Patent Publication Number: US-5833854-A

Title: Semipermeable composite membrane and a method for the preparation of such a membrane

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a semipermeable composite membrane comprising a microporous substrate provided with a semipermeable microporous substrate membrane which is provided, on at least one side, with a water-permeable polymer layer. 
     Such a membrane is disclosed in U.S. Pat. No. 4,885,091. More in particular, this publication relates to a clorine-resistant semipermeable membrane which has a polyamide layer formed by interfacial condensation of an aromatic carboxylic acid chloride with an aromatic polyamine which has a chlorine substituent on the aromatic ring and is selected from 4-chloro-m-phenylenediamine, 5-chloro-m-phenylenediamine. Such a membrane is suitable for the desalination of brackish water or sea water. 
     A composite membrane of the abovementioned type is also disclosed in U.S. Pat. No. 4,876,009, the polymer layer containing the reaction product of a tetrakis-aminomethyl compound having the formula C (NHMe) 4  with an aromatic polyacyl halide having the formula Ar(COX) n , in which Me: methyl, Ar: aryl, X: halide and n=2 or 3. 
     A composite membrane comprising a microporous substrate and a polymer layer applied thereto is also known from U.S. Pat. No. 4,828,708, the polymer layer having been formed by interfacial condensation of an aromatic polycarboxylic acid and a mixture of isophthaloyl chloride and trimesoyl chloride. Such a membrane can be used in reverse-osmosis systems. 
     SUMMARY OF THE INVENTION 
     A novel group of semipermeable composite membranes has now been found, which are provided with a polymer layer of such a composition that the membrane can be used for reverse osmosis or nanofiltration purposes. 
     The semipermeable composite membrane according to the invention is characterized in that the polymer layer comprises the interfacial-polymerization product of an amine-terminal dendrimer and a compound polymerizing therewith. 
     It should be noted that the term dendrimer refers to a macromolecule which has an even higher concentration of functional groups per unit volume of the polymer macromolecule, and also a more uniform distribution of these functional polymers in the external regions of the macromolecule than is the case with other known star-shaped polymers. 
     Expediently, according to the invention, poly(alkylenimine) dendrimers are used as amine-terminal dendrimers. These dendrimers preferably have terminal primary and/or secondary amine groups. 
     The polymer layer in the composite membrane according to the invention is preferably based on an amine-terminal poly(iminopropane-1,3-diyl) dendrimer, more in particular a 1,4-diaminobutane  4!: (1-azabutylidene) x-4  : propylamine or propyl-N-alkyl-amine, wherein x=4, 8, 16, 32 or 64. 
     Reverse-osmosis composite membranes of this composition have a flux of at least 40 l/m 2 .h and a salt retention of at least 95% under standard condition. 
     Expediently, the compound which polymerizes with the dendrimer is selected from the group consisting of a carboxylic acid or carboxylic acid derivative, a sulphonic acid or sulphonic acid derivative, and an isocyanate compound, being reactive with an amine. 
     The carboxylic acid derivative used is selected, according to an expedient embodiment of the invention, from the group consisting of isophthaloyl chloride, tere-phthaloyl chloride, trimesoyl chloride and thiadiazole polyacid derivatives such as thiadiazoledicarboxylic acid halides. The thiadiazoledicarboxylic acid halide used is preferably 1,2,5-thiadiazole-3,4-dicarboxylic acid chloride. Obviously it is also possible to use other polyfunctional acid halides, such as, for example, thionyl chloride. 
     If a sulphonic acid derivative is used, this is preferably selected from the di- or trifunctional sulphonyl chlorides, in particular 1,3,5-benzenetrisulphonyl chloride or 4,4&#39;-diphenyldisulphonyl chloride. 
     Instead of a carboxylic acid derivative or sulphonic acid derivative it is also possible, however, to use a polyfunctional isocyanate compound, such as toluene diisocyanate. 
     The semipermeable microporous substrate membrane for the top layer is preferably a polysulphone membrane, a polyethersulphone membrane, a polyacrylonitrile membrane or a poly(vinylidene fluoride) membrane. This list should not, however, be regarded as limiting. Substrate membranes formed from polycarbonate, polyamides, copoly-mers of acrylonitrile and vinyl chloride, polyacetal, polyacrylates, polyelectrolyte complexes and crosslinked poly(vinyl alcohols) can likewise be used, as can the membranes formed, for example, from cellulose nitrate, cellulose acetate, acrylic copolymers, poly(vinyl chloride) and polytetrafluoroethylene. 
     Expediently, the semipermeable substrate membrane has a layer thickness from 20 to 200 μm, in particular about 100 μm. The polyamide top layer, in contrast, is preferably ultrathin, i.e. has a thickness of from 0.05 to 1.0 μm. 
     The invention further relates to a method for the preparation of a semipermeable composite membrane, at least one surface of a substrate material being provided with a water-permeable polymer layer. 
     Such a method is disclosed in U.S. Pat. No. 4,885,091. The polyamide top layer formed in this case is formed, however, by interfacial polymerization of 4- (or 5-)chloro-m-phenylenediamine with an aromatic polycarboxylic acid chloride. 
     According to the method according to the invention, a polymer layer is formed, however, by means of an interfacial polymerization reaction from an aliphatic amine-terminal dendrimer and a compound polymerizing therewith. 
     The terminal amine groups of the dendrimer involved can be primary or secondary or a combination thereof. 
     Preferably, the dendrimer used is a poly(alkylenimine) dendrimer, in particular a poly(propylenimine) dendrimer. 
     A poly(propylenimine) dendrimer having primary terminal amine groups can be obtained, for example, by 1,4-diaminobutane being reacted with acrylonitrile, as described in WO 95/02008. By means of reductive alkylation, such a dendrimer can be converted, in part or in its entirety, into dendrimers having secondary terminal amine groups. 
     Preferably, the dendrimer used is a 1,4-diaminobutane  4!: (1-azabutylidene) x-4  : propylamine or propyl-N-alkyl-amine, wherein x=4, 8, 16, 32 or 64. 
     The compound which polymerizes with the dendrimer is preferably selected from the group consisting of a carboxylic acid or carboxylic acid derivative, a sulphonic acid or sulphonic acid derivative, and an isocyanate compound, being reactive with an amine. 
     The carboxylic acid derivative to be used according to the invention is expediently isophthaloyl chloride, terephthaloyl chloride, trimesoyl chloride or a thiadiazole polyacid derivative such as a thiadiazole dicarboxylic acid halide, in particular 1,2,5-thiadiazole-3, 4-dicarboxylic acid chloride. 
     The sulphonic acid derivative to be used according to the method of the invention is expediently selected from the di- and trifunctional sulphonyl chlorides, preferably 1,3,5-benzenetrisulphonyl chloride or 4,4&#39;-(phenoxybenzene)-disulphonyl chloride or 4,4&#39;-diphenyldisulphonyl chloride. 
     The isocyanate compound to be used according to the method of the invention expediently is an aromatic isocyanate, preferably toluene diisocyanate. 
     The semipermeable substrate membrane used is preferably a polysulphone membrane, a polyethersulphone membrane, a polyacrylonitrile membrane or a poly(vinylidene fluoride) membrane. This list should not, however, be seen as limiting, other polymers can likewise be used as explained hereinabove. 
     The invention further relates to the use of a 1,4-diaminobutane  4!: (1-azabutylidene) x-4  : propylamine or propyl-N-alkyl-amine dendrimer, wherein x=4, 8, 16, 32 or 64 for the preparation of a composite membrane. 
     Such a composite membrane can be used, in particular, as a membrane for nanofiltration or for reverse osmosis. 
     Surprisingly it was found that the composite membranes according to the invention, when the pH is lowered (pH&lt;5), exhibit higher fluxes, compared with known membranes, while the retention remained comparable. 
     It should be noted that the invention is not limited to a particular type of membrane but that any type can be used such as a flat membrane, hollow fibre membrane or tubular membrane. 
    
    
     BRIEF DESCRIPTION OF FIGURE 
     The sole figure represents the effect of pH in membrane flux for the membrane of the invention and a known membrane. 
    
    
     DETAILED DESCRIPTION 
     The invention will now be illustrated by means of a number of exemplary embodiments. 
     Example 1 
     Two asymmetric, microporous tubular polysulphone substrate membranes having a length of 2 meters were submerged in a 0.25 wt % aqueous solution of DAB-dendri-(NH 2 ) 8  (=DAB-8, brand name of DSM Fine Chemicals B.V., Sittard, NL, for a diaminobutane (amine) 8  dendrimer), to which 0.1 wt % of sodium dodecylsulphate (SDS) had been added as a surfactant. 
     The excess solution was allowed to drip off the substrate membranes. They were then immersed for 30 seconds at room temperature in a 0.5 wt % solution of trimesoyl chloride (TMC) in hexane. This resulted in the formation of a thin polyamide film on the microporous surface of the polysulphone substrate. The composite membranes obtained then underwent a heat treatment in an oven for 10 min at 80° C. 
     The membranes thus formed were tested in testing modules having a length of 1.8 meters at a pressure of 40 bar and a temperature of 25° C., at pH=6, with water containing 0.35% of NaCl. 
     The test results showed an average flux of 64 l/m 2 .h and an average NaCl retention of 98%. 
     Example 2 
     Two different composite membranes were prepared in the same way as in Example 1, except that instead of a solution containing 0.25% of DAB-8, an aqueous solution was used which contained 0.75 wt % of DAB-8, 0.5 wt % of poly(vinylmethylamine) (PVMA) and 0.02% of sodium dodecyl sulphate (SDS). 
     Test results of these membranes, at pH 6.5 and 25° C., gave an average flux of 84 l/m 2 .h at 40 bar with an NaCl retention of 98.0%. 
     The use of poly(ethylenimine) (PEI) instead of PVMA gave similar results. 
     Example 3 
     This example relates to the use of DAB-dendri-(NH 2 ) 4  (=DAB-4; 1,4-diaminobutane 4!: propylamine). 
     Two composite membranes were prepared in essentially the same way as in Examples 1 and 2. 
     The aqueous solution in this case contained 0.75 wt % of DAB-4 (brand name of DSM Fine Chemicals), 0.5 wt % of PVMA and 0.02 wt % of sodium dodecylsulphate (SDS), and the organic solution contained 1.6 wt % of isophthaloyl chloride (IPC) and 0.06% of trimesoyl chloride (TMC). 
     Test results under the abovementioned conditions gave an average flux of 40 l/m 2 .h and an NaCl retention of 98.9%. 
     Example 4 
     This example illustrates both the effect of the pH of the liquid to be treated on the flux and the NaCl retention values of membranes. 
     Membranes according to Example 2 show the following behaviour as a function of pH: 
     
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         Flux (1/m.sup.2.h)/Retention                                     
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pH  4.4    147/95.4%                                                      
pH  6.5    84/98.0%                                                       
pH 10.9    58/88.3%                                                       
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     Composite membranes according to the invention therefore, surprisingly, exhibit a special behaviour in their reaction with respect to the degree of acidity of the liquid to be treated. It was found that, in particular, there is a strong effect with respect to the water flux. Thus, when the pH is lowered (pH&lt;5) an unexpectedly larger flux is found to occur with such membranes, compared with conventional membranes, while the effect of the degree of acidity on the retention value is limited. 
     For example, in the accompanying figure a membrane according to the invention has been compared with a WFC 0995 membrane, as described in NL-A-84.02647 (corresponding with U.S. Pat. No. 4,721,568). 
     A membrane according to the invention is therefore particularly suitable for treating acidic feeds.