GB2038232A

GB2038232A - Thin walled hollow fibre dialysis membrane

Info

Publication number: GB2038232A
Application number: GB7934087A
Authority: GB
Original assignee: Akzo NV
Current assignee: Akzo NV
Priority date: 1978-10-02
Filing date: 1979-10-02
Publication date: 1980-07-23
Also published as: FR2444486A1; IT7950417A0; GB2038232B; NL7907214A; IT1164715B; LU81737A1

Abstract

A hollow fibre of regenerated cellulose for use as a dialysis membrane has a wall thickness in the range of from 0.5 to 8 mu m. Preferably the cellulose is coagulated from cuprammonium solution.

Description

SPECIFICATION Thin walled hollow fibre dialysis membrane The present invention relates to a hollow fibre of regenerated cellulose, for use as a dialysis membrane.

Regenerated cellulose, which is suitable for the invention, is a cellulose which was coagulated from cellulose solutions by means of regeneration agents. Wherein the cuprammonium process is most preferably used because it leads to hollow fibres with the best membrane characteristics.

Dialysis membranes, having a form according to the invention, are used predominately for haemodialysis.

However, they can also be used for reverse osmosis.

Until now, it has only been known to use dialysis membranes, having a hollow fibre form with relatively thick walls, since it was feared that if thin walls were used the hollow fibre would collapse and thereby present the passage of the blood. Using supporting layers, thinner - walled dialysis membranes have already been produced in hollow fibre form. In order to produce these, complicated spinnerets are required and the process has to be carried out with extreme precision.

Up till now, with dialysis membranes of regenerated cellulose, in hollow fibre form, a minimum wall thickness of more than 10 um has been adhered to, whereby the lumen diameter usually amounted to 300 um.

An object of the present invention was the application of hollow fibres of regenerated cellulose as dialysis membranes with extremely thin wall thicknesses with the largest possible lumen diameter.

According to the present invention there is provided a hollow fibre of regenerated cellulose for use as a dialysis membrane the fibre having a wall thickness in the range of from 0.5 to 8 um.

The hollow fibre wall is preferably composed of a single layer. Such hollow fibres are produced according to the known cuprammonium process by means of known hollow fibres nozzles. As a coagulation agent, sulphuric acid, diluted caustic soda lye or warm water can be used. Particularly the process described in Example 10 of German Patent Specification No. 736 321, is most suitable for the production of thin-walled hollow fibres of regenerated cellulose. By variation of the discharge quantity of cellulose - cuprammonium solution and of hollow - forming fluid with respect to the drawing - off velocity, practically every dimension, which lies in the framework of the invention, can be produced without difficulty.

It was also an object of the present invention to provide a process for spinning thin-walled hollow fibres of regenerated cellulose for semi-permeable membranes, which -allows the production of hollow fibres with exact cross-sectional form given by the spinneret shape, which are at the same time superior in their outstanding characteristics as semi-permeable membranes and whose mechanical properties are not impaired.

Thus according to a further aspect of the invention a process is provided for producing hollow fibres according to the invention wherein a hollow fibre spinneret is immersed in an aqueous caustic soda lye and the ratio of the drawing-off velocity of the hollow fibre at a first drawing-off roller to the discharge velocity of the cellulose-cuprammonium-solution from a ring slit of the hollow fibre spinneret is from 1.00 to 1.05, and the direction of the fibre run from the hollow fibre spinneret to the first drawing-off roller forms an acute angle with the axis of the hollow fibre spinneret openings.

By the use of hollow fibre spinnerets with an orbicular cross-section, one obtains hollow fibres with an exactly orbicular cross-section with such slight deviation from the ideal circular form as not thought possible up till now. If the standard deviations are determined for the deviations in the wall thickness of a hollow fibre according to the invention with a concentric circle cross-section, then values of less than 5% are obtained.

If hollow fibre spinnerets are used for hollow fibres with a profiled or eccentric cross-section, then in contrast to the processes according to the prior art, no levelling of the profile or decrease of the eccentricity, is obtained.

With regard to the eccentricity it should be said that this can even be increased with the process according to the invention, even when the fibre is drawn-off in such a way that an acute angle which is formed with the axis of the hollow fibre spinneret openings lies on the side of greater wall thickness. It should have been expected from this that a lessening of the eccentricity would then result.

It is in principle also possible with the process of the invention to arrange the hollow fibre spinneret on the floor of the coagulation bath and to spin the fibre from above. Because of the great technical difficulties which result with such an arrangement in changing the nozzle in the seal and the initiating of spinning, this embodiment will be less significant for carrying out the process than the arrangement in which the hollow fibre spinneret is on the surface of the coagulation bath.

With the process of the invention the hollow fibre spinneret is preferably immersed to a depth of from 5 to 10 mm In the aqueous caustic soda lye. This immersion depth is just adequate for the fibre to coagulate fast enough, whereby, the hollow fibre spinneret opening can still be clearly observed in the caustic soda lye which is coloured deep blue by the cuprammonium solution.

The first drawing-off roller is arranged according to the invention such that the spun hollow fibre, when it issues from the hollow fibre spinneret, it is not carried down vertically but is removed to such a distance and arranged in such a manner that the direction of the fibre run from the hollow fibre spineret to the first drawing-off roller forms an acute angle with the axis of the hollow fibre spinneret openings. This acute angle has a value of from 15 and 70 In the coagulation bath, by the process of to the invention, the newly spun hollow fibre is transported under the application of only very slight strains. The circumferential velocity of the second drawing-off roller which is set up behind the first drawing-off roller, is preferably only 90 to 98% of the circumferential velocity of the first drawing-off roller.By this means, a small shrinking of the newly spun hollow fibre results, while with the processes according to the prior art, the hollow fibre is stretched immediately after leaving the spinneret.

Up till now, it has been thought that hollow fibres, especially those which are used as semipermeable membranes, which have diameters of approximately 50 to 1000 um, with wall thickness of approximately 10 to 200 um, could only be produced with hollow fibre spinnerets, whose dimensions are a multiple, for example of from 10 to 50 times the hollow fibre dimensions. Within the framework of the invention, hollow fibre spinnerets are preferably used in which the dimensions of the ring slit of the hollow fibre spinneret are from 2.5 to 6 times the dimensions of the finished hollow fibre.

The cellulose content of the cellulose-cuprammonium solution does not generally deviate from the cellulose content which the cellulose-cuprammonium-spinning solutions have, which are usually used for the regeneration of cellulose. However, the cellulose content preferably amounts to 6 to 10% by weight, of the solution. The NaOH-content of the caustic soda lye can fluctuate within greater limits. However it should preferably lie between 10 and 20% by weight to guarantee a sufficiently fast formation of the Normann-cellulose, which introduces the strain-hardening of the hollow fibre.

Inasmuch as the hollow fibre, which is produced according to the invention is or has to be stretched, this stretching is advantageously carried out by passing it thorough after treatment baths.

Thin walled hollow fibres of regenerated cellulose often lie flat when they are dry, and while they are flat they can be rolled up and converted into flat forms. By soaking, the Iaid-outflathollowfibresstraighten out again, and a flat, round lumen cross-section forms, which makes it possible to provide an uninterupted passage for the blood.

In a development of the invention, the luman has a round cross-section, whereby the diameter of the lumen is from 50 to 500 um.

Every cross-sectional form should be understood as being rounded, having no corners or projections, and curved outwards from the fibre axis. Preferred cross-sectonal forms of the lumen are circular and elliptic.

The wall thickness of the hollow fibre is preferably from 1 to Sum. In an embodiment of the invention, the wall thickness remains constant both over the circumference and over the length. In another embodiment of the invention, the wall thickness changes regularly and uniformly over the circumference and/or the length.

Such changes of the wall thickness over the length and/or over the circumference serve the purpose of avoiding the so called glass plate effect, which results in the hollow fibres sticking together and thereby presenting the dialysis fluid from coming into contact with the total membrane surface.

Wall thicknesses of 1.0 um, 20 um, 3.5 um, 5 um and 7 um were realised in preference and a wall thickness of 8.5 um. did not present any difficulties in the production. The hollow fibres have good characteristics as dialysis membranes, if their wall thickness changes regularly and uniformly over the circumference in the ratio of up to 1:3. The wall thickness mentioned in the previous sentence then present the dimentions of the smallest wall thickness. In the following table, dimensions of hollow fibres of regenerated cuprammoniumcellulose have been assembled wherein D/a is the ratio of lumen diameter to wall thickness.

TABLE 1 Nr. lumen diameter Wall thickness Ratio D/a 1. 30 um 0.5 um 60 2. 50 um 2.0 um 25 3. 90 um 3.0 um 30 4. 150 um 3.0 um 50 5. 160um 4.out 40 6. 210 um 3.5 um 60 7. 285 um 2.5 um 114 8. 290 um 5.0 um 58 9. 380 um 7.0 um 54 10. 390 um 3.0 um 130 11. 420 um 6.0 um 70 12. 430 um 2.0 um 215 The hollow fibres, the dimentions of which are presented in Table 1, were installed in a test dialyzer and were examined in vitro under test conditions.

The remaining method is described in "Evaluation of Hemodialysers and Dialysis Membranes" Report of a study group for the artificial kidney - chronic uremia program 1977; Elias Elein et al. US-Dept-artment of Health Education and Welfare. Bethesda Maryland 20014. Publication No. NIH 77-1294.

With a solution flow of 200 mI/min. m2 and with a dialyzate flow of 500 ml/min. m2, a urea-clearence of up to 195 mllmin was reached. From this, the outstanding suitability of the thin walled dialysis membrane hollow fibres is shown for use in artificial kidneys.

In the accompanying drawings, Figure 1 and 2 show a first embodiment of the invention, Figure 3 shows a a second embodiment and Figure 4 illustrates the process of to the invention is presented schematically.

Figure 1 shows a cut fibre in a perspective presentation, while Figure 2 shows a cross section perpendicularto the fibre axis the dimentional ratios of which being shown greatly distorted for the purpose of illustration. In these Figures, A is the hollow fibre having a wall B comprising regenerated cellulose. The lumen diameter is indicated by D and the wall thickness by a.

Figure 3 shows a cross section perpendicular to the fibre axis, the dimentional ratios of which being shown greatly distorted for the purpose of illustration in which the wall thickness changes regularly and uniformly over the circumference. In this, Figure A is a hollow fibre having a wall B comprising regenerated cellulose.

The lumen diameter is indicated by D and the wall thickness by a.

Figure 4 shows an embodiment of a process scheme. A cellulose-cuprammonium-spinning solution 1 and the hollow forming fluid 2, for example isopropryl myristate of paraffin oil are introduced into the hollow fibre spinneret 3. This hollow fibre spinneret 3 is immersed in the aqueous caustic soda lye conducted through a coagulation bath 4. The hollow fibre A which comes out of the hollow fibre spinneret 3 diverted at the first drawing-off roller 6 and carried over the second drawing-off roller 7 to after treatment baths. The direction of transport of the fibre between the first drawing-off roller 6 and the hollow fibre spinneret 3 makes an acute angle with the axis of the hollow fibre spinneret openings. The after treatment baths are suitably constructed as tubs, of which two 8, 14 are shown by way of example in the Figure. Diversion rollers 9 are arranged in the after treatment baths. The driving rollers 10, 11, 12 and 13 are run with increasing circumferential velocity, whereby the hollow fibre A is stretched out to the required length. The washed hollow fibre A is carried over a last diversion roller into a dryer 15, is dried there and roller up on roller 16 to be rinsed.

For the after treatment baths, usually diluted caustic soda lye, water, diluted sulphuric acid, "acidulous water" and pure water are used in succession. Before being dried the hollow fibres are suitably treated with glycerine.

Claims

1. A hollow fibre of regenerated cellulose for use as a dialysis membrane the fibre having a wall thickness in the range of from 0.5 to 8 um. D

2. A fibre according to claim 1, wherein the hollow fibre wall is composed of one layer.

3. Afibre according to claims 1 to 2, wherein the lumen of the hollow fibre has a round cross-section perpendicular to the axis of the fibre, and the diameter of the lumen is from 50 to 500 um.

4. A fibre according to any one of claims 1 to 3 wherein the wall thickness of the hollow fibre is from 1 to 5 um.

5. A fibre according to any one of claims 1 to 4, wherein the wall thickness remains constant over the circumference and the length of the hollow fibre.

6. A fibre according to any one of claims 1 to 4, wherein the wall thickness varies regularly and uniformly over the circumference and/or the length of the hollow fibre.

7. A fibre according to any of claims 1 to 6, wherein the ratio of lumen diameter to the wall thickness is from 20 to 280.

8. A hollow fibre of regenerated cellulose for used as a dialysis membrane substantially as herein described with reference to any of Figures 1 to 3 of the accompanying drawings.

9. A process for spinning a hollow fibre of regenerated cellulose for use as a dialysis membrane according to any one of claims 1 to 7, the process comprising: forcing a cellulose-cuprammonium-solution through a ring slit of a hollow fibre spinneret into aqueous caustic soda lye, and a hollow forming fluid through the inner bore of the hollow fibre spinneret, wherein the hollow fibre spinneret is immersed in the aqueous caustic soda lye and the ratio of the drawing-off velocity of the hollow fibre at a first drawing-off roller to the discharge velocity of the cellulose-cuprammonium solution from a ring slit of the hollow fibre spinneret is from 1.00 to 1.05, and the direction of the fibre run from the hollow fibre spinneret to the first drawing-off roller forms an acute angle with the axis of the hollow fibre spinneret openings.

10. A process according to claim 9 wherein the hollow fibre spinneret is immersed in the aqueous caustic soda lye to a depth of from 5 to 10 mm.

11. A process according to claim 9 or 10 wherein the angle formed between the fibre run direction from the hollow fibre spinneret to first drawing-off roller and the axis of the hollow fibre spinneret opening, is from 15 to 70".

12. A process according to any of claims 9 to 11 wherein the circumferential velocity of the second drawing-off roller which is arranged behind the first drawing-off roller is from 90 to 98% of the circumferential velocity of the first drawing-off roller.

13. A process according to any of claims 9 to 12 wherein the dimensions of the ring slit of the holiowfibre spinneret are from 2.5 to 6 times the dimentions of the finished hollow fibre.

14. A process according to claims 9 to 13 wherein the cellulose content of the cuprammonium solution is from 6 to 10% by weight and the NaOH - content of the aqueous caustic soda lye is from 10 to 20%, by weight each corresponding to the weight of the respective solutions.

15. A process according to any one of claims 9 to 14 wherein the hollow fibre formed is after treated.

16. A process according to claim 15, wherein the hollow fibre is stretched while passing through after treatment baths.

17. A process for spinning a hollow fibre of regenerated cellulose for use as dialysis membrane as claimed in any of claims 1 to 8, substantially as herein described with reference to Figure 4 of the accompanying drawings.