Process for preparation of hollow fibers

Abstract: A hollow fiber having longitudinal fins on its periphery is produced by melt-spinning a liquid which, when extruded from a spinning novel, has a viscosity in the range 100-10,000 P.

Description: The present invention relates to a process for the preparation of a hollow fiber having fins on the periphery thereof, for use in blood dialysis. Fluid separators comprising hollow fibers having a selective permeability have been used for reverse osmosis or hemodialysis. Especially, hollow fiber type blood dialyzers have been vigorously used for purifying the blood of patients suffering from renal insufficiency. Hollow fibers to be used for such blood treatments should allow selective permeation of specific solutes according to the intended use. The capacity of a hollow fiber depends on the material, the porosity (pore size and pore number) and the membrane thickness of the hollow fiber. It has been proposed to crimp hollow fibers or to wind cover yarns around them to prevent contact of the hollow fibers with one another and to increase the efficiency of the transport of the solute by the flow of the dialyzing solution. The hollow fibers are generally produced by extrusion and spinning, such as wet spinning, semi-wet, semi-dry spinning or melt spinning, of a mixture of a thermoplastic polymer and an additive which is later removed so as to form pores. FR-A-2437858 discloses a process in which fibers are produced through coagulation of a cellulose-cuprammonium solution extruded from a hollow fiber nozzle directly into a caustic soda lye and are subsequently drawn off to undergo further processing treatments, for example to be stretched. However, such methods are still insufficient and further improvements are desired. FR-A 2451952 discloses the use of hollow fibres in heat transfer apparatus and FR-A 2506792 discloses a method for preparation of hollow fibres for dialysis by extrusion of a cellulosic spinning dope into a liquid bath and subsequently into a coagulating liquid. An object of the present invention is to provide an improved process for the preparation of a finned hollow fiber for use in blood dialysis. Accordingly the invention provides a process for preparing, by melt-spinning, a selective-permeable hollow fiber for blood dialysis having an outer diameter of about 100 to 500 µm, a hollow portion being formed in the longitudinal direction and 1 to 10 fins extending in the longitudinal direction on the periphery thereof, the fins having a height H to width W ratio H/W of at least 0.5, characterised in that a spinning liquid containing at least one cellulose ester polymer having an average degree of polymerization of at least 150 and at least one additive for forming pores is extruded into the gas phase from a spinning nozzle for said hollow fibre having an annular double slit and 1 to 10 notches formed on an outer side of the double slit, the viscosity of the spinning liquid at said nozzle being 200 to 5,000 P, the distance between the end face of the spinning nozzle and the point where the hollow fiber extruded from the spinning nozzle is solidified in the gas phase being between more than 10 and 40 cm, the extruded fiber is cooled by air at room temperature and has a draft ratio of from 20 to 200, whereby a stable hollow fiber is obtained, and the solidified hollow fiber is treated so that it has selective permeable properties. In the accompanying drawings, Figures 1, 2, 3, 4, and 5 are enlarged sectional views illustrating hollow fibers obtained by the process according to the present invention, and Figures 6, 7, and 8 are diagrams illustrating examples of the shape of the nozzle for spinning hollow fibers according to the process of the present invention. The present invention will now be described in detail. Selective-permeable finned hollow fibers valuably used for dialyzers are prepared by extruding a spinning liquid from a spinning nozzle for finned hollow fibers, which has an appropriate configuration, so that the viscosity of the liquid at the nozzle is 200 to 5000 poise (hereinafter referred to as P). The spinning solution for hollow fibers is in the liquid state and, after it is extruded from the spinning nozzle, the liquid of the fin portion tends to gather toward the root because of surface tension. In order to control this tendency, it is necessary that the viscosity of the spinning liquid at the time of extrusion from the nozzle should be increased and the time required for formation of the shape of the hollow fiber should be shortened. As the result of research made with a view to solving this problem, it was found that in order to obtain non-circular finned hollow fibers which can be used for purification of blood and have such a shape that, when used for hemodialysis, the flow of the dialyzing solution is uniform and a sufficient dialyzing effect is attained without substantial reduction of the effective area of the hollow fiber membrane, it is important that the melt viscosity of the spinning liquid at the spinning nozzle should be 200 to 5000 P. A spinneret shown in Figure 6 is ordinarily used for spinning of finned hollow fibers. A gas or liquid for formation of the hollow portion is extruded from a portion 1, and the spinning liquid of the hollow fiber membrane-forming material is extruded from a portion 2. Then, the extruded spinning liquid is cooled and solidified and the shape of the hollow fiber is fixed. Nozzles as shown in Figures 7 and 8 may be used for narrowing the root of the fin portion. If the viscosity of the spinning liquid is lower than 200 P, the shape of the fin is not normally fixed and the intended object is not sufficiently attained. On the other hand, if the melt viscosity of the spinning liquid is too high and exceeds 5000 P, the pressure loss at the extrusion hole of the spinneret is increased, a mechanical problem arises and it is impossible to impart smooth drafting (drawing) to the extrudate. Accordingly, breaking is readily caused during spinning, unevenness in thickness is caused, and stable spinning is impossible. It is especially preferred that the viscosity of the spinning liquid at the spinning nozzle be 300 to 2,000 P, particularly 500 to 1,500 P. If the viscosity of the spinning liquid is within this range, the shape of the fins is correctly fixed and spinning can be performed stably. If the time required for solidification of the extruded hollow fiber is too long even though the viscosity of the spinning liquid is within an appropriate range at the extrusion from the nozzle, deformation is promoted by the surface tension of the spinning liquid and a sharp shape cannot be given to the fin. In order to sharply fix the shape of the extrudate, it is preferred that the extrudate be promptly cooled and solidified after the extrusion. In ordinary melt spinning, the so-called solidification point where the extrudate loses flowability and attenuation by the winding tension is not further advanced about 10 cm from the nozzle. However, it was found that in order to obtain a finned hollow fiber for purification of blood, as intended in the present invention, it is preferred that the solidification point be located within 40 cm from the top end of the nozzle, particularly 10 to 25 cm. If the distance is within this range, spinning can be performed very smoothly. The draft ratio as another spinning condition is 20 to 200 and preferably 40 to 100 in the present invention. If the draft ratio is within this range, a hollow fiber provided with fins having a predetermined shape can be obtained stably. The spinning liquid used in the preparation process of the present invention contains a thermoplastic cellulose ester polymer, but additives may be incorporated in the spinning liquid. As the additive, there can be mentioned polyols such as polyalkylene glycols, glycerol and glycols having ethylene and/or propylene chain in the molecule, plasticizers such as sulforane, caprolactone, dimethylsulfoxide and mixtures thereof, and thermoplastic polymers. The composition of the spinning liquid is appropriately selected according tot the required selective permeability and the viscosity at the spinning step. In the process of the present invention, the selective permeability is manifested by drawing of the spun and solidified hollow fiber, extraction removal of the additives or chemical treatment such as saponification. An appropriate treatment is selected according to the composition of the spinning liquid, but any of customary treatments may be adopted and the treatment method is not particularly critical. The shape of size of the finned hollow fiber of the present invention is not particularly critical. The number of fins is 1 to 10 and preferably 2 to 7, and the outer diameter d is 100 to 500 µm and preferably 200 to 300 µm. The thickness (h) of the fin-free portion is 5 to 50 µm and preferably 5 to 40 µm. The height H of the fin is practically in the range of from 10 to 65 µm. In accordance with the present invention, there is provided a cellulose type hollow fiber that is very suitably used for blood dialysis. This fiber is a cellulose type hollow fiber having on the periphery thereof fins extended in the longitudinal direction and having a selective permeability, wherein the average degree of polymerization of a cellulose type ester polymer as the main constituent is at least 150 and the ratio H/W of the fin height H to the fin width W is at least 0.5. The selective-permeable cellulose type hollow fiber of the present invention comprises a cellulose type ester polymer as the main constituent. As the cellulose type ester polymer, there can be mentioned cellulose acetates such as cellulose diacetate and cellulose triacetate, cellulose propionate, cellulose butyrate, cellulose acetate butyrate, and cellulose nitrate. These cellulose type polymers may be used singly or in the form of a mixture of two or more of them. Cellulose esters such as cellulose diacetate and cellulose triacetate are preferred. The cellulose ester type polymer used in the present invention is characterised in that the average degree of polymerization is at least 150. By the average degree of polymerization referred to herein is meant a value obtained by dividing the weight mean molecular weight by the molecular weight of the recurring unit of the polymer. As means for determination of the weight mean molecular weight, there can be mentioned gel filtration chromatography, the measurement of the coefficient of viscosity, the measurement of the sedimentation equilibrium and the light scattering method. If the average degree of polymerization of the cellulose type ester polymer is lower than 150, a selective-permeable cellulose type hollow fiber having fins having a large height can hardly be obtained, and a hollow fiber having a low re-wet elongation can hardly be obtained. It is preferred that the average degree of polymerization be at least 160, especially at least 170. The upper limit of the average degree of polymerization is not particularly critical but, from the practical viewpoint, it is preferred that the degree of polymerization be lower than 230, especially lower than 200. The cellulose type hollow fiber of the present invention comprises as the main constituent a cellulose ester type polymer as described above. The content of the polymer is at least 50, preferably at least 70%. Other component is not particularly critical, and for example, there can be mentioned organic polymers such as polymethyl methacrylate, polyethylene, polypropylene, polyacrylonitrile, and polyvinyl alcohol. The cellulose type hollow fiber of the present invention has on the periphery thereof fins extended in the longitudinal direction, and the hollow fiber is characterized in that the ratio H/W of the fin height H to the fin width W is at least 0.5. If the ratio H/W is lower than 0.5, contact of hollow fibers with one another can hardly be prevented when they are filled in the fluid separator. It is Preferred that the ratio H/W be at least 0.8. Incidentally, the fin width W is the width of the fin at a position of 1/2 of the fin height H. The number x of fins in the hollow fiber of the present invention is at least 1. However, if the number x is 11 or larger, reduction of the effective membrane area at the root portions of the fins becomes prominent, and the solute dialyzing capacity and water permeability of the blood dialyzer are drastically reduced. Accordingly, the number x is in the range of from 1 to 10, and it is preferred that the number x be 2 to 8, especially 3 to 7. It is preferred that the outer diameter d of the hollow fiber of the present invention be 100 to 500 µm, especially 200 to 300 µm, and that the thickness h of the fin-free portion of the hollow fiber be 5 to 40 µm, especially 5 to 30 µm, particularly especially 10 to 25 µm. Furthermore, it is preferred that the fin height H be 5 to 100 µm, especially 10 to 65 µm, and that the ratio H/d be 0.5 to 3, especially 1 to 2. Moreover, it is preferred that the ratio H/d of the average height H (µm) of fins to the average outer diameter d of the hollow fiber exclusive of the fin portion be 0.01 to 1, especially 0.02 to 0.5, particularly especially 0.03 to 0.2. It is preferred that the width of the root portion of the fin be narrower than that of the upper portion. However, ordinarily, the root portion is readily expanded by the surface tension of the spinning liquid after extrusion of the spinning liquid from the nozzle. The width of the root portion is 15 to 50 µm and preferably 20 to 40 µm. If the width of the root portion is within this range, the circularity of the hollow fiber is good and coagulation of blood is hardly caused in the hollow fiber or blood is hardly left in the hollow fiber. If one hollow fiber has a plurality of fins, the respective fins may be the same or different in the height H or width W. Furthermore, at least parts of the fins may be spirally extended on the outer surface of the hollow fibers, or the fins may be partially discontinuous. The cellulose type hollow fiber of the present invention has a selective permeability to fluids. For example, when the cellulose type hollow fiber is used for hemodialysis, it is preferred that the water permeation capacity UFR of the hollow fiber exclusive of the fin portion be 1.0 to 30.0 ml/(m².hr.mmHg), especially 3.0 to 10.0 ml/(m².hr.mmHg). The cellulose type hollow fiber of the present invention is especially suitable for separation utilizing difference of the diffusion speeds, such as dialysis in which a good flow state of the fluid outside the hollow fiber is required, or blood treatment in which smooth flow within the hollow fiber is necessary. In the hollow fiber of the present invention, other membranes having a fluid separating capacity may be formed as a supporting membrane on the inner or outer membrane surface. The process for the preparation of the cellulose type hollow fiber according to the present invention is characterized in that a liquid mixture comprising a cellulose ester polymer having an average degree of polymerization of at least 150 and other additives is used as the spinning liquid, and the spinning liquid is extruded together with a core agent from a spinneret for a finned hollow fiber in the state where the viscosity is in the range of from 200 to 5,000 P, especially 500 to 3,000 P, in the case of melt spinning. The melt viscosity is measured by using a flow tester, and the liquid viscosity is measured by a rotational viscometer. In the melt spinning process, the spinning liquid is a heated melt, and the melt is extruded from the spinneret and cooled to form a solidified finned hollow fiber. Then, at least a part of the additive in the finned hollow fiber is extracted and removed by a solvent incapable of dissolving the cellulose ester polymer but capable of dissolving the additive, and if necessary, the hollow fiber is subjected to an alkali treatment. Thus, a finned cellulose type hollow fiber having a selective permeability can be obtained. In this melt spinning process, if the time required for substantial solidification of the hollow fiber by cooling from the point of extrusion of the heated solution from the spinneret is less than 10 seconds, preferably less than 5 seconds, spinning can be performed stably while retaining a good shape of the fin. By the term "solidification" used herein is meant the state where the spun fiber loses the flowability and attenuation is not advanced any more by the winding tension. The solidification is determined by taking out a fiber being spun by a clipper and measuring the point of termination of the attenuation. A specific example of the melt spinning process will now be described. A heated spinning liquid (melt or solution) is extruded in a gas or spinning bath according to customary procedures, for example, from a hollow hole of a spinneret having a notch in the periphery of a double annular portion, and spinning is conducted while retaining the hollow portion by filling a gas or liquid substantially incapable of dissolving the membrane material or reacting therewith into the central hollow portion. For example, a plasticizer such as polyethylene glycol is added as the additive to flakes of cellulose diacetate, and the mixture is molten and the melt is extruded into air from a hollow hole of the spinning nozzle. Then, nitrogen gas is blown into the central portion and simultaneously, the extrudate is cooled. The plasticizer is removed from the obtained hollow fiber extrudate. If necessary, a saponification treatment is carried out with caustic soda. Thus, a finned hollow fiber having a solute selective permeability is prepared. As the plasticizer, there can be mentioned polyalkylene glycols such as polyethylene glycol, glycols having an ethylene/propylene chain in the molecule, glycerol, diglycerol, sulforanes, caprolactones, and dimethylsulfoxide. The amount of the plasticizer added to the spinning solution is appropriately determined according to the intended use of the hollow fiber. For example, when the hollow fiber is used for hemodialysis, it is preferred that the amount of the plasticizer be 20 to 70% by weight, especially 25 to 60% by weight. In the process for the preparation of the hollow fiber of the present invention, it is sometimes preferred that the spinning be carried out at a winding speed higher than the extrusion speed. In this case, it is preferred that the draft ratio of the melt spinning process be 30 to 200. In melt spinning, the draft ratio refers to that before the solidification of the spun filament. Optionally, the spun filament may further be drafted after the solidification thereof, but usually, such further drafting is not necessary. The present invention will now be described in detail with reference to the following examples. Examples 1 to 10 and Comparative Examples 1 to 3 To 100 parts of cellulose diacetate was added polyethylene glycol (hereinafter referred to as "PEG") (having a molecular weight of 300) in an amount shown in Table 1, and the mixture was molten at 200 to 240 o C and extruded at a temperature shown in Table 1 at the extrusion opening through a nozzle shown in Figure 6 (the inner diameter of the annular double slit was 1.6 mm, the outer diameter was 2 mm, the notch width was 0.15 mm and the notch height was 0.7 mm). The extrudate was cooled by air fed at a rate shown in Table 1 at room temperature, followed by winding. The melt viscosity of the polymer melt at each temperature was separately measured by a flow tester. The distance between the solidification point and the top end of the nozzle was measured while clipping the fiber being spun by a clipper, and the point where attenuation terminates was regarded as the solidification point. At a nozzle temperature of 190 o C, the viscosity of the polymer melt was too high and yarn breakage was readily caused, and hence, winding could not be carried out smoothly. Hollow fibers wound were saponified by an alkali to convert cellulose diacetate to cellulose and obtain finned hollow fibers as shown in Figure 6 (the thickness of the fin-free portion was about 20 to about 25 µm). Dialyzers were assembled by filling these hollow fibers in shells at substantially the same filling ratio. The in vitro dialysis performance was determined according to the dialyzer performance evaluation standard stipulated at the meeting of the Japanese Artificial Organ Association held in September 1982. The obtained results are shown in Table 1. In the hollow fibers obtained according to the present invention, the shape of the fins was sharp and the width of the root portion of the fins was narrow, and the dialyzer comprising the hollow fibers of the present invention had a high performance value. On the other hand, in case of the comparative hollow fibers outside the scope of the present invention, the shape of the fins was rounded and fins having a sufficient height could not be formed.

Claims: A process for preparing, by melt-spinning, a selective-permeable hollow fiber for blood dialysis having an outer diameter of about 100 to 500 µm, a hollow portion being formed in the longitudinal direction and 1 to 10 fins extending in the longitudinal direction on the periphery thereof, the fins having a height H to width W ratio H/W of at least 0.5, characterised in that a spinning liquid containing at least one cellulose ester polymer having an average degree of polymerization of at least 150 and at least one additive for forming pores is extruded into the gas phase from a spinning nozzle for said hollow fiber having an annular double slit and 1 to 10 notches formed on an outer side of the double slit, the viscosity of the spinning liquid at said nozzle being 200 to 5,000 P, the distance between the end face of the spinning nozzle and the point where the hollow fiber extruded from the spinning nozzle is solidified in the gas phase being between more than 10 and 40 cm, the extruded fiber is cooled by air at room temperature and has a draft ratio of from 20 to 200, whereby a stable hollow fiber is obtained, and the solidified hollow fiber is treated so that it has selective permeable properties. A process according to Claim 1, wherein a plasticizer of cellulose ester polymer is contained in the spinning liquid. A process according to Claim 1 or 2, wherein the viscosity of the spinning liquid is 300 to 2,000 P. A process according to any one of the preceding claims, wherein the solidified hollow fiber is treated by extraction to remove the additive. A process according to any one of the preceding claims, wherein the solidified hollow fiber is treated with an alkali.

Inventors: MIZUTANI SHOJI, TANEDA NOBUO, MIZUTANI, SHOJI, TANEDA, NOBUO
Applicants: TEIJIN LTD, TEIJIN LIMITED
Application year: 1989
Main classification: B01D69
IPC codes: D01D5, B01D69, B01D63