JPH02131126A - Production of porous film - Google Patents

Abstract

PURPOSE:To form a porous film free from pinhole by forcibly supplying saturated steam on the surface of a thin filmlike material in such a state that the water content in the thin filmlike material of a formed polymer is not increased and coagulating the polymer. CONSTITUTION:A polymer capable of molding a film is dissolved in a good solvent and also the water content of the solvent soln. is regulated to 0.5wt.% or below. After a thin filmlike material is formed by using this polymer soln., saturated steam or a steam contg. mists is forcedly supplied on at least single surface of the thin filmlike material in such a state that the water content contained in this thin filmlike material is not substantially increased and the polymer is coagulated to produce a porous film. As this polymer, polyether imide having the repeated unit shown in a formula I ((n) shows an integer of 1-7) is preferably utilized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for manufacturing a porous membrane that is effective for filtering fine particles present in a fluid.

[Conventional technology]

Various methods have been known to produce porous membranes, but in recent years, a simple process has been proposed in which a polymer solution is brought into contact with a thin film of water vapor to solidify the polymer. Enter.

[Problem to be solved by the invention]

However, in the film forming method using water vapor, etc., defects such as Binho V occur when the polymer concentration in the polymer solution during film forming is lowered in order to increase the porosity and obtain a highly permeable membrane. The problem is that it is easy to do.

[Means to solve the problem]

In order to solve the above problems, the present inventors have conducted extensive studies and discovered a method for stably manufacturing a highly permeable membrane without defects such as pinholes by controlling the water content in the membrane forming solution. , we have completed the present invention.

The gist of the present invention is to form a thin film using a polymer solution comprising a film-formable polymer and its good solvent and having a moisture content of (L5 weight coefficient or less), and then The present invention provides a method for producing a porous membrane in which a polymer is coagulated by forcibly supplying saturated steam or steam containing mist to at least one surface of the thin film while substantially increasing the coagulation rate.

The film-formable polymer in the present invention refers to a polymer that is soluble in organic solvents and insoluble in water, and whose solution can be cast. Examples include polyetherimide, polyimide, polyhynylidene fluoride, polytetrafluoroethylene copolymer, fluorine polymers such as trifluoroethylene, polyμphone, polyethersulfone, polycarbonate, polyethylene terephthalate, polymethyl methacrylate, and polybutylene. Poly(meth)acrylic acid esters such as V(meth)acrylate, polyacrylonitrile, cellulose esters such as lulose acetate, cellulose nitrate, polyolefins such as polyethylene, poly-4-methyl-1-pentene, polybutadiene, polyvinyl acetate , polystyrene, polyα-methylene V-styrene, poly-4-vinylpyridine, polyvinylpyrrolidone, polyvinyl chloride, polyvinylidene chloride, silicone thread polymer, polyphenylene oxide, or copolymers thereof. However, a polymer suitable for each purpose can be used in consideration of heat resistance, chemical resistance, etc.

Among such polymers, the following general formula [where n is 1
[representing an integer of ~7] A polyetherimide having a repeating unit represented by
Alternatively, polyimide having a repeating unit represented by the following structural formula has excellent mechanical properties, solvent resistance, and heat resistance, and is a suitable polymer as a material for porous membranes that require these properties.

In the present invention, a film is formed by forming a thin film of a polymer solution consisting of the polymer and its good solvent, and forcibly bringing saturated steam or water vapor containing mist into contact with at least one surface of the thin film. method (steam coagulation method) is adopted.

Compared to the wet film forming method, this steam coagulation method can shorten the time between the start of phase separation of the polymer-solvent system in a thin film and the subsequent start of coagulation of the polymer. It is believed that a porous membrane without a non-porous layer (dense layer) can be obtained because the solidification rate of coalescence is slowed down.

Known good solvents for polymers can be used, but for polyetherimide and polyimide, N-methylpyrrolidone, dimethylacetamide, dimethylsulfoxide, 1,4-dioxane, trichlorethylene, etc. can be used. First, a polymer solution is prepared by dissolving the polymer in these solvents.

The content of the polymer in the polymer solution affects the porosity, pore size distribution, etc. of the porous membrane, and the optimum content varies depending on the type of solvent, but it is approximately 2 to 40% by weight. Preferably b6 and approximately 1 to obtain a highly permeable membrane.
The content is more preferably 5% by weight or less, while the content is more preferably 8% by weight or more in order to obtain a film with practical mechanical strength.

In the present invention, a thin film is formed using a polymer solution having a water content of 0.5% by weight or less, and then water vapor is supplied to the surface of the thin film without substantially increasing the water content. If the water content in the polymer solution is higher than this value, local precipitation may occur before solidification due to water vapor, or defects such as pinholes may occur due to loss of part of the three-dimensional network structure formed during solidification. This is likely to occur.

The water content in the polymer solution can be controlled by various methods, such as a method in which a good solvent is dehydrated in advance and then the polymer is dissolved in a dry atmosphere, depending on the need.

The thickness of the thin film formed from the polymer solution may be appropriately set depending on the thickness of the intended porous membrane, but in normal cases it may be approximately 50 to 2000 μm. It can be obtained by casting, coating, etc. on objects with smooth surfaces such as metal plates, polymer films, rotating drums, endless belts, etc., but as long as the smoothness of the thin film is not impaired, porous It is also possible to incorporate porous objects such as polymeric films.

Normally, a thin film-like material is brought into contact with water vapor immediately after it is produced, but it may be brought into contact with water vapor after some time has elapsed. However, in the latter case, it is preferable to form the thin film in a low humidity atmosphere such as dry air in order to suppress moisture absorption from the surface layer of the thin film as much as possible. Water vapor is supplied by adjusting the temperature and concentration using a known device, but usually a method is adopted in which saturated water vapor of several atmospheres is jetted out from a nozzle to cover the surface of a thin film.

The solidification rate and solidification behavior of the polymer differ depending on the concentration of the polymer solution, the thickness of the thin film, the boiling point of a good solvent, the compatibility between a good solvent and water, etc., so the temperature, concentration, supply rate, and supply of water vapor are By appropriately selecting conditions such as time, the pore diameter, pore diameter distribution, porosity, etc. can be controlled to preferable values.

The amount of saturated steam or steam containing mist supplied to the surface of the thin film is approximately α1 to 1000'lli/sθC
-12 is preferable, approximately cL5 to 10
More preferably, it is about 0'III/5ec-1. Further, the amount of water vapor etc. supplied within about 10 minutes is sufficient. Further, the direction in which the water vapor is supplied to the surface of the thin film-like material is not particularly limited, but in consideration of the supply efficiency of the water vapor, it is preferable to supply the water vapor from the vertical direction.

By supplying water vapor to the surface of the thin film of polymer solution in this manner, the polymer component is solidified and a porous structure is formed. In this case, from the viewpoint of promoting coagulation of the polymer and preventing re-dissolution, it is preferable to remove a good solvent from the thin film-like material or the porous polymer during or after supplying water vapor. The good solvent can be removed by evaporation or by running off as an aqueous solution of condensed water.

Incidentally, if a good solvent or the like remains inside the porous membrane obtained after coagulating the polymer, it is preferable to remove it by drying, washing with water, etc. as necessary.

〔Example〕

The present invention will be explained below with reference to Examples. In the Examples, water flux etc. were measured by the following method. Moreover, all amounts (parts) of each component used are parts by weight.

(1) For water flux, a porous membrane punched to a diameter of 25+a+ is immersed in Ethano-V, then replaced with water to introduce the water into the micropores, and then installed in a membrane filter V holder and filled with clear water at 20℃. Transmembrane pressure differential 10 p, s, i
, and each ten points were measured using the transmission point method. A porous membrane punched to a diameter of 25 cm was immersed in mineral oil for 5 minutes, then assembled into a membrane filter holder and heated to 20°C while increasing pressure linearly at a rate of 1 to 97 i per minute on one side of the membrane. Clean air was supplied under pressure, and the pressure PL when air began to permeate to the opposite side of the membrane was measured, and this value was taken as the leak point.

In addition, the pressure (kg/cm'') is expressed as the horizontal axis K, and the air permeation amount (
In a graph plotted with 47m1n) as the vertical axis, the amount of air permeation increases exponentially and the slope of the tangent line is αOs (z/m1n)/(same/known).

△p However, γ is the surface tension of the liquid (34
dine/m ), θ is the contact angle.

(3) The moisture content in the polymer solution was measured by Karl Fischer method.

Example 1 Polyetherimide (G, R
After dissolving 100 parts of ULTEM 1000) in 900 parts of dimethylacetamide, it was cast onto a glass plate to a thickness of 254 μm using an applicator for film production.
A thin film of polymer solution was formed. Next, the pulp of the pipe containing saturated steam was opened, and saturated steam was applied to the surface of the thin film from a nozzle with a diameter of 12+wφ at 167 r/min.
The polymer was coagulated by feeding for 1 minute.

The thin film material was placed vertically at a distance of 30 crn from the nozzle.

Water vapor was supplied under the same conditions, and 1 c
When the temperature at the tn position was measured, it was 60°C. Further, the rapidly solidified polymer was peeled off from a glass plate, washed with running water for about 1 hour, and then dried with ventilation at 45° C. for about 12 hours.

The water content was measured and shown in Table 1 along with the water content of the polymer solution.

Further, when observed with a scanning electron microscope, a substantially porous sponge-like structure was observed on the surface, and a finger-shaped structure was observed from the inside to the back surface.

Examples 2 and 3 Film formation was carried out in the same manner as in Example 1, except that the polymer concentration in the polymer solution was changed to 11 or 12, respectively. The measurement results are shown in Table 1.

Comparative Examples 1 to 3 Using dimethylacetamide with a high moisture content, the concentration of the polymer solution was 10, 11, or 12tlllK, respectively:
The film was formed in the same manner as in Example 1, except that the first
Obtained the results in the table.

Examples 4 to 6 Polyimide resin (Ciba Geigy XU-
218), film formation was carried out in the same manner as in Example 1, except that dimethylformamide was used as a good solvent and the polymer concentration was set to the value shown in Table 1, and the results shown in Table 1 were obtained.

Comparative Examples 4 to 6 Films were formed in the same manner as in Examples 4 to 6, except that dimethylformamide having a high moisture content was used, and the results shown in Table 1 were obtained.

〔Effect of the invention〕

As is clear from the results of the Examples, according to the method of the present invention, it is possible to obtain a porous membrane with almost no difference between the values of PL and Pl, that is, a porous membrane without Binho V.

Claims (3)

[Claims] (1) Form a thin film using a polymer solution consisting of a film-formable polymer and its good solvent and having a moisture content of 0.5% by weight or less, and then reduce the water content in the thin film to a substantial amount. A method for producing a porous membrane, which comprises coagulating a polymer by forcibly supplying saturated steam or steam containing mist to at least one surface of the thin film without increasing the amount of water. (2) As a polymer, there are the following general formulas ▲ Numerical formulas, chemical formulas, tables, etc. ▼ [However, n represents an integer from 1 to 7] The method described in Section 1. (3) The method according to claim 1, which uses a polyimide having a repeating unit represented by the following structural formula (numerical formula, chemical formula, table, etc.) as a polymer.