Stock material or miscellaneous articles – Structurally defined web or sheet – Longitudinal or transverse tubular cavity or cell
Reexamination Certificate
2002-05-03
2004-09-07
Loney, Donald J. (Department: 1772)
Stock material or miscellaneous articles
Structurally defined web or sheet
Longitudinal or transverse tubular cavity or cell
C428S156000, C264S177140, C264S17800F, C264S209100, C264S211130, C156S244130, C055S520000, C055S524000, C055S498000
Reexamination Certificate
active
06787216
ABSTRACT:
The present invention relates to a method for manufacturing multiple channel membranes by extrusion of a solution of a polymer which can form a semi-permeable membrane after coagulation. The invention further relates to membranes that can be obtained using this method and to the use of said membranes in separation, filtration and purification techniques.
Membranes of semi-permeable materials that are provided with several continuous channels are known.
FR 2,616,812 A relates to a method for manufacturing a porous organic material, particularly an organic semi-permeable membrane, by extruding a solution of a polymer and coagulating it, to an extrusion nozzle for carrying out said method, to the membranes obtained and to filtration modules containing such membranes. According to FR 2,616,812 A a polymer solution is extruded through an extrusion nozzle which is provided with several separated pipes through which a liquid is injected so that an extrudate is formed having several longitudinal channels, and subsequently coagulation is carried out in order to form the porous organic material. By using a non-solvent for the polymer as liquid to be injected, and guiding the extrudate immediately after leaving the extrusion nozzle into a bath containing a non-solvent, an active layer consisting of small pores is formed both in the channels and on the outer surface of the membrane. According to FR 2,616,812 A by first running the extrudate through an air gap prior to guiding it into a bath with a non-solvent, a membrane is obtained having only an active layer in the channels, and by injecting a liquid which does not precipitate the polymer, and guiding the extrudate immediately after leaving the extrusion nozzle into a bath of non-solvent, a membrane having an active layer on the outer surface is obtained. The multiple channel membranes of this reference may for instance be flat or cylindrical.
EP 0,375,003 A1 and EP 0,375,004 A1 relate to the manufacturing of organic semi-permeable membranes provided with several separated channels by means of the method of FR-2,616,812 A. Said references describe in particular the dimensions of the extrusion nozzles, the needles present in them, the dimensions of the channels and the wall thickness of the extruded membrane, the viscosity and the volume of the polymer solution to be extruded and of the injected liquid and the length of the air gap.
The larger mechanical strength, the easy handling and the higher production speed as well as the easy use in filtration modules, are mentioned in FR 2,616,812 A as advantages of the multiple channel membranes with respect to the known hollow-fibre membranes.
FR 2,437,857 A relates to cellulose dialysis membranes in the shape of hollow fibres, in which two or more hollow fibres are connected to each other parallel to the fibre axes. Said membranes are obtained by using an extrusion nozzle provided with conduits through which a liquid is injected which forms the channels.
WO 81/02750 relates to the manufacturing of a membrane unit of a semi-permeable synthetic material provided with a number of parallel tubular channels wherein the synthetic material is extruded through an extrusion nozzle which is provided with a number of thin metal threads or a number of conduits through which a liquid is injected.
DE 3,022,313 A1 relates to multiple hollow fibres, in which the hollow fibres have several separated cavities which extend in the length of the hollow fibre. The multiple hollow fibres are made either by adhering a number of hollow fibres having one cavity to each other, or by extruding a hollow fibre having several cavities, preferably no more than four. The hollow fibres are intended for dialysis.
Above-mentioned dialysis membranes, particularly membranes for kidney dialysis consist of cellulose derivatives. The characterizing feature of said membranes is that the membrane wall is homogeneous and therefore in itself responsible for the resistance against liquid permeability. Because of this the wall is made as thin as possible, usually in the order of 0.15 &mgr;m. Because in dialysis no or hardly any pressure difference is exerted over the membrane said thin wall is no problem. In for instance ultra-filtration and micro-filtration there is indeed a pressure difference and the membrane will have to be able to resist a pressure of at least 3 bar. The thin walls of dialysis membranes are not resistant to such a pressure.
The known semi-permeable membranes are guided into a coagulation bath after extrusion either directly or after running through an air gap. In the first case a separating layer is always formed at the outer surface of the membrane in addition possibly to a separating layer formed in the channels. Using an air gap makes it possible that a membrane is formed which only has a separating layer at the channel side. The length of said air gap should be such that the structure of the membrane is sufficiently fixed by the coagulation liquid which diffuses from the channels into the extruded membrane material, before the membrane enters a coagulation bath for further removal of the soluble components. Because of the length of the air gap the membrane can sag as a result of its own weight while it is still in a substantially liquid condition. As a result it is necessary to use polymer solutions of a high viscosity, such as for instance is described in FR 2,616,812 A, EP 0,375,003 A1 and EP 0,375,001 A1. In order to obtain a polymer solution of a high viscosity, a high concentration of polymer and/or polymer additives are used. As a result the coagulation is slowed down whereas said additives are hard to rinse out. A high concentration of polymer in the solution also gives a membrane having a low flux. Moreover a longer air gap may cause a shape made in the outer surface to disappear due to flow under the influence of surface tension.
By using the methods described above it is not possible to manufacture a membrane of a complex shape, such as a flat multiple channel membrane having recessed portions parallel to the channels, in which an active layer is formed in the channels only.
Methods in which coagulation from one side is effected so that the membrane structure is fixed before the membrane reaches the coagulation bath, suffer from the drawback that no larger wall thicknesses can be produced so that the diameters of the channels are strongly limited.
An object of the invention is therefore to provide a method for the manufacturing of multiple channel membranes which do not entail the above-mentioned drawbacks.
Said objective is achieved according to the invention by a method for manufacturing multiple channel membranes, wherein a solution of a polymer which forms a semi-permeable membrane after coagulation, is extruded through an extrusion nozzle wherein several hollow needles are arranged, a gas containing coagulating vapour or a coagulating liquid is injected through the hollow needles into the extruded material during extrusion, so that parallel continuous channels extending in extrusion direction are formed in the extruded material, and the outer surface of the membrane is brought into contact with coagulation agents, characterized in that the outer surface of the membrane after it leaves the extrusion nozzle is first brought into contact with a mild coagulation agent such that the shape of the membrane is fixed without an active layer being formed on the outer surface of the membrane and subsequently the membrane is brought into contact with a strong coagulation agent.
By using the method according to the invention it is possible to control the pore size on the outer surface of the membrane and those in the channels independent from each other. As a result a membrane can be obtained having a separating layer in the channels in which the outer surface with respect to the active layer has no or hardly any resistance against liquid flows in for instance micro- or ultra-filtration.
In the method according to the invention coagulation takes place from two sides, which results in the coagulation distances b
inge AG
Loney Donald J.
Young & Thompson
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