GB2090187A

GB2090187A - Hollow fibre dialysis membrane

Info

Publication number: GB2090187A
Application number: GB8200684A
Authority: GB
Original assignee: Akzo NV
Current assignee: Akzo NV
Priority date: 1978-10-02
Filing date: 1979-10-02
Publication date: 1982-07-07
Also published as: GB2090187B

Abstract

A hollow fibre of cellulose generated from cuprammonium solutions for use as a dialysis membrane has a continuous cavity and a surface which has projections arranged periodically in circumferential direction thereof. The cross section of the cavity may be circular or elliptical.

Description

SPECIFICATION Hollow fibre dialysis membrane The present invention relates to a hollow fibre having a continuous cavity which is composed of cellulose regenerated from cuprammonium solutions for use as a dialysis membrane.

A hollow fibre of regenerated cellulose which has been regenerated from cuprammonium solutions, having a continuous cavity is known from German Patent Specification No. 736,321.

It is known from U.S.A. Patent Specification No.

3,228,877 that the hollow fibres produced according to the above mentioned German Patent No. 736321 are suitable as dialysis membranes and as membranes for reverse osmosis.

From U.S.A. Patent Specification No.

3,888,771 hollow fibres of cellulose which have been regenerated from cuprammonium solutions are known, which have a definite membrane structure and have along the entire axis thereof a regular wall thickness and an exactly circular cross-section.

In dialysis, for example haemodialysis, it is necessary for the membrane walls to be washed by the completely dialysis fluid without any obstacles. If dialysis membranes are used, in the form of hollow fibres which are assembled together in bundles, then hollow fibres with exactly circular cross sections in the bundles of several thousand hollow fibres with a fibre thickness of about 500 to 1,000 per cm2, tend to fit easily in a firm and snug fashion to each other for their entire length in similar fashion to the glass plate effect of two plane parallel plates.

Due to this adhesion between the fibres, access to the gaps between the hollow fibres for the dialyzate fluid is made difficult, and the spaces, in which the hollow fibres stick to each other, remain unused for the material exchange, whereby the efficacy of the hollow fibre module is diminished.

An object of the present invention was to form hollow fibres for dialysis from regenerated cellulose in such a manner that the adhesion between the hollow fibres is substantially prevented.

According to the present invention, there is provided a hollow fibre of cellulose regenerated from cuprammonium solutions, for use as a dialysis membrane, the hollow fibre having a continuous cavity, and an external surface on which projections are arranged periodically in the circumferential direction thereof.

The projections extend from the wall at preferably 1 to 3 times the wall thickness of the hollow fibres. The wall thickness of the hollow fibres according to the invention amounts to 1 to 100 Flm in a manner known per se, preferably 5 to 50,um. The inner diameter of the hollow fibres amounts to 10 to 1000 ym and is preferably 20 to 600 cm.

In general, the inner cavity cross-section is circular, because such a fibre cross section presents fewer difficulties when spinning cellulosecuprammonium-solutions.

The efficacy of the dialysis membrane provided by the hollow fibres according to the invention increases substantially if the inner cavity crosssection is developed so that it is elliptic. By this means, the exchange surface of the dialysis membrane for the same blood volume is greatly enlarged, and for this reason the effectiveness of the metabolite exchange is substantially improved.

Because of the small blood film thickness in the dialysis membrane, provided by the hollow fibres according to the invention, a further improvement is achieved in the efficacy of the dialysis. The known effect of the adhesion of the fibres of circular dialysis fibres has not been observed.

The hollow fibres according to the invention are produced by coagulation of a cellulosecuprammonium-solution extruded from a hollow fibre spinneret, in diluted caustic soda lye, wherein while avoiding as much as possible, stretching and orientation of the fibres, the spinning solution is carried into the coagulation bath immediately after leaving the spinneret. In order to form the continuous cavity, a hollow forming fluid is passed through the central bore in a known manner; examples of hollow forming fluids are halogenated hydrocarbons, hydrocarbons and ester, wherein isopropyl myristate has proved to be favourable. It has also been shown that water and aqueous solutions, for example solutions of carboxymethyl cellulose salts are extremely well suited as hollow forming fluids.

The development of the hollow fibre form according to the invention is obtained through application of a hollow fibre nozzle with a correspondingly profiled spinning slit and a central bore for admitting the hollow-forming fluid. In order to achieve the desired form, the profile of the spinneret is formed so it is more strongly defined than the corresponding cross-sectional profile for the hollow fibre desired.

It is also possible for the dialysis membrane formed by the hollow fibre according to the invention that the wall of the hollow fibre be composed of two or more layers as is described in detail in the German Patent Applications P P 26 27 858, P 27 05 735 and P 27 05 733 in order to increase the leak proof resistance, or to obtain a surface layer which contains chemically modified cellulose, or to store adsorbents in the membrane wall. Thus, one embodiment of the present invention, the hollow fibre comprises two or more self-adhesive permeable layers of regenerated cellulose which stick together firmly.

In a preferred embodiment, one of these layers contains from 1 to 95% by weight of adsorbents.

In order to spin hollow fibres with the exact cross-sectional form given by the spinneret form, which at the same time have outstanding characteristics as semi-permeable membranes but whose mechanical properties are not thereby impaired, a process was found, which is characterised in that the spinneret is immersed in an aqueous caustic soda lye, and the ratio of drawing-off velocity of the hollow fibre at a first drawing-off roller to discharge velocity of a cellulose-cuprammonium-solution from a ring slit of the hollow fibre spinneret is 1.00 to 1.05 and the direction of the fibre run from the hollow fibre spinneret to the first drawing-off roller makes an acute angle with the axis of the hollow fibre spinneret openings.

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

It is in principle also possible, by the present process for forming fibres according to 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 the change of nozzle, with the sealing and initiating of spinning this embodiment will be less significant for carrying out the process than the arrangement of the hollow fibre spinneret on the surface of the coagulation bath.

The hollow fibre spinneret is preferably immersed to z depth of from 5 to 10 mm in an aqueous caustic soda lye in the present process.

This immersion depth is just adequate to cause the fibre to coagulate fast enough, wherein 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 drawing-off roller is arranged such that a spun hollow fibre, when it issues from the hollow fibre spinneret, is not carried downwards vertically, but having been removed to a certain distance is arranged so that the direction of the fibre run from the hollow fibre spinneret to the first drawing-off roller makes an acute angle with the axis of the hollow fibre spinneret openings. This acute angle is preferably from 150 to 700.

In the coagulation bath, by the present process, the newly spun hollow fibre is transported only under the application of very slight strains. The circumferential velocity of the second drawing-off roller, which is positioned after the first drawing off roller, is preferably only 90 to 98% of the circumferential velocity of the first drawing-off roller. By this means, only a small shrinking of the newly spun hollow fibre results, while in the processes according to the prior art, the hollow fibre is stretched out immediately after leaving the spinneret.

Up till now, it has been thought that for the production of hollow fibres, especially those which are used as semi-permeable membranes, and which have diameters of from approximately 50 to 1 000 ym with wall thicknesses of from 10 to approximately 200 ym could only be produced with hollow fibre spinnerets whose dimensions are a multiple of, for example, from 10 to 50 times, the hollow fibre dimensions. In the scope of the invention, hollow fibre spinnerets are preferably used for which the ratio of the dimensions of the ring slit of the hollow fibre spinneret is from 2.5 to 6 times the dimension of the finished hollow fibre.

The cellulose content of the cellulosecuprammonium solution does not generally deviate from the cellulose content of the cellulosecuprammonium-spinning solutions which are usually used for the regeneration of cellulose.

However, the cellulose content is preferably from 6 to 10 percent by weight with respect to the weight of the solution. The NaOH-content of the caustic soda lye, can fluctuate within greater limits, however, it should preferably lie in the range from 10 to 20 percent by weight to guarantee a sufficiently fast formation of the Normann-cellulose, which introduces strain hardening for 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 the fibre through aftertreatment baths.

In the accompanying drawings; Figures 1 to 5 show cross-sections of embodiments of a hollow fibre according to the invention.

Figure 6 illustrates a process for producing hollow fibres according to the invention.

In Figure 1, a hollow fibre A is shown having a circular cavity cross section and which has two symmetrically arranged longitudinally running ribs.

In Figure 2 a hollow fibre A is shown having a circular cavity cross-section and which has four symmetrical arranged longitudinally running ribs.

In Figure 3 a hollow fibre A is shown having a circular cavity cross-section and which has an external profile having the form of a three pronged star.

In Figure 4 a hollow fibre A is shown having a circular cavity cross-section and which has an external profile having the form of a five pronged star.

In Figure 5 a hollow fibre A is shown having an elliptical cavity cross-section and which has two ribs running symmetrically along the hollow fibres at either end of the major axis of the ellipse.

In Figure 6 a process for producing hollow fibres according to the invention is presented schematically.

A ceilulose-cuprammonium-spinning solution 1 and a hollow forming fluid 2, for example, isopropyl myristate or paraffin oil, are introduced into a hollow fibre spinneret 3. This hollow fibre spinneret 3 is immersed in the aqueous caustic soda lye which is guided through a coagulation bath 4. A hollow fibre A which issues from the hollow fibre spinneret 3 is diverted at a first drawing-off roller 6 and carried over a 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 forms an acute angle with the axis of the openings of the hollow fibre spinneret 3. The after treatment baths are suitably constructed as tubs, of which two are shown (8, 14) by way of example in the Figure. In the after treatment baths, diversion rollers 9 are arranged. Driving rollers 10, 11, 12 and 13 are run with increasing circumferential speed, whereby the hollow fibre 5 is stretched out to a desired extent. The washed hollow fibre is carried over a last diversion roller into a drier 15, is dried there and rolled up on a 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.

EXAMPLE 1 Production of a hollow fibre according to the invention.

A cuprammonium-cellulose-solution was forced out of a hollow fibre nozzle, the spinning slit of which was outwardly extended symmetrically at two opposite sides, therein the length of the extension corresponded to three times the spinning slit width of the extension corresponded approximately to 1.5 times the spinning slit width.

The cellulose content of the cuprammoniumcellulose-solution amounted to 9.2% Isopropyl myristate as the hollow forming fluid was forced through the inner bore of the hollow fibre nozzle, the diameter of which corresponded to three times the spinning slit width.

The nozzle was placed such that the outlet openings were 5 mm beneath the surface of the regenerating bath of 12.5% NaOH.

The hollow fibre forming material, issuing from the nozzle, was carried at an angle of 400 to the axis of the hollow fibre spinneret opening to a first drawing-off roller situated in the regenerating bath, and after emerging behind this roller, was carried over a second drawing-off roller. The discharge velocity of the hollow fibre forming cellulose material was 30.9 m/min, the circumferential velocity of the first roller was 30.9 m/min and the circumferential velocity of the second roller was 30.26 m/min.

After this, the hollow fibre was carried through the usual following baths to remove the copper.

After a further caustic soda lye bath, followed a water wash, a sulphuric acid wash, and further water washes.

After being treated with glycerine and after having been dried, a hollow fibre was obtained with an inner diameter of 21 5 ym and a wall thickness of 16 ym. On each of two opposite sides of the outer wall it had a rib like protrusion in exterior of the hollow fibre wall of depth 1.5 times the wall thickness. The inner cross-section of the hollow fibre was circular.

The hollow fibre had a tensile strength of 22.103 cN/mm2, measured at 50% humidity and 230C, and an elongation of 26to.

EXAMPLE 2 Application of the hollow fibre according to the invention to haemodialysis.

The high efficacy for haemodialysis of the hollow fibre according to the invention, produced according to Example 1, is shown by the following results from dialysis tests.

Bundles consisting of 6000 of these profiled hollow dialysis fibres showed an extremely loose construction, also when they were wet. They were installed in a hollow fibre test dialyzer and examined under standardised conditions. The following results were obtained at a solution flow of 200 ml/min.m2 and a dialyzate flow of 500 ml/min.m2; Ultrafiltration rate 2.7 ml/h m2.mmHg. Urea clearance 138 ml/min and Vitamin B1 2 clearance 28 ml/min.

Claims

1. A hollow fibre of cellulose regenerated from cuprammonium solutions, for use as a dialysis membrane, the hollow fibre having a continuous cavity, and an external surface on which projections are arranged periodically in the circumferential direction thereof.

2. A fibre according to claim 1, wherein the hollow fibre has one or more longitudinally extending rib-like thickenings on the external surface thereof.

3. A fibre according to claim 1 or 2, wherein the cross-section of the hollow fibre has the external profile of a 3 or more pronged star.

4. A fibre according to any one of claims 1 to 3, wherein the cavity cross-section is circular.

5. A fibre according to any one of claims 1 to 3, wherein the cavity cross-scction is elliptical.

6. A fibre according to claims 1 to 5, wherein the hollow fibre comprises two or more selfadhesive permeable layers of regenerated cellulose which stick together firmly.

7. A fibre according to claim 6, wherein one layer of the hollow fibre contains from 1 to 95% by weight of adsorbents.

8. A hollow fibre of cellulose regenerated from cuprammonium solution for use as a dialysis membrane substantially as hereindescribed with reference to Figures 1 to 5 of the accompanying drawings.

9. A process for the production of a hollow ribre according to any of claims 1 to 8 comprising forcing a cellulose cuprammonium solution through the ring slit of a hollow fibre spinneret into aqueous caustic soda lye, and a hollow forming fluid through an inner bore of the hollow-fibre spinneret, wherein the hollow fibre spinneret is immersed in the aqueous caustic soda lye, and the ratio of drawing-off velocity of the hollow fibre at a first drawing-off roller to the discharge ve!ocity of the cellulose-cuprammonium solution from the 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 makes 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 to a depth of from 5 to 10 mm in the aqueous caustic soda lye.

11. A process according to claim 9 or 10 wherein the angle which is made by the direction of the fibre run from the hollow fibre spinneret to the first drawing-off roller with the axis of the hollow fibre spinneret openings, is in the range from 150 to 170.

12. A process according to any one 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 one of claims 9 to 12, wherein the dimensions of the ring split of the hollow fibre spinneret are from 2.5 to 6 times the dimensions of the finished hollow fibre.

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

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 when passing through after treatment baths.

17. A process for the production of a hollow fibre according to any one of claims 1 to 8 substantially as hereindescribed with reference to Figure 6 of the accompanying drawings.

1 8. A process for the production of a hollow fibre according to claim 1, substantially as herein described in the Example 1.