Membrane-forming dope for carbon membranes and a method for producing a carbon hollow fiber membrane using the same

Abstract

A membrane-forming dope for carbon membranes, comprising polyphenylene oxide in an amount giving a concentration of 15 to 40 wt. % in the membrane-forming dope, and sulfur in an amount giving a ratio of 0.1 to 5.0 wt. %, preferably 0.2 to 3.0 wt. %, of the total weight of the polyphenylene oxide and the sulfur, both of which are dissolved in a solvent that can dissolve these components. A hollow fiber carbon membrane is produced by molding the membrane-forming dope for carbon membranes in a hollow shape by means of a wet or dry-wet spinning method using a double tubular nozzle, subjecting the molded product to an infusibilization treatment by heating at 150 to 350? C. in the air, and then subjecting it to a carbonization treatment by heating at 600 to 800? C. in an inert atmosphere or under vacuum. When the product molded in a hollow shape by means of a wet or dry-wet spinning method is subjected to an infusibilization treatment by heating in the air while stretching the product with a stress of 0.002 to 0.005 MPa, a hollow fiber carbon membrane having excellent gas permeability and a further high gas separation factor (He/CH.sub.4) can be obtained.

Claims

1. A membrane-forming dope for carbon membranes, comprising polyphenylene oxide in an amount giving a concentration of 15 to 40 wt. % in the membrane-forming dope, and sulfur in an amount giving a ratio of 0.1 to 5.0 wt. % of the total weight of the polyphenylene oxide and the sulfur, both of which are dissolved in a solvent that can dissolve these components.

2. The membrane-forming dope for carbon membranes according to claim 1, further comprising an organic peroxide, a phenol resin crosslinking agent, or a quinone dioxime crosslinking agent.

3. A method for producing a hollow fiber carbon membrane, comprising molding the membrane-forming dope for carbon membranes according to claim 1 in a hollow shape by means of a wet or dry-wet spinning method using a double tubular nozzle, subjecting the molded product to an infusibilization treatment by heating at 150 to 350? C. in the air, and then subjecting it to a carbonization treatment by heating at 600 to 800? C. in an inert atmosphere or under vacuum.

4. The method for producing a hollow fiber carbon membrane according to claim 3, wherein the product molded in a hollow shape by means of a wet or dry-wet spinning method is subjected to an infusibilization treatment by heating in the air while stretching the product with a stress of 0.002 to 0.005 MPa.

5. A hollow fiber carbon membrane produced by the production method according to claim 3.

6. The hollow fiber carbon membrane according to claim 5, wherein the surface is further subjected to chemical vapor deposition using hydrocarbon gas.

7. A hollow fiber carbon membrane produced by the production method according to claim 4.

8. The hollow fiber carbon membrane according to claim 7, wherein the surface is further subjected to chemical vapor deposition using hydrocarbon gas.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1: A figure showing a magnified SEM image (?55) of the hollow fiber membrane obtained after spinning and drying in Example 1.

(2) FIG. 2: A figure showing a magnified SEM image (?100) of the hollow fiber membrane obtained after spinning and drying in Example 1.

(3) FIG. 3: A figure showing a magnified SEM image (?170) of the hollow fiber carbon membrane obtained after treatments of infusibilization and carbonization in Example 1.

(4) FIG. 4: A figure showing a magnified SEM image (?55) of the hollow fiber membrane obtained after spinning and drying in Comparative Example 1.

(5) FIG. 5: A figure showing a magnified SEM image (?100) of the hollow fiber membrane obtained after spinning and drying in Comparative Example 1.

(6) FIG. 6: A figure showing a magnified SEM image (?130) of the hollow fiber carbon membrane obtained after treatments of infusibilization and carbonization in Comparative Example 1.

(7) FIG. 7: A figure graphing the results of a gas permeability test conducted in Examples 1 and 2, and Comparative Examples 1 and 2 in the relationship between the gas molecule diameter and the gas permeability rate.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

(8) As the polyphenylene oxide, a commercial product, such as PP0646 (produced by SABIC), or PX100F or PX100L (produced by Mitsubishi Engineering-Plastics Corporation), can be used as they are. The polyphenylene oxide is used at a ratio of about 15 to 40 wt. %, preferably about 20 to 35 wt. %, in the membrane-forming dope. If the concentration of polyphenylene oxide is higher than this range, the membrane-forming dope is separated, and spinning cannot be performed. In contrast, if the concentration of polyphenylene oxide is lower than this range, the membrane is fragile during calcination, and an excellent carbon membrane cannot be obtained in some cases.

(9) To the membrane-forming dope, sulfur is further added in an amount giving a ratio of 0.1 to 5.0 wt. %, preferably 0.2 to 3.0 wt. %, of the total weight of the polyphenylene oxide and the sulfur. If the ratio of sulfur is higher than this range, the membrane-forming dope is separated, and spinning cannot be performed. In contrast, if the ratio of sulfur is lower than this range, the effects obtained by the addition of sulfur, such as high gas separation performance, cannot be exhibited.

(10) In the membrane-forming dope, a component having a crosslinking effect can also be used by adding in combination with sulfur. Examples of the component include organic peroxides, such as dialkyl-based peroxides (e.g., di(2-tert-butylperoxyisopropyl)benzene, dicumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3), diacyl-based peroxides (e.g., diisobutyryl peroxide), phenol resin crosslinking agents, quinone dioxime crosslinking agents, and the like.

(11) The preparation of the membrane-forming dope for carbon membranes is performed by dissolving polyphenylene oxide and sulfur (and an additive) in a solvent that can dissolve these components. Dissolution is performed in such a manner that sulfur is first dissolved in a solvent, and polyphenylene oxide is then dissolved. Examples of the solvent include methanol, ethanol, tetrahydrofuran, N, N-dimethylacetamide, N-methyl-2-pyrrolidone, and the like; aprotic polar solvents, such as N, N-dimethylacetamide and N-methyl-2-pyrrolidone, are preferably used.

(12) Here, if the spinning dope undergoes phase separation during spinning, stable spinning cannot be performed. Therefore, as the membrane-forming solution at the time of spinning and membrane formation, a phase-stabile with a phase-stable temperature, preferably one with, the absolute value calculated by [the temperature during membrane formationthe temperature at which phase separation occurs] of 10? C. or above is used.

(13) The prepared membrane-forming dope is extruded by a wet spinning method or a dry-wet spinning method into a coagulation bath directly or through free running from the outer tube of a hollow fiber spinning nozzle having a double tubular structure. If necessary, a core liquid that is immiscible with polymer of the membrane-forming dope is simultaneously extruded from the inner tube of the spinning nozzle. Thus, a polyphenylene oxide hollow fiber membrane is formed by a non-solvent induced phase separation method. The core liquid and the coagulation bath used herein are solvents (e.g., water or ethylene glycol) that are miscible with the solvent of the membrane-forming dope, but immiscible with the polyphenylene oxide. Moreover, the temperatures of the core liquid and the coagulation bath in this case are generally about-20 to 60? C., preferably about 0 to 30? C.

(14) The obtained polyphenylene oxide hollow fiber membrane is washed with water, if necessary, and then dried; that is, the moisture content is removed from the polyphenylene oxide portion in the hollow fiber membrane. The conditions of drying are not particularly limited, as long as the polyphenylene oxide hollow fiber membrane is completely dried. In general, drying is performed at about 20 to 80? C., preferably about 25 to 35? C., for about 0.5 to 4 hours. In addition, the hollow fiber membrane after spinning is, for example, wound on a bobbin while blowing hot air at about 60 to 240? C. Here, in the cross-sectional structure of the hollow fiber membrane dried after spinning, when sulfur is not added, a void structure (voids) of a size of several tens of microns is partially observed (see FIGS. 4 to 6 relating to Comparative Example 1), whereas when sulfur is added to the membrane-forming dope, such a void structure is not observed (see FIGS. 1 to 3 relating to Example 1).

(15) The dried polyphenylene oxide hollow fiber membrane is subjected to an infusibilization treatment prior to a carbonization treatment. The infusibilization treatment is performed by heating at about 150 to 350? C. for about 1 minute to 2 hours. Due to the infusibilization treatment, a dense structure in which no pores are observed by a SEM at a magnification of 5000 can be obtained.

(16) In addition to the above-mentioned infusibilization treatment without the application of stress, an infusibilization treatment can also be performed by heating at a temperature of about 250 to 350? C., which is lower than the carbonization temperature, for about 0.5 to 4 hours, while applying stress to the hollow fiber membrane. The stress applied to the hollow fiber membrane during the infusibilization treatment is applied, for example, by stretching the spun hollow fiber membrane during winding on a bobbin. The degree of stress varies depending on the membrane thickness and size of the hollow fiber membrane. For example, a stress of about 0.002 to 0.005 MPa is applied to a hollow fiber membrane having an outer diameter of about 400 to 500 ?m and an inner diameter of about 350 to 450 ?m. According to a conventional method, heating is generally performed without the application of stress in the air, that is, in a state in which no force in the stretching direction is applied; however, according to the present invention, the infusibilization treatment performed while applying stress contributes to excellent effects that porous pores of 1 nm or less can be formed in the carbon hollow fiber membrane, and the resulting hollow fiber carbon membrane has excellent gas permeability and a high separation factor.

(17) Patent Documents 5 and 6 disclose that, after membrane formation, the resulting membrane is stretched while heating for pore formation or expansion; however, in these methods, the membrane is not stretched during the infusibilization treatment. Patent Document 5 indicates that a cellulose-based membrane is stretched to adjust the pore size to 10 ?m or less. Patent Document 6 indicates that the pore size of a crystalline membrane is adjusted to 0.1 to 1.0 ?m. However, in both inventions, a membrane comprising a mixture of crystals and non-crystals is merely stretched to adjust the pore size in the non-crystal portion. These inventions are significantly different from an invention in which the pore size of porous pores in a carbon membrane is adjusted to 1 nm or less by stretching during the infusibilization treatment, as in the present invention.

(18) The carbonization treatment is performed by heating a precursor polymer hollow fiber membrane by a known method wherein, for example, the precursor polymer hollow fiber membrane is housed in a container, and heated at a reduced pressure of 10.sup.?4 atm or less (about 10 Pa or less) or in an inert gas atmosphere replaced by helium, argon gas, nitrogen gas, or the like. The heating conditions vary depending on the type of material that constitutes the precursor polymer, the amount thereof, etc. In general, conditions of about 600 to 800? C. and about 0.5 to 4 hours are applied at a reduced pressure of above 10.sup.?4 atm or less (about 10 Pa or less) or in an inert gas atmosphere mentioned above.

(19) In order to further improve the separation performance of the obtained hollow fiber carbon membrane, the surface thereof can be subjected to chemical vapor deposition (CVD), which is a known technique (see Patent Document 7, etc.), preferably a CVD treatment using hydrocarbon gas, such as propylene, butane, or cyclohexane. Due to the CVD treatment, a carbon membrane obtained by adding sulfur to the spinning dope achieves higher separation properties compared with those of a carbon membrane to which sulfur is not added.

EXAMPLES

(20) The following describes the present invention with reference to Examples.

Example 1

(21) A spinning dope comprising 28 parts by weight of polyphenylene oxide resin (PP0646, produced by SABIC), 0.6 parts by weight of sulfur (produced by Kanto Chemical Co., Inc.), and 71.4 parts by weight of dimethylacetamide was prepared by first dissolving the sulfur in the dimethylacetamide, and then dissolving the polyphenylene oxide resin.

(22) The prepared spinning dope was heated to 150? C., and extruded into a water coagulation bath using a spinning nozzle having a double tubular structure while using ethylene glycol as a core liquid, and dry-wet spinning was performed at a spinning rate of 15 m/min. Thereafter, the resultant was dried in an oven at 60? C., thereby obtaining a porous polyphenylene oxide hollow fiber membrane having an outer diameter of 1060 ?m and an inner diameter of 930 ?m. FIGS. 1 and 2 show magnified SEM images of the cross-section of the hollow fiber membrane after spinning and drying.

(23) Subsequently, the obtained hollow fiber membrane was inserted into a tube made of perfluoroalkoxyalkane resin (PFA: tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin), and subjected to an infusibilization treatment by heating at a temperature of 290? C. for 1 hour in the air. Further, the infusibilized hollow fiber membrane was inserted into a quartz tube, and subjected to a carbonization treatment by heating at a temperature of 650? C. for 1 hour in a nitrogen atmosphere, thereby obtaining a hollow fiber carbon membrane having an outer diameter of 430 ?m and an inner diameter of 370 ?m. FIG. 3 shows a magnified SEM image of the cross-section of the obtained hollow fiber carbon membrane. Further, a gas permeability test was conducted using the obtained carbon membrane.

(24) Gas permeability test: One end of the carbon membrane was sealed by adhesion with an epoxy resin, and the other end was inserted into a piping part of a metal gasket gland (6LV-4-VCR-3S-6MTB7, produced by Swagelok) for 10 mm. The gap between the inserted part of the carbon membrane of up to 5 mm and the pipe of the gland was bonded with an epoxy resin, thereby producing a mini module for evaluating gas separation. The mini module was attached to a gas separation device, and different gases were flown on the outside of the carbon membrane at a pressure of 200 kPa g. The flow rate of the gas penetrating to the tube side was measured by a mass flow controller, and the obtained gas flow rate was divided by the membrane area, time, and pressure to calculate the gas permeability rate.

Example 2

(25) The hollow fiber carbon membrane obtained in Example 1 was subjected to a CVD treatment at 650? C. for 2 minutes in a propylene gas atmosphere. The magnified SEM image of the cross-section of the obtained hollow fiber carbon membrane was the same as that of Example 1 (FIG. 3). Further, the gas permeability test was also conducted on the hollow fiber carbon membrane after the CVD treatment.

Comparative Example 1

(26) In Example 1, dry-wet spinning was performed using a spinning dope that was prepared without using sulfur and by changing the amount of dimethylacetamide to 72 parts by weight. After drying, a porous polyphenylene oxide hollow fiber membrane having an outer diameter of 800 ?m and an inner diameter of 650 ?m was obtained. Further, after an infusibilization treatment and a carbonization treatment, a hollow carbon membrane having an outer diameter of 435 ?m and an inner diameter of 375 ?m was obtained. FIGS. 4 and 5 show magnified SEM images of the cross-section of the hollow fiber membrane after spinning and drying, and FIG. 6 shows a magnified SEM image of the cross-section of the hollow fiber carbon membrane after the infusibilization treatment and the carbonization treatment.

Comparative Example 2

(27) The hollow fiber carbon membrane obtained in Comparative Example 1 was subjected to a CVD treatment at 650? C. for 2 minutes in a propylene gas atmosphere. The magnified SEM image of the cross-section of the obtained hollow fiber carbon membrane was the same as that of Comparative Example 1 (FIG. 6). Further, the gas permeability test was also conducted on the hollow fiber carbon membrane after the CVD treatment.

(28) Table 1 below shows the results of the gas permeability test obtained in the above Examples and Comparative Examples, together with the separation factor. Further, FIG. 7 (?: Example 1, ?: Example 2, ?: Comparative Example 1, and ?: Comparative Example 2) shows a graph of the relationship between the gas molecule diameter and the gas permeability rate.

(29) TABLE-US-00001 TABLE 1 Separation Gas permeability rate (mole/m.sup.2 .Math. sec .Math. Pa) factor ? Example He CO.sub.2 N.sub.2 CH.sub.4 (He/CH.sub.4) Ex. 1 8.3 ? 10.sup.?9 2.8 ? 10.sup.?9 8.2 ? 10.sup.?11 4.3 ? 10.sup.?11 193 Ex. 2 2.0 ? 10.sup.?9 1.0 ? 10.sup.?11 1.7 ? 10.sup.?12 5.6 ? 10.sup.?13 3571 Comp. 3.8 ? 10.sup.?9 3.3 ? 10.sup.?9 8.8 ? 10.sup.?11 5.2 ? 10.sup.?11 73 Ex. 1 Comp. 3.1 ? 10.sup.?9 4.2 ? 10.sup.?11 9.8 ? 10.sup.?12 4.3 ? 10.sup.?12 721 Ex. 2

Example 3

(30) The spinning dope prepared in Example 1 was heated to 150? C., and extruded into a water coagulation bath using a spinning nozzle having a double tubular structure while using ethylene glycol as a core liquid, and dry-wet spinning was performed at a spinning rate of 15 to 20 m/min. Thereafter, the resultant was wound on a bobbin while drying by blowing hot air of 220? C., thereby obtaining a porous hollow fiber membrane having an outer diameter of 450 ?m and an inner diameter of 395 ?m.

(31) Subsequently, while applying a stress of 0.004 MPa, the hollow fiber membrane was subjected to an infusibilization treatment by heating at a temperature of 320? C. for 1 hour in the air. Further, the infusibilized hollow fiber membrane was inserted into a quartz tube, and subjected to a carbonization treatment by heating at a temperature of 700? C. for 45 minutes in a nitrogen atmosphere. Finally, a CVD treatment was performed at a temperature of 700? C. for 5 minutes in a nitrogen and propylene gas atmospheres, thereby obtaining a porous hollow fiber carbon membrane having an outer diameter of 265 ?m and an inner diameter of 220 ?m. The same gas permeability test as described above was conducted using the obtained carbon membrane.

Example 4

(32) In Example 3, the stress applied to the hollow fiber membrane during the infusibilization treatment was changed to 0.005 MPa, and a porous hollow fiber carbon membrane having an outer diameter of 200 ?m and an inner diameter of 165 ?m was obtained.

Example 5

(33) In Example 3, the infusibilization treatment was performed, without applying stress to the hollow fiber membrane, by heating at a temperature of 320? C. for 1 hour in the air, and a porous hollow fiber carbon membrane having an outer diameter of 315 ?m and an inner diameter of 270 ?m was obtained.

Comparative Example 3

(34) In Example 3, when the stress applied to the hollow fiber membrane during the infusibilization treatment was changed to 0.006 MPa, the hollow fiber membrane was broken, and a porous hollow fiber carbon membrane was not able to be obtained.

(35) Table 2 below shows the results obtained in Examples 3 to 5 above, together with the stress applied to the hollow fiber membranes during the infusibilization treatment, and the membrane thickness of the hollow fiber carbon membranes.

(36) TABLE-US-00002 TABLE 2 Stress Membrane Separation applied thickness Gas permeability rate (mole/m.sup.2 .Math. sec .Math. Pa) factor Example (MPa) (?m) He CO.sub.2 N.sub.2 CH.sub.4 ? (He/CH.sub.4) Ex. 3 0.004 22.5 5.1 ? 10.sup.?9 3.1 ? 10.sup.?11 1.6 ? 10.sup.?12 6.9 ? 10.sup.?13 7385 Ex. 4 0.005 17.5 6.1 ? 10.sup.?9 9.8 ? 10.sup.?12 9.9 ? 10.sup.?13 3.9 ? 10.sup.?13 15649 Ex. 5 0 22.5 4.3 ? 10.sup.?9 4.9 ? 10.sup.?11 1.8 ? 10.sup.?12 7.1 ? 10.sup.?13 6085

INDUSTRIAL APPLICABILITY

(37) The carbon membrane according to the present invention can be effectively used for the purpose of removing only hydrogen or helium from a mixed gas of a beneficial low-molecular gas, such as hydrogen or helium, and an organic gas, such as methane or toluene, and used as a membrane for vapor separation and membrane surface distillation, etc. Furthermore, due to the excellent heat resistance and chemical resistance, the carbon membrane according to the present invention can also be used for applications for which organic membranes cannot be used.