JPH09104775A - Production of microporous polyolefin film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently obtain a microporous polyolefin film which is easy to handle by extruding and quenching a soln. contg. an ultrahigh-mol.-wt. polyolefin and a polyolefin having a specified density, stretching the resultant molded gel, and removing a solvent from the gel. SOLUTION: A soln. is prepd. which comprises 90-50wt.% solvent and 10-50wt.% polyolefin compsn. comprising 1wt.% or higher ultrahigh-mol.-wt. polyolefin having a wt.-average mol.wt. of 5×10<5> or higher and another polyolefin having a wt.-average mol.wt. of 1×10<4> or higher but lower than 1×10<5> and a density of 0.945g/cc or higher. The soln. is extruded through a die and quenched to form a gel compsn., which is stretched at or below the temp. 10 deg.C higher than the m.p. of the polyolefin compsn. and freed of the remaining solvent, thus giving a microporous polyolefin film. Since the gel compsn. in this process has high hardness and scratch resistance, does not cause film breaking due to notches, and sags little, it can be easily held by a fastener of a stretching machine and enables an efficient production of the microporous film.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a polyolefin microporous membrane, and more particularly to a method for producing a polyolefin microporous membrane which is easy and efficient to handle a gel-like sheet during the film-forming process.

[0002]

2. Description of the Related Art In recent years, an ultrahigh molecular weight polyolefin which can be formed into a film having high strength and high elasticity has been developed, and various methods for producing a high strength microporous membrane using the same have been proposed.

For example, JP-A-60-242035 discloses
A gel-like sheet is formed from a solution obtained by heating and dissolving ultrahigh molecular weight polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more in a solvent, the solvent of the gel-like sheet is desolvated to 10 to 80% by weight, and then heated. A method for producing a polyethylene microporous membrane by removing the residual solvent after stretching is disclosed. However, in this method, in order to biaxially stretch the ultra-high molecular weight polyolefin, it is necessary to prepare a dilute solution of the polyolefin, and thus the resulting solution has a large swell or neck-in at the die exit for sheet molding, Molding is difficult, and since the solvent is contained in excess in the sheet, the desired microporous membrane cannot be obtained by stretching as it is, so it is necessary to desolvate and adjust the amount of solvent in the sheet. There was a problem in productivity.

On the other hand, for the purpose of solving such a problem, JP-A-3-64334 discloses a composition containing an ultra high molecular weight polyolefin and having a weight average molecular weight / number average molecular weight value in the range of 10 to 300. Disclosed is a method for producing a microporous polyolefin membrane using a product. According to this method, it is possible to efficiently produce a microporous polyolefin membrane from a polyolefin composition that has good stretchability and can be made into a high-concentration solution.

However, according to the above-mentioned method, it is possible to form a film without adjusting the amount of solvent of the gel-like sheet, but since the gel-like sheet contains a large amount of solvent, it is soft and easily scratched, and it is difficult to handle. Is difficult. Also, the scratches become notches and cause film rupture during stretching. For this reason, it is desired to increase the hardness of the gel-like sheet and more efficiently produce a polyolefin microporous film containing an ultrahigh molecular weight component.

[0006]

An object of the present invention is to improve the hardness of a gel composition composed of a polyolefin composition to prevent scratches during the process of producing a microporous membrane, and to make the handling easy and efficient. A method for producing a microporous polyolefin membrane is provided.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies in view of the above problems, and as a result, if an ultrahigh molecular weight polyolefin is contained in a predetermined amount or more and the density of other polyolefin components is a predetermined value or more, The inventors have completed the present invention by finding that the hardness of the gel-like sheet can be enhanced and the gel-like sheet can be easily handled to efficiently produce a microporous polyolefin membrane.

That is, in the method for producing a microporous polyolefin membrane of the present invention, 1% by weight or more of an ultrahigh molecular weight polyolefin having a weight average molecular weight of 5 × 10 ⁵ or more and a weight average molecular weight of 1 × 10 ⁴ or more 5 × 10 ⁵ The density is less than 0.9
10 to 50% by weight of a polyolefin composition comprising another polyolefin component of 45 g / cc or more and a solvent 50
To 90% by weight, a solution is extruded from a die and cooled to form a gel composition, and the gel composition is stretched at a temperature not higher than the melting point of the polyolefin composition + 10 ° C., After that, the residual solvent is removed.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the ultrahigh molecular weight polyolefin has a weight average molecular weight of 5 × 10 ⁵ or more, preferably 1 × 10 ⁶ to 15 × 10 ⁶ . When the weight average molecular weight is less than 5 × 10 ⁵ , the maximum draw ratio is low and the desired microporous membrane cannot be obtained. On the other hand, the upper limit is not particularly limited, but if it exceeds 15 × 10 ⁶ , moldability is poor in forming a gel-like molded product.

Examples of such ultra-high molecular weight polyolefins include crystalline homopolymers or copolymers obtained by polymerizing ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and the like. Of these, ultra high molecular weight polyethylene, particularly high density ultra high molecular weight polyethylene is preferred.

The content of the above ultrahigh molecular weight polyolefin in the polyolefin composition is 1% by weight or more, preferably 10 to 70% by weight, based on 100% by weight of the entire polyolefin composition. When the content of the ultrahigh molecular weight polyolefin is less than 1% by weight, the entanglement of the molecular chains of the ultrahigh molecular weight polyolefin, which contributes to the improvement of the stretchability, is hardly formed, and a high-strength microporous membrane cannot be obtained.
On the other hand, the upper limit is not particularly limited, but 70% by weight is preferable from the viewpoint of achieving a high concentration of the solution.

The polyolefin other than the ultrahigh molecular weight polyolefin in the polyolefin composition has a weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ , and its density is 0.945 g / cc or more, preferably 0. .950 g
/ Cc or more. The use of a polyolefin having a weight average molecular weight of less than 1 × 10 ⁴ is not preferable because breakage easily occurs during stretching and fine pores cannot be obtained. Also,
When the density is less than 0.945 g / cc, the gel composition made of polyolefin has a low hardness and is not easy to handle, and the microporous membrane cannot be produced efficiently, which is not preferable. In particular, the weight average molecular weight is 1 × 10 ⁵ or more and 5 × 10 5.
^It is preferable to add 30 to 90% by weight of a polyolefin having a density of less than ⁵ and a density of 0.950 g / cc or more to the ultrahigh molecular weight polyolefin.

Examples of such a polyolefin include those which satisfy the above-mentioned density and are of the same kind as the ultra-high molecular weight polyolefin. Among these, polyethylene, particularly high-density polyethylene satisfying the above-mentioned density is preferable.

Further, the weight average molecular weight / number average molecular weight used as a measure of the molecular weight distribution of the polyolefin composition is preferably 10 to 300 from the viewpoint of preparing a high-concentration polyolefin solution, and more preferably 12 to 250.
It is.

It should be noted that various additives such as an antioxidant, an ultraviolet absorber, a lubricant, an antiblocking agent, a pigment, a dye and an inorganic filler may be added to the polyolefin composition, if necessary, without impairing the object of the present invention. It can be added in a range.

The solution of the polyolefin composition used as a raw material in the present invention is prepared by heating and dissolving the above polyolefin composition in a solvent.

The solvent is not particularly limited as long as it can sufficiently dissolve the polyolefin composition. For example, nonane, decane, undecane, dodecane, aliphatic or cyclic hydrocarbons such as liquid paraffin, or a mineral oil fraction having a boiling point corresponding to these, etc.,
A non-volatile solvent such as liquid paraffin is preferable in order to obtain a gel-like molded product having a stable solvent content.

The heating dissolution is carried out by vigorously stirring or kneading with an extruder at a temperature at which the polyolefin composition is completely dissolved in the solvent. The temperature is, for example, 140 to 250
C. is preferred. The concentration of the polyolefin composition solution is 10 to 50% by weight, preferably 10 to 40% by weight. If the concentration is less than 10% by weight, not only is the amount of solvent used large and it is not economical, but also when forming into a sheet, swell and neck-in are large at the die outlet, making it difficult to form the sheet. On the other hand, if the concentration exceeds 50% by weight, it becomes difficult to prepare a uniform solution. In addition, it is preferable to add an antioxidant in order to prevent the polyolefin composition from being oxidized during heating and dissolution.

Next, the heated solution of the polyolefin composition is preferably extruded from a die to be molded. As the die, a sheet die having a rectangular base shape is usually used, but a double cylindrical inflation die or the like can also be used. When a sheet die is used, the die gap is usually 0.1 to 5 mm, and the extrusion molding temperature is 140 to 250 ° C. At this time, the extrusion speed is usually 20 to 30 cm / minute and 2 to 3 m / minute.

The solution thus extruded from the die is cooled to form a gel-like molded product. Cooling is preferably performed at a rate of 50 ° C./min or more up to at least the gelation temperature. Generally, if the cooling rate is slow,
The higher-order structure of the obtained gel-like molded product becomes coarse and the pseudo-cell units forming it become large, but when the cooling rate is fast, it becomes dense cell units. When the cooling rate is less than 50 ° C./minute, the crystallinity increases and it is difficult to form a gel-like molded product suitable for stretching. Therefore, by adjusting the cooling rate, the pore diameter of the obtained microporous membrane can be changed. it can.

As a cooling method, a method of directly contacting with cold air, cooling water, or other cooling medium, a method of contacting with a roll cooled with a refrigerant, or the like can be used. In addition,
The solution extruded from the die may be taken up at a take-up ratio of 1 to 10, preferably 1 to 5 before or during cooling. When the take-up ratio is 10 or more, neck-in becomes large, and breakage easily occurs during stretching, which is not preferable.

Next, the gel-like molded product is stretched. The stretching is carried out by heating the gel-like molded product and using a normal tenter method, roll method, inflation method, rolling method or a combination of these methods at a predetermined magnification.
The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. Further, in the case of biaxial stretching, either longitudinal / transverse simultaneous stretching or sequential stretching may be used.

The stretching temperature is preferably a melting point + 10 ° C. or lower, more preferably a crystal dispersion temperature or higher and a melting point or lower. For example, polyethylene is 90 to 140
C is preferred. When the stretching temperature exceeds the melting point + 10 ° C., it is not preferable from the viewpoint of orientation of the molecular chain due to stretching due to melting of the resin.

Although the draw ratio varies depending on the thickness of the material,
In uniaxial stretching, it is preferably 2 times or more, more preferably 3 to
It is 30 times. In the biaxial stretching, the surface magnification is preferably 10 times or more, more preferably 15 to 400 times. If the surface magnification is less than 10 times, the stretching is insufficient and a highly elastic and high-strength microporous membrane cannot be obtained. On the other hand, if the areal magnification exceeds 400 times, there are restrictions on the stretching device, stretching operation, and the like.

The stretched molded product obtained is washed with a solvent to remove the residual solvent. Examples of the cleaning solvent include hydrocarbons such as pentane, hexane, and heptane; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride; fluorinated hydrocarbons such as ethane trifluoride; and ethers such as diethyl ether and dioxane. Volatile ones can be used.
These solvents are appropriately selected according to the solvent used for dissolving the polyolefin composition, and used alone or as a mixture.
The washing method can be performed by a method of immersing in a solvent for extraction, a method of showering the solvent, or a combination thereof.

The above-mentioned washing is carried out until the residual solvent in the stretch-molded product is less than 1% by weight. After that, the washing solvent is dried. The washing solvent can be dried by heating, drying with hot air, contact with a heating roll, immersion in a heating medium, or the like.

The dried stretched molded product is preferably heat-set in the temperature range of the crystal dispersion temperature to the melting point. If the heat setting temperature exceeds the melting point, the resin will melt. The time for heat setting treatment varies depending on the heat setting temperature, but is 10 seconds to 10
It is preferable to carry out for minutes.

The thickness of the polyolefin microporous film is
The thickness is appropriately selected depending on the application, but is preferably 0.1 to 50 μm, more preferably 1 to 25 μm. If the thickness is less than 0.1 μm, it is difficult to put the film into practical use because the mechanical strength of the film is insufficient. On the other hand, when it exceeds 50 μm, it is not preferable because it is too thick and the effective resistance becomes large.

In the above-mentioned method for producing a microporous polyolefin membrane, the sheet made of the gel-like composition contains a large amount of solvent in the production process, and it is not affected by the melt index. It has high hardness, is not easily scratched, is easy to handle, and can efficiently produce a microporous membrane.

The obtained polyolefin microporous membrane has a tensile breaking strength of preferably 200 kg / cm ² or more, and an air permeability of preferably 1000 seconds / 100 cc or less,
The average through-hole diameter is preferably 0.005 to 1 μm, which is good.

Although the reason why such an effect is obtained is not always clear, in the method of the present invention, a polyolefin having a high density of a predetermined value or more is used as the polyolefin other than the ultrahigh molecular weight polyolefin in the polyolefin composition. It is considered that the mechanical properties are improved by increasing the crystallinity of polyolefin and increasing the rigidity and tensile strength.

[0032]

EXAMPLES Examples of the present invention will be shown below, but the present invention is not limited to the following examples. In addition, the test method in an Example is as follows. (1) Weight average molecular weight: GPC manufactured by Waters Co., Ltd.
Measured by gel permeation chromatography (GPC) method at a temperature of 135 ° C. and a flow rate of 1.0 ml / min using GMH-6 manufactured by Tosoh Corporation as a column and 0-dichlorobenzene as a solvent. did. (2) Hardness: Measured according to JIS K6301. (C
Method) (3) Melt index (MI): JIS K676
Measured according to 0. (190 ° C, 2.16kg load, g
/ 10 minutes) (4) Slack: 20 mm × 20 mm at one end of a 120 mm × 20 mm × 1.5 mm gel-like sheet was horizontally fixed to a stand, and the slack from the horizontal extension line at the other end of the gel-like sheet was measured. . (Mm) (5) Film thickness: The cross section was measured with a scanning electron microscope.
(Μm) (6) Tensile breaking strength: The breaking strength of a strip test piece having a width of 15 mm was measured according to ASTM D882. (Kg
/ Cm ² ) (7) Air permeability: Measured according to JIS P8117. (Sec / 100 cc) (8) Average through-hole diameter: Measured with a nitrogen adsorption / desorption type pore size measuring instrument [manufactured by Nikkaki Co., Ltd.]. (Μm)

Examples 1 to 4, Comparative Examples 1 to 4 Ultra high molecular weight polyethylene (UHMWP) as shown in Table 1
E) and polyethylene (PE) as the other components are blended, and 0.375 parts by weight of an antioxidant is added to 100 parts by weight of this composition, and this composition is blended with liquid paraffin as shown in Table 1. Mixed in ratio.

This mixed solution was filled in an autoclave equipped with a strong power stirrer and stirred at 200 ° C. for 90 minutes,
A homogeneous solution was obtained. This solution was extruded from a T-die at 200 ° C. by an extruder having a diameter of 45 mm, and a gel-like sheet was formed while being taken up by a cooling roll cooled to 20 ° C.

With respect to each of the obtained gel-like sheets, the hardness,
The melt index and sag were evaluated, and the results are shown in Table 1.

Then, the gel-like sheets were set in a biaxial stretching machine, respectively, and simultaneously biaxially stretched 5 × 5 times at a temperature of 115 ° C. and a film forming speed of 5 m / min. At this time, Table 1 also shows the number of times the gel-like sheet did not enter the chuck of the stretching machine due to the slack of the gel-like sheet.

The stretched membranes thus obtained were washed with methylene chloride to extract and remove the remaining liquid paraffin, dried at room temperature, and then heat-set at 120 ° C. for 30 seconds to obtain a polyethylene microporous membrane. It was The results are also shown in Table 1.

[0038]

[Table 1]

[0039]

As described above in detail, the method for producing a microporous polyolefin membrane of the present invention uses a polyolefin having a density of a predetermined value or more of the other polyolefin to be blended with the ultrahigh molecular weight polyolefin. In the gel composition, the hardness of the gel composition is high, scratches are less likely to occur, there is no film breakage due to notches, and the sagging is small, so that the polyolefin microporous film can be efficiently manufactured, such as being easy to enter the chuck of the stretching machine.

The polyolefin microporous membrane obtained has good strength, air permeability, and average through-hole diameter,
It is suitable for various applications, especially as a separator for batteries and capacitors.

Claims (2)

[Claims] 1. An ultrahigh molecular weight polyolefin having a weight average molecular weight of 5 × 10 ⁵ or more and 1% by weight or more, a weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ and a density of 0.
Solvent 5 and 10 to 50% by weight of a polyolefin composition comprising another polyolefin component of 945 g / cc or more
0 to 90% by weight of a solution is prepared, the solution is extruded from a die and cooled to form a gel composition, and the gel composition is stretched at a melting point of the polyolefin composition + 10 ° C. or less. A method for producing a polyolefin microporous membrane, which comprises thereafter removing the residual solvent. 2. The polyolefin composition comprises 10 to 70% by weight of a polyolefin having a weight average molecular weight of 1 × 10 ⁶ or more, a weight average molecular weight of 1 × 10 ⁵ or more and less than 5 × 10 ⁵ , and a density of 0.945 g / The method for producing a polyolefin microporous membrane according to claim 1, wherein the content of the other polyolefin component is cc or more and 30 to 90% by weight.