WO2014142447A1 - Method for manufacturing fine-porous film and separation membrane for secondary battery using same - Google Patents

Abstract

The present invention relates to a method for manufacturing a fine-porous film by a dry process, which includes the steps of: obliquely drawing a fine-porous film with the elongation axis being oblique to the longitudinal direction of the film so as to prepare a film through a high temperature elongation process; and laminating two or more of the high-temperature obliquely drawn films with the elongation axis of one layer crossing that of the adjacent layer (almost perpendicularly) so as to ensure that the longitudinal physical properties of the film are uniform with the transverse physical properties, and applying the fine-porous film as a separation membrane for a secondary battery.

Description

Separation membrane for secondary battery using a method for producing a microporous membrane and the membrane prepared by the above method

The present invention relates to a method for producing a microporous membrane and a separator for a secondary battery to which the microporous membrane prepared by the above method is applied.

Microporous membranes can be produced by a variety of processes, and the processes by which the membranes are made are known to have a significant impact on the physical properties of the membranes. Commercially used processes can be largely divided into three processes. That is, it may be classified into a dry process, a wet process, and a particle stretching process.

The dry process is produced by extruding a nonporous precursor, and then controlling the orientation of the lamellae by heat treatment such as annealing, and stretching between rolls in a uniaxial direction. Refers to the method of forming pores. (See US Patent 3,426,754, US Patent 4,620,956, US Patent 5,013,439). Generally, as mentioned in the above patents, pores are formed by low temperature stretching and high temperature stretching, and finally, a microporous membrane is finally manufactured through a process such as heat setting. Such a dry process is eco-friendly and price competitive because it does not use an extraction solvent compared to a wet process. However, since the pores are formed by uniaxial stretching in terms of mechanical properties, there is a problem that the tensile strength in the longitudinal direction is lowered.

The wet process involves mixing polymeric raw materials with processing oils (sometimes called plasticizers) and extruding the mixture into sheets to form pores while removing processing oils from the sheets. Say The oil may be stretched before or after the oil is removed, and oil is generally removed after biaxial stretching (see US Pat. No. 5,641,565, US Pat. No. 7,081,321). Therefore, the microporous membrane produced by the wet process maintains the balance between the longitudinal and transverse physical properties due to the biaxial stretching characteristics of the process, and has excellent tensile strength in the longitudinal direction compared to the microporous membrane of the dry process.

The particle stretching process is a method of extruding a sheet by mixing a polymeric raw material and a particulate, and stretching the sheet to form pores around the fine particles as the interface between the polymer and the fine particles is destroyed in the stretching process. . The dry process described above may be classified into a dry process, but the above-described dry process is a process of forming pores between lamellae in a lamellar crystal structure, and the stretching process of particles is a process of forming pores at the interface between the fine particles and the polymer. to be. Particle stretching processes may also be used commercially in combination with wet processes. Both uniaxial stretching and biaxial stretching can be used as a stretching method of the particle stretching step.

As described above, the dry process has advantages and cost competitiveness of not using a solvent in the process, but there is a problem in that the tensile strength in the longitudinal direction is lower than that in the transverse direction. In particular, due to these characteristics, the problem of deterioration of puncture strength and process problems due to breakage in the battery assembly process may be caused. In order to overcome this problem, the following technology development has been progressed.

Korean Patent Laid-Open Publication No. 2008-0085922 proposes a method of improving the longitudinal tensile strength and the transverse tensile strength of a stretched microporous film by introducing a biaxial stretching method in a drying method, but using a laminated lamellar structure of a precursor film ( Due to the stacked lamellar structure, it is very difficult to secure a stretched product that can be rolled up without breaking the film when longitudinal stretching or simultaneous stretching is applied after transverse stretching. U.S. Patent No. 5,667,911 also discloses a precursor film of cylindrical shape, annealing, low temperature stretching and high temperature stretching to produce a cylindrical microporous membrane, and finally slitting while rotating the cylindrical microporous membrane. And the method of winding inclinedly the longitudinal direction and extending | stretching orientation axis | shaft of a film in roll form is proposed. However, this winding method requires a separate winding equipment, and since the moving direction and the winding direction of the film exist diagonally, the tension according to the position of the winding roll during the winding process is uniform, so that wrinkles of the winding process are necessarily inevitable. It is very difficult to generate and deodorize the microporous membrane in roll form.

The present invention has been made to solve the problem of lowering the longitudinal tensile strength and the puncture strength of the microporous membranes according to the conventional techniques described above, and an object of the present invention is to introduce a gradient stretching method into a high temperature stretching process of a dry process, and By laminating the films produced by the method in such a manner that two or more layers of the stretching axes are alternately arranged (the stretching axes are substantially perpendicular), it is possible to secure the balance between the longitudinal and transverse physical properties of the film, that is, the continuous state in the roll state. It is to provide a method for producing a microporous membrane which can control the variation of the physical properties in the longitudinal direction and the transverse direction of the microporous membrane in the state.

Still another object of the present invention is to provide a separator for a secondary battery, which is manufactured by the method of the present invention and employs a microporous membrane having an improved tensile strength in the longitudinal direction compared to a microporous membrane prepared by conventional uniaxial stretching.

The method for producing a microporous membrane according to the invention is characterized in that it comprises the following steps:

(1) extrusion molding a polymer compound or a composition comprising the same to form a precursor film,

(2) annealing the precursor film,

(3) cold drawing the annealed film,

(4) high temperature diagonal stretching of the low temperature stretched film, and

(5) laminating the hot drawn film alternately in two or more layers.

The polymer compound used in the step (1) is preferably a semicrystalline polymer, for example, polyolefin, polyfluorocarbon, polyamide, polyester, polyacetal, polysulfide, polyvinyl alcohol, these It may be a polymer compound selected from the group consisting of copolymers and combinations thereof.

In the step (1), during the extrusion molding of the polymer compound, the reinforcing material, the filler, the antioxidant, the neutralizer, the heat stabilizer, the weather stabilizer, the antistatic agent, the lubricant, and the slip agent within a range that does not interfere with battery operation even when applied to the secondary battery. Various additives, such as a pigment and the like, can be added. The additive is not particularly limited as long as it is a substance known in the art. Among these additives, it is preferable to add an antioxidant to ensure long-term heat resistance and oxidation stability.

In the step (1), a precursor film may be prepared by extrusion molding a resin composition containing a high molecular compound and fine particles as in the particle stretching step.

In the step (1), it is preferable that the lamellae of the precursor film is oriented perpendicular to the machine direction (longitudinal direction) during film formation, and the lamellars are laminated along the machine direction.

The extrusion method for preparing the precursor film is not particularly limited, but a polymer compound or a resin composition may be melted and formed into a film by using a T die or a cyclic die using an extruder of a single screw or a twin screw, and the discharged resin. The air can be injected through the air knife or air ring for the purpose of controlling the temperature of the film and improving the manufacturing state of the film.

In step (2), the non-porous precursor film extruded in step (1) is subjected to an annealing process before stretching, and the annealing is a non-limiting example by putting a film roll in an oven where tropical flow occurs or The method may be performed by applying heat to the film through contact with a heating roll, hot air in a tender, or an IR heater. After annealing, the precursor film preferably has an elastic recovery rate of 70% or more. If the elastic recovery rate is less than 70%, lamellar fracture occurs in a subsequent stretching process, and sufficient pore formation is difficult. The annealing temperature can be treated at a temperature lower than the melting temperature of the semicrystalline polymer compound. Annealing temperature and annealing time may be adjusted according to the elastic recovery rate of the precursor film to be produced.

In the step (3), the film annealed in the step (2) is low-temperature stretching, the low-temperature stretching process can be stretched in one axis (longitudinal direction) using a stretching roll (non-limiting example). The temperature of the low temperature stretching process may be set to a temperature at which cracks may be formed in the amorphous region according to the kind of semicrystalline polymer compound which is a component of the precursor film. For example, the glass transition temperature (Glass Transition Temperature) of the high-molecular compound used is -70 ℃ to glass transition temperature (Glass Transition Temperature) + 70 ℃ is appropriate. Glass transition temperature-less than 70 ℃ to form a crack in the precursor film is difficult, at the temperature of more than glass transition temperature + 70 ℃ occurs a phenomenon that the crack formed again by the thermal motion of the polymer. In the low temperature stretching process, the preferred elongation is 10 to 70%. In the case of low temperature stretching below 10%, there is a problem that the air permeability is lowered after the high temperature stretching because the crack is not sufficiently formed in the amorphous region, and in the case of low temperature stretching above 70%, the air permeability decreases again after the high temperature stretching.

In the step (4), the low-temperature stretched film is stretched at high temperature in step (3), but the high-temperature oblique stretching process is performed by stretching the low-temperature stretched precursor film in an oblique direction as illustrated in FIG. 1, for example. do. The temperature of the high temperature stretching process is appropriate between the melting temperature (Melting Temperature)-40 ℃, and at the temperature below the melting temperature-40 ℃, no pores are formed in the crack of the low temperature stretched film, Pore formation occurs due to the phenomenon of stretching in the longitudinal direction and contraction in the transverse direction, and the formation of pores is difficult due to melting and closing of pores at a temperature above the melting temperature. The diagonal stretching process illustrated in FIG. 1 is a method of holding and stretching a low temperature stretched precursor film with a clip, in which the low temperature stretched precursor film is stretched while the speed of the clip increases while traveling. This stretching method may use a method known as Simultaneous Stretching. Such simultaneous stretching technology can be implemented using components such as pantograph, spindle, and linear motors.

Referring to the high temperature gradient drawing step in more detail, as shown in FIG. 1, (i) a precursor film that has been low-stretched in the direction of arrow (a) is introduced, and (ii) the film is stretched in a diagonal direction at a high temperature. (iii) The film drawn in diagonal direction at high temperature is wound while being conveyed in the direction of arrow (b). As shown in FIG. 1, the width w1 of the inlet and the width w2 of the outlet may be the same or the width of the outlet may be reduced within 10% of the width w1 of the inlet. As pointed out in the problem of the dry biaxial stretching method, the film breakage problem may occur when the outlet width is larger than the width of the inlet. The angle (θ) of the inclined stretching in the high temperature gradient stretching step can be adjusted according to the characteristics of the product, the angle of the inclined stretching is preferably 20 to 65 degrees, more preferably to maintain the 30 to 55 degrees It is advantageous for balancing the physical properties of the product. If the angle of inclined stretching is less than 20 °, the difference in the physical properties in the longitudinal direction and the transverse direction of the separator in which two or more layers are laminated alternately occurs, and it is difficult to simultaneously improve the properties in the longitudinal direction and the transverse direction. In the case of more than ˚, the possibility of breakage in the oblique stretching process increases.

FIG. 2 is a manner in which a preheating section is additionally installed in FIG. 1 to improve stability of high temperature gradient drawing. Specifically, (i) the precursor film low-stretched in the direction of arrow (a) is introduced, (i) the film is preheated while advancing in the direction of arrow (a), and (iii) the arrow (b) in the direction of arrow (a) Direction is stretched in the diagonal direction at high temperature, (iv) the film stretched in the diagonal direction at high temperature is wound while being transported in the direction of the arrow (b). As shown in FIG. 2, the width w1 of the inlet and the width w2 of the outlet may be the same or the width of the outlet may be reduced within 10% of the width w1 of the inlet in the high temperature drawing process.

Optionally the hot stretched film may be heat setting after stretching, as is well known.

In the step (5), as a step of laminating two or more layers of the high temperature gradient stretched film in step (4), an example thereof is illustrated in FIG. FIG. 3 is a method of laminating two layers of films alternately, in which the alignment axes of the hot diagonally stretched films are arranged to cross each other (almost vertically). In the case of lamination of two or more layers, the lamination can be performed while the alignment axes of the films intersect with each other, and in particular, a film made of a polymer compound capable of being shut down at a low temperature in a middle layer is laminated to form a fine multilayer structure. Porous membranes may also be prepared.

The porosity of the microporous membrane prepared by the method of the present invention as described above is preferably 20 to 80%. If the porosity is less than 20%, the ion conductivity of the separator is low, so it is not efficient to use it as a separator. If the porosity is greater than 80%, the mechanical properties of the separator may be reduced.

The separator for a secondary battery, in particular a lithium secondary battery, according to the present invention comprises a microporous membrane prepared by the method of the present invention described above.

According to the method for producing a microporous membrane of the present invention, a high temperature oblique stretching process is introduced to a high temperature stretching process of a dry process, and thus, the stretching axes are alternately arranged in two or more layers of the film produced by high temperature oblique stretching. By laminating to obtain a balance between the longitudinal and transverse physical properties of the film, the microporous membrane having improved tensile strength and puncture strength in comparison with the microporous membrane prepared by conventional uniaxial stretching can be manufactured. . The microporous membrane prepared by the method of the present invention can be usefully applied as a water treatment separation membrane, a breathable film, a secondary battery separator.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows an example of high temperature diagonal stretch in the manufacturing method of the microporous membrane of this invention.

2 is a view showing another example of high temperature oblique stretching in the method for producing a microporous membrane of the present invention.

3 is a view showing an example of the lamination method of a high temperature gradient stretched film in the method for producing a microporous membrane of the present invention.

Hereinafter, the present invention will be described in more detail with reference to the following examples, but the scope of the present invention is not limited to the following examples.

Measuring methods of the overall physical properties of the films prepared in the following Examples and Comparative Examples are as follows.

(1) thickness

The thickness of the membrane is measured according to ASTM D374.

(2) tensile strength

It is measured by an Instron Universal Testing Machine (UTM) according to ASTM D3763.

(3) Gurley

According to the Gurley method of Japanese Industrial Standard (JIS), the time (in seconds) for 100 mL of air to pass through 1 square inch (inch ² ) of microporous film at a constant pressure of 4.8 inch H ₂ O at room temperature is measured.

(4) sting strength

Using the KES-G5 instrument of Kato Tech, Japan, the puncture strength was measured at a speed of 10 mm / sec using a tip of 1 mm in diameter at the distal end.

Example

0.05 parts by weight of antioxidant IRGANOX-1010, 0.05 parts by weight of antioxidant IRFOS-168, and 0.05 parts by weight of calcium stearate as neutralizing agent based on 100 parts by weight of polypropylene (Samsung Total, MFI = 3g / 10min, Tm = 165 ° C) Compounding at 250 ℃ to prepare a polypropylene composition. The compounded pellet is put into a hopper of an extruder with a Ti-die, the extruder temperature is set to 230 ° C., the first filter (hole size 20 μm) set to 230 ° C., and the second filter in the gear pump. The polypropylene composition was extruded by setting the temperature up to (the pore size of 70 μm) at 215 ° C., and the temperature from 200 ° C. immediately after the second filter to the Ti-die. The polypropylene extrudate was adjusted to an extrusion rate such that a draw down ratio was 90 on a casting roll set at 90 ° C. to prepare a precursor film having a thickness of 15 μm.

The prepared precursor film roll was annealed in an oven at 150 ° C. for 24 hours to prepare a film having an elastic recovery rate of 88%.

After stretching at 50% low temperature at room temperature using a uniaxial roll stretching machine, a microporous membrane having a thickness of 13 μm was prepared by stretching at 180 ° C. at 150 ° C. using a tenter type gradient drawing machine as shown in FIG. 1. The prepared microporous membrane was laminated as shown in FIG. 3 to prepare a microporous membrane having a laminated structure having a total thickness of 26 μm.

The air permeability of the microporous membrane was 280 sec / 100ml, the longitudinal tensile strength was 1310 kgf / cm ² , the transverse tensile strength was 1290 kgf / cm ² , and the stabbing strength was 530gf.

Comparative Example 1

Using the same polypropylene composition and conditions as in Example, a precursor film having a thickness of 15 μm was prepared, stretched by 50% at low temperature using a uniaxial roll stretching machine, and then stretched by 180% at 150 ° C. using the same uniaxial roll stretching machine. A 13 micron thick microporous membrane was prepared. The prepared microporous membrane was laminated as shown in FIG. 3 to prepare a microporous membrane having a total thickness of 26 μm. The air permeability of the microporous membrane was 280 sec / 100ml, the longitudinal tensile strength was 1370 kgf / cm ² , the transverse tensile strength was 150 kgf / cm ² , and the stinging strength was 350 gf.

Comparative Example 2

Using the same polypropylene composition and conditions as in Example, a precursor film having a thickness of 29 μm was prepared, stretched by 50% at low temperature using a uniaxial roll stretching machine, and then stretched by 180% at 150 ° C. using the same uniaxial roll stretching machine. A 26 micron thick microporous membrane was prepared. The air permeability of the microporous membrane was 270 sec / 100ml, the longitudinal tensile strength was 1340 kgf / cm ² , the transverse tensile strength was 130 kgf / cm ² , and the stabbing strength was 310gf.

Comparing the above Example and Comparative Example 1, it can be confirmed that the structure in which the alignment axes are laminated in the same manner can not improve the transverse tensile strength and the puncture strength. When the post lamination is applied, it can be seen that the transverse tensile strength and the puncture strength are improved compared to the microporous membrane prepared by the general dry method. In addition, the embodiment of the present invention provides a microporous membrane in which the longitudinal and transverse physical properties are secured in a continuous roll form, that is, the tensile strength and the puncture strength of the longitudinal direction are improved compared to the microporous membrane prepared by conventional uniaxial stretching. It can be confirmed that.

Claims (6)

1. A method of making a microporous membrane, comprising the following steps:

   (1) extrusion molding a polymer compound or a composition comprising the same to form a precursor film,

   (2) annealing the precursor film,

   (3) cold drawing the annealed film,

   (4) high temperature diagonal stretching of the low temperature stretched film, and

   (5) alternately laminating the hot diagonal stretched film in two or more layers.
2. The method of claim 1, wherein the polymer compound is a semicrystalline polymer compound.
3. The method of claim 2, wherein the polymer compound is selected from the group consisting of polyolefins, polyfluorocarbons, polyamides, polyesters, polyacetals, polysulfides, polyvinyl alcohols, copolymers thereof, and combinations thereof. A method of producing a microporous membrane.
4. The method of manufacturing a microporous membrane according to claim 1, wherein the inclined stretching angle at the time of high temperature gradient stretching in the step (4) is 20 to 65 °.
5. The method of claim 1, wherein the porosity of the microporous membrane is 20 to 80%.
6. Separation membrane for a secondary battery to which the microporous membrane prepared by the method of any one of claims 1 to 5.

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