JPH0761432B2 - Method for producing highly functional asymmetric membrane - Google Patents

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 separation membrane for use in a membrane separation process for separating an organic solution containing at least one solute, an organic liquid mixture, a gas mixture or the like. . More specifically, it is a non-porous (diffusion permeable) asymmetric membrane (reverse osmosis) with organic solvent resistance used in the concentration, separation and purification processes in the chemical industry, petroleum industry, food, pharmaceuticals, brewing industry, etc. Membranes, pervaporative membranes, gas separation membranes) by a phase conversion method.

[0002]

2. Description of the Related Art Among various membrane separation processes, what is practically used on an industrial scale is desalination of seawater, desalination, production of ultrapure water, wastewater treatment, recovery of useful substances such as acids and alkalis. The main components are so-called aqueous solution separation such as concentration of beverages and foods, and gas mixture separation such as hydrogen recovery, and organic solution-based membrane separation processes such as organic liquid mixtures have not yet been put to practical use. This is because polyimide, which exhibits solvent resistance, heat resistance such as polyacrylonitrile, etc., and it is generally quite difficult to form a film from a high-molecular polymer having chemical resistance, so that a practical separation membrane has been obtained so far. Because it wasn't. Conventionally, polymer materials such as cellulose acetate, polysulfone, and polyvinyl chloride, which have been widely used for separation membranes for aqueous systems, can be relatively easily formed into a membrane by a phase conversion method. Does not have sufficient solvent resistance in a practical sense.

Up to now, several methods for producing a porous type ultrafiltration membrane for the purpose of membrane separation of an organic solution containing an organic solute or a solvent have been reported. For example, JP-A-54-71785 and JP-A-54-61359 are available. However, an organic solution mainly containing a low molecular weight substance as a solute, or an organic liquid mixture, or a gas mixture, etc. is to be separated, a non-porous high-performance separation membrane having practical solvent resistance, That is, a method for producing a non-aqueous reverse osmosis membrane, a pervaporation membrane, a gas separation membrane and the like by a phase inversion method has not been reported so much, and development of such a separation membrane is strongly demanded.

[0004]

As a method for producing a separation membrane, an active layer (a dense surface layer, which has a dense surface layer, has a large separation function by utilizing a phase change of a polymer solution from sol to gel). Control) and a porous support layer that supports it at the same time by a so-called phase inversion method, or by using a ready-made porous polymer membrane as a support on the surface of which an active layer is formed by a lamination method or a plasma polymerization method. A typical example is a composite film method that is separately formed. However, in these conventional film forming methods, the following problems remain unsolved when trying to achieve the above object.

First, when an asymmetric membrane having a dense active layer is to be produced by a conventional phase inversion method, it is easy to adjust the components of the membrane-forming stock solution and improve the membrane characteristics. In most cases, solid high-molecular polymers in the form of powder or granules are used. The reason for this is that, in general, high molecular weight polymers that are commercially available in the form of a solution often contain impurities such as low boiling point organic compounds in the high molecular weight polymer itself, or the solvent in which it is dissolved. , thereby forming an active layer with a very dense and uniform structure during the film was Tsu der because it is very difficult.

[0006] However, since membrane materials exhibiting solvent resistance such as polyimide polymers are often commercially available in the form of a solution, the above-mentioned known phase inversion method is a porous membrane that uses ultrafiltration. It was substantially difficult to form a so-called diffusion transmission type separation membrane other than the membrane. Further, since such a polymer material has a considerably rigid structure as compared with a hydrophilic polymer, it is said that the permeation flux of the obtained membrane is generally extremely small even if the membrane can be formed.

Secondly, the film produced by the conventional phase inversion method, especially the film produced by using the film forming stock solution in the form of a solution,
In general, gelation occurs rapidly, so solvent resistance,
In many cases, the mechanical strength and the like are low, and it was not possible to obtain a so-called diffusion-permeation type separation membrane such as a reverse osmosis membrane or a pervaporation membrane.

Thirdly, in the composite membrane method, it is necessary to develop a polymer material having a high separating function to be laminated on a support, and further, it is highly required to form a thin film layer to be an active layer and its adhesion method. However, the manufacturing process becomes extremely complicated.

[0009]

Against this background, the present inventors have diligently studied and made various attempts to improve the known phase inversion method, and as a result, a high molecular polymer giving a solvent resistant film was obtained. The small amount of water (about 0.5 to several weight%) contained in the composition greatly affects the deterioration of the membrane performance, and the solution of the high molecular polymer is heated and concentrated in advance so that the water content becomes substantially zero. By doing so, it was found that an active layer having a very dense and uniform structure can be formed by the phase inversion method, and the present invention was completed based on this finding. INDUSTRIAL APPLICABILITY The present invention can use a high molecular polymer having heat resistance, chemical resistance, etc., such as a polyimide polymer as a membrane material regardless of its commercially available form, and accompany a known phase inversion method with a simple treatment operation. According to the present invention, there is provided a method capable of easily producing a solvent-resistant non-porous high-performance asymmetric membrane.

That is, the present invention relates to a polyimide polymer or a polymer.
The solution of riacrylonitrile polymer is heated and concentrated to form a solution.
The moisture content in the product is 0.3% by weight (hereinafter, weight% is simply referred to as%).
The production of a highly functional asymmetric membrane characterized in that after the following steps, a membrane-forming undiluted solution having a predetermined composition is prepared, and the membrane-forming undiluted solution is used to form an asymmetric membrane by the phase inversion method. It is about the method. The method for producing a highly functional asymmetric membrane of the present invention is a non-porous type asymmetric membrane even when a film material such as a polyimide polymer which is commercially available in a solution form, which has been extremely difficult to form by the conventional phase inversion method, is used as a membrane material. It is possible to manufacture Further, a water-containing membrane material which is a solid powder like a polyacrylonitrile polymer but has a water content of 5 % or more in a normal state, and in some cases 20 % or more, which is extremely difficult to completely dehydrate. By using, it is possible to easily produce a non-porous type asymmetric membrane.

As the high molecular weight polymer having heat resistance and chemical resistance used in the present invention, a commercially available wholly aromatic polyimide polymer (PI2080) having a structure represented by the following formula, and conventionally known polyimide type Coalescence can also be used. Moreover, a commercially available polyacrylonitrile polymer can also be used.

[0012]

[Chemical 1]

These membrane materials are in the form of solid such as powder or granules, or liquid such as varnish, or solution in which they are dissolved in a suitable solvent (for example, a solution obtained by purifying a synthesis reaction solution of a high molecular polymer to some extent). Etc.) is commercially available.

A method for forming an asymmetric membrane by using the present invention, for example, using a polyimide polymer as a membrane material by the phase conversion method of the present invention will be described below. First, the solid polymer is dissolved in an appropriate solvent, and the polymer in solution is also completely dissolved by adding an appropriate solvent if necessary. As the solvent, a solvent having a compatibility with a swelling agent and a coagulating solvent (gelling agent), which will be described later, is used in addition to dissolving a high-molecular polymer, and N-methyl-2-pyrrolidone, dimethylacetamide, dimethyl are used. Examples include formamide, dimethyl sulfoxide, γ-butyl lactone, and the like. The concentration of the high molecular weight polymer is 10 to 35% by weight (hereinafter, all% representing composition represents% by weight unless otherwise specified), preferably 15 to 25%, depending on the film thickness to be formed. Is. The concentration and viscosity of the solution at this time are appropriately selected so that the self-supporting property (film forming ability) during film formation, the uniformity of the film forming stock solution, and the like are good.

An important technical means in the present invention is to heat-concentrate the high-molecular polymer solution in advance before preparing the membrane-forming stock solution, whereby the water content and low boiling point contained in the solution are reduced. It is to remove impurities such as compounds. As described above, generally commercially available polymer materials often contain about several percent of such impurities in the polymer itself or the solvent in which it is dissolved. In particular, it is considered that the content of the solution type high molecular weight polymer is higher than that of the solid state type polymer. Even if the polymer material itself does not contain any impurities, the commercially available polar solvent that is separately added as a solvent is hydrophilic and often contains a considerable amount of water or the like. Moisture, low boiling point organic compounds, etc. present in the stock solution for film formation generally accelerate the gelation of the solution, resulting in non-uniform microstructure of the dense active layer formed on the surface of the formed film, leading to local defects ( Pinholes) are easily formed, and the film performance is deteriorated. Therefore, it is extremely important to remove impurities, especially water, from the stock solution for film formation when producing an asymmetric film.

The heating and concentrating treatment of the polymer solution is
The solution of the polymer or the like is heated at a relatively high temperature of at least 60 ° C. or higher, preferably 100 ° C. or higher, with slow stirring so as to be substantially free of water, that is, at least 0.3% or less, preferably Concentration is continued until it is below 0.1%. A membrane-forming stock solution having a predetermined composition is prepared from the solution thus treated, and by using this, a membrane having a high separation function is easily formed. The heating temperature varies depending on the solvent, but if it is too low, it takes time to concentrate, and if it is too high, the high-molecular polymer deteriorates. The heating time varies depending on the solvent and the amount of the solution, but is usually 6 to 4
About 8 hours is preferable.

When the water content becomes substantially zero, a swelling agent or the like is added to the polymer solution, if necessary, to prepare a stock solution for film formation. The swelling agent is compatible with the solvent and coagulating solvent, and is a hydrophilic organic solvent such as dioxane, acetone, or tetrahydrofuran, or lithium chloride (LiC
Inorganic salts such as l) can be mentioned as preferable ones. In particular, when a high polymer having high solvent resistance such as polyimide or polyacrylonitrile as described above is used as a membrane material, a compound of a ketone such as acetone or dioxane or an oxygen-containing cyclic compound is preferable.

Next, the stock solution for film formation thus prepared is cast on an appropriate support to a predetermined thickness. Further, in the case of producing a hollow fiber membrane, casting is performed directly from a concentric double structure nozzle (base) without using a support. It is desirable that these castings are performed in an atmosphere in which the relative humidity is controlled to be at least 40% or less. The support to be used is not particularly limited, but a flat plate or tube having a smooth surface such as glass, aluminum, or plastic can be used, and a sheet-like one such as nonwoven fabric can also be used. As a means for casting in a flat plate shape, a conventionally known coating means may be used, and in the laboratory, for example, a doctor knife, a doctor blade, a bar coder or the like can be used. The thickness cast on the support varies depending on the intended use of the membrane, but is usually about 80 to 30.
About 0 μm is preferable. The film-forming stock solution cast on the support is usually immediately or within a few minutes immersed in the coagulating solution to substantially exchange the solvent and the coagulating solution in the film-forming stock solution,
The high-molecular polymer is phase-converted and solidified (gelled) to form a film.

The coagulating liquid (gelling liquid) is compatible with a solvent for dissolving a high molecular polymer, a swelling agent, etc., and has no dissolving power for the high molecular polymer, such as water, acetone, Benzene, hexane, or alcohol such as methanol or ethanol can be used. The temperature at the time of forming a gel by dipping the film-forming stock solution in a coagulating liquid is not particularly limited as long as it is equal to or lower than the boiling point of the coagulating liquid.
Generally, about 0 to 40 ° C is preferable. Although the immersion time varies depending on the immersion temperature, it is generally preferably 1 hour or more.

In the present invention, it is preferable to heat-treat the film produced by the phase inversion method as described above under vacuum to improve its mechanical strength, solvent resistance and the like. The heat treatment of the film to increase its mechanical strength is a conventionally known means, but in the present invention, this treatment is performed under vacuum to obtain not only mechanical strength but also chemical stability (solvent resistance, etc.). ) Can also be increased. This is because when heat treatment is performed under vacuum, the water remaining in the film after film formation (including water that is generated by crosslinking reactions such as dehydration condensation by heat treatment) and trace amounts of coagulation solvents, etc. This is because even such impurities are removed, and structural changes (crosslinking, etc.) of the polymer material for increasing mechanical strength and chemical stability are significantly promoted. It is difficult to remove such a trace amount of impurities simply by raising the temperature. Therefore, the heat treatment in the present invention is
It is particularly effective when heat-resistant polymers such as polyimide and polyacrylonitrile are used as the film material, and the heat treatment efficiency is higher than the conventional heat treatment, and the progress of the crosslinking reaction is significantly large. It is possible to enhance the function of the asymmetric film made of.

In order to carry out such a heat treatment after film formation, first, the film produced by the above method is taken out from the coagulation liquid and sequentially immersed in an appropriate solvent to remove the coagulation liquid in the film. Moisture etc. are removed sequentially. In particular, when water is used as the coagulating liquid, it is preferable to sequentially immerse it in ethanol, isopropyl alcohol, etc. to perform dehydration while performing solvent replacement, and finally, a solvent having a low surface tension and a low boiling point such as hexane. It is preferable that the membrane is taken out, air dried at room temperature, and then further heat-treated in a vacuum dryer.
The heating temperature is preferably not higher than the glass transition point of the high molecular weight polymer that is the film material, and is usually 100 to 350.
℃ range, especially in the case of polyimide polymer film 20
A high temperature of about 0 to 300 ° C. is preferable. The heating time is usually about 1 to 8 hours, preferably about 3 to 6 hours.
At this time, the degree of vacuum is at least 1 Torr or less, 10 ^-1 to 1
0 ^-3 Torr is preferred. If necessary, the vacuum degree can be further reduced to obtain a film with higher functionality. As described above, the method for producing a highly functional asymmetric membrane of the present invention comprises preparation of a membrane-forming stock solution, membrane formation by a phase inversion method, and functionalization of a membrane by heat treatment.

[0022]

The present invention will be described in more detail based on the following examples. However, these examples do not limit the present invention.

Example 1 Three 100 g of a dimethylformamide solution containing 25% of polyimide (trade name PI2080, manufactured by Dow Chemical Co.)
The mixture was dispensed into a 00 ml eggplant-shaped flask, stirred while slowly rotating on an oil bath, and heated and concentrated at 120 ° C. for 24 hours. The water content of the polyimide resin solution before the heat treatment was about 0.8%, but after the above operation, the value was 0.09%. In this case, the water content of the polyimide solution was measured with a Karl Fischer moisture meter (manufactured by Mitsubishi Kasei). Dimethylformamide as a solvent and dioxane as a swelling agent were added to this concentrated polyimide solution to prepare a stock solution for film formation. However, the composition of the film-forming stock solution was polyimide 22.5%, solvent 62%, and swelling agent 15.5%. This film-forming stock solution is uniformly cast (applied) to a thickness of 250 μm on a glass plate using a doctor plate in a nitrogen gas atmosphere (relative humidity is 35% or less) at a temperature of 25 ° C. for about 30 seconds. The solution was retained in the above to evaporate a part of the solvent, and then immersed in cold water at 2 ° C. for gelation.
The produced asymmetric membrane was taken out from water, ethanol,
The film was dehydrated and the solvent was replaced by immersing the solution in isopropyl alcohol and hexane for 3 days respectively. The film thus obtained is referred to as Sample No. 1. The film was air dried and placed in a vacuum dryer for 6 hours under a vacuum of 10 ^-2 Torr or less,
The film obtained by heat treatment at 250 ° C. is designated as sample No.2.

The separation performance of the membrane samples No. 1 and No. 2 produced in the above-mentioned Example 1 was 9 at 25 ° C. and a secondary pressure of 1 Torr or less.
It was evaluated by measuring the pervaporation characteristics (membrane permeation flux Q, separation coefficient α (H ₂ O / EtOH)) for a 5% by volume ethanol aqueous solution. The results are shown in Table 1. In addition, regarding the film (Sample No. 1) which was not subjected to heat treatment after film formation,
Ultrafiltration characteristics were also measured (25 ° C, operating pressure 3 kg / c
m ² ). As a result, this membrane has a rejection of 90% for polyethylene glycol having a molecular weight of 6000 and Q = 0.5 t / m ^2.
-D performance was shown. From this, by performing the film formation according to the present invention, without performing any heat treatment after the film formation,
It can be seen that an ultrafiltration membrane having sufficient performance can be obtained.

The film sample No. 1 produced in Example 1 was
After heat treatment under each temperature condition from 00 ℃ to 300 ℃ or more,
The tensile strength was measured. The results are shown in Fig. 1. Further, the chemical stability of the film was evaluated by immersing the samples No. 1 and 2 in various solvents at 25 ° C. for 48 hours and then measuring the tensile strength. The results are shown in Table 2. Figure 1,
As shown in Table 1 and Table 2, by heat-treating the film produced by the phase inversion method under vacuum, the separation characteristics, mechanical strength, and chemical stability were improved.

[0026]

[Table 1]

[0027]

[Table 2]

Comparative Example 1 An asymmetric membrane was formed in the same manner as in Example 1 except that the heating and concentration operation was not performed. The produced film had many defects (pinholes), the dense layer was insufficient, and the film had substantially no separation performance. Therefore, it can be seen that it is very difficult to form a film from the polyimide PI2080 without adjusting the water content in the stock film forming solution, which is a technical feature of the present invention.

Example 2 An asymmetric film sample No. 3 was prepared in the same manner as the sample No. 2 of Example 1 except that the heat treatment temperature after film formation was 300 ° C.
Was prepared and its pervaporation characteristics were measured. The results are shown in Table 1.

Example 3 The composition of the film-forming stock solution was 38.75% solvent and 38.75 swelling agent.
%, And untreated and heat-treated asymmetric membrane samples Nos. 4 and 5 were prepared in the same manner as in Example 1 except that the heat treatment temperature was 300 ° C., and the pervaporation characteristics thereof were measured. The results are shown in Table 1. This membrane has pervaporation characteristics at 60 ° C of α = 900, Q = 1.0 kg / m ²
・ It was h, and it had an extremely high separation function. Also,
Ultrafiltration characteristics were also measured (25 ° C., operating pressure 3 kg / cm ² ) for the membrane (Sample No. 4) which was not subjected to heat treatment after film formation. As a result, the membrane had a rejection of 97% for polyethylene glycol having a molecular weight of 2000, and Q =
The performance was 0.2 t / m ² · D. It can be seen that by forming a membrane according to the present invention, an ultrafiltration membrane having sufficient performance can be obtained without any heat treatment after the membrane formation.

Example 4 The composition of the stock solution for film formation was 31% solvent and 46.5% swelling agent.
Untreated and heat-treated asymmetric membrane samples No. 6 and 7 were prepared in the same manner as in Example 1 except that the temperature for heat treatment was 300 ° C., and the pervaporation characteristics thereof were measured. The results are shown in Table 1. The reverse osmosis characteristics were also measured (25 ° C., operating pressure 40 kg / cm ² ) for the film (Sample No. 6) which was not subjected to heat treatment after film formation. As a result, this film exhibited a rejection of 99% against sodium chloride and a performance of Q = 0.7 t / m ² · D. It can be seen that by forming a film according to the present invention, a reverse osmosis membrane having sufficient performance can be obtained without any heat treatment after film formation.

[0032]

According to the method of the present invention, the following operational effects can be obtained and the industrial utility is extremely high. First, since a high-molecular polymer solution is heated and concentrated before the film-forming stock solution is prepared to remove impurities, a non-porous type asymmetric structure can be easily obtained by the phase inversion method regardless of the commercially available form of the material. Membranes can be made. Secondly, by appropriately heat-treating the produced film in a vacuum, not only mechanical strength but also chemical stability can be imparted to the asymmetric film in a relatively short time. Thirdly, a highly functional pervaporation membrane for ethanol separation can be produced using wholly aromatic polyimide (PI2080) as a material.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the mechanical strength and the heat treatment temperature of an asymmetric film produced according to the present invention.

Claims (2)

[Claims] 1. A polyimide polymer or polyacrylonite
The ril polymer solution is heated and concentrated to reduce the water content in the solution.
A highly functional asymmetric membrane characterized by producing a film-forming undiluted solution having a predetermined composition after the content of 0.3% by weight or less, and using this film-forming undiluted solution to form an asymmetric film by the phase inversion method. Membrane manufacturing method. 2. The asymmetric membrane produced is less than 1 Torr.
Characterized by heat treatment at a temperature of 60 ° C. or higher in vacuum
The method for producing a highly functional asymmetric membrane according to claim 1.