SUPER-HYDROPHILIC SURFACE TREATMENT METHOD OF FILTRATION MEDIUM, SUPER-HYDROPHILIC FILTER FOR OIL-WATER SEPARATION AND METHOD OF FABRICATING THE SAME

Abstract

A super-hydrophilic surface treatment method of a filter medium of a filter for oil-water separation according to the present invention includes preparing a filter medium or a filter including the filter medium using a polymer base or a metal base, and forming a hydrophilic coating layer to the filter medium or the filter including the filter medium by cross-linking bis-acrylamide (N,N-methylenebisacrylamide).

Claims

1. A filter for oil-water separation comprising a hydrophilic coating layer formed by cross-linking bis-acrylamide (N,N-methylenebisacrylamide) to a surface of a filter medium, wherein the filter has super-hydrophilicity with a contact angle of 10° or less with respect to water in the air.

2. The filter for oil-water separation of claim 1, wherein the filter has a contact angle of 150° to 180° with respect to oil in water.

3. The filter for oil-water separation of claim 1, wherein the filter selectively separates only water from an oil-water mixture.

4. The filter for oil-water separation of claim 1, wherein the filter medium comprises a polymer base or a metal base.

5. The filter for oil-water separation of claim 4, wherein the polymer base comprises one or more selected from the group consisting of polypropylene (PP), polyethylene (PE), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE).

6. The filter for oil-water separation of claim 4, wherein the metal base comprises one or more selected from the group consisting of stainless steel (STS), aluminum (Al), and copper (Cu).

7. A method of fabricating a super-hydrophilic filter for oil-water separation, comprising: preparing a filter medium or a filtration filter comprising the filter medium using a polymer base or a metal base; and forming a hydrophilic coating layer to the filter medium or the filtration filter comprising the filter medium by cross-linking bis-acrylamide (N,N-methylenebisacrylamide).

8. The method of claim 7, wherein the forming the coating layer comprises performing a cross-linking polymerization reaction using a cross-linking solution comprising a solvent, a cross-linking agent, and an oxidizing catalyst.

9. The method of claim 8, wherein the forming the coating layer comprises performing a cross-linking polymerization reaction using a cross-linking solution comprising a solvent, a bisacrylamide (N,N-Methylenebisacrylamide, BIS), and ammonium persulfate (APS).

10. The method of claim 8, wherein the forming the coating layer comprises immersing the filter medium or the filtration filter comprising the filter medium in ethanol, followed by immersion in the cross-linking solution.

11. A super-hydrophilic surface treatment method comprising: preparing a filter medium or a filtration filter comprising the filter medium using a polymer base or a metal base; and forming a hydrophilic coating layer to the filter medium or the filtration filter comprising the filter medium by cross-linking a bis-acrylamide (N,N-methylenebisacrylamide).

12. The super-hydrophilic surface treatment method of claim 11, wherein the forming the coating layer comprises performing a cross-linking polymerization reaction using a cross-linking solution comprising a solvent, a cross-linking agent, and an oxidizing catalyst.

13. The super-hydrophilic surface treatment method of claim 12, wherein the forming the coating layer comprises performing a cross-linking polymerization reaction using a cross-linking solution comprising a solvent, a bis-acrylamide (N,N-Methylenebisacrylamide, BIS), and ammonium persulfate (APS).

14. The super-hydrophilic surface treatment method of claim 12, wherein the forming the coating layer comprises immersing the filter medium or the filtration filter comprising the filter medium in ethanol, followed by immersion in the cross-linking solution.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] FIG. 1 is a schematic view schematically showing that a conventional filter is modified into a super-hydrophilic filter according to a super-hydrophilic surface treatment method according to an exemplary embodiment of the present invention.

[0056] FIGS. 2A, 2B, and 2C show a comparison between a conventional polyethylene filter and a super-hydrophilic polyethylene (PE) filter surface-treated according to the super-hydrophilic surface treatment method according to an exemplary embodiment of the present invention, FIG. 2A is a graph obtained by Fourier transform infrared spectroscopy, FIG. 2B is an SEM photograph showing polymer fibers of a conventional polyethylene filter and pores formed by cross-linking the polymer fibers, and a photograph showing a degree of surface wettability of the polyethylene filter, and FIG. 2C is an SEM photograph showing polymer fibers of a super-hydrophilic polyethylene filter surface-treated according to an exemplary embodiment of the present invention and pores formed by cross-linking the polymer fibers, and a photograph showing a degree of surface wettability of the super-hydrophilic polyethylene filter.

[0057] FIG. 3 is a diagram showing a process of polymerizing a cross-linking agent with an oxidant during a coating process of the super-hydrophilic surface treatment method according to an exemplary embodiment of the present invention to form a hydrophilic cross-linkable group.

[0058] FIG. 4 is a photograph of contact angles of super-hydrophilic filters, which are fabricated by subjecting various polymer bases, metal bases, and super-hydrophobic bases to the super-hydrophilic surface treatment method according to an exemplary embodiment of the present invention, with respect to a water drop.

[0059] FIG. 5 is a photograph showing a larger super-hydrophilic filter with a size of 400 mm×1,000 mm, which is fabricated using the super-hydrophilic surface treatment method according to an exemplary embodiment of the present invention.

[0060] FIG. 6 is a diagram shown to explain one example in which the super-hydrophilic surface treatment method according to an exemplary embodiment of the present invention is applied to a roll-to-roll process.

[0061] FIGS. 7A, 7B, and 7C are photographs showing the results of evaluating the wettability of water to oil in the super-hydrophilic filter fabricated by subjecting a polyethylene filter to the super-hydrophilic surface treatment method according to an exemplary embodiment of the present invention.

[0062] FIGS. 8A, 8B, and 8C are graphs showing the results of evaluating the durability and stability of the super-hydrophilic filter fabricated using the super-hydrophilic surface treatment method according to an exemplary embodiment of the present invention.

[0063] FIGS. 9A and 9B are photographs showing a self-cleaning ability of the super-hydrophilic filter, which is fabricated using the super-hydrophilic surface treatment method according to an exemplary embodiment of the present invention, with respect to oil.

[0064] FIG. 10 is a photograph showing an oil-water mixture separated through the super-hydrophilic polyethylene filter fabricated using the super-hydrophilic surface treatment method according to an exemplary embodiment of the present invention.

[0065] FIG. 11 is a graph showing the oil-water mixture separation efficiency and processing speed of the super-hydrophilic polyethylene filter, which is fabricated using the super-hydrophilic surface treatment method according to an exemplary embodiment of the present invention, with respect to various types of oils.

[0066] FIG. 12 is a graph showing the result of evaluating the reusability of the super-hydrophilic polyethylene filter fabricated using the super-hydrophilic surface treatment method according to an exemplary embodiment of the present invention.

[0067] FIG. 13 is a graph showing the results of measuring the purities of recovered types of water in the reusability evaluation shown in FIG. 12.

[0068] FIG. 14 is a schematic view schematically showing a separation mechanism of an emulsion.

[0069] FIGS. 15A, 15B, 15C, and 15D are photographs and graphs showing an emulsion stabilized with a surfactant before and after the emulsion is processed with the super-hydrophilic filter fabricated using the super-hydrophilic surface treatment method according to an exemplary embodiment of the present invention.

[0070] FIG. 16 is a graph showing the results of measuring the emulsion separation efficiency and processing speed of the super-hydrophilic filter, which is fabricated using the super-hydrophilic surface treatment method according to an exemplary embodiment of the present invention, after washing and repeatedly using the super-hydrophilic filter.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0071] The present invention will be described in further detail with reference to the following exemplary embodiments thereof. However, it should be understood that the exemplary embodiments are not intended to limit the scope of the present invention.

Example 1: Fabrication of Super-Hydrophilic Filter According to Single-Step

[0072] Coating Process

[0073] 1-1: Preparation of Filtration Filter

[0074] To fabricate a super-hydrophilic filter, a commercially available polyethylene (PE) filter (a membrane filter having a diameter of 47 mm and a nominal pore size of 10 μm; Pall Life Science (USA)) which was not surface-treated was prepared. An SEM photograph of the polyethylene filter before surface treatment is shown in FIG. 2B. Here, polymer fibers and pores formed by cross-linking the polymer fibers are shown. Such a general commercial PE filter has hydrophobicity due to the presence of a methyl group having a low surface tension with respect to the microsized fibers.

[0075] 1-2: Formation of Hydrophilic Coating Layer

[0076] The prepared polyethylene (PE) filter was immersed in an aqueous ethanol solution at 20° C. for 10 seconds, and then immersed in a mixed solution including bis-acrylamide (N,N-methylenebisacrylamide) as a cross-linking agent and ammonium persulfate as an oxidant at 60° C. for an hour. The mixed solution was prepared, using water as a solvent, by dissolving 30 mM bis-acrylamide (N,N-methylenebisacrylamide) as the cross-linking agent and dissolving ammonium persulfate as the oxidant at a weight ratio of 1%. In the coating process, the oxidant formed radicals to break a double bond of the cross-linking agent, thereby forming radicals in the cross-linking agent. The radicals formed on the chain of the cross-linking agent bound to another chain of cross-linking agent to form a hydrophilic layer while surrounding fibers made of a filter base. During the coating process, a process of polymerizing the cross-linking agent by the oxidant to form a hydrophilic cross-linkable group is shown in FIG. 3.

[0077] The hydrophilic coating layer formed in a micro-/nano-structure in which the filter fibers were formed allowed the filter to have super-hydrophilicity. An SEM photograph of the polyethylene filter on which the super-hydrophilic surface treatment is completed is shown in FIG. 2C. As such, when the polyethylene filter is treated using a coating method of the present invention, the thickness and pore size of polymer fibers were not changed, but the surface wettability was altered. Thus, the super-hydrophilic filter was formed.

Example 2: Evaluation of Characteristics of Polymer Filters During Formation of Coating Layer

[0078] 2-1: Spectroscopic Analysis of Filter

[0079] For a conventional polyethylene (PE) filter and a filter composed of polyethylene fibers having a surface product obtained by the process, it was evaluated whether a functional group was formed using Fourier transform infrared spectroscopy.

[0080] Based on the results of evaluating the formation of the functional group using Fourier transform infrared spectroscopy as obtained in FIG. 2A, the conventional untreated PE filter had characteristic peaks at 1,472, 2,847, and 2,914 cm.sup.−1, as widely known in the art. After the single-step coating, the characteristic peaks were further observed at 1,538, 1,652, and 3,296 cm.sup.−1, indicating the hydrophilic functional groups, for example, C═O, C═O, and N—H bonds.

[0081] 2-2: Evaluation of Hydrophilicity of Filter

[0082] A polymer base, a metal base, and a super-hydrophobic base were subjected to the coating method according to the present invention to fabricate super-hydrophilic filters and contact angles of the super-hydrophilic filters were measured. The results are shown in FIG. 4. Specifically, the contact angle was measured with respect to 5 μL of pure water (deionized water) at room temperature in the air using a contact angle measuring equipment (SmartDrop; FemtoFab, Inc.). Super-hydrophilic filters were fabricated using PE, PP, and PTFE as the polymer base, super-hydrophilic filters were fabricated using STS, Al, and Cu as the metal base, and a super-hydrophilic filter was fabricated using aluminum (Al) having a super-hydrophobic surface. The numbers in parentheses refer to nominal pore sizes of respective filters, and their units are in μm.

[0083] Here, membrane filters having a diameter of 47 mm and a nominal pore size of 10 μm (Pall Life Science (USA)) were used as a PE filter 10 and a PP filter 10. Membrane filters having a diameter of 47 mm and a nominal pore size of 0.1 μm (GVA Filter Technology (USA)) were used as PP filter 0.1 and a PTFE filter. Meshes (TWP Inc. (USA)) were used as STS, Al, and Cu meshes. These filters are all commercially available products, that is, filters made by cross-linking polymer fibers or metal wires to form pores.

[0084] The super-hydrophobic aluminum base is a super-hydrophobic aluminum base fabricated by forming a micro-/nano-structure on the aluminum mesh (TWP Inc.) and coating the micro-/nano-structure with a hydrophobic material. The micro-structure was formed by immersing an aluminum mesh (TWP Inc.) in a 1 M aqueous sodium hydroxide solution at 25° C. for a minute and immersing the aluminum mesh in a 2 M aqueous hydrochloric acid solution at 25° C. for 2 minutes. Then, the aluminum mesh was immersed in a 1 M aqueous sodium hydroxide solution at 25° C. for 5 seconds, and then immersed in boiling water for 5 minutes to form a nano-structure on the micro-structure. The aluminum mesh on which the micro-/nano-structure was formed was immersed in a super-hydrophobic coating solution (prepared by diluting heptadecaperfluorosilane with hexane at a volume ratio of 0.1%) for 10 minutes, and then dried for 60 minutes in a 80° C. oven to fabricate a super-hydrophobic aluminum base.

[0085] All the bases as described above were subjected to the super-hydrophilic coating method as described in Example 1-2 to fabricate super-hydrophilic filters.

[0086] A hydrophobic base having a contact angle of 90° or more before treatment and a super-hydrophobic base having a contact angle of 150° or more were converted into super-hydrophilic filters through a single coating step according to the present invention. The experimental results show that the coating method of the present invention was applicable regardless of the characteristics of the material and the pore size of the material, which makes it possible to fabricate a super-hydrophilic filter.

Example 3: Fabrication of Large Super-Hydrophilic Filter

[0087] A photograph of a large super-hydrophilic filter with a size of 400 mm×1,000 mm fabricated by the surface treatment process according to the present invention is shown in FIG. 5. The large super-hydrophilic filter was fabricated using a stainless steel mesh (TWP Inc.) as the base, and a mesh base with a length of 400 mm and a width of 1,000 mm was identical to the stainless steel mesh used in Example 2. Because the surface treatment process was applied to a simple immersion method as a single coating process, a large super-hydrophilic filter was able to be easily fabricated.

Example 4: Evaluation of Oleophobicity of Filter with Respect to Oil

[0088] The wettability of water with respect to oil was evaluated for the super-hydrophilic filter obtained in Example 1 using the commercial PE filter. The results are shown in FIGS. 7A and 7B. In this case, the oil used was diesel. The super-hydrophilic filter fabricated as in FIG. 7A had an excellent level of hydrophilicity to completely absorb water in 3.7 seconds when the filter was in a dried state. Because the contact of the filter with oil was blocked when the filter was wetted with water, the filter had a very high contact angle of 157.9° with respect to oil in water, as shown in FIG. 7B. Also, when the oil was forcedly attached and released in water, a surface of the filter was not stained with the oil. From the results, it can be seen that the fabricated super-hydrophilic filter had a very high repulsive force to the oil in water. The measurement of the contact angle was performed using SmartDrop (FemtoFab, Inc.). In this case, 5 μL of droplets were measured 5 times and an average value was then calculated. The contact angle of oil in water was obtained by measuring a contact angle between the filter and the oil in a state in which the filter was immersed in water.

Example 5: Evaluation of Durability and Stability of Filter

[0089] The durability and stability of the super-hydrophilic filter obtained in Example 1 using the fabrication method according to the present invention were evaluated. The results are shown in FIGS. 8A to 8C.

[0090] FIG. 8A is a graph showing the contact angles measured after the fabricated filter was processed with ultrasonic waves for 300 minutes. (Ultrasonic treatment equipment: 5510E-DTH, BRANSON, USA) When the hydrophilic layer was weakly bound to a surface of the base after the coating, the hydrophilic layer was detached from the base by ultrasonic waves, which resulted in degraded super-hydrophilicity of the filter. However, even when the fabricated super-hydrophilic filter was treated with ultrasonic waves for 300 minutes, the super-hydrophilicity and super-oleophobicity in water were not changed because the hydrophilic layer is firmly attached to the base. (After treatment with ultrasonic waves for 300 minutes, the contact angle with respect to water was 0°, and the contact angle with respect to oil in water was 159.8°.)

[0091] Also, even when a surface of the super-hydrophilic filter was rubbed and worn with sandpaper as shown in FIG. 8B, no super-hydrophilicity and super-oleophobicity in water were changed at all. (After a wear length of 1,500 mm, the contact angle with respect to water was 0°, and the contact angle with respect to oil in water was 159.3°.) Because the fabricated super-hydrophilic filter had stability with respect to a strong acid-weak base solution, the super-oleophobicity was maintained with a contact angle of 150° or more with respect to the oil in the solution (pH 3 to pH 9), as shown in FIG. 8C. The super-hydrophilic filter exhibited excellent wettable characteristics even in the solution (pH 11), but the contact angle with respect to the oil was not measurable because the oil particles were stabilized due to the strong interaction between the strong base solution and the oil. Based on the results, it was identified that the fabricated super-hydrophilic filter was able to be used under poor environments because the super-hydrophilic filter had very excellent durability.

Example 6: Evaluation of Self-Cleaning Ability of Filter

[0092] The self-cleaning ability of the super-hydrophilic filter, which was obtained in Example 1 by the fabrication method according to the present invention, with respect to the oil was tested. The results are shown in FIGS. 9A and 9B.

[0093] Unlike the filter previously wetted with water, the dried filter was easily contaminated with oil. However, even when the super-hydrophilic filter was contaminated with oil, the super-hydrophilic filter repelled the oil with a strong interaction between water and filter in water, and was then self-cleaned as the oil was detached from the filter. When the commercial PE filter was in a dry state, the commercial PE filter was easily contaminated with the oil, and the oil was not washed out, as shown in FIG. 9A. On the other hand, it can be seen that the super-hydrophilic PE filter coated according to the present invention was easily contaminated with the oil when the super-hydrophilic PE filter was in a dry state, but was self-cleaned within 10 seconds because the oil was easily detached in water, as shown in FIG. 9B. The oil used was diesel, and dyed with a red color in order to enhance visibility.

Example 7: Implementation of Oil-Water Separation Using Filter

[0094] 7-1: Experiment for Separation of Oil-Water Mixture

[0095] A photograph showing that 200 mL of an oil-water mixture (water:oil=1:1 volume ratio) is separated using the super-hydrophilic PE filter obtained in Example 1 by the surface treatment process according to the present invention is shown in FIG. 10. Because only water passed through the super-hydrophilic filter, pure water was recovered, and the oil was heaped on the filter because the oil did not pass through the filter. The super-hydrophilic filter used was fabricated using a PE filter having a nominal pore size of 10 μm as the base. Also, this filter was used in later experiments to evaluate the oil-water mixture separation (see FIGS. 11 to 13).

[0096] 7-2: Experiment on Oil-Water Mixture Separation Efficiency and Processing Speed

[0097] The oil-water mixture separation efficiency and processing speed with respect to various types of oils were calculated using the super-hydrophilic PE filter obtained in Example 1. The results are shown in the graph of FIG. 11. The separation efficiency was calculated using an amount of the finally recovered water in the oil-water mixture according to the following Equations 1 and 2, and the processing speed was calculated using the time taken to separate 200 mL of the oil-water mixture (water:oil=1:1 volume ratio), and the area of the filter.

[00001]$\begin{array}{cc}\mathrm{Processing}\mathrm{speed}=\frac{V}{A\Delta t}& \left[\mathrm{Equation}1\right]\\ \mathrm{Separation}\mathrm{efficiency}=\frac{m_1}{m_0}\times 100\%& \left[\mathrm{Equation}2\right]\end{array}$

[0098] (V: an amount of recovered water, A: an effective area of a filter, Δt: a time taken to recover water, m.sub.0: a weight of water in an oil-water mixture, mi: a weight of finally recovered water) The types of oils used were diesel, hexane, xylene, and benzene, the separation efficiencies of the oils were 99.2, 99.5, 99.3, and 99.5%, respectively, and the processing speeds were 3,020, 2,815, 2,564, and 3,112 Lm.sup.−2h.sup.−1, respectively. Based on the experimental results, it can be seen that the fabricated super-hydrophilic filter was very effective in separating the oil-water mixture because it had both the high oil-water mixture separation efficiency and processing speed.

Example 8: Evaluation of Reusability of Filter

[0099] The reusability of the filter was evaluated using the super-hydrophilic PE filter obtained in Example 1. The results are shown in FIG. 12. The fabricated super-hydrophilic filter was reusable because the super-hydrophilic filter was simply washed by immersion in water for 30 seconds after it was used to separate the oil-water mixture. To evaluate the reusability of the super-hydrophilic filter, diesel was selected as the representative oil to measure the separation efficiency and processing speed. As a result, it can be seen that the separation efficiency and the processing speed of the super-hydrophilic filter were maintained at high levels with 99.4% and 2,896 Lm.sup.−2h.sup.−1, respectively, even when the super-hydrophilic filter was repeatedly used 10 times. From the results, it was proven that the filter was simply washed with water and then repeatedly used to process the oil-water mixture.

Example 9: Measurement of Purity of Recovered Water

[0100] The purity of water recovered in the experiment of Example 8 was measured. The results are shown in FIG. 13. It was confirmed that, even when the filter was washed and then reused for oil-water separation to repeatedly separate the oil-water mixture 10 times, very clean water was recovered so that an amount of the oil in water was less than or equal to 5 ppm. From the results, it was confirmed that the recovered water had very high purity, and the oil-water mixture separation performance was not degraded even when the super-hydrophilic filter was washed and repeatedly used.

Example 10: Emulsion Separation Performance of Filter

[0101] A separation mechanism of an emulsion is schematically shown in FIG. 14. The oil particles stabilized with a surfactant formed a filter cake on the super-hydrophilic filter without passing through the filter. Because such a filter cake served to catch very small oil particles, the oil drops did not pass through the filter. On the other hand, because water easily passed through pores of the filter, only the water was able to be selectively recovered from the emulsion stabilized with the surfactant.

[0102] A photograph and a graph shown before and after the emulsion stabilized with the surfactant is processed with the super-hydrophilic filter using the surface treatment process according to the present invention are shown in FIGS. 15A to 15D. The emulsion was prepared by mixing 0.2 g of sodium dodecyl sulfate (SLS) with 99 g of water and 1 g of oil and ultrasonicating the resulting mixture for an hour. The super-hydrophilic filter was fabricated by the method of forming a hydrophilic coating layer as described in Example 1-2 using a PP filter having a nominal pore size of 0.1 μm as the base.

[0103] As shown in FIGS. 15A and 15B, a trace of large oil particles observed even under an optical microscope were present in the emulsion stabilized with the surfactant, and the emulsion was mainly composed of oil particles having a size of 100 to 1,000 nm. When the emulsion was processed with the fabricated filter, most of the oil particles were filtered by the filter pores and the filter cake, as shown in FIGS. 15C and 15D. Therefore, only the fine oil particle (approximately 10 nm) passed through the filter, which makes it possible to recover very clean water.

Example 11: Emulsion Separation Performance According to Repeated Use of Filter after Washing

[0104] A graph showing the emulsion separation efficiency and processing speed measured after the super-hydrophilic filter obtained by the surface treatment process according to the present invention was washed and repeatedly used is shown in FIG. 16. The super-hydrophilic PP filter obtained in Example 10 was used as the super-hydrophilic filter.

[0105] The processing speed was calculated as described in Example 7-2, and the separation efficiency was calculated according to the following Equation 3 using a content of oil in raw water and a content of oil in recovered water.

[00002]$\begin{array}{cc}\mathrm{Emulsion}\mathrm{separation}\mathrm{efficiency}=\left(1-\frac{C_1}{C_0}\right)\times 100\%& \left[\mathrm{Equation}3\right]\end{array}$

[0106] (C.sub.0: a content of oil in raw water, and C.sub.1: a content of oil in recovered water)

[0107] It was confirmed that the separation efficiency and the processing speed were maintained at high levels with 99.7% and 104 Lm.sup.−2h.sup.−1, respectively, even after the filter was washed and repeatedly used to separate the oil-water mixture 10 times. From the results, it was proven that the filter was repeatedly used to process the emulsion stabilized with the surfactant even when the filter was simply washed with water.

[0108] The oil-water mixture separation performance and the emulsion separation performance were able to be controlled through the nominal size of the filter. As described in Example 2-2, because the coating method of the present invention was applicable to the various materials and bases having various nominal size, the oil-water separation performance (separation efficiency or processing speed) was able to be controlled, indicating that the coating method of the present invention may be effectively used in the industries requiring the oily wastewater treatment.

[0109] Although exemplary embodiments of the present invention have been described above in detail, the exemplary embodiments are not intended to limit the scope of the present invention, and various changes and modifications made by those skilled in the art to which the present invention pertains also fall within the scope of the present invention.