CA1090520A - Hollow fiber having selective gas permeability and process for preparation thereof - Google Patents
Hollow fiber having selective gas permeability and process for preparation thereofInfo
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- CA1090520A CA1090520A CA261,993A CA261993A CA1090520A CA 1090520 A CA1090520 A CA 1090520A CA 261993 A CA261993 A CA 261993A CA 1090520 A CA1090520 A CA 1090520A
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Abstract
ABSTRACT OF THE DISCLOSURE
An asymmetric hollow fiber having a selective gas permeability, said fiber having an outer diameter of 0.2 to 3 mm, a ratio of outer diameter/
inner diameter in the range of from 1.1/1.0 to 3.0/1.0 and a nitrogen gas permeability of from 5 % 10-4 to 0.1 cm3 (STF)/cm2?sec?cmHg, and wherein the permeability of the fiber to different gases is proportional to the reciprocals of the square roots of the molecular weights of the respective gases. There is also provided a coated hollow fiber as aforesaid having an external coating film of a film-forming polymer, said film having a thickness of from 0.1 to 300 microns. The hollow fiber is prepared by extruding through the outer tube of a double tube spinneret a solution containing 22 to 33% by weight of a cellulose derivative in a solvent and simultaneously flowing a aqueous liquid or gas through the inner-tube of the spinneret.
The spinning solution is extruded into as aqueous coagulating both having a temperature of 0° to 45°C, and after removal therefrom is dipped in water having a temperature of 30° to 100°C. Then free water is removed from the fiber and the fiber is subjected to freeze-drying under vacuum. The coated fiber is prepared by immersing the above-mentioned fiber in a bath containing 0.2 to 5% by weight of the polymer dissolved in a solvent which is a non-solvent for the cellulose derivative.
An asymmetric hollow fiber having a selective gas permeability, said fiber having an outer diameter of 0.2 to 3 mm, a ratio of outer diameter/
inner diameter in the range of from 1.1/1.0 to 3.0/1.0 and a nitrogen gas permeability of from 5 % 10-4 to 0.1 cm3 (STF)/cm2?sec?cmHg, and wherein the permeability of the fiber to different gases is proportional to the reciprocals of the square roots of the molecular weights of the respective gases. There is also provided a coated hollow fiber as aforesaid having an external coating film of a film-forming polymer, said film having a thickness of from 0.1 to 300 microns. The hollow fiber is prepared by extruding through the outer tube of a double tube spinneret a solution containing 22 to 33% by weight of a cellulose derivative in a solvent and simultaneously flowing a aqueous liquid or gas through the inner-tube of the spinneret.
The spinning solution is extruded into as aqueous coagulating both having a temperature of 0° to 45°C, and after removal therefrom is dipped in water having a temperature of 30° to 100°C. Then free water is removed from the fiber and the fiber is subjected to freeze-drying under vacuum. The coated fiber is prepared by immersing the above-mentioned fiber in a bath containing 0.2 to 5% by weight of the polymer dissolved in a solvent which is a non-solvent for the cellulose derivative.
Description
10~520 BACKGROUND OT THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a membrane for separsting gases, said membrane being composed of a cellulose fiber that posseæses different permeabilities to different gases, and to a process for preparing that membrsne.
More particularly, the present invention relates to a hollow fiber having a high gas permeation speed, a selective gas permeability snd a high pressure resistance, a hollow fiber as aforesaid coated with a polymeric substance, and to processes for the preparation ~f such hollow fibers.
As gas-separating polymeric membranes, there are known a so-called homogeneous film, a porous film consisting of sn entirely porous structure a~d a so-called asymmetric film comprised of a very thin homogeneous layer (0.3 to 0.7 ~) present as a surface layer on a porous ~upporting layer.
Mbmbranes composed of a homogeneous film or an asymmetric fil~ have the in-herent selective permeability of the polymer of which the film is msde. In membrsnes of this type, it is known that the gss permeation speed or gas permeability is generally proportional to the film srea snd the pressure and is inversely proportional to the film thickness (the thickness of the very thin homogeneous layer in case of the asy~metric film).
In the case of a porous film, the gas permestion speed is proportional to the reciprGcal of the square root of the molecular weight of the gas, snd it is much higher than that of the homogeneous film. According to our experiments, it has been found that the gas permeation speed of a porous film is 104 to 107 times the gas permeation speed of a homogeneous film.
In view of the foregoing, it is recognized that in orter to perform gas separation effectively, it is necessary to increase the permeation area and the pressure difference and to reduce the film thicknes6, when a homogeneous film is employed.
When a planar permeation membrane is employed, in order to enab1e the membrane to withstand a high pre6sure it is necessary to increase the membrane thickness and to provide a porous supporting layer.
These requirements can be satisfied by using a hollow fiber including an asymmetric film. More specifically, a hollow fiber having a small diameter has a higher pressure resistance and provides a larger surface area in comparison with the same weight of a planar film. Further, if the thickness of the homogeneous portion of the asymmetric film is reduced, the permeation speed can be increased.
However, it is very difficult to prepare an asymmetric hollow fiber, and in connection with all polymeric sub6tances, asymmetric hollow fibers having a good selective gas permeability have hardly been known in the art, Thiæ invention relates to an asymmetric hollow fiber made of a fiber-forming cellulose ester or a fiber-forming cellulose ether, said hollow fiber having an outer diameter of from 0.2 to 3 mm, a ratio of outer diameter/inner diameter in the range of from 1.1/1.0 to 3.0/1.0, and a nitrogen gas permeability in the range of from 5 x 10 4 to 0.1 cm3 at standard temperature and pressure/cm2 sec cmHg, the relative permeability of said fiber to varioufi ga~es being sub6tantially in proportion to the reciprocals of the square roots of the molecular weights of the respective gases.
This invention also relates to a process for preparing a hollow fiber which comprises extruding through the annular space between the inner and outer tubes of a double tube spinneret, a solution containing from about 22 to about 33 percent by weight of a fiber-forming cellulose ester or a fiber-forming cellulose ether dissolved in an organic solvent and having a viscosity of from about 300 to about 1500 poise at a temperature of 20C
and simultaneously flowing through the inner tube of said spinneret an aqueous liquid or an inert gas, said spinneret being submerged in an aqueous coagulating liquid bath having a temperature of from zero to 45 C whereby to coagulate said solution directly in said bath to produce a hollow fiber;
removing the fiber from said coagulating bath and then, without permitting said fiber to dry, immersing it in water having a temperature of 30 to 100 C;
then reving free water present on the exterior and interior surfaces of said fiber; then freezing and vacuum-drying the fiber.
According to a first embodiment of the present invention, there i8 provided a membrane composed of a hollow fiber of a cellulose derivative, which fiber has a very high gas permeation speed and a separating capacity substantially in proportion to the reciprocal of the square root of the -3a-molecular weight of the gas, and a process for preparing such a fiber.
However, it has been found that it is difficult to separate a mixture of gases in which the difference of the molecular weights is small, into respective gases at a high efficiency by using this membrane.
According to a second embodiment of the invention, there is provided a hollow ffber as in the above-mentioned first embodiment which is coated on its surface with a very thin homogeneous polymer film having a high gas permeability, and a process for preparing same. The fiber of the second embodiment is unexpectedly superior to the fiber of the first embodiment because it can effect separation of a mixture of gases having similar molecular weights with high efficiency.
Various semi-permeable membranes having a two-layer structure are known in the art. For example, there is known a hollow fiber semi-permeable membrane having a good reverse osmosis capacity, which is made by the con~en-tric sheath-core composite spinning method and which com-prises a core of a polymer composition providing a porous structure and a sheath of a polymer composition providing a compact structure. According to this method, however, a complicated apparatus must be used, and it is expected that a high operation control will be required. Fbrther, there is known a semipermeable membrane having a two-layer structure which is formed by coating a polymer solution on a polyolefin film having dispersed therein a salt which is soluble in water 109~520 :
or the like and then removing the soluble salt by extrac-tion. This method is different from our method in the point that in this conventional method, if the sequence of the step of coating the polymer solution and the step of removing the soluble salt by extraction is reversed, clogging occurs in the porous structure and a fiber having the intended properties cannot be obtained. There is also-known a method in which a thin film is crosslinked and coated on a porous film by plasma polymerization. In view of difficulties involved in the apparatus and operation of this process, it is considered that this method is lacking in practical utility.
We have discovered, according to the first embodiment of the invention, a process in which hollow fibers having a selective gas perme~bi~ity and possessing various addi-tional properties can be obtained by a simple procedure.
According to the second embodiment of the invention, . we have discovered that an unexpectedly improved hollow fiber having a selective gas permeability can be prepared by coating a polymer solution having a prescribed concen-.
tration onto the hollow fiber of the first embodiment, by dipping or the li~e, as illustrated in the Examples given hereinafter.
. More specifically,. the first embodiment of the present invention provides a process for preparing hollow fibers having a selective gas permeability which comprises the steps of dissolving a cellulose derivative in a solvent to form a solution having a solid content of 22 to 33% by weight, extruding said solution from the annular space between the concentric inner and outer tubes of a double tube-type lO9G520 spinneret and simultaneously feeding an aqueous liquid or a gas from the interior of the inner tube of said spinneret directly into an aqueous coagulating bath maintained at 0 to 45C to form a hollow fiber, dipping the hollow fiber in warm water maintained at 30 to 100C without drying, taking the hollow fiber out of the water bath, removing water from the hollow fiber inclusive of the water present in interior hollow portion, and immediately freeze-drying the hollow fiber under vacuum to obtain the fiber of the first embodiment of the invention.
The fiber of the second embodiment of the invention is prepared by coating the exterior surface of the thus-obtained hollow fiber wi~h a solution formed by dissolving a polymer in a solvent which is incapable of dissolving said cellulose derivative so that the solid (polymer) content of this coating solution is 0.2 to 5.0~ by weight, thereby to form on the surface of the hollow fiber a homogeneous and thin coating film of the polymer,said coating film having a thick-ness of 0.1 to 300 ~. The hollow fiber of the first embodi-ment of the invention prepared according to the above-described process, namely, a hollow fiber having a æelective gas permeability consists essentially of an asymmetric hollow fiber of a cellulose derivative, said asymmetric hollow fiber ~a) having an outer ~iameter of from 0.2 to 3 mm and an outer diameter/inner diameter ratio of from 1.1 to 3.0/1 and ~b) having a nitrogen gas permeability of 5 x 10 4 r~ 3 2 ~ ~ ~
to 0.1 (cm (STP)/cm sec cmHg), ~horoin its permeability to various gases is substantially in proportion to the reciprocal of the square roots of the moiecular weights of the respective gases.
lO~C-SZO
The hollow fiber of the second embodiment of the in-vention consists essentially of the hollow fiber of the first embodiment additionally coated on its exterior surface with a coating film of a polymer, said film having a thickness of from 0.1 to 300 ~.
As the cellulose derivative that i5 used in the present invention, there can be mentioned, for example, cellulose acetate, cellulose acetate butyrate~cellulose propionate, cellulose acetate propionate, cellulose nitrate and ethyl cellulose having a substitution degree of from 2.1 to 2.6.
Acetone-soluble cellulose acetate is especially preferred.
Acetone alone or a mixture of acetone wfth one or more additional solvents such as, for example, dimethyl formamide, formamide, 2-methoxyethyl acetate, 1,4-dioxane or 1,3-dioxolan can be used for dissolving such cellulose derivative to form the starting solution for the extrusion step. In order to form fine pores, a swelling agent or an inorganic salt such as potassium hypochlorite may be added to the solvent. The use of an acetone-formamide mixed solvent or acetone-1,3-dioxane mixed solvent is preferred. A cellulose derivative such as mentioned above is dissolved in such solvent so that its concentration in the solution is from 22 to 33~ by weight, which provides a viscosity of 300 to 1500 poises (measured at 20C) suitable for the formation of fibers. The solution is allowed to stand still to remoYe - gases sufficiently. Then, the solution is fed quantitatively to the ar~ular space between the concentric outer and inner tubes of a spinneret of a double tube structure by a gear pump, and simultaneously, an aqueous liquid compatible with, the solvent of the solution, or an lnert gas, is fed to the inner tube of the spinneret by a metering pump or the like, lOg~S20 and thus, composite spinning of the solution, as the sheath, and the aqueous liquid or gas, as the core, is performed.
In order to facilitate the spinning operation, it is neces-sary to maintain the viscosity of the solution within a certain range. If the viscosity is lower than 300 poises, the solution does not possess a fiber-forming property and because the extrudate is readily broken by a very slight change of the tension, it is impossible to conduct con-tinuous spinning. If the viscosity of the solution is higher than 15~0 poises, because the pressure of the spinning solution must be very high before it exits from the spin-neret, it is difficult to feed the-solution by an ordinary spinning gear pump, and because the extruded fiber cannot be drawn, it is impossible to conduct the spinning operation in a good condition.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a diagrammatic illustration of a portion of the process.
Figs. 2, 3 and 4 are scanning electron microscopic photographs lmagnification: 10000 x) of the hollow fibers of Comparative Examples 1 and 4 and Example 1, respectively).
Referring now to Fig. 1 which illustrates a part of the process for preparing the ho~low fiber, a cellulose acetate dope formed by dissolving cellulose acetate in a solvent and removing gas therefrom is fed to an outer tube - 4 of a spinneret 3 from a tan~ 1 by a gear pump 2 and is spun into a coagulating bath 14. An aqueous liquid is fed into the central portion of the fiber from a tank 5 through an inner tube 7 of the spinneret by a metering pump 6 to form the core of the hollow fi~er.
In this step, a gas can be used instead of or to-gether with the aqueous liquid. The spun and coagulated fiber 8 is wound on a bobbin 13 after passing around a guide 9. I~hen a prescribed length of the fiber is wound on the bobbin, the fiber in the wet state is transferred to the next step of the warm water dipping treatment.
Solvent-free water maintalned at a prescribed temperature is supplied to the coagulating bath tank 10 from a supply coagulating water inlet 11, while a part of the coagula-ting liquid is discharged by a siphon 1~ so that an increaseof the solvent concentration is prevented.
As described above, the cellulose derivative solution (dope) is directly extruded into an aqueous coagulating solution to form a hollow fiber. In this point, the process of the present invention is different from the majority of conventional methods in which the technique of extrusion into air is adopted.
In the present invention, especially good results are obtained with respect-to the gas permeation speed when the spinneret is submerged in the aqueous coagulating bath, namely, the extruded fiber is not exposed to air, and the value of the gas permeation speed in this case is more than ', several hundred times greater than the values attainable according to the conventional methods.
lhe temperature of the cellulose derivative solution supplied to the spinneret can be a temperature approximating ambient temperature (15 to 30C), and the temperature of the aqueous coagulating solution into which the cellulose derivative solution is extruded is within the range of 0 to 45C. When the temperature of the aqueous coagulating 1C~9~5ZO
solution is higher than 50C, the gas permeability of the resulting hollow fiber is reduced. Accordingly, in the pr-esent invention, it is preferred that the temperature of the aqueous coagulating solution be within a range of 0 to 45C.
It is preferred that the temperature of the aqueous injection liquid extruded, as the core, from the inner tube of the double tube spinneret be higher than 0C, and in general, temperatures approximating ambient temperature are employed. Hollow fibers differing in the ratio of inner diameter/outer diameter can be prepared by changing the amount of the aqueous injection liquid. I
As pointed out hereinbefore, gases such as air and ?
nitrogen can be injected instead of the aqueous injection liquid, under the same temperature conditions.
Both the injection liquid and the-coagulating solution are composed mainly of water, and various acids, salts, bases of surface active agents can be incorporated if needed.
After the spun hollow fiber has been allowed to stand still in the coagulating solution for preferably at least 10 minutes, it is dipped in a bath of warm water maintained at 30 to 100C for at least 3 minutes, and it is then taken out from the water bath and water present on the outside and interior surfaces of the hollow fiber is removed by, for example, wiping away water present on the outside of the hollow fi~er and blowing away water present in the interior of the hollow fiber by using high pressure air or nitrogen gas. Immediately thereafter, the hollow fiber is frozen and vacuum-dried. More specifically, the hollow fiber is dipped in a coolant cooled below -40C. The coolant is a non-solvent for the cellulose derivative and is immiscible with water, for example, gasoline, fluorodichloromethane, pentane, cyclopentane, hexane or heptane. The hollow fiber is thereby frozen quickly, and the frozen hollow fiber is dried under a high vacuum of less than 0.5 mm Hg for more than 12 hours in a freeze-drier maintained below -30VC, preferably -35C. Thus, there is obtained an asymmetric hollow fiber having an outer diameter of 0.2 to 3 mm and an outer diameter/inner diameter ratio of 1.1 to 3.0/1Ø
Properties of the gas-separating hollow fiber of the present invention will now be described by reference to the hollow fiber obtained in Example15given hereinafter.
The outer diameter/inner diameter ratio is 1.14, and the nitrogen gas permeation speed is 3.53 x 10 3 cc/cm2 sec -cmHg. The nitrogen gas permeation speed of a prior art asymmetric cellulose acetate film is, for example, from O.06 x 10 5 to 0.3I x 10 5 cc/cm2-sec-mmHg as disclosed in Journal of Plastics, 24, 12, p. 19. Accordingly, it is seen that the hollow fiber of the pFesent invention has a nitrogen gas permeation speed at least several hundred times greater than the gas permeation speed of the conventional product according to the prior art. With respect to-this hollow fiber, when gas permeation speeds for various gases are measured and their ratios to the nitrogen gas permea-tion speed, namely the separation coefficients, are calculated, it is found that the permeabilities of the hollow fiber of the present invention to various gases are substantially in proportion to the reciprocal of the square root of the molecular weight of the respective gases.
According to the second embodiment of the invention, a film of a film-forming polymer is coated on the exterior surface of the thus-prepared hollow fiber as the substrate.
When the hollow fiber is composed of acetone-soluble cellulose acetate, a solution containing 0.2 to 5~ by-weight of a polymer such as a silicone polymer, a cellulose derivative (for example, ethyl cellulose having a substitution degree of from 2.1 to 2.6), natural rubber, polyisoprene, polybutadiene, polysuifone,other polymeric compound,copolymers such as butadienestyrene copolymer or polymer blends, optionally containing a plasticizer or other additive, is prepared by dissolving the polymer in a solvent having reduced dissolving and swelling properties with respect to the hollow fiber, preferably, pentane, gasoline, xylene, toluene or benzene. The hollow fiber, both ends of which are sealed so that the solution does not enter into the interior of the fiber, is dipped in the polymer solution or is coated with the polymer solution. After the dipping or coating treatment, the fiber is dried whereby to form on the external surface of the fiber a polymer coating film having a thickness of 0.1 to 300 ~.
When the concentration of the polymer solution is less than 0.2~ by weight, no improvement of the gas-separating capacity is attained by the coating treatment, as specifi-cally illustrated hereinafter. If-the concentrat~on is higher tAan 5% by weight, the thickness of the film ~s too large and the gas permea~ility is reduced. If the thickness of the coating film is increased, the separation capacity is slightly improved, but this is not preferred from the practical viewpoint because the permeation speed is drastically lowered.
Fr~m the data of gas permeabilities of ethyl cellulose and silicone disclosed in "Handbook of Materials and nater Contents" compiled by Co~mittee of Polymers and Hygro-. scopicity, Polymer Academy and published by Kyoritsu Shuppan, the gas permeation speeds of films of hollow fibers of the same shape prepared from these polymers are calculated, and the calculated values are compared with the gas permeation speeds of the coated hollow fibers of the present invention to obtain results shown in Table 1.
It is seen that in case of ethyl cellulose, the coated :
hollow fiber of the present invention has a gas permeation speed about 50 times as high as the comparative sample and in case of silicone the coated hollow fiber Gf the present invention has.a g~s permeation speed ? to 3 times . as high as the gas permeation speed of the comparative 1 sample. Based on the suppos tion that the gas permeation s~eed is controlled by the coated polymer, the thickness of the ethyl cellulose film and the thickness of the silicone film are calculated to be 1.8 ~ and 40 ~, respectively.
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1~9C520 The present invention will row be further described in detail by reference to the following illustrative Examples, in which all references to "%" are percent by weight.
Example 1 and Comparative Examples 1-4 A solution (dope) consisting of 26% by weight of cellu-lose acetate (Eastman Kodak's E 400-25), 49~ by weight of acetone and 25% by weight of formamide was subjected to spinning and hot water immersion under the conditions specified in Table 1 by using the apparatus of Fig. 1, followed by immersion in gasoline cooled to -50C for quick freezing the fiber and then vacuum drying the fiber at -40C
and 0.01 mmHg for 24 hours. The nitrogen gas permeability of the obtained dry hollow fibers was measured by placing the fiber in a pressure container, introducing pure nitrogen gas into the vessel externally of the fiber and measuring the flow rate of the nitrogen gas that permeates into the hollow por-tion to determine gas permeability. The outer and inner diameters of the obtained hollow fibers are also shown in Table 2.
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lOgO520 The permeation rates of various kinds of gases were measured for the respective fibers obtained in Example 1 and Comparative Example 4, and the ratio of such permeation rate of each gas to that of nitrogen, that is, the separa-tion factor, was determined. The results are shown in Table 3. The film remained free of any damage in each case e~en when the gas feed pressure at the time of measurement of the gas permeation rate was raised to 10 kg/cm2. This shows -the existence of a linear relationship between permeability and feed pressure.
Table 3 Separation Factor Specimen H2/N2 He/N2 2/N2 2 Comparat ve4 40 4.20 0.960 0.813 Example 13.40 2.40 0.916 0.854 Value (Note) 2.64 0.935 0.837 (Note) "Theoretical value" is the ratio cf the reciprocals of the square roots of the molecular weights of the respective gases.
The scanning electron microscopic photographs (magnifi-cation : lOOOOX) of sections of the hollow fibers obtained in Comparative Examples 1 and 4 and Example 1 are shown in Figs.
FIELD OF THE INVENTION
The present invention relates to a membrane for separsting gases, said membrane being composed of a cellulose fiber that posseæses different permeabilities to different gases, and to a process for preparing that membrsne.
More particularly, the present invention relates to a hollow fiber having a high gas permeation speed, a selective gas permeability snd a high pressure resistance, a hollow fiber as aforesaid coated with a polymeric substance, and to processes for the preparation ~f such hollow fibers.
As gas-separating polymeric membranes, there are known a so-called homogeneous film, a porous film consisting of sn entirely porous structure a~d a so-called asymmetric film comprised of a very thin homogeneous layer (0.3 to 0.7 ~) present as a surface layer on a porous ~upporting layer.
Mbmbranes composed of a homogeneous film or an asymmetric fil~ have the in-herent selective permeability of the polymer of which the film is msde. In membrsnes of this type, it is known that the gss permeation speed or gas permeability is generally proportional to the film srea snd the pressure and is inversely proportional to the film thickness (the thickness of the very thin homogeneous layer in case of the asy~metric film).
In the case of a porous film, the gas permestion speed is proportional to the reciprGcal of the square root of the molecular weight of the gas, snd it is much higher than that of the homogeneous film. According to our experiments, it has been found that the gas permeation speed of a porous film is 104 to 107 times the gas permeation speed of a homogeneous film.
In view of the foregoing, it is recognized that in orter to perform gas separation effectively, it is necessary to increase the permeation area and the pressure difference and to reduce the film thicknes6, when a homogeneous film is employed.
When a planar permeation membrane is employed, in order to enab1e the membrane to withstand a high pre6sure it is necessary to increase the membrane thickness and to provide a porous supporting layer.
These requirements can be satisfied by using a hollow fiber including an asymmetric film. More specifically, a hollow fiber having a small diameter has a higher pressure resistance and provides a larger surface area in comparison with the same weight of a planar film. Further, if the thickness of the homogeneous portion of the asymmetric film is reduced, the permeation speed can be increased.
However, it is very difficult to prepare an asymmetric hollow fiber, and in connection with all polymeric sub6tances, asymmetric hollow fibers having a good selective gas permeability have hardly been known in the art, Thiæ invention relates to an asymmetric hollow fiber made of a fiber-forming cellulose ester or a fiber-forming cellulose ether, said hollow fiber having an outer diameter of from 0.2 to 3 mm, a ratio of outer diameter/inner diameter in the range of from 1.1/1.0 to 3.0/1.0, and a nitrogen gas permeability in the range of from 5 x 10 4 to 0.1 cm3 at standard temperature and pressure/cm2 sec cmHg, the relative permeability of said fiber to varioufi ga~es being sub6tantially in proportion to the reciprocals of the square roots of the molecular weights of the respective gases.
This invention also relates to a process for preparing a hollow fiber which comprises extruding through the annular space between the inner and outer tubes of a double tube spinneret, a solution containing from about 22 to about 33 percent by weight of a fiber-forming cellulose ester or a fiber-forming cellulose ether dissolved in an organic solvent and having a viscosity of from about 300 to about 1500 poise at a temperature of 20C
and simultaneously flowing through the inner tube of said spinneret an aqueous liquid or an inert gas, said spinneret being submerged in an aqueous coagulating liquid bath having a temperature of from zero to 45 C whereby to coagulate said solution directly in said bath to produce a hollow fiber;
removing the fiber from said coagulating bath and then, without permitting said fiber to dry, immersing it in water having a temperature of 30 to 100 C;
then reving free water present on the exterior and interior surfaces of said fiber; then freezing and vacuum-drying the fiber.
According to a first embodiment of the present invention, there i8 provided a membrane composed of a hollow fiber of a cellulose derivative, which fiber has a very high gas permeation speed and a separating capacity substantially in proportion to the reciprocal of the square root of the -3a-molecular weight of the gas, and a process for preparing such a fiber.
However, it has been found that it is difficult to separate a mixture of gases in which the difference of the molecular weights is small, into respective gases at a high efficiency by using this membrane.
According to a second embodiment of the invention, there is provided a hollow ffber as in the above-mentioned first embodiment which is coated on its surface with a very thin homogeneous polymer film having a high gas permeability, and a process for preparing same. The fiber of the second embodiment is unexpectedly superior to the fiber of the first embodiment because it can effect separation of a mixture of gases having similar molecular weights with high efficiency.
Various semi-permeable membranes having a two-layer structure are known in the art. For example, there is known a hollow fiber semi-permeable membrane having a good reverse osmosis capacity, which is made by the con~en-tric sheath-core composite spinning method and which com-prises a core of a polymer composition providing a porous structure and a sheath of a polymer composition providing a compact structure. According to this method, however, a complicated apparatus must be used, and it is expected that a high operation control will be required. Fbrther, there is known a semipermeable membrane having a two-layer structure which is formed by coating a polymer solution on a polyolefin film having dispersed therein a salt which is soluble in water 109~520 :
or the like and then removing the soluble salt by extrac-tion. This method is different from our method in the point that in this conventional method, if the sequence of the step of coating the polymer solution and the step of removing the soluble salt by extraction is reversed, clogging occurs in the porous structure and a fiber having the intended properties cannot be obtained. There is also-known a method in which a thin film is crosslinked and coated on a porous film by plasma polymerization. In view of difficulties involved in the apparatus and operation of this process, it is considered that this method is lacking in practical utility.
We have discovered, according to the first embodiment of the invention, a process in which hollow fibers having a selective gas perme~bi~ity and possessing various addi-tional properties can be obtained by a simple procedure.
According to the second embodiment of the invention, . we have discovered that an unexpectedly improved hollow fiber having a selective gas permeability can be prepared by coating a polymer solution having a prescribed concen-.
tration onto the hollow fiber of the first embodiment, by dipping or the li~e, as illustrated in the Examples given hereinafter.
. More specifically,. the first embodiment of the present invention provides a process for preparing hollow fibers having a selective gas permeability which comprises the steps of dissolving a cellulose derivative in a solvent to form a solution having a solid content of 22 to 33% by weight, extruding said solution from the annular space between the concentric inner and outer tubes of a double tube-type lO9G520 spinneret and simultaneously feeding an aqueous liquid or a gas from the interior of the inner tube of said spinneret directly into an aqueous coagulating bath maintained at 0 to 45C to form a hollow fiber, dipping the hollow fiber in warm water maintained at 30 to 100C without drying, taking the hollow fiber out of the water bath, removing water from the hollow fiber inclusive of the water present in interior hollow portion, and immediately freeze-drying the hollow fiber under vacuum to obtain the fiber of the first embodiment of the invention.
The fiber of the second embodiment of the invention is prepared by coating the exterior surface of the thus-obtained hollow fiber wi~h a solution formed by dissolving a polymer in a solvent which is incapable of dissolving said cellulose derivative so that the solid (polymer) content of this coating solution is 0.2 to 5.0~ by weight, thereby to form on the surface of the hollow fiber a homogeneous and thin coating film of the polymer,said coating film having a thick-ness of 0.1 to 300 ~. The hollow fiber of the first embodi-ment of the invention prepared according to the above-described process, namely, a hollow fiber having a æelective gas permeability consists essentially of an asymmetric hollow fiber of a cellulose derivative, said asymmetric hollow fiber ~a) having an outer ~iameter of from 0.2 to 3 mm and an outer diameter/inner diameter ratio of from 1.1 to 3.0/1 and ~b) having a nitrogen gas permeability of 5 x 10 4 r~ 3 2 ~ ~ ~
to 0.1 (cm (STP)/cm sec cmHg), ~horoin its permeability to various gases is substantially in proportion to the reciprocal of the square roots of the moiecular weights of the respective gases.
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The hollow fiber of the second embodiment of the in-vention consists essentially of the hollow fiber of the first embodiment additionally coated on its exterior surface with a coating film of a polymer, said film having a thickness of from 0.1 to 300 ~.
As the cellulose derivative that i5 used in the present invention, there can be mentioned, for example, cellulose acetate, cellulose acetate butyrate~cellulose propionate, cellulose acetate propionate, cellulose nitrate and ethyl cellulose having a substitution degree of from 2.1 to 2.6.
Acetone-soluble cellulose acetate is especially preferred.
Acetone alone or a mixture of acetone wfth one or more additional solvents such as, for example, dimethyl formamide, formamide, 2-methoxyethyl acetate, 1,4-dioxane or 1,3-dioxolan can be used for dissolving such cellulose derivative to form the starting solution for the extrusion step. In order to form fine pores, a swelling agent or an inorganic salt such as potassium hypochlorite may be added to the solvent. The use of an acetone-formamide mixed solvent or acetone-1,3-dioxane mixed solvent is preferred. A cellulose derivative such as mentioned above is dissolved in such solvent so that its concentration in the solution is from 22 to 33~ by weight, which provides a viscosity of 300 to 1500 poises (measured at 20C) suitable for the formation of fibers. The solution is allowed to stand still to remoYe - gases sufficiently. Then, the solution is fed quantitatively to the ar~ular space between the concentric outer and inner tubes of a spinneret of a double tube structure by a gear pump, and simultaneously, an aqueous liquid compatible with, the solvent of the solution, or an lnert gas, is fed to the inner tube of the spinneret by a metering pump or the like, lOg~S20 and thus, composite spinning of the solution, as the sheath, and the aqueous liquid or gas, as the core, is performed.
In order to facilitate the spinning operation, it is neces-sary to maintain the viscosity of the solution within a certain range. If the viscosity is lower than 300 poises, the solution does not possess a fiber-forming property and because the extrudate is readily broken by a very slight change of the tension, it is impossible to conduct con-tinuous spinning. If the viscosity of the solution is higher than 15~0 poises, because the pressure of the spinning solution must be very high before it exits from the spin-neret, it is difficult to feed the-solution by an ordinary spinning gear pump, and because the extruded fiber cannot be drawn, it is impossible to conduct the spinning operation in a good condition.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a diagrammatic illustration of a portion of the process.
Figs. 2, 3 and 4 are scanning electron microscopic photographs lmagnification: 10000 x) of the hollow fibers of Comparative Examples 1 and 4 and Example 1, respectively).
Referring now to Fig. 1 which illustrates a part of the process for preparing the ho~low fiber, a cellulose acetate dope formed by dissolving cellulose acetate in a solvent and removing gas therefrom is fed to an outer tube - 4 of a spinneret 3 from a tan~ 1 by a gear pump 2 and is spun into a coagulating bath 14. An aqueous liquid is fed into the central portion of the fiber from a tank 5 through an inner tube 7 of the spinneret by a metering pump 6 to form the core of the hollow fi~er.
In this step, a gas can be used instead of or to-gether with the aqueous liquid. The spun and coagulated fiber 8 is wound on a bobbin 13 after passing around a guide 9. I~hen a prescribed length of the fiber is wound on the bobbin, the fiber in the wet state is transferred to the next step of the warm water dipping treatment.
Solvent-free water maintalned at a prescribed temperature is supplied to the coagulating bath tank 10 from a supply coagulating water inlet 11, while a part of the coagula-ting liquid is discharged by a siphon 1~ so that an increaseof the solvent concentration is prevented.
As described above, the cellulose derivative solution (dope) is directly extruded into an aqueous coagulating solution to form a hollow fiber. In this point, the process of the present invention is different from the majority of conventional methods in which the technique of extrusion into air is adopted.
In the present invention, especially good results are obtained with respect-to the gas permeation speed when the spinneret is submerged in the aqueous coagulating bath, namely, the extruded fiber is not exposed to air, and the value of the gas permeation speed in this case is more than ', several hundred times greater than the values attainable according to the conventional methods.
lhe temperature of the cellulose derivative solution supplied to the spinneret can be a temperature approximating ambient temperature (15 to 30C), and the temperature of the aqueous coagulating solution into which the cellulose derivative solution is extruded is within the range of 0 to 45C. When the temperature of the aqueous coagulating 1C~9~5ZO
solution is higher than 50C, the gas permeability of the resulting hollow fiber is reduced. Accordingly, in the pr-esent invention, it is preferred that the temperature of the aqueous coagulating solution be within a range of 0 to 45C.
It is preferred that the temperature of the aqueous injection liquid extruded, as the core, from the inner tube of the double tube spinneret be higher than 0C, and in general, temperatures approximating ambient temperature are employed. Hollow fibers differing in the ratio of inner diameter/outer diameter can be prepared by changing the amount of the aqueous injection liquid. I
As pointed out hereinbefore, gases such as air and ?
nitrogen can be injected instead of the aqueous injection liquid, under the same temperature conditions.
Both the injection liquid and the-coagulating solution are composed mainly of water, and various acids, salts, bases of surface active agents can be incorporated if needed.
After the spun hollow fiber has been allowed to stand still in the coagulating solution for preferably at least 10 minutes, it is dipped in a bath of warm water maintained at 30 to 100C for at least 3 minutes, and it is then taken out from the water bath and water present on the outside and interior surfaces of the hollow fiber is removed by, for example, wiping away water present on the outside of the hollow fi~er and blowing away water present in the interior of the hollow fiber by using high pressure air or nitrogen gas. Immediately thereafter, the hollow fiber is frozen and vacuum-dried. More specifically, the hollow fiber is dipped in a coolant cooled below -40C. The coolant is a non-solvent for the cellulose derivative and is immiscible with water, for example, gasoline, fluorodichloromethane, pentane, cyclopentane, hexane or heptane. The hollow fiber is thereby frozen quickly, and the frozen hollow fiber is dried under a high vacuum of less than 0.5 mm Hg for more than 12 hours in a freeze-drier maintained below -30VC, preferably -35C. Thus, there is obtained an asymmetric hollow fiber having an outer diameter of 0.2 to 3 mm and an outer diameter/inner diameter ratio of 1.1 to 3.0/1Ø
Properties of the gas-separating hollow fiber of the present invention will now be described by reference to the hollow fiber obtained in Example15given hereinafter.
The outer diameter/inner diameter ratio is 1.14, and the nitrogen gas permeation speed is 3.53 x 10 3 cc/cm2 sec -cmHg. The nitrogen gas permeation speed of a prior art asymmetric cellulose acetate film is, for example, from O.06 x 10 5 to 0.3I x 10 5 cc/cm2-sec-mmHg as disclosed in Journal of Plastics, 24, 12, p. 19. Accordingly, it is seen that the hollow fiber of the pFesent invention has a nitrogen gas permeation speed at least several hundred times greater than the gas permeation speed of the conventional product according to the prior art. With respect to-this hollow fiber, when gas permeation speeds for various gases are measured and their ratios to the nitrogen gas permea-tion speed, namely the separation coefficients, are calculated, it is found that the permeabilities of the hollow fiber of the present invention to various gases are substantially in proportion to the reciprocal of the square root of the molecular weight of the respective gases.
According to the second embodiment of the invention, a film of a film-forming polymer is coated on the exterior surface of the thus-prepared hollow fiber as the substrate.
When the hollow fiber is composed of acetone-soluble cellulose acetate, a solution containing 0.2 to 5~ by-weight of a polymer such as a silicone polymer, a cellulose derivative (for example, ethyl cellulose having a substitution degree of from 2.1 to 2.6), natural rubber, polyisoprene, polybutadiene, polysuifone,other polymeric compound,copolymers such as butadienestyrene copolymer or polymer blends, optionally containing a plasticizer or other additive, is prepared by dissolving the polymer in a solvent having reduced dissolving and swelling properties with respect to the hollow fiber, preferably, pentane, gasoline, xylene, toluene or benzene. The hollow fiber, both ends of which are sealed so that the solution does not enter into the interior of the fiber, is dipped in the polymer solution or is coated with the polymer solution. After the dipping or coating treatment, the fiber is dried whereby to form on the external surface of the fiber a polymer coating film having a thickness of 0.1 to 300 ~.
When the concentration of the polymer solution is less than 0.2~ by weight, no improvement of the gas-separating capacity is attained by the coating treatment, as specifi-cally illustrated hereinafter. If-the concentrat~on is higher tAan 5% by weight, the thickness of the film ~s too large and the gas permea~ility is reduced. If the thickness of the coating film is increased, the separation capacity is slightly improved, but this is not preferred from the practical viewpoint because the permeation speed is drastically lowered.
Fr~m the data of gas permeabilities of ethyl cellulose and silicone disclosed in "Handbook of Materials and nater Contents" compiled by Co~mittee of Polymers and Hygro-. scopicity, Polymer Academy and published by Kyoritsu Shuppan, the gas permeation speeds of films of hollow fibers of the same shape prepared from these polymers are calculated, and the calculated values are compared with the gas permeation speeds of the coated hollow fibers of the present invention to obtain results shown in Table 1.
It is seen that in case of ethyl cellulose, the coated :
hollow fiber of the present invention has a gas permeation speed about 50 times as high as the comparative sample and in case of silicone the coated hollow fiber Gf the present invention has.a g~s permeation speed ? to 3 times . as high as the gas permeation speed of the comparative 1 sample. Based on the suppos tion that the gas permeation s~eed is controlled by the coated polymer, the thickness of the ethyl cellulose film and the thickness of the silicone film are calculated to be 1.8 ~ and 40 ~, respectively.
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1~9C520 The present invention will row be further described in detail by reference to the following illustrative Examples, in which all references to "%" are percent by weight.
Example 1 and Comparative Examples 1-4 A solution (dope) consisting of 26% by weight of cellu-lose acetate (Eastman Kodak's E 400-25), 49~ by weight of acetone and 25% by weight of formamide was subjected to spinning and hot water immersion under the conditions specified in Table 1 by using the apparatus of Fig. 1, followed by immersion in gasoline cooled to -50C for quick freezing the fiber and then vacuum drying the fiber at -40C
and 0.01 mmHg for 24 hours. The nitrogen gas permeability of the obtained dry hollow fibers was measured by placing the fiber in a pressure container, introducing pure nitrogen gas into the vessel externally of the fiber and measuring the flow rate of the nitrogen gas that permeates into the hollow por-tion to determine gas permeability. The outer and inner diameters of the obtained hollow fibers are also shown in Table 2.
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lOgO520 The permeation rates of various kinds of gases were measured for the respective fibers obtained in Example 1 and Comparative Example 4, and the ratio of such permeation rate of each gas to that of nitrogen, that is, the separa-tion factor, was determined. The results are shown in Table 3. The film remained free of any damage in each case e~en when the gas feed pressure at the time of measurement of the gas permeation rate was raised to 10 kg/cm2. This shows -the existence of a linear relationship between permeability and feed pressure.
Table 3 Separation Factor Specimen H2/N2 He/N2 2/N2 2 Comparat ve4 40 4.20 0.960 0.813 Example 13.40 2.40 0.916 0.854 Value (Note) 2.64 0.935 0.837 (Note) "Theoretical value" is the ratio cf the reciprocals of the square roots of the molecular weights of the respective gases.
The scanning electron microscopic photographs (magnifi-cation : lOOOOX) of sections of the hollow fibers obtained in Comparative Examples 1 and 4 and Example 1 are shown in Figs.
2, 3 and 4, respectively. It is noted from these photographs that the~dense layer A of the external surface, which is considered to perform the separating action-, is thicker in the order of Fig. 2, Fig. 3 and Fig. 4, and that the struc-ture of ths porous layer B supporting such dense layer to lf~)9C520 maintain the mechanical strength becomes better in porosity and pore uniformity correspondingly.
~ Examples 2-6 and Comparative Example 5 J 24% by weight of cellulose acetate (Daicel's RO-CA
5430, acetylation degree 54~, viscosity 30 sec), 51% by weight of acetone and 25% by weight of 1,4-dioxane were mixed well, and after dissolving, filtration and deaeration, - the mixture was subjected to spinning at the rate of 13.Sm/min by extruding the mixture along with injected 1~ water having a temperature of 20C from a spinneret immersed in wa.ter at 5C as a coagulating liguid (water) by using the apparatus of Fig. 1. This was followed by immersion for 15 minutes in water having the temperature shown in Table .3 and then vacuum drying in the same way as described in Example 1. The diameters and gas permeability of the obtained dry hollow fibers are shown in Table 4.
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~0~520 Examples 7-9 and Comparative ExampIe 6 A spinning dope prepared in the same way as described in Example 1 was subjected to spinning at the rate of 14.8 m/min from the outer tube of a double-tube spinneret immersed in a coagulating liquid (water) having the tempera-ture shown in Table 4 while forcing out the injected water of 20C from the inner tube. Each obtained product was then immersed in 70C hot water for 10 minutes, removing the water present on both the interior and exterior of the fibers and then immersing the fibers in -60C gasoline for quick freezing. Then the product was put into a vacuum dryer cooled to -40C and dried therein at 0.02 mm~g for 4g hours. The gas permeability of each of the obtained dry fibers was measured by passing nitrogen and argon gas therethrough to determine the separation factor Ar/N2. The results are shown in Table 5.
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Examples 10 and 11 The same spinning dope as used in Example 1 was spun in the same way except that the amount of injected water was changed to obtain hollow fibers with different inner and outer diameter ratios. After immersion in 70C hot water for more than 10 minutes and removal of water from both the interior and exterior of the fibers, the obtained fibers were further immersed in -55C pentane for 2 minutes for quick freezing. The thus treated fiber mass was then put into a -40C vacuum dryer and dried therein at 0.01 mmHg for 24 hours. The gas permeability of the resultant dry -fibers was measured by using N2 and Ar gas to determine the separation factor Ar/N2.
The results are shown in Table 6 ~, .
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Examples 12 and 13 18% by weight of cellulose acetate (Daicel's RO-CA~
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solving, fil~ration and deaeration, the mixture was subjected to spinning at the rate of 135 m/min, with the injected water being at a temperatureof 5C and the coagulation bath temperature being 3C and with the spinneret immersed in water, by using the apparatus of Fig. 1. The obtained pro-ducts were then immersed in water at 70C for 10 minutes, ! followed by freezing and drying under the conditions specified in the following table, and then the gas permea-bility of each obtained fiber was measured. The results are shown in Table 7. Some products of Example 13 had wide variations in performance and could not be put to practical use. ~ j - I
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~0~520 Examples 7-9 and Comparative ExampIe 6 A spinning dope prepared in the same way as described in Example 1 was subjected to spinning at the rate of 14.8 m/min from the outer tube of a double-tube spinneret immersed in a coagulating liquid (water) having the tempera-ture shown in Table 4 while forcing out the injected water of 20C from the inner tube. Each obtained product was then immersed in 70C hot water for 10 minutes, removing the water present on both the interior and exterior of the fibers and then immersing the fibers in -60C gasoline for quick freezing. Then the product was put into a vacuum dryer cooled to -40C and dried therein at 0.02 mm~g for 4g hours. The gas permeability of each of the obtained dry fibers was measured by passing nitrogen and argon gas therethrough to determine the separation factor Ar/N2. The results are shown in Table 5.
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Examples 10 and 11 The same spinning dope as used in Example 1 was spun in the same way except that the amount of injected water was changed to obtain hollow fibers with different inner and outer diameter ratios. After immersion in 70C hot water for more than 10 minutes and removal of water from both the interior and exterior of the fibers, the obtained fibers were further immersed in -55C pentane for 2 minutes for quick freezing. The thus treated fiber mass was then put into a -40C vacuum dryer and dried therein at 0.01 mmHg for 24 hours. The gas permeability of the resultant dry -fibers was measured by using N2 and Ar gas to determine the separation factor Ar/N2.
The results are shown in Table 6 ~, .
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solving, fil~ration and deaeration, the mixture was subjected to spinning at the rate of 135 m/min, with the injected water being at a temperatureof 5C and the coagulation bath temperature being 3C and with the spinneret immersed in water, by using the apparatus of Fig. 1. The obtained pro-ducts were then immersed in water at 70C for 10 minutes, ! followed by freezing and drying under the conditions specified in the following table, and then the gas permea-bility of each obtained fiber was measured. The results are shown in Table 7. Some products of Example 13 had wide variations in performance and could not be put to practical use. ~ j - I
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drying therein.under vacuum of 0.5 mmHg for 24 hours. The gas permeability of the thus-obtained dry film was measured.
The re-ults are a~ shown in ~able 8.
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Example 15 A solution (dope) comprising 26% by weight of cellu-lose acetate (E 400-25 manufactured by Eastman Kodak), 49 by weight of acetone and 25% by weight of formamide was spun at a spinning speed of 15 m/min in the apparatus shown in Fig. 1 through a spinneret having an internal diameter of the outer tube of 1 mm immersed in water maintained at 2C, while water at 20C was injected through the inner tube of the spinneret, which inner tube had an outer dia-meter of 0.3 mm. The extrudate was immersed in warm water - maintained at 70C for 15 minutes without drying, and the extrudate was then taken out of the water bath and the water on the exterior and in the interior of the fiber was removed. Immediately, the hollow extrudate was dipped in gasoline maintained at -60C to freeze it ~uickly and then the fiber was vacuum-dried at -40C and 0.01 mm ~g for 12 hours. The resulting hollow fiber had an outer diameter of 0.8 mm and an outer diameter/inner diameter ratio of 1.14. The nitrogen gas permsation speed of t'ne hollow fiber was 3.53 x 10 3 cm3 (STP)/cm2-sec cmHg. Then, ethyl cellulose (N-50 or N-100 manufactured by Hercules) was coated on the hollow fiber. More specifically, ethyl cellulose was dissolved in xylene at a concentration indi-cated in Table 9, and the hollow fiber having both its ends sealed was dipped in the solution and then dried. The gas permeability of the resulting coated hollow fiber was measured by placing the sample in a pressure vessel, intro-ducing a pure gas into the vessel externally of the fiber, removing from the hollow portion the gas that permeated through the fiber and measuring the amount of the permeating lO9GS20 gas by using a flow meter. The results obtained are shown in Table 9. The gas permeabilities of the cellulose acetate hollow fiber before coating are also shown in Table 9.
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Example 16 The cellulose acetate hollow fiber used in Example 15 was coated by using 102 RTV, 103 RTV, Primer U, XR-251 or KR-255 (manufactured by Shinetsu Chemicals) as the silicone.
In the case of 102 RTV and 103 RTV, 0.5% of Cat-RH and 5% of Cat-103 were added as the catalyst, respectively, and the mixtures were dissolved in xylene at the concentrations indlcated in Table 10.
Primer U was used directly in the commercially avail-able form, and KR-251 and KR-255 were dissolved in xylene at concentrations indicated in Table ~0.
Cellulose acetate hollow fibers prepared in the same manner as described in Example 15 were dipped in these solutions and dried, and the gas permeability was determined in the same manner as in Example lS. The results shown in Ta~le 10 were obtained.
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Example 17 .
Natural rubber was dissolved in xylene at a concentra-tion of 1.00% and the solution was coated on a cellulose acetate fiber prepared in the same manner as described in Example 15. The gas permeability was determined to obtain results shown in Table ll. .
. Table 11 Gas Permeabilities of Natural Rubber-Coated Hollow Fiber l.0 Gas Natural Rubber, 1.0%
. AG
N2 3.46 x 10-6 H2 2.72 x 10 5 7.89 He 2.49 x 10 5 7.19 2 5.01 x 10 6 1.45 ! Ar 3.79 x 10 6 l.lO
C2 1.72 x lO 5 4.98 CH4 2.82 x 10 6 0.77 C3H6 9'05 x lO 6 2.47 2~ Note AG and ~ are as defined in "Notes" of Table 9.
Example 18 Ethyl cellulose (N-60 manufactured by Hercules~ was dissolved in xylene at a concentration of Q.1% or 0.2%, and in the same manner as in Example 15,cellulose hollow fibers were immersed in the thus obtained solutions from l to 3 times. The gas permeabilities of the resulting coated hollow fibers were measured to ob~ain results shown in Table 12.
The gas permeabilities of the coated hollo~ fiber obtained by dipping the hollow fiber in a 3~ solution of ethyl cellulose are also shown in Table 12.
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ccY~e As a~oilli~ polymer for coating of hollow fiber, there may be used products made by Shin-ets Kagaku Kogyo K. K., namely KE-102RTV, KE103RTV, Primer U, KR-251 and KR-255.
~hese silicone polymers have properties as shown in Table 13.
. . . . . I
. KE-102 KE-103 KR-251* KR-25 5* rimer-U
RTV* P.~J*
property unit .
ransparent transparent transparent ransparen1 transparent APPEARA~CE olorless cololess light-yello~ light- olorless iquid liquid ~i~uidyellow iquid . . _ li~uid viscosity at 25AC poise13 11 0.05 - 0.25 0.7 - 1.5 specific .
gravity-- . l.Ol O. 97 0. 921. 02 _ average ~
curing our/oc 8/2 5 16/2 5 - _ _ i t~ time .
,1tiinY~mUtYl ~ j ~ ~ - _ volumetric Q cm xlOl48xlOl4 _lOl50r mor .
curing . ondensat- addition _ _ _ i mechanlsm on .
_ ..
solvent _ _ txy;nené D li~ .
* Note*: These polymer has a structure as - O - si - o - si - o - si - o - si -R R ¦ R
--O - si - o - si - o - si - o - si -R O R O
in which R stands for phenyl or methyl.
,5 ~ 520 SUPPLEMENTA.~ DISCLOSURE
In the preparation of aqueous reverse osmosis membranes, it is known to use an aqueous solution of sodium chloride as an aqueous coagulating medium to form voids, and the results of experiments made on applications of this known technique using an aqueous solution of sodium chloride as the aqueous coagulating medium for improving the rupture strength in hollow fiber membranes for aqueous reverse osmosis have been reported (see D. S. Cleveland, M. Rambean, A C~ernicki and T. R. Rich; Evaluation of Asymmetric Hollow Fibers for Desalination by Reverse Osmosis, United States Department, Interior Office of Saline Water, Research and Development Progress Report No. 856 (1973).
It has been discovered tha~ the gas separation capacity of the fibers described in the principal disclosure is improved by using an aqueous solution of various water-soluble salts, such as sodium chloride, instead of water, as the injection medium fed to the central opening of the hollow fibers during spinning of the fibers. By this procedure a compact layer also is formed on the interior surface of the hollow fiber whereby the hollow fiber has com-pact layers including capillary vessels on both the outer and the inner sur-faces of the hollow fiber.
As the cellulose derivative that can be used in the present invention there can be mentioned, for example, cellulose acetate, cellulose acetate butyrate, cellulose propionate and ethyl cellulose. Use of acetone-soluble cellulose acetate is especially preferred.
Acetone alone can be used as the solvent for dissolving the cellu-lose derivative, but mixtures of acetone with other solvents such as dimethyl-formamide, formamide, 2-methoxyethyl acetate, 1,4-dioxane and 1,3-dioxolan can also be employed. In order to form fine voids, a swelling agent or an inorganic salt such as potassi~m hypochlorite can be incorporated into such solvent. An acetone-formamide luxture is preferred as the mixed solvent.
me cellulose derivative is dissolved in such solvent to form a solution SZO
having a concentration of 22 to 33 percent by weight which provides a vis-cosity suitable for the formation of fibers, namely, 300 to 1500 poises as measured at 20 C.
The solution is fed to the outer tube portion of a spinneret of the double tube structure at a fixed rate by a gear pump after sufficient defoam-ing by standing, and simultaneously, an aqueous injection liquid miscible with the above-mentioned solvent is fed to the inner tube of the spinneret by a gear pump and is extruded therefrom together with the spinning dope to effect spinning. The spinneret is immersed in water maintained at 0 to 30 C.
.
Simultaneously with the extrusion of the spinning dope from the spinneret, the outer surface of the extrudate contacts the water and is coagulated while the solvent in the dope is extracted therefrom. Since the osmotic pressure is enhanced by dissolving the salt into the injection liquid that is injected into the hollow core of the fiber, diffusion of the solvent from the inner wall of the fiber is delayed and also the speed of diffusion of water in the injection liquid into the interior of the fiber is reduced, whereby a compact layer is formed on the inner surface of the hollow fiber. From the results of numerous experiments, it was found that the salt concentration in the injection liquid must be from 1 to 20 percent by weight, preferably 4 to 20 percent by weight, especially preferably 5 to 10 percent by weight. As the salt that is used in the present invention, there can be mentioned, for example, inorganic salts such as sodium chloride, Na2C03, Na2S04 and KH2P04 and organic acid salts such as CH3COONa.
The thus-spun hollow fibers are maintained immersed in the coagula-ting liquid for at least 20 minutes and then they are immersed in warm water maintained at 50 to 100C for at least 3 minutes to replace the injection liquid present in the hollow core thereof by pure water and to dissolve out any solvent, such as acetone, remaining in the fiber wall. Then, the free water present as a film or droplets on the inside and the outside of the fiber lOg~SZO
is removed by suitable means such as blowing out by compressed air. Then, the fibers are dipped in a cooling medium (a non-solvent for the cellulose deriva-tive and also insoluble in water, for example, gasoline, fluorodichloromethane, cyclopentane and hexane) cooled below -40C without drying to rapidly cool the fibers and free~e the water contained in fine voids of the fibers, and in this state, the frQzen fibers are placed in a vacuum freeze drier cooled below -20 C, preferably below -35 C, and allowed to stand under high vacuum of 0.5 mm Hg or lower for at least 12 hours to sublimate fine crystals of water and thereby form fine voids in the fibers, whereby the hollow fibers of the pre-sent invention are prepared.
It was found that the thus-prepared hollow fibers have a much higher mixed gas separation capacity than hollow fibers formed by using water as the injection liquid, even though the gas permeation speed is lowered to 1/10 to 1/100.
The improved process will now be further described in detail with reference to the following illustrative examples.
Examples 19-22 according to the invention and Comparative Example 7 A mixture comprising 23 percent by weight of cellulose acetate t C (R0-CA5430 manufactured by Daicel~tDegree of acetylation = 54%, viscosity =
30 seconds), 52% by weight of acetone and 25% by weight of 1,4-dioxane was thoroughly blended to~form a solution, and the solution was filtered, defoamed and injected into an outer tube of a spinneret of the double tube structure by means of a gear pump. Simultaneously, an aqueous solution containing sodium chloride at a concentration of 1 to 20% was fed to the inner tube of the spin-neret by a metering pump. Both the solutions were extruded into a coagulating liquid (water) maintained at 3C and the extrudate was wound at a rate of 12m/min in the water. Then, the extrudate was allowed to stand in the coag-ulating liquid (water) for at least 30 minutes, during which time one end of the extruded fiber was hung down from the vessel to permit the liquid in the l~CSZO
hollow core to flow out by capillary action, so as to replace the aqueous solution of sodium chloride in the hollow core by pure water. Then, the fiber was dipped in water maintained at 80 C for 15 minutes and then free water was removed from the outside and inside of the fiber. Then, the fiber was immersed in gasoline maintained at -50C to rapidly freeze the fiber and the fiber was then placed in a vacuum drier maintained at -~0C and 0.01 mm Hg for 24 hours to effect vacuum drying. The thus-obtained dry hollow fiber membrane was placed in a pressure vessel and a gaseous mixture comprised of 99 vol.% of nitrogen and 1 vol.% of krypton was flowed into the vessel under a pressure of 5 Kg/cm . In this state, the permeated gas was sampled from the hollow core of the hollow fiber at prescribed time intervals and the volume ratio ~ of nitrogen/krypton was determined by gas chromatogram. The volume ratio Ko of nitrogen/krypton in the starting gas was similarly de-termined and the degree of R of separation of krypton from nitrogen was cal-culated from the following equation:
Ko (1) The flow rate of the permeated gas from the hollow portion was determined from the collected amount of sample gas. The results obtained are shown in Table 13.
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Examples 23 to 26 Spinning was carried out in the same manner as described in Examples 18 to 22 except that a solution obtained by dissolving sodium sulfate, sodium carbonate~ acidic sodium phosphate or sodium acetate in water at a concentra-tion of 5 percent by weight? instead of sodium chloride, was used as the aqueous injection liquid fed into the hollow core. Post treatments were con-ducted in the same manner as described in Examples 18 to 22 to obtain hollowfibers, and the gas permeability and the krypton separation degree were de-termined. The results obtained are shown in Table 14.
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drying therein.under vacuum of 0.5 mmHg for 24 hours. The gas permeability of the thus-obtained dry film was measured.
The re-ults are a~ shown in ~able 8.
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Example 15 A solution (dope) comprising 26% by weight of cellu-lose acetate (E 400-25 manufactured by Eastman Kodak), 49 by weight of acetone and 25% by weight of formamide was spun at a spinning speed of 15 m/min in the apparatus shown in Fig. 1 through a spinneret having an internal diameter of the outer tube of 1 mm immersed in water maintained at 2C, while water at 20C was injected through the inner tube of the spinneret, which inner tube had an outer dia-meter of 0.3 mm. The extrudate was immersed in warm water - maintained at 70C for 15 minutes without drying, and the extrudate was then taken out of the water bath and the water on the exterior and in the interior of the fiber was removed. Immediately, the hollow extrudate was dipped in gasoline maintained at -60C to freeze it ~uickly and then the fiber was vacuum-dried at -40C and 0.01 mm ~g for 12 hours. The resulting hollow fiber had an outer diameter of 0.8 mm and an outer diameter/inner diameter ratio of 1.14. The nitrogen gas permsation speed of t'ne hollow fiber was 3.53 x 10 3 cm3 (STP)/cm2-sec cmHg. Then, ethyl cellulose (N-50 or N-100 manufactured by Hercules) was coated on the hollow fiber. More specifically, ethyl cellulose was dissolved in xylene at a concentration indi-cated in Table 9, and the hollow fiber having both its ends sealed was dipped in the solution and then dried. The gas permeability of the resulting coated hollow fiber was measured by placing the sample in a pressure vessel, intro-ducing a pure gas into the vessel externally of the fiber, removing from the hollow portion the gas that permeated through the fiber and measuring the amount of the permeating lO9GS20 gas by using a flow meter. The results obtained are shown in Table 9. The gas permeabilities of the cellulose acetate hollow fiber before coating are also shown in Table 9.
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Example 16 The cellulose acetate hollow fiber used in Example 15 was coated by using 102 RTV, 103 RTV, Primer U, XR-251 or KR-255 (manufactured by Shinetsu Chemicals) as the silicone.
In the case of 102 RTV and 103 RTV, 0.5% of Cat-RH and 5% of Cat-103 were added as the catalyst, respectively, and the mixtures were dissolved in xylene at the concentrations indlcated in Table 10.
Primer U was used directly in the commercially avail-able form, and KR-251 and KR-255 were dissolved in xylene at concentrations indicated in Table ~0.
Cellulose acetate hollow fibers prepared in the same manner as described in Example 15 were dipped in these solutions and dried, and the gas permeability was determined in the same manner as in Example lS. The results shown in Ta~le 10 were obtained.
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Example 17 .
Natural rubber was dissolved in xylene at a concentra-tion of 1.00% and the solution was coated on a cellulose acetate fiber prepared in the same manner as described in Example 15. The gas permeability was determined to obtain results shown in Table ll. .
. Table 11 Gas Permeabilities of Natural Rubber-Coated Hollow Fiber l.0 Gas Natural Rubber, 1.0%
. AG
N2 3.46 x 10-6 H2 2.72 x 10 5 7.89 He 2.49 x 10 5 7.19 2 5.01 x 10 6 1.45 ! Ar 3.79 x 10 6 l.lO
C2 1.72 x lO 5 4.98 CH4 2.82 x 10 6 0.77 C3H6 9'05 x lO 6 2.47 2~ Note AG and ~ are as defined in "Notes" of Table 9.
Example 18 Ethyl cellulose (N-60 manufactured by Hercules~ was dissolved in xylene at a concentration of Q.1% or 0.2%, and in the same manner as in Example 15,cellulose hollow fibers were immersed in the thus obtained solutions from l to 3 times. The gas permeabilities of the resulting coated hollow fibers were measured to ob~ain results shown in Table 12.
The gas permeabilities of the coated hollo~ fiber obtained by dipping the hollow fiber in a 3~ solution of ethyl cellulose are also shown in Table 12.
- ~0~520 co o ~
. . CO .
O --I N ~ 'r `1 . I
~ .~ t~
. ~ o~O ~ l l l l l . I
,~ O O 0, o o o a~ a~ ~ 1 C,) ~ 1: ~1 X X X X X
O ~ ~ In _I~
.C ~ a~
li3 Z
~1 ~ _~ , ~- I . I
~: tO ~1 ~
'' Q~-~ O. . .
~ ~ ~ U~ D
~Id a) o o . ,.
~ S~ ~ ~ X X
P .C ~ C~ I I
, ~
S ~ d ~-~ ~ , , ~O~ . z P. ~
3 Q~ a) ~ u~
N O U~ U~
`_1 _1 ~ ,1 3o O O
O ~ :~ ~ X X
~:~ _1 ~3 Q I ~I I I .
J ~ .
E~ ~ ~ ~ ~ ~ .
C~ ~ .C _~ ~
~oW W ' I O _~ ~0~ U7 . ~ .
tO O ~I N O O _i ~.~ 'Q . .
~ 1010 10 10 10 . . I
3 ~ o ~ ~o ~ ~ x x ~ o . . . . .
' ' ~ D ~ ~ ~ . .
3 . .
r~
~q o 1`
~ Q~ ~1 ~ ~` ~
- . o I ~ i o _~
:5~ Q .
~ ~: ~a u~
Uo ~ 1~ 1~o 1~o 1~o 1~o _I~ O ~ X X X 'X X
:~ O ~ ~1 ~`J ~ ~ l~
s~ ~ ') ~O ~ Q) C~ Z ~ o ~ e~
ccY~e As a~oilli~ polymer for coating of hollow fiber, there may be used products made by Shin-ets Kagaku Kogyo K. K., namely KE-102RTV, KE103RTV, Primer U, KR-251 and KR-255.
~hese silicone polymers have properties as shown in Table 13.
. . . . . I
. KE-102 KE-103 KR-251* KR-25 5* rimer-U
RTV* P.~J*
property unit .
ransparent transparent transparent ransparen1 transparent APPEARA~CE olorless cololess light-yello~ light- olorless iquid liquid ~i~uidyellow iquid . . _ li~uid viscosity at 25AC poise13 11 0.05 - 0.25 0.7 - 1.5 specific .
gravity-- . l.Ol O. 97 0. 921. 02 _ average ~
curing our/oc 8/2 5 16/2 5 - _ _ i t~ time .
,1tiinY~mUtYl ~ j ~ ~ - _ volumetric Q cm xlOl48xlOl4 _lOl50r mor .
curing . ondensat- addition _ _ _ i mechanlsm on .
_ ..
solvent _ _ txy;nené D li~ .
* Note*: These polymer has a structure as - O - si - o - si - o - si - o - si -R R ¦ R
--O - si - o - si - o - si - o - si -R O R O
in which R stands for phenyl or methyl.
,5 ~ 520 SUPPLEMENTA.~ DISCLOSURE
In the preparation of aqueous reverse osmosis membranes, it is known to use an aqueous solution of sodium chloride as an aqueous coagulating medium to form voids, and the results of experiments made on applications of this known technique using an aqueous solution of sodium chloride as the aqueous coagulating medium for improving the rupture strength in hollow fiber membranes for aqueous reverse osmosis have been reported (see D. S. Cleveland, M. Rambean, A C~ernicki and T. R. Rich; Evaluation of Asymmetric Hollow Fibers for Desalination by Reverse Osmosis, United States Department, Interior Office of Saline Water, Research and Development Progress Report No. 856 (1973).
It has been discovered tha~ the gas separation capacity of the fibers described in the principal disclosure is improved by using an aqueous solution of various water-soluble salts, such as sodium chloride, instead of water, as the injection medium fed to the central opening of the hollow fibers during spinning of the fibers. By this procedure a compact layer also is formed on the interior surface of the hollow fiber whereby the hollow fiber has com-pact layers including capillary vessels on both the outer and the inner sur-faces of the hollow fiber.
As the cellulose derivative that can be used in the present invention there can be mentioned, for example, cellulose acetate, cellulose acetate butyrate, cellulose propionate and ethyl cellulose. Use of acetone-soluble cellulose acetate is especially preferred.
Acetone alone can be used as the solvent for dissolving the cellu-lose derivative, but mixtures of acetone with other solvents such as dimethyl-formamide, formamide, 2-methoxyethyl acetate, 1,4-dioxane and 1,3-dioxolan can also be employed. In order to form fine voids, a swelling agent or an inorganic salt such as potassi~m hypochlorite can be incorporated into such solvent. An acetone-formamide luxture is preferred as the mixed solvent.
me cellulose derivative is dissolved in such solvent to form a solution SZO
having a concentration of 22 to 33 percent by weight which provides a vis-cosity suitable for the formation of fibers, namely, 300 to 1500 poises as measured at 20 C.
The solution is fed to the outer tube portion of a spinneret of the double tube structure at a fixed rate by a gear pump after sufficient defoam-ing by standing, and simultaneously, an aqueous injection liquid miscible with the above-mentioned solvent is fed to the inner tube of the spinneret by a gear pump and is extruded therefrom together with the spinning dope to effect spinning. The spinneret is immersed in water maintained at 0 to 30 C.
.
Simultaneously with the extrusion of the spinning dope from the spinneret, the outer surface of the extrudate contacts the water and is coagulated while the solvent in the dope is extracted therefrom. Since the osmotic pressure is enhanced by dissolving the salt into the injection liquid that is injected into the hollow core of the fiber, diffusion of the solvent from the inner wall of the fiber is delayed and also the speed of diffusion of water in the injection liquid into the interior of the fiber is reduced, whereby a compact layer is formed on the inner surface of the hollow fiber. From the results of numerous experiments, it was found that the salt concentration in the injection liquid must be from 1 to 20 percent by weight, preferably 4 to 20 percent by weight, especially preferably 5 to 10 percent by weight. As the salt that is used in the present invention, there can be mentioned, for example, inorganic salts such as sodium chloride, Na2C03, Na2S04 and KH2P04 and organic acid salts such as CH3COONa.
The thus-spun hollow fibers are maintained immersed in the coagula-ting liquid for at least 20 minutes and then they are immersed in warm water maintained at 50 to 100C for at least 3 minutes to replace the injection liquid present in the hollow core thereof by pure water and to dissolve out any solvent, such as acetone, remaining in the fiber wall. Then, the free water present as a film or droplets on the inside and the outside of the fiber lOg~SZO
is removed by suitable means such as blowing out by compressed air. Then, the fibers are dipped in a cooling medium (a non-solvent for the cellulose deriva-tive and also insoluble in water, for example, gasoline, fluorodichloromethane, cyclopentane and hexane) cooled below -40C without drying to rapidly cool the fibers and free~e the water contained in fine voids of the fibers, and in this state, the frQzen fibers are placed in a vacuum freeze drier cooled below -20 C, preferably below -35 C, and allowed to stand under high vacuum of 0.5 mm Hg or lower for at least 12 hours to sublimate fine crystals of water and thereby form fine voids in the fibers, whereby the hollow fibers of the pre-sent invention are prepared.
It was found that the thus-prepared hollow fibers have a much higher mixed gas separation capacity than hollow fibers formed by using water as the injection liquid, even though the gas permeation speed is lowered to 1/10 to 1/100.
The improved process will now be further described in detail with reference to the following illustrative examples.
Examples 19-22 according to the invention and Comparative Example 7 A mixture comprising 23 percent by weight of cellulose acetate t C (R0-CA5430 manufactured by Daicel~tDegree of acetylation = 54%, viscosity =
30 seconds), 52% by weight of acetone and 25% by weight of 1,4-dioxane was thoroughly blended to~form a solution, and the solution was filtered, defoamed and injected into an outer tube of a spinneret of the double tube structure by means of a gear pump. Simultaneously, an aqueous solution containing sodium chloride at a concentration of 1 to 20% was fed to the inner tube of the spin-neret by a metering pump. Both the solutions were extruded into a coagulating liquid (water) maintained at 3C and the extrudate was wound at a rate of 12m/min in the water. Then, the extrudate was allowed to stand in the coag-ulating liquid (water) for at least 30 minutes, during which time one end of the extruded fiber was hung down from the vessel to permit the liquid in the l~CSZO
hollow core to flow out by capillary action, so as to replace the aqueous solution of sodium chloride in the hollow core by pure water. Then, the fiber was dipped in water maintained at 80 C for 15 minutes and then free water was removed from the outside and inside of the fiber. Then, the fiber was immersed in gasoline maintained at -50C to rapidly freeze the fiber and the fiber was then placed in a vacuum drier maintained at -~0C and 0.01 mm Hg for 24 hours to effect vacuum drying. The thus-obtained dry hollow fiber membrane was placed in a pressure vessel and a gaseous mixture comprised of 99 vol.% of nitrogen and 1 vol.% of krypton was flowed into the vessel under a pressure of 5 Kg/cm . In this state, the permeated gas was sampled from the hollow core of the hollow fiber at prescribed time intervals and the volume ratio ~ of nitrogen/krypton was determined by gas chromatogram. The volume ratio Ko of nitrogen/krypton in the starting gas was similarly de-termined and the degree of R of separation of krypton from nitrogen was cal-culated from the following equation:
Ko (1) The flow rate of the permeated gas from the hollow portion was determined from the collected amount of sample gas. The results obtained are shown in Table 13.
_ 40 -1~9C`520 o G 3~ d 2 0 0 ~ ~
., o o o o o Ul ~ h R U~ ~
to to to to 9`-1 x x x x x ~ ~ O ~ ,~ u) ,~
O
~qc~ o o o _ ~ ~ ~ o ~o 1~ h .~ ~ O O O O O
.~ L~ ~0 R ~
~ ~ 0 0 0 ~D 0 0 r~OI O O O O O O
O O
d o ~1 ~ u~
o o Ei ~i ~ O rl o O
D~ 0 o O~ O
~ X X ~ X X
O ~
_ 41 --~9,t When the sodium chloride concentration was higher than 20 percent, reduction of the permeation speed was extreme and the resulting fibers had no practical utility.
Examples 23 to 26 Spinning was carried out in the same manner as described in Examples 18 to 22 except that a solution obtained by dissolving sodium sulfate, sodium carbonate~ acidic sodium phosphate or sodium acetate in water at a concentra-tion of 5 percent by weight? instead of sodium chloride, was used as the aqueous injection liquid fed into the hollow core. Post treatments were con-ducted in the same manner as described in Examples 18 to 22 to obtain hollowfibers, and the gas permeability and the krypton separation degree were de-termined. The results obtained are shown in Table 14.
-lV9~520 ~d ~
~ ~ OD a~
o ~ o o o o ~3 ~ ~ I to to t~ to X ~C X
c~ ~ ~1 ~r C`~ ~ _, o o o Z; ~
o ~ ~
.,, ,ç~ V o o o o .,1 Q~ ~ 0~ ~
O Z ~
X
~Z C~
~,~,i
Claims (23)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An asymmetric hollow fiber made of a fiber-forming cellulose ester or a fiber-forming cellulose ether, said hollow fiber having an outer diameter of from 0.2 to 3 mm, a ratio of outer diameter/inner diameter in the range of from 1.1/1.0 to 3.0/1.0, and a nitrogen gas permeability in the range of from 5 x 10-4 to 0.1 cm3 at standard temperature and pressure/
cm2?sec?cmHg, the relative permeability of said fiber to various gases being substantially in proportion to the reciprocals of the square roots of the molecular weights of the respective gases.
cm2?sec?cmHg, the relative permeability of said fiber to various gases being substantially in proportion to the reciprocals of the square roots of the molecular weights of the respective gases.
2. An asymmetric hollow fiber as claimed in claim 1 in which said fiber is made of a substance selected from the group consisting of cellulose acetate, cellulose acetate butyrate, cellulose propionate and ethyl cellulose.
3. An asymmetric hollow fiber as claimed in claim 1 in which said fiber is made of acetone-soluble cellulose acetate.
4. A process for preparing a hollow fiber which comprises extruding through the annular space between the inner and outer tubes of a double tube spinneret, a solution containing from about 22 to about 33 percent by weight of a fiber-forming cellulose ester or a fiber-forming cellulose ether disssolved in an organic solvent and having a viscosity of from about 300 to about 1500 poise at a temperature of 20° C and simultaneously flowing through the inner tube of said spinneret an aqueous liquid or an inert gas, said spinneret being submerged in an aqueous coagulating liquid bath having a temperature of from zero to 45° C whereby to coagulate said solution directly in said bath to produce a hollow fiber; removing the fiber from said coagulating bath and then, without permitting said fiber to dry, immersing it in water having a temperature of 30° to 100°C; then removing free water present on the exterior and interior surfaces of said fiber;
then freezing and vacuum-drying the fiber.
then freezing and vacuum-drying the fiber.
5. A process as claimed in claim 4 in which said solution has a temperature of about ambient temperature and the aqueous liquid or the inert gas has a temperature of from zero°C to about ambient temperature, the extruded fiber is maintained in the coagulating bath for at least about 10 minutes and then is immersed in said water at 30° to 100°C for at least 3 mintues, said fiber is frozen by applying thereto a coolant having a temperature of below minus 40°C, said coolant being a non-solvent for said cellulose ether or said cellulose ester and being immiscible with water and said fiber is vacuum-dried under a pressure of less than 0.5 mmHg absolute, for more than 12 hours at a temperature of below minus 30°C.
6. A process as claimed in claim 5 in which the solute in said solution is a cellulose ester selected from the group consisting of cellulose acetate, cellulose acetate butyrate and cellulose propionate.
7. A process as claimed in claim 5 in which the solute in said solution consists of acetone-soluble cellulose acetate.
8. A process as claimed in claim 5 in which the solute in said solution consists of ethyl cellulose.
9. A process as claimed in claim 5 in which said organic solvent is acetone or a mixture of acetone and one or more substances selected from the group consisting of dimethyl formamide, formamide, 2-methoxyethyl acetate, 1,4-dioxane and 1,3-dioxolane.
10. An asymmetric hollow fiber made of a fiber-forming cellulose ester or a fiber-forming cellulose ether, said hollow fiber having an outer diameter of from 0.2 to 3 mm, a ratio of outer diameter/inner diameter in the range of from 1.1/1.0 to 3.0/1.0, and a nitrogen gas permeability in the range of from 5 x 10-4 to 0.1 cm3 at standard temperature and pressure/cm2?sec?cmHg, the relative permeability of said fiber to various gases being substantially in proportion to the reciprocals of the square roots of the molecular weights of the respective gases, said fiber being coated on its exterior surface with a film of a film-forming polymer, said film having a thickness of from 0.1 to 300 microns.
11. An asymmetric hollow fiber as claimed in claim 10 in which said fiber is made of a substance selected from the group consisting of cellulose acetate, cellulose acetate butyrate, cellulose propionate and ethyl cellulose.
12. An asymmetric hollow fiber as claimed in claim 10 in which said fiber is made of acetone-soluble cellulose acetate.
13. A process for preparing a hollow fiber which comprises extruding through the annular space between the inner and outer tubes of a double tube spinneret a solution containing from about 22 to about 33 percent by weight of a fiber-forming cellulose ester or a fiber-forming cellulose ether dissolved in an organic solvent and having a viscosity of from about 300 to about 1500 poise at a temperature of 20°C and simultaneously flowing through the inner tube of said spinneret an aqueous liquid or an inert gas, said spinneret being submerged in an aqueous coagulating liquid bath having a temperature of from zero to 45°C whereby to coagulate said solution directly in said bath to produce a hollow fiber; removing the fiber from said coagulating bath and then, without permitting said fiber to dry, immersing it in water having a temperature of 30° to 100°C; then removing free water present on the exterior and interior surfaces of said fiber; then freezing and vacuum-drying the fiber, and then immersing said fiber in a solution containing from 0.2 to 5 percent by weight of a film-forming polymer dissolved in a liquid which is a solvent for said polymer and is a non-solvent for said cellulose ester or cellulose ether to form on the exterior surface of the fiber a homogeneous thin film of said polymer having a thickness of from 0.1 to 300 microns.
14. A process as claimed in claim 13 in which said solution has a temperature of about ambient temperature and the aqueous liquid or the inert gas has a temperature of from zero°C to about ambient temperature, the extruded fiber is maintained in the coagulating bath for at least about 10 minutes and then is immersed in said water at 30° to 100°C for at least 3 minutes, said fiber is frozen by applying thereto a coolant having a temperature of below minus 40°C, said coolant being a non-solvent for said cellulose ether or said cellulose ester and being immiscible with water and said fiber is vacuum-dried under a pressure of less than 0.5 mmHg absolute, for more than 12 hours at a temperature of below minus 30°C.
15. A process as claimed in claim 14 in which the solute in said solution is a cellulose ester selected from the group consisting of cellulose acetate, cellulose acetate butyrate and cellulose propionate.
16. A process as claimed in claim 14 in which the solute in said solution consists of acetone-soluble cellulose acetate.
17. A process as claimed in claim 14 in which the solute in said solution consists of ethyl cellulose.
18. A process as claimed in claim 14 in which said organic solvent is acetone or a mixture of acetone and one or more substances selected from the group consisting of dimethyl formamide, formamide, 2-methoxyethyl acetate, 1,4-dioxane and 1,3-dioxolane.
19. A process as claimed in claim 14 in which said polymer is selected from the group consisting of silicone polymer, cellulose esters, cellulose ether and natural rubber and the liquid in which said polymer is dissolved is selected from the group consisting of pentane, gasoline, xylene, toluene and benzene.
Claims Supported By Supplementary Disclosure
Claims Supported By Supplementary Disclosure
20. A process for the preparation of a hollow fiber comprising dissolving a fiber-forming cellulose ester or a fiber-forming cellulose ether into an organic solvent to form a solution having a solid content of 22 to 33 percent by weight and a viscosity of from about 300 to about 1500 poises at a temperature of 20°C, extruding the resulting solution through the annular space between the inner and outer tubes of a double tube type spinneret and flowing an aqueous injection liquid through the inner tube of the spinneret, said spinneret being submerged in an aqueous coagulating liquid maintained at 0 to 30°C to coagulate said solution directly and form the hollow fiber, dipping the hollow fiber into warm water maintained at 50 to 100°C without drying, taking out the fiber from warm water, removing water from the inside and outside of the fiber, immediately freezing them and subjecting them to vacuum drying, said process being characterized in that as the aqueous injection liquid to be injected into the inner tube of the spinneret, an aqueous solution formed by dissolving a salt in water at a concentration of 1 to 20 percent by weight is used.
21. A process as claimed in claim 20 wherein said fiber-forming cellulose ester is selected from the group consisting of cellulose acetate, cellulose acetate butyrate cellulose propionate and acetone-soluble cellulose acetate.
22. A process as claimed in claim 20 wherein said salt in water concentration is from 5 to 10 percent by weight.
23. A process as claimed in claims 20 or 22 wherein said salt is sodium chloride, Na2CO3, Na2SO4, KH2PO4 or CH3COONa.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP13198475A JPS5255719A (en) | 1975-10-31 | 1975-10-31 | Hollow fibers having selective gas permeability and production thereof |
| JP131984/75 | 1975-10-31 | ||
| JP686076A JPS5289574A (en) | 1976-01-23 | 1976-01-23 | Production of hollow filament used for separation |
| JP6860/76 | 1976-01-23 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1090520A true CA1090520A (en) | 1980-12-02 |
Family
ID=26341068
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA261,993A Expired CA1090520A (en) | 1975-10-31 | 1976-09-24 | Hollow fiber having selective gas permeability and process for preparation thereof |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1090520A (en) |
-
1976
- 1976-09-24 CA CA261,993A patent/CA1090520A/en not_active Expired
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