Method for coating with water-dispersible polyurethane for adjusting pore size of UF hollow fiber membrane, and UF hollow fiber membrane produced thereby

Abstract

The present invention relates to a method for coating an ultrafiltration (UF) hollow fiber membrane with water-dispersible polyurethane, and to a UF hollow fiber membrane produced by the method. By coating the UF hollow fiber membrane with the water-dispersible polyurethane to narrow the pores of the hollow fiber membrane, the present invention maximizes the contaminant filtration capability when applied to a water purifier filter.

Description

Method for coating a water-dispersible urethane for controlling the pore size of a UF hollow fiber membrane and a UF hollow fiber membrane manufactured by the method

[0001] The present invention relates to a method for coating a water-dispersible urethane to control the pore size of an ultrafiltration (UF) hollow fiber membrane and to a UF hollow fiber membrane manufactured by the method. More specifically, the invention relates to a method for coating a UF hollow fiber membrane with water-dispersible polyurethane (PUD) to narrow the pores of the hollow fiber membrane by coating the UF hollow fiber membrane with water-dispersible polyurethane (PUD), thereby maximizing the ability to filter foreign substances when applied to a water purifier filter, and to a UF hollow fiber membrane manufactured by the method.

[0002]

[0003] Water purification filters play a crucial role in water purification and the removal of impurities, and the UF hollow fiber membranes used for this purpose provide high-efficiency filtration performance due to their fine pore structure. UF hollow fiber membranes are primarily manufactured using polymer materials, and water purification performance can be improved by controlling the pore size of the membrane. However, conventional UF hollow fiber membranes often face limitations in filtration performance due to inconsistent pore sizes or non-uniform pore structures during manufacturing. To address this, various studies have been conducted on coating technologies that improve filtration performance by forming an additional coating layer on the surface of UF hollow fiber membranes using polymer materials to uniformly control pore size.

[0004]

[0005] In particular, water-dispersible polyurethane (PUD) is used as a coating material and is evaluated as a suitable material for improving the performance of UF hollow fiber membranes due to its excellent water dispersibility, mechanical strength, and chemical resistance. Water-dispersible polyurethane (PUD) is a form in which polyurethane polymers are dispersed in water, and it can harmoniously combine hydrophilicity and hydrophobicity to provide a uniform coating on the membrane surface. This enables applications that control pore size on the surface of UF hollow fiber membranes, increase durability, and maximize filtration efficiency.

[0006]

[0007] However, conventional coating technologies using water-dispersible polyurethane (PUD) have had limitations in precisely and uniformly controlling the pore size of UF hollow fiber membranes. For instance, problems such as reduced surface uniformity or insufficient durability of the coating layer were frequently observed depending on the selection of coating materials or application methods. Therefore, the development of coating materials and processes suitable for the surface of UF hollow fiber membranes is essential, and technological innovation is required to simultaneously improve filtration performance while satisfying the economic efficiency and durability of UF hollow fiber membranes.

[0008]

[0009] The present invention proposes a technology for synthesizing a highly water-dispersible water-dispersible polyurethane (PUD) using polycarbonate diol and isophorone diisocyanate (IPDI) and coating it onto a hollow fiber membrane to precisely control pore size and improve the performance of the UF hollow fiber membrane. In particular, the invention aims to solve the problems of existing technologies by providing a technology that prevents bonding between UF hollow fiber membranes and promotes the formation of a uniform coating layer through methods such as introducing hydrophilicity to the water-dispersible polyurethane (PUD) using dimethylolpropionic acid (DMPA) and triethylamine (TEA), or through squeezing and hot-air drying processes during the coating process.

[0010]

[0011] The present invention aims to provide a UF hollow fiber membrane suitable for high-performance filtering applications, such as water purification filters, by effectively controlling the pore size of the UF hollow fiber membrane. In particular, it seeks to solve the problem of significantly improving impurity filtration performance and divalent ion filtration performance by narrowing the pore size of the membrane surface and forming a uniform coating layer on the surface of the UF hollow fiber membrane using water-dispersible polyurethane (PUD). Furthermore, it prevents the formation of non-uniform coating layers and coating peeling phenomena that may occur in conventional coating methods, and implements functions such as reducing the aqueous conductivity of the UF hollow fiber membrane and controlling the permeation flow rate. In addition, it aims to resolve the environmental friendliness of the production process and the problem of preventing surface bonding between UF hollow fiber membranes through the use of an eco-friendly catalyst and a hot air drying process, and to achieve optimal coating performance by introducing a precise dilution ratio.

[0012]

[0013] To achieve the above objectives, the present invention is characterized by providing a method for coating a water-dispersible polyurethane (PUD) to improve filtering performance by controlling the pore size of a UF hollow fiber membrane, and a UF hollow fiber membrane manufactured by said method.

[0014]

[0015] The present invention will be described in detail below.

[0016]

[0017] The method for manufacturing a hollow fiber membrane by a wet spinning method using polysulfone and polyphenylsulfone as polymer resins according to the present invention is described as follows.

[0018]

[0019] Preparation of radiation source solution

[0020] First, a polymer solution (dope) is prepared by mixing polysulfone and polyphenylsulfone using N-methyl-2-pyrrolidone (NMP) as a solvent.

[0021]

[0022] At this time, additives are used to control the hydrophilicity and pore size of the hollow fiber membrane, and the additives may include water, polyethylene glycol (PEG) 200, polyvinylpyrrolidone (PVP), etc. These additives are added to the dope to adjust the viscosity of the spinning solution, and the viscosity of the prepared spinning solution is 500 to 50,000 cps, preferably 800 to 30,000 cps, at 25°C.

[0023]

[0024] The viscosity of the spinning solution is adjusted to suit the desired film characteristics, and this viscosity may vary depending on the mixing ratio of the polysulfone resin and the solvent. A person skilled in the art can appropriately set and use this mixing ratio within the technical scope of the present invention.

[0025]

[0026] Preparation of internal coagulant

[0027] The internal coagulation solution is used to form hollow and internal pores of the hollow fiber membrane and is prepared by mixing water, a nonsolvent, and dimethylacetamide (DMAc), a solvent. To significantly control the internal pore size of the hollow fiber membrane, it is preferable to set the ratio of the nonsolvent to the solvent in the internal coagulation solution to 10:90 to 1:99 by weight. At this ratio, if the solvent content is less than 90% by weight, it may be difficult to form a network structure on the inner surface, while conversely, if it exceeds 99% by weight, the formation of the hollow fiber membrane may become unstable. The viscosity of the prepared internal coagulation solution is set to a range of 0.5 to 100 cps at 25°C.

[0028]

[0029] Manufacturing of hollow fiber membranes

[0030] The spinning solution is extruded into the air along with an internal coagulant through a spinneret equipped with a double tubular nozzle. During this process, as the internal coagulant passes through the center of the spinning solution, a hollow structure is formed, and the spinning solution begins to solidify from the hollow surface.

[0031] Subsequently, the spun solution is immersed in an external coagulation solution, which may include water, N-methyl-2-pyrrolidone, dimethylformamide (DMF), dimethylacetamide (DMAc), chloroform, tetrahydrofuran (THF), polyethylene glycol, propylene glycol, ethylene glycol, glycerin, or a mixture thereof. The external coagulation solution induces rapid coagulation of the spun solution, thereby producing a hollow fiber membrane with a separation filtration layer and a hollow structure.

[0032] It is desirable to control the phase separation rate of the spinneret and the internal coagulant by adjusting the distance between the discharge port of the spinneret and the external coagulant bath. This allows for precise control of the structural characteristics and pore distribution of the hollow fiber membrane.

[0033]

[0034] As such, the hollow fiber membrane manufactured according to the above method forms a separation filtration layer composed of micropores on its outer surface, and a network structure is formed inside the separation filtration layer. The pore size gradually increases toward the hollow side, and in particular, large pores ranging from 10 to 200 µm exist on the inner surface, providing high permeability and pressure resistance suitable for external pressure applications. More specifically, the average diameter of the internal circular pores on the inner surface is set to 30 to 100 µm, which is suitable for water treatment separation membrane applications requiring ultrafiltration and microfiltration. Thanks to these structural characteristics, the hollow fiber membrane of the present invention provides an excellent separation effect capable of efficiently removing impurities of 5.0 µm or less.

[0035]

[0036] Next, a step is carried out to synthesize a polyurethane with high water dispersibility by reacting polycarbonate diol with isophorone diisocyanate (IPDI) to form a polyurethane dispersion (PUD). This step is characterized by forming a polyurethane dispersion (PUD) with high dispersibility and hydrophilicity through the reaction of polycarbonate diol and isophorone diisocyanate, thereby enabling stable design of the pore size of the UF hollow fiber membrane.

[0037]

[0038] The water-dispersible polyurethane (PUD) used in this invention is designed to have a low glass transition temperature (Tg). This is because using a water-dispersible polyurethane (PUD) with a high glass transition temperature (Tg) can cause adhesion between hollow fiber membranes after coating, which may negatively affect the quality and performance of the product. Therefore, in this invention, a water-dispersible polyurethane (PUD) with a low glass transition temperature (Tg) is synthesized and applied to hollow fiber membrane coating to maximize the impurity filtration performance.

[0039]

[0040] The main raw materials and processes for manufacturing a water-dispersible polyurethane (PUD) according to the present invention, which provides water dispersibility while maintaining the special elasticity and durability of the polyurethane, are as follows.

[0041]

[0042] Composition of main raw materials

[0043] ① Polyol

[0044] The physical properties of a polymer are determined by the molecular weight of the polyol; when the molecular weight is high, fluidity improves but adhesion weakens, while when the molecular weight is low, hardness increases excessively. Accordingly, in this invention, a water-dispersible polyurethane (PUD) with excellent elasticity and a low glass transition temperature (Tg) can be obtained by using a polycarbonate diol with a molecular weight of 1000 g / mol. That is, the glass transition temperature (Tg) of the water-dispersible polyurethane (PUD) is designed to be low so that the coated water-dispersible polyurethane (PUD) membrane possesses flexibility. Designing a low glass transition temperature (Tg) prevents deformation even under stress changes caused by water flow during filtering of the UF hollow fiber membrane, and ensures sustainable and stable performance.

[0045]

[0046] ② Polyisocyanate

[0047] In this invention, isophorone diisocyanate (IPDI), an aliphatic isocyanate, is used to maintain resistance to yellowing and transparency during polyurethane synthesis. IPDI belongs to the aliphatic isocyanate family and provides non-yellowing properties, thereby enabling the maintenance of excellent optical transparency even in long-term usage environments. In particular, IPDI possesses the characteristic of being able to control the glass transition temperature (Tg) of the polyurethane structure to a low level, which aligns with the primary design objective of the water-dispersible polyurethane (PUD) synthesized in this invention. A low glass transition temperature (Tg) prevents adhesion between coated hollow fiber membranes and contributes to simultaneously ensuring filtration performance and physical stability. Therefore, IPDI has been adopted as a key raw material that plays a crucial role in controlling the physical properties and improving the performance of the water-dispersible polyurethane synthesized in this invention.

[0048]

[0049] ③ Hydrophilic Monomer

[0050] DMPA (Dimethylolpropionic Acid) is added to impart water dispersibility.

[0051]

[0052] ④ Catalyst

[0053] The previously used DBTDL (Dibutyltin Dilaurate) has been banned due to environmental issues, and an environmentally friendly bismuth catalyst is used to replace it.

[0054]

[0055] ⑤ Neutralizer

[0056] Triethylamine (TEA) is used to neutralize hydrophilic carboxylic acid groups.

[0057]

[0058] ⑥ Solvent

[0059] Methyl ethyl ketone (MEK) is used as a solvent to control dispersibility and reaction rate during the initial synthesis process of water-dispersible polyurethane (PUD). The use of methyl ethyl ketone (MEK) plays an important role in increasing the uniformity of the water-dispersible polyurethane (PUD) synthesis reaction and maintaining the quality stability of the coating solution through reaction rate control.

[0060]

[0061] ⑦ Distilled water

[0062] High-purity deionized water is used in reaction and dispersion processes.

[0063]

[0064] manufacturing process

[0065] ① Prepolymer Synthesis

[0066] The reactor is set to prevent oxidation under a nitrogen (N2) atmosphere, the reaction temperature is set to 60~80℃, and the stirring speed is set to 700~800rpm.

[0067]

[0068] Polycarbonate diol and IPDI are added to set the NCO ratio to 1.1 to 1.2, and the reaction is carried out while forming an appropriate crosslink.

[0069]

[0070] To introduce hydrophilicity, dimethylolpropionic acid (DMPA) is added at 70°C to introduce hydrophilic groups (-COOH) into the polymer structure, and the mixture is stirred sufficiently for a uniform reaction. This hydrophilicity enables the improvement of the efficiency of the UF hollow fiber membrane coating performance. This allows for securing the water purification filtering efficiency and pore size control characteristics of the UF hollow fiber membrane.

[0071]

[0072] The reaction rate is controlled by adding a bismuth catalyst (concentration: 0.05~0.1%), and the completion of the reaction is checked by verifying the NCO content using the dibutylamine method.

[0073]

[0074] ② Neutralization

[0075] To ensure water dispersibility, triethylamine (TEA) is added as a neutralizing agent at 40–50°C to neutralize the carboxylic acid groups of dimethylolpropionic acid (DMPA) and adjust the pH to 7–8. By using triethylamine (TEA) to stabilize the carboxylic groups of dimethylolpropionic acid (DMPA), water stability of the water-dispersible polyurethane (PUD) is ensured, and a high-quality coating film is realized while maintaining ideal viscosity during surface coating of UF hollow fiber membranes.

[0076]

[0077] ③ Dispersion

[0078] The prepared prepolymer is stirred at 500–800 rpm while gradually adding distilled water to control the particle size to 50–200 nm. Dispersion stability is examined by checking for particle aggregation.

[0079]

[0080] ④ Volume Growth (Chain Extension)

[0081] Ethylene diamine (EDA) is slowly added at 30–40°C to increase the molecular weight of the polymer and induce volume growth. This process affects the transparency and mechanical strength of the water-dispersible polyurethane (PUD).

[0082]

[0083] ⑤ Solvent Removal

[0084] The used triethylamine (TEA) is removed through vacuum distillation or heat treatment to minimize the volatile organic compound (VOC) content. In this process, methyl ethyl ketone (MEK) is removed by dilution using a neutralizing agent and distilled water, followed by heating and a vacuum pump.

[0085]

[0086] ⑥ Solid Content Adjustment

[0087] Additional distilled water is added to adjust the final viscosity to 50–200 mPa·s and the solid content to 30–40%.

[0088]

[0089] The water-dispersible polyurethane (PUD) produced according to the above manufacturing method provides high transparency and excellent durability by precisely controlling particle size, dispersion stability, and volume growth processes.

[0090]

[0091] Next, the synthesized water-dispersible polyurethane (PUD) is diluted with water to form a coating solution suitable for application to hollow fiber membranes. In this step, the water-dispersible polyurethane (PUD) is diluted to an appropriate ratio of water to obtain a coating solution with viscosity and adhesion properties suitable for the surface of the UF hollow fiber membrane, and the diluted solution enables uniform application to the surface of the UF hollow fiber membrane. This lays the foundation for more precise control of pore channels. Meanwhile, when coating UF hollow fiber membranes, it is important to use a coating solution diluted with water-dispersible polyurethane (PUD) and water at a weight ratio of 1:450 to maximize coating efficiency. This ratio is adjusted to form a uniform, thin coating layer on the UF hollow fiber membrane, providing both durability and filtering capabilities for the coating film.

[0092]

[0093] Subsequently, as a method to impart functionality by uniformly coating the surface of a hollow fiber membrane with water-dispersible polyurethane (PUD), the coating process proceeds as follows.

[0094]

[0095] Coating process

[0096] The manufactured hollow fiber membrane is dipped in a coating solution to apply water-dispersible polyurethane (PUD) to the surface of the hollow fiber membrane. The viscosity and concentration of the coating solution can be adjusted according to the pore size and surface characteristics of the hollow fiber membrane. In the present invention, the solid content of the water-dispersible polyurethane (PUD) is set to 30-40% to form a uniform coating layer.

[0097]

[0098] The process of twisting hollow fiber membranes

[0099] A process of hand-twisting the coated hollow fiber membrane is performed to allow water-dispersible polyurethane (PUD) to permeate evenly into the pores of the hollow fiber membrane.

[0100]

[0101] Drying process

[0102] The hollow fiber membrane that has undergone the above twisting process is dried at 70°C for 4 hours using a hot air drying method. During this process, moisture and residual solvent within the coating layer are removed, and a coating layer with improved strength and stability is finally formed.

[0103]

[0104] The coating layer dried in this manner is uniformly formed on the surface of the hollow fiber membrane, enhancing durability and chemical resistance to the external environment. Furthermore, this coating layer not only improves filtering performance by controlling pore size and distribution but also imparts hydrophilic and anti-fouling properties.

[0105]

[0106] In addition, by performing drying at 70°C for 4 hours under the above hot air drying conditions, coalescence or adhesion of the UF hollow fiber membranes can be prevented. This maintains the physical independence between individual UF hollow fiber membranes and minimizes deformation or damage to the coated layer.

[0107]

[0108] The final pore size of the UF hollow fiber membrane is controlled within the range of 0.01 to 0.1㎛, and by designing a narrow pore size, it provides powerful filtering performance capable of effectively removing even fine particles or foreign substances. This leads to improved water purification capacity and stable filtering efficiency compared to conventional UF hollow fiber membranes.

[0109]

[0110] In addition, surface SEM analysis results of the UF hollow fiber membrane provided in the present invention confirm that it consists of a uniform membrane after coating, thereby enabling it to exhibit optimal performance as a filter.

[0111]

[0112] UF hollow fiber membranes coated in the manner described above are designed to enable divalent ion filtering and exhibit characteristics that reduce electrical conductivity. Through this, UF hollow fiber membranes can play an important role in water purification in water purification filtering applications.

[0113]

[0114] Finally, the present invention is characterized by using a bismuth catalyst instead of a tin-based catalyst during synthesis to form a coating layer of a UF hollow fiber membrane, with consideration for environmental friendliness. This reduces harmful environmental pollution and enables sustainable, eco-friendly technology.

[0115]

[0116] The present invention, with the above-described configuration, can achieve a high-level water purification filtering function by coating a UF hollow fiber membrane with water-dispersible polyurethane (PUD) to control the pore size of the UF hollow fiber membrane and enhance the filtration performance of foreign substances.

[0117]

[0118] In addition, a water-dispersible polyurethane (PUD) coating prepared using polycarbonate diol and isophorone diisocyanate (IPDI) enables the pore size of the UF hollow fiber membrane to be controlled to a range of 0.01 to 0.1 μm, thereby enabling the treatment of high-purity water requiring microfiltration, and has the effect of maximizing the water treatment efficiency of the UF hollow fiber membrane through the hydrophilic properties of dimethylolpropionic acid (DMPA) introduced during the coating process and the carboxylic acid group neutralization reaction using triethylamine (TEA).

[0119]

[0120] The present invention can maintain the stability of the coating solution by simultaneously controlling the reaction rate and dispersibility of the water-dispersible urethane using methyl ethyl ketone (MEK) as a solvent, and contributes to high coating quality and prevention of coating peeling by uniformly coating a UF hollow fiber membrane in a single layer.

[0121]

[0122] In addition, by setting the twisting process of the coated hollow fiber membrane and hot air drying conditions (4 hours at 70°C), membrane bonding caused by contact between UF hollow fiber membranes is prevented, and a uniform coating surface (confirmed by SEM analysis) is created, thereby improving filter quality and durability.

[0123]

[0124] The UF hollow fiber membrane of the present invention ensures flexibility and stability through a water-dispersible polyurethane (PUD) coating layer designed to have a low glass transition temperature (Tg) value during the coating process, thereby minimizing changes in performance even during long-term use.

[0125]

[0126] In addition, by combining the hydrophilicity of the UF hollow fiber membrane with the hydrophobicity of the water-dispersible polyurethane (PUD) coating layer in a balanced manner, high ion rejection rates and permeation flow rate control functions are achieved, enabling utilization as a general water purification filter and a high-performance desalination filtering device.

[0127]

[0128] Furthermore, the present invention can maintain stable water quality in electrolyte treatment and water treatment systems by reducing the electrical conductivity of the coated UF hollow fiber membrane, and provides an eco-friendly manufacturing process by introducing the use of an eco-friendly catalyst (bismuth catalyst) compared to existing UF filters.

[0129]

[0130] Figure 1 is an SEM electron microscope image of the surface of a 150x UF hollow fiber membrane before and after coating with water-dispersible polyurethane (PUD).

[0131] Figure 2 is an SEM electron microscope image of the surface of a 500x UF hollow fiber membrane before and after coating with water-dispersible polyurethane (PUD).

[0132] Figure 3 is an SEM electron microscope image of the surface of a 2,000x UF hollow fiber membrane before and after coating with water-dispersible polyurethane (PUD).

[0133] Figure 4 is an SEM electron microscope image of a cross-section of a 400x UF hollow fiber membrane before and after coating with water-dispersible polyurethane (PUD).

[0134] Figure 5 is an SEM electron microscope image of a cross-section of a 1,000x UF hollow fiber membrane before and after coating with water-dispersible polyurethane (PUD).

[0135] Figure 6 is an SEM electron microscope image of a cross-section of a 5,000x UF hollow fiber membrane before and after coating with water-dispersible polyurethane (PUD).

[0136]

[0137] The present invention will be explained in more detail below through examples. However, the following examples do not limit the scope of the present invention, and ordinary modifications by those skilled in the art are possible within the scope of the technical concept of the present invention.

[0138]

[0139] <Example 1>

[0140] Manufacturing of hollow fiber membranes

[0141] Polysulfone was used as the main polymer, and polyvinylpyrrolidone (PVP) and propylene glycol were dissolved in N-methyl-2-pyrrolidone (NMP) solvent as additives to prepare a spinning solution. The viscosity of the prepared spinning solution was measured to be 2,000 cps at 25°C.

[0142] The internal coagulation solution consists of a mixed solvent of water and NMP, and the mixing ratio of water to NMP was set to 10:90 by weight. This composition is intended to optimize the formation of internal pores and structural control of the hollow fiber membrane.

[0143] A hollow fiber membrane was manufactured by extruding the spinning solution and internal coagulation solution into an external coagulation solution through a spinneret equipped with a double tubular nozzle. The external coagulation solution consists of a mixture of water and NMP and induces the coagulation of the spinning solution during the spinning process.

[0144] At this time, the nozzle diameters of the spinnerets used were set to Φ400㎛ (inner nozzle) and Φ1,200㎛ (outer nozzle), respectively, based on the discharge port of the spinneret. The distance between the spinneret and the external coagulation solution was maintained at 15cm to control the phase separation rate of the spinneret. The temperature of the coagulation bath was maintained at 35℃ to promote uniform coagulation and structure formation.

[0145]

[0146] <Example 2>

[0147] Manufacturing of water-dispersible polyurethane (PUD)

[0148] The reactor was set in a nitrogen (N2) atmosphere to prevent oxidation. The reaction conditions were maintained at a temperature of 60–80°C and a stirring speed of 700–800 rpm.

[0149] First, 1,000 g of polycarbonate diol with a molecular weight of 1,000 g / mol and 244 g of aliphatic isocyanate (Isophorone Diisocyanate, IPDI) were introduced into a reactor to set the NCO ratio to 1.1.

[0150] 370g of methyl ethyl ketone (MEK) was added as a solvent for the initial synthesis.

[0151] Subsequently, 134.2 g of dimethylolpropionic acid (DMPA) was added at 70°C to introduce hydrophilic groups (-COOH) into the polymer structure, and the mixture was stirred sufficiently for a uniform reaction. To control the reaction rate, a bismuth catalyst (concentration: 0.05–0.1%) was added, and the completion of the reaction was confirmed by measuring the NCO content using the dibutylamine method.

[0152] In the next step, 101.2 g of triethylamine (TEA) was added at 40–50°C to neutralize the carboxylic acid groups of DMPA, and the pH of the reaction solution was adjusted to 7–8 to ensure water dispersibility.

[0153] Afterwards, 1,479.4 g of the prepared prepolymer was stirred at 700 rpm and distilled water was gradually added to adjust the particle size to 100 nm, and dispersion stability was examined by checking for particle aggregation.

[0154] Volume growth was induced by slowly adding 30g of ethylene diamine (EDA) at 35℃ to increase the molecular weight of the polymer. Subsequently, the used MEK was removed through heat treatment to minimize the volatile organic compound (VOC) content.

[0155] Finally, 2,803 g of distilled water was added to adjust the viscosity to 150 mPa·s and the solid content to 35%.

[0156]

[0157] <Example 3>

[0158] Preparation of hollow fiber membranes coated with water-dispersible polyurethane (PUD) diluted with distilled water

[0159] 3-1. Preparation of Coating Solution

[0160] 10 g of the water-dispersible polyurethane (PUD) prepared in Example 2 above was diluted in 4,500 ml of distilled water to adjust the dilution ratio to approximately 0.222%. Through this, a coating solution with a total weight of 4,510 g was prepared.

[0161]

[0162] 3-2. Coating of Hollow Fiber Membranes

[0163] 300g of the hollow fiber membrane prepared in Example 1 above was dipped into 4,510g of the coating solution prepared in Example 3-1 above, and water-dispersible polyurethane (PUD) was uniformly coated on the surface of the hollow fiber membrane.

[0164]

[0165] 3-3. Drying of Coated Hollow Fiber Membranes

[0166] The hollow fiber membrane coated in Example 3-2 above was dried using a hot air drying method at 70°C for 4 hours to finally produce a hollow fiber membrane coated with water-dispersible polyurethane (PUD).

[0167]

[0168] <Example 4>

[0169] Preparation of hollow fiber membranes coated with water-dispersible polyurethane (PUD) diluted with distilled water

[0170] 4-1. Preparation of Coating Solution

[0171] 10 g of the water-dispersible polyurethane (PUD) prepared in Example 2 above was diluted in 4,500 ml of distilled water to adjust the dilution ratio to approximately 0.222%. Through this, a coating solution with a total weight of 4,510 g was prepared.

[0172]

[0173] 4-2. Coating of Hollow Fiber Membranes

[0174] 300g of the hollow fiber membrane prepared in Example 1 above was dipped into 4,510g of the coating solution prepared in Example 4-1 above, and water-dispersible polyurethane (PUD) was uniformly coated on the surface of the hollow fiber membrane.

[0175]

[0176] 4-3. The Wringing Process of Hollow Fiber Membranes

[0177] In order to ensure that water-dispersible polyurethane (PUD) permeates evenly into the pores of the hollow fiber membrane, the hollow fiber membrane coated in Example 4-2 was subjected to a process of being twisted and squeezed by hand.

[0178]

[0179] 4-4. Drying of Coated Hollow Fiber Membranes

[0180] The hollow fiber membrane coated in Example 4-3 above was dried using a hot air drying method at 70°C for 4 hours to finally produce a hollow fiber membrane coated with water-dispersible polyurethane (PUD).

[0181]

[0182] <Comparative Example 1>

[0183] Manufacturing of hollow fiber membranes coated with water-dispersible polyurethane (PUD) stock solution

[0184] A hollow fiber membrane coated with a water-dispersible polyurethane (PUD) stock solution was prepared in the same manner as in Example 3, except that the water-dispersible polyurethane (PUD) was used in its stock solution state without diluting it with distilled water.

[0185]

[0186] <Comparative Example 2>

[0187] Preparation of hollow fiber membranes coated with water-dispersible polyurethane (PUD) diluted with distilled water

[0188] A hollow fiber membrane coated with water-dispersible polyurethane (PUD) was prepared using the same method as in Example 3, except that 5g of water-dispersible polyurethane (PUD) was used and the dilution ratio was adjusted to about 0.111%.

[0189]

[0190] <Comparative Example 3>

[0191] Preparation of hollow fiber membranes coated with water-dispersible polyurethane (PUD) diluted with distilled water

[0192] A hollow fiber membrane coated with a water-dispersible polyurethane (PUD) stock solution was prepared in the same manner as in Example 3, except that 15g of water-dispersible polyurethane (PUD) was used and the dilution ratio was adjusted to approximately 0.332%.

[0193]

[0194] <Comparative Example 4>

[0195] Preparation of hollow fiber membranes coated with water-dispersible polyurethane (PUD) diluted with distilled water

[0196] A hollow fiber membrane coated with a water-dispersible polyurethane (PUD) stock solution was prepared in the same manner as in Example 3, except that 20g of water-dispersible polyurethane (PUD) was used and the dilution ratio was adjusted to approximately 0.443%.

[0197]

[0198] <Test Example 1>

[0199] Hollow fiber membrane flow measurement based on water-dispersible polyurethane (PUD) coating

[0200] To evaluate the effect of a water-dispersible polyurethane (PUD) coating layer on the flow rate and filtration performance of hollow fiber membranes, comparative experiments were conducted based on whether or not a water-dispersible polyurethane (PUD) coating was applied. The subjects of comparison were a hollow fiber membrane without a water-dispersible polyurethane (PUD) coating (Example 1) and a hollow fiber membrane with a water-dispersible polyurethane (PUD) coating (Comparative Example 1). Flow rate measurements were performed using a standard hydraulic pressure gauge used for product inspection, and the hydraulic pressure conditions were set to 1.5 kg / cm² (flow rate at low hydraulic pressure is used to evaluate the initial performance and suitability of the hollow fiber membrane), 3 kg / cm² (flow rate at medium hydraulic pressure is important for evaluating the stability and continuous performance of the hollow fiber membrane in actual usage environments), and 5 kg / cm² (flow rate at high hydraulic pressure is used to test the maximum capacity and durability of the hollow fiber membrane). Ordinary tap water was used as the solution for flow rate measurement, and the permeate flow rate was measured under each hydraulic pressure condition, with the results shown in Table 1 below.

[0201] The unit in Table 1 below is "LMH (L / m²) 2 ·h” is a unit representing the permeate flow rate of a hollow fiber membrane, 1 square meter (m²). 2 ) refers to the flow rate passing through per unit area per hour (L / h).

[0202]

[0203] Milk Powder Pressure (kg / ㎠) Flow Rate (LMH) Optimal Flow Rate (LMH) Example 1 (Before PUD Coating) 1.5 200 100~300 3.0 400 200~500 5.0 5 50 400~700 Comparative Example 1 (After PUD Stock Coating) 1.5 5.2 6 100~300 3.0 11.6 200~500 5.0 21.6 400~700

[0204]

[0205] As can be seen in Table 1 above, in Example 1 (before PUD coating), the flow rate was outside the range of the appropriate flow rate according to the hydraulic pressure, whereas in Comparative Example 1 (after PUD undiluted solution coating), the result fell far short of the range of the appropriate flow rate according to the hydraulic pressure. This indicates that when water-dispersible polyurethane (PUD) is used as is to coat a hollow fiber membrane, it results in most of the pores of the hollow fiber membrane becoming clogged.

[0206]

[0207] <Test Example 2>

[0208] Evaluation of Flow Rate and Filtration Performance of Hollow Fiber Membranes According to Dilution Ratio of Water-Dispersible Polyurethane (PUD) Coating Layer

[0209] To evaluate the effect of the water-dispersible polyurethane (PUD) coating layer on the flow rate and filtration performance of the hollow fiber membrane, comparative experiments were conducted according to the dilution ratio of the water-dispersible polyurethane (PUD) with distilled water. The subjects of comparison were set as a hollow fiber membrane with a dilution ratio of approximately 0.222% (Example 3), a hollow fiber membrane with a dilution ratio of approximately 0.111% (Comparative Example 2), a hollow fiber membrane with a dilution ratio of approximately 0.332% (Comparative Example 3), and a hollow fiber membrane with a dilution ratio of approximately 0.443% (Comparative Example 4). Flow rate measurements were performed using a standard hydraulic gauge used for product inspection, and hydraulic conditions were set to 1.5 kg / cm² (flow rate at low hydraulic pressure is used to evaluate the initial performance and suitability of the hollow fiber membrane), 3 kg / cm² (flow rate at medium hydraulic pressure is important for evaluating the stability and continuous performance of the hollow fiber membrane in actual usage environments), and 5 kg / cm² (flow rate at high hydraulic pressure is used to test the maximum capacity and durability of the hollow fiber membrane). The solution used for flow rate measurement was ordinary tap water, and the permeate flow rate was measured under each hydraulic pressure condition, with the results shown in Table 2 below.

[0210]

[0211] Milk Powder Pressure (kg / ㎠) Flow Rate (LMH) Optimal Flow Rate (LMH) Example 3 (PUD 10g) 1.58 5 100~300 3.01 70 200~500 5.03 50 400~700 Comparative Example 2 (PUD 5g) 1.53 10 100~300 3.05 50 200~500 5.07 40 400~700 Comparative Example 1 (PUD 15g) 1.51 5 100~300 3.01 00 200~500 5.02 20 400~700 Comparative Example 4 (PUD 20g) 1.50 100~300 3.07 200~500 5.01 10 400~700

[0212]

[0213] As can be seen in Table 2 above, Comparative Examples 1, 2, and 4 fall outside the range of appropriate flow rates according to hydraulic pressure, whereas Example 3 shows a flow rate result that is somewhat smaller than the range of appropriate flow rates according to hydraulic pressure. This is intended to maximize the effect of filtering foreign substances (heavy metals, etc.) by partially blocking the pores of the hollow fiber membrane as originally intended.

[0214] In addition, as an evaluation of the effect of changes in the dilution ratio of the water-dispersible polyurethane (PUD) coating layer on the physical performance of the hollow fiber membrane, it is preferable to dilute the water-dispersible polyurethane (PUD) in distilled water rather than using the undiluted solution, and it can be seen that the most appropriate ratio is water : water-dispersible polyurethane = 4,500 ml : 10 g.

[0215]

[0216] <Test Example 3>

[0217] Effect of Water-Dispersible Polyurethane (PUD) Coating Layer on Flow Rate and Filtration Performance of Hollow Fiber Membranes: Evaluation of the Effect of Squeezing

[0218] To evaluate the effect of a water-dispersible polyurethane (PUD) coating layer on the flow rate and filtration performance of a hollow fiber membrane, a comparative experiment was performed on hollow fiber membranes (Examples 3 and 4) in which the dilution ratio of the water-dispersible polyurethane (PUD) with distilled water was approximately 0.222%, depending on whether or not they were squeezed.

[0219] The experimental conditions are as follows.

[0220] ■ Example 3: A hollow fiber membrane with a dilution ratio of approximately 0.222%, in a state where no squeezing action was performed.

[0221] ■ Example 4: A hollow fiber membrane with a dilution ratio of approximately 0.222%, in a state where the squeezing action has been performed.

[0222]

[0223] Flow rate measurements were performed using a standard hydraulic gauge used for product inspection, and the hydraulic conditions were set to 1.5 kg / cm² (flow rate at low hydraulic pressure is used to evaluate the initial performance and suitability of the hollow fiber membrane), 3 kg / cm² (flow rate at medium hydraulic pressure is important for evaluating the stability and continuous performance of the hollow fiber membrane in actual usage environments), and 5 kg / cm² (flow rate at high hydraulic pressure is used to test the maximum capacity and durability of the hollow fiber membrane). The solution used for flow rate measurement was ordinary tap water, and the permeate flow rate was measured under each hydraulic pressure condition, with the results shown in Table 3 below.

[0224]

[0225] Milk Powder Pressure (kg / ㎠) Flow Rate (LMH) Optimal Flow Rate (LMH) Example 3 (Squeezing: X) 1.585 100~300 3.0170 200~500 5.0350 400~700 Example 4 (Squeezing: ○) 1.580 100~300 3.0164 200~500 5.0330 400~700

[0226]

[0227] As can be seen in Table 3 above, the flow rate of Example 4, which includes the squeezing process, was lower than that of Example 3, which does not include the squeezing process. This is believed to be because the water-dispersible polyurethane (PUD) coating solution seeped more evenly into the pores of the hollow fiber membrane through the squeezing process, thereby reducing the pore size.

[0228]

[0229] <Test Example 4>

[0230] Evaluation of Glass Transition Temperature (Tg) and Adhesion Phenomenon of Water-Dispersible Polyurethane (PUD) Coating Layers

[0231] In this invention, a water-dispersible polyurethane (PUD) with elasticity and a low glass transition temperature (Tg) was designed using polycarbonate diol, and IPDI was additionally applied to maintain a low glass transition temperature (Tg) along with non-yellowing properties. After applying this water-dispersible polyurethane (PUD) to hollow fiber membrane coatings, the occurrence of adhesion between the coated hollow fiber membranes was evaluated.

[0232]

[0233] Experiments were conducted on the following four hollow fiber membrane conditions through visual observation:

[0234] ■ Example 3: Dilution ratio of approximately 0.222%, without squeezing.

[0235] ■ Example 4: Dilution ratio of approximately 0.222%, state after squeezing.

[0236] ■ Comparative Example 1: Hollow fiber membrane coated with PUD stock solution (undiluted).

[0237] ■ Comparative Example 4: Hollow fiber membrane with a dilution ratio of approximately 0.443%.

[0238]

[0239] The results are shown in Table 4.

[0240]

[0241] Classification Observation Results Example 3: Weak adhesion Example 4: Weak adhesion Comparative Example 1: Very severe adhesion Comparative Example 4: Severe adhesion

[0242]

[0243] As can be seen in Table 4 above, in Comparative Examples 1 and 4, adhesion between hollow fiber membranes occurred between the hollow fiber membrane coated with an undiluted water-dispersible polyurethane (PUD) stock solution (Comparative Example 1) and the hollow fiber membrane coated with a dilution ratio of approximately 0.443% (Comparative Example 4). This suggests that adhesion was induced due to the limitations in controlling the glass transition temperature (Tg) when the dilution ratio was too low or too high.

[0244] On the other hand, in Examples 3 and 4, no adhesion was observed in the hollow fiber membranes coated with a dilution ratio of approximately 0.222% (Examples 3 and 4), and the coating condition was good. In particular, it was found that adhesion was prevented even in Example 4, where a squeezing process was performed.

[0245]

[0246] These results confirm that the water-dispersible polyurethane (PUD) designed in this invention contributes to preventing adhesion between hollow fiber membrane coating layers and improving coating quality by maintaining a low glass transition temperature (Tg) through the use of polycarbonate diol and IPDI. Furthermore, this suggests that it plays an effective role in simultaneously ensuring filtration performance and physical stability.

[0247]

[0248] <Test Example 5>

[0249] Evaluation of Divalent Metal Filtering Performance of Hollow Fiber Membranes Using Aqueous Magnesium Sulfate Solution

[0250] This test was conducted to evaluate the filtering performance of divalent metal ions (magnesium) in an aqueous magnesium sulfate (MgSO4) solution using a hollow fiber membrane. To this end, an aqueous solution of magnesium sulfate was prepared at a certain concentration (20%), and then filtered by passing it through the hollow fiber membrane of Example 1 (before PUD coating) and the hollow fiber membrane of Example 3 (after PUD coating).

[0251] The filtering performance of the hollow fiber membrane was evaluated by the change in electrical conductivity before and after filtration, and a decrease in electrical conductivity indicates that the hollow fiber membrane effectively removed divalent metal ions (magnesium) from the aqueous solution. Electrical conductivity was measured using a TDS (Total Dissolved Solids) measuring device.

[0252] The results are shown in Table 5.

[0253]

[0254] Classification Electrical Conductivity (mS / cm) Magnesium Sulfate Aqueous Solution Alone 530 Example 1507 Example 3422

[0255] As can be seen from Table 5 above, in the case of the PUD-coated hollow fiber membrane (Example 3), the electrical conductivity was the lowest, indicating that magnesium, a divalent ion, was filtered through the hollow fiber membrane.

[0256]

[0257] <Test Example 6>

[0258] The surface morphology of the hollow fiber membrane of Example 1 (before PUD coating) and the hollow fiber membrane of Example 3 (after PUD coating) was observed using a scanning electron microscope (SEM), and the results are shown in Fig. 1 (150x magnification), Fig. 2 (500x magnification), and Fig. 3 (2,000x magnification), respectively.

[0259]

[0260] As can be seen in Figure 1, the surface of the hollow fiber membrane before coating has a relatively smooth and uniform structure with widely distributed pores. On the other hand, the surface of the hollow fiber membrane after coating shows that the pores have narrowed and the surface has become more uniform due to the water-dispersible polyurethane (PUD) coating. These changes contribute to maximizing the foreign substance filtration capacity of the hollow fiber membrane and improve its performance as a water purifier filter.

[0261]

[0262] As can be seen in Figure 2, the surface of the hollow fiber membrane before coating has numerous fine pores, whereas the surface of the hollow fiber membrane after coating has narrowed pores. This change in pores is achieved by a water-dispersible polyurethane (PUD) coating, which contributes to maximizing the ability to filter impurities when used as a water purifier filter.

[0263]

[0264] As can be seen in Figure 3, the surface of the hollow fiber membrane before coating has an irregular surface structure due to the presence of numerous pores. On the other hand, it can be seen that the surface of the hollow fiber membrane after coating has significantly reduced pores, forming a smoother and more uniform surface structure. This change suggests that the pores of the hollow fiber membrane are narrowed through the water-dispersible polyurethane (PUD) coating, thereby maximizing the ability to filter foreign substances.

[0265]

[0266] <Test Example 7>

[0267] The cross-sectional morphology of the hollow fiber membrane of Example 1 (before PUD coating) and the hollow fiber membrane of Example 3 (after PUD coating) was observed using a scanning electron microscope (SEM), and the results are shown in Fig. 4 (400x magnification), Fig. 5 (1,000x magnification), and Fig. 3 (5,000x magnification), respectively.

[0268]

[0269] As can be seen in Figure 4, the hollow fiber membrane before coating has pores that are relatively widely distributed, whereas the hollow fiber membrane after coating has pores that are narrowed by water-dispersible polyurethane (PUD). This change in pores improves the filtration performance of the hollow fiber membrane and contributes to maximizing the ability to filter foreign substances. The surface of the hollow fiber membrane after coating has a more uniform and dense structure, which acts as an important factor in improving performance as a water purifier filter.

[0270]

[0271] As can be seen in Figure 5, the hollow fiber membrane before coating has a porous structure and relatively large pore sizes. On the other hand, it can be seen that the pores of the hollow fiber membrane after coating have narrowed due to water-dispersible polyurethane (PUD). The surface of the hollow fiber membrane after coating has a more uniform and smooth structure, which helps to reduce clogging during the filtering process.

[0272]

[0273] As can be seen in Figure 6, the structural difference between the hollow fiber membrane before and after coating is clearly evident. The hollow fiber membrane before coating exhibits a distinct porous structure with relatively large pore sizes. In contrast, the hollow fiber membrane after coating shows narrowed pores due to the water-dispersible polyurethane (PUD) coating. This reduction in pore size improves the filtration performance of the hollow fiber membrane and contributes to maximizing the ability to filter impurities. The surface of the hollow fiber membrane after coating possesses a more uniform and dense structure, which acts as an important factor in enhancing its performance as a water purifier filter.

Claims

In a coating method for designing a narrow pore size of a hollow fiber membrane by coating a UF hollow fiber membrane with water-dispersible polyurethane (PUD), a) A step of synthesizing a highly water-dispersible polyurethane by reacting polycarbonate diol and isophorone diisocyanate (IPDI) to form a water-dispersible polyurethane (PUD); b) a step of diluting the water-dispersible polyurethane (PUD) from step a) with distilled water to form a coating solution applicable to a UF hollow fiber membrane; c) a step of coating the surface of a UF hollow fiber membrane with the coating solution of step b) above, and then twisting and squeezing the coated UF hollow fiber membrane; and d) A coating method characterized by including a step of fixing the coated UF hollow fiber membrane of step c) using hot air. In claim 1, The above step b) is a coating method characterized by diluting water-dispersible polyurethane (PUD) and distilled water at a weight ratio of 1:450 to improve coating performance. In claim 1, A coating method characterized in that the water-dispersible polyurethane (PUD) of step b) above has a low glass transition temperature (Tg), thereby preventing adhesion between UF hollow fiber membrane coating layers and improving coating quality. UF hollow fiber membrane coated with water-dispersible polyurethane (PUD) manufactured by the method of claim 1. UF hollow fiber membrane coated with water-dispersible polyurethane (PUD) manufactured by the method of claim 2. UF hollow fiber membrane coated with water-dispersible polyurethane (PUD) prepared by the method of claim 3.

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