phase and by reacting this solution with the dichloride or with the mixtures of the dichlorides of the dicarboxylic acids dissolved in another liquid phase which is immiscible with the first.

The polycondensate ion temperature can vary from the freezing point of the higher freezing phase up to the decomposition temperature of the reactants. The preferred temperatures are between -10 ° and + 70 ° C.

Particularly suitable solvents for the dichlorides of the acids are: benzene, chlorobenzene, dichloromethane, chloro-

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6 form, toluene, xylene, carbon tetrachloride and cyclohexanone.

The concentration of the dichlorides of the acids and of the piperazine can vary within a wide range of values ranging from the pure reactants to very dilute solutions (0.001 mol / liter).

In general, the boundary 1-ade polycondensation is carried out using an hydrochloric acid acceptor which is formed during the reaction. The acceptor may be either an organic or inorganic base or the piperazine itself; particularly suitable are: sodium hydroxide, potassium hydroxide, sodium carbonate, magnesium oxide and triethylamine.

When the polycondensation is carried out in low temperature solution, the same solvents are used both for piperazine and for the acid dichloride.

The polycondensation is carried out by mixing the solutions of the individual reactants. When mixtures of two or more dichlorides are used, the latter can be added successively.

As solvents, dichloromethane, chloroform, N-methylpyrrolidone and dimethyl acetate amide are preferably used.

The low temperature polycondensation may also be carried out in the presence of an organic acceptor such as trimethylamine, dimethylaniline, n-methylmorpholine, pyridine 25 or the piperazine itself used as a reactant in the polycondensation. The concentration of the monomers can vary from the reactants with a purity of 100? Up to very dilute solutions. The temperature can range from -20 ° to + 25 ° C.

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Such polypiperazine amides are characterized by a logarithmic viscosity number (measured at 30 ° C in a solution of 0.5 g of polymer in 99.5 g of a 98 ° t 2 SO 2), which is generally greater than 0.5, but preferably between 1 and 6.

The membranes of the invention exhibit an anisotropic structure, characterized by a thick and homogeneous layer which ensures a high salt retention capacity, and a porous substructure which serves as a support or support.

The membranes of the type described above can be prepared by following a method which is also an object of the invention, which is characterized by the characterizing part of claim 3. The process comprises, in the order indicated, the following steps: 15 'steps: preparation of a solution of the polypiperazine amide in a suitable organic solvent, 2' steps: spreading the solution over a flat glass plate and the resulting formation of the membrane, 3 'steps: the partial evaporation of the solvent, 4' steps: the coagulation of the membrane by dipping it in water, and 5 'steps: the thermal treatment of the membrane.

U.S. Patent No. 3,696,031 does not disclose starting materials that may lead to the production of anisotropic membranes, as is evident from the following experiments.

Experiment 1. Membrane A (according to the invention) is prepared by reproducing the following Example 1 and then reproducing the following Examples 3 and 4 so that the final product was the heat treated membrane of Example 4. The thickness of the membrane was 80 µm. .

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Experiment 2. Membrane B (Prior Art) was prepared by reproducing the method described in Example 1 (A) of U.S. Patent No. 3,696,031 using poly (piperazine isophthalamide) polymer having a logarithmic viscosity number. of 2.1 dl / g (determined at 30 ° C in a solution of 0.5 g of polymer in 99.5 g of 98% sulfuric acid) and formic acid as solvent. The thickness of the membrane was 70 µm.

The membranes A and B were now tested under conditions of use, cf. page 14, line 1 from above to page 14 line 10 20, as follows:

Sample a

Positive side facing the solution to desalt

Membrane A Membrane B

2

Water flow, 1 / m / day 380 2.30 15 Salt retention,% 98 99.1

Sample b

Negative side facing the solution to desalt

Membrane A Membrane B

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Water flow, 1 / m / day 270 2.30 20 Salt retention,% negligible 99.1

These tests show that the membrane according to the invention is anisotropic, while this is not the case for the membrane according to US Patent No. 3,696,031. Thus, the superiority of the invention in comparison with prior art is also documented, cf. the technical advantages of an anisotropic gel type membrane as compared to the prior art non-anisotropic type membranes.

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1 'step

The concentration of the polyamide in the solution can vary widely within a wide range, generally between 5 and 60% by weight, but preferably between 8 and 25% by weight, 5 by weight of the solution.

For the preparation of the solution, water-soluble polar solvents of the organic type, belonging to the m-class of the solvents forming hydrogen bonds (mH bond group) and having a solubility parameter 8 (cal / cm) 1/2, are used. for partial classification given by H. Burreli in "Polymer Handbook", IV - 341, J. Brandrup, EN Immergut, Editor Interscience, N.V.

Examples of such solvents are: dimethylformamide, dimethylacetamide, diethylformamide, diethylacetamide, dimethylsulfoxide, N-methylpyrrolidone, tetramethylsulfone and the like.

Preferred solvents are: N-methylpyrrolidone and dimethylacetamide. A salt that is soluble in water and in the organic solvent may be present as the third component of the solution. Examples of such salts are: LiCl, LiNO 2, LiBr, CaCl 2, ZnC

MgClO 2 and the like.

In addition to the salt-like component, water may sometimes be present as a fourth component of the solution. In general, the salt may be present in the solution even in a large amount, in a polymer / salt weight ratio between 1 and 2.

The solution can be prepared in various ways, e.g.

30 using a mechanical "Werner & Pfleiderer" mixer 10

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and then filtering through a porous diaphragm or filter membrane or using various filtration systems.

The properties of the membranes resulting from this solution depend to a large extent on the amount of the solution. Particularly advantageous results are obtained by treating the solution - however manufactured - at a temperature greater than 70 ° C and generally at a temperature between 80 ° C and the boiling point or solvent decomposition point easily. According to a preferred method, N-methylpyrrolidone is used as the solvent and the solution is treated at a temperature greater than 160 ° C.

15 21 steps

The solution obtained as a result of the 1 'step is spread over a flat glass plate, e.g. using a film spreader to form a film. The spreading is generally carried out at room temperature, and the thickness of the resulting film varies within a wide range generally ranging from 0.002 to 0.2 era.

To increase the abrasiveness of the solution during the dispersion step, the solution can sometimes be heated to a temperature greater than room temperature. In this way, it is possible to obtain membranes that will be flat in the final step. As a forming support, besides glass, any other suitable material may be used, such as e.g. a metal plate, a polyethylene terephthalate film, a polytetrafluoroethylene film, porous substrates, woven and nonwoven fabrics, paper and other similar materials, either in flat, tubular or other form.

3 'steps

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The membrane molded on the glass plate is heated to evaporate part of the solvent. The time and temperature of the partial evaporation of the solvent 5 can vary within a wide range, depending on the type of solvent used, the composition of the solution and the thickness of the membrane one wishes to obtain. The temperature of the partial evaporation of the solvent is between 70 and 200 ° C, 10 but preferably between 80 and 180 ° C. The evaporation time is generally between 1 minute and 3 hours, but preferably between 3 and 30 minutes.

4 'steps

After the partial evaporation of the solvent, the membrane is coagulated to a gel-type structure by immersion in water. The temperature of the coagulation bath is generally between 0 and 30 ° C, but preferably it is between 0 and 5 ° C. Sometimes it can be useful as a coagulation bath to use an aqueous saline solution.

The salts which can be dissolved in water are: NaCl, CaCl3 and the like. Sometimes water-soluble organic solvents such as alcohols, acetone and the like can be added to the coagulation bath.

Coagulation time can vary within a very wide range. In general, the membrane is kept in the coagulation bath at a temperature between 0 and 5 ° C for approx. 60 minutes; they are then kept in water at room temperature.

5 'step 12

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The membranes according to the invention sometimes - as they are as a result of the 4 'stage - do not at all have any satisfactory properties with respect to reverse osmosis; the flow rate is very high, 2 generally greater than 500 l / m, but the salt retention capability is generally below 50¾.

The thermal treatment provided as the 5 'step of the process of the invention eliminates these 10 drawbacks and provides a significant and sustained increase in the desalination capacity of the membrane.

The thermal treatment of the membrane can be carried out in various ways. According to a preferred embodiment of the invention, the membrane is placed in hot water 15 for a period of time between 1 minute and 2 hours, at a temperature between 60 and 100 ° C.

In choosing the treatment time and temperature, it must be taken into account that in general, an increase in salt retention capacity and a reduction in water flow occurs when both time and temperature are increased.

The present invention relates mainly to flat anisotropic gel type membranes. In each case, it is to be understood that the membrane of the invention may also be made in tubular form or as hollow fibers, utilizing a technique known to those skilled in the art.

In accordance with the invention, membranes of the "anisotropic gel" type have a surface capable of repelling the salts of a reverse osmosis, 30 and having a porous substructure which will allow a greater flow of water .

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The anisotropic structure of such membranes can be documented by the two reverse osmosis experiments. In the first experiment, the membrane is arranged in the reverse osmosis cell with the dense and homogeneous surface facing the saline solution to be treated. The membrane will exhibit a large flow of water and a large salt content 1 capacity.

In the second experiment, the same membrane is placed in the inverted osmosis cell with the porous surface facing in towards the saline solution to be treated. In this case, the membrane still exhibits a large flow of water, but the retention of salt is practically zero. The "gel" structure of the membrane is documented by the high content of water in the membranes - above 20% by weight - and 15 generally between 40 and 80% by weight.

The permeability to water of the membranes' can be defined as follows: .... water passed (liters)

Water flow = -c ----— / ... / 2, s membrane surface x time llter / m. day) (J} {dayB) or it can be defined as the constant of membrane A in the following way:

Membrane constant = A = water flow (liter / m2 day) actual pressure (atm.) Where by "actual pressure applied" is meant the difference (^ P -> where Δ P is the difference of the hydraulic pressure on both sides of the membrane and where 4 is the difference of osmotic pressure between the applied solution and the solution passing through the membrane.

The membranes of the invention generally exhibit a high membrane constant.

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C

For example, according to the invention, a membrane having a salt retention capacity exceeding 98%, which enables desalination of seawater in a single passage, can be prepared with a membrane constant greater than 3.2 liters / liter. m day. atm. (which with an applied pressure of 80 atm. and with an added amount of NaCl of 2 35,000 ppm corresponds to a flow of about 200 liters / m day); or one can prepare membranes suitable for desalination of brackish water with a salt retained capacity of over 90%, with a membrane constant greater than 8.3 liters / m day. atm. (which with a pressure of 80 atm and with an added amount of NaCl of 10,000 ppm 2 corresponds to a flow of about 600 liters / m day).

The osmotic pressure (in atm) of a solution of NaCl can be calculated approximately using the formula Ψ = 8.2 x Cr where C C is the salt concentration of the solution in wt%.

As is known, the higher the efficiency of a membrane, the greater the membrane constant and the salt-retaining capacity.

The membranes according to the invention allow desalinated water to be prepared in a single passage (containing salts less than 500 ppm), starting from brackish water or sea water, using water flow rates which make this application extremely suitable. .

In addition, for certain types of treatments, it may be appropriate to prepare membranes with much higher flow rates and with a lower salt retention capacity.

Thus, it is possible to obtain membranes with a constant 2 A ', which are between 50 and 90 liters / m day. atm. and with a salt retention capacity of between 50 and 90¾.

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The membranes according to the invention exhibit a particular resistance to compression due to the application of the indicated pressure, which resistance ensures a long life of the membrane. This particular resistance to compression 5 makes these membranes particularly suitable for desalination of seawater, making use of fairly high pressures.

In addition, the membranes of the invention are particularly effective in various separation and concentration processes where the reverse osmosis principles can be applied, such as e.g. purification of contaminated wastewater or sewage, recovery of undissolved organic matter; recovery of dissolved, inorganic substances, processing of solutions of foods such as milk, coffee, tea, grapefruit juice, whey, tomato juice, sugary solutions, separation of azeotropic products, such as hormones, proteins, vitamins, antibiotics, vaccines, amino acids, and in many other processes of similar nature.

The following Examples 3-6 are intended to illustrate the essential features of the invention while Examples 1 and 2 illustrate the preparation of polypiperazine amides.

EXAMPLE 1

Preparation of a Co-Polypiperazinamide of Furan-2,5-Dicarboxylic Acid with Thiofurazan-3,4-Dicarboxylic Acid 25 and 36.54 g of trans-2,5-dimethylpiperazine. A solution of 150 ml of dichloromethane containing 16.88 g of dichloride of thiofurazan-3,4-dicarboxylic acid was then mixed with the diamine solution over a period of 1 hour.

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This mixture was then stirred for 1 hour then 150 ml of dichloromethane containing 15.44 g of dichloride of furan-2,5-dicarboxylic acid was added.

After 1 hour, the solution was poured into 3 liters of 5 n-hexane and the polymer separated therefrom was filtered. After washing, the polymer was dried in an oven at 60 ° under vacuum. The yield was 95¾ and the logarithmic viscosity number (determined in h₂SO ^ at a concentration of 98¾ at a concentration of 0.5 10 g / 100 ml of solvent, at 30 °) was equal to 1.85.

EXAMPLE 2

Preparation of a polypiperazinamide of furan-2,5-di-carboxylic acid

A solution consisting of 400 ml of water, 4.56 g of trans-2.5-15 dimethyl-piperazine and 10.18 g of anhydrous £ 00 00 was poured into a 1.9 liter glass reactor under external cooling. This solution was then kept under vigorous stirring and through a lateral funnel was rapidly added a solution of 100 ml of dichloromethane and 7.72 g of dichloride of furan-2,5-dicarboxylic acid.

After 15 minutes, stirring was stopped and the dichloromethane was evaporated in vacuo and the resulting polymer was ground and then suspended in water with vigorous stirring. This suspension was then dried under vacuum at 50 ° C.

The yield was approx. 95 *, but the logarithmic viscosity number (determined in f 2 SO 2 with the concentration 98 wt *, with a concentration of 0.5 polymer / 100 ml of solvent, at 30 ° C) was equal to 3.27.

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Example 3 13.5 g of the copolypiperazinamide prepared according to Example 1 was suspended at room temperature in 86.5 g of a solution consisting of 82.15 g of N-methylpyrrolidone and 4.35 g of LiCl. This suspension was then brought to a temperature of 140 ° C with stirring. Thereby, there was a rapid formation of a transparent and clear solution which, at a temperature of 95 ° C, was filtered through a filter having a porosity of 5 µm.

The solution was then degassed, heated to 50 ° C and finally spread over a flat glass plate with a thickness of 0.2 cm.

This glass plate was then heated to 120 ° C on a Pia two term type electric heating organ (Bicase-15 Milano) for 10 minutes. The glass plate was then cooled to room temperature over 90 seconds and then dipped in a container containing water and ice. After approx. Ten minutes of immersion in this mixture of water and ice, the membrane was peeled off the glass plate. Membrane 20 was then kept in water and ice for an additional 50 minutes and then stored in water at room temperature.

This membrane had a positive and a negative side. By "positive side" is meant the side of the membrane opposite to the glass plate during its manufacture. By "negative side 25" is meant the surface of the membrane in contact with the glass plate during manufacture.

The water content of this membrane is approx. 61%. Deinne membrane was placed in a standard reverse osmosis cell in which an aqueous solution containing 10,000 30 ppm sodium chloride was circulated. The diaphragm was placed in the cell, making sure that it was on page 18

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facing the solution to be desalted should be the one opposite to the glass plate (the positive side) during the spread.

The saline solution was circulated in the cell under a pressure of 80 atm. The resulting water flow amounted to 2,000 liters / m day, while the salt retaining capacity was 51.5¾.

EXAMPLE 4

A membrane prepared according to Ex.

10 3, was subjected to a thermal treatment by immersion in water at 80 ° C for 15 minutes. This membrane was then placed in a reverse osmosis cell, according to the methods described in Example 3, to obtain a water flow of 380 liters / m day and a salt-retaining capacity of 98¾.

EXAMPLE 5 10 g of the polyamide whose preparation is set forth in Example 2 were suspended at room temperature in a solution consisting of 85.5 g of N-methylpyrrolidone and 4.5 g of LiCl. This suspension was then treated according to the methods described in Example 3, and a solution was obtained which was filtered through a 5 µm porosity filter.

The solution was then degassed, heated to 50 ° C and spread over a glass plate of 0.2 cm thickness to form a film of 0.03 cm thickness.

The glass plate was then heated on a Platoterm-type electric heater (Bicasa - Milan) for 8 minutes at 120 ° C and then cooled to room temperature over 90 seconds, and dipped in a container, for 19 minutes.

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containing water and ice.

After approx. After 10 minutes immersion in water and ice, the membrane could be removed from the glass plate. The membrane was then kept in water and ice for an additional 50 minutes, and finally preserved in water at room temperature.

The water content of the membrane was 64%. The membrane was placed in a reverse osmosis cell according to the methods set forth in Example 3, resulting in a water flow of 1200 liters / m day and a salt retained capacity of 92%.

EXAMPLE 6 According to the methods set forth in Example 5, a solution containing 10 g of poly (trans-2,5-dimethylpiperazine-3,4-furanamide) was prepared, which was characterized by λ = 4.

The solution was then spread over a 0.2 cm thick glass plate to form a film of 0.045 cm thick. The glass plate was then heated on a Platoterm-type electric heater (Bicasa - Milan) for 20 minutes at 1000 ° C. The glass plate was then cooled to room temperature over 90 seconds and then immersed in a container containing a mixture of water and ice.

After approx. 10 minutes of immersion in water and ia the membrane could be removed from the glass plate. The membrane was kept in water and ice for a further 50 minutes and then stored in water at room temperature. When the membrane was placed in a reverse osmosis cell according to the procedure of Example 3, a water flow of 690 liters / m day was obtained and a salt retention capacity of 96%.

Claims (11)

1. Anisotropic reverse osmosis polypeptazine amide gel type membrane, characterized in that the membrane consists of polypiperazine amides having a logarithmic viscosity of 0.5-6 dl / g (measured at 30 ° C in a solution of 0 5 g of polyamide in 99.5 gh of SO 2 with a concentration of 98%) and a structure between the end groups defined by the general formula (I): - * - Q - m where n is an integer sufficiently large to give the polymer a molecular weight suitable for the formation of a membrane, whereby the group defined by the general formula (II) / ~ Λ, - Η N - (II) rV7 Rx is a divalent organic group which is derived from piperazine, where x is an integer between 0 and 8, and hyo R is a alkyl, especially methyl or ethyl, wherein the -K groups represent groups derived from dicarboxylic acids, there for at least 10 mole percent has a structure defined by the general formula (III): DK 157978 B R. R "/ CQ (III) II il - OC - C0 "where V is an oxygen or sulfur liom, wherein R 'and R" which are identical or mutually different, are hydrogen, alkyl and / or an aryl group and have, for the remainder, a maximum of 90 mole percent, respectively, a structure selected from -OC - CH CH - CO - derived from fumaric acid -OC - CH II - C - CO - derived from mesaconic acid f derived from ortho- or isophthalic acid or terephthalic acid cou ^ ° ~ J derived from thiofurazan- 3,4-di- "carboxylic acid. N N \ / S Membrane according to claim 1, characterized in that the polypiperazine amide is prepared by polycondensation of 1 equivalent of substituted = piperazine of the formula: H - N / H y_y R x DK 157978B '' 'S .. and 1 equivalent of a dicarboxylic acid dichloride of the formula: R 1 - Nc - c II II C 10 C - cc - co-ci or 1 equivalent of a mixture of at least 10 mole-o of the dichloride and with the remaining amount up to 90 5 mole-Jo of one of the dicarboxylic acid dichlorides of the formulas: CIO C - CH CIO C ~ CH CH-COC1 * CH -C-C0-C1 * O acq COC1 foci \ _ / @ -cocx - \ J where x, R, R 'and R "have the same meaning as previously stated. Process for the preparation of the membrane according to claim 1 or 2, characterized in that the process comprises the following steps in the order indicated: 1. preparation of a solution of polypiperazine of the type defined in claim 1, in a suitable organic solvent, 5 2 ': spreading the solution over a flat plate, 3. partial evaporation of the solvent, 4. coagulation of the membrane by immersion in water, and finally preferably 5': thermal treatment of the membrane. Process according to claim 3, characterized in that the solution of the polymer in an organic solvent is carried out in the presence of a salt which is soluble in water and in the organic solvent. Process according to claim 3, characterized in that the organic solvent is selected from the group consisting of dimethylformamide, dimethylacetamide, diethylformamide, diethylacetamide, drroethylsulfoxide, N-methylpyrrolidone and tetramethylsulfone. Process according to claims 4 and 5, characterized in that the indicated salt, which is soluble in water and in the specified organic solvent, is an organic salt selected from the group consisting of LiCl, LiNO , LiBr, CaCl 2, ZnCl 2, MgCl 2 and MgClO 2. Process according to claims 3-6, characterized in that the solution is heated to temperatures exceeding 70 ° C, but preferably between 80 ° C and the boiling point of the solvent used. Process according to claims 3-7, characterized in that the spreading of the solvent is carried out on a flat glass plate. DK 157978 B Process according to claims 3-8, characterized in that the partial evaporation of the solvent is carried out at a temperature between 70 ° C and 200 ° C for a period ranging from 1 minute to 3 hours. Process according to claims 3-9, characterized in that the coagulation is carried out by dipping the membrane in water at a temperature between 0 and 300 ° C. Process according to claims 3-11, characterized in that the thermal treatment is carried out by dipping the membrane in hot water at a temperature between 60 and 100 ° C for a period between 1 minute and 5 hours.