KR20130120190A - Laminated porous film, preparation method thereof and separator for secondary cell comprising same - Google Patents

Abstract

A laminate porous film having a structure in which a first resin layer including poly propylene resin and β crystal nucleation agent and a second resin layer including poly propylene resin and wax having crystallinity of 80% or greater and melting temperature of 90 deg. C or greater are laminated in two or more layers has improved shutdown, breaking, and air transmission rate, thereby being used for a separator for a secondary cell and the likes.

Description

Laminated porous film, a method for manufacturing the same, and a separator for a secondary battery including the same {LAMINATED POROUS FILM, PREPARATION METHOD THEREOF AND SEPARATOR FOR SECONDARY CELL COMPRISING SAME}

The present invention relates to a laminated porous film having excellent shutdown (SD) and breakdown (BD) characteristics, and air permeability, a method of manufacturing the same, and a separator for a secondary battery including the same.

Battery separators are largely divided into polypropylene (PP) monolayer separators, polyethylene (PE) monolayer separators, and PE / PP laminate separators. In the case of PP monolayer separators, the shutdown properties are relatively low, while the fracture properties are good. In contrast, PE monolayer membranes exhibit good shutdown characteristics and poor fracture characteristics.

On the other hand, since the membrane of the PE / PP laminated structure has both the breaking performance of the PP and the shutdown performance of the PE, it has been used in recent years as a product to complement the disadvantages of the two products. Japanese Unexamined Patent Publication Nos. 2010-61973, 2010-61974, and 2010-120217 are mainly composed of a PP resin and a PE resin using a β crystal nucleating agent, and a modified polyolefin resin, an alicyclic saturated hydrocarbon resin, or modified form thereof. A laminated porous film comprising at least one selected from a sieve, an ethylene copolymer and a wax is disclosed.

However, these conventional films are different from each other by the characteristics of the type of thermoplastic resin added to the PE resin layer it is difficult to secure sufficient air permeability required for the separation membrane when applied in the same manner as the conventional, there is a limitation in the manufacturing process.

Japanese Patent Publication No. 2010-61973 Japanese Patent Publication No. 2010-61974 Japanese Patent Publication No. 2010-120217

Accordingly, it is an object of the present invention to provide a laminated porous film having excellent air permeability with shutdown and breaking characteristics, a method of manufacturing the same, and a separator for a secondary battery including the same.

In accordance with the above object, the present invention is 1) a first resin layer comprising a polypropylene resin and β crystal nucleating agent; And 2) a laminated porous film comprising a polyethylene resin and a second resin layer including a wax having a melting temperature (mp) of 90 ° C. or higher and a crystallinity of 80% or more, in a laminated structure of two or more layers. To provide.

In addition, the present invention comprises the steps of a) adding a β crystal nucleating agent to the polypropylene resin and melt kneading to prepare a first resin; b) adding a wax having a melting temperature of 90 ° C. or higher and a crystallinity of 80% or more to the polyethylene resin and melt kneading to prepare a second resin; c) extruding and laminating the prepared first and second resins, respectively, to form an unstretched sheet in which the first resin layer and the second resin layer are laminated; And d) provides a method for producing the laminated porous film comprising the step of stretching the prepared unstretched sheet.

In another aspect, the present invention provides a secondary battery separator comprising the laminated porous film.

The laminated porous film according to the present invention has excellent air permeability with shutdown and breaking characteristics, and can be usefully applied to a separator for a lithium secondary battery.

Hereinafter, the present invention will be described more specifically.

Laminated porous film of the present invention, 1) a first resin layer made of a polypropylene resin containing a β crystal nucleating agent; And 2) a second resin layer made of a polyethylene resin comprising a wax having a melting temperature of 90 ° C. or higher and a crystallinity of 80% or more in a laminated structure of two or more layers.

In the present invention, the β crystal nucleating agent included in the first resin layer is N, N'-dicyclohexyl-2,6-naphthalenedicarboxamide, N, N'-dicyclohexyl terephthalamide, N, N'- Dicyclohexanecarbonyl-p-phenylenediamine, N, N'-dibenzoyl-1,5-diaminonaphthalene, N, N'-dicyclohexanecarbonyl-1,4-diaminocyclohexane, N- It may be selected from the group consisting of cyclohexyl-4-benzamide and mixtures thereof, and preferably N, N'-dicyclohexyl-2,6-naphthalenedicarboxamide can be used.

The β-crystal nucleating agent is preferably included in 0.0001 to 5 parts by weight, more preferably 0.001 to 1 part by weight based on 100 parts by weight of polypropylene resin.

The polypropylene resin used in the present invention preferably has a melt index (MI) of 1 to 20 g / 10 minutes.

In the present invention, the second resin layer can form a porous structure by adding wax to the polyethylene resin. The wax has a melting temperature of 90 ° C. or higher, more preferably 95 ° C. or higher, for example, 90 ° C. to 150 ° C., and a crystallinity of 80% or higher, more preferably 85% or higher. Due to this, it can represent the air permeability that can be used as a separator for a lithium secondary battery.

If the melting temperature of the wax is less than 90 ° C., the wax melts in the longitudinal (MD) stretching process, preventing the formation of the pore structure of the adjacent polypropylene resin surface, thereby preventing sufficient air permeability of the laminated porous film. In this case, in order to ensure sufficient air permeability, the longitudinal stretching temperature must be treated below 90 ° C., but the breakage property of the film increases at a temperature below 90 ° C., which causes a considerable problem in productivity and film forming stability. Therefore, the melting temperature of the wax used by this invention needs to be 90 degreeC or more.

In addition, when the crystallinity of the wax is less than 80%, stretching occurs together with the polyethylene layer in the stretching step, so that sufficient porous structure of the polyethylene layer cannot be formed and thus sufficient air permeability cannot be exhibited. Therefore, the crystallinity of the wax used in the present invention needs to be 80% or more.

The wax component having such melting temperature and crystallinity may be a polar wax or a nonpolar wax, and specific examples thereof include polypropylene wax, polyethylene wax, castor oil wax, microcrystalline wax, paraffin wax, monte wax, ceresin wax, and substitution Amide waxes, or mixtures thereof, are possible.

Such wax is preferably included in 5 to 30% by weight, more preferably in the range of 5 to 20% by weight based on the weight of the second resin layer.

The polyethylene resin used in the present invention preferably has a melt index (MI) of 1 to 20 g / 10 minutes, for example, low density polyethylene, linear low density polyethylene, linear ultra low density polyethylene, medium density polyethylene, high density polyethylene, A resin selected from the group consisting of ultra high density polyethylene, copolymers containing ethylene as a main component, and blend resins thereof can be used, and among these, high density polyethylene resins are preferably used.

In addition, the second resin layer may further include an α crystal nucleating agent. The α crystal nucleating agent may be a nucleus of crystal growth when the polyethylene resin is cooled in a molten state, thereby enabling rapid crystal formation, and thus may play a role of generating many small and uniform crystals. The α crystal nucleating agent may be included in an amount of preferably 0.0001 to 5 parts by weight, more preferably 0.001 to 1 part by weight based on 100 parts by weight of polyethylene resin. Possible α-nucleating agents include dibenzylidine sorbitol; Aliphatic / alicyclic / aromatic carboxylic acids; Aliphatic amides; Inorganic particles such as silica, talc, kaolin and calcium carbide; Glycerol; Higher fatty acid esters such as glycerin monoester; And mixtures thereof.

The laminated porous film of the present invention is two or more layers, for example, may be 2 to 5 layers.

Hereinafter, the manufacturing method of the laminated porous film of the present invention will be described.

The laminated porous film of the present invention comprises the steps of: a) adding a β crystal nucleating agent to a polypropylene resin and melt kneading to prepare a first resin; b) adding a wax having a melting temperature of 90 ° C. or higher and a crystallinity of 80% or more to the polyethylene resin and melt kneading to prepare a second resin; c) extruding and laminating the prepared first and second resins, respectively, to form an unstretched sheet in which the first resin layer and the second resin layer are laminated; And d) stretching the prepared unstretched sheet.

As a specific example, in step a), β crystal nucleating agent (for example, N, N'-dicyclohexyl-2,6-naphthalenedicarboxa) in 100 parts by weight of polypropylene resin having a melt index of 1 to 20 g / 10 minutes Mead) may be melt-kneaded after adding 0.0001 to 5 parts by weight to prepare a first resin. At this time, the melt kneading temperature is preferably 250 ° C or less, for example, may be 200 to 230 ° C, the prepared first resin may be cooled pelletized.

In step b), a second resin may be prepared by adding a wax and an α crystal nucleating agent having a melting temperature of 90 ° C. or higher and a crystallinity of 80% or more to a polyethylene resin having a melt index of 1 to 20 g / 10 minutes, followed by melt kneading. have. At this time, the melt kneading temperature is preferably 230 ℃ or less, for example, may be 180 to 220 ℃, the second resin produced can be cooled pelletized.

In step c), the first resin and the second resin prepared in steps a) and b) are melt-extruded and cooled and solidified in a casting roll to obtain an unstretched sheet in which the first resin layer and the second resin layer are laminated. can do. At this time, the melt extrusion temperature is preferably less than 250 ℃, for example, may be 200 to 230 ℃, the temperature of the casting roll is preferably 100 ℃ or more, for example may be 110 to 130 ℃.

In step d), the laminated sheet produced in step c) can be filmed by successive biaxial stretching. The stretching step may be stretched 2 to 10 times in the longitudinal and transverse directions, respectively, in a temperature range of 90 ° C. or higher. At this time, when the stretching temperature is less than 90 ℃, since the breakability increases and there is a significant problem in film forming stability, the stretching temperature according to the production method of the present invention is preferably 90 ℃ or more, for example, contained in the second resin layer It may be a temperature of more than 90 ℃ does not exceed the melting temperature of the wax to be. In one example, the laminated sheet may be stretched 2 to 10 times in the longitudinal direction at 95 ° C., and then stretched 2 to 10 times in the transverse direction at 100 ° C. to form a film.

The laminated porous film obtained by the above method may implement dimensional stability through a heat treatment process at 100 ℃ or more.

The laminated porous film according to the present invention may exhibit a time at which 100 mL air passes through a film width of 645 mm 2 at a temperature of 25 ° C. measured according to ASTM D726, that is, 25 ° C. Gurley air permeability of 500 seconds / 100 mL or less. .

In addition, the laminated porous film according to the present invention has a Gurley air permeability after heating for 30 seconds in a 135 ℃ oven measured in accordance with ASTM D726 for 30 seconds after the 100 mL air passes through the film width 645 mm 2, that is, heating at 135 ℃ 30 seconds It may exceed 40,000 seconds / 100 mL. It can be seen that the film of the present invention has excellent shutdown properties.

As described above, the porous polypropylene film of the present invention not only shows excellent fracture and shutdown properties but also has excellent air permeability, and thus may be usefully used in secondary battery separators.

Accordingly, the present invention provides a secondary battery separator including the laminated porous film. Preferably, the secondary battery is a lithium secondary battery.

The laminated porous film according to the present invention preferably has a total thickness of 10 to 50 μm in order to be used as a separator in a secondary battery.

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

Example 1 Preparation of Laminated Porous Film

Polypropylene prepared by adding 0.2 part by weight of N, N'-dicyclohexyl-2,6-naphthalenedicarboxamide as a β crystal nucleating agent based on 100 parts by weight of polypropylene resin (melt index (MI) 3.0 g / 10 min) The propylene resin composition was melt kneaded at 220 ° C. and cooled pelletized at room temperature to prepare Chip A.

10 parts by weight of a microcrystalline wax having a melting temperature of 91 ° C. and a crystallinity of 85% based on 100 parts by weight of a polyethylene resin (MI 5.5 g / 10 minutes), and 0.3 part by weight of an α crystal nucleating agent were added. The prepared polyethylene resin composition was melt kneaded at 220 ° C., and cooled pelletized at room temperature to prepare Chip B.

Chip A and Chip B prepared above were extruded into layer A and layer B respectively at a temperature of 220 ° C. in an extruder, and stacked in three layers (A / B / A = 3: 1: 3 thickness ratio) using a feed block. After cooling solidified in a casting roll at 120 ℃ to prepare a laminated sheet.

The laminated sheet prepared above was drawn four times in the longitudinal direction at 95 ° C., and then three times in the transverse direction at 100 ° C. to prepare a laminated porous film.

Example 2: Preparation of Laminated Porous Film

A laminated porous film was prepared in the same manner as in Example 1, except that the polyethylene resin composition was prepared using a microcrystalline wax having a melting temperature of 101 ° C. and a crystallinity of 90%.

Example 3: Preparation of Laminated Porous Film

A laminated porous film was prepared in the same manner as in Example 1, except that the polyethylene resin composition was prepared using a microcrystalline wax having a melting temperature of 116 ° C. and a crystallinity of 92%.

Example 4: Preparation of Laminated Porous Film

A laminated porous film was prepared in the same manner as in Example 1, except that the polyethylene resin composition was prepared using a microcrystalline wax having a melting temperature of 128 ° C. and a crystallinity of 87%.

Comparative Example 1: Preparation of Laminated Porous Film

A laminated porous film was prepared in the same manner as in Example 1, except that the polyethylene resin composition was prepared using a microcrystalline wax having a melting temperature of 84 ° C. and a crystallinity of 87%.

Comparative Example 2: Preparation of Laminated Porous Film

A laminated porous film was prepared in the same manner as in Example 1, except that the polyethylene resin composition was prepared using a microcrystalline wax having a melting temperature of 109 ° C. and a crystallinity of 65%.

The performance of the films prepared in Examples 1 to 4 and Comparative Examples 1 and 2 were evaluated in the following manner, and the results are shown in Table 1 below.

Test Example 1: Air Permeability (25 ° C)

Gurley measuring equipment (Gurley type densometer G-B3C, Toyoseiki Co., Ltd.) according to ASTM D726 at a temperature of 25 ℃ was measured the time for 100mL air to pass through the film width 645mm2.

Test Example 2: Air Permeability (135 ° C)

After heat treatment in an oven at 135 ° C. for 30 seconds, 100 mL air was passed through a film width of 645 mm 2 using a Gurley measuring instrument (Gurley type densometer G-B3C, Toyoseiki Co., Ltd.) according to ASTM D726. .

Item unit Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Film total thickness ㎛ 36 38 39 38 35 32 Wax melting temperature ℃ 91 101 116 128 84 109 Wax crystallinity % 85 90 92 87 87 65 25 ℃ air permeability Seconds / 100mL 493 299 342 365 1060 6375 135 ℃ air permeability
(shut down) Seconds / 100mL 40000 <
shut down 40000 <
shut down 40000 <
shut down 40000 <
shut down 40000 <
shut down 40000 <
shut down

As shown in Table 1, the shutdown performance was effective for both the films of the examples and the comparative examples. However, it can be seen that only the films of Examples 1 to 4 using waxes having a melting temperature of 90 ° C. or higher and a crystallinity of 80% or more showed suitable air permeability as a separator.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, It is to be understood that the invention may be practiced within the scope of the appended claims.

Claims (12)

1) a first resin layer containing a polypropylene resin and a β crystal nucleating agent; And
2) A laminated porous film comprising a polyethylene resin and a second resin layer containing a wax having a melting temperature (mp) of 90 ° C. or higher and a crystallinity of 80% or more in a laminated structure of two or more layers. .
The method of claim 1,
The β-nucleating agent is N, N'-dicyclohexyl-2,6-naphthalenedicarboxamide, N, N'-dicyclohexyl terephthalamide, N, N'-dicyclohexanecarbonyl-p-phenylene Diamine, N, N'-dibenzoyl-1,5-diaminonaphthalene, N, N'-dicyclohexanecarbonyl-1,4-diaminocyclohexane, N-cyclohexyl-4-benzamide and their Laminated porous film, characterized in that selected from the group consisting of a mixture.
The method of claim 1,
The first resin layer is characterized in that the β crystal nucleating agent based on 0.0001 to 5 parts by weight based on 100 parts by weight of the polypropylene resin, laminated porous film.
The method of claim 1,
Laminated porous film, characterized in that the polypropylene resin has a melt index (MI) of 1 to 20 g / 10 minutes.
The method of claim 1,
Laminated porous film, characterized in that the wax is selected from the group consisting of polypropylene wax, polyethylene wax, castor oil wax, microcrystalline wax, paraffin wax, montene wax, ceresin wax, substituted amide wax and mixtures thereof.
The method of claim 1,
Laminated porous film, characterized in that the laminated porous film comprises the wax in the range of 5 to 30% by weight based on the weight of the second resin layer.
The method of claim 1,
Laminated porous film, characterized in that the polyethylene resin has a melt index (MI) of 1 to 20 g / 10 minutes.
The method of claim 1,
The second resin layer is selected from the group consisting of dibenzylidene sorbitol, aliphatic carboxylic acid, alicyclic carboxylic acid, aromatic carboxylic acid, aliphatic amide, silica, talc, kaolin, calcium carbide, glycerol, higher fatty acid esters and mixtures thereof Laminated porous film, characterized in that it further comprises a material to be α crystal nucleating agent.
The method of claim 1,
Laminated porous film, characterized in that the laminated porous film has an air permeability of less than 500 seconds / 100mL measured at 25 ℃ according to ASTM D726.
a) adding a β crystal nucleating agent to the polypropylene resin and melt kneading to prepare a first resin;
b) adding a wax having a melting temperature of 90 ° C. or higher and a crystallinity of 80% or more to the polyethylene resin and melt kneading to prepare a second resin;
c) extruding and laminating the prepared first and second resins, respectively, to form an unstretched sheet in which the first resin layer and the second resin layer are laminated; And
d) stretching the prepared non-stretched sheet, the method of producing a laminated porous film of claim 1.
11. The method of claim 10,
The stretching process of step d) is characterized in that the biaxial stretching successively biaxially stretched 2 to 10 times in the longitudinal and transverse directions in the temperature range of 90 ℃ or more, respectively.
A separator for secondary batteries comprising the laminated porous film according to any one of claims 1 to 9.