Process for the production of cellulosic moulded bodies

Details Process for the production of cellulosic moulded bodies Process for the production of cellulosic moulded bodies

2000-01-28

2003-02-11

Eashoo, Mark (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Forming continuous or indefinite length work

Shaping by extrusion

C264S211130, C264SDIG006

Reexamination Certificate

active

06517758

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a process for the production of cellulosic moulded bodies, flat and tubular films and cellulosic membranes in the form of flat or tubular films whereby a solution of cellulose in an aqueous tertiary amine oxide is extruded using an extrusion nozzle whereby the solution can as the case may be also contain additives such as stabilising and softening agents, pore formers and so on whereby a moulded solution results which has two surfaces and the extruded moulded solution is led over a gap into a precipitation bath and drawn off from the precipitation bath.
Processes for the production of cellulosic moulded bodies from solutions of cellulose in an aqueous tertiary amine oxide are well known and are described for example in U.S. Pat. No. 4,246,221 and PCT-WO 93/19230 for example. This well known process is described in the following as the “amine oxide process”.
With cellulosic films the pore structure of the film represents a special property. It has a decisive influence on the suitability of the film for various applications, such as for example as a packaging film or for use as a membrane.
In particular flat and tubular films of cellulose with an asymmetric structure are suitable for packaging films with breathing properties and for use as membranes with pressure-driven separation processes such as ultrafiltration, nanofiltration and reverse osmosis. These are characterised in that they are made up of a macroporous layer and a thin fine pored respectively dense skin. When using these as a membrane the macroporous layer assumes a supporting function whilst the actual separation effect is performed via the thin fine pored layer. The comparatively slight thickness of this layer leads to the high permeability of the films, and at the same time high strengths.
Cellulosic flat or tubular films manufactured according to well known processes such as the viscose process, the cuoxam process or more recently the amine oxide process, do not normally have the above-named asymmetric structure.
Thus U.S. Pat. No. 4,354,938 describes a process for the production of tubular films for use as dialysis membranes according to the viscose process in which a film moulded in the form of a tube is stretched by between 40% and 120% prior to drying in the diagonal direction by blowing it up with air which leads to a membrane with the same orientation in the longitudinal and diagonal direction. When converting the dried membranes to the wet state the membranes depicted in this way undergo shrinkage of 0.5-10% in the longitudinal and diagonal direction.
In this document, however, nothing is said about the asymmetric structure of the film.
WO 96/20301 describes a process for the production of cellulosic moulded bodies which is characterised in that a solution, containing cellulose dissolved in amine oxides, is formed in a nozzle and the moulded solution is led through at least two precipitating media, following an air gap, one after the other whereby it is mentioned as an essential point that in the first precipitation medium at least the cellulose coagulates more slowly than in comparison with the last precipitation medium. Solvents are used for the precipitation media which cannot be mixed with each other and can be layered one above the other due to the differences in density.
The moulded bodies depicted in this way have, in relation to their cross-section, an inner area with a high supramolecular order in the form of finely disperse pores whilst an outer area surrounding the inner area has a low supramolecular order with larger heterogeneous voids compared to the inner area. When the moulded bodies are used as a membrane, e.g. in the form of follow fibres or flat membranes, the outer coarsely disperse area acts as a supporting layer whilst the finely disperse inner area performs the actual separation. The comparatively low thickness of the inner area results in the high permeability of the moulded body.
Films with a finely pored core and coarsely disperse outer layers are only poorly suited to separation processes such as ultrafiltration and reverse osmosis since this can very quickly result in the blocking of the pores on the feed side. In these processes it is common to use an asymmetric membrane comprising a dense skin and a large pored body whereby the dense skin always faces the feed side. In pressure-driven processes of this kind a corresponding Cross-Flow minimises the formation of a polarisation of the concentration and therefore a layer of gel on the membrane surface by means of which the flow through the membrane can be kept at a high level over a long period of time. In the event that the outer layer of the membrane turned to the feed side has a coarser structure than the core, a Cross-Flow is not effective. The particles to be separated cannot be transported away from the membrane surface and they choke the pores.
Keeping the NMMO content of the precipitation bath constant during the process is well known in the amine oxide process, which is performed in practise by the permanent supply of a corresponding amount of fresh precipitation agent respectively wash water. This would appear to be very difficult respectively very complicated in the process described as a result of the use of different precipitation media and the fact that they are superimposed.
DE 44 21 482 C2 describes a process for the production of oriented asymmetric cellulose films by spinning non-derivatised cellulose dissolved in amine oxides through a ring nozzle into a precipitation bath whereby the film is blown up by a blowing gas. The asymmetric structure is achieved as a result of the extruded tube being precipitated on the inner and outer side via difference precipitation agents. This produces different coagulation behaviour on the inner and outer side which leads to the asymmetric structure of the films.
The use of different precipitation agents is, however, also costly particularly when other precipitation agents are used as water or additives are added to the precipitation agents.
EP 0 042 517 B1 describes a process for the production of dialysis membranes of regenerated cellulose in the form of flat films, tubular films and hollow fibres, in which a mixture of completely or partly substituted cellulose, tertiary amine oxide and as the case may be up to 25 weight percentage of a non cellulose dissolving diluting agent and up to 10 weight percentage of usual additives was dissolved in a double screw extruder in less than 8 min at temperatures between 80 and 150° C., the solution is extruded into a precipitation bath using a corresponding forming tool and the coagulated moulded body is dried after washing and treating with glycerine taking care to avoid shrinkage. The dialysis membranes described have a high dialytic permeability in the middle molecular range which remains intact in the main even when the hydraulic permeability for water is drastically reduced by a subsequent treatment. This subsequent treatment is carried out by the membrane being exposed to a raised temperature for some time in aqueous liquids prior to drying.
However, no asymmetric structure is described of the products manufactured.
In EP 0 807 460 A1 a process is described for the production of cellulosic dialysis membranes in the form of flat, tubular or hollow fibre membranes by spinning a solution of cellulose and/or modified cellulose in an aqueous tertiary amnine oxide and other additives such as stabilising agents. pore formers etc. whereby the drawing-off speed equals at least 30 m/min and according to this membranes can be produced for the high, middle and low flux area. The deliberate influencing of the pore structure of the membranes is achieved in this process primarily by using substituted cellulose respectively the addition of various additives and a subsequent treatment and is thus costly.
The production of asymmetric membranes according to this process is not, however, described.
SUMMARY OF THE INVENTION
It is an object of the present invention to make a process available with whi

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