CN113648844A - Polyvinylidene fluoride hollow fiber membrane with reticular pore structure and preparation method thereof - Google Patents
Polyvinylidene fluoride hollow fiber membrane with reticular pore structure and preparation method thereof Download PDFInfo
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- 239000002033 PVDF binder Substances 0.000 title claims abstract description 107
- 229920002981 polyvinylidene fluoride Polymers 0.000 title claims abstract description 107
- 239000012510 hollow fiber Substances 0.000 title claims abstract description 59
- 239000011148 porous material Substances 0.000 title claims abstract description 58
- 238000002360 preparation method Methods 0.000 title claims abstract description 29
- 239000003085 diluting agent Substances 0.000 claims abstract description 47
- 238000005266 casting Methods 0.000 claims abstract description 42
- 238000005191 phase separation Methods 0.000 claims abstract description 30
- 238000001816 cooling Methods 0.000 claims abstract description 18
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- 239000000654 additive Substances 0.000 claims abstract description 11
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- 238000000034 method Methods 0.000 claims description 35
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- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Chemical compound O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 27
- DOIRQSBPFJWKBE-UHFFFAOYSA-N dibutyl phthalate Chemical compound CCCCOC(=O)C1=CC=CC=C1C(=O)OCCCC DOIRQSBPFJWKBE-UHFFFAOYSA-N 0.000 claims description 18
- 239000008367 deionised water Substances 0.000 claims description 15
- 229910021641 deionized water Inorganic materials 0.000 claims description 15
- QZCLKYGREBVARF-UHFFFAOYSA-N Acetyl tributyl citrate Chemical compound CCCCOC(=O)CC(C(=O)OCCCC)(OC(C)=O)CC(=O)OCCCC QZCLKYGREBVARF-UHFFFAOYSA-N 0.000 claims description 9
- 239000000203 mixture Substances 0.000 claims description 7
- YEJRWHAVMIAJKC-UHFFFAOYSA-N 4-Butyrolactone Chemical compound O=C1CCCO1 YEJRWHAVMIAJKC-UHFFFAOYSA-N 0.000 claims description 6
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims description 6
- 239000003795 chemical substances by application Substances 0.000 claims description 4
- PYGXAGIECVVIOZ-UHFFFAOYSA-N Dibutyl decanedioate Chemical compound CCCCOC(=O)CCCCCCCCC(=O)OCCCC PYGXAGIECVVIOZ-UHFFFAOYSA-N 0.000 claims description 3
- 239000000110 cooling liquid Substances 0.000 claims description 3
- 229910052757 nitrogen Inorganic materials 0.000 claims description 3
- RUOJZAUFBMNUDX-UHFFFAOYSA-N propylene carbonate Chemical compound CC1COC(=O)O1 RUOJZAUFBMNUDX-UHFFFAOYSA-N 0.000 claims description 3
- 239000002994 raw material Substances 0.000 claims description 2
- 229920000642 polymer Polymers 0.000 abstract description 29
- 239000012982 microporous membrane Substances 0.000 abstract description 6
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
- B01D67/0002—Organic membrane manufacture
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
- B01D67/0002—Organic membrane manufacture
- B01D67/0009—Organic membrane manufacture by phase separation, sol-gel transition, evaporation or solvent quenching
- B01D67/0011—Casting solutions therefor
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
- B01D69/02—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor characterised by their properties
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
- B01D69/08—Hollow fibre membranes
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
- B01D71/06—Organic material
- B01D71/30—Polyalkenyl halides
- B01D71/32—Polyalkenyl halides containing fluorine atoms
- B01D71/34—Polyvinylidene fluoride
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2325/00—Details relating to properties of membranes
- B01D2325/02—Details relating to pores or porosity of the membranes
- B01D2325/021—Pore shapes
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2325/00—Details relating to properties of membranes
- B01D2325/24—Mechanical properties, e.g. strength
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- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Engineering & Computer Science (AREA)
- Manufacturing & Machinery (AREA)
- Dispersion Chemistry (AREA)
- Separation Using Semi-Permeable Membranes (AREA)
- Artificial Filaments (AREA)
Abstract
The invention discloses a polyvinylidene fluoride hollow fiber membrane with a reticular pore structure and a preparation method thereof, wherein the preparation method comprises the steps of uniformly mixing common molecular weight polyvinylidene fluoride, an additive and a diluent under the heating condition to obtain a membrane casting solution; extruding the casting solution through a spinneret plate, cooling, winding, extracting, drying, and performing solid-liquid phase separation to obtain a polyvinylidene fluoride hollow fiber membrane with a mesh pore structure; wherein the additive is ultra-high molecular weight polyvinylidene fluoride. The invention mixes a small amount of polyvinylidene fluoride with ultrahigh molecular weight as an additive with polyvinylidene fluoride with common molecular weight, and obtains the microporous membrane with thin and porous skin layer and a through net-shaped pore structure as a supporting layer by using a single diluent at a temperature lower than the melting temperature of a polymer.
Description
Technical Field
The invention belongs to the technical field of separation membrane preparation, and particularly relates to a polyvinylidene fluoride hollow fiber membrane with a mesh pore structure and a preparation method thereof.
Background
Polyvinylidene fluoride (PVDF) is a crystalline polymer material with strong polarity, and is widely used as a separation membrane material in the fields of water treatment, membrane contactors, medical tissue engineering and the like due to its excellent properties in weather resistance, heat resistance, mechanical properties and the like. The non-solvent induced phase separation method and the thermal induced phase separation method are the most commonly used techniques for preparing porous films at present. Compared with a non-solvent induced phase separation technology, the thermally induced phase separation technology has the advantages of easy regulation and control of a pore structure, high membrane mechanical property, capability of dry-state storage of the membrane and the like, is particularly suitable for preparing a polymer porous membrane without a solvent at room temperature, and the polyvinylidene fluoride porous membrane for water treatment is prepared by adopting the method at present.
Thermally induced phase separation technology is a process proposed by Castro in the 80's of the 20 th century to produce microporous membranes by changing the temperature to cause phase separation (US Patent, 4247498,1981). The technology is that a polymer and a specific diluent form a homogeneous solution at high temperature, the diluent is a good solvent of the polymer at high temperature and is a non-solvent of the polymer at low temperature, when the temperature is reduced, the solution is subjected to solid-liquid (S-L) or liquid-liquid (L-L) phase separation, and after the diluent is extracted from a membrane, micropores are formed in the space occupied by the diluent in the system, so that the membrane material containing a micropore structure is obtained. The ideal structure of the separation membrane is that the skin layer is thin, and the supporting layer is a through mesh hole structure. When L-L phase separation occurs, a mesh-like pore structure is to be obtained, a binary mixed diluent system is usually required to be used, so that the interaction between a polymer and a mixed diluent is moderate, a bicontinuous polymer lean phase and a polymer rich phase are formed in the cooling process, and a through mesh-like pore structure is formed after the diluent is extracted, but the preparation conditions are very strict, for example, the mass ratio of the diluent to the non-diluent needs to be strictly measured, the temperature of a formed solution is high (220 ℃ and about 50 ℃ higher than the melting temperature of the polymer), the regulation difficulty is high, and the polymer is easily degraded at the temperature. The S-L phase separation has low requirement on temperature and is easy to operate, but the direct crystallization of the polymer in the S-L phase separation process can form a structure with thick and compact skin layer and spherical supporting layer, the compact thick skin layer increases the filtration resistance of the membrane, even the membrane has almost no flux and cannot meet the requirement of industrial application, but the single diluent found at present can only form an S-L phase separation system with PVDF.
The ultra-high molecular weight polyvinylidene fluoride has the characteristics of high molecular weight, long chain section, high viscosity, low molecular motion speed and the like, can influence the crystallization process of the polymer, and can be mixed with other polymers to be used in final applications and products. At present, a film preparation method for blending ultra-high molecular weight polyethylene and polyvinylidene fluoride (lina and the like, enhanced ultra-high molecular weight polyethylene and polyvinylidene fluoride binary blend films and preparation methods thereof, CN 106492645 a, 2016) is available, but solid-solid phase separation interface holes appear in the blend films in the cooling process due to incompatibility of two polymers and different cooling shrinkage rates, and the film hole structure, the film strength and the like are directly influenced.
Disclosure of Invention
This section is for the purpose of summarizing some aspects of embodiments of the invention and to briefly introduce some preferred embodiments. In this section, as well as in the abstract and the title of the invention of this application, simplifications or omissions may be made to avoid obscuring the purpose of the section, the abstract and the title, and such simplifications or omissions are not intended to limit the scope of the invention.
The present invention has been made keeping in mind the above and/or other problems occurring in the prior art.
One of the purposes of the invention is to provide a preparation method of a polyvinylidene fluoride hollow fiber membrane with a reticular pore structure, wherein a microporous membrane with a thin and porous skin layer and a through reticular pore structure as a supporting layer can be obtained by using a single diluent at a temperature lower than the melting temperature of a polymer.
The invention mixes a small amount of polyvinylidene fluoride with ultrahigh molecular weight as an additive with polyvinylidene fluoride with common molecular weight, and the PVDF hollow fiber membrane with thin skin layer and good surface openness and the reticular pore structure as a supporting layer is prepared by the system through S-L phase separation of a thermally induced phase separation method, thereby not only avoiding the thick skin layer and the spherical particle structure formed by the S-L phase separation membrane, but also avoiding the defects of harsh preparation conditions and high temperature when the L-L phase separation occurs, and simultaneously realizing the advantages of low requirement on the S-L phase separation temperature and the penetrating reticular pore structure formed by the L-L phase separation.
In order to achieve the above purpose of the present invention, the present invention provides the following technical solutions: a preparation method of a reticular pore structure polyvinylidene fluoride hollow fiber membrane is characterized by comprising the following steps: comprises the steps of (a) preparing a mixture of a plurality of raw materials,
uniformly mixing common molecular weight polyvinylidene fluoride, an additive and a diluent under a heating condition to obtain a casting solution;
extruding the casting solution through a spinneret plate, cooling, winding, extracting, drying, and performing solid-liquid phase separation to obtain a polyvinylidene fluoride hollow fiber membrane with a mesh pore structure;
wherein the additive is ultra-high molecular weight polyvinylidene fluoride.
As a preferable scheme of the preparation method of the reticular pore structure polyvinylidene fluoride hollow fiber membrane of the invention, wherein: the weight average molecular weight of the ultra-high molecular weight polyvinylidene fluoride is not less than 100 ten thousand, and the weight average molecular weight of the common molecular weight polyvinylidene fluoride is 10-70 ten thousand.
As a preferable scheme of the preparation method of the reticular pore structure polyvinylidene fluoride hollow fiber membrane of the invention, wherein: the casting film liquid comprises, by mass, 2-10 parts of ultra-high molecular weight polyvinylidene fluoride, 15-40 parts of common molecular weight polyvinylidene fluoride and 50-83 parts of a diluent.
As a preferable scheme of the preparation method of the reticular pore structure polyvinylidene fluoride hollow fiber membrane of the invention, wherein: the diluent comprises one or more of dibutyl phthalate, dibutyl sebacate, acetyl tributyl citrate, propylene carbonate and gamma-butyrolactone.
As a preferable scheme of the preparation method of the reticular pore structure polyvinylidene fluoride hollow fiber membrane of the invention, wherein: the polyvinylidene fluoride is uniformly mixed under the heating condition, and the heating temperature is lower than the melting temperature of the common molecular weight polyvinylidene fluoride.
As a preferable scheme of the preparation method of the reticular pore structure polyvinylidene fluoride hollow fiber membrane of the invention, wherein: the heating temperature is 150-170 ℃.
As a preferable scheme of the preparation method of the reticular pore structure polyvinylidene fluoride hollow fiber membrane of the invention, wherein: and extruding the casting solution through a spinneret plate to form a core solution which is adopted by the hollow fiber inner cavity and is one of a diluent or nitrogen.
As a preferable scheme of the preparation method of the reticular pore structure polyvinylidene fluoride hollow fiber membrane of the invention, wherein: and extruding the mixture through a spinneret plate, wherein the temperature of the core liquid is 60-100 ℃, and/or the flow rate of the core liquid is 0.01-0.1L/min.
As a preferable scheme of the preparation method of the reticular pore structure polyvinylidene fluoride hollow fiber membrane of the invention, wherein: cooling, wherein the cooling liquid is deionized water at 25 ℃; and/or extracting, wherein the extracting agent is ethanol with the purity of not less than 99%.
Another object of the present invention is to provide a reticular pore structure polyvinylidene fluoride hollow fiber membrane obtained by the above preparation method, which has high strength, good membrane pore penetration and excellent permeability.
Compared with the prior art, the invention has the following beneficial effects:
(1) compared with other methods for adding complex multi-component mixed diluent systems, the method can obtain an ideal thin skin layer with high surface aperture ratio and a membrane structure with a supporting layer of reticular pores only by using a single diluent at a lower temperature (not higher than the melting temperature of a polymer), has low requirement on temperature control, does not need to add an antioxidant, and has simple process and convenient actual production;
(2) compared with the preparation of the ultra-high molecular weight polyethylene and polyvinylidene fluoride composite hollow fiber membrane, only the same polymer with different molecular weights needs to be added, so that the compatibility of the system is better, macroscopic phase separation and interface defect holes in the cooling process due to poor compatibility of the two polymers are avoided, the interception precision of the membrane is higher, and the mechanical property is good;
(3) compared with the thick and compact spherical particle structure hollow fiber membrane prepared by S-L phase separation of pure common molecular weight polyvinylidene fluoride, the membrane prepared by the invention forms a structure with thin skin layer and more open pores and a support layer of through mesh holes by obviously hindering the crystallization of the polymer, so the membrane has higher mechanical strength, is not easy to cause yarn breakage and leakage in the using process, and has better permeability.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without inventive exercise. Wherein:
FIG. 1 is a scanning electron microscope image of a hollow fiber membrane prepared in example 1 of the present invention; wherein, (a) is an SEM image of the section of the polyvinylidene fluoride microporous membrane close to the outer skin layer, and (b) is an SEM image of the section supporting layer of the polyvinylidene fluoride microporous membrane;
FIG. 2 is a scanning electron microscope photograph of a hollow fiber membrane prepared in comparative example 1 of the present invention; wherein, (a) is an SEM image of the section of the microporous membrane close to the outer skin layer, and (b) is an SEM image of the section support layer of the microporous membrane.
Detailed Description
In order to make the aforementioned objects, features and advantages of the present invention more comprehensible, specific embodiments thereof are described in detail below with reference to examples of the specification.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, but the present invention may be practiced in other ways than those specifically described and will be readily apparent to those of ordinary skill in the art without departing from the spirit of the present invention, and therefore the present invention is not limited to the specific embodiments disclosed below. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially.
Furthermore, reference herein to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation of the invention. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments.
In view of the problems of poor pore forming property, thick and compact skin layer and low flux of the conventional solid-liquid phase separation membrane preparation in the prior art, the invention provides a preparation method of a polyvinylidene fluoride hollow fiber membrane with a reticular pore structure, and aims to solve the technical problems of the existing product.
Specifically, a small amount of polyvinylidene fluoride with ultrahigh molecular weight is used as an additive to be mixed with polyvinylidene fluoride with common molecular weight, and the PVDF hollow fiber membrane with thin skin layer, good surface openness and a reticular pore structure as a supporting layer is prepared by S-L phase separation of a thermally induced phase separation method, so that the thick skin layer and a spherical particle structure formed by the S-L phase separation membrane are avoided, the defects of harsh preparation conditions and high temperature of the L-L phase separation are avoided, and the advantages of low requirement on the S-L phase separation temperature and the penetrating reticular pore structure formed by the L-L phase separation are realized, and the preparation method can refer to the following steps:
uniformly mixing common molecular weight polyvinylidene fluoride, an additive and a diluent under a heating condition to obtain a casting solution;
extruding the casting solution through a spinneret plate, cooling, winding, extracting, drying, and performing solid-liquid phase separation to obtain a polyvinylidene fluoride hollow fiber membrane with a mesh pore structure;
wherein the additive is ultra-high molecular weight polyvinylidene fluoride.
The weight average molecular weight of the ultra-high molecular weight polyvinylidene fluoride is not less than 100 ten thousand, for example, but not limited to 120, 130, 150, or 200 ten thousand, and the weight average molecular weight of the ordinary molecular weight polyvinylidene fluoride is 10 to 70 ten thousand, for example, but not limited to 10, 30, 50, or 70 ten thousand.
Specifically, the casting solution comprises, by mass, 2-10 parts of ultra-high molecular weight polyvinylidene fluoride, for example, but not limited to, 2, 3, 4, 5, 6, 7, 8, 9 or 10 parts; the common molecular weight polyvinylidene fluoride is 15-40 parts, for example, but not limited to, 15, 20, 25, 30, 35 or 40 parts; the diluent is 50 to 83 parts, and may be, for example, but not limited to, 50, 57, 63, 72, 78, or 83 parts.
In the above reaction step, the diluent comprises one or more of dibutyl phthalate, dibutyl sebacate, acetyl tributyl citrate, propylene carbonate, and gamma-butyrolactone.
In the above reaction step, the mixture is mixed under heating condition, and the heating temperature is lower than the melting temperature of the common molecular weight polyvinylidene fluoride; specifically, the heating temperature is 150-170 ℃, for example, but not limited to 150, 155, 160, 165 or 170 ℃.
In the above reaction step, extruding the casting solution through a spinneret plate, and extruding the casting solution and a heated core solution through the spinneret plate, wherein the core solution is one of a diluent or nitrogen; specifically, the temperature of the core liquid is 60-100 ℃, for example, but not limited to, 60, 70, 80, 90 or 100 ℃; and/or the bore fluid flow rate is 0.01 to 0.1L/min, for example, but not limited to, 0.01, 0.03, 0.05, 0.08, or 0.1L/min.
In the above reaction step, the cooling is carried out, and the cooling liquid is deionized water with the temperature of 25 ℃; and/or extracting, wherein the extracting agent is ethanol with the purity of not less than 99%.
The polyvinylidene fluoride hollow fiber membrane with the reticular pore structure prepared by any method has the advantages of high strength, good membrane pore connectivity, excellent permeability and the like.
Example 1
(1) Adding the ultra-high molecular weight polyvinylidene fluoride (molecular weight 130 ten thousand)/common molecular weight polyvinylidene fluoride (weight average molecular weight 33 ten thousand)/diluent (dibutyl phthalate) into a reaction kettle according to the mass ratio of 6/20/74, mixing and stirring for 3 hours at the temperature of 170 ℃, and then defoaming for 3 hours in vacuum to obtain a uniform membrane casting solution;
(2) taking a diluent (dibutyl phthalate) as a core liquid, wherein the temperature of the core liquid is 80 ℃, the flow rate of the core liquid is 0.03L/min, metering a casting film liquid by a spinning pump, continuously extruding the casting film liquid from a hollow fiber spinning nozzle at the flow rate of 0.05L/min, immersing the casting film liquid into a 25 ℃ solidification bath (deionized water) for cooling and forming after passing through an air gap of 0.5cm, then immersing membrane filaments in absolute ethyl alcohol for 48h, extracting the residual diluent in the membrane, immersing the membrane filaments in the deionized water for 24h to remove the residual ethyl alcohol, and finally naturally airing the membrane filaments to obtain the polyvinylidene fluoride hollow fiber membrane with the reticular pore structure.
The scanning electron microscope picture of the hollow fiber membrane obtained in example 1 is shown in fig. 1, and it can be seen that the membrane structure is thin in the outer skin layer, free of the inner skin layer, and the supporting layer is a mesh-like pore structure.
And (3) testing the water flux: taking 300mm membrane wires for end sealing casting, testing for 2 hours by using pure water at 25 ℃ under the pressure of 0.1MPa, then calculating the pure water flux under the conditions of unit area and time, and taking an average value after five times of testing; the test result shows that the pure water flux is 784L/(m)2·h)。
And (3) testing the breaking strength: taking a 100mm fiber membrane, performing a tensile test on a universal testing machine at a linear speed of 10mm/s, recording the maximum force when a membrane wire is broken, and performing the test for five times to obtain an average value; the test results showed that the breaking strength was 13.5 MPa.
Example 2
(1) Adding 4/22/74 mass ratio of ultra-high molecular weight polyvinylidene fluoride (molecular weight 130 ten thousand)/common molecular weight polyvinylidene fluoride (weight average molecular weight 33 ten thousand)/diluent (dibutyl phthalate) into a reaction kettle, mixing and stirring for 3h at 160 ℃, and then defoaming for 3h in vacuum to obtain a uniform membrane casting solution;
(2) taking a diluent (dibutyl phthalate) as a core liquid, wherein the temperature of the core liquid is 80 ℃, the flow rate of the core liquid is 0.03L/min, metering a casting film liquid by a spinning pump, continuously extruding the casting film liquid from a hollow fiber spinning nozzle at the flow rate of 0.05L/min, immersing the casting film liquid into a 25 ℃ solidification bath (deionized water) for cooling and forming after passing through an air gap of 0.5cm, then immersing film filaments in absolute ethyl alcohol for 48h, extracting the residual diluent in the film with 99% of purity ethanol, then immersing the film filaments in the deionized water for 24h to remove the residual ethanol, and finally naturally airing the film filaments to obtain the polyvinylidene fluoride hollow fiber film with the mesh-hole structure.
Through determination, the membrane structure obtained in example 2 is a thin outer skin layer, no inner skin layer and the supporting layer is a reticular pore structure, the performance tests of the obtained reticular pore structure polyvinylidene fluoride hollow fiber membrane in example 1 are respectively carried out, and the test result shows that the breaking strength is 11.2 MPa; the pure water flux of the membrane was 836L/(m) at 0.1MPa and 25 deg.C2·h)。
Example 3
(1) Adding the ultra-high molecular weight polyvinylidene fluoride (molecular weight 130 ten thousand)/common molecular weight polyvinylidene fluoride (weight average molecular weight 33 ten thousand)/diluent (acetyl tributyl citrate) into a reaction kettle according to the mass ratio of 8/18/74, mixing and stirring for 3 hours at the temperature of 170 ℃, and then defoaming for 3 hours in vacuum to obtain a uniform membrane casting solution;
(2) taking a diluent (acetyl tributyl citrate) as a core solution, wherein the temperature of the core solution is 80 ℃, the flow rate of the core solution is 0.03L/min, metering a casting film solution by a spinning pump, continuously extruding the casting film solution from a hollow fiber spinning nozzle at the flow rate of 0.05L/min, immersing the casting film solution into a 25 ℃ coagulating bath (deionized water) for cooling and forming after passing through an air gap of 0.5cm, then immersing membrane filaments in absolute ethyl alcohol for 48h, extracting the residual diluent in the membrane, then immersing in the deionized water for 24h to remove the residual ethyl alcohol, and finally naturally airing the membrane filaments to obtain the reticular pore structure polyvinylidene fluoride hollow fiber membrane.
It was determined that the film structure obtained in example 3 had a thin outer skin layer, no inner skin layer, and a reticulated pore structure as the support layer. The obtained polyvinylidene fluoride hollow fiber membrane with the reticular pore structure is subjected to the performance test as described in the embodiment 1, and the test result shows that the breaking strength is 14.6 MPa; the pure water flux of the membrane was 757L/(m) at 0.1MPa and 25 deg.C2·h)。
Example 4
(1) Adding 4/22/74 mass ratio of ultra-high molecular weight polyvinylidene fluoride (molecular weight 150 ten thousand)/33 ten thousand common molecular weight polyvinylidene fluoride (weight average molecular weight)/33 ten thousand diluent (acetyl tributyl citrate) into a reaction kettle, mixing and stirring for 3h at 170 ℃, and then defoaming for 3h in vacuum to obtain a uniform membrane casting solution;
(2) taking a diluent (acetyl tributyl citrate) as a core solution, wherein the temperature of the core solution is 80 ℃, the flow rate of the core solution is 0.03L/min, metering a casting film solution by a spinning pump, continuously extruding the casting film solution from a hollow fiber spinning nozzle at the flow rate of 0.05L/min, immersing the casting film solution into a 25 ℃ coagulating bath (deionized water) for cooling and forming after passing through an air gap of 0.5cm, then immersing membrane filaments in absolute ethyl alcohol for 48h, extracting the residual diluent in the membrane, then immersing in the deionized water for 24h to remove the residual ethyl alcohol, and finally naturally airing the membrane filaments to obtain the reticular pore structure polyvinylidene fluoride hollow fiber membrane.
It was determined that the film structure obtained in example 4 had a relatively thin outer skin layer, no inner skin layer, and a supporting layer having a network pore structure. The obtained polyvinylidene fluoride hollow fiber membrane with the reticular pore structure is subjected to the performance test as described in the embodiment 1, and the test result shows that the breaking strength is 12.2 MPa; the pure water flux of the membrane was 824L/(m) at 0.1MPa and 25 deg.C2·h)。
Comparative example 1
(1) Adding common molecular weight polyvinylidene fluoride (with the weight average molecular weight of 33 ten thousand)/diluent (dibutyl phthalate) into a reaction kettle according to the mass ratio of 26/74, mixing and stirring for 3 hours at the temperature of 160 ℃, and then defoaming for 3 hours in vacuum to obtain uniform membrane casting liquid;
(2) taking a diluent (dibutyl phthalate) as a core liquid, wherein the temperature of the core liquid is 80 ℃, the flow rate of the core liquid is 0.03L/min, metering a casting film liquid by a spinning pump, continuously extruding the casting film liquid from a hollow fiber spinning nozzle at the flow rate of 0.05L/min, immersing the casting film liquid into a 25 ℃ solidification bath (deionized water) for cooling and forming after passing through an air gap of 0.5cm, then immersing membrane filaments in absolute ethyl alcohol for 48h, extracting the residual diluent in the membrane, immersing the membrane filaments in the deionized water for 24h to remove the residual ethyl alcohol, and finally naturally airing the membrane filaments to obtain the polyvinylidene fluoride hollow fiber membrane.
The scanning electron microscope picture of the hollow fiber membrane obtained in comparative example 1 is shown in fig. 2, and it can be seen that the membrane structure obtained in comparative example 1 is thick in the outer skin layer, has no inner skin layer, and has a spherical particle structure in the support layer.
The polyvinylidene fluoride hollow fiber membranes obtained are respectively subjected to the performance test as described in example 1, and the test result shows that the breaking strength is 8.9 MPa; the pure water flux of the membrane was 8L/(m) at 0.1MPa and 25 deg.C2·h)。
Comparative example 2
(1) Adding common molecular weight polyvinylidene fluoride (with the weight average molecular weight of 33 ten thousand)/diluent (acetyl tributyl citrate) into a reaction kettle according to the mass ratio of 26/74, mixing and stirring for 3 hours at the temperature of 160 ℃, and then carrying out vacuum defoaming for 3 hours to obtain a uniform membrane casting solution;
(2) taking a diluent (acetyl tributyl citrate) as a core solution, wherein the temperature of the core solution is 80 ℃, the flow rate of the core solution is 0.03L/min, metering a casting film solution by a spinning pump, continuously extruding the casting film solution from a hollow fiber spinning nozzle at the flow rate of 0.05L/min, immersing the casting film solution into a 25 ℃ solidification bath (deionized water) for cooling and forming after passing through an air gap of 0.5cm, then immersing membrane filaments in absolute ethyl alcohol for 48h, extracting the residual diluent in the membrane, then immersing in the deionized water for 24h to remove the residual ethyl alcohol, and finally naturally airing the membrane filaments to obtain the polyvinylidene fluoride hollow fiber membrane.
It was determined that the membrane structure obtained in comparative example 2 was thick with no inner layer and the supporting layer was a spherical particle structure. The performance tests of the obtained polyvinylidene fluoride hollow fiber membrane with the reticular pore structure are respectively carried out as described in the embodiment 1, and the test results show that the breaking strength is 8.6 Mpa;the pure water flux of the membrane was 11L/(m) at 0.1MPa and 25 deg.C2·h)。
The principle of obtaining the thin skin layer and the reticular hole supporting layer is as follows: when the polyvinylidene fluoride with common molecular weight is used for preparing the membrane by a thermally induced phase separation method, S-L phase separation is generated to form a compact thick skin layer and a spherical particle structure, and the main reasons are as follows: the film-making temperature is lower than the polymer melting temperature, when the casting solution is reduced to a certain temperature, the polymer is directly crystallized, the common molecular weight polyvinylidene fluoride molecular chain segment moves too fast, the crystallization rate is fast, and when the polymer solution is contacted with quenching agent (water) with strong heat conduction capability, the temperature of the polymer solution drops suddenly to form large temperature gradient, so that the polymer is rapidly crystallized to form a thick compact skin layer and spherical particle structure, and the film has almost no flux.
A small amount of polyvinylidene fluoride with ultrahigh molecular weight is added into the system, and due to the large molecular weight, long chain segment, poor flexibility and low molecular motion speed, on one hand, the viscosity of the casting solution system can be increased, and the heat transfer rate is slowed down, so that the crystallization rate is reduced, and the thickness of a skin layer is reduced; on the other hand, the polyvinylidene fluoride with ultrahigh molecular weight and the polyvinylidene fluoride with common molecular weight belong to the same substance, so that the compatibility is good, chain segment winding occurs, the chain segment movement speed of the polyvinylidene fluoride with common molecular weight in the crystallization process is remarkably slowed down, the crystallization rate is slowed down, the crystallization process of the polyvinylidene fluoride is hindered, the generation of a spherical particle structure and a thick and compact skin layer is hindered, and a through mesh hole structure surrounded by imperfect crystallization serial crystals or lamella crystals is finally formed. Because the molecular weight of ultra-high molecular weight polyvinylidene fluoride is too large, not only is it difficult to process by itself, but also the solid content of the polymer cannot be so high that the film shrinks too much and has almost no pressure resistance, so that the ultra-high molecular weight polyvinylidene fluoride film cannot be used alone.
Compared with other methods for adding complex multi-element mixed diluent systems, the method can obtain an ideal thin skin layer at a lower temperature (not higher than the melting temperature of the polymer) by using a single diluent, has high surface aperture ratio and a supporting layer with a reticular pore membrane structure, has low requirement on temperature control, does not need to add an antioxidant, and has simple process and convenient actual production; compared with the preparation of the ultra-high molecular weight polyethylene and polyvinylidene fluoride composite hollow fiber membrane, only the same polymer with different molecular weights needs to be added, so that the compatibility of the system is better, macroscopic phase separation and interface defect holes in the cooling process due to poor compatibility of the two polymers are avoided, the interception precision of the membrane is higher, and the mechanical property is good; compared with the thick and compact spherical particle structure hollow fiber membrane prepared by S-L phase separation of pure common molecular weight polyvinylidene fluoride, the membrane prepared by the invention forms a structure with thin skin layer and more open pores and a support layer of through mesh holes by obviously hindering the crystallization of the polymer, so the membrane has higher mechanical strength, is not easy to cause yarn breakage and leakage in the using process, and has better permeability.
It should be noted that the above-mentioned embodiments are only for illustrating the technical solutions of the present invention and not for limiting, and although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions may be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, which should be covered by the claims of the present invention.
Claims (10)
1. A preparation method of a reticular pore structure polyvinylidene fluoride hollow fiber membrane is characterized by comprising the following steps: comprises the steps of (a) preparing a mixture of a plurality of raw materials,
uniformly mixing common molecular weight polyvinylidene fluoride, an additive and a diluent under a heating condition to obtain a casting solution;
extruding the casting solution through a spinneret plate, cooling, winding, extracting, drying, and performing solid-liquid phase separation to obtain a polyvinylidene fluoride hollow fiber membrane with a mesh pore structure;
wherein the additive is ultra-high molecular weight polyvinylidene fluoride.
2. The method for preparing a reticular pore structure polyvinylidene fluoride hollow fiber membrane according to claim 1, wherein: the weight average molecular weight of the common molecular weight polyvinylidene fluoride is 10-70 ten thousand, and the weight average molecular weight of the ultrahigh molecular weight polyvinylidene fluoride is not less than 100 ten thousand.
3. The method for preparing a reticular pore structure polyvinylidene fluoride hollow fiber membrane according to claim 1 or 2, wherein: the casting film liquid comprises, by mass, 15-40 parts of common molecular weight polyvinylidene fluoride, 2-10 parts of ultrahigh molecular weight polyvinylidene fluoride and 50-83 parts of a diluent.
4. The method for preparing the polyvinylidene fluoride hollow fiber membrane with the reticular pore structure as claimed in claim 3, wherein the method comprises the following steps: the diluent comprises one or more of dibutyl phthalate, dibutyl sebacate, acetyl tributyl citrate, propylene carbonate and gamma-butyrolactone.
5. The method for preparing a reticular pore structure polyvinylidene fluoride hollow fiber membrane according to any one of claims 1, 2 or 4, wherein: the polyvinylidene fluoride is uniformly mixed under the heating condition, and the heating temperature is lower than the melting temperature of the common molecular weight polyvinylidene fluoride.
6. The method for preparing a reticular pore structure polyvinylidene fluoride hollow fiber membrane according to claim 5, wherein: the heating temperature is 150-170 ℃.
7. The method for preparing a reticular pore structure polyvinylidene fluoride hollow fiber membrane according to any one of claims 1, 2, 4 or 6, wherein: and extruding the casting solution through a spinneret plate, wherein the adopted core solution is one of diluent or nitrogen.
8. The method for preparing a reticular pore structure polyvinylidene fluoride hollow fiber membrane according to claim 7, wherein: and extruding the mixture through a spinneret plate, wherein the temperature of the core liquid is 60-100 ℃, and/or the flow rate of the core liquid is 0.01-0.1L/min.
9. The method for preparing a reticular pore structure polyvinylidene fluoride hollow fiber membrane according to any one of claims 1, 2, 4, 6 or 8, wherein: cooling, wherein the cooling liquid is deionized water at 25 ℃; and/or extracting, wherein the extracting agent is ethanol with the purity of not less than 99%.
10. A reticular pore structure polyvinylidene fluoride hollow fiber membrane obtained by the preparation method of any one of claims 1 to 9.
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