JP7716206B2 - Forward osmosis membrane manufacturing method - Google Patents

Description

The present invention relates to a method for manufacturing a forward osmosis membrane.

Semipermeable membranes are useful for selectively separating specific components from liquid or gas mixtures, and are ideally suited for use, for example, in producing high-purity water or separating specific solutes from a solution.

Of the two membrane separation methods that utilize semipermeable membranes, reverse osmosis and forward osmosis, the reverse osmosis method, which has traditionally been the mainstream, exposes the reverse osmosis membrane to high pressure. Therefore, in order to achieve high strength, the mainstream reverse osmosis membranes are composite semipermeable membranes that are made by laminating, in this order, a porous support (e.g., nonwoven fabric), a porous polymer layer (e.g., polysulfone layer), and a layer that actually functions as the semipermeable membrane (also known as a skin layer, separation layer, etc.).

In contrast, forward osmosis uses the osmotic pressure generated between aqueous solutions of different solute concentrations separated by a forward osmosis membrane as the driving force, moving water from a low-salt feed solution (such as freshwater) to a high-salt draw solution (such as seawater). This means that there is no need for high pressure or membrane strength to overcome the osmotic pressure required in reverse osmosis, and it offers potential advantages such as excellent energy savings and the ability to simplify the osmosis membrane structure.

However, forward osmosis has the problem of low permeation flux (see, for example, Patent Document 1), and when using forward osmosis membranes to treat large amounts of water for agricultural use or water purification, there is a need for further improvement in permeation flux from the low-salt concentration side to the high-salt concentration side.

Meanwhile, the present inventor has disclosed a forward osmosis membrane with excellent permeation flow rate, using, for example, a semipermeable membrane having a polyether ketone structure (see, for example, Patent Document 2). Furthermore, in the examples of Patent Document 2, a varnish containing a polymer having the aforementioned polyether ketone structure is used as a release film, and a free-standing membrane is obtained by a casting method.

Japanese Patent Application Laid-Open No. 2014-213262 International Publication No. 2018-79733

According to the inventors' research, forward osmosis membranes with a high water permeation flux from the feed solution, such as those disclosed in Patent Document 2, tend to have a large amount of back-diffusion of salt from the draw solution. It has also been found that the amount of back-diffusion of salt per water permeation flux is not necessarily low. Based on this perspective, the inventors have developed a forward osmosis membrane that can achieve both high water permeability and high salt rejection. It will be necessary to continue to improve the performance required of such forward osmosis membranes.

In light of the above issues, the inventors conducted research and found that the above-mentioned cast films sometimes had relatively more irregularities on the peeled surface. Also, in some cases, visible streaks could be observed on the peeled surface. It was thought that such shapes may be the cause of the back diffusion of salts mentioned above.

As a result of research conducted by the inventors based on the above perspective, they discovered that by manufacturing a forward osmosis membrane using a method that uses a specific release film, it is possible to obtain a membrane with a smooth surface and further suppress the back diffusion of salts, leading to the completion of the present invention.

That is, the present invention relates to the following [1] to [4].
[1] an aromatic structure;
a protonic acid structure;
A method for producing a forward osmosis membrane that is a self-supporting membrane, comprising the steps of: casting a varnish obtained by dissolving a resin (A) containing a structure selected from an ether structure (—O—) and a ketone structure (—CO—) in an organic compound (S) onto a film having a surface with a contact angle of 10° to 60° with the organic compound (S), and then drying the resulting varnish.

[2] The method for producing a forward osmosis membrane according to [1], wherein the resin (A) has a structural unit (1) represented by the following formula (1) and a structural unit (2) represented by the following formula (2):
[In formulas (1) and (2),
R ¹ to R ¹⁰ each independently represent H, Cl, F, CF ₃ or C _m H _2m+1 (m represents an integer of 1 to 10);
At least one of R ¹ to R ¹⁰ is C _m H _2m+1 (m is an integer of 1 to 10),
Two or more of each of R ¹ to R ¹⁰ may be present in an aromatic ring, and when two or more C _m H _2m+1 groups are present in one aromatic ring, the C _m H _2m+1 groups may be the same as or different from each other.
X ¹ to X ⁵ are each independently H, Cl, F, CF ₃ or a protonic acid group;
At least one of X ¹ to X ⁵ is a protonic acid group;
Two or more of each of X ¹ to X ⁵ may be present in an aromatic ring, and when two or more protonic acid groups are present in one aromatic ring, the respective protonic acid groups may be the same or different.
A ¹ to A ⁶ each independently represent a direct bond, —CH ₂ —, —C(CH ₃ ) ₂ —, —C(CF ₃ ) ₂ —, —O— or —CO—, and at least one of A ¹ to A ⁶ is —O— or —CO—.
i, j, k, and l each independently represent 0 or 1.

[3] The method for producing a forward osmosis membrane according to [1], wherein the resin (A) comprises a protonic acid group-containing resin (A1) and a resin (A2) having a molar fraction of protonic acid group-containing structural units different from that of the resin (A1).

[4] The method for producing a forward osmosis membrane according to [3], wherein the semipermeable membrane is a laminate including a layer containing the resin (A1) and a layer containing the resin (A2).

The present invention makes it possible to produce forward osmosis membranes that have an excellent balance of water permeability and salt rejection, specifically forward osmosis membranes that have high water permeability and high salt rejection.

FIG. 1 is a schematic diagram of an apparatus used to evaluate the separation performance of a forward osmosis membrane in the examples.

The present invention will be specifically described below.
[Forward osmosis membrane]
In a preferred embodiment, the forward osmosis membrane of the present invention comprises a semipermeable membrane and a porous substrate disposed on at least one surface of the semipermeable membrane, wherein the semipermeable membrane contains a resin (A) (hereinafter sometimes simply referred to as resin (A)) containing an aromatic structure, a protonic acid group-containing structure, and a structure selected from an ether group and a ketone group.

The semipermeable membrane used in the forward osmosis membrane is often obtained as a freestanding membrane, but preferably, regardless of whether it is a freestanding membrane, it has the porous substrate disposed on at least one side thereof, which allows it to achieve high water permeability and high salt rejection.

<Method of manufacturing semipermeable membrane>
The semipermeable membrane contains a resin (A) having a protonic acid group-containing structural unit, an aromatic structure, and a structure selected from an ether group and a ketone group. Examples of the resin include a protonic acid group-containing resin having a structural formula such as formula (1) above, and a protonic acid group-containing resin having both structural formulas (1) and (2). The latter is preferred.

A preferred embodiment of the resin (A) is a structure containing at least a protonic acid group-containing resin (A1) and a resin (A2) having a higher molar fraction of protonic acid group-containing structural units than the resin (A1). From the viewpoint of a balance between water permeability and salt rejection rate, the absolute value of the difference between the molar fraction of protonic acid groups contained in the resin (A1) and the molar fraction of protonic acid groups in the resin (A2) is preferably 0.03 to 0.6. A more preferred lower limit is 0.06, even more preferably 0.1, and particularly preferably 0.15. On the other hand, a more preferred upper limit is 0.5, even more preferably 0.45, and particularly preferably 0.4.

Hereinafter, the resins (A), (A1), and (A2) will also be referred to simply as "resin (A)," "resin (A1)," and "resin (A2)," respectively. Details of these resins will be provided later.

In the present invention, the protonic acid group refers to a functional group that easily releases a proton or a group in which the hydrogen atom thereof has been substituted with Na or _{K. Examples of such groups include a sulfonic acid group (-SO3H), a carboxylic acid group (-COOH), a phosphonic acid group (-PO3H2), an alkylsulfonic acid group (-(CH2)nSO3H ) , an alkylcarboxylic acid} group (-( _CH2 ) _nCOOH ), an alkylphosphonic acid group (- _{( CH2} ) _nPO3H2 ), a hydroxyphenyl group ( _-C6H4OH ) _, and groups in which the terminal hydrogen atom of these groups has been substituted with Na or K. n is an integer of 1 to ₁₀ .

The molar fraction of protonic acid group-containing structural units is the ratio of the number of protonic acid group-containing structural units to the total number of structural units constituting the resin.

The resin (A) has excellent water permeability. When at least two resins with different molar fractions of protonic acid group-containing structural units, i.e., resin (A1) and resin (A2), are used, the resin (A2), which has a higher molar fraction of protonic acid group-containing structural units, tends to improve water permeability, while the resin (A1), which has a lower molar fraction of protonic acid group-containing structural units, tends to suppress back-diffusion of salts.

The method for producing a semipermeable membrane of the present invention is characterized by comprising the steps of casting a varnish, in which a resin (A) is dissolved in an organic compound (S), onto a film having a surface with a contact angle of 10° to 60° with the organic compound (S), and then drying the film.

(Varnish of Resin (A))
Resin (A) has a basically linear molecular structure and, even if it has a crosslinked structure derived from the polyfunctional compound, the amount of the crosslinked structure is small, so that it has excellent solvent solubility, and therefore can be made into a varnish by dissolving resin (A) in a solvent.

Examples of organic compounds (S) used as so-called solvents to form the varnish include, but are not limited to, alcohols such as methanol, ethanol, 1-propanol, 2-propanol, and butanol; hydrocarbons such as toluene and xylene; halogenated hydrocarbons such as methyl chloride and methylene chloride; ethers such as dichloroethyl ether, 1,4-dioxane, and tetrahydrofuran; fatty acid esters such as methyl acetate and ethyl acetate; ketones such as acetone and methyl ethyl ketone; cellosolves such as 2-methoxyethanol and 2-ethoxyethanol; and aprotic polar solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, and dimethyl carbonate. These can be used alone or in combination. Among these, lower alcohols (methanol, ethanol, 1-propanol, 2-propanol, t-butyl alcohol, etc.), tetrahydrofuran, N,N-dimethylformamide, N-methyl-2-pyrrolidone, etc. are preferred due to their water solubility, and mixed solvents of these with water are also preferred. The resin concentration in the varnish can be selected depending on how the varnish is used, but is preferably 1% by mass or more and 80% by mass or less.

(film)
The film (so-called release film) used in the casting method of the present invention is a film whose surface exhibits a contact angle of 10° to 60° with the organic compound (S). The film of the present invention also includes forms called sheets and plates. When the organic compound (S) is a mixture of multiple compounds, the contact angle is preferably the contact angle of the mixture, but the contact angle of the compound with the highest polarity may also be within the above-mentioned angle range. The contact angle of the present invention is determined by the method shown in the examples.

Furthermore, the organic compound (S) used to measure the contact angle is, of course, one of the organic compounds (S) contained in the varnish that is cast directly onto the film.

There are no particular restrictions on the organic materials (resins) and inorganic materials such as glass that are used to make the film, as long as they meet the above requirements. Furthermore, the contact angle will vary depending on the smoothness of the film's surface and the surface material (a typical example is a release layer), but there are no restrictions on these as long as they meet the above contact angle range.

Typical examples of the film material mentioned above include glass plates, and polyester films such as PET film and PBT film. Other examples include polyolefin films such as polyethylene film, polypropylene film, and poly-4-methyl-1-pentene film (trade name: TPX (registered trademark)); halogen-containing films such as polyvinyl chloride film, polyvinylidene chloride film, and polyvinylidene fluoride film; poly(meth)acrylate films such as polyacrylic acid esters and polymethacrylic acid esters; polyamide films such as various nylon films; olefin films containing polar groups such as (meth)acrylic acid, (meth)acrylic acid esters, and vinyl acetate; and films containing hydroxy groups such as polyvinyl alcohol.

Among the above, polyester film and polyamide film are preferred, taking into consideration the ease of drying at high temperatures and costs, and polyester film is particularly preferred.
Furthermore, it is preferable that the surface shape is as smooth as possible. Furthermore, it is preferable that the film has a release layer on the surface. As the raw material for the release layer, known materials such as silicone-based release agents can be used as long as they satisfy the above-mentioned contact angle requirement.

The contact angle range of 10 to 60° is the range observed for the organic compound (S). The preferred lower limit of the contact angle is 12°, more preferably 13°, even more preferably 14°, and particularly preferably 15°. On the other hand, the preferred upper limit is 50°, more preferably 45°, even more preferably 42°, and particularly preferably 40°.

The method for producing a forward osmosis membrane (semipermeable membrane) of the present invention can effectively suppress the back diffusion of salts. It also tends to make it easier to obtain membranes with high surface smoothness.

The reason why the forward osmosis membrane (semipermeable membrane) obtained by the above method exhibits the above effects is unclear, but the inventors speculate as follows. Generally, it is preferable for semipermeable membranes to have few irregularities and little thickness variation, but this is mainly due to factors such as strength. The inventors believe that the uneven thickness of the semipermeable membrane may have a larger effect than expected on the back diffusion of salt. In other words, the inventors believe that if there are even slightly thinner areas, salt may migrate to those areas and become unevenly distributed, causing structural changes in the membrane and making it easier for back diffusion of salt to occur.

Semipermeable membranes obtained using the manufacturing method of the present invention tend to have particularly excellent surface smoothness. This is likely why they exhibit the inherent effect of semipermeable membranes in inhibiting back-diffusion of salt.

The following describes the resin (A) of the present invention, which contains a protonic acid group structure, an aromatic structure, and a structure selected from an ether structure and a ketone structure, as well as its preferred embodiments, resin (A1) and resin (A2).

The resins (A), (A1), and (A2) of the present invention preferably have a structure represented by the following formula (1), or a structure represented by the following formulas (1) and (2).

In formulas (1) and (2),
i, j, k and l each independently represent 0 or 1.

R ¹ to R ¹⁰ are each independently H, Cl, F, CF ₃ or C _m H _2m+1 (m is an integer of 1 to 10), and examples of C _m H _2m+1 include a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group and a hexyl group.

At least one of R ¹ to R ¹⁰ is C _m H _2m+1 (m is an integer of 1 to 10). Specifically, i, j, k, and l, as well as R ¹ to R ¹⁰ , are selected so that the structural unit (1) and/or the structural unit (2) have at least one group represented by C _m H _2m+1 . That is, when i, j, k, and l are all 1, at least one of R ¹ to R ¹⁰ is C _m H _2m+1 , but when, for example, i = 0, j = 1, k = 0, and l = 1, at least one of R ¹ to R ³ , R ⁵ to R ⁸ , and R ¹⁰ is C _m H _2m+1 .

Two or more of each of R ¹ to R ¹⁰ may be present in an aromatic ring, and when two or more C _m H _2m+1 groups are present in one aromatic ring, the C _m H _2m+1 groups may be the same as or different from each other.

X ¹ to X ⁵ are each independently H, Cl, F, CF ₃ or a protonic acid group.

At least one of ^X1 to ^X5 is a protonic acid group. Specifically, i, j, and ^X1 to ^X5 are selected so that the structural unit (1) has at least one protonic acid group. That is, when i=1 and j=1, at least one of ^X1 to ^X5 is a protonic acid group, but when j=0, at least one of ^X1 to ^X3 is a protonic acid group, and when i=0 and j=1, at least one of ^X1 to ^X3 and ^X5 is a protonic acid group.

Two or more of each of X ¹ to X ⁵ may be present in an aromatic ring, and when two or more protonic acid groups are present in one aromatic ring, the respective protonic acid groups may be the same or different.

A ¹ to A ⁶ are each independently a direct bond, —CH ₂ —, —C(CH ₃ ) ₂ —, —C(CF ₃ ) ₂ —, —O— or —CO—, and at least one of A ¹ to A ⁶ is preferably —CO—.

The lines at both ends of formulas (1) and (2) indicate bonds between adjacent structural units.

In the present invention, the protonic acid group refers to a functional group that easily releases a proton or a group in which the hydrogen atom has been substituted with Na or _{K. Examples of such a group include a sulfonic acid group (-SO3H), a carboxylic acid group (-COOH), a phosphonic acid group (-PO3H2), an alkylsulfonic acid group (-(CH2)nSO3H ) , an alkylcarboxylic acid} group (-( _CH2 ) _nCOOH ), an _{alkylphosphonic} acid group (-( _CH2 ) _{nPO3H2 )} , a hydroxyphenyl group ( _-C6H4OH ), and groups in which the terminal hydrogen atom has been substituted with Na or _K. n is an integer of 1 to 10. The protonic acid group is preferably _{-Cn'H2n' - SO3Y} (n' is an integer of 0 to 10, preferably 0, and Y is H, Na, or K).

Examples of the structural unit (1) include structural units represented by the following formula (1-1) or (1-2), and in these structures, A ¹ is preferably —CO—.

More specifically, structural units represented by the following formula (1-3) or (1-4) can be mentioned.

(In formulas (1-1) to (1-4), each Z is independently the protonic acid group, each s is independently an integer of 0 to 3, each t is independently an integer of 0 to 4, and the lines at both ends represent bonds to adjacent structural units. The definitions of symbols other than these are as described above.)

Furthermore, examples of structural unit (2) include structural units represented by the following formula (2-1) or (2-2):

More specifically, structural units represented by the following formula (2-3) or (2-4) can be mentioned.

(In formulas (2-1) to (2-4), t's each independently represent an integer of 0 to 4, and the lines at both ends represent bonds to adjacent structural units. The definitions of the symbols other than t are as described above.)

In the entire resin (A), the molar fraction of the structural unit (1) relative to the total amount of the structural unit (1) and the structural unit (2) is preferably 0.1 or more, more preferably 0.2 or more, and even more preferably 0.25 or more, since a semipermeable membrane with high water permeability can be formed; and is preferably 0.9 or less, more preferably 0.7 or less, and even more preferably 0.6 or less, since a semipermeable membrane with high salt rejection and no gelation can be formed.

In terms of the molar fraction of the structural unit (1) relative to the total amount of the structural unit (1) and the structural unit (2), from the viewpoint of the balance between water permeability and salt rejection rate, the absolute value of the difference between the molar fraction of the resin (A1) and the molar fraction of the resin (A2) is preferably 0.03 to 0.6, more preferably 0.1 to 0.4.

Note that, in the above-mentioned definition of the difference in molar fraction, the number of resins contained in resin (A) is not limited to two. When resin (A) contains three or more resins with different molar fractions of structural unit (1), the above-mentioned definition of the difference in molar fraction of structural unit (1) needs to be satisfied by at least one set of "two resins."

Resin (A) may further contain structural units derived from a polyfunctional compound, as described below. Resin (A) may be crosslinked or non-crosslinked.

The weight-average molecular weight (Mw) of resin (A), measured using GPC (Gel Permeation Chromatography) under the following conditions (1) to (6), is preferably 70,000 or more, more preferably 80,000 or more, and even more preferably 90,000 or more. When the molecular weight is within this range, the resulting semipermeable membrane has high mechanical properties and is less likely to tear during membrane formation or use. Furthermore, from the perspective of gel generation rate, the weight-average molecular weight is preferably 180,000 or less.

(1) Measurement temperature: 40℃
(2) Developing solvent: N,N-dimethylformamide (DMF)
(3) Flow rate: 1.0ml/min
(4) Injection volume: 500μl
(5) Detector: UV detector
(6) Molecular weight standard: standard polystyrene The weight average molecular weight can be controlled by adjusting the molar ratio of raw material monomers and the amount of end-capping agent when producing the resin (A).

The protonic acid group equivalent of resin (A), i.e., the mass of resin (A) per mole of protonic acid groups, is preferably 200 g/mol or more, since this results in a semipermeable membrane that is insoluble in water or methanol, has reduced swelling, and exhibits a low amount of electrolyte permeation through the membrane. Furthermore, this is preferably 5,000 g/mol or less, more preferably 1,000 g/mol or less, since this results in a semipermeable membrane that is highly water permeable and economical.

In particular, if both resin (A1) and resin (A2) have the structure of formula (1), or the structure of formula (1) and the structure of formula (2), it is believed that these resins will disperse well. Therefore, it is believed that dense, multi-stage, continuous layers of pseudo-(A1)/(A2) units are formed, which likely allows for efficient water permeability and the effect of suppressing back-diffusion of salts.

In one embodiment, the semipermeable membrane preferably has a concentration gradient of protonic acid groups derived from resin (A) in the thickness direction of the semipermeable membrane. The concentration gradient may be such that the concentration increases continuously or in a stepwise manner. One example of such a configuration is a laminated membrane having a layer (L1) containing resin (A1) and a layer (L2) containing resin (A2). The thickness ratio of layer (L1) to layer (L2) (thickness of L1:thickness of L2) is typically 5:95 to 80:20, preferably 20:80 to 60:40. Such a multilayer structure provides higher water permeability and salt rejection.

This configuration is preferable from the viewpoint of more reliably realizing the effects of the (A1)/(A2) unit configuration.

Here, the concentration gradient of protonic acid groups derived from resin (A) refers to the change in the concentration of protonic acid groups contained in resin (A) in the thickness direction of the semipermeable membrane. When the protonic acid groups are sulfonic groups or groups in which the terminal hydrogen atoms are substituted with Na or K, the concentration gradient can be confirmed by analyzing the concentration distribution of sulfur atoms in the cross section of the semipermeable membrane using elemental mapping.

When using a forward osmosis membrane, it is preferable to position the semipermeable membrane so that the side with the higher protonic acid group concentration faces the feed solution and the side with the lower protonic acid group concentration faces the draw solution. For example, it is preferable to position the semipermeable membrane so that the layer (L2) containing resin (A2) faces the feed solution and the layer (L1) containing resin (A1) faces the draw solution. This configuration results in higher water permeability and salt rejection.

In one embodiment, the semipermeable membrane is preferably formed from a mixture containing at least resin (A1) and resin (A2). For example, the semipermeable membrane has a structure separated into at least two phases corresponding to resin (A1) and resin (A2). This phase-separated structure results in a higher water flux and salt rejection. In the mixture, the mass ratio (A1:A2) of resin (A1) to resin (A2) is typically 10:90 to 90:10, preferably 30:70 to 70:30.

Such a phase-separated structure can be confirmed by observation with an electron microscope (TEM or SEM). Possible phase-separated structures include a sea-island structure and an interpenetrating structure, with the interpenetrating structure being more preferable from the viewpoint of preventing back-diffusion of salt from the draw solution. In the case of a sea-island structure, it is preferable that resin (A1) with a small molar fraction of protonic acid group-containing structural units forms the sea phase, and resin (A2) with a large molar fraction forms the island phase.

In the present invention, a semipermeable membrane can be produced as a free-standing membrane using resin (A) as a raw material, and by laminating this semipermeable membrane with a porous substrate, a forward osmosis membrane with high water permeability and a high salt rejection rate can be obtained.

The semipermeable membrane consists essentially of resin (A) alone, but may contain small amounts of other components (e.g., 1% by mass or less, or 0.1% by mass or less) that do not impair the effects of the present invention.

The thickness of the semipermeable membrane is typically 3.0 μm or less, preferably 0.01 to 3.0 μm, and more preferably 0.01 to 1.5 μm. A forward osmosis membrane using a semipermeable membrane with a thickness within this range has sufficient membrane strength and exhibits water permeability high enough for practical use. The thickness of the semipermeable membrane can be controlled by the manufacturing conditions of the semipermeable membrane, such as the temperature and pressure during press molding, and the varnish concentration and coating thickness during casting.

It is preferable that the semipermeable membrane have as little unevenness in thickness as possible. In other words, it is preferable that the semipermeable membrane surface has as little unevenness as possible. Such unevenness can be determined by the surface roughness (SA value) measured by the method shown in the examples. In the present invention, the surface roughness is preferably 10 nm or less, more preferably 8 nm or less, and even more preferably 5 nm or less. The preferred lower limit is of course zero, but a realistic and preferred lower limit is approximately 0.5 nm.

Here, the structural units (1) in these resins (A1) and (A2) may be the same or different, and the structural units (2) in resins (A1) and (A2) may be the same or different.

(Method for producing resin (A))
Resin (A) can be obtained by condensing monomers having an aromatic ring according to a conventionally known method (for example, the method described in WO 2003/33566).

and a monomer having a halogen substituent represented by the following formulae (1b) and (2b):

The resin can be obtained by condensation polymerization with a monomer having a hydroxyl group represented by the following formula:

In the formula, Y is a halogen atom, and the meanings of the other symbols are the same as those of the same symbols used in formulas (1) and (2) above. Preferred examples of the halogen atoms include fluorine and chlorine.

The condensation is preferably carried out using a basic catalyst such as K ₂ CO ₃ .

Examples of the monomer represented by the above formula (1a) (wherein at least one of X ¹ and X ² is a protonic acid group) include protonic acid group-containing aromatic dihalide compounds such as 5,5'-carbonylbis(sodium 2-fluorobenzenesulfonate) and 5,5'-carbonylbis(sodium 2-chlorobenzenesulfonate).

Examples of the monomer represented by the formula (2a) include:
Aromatic dihalide compounds such as 4,4'-difluorobenzophenone, 3,3'-difluorobenzophenone, 4,4'-dichlorobenzophenone, 3,3'-dichlorobenzophenone, 4,4'-difluorobiphenyl, 4,4'-difluorodiphenylmethane, 4,4'-dichlorodiphenylmethane, and 4,4'-difluorodiphenyl ether; and alkyl group-containing aromatic dihalide compounds such as 3,3'-dimethyl-4,4'-difluorobenzophenone, 3,3'-diethyl-4,4'-difluorobenzophenone, 3,3',5,5'-tetramethyl-4,4'-difluorobenzophenone, 3,3'-dimethyl-4,4'-dichlorobenzophenone, and 3,3',4,4'-tetramethyl-5,5'-dichlorobenzophenone.

Examples of the monomer represented by the formula (2b) (or the monomer represented by the formula (1b) (wherein X ³ to X ⁵ are all hydrogen atoms)) include:
Aromatic dihydroxy compounds such as 4,4'-dihydroxybiphenyl, 4,4'-dihydroxydiphenylmethane, 4,4'-dihydroxydiphenyl ether, 4,4'-dihydroxybenzophenone, 2,2-bis(4-hydroxyphenyl)propane, 1,1,1,3,3,3-hexafluoro-2,2-bis(4-hydroxyphenyl)propane, 1,4-bis(4-hydroxyphenyl)benzene, α,α'-bis(4-hydroxyphenyl)-1,4-dimethylbenzene, α,α'-bis(4-hydroxyphenyl)-1,4-diisopropylbenzene, α,α'-bis(4-hydroxyphenyl)-1,3-diisopropylbenzene, 1,4-bis(4-hydroxybenzoyl)benzene, and 3,3'-difluoro-4,4'-dihydroxybiphenyl; and 3,3'-dimethyl-4,4'-dihydroxybiphenyl, 3,3',5,5'-tetramethyl-4,4'-dihydroxybiphenyl, 3,3'-dimethyl-4,4'-dihydroxydiphenylmethane, 3,3',5,5'-tetramethyl-4,4'-dihydroxydiphenylmethane (also known as bis(3,5-dimethyl-4-hydroxyphenyl)methane), 3,3',5,5'-tetraethyl-4,4'-dihydroxydiphenylmethane, 3,3'-dimethyl-4,4'-dihydroxydiphenyl ether, 3,3',5,5'-tetramethyl-4,4'-dihydroxydiphenyl ether, 2,2-bis(3-methyl-4-hydroxydiphenyl) alkyl group-containing aromatic dihydroxy compounds such as 2,2-bis(3-ethyl-4-hydroxyphenyl)propane, 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane, α,α'-bis(3-methyl-4-hydroxyphenyl)-1,4-diisopropylbenzene, α,α'-bis(3,5-dimethyl-4-hydroxyphenyl)-1,4-diisopropylbenzene, α,α'-bis(3-methyl-4-hydroxyphenyl)-1,3-diisopropylbenzene, and α,α'-bis(3,5-dimethyl-4-hydroxyphenyl)-1,3-diisopropylbenzene.

A polyfunctional compound may be copolymerized with the monomers described above, provided that the solvent solubility of resin (A) is not impaired. By copolymerizing a polyfunctional compound, resin (A) can have a micro-crosslinked structure. Examples of polyfunctional compounds include those having three or more hydroxyl groups per molecule, such as (2,4-dihydroxyphenyl)(4-hydroxyphenyl)methanone, 4-[1-(4-hydroxyphenyl)-1-methylethyl]-1,3-benzenediol, 4-[(2,3,5-trimethyl-4-hydroxyphenyl)methyl]-1,3-benzenediol, 4-[(4-hydroxyphenyl)methyl]-1,2,3-benzenetriol, (4-hydroxyphenyl)(2,3,4-trihydroxyphenyl)methanone, and 4-[(3,5-dimethyl- Examples include [(4-hydroxyphenyl)methyl]-1,2,3-benzenetriol, 4-[(2,3,5-trimethyl-4-hydroxyphenyl)methyl]-1,2,3-benzenetriol, 4,4'-[1,4-phenylenebis(1-methylethylidene)]bis[benzene-1,2-diol], 5,5'-[1,4-phenylenebis(1-methylethylidene)]bis[benzene-1,2,3-triol], α,α,α'-tris(4-hydroxyphenyl)-1-ethyl-4-isopropylbenzene, phloroglucin, and pyrogallol.

From the viewpoint of preventing a decrease in the solvent solubility of resin (A), a decrease in the fluidity of the resin during membrane formation, and a decrease in the elongation of the semipermeable membrane, the copolymerization amount is preferably 0 to 8 mol%/total OH equivalent (i.e., of the total number (100 mol%) of OH groups possessed by the monomer of formula (1b), the monomer of formula (2b), and the polyfunctional monomer, 0 to 8 mol% are OH groups derived from the polyfunctional monomer), and more preferably 0 to 5 mol%/total OH equivalent.

Tensile modulus, tensile strength at break, and elongation
The tensile modulus of the semipermeable membrane is preferably 0.8 to 2.0 GPa, more preferably 1.2 to 1.6 GPa. The tensile breaking strength of the semipermeable membrane is preferably 40 MPa or more, with an upper limit of, for example, 100 MPa. The elongation of the semipermeable membrane is preferably 40% or more, with an upper limit of, for example, 200%. These tensile modulus, tensile breaking strength, and elongation are measured under the following conditions.

A test piece measuring length x width x thickness = 100 mm x 10 mm x 5 μm is prepared and pulled at a rate of 50 mm/min using a tensile testing machine. The strength (tensile load value divided by the cross-sectional area of the test piece) and elongation are determined when the test piece breaks (fractures).

The elongation is calculated using the following formula:
Elongation (%) = 100 x (L - L ₀ ) / L ₀
(L ₀ : length of test piece before test, L: length of test piece at break)
The tensile modulus is determined by dividing the load at break by the cross-sectional area of the test piece and the strain at break, ie, (LL ₀ )/L ₀ .

The tensile strength at break and the elongation can be increased, for example, by increasing the molecular weight of resin (A).

<Solubility and mass loss rate>
The solubility of the semipermeable membrane in dimethyl sulfoxide (hereinafter also referred to as "DMSO") and water can be evaluated by the following mass loss rate, which is preferably less than 2% by mass, more preferably 1.0% by mass or less, and particularly preferably 0.5% by mass or less.

The semipermeable membrane is left to dry under a nitrogen atmosphere at 150°C for 4 hours and then weighed. The semipermeable membrane is immersed in DMSO or water and left to dry at 25°C for 24 hours. The semipermeable membrane is removed from the DMSO or water and left to dry under a nitrogen atmosphere at 150°C for 4 hours and then weighed. The mass loss rate is calculated using the following formula based on the mass of the semipermeable membrane before and after immersion.

Mass reduction rate=(mass of semipermeable membrane before immersion−mass of semipermeable membrane after immersion)/mass of semipermeable membrane before immersion×100
The dissolution amount can be reduced by, for example, crosslinking the resin (A), and therefore, even if the protonic acid group equivalent is small, the dissolution amount can be kept within the above range.

(Method for producing semipermeable membrane)

In the present invention, the semipermeable membrane is produced from the aforementioned varnish by a casting method. That is, the varnish is applied to the aforementioned film, and the solvent is removed by evaporation to obtain a semipermeable membrane. Furthermore, the semipermeable membrane can be peeled off from the film to form a free-standing membrane.

If organic solvents or the like remaining in the semipermeable membrane during casting remain, the mechanical strength of the semipermeable membrane may decrease, making it more susceptible to breakage. Therefore, it is preferable to thoroughly dry and/or wash the semipermeable membrane produced by the casting method with water, an aqueous sulfuric acid solution, hydrochloric acid, or the like.

In addition, when the protonic acid group possessed by the resin (A) used to produce the semipermeable membrane is a functional group that easily releases protons, in which a hydrogen atom has been substituted with Na or K, a membrane of the resin (A) may be formed and then the membrane may be brought into contact with an aqueous solution of hydrochloric acid, sulfuric acid, or the like, to substitute the Na or K possessed by the protonic acid group with a hydrogen atom.

A semipermeable membrane, specifically a laminated membrane, in which the concentration of the above-mentioned protonic acid groups has a gradient in the thickness direction of the semipermeable membrane can be obtained, for example, by forming a layer (L1) containing resin (A1) and then forming a layer (L2) containing resin (A2) on the layer (L1). Alternatively, it can also be obtained by forming a layer (L2) containing resin (A2) and then forming a layer (L1) containing resin (A1) on the layer (L2).

It is desirable that the organic compound (S) contained in the varnish used to form the L2 layer be a compound in which the L1 layer is insoluble or poorly soluble.

In the lamination process, any step (e.g., the step of forming the first film, the step of forming the second film to form a laminate structure, etc.) can be subjected to light irradiation such as UV or plasma treatment. It is particularly preferable to irradiate the layer containing resin (A2) with the light.

Furthermore, in a semipermeable membrane formed from a mixture containing at least the above-mentioned resins (A1) and (A2), the dispersion diameter of the phase-separated structure can be adjusted, for example, by adjusting the stirring speed of the mixed resin or varnish.

<Porous base material>
The semipermeable membrane of the present invention is preferably used in combination with a layer of a porous substrate or a mesh material from the viewpoint of handling and the like.

The porous substrate preferably has an air permeability of 100 to 400 ^{cm 3} /cm ² /s, which is measured in accordance with Method A (Fragile type method) of JIS L 1096 as follows.

Attach a 20cm x 20cm test piece to the testing machine, adjust the suction fan and air holes so that the inclined barometer reads 125 Pa, and measure the pressure indicated by the vertical barometer. Calculate the amount of air passing through the test piece using the conversion table provided with the testing machine from the measured pressure and type of air hole.

The thickness of the porous substrate is typically 50 to 700 μm, preferably 80 to 600 μm, and more preferably 100 to 500 μm.

Forward osmosis membranes preferably use thin porous substrates with high air permeability. This suppresses concentration polarization that occurs inside the porous substrate when the forward osmosis membrane is in use, and reduces fluid resistance when water passes through the forward osmosis membrane, thereby increasing the water permeability of the forward osmosis membrane. The air permeability and thickness of the porous substrate can be controlled by conventional methods.

Preferred materials for the porous substrate include synthetic resins and natural fibers.

Synthetic resins include thermoplastic resins and thermosetting resins, with thermoplastic resins being preferred. Specific examples of thermoplastic resins include olefin polymers, polyester resins, polyamide resins, polyvinyl chloride, and (meth)acrylic resins. Of these, olefin polymers and polyester resins are preferred, with olefin polymers being particularly preferred.

Specific examples of olefin polymers include homopolymers or copolymers of α-olefins, or copolymers of α-olefins with other monomers. Examples of α-olefins include α-olefins having 2 to 8 carbon atoms, such as ethylene, propylene, and 1-butene. Olefin polymers include polyolefin resins such as polypropylene, polyethylene, poly-1-butene, poly-4-methyl-1-pentene, ethylene-propylene copolymer, ethylene-1-butene copolymer, ethylene-4-methyl-1-pentene copolymer, propylene-1-butene copolymer, and 4-methyl-1-pentene-1-decene copolymer, as well as copolymers of α-olefins with other monomers, such as ethylene-vinyl alcohol copolymer.

Examples of polyester resins include polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate. Examples of polyamide resins include nylon 6 and nylon 66.

Natural fibers include plant fibers such as cotton and hemp, and animal fibers such as silk and wool. Of these, cotton and silk are preferred.

Examples of the porous substrate include fabric substrates such as woven fabric, nonwoven fabric, and knitted fabric, as well as foam sheets. Of these, fabric substrates are preferred, with woven fabric and nonwoven fabric being more preferred, and nonwoven fabric being particularly preferred.

Examples of nonwoven fabrics include long-fiber nonwoven fabrics made by the spunbond method, short-fiber nonwoven fabrics made by the meltblown method, long-fiber nonwoven fabrics such as flash-spun nonwoven fabrics, spunlace nonwoven fabrics, airlaid nonwoven fabrics, thermal-bonded nonwoven fabrics, needle-punched nonwoven fabrics, and chemical-bonded nonwoven fabrics, with spunbonded nonwoven fabrics and meltblown nonwoven fabrics being preferred. Of these, spunbonded nonwoven fabrics are preferred, with spunbonded nonwoven fabrics made of polyethylene terephthalate or polypropylene being particularly preferred.

The nonwoven fabric may be made of composite fibers made of different resins, and may be a core-sheath or side-by-side composite fiber in which a different resin is arranged on the outer surface of the fiber. Examples of composite fibers include polyethylene/polypropylene core-sheath or side-by-side composite fibers.

The nonwoven fabric may be a laminated nonwoven fabric. Examples of laminated nonwoven fabrics include laminated nonwoven fabrics containing spunbonded nonwoven fabric and meltblown nonwoven fabric, such as a laminated nonwoven fabric of spunbonded nonwoven fabric and meltblown nonwoven fabric (SM), and a laminated nonwoven fabric of spunbonded nonwoven fabric, meltblown nonwoven fabric, and spunbonded nonwoven fabric (SMS), in that order.

To obtain such a laminated nonwoven fabric, a spunbond nonwoven fabric and a meltblown nonwoven fabric are laminated together to form an integrated fabric. Examples of integration methods include a method in which the spunbond nonwoven fabric and the meltblown nonwoven fabric are layered together and heated and pressurized, a method in which the two are bonded together using an adhesive such as a hot melt adhesive or a solvent-based adhesive, or a method in which fibers are deposited on top of the spunbond nonwoven fabric using the meltblown method and then thermally fused.

The nonwoven fabric may be a laminated nonwoven fabric in which a porous film with micropores is sandwiched between layers of nonwoven fabric. Examples of such laminated nonwoven fabrics include a polypropylene (PP) nonwoven fabric, a porous PP film, and a PP nonwoven fabric (SFS) laminated in this order, and a PP nonwoven fabric, a porous PP film, and a rayon PP nonwoven fabric (SFR) laminated in this order.

The weight of the nonwoven fabric (in the case of a laminated nonwoven fabric, the weight of the laminated nonwoven fabric) is preferably about 10 to 80 g/m ² , more preferably about 20 to 40 g/m ² .

The forward osmosis membrane of the present invention may include the porous substrate on both sides of the semipermeable membrane. In this embodiment, the strength of the forward osmosis membrane is further improved.

The forward osmosis membrane of the present invention may include a layer other than the porous substrate on one or both sides of the semipermeable membrane in the order semipermeable membrane/layer other than porous substrate/porous substrate, but in order to obtain good permeability, it is preferable not to include any layer other than the porous substrate. A polyamide layer may be included as a layer other than the porous substrate, but its thickness is preferably 100 μm or less.

<Method of manufacturing a forward osmosis membrane>
The forward osmosis membrane can be produced, for example, by producing the semipermeable membrane as a free-standing membrane and sandwiching the semipermeable membrane between two of the porous substrates from both sides.

Specifically, the forward osmosis membrane can be manufactured by the following procedure. For example, a dilute solution of resin (A) is applied to a substrate such as PET so that the desired thickness is achieved after drying, and the resulting membrane is then peeled off as a free-standing membrane after drying. In one preferred embodiment of the present invention, the light irradiation step or plasma treatment step described above is included in the process from application to formation of the free-standing membrane. Among these, it is also preferable to perform the light irradiation step after the drying step and before the step of peeling off the free-standing membrane.

The present invention is a method for producing a forward osmosis membrane by attaching one or two porous substrates, such as nonwoven fabrics, to one or both sides of the freestanding membrane. Furthermore, the light irradiation step of the present invention may be included in a process for forming a forward osmosis membrane using this porous membrane and the semipermeable membrane, in addition to the membrane formation process described above.

If the area of the forward osmosis membrane is large, the semipermeable membrane and the porous substrate may be bonded together using an adhesive to maintain the distance between them, to the extent that this does not affect permeability.

The forward osmosis membrane can also be produced by producing the semipermeable membrane as a free-standing membrane and laminating it on the porous substrate. In this case, the two may be bonded together using an adhesive.

According to this manufacturing method, the semipermeable membrane described above is first manufactured as a free-standing membrane, which is then laminated on a porous substrate (such as a nonwoven fabric) or sandwiched between two porous substrates (such as a nonwoven fabric) to produce a forward osmosis membrane. This allows for the easy production of a forward osmosis membrane with a controlled semipermeable membrane thickness that exhibits high water permeability and high salt rejection.

[Uses of forward osmosis membranes]
The forward osmosis membrane element of the present invention comprises the forward osmosis membrane of the present invention and a spacer. In other words, the forward osmosis membrane element of the present invention is a conventional forward osmosis membrane element comprising a forward osmosis membrane and a spacer, but using the forward osmosis membrane of the present invention as the forward osmosis membrane. As the spacer, a conventionally known spacer used in forward osmosis membrane elements can be used.

The forward osmosis membrane module of the present invention comprises a forward osmosis membrane element of the present invention housed in a container. In other words, the forward osmosis membrane module of the present invention is a conventional forward osmosis membrane element in which a forward osmosis membrane element is housed in a container, but in which the forward osmosis membrane element of the present invention is used as the forward osmosis membrane element. Any conventional container known for use in forward osmosis membrane modules can be used as the container.

The system of the present invention includes a forward osmosis membrane module of the present invention and a drive pump for passing a feed solution (FS) on one side of the forward osmosis membrane of the present invention included in the forward osmosis membrane module, and a draw solution (DS) having a higher concentration of solutes such as salts than the feed solution on the other side, and is configured to transfer water contained in the feed solution to the draw solution through the forward osmosis membrane.

With such an apparatus, it is also possible to apply pressure from the FS side to the DS side. In some embodiments of the present invention, it may be preferable to apply pressure from the FS side to the DS side. For example, it may be preferable to apply a pressure of preferably 15 kPa or less, more preferably 12 kPa or less, and even more preferably 10 kPa or less from the FS side to the DS side.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.
The abbreviations used in the present examples and comparative examples are as follows:

(1) Solvents DMSO: dimethyl sulfoxide NMP: N-methyl-2-pyrrolidone DMF: N,N-dimethylformamide

(2) Components of aromatic polyether DFBP: 4,4'-difluorobenzophenone DSDFBP: 5,5'-carbonylbis(sodium 2-fluorobenzenesulfonate)
TMBPF: 3,3',5,5'-tetramethyl-4,4'-dihydroxydiphenylmethane

Various measurement methods (contact angle measurement of film surface)
The contact angle of each substrate surface used in the examples and comparative examples was measured using the following device. One drop (approximately 0.5 ml) of the following test solvent (mixed solvent) was dropped onto the surface of the following sample (substrate) using a syringe dedicated to the device and left to stand for 1 minute. The contact angle of the droplet with each substrate surface was then measured to evaluate the wettability of the film surface. The results are shown in Table 2.
Measurement device: DM500 type device manufactured by KYOWA Test solvent: mixed solvent of DMF and toluene (weight ratio 55:45. Varnish solvent used in Examples and Comparative Examples)
Samples: Film 1 used in the examples and Film 2 used in the comparative examples (described later)

(Peel strength measurement)
In the cast molding of the Examples and Comparative Examples, the 180° peel strength was measured when peeling the semipermeable membrane from the substrate film. The measurement was carried out using the following equipment and conditions. The results are shown in Table 3.
Measuring device: Strograph E-S type device manufactured by TOYOSEIKI Measuring speed: 50 mm/min
Measurement range: 2.5N
Sample size: Width 15 mm, length approximately 6 cm
Number of measurements N: 3 (average value was used as the measured value)

(Surface observation and roughness measurement of semipermeable membrane peeling surface)
The surface condition of the semipermeable membrane peeled from the substrate was visually evaluated, and the surface roughness was measured. The surface roughness was measured using a VS1800 model device manufactured by Hitachi High-Tech Corporation in Wave mode. The results are shown in Table 4.

(Method for measuring film thickness)
Measurements were carried out using a Nikon Digimicro MH-15M (contact film thickness measuring device: minimum reading: 0.01 μm) according to the following method. The results are shown in Table 5.
After the varnish was applied to the substrate and dried, i.e., at the stage of the "substrate/coating film" laminate, a portion was cut out as a sample for thickness measurement. The thickness was measured using the above-mentioned device (thickness A (μm)). Next, the film at the measurement point was peeled off from the substrate, and the thickness of the substrate at that point was measured (thickness B (μm)).

From the above results, the value was calculated using the formula "Film Thickness = Thickness A - Thickness B." Measurements were taken at five or more locations, and the average value was determined as the film thickness. For semipermeable membranes with a laminated structure of two or more layers, the above procedure was repeated for each layer formed (average value of five or more measurement locations). The film thickness of each layer was determined from the difference between the individual measurement values.

(Performance evaluation of forward osmosis membrane)
The performance of a forward osmosis membrane was evaluated using an apparatus 10 as shown in FIG. 1 by the following method. The apparatus 10 includes a feed solution tank 1, a flow path 2, a pump 3, a draw solution tank 11, a flow path 12, a pump 13, and an evaluation cell 21. The feed solution tank 1 is placed on a balance 4, and the draw solution tank 11 is equipped with an electrical conductivity meter 15. At the start of the evaluation, the feed solution tank 1 contains 500 g of Milli-Q water having an electrical conductivity of 50 μS/cm or less as a feed solution FS (feed solution), and the draw solution tank 11 contains 800 g of a 0.6 M aqueous ammonium sulfate solution as a draw solution DS (draw solution). The feed solution in the feed solution tank 1 flows through flow path 2 at a rate of 0.6 L/min using pump 3, and the draw solution in the draw solution tank 11 flows through flow path 12 at a rate of 0.6 L/min using pump 13. The feed solution flowing through flow path 2 and the draw solution flowing through flow path 12 are in contact with each other inside evaluation cell 21 via forward osmosis membrane 22 with an effective membrane area of 0.0042 ^m2 . Through forward osmosis membrane 22, a portion of the feed solution moves into the draw solution, and a portion of the salt (NaCl) in the draw solution moves (backflows) into the feed solution.

<Water permeation flux (Jw)>
The weight loss of the feed solution was measured every minute using the balance 4, and the weight loss per unit time and per unit area of the forward osmosis membrane was defined as the water permeation flux (L/( ^m2 ·h)). The results are shown in Table 5.

<Salt Reverse Diffusion Permeability Factor (SRSF)>
The change in electrical conductivity of the draw solution was measured every minute using the electrical conductivity meter 15, and this was converted into the change in weight of the draw solution. The change in weight per unit time per unit area of the forward osmosis membrane was defined as the salt permeation rate (g/( ^m2 ·h)). This value was then divided by the water permeation flux (Jw) to calculate the SRSF value (g/L). The results are shown in Table 5.

(Resin Synthesis Example 1)
40.1 g (0.095 mol) of D- SDFBP, 62.2 g (0.285 mol) of DFBP, 97.4 g (0.380 mol) of TMBPF, and 65.7 g (0.475 mol) of potassium carbonate were weighed into a five-neck reactor equipped with a nitrogen inlet tube, a thermometer, a reflux condenser, and a stirrer. 783.4 g of DMSO and 261.1 g of toluene were added, and the mixture was stirred under a nitrogen atmosphere and heated at 130°C for 12 hours. The generated water was removed from the system, and the toluene was then distilled off.

The reaction was continued at 160°C for 12 hours, yielding a viscous polymer solution. The resulting solution was diluted with 570 g of toluene and then poured into 2,400 g of methanol. The precipitated polymer powder was filtered, washed, and dried at 150°C for 4 hours to yield 171.6 g of polyether ketone powder (Resin 1). Resin (1) contains the structural unit (1):

As the structural unit (2),

The content of the structural unit (1) is 25 mol % (where the total of the structural unit (1) and the structural unit (2) is 100 mol %, the same applies hereinafter).

(Resin Synthesis Example 2)
The resin raw materials and solvent were 48.1 g (0.114 mol) of DSDFBP, 58.0 g (0.265 mol) of DFBP, 97.4 g (0.380 mol) of TMBPF, 65.7 g (0.475 mol) of potassium carbonate, and 814.4 g of DMSO and 271.5 g of toluene. Except for this, the procedure of Example 1 was repeated to obtain 175.2 g of polymer powder (Resin 2). Resin 2 contained 30 mol% of the structural unit (1).

(Resin Synthesis Example 3)
The resin raw materials and solvent were 80.2 g (0.190 mol) of DSDFBP, 41.5 g (0.190 mol) of DFBP, 97.4 g (0.380 mol) of TMBPF, and 65.7 g (0.475 mol) of potassium carbonate, and 876.4 g of DMSO and 292.1 g of toluene. Except for this, 174.6 g of polymer powder (Resin 3) was obtained in the same manner as in Resin Synthesis Example 1. Resin 3 had a content of the structural unit (1) of 50 mol%.

(Resin Synthesis Example 4)
The resin raw materials and solvent were 96.3 g (0.228 mol) of DSDFBP, 33.2 g (0.152 mol) of DFBP, 97.4 g (0.380 mol) of TMBPF, and 65.7 g (0.475 mol) of potassium carbonate, and 907.5 g of DMSO and 302.5 g of toluene. Except for this, 173.6 g of polymer powder (Resin 4) was obtained in the same manner as in Resin Synthesis Example 1. Resin 4 had a content of the structural unit (1) of 60 mol%.

The constituent raw materials and sulfonic acid unit content of the polymers synthesized in Resin Synthesis Examples 1 to 4 are shown in Table 1.

[Example 1]
Resin 1 produced in Resin Synthesis Example 1 was dissolved in a mixed solvent of DMF and toluene (DMF/toluene = 55/45 (weight ratio)) to prepare a varnish, which was then cast onto a PET substrate ("PLD38TH" manufactured by Lintec Corporation; Film 1) having a release agent layer and dried at 110°C for 3 minutes to obtain a film of Resin 1. A portion of this film was peeled off from Film 1, and the appearance and surface roughness of the peeled surface were measured. The results are shown in Table 4.

Next, Resin 3 produced in Resin Synthesis Example 3 was dissolved in a methyl cellosolve solvent to prepare a varnish. This varnish was cast onto the Resin 1 film and dried at 160°C for 3 minutes to obtain a two-layer film, which was then peeled off from Film 1 to form a free-standing film, yielding Film 1.

[Example 2]
Resin 1 produced in Resin Synthesis Example 1 was dissolved in a mixed solvent of DMF and toluene (DMF/toluene = 55/45 (weight ratio)) to prepare a varnish, which was then cast onto Film 1 and dried at 110°C for 3 minutes to obtain a film. A portion of this film was peeled off from Film 1, and the appearance and surface roughness of the peeled surface were measured. The results are shown in Table 4.

Next, Resin 4 produced in Resin Synthesis Example 4 was dissolved in a methyl cellosolve solvent to prepare a varnish. This varnish was cast onto the Resin 1 film and dried at 160°C for 3 minutes to obtain a two-layer film, which was then peeled off from Film 1 to form a free-standing film, yielding Film 2.

[Example 3]
Resin 2 produced in Resin Synthesis Example 2 was dissolved in a mixed solvent of DMF and toluene (DMF/toluene = 55/45 (weight ratio)) to prepare a varnish, which was then cast onto Film 1 and dried at 110°C for 3 minutes to obtain a film of Resin 2. A portion of this film was peeled off from Film 1, and the appearance and surface roughness of the peeled surface were measured. The results are shown in Table 4.

Next, Resin 3 produced in Resin Synthesis Example 3 was dissolved in a methyl cellosolve solvent to prepare a varnish. This varnish was cast onto the film of Resin 2 and dried at 160°C for 3 minutes to obtain a two-layer film, which was then peeled off from Film 1 to form a free-standing film, yielding Film 3.

[Example 4]
The resin produced in Resin Synthesis Example 2 was dissolved in a mixed solvent of DMF and toluene (DMF/toluene = 55/45 (weight ratio)) to prepare a varnish, which was then cast onto Film 1 and dried at 110°C for 3 minutes to obtain a film of Resin 2. A portion of this film was peeled off from Film 1, and the appearance and surface roughness of the peeled surface were measured. The results are shown in Table 4.

Next, Resin 4 produced in Resin Synthesis Example 4 was dissolved in a methyl cellosolve solvent to prepare a varnish. This varnish was cast onto the Resin 2 film and dried at 160°C for 3 minutes to obtain a two-layer film, which was then peeled off from Film 1 to form a free-standing film, yielding Film 4.

[Preparation of forward osmosis membrane]
Two sheets of polypropylene nonwoven fabric ("Syntex (registered trademark) PS-105" manufactured by Mitsui Chemicals, Inc.) having an air permeability of 300 m ³ /cm ² /s as measured by Method A (Fragile method) according to JIS L 1096, a thickness of 290 μm were prepared, and the membranes 1 to 4 were sandwiched between these nonwoven fabrics and integrated to obtain forward osmosis membranes 1 to 4. The evaluation results of Examples 1 to 4 are shown in Table 5.

[Comparative Example 1]
As in Example 1, Resin 1 produced in Resin Synthesis Example 1 was dissolved in a mixed solvent of DMF and toluene (DMF/toluene = 55/45 (weight ratio)) to prepare a varnish, which was then cast onto a smooth PET substrate (Film 2) without a release layer and dried at 110°C for 3 minutes to obtain a film. A portion of this film was peeled off from Film 2, and the appearance and surface roughness of the peeled surface were measured. The results are shown in Table 4.

Next, Resin 3 produced in Resin Synthesis Example 3 was dissolved in a methyl cellosolve solvent to prepare a varnish. This varnish was cast onto the membrane and dried at 160°C for 3 minutes to obtain a two-layer membrane. This was then peeled off from Film 2 to form a free-standing membrane, which was then sandwiched between two sheets of nonwoven fabric in the same manner as above to obtain forward osmosis membrane C1. The evaluation results are shown in Table 5.

The above-mentioned film 2 is the same film as the release film used in the examples of publications relating to forward osmosis membrane inventions, such as WO 2017/079733, WO 2019/189547, WO 2019/189548, and WO 2020/032019.

[Comparative Example 2]
In the same manner as in Example 2, Resin 1 produced in Resin Synthesis Example 1 was dissolved in a mixed solvent of DMF and toluene (DMF/toluene = 55/45 (weight ratio)) to prepare a varnish, which was then cast onto Film 2 and dried at 110°C for 3 minutes to obtain a film. A portion of this film was peeled off from Film 2, and the appearance and surface roughness of the peeled surface were measured. The results are shown in Table 4.

Next, Resin 4 produced in Resin Synthesis Example 4 was dissolved in a methyl cellosolve solvent to prepare a varnish. This varnish was cast onto the membrane and dried at 160°C for 3 minutes to obtain a two-layer membrane. This was then peeled off from Film 2 to form a free-standing membrane, which was then sandwiched between two sheets of nonwoven fabric in the same manner as above to obtain forward osmosis membrane C2. The evaluation results are shown in Table 5.

[Comparative Example 3]
In the same manner as in Example 3, Resin 2 produced in Resin Synthesis Example 2 was dissolved in a mixed solvent of DMF and toluene (DMF/toluene = 55/45 (weight ratio)) to prepare a varnish, which was then cast onto Film 2 and dried at 110°C for 3 minutes to obtain a film. A portion of this film was peeled off from Film 2, and the appearance and surface roughness of the peeled surface were measured. The results are shown in Table 4.

Next, Resin 3 produced in Resin Synthesis Example 3 was dissolved in a methyl cellosolve solvent to prepare a varnish. This varnish was cast onto the membrane and dried at 160°C for 3 minutes to obtain a two-layer membrane. This was then peeled off from Film 2 to form a free-standing membrane, which was then sandwiched between two sheets of nonwoven fabric in the same manner as above to obtain forward osmosis membrane C3. The evaluation results are shown in Table 5.

[Comparative Example 4]
In the same manner as in Example 4, Resin 2 produced in Resin Synthesis Example 2 was dissolved in a mixed solvent of DMF and toluene (DMF/toluene = 55/45 (weight ratio)) to prepare a varnish, which was then cast onto Film 2 and dried at 110°C for 3 minutes to obtain a film. A portion of this film was peeled off from Film 2, and the appearance and surface roughness of the peeled surface were measured. The results are shown in Table 4.

Next, Resin 4 produced in Resin Synthesis Example 4 was dissolved in a methyl cellosolve solvent to prepare a varnish. This varnish was cast onto the membrane and dried at 160°C for 3 minutes to obtain a two-layer membrane. This was then peeled off from Film 2 to form a free-standing membrane, which was then sandwiched between two sheets of nonwoven fabric in the same manner as above to obtain forward osmosis membrane C4. The evaluation results are shown in Table 5.

The measurement results in Table 2 show that Film 2 used in Comparative Examples 1 to 4 had a contact angle with the solvent (a mixed solvent of DMF and toluene) of less than 10°, while Film 1 used in Examples 1 to 4 had a contact angle with the solvent (a mixed solvent of DMF and toluene) of 10° or more.

The measurement results in Table 3 show that Examples 1 to 4 had lower peel strengths than Comparative Examples 1 to 4. This is thought to be because there was less damage to the film when it was peeled off, leading to improvements in surface roughness and appearance, as described below.

The results in Table 4 show that the film surfaces of Examples 1 to 4 were visually more uniform than those of Comparative Examples 1 to 4. Furthermore, the surface roughness (SA value) of the Examples was 10 nm or less, a lower value than that of the Comparative Examples (greater than 10 nm).

From the results in Table 5, it can be said that the forward osmosis membrane obtained by the forward osmosis membrane manufacturing method of the present invention has an excellent balance between permeation flux and suppression of salt back-diffusion.

REFERENCE SIGNS LIST 1 Feed solution tank 2, 12 Flow path 3, 13 Pump 4, 14 Balance 10 Evaluation device 11 Draw solution tank 15 Electrical conductivity meter 21 Evaluation cell 22 Forward osmosis membrane DS Draw solution FS Feed solution

Claims (3)

an aromatic structure;
a protonic acid structure;
a step of casting a varnish obtained by dissolving a resin (A) containing a structure selected from an ether structure (—O—) and a ketone structure (—CO—) in an organic compound (S) onto a film made of an organic material, the film having a surface with a contact angle of the organic compound (S) of 10° to 40 °, and then drying the film ;
The method for producing a forward osmosis membrane that is a self-supporting membrane, wherein the resin (A) has a structural unit (1) represented by the following formula (1) and a structural unit (2) represented by the following formula (2) :
[In formulas (1) and (2),
R ¹ to R ¹⁰ each independently represent H, Cl, F, CF ₃ or C _m H _2m+1 (m represents an integer of 1 to 10);
At least one of R ¹ to R ¹⁰ is C _m H _2m+1 (m is an integer of 1 to 10),
Two or more of each of R ¹ to R ¹⁰ may be present in an aromatic ring, and when two or more C _m H _2m+1 groups are present in one aromatic ring, the C _m H _2m+1 groups may be the same as or different from each other.
X ¹ to X ⁵ are each independently H, Cl, F, CF ₃ or a protonic acid group;
At least one of X ¹ to X ⁵ is a protonic acid group;
Two or more of each of X ¹ to X ⁵ may be present in an aromatic ring, and when two or more protonic acid groups are present in one aromatic ring, the respective protonic acid groups may be the same or different.
A ¹ to A ⁶ each independently represent a direct bond, —CH ₂ —, —C(CH ₃ ) ₂ —, —C(CF ₃ ) ₂ —, —O— or —CO—, and at least one of A ¹ to A ⁶ is —O— or —CO—.
i, j, k, and l each independently represent 0 or 1. 2. The method for producing a forward osmosis membrane according to claim 1, wherein the resin (A) comprises a protonic acid group- containing resin (A1) and a resin (A2) having a molar fraction of protonic acid group-containing structural units different from that of the resin (A1). 3. The method for producing a forward osmosis membrane according to claim 2 , wherein the forward osmosis membrane is a laminate including a layer containing the resin (A1) and a layer containing the resin (A2).