KR100529719B1 - Microporous composite membrane made by dry uniaxial stretching followed by surface modification process and making the same - Google Patents

Abstract

The present invention relates to a microporous composite membrane used in a secondary battery and a method for manufacturing the same, and more particularly, by using a polymer material processed by uniaxial stretching as a base, to shorten the processing process and to modify the surface of the base. The present invention relates to a multicomponent microporous composite membrane having improved interfacial adhesion, and a method of manufacturing the same. To this end, the multicomponent microporous composite membrane of the present invention includes a base portion obtained by dry uniaxial stretching and surface modification treatment of a polyolefin-based polymer material, and a coating film obtained by adhering a fluoropolymer material to both sides of the base portion. In addition, the present invention is a process for producing a polymer mixture for the base and the coating film, a process for producing a base film, and performing a uniaxial stretching to improve the mechanical direction (MD) tensile strength of the base film and It includes a process of surface modification to both sides of the base film to have a hydrophilicity, and the step of adhering the polymer compound for coating film on both sides of the base film.

Description

Microporous composite membrane made by dry uniaxial stretching followed by surface modification process and making the same} Dry uniaxial stretching / surface modified multicomponent microporous composite membrane

The present invention relates to a microporous composite membrane used in a secondary battery and a method for manufacturing the same, and more particularly, by using a polymer material processed by uniaxial stretching as a base, to shorten the processing process and to modify the surface of the base. The present invention relates to a multicomponent microporous composite membrane having improved interfacial adhesion, and a method of manufacturing the same.

Microporous membranes are widely used in various fields such as medical dialysis, environmental filtration, food purification, etc. Recently, microporous membranes are widely used as separators of lithium secondary batteries (lithium ion batteries, lithium polymer batteries, etc.). In particular, in the lithium polymer battery, the microporous membrane functions not only as a separator for the anode and the cathode but also as a medium for ion conduction, that is, an electrolyte. As described above, the microporous membrane used as the separator and the electrolyte of the battery may be mainly implemented using a polyolefin resin. The polyolefin resin including polyethylene and polypropylene has a high crystallinity and may be used as a separator of a lithium secondary battery. In this case, not only the tensile strength, the stiffness and the impact strength can be improved, but also a large improvement in the ion permeability can be expected.

The method for producing a microporous membrane using the polyolefin resin described above is briefly described. A precursor film is prepared to anneal the original film, followed by stretching to form fine pores, followed by hot setting. ) Therefore, the microporous membrane manufacturing method using the original film is largely classified into MSS (stretching method: 1 phase), TIPS (thermal induction phase separation: 2 phases), phase invesion (three phase separation: 3 phases). The dry manufacturing method, the MSS method using only stretching), and the wet manufacturing method, the phase invesion method using a solvent, are mainly used.

By the way, the dry manufacturing method (hereinafter referred to as dry uniaxial stretching method) to impart fine pores by uniaxial stretching (1 axis stretching) of the dry manufacturing method is the most economical because the production process is simple and mass production is possible, MD (machine direction) tensile strength can be improved and there is an advantage of an eco-friendly manufacturing method by using no organic solvent. However, on the other hand, there was a problem that the tensile strength of the TD (product width direction) was low and it was difficult to secure the permeability strength due to dendritic crystals when used in a battery. Therefore, a dry manufacturing method (hereinafter, referred to as dry biaxially) in which fine pores are provided by biaxially oriented stretching to improve MD (machine direction) and TD (product width direction) tensile strength without using a solvent. A method of producing a microporous membrane by a new method has been devised. However, although the microporous membrane by the dry biaxial stretching method has achieved the purpose of improving MD and TD tensile strength, it has to have two unit processes of biaxial stretching rather than a single process of uniaxial stretching. As a result, production economy was more problematic than uniaxial stretching. In other words, the manufacturing by the dry biaxial stretching method requires a complex Tentering M / C for stretching the product width direction (TD), and in parallel with this, by stretching the machine direction (MD) with the product width direction (TD) Since there is a need to control, there was a problem in that there is a limitation in the large facilities and productivity, ease of operation.

On the other hand, the wet manufacturing method is a method for producing a microporous composite membrane using a solvent, such a wet manufacturing method has an environmental problem due to the problem of using an organic solvent. In addition, such a wet manufacturing method has a problem in that high ionic conductivity cannot be expected due to the limitation of the residual solvent, solvent, and pore size determined thereby.

Therefore, in order to improve the ion conductivity, as shown in FIG. 1, as a microporous composite membrane having a triple-layer structure, a porous first polymer (even though it does not absorb liquid electrolyte or absorbs liquid electrolyte, absorbs only a small amount of liquid and only liquids in pores) A microporous composite comprising a polymer filling the electrolyte as a base 100 and a gelling second polymer (a polymer that is gelled and swollen by absorbing itself when contacted with a liquid electrolyte) as a coating film (first coating film / second coating film, 102,104). It has been proposed to use membranes as separators for batteries (US Pat. Nos. 5,639,573, 5,716,421, 5,631,103, 5,849,433, and European Patent Publication No. 0933824A2). The porous first polymer used for the base portion 100 is a material that hardly swells by limiting the absorption of the liquid electrolyte into the pores. Polyethylene, polypropylene, polytetrafluoroethylene, polyethylene Terephthalate (polyethylene terephthalate), polybutylene terephthalate (polybutyleneterephthalate), polyethylene naphthalate (polythylenenaphthalate) and the like. In addition, the gelling second polymer used in the first and second coating layers 102 and 104 may be polyvinylidene fluoride (PVDF), polyvinylidene fluoride hexafluoropropylene copolymer, or polyvinylidene fluoride-hexafluorofluoropropylene copolymer. ), Polyethylene oxide, polypropylene oxide, polybutylene oxide, polyacrylonitrile, polyacrylic acid, polysulfone, polycarbonate ( polycarbonate, polyphenylene oxide, polyvinylidene chloride, and derivatives thereof, which have been used as one or more materials selected from the group consisting of.

 Such a structure can improve the mechanical strength as desired, but due to the resistance of the second gelling polymer of the coating membranes 102 and 104 to the ionic conductivity of only the porous first polymer layer of the base portion 100 and the liquid electrolyte incorporated therein. It has lower ionic conductivity than that. Therefore, low molecular weight plasticizers such as dibutyl phthalate are required to improve mechanical strength and improve ionic conductivity (US Pat. No. 5,631,103), which also faces environmental problems. . In addition, in the case of the multi-layered film produced by the above method, the coating films 102 and 104 of the gelled second polymer layer existing on the base portion 100 of the porous first polymer layer become dense structures without pores, thereby increasing ion conductivity resistance. It was large and lacked the interfacial adhesion between the two layers, there was a problem in maintaining the long-term performance of the battery.

Accordingly, there has been a demand for a microporous composite membrane that is both environmentally friendly and economical, such as dry uniaxial stretching, and that satisfies mechanical strength and ion conductivity simultaneously, such as a triple membrane structure using a porous first polymer, a gelled second polymer, and a plasticizer. In addition, in the conventional microporous composite membrane manufacturing of the triple layer structure, since the base and the coating film are adhered to each other by physical adsorption method, the interfacial adhesion between the base and the coating film is lowered, which leads to various disadvantages such as battery assembly and battery performance deterioration. I had.

The present invention has been proposed to solve the above problems, unlike the conventional method of using a porous first polymer on the base of the microporous composite membrane having a triple layer structure without stretching processing, the base is a dry monoaxial It is processed by stretching to secure the product width direction (TD) tensile strength and puncture strength, and at the same time to apply a coating film on the base of the dry uniaxially stretched, as a result, to secure the MD, TD tensile strength at the same time. In addition, an object of the present invention is to propose a chemical surface modification method for improving the interfacial adhesion between the base and the composite film.

In order to achieve the above object, the multi-component microporous composite membrane of the present invention is a polyolefin-based polymer material, which is manufactured by a dry uniaxial stretching method for improving the mechanical direction (MD) tensile strength, and the fluorine-based polymer material. It includes a coating film adhered to both sides of the base. The multi-component microporous composite membrane is a process for preparing a polymer formulation for the base by blending a polyolefin-based high molecular material to be used in the base, and preparing a polymer blend for the coating membrane by mixing a fluorine-based polymer material to be used in the coating membrane, Producing a base film extruded and molded to a predetermined size, and performing a uniaxial stretching to improve the machine direction (MD) tensile strength of the base film, and the uniaxial stretching Surface modification treatment of both sides of the base film of the inert, hydrophobic component to have a hydrophilicity, and the step of adhering the prepared polymer blend for coating film on both sides of the base film to produce a microporous composite membrane.

Hereinafter, the detailed description of the preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the following description of the reference numerals to the components of the drawings it should be noted that the same reference numerals as possible even if displayed on different drawings. In addition, in describing the present invention, when it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted. In addition, terms to be described below are terms defined in consideration of functions in the present invention, which may vary according to general practices of those belonging to the technical field of the present invention, and the definitions should be made based on the contents throughout the present specification.

FIG. 2 is a diagram illustrating a configuration block of a multicomponent microporous composite membrane according to the present invention. FIG. (Hereinafter, the multicomponent microporous composite membrane is a dry uniaxial stretching method, unlike the conventional microporous composite membrane which uses the porous first polymer as it is without the stretching process as a base material as shown in Fig. 1). The microporous composite membrane of the present invention having a base processed by is referred to as a multicomponent microporous composite membrane)

The multi-component microporous composite membrane according to the present invention is composed of a base portion 200 and first and second coating membranes 202 and 204. Compared with the conventional microporous composite membrane shown in FIG. .

First, the base portion 100 shown in FIG. 200) is characterized in that the polyolefin system is stretched by a dry uniaxial stretching method. Polyolefin-based polymer material to be used in the base portion 200 of the multi-component microporous composite membrane according to the present invention, the polyolefin is a high molecular weight polyethylene (Ultra high molecular weight polyethylene), high molecular weight polyethylene (High molecular weight polyethylene), high density polyethylene (High density polyethylene), Low density polyethylene, Linear low density polyethylene, Polypropylene, High crystalline polypropylene, Polyethylene-proopylene Co and at least one of the polymers, and mixtures thereof, is used alone or in combination to be subjected to dry uniaxial stretching. Scanning electron micrographs of the base surfaces 212 and 214 made of the material are shown in FIG. 3, as shown in the drawing. Ultra-high molecular weight polyethylene powder, high density polyethylene wax, low density polyethylene wax, linear low density polyethylene wax, ultra low density polyethylene, etc. should be added in order to induce crystallization of the crystals and amorphous regions that make the pore size constant. Since the production method is a known fact, it will be obvious to those skilled in the art, and thus a detailed description thereof will be omitted.

Polyfluoride is used as a material for the first and second coating layers 202 and 204 bonded to both sides 212 and 214 of the base, and the polyfluoride is polyvinylidene fluoride (PVDF) or polyvinylidene fluoride. Polyvinylidene fluoride hexafluoride propylene co-polymer, polyethylene oxide, polypropylene oxide, polybuthylene oxide, polyurethane, polyacrylo Preference is given to at least one material selected from nitriles, polyacrylates, polyvinylidene chlorides and derivatives thereof.

In addition, as a solvent for the polymer material used in the first and second coating films 202 and 204, it may be selected according to each material, but generally acetone, ethanol, 1methyl-2 picolidon ( 1methyl-2pyrrolidone, n-butanol, n-propanol, n-propanol, n-hexane, n-hexane, acetic acid, ethyl acetate, dimethyl ether ether, dimethyl acetamide (DMAc), dioxane, tetrahydrofuran (THF), benzene, toluene, xylene, water, and mixtures thereof It is preferable that at least one selected from the group consisting of

On the other hand, when comparing the conventional microporous composite membrane shown in Figure 1 and the multi-component microporous composite membrane according to the present invention shown in Figure 2, the second feature, between the base portion 200 and the coating (202, 204) film (212,214) Has an improved interfacial adhesion. In the conventional microporous composite membrane, since the base and the coating membrane are adhered to each other by physical adsorption, there are various problems such as battery assembly difficulty and performance degradation. Accordingly, the multi-component microporous composite membrane of the present invention performs chemical surface modification of the base 200 and the coating films 202 and 204 instead of physical adsorption, thereby increasing the adhesion between the base and the coating film, thereby facilitating battery assembly and improving performance. It can be planned. The surface modification treatment is performed by corona discharge treatment on the surfaces 212 and 214 of the base portion 200 to improve hydrophilicity. This surface modification treatment process will be described later in detail at step S408 of FIG. 4.

4 is a flowchart illustrating a process of manufacturing the multicomponent microporous composite membrane.

Referring to Figure 2, in order to produce a multi-component microporous composite film according to the present invention, first, a step (S402) of mixing the polymer material to be used in the base portion 200 and the coating film (202, 204). In other words, a compound having a lower molecular weight has a process of compounding a higher molecular weight material so that it can be used in the base and the coating film.

Base portion 200 is a polyolefin-based resin as described above in Figure 2, ultra high molecular weight polyethylene (Ultra high molecular weight polyethylene), high molecular weight polyethylene (High molecular weight polyethylene), high density polyethylene (Low density polyethylene), low density polyethylene (Low) density polyethylene, linear low density polyethylene, polypropylene, high crystalline polypropylene, polyethylene-proopylene co-polymer and mixtures thereof may be used. However, in the embodiment of the present invention, a process of generating a polymer material to be used at the bottom by blending ultra high molecular weight polyethylene (Ultra high molecular weight polyethylene) and high density polyethylene (high density polyethylene) will be described as an example. In other words, the polymer material used for the base part is 5% by weight of ultra high molecular weight polyethylene (2.5 million molecular weight), 90% by weight of high density polyethylene (molecular weight 400,000) using Henschel mixer, Bambari mixer, and French mixer. 1010 3% by weight, polyethylene wax 2% by weight is produced by mixing for 1 hour at room temperature. 2% by weight of polyethylene wax in the above was added to the pore generation of the base has already been described in FIG. It will be apparent that the polymer material blending process and blending ratio to be used in such a base can be made by various methods known in addition to the above method.

Meanwhile, the polymer material to be used for the coating films 202 and 204 bonded to both sides 212 and 214 of the base 200 is a polyfluoric resin, as described above with reference to FIG. 2, and includes polyvinylidene fluoride (PVDF) and polyvinyl resin. Polyvinylidene fluoride hexafluoride propylene co-polymer, polyethylene oxide, polypropylene oxide, polybuthylene oxide, polyurethane, poly Acrylonitrile (polyacrylonitrile), polyacrylate (polyacrylate), polyvinylidene chloride (polyvinylidene chloride) and derivatives thereof may be used, as an embodiment of the present invention polyvinylidene fluoride ( The process of compounding the coating film using PVDF) will be described.

For example, preparing a compound to be used for a coating film using polyvinylidene fluoride is to use a solvent mixer by weighing 10 to 27 WT% by weight of polyvinylidene fluoride (PVDF) of 100,000 to 170,000 molecular weight As a weight part 90-73WT% of 1 methyl-2pyrrolidone (NMP) is added and blended for 3 hours at a 60 ° mixing temperature, and then the air bubbles are removed for 1 hour under vacuum to produce a first blend. Likewise, polyvinylidene fluoride homopolymer (PVDF homopolymer) having a molecular weight of 100,000 to 170,000 was weighed by 8 to 16 WT%, and added to 1 part by weight of 92 to 84 WT% of 1methyl-2pinolidon (NMP). After mixing for 3 hours at a blending temperature of 60 degrees, bubbles are removed under vacuum for 1 hour to produce a second blend. Thereafter, 20 to 40 WT% by weight of the produced first mixture and 60 to 80 WT% by weight of the produced second mixture are mixed and kneaded under the same compounding conditions as above for 2 hours to remove bubbles, thereby coating the final formulation. To produce a polymeric material to be used.

On the other hand, when the polymer material to be used in the base (hereinafter referred to as the base polymer blend) and the polymer material to be used in the coating film (called the polymer blend for coating film) is produced through the blending process (S402), the base polymer blend is separated The base polymer blended to perform a role is extruded and molded into a 100 um thick film to produce a base film (S404). The extrusion and molding are extruded into a precursor film having a thickness of 100 um using a coat hanger type T-dies in a L / D (Length / diameter) 38: 1 or more twin screw extruder. At this time, the temperature of the die is 210 degrees, and the melt-pump is installed for uniformity of the discharge amount, and the hot air circulation tank is continuously passed to induce orientation and crystallization.

After the step (S404) of extruding and molding the base polymer compound in the form of a 100 μm thick base film as described above (S404), the base 100 μm thick film is stretched to form a 25 μm thick film in a uniaxial direction. It has a uniaxial stretching process (S406) to increase the strength. The polymer material used in the base of the present invention is stretched by a dry uniaxial stretching method, the stretching process (S406) is carried out uniaxial stretching in the machine direction (MD). Therefore, cold drawing for cleavage of the lamella layer at room temperature is carried out using a roll drawing machine at first, heat-setting, and continuously hot-heating at a temperature below the melting point of the polymer in a hot-air heating bath, and then heat-setting at a lower temperature to a base film having a thickness of 25 um. Create By having the dry uniaxial stretching process, the multicomponent microporous composite membrane of the present invention can be economically and environmentally friendly by uniaxially stretching the monocomponent system in the machine direction (MD). On the other hand, the extrusion, molding process and uniaxial stretching process is known in the art it will be apparent that the extrusion, molding / stretching can be made in a variety of methods in addition to the above method.

After the base film is formed through the extrusion, molding (S404) and stretching (S406), the surface of the base film is modified (S408). The modification treatment refers to a chemical surface modification process for improving the interfacial adhesion of the base film in order to improve the adhesion between the base film and the coating film. In the conventional microporous composite membrane, since the surface of the base film and the coating membrane are adhered to each other by physical adsorption, there are various problems such as battery assembly difficulty and battery performance deterioration. Accordingly, the multi-component microporous composite membrane of the present invention performs chemical surface modification of the base film and the coating film instead of physical adsorption, thereby increasing the adhesive strength of the base film and the coating film, thereby facilitating battery assembly and improving battery performance. It is.

The surface modification process S408 is performed by corona discharge treatment on the surface of the base film. The corona discharge is a process in which a strong portion of the electric field emits light and has conductivity when a high voltage is applied to the base film, and a partial discharge is generated as light or sound in a portion where the electric field is strengthened. In general, since the surface of the polyolefin polymer film exhibits inertness and strong hydrophobicity, it is necessary to hydrophilize both sides of the base film in order to improve adhesion between the coating film and the base film, which are fluorinated monomers. Corona discharge is performed as a pre-process prior to the adhesion of the fluorine-based coating film for this hydrophilization treatment.

After the surface modification process S408 called corona discharge of the base film, the coating film is adhered to the corona discharged base film (S410). That is, the final compound produced by fusing the first compound and the second compound in the blending process for use in the fluorine-based polymer solution, that is, the coating film on both sides of the base film using slot dies, the first coating film and the second coating film After 2μm of each coating thickness, ultrapure water was added and phase separation was carried out, followed by drying and volatilization in a hot air drying furnace at 65 degrees. In the above, the mutual adhesion of both sides of the first coating film, the second coating film, and the base film is improved and adhered to the prior art. This is because the base film having inertness and strong hydrophobicity is subjected to a surface modification process and thus has high hydrophilicity, thereby having high adhesive strength between them. It can be.

On the other hand, the phase separation (phase separation) is made of the same method as the phase separation applied in the three-component phase invesion (three phase separation: 3 Phase) method consisting of polymer + solvent + non solvent in the conventional microporous composite membrane manufacturing method, Phase separation is the addition of ultrapure distilled water (non-solvent) to a homogeneous polymer solution consisting of polymer and solvent, resulting in unstable conditions, resulting in the formation of pores in the coating film. It is to be.

After completion of the coating film adhesion process (S410) has a process of roll pressing (S412) to more secure the adhesion of the base film and the coating film. This roll-pressing process is similar to the method of adhering the coating film when the conventional microporous composite film is produced. In order to secure the surface smoothness, thickness, and adhesion of the coating film adhered to the base film, the roll surface temperature is 90 degrees, and the gap between the roll and the roll. Roll pressing is performed at a gap of 24 um (S412). The coating film surface (PVDF coating film surface) of the multi-component microporous composite film produced through each process is shown in FIG. 5.

As a result, the multicomponent microporous composite membrane, which is finally finished through the above processes, exhibits 1700 kg / cm 2 and 300 kg / cm 2 in machine direction (MD) and product width direction (TD) tensile strength, respectively, and the pore size of the base is 0.05. ~ 0.1um, the pore size of the coating film on both sides of the base is 0.2 ~ 0.5um, the porosity is 45% of the base, 51% of the coating film on both sides. Therefore, it can be seen that the mechanical strength (MD) and product width (TD) tensile strengths are improved compared to the conventional microporous composite membrane.

Although the technical spirit of the present invention has been described in detail according to the above-described preferred embodiment, the above-described embodiment is for the purpose of description, and various ordinary embodiments of the present invention may be made by those skilled in the art. I can understand.

As described above, the present invention, by preparing the base portion of the microporous composite membrane by dry uniaxial stretching, and by adhering the coating film it is possible to secure the tensile strength and to improve the productivity, environmentally friendly. In addition, by improving the interfacial adhesion of the coating film to the base surface by chemically modifying the surface on both sides of the base part, it is possible to produce a battery easily and improve battery performance when the microporous composite membrane is used in a lithium secondary battery or the like.

1 is a view showing the internal configuration of a conventional microporous composite membrane.

Figure 2 is a diagram showing the internal configuration of the multi-component microporous composite membrane according to the present invention.

3 is an electron micrograph of the base surface processed by dry uniaxial stretching according to the present invention.

4 is a flowchart illustrating a process of generating a multicomponent microporous composite membrane according to the present invention.

Figure 5 is an electron micrograph of the surface of the outer coating film of the multi-component microporous composite membrane of the present invention.

* Description of the symbols for the main parts of the drawings *

S402: Generation of polymer formulation for base and coating membrane

S404: Extrusion, molding of polymer compound for base

S406: Base film uniaxially stretched

S408: Surface Modification of Base Film

S410: Bonding the coating film to the base film

S412: roll pressing treatment

Claims (7)

In the microporous composite membrane consisting of a base and a coating film attached to both sides of the base to serve as a separator and an electrolyte, A base made of a polyolefin-based polymer material which is processed and manufactured by a dry uniaxial stretching method for improving the mechanical strength (MD) tensile strength; Coating film adhered to both sides of the base as a fluoropolymer material Dry uniaxial stretching / surface modified multi-component microporous composite membrane comprising a. The method of claim 1, The base portion is a multi-component microporous composite membrane, characterized in that the surface modification treatment to increase the hydrophilicity of the base portion in order to improve the interfacial adhesion. The method of claim 2, The hydrophilicity of the base portion is a multi-component microporous composite membrane, characterized in that the surface modification treatment through a corona discharge. The method according to any one of claims 1 to 3, The base polyolefin-based polymer material is ultra high molecular weight polyethylene (Ultra high molecular weight polyethylene), high molecular weight polyethylene (High molecular weight polyethylene), high density polyethylene (Low density polyethylene), low density polyethylene (Linear low density polyethylene) Linear low density polyethylene, polypropylene, high crystalline polypropylene, polyethylene-proopylene co-polymer, and mixtures thereof Multi-component microporous composite membrane, characterized in that the material. In the method for producing a microporous composite membrane consisting of a base and a coating film attached to both sides of the base to serve as a separator and an electrolyte, Preparing a polymer blend for the base by blending a polyolefin-based polymer material to be used in the base, and preparing a polymer blend for the coating by blending a fluorine-based polymer to be used in the coating film; A process of producing a base film extruded and molded to a predetermined size for the prepared base polymer blend, Performing uniaxial stretching to improve the machine direction (MD) tensile strength of the base film; Surface modification treatment of both sides of the uniaxially-stretched inert, hydrophobic base film with hydrophilicity, A process for producing a microporous composite membrane by adhering the prepared polymer blend for coating film on both sides of the base film Method for producing a dry uniaxial stretching / surface modified multi-component microporous composite membrane comprising a. The method of claim 5, The surface modification process is a method for producing a dry uniaxially stretched / surface modified multi-component microporous composite membrane, characterized in that the treatment to be hydrophilic through the corona discharge. The method according to claim 5 or 6, After finishing the process of producing the microporous composite membrane dry uniaxial stretching / surface modification further comprising the step of a roll pressing process to a predetermined thickness in order to ensure the smoothness, thickness, adhesion of the microporous composite membrane To prepare a multicomponent microporous composite membrane.