CN100544809C - A method for hydrophilic modification of polyvinylidene fluoride membrane - Google Patents

Abstract

The invention relates to a method for hydrophilic modification of a polyvinylidene fluoride membrane. The process is as follows: 1. Prepare a casting liquid: prepare a blend liquid according to the following mass concentration formula: polyvinylidene fluoride 10-18%; polyvinyl alcohol 2-10%; dimethyl sulfoxide 80%, put the blended solution in a 98°C water bath, stir for 6 hours, and then let it stand for defoaming to obtain a uniform casting solution; 2. Prepare the membrane: the After the casting liquid is scraped into a flat film or wet-spun hollow fiber membrane at room temperature, it is immediately immersed in a coagulation bath, and kept for 24 hours to solidify and form; the coagulation bath is water, an ethanol aqueous solution with a mass concentration of 20%, or 5mol/ One of L sodium chloride aqueous solutions; after the formed film is rinsed with water, soaked in 60% glycerin aqueous solution for 12 hours, taken out and dried naturally in the air, it can be made into polyvinylidene fluoride with good hydrophilicity Vinyl flat sheet membrane or hollow fiber membrane.

Description

A method for hydrophilic modification of polyvinylidene fluoride membrane

technical field

The invention relates to a modification technology of membrane products, specifically a method aimed at improving the hydrophilicity of polyvinylidene fluoride membranes, and the international patent classification number is proposed to be Int.Cl.C08J7/12(2006.01)I.

Background technique

Polyvinylidene fluoride is a semi-crystalline polymer. Because of its short C-F bond length and strong bond energy (486kJ/mol), it is heat-resistant, corrosion-resistant, radiation-resistant, and has high strength and good toughness. It is the preferred material for preparing membranes. However, its strong hydrophobicity will lead to two problems: one is that the separation process requires a large driving force; the other is that it is easy to generate adsorption pollution, which reduces the two main separation indicators of membrane flux and rejection rate, and shortens the service life of the membrane. This restricts its application in aqueous systems such as biochemical pharmacy, food and beverage, and water purification. Therefore, the hydrophilic modification of polyvinylidene fluoride membrane has important practical significance.

When preparing hydrophilic polymer materials, the following three methods can be used: (1). Small molecules with hydrophilic groups are polymerized to obtain hydrophilic polymers; (2). The polymer skeleton is hydrophilized, so that the polymer chain is suspended with hydrophilic groups; (3). Physical modification of ordinary polymer materials to make them hydrophilic. The more commonly used hydrophilic modification methods include surface modification, blending, grafting, copolymerization and crosslinking. Comparing the above three methods, the first one is the most difficult to realize, because it has to go through complex organic synthesis reactions and polymerization reactions of monomers to gradually obtain the final product, during which the protection and deprotection of functional groups may be required; the second one Although the method is easier to implement than the first method, there are many technical problems that are difficult to solve in the hydrophilization reaction of polymers, and the functional materials obtained by this method have uneven distribution of functional groups and do not meet the theoretical requirements. content; and the third method is easier to realize than the first two methods both in research and in industrial application.

Many studies have shown that blending membranes is a simple method to improve membrane performance based on existing materials. Blending a polymer with other polymers can comprehensively balance the performance of each polymer component, learn from each other’s strengths, eliminate the weakness of each single polymer component’s performance, and, through blending, the processing performance of the polymer can also be improved , for some engineering plastics with excellent performance but expensive, the cost of raw materials can be reduced by blending without affecting the use requirements. At present, the commonly used blending system for polyvinylidene fluoride to prepare membrane materials is polyvinylidene fluoride/polymethyl methacrylate (see S.P.Nunes, etc., PVDF/PMMA blended ultrafiltration membrane, Membrane Science Journal. 73 (1992) 25), polyvinylidene fluoride/polyacrylonitrile (see Yu Zhihui et al., Research on polyvinylidene fluoride/polyacrylonitrile blend ultrafiltration membrane, Membrane Science and Technology. 20 (2000) 10), polyvinylidene fluoride /Sulfonated polysulfone (see Sun Liqing et al., Polyvinylidene fluoride as polysulfone blend compatibility and ultrafiltration membrane research, Membrane Science and Technology. 21 (2001) 1) and polyvinylidene fluoride/polyvinyl acetate (see F.M.Ksei etc., the method for preparing porous film.US Patent4,772,440,1998) etc. The above technical literature only discusses the effect on membrane performance from the perspective of hydrophilicity, while ignoring the influence of interface pore structure on membrane performance.

Polyvinyl alcohol is a good hydrophilic polymer. It is used as a blending component to improve the interfacial pore structure of polyvinylidene fluoride membranes and combined with hydrophilic modification. There has been no report so far. .

Contents of the invention

Aiming at the deficiencies of the prior art, the technical problem to be solved by the present invention is to provide a method for hydrophilic modification of polyvinylidene fluoride membrane, which adopts wet phase inversion method to prepare polyvinylidene fluoride/polyvinyl alcohol The meta-blend membrane can effectively improve the hydrophilicity of the polyvinylidene fluoride membrane, and at the same time, the pore structure of the membrane is enriched by using the phase separation principle of the polymer blend interface, and it has a simple process, no special requirements for equipment conditions, and a relatively low cost. Low, easy for industrial applications.

The technical solution of the present invention to solve the technical problem is: design a method for hydrophilic modification of polyvinylidene fluoride membrane, and its process is as follows:

1. Prepare the casting solution: prepare the blend solution according to the following mass concentration formula:

Polyvinylidene fluoride 10-18%;

Polyvinyl alcohol 2-10%

Dimethyl sulfoxide 80%,

The blended solution was placed in a water bath at 98°C, stirred for 6 hours, and left to defoam to obtain a uniform casting solution;

2. Preparation of membrane: after scraping the flat membrane or wet-spun hollow fiber membrane with the casting solution at room temperature, immediately immerse in the coagulation bath, and keep for 24h to solidify and form; the coagulation bath is water, and the mass concentration is One of 20% ethanol aqueous solution or 5mol/L sodium chloride aqueous solution; the formed film is rinsed with water, then soaked in 60% glycerin aqueous solution for 12 hours, taken out and dried naturally in the air, and can be made Polyvinylidene fluoride flat membrane or hollow fiber membrane with good hydrophilicity.

Compared with the prior art, the method of the present invention combines the hydrophilic modification with the interface pore structure for the first time, and studies its influence on the hydrophilic performance of the membrane, which can not only effectively improve the hydrophilicity of the polyvinylidene fluoride membrane, but also utilize The principle of interfacial phase separation enriches the pore structure of the membrane, and the process is simple, there is no special requirement for equipment conditions, the cost is low, and it is convenient for industrial application.

Detailed ways

Below in conjunction with specific embodiment, the present invention is further elaborated:

The present invention designs a kind of method (hereinafter referred to as method) of polyvinylidene fluoride film hydrophilic modification, and its technique is as follows:

1. Prepare the casting solution: prepare the blend solution according to the following mass concentration formula:

Polyvinylidene fluoride 10-18%;

Polyvinyl alcohol 2-10%;

Dimethyl sulfoxide 80%,

The blended solution was placed in a water bath at 98°C, stirred for 6 hours, and left to defoam to obtain a uniform casting solution;

2. Membrane preparation: After scraping the flat membrane or wet-spun hollow fiber at room temperature, the casting solution was immediately immersed in a coagulation bath, and kept for 24 hours to solidify and form; the coagulation bath was water with a mass concentration of 20 % ethanol aqueous solution or 5mol/L sodium chloride aqueous solution; after the formed film is rinsed with water, soaked in 60% glycerin aqueous solution for 12 hours, taken out and dried naturally in the air, it can be made into a pro- Polyvinylidene fluoride flat membrane or hollow fiber membrane with good water resistance.

The base membrane material or matrix phase of the membrane described in the method of the present invention is polyvinylidene fluoride.

The polyvinylidene fluoride membrane of the present invention includes a flat sheet membrane and a hollow fiber membrane. A key technology of its hydrophilic modification method is to reasonably select a hydrophilic substance to blend with it. The basis for its selection is: ① it is non-thermodynamically compatible or partially compatible with the matrix phase polyvinylidene fluoride; ② after blending with polyvinylidene fluoride, the casting solution has good film or fiber forming and processing properties; No physical or chemical damage to the membrane. Accordingly, the blending component of the film of the present invention selects polyvinyl alcohol, a hydrophilic substance that is partially compatible or incompatible with polyvinylidene fluoride. Tests show that in the formula of the present invention, within a certain range, with the increase of polyvinyl alcohol content, the water flux of the resulting blend membrane first increases and then decreases, and the ovalbumin retention rate first decreases and then increases . Studies have shown that although the mass concentration ratio of polyvinyl alcohol in the blend can be 2-10%, it is preferably 3-5%, and the effect is ideal.

In order to further increase the flux of the membrane, or to meet the requirements of different applications, a further feature of the method of the present invention is that a third component or additive can be added to the blend solution formulation. The basis for the selection of the third component of the present invention is: 1. After preparing and blending the casting film liquid with the polyvinylidene fluoride/polyvinyl alcohol, it should have good film or fiber forming and processing performance; 2. After film formation It can be removed to make the membrane form a porous structure; ③There is no physical or chemical damage to the final formed membrane. The third component or additive in the blend formulation of the present invention is polyethylene glycol 600. After adding the third component, the mass concentration formula of the blended liquid is adjusted to:

Polyvinylidene fluoride 10-18%;

Polyvinyl alcohol 2-10%;

Polyethylene glycol 600 3-15%

Dimethyl sulfoxide 65-77%,

Although the mass concentration ratio of the additives in the blend solution is 3-15%, it is generally preferably 3-10%. Excessively high additive mass concentration ratios will affect the viscosity of the casting solution.

It should be noted that although the additives described in the present invention can be added to increase the water flux of the obtained membrane, they are not necessary. The present invention can completely solve the technical problem to be solved by the present invention without adding any other components during the film making process.

The method of the invention investigates the effects of polyvinylidene fluoride/polyvinyl alcohol blending ratio, solid content, additive concentration, coagulation conditions, post-treatment and other process conditions or parameters on the hydrophilic performance of the membrane. Studies have shown that when the mass concentration formula of the blend solution is designed as follows, the overall performance of the prepared membrane is relatively ideal: the mass ratio of polyvinylidene fluoride/polyvinyl alcohol is 80/20, the mass concentration ratio is 20%, polyvinylidene fluoride The mass concentration ratio of ethylene glycol 600 is 6%, the mass concentration ratio of dimethyl sulfoxide is 74%, and the coagulation bath is water at 20°C. The study also showed that the membrane was treated with water at 40°C for 10 minutes, and the rejection rate was significantly improved, while the water flux was significantly increased when the membrane was treated with alcohol for 180 minutes, but the rejection rate was basically unchanged.

The method of the present invention is further characterized in that a glass plate is used as a support in the process of preparing or scraping the flat film. Due to the strong interaction between polyvinyl alcohol and glass, the upper and lower surfaces of the formed flat film have hydrophilic differences. Experiments show that the hydrophilicity of the lower surface of the blended flat membrane prepared by this method is better than that of the upper surface.

The method of the present invention adopts the wet phase inversion method to prepare the polyvinylidene fluoride/polyvinyl alcohol binary blend film. The blending of polyvinylidene fluoride and polyvinyl alcohol can balance the performance of the two, learn from each other, and eliminate the polyvinylidene fluoride affinity. The weak point of poor water, and reduce the cost of raw materials without affecting the use requirements; at the same time, polyvinylidene fluoride/polyvinyl alcohol is used as an incompatible system, and interfacial phase separation occurs after blending to form interfacial pores. The method of the invention combines the hydrophilic modification of the membrane with the interface pore structure for the first time to study its influence on the performance of the membrane, and has the characteristics of simple process, no special requirements for equipment conditions, and convenient industrial application.

Specific examples of the present invention are given below, but the specific examples do not limit the present invention.

Example 1:

Mix polyvinylidene fluoride/polyvinyl alcohol with a mass concentration ratio of 20% and dimethyl sulfoxide with a mass concentration ratio of 80%; place the above mixture in a water bath at 98°C, stir for 6 hours, and let it stand Degassing to obtain a uniform casting solution; after scraping the casting solution with a glass plate at room temperature to make a flat film, immediately immerse it in 20°C and 20% alcohol aqueous solution to solidify and form; after the film falls off by itself, take out the glass plate , the film was kept in the coagulation bath for 24 hours; after the formed flat film was rinsed with water, soaked in a 60% glycerin aqueous solution for 12 hours, it was taken out and dried naturally in the air to make PVDF with good hydrophilicity. flat film.

The mass ratios of polyvinylidene fluoride/polyvinyl alcohol described in the above method are respectively 100/0, 90/10, 80/20, 70/30, 50/50, 0/100, and the scraped flat film is heated at 20°C , 0.1Mpa, the water flux and the retention rate of ovalbumin were measured respectively, and the corresponding values of the obtained water flux (mL·cm ^-2 ·h ^-1 ) were 0.61, 1.88, 4.13, 2.62, 0.22, 0.10 ; The corresponding values for ovalbumin retention (%) were 83.8, 72.6, 63.3, 70.74, 87.1, 91.2 (%) respectively. The results show that when the mass ratio of polyvinylidene fluoride/polyvinyl alcohol flat film is 80/20, the overall performance of the film is better.

The anti-pollution properties of polyvinylidene fluoride flat film and polyvinylidene fluoride/polyvinyl alcohol flat film with a mass ratio of 80/20 to ovalbumin were further measured, and the resistance increase coefficients (%) were 42.42 and 22.78 respectively. . The results show that the anti-pollution property of the modified polyvinylidene fluoride is obviously improved.

Example 2:

Prepare the casting solution according to the method described in Example 1; after the casting solution is scraped with a glass plate at room temperature to make a flat film, place it in the air at 20°C to solidify and form, and use the solvent evaporation method to make the film, which can avoid the surface micropore docking The influence of tentacles; after forming, immerse in clean water, and after the film falls off by itself, take out the glass plate, and keep the film in clean water for 24 hours; take it out and dry it naturally in the air. The contact angles of the upper surface and the lower surface of the flat film made by measuring the mass ratio of polyvinylidene fluoride/polyvinyl alcohol as 100/0, 90/10, 80/20, 70/30, 50/50, and 0/100 were respectively determined. The corresponding values of contact angles (°) on the upper surface of the film were 98.2, 96.8, 92.1, 71.2, 67.3, 38.7; The results show that when the mass ratio of polyvinylidene fluoride/polyvinyl alcohol is 80/20, the contact angle is smaller and the hydrophilicity is better, and there is a difference in the contact angle between the upper and lower surfaces of the film, so there is delamination.

Example 3:

Mix polyvinylidene fluoride/polyvinyl alcohol (mass ratio 80/20) with a mass concentration ratio of 20% and dimethyl sulfoxide with a mass concentration of 80%; mix and defoam the above mixture according to the method in Example 1 Immediately after scraping the film, immerse in the following coagulation bath to solidify and form; after the film falls off automatically, take out the glass plate, and keep the film in the coagulation bath for 24 hours; the formed film is made into a dry film according to the method in Example 1. The coagulation baths are respectively 20°C water, 20°C, 20% alcohol aqueous solution and 20°C, 5mol/L NaCl aqueous solution. Measure the water flux and ovalbumin rejection of the flat membranes prepared in three coagulation baths at 20°C and 0.1Mpa, and the corresponding values of water flux (mL·cm ^-2 ·h ^-1 ) can be obtained, respectively. 2.67, 4.13, 2.45; the corresponding values for ovalbumin retention (%) were 80.0, 63.3, 81.0 (%) respectively. The results show that the comprehensive performance of the membrane is better when the water at 20℃ is selected as the coagulation bath.

Example 4:

The temperature (°C) of the coagulation bath water is designed to be 0, 20, 40 and 60, and the remainder is the same as in Example 3, and the prepared flat membrane measures its water flux and the ovalbumin retention rate respectively at 20°C and 0.1Mpa, The corresponding values of water flux (mL·cm ^-2 ·h ^-1 ) were 0.63, 2.67, 3.54 and 8.75; the corresponding values of ovalbumin retention (%) were 87.8, 80.0, 50.9 and 22.1. The results show that the comprehensive performance of the membrane is better when the water at 20℃ is selected as the coagulation bath.

Example 5:

After the film was formed according to the method of Example 3, the film was heat-treated at the following temperature for 10 minutes. The temperature (°C) of the heat treatment is 20, 40, 50, 60 and 80 respectively, and the water flux and the retention rate of ovalbumin are respectively measured at 20°C and 0.1Mpa for the prepared flat membrane at different temperatures, The corresponding values of water flux (mL·cm ^-2 ·h ^-1 ) were 2.67, 1.98, 1.91, 1.87 and 3.27; 65.2. The results showed that the heat treatment could make the membrane structure denser, and the rejection rate of the membrane increased significantly after the membrane was treated with water at 40°C for 10 minutes, while the water treatment at 80°C made the membrane structure unstable.

Embodiment 6:

After the film was prepared according to the method described in Example 3, the film was treated with alcohol at room temperature. The alcohol treatment time (min) was 0, 30, 60, 120, 180 and 300, and the flat membranes obtained at different treatment times were measured at 20°C and 0.1Mpa to measure their water flux and ovalbumin retention rate respectively, and the water flux was obtained. The corresponding values of volume (mL·cm ^-2 ·h ^-1 ) were 2.67, 2.93, 3.00, 3.27, 3.28 and 3.30; the corresponding values of ovalbumin retention (%) were 80.0, 80.2, 80.1, 80.9, 81.2 and 80.3. The results showed that alcohol treatment could significantly increase the water flux, but did not affect the retention rate. When the alcohol treatment time was 180 minutes, the water flux increased significantly.

Embodiment 7:

The mass concentration ratio is 15,20 and 25 (%) polyvinylidene fluoride/polyvinyl alcohol (mass ratio 80/20), the mass concentration ratio is 6% polyethylene glycol 600, and the mass concentration ratio Mix for 79, 74 and 69 (%) dimethyl sulfoxide; mix the above mixture according to the method in Example 1, defoam and make a dry film. Measure the water flux and the retention rate of ovalbumin respectively at 20°C and 0.1Mpa with flat membranes with polyvinylidene fluoride/polyvinyl alcohol mass concentrations of 15, 20, and 25 (%) to obtain the water flux (mL The corresponding values of ·cm ^-2 ·h ^-1 ) were 7.09, 4.57, 0.76; the corresponding values of ovalbumin retention (%) were 30.2, 68.5, 79.7. The results show that when the mass ratio of polyvinylidene fluoride/polyvinyl alcohol is 80/20 and the mass concentration ratio is 20%, the overall performance of the membrane is better.

Embodiment 8:

The mass concentration ratio is 20% polyvinylidene fluoride/polyvinyl alcohol (mass ratio 80/20), the mass concentration ratio is 0,3,6,10,15(%) polyethylene glycol 600, and Mix dimethyl sulfoxide with a mass concentration ratio of 80, 77, 74, 70 and 65 (%); mix the above mixture according to the method in Example 1, defoam and make a dry film. The polyethylene glycol 600 mass concentration ratio is 0, 3, 6, 10, 15 (%) of the flat membrane at 20 ℃, 0.1Mpa to measure its water flux and the retention rate to ovalbumin respectively, to obtain the water flux The corresponding values of volume (mL·cm ^-2 ·h ^-1 ) were 2.67, 2.93, 4.57, 3.68, 1.27; the corresponding values of ovalbumin retention (%) were 80.0, 78.2, 68.5, 73.77, 83.11. The results show that when the mass concentration ratio of the third component polyethylene glycol 600 is 6%, the overall performance of the membrane is better.

Embodiment 9:

The mass concentration ratio is 20% polyvinylidene fluoride/polyvinyl alcohol, the mass concentration ratio is 6% polyvinyl alcohol 600, and the mass concentration ratio is 74% dimethyl sulfoxide; The above mixture was placed in a 98°C water bath and stirred for 12 hours; the casting solution was added to a wet spinning autoclave, and after vacuum defoaming, the hollow fiber was wet spun. The spinning temperature was controlled at 78°C, and the coagulation bath was 20°C water. The hollow fiber membranes prepared with polyvinylidene fluoride/polyvinyl alcohol mass ratios of 100/0, 90/10, 80/20, and 70/30 were measured at 20°C and 0.1Mpa to measure their water flux and Ovalbumin retention rate, the corresponding values of water flux (mL·cm ^-2 ·h ^-1 ) were 2.78, 3.67, 12.76, 7.78; the corresponding values of ovalbumin retention rate (%) were 80.4, 71.3, 54.2, 55.8. The results show that when the mass ratio of polyvinylidene fluoride/polyvinyl alcohol is 80/20 and the polymer mass concentration ratio is 20%, the overall performance of the obtained hollow fiber membrane is better.

Claims (7)

1. polyvinylidene fluoride film hydrophilically modified method, its technology is as follows:

(1). preparation casting solution: press following mass concentration formulation blended liquid:

Kynoar 10-18%;

Polyvinyl alcohol 2-10%;

Dimethyl sulfoxide (DMSO) 80%,

Above-mentioned blended liquid is placed 98 ℃ of water-baths, stir after 6 hours, standing and defoaming promptly obtains even casting solution;

(2). preparation film: described casting solution behind striking flat sheet membrane or wet method spinning hollow-fibre membrane under the room temperature, is immersed in the coagulating bath immediately, keep the 24h solidified forming; Described coagulating bath is that water, mass concentration are a kind of in 20% ethanol water or the 5mol/L sodium-chloride water solution; After the film that is shaped washed with clear water, being immersed in mass concentration again was 12h in 60% glycerine water solution, takes out air dry in air, promptly can be made into hydrophily good polyvinylidene fluoride flat film or hollow-fibre membrane.

2. polyvinylidene fluoride film hydrophilically modified method, its technology is as follows:

(1). preparation casting solution: press following mass concentration formulation blended liquid:

Kynoar 10-18%;

Polyvinyl alcohol 2-10%;

Macrogol 600 3-15%;

Dimethyl sulfoxide (DMSO) 65-77%,

(2). preparation film: described casting solution behind striking flat sheet membrane or wet method spinning hollow-fibre membrane under the room temperature, is immersed in the coagulating bath immediately, keep the 24h solidified forming; Described coagulating bath is that water, mass concentration are a kind of in 20% ethanol water or the 5mol/L sodium-chloride water solution; After the film that is shaped washed with clear water, being immersed in mass concentration again was 12h in 60% glycerine water solution, takes out air dry in air, promptly can be made into hydrophily good polyvinylidene fluoride flat film or hollow-fibre membrane.

3. polyvinylidene fluoride film hydrophilically modified method according to claim 2 is characterized in that the Macrogol 600 mass concentration proportioning in the described blended liquid is 3-10%.

4. polyvinylidene fluoride film hydrophilically modified method according to claim 2, the mass concentration prescription that it is characterized in that described blended liquid is: the concentration proportioning of Kynoar and polyvinyl alcohol quality is 20%, wherein the mass ratio of Kynoar/polyvinyl alcohol is 80/20, Macrogol 600 mass concentration proportioning is 6%, and dimethyl sulfoxide (DMSO) mass concentration proportioning is 74%; Described coagulating bath is 20 ℃ a water.

5. according to each described polyvinylidene fluoride film hydrophilically modified method among the claim 1-4, it is characterized in that prepared polyvinylidene fluoride flat film or hollow-fibre membrane with 40 ℃ water treatment 10min, or handle 180min with alcohol.

6. according to each described polyvinylidene fluoride film hydrophilically modified method among the claim 1-4, it is characterized in that in striking flat sheet membrane process, the employing glass plate is a supporter.

7. polyvinylidene fluoride film hydrophilically modified method according to claim 5 is characterized in that in striking flat sheet membrane process, the employing glass plate is a supporter.