CA2313213A1

CA2313213A1 - Reactive fiber-like cellulose coagulates

Info

Publication number: CA2313213A1
Application number: CA002313213A
Authority: CA
Inventors: Rainer Dolfel; Werner Schiffer; Joachim Kahlke; Bernd Riedel; Eberhard Taeger; Klaus Heinemann
Original assignee: Individual
Current assignee: Melitta Haushaltsprodukte GmbH and Co KG
Priority date: 1999-07-01
Filing date: 2000-06-30
Publication date: 2001-01-01
Also published as: EP1065301A1; BR0002277A

Abstract

The present invention relates to reactive fiber-like cellulose coagulates, to a method of producing and using the same, and to flat structures comprising the same. The method of the invention for producing the reactive fiber-like cellulose coagulates comprises the steps of (a) providing a solution of cellulose in a mixture containing water and NMMNO, and (b) treating the solution provided in step (a) in a precipitation bath containing water and NMMNO, with a shear field. The flat structure according to the invention comprises a substrate and a reactive fiber-like cellulose coagulate according to the invention.

Description

REACTIVE FIBER-LIKE CELLULOSE COAGULATES
The present invention relates to reactive fiber-like cellulose coagulates, to a method of producing and using the same and to flat structures comprising the same.
Cellulose fibers or films and the methods of producing the same are known.
US patent specification 5,417,909 describes a method of producing cellulose fibers or films, wherein an aqueous cellulose solution in a tertiary amine-N-oxide is extruded through nozzles into a medium which does not precipitate cellulose, for instance air. The cellulose molecules are pre-oriented by a shear force gradient in the nozzle and are then oriented by stretching within the medium that does not precipitate cellulose.
The stretched cellulose filaments are then brought into contact with a precipitation bath, whereupon no further stretching operation is carried out. The precipitation bath may be a diluted aqueous amine oxide solution.
WO 98/07911 describes a method of producing lyocell fibers. Lyocell fibers are fibers made from cellulose/N-methylmorpholine-N-oxide solutions (hereinafter designated as cellulose/NMMNO solutions). To this end a cellulose/NMMNO solution is prepared and sprayed by centrifugal spinning in air, whereby liquid filaments are drawn.
These are then introduced into a precipitation bath which may consist of water, a low-aliphatic alcohol or mixtures thereof.

WO 97/01660 discloses a method of producing a mixture consisting of fibers and microfibers of cellulose. To this end a solution of cellulose is prepared in N-methylmorpholine-N-oxide (hereinafter designated as NMMNO). The solution is then extruded from a nozzle and defibrated with a strong air current in accordance with the melt-blow principle. The defibrated extruded mixture is then directly blown into a precipitation bath, e.g. water.
In the above-mentioned methods, the cellulose/NMMNO solution must first be prestressed, oriented or defibrated prior to precipitation in a medium which does not precipitate cellulose, and this state must then be fixed in the precipitation bath. As a consequence, the methods are expensive and troublesome. Moreover, only low-viscosity solutions, i.e. economically disadvantageous cellulose concentrations, can be processed in centrifugal spinning and melt-blow spinning, respectively.
DE-A-196 22 476 describes a method for producing microfibers from cellulose. A
solution of cellulose in hydrated cyclic amine oxides or in mixtures consisting of hydrated cyclic amine oxides and organic diluents is added to a desolvatation medium and is deformed into microfibers in one or a plurality of shear fields at the same time and/or successively. The desolvatation medium is a solution of a polymer or polymer mixture in a solvent or solvent mixture. Solvents may be dimethylformamide, dimethylacetamide and N-methyl-pyrrolidone.
DE-A-196 32 540 describes a further method of producing microfibers from cellulose.
The difference with respect to the method described in DE-A-196 22 476 is that solvent-free polymers with urethane, amide and/or urea groups as desolvatation media are used.
The two above-described methods have the drawback that the desolvatation media are expensive, have an adverse effect on the environment and cause costs when recovered e.g. by way of distillation or ultrafine filtration. A replacement of said desolvatation media by environmentally acceptable precipitants, such as water or alcohol, will not yield reactive fiber-like cellulose coagulates. Said precipitants are quick-acting precipitants which when used yield coarse-titer products which are only fiber-like in part.
It is the object of the present invention to provide a method in which cellulose/NMMNO
solutions having a high content of cellulose can be processed at low costs and in an economic way and can optionally be converted into flat structures.
This object is achieved with a method of producing reactive fiber-like cellulose coagulates comprising the steps of:
(a) providing a solution of cellulose in a mixture containing water and NMMNO, and (b) treating the solution provided in step (a) in a precipitation bath containing water and NMMNO, with a shear field.
The method has the advantage that no stretching or drawing in air or in another non-precipitating medium is required for orienting the cellulose molecules along the fiber axis.
The coagulates can directly be obtained in a precipitation path using a shear field generator. With the method according to the invention, it is possible to process cellulose/NMMNO solutions with a high content of cellulose. Moreover, reactive cellulose coagulates can be obtained thereby, i.e. amorphous cellulose coagulates having a highly active structure which is not characterized yet by predominantly closed internal hydrogen bridges.
The provision of a solution of cellulose in a mixture containing water and NMMNO in step (a) of the method of the invention can be carried out in a known manner, e.g.
by preparation of the solution. For instance, a cellulose/NMMNO solution can be prepared by kneading a mixture of cellulose, NMMNO and water. The content of water is preferably 7 to 16% and that of NMMNO 84 to 93%, based on the mixture of water and NMMNO.
The cellulose can also be dispersed in a mixture consisting of water and 40 to 70% by wt.
of NMMNO, based on the mixture of water and NMMNO, in particular 40 to 65% by wt.
The dispersion obtained will then be subjected under kneading to vacuum distillation for removing water from the mixture until a cellulose/NMMNO solution is obtained.
The content of cellulose in the solution provided in step (a) is preferably 2 to 16% by wt., based on the total weight of the solution. Preferably, the cellulose/NMMNO
solution has a viscosity of 1,000 to 8,000 Pa~s at processing temperature.
In the method of the invention pulp having more than 90% a-cellulose is preferably used for providing the solution in step (a).
Moreover, the cellulose/NMMNO solution may contain up to 200% by wt., based on the total weight of the cellulose, of at least one additional active agent.
Preferred agents are adsorbents, ion exchangers, dyes, bactericidal or fungicidal substances, electrically conductive materials, magnetically acting particles or activated carbon. The active agent is preferably present in the form of particles having a diameter of up to 300 Nm.
Moreover, the cellulose/NMMNO solution may contain at least one additional polymer that is soluble in NMMNO. Preferred polymers are polysaccharides and the derivatives thereof.
In step (b} the cellulose/NMMNO solution is treated with a shear field in a precipitation bath containing water and NMMNO. A reactive fiber-like cellulose coagulate is thereby obtained.

The precipitation bath comprises water and NMMNO, preferably in the presence of 40 to 70% by wt., in particular 50 to 65% by wt., of NMMNO, based on the total weight of the precipitation bath. In general, the preferred content of NMMNO in the precipitation bath depends on the content of cellulose in the solution. When the content is low, e.g. < 10%
of cellulose, the content of NMMNO is preferably also low, e.g. 50 to 60% by wt. If the content of cellulose in the solution is high, e.g. > 10% cellulose, the content of NMMNO in the precipitation bath is preferably high, e.g. 60 to 70% by wt., in particular 65% by wt.
Step (b) is preferably carried out at a temperature of 45 to 100°C, in particular 90 to 98°C. When step (b) is carried out at an elevated temperature, additional additives are preferably added to the cellulose/NMMNO solution for thermal and/or hydrolytic stabilization to avoid a thermal and/or hydrolytic degradation of the cellulose in the heated cellulose/NMMNO solution. Such additives are preferably alkali hydroxides.
The shear field can be produced by surfaces that are closely arranged side by side and are movable relative to one another. Preferably, the shear field is produced with an apparatus operating in accordance with the rotor-stator principle, as will be explained below with reference or figures 1 and 2 or is described in EP-A-0 807 698. The surfaces which are closely arranged side by side and are movable relative to one another and by which the shear field is produced may be interrupted or non-interrupted surfaces. In particular, the shear field may be produced by means of cylindrical, non-interrupted rims, perforated rims or gear rims or combinations thereof.
Preferably, the precipitation bath can be separated in a further step (c) from the dispersion obtained after step (b), which consists of reactive fiber-like cellulose coagulate and precipitation bath. The separation process can be carried out by way of filtration, resulting in a filter cake and a filtrate. The resulting filter cake consisting of reactive fiber-like cellulose coagulate and residues of the precipitation bath can optionally be washed with water to wash out the residues of the precipitation bath. Preferably, the water distilled off in the preparation of the cellulose/NMMNO solution in step (a) can be used as wash water. This results in an aqueous dispersion consisting of reactive fiber-like cellulose coagulate and water, the ratio of cellulose coagulate and water being preferably about 1:10 to 1:100. Said dispersion can be dried. The wash water used, which contains water and the residues of the precipitation bath, can preferably be combined with the filtrate obtained after step (c).
The filtrate obtained in step (c) during separation can be used either as such or after addition of the wash water again in step (b) as precipitation bath or in step (a) for preparing a cellulose/NMMNO solution. If it is used in step (a) a dispersion of precipitation bath and cellulose is first prepared in which the water content is then reduced to such a degree that a solution of cellulose is obtained.
Preferably, the precipitation bath separated in step (c) can be divided into two streams, of which one is used as precipitation bath in step (b) and the other for preparing the cellulose/NMMNO solution in step (a). The weight ratio of the two streams to one another is preferably about 6:1 to 60:1.
It is thereby possible to keep the consumption of precipitation bath and solvent, respectively, as well as the consumption of water, as low as possible in the method of the invention, whereby the latter becomes inexpensive and environmentally acceptable.
Reactive fiber-like cellulose coagulates are obtained with the method of the invention.
These have an amorphous structure. The cellulose coagulates contain a very great number of free OH groups that have not been deactivated yet by hydrogen bonds.
As a result, the coagulates are reactive and can be connected, e.g. in case of a flat deposition at the crossing points, to obtain a coherent film through the formation of hydrogen bonds between the coagulates.
Examples of coagulates according to the invention are shown in Figs. 3 to 6.
The coagulates of the invention may have a diameter of 0.1 to 200 Nm, preferably 1 to 10 Nm.
The ratio of the length to the diameter of the coagulates is preferably 1:10 to 1:1000. The coagulates of the invention are preferably micro-coagulates, i.e. superfine coagulates whose diameter amounts up to 10 Nm.
Optionally, the water-washed cellulose coagulates of the invention can be converted into a high-viscosity aqueous paste having a content of reactive fiber-like cellulose coagulates of preferably 3 to 10% by wt., which facilitates a later further processing in wet processes.
An apparatus for pertorming the method of the invention shall now be described with reference to figures 1 and 2, of which Fig. 1 is a schematic vertical section and Fig. 2 a schematic horizontal section through the apparatus. The apparatus has an inlet 5 for the cellulose/NMMNO solution, the inlet 5 terminating in two nozzles 1,2.
Moreover, it comprises two perforated rims 3, 4, of which one forms part of a rotor 3 and the other one forms part of a stator 4. Said pertorated rims include penetrations or openings 8, 9.
An inlet 6 serves to fill in the precipitation bath. The inlet 6 encompasses the inlet 5. The nozzles 1,2 are arranged in the interior of the perforated rim 3 and are oriented radially outwards towards the inner wall of the pertorated rim. Finally, the apparatus comprises an outlet 7.
During operation the cellulose/NMMNO solution is introduced via the inlet 5 to the nozzles 1, 2. The precipitation bath has previously been filled via the inlet 6, thereby filling up the space within stator and rotor. The cellulose/NMMNO solution is sprayed through the nozzles 1, 2 into the precipitation bath, thereby passing through the penetrations into the gap between rotor 3 and stator 4 where it is exposed to an intensive shear action because of the relative movement of the neighboring surfaces and is precipitated by the precipitation bath. Reactive fiber-like cellulose coagulates according to the invention are thereby obtained.
The reactive fiber-like cellulose coagulates according to the invention can be used in the production of flat structures. Hence, such a flat structure comprises a reactive fiber-like cellulose coagulate according to the invention.
In one embodiment the reactive fiber-like cellulose coagulate according to the invention is evenly distributed within the flat structure. Such flat structures according to the invention can e.g. be obtained by adding a reactive fiber-like cellulose coagulate in paper production or wet nonwoven production and in the production of spun-laced nonwoven fabrics.
In another embodiment the reactive fiber-like cellulose coagulate according to the invention is applied to a substrate. Flat structures are thereby obtained in which the reactive fiber-like cellulose coagulate of the invention is present at at least one of the sides of the substrate. Preferred substrates are paper, wet nonwoven, air-laid material, textiles, fibers based on synthetic materials, biodegradable substances, nonwoven fabrics, cellulose or mixtures thereof.
When viscose fibers, cellulose fibers or other natural fibers are used as a flat structure or substrate, a flat structure according to the invention will be obtained that is fully biodegradable or compostable. Substrates with fibers based on synthetic materials may also be used. Fig. 7 shows a flat structure according to the invention.

The amount of added reactive fiber-like cellulose coagulate is preferably 0.5%
by wt. to 60% by wt., based on the total weight of the flat structure. Whenever a higher amount is used it may be more difficult to dewater a paper web or wet nonwoven web in the production plant. In such a case the reactive fiber-like cellulose coagulate would clog almost all pores of the paper or the wet nonwoven, whereby air permeability and particle depositing pertormance would no longer be guaranteed to an adequate degree.
Flat structures of the invention in which the reactive fiber-like cellulose coagulate according to the invention is present at at least one of the sides of the substrate can be produced by applying an aqueous slurry of the reactive fiber-like cellulose coagulates of the invention to one or both sides of a substrate. The content of reactive fiber-like cellulose coagulate in the slurry is preferably 0.01 to 10% by wt., based on the total weight of the slurry. Preferably, the slurry used is the aqueous dispersion which, as has been described above, is obtained by washing the filter cake obtained after step (c) with water. The slurry can preferably be applied by means of spraying units. After the slurry has been applied to the substrate, the flat structure is dried. This yields a flat structure with an excellent filtering performance that exhibits a higher mechanical strength than a comparable flat structure made from a melt-blow nonwoven fabric, as described in DE-A-4443158.
Moreover, the reactive fiber-like cellulose coagulate can also be applied in the form of a suspension by means of conventional coating or impregnating methods.
The flat structure according to the invention may additionally comprise binders quantitatively and qualitatively corresponding to those of the prior art. The flat structures according to the invention may be used as filters, cleaning cloths, dust filters or in combination with already known materials for many applications.

The invention shall now be explained with reference to an example:
In a heatable 5-liter stirrer vessel, 0.11 kg cellulose with an average degree of polymerization (DP) of 486 and a content of 93% a-cellulose is mashed together with 2.352 kg of a 54% N-methylmorpholine-N-oxide solution and heated to 85°C, and 0.886 kg water is distilled off from the solution by slowly lowering the pressure within 3 hours. This yields 1.576 kg of a clear cellulose/NMMNO solution. Said cellulose/NMMNO
solution having a temperature of 85°C is supplied at a delivery rate of 30 g/min together with 15.760 kg of a 92°C-hot 54% aqueous NMMNO solution in separate conduits to an Ultraturrax UTR IL-F/mixing chamber DK 25,11 rotating at 20,000 rpm, the solution is deformed, and the resulting fine-titer fiber-like products are coagulated. One obtains a total of 17.36 kg of an approximately 0.56% cellulose coagulation dispersion from the shear field generator. On a wire screen having 1,500 Ma/cm2, the dispersion is filtered off at a negative pressure of about 80 mbar, and the remaining filter cake is washed with 1.786 kg water in several steps to free it from NMMNO. The resulting filter cake contains 0.1 kg of a reactive fiber-like cellulose coagulate and about 0.9 kg water.
Drying in air will yield a hard transparent film that cannot be redispersed. The wash water and the filtrate obtained during filtration of the dispersion are combined. This yields 18.122 kg of a mixture of which 2.352 kg are again used for preparing a cellulose/NMMNO
solution and 15.76 kg as a precipitation bath in a further production method. The non a-cellulosic components which are introduced by the 93% a-cellulose and amount to about 0.01 kg and which are present in the combined filtrate are removed from the cycle by filtration with subsequent ion exchanger passage.

Claims

1. A method of producing reactive fiber-like cellulose coagulates, comprising the steps of:
(a) providing a solution of cellulose in a mixture containing water and NMMNO, and (b) treating the solution provided in step (a) in a precipitation bath containing water and NMMNO, with a shear field.

2. The method according to claim 1, wherein in a further step (c) the precipitation bath is separated from the dispersion which has been obtained in step (b) and consists of reactive fiber-like cellulose coagulate and precipitation bath.

3. The method according to claim 2, wherein the reactive fiber-like cellulose coagulate obtained after separation of the coagulate in step (c) is washed with water.

4. The method according to claim 3, wherein the water used is combined with the filtrate obtained after step (c).

5. The method according to any one of claims 2 to 4, wherein the precipitation bath separated in step (c) is used as such or after addition of the used water for preparing the solution of cellulose in step (a) and/or as the precipitation bath in step (b).

6. The method according to any one of the preceding claims, wherein the solution of cellulose in step (a) is provided in that cellulose is introduced into a mixture containing water and 40 to 70% by wt. of NMMNO, based on the total weight of the mixture, and the content of NMMNO is then increased in the mixture in such a manner that a solution of cellulose is obtained.

7. The method according to any one of the preceding claims, wherein the solution of cellulose provided in step (a) has a viscosity of about 1000 to 8000 Pa~s at processing temperature.

8. The method according to any one of the preceding claims, wherein the solution of cellulose provided in step (a) comprises additional additives for thermal and/or hydrolytic stabilization.

9. The method according to any one of the preceding claims, wherein the solution of cellulose provided in step (a) contains at least one additional active agent in the form of particles having a diameter of up to 300 µm and an amount of up to 200%
by wt., based on the total weight of cellulose.

10. The method according to any one of the preceding claims, wherein the solution of cellulose provided in step (a) contains at least one additional, NMMNO-soluble polymer which amounts up to 200% by wt., based on the total weight of cellulose.

11. The method according to any one of the preceding claims, wherein the shear field in step (b) is generated by surfaces that are closely arranged side by side and are movable relative to one another.

12. The method according to any one of the preceding claims, wherein the precipitation bath in step (b) comprises a mixture of water and 40 to 70% by wt. of NMMNO, based on the total weight of the mixture.

13 13. The method according to any one of the preceding claims, wherein step (b) is carried out at about 45°C to 110°C.

14. A reactive fiber-like cellulose coagulate obtainable by a method according to any one of the preceding claims.

15. The reactive fiber-like cellulose coagulate according to claim 14, wherein the diameter of the coagulate is about 0.1 to about 200 µm.

16. Use of the reactive fiber-like cellulose coagulate according to claim 14 or 15 in the production of flat structures.

17. A flat structure comprising a reactive fiber-like cellulose coagulate according to claim 14 or 15.

18. The flat structure according to claim 17, wherein the reactive fiber-like cellulose coagulate is uniformly distributed within the flat structure.

19. The flat structure according to claim 17, wherein the reactive fiber-like cellulose coagulate is applied to a substrate selected from the group consisting of paper, wet nonwoven, air-laid material, textiles, fibers based on synthetic materials, biodegradable substances, cellulose, nonwoven fabric or mixtures thereof.

20. The flat structure according to claims 17 and 19, wherein the reactive fiber-like cellulose coagulate is present at at least one of the sides of the substrate.

21. The flat structure according to claim 17 to 20, wherein binder is additionally present.

22. The flat structure according to claims 17 to 21, wherein 0.5% by wt. to 60% by wt.
of reactive fiber-like cellulose coagulate, based on the total weight of the flat structure, is present.

23. The flat structure according to claims 17 to 22, wherein the flat structure is a filter, a cleaning cloth or a dust filter.

24. A method of producing a flat structure according to claim 20, wherein a reactive fiber-like cellulose coagulate according to any one of claims 14 or 15 is added in paper production, wet nonwoven production or spunlaced nonwoven fabric production.

25. The method of producing a flat structure according to any one of claims 21 to 23, wherein a slurry of reactive fiber-like cellulose coagulate according to any one of claims 14 or 15 and of water is applied to at least one of the sides of the substrate.

26. The method according to claim 25, wherein the slurry is applied by means of spraying units.