Abstract:
Composite RO membranes having good resistance to halogen attack and extraordinarily high fluxes are disclosed, the composite membranes comprising a microporous polymeric support with an interfacially polymerized salt-rejecting layer thereon comprising an aromatic aliphatic diamide/diimide.

Description:
The government has rights in this invention under Department of Defense Contract No. DAAK-70-85-C-0059. 
     This invention relates to membranes for reverse osmosis that are resistant to attack by halogens and that have extraordinarily high fluxes. 
    
    
     BACKGROUND OF THE INVENTION 
     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. Pat. No. 4,039,440. 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 &#34;Novel Composite Membranes&#34; by Eric K. L. 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 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 
     
         R.sub.2 HNR.sub.1 --Ph--R.sub.1 NHR.sub.2                  (I) 
    
     where Ph is a para-substituted phenylene group, R 1  is an alkylene group containing from 1 to 2 carbon atoms, and R 2  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 2 to 6 mils thick. 
     Preferred hollow fiber supports typically have an internal diameter of 260 microns and an external diameter of 620 microns. 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 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: ##STR1## 
     The general class of composite RO membranes prepared according to the present invention and having the structure (I) exhibit salt rejections in excess of 95%, good chlorine resistance and extraordinary fluxes as high as 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 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 trade name &#34;Iconol NP-9&#34; (BASF Wyandotte Corporation of Parsippany, N.J.), 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 an 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 gallons per square foot of membrane surface area per day (gfd), are shown in Table I. 
     
                       TABLE I______________________________________Membrane/       Flux   RejectionExample No.     (gfd)  (%)______________________________________1               19.5   96.42               23.4   95.83               20.9   96.94               23.5   96.65               23.1   96.5______________________________________ 
    
     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. 
     
                       TABLE II______________________________________Membrane/    pXDA    TMC     Heat Cure                            Flux RejectionExample No.    (wt %)  (wt %)  (°C.)                            (gfd)                                 (%)______________________________________6        2       0.1     100     20.5 99.07        2       0.5     100     22.1 96.48        2       0.1     60      26.6 98.89        2       0.5     60      21.6 98.110       2       0.05    none    20.6 97.511       0.5     0.1     60      25.5 97.212       2       0.1     60      36.6 97.113       1       0.1     60      42.8 96.4______________________________________ 
    
     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 &#34;on/off&#34; scheme of six hours of chlorine-containing feed/18 hours of chlorine-free 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. 
     
                       TABLE III______________________________________    Chlorine      Flux   RejectionDay No.  (ppm)         (gfd)  (%)______________________________________0        0             14.5   97.25        10            3.6    98.210       10            3.8    98.817       10            2.8    98.619       0.5           4.2    98.024       10            4.5    98.128       0.5           3.4    97.035       10            3.5    97.040       10            3.3    97.0______________________________________ 
    
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.