Preparation method of nano-salt porous waterproof and breathable membrane

Abstract

This invention relates to the field of polymer materials technology, specifically to a method for preparing a nano-salt-based porous waterproof and breathable membrane. The method includes: S1, water-soluble inorganic salts are pulverized, surface-modified, and wet-ball-milled, followed by centrifugal precipitation. The particle size distribution index of the nano-salt particles is controlled to be between 0.10 and 0.22 through gradation design, and then gently treated in a mixture of water and low molecular weight alcohol; S2, the nano-salt slurry is added to a polymer film-forming liquid, and dispersed by high-speed shearing and vacuum degassing to obtain a casting liquid, with the viscosity controlled at 5000-20000 cP at 25°C; S3, the casting liquid is coated onto a substrate and pre-dried using a temperature gradient; S4, water-soluble inorganic salts are dissolved in hot water; S5, drying. The core of this invention lies in introducing and defining the pore size conversion coefficient K as the core control target. The K value is controlled through active intervention throughout the entire process of water-soluble inorganic salt preparation, mixing, and film formation, achieving refined and quantitative adjustment of the final membrane pore size and pore size distribution.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to a method for preparing a nano-salt porous waterproof and breathable membrane. Background Technology

[0002] In fields such as outdoor sportswear, medical protective clothing, and special work clothes, there are extremely high requirements for the waterproof and breathable (also known as "breathable") performance of fabrics. Waterproof and breathable membranes are key materials that balance the expulsion of human sweat vapor and the blocking of external wind and rain. Their pore size is between that of water vapor molecules (about 0.0004 nanometers) and liquid water droplets (usually with a diameter greater than 100 micrometers). Theoretically, they allow water vapor to pass through while blocking liquid water.

[0003] Existing mainstream technologies have inherent limitations: polytetrafluoroethylene (PTFE) stretched membranes involve perfluorinated compounds, raising environmental concerns and involving complex production processes; polyurethane (PU) hydrophilic membranes experience a significant decrease in permeability under high humidity conditions. In the field of microporous membranes, existing technologies for controlling membrane pore size largely rely on the inherent properties of raw materials or indirect control during phase separation processes, resulting in insufficient precision and poor predictability.

[0004] Existing patents such as CN108499361A disclose methods using soluble molecules (such as organometallic coordination compounds) as pore-forming agents. While such methods can create pores, the pore size is indirectly determined by the molecular size, the removal process relies on chemical dissolution, and there is a lack of a direct and quantitative mapping relationship between the pore size and the size of the pore-forming agent. This makes it difficult to achieve precise directional design of the final membrane performance (the balance between moisture permeability and water pressure resistance), especially in applications requiring nanoscale precision control. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing a nano-salt-induced porous waterproof and breathable membrane, which is green, has precise pore size control, and allows for quantitative design of performance over a wide range.

[0006] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0007] A method for preparing a nano-salt-based porous waterproof and breathable membrane includes the following steps:

[0008] S1. Preparation and K-coefficient pre-control of nano-salt slurry: Water-soluble inorganic salts were selected and initially processed into micron-sized initial products. These initial nano-salt slurries were obtained by wet ball milling with a surface modifier in an organic solvent. After centrifugation and sedimentation, the particle size distribution index of the nano-salt particles was controlled between 0.10 and 0.22 through gradation design. Then, the nano-salt particles were placed in a mixed medium of water and low molecular weight alcohol for mild treatment to obtain nano-salt slurry.

[0009] S2. Preparation of casting solution and control of K coefficient process: The polymer film-forming material is dissolved in an organic solvent to form a uniform solution. The nano salt slurry obtained in step S1 is added at a dry weight of 20% to 80% of the dry weight of the polymer. The casting solution is obtained by high-speed shear dispersion and vacuum degassing. The viscosity at 25℃ is controlled at 5000-20000 cP.

[0010] S3. Casting and Preliminary Drying: The casting solution obtained in step S2 is coated onto the substrate under constant temperature and humidity and low disturbance environment, and the precursor film is obtained by preliminary drying using a temperature gradient; the constant temperature is preferably 20℃-30℃, and the constant humidity is preferably 40%-60% relative humidity.

[0011] S4. Hot water dissolution of water-soluble inorganic salts: The dried precursor membrane is immersed in constant temperature hot water using a multi-stage countercurrent rinsing process to dissolve nano-salts.

[0012] S5. Drying yields a nano-salt porous waterproof and breathable membrane.

[0013] The average pore size d of the nano-salt porous waterproof and breathable membrane and the average particle size D of the nano-salt slurry satisfy the following relationship: d=K×D, where K is the pore size conversion coefficient. Through the above adjustment, the value of K is precisely controlled within the range of 0.9 to 1.1.

[0014] Preferably, the water-soluble inorganic salt is sodium chloride, potassium chloride, or sodium sulfate.

[0015] Preferably, the low molecular weight alcohol is ethanol or isopropanol.

[0016] Preferably, in step S1, the volume ratio of water to low molecular weight alcohol is between 3:7 and 7:3, the temperature is 20℃-50℃, and the time is 0.5-3 hours. This induces a slight "dissolution-redeposition" dynamic equilibrium on the surface of the nano-salt particles, thereby blunting surface edges and defects and obtaining nano-salt particles with more uniform surface energy and more regular geometry. Improved surface uniformity contributes to the stability of the K value.

[0017] Preferably, the surface modifier is a silane coupling agent, a titanate coupling agent, or a nonionic surfactant. By precisely controlling the concentration of the surface modifier (0.5%-3.0% of the salt mass), the reaction temperature (50-80°C), and the time (1-4 hours), the aim is to form a uniform and complete monolayer to nanometer-scale coating layer on the surface of the salt particles. The integrity of this coating layer is key to controlling the interfacial bonding strength between the salt particles and the polymer: the more complete and uniform the coating, the weaker and more consistent the interfacial bonding, the closer the pore morphology of the salt particles after dissolution is to its original template shape, and the closer the K value is to 1.

[0018] Preferably, the polymeric film-forming material is selected from thermoplastic polyurethane, polyvinylidene fluoride, or thermoplastic polyester elastomer. The concentration, molecular weight, and solvent type of the polymeric film-forming material solution jointly determine the viscosity of the casting solution. A casting solution viscosity of 5000-20000 cP at 25°C can effectively inhibit the sedimentation of nano-salt particles and the excessive embedding and compression of the salt particle template by the polymer chains during the drying process, which is beneficial to the maintenance of the pore structure after drying and makes the K value approach 1.

[0019] Preferably, the initial drying via a temperature gradient involves first slowly evaporating most of the solvent at 30°C-40°C to form a nascent gel, and then raising the temperature to 50°C-70°C to completely remove any remaining solvent. This temperature gradient drying process avoids the formation of a dense skin layer on the film surface or the generation of uneven stress inside due to excessively rapid solvent evaporation, thereby ensuring the uniformity of the K value in the film thickness direction.

[0020] Preferably, the multi-stage countercurrent rinsing process specifically comprises: a first stage in warm water containing 0.01wt%-0.1wt% surfactant to promote salt particle detachment, followed by soaking in warm water, and then rinsing with pure water; the dissolution temperature and time are matched with the thermodynamic properties of the polymer film-forming material to ensure complete salt removal while preventing the pore structure from collapsing due to excessive thermal relaxation, which would lead to an unintended decrease in the K value. The preferred warm water temperature is 40℃-60℃.

[0021] Preferably, the surfactant is sodium dodecyl sulfate.

[0022] Preferably, the drying temperature in step S5 is 60℃-80℃.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) Pore size is precise, controllable and predictable: By defining and adjusting the K coefficient, a quantitative and predictable mapping relationship from the nano salt template particle size D to the final membrane pore size d was established, realizing the transformation from empirical exploration to quantitative design.

[0025] (2) Excellent performance and customizable: By precisely controlling the K value and template gradation, films with high pore size uniformity (narrow pore size distribution) can be prepared. For example, when the target pore size is designed to be around 200 nm (K≈1), high moisture permeability (>8000 g / m²·24h), high water pressure resistance (>20 kPa) and excellent wind resistance can be achieved at the same time.

[0026] (3) Green and environmentally friendly process: Inorganic salts and water are used as the core pore-forming and dissolution media, and the whole process is environmentally friendly.

[0027] The core of this invention lies in introducing and defining the "pore size conversion coefficient K" as the core control target. The K value is controlled by active process intervention throughout the entire process of water-soluble inorganic salt preparation, mixing, and film formation, thereby achieving refined and quantitative adjustment of the final membrane pore size and pore size distribution, breaking away from the passive and single control mode that relies solely on the initial particle size. Detailed Implementation

[0028] Example 1

[0029] This embodiment provides a method for preparing a nano-salt-based porous waterproof and breathable membrane, including the following steps:

[0030] S1. Preparation and K-coefficient pre-control of nano-salt slurry: 100g of analytical grade sodium chloride was pulverized into micron-sized initial product by air jet milling. Then, it was reacted with 1.5g of silane coupling agent KH-550 in 200ml of anhydrous ethanol at 80℃ for 2 hours for surface modification. Subsequently, it was wet-milled with zirconia beads for 12 hours to obtain the initial nano-salt slurry. The nano-salt slurry was centrifuged, and the precipitate was redispersed in a water:ethanol = 1:1 (volume ratio) mixed solvent. The mixture was gently stirred at 35℃ for 1 hour. The nano-salt slurry was controlled by gradation design to achieve a particle size distribution index of 0.18, an average particle size D of 200nm, and uniform surface coating.

[0031] S2. Preparation of Casting Solution and Control of K-coefficient: 100 g of thermoplastic polyurethane (TPU) particles were dissolved in 900 g of N,N-dimethylformamide (DMF) to form a homogeneous solution. The nano-salt slurry obtained in step S1 was added at 50% of the dry weight of the polymer. The mixture was dispersed by high-speed shearing at 6000 rpm for 45 minutes, and the viscosity was controlled at 15000 cP at 25°C. Vacuum degassing was then performed to obtain the casting solution. Note that in some other examples, DMF can be replaced with tetrahydrofuran (THF) or acetone, and TPU can be replaced with polyvinylidene fluoride (PVDF) or thermoplastic polyester elastomer.

[0032] S3. Casting and Preliminary Drying: Under constant temperature of 25℃, constant humidity of 50%, and low disturbance, the casting solution is coated onto the release paper with a doctor blade gap of 300 micrometers. Then, it is placed in a programmable drying oven for preliminary drying using a temperature gradient: first, it is allowed to stand at 35℃ for 5 minutes to slowly evaporate most of the solvent and form a primary gel, and then it is raised to 60℃ and dried for 10 minutes to completely remove the residual solvent, thus obtaining the precursor film.

[0033] S4. Hot water dissolution of water-soluble inorganic salts: The precursor membrane is countercurrently rinsed in an aqueous solution containing 0.05wt% sodium dodecyl sulfate at 60℃ for 1 hour to promote the detachment of salt particles; then it is immersed in a constant temperature water bath at 60℃ for 2 hours and gently stirred to completely dissolve the salt; then it is rinsed 3 times with room temperature deionized water.

[0034] S5. Place in a 65℃ oven to dry thoroughly to obtain a nano-salt porous waterproof and breathable membrane.

[0035] Example 2

[0036] The steps are basically the same as in Example 1, except that the ball milling and classification processes are adjusted in step S1 to obtain a nano-salt slurry with a particle size distribution index of 0.15, an average particle size D of 80 nm, and a uniform surface coating.

[0037] Comparative Example 1

[0038] The main difference from Example 1 is that the surface modification step of silane coupling agent KH-550 is omitted in step S1. After wet ball milling and mild treatment with the same parameters, unmodified NaCl nanoparticles are obtained with an average particle size of about 200 nm and a PDI particle size distribution index of about 0.25. In step S2, the mixture is mixed with TPU / DMF solution, and the viscosity of the casting solution is about 4000 cP. Due to the partial agglomeration of unmodified salt particles, the dispersion and viscosity are affected. The subsequent film formation and dissolution steps are the same as in Example 1.

[0039] Comparative Example 2

[0040] The main difference from Example 1 is that the concentration of the polymer solution is significantly reduced. Specifically, 50 grams of TPU particles are dissolved in 950 grams of DMF (solid content approximately 5%), and the dry weight of the nano-salt slurry is the same as in Example 1. The viscosity of the resulting casting solution is approximately 800 cP. The subsequent film formation and dissolution steps are the same as in Example 1.

[0041] Performance tests were conducted on Examples 1-2 and Comparative Examples 1-2, and the results are shown in Table 1.

[0042] Table 1: Test results of waterproof and breathable membranes of Examples 1-2 and Comparative Examples 1-2 of the present invention

[0043]

[0044] The average pore size d of the nano-salt porous waterproof and breathable membrane satisfies the relationship D between the average particle size D of the nano-salt slurry: d = K × D, where K is the pore size conversion coefficient. Through the above adjustment, the value of K is precisely controlled within the range of 0.9 to 1.1, thereby achieving precise designability of the final pore size d from 50 nanometers to 800 nanometers. This pore size range can efficiently allow water vapor molecules to pass through while effectively blocking the penetration of liquid water droplets and wind.

[0045] The foregoing has shown and described the basic principles, main features and advantages of this invention. Those skilled in the art should understand that this invention is not limited to the above embodiments. The embodiments and descriptions in the specification are only illustrative of the principles of this invention. Various changes and modifications can be made to this invention without departing from the spirit and scope of this invention. All such changes and modifications fall within the scope of this invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a nano-salt-based porous waterproof and breathable membrane, characterized in that, Includes the following steps: S1. Preparation and K-coefficient pre-control of nano-salt slurry: Water-soluble inorganic salts were selected and initially processed into micron-sized initial products. These initial nano-salt slurries were obtained by wet ball milling with a surface modifier in an organic solvent. After centrifugation and sedimentation, the particle size distribution index of the nano-salt particles was controlled between 0.10 and 0.22 through gradation design. Then, the nano-salt particles were placed in a mixed medium of water and low molecular weight alcohol for mild treatment to obtain nano-salt slurry. S2. Preparation of casting solution and control of K coefficient process: The polymer film-forming material is dissolved in an organic solvent to form a uniform solution. The nano salt slurry obtained in step S1 is added at a dry weight of 20% to 80% of the dry weight of the polymer. The casting solution is obtained by high-speed shear dispersion and vacuum degassing. The viscosity at 25℃ is controlled at 5000-20000 cP. S3. Casting and Preliminary Drying: The casting solution obtained in step S2 is coated onto the substrate under constant temperature and humidity and low disturbance environment, and the precursor film is obtained by preliminary drying using a temperature gradient. S4. Hot water dissolution of water-soluble inorganic salts: The dried precursor membrane is immersed in constant temperature hot water using a multi-stage countercurrent rinsing process to dissolve nano-salts. S5. Drying yields a nano-salt porous waterproof and breathable membrane. The average pore size d of the nano-salt porous waterproof and breathable membrane and the average particle size D of the nano-salt slurry satisfy the following relationship: d=K×D, where K is the pore size conversion coefficient. Through the above adjustment, the value of K is precisely controlled within the range of 0.9 to 1.

1.

2. The method for preparing a nano-salt-based porous waterproof and breathable membrane according to claim 1, characterized in that: The water-soluble inorganic salt is sodium chloride, potassium chloride, or sodium sulfate.

3. The method for preparing a nano-salt-based porous waterproof and breathable membrane according to claim 1, characterized in that: The low molecular weight alcohol is ethanol or isopropanol.

4. The method for preparing a nano-salt-based porous waterproof and breathable membrane according to claim 3, characterized in that: In step S1, the volume ratio of water to low molecular weight alcohol is between 3:7 and 7:3, the temperature is 20℃-50℃, and the time is 0.5-3 hours.

5. The method for preparing a nano-salt-based porous waterproof and breathable membrane according to claim 1, characterized in that: The surface modifier is a silane coupling agent, a titanate coupling agent, or a nonionic surfactant.

6. The method for preparing a nano-salt-based porous waterproof and breathable membrane according to claim 1, characterized in that: The polymeric film-forming material is selected from one of thermoplastic polyurethane, polyvinylidene fluoride, or thermoplastic polyester elastomer.

7. The method for preparing a nano-salt-based porous waterproof and breathable membrane according to claim 1, characterized in that: The preliminary drying process using the temperature gradient involves first slowly evaporating most of the solvent at 30℃-40℃ to form a nascent gel, and then raising the temperature to 50℃-70℃ to completely remove any remaining solvent.

8. The method for preparing a nano-salt-based porous waterproof and breathable membrane according to claim 1, characterized in that: The multi-stage countercurrent rinsing process is as follows: the first stage is carried out in warm water containing 0.01wt%-0.1wt% surfactant to promote the removal of salt particles, followed by soaking in warm water, and then rinsing with pure water; the dissolution temperature and time are matched with the thermodynamic properties of the polymer film-forming material.

9. The method for preparing a nano-salt-based porous waterproof and breathable membrane according to claim 8, characterized in that: The surfactant is sodium dodecyl sulfate.

10. The method for preparing a nano-salt-based porous waterproof and breathable membrane according to claim 1, characterized in that: The drying temperature in step S5 is 60℃-80℃.

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