CA2923780A1

CA2923780A1 - Method for producing a forming solution

Info

Publication number: CA2923780A1
Application number: CA2923780A
Authority: CA
Inventors: Andreas Diener; Andreas Grundei; Oliver Tretzack
Original assignee: List Holding AG
Current assignee: List Holding AG
Priority date: 2013-10-04
Filing date: 2014-09-26
Publication date: 2015-04-09
Also published as: TW201529916A; BR112016006237A2; WO2015049040A1

Abstract

The invention relates to a method for producing moulded bodies from a base substance which is mixed with a solvent for producing a moulding solution and subsequently said solvent is removed at least partially from the moulding solution which is then guided to a moulding device. Said moulding solution is guided to a thick layer solvent which is cylindrical or shaped as a figure of eight in the cross section or to a combination consisting of a thin layer evaporator which is cylindrical in the cross section and to a thin layer solvant which is cylindrical or shaped as a figure of eight.

Description

METHOD FOR PRODUCING A FORMING SOLUTION
The present invention relates to a method for producing a forming solution according to the preamble of claim 1.
PRIOR ART
In the present case, the term "formed bodies"
subsumes all possible bodies produced from a natural or artificial base substance. This is generally accomplished with the aid of a forming tool whereby the base substance is brought into a form for the formed bodies. An example which may be mentioned, purely illustratively and by no means exhaustively, is that of the lyocell and viscose fibers. Lyocell/viscose fibers are fibers consisting of cellulose as base material and industrially produced via a dissolution process with subsequent spinning. Some of the formed bodies require a high-grade quality of forming solution in order to fulfill the quality standards required of the formed body, for example the filament. The chemical nature of viscose/lyocell fibers resembles that of cotton fibers.
The formed bodies are for example staple fibers, non-wovens such as batting, hybrid fibers, functionalized fibers and filaments (fibers having a length of far more than 1 meter). They consist 10(A of cellulose and just like viscose fibers are produced from natural pulp. Formed AMENDED SHEET

- 2 -bodies such as filaments are usable for example as precursor material for further processing into carbonized fibers. Hybrid fibers or hybrids are fibers produced from a mixture of various starting materials, such as cellulose and PAN (polyacrylonitrile), cellulose and thermoplastics, or cellulose and polymers. These starting materials may either be dissolved together as forming solution or merely mixed. In the case of functionalized fibers, functional materials, such as silver, paraffin or carbons, are admixed before, during or after the dissolution process in order to influence the property of the fiber.
For the lyocell fibers, the nontoxic solvent NMMO
(N-methylmorpholine N-oxide) is used to dissolve the pulp directly and in unchanged form, without prior reaction with caustic soda and derivatization to the xanthate.
Lyocell fibers are spun in a dilute aqueous NMMO bath by going below the solubility limit of cellulose and thereby forming a thread. For this purpose, the corresponding forming solution is pressed through spinneret dies. This lyocell process is described for example in US-A-3 447 939 or WO 94/06530. The suitable forming solution is produced for example in a vertically operating cylindrical thin-film dissolver (a filmtruder), as described for example in WO 94/06530, US 5 603 883 A or US 5,888,288,A, or in a horizontally operating thick-layer dissolver (a kneading reactor), as described in DE 198 37 210 WO 2011/154134 Al or WO 02/20885 Al.
AMENDED SHEET

- 3 -In these devices and known methods, the forming solution is produced in the further processable viscosity needed for the spinning process and in the cellulose concentration associated therewith.
Neither device for generating the forming solution for the production of lyocell fibers is ideal for the entire process of dissolving the pulp base material in the NMMO solvent. The vertical thin-film dissolver has a good heat-transfer performance, but a short residence time, thereby failing to ensure the requisite swelling of the natural fibers and the requisite homogenization for a perfect forming solution. The horizontal thick-layer dissolver does provide a longer residence time, which leads to good penetration of the solvent into the fiber and hence to good homogenization for a very good forming solution.
Nonetheless, however, both devices are currently being used in the industry to generate the forming solution for lyocell fibers. Owing to the suboptimal conditions described above, the dissolvers for both processes are becoming larger and larger and are limited by their maximum size of construction. Comparatively large line capacities of more than 50 metric tons of fibers a day are impossible to realize with these devices in this way. Capacities of 100 metric tons of fibers a day per production line are needed to make this technology more efficient, and hence competitive with the viscose fiber or with the modal fiber, in the long term.
AMENDED SHEET

- 4 -Some formed bodies, for example filaments, hybrid fiber and functionalized fibers, require a very high-quality forming solution that cannot be generated by a thin-film evaporator alone.
PROBLEM
The problem addressed by the present invention is that of optimizing the abovementioned method such that larger capacities of, for example, more than 100 metric tons of fibers per day per production line may be achieved.
The problem addressed by the present invention is additionally that of generating forming solutions in a quality to produce filaments, precursors to carbonized fibers, hybrid fibers and functionalized fibers.
SOLUTION TO PROBLEM
The problem is solved with the features of the characterizing part of claim 1.
A thin-film evaporator is an evaporator which evaporates solutions in a thin film. It is normally operated under reduced pressure and is therefore suitable for the gentle separation of mixtures. Its mode of operation is as follows:
A liquid is guided from above onto a perpendicular evaporator surface. The evaporator surface is a pipe AMENDED SHEET

- 5 -having a mechanical stirring system in the middle. The thin film of solution is formed by the stirring system, with fixed or moving stirring paddles, which scrape along the heating surface. The evaporation energy needed is introduced into the film of solution via an external jacket. [1]
http://de.wikipedia.org/wiki/D%C3 I.BCnnschichtverdampfer#ci te note-1 _ Since the apparatus can be operated very effectively under reduced pressure, it is suitable for gentle distillation at low temperatures. For crystallizing processes, thin-film evaporators with moving stirring paddles are suitable.
A thin-film dissolver is a derived term for a thin-film evaporator. In the case of the dissolver, in addition to the evaporation, one substance is dissolved in another, such as cellulose in a solvent, for example.
The term thick-layer dissolver is intended to refer to a mixer-kneader in which in contrast to the thin-film dissolver the medium for evaporation is evaporated not in a thin film but instead by way of surface renewal of a thick layer. The terminology is selected in order to mark the contrast with a thin-film dissolver. Solvent and medium (cellulose) are brought together by a kneading and shearing action.
Preferred is a horizontal, cross-sectionally cylindrical or eight-shaped thick-layer dissolver, or a combination of a vertical cross-sectionally cylindrical AMENDED SHEET

- 6 -thin-film evaporator and a horizontal cross-sectionally cylindrical or eight-shaped thick-layer dissolver. The thick-layer dissolver is in particular of the kind known as mixer-kneaders, which may be single-shaft or twin-shaft. A distinction is made essentially between single-shaft and twin-shaft mixer-kneaders. A single-shaft mixer-kneader with a horizontally disposed shaft is described in EP 91 405 497.1, for example. Multishaft mixing and kneading machines having a cross-sectionally eight-shaped housing are described in CH-A 506 322, in EP 0 517 068 B, in DE 199 40 521 Al or in DE 101 60 535. Located on a horizontally disposed shaft there are radial disk elements, and axially aligned kneading bars disposed between the disks. Engaging between these disks are mixing and kneading elements formed in framelike fashion, from the other horizontally disposed shaft. These mixing and kneading elements clean the disks and kneading bars of the first shaft. The kneading bars on both shafts, in turn, clean the inner wall of the housing.
The dissolution process of the pulp base material in NMMO was analyzed in process-engineering terms on the basis of the two known devices. It was determined that the dissolution process can in principle be subdivided into three sections that require very different processing conditions. The first section is where the water evaporates from a pulp-solvent suspension (also called slurry) up to the point where the pulp starts to dissolve, which corresponds to the reaching of the dissolution AMENDED SHEET

- 7 -window, i.e., those conditions under which the cellulose dissolves in the aqueous NMMO, and hence approximately to the 2.5 hydrate of the NMMO. This section requires a lot of thermal energy to evaporate the water, but does not need any additional residence time, since the pulp does not as yet dissolve, and the viscosity of the suspension is low.
After reaching the dissolution window, the second section is where the main dissolution with a pronounced increase in viscosity and the low rate of water evaporation needed for this, down to the approximately 1.5 hydrate of NMMO, takes place.
The third section is governed by the homogenization of the spinning solution and a likewise lower rate of water evaporation down to the approximately 0.8 to 1.0 hydrate depending on pulp concentration.
The process-engineering analysis, then, shows in connection with the devices used for the dissolving stage that the thin-film evaporator is by virtue of its good heat-transfer performance very highly suitable for a high rate of water evaporation at low viscosity and short residence time in the first section and that the thick-layer evaporator or kneading reactor is by virtue of its very good homogenization performance, the longer residence times and also the processing of higher viscosities and the lower rate of water evaporation very highly suitable for the second and third sections.
AMENDED SHEET

- 8 -The two devices are linked such that the product spaces are in direct communication, as a result of which the transfer point, being a complicated interface, is located on the inside and hence the transfer of partly changing product consistencies is eliminated. Variations due to a low hold-up on the part of the thin-film dissolver are easily rectifiable by the thick-layer evaporator or kneading reactor. In some cases, however, it is necessary for the apparatus to be separate, in order to allow different operating conditions (e.g., reduced pressures) to be realized.
In a preferred exemplary embodiment of the invention, there is also a mixer upstream of the thin-film evaporator, this mixer carrying out preliminary mixing of the base substance - that is, in particular, cellulose -with the solvent. As a result, an intensively commixed slurry is formed. The mixer used here is preferably a horizontal kneading reactor.
For the complete dissolution of the pulp in NMMO, it has been found that the dissolution procedure requires a long residence time, and intensive commixing and introduction of shear in order to generate a high-quality forming solution. Dissolving with pulp concentrations of up to 36 wt % (concentration of the pulp in the forming solution) additionally supports the generation of a high quality of forming solution. As a result of the MasterConti process, the appropriately optimum viscosity AMENDED SHEET

- 9 -for shaping (e.g., for spinning) can be brought about by dilution.
The process-engineering analysis, then, shows in connection with the devices used for the dissolving stage that the above-described combination of thin-film and thick-layer dissolver is outstandingly suitable for the generation of qualitatively high-grade forming solution.
To further enhance the capacity of the process, the concentrated solution already explained in WO
2009/098073 was included for consideration. This makes it possible to combine the two-stage process of producing the spinning solution with a concentrated solution of the cellulose with subsequent recovery and thereby achieve a still further increase in efficiency.
As already in the case of WO 2009/098073, the concentration of the forming solution and/or of the thinner shall be policed via the optical index (refractive index). This is accomplished in the case of the thinner before incorporation into the forming solution and/or in the case of the forming solution after dilution. What is desired is an optical index for the thinner and/or the forming solution that is in the range from 1.45 to 1.52.
The solvent and/or diluent used is preferably an aqueous tertiary amine oxide. However, the invention shall not be restricted thereto. Nor is the invention restricted to pulp, but also comprehends substances such as proteins, polylactides, polyacrylonitrile (PAN) or starch or a mixture thereof, producing hybrid fibers or composites.
AMENDED SHEET

- 10 -Which formed body is produced is of minor importance with the present method. Preference is given to producing filaments, fibrous nonwoven webs and/or filament yarn. However, it is also possible to produce hybrid fibers, functionalized fibers, films, hollow fibers, membranes or the like. The forming of the solution into a desired cellulosic formed body may be effected using known spinneret dies for producing fibers, slot dies or hollow-fiber spinneret dies. After forming, i.e., prior to being introduced into the coagulation bath, the formed solution may also be drawn.
To generate the desired quality of the formed body it is possible for the fiber after forming not to be dried, or not fully dried, and to be drawn a little, or even not at all. This production feature after the creation of the forming solution allows the formed body to dry out without tension.
AMENDED SHEET

- 11 -FIGURE DESCRIPTION
Further advantages, features and details of the present invention will become apparent from the ensuing description of preferred exemplary embodiments and also from the drawing; this shows in its single figure a schematic block diagram of a method for producing formed bodies from a base substance, in particular from renewable raw materials, in the manner of the present invention.
The cellulose needed for this or, to be more precise, the premixed cellulose slurry is fed via supply line 1 to a thin-film evaporator 2. These kinds of vertical cylindrical apparatus are known from GB
08/875,437 or US 5,888,288 for example.
The thin-film evaporator 2 concentrates the suspension. From there the concentrated suspension transfers directly into a horizontal kneading reactor 4.
These kneading reactors are known from DE 199 40 521 Al or DE 41 18 884 for example.
In the present exemplary embodiment, the renewable raw material is treated by means of a solvent, preferably an aqueous tertiary amine oxide, already mixed previously as a cellulose slurry and accordingly fed in supply line 1 to the thin-film evaporator.
In the thin-film evaporator 2, heat is added to effect water evaporation from the suspension to a degree which falls short of leading to the dissolution of the pulp.
AMENDED SHEET

,

- 12 -In the kneading reactor 4, heat is added to effect intensive commixing of the raw material with the solvent, some partial evaporation of the water from the solvent and hence the dissolution of the pulp to obtain a relatively high-viscosity forming solution. This forming solution is then fed via a discharge means 5 to a means for forming, as for example to a spinning rig.
Generating a forming solution in the combination of thin-film evaporator and kneading reactor gives a high-quality base substance for subsequent spinning into a filament, staple fiber or nonwoven web by a wet-spinning process which takes place primarily but not necessarily through an air gap.
Before finally being processed into fibers, the relatively high-viscosity forming solution may be diluted to form a spinnable forming solution. This is accomplished in the discharge means 5 via a supply line 6 or even upstream of the discharge means 5 in the kneading reactor 4 at any point and/or with splitting. This procedure was confirmed in trials. Combining the two sites of addition is also conceivable if the dilution ratio is very high.
A pump 7 is positioned downstream of the discharge means 5 and upstream of the forming means 8 to back up the forming solution after discharge.
The method of the present invention is carried out as follows:
Supply line 1 sends the suspension, consisting of the base substance, in particular the renewable raw AMENDED SHEET

- 13 -material, and the solvent, into the thin-film evaporator 2. Heat is added from the outside via a heating jacket to effect intensive evaporation of water from the suspension up to the dissolution window without starting the dissolution of the base substance.
The concentrated suspension leaves the thin-film evaporator 2 through a direct transition 3 directly or via a pump or valve, and passes into the kneading reactor 4.
In the kneading reactor 4, intensive commixing takes place, accompanied where appropriate by the evaporation of water until the dissolution window is reached, while the heat which is added may be added from the outside by means of a heating jacket, through heated kneading shafts and/or through heated kneading elements (disk elements). There is a further mechanical input of heat in the course of the commixing itself, through the shearing energy of commixing. As a result of the intensive commixing and the input of shearing energy, base substance is fully dissolved and a forming solution of high quality is produced.
As some of the solvent evaporates, the suspension _ transforms into a forming solution (molding solution) and becomes further concentrated, such that it comprises approximately a base substance fraction of from 8 to 36%
at the downstream end of the kneading reactor 4 just upstream of the discharge means 5. This forming solution is possibly too viscous for later spinning. It is then, if the spinning requirements dictate, thinned with a thinner AMENDED SHEET

- 14 -which is supplied via supply line 6. In the process, the concentration of the forming solution before and/or after addition of the thinner is policed via the optical index.
This optical index is also called the refractive index. It characterizes the refraction (change of direction) and the reflection characteristics (partial reflection and total reflection) of electromagnetic waves on encountering a boundary layer between two media.
It is further conceivable for an additive to be additionally provided to the mixture/forming solution upstream of the discharge or into the discharge, optionally also via supply line 6. An additive or additive mix may also be supplied together with the thinner.
Vapors produced in the thin-film evaporator 2 and/or the kneading reactor 4 are fed via a gas space connector 9 to a condenser 10.
AMENDED SHEET

- 15 -WeiS, Arat & Partner mbB
German Patent Attorneys European Patent Attorney Docket: P 4782 PCT Date: Sept. 26, 2014 W/ST
List of reference signs 1 supply line 2 thin-film evaporator 3 direct transition 4 kneading reactor discharge device 6 supply line 7 pump 8 forming means 9 gas space connector condenser AMENDED SHEET

Claims

What is claimed is:-

1. A method for producing formed bodies from a base substance which is mixed with a solvent to produce a forming solution and subsequently this solvent is partly removed from the forming solution, and the forming solution is fed to a means (8) for forming, characterized in that the forming solution is fed to a combination of a cross-sectionally cylindrical thin-film evaporator (2) and a cylindrical or eight-shaped kneading reactor (4).

2. The method as claimed in claim 1, characterized in that the solvent is an aqueous tertiary amine oxide.

3. The method as claimed in claim 1 and 2, characterized in that the mixture of base substance and solvent is stored in a stirred container and the container functions as metering stock reservoir vessel.

4. The method as claimed in claim 1 to 3, characterized in that the base substance is a renewable raw material, such as cellulose, starch or the like or a mixture of the renewable raw material and a synthetic material, such as PAN (polyacrylonitrile), TPE
(thermoplastic elastomer) or polymer.

5. The method as claimed in claim 1 to 4, characterized in that the suspension of base substance and solvent is received by the thin-film evaporator (2) and concentrated therein by water evaporation to a pre-dissolution state corresponding to an approximately 2.5 hydrate of a tertiary amine oxide.

6. The method as claimed in at least one of claims 1 to 4, characterized in that the suspension passes into the kneading reactor (4) from the thin-film evaporator (2) directly and is concentrated to 0.6 to 1.5 hydrate, dissolved and homogenized into a forming solution by water evaporation.

7. The method as claimed in at least one of claims 1 to 5, characterized in that the thin-film evaporator (2) and/or the kneading reactor (4) is/are operated at 80° to 180°C, preferably at 100° to 150°C.

8. The method as claimed in at least one of claims 1 to 6, characterized in that the thin-film evaporator (2) and/or the kneading reactor (4) is/are operated under vacuum of 20 to 200 mbar absolute, preferably under 30 to 100 mbar absolute.

9. The method as claimed in at least one of claims 1 to 7, characterized in that temperature is permanently policed along the axes of thin-film evaporator (2) and/or of kneading reactor (4).

10. The method as claimed in at least one of claims 1 to 9, characterized in that the forming solution is back-diluted.

11. The method as claimed in at least one of claims 1 to 10, characterized in that the forming solution is formed by a wet-spinning process wherein the forming solution is spun through primarily but not necessarily an air gap or directly from the spinbath.

12. The method as claimed in at least one of claims 1 to 11, characterized in that the thinner is an aqueous tertiary amine oxide.

13. A device for carrying out the method as claimed in at least one of claims 1 to 11, characterized in that the thin-film evaporator (2) and/or the kneading reactor (4) are in direct communication with each other via their product spaces or are operated separately in the optimum range of vacuum.

14. The device as claimed in claim 10, characterized in that the thin-film evaporator (2) and/or the kneading reactor (4) are in direct communication with each other via their gas spaces or are separated from each other via a suitable separating integer such as a valve or a pump.