JPH01223136A - Production of microcellular polyolefin film - Google Patents

Abstract

PURPOSE:To obtain the subject film suitable as separators in electrolytic capacitors, cells or batteries, etc., without fish eyes, by adding two kinds of low- and high-melting organic solids to a polyolefin, melt extrusion molding the resultant mixture and extracting the low-melting organic solid. CONSTITUTION:(A) An organic solid (preferably dicyclohexyl phthalate or triphenyl phosphite) having 35-100 deg.C, preferably 45-80 deg.C melting point, 200-1,000mol.wt., polarity and polar groups in the molecular structure in an amount of 80-180pts.vol., preferably 90-160pts.vol. and (B) an organic solid (e.g., phthalic anhydride) having >=105 deg.C melting point which is within the range of <=the melting point of the above-mentioned polyolefin +50 deg.C, preferably >=the melt crystallization temperature of the afore-mentioned polyolefin and <=the melting point of the polyolefin in an amount of 0.01-20pts.vol., preferably 0.05-10pts.vol. are added to 100pts.vol. polyolefin and the resultant mixture is melt extrusion molded. At least the component (A) is then extracted to afford the aimed film having continuous micropores.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing a polyolefin microporous membrane suitable as a separator or microfilter for electrolytic capacitors, electric double layer capacitors, batteries, etc.

The term "microporous membrane" as used herein refers to a film-like, tube-like, or hollow fiber-like film having micropores continuous at least in the thickness direction.

[Prior Art] Conventionally, a method for manufacturing a polyolefin microporous membrane involves adsorbing a room temperature organic liquid such as a plasticizer onto inorganic fine particles, adding it to the polyolefin, and after forming a sheet, the room temperature organic liquid or the inorganic fine particles are added to the polyolefin. Extraction method (Special Publication Showa 5B-32
171 etc.) are known.

[Problems to be Solved by the Invention] However, this method had the following problems. In other words, in the method of adding an organic liquid to polyolefin, a specific amount of inorganic fine particles is essential as a carrier, so thixotropic materials with many branched side chains such as medium density polyethylene, low density polyethylene, polypropylene, poly(4-methylpentene) etc. When applying this technology to polymers with strong oxidation, the addition of inorganic particles greatly increases thixotropy, making it impossible to obtain extrusion stability, resulting in poor thickness uniformity, and in particular, it is difficult to obtain thin microporous membranes. I can't do it,
Furthermore, since inorganic particles are added, even if extraction is performed in a subsequent process, it takes a long time to complete the extraction process, and there are problems in that there are void-like defects that are thought to be caused by aggregates of inorganic particles. Ta.

[Means for Solving the Problem] The present invention is a method for producing a polyolefin microporous membrane having continuous micropores, in which 100 parts by volume of a polyolefin having a melting point of 35 to 100°C and a molecular weight of 200 to 100, and in the molecular structure (80 to 180 parts by volume of an organic solid containing polarizability and polar groups, and a melting point of 105°
C or above, relates to a method for producing a polyolefin microporous membrane, which comprises adding 0.01 to 20 parts by volume of an organic solid (II) having a melting point of the polyolefin +50°C or less and extracting at least the organic solid (1) after melt extrusion molding. It is.

In the present invention, polyolefins include α-olefin polymers such as high-density polyethylene, medium-density polyethylene, low-density polyethylene, polypropylene, polybutene, polymethylpentene, and polymethylbutene, and copolymers and blends thereof.

Among these, polyethylene resins and polypropylene resins are preferred, as they have well-balanced mechanical properties, chemical resistance, and cost performance, and have a weight average molecular weight (Mw) of more than 30x104.

In particular, polypropylene resin (hereinafter abbreviated as PP resin) is preferable because it has excellent heat resistance.

In order to have good mechanical strength and uniform micropores, the intrinsic viscosity of the PP resin (hereinafter abbreviated as [η]) must be 2.
．． 1 to 3.3 dl/a, preferably 2°3 to 3.0 d
110, melt index (MI) is 0.1-3Q/
Those within the 10 minute range are excellent.

Further, the isotactic index (hereinafter abbreviated as II) of the PP resin is preferably 93% or more, more preferably 97% or more. If II is within this range, not only will the pore diameter and porosity be good, but the extraction rate of the added organic solid will be high, the extraction time will be short, and the cost will be excellent.

In addition, the density of the polyethylene resin is 0093G/
Those having a diameter of cm3 or more are preferable because they have excellent solvent resistance, dimensional stability, and microporosity.

Next, the organic solid (I) used in the present invention has a melting point of 35
-100°C, preferably 45°C
~80°C. If the melting point is too low, it will cause slippage in the feed section of the extruder screw when blending with PP, making it virtually impossible to blend, making it necessary to add inorganic fine particles, and failing to achieve the purpose of the present invention.

On the other hand, if the melting point is too high, the extraction rate will be slow, causing cost problems, and when the blend with polyolefin is cast, the phase separation structure will be non-uniform, resulting in a widened pore size distribution and the inability to stretch. Also occurs.

Further, the molecular weight of the organic solid (1) needs to be 200 to 1000, preferably 300 to 700. If the molecular weight is too small, the solubility in the polyolefin will increase, resulting in only small porosity and pore diameter, which will not achieve the purpose of the present invention. On the other hand, if the molecular weight is too large, the dispersion diameter of the organic solid (1) in the polyolefin becomes large, the pores formed are large, and the pore size distribution is widened, so that the purpose of the present invention is not achieved.

Furthermore, the extraction time becomes longer, which causes an increase in cost.

Furthermore, it is necessary that the molecular structure of the organic solid (I) contains polarizable and polar groups. Here, the term "polarizable group" refers to a group containing an aromatic ring such as a benzene ring or a naphthalene ring, and the term "polar group" refers to a polar molecule, such as that described in Goiba Shoten's "Rikagaku Dictionary". The constituent elements are carbonyl groups, amino groups, sulfone groups,
Examples include hydroxyl group. Organic solids (such as low molecular weight polystyrene that contain only polarizable groups in I) have extremely high dispersibility in polyolefins, so even if a large amount is added and extracted, the pore size, Only those with low porosity can be produced, and the resistance increases when impregnated with an electrolyte.On the other hand, organic solids (■) containing only polar groups, for example.

When fatty acid esters are added in large quantities to polyolefins, the dispersed diameter of the phase-separated structure increases, making the cast sheets or tubes extremely brittle, making subsequent processes virtually impossible. be. Of course, with substances that do not contain either polar groups or polarizable groups, polyolefin (not only will it not be possible to form a uniform phase separation structure when added, but the extraction solvent will be limited to those that easily dissolve the polyolefin). Extraction becomes difficult.

Among the organic solids (I) having the above characteristics, phthalate esters, phosphate esters, etc., which are used as plasticizers for vinyl chloride, etc., are excellent, and in particular, dicyclohexyl phthalate (DCHP), Preferably, it is at least one selected from triphenyl phosphate (TPP).

The amount of the organic solid (I> added is 10% of the polyolefin resin
80~.0 volume part. It is necessary that the amount is 180 parts by volume, and more preferably 90 to 160 parts by volume. If the amount added is small, continuous micropores will not be formed, and when used as a separator for electrolytic capacitors, etc., the resistance of the electrolyte will increase, making it unusable. on the other hand,
If the amount added is too large, discharge fluctuations will occur when the raw material is supplied to the extruder, and not only will a uniform film not be obtained, but the mechanical strength of the film will decrease, making it unusable.

Roughly speaking, in the present invention, the organic solid (II) having a melting point of 105° C. or more, the melting point of the polyolefin + 50° C. or less, preferably - the melt crystallization temperature of the polyolefin or more and the melting point of the polyolefin or less, is used as polyolefin 10.
0.01 to 20 parts by volume, preferably 0.0 parts by volume.
It is necessary to add 0.5 to 10 parts by volume.

If the melting point of the organic solid (I) is less than 105°C, the dispersion uniformity of the organic solid (1) after melt molding will be affected, resulting in a wide distribution of the formed micropores. On the other hand, if the melting point exceeds the melting point of the polyolefin +50°C, it causes fish eyes and voids, and when used as a separator for electrolytic capacitors, etc., it causes short circuits.

Further, the organic solid (II) has a molecular weight (Mn) of 100 to
6000, and the degree of crystallinity by X-ray diffraction is 50% or more, which is preferable in order to improve the uniformity of the formed micropores. It is preferable that it has excellent compatibility with at least one of the organic solids. Examples of organic solids having these characteristics include phthalic anhydride, phthalic acid, and low molecular weight polyolefins.
If the amount added is less than 0.01 parts by volume, the cooling conditions during molding will be too narrow for uniformly dispersing the organic solid (I), while if it exceeds 20 parts by volume, extrudability will become unstable.

Further, a heat stabilizer, an antioxidant, an organic or inorganic lubricant, an antistatic agent, etc. may be added to the polyolefin resin, and in particular, a metal salt of fatty acid such as calcium stearate may be added at 0.01% of the total composition. If ~5 parts by weight is added,
This is preferable because it improves discharge properties and the like.

Regarding the addition of inorganic fine particles, it is permissible to add them to the extent that extrudability does not deteriorate due to the addition, but it is preferable not to add them as much as possible. The amount is preferably 5 parts by volume or less.

Next, the temperature at which the above-mentioned blend is melt-extruded and molded is equal to or higher than the melting point (Tml) of the organic solid (1) and lower than the melt crystallization temperature (TllIc) of the polyolefin. This is preferable because phase separation from the solid (1) proceeds uniformly and continuous micropores are formed.

In general, it is preferable that the cooling rate during cooling is as high as possible, and it is further preferable that the front and back surfaces of the extruded sheet/film material, tube, or hollow fiber are cooled uniformly. From this point of view, it is preferable to cool these melt extrudates by introducing them into a water bath maintained at the above-mentioned temperature.

Here, the liquid used for cooling is usually water, but if necessary, a liquid with a high boiling point such as ethylene glycol or diethylene glycol may be mixed with water, and the liquid is not limited to this. It's not something you can do.

Furthermore, when the melt is formed into a film or sheet using a T-die, a cooling rate equivalent to that of the water bath casting method can be obtained by applying sufficient pressure to adhere the melt using the electrostatic application method.

As described above, it is preferable to set the draft ratio at the time of molding to 4 or more in order to improve the uniformity of the phase separation structure and the continuous through-hole property, and it is more preferable to set the draft ratio to 6 or more.

The molded article obtained as described above is preferably 95% or more of the organic solid (I>, more preferably 95% or more, and more preferably A microporous membrane is obtained by extracting 99% or more of this.The solvent used here includes trichloromethane, trichloroethane, acetone, methyl ethyl ketone, ethyl acetate, methanol, toluene, xylene, etc. Among them, halogenated solvents such as trichloroethane and trichlorethylene are often used industrially because they have high extraction ability and are free from the risk of ignition.

In addition, a cast molded product is preferred because mechanical properties and continuous through-hole properties of fine pores are improved by stretching. When stretching here, it is preferable to remove 95% or more, preferably 99% or more of the added organic solid (1) by extraction before stretching in order to improve the continuous through-hole property. It is not necessary to perform extraction before stretching, and methods such as performing extraction after stretching, performing extraction while stretching, partial extraction, stretching, and then complete extraction are possible.

Here, the stretching is (glass transition point of the polyolefin) ~
(The melting point of the polyolefin -10°C) It is preferable to stretch at least uniaxially 1.5 to 10 times, preferably 1.7 to 7 times, in order to improve mechanical properties and continuous through-hole properties. . At this time, if at least some of the stretching steps include a low-speed stretching step in which the stretching speed is 500%/min or less, preferably 2000%/min or less, and more preferably 1000%/min or less, continuous penetration may occur. This method is preferable because the porosity becomes good, and for example, a method in which a plurality of stretching zones are provided in -axial stretching and the above-mentioned low-speed stretching is performed in at least one of the stretching zones, or a method in which two wheels are stretched in any one direction. The above-mentioned low-speed stretching step may be included in the stretching step. Here, as a method of stretching the two wheels, after stretching in the longitudinal direction at an arbitrary stretching speed of 1.5 to 10 times, in the width direction by the above-mentioned low speed stretching method,
twice, preferably 1.1 to 2 times, more preferably 1.
By stretching at 2-1°5 times, the area magnification is 1.7-1.
A range of 15 times, preferably 2 to 10 times, is preferred because mechanical properties, continuous through-hole properties, and uniformity of pore shape are all improved.

After the above-mentioned stretching, extraction is carried out again if necessary to extract 95% or more, preferably 99% or more of the added organic solid (1), and then the polyolefin has a melt crystallization temperature or higher, a melting point 0 to 20 at temperatures below -5℃
%, preferably 1 to 10%, is preferable because thermal dimensional stability is improved.

Roughly, in the present invention, if necessary, the microporous membrane may be subjected to surface treatment such as ultraviolet irradiation treatment, corona discharge treatment, or low-temperature plasma treatment, coating with a surfactant, or hydrophilization by sulfonation or grafting treatment with methyl methacrylate, etc. Processing may be performed. In particular, when the microporous membrane obtained according to the present invention is used as an aqueous microfilter or an electrolyte separator, it is preferable to perform a hydrophilic treatment.

In the present invention, when stretching is performed after extraction, the above-mentioned hydrophilic treatment can be performed before or during stretching. This is preferable because it not only improves cost efficiency but also enables uniform processing.

The film obtained in the above manner usually has an average pore size of 0.
．． 05-5μm1 porosity is in the range of 50-85%,
Not only does it have excellent pore uniformity, but due to the manufacturing process, no void-like defects occur, and it has good pinhole resistance, making it useful as an electrolyte separator for microfilters, electrolytic capacitors, lithium batteries, etc. Not only that, but by using this film as a base film and coating the surface with a functional membrane that has separation ability such as gas separation ability, it can also be used for applications such as oxygen enrichment membranes and pervaporation. .

[Method of Measuring Characteristics and Method of Evaluating Effects 1] Next, the measuring methods and evaluation methods related to the present invention will be summarized.

(1) Intrinsic viscosity of PP ([η]) According to ASTM D 1601, 0.10 of the sample was completely dissolved in 100 ml of tetralin at 135°C, and this solution was measured with a viscometer in a constant temperature bath at 135°C. The limiting viscosity is determined from the specific viscosity S according to the following formula.

[η]-8/(0,1x (1+0.22xS))(2
) Melt index (M I ) P E : A
ST) 1-D1238-651.

PP: Measured according to ASTH-01238-62T.

All units are g/10 minutes (3) Isotactic index of PP (II) The sample is vacuum dried at 130° C. for 2 hours. From now on weight W
A sample of (111 g) was taken, placed in a Soxhlet extractor, and extracted with boiling ln-hebutane for 12 hours.

Next, take out this sample, wash it thoroughly with acetone, and then
Vacuum drying at 130°C for 6 hours, then weight W' (1
11g> is measured and calculated using the following formula.

1N%) - (W' /W) 2
Measurement is performed from room temperature at 0°C/min, and the endothermic peak temperature accompanying melting is defined as the melting point.

Subsequently, the temperature was raised to 280℃ and held for 5 minutes.
In the process of lowering the temperature at a rate of drop per minute, the peak temperature of latent heat accompanying crystallization of the polyolefin is defined as the melt crystallization temperature.

(5) Melting point of organic solid (I) Measured according to ASTM-E-28.

(6) Molecular weight of organic solid (If> Molecular weight (Mn) measured by vapor pressure osmotic pressure method.

(7) MDC longitudinal direction) strength Sample longitudinal direction (MD) breaking strength of JISK678
2 and expressed in kg/15mm+.

<8) ESR (Equivalent Series Resistance) Based on the method described in JP-A-61-187221, γ-butyrolactone + triethylamine + 7-talic acid was dissolved to prepare an electrolytic solution of 3.1 mS/cm (25°C). . The sample was cut into 30-35 mm squares, impregnated with the electrolyte solution, and approximately 1.0% of the pores were replaced with the electrolyte solution.The samples were then stacked as 5 sheets and measured between platinum black-treated platinum electrodes, and the ESR of each sheet was measured. I asked for it.

Here, electrolytic capacitor paper (v2'
51 MER2, 5-50) (7) Using the ESR value as a comparison value, a microporous membrane having an ESR of less than the manila paper was evaluated as 01. A microporous membrane having an ESR of more than the manila paper was evaluated as X.

Note that the measurement conditions were as follows.

(a) Electrode: Platinum black treated platinum electrode (25 mm square) Measuring load: 240 g (b) Impedance measuring device AG-4311LCRMETER (manufactured by Kuuken Denki Co., Ltd.) Measurement conditions: 1 kHz, 25°C (9) Average pore diameter mercury porosimeter ( The distribution of micropore diameters was measured using HICRO)lER, PORESIZER9300 (manufactured by ITIcs), and the number average was taken as the average pore diameter.

6φ Porosity (P'') A sample (10 x 10 cm) was immersed in liquid paraffin for 24 hours, the weight (W2) was measured after wiping off the liquid paraffin on the surface layer, and the weight of the sample before immersion (Wl ) The pore volume (Vo) is determined from the density (ρ) of liquid paraffin using the following formula.

Vo - (W2-Wl)#) The porosity (P'') is calculated from the apparent volume (a value calculated from thickness and dimensions) ■ and the pore volume ■O.

Pr-Vo/VXI00 (%) 01) Draft ratio The ratio of the cross-sectional area of the extruder die (Sl) to the cross-sectional area of the cast sheet or tube-like product (S2) Sl /
Defined in 32.

(b) Composition ratio of blends in Examples The composition ratios of blends in Examples are calculated from the weight ratio and true specific gravity of each blend composition.

01 Weight average molecule of polyolefin! (MW) by gel permeation chromatography.

Apparatus: GPC-150C (manufactured by WATER3) Column: 5hodex KF-80M (manufactured by Showa Denko) Solvent 20-dichlorobenzene Molecular weight calibration: Monodisperse polystyrene (b) Melting point of organic solid (II): Previous DSC method Measured by

[Example] Next, the present invention will be explained based on examples.

(Preparation of raw materials) As shown in Table 1, polypropylene (PP) powder (
Both are Mitsui Noprene Powder), organic solid (I>
as dicyclohexyl phthalate (DCHP), triphenyl phosphate (TPP), DOP (dioctyl phthalate).

Low molecular weight polystyrene (“Himer” 5T-75゜ manufactured by Sanyo Kasei Co., Ltd.), phthalic anhydride as organic solid (II), low molecular weight polypropylene (Viscol 660-P)
were prepared and melt-blended using a twin-screw extruder to form pellets. In addition, when adding DOP,
Inorganic fine particles (“Aerosil” 200 manufactured by Nippon Aerosil Co., Ltd.) are preliminarily applied.

P was adsorbed and added to the resin powder.

Examples 1 to 3 Using raw material A, melt-extrude it into a sheet using a T-die from a 4φmm extruder, and apply a DC voltage of 5 kV to a tungsten wire on a casting drum at 90°C at a draft ratio of 6 to make it adhere. The mixture was cooled and solidified (electrostatic application method, Example 1). In addition, it was cooled and solidified under the same conditions without electrostatic application (Example 2).

Furthermore, the molten sheet was introduced into a water tank (50°C, water) and cooled and solidified while being introduced at a draft ratio of 7 (Example 3)
.

The cast film obtained in the above manner was heated to 1°C at 40°C.
-1-1 The sample was introduced into a trichloroethane extraction tank, and 99% or more of the added DCHP was extracted while maintaining an extraction time of 2 minutes. After drying the solvent at 100°C, the sample was rolled up.

The extracted film thus obtained was stretched 3.5 times in the longitudinal direction (MD) at 130°C using a roll-type stretching device, and then introduced into a stenter and stretched in the transverse direction (TD) at a temperature of 135°C and a stretching speed of 500. The film was stretched 1.4 times at a rate of %/min, heat-set at 140° C. while allowing 5% relaxation in the width direction, and then wound up.

The manufacturing conditions and properties of the film obtained as described above are summarized in Table 2, and it can be seen that it is a microporous film with excellent mechanical properties and resistance properties superior to electrolytic capacitor paper. .

Comparative Example 1 C (PP+DOP+Aerosil 200) in Table 1 was used as raw material.
) and tried cooling and solidifying it in a casting drum using the extrusion device used in Example 1, but a thin cast film of 250 μm or less could not be obtained due to poor extrusion stability of the melt (draft ratio 2). In addition, since the drum adhesion was poor, the molding was performed while lightly pressing the drum surface with a small diameter roll. Next, the DOP was introduced into the extraction tank used in Example 1, and in order to extract more than 99% of the added DOP,
It was necessary.

The thus obtained extraction film was stretched in the same manner as in Examples 1-3. However, in order to improve the stretchability, the stretching temperature was set to 135°C (MD stretching) and 145°C (TD stretching), respectively.
And so.

The properties of the film thus obtained are shown in Table 2, and MD
It can be seen that the strength is small, the surface is uneven, the ESR is poor, and the continuity of micropores is poor. In general, there were some void-like defects due to aggregates of inorganic fine particles.

Example 4 Using B in Table 1 as the raw material, at a draft ratio of 20, 68
The mixture was introduced into a water bath at 0.degree. C. and cooled and solidified to obtain a cast film with a thickness of 60 .mu.m.

Subsequently, in the extraction apparatus of Example 1, 99% of the added DCHP was extracted.
% or more was extracted, dried and rolled up.

The extracted film thus obtained was stretched 3.0 times by 200° in the longitudinal direction (MD) at 135°C using a roll-type stretching device.
The film was stretched at a stretching speed of 0%/min, heat-set at 150° C. while allowing 5% relaxation in the longitudinal direction, and wound up. The properties of the film thus obtained are shown in Table 2, and the ES
It can be seen that it has excellent R and continuous through-hole properties.

Comparative Example 2 D was used as the raw material in Table 1, and the material was cooled, solidified, and wound in the same manner as in Example 3, except that the water tank temperature was 80°C. Continued 1
-1-1 The mixture was introduced into a trichloroethane extraction tank and extracted, but it took 20 minutes to extract more than 99% of the added low molecular weight polystyrene. Next, the extracted film thus obtained was stretched to 3.0 times at 138°C and then stretched to 14
It was heat-set at 5° C. while allowing 5% relaxation, and then rolled up.

The properties of the microporous film thus obtained are shown in Table 2, and it can be seen that the ESR is poor and the continuous pores are poor.

[Effects of the Invention] The present invention provides a method for obtaining a polyolefin microporous membrane, including:
A method is adopted in which the molecular structure of the organic solid (I) to be added to the polyolefin is defined, and an organic solid (II) having a melting point of 105°C or higher and a melting point of the polyolefin or lower is added and at least organic solid (1) is extracted. By doing so, the following effects can be achieved.

(1) Additive materials such as inorganic fine particles are no longer required due to the addition of re-extracted substances, and the microporous membrane that can be produced not only does not have fissure-like defects, but also has thixotropy during melt extrusion compared to base polyolefin. It can be applied to many polyolefins without causing significant damage.

(2) Due to the excellent uniformity and continuity of the phase separation structure during casting, it has excellent uniformity in the size of the micropores that can be produced and continuous pores in the thickness direction, and is used as separators for electrolytic capacitors, batteries, etc. It exhibits excellent characteristics with low internal resistance when

(3) It has become possible to produce a uniform microporous membrane with good reproducibility, in which the dependence of microporous properties such as pore diameter and porosity on casting temperature is small.

Claims (6)

[Claims] (1) A method for producing a polyolefin microporous membrane having continuous micropores, the method comprising: adding 100 parts by volume of polyolefin to
The melting point is 35-100°C, the molecular weight is 200-1000,
and 80 to 180 parts by volume of an organic solid (I) containing polarizable and polar groups in its molecular structure, and a melting point of 105°C.
As mentioned above, after adding 0.01 to 20 parts by volume of organic solid (II) having a melting point of the polyolefin +50°C or less, and melt extrusion molding,
A method for producing a polyolefin microporous membrane for extracting at least an organic solid (I). (2) The method for producing a polyolefin microporous membrane according to claim 1, wherein cooling and solidification after melt extrusion are performed in a water bath. (3) The polyolefin micropores according to claim 1, wherein the melt extrusion is performed using a T-shaped die, cooling and solidification are performed on a drum, and the melt is pressed onto the drum by electrostatic application. Method for producing sexual membranes. (4) The method for producing a polyolefin microporous membrane according to any one of claims 1 to 3, wherein the draft ratio during melt extrusion is 4 or more. (5) The method for producing a microporous membrane according to claim 1, further comprising the step of stretching at least uniaxially by 1.5 to 10 times after extracting at least the organic solid (I). (6) The method for producing a polyolefin microporous membrane according to claim 5, wherein the stretching speed during stretching is 5000%/min or less.