A hydrophilic polyolefin membrane

Abstract: The present invention is directed to a hydrophilic polyolefin article comprising a polyolefin article having a coating containing a surfactant and an ethylene vinyl alcohol (EVOH) copolymer.

Description: Field of the Invention The instant invention is a hydrophilic polyolefin membrane in which a hydrophobic polyolefin membrane is coated with a mixture of surfactant and ethylene vinyl alcohol copolymer (EVOH). Background of the Invention Hydrophilic refers to the ability to "wet-out" a liquid. "Wet-out" refers to the ability to cause a liquid (e.g., an aqueous solution) to penetrate more easily into, or spread over the surface of another material. Generally, polyolefins are considered hydrophobic. Hydrophobic refers to the inability to "wet-out" a liquid. Various methods to make polyolefins more hydrophilic generally include: coating with a surfactant (or surface active agent or wetting agent); coating with a polymer(s), the polymer(s) having different (i.e., better) surface active properties than the polyolefin; surface activation (e.g., by plasma treatment); surface roughing to increase surface area (e.g., foaming the surface); and blending the polyolefins with another polymer(s), the blend having different (i.e., better) surface active properties than the polyolefin. Examples of the foregoing are illustrated in Japanese Kokai Nos.: 2-133608 (published May 22, 1990); 2-133607 (May 22, 1990); 3-55755 (March 11, 1991); 4-346825 (December 2, 1992); and 5-106112 (April 17, 1993); and European Patent Publication Nos. 498,414 A2 (August 12, 1992); 634,802 A1 (January 18, 1995); and 203,459 A2 (December 3, 1986). EP 0 203 459 A describes a hydrophilic polyolefin membrane comprising a polyolefin porous membrane having a coating thereon consisting of an ethylene vinyl alcohol copolymer. The membrane advantageously does not contain any undesired malters, such as a surfactant. US 4 501 793 A describes a hydrophilic polyolefin membrane comprising a polyolefin microporous membrane impregnated with a nonionic surfactant of the class of alkylphenoxy poly(ethyleneoxy) ethanol compounds, e.g. nonylphenoxy poly(ethyleneoxy) ethanol. In the foregoing methods, the hydrophilicity of the polyolefin may degrade over time. Accordingly, there is a need for a permanently hydrophilic polyolefin. Summary of the Invention The present invention is directed to a hydrophilic (polyolefin membrane as claimed in claim 1 comprising a polyolefin membrane having a coating containing a surfactant and an ethylene vinyl alcohol (EVOH) copolymer, and the use of the membrane as a battery separator as claimed in claim 6. Detailed Description of the Invention Polyolefins are a class or group of thermoplastic polymers derived from simple olefins. Polyolefins generally include polyethylene, polypropylene, polybutylene, polymethyl pentene, and copolymers thereof. Polyolefins membranes generally include fibers and films, but also include microporous films and microporous hollow fibers. Microporous refers to a membrane which has a plurality of pores with effective diameters of 1 micron or less. Hydrophobic polyolefins refer to polyolefins having surface energies equivalent to or less than the surface energy of polyethylene. The polyolefin membrane is made hydrophilic by a coating. The coating contains a surfactant and an ethylene vinyl alcohol (EVOH) copolymer. Preferably, the coating is applied as a mixture of the surfactant and EVOH, but the coating may also be applied serially, e.g. first apply the surfactant, and then apply the EVOH. Preferably, the coating is added onto the membrane at an average weight percentage in the range of 10-25, preferably 10-18 based upon the weight of the membrane. It is believed that the addition of the surfactant to the EVOH facilitates the adhesion of the EVOH to the article, and thereby increases the permanency of the coating. The coating is preferably applied to the surface in solution form. The solution may comprise an alcohol/water (preferably 60:40 propanol:water) and no more than 0.9% by weight surfactant and between 1.5-3% by weight EVOH. The amount of surfactant is balanced so that the surfactant does not cause wetting. Of the surfactants named herein, no more than 0.9% should be use. Preferably, the range should be from 0.7 to 0.9% by weight. The amount of EVOH is chosen to insure consistent wetting of the surface but to avoid pore blockage. At less than 1.5% wetting becomes erratic. At greater than 3.0% pore blockage is likely to occur. Preferably, the range should be from 2.0 to 3.0% by weight. After application, any application method is appropriate, the alcohol/water portion is driven off, e.g., by drying including forced hot air dying, and the membrane is ready for use. The surfactant is capable of raising the surface energy of the polyolefin to about 0.048 N/m (48 dynes/cm), and is selected from the group consisting of polyethylene glycol dioleate, nonylphenoxypoly(ethyleneoxy) ethanol, triethylene glycol divinyl ether, and combinations thereof. Polyethylene glycol dioleate is available as MAPEG® D0400 from PPG Industries, Inc. of Gurnee, IL. Triethylene glycol divinyl ether is available as Rapidcure® DVE-3 from ISP Technologies Inc. of Wayne, NJ. Nonylphenoxypoly (ethyleneoxy) ethanol is available as Igepal® CO 660 from Rhone-Poulenc Surfactants and Specialties, Inc. of Cranberry, NJ. The EVOH may be any commercially available EVOH copolymer. Preferably, the EVOH has about 38 mole % of ethylene monomer. Suitable EVOH copolymers include: Soarnol® K3835N from Nippon Goshei of Osaka, Japan; and EVAL® F104 from Eval Company of America, Lisle, IL. The foregoing hydrophilic polyolefin membranes may be used in any application where a hydrophilic polyolefin is necessary or desirable, for example, air filtration, air cleaning, water filtration, water cleaning, water purification, medical equipment, separation equipment, semiconductor manufacture, battery cell separator (particularly for batteries having aqueous based electrolytes), ultrafiltration equipment and the like. With regard to battery cell separators, the coating is applied to one or preferably both surfaces of the microporous polyolefin membrane. Such membranes are commercially available from: CELGARD LLC, Charlotte, NC; Tonen KK, Tokyo, Japan; Asahi Chemical Industry Co., Ltd., Tokyo, Japan; Nitto Denko Corp., Osaka, Japan; and Ube Industries Ltd., Tokyo, Japan. This coated separator is particularly suitable for aqueous alkaline secondary battery. Such batteries include, but are not limited, nickel metal hydride (NiMH), nickel cadmium (NiCd), and zinc air batteries. In such uses, preferably about 50-90% (most preferably 60-80%) of the separator's surface is coated. Partial coating is preferred because it facilitates gas permeability. During the recharging of alkaline secondary batteries, gases may be formed on the electrode surfaces, these gases must be able to permeate the separator to recombine. With regard to the separation, filtration, cleaning, and purification equipment, particularly where microporous polyolefin hollow fibers or flat sheet membranes are used, higher flux rates all obtainable by use of the coatings. Suitable hollow fibers and membrane contactors (e.g., LIQUI-CEL® contactors) are available from CELGARD LLC, Charlotte, NC. The following examples serve only to further illustrate aspects of the present invention and should not be construed as limiting the present invention. The Gurley value is measured using a Model 4120 or 4150 Gurley Densometer from Teledyne Gurley, and is the time in seconds required to pass 10 cc of air through 0.000645 m 2 (1 inch 2 ) of membrane at a constant pressure of 0.03039 bar (12.2 inch of water). [Ref. ASTM D726(B)] The electrical resistance (or resistivity (ER) is measured as follows: A R.A.I. AC Milliohm Resistance Meter, Model 2401 and R.A.I. test cell electrode (from RAI Research Corp., Hauppauge, NY) are used. A 31% by wt KOH solution is used to wet the sample. Samples should not be dry when tested. The results, reported in milliohm-inch 2 (1 milliohm - inch 2 = 1 milliohm - 0.000645 m 2 ). EXAMPLES Example 1 A solution of the coating was prepared by adding 2g Soarnol® K3835N resin to a 60:40 2-propanol/water (i.e., 60 ml 2-propanol, 40 ml demineralized water). The EVOH was dissolved by stirring at 70°C. MAPEG® D0400 is added and stirred for 10 minutes. Example 2 Circular samples (55 mm diameter) of microporous polypropylene membrane (CELGARD® 2500, 2402, 2400, & 2456) was immersed in the solution (Example 1) for up to 10 minutes 40-50°C with stirring. After removal from the solution, excess solution was removed from the sample with a paper towel. The sample was dried in a hot air oven (50°C) for 30 minutes. Example 3 The coated samples were tested as set forth in TABLE 1. SAMPLE ID GURLEY COAT TIME % ADD ON ER WATER WETTABLE PORE WIDTH PORE LENGTH 2500 Control 8.8 0.0551 0.2059 2500 Coated with MAPEG® 8.3 Dip coat 4.1 9.8 -infinity No 2500 Coated with Mixture 10.1 Dip coat 15.6 5.5 Instantly 2402 Control 27 0.0499 0.1376 2402 Coated with MAPEG® 37.8 10 min 3.1 54 -infinity No 2402 Coated with Mixture 76.7 10 min 14.2 12.4 Instantly 2400 Control 25.1 0.0433 0.1228 2400 Coated with MAPEG® 28 10 min 3.9 600-infinity No 2400 Coated with Mixture 40.7 10 min 13.2 9.7 Slowly 2456 Control 17.3 0.0440 0.1390 2456 Coated with MAPEG® 15.6 10 min 13.2 9.7-infinity No 2456 Coated with Mixture 29.4 10 min 14.5 21.2 Slowly Example 4 A coating of 3% Soarnol® K3835N and 0.7% Mapeg® D0400 was pattern coated on to a microporous polypropylene membrane (CELGARD® 2502). The pattern was formed by dropping 20 microliters of the coating solution at a distance of 1.27 cm (one half inch) between drops. The coated membrane was air dried and then dried in a hot air oven (50°C) for 30 minutes. The dried coated film had an ER of 46 milliohm/0.000645 m 2 (46 milliohm/in 2 ) (due to ionic conduction through the hydrophilic region) and gas flux of 12 cc/ 0.0689 bar/min (12cc/psi/min) (due to gas flow through the hydrophobic regions ). Example 5 The surface energy of a polypropylene film was modified by polymerizing triethylene glycol divinyl ether to improve its hydrophilicity by raising its surface energy value to 0.048 N/m (48 dynes/cm) (as measured by a Dyne test solution kit from Pillar Technologies, Inc. of Hartland, WI). The modified film was then coated with a solution of 1.5% EVOH (EVAL® F104) in 60/40 2-propanol/water followed by drying in a hot air oven (50°C) for 30 minutes. The final film was wetted out by water instantly. Example 6 The surface energy of polypropylene film was modified to 0.048 N/m N/m (48 dynes/cm) (as measured by a Dyne test solution kit) by coating film surface with a solution of 0.7% polyethylene glycol dioleate (Mapeg® DO 400). The modified film was then immersed in a solution of 2% Soarnol® K3835N in 60/40 2-propanol/water for 30 seconds and dried in a hot oven at 50°C for 30 minutes. The final film was water wettable. Example 7 Polypropylene hollow fiber membranes (HF X20-305 from CELGARD LLC of Charlotte NC) were immersed in a mixture solution (Example 1) of 0.7% polyethylene glycol dioleate and 2% Soarnol® K3835N for 2 minutes. The fibers were dried in a hot air oven (50°C) for 30 minutes. The final fibers were water wettable. Example 8 An amount of 0.9% nonylphenoxypoly (ethyleneoxy) ethanol (Igepal® CO 660) was added to 2% Soarnol K3835N solution and the mixture was used to coat on the polypropylene control. The results of Examples 5-8 are presented in Table 2-5. Coating of a Mixture of MAPEG® DO 400 and Soarnol® K3835N on Control Polypropylene Sample ID Gurley % of Weight Gain ER (mOhm-in 2 ) Water Wettability Polypropylene Control 8.8 -- 2.7 primed w/MeOH no Polypropylene Coated with 0.7% MAPEG® 8.3 4.1 9.8-infinity No Polypropylene Coated with 2% Soarnol® K3835 11.6 14.0 Infinite No Double Coat 10.1 15.6 5.5 Yes Single Coat (mixture) 11.5 15.2 4.1 Yes Coating of a Mixture of Surfactant and Soarnol® K3835N on Polypropylene Film Surfactant EVOH (Soarnol) Gurley (sec) Weight Gain (%) ER (*) 0.7% MAPEG® D0400 2% 10.1 15.6 5.5 0.9% IGEPAL® C0660 2% 12.6 17.3 4.4 0.9% IGEPAL® ALONE 161 Coating of a Mixture of 0.7% MAPEG® DO 400 and 2% Soarnol® K3835N Weight Gain (%) Gurley (sec) Water Flux (L/m 2 /hr) Coated with 0.7% MAPEG® 3.1 13.4 32 Coated with Mixture 13.6 20.2 119 2500 Grafted Polymerization With Rapidcure® DVE-3 and Coated With 2% Soarnol® K3835N Gurley (sec) Weight Gain (%) Water Wettability 2500 Polymerization with 20% DV-3 9.1 19.2 No DVE-3 2500 Coated with 15% EVAL® F104 11.1 24.8 yes Example 9 A sample coated as set forth in Example 1 was immersed in at 31% KOH solution at room temperature for 3 months. Every two weeks, the film was removed and dried in a hot air oven (50°C) for 2 hours, at all times the KOH solution re-wetted immediately the same. Example 10 A solution of 3% Soarnol® K3835N and 0.7% MAPEG® D0400 in 60:40 propanol/water was prepared. The solution was pumped through the lumen side of a LIQUI-CEL® membrane contactor (approximately 1.12 square meters lumen side surface area) having X20-240 CELGARD® hollow fibers at a pressure of 0.2068 bar (3 psi) for 20 minutes. The contactors were dried in a hot air oven (50°C) for 12 hours. (In commercial operation, drying time may be reduced to 5 minutes.) The contactors were very hydrophilic. Water flow (from shellside) at various pressures is set forth in Table 6. Pressure-in (psi) Pressure-out (psi) Flow (cc/min) Contactor 1 5 0.5 440 10 1.5 940 15 1.75 1240 20 2.5 1575 25 3.4 1800 30 4.1 2160 35 5.1 2350 Contactor 2 5 0.5 500 10 1.7 1080 15 2.3 1400 20 3.4 1750 25 4.6 2180 30 5.5 2440 35 6.5 2600 (1 psi = 0.0689 bar) The contactors were then dried for 24 hours and left at room temperature for three weeks. The contactors were still very hydrophilic. Water flow (from shellside) at various pressure is set forth in Table 7. Pressure-in (psi) Pressure-out (psi) Flow (cc/min) Contactor 1 5 0.5 475 10 1.2 996 15 1.75 1400 20 2.75 1850 25 3.8 2000 30 4.8 2360 35 6.2 2660 Contactor 2 5 1.0 740 10 1.7 1320 15 2.8 1650 20 3.8 2000 25 4.9 2380 30 6.8 2652 35 7.7 2950 (1 psi = 0.0689 bar)

Claims: A hydrophilic polyolefin membrane characterized by a polyolefin microporous membrane and a coating thereon containing an ethylene vinyl alcohol copolymer and a surfactant capable of raising the surface energy of said membrane to 0.048 N/m (48 dynes/cm) and being selected from the group consisting of polyethylene glycol dioleate, nonylphenoxypoly (ethyleneoxy) ethanol, triethylene glycol divinyl ether, and combinations thereof, and said coating preferably being applied via a solution wherein said ethylene vinyl alcohol copolymercomprises between 1.5 and 3% by weight of said solution and said surfactant comprises no more than 0.9% by weight of said solution. The membrane of Claim 1 wherein said polyolefin being selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, and copolymers thereof. The membrane of Claim 2 wherein said polyolefin being polypropylene and copolymers thereof. The membrane of Claim 1 wherein said coating covers 50-90% of the membrane's surface. The membrane of Claim 1 wherein said coating comprises 10-18% by weight of said membrane. The use of the membrane of Claims 1-5 as a battery separator.

Inventors: CALLAHAN ROBERT W, NGUYEN KHUY V, CALLAHAN, ROBERT W., NGUYEN, KHUY V.
Applicants: CELGARD INC, CELGARD INC.
Application year: 1999
Main classification: H01M2
IPC codes: B01D69, C08J7, H01M10, H01M2, C08J9, D06M15, D06M101, C08J5, B01D71, B01D67, D01F6