COMPOSITE POROUS MEMBRANE AND PREPARATION METHOD THEREFOR AND USE THEREOF

Abstract

A composite porous membrane contains at least one porous base layer and at least one uniaxially stretched coating layer located on at least one side surface of the porous base layer. For example, the composite porous membrane comprises at least one porous base layer and at least one nanofiber-like non-polyolefin polymer porous layer oriented along the transverse stretching direction of the composite porous membrane and located on one or two side surfaces of the porous base layer, or the composite porous membrane comprises a biaxially stretched polypropylene porous base layer and a uniaxially stretched coating layer located on at least one side surface of the porous base layer. The composite porous membrane is coated with a coating solution prior to transversely stretching. The nanofiber-like non-polyolefin polymer porous layer may reduce cracking of the composite porous membrane in the machine direction.

Claims

1. A composite porous membrane, which comprises at least one porous base layer and at least one uniaxially stretched coating layer located on at least one side surface of the porous base layer.

2. The composite porous membrane of claim 1, wherein said composite porous membrane comprises a biaxially stretched porous base layer and a uniaxially stretched coating layer located on at least one side surface of the porous base layer. Preferably, said composite porous membrane further comprises a biaxially stretched coating layer located on at least one side surface of said porous base layer. Preferably, said composite porous membrane further comprises a biaxially stretched coating layer located on at least one side surface between said porous base layer and said uniaxially stretched coating layer. Preferably, said uniaxially stretched coating layer located on at least one side surface of said porous base layer is a porous coating layer, or a non-porous dense coating layer. Preferably, said biaxially stretched coating layer located on at least one side surface of said porous base layer is a porous coating layer, or a non-porous dense coating layer.

3. The composite porous membrane of claim 1, wherein said composite porous membrane comprises at least one porous base layer and at least one nanofiber-like non-polyolefin polymer porous layer oriented along the transverse stretching direction of the composite porous membrane and located on one or two side surfaces of said porous base layer. Preferably, said composite porous membrane comprises at least one porous base layer and at least one nanofiber-like non-polyolefin polymer porous layer oriented along the transverse stretching direction of the composite porous membrane; preferably, said composite porous membrane comprises a porous base layer and a nanofiber-like non-polyolefin polymer porous layer oriented along the transverse stretching direction of the composite porous membrane; said nanofiber-like non-polyolefin polymer porous layer oriented along the transverse stretching direction of said composite porous membrane is located on one side surface of said porous base layer; or said composite porous membrane comprises a porous base layer and two nanofiber-like non-polyolefin polymer porous layers oriented along the transverse stretching direction of the composite porous membrane; the nanofiber-like non-polyolefin polymer porous layers oriented along the transverse stretching direction of said composite porous membrane are located on two side surfaces of said porous base layer. Preferably, said non-polyolefin polymer is preferably selected from the group consisting of non-polyethylene polymer, non-polypropylene polymer; preferably, the non-polyolefin polymer is preferably selected from non-polypropylene polymer. Preferably, the diameters of said nanofibers are 10-500 nm, preferably 15-250 nm.

4. The composite porous membrane of claim 1, wherein, said porous base layer is selected from porous polyolefin base layers, and used as the porous polyolefin base layer. In particular, it is advantageous to use a porous polyethylene base layer, a porous polypropylene base layer or a porous polypropylene/polyethylene/polypropylene tri-layer composite base layer. More preferably, the porous base layer is selected from porous polypropylene based layers.

5. The method for producing the composite porous membrane of claim 1, comprising the following steps of: (S1) adding a nucleating agent into polypropylene to promote the formation of -crystal; (S2) melting and extruding polypropylene mixed with the nucleating agent in step (S1), moulding to obtain a film with high content of -crystal; (S3) stretching the film with high content of -crystal in the machine direction to obtain an axial stretching film, and coating a coating solution on one or two side surfaces of said axial stretching film; (S4) transversely stretching the axial stretching film coated with a coating solution to obtain said composite porous membrane. Preferably, the following steps are also comprised between step (S2) and step (S3) of said method for producing the composite porous membrane: (S2) coating the coating solution on one or two side surfaces of the film with high content of -crystal obtained in step (S2), to obtain a film with high content of -crystal coated with a coating solution. Preferably, in step (S3) and step (S2), said coating solution comprise a solution using an organic solvent as a medium, or a solution or a dispersion using water as a medium; said solution using an organic solvent as a medium comprises a solution formed by dissolving polymer or polymer compositions with an organic solvent; said solution using water as a medium comprises a solution formed by dissolving water-soluble polymer or water-soluble polymer compositions with water; the dispersion using water as a medium comprise polymer or polymer composition emulsions obtained by the emulsion polymerization of water-insoluble polymer or water-insoluble polymer compositions, or aqueous dispersion containing polymer or polymer compositions obtained by grinding or pulverizing water-insoluble polymer or water-insoluble polymer compositions. Preferably, said solution using an organic solvent as a medium or said solution or dispersion using water as a medium further comprises inorganic fillers. Preferably, in said solution using an organic solvent as a medium, said polymer or the polymer compositions comprise one or more of polyvinylidene fluoride, poly(vinylidene fluoride-co-hexafluoropropylene) copolymer, polyacrylonitrile, poly(vinylidene chloride), polymethacrylate, polyethylene, polyethylene wax, chlorinated polyethylene, chlorinated polypropylene, poly(vinyl alcohol), polyurethane, poly(methyl methacrylate-co-acrylonitrile) copolymer, poly(oxyethylene ether), sodium alginate, cellulose derivatives, polydimethylsiloxane, polyimide, polyurethane, poly(sodium styrenesulfonate), sulfonated poly(ether ether ketone), poly(vinyl alcohol)-graft-poly(vinyl sulfonic acid) copolymer, sulfonated polysulfone, sulfonated polybenzimidazole, sulfonated poly(phenylquinoline), perfluorosulfonic acid polymer (e.g., Nafion), etc. Preferably, in said solution using an organic solvent as a medium, said organic solvent is selected from ketone solvent such as acetone, butanone, etc., alcohol solvent such as methanol, ethanol, etc., halogenated hydrocarbon solvent such as chloromethane, dichloromethane, chloroform, carbon tetrachloride, etc., amide solvent such as N,N-dimethylformamide, N,N-dimethylacetamide, etc., aromatic solvent such as benzene, toluene, xylene, etc., tetrahydrofuran, N-methylpyrrolidone, dimethyl sulfoxide, or diethyl ether, etc. Preferably, in said solution using water as a medium, said water-soluble polymer or the water-soluble polymer compositions comprise one or more of poly(vinyl alcohol), poly(oxyethylene ether), sodium alginate, sodium carboxymethyl cellulose, polyacrylamide, chitosan, konjac glucomannan, etc. Preferably, in said dispersion using water as a medium, said water-insoluble polymer or the water-insoluble polymer compositions comprise one or more of poly(vinylidene fluoride), poly(vinylidene chloride), polytetrafluoroethylene, poly(vinylidene fluoride-co-hexafluoropropylene) copolymer, polystyrene, poly(methyl methacrylate), polyurethane, perfluorosulfonic acid polymer (e.g., Nafion), etc.

6. The method for producing the composite porous membrane of claim 1, comprising the following steps of: coating a coating solution on one or two side surfaces of the axial stretching film, and then transversely stretching to obtain said composite porous membrane. Preferably, after said axial stretching film is coated, it is then transversely stretched, wherein the axial stretching film is treated with the transversely stretching, i.e., to obtain said porous base layer of the present invention; wherein the axial stretching film coated with the coating solution is treated with the transversely stretching, i.e., to obtain said nanofiber-like non-polyolefin polymer porous layer oriented along the transverse stretching direction of the composite porous membrane. Preferably, the porosity of said axial stretching film is 5-35%. More preferably, the porosity of said axial stretching film is 15-35%. Preferably, the thickness of said axial stretching film is 16-80 m; more preferably, the thickness of said axial stretching film is 20-60 m. Preferably, said composite porous membrane is prepared by the following method of: 1) adding a nucleating agent into polypropylene to promote the formation of -crystal; melting and extruding the nucleating agent contained polypropylene, moulding to obtain a film with high content of -crystal; 2) stretching the film with high content of -crystal in the machine direction to obtain an axial stretching film, coating a coating solution on one or two side surfaces of the axial stretching film, and then transversely stretching to obtain said composite porous membrane. Preferably, the thickness of said film with high content of -crystal is 80-300 and more preferably, the thickness of said film with high content of -crystal is 100-200 Preferably, in step (S3) and step 2), said axial stretching temperature is 60-120 C., preferably, said axial stretching temperature is 80-110 C.; said axial stretching ratio is 2.5-5.5 times, preferably, said axial stretching ratio is 3-5 times; after said coating solution is dried, the thickness of the obtained coating layer is 0.3-10 m; preferably, after said coating solution is dried, the thickness of the obtained coating layer is 2-10 more preferably 3-5 Preferably, in step (S4) and step 2), said transverse stretching temperature is 120-150 C.; said transverse stretching ratio is 2-5 times; more preferably, said transverse stretching temperature is 130-140 C.; said transverse stretching ratio is 2.5-4.5 times. Preferably, said composite porous membrane is prepared by the following method of: a) melting and extruding polypropylene, drawing at a high speed for oriented moulding to obtain a polypropylene precursor film; b) annealing the polypropylene precursor film at a high temperature to obtain a polypropylene pre-stretched film; c) stretching the polypropylene pre-stretched film by using a dry uniaxial stretching process to obtain an axial stretching film, coating a coating solution on one or two side surfaces of the axial stretching film, and then transversely stretching to obtain the composite porous membrane. According to the present invention, step c) specifically comprises the following steps of: c) multilayer laminating the polypropylene pre-stretched film, after pre-stretching 20-50% at 20-50 C., continuously stretching 30-80% at 100-130 C., and optionally peeling the multilayer laminated axial stretching film after stretching, to obtain an axial stretching film; after that, coating a coating solution on one or two side surfaces of the axial stretching film, and then transversely stretching to obtain said composite porous membrane. Preferably, in step c), said multilayer lamination process is carried out by laminating at least one layer of the polypropylene pre-stretched film, and after stretching, an axial stretching film comprising at least one layer is obtained; more preferably, 8-16 layers of the polypropylene pre-stretched film are carried out with multilayer lamination treatment, and after stretching, an axial stretching film comprising 1-4 layers is obtained by interlaminar peeling. Preferably, said coating solution is a dispersion using water as a medium; said coating solution comprises water-insoluble polymer or water-insoluble polymer compositions; said water-insoluble polymer or said water-insoluble polymer compositions are dispersed in the system in the form of particles, which have an average diameter of 0.01-3 m, more preferably 0.1-1 m. Preferably, said dispersion in aqueous medium is a polymer or polymer composition emulsion obtained by the emulsion polymerization of water-insoluble polymer or water-insoluble polymer compositions, or an aqueous polymer or polymer composition dispersion obtained by grinding or pulverizing water-insoluble polymer or water-insoluble polymer compositions. Preferably, the glass transition temperatures or melting points of said water-insoluble polymer or said water-insoluble polymer compositions are lower than the transverse stretching temperatures. Preferably, said water-insoluble polymer or said polymer compositions comprise one or more of poly(vinylidene fluoride), poly(vinylidene chloride), poly(vinylidene fluoride-co-hexafluoropropylene) copolymer, polystyrene, poly(methyl methacrylate), poly(butylene succinate), polyurethane, perfluorosulfonic acid polymer (e.g., Nafion), etc. Preferably, the mass percent of said water-insoluble polymer or said water-insoluble polymer compositions in the dispersion using water as a medium is 5-50%, more preferably 10-30%. Preferably, the mass percent of said water in the dispersion using water as a medium is 50-95%, more preferably 70-90%. Preferably, said coating solution also comprises organic fillers or inorganic fillers; said inorganic fillers comprise one or more of silica, titanium dioxide, cerium oxide, zirconium dioxide, aluminum oxide, barium sulfate, calcium carbonate, carbon nitride, boehmite, silicon carbide, molecular sieves, talc, montmorillonite. Preferably, said organic fillers comprise one or more of high-temperature resistant polymer such as polytetrafluoroethylene, poly(ether ether ketone), polyethersulfone, poly(phenylene oxide), etc. Preferably, the added amount of said organic fillers is 5-10 wt % of the coating solution; and the added amount of said inorganic fillers is 5-20 wt % of the coating solution. Preferably, said coating solution also comprises a binder; said binder comprises a polyacrylate emulsion, a polyurethane emulsion, a poly(butadiene-styrene) emulsion, poly(vinyl alcohol), carboxymethyl cellulose, etc.; the added amount of said binder is 2-8 wt % of the coating solution; preferably 3-6 wt %.

7. A separator for lithium batteries, comprising any of the composite porous membranes of claim 1.

8. A gas separation membrane, comprising any of the composite porous membranes of claim 1.

9. A facility is used to produce any of the composite porous membranes of claim 1, comprising a feeding and extrusion system, a film casting system, an axial stretching system, a second coating system, and a transverse stretching system; said feeding and extrusion system is connected to the axial stretching system via the film casting system, and said axial stretching system is connected to the transverse stretching system via the second coating system. Preferably, said feeding and extrusion system comprises feeding equipment, a screw extruder, a first melt pipe, a filter and a second melt pipe; said second melt pipe is connected with the film casting system; said feeding equipment is located above the feeding port of the screw extruder; said screw extruder is connected to the filter via the first melt pipe; the filter is connected to the film casting system via the second melt pipe. Preferably, a metering pump is also arranged in said first melt pipe. Preferably, said film casting system comprises a slit die and a casting roller; said slit die is selected from an automatically thickness-adjusted slit die, and said casting roller is selected from an accurately temperature-controlled casting roller. Preferably, said facility also comprises a first coating system; said first coating system is arranged between the film casting system and the axial stretching system. Preferably, said first coating system comprises a first coating part; said second coating system comprises a second coating part. Preferably, said axial stretching system comprises preheating through precise temperature control, stretching and a set of heat setting rollers; said transverse stretching system comprises a rail, a chain, an oven and a driving mechanism. Preferably, said facility also comprises a traction winding system, the purpose of which is to pass the prepared composite porous membrane through said traction winding system to obtain a roll of the composite porous membrane. Preferably, said facility also comprises a thickness feedback control system; said thickness feedback control system comprises a thickness gauge and a control system; said thickness gauge online measures film thicknesses and controls the automatically adjusted the slit die by using the control system to realize the automatic control of the thickness of the porous membrane. Preferably, said thickness feedback control system comprises the first thickness feedback control system and the second thickness feedback control system; said first thickness feedback control system is arranged between the film casting system and the axial stretching system; said second thickness feedback control system is arranged after the transverse stretching system. Preferably, said first thickness feedback control system comprises a film thickness gauge, which is used to measure and control the thickness of a non-stretched film or to measure and control the thickness of a non-stretching film coated on at least one side with a coating solution. Preferably, said second thickness feedback control system comprises a film thickness gauge, which is used to measure and control the thickness of a stretching film. Preferably, the facility also comprises an automatic control system; the automatic control system comprises a pressure and temperature control module, a PLC, a frequency converter and a sensor for controlling temperature, pressure, tension and speed of the facility.

10. A method for producing the composite porous membrane, which is prepared by using the facility of claim 9, and the method comprises the following steps of: (a) melting polypropylene containing a nucleating agent, which is capable of promoting the formation of -crystalline form, via the feeding equipment into the screw extruder, the melt flows through the first melt pipe and the metering pump, and after accurately measured and filtered through the filter, enters the second melt pipe; (b) casting the melt of step (a) through the slit die onto the casting roller to crystallize and form a film with high content of -crystal; (c) peeling the film with high content of -crystal of step (b) from the casting roller; optionally, coating a coating solution on one or two side surfaces of the film through the first coating system; optionally, monitoring the thickness of the film with high content of -crystal by using the film thickness gauge, and measuring the thickness of the non-stretching film or the thickness of the non-stretching film coated on at least one side with the coating solution; (d) axially stretching the non-stretching film or the non-stretching film coated on one or two side surfaces with a coating solution of step (c) through the axial stretching system to obtain an axial stretching film; (e) coating the coating solution on one or two side surfaces of the axial stretching film of step (d) through the second coating system; (f) passing the film of step (e) through the transverse stretching system to obtain a transverse stretching film, and thus obtain said composite porous membrane. According to the invention, the method for producing the composite porous membrane also comprises the following steps of: (g) passing the composite porous membrane of step (f) through the traction winding system to obtain a roll of the composite porous membrane; optionally, before passing through the traction winding system, monitoring the thickness of the composite porous membrane by using the film thickness gauge.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0127] FIG. 1 is a scanning electron micrograph of the composite porous membrane of Example 1 of the present invention.

[0128] FIG. 2 is a scanning electron micrograph of the composite porous membrane of Comparative Example 1 of the present invention.

[0129] FIG. 3 is a view of the facility for preparing the composite porous membrane of the present invention;

[0130] the markings in the drawing are described as follows, 1 is a feeding equipment; 2 is a screw extruder; 3 is a metering pump; 4 is a filter; 5 is a second melt pipe; 6 is a slit die; 7 is a film casting roller; 8 is a first coating part; 9 is a film thickness gauge; 10 is an axial stretching system; 11 is a second coating part; 12 is a transverse stretching system; 13 is a film thickness gauge; 14 is a traction winding system; 15 is an automatic control system.

EXAMPLES

[0131] In order to make the objects, technical schemes and advantages of the present invention clearer, the present invention is further described in detail hereinafter with reference to the specific embodiments and accompanying drawings. However, it is understood by those skilled in the art that the present invention is not limited to the drawings and the following embodiments.

Example 1

[0132] A polypropylene homopolymer resin having a melt flow index of 2.5 g/10 min and a -crystal nucleating agent of N,N-dicyclohexyl-2,6-naphthalenediamide which was 0.03 wt % of the polypropylene homopolymer resin were uniformly mixed. After melting at a temperature of 200-250 C., a polypropylene layer melt was formed; After the polypropylene layer melt was extruded through a T-slit die, it was cooled and crystallized on a 128 C. casting roller to obtain a polypropylene film containing crystal, i.e., a film with high content of -crystal. After the film was stretched 4.5 times in the machine direction at 100 C., two side surfaces of the film were coated with an aqueous dispersion with a solid content of 20% and poly(vinylidene fluoride) with a particle size of 200 nm to form a coating layer having a thickness of 6 m, respectively, and then the film entered into the transverse stretching system and was transversely stretched 3.0 times at 135 C. to obtain a 20 m thick composite polypropylene porous membrane. Among them, the thicknesses of the two poly(vinylidene fluoride) layers were 2 respectively, and the thickness of the core polypropylene porous base layer was 16 The diameters of the polyvinylidene fluoride fibers in the two surfacial poly(vinylidene fluoride) layers were 30-70 nm.

[0133] FIG. 1 is a scanning electron microscope image of the composite porous membrane of Example 1 of the present invention.

Example 2

[0134] A polypropylene homopolymer resin having a melt flow index of 2.5 g/10 min was melted at 200-250 C. through a single screw extruder, extruded through a T-slit die, and wound up at a speed of 50 m/min to obtain a highly oriented polypropylene film. After the polypropylene film was annealed and crystallized at 125 C., a polypropylene pre-stretched film was obtained. After the polypropylene pre-stretched film was pre-stretched by 40% at 30 C., an axial stretching film having a porosity of 30% was obtained by continuously stretching by 50% at 125 C. Two side surfaces of the film were coated with an aqueous dispersion with a solid content of 20% and poly(vinylidene fluoride) with a particle size of 200 nm to form a coating layer having a thickness of 6 respectively, and then the film entered into the transverse stretching system and was transversely stretched 4.0 times at 140 C. to obtain a 20 m thick composite polypropylene porous membrane. Among them, the thicknesses of the two poly(vinylidene fluoride) layers were 2 respectively, and the thickness of the core polypropylene porous base layer was 12 The diameters of the poly(vinylidene fluoride) nanofibers in the two surfacial poly(vinylidene fluoride) layers were 30-50 nm.

Example 3

[0135] In Example 3, except that the aqueous dispersion with a solid content of 15% and poly(methyl methacrylate) with a particle size of 500 nm, the rest was the same as in Example 1 to obtain that the thicknesses of the two poly(methyl methacrylate) layers were 2 respectively, and the thickness of the core polypropylene porous base layer was 16 The diameters of the poly(methyl methacrylate) nanofibers in the two skin layers were 80-120 nm.

Comparative Example 1

[0136] A polypropylene homopolymer resin having a melt flow index of 2.5 g/10 min and the polypropylene with a -nucleating agent of N,N-dicyclohexyl-2,6-naphthalenediamide which was 0.03 wt % of the polypropylene homopolymer resin were uniformly mixed. After melting at a temperature of 200-250 C., a polypropylene layer melt was formed;

[0137] After the polypropylene layer melt was extruded through a T-slit die, it was cooled and crystallized on a 128 C. casting roller to obtain a polypropylene film containing crystal, i.e., a film with high content of -crystal. After the film was stretched 4.5 times in the machine direction at 100 C., it entered into the transverse stretching system and was transversely stretched 3.0 times at 135 C. to obtain a 16 m thick composite polypropylene base film. Two side surfaces of the polypropylene base film were coated with an aqueous dispersion with a solid content of 20% and poly(vinylidene fluoride) with a particle size of 200 nm to form a coating layer having a thickness of 2 m, respectively, and dried to obtain a 20 m thick composite polypropylene porous membrane.

[0138] FIG. 2 is a scanning electron microscope image of the composite porous membrane of Comparative Example 1 of the present invention.

[0139] As shown in FIG. 1, by using the method of the present invention, after the poly(vinylidene fluoride) coating layer, spherical particles of 200 nm size aggregation obtained by coated on the surface of the axial stretching polypropylene film, was stretched accompanying with the transversely stretching of the substrate, the spherical poly(vinylidene fluoride) particles were deformed to nanofibers oriented along the transverse stretching direction, the nanofibers having a diameter of about 10-70 nm. As shown in FIG. 2, without using method of the present invention, the coating was not transversely stretched, poly(vinylidene fluoride) coating layer with spherical particles aggregation obtained.

Example 4

[0140] A polypropylene homopolymer resin having a melt flow index of 2.5 g/10 min and a -crystal nucleating agent of N,N-dicyclohexyl-2,6-naphthalenediamide which was 0.03 wt % of the polypropylene homopolymer resin were uniformly mixed, and added into the single screw extruder 2 via the feeding equipment 1. After being melt at 160-230 C., it was measured with the metering pump 3, passed through the filter 4, and entered the T-slit die 6 through the second melt pipe 5 and extruded. It was cooled down on the casting roller 7 to obtain a polypropylene film with a -crystal nucleating agent. The thickness of the polypropylene film with the -crystal nucleating agent was 140 m.

[0141] The film entered into the axial stretching system 10 via the film thickness gauge 9, and was stretched 4.5 times at 100 C. in the machine direction to obtain an axial stretching film.

[0142] Two side surfaces of the axial stretching film were coated with an aqueous dispersion of poly(methyl methacrylate) with a solid content of 25 wt % through the second coating system, and the thickness of the coating after drying was 6 m. The coated axial stretching film was transversely stretched 3.0 times at 135 C. in a transverse stretching system to obtain a composite polypropylene porous membrane, which was coated on two side surfaces with poly(methyl methacrylate) and exhibited porous structure on the sides.

[0143] The thickness of the coating layer was 2 m; the thickness of the composite porous membrane was 20 m.

[0144] When the composite polypropylene porous membrane prepared as described above was used as lithium ion battery separators, the wettability and the absorbency of the separators towards lithium ion battery electrolytes were obviously improved compared to those of single-layer polypropylene separators.

Example 5

[0145] Example 5 was the same as Example 4 except that the axial stretching film was coated on one side with a Nafion solution (purchased from DuPont Company) through the coating system (II), the thickness of the coating after drying was 3 m, and then it was transversely stretched to obtain a composite polypropylene porous membrane, which was coated on one side with Nafion and exhibited non-porous dense structure on the side.

[0146] The thickness of the coating layer was 1.0 m; the thickness of the composite polypropylene porous membrane was 19 m.

[0147] When the composite polypropylene porous membrane prepared as described above was used as lithium ion battery separators, due to the non-porous dense structure of the Nafion layer on the surface, polysulfide dissolved in electrolytes could not be transported through the separators, but lithium ions could be bound to sulfonic acid groups of Nafion to be transported. Therefore, the composite polypropylene porous membrane could alleviate polysulfide shuttling effect in lithium-sulfur batteries and improve the cycling performance of lithium-sulfur batteries.

Example 6

[0148] Example 6 was the same as Example 4 except that the axial stretching film was coated on one side with a solution of polydimethylsiloxane in xylene through the second coating system, the thickness of the coating after drying was 6 m, and then it was transversely stretched to obtain a composite polypropylene porous membrane, which was coated on one side with polydimethylsiloxane and exhibited non-porous dense structure on the side.

[0149] The thickness of the coating layer was 2 m; the thickness of the composite polypropylene porous membrane was 20 m.

[0150] When the composite polypropylene porous membrane prepared as described above was used as a gas separation membrane, the separation of ethanol/water solution by pervaporation achieved good effect.

Example 7

[0151] A facility was used to produce the composite polypropylene porous membranes in Examples 4-6, which comprised a feeding and extrusion system, a film casting system, an axial stretching system, a second coating system, and a transverse stretching system;

[0152] the feeding and extrusion system was connected to the axial stretching system via the casting system, and the axial stretching system was connected to the transverse stretching system via the second coating system.

[0153] In a preferred embodiment of the present invention, the feeding and extrusion system comprises a feeding equipment 1, a screw extruder 2, a first melt pipe, a filter 4 and a second melt pipe 5;

[0154] the second melt pipe 5 is connected with the film casting system; the feeding equipment 1 is located above the feeding port of the screw extruder 2; the screw extruder 2 is connected to the filter 4 via the first melt pipe; the filter 4 is connected to the film casting system via the second melt pipe 5; the metering pump 3 is also arranged in the first melt pipe.

[0155] In a preferred embodiment of the present invention, the film casting system includes a slit die 6 and a casting roller 7; the slit die 6 is selected from an automatically thickness-adjusted slit die; the casting roller 7 is selected from an accurately temperature-controlled casting roller.

[0156] In a preferred embodiment of the present invention, the facility also comprises a first coating system; the first coating system is arranged between the film casting system and the axial stretching system; the first coating system comprises a coating part 8; the second coating system comprises a second coating part 11.

[0157] In a preferred embodiment of the present invention, the axial stretching system 10 comprises preheating through precise temperature control, stretching and a set of heat setting rollers; the transverse stretching system 12 comprises a rail, a chain, an oven and a driving mechanism.

[0158] In a preferred embodiment of the present invention, the facility also comprises a traction winding system 14.

[0159] In a preferred embodiment of the present invention, the facility also comprises a thickness feedback control system; the thickness feedback control system comprises a thickness gauge and a control system; the thickness gauge online measures film thicknesses and controls the automatically adjusted the slit die by using the control system to realize the automatic control of the thickness of the polypropylene porous membrane.

[0160] In a preferred embodiment of the present invention, the thickness feedback control system comprises a first thickness feedback control system and a second thickness feedback control system; the first thickness feedback control system is arranged between the film casting system and the axial stretching system; the first thickness feedback control system is arranged between the first coating system and the axial stretching system; the second thickness feedback control system is arranged after the transverse stretching system; the second thickness feedback control system is arranged between the transverse stretching system and the traction winding system 14.

[0161] The first thickness feedback control system comprises a film thickness gauge 9; the second thickness feedback control system comprises a film thickness gauge 13, the purpose of which is to measure and control the thickness of a stretching film.

[0162] In a preferred embodiment of the present invention, the facility also comprises an automatic control system 15; the automatic control system 15 comprises a pressure and temperature control module, a PLC, a frequency converter and a sensor.

[0163] The embodiments of the present invention are described above. However, the present invention is not limited to the above embodiments. Any modification, equivalent alternative, improvement, etc., falling within the spirit and scope of the present invention, are intended to be comprised within the scope of the present invention.