SURFACE FLUORINATION OF MEMBRANES

Abstract: Water can be passed through a polysulfone membrane at an improved rate of exposing the membrane to elemental fluorine in admixture with an inert gas in the substantial absence of oxygen.

Description: SURFACE FLUORINATION OF MEMBRANES The invention relates to improving the rate at which water can be transferred through a membrane, as 1n filtration or reverse osmosis. More particularly, the invention relates to the treatment of polysulfone membranes used for ultraf1ltrat1on. It has been recognized that while many polymers are hydrophoblc, formation of a hydrophlUc surface on such polymers would be beneficial where aqueous liquids are to be processed through a polymer membrane. The art 1s discussed 1n U.S. patent 4,618,533, which discloses a method of depositing a hydrophlUc coating on a membrane by in situ polymerization. The surface fluorination of various polymers is also known. D1xon et al. in U.S. patent 4,020,223 disclose treatment of polyolefln and polyacrylonitrile fibers with fluorine 1n the presence of small amounts of oxygen to increase the amount of fluorlnation. The patentees state that improved water absorption results from the treatment. The purpose of the treatment was related to Improvement in the performance of fabrics made from such treated fibers. Fluorlnation of various polymers, particularly hydrocarbon polymers, in the absence of oxygen but using a cold plasma was disclosed by Anand et al. in U.S. patent 4,264,750. Functional1zed polymers containing pendant ester, carboxyl1c add, acid halide, or add anhydride groups were fluorlnated by fluorine-1nert gas mixtures by Lagow as shown 1n U.S. patent 4,076,916. Boultlnghouse, in U.S. patent 4,296,151 discloses the fluorlnation of plastic articles with fluorine 1n an inert gas to make the surfaces more wettable with water and thus to be receptive to various coatings, Inks, paints, etc. The polymers were based on hydrocarbons such as olefins, dienes, and vinyl- substituted aromatics. More recently, McG1nn1ss et al. 1n U.S. patent 4,491,653 examined the surface-fluorination of polymers and determined that the oxygen content should be restricted and that partial fluori- nation of the surface to produce -CHF- groups was desirable. Various types of polymers were suggested as being benefited by such fluorination, including polysulfone, although no working example was provided. Polysulfone membranes as produced are hydrophobic and will pass very little water even under significant pressure differentials because the pores are so small that the capillary pressures are too great to permit water to be forced through them. It has been necessary to pretreat such membranes in order to obtain a high flux rate through the membranes. For example, by installation and subsequent carboxylation (see Gulrer et al., Abstracts of Papers - Third Chemical Congress of North America Part I, Toronto, Canada, June 5-10, 1988 I&EC #63). The applicant has found an improved method for providing a more hydrophlUc surface and increasing the rate at which water can be passed through such membranes. Summary of the Invention The rate at which water can be passed through a polysul¬ fone membrane is Improved by exposing the membrane to elemental fluorine 1n admixture with an inert gas in the substantial absence of oxygen. Such exposure will be carried out at temperatures in the range of about -45° to 70°C and with a partial pressure of fluorine of about 0.01 to 10 kPa, preferably 1n the range of about 0.2 to 2.0 kPa. The preferred inert gas is helium. Description of the Preferred Embodiments Membranes Menbranes have been extensively Investigated in recent years and have been used for many separations. Polysulfones have been used widely for their chemical resistance. They may serve as a base for thin films which carry out the actual process of ultrafiltratlon or they may serve as the membrane. Polysulfone is a generic name for a family of high molecular weight polymers containing sulfone groups and aromatic nuclei in the main chain. Of particular commercial importance is the condensation product of bisphenol A and dichlorodiphenyl sulfone. Since it has considerable resistance to temperature and hydrolysis, it is employed in various uses where these properties are useful, such as ultrafiltratlon in the food industry or as a support in preparation of desalination membranes. Of particular Interest to the present inventors is its application as a porous support for a thin membrane deposited on it or as a membrane Itself. It is relatively hydrophobic so that Its use where it 1s exposed to an aqueous fluid must account for this property. As will be seen in the examples below, even with substantial pressures applied a commercial polysulfone membrane passed only a very small amount of water. However, if pretreated with a methanol solution or certain surfactant solutions the flow of water 1s substantially Increased. Such a treatment must be considered a temporary expedient since methanol or surfactants would be removed by water passing through the membrane so that unless continuously in contact with water the Increased rate would not be maintained. In addition to the bisphenol A sulfone polymers related sulfone polymers such as polyphenyl suIfones, polyether suIfones, and poly aryl ether suIfones may be subjected to the treatment of the Invention. When polysulfones are used to make reverse osmosis or ultraflltra ion membranes, they are either solution cast and then precipitated 1n a non-solvent or melt extruded in the presence of pore forming materials. Other polymers that are comnonly employed 1n the manufacture of synthetic membranes are polyacryUcs, polyamldes, and polyacrylonltrϋe. These materials would benefit also from fluorlnation according to invention. Fluorine Treatment In view of the aggressive nature of fluorine, 1t is usually diluted with an Inert gas, preferably helium. It has been found that the fluorine content may be up to about 10 volume percent, but lower amounts are preferred. Generally, atmospheric or sub-atmospher1c pressures are preferred over above-atmospheric pressures. The partial pressure of fluorine used will be about 0.01 to 10 kPa, preferably about 0.2 to 2 kPa. The temperature used in the fluorination treatment may be from about -45°C to +70°C, with ambient temperature generally preferred. The reaction may be initiated by ultraviolet radiation as shown in the examples below, but its use is not considered essential. The fluorination process is rapid and exposure time is believed less Important than the partial pressure of the fluorine. Typical exposure times would be up to two hours, with much shorter times being preferred. The fluorination treatment of the invention is independent of the membrane configuration. Flat sheet membranes are available in a range of pore sizes, depending on whether they are used for hyperfiltration, ultrafiltration, microfiltration, or particle filtration. Fluorination of the finished membrane products by the process of the invention gives beneficial properties such as hydrophllidty to the membrane surface without damage to the bulk membrane properties. When these membranes are further assembled into plate and frame, spiral wound, or hollow fiber units, the benefits of the treatment will still be present. The utility of the invention 1n treating preformed structures will be apparent. The process may be carried out either batchwise or in a continuous manner, depending upon the type and amount of membrane being treated. Example 1 Comparative A Type A polysulfone ultrafiltratlon membrane made by UOP Fluid Systems was selected for comparison testing. This asymmetric membrane 1s made by depositing polysulfone as a viscous solution on a porous non-woven polyester backing. The thin polysulfone membrane has a nominal pore size of 0.01 μm. A 25.4 mm x 76.2 mm sample of the membrane 1s placed in a closed loop test system which recirculates both the fluid which passes through the membrane (permeate) and the fluid which does not pass through the membrane (retentate). As prepared, the membrane was capable of passing almost no water (about 61.12 1/m /day) even when the pressure differential across the membrane was 689.5 kPa. When a sample of the membrane is pretreated by immersion in a 50% aqueous methanol solution a water flux of about 4074 1/πr/day was obtained with a differential pressure of 689.5 kPa. Example 2 Samples of the membrane of Example 1 were fluorlnated by exposure to a flowing mixture of elemental fluorine in helium while Irradiated with ultraviolet light. The membrane samples were supported on a Monel sheet mounted in a 101.6 mm o.d. Monel cylinder 152.4 mm long with a top having a 50.8 mm square quartz window coated with 0.25 mm of fluorlnated ethylene-propylene polymer film. The ultraviolet light generated by a 750 watt mercury arc lamp passes through the quartz window onto the membrane sample undergoing fluorlnation. A predetermined mixture of helium and 0.05 vol % fluorine 1s passed through the Monel cylinder continuously. The fluorine used was 97% technical grade material supplied by Air Products and Chemicals Inc. The helium gas was 99.9% pure and was scrubbed of any trace oxygen by Oxisorb cartridges supplied by Matheson Gas Products. The conditions of fluorlnation were about 20°C and 101.3 kPa. A sample of the fluorlnated membrane was tested in the equipment of Example 1 and a water flux of 3300 l/m 2 /day was obtained at a pressure differential of 689.5 kPa. The Increase in water flux was comparable to that obtained with methanol treatment. The fluorine treatment, however, is considered durable while the methanol would be expected to be removed in use so that should the membrane be dried, it would lose much of its ability to pass water and require re-conditioning with methanol. Exa ple 3 Samples of the membrane of Example 1 were treated with varying amounts of fluorine in a helium carrier as described in Example 2. The contact angle of water on membrane was determined by observation of a water droplet freshly deposited on the membrane surface. The results of these tests are given in the following table. Table A Water Vol % of Fluorine Exposure, Ultraviolet Contact 1n Helium minutes Exposure Anqle untreated - - 62 0 15 yes 58 0 30 yes 55 0.025 15 yes 51 0.025 30 yes 48 0.05 30 no 37 0.05 30 no 33 0.05 30 yes 30 2 15 no 0 2 120 no 0 It will be seen that the contact angle decreased with increased severity of the fluorine treatment. At a concentration of 2 volume percent fluorine complete wetting was observed. However, deterioration of the surface due to fluorine attack makes the lower concentrations preferred. Example 4 Polysulfone membranes of Example 1 were compared with various treatments as described in the previous examples, with the results of water flux testing given in the following table. Table B Water Flux (1 Exposed to Pressure Treated Exposed 0.05% F 2 Differential Untreated with to He in He for 30 kPa Polysulfone 50% MeOH only + UV min. + UV 690 61.1 4237 61.1 3300 0 0 0 0 0 172 0 1426 0 1263 345 28.5 2648 36.7 2241 690 44.8 4319 81.5 3422

Claims: CLAIMS 1. A process for improving the rate at which water can be passed through a polysulfone membrane comprising enhancing the hydrophilic property of said membrane by exposing said membrane to elemental fluorine in admixture with an inert gas in the substantial absence of oxygen. 2. The process of Claim 1 wherein the exposure of said membrane to fluorine is carried out at about -45° to 70°C and at atmospheric or sub-atmospheric pressure. 3. The process of Claim 1 wherein the fluorine partial pressure is about 0.01 to 10 kPa. 4. The process of Claim 1 wherein said polysulfone membrane is an asymmetric membrane from the condensation polymer of bisphenol A and dichlorodiphenyl sulfone on a support. 5. The process of Claim 1 wherein the inert gas is helium. 6. The process of Claim 1 wherein the membrane is exposed to fluorine in the presence of ultraviolet radiation.

Inventors: TAYLOR DONALD ROBERT, WHITE LLOYD STEVEN, TAYLOR, DONALD, ROBERT, WHITE, LLOYD, STEVEN
Applicants: ALLIED SIGNAL INC, ALLIED-SIGNAL INC.
Application year: 1990
Main classification: B01D71
IPC codes: B01D67, B01D71, C08J7