JP2000072917A - Production of polyethylene fine porous membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel technique of structural control intended for the liquid-liquid phase separation membrane of polyethylene fine porous membranes. SOLUTION: In the process for the preparation of polyethylene fine porous membranes by kneading a mixture of a polyethylene and two or more plasticizers at a temperature not-lower-than the melting point of the polyethylene, cooling and solidifying the kneaded mass at a temperature of not higher than the crystallization temperature of the polyethylene and then, removing the plasticizers, are used such not less than two plasticizers that the phase separation temperature of the kneaded mass can exhibit nonlinearity to the weight ratio of each component of the plasticizers.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a microporous polyethylene membrane.

[0002]

2. Description of the Related Art A microporous polyethylene membrane is prepared by kneading a polyethylene and a plasticizer at a melting point or higher, cooling and solidifying the polyethylene at a temperature lower than the crystallization temperature of the polyethylene, and then removing the plasticizer. Such a manufacturing method is called heat-induced phase separation, and is further classified into a solid-liquid phase separation membrane and a liquid-liquid phase separation membrane according to the generation form of the phase separation. The solid-liquid phase separation membrane is a membrane having a relatively small pore diameter using pores of polyethylene microcrystals as pores, and is mainly used as a battery separator or the like. On the other hand, a liquid-liquid phase separation membrane is a membrane having a relatively large pore diameter using various fine structures such as a percolation structure or a cluster structure as pores, and is used for a microfiltration membrane or the like. The method of controlling the structure of the liquid-liquid phase separation membrane is roughly classified into a physical method and a chemical method. Physical methods include control by cooling rate and polymer / plasticizer ratio, and by increasing these, the same small pore size can be obtained.

On the other hand, as a chemical method, an affinity between a plasticizer and a polymer can be mentioned, and by increasing the same, a small pore size can be obtained. By the way, the use of two or more kinds of plasticizers (hereinafter, referred to as a mixed plasticizer) has been attempted as a part of the latter chemical structure control method. For example, Japanese Patent Application Laid-Open No. H5-156058 discloses a method of mixing a solid-liquid phase separation plasticizer and a liquid-liquid phase separation plasticizer, and US Pat. No. 4,594,207 discloses a method of mixing a good solvent and a poor solvent. Is disclosed.

[0004] However, in these methods, the overall affinity and other properties are adjusted on average by mixing a plurality of plasticizers having different affinities. As compared with the case where a single plasticizer having the same is used, no great effect has been found which affects the control of the film structure.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a new structure control method for a liquid-liquid phase separation membrane of polyethylene.

[0006]

In order to solve the above-mentioned problems, the present inventors have conducted intensive studies on a plurality of polymer / mixed plasticizers. The present inventors have found that a method for controlling the structure can be provided, and have accomplished the present invention. That is, the present invention provides (1) a mixture of polyethylene and two or more types of plasticizers, which is kneaded at a temperature higher than the melting point of the polyethylene; In the method for producing a porous membrane, a polyethylene microporous membrane is characterized in that two or more kinds of plasticizers are used such that the phase separation temperature of the kneaded material shows nonlinearity with respect to the weight ratio of each component of the plasticizer. 2. The method according to claim 1, wherein (2) the two or more plasticizers are a combination of N, N-bis (2-hydroxyethyl) alkylamine and diethylene glycol nonylphenyl ether. And a method for producing the same.

Hereinafter, the present invention will be described in detail. The liquid-liquid phase separation according to the present invention means that a homogeneous solution of a polymer / plasticizer is first prepared at a sufficiently high temperature, and then the homogeneous solution is cooled to a thermodynamically unstable state or a metastable state in a process of cooling the homogeneous solution. (Hereinafter referred to as liquid-liquid phase separation). As a result, a phenomenon in which a fine structure such as a percolation structure or a cluster structure is generated and the polymer is separated into a polymer rich phase and a polymer dilute phase is referred to. Immobilized by crystallization of the polymer,
By removing the plasticizer from the inside, a microporous membrane can be obtained.

On the other hand, when the homogeneous solution is stable and does not exhibit liquid-liquid phase separation even at the crystallization temperature of the polymer, only crystallization occurs without formation of a fine structure. This phenomenon is called solid-liquid phase separation. . The crystal of the polymer obtained at this time has many gaps between the crystals unlike the crystal from the melt, and it is necessary to obtain a microporous membrane with pores between the crystals by removing the plasticizer from the inside. Can be done.

[0009] The phase separation temperature depends on the affinity of the plasticizer for the polymer. In other words, the higher the affinity, the lower the phase separation temperature, so that solid-liquid phase separation is more likely to occur. The lower the affinity, the higher the phase separation temperature, so that the liquid-liquid phase separation is more likely to develop. For example, high-affinity plasticizers such as liquid paraffin and decalin are known as solid-liquid phase-separated plasticizers for polyethylene, and low-affinity plasticizers such as dioctyl phthalate and diphenyl ether are known as liquid-liquid phase-separated plasticizers. Since the affinity of the plasticizer plays an important role in thermally induced phase separation, attempts have been made to mix two or more plasticizers for the purpose of adjusting the affinity. An example is described in JP-A-5-15 / 15.
6058, U.S. Pat. No. 4,594,207, and JP-A-5-21050 and JP-A-8-21050 using a mixed plasticizer of dioctyl phthalate and liquid paraffin.
-64194 is known.

However, in each case, the mixing of a plasticizer has always resulted in obtaining an average affinity (linearity), and although the advantage of widening the range of selection of the phase separation temperature is recognized, each of them is used alone. However, no significant effect has been found that is within the range sandwiched by the phase separation temperatures in the case where the use of the mixed plasticizer makes it difficult to recover the plasticizer in the production line.

On the other hand, in the present invention, the expression that the phase separation temperature of the kneaded material exhibits nonlinearity with respect to the weight ratio of each component of the plasticizer means that the mixed plasticizer system (polymer / plasticizer 1 / In the plasticizer 2), the dependence of the phase separation temperature on the mixing ratio of the plasticizer at a constant polymer concentration depends on the phase separation of the single plasticizer system (polymer / plasticizer 1 and polymer / plasticizer 2) located at both ends of the figure. It deviates from the ideal behavior of connecting the temperature with a straight line (this behavior is referred to as linearity), and shows a clear maximum or minimum value for some reason.

In the conventional control based on linearity, as described above, there was nothing but the complementary action between the plasticizer having a high phase separation temperature and the plasticizer having a low phase separation temperature. Since the desired phase separation temperature can be obtained by a non-linear action regardless of the phase separation temperature, the range of choice of the plasticizer can be further expanded. In addition, this allows viscosity, volatility,
Since properties such as density and solubility, which greatly affect the structure control, can be maintained or changed independently of the phase separation temperature, the applicable range of the manufacturing process can be expanded.

The raw material polymer used in the present invention, polyethylene, is a high-density polyethylene which is a crystalline polymer mainly composed of ethylene, or α such as propylene, butene, pentene, hexene, octene or the like per ethylene unit. A copolymer (linear copolymer polyethylene) containing olefin units in a proportion of 4 mol% or less may be used.
Further, a polyolefin such as polypropylene, medium density polyethylene, linear low density polyethylene, low density polyethylene, ethylene-propylene-rubber, etc.
It may be blended at a ratio of not more than weight%. The polyethylene has a weight average molecular weight of 100,000 to 4,000,000, preferably 200,000 to 700,000, more preferably 250,000 to 5,000,000.
100,000. If the molecular weight is less than 100,000, it is easy to break during stretching, and if it is more than 4,000,000, it becomes difficult to produce a hot solution, which is not preferable. Further, the weight average molecular weight may be adjusted to a preferable range by means such as blending or multi-stage polymerization.

Next, a method for producing the microporous polyethylene membrane of the present invention will be described. The polyethylene microporous membrane is formed by mixing a mixture of polyethylene and a plasticizer at a temperature higher than the melting point of polyethylene and lower than the decomposition temperature of polyethylene, preferably 250 ° C.
It is prepared by kneading at a temperature of not more than ℃, cooling and solidifying the kneaded product at a temperature lower than the crystallization temperature of polyethylene, usually at room temperature, and then removing the plasticizer. Here, the step until cooling is called a film forming step, and the step of removing the plasticizer is called an extraction step.
After the film formation step, uniaxial or biaxial stretching can be performed before and after the extraction step or both.

[0015] The ratio of the plasticizer in the kneaded material is not particularly limited, but is 20 to 90% by weight, preferably 50 to 70% by weight. If it is less than 20% by weight, it is difficult to obtain a microporous film having an appropriate porosity,
Above, the viscosity of the hot solution decreases and continuous forming of the sheet becomes difficult. As the mixed plasticizer, any plasticizer can be used as long as the mixing ratio dependency of the phase separation temperature shows nonlinearity in the combination.
It is preferable to use a combination of -bis (2-hydroxyethyl) alkylamine (hereinafter, referred to as TA) and diethylene glycol nonylphenyl ether (hereinafter, referred to as NS). N, N-bis (2-hydroxyethyl) alkylamine is mainly C1
It can be synthesized by reacting ethylene oxide with a saturated aliphatic primary amine of 2 to C18.

Although there is no particular limitation on the film forming method,
For example, a linear copolymerized polyethylene powder and a plasticizer are supplied to an extruder, melt-kneaded at a temperature of about 200 ° C., and then cast from a usual hanger coat die onto a cooling roll from several tens μm to several mm. Can be continuously formed. In the present invention, since ultra-high molecular weight polyethylene is not an essential component, no special heating and melting equipment is required, and it is extremely easy to prepare a homogeneous sheet simply by adding polyethylene and a mixed plasticizer to an extruder. It is possible.

Further, when using an extruder, a plurality of plasticizer supply units are provided in the barrel of the extruder, and each component of the mixed plasticizer is injected from each supply unit to thereby reduce the kneaded material inside the extruder. It is also possible to control the phase separation state. Next, an extraction film is obtained by extracting and removing the plasticizer from the sheet. The extraction method is not particularly limited, but when using paraffin oil or dioctyl phthalate, after extraction with an organic solvent such as methylene chloride or MEK,
It can be removed by heating and drying below the fuse temperature of the selected extraction membrane. When a low boiling point compound such as decalin is used as the plasticizer, it can be removed only by heating and drying at a temperature lower than the fuse temperature of the microporous film.

In any case, it is preferable to restrain the film in order to prevent a decrease in physical properties due to contraction of the film. Further, when it is desired to produce a high-strength microporous membrane, uniaxial or biaxial stretching can be appropriately performed before and after the extraction step or both. The microporous membrane obtained by the above-mentioned production method can be subjected to heat treatment such as heat setting or thermal relaxation, as needed, in order to improve permeability or enhance dimensional stability.

[0019]

Next, the present invention will be described in more detail by way of examples. The test method shown in the examples is as follows.・ Phase separation temperature A sample was cut out into a size of about 3 mm square, sandwiched between two slide glasses, and then set on a hot stage (FP-80 / 82 manufactured by METTLER COMPANY), and the optical path length was 2 mm.
The phase separation temperature was measured with an optical microscope (Olympus) under the conditions of an initial temperature of 220 ° C. and a cooling rate of 10 ° C./min.・ Film thickness dial gauge (Ozaki Seisakusho: PEACOCK No.25)
Was measured. -Porosity A sample of 20 cm square was cut out from the microporous membrane, and its volume and weight were obtained. From the obtained results, calculation was made using the following equation. Porosity (%) = 100 × {(volume (cm ³ ) −weight (g) /0.95)} / volume (cm ³ ) In the following table, “parts” represents parts by weight.

[0020]

Example 1 High-density polyethylene having a weight average molecular weight of 250,000 and a density of 0.956 (manufactured by Asahi Kasei Corporation: SH-80)
0 melting point: 136 ° C) and TA (manufactured by Kao Akzo Co., Ltd.):
Armostat 210) and NS (Nippon Yushi Co., Ltd.)
Plasticizer composed of NS-202) and a heat stabilizer (Irganox 245, manufactured by Ciba Geigy Co., Ltd.) under the compositions shown in Tables 1 and 2, respectively, in a batch-type melt kneader (Toyo Seiki Co., Ltd.) The mixture was kneaded at 200 ° C. and 50 rpm for 10 minutes using a Labo Plastomill (manufactured by K.K.). FIG. 2 shows the dependence of the phase separation temperature on the mixing ratio.

[0021]

[Table 1]

[0022]

[Table 2]

As is clear from FIG.
Under a constant condition of 0% by weight, each of these plasticizers alone exhibits a phase separation temperature of around 200 ° C., but around a specific mixing ratio of 25% by weight of NS, the phase separation temperature shows a large nonlinearity. The phase separation temperature drops to about 150 ° C.

[0024]

Comparative Example 1 TA and dioctyl phthalate (hereinafter referred to as
Called DOP. ) Was measured under the same conditions as in Example 1 except that the mixed plasticizer was used. Table 3 shows the composition ratio of the plasticizer used, and the results are shown in FIG.

[0025]

[Table 3]

[0026]

Example 2 A, C, and F of Example 1 were molded by a hot press at 200 ° C., and then cooled by a water-cooled press to obtain a thickness of 1000.
The raw material was μm. Thereafter, TA and NS were removed by extraction with methylene chloride. The porosity of the obtained polyethylene microporous membrane was 55%, 53%, and 55%, respectively.

[0027]

Example 3 A, C, and F of Example 1 were molded by a hot press at 200 ° C., and then cooled by a water-cooled press to obtain a film having a thickness of 1,000.
The raw material was μm. The pore diameter of A consisting of TA alone was smaller than that of F consisting of NS alone. On the other hand, from C using the mixed plasticizer, a pore diameter smaller than A was obtained as shown in Table 4 below due to the specificity of the phase separation temperature.

[0028]

[Table 4]

[0029]

According to the method for producing a microporous polyethylene membrane of the present invention, it is possible to obtain a desired phase separation temperature by a non-linear action regardless of the phase separation temperature when individual plasticizers are used. became. This makes it possible to produce a microporous polyethylene membrane having a wide range of pore sizes without changing the plasticizer, which is extremely useful in industry.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a phase separation temperature and a mixing ratio of a plasticizer in a mixed plasticizer system (polymer / plasticizer 1 / plasticizer 2) at a constant polymer concentration.

FIG. 2 is a graph showing the relationship between the phase separation temperature of a kneaded product of polyethylene and a non-linear mixed plasticizer and the mixing ratio of the plasticizer in Example 1.

FIG. 3 is a graph showing a relationship between a phase separation temperature of a kneaded product of polyethylene and a mixed plasticizer and a mixing ratio of a plasticizer in Comparative Example 1.

Continued on the front page F term (reference) 4F074 AA17 AA18 AA19 AA20 AA21 AA24 AA25 AA98 AD05 AD13 AG02 CB16 CC05X CC22X DA14 DA43 4J002 BB031 BB041 BB151 ED017 EN026 EN106 FD026 FD027 GD05

Claims (2)

[Claims] A mixture of polyethylene and two or more plasticizers is kneaded at a temperature higher than the melting point of the polyethylene, and the kneaded product is cooled and solidified at a temperature lower than the crystallization temperature of the polyethylene. A method for producing a microporous polyethylene membrane, comprising using two or more plasticizers whose phase separation temperature of the kneaded product shows non-linearity with respect to the weight ratio of each component of the plasticizer. 2. The microporous polyethylene membrane according to claim 1, wherein the two or more plasticizers comprise a combination of N, N-bis (2-hydroxyethyl) alkylamine and diethylene glycol nonyl phenyl ether. Manufacturing method.