Abstract:
Supported anisotropic reverse osmosis membrane based on synthetic polyamides are prepared by first preparing a support therefor for treating a material suited for serving as a support for a polyamide membrane, with a heat resistant water soluble polymer which is insoluble in the polar solvents to be used for preparing the polyamide solution described below; preparing a solution of the polyamide in an organic polar solvent, in the presence of a saline compound; spreading the thus prepared polyamide solution onto the prepared support; partially evaporating the solvent by heating; coagulating the membrane in an aqueous medium; and optionally, thermally treating the membrane.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 708,969, filed July 26, 1976, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to supported anisotropic reverse osmosis membranes based on synthetic polyamides and processes for their preparation. 
     2. The Prior Art 
     It is known that many synthetic polyamides may be used with success in the preparation of anisotropic membranes for reverse osmosis, which membranes have good salt rejection characteristics and a high water flux. 
     Unfortunately, such membranes do not possess altogether satisfactory mechanical properties. In general, serious difficulties are encountered in assembling these membranes into particular types of reverse osmosis modules such as tubular, spiral and the like modules. 
     In order to overcome these disadvantages, numerous procedures have been suggested. One such procedure involves the use of supported polyamide membranes, namely membranes containing homogeneously incorporated therein a support such as cloth, fabrics or fibers of various types, shapes and sizes, for improving the physical-mechanical characteristics of the membranes and for facilitating their assembly into reverse osmosis modules of any type, shape and dimension. 
     This procedure, however, is not altogether free of disadvantages. In fact, in the case of processes for the preparation of membranes in which the solution is spread on one face of the support or carrier, or the support is immersed or dipped into the polyamide solution, it has been observed that a thick layer of polymeric material, which will seriously compromise the subsequent working of the membrane, forms on both faces of the carrier. In fact, such a membrane initially exhibits low water flux and high salt rejection and then, due to the accumulation of salt around the membrane, the phenomenon is reversed and it exhibits low saline rejection and high water flux. 
     In the case of processes consisting of laying the support or carrier on a glass sheet and subsequently spreading over the glass sheet a polyamide solution (which is then followed by evaporation of the solvent and the coagulation in an aqueous medium of the membrane) the polyamide solution penetrates into the meshes of the support at the moment of the spreading, this, of course, causes the same disadvantages as in the previously noted situation. Moreover, part of the air trapped between the glass sheet and the support remains incorporated in the polyamide solution and leads to the emergence of air bubbles which, in turn, produce superficial faults in the membrane during the evaporation of the solvent. This causes a serious deterioration in the flux rate and/or saline rejection characteristics of the membranes. 
     It is an object of the present invention to provide a process for the preparation of supported anisotropic reverse osmosis membranes based on synthetic polyamides, and the membranes themselves, which are free of the above-mentioned disadvantages. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, there are provided supported anisotropic reverse osmosis membranes based on synthetic polyamides, and processes for their preparation. The process according to the invention comprises, in the order given, the following steps: 
     1. preparation of supports by treating materials suitable for use as a support for polyamide membranes, with water soluble polymers that are resistant to high temperatures, in particular to temperatures between 80° and 140° C., and which are insoluble in the polar solvents used for the preparation of the polyamide solutions; 
     2. preparation of a polyamide solution in an organic polar solvent, in the presence of saline components; 
     3. spreading of the polyamide solutions on the supports prepared according to step 1; 
     4. partial evaporation of the solvent by heating; 
     5. coagulation of the membranes in an aqueous medium; and finally, preferably, but not necessarily; 
     6. thermal treatment of the thus obtained membranes. 
     FIRST PHASE 
     The material of which the support consists may be selected from a wide range of synthetic, natural or artificial products such as polypropylene, rayon, polymers or copolymers of vinylidene chloride, polymers or copolymers of acrylonitrile, glass fibers, asbestos fibers, cotton, polyester fibers and the like. 
     All of these materials may be in the form of fabrics, cloth or other manufactured flat, tubular (or any other desirable shape) products. 
     All of these materials must be capable of resisting, at high temperatures, the solvents used in the casting of the polyamide membrane. Moreover, the above materials must not show any appreciable variations in volume with varying temperatures in the range of 20° C. to 100° C. 
     In order to insure good working of the polyamide membranes in reverse osmosis processes, the material used as the support must be inexpensive and moreover must: 
     be resistant to the pressures used in reverse osmosis processes without breaking or excessively clogging; 
     have at least one smooth face with a homogeneous structure so that the membrane is not locally damaged when it is under pressure; 
     have a sufficiently high porosity so as not to produce excessive load losses; and finally, it must 
     possess good chemical resistance in acid and alkaline media and be resistant to oxidizing agents. In any event, it must have a mean life span greater than that of the membrane itself. 
     The polymers suitable for use in the treatment of the supports may be selected from a wide range of natural, synthetic and artificial water soluble polymeric materials, resistant to high temperatures, and more particularly to temperatures between 80° and 140° C., and must be insoluble in the polar solvents used for the preparation of the polyamide solutions. 
     Typical examples of such materials are water soluble acrylic and methacrylic polymers and copolymers, such as homopolymers of acrylic and methacrylic acid; copolymers of acrylic acid with vinylpyrrolidone; polyvinyl-methylether, polyacrylamides, polymethacrylamides, copolymers of acrylamide with amides such as dimethylamino-acrylamide and dimethylamino-propylamide; water soluble derivatives of cellulose such as hydroxyalkylcellulose, carboxyalkylcellulose and the like. 
     The cloth, fabric or other manufactured articles to be used as supports for the spreading of the membrane are homogeneously coated with a thin layer of the water soluble polymer. 
     This spreading operation may be carried out in various ways; however, a preferred technique comprises: 
     preparing an aqueous solution of the water soluble polymer; 
     dipping the support in or spreading said solution on it at a suitable temperature which, in general, is about room temperature, for a period varying from 1 second to 1 hour, preferably between 10 seconds and 30 minutes; and finally, 
     drying the support which has been thus treated with the solution, in an oven at a temperature between 40° C. and 140° C., for a period of from 1 minute to 1 hour, preferably between 1 minute and 40 minutes. 
     This treatment of the support with water soluble materials allows one to avoid the evaporation of the film-generating solvent (solvent used for dissolving the polyamide) from the side of the support; in this way a differentiated evaporation takes place, wherefore, after coagulation of the membrane in an aqueous medium, an asymmetrical structure is formed. 
     Such a treatment offers, moreover, the advantage of hindering the passage of the casting solution through the support during evaporation of the solvent at high temperature, due to the consistent decrease in viscosity of said solution with varying temperature (from 20° C. to about 140° C.). 
     SECOND PHASE 
     The process according to the invention may be applied to the preparation of supported membranes based on any type of polyamide. At any rate, it is particularly suited in the case of polyamides having good solubility in organic polar, water soluble solvents, belonging to class m of the solvents that form hydrogen bonds (m-H bonding group) with a solubility parameter δ&gt;8 (cal/cc) 1/2  according to the classification system of H. Burrel, in &#34;Polymer Handbook IV&#34; page 341, J. Brandrup, E. N. Immergut, Editor, Interscience, N.Y. 
     Examples of such solvents are dimethylformamide, dimethylacetamide, diethylformamide, diethylacetamide, dimethylsulphoxide, N-methyl-pyrrolidone, tetramethylsulphone, and the like. 
     These solvents may be used alone or in admixture with smaller amounts of other solvents belonging to class s according to the above indicated classification system. 
     Illustrative but not limiting examples of such polyamides are the (co)polypiperazinamides, i.e., the polycondensation products of piperazine or its derivatives substituted in the nucleus (possibly in admixture with other diamines) with anhydrides or dichlorides of aromatic and heterocyclic, saturated or unsaturated dicarboxylic acids such as for instance fumaric, mesaconic, adipic, phthalic, isophthalic acid, phthalic acids substituted in the aromatic nucleus and heterocyclic acids derived from furazane, thiofurazane, pyridine, furan, thiophene and the like. 
     Other examples of polyamides are those described in U.S. Pat. Nos. 3,567,632 and 3,518,234 and in particular the polyamides formed from phthalic acids (ortho, iso and terephthalic) and phenyldiamines (meta and para) and those containing simple or substituted benzimidazolic groups. 
     A water soluble salt which is also soluble in the organic solvent is present as a third component in the polyamide solution. Examples of such salts are LiCl, LiNO 3 , LiBr, CaCl 2 , ZnCl 2 , MgCl 2 , Mg(ClO 4 ) 2  and other like salts. In addition to this saline component water may also be present in the solution. 
     In general, the salt is present in the solution in considerable quantities in comparison with the polyamide; in general the weight ratio of polyamide/salt is between 1 and 10. 
     The polyamide concentration in the solution may vary within a wide range, in general between 5% and 60% and preferably between 8% and 25% by weight, with respect to the weight of the solution. 
     The solution may be prepared following different methods; for example, in one such method the solvent+polyamide mixture is subjected to mechanical stirring, eventually heating the mixture at temperatures which, depending on the type of polyamide, solvent and the polyamide/solvent weight ratio range from 20° C. to the boiling temperature or the degrading temperature of the solvent, and in general at temperatures between 40° and 180° C.; after which, the thus obtained solution is filtered through a porous filter or a filtering membrane or through other filtering systems. 
     The properties of the membranes that are obtained from this solution depend largely on the quality of the prepared solution. 
     THIRD PHASE 
     The solution obtained in the second phase is spread on the support or carrier prepared in the first phase. 
     The spreading may be achieved in various ways. For example, the solution may be spread on the support by means of a film spreader so as to form a thin layer of solution on the support itself. 
     The spreading is carried out on flat supports if one wishes to obtain supported membranes of a flat shape. If supported membranes of a tubular shape are desired, the polyamide solution will be extruded into the inside of the tube that acts as the support and which is rotated about its axis and into which is fed a nitrogen current. 
     In the case where the polyamide solution is spread on the outside surface of the supporting tube, the latter (closed at one or both of its ends) is immersed into the polyamide solution or alternatively, the latter is uniformly spread on the outer surface of the tube, for example, by extrusion. The thickness of the cast film may vary over a wide range and is generally between 0.02 and 0.8 mm, depending on the characteristics of the support. 
     FOURTH PHASE 
     The thin layer of polyamide solution, spread on the support of suitable shape, is placed into an oven to partially evaporate the solvent. The evaporation time and temperature depend on the type of solvent, on the composition of the casting solution, on the type of polyamide and on the type of water soluble polymer used. 
     The evaporation temperature in general is between 40° C. and 180° C., while the evaporation time is between 1 minute and 3 hours. 
     FIFTH PHASE 
     The film from solution as obtained in the fourth phase, is coagulated either in water or in an aqueous saline or alcoholic solution (i.e., containing small quantities of inorganic salt and/or organic compounds containing one or more OH groups, such as lower aliphatic alcohols, glycols or ethers, at a temperature between 0° C. and 30° C. and for a period of time between 1 minute and 60 minutes. In this phase the real formation of the semipermeable membrane occurs. Moreover, during this phase, the water soluble polymer layer which had been spread on the support during the first phase goes into solution and the membrane automatically and homogeneously adheres to the support. 
     SIXTH PHASE 
     The membranes, as obtained from the fifth phase, do not always show completely satisfactory reverse osmosis characteristics; the flux in general is very high, usually greater than 500 liters/sq.m. day, but the saline rejection is in general low and usually below 50%. 
     The thermal treatment, i.e., that of phase 6 of the process according to the invention, obviates these disadvantages and causes a considerable and long lasting increase in the desalting capacity of the membrane. 
     The thermal treatment may be conducted in various ways; according to a preferred method the membranes are placed into hot water for a period of time between 1 minute and 5 hours, preferably between 5 minutes and 2 hours, at a temperature between 40° C. and 100° C. 
     The &#34;gel&#34; structure of the membrane is evidenced by the high water content of the membranes, greater in fact than 20% by weight, and generally between 40% and 80% by weight. 
     The permeability to water of the membranes may be defined by the following equation: ##EQU1## 
     It may also be defined as a constant A of the membrane, by the following equation: ##EQU2## wherein the expression &#34;applied effective pressure&#34; means the difference (ΔP-Δπ), wherein ΔP is the difference in hydraulic pressure applied on the two faces of the membrane and Δπ is the difference in osmotic pressure between the feeding solution and the solution crossing the membrane. 
     The membranes of the invention have a membrane constant that, in general, is rather high. 
     For exemplification purposes only, it is noted that membranes having a saline rejection greater than 98% and which may be used for desalting of sea water in a single passage, may be obtained according to the invention with a membrane constant greater than 3.2 l/sq.mt.d.atm. (which, with an applied pressure of 80 atm. and a feed of 35,000 ppm of NaCl, corresponds to a flux of about 200 l/sq.mt.d). In addition, membranes suited for the desalting of brackish waters, with a saline rejection greater than 90% may be obtained with a membrane constant greater than 8.3 l/sq.mt.d.atm. which with a pressure of 80 atm. and with a feed of 10,000 ppm of NaCl corresponds to a flux of about 600 l/sq.mt.d.). 
     The osmotic pressure (in atm.) for an NaCl solution may be calculated approximately from the equation π=8.2×C 1 , wherein C 1  is the saline concentration of the solution in % by weight. As is known, the efficiency of a membrane increases as its membrane constant and saline rejection increase. 
     The membranes of the invention permit one to obtain in a single passage desalted waters (with a salt content of less than 500 ppm) starting from brackish or sea waters, with water flux values that make this procedure extremely convenient. 
     Moreover, for some types of treatments, it may be more convenient to obtain membranes with very high fluxes and a somewhat lower saline rejection. 
     Thus, there may be obtained membranes with a constant A between 50 and 90 l/sq.mt.d.atm. and a saline rejection between 50% and 90%. 
     The membranes of the invention exhibit a particular resistance against packing resulting from the applied pressure. This results in a long working life for the membrane; this particular resistance to packing makes these membranes particularly suitable for the desalting of sea water, in the treatment of which, very high pressures are generally used. 
     The membranes of the invention are, moreover, particularly effective in various separation and concentration processes in which the principle of reverse osmosis may be applied, such as, purification of polluted water drains; recovery of organic solutes; treatment of food solutions such as milk, coffee, tea, citrus juices, whey, tomato juice, sugar solutions; separation of azeotropes; separation and concentration of biological and pharmaceutical products such as hormones, proteins, vitamins, antibiotics, vaccines, aminoacids; and other similar processes. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following examples are given for illustrative purposes without, however, limiting the invention as defined in the claims. 
     EXAMPLE 
     (A) PREPARATION OF SUPPORTS 
     A series of supports consisting of cloths of various types, the characteristics of which are reported in Table I, were treated with water soluble polymers having the characteristics recorded in Table II. 
     Thereafter, an aqueous solution of water soluble polymer was prepared, operating at room temperature and under vigorous stirring. This solution was then filtered on a filter with a porosity of 0.5 micron, and the filtered solution was left to deareate overnight. 
     The flat-shaped supports thus treated were then spread with the aqueous solutions of the water soluble polymeric materials, at room temperature. Subsequently the thus treated supports were dried in an oven under heavy ventilation, for the times and temperatures given in Table III. 
     (B) PREPARATION OF SUPPORTED MEMBRANES 
     For preparing the solutions of polymeric materials to be spread on the supports prepared according to the procedures described above, polyamides and copolyamides of various types were used. 
     Tables IV and V list the characteristics of said polyamides and copolyamides, the solutions obtained therefrom and the spreading and drying conditions maintained during the preparation of the supported membranes. 
     In practice, the solutions were prepared using the components indicated in Tables IV and V, and subjecting the polyamide+solvent+salt mixture to stirring until clear solutions were obtained, which solutions were then filtered on filters of 5 and 2 microns and by then spreading these mixtures on the supports prepared according to the procedure described above under (A). 
     The thin layer of polyamide solution spread onto the support was partially dryed in an oven heated from the bottom, at temperatures and for the times indicated in Tables IV and V. Thereafter, the film and the support adhering thereto were immersed into ice water kept under stirring, so that the salt, the residual solvent and the water soluble polymer were thus removed. 
     The asymmetric membranes thus obtained, before being used in reverse osmosis processes, were kept in water at 20° C. for at least 24 hours. 
     The reverse osmosis tests, the results of which are recorded in Tables IV and V, were carried out over a period of time greater than 2 days, under the following conditions: 
     
         ______________________________________operational pressure            =           60 kg./sq.cm.NaCl concentration in the feed            =           10,000 ppmtemperature      =           25° C.______________________________________ 
    
     
                                           TABLE I__________________________________________________________________________Characteristics of the Cloths Used in the Preparation of the Supports            CHARACTERISTICS                        Number of                        Threads perREINFORCING CLOTH             OF THREAD  cm.    Thickness ofType   Chemical Nature            Warp  Weft  Warp                            Weft                               Cloth__________________________________________________________________________A  Cotton        65 denier                  79 denier                        53  35 160 micronB  Polyethylenterephthalate*            50 denier                  70 denier                        53  35 160 micronC  Polypropylene**            100 denier                  100 denier                        120 56 300 micronD  Polyvinylidenchloride            91 denier                  91 denier                        59  59 100 micronE  Polyethylenterephthalate**            70 denier                  70 denier                        59  59 100 micronF  Polypropylene**            106 denier                  106 denier                        47  47 105 micronG  Polyethylenterephthalate***            125 denier                  125 denier                        42  33 150 micron__________________________________________________________________________  *texturized  **continuous filament  ***calendered 
    
     
                                           TABLE II__________________________________________________________________________Characteristics of Water Soluble Polymeric Materials UsedFor the Treatment of the Reinforcing ClothsType of Polymeric Material             Initials                    Producer__________________________________________________________________________Polyacrylic acid with molecu-             PA     Cofaltz &amp; Farbenfabrikenlar weight of about 50,000                    Bayer - GermanyMethylcellulose with a viscos-             MC     Fluka - Germanyity of 4,000 cp aμ at 25° C.Separarne NP 10 - copolymer of             SNP-10 Dow Chem. GmbH - Germanyacrylamide and acrylic acidSepararne AP-45 - copolymer of             S-AP-45                    Dow Chem. GmbH - Germanyacrylamide and acrylic acidPolyvinylpyrrolidone             PVP    General Aniline &amp; Film                    Corporation - GermanyPolyvinylalcohol with molecu-             PVA    BDH Chem. Ltd. - Greatlar weight of 14,000     BritainVinylpyrrolidone-vinyl acetate             E-735  G.A.F. S.r.l. - Italycopolymers        E-335__________________________________________________________________________ 
    
     
                                           TABLE III__________________________________________________________________________Treatment Conditions of the Supporting ClothsWith Water Soluble Polymer Materials     Type of Polymer Concen-                      Thickness of Coat-                                DryingType ofType of     Water Soluble             tration in                      ing of Cloth in                                ConditionsSupportCloth(1)     Polymer(2)             Water (wt. %)                      microns   °C.                                   Minutes__________________________________________________________________________1    A    PA      50       100       70 82    B    PA      50       100       70 83    C    PA      50       200       70 104    D    PA      50       80        70 55    E    PA      50       80        70 56    F    PA      50       80        70 57    G    PA      50       100       70 88    A    MC      1.7      100       70 89    B    MC      1.7      100       70 810   C    SNP-10  1.0      200       70 1011   D    SAP-45  1.0      80        70 512   E    PVA     1.7      80        70 513   F    E-735   1.7      80        70 514   G    E-335   1.7      80        70 5__________________________________________________________________________  (1)See Table I  (2)See Table II 
    
     
                                           TABLE IV__________________________________________________________________________Osmotic Performance of Supported Anisotropic Membranes Based onPoly(trans-2,5-dimethylpiperazine-3,4-thiofurazanamide)(1) in Function of Supports of Various Types              Performance of Membrane                                Breaking LoadType of Type of               Resistance                                at MoistureMembrane Support      Flux    Saline Rejection                       (2)      (2)__________________________________________________________________________1     1-A  519         1/sq.m.d              96.0%    -- kg./sq.cm.                                   kg./sq.cm.2     2-B  508         &#34;    98.2%    &gt;9 &#34;     530                                   &#34;3     9-B  556         &#34;    98.7%    &gt;9 &#34;     530                                   &#34;4     10-C 456         &#34;    90.8%    &gt;9 &#34;     345                                   &#34;5     11-D 780         &#34;    88.7%    &gt;9 &#34;6     12-E 480         &#34;    95.8%    &gt;9 &#34;     995                                   &#34;7     13-F 565         &#34;    90.9%    &gt;9 &#34;     300                                   &#34;8     14-G 1043         &#34;    88.2%    &gt;9 &#34;     860                                   &#34;9     7-G  789         &#34;    94.6%    &gt;9 &#34;     860                                   &#34;10 (3) --   600         &#34;    98.7%    1.0                          &#34;     66 &#34;__________________________________________________________________________ (1) A polyamide with a viscosity η.sub.in = 3.7 dl/g was used; the spreading solution was prepared in Nmethylpyrrolidone; polyamide concentration = 10.5% by weight with respect to the solvent with about 3.5% by weight of LiCl, the viscosity of the solution at 30° C. wa 300 poises; the thickness of the coating was 600 microns and the evaporation conditions were 120° C. for 12 minutes. (2) According to ASTM D 638. (3) These data concerning nonsupported membranes are recorded for purpose of comparison. 
    
     
                                           TABLE V__________________________________________________________________________Osmotic Performance of Supported Anisotropic Membranes Based on VariousPolyamides Depending on Type of Support, Type of Polyamideand the Preparation Conditions of the Membranes Themselves             Conditions For Preparation of Membranes                                   Evapora-    Osmotic Prop-Type     Type           wt % of    Coating                                   tion Tem-   tiesof  Type of  Inherent             LiCl wt %                   Polyamide                         Type Thick-                                   perature          SalineMem-    of   Poly-        Viscosity             Based on                   Based on                         of   ness and time                                         Coagula-    rejec-brane    Support    amide        in dl/g             Polyamide                   Solvent                         Solvent                              (microns)                                   (minutes)                                         tion  Flux  tion__________________________________________________________________________11  2-B  (1) 3.60 29    11    NMP(5)                              600  120°/12                                         in water                                               540 1/m.sup.2                                                       95 %                                         at 0° C.12  2-B  (2) 3.79 29    11    NMP  600  120°/13                                         in water                                               400                                                     97.9%                                         at 0° C.13  2-B  (3) 1.9  21    15    NMP  400  130°/10                                         in CH.sub.3 OH                                               170                                                     93.9%                                         at 30° C.14  2-B  (4) 0.8  30    15    DMA(6)                              400  110°/15                                         in water                                               600                                                     94.5%                                         at 0° C.__________________________________________________________________________ (1) Copolyamide of 3,4thiofurazan-dicarboxylic acid (80 mol%) and isophthalic acid (20 mol%) with trans 2,5dimethylpiperazine. (2) Copolyamide as in (1) in which the molar ratio between the two acids is equal to 50/50 instead of 80/20. (3) Polyamidobenzimidazol obtained by condensation of terephthaloyl chloride and 2,4diamino-diphenylamine and subsequent heating at 280° -300° C. for 3 hours. (4) Copolyamide of mphenylendiamine and isophthalic and terephthalic acid (molar ratio: 70/30). (5) NMP = Nmethyl-pyrrolidone. (6) DMA = Dimethylacetamide. 
    
     From the data reported in Tables IV and V it will be readily noted that the supported membranes according to the invention possess excellent mechanical resistance characteristics combined with excellent osmotic performances (high flux and high salt rejection). 
     Variations and modifications can, of course, be made without departing from the spirit and scope of our invention. 
     Having thus described our invention what we desire to secure by Letters Patent and hereby claim is: