High-flux PVDF hollow fiber ultrafiltration membrane based on PVP regulation and control and preparation method of high-flux PVDF hollow fiber ultrafiltration membrane

Abstract

The invention discloses a high-flux PVDF (polyvinylidene fluoride) hollow fiber ultrafiltration membrane based on PVP (polyvinyl pyrrolidone) regulation and control and a preparation method of the high-flux PVDF hollow fiber ultrafiltration membrane. By optimizing a PVDF / PVP / PEG ternary membrane casting solution system and combining a glycerol-assisted coagulating bath and a gradient ethanol replacement process, a PVDF / PVP composite layer with gradient pore size distribution (the gradient pore size distribution from the surface layer to the inner layer is from 0.02 mu m to 0.05 mu m) is formed on the surface of the PET braided tube, and the performance breakthrough that the porosity is greater than or equal to 80% and the pure water flux is greater than or equal to 1500L / (m < 2 >. H) (0.1 MPa, 25 DEG C) is realized. Through the synergistic effect of a hydroxyl (. OH) hydrogen bond network of PVP and PVDF, the problems that a traditional PEI modified membrane is low in porosity (less than or equal to 75%) and fast in flux attenuation (gt; and the method is suitable for seawater desalination pretreatment and high-turbidity wastewater treatment.

Description

Technical Field

[0001] The present invention belongs to the technical field of membrane separation, and particularly relates to a high-flux PVDF hollow fiber ultrafiltration membrane regulated by PVP and a preparation method thereof, which are applicable to the separation of high-turbidity fluids and the pretreatment of seawater desalination. Background Art

[0002] Currently, polyvinylidene fluoride (PVDF) ultrafiltration membranes face multiple technical bottlenecks in practical applications: First, the inherent contradiction between flux and porosity: In order to pursue a high porosity (>75%) for PVDF membranes prepared by traditional phase separation methods, it is often necessary to increase the average pore size or the pore opening rate, but this will lead to a loose pore structure and a discrete pore size distribution (0.01 - 0.1 μm). During long-term operation, pollutants are likely to block the large pore channels, resulting in a rapid decline in flux (>20% / 200h). The PVDF ultrafiltration membranes disclosed in the prior art (such as CN201510123456.7) have a porosity ≤75% and a flux decay rate >15%, but the problem of gradient pore size regulation has not been solved. Second, the limitation in the selection of hydrophilic modifiers: Although modifiers represented by polyethyleneimine (PEI) can improve the hydrophilicity of the membrane surface, their small molecular segments are prone to migrate in the coagulation bath, resulting in uneven distribution within the membrane layer. Excessive addition (>5wt%) will also cause a sharp increase in the viscosity of the casting solution (>4500 mPa·s), affecting the stability of the coating process. In addition, the functional defects of the homogeneous structure: Most existing PVDF membranes adopt a single pore size design (such as 0.02 - 0.03 μm). When dealing with high-turbidity fluids (turbidity >100 NTU), although the dense surface layer can intercept large particle pollutants, it sacrifices the permeation flux. While the loose structure has a high flux but is easily deeply contaminated by fine particles, making it difficult to balance the rejection rate and the operating efficiency.

[0003] In view of the above problems, the industry urgently needs a new membrane structure design method that can break through the "ternary contradiction" of flux - porosity - rejection rate. Through material system innovation and process optimization, the present invention proposes a gradient pore size regulation strategy, providing a new solution for the development of high-performance ultrafiltration membranes. Summary of the Invention

[0004] [Technical Problem]

[0005] Based on the above background art, in view of the "ternary contradiction" among the flux, porosity, and rejection rate of existing PVDF ultrafiltration membranes, the present invention proposes a high-flux PVDF hollow fiber ultrafiltration membrane regulated by PVP and a preparation method thereof. The core innovation lies in: through the synergistic pore formation of PVP (polyvinylpyrrolidone) hydrogen bond network and glycerol gradient coagulation bath, a PVDF / PVP composite layer with a gradient pore size distribution is formed on the surface of the PET braided tube, realizing the synergistic optimization of high porosity (≥80%), high-flux stability (flux decay rate ≤5% / 200h), and multi-stage rejection function of the membrane structure.

[0006] [Technical Solution]

[0007] To achieve the above object, a high-throughput PVDF hollow fiber ultrafiltration membrane regulated by PVP and a preparation method thereof, the preparation scheme includes the following steps:

[0008] (1) Heat the DMAc solvent, and sequentially add PVDF powder, PVP, and PEG-400 under stirring conditions, stir until completely dissolved, and then perform vacuum degassing treatment to obtain a casting solution. Use an annular spinneret with an inner diameter of 1.2 mm to uniformly coat the casting solution on the outer surface of the PET braided tube. After pre-evaporation in the air section, enter the gradient coagulation bath. The outer layer: pure water, the middle layer: glycerol aqueous solution, the inner layer: glycerol aqueous solution, and the membrane filaments stay for several seconds. The volume ratio of the three layers is 1:2:1.

[0009] (2) Immerse the coagulated membrane filaments in deionized water for several hours, and then immerse them in ethanol solutions with different concentrations for 5 hours each. Before immersing in ethanol solutions with different concentrations, drain the previous solution to avoid cross-contamination. Finally, shape and dry them in an oven for several hours.

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

[0011] In the 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 pieces / cm 2 , the outer diameter is 1.9 - 2.1 mm; pre-evaporation in the air section (distance 10 - 20 cm, temperature 25 - 30 °C, relative humidity 40 - 60%); the gradient of glycerol is 5 - 12%; the gradient coagulation bath is divided into three layers along the traveling direction of the membrane filaments: the outer layer of pure water (25 - 30 °C), the middle layer containing 8% glycerol (30 - 35 °C), the inner layer containing 10 - 15% glycerol (35 - 40 °C), and the residence time of the membrane filaments is 30 s - 50 s;

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

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

[0014] A high-throughput PVDF hollow fiber ultrafiltration membrane regulated by PVP, prepared by the above preparation method.

[0015] Material source

[0016] The weft density of the PET braided tube is 1250 - 1300 roots / cm 2 , with an outer diameter of 1.9 - 2.1 mm, no pre-treatment required, purchased from Jiangsu Hengli Group; PVDF powder purity ≥ 99%, purchased from Shandong Dongyue Group; PVP, K15 type (molecular weight 15,000), K60 type (molecular weight 55,000), K60 type (molecular weight 60,000), K90 type (molecular weight 1,000,000), all purchased from Hubei New Blue Sky New Materials; PEG - 400 with a molecular weight of 380 - 420, purchased from Liaoning Aoke Chemistry; DMAc analytical pure, purity ≥ 99.9%, purchased from Zhejiang Jiangshan Chemical Industry; ethanol; concentration ≥ 95%, purchased from Henan Tianguan Group; glycerol, analytical pure, purity ≥ 99.9%, purchased from Henan Tianguan Group; dry-wet spinning machine (including air section pre-evaporation device, distance 10 - 20 cm).

[0017] [Beneficial effects]

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

[0019] High molecular weight polyvinylpyrrolidone (PVP, 20,000 - 60,000) forms a hydrogen bond cross-linked network with polyvinylidene fluoride (PVDF), and its hydroxyl group (·OH) has a strong interaction with the fluorine atom (F) of PVDF, endowing the membrane surface with lasting hydrophilicity (contact angle ≤ 55°). The long-chain molecular structure of PVP slowly precipitates in the coagulation bath, forming uniformly distributed hydrophilic channels. During long-term operation, the hydrophilicity attenuation rate < 5% (200 h), and there is no performance deterioration caused by the migration of small molecules; by regulating the non-solvent diffusion kinetics through a glycerol gradient coagulation bath (outer layer pure water, middle layer 8% glycerol, inner layer 15% glycerol), a gradient pore size distribution from the surface to the inside is formed (surface layer 0.02 μm, transition layer 0.03 - 0.04 μm, inner layer 0.05 μm). Through the synergistic effect of the gradient pore size regulation strategy and the PVP hydrogen bond network design, all-round breakthroughs are achieved in core performance indicators such as hydrophilicity, flux, anti-pollution ability, and mechanical strength. At the same time, it has environmental protection and multi-scenario adaptation capabilities, providing an efficient and reliable solution for the upgrading of ultrafiltration membrane technology in fields such as water treatment and biomedicine, including: significantly improving the anti-pollution ability, reducing the adsorption of organic matter on the membrane surface, extending the chemical cleaning cycle (> 30 days), reducing the operation and maintenance cost; high flux, porosity ≥ 80%, pure water flux ≥ 1500 L / (m 2 ·h) (0.1 MPa, 25 °C), an 80% increase compared to the homogeneous membrane in Comparative Example 2 (750, 1500 L / (m 2· h)); High rejection rate, the dense surface layer intercepts large particle pollutants (turbidity removal rate ≥ 99%), and the loose inner layer maintains high flux; Anti-pollution, the gradient pore size delays the deep penetration of pollutants, and the flux decay rate ≤ 5% (200-hour operation); No secondary pollution, avoiding the risk of nanoparticle shedding, applicable to drinking water treatment, hygienic and safe, no toxic solvent residues, meeting the hygienic standards for food separation scenarios such as dairy products and fruit juices. Description of the Drawings

[0020] Figure 1 It is a comparison chart of the pure water fluxes of the hollow fiber membranes obtained in all examples and comparative examples.

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

[0022] Figure 3 It is a comparison chart of the flux decay (200 h) of the hollow fiber membranes obtained in all examples and comparative examples. Detailed Description of the Invention

[0023] In the present invention, the pure water flux test: Ultra-pure water at 25°C is introduced under a pressure of 0.1 MPa. After stabilizing for 30 min, the permeate is collected, and the flux is calculated according to the formula:

[0024]

[0025] (V: volume of permeate (L), A: membrane area (m 2 ), t: time (h))

[0026] In the present invention, the porosity test: The sample is weighed after drying (Wd), and the mercury intrusion porosimeter measures the pore volume under a pressure of 0.1 - 400 MPa, and the porosity is calculated:

[0027]

[0028] (Vpore: volume of mercury pressed in, Vtotal: geometric volume of the sample)

[0029] In the present invention, the flux decay rate test: Continuously operate for 200 hours, test conditions: 0.1 MPa, 25°C, 200 mg / L bovine serum albumin solution. According to the initial flux (J 0 ) and the flux after 200 hours (J 200 ), the decay rate is calculated:

[0030]

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

[0032] In order to enable those skilled in the art to understand the present invention more clearly, the present invention will be further described in detail below in conjunction with embodiments. However, it should be understood that the following embodiments are only the preferred embodiments of the present invention, and the scope of the present invention to be protected is not limited thereto.

[0033] Example 1:

[0034] A PVP-regulated high-throughput PVDF hollow fiber ultrafiltration membrane and its preparation method, characterized in that it comprises the following steps:

[0035] (1) Heat the DMAc solvent to 70 °C, and sequentially add PVDF powder (18 wt%), PVP (K60 type, molecular weight 55,000, 6 wt%), and PEG-400 (12 wt%) under stirring conditions, and stir for 8 hours until completely dissolved, then perform vacuum degassing treatment to obtain a casting solution (viscosity at 25 °C is 3200 mPa·s). Using an annular spinneret with an inner diameter of 1.2 mm, uniformly coat the casting solution on the outer surface of the PET braided tube (weft density 1280 roots / cm 2 , outer diameter 2.0 mm). After pre-evaporation in a 15 cm air section, enter a gradient coagulation bath: outer layer: pure water (25 °C), middle layer: 8% glycerol aqueous solution (30 °C), inner layer: 15% glycerol aqueous solution (35 °C), the residence time of the membrane filaments is 40 s, and the volume ratio of the three layers is 1:2:1.

[0036] (2) Immerse the coagulated membrane filaments in deionized water at 60 °C for 30 hours, and then immerse them in 30%, 50%, and 70% ethanol solutions for 5 hours each. Before each gradient immersion, drain the previous solution to avoid cross-contamination. Finally, shape the membrane in an oven at 65 °C (wind speed 1.0 m / s, humidity 20%) and dry it for 12 hours.

[0037] Example 2:

[0038] A PVP-regulated high-throughput PVDF hollow fiber ultrafiltration membrane and its preparation method, which is different 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 regulated by PVP and its preparation method, which is different from Example 1 in that: in step (1), the molecular weight of PVP is 15,000 (type K15), and the remaining conditions remain unchanged.

[0041] Comparative Example 2:

[0042] A high-flux PVDF hollow fiber ultrafiltration membrane regulated by PVP and its preparation method, which is different from Example 1 in that: in step (1), no PVP is added, and the addition amount of PEG-400 is 18 wt% (the same as the total additive content in Example 1), and the remaining conditions remain unchanged.

[0043] Comparative Example 3:

[0044] A high-flux PVDF hollow fiber ultrafiltration membrane regulated by PVP and its preparation method, which is different from Example 1 in that: in step (1), the molecular weight of PVP is 1,000,000 (type K90), and the remaining conditions remain unchanged.

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

[0046]

[0047] As can be seen from Table 1, in the examples, by optimizing the molecular weight of PVP (55,000 - 60,000), the prepared ultrafiltration membrane presents a gradient pore size (0.02 - 0.05 μm), the pure water flux reaches 1500 - 1580 L / (m 2 ·h), the porosity is 80% - 82%, and the flux attenuation rate ≤ 6.2% (200 h), verifying the improvement of the performance by the hydrogen bond network synergistic gradient phase separation of high molecular weight PVP and PVDF. The comparative examples show that: ① Low molecular weight PVP (15,000) leads to a sudden drop in porosity (70%) and flux (980 L / (m 2 ·h)), and the attenuation rate rises to 18%; ② When there is no PVP, the pore-forming performance of only PEG is even worse (flux 750 L / (m 2 ·h), attenuation rate 25%); ③ Ultra-high molecular weight PVP (1,000,000) has a flux (1420 L / (m 2 ·h)) and porosity (78%) inferior to those of the examples due to the too high viscosity of the casting solution (4600 mPa·s). Conclusion: The synergistic design of the PVP molecular weight (20,000 - 60,000) and the gradient coagulation bath is the core to break through the contradiction among flux - porosity - anti-pollution performance.

[0048] In summary, by optimizing the PVDF / PVP / PEG ternary casting solution system, combining with a glycerol gradient coagulation bath (pure water in the outer layer, 8% glycerol in the middle layer, and 15% glycerol in the inner layer) and a gradient ethanol replacement process, a gradient pore size structure (0.02μm, 0.05μm) from the surface to the inside is formed on the surface of the PET braided tube, achieving a synergistic breakthrough with a porosity ≥ 80%, a pure water flux ≥ 1500L / (m 2 ·h) and a flux decay rate ≤ 5%. The present invention utilizes the hydrogen bond network of high molecular weight PVP (55,000) and PVDF to construct a persistent hydrophilic channel, and regulates the membrane pore distribution through gradient phase separation, effectively solving the contradiction of traditional membrane flux - porosity - anti-pollution, and is applicable to the fields of seawater desalination pretreatment, high turbidity wastewater treatment and food industry separation, with the technical advantages of high flux, low attenuation and long-term stability.

Claims

1. A high-throughput PVDF hollow fiber ultrafiltration membrane based on PVP regulation and a preparation method thereof, characterized in that: The preparation method steps are as follows: (1) Heat the DMAc solvent, add PVDF powder, PVP, and PEG-400 in sequence under stirring conditions, stir until completely dissolved, and then vacuum degassing to obtain a casting solution. Use a ring spinneret with an inner diameter of 1.2 mm to evenly coat the casting solution on the outer surface of the PET braided tube. After pre-evaporation in the air section, enter the gradient coagulation bath, outer layer: pure water, middle layer: glycerol aqueous solution, inner layer: glycerol aqueous solution, the membrane filaments stay for a few seconds, and the volume ratio of the three layers is 1:2:

1. (2) The solidified membrane fibers are immersed in deionized water for several hours, and then immersed in ethanol solutions of different concentrations for 5 hours each. The previous solution needs to be drained before immersing in ethanol solutions of different concentrations to avoid cross contamination. Finally, the membrane fibers are shaped and dried in an oven for several hours.

2. A high-throughput PVDF hollow fiber ultrafiltration membrane based on PVP regulation and a preparation method thereof according to claim 1, characterized in that: In the step (1), DMAc is heated to 60-80° C.; the rotation speed is 200-300 rpm and the stirring is performed for 6-8 hours; the molecular weight of PVP is 20,000-60,000, and the viscosity of the casting solution at 25° C. is 2800-3800 mPa·s; and the mass fraction ratio of PVDF, PVP and PEG-400 is 16-20 wt %: 4-8 wt %: 10-15 wt %.

3. A high-throughput PVDF hollow fiber ultrafiltration membrane based on PVP regulation and a preparation method thereof according to claim 1, characterized in that: In the step (1), the inner diameter of the annular spinneret is 1.0 to 1.5 mm; the weft density of the PET braided tube is 1250 to 1300 strands / cm 2 , outer diameter 1.9~2.1mm; air segment pre-evaporation (distance 10~20cm, temperature 25~30℃, relative humidity 40~60%); glycerol gradient 5~12%; gradient coagulation bath is divided into three layers along the membrane filament moving direction: outer layer pure water (25~30℃), middle layer containing 8% glycerol (30~35℃), inner layer containing 10~15% glycerol (35~40℃), membrane filament residence time is 30s~50s.

4. A high-throughput PVDF hollow fiber ultrafiltration membrane based on PVP regulation and a preparation method thereof according to claim 1, characterized in that: In the step (2), the temperature of the deionized water is 50-70° C.; the soaking time is 24-36 hours; the concentration of the ethanol solution is 30%-70%, and each gradient soaking time is 4-6 hours.

5. A high-throughput PVDF hollow fiber ultrafiltration membrane based on PVP regulation and a preparation method thereof according to claim 1, characterized in that: In the step (2), the oven temperature is 60-70° C., the wind speed is 0.8-1.2 m / s, and the drying time is 10-14 hours.

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