P-XYLYLENEDIAMIDE/DIIMIDE COMPOSITE RO MEMBRANES

Abstract: Composite RO membranes having good resistance to halogen attack and extraordinarily high fluxes comprise a micro-porous polymeric support with an interfacially polymerized salt-rejecting layer thereon comprising an aromatic aliphatic diamide/diimide.

Description: BACKGROUND OF THE INVENTION Composite reverse osmosis (RO) membranes are known but there is still a need in the art for RO membranes that are resistant to degradation by halogens such as chlorine, which is often present in the feed waters of an RO process stream. There is also an ongoing need in the art for membranes with high salt rejection and flux rates. One method of making RO membranes is by interfacial polymerization of a salt-rejecting layer on a porous polymeric support so as to form a composite membrane. See, for example, U.S. Patent Nos. 4,039,440 and 4,277,344. Meta -xylylenediamine has been condensed with trimesoyl chloride in an interfacial polymerization reaction on a polysulfone support so as to form a meta -xylylenediamide composite RO membrane. See NTIS Report No. PB83-243170 entitled "Novel Composite Membranes" by Eric K. Ln. Lee, et al. dated April 1983. However, the membranes so prepared were not useful for practical desalination applications due to their poor chlorine resistance and low fluxes (on the order of 294 to 489 litres/sq.metre/ day [6 to 10 gfd] for those with salt rejections of 95% or better). SUMMARY OF THE INVENTION The novel RO membranes of the present invention are composite membranes comprising a para -aromatic aliphatic diamide or diimide salt-rejecting layer interfacially polymerized on a microporous polymeric support. DETAILED DESCRIPTION OF THE INVENTION According to the present invention, novel composite RO membranes are provided that possess good chlorine resistance, high salt rejection and extraordinarily high fluxes, the membranes comprising an interfacially-polymerized salt rejection layer on a microporous polymeric support. The salt-rejecting layer is the interfacial polymerization reaction product of an aromatic polyacylhalide cross-linking agent and an aromatic diamine or diimine of the general formula R₂HNR₁-Ph-R₁NHR₂ where Ph is a para -substituted phenylene group, R₁ is an alkylene group containing from 1 to 2 carbon atoms, and R₂ is hydrogen or an alkyl group containing from 1 to 2 carbon atoms. Examples of suitable aromatic polyacylhalides are isophthaloyl chloride (IPC), trimesoyl chloride (TMC) and terephthaloylchloride (TPC). The microporous polymeric support may be in the form of flat sheets or hollow fibers and may be selected from polysulfones, polyethersulfones, polyphenylsulfones, polyvinylidenehalides, including polyvinyl chloride, polyimides and polyetherimides, polyamides, polyesters, and polycarbonates. Such supports are available commercially in flat sheets typically 0.05 to 0.15 mm (2 to 6 mils) thick. Preferred hollow fiber supports typically have an internal diameter of 260 µm and an external diameter of 620 µm. The salt-rejecting layer may be formed on either the inner or lumens side of such hollow fibers or on the outer or external side 10 of the fiber. Flat sheet support membranes may be prepared by machine casting in accordance with known prior art methods. Hollow fiber support membranes may also be prepared according to known methods of hollow fiber spin casting. The composite membrane of the present invention is typically prepared by first immersing the support membrane in an aqueous solution of the diamine or diimine for a short period of time, preferably 1 to 3 minutes, removing it from the aqueous and draining it of excess solution, then immersing it in a dilute solution of a cross-linking agent in an organic solvent at ambient temperatures for less than one minute to form the interfacial polymer on the support. The composite membrane so formed is air-dried, then heat-cured at temperatures in the range of 40°C to 120°C for about 10 minutes to complete the reaction. The aqueous solution should contain from about 0.25 wt% to about 5 wt% of the amine or imine, 0.5 wt% to 2 wt% being preferred. The organic phase should contain from about 0.01 wt% to about 1.0 wt% of the cross-linking agent, preferably 0.05 wt% to about 0.5 wt%, together with additives of triethylamine (functioning as an acid scavenger), and a surfactant. Two of the most preferred reactants in forming the salt-rejecting layer of the composite membrane of the present invention are TMC and para -xylylenediamine (pXDA), which produce a salt rejection layer having the chemical structure shown below: The general class of composite RO membranes prepared according to the present invention using an aromatic diamine or diimine having the structure (I) exhibit salt rejections in excess of 95%, good chlorine resistance and extraordinary fluxes as high as 980 to 1960 litres/sq.metre/day (20 to 40 gfd), making them especially suitable for desalination of sea water by RO. Examples 1-5 A series of composite membranes of the present invention were prepared as follows. An asymmetric microporous polysulfone support flat sheet membrane approximately 0.10 mm (4 mils) thick and having microporosity on one side was immersed in a 2 wt% aqueous solution of pXDA containing 0.5 wt% triethylamine (as an acid scavenger) and 0.01 wt% of a surfactant sold under the trademark "Iconol NP-9" (BASF) Wyandotte Corporation of Parsippany, New Jersey), drained of excess solution, then immersed in a 0.5 wt% solution of TMC in hexane for 30 seconds at room temperature, causing formation of a para -xylylenediamide film on the microporous surface of the polysulfone support. The resulting composite membrane was air-dried at room temperature overnight, then heat-cured for 10 minutes at 100°C. The so-formed membrane was then tested in a 5515 kPa (800 psi) RO test, the feed water containing 0.5 wt% NaCl at pH 6.0 and 25°C. The results, with flux being expressed in litres per square metre of membrane surface area per day and, in parenthesis, gallons per square foot of membrane surface area per day (gfd), are shown in Table I. Examples 6-13 Composite membranes prepared in essentially the same fashion as those in Examples 1-5 with the exceptions noted were prepared in batches of three membranes at a time, and RO-tested, the average results of each group of three being as noted in Table II. Example 14 Chlorine resistance of an exemplary membrane prepared in essentially the same fashion as in Examples 1-5 was tested by subjecting the membrane to the same type of RO test conditions as set forth in Examples 6-13 for 40 days except that the pH was 4.5 and the feed water contained either 0, 0.5 or 10 ppm free chlorine in the form of NaOCl. In order to accelerate the effects of chlorine on the performance of the membrane, the test was run in an "on/off" scheme of six hours of chlorine-containing feed/18 hours of chlorinefree feed with a rinse of the membrane after each chlorination cycle. The results, taken in each instance after exposure to chlorine, are shown in Table III.

Claims: 1. A composite membrane comprising: (a) a microporous polymeric support; and (b) a membrane formed on said support; characterised in that the membrane on said support is an aromatic aliphatic diamide/diimide membrane constituting the product of interfacial polymerization of an aromatic polyacylhalide and an aromatic aliphatic amine/imine of the formula R₂HNR₁-Ph-R₁NHR₂ where Ph is a para -substituted phenylene group, R₁ is an alkylene group containing from 1 to 2 carbon atoms, and R₂ is hydrogen or an alkyl group containing from 1 to 2 carbon atoms. 2. The membrane of claim 1 wherein the microporous polymeric support is selected from polysulfones, polyethersulfones, polyphenylsulfones, polyvinylidenehalides, polyimides and polyetherimides, polyamides, polyesters, and polycarbonates. 3. The membrane of claim 1 or 2 wherein the aromatic polyacylhalide is trimesoyl chloride, the aromatic aliphatic amine/imine is para -xylylenediamine, and R₂ is hydrogen. 4. The membrane of any of the preceding claims wherein the support is selected from flat sheets and hollow fibers. 5. The membrane of any of the preceding claims wherein the support is a hollow fiber and the membrane is formed on the inside of said hollow fiber. 6. The membrane of any of claims 1 to 5 wherein the support is a hollow fiber and said membrane is formed on the outer surface of said hollow fiber. 7. A process for desalination of saline water by reverse osmosis comprising contacting the saline water under pressure with the membrane of claim 1, 2, 3 or 4.

Inventors: MCCRAY, SCOTT B.
Applicants: BEND RESEARCH, INC.
Application year: 1988
Main classification: B01D69
IPC codes: B01D71, C08G69, B01D69, D01F8, C02F1, D01F6