JPS63194701A - Composite membrane - Google Patents

Abstract

PURPOSE:To obtain the title membrane free of pinholes by forming a thin membrane consisting of a cross-linked cellulosic resin on the surface of the dense layer of a porous membrane. CONSTITUTION:A resin soln. is coated on the surface of the dense layer of the porous membrane, the org. solvent is evaporated, and the cellulosic rein is simultaneously cross-linked by a cross-linking agent to form the thin cross- linked cellulose membrane. The thin membrane is repeatedly formed, and a cross-linked thin layer free of pinholes can be obtained. Although the thickness of the cross-liked thin layer of the cellulosic resin depends on the concn. of the resin soln and the thickness of the this layer of the resin soln. formed o the dense layer, the thickness is usually controlled to 0.05-10mum. When the cross-linked membrane is excessive thickened, the gas permeability is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a composite membrane, and more particularly, to a composite membrane that can be suitably used for gas concentration separation treatment.

Dependency boiling generally involves selectively concentrating specific components in a gas mixture,
In recent years, separation methods using thin films made of polymers have attracted attention. For example, thin films made of cellulose acetate such as diacetate and triacetate have excellent separation properties for certain gases. It has been known.

However, in general, thin films made of cellulose resin have low mechanical strength, and therefore, for practical use in membrane separation, it is necessary to increase the film thickness to about 100 μm or more. However, since the permeability of a membrane is inversely proportional to the membrane thickness, increasing the membrane thickness reduces the permeability, which determines economic efficiency in membrane separation, even if the membrane itself has a large permeability coefficient for gas.

In order to solve these problems, a thin film made of cellulose acetate has already been formed on a porous substrate made of polyimide, for example, to supplement the mechanical strength of the cellulose acetate film and to reduce its thickness. It has been proposed to increase the gas permeability. However, even in such a composite film, if the thickness of the thin film is reduced, a decrease in mechanical strength is unavoidable, and pinholes are likely to occur in the thin film.

5 Points to be Solved by the Invention The present invention was made to solve the above-mentioned various problems in composite membranes. An object of the present invention is to provide a composite membrane that includes a thin membrane made of resin and can be suitably used for gas separation, particularly for the separation of carbon dioxide and methane.

Means for Solving Problem 2 The composite membrane according to the present invention has a porous membrane having an anisotropic structure in which a dense layer on the surface is integrally supported by a continuous porous layer. , is characterized in that a thin film made of crosslinked cellulose resin is formed.

Such a composite membrane consists of a dense layer of a porous membrane having an anisotropic structure, in which a dense layer on the surface is integrally supported by a continuous porous layer; Alternatively, a solution obtained by dissolving a cellulose resin having an average molecular weight larger than this and a crosslinking agent for crosslinking the cellulose resin in an organic solvent is brought into contact with each other, and then the organic solvent is evaporated and removed to form the porous structure. It is manufactured by forming a thin film of crosslinked cellulose resin on a dense layer of the membrane.

In the present invention, the cellulose resin generally includes cellulose, which is a type of polysaccharide having a composition formula represented by C6H100S, cellulose ester derivatives such as nitrocellulose, acetyl cellulose, cellulose acetate propionate, and cellulose acetate butyrate. , xanthate derivatives such as cellulose dithiocarbonate, methylcellulose, ethylcellulose,
Cellulose ether derivatives such as hydroxyethylcellulose, carboxymethylcellulose, methylhydroxyethylcellulose, ethylhydroxyethylcellulose, methylhydroxypropylcellulose, cyanoethylcellulose, and benzylcellulose can be used. However, among these, cellulose acetate and ethyl cellulose can be particularly preferably used.

As will be described later, the composite membrane according to the present invention includes a cellulose resin as described above and a crosslinking agent for crosslinking the cellulose resin on the dense layer of a porous membrane having a dense layer as a base membrane. By contacting a solution formed by dissolving in a solvent, and then preferably evaporating and removing the organic solvent by heating, and crosslinking the cellulose resin on the dense layer of the porous membrane to form a thin film. Obtainable.

As the crosslinking agent, a bifunctional or more polyfunctional compound or polymer having a functional group that reacts with the hydroxyl group, ether group, ester group, etc. contained in the cellulose resin is used, depending on the cellulose resin. Suitable examples of the above-mentioned functional groups include amino groups, isocyanate groups, epoxy groups, acid halogen groups (halocarbonyl groups), etc. Therefore, in the present invention, one or more of these functional groups Any compound or polymer having the above may be used. However, diamines, diisocyanates, epoxy resins, etc. are particularly preferably used.

In order to produce the composite membrane according to the present invention, an organic solution (hereinafter simply referred to as resin solution) prepared by dissolving a cellulose resin and a crosslinking agent for crosslinking the cellulose resin is added to the porous membrane as described above. Although it is applied onto the dense layer, it is preferable that the crosslinking agent does not crosslink the cellulose resin at the time of application, but crosslinks it by heating. That is, if the cellulose resin and the crosslinking agent react at room temperature, the resin solution may gel or the crosslinked resin may precipitate when the resin solution is prepared or applied onto the dense layer of the porous membrane. In such a case, a homogeneous thin film with excellent gas separation properties cannot be formed.

In the present invention, the cellulose resin does not need to be crosslinked through all of its functional groups, and it is sufficient that it is crosslinked to the extent that it does not dissolve in an organic solvent. The degree of crosslinking of the cellulose resin can be determined by selecting the amount of the cellulose resin and the crosslinking agent in the resin solution, the heating temperature, and the like. When using diamine, diisocyanate or acid halide as the crosslinking agent, for example, 0.1 to 50 parts by weight, preferably 1 to 50 parts by weight of the crosslinking agent per 100 parts by weight of cellulose resin.
It is used in a range of 20 parts by weight.

When producing the composite membrane according to the present invention, when the resin solution containing the cellulose resin is brought into contact with the dense layer of the porous membrane, if the cellulose resin permeates into the interior of the homogeneous membrane, the obtained This may reduce the permeability of the composite membrane. Therefore, it is preferable that the average molecular weight of the cellulose resin used is equal to or larger than the molecular weight cutoff of the porous membrane used. and is selected to satisfy the above conditions, usually 700
Cellulose resins having an average molecular weight of 00 to aooooo are preferred. In addition, in the present invention, the average molecular weight is
It refers to the weight average molecular weight determined by GPC method.

In the present invention, the organic solvent for forming the resin solution containing dissolved cellulose resin and crosslinking agent is as follows:
For example, ketone solvents, ether solvents, ester solvents, cellosolve solvents, chlorinated hydrocarbon solvents, or mixtures thereof are preferably used. More specifically,
For example, acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, tetrahydrofuran,
Dioxane, methyl acetate, ethyl lactate, methyl cellosolve acetate, methyl cellosolve, tetrachloroethane, methylene chloride, acetone-methanol mixed solvent, ethyl acetate-ethanol mixed solvent, methylene chloride-methanol mixed solvent, etc. are used.

However, it is not limited to these.

A composite membrane according to the present invention is obtained by bringing a resin solution containing such a solvent into contact with the surface of the dense layer of the anisotropic membrane, evaporating the solvent, and crosslinking the cellulose resin. A resin solution may be applied to the surface of the dense layer, or the surface of the dense layer may be immersed in the resin solution. Also,
It is also possible to use a spin code method.

In the present invention, the anisotropic porous membrane used as the base material is one that does not dissolve in the solvent of the resin solution. Therefore, for example, an anisotropic porous membrane made of fluororesin, polyimide resin, polyamide resin, polyamideimide resin, etc. is preferably used. Further, the shape of the porous membrane is not limited in any way, and may be arbitrary, such as a flat membrane, a cylindrical shape, a fiber shape, etc.

In the present invention, the polyimide resin that can be particularly suitably used as the anisotropic porous membrane is substantially composed of polyimide resins represented by the following general formula (wherein R1 is a divalent organic group). Become. Its intrinsic viscosity [η] (30℃
Measured as a N-methyl-2-pyrrolidone solution in
same as below. ) is preferably in the range of 0.5 to 2. This is because if the intrinsic viscosity is too small, the film-forming ability will be poor, while if it is too large, it will be difficult to dissolve in a solvent and the film-forming workability will be poor. As is already known, this anisotropic membrane made of polyimide resin has an anisotropic structure in which a dense layer on the surface is integrally supported by a continuous porous layer. 1972-
According to the wet membrane forming method described in No. 152507 and JP-A-56-139104, an anisotropic membrane having ultrafiltration properties can be obtained.

In the above general formula, R1 is a divalent organic group, and is preferably a hydrocarbon group, for example, a hydrocarbon group containing an ether bond, a sulfone group, or the like.

In the present invention, the anisotropic membrane used as the base material preferably has a molecular weight cut-off of 1,000 to 100,000. In the case of an anisotropic membrane with a molecular weight cut-off of less than 1000, the permeability of the resulting composite membrane may be sufficiently high (
On the other hand, when the molecular weight cutoff is greater than 100,000,
This is because, as will be described later, the cellulose resin solution may permeate into the porous membrane during the formation of the crosslinked cellulose resin thin film, reducing the permeability of the resulting composite membrane.

Here, the molecular weight fraction can be determined by measuring the exclusion rate of an anisotropic membrane for a solute with a known molecular weight, and in the present invention, polyethylene glycol having a monodisperse molecular weight distribution is used as a solute and ppm f
A toluene solution containing M degrees was heated at a temperature of 25°C and a pressure of 4k.
The molecular weight of polyethylene glycol that is supplied to the membrane surface at a rate of g/cIiW and has an exclusion rate of at least 90% is defined as the molecular weight cutoff of the membrane. An anisotropic membrane having such a molecular weight cut-off is generally referred to as an ultrafiltration membrane in liquid separation, so in the present invention, it may also be referred to as an ultrafiltration membrane instead of an anisotropic membrane.

It is desirable that the cellulose resin thin film thus formed on the surface of the dense layer of the anisotropic film be as thin as possible.

That is, as described above, the gas permeability of the resulting composite membrane is substantially controlled by the thickness of the thin film, and the thinner the membrane, the greater the gas permeability.

As described above, the composite membrane of the present invention is produced by applying a resin solution to the surface of the dense layer of the porous membrane, preferably heating it to evaporate the organic solvent, and crosslinking the cellulose resin with a crosslinking agent. This can be obtained by forming a crosslinked cellulose thin film, and in particular, by repeatedly forming such a thin film, a pinhole-free crosslinked thin film can be obtained. Therefore, it is desirable that the cellulose resin solution is relatively dilute, and usually the cellulose resin concentration is 0.01 to 20%, preferably 0.
．． The amount is adjusted to 0.05 to 10% by weight. When the cellulose resin concentration is less than 0.01% by weight, pinholes may occur even if cross-linked thin films are repeatedly formed; on the other hand, when it is higher than 20% by weight, the thin film formed The thickness of the cross-linked film is too large, especially when forming cross-linked thin films repeatedly, and the cross-linked film becomes unnecessarily thick.
Practical gas permeability cannot be obtained.

In this way, the crosslinked thin film of cellulose resin formed on the surface of the dense layer of the anisotropic porous membrane depends on the concentration of the resin solution and the thickness of the thin layer of resin solution formed on the dense layer. is usually preferably 0.05 to 10 μm,
Moreover, according to the present invention, a crosslinked film having such a film thickness can be easily formed.

As mentioned above, the composite film of the present invention has a thin film made of a crosslinked cellulose resin formed on an anisotropic film. It has excellent resistance to organic solvents and selective separation of gases, and can be suitably used for concentrating and separating various gases.

In particular, according to the present invention, by repeatedly forming a crosslinked thin film of cellulose resin on a porous membrane, a composite membrane with no pinholes, excellent strength, and excellent selective separation of gases can be obtained. be able to.

However, the composite semipermeable membrane according to the present invention is not limited in its use, and it goes without saying that it can be used for separation treatment of aqueous liquid mixtures.

EXAMPLES The present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples in any way.

Example 1 Polyimide whose repeating unit is (intrinsic viscosity [η] is 0.91 dl/g
) 22% by weight and diethylene glycol 22% by weight.
A film was formed according to the conventional phase transformation method described in Publication No. 152507, and the thickness was 200 μm and the molecular weight cutoff was 2000.
An ultrafiltration membrane with an anisotropic structure of 0 was prepared.

Using this ultrafiltration membrane, the permeability of carbon dioxide and methane was measured by the high vacuum method at a pressure difference of 0.2 atm and a temperature of 25°C, and the permeability of carbon dioxide was 6.44.
X 10-”cc (STP) / ctA・sec・cm
The permeability of Hg and methane is 9.84 x 10-”cc (
STP) / ctA ・sec・cmHg, which is the separation coefficient α calculated from carbon dioxide permeability/methane permeability
(CO□/CHt) was 0.65.

Separately, 4.0 parts by weight of cellulose diacetate with an average molecular weight of 170,000 was dissolved in 96.0 parts by weight of methyl cellosolve acetate, and further, 10 parts by weight of trimesoyl chloride was added to 100 parts by weight of the cellulose diacetate, A resin solution was prepared by dissolving.

The resin solution is uniformly applied to the surface of the dense layer of the polyimide ultrafiltration membrane using an applicator to a coating thickness of 45 μm, and then left in a dryer at 110° C. for 1 hour to evaporate the solvent. A thin film made of crosslinked cellulose diacetate was formed on the surface of the dense layer of a polyimide ultrafiltration membrane to obtain a composite membrane according to the present invention. When this composite film was observed using an electron microscope, the thickness of the diacetate thin film was 2.2 μm.

Regarding this composite membrane, the permeability of carbon dioxide and methane was measured under the same conditions as above, and the results were 2.
0 x 10-'cc (STP) / c+a-second・C
mHg and 9.8 X 10-8cc (STP) /
cf-sec・CmHg, separation coefficient α (CO□/C)lt obtained from carbon dioxide permeability/methane permeability
) was 21.

Example 2 Ethylcellulose (ETCEL 5TD manufactured by Dow Chemical Company)
-100) 4.0 parts by weight are dissolved in 96.0 parts by weight of acetone, and further, 20 parts by weight of tolylene diisocyanate is added as a crosslinking agent to 100 parts by weight of the above ethyl cellulose, and dissolved to form a resin solution. Prepared.

Using this resin solution, a composite membrane was obtained in the same manner as in Example 1. The thickness of the cross-linked ethyl cellulose thin film in this composite membrane is 3.1 μm, and the permeability of carbon dioxide and methane is 1.2X 10''cc, respectively.
(STP) / c+a ・sec・cml (g and 6.
OX 10-7cc (STP) /cJ-sec・cmlg
The separation coefficient αb (002/C114) was 20.

Example 3 Cellulose acetate with an average molecular weight of 170,000 was dissolved in methyl cellosolve acetate to prepare a solution having a resin concentration shown in Table 1, and further, 10 parts by weight of trimesoyl chloride was added to 100 parts by weight of this cellulose acetate solution. A resin solution was prepared by adding and dissolving the resin.

This resin solution was applied to the surface of the dense layer of the same polyimide ultrafiltration membrane as described above using an applicator, and then the cellulose resin was crosslinked to form a thin film in the same manner as in Example 1. This thin film formation was repeated two more times to obtain a composite film according to the present invention.

For each composite membrane obtained in this way, the thickness of the crosslinked resin thin film, the permeability of carbon dioxide, and the separation coefficient α
(C02/C114) is shown in Table 1.

Comparative Example 1 A composite membrane was prepared in the same manner as in Example 1 without adding a crosslinking agent to the 1.8 wt% methyl acetate cellulose solution b of the same cellulose diacetate used in Example 1. did.

The carbon dioxide permeability and separation coefficient α of this composite membrane are 8.8 x 10-5 (cc (STP)/cm), respectively.
H seconds cmHg) and 1.3, thus,
The reason for the significantly lower separation factor is that the composite membrane has a large number of pinholes.

Claims (3)

[Claims] (1) A thin film made of cross-linked cellulose resin is formed on the surface of the dense layer of a porous membrane having an anisotropic structure in which the dense layer on the surface is integrally supported by a continuous porous layer. A composite membrane characterized by: (2) Molecular weight cutoff of the porous membrane is 1000 to 100000
The composite membrane according to claim 1, wherein the composite membrane is within the range of . (3) The composite membrane according to claim 1, wherein the average molecular weight of the cellulose resin is equal to or larger than the molecular weight cutoff of the porous membrane.