Method for forming a dense skin layer on the surface of a PP or PMP hollow fiber membrane

Abstract

The present invention discloses a method for forming a dense skin layer on the surface of a PP or PMP hollow fiber membrane, comprising: mixing a polymerizable siloxane, a radical initiator and a solvent, heating and stirring, cooling to room temperature for standby after complete reaction, adding polyalkylsiloxanes with different chain lengths, and stirring evenly at room temperature to obtain a skin layer solution; immersing the PP or PMP hollow fiber membrane in the skin layer solution and staying for 0.5 - 3 h for skin layer coating; preparing an aqueous solution with a pH of 2.5 - 3.6, heating and immersing the hollow fiber membrane after skin layer coating in the aqueous solution for skin layer alcoholysis; moving the hollow fiber membrane after skin layer alcoholysis into a high-temperature environment for dehydration crosslinking to obtain a hollow fiber membrane with a dense skin layer formed on the surface. The hollow fiber membrane prepared by the method of the present application has significantly improved membrane anti-wetting performance without significantly reducing the gas flux.

Description

Technical Field

[0001] The present invention relates to the technical field of hollow fiber membranes, and particularly to a method for forming a dense skin layer on the surface of a PP or PMP hollow fiber membrane. Background Art

[0002] At present, the hollow fiber membrane contactors produced industrially generally use PP or PMP membrane materials prepared by the melt stretching method or the thermally induced phase separation method as the contact medium. During the preparation process, a large number of dense pores will be formed on their surfaces for gas transmission. Such materials are all hydrophobic to prevent liquid transmembrane penetration and wetting. However, in actual use, since the liquid processed by the membrane contactor is not pure water and may be mixed with impurities such as a small amount of organic substances such as alcohols, esters, hydrocarbons, or bacteria and microorganisms, the membrane is likely to be wetted and lose its ability to degas or aerate. Reducing the membrane pore size can improve the anti-wetting ability of the membrane. However, if the membrane pore size is reduced, the gas flux of the membrane will decrease, and the ability to degas or aerate will decline.

[0003] Therefore, how to improve the anti-wetting performance of the PP or PMP hollow fiber membrane without significantly reducing the gas flux of the membrane is an urgent problem to be studied. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for forming a dense skin layer on the surface of a PP or PMP hollow fiber membrane to solve the problems raised in the above background art.

[0005] To achieve the above purpose, the present invention provides the following technical solution: A method for forming a dense skin layer on the surface of a PP or PMP hollow fiber membrane, comprising:

[0006] S1. Preparation of the skin layer solution:

[0007] S1.1 Polymerization reaction: Mix the polymerizable siloxane, free radical initiator, and solvent, heat to 60 - 80 °C and stir. After reacting for 7 - 9 h, cool to room temperature for standby;

[0008] S1.2 Crosslinking reaction: Add polysiloxanes with different chain lengths to the solution obtained in step S1.1, and stir at room temperature for 1 - 3 h to obtain the skin layer solution;

[0009] S2. Skin layer coating: Immerse the PP or PMP hollow fiber membrane in the skin layer solution obtained in step S1 and stay for 0.5 - 3 h for skin layer coating;

[0010] S3. Skin layer alcoholysis: Prepare an aqueous solution with a pH of 2.5 - 3.6, heat to 75 - 85 °C, immerse the PP or PMP hollow fiber membrane after skin layer coating in step S2 and stay for 0.5 - 1 h for skin layer alcoholysis;

[0011] S4. High-temperature dehydration and polymerization: Transfer the PP or PMP hollow fiber membrane after the cortical alcoholysis in step S3 to a high-temperature environment of 75 - 85 °C for dehydration cross-linking to obtain a PP or PMP hollow fiber membrane with a dense cortex formed on the surface.

[0012] In this application, a polymerizable siloxane is used as a monomer in a solvent for a free radical polymerization reaction to obtain polymer A. A cross-linking agent, polyalkylsiloxane, is added and dip-coated on the hollow fiber membrane. An aqueous solution of an acid is used to cause an alcoholysis and cross-linking reaction between polymer A and polyalkylsiloxane. When using a solvent slightly soluble in water, it is more conducive to the full progress of the alcoholysis and cross-linking reaction at the interface. After drying at a high temperature, further cross-linking is completed to form a dense cortex covering the surface of the PP or PMP hollow fiber membrane. After testing, the membrane anti-wetting performance of the PP or PMP hollow fiber membrane prepared by the method of this application is significantly improved without significantly reducing the gas flux.

[0013] Preferably, in step S1, the polymerizable siloxane is a methoxy- or ethoxy-containing silane with a polymerization site, and the polymerization site is a vinyl, propenyl, or acryloyloxy group that can form a high polymer through a free radical reaction.

[0014] Preferably, in step S1, the polyalkylsiloxane is a methoxy- or ethoxy-containing silane with different carbon numbers.

[0015] Preferably, in step S1, the free radical initiator is an azo initiator.

[0016] Preferably, in step S1, the solvent is an ester solvent.

[0017] Preferably, the polymerizable siloxane includes one or more of vinyltrimethoxysilane, vinyltriethoxysilane, propenyltrimethoxysilane, propenyltriethoxysilane, acryloyloxyethyltrimethoxysilane, acryloyloxyethyltriethoxysilane, 3-acryloyloxypropyltrimethoxysilane, or 3-acryloyloxypropyltriethoxysilane.

[0018] Preferably, the polyalkylsiloxane includes one or more of methyltrimethoxysilane, ethyltrimethoxysilane, propyltrimethoxysilane, butyltrimethoxysilane, octyltrimethoxysilane, heptyltrimethoxysilane, dodecyltrimethoxysilane, tetradecyltrimethoxysilane, hexadecyltrimethoxysilane, octadecyltrimethoxysilane, methyltriethoxysilane, ethyltriethoxysilane, propyltriethoxysilane, butyltriethoxysilane, octyltriethoxysilane, heptyltriethoxysilane, dodecyltriethoxysilane, tetradecyltriethoxysilane, hexadecyltriethoxysilane, or octadecyltriethoxysilane.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: The anti-wetting performance of the PP or PMP hollow fiber membrane prepared by the method of the present application is significantly improved without significantly reducing the gas flux. The pores on the surface of the hollow fiber membrane can be reduced, and the pores inside the hollow fiber are not affected, thereby improving the radial asymmetry of the hollow fiber membrane. Description of the Drawings

[0020] Figure 1 It is a schematic diagram comparing the contact angles before and after the formation of a dense skin layer on the surface of the PP or PMP hollow fiber membrane in the embodiment of the present invention;

[0021] Figure 2 It is an electron micrograph of the surface of the PP or PMP hollow fiber membrane before the formation of a dense skin layer in Embodiment 1 of the present invention;

[0022] Figure 3 It is an electron micrograph of the surface of the PP or PMP hollow fiber membrane after the formation of a dense skin layer in Embodiment 1 of the present invention. Detailed Embodiments

[0023] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0024] The present invention provides a method for forming a dense skin layer on the surface of a PP or PMP hollow fiber membrane, including:

[0025] S1. Preparation of the skin layer solution:

[0026] S1.1 Polymerization reaction: Mix the polymerizable siloxane, free radical initiator, and solvent, heat to 60 - 80 °C and stir. After reacting for 7 - 9 h, cool to room temperature for standby. Among them, the polymerizable siloxane is a methoxy- or ethoxy-containing silane with a polymerization site, and the polymerization site is a vinyl, propenyl, or acryloyloxy group that can form a high polymer through a free radical reaction, such as vinyltrimethoxysilane, vinyltriethoxysilane, propenyltrimethoxysilane, propenyltriethoxysilane, acryloyloxyethyltrimethoxysilane, acryloyloxyethyltriethoxysilane, 3-acryloyloxypropyltrimethoxysilane, or 3-acryloyloxypropyltriethoxysilane; the free radical initiator is an azo initiator; the solvent is an ester solvent.

[0027] S1.2, Crosslinking reaction: Add polyalkylsiloxanes with different chain lengths to the solution obtained in step S1.1, and stir for 1 - 3 h at room temperature to obtain a skin layer solution. Among them, the polyalkylsiloxane is a methoxy or ethoxy silane containing different numbers of carbon atoms, such as methyltrimethoxysilane, ethyltrimethoxysilane, propyltrimethoxysilane, butyltrimethoxysilane, octyltrimethoxysilane, heptyltrimethoxysilane, dodecyltrimethoxysilane, tetradecyltrimethoxysilane, hexadecyltrimethoxysilane, octadecyltrimethoxysilane, methyltriethoxysilane, ethyltriethoxysilane, propyltriethoxysilane, butyltriethoxysilane, octyltriethoxysilane, heptyltriethoxysilane, dodecyltriethoxysilane, tetradecyltriethoxysilane, hexadecyltriethoxysilane or octadecyltriethoxysilane.

[0028] S2, Skin layer coating: Immerse the PP or PMP hollow fiber membrane in the skin layer solution obtained in step S1, and stay for 0.5 - 3 h for skin layer coating;

[0029] S3, Skin layer alcoholysis: Prepare an aqueous solution with a pH of 2.5 - 3.6, heat it to 75 - 85 °C, and immerse the PP or PMP hollow fiber membrane after skin layer coating in step S2 and stay for 0.5 - 1 h for skin layer alcoholysis;

[0030] S4, High-temperature dehydration polymerization: Transfer the PP or PMP hollow fiber membrane after skin layer alcoholysis in step S3 to a high-temperature environment of 75 - 85 °C for dehydration crosslinking to obtain a PP or PMP hollow fiber membrane with a dense skin layer formed on the surface.

[0031] Taking vinyltriethoxysilane as an example, the reaction process of step S1.1 is as follows:

[0032] 。

[0033] The reaction process of step S1.2 is as follows:

[0034] 。

[0035] The reaction process of step S3 is as follows:

[0036] 。

[0037] The reaction process of step S4 is as follows:

[0038] 。 Example 1

[0039] Add the following reagents into a three-necked flask: 10 g of vinyltriethoxysilane, 0.1 g of azobisisobutyronitrile, and 100 g of ethyl acetate. Heat the mixture to 70 °C and stir. After reacting for 8 h, cool it to room temperature for standby; then continue to add 5 g of octadecyltriethoxysilane into the three-necked flask and stir for 1 h to obtain the skin layer solution; immerse the PP or PMP hollow fiber membrane into the skin layer solution and let it stay for 3 h for skin layer coating; prepare an aqueous solution with a pH of 3, heat it to 70 °C, immerse the PP or PMP hollow fiber membrane after skin layer coating and let it stay for 3 h for skin layer alcoholysis; transfer the PP or PMP hollow fiber membrane after alcoholysis to an oven at 80 °C and let it stay for 1 h for dehydration crosslinking to obtain a PP or PMP hollow fiber membrane with a dense skin layer formed on the surface. Example 2

[0040] Add the following reagents into a three-necked flask: 15 g of vinyltrimethoxysilane, 0.1 g of azobisisobutyronitrile, and 100 g of triethyl phosphate. Heat the mixture to 80 °C and stir. After reacting for 8 h, cool it to room temperature for standby; then continue to add 3 g of n-decyltriethoxysilane into the three-necked flask and stir for 1 h to obtain the skin layer solution; immerse the PP or PMP hollow fiber membrane into the skin layer solution and let it stay for 3 h for skin layer coating; prepare an aqueous solution with a pH of 3, heat it to 70 °C, immerse the PP or PMP hollow fiber membrane after skin layer coating and let it stay for 3 h for skin layer alcoholysis; transfer the PP or PMP hollow fiber membrane after alcoholysis to an oven at 80 °C and let it stay for 1 h for dehydration crosslinking to obtain a PP or PMP hollow fiber membrane with a dense skin layer formed on the surface. Example 3

[0041] Add the following reagents into a three-necked flask: 12 g of acryloxyethyltrimethoxysilane, 0.1 g of azobisisobutyronitrile, and 100 g of butyl acetate. Heat the mixture to 60 °C and stir. After reacting for 8 h, cool it to room temperature for standby; then continue to add 6 g of ethyltrimethoxysilane into the three-necked flask and stir for 1 h to obtain the skin layer solution; immerse the PP or PMP hollow fiber membrane into the skin layer solution and let it stay for 0.5 h for skin layer coating; prepare an aqueous solution with a pH of 3, heat it to 70 °C, immerse the PP or PMP hollow fiber membrane after skin layer coating and let it stay for 3 h for skin layer alcoholysis; transfer the PP or PMP hollow fiber membrane after alcoholysis to an oven at 80 °C and let it stay for 1 h for dehydration crosslinking to obtain a PP or PMP hollow fiber membrane with a dense skin layer formed on the surface. Example 4

[0042] Add the following reagents into a three-necked flask: 16 g of 3-acryloxypropyltrimethoxysilane, 0.12 g of azobisisobutyronitrile, and 100 g of butyl acetate. Heat the mixture to 60 °C and stir. After reacting for 8 h, cool it to room temperature for standby. Then, continue to add 3 g of tetradecyltrimethoxysilane into the three-necked flask and stir for 1 h to obtain the skin layer solution. Immerse the PP or PMP hollow fiber membrane into the skin layer solution and leave it for 0.5 h for skin layer coating. Prepare an aqueous solution with a pH of 3, heat it to 70 °C, immerse the PP or PMP hollow fiber membrane after skin layer coating and leave it for 3 h for skin layer alcoholysis. Transfer the PP or PMP hollow fiber membrane after skin layer alcoholysis to an oven at 80 °C and leave it for 1 h for dehydration cross-linking to obtain the PP or PMP hollow fiber membrane with a dense skin layer formed on the surface. Example 5

[0043] Add the following reagents into a three-necked flask: 16 g of vinyltrimethoxysilane, 0.12 g of azobisisobutyronitrile, and 100 g of butyl acetate. Heat the mixture to 60 °C and stir. After reacting for 8 h, cool it to room temperature for standby. Then, continue to add 6 g of tetradecyltrimethoxysilane into the three-necked flask and stir for 1 h to obtain the skin layer solution. Immerse the PP or PMP hollow fiber membrane into the skin layer solution and leave it for 0.5 h for skin layer coating. Prepare an aqueous solution with a pH of 3, heat it to 70 °C, immerse the PP or PMP hollow fiber membrane after skin layer coating and leave it for 3 h for skin layer alcoholysis. Transfer the PP or PMP hollow fiber membrane after skin layer alcoholysis to an oven at 80 °C and leave it for 1 h for dehydration cross-linking to obtain the PP or PMP hollow fiber membrane with a dense skin layer formed on the surface.

[0044] Detect the contact angle, air flux, and water immersion pressure of the PP or PMP hollow fiber membranes with dense skin layers formed in Examples 1 to 5, and the results are shown in the following table:

[0045] 。

[0046] Such as Figures 1-3As shown, dense hydrophobic surfaces with different properties can be prepared using different polymerizable siloxanes, initiators, and polyalkylsiloxanes. Generally speaking, polyalkylsiloxanes with longer carbon chains can provide a stronger hydrophobic effect because the long polyalkyl carbon chains have a lower surface energy and a greater repulsive force to water, resulting in a better hydrophobic effect. At the same time, increasing the dosage of the polymerizable siloxane and polyalkylsiloxane, or increasing the crosslinking temperature, can form polymers with a larger molecular weight, which is more conducive to the formation of a dense crosslinked structure for this coating. Compared with the unmodified PP hollow fibers, the hollow fiber membranes prepared by this method show a stronger hydrophobic effect. The surface water contact angle has increased significantly from 97° to about 128° at most, forming a superhydrophobic surface. At the same time, this method also increases the water immersion pressure without significantly reducing the gas flux, which is also valid evidence for the formation of a dense layer by the hydrophobic coating.

[0047] In summary, for the PP or PMP hollow fiber membranes treated by this method, a dense hydrophobic cortex is formed on the surface of the fiber membrane, increasing the water immersion pressure, making it more difficult for pollutants to infiltrate the membrane filaments, and significantly enhancing the hydrophobicity. At the same time, without significantly reducing the gas flux, the growth of this dense cortex does not spread deep into the radial direction of the hollow fiber membrane, and the originally numerous and dense gas channels are retained. According to the test results, in this application, without reducing the pore size of the hollow fiber membrane, a hydrophobic dense cortex is coated on its surface, which can effectively reduce the membrane pore size on the outer side of the membrane without affecting the pore channels in the radial direction of the hollow fiber membrane. Such a cortex should have a high degree of density, a high gas permeability, be not easy to fall off, and not penetrate into the interior of the membrane.

[0048] The polymerizable siloxane has polymerizable vinyl, propenyl, and acryloyloxyethyl groups, which can easily and quickly undergo free radical polymerization reactions to generate polymers. After the polymerization reaction is completed, polyalkylsiloxane is used for mixing. Under acidic conditions, the methoxy and ethoxy groups of the polymer and polyalkylsiloxane undergo hydrolysis to generate crosslinkable silanol bonds and methanol and ethanol. The silanol bonds undergo polycondensation and dehydration with each other to form siloxane-siloxane bonds and crosslink tightly. During the drying process, the silanol further undergoes dehydration polycondensation, and the degree of crosslinking is further increased. At the same time, the polyalkyl chains also provide hydrophobicity, making the coating layer more hydrophobic and further increasing the contact angle. Due to the crosslinking reaction, the polymers and crosslinking agents between different molecular chains are fixed to each other and will not fall off during use.

[0049] It is obvious to those skilled in the art that the present invention is not limited to the details of the above-described exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be embraced within the present invention. Any reference signs in the claims should not be construed as limiting the claims involved.

Claims

1. A method for forming a dense cortex on the surface of a PP or PMP hollow fiber membrane, characterized in that: include: S1. Preparation of cortical solution: S1.1, polymerization reaction: mix the polymerizable siloxane, azo initiator and solvent, heat to 60-80°C and stir, react for 7-9 hours, then cool to room temperature for use; S1.2, cross-linking reaction: add polyalkylsiloxane to the solution obtained in step S1.1, and stir at room temperature for 1-3 hours to obtain a cortex solution, wherein the polyalkylsiloxane includes octadecyltriethoxysilane or tetradecyltrimethoxysilane; S2, skin coating: immerse the PP or PMP hollow fiber membrane in the skin solution obtained in step S1, and leave it for 0.5-3h to perform skin coating; S3, cortical alcoholysis: prepare an aqueous solution with a pH of 2.5-3.6, heat it to 75-85°C, immerse the PP or PMP hollow fiber membrane coated with the cortical layer in step S2 and leave it for 0.5-1h to perform cortical alcoholysis; S4, high temperature dehydration polymerization: the PP or PMP hollow fiber membrane after the alcoholysis of the cortex in step S3 is placed in a high temperature environment of 75-85°C for dehydration and crosslinking to obtain a PP or PMP hollow fiber membrane with a dense cortex formed on the surface.

2. The method for forming a dense cortex on the surface of a PP or PMP hollow fiber membrane according to claim 1, characterized in that: The polymerizable siloxane in step S1 is a methoxy- or ethoxy-containing silane with a polymerization site, wherein the polymerization site is a vinyl, acryl or acryloxy group that can form a polymer through a free radical reaction.

3. The method for forming a dense cortex on the surface of a PP or PMP hollow fiber membrane according to claim 1 or 2, characterized in that: In step S1, the solvent in step S1.1 is an ester solvent.

4. The method for forming a dense cortex on the surface of a PP or PMP hollow fiber membrane according to claim 2, characterized in that: The polymerizable siloxane includes one or more of vinyltrimethoxysilane, vinyltriethoxysilane, acryltrimethoxysilane, acryltriethoxysilane, acryloxyethyltrimethoxysilane, acryloxyethyltriethoxysilane, 3-acryloxypropyltrimethoxysilane or 3-acryloxypropyltriethoxysilane.

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