JP2000256499A - Porous film and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous film which is reduced in heat shrinkage and has excellent low temperature shut-down properand a high porosity, and a process for producing the same. SOLUTION: The porous film is composed of a high-molecular weight polyolefin having a weight average molecular weight of nor smaller than 1×106 and another resin having a melting point or a softening point of not higher than the melting point of the high-molecular weight polyolefin, and have a shut-down temperature of not higher than 135 deg.C, a porosity of not lower than 40%, and a shrinkage factor of not greater than 30%. A process for producing porous films comprises the steps of kneading a resin composition composed of a high-molecular weight polyolefin having a weight average molecular weight of not smaller than 1×106, another resin having a melting point or a softening point of not higher than the melting point of the high-molecular weight polyolefin, and a solvent, molding the composition into the form of a sheet, effecting stretching and desolvation treatment and then, impregnating the resulting film with a poor solvent having a temperature of the melting point of the high molecular weight polyolefin minus 15 deg.C to that plus 5 deg.C to conduct heat setting treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a porous film and a method for producing the same. More specifically, the present invention relates to a porous film which is preferably used as a battery separator or the like which is disposed between a positive electrode and a negative electrode of a battery and isolates them, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, lithium batteries that are lightweight, have high electromotive force and high energy, and have low self-discharge have been attracting attention in order to cope with cordless electronic devices. Between the positive and negative electrodes of this lithium battery,
A separator is provided to prevent a short circuit between the positive electrode and the negative electrode. As the separator, a porous film formed with a large number of micropores is used to secure the permeability of ions between the positive electrode and the negative electrode.

[0003] On the other hand, as the capacity of the battery has been increased and the production speed has been increased, a thin and high-strength porous film has been desired, and various porous films using a high-molecular-weight polyolefin as a material for such a film have been proposed. Have been.

[0004] Further, a film used as a battery separator is required to have a shutdown property of closing a hole and shutting down a current when an electrode is short-circuited and the temperature inside the battery rises.

Japanese Patent No. 2,657,434 discloses an ultrahigh molecular weight polyethylene 1 having a weight average molecular weight of 7 × 10 ⁵ or more.
-69% by weight and high-density polyethylene 98-1% by weight
And forming a gel composition from a solution obtained by heating and dissolving a mixture containing 1 to 30% by weight of a low-density polyethylene as a low-melting resin in a solvent, heating and stretching, removing the remaining solvent, and drying. We report that low-temperature shutdown performance can be obtained for a microporous polyethylene membrane obtained by heat setting.

[0006] However, the above-mentioned Japanese Patent No. 2,657,4 is disclosed.
According to No. 34, it is desirable to heat-set in a temperature range from the crystal dispersion temperature to the melting point, and heat-fixing is performed at a temperature lower than the melting point of low-density polyethylene for 30 seconds. The temperature and time at which the thermal relaxation of the high molecular weight polyethylene and the high density polyethylene are performed are insufficient, and it is estimated that the film shrinks greatly.

[0007]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a porous film which is less likely to be thermally shrunk, has excellent low-temperature shutdown characteristics and high porosity, and a method for producing the same.

[0008]

The gist of the present invention is as follows.
It is composed of a high-molecular-weight polyolefin having a weight-average molecular weight of 1 × 10 ⁶ or more and another resin having a melting point or a softening point equal to or lower than the melting point of the high-molecular-weight polyolefin, a shutdown temperature of 135 ° C. or less, and a porosity of A porous film having a heat shrinkage of at least 40% and a heat shrinkage of at most 30%, and (2) a weight average molecular weight of 1 × 10
⁶ or more high molecular weight polyolefin, the melting point or softening point is kneaded with a resin composition containing another resin and a solvent at a temperature equal to or lower than the melting point of the high molecular weight polyolefin, formed into a sheet, stretch and desolvation treatment. A method of producing a porous film, comprising: impregnating a poor solvent having a melting point of −15 ° C. or more and a melting point of + 5 ° C. or less with the high-molecular-weight polyolefin and performing a heat-setting treatment.
About.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to a resin composition containing a high-molecular-weight polyolefin, a resin having a melting point or a softening point lower than the melting point of the high-molecular-weight polyolefin and a solvent, and kneading the mixture to form a sheet. Then, after performing the stretching and the desolvation treatment, the porous film of the present invention is obtained by performing a heat setting treatment under predetermined conditions.

The high-molecular-weight polyolefin that can be used in the present invention includes homopolymers and copolymers of olefins such as ethylene, propylene, 1-butene, 4-methyl-1-pentene and 1-hexene; Blends and the like can be mentioned. Among them, it is preferable to use ultrahigh molecular weight polyethylene from the viewpoint of increasing the strength of the obtained porous film.

Weight average molecular weight of high molecular weight polyolefin
Is 1 × 10⁶Above, preferably 1.5 × 10⁶Above
is there. Weight average molecular weight of high molecular weight polyolefin is 1 ×
10 ⁶If less than the film strength at the temperature above the melting point
But weak in heat resistance. Used for the porous film of the present invention
Resin contains the high molecular weight polyolefin.
High strength due to entanglement of high molecular weight polyolefin
Can be obtained effectively.

Other resins that can be used in the present invention include, for example, low-density polyethylene, linear low-density polyethylene, ethylene-butadiene copolymer, ethylene-methacrylic acid ester copolymer, methacrylic acid ester-grafted ethylene, Olefinic thermoplastic elastomers such as ethylene propylene copolymerized elastomers, hydrogenated styrene-isoprene-styrene copolymers, styrene-butadiene rubber, and the like. Among these, the reactivity inside the battery is low, and the high molecular weight polyolefin is used. An olefin resin having a relatively good miscibility with the olefin resin is preferred.

The other resin is a resin having a low melting point or a low softening point, the melting point or the softening point of which is lower than the melting point of the high molecular weight polyolefin, and the temperature lower than the melting point of the high molecular weight polyolefin minus 10 ° C. Preferably, the temperature is 100 ° C. or higher from the viewpoint of maintaining the porous structure well. If the melting point or softening point of the other resin exceeds the melting point of the high molecular weight polyolefin, it will not contribute to lower shutdown. In the present invention, the melting point is an onset temperature in differential scanning calorimetry, and the softening point is TMA.
Using a measuring device, the temperature was raised at a rate of 10 ° C./min, and the load was 3 g.
Is the temperature at which the 0.5 mm needle starts to penetrate under the following conditions.

The mixing ratio of the high molecular weight polyolefin to the other resin is preferably 50/50 to 90/10 (weight ratio), more preferably 60/40 to 90/10 (weight ratio). Ratio). In order to give sufficient film strength to the obtained porous film, the compounding amount of the high molecular weight polyolefin is preferably 50% by weight or more as a resin, and in order to lower the shutdown temperature, the compounding amount of the high molecular weight polyolefin is 90% by weight or less is preferable as the resin.

As the solvent that can be used in the present invention, known solvents that are generally used can be used without any limitation as long as they are excellent in solubility of high-molecular-weight polyolefin and other resins. For example, aliphatic or cyclic hydrocarbons such as nonane, decane, undecane, dodecane, decalin, and liquid paraffin, and mineral oil fractions whose boiling points correspond to these, and the like, among these, non-volatile such as liquid paraffin Solvents are preferred.

The mixing ratio of the high molecular weight polyolefin, the other resin and the solvent is not particularly limited as long as the obtained slurry-like resin composition can be kneaded and formed into a sheet. For example, the total weight of the high molecular weight polyolefin and the other resin is preferably 5 to 50% by weight of the resin composition, more preferably 10 to 50% by weight. The solvent preferably accounts for 50 to 95% by weight of the resin composition, and more preferably 50 to 90% by weight. In order to prevent neck-in at the die exit when the resin composition is formed into a sheet and to facilitate molding, the content of the high-molecular-weight polyolefin and other resins is preferably 5% by weight or more, and the melting of the resin composition In order to adjust the viscosity and knead uniformly, the content of the high-molecular-weight polyolefin and other resins is preferably 50% by weight or less.

Incidentally, additives such as an antioxidant and an ultraviolet absorber can be added to the resin composition, if necessary, as long as the object of the present invention is not impaired.

The step of kneading the resulting resin composition and forming it into a sheet can be carried out by a commonly used known method. For example, the resin composition may be kneaded in batches using a Banbury mixer, a kneader, or the like, and then sandwiched between cooled metal plates and rapidly cooled to form a sheet-like molded product by rapid cooling crystallization. A sheet-like molded product may be obtained using an attached extruder or the like.

The kneading of the resin composition may be carried out under suitable temperature conditions, and is not particularly limited. Preferably, the temperature is in a range from a temperature at which the solvent starts dissolving the high molecular weight polyolefin (dissolution starting temperature) to + 60 ° C. It is more preferable to perform the dissolution at a melting start temperature of + 20 ° C. to + 50 ° C. from the viewpoint of obtaining a structure suitable for sheeting in the subsequent sheeting step.

When forming into a sheet, the sheet-shaped product coming out of an extruder or the like may be further rapidly cooled. At this time, it is more preferable to perform rapid cooling under the condition that the degree of supercooling (ΔT) becomes 20 ° C. or more. By performing the quenching operation, the film strength can be further increased.

In this manner, a sheet-like molded product of the resin composition containing the high-molecular-weight polyolefin and another resin can be obtained. The thickness of the sheet-like molded product obtained here is not particularly limited, but is preferably 1 to 20 mm, more preferably 3 to 15 mm.

Next, the sheet-like molded product is stretched and desolvated. The method of the stretching treatment is not particularly limited, and may be an ordinary tenter method, a roll method, an inflation method or a combination of these methods,
Further, any method such as uniaxial stretching and biaxial stretching can be applied. In the case of biaxial stretching, either vertical or horizontal simultaneous stretching or sequential stretching may be used. Further, in the present invention,
Prior to the stretching treatment, the sheet-like molded product may be subjected to a rolling treatment.

The temperature during the stretching treatment is preferably a temperature not higher than the melting point of the high molecular weight polyolefin + 5 ° C. As other stretching treatment conditions, commonly used known conditions can be adopted.

The desolvation treatment is a step of forming a porous structure by removing a solvent from a sheet-like molded product.
It can be carried out by washing the sheet-like molded product with a solvent to remove the residual solvent. Examples of the solvent include hydrocarbons such as pentane, hexane, heptane, and decane; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride; fluorinated hydrocarbons such as ethane trifluoride; and ethers such as diethyl ether and dioxane. Easily volatile solvents can be mentioned, and these can be used alone or in combination of two or more. The washing method using such a solvent is not particularly limited, and examples thereof include a method of immersing the sheet-like molded product in the solvent to extract the solvent, a method of showering the solvent to the sheet-like molded product, and the like.

In the present invention, the solvent removal treatment may be appropriately performed before and after stretching. For example, the sheet-shaped molded product may be subjected to a desolvation process and then subjected to a stretching process, or the sheet-shaped molded product may be directly subjected to a stretching process and then subjected to a desolvation process. Alternatively, the solvent may be removed before the stretching, and the solvent may be removed again after the stretching to remove the residual solvent.

Next, the molded article having a porous structure obtained by the above-described steps is subjected to a heat setting treatment. In the present invention, the heat setting treatment is performed by impregnating the molded article having a porous structure obtained in the above-mentioned step with a high boiling poor solvent having a boiling point not lower than the heat setting temperature.

The term "poor solvent" refers to a solvent having high solubility of 1 or less in high molecular weight polyolefin and other resins. The solubility is determined by taking a predetermined amount (weight is “A (g)”) of a high molecular weight polyolefin (or other resin) as a sample, adding a 1000-fold (weight) solvent thereto, and heating at a temperature of 100 ° C. After heating for 1 hour at, the sample was taken out, washed with acetone at room temperature (25 ° C.), and then left at room temperature for 1 hour.
The weight is weighed (the weight at this time is referred to as “B (g)”), and can be calculated by the following equation. Solubility = [(AB) / A] × 100 When the solubility is determined by this method, the high molecular weight polyolefin and other resins are hardly soluble in the solvent, so they swell with the solvent, and are washed with acetone. Solvent may remain, and the solubility may show a negative value in this case. However, it is preferable that swelling is small, and a solvent showing a value of “−1 or more” in the above formula is preferable. In addition, as such a poor solvent, N-methylpyrrolidone, propylene carbonate, n-dodecanol, etc. are mentioned, for example.

In the heat setting treatment, the temperature of the poor solvent is the melting point of the high molecular weight polyolefin −15 ° C. or higher and the melting point + 5 ° C. or lower, preferably the temperature of the high molecular weight polyolefin melting point −10 ° C. or higher and lower than the melting point. It is. When the temperature of the poor solvent during the heat setting treatment is lower than the melting point of the high molecular weight polyolefin −15 ° C., the heat shrinkage of the obtained porous film increases, and when the temperature exceeds the melting point of the high molecular weight polyolefin + 5 ° C., the temperature decreases. The flow of another resin having a melting point or a low softening point occurs, and the porous structure cannot be maintained.

The heat setting treatment time varies depending on the treatment temperature and the like, and cannot be unconditionally determined. However, it is preferable that the heat shrinkage time of the obtained porous film is sufficient to reduce the heat shrinkage to 30% or less. , Preferably about 30 seconds to 1 hour, more preferably about 5 minutes to 1 hour, and 10 minutes to
About one hour is particularly preferable.

After the heat setting, it is preferable to wash the poor solvent. Suitable cleaning agents also differ depending on the poor solvent used. For example, acetone is preferable in the case of N-methylpyrrolidone, and methanol is preferable in the case of n-dodecanol.

In the present invention, the heat setting may be carried out after preheating at a temperature not higher than the melting point of the high-molecular-weight polyolefin minus 20 ° C. in advance.

The shutdown temperature of the porous film of the present invention thus obtained is 135 ° C. or lower, preferably 130 ° C. or lower.

The porosity of the porous film is 40%
As described above, the content is preferably 45 to 65%. 40% porosity
If it is less than 1, the film resistance increases, and the efficiency of the battery during charging and discharging decreases.

The heat shrinkage of the porous film is 30
%, Preferably 25% or less. Heat shrinkage is 30
%, The internal short circuit occurs due to shrinkage when the temperature of the battery rises due to an external short circuit or the like, and the risk of ignition increases. In addition, the heat shrinkage rate here means that the porous film has a thickness of 10% as described in Examples described later.
Heat shrinkage when heated at 5 ° C for 1 hour.

The thickness of the porous film suppresses the membrane resistance,
And from the viewpoint of securing strength, preferably 10 to
It is 60 μm, more preferably 15 to 45 μm.

As described above, the porous film of the present invention is impregnated with a poor solvent having a melting point of −15 ° C. or more and a melting point of + 5 ° C. or less of the high-molecular-weight polyolefin in the manufacturing process, and is subjected to a heat setting treatment. Therefore, it is a film that has excellent low-temperature shutdown characteristics despite being a film that is not easily thermally contracted and has a high porosity. That is, in the present invention, in a state in which the poor solvent is impregnated inside the pores, the heat set treatment is performed, so that the structure shrinkage, particularly the shrinkage in the thickness direction, is suppressed, and other resins having a low melting point or a low softening point are used. Is suppressed, and the porous structure is maintained. On the other hand, it is considered that pores are closed when the temperature inside the battery rises because the surface pressure is applied at the time of mounting the battery, and it is considered that the high molecular weight polyolefin is subjected to thermal relaxation at a high temperature, so that it does not easily undergo thermal contraction. Therefore, the porous film of the present invention can be suitably used not only as a battery separator, but also as various filters, diaphragms for electrolytic capacitors, and the like.

[0037]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

Various characteristics are measured in the following manner.

(1) Melting point Differential scanning calorimeter “D” manufactured by Seiko Denshi Kogyo Co., Ltd.
Using an "SC-200" at a heating rate of 10 ° C./min, the onset temperature is defined as the melting point.

(2) Porosity The porous film to be measured is cut into a circular shape having a diameter of 6 cm, the volume and weight are obtained, and the obtained result is calculated by the following equation.

Porosity (% by volume) = 100 × [volume (cm)
³ )-Weight (g) / density of polyolefin (g / c)
m ³ )] / volume (cm ³ )

(3) Heat Shrinkage A porous film to be measured is cut into a circular shape having a diameter of 6 cm, and is put into an oven at 105 ° C. for 1 hour in a tensionless state. Next, the number of pixels before and after heating is measured using a computer and a scanner, and the heat shrinkage is calculated from the following equation.

Heat shrinkage (%) = 100 × (number of pixels after heating) / (number of pixels before heating)

(4) Shutdown temperature A separator impregnated with an electrolytic solution sandwiched between Pt electrodes of φ20 mm and φ10 mm in a cell is fastened at a low pressure, and the change in internal resistance at a temperature rising rate of 10 ° C./min is recorded. The temperature when the resistance reaches 100 Ω · cm ² is defined as the shutdown temperature.

Example 1 Ultra high molecular weight polyethylene (weight average molecular weight: 2 × 1)
0 ^6, melting point: 133 ° C.) 12 parts by weight, an olefin-based thermoplastic elastomer (Sumitomo Chemical Co., Ltd., TPE821,
(Softening point: 103 ° C.) 3 parts by weight and 85 parts by weight of liquid paraffin were uniformly mixed in a slurry state, and the obtained resin composition was kneaded at a temperature of 160 ° C. for about 60 minutes using a small kneader. Thereafter, the obtained kneaded material was sandwiched between metal plates cooled to 0 ° C., and rapidly cooled into a sheet. These quenched and crystallized sheet-like molded products are heat-pressed at 120 ° C. until the sheet thickness becomes 0.8 mm, and are simultaneously biaxially stretched at 120 ° C. 3.5 × 3.5 times in length and width. A solvent was removed using heptane to form a film. Then 125
After performing heat set treatment by impregnating in N-methylpyrrolidone heated to 10 ° C. for 10 minutes, washing with acetone,
A porous film was obtained.

Example 2 Example 1 was repeated except that the heat setting treatment was carried out by impregnating in N-methylpyrrolidone heated to 125 ° C. for 1 hour.
A porous film was obtained in the same manner as described above.

Example 3 The amount of the ultra-high molecular weight polyethylene was changed to 10 parts by weight, and the amount of the olefinic thermoplastic elastomer was changed to 5 parts by weight.
Except having changed each, it carried out similarly to Example 2, and obtained the porous film.

Example 4 Ultra high molecular weight polyethylene (weight average molecular weight: 2 × 1)
0 ^6, melting point: 133 ° C.) 11 parts by weight, low-density polyethylene (melting point: 105 ° C.) 4 parts by weight of liquid paraffin 85
Using a weight part, a film was formed in the same manner as in Example 1 except that the film was biaxially stretched 3.5 × 3.5 times in length and width at 115 ° C. at the same time. After desolvation treatment, N was heated to 120 ° C. After performing heat setting treatment by impregnating in -methylpyrrolidone for 10 minutes, the film was washed with acetone to obtain a porous film.

Comparative Example 1 Example 1 was repeated except that the heat-setting treatment was performed by impregnating N-methylpyrrolidone heated at 115 ° C. for 1 hour.
A porous film was obtained in the same manner as described above.

Comparative Example 2 Example 3 was repeated except that the heat-setting treatment was performed by impregnating N-methylpyrrolidone heated to 140 ° C. for 1 hour.
A porous film was obtained in the same manner as described above. The obtained porous film was translucent and increased in weight before the heat setting treatment. The hole in the film is closed, and some N
-It is believed that methylpyrrolidone was contained.

Comparative Example 3 A film was formed in the same manner as in Example 1, and after the solvent was removed, a heat treatment was not performed to obtain a porous film.

Comparative Example 4 Ultra high molecular weight polyethylene (weight average molecular weight: 2 × 1)
0 ^6, melting point: 133 ° C.) 12 parts by weight, low-density polyethylene (melting point: 105 ° C.) 3 parts by weight of liquid paraffin 85
Using a weight part, a film was formed in the same manner as in Example 1 except that the film was biaxially stretched 3.5 × 3.5 times at 115 ° C. simultaneously and desolvation treatment was performed. For 30 seconds to obtain a porous film.

Table 1 shows the thickness, porosity, heat shrinkage, and shutdown temperature of the porous films obtained in Examples and Comparative Examples.

[0054]

[Table 1]

From the above results, since the porous films of Examples 1 to 4 were sufficiently heat-relaxed in the poor solvent heated to a predetermined high temperature during the heat setting treatment, they were not thermally contracted. It can be seen that the film has excellent low-temperature shutdown characteristics despite its low porosity and low porosity. On the other hand, the porous film of Comparative Example 1 in which the heat setting treatment was performed at a temperature lower than the melting point of the high molecular weight polyolefin −15 ° C. had a large heat shrinkage, and the heat setting treatment was performed at a temperature higher than the melting point of the high molecular weight polyolefin + 5 ° C. It can be seen that the porous film of Comparative Example 2 carried out at a high temperature is closed due to the poor solvent being enclosed inside the pores and the porous structure cannot be maintained. Furthermore, the porous films of Comparative Example 3 in which the heat setting treatment was not performed and Comparative Example 4 only held in low-temperature warm air both had high porosity and exhibited low-temperature shutdown characteristics, It can be seen that the heat shrinkage is significantly increased.

[0056]

According to the present invention, it has become possible to provide a porous film which does not easily undergo heat shrinkage and has excellent low-temperature shutdown characteristics and high porosity.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C08L 23:00 101: 00 (72) Inventor Mutsuko Yamaguchi 1-1-2 Shimohozumi, Ibaraki-shi, Osaka Nitto Denko (72) Inventor Kazunari Yamamoto 1-1-2 Shimohozumi, Ibaraki-shi, Osaka Nitto Denko Corporation (72) Inventor Yoshihiro Uetani 1-1-2 Shimohozumi, Ibaraki-shi, Osaka Nitto Denko Corporation F term (reference) 4F074 AA16 AA17 AB03 AD02 CA03 CB31 CC02Y CC04Z CC32Z CC36Z DA02 DA22 DA49 5H021 BB01 BB04 BB05 BB12 BB13 CC00 EE01 EE04 EE15 EE23 HH01 HH02 HH06 HH07

Claims (5)

[Claims] 1. A high-molecular-weight polyolefin having a weight-average molecular weight of 1 × 10 ⁶ or more and another resin having a melting point or a softening point lower than the melting point of the high-molecular-weight polyolefin, and having a shutdown temperature of 135 ° C. or lower. And a porosity of 40% or more and a heat shrinkage of 30% or less. 2. A resin composition containing a high molecular weight polyolefin having a weight average molecular weight of 1 × 10 ⁶ or more, another resin having a melting point or a softening point lower than the melting point of the high molecular weight polyolefin, and a solvent. Forming a sheet, stretching and desolvating, impregnating with a poor solvent having a melting point of −15 ° C. or more and a melting point of + 5 ° C. or less of the high-molecular-weight polyolefin, and performing a heat-setting process. A method for producing a porous film. 3. The method according to claim 2, wherein the obtained porous film has a shutdown temperature of 135 ° C. or less, a porosity of 40% or more, and a heat shrinkage of 30% or less. 4. The mixing ratio of the high molecular weight polyolefin and the other resin is 50/50 in the high molecular weight polyolefin / other resin.
The method according to claim 2 or 3, wherein the ratio is 50 to 90/10 (weight ratio). 5. The method according to claim 2, wherein the resin composition comprises 5 to 50% by weight of a high molecular weight polyolefin and another resin, and 50 to 95% by weight of a solvent.