CN114345138B - Casting solution of asymmetric flat microporous membrane and preparation method of PVDF flat microporous membrane - Google Patents
Casting solution of asymmetric flat microporous membrane and preparation method of PVDF flat microporous membrane Download PDFInfo
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- CN114345138B CN114345138B CN202210033992.2A CN202210033992A CN114345138B CN 114345138 B CN114345138 B CN 114345138B CN 202210033992 A CN202210033992 A CN 202210033992A CN 114345138 B CN114345138 B CN 114345138B
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Abstract
The invention discloses a casting solution of an asymmetric flat microporous membrane and a preparation method of a PVDF flat microporous membrane, which comprises the following steps: determining the surface tension of a high polymer material for preparing the microporous membrane to be sigma; if the pagoda-shaped asymmetric microporous membrane is prepared, selecting a solvent with the surface tension larger than sigma as a casting solution solvent; if the funnel-shaped asymmetric microporous membrane is prepared, selecting a solvent with the surface tension less than sigma as a casting solution solvent; the invention adds high molecular material, first balance additive and second balance additive into the casting solution solvent, and the invention has the following beneficial effects: by utilizing the thermodynamic principle, the surface tension ratio between the solvent of the membrane casting solution and the high molecular material is changed by adjusting the types and the adding proportions of the solvent, the first balance additive and the second balance additive in the membrane casting solution, so that the membrane casting solution has positive adsorption or negative adsorption performance, the distribution state of the high molecular material and the mass transfer behavior of the solvent in the phase conversion process of the membrane casting solution are regulated and controlled, and the control on the morphological structure of the membrane pores is realized.
Description
Technical Field
The invention relates to the technical field of membranes, in particular to a membrane casting solution of an asymmetric flat microporous membrane and a preparation method of a PVDF flat microporous membrane.
Background
The microporous membrane has multiple purposes, different use scenes require that the microporous membrane has different micro-morphology structures, wherein the micro-morphology structures comprise membrane surface pore structures and membrane section pore structures, such as the shapes, the number, the sizes and the like of pores, organic connection exists among the micro-morphology structures, and the use function of the microporous membrane is obviously influenced. When the microporous membrane is used for filtration, the microporous membrane is required to have stable filtration precision, high flow rate and good resistance to pollution; also, when the microporous membrane is used for a battery separator, the microporous membrane is required to have low resistance, small and uniform surface pores, good liquid retention ability, and the like.
The control of the microstructure of microporous membranes has been developed by techniques and technologies, such as stretching, thermal phase inversion, sintering, solvent phase inversion, etc., which are well known in the literature and patents. The shape structure of the microporous membrane hole prepared by the stretching process, the thermal phase transformation process and the sintering process is mainly a symmetrical structure; the solvent phase inversion process can realize the asymmetric structure of the membrane, but research and development mainly focus on the control of the preparation process of the microporous membrane, so that the process is complex, the requirement on the environment is strict, the production efficiency is low, and simultaneously, the defects that the quality of the membrane is unstable, particularly the surface pore size distribution and the section structure are unstable, so that the preparation cost of the microporous membrane is higher, and the like exist.
In view of this, how to develop a simple method capable of controlling the micro-morphology structure of the asymmetric PVDF and PES flat microporous membrane to overcome the defects of complex process, severe environmental requirements and low production efficiency of the existing asymmetric PVDF and PES flat microporous membrane becomes a technical problem to be solved urgently in the industry.
Disclosure of Invention
The invention aims to disclose a membrane casting solution of an asymmetric flat microporous membrane and a preparation method of a PVDF flat microporous membrane.
The first purpose of the invention is to develop a preparation method of a casting solution of an asymmetric flat microporous membrane.
The second invention aim at developing a preparation method of the asymmetric PVDF flat microporous membrane.
In order to achieve the first object, the invention provides a method for preparing a casting solution of an asymmetric flat microporous membrane, which comprises the following steps:
determining the surface tension of a high polymer material for preparing the microporous membrane to be sigma;
if a pagoda-shaped asymmetric microporous membrane is prepared, selecting a solvent with the surface tension larger than sigma as a membrane casting solution solvent;
if the funnel-shaped asymmetric microporous membrane is prepared, selecting a solvent with the surface tension less than sigma as a casting solution solvent;
adding a polymeric material, a first balancing additive and a second balancing additive to the casting solution solvent.
Preferably, the polymer material is PES.
Preferably, the polymer material is PVDF, the solvent with surface tension greater than sigma is one or a mixture of N, N-dimethylacetamide, N-methylpyrrolidone and dimethyl sulfoxide, and the solvent with surface tension greater than sigma has surface tension greater than 26.5dyn/cm.
Preferably, the polymer material is PVDF, the solvent with surface tension less than sigma is one or a mixture of N, N-dimethylacetamide, N-methylpyrrolidone and dimethyl sulfoxide, and one or a mixture of acetone and butanone, and the solvent with surface tension less than sigma has surface tension less than 23.5dyn/cm.
Preferably, the polymer material is PVDF, and the first balancing additive is an inorganic salt containing not less than a divalent positively charged ion.
Preferably, the inorganic salt containing not less than divalent positively charged ions is one of calcium chloride, aluminum chloride, calcium nitrate, aluminum nitrate or a mixture thereof.
Preferably, the polymer material is PVDF, and the second balance additive is an inorganic acid containing a plurality of oxygen elements.
Preferably, the inorganic acid containing multiple oxygen elements is one or a mixture of sulfuric acid, nitric acid and phosphoric acid.
Preferably, the preparation temperature of the membrane casting solution is 20-120 ℃, the preparation time is 2-30 h, and the membrane casting solution is cured for later use.
In order to achieve the second object, the invention provides a preparation method of an asymmetric PVDF flat microporous membrane, which comprises the following steps:
preparing a membrane casting solution according to the method created by the first invention, and coating the membrane casting solution on one of a non-woven fabric, a metal belt or a polymer belt through a membrane casting machine to form a flat microporous membrane;
blade coating, wherein the blade distance is controlled to be 30-900 μm during blade coating;
gas phase mass transfer, wherein the ambient temperature of the gas phase mass transfer is controlled to be 15-90 ℃, the relative humidity is controlled to be 10-95%, and the flowing speed of gas is controlled to be 0.3-5.0 m/s;
liquid-phase mass transfer is carried out, and the temperature of the liquid-phase mass transfer is controlled to be 5-50 ℃;
washing, wherein the washing temperature is controlled to be 10-70 ℃;
heat treatment, wherein the temperature of the heat treatment is controlled to be 75-100 ℃;
drying at 50-80 deg.c;
the non-woven fabric is made of one of polyester, polypropylene, chinlon, spandex and acrylon;
the metal belt is made of one of stainless steel, copper and aluminum;
the material of the polymer belt is one of polyester and polypropylene.
Compared with the prior art, the invention has the beneficial effects that:
(1) By utilizing the thermodynamic principle, the surface tension ratio between the solvent of the membrane casting solution and the high molecular material is changed by adjusting the types and the adding proportions of the solvent, the first balance additive and the second balance additive in the membrane casting solution, so that the membrane casting solution has positive adsorption or negative adsorption performance, the distribution state of the high molecular material and the mass transfer behavior of the solvent in the phase conversion process of the membrane casting solution are regulated and controlled, and the control on the morphological structure of the membrane pores is realized.
(2) When the high polymer material is PVDF, when the membrane casting solution has positive adsorption performance, namely the surface tension of a membrane casting solution solvent is greater than that of PVDF, the surface concentration of PVDF in the membrane casting solution is higher than that of a main body, so that the pagoda-shaped asymmetric PVDF microporous membrane is prepared; when the casting solution has negative adsorption performance, that is, the surface tension of the solvent of the casting solution is less than that of PVDF, the surface concentration of PVDF in the casting solution is lower than that of the main body, so that the funnel-shaped asymmetric PVDF microporous membrane can be prepared.
Drawings
FIG. 1 is a scanning electron micrograph of a film cross section of example V of the present invention;
FIG. 2 is a scanning electron micrograph of a film surface according to a fifth embodiment of the present invention;
FIG. 3 is a scanning electron micrograph of a cross section of a film according to a sixth embodiment of the present invention;
FIG. 4 is a scanning electron micrograph of a film surface according to a sixth embodiment of the present invention;
FIG. 5 is a scanning electron micrograph of a film cross section of a seventh embodiment of the present invention;
FIG. 6 is a SEM photograph of the surface of a film according to a seventh embodiment of the present invention;
FIG. 7 is a scanning electron micrograph of a film cross section of example eight of the present invention;
FIG. 8 is a scanning electron micrograph of a film surface of example eight of the present invention.
Detailed Description
The present invention is described in detail with reference to the embodiments shown in the drawings, but it should be understood that these embodiments are not intended to limit the present invention, and that functional, methodological, or structural equivalents thereof, which are equivalent or substituted by those of ordinary skill in the art, are within the scope of the present invention.
In the description of the present invention, it is to be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", and the like, indicate orientations or positional relationships based on those shown in the drawings, merely for convenience of description and simplification of the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed in a particular orientation, and be operated, and thus, are not to be construed as limiting the present invention.
The following describes a specific implementation process of the present invention by using a plurality of embodiments.
The first embodiment is as follows:
the embodiment discloses a preparation method of a casting solution of an asymmetric flat microporous membrane, which comprises the following steps when a high polymer material is PVDF: determining the surface tension of the PVDF as sigma, specifically, the surface tension sigma of the PVDF is 25dyn/cm; if a pagoda-shaped asymmetric PVDF microporous membrane is prepared, selecting a solvent with the surface tension larger than sigma as a membrane casting solution solvent, and enabling the surface concentration of PVDF in the membrane casting solution to be higher than the main concentration, specifically, preparing the pagoda-shaped asymmetric PVDF microporous membrane, wherein the membrane casting solution solvent needs to be prepared into a positive adsorption solution, and selecting a solvent which can fully dissolve PVDF and has the surface tension larger than sigma as the membrane casting solution solvent, wherein the membrane casting solution solvent is a single solvent or a mixed solvent; if the funnel-shaped asymmetric PVDF microporous membrane is prepared, selecting a solvent with the surface tension less than sigma as a membrane casting solution solvent, and enabling the surface concentration of PVDF in the membrane casting solution to be lower than the main concentration, specifically, preparing the funnel-shaped asymmetric PVDF microporous membrane, wherein the membrane casting solution solvent needs to be prepared into a negative adsorption solution, and selecting a solvent which can fully dissolve PVDF and has the surface tension less than sigma as the membrane casting solution solvent, wherein the membrane casting solution solvent is a mixed solvent; adding PVDF, a first balance additive and a second balance additive into a casting solution solvent, wherein the casting solution comprises the solvent and a high-molecular material PVDF, and the first balance additive and the second balance additive are specifically included, the first balance additive is used for adjusting the charge balance and the coordination balance of the casting solution, and the second balance additive is used for adjusting the phase inversion balance and the proton balance of the casting solution.
It is further explained that the phase inversion balance is the basis of phase inversion, and the concentration of the polymer material PVDF and the kind of the solvent significantly affect the result of the mass transfer process of the casting solution; the charge balance refers to the degree that the total concentration of the unit positive charges charged by various charged types corresponds to the total concentration of the unit negative charges charged in a casting solution chemical balance system; the coordination balance means that fluorine with extremely strong electronegativity and electron-deficient hydrogen in the PVDF polymer material need to be coordinated with a first balance additive; proton balance, forming 'acid-base pair' between the second balance additive and the casting solution solvent, so as to effectively adjust the buffering performance of the solvent on PVDF dissolution, and enable the microcell structure formed by PVDF in the casting solution to reach a metastable state close to phase separation and have better buffering capacity.
It should be further explained that, the pagoda-shaped asymmetric PVDF microporous membrane means that the shape of micropores on the section of the microporous membrane is pagoda-shaped, that is, the microporous membrane has a structural morphology with small surface pore size and large internal pore size; the funnel-shaped asymmetric PVDF microporous membrane refers to a structural morphology that micropores of the cross section of the microporous membrane are funnel-shaped, namely the surface aperture of the microporous membrane is large and the internal aperture is small; the solvent having a surface tension greater than σ or less than σ is selected, where the solvent is the total surface tension of the solvent of the casting solution that has been formulated.
Compared with the prior art, the beneficial effect of this embodiment is:
(1) By utilizing the thermodynamic principle, the surface tension ratio between the solvent of the membrane casting solution and PVDF is changed by adjusting the types and the adding proportions of the solvent, the first balance additive and the second balance additive in the membrane casting solution, so that the membrane casting solution has positive adsorption or negative adsorption performance, the distribution state of PVDF and the mass transfer behavior of the solvent in the phase conversion process of the membrane casting solution are regulated and controlled, and the control on the morphology structure of membrane pores is realized.
(2) When the casting solution has positive adsorption performance, namely the surface tension of a solvent of the casting solution is greater than that of PVDF, the surface concentration of PVDF in the casting solution is higher than the main concentration, so that the pagoda-shaped asymmetric PVDF microporous membrane is prepared; when the casting solution has negative adsorption performance, that is, the surface tension of the solvent of the casting solution is less than that of PVDF, the surface concentration of PVDF in the casting solution is lower than that of the main body, so that the funnel-shaped asymmetric PVDF microporous membrane can be prepared.
The second embodiment:
the embodiment discloses a preparation method of a casting solution of an asymmetric flat microporous membrane, which comprises the following steps when a high polymer material is PVDF:
determining the surface tension of the PVDF as sigma, wherein the sigma is 25dyn/cm;
preparing a pagoda-shaped asymmetric PVDF microporous membrane, selecting a solvent with surface tension larger than sigma as a casting solution solvent, specifically, preparing a casting solution solvent with positive adsorption property, specifically, selecting a solvent which can fully dissolve PVDF and selects a solvent with surface tension larger than 25dyn/cm to ensure that the surface concentration of PVDF in the casting solution is higher than the main concentration, wherein the surface tension sigma refers to the total surface tension of the final casting solution solvent, specifically, one or a mixture of N, N-dimethylacetamide with surface tension of 25.3dyn/cm, N-methylpyrrolidone with surface tension of 33.7dyn/cm and dimethyl sulfoxide with surface tension of 43.6dyn/cm is selected, and the surface tension of the finally prepared casting solution solvent is larger than 26.5dyn/cm, for example, the surface tension of the casting solution solvent is 28dyn/cm, 30N/cm, 35dyn/cm and the like;
adding PVDF, a first balancing additive and a second balancing additive to a casting solution solvent, wherein the first balancing additive is an inorganic salt containing ions with positive charge not less than divalent, such as one or a mixture of calcium chloride, aluminum chloride, calcium nitrate and aluminum nitrate, the first balancing additive is used for adjusting charge balance and coordination balance of the casting solution, when calcium chloride capable of being fully dissolved in the casting solution solvent is selected as the first balancing additive, divalent positively charged calcium ions form coordination with lone-pair electrons on extremely electronegative fluorine in PVDF molecules, the negatively charged chlorine ions with a positive tetrahedral structure form hydrogen bond with electron-deficient hydrogen in PVDF molecules, PVDF molecules form self-assembly while being fully dispersed in the solvent under the action of calcium chloride to form a structurally stable microcellular morphology, the addition amount of calcium chloride can be calculated and adjusted according to the molar ratio of calcium ions to fluorine elements to form coordination, the second balancing additive is an inorganic acid containing multiple oxygen elements, such as one or a mixture of sulfuric acid, nitric acid and phosphoric acid, the second balancing additive is used for adjusting the molar ratio of calcium ions to fluorine elements to form a molar ratio of weak nitric acid buffer groups in PVDF molecules, and the additive can be calculated and the additive can be adjusted according to the metastable acetamide-base buffer solution forming a fully dissolved in the casting solution, and the method for adjusting the nitrate-forming buffer solution.
As a preferred embodiment, the preparation temperature of the casting solution is 20-120 ℃, the preferred preparation temperature is 20 ℃, 30 ℃, 50 ℃, 80 ℃,100 ℃ and 120 ℃, the preparation time is 2-30 h, the preferred preparation time is 2h, 8h, 10h, 15h, 20h, 25h and 30h, and the casting solution is cured for later use.
Compared with the prior art, the beneficial effect of this embodiment is:
(1) By utilizing the thermodynamic principle, the surface tension ratio between the solvent of the membrane casting solution and PVDF is changed by adjusting the types and the adding proportions of the solvent, the first balance additive and the second balance additive in the membrane casting solution, so that the membrane casting solution has positive adsorption performance, the distribution state of PVDF and the mass transfer behavior of the solvent in the phase conversion process of the membrane casting solution are regulated and controlled, and the control on the morphology structure of membrane pores is realized.
(2) The prepared casting solution has positive adsorption performance, namely when the surface tension of a casting solution solvent is greater than that of PVDF, the surface concentration of PVDF in the casting solution is higher than that of a main body, so that preparation is made for preparing the asymmetric PVDF flat microporous membrane with the pagoda-shaped micro-morphology structure.
Example three:
the difference from the second example is that the third example prepares funnel-shaped asymmetric PVDF microporous membrane, selects a solvent with surface tension less than sigma as the casting solution solvent, makes the surface concentration of PVDF in the casting solution less than the bulk concentration, specifically, prepares the casting solution solvent with negative adsorption performance, specifically selects a solvent capable of fully dissolving PVDF, and selects a solvent with surface tension less than 25dyn/cm, where surface tension sigma refers to the total surface tension of the final casting solution solvent, specifically selects one or a mixture of N, N-dimethylacetamide with surface tension of 25.3dyn/cm, N-methylpyrrolidone with surface tension of 33.7dyn/cm, dimethyl sulfoxide with surface tension of 43.6dyn/cm, and one or a mixture of acetone with surface tension of 18.8dyn/cm, butanone with surface tension of 21.0dyn/cm, and finally prepares the casting solution solvent with surface tension of less than 23.5dyn/cm, such as the casting solution solvent with surface tension of 23dyn/cm, 20dyn/cm, 19dyn/cm and the like.
Compared with the prior art, the beneficial effect of this embodiment is:
(1) By utilizing the thermodynamic principle, the surface tension ratio between the solvent of the membrane casting solution and PVDF is changed by adjusting the types and the adding proportions of the solvent, the first balance additive and the second balance additive in the membrane casting solution, so that the membrane casting solution has negative adsorption performance, the distribution state of PVDF and the mass transfer behavior of the solvent in the phase conversion process of the membrane casting solution are regulated and controlled, and the control on the morphology structure of the membrane pores is realized.
(2) The prepared membrane casting solution has negative adsorption performance, namely when the surface tension of a membrane casting solution solvent is less than that of PVDF, the surface concentration of PVDF in the membrane casting solution is lower than that of a main body, so that preparation is made for preparing the asymmetric PVDF flat microporous membrane with a funnel-type micro-morphology structure.
Example four:
the embodiment discloses a preparation method of an asymmetric flat microporous membrane, which comprises the following steps when a high polymer material is PVDF:
preparing a casting solution by the method described in the first embodiment, the second embodiment or the third embodiment, and coating the casting solution on one of a non-woven fabric, a metal belt or a polymer belt through a casting machine to form a flat microporous membrane;
blade coating, wherein the blade distance is controlled to be 30-900 mu m during blade coating so as to control the thickness of the microporous membrane;
gas-phase mass transfer, wherein the ambient temperature of the gas-phase mass transfer is controlled to be 15-90 ℃, the relative humidity is controlled to be 10-95%, and the flow speed of gas is controlled to be 0.3-5.0 m/s, and the step aims to regulate and control the phase conversion speed through the evaporation of a solvent on the surface of the casting solution and the introduction of water molecules, so as to form a large number of surface hole prototypes with uniform size;
liquid phase mass transfer, wherein the temperature of the liquid phase mass transfer is controlled to be 5-50 ℃, and the step aims at solidifying the surface pore structure, accelerating the mass transfer speed of the internal solvent and the external non-solvent of the membrane, and forming and solidifying the internal pore structure of the membrane;
washing, wherein the washing temperature is controlled to be 10-70 ℃, and the step aims to remove the solvent, the first balance additive and the second balance additive;
heat treatment, the temperature of the heat treatment is controlled to be 75-100 ℃, the purpose of the step is to carry out molecular rearrangement on PVDF, eliminate stress generated in the formation process of a membrane pore structure and improve the uniformity of the membrane pore structure;
drying at 50-80 deg.c;
the non-woven fabric is made of one of polyester, polypropylene, chinlon, spandex and acrylon;
the metal belt is made of one of stainless steel, copper and aluminum;
the material of the polymer belt is one of polyester and polypropylene.
Example five:
the embodiment discloses a preparation method of an asymmetric PVDF flat microporous membrane, which comprises the following steps:
firstly, preparing materials, namely adding 10KgN, N-dimethylacetamide, 15 KgN-methylpyrrolidone and 9Kg PVDF into a 50L stirring kettle, uniformly stirring, swelling until the PVDF is transparent, heating to 105 ℃, and stirring for 4.5 hours to prepare a first component; adding 8Kg of N, N-dimethylacetamide, 1.2Kg of calcium chloride, 1.05Kg of nitric acid and 1.05Kg of glycerol into a 15L stirring kettle, heating to 40 ℃, and stirring for 2hr to obtain a second component; and slowly adding the second component into the stirred first component, continuously stirring for 2hr after the addition is finished, standing at 65 ℃ for defoaming for 10h to obtain the membrane casting solution.
Through calculation, the surface tension of the mixed solvent consisting of the N, N-dimethylacetamide and the N-methylpyrrolidone is greater than the surface tension of 25.0dyn/cm of PVDF, and the prepared casting solution is a high-molecular solution which is absorbed positively by thermodynamics.
Taking non-woven fabric for later use, wherein the non-woven fabric is made of polyester, has a thickness of 155 mu m and a gram weight of 100g/m 2 。
The second step, preparing the microporous membrane, coating the casting film liquid on the non-woven fabric through a film casting machine to form a flat microporous membrane, specifically, producing on an automatic film scraping machine, wherein the film scraping machine is a continuous production process and consists of 7 subsystems of a non-woven fabric unreeling machine frame, a casting film liquid single-side blade coating device, a first gas-phase mass transfer evaporation chamber, a second liquid-phase mass transfer water tank, a washing heat treatment box, a drying box and a membrane reeling machine frame; the conveying speed of the non-woven fabric is controlled to be 2.4m/min in the preparation process.
The clearance between the knife coating cutter and the supporting roller is controlled to be 750 mu m, the temperature control value of the casting film liquid is 65 ℃, the temperature control value of the evaporation chamber (the temperature of the environment of gas-phase mass transfer) is 35 ℃, the relative humidity control value is 20 percent, and the flow speed control value of the gas is 1.5m/s; the liquid phase mass transfer medium is purified water, and the temperature control value is 30 ℃; the washing and heat treatment medium is purified water, and the temperature control value is 75 ℃; the drying temperature control value is 65 ℃.
Step three, product detection, wherein the microporous membrane prepared by the process is tested, and the result shows that: the thickness of the film is 220 +/-10 mu m,
the absolute ethyl alcohol is used as a medium to test that the first bubble point pressure is 520KPa-550KPa, and the pure water flow is 1860L/m when the pressure is 100KPa after the membrane is wetted by the ethyl alcohol 2 .h。
The scanning electron microscope analysis of the microporous membrane prepared by the above process specifically refers to fig. 1 and 2, fig. 1 is a scanning electron microscope photograph of a membrane section, fig. 2 is a scanning electron microscope photograph of a membrane surface, and the result shows that: the microporous membrane has a large number of surface pores, small pore diameter, good uniformity and good aperture ratio; the section of the film presents a standard pagoda-shaped asymmetric structure shape which gradually increases from the surface of the film to the non-woven fabric.
Example six:
the embodiment discloses a preparation method of an asymmetric PVDF flat microporous membrane, which comprises the following steps:
firstly, preparing materials, namely adding 20Kg of dimethyl sulfoxide into a 30L batching kettle, sequentially adding 0.25Kg of calcium nitrate, 0.25Kg of calcium chloride, 0.85Kg of glycerol and 0.76Kg of phosphoric acid in the stirring process, heating to 50 ℃ after the addition is finished, stirring for 1.5h, then adding 2.5Kg of PVDF, heating to 85 ℃ after the PVDF is swelled to be transparent, stirring for 3.0h, naturally cooling to 45 ℃, heating to 100 ℃, stirring for 3.5h, standing for 2.5h, then cooling to 30 ℃ at the speed of 0.5 ℃/min, and preparing the PVDF casting solution with the mass fraction of 10.2% after defoaming.
Through measurement and calculation, the surface tension of the dimethyl sulfoxide solvent is 25.0dyn/cm higher than that of PVDF, and the prepared casting solution is a high-molecular solution which is absorbed positively by thermodynamics.
Taking non-woven fabric for later use, wherein the non-woven fabric is made of polypropylene, the thickness is 85 μm, and the gram weight is 40g/m 2 。
The second step, preparing the microporous membrane, coating the casting solution on the non-woven fabric through a film casting machine to form a flat microporous membrane, specifically, producing on an automatic film scraping machine, wherein the film scraping machine is a continuous production process and comprises 6 subsystems of a non-woven fabric unreeling machine frame, a casting solution dipping and blade coating device, a second liquid-phase transmission water tank, a washing heat treatment box, a drying box and a membrane reeling machine frame; the conveying speed of the non-woven fabric is controlled to be 3.5m/min in the preparation process.
Controlling the clearance between the knife coating cutter and the supporting roll to be 250 mu m, controlling the temperature of the cast film liquid to be 35 ℃ and controlling the water inlet angle to be 60 ℃; the liquid phase mass transfer medium is purified water, and the temperature control value is 35 ℃; the washing and heat treatment medium is purified water, and the temperature control value is 86 ℃; the drying temperature control value is 65 ℃.
Step three, product detection, wherein the microporous membrane prepared by the process is tested, and the result shows that: the thickness of the film is 125 +/-5 mu m,
the first bubble point pressure is 30KPa-40KPa when absolute ethanol is used as a medium, and the pure water flow rate is 3200L/m when the pressure after the membrane is wetted by ethanol is 20KPa 2 H, an air flux of 865m3/m at an intake pressure of 100KPa and a pressure difference of 10KPa 2 H, efficiency was measured using an aerosol with an average particle size of 0.01 μm, and the rejection of the membrane was 99.99995% measured at a flow rate of 5.3 cm/s.
Prepared by the above processThe microporous membrane is analyzed by a scanning electron microscope, specifically referring to fig. 3 and 4, fig. 3 is a scanning electron microscope photograph of a membrane section, fig. 4 is a scanning electron microscope photograph of a membrane surface, and the result shows that: the pores on the membrane surface are uniformly distributed, and the number is about 5.0 x 10 9 Per cm 2 The aperture ratio is good, the aperture of the membrane surface is 0.18-0.25 μm, and the absolute ethyl alcohol is used as a medium to test that the first bubble point pressure value is 30-40 KPa, which is 3-4 times lower than the bubble point pressure calculated by a capillary model; the compact layer of the membrane section is very thin, the pore size of the compact layer is greatly different from that of the lower supporting layer of the compact layer by more than one order of magnitude, and the membrane section presents a pagoda-shaped asymmetric structure shape which is enlarged from the surface of the membrane to the middle.
Example seven:
the embodiment discloses a preparation method of an asymmetric PVDF flat microporous membrane, which comprises the following steps:
the first step is to prepare materials, 16.0KgN, N-dimethylacetamide and 6.6KgPVDF are added into a 20L material mixing kettle, stirred and mixed evenly, and then are heated to 105 ℃ and kept warm for 2.0h, and stirred at 100 ℃ for 4.5h to prepare a first component, 3.6KgN, N-dimethylacetamide and 35.0Kg of acetone are added into a 50L material mixing kettle, and stirred at normal temperature for 10min to prepare a second component, the first component is transferred into a 100L material mixing kettle at 100 ℃, cooled to 80 ℃, and slowly added with the second component under stirring, and after the addition is finished, the stirring is continued for 1.5h, and the curing is carried out for 6.0h to prepare the PVDF casting solution.
According to calculation, the surface tension of the solvent consisting of the N, N-dimethylacetamide and the acetone is less than 25.0dyn/cm of the surface tension of the PVDF, and the prepared casting solution is a high molecular solution with thermodynamic negative adsorption.
The second step, preparing the microporous membrane, coating the casting solution on the polyester belt through a film casting machine to form a flat microporous membrane, specifically, producing on an automatic film scraping machine, wherein the film scraping machine is a continuous production process and consists of 8 subsystems of a polyester belt unreeling rack, a casting solution single-side blade coating device, a gas phase mass transfer evaporation box, a liquid phase mass transfer water tank, a gas phase heat treatment box, a washing heat treatment box, a drying box, the polyester belt and a membrane reeling rack; the conveying speed of the polyester belt is controlled to be 1.85m/min in the preparation process.
The clearance between the knife coating cutter and the supporting roll is controlled to be 350 mu m, the temperature control value of the cast film liquid is 45 ℃, and the water inlet angle is controlled to be 60 ℃; the control value of the temperature (the temperature of the environment of gas phase mass transfer) of the gas phase mass transfer evaporation box is 48 ℃, and the control value of the relative humidity is 80 percent; the liquid phase mass transfer medium is purified water, and the temperature control value is 50 ℃; the washing and heat treatment medium is purified water, and the temperature control value is 86 ℃; the drying temperature control value is 70 ℃.
And step three, product detection, wherein the microporous membrane prepared in the process is tested, and the result shows that: the thickness of the film is 75 +/-5 mu m,
the first bubble point pressure is 10KPa-12KPa when absolute ethyl alcohol is used as a medium, and the pure water flow rate is 26.5ml/cm when the pressure is 2KPa after the membrane is wetted by ethyl alcohol 2 Min, the retention rate of the membrane on 5 μm standard particles when filtering liquid is above 99.995%.
The microporous membrane prepared by the above process is analyzed by a scanning electron microscope, specifically referring to fig. 5 and 6, fig. 5 is a scanning electron microscope photograph of a membrane section, and fig. 6 is a scanning electron microscope photograph of a membrane surface, and the results show that: the aperture of the membrane surface is about 10 mu m, the aperture of the section hole is about 5 mu m, the section holes are regular and consistent, and the membrane section presents a funnel-shaped asymmetric structure shape which is gradually reduced from the membrane surface to the middle.
Example eight:
the embodiment discloses a preparation method of a casting solution of an asymmetric flat microporous membrane, wherein the high polymer material is PES, and the preparation method comprises the following steps:
determining the surface tension of PES to be sigma, specifically, the surface tension of PES to be 50dyn/cm;
the surface tension of PES is relatively large, and the surface tension of a solvent capable of dissolving PES is relatively small, so that the funnel-shaped asymmetric PES microporous membrane is more convenient and richer in solvent selection, and specifically, the funnel-shaped asymmetric PES microporous membrane is prepared by preparing a casting solution solvent into a negative adsorption solution, selecting a solvent capable of fully dissolving PES and having a surface tension smaller than sigma as the casting solution solvent, so that the surface concentration of PES in the casting solution is lower than the main concentration, and in order to prepare a membrane with excellent comprehensive performance, the casting solution solvent is a mixed solvent which is suitable for being used, wherein the specific solvent can be a mixed solvent consisting of N, N-dimethylacetamide, N-methylpyrrolidone and butanone.
The preparation method of the asymmetric PES flat microporous membrane comprises the following steps:
the first step, material preparation, add 12.5KgN, N-dimethylacetamide, 12.5 KgN-methyl pyrrolidone, 0.6Kg polyvinylpyrrolidone and 1.2Kg glycerol into a 30L compounding kettle, stir until polyvinylpyrrolidone is fully dissolved, add 4.5KgPES, heat to 60 ℃ after adding, stir for 1.5h, heat to 95 ℃ and stir for 4.0h after PES swells to be transparent, cool to 45 ℃, drip 1.25Kg butanone under stirring, continue stirring for 3.0h after dripping is finished, stand for 3.0h, prepare casting solution for subsequent use after defoaming.
Through calculation, the surface tension of the mixed solvent is less than 50dyn/cm of the surface tension of PES, and the prepared casting solution is a high molecular solution with thermodynamic negative adsorption.
Step two, preparing a microporous membrane, namely coating the membrane casting solution on a stainless steel belt through a membrane casting machine to form a flat microporous membrane, specifically, producing on an automatic membrane scraping machine, wherein the membrane scraping machine is a continuous production process and consists of 7 subsystems of a stainless steel belt rotating mechanism, a membrane casting solution scraping device, a first gas phase mass transfer box, a second liquid phase mass transfer water tank, a washing heat treatment box, a drying box and a membrane winding machine frame; the conveying speed of the stainless steel band is controlled to be 0.9m/min in the preparation process.
The clearance between the knife coating cutter and the stainless steel band is controlled to be 250 mu m, the temperature control value of the cast film liquid is 55 ℃, the temperature control value of the gas phase mass transfer is 65 ℃, and the humidity control value is 40%; the liquid phase mass transfer medium is purified water, and the temperature control value is 45 ℃; the washing and heat treatment medium is purified water, and the temperature control value is 90 ℃; the drying temperature control value is 65 ℃.
And step three, product detection, wherein the microporous membrane prepared in the process is tested, and the result shows that: the thickness of the film is 125 +/-5 mu m,
the first bubble point pressure is 135KPa-145KPa when absolute ethyl alcohol is used as a medium, and the pressure is 20KPa after the membrane is wetted by the ethyl alcoholThe pure water flow rate of the test is 2360L/m 2 .h。
The scanning electron microscope analysis of the microporous membrane prepared by the above process specifically refers to fig. 7 and 8, fig. 7 is a scanning electron microscope photograph of a membrane section, and fig. 8 is a scanning electron microscope photograph of a membrane surface, and the result shows that: the pore distribution on the surface of the membrane is uniform, the aperture ratio is very high, the pore diameter on the surface of the membrane is 1.5-3.5 μm, and the absolute ethyl alcohol is used as a medium to test that the first bubble point pressure value is 135-145 KPa, which is more than 4 times higher than the bubble point pressure calculated by adopting a capillary model; the section of the membrane presents a standard funnel-shaped asymmetric structural morphology gradually reduced from the surface of the membrane to the middle.
Claims (7)
1. The preparation method of the asymmetric PVDF flat microporous membrane is characterized by comprising the following steps:
determining the surface tension of a high polymer material for preparing the microporous membrane to be sigma;
if the pagoda-shaped asymmetric microporous membrane is prepared, selecting a solvent with the surface tension larger than sigma as a casting solution solvent;
if the funnel-shaped asymmetric microporous membrane is prepared, selecting a solvent with the surface tension less than sigma as a casting solution solvent;
adding a high polymer material, a first balance additive and a second balance additive into a casting solution solvent, coating the casting solution on one of a non-woven fabric, a metal belt or a high molecular belt through a film casting machine to form a flat microporous film, wherein the first balance additive is an inorganic salt containing divalent positive charge ions or more, and the second balance additive is an inorganic acid containing a plurality of oxygen elements;
the high polymer material is PVDF;
selecting a solvent with surface tension larger than sigma or a solvent with surface tension smaller than sigma, wherein the solvent is a prepared casting solution solvent;
by utilizing the thermodynamic principle, the surface tension ratio between the solvent of the membrane casting solution and the high molecular material is changed by adjusting the types and the adding proportions of the solvent, the first balance additive and the second balance additive in the membrane casting solution, so that the membrane casting solution has positive adsorption or negative adsorption performance, the distribution state of the high molecular material and the mass transfer behavior of the solvent in the phase conversion process of the membrane casting solution are regulated and controlled, and the control on the morphological structure of a membrane pore is realized;
when the casting solution has positive adsorption performance, namely the surface tension of a solvent of the casting solution is greater than that of PVDF, the surface concentration of PVDF in the casting solution is higher than the main concentration, so that the pagoda-shaped asymmetric PVDF microporous membrane is prepared; when the casting solution has negative adsorption performance, that is, the surface tension of the solvent of the casting solution is less than that of PVDF, the surface concentration of PVDF in the casting solution is lower than that of the main body, so that the funnel-shaped asymmetric PVDF microporous membrane can be prepared.
2. The method of claim 1, wherein the solvent with surface tension greater than σ is one or a mixture of N, N-dimethylacetamide, N-methylpyrrolidone, and dimethylsulfoxide, and the solvent with surface tension greater than σ has a surface tension greater than 26.5dyn/cm.
3. The method for preparing an asymmetric PVDF flat microporous membrane as in claim 1, wherein the solvent with surface tension less than σ is a casting solution solvent prepared from one or a mixture of N, N-dimethylacetamide, N-methylpyrrolidone and dimethyl sulfoxide, and one or a mixture of acetone and butanone, and the surface tension of the solvent with surface tension less than σ is less than 23.5dyn/cm.
4. The method for preparing an asymmetric PVDF flat microporous membrane as claimed in claim 1, wherein said inorganic salt containing not less than divalent positively charged ions is one or a mixture of calcium chloride, aluminum chloride, calcium nitrate and aluminum nitrate.
5. The method for preparing the asymmetric PVDF flat microporous membrane as claimed in claim 1, wherein the inorganic acid containing multiple oxygen elements is one or a mixture of sulfuric acid, nitric acid and phosphoric acid.
6. The method for preparing an asymmetric PVDF flat microporous membrane as in any one of claims 1-5, wherein the preparation temperature of the membrane casting solution is 20-120 ℃, the preparation time is 2-30 h, and the membrane casting solution is used after being treated by a curing process.
7. The method of making an asymmetric PVDF flat microporous membrane as in any of claims 1 to 5, further comprising the steps of,
blade coating, wherein the blade distance is controlled to be 30-900 μm during blade coating;
liquid-phase mass transfer, wherein the temperature of the liquid-phase mass transfer is controlled to be 5-50 ℃;
washing, wherein the washing temperature is controlled to be 10-70 ℃;
heat treatment, wherein the temperature of the heat treatment is controlled to be 75-100 ℃;
drying at 50-80 deg.c;
the non-woven fabric is made of one of polyester, polypropylene, chinlon, spandex and acrylon;
the metal belt is made of one of stainless steel, copper and aluminum;
the material of the polymer belt is one of polyester and polypropylene.
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