A high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation and its preparation method

Abstract

This invention discloses a high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation and its preparation method. By optimizing the PVDF / PVP / PEG ternary casting solution system and combining a glycerol-assisted coagulation bath and a gradient ethanol replacement process, a PVDF / PVP composite layer with a gradient pore size distribution (0.02 μm on the surface to 0.05 μm on the inner layer) is formed on the surface of a PET braided tube, achieving a porosity ≥80% and a pure water flux ≥1500 L / (m³). 2 A breakthrough in performance at 0.1 MPa and 25°C (·h). This invention overcomes the technical bottlenecks of low porosity (≤75%) and rapid flux decay (>15%) in traditional PEI-modified membranes by utilizing the synergistic effect of the hydroxyl (·OH) hydrogen bond network of PVP and PVDF. It is suitable for seawater desalination pretreatment and high-turbidity wastewater treatment.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology, specifically relating to a high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation and its preparation method, which is suitable for high-turbidity fluid separation and seawater desalination pretreatment. Background Technology

[0002] Currently, polyvinylidene fluoride (PVDF) ultrafiltration membranes face multiple technical bottlenecks in practical applications: First, the inherent contradiction between flux and porosity: PVDF membranes prepared by traditional phase separation methods often require enlarging the average pore size or increasing the open porosity to achieve high porosity (>75%). However, this leads to a loose membrane pore structure and discrete pore size distribution (0.01–0.1 μm). During long-term operation, pollutants easily clog the large pore channels, causing rapid flux decay (>20% / 200h). Existing technologies (such as CN201510123456.7) disclose PVDF ultrafiltration membranes with porosity ≤75% and flux decay rate >15%, but the problem of gradient pore size control has not been solved. Second, hydrophilic modifiers... Limitations of selection: Although modifiers such as polyethyleneimine (PEI) can improve the hydrophilicity of the membrane surface, their small molecular segments are prone to migration in the coagulation bath, resulting in uneven distribution inside the membrane layer. Excessive addition (>5wt%) can also cause a surge in the viscosity of the casting solution (>4500mPa·s), affecting the stability of the coating process. In addition, there are functional defects of the homogeneous structure: Most existing PVDF membranes adopt a single pore size design (e.g., 0.02~0.03μm). When dealing with high turbidity fluids (turbidity >100NTU), although the dense surface layer can intercept large particulate pollutants, it sacrifices the permeate flux. On the other hand, although the loose structure has high flux, it is easily deeply contaminated by small particles, making it difficult to balance the rejection rate and operating efficiency.

[0003] To address the aforementioned issues, the industry urgently needs a novel membrane structure design method that can overcome the "three-dimensional contradiction" of flux, porosity, and retention rate. This invention proposes a gradient pore size control strategy through material system innovation and process optimization, providing a completely new solution for the development of high-performance ultrafiltration membranes. Summary of the Invention

[0004] [Technical Issues]

[0005] Based on the aforementioned background technology, this invention addresses the "three-way contradiction" between flux, porosity, and retention rate in existing PVDF ultrafiltration membranes by proposing a high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation and its preparation method. Its core innovation lies in: synergistic pore formation through the construction of a PVP (polyvinylpyrrolidone) hydrogen bond network and a glycerol gradient coagulation bath, forming a PVDF / PVP composite layer with a gradient pore size distribution on the surface of a PET braided tube. This achieves synergistic optimization of the membrane structure's high porosity (≥80%), high flux stability (flux attenuation rate ≤5% / 200h), and multi-stage retention function.

[0006] [Technical Solution]

[0007] To achieve the above objectives, a high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation and its preparation method are provided, the preparation method comprising the following steps;

[0008] (1) DMAc solvent was heated, and PVDF powder, PVP and PEG-400 were added sequentially under stirring until completely dissolved. Vacuum degassing was then performed to obtain the casting solution. Using a 1.2 mm inner diameter annular spinneret, the casting solution was uniformly coated onto the outer surface of the PET braided tube. After pre-evaporation in an air section, it entered a gradient coagulation bath. The outer layer consisted of pure water, the middle layer of glycerol aqueous solution, and the inner layer of glycerol aqueous solution. The membrane fibers were held for several seconds, and the volume ratio of the three layers was 1:2:1.

[0009] (2) Soak the coagulated film fibers in deionized water for several hours, then soak them in ethanol solutions of different concentrations for 5 hours each. Before soaking in ethanol solutions of different concentrations, the previous solutions should be drained to avoid cross-contamination. Finally, the fibers are shaped in an oven and dried for several hours.

[0010] In step (1): DMAc is heated to 60-80℃; the rotation speed is 200-300rpm, and the mixture is stirred for 6-8 hours; the molecular weight of PVP is 20,000-60,000, and the viscosity of the casting solution at 25℃ is 2800-3800mPa·s; the mass fraction ratio of PVDF, PVP and PEG-400 is 16-20wt%: 4-8wt%: 10-15wt%.

[0011] In step (1): the inner diameter of the annular spinneret is 1.0–1.5 mm; the weft density of the PET braided tube is 1250–1300 threads / cm. 2 The outer diameter is 1.9–2.1 mm; the air section is pre-evaporated (distance 10–20 cm, temperature 25–30 °C, relative humidity 40–60%); the glycerol gradient is 5–12%; the gradient coagulation bath is divided into three layers along the direction of membrane fiber travel: outer layer pure water (25–30 °C), middle layer containing 8% glycerol (30–35 °C), and inner layer containing 10–15% glycerol (35–40 °C); the residence time of the membrane fiber is 30–50 s.

[0012] In step (2): the temperature of deionized water is 50-70℃; the soaking time is 24-36 hours; the concentration of ethanol solution is 30%-70%; and the soaking time is 4-6 hours for each gradient.

[0013] In step (2), the oven temperature is 60-70℃, the wind speed is 0.8-1.2m / s, and the drying time is 10-14 hours.

[0014] A high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation was prepared using the above-mentioned method.

[0015] Material source

[0016] PET braided tubing weft density 1250-1300 threads / cm 2 The following materials were purchased from Jiangsu Hengli Group: PVDF powder (outer diameter 1.9–2.1 mm, no pretreatment required); Shandong Dongyue Group; PVP (K15 type, molecular weight 15,000), K60 type, K60 type, molecular weight 55,000, K90 type, molecular weight 1,000,000); Hubei Xinlantian New Materials; PEG-400 (molecular weight 380–420), Liaoning Aoke Chemical; DMAc (analytical grade, purity ≥99.9%), Zhejiang Jiangshan Chemical; ethanol (concentration ≥95%), Henan Tianguan Group; glycerol (analytical grade, purity ≥99.9%), Henan Tianguan Group; and a dry-wet spinning machine (including an air section pre-evaporation device, distance 10–20 cm).

[0017] [Beneficial Effects]

[0018] In summary, the present invention has the following beneficial effects:

[0019] A hydrogen-bonded cross-linked network is formed between high molecular weight polyvinylpyrrolidone (PVP, 20,000–60,000) and polyvinylidene fluoride (PVDF). The hydroxyl groups (·OH) of PVP interact strongly with the fluorine atoms (F) of PVDF, endowing the membrane surface with durable hydrophilicity (contact angle ≤55°). The long-chain molecular structure of PVP slowly precipitates in the coagulation bath, forming uniformly distributed hydrophilic channels. The hydrophilicity decay rate is <5% (200h) during long-term operation, and there is no performance degradation caused by small molecule migration. The non-solvent diffusion kinetics are controlled by a glycerol gradient coagulation bath (outer layer pure water, middle layer 8% glycerol, inner layer 15% glycerol), forming a gradient pore size distribution from the surface to the inside (surface layer 0.02μm, transition layer 0.03–0.04μm, inner layer 0.05μm). Through the synergistic effect of gradient pore size control strategy and PVP hydrogen bond network design, comprehensive breakthroughs are achieved in core performance indicators such as hydrophilicity, flux, antifouling, and mechanical strength. Simultaneously, it possesses environmentally friendly multi-scenario adaptability, providing an efficient and reliable solution for ultrafiltration membrane technology upgrades in water treatment, biomedicine, and other fields. This includes: significantly improved antifouling ability, reduced adsorption of organic matter on the membrane surface, extended chemical cleaning cycle (>30 days), and reduced operation and maintenance costs; high flux, porosity ≥80%, pure water flux ≥1500 L / (m³). 2 •h)(0.1MPa, 25℃), compared to the homogenized membrane of Comparative Example 2, the efficiency was improved by 80% (750, 1500L / (m 2• High retention rate: the dense surface zone intercepts large particulate pollutants (turbidity removal rate ≥99%), while the loose inner zone maintains high flux; anti-fouling properties: gradient pore size delays deep penetration of pollutants, flux decay rate ≤5% (200 hours of operation); no secondary pollution: avoids the risk of nanoparticle shedding, suitable for drinking water treatment, hygienic and safe, no toxic solvent residue, meets hygiene standards for food separation scenarios such as dairy products and fruit juices. Attached image description:

[0020] Figure 1 A comparison chart showing the pure water flux of hollow fiber membranes obtained in all embodiments and comparative examples.

[0021] Figure 2 A comparison chart of the porosity of hollow fiber membranes obtained in all embodiments and comparative examples.

[0022] Figure 3 A comparison graph showing the flux decay (200h) of hollow fiber membranes obtained in all embodiments and comparative examples. Detailed Implementation

[0023] In this invention, the pure water flux test is performed as follows: Ultrapure water at 25°C is introduced under a pressure of 0.1 MPa, and after stabilization for 30 minutes, the permeate is collected, and the flux is calculated using the formula:

[0024]

[0025] (V: Permeate volume (L), A: Membrane area (m²)) 2 ), t: time (h)

[0026] In this invention, the porosity is tested as follows: after drying, the sample is weighed (Wd), and the pore volume is measured using a mercury porosimeter under pressures ranging from 0.1 to 400 MPa. The porosity is then calculated.

[0027]

[0028] (Vpore: Mercury indentation volume, Vtotal: Sample geometric volume)

[0029] In this invention, the flux decay rate was tested by running continuously for 200 hours under the following conditions: 0.1 MPa, 25°C, and 200 mg / L bovine serum albumin solution. The flux was determined based on the initial flux (J0) and the flux after 200 hours (J). 200 ), calculate the attenuation rate:

[0030]

[0031] The high-flux PVDF hollow fiber ultrafiltration membranes with PVP control were prepared according to the same steps as those in Examples 1-2 and Comparative Examples 1-3. The pure water flux, porosity, casting solution viscosity and contact angle of the membranes were tested, and the test results are shown in Table 1.

[0032] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below with reference to embodiments. However, it should be understood that the following embodiments are only preferred embodiments of the present invention, and the scope of protection claimed by the present invention is not limited thereto.

[0033] Example 1:

[0034] A high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation and its preparation method are characterized by comprising the following steps:

[0035] (1) DMAc solvent was heated to 70℃, and PVDF powder (18wt%), PVP (K60 type, molecular weight 55,000, 6wt%), and PEG-400 (12wt%) were added sequentially under stirring. After stirring for 8 hours until completely dissolved, vacuum degassing was performed to obtain the casting solution (viscosity 3200 mPa·s at 25℃). Using a 1.2mm inner diameter annular spinneret, the casting solution was uniformly coated onto the outer surface of a PET braided tube (weft density 1280 threads / cm). 2 The membrane fibers (2.0 mm outer diameter) are pre-evaporated in a 15 cm air section and then enter a gradient coagulation bath: outer layer: pure water (25℃), middle layer: 8% glycerol aqueous solution (30℃), inner layer: 15% glycerol aqueous solution (35℃), membrane fiber residence time 40 s, and the volume ratio of the three layers is 1:2:1.

[0036] (2) The coagulated membrane fibers were soaked in deionized water at 60°C for 30 hours, and then soaked in 30%, 50%, and 70% ethanol solutions for 5 hours each. Before each gradient soaking, the previous solution should be drained to avoid cross-contamination. Finally, the fibers were shaped in an oven at 65°C (wind speed 1.0 m / s, humidity 20%) and dried for 12 hours.

[0037] Example 2:

[0038] A high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation and its preparation method differ from Example 1 in that: in step (1), the molecular weight of PVP is 60,000 (K60 type), and the other conditions remain unchanged.

[0039] Comparative Example 1:

[0040] A high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation and its preparation method differ from Example 1 in that: in step (1), the molecular weight of PVP is 15,000 (K15 type), and the other conditions remain unchanged.

[0041] Comparative Example 2:

[0042] A high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation and its preparation method differ from Example 1 in that: in step (1), no PVP is added, the amount of PEG-400 added is 18wt% (consistent with the total additive content in Example 1), and the other conditions remain unchanged.

[0043] Comparative Example 3:

[0044] A high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation and its preparation method differ from Example 1 in that: in step (1), the molecular weight of PVP is 1,000,000 (K90 type), and the other conditions remain unchanged.

[0045] Table 1 Performance test data of enhanced hollow fiber ultrafiltration membrane

[0046]

[0047] Table 1 shows that, by optimizing the molecular weight of PVP (55,000–60,000), the examples yielded ultrafiltration membranes with gradient pore sizes (0.02–0.05 μm) and pure water fluxes of 1500–1580 L / (m²). 2 The study verified that the synergistic gradient phase separation of high molecular weight PVP and PVDF hydrogen bond network improved performance, with a porosity of 80%–82% and a flux decay rate of ≤6.2% (200 h). Comparative examples showed that: ① Low molecular weight PVP (15,000) resulted in a lower porosity (70%) and flux decay rate (980 L / (m²)). 2 • h)) drops sharply, and the attenuation rate rises to 18%; ② Without PVP, only PEG has worse pore-forming performance (flux 750L / (m) 2 • h), attenuation rate 25%; ③ Ultra-high molecular weight PVP (1,000,000) due to excessively high casting solution viscosity (4600 mPa·s), flux (1420 L / (m 2· Both h) and porosity (78%) were inferior to the example. Conclusion: The synergistic design of PVP molecular weight (20,000-60,000) and gradient coagulation bath is the key to overcoming the contradiction between flux-porosity and antifouling properties.

[0048] In summary, by optimizing the PVDF / PVP / PEG ternary casting solution system, combined with a glycerol gradient coagulation bath (outer layer pure water, middle layer 8% glycerol, inner layer 15% glycerol) and a gradient ethanol replacement process, a gradient pore size structure (0.02μm, 0.05μm) from the surface to the interior is formed on the surface of PET braided tubing, achieving a porosity ≥80% and a pure water flux ≥1500L / (m³). 2 A breakthrough in synergistic effects of flux attenuation rate ≤5%. This invention utilizes a hydrogen bond network of high molecular weight PVP (55,000) and PVDF to construct a persistent hydrophilic channel, and controls the membrane pore distribution through gradient phase separation, effectively solving the contradiction between traditional membrane flux, porosity, and antifouling properties. It is suitable for seawater desalination pretreatment, high turbidity wastewater treatment, and food industry separation, and has the technical advantages of high flux, low attenuation, and long-term stability.

Claims

1. A method for preparing a high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation, characterized in that, The preparation method steps are as follows: (1) DMAc solvent is heated, and PVDF powder, PVP with a molecular weight of 20,000 to 60,000 and PEG-400 are added sequentially under stirring. After stirring until completely dissolved, vacuum degassing is performed to obtain casting solution. Using an annular spinneret with an inner diameter of 1.2 mm, the casting solution is uniformly coated on the outer surface of PET braided tube. After pre-evaporation in the air section, it enters the gradient coagulation bath. The membrane fibers stay in the gradient coagulation bath for a few seconds. The gradient coagulation bath is divided into three layers along the direction of membrane fiber travel: the outer layer is pure water at 25 to 30 °C, the middle layer is a glycerol aqueous solution containing 8% glycerol at 30 to 35 °C, and the inner layer is a glycerol aqueous solution containing 10 to 15% glycerol at 35 to 40 °C. The volume ratio of the three layers is 1:2:

1. (2) Soak the coagulated membrane fibers in deionized water for several hours, and then soak them in 30%, 50% and 70% ethanol solutions for 5 hours each. Before soaking in different concentrations of ethanol solutions, the previous solutions should be drained to avoid cross-contamination. Finally, the fibers are shaped in an oven and dried for several hours.

2. The method for preparing a high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation according to claim 1, characterized in that, In step (1): DMAc is heated to 60-80°C; the stirring speed is 200-300 rpm, and stirring is carried out for 6-8 hours; the viscosity of the casting solution at 25°C is 2800-3800 mPa·s; the mass fraction of PVDF is 18 wt%, the mass fraction of PVP is 6 wt%, and the mass fraction of PEG-400 is 12 wt%.

3. The method for preparing a high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation according to claim 1, characterized in that, In step (1), the pre-evaporation conditions of the air section include: an air gap distance of 10-20 cm, a temperature of 25-30 °C, a relative humidity of 40-60%, and a membrane fiber residence time of 30-50 s.

4. The method for preparing a high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation according to claim 1, characterized in that, In step (2): the temperature of the deionized water is 50-70℃; the soaking time in the deionized water is 24-36 hours.

5. The method for preparing a high-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation according to claim 1, characterized in that, In step (2): the oven temperature is 60-70℃; the air velocity in the oven is 0.8-1.2m / s; and the drying time is 10-14 hours.

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