JPH02251545A - Porous film of ultra-high-molecular weight polyethylene and production thereof - Google Patents

Abstract

PURPOSE:To provide the subject film having a specific porous structure, exhibiting high elongation to prevent unexpected failure under practical stress, having excellent handleability and producible without difficulty. CONSTITUTION:An ultra-high-molecular weight polyethylene having a viscosity- average molecular weight of >=500,000 is dissolved in a good solvent to obtain a dope. A film produced by using the dope is immersed into a poor solvent to remove the good solvent from the film and the obtained porous film is drawn to obtain the objective film having a porous layer with a number of pores and two dense layers with a number of fine pores of a smaller pore diameter than the pore of the porous layer, wherein said dense layers are formed continuous to both faces of the porous layer. The film is used in the form of a laminated film produced by laminating two or more films.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to ultra-high molecular weight polyethylene (hereinafter referred to as UHPg-1) porous film, its production by a method and the superimposition of two or more sheets of the porous film. The present invention relates to a laminated porous film IC that can fit into a cell.

(Prior Art) Porous films are sometimes used for battery diaphragms, capacitor diaphragms, various separation membranes, etc.9, and one made of uHpa is known as one of these porous films.

This U HP E porous film. Fill the mold with UIII'E powder. Under pressurizing condition 'F, U HP
vr: above the melting point of g, sintered by heating 1° to form a block-like body, first cooled, and taken out from the mold 6
After -t', the block-shaped body is cut to a predetermined thickness along the circumferential direction IC.

The porous structure of the film obtained by the above method can be determined by cutting the film at a predetermined position 1-2 and observing its cross section with an electronic microscope (magnification is about 200x).
,! : D sureg able Z).

The present inventor observed that J: m"1.B. It was found that the above conventional product had a large number of fine pores having approximately the same diameter and uniform pressure distribution over the entire cross section.

Although the uniform pore size and uniform distribution of micropores in the conventional 1JHPE porous fiber are favorable, the film has the disadvantage that it has low elongation and is easy to cut. There are concerns about industrial use.

for example. When UNiPE porous film is used as a lithium battery pallet. IC porous on negative electrode [Film fully wound, additional l/i: Positive electrode is placed on the film J: 1
-/H1ren f1. tCoos tF:, 1iI may occur, but the stress MCJ:l that acts when wrapping it on the negative electrode or when inserting it into the case may cause the porous film to break. Ta.

Also. The porosity of the above-mentioned method VC and the resulting porous film is about 40%, and the porosity of the porous film is about 40%. 'fx, katsuroku・
2. Such a low pore book. When r,, tl, (℃) is incorporated into a battery as a palletator, it causes an increase in internal resistance.

Therefore, the present invention has a large elongation. Moreover, U with high porosity
The purpose of the present invention is to provide a HPff porous shell film and a method for producing the same.

(Means for solving the problem fE) The VC of the present invention and the UPiPg porous film have a porous layer having a large number of pores. Hollow hole! : The porous layer has a large number of micropores with small pore diameters, and both sides 1-(7) of the porous layer are composed of dense layers.

below. The present invention will be described in detail with reference to the drawings. The drawings show a porous film according to the present invention cut in the thickness direction at predetermined positions. This is a schematic diagram of a multi-hole structure 1Z with a cross section of 1,000 mm.

In this drawing, reference numeral 61 denotes a porous layer that is dimpled approximately at the center in the thickness direction of the film. Compact layers 2 and 3 are formed on both sides of the nuclear layer 1.

Porous layer 1. The dense layer 283 has a common feature in that it has a porous structure having a large number of pores. The pore size of the pores 4 in the porous layer 1 is larger than the pore size of the pores 5°6 in both dense NJs 2 and 3. Although the pore size may vary depending on the application of the porous film.

usually. The pore diameter of pore 4 is approximately 2=50μ. The pore size of the pores 5.6 is about 0.1 to 10 μm.

This porous film (usually having a thickness of 5 to 200 layers) is as described above. Although the pore diameters of the porous layer 1 and the dense layer 2.3 are different, in the IC of the present invention, the pore size is 1! tI
The ratio (Da/D
a) t: 1 case, preferably 3 or more, more preferably 4 to 2
There are 0 and 1.

Also, the density of this porous film is high! The thickness of the layer 2.3 may vary depending on the manufacturing conditions of the film, but is usually 1. Nagi 1. i! are about 1-10 μm each.

In addition, the UI ball PE, which is the constituent material of the porous film, has a viscosity average molecular weight (hereinafter referred to as 8 molecules, referred to as 1) of 500,000 or more. This is larger than that of general polyethylene, which is about 300,000 or less. As such IJHPE. For example, manufactured by Mikata Petrochemical Industry Co., Ltd.

Product name: Hi-Zex Million. Manufactured by Hechust. Commercially available products such as the trade name Hostalen GUR can be used.

Is Jin's porous film superimposed on at least two layers?
7 can also be used as a laminated porous film. For ease of handling, it is preferable for this laminated film to integrate two or more porous films in close contact with each other, e.g. This can be achieved by passing the porous film (2 or more sheets) between a pair of pressure rolls under heating conditions (but above the melting point of the porous film). It is preferable to improve adhesion strength.

In addition. When a laminated porous film is used as a pallet for IE Ikegawa (in consideration of strength and electrical resistance, it is preferable that the thickness after lamination is approximately 15-=4 Oμ).

Next, a method for manufacturing a porous film, which is another embodiment of the present invention, will be described. This method includes the following four steps: a) A step of dissolving UHPE with a molecular weight of 500,000 or more in a good solvent.

b) Forming a film using this solution.

C) A step of making the film porous by immersing the film in a poor solvent and removing the good solvent from the film.

d] stretching the porous 1JIi film.

In step a) of this method, the UHPE is JL solvent V
C is dissolved. UHPgd has a high molecular weight as mentioned above.

Since it is relatively poorly soluble, it is preferable to use a heating dissolution method. For example, a method can be applied in which UHPg powder and a good solvent are placed in a predetermined container, the temperature is gradually raised while stirring, and the final temperature at the time of hot melting is usually 100 to 180℃. Yes, it takes about 10 minutes
It is 10 hours.

The solvent used in this step is not particularly limited as long as it can be dissolved in UHPEf:WI, such as medium silene, decalin,
Nonane, decane, undecane, etc. can be used.

In addition, in the UHPff solution obtained in the above dissolution step (DUHPff11fft!, about 0.1 to 20% by weight)
,! : Sur 17) has been found to be preferable. The upper limit of concentration is determined by the freshness of UHPE to good fns. Furthermore, if the concentration of this solution is too low, it will be difficult to obtain a porous film with high zero mechanical strength.

The ft-UHPE fs liquid obtained by step a) above is then subjected to film forming step b) K.

Film forming from UHPffll liquid is 0, usually about 1
It is carried out under heating conditions of 00 to 180°C. The film forming method does not need to be any special method, and any known plastic film forming technique such as T-die extrusion method or flake wrapping method can be used.

The JILIIF medium-containing film storage obtained by step C) is subjected to dipping step C).

The poor solvent used here does not dissolve UHPff.

Any %hO that is compatible with the jL solvent may be used, and water and various organic solvents can be used, but alcohols, particularly methanol, are preferred. These solvents can also be used in combination of two or more.

By this immersion, the good solvent in the film-like material is extracted and removed by the poor solvent, and as a result, the film-like material becomes porous. This film-like material consists of a porous layer having many large pores in the central part in the thickness direction, and a dense layer having many small pores formed on both sides of the porous layer. It exhibits a unique porous structure.

The reason for this is not necessarily clear, but there is a difference in the rate of extraction into the poor ms between the JL mortar present near the surface of the film and the good solvent present in the center. It is inferred that the pores in the porous layer formed in the center of the film and the pores in the dense layer formed on both sides of the film are different in pore size.

For example, near the surface of a film-like material, the amount of poor solvent is large compared to the amount of good solvent (poor solvent rich), and the jL latent medium is rapidly extracted and removed, resulting in the formation of pores with relatively small pore diameters. On the other hand, extraction at the center of the film-like material (center in the thickness direction) is performed under a small amount of poor solvent that gradually permeates from one surface (rich in good solvent). It is thought that the extraction rate of is slow, and as a result, pores with a relatively large pore size are formed.

As mentioned above, by performing step C), the porous layer is
It has been found that variations due to manufacturing conditions such as the molecular weight of UHPE, the type of solvent used, and the total thickness of the film are relatively small and are usually about 1 to 10 microns each.

The pore diameters of the pores in the porous layer and the dense layer formed by step C) are mainly determined by the rate of extraction of a good solvent into a poor solvent, and the pore diameter of young pores with a high extraction rate becomes small.

Note that, if other conditions are the same, the larger the molecular weight of the Ul(Pg used), the smaller the pore diameters of the pores formed in both the porous layer and the dense layer tend to be.

Therefore, by appropriately selecting the molecular weight of UIIPE, it is possible to control the maximum pore diameter ratio f of pores formed in both layers.

As described above, the immersion is a method for forming a porous structure, and the immersion time in the poor solvent is not particularly limited as long as the bathing medium can be extracted and removed, but it is usually about 0.1 to 30 minutes. be.

In the method of the present invention, the film material made porous in step C) is then subjected to stretching step d). This stretching is carried out for the purpose of increasing the porosity and film strength by enlarging the pore diameters of the porous layer and the dense layer. Stretched ore is usually uniaxially stretched, but biaxially stretched may also be used.The stretching temperature may be lower than the melting point of UHPE.

Practically, it is about lθ to 130°C. The housing and stretching ratio are usually 20% or more, preferably Fi50 to 500%.

The UHPID porous sheet obtained in this way.

The thickness is about 5-200μ, and the porosity is about 20-90%.

Further, this porous sheet may be drawn even if it is crosslinked, and the crosslinking can be performed, for example, before or after the stretching step.

(Example) Hereinafter, the present invention will be explained with reference to Examples. will be explained in detail.

Example 1 Place 100% of Nakasilene in a glass container and 100% of Decalin.
Parts by weight and UHPE powder (molecular weight 2,000,000, particle size 30
Add 10 parts by weight of μsm), and while stirring, raise the liquid temperature to 25℃.
The temperature was increased from 140°C to 140°C, and stirring was continued for 1 hour at the same temperature to dissolve UHPEflW.

This UHPji2! The liquid is extruded into a film with a thickness of 60 μm using a T-die (temperature: 135° C.) and cooled to room temperature.

Next, this film-like material is immersed in methanol (liquid temperature: 25° C.) for 5 minutes, and xylene and decalin are extracted and removed with methanol, thereby forming a porous layer and a dense layer, which are then pulled up and air-dried.

Next, ``t'', temperature 100'C, speed 500m/
A porous film was obtained by stretching 200% in the uniaxial direction under conditions of 0 minutes.

Example 2 The same procedure as in Example 1 was carried out except that UHPE powder with a molecular weight of 600,001 and a particle size of 150IIs was used to obtain a porous film.

Example 3 100 parts of xylene was placed in a glass container, 5 parts by weight of Decalymp and UHPE powder (molecular weight 1.5 million, particle size 50 μm)
Add 8 parts by weight of inertia) and lower the liquid temperature from 25°C to 1°C without stirring.
The temperature was raised to 40°C, and stirring was continued for 1 hour at the same temperature to dissolve U) IPff.

This UHPEM liquid was extruded into a 60 μm thick film using a T-die (temperature: 135° C.) and cooled to room temperature.

Next, this film-like material was immersed in methanol (liquid temperature 25°C) for 10 minutes. ? Silene and decalin are extracted and removed with methanol to form a porous layer and a dense layer, which are pulled up and air-dried.

Next, this is stretched 300% in the uniaxial direction at a temperature of 100° C. and a speed of 500 m/min to obtain a porous film with a thickness of 25 μm.

Then, these two porous films were stacked together.

The films are passed through a pair of pressure rolls (one is a metal cylinder roll, the other is a rubber roll), and the films are tightly integrated into each other to a thickness of 2.
A laminated porous film with a thickness of 5 μm was obtained.

Note that the metal rolls were heated to 105° C. and the pressure between the rolls was set to a linear pressure of 0.3 ky/ca.

Comparative Example 1 kg of the same UHPE powder used in Example 1 was placed in a mold (
A mandrel with an inner diameter of 120111 and a length of 50 m is set in the center), and further pressure is applied until the filling height is approximately 1.
Adjust to 9 CIl (apparent density 0.55).

Next, it was heated and pressed for 4 hours at a temperature of 160°C and a pressure of 5017/-, and then left to cool at room temperature (25°C) for 8 hours, taken out from the mold to obtain a cylindrical porous body, and further turned into a cylindrical porous body with a lathe. It was cut into a film with a thickness of 50 μm.

The following tests were conducted on the porous films obtained in Examples and Comparative Examples, and the results are shown in Table M1.

[B] Film thickness Measured using a dial gauge (1/1000sn).

[Note] The thickness and maximum pore diameter of the porous layer and dense layer were measured by observing a cross section of the film using a scanning electron microscope @I/C (magnification: 2000x). In addition, since the thickness and maximum pore diameter of both dense layers in each porous film are almost the same,
Table 1 shows the data for one of the dense layers and the ratio.

[C] Porosity Cut the porous film so that the vertical and horizontal dimensions are 25 am each, measure the thickness (total), and make 1 set (
1) Appearance rl! Calculate i degrees (2).

Furthermore, due north of UHPff! Calculate the porosity using Equation 1 (II) using 1 (G).

k゛ In addition, the units of D and E in Formula 1 (1) are as follows.

"μ髫" and "y".

[2] Elongation and tensile strength Samples with a width of 20 degrees were prepared and measured according to the method specified in ASTM-D-882.

[E] Internal resistance Measured according to JIS-C-2313 (at 25°C). In addition, the electrolyte is propylene carbonate and 1,
3-dioxolane was mixed at a weight ratio of 1:1, and 1 mol/I of lithium perchlorate was dissolved therein as an electrolyte.

The following is a margin: Table 1 The film has a high elongation and is easy to use as it will not break accidentally even under the stress that is applied when it is put into practical use. A porous film having the following properties can be easily produced.

[Brief explanation of drawings]

The drawing is a schematic diagram of the porous structure of the UIIPff porous film according to the present invention. l... Porous layer 2.3... Dense layer 4.5.6.
... In Example 3, the "film thickness" and "internal resistance" are the data of the laminated porous film, and the other data are the data of the porous film used to create this laminated MJ film. (Effects of the Invention) As shown in the above embodiments and teaching examples.

Claims (3)

[Claims] (1) An ultra-high molecular weight polyethylene porous film consisting of a porous layer having a large number of pores and a dense layer having a large number of micropores with a smaller pore diameter than the pores and formed on both sides of the porous layer. . (2) The following four steps: (a) A step of dissolving ultra-high molecular weight polyethylene having a viscosity average molecular weight of 500,000 or more in a good solvent. (b) Forming a film using this solution. (c) A step of making the film porous by immersing the film in a poor solvent and removing the good solvent from the film. (d) A step of stretching the porous film. A method for producing an ultra-high molecular weight polyethylene porous film comprising: (3) An ultra-high molecular weight polyethylene porous film consisting of a porous layer having a large number of pores and a dense layer having a large number of micropores with a smaller pore diameter than the pores and formed on both sides of the porous layer. A laminated porous film formed by overlapping at least two sheets of.