A method for preparing high boric acid rejection reverse osmosis membranes using ultraviolet light modification.

Abstract

This invention relates to a method for preparing a high boric acid rejection reverse osmosis membrane using a UV-modification process. The method involves immersing a supporting membrane in an aqueous solution containing 2-sulfoethyl methacrylate additive and a UV photoinitiator; immersing the membrane in an oil-phase solution for interfacial polymerization during this process, while irradiating the membrane under a UV lamp; washing the membrane surface with a solution containing n-hexyl methacrylate, followed by UV irradiation; and heat-treating the membrane at 70–90°C to obtain a reverse osmosis membrane containing the UV polymerization product of 2-sulfoethyl methacrylate and chemically grafted n-hexyl methacrylate. The reverse osmosis membrane prepared by this method exhibits an H3BO3 rejection rate increased from 83.82% to 92.47%, and a NaCl rejection rate increased from 98.89% to 99.19%. This preparation method is simple, efficient, and easy to implement, and can significantly improve the boric acid rejection rate of the prepared membrane while maintaining water flux.

Description

Technical Field

[0001] This invention relates to a method for preparing a high boric acid rejection reverse osmosis membrane using ultraviolet light modification technology. It combines ultraviolet-enhanced interfacial polymerization and ultraviolet grafting modification processes to optimize the main body and surface area of ​​the reverse osmosis membrane separation layer in a targeted manner, thereby improving the boric acid rejection performance of the reverse osmosis membrane. This invention belongs to the field of composite reverse osmosis membrane preparation. Background Technology

[0002] Reverse osmosis membrane technology, due to its stability, high efficiency, mild operating conditions, and ease of integration and scale-up, has been widely applied in seawater desalination, wastewater / sewage treatment, food processing, and drinking water purification. Seawater desalination is the most common application of reverse osmosis membrane technology. Generally, brine cannot be directly used in human production and daily life; appropriate desalination methods are needed to convert unusable brine into usable freshwater. In recent years, many researchers have focused on improving the water permeability coefficient and sodium chloride rejection rate of reverse osmosis membranes. For membrane-based seawater desalination technology, which uses reverse osmosis membranes as its core, to ensure that the desalinated water meets the upper limits of boron content in drinking water as stipulated by my country and some other countries, as well as the water quality requirements for irrigation water for some crops, the reverse osmosis membrane needs to remove more than 90% of the boron from seawater. Therefore, the preparation of reverse osmosis membranes with both high salt rejection rates and high boric acid rejection rates is of great significance. In seawater, boron mainly exists in the form of boric acid (H3BO3). However, due to the weak hydrolysis ability and small molecular size of H3BO3, it is difficult for commonly used reverse osmosis membranes to effectively retain H3BO3 through electrostatic repulsion and steric hindrance effects. This hinders the further promotion and application of reverse osmosis membrane technology.

[0003] Currently, common methods for improving the boric acid rejection rate of reverse osmosis membranes can be divided into two categories. One is to enhance the density of the reverse osmosis membrane, thereby increasing the transmembrane mass transfer resistance of boric acid molecules; the other is to rationally regulate the interaction between boric acid molecules and the reverse osmosis membrane. Both can improve the H3BO3 rejection performance of the reverse osmosis membrane to a certain extent. Therefore, combining these two methods is expected to more effectively improve the H3BO3 rejection performance of the reverse osmosis membrane. Research has found that, in addition to having a dense structure, the ideal high H3BO3 rejection performance reverse osmosis membrane should also have a strong interaction with H3BO3 to slow down the transfer rate of H3BO3 within the membrane. Simultaneously, an ideal high H3BO3 rejection performance reverse osmosis membrane should have a non-polar or weakly polar surface to reduce the interaction between the membrane surface and H3BO3 containing polar groups in the feed solution, preventing H3BO3 accumulation on the membrane surface. Li et al. employed a "swelling-intercalation-shrinkage" strategy to embed 4-nitrobenzenesulfonyl chloride (NBS), containing sulfonic acid groups, into the polyamide (PA) separation layer of a reverse osmosis membrane. First, an ethanol solution containing NBS was poured onto the surface of the reverse osmosis membrane. Ethanol has a strong affinity for PA materials, thus swelling the PA separation layer and increasing the size of the inter-chain gaps within the separation layer. After NBS entered the swollen separation layer, the ethanol gradually evaporated, causing the PA chains to shrink and ultimately fixing the NBS within the membrane separation layer. The intercalation of NBS reduced the size of the inter-chain gaps within the membrane separation layer. Furthermore, the sulfonic acid groups within the membrane can inhibit the rapid diffusion of H3BO3 within the membrane through strong intermolecular interactions. Under the influence of these two factors, after embedding NBS into the PA separation layer of the reverse osmosis membrane, the H3BO3 rejection rate of the reverse osmosis membrane increased from 82.12% to 93.10%, and the NaCl rejection rate increased from 99.20% to 99.57%. (Li Y.,Wang S.,Song X.,Zhou Y.,Shen H.,Cao X.,Zhang P.,Gao C.,Highboron removal polyamide reverse osmosis membranes by swelling induce dembedding of a sulfonyl molecular plug,Journal of Membrane Science[J],2020,597:117716.). However, NBS containing a rigid benzene ring structure easily clogs the water transport channels inside the membrane, thus reducing the water flux of the reverse osmosis membrane from ~33.8 L·m -2 ·h -1 Decreased to ~16.9 L·m -2 ·h -1(The feed solution was an aqueous solution of 32000 mg / L sodium chloride and 5 mg / L boric acid, and the test pressure was 5.5 MPa). Li et al. used a strategy similar to "swelling-intercalation-shrinkage" to intercalate decanoic acid molecules containing nonpolar alkyl chains into the PA separation layer of the reverse osmosis membrane, thus preparing a reverse osmosis membrane with high H3BO3 rejection performance. After the nonpolar decanoic acid molecules were intercalated into the reverse osmosis membrane, the polarity of the membrane surface was significantly reduced, and the density of the separation layer was improved, thereby improving the H3BO3 rejection rate of the reverse osmosis membrane (Li Yunhao, Li Aiai, Yang Binbin, Yu Junjie, Wang Kaizhen, Zhou Yong, Gao Congjie, Preparation of high boron removal reverse osmosis membrane by swelling and intercalation of fatty acid molecules, Journal of Chemical Industry and Engineering [J], 2020, 71: 1343-1351+1896). Under optimal membrane fabrication conditions, the H3BO3 rejection rate of the fabricated reverse osmosis membrane increased from 47.85% to 77.32%, and the NaCl rejection rate increased from 90.36% to 96.46%. However, due to the incorporation of nonpolar caprylic acid molecules, the hydrophilicity of the membrane surface decreased. Consequently, the water flux of the reverse osmosis membrane decreased from ~37.6 L·m -2 ·h -1 Reduced to ~28.0 L·m -2 ·h -1 (The feed solution is an aqueous solution of 2000 mg / L sodium chloride and 5 mg / L boric acid, and the test pressure is 1.55 MPa). It is worth noting that swelling the PA separation layer of the reverse osmosis membrane may cause some low-molecular-weight PA materials to dissolve from the membrane separation layer, potentially causing irreparable damage to the membrane structure. In summary, improving the boric acid rejection rate of a reverse osmosis membrane without compromising the permeate flux is quite challenging. However, the method for preparing a high boric acid rejection rate reverse osmosis membrane provided by this invention, combining a UV-enhanced interfacial polymerization process and a UV grafting modification process, can significantly improve the boric acid rejection rate while ensuring the permeate flux of the prepared membrane. Summary of the Invention

[0004] This invention provides a method for preparing a high boric acid rejection reverse osmosis membrane using an ultraviolet light modification process. This method can improve the density of the reverse osmosis membrane while optimizing the interaction between the membrane material and boric acid molecules. This method can improve the boric acid and sodium chloride rejection rates of the reverse osmosis membrane while ensuring the permeate flux of the prepared membrane. This invention is achieved through the following technical solutions: Figure 2 As shown:

[0005] The present invention discloses a method for preparing a high boric acid rejection reverse osmosis membrane using an ultraviolet light modification process, comprising the following steps:

[0006] 1) Immerse the support membrane in an aqueous solution containing 2-sulfoethyl methacrylate additive and ultraviolet photoinitiator;

[0007] 2) The membrane is immersed in an oil phase solution to carry out interfacial polymerization reaction, and the membrane is irradiated under a UV lamp during the process;

[0008] 3) Rinse the film surface with a solution containing n-hexyl methacrylate, and then apply ultraviolet light irradiation;

[0009] 4) The membrane is heat-treated at 70-90℃ to obtain a reverse osmosis membrane containing the UV polymerization product of 2-sulfoethyl methacrylate and the chemical grafting of n-hexyl methacrylate.

[0010] The mass fraction of 2-sulfoethyl methacrylate in the aqueous solution of step 1) is 0.1% to 1.5%.

[0011] In step 1), the membrane is immersed in an aqueous solution for 10–90 s.

[0012] The ultraviolet photoinitiator in step 1) is 2,2-diethoxyacetophenone, with a mass fraction of 0.02% to 0.5%.

[0013] The oil phase solution in step 2) is a heptane solution containing trimesoyl chloride, with a mass fraction of 0.15% trimesoyl chloride.

[0014] In step 2), the membrane is immersed in the oil phase solution for 10–90 s to carry out the interfacial polymerization reaction.

[0015] Step 3) Irradiate with ultraviolet light for 10-90 seconds.

[0016] The mass fraction of n-hexyl methacrylate in the solution containing n-hexyl methacrylate in step 3) is 0.1% to 8.0%.

[0017] Step 4) involves heat treatment for 3 to 10 minutes.

[0018] Using the above technical solution, a reverse osmosis membrane is prepared using a polysulfone ultrafiltration membrane as the substrate via interfacial polymerization between an aqueous solution containing m-phenylenediamine and an oil solution containing trimesoyl chloride in n-heptane. The aqueous solution used in the interfacial polymerization process includes 2-sulfoethyl methacrylate, m-phenylenediamine, (±)-camphor-10-sulfonic acid, triethylamine, sodium dodecyl sulfonate, and 2,2-diethoxyacetophenone; the oil solution consists entirely of n-heptane containing only 0.15% trimesoyl chloride by mass. 2-sulfoethyl methacrylate (SMA), which exhibits UV-initiated polymerization activity, is selected as the aqueous additive. During the UV-enhanced interfacial polymerization process, taking advantage of the characteristic that SMA containing sulfonic acid groups is difficult to diffuse into the deep interfacial polymerization reaction zone of the oil phase, the SMA polymerization product is successfully enriched in the main body of the PA separation layer. This method improves the density of the separation layer while utilizing the strong intermolecular interaction between sulfonic acid groups and H3BO3 to enhance the interaction between the separation layer and H3BO3 in the membrane, thereby slowing down the transfer rate of H3BO3 in the membrane and optimizing the main body of the PA separation layer.

[0019] Following the interfacial polymerization reaction, hexyl methacrylate (HMA), which exhibits UV-initiated polymerization activity, was added to the hexane solvent used to rinse the reverse osmosis membrane surface. A UV grafting modification process was then employed to graft HMA containing non-polar alkyl chains onto the surface region of the PA separation layer. This method significantly weakened the polarity of the separation layer surface, inhibited the enrichment of H3BO3 containing polar groups on the membrane surface, and optimized the surface region of the PA separation layer.

[0020] Combining the above strategies, a reverse osmosis membrane with optimized bulk and surface was prepared. The H3BO3 rejection rate of the membrane prepared by this method increased from 83.82% to 92.47%, and the NaCl rejection rate increased from 98.89% to 99.19%. (The feed solution was an aqueous solution containing 32000 mg / L sodium chloride and 5 mg / L boron; the operating pressure was 5.50 MPa; and the test temperature was 25°C). This invention provides a method for preparing high boric acid rejection reverse osmosis membranes using ultraviolet light modification, which can improve the boric acid rejection rate of the prepared membrane without compromising the permeate flux and sodium chloride rejection rate. The advantages of this invention are: simple operation, time-saving and efficient, mild modification conditions, and minimal damage to the separation layer structure of the reverse osmosis membrane. Furthermore, the prepared high boric acid rejection reverse osmosis membrane combines high permeate flux and high sodium chloride rejection rate. This invention is applicable not only to the preparation of reverse osmosis membranes, but also to the development of other separation membranes prepared based on interfacial polymerization and grafting modification processes. Attached Figure Description

[0021] Figure 1 This is a scanning electron microscope image of the surface structure of the reverse osmosis membrane prepared in Example 4.

[0022] Figure 2 This is a schematic diagram of the membrane fabrication process for the high boric acid rejection reverse osmosis membrane of the present invention. Detailed Implementation

[0023] The present invention will be further described in detail below with reference to specific embodiments. A reverse osmosis membrane is prepared using a polysulfone ultrafiltration membrane as a substrate via an interfacial polymerization process between an aqueous solution containing m-phenylenediamine and an oil solution containing trimesoyl chloride in n-heptane. The aqueous solution used in the interfacial polymerization process includes 2-sulfoethyl methacrylate, m-phenylenediamine, (±)-camphor-10-sulfonic acid, triethylamine, sodium dodecyl sulfonate, and 2,2-diethoxyacetophenone; the oil solution is an n-heptane solution containing only 0.15% trimesoyl chloride by mass. 2-Sulfoethyl methacrylate (SMA), which exhibits UV-initiated polymerization activity, was selected as an aqueous phase additive. During UV-enhanced interfacial polymerization, the SMA, containing sulfonic acid groups, was difficult to diffuse into the deep interfacial polymerization reaction zone of the oil phase, thus enriching the SMA polymerization product in the main body of the PA separation layer. Subsequently, through a UV grafting modification process, HMA was chemically grafted onto the surface of the reverse osmosis membrane separation layer, thereby weakening the polarity of the membrane surface and reducing the attraction of the membrane surface to H3BO3, thus optimizing the surface region of the separation layer. It is expected that by combining the UV-enhanced interfacial polymerization process and the UV grafting modification process, the main body and surface regions of the reverse osmosis membrane separation layer can be optimized separately to improve the H3BO3 retention performance of the reverse osmosis membrane.

[0024] Example 1

[0025] 1) Prepare an aqueous solution containing 0.20% 2-sulfoethyl methacrylate, 3.0% m-phenylenediamine, 3.9% (±)-camphor-10-sulfonic acid, 1.9% triethylamine, 0.15% sodium dodecyl sulfonate and 0.02% 2,2-diethoxyacetophenone by mass fraction. Immerse the support membrane in the aqueous solution for 10 seconds, then remove the aqueous solution and dry.

[0026] 2) Prepare an oil phase solution of n-heptane containing 0.15% trimesoyl chloride by mass, and then immerse the membrane obtained in step 1) in the oil phase solution for 10s to carry out interfacial polymerization reaction. During this process, the membrane is irradiated under a UV lamp.

[0027] 3) Prepare a hexane solution containing 0.1% n-hexyl methacrylate by mass, then rinse the film surface with the hexane solution containing 0.1 wt% n-hexyl methacrylate, and then apply ultraviolet light irradiation for 10 seconds.

[0028] 4) The membrane obtained in step 3) is heat-treated at 70°C for 3 minutes to obtain a composite reverse osmosis membrane with high boric acid rejection rate.

[0029] The permeate flux of the prepared reverse osmosis membrane was tested at 5.50 MPa and 25 °C using an aqueous solution containing 5 mg / L boric acid and 32000 mg / L sodium chloride. The results showed that the permeate flux was less than 44.69 L / (m²). 2 (h), the sodium chloride retention rate exceeds 98.89%, and the boric acid retention rate exceeds 83.82%.

[0030] Example 2

[0031] 1) Prepare an aqueous solution containing 1.5% 2-sulfoethyl methacrylate, 3.0% m-phenylenediamine, 3.9% (±)-camphor-10-sulfonic acid, 1.9% triethylamine, 0.15% sodium dodecyl sulfonate and 0.5% 2,2-diethoxyacetophenone by mass fraction. Immerse the support membrane in the aqueous solution for 90 seconds, then remove the aqueous solution and dry.

[0032] 2) Prepare an oil phase solution containing 0.15% trimesoyl chloride in n-heptane, and then immerse the membrane obtained in step 1) in the oil phase solution for 90s to carry out interfacial polymerization reaction. During this process, the membrane is irradiated under a UV lamp.

[0033] 3) Prepare a hexane solution containing 8% n-hexyl methacrylate by mass, then rinse the film surface with the hexane solution containing 8 wt% n-hexyl methacrylate, and then apply ultraviolet light irradiation for 90 seconds.

[0034] 4) The membrane obtained in step 3) is heat-treated at 90°C for 10 min to obtain a composite reverse osmosis membrane with high boric acid rejection rate.

[0035] The permeate flux of the prepared reverse osmosis membrane was tested at 5.50 MPa and 25 °C using an aqueous solution containing 5 mg / L boric acid and 32000 mg / L sodium chloride. The results showed that the permeate flux was less than 40.95 L / (m²). 2 (h), the sodium chloride retention rate is less than 99.19%, and the boric acid retention rate is less than 92.47%.

[0036] Example 3

[0037] 1) Prepare an aqueous solution containing 0.10% 2-sulfoethyl methacrylate, 3.0% m-phenylenediamine, 3.9% (±)-camphor-10-sulfonic acid, 1.9% triethylamine, 0.15% sodium dodecyl sulfonate and 0.04% 2,2-diethoxyacetophenone by mass fraction. Immerse the support membrane in the aqueous solution for 60 seconds, then remove the aqueous solution and dry.

[0038] 2) Prepare an oil phase solution of n-heptane containing 0.15% trimesoyl chloride by mass, and then immerse the membrane obtained in step 1) in the oil phase solution for 60s to carry out interfacial polymerization reaction. During this process, the membrane is irradiated under a UV lamp.

[0039] 3) Prepare a hexane solution containing 6% n-hexyl methacrylate by mass, then rinse the film surface with the hexane solution containing 6 wt% n-hexyl methacrylate, and then apply ultraviolet light irradiation for 60 seconds.

[0040] 4) The membrane obtained in step 3) is heat-treated at 80°C for 5 minutes to obtain a composite reverse osmosis membrane with high boric acid rejection rate.

[0041] The permeate flux of the prepared reverse osmosis membrane exceeded 38.64 L / (m³) at 5.50 MPa and 25 °C using an aqueous solution containing 5 mg / L boric acid and 32000 mg / L sodium chloride. 2 (h), the sodium chloride retention rate exceeds 98.89%, and the boric acid retention rate exceeds 83.82%.

[0042] Example 4

[0043] 1) Prepare an aqueous solution containing 0.20% 2-sulfoethyl methacrylate, 3.0% m-phenylenediamine, 3.9% (±)-camphor-10-sulfonic acid, 1.9% triethylamine, 0.15% sodium dodecyl sulfonate, and 0.02% 2,2-diethoxyacetophenone by mass fraction; immerse the support membrane in the aqueous solution for 60 s, then remove the aqueous solution and dry.

[0044] 2) Prepare an oil phase solution of n-heptane containing 0.15% trimesoyl chloride by mass, and then immerse the membrane obtained in step 1) in the oil phase solution for 60s to carry out interfacial polymerization reaction. During this process, the membrane is irradiated under a UV lamp.

[0045] 3) Prepare a hexane solution containing 6% n-hexyl methacrylate by mass, then rinse the film surface with the hexane solution containing 6 wt% n-hexyl methacrylate, and then apply ultraviolet light irradiation for 60 seconds.

[0046] 4) The membrane obtained in step 3) was heat-treated at 80℃ for 5 minutes to obtain a composite reverse osmosis membrane with high boric acid rejection rate. The electron microscope image of the membrane surface is shown below. Figure 1 As shown.

[0047] The permeate flux, sodium chloride rejection rate, and boric acid rejection rate of the prepared reverse osmosis membrane were tested at 5.50 MPa and 25℃ using an aqueous solution containing 5 mg / L boric acid and 32000 mg / L sodium chloride. The results showed that these parameters were 40.95 L / (m³). 2 (·h), 99.19% and 92.47%.

[0048] Comparative Example

[0049] The reverse osmosis membrane prepared by Li et al. (Li Y., Wang S., Song X., Zhou Y., Shen H., Cao X., Zhang P., Gao C., High boron removal polyamide reverse osmosis membranes by swelling induce dembedding of a sulfonyl molecular plug, Journal of Membrane Science[J], 2020, 597:117716.) was tested at 5.50 MPa and 25 °C using an aqueous solution containing 5 mg / L boric acid and 32000 mg / L sodium chloride. The permeate flux, sodium chloride rejection rate, and boric acid rejection rate of the prepared reverse osmosis membrane were found to be 16.90 L / (m²). 2 (·h), 99.57% and 93.10%.

[0050] Examples 1-4, using the method of the present invention, prepared reverse osmosis membranes with optimized substrates and surfaces. Under the premise of a slight loss in water flux, both sodium chloride rejection and boric acid rejection were improved. The optimal concentration of the additive was 0.2 wt%, and the optimal concentration of n-hexyl methacrylate in n-hexane solution was 6.0 wt%. As shown in Example 4, the permeate flux, sodium chloride rejection, and boric acid rejection of the reverse osmosis membrane prepared under these conditions were 40.95 L / (m³). 2 The water flux of Example 4 was 99.19% and 92.47%, respectively. Compared with the comparative example, both showed higher sodium chloride and boric acid rejection rates, but Example 4 exhibited superior water flux. In summary, the high boric acid rejection rate composite reverse osmosis membrane prepared by combining UV-enhanced interfacial polymerization and UV grafting modification processes of this invention exhibits superior water flux, sodium chloride rejection rate, and boric acid rejection rate.

[0051] This invention relates to a method for preparing a high boric acid rejection reverse osmosis membrane using a UV-modification process. It combines UV-enhanced interfacial polymerization and UV grafting modification to specifically optimize the main body and surface areas of the reverse osmosis membrane separation layer. An aqueous solution containing both 2-sulfoethyl methacrylate additive and a UV photoinitiator is prepared. A supporting membrane is immersed in the aqueous solution to ensure thorough wetting. After removing excess aqueous solution from the membrane surface, the membrane is immersed in an oil-phase solution for interfacial polymerization, and the membrane surface is irradiated with UV light. Excess oil-phase solution is removed from the membrane surface, and the membrane surface is rinsed with a solution containing n-hexyl methacrylate, followed by UV irradiation. After drying, a reverse osmosis membrane with a high boric acid rejection rate is obtained. This method is simple, efficient, and easy to implement, and can significantly improve the boric acid rejection rate of the prepared membrane while maintaining water flux.

[0052] This invention discloses a method for preparing high boric acid rejection reverse osmosis membranes using an ultraviolet light modification process. Those skilled in the art can implement this method by appropriately modifying conditions and procedures, based on the content of this document. Although the method and preparation technique of this invention have been described through preferred embodiments, those skilled in the art can obviously modify or recombine the methods and techniques described herein without departing from the content, spirit, and scope of this invention to achieve the final preparation technique. It is particularly important to note that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included within the spirit, scope, and content of this invention.

Claims

1. A method for preparing a high boric acid rejection reverse osmosis membrane using an ultraviolet light modification process, characterized in that it includes... The process is as follows: 1) Immerse the support membrane in an aqueous solution containing 2-sulfoethyl methacrylate additive and ultraviolet photoinitiator; 2) The membrane is immersed in an oil phase solution to carry out interfacial polymerization reaction, and the membrane is irradiated under a UV lamp during the process; 3) Rinse the film surface with a solution containing n-hexyl methacrylate, and then apply ultraviolet light irradiation; 4) The membrane is heat-treated at 70-90℃ to obtain a reverse osmosis membrane containing the UV polymerization product of 2-sulfoethyl methacrylate and the chemical grafting of n-hexyl methacrylate.

2. The method as described in claim 1, characterized in that, The mass fraction of 2-sulfoethyl methacrylate in the aqueous solution of step 1) is 0.1% to 1.5%.

3. The method as described in claim 1, characterized in that, In step 1), the membrane is immersed in an aqueous solution for 10–90 s.

4. The method as described in claim 1, characterized in that, The ultraviolet photoinitiator in step 1) is 2,2-diethoxyacetophenone, with a mass fraction of 0.02% to 0.5%.

5. The method as described in claim 1, characterized in that, The oil phase solution in step 2) is a heptane solution containing trimesoyl chloride, with a mass fraction of 0.15% trimesoyl chloride.

6. The method as described in claim 1, characterized in that, In step 2), the membrane is immersed in the oil phase solution for 10–90 s to carry out the interfacial polymerization reaction.

7. The method as described in claim 1, characterized in that, Step 3) Irradiate with ultraviolet light for 10-90 seconds.

8. The method as described in claim 1, characterized in that, The mass fraction of n-hexyl methacrylate in the solution containing n-hexyl methacrylate in step 3) is 0.1% to 8.0%.

9. The method as described in claim 1, characterized in that, Step 4) involves heat treatment for 3 to 10 minutes.

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