JPH0257575B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to porous films.
The porous film is made of a single polymer, or a single polymer is homogeneously mixed with other polymers or compounds to improve its physical properties, or a single polymer film is coated with other polymers or compounds. There are things that have been done. The known methods for producing porous films include adding a compound that is a non-solvent for the polymer to a polymer solution, molding it, and then extracting the compound, and adding a solid to the polymer solution and molding it. rear,
There are methods for extracting the solid, and methods for controlling void formation by wet coagulation of a polymer solution, but both require complicated manufacturing conditions and are difficult to control the pore size and porosity. Films obtained by these methods essentially consist of extremely small pores, and for this reason, the film has poor heat resistance and pressure resistance, and a decline in performance during use is unavoidable. . This is a major drawback, especially when an acrylic polymer or cellulose polymer is used as the material. Other methods include neutron beam irradiation-etching method, sintering method, stretching-embossing method, etc.
The manufacturing method is not common, and a membrane with good performance has not been obtained. The inventors of the present invention have carried out extensive research in order to improve these drawbacks, and have previously proposed a porous membrane having a completely new structure in Japanese Patent Application No. 175530/1983. The present inventors conducted further studies and completed a microporous membrane that has excellent air permeability and water permeability and excellent separation performance. An object of the present invention is to provide a microporous film that is less compacted, has excellent heat resistance, and can easily be provided with a variety of functions. The present invention comprises a film-forming polymer A and at least one polymer B that is miscible but incompatible with the polymer A, and the polymer B is dispersed in the polymer A, and ( (b) A microporous film stretched in at least one direction, containing large pores due to phase separation at the interface between polymer A and polymer B, and (b) a large number of micropores communicating in polymer A. be. The film-forming polymer A is not particularly limited as long as it can form a film, but includes, for example, acrylic polymers, modacrylic polymers, cellulose acetate, polyesters, polystyrene, polyolefins, vinyl chloride polymers, polyamides, and polycarbonates. , vinylidene chloride polymer, fluororesin, polysulfone, polyvinyl alcohol, etc. Among these, acrylic polymers, modacrylic polymers, and cellulose acetate are preferred. An essential condition is that polymer B is miscible with polymer A, but incompatible with it. Miscible refers to the property that polymers or polymer solutions can be mixed well without agglomeration or gelation, and immiscible refers to the property that polymers A and B solutions can be mixed well. When mixed, the two do not dissolve and phase separate, or polymer A and polymer B phase separate during solvent removal and molding, or polymer A and polymer B are mixed and melted and kneaded. This means that they are not evenly blended with each other and the phases separate. As for the state of phase separation, it is generally preferable that the polymer B has a spherical shape or a rotating ellipsoidal shape, and more preferably a spherical shape having a more uniform size. It is also possible to use two or more types of polymer B, but in this case as well, it is necessary that the polymer B be miscible with and incompatible with the polymer A. Polymer B is not particularly limited as long as it is miscible and incompatible with polymer A, but for example, a combination of polymer A and polymer B is an acrylic polymer or a modacrylic polymer; Cellulose acetate, styrene polymers, vinyl acetate polymers, vinyl chloride polymers, polyvinyl acetals, polyvinyl alcohol derivatives such as cyanoethylated polyvinyl alcohol, methacrylic acid alkyl ester polymers, polyurethane or butadiene polymers, etc. combination with at least one selected species;
Cellulose acetate, acrylic polymers, modacrylic polymers, acrylic acid alkyl ester polymers, methacrylic acid alkyl ester polymers, styrene polymers, polyvinyl alcohol derivatives, vinyl acetate polymers, vinyl chloride polymers A combination with at least one selected from the group consisting of a combination of polyvinyl acetate and an acrylic polymer or a modacrylic polymer, a combination of polyethylene and a polystyrene polymer, a combination of a butadiene polymer and polyvinyl alcohol, etc. Any combination of incompatible polymers can be used. The amount of polymer A is preferably at least 50% by weight; if it is less than 50% by weight, it will be difficult for polymer B to be dispersed in a spherical shape in polymer A during film formation, resulting in uneven pore size. is likely to occur. In order to obtain a more stable polymer dispersion state and thus a microporous film without pinholes, the amount is preferably 60% by weight or more, particularly 60 to 99% by weight. The amount of polymer B can be varied as appropriate within a range of up to 50% by weight depending on the intended use and performance. In order to improve performance such as consolidation of the microporous film, the amount of polymer B is 0.5% by weight or more, more preferably 1% by weight or more, and most preferably 2% by weight or more. The size of giant pores due to phase separation of Polymers A and B is usually about 0.5 μm or more (see Table 2 of Example 2), and the physical properties of Polymers A and B, the mixing ratio of the polymers, etc. It changes with When the purpose is to reduce the porosity or obtain a smaller pore size, the amount of polymer B is better to be small; when the purpose is to increase the porosity or obtain a larger pore size, the amount of polymer B is The more the better. However, the size of the pores can be easily changed by other methods such as changing the stirring force or adding a dispersion improver. The dispersibility improver has the ability to enhance the affinity between polymers, and block polymers, graft polymers, or copolymers containing each polymer component can generally be used. As for the film forming method, conventionally known porous film forming methods can be applied. In order to generate giant pores in the formed film due to phase separation of polymers A and B, it is necessary to stretch the film in at least one direction, and pores are generated only by stretching. In the film, polymer B exists in a phase-separated state within polymer A, and since the interaction between these two phases is smaller than the force required to deform B, when the film is stretched, A and B It is thought that pores are generated at the interface of These pores are generated in the same way at all interfaces between A and B, and the amount and size of the pores can be changed as appropriate depending on the size of the dispersion area of B and the stretching ratio. The size of the pores obtained here is within the range expressed by equation (1). l≦(λ−1)D (1) where l: maximum length of pores in the stretching direction D: maximum length of polymer B in the stretching direction λ: stretching ratio, usually 7 or less The shape of the pores will be explained in detail. FIG. 1 is an enlarged explanatory view of the stretched film after molding. 1 indicates polymer A, 2 indicates polymer B, and 3 indicates pores, indicating that polymer B is spherically dispersed in polymer A. 2 and 3 are explanatory diagrams of the pores of the porous film according to the present invention, and FIG. 2 shows an unstretched film that has been uniaxially stretched in the horizontal direction in the figure. The figure shows biaxial stretching in the horizontal and vertical directions. 3 indicates pores formed at the interface of polymers A and B by stretching, +D or +
D' is the maximum length of the pores and the polymer B in the stretching direction, and D or D' is the maximum length of the polymer B in the stretching direction. The shape of the pores is clearly different from the conventional spherical, cylindrical, or irregular shape, and has a smooth shape, and is characterized by a large surface area relative to the volume of the pores. Polymer B is not significantly deformed during the stretching process and maintains a spherical or nearly spherical shape. This polymer B
This seems to support the giant pores caused by phase separation, and also serves as a resistance to compaction of the membrane. In addition to the above-mentioned giant pores, the film of the present invention has minute pores that communicate with the polymer A. The micropores in Polymer A have a pore diameter of less than 0.2 μm, which is much smaller than the giant pores caused by phase separation, and are indicated by diagonal lines in the figure. Micropores can be created by adding an additive to a polymer, removing the additive after forming a film, and generating micropores in the area where the additive was removed, or by introducing gas into the polymer through chemical reaction or thermal decomposition. A method of adding additives that cause micropores to be generated, and a method of generating micropores by changing the conditions of the drying to solvent removal (coagulation) steps during film forming from a polymer solution can be adopted. Among these methods, the method of adding additives to a polymer solution and removing the additives after forming a film is preferred in that it is easy to create interconnected micropores. When an acrylic polymer is used as polymer A,
A more preferable additive is funnel oil. These minute pores contribute to improving the air permeability and water permeability of the microporous film, as well as improving its separation performance. Microporous films consisting only of minute pores have been known for some time, but they have drawbacks such as low heat resistance, changes in performance when dried, and pores that are easily crushed when large pressure is applied. The combination of the invention's "microporosity + giant pores due to phase separation" resulted in a membrane that improved on these drawbacks. Next, an example of a method for manufacturing the film of the present invention will be described. The method of mixing polymer A and polymer B is not particularly limited, but a method of mixing solutions of each or melt-mixing them, etc. is adopted. However, in order to industrially easily and efficiently obtain the target porosity with higher precision, it is preferable to mix polymer A and polymer B in a solution state. When polymer B is soluble in the solvent of polymer A, a common solvent is used for both. For example, if polymer A is an acrylic polymer, dimethylformamide (hereinafter referred to as
(abbreviated as DMF), dimethylacetamide, etc. When polymer B is poorly soluble in the solvent of polymer A, it is necessary to use a different solvent, and in this case it is preferable that the two solvents are incompatible with each other. A molding stock solution can be easily prepared by stirring and mixing the solutions of Polymer A and Polymer B. For example, if a DMF solution of an acrylic polymer is used as the polymer A and a DMF solution of cellulose acetate or polyvinyl butyral is used as the polymer B, it can be easily stabilized by simple stirring. A molding solution may be prepared. For materials with low miscibility or high viscosity, it is preferable to use a stirrer with high speed rotation or large shear stress, such as a homomixer or a grinder. A molding stock solution can also be prepared by mixing thermoplastic polymers by kneading them in a molten state. The prepared molding stock solution is generally cloudy due to the incompatibility of polymers A and B, which provides a qualitative indication of the dispersion state. In order to obtain more precise porosity, it is preferable to set the mixing and stirring conditions while constantly checking the dispersion state using a phase contrast microscope or the like. As methods for forming films from stock solutions, conventional methods such as the solvent drying method and coagulation method are employed for polymer solutions, and the T-die method and inflation method for polymer melts. After forming the film, the film is stretched in at least one direction in order to generate giant pores due to phase separation. When forming a porous film from a polymer solution, the coagulation method depends on the coagulation bath composition, the coagulation bath temperature, or the addition of a precipitant or filler to the polymer solution, and the solvent drying method depends on the drying time and temperature. and the addition of fillers to the polymer solution allows the formation of micropores. When forming a film from a polymer melt, micropores can be formed by adding liquid or fine powder of an organic or inorganic substance that does not decompose at the melting temperature to the melt. The above-mentioned additives can be extracted to form minute pores by extracting them before or after film formation and stretching. The amount of micropores is also adjusted by changing the above-mentioned conditions depending on the intended use. The film is preferably heat-treated to completely remove solvent and to stabilize heat resistance. The heat treatment is usually carried out in hot water, steam, or a heating medium, but it is preferable to fix the film in order to prevent the film from becoming bent or distorted. Further, it is a matter of course that the heat treatment is carried out at a temperature that does not eliminate the pores generated by the stretching.
Heat treatment is a preferred operation for morphological stability and strength elongation reinforcement of the film. In this way, the microporous film of the present invention has (a) a mixture of incompatible polymers, which has giant pores due to phase separation at the interface, and (b) microporous pores connected to the polymer A. It has pores and is easy to manufacture; it is easy to adjust the performance such as the diameter and amount of pores; it is possible to select a wide range of raw materials; and by selecting polymers A and B. It has excellent characteristics such as being able to provide a variety of functions and having good heat resistance and compaction resistance despite having a large number of minute pores. Note that the film can take the form of a microporous film alone, a porous film or a woven or knitted film coated on it, or a composite film or a laminated film sandwiched between the microporous film and the porous film or woven or knitted film. A more detailed explanation will be given below with reference to Examples, where parts and % indicate parts by weight and % by weight unless otherwise specified. Further, capacity % is abbreviated as Vol%. The size of the pores was determined by observation using an optical microscope or an electron microscope, and in the examples, the maximum pore size was expressed as l in the above formula (1). In addition, the porosity was determined using equation (2). Porosity V (volume %) = (1-w/s×d×o)×100 (2) where s: Surface area of one side of the film (cm ² ) d: Thickness of the film (cm) o: No pores Example 1: Acrylic polymer having a composition of acrylonitrile: methyl acrylate: sodium allylsulfonate = ^90.0 :9.3:0.7 (%) is referred to as A-1, and dimethylformamide (hereinafter referred to as DMF) with a polymer concentration of 20% is used.
) solution was prepared. On the other hand, cellulose acetate (degree of acetylation 55%, average degree of polymerization 130) was designated as B-1,
A DMF solution with a polymer concentration of 15% was prepared. Furthermore, a DMF solution with a polymer concentration of 15% was prepared using vinyl chloride-vinyl acetate copolymer (vinyl acetate content, 8% average degree of polymerization 1150) as B-2. Each solution was stirred and mixed in a homomixer so that the polymer ratio shown in Table 1 was obtained, and a 50% DMF solution of funnel oil was added to the mixed solution, and the mixture was further stirred and mixed to obtain a stock solution for film preparation. The stock solution was cast onto a glass plate, and DMF was evaporated in a hot air dryer at 85° C. to obtain a film with a thickness of 20 to 30 μm. The obtained film is uniaxially stretched 1.5 times in hot water at 85 to 95°C. This stretched film was treated with ethanol to completely extract the funnel oil in the film. Table 1 shows the pore morphology and water permeability of the film.

[Table] Example 2 Using A-1 solution and B-1 solution of Example 1, B
-1/A-1 ratio was changed and funnel oil was added in an amount equal to the weight of the polymer, and after stirring and mixing in a homomixer, a stock solution for film preparation was obtained.
The preparation and stretching of the film and the extraction of funnel oil were carried out in the same manner as in Example 1. The results are shown in Table 2.

[Table] Example 3 Cellulose acetate (degree of acetylation 56%,
Using an average degree of polymerization of 170), the polymer was dissolved in DMF to a polymer concentration of 15%. Polystyrene with a degree of polymerization of 5000 was used as Polymer B, and this was dissolved in DMF to a polymer concentration of 15%. The ratio of polymer A to B is 8:
Each solution was mixed and stirred so that the total amount was 2. Pour the mixed solution through a nozzle with a slit width of 0.05 mm.
The mixture was poured into a coagulation bath containing DMF:water = 30:70 (%) to obtain a 30μ film. This film was stretched 1.5 times in the width direction in water at 50°C, and then the solvent was thoroughly removed in water at 70°C. The resulting film had a thickness of 22μ and was composed of polymer A.
Polymer A had giant pores at the interface between polymer A and B, and had a porosity of 75%, and had very good water permeability and good compaction resistance. Example 4 The acrylic polymer of Example 1 was used as Polymer A and dissolved in an aqueous zinc chloride solution so that the polymer concentration was 10%. Polybutadiene (average molecular weight: 130,000) was used as polymer B, and a toluene solution with a polymer concentration of 10% was prepared. Take each solution so that the polymer ratio is A/B=8/2, stir and mix using a homomixer to obtain a stock solution for film production. The stock solution forms an emulsion and is cloudy. The stock solution was cast onto a glass plate, dried for 30 minutes in a hot air dryer at 100°C, then immersed in warm water to remove subchloride, and then uniaxially stretched.
A microporous film was obtained by multiplying by 1.5 times. The film is made by dispersing polybutadiene in an acrylic polymer, and contains giant pores at the interface and micro pores in the acrylic polymer.It has excellent water permeability, and has a filler effect of hydrophobic polybutadiene. Therefore, it had excellent compaction resistance.

[Brief explanation of drawings]

Figure 1 is an enlarged explanatory diagram of the stretched film after molding.
FIG. 2 is an explanatory view showing the state of holes in a uniaxially stretched film, and FIG. 3 is a biaxially stretched film.

Claims (1)

[Scope of Claims] 1. A film-forming polymer A, and at least one polymer B that is miscible with but incompatible with the polymer A.
Therefore, polymer B is dispersed in polymer A and
(a) A microporous film stretched in at least one direction, containing giant pores due to phase separation at the interface between polymer A and polymer B, and (b) a large number of micropores communicating with polymer A. . 2. The film according to claim 1, wherein the polymer A is an acrylic polymer. 3. The film according to claim 1, wherein the polymer A is a modacrylic polymer containing a halogen-containing monomer. 4. The film of claim 1, wherein the polymer A is at least 50% by weight. 5. The film according to claim 1, wherein the polymer A is 60 to 99% by weight. 6. The film according to claim 1, wherein polymer B is soluble in the solvent of polymer A. 7. The film according to claim 1, wherein the polymer B is cellulose acetate, a vinyl acetate polymer, a vinyl chloride polymer, or a polyvinyl alcohol derivative.