SUPPORTED REVERSE OSMOSIS MEMBRANES

Abstract: Preparation of supported reverse osmosis membranes on an unsized support using dimethylacetamide solvent and a lithium nitrate or chloride or magnesium chloride salt.

Description: TITLE Supported Reverse Osmosis Membranes Background of the Invention Asymmetric membranes of aromatic polyamides have long been used in ultrafiltration and reverse osmosis applications. However, the mechanical properties of the membranes often require reinforcement or support, for example, with polyester fabrics or fibers, in order to prepare satisfactory reverse osmosis structures based on these membranes. Typically, the reverse osmosis membrane is formed by coating an appropriate polymer solution onto the supporting fabric. Difficulties have previously been encountered in providing a supported membrane that has both good flux and high salt rejection. In addition, with the coating of extremely thin membranes, continuity of the membrane on the supporting surface is difficult to obtain. Moreover, the lack of proper adhesion between the supporting fabric and the reverse osmosis membrane can lead to delamination in use which results in the formation of blisters between the supporting fabric and the reverse osmosis membrane. SUMMARY OF THE INVENTION The instant invention provides an improved process for the preparation of supported reverse osmosis membranes which results in a product having excellent flux and salt rejection characteristics, membrane continuity and low blistering in operation. Specifically, the instant invention provides, in a process for the preparation of a reverse osmosis membrane by coating a solution of aromatic polyamide, solvent, and modifying salt onto a porous support, evaporating a portion of the volatile components and coagulating the polymer in the resulting membrane in a nonsolvent for the polymer, the improvement wherein the porous support is a woven, unsized, polyester or polyamide fabric, the solvent comprises at least about 80 percent dimethylacetamide, and the modifying salt comprises about from 20 to 60 weight percent, based on the polymer, of lithium nitrate, lithium chloride or magnesium chloride. DETAILED DESCRIPTION OF THE INVENTION The present invention is based on the discovery of the unique compatibility of a woven, unsized polyester fabric, dimethylacetamide solvent and lithium nitrate, lithium chloride or magnesium chloride salts in the coating solution. The polymers which can be used in the present invention can be those nitrogen-containing polymers described, for example, in Richter et al. U.S.-Patent 3,567,632, hereby incorporated by reference. Of the polymers disclosed in that patent, those having a sulfur content of about from 2.0 to 4.5 percent are preferred, as are those polymers prepared from metaphenylenediamine, metaphenylenediamine-4-sulfonic acid and isophthalic and terephthalic acids in which the ratio of isophthalic acid to terephthalic acid is about from 50:50 to 80:20. Within the solvent and salt requirements of the present invention, the coating solutions can be made and the membranes formed according to the usual procedures described therein. The solvent used in the present casting solutions is dimethylacetamide. The solvent should comprise at least about 80 percent dimethylacetamide, and substantially pure dimethylacetamide is preferred. Up to about 20 percent of a wide variety of other solvent components can be present, including residual water and other solvents such as dimethylformamide, dimethylsulfoxide and dimethylpyrrolidone. The modifying salts used in the coating solutions in the instant process should be lithium nitrate, lithium chloride, magnesium chloride or combinations thereof. These modifying salts should generally make up about from 20 to 60 weight percent based on the polymer. The membrane support used in the instant process is a woven, unsized polyester or polyaramide fabric. Fabrics used as a support in reverse osmosis applications are preferably low in cost, stable under the operating conditions encountered in reverse osmosis operations and highly porous so as to retain high flux in operation. The support material used in the present invention is generally prepared by weaving a yarn of polyester or aromatic polyamide which has been sized with poly(acrylic acid) or poly(vinyl alcohol). After weaving into a fabric, the fabric is scoured in water at elevated temperatures, substantially completely removing the sizing originally present on the yarn. The woven cloth is then dried, heat set and calendered and used as the reverse osmosis membrane substrate without further sizing treatment. A particularly satisfactory woven fabric for this purpose is that commercially available from Texlon, Inc. as "Texlon" 0715 sailcloth, woven from continuous filament "Dacron" polyester fiber. Other fabrics that have been found particularly satisfactory are "Monotex"- fabric prepared from polyester monofilament and commercially available from Advance Process Supply of Pennsauken, New Jersey and fabric prepared from continuous filament "Nomex" aramid fiber and commercially available from Stern & Stern Textiles, Inc. The final polymer solution generally contains about from 16 to 21 percent polymer, based on the total solution, and further comprises about from 60 to 20 percent of the modifying lithium salt, based on weight of the polymer. In general, increasing concentrations of salt result in higher flux in the finished membrane, while lessen amounts of salt increase the rejection rate of the membrane. While the viscosity of the polymer coating solution can vary widely, it is generally about from 100 to 400 poise to provide convenient handling characteristics and little tendency to permeate the support fabric. The coating solution further comprises minor quantities of additives typically used in such polymer coating solutions. For example, about 800 parts per million of polyethylene glycol monostearate typically improves the flux of the final membrane without significant depreciation of selectivity. After coating the solution onto the woven substrate, evaporating a portion of the volatile components and coagulating the polymer in a nonsolvent according to usual techniques, the resulting membrane can be further improved by chemical treatment, such as contacting the membrane with a dilute aqueous solution of about 100 parts per million of polyvinyl methyl ether. After coating the reverse osmosis membrane onto the porous support, it is preferably annealed to improve the stability of the supported membrane in use. Despite an initial reduction in flux resulting from such annealing, less change in flux over extended operation is realized. Annealing is typically conducted in water at elevated temperatures of about from 50 to 750C for a period of about from 15 to 60 minutes. The membranes resulting from the process of the instant invention exhibit an excellent combination of flux and salt rejection, combined with a low propensity toward delamination or blister formation. In this regard, the membranes resulting from the instant process are substantially better in both flux and salt rejection than unsized nonwoven supports and fully equivalent to sized nonwoven supports without the inconvenience and expense of an additional sizing step. The present membranes can exhibit a flux as high as 3.7 m/sTPa or greater and a salt rejection as high as 97 percent or more. By contrast, high flux and salt rejection do not seem to be realized when different solvents or different salts are used in the membrane coating solution with the same woven support fabrics. The invention is further illustrated by the following specific examples. In these examples, the polymer used was an aromatic copolyamide of metaphenylenediamine and isophthalic and terephthalic acids according to the formulations and procedures described in Richter et al., U.S. Patent 3,567,632. EXAMPLES 1 TO 14 AND COMPARATIVE EXAMPLES A TO K In Examples 1 to 14 and Comparative Examples A to K, polymers were used which were prepared from metaphenylenediamine (MPD), metaphenylenediamine-4sulfonic acid (HSMPD), isophthalylyl chloride (ICI) and terephthalylyl chloride (TC1). The ratios of MPD and ESMPD were varied to provide a sulfur content in the final polymer of 2.5 to 3.4 weight percent, while the ratio of ICl and TCl was varied to provide a ratio of between 50:50 and 80:20. The polymer composition used in Examples 1 to 14 and Comparative Examples A to K is indicated in Table lA. TABLE 1A ICI/TCl Ratio MPD/HSMPD Ratio Example (moles) %Sulfur (moles) 1 50:50 3.4 72/28 2 50:50 3.4 72/28 3 50:50 3.4 72/28 4 60:40 2.5 80/20 5 60:40 2.5 80/20 6 60:40 2.5 80/20 7 70:30 2.5 80/20 8 70:30 2.5 80/20 9 80:20 3.0 76/24 10 80:20 3.0 76/24 11 80:20 3.0 76/24 12 80:20 2.5 80/20 13 80:20 2.5 80/20 14 80:20 2.5 80/20 A 70:30 2.5 80/20 B 70:30 2.5 80/20 C 70:30 2.5 80/20 D ) 70:30 2.5. 80/20 E 70:30 2.5 80/20 F 70:30 2.5 80/20 G 70:30 2.5 80/20 E 70:30 2.5 80/20 I v 70:30 2.5 80/20 J 70:30 2.5 80/20 K 70:30 2.5 80/20 Membrane coating solutions were prepared from the polymer, solvent and modifying salt in the concentrations indicated in Table 1B. The solutions were coated onto a fabric support of the type indicated in Table lB and the resulting membrane was dried, cooled, gelled and substantially fully extracted. In Example 7, the membrane was annealed at 500C for 30 minutes and post-treated with a dilute solution of polyvinyl methyl ether. The supported membranes were evaluated for flux and salt rejection characteristics. The results of these tests, together with the process conditions used for each example, are summarized in Table IB. TABLE IB Coated Dry- Dry- At Polymer Salt Film ing ing 420 psi Conc. Conc. Thick Temp. Time kw Rej. Example Base Sizing Solvent Wt% Salt Wt%(BOP) mils. *F min. m/s. TPa % Comment 1 Texlon 1 None DMACa 20 LiNO3 25 9.7 215 12 3.2 83.9 2 Texlon None DMAC 20 LiNO3 50 8.9 215 12 5.2 89.7 3 Texlon None DMAC 20 LiNO3 40 9.1 215 12 3.7 92.2 4 Texlon None DMAC 18 LiNO3 10 8.3 215 12 .5 98.2 tested 800 psi 5 Texlon None DMAC 18 LiNO3 50 8.1 215 12 3.8 93.1 6 Texlon None DMAC 18 LiNO3 40 9.5 210 12 3.2 95.5 7 Texlon None DMAC 17 LiNO3 49 6.7 215 5.0 4.15 94.8 No blisters 3.1 95.4 Annealed 30 /50 C 1.9 97.8 Chem.Post treatement 8 Texlon None DMAC 17 LiNO3 49 7.5 215 3.4 3.46 95.2 No blisters 9 Texlon None DMAC 20 LiNO3 60 10.2 215 12 4.1 90.3 10 Texlon None DMAC 20 LiNO3 40 8.8 215 12 3.6 87.2 11 Texlon None DMAC 20 LiNO3 25 8.9 215 12 2.6 91.9 12 Texlon None DMAC 20 LiNO3 25 7.9 215 12 2.4 95.6 13 Texlon None DMAC 20 LiNO3 40 8.3 215 12 3.8 93.6 14 Texlon None DMAC 20 LiNO3 60 9.1 215 12 4.7 88.9 TABLE IB(continued) Wet Dry- Dry Polymer Salt Film ing ing Conc. Conc. Thick Temp. Time kw Rej. Example Base Sizing Solvent Wt% Salt Wt%(BOP) mils. *F min. m/s. TPa % Comment A Texlon None DMF10 16 MgCl2 39B 6.9 210 3.4 2.8 84.7 No blisters B 3329ê PAA7 DMAC 17 LiNO3 48 7.0 191 4.2 4.1 90.7 No blisters 3.0 93.7 Annealed 30 /55 C C 3329 None DMAC 17 LiNO3 48 7.0 191 4.2 - - Profuse blisters D 3329 PAA DMAC 16.94 LiNO3 49.53 6.8 212 4.5 3.4 96.7 No blisters 2.9 97.1 Annealed E 3329 PAA7 DMF 17.78 MgCl2 30.468 6.6 192 4.2 5.4 92.9 No blisters 4.8 93.2 Annealed F 3329 None DMF 16.18 MgCl2 39.058 7.9 193 3.4 0.84 35.5 No blisters G Pellon None DMAC 18.0 LiNO3 50.0 5.9 194 2.4 2.6 84.5 Blistered H 3329 None DMAC 18.0 LiNO3 50.0 7.9 193 3.4 5.78 91.9 Blisters I 33614 None DMAC 18.0 LiNO3 50.0 7.9 193 3.4 2.78 93.4 Blisters J 32645 None DMAC 16.7 LiNO3 49.61 7.8 203 3.4 4.1 95.0 Blisters K Powell6 None DMAC 16.7 LiNO3 49.61 6.9 203 3.4 - Profuse blisters Notes to Table IB: 1 "Texlon" 0715 calendered sailcloth; commercially available from Texlon, Inc. 2 "Hollytex" 3329, calendered spunbonded polyester; commercially available from Eaton-Dikeman 3 "Pellon" 6800, resin-bonded polyester; commercially available from Pellon 4 "Hollytex 3361, calendered spunbonded polyester; commercially available from Eaton-Dikeman 5 "Hollytex" 3254, calendered spunbonded polyester; commercially available from Eaton-Dikeman 6 Powell High Density Fabric, resin-bonded polyester; commercially available from Powell 7 Polyacrylic acid sodium salt neutralized to pH 5.0, molecular weight less than 50,000, wAcrysol A-l", commercially -available from Rohm and Haas 8 Anhydrous MgC12 concentration; salt added as hexahydrate 9 Dimethylacetamide 10 Dimethylformamide The data demonstrate that excellent supported membranes can be prepared on unsized supports, in which the membranes exhibit excellent flux and salt rejection with no blister formation. The use of magnesium chloride salt and dimethylformamide in Comparative Example A instead of the lithium salt resulted in low flux and salt rejection. The use of a sized support fabric in Comparative Example B resulted in a satisfactory membrane with no blister formation with high water flux but only fair to good salt rejection. The use of unsized nonwoven support fabric in Comparative Example C resulted in profuse blistering with very low salt rejection. The use of a similar, but sized support in Comparative Example D resulted in high flux and high salt rejection. In Comparative Example E, a membrane was produced having good salt rejection and high flux, but with the necessity of a sizing for the nonwoven support fabric. Elimination of the sizing in Comparative Example F resulted in a product having very low flux and salt rejection characteristics. In Comparative Examples G to K, the use of unsized nonwoven support fabrics resulted in blistering in each case. EXAMPLES 15 TO 29 AND COMPARATIVE EXAMPLES L TO Q In Examples 15 to 29 and Comparative Examples L to Q, the general procedure of Examples 1 to 14 was repeated, using the Texlon unsized base with DMAC solvent. Lithium nitrate salt was used in the coating solution at the concentrations indicated. The ratio of MPD to HSMPD in the polymer preparation was varied to provide a sulfur content in the final polymer ranging from 2.5 to 4.3 weight percent. The resulting polymers were coated onto a fabric support and evaluated as in Examples l to 14. The results are summarized in Table II. TABLE II Coated Dry- Dry- At Polymer Salt Film ing ing 420 psi Conc. Wt.% % MPD/HSMPD Thick Temp. Time kw Rej. Example Wt.% (BOP) Sulfur Ratio mils F min m/s. TPa % Comment 15 17 50 2.5 80/20 10.5 215 10 3.9 95.8 16 17 50 2.5 80/20 6.1 175 10 3.9 95.3 17 17 50 2.5 80/20 6.3 175 15 3.9 95.8 18 17 50 2.5 80/20 6.5 215 5 2.1 95.6 19 17 50 2.5 80/20 7.1 215 5 3.7 95.1 20 18 50 2.5 80/20 9.5 215 12 4.9 87.6 21 18 40 3.0 76/24 9.4 215 12 3.3 95.0 22 18 25 3.4 72/28 10.1 215 12 3.0 92.5 23 18 40 3.4 72/28 9.3 215 12 3.4 91.8 24 18 60 3.4 72/28 8.7 215 12 4.6 86.8 25 20 25 4.3 64/36 9.9 215 12 1.9 85.1 26 20 50 4.3 64/36 9.3 215 12 3.3 74.2 27 20 25 4.3 64/36 8.7 215 12 2.1 96.1 made with poly28 20 40 4.3 64/36 9.2 215 12 4.9 95.2 mer 50% neutra29 20 25 4.3 64/36 9.5 215 10 2.9 96.2 lized with CaO L 18 10 2.5 80/20 8.2 215 12 .5 96.1 tested 800 psi M 18 75 2.5 80/20 10.5 215 12 5.1 86.1 N 18 10 3.0 76/24 7.3 215 12 1.5 85.5 O 18 75 3.0 76/24 9.4 215 12 11.4 29.7 tested 600 psi P 18 10 3.4 72/28 7.4 215 12 .5 96.1 tested 800 psi Q 18 75 3.4 72/28 8.9 215 12 fast 15 EXAMPLES 30 TO 36 The general procedure of Examples l to 14 is repeated, except that in Examples 30 to 32 the polymer was coated onto an unsized sheet of a fabric prepared from continuous filament "Nomex" aramid fiber, and in Examples 33 to 36 the polymer was coated onto an unsized sheet of "Monotex" fabric prepared from polyester monofilament. The solvent used was DMAC and lithium nitrate salt was added to the solution. The resulting supported membranes were evaluated for flux and salt rejection characteristics. The polymer composition and test -results are summarized in Table Ill. TABLE III Coated Dry- Dry- At Polymer Salt Conc. Film ing ing 420 psi Conc. Wt.% % MPD/HSMPD Thick Temp. Time kw Rej. Example Wt.% (BOP) Sulfur Ratio mils F min m/s. TPa % 30 18 40 3.0 76/24 16.2 215 15 3.0 94.5 31 20 40 4.3 64/36 18.0 215 15 3.6 92.8 32 18 40 3.4 72/28 15.1 215 15 2.9 89.6 33 17 50 2.5 80/20 5.3 215 15 4.3 91.1 34 17 50 2.5 80/20 5.2 215 15 3.4 94.9 35 17 50 2.5 80/20 7.7 215 15 - 36 17 50 2.5 80/20 5.4 215 15 2.9 92.6 EXAMPLES 37 to 42 In Examples 37 to 42, the general procedure of Examples 1 to 14 was repeated, except that the polymers used are those shown in Table IVA. TABLE IVA EMI17.1 ICI/TCI <tb> Example <SEP> Amine(s) <SEP> Amine <SEP> Ratio <SEP> Acid <SEP> (s) <SEP> Ratio <tb> <SEP> O <SEP> O <SEP> O <tb> <SEP> # <SEP> # <SEP> # <tb> 37 <SEP> H2N <SEP> # <SEP> CNHNH2 <SEP> H2NHN <SEP> C <SEP> # <SEP> C <SEP> NHNH2 <SEP> 50/50 <SEP> ICI/TCI <SEP> 70/30 <tb> <SEP> # <SEP> # <tb> <SEP> O <tb> <SEP> # <tb> 38 <SEP> H2N <SEP> # <SEP> CNHNH2 <SEP> - <SEP> ICI <SEP> <SEP> # <tb> <SEP> O <SEP> O <tb> <SEP> # <SEP> # <tb> 39 <SEP> H2NNHC <SEP> # <SEP> O-CH2CH2CH2-O-#-C-NHNH2 <SEP> - <SEP> ICI/TCI <SEP> 70/30 <tb> <SEP> # <SEP> # <tb> <SEP> O <SEP> O <tb> <SEP> # <SEP> # <tb> 40 <SEP> H2NNHC <SEP> # <SEP> O-CH2CH2-O- <SEP> # <SEP> -C-NHNH2 <SEP> - <SEP> ICI/TCI <SEP> 70/30 <tb> 41 <SEP> NH2 <SEP> # <SEP> NH2 <SEP> - <SEP> TCI <SEP> <SEP> # <tb> 42 <SEP> H2NHN <SEP> # <SEP> NHNH2 <SEP> H2N#-O-#-NH2 <SEP> 50/50 <SEP> ICI/TCI <SEP> 70/30 <tb> The polymers were coated onto unsized "Texlon" fabric using DMAC solvent and lithium nitrate salt. The supported membranes were evaluated for flux and salt rejection characteristics. The results of these tests, together with the process conditions used for each example, are summarized in Table IV. TABLE IV Coated Dry- Dry- At Polymer Salt Conc. Film ing ing 420 psi Conc. Wt.% Thick Temp. Time Rej. kw Example Wt.% (BOP) mils F min % m/s. TPa Comments 37 18.9 30 11.0 215 12 93.1 3.4 38 20 40 8.5 215 12 42.5 6.1 39 16.6 50 10.1 215 12 94.9 .9 40 16.6 30 7.5 215 12 98.9 1.5 41 20 40 10.7 215 12 89.2 1.9 42 20 40 8.1 215 12 - - No rejection

Claims: CLAIMS: 1. In -a process for the preparation of a reverse osmosis membrane by coating a solution of aromatic polyamide, solvent, and modifying salt onto a porous support, evaporating a portion of the volatile components and coagulating the polymer in the resulting membrane in a nonsolvent for the polymer, the improvement wherein the porous support is a woven, unsized, polyester or aromatic polyamide fabric, the solvent comprises at least about 80 percent dimethylacetamide, and the modifying salt comprises about from 20 to 60 weight percent based on the polymer, of lithium nitrate, lithium chloride or magnesium chloride. 2. A process of Claim 1 wherein the nonwoven fabric is polyester. or Claim 2 3. A process of Claim wherein the modifying salt consists essentially of lithium nitrate. any one of to 3 4. A process of /Claimsl/further comprising annealing the resulting supported membrane in water at a temperature of about from 50 to 750C for a period of about from 15 to 60 minutes. 5. A procesasflYoof9eCloafimsA further comprising contacting the supported membrane with an aqueous solution of about 100 ppm polyvinyl methyl ether.

Inventors: BUCKFELDER, JOHN JOSEPH, SCHLEINITZ, HENRY MAX
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1982
Main classification: B01D13
IPC codes: B01D69, B01D71