Technical Field The present disclosure relates to a continuous process for preparing a spinning solution for dialysis membranes which involves a multiple screw extruder.

Description of the Related Art

Spinning solutions for the production of dialysis membranes conventionally are prepared by providing multiple polymer components, solvents, and additives to a mixing vessel and stirring the mixture until a homogeneous solu- tion is obtained. This process usually requires 2-3 hours. The solution is subsequently degassed in a sepa ^¬ rate unit to remove any dissolved air from the solution and transferred to a storage tank. The extended time re ^¬ quired to prepare the spinning solution is a drawback of this process. As it is a batch process, it also does not allow for a rapid change of the composition of the spinning solution during membrane production. The recipe of the spinning solution can only be changed when the storage tank has been emptied; i.e. from batch to batch.

It would be desirable to have a process for the produc ^¬ tion of spinning solutions for dialysis membranes which is faster and more flexible. JP 1234425 A describes a process for continuously prepar ^¬ ing a spinning stock solution by kneading an acrylo- nitrile-based polymer in an engaging type multiple screw extruder with a solvent for the polymer.

DE 4027888 Al discloses a process for the continuous dis ^¬ solution of gelatine wherein gelatine powder and a sol ^¬ vent are continuously fed to a screw extruder and mixed by applying shear force at elevated temperature and pres- sure to produce a liquid gelatine solution.

US 4784820 A teaches a process for the preparation of ho ^¬ mogeneous solutions of ultrahigh molecular weight poly- olefins which comprises forming a slurry of particles of the polymer in a solvent for the polymer, pumping the slurry under pressure to a single screw extruder and maintaining the slurry in the screw extruder until a homogeneous solution is formed. Up to now, extruders have not been used to prepare multi- component spinning solutions such as those required for the production of dialysis membranes which typically com ^¬ prise a multitude of solid and liquid constituents dif ^¬ fering in physical properties such as viscosity, solubil- ity, or boiling temperature.

Summary

It has now been found that spinning solutions for dialy- sis membranes can be prepared continuously using a multi ^¬ ple screw extruder.

A principal aim of the present invention is to provide a flexible and fast process for the production of spinning solutions for dialysis membranes. The process of the invention involves continuously feed ^¬ ing at least one hydrophobic polymer, at least one hydro ^¬ philic polymer, a solvent for the polymers, and, optio- nally, a non-solvent for the polymers to a multiple screw extruder and mixing the components in the extruder to form a homogeneous solution.

Further characteristics and advantages of the present in vention will better emerge from the detailed description that follows of at least a preferred embodiment of the invention, illustrated purely in the form of a non- limiting example. Detailed Description

The present invention provides a continuous process for preparing a spinning solution comprising

i) continuously introducing into the feed zone of a multiple screw extruder

a) at least one hydrophobic polymer,

b) at least one hydrophilic polymer,

c) at least one solvent for the hydrophobic polymer and the hydrophilic polymer, and d) optionally, at least one non-solvent for the hydrophobic polymer;

ii) continuously mixing a) to d) in the multiple screw extruder to form a homogeneous solution; iii) continuously discharging the solution formed in step ii) from the multiple screw extruder.

The at least one hydrophobic polymer is selected from the group consisting of polysulfones , polyethersulfones , polyamides, polycarbonates, polyethyleneimines , and cycloolefin copolymers. In one embodiment of the process, the hydrophobic polymer is a polysulfone. In another em ^¬ bodiment of the process, the hydrophobic polymer is a polyethersulfone . The at least one hydrophilic polymer is selected from the group consisting of polyvinylpyrrolidones, polyethylene- glycols, and polyoxazolines . In one embodiment of the process, the hydrophobic polymer is a polyvinylpyrroli ^¬ done. In another embodiment of the process, the hydropho- bic polymer is a mixture of a polyvinylpyrrolidone having a molecular weight of less than 100,000 g/mol and a poly ^¬ vinylpyrrolidone having a molecular weight of at least 100,000 g/mol. The solvent is selected from the group consisting of N-alkyl-2-pyrrolidones, dimethylacetamide, dimethyl- sulfoxide, tetrahydrofurane, gamma-butyrolactone, and di- methylformamide . In one embodiment of the process, the solvent is N-methyl-2-pyrrolidone .

In one embodiment of the process, the non-solvent is se ^¬ lected from the group consisting of water and aliphatic alcohols . In one embodiment of the process, the multiple screw ex ^¬ truder is a co-rotating twin screw extruder. In one embodiment of the process, the working length of the multi ^¬ ple screw extruder is 40D to 80D. In one embodiment of the process, the multiple screw extruder is operated at 50 to 250 rpm. In one embodiment of the process, the screws of the multiple screw extruder predominantly com ^¬ prise mixing elements (kneading elements) , the other ele ^¬ ments being selected from conveying elements and comminu ^¬ ting elements. In another embodiment of the process, the screws of the multiple screw extruder exclusively com- prise mixing elements. In one embodiment of the inven ^¬ tion, the screws of the multiple screw extruder have a thread pitch of less than 30 mm. In one embodiment of the process, the temperature in the mixing zones of the mul- tiple screw extruder is in the range of from 20°C to

210°C, for instance in the range of from 40 to 100°C. In one embodiment of the process, the residence time in the multiple screw extruder is from 1 to 5 min. In one embodiment of the process, the solution formed in step ii) is degassed prior to being discharged from the multiple screw extruder. In one embodiment of the proc ^¬ ess, the pressure in the end zone of the multiple ex ^¬ truder is reduced below atmospheric pressure to effect degassing of the solution formed in step ii) .

In one embodiment of the process, .

It will be understood that the features mentioned above and those described hereinafter can be used not only in the combination specified but also in other combinations or on their own, without departing from the scope of the present invention. The present invention will now be described in more de ^¬ tail in the examples below. It is to be understood that the examples are not intended to limit the scope of the present invention and are merely an illustration of a preferred embodiment of the invention. Examples

The viscosity of the polymer solution is the dynamic vis ^¬ cosity, if not otherwise indicated. To measure the vis- cosity of the polymer solution, a commercial rheometer (MCR 101, Anton Paar GmbH) was used. The polymer solution is placed between two temperature-controlled plates. The measurement is performed at 22°C. All other measurement conditions are according to the manufacturer's instruc- tions.

Starting materials

• PES: polyethersulfone Ultrason ^® E 6020 P, BASF S.E.;

M _w = 46-55 kDa; M _w /M _n = 3.0-4.0;

· PVP-K30: polyvinylpyrrolidone Luvitec ^® K30, BASF

S.E.; M _n = 14 kDa, M _w = 50 kDa;

• PVP-K85: polyvinylpyrrolidone Luvitec ^® K85, BASF S.E.; M _n = 250 kDa, M _w = 1,100 kDa.

Example 1

PES, PVP-K30, and PVP-K85 were continuously fed to the feeding zone of a co-rotating twin-screw extruder having a working length of 40 D (ZSE 27 HP-PH 40D, Leistritz Ex- trusionstechnik GmbH, D-90459 Nurnberg) using screw feeders. Simultaneously, NMP and water were continuously fed to the feeding zone of the extruder. The weight ratio of the individual feed streams was 14 PES/2 PVP-K85/5 PVP- K30/76 NMP/3 water. The extruder was thermostatted at 70 °C and operated at 100 rpm. The screws of the extruder comprised mixing elements only. A polymer solution comprising small agglomerates was obtained. Example 2

PES was continuously fed to the feeding zone of a co- rotating twin-screw extruder having a working length of 40 D (ZSE 27 HP-PH 40D, Leistritz Extrusionstechnik GmbH, D-90459 Nurnberg) using a screw feeder. Simultaneously, NMP and water were continuously fed to the feeding zone of the extruder. PVP-K30 and PVP-K85 were continuously fed to the extruder at between 24D and 28D using screw feeders. The weight ratio of the individual feed streams was 14 PES/2 PVP-K85/5 PVP-K30/76 NMP/3 water. The ex ^¬ truder was thermostatted at 78 °C. Several experiments were conducted in which the extruder was operated at dif ^¬ ferent velocities in the range of from 150 rpm to 350 rpm. The screws of the extruder comprised mixing elements only. The polymer solutions obtained still comprised some agglomerates, but less than in Example 1.

Example 3

PES, PVP-K30, and PVP-K85 were continuously fed to the feeding zone of a co-rotating twin-screw extruder having a working length of 60 D (ZSE 27 60D, Leistritz Extrusionstechnik GmbH, D-90459 Nurnberg) using screw feeders. Simultaneously, NMP and water were continuously fed to the feeding zone of the extruder. The weight ratio of the individual feed streams was 14 PES/2 PVP-K85/5 PVP-K30/76 NMP/3 water. The extruder was thermostatted at 78 °C and operated at 150 rpm. The screws of the extruder comprised only mixing elements and had a screw thread pitch of 30 mm. A polymer solution comprising sporadic agglomerates was obtained. Example 4

PES, PVP-K30, and PVP-K85 were continuously fed to the feeding zone of a co-rotating twin-screw extruder having a working length of 60 D (ZSE 27 60D, Leistritz Extru- sionstechnik GmbH, D-90459 Nurnberg) using screw feeders. Simultaneously, NMP and water were continuously fed to the feeding zone of the extruder. The weight ratio of the individual feed streams was 14 PES/2 PVP-K85/5 PVP-K30/76 NMP/3 water. The extruder was thermostatted at 78 °C and operated at 150 rpm. The screws of the extruder comprised only mixing elements and had a screw thread pitch of less than 30 mm. No particles could be detected in the polymer solution obtained. The solution contained numerous air bubbles. The viscosity of the solution was measured to be 6,200 mPa*s.

Example 5

PES, PVP-K30, and PVP-K85 were continuously fed to the feeding zone of a co-rotating twin-screw extruder having a working length of 60 D (ZSE 27 60D, Leistritz Extru- sionstechnik GmbH, D-90459 Nurnberg) using screw feeders. Simultaneously, NMP and water were continuously fed to the feeding zone of the extruder. The weight ratio of the individual feed streams was 14 PES/2 PVP-K85/5 PVP-K30/76 NMP/3 water. The extruder was thermostatted at 78 °C and operated at 200 rpm. The screws of the extruder comprised only mixing elements and had a screw thread pitch of less than 30 mm. In the end zone of the extruder, pressure was adjusted to 200 mbar using the vacuum venting system of the extruder. A clear solution containing neither particles nor air bubbles was obtained. The viscosity of the solution was measured to be 18,500 mPa*s.