JPH042292B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for producing a reverse osmosis membrane made of a cellulose derivative with excellent liquid separation performance, and more specifically to a method for producing a reverse osmosis membrane using an improved membrane-forming solution. It provides technology.

[Prior art] Reverse osmosis membranes are being applied not only to desalination of seawater, brine, etc., but also to the production of ultrapure water and wastewater treatment, and their applications are expanding more and more.

Among cellulose-based reverse osmosis membranes, an asymmetric membrane forming method was invented by Robb et al. in 1961, and in 1966, Manjikian et al. developed cellulose derivatives and microporous forming agents that improve water permeability without impairing their solvent and salt removal performance. That is, since a method for forming an asymmetric membrane from a membrane forming solution containing additives was invented and put into practical use, it has steadily achieved results in various fields. In particular, cellulose acetate-based reverse osmosis membranes have excellent practical performance such as oxidation resistance, especially chlorine resistance, and improvements in membrane performance have been actively promoted, and to date, many improved cellulose-based reverse osmosis membranes and Membrane forming solutions for reverse osmosis membranes have been proposed.

As an example of such a membrane-forming solution, Japanese Patent Publication No. 49-27269 discloses a method in which organic amine salts are added as a microporous agent to a membrane-forming solution using cellulose acetate as a membrane material. Moreover, it cannot be said that a material with high water permeability has been obtained.

Furthermore, in Japanese Patent Publication No. 59-40482, using cellulose acetate with an average degree of acetylation of 41.5% by weight or more,
A film-forming solution containing a mixture of an ester of an oxycarboxylic acid and an alcohol with a solubility in water of 5% by weight or more at room temperature and a monohydric or polyhydric alcohol as an additive has been proposed, but the salt rejection rate is low. is 98
It is difficult to obtain a membrane with a high-temperature rejection rate of % or more.

Therefore, with the prior art, it is currently difficult to obtain a reverse osmosis membrane that has a high salt rejection rate and high water permeability.

[Problems to be Solved by the Invention] Water-soluble organic solvents are preferably used as solvents for membrane-forming solutions for conventional cellulose derivative reverse osmosis membranes because water is used in the coagulation liquid in practice. ,
The present inventors have widely explored the effects of solvents without such constraints, and have discovered that membrane performance can be significantly improved even if the membrane is not particularly water-soluble, as long as certain conditions are met. That is, an object of the present invention is to provide a membrane forming solution for a reverse osmosis membrane that has a high salt rejection rate and water permeability.

[Means for Solving the Problem] The present invention has the following configuration to achieve the above object.

In a method for producing a reverse osmosis membrane using a membrane forming solution comprising a cellulose derivative, a solvent, and an additive, the solvent includes at least a solvent A that is water-soluble and a good solvent for cellulose derivatives, and a solvent A that is poorly water-soluble and is a good solvent for cellulose derivatives. It is a mixed solvent containing solvent B, which is a good solvent for derivatives, and the amount of solvent B is 100% of the film forming solution.
A method for producing a reverse osmosis membrane, characterized in that the additive is in the range of 5 to 13 parts by weight based on the weight part, and further comprises maleic acid and/or butanetetracarboxylic acid.

A reverse osmosis membrane made of a cellulose derivative as used in the present invention refers to a membrane formed using cellulose acetate, cellulose acetate butyrate, methyl cellulose, ethyl cellulose, or the like as a main polymer component. Among these, cellulose acetate is particularly preferred. Cellulose acetate herein refers to cellulose diacetate, cellulose triacetate, acetate with an intermediate degree of substitution, or a blend of cellulose diacetate and cellulose triacetate. Practical acetyl group concentration in cellulose acetate is usually 39.8 to 43.2%.

Next, the film forming solution in the present invention will be explained. The film-forming solution is applicable to various film-forming solutions consisting of polymers, solvents, and additives, and is not particularly limited except for the mixed solvent according to the present invention.
As mentioned above, cellulose acetate is preferred as the polymer, and a mixed composition of so-called cellulose triacetate and cellulose diacetate can also be preferably used. Moreover, the additive consists of maleic acid and/or butanetetracarboxylic acid.

Next, the mixed solvent according to the present invention will be explained.
The mixed solvent consists of solvent A, which is water-soluble and a good solvent for cellulose derivatives, and solvent B, which is poorly water-soluble and is a cellulose derivative. Here, the water-soluble solvent refers to a solvent that can be mixed with water in any proportion. A poorly water-soluble solvent refers to a solvent that dissolves only in a certain amount in water, and also includes a solvent that does not dissolve in water at all.

Preferred solvents A include dioxane, acetone, tetrahydrofuran, dimethylformamide, formamide, and diacetone alcohol.

Mixed solvents of these are also preferably used.

Preferable examples of solvent B include ketones such as mesityl oxide, methyl ethyl ketone, and methyl isobutyl ketone, fatty acid esters such as methyl acetate and ethyl acetate, halogenated hydrocarbons such as methylene chloride and ethylene dichloride, and nitroethane.

The reason why these solvents are preferable is not clear, but it has been confirmed by experimental facts.

As described above, the present invention is characterized in that a water-soluble solvent (solvent A) and a poorly water-soluble solvent (solvent B) are used as a mixture of good solvents for cellulose. The water-soluble solvent A is an essential component since water is used in the coagulation step during film formation. On the other hand, poorly water-soluble solvent B is usually not used as a film forming solvent. The reason for this is that water is usually used in the coagulation bath, but if we consider the actual film-forming process, film-forming involves applying a film-forming solution onto a support and evaporating a certain amount of solvent. This is done by immersing the membrane in a large amount of water, which is a coagulating solvent, and it is thought that even poorly water-soluble solvents can be used as an additive solvent in the membrane-forming solution as long as the amount added and the evaporation conditions are appropriate.

As a result of conducting research from this perspective, the present inventors discovered that by combining water-soluble and poorly water-soluble solvents, membrane performance was improved compared to when only water-soluble solvents were used. It came to this.

The amount of water-soluble solvent A and poorly water-soluble solvent B to be added depends on each solvent's ability to dissolve the polymer, its poor water solubility, the volatility of solvent B, its solubility in water, etc., but it cannot be determined unconditionally. Solvent B is used in an amount of 5 to 13 parts by weight based on 100 parts by weight of the film forming solution. If it exceeds 13 parts by weight, the amount remaining in the cast film immediately before solidification increases, making the solidified film brittle and prone to defects. On the other hand, if it is less than 5 parts by weight, the effect of improving membrane performance due to addition is small.

Although the mechanism by which such a mixed solvent system improves membrane performance is not clear, it can be considered as follows.

The mixed solvent according to the present invention is a good solvent for polymers, but has low compatibility with water. Therefore, during casting in the film forming process, even if some of the solvent evaporates and the polymer concentration on the film surface increases, the aggregation and precipitation of polymers is suppressed, and as a result, the film has uniform pores. This means that a dense layer is likely to be formed. On the other hand, when the membrane is immersed in water during solidification, the slightly water-soluble solvent B remains in the membrane, accelerating the polymer precipitation rate, resulting in the formation of many small pores in the dense layer of the membrane.

[Examples] The present invention will be explained below by using Examples and comparisons.

The performance of reverse osmosis membranes is expressed in salt rejection rate (Rej) and water permeation rate (Flux), but the performance of membranes with different values cannot be directly compared. However, in general, the performance of reverse osmosis membranes is such that if the membrane forming solution is constant, the salt rejection rate and water permeation amount will change contradictoryly with changes in manufacturing conditions (e.g. heat treatment temperature), and A ³ /B as described next. It is known that the value of is almost constant. A is the water permeability coefficient (g/cm ²・sec.
atm), B is the salt permeability coefficient (cm/sec), and the higher the salt rejection rate and the higher the water permeability, the higher the performance.
The value of A ³ /B will be large. Therefore, by using this index A ³ /B, membrane performance can be compared.

A and B are salt rejection rates (Rej) under actual operating conditions
It is calculated using the following formula using and water permeation amount (Flux).

A=Flux/(△p-σ×△π ⁾ ×119.6× ^10-5 B=(100-Rej)/Rej×Flux×115.7×10-5 △P is the actual operating pressure (Kg/ ^cm2 ), Δπ is the osmotic pressure (Kg/cm ² ) determined by the salt concentration. σ is a quantity called a reflection coefficient, and can be set to 1 in the case of a reverse osmosis membrane.

Membrane performance is 30kg/30kg of saline solution containing 1500ppm salt.
The measurement was carried out by applying a pressure of cm ² and flowing it onto the membrane surface at a speed of 10 m/min.

Example 1 14 parts by weight of a mixture of cellulose triacetate with a degree of acetylation of 43.2% and cellulose diacetate with a degree of acetylation of 39.8% in a ratio of 1:1 by weight, 43 parts by weight of dioxane as water-soluble solvent A,
To a solution obtained by adding and dissolving 27 parts by weight of acetone and 7 parts by weight of ethyl acetate as poorly water-soluble solvent B, a solution obtained by dissolving 3 parts by weight of maleic anhydride in 6 parts by weight of methanol was added to obtain a membrane forming solution. To form a film, a solution with a thickness of 0.2 mm is cast onto a glass plate using an applicator in an atmosphere of 23°C, and after drying for about 5 minutes, it is coated with a glass plate for about 1 inch.
This was done by immersing the glass plate in water at 20°C for an hour and peeling the film off the glass plate. The obtained membrane was placed in a hot water bath (80
℃) for 5 minutes. The membrane thus obtained has a salt rejection rate of 98.0% and a water permeation rate of 0.61 m ³ /m ²
The value of A ³ /B, which is an index of membrane performance, is
It was 11.286×10 ^-10 .

Comparative Example 1 The salt rejection rate of a membrane obtained in the same manner as in Example 1 except that 34 parts by weight of acetone was used without adding ethyl acetate, which is poorly water-soluble, was
97.7%, water permeation amount is 0.59m ³ /m ² days, A ³ /B
The value of is 9.153×10 ^-10 .

Using this value of A ³ /B, the amount of water permeation at a salt rejection rate of 98.0% is determined to be 0.55 m ³ /m ² days.

In other words, by using a membrane formed using a membrane forming solution to which ethyl acetate has been added, the amount of water permeation increases by 11% at the same salt rejection rate, which leads to an 11% cost reduction.

Comparative Example 2 A film was formed in exactly the same manner as in Example 1, except that 35 parts by weight of dioxane, 22 parts by weight of acetone, and 20 parts by weight of ethyl acetate were used, and the performance was evaluated. As a result, the membrane of this comparative example had many defects and was significantly inferior in salt removal performance. The defect was presumed to be that droplets containing ethyl acetate as a main component, which is a component that is difficult to dissolve in the water of the coagulation bath, were generated in the film during coagulation.

Example 2 10 parts by weight of cellulose triacetate with a degree of acetylation of 43.2%, 20 parts by weight of acetone and 44 parts by weight of dioxane as a water-soluble solvent A, and 12 parts by weight of methyl ethyl ketone as a slightly water-soluble solvent B were dissolved, and butane tetraacetate was added to the solution. A solution prepared by dissolving 4 parts by weight of carboxylic acid in 10 parts by weight of methanol was added to obtain a membrane forming solution. To form a film, use an applicator to cast the solution onto a glass plate to a thickness of 0.2 mm in an atmosphere of 23°C, dry it for about 10 minutes, and then immerse it together with the glass plate in water at 30°C for about 1 hour to peel it off from the glass plate. A semipermeable membrane was obtained. The membrane thus obtained had a salt rejection rate of 98.3% and a water permeation rate of 0.50 m ³ /m ² days, which are indicators of membrane performance.
The value of A ³ /B was 8.942×10 ^-10 .

Comparative Example 3 A membrane obtained in the same manner as in Example 2 except that methyl ethyl ketone, which is a poorly water-soluble solvent, was not added and 32 parts by weight of acetone was used had a salt rejection rate of 98.2% and a water permeation rate of 98.2%. 0.47m ³ /m ² days,
The value of A ³ /B is 7.460×10 ^-10 . Using this value of A ³ /B, the amount of water permeation at a salt rejection rate of 98.3% is determined to be 0.46 m ³ /m ² days.

Example 3 To 20 parts by weight of a mixture of cellulose triacetate with a degree of acetylation of 43.2% and cellulose diacetate with a degree of acetylation of 39.8% in a weight ratio of 2:3, 41 parts by weight of dioxane as water-soluble solvent A,
To a solution obtained by adding and dissolving 14 parts by weight of acetone and 13 parts by weight of ethyl acetate as poorly water-soluble solvent B, a solution obtained by dissolving 3 parts by weight of maleic acid in 9 parts by weight of methanol was added to obtain a membrane forming solution.

To form a film, use an applicator to cast a solution to a thickness of 0.2 mm onto a glass plate in a 25°C atmosphere, dry it for about 1 minute, and then immerse it together with the glass plate in cold water at 10°C for about 20 minutes to remove the film from the glass plate. This was done by peeling it off. The obtained membrane was heat treated in a hot water bath (80°C) for 5 minutes. The salt rejection rate of the membrane thus obtained is
98.0%, the water permeation amount was 0.62 m ³ /m ² days, and the value of A ³ /B, which is an index of membrane performance, was 11.659×10 ⁻¹⁰ .

Comparative Example 4 The salt rejection rate of a membrane obtained in the same manner as in Example 3 except that methyl acetate, which is a poorly water-soluble solvent, was not added and acetone was changed to 27 parts by weight was as follows.
97.9%, water permeation amount is 0.59m ³ /m ² days, A ³ /B
The value of is 10.045×10 ^-10 . Using this value of A ³ /B to calculate the amount of water permeation at a salt rejection rate of 98%,
0.58m ³ /m ² days.

Example 4 Cellulose diacetate with a degree of acetylation of 39.8%
25 parts by weight, 31 parts by weight of acetone as a water-soluble solvent A, 30 parts by weight of formamide, and 5 parts by weight of mesityl oxide as a poorly water-soluble solvent were added and dissolved, and 3 parts by weight of maleic acid was dissolved in 6 parts by weight of methanol. A membrane forming solution was obtained by adding the dissolved liquid. Film formation at 20℃
0.2 thickness on a glass plate with an applicator in an atmosphere of
The solution was cast onto a glass plate, dried for about 30 seconds, and then immersed together with a glass plate in cold water at 2°C for about 1 hour to peel the film from the glass plate. The resulting membrane was heat treated in a water bath (85°C) for 5 minutes. The membrane obtained in this way has a salt rejection rate of 98.0% and a water permeation rate of 0.58 m ³ /m ² days, which is ^an indicator of membrane performance.
The value of B was 10.203×10 ^-10 .

Comparative Example 5 The performance of the membrane obtained in the same manner as in Example 4 except that mesityl oxide, which is a poorly water-soluble solvent, was not added and the amount of acetone was changed to 36 parts by weight was as follows: salt rejection rate of 97.8%; The amount of water permeation is 0.58 m ³ /m ² days, and the value of A ³ /B is 9.257×10 ^-10 . this
Using the value of A ³ /B, the amount of water permeation at a salt rejection rate of 98.0% is determined to be 0.55 m ³ /m ² days.

[Effects of the Invention] In the present invention, by using a mixture of a water-soluble solvent and a poorly water-soluble solvent among good solvents for cellulose derivatives as a solvent for a film-forming solution, the solubility of the polymer during film-forming is improved. As the precipitation rate during solidification is improved and the precipitation rate during solidification is accelerated, the dense layer of the resulting membrane is uniform and has many small pores, making it possible to obtain a membrane with unprecedented high performance.

Furthermore, since the present invention does not particularly significantly change the conventional manufacturing process, it has the remarkable effect of being able to manufacture high quality membranes without increasing manufacturing costs. Furthermore, it is clear that the use of a solvent that is a good solvent and a poorly water-soluble solvent for the polymer of the present invention in the membrane forming solution is also applicable to polymers other than cellulose derivatives.

Claims (1)

[Claims] 1. A method for producing a reverse osmosis membrane using a membrane-forming solution consisting of a cellulose derivative, a solvent, and an additive, in which the solvent includes at least a solvent A that is water-soluble and a good solvent for cellulose derivatives, and a solvent A that is poorly water-soluble and is a good solvent for cellulose derivatives. It is a mixed solvent containing solvent B, which is a good solvent for cellulose derivatives, and the amount of solvent B is the same as that of the film forming solution.
It is in the range of 5 to 13 parts by weight per 100 parts by weight,
A method for producing a reverse osmosis membrane, further characterized in that the additive comprises maleic acid and/or butanetetracarboxylic acid.