CN112090285A

CN112090285A - Method for preparing hydrophilic PVC flat membrane and hydrophilic PVC flat membrane prepared by method

Info

Publication number: CN112090285A
Application number: CN201910528610.1A
Authority: CN
Inventors: 贾建东; 朱杭城; 陈锋
Original assignee: Hangzhou Cobetter Filtration Equipment Co Ltd
Current assignee: Hangzhou Cobetter Filtration Equipment Co Ltd
Priority date: 2019-06-18
Filing date: 2019-06-18
Publication date: 2020-12-18

Abstract

The invention relates to a method for preparing a hydrophilic PVC flat membrane, which comprises the following steps: s1: preparing a membrane casting solution, mixing 8-15 wt% of polyvinyl chloride, 5-15 wt% of hydrophilic pore-forming agent, 5-10 wt% of high-volatility solvent and 65-82 wt% of conventional solvent, fully stirring uniformly, defoaming, standing and cooling to room temperature; s2: coating the casting solution on a base layer for performing, placing a preformed product in an air section for evaporation induced phase transformation to form a primary finished film, wherein the performing is carried out until the non-volatile high-volatility solvent in the casting solution accounts for 20-90% of the total amount of the original high-volatility solvent; s3: carrying out non-solvent induced phase inversion and simultaneous curing, and immersing the primary finished film formed in the step S2 into a coagulating bath at a constant speed for curing; s4: and (5) cleaning and drying. The invention aims to provide a hydrophilic PVC flat membrane with an asymmetric structure, hydrophilicity, high strength, high porosity and stable structure and performance and a preparation method thereof.

Description

Method for preparing hydrophilic PVC flat membrane and hydrophilic PVC flat membrane prepared by method

Technical Field

The invention relates to a flat membrane, in particular to a hydrophilic PVC flat membrane.

Background

The membrane separation technology has the characteristics of simplicity, practicability, environmental protection, high efficiency and the like, is continuously developed in recent years, and is widely applied to the fields of electronic semiconductors, medical pharmacy, food and beverage, air separation, wastewater treatment and the like. At present, commercial membranes are mainly prepared from materials such as polyether sulfone, polysulfone, polyvinylidene fluoride, polytetrafluoroethylene, nylon and the like. The materials have respective characteristics and advantages due to different characteristics, and the application fields are naturally different. In the wastewater treatment industry, because the required quantity is the largest and no special requirements are made on the membrane material, the search and realization of the preparation of low-cost and high-performance polymers from the two aspects of material selection and membrane forming methods are the main way of technical development.

Polyvinyl chloride is one of three synthetic resins with the largest output, has low price, rich sources, acid and alkali resistance, chemical corrosion resistance and good chemical stability, and is an ideal synthetic membrane material. The nature of the membrane surface and the structure of the membrane are critical in determining the anti-fouling and separation performance of the membrane.

However, the polyvinyl chloride material has surface tension of 33-38dynes/cm, so that the polyvinyl chloride material is not good in hydrophilicity and is easy to be polluted, thereby reducing the water flux of the membrane and influencing the service performance of the membrane. At present, the main measures which can be taken for improving the hydrophilicity of the surface of the polyvinyl chloride film are as follows: 1 surface chemical grafting modification, grafting some hydrophilic monomer on the membrane by chemical treatment to convert the surface from non-polar to polar. 2 surface radiation grafting modification, which can generate active free radicals on the surface by using ultraviolet, plasma or electron beams with higher energy and other radiation methods, and the free radicals react with some hydrophilic monomers to improve the hydrophilic performance of the hydrophilic monomers. The two methods can achieve the hydrophilic effect, but the problems are more when the two methods are actually applied to industrial production, and the production cost is high.

The structure of the membrane is mainly determined by the formula of the membrane casting solution and the phase separation process. The main methods for preparing the polyvinyl chloride filtration membrane are divided into two types. The membrane produced by the method has the advantages that the internal structure of the membrane is mostly large finger-shaped holes, the probability of generating defects is high, the strength of the membrane is low, and a plurality of problems exist in practical application. 2 thermal phase inversion method, melting and blending polyvinyl chloride and thinner at 170 ℃, degassing and preparing membrane. In the prior art, polyvinyl chloride and diphenyl ether are melted and blended at 140 ℃ to prepare the polyvinyl chloride microporous membrane. The membranes prepared by the two methods are homogeneous membranes, have much lower permeation flux than asymmetric membranes, and have much poorer stain resistance than asymmetric membranes. Moreover, polyvinyl chloride becomes a viscoelastic body at 130 ℃, the polyvinyl chloride per se exists in a viscous state at 160-180 ℃, and the polyvinyl chloride per se is very unstable and is easy to break molecular chains, so that the service performance of the polyvinyl chloride is influenced.

Different from the scheme, the invention changes the property of the membrane surface by adding the hydrophilic high molecular substance, greatly improves the hydrophilic performance of the microfiltration membrane, obviously improves the anti-fouling capability, and does not degrade the hydrophilicity in the using process; and on the basis of full experiments and pilot experiments, the hydrophilic PVC flat membrane is obtained. It has the advantages of asymmetric structure, hydrophilicity, high strength, high porosity, stable structure and performance, etc.

Disclosure of Invention

The invention aims to provide a hydrophilic PVC flat membrane with an asymmetric structure, hydrophilicity, high strength, high porosity and stable structure and performance and a preparation method thereof.

In order to achieve the purpose, the invention adopts the following technical scheme: a method of making a hydrophilic PVC flat sheet membrane, the method comprising: s1: preparing a membrane casting solution, mixing 8-15 wt% of polyvinyl chloride, 5-15 wt% of hydrophilic pore-forming agent, 5-10 wt% of high-volatility solvent and 65-82 wt% of conventional solvent, fully stirring uniformly, defoaming, standing and cooling to room temperature; s2: coating the casting solution on a base layer for performing, placing a preformed product in an air section for evaporation induced phase transformation to form a primary finished film, wherein the performing is carried out until the non-volatile high-volatility solvent in the casting solution accounts for 20-90% of the total amount of the original high-volatility solvent; s3: carrying out non-solvent induced phase inversion and simultaneous curing, and immersing the primary finished film formed in the step S2 into a coagulating bath at a constant speed for curing; s4: cleaning and drying; the polymerization degree of the polyvinyl chloride is set to be between 800-2000, the hydrophilic pore-foaming agent is polyethylene glycol and polyvinylpyrrolidone which are added simultaneously, and the conventional solvent is any one of N, N '-dimethylformamide, N' -dimethylacetamide and N-methylpyrrolidone; the coagulating bath comprises 0-30 wt% of water and 70-100 wt% of solvent, wherein the solvent is any mixture of three of N, N '-dimethylformamide, N' -dimethylacetamide and N-methylpyrrolidone or one of the same as a conventional solution.

Further, in the step S2, when the concentration of the volatile substance at the position 40cm-60cm away from the surface of the flat membrane is less than or equal to 6000ppm, the pre-phase separation is finished.

Further, in the step S2, the humidity of the air section is set to be greater than 40%, the temperature is set to be between 25 ℃ and 30 ℃, the length of the air section is set to be between 1cm and 50cm, and the residence time in the air section is set to be between 5S and 210S.

Further, the air section blows air to the preformed primary product film.

Further, the air blowing is performed above the film in parallel, the air blowing direction is along the reverse direction of the liquid film casting direction, and the air speed is set between 0m/s and 8 m/s.

Further, the molecular weight of the polyethylene glycol is set to be between 200 and 8000, and the molecular weight of the polyvinylpyrrolidone is set to be between 10000 and 2000000.

Further, in the step S4, the membrane is immersed in hot water at a temperature of 50 ℃ to 80 ℃, washed for 10 minutes to 30 minutes, and then dried.

Furthermore, the invention also comprises a hydrophilic PVC flat membrane prepared by the method, which comprises a base layer and a filter layer with micropores, and is characterized in that: the pore diameter of the micropores is set to be between 0.1 and 20 microns, the porosity is larger than 80 percent, the cross section of the membrane is in a spongy asymmetric structure, and the pore diameter of the micropores tends to gradually decrease/increase towards one side of the base layer in the cross section direction of the membrane.

Further, the thickness of the flat film is set to be between 50 μm and 500 μm.

Further, the base layer is a non-woven fabric layer.

In the scheme of the invention, polyvinyl chloride is a main material for film formation, is abbreviated as PVC, is one of three synthetic resins with the largest output, has low price, rich sources, acid and alkali resistance, chemical corrosion resistance and good chemical stability, and is an ideal synthetic film material; polyethylene glycol is a substance which is nontoxic, nonirritating and slightly bitter in taste, is abbreviated as PEG, has good water solubility, has good intermiscibility with a plurality of organic matter components, has excellent lubricity, moisture retention, dispersibility, adhesive, antistatic agent, softening agent and the like, and has very wide application in industries such as cosmetics, pharmacy, chemical fiber, rubber, plastics, papermaking, paint, electroplating, pesticides, metal processing, food processing and the like; polyvinylpyrrolidone is a synthetic water-soluble high molecular compound, is abbreviated as PVP, and has the general properties of the water-soluble high molecular compound, such as colloid protection, film-forming property, cohesiveness, hygroscopicity, solubilization or coacervation; tetrahydrofuran is a colorless, water-miscible, less viscous organic liquid at normal temperature and pressure, abbreviated as THF, and is a commonly used medium-polarity aprotic solvent; acetone, also called dimethyl ketone, is the simplest saturated ketone, and is abbreviated as acetone, which is a colorless transparent liquid, has special spicy smell, is easily soluble in water and organic solvents such as methanol, ethanol, ether, chloroform, pyridine and the like, and has the characteristics of flammability, easy volatilization and active chemical property; n, N' -dimethylformamide is a colorless transparent liquid, is abbreviated as DMF and is a good solvent with wide application range; n, N' -dimethylacetamide is a colorless transparent liquid, is abbreviated as DMAC and is a solvent with high boiling point, high flash point, high thermal stability and stable chemistry; the N-methylpyrrolidone is a colorless transparent oily liquid, can be mutually dissolved with water, alcohol, ether, ester, ketone, halogenated hydrocarbon, aromatic hydrocarbon and castor oil, has low volatility and good thermal stability and chemical stability, and is abbreviated as NMP.

Compared with the prior art, the finished hydrophilic PVC flat membrane has larger aperture, better hole-to-hole connectivity and better hydrophilicity and pollution resistance, and structurally, the finished membrane has higher structural strength and larger water flux. Compared with the prior art, the method for preparing the hydrophilic PVC flat membrane has stronger controllability, can control the aperture size of the finished membrane, can prepare the finished membrane with larger aperture compared with the prior art, contains less unnecessary substances in the membrane casting solution, and can improve the purity of the finished membrane.

Drawings

The invention will be further described with reference to the accompanying drawings in which:

FIG. 1 is an electron microscope image of the pore diameter of the upper surface of a hydrophilic PVC flat membrane;

FIG. 2 is an electron microscope image of the aperture of the lower surface of a hydrophilic PVC flat membrane;

FIG. 3 is an electron microscope image of the cross section of the hydrophilic PVC flat membrane.

Detailed Description

In order that the above objects, features and advantages of the present invention can be more clearly understood, a more particular description of the invention will be rendered by reference to the appended drawings. It should be noted that the embodiments and features of the embodiments of the present application may be combined with each other without conflict.

In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, however, the present invention may be practiced in other ways than those specifically described herein, and therefore the scope of the present invention is not limited by the specific embodiments disclosed below.

A hydrophilic PVC flat membrane comprises a base layer and a filter layer with micropores, wherein the filter layer is arranged on one side of the base layer, and for the convenience of our, the side of the whole flat membrane close to the base layer is called a lower surface, and the side of the whole flat membrane close to the filter layer is called an upper surface. The filter layer is provided with a large number of micropores with the pore diameter of 0.1-20 μm, the porosity is more than 80%, and the micropores are regularly arranged. The pore diameter of the micropores is from the upper surface to the lower surface of the hydrophilic PVC flat membrane, the micropores are arranged in a gradually decreasing/increasing trend, and a positive V-shaped or reverse V-shaped structure is approximately presented when viewed from the section; i.e. the pore size of the micropores as a whole has such a tendency to vary, but does not mean that the pore size of the micropores close to the upper surface must be larger than the pore size of the micropores close to the lower surface, see in particular fig. 1-3.

In the prior art, the aperture of the hydrophilic PVC flat membrane is generally about 0.1-1 μm and cannot be larger; and the porosity is generally less than 80%, and the section is observed by an electron microscope picture, and the micropores basically present a finger shape or a symmetrical shape. The following table I shows the comparison between the performance data of the hydrophilic PVC flat membrane in the prior art and the performance data of different samples of the hydrophilic PVC flat membrane in the invention.

Table one:

by combining the relevant experimental data in the first table, we can easily find that the hydrophilic PVC flat membrane has higher flux, higher strength and better hydrophilicity under the same pore diameter parameter compared with the flat membrane in the prior art; and under the same aperture parameters, the cross section of the material has the same or similar effect regardless of the positive V-shaped or reverse V-shaped structure.

The invention further provides a method for preparing the hydrophilic PVC flat membrane, which comprises the following steps:

s1: preparing a membrane casting solution, mixing 8-15 wt% of polyvinyl chloride, 5-15 wt% of hydrophilic pore-forming agent, 5-10 wt% of high-volatility solvent and 65-82 wt% of conventional solvent at 25-80 ℃, fully stirring uniformly, defoaming, standing and cooling to room temperature; whether the casting solution is fully and uniformly mixed can be known through visual observation. The selection of the mixing temperature can prevent the situation that the high-volatility solvent is volatilized in advance due to the excessively high-temperature mixing, ensure that the relative activity of molecules is not influenced by the extremely low temperature, and fully and uniformly mix the casting solution. In the method, the polymerization degree of the polyvinyl chloride is set between 800-2000; the hydrophilic pore-forming agent is specifically two substances of polyethylene glycol and polyvinylpyrrolidone, and the polyethylene glycol and the polyvinylpyrrolidone are added simultaneously for use. And further limiting the molecular weight of the polyethylene glycol to be between 200-8000 and the molecular weight of the polyvinylpyrrolidone to be between 10000-2000000. Because the molecular weight of the polyethylene glycol is obviously less than that of the polyvinylpyrrolidone, when the phase separation is carried out and the non-solvent diffuses to the coagulation bath, the exchange speed of the polyethylene glycol is far lower than that of the polyvinylpyrrolidone, namely, the polyethylene glycol firstly enters the coagulation bath, and the polyvinylpyrrolidone slowly enters the coagulation bath, and micropores formed in such a way have better connectivity in the radial direction of the radial section of the fiber monomer. If only a single porogen is disposed in the casting solution, the pore connectivity is greatly reduced, resulting in a very slow flow rate. In the process of the present invention, the highly volatile solvent is tetrahydrofuran or acetone; the conventional solvent is any one of N, N '-dimethylformamide, N' -dimethylacetamide and N-methylpyrrolidone.

S2: coating the casting solution on a substrate for preforming, (in this embodiment, the substrate is selected as a non-woven fabric substrate), and placing the preformed product in an air section for evaporation-induced phase inversion to form a primary film. When the initial finished film coated with the casting film liquid is preformed in an air section, the high-volatility solvent (tetrahydrofuran or acetone) in the casting film liquid is volatilized until the non-volatilized high-volatility solvent in the casting film liquid accounts for 20-90% of the total amount of the original high-volatility solvent. Since the detection of the high volatile solvent content from the casting solution is troublesome in the whole production process, the concentration of the volatile substances detected in the air is detected by a detection instrument to correspond to the high volatile solvent content interval in the casting solution. I.e., specifically when the concentration of volatile substances in the air is detected to be at most 6000ppm, the whole step of S2 is ended. If the concentration of volatile substances in the air is more than 6000ppm, the surface of the membrane begins to shrink, so that the normal shape of the membrane is influenced, and the normal production and the application of actual products are not facilitated. Of course, in the course of the specific implementation of the process, it is also possible to end the entire step S2 in the case where volatile substance concentrations of 5400ppm, 4500ppm, etc. in the air are detected to be less than 6000 ppm. The critical point is more visually expressed by observing the state of the film surface of the primary product until the film surface of the primary product slightly changes to be turbid. In order to ensure that the concentration of volatile substances in the air is within 6000ppm, a certain limitation is carried out on an air section in a preforming process, the humidity of the air section is limited to be more than 40%, the temperature is between 25 and 30 ℃, the length of the air section is set to be between 1 and 50cm, the residence time of a primary finished product film in the primary finished product film is 5 to 210 seconds, and the production under the condition that the concentration of high-volatility solvents in the air section is less than 6000ppm can be realized correspondingly, namely, the non-volatile high-volatility solvents in the corresponding casting film liquid account for 20 to 90 percent of the total amount of the original high-volatility solvents.

The second table below shows the hydrophilic PVC hollow fiber membrane prepared by the method, which uses a casting solution composed of polyvinyl chloride with a polymerization degree of 1200 and a weight ratio of 10%, polyethylene glycol with a molecular weight of 400 and a weight ratio of 5%, polyvinylpyrrolidone with a molecular weight of 100000 and a weight ratio of 5%, tetrahydrofuran with a weight ratio of 10%, and N, N' -dimethylformamide with a weight ratio of 70%; fully and uniformly stirring at 30 ℃, defoaming, standing and cooling to room temperature; and (3) under the condition that the casting solution is immersed into a coagulating bath consisting of 70 wt% of N, N' -dimethylformamide and 30 wt% of water for solidification after pre-phase separation, the high-volatility solvent which is not volatilized in the casting solution respectively accounts for 40%, 50%, 60%, 70%, 80% and 90% of the total amount of the original high-volatility solvent.

Table two:

meanwhile, the higher the concentration of the high-volatility solvent volatilized in the air is, the larger the volatilized amount is, and the pore diameter of the pores on the primary finished product membrane can be larger in the volatilizing process. Of course, this is only a factor affecting the pore size of the final hydrophilic PVC flat membrane, and the pore size of the final hydrophilic PVC flat membrane is formed by combining multiple aspects. In specific operation process, in order to improve the speed of preforming, improve holistic production efficiency, can use the equipment of blowing to blow to the primary product membrane in the air section of this step to increase the flow velocity of air, do benefit to the volatilization of high volatility solvent, thereby reduce production time, improve production efficiency. The specific blowing related settings including wind speed, wind direction and blowing position are obtained through a large number of experiments, and the lower table III is the specific experimental condition of the influence of different blowing settings on the preforming time when micropores with the aperture of 0.2 mu m are formed under the condition that the air sections have the same blowing distance when the method of the invention is used for manufacturing the hydrophilic PVC flat membrane. Because the primary finished membrane is a flat membrane, namely a plane, a planar blowing mode is selected for blowing. If the air blow is directly directed to the surface of the primary product film, the microporous structure of the surface of the primary product film is greatly damaged under the condition of high wind speed, so that a planar air blow parallel to the surface of the primary product film is selected. The blowing angle in the third table is the included angle between the blowing direction and the casting direction of the primary finished film. The position of the blowing also has an influence on the preforming time, i.e. the distance between the blown-off sheet wind and the film as a starting product. If the distance is too far, the efficiency is not obviously improved, and if the distance is too close, the air blowing can damage the original porous structure on the surface of the primary product film. Therefore, we further perform related experiments to obtain the optimal blowing distance. The fourth table is the specific experimental data of the influence of different blowing distances in the air section on the pre-phase separation time when micropores with the pore diameter of 0.2 mu m are formed under the condition that the relative wind speeds are 2m/s, 4m/s, 6m/s, 8m/s and the blowing angles are all 180 degrees when the hydrophilic PVC flat sheet membrane is manufactured by using the method of the invention.

Table three:

table four:

through different groups of experiments in the third table, we find that the main factor influencing the preforming time is the wind speed, the larger the wind speed, the faster the air flow speed, and the higher the volatilization efficiency, so the time for reaching the same volatile substance concentration in the air is shorter, but if the wind speed exceeds 8m/s, the shape of the primary product film is easily influenced, the microporous structure on the surface of the primary product film is damaged, and a defective product film is easily formed. Under the same wind speed, different blowing angles have a certain slight influence on the overall pre-phase separation time, and when the blowing direction is opposite to the casting direction of the primary product film, namely the blowing angle in the upper table is 180 degrees, the pre-phase separation efficiency is highest. Through different experimental groups in the fourth table, we find that the blowing distance and the pre-phase separation time are also closely related, and under the same condition, the closer the blowing distance is, the shorter the pre-phase separation time is, and the higher the production efficiency is, but considering the actual situation of equipment installation in production, the blowing distance is preferably selected to be between 4cm and 6 cm.

S3: and (4) carrying out phase inversion induced by a non-solvent and curing simultaneously, and immersing the primary product film formed in the step S2 into a coagulating bath at a constant speed for curing. The coagulating bath in this step comprises water in a weight ratio of between 0% and 30% and a solvent in a weight ratio of between 70% and 100%, wherein the solvent is any mixture of three of N, N '-dimethylformamide, N' -dimethylacetamide and N-methylpyrrolidone or one of the same as a conventional solution.

S4: and (3) cleaning, namely cleaning and drying the cured finished membrane, more specifically, immersing the finished hydrophilic PVC flat membrane in hot water at the temperature of between 50 and 80 ℃, cleaning for 10 to 30 minutes, and drying again.

In order to prove that the pores on the membrane have better connectivity by simultaneously adding two hydrophilic porogenic agents, namely the polyethylene glycol and the polyvinylpyrrolidone, in the scheme of the invention, the following experiments are carried out under the condition that other steps are the same. Experimental group 1 was selected: the hydrophilic pore-foaming agent is polyethylene glycol and polyvinylpyrrolidone which are added simultaneously, and the method is used for preparing the hydrophilic PVC hollow fiber membrane; experimental group 2: only polyethylene glycol is added into the hydrophilic pore-foaming agent, and the method is used for preparing the hydrophilic PVC hollow fiber membrane; experimental group 3: only polyvinylpyrrolidone is added into the hydrophilic pore-foaming agent, and the method is used for preparing the hydrophilic PVC hollow fiber membrane; control group 1: the hydrophilic pore-foaming agent is only added with polyethylene glycol, and the method is not used for preparing the hydrophilic PVC hollow fiber membrane; control group 2: the hydrophilic pore-foaming agent is only added with polyvinylpyrrolidone, and the method is not used for preparing the hydrophilic PVC hollow fiber membrane; control group 3: the hydrophilic pore-forming agent is selected from other materials such as hydrophilic nano titanium dioxide, and the method is used for preparing the hydrophilic PVC hollow fiber membrane. Six different hydrophilic PVC flat sheet membranes are respectively prepared through an experimental group 1, an experimental group 2, an experimental group 3, a control group 1, a control group 2 and a control group 3, and the relevant performances of the membranes are tested, and the specific test results are shown in the following table five.

Table five:

specific experimental data are compared to find that the flux of the hydrophilic PVC flat membrane prepared in the experimental group 1 is obviously greater than that of the rest experimental groups and the control group; the breaking strength of the hydrophilic PVC flat membrane prepared by the experimental group 1 is obviously greater than that of the rest experimental groups and the control group; meanwhile, the hydrophilic PVC flat membrane prepared by the experimental group 1 has the smallest water static contact angle. Therefore, the fact that polyethylene glycol and polyvinylpyrrolidone are added simultaneously as hydrophilic pore-forming agents is proved, compared with a scheme of only adding one of the hydrophilic pore-forming agents or adding the rest of the hydrophilic pore-forming agents, the finished membrane obviously has better permeability, hydrophilicity and higher strength.

In step S2, the preforming step is continued up to 6000ppm of volatile substances detected in the air, and the whole step S2 is ended. This critical value is the conclusion that after a large number of experiments, different experimental groups were set up and the method of the present invention was used to prepare hydrophilic PVC flat sheet membranes. The difference between the different experimental groups was only in the duration of the preforming in step S2, i.e. the content of volatile substances in the casting solution during the preforming, more specifically the concentration of volatile substances in the air, specifically the concentration of volatile substances in the air measured at a distance of 50cm from the membrane surface; of course, the concentration of the volatile substance in the closed space above the membrane body with the height of 0.5 meter, the width of 0.5 meter and the length of 3 meters can be detected. The detailed relevant experimental data and parameters are referenced in table six. In the experimental group a, volatile substances in the air are close to 0ppm, namely the primary finished product film is basically not volatilized in the air section and then enters the step S3; experimental group b is 1500ppm of volatile substances in the air; the experimental group c shows that the volatile substance in the air is 3000 ppm; experimental group d was 4500ppm of volatile substances in air; the experimental group e is 6000ppm of volatile substances in the air; experimental group f is 7500ppm of volatile matter in air.

Table six:

according to the data in the table, the actual use condition is comprehensively considered, and 6000ppm is finally selected as a critical value, if the value is exceeded, the aperture of the surface of the membrane cannot be expanded continuously, the membrane can be contracted, the thickness of the membrane is increased, the normal form of the membrane is influenced, and the membrane has no strong practicability in specific application, or even has no actual application prospect.

While the preferred embodiments of the present invention have been illustrated and described in detail, it should be understood that various changes and modifications of the invention can be effected therein by those skilled in the art after reading the above teachings of the invention. Such equivalents are intended to fall within the scope of the claims appended hereto.

Claims

1. A method for preparing a hydrophilic PVC flat sheet membrane, characterized in that the method comprises:

s1: preparing a membrane casting solution, uniformly mixing 8-15 wt% of polyvinyl chloride, 5-15 wt% of a hydrophilic pore-forming agent, 5-10 wt% of a high-volatility solvent and 65-82 wt% of a conventional solvent, defoaming, standing and cooling to room temperature;

s2: coating the casting solution on a base layer for performing, placing a preformed product in an air section for evaporation induced phase transformation to form a primary finished film, wherein the performing is carried out until the non-volatile high-volatility solvent in the casting solution accounts for 20-90% of the total amount of the original high-volatility solvent;

s3: carrying out non-solvent induced phase inversion and simultaneous curing, and immersing the primary finished film formed in the step S2 into a coagulating bath at a constant speed for curing;

s4: cleaning and drying;

the polymerization degree of the polyvinyl chloride is set to be between 800-2000, the hydrophilic pore-foaming agent is polyethylene glycol and polyvinylpyrrolidone which are added simultaneously, and the conventional solvent is any one of N, N '-dimethylformamide, N' -dimethylacetamide and N-methylpyrrolidone;

the coagulating bath comprises 0-30 wt% of water and 70-100 wt% of solvent, wherein the solvent is any mixture of three of N, N '-dimethylformamide, N' -dimethylacetamide and N-methylpyrrolidone or one of the same as a conventional solution.

2. The method of hydrophilic PVC flat sheet membrane according to claim 1, wherein the preliminary phase separation is terminated at a volatile substance concentration of 6000ppm or less at a position 40cm to 60cm from the surface of the flat sheet membrane in the step of S2.

3. The method of hydrophilic PVC flat sheet membrane according to claim 1, wherein the humidity of the air section is set to be more than 40% in the step of S2, the temperature is set to be between 25 ℃ and 30 ℃, the length of the air section is set to be between 1cm and 50cm, and the residence time in the air section is set to be between 5S and 210S.

4. A method of hydrophilic PVC flat sheet membrane according to claim 1, 2 or 3, characterized in that the pre-formed as-manufactured membrane is blown in the air section.

5. The method of claim 4, wherein the air blowing is performed in parallel with the membrane, and the speed of the air relative to the membrane is set between 0m/s and 8 m/s.

6. The method as claimed in claim 1, wherein the molecular weight of the polyvinyl alcohol is set to 200-8000 and the molecular weight of the polyvinylpyrrolidone is set to 10000-2000000.

7. The method of hydrophilic PVC flat sheet membrane according to claim 1, wherein in the step of S4, the membrane is immersed in hot water at 50-80 ℃, washed for 10-30 minutes, and dried again.

8. A hydrophilic PVC flat sheet membrane prepared by the preparation method according to any one of claims 1 to 7, comprising a base layer and a filter layer having micro-pores, wherein: the pore diameter of the micropores is set to be between 0.1 and 20 microns, the porosity is larger than 80 percent, the cross section of the membrane is in a spongy asymmetric structure, and the pore diameter of the micropores tends to gradually decrease/increase towards one side of the base layer in the cross section direction of the membrane.

9. The hydrophilic PVC flat sheet membrane according to claim 1, wherein the flat sheet membrane thickness is set to be between 50 μm and 500 μm.

10. The hydrophilic PVC flat sheet membrane according to claim 1, wherein: the base layer is a non-woven fabric layer.