Composite hollow fiber membrane and method for producing same

Abstract

Problems that the invention is to solve is to provide a composite hollow fiber membrane being excellent in separation performance and permeation performance, having high membrane strength, and capable of being easily produced, and a method for producing the same. The present invention relates to a composite hollow fiber membrane including at least a layer (A) and a layer (B), in which the composite hollow fiber membrane has an outer diameter of 20 to 350 m and an inner diameter of 14 to 250 m, the tensile modulus of the composite hollow fiber membrane is from 1,000 to 6,500 MPa, the layer (A) contains a cellulose ester, the thickness of the layer (A) is from 0.01 to 5 m, and the open pore ratio H.sub.A of the layer (A) and the open pore ratio H.sub.B of the layer (B) satisfy H.sub.A<H.sub.B.

Claims

1. A composite hollow fiber membrane comprising at least a layer (A) and a layer (B), wherein: the composite hollow fiber membrane has an outer diameter of 20 to 350 m and an inner diameter of 14 to 250 m, a tensile modulus of the composite hollow fiber membrane is from 1,000 to 6,500 MPa, the layer (A) contains a cellulose ester, a thickness of the layer (A) is from 0.01 to 5 m, and an open pore ratio H.sub.A of the layer (A) and an open pore ratio H.sub.B of the layer (B) satisfy H.sub.A<H.sub.B, the open pore ratio H.sub.A of the layer (A) is from 0 to 10%, and the open pore ratio of H.sub.B of the layer (B) is from 5 to 55%.

2. The composite hollow fiber membrane according to claim 1, wherein an outermost layer of the composite hollow fiber membrane is the layer (A).

3. The composite hollow fiber membrane according to claim 1, wherein the layer (B) contains a cellulose ester.

4. The composite hollow fiber membrane according to claim 1, wherein the cellulose ester contained in the layer (A) is at least one compound selected from the group consisting of cellulose acetate propionate and cellulose acetate butyrate.

5. The composite hollow fiber membrane according to claim 1, wherein the cellulose ester contained in the layer (B) is at least one compound selected from the group consisting of cellulose acetate propionate and cellulose acetate butyrate.

6. The composite hollow fiber membrane according to claim 1, wherein the layer (B) has a continuous pore.

7. The composite hollow fiber membrane according to claim 1, having a stress at 5% elongation in a longitudinal direction thereof of 30 MPa or more.

8. The composite hollow fiber membrane according to claim 1, having a salt rejection ratio of from 90.0 to 99.9% at the time of filtrating an aqueous solution having a sodium chloride concentration of 500 mg/l at 25 C. and a pressure of 0.5 MPa.

9. A method for producing the composite hollow fiber membrane of claim 1 comprising the following steps 1 to 3: 1. a step of heating and thereby melting a resin composition constituting each of the layers of the composite hollow fiber membrane, in which a resin composition constituting at least one of the layers contains the cellulose ester, 2. a step of combining and thereby compounding the melted resin compositions of respective layers within a spinneret having a multi-annular nozzle where a gas channel is arranged in a central part thereof, and 3. a step of either winding the compounded resin composition at a draft ratio of 200 to 1,000 while discharging it into air from the multi-annular nozzle, or spinning out the compounded resin composition at a draft ratio of 10 to 200 to obtain a hollow fiber membrane, drawing the hollow fiber membrane at a ratio of 1.1 to 2.5 times, and subsequently winding the hollow fiber membrane.

Description

EXAMPLES

(1) The present invention is more specifically described below by referring to Examples, but the present invention should not be construed as limited thereto in any way.

(2) [Measurement and Evaluation Methods]

(3) The present invention is more specifically described below by referring to Examples. Respective characteristic values in Examples were determined by the following methods. The present invention is not limited to these Examples. In (3) to (7) and (10) to (12) below, the separation membrane was measured and evaluated in the state of being vacuum-dried at 25 C. for 8 hours.

(4) (1) Average Degree of Substitution for Cellulose-Mixed Ester

(5) The method for calculating the average degree of substitution for a cellulose-mixed ester in which an acetyl group and an acyl group are bonded to cellulose is as follows.

(6) 0.9 g of a cellulose-mixed ester dried at 80 C. for 8 hours was weighed and dissolved by adding 35 ml of acetone and 15 ml of dimethylsulfoxide and thereafter, 50 ml of acetone was further added. The resulting solution was saponified for 2 hours by adding 30 ml of an aqueous 0.5 N-sodium hydroxide solution with stirring. After adding 50 ml of hot water to wash the side surface of flask, titration was performed with 0.5 N-sulfuric acid by using phenolphthalein as an indicator. Separately, a blank test was performed by the same method as that for the sample. A supernatant of the solution after the completion of titration was diluted to 100 times, and the compositions of organic acids were measured using an ion chromatograph. From the measurement results and the results of acid composition analysis by ion chromatograph, the degrees of substitution were calculated according to the following formulae.
TA=(BA)F/(1000W)
DSace=(162.14TA)/[{1(Mwace(16.00+1.01))TA}+{1(Mwacy(16.00+1.01))TA}(Acy/Ace)]
DSacy=DSace(Acy/Ace)

(7) TA: Total organic acid amount (ml)

(8) A: Sample titration amount (ml)

(9) B: Blank test titration amount (ml)

(10) F: Titer of sulfuric acid

(11) W: Sample weight (g)

(12) DSace: Average degree of substitution of acetyl group

(13) DSacy: Average degree of substitution of another acyl group

(14) Mwace: Molecular weight of acetic acid

(15) Mwacy: Molecular weight of another organic acid

(16) Acy/Ace: Molar ratio of acetic acid (Ace) and another organic acid (Acy)

(17) 162.14: Molecular weight of repeating unit of cellulose

(18) 16.00: Atomic weight of oxygen

(19) 1.01: Atomic weight of hydrogen

(20) (2) Weight Average Molecular Weight (Mw) of Cellulose Ester

(21) The sample for GPC measurement was prepared by completely dissolving a cellulose ester in tetrahydrofuran at a concentration of 0.15 wt %. Using this sample, GPC measurement was performed by Waters 2690 under the following conditions to determine the weight average molecular weight (Mw) in terms of polystyrene.

(22) Column: TSK gel GMHHR-H, manufactured by Tosoh Corp., two columns connected

(23) Detector: Waters 2410, differential refractometer RI

(24) Solvent for mobile phase: Tetrahydrofuran

(25) Flow speed: 1.0 ml/min

(26) Injection amount: 200 l

(27) (3) Outer Diameter (m) of Composite Hollow Fiber Membrane

(28) Cross-sections in a direction (radial direction) perpendicular to the longitudinal direction of the composite hollow fiber membrane and in the thickness direction of the membrane were observed and photographed by an optical microscope, and the outer diameter (m) of the composite hollow fiber membrane was calculated. Here, as for the outer diameter of the composite hollow fiber membrane, the outer diameter was calculated using 10 composite hollow fiber membranes, and the average value thereof was employed.

(29) (4) Inner Diameter (m) of Composite Hollow Fiber Membrane

(30) Cross-sections in a direction (radial direction) perpendicular to the longitudinal direction of a composite hollow fiber membrane and in the thickness direction of the membrane were observed and photographed by an optical microscope, and the inner diameter (m) of the composite hollow fiber membrane was calculated. Here, as for the inner diameter of the composite hollow fiber membrane, the inner diameter was calculated using 10 composite hollow fiber membranes, and the average value thereof was employed.

(31) (5) Thickness (m) of Layer (A)

(32) The composite hollow fiber membrane was cooled in liquid nitrogen and fractured in a direction (radial direction) perpendicular to the longitudinal direction and in the thickness direction of the membrane by applying a stress. The cross-sections thereof were observed and photographed by a scanning electron microscope, and the thickness (m) of the layer (A) was calculated. Here, as for the thickness of the layer (A), the thickness was calculated by observing arbitrary 10 points, and the average value thereof was employed.

(33) (6) Open Pore Ratios H.sub.A and H.sub.B (%)

(34) The surface (outer surface or inner surface) or cross-section of each of the layer (A) and the layer (B) of the composite hollow fiber membrane was observed and photographed at a magnification of 30,000 times by a scanning electron microscope, a transparent film or sheet was overlaid on the obtained photograph, and the portion corresponding to a fine pore was filled with oil-based ink, etc. The area of the region corresponding to a fine pore was then determined using an image analyzer. This measurement was performed on arbitrary 30 fine pores, and the average pore area S (m.sup.2) was calculated by number-averaging the measured areas. Subsequently, the number of fine pores per 3 m square in the photograph where the average pore area S was calculated, was counted and converted to the number of fine pores per 1 m.sup.2, and the fine pore density (pores/m.sup.2) was thereby calculated. The open pore ratio was calculated and determined from the obtained average pore area S and fine pore density according to the following formula. Here, for the calculation of the open pore ratio, fine pores having a fine pore diameter (in the case of an elliptic or bar-like shape, a short diameter) of 1 nm or more are used.
Open pore ratio (%)=(average pore area S)(fine pore density)100

(35) Incidentally, in the case where the surfaces of the layer (A) and the layer (B) are exposed and can be observed, fine pores in the surface are observed in principle. However, in the case where the surfaces are not exposed or where the diameter of the hollow fiber membrane is small and the inner surface cannot be observed, the cross-section is observed. In both cases, the observation surface of the layer (A) should be matched with that of the layer (B). The cross-section was prepared by the same method as that described in (5), and observation of the cross-section was performed in the central part in the thickness direction of each layer. The number of fine pores for calculating the average pore area S and the area in the photograph for calculating the fine pore density can be arbitrarily changed from the above-exemplified numerical values according to the sample shape or the pore shape.

(36) In addition, in the case where the number of fine pores can be hardly calculated, for example, where the pore has a bar-like shape, the surface (outer surface or inner surface) or cross-section was observed and photographed at a magnification of 30,000 times by a scanning electron microscope, a transparent film or sheet was overlaid on the obtained photograph, and all portions corresponding to a fine pore were filled with oil-based ink, etc. The area of the region corresponding to a fine pore was then determined using an image analyzer. The open pore ratio was calculated directly from the determined area and the area of the photograph used for evaluation.

(37) (7) Pore Diameter (m) of Layer (B)

(38) With respect to the cross-section of the layer (B), the pore diameter was determined according to the following formula by using the average pore area S (m.sup.2) calculated by the method described in (6).
Pore diameter (m) of layer (B)=(2(S/).sup.1/2)10.sup.6

(39) In the case where the number of fine pores can be hardly calculated and the average pore area S cannot be determined, for example, where the pore has a bar-like shape, the pore diameter was calculated by the following method.

(40) The composite hollow fiber membrane was cooled in liquid nitrogen and fractured in a direction (radial direction) perpendicular to the longitudinal direction and in the thickness direction of the membrane by applying a stress. The central part of the layer (B) in the thickness direction of the resulting cross-section was observed by a scanning electron microscope, the obtained photograph was Fourier-transformed, a maximum wave number was determined when plotting the wave number on the abscissa and the strength on the ordinate, and the pore diameter was obtained from the inverse number thereof. At this time, the image size of the scanning electron micrograph is a square having a side length of 20 to 100 times the pore diameter.

(41) (8) Permeation Performance (Membrane Permeation Flux (L/m.sup.2/day))

(42) The composite hollow fiber membrane hydrophilized by immersion in a 10 wt % aqueous solution of isopropyl alcohol for 1 hour. An aqueous sodium chloride solution adjusted to a concentration of 500 mg/l, a temperature of 25 C. and a pH of 6.5 was fed thereto at an operation pressure of 0.50 MPa, thereby performing a membrane filtration treatment. Based on the permeate amount obtained, the membrane permeation flux was determined according to the following formula:
Membrane permeation flux (L/m.sup.2/day)=permeate amount per day/membrane area
(9) Separation Performance (Salt Rejection Ratio (%))

(43) A membrane filtration treatment was performed under the same conditions as in the case of membrane permeation flux, and the salt concentration of the obtained permeate was measured. From the measured salt concentration of permeate and the salt concentration of feed water, the salt rejection ratio was determined based on the following formula. Incidentally, the salt concentration of permeate was determined from the measured value of electric conductivity.
Salt rejection ratio (%)=100{1(sodium chloride concentration in permeate/sodium chloride concentration in feed water)}

(44) Here, in (7) and (8) above, the membrane filtration treatment was performed by manufacturing a small-sized module as follows.

(45) Composite hollow fiber membranes were bundled, inserted into a plastic pipe, and sealed by injecting a thermosetting resin into the pipe and curing it at the ends. Opening faces of hollow fiber membranes were obtained by cutting the ends of the sealed hollow fiber membranes to prepare the small-sized module for evaluation, having a membrane area on an outer diameter basis of about 0.1 m.sup.2.

(46) (10) Tensile Modulus (MPa)

(47) In an environment at a temperature of 20 C. and a humidity of 65%, using a tensile tester (Tensilon UCT-100, manufactured by Orientec Co., Ltd.), measurement was performed under conditions of a sample length of 100 mm and a tension rate of 100 mm/min and as for the rest, according to the method prescribed in JIS L 1013: 2010, Testing methods for man-made filament yarns, 8.10 Initial tensile resistance. The apparent Young's modulus calculated from the initial tensile resistance was taken as the tensile modulus (MPa). The number of measurements was 5, and the average value thereof was employed.

(48) (11) Stress (MPa) at 5% Elongation

(49) In an environment at a temperature of 20 C. and a humidity of 65%, using a tensile tester (Tensilon UCT-100, manufactured by Orientec Co., Ltd.), measurement was performed under conditions of a sample length of 100 mm and a tension rate of 100 mm/min and as for the rest, according to the method prescribed in JIS L 1013: 2010, Testing methods for man-made filament yarns, 8.5 Tensile strength and elongation percentage, and the stress (MPa) at 5% elongation was thereby measured. The number of measurements was 5, and the average value thereof was taken as the stress at 5% elongation.

(50) (12) Tensile Strength (MPa)

(51) In an environment at a temperature of 20 C. and a humidity of 65%, using a tensile tester (Tensilon UCT-100, manufactured by Orientec Co., Ltd.), measurement was performed under conditions of a sample length of 100 mm and a tension rate of 100 mm/min and as for the rest, according to the method prescribed in JIS L 1013: 2010, Testing methods for man-made filament yarns, 8.5 Tensile strength and elongation percentage, and the tensile strength (breaking strength) (MPa) was calculated from the tensile tenacity. The number of measurements was 5, and the average value thereof was taken as the tensile strength.

(52) [Cellulose Ester (C)]

(53) (C1)

(54) 240 Parts by weight of acetic acid and 67 parts by weight of propionic acid were added to 100 parts by weight of cellulose (cotton linter) and mixed at 50 C. for 30 minutes. The mixture was cooled to room temperature and then 172 parts by weight of acetic anhydride cooled in an ice bath and 168 parts by weight of propionic anhydride were added as esterifying agents, and, 4 parts by weight of sulfuric acid was added as an esterifying catalyst and stirred for 150 minutes. The esterification reaction was thus performed. In the esterification reaction, when the temperature exceeded 40 C., the system was cooled in a water bath. After the reaction, a mixed solution of 100 parts by weight of acetic acid and 33 parts by weight of water as a reaction terminator was added over 20 minutes to hydrolyze an excessive anhydride. Thereafter, 333 parts by weight of acetic acid and 100 parts by weight of water were added, and the mixture was stirred at 80 C. for 1 hour. After the completion of reaction, an aqueous solution containing 6 parts by weight of sodium carbonate was added, and the precipitated cellulose acetate propionate was separated by filtration, subsequently washed with water, and then dried at 60 C. for 4 hours. In the obtained cellulose acetate propionate, the average degrees of substitution of an acetyl group and a propionyl group were 1.9 and 0.7, respectively, and the weight average molecular weight (Mw) thereof was 178,000.

(55) (C2)

(56) Cellulose acetate (LT35) produced by Daicel Corporation, degree of substitution: 2.90

(57) [Plasticizer (P) for Cellulose Ester]

(58) (P1)

(59) Polyethylene glycol having a weight average molecular weight of 600

(60) [Antioxidant (O)]

(61) (O1)

(62) Bis (2,6-di-tert-butyl-4-methylphenyl) pentaerythritol diphosphite

(63) [Hydrophilic Resin (H)]

(64) (H1)

(65) Polyethylene glycol having a weight average molecular weight of 8,300

(66) (H2)

(67) Polyvinylpyrrolidone, K17

(68) [Production of Composite Hollow Fiber Membrane]

Example 1

(69) 74 wt % of cellulose ester (C1), 25.9 wt % of polyethylene glycol (P1) having a weight average molecular weight of 600, and 0.1 wt % of bis(2,6-di-tert-butyl-4-methylphenyl)pentaerythritol diphosphite (O1) as an antioxidant were melt-kneaded at 240 C. in a twin-screw extruder, homogenized and then pelletized to obtain a resin composition (a) for the layer (A). The pellet was vacuum-dried at 80 C. for 8 hours.

(70) In addition, 74 wt % of cellulose ester (C1), 17.9 wt % of polyethylene glycol (P1) having a weight average molecular weight of 600, 0.1 wt % of bis(2,6-di-tert-butyl-4-methylphenyl)pentaerythritol diphosphite (O1) as an antioxidant, and 8 wt % of polyethylene glycol (H1) having a weight average molecular weight of 8,300 were melt-kneaded at 240 C. in a twin-screw extruder, homogenized and then pelletized to obtain a resin composition (b) for the layer (B). The pellet was vacuum-dried at 80 C. for 8 hours.

(71) The dried pellet of the resin composition (a) for the layer (A) and the dried pellet of the resin composition (b) for the layer (B) were each fed to separate twin-screw extruders, melt-kneaded at 230 C., adjusted by a gear pump to a discharge rate of 2.4 g/min for the resin composition (a) and 24 g/min for the resin composition (b). Then the pellets were introduced into a spinneret with a multi-annular nozzle having a gas channel arranged in the central part thereof, such that the outer layer becomes the layer (A) and the inner layer becomes the layer (B), and compounded within the spinneret. Thereafter the compounded composition was spun downward from spinneret holes (outer diameter: 4.6 mm, inner diameter: 3.7 mm, slit width: 0.45 mm, number of holes: 4). The spun-out hollow fiber membranes were introduced into a cooling apparatus (chimney) such that the distance L from a lower surface of the spinneret to an upper end of the cooling apparatus becomes 30 mm, cooled with cooling air at 25 C. and an air speed of 1.5 m/sec, and bundled by applying an oil solution, and the spun-out yarn was then wound by a winder at a draft ratio of 400. The structure and physical properties of the obtained composite hollow fiber membrane are shown in Table 1.

(72) Incidentally, in the composite hollow fiber membrane of this Example, it was confirmed from the change in weight between before and after immersion in a 10 wt % aqueous solution of isopropyl alcohol for 1 hour that the whole amount of each of polyethylene glycol having a weight average molecular weight of 600 added as a plasticizer at the time of melt spinning and polyethylene glycol having a weight average molecular weight of 8,300 was eluted from the composite hollow fiber membrane.

Example 2

(73) A composite hollow fiber membrane was obtained in the same manner as in Example 1 except that as the resin composition (b) for the layer (B), 74 wt % of cellulose ester (C1), 13.9 wt % of polyethylene glycol (P1) having a weight average molecular weight of 600, 0.1 wt % of bis(2,6-di-tert-butyl-4-methylphenyl) pentaerythritol diphosphite (O1) as an antioxidant, and 12 wt % of polyethylene glycol (H1) having a weight average molecular weight of 8,300 were used. The structure and physical properties of the obtained composite hollow fiber membrane are shown in Table 1.

Example 3

(74) A composite hollow fiber membrane was obtained in the same manner as in Example 2 except that as the resin composition (a) for the layer (A), 74 wt % of cellulose ester (C1), 21.9 wt % of polyethylene glycol (P1) having a weight average molecular weight of 600, 0.1 wt % of bis(2,6-di-tert-butyl-4-methylphenyl) pentaerythritol diphosphite (O1) as an antioxidant, and 4 wt % of polyethylene glycol (H1) having a weight average molecular weight of 8,300 were used. The structure and physical properties of the obtained composite hollow fiber membrane are shown in Table 1.

Example 4

(75) A composite hollow fiber membrane was obtained in the same manner as in Example 1 except that the draft ratio was changed to 800. The structure and physical properties of the obtained composite hollow fiber membrane are shown in Table 1.

Example 5

(76) A composite hollow fiber membrane was obtained in the same manner as in Example 1 except that the draft ratio was changed to 200. The structure and physical properties of the obtained composite hollow fiber membrane are shown in Table 1.

Example 6

(77) 74 wt % of cellulose ester (C1), 25.9 wt % of polyethylene glycol (P1) having a weight average molecular weight of 600, and 0.1 wt % of bis(2,6-di-tert-butyl-4-methylphenyl)pentaerythritol diphosphite (O1) as an antioxidant were melt-kneaded at 240 C. in a twin-screw extruder, homogenized and then pelletized to obtain a resin composition (a) for the layer (A). The pellet was vacuum-dried at 80 C. for 8 hours.

(78) In addition, 50 wt % of cellulose ester (C1), 19.9 wt % of polyethylene glycol (P1) having a weight average molecular weight of 600, 0.1 wt % of bis(2,6-di-tert-butyl-4-methylphenyl)pentaerythritol diphosphite (O1) as an antioxidant, and 30 wt % of polyvinylpyrrolidone (H2) were melt-kneaded at 240 C. in a twin-screw extruder, homogenized and then pelletized to obtain a resin composition (b) for the layer (B). The pellet was vacuum-dried at 80 C. for 8 hours.

(79) The dried pellet of the resin composition (a) for the layer (A) and the dried pellet of the resin composition (b) for the layer (B) were each fed to separate twin-screw extruders, melt-kneaded at 230 C., adjusted by a gear pump to a discharge rate of 2.4 g/min for the resin composition (a) and 24 g/min for the resin composition (b) Then the pellets were introduced into a spinneret with a multi-annular nozzle having a gas channel arranged in the central part thereof, such that the outer layer becomes the layer (A) and the inner layer becomes the layer (B), and compounded within the spinneret at 190 C. The compounded composition was thereafter spun downward from spinneret holes (outer diameter: 4.6 mm, inner diameter: 3.7 mm, slit width: 0.45 mm, number of holes: 4). The spun-out hollow fiber membranes were introduced into a cooling apparatus (chimney) such that the distance L from a lower surface of the spinneret to an upper end of the cooling apparatus becomes 30 mm, cooled with cooling air at 25 C. and an air speed of 1.5 m/sec, and bundled by applying an oil solution, and the spun-out yarn was then wound by a winder at a draft ratio of 400. The structure and physical properties of the obtained composite hollow fiber membrane are shown in Table 1.

(80) Incidentally, in the composite hollow fiber membrane of this Example, it was confirmed from the change in weight between before and after immersion in a 10 wt % aqueous solution of isopropyl alcohol for 1 hour that the whole amount of each of polyethylene glycol having a weight average molecular weight of 600 added as a plasticizer at the time of melt spinning and polyvinylpyrrolidone was eluted from the composite hollow fiber membrane.

Example 7

(81) A spun-out yarn was obtained in the same manner as in Example 1 except that the draft ratio was changed to 100. This spun-out yean was heated to 120 C. by passing it through a dry heat oven, drawn at a draw ratio of 1.8 times by utilizing a peripheral speed difference between rolls, and wound to obtain a composite hollow fiber membrane. The structure and physical properties of the obtained composite hollow fiber membrane are shown in Table 1.

Example 8

(82) A spun-out yarn was obtained in the same manner as in Example 6 except that the draft ratio was changed to 100. This spun-out yean was heated to 120 C. by passing it through a dry heat oven, drawn at a draw ratio of 1.8 times by utilizing a peripheral speed difference between rolls, and wound to obtain a composite hollow fiber membrane. The structure and physical properties of the obtained composite hollow fiber membrane are shown in Table 1. Incidentally, in the composite hollow fiber membrane of this Example, it was confirmed from the change in weight between before and after immersion in a 10 wt % aqueous solution of isopropyl alcohol for 1 hour that the whole amount of each of polyethylene glycol having a weight average molecular weight of 600 added as a plasticizer at the time of melt spinning and polyvinylpyrrolidone was eluted from the composite hollow fiber membrane.

Comparative Example 1

(83) A composite hollow fiber membrane was obtained in the same manner as in Example 1 except that the resin composition (b) for the layer (B) was not used and the resin composition (a) was adjusted by a gear pump to a discharge rate of 26.4 g/min. The structure and physical properties of the obtained composite hollow fiber membrane are shown in Table 1.

Comparative Example 2

(84) A composite hollow fiber membrane was obtained in the same manner as in Example 3 except that the draft ratio was changed to 80. The structure and physical properties of the obtained composite hollow fiber membrane are shown in Table 1.

(85) Incidentally, in the composite hollow fiber membrane of this Example, it was confirmed from the change in weight between before and after immersion in a 10 wt % aqueous solution of isopropyl alcohol for 1 hour that the whole amount of each of polyethylene glycol having a weight average molecular weight of 600 added as a plasticizer at the time of melt spinning and polyethylene glycol having a weight average molecular weight of 8,300 was eluted from the composite hollow fiber membrane.

Comparative Example 3

(86) Spinning was attempted in the same manner as in Comparative Example 1 except that the draft ratio was changed to 800, but yarn breakage occurred between the spinneret and the winder, and a composite hollow fiber membrane could not be obtained.

Comparative Example 4

(87) A spun-out yarn was obtained in the same manner as in Comparative Example 1 except that the draft ratio was changed to 100. This spun-out yean was heated to 120 C. by passing it through a dry heat oven and attempted to be drawn at a draw ratio of 1.8 times by utilizing a peripheral speed difference between rolls, but yarn breakage occurred during drawing, and a composite hollow fiber membrane could not be obtained.

Comparative Example 5

(88) 41 wt % of cellulose ester (C2), 49.9 wt % of N-methyl-2-pyrrolidone, 8.8 wt % of ethylene glycol and 0.3 wt % of benzoic acid were dissolved at 180 C. The obtained solution was defoamed under reduced pressure, then spun downward from spinneret holes (a type of forming one discharge hole by arranging 3 arcuate slit parts) at 160 C. and after a time of exposure to air of 0.03 seconds, solidified in a bath at 12 C. containing N-methyl-2-pyrrolidone/ethylene glycol/water=4.25 wt %/0.75 wt %/95 wt %, followed by washing in water. Thereafter, a heat treatment was performed in water at 60 C. for 40 minutes to obtain a hollow fiber membrane having an outer diameter of 167 m and an inner diameter of 83 m.

(89) The obtained hollow fiber membrane has the membrane permeation flux of 54.7 L/m.sup.2/day, the salt rejection ratio of 95.8%, the tensile modulus of 1,435 MPa, the stress at 5% elongation of 28 MPa, and the tensile strength of 72 MPa. In addition, the obtained hollow fiber membrane was not a composite hollow fiber membrane but was a so-called asymmetric membrane having a heterogeneous cross-section structure.

(90) Incidentally, in the hollow fiber membranes of Examples 2 to 5 and 7 and Comparative Examples 1 and 2, it was confirmed from the change in weight between before and after immersion in a 10 wt % aqueous solution of isopropyl alcohol for 1 hour that the whole amount of each of polyethylene glycol having a weight average molecular weight of 600 added as a plasticizer at the time of melt spinning and polyethylene glycol having a weight average molecular weight of 8,300 was eluted from the composite hollow fiber membrane.

(91) In addition, the cross-sections of the hollow fiber membranes of Examples 1 to 8 and Comparative Examples 1 and 2 showed that the cross-section structure was homogeneous in all layers.

(92) In the composite hollow fiber membranes of Examples 1 to 8, the membrane permeation flux was high and the permeation performance was good. Furthermore, the salt rejection ratio was high to provide good separation performance, and the tensile strength was also high to provide good membrane strength.

(93) On the other hand, in the composite hollow fiber membrane of Comparative Example 1 where the layer (B) is not formed and the thickness of the layer (A) is large and outside the scope of the present invention, the permeation performance was poor compared with Examples. In the composite hollow fiber membrane of Comparative Example 2 where the tensile modulus is less than 1,000 MPa and outside the scope of the present invention, the salt rejection ratio was low compared with Examples, resulting in poor separation performance. Furthermore, in the composite hollow fiber membrane of Comparative Example 2, the tensile strength was also low, resulting in poor membrane strength.

(94) TABLE-US-00001 TABLE 1 Example 1 Example 2 Example 3 Example 4 Resin Layer (A) Cellulose ester kind C1 C1 C1 C1 composition wt % 74 74 74 74 Plasticizer for cellulose kind P1 P1 P1 P1 ester wt % 25.9 25.9 21.9 25.9 Antioxidant kind O1 O1 O1 O1 wt % 0.1 0.1 0.1 0.1 Hydrophilic resin kind H1 wt % 4 Layer (B) Cellulose ester kind C1 C1 C1 C1 wt % 74 74 74 74 Plasticizer for cellulos kind P1 P1 P1 P1 ester wt % 17.9 13.9 13.9 17.9 Antioxidant kind O1 O1 O1 O1 wt % 0.1 0.1 0.1 0.1 Hydrophilic resin kind H1 H1 H1 H1 wt % 8 12 12 8 Production conditions Spinning temperature C. 230 230 230 230 Draft ratio 400 400 400 800 Drawing temperature C. Draw ratio Structure of Layer (A) Cross-section structure homogeneous homogeneous homogeneous homogeneous composite hollow Thickness m 1.4 1.4 1.4 0.5 fiber membrane Open pore ratio H.sub.A % 0.0 0.0 3.5 0.0 Layer (B) Cross-section structure homogeneous homogeneous homogeneous homogeneous Open pore ratio H.sub.B % 6.8 10.4 10.7 8.8 Pore diameter m 0.09 0.10 0.10 0.06 Continuous pore none none none none Layer configuration A/B A/B A/B A/B (outer layer/inner layer) Outer diameter m 90 95 94 38 Inner diameter m 52 58 56 26 Physical Membrane permeation flux L/m.sup.2/day 15.7 16.9 30.4 33.3 properties of Salt rejection ratio % 93.7 92.2 94.5 96.8 composite hollow Tensile modulus MPa 1,588 1,503 1,430 2,055 fiber membrane Stress at 5% elongation MPa 39 34 30 46 Tensile strength MPa 96 89 82 115 Example 5 Example 6 Example 7 Example 8 Resin Layer (A) Cellulose ester kind C1 C1 C1 C1 composition wt % 74 74 74 74 Plasticizer for cellulose kind P1 P1 P1 P1 ester wt % 25.9 25.9 25.9 25.9 Antioxidant kind O1 O1 O1 O1 wt % 0.1 0.1 0.1 0.1 Hydrophilic resin kind wt % Layer (B) Cellulose ester kind C1 C1 C1 C1 wt % 74 50 74 50 Plasticizer for cellulose kind P1 P1 P1 P1 ester wt % 17.9 19.9 17.9 19.9 Antioxidant kind O1 O1 O1 O1 wt % 0.1 0.1 0.1 0.1 Hydrophilic resin kind H1 H2 H1 H2 wt % 8 30 8 30 Production conditions Spinning temperature C. 230 230 230 230 Draft ratio 200 400 100 100 Drawing temperature C. 120 120 Draw ratio 1.8 1.8 Structure of Layer (A) Cross-section structure homogeneous homogeneous homogeneous homogeneous composite hollow Thickness m 3.1 1.1 2.2 2.0 fiber membrane Open pore ratio H.sub.A % 0.0 0.0 0.0 0.0 Layer (B) Cross-section structure homogeneous homogeneous homogeneous homogeneous Open pore ratio H.sub.B % 6.1 26.8 7.5 24.2 Pore diameter m 0.10 0.04 0.08 0.05 Continuous pore none formed none formed Layer configuration A/B A/B A/B A/B (outer layer/inner layer) Outer diameter m 239 85 150 148 Inner diameter m 170 60 102 99 Physical Membrane permeation flux L/m.sup.2/day 8.4 31.2 10.2 20.9 properties of Salt rejection ratio % 91.0 93.4 99.5 99.1 composite hollow Tensile modulus MPa 1,291 1,514 6,340 5,847 fiber membrane Stress at 5% elongation MPa 31 37 85 80 Tensile strength MPa 84 95 213 187 Comparative Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 4 Resin Layer (A) Cellulose ester kind C1 C1 C1 C1 composition wt % 74 74 74 74 Plasticizer for cellulose kind P1 P1 P1 P1 ester wt % 25.9 21.9 25.9 25.9 Antioxidant kind O1 O1 O1 O1 wt % 0.1 0.1 0.1 0.1 Hydrophilic resin kind H1 wt % 4 Layer (B) Cellulose ester kind C1 wt % 74 Plasticizer for cellulose kind P1 ester wt % 13.9 Antioxidant kind O1 wt % 0.1 Hydrophilic resin kind H1 wt % 12 Production conditions Spinning temperature C. 230 230 230 230 Draft ratio 400 80 800 100 Drawing temperature C. 120 Draw ratio 1.8 Structure of Layer (A) Cross-section structure homogeneous homogeneous yarn breakage yarn breakage composite hollow Thickness m 19.0 4.6 fiber membrane Open pore ratio H.sub.A % 0.0 3.4 Layer (B) Cross-section structure homogeneous Open pore ratio H.sub.B % 10.2 Pore diameter m 0.15 Continuous pore none Layer configuration A A/B (outer layer/inner layer) Outer diameter m 104 345 Inner diameter m 66 242 Physical Membrane permeation flux L/m.sup.2/day 1.3 5.3 properties of Salt rejection ratio % 93.9 78.5 composite hollow Tensile modulus MPa 1,638 974 fiber membrane Stress at 5% elongation MPa 38 23 Tensile strength MPa 97 58

(95) While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention. This application is based on Japanese Patent Application (Patent Application No. 2015-091194) filed on Apr. 28, 2015, the entirety of which is incorporated herein by way of reference.

INDUSTRIAL APPLICABILITY

(96) The present invention provides a composite hollow fiber membrane that is excellent in permeation performance and separation performance, has high membrane strength, and can be easily produced, and a method for producing the same. The hollow fiber membrane of the present invention can be used as a water treatment membrane for producing industrial water or drinking water from seawater, brine water, sewage water, wastewater, etc., a medical membrane for artificial kidney, plasma separation, etc., a membrane for food-beverage industry such as fruit juice concentration, a gas separation membrane for separating exhaust gas, carbonic acid gas, etc., and a membrane for electronic industry such as fuel cell separator. As for the type of the water treatment membrane above, the present invention can be preferably used for a microfiltration membrane, an ultrafiltration membrane, a nanofiltration membrane, a reverse osmosis membrane, a forward osmosis membrane, etc.