JPH05310989A - Polyethylenic porous film - Google Patents

Abstract

PURPOSE:To obtain a separator for cells and batteries having self locking properties in overheating. CONSTITUTION:The objective polyethylenic porous film is a porous film composed of an ultra-high-molecular weight polyethylene having >=500,000 viscosity- average molecular weight (MV) and characterized by providing >=1000sec/100cc air permeability in heat treatment at a temperature above the melting point of the polyethylene.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyethylene porous membrane, and more particularly, it is suitable for ultra-fine filtration membranes and membranes for battery separators, which are excellent in gas, liquid and ion permeability and maintain shape of the membrane at high temperature. Polyethylene porous membrane.

[0002]

2. Description of the Related Art With the development of small portable devices, small size and high performance batteries have been required. The lithium battery is highly useful as an electrode material for a small and high-performance battery because it has a high electromotive voltage due to the use of the base metal, lithium. However, since lithium is highly reactive, mishandling causes a serious accident. As for lithium batteries, cases such as ignition accidents have occurred in the past, and ensuring safety is an important issue.

[0003]

If the separator does not have sufficient high-temperature film shape retention characteristics, a large current will flow in a short time due to a short-circuit accident, etc., and the lithium battery will generate heat, causing internal short circuit due to damage to the separator due to heat. There is a risk of explosion and fire accident. When the internal temperature of the battery rises in the separator, the pores of the separator are automatically closed by heat (self-closing property) and the property of maintaining the film shape and separating the electrodes even at high temperatures (maintaining the high temperature film shape) Characteristics) are required.

The polypropylene separator film is excellent in shape retention at high temperature, but when used as a lithium battery separator, the temperature at which self-closing property is exhibited is about 175 ° C., which is close to the ignition temperature of lithium of 180 ° C. And dangerous. In addition, the separator film is usually stretched to improve the strength, but the stretched film has a low high-temperature film shape retention property, and is 150 to 160 when made of polyethylene.
In the case of polypropylene made of ℃ or polypropylene, it breaks at around 180 ℃, causing a problem in the isolation property of the electrode.

[0005]

The inventors of the present invention have conducted extensive studies to solve the above problems, and as a result, have a high low-temperature occluding property, improve the high-temperature film shape-maintaining property, and have a basic structure as a separator. The inventors have invented a film satisfying the characteristics. The present invention has a viscosity average molecular weight (Mv) of 500,00.
A porous film composed of 0 or more ultra high molecular weight polyethylene (a) thickness 10 to 50 μm, (b) air permeability 20 to 100
0 sec / 100 cc, (c) Porosity 25-80%, (d)
Breaking strength is 100 Kg / cm ² or more in both the vertical and horizontal directions, (e) Bubble point (BP value), 2 to 5 Kg / c
m ² , (f) Water permeability 100 liter / hr ・ m ²・ atm
As described above, (g) a polyethylene porous film that blocks 50% or more of 0.091 μm styrene latex particles, and has an air permeability of 1000 seconds / 100 cc or more when heat-treated at a temperature of not less than the melting point of polyethylene and not more than 200 ° C. The polyethylene porous membrane is characterized by

The present invention will be described in detail below. The polyethylene porous membrane of the present invention has a thickness of 10 to 50 μm, more preferably 15 to 30 μm. A film thinner than 10 μm is not preferable because the absolute strength is small, and breakage during film formation or film breakage after battery processing is likely to occur. Further, when the film thickness exceeds 50 μm, there are problems that the amount of water permeation becomes small and the ratio of the separator occupying in the battery becomes large and the capacity of the battery decreases. Air permeability is 20 to 1000 seconds / 10
It is 0 cc and preferably 50 to 300 seconds / 100 cc. If the air permeability is less than 20 seconds / 100 cc, the ratio of pores to the surface area of the film (opening ratio) increases and the strength of the film decreases. If it is more than 1000 seconds / 100 cc, the permeation resistance of ions becomes so large that it cannot be used as a separator.

However, the melting point of the polyethylene (usually 135
(About ℃) or higher, usually 1000 ℃ / 100cc or more by heating to a temperature of 200 ℃ or less for about 1 to 2 minutes, it is possible to cut off the ionic current, and it is safe even if the battery heats up due to a short circuit accident. The electrode reaction can be stopped. If the air permeability is greater than 1000 sec / 100 cc, the filtration resistance is too high for a filtration membrane, which is not practical.

The porosity is 25 to 80%. Porosity is 2
If it is less than 5%, the pore structure is too dense, which causes problems in filtration and ion permeation. If it is more than 80%, the amount of polyethylene occupying the unit volume becomes too small and the strength is lowered, which is not preferable. Breaking strength is 100 in both vertical and horizontal directions
Kg / cm ² or more is required. If it is less than this range, the film is likely to be broken at the time of membrane production or when the filtration membrane is processed into a cartridge, resulting in poor workability. Bubble point is 2 to 5 kg / cm
² or more. If the bubble point is less than ² Kg / cm ² , the pore structure becomes sparse and it is not practical. On the other hand, if it is larger than 5 kg / cm ² , the pore structure becomes too dense and resistance to filtration and ion permeation is not preferable.

Water permeability is 100-1500 liters / hr
・ M ²・ atm. Water permeability is 100 liters / hr
・ If it is less than m ² · atm, the filtration rate is too slow to be practical.
If it is more than 1500 liters / hr · m ² · atm, the pore structure becomes sparse and there is a risk that the electrodes contact each other when used as a battery separator. Styrene latex particles of 0.091 μm need to block 50% or more.
If it is less than 50%, unnecessary particles cannot be sufficiently filtered in terms of filtration performance.

A preferred method for obtaining the porous membrane of the present invention is obtained by melt extruding a composition comprising ultrahigh molecular weight polyethylene and a plasticizer to obtain a sheet, and then removing the plasticizer from the sheet. A porous sheet is provided. A heat roll method, a tenter method, or the like can be used for the heat treatment. The structure of the porous film thus obtained is characterized in that it has a network structure composed of fibrils.

The polyethylene used in the present invention is a so-called ultra-high molecular weight polyethylene having a weight average molecular weight of 500,000 or more, and particularly a viscosity average molecular weight of 1 × 10.
^{6 to} 3.0 × 10 ⁶ is preferable. Further, other polyolefin having a molecular weight of 5 × 10 ⁵ to 2 × 10 ⁶ and a modified polyolefin containing 50% or more of the polyethylene may be contained.

If the molecular weight is too low, it is difficult to uniformly knead with the plasticizer, and a porous membrane having a fine pore structure cannot be obtained. In addition, stable sheet formation becomes impossible. Next, the plasticizer has good compatibility with polyethylene, has a boiling point higher than the melt-forming temperature of polyethylene (up to 250 ° C), and has a low vapor pressure so that evaporation does not easily occur during sheet molding. ..

Further, in consideration of the stability of a sheet made of polyethylene and a plasticizer obtained in the intermediate stage of the product and the ease of handling, liquid paraffin, solid paraffin, stearyl alcohol, cetyl alcohol and the like are specifically desirable. In particular, those that are solid at room temperature are very useful for handling. These plasticizers and ultra-high-molecular-weight polyethylene are mixed in an ordinary mixer, then once melt-kneaded, uniformly kneaded, pelletized, and then subjected to sheet forming. In particular, stearyl alcohol uses fine granular products. In addition, it is easy to mechanically blend with the powdery ultra-high molecular weight polyethylene, and stable extrusion molding is possible by directly feeding the powder to the extruder feeding section. Further, a heat stabilizer, an antioxidant, a coloring agent, etc. may be added to the composition.

The mixing ratio of ultrahigh molecular weight polyethylene and plasticizer is usually ultrahigh molecular weight polyethylene / plasticizer = 10/90 to 40/60, preferably 15 /
The range is 85 to 35/65. If the proportion of polyethylene is too low, the extruded state in the extruder becomes unstable and a good sheet cannot be obtained. On the other hand, if the proportion of polyethylene is too high, the viscosity becomes too high, and the flow at the die becomes unstable, making it impossible to obtain a stable sheet. A composition obtained by once melt-kneading these compositions into pellets can prevent the polyethylene and the plasticizer from being classified during sheet molding, which leads to improvement in molding stability.

The sheet is formed by feeding pellets obtained by melt-kneading polyethylene and a plasticizer or a mixture obtained by mechanically blending polyethylene and a plasticizer to an extruder, and then bringing the mixture into a uniform molten state, and then a sheet is formed from an appropriately selected die. It is done by extruding into a shape. Usually, the thickness of the T-die molded product sheet is 0.03 to 0.5 mm, preferably 0.03 to 0.5 mm.
It is 0.08 mm.

At this time, it is desirable that the sheet is not stretched and molded so as not to cause molecular orientation as much as possible. The removal (extraction) of the plasticizer performed next has a high solubility of the plasticizer,
An extraction method using an easily volatile solvent is preferable. Hydrocarbon solvents such as pentane, hexane, heptane, etc.
Examples include halogenated hydrocarbons such as methylene chloride, chloroform, carbon tetrachloride, ethane trifluoride, alcohols such as methanol, ethanol, propanol, etc. to remove all plasticizers, and then to remove volatile solvents by drying. Thus, a porous sheet is obtained. The content of the plasticizer remaining in this porous sheet is preferably less than 1% by weight. It is desirable to remove the plasticizer at room temperature or higher in order to improve the removal efficiency.

The above-mentioned porous sheet can be sufficiently porous and used as a separator as it is, but it can be further heat-treated in order to prevent shrinkage due to temperature. Industrially, there are a heating roll method, a tenter method, and the like, and it is desirable that the heat treatment temperature is high. However, if the temperature is higher than the melting point of the polyethylene, the pores are blocked and the air permeability is greatly increased, which is not preferable. In the case of the heating roll method, it is preferable to perform the heat treatment at a melting point or lower, preferably 130 ° C. or lower.

[0018]

EXAMPLES The present invention will now be described in detail with reference to examples, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded. The test method in the examples is as follows. 1. Air permeability (unit: second / 100cc) JIS P81
17 2. Porosity (%) = pore volume / porous membrane volume × 100% 3. Breaking strength (unit: Kg / cm ² ) JIS K67
81 4. Bubble point (BP) JIS K3832 5. Permeability (unit: liter / hr ・ m ²・ atm)
Using Amicon 8010 type cell, differential pressure 1kg / c
Measured at m ² temperature of 23 ° C. 6. Pore Size Measurement (Styrene Latex Rejection Rate) Dow's weight average particle diameters of 0.091 μm and 0.212 μm styrene latex particles are dispersed in water to obtain 80 from Amicon.
A 10-type cell was used to conduct a permeation test at a differential pressure of 1 kg / cm ^2, and the styrene latex concentration before and after the permeation test was measured with a UV meter, and the blocking rate was determined by the following formula.

[0019]

## EQU1 ## Rejection rate (%) = (concentration before permeation−concentration after permeation) / (concentration before permeation) × 100 Example 1 Polyethylene powder having a viscosity average molecular weight of 2 × 10 ⁶ (melting point 135 ° C.) 20 weight Parts and granulated stearyl alcohol 80 parts by weight of dry blend were fed to the extruder for 24
While kneading at 0 ° C, it is continuously extruded from a T-die with a width of 550 mm and a die clearance of 0.2 mm to a thickness of 0.07.
A sheet of mm was obtained.

Stearyl alcohol was extracted from this sheet in an isopropyl alcohol bath at 60 ° C. to obtain a polyethylene porous membrane. The physical properties of this film are as follows: (a) film thickness 47 μm (b) air permeability 105 seconds / 100 cc (c) porosity 67% (d) breaking strength 170 Kg / cm ² (vertical direction), 1
20 Kg / cm ² (horizontal direction) (e) Bubble point 3.4 Kg / cm ² (f) Water permeability 400 liters / hr · m ² · atm (g) Styrene latex blocking rate (SR blocking rate) 98
% Or more.

This membrane was placed in a hot air circulation oven at 150 ° C. for 1 hour.
The air permeability of the sample heated for 1 minute cannot be measured (1200 seconds / 1
00cc or more). Furthermore, this membrane is 1
The film shape was maintained even after the heat treatment for a minute. Example 2 The polyethylene porous film having a film thickness of 47 μm obtained in Example 1 was heated to 30 ° C. using a heating pinch roll having a surface temperature of 120 ° C.
It was heat-treated for a second to form a 33 μm film, and a porous film was obtained.
The physical properties of this film are shown in Table 1. Comparative Example 1 Polyethylene having a viscosity average molecular weight of 4.5 × 10 ⁵ (melting point 1
A film was prepared in the same manner as in Example 1 except that (28 ° C.) was used. The 0.091 μm styrene latex removal rate of this film was 45%. Comparative Example 2 The surface temperature of the porous film prepared in Example 1 was 140 ° C. (melting point +
Heat treatment was performed for 30 seconds using a heating roll at 5 ° C.
The physical properties of this film are shown in Table 1.

[0022]

[Table 1]

[0023]

EFFECTS OF THE INVENTION According to the present invention, it is possible to prepare a porous film which is excellent in low-temperature occluding property and retains the film shape even at high temperatures. This membrane can provide a battery separator with excellent safety.

Claims (4)

[Claims] 1. Viscosity average molecular weight (Mv) 500,000
A porous membrane made of the above ultra high molecular weight polyethylene (a)
Thickness 10 to 50 μm, (b) Air permeability 20 to 1000 seconds /
100 cc, (c) Porosity 25 to 80%, (d) Breaking point strength is 100 Kg / cm ² or more in both the longitudinal and transverse directions,
(E) Bubble point (BP value) ^{2 to 5} Kg / cm ² ,
(F) A polyethylene porous membrane which blocks 50% or more of styrene latex particles having a water permeation rate of 100 liter / hr · m ² · atm or more and (g) 0.091 μm at a temperature of not less than the melting point of polyethylene and not more than 200 ° C. A porous polyethylene membrane characterized by having an air permeability of 1000 seconds / 100 cc or more when heat-treated. 2. The polyethylene porous film according to claim 1, wherein the film shape is maintained when heat-treated at 175 ° C. 3. The polyethylene porous membrane according to claim 1, which is used as a filtration membrane. 4. The polyethylene porous membrane according to claim 1, which is used as a battery separator.